Tracing Variation in Diagenesis in Concretions: Implications from a Raman Spectroscopic Study

Abstract

Concretions represent an exceptional mode of fossil preservation. This is attributed to their mineralized outer mantle, which exhibits low permeability and porosity, thereby limiting diagenetic alteration. The present research employs microscopic Raman spectroscopy to assess the thermal maturity of kerogen—a highly sensitive organic material—within concretions from northeast Taiwan. Comparative analysis of kerogen from the concretion’s core, rim, and surrounding matrix reveals differential preservation states. The organic matter in the core remains relatively unaltered, whereas the rim exhibits partial graphitization, albeit to a lesser extent than the surrounding matrix. These findings indicate a progressive diagenetic gradient, with the core influenced by the least thermal alteration, followed by the rim, and the surrounding matrix that experiences the highest degree of graphitization. Therefore, the present research underscores the role of concretionary encapsulation in mitigating diagenetic modification and enhancing organic matter preservation.

1. Introduction

Modes of fossil preservation can be broadly categorized into three groups: normal, selective, and exceptional status [1,2]. In the ‘normal’ fossil preservation status, soft tissues of deceased organisms are predominantly degraded through the burial and/or diagenesis is processes, whereas any remaining materials are often replaced by minerals through biomineralization [1]. ‘Selective’ fossil preservation involves the fossilization of resistant and persistent tissues or biopolymers that withstand decay [1,2]. In contrast, ‘exceptional’ fossil preservation refers to the rare case in which soft tissues are only partially degraded, allowing some organic matter and biomarkers to persist [1,2,3]. This form of fossil preservation provides critical and essential insights, as it is anticipated to offer clues of paleoenvironments and extinction [1,2].

The formation of concretion is one of the ‘exceptional’ types of fossil preservations [1,2,3]. These spheroidal or ellipsoidal mineral masses are characterized by low permeability and porosity [4,5,6], often found in fine-grained marine sedimentary rocks such as shales and mudstones, and sometimes encapsulate fossils with remarkably preserved soft tissues [3,4,5,6,7,8]. Recent studies [4,5] have demonstrated that concretion can form rapidly during the post-mortem stage, providing the essential protection that prevents organic matter from extensive degradation. As pointed out by Grice et al. (2019) [1], concretion plays a vital role as a shield for the enclosed organic material, which is a necessary condition for organic matter preservation hence forming ‘exceptional’ fossil deposits [2].

Although numerous studies [1,2,3,4] have been conducted on the formation pathways and the effects of microbial activities regarding concretions, little is known about how their outer mineral-composed mantle can isolate the organic matter from the external environment. In order to explore the mechanisms that create this ‘sealing phenomenon’, this study investigates the spatial variation in kerogen throughout the rim, core, and surrounding matrix of a concretion as assessed through micro-Raman spectroscopy.

Raman spectroscopy, a molecular-level, non-destructive vibrational technique, has become an increasingly prominent tool in the study of fossil-bearing concretions [8,9]. Since the pioneering work of Tuinstra and Koenig (1970) [10], the Raman spectra of kerogen have been extensively analyzed to assess structural and chemical changes related to thermal maturation [10,11,12,13,14,15]. Two distinct bands of the graphite band (G band; ~1580 cm⁻¹) and the D band (~1350 cm⁻¹) are identified for kerogen. The G band is commonly found in natural graphite and is associated with the in-plane C-C stretching vibration of carbon atoms in graphene sheets with E_2g2 symmetry [10,11,12,13]. The D band originates from lattice discontinuities or structural defects of organic matter [11,13,14]. Numerous studies [11,12,13,14,15] have demonstrated that the spectral features of kerogen change significantly in response to temperature and pressure variation during diagenesis, catagenesis and metagenesis. For instance, the G band becomes distinctly sharper as pressure and temperature increases, which reflects the maturation level for hydrocarbon or graphitization [10,12,13,14,16].

Due to this property, Raman spectroscopy analysis for kerogen is widely used in geothermometers in source rock analysis [12,13,15,16]. Building on this foundation, we propose that Raman spectral analysis of kerogen within and around concretions can serve as a proxy to evaluate variations in the degree of diagenesis. This approach may offer new insights into the sealing function of concretions and the conditions that contribute to exceptional fossil preservation.

2. Materials and Methods

The concretion sample analyzed in this study was collected from the coastal area of Nanya, located in the northeastern region of Taiwan (Figure 1). This area is part of the Pliocene Kueichulin Formation (6.5–4 Ma), which consists of thick interbedded sequences of sandstone, argillaceous sandstone, and shale. These sediments were deposited in wave- and tide-dominated shallow marine to deltaic environments [17,18,19]. Based on lithological characteristics, the lower portion of the Kueichulin Formation is also referred to as the Tapu Formation. This unit is characterized by thick-bedded, dark yellowish sandstone and greyish, fine-grained argillaceous sandstone interbedded with shale layers [17]. Marine fossils, including Ditrupa species, are commonly found in this interval. The top boundary of the Tapu Formation is notably enriched with ferric oxide concretions, from which the sample used in this study was obtained [17,20]. The upper portion of the Kueichulin Formation is known as the Erchiu Formation. It is composed primarily of interbedded, thick, greenish-grey argillaceous sandstone and siltstone, along with shale [17]. Subsequent tectonic activity during the Pleistocene, driven by the subduction of the Philippine Sea Plate beneath the Ryukyu arc, resulted in the folding of the Kueichulin Formation and overlying strata into the Bitou Syncline and the Wentzukeng Anticline [18].

The concretion sample was sectioned along a profile shown in Figure 2a, and the middle portion (Figure 2b) was extracted and polished with progressively finer sandpaper and ultrasonically cleaned to remove any residual particles throughout each step before analysis. The central portion of the concretion was further subdivided into three distinct regions for analysis: (1) Region A, the rim, representing the exposed and protruding outer part; (2) Region B, the core, representing the internal, non-exposed portion; and (3) the surrounding matrix (Region M), which borders both sides of the concretion (Figure 2b). Micro-Raman spectroscopy was employed to characterize the mineral composition of these three regions.

Measurements were conducted using a Micro-RAM300 micro-Raman spectrometer (Lambda Vision, Sagamihara, Japan). The excitation wavelength was set to 532 nm, and a 20×/0.45 objective lens (Nikon, Tokyo, Japan) was used. Laser power was adjusted depending on the specific component being analyzed. The exposure time ranged from 5 to 10 s, with 5 to 10 accumulations per measurement. A diffraction grating of 1800 grooves/mm was used. All spectral data were processed using OriginPro 2022, including baseline correction and spectral smoothing as necessary.

3. Results

The five-times-magnified microscopic image of the rim (region A, Figure 3a) reveals that the concretion sample is primarily composed of a brown matrix, interspersed with clustered distributions of course angular transparent white to grey mineral grains. Some white grains displayed a yellowish coating along the grain boundaries (Figure 3b). Few dark grey angular grains are also noted. The Raman spectra obtained from the brown matrix region exhibits characteristic shifts at 612 cm⁻¹ (E_g), 405 cm⁻¹ (E_g), 290 cm⁻¹ (E_g), and 223 cm⁻¹ (A_1g), which are attributed to hematite (Figure 3b; [22,23,24,25]). For the yellowish grain boundary material (Figure 3c), the presence of Raman shifts at 640 cm⁻¹ (E_g), 514 cm⁻¹ (A_1g), 395 cm⁻¹ (B_1g), 149 cm⁻¹ (E_g) indicates the existence of anatase [8,26]. The clear white and grey grains (Figure 3d) revealed Raman shifts at 810 cm⁻¹ (E₁), 467 cm⁻¹ (A₁), 365 cm⁻¹ (A₁), 270 cm⁻¹ (E_t + E₁), and 126 cm⁻¹ (E_t + E₁), which can be assigned to quartz [23,27]. The dark grey grain exhibits prominent D and G bands at approximately 1350 cm⁻¹ and 1585 cm⁻¹, respectively, which are characteristic of kerogen, along with an additional signal at 1035 cm⁻¹ associated with other organic or mineral phases. These results indicate that the mineral composition of the concretion consists of aggregates of quartz grains, an authigenic anatase coating on quartz grains [28], and organic carbon fragments. These sedimentary grains are cemented with a hematite matrix forming the concretion.

Following grain composition identification, the distribution and characterization of organic carbon material (kerogen-bearing) within the dark grey gains across the rim (A), core (B) of the concretion, and the surrounding matrix (M) are compared and contrasted. The first-order Raman spectra (1800–1000 cm⁻¹) from each region are presented in Figure 4. Distinct D (~1350 cm⁻¹) and G (~1585 cm⁻¹) bands were observed in all three regions. Compared to the concretion interior, the surrounding matrix (M) showed high-intensity but wider peaks that are connected without complete separation. For the concretion portion, sharp peaks are measured. The core (B) showed narrow, clear, isolated peaks. Of the three analyses, all showed a lower intensity for D bands and a higher intensity for G bands. The rim (A) portion, however, showed a lower intensity for both the D band and the G band compare to the core region. Two of the three analyses showed a higher intensity for the D band than for the G band. Furthermore, the second-order Raman region (3500–2400 cm⁻¹) was also measured but did not exhibit any significant or discernible bands.

The degree of separation between the D and G bands is indicative of the structural ordering of kerogen (Figure 5). Among the regions analyzed, the core (B) showed the least band separation, suggesting the lowest degree of structural order. The rim (A) exhibited greater separation, indicating moderate ordering, while the matrix (M) demonstrated the highest degree of ordering, as inferred from the sharpness and separation of the D and G peaks.

Although no microscopic fossils can be identified within the concretion, coexistence the of quartz, kerogen and a minor amount of aragonite (C–O symmetric stretching: 1085 cm⁻¹, in-plane bending: 705 cm⁻¹, 701 cm⁻¹) [29,30] is observed at a specific location within the core region, suggesting the possibility of shell fragments within the sediment clasts (Figure 6).

4. Discussion

Concretions are commonly defined as compact masses formed by the precipitation of mineral cement within the pore spaces of clastic sediments such as sand or mud [3,6]. In this study, the concretion sample is characterized by the widespread presence of brown material exhibiting Raman shifts that are consistent with hematite. These hematite-rich regions are distributed across both the rim (A) and core (B) of the concretion (Figure 3a), allowing this specimen to be classified as an Fe₂O₃ concretion. Additionally, kerogen was found in a clustered distribution within the concretion, which is in agreement with previous findings by Lai and Huang [17], who reported ferric oxide concretions in this coastal area of Nanya. The quartz detected in the sample likely originated from detrital clastic sediments, such as sand.

The absence of discernible Raman bands in the second-order region (3500–2400 cm⁻¹) suggests that kerogen in this sample has undergone only diagenesis and not metamorphism [13]. Furthermore, the D3 band (~1500 cm⁻¹), typically associated with out-of-plane vibrations due to defects and heteroatoms, is absent in the kerogen from the rim (A) and the matrix (M). Its presence only in the core (B) suggests a lower degree of diagenetic alteration, as the D3 band tends to disappear progressively with increasing diagenesis or graphitization [13,16]. This observation supports the interpretation that the kerogen in the core retains a higher proportion of original organic material than that in the rim and matrix.

The key distinction between kerogen in the rim (A) and matrix (M) lies in the shape and sharpness of the G band. Based on the measured average full width at half maximum (FWHM), the G band from the rim exhibits a broader width (~25.4 cm⁻¹) than that from the matrix (~21.5 cm⁻¹), indicating a lower degree of graphitization in the rim relative to the matrix [12,13,15,16]. These results support the following interpretation of the progression of kerogen alteration due to diagenesis and graphitization: core (B) < rim (A) < matrix. The relatively well-preserved state of organic matter in the core is likely due to a “sealing effect” created by the dense, low-porosity hematite mantle, which limits exposure to external environmental influences such as fluid migration. In contrast, the kerogen in the rim and matrix shows greater structural ordering, reflecting a higher degree of alteration.

The Nanya coastal area is known for its weathered sandstone formations, suggesting that the weathering processes may have also influenced the kerogen in this concretion. Petsch et al. [31] reported that weathering can lead to the loss of specific types of organic matter in kerogen. We propose that the absence of the D3 band in the kerogen from the rim might be due to the weathering effect since the rim region is more exposed to the atmosphere than the core in our sample. Our results demonstrated that both diagenesis and weathering contributed to the degradation of organic matter in the rim and matrix.

An unexpected finding in this study is the presence of aragonite in the core (Figure 6). Aragonite, a metastable form of CaCO₃, typically forms in high-pressure, low-temperature environments such as marine settings [32,33]. It is generally expected to transform into calcite during shallow marine burial due to decreased pressure [29,34]. Another possible source of aragonite is the shell fragments of marine mollusks [29,35]. Since there is no clear shape, we can not determine the true origin of the aragonite. The preservation of aragonite in the core, however, indicates that the concretion provided protection against diagenetic transformation, further supporting the notion of the mineral mantle’s protective function.

5. Conclusions

The present study demonstrates the protective role of the mineral-rich mantle in preserving organic matter within a ferric oxide concretion collected from the Nanya, northeastern coast of Taiwan. Raman spectroscopic analysis reveals that the degree of graphitization and diagenesis of kerogen varies spatially across the concretion: core (B) < rim (A) < matrix (M). The presence of the D3 band in the core and its absence in the rim and matrix suggest that the core kerogen has undergone significantly less alteration and retains more of its original organic structure. The dense and compact hematite mantle contributes to the preservation of the core by providing a barrier to external diagenetic and weathering influences. In contrast, kerogen in the rim and matrix exhibits a higher degree of structural ordering and graphitization, indicating more advanced diagenetic transformation. Weathering may also play a role in the degradation of kerogen, particularly in the rim and matrix, as evidenced by the loss of the D3 band. The detection of aragonite in the core provides further evidence of the mantle’s protective effect, as it inhibited the typical transformation of aragonite to calcite in shallow marine sediments. This finding also raises the possibility that the preserved organic matter in the core may be biologically derived, potentially from mollusks.