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Review

231Pa in the Ocean: Research Advances and Implications for Climate Change

by
Pu Zhang
1,*,† and
Zhe Zhang
2,*,†
1
International Center for Planetary Science, College of Earth Sciences, Chengdu University of Technology, Chengdu 610059, China
2
College of Environmental Science and Engineering, Nankai University, Tianjin 300350, China
*
Authors to whom correspondence should be addressed.
†
These authors contributed equally to this work.
Atmosphere 2025, 16(9), 1018; https://doi.org/10.3390/atmos16091018
Submission received: 1 July 2025 / Revised: 21 August 2025 / Accepted: 27 August 2025 / Published: 28 August 2025
(This article belongs to the Section Climatology)
Versions Notes

Abstract

Protactinium-231 (231Pa), a particle-reactive radionuclide derived from 235U decay, serves as a pivotal tracer in marine geochemistry and paleoceanography, offering unique insights into particle scavenging, deep ocean circulation, and sedimentary processes. This review synthesizes significant advances in 231Pa research. A core application lies in utilizing the 231Pa/230Th ratio as a sensitive proxy for reconstructing past Atlantic Meridional Overturning Circulation (AMOC) intensity, with compelling evidence indicating a substantially weakened AMOC during the Last Glacial Maximum compared to the Holocene. Major technological breakthroughs, particularly the advent of high-precision ICP-MS and TIMS methodologies, have revolutionized the quantification of 231Pa in both dissolved and particulate phases, enabling spatial and temporal resolution. Looking forward, the integration of high-resolution sediment core analyses—featuring refined 231Pa/230Th chronologies—with advanced climate models offers a powerful pathway to significantly enhance our mechanistic understanding of the ocean’s role in global climate regulation. This synergistic approach will help constrain the dynamics of oceanic overturning circulation and its critical functions in carbon sequestration and heat redistribution across past, present, and future climate scenarios.

1. Introduction

Protactinium-231 (231Pa) is a rare radioactive isotope that plays a critical role in marine and environmental science due to its isotopic characteristics and its use as a tracer in studying oceanic processes [1,2,3,4,5,6,7,8,9]. As a decay product of uranium-235 (235U), 231Pa is an intermediate member of the uranium decay series, which eventually decays to form thorium-231 (231Th) [10]. Over the past several decades, research on 231Pa has significantly advanced, with studies focusing on its properties, distribution, and applications in understanding biogeochemical cycles and ocean circulation [5,6,7,8,9]. This isotope has proven to be a valuable tool in paleoceanography, sedimentology, and marine geochemistry, particularly through its application in reconstructing past climate and oceanic circulation patterns.
231Pa is a member of the actinide series, characterized by its radioactive properties and relatively long half-life of approximately 32,760 years [10]. Due to this half-life, it remains stable over geological timescales, allowing its effective use as a tracer for long-term oceanographic and sedimentary processes. 231Pa is chemically similar to thorium (another actinide), and as a result, both elements often exhibit similar behaviors in marine systems, especially when it comes to adsorption to particles and interaction with organic matter [11,12]. However, 231Pa is less soluble in seawater compared to other radionuclides like radium, which is a key factor in its application as a particle-reactive tracer. The marine chemistry of 231Pa makes it particularly useful in studying particle dynamics. In seawater, 231Pa preferentially adsorbs onto sinking particles, which play a crucial role in oceanic carbon cycling and nutrient transport [11,12,13]. Its distribution is influenced by ocean circulation, biological activity, and particle fluxes, with higher concentrations typically found in deep waters and sediments compared to the upper water column [1,2,3,4,5,6,7,8,9,14]. The uranium decay chain results in the formation of 231Pa, which is then transported into the oceans via various natural processes, including river runoff, atmospheric deposition, and hydrothermal vent activity. Once in the ocean, 231Pa behaves as a “particle-reactive” isotope, meaning that it is strongly associated with particles in the water column, especially those composed of biogenic materials, clays, and organic matter [1,4,5,6,7,14]. This property allows 231Pa to be used as a tracer for particle dynamics in the water column and sedimentation rates on the seafloor. The distribution of 231Pa in the ocean is non-uniform and highly variable, influenced by several factors including ocean circulation patterns, biological productivity, and particle flux [1,2,3,4,5,6,7,8,9]. High-latitude regions and deep-ocean areas typically show elevated concentrations of 231Pa due to increased particle scavenging and long residence times in deep waters. In contrast, surface waters, particularly in productive upwelling zones, often have lower concentrations of 231Pa due to enhanced particle removal and biological cycling [15]. The distribution of 231Pa is also closely tied to that of thorium-230 (230Th), another isotope in the uranium decay series. The ratio of 231Pa to 230Th in marine sediments has been widely used to assess past ocean circulation and sedimentation rates, providing insights into the efficiency of the biological pump and the nature of ocean mixing in different time periods [2,16]. Since 231Pa is more mobile than 230Th and preferentially adsorbs to sinking particles, it is often used in conjunction with 230Th to reconstruct past oceanographic conditions. The use of the 231Pa/230Th ratio, for example, has revolutionized paleoceanographic studies, providing an effective method to trace past shifts in ocean circulation and to quantify oceanic processes such as the biological pump and particle flux.

2. Importance in Marine Geochemistry

The study of 231Pa in marine environments dates back to the mid-20th century, but its significance has only been fully realized in the past few decades (Table 1). Early studies in the 1960s and 1970s focused on the basic geochemical properties of protactinium and its behavior in different geological environments, including marine sediments. Researchers initially recognized that 231Pa could make it a powerful tracer for oceanic processes. The breakthrough in using 231Pa to trace past ocean circulation and sedimentation came in the 1980s [17,18,19]. The 231Pa/230Th ratio provided the first concrete evidence of how this ratio could be used to estimate past ocean circulation rates and sedimentation dynamics. This discovery established 231Pa as a key tool for paleoceanographic reconstructions, particularly for understanding changes in deep-ocean circulation during glacial-interglacial transitions. By the late 1990s, the advancement of mass spectrometry and other high-precision analytical techniques allowed for more accurate measurements of 231Pa concentrations in marine sediments. These technological improvements made it possible to trace the behavior of 231Pa in increasingly fine temporal and spatial scales, leading to a better understanding of ocean circulation during the last ice age. Researchers also began to incorporate 231Pa in studies of the biological pump, further enhancing its application in modern marine geochemistry. The 2000s saw further progress in the application of 231Pa in oceanographic studies, especially with its integration into studies on particle scavenging and its relationship to other radionuclides like 230Th. This work solidified 231Pa’s role in both contemporary oceanography and paleoclimate reconstructions. Recent decades have focused on the role of 231Pa in tracing modern-day oceanic carbon cycling and understanding its potential as a tool for modeling future changes in ocean circulation under climate change. New models incorporating the behavior of 231Pa have been developed to assess the stability of the ocean’s carbon sink and its response to warming temperatures and increased carbon dioxide concentrations.

3. The Oceanic Behavior of Protactinium-231

3.1. Sources and Sinks of 231Pa in the Marine Environment

3.1.1. Cosmogenic Production and Thorium Decay

231Pa is a key radioactive isotope in marine geochemistry, predominantly used as a tracer in oceanic and sedimentary processes. The behavior of 231Pa in the ocean is influenced by its sources, its interaction with marine particles, and its removal processes, which ultimately control its concentration in different marine environments. Understanding the sources and sinks of 231Pa provides insights into ocean circulation, sedimentation rates, and the overall biogeochemical cycling of key trace elements and nutrients in the ocean. This isotope is primarily sourced from cosmogenic production, thorium decay, and various terrestrial inputs, including rivers, sediments, and hydrothermal vents. One of the primary sources of 231Pa in the marine environment is cosmogenic production [20,21]. This process occurs in the upper atmosphere when cosmic rays interact with nitrogen and oxygen to produce radionuclides, including 231Pa. These radionuclides are then transported through the atmosphere to the Earth’s surface, where they become part of the global carbon and water cycles. In the oceans, 231Pa is produced as part of the decay chain of uranium-235 (235U), which undergoes alpha decay to form 231Pa. This production occurs both in the atmosphere, where cosmic rays influence nitrogen nuclei, and in the upper ocean layers. In addition to cosmogenic production, 231Pa is also generated by the decay of Uranium-235 (235U), a process that occurs within the uranium decay series [10]. While thorium-230 itself is particle-reactive and tends to remain attached to particulate matter, 231Pa behaves similarly but has a slightly different solubility and particle affinity, which influences its distribution in the marine environment [11,12]. The decay of 235U in the water column results in the generation of 231Pa, which is then subject to removal via particle scavenging. As 231Pa decays further, it contributes to the formation of 227Ac, creating an important tracer pair for studying ocean circulation and particle fluxes.

3.1.2. Input from Rivers, Sediments, and Hydrothermal Vents

In addition to cosmogenic production and decay processes, 231Pa enters the ocean through several terrestrial inputs, including river runoff, sediment resuspension, and hydrothermal vent activity. These inputs are vital in understanding the sources of 231Pa in marine systems, as they directly influence its distribution and concentration in the oceans. Rivers are one of the most significant sources of 231Pa to the ocean. They transport 231Pa from the terrestrial environment into the ocean via surface runoff. The concentration of 231Pa in rivers can vary depending on the geology and the weathering of uranium-rich rocks in the river basin. As these rivers discharge into coastal waters, 231Pa mixes with seawater and contributes to the pool of dissolved and particulate 231Pa in coastal zones. This process plays a significant role in the coastal and estuarine behavior of 231Pa, with concentrations in riverine environments often reflecting the geochemical signature of local catchment areas. Sediments, particularly those in continental margins, represent another important source of 231Pa. The resuspension of sediments and the associated particle dynamics in the water column can release 231Pa into the overlying water. This process is especially important in areas with high sedimentation rates or in regions affected by bottom currents that stir up sediments. As particles containing 231Pa are transported in the water column, the isotope becomes available for scavenging by other particles, further influencing its distribution. Additionally, the dissolution of sediment-bound 231Pa into the water column can provide a source of dissolved 231Pa to the ocean, particularly in areas of active sedimentation or sediment-water exchange. Hydrothermal vents on the ocean floor also serve as an important source of 231Pa, although to a lesser extent compared to rivers and sediments. The activity of hydrothermal vents results in the release of dissolved trace elements and radionuclides, including 231Pa, into the surrounding seawater. The chemical composition of hydrothermal fluids, which includes high concentrations of dissolved uranium and thorium, facilitates the production of 231Pa. This input from hydrothermal systems is particularly relevant in deep-ocean environments and can influence the distribution of 231Pa in deep-sea water masses. Furthermore, hydrothermal vent systems are often located near areas of intense biogeochemical cycling, providing a link between 231Pa behavior and processes like organic carbon remineralization. Once in the ocean, 231Pa’s removal from the water column occurs primarily through particle scavenging. The most significant sink for 231Pa in the ocean is its adsorption onto sinking particulate matter, which includes both inorganic particles, such as clays, and biogenic particles, such as organic matter and phytoplankton detritus [14]. The efficiency of this scavenging process is strongly influenced by the concentration and size of particles in the water column. In high-productivity regions, such as upwelling zones, the biological pump enhances particle flux, leading to more efficient scavenging of 231Pa from the surface layers to the deep ocean [5,6,7,8,9]. In contrast, in oligotrophic or nutrient-depleted regions, particle flux is lower, resulting in slower removal rates for 231Pa. Additionally, the distribution of 231Pa in marine sediments reflects the efficiency of particle scavenging and can be used as a tracer for reconstructing past ocean circulation patterns. 231Pa’s removal through particle scavenging results in its accumulation in deep-sea sediments. In regions with high sedimentation rates, 231Pa is more likely to be trapped in the sediment column, where it can be preserved in sediment cores and used to infer past oceanic conditions [2,16]. The relationship between 231Pa and its decay product, 230Th, provides further insights into the efficiency of particle scavenging and vertical particle flux in marine environments. In addition, Serpentinization profoundly influences the marine cycle of 231Pa by markedly enhancing the scavenging efficiency of dissolved 231Pa from the water column through the production of abundant, highly reactive particulate matter, such as iron oxides and serpentine-group minerals [22]. This water-rock interaction process, prevalent in slow- to ultra-slow-spreading oceanic basins, generates hydrothermal plumes enriched in these fine particles. Upon their injection and dispersal into the surrounding water mass, these particles provide vast surface areas for adsorption, thereby substantially facilitating the transfer of dissolved 231Pa to the particulate phase and its subsequent removal via sedimentation. This augmented abiotic scavenging pathway not only locally shortens the residence time of 231Pa in the water column and increases its vertical flux to the seafloor but, more critically, introduces a potential complicating factor in the interpretation of paleoceanographic proxies like the 231Pa/230Th ratio. Consequently, observed ratio variations may not solely reflect changes in past oceanographic conditions, such as flow strength or productivity, but could be significantly overprinted by the intense particle-scavenging effect dominated by serpentinization, necessitating careful consideration when reconstructing paleoenvironments in affected regions.

3.2. Dissolution and Particle-Reactive Behavior

231Pa is strongly influenced by its interaction with marine particles. Unlike some other radionuclides, 231Pa has a low solubility in seawater, causing it to primarily exist in particulate form after adsorbing to organic matter, inorganic particles, and mineral surfaces. This nature makes 231Pa a valuable tracer in oceanographic research, particularly in studies of particle dynamics, sedimentation, and biogeochemical cycling [23]. The process of adsorption/desorption plays a critical role in determining the distribution of 231Pa in the water column and its transport to the seafloor. Adsorption refers to the attachment of 231Pa to particles, which then sink through the water column, while desorption refers to the release of 231Pa from particles back into the dissolved phase. The balance between these processes is influenced by several factors, including the chemical composition of the particles, the concentration of dissolved 231Pa, the physicochemical properties of the water column (such as pH and temperature), and the presence of other dissolved ions that may compete for binding sites [4]. Recent studies have shown that the adsorption of 231Pa onto particles is a rapid process, with the isotope showing a strong affinity for sinking particles, particularly those that are rich in organic matter or clay minerals [24,25]. However, 231Pa can also undergo desorption under certain conditions, such as changes in temperature, salinity, or the chemical composition of the surrounding water. The reversibility of this process is an important factor in determining the residence time of 231Pa in the water column and its potential to cycle through different oceanic layers. The dynamics of 231Pa adsorption/desorption have significant implications for our understanding of ocean circulation and particle flux. For instance, in areas of high particle flux, such as upwelling zones [26], the rapid scavenging of 231Pa onto sinking particles results in relatively high concentrations in deep waters and sediments. Conversely, in regions of low particle flux, such as oligotrophic open-ocean areas [27], the desorption process may release 231Pa back into the dissolved phase, leading to lower concentrations of 231Pa in the water column.

3.3. Biogeochemical Cycling of 231Pa

3.3.1. Interactions with Organic and Inorganic Matter

231Pa tends to adsorb onto particles, both biogenic (organic matter) and lithogenic (inorganic minerals), once it is introduced into the marine environment [28]. This behavior is one of the key aspects that govern its distribution and cycling within the ocean and is integral to its use as a tracer for understanding ocean circulation, sedimentation rates, and biogeochemical cycles. The interaction of 231Pa with organic matter primarily involves its adsorption onto the surfaces of organic particles, such as phytoplankton, detritus, and bacterial biomass. This process is crucial because organic particles, which are key components of the biological pump, are actively involved in the vertical transport of carbon and nutrients. As organic particles sink from the surface ocean to the deep ocean, they carry with them various trace elements, including 231Pa, which plays a role in the sequestration of carbon in the deep ocean. The rate at which 231Pa is adsorbed onto these particles and subsequently removed from the water column is governed by the composition of the particles, the concentration of dissolved 231Pa, and environmental factors like temperature, salinity, and biological activity. In addition to organic matter, 231Pa also interacts with inorganic particles, such as clay minerals and silicate materials [29]. These particles act as sinks for 231Pa in the water column, promoting its removal via the process of particle scavenging. The strength of the adsorption between 231Pa and inorganic particles depends on the specific surface area and reactivity of the particles, as well as the chemical composition of the water [6]. For example, regions with high concentrations of fine clay particles tend to have a more efficient scavenging of 231Pa, leading to its accumulation in deeper waters and sediments. This interaction with inorganic particles contributes to the global cycling of 231Pa and affects its distribution and behavior in different oceanic zones. The interactions of 231Pa with both organic and inorganic particles provide valuable information about the efficiency of the ocean’s biological and physical pumps, particularly in how trace elements are cycled through the ocean. These interactions are not only key to understanding the fate of 231Pa in the ocean but also provide insights into broader processes such as nutrient cycling, particle flux, and the efficiency of carbon sequestration in the ocean.

3.3.2. Role in Carbon Cycling

The role of 231Pa in carbon cycling is deeply intertwined with its nature and its interactions with organic and inorganic particles. As 231Pa is preferentially adsorbed onto sinking particles, particularly organic matter, it becomes a carrier of carbon to the deep ocean. This is especially relevant in the context of the biological pump, a process by which carbon from the atmosphere is transferred to the deep ocean via sinking particles. The biological pump is a key mechanism for regulating atmospheric CO2 levels and thus plays a crucial role in the Earth’s climate system [30]. As particles containing 231Pa sink through the ocean, they contribute to the sequestration of carbon in the deep ocean. This process is particularly important in high-productivity regions such as upwelling zones [26], where biological activity is high and large quantities of organic matter are produced. In these regions, the removal of 231Pa from surface waters by particle scavenging is more efficient, and the isotope is transported downward with the organic carbon. Once in the deep ocean, this carbon can remain sequestered for centuries to millennia, effectively removing it from the atmosphere and mitigating climate change. The 231Pa/230Th ratio is commonly used as a tool to study the efficiency of the biological pump and the rates of particle flux in the ocean [31,32]. Since 231Pa is more particle-reactive than 230Th, it is preferentially removed by sinking particles. The difference in the removal rates of 231Pa and 230Th provides a measure of the efficiency of the biological pump, as the ratio is higher in regions where particle flux is more effective. This ratio also helps to quantify past oceanic conditions and climate changes, providing a historical record of the efficiency of the biological pump and its contribution to the long-term regulation of atmospheric CO2. In addition to its role in carbon sequestration, the behavior of 231Pa in the ocean also provides insights into the cycling of other trace elements and nutrients. As 231Pa is part of the uranium-thorium decay series, its interactions with other isotopes like 230Th and 234U help researchers understand the global cycling of these elements, particularly in terms of their role in oceanic processes like mixing, particle flux, and deep-water formation. The distribution of 231Pa in marine sediments also serves as a valuable tool for reconstructing past ocean circulation patterns and assessing how changes in ocean circulation and carbon cycling might have impacted Earth’s climate over time [18,33].

4. 231Pa as a Paleoceanographic Proxy

4.1. Theoretical Basis for 231Pa/230Th as a Proxy

4.1.1. Fractionation Processes

231Pa and 230Th are both members of the uranium decay series, and their isotopic ratio, 231Pa/230Th, has become a powerful tool in paleoceanography [18,33]. This ratio is widely used to reconstruct past ocean circulation, sedimentation rates, and the efficiency of the biological pump. The theoretical basis for using the 231Pa/230Th ratio as a proxy lies in the distinct behaviors of these two isotopes in the ocean, particularly their differing particle-reactive properties. Both 231Pa and 230Th are produced in the ocean primarily through the decay of uranium isotopes dissolved in seawater. However, their subsequent behavior in the water column differs significantly. 230Th, being relatively more soluble in seawater, remains largely in the dissolved phase, while 231Pa is preferentially removed from the water column by adsorption to sinking particles, such as organic matter, biogenic detritus, and clays. This difference in reactivity allows the 231Pa/230Th ratio in marine sediments to serve as an indicator of past particle flux and oceanic circulation patterns. The 231Pa/230Th ratio is particularly useful because the removal of 231Pa by particle scavenging is more efficient than that of 230Th, which leads to a relative enrichment of 231Pa in surface waters compared to 230Th. This ratio is preserved in marine sediments, allowing scientists to reconstruct sedimentation rates and infer past changes in ocean circulation. The efficiency of 231Pa removal is highly sensitive to ocean productivity, particle flux, and water column processes, making it a sensitive proxy for studying past marine environments. The fractionation processes between 231Pa and 230Th are fundamental to the use of the 231Pa/230Th ratio as a paleoceanographic proxy. These processes occur as a result of differences in the behavior of 231Pa and 230Th in the water column. While both isotopes are produced in seawater through the decay of uranium, the fractionation between them arises because of their contrasting affinities for particulate matter. 231Pa is preferentially adsorbed onto sinking particles and removed from the water column at a faster rate. This results in a larger fraction of 231Pa being deposited in the sediments compared to 230Th, leading to a fractionation between the two isotopes in the water column. The extent of this fractionation depends on several factors, including the concentration and composition of the particles in the water column, the rate of particle sinking, and the ocean’s circulation patterns. The fractionation between 231Pa and 230Th is also influenced by particle flux—the rate at which particles sink through the water column [34]. In areas of high particle flux, such as coastal upwelling zones or regions of intense biological productivity, the scavenging of 231Pa is more efficient, and thus the 231Pa/230Th ratio in marine sediments tends to be higher. Conversely, in low-productivity areas with lower particle flux, the fractionation between 231Pa and 230Th is less pronounced, and the ratio in sediments is lower. These fractionation processes provide valuable information about past ocean circulation, sedimentation rates, and the efficiency of the biological pump. By analyzing the 231Pa/230Th ratio in sediment cores, researchers can gain insights into past shifts in ocean productivity, the efficiency of particle scavenging, and changes in deep-water circulation.

4.1.2. Influence of Particle Flux and Water Column Processes

The influence of particle flux and water column processes is central to the behavior of 231Pa in the ocean and its use as a paleoceanographic proxy [34]. Particle flux refers to the vertical transport of particles, which includes both organic particles (e.g., phytoplankton, zooplankton) and inorganic particles (e.g., clays, dust). The efficiency with which 231Pa is scavenged from the water column depends on the amount of particulate material present and the rate at which particles sink. In high-productivity regions, such as coastal upwelling zones, the biological pump enhances the flux of particles and leads to increased scavenging of 231Pa. In these areas, the rapid removal of 231Pa from surface waters results in an elevated concentration of 231Pa in deep waters and sediments, providing a more reliable signal of past productivity and ocean circulation. Conversely, in oligotrophic or low-productivity regions, where particle flux is slower, 231Pa scavenging is less efficient, and the 231Pa/230Th ratio is lower in marine sediments. Water column processes, including ocean circulation and mixing, also play a key role in the distribution of 231Pa [35]. Changes in ocean circulation, such as shifts in the Atlantic Meridional Overturning Circulation (AMOC) or the Southern Ocean’s overturning circulation, can alter the rate of particle flux and the distribution of 231Pa in the water column. For example, during periods of strong ocean circulation, the increased mixing and upwelling of deep waters may result in higher particle flux and enhanced scavenging of 231Pa. Conversely, during periods of weak circulation or stratified ocean conditions, particle flux may decrease, leading to less efficient scavenging and a lower 231Pa/230Th ratio in sediments. The ability to study the relationship between particle flux, water column processes, and the 231Pa/230Th ratio provides valuable insights into past changes in oceanic and climatic conditions. By reconstructing past particle flux and ocean circulation patterns, researchers can gain a deeper understanding of how changes in ocean dynamics influenced global climate over time.

4.2. Applications in Reconstructing Past Ocean Circulation

4.2.1. Thermohaline Circulation and Meridional Overturning

The thermohaline circulation (THC), or the meridional overturning circulation (MOC), is a fundamental component of the global ocean circulation, driven by differences in water temperature and salinity. It plays a crucial role in regulating the Earth’s climate by controlling the distribution of heat, nutrients, and carbon across ocean basins. The study of past changes in thermohaline circulation, particularly during periods of abrupt climate transitions, is essential for understanding how ocean circulation influences global climate patterns. The 231Pa/230Th ratio has proven to be a valuable proxy for studying past changes in the strength and structure of the MOC [35]. 231Pa is preferentially removed from the surface ocean by sinking particles and transported downward. In regions with strong ocean circulation and vertical mixing, 231Pa is scavenged more efficiently, leading to higher concentrations in deep-water sediments. Conversely, in areas with weak circulation, the scavenging of 231Pa is less efficient, and the concentration in sediments is lower. By examining the ratio of 231Pa to 230Th in sediment cores, scientists have been able to track changes in the strength of the MOC and the distribution of deep-water formation in the past. For example, during the Last Glacial Maximum (LGM), the strength of the MOC was likely weaker than today, which has been inferred from sediment core data showing a reduced 231Pa/230Th ratio in regions influenced by deep-water formation, such as the North Atlantic. Changes in the MOC during past warming events, such as the deglaciation period, are also visible in the 231Pa/230Th ratio, providing insight into how ocean circulation has responded to climate change. This has important implications for understanding how oceanic circulation processes may respond to future climate shifts, especially in the context of ongoing global warming. Similar studies in the Arctic Ocean have highlighted the importance of 231Pa in tracing sediment transport and lateral export processes, with evidence suggesting that 231Pa is persistently exported from the Arctic through deep-water exchange via the Fram Strait. Recent research has also revealed the influence of hydrothermal processes on 231Pa distribution. Intense hydrothermal scavenging in regions such as the deep Southeast Pacific has been identified as a significant mechanism affecting the removal and redistribution of 231Pa and 230Th in the ocean. Additionally, boundary scavenging, where 231Pa is preferentially removed along continental margins due to increased particle flux, has been demonstrated as a key process in shaping its global distribution.

4.2.2. Paleo-Productivity and Sedimentation Rates

The 231Pa/230Th ratio also serves as a proxy for studying past paleo-productivity and sedimentation rates in the ocean. Since 231Pa is preferentially removed from the water column by adsorption to particles, its distribution in marine sediments reflects the efficiency of the biological pump and the rate of particle flux. The biological pump is responsible for transporting carbon and other nutrients from the surface ocean to the deep ocean via sinking organic matter, making it a critical process for regulating atmospheric CO2 levels. Higher paleo-productivity in the surface ocean typically results in increased biological activity and higher fluxes of organic material to the deep ocean. As a result, the scavenging of 231Pa increases, leading to a higher 231Pa/230Th ratio in sediments. Conversely, periods of low productivity, often linked to colder or more stratified conditions, are marked by lower concentrations of 231Pa in deep-sea sediments due to reduced particle flux. By analyzing these ratios, researchers can infer past changes in ocean productivity and reconstruct past climate conditions. Additionally, the 231Pa/230Th ratio is used to estimate past sedimentation rates. When particle flux is high, particles—including those carrying 231Pa—sink more rapidly to the seafloor, leading to an increase in sedimentation rates and a corresponding increase in the 231Pa/230Th ratio. Lower fluxes, on the other hand, are associated with slower particle sinking and lower sedimentation rates. This relationship allows for the reconstruction of past sedimentation rates, which in turn provide insights into the efficiency of the biological pump and the rates of nutrient recycling in the ocean. The Arctic Ocean presents a unique case for 231Pa cycling, with studies showing spatial variability in sedimentary 231Pa/230Th ratios, influenced by changes in sea-ice coverage, biological productivity, and sediment transport mechanisms. Furthermore, the interaction of 231Pa with biogenic and lithogenic particles has been a focal point of recent research, with evidence suggesting that different particle types, including opal, organic carbon, and clays, exhibit distinct scavenging efficiencies for 231Pa [9]. This has implications for understanding particle dynamics and nutrient cycling in various oceanic regimes. The use of 231Pa to assess past productivity and sedimentation has been particularly valuable in understanding the role of ocean ecosystems in regulating atmospheric CO2 levels during past climate changes. For instance, studies have shown that during glacial periods, ocean productivity was generally higher, with increased nutrient supply to the surface ocean through upwelling and stronger ocean circulation. Conversely, interglacial periods are often marked by lower oceanic productivity, as evidenced by reduced 231Pa/230Th ratios in marine sediments. In addition, the 231Pa/230Th ratio—which serves as a robust tracer of particle scavenging efficiency and water mass transit time—exhibits strong complementarity with proxies like δ13C and Cd/Ca (reflecting nutrient cycling and biogeochemical processes [36]) and εNd (tracing water mass sources and mixing [37,38]). The synergistic use of these proxies helps to mitigate the interpretative ambiguities inherent in single-proxy approaches, thereby enabling a more comprehensive reconstruction of past ocean circulation patterns, productivity changes, and biological pump efficiency. For instance, correlating high 231Pa/230Th ratios with depleted δ13C values and enriched εNd signatures can provide more robust evidence for identifying intense particle adsorption-transport processes and aged water masses formed under high-productivity conditions. This multi-proxy comparison and integration framework significantly enhances the reliability of interpreting paleoenvironmental signals in sedimentary records and highlights the unique value of 231Pa/230Th in multi-proxy reconstruction studies.

5. Analytical Methods in 231Pa Research

5.1. Sampling Strategies

Seawater, Sediment Cores, and Particulates

In the study of 231Pa in marine environments, robust sampling strategies are essential for accurately characterizing the distribution and cycling of this isotope. 231Pa’s concentration and distribution depend on its interactions with particulate material, such as organic matter and sediments, as well as its behavior in the dissolved phase in seawater. The isotopic ratio of 231Pa/230Th has become a critical proxy for reconstructing past ocean circulation, productivity, and sedimentation rates, making the choice of sampling strategy vital for obtaining reliable data. This section focuses on the primary sampling strategies used in 231Pa research across seawater, sediment cores, and particulates (Table 2).
(1) Seawater sampling. Sampling seawater for 231Pa analysis typically involves collecting water at various depths in different marine environments to study the vertical distribution of the isotope. 231Pa’s concentration in the dissolved phase is typically low. As such, sampling strategies focus not only on the dissolved phase but also on the particulate matter to understand the partitioning between the two. Seawater samples are typically collected using trace-metal clean Niskin bottles or rosette samplers mounted on CTD (conductivity, temperature, and depth) profiling systems, ensuring minimal contamination from the surrounding environment. Multiple depths are sampled to capture vertical gradients in 231Pa concentration. The samples are then filtered on-site to separate dissolved and particulate fractions, with particulate matter often being filtered through membranes with pore sizes ranging from 0.2 µm to 1 µm, depending on the focus of the study. Once filtered, the dissolved phase of 231Pa is extracted using standard separation techniques, often involving solid-phase extraction (SPE) or liquid–liquid extraction methods. The particulate fraction is then analyzed for 231Pa content, with attention paid to its association with specific particle types, such as organic matter, minerals, and biogenic particles, which can affect the scavenging and removal efficiency of 231Pa. The isotopic analysis of seawater samples typically involves high-resolution mass spectrometry techniques like inductively coupled plasma mass spectrometry (ICP-MS) or thermal ionization mass spectrometry (TIMS) [26,39]. This precise measurement is critical in understanding the small concentrations of 231Pa in the marine environment and for assessing the role of ocean processes such as circulation and biological productivity.
(2) Sediment core sampling. Sediment cores are invaluable in studying the historical deposition and cycling of 231Pa, as they provide direct records of past oceanographic conditions [18]. Given that 231Pa is preferentially removed from the water column through adsorption to sinking particles, sediments are the primary sink for 231Pa. Sediment cores are collected from various oceanic environments using piston corers, multi-corer systems, or gravity corers, depending on the sediment depth and desired resolution of the record. The strategy for core sampling is often based on regions where particle flux is high, such as near continental margins, upwelling zones, or deep-sea environments with intense sedimentation rates. These locations typically yield more accurate reconstructions of past ocean processes and climate conditions. Cores are sliced at intervals (typically 1 cm or less) to maintain high temporal resolution, with careful processing to avoid contamination. After extraction, each sediment slice is treated to separate the particulate fraction from the pore water, allowing for precise analysis of the 231Pa/230Th ratio at different depths. Once separated, the sediment samples undergo chemical digestion to extract 231Pa, often employing ion-exchange chromatography or solid-phase extraction methods [40]. The concentration of 231Pa and its ratio to 230Th can then be used to infer sedimentation rates, past productivity, and ocean circulation dynamics. The 231Pa/230Th ratio is particularly useful for studying sedimentation rates and the efficiency of the biological pump, as 231Pa’s preferential scavenging onto sinking particles is sensitive to changes in ocean productivity and circulation.
(3) Particulate matter sampling. In addition to seawater and sediment core samples, particulate matter plays a central role in 231Pa research. Understanding 231Pa’s interaction with particles in the ocean is crucial for accurate reconstructions of past ocean conditions. Particulate samples are collected using in situ filtration systems or deck-mounted pumps, often employing large-volume filtration techniques to capture particles at different depths in the water column. The sampling of particulates typically targets both organic and inorganic particles, as 231Pa is known to associate with both types. Organic particles, such as planktonic detritus, tend to dominate in high-productivity regions, while inorganic particles, like clays and silicates, are more prevalent in regions with strong physical forcing, such as near river mouths or in the deep ocean. The filter pore size typically ranges from 0.2 µm to 1 µm, with smaller sizes used to capture finer particulate matter that may play a role in 231Pa scavenging. Once collected, the particles are analyzed for 231Pa content by dissolving the material and using ICP-MS or TIMS to measure the isotope. Understanding the relationship between 231Pa and different particle types helps determine the efficiency of particle flux and the role of various particles in the oceanic cycling of 231Pa.

5.2. Techniques for Measurement and Quantification

A new technique was developed to measure extremely low concentrations of 231Pa in seawater using isotope-dilution TIMS [41]. The new technique has a very low procedural blank, high ionization efficiency, and can rapidly measure 231Pa. The technique can detect 231Pa with high N2 precision in very small volumes of seawater, down to 2 L for particulate matter and less than 0.1 L for dissolved 231Pa. A new technique was developed to quantify ultra-trace levels of 231Pa in seawater using MC-ICP-MS [42]. The study reported background levels of 231Pa in the water column and in large and small suspended particles. The new technique has high precision and low detection limits, allowing 231Pa to be measured in relatively small volumes of seawater. The new method has high extraction efficiency (>80%) and very low procedural blanks, allowing for the measurement of very low concentrations of 232Th, 230Th, and 231Pa in seawater [43]. The method provides highly precise measurements of the three isotopes, enabling reliable measurements even in low-concentration samples. The accuracy of the new method was validated through the analysis of standard solutions and seawater samples.

5.3. Challenges and Limitations

For TIMS measurement, the high procedural blank and low ionization efficiency, which could limit sensitivity and accuracy; a relatively large amount of 231Pa isotope is required to achieve reasonable uncertainty, which could limit sample size; non-trivial sample size requirements, even if less than previous techniques. More studies are needed to further examine the influence of natural organic matter on the fractionation of Th/Pa ratios. For MC-ICP-MS measurement, the method has a non-negligible process blank value, which could limit the minimum detectable concentration. The ionization efficiency is relatively low, which could limit the sensitivity and precision of the measurements. The wide range of 231Pa amounts required to achieve different levels of uncertainty could limit the consistency and reliability of the measurements. In continental margin seas, large particulate matter delivered by rivers (such as suspended river particles and resuspended shelf sediments) contributes significantly to the scavenging flux of 231Pa and 230Th. This flux can exhibit substantial temporal variability due to events such as flooding and sea-level changes, introducing considerable complexity into the interpretation of 231Pa/230Th ratios in these regions. If overlooked, elevated ratios could be misinterpreted as reflecting changes in ocean circulation rather than localized particulate inputs. To address this issue, multi-proxy approaches and phase-specific partitioning are commonly employed to evaluate and correct for such influences. Standard methods include comparing isotopic ratios between dissolved and particulate phases, or distinguishing between authigenic (seawater-derived) and detrital (terrigenous-derived) fractions. For example, concurrent analysis of neodymium (Nd) or strontium (Sr) isotope ratios (εNd, 87Sr/86Sr) in particles helps trace and quantify the proportion of terrestrial material [44,45]. Furthermore, flood events and intense alluvial sedimentation can lead to rapid scavenging and burial of 231Pa in nearshore or shelf environments before it reaches deep-sea depositional sites, resulting in a “loss” of signal. Therefore, caution is warranted when interpreting 231Pa/230Th ratios from near-coastal sediment cores. This proxy is generally considered more reliable in open-ocean settings, where water-column processes dominate isotope distribution; applications in marginal seas require corrections based on independent indicators of terrestrial input.

6. Conclusions

231Pa is a particle-reactive isotope produced through the decay of 235U, playing a crucial role in marine geochemistry and paleoceanographic reconstructions. Over the past few decades, research on 231Pa has expanded significantly, focusing on its sources, sinks, and its application in tracing oceanic circulation and particle dynamics. The behavior of 231Pa in marine environments is primarily governed by its strong affinity for sinking particles, making it a valuable tracer for studying deep-water formation and sedimentation rates. One of the key insights from 231Pa research is its application in reconstructing past ocean circulation, particularly through the 231Pa/230Th ratio. This ratio has been used extensively to infer the strength of the AMOC and deep-water formation rates. Studies have shown that during the LGM, AMOC was weaker than in the Holocene, as evidenced by lower 231Pa/230Th ratios in sediments from the North Atlantic. Advancements in analytical techniques have significantly enhanced our ability to measure 231Pa with higher precision and resolution. The development of ICP-MS and TIMS has allowed for more accurate quantification of 231Pa in both dissolved and particulate phases. Future studies are expected to leverage these technologies to improve spatial and temporal resolution in 231Pa measurements, enabling more detailed reconstructions of past and present ocean circulation. High-resolution sediment core analyses, coupled with improved dating methods such as uranium-series disequilibrium dating, will refine the chronology of past oceanographic events. Furthermore, the integration of 231Pa data with climate models will enhance our understanding of the interactions between ocean circulation and global climate change. By combining geochemical proxies with numerical simulations, researchers can better constrain the role of oceanic overturning circulation in regulating carbon and heat transport.

Author Contributions

Conceptualization, P.Z.; methodology, P.Z. and Z.Z.; writing—original draft preparation, P.Z.; writing—review and editing, P.Z. and Z.Z.; supervision, P.Z.; funding acquisition, P.Z. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the National Natural Science Foundation of China (Nos. 42173027 and 41888101) and the Everest Talent Plan Project (Grant No. 10912-KYQD2022-09482).

Data Availability Statement

Not applicable.

Acknowledgments

We express our sincere gratitude to all authors for their diligent efforts in literature collection, compilation, organization, and the completion of this paper. Additionally, we would like to extend our appreciation to the editors and reviewers for their invaluable and constructive suggestions.

Conflicts of Interest

The authors declare no conflicts of interest.

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Table 1. Key milestones in the development and application of 231Pa in marine geochemistry.
Table 1. Key milestones in the development and application of 231Pa in marine geochemistry.
PeriodKey DevelopmentsPrimary Applications
1960s–1970sInvestigation of basic geochemical properties and behavior in marine sediments.Fundamental marine geochemistry studies.
1980sBreakthrough in using 231Pa/230Th as a tracer for ocean processes.Paleocean circulation and sedimentation dynamics.
Late 1990sAdvancements in mass spectrometry enabling precise measurement.High-resolution ocean circulation and biological pump studies.
2000sIntegration into particle scavenging and multi-radionuclide studies.Modern oceanography and paleoclimate reconstructions.
Recent DecadesDevelopment of models for ocean carbon cycling under climate change.Tracing modern carbon cycles and modeling future changes.
PeriodKey DevelopmentsPrimary Applications
Table 2. Summary of analytical approaches, key features, and challenges in the measurement and application of 231Pa in marine environmental research.
Table 2. Summary of analytical approaches, key features, and challenges in the measurement and application of 231Pa in marine environmental research.
Method TypeKey MethodsFeatures and ApplicationsLimitations
Seawater SamplingMulti-depth sampling; on-site filtration (0.2–1 µm)Measures dissolved/particulate 231Pa via ICP-MS/TIMS; studies vertical distribution and scavengingVery low concentration; contamination risk; large sample volume needed
Sediment Core SamplingCoring in high-flux zones; fine slicing (e.g., 1 cm)231Pa/230Th ratio reveals sedimentation and circulation historySensitive to terrestrial input; requires correction; limited temporal resolution
Particulate SamplingIn situ filtration; multi-depth particle collectionAnalyzes particle-bound 231Pa; clarifies scavenging efficiencySampling bias; complex logistics; time-representative limitation
TIMS MeasurementIsotope dilution; low blank designHigh precision with small sample volumes (≥2 L particulates; <0.1 L dissolved)High sample requirement; variable ionization efficiency
MC-ICP-MS MeasurementHigh extraction efficiency (>80%); low blanksPrecise ultra-trace 231Pa/230Th/232Th measurement in seawaterMatrix sensitivity; requires calibration; moderate ionization efficiency
General CorrectionsMulti-proxy (εNd, Sr isotopes); phase partitioningCorrects terrestrial inputs in marginal seasNearshore signal loss; best for open ocean
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Zhang, P.; Zhang, Z. 231Pa in the Ocean: Research Advances and Implications for Climate Change. Atmosphere 2025, 16, 1018. https://doi.org/10.3390/atmos16091018

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Zhang P, Zhang Z. 231Pa in the Ocean: Research Advances and Implications for Climate Change. Atmosphere. 2025; 16(9):1018. https://doi.org/10.3390/atmos16091018

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Zhang, P., & Zhang, Z. (2025). 231Pa in the Ocean: Research Advances and Implications for Climate Change. Atmosphere, 16(9), 1018. https://doi.org/10.3390/atmos16091018

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