Arsenic enrichment patterns are defined by microbialite morphology, fabric, and accretion mechanism

Domain: earth_science
Published: 2025-11-20
Editorial outcome: minor_revision

Abstract
Microbialites accrete through microbe-environment interactions and incorporate elements like arsenic, creating enrichment patterns that can preserve evidence of ancient microbial activity. However, the effects of morphology, fabric, and accretion mechanism on arsenic incorporation in microbialites is poorly understood, complicating the use of arsenic enrichment patterns as a chemical biosignature. By analyzing arsenic concentrations in actively accreting microbialites with diverse architectures from Hamelin Pool, Australia, we document the effects of morphology, fabric, and accretion mechanism on arsenic enrichment patterns. Our results demonstrate that arsenic enrichment patterns originate from microbial activity, sedimentary inputs, and seawater chemistry, the proportions of which vary with changing aspects of microbialite morphogenesis. Here we show that initial microbialite architecture is a fundamental yet underexplored factor that controls the geochemical composition of microbialites through geological time.

Introduction
Dynamic interplay between microbial communities and early Earth environments shaped the evolution of major biogeochemical cycles and facilitated the habitability of our planet. Evidence of the interaction between life and environment exists in organosedimentary deposits formed by micrite precipitation and/or trapping and binding of sediments by microbial communities, called microbialites1. Morphological and textural variations in microbialites have been used to reconstruct microbe-environment interactions2–7, but less is known about how morphological and textural characteristics influence the incorporation and preservation of elemental enrichment patterns, and thus their utility as chemical biosignatures.

Metalloid enrichment patterns in microbialites, especially arsenic, have received recent attention8–14 because arsenic-rich, cell-like globules from the 2.7 Ga Tumbiana microbialites12 suggest they may be enduring geochemical proxies for early metabolisms15. Variable spatial correlations between arsenic and other micro-essential trace elements (i.e., Fe, Mn, Zn, Cu) led to the interpretation of arsenic enrichment as a chemical biosignature in both modern and ancient microbialites9–12. A key assumption is the existence of mechanistic links between arsenic enrichment patterns and biological activity irreplicable by physical or chemical processes alone16–18. Arsenic can be influenced by microbial communities through arsenotrophy, which involves arsenate (AsO₄³⁻) respiration and photosynthetic arsenite (AsO₃³⁻) oxidation9,10,12,14, and detoxification mechanisms13. Microbial arsenic cycling occurs in environments with both high and low arsenic concentrations, and the associated mats accrete microbialites with varying morphologies and textures9,10,13,14,19–22.

Microbial communities also influence the accretion of microbialites. Major aspects of microbialite morphogenesis can be defined at three scales of observation5 (“Microbialite Balancing Act”; Fig. 1), each of which may influence arsenic incorporation. Variations in the three-dimensional shape of microbialites at the meter-scale are described by microbialite morphology5,23,24 (Fig. 1 and Supplementary Fig. 6). At the centimeter scale, internal textures such as clotting or lamination produce microbialite fabrics that are visible in hand sample7,25–27 (Fig. 1 and Supplementary Fig. 7). Finally, changes in accretion mechanism drive microscopic variability in the proportion of micrite, organic matter, and agglutinated grains6,25–27 (Fig. 1 and Supplementary Fig. 8). Microbialite morphogenesis has changed over geological time in response to changes in microbial community composition, the evolution of new metabolic pathways, and environmental changes2,3,5–7,24,28. Since microbialite morphogenesis influences the production, sequence, and arrangement of organic matter, micrite, and agglutinated grains5, morphogenetic processes likely control the incorporation rate and partitioning of arsenic and other trace elements, but such effects have yet to be investigated.

To examine how microbialite morphology, fabric, and accretion mechanism affect the incorporation and partitioning of arsenic in microbialites, we examined a well-constrained, modern microbialite depositional system in Hamelin Pool, Shark Bay, Western Australia. Hamelin Pool is an ideal study system because it hosts a diversity of microbialite architectures5,25–27,29–34 (Fig. 1). Morphologies range from unlithified sheet mats to lithified buildups that are accreted by microbial communities29–31,33,34, some of which engage in arsenic-related metabolic processes19–22. Microbialite fabrics exhibit both laminated and clotted textures5,25,26,29 (Fig. 1), and both agglutinated and micritic accretion mechanisms have been observed25–27 (Fig. 1). Here, we demonstrate the important influence of microbialite morphogenesis on arsenic enrichment patterns in Hamelin Pool microbialites. Integrating elemental analyses of microbialites, sediment, seawater, and groundwater samples, we show that morphology, fabric, and accretion mechanism predictably correspond to changes in arsenic enrichment patterns in organic matter and micrite. Some enrichment patterns originate from variations in seawater chemistry, while others require the involvement of microbial activity. Thus, microbialite morphogenesis is a fundamental factor to consider when interpreting the significance of elemental enrichment patterns in ancient and modern microbialites.

Results
Microbialites from Hamelin Pool consist of three major geochemical components, the proportions of which vary in different microbialite types: (1) an organic matter fraction, (2) bulk carbonate —including micrite and carbonate allochems— and (3) a 0.35M HNO₃ acid-insoluble siliciclastic fraction (Figs. 2, 3, and 5). Microbialite components were leached quantitatively and independently (see Supplementary Methods), enabling measurements of both absolute arsenic concentration and the size of each compositional fraction (e.g., micrite, trapped-and-bound allochems, or organic matter), where size refers to the relative mass contribution of each fraction to the total sample mass. A mixing model was used to subdivide bulk carbonate into micrite and trapped-and-bound carbonate allochems fractions. Of the four fractions present, organic matter and micrite are produced by the accreting mat, while carbonate allochems and siliciclastic grains are trapped and bound during accretion. The bulk carbonate is comprised of ~95% aragonite, 4% high-magnesium calcite (HMC), and 1% low-magnesium calcite (LMC), while the siliciclastic fraction is primarily composed of quartz (see Supplementary Discussion), likely sourced from the Peron Sandstone on the western coast of Hamelin Pool35. Because siliciclastic-associated arsenic does not originate from microbial activity or modern seawater chemistry, it was excluded from further consideration.

In the following, we examine the size and arsenic concentrations of the organic matter, micrite, and trapped-and-bound allochem fractions in microbialites with different morphologies, fabrics, and accretion mechanisms (Figs. 2–5). We explore how microbial activity, agglutination, and seawater chemistry interact to produce unique arsenic enrichment patterns (Figs. 6–8; Table 1). Finally, we evaluate the utility of these patterns as chemical biosignatures in modern sedimentary environments and consider their preservation potential in the geological record (Fig. 9).

Control of microbialite morphology on arsenic incorporation
Microbialite morphology influences the size of the organic matter and micrite fractions, but not the allochem and siliciclastic fractions (Fig. 2A). Sheet mats contained ~5.6× more organic matter (21.3±3.4 wt%) than discrete microbial buildups (3.7±1.6 wt%; Fig. 2A) and less micrite (45.3±1.6 wt% vs. 71.9±17.0 wt%; Fig. 2A). Consequently, the proportion of organic matter should be expected to have a greater influence on overall arsenic enrichment patterns in sheet mats compared to discrete microbial buildups (Fig. 9). The mass of the allochem fraction did not differ between sheet mats and discrete microbial buildups (Fig. 2A), suggesting that this fraction contributes equally to arsenic enrichment patterns in these microbialite morphologies. High organic matter production in sheet mats could be related to differences in environmental settings. Sheet mats dominate the supratidal zone29,31,33,34 (Fig. 1), where high evaporation rates increase osmotic stress19, stimulating the production of extracellular polymeric substances (EPS) that offer protection against environmental stressors33,36. Thick EPS layers bind Ca²⁺ and inhibit carbonate precipitation37,38, resulting in low micrite abundance. In contrast, discrete microbial buildups form in intertidal and subtidal zones25,26,29,31–34 (Fig. 1), where lower environmental stress allows faster EPS consumption by heterotrophic microbial communities19, promoting the release of Ca²⁺ from EPS, enhanced micrite precipitation39, and a larger proportion of micrite (Fig. 2A).

Microbialite morphology also affects arsenic concentrations in organic matter and micrite. Arsenic concentrations were higher in sheet mats than in discrete microbial buildups, both in micrite (2.3±1.3 mg/kg vs. 1.0±0.6 mg/kg; Fig. 2B) and organic matter (10.2±1.9 mg/kg vs. 5.3±3.8 mg/kg; Fig. 2D). Environmental conditions unique to each morphology may be explanatory: Sheet mats form in supratidal zones31,33 (Fig. 1), where evaporation increases seawater arsenic concentrations19 and reduces rates of sulfate reduction and methanogenesis40. Arsenic-related metabolisms are necessary adaptations in these settings. More arsenic-related genes occur in sheet mats that facilitate energy production in localized anoxic niches, while others encode for detoxification19,20,22. Negatively charged EPS can immobilize arsenic in alkaline conditions41,42 like in Hamelin Pool43 (pH ~8.1). Arsenic is thus accumulated in higher concentrations in organic matter and micrite produced by sheet mats that form in challenging environmental conditions. When the mat disaggregates, micritic grains can be released to the environment26,27,31,33, providing an arsenic-rich allochem supply that can be trapped by other microbialites. No significant difference in arsenic concentration was observed in the allochem fraction of different microbialite morphologies (Fig. 2C), suggesting that sediment compositions are relatively consistent in the supratidal and inter/subtidal zones, where sheet mats and discrete microbial buildups occur (Fig. 1).

Control of microbialite fabric on arsenic incorporation
Changes in microbialite fabric do not influence the relative sizes of geochemical fractions. Microbialites with clotted and laminated fabrics exhibit no differences in the proportion of organic matter, micrite, or the allochem and siliciclastic fractions (Fig. 3A). Clotted and laminated fabrics accrete primarily by micrite precipitation25,26 and thus have abundant micrite and relatively low organic matter content (Figs. 3A and 9). Variations in micrite arrangement have been linked to light availability in different tidal zones25,29 (Fig. 1). Laminated fabrics form in lower intertidal and subtidal zones with lower light (Fig. 1), prompting smaller Entophysalis colonies to align in layers that become micritic laminae25. In contrast, clotted fabrics form in higher-light upper intertidal settings (Fig. 1), where larger Entophysalis colonies form clusters resulting in clots25. Although variation in colony arrangement alters the fabric, it does not change the overall proportions of micrite, organic matter, allochems, or siliciclastic grains (Fig. 3A).

Arsenic concentrations in organic matter, micrite and allochem fractions in clotted and laminated fabrics exhibited unique trends. The arsenic concentration of organic matter from clotted and laminated microbialites was identical (Fig. 3D), while more arsenic was observed in micrite from laminated fabrics (1.1±0.4 mg/kg) compared to clotted fabrics (0.7±0.4 mg/kg; Fig. 3B). In contrast, higher arsenic concentrations were measured in the allochem fraction trapped and bound by microbialites with clotted fabrics (1.9±0.5 mg/kg) compared to those with laminated fabrics (1.3±0.8 mg/kg; Fig. 3C). These results suggest that the composition of the sedimentary environment differs between the upper intertidal zone, where clotted microbialites occur, and the lower intertidal/subtidal zones, where laminated microbialites are found (Fig. 1).

The relationship between arsenic concentrations in organic matter and micrite varied between fabrics (Fig. 4). Laminated fabrics exhibited a positive correlation between arsenic in organic matter and micrite (Fig. 4A), suggesting that the amount of arsenic incorporated into the micrite of laminated microbialites depends on the arsenic accumulated in the co-occurring organic matter (Eq. 1):

[As]micrite = 0.12 (±0.03) × [As]organics + 0.79 (±0.18) (1)

In contrast, clotted microbialites exhibited no correlation between arsenic content in the organic matter and micrite (Fig. 4B). Therefore, there is no evidence that organic matter arsenic concentrations influence the amount of arsenic in the micrite produced in clotted fabrics.

Differences in arsenic concentrations between organic matter and micrite in laminated and clotted microbialites are likely influenced by distinct community metabolisms. Laminated fabrics are accreted by mats in which energy production is driven by sulfate reduction19. In deep, anoxic layers of the mat, sulfate reduction generates bicarbonate, raising local pH and promoting carbonate precipitation44–47, while arsenic cycling associated with sulfur-based metabolic pathways14 enhances arsenic accumulation in organic matter9,10,48. The spatial overlap between arsenic-related metabolic activities and locus of micrite precipitation in laminated microbialites facilitates the transfer of arsenic from organic matter to micrite (Fig. 4A). The slope of the regression in Fig. 4A describes this transfer rate, suggesting that 12% of organic matter-associated arsenic is incorporated into micrite forming laminated fabrics (Fig. 4A; Eq. 1). Conversely, micrite in clotted fabrics is accreted by mats dominated by photosynthetic genes19,25, which increase alkalinity and promote carbonate precipitation in oxygenated microenvironments near the mat surface44,45. The spatial decoupling of micrite precipitation near the mat surface and arsenic-related microbial activities occurring in deeper anoxic niches result in no correlation between the arsenic content of organic matter and micrite in clotted fabrics (Fig. 4B).

Control of accretion mechanism on arsenic incorporation
Accretion mechanism influences arsenic incorporation by altering the relative proportions of the micrite and trapped-and-bound fractions, leaving the size of the organic matter fraction unchanged (Fig. 5A). Agglutinated microbialites contained less micrite (65.3±18.4 wt%) and more trapped-and-bound material, including allochems (17.1±9.8 wt%) and siliciclastic minerals (14.3±9.2 wt%) than micritic microbialites (Fig. 5A), consistent with petrographic descriptions25,26. Higher abundances of trapped-and-bound grains means that sedimentary allochems, some of which have high arsenic concentrations (Table 1), exert a stronger influence on the total arsenic concentration of agglutinated microbialites. In contrast, micritic microbialites are dominated by micrite (78.8±12.4 wt%) and contain lower proportions of trapped-and-bound grains (Fig. 5A). Variations in accretion mechanism in Hamelin Pool have been proposed to be related to changes in the sediment supply across tidal zones (Fig. 1), which influence the proportions of micrite and carbonate allochems in the accreting microbialite, and thus the pathways of arsenic incorporation25–27,29–32.

Variations in accretion mechanism do not impact the arsenic content of micrite and organic matter fractions in Hamelin Pool’s microbialites. Micrite arsenic concentrations were similar in agglutinated (1.0±0.5 mg/kg) and micritic microbialites (0.9±0.4 mg/kg; Fig. 5B), as were arsenic concentrations in organic matter (6.5±4.3 mg/kg and 4.0±2.7 mg/kg, respectively; Fig. 5D). No influence of accretion mechanism on arsenic concentrations in micrite and organic matter contrasts with the trends produced by variation in morphology and fabric (Figs. 2B, D and 3B, D), which were linked to changes in the metabolic profiles of the mats19. By contrast, nearly identical metabolic profiles of mats producing agglutinated and micritic microbialites19 indicate that similar processes likely govern organic matter and micrite production.

The trapped-and-bound carbonate allochem fraction of agglutinated and micritic microbialites exhibited similar arsenic concentrations (1.5±0.8 mg/kg and 1.7±0.7 mg/kg, respectively; Fig. 5C), reflecting the non-selective nature of the trapping and binding process6. The concentration of arsenic in the trapped-and-bound allochem fraction is controlled by the sediment arsenic content, which varied across provinces (see Supplementary Table 4), reaching a maximum in Nilemah (2.3±0.2 mg/kg), and a minimum in Carbla (1.6±0.3 mg/kg). Variations in bulk sediment arsenic concentrations reflect changes in the abundance of micritic grains, coated grains, bivalves, foraminifera, and gastropods, which are influenced by ecological niche requirements, transport, and redeposition29–34. Arsenic concentrations of sedimentary allochems ranged from 0.4±0.1 mg/kg (Fragum erugatum) to 2.8±0.2 mg/kg (micritic grains; Table 1). Due to their high abundance31,33 and elevated arsenic concentrations, micritic grains are the dominant source of arsenic in the trapped-and-bound fraction throughout the basin, contributing 78.2±16.8% of bulk sediment arsenic concentrations (see Supplementary Table 4; Fig. 6).

Our results indicate that microbialites with different accretion mechanisms can exhibit similar overall arsenic concentrations (Fig. 5B–D) but the mechanisms driving arsenic enrichment patterns differ (Fig. 6). In micritic fabrics, 80.6±16.2% of the arsenic in the bulk carbonate is associated with micrite (Fig. 6A), indicating that arsenic enrichment patterns are driven primarily by differences in arsenic transfer efficiency from organic matter to micrite (Fig. 4A). In contrast, an average of 32.0±24.1% of total arsenic is associated with trapped-and-bound allochems in agglutinated microbialites, though this contribution varies widely across the basin (1–85%). Consequently, arsenic concentrations in agglutinated microbialites are more influenced by the quantity and type of allochems that are trapped and bound (Fig. 6B) compared to micritic microbialites. Given that micritic grains eroded from sheet mats are the most arsenic-enriched and the most abundant allochems (Table 1), they play a crucial role in arsenic enrichment patterns in agglutinated microbialites (Fig. 6B), occasionally surpassing micrite precipitated by the accreting mat as the dominant source of arsenic.

Origin of arsenic enrichment patterns across microbialite architectures
Morphology, fabric, and accretion mechanism each affect arsenic incorporation by influencing the size and arsenic content of the organic matter, micrite, and trapped-and-bound allochem fractions in microbialites. Variations in these aspects of morphogenesis produce distinct arsenic enrichment patterns within the microbialite architectures of Hamelin Pool (Figs. 2, 3, 4, 5, 6). Like the “Microbialite Balancing Act”, in which interplay between environment and microbes determines the initial microbialite architecture5, similar dynamics drive the development of arsenic enrichment patterns. The extent to which these patterns can be interpreted as chemical biosignatures depends on their likelihood of forming through biotic versus abiotic mechanisms16–18. Thus, interpreting the origin of arsenic enrichment patterns requires the quantification of all mechanisms contributing to arsenic incorporation in each microbialite architecture. Spatio-temporal variations in seawater arsenic concentration can modulate baseline arsenic incorporation, whether driven by biotic or abiotic processes13,49. In Hamelin Pool, seawater arsenic concentrations increased from 3.9±0.2 µg/kg in the northern provinces to 6.9±0.2 µg/kg in the southernmost Nilemah Province (Fig. 7; Student’s t-test, t(5)=-6.777, p<0.001). In the following, we evaluate the degree to which arsenic enrichment patterns in the microbialite architectures of Hamelin Pool are influenced by microbial and environmental processes (Fig. 8) and assess the utility of these arsenic enrichment patterns as chemical biosignatures in the rock record (Fig. 9).

Datasets presented here (Eq. 1, Fig. 4A) provide an opportunity to examine how environmental and microbial processes contribute to arsenic enrichment patterns in micrite precipitated by microbial mats in variable environmental conditions (Fig. 8). Carbonate minerals can incorporate arsenic from seawater without microbial influence through surface adsorption or carbonate group substitution50 (CO₃²⁻ ↔ HAsO₃³⁻). At equilibrium, this abiotic incorporation of arsenic can be described by a distribution coefficient, previously estimated for calcite to be 490 L/kg for arsenate and <0.23 L/kg for arsenite50. Similar incorporation mechanisms likely apply to aragonite51, but the absence of known partition coefficients limits estimates of arsenic incorporation from seawater into the aragonitic micrite of Hamelin Pool microbialites. Without arsenic transfer from organic matter ([As]org = 0; Eq. 1), micrite in laminated microbialites is predicted to contain 0.79±0.18 mg As/kg. We propose that this estimate approximates inorganic arsenic incorporation from seawater into aragonitic micrite in Hamelin Pool. Normalizing this value to the average arsenic concentration in Hamelin Pool seawater (Fig. 7) yields an “inorganic” enrichment factor (EFinorg) of 120±38 L/kg (or 1.13±0.29 normalized to the sum of Ca and Mg concentrations in the aragonite and seawater), aligning with the range of expected partition coefficients for calcite described above. Although not a true partition coefficient because microbial activity is responsible for the formation of the micrite incorporating the arsenic25,27,29,33, the EFinorg provides an initial point of reference. The amount of arsenic incorporated into the micrite from seawater, with minimal microbial influence, is expected to increase linearly with rising arsenic concentrations in seawater (Eq. 2; Fig. 8, blue line):

[As]micrite from seawater = 0.12±0.04 × [As]SW (2)

Equation 1 also describes the contribution of microbial processes to arsenic incorporation. The first term of the equation suggests that ~12% of arsenic incorporated into organic matter is transferred to micrite in laminated discrete microbial buildups (Fig. 4A). This contribution of arsenic from organic matter to micrite, isolated in Eq. 3, depends on organic matter arsenic concentrations, which vary across Hamelin Pool provinces (Kruskal-Wallis test; H(6)=9.744, p=0.045) and correlate exponentially with seawater arsenic concentrations (Pearson correlation after linearization; r=0.789, p=0.035; Supplementary Fig. 10).

[As]micrite from organic matter = 0.12±0.03 × [As]org = 0.12±0.03 × 0.684e0.355[As]SW (3)

The transfer of arsenic from organic matter to micrite in laminated fabrics (Eq. 3; Supplementary Fig. 10) supplements the inorganic incorporation of arsenic from seawater with minimal microbial influence (Eq. 2; Fig. 8, blue line). The total arsenic concentrations incorporated into micrite through this combined input are expected to increase exponentially as seawater arsenic concentrations rise (Fig. 8A, green line).

Mixed origins for arsenic enrichment patterns in laminated buildups
Arsenic enrichment patterns in micrite from laminated fabrics originate from a combination of organic matter degradation and seawater, with contributions from both sources modulated by seawater arsenic concentrations (Fig. 9). This interpretation is evidenced by the average micrite arsenic concentrations in laminated buildups collected across six provinces, which exhibit an exponential relationship with seawater arsenic concentrations (Fig. 8A; yellow triangles; Pearson regression after linearization; r=0.879, p=0.021). Laminated micrite arsenic enrichment factors (EF) consistently surpass the baseline for inorganic arsenic incorporation (EFinorg), with notable regional variability (ANOVA; F(5,16)=3.052, p=0.043). EF increases southward, from 1.5±0.5 at Nanga to 2.1±0.2 at Flagpole North, reflecting a gradual increase in the rate of arsenic input from organic matter degradation. Arsenic from organic matter degradation accounts for ~42% of arsenic incorporated in micrite in relatively arsenic-poor northern provinces, and up to ~54% in the arsenic-rich southern provinces (Fig. 8A). The contribution of arsenic from organic matter degradation is never negligible in laminated fabrics, and arsenic concentrations consistently exceed what inorganic incorporation alone can explain, thereby satisfying the criteria for a chemical biosignature16,17 (Fig. 9).

Arsenic enrichment patterns in the micrite of laminated microbial buildups beyond Hamelin Pool likely originate from a similar combination of organic and inorganic contributions. We make this interpretation because laminated buildups are known to be constructed by mats dominated by microbial communities with heterotrophic metabolisms46,47. Heterotrophic metabolisms are required to transfer arsenic from organic matter into micrite (Fig. 4A). Actively accreting laminated microbial buildups from Laguna Diamante (Argentina) provide evidence in support of this interpretation, because heterotrophic degradation in deeper layers of the mat was interpreted to facilitate arsenic incorporation into micrite10. Our data also suggest that the arsenic-rich lake water conceivably increases micrite arsenic concentrations (Eq. 2), suggesting that both inorganic and organic processes likely influence arsenic concentrations in micrite from microbialites in Laguna Diamante. Thus, such a dual origin of arsenic may be applicable to laminated microbial buildups more broadly.

Sheet mat arsenic enrichment patterns originate from organic matter
Arsenic concentrations in sheet mat micrite originate both from organic matter degradation and seawater (Fig. 8A), suggesting that micrite in sheet mats forms via similar processes to those in laminated buildups. Similar micrite-to-organic matter arsenic concentration ratios in sheet mats (3.6±1.1) and laminated buildups (3.4±2.4; Mann-Whitney U-test, W=28, p=0.648) suggest that arsenic transfer occurs at comparable rates. However, because sheet mats form in arsenic-enriched supratidal zones5,31,33,34, they accumulate more arsenic in the organic matter (Figs. 2D and 3D), likely arising from the increased prevalence of genes encoding arsenic cycling and resistance19. Correspondingly, micrite arsenic concentrations in sheet mats are consistently high (Fig. 8A), with an average EF of 3.6±1.2, exceeding the inorganic baseline (EFinorg). Arsenic concentrations in sheet mat micrite are 2.4±1.2× higher than in laminated discrete microbial buildups (Figs. 2B and 3B). Notably, heterotrophic transfer accounts for 71% of total arsenic in sheet mat micrite, surpassing both seawater contributions (29%; Fig. 8A) and heterotrophic transfer in laminated buildups (42–54%; Fig. 8A). Consequently, arsenic enrichment patterns in sheet mat micrite are consistent with the definition of a robust chemical biosignature, because microbial activity induces the conditions for enhanced arsenic incorporation superseding inorganic processes alone16,17 (Fig. 9). Although our analysis reveals consistent trends, the five sheet mats specimens examined here may not encompass the full environmental heterogeneity of Hamelin Pool, and future studies with broader sampling will be essential to evaluate the robustness of this interpretation.

Beyond Hamelin Pool, unlithified or weakly lithified microbialites often contain strong arsenic enrichment in the micrite, a characteristic previously linked to microbial activity9,10,13,14. High evaporation and high concentrations of toxic metals characteristic of the supratidal zone can select for this type of microbialite morphology via the enhancement of organic matter production, which accumulates arsenic. Based on our results, sheet mats should be expected to have higher arsenic enrichment patterns than co-occurring discrete microbial buildups (Fig. 9), with more arsenic originating from the degradation of organic matter (71%) than from seawater (29%). The origin of arsenic enrichment patterns in sheet mat micrite are thus easier to determine because the arsenic concentration of the micrite in these microbialites exhibits a higher dependency on the degradation of arsenic-rich organic matter (Fig. 8A).

The geological preservation potential of the morphological characteristics of sheet mats is generally low since the organic matter is rapidly decomposed5,11,33. With decomposition, micritic components of sheet mats are reworked29–31,33 (Fig. 9), potentially accumulating into extensive sedimentary deposits without conventional morphological biosignatures that indicate their microbial origin54,55. Thus, high arsenic enrichment in micritic grains provides evidence for past microbial activity that confirms the origins of the micrite and may be interpreted as a chemical biosignature (Fig. 9). Notably, exceptions to poor preservation exist when early silicification has protected organic residues, as observed in ancient mats from the Pilbara Craton56 (Australia) and the Barberton Greenstone Belt57,58 (South Africa). Such early silicification may enhance the preservation of trace metal enrichments in organic matter, and comparison of arsenic concentrations between putative mineralized mats and surrounding sediments could help confirm their biogenicity.

Clotted buildups record seawater arsenic concentrations
Arsenic enrichment patterns in clotted discrete microbial buildups in Hamelin Pool appear to be controlled by seawater chemistry, indicating they may serve as a useful proxy for reconstructing environmental conditions. Measured EF values of clotted microbialite micrite are 1.1±0.6, which is indistinguishable from EFinorg, suggesting that clotted micrite arsenic enrichment patterns originate from seawater (Fig. 8B). Further, micrite associated with clotted fabrics has 1.7±1.2× less arsenic than that associated with laminated fabrics (Fig. 3B). However, the relationship between micrite arsenic concentrations and seawater from clotted microbialite samples collected from four provinces varied (Fig. 8B). Samples from Nanga and Hutchison provinces overlap the 95% confidence intervals for predicted values based on seawater arsenic concentrations, but the regression overestimates values for samples from Flagpole North and underestimate values for samples from Flagpole South (Fig. 8B). Compared to trends in laminated microbialites, this variability suggests that seawater arsenic concentrations in these settings may be more temporally variable than captured in our dataset (Fig. 7). Changing contributions of arsenic-rich groundwater may be responsible (see Supplementary Discussion), but such trends are beyond the scope of this study. Since estimates of EFinorg are sufficient to explain arsenic enrichment patterns in clotted fabrics, it is unlikely that microbial activity is required to incorporate arsenic in these microbialites.

Outside of Hamelin Pool, carbonate precipitation in microbialites with clotted fabrics has been attributed to both phototrophic and heterotrophic metabolisms59–61. Modern microbialites with clotted fabrics from Laguna La Brava, Chile, similarly lack arsenic enrichments in micrite, despite significant arsenic accumulation in organic matter9. Low arsenic concentrations of micrite should be expected in phototrophic mats because while phototrophy is known to enhance micrite precipitation19,45, our results indicate no arsenic transfer from organic matter into carbonate in clotted fabrics (Fig. 4B). The geological preservation potential of clotted microbialites is high because early micritization provides structural stability and enhances structure lithification1,3,5,62. Arsenic enrichment patterns in clotted buildups likely hold limited value as chemical biosignatures but may provide an indication of environmental arsenic concentrations. Reconstructions could be attempted using the proposed EFinorg for aragonite, or the partition coefficient for calcite50. Well-preserved ancient examples, such as the 2.4 Ga Turee Creek Group thrombolites63 (Australia), offer promising sites to test this model because four distinct clotted fabrics were interpreted to form under varying paleoenvironmental conditions, which may have influenced arsenic enrichment patterns.

Agglutinated microbialites contain intrinsic and extrinsic biosignatures
Arsenic enrichment patterns in agglutinated microbialites largely arise from the composition of the trapped-and-bound allochems (Table 1; Fig. 9). Micrite produced by agglutinated mats has a higher EF (1.5±0.7, n=22) than Fragum erugatum bivalves (0.6±0.1, n=3; Welch’s t-test, t(20.7)=4.934, p<0.001) and gastropods (0.9±0.1, n=3, Welch’s t-test; t(19.9)=3.195, p=0.005). Both calcifiers regulate trace element uptake, often excluding toxic elements like arsenic64,65. In contrast, foraminiferal tests have EF (1.6±0.2, n=3) comparable to micrite (Welch’s t-test; t(16.5)=-0.961, p=0.350), because they are deposit-feeding benthic species that ingest arsenic-rich organic detritus and clay66. Micritic grains, produced by sheet mats33, have higher EF (5.4±0.2, n=3) than micrite formed by mats producing agglutinated microbialites (Student’s t-test; t(23)=-9.042, p<0.001; Fig. 2B). Our results show that each allochem in Hamelin Pool has a unique EF, which can be lower, similar to, or higher than those in the micrite precipitated by the mat accreting agglutinated buildups. Therefore, trapped-and-bound allochems may attenuate, mimic, or amplify the EF generated by the accreting mat. Notably, the high EF of trapped-and-bound micritic grains could be easily misinterpreted as a product of the activity of the microbial mat that accreted the ancient agglutinated microbialites, if such trends go undetected.

Our results demonstrate the need for careful sedimentological description of samples before geochemical analysis and interpretation of EF. Outside Hamelin Pool, modern agglutinated microbialites incorporate grains of variable origins reflecting local sedimentary environments6,27. For instance, Bahamian microbialites predominantly trap and bind ooids5,67 which are also influenced by microbial activity68, and thus have the potential to impart an EF that is distinct from the micrite produced by the accreting mat. Ancient agglutinated microbialites often co-occur with micritic grains54,55, like in Hamelin Pool32,34,35, creating scenarios where trapped-and-bound allochems could mask or mimic arsenic enrichment patterns associated with micrite formed by the accreting mat (Fig. 9). The composition of allochems also varies through geologic time: Phanerozoic microbialites commonly incorporate skeletal grains such as bivalves, foraminifera, and other bioclasts, whereas Precambrian microbialites lack such components due to the absence of carbonate-producing metazoans3,6. Temporal changes in allochem composition likely affect arsenic enrichment patterns in microbialites accreted at different periods. Careful petrographic analysis, sequential leaching experiments, and mixing models integrating measurements of microbialites and sediments are critical for disentangling the contributions of micrite and allochems. By doing so, well-preserved agglutinated microbialites can serve as geochemical archives for both microbial processes and sedimentary influences (Fig. 9).

Implications for the interpretation of ancient microbialites
Our findings demonstrate that arsenic enrichment patterns provide a geochemical tool for evaluating the biogenicity of ancient carbonate deposits. Most notably, we observe systematic variability in arsenic enrichment patterns among distinct microbialite architectures accreting in the same depositional environment. Attributing such structured geochemical variability to abiotic processes would require multiple finely-tuned geochemical regimes operating independently yet synchronously. In contrast, spatial diversity in microbial communities readily accounts for these patterns. This is particularly relevant for the interpretation of ancient microbialite-bearing successions, which commonly preserve a range of morphologies, fabrics, and accretion mechanisms4,57,63,69. Detecting shifts in arsenic enrichment patterns that correlate with changes in microbialite morphogenesis, such as across the seven distinct stromatolite facies of the Strelley Pool Formation4, may provide strong evidence that these transitions of microbialite architectures reflect biological, rather than abiotic, processes.

Interpreting arsenic enrichment patterns in ancient microbialites requires careful consideration of post-depositional processes that are capable of modifying the original geochemical signatures. Diagenetic alteration such as cementation, dissolution, and recrystallization may redistribute arsenic, potentially changing the original enrichment patterns, as observed for other redox-sensitive elements (see review by Swart70). Identifying signs of diagenetic overprinting, including changing textures, syntaxial cements, or coupled variations in redox-sensitive elements, should be carefully assessed through combined petrographic and geochemical screening. In metamorphosed settings, substantial caution is warranted, since original trace element patterns can be obliterated52. Despite these challenges, pristine examples confirm the possibility for long-term preservation. In the Tumbiana Formation (2.7 Ga), arsenic-rich carbonaceous globules interpreted as micritized microbial cells suggest retention of arsenic enrichment patterns during early diagenetic alteration12. Likewise, Chraiki et al.69 reported not only arsenic enrichment, but also contrasting arsenic distributions across stromatolites and thrombolites in the Ediacaran Anti-Atlas (571 Ma), demonstrating that deep time architecture-specific biosignatures can persist in the geological record (Fig. 9).

In conclusion, three fundamental aspects of microbialite morphogenesis—morphology, fabric, and accretion mechanism—control arsenic incorporation into actively accreting microbialites from Hamelin Pool. Microbialite morphology is influenced by environmental conditions, which can select for genes involved in arsenic cycling and resistance, causing substantial differences in arsenic enrichment between sheet mats and discrete microbial buildups. Microbialite fabric is a product of the metabolic pathways driving micrite precipitation. The transfer of arsenic from organic matter to micrite is governed by the predominance of heterotrophic metabolisms, a factor which distinguishes arsenic enrichment patterns in clotted versus laminated microbialites. Microbialite accretion mechanism impacts the proportions of micrite and trapped-and-bound allochems comprising microbialites, altering the sources of arsenic incorporated in agglutinated and micritic microbialites. In concert, our results show that arsenic enrichment patterns are not created equally in different types of microbialites. Each microbialite architecture is influenced to a different extent by microbial processes, sedimentary inputs, and seawater chemistry, each of which can change through space and time, resulting in the diversity of microbialite deposits observed in the geological record. With the process-based understanding of arsenic incorporation produced here, our results allow for comparison of the origin of arsenic enrichment patterns across microbialite architectures. Sheet mats can be studied to reconstruct microbial activity, clotted microbialites to infer seawater arsenic concentrations, laminated fabrics provide information about the metabolic profile of the mat, and agglutinated fabrics can be used to characterize sedimentary inputs. Leveraging the unique origins of arsenic enrichment patterns in different microbialite architectures enables a more continuous reconstruction of dynamic interactions between life and the environment that have induced microbialite accretion over billion of years of Earth history.

Methods
Samples election
This study investigated 47 previously collected microbialite samples from 12 transects distributed around the coastline of Hamelin Pool in Shark Bay, Western Australia (Fig. 7 and Supplementary Table 3). The complete dataset, including field metadata, high-resolution slab photographs, and thin section images for each microbialite buildup is publicly available71. Sheet mat samples (n=5) were collected from the Spaven, Nilemah, and Hutchison provinces, near sampling sites T2b, T6, and T11, respectively (Fig. 7 and Supplementary Fig. 6A). Discrete microbial buildups (n=42) were sampled along each of the 12 transects, with 2–5 microbialites collected per transect across a range of water depths corresponding to distinct tidal zones (Figs. 1, 7 and Supplementary Fig. 6; see Suosaari et al.71 for field sampling metadata).

In the laboratory, each microbialite buildup was cut in half (Supplementary Fig. 7), and thin sections were prepared (Supplementary Fig. 8; see Suosaari et al.71 for repository of slab images, thin section locations, and high-resolution scans of each thin section). On this basis, the internal texture of each sheet mat and buildup was investigated from meso- to nanoscale using petrographic analysis (Supplementary Fig. 8) and SEM imaging, with descriptions of morphology, fabrics, and accretion mechanisms provided in previously published studies5,25,26,31–33. The present sampling strategy builds on these prior publications by sampling the same microbialites and integrating their published fabric descriptions25,26,33. Microbialite architectures sampled include sheet mats (n=5) and discrete microbial buildups (n=42), which were further subdivided into laminated (n=27) and clotted (n=15) fabrics, as well as micritic (n=20) and agglutinated (n=22) microbialites. A low-speed hand drill was used to remove the surface layer of the microbialite slab, and ~100 mg of material was collected from each sampling location. Equivalent quantities of 12 bulk sediment samples were collected from each microbialite transect in Hamelin Pool. Individual sedimentary allochems, including triplicates of micritic grains, foraminifera, gastropods, bivalves (Fragum erugatum), and ooids were also handpicked to create triplicate 25 mg samples for each allochem. All microbialite, sediment, and allochem samples were manually homogenized using an agate mortar and pestle.

Sample digestion and analysis
Approximately 100 mg of powdered samples (microbialite and sediment) were weighed into 5 mL Eppendorf tubes for sequential leaching in an ISO Level 7 clean room equipped with a Class 100 UL1805 certified polypropylene trace metal workstation. The protocol employed here (see Supplementary Methods) was conducted in seven individual steps. In summary, a two-step leaching of the organic fraction was first conducted using H₂O₂, designated as steps H1 and H2, followed by leaching of the adsorbed fraction (N1) with ammonium acetate, and finally, a four-step leaching of the bulk carbonate phase (S1, S2, S3, and S4) using nitric acid. The leaching of the organic fraction was conducted using 5 mL of pH-buffered 1.49 M distilled H₂O₂ (VWR, USA). To neutralize the H₂O₂ acidity and prevent leaching of carbonate phases, 0.10 M NaOH (EMD, Germany) was added at a ~1:14 ratio with the H₂O₂ to maintain a pH of ~8, verified before each reaction using standard pH strips. Subsequently, the samples were treated with 1.5 mL of 0.23 M ammonium acetate (Aldrich, USA) to remove easily exchanged ions adsorbed on the surface of clay minerals or Fe-Mn oxides (adsorbed fraction). Subsequently, the bulk carbonate phase was leached using 0.35 M distilled nitric acid (Savillex Acid Purification System, DST-1000), with the volume calibrated to leach ~25% of the sample’s carbonate content in each step. The effectiveness of the sequential leaching protocol in selectively leaching only the targeted phase, without contamination from other phases, was monitored throughout the process (see Supplementary Methods). All procedural and reagent blanks assessed (n=6) were confirmed to have arsenic, calcium, and magnesium concentrations beneath the methodological limit of detection.

After introducing a reagent at each step, samples were vortexed and ultrasonicated at room temperature (30, 15, and 60 min for the organic fraction, adsorbed fraction, and bulk carbonate phase, respectively). Following reaction completion, samples were centrifuged at 4500 × g for 30 min, and the supernatant was transferred to 5 mL Eppendorf tubes. The residual material was dried uncapped overnight at 60 °C on a metal-free hot plate (HPX-200, Savillex, USA) in the trace metal workstation. After evaporating to dryness, the mass of the residue was measured to quantify the mass loss, allowing elemental concentrations to be reported as absolute values. Before each reagent change, samples were rinsed with Milli-Q water (Milli-Q IQ 7003, Millipore Sigma, USA). Finally, all supernatants were evaporated, resuspended in 0.16 M HNO₃ and transferred to 15 mL centrifuge tubes for elemental analysis.

In parallel, the individual sedimentary allochems, including triplicates of micritic grains, foraminifera, bivalves (Fragum erugatum), gastropods, and coated grains, were subjected to a bulk digestion process, during which 25 mg of the material was treated with 0.79 M HNO₃ for an hour at room temperature in an ultrasonic bath. The resulting solution was then transferred into 15 mL centrifuge tubes and diluted to 0.16 M HNO₃ for elemental analysis.

Arsenic, calcium, and magnesium concentrations were determined using triple quadrupole inductively coupled plasma mass spectrometry (Agilent 8900 ICP-QQQ, Agilent, Santa Clara, CA). The first organic leachate (H1) and the second carbonate leachate (S2) were selected for analysis because they were considered the most representative of the organic fraction and bulk carbonate phase, respectively (see Supplementary Methods). For arsenic measurements, the instrument was operated in MS/MS mode using O₂ gas in the collision-reaction cell (CRC) and analyzed the O₂ mass-shifted analyte in the third quadrupole at mass 91. For calcium and magnesium measurements, the instrument was operated in MS/MS mode using H₂ gas in the CRC and analyzed at masses 40 and 24, respectively. Multiple digestions of certified reference materials (CRMs)—IAEA-413, IAEA-392, IAEA-V8, DOLT-5 for organic leachates, and NCS-DC-73340, FEBS-1, BHVO-2, and AGV-2 for carbonate leachates—were used for method validation and accuracy estimates in repeat analyses conducted during each analytical batch. Average recoveries for each CRM were >90% of the certified values for arsenic, calcium, and magnesium. Further details on analytical methods and CRM recovery can be found in the Supplementary Methods. The arsenic concentrations of the adsorbed fraction (leachate N1) in the analyzed running solution were, on average, 10 times lower than those in the organic and carbonate leachates. Due to these lower concentrations, the adsorbed fraction was excluded from further analyses.

Seawater analysis
Open water samples (n=32) were collected in the Hamelin Pool basin in November 2017 (Fig. 7). Salinity of each sample was determined using a refractometer and arsenic concentrations were determined using a modification of the Standard Addition Calibration method13,72. A 50 µg/L arsenic spike solution was prepared from a NIST-traceable Spex Certiprep solution 2A (USA) diluted in 0.16 M HNO₃. The certified seawater reference material, CASS-6, which naturally contains 0.1 µg/L of arsenic, was diluted proportional to its salinity in 0.16 M ultra-trace HNO₃. Afterward, the spike solution was added to aliquots of the diluted CASS-6 to produce a series of seven solutions containing a range of 0.1–1.8 µg/L arsenic. All samples were analyzed on an Agilent 8900 ICP-QQQ, with identical analytical settings and quality control standards as the microbialite and sediment samples (See Supplementary Methods). The regression of counts versus expected concentrations for arsenic showed a strong linear relationship (R²=0.999). The x-axis intercept, along with its standard error, was used to estimate the arsenic concentration in the unspiked CASS-6 solution, yielding a recovery of 94±4%. Counts from each water sample were then converted to arsenic concentrations using this calibration equation. The concentrations of Ca and Mg in the seawater were determined using a 10-point external calibration curve, similar to the approach used for the organic and carbonate leachates from the microbialites (see Supplementary Methods). Multiple replicates of each CRM (IAPSO) were analyzed with each batch of samples, and the recovery percentages were 97±2% and 96±1% for calcium and magnesium, respectively. A map of arsenic concentrations in the surface waters of Hamelin Pool was generated in ArcGIS Pro (version 3.1.3, Esri Inc., USA) using the Kriging interpolation model (Fig. 7).

Mineralogy
Mineralogy was determined using a PANalytical X’pert PRO X-ray Diffractometer (PANalytical Inc, Almelo, The Netherlands). Samples were powdered using a mortar and pestle, hydrated with MilliQ water, and spread onto a glass slide. Smear slides were allowed to air dry overnight before analysis. To measure the relative abundance of carbonate polymorphs, samples were scanned between 23° and 32° 2Ɵ, using a step interval of 0.01° and a count time of 1 s for each step. The relative concentrations of aragonite, HMC, LMC (and dolomite, if present) were calculated using the model of Melim et al.73, which calculates carbonate mineral concentrations based on peak area ratios assuming the sample only contains HMC, LMC, aragonite, and dolomite. Thus, the presence of other minerals, such as quartz or detrital clays, are excluded when calculating the proportion of carbonate minerals in the sample. Based on prior repeat measurements, the standard error of the analysis is ~3%74. Sixteen samples were also scanned from 4 to 70° 2Ɵ to identify the presence of mineral phases other than carbonates. The obtained diffractograms were processed using HighScorePlus software (version 3.0.5, PANalytical, The Netherlands), which facilitated background subtraction and identification of the main reflection peaks. The positions of the 2Ɵ peaks were compared with those listed in the PDF-2 2003 database (ICDD) to identify mineral phases present in the samples.

Mass balance
Mass balance calculations were conducted to determine the relative size of the micrite and trapped-and-bound allochem fractions within the microbialite samples. During the sequential leaching protocol, the carbonate leaching steps extracted the bulk carbonate phase of the microbialites, which encompasses two distinct fractions: the micrite produced by microbes within the accreting mat, and the trapped-and-bound carbonate allochems (Eq. 4).

wMcarb = wMmicrite + wMallochems (4)

where, wMcarb denotes the mass fractions of the bulk carbonate phase in the microbialites, including both the micrite fraction (wMmicrite), and the trapped-and-bound allochem fraction (wMallochems).

When microbialites incorporate trapped-and-bound carbonate allochems, they also incorporate siliciclastic grains in a ratio similar to that of the surrounding sediment. This occurs because microbialites do not differentiate between carbonate allochems and siliciclastic grains during the trapping and binding process6. This relationship can be expressed as:

wMallochems / wMsiliciclastic = wSallochems / wSsiliciclastic (5)

Here, wSallochems represents the mass fraction of allochems in the sediment. Unlike microbialites, in which carbonate leachates include both micrite and allochems, the carbonate leachate extracted from the sediments consists solely of allochems. Therefore, wSallochems is equivalent to the mass loss measured during the carbonate leaching step of the sediment sample. wMsiliciclastic and wSsiliciclastic denote the mass fractions of siliciclastic grains in the microbialite and sediment samples, respectively. These values correspond to the mass of unleached residue remaining at the end of the leaching protocol. Using these information, Eq. 5 can be rewritten to derive the allochem mass fraction within the microbialites:

wMallochems = wMsiliciclastic × (wSallochems / wSsiliciclastic) (6)

By using this relationship and deriving Eq. 4, the micrite mass fraction within the microbialites can also be determined:

wMmicrite = wMcarb - wMallochems (7)

Mixing model
Based on the mass balance calculations, a mixing model was implemented to estimate the arsenic concentrations in the micrite fraction and the trapped-and-bound allochem fraction that together comprise the bulk carbonate phase extracted during the sequential leaching protocol. The arsenic concentrations of the bulk carbonate phase correspond to the combined contributions of the micrite and the trapped-and-bound allochem fractions, expressed as:

[As]Mcarb = ([As]Mmicrite × wMmicrite) + ([As]Mallochems × wMallochems) (8)

Here [As]Mcarb represents the arsenic concentration in the bulk carbonate phase, which corresponds to the arsenic concentration measured in the carbonate leachate obtained during the sequential leaching experiment. [As]Mmicrite and [As]Mallochems denote the arsenic concentration in the micrite and allochem fraction of the microbialites, respectively.

Since microbialites were assumed to not differentiate between grain types when trapping and binding sediments, the arsenic concentration of allochems trapped and bound in the microbialites should be equivalent to that of allochems isolated from sediment samples surrounding the microbialite. Starting from this assumption and by deriving Eq. 8, it is thus possible to calculate the arsenic concentration in the micrite fraction comprising the microbialites:

[As]Mmicrite = ([As]Mcarb - ([As]Mallochems × wMallochems)) / wMmicrite (9)

The concentrations of calcium and magnesium in the micrite of the microbialites were calculated using the same procedure, based on the measured concentrations of these elements in the bulk carbonate of the microbialites and the sediment.

Statistical analysis
Statistical analyses were performed using R (R Core Team, 2025). Prior to conducting group comparisons, we assessed the assumptions of normality and homogeneity of variances. Normality within each group was tested using the Shapiro–Wilk test, with a p>0.05 indicating no significant deviation from a normal distribution. Homogeneity of variances across groups was evaluated using Levene’s test, where a p>0.05 indicated equal variances among groups. Based on these assumptions, we selected appropriate statistical methods for pairwise comparisons. When both normality and homogeneity of variances were met, we applied a Student’s t-test. If normality was satisfied but variances were unequal, we used Welch’s t-test. If either group deviated from normality, we applied the Mann–Whitney U test (Wilcoxon rank-sum test) as a non-parametric alternative. For comparisons involving more than two groups, a one-way ANOVA was used when all assumptions were met, whereas the Kruskal–Wallis test was used when normality was violated in one or more groups. All statistical tests were two-tailed, and results were considered statistically significant at p<0.05.

To determine correlation between X and Y when individual measurements for each pair of X and Y corresponded to one sample, we used the “correlation_test” function in R to calculate the correlation coefficient (r). In instances where X and Y were provided as averages with known standard deviations, we conducted a weighted linear regression analysis75. This analysis, performed using the modified “lm()” function in R, provided estimates for the correlation coefficients r. To facilitate correlation analysis of exponential relationships, we employed a linearization technique76. After linearizing the variables, we conducted correlation and regression analyses using the same statistical tools as for linear relationships. In every scenario, statistical significance of the correlations was tested using t-statistics with a p value threshold of less than 0.05.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "Review of Pollier of Pollier of Pollier et al. “Arsenic enrichment patterns are defined by microbialite morphology, fabric, and accretion mechanism”\nPollier et al. present a very detailed treatment of the mechanisms responsible for As enrichments in Hamelin Pool microbialites, showing that the combined effects of microbialite morphology, fabric, and accretion mechanism can influence the enrichment of As within recent microbialites. Perspectives for extending this finding to ancient microbialites are also considered. I find this to be a well-structured and well-reasoned manuscript and have generally only relatively minor comments for its improvement.\nThe relationships devised in the discussion between the above three factors are particularly useful and represent a novel contribution to understanding trace element incorporation in microbialites both modern and ancient. The summary Figure 9 is particularly effective in outlining the contributions made to modern systems and the implications of these findings for As enrichment research in fossil microbialites.\nMy main recommendations for improvement are:\n1) It is necessary to see more complete details and characterisation of the locality from which these microbialites were collected. A panel (ideally in the main manuscript but in the supplementary materials if space does not allow) should show a map location of the sampling site, field photographs of the microbialites sampled, and ideally photographs of the sample prior to analysis. I appreciate that the map will slightly repeat what is shown in Figure 7, but since that related to specific characteristics of the water (As concentration), you could choose a satellite map (e.g. SENTINEL) for this image. It is particularly important to show these images as you note , for example in L583, that “[your] results demonstrate the need for careful sedimentological description of samples before geochemical analysis and interpretation”.\n2) When discussing the application of As enrichments to the fossil record, it is important to summarise how taphonomy and diagenesis might play a role in influencing the final enrichments observed. I have noted this in more detail in my comment to L610 below.\nL69 – I strongly recommend removing the Nutman et al. (2016) reference as these features have since been convincingly shown to be metamorphic, see e.g. Allwood et al. (2018, Nature) and Zawaski et al. (2020, EPSL).\nL97 – you could note here “…arsenic and other trace elements, but such effects…”\nL152 – I find the phrase “size of the organic matter and micrite fractions” slightly confusing. If this could be defined once in the manuscript, perhaps as “relative fractional contribution to the total mass of the sample” or similar, this would be beneficial.\nL184 – typo, should be “…sheet mats than discrete microbial buildups…”\nL249 – should read “…no evidence that organic matter…”\nL260 – This is perhaps slightly too early in the manuscript to state that differences in As concentrations likely result from differences in mat community. This could be reworded as “…are likely influenced by distinct community metabolisms” or similar. The evidence provided in the next few sentences certainly supports this.\nL270 – typo, should be “promote”.\nL271 – rewrite as “…near the mat surface…”\nL419 – I agree that looking for trace element patterns in Archaean materials is an appropriate and underutilised approach to considering their biogenicity (when this is in doubt). Additional descriptive details would help to clarify how exactly other researchers might search for these patterns in their samples. I strongly recommend the inclusion of some discussion here to propose patterns of enrichment that might be examined in the Isua and Strelley Pool examples that you highlight. For the Strelley Pool, you might refer to the different morphotypes outlined in Allwood et al. (2006, Nature). I realise that you have included some ideas addressing this in the “fossil record” row of Figure 9, so a few brief lines supporting these patterns in the text should suffice.\nL524 – From experience, I agree that organic materials in ancient carbonates often has a poor record. This is not necessarily the case when these materials have undergone early silicification (see Hickman-Lewis et al., 2024, GPL). You might mention that these exceptions do exist and could be good targets for testing your approach.\nAs a complementary point of discussion, it would also be useful to indicate what might be examined in non-precipitating mats, e.g. stratiform microbial mats from the Barberton greenstone belt (see reviews in Homann et al., 2019, Earth-Science Reviews and Hickman-Lewis and Westall, 2021, South African Journal of Geology). Since these are often silicified during deposition and earliest diagenesis, the possibility for preservation of trace metal signatures in organics is perhaps higher.\nL558 – another possibility to test this in deeper time would be the 2.4 Ga Turee Creek Group thrombolites (Nomchong and Van Kranendonk, 2020, Precambrian Research).\nL578 – it would be useful here to briefly note that the nature of allochems will change depending on whether the studied samples are Precambrian or Phanerozoic. This will obviously have some influence on the amplification or attenuation of the As signal, but it is probably beyond the scope of this paper to discuss these in turn for both periods.\nL610 – in studies of ancient microbialites, the questions of taphonomy, diagenesis and metamorphism are crucial. In the current version of the manuscript, you only mention taphonomy and diagenesis once, in the caption to Figure 9, but these will be important factors to consider in every study of non-modern materials. It would be highly useful to include a subsection at the end of the discussion to outline some of the taphonomic and diagenetic phenomena that might influence As enrichment in fossil microbialites. For example, which long-term processes might significantly increase or decrease As concentrations in biogenic carbonates outside of the expected ranges, and how might these be traced in geological samples? In my view, this is essential to explain how you arrived at the fossil record As enrichment models shown in Figure 9. I do not believe that this needs to be an exhaustive sub-section, but at least some reference to these phenomena should be made.\nL643 – if available, specific sampling localities (GPS locations if available) should be provided.\nI hope that the authors find my comments useful in their revisions of this manuscript and I commend them on producing a very interesting study with implications for modern and ancient microbialites.\nKeyron Hickman-Lewis\nBirkbeck, University of London", "reviewer_2": "The manuscript examines the incorporation of arsenic in microbialites, which can serve as a marker of ancient microbial activity, and enhance our understanding about the distribution and preservation of arsenic in the fossil records. The authors have found that arsenic enrichment depends on microbial activity, sediment inputs and seawater chemistry, all of which vary with the microbialites’ structure. It is an extremely interesting work, and robust statistical and technical methods have been used to show the key role of microbialites’ fabrics and structure in controlling their geochemical composition. This work therefore seems to me to be appropriate in the chosen journal. The article is well written, easy to read, and the authors' reasoning is well explained. I have no complaints and I consider the article ready for acceptance, provided the other reviewers agree, of course.\nSome minor comments:\n1- Including a figure with images illustrating the different microbialite types would greatly enhance the understanding of the discussed morphotypes. Such visual representation would clarify their morphological features and help highlight potential relationships with arsenic distribution, contributing to a more comprehensive interpretation.\n2- It would be valuable to include a paragraph comparing your findings with those from the fossil record, particularly in relation to whether different microbialite morphotypes in ancient environments exhibited similar arsenic distribution patterns. I suggest referencing the Ediacaran example from Amane Tazgart (Chraiki et al., 2023 doi: 10.1038/s41598-023-36031-5), where arsenic distribution was studied in both thrombolites and stromatolites.\n3- Is the number of sheet mats specimens studied (n=5) enough for a statistical approach?\n4- The authors discuss the size and concentration of arsenic in the different specimens (e.g., L144). While concentration is very well represented in the manuscript, “size” could be misleading. I think it should be replaced by “repartition”, “distribution” or “pattern”.\n5- L184: replace “that” with “than in”."}, "author_response": "Thank you for your thoughtful review of the manuscript. We have responded to each of your specific comments below.\n\nMy main recommendations for improvement are:\n1) It is necessary to see more complete details and characterisation of the locality from which these\nmicrobialites were collected. A panel (ideally in the main manuscript but in the supplementary\nmaterials if space does not allow) should show a map location of the sampling site, field photographs\nof the microbialites sampled, and ideally photographs of the sample prior to analysis. I appreciate\nthat the map will slightly repeat what is shown in Figure 7, but since that related to specific\ncharacteristics of the water (As concentration), you could choose a satellite map (e.g. SENTINEL)\nfor this image. It is particularly important to show these images as you note , for example in L583,\nthat “[your] results demonstrate the need for careful sedimentological description of samples before\ngeochemical analysis and interpretation”.\nWe thank the reviewer for this constructive suggestion. In an effort to be concise, we had referenced\nprior studies from our team which provide this information (see Suosaari et al., 2022; Vitek et al.,\n2022, 2023). We agree that the reader would benefit from additional detail about the sample\ncollection and characterization effort, and are grateful for the reviewer’s request to expand this\nsection of the manuscript. In response, we now explicitly indicate the locations of the 12 microbialite\nsampling transects previously referenced in the manuscript. To address the comment without adding\nan additional figure to the main text (due to space constraints), we incorporated this information\ndirectly into the figure with a satellite image-based map shown in Figure 7. As the reviewer noted,\nthis is particularly appropriate since these sampling localities also relate to the spatial distribution of\nseawater arsenic concentrations, already presented in that figure. We have also added Table S3 in\nthe Supplementary Information, which lists the coordinates for each sampling transect.\nAdditionally, we have included three new supplementary figures which provide additional visual and\nspatial context for each sample:\n• Figure S6 presents field photographs of representative microbialites taken immediately prior\nto sampling, illustrating morphological diversity across different tidal zones.\n• Figure S7 shows corresponding slab images of microbialites prior to subsampling illustrating\ndifferences in internal fabrics\n• Figure S8 shows corresponding thin sections images documenting the variable accretion\nmechanisms encountered\nFinally, we also substantially revised the Methods section to clarify that all geochemical analyses in\nthis study are based on samples whose fabrics and accretion mechanisms have been thoroughly\ncharacterized previously by our research group in a series of detailed studies (Suosaari et al., 2022;\nVitek et al., 2021, 2022; Reid et al., 2024). The complete dataset, including field sampling metadata,\nhigh-resolution slab photographs, and thin section images for each of the 47 microbialite samples\nanalyzed, is publicly available via an open-access online database:\nhttps://data.mendeley.com/datasets/bhrp5hrvyn/1 (DOI: 10.17632/bhrp5hrvyn.1) which is now\nreferenced in the methods section (Suosaari et al., 2023).\n2) When discussing the application of As enrichments to the fossil record, it is important to\nsummarise how taphonomy and diagenesis might play a role in influencing the final enrichments\nobserved. I have noted this in more detail in my comment to L610 below.\nWe thank the reviewer for raising this important point. A discussion of the potential effects of\ntaphonomy and diagenesis on arsenic enrichment patterns has been added to \" Implications for the\ninterpretation of ancient microbialites\" section on lines 743 - 758. Further details are outlined in our\nresponse to Comment L610 below.\nL69 – I strongly recommend removing the Nutman et al. (2016) reference as these features have\nsince been convincingly shown to be metamorphic, see e.g. Allwood et al. (2018, Nature) and\nZawaski et al. (2020, EPSL).\nThank you for this feedback. The reference to Nutman et al. (2016) has been removed.\nL97 – you could note here “…arsenic and other trace elements, but such effects…”\nThank you for the recommendation, we have added the suggested text to this sentence.\nL152 – I find the phrase “size of the organic matter and micrite fractions” slightly confusing. If this\ncould be defined once in the manuscript, perhaps as “relative fractional contribution to the total mass\nof the sample” or similar, this would be beneficial.\nThank you for your helpful suggestion. In response, we have revised the manuscript to define “size”\nclearly at first mention in the previous paragraph at line 145. Specifically, we now state: “...enabling\nmeasurements of both absolute arsenic concentrations and the size of each compositional fraction\n(e.g. micrite, trapped-and-bound allochems, or organic matter), where size refers to the relative\nmass contribution of each fraction to the total sample mass.”\nL184 – typo, should be “…sheet mats than discrete microbial buildups…”\nThe typo has been resolved.\nL249 – should read “…no evidence that organic matter…”\nThe typo has been resolved.\nL260 – This is perhaps slightly too early in the manuscript to state that differences in As\nconcentrations likely result from differences in mat community. This could be reworded as “…are\nlikely influenced by distinct community metabolisms” or similar. The evidence provided in the next\nfew sentences certainly supports this.\nWe have revised the sentence at L297 to read: “Differences in arsenic concentrations between\norganic matter and micrite in laminated and clotted microbialites are likely influenced by distinct\ncommunity metabolisms.”\nL270 – typo, should be “promote”.\nThe typo has been resolved.\nL271 – rewrite as “…near the mat surface…”\nThank you for this suggestion. The original sentence has been re-written as \"..near the mat\nsurface..\".\nL419 – I agree that looking for trace element patterns in Archaean materials is an appropriate and\nunderutilised approach to considering their biogenicity (when this is in doubt). Additional descriptive\ndetails would help to clarify how exactly other researchers might search for these patterns in their\nsamples. I strongly recommend the inclusion of some discussion here to propose patterns of\nenrichment that might be examined in the Isua and Strelley Pool examples that you highlight. For the\nStrelley Pool, you might refer to the different morphotypes outlined in Allwood et al. (2006, Nature). I\nrealise that you have included some ideas addressing this in the “fossil record” row of Figure 9, so a\nfew brief lines supporting these patterns in the text should suffice.\nWe thank the reviewer for this excellent suggestion and for providing the key reference useful to\nhighlight. To address the comment, we have expanded our discussion to clarify how researchers\nmight apply arsenic geochemistry to the fossil record. Specifically, we now propose that a practical\napproach involves comparing arsenic concentrations in laminated carbonates suspected of\nbiological origin to those in coeval non microbial sediments, as demonstrated in Hamelin Pool.\nAdditionally, we emphasize that the spatial localization of arsenic—along laminae or within micritized\ncells—can further support biologically mediated incorporation of arsenic. These additions can be\nfound on lines 573-578 of the revised manuscript.\nWe also added a new paragraph in the final section of the Discussion (lines 730 to 741) highlighting\nthat specific differences in arsenic enrichment patterns between microbialite architectures may serve\nas confirmation of a biosignature. To address the reviewer’s request for concrete examples, we have\nadded new text referencing the seven stromatolite morphotypes described by Allwood et al. (2006)\nin the Strelley Pool Formation. We suggest that if these distinct morphotypes display different\narsenic enrichment levels—similar to what we observe across microbialite architectures in Hamelin\nPool—it would provide strong evidence that these morphological differences were shaped by\nbiological rather than abiotic processes. This idea was summarized visually in the \"fossil record\"\npanel of Figure 9, and which supports the reviewer suggestion very well.\nL524 – From experience, I agree that organic materials in ancient carbonates often has a poor\nrecord. This is not necessarily the case when these materials have undergone early silicification (see\nHickman-Lewis et al., 2024, GPL). You might mention that these exceptions do exist and could be\ngood targets for testing your approach.\nAs a complementary point of discussion, it would also be useful to indicate what might be examined\nin non-precipitating mats, e.g. stratiform microbial mats from the Barberton greenstone belt (see\nreviews in Homann et al., 2019, Earth-Science Reviews and Hickman-Lewis and Westall, 2021,\nSouth African Journal of Geology). Since these are often silicified during deposition and earliest\ndiagenesis, the possibility for preservation of trace metal signatures in organics is perhaps higher.\nWe thank the reviewer for this thoughtful suggestion. As recommended, we now explicitly discuss\nearly silicification as a mechanism that can enhance the preservation of microbial textures and\norganic residues, even in non-precipitating mats. We have added a paragraph referencing the\nPilbara Craton and Barberton Greenstone Belt examples with appropriate references (Hickman-\nLewis et al., 2024; Hickman-Lewis and Westall, 2021, Homann, 2019), emphasizing their relevance\nfor testing arsenic-based biosignatures. We also highlight the potential value of comparing arsenic\nenrichment patterns between putative microbial mats and their sedimentary matrix in these settings\nto evaluate biogenicity. This new discussion appears on lines 633–639 of the revised manuscript.\nL558 – another possibility to test this in deeper time would be the 2.4 Ga Turee Creek Group\nthrombolites (Nomchong and Van Kranendonk, 2020, Precambrian Research).\nWe thank the reviewer for this helpful suggestion. We have now incorporated the Turee Creek\nGroup thrombolites into the discussion as a relevant deep-time example where our model could be\ntested. Specifically, we highlight that this site hosts four distinct clotted fabric types deposited under\ndifferent paleoenvironmental conditions, offering an ideal opportunity to assess whether variations in\nenvironmental arsenic concentrations are reflected in micritic arsenic signals. This discussion\nappears on lines 679–683 of the revised manuscript.\nL578 – it would be useful here to briefly note that the nature of allochems will change depending on\nwhether the studied samples are Precambrian or Phanerozoic. This will obviously have some\ninfluence on the amplification or attenuation of the As signal, but it is probably beyond the scope of\nthis paper to discuss these in turn for both periods.\nWe agree that the nature of allochems differs significantly between Precambrian and Phanerozoic\nmicrobialites, which may influence arsenic enrichment patterns. To address this, we have added a\nclarification in the revised manuscript noting that Phanerozoic microbialites are more likely to\nincorporate skeletal grains such as foraminifera and shell fragments, while Precambrian examples\nlack such bioclasts due to the absence of carbonate-producing metazoans. While a full comparison\nof how this difference might affect arsenic signatures lies beyond the scope of our current study,\nwe acknowledge that this variability in allochem type will likely modulate the expression of arsenic\nenrichment patterns in microbialites and merits further investigation. This discussion appears on\nlines 719–723 of the revised manuscript\nL610 – in studies of ancient microbialites, the questions of taphonomy, diagenesis and\nmetamorphism are crucial. In the current version of the manuscript, you only mention taphonomy\nand diagenesis once, in the caption to Figure 9, but these will be important factors to consider in\nevery study of non-modern materials. It would be highly useful to include a subsection at the end of\nthe discussion to outline some of the taphonomic and diagenetic phenomena that might influence As\nenrichment in fossil microbialites. For example, which long-term processes might significantly\nincrease or decrease As concentrations in biogenic carbonates outside of the expected ranges, and\nhow might these be traced in geological samples? In my view, this is essential to explain how you\narrived at the fossil record As enrichment models shown in Figure 9. I do not believe that this needs\nto be an exhaustive sub-section, but at least some reference to these phenomena should be made.\nWe appreciate the reviewer’s observation regarding the importance of taphonomic, diagenetic, and\nmetamorphic processes in shaping trace element distributions in ancient microbialites. In response,\nwe have added a new subsection at the end of the Discussion (lines 743 to 758) to explicitly address\nthese concerns. The revised paragraph outlines how diagenetic alterations (e.g., cementation,\nrecrystallization, dissolution) and long-term fluid-rock interactions can overprint primary arsenic\nsignatures. Indicators of such overprinting, such as changes in texture and redox-sensitive trace\nelement patterns, are noted as essential for geochemical and petrographic screening. Furthermore,\nwe emphasize the limitations posed by metamorphism in preserving primary geochemical signals.\nTo support the applicability of our approach despite these challenges, we now cite relevant\nexamples from the fossil record, including 2.7 Ga old arsenic-bearing organic globules in the\nNeoarchean Tumbiana stromatolites in Australia (Sforna et al., 2014) and architecture-specific\narsenic patterns in Ediacaran microbialites from the Anti-Atlas in Morocco (Chraiki et al., 2023).\nThese examples illustrate the potential for arsenic-based biosignatures to persist over geological\ntimescales and support the framework presented in Figure 9.\nL643 – if available, specific sampling localities (GPS locations if available) should be provided.\nWe have added Supplementary Table 3 in the Supplementary Information, which provides the\ngeographic coordinates (latitude and longitude in decimal degrees) for each of the 12 microbialite\nsampling transects referenced in the study. These locations are also shown on the satellite map in\nFigure 7 to facilitate spatial context and cross-reference with water chemistry data.\n\nReviewer #2 (Remarks to the Author):\nThe manuscript examines the incorporation of arsenic in microbialites, which can serve as a marker\nof ancient microbial activity, and enhance our understanding about the distribution and preservation\nof arsenic in the fossil records. The authors have found that arsenic enrichment depends on\nmicrobial activity, sediment inputs and seawater chemistry, all of which vary with the microbialites’\nstructure. It is an extremely interesting work, and robust statistical and technical methods have been\nused to show the key role of microbialites’ fabrics and structure in controlling their geochemical\ncomposition. This work therefore seems to me to be appropriate in the chosen journal. The article is\nwell written, easy to read, and the authors' reasoning is well explained. I have no complaints and I\nconsider the article ready for acceptance, provided the other reviewers agree, of course.\nThank you for your thoughtful review of the manuscript. We have responded to your comments\nbelow.\nSome minor comments:\n1- Including a figure with images illustrating the different microbialite types would greatly enhance\nthe understanding of the discussed morphotypes. Such visual representation would clarify their\nmorphological features and help highlight potential relationships with arsenic distribution,\ncontributing to a more comprehensive interpretation.\nWe agree with the reviewer that visual documentation of microbialite types enhances the reader’s\nunderstanding of morphotypes and their potential relationship to arsenic distribution. In response, we\nhave added three new supplementary figures:\n– Figure S6 shows field photographs of representative microbialites sampled in Hamelin Pool,\nillustrating the diversity of microbialite morphologies\n– Figure S7 presents slab images of the same microbialites, highlighting variations in internal fabrics\nsuch as laminated and clotted structures.\n– Figure S8 provides thin section images corresponding to these samples, illustrating the two main\naccretion mechanisms observed—micrite precipitation and grain agglutination.\nThese figures are cross-referenced with our morphogenetic framework (Fig. 1), providing an\nintegrated visual context for the different microbialite architectures analyzed in this study.\n2- It would be valuable to include a paragraph comparing your findings with those from the fossil\nrecord, particularly in relation to whether different microbialite morphotypes in ancient environments\nexhibited similar arsenic distribution patterns. I suggest referencing the Ediacaran example from\nAmane Tazgart (Chraiki et al., 2023 doi: 10.1038/s41598-023-36031-5), where arsenic distribution\nwas studied in both thrombolites and stromatolites.\nWe appreciate the reviewer’s thoughtful suggestion. In response, we have added a new paragraph\nto the Discussion section that explicitly compares our findings from Hamelin Pool with patterns\nobserved in the fossil record. We now reference the Ediacaran microbialites from Amane Tazgart\n(Chraiki et al., 2023), where contrasting arsenic distributions were documented between\nstromatolites and thrombolites, highlighting the potential for architecture-specific geochemical\nsignatures to persist over geological timescales. To further contextualize our results, we also draw\nattention to ancient microbialite-bearing successions that display extensive variability in morphology,\nfabric, and accretion mechanisms—such as the Strelley Pool Formation (Allwood et al., 2006),\nthe Tumbiana Formation (Sforna et al., 2014), and other successions discussed by Nomchong and Van\nKranendonk (2020) and Hickman-Lewis et al. (2021). These examples reinforce the relevance of our\napproach and support the idea that systematic differences in arsenic enrichment across microbialite\narchitectures—when observed in ancient successions—may serve as a robust biosignature. The\nnew paragraph has been incorporated at the end of the discussion (lines 729–741)\n3- Is the number of sheet mats specimens studied (n=5) enough for a statistical approach?\nWe thank the reviewer for raising this important point. Unfortunately, the number of sheet mat\nsamples collected and appropriately stored for geochemical analyses (e.g. without fixatives) was\nrelatively limited, given the initial focus on petrographic characterization of these samples. However,\nthe arsenic concentrations of the five sheet mat samples were compared against 42 discrete\nmicrobialite buildups collected across a wide range of tidal zones and morphotypes. The magnitude\nand consistency of arsenic enrichment observed in the sheet mats—up to sixfold higher than in\nclotted fabrics and significantly elevated compared to sediment samples—support the robustness of\nthe observed pattern even in the absence of large sample size. Moreover, the sample size is\nsufficient for pairwise statistical testing, with differences in arsenic concentrations yielding significant\nresults (p < 0.05). Nonetheless, we understand the concern that five specimens may not fully\ncapture the environmental heterogeneity experienced by sheet mats in Hamelin Pool, and now\nencourage future studies to expand on these observations with additional sampling. New text to this\neffect can be found on lines 611-614 in the revised manuscript. We agree that future studies\nincorporating a broader spatial and temporal range of sheet mats will be valuable to further validate\nand expand upon these findings.\n4- The authors discuss the size and concentration of arsenic in the different specimens (e.g., L144).\nWhile concentration is very well represented in the manuscript, “size” could be misleading. I think it\nshould be replaced by “repartition”, “distribution” or “pattern”.\nWe agree that the term “size” could be ambiguous if not properly defined. In response, we have\nclarified its meaning in the manuscript where it is first introduced. Specifically, we now state:\n“Microbialite components were leached quantitatively and independently (see Supplementary\nMethods), enabling measurements of both absolute arsenic concentrations and the size of each\ncompositional fraction (e.g. micrite, trapped-and-bound allochems, or organic matter), where size\nrefers to the relative mass contribution of each fraction to the total sample mass\" (line 145). We\nbelieve this definition addresses the potential confusion while preserving the term, which is used\nconsistently throughout the manuscript.\n5- L184: replace “that” with “than in”.\nThank you, the typo has been resolved."}, {"round": 2, "reviewer_comments": {"reviewer_1": "The authors have addressed all of my comments on the manuscript and included appropriate information to answer my main queries, i.e. further details about the sampling localities including GPS locations, discussion of taphonomic and diagenetic phenomena that may influence As distributions, and a description and justification of As distributions to consider in the fossil record. The addition of further images of the materials in the Supplementary Materials is particularly useful.\nI have no further comments and re-iterate that I find this to be a very thorough and interesting study with broad appeal to geobiologists interested in both the modern and fossil records.\nKeyron Hickman-Lewis\nBirkbeck, University of London"}, "author_response": ""}]

Decision letter