Radioisotopic Approaches to Understanding Lake Sediment History

Abstract

Radioisotopic techniques provide powerful tools for reconstructing the history of lake sediments, offering critical insights into past environmental changes and human impacts. These techniques have contributed significantly to our understanding of past environmental change and have implications for current environmental management practices. This review comprehensively examines various radiometric dating techniques used for lake sediments, with a focus on natural, cosmogenic, and artificial radionuclides, including ²¹⁰Pb, ¹³⁷Cs, ²⁴¹Am, ⁷Be, ³H, and ¹⁴C. The review highlights the widespread use of radionuclides in establishing sediment chronologies across different time scales, from short-term processes (days to decades) to long-term environmental reconstructions spanning thousands of years. Moreover, applications in limnological research are explored, including sedimentation rate estimation, reconstruction of pollution history of trace elements, nutrients, microplastics, and organic compounds, and assessment of anthropogenic impacts and catchment changes. The integration of radioisotopic methods with multiproxy paleolimnological approaches is emphasized as a powerful framework for reconstructing past environmental and ecological conditions. Despite their effectiveness, radioisotopic methods are exposed to several sources of uncertainty, including dispersion in atmospheric isotope flux, post-depositional processes, reservoir effects, and model assumptions. These challenges highlight the importance of careful methodological selection, site-specific evaluation, and rigorous uncertainty assessment in radioisotopic studies of lake sediments. Future research should emphasize refining sediment age-model calibration using region-specific sedimentation parameters and standardized validation procedures, and integrating radiometric techniques with geochemical, biological, and paleolimnological proxies to improve the reconstruction of environmental change in lacustrine systems. Such developments would enhance the interpretation of historical pollution records, sediment accumulation patterns, eutrophication history, and ecological variability, thereby providing scientifically robust information to support evidence-based lake management, restoration programs, and long-term conservation strategies.

1. Introduction

Radioisotopic techniques have become essential tools in limnological and environmental studies because they provide reliable chronological frameworks for reconstructing historical ecosystem changes and sedimentation processes in lake ecosystems. The analysis of natural and anthropogenic radionuclides, particularly ²¹⁰Pb, ¹³⁷Cs, and plutonium isotopes, has been widely applied to determine sediment accumulation rates and establish temporal records preserved in lake sediments [1]. These methods are especially valuable for assessing long-term environmental changes because lake sediments act as natural archives that preserve evidence of anthropogenic activities and natural processes over time. Consequently, radioisotopic dating has been extensively used to investigate the historical deposition of heavy metals, organic contaminants, and radioactive pollutants, as well as to evaluate variations in water quality associated with atmospheric deposition, industrialization, agricultural activities, and land-use change [1,2]. In addition, radiometric analyses contribute to understanding sediment transport dynamics, bioturbation processes, radionuclide mobility, and geological events such as floods and landslides, while also supporting the estimation of radionuclide diffusion and sediment dispersion within aquatic systems [3,4].

The widespread application of radiometric dating across diverse lake environments demonstrates the adaptability and scientific significance of these techniques under varying climatic, hydrological, and sedimentological conditions. Studies conducted in Lake Qattinah (Syria), Red Lake (Romania), and several Swiss lakes, including Brienz, Thun, Biel, and Lucerne, successfully utilized radionuclide-based chronologies to reconstruct sedimentation histories and evaluate environmental changes driven by both natural and anthropogenic influences [5,6,7].

Similarly, investigations in Lake Burullus and Lake Qarun in Egypt demonstrated the effectiveness of ²¹⁰Pb and ¹³⁷Cs techniques for assessing sediment accumulation patterns, pollution histories, and ecological degradation in semi-arid lacustrine systems [8,9,10]. Collectively, these studies highlight the importance of site-specific calibration and methodological adaptation in obtaining reliable sediment chronologies and ecosystem reconstruction, rather than merely serving as isolated regional examples.

Over the past century, combined ²¹⁰Pb and ¹³⁷Cs measurements have become among the most widely used approaches for reconstructing recent sedimentation histories and estimating sediment accumulation rates in aquatic environments [11,12,13]. The complementary behavior of these radionuclides improves chronological accuracy and enables validation of sediment dating models. Consequently, numerous studies have applied ²¹⁰Pb, ¹³⁷Cs, and plutonium isotopes not only to date sediments but also to identify pollution sources, examine sediment-source relationships, and evaluate ecosystem responses to anthropogenic disturbances [14,15,16,17,18]. These investigations have provided critical insights into the temporal evolution of environmental contamination and the dynamics of sedimentation processes in lakes worldwide [19,20,21].

In addition to their chronological applications, radioisotopes have increasingly been used as paleoenvironmental proxies to reconstruct past climatic and environmental conditions. Previous studies have demonstrated the usefulness of radiogenic isotopes in sedimentary investigations, paleoclimatic reconstructions, and the interpretation of past environmental and climatic processes [22,23,24,25]. Furthermore, paleolimnological studies based on sediment records are increasingly being used to establish baseline environmental conditions that support restoration planning and evidence-based lake management strategies [26]. For example, ²¹⁰Pb dating has proven particularly effective for reconstructing environmental changes occurring during the last two centuries, whereas plutonium isotopes have shown high reliability for dating sediments affected by atmospheric fallout from nuclear activities and weapons testing [15,17].

Over recent decades, many lakes worldwide have experienced substantial changes in sedimentation patterns and environmental quality due to increasing anthropogenic pressures and climate-related influences [27]. Therefore, understanding the spatial and temporal distribution of contaminants and sedimentary processes through radioisotopic analyses has become increasingly important for identifying the mechanisms responsible for lake degradation and distinguishing between historically accumulated sediment contamination and ongoing pollutant inputs from surrounding human activities. Such information provides a scientifically robust basis for environmental assessment, ecosystem restoration, pollution mitigation, and long-term lake conservation and management strategies.

Despite the extensive application of radionuclides in lake sediment chronology, several methodological and interpretative challenges remain unresolved. Uncertainties associated with post-depositional radionuclide mobility, sediment mixing, spatial variability in atmospheric fallout, reservoir effects, and age-depth model assumptions can significantly influence chronological accuracy and environmental interpretation. In addition, differences in lake hydrology, sedimentation regimes, and geochemical conditions complicate the direct comparison of chronologies among lacustrine systems. Although numerous studies have investigated individual radionuclides and site-specific applications, comprehensive evaluations integrating methodological limitations, uncertainty analysis, and multi-isotope approaches across diverse lake environments remain limited.

Therefore, this review aims to critically synthesize the principles, applications, advantages, and limitations of major radioisotopic dating techniques used in lake sediments, including natural, cosmogenic, and artificial radionuclides. Particular emphasis is placed on methodological uncertainties, chronological modeling approaches, historical reconstruction, and the integration of radioisotopic methods with complementary paleolimnological proxies, including geochemical indicators, diatom assemblages, pollen records, and biomarker analyses. By evaluating both the strengths and limitations of existing approaches, this review seeks to provide a more comprehensive framework for improving lake sediment chronologies and supporting future limnological and environmental research.

2. Methodology

2.1. Systematic Literature Review Design

This study employed a systematic literature review methodology grounded in the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) framework to identify, screen, appraise, and synthesize published research about radioisotopic dating techniques applied to lacustrine sediments. The adoption of the PRISMA protocol was intended to ensure transparency, methodological reproducibility, and scientific rigor throughout the review process. The scope of the review encompassed the application of natural, cosmogenic, and fallout-derived radionuclides—specifically ²¹⁰Pb, ¹³⁷Cs, ²⁴¹Am, ⁷Be, ³H, and ¹⁴C, as well as their integration within paleolimnological and multiproxy analytical frameworks for the reconstruction of environmental change and sediment accumulation histories.

2.2. Literature Search Strategy

A comprehensive database search was conducted across three major international scientific repositories: Scopus, Web of Science, and Google Scholar. These platforms were selected for their extensive multidisciplinary coverage of peer-reviewed scientific literature and their recognized capacity to facilitate access to high-quality publications in geochronology, limnology, environmental radioactivity, and sedimentology. The temporal scope of the search spanned the period from 1963 to 2026, corresponding to the principal era of global atmospheric fallout radionuclide deposition associated with nuclear weapons testing programs and subsequent anthropogenic nuclear events, including the Chornobyl (1986) and Fukushima Daiichi (2011) nuclear disasters.

The search strategy was constructed using combinations of discipline-specific keywords and Boolean operators, including: “²¹⁰Pb dating,” “¹³⁷Cs chronology,” “²⁴¹Am,” “fallout radionuclides,” “lake sediments,” “sediment accumulation rates,” “paleolimnology,” “radioisotope geochronology,” “sediment core analysis,” and “environmental reconstruction.” Duplicate records retrieved across multiple databases were systematically identified and removed prior to the screening stage.

Eligibility criteria were defined a priori to ensure methodological consistency. Studies were considered eligible for inclusion if they constituted peer-reviewed articles reporting on the application of radionuclide-based dating techniques to lacustrine, lagoonal, reservoir, or wetland sediment archives, including associated paleolimnological analyses. Studies were excluded from consideration under the following conditions: primary focus on heavy metal contamination without geochronological interpretation; non-peer-reviewed document types (e.g., technical reports, book chapters without rigorous review); conference abstracts; and studies lacking sufficient methodological relevance to sediment chronology.

The systematic literature search and screening process followed the PRISMA 2020 guidelines (Figure 1), encompassing four sequential stages: identification, screening, eligibility assessment, and final inclusion. An initial corpus of 180 records was retrieved through structured database searches conducted across Web of Science, Scopus, PubMed, ScienceDirect, and Google Scholar. After removing five duplicate entries, 175 unique records proceeded to the title and abstract screening stage. Of these, 157 articles satisfied the preliminary inclusion criteria and were advanced to full-text eligibility assessment based on demonstrated relevance to radioisotopic dating methodologies and lacustrine sediment chronology. During the eligibility assessment stage, 35 records were excluded for the following reasons: irrelevance to the review scope (n = 17), insufficient quantitative or methodological data (n = 12), inappropriate study design or analytical approach (n = 4), and absence of a lake sediment focus (n = 2). Ultimately, 122 peer-reviewed studies satisfied all predefined inclusion criteria and were incorporated into the qualitative synthesis.

Figure 1. PRISMA flow diagram showing the literature search, screening, eligibility assessment, and selection of studies included in this review of using radioisotopes for lake sediment dating.

The included studies were classified according to the primary radioisotopic method employed. The largest proportion utilized ²¹⁰Pb dating and associated models (n = 49; 40.2%), followed by ¹³⁷Cs dating and fallout marker analysis (n = 29; 23.8%), ¹⁴C dating (n = 16; 13.1%), ²⁴¹Am dating (n = 10; 8.2%), other radioisotopes and combined multi-proxy approaches (n = 8; 6.6%), ⁷Be tracing (n = 6; 4.9%), and ³H (Tritium) dating (n = 4; 3.3%). This structured and transparent review methodology facilitated a rigorous, reproducible evaluation of methodological advancements, practical field applications, principal sources of analytical uncertainty, and the broader environmental implications of applying radioisotopic techniques in limnological and paleolimnological research.

3. Principles of Radioisotopic Dating

Radioisotopes are isotopes of an element with unstable nuclei that release excess energy by emitting radiation as alpha, beta, and gamma rays. Radioactivity decays over time, and scientists use the term “half-life” to measure this decay. The half-life of a radioisotope (shown in Figure 2) is the time required for half of the radioactive atoms present to decay. The half-lives of various radioisotopes vary, ranging from a fraction of a second to millions of years. For instance, radon-220 has a very short half-life, while thorium-232 has a half-life of millions of years.

Figure 2. Half-life of cobalt-60 [28].

Radionuclides with different half-lives can originate from various source materials, making them ideal tools for tracing geochemical processes, determining rock ages, and constructing the chronology of sediment profiles [29]. The usefulness of radioactive decay in geochronology arises from the ability to detect either the accumulation of decay products or the decay of parent radioactive isotopes.

Radionuclide dating techniques rely on naturally occurring radionuclides that decay into stable daughter nuclides. First of all, radioactive decay processes are not caused by chemical interactions between the presence or absence of other atoms. On the other hand, decay is an ongoing phenomenon with a constant rate of occurrence per unit of time [30]. The dating of sediments using radiometric methods is the only widely applicable technique that provides absolute age estimates and has greatly enhanced research on sedimentary processes [31]. Numerous radiometric dating techniques, including Re–Os, Rb–Sr, K–Ar, U–Pb, and Sm–Nd, are frequently employed to determine the ages of minerals and rocks over long time spans [32].

Furthermore, throughout the Quaternary Period, similar techniques have been used to date creation or deposition in a variety of natural archives, including foraminiferal tests, sediments, corals, speleothems, and ice cores [33]. The main chronological techniques used in environmental research over the most recent geological period, which began 11,700 years ago, are summarised and briefly described in Figure 3. The figure highlights radionuclides applicable for chronological reconstruction and their corresponding age ranges.

Figure 3. Overview of radiometric dating methods and their corresponding age ranges for chronological applications (modified from [30]).

4. Radiometric Dating Methods in Lake Sediments

Radionuclide dating techniques use the nuclear decay of radioactive elements to calculate the ages of minerals, sediments, and rocks [34]. These methods can be divided into three categories: natural radionuclides, cosmogenic radionuclides, and artificial radionuclides that emerged during the atomic age (post-1945). Figure 4 and Table 1 present chronologic techniques and their corresponding age ranges, which are widely applied in marine sediment research to determine the timing of depositional events and environmental changes. These methods include several radiometric and stratigraphic approaches, each suitable for specific time scales depending on the isotopes’ half-lives or the nature of the sedimentary record. By selecting appropriate dating techniques, researchers can establish reliable age models for marine sediment cores and reconstruct past oceanographic and climatic conditions [35,36].

Figure 4. Chronological techniques and age ranges used in marine studies (modified after [35,36]).

Table 1. Radioisotopic dating methods in lake sediments: materials, age ranges, and typical applications.

Radioisotope	Material Dated	Approximate Dating Range	Typical Use in Lake Sediments
7Be (Beryllium-7)	Surface sediment	Days–~1 year	Tracing very recent sediment deposition and mixing
137Cs (Cesium-137)	Sediment particles	~1950s–present (~70 years)	Identifying nuclear fallout markers (1963 peak, 1986 Chernobyl)
210Pb (Lead-210)	Fine sediments, organic matter	~0–150 years	Determining recent sediment accumulation rates
239+240Pu (Plutonium isotopes)	Sediment particles	~1950s–present	Nuclear weapons testing fallout chronostratigraphy
241Am (Americium-241)	Sediment	~1960s–present	Additional fallout marker linked to 239Pu
14C (Radiocarbon)	Organic matter, plant macrofossils, carbonates	~300–50,000 years	Long-term lake sediment chronology
234Th (Thorium-234)	Suspended or very recent sediments	Days–months	Short-term sediment flux and particle dynamics

4.1. Natural Radionuclides

Isotopic disequilibrium within the decay series, arising from the contrasting geochemical properties of constituent isotopes, forms the basis for several dating methods. Thompson, 2007 and Dickin, 2018 [37,38] discussed methods employing the ²³⁸U and ²³⁵U decay series, where chronologies are established through isotopic excesses or deficiencies. In younger carbonate sediments (less than 104 years old), the ²³⁴U(²³⁰Th)−²²⁶Ra_ex system was applied. This system, as highlighted by [39], assumes that only uranium and radium coprecipitate during mineralization. Previous applications of the ²²⁶Ra_ex chronology in sediment dating have been reported by [40,41,42]. Additionally, this group of methods includes the ²¹⁰Pb_ex chronology, which utilizes a naturally occurring isotope (half-life ~22.3 years; [43]) derived from the uranium-238 decay series. [44,45] Krishnaswamy, 1971 and Persson and Holm, 2011 have employed this method for dating lake sediments.

Similarly, radioactive disequilibria within the Thorium-232 decay series offer a chronological tool applicable to a range of recent biological and geological materials. The activity ratio of thorium-228 to radium-228 has been utilized to investigate growth rates in marine bivalve shells from hydrothermal vents [46] and the lifespans of specific plant tissues [47] and has been suggested to estimate the time elapsed since death in bone tissue [48]. The chronology of young marine sediments can be constrained using the Th-228/Th-232 activity ratio [40], in addition to analyses of hydrothermal minerals from mid-ocean ridge chimney systems [49,50] and geothermal precipitates [51]. Moreover, excess Th-228 in surface sediments of the China Sea [52] has been utilized to quantify particle mixing rates within ecological zones of the northeastern tropical Atlantic [53].

The principle of the ²¹⁰Pb method is to quantify the disequilibrium in sediments between ²¹⁰Pb and its parent isotope, ²²⁶Ra. A portion of the measured ²¹⁰Pb originates from the ongoing decay of ²²⁶Ra and is classified as ‘supported’ ²¹⁰Pb, exhibiting constant activity over time. A further component is produced by the decay of atmospheric ²²²Rn to ²¹⁰Pb, known as ‘unsupported’ or ‘excess’ ²¹⁰Pb. Once deposited on land or water surfaces, it is quickly eliminated from the water column by scavenging mechanisms [54,55]. This removal process occurs through adsorption onto suspended particles, which can subsequently become embedded within the Sediment (Figure 5). An analysis of the lowermost sediment layer revealed the presence of both supported and unsupported ²¹⁰Pb. Sedimentation rates over the past century have been determined by a well-established method that analyzes sediment core samples for ²¹⁰Pb and ¹³⁷Cs [12,13]. Importantly, these radioactive isotopes can also be used to track sediment movement across different bodies of water, from oceans [11] to lakes and rivers [56,57]. Researchers have established three primary dating models for analyzing ²¹⁰Pb_ex [58]. These models account for the integrated effects of anthropogenic activities, geological processes, and sediment dynamics. The three models are constant-flux constant-sedimentation (CF: CS) [44,59], constant initial concentration (CIC) [60], and constant rate supply (CRS) [61]. These models rely on two key assumptions: (i) that atmospheric fallout is the primary source of ²¹⁰Pb_ex and (ii) that the contribution of eroded sediment containing ²¹⁰Pb_ex to the flux is negligible [62].

Figure 5. The principle of ²¹⁰Pb dating (a) [32] and (b) [30].

4.2. Cosmogenic Radionuclides

4.2.1. Radiocarbon (¹⁴C)

Radiocarbon (¹⁴C) is a radioactive form of carbon that occurs naturally in the atmosphere through the influence of secondary cosmic rays, as shown in Figure 6. In 1940, Martin Kamen and Sam Ruben, two American physicists, made groundbreaking discoveries by identifying the long-lived radioisotope ¹⁴C, which has a half-life of 5730 ± 40 years. They harnessed their potential as tracers in biological systems, revolutionizing the field [32]. The discovery of ¹⁴C and the subsequent establishment of the current radiocarbon dating method were the culmination of collaborative efforts across multiple disciplines during the twentieth century [63]. The immediate applicability and success of the method stem from two main factors. First, carbon-14 is naturally incorporated into all carbon-based materials, whether organic or inorganic, as part of the global carbon cycle. Second, its long half-life of 5730 years enables dating a wide range of materials formed over the past 55,000 years. Radiocarbon dating has been instrumental in fields such as anthropology, archaeology, paleoclimatic research, environmental science, biomedicine, cultural heritage management, art authentication, and forensic investigations. For example, carbon can reside in the deep ocean for extended periods, allowing radioactive decay to occur and producing apparent ages exceeding 2000 years in the deep North Pacific. Ocean circulation processes, including upwelling and lateral transport, affect the ¹⁴C content of surface waters, while gradual exchange with the atmosphere results in an average surface ocean age of approximately 400–500 years [64].

Figure 6. Radiocarbon dating method [32].

Radiocarbon (¹⁴C) dating of lake sediments is commonly based on the assumption that the measured ¹⁴C age reflects the timing of sediment deposition [65]. However, this assumption is often complicated by the heterogeneous composition of sediments, which typically consist of complex mixtures containing limited amounts of organic carbon derived from multiple sources [65]. Several studies have identified key sources of error associated with this method. For instance, the incorporation of “fossil” or old carbon (e.g., graphite or lignite) into younger sediments can significantly bias age estimates, particularly in low-carbon environments [66]. In addition, the reworking and transport of older terrestrial materials into lacustrine systems can result in anomalously old radiocarbon ages [67]. Furthermore, autochthonous organic matter produced within lakes may assimilate dissolved inorganic carbon derived from the dissolution of ancient carbonates, thereby introducing additional age offsets [65].

Despite these limitations, bulk-sediment ¹⁴C dating remains a widely used geochronological tool, especially in settings where terrestrial macrofossils are scarce. Alonso-Hernandez et al., 2006 [13] noted that, to address systematic offsets, some studies apply a reservoir age correction, typically estimated from surface sediment dating or extrapolated from down-core age trends. However, evidence suggests that reservoir ages may vary over time, and assuming a constant correction can introduce additional uncertainties [68].

To improve the reliability of bulk-sediment ¹⁴C dating, several methodological approaches have been developed. One such approach involves the thermal fractionation of organic carbon based on its stability. Reference [69] McGeehin et al., 2001 introduced a stepped-combustion technique that allows selective oxidation of younger, more labile carbon fractions at relatively low temperatures (approximately 400 °C). This method has subsequently been applied in other studies (e.g., [70,71]), demonstrating that low-temperature combustion can reduce the influence of older, refractory carbon and yield ages that more closely approximate the true depositional age. Nevertheless, even these refined techniques may still yield maximum-age estimates, as the complete exclusion of older carbon components remains challenging.

4.2.2. Beryllium-7 (⁷Be) and Tritium (³H)

⁷Be is a radioactive isotope with a short half-life that originates in the Earth’s stratosphere, where cosmic rays split nitrogen and oxygen atoms, creating ⁷Be [72]. This isotope, with a half-life of approximately 53.22 days, preferentially binds to airborne particles. Precipitation serves as the primary removal mechanism for ⁷Be from the atmosphere [73]. As shown in Figure 7, this process is critical for geochronological investigations, such as calculating soil erosion rates [74] and sedimentation processes associated with episodic severe rainfall events [75]. In oceanographic investigations, ⁷Be is commonly used as a tracer to analyze water flow and mixing in the surface ocean. The presence of ⁷Be in young sediments [76] enables researchers to study mixing mechanisms, sediment dynamics, and accumulation patterns [77,78].

The cosmogenic radioisotope ³H has a half-life of 12.32 years, with an uncertainty of 2 years [79]. Tritium and oxygen combine to form water molecules, which precipitate onto Earth’s surface. Because of its unique properties, tritium is an important tracer for studying ocean circulation patterns [80].

Figure 7. Schematic diagram of ⁷Be production and fallout [81].

Figure 7. Schematic diagram of ⁷Be production and fallout [81].

4.3. Artificial Radionuclides

Cesium-137 (¹³⁷Cs), a common fission byproduct with a half-life of approximately 30.05 years, serves as a valuable chronological tool (chronometric) in modern sediment studies. Nuclear bomb testing in the 1950s released ¹³⁷Cs globally, establishing a distinct reference layer (initial appearance horizon) in undisturbed sediments corresponding to that period [82]. Peak deposition is estimated at approximately 1963, as exemplified in Figure 8 for the Aztec Reservoir, New Mexico [83]. Subsequent significant releases occurred following the Chornobyl nuclear disaster in May 1986 and the Fukushima Daiichi nuclear incident in March 2011. A study by [84] documented that Cesium-134 (¹³⁴Cs) had reached a depth of around 10 cm in a sediment core from Lake Sniardwy, Poland, within 4 months following the Chornobyl fallout. Furthermore, ¹³⁴Cs was detected at depths of 14–24 cm in cores collected from distinct locations 1 year later. Ref. [85] Holby and Evans,1996 reported similar findings of extensive Cesium isotope infiltration within Baltic Sea sediments, attributing this phenomenon to a mechanism other than bioturbation or diffusion.

Figure 8. ¹³⁷Cs age dating of Aztec reservoir, New Mexico, USA (known dates: 1947, 1963, and 2017) [83].

In the same context, strontium-90 (⁹⁰Sr) is a highly abundant long-lived radionuclide in global radioactive fallout from nuclear weapon testing in open environments. It has a half-life of 28.8 years and is widely used in radiometric dating. From 1953 to 1964, the Great Lakes region received strontium-90 deposits from fallout and precipitation resulting from nuclear testing. Nonetheless, aboveground nuclear weapon testing ceased after 1964, leading to a decrease in the annual deposition of Sr-90 in the region [30]. Americium-241 (²⁴¹Am), characterized by a half-life of approximately 432.6 years, originates primarily from the radioactive decay of Plutonium-241 (²⁴¹Pu), which is found in nuclear fallout. Both low-energy gamma-ray and alpha-particle spectrometry techniques are suitable for its analysis. Research by [86] indicates that ²⁴¹Am exhibits lower mobility within lake sediments than cesium-137 (¹³⁷Cs). This property makes it a valuable chronometer, particularly for the period following the early 1960s, when the reliability of ¹³⁷Cs as a dating tool diminished. Ref. [87] Aston and Stanners, 1981 reported the efficient removal of americium from seawater, resulting in its deposition in sediments. The successful application of ²⁴¹Am as a complementary chronometer for validating chronologies established using lead-210 (²¹⁰Pb) has been documented in several studies [86,88,89].

5. Recent Lake Sediment Chronology Using Fallout Radionuclides (²¹⁰Pb, ¹³⁷Cs, and ²⁴¹Am)

5.1. ²¹⁰Pb Numerical Models

The initial application of the ²¹⁰Pb method to lacustrine sediments for reconstructing depositional history and assessing contaminant input and burial was conducted by [44]. This method has proven to be a valuable tool for dating recent sedimentary deposits spanning up to approximately a century and has quickly gained widespread acceptance. Currently, ²¹⁰Pb dating is commonly employed as one of several geochronometric techniques in most oceanographic or limnological studies of recent sedimentation [90,91]. Each dating model relies on environmental assumptions concerning the flux of excess ²¹⁰Pb at the sediment–water interface and the sedimentation rate. Specifically, these models generally assume that: (1) the processes controlling ²¹⁰Pb flux to the interface (e.g., atmospheric fallout, focusing, weather, and ocean circulation) have remained constant over the past century; (2) sedimentation processes (such as suspended matter sources, particle scavenging, and settling) have been stable during this period; and (3) ²¹⁰Pb activity decreases over time according to radioactive decay and is not altered by post-depositional redistribution [92]. These models are typically used to (i) establish an age–depth relationship, where age (t) is expressed as a function of depth (z), and (ii) estimate accumulation rates, including sediment accumulation rate (SAR, si) and mass accumulation rate (MAR, ri).

5.1.1. Constant Initial Concentration (CIC) Model

The constant initial concentration (CIC) model assumes a constant initial activity of unsupported ²¹⁰Pb (²¹⁰Pb_ex) regardless of fluctuations in sediment accumulation rate [58,60]. However, these ideal conditions are uncommon, and most sediment environments experience variations in accumulation rates driven by natural or human factors. Modified CIC models can be employed under specific circumstances [88,93,94]. Nonetheless, the standard CIC model remains valid only if two criteria are met: (i) unsupported ²¹⁰Pb activity decreases continuously with increasing depth, and (ii) the total inventories of ²¹⁰Pb (²¹⁰Pb_ex) in sediment cores from the same lake are roughly proportional to the average sedimentation rates within each core [59]. Age determination can be expressed as

$$t_i(yr.)={\lambda }^{-1}\mathit{ln}\left(C_0/C_i\right)$$

(1)

where $\mathsf{\lambda }=\ln 2/{\mathrm{T}}_{1/2}$ represents the radioactive decay constant of ²¹⁰Pb; C_i and C₀ denote the excess ²¹⁰Pb concentrations at layer i and at the sediment surface, respectively. Estimation of accumulation rates (s_i;, r_i;) requires determining layer ages.

The mean sediment accumulation rate (s_i, cm yr⁻¹):

$$s_i=\left[z\left(i+1\right)-z\left(i\right)\right]/{∆t}_i$$

(2)

where z(i) is the depth of the layer experimentally determined.

The mean mass accumulation rate (r_i, g cm ⁻² yr ⁻¹) is defined as

$$r_i=\left[m\left(i+1\right)-m\left(i\right)\right]/{∆t}_i$$

(3)

where m (i) is the mass depth of layer (i) represented by

$$m\left(i\right)(kg m^{-2})={\displaystyle {\sum }_{j=1}^{j=i}}(∆m_i/S^{/})$$

(4)

where $∆m_i$ is the dry mass of section i (kg) and S^/ is the cross-sectional area of the core.

Calculating uncertainty (u) in t_i, s_i, r_i, m(i), $\Delta t_i$, and t(i) are calculated according to the equations presented by [62]:

$${u(t}_i)={\lambda }^{-1}\sqrt{\left({\left(u\left(\lambda \right)t_i\right)}^2+{\left({\displaystyle \frac{u\left(c_0\right)}{c_0}}\right)}^2+{\left({\displaystyle \frac{u\left(c_i\right)}{c_i}}\right)}^2\right)}$$

(5)

$$u\left(s\right)=s\left[u(∆t)/∆t\right]$$

(6)

$$u\left(r\right)=r\left[u(∆t)/∆t\right]$$

(7)

5.1.2. Constant Rate of Supply (CRS) Model

A key assumption for the constant rate of supply (CRS) model is a steady flux of ²¹⁰Pb to the sediment, while the rate of sediment accumulation can fluctuate [60]. The ²¹⁰Pb (unsupported) integrated through the sediment core’s mass-depth profile aligns with the constant-flux integrated over the corresponding timescale. The accuracy of the CRS model hinges on a reliable estimate of the total unsupported ²¹⁰Pb inventory, which must be in equilibrium with atmospheric inputs in each sediment core. It is based on comparing ²¹⁰Pb_ex inventories below a given depth with the overall ²¹⁰Pb_ex inventory in the sediment core. According to the CRS model, layer age (t_i) at depth i, MAR (r, g cm⁻¹ yr⁻¹), and SAR (s, cm yr⁻¹) can be estimated using the following equations [95]:

$$t\left[\left(i\right),yr\right]={\displaystyle \frac{1}{\lambda }}\mathit{ln}{\displaystyle \frac{\mathrm{A}\left(0\right)}{\mathrm{A}\left(\mathrm{i}\right)}}$$

(8)

$$\mathrm{r} (i)={\displaystyle \frac{\lambda \mathrm{A}\left(\mathrm{i}\right)}{C\left(i\right)}}$$

(9)

$$s\left(i\right)={\displaystyle \frac{r\left(\mathrm{i}\right)}{\rho \left(i\right)}}\ast 100$$

(10)

where A(i) is the accumulated ²¹⁰Pb_ex inventory below layer (i), C(i) is the ²¹⁰Pb_ex concentration in layer (i) and $\mathsf{\rho }\left(\mathrm{i}\right)$ is the mean dry bulk density of a certain layer, $∆{\mathrm{z}}_{\mathrm{i}}$ is the width of section i (m).

The accumulated ²¹⁰Pb_ex inventory below layer (i) is represented by

$$A\left(i\right)={\displaystyle {\sum }_{j=i+1}^{j=\infty }}∆A_j$$

(11)

$∆{\mathrm{A}}_{\mathrm{i}}$ An accumulated ²¹⁰Pb_ex deposit below section (i) and given by

$$∆A_i=C_i[∆m_i/S^{/}]$$

(12)

The equations below [62] were used to calculate the uncertainties (u) associated with t_i, s_i, r_i, and $∆{\mathrm{A}}_{\mathrm{i}}$:

$$\mathrm{u}(\mathrm{t}(\mathrm{i}))={\lambda }^{-1}\sqrt{\left({\left(u\left(\lambda \right)t\left(i\right)\right)}^2+{\left({\displaystyle \frac{u\left(A\right(0\left)\right)}{A\left(0\right)}}\right)}^2+\left[\left(1-{\displaystyle \frac{2A\left(i\right)}{A\left(0\right)}}\right){\left({\displaystyle \frac{uA\left(i\right)}{A\left(i\right)}}\right)}^2\right]\right)}$$

(13)

$$u (r(i))=r\left(i\right)\sqrt{\left({\left({\displaystyle \frac{u\left(\lambda \right)}{\lambda }}\right)}^2+{\left({\displaystyle \frac{u\left(A\left(i\right)\right)}{A\left(i\right)}}\right)}^2+{\left({\displaystyle \frac{u\left(C\left(i\right)\right)}{C\left(i\right)}}\right)}^2\right)}$$

(14)

$$\mathrm{u}(s(\mathrm{i}))=s(i)\sqrt{\left({\left({\displaystyle \frac{u\left(r\right(i\left)\right)}{r\left(i\right)}}\right)}^2+{\left({\displaystyle \frac{u\left(\rho \right(i\left)\right)}{\rho \left(i\right)}}\right)}^2\right)}$$

(15)

$$u (∆(A_i))=A_i\sqrt{{\left(u\left(c_i\right)/c_i\right)}^2+{\left(u({\displaystyle \frac{∆m_i}{S^{/}}})/{\displaystyle \frac{∆m_i}{S^{/}}}\right)}^2}$$

(16)

²¹⁰Pb_ex fluxes can be calculated by the following equation:

$$\mathrm{F}=\lambda A_0$$

(17)

where A₀ is the total inventory of ²¹⁰Pb_ex.

5.1.3. Constant Flux Constant Sedimentation Rate (CFCS) Model

The constant flux, constant sedimentation (CFCS) model assumes a static environment with minimal sediment disturbance. This is achieved through mixing and lead (Pb) diffusion within the sediment layer [44,96]. The model hinges on two key conditions: consistent atmospheric deposition of a specific lead isotope (²¹⁰Pb_ex) and unwavering sediment accumulation rates. These requirements render the CFCS model most applicable to environments with remarkable stability, where the influence of external factors is minimal [58,97]. Studies have demonstrated the effectiveness of the CFCS model in determining reliable sediment ages for lakes exhibiting constant sedimentation rates and consistent ²¹⁰Pb_ex fluxes [98,99]. The CF–CS model assumes a constant flux to the sediment surface and a constant mass accumulation rate [59], which implies that the concentration C_i should decrease exponentially with depth. The mass accumulation rate (MAR) can be determined by performing a linear regression [100] of ln Ci against the mass depth mi using the following equation:

$$r = -{\displaystyle \frac{\lambda }{slope }}$$

(18)

$$u(r)=r\sqrt{\left({\left(u\left(\lambda \right)/\lambda \right)}^2+{\left(u\left(slope\right)/slope\right)}^2\right)}$$

(19)

We can also determine sediment accumulation rate SAR (s, cm yr⁻¹) from a linear regression analysis by using the section depth (z_i) instead of mass depth (m_i) [62,101]:

$$s=-{\displaystyle \frac{\lambda }{slope}}$$

(20)

$$u(s)=s\sqrt{\left({\left(u\left(\lambda \right)/\lambda \right)}^2+{\left(u\left(slope\right)/slope\right)}^2\right)}$$

(21)

Using the CF-CS model, the sediment age t at a depth x can be calculated using the equation:

$$t={\displaystyle \frac{m_i}{r}}$$

(22)

$$u(t)= t\sqrt{\left({\left(u\left(m_i\right)/m_i\right)}^2+{\left(u\left(r\right)/r\right)}^2\right)}$$

(23)

where $m_i$ is cumulative mass depth for section i.

5.2. ¹³⁷Cs Method

The artificial radioisotope Cesium-137 (¹³⁷Cs) serves as a well-established marker for dating recent sediments. Produced through nuclear fission, it was extensively released into the environment during atmospheric nuclear weapon testing programmes of the 1950s and 1960s, with a peak in fallout occurring in approximately 1963 [102]. This historical release is reflected in the environmental record by a three-peak pattern of ¹³⁷Cs activity: (a) an initial peak corresponding to the aforementioned weapon testing period, with a maximum at approximately 1963 [103,104,105]; (b) a subsequent peak attributable to the Chernobyl nuclear accident of 1986; and (c) the most recent peak associated with the 2011 Fukushima Daiichi nuclear disaster [106]. As an example, the radiocaesium profile measured in a sediment core retrieved from the southern basin of Loch Lomond, UK (Figure 9) [94,107,108] clearly demonstrates this pattern: an initial appearance of ¹³⁷Cs in approximately 1954 (at a depth of 9.5–10 cm), a peak in activity in approximately 1964 (at 5.7–6.0 cm) corresponding to weapons testing fallout, and a final peak at a depth of 1.8–2.1 cm attributable to the Chernobyl accident [107].

Figure 9. ¹³⁷Cs profiles for the Loch Lomond sediment core [94,107,108].

Several limitations restrict the application of the ¹³⁷Cs dating method:

• Temporal Scope: This technique is inapplicable to studies focusing on the 1950s, which predates the significant fallout events used as references.
• Geographic Bias: Uneven atmospheric circulation patterns and the placement of test locations during the nuclear era resulted in a pronounced disparity in ¹³⁷Cs deposition between the Northern and Southern Hemispheres.

Concentrations in the Northern Hemisphere are roughly three times those in the Southern Hemisphere, hindering accurate dating in the latter region [109]. The concentration of ¹³⁷Cs is corroborated by research across seven wetland sites in the United States [110], suggesting the potential obsolescence of ¹³⁷Cs dating for wetland and sediment studies in this region. However, exceptions may exist in areas with exceptionally high ¹³⁷Cs deposition or where the Chornobyl fallout peak is identifiable [111,112].

• Stand-Alone Usage: Sole reliance on ¹³⁷Cs dating has long been discouraged by the scientific community [113], and its current use as a sole technique is no longer considered acceptable.
• Combined Dating Challenges: Even when employed in conjunction with ²¹⁰Pb dating, this approach may yield ambiguous results due to frequent inconsistencies between the two methods in establishing a definitive chronology.

When the assumptions underlying the constant rate of supply (CRS) model are not satisfied, an alternative approach, the composite CRS (C-CRS) or correct CRS model, can be employed. This method, outlined by [9], utilizes independent age markers, such as cesium-137 (¹³⁷Cs), to address these limitations. The C-CRS model is particularly advantageous when calculations based on the standard CRS model yield significant discrepancies, especially within sediment depth layers. These discrepancies can lead to an overestimation of the age of dated sediments when the CRS model is applied with a fixed time interval [9]. In other words, the C-CRS model addresses the issue of inaccurate age determination arising from inconsistencies in the standard CRS model calculations, particularly at specific sediment depths. As described by [9], the C-CRS model addresses this challenge by estimating a constant flux rate (P) of ²¹⁰Pb within a defined section between two known dates (t₁ and t₂):

$$\mathrm{P}={\displaystyle \frac{\mathrm{ʎ}∆\mathrm{A}}{{\mathrm{e}}^{-\mathrm{ʎ}{\mathrm{t}}_1}-{\mathrm{e}}^{-\mathrm{ʎ}{\mathrm{t}}_2}}}$$

(24)

where the total inventory of ²¹⁰Pb_ex between the reference depths x₁ and x₂, corresponding to the ages t₁ and t₂ (t₁ is the sampling time and t₂ is the marker of Cs-137) is denoted as ∆A. The age t at any depth x between x₁ and x₂ can be determined by solving Equation [24] as

$$\mathrm{t}={\displaystyle \frac{-1}{\mathrm{ʎ}}}\mathrm{l}\mathrm{n}({\mathrm{e}}^{-\mathrm{ʎ}{\mathrm{t}}_1}-∆{\mathrm{A}}_{\mathrm{X}1-\mathrm{X}}{\displaystyle \frac{\mathrm{ʎ}}{\mathrm{P}}})$$

(25)

where ∆A_x1−x is the total inventory of ²¹⁰Pb_ex between depths x₁ and x. The sedimentation rate r of the ith layer at time t can be calculated using the following formula:

$$\mathrm{r}={\displaystyle \frac{{\mathrm{P}\mathrm{e}}^{-\mathrm{ʎ}\mathrm{t}}}{{\mathrm{C}}_{\mathrm{i}}}}$$

(26)

5.3. ²⁴¹Am Method

The presence of Americium-241 (²⁴¹Am), a byproduct of nuclear weapon testing, has been confirmed in sediment cores. This identification is based on the characteristic gamma radiation emitted by ²⁴¹Am at 59.5 keV, as measured with low background counting systems. Recent research on sediments from lakes with varying pH levels indicates that ²⁴¹Am exhibits lower mobility than cesium-137 (¹³⁷Cs). This property makes ²⁴¹Am a more reliable tool for dating sediments originating from the early 1960s, a period where ¹³⁷Cs records may be unreliable. Furthermore, ²⁴¹Am can be employed to validate the accuracy of ¹³⁷Cs-based dating methods, particularly in instances where sediment profiles have been disturbed. A comprehensive record of ²⁴¹Am deposition up to 1987 is documented by [114]. Although ²⁴¹Am exhibits lower activity levels than ¹³⁷Cs, its distribution patterns within sediment cores provide a more accurate marker for the peak period of radioactive fallout, as demonstrated by [86].

The anthropogenic radionuclide ¹³⁷Cs, produced during nuclear fission and released mainly through atmospheric nuclear weapons testing in the 1950s–1960s with a global maximum in 1963, as well as through nuclear accidents such as Chornobyl and Fukushima, is widely used as a chronostratigraphic marker in sediment studies [102,115]. In combination with ²¹⁰Pb dating, ¹³⁷Cs provides an important independent time marker that improves sediment chronology, particularly for the last ~60 years, as confirmed by interlaboratory comparisons of ²¹⁰Pb methods [116].

Multiple studies demonstrate both the strengths and limitations of ¹³⁷Cs stratigraphy in different environmental settings. In Lake Bosten (China), combined ²¹⁰Pb and ¹³⁷Cs dating revealed long-term hydrological variability driven by climatic and anthropogenic influences [117], while Yangtze River basin lakes showed increased post-1950s sedimentation rates consistent with nuclear fallout signals [118]. In North Indian lakes, ¹³⁷Cs profiles generally follow the expected fallout sequence (early 1950s onset, late 1950s secondary peak, and dominant 1963–64 maximum), although early peaks may be missing due to high sedimentation rates or core limitations [119].

However, several studies highlight complications arising from post-depositional redistribution and overlapping fallout sources. In European mountain lakes such as Nižné Terianske Pleso, very high surficial ¹³⁷Cs concentrations indicate dominance of Chornobyl-derived fallout (1986), which likely obscured the original weapons-testing signal due to downward migration processes [9]. In the same core, elevated ²⁴¹Am activity between 0.2 and 1.0 cm identifies sediments corresponding to peak atmospheric weapons testing in the 1960s, providing a more stable marker of historical fallout. Similarly, in Redó Lake, a skewed ¹³⁷Cs peak centered at ~1 cm depth reflects Chornobyl fallout, while deeper sediments (0.8–2.5 cm) containing both ¹³⁷Cs and ²⁴¹Am record partially overprinted signals from weapons fallout, limiting the precise identification of the 1963 horizon.

In contrast, studies from southern Chile demonstrate more severe limitations, where no well-defined ¹³⁷Cs peak was observed despite its use as a chronological constraint. In these systems, ²⁴¹Am was detected only in low-energy distal sediments and confirmed a mid-1960s fallout signal, highlighting its greater stability under low sedimentation conditions [120].

Table 2 synthesizes the comparative performance of the CIC, CRS, and CFCS ²¹⁰Pb dating models across a wide range of lacustrine environments, including natural, volcanic, deltaic, reservoir, and anthropogenically impacted systems. The compiled studies show sedimentary conditions, hydrodynamic variability, and the availability of independent validation markers such as ¹³⁷Cs and ²⁴¹Am strongly control the model performance. Overall, the CRS model consistently demonstrates the highest and most stable reliability across most environments, particularly in systems with variable sedimentation rates, where reported age errors are typically ~5–20% and may improve to ~5–10% when constrained by independent stratigraphic markers [9,10,121,122,123,124,125]. In contrast, the CIC model shows more variable performance, with moderate reliability under stable conditions but substantially higher uncertainty in disturbed or dynamic environments, where age errors commonly range from ~10–45% or more due to sensitivity to changes in sedimentation and initial radionuclide concentration assumptions [10,61,121,125]. The CFCS model exhibits the least applicability, with moderate performance only in select environments and typical uncertainties of ~20–40%, reflecting its restrictive assumptions of constant flux and constant sedimentation rate [120]. Across all datasets, the best agreement between modeled ages and independent ¹³⁷Cs/²¹⁰Pb markers is observed in stable, stratified lakes, whereas the largest discrepancies occur in anthropogenically influenced, volcanic, or high-energy sedimentary systems. Overall, the synthesis confirms that CRS is the most robust and widely applicable ²¹⁰Pb dating approach, whereas CIC and CFCS are more conditionally reliable and strongly dependent on environmental stability.

Table 2. Comparative performance of ²¹⁰Pb dating models (CIC, CRS, and CF-CS) across contrasting lacustrine environments: quantitative age error ranges in different countries.

Study	Environment Type	Validation Marker	CIC Reliability	CRS Reliability	CFCS Reliability	Quantitative Performance Range (Age Error vs.137Cs/210Pb)	References
North Indian lakes, India	Natural and artificial lakes	137Cs (1963 peak)	-	⭐ Mod–High	⭐ Mod–High	-	[119]
European mountain lakes	Mountain lakes	137Cs and 241Am + varves	❌ Low	⭐ High	-	CIC: 10–45%, CRS: 5–15%	[9]
Chilean lakes, South America	Volcanic lakes	137Cs, 241Am	-	-	⚠ Moderate	CFCS: 20–40%	[120]
Canadian lakes	lacustrine sediment	137Cs + historical markers	⚠ Moderate	⭐ Mod–High	-	CIC: 10–45%, CRS: 5–20%	[121]
Mining-impacted lake, USA	Disturbed lake	137Cs	❌ Low	⚠ Moderate	-	CIC: 25–50%, CRS: 10–25%	[61]
St. Ana Lake (Romania)	Temperate stratified lake	137Cs -	-	⭐ High	-	CRS: 5–12%	[123]
Shinji-ko & Nakaumi, Japan	Coastal/brackish lakes	137Cs + historical records	⚠ Moderate	⚠ Moderate	⚠ Moderate	-	[126]
Apipucos Reservoir, Brazil	Reservoir	-	⚠ Moderate	⭐ High	-	CIC: 15–30%, CRS: 5–15%	[127]
Danube Delta lakes, Romania	Deltaic system	137Cs	-	⭐ Mod–High	-	CRS: 10–20%	[124]
Tanzania Lake, Tanzania	Semi-arid	137Cs	-	⚠ Moderate	-	CRS: 10–25%	[128]
Lake Qarun, Egypt	Closed lake	137Cs + proxies	⚠ Moderate	⭐ High	-	CIC: 10–30%, CRS: 5–15%	[10]
Burullus Lagoon, Egypt	Agricultural reservoir	137Cs	❌ Low–Mod	⭐ High	❌ Low	CIC: 20–35%, CRS: 5–18%	[129]
Lake Edku, Egypt	Anthropogenic shallow lake	137Cs	❌ Low–Mod	⭐ High	❌ Low	CIC: 20–40%, CRS: 5–15%	[125]
Red Lake, Romania	Eutrophic lake	137Cs	-	⭐ High	-	CRS: 5–15%	[122]

❌ Low. ⚠ Moderate. ⭐ High.

6. Applications in Limnological Research

6.1. Sedimentation Processes in Lakes

Lake sediments are characterized by rapid accumulation and continuous deposition, making them valuable archives of past climate, environmental conditions, and human activities within lakes and their upstream watersheds [100]. Sedimentation refers to the complete process involving the transport of particles to the seabed, their deposition, possible redistribution, and long-term preservation [130]. Sediments are transported into lakes through streams, rivers, and overland flow. The rate at which sediments accumulate on the lakebed varies across different parts of the lake, depending on (i) the amount of sediment supplied by surrounding tributaries and the watershed, and (ii) internal circulation patterns influenced by wind and lake currents [131]. In addition, human activities such as deforestation, agriculture, and domestic waste disposal have significantly accelerated sedimentation rates. Once inside the lake, sediments gradually settle on the bottom, reducing water storage capacity, limiting light penetration, and promoting biological growth, all of which affect the overall functioning of the lake ecosystem [132]. In recent years, increasing anthropogenic pressures have severely altered the hydrological regimes of lakes across the country. The influx of eroded materials and pollutants from surrounding catchments has intensified both eutrophication and siltation processes [132].

Sedimentation rates can be accurately determined using radiometric techniques, which provide spatially detailed estimates across different parts of a water body, unlike conventional methods that yield only overall averages. These rates are typically derived by plotting radionuclide activity against sediment depth [131]. From such profiles, sedimentation rates can be calculated either by dividing sediment depth by the radionuclide time marker and the elapsed time since deposition [119] or by fitting a regression line to the activity data, depending on the radionuclide used. Elevated sedimentation rates have reduced the effective lifespan of many small lakes, with some shrinking at an alarming pace. This issue has become a major concern for authorities, underscoring the need to accurately determine sedimentation rates and identify their underlying causes to implement appropriate management strategies. Furthermore, estimates of lake lifespan provide insight into how long a lake remains functional for water-related uses before becoming significantly degraded [2].

6.2. Human Impacts and Catchment Change

Human activities, such as deforestation, agricultural practices, and domestic waste disposal, have significantly increased sedimentation rates in lakes. Sediments entering the lake gradually settle on the lakebed through natural processes, reducing water storage capacity, decreasing light penetration, and enhancing biological growth, ultimately affecting the overall functioning of the lake ecosystem [132]. In particular, sediments derived from agricultural lands and hilly terrains represent major sources of pollutants in rivers, lakes, and reservoirs [133].

In recent years, increasing anthropogenic pressures have severely altered the hydrological regimes of lakes across the country. The inflow of eroded materials and other contaminants from surrounding catchments has intensified eutrophication and accelerated siltation processes [132]. Ref. [118] Xiang et al., 2002 reported that elevated sedimentation rates in erosion-prone areas are largely associated with land reclamation for agriculture. Similarly, studies in the Jianghan Plain and Dongting region indicate that land reclamation since the 1950s has increased sediment deposition, resulting in reduced lake area and diminished flood control capacity, thereby negatively impacting the environment [134].

Conversely, the construction of water control structures, such as sluice gates, can also influence sedimentation patterns by regulating sediment inflow and outflow and by modifying hydrodynamic conditions through changes in water flow (2010). It has further been observed that sediment accumulation rates in large lakes of the middle and lower Yangtze Basin (MLYB) do not always reflect catchment erosion history, whereas smaller lakes tend to be more sensitive to catchment disturbances [135]. In recent decades, the growing impact of human activities on the environment has received global attention, particularly in geology, ecology, and environmental sciences [136]. Metal pollution in aquatic systems poses serious risks to ecosystem health and human well-being due to its toxicity, persistence, and tendency to accumulate [137]. These metals originate from both natural sources—such as rock weathering and atmospheric deposition—and anthropogenic sources, including agricultural runoff, municipal discharge, and industrial effluents [138,139].

Radiometric techniques, particularly lead-210 (²¹⁰Pb) dating, have proven to be reliable tools for reconstructing historical environmental changes [27,68,140]. Sediment core analysis using ²¹⁰Pb has been widely applied to trace historical trends of heavy metal contamination in estuaries [141], lakes [142], river catchments [143], and reservoirs [144]. Over recent decades, human activities such as mining, industrial discharge, fertilizer application, and enhanced soil erosion have intensified heavy metal pollution [30]. Therefore, identifying the sources of heavy metal contamination is essential for developing effective prevention, control, and remediation strategies [145].

6.3. Integration with Other Paleolimnological Proxies

Paleolimnology is the scientific discipline concerned with reconstructing past environmental conditions by analyzing lake (lacustrine) sediments [146]. This method uses a variety of physical, geochemical, and biological proxies in conjunction with radioisotope dating techniques like ¹³⁷Cs, ²¹⁰Pb, and ¹⁴C to establish a trustworthy chronological framework for interpreting environmental changes in lake systems and their surrounding catchments [60,147,148]. As a well-developed field, paleolimnology utilizes a wide range of abiotic proxies—such as bulk density, dry mass, radioactive isotopes, and mineral composition—and biotic proxies, including fossils, species abundance, presence/absence, and pigments preserved in sediments. These indicators enable the reconstruction of ecosystem changes over timescales ranging from years and decades to centuries and millennia [149,150]. Intact sediment cores, varying from a few centimeters to several hundred meters in length, can be retrieved from depositional basins using coring devices or drilling systems [151].

Following core collection, paleoecological analysis focuses on examining these proxies to develop detailed reconstructions of past environmental conditions. This process forms the foundation of paleolimnology and provides essential context for related fields such as resurrection ecology [152]. In paleolimnological research, proxies are frequently categorized according to their relationship to lake trophic conditions. Because primary productivity and organic matter sources directly affect geochemical indicators like total organic carbon (TOC), total nitrogen (TN), total phosphorus (TP), carbon-to-nitrogen ratios (C/N), and stable isotopes (δ¹³C and δ¹³N), they are trustworthy indicators of trophic evolution [153,154,155,156]. By contrast, biological proxies reflect nutrient changes indirectly by responding to shifts in habitat conditions, resource availability, and ecological dynamics. Among these, diatoms, fossil pigments, chironomids, and sedimentary DNA (sedDNA) are widely used to investigate human-induced eutrophication [157,158,159,160,161,162,163]. Information on the presence, abundance, morphology, and preservation state of these proxies provides critical insights into past environmental conditions, depositional settings, and ecological dynamics.

A particularly robust approach in paleolimnology is multiproxy analysis (Figure 10), which involves the integrated study of several physical, chemical, and biological indicators within sediment cores [164]. This method provides a more comprehensive understanding of the timing, magnitude, and drivers of environmental change, including atmospheric deposition, habitat alteration, eutrophication, and associated food-web dynamics [26]. For example, studies of Australian billabongs have demonstrated how multiproxy analyses can reveal the impacts of eutrophication on diatom, macrophyte, Cladocera, and chironomid communities [165].

Figure 10. Integrated framework of paleolimnological methodology: from sediment recovery to ecosystem reconstruction [26,146,152,164].

7. Methodological Challenges and Uncertainty

Radiometric dating has made a revolutionary contribution to the study of sedimentary processes [166]. Variability in atmospheric isotope flux plays a critical role in the accuracy and reliability of several radiometric dating techniques, including fallout radionuclides (²¹⁰Pb, ¹³⁷Cs, ²⁴¹Am), radiocarbon (¹⁴C), and cosmogenic isotopes such as ⁷Be and ³H. These methods rely on assumptions about the constancy or predictability of isotope input to environmental systems; however, both natural and anthropogenic processes introduce significant temporal and spatial variability.

For ²¹⁰Pb dating, atmospheric supply originates from the decay of radon (²²²Rn), which is emitted from terrestrial surfaces and subsequently deposited via precipitation. The ²¹⁰Pb fallout rate is locally governed by wet deposition and varies considerably over time, from hours to years, with annual fallout fluxes globally ranging from 0.1 to 360 Bq m⁻² y⁻¹ [167]. Early work by [60] established models such as the Constant Rate of Supply (CRS), which assume a steady atmospheric flux. However, later studies showed that climatic variability, including changes in precipitation and atmospheric circulation, can significantly alter ²¹⁰Pb deposition rates, introducing uncertainties in sediment chronologies [1]. These variations may result in inaccurate estimates of sediment accumulation rates if they are not adequately considered. While periodic (harmonic) and random fluctuations in atmospheric flux tend to balance each other and thus have limited influence on CRS-derived age estimates, they can nevertheless introduce artificial variability into sediment accumulation rates (SARs). In contrast, long-term (persistent) changes in flux lead to cumulative errors in CRS ages that cannot be adequately corrected, even with piecewise CRS models. Despite these limitations, both raw and piecewise CRS approaches remain useful tools when their assumptions and conditions of applicability are properly understood [31].

In the same context, most ²¹⁰Pb dating models rely on three key assumptions: (i) the sediment sequence is continuous, with no missing layers due to erosion or major episodic deposition; (ii) excess ²¹⁰Pb acts as a particle-bound tracer, deposited at the sediment–water interface (SWI) onto existing material; and (iii) no post-depositional redistribution occurs. However, in natural settings, these assumptions are often violated. Processes such as mixing and diffusion have been widely observed and modeled (e.g., [14,168,169,170,171]). Additionally, non-ideal deposition can occur in sediments with high surface porosity [172], while erosional and episodic depositional events are common in dynamic environments (e.g., [130,173]). Even when these assumptions are approximately met, they may still be insufficient to establish a reliable chronology.

Similarly, anthropogenic radionuclides such as ¹³⁷Cs and ²⁴¹Am are strongly influenced by episodic atmospheric inputs. These isotopes were primarily introduced during mid-20th-century nuclear weapons testing and events such as the Chornobyl disaster. Atmospheric nuclear weapons testing is the main source of radioactive contamination in the Arctic region. It is estimated that approximately 75% of ¹³⁷Cs and 95% of ⁹⁰Sr inventories in the North Atlantic originate from global fallout resulting from atmospheric nuclear weapons testing [174]. Research by [175] highlighted the high spatial variability in ¹³⁷Cs deposition, driven by differences in rainfall patterns and local environmental conditions. As a result, while ¹³⁷Cs is often used as a chronological marker (e.g., the 1963 fallout peak), its reliability can be compromised by post-depositional redistribution and uneven initial deposition. ²⁴¹Am, derived from plutonium decay, is often used to validate ¹³⁷Cs chronologies, but it is similarly affected by regional variability in fallout patterns [176]. Overall, these studies highlight that while ²¹⁰Pb dating remains a powerful tool for reconstructing recent environmental change, its accuracy depends heavily on environmental stability and model selection. In dynamic, human-impacted systems, integrating multiple tracers (e.g., ¹³⁷Cs) and complementary geochemical approaches is essential to reduce uncertainty and enhance the robustness of chronological interpretations [1,125,129]. This approach has proven especially valuable in studies of Lake Edku and Burullus Lagoon [125,129], where cross-referencing helped ensure robust geochronologies despite environmental variability.

Radiocarbon dating is subject to several limitations that affect its accuracy and reliability. One of the main challenges is the reservoir effect, where different carbon reservoirs such as oceans, lakes, and the atmosphere exhibit varying ¹⁴C concentrations, leading to apparent age offsets in samples. Additionally, atmospheric ¹⁴C levels are not constant over time; they fluctuate due to changes in solar activity, geomagnetic field strength, and carbon cycle dynamics, making calibration essential for accurate age determination. Anthropogenic influences further complicate the method, as nuclear weapons testing has increased atmospheric ¹⁴C (the “bomb effect”), while fossil fuel combustion has diluted it (the “Suess effect”), both altering the natural baseline. Calibration studies led by Minze Stuiver and Paula J. Reimer have demonstrated that uncalibrated radiocarbon ages can deviate significantly from true calendar ages [177]. Consequently, the use of calibration curves is essential to correct for these temporal variations in atmospheric ¹⁴C production.

Another important issue is isotope fractionation, where physical, chemical, and biological processes preferentially select lighter isotopes, thereby affecting measured ratios and requiring correction. Furthermore, contamination from external carbon sources, such as dissolved ancient carbonates or organic matter, can introduce older carbon into samples and yield erroneously old ages. Regional differences in atmospheric mixing and hemispheric variations also contribute to inconsistencies in radiocarbon ages. Finally, analytical uncertainties related to sample preparation, measurement techniques, and instrument precision, particularly for older samples, can further reduce dating accuracy. Overall, these factors demonstrate that radiocarbon dating requires careful calibration and methodological control to produce reliable chronological results [177].

Cosmogenic isotopes such as ⁷Be and ³H are particularly sensitive to short-term atmospheric processes. ⁷Be, with a half-life of approximately 53 days, shows strong seasonal variability linked to atmospheric circulation and precipitation patterns. Studies by [178] indicate that ⁷Be deposition is highly episodic, limiting its use to short-term environmental tracing rather than absolute dating. Tritium (³H), by contrast, experienced a dramatic increase during the 1950s and 1960s due to nuclear weapons testing. According to [179], accurate use of tritium in hydrological studies requires detailed knowledge of historical atmospheric input functions, as its variability directly affects age interpretations of groundwater systems.

The literature consistently demonstrates that variability in atmospheric isotope fluxes introduces significant uncertainty into radiometric dating methods. These uncertainties arise from deviations in initial isotope concentrations, spatial heterogeneity in deposition, and temporal fluctuations in production rates. To mitigate these effects, modern approaches incorporate calibration models, independent chronological markers, and multi-proxy analyses. Such strategies are essential for improving the robustness and accuracy of radiometric dating in environmental and geological studies.

Table 3 provides a structured comparative overview of the key radioisotopic dating techniques, summarizing their reliability, principal methodological limitations, and conditions under which their application may fail. Overall, fallout radionuclide-based approaches, particularly ²¹⁰Pb, ¹³⁷Cs, and ²⁴¹Am, remain the most widely applied tools for reconstructing recent sediment chronologies; however, their accuracy is strongly dependent on stable depositional conditions, well-constrained radionuclide input functions, and minimal post-depositional redistribution. The ²¹⁰Pb method offers high-resolution age–depth reconstructions over the past ~100–150 years, though its reliability depends on appropriate model selection (CIC, CRS, or CF-CS), particularly in environments with variable sedimentation regimes. The ¹³⁷Cs and ²⁴¹Am tracers provide valuable independent chronostratigraphic markers for validating ²¹⁰Pb-derived chronologies, but their effectiveness may be compromised by low fallout intensity, sediment mixing, and post-depositional mobility. In contrast, cosmogenic and radiocarbon-based methods (¹⁴C, ⁷Be, and ³H) extend the temporal applicability of sediment dating but introduce additional uncertainties associated with reservoir effects, short half-lives, and variability in atmospheric input histories. Collectively, the table highlights that no single isotopic method is universally applicable, and robust chronological interpretation requires the integrated use of multiple tracers, along with a clear understanding of site-specific environmental and sedimentological conditions.

Table 3. Comparative overview of radioisotopic dating methods: reliability, principal limitations, and conditions of potential failure.

Method	Reliability & Accuracy	Principal Limitations	Conditions of Potential Failure
210Pb dating Fallout radionuclide	Widely regarded as one of the most reliable methods for reconstructing recent sediment chronologies (~100–150 years), providing high-resolution age–depth relationships and sediment accumulation rates through models such as CIC, CRS, and CF-CS.	Accuracy depends on constant radionuclide flux, continuous sedimentation, and minimal post-depositional disturbance; sediment mixing, erosion, diffusion, and variable fallout can reduce reliability.	Unreliable in highly dynamic environments with bioturbation, episodic deposition, sediment resuspension, or variable sedimentation rates, where inappropriate model selection can cause significant chronological errors.
137Cs dating: Fallout radionuclide	137Cs is an effective independent chronological marker linked to major fallout horizons (1954 onset, 1963 weapons-testing peak, 1986 Chernobyl, and 2011 Fukushima) and is widely used to validate 210Pb chronologies.	The 137Cs method is limited by uneven fallout distribution, post-depositional migration, low radionuclide activities, and poor resolution of multiple fallout peaks; therefore, its stand-alone application is generally not recommended.	It may be unreliable in low-fallout regions, disturbed or mixed sediments, and where fallout peaks are poorly preserved, particularly in the Southern Hemisphere and older pre-1950s sediments.
241Am dating Fallout radionuclide	241Am is generally more geochemically stable and less mobile than 137Cs in sediments, making it a reliable tracer for validating fallout chronologies and identifying early-1960s fallout horizons.	Activity concentrations are generally lower than those of 137Cs, reducing analytical sensitivity and requiring high-resolution gamma spectrometry.	Low radionuclide concentrations, limited detector sensitivity, and regional fallout variability may reduce analytical reliability and complicate interpretation.
14C dating Cosmogenic	Effective for long-term sediment chronologies beyond the range of fallout radionuclides and widely used in paleolimnological and paleoenvironmental reconstructions.	Subject to reservoir effects, old carbon input, isotopic fractionation, and temporal variability in atmospheric 14C, accurate dating requires calibration using established calibration curves.	Chronological uncertainty increases in carbonate-rich lakes, low-organic sediments, and deposits influenced by reworked or fossil carbon. In such settings, uncalibrated ages may show significant deviations from true depositional ages.
7Be tracing Cosmogenic	Particularly useful for tracing short-term sedimentation dynamics, surface erosion, and recent depositional events owing to its short half-life (ca. 53 days).	Activity profiles are strongly influenced by seasonal atmospheric variability and precipitation patterns. Applicable exclusively to very recent sedimentary deposits.	Not suitable for long-term chronology or environments with variable atmospheric input and rapid sediment redistribution.
3H (tritium) dating Cosmogenic	Useful tracer for hydrological investigations and water mass circulation studies. Can provide chronological constraints for modern hydrological systems.	Atmospheric tritium (3H) concentrations vary significantly over time due to nuclear weapons testing and climatic influences, requiring well-constrained historical input functions for reliable dating applications.	Chronological interpretations become uncertain when atmospheric input histories are poorly constrained or unavailable, thereby increasing ambiguity in age estimates.

8. Recommendations

The application of radioisotopic techniques in lake sediment studies has been widely recognized as a robust approach for reconstructing environmental histories. Based on the reviewed literature, several key recommendations emerge that can enhance the reliability, applicability, and integration of these methods within limnological and environmental research frameworks.

A consistent finding across previous studies is the need to employ multi-isotope approaches rather than relying on a single radionuclide. Numerous authors have demonstrated that combining isotopes such as ²¹⁰Pb, ¹³⁷Cs, and ²⁴¹Am improves chronological control and allows cross-validation of sediment age models. While ²¹⁰Pb remains the most widely used method for reconstructing recent (the last ~100–150 years) sedimentation histories, its assumptions are often challenged by environmental variability. In this context, ¹³⁷Cs and ²⁴¹Am serve as independent time markers associated with known historical events (e.g., nuclear weapons testing and nuclear accidents), thereby strengthening chronological interpretations.

In parallel, the literature strongly supports integrating radiometric dating with multiproxy paleolimnological approaches. Studies in paleolimnology emphasize that chronological frameworks derived from radioisotopes are most effective when combined with biological, geochemical, and physical indicators preserved in sediments. Such integration enables more comprehensive reconstructions of past environmental conditions, including nutrient enrichment, pollution history, and ecological shifts. This multiproxy strategy is increasingly considered essential for disentangling complex interactions between natural variability and anthropogenic impacts.

Another critical aspect highlighted in previous research is the importance of selecting appropriate numerical models for ²¹⁰Pb dating. Models such as Constant Initial Concentration (CIC), Constant Rate of Supply (CRS), and Constant Flux–Constant Sedimentation (CF-CS) are widely used; however, their applicability depends heavily on site-specific conditions. Several studies have shown that violations of model assumptions—such as variable sedimentation rates or fluctuating atmospheric inputs—can lead to significant chronological errors. Consequently, recent approaches advocate for the use of modified or composite models (e.g., C-CRS), supported by independent chronostratigraphic markers.

The literature also emphasizes the need for a thorough understanding of local environmental processes prior to applying radiometric techniques. Factors such as sediment mixing (bioturbation), diffusion, erosion, and episodic deposition can alter radionuclide distributions within sediment profiles. These processes may compromise the fundamental assumptions of radiometric dating models, leading to inaccurate age-depth relationships. Therefore, site-specific investigations and sedimentological assessments are essential components of reliable geochronological studies.

A recurring theme in global research is the significance of regional calibration and atmospheric deposition data. Variability in radionuclide fallout, particularly for ²¹⁰Pb and ¹³⁷Cs, has been documented across different climatic and geographic settings. The development of regional databases for radionuclide deposition is therefore recommended to improve chronological accuracy and reduce uncertainties associated with global generalizations. Variability in radionuclide fallout is particularly relevant for regions such as Egypt, where localized studies remain limited.

Furthermore, the literature consistently highlights the importance of quantifying and reporting uncertainties in radiometric dating. Sources of uncertainty include analytical measurement errors, variability in isotope flux, model assumptions, and post-depositional processes. Advanced statistical approaches and uncertainty-propagation methods are increasingly incorporated into sediment chronology studies to enhance transparency and reliability.

Table 4 presents a comprehensive synthesis of recommended radioisotopic dating methods and numerical models based on lake characteristics and prevailing environmental conditions. The table demonstrates that the reliability of sediment chronologies is primarily determined by depositional dynamics, sediment mixing intensity, geochemical conditions, availability of independent stratigraphic markers, and temporal scope of the investigated sediments. Among the evaluated approaches, the CRS model is identified as the most broadly applicable framework for environments with variable sedimentation rates. In contrast, the CIC and CF-CS models are most suitable for relatively stable depositional conditions in which their assumptions hold. The table also highlights the importance of integrating independent fallout radionuclides such as ¹³⁷Cs and ²⁴¹Am, especially in disturbed or heterogeneous sedimentary systems where standalone ²¹⁰Pb chronologies may be uncertain.

Table 4. Recommended radioisotopic dating methods and numerical models based on lake characteristics and prevailing environmental conditions.

Lake Characteristic	Environmental Condition	Recommended Method/Model	Scientific Rationale	Principal Limitations
Sedimentation rate Depositional dynamics
Constant sedimentation	Stable, continuous sediment accumulation over the dating interval	CF-CS model Constant Flux–Constant Sedimentation	Appropriate where both sediment accumulation rate and unsupported 210Pb flux remain relatively invariant through time.	Sensitive to fluctuations in sediment input and episodic depositional events that violate model assumptions.
Constant sedimentation with stable initial activity	Uniform sediment accumulation; constant initial unsupported 210Pb concentration at deposition	CIC model Constant Initial Concentration	Assumes a constant initial concentration of unsupported 210Pb at the sediment–water interface, allowing sediment ages to be estimated from the exponential decrease in activity with depth.	Highly sensitive to deviations from the constant initial concentration assumption, potentially leading to age underestimation under variable sedimentation or sediment focusing conditions.
Variable sedimentation	Temporally fluctuating sediment accumulation rates	CRS model Constant Rate of Supply	Accommodates variable sediment accumulation rates under the assumption of a constant unsupported 210Pb atmospheric supply.	Chronological errors increase if the radionuclide flux varies significantly through time.
Highly irregular or disturbed sedimentation	Episodic deposition, erosion, or sediment disturbance	Composite CRS (C-CRS) + 137Cs/241Am validation	Improves chronological reconstruction in dynamic depositional environments; independent fallout markers provide additional stratigraphic constraints.	Requires identifiable independent chronological markers; interpretation is inherently complex.
Bioturbation intensity: Sediment mixing
Low bioturbation	Minimal biological mixing; intact sediment stratigraphy	Standard 210Pb and 137Cs models	Sediment profiles are expected to preserve clear radionuclide stratigraphy amenable to standard dating approaches.	Minor diffusion effects may still occur and should be assessed.
Moderate bioturbation	Partial mixing of surficial sediments	CRS + multiproxy validation	Cross-validation with independent biological or geochemical proxies enhances reliability where partial mixing has occurred.	Peak broadening in radionuclide profiles may reduce chronological precision.
High bioturbation or pervasive mixing	Severe disruption of the primary sediment fabric	Multiproxy approach 210Pb + 137Cs + 241Am + sedimentological proxies	Integration of multiple radionuclides and independent proxies reduces uncertainty arising from radionuclide redistribution and stratigraphic disturbance.	Age–depth relationships may remain unreliable in severely mixed or homogenized sediment intervals.
Lake pH and geochemistry: Geochemical context
Acidic lakes (pH < 6)	Low pH promotes differential radionuclide mobility	241Am-supported chronologies	241Am exhibits lower geochemical mobility than 137Cs under acidic conditions, offering greater stratigraphic stability.	Low activity concentrations may complicate detection and reduce analytical sensitivity.
Neutral lakes (pH 6–8)	Circumneutral conditions are favorable to radionuclide adsorption	210Pb + 137Cs combined models	Circumneutral conditions are most conducive to stable radionuclide preservation and consistent particle adsorption.	A reliable application requires identifiable and well-preserved fallout horizons.
Alkaline or carbonate-rich lakes (pH > 8)	Elevated pH with potential geochemical interference	210Pb with caution; integrate with 14C or sedimentological proxies	Carbonate–radionuclide interactions and enhanced geochemical mobility necessitate supplementary dating approaches.	Radionuclide remobilization and reservoir effects may introduce substantial chronological uncertainty.
Availability of independent time markers: Stratigraphic validation
Clear fallout peaks are identifiable	Distinct 137Cs horizons preserved in the sediment record	210Pb validated with 137Cs/241Am	Known fallout horizons (1963 weapons-testing peak; 1986 Chornobyl; 2011 Fukushima) provide robust independent chronological constraints.	Superimposition of multiple fallout peaks may complicate stratigraphic interpretation.
Weak or absent fallout peaks	Insufficient activity for fallout-based validation	210Pb + sedimentological and geochemical proxies	Supplementary proxies are necessary where fallout radionuclide activities are too low to provide reliable independent validation.	Chronological uncertainty is elevated in the absence of independent stratigraphic markers.
Geographic location Spatial context
Northern Hemisphere	High atmospheric fallout deposition	137Cs and 241Am are highly applicable	Greater fallout inventories from atmospheric nuclear weapons testing and major nuclear events enhance radionuclide detectability and stratigraphic resolution.	Post-depositional redistribution processes may still compromise peak preservation.
Southern Hemisphere	Reduced atmospheric fallout deposition	Greater reliance on 210Pb and multiproxy methods	Lower 137Cs fallout inventories reduce the reliability of fallout-based chronological validation, necessitating alternative approaches.	Weak radionuclide signals may limit chronological resolution and precision.
Target sediment age range, Temporal scope.
Very recent deposits (<1 year)	Tracing of contemporary erosion and deposition	7Be tracing Beryllium-7	The short half-life of 7Be (ca. 53 days) renders it uniquely suited to tracing short-term depositional and erosional processes at sub-annual timescales.	Applicability is inherently constrained by its short half-life; it is unsuitable for longer-term chronological reconstruction.
Recent sediments (ca. 100–150 years)	Modern lake chronology within the fallout era	210Pb + 137Cs/241Am	The most widely applied and methodologically validated framework for constructing modern lacustrine sediment chronologies.	Susceptible to post-depositional disturbance; model selection must be carefully justified.
Older sediments (>150 years)	Pre-fallout era; long-term paleoenvironmental records	Radiocarbon dating (14C) + paleolimnological proxies	Extends chronological coverage beyond the temporal range of fallout radionuclides; multiproxy integration strengthens age–depth model reliability.	Reservoir effects, old carbon contamination, and calibration curve uncertainties may introduce significant chronological error.

Additionally, the synthesis recognizes the influence of bioturbation, sediment redistribution, pH-dependent radionuclide mobility, and regional fallout variability on chronological reliability. The inclusion of geographic considerations, particularly the reduced fallout inventories in the Southern Hemisphere, further enhances the practical applicability of the proposed framework. Collectively, the table presents a scientifically robust and well-structured decision-support scheme that clearly demonstrates the necessity of site-specific model selection and multiproxy validation to achieve reliable sediment chronologies across contrasting lacustrine environments.

9. Conclusions

This review highlights the fundamental role of radioisotopic techniques in advancing the understanding of lake sediment dynamics, sediment chronology, and historical environmental change in lacustrine systems. The application of natural, cosmogenic, and artificial radionuclides, particularly ²¹⁰Pb, ¹³⁷Cs, ²⁴¹Am, ⁷Be, ³H, and ¹⁴C, has provided reliable chronological frameworks across a wide range of temporal scales, from short-term sedimentary processes to long-term paleoenvironmental reconstructions extending over thousands of years. These techniques have significantly contributed to the reconstruction of sedimentation rates, contaminant accumulation histories, nutrient enrichment, eutrophication trends, and broader anthropogenic impacts on lake ecosystems. Consequently, radioisotopic approaches have become indispensable tools in limnological and environmental investigations aimed at understanding ecosystem evolution and supporting evidence-based environmental management.

The reviewed studies demonstrate that the effectiveness of radioisotopic methods is substantially enhanced when multiple radionuclides are integrated within a multiproxy framework that includes geochemical, biological, and sedimentological indicators. Such integration improves chronological reliability and strengthens the interpretation of environmental signals preserved in lake sediments. In addition, the use of appropriate numerical dating models and uncertainty assessment procedures is crucial for improving the accuracy and robustness of sediment age reconstructions. The reviewed literature further confirms that radioisotopic methods are highly valuable for correlating sediment records with documented historical events, thereby enabling more precise evaluation of environmental disturbances associated with industrialization, land-use changes, atmospheric deposition, and climate variability.

Despite these advantages, several methodological and interpretational limitations remain associated with the application of radioisotopic techniques in lake sediment studies. The reliability of sediment chronologies depends strongly on the validity of model assumptions, the relative stability of radionuclide supply, and the preservation of undisturbed sedimentary sequences. Processes such as sediment mixing, bioturbation, erosion, post-depositional radionuclide mobility, and spatial heterogeneity in atmospheric deposition may introduce considerable uncertainties into age determinations and sedimentation rate estimates. Furthermore, reservoir effects, variations in isotope fluxes, and analytical uncertainties can affect the consistency and comparability of results among different studies and environmental settings.

In addition to the methodological limitations of the techniques themselves, this review is also subject to certain limitations that should be acknowledged. The available literature is not evenly distributed geographically, with a substantial proportion of studies originating from well-investigated regions such as Europe and North America, whereas tropical, arid, and developing regions remain comparatively underrepresented. This regional imbalance may limit the broader generalization of some findings to lakes with different climatic, geological, and hydrological characteristics. Moreover, the depth of discussion among radionuclides is inherently uneven because some isotopes, particularly ²¹⁰Pb and ¹³⁷Cs, have received considerably greater scientific attention and methodological development than less commonly applied tracers. Differences in study objectives, sampling strategies, analytical procedures, and reporting standards across the reviewed literature may also affect the comparability of studies.

Future research should therefore prioritize refining and standardizing radioisotopic methodologies, particularly through the development of region-specific calibration datasets, improved uncertainty quantification methods, and the application of advanced numerical and statistical models. Greater emphasis should also be placed on interdisciplinary investigations that integrate radiometric techniques with complementary paleolimnological, geochemical, ecological, and remote sensing approaches to provide more comprehensive reconstructions of environmental change. Expanding research efforts in understudied regions and lake systems will further improve the global applicability of radioisotopic interpretations and strengthen understanding of regional environmental variability.

The synthesis presented in this review demonstrates that radioisotopic techniques, when appropriately applied and integrated with complementary proxies, provide robust but method-dependent reconstructions of lake sediment histories. Accuracy is strongly controlled by site-specific conditions and modeling assumptions, underscoring the need for continued methodological refinement and regionally tailored applications.