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Seamounts are ubiquitous topographic units in the global oceans, and the Caiwei Guyot in the Magellan Seamounts of the western Pacific is a prime example. In this study, we analyzed a well-dated sediment core using magnetic properties, sediment grain size, and metal enrichment to uncover regional ventilation history during the middle Pleistocene and explore potential linkages to global climate changes. Our principal findings are as follows: (1) The median grain size is 3.3 ± 0.2 mm, and clay and silt particles exhibit minimal variation, with average values of 52.8 ± 1.8% and 38.2 ± 1.6%, respectively, indicating a low-dynamic process; (2) Three grain-size components are identified, characterized by modal patterns of ~3 mm (major one), ~40 mm, and 400-500 mm, respectively; (3) Magnetic coercivity of the deep-sea sediments can be classified into three subgroups, and their coercivity values are 6.1 ± 0.5 mT, 25.7 ± 1.0 mT, and 65.2 ± 2.1 mT. Based on these results, we propose a close linkage between magnetic coercivity and metal enrichment, correlating with changes in deep-sea circulation intensity. Conversely, sediment grain-size changes seem to be more strongly influenced by eolian inputs. Consequently, we suggest that regional ventilation has weakened since ~430 ka, likely linked to a reduction in Antarctic bottom water formation. [ABSTRACT FROM AUTHOR]

Phosphorite deposit is a kind of marine sedimentary phosphate deposit with the francolite as the main ore mineral. These deposits were formed in the upwelling areas during six historical periods after Paleoproterozoic, and are generally related to the biological activity and the intensity of continental weathering. The formation of phosphorite deposits is controlled by marine transgressions and regressions, microbial activities and physical and chemical conditions of sediment pore water. The key factors for the formation of phosphorite deposits with high rare earth elements (REEs) content are low sedimentation rate and periodic submarine storms. Some phosphorite deposits are rich in medium and heavy REEs, which are potential important REE resources. The main host mineral of REEs in phosphorite is francolite. The REEs in francolite are mainly derived from seawater in sedimentary period and sediment pore water in early diagenetic period, and are incorporated into francolite via adsorption or lattice substitution. REEs can be adsorbed on the surface of marine particles by forming as REEs-carbonate complex and released by the marine particles decomposing that is induced by the change of redox conditions in the sediment pore water. Through the previous research, it is found that different marine particles preferentially adsorb different REEs, which results in the formation of different REEs patterns of pore water and then affects the patterns of francolite in phosphorite deposits. The enrichment of REEs in francolite is equal for light, medium and heavy REEs and different REEs patterns in francolite are derived from contrasting fluid sources. In general, seawater can produce a HREEs-enriched pattern, and pore water might yield a MREEs-bulge pattern (enrichment of medium REEs). As the phosphorite deposits with significant REEs contents are characterized by MREEs-bulge patterns that are consistent with Fe-Mn (oxyhydr) oxides input by the submarine hydrothermal process and the pore water in the early diagenetic process, which may indicates the influence of the submarine hydrothermal activities and the early diagenetic process on the REEs enrichment mechanism of phosphorite deposits. In this paper, the metallogenic process of phosphorite deposits is summarized from the characteristics, spatiotemporal distribution and metallogenic control factors of phosphorite deposits; the enrichment behavior and differentiation behavior of REEs in the process of phosphorite mineralization are analyzed from the content and anomaly of REEs in phosphorite deposits; and the possible enrichment mecha-nism of REEs in phosphorite rocks with abnormal enrichment of REEs is revealed. The study of metallogenic process and REEs enrichment mechanism of phosphorite deposits is of great significance for the prospecting and ex-ploitation of phosphorite deposits that are highly enriched in REEs. [ABSTRACT FROM AUTHOR]

Organic-rich shales from the uppermost Doushantuo Fm. (South China) record one of the most negative carbonate carbon isotopic excursions in Earth's history, known as the Shuram excursion, and contain meter to micro-size spheroids. In this study, we use Raman and energy dispersive spectroscopy to identify and describe the most common diagenetic spheroids to refine our understanding of the profound perturbations of the carbon cycle and the evolution of pore fluid chemistry imprinted in the sedimentary Precambrian record, especially in the late Ediacaran. The presence of 13C-depleted carbonate concretions or organic matter (OM) enclosed by lenticular dolomitic structures within the host shale unit suggests OM remineralisation and anaerobic oxidation, resulting in authigenic carbonate precipitation during the earliest stages of sediment diagenesis. Other mineralogical features, however, point to high levels of primary production, such as apatite bands that host spheroidal microfossils with highly fluorescent quartz and OM within abiotic concretions. These observations highlight the importance of considering co-occurring biotic and abiotic processes in explaining the formation of diagenetic spheroids in ancient sedimentary environments. From an astrobiology perspective, the interplay of biotic and abiotic processes reflects the complexity of early life systems and the environments that may exist on other terrestrial planets. Understanding the signatures of biotic and abiotic interactions in the Doushantuo Fm. is crucial for identifying potential biosignatures in extraterrestrial materials, thereby enhancing our understanding of life's universality and adaptability in diverse and extreme environments. [ABSTRACT FROM AUTHOR]

Shelf seas are experiencing a rise in shallow gas leaks, primarily methane, raising concerns due to their environmental impact. However, the effect of the leaks on early diagenesis remains poorly understood. This study analyzes sediment lithology, organic geochemistry and porewater geochemistry of two short cores collected nearby the pockmarks in the muddy inner shelf of the East China Sea. Our findings clearly demonstrate the impact of methane leakage on early diagenesis, evidenced by the shallower position of the SMTZ (sulfate-methane transition zone), higher concentrations of uranium (U), vanadium (V), and manganese (Mn) in the porewater near and above the SMTZ, and downcore decrease in Mg2+, Ca2+, and Sr2+ concentrations versus increase in Mg2+/Ca2+ and Sr2+/Ca2+ ratios. Their profile variations and the difference between two cores are determined by the intensity of methane leakage. The estimated methane diffusive flux of 619 mmol m-2 yr-1 at YEC7-2 is roughly 8.5 times that at YEC7-1 (73 mmol m-2 yr-1), consistent with a shorter distance of YEC7-2 to the pockmark with active methane leakage. A schematic model is summarized to demonstrate the response of early diagenesis processes to the increasing methane leakages in response to changing sedimentation regimes from accretion to severe erosion. This study undoubtedly improves our understanding mutual promotion effect between seafloor erosion and gas leakage, and their impact on early diagenesis processes and resultant porewater geochemical changes and authigenic mineral records. [ABSTRACT FROM AUTHOR]

Methane release from marine sediments strongly influences the local seafloor environment and ecosystems, and may impact Earth's climate system. Recent studies revealed anomalous molybdenum (Mo) enrichment in seep sediments, which was linked to methane release events. Marine seep sediments are a potential sink of Mo for the global ocean, while the mechanisms leading to local Mo enrichment are not fully understood. The sediments from a gas hydrate-bearing area of the South China Sea analyzed herein reveal authigenic Mo (Mo auth) contents ranging from 0 and 31.4 µg/g and δ98Mo auth values ranging from 0.18 ‰ to 3.31 ‰. The range of δ98Mo auth values of seep sediments is therefore similar to values of modern iron-rich sediments with low concentrations of dissolved porewater hydrogen sulfide and sediments deposited under weakly euxinic sediments. Among the obtained South China Sea data, the more positive δ98Mo auth values (>ca. 1.5 ‰) are interpreted to reflect diffusion of seawater Mo into the sediment at moderate seepage rates and Mo isotope fractionation during the formation of thiomolybdates in the sulfidic seep environment. The lower δ98Mo auth values (

Mercury (Hg) enrichment relative to total organic carbon (TOC) in sedimentary records has been widely used as a volcanism proxy. However, the depositional and diagenetic effects on Hg burial are not well understood, limiting the reliability of the proxy. Here, we report a systematic investigation of Hg sedimentation under well‐oxygenated bottom water. Most of the studied cores show total mercury content (THg) to TOC ratio between 50 and 300 ppb/%, including northwest Pacific ODP Site 1208, East Equatorial Pacific ODP Sites 677 and 1241, the Antarctic Zone ODP Site 1094, and East China Sea sediment core EC2005. The consistent THg/TOC ratio confirms the strong coupling between Hg and particulate organic matter (POM) despite large differences in geographic locations, sedimentation rates, TOC content, POM sources, and early diagenetic environments. Nevertheless, the THg/TOC ratio is higher in sediments under well‐oxygenated bottom water than in those under oxygen‐depleted waters, probably as a result of a higher degree of organic matter degradation in oxygenated sediments during early diagenesis. The THg in the high TOC variability section at Site 1208 is abnormally high, resulting in decoupling between Hg and POM. Mercury in this section is still leachable by oxidizing solution and has a similar trend of variability with easily reducible iron content, implying that the metal oxide‐organic matter association might act as Hg bounding phase. We suggest that the enriched Hg is probably supplied by the neighboring high TOC sediment. Therefore, while THg/TOC > 300 ppb/% could be considered as distinct Hg enrichment in sedimentary records, the diagenetic mobilization effect should be first excluded as the possible cause as demonstrated by the records of site 1208. Our study therefore provides new insights into the Hg cycle in the modern ocean and the utilization of Hg enrichment as a volcanism proxy. Key Points: Strong coupling between sedimentary Hg and particulate organic matter in an oxygenated marine environmentHg is relatively enriched compared to particulate organic matter during its aerobic degradationLarge change in the content of particulate organic matter causes its decoupling with Hg [ABSTRACT FROM AUTHOR]

To study the processes that govern the post-depositional mobility of metals in the estuarine sediment, five sediment cores were sampled in the Krka River estuary (Croatia). The obtained concentration ranges in the pore water were 0.057–49.7 μM for Fe, 0.310–100 μM for Mn, 0.068–26.8 nM for Co, 0.126–153 nM for Cu, 11.5–2793 nM for Zn, 0.222–31.3 nM for Pb, 4.09–59.4 nM for U, 38.8–2228 nM for Mo, and 0.065–2.29 nM for As. The vertical distribution of metals in the dissolved and solid fraction of the sediment, coupled with other diagenetic tracers (e.g., dissolved sulphide), demonstrate the importance of early diagenetic reactions, in particular Fe and Mn oxyhydroxide and sulphate reduction, for the cycling of metals in the sediment. The redox zonation in the sediment was compressed, and the suboxic zone occurs immediately below the sediment–water interface. The estimated benthic fluxes in the estuary were 5220 kg y−1 for Fe, 27,100 kg y−1 for Mn, 6.00 kg y−1 for Co, 20.5 kg y−1 for Cu, 5.16 kg y−1 for Pb, 111 kg y−1 for Mo, and 87.3 kg y−1 for As. The riverine input was more important than the benthic flux, except in the case of Mn and Fe. [ABSTRACT FROM AUTHOR]

Organic-rich shales from the uppermost Doushantuo Fm. (South China) record one of the most negative carbonate carbon isotopic excursions in Earth’s history, known as the Shuram excursion, and contain meter to micro-size spheroids. In this study, we use Raman and energy dispersive spectroscopy to identify and describe the most common diagenetic spheroids to refine our understanding of the profound perturbations of the carbon cycle and the evolution of pore fluid chemistry imprinted in the sedimentary Precambrian record, especially in the late Ediacaran. The presence of 13C-depleted carbonate concretions or organic matter (OM) enclosed by lenticular dolomitic structures within the host shale unit suggests OM remineralisation and anaerobic oxidation, resulting in authigenic carbonate precipitation during the earliest stages of sediment diagenesis. Other mineralogical features, however, point to high levels of primary production, such as apatite bands that host spheroidal microfossils with highly fluorescent quartz and OM within abiotic concretions. These observations highlight the importance of considering co-occurring biotic and abiotic processes in explaining the formation of diagenetic spheroids in ancient sedimentary environments. From an astrobiology perspective, the interplay of biotic and abiotic processes reflects the complexity of early life systems and the environments that may exist on other terrestrial planets. Understanding the signatures of biotic and abiotic interactions in the Doushantuo Fm. is crucial for identifying potential biosignatures in extraterrestrial materials, thereby enhancing our understanding of life’s universality and adaptability in diverse and extreme environments.

Reconstructing the oxygen content at the ocean floor provides insight into ocean circulation, ventilation, and carbon storage in the deep sea. The microbial breakdown of organic carbon within marine sediment through aerobic respiration consumes oxygen in the pore fluid and releases dissolved inorganic carbon. The offset in the carbon isotopic composition, δ13C, of epifaunal and infaunal foraminifera, Δδ13C, is considered to reflect the aerobic respiration of organic carbon and can be used to reconstruct the oxygen content of the bottom water. Previous work provided an empirical calibration that was suggested to be valid for oxygen reconstructions between 55 and 235 μmol kg−1. In this study, we apply a biogeochemical reactive transport model (RTM) to extend and update this calibration, allowing for the reconstruction of oxygen concentrations ([O 2 ]) up to ∼300 μmol kg−1. Using the RTM and new bottom water [O 2 ] and pore fluid measurements from Iberian Margin sediment cores, we also demonstrate that the calibration between the Δδ13C and bottom water [O 2 ] must account for the coupled changes in the carbon system due to the respiration of organic carbon in the overlying ocean including the concentration and carbon isotopic composition of the dissolved inorganic carbon, and the δ13C of the organic carbon within the sediment column. We apply the improved calibration to reconstruct the changes in oxygen content at International Ocean Discovery Program (IODP) Site U1385 in the deep North Atlantic over the past 1.5 Myr. [ABSTRACT FROM AUTHOR]

The quantification of anaerobic oxidation of organic matter in the global seabed is to a large extent based on transport-reaction modeling of pore water ions involved in the mineralization processes. As the predominant of these processes, sulfate reduction can be modeled from the depth distribution of sulfate and other relevant solutes. An active organoclastic sulfate reduction is typically characterized by sulfate profiles with a concave-down shape. We use here a comprehensive database to show that half of all competent sulfate profiles have an opposite, concave-up shape, which may appear inconsistent with this concept. We also show that there is a strong discrepancy between sulfate profiles obtained by gravity coring and by IODP coring, with generally much higher sulfate flux and shallower sulfate penetration depth in the gravity cores (the term IODP here covers the international ocean drilling programs, DSDP, ODP and IODP). The discrepancy is correlated with a selectivity for coring sites in the published studies, by which gravity coring is focused towards the ocean margins and continental shelf and towards high surface water productivity and high sedimentation rate. In contrast, IODP coring is more frequently done in low sedimentation regions and is largely absent from the continental shelf. As a result, estimates of sulfate reduction in the global seabed may be biased by a selection of only IODP core data for the modeling. Furthermore, the modeling of pore water profiles shows net rates of sulfate reduction, while 35S-radiotracer experiments show the higher gross rates of the process. Estimates from 35S-radiotracer measurements suggest that dissimilatory sulfate reduction oxidizes 77 Tmol yr−1 of organic carbon globally, which is 3- to 4-fold higher than recent data based on modeling of IODP data. The potential biases by global estimates of sulfate reduction in marine sediments are discussed. [ABSTRACT FROM AUTHOR]

The study on the origin of quartz and silica in Xuanwei Formation coal in Northwest Yunnan, China, is helpful to understand the relationship between quartz and silica and the high incidence of lung cancer from the root. To address these questions, the mineralogy and microscopic studies of silica in Xuanwei Formation coal were performed. The following results were obtained: (1) silica in the late Permian Xuanwei Formation coal seams originated from detrital input, early diagenesis, and late diagenesis. (2) A more significant contribution comes from early diagenesis, which contains abundant authigenic quartz and amorphous silica. (3) Quartz and silica from inorganic silicon are more symbiotic with kaolinite and from biogenic silicon with chamosite. (4) Three silica polymorphs in coal samples have been identified: opal-A (amorphous silica), opal-CT/-C (cristobalite/tridymite), and α quartz. (5) Opal-A is ubiquitous, while opal-CT/-C and α quartz are rare. (5) Opal-A is an amorphous and nontoxic ordinary silica. (6) Since the toxicity of amorphous silica and its presence in coal is an emerging topic, it should be continuously monitored. [ABSTRACT FROM AUTHOR]

Hemipelagic calcareous ooze is a mixture of biogenic carbonate ooze and detrital terrigenous materials from continental weathering. Understanding its early diagenetic features is critical for geophysical measurement and geo-hazard assessment. However, the early diagenesis of hemipelagic calcareous ooze has rarely been studied due to a lack of samples and in-situ measurement in shallow strata. During IODP Expedition 368, hemipelagic calcareous oozes, up to hundreds of meters thick, were encountered in the shallow subsurface (< 400 m) at two sites (U1501 and U1505) close to the continent-ocean transition zone in the northern South China Sea margin. Here, we use coring and well logging data to perform rock physics diagnostics to characterize early diagenetic processes in hemipelagic calcareous ooze. The calcareous ooze in the study area is primarily chalky marl with the ratio of carbonate to carbonate plus siliciclastic minerals ranging 30% ~ 70%, according to a mineralogy-based lithofaices classification scheme. In the chalky marl interval, pyrite accounts for around 15% of the total mineral composition and this existence may be related to significant post-rift magmatic activity that produced significant amouts of volcanic material. The porosity reduction with depth in the chalky marl interval has three phases and carbonate content is suggested to be the controlling factor affecting porosity reduction. The varying carbonate content in the oozes can result in contrasting mechanical properties and porosities, potentially serving as weak layers for submarine landslides. Additionally, the interface between calcareous ooze and siliciclastic sediment is also a potential failure plane. Therefore, we propose that the development of hemipelagic calcareous ooze has significant implications for seafloor instability on continental margins. [ABSTRACT FROM AUTHOR]

Authigenic molybdenum (Moauth) in marine sediments holds great potential to archive the Mo isotopic composition of seawater and biogeochemical processes. However, the factors that control authigenic Mo isotope (δ98Moauth) distribution patterns remain poorly constrained. Here, we report Mo abundances and δ98Mo compositions for bulk‐rock (bulk) and sequentially extracted fractions—including total authigenic (auth; i.e., non‐lithogenic fraction), carbonate (carb), iron and manganese oxyhydroxides, pyrite (py), and organic fractions (OM)—of authigenic carbonates recovered from various hydrocarbon seep sites in the Gulf of Mexico and the South China Sea. Extracted pyrite fractions exhibit Mo contents varying from 0.1 to 23.4 μg/g and generally dominate the Mo budget of seep carbonate rocks. Our data indicate large ranges of δ98Mobulk and δ98Moauth values relative to NIST 3134 (0.25‰), varying from 1.02 to 1.98‰ (n = 4) and from 0.15 to 3.07‰ (n = 34), respectively. The difference in δ98Mo values between carbonate and pyrite fractions of seep carbonate rocks formed under sulfidic conditions increases with higher Moauth contents, suggesting a control of dissolved hydrogen sulfide concentrations on Mo isotope fractionation during carbonate precipitation. Compared with δ98Moauth and δ98Mopy, δ98Mocarb of seep carbonate rocks formed under sulfidic conditions shows a relatively narrow range with an average of 1.98 ± 0.31‰ (1 SD; n = 10), providing constraints on the δ98Mo composition of seawater in the course of Earth history. Overall, our findings show that the δ98Mo composition of sequentially extracted phases of carbonate‐rich sedimentary rocks can provide insights into seawater‐sediment interactions and biogeochemical pathways of Mo during early diagenesis. Plain Language Summary: Sulfate reduction has created widespread euxinic conditions in the water column of ancient oceans and prevalent sulfidic conditions in subseafloor pore‐water environments of the modern ocean. Ancient seawater δ98Mo compositions have been considered to reflect the overall extent of oxic versus euxinic conditions of the global ocean, highlighting the importance of a robust reconstruction of the marine Mo isotope inventory largely unaffected by diagenesis. Under sulfidic conditions, the seawater δ98Mo value can be archived by authigenic Mo in marine carbonate sediments formed during early diagenesis. However, the conditions of capturing the seawater δ98Mo signature remain poorly constrained. Using a sequentially chemical leaching procedure, we investigated Mo contents and isotopic compositions of different fractions of modern seafloor seep carbonate rocks. Our results show that a δ98Mo average value of 1.98 ± 0.31‰ (1 SD; n = 10) of the carbonate mineral phase results from small or no Mo isotope fractionation during near‐quantitative authigenic Mo sequestration under sulfidic conditions, potentially capturing seawater δ98Mo. Our study shows that δ98Mo of sequentially extracted phases of sedimentary carbonate rocks is a robust tool to constrain seawater and pore fluid properties. Key Points: δ98Mo of sequentially extracted phases of carbonates is useful to trace Mo isotope fractionation during early diagenesisDegree of Mo isotope fractionation during carbonate precipitation is controlled by hydrogen sulfide concentrationsAuthigenic Mo sequestration into the carbonate phase under sulfidic conditions generates small Mo isotope fractionation [ABSTRACT FROM AUTHOR]

Abstract Mercury (Hg) enrichment relative to total organic carbon (TOC) in sedimentary records has been widely used as a volcanism proxy. However, the depositional and diagenetic effects on Hg burial are not well understood, limiting the reliability of the proxy. Here, we report a systematic investigation of Hg sedimentation under well‐oxygenated bottom water. Most of the studied cores show total mercury content (THg) to TOC ratio between 50 and 300 ppb/%, including northwest Pacific ODP Site 1208, East Equatorial Pacific ODP Sites 677 and 1241, the Antarctic Zone ODP Site 1094, and East China Sea sediment core EC2005. The consistent THg/TOC ratio confirms the strong coupling between Hg and particulate organic matter (POM) despite large differences in geographic locations, sedimentation rates, TOC content, POM sources, and early diagenetic environments. Nevertheless, the THg/TOC ratio is higher in sediments under well‐oxygenated bottom water than in those under oxygen‐depleted waters, probably as a result of a higher degree of organic matter degradation in oxygenated sediments during early diagenesis. The THg in the high TOC variability section at Site 1208 is abnormally high, resulting in decoupling between Hg and POM. Mercury in this section is still leachable by oxidizing solution and has a similar trend of variability with easily reducible iron content, implying that the metal oxide‐organic matter association might act as Hg bounding phase. We suggest that the enriched Hg is probably supplied by the neighboring high TOC sediment. Therefore, while THg/TOC > 300 ppb/% could be considered as distinct Hg enrichment in sedimentary records, the diagenetic mobilization effect should be first excluded as the possible cause as demonstrated by the records of site 1208. Our study therefore provides new insights into the Hg cycle in the modern ocean and the utilization of Hg enrichment as a volcanism proxy.

FurongianâTremadocian phosphorites of Estonia are sandstone rich in biogenic apatite, represented by brachiopod detritus. The study focuses on the mineralogical and micro-analytical characterisation of phosphorites from the Aseri, Toolse, and Kabala deposits based on FE-SEM and EPMA analyses. The shell fragments are composed of alternating compact and porous laminae, but with considerably poor preservation of pristine textures, superseded by the formation of authigenic CAF-apatite during the early diagenesis. In all settings, the shells showed preferential uptake of Sr into the porous cryptocrystalline laminae. The altered areas are composed of massive apatite crystallites with Mn-enriched layers. They are frequently covered with pyrite, indicating progressive recrystallisation under the influence of interstitial fluids and fluctuations in redox gradients in coastal environments.

Abstract Authigenic molybdenum (Moauth) in marine sediments holds great potential to archive the Mo isotopic composition of seawater and biogeochemical processes. However, the factors that control authigenic Mo isotope (δ98Moauth) distribution patterns remain poorly constrained. Here, we report Mo abundances and δ98Mo compositions for bulk‐rock (bulk) and sequentially extracted fractions—including total authigenic (auth; i.e., non‐lithogenic fraction), carbonate (carb), iron and manganese oxyhydroxides, pyrite (py), and organic fractions (OM)—of authigenic carbonates recovered from various hydrocarbon seep sites in the Gulf of Mexico and the South China Sea. Extracted pyrite fractions exhibit Mo contents varying from 0.1 to 23.4 μg/g and generally dominate the Mo budget of seep carbonate rocks. Our data indicate large ranges of δ98Mobulk and δ98Moauth values relative to NIST 3134 (0.25‰), varying from 1.02 to 1.98‰ (n = 4) and from 0.15 to 3.07‰ (n = 34), respectively. The difference in δ98Mo values between carbonate and pyrite fractions of seep carbonate rocks formed under sulfidic conditions increases with higher Moauth contents, suggesting a control of dissolved hydrogen sulfide concentrations on Mo isotope fractionation during carbonate precipitation. Compared with δ98Moauth and δ98Mopy, δ98Mocarb of seep carbonate rocks formed under sulfidic conditions shows a relatively narrow range with an average of 1.98 ± 0.31‰ (1 SD; n = 10), providing constraints on the δ98Mo composition of seawater in the course of Earth history. Overall, our findings show that the δ98Mo composition of sequentially extracted phases of carbonate‐rich sedimentary rocks can provide insights into seawater‐sediment interactions and biogeochemical pathways of Mo during early diagenesis.

To study the processes that govern the post-depositional mobility of metals in the estuarine sediment, five sediment cores were sampled in the Krka River estuary (Croatia). The obtained concentration ranges in the pore water were 0.057–49.7 μM for Fe, 0.310–100 μM for Mn, 0.068–26.8 nM for Co, 0.126–153 nM for Cu, 11.5–2793 nM for Zn, 0.222–31.3 nM for Pb, 4.09–59.4 nM for U, 38.8–2228 nM for Mo, and 0.065–2.29 nM for As. The vertical distribution of metals in the dissolved and solid fraction of the sediment, coupled with other diagenetic tracers (e.g., dissolved sulphide), demonstrate the importance of early diagenetic reactions, in particular Fe and Mn oxyhydroxide and sulphate reduction, for the cycling of metals in the sediment. The redox zonation in the sediment was compressed, and the suboxic zone occurs immediately below the sediment–water interface. The estimated benthic fluxes in the estuary were 5220 kg y−1 for Fe, 27,100 kg y−1 for Mn, 6.00 kg y−1 for Co, 20.5 kg y−1 for Cu, 5.16 kg y−1 for Pb, 111 kg y−1 for Mo, and 87.3 kg y−1 for As. The riverine input was more important than the benthic flux, except in the case of Mn and Fe.

Excess phosphorus abundance often drives eutrophication and affects surface water quality. Formation of vivianite (Fe3(PO4)2 • 8H2O) in aquatic sediments acts as a significant sink for phosphate (P), crucial for resorting surface waters. Authigenic vivianite formation, however, can be limited by other ferrous iron containing phases, in particular iron sulfides. Although thermodynamically feasible under suitable conditions, the formation of vivianite from mackinawite has been widely disregarded for authigenic phosphate mineral formation. Here we investigated the formation of vivianite from mackinawite (FeS) in batch experiments in which dissolved sulfide was continuously removed, at P levels between 0 – 5 mM in a pH of 6 to 8. Solid characterizations by electron microscopy, X-ray diffraction as well as Mössbauer and X-ray absorption spectroscopy demonstrates that vivianite was formed at all pH values in P amended experiments. The temporal evolution of dissolved Fe(II) concentrations indicates that the transformation proceeds via the release of the dissolved Fe(II) by FeS dissolution and subsequent vivianite precipitation, over time scales of days. The kinetics of the transformation are controlled by the dissolution rates of FeS. Aging and transformation of FeS, however, compete with vivianite formation. Aging is more pronounced at higher pH but is inhibited by P adsorption. Hence, the effect of pH and P concentration on aging is the main reason for the influence on these parameters on the rates and extent of vivianite formation. Our findings demonstrate that FeS can be an effective iron source for vivianite formation in aquatic sediments when sulfide concentrations decrease due to, for example, changes in external forcing or microbial sulfide oxidation. Formation of vivianite from FeS as an Fe source can also open new perspectives in P recovery in water treatment, for example when Fe is added to digesters to bind H2S.

Ferromanganese nodules are a potential energy resource because of their high contents of economically interesting elements (i.e. Mn, Ni, Cu, and Zn). These are higher in diagenetic layers than in hydrogenetic layers. The study of the causes of elemental accumulation in the diagenetic layer is useful for the exploration metal-rich nodules. A diagenetic-dominant ferromanganese nodule, from the central Clarion–Clipperton Fracture Zone of the eastern Pacific Ocean was studied from core to rim. It was divided into four layers and seven sublayers, of the four typical diagenetic sublayers (L1, L2-1, L2-2, and L3-1). Differences were observed in these diagenetic sublayers. L1 presents the highest Mn/Fe ratio (54), the lowest Co content (2262 ppm), and a positive Ce anomaly. L2-1 exhibits high Co (3122 ppm) and Ba contents (4020 ppm), a positive Ce anomaly, and an obvious peak for 10 Å manganate minerals. L2-2 contains the lowest Ni+Cu contents (3.2 wt%), the highest Ba and Co contents (5110 ppm), and the strongest positive Ce anomaly. In L2-2, the δCe value can be positively correlated to the Mn/Fe ratio and a pronounced peak for 10 Å manganate minerals indicates that this layer has the highest mineral crystallinity. L3-1 shows the highest Ni+Cu contents (5.4 wt%), the lowest Ba (1247ppm), and Co (1725 ppm), a weakly positive Ce anomaly, and the poorest mineral crystallization. Diagenetic- and hydrogenetic-endmember mixing models reveal that hydrogenetic input contributes minimally to these chemical changes, whereas diagenetic input contributes greatly. The changes in diagenetic input may be caused by the changes in primary productivity brought about by movement of tectonic plates and the intense activity of the diagenetic pore fluid. The activity may provide a metal source for the diagenetic sublayer (anomalously high Co and Ce content) via the incorporation of metals released from dissolved buried nodules and micronodules under a suboxic or reducing environments. [ABSTRACT FROM AUTHOR]

Volcanism is a dominant natural source of mercury (Hg) to the atmosphere, biosphere, ocean and sediments. In recent years, sedimentary Hg contents have emerged as a tool to reconstruct volcanic activity, and particularly activity of (subaerially emplaced) large igneous provinces in geological deep time. More specifically, Hg has shown potential as a useful proxy to illuminate the previously elusive impact of such large-scale volcanism on marine and terrestrial paleo-environments. While Hg is now widely applied as volcanism tracer, non-volcanic factors controlling sedimentary Hg content are generally not well constrained. Part of this uncertainty stems from our inability to directly observe a natural unperturbed "steady-state" environment as a baseline, as the modern Hg cycle is heavily influenced by anthropogenic activity. Here we focus on the effects of ambient redox conditions in the water column and shallow sediments (early diagenesis), quantify their influence on the geological Hg record and thereby contribute to constraining their potential impact on the use of Hg as a proxy for deep-time volcanic activity. Constraining these factors is of critical importance for the application of Hg as such a proxy. Many periods in the geological past for which records have been generated, such as the Mesozoic Oceanic Anoxic Events, are marked by a variety of high-amplitude environmental perturbations, including widespread deoxygenation and deposition of organic-rich sediments. We estimate the impact of redox changes and early diagenesis on the geological Hg record using a suite of (sub)recent–Pleistocene and Upper Cretaceous sediments representing oxic to euxinic marine conditions. Our sample set includes a transect through an oxygen minimum zone and cores that record transient shifts in oxygenation state, as well as post-depositional effects – all unrelated to volcanism, to the best of our knowledge. We find substantial alterations to the Hg record and the total organic carbon and total sulfur content, which are typically assumed to be the most common carrier phases of Hg in marine sediments. Moreover, these biases can lead to signal alteration on a par with those interpreted to result from volcanic activity. Geochemical modifications are ubiquitous and their potential magnitude implies that the factors leading to biases in the geological record warrant careful consideration before interpretation. Factors of particular concern to proxy application are (1) the disproportionate loss of organic carbon and sulfur relative to Hg during oxidation that strongly modulates normalized Hg records, (2) the evasion of Hg in anoxic and mildly euxinic sediments and (3) sharp focusing of Hg during post-depositional oxidation of organic matter. We suggest that paired analyses of additional redox-sensitive trace elements such as molybdenum, and organic-matter characteristics, particularly the type of organic matter, could provide first-order constraints on the role that redox and diagenetic changes played in shaping the Hg record as part of checking the attribution of enrichments to volcanic activity. [ABSTRACT FROM AUTHOR]

The Sturtian (720–670 Ma) glacial–interglacial transition period was an important interval for sedimentary manganese metallogenesis, including the Mn oxide deposit in the Otjosondu region in Namibia and Mn carbonate deposits in the Datangpo Formation in the south-eastern Yangtze Platform, South China. During this period, Earth experienced the breakup of Rodinia, the Sturtian glaciation, and the Neoproterozoic oxygenation event. In this study, we investigate scenarios that might have provided geologically and geochemically favorable conditions for Mn metallogenesis. In these scenarios, the global recovery of microorganisms enhanced marine primary productivity and O2 levels of the hydrosphere and atmosphere during the Sturtian glacial–interglacial transition. However, the water column was not completely oxidized, maintaining redox stratification. Transgression–regression cycles or O2-rich downwelling drove the exchange of oxygenated topwater and anoxic deep water in rift-related basins that developed due to Rodinia's breakup. The coupling of these processes precipitated existing dissolved Mn(II) at the margins of basins (Otjosondu region) or at their centers (Yangtze Platform). In the latter case, precursor Mn oxides were further converted into Mn carbonates via the reduction of Mn oxides coupled with organic matter oxidation during early diagenesis. A brief review of Mn metallogenesis in the geological record reveals that Mn metallogenic processes typically occur under geo-environmental conditions that, in concert, produce favorable conditions for Mn sourcing, concentration, and sedimentation. [ABSTRACT FROM AUTHOR]

Simple Summary: A comprehensive geochemical, mineralogical and microbiological analysis of bottom sediments was carried out in five main areas of the littoral zone of Lake Issyk-Kul, one of the largest mountain lakes in the world. The aim of this work is to study the microbial diversity of bottom sediments of Lake Issyk-Kul and to establish the role of microbiota in biogeochemical processes, in particular, its involvement in authigenic mineral formation. Aerobic and anaerobic oxidation of organic matter begins at the water-sediment interface and is the main source of carbon dioxide which provides the formation of carbonate minerals. Oxygen consumption during organic carbon oxidation triggers anaerobic sulphate reduction processes, leading to the formation of authigenic sulphide minerals. This work is the initial stage in the study of the microbiology and biogeochemistry of the ecosystem of Lake Issyk-Kul, and the data obtained will allow a better understanding of the processes of C, N and S cycling in the lake and the mechanisms of formation of modern bottom sediments. This article presents geochemical, mineralogical and microbiological characteristics of five samples of modern bottom sediments in the littoral zone of the high-mountain salty lake Issyk-Kul. The 16S rRNA gene sequencing method shows that the microbial community consists of organic carbon degraders (representatives of phyla: Proteobacteria, Chloroflexi, Bacteroidota and Verrucomicrobiota and families Anaerolineaceae and Hungateiclostridiaceae), photosynthetic microorganisms (representatives of Chloroflexi, phototrophic Acidobacteria, purple sulphur bacteria Chromatiaceae and cyanobacteria) and bacteria of the reducing branches of the sulphur biogeochemical cycle (representatives of Desulfobacterota, Desulfosarcinaceae and Desulfocapsaceae). The participation of microorganisms in processes in the formation of a number of authigenic minerals (calcite, framboidal pyrite, barite and amorphous Si) is established. The high diversity of microbial communities indicates the presence of labile organic components involved in modern biogeochemical processes in sediments. The active destruction of organic matter begins at the water-sediment interface. [ABSTRACT FROM AUTHOR]

In the marine sediment record, concentrations and isotope ratios of chromium (Cr) can be used to reconstruct ocean biogeochemical conditions. These reconstructions rely on a detailed understanding of the chemical pathways that Cr undergoes as it is transferred from the water column to the sediment record. We examined Cr concentrations in marine pore fluids and sediments from six continental margin sites, which can be grouped into two basic environments: (1) sites where sediments are oxygenated and rich in solid phase Mn (herein termed oxic), and (2) sites where sediments are organic C (C org)-rich and oxygen is depleted (anoxic). We found Cr concentrations to be lower (maximum of 12 nM in pore fluids and 124 ppm sediment solid phase) at oxic sites compared with anoxic sites (maximum of 77 nM and 184 ppm). Our findings confirm previously published interpretations of dissolved Cr in pore fluids (Brumsack and Gieskes, 1983; Shaw et al., 1990). In oxic surface sediments, particulate Cr(III) can be oxidised by Mn oxides, which leads to elevated concentrations of dissolved Cr co-occurring at the same depth as elevated Mn concentrations in the sediment. Under these oxidising conditions, down-core sediments contain relatively low solid-phase Cr concentrations. In oxic sediments, Cr speciation reveals that most of the pore fluid Cr is in the Cr(VI) state. At the site where Mn oxide-rich sediments rest below an oxic water column, oxidative loss of Cr from the sediment to the bottom water leads to the lowest estimated Cr burial efficiency of the sites examined here. Under anoxic C org -rich conditions, both pore fluids and sediment solid phases contain high Cr concentrations, with 40–80% of dissolved pore fluid Cr present as Cr(III). This enrichment of Cr appears to be tightly linked to the presence of high total organic carbon (TOC) content and scavenging of Cr by (organic) particles in the water column. Combined, these data highlight the strong dependence of Cr on both sedimentary redox conditions as well as biological productivity. Based on the data from modern continental margin sediments, we propose that Cr concentrations and isotope compositions of the authigenic sediment fraction may record a combination of redox conditions and biological productivity in the water column. If confirmed by Cr isotope analyses, these findings will add support for the notion that Cr may serve as a proxy for ocean biological and chemical sedimentological conditions. Thus, careful assessment of the impact of organic matter on Cr is required for reconstructions of redox conditions with sedimentary records. [ABSTRACT FROM AUTHOR]

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We report the measurement of the Fluxes of Dissolved Compounds at the Sediment-Water Interface (DFSWIs), obtained by analyzing data collected in the Italian Exclusive Economic Zone (EEZ) of the Northern and Central Adriatic Sea from 1982. We also describe the methods, honed by Italy’s Consiglio Nazionale delle Ricerche (CNR), employed to measure and calculate DFSWIs, including benthic chambers and landers, sediment-water interface micro-profiling, on-board incubation, and pore water modeling. Data analysis demonstrated that in situ measurements are the most reliable approaches, but that on-board collected data also supply interesting results. The DFSWI data allowed to divide the Adriatic seafloor into areas with similar DFSWI types and intensities and to define the role of sediments as sources or sinks of chemicals such as nutrients, Dissolved Inorganic Carbon (DIC), and metals. DFSWIs mainly depend on dissolved and solid river inputs, Marine Organic Matter (mOM) production, and sediment reworking. They show a seasonal temperature-related trend, which in summer induces increased chemical reaction kinetics and microbial activity. DFSWIs decline from the Po River mouths southward, along the Holocene mud wedge. North of the Po River Delta, the DFSWIs are weaker and confined to the areas in front of the major river mouths, due to poor Organic Matter (OM) inputs and strong reworking of bottom sediments. The area south of the Po Delta cusp is characterized by strong DFSWIs, due to the high deposition of solid inputs from the Po River, strong primary production, and protection from the Western Adriatic Current and Bora storms; in summer, higher temperatures, and calm hydrodynamic conditions generate near-bottom hypoxic to anoxic environments in this area. In Northern and Central Adriatic offshore areas, negative DIC and phosphate fluxes are due to poorly reactive OM reaching this area and to the mainly carbonate composition and oxic environment of local bottom sediments. DFSWI data analysis highlighted the important contribution of sediments to marine carbon and nutrient cycles. This is particularly important for carbon, which plays a major role in seawater acidification and global climate change. The study also provides average DFSWI data for each diagenetic area, which allow calculating the carbon and nutrient budgets in the Adriatic Sea.

Thirteen Short sediment cores (30-50 cm) were collected from Bohai Sea, Yellow Sea and Changjiang Estuary in China, and the early diagenesis of several redox sensitive metals (Fe, Mn, Mo, U and V, referring to as RSMs) in sediment were studied. The recycling process of Mo and Mn was closely correlated with each other, generating benthic fluxes diffusing upward from sediment to overlying water column, and the flux rates are related to the organic carbon oxidation rates. The recycling of U and V were more tightly coupled with Fe oxides, generating benthic fluxes going downward into the sediment in most cores. Significant authigenic accumulation of U, in contrary to little to no accumulation of Mo and V, were found in the study region, even in Changjiang Estuary where hypoxic condition was often found during summer. Benthic diffusive fluxes were compared with authigenic mass accumulate rates (MAR), which indicated that, besides the benthic diffusion process, there are other processes controlling the authigenic accumulation of the RSMs. The close relationships between authigenic accumulation of RSMs with OCburial and OCburial with Sburial, indicating the authigenic accumulation of RSMs is a consequence of redox environment in shelf sediment, which directly influencing the organic carbon degradation process. Compared with other continental margin, moderate enrichment of U was found in China continental sediment. The authigenic U accumulation in BS and NYS sediments accounted for 20 - 68% of the Yellow River input, whilst in SYS sediments accounted for ~ 64% of the Yellow River and Changjiang River input, which acting as important U sinks that cannot be ignored.

Upward diffusing methane in gas hydrate geological systems is consumed in the sediment and water column by a series of biogeochemical reactions, which not only affect living foraminifera but also lead to early diagenetic alteration of buried foraminiferal tests. Previous studies of the impact of methane release events on fossil foraminifera focused mainly on carbon and oxygen isotopes, with little attention given to other geochemical proxies to date. Here, we examine the test wall microstructure and analyze the elemental and stable carbon-oxygen isotopic compositions of buried foraminifera from gas hydrate-bearing sediments at Site GMGS2-16 in the northern South China Sea. Our results show that diagenetic alteration of foraminiferal tests at Site GMGS2-16 is mainly linked to high-Mg calcite overgrowths. Test δ13C covaries negatively with the degree of diagenetic alteration (based on petrographic characteristics) but positively with Mg/Ca ratios. With increasing diagenetic alteration, Ba/Ca, Mn/Ca, Fe/Ca, Mo/Ca, U/Ca, V/Ca, Ni/Ca and Co/Ca also generally increase, but Sr/Ca and Cu/Ca exhibit variable changes. We infer that foraminiferal tests are highly susceptible to alteration by methane-bearing fluids, and that the elemental ratios of diagenetically altered tests are potentially reliable proxies for paleo-methane release events. At Site GMGS2-16, the overall pattern of diagenetic alteration of foraminiferal tests records frequent vertical fluctuations of the sulfate-methane transition zone (SMTZ) caused by variable fluid flux and methane release rates, with two discrete large-scale methane release events having maximum ages of ~0.47 ka and ~170 ka.

The biogeochemical cycles of carbon and silicon are tightly coupled in modern marine environments due to the pivotal role of planktonic diatoms. Once diatoms are sedimented, the fate of organic matter and biogenic silica is initially governed by bottom water oxygen levels and bacterial communities involved in remineralization processes. The early diagenesis of biosiliceous sediments may result in drastic changes in composition, sometimes hampering palaeoenvironmental reconstruction. Sedimentary successions deposited in the Mediterranean region during the Messinian salinity crisis (5.97–5.33 Ma) allow to explore the early diagenetic transformation of organic matter and biogenic silica in a restricted basin experiencing a severe palaeoceanographic turnover. Sedimentological and petrographic observations coupled to elemental, mineralogical, inorganic and organic geochemical analyses were carried out on biosiliceous (diatomaceous shales and diatom‐bearing mudstones) and associated clay‐rich and dolomite‐rich sediments (dolomitic mudstones) interbedded with primary gypsum layers in the Piedmont Basin (north‐west Italy). The state of preservation of biogenic silica was governed by different pathways of organic matter remineralization, depending on bottom and pore water redox conditions, which were controlled by the structure of the water column. Aerobic respiration of organic matter in a mixed water column and an oxygenated seafloor promoted biogenic silica preservation. In contrast, bottom water anoxia induced by permanent stratification of the water column favoured the concomitant formation of dolomite and authigenic clays. In particular, organic matter degradation through bacterial sulphate reduction increased pore water alkalinity, promoting the precipitation of dolomite. At the same time, the rise in pH promoted the dissolution of biogenic silica which, reacting with pore water cations, ultimately caused the formation of authigenic clays. This study suggests that the apparent annihilation of Mediterranean marine biota during the Messinian salinity crisis partially reflects an early diagenetic bias produced by interactions of the carbon, silicon and sulphur biogeochemical cycles in a restricted basin. [ABSTRACT FROM AUTHOR]

The early diagenetic interplay between reactive iron, sulfur, and organic matter in the bathymetrically isolated Santa Monica Basin (SMB) sediments are investigated in this study. We explore solid-phase and porewater profiles from the basin, supplemented with a transect from 71 to 907 m water depth that includes oxygenated (>60 μM O 2) bottom waters near the coast and oxygen-deficient waters (∼4 µM O 2) in the basin. The geochemical data of the basin sediments are further scrutinized by means of reactive transport modeling. The results show that the basin sediments do not follow the traditional geochemical signatures of oxygen-deficient settings. A lack of dissolved sulfide accumulation and sulfurized iron persists despite the sediments being deposited under reducing conditions (without bioturbation/bioirrigation), strong organic carbon input (TOC up to 5.0 wt%), and active dissimilatory sulfate reduction. Not only did we find an exceptional enrichment in highly reactive Fe in the surface sediments (∼45 % of total Fe), but the enrichment of reactive Fe, including ferrihydrite, persists downcore and coexists with high levels of dissolved Fe. The enhanced preservation of Fe oxides and lack of iron-sulfide precipitation is in part explained by detection via Mössbauer spectra of iron oxides bounded to organic matter (Fe[III]-OM coprecipitates). The modeled Fe budget shows that most of the Fe oxides in the surface sediments are internally recycled by upward diffusion and subsequent oxidation of Fe2+. Sulfide oxidation coupled to Fe reduction effectively precludes sulfide accumulation while enhancing build-up of dissolved Fe, fueling the Fe cycle within the first 5 cm depth. Continuous reoxidation of Fe2+ enhances the formation of Fe(III)-OM coprecipitates, limiting the amount of reactive organic matter. In the unavailability of labile organic matter, other than within the uppermost layers, the organic-rich sediment profiles are dominated by Fe cycling that limits the production and preservation of sulfides and enhances the preservation of Fe oxides and organic carbon. This study highlights key local controls on Fe availability in marginal basins and describes an intricate biogeochemical C-Fe-S cycling in modern and possibly ancient marine systems with important implications for Fe availability in the marine realm. [ABSTRACT FROM AUTHOR]

Furongian–Tremadocian phosphorites of Estonia are sandstone rich in biogenic apatite, represented by brachiopod detritus. The study focuses on the mineralogical and micro-analytical characterisation of phosphorites from the Aseri, Toolse, and Kabala deposits based on FE-SEM and EPMA analyses. The shell fragments are composed of alternating compact and porous laminae, but with considerably poor preservation of pristine textures, superseded by the formation of authigenic CAF-apatite during the early diagenesis. In all settings, the shells showed preferential uptake of Sr into the porous cryptocrystalline laminae. The altered areas are composed of massive apatite crystallites with Mn-enriched layers. They are frequently covered with pyrite, indicating progressive recrystallisation under the influence of interstitial fluids and fluctuations in redox gradients in coastal environments. [ABSTRACT FROM AUTHOR]

We report the measurement of the Fluxes of Dissolved Compounds at the Sediment-Water Interface (DFSWIs), obtained by analyzing data collected in the Italian Exclusive Economic Zone (EEZ) of the Northern and Central Adriatic Sea from 1982. We also describe the methods, honed by Italy's Consiglio Nazionale delle Ricerche (CNR), employed to measure and calculate DFSWIs, including benthic chambers and landers, sediment-water interface micro-profiling, on-board incubation, and pore water modeling. Data analysis demonstrated that in situ measurements are the most reliable approaches, but that on-board collected data also supply interesting results. The DFSWI data allowed to divide the Adriatic seafloor into areas with similar DFSWI types and intensities and to define the role of sediments as sources or sinks of chemicals such as nutrients, Dissolved Inorganic Carbon (DIC), and metals. DFSWIs mainly depend on dissolved and solid river inputs, Marine Organic Matter (mOM) production, and sediment reworking. They show a seasonal temperature-related trend, which in summer induces increased chemical reaction kinetics and microbial activity. DFSWIs decline from the Po River mouths southward, along the Holocene mud wedge. North of the Po River Delta, the DFSWIs are weaker and confined to the areas in front of the major river mouths, due to poor Organic Matter (OM) inputs and strong reworking of bottom sediments. The area south of the Po Delta cusp is characterized by strong DFSWIs, due to the high deposition of solid inputs from the Po River, strong primary production, and protection from the Western Adriatic Current and Bora storms; in summer, higher temperatures, and calm hydrodynamic conditions generate near-bottom hypoxic to anoxic environments in this area. In Northern and Central Adriatic offshore areas, negative DIC and phosphate fluxes are due to poorly reactive OM reaching this area and to the mainly carbonate composition and oxic environment of local bottom sediments. DFSWI data analysis highlighted the important contribution of sediments to marine carbon and nutrient cycles. This is particularly important for carbon, which plays a major role in seawater acidification and global climate change. The study also provides average DFSWI data for each diagenetic area, which allow calculating the carbon and nutrient budgets in the Adriatic Sea. [ABSTRACT FROM AUTHOR]

Diagenesis is a double-edged sword of geochemical recordings. It makes us always doubt about the representativeness of many geochemical indicators, especially the isotope and mineral related. It also provides a window to explore the biogeochemical processes at the water–rock interface, which are related to the interactions between the hydrosphere, biosphere, and lithosphere. In this study, we identified microbial early diagenesis in lacustrine ostracods from the Songliao Basin 91.35 million years ago by using in situ mineralogical and carbon isotope analytical methods. Our results suggest multiple biological early-diagenesis processes and the formation of a ferric and methane transition zone (FMTZ) in the sulfate-poor pore water, which are conducive to the formation of dolomite and ankerite. These secondary carbonate minerals related to dissimilatory iron reduction and methanogenesis have heavier carbon isotopic compositions than the calcified ostracod shell in the water column and might bring interferences to the geochemical parameters of ostracods. [ABSTRACT FROM AUTHOR]

The phylum “Candidatus Patescibacteria” (or Candidate Phyla Radiation [CPR]) accounts for roughly one-quarter of microbial diversity on Earth, but the presence and diversity of these bacteria in marine sediments have been rarely charted. Here, we investigate the abundance, diversity, and metabolic capacities of CPR bacteria in three sediment sites (Mohns Ridge, North Pond, and Costa Rica Margin) with samples covering a wide range of redox zones formed during the early diagenesis of organic matter. Through metagenome sequencing, we found that all investigated sediment horizons contain “Ca. Patescibacteria” (0.4 to 28% of the total communities), which are affiliated with the classes “Ca. Paceibacteria,” “Ca. Gracilibacteria,” “Ca. Microgenomatia,” “Ca. Saccharimonadia,” “Ca. ABY1,” and “Ca. WWE3.” However, only a subset of the diversity of marine sediment “Ca. Patescibacteria,” especially the classes “Ca. Paceibacteria” and “Ca. Gracilibacteria,” can be captured by 16S rRNA gene amplicon sequencing with commonly used universal primers. We recovered 11 metagenome-assembled genomes (MAGs) of CPR from these sediments, most of which are novel at the family or genus level in the “Ca. Paceibacteria” class and are missed by the amplicon sequencing. While individual MAGs are confined to specific anoxic niches, the lack of capacities to utilize the prevailing terminal electron acceptors indicates that they may not be directly selected by the local redox conditions. These CPR bacteria lack essential biosynthesis pathways and may use a truncated glycolysis pathway to conserve energy as fermentative organotrophs. Our findings suggest that marine sediments harbor some novel yet widespread CPR bacteria during the early diagenesis of organic matter, which needs to be considered in population dynamics assessments in this vast environment. IMPORTANCE Ultrasmall-celled “Ca. Patescibacteria” have been estimated to account for one-quarter of the total microbial diversity on Earth, the parasitic lifestyle of which may exert a profound control on the overall microbial population size of the local ecosystems. However, their diversity and metabolic functions in marine sediments, one of the largest yet understudied ecosystems on Earth, remain virtually uncharacterized. By applying cultivation-independent approaches to a range of sediment redox zones, we reveal that “Ca. Patescibacteria” members are rare but widespread regardless of the prevailing geochemical conditions. These bacteria are affiliated with novel branches of “Ca. Patescibacteria” and have been largely missed in marker gene-based surveys. They do not have respiration capacity but may conserve energy by fermenting organic compounds from their episymbiotic hosts. Our findings suggest that these novel “Ca. Patescibacteria” are among the previously overlooked microbes in diverse marine sediments. [ABSTRACT FROM AUTHOR]

Diagenetic models help explain reservoir quality of siliciclastic strata, which are globally important exploration targets for groundwater, petroleum and carbon sequestration. Siliciclastics occur in salt provinces worldwide but remain poorly studied despite documented deviation from basic eodiagenetic models. A new variation is explained based on evidence from Cumberland Basin strata (Grande Anse Formation, Pennsylvanian, Canada). These strata were deposited in a fluvial setting under semi‐humid to humid climates and represent an unconformable cover sequence over a salt wall. Optical microscopy and scanning electron microscopy, both employing cathodoluminescence, reveal that widespread early diagenetic phases are mostly deleterious to reservoir quality. These pre‐compactional eodiagenetic phases are: (i) variable dissolution/alteration of feldspars, micas and carbonate/evaporite clasts; clay and grain staining iron oxides; and pore‐filling kaolinite; (ii) microcrystalline calcite; quartz overgrowths, prismatic quartz, quartzine chalcedony and blocky quartz, the latter two pseudomorphous after gypsum; and barite; (iii) framboidal pyrite and locally extensive Mg, Fe and Mn‐bearing blocky calcite. The first phase is attributed to a seasonally wetted vadose zone under relatively low pH and positive Eh conditions characteristic of humid climates. In contrast, the second phase is usually associated with evaporitic conditions and arid climates, where authigenesis occurs in the capillary fringe to just below the water table, and where porewaters are much more saline. The third phase is related to authigenesis from shallow burial, anoxic, phreatic porewaters. How is the aforementioned humid/arid dichotomy deciphered? Important may be the distinction between vadose and circum‐water‐table phases. Beneath vadose deposits of the humid climate, the proximity of evaporite rock in the salt wall allows for its localized dissolution, providing abundant Ca, CO3 and SO4 ions to produce anomalously saline, shallow groundwater. Barite and transient gypsum precipitated, the latter subsequently replaced by chalcedony and pseudomorphous silica during sporadic events where flushing of meteoric water temporarily lowered pH. [ABSTRACT FROM AUTHOR]

Methods of mathematical statistics were used in this article to study the processes of sedimentogenesis and early diagenesis of surface sediments in the Kara Sea. Correlation analysis established two groups of oxides associated with certain particle-size types of deposits. The one-way analysis of the variance revealed the regularity of the sand, silt, and pelite distribution in accordance with sedimentation conditions by differences in their mean contents. These data were used to compile maps of the sand, silt, and pelite distribution within the studied area of the Kara Sea. Some features of the main factors of sedimentogenesis in this region were revealed on the basis of statistical data. The relationship between the Mn/Al, Mn/Fe, and MnO/SiO2 ratios and depth established by the univariate regression analysis enabled us to study the intensity of early oxidative diagenesis within the shelf zone of the Kara Sea. [ABSTRACT FROM AUTHOR]

• Porewater Ni isotope composition (δ60Ni) can be heavier than deep seawater. • Dissolution of detrital materials in marine sediments may supply dNi to pore fluids. • In marine sediments, porewater Ni isotopes are fractionated by active Mn cycling. • Continental margin sediments can supply dissolved Ni with heavy δ60Ni to the ocean. • Heavy benthic δ60Ni may help to resolve the global ocean mass balance of Ni. Nickel (Ni) is a bio-essential element for phytoplankton in the modern oceans, and yet, the global ocean mass balance of Ni has puzzled scientists for decades. Many estimates of total Ni output flux are larger than the total Ni input flux to the ocean. The measurement of Ni stable isotopes (δ60Ni) in earth materials may inform our understanding of ocean biogeochemistry and redox conditions in both the modern ocean and the geological past. An ocean Ni stable isotope imbalance has also concerned scientists, with a global ocean δ60Ni of +1.4 ‰ which is notably heavier than the major source of Ni to the oceans from rivers. Recent efforts to resolve Ni and δ60Ni imbalances have focused on the role which manganese (Mn) oxides play in marine Ni cycling, both in the water column and in marine sediments. Manganese oxide rich marine sediments can supply isotopically heavy dissolved Ni to porewater fluids and seawater, but the source of the isotopically heavy porewater Ni remains unclear. Here we present porewater trace metal concentrations and δ60Ni from two sites from the continental margin off southern California. Porewater Ni concentrations are up to roughly 100-fold higher than deep seawater concentrations (∼1 μM compared to 10 nM, respectively). Porewater δ60Ni is near 0 ‰ in the subsurface region where Ni concentrations are highest, suggesting dissolution of lithogenic material from sediments. Porewater δ60Ni increases dramatically towards the sediment-water interface with values of up to +2.66 ‰, which to our knowledge are the heaviest Ni isotope compositions ever reported for natural materials. Measurements of porewater and sediment Ni and Mn concentrations suggest that Ni is released to porewaters in the Mn-reducing zone, and then removed by newly precipitated Mn oxides as Mn and Ni move towards the oxygenated zone of sediments. A simple Rayleigh model suggests a Ni isotope fractionation of factor of -0.61 ‰ for Ni sorption onto Mn oxides, while a diffusion-reaction model suggests a Ni isotope fractionation of -1.80 to -0.96 ‰, always with preferential adsorption of lighter Ni isotopes. This flux of Ni toward the sediment-water interface, coupled with preferential removal of lighter Ni isotopes in marine sediments, provides evidence supporting an isotopically heavy benthic source of new Ni to the oceans, which we estimate has a magnitude of 0.65 × 108 to 1.66 × 108 moles/year. We find that a benthic flux such as measured here over just 0.27-2.70 % of the ocean seafloor could mix with the river δ60Ni of +0.8 ‰ to achieve the observed deep ocean δ60Ni of +1.4 ‰. This study reveals the similarities in how rivers and continental margin sediments supply heavy Ni isotopes to the ocean. In both continental rivers and margin sediments, terrigenous Ni is released with an δ60Ni near 0.1 ‰, but lighter isotopes are captured by precipitation onto oxides so that the dissolved flux to the ocean is isotopically heavier than the terrigenous material from which Ni was released, and in the case of margin sediments may be heavier than even bulk seawater δ60Ni. We thus recognize continental margin sediments with active Mn cycling as a 'new' source of isotopically heavy dissolved Ni to the ocean. [ABSTRACT FROM AUTHOR]

Sedimentary nitrogen isotopic ratios are used as a proxy for ancient biogeochemical cycles on Earth's surface. It is generally accepted that sediment hole tops record primary signatures because organic nitrogen (ON) is predominant in this part of the hole. In contrast to such early to middle diagenetic stages, it is well known that heavier nitrogen isotope 15N tends to enrich in sedimentary rocks during later diagenetic and metamorphic stages. However, there are some critical gaps in our understanding of nitrogen isotopic alteration associated with abiotic processes during early-middle diagenesis. In this study, we examined the isotope ratios of ammonium nitrogen in interstitial water (IW) and total nitrogen (TN), including exchangeable ammonium and mineral nitrogen, in the solid-phase of organic-rich-sediment recovered by International Ocean Discovery Program (IODP) Expedition 385 cores drilled in the Guaymas Basin, Gulf of California, that contained ammonium-rich IW. The isotopic ratios (δ15N value) of TN are the most variable with depth compared to any other type of nitrogen. This variation can be interpreted as reflecting changes in the water mass environment in the basin caused by glacial–interglacial climate changes, modifying the δ15N values of the marine primary producers. Thus, the δ15N value of TN is a proxy for environmental change in the basin, while each component of TN shows different trends. The δ15N values of IW and exchangeable ammonium did not exhibit significant changes with depth, but the latter values are about 3 ‰ enriched in 15N. This may be due to advective transport of solute into adjacent layers followed by the formation of an isotopic equilibrium between IW and exchangeable ammonium in the case of fast sediment accumulation rate. The δ15N value of exchangeable ammonium is the highest among the other types of nitrogen with one exception, where the δ15N value of TN is the highest. The calculated δ15N values of ON based on mass balance are almost the same as those of associated TN in the shallow sediment layers (< 150 m below seafloor), but the difference in the δ15N values of TN and ON are significant in the deeper layers, where proportions of ON contents are <50%. In particular, in the layer where the δ15N value of TN is the highest, that of ON shows an even higher value and the difference reaches 3.5 ‰. The δ15N values of mineral nitrogen are similar to that of IW ammonium except the surface layers. Under such conditions, when δ15N value of TN is intermediate between those of mineral nitrogen and exchangeable ammonium, calculated δ15N value of ON is close to that of TN. On the other hand, if δ15N value of TN is out of the range between mineral nitrogen and exchangeable ammonium, it causes further difference in δ15N value of ON. It means that the fluctuation of δ15N values of TN is reduced relative to those of ON through depth. It has been considered that δ15N value of TN in sediment is similar to that of ON, and changes in the δ15N value of TN due to diagenesis are limited, but in such environment ON fluctuations over depth may be slightly underestimated. • IODP Ex.385 core sediments drilled in Guaymas Basin were used for geochemical study. • Climate change significantly fluctuate in δ15N values of total nitrogen in sediments. • High accumulation rate of sediment homogenizes the δ15N values of NH 4 + in interstitial water through depth. • Downhole profiles for δ15N of exchangeable and interstitial water NH 4 + are parallel and the former is enriched in 15N. • Under the above condition, fluctuation of δ15N values of TN is reduced relative to those of ON through depth. [ABSTRACT FROM AUTHOR]

Marine calcium cycling is closely linked with carbon cycling in the ocean, in which authigenic carbonates precipitated in sediments play a non-negligible role. However, calcium cycling during authigenic carbonate precipitation in organic-rich, shaly sediments in geological history remains underexplored. This study focuses on carbonate concretions (aggregates of authigenic carbonates) in the lower Cambrian Niutitang Formation, South China, to provide insights into calcium cycling during their growth. Sedimentological and mineralogical observations suggest that these concretions were formed through concentric growth by authigenic calcite and pyrite precipitation during the early diagenetic stage. Geochemical analyses reveal internal variations in "M-shaped" δ13C carb trends (from −11.9 ‰ to −4.4 ‰) and diverse δ34S pyr trends (from 4.7 ‰ to 14.0 ‰) along core-to-rim transects. These findings suggest formation through microbial sulfate reduction by organic matter in a shallow depth beneath the sediment–water interface. In contrast to the dynamic δ13C carb and δ34S pyr variations and multi-stage concentric growth, these carbonate concretions display nearly uniform δ44/40Ca carb values (from 0.80 ‰ to 1.03 ‰, average 0.96 ± 0.06 ‰, 1SD) and consistent internal trends, which are further attributed to strongly seawater-buffered porewater calcium geochemistry and small calcium isotope fractionation due to calcite precipitation at slow rates. This study confirms that early diagenetic carbonate concretions in the lower Cambrian Niutitang Formation are characterized by much heavier δ44/40Ca values compared to coeval shallow platform carbonates. In light of abundant authigenic carbonates observed in the lower Cambrian successions, their roles in calcium isotope mass balance in the early Cambrian ocean warrant further investigation in the future. Therefore, early diagenetic carbonate concretions in black shales could provide valuable insights into porewater and seawater calcium isotope signals, as well as early diagenetic and marine calcium cycling in geological history. • Early diagenetic carbonate concretions were formed in the lower Cambrian black shales. • The concretions display dynamic C–S isotopic variations and uniform δ44/40Ca values. • Seawater-buffered porewater and calcite precipitation rates control δ44/40Ca values. • Shale-hosted carbonate concretions provide insights into marine calcium cycling. [ABSTRACT FROM AUTHOR]

In the marine environment, nickel distributions are linked to the biogeochemical cycling of both organic matter and manganese. Thus, Ni and its isotopes have the potential to be used to reconstruct changes of bioproductivity and oxygenation in paleoenvironments. However, their utility relies on an understanding of the behaviour of Ni in modern marine environments. Here we investigate the distribution of Ni under a range of oxidation-reduction conditions, with organic carbon (C org) burial rates that range from approximately 0.08 to 8.4 mmol m−2 d−1, none of the sites show deep (> 5 cm) penetration of dissolved oxygen into the sediment. We present Ni concentrations for sediments and pore fluids, as well as Ni isotope compositions from pore fluids, from the California and Mexico continental margins. The sites are sufficiently reducing that solid-phase Mn concentrations are typically <1000 ppm, but two of the stations, where oxygenated bottom water bathes the underlying sediment, have near-surface sediment Mn concentrations that reach up to 2.3%. Dissolved Ni is lower in pore fluids for stations with high solid phase C org compared to those stations with oxygenated bottom water and high solid phase Mn concentrations. The calculated benthic fluxes of Ni at all stations are small relative to their burial rates, which implies a high Ni burial efficiency (> 85%). Pore fluid δ60Ni values range from approximately −0.39 to +2.36 ‰, with the higher δ60Ni values occurring at the C org -rich station, and the lower values at the Mn-rich stations. At the station with the highest C org content, the distribution of solid-phase Ni as a function of C org content is consistent with the strong association of Ni with C org seen at open-ocean upwelling margins globally. In contrast, the other stations investigated here clearly do not show such an association. Our data offer support for the notion that Ni accumulation within sediments is linked to organic matter accumulation in regions of high photic zone productivity and where the sediments are reducing. However, at the sites in our study where Mn oxidation-reduction reactions occur near the sediment-water boundary, any simple relationship between Ni and C org burial is obfuscated. With a sufficiently deep oxygen penetration depth and the formation of a solid-phase Mn oxide layer, Ni burial within the sediment can be highly efficient. Importantly, Ni is well preserved in sediments deposited under the full range of conditions studied here. This observation of high Ni preservation is an important constraint if Ni is to be used as a proxy to reconstruct paleoenvironmental conditions. • Ni accumulation in organic-rich, reducing sediments is driven by organic matter. • Ni is well-preserved in sediments under oxidising and highly reducing conditions. • Intermediate redox conditions, with Mn and organic matter burial, are more complex. • These situations drive small fluxes back to the water column. [ABSTRACT FROM AUTHOR]

• Comparable metal isotope records in carbonates from the Bahamas and South China Sea. • Carbonate diagenetic extents and regimes are distinct for specific isotope systems. • Fluid chemistry and depositional conditions affect compositions of carbonate rocks. • Carbonate early diagenesis can be temporally and spatially correlated. Early marine and meteoric diagenesis is commonly proposed to alter the primary geochemical signals of shallow-marine carbonates, hindering their use as a reliable archive for ancient seawater chemistry. However, there are still uncertainties on the extent and controlling factors of early diagenetic alteration for shallow-marine carbonates. In this study, we compile and compare radiogenic Sr, Li, and U isotope data of carbonate island reefs and platforms from the South China Sea and the Bahamas to better constrain the early diagenetic effects on geochemical signals in carbonates. Multiple diagenetic zones are identified in carbonate drillcores from the South China Sea, and temporally correlated with similar zones in the Bahamas. The radiogenic Sr, Li and U isotope records from shallow-marine carbonates in these two geographically distant regions are affected by multiple factors, including compositions of diagenetic fluids, local depositional environments, fluid advection/diffusion rates, and carbonate recrystallization rates. These factors ultimately determined the diagenetic extents and regimes (fluid-buffered or sediment-buffered) of shallow-marine carbonates, which vary across different isotopic systems. A full understanding of early diagenetic history and its effects on different isotopic proxies will validate the use of shallow-marine carbonate as an archive for ancient seawater information, and contribute to tracking the temporal changes in paleo-climate and -environment. [ABSTRACT FROM AUTHOR]

• First application of Rock-Eval® 7S to recent lacustrine sediments. • Lake Dziani Dzaha is a modern analog for Type I-S petroleum source rock depositional environment. • Increase of hydrogen index linked to organic-sulfur enrichment. • Pyrolysis thermal stability of sulfurized organic matter increases with depth. • Pyrolysis organic sulfur is released either with CO 2 or HC. The Dziani Dzaha (Mayotte Islands, Indian Ocean) is a small, shallow, saline and hyperalkaline maar lake. Its surface waters are characterized by intense primary production, inducing the waters below 2 m depth to remain aphotic and anoxic all year round, at least until 2017, when a ∼5 m-long core was taken from the center of the lake. The recovered sediments were described and sampled at high resolution. They consist of laminated microbial mats, mixed with carbonate lenses or nodular beds, and rare silty detrital facies. The inorganic and organic carbon contents of 160 samples were analyzed using the Rock-Eval® method, including the Rock-Eval® 7S device which quantifies both total organic sulfur and total sulfur content, in addition to conventional Rock-Eval parameters. The Dziani Dzaha sediments are characterized by high TOC content (8.5 wt% on average and up to 27.9 wt%) and variable inorganic carbon content. HI values average 630 mg HC/g TOC and reach up to 834 mg HC/g TOC. TS content varies from 0.3 to 3.6 wt%, with TSorg/TOC ratios close to 0.01 at the top of the core and fluctuating downcore between 0.02 and 0.05. Interestingly, the pyrolysis thermal stability of sulfurized organic matter increases with depth, and the highest HI values are associated with highest Sorg content, sulfurization of organic matter being generally accompanied by reductive processes. The hydrogen- and organic-sulfur-rich sediments of Dziani Dzaha can be considered modern analogues of Type I and I-S petroleum source rock deposits, such as the Eocene Green River shales and Kimmeridgian Orbagnoux laminites, with remarkable facies and geochemical similarities. (258 words) [ABSTRACT FROM AUTHOR]

Barium (Ba) isotopes are a promising new tracer for riverine freshwater input to the ocean and marine biogeochemical cycling. However, many processes that affect Ba cycling at continental margins have not yet been investigated with respect to Ba isotope fractionation. Here, we present a comprehensive data set of Ba concentration and isotope data for water column, pore water and sediment samples from Kiel Bight, a seasonally stratified and hypoxic fjord in the southwestern Baltic Sea. The surface water Ba concentration and Ba isotope inventory of the water column can generally be explained by mixing of riverine freshwater and Atlantic seawater. However, the deep-water below the seasonal pycnocline (10 - 15 m water depth) is characterized by a pronounced positive Ba concentration anomaly (up to 915 nM) that is accompanied by a δ138Ba of ~+0.25 ‰, which is lighter than expected from the seawater-freshwater mixing line (Ba: 77 nM, δ138Ba: +0.32 ‰ at a salinity of 18). Pore water profiles indicate a Ba flux across the sediment-water interface, which contributes to the enrichment in isotopically light Ba in the deep-water. Pore waters of surface sediments and deep-waters are oversaturated with respect to barite. Therefore, barite dissolution is unlikely to account for the benthic Ba flux. Water column Ba concentrations closely correlate with those of the nutrients phosphate and silica, which are removed from surface waters by biological processes and recycled from the sediment by diffusion across the sediment-water interface. As nutrient-to-Ba ratios differ among sites and from those observed in open-marine systems, we propose that Ba is removed from surface waters by adsorption onto biogenic particles (rather than assimilation) and regenerated within surface sediments upon organic matter degradation. Pore water data for subsurface sediments in Kiel Bight indicate preferential transfer of isotopically heavy Ba into an authigenic phase during early diagenesis. Quantifying the burial flux associated with this authigenic Ba phase along continental margins could potentially help to settle the isotopic imbalance between known Ba source and sink fluxes in the ocean.

Glacier meltwater supplies silicon (Si) and iron (Fe) sourced from weathered bedrock to downstream ecosystems. However, the extent to which these nutrients reach the ocean is regulated by the nature of the benthic cycling of dissolved Si and Fe within fjord systems, given the rapid deposition of reactive particulate fractions at fjord heads. Here, we examine the benthic cycling of the two nutrients at four Patagonian fjord heads through geochemical analyses of sediment pore waters, including Si and Fe isotopes (δ30Si and δ56Fe), and reaction‐transport modeling for Si. A high diffusive flux of dissolved Fe from the fjord sediments (up to 0.02 mmol m−2 day−1) compared to open ocean sediments (typically <0.001 mmol m−2 day−1) is supported by both reductive and non‐reductive dissolution of glacially‐sourced reactive Fe phases, as reflected by the range of pore water δ56Fe (−2.7 to +0.8‰). In contrast, the diffusive flux of dissolved Si from the fjord sediments (0.02–0.05 mmol m−2 day−1) is relatively low (typical ocean values are >0.1 mmol m−2 day−1). High pore water δ30Si (up to +3.3‰) observed near the Fe(II)‐Fe(III) redox boundary is likely associated with the removal of dissolved Si by Fe(III) mineral phases, which, together with high sedimentation rates, contribute to the low diffusive flux of Si at the sampled sites. Our results suggest that early diagenesis promotes the release of dissolved Fe, yet suppresses the release of dissolved Si at glaciated fjord heads, which has significant implications for understanding the downstream transport of these nutrients along fjord systems. Plain Language Summary: Through physical grinding and chemical reactions, glaciers are capable of releasing nutrients such as silicon and iron from the underlying rocks. In particular, there is a growing interest in the supply of these glacier‐sourced nutrients to sustain the growth of marine life in high‐latitude estuaries (fjords) and the adjacent coastal ocean. However, a substantial portion of the glacier‐sourced nutrients may be removed by rapid settling of debris (sediments) at the heads of these fjords, before they can be used by marine life. This study analyzes the chemistry of fluids within sediments collected from Patagonian fjord near river and glacial inputs to investigate the fate of glacier‐sourced nutrients (silicon and iron) that have been transferred to the fjord floor. Our laboratory analysis and calculations reveal that biological, chemical and geological processes in these sediments release significant amounts of iron back into the overlying fjord waters, but these high levels of iron chemically bind silicon, restricting its release into fjord waters. The disproportionate release of the two nutrients from fjord sediments found in this study advances our understanding of the modes of supply of glacier‐sourced nutrients to fjords and the coastal ocean. [ABSTRACT FROM AUTHOR]

The speciation of iron in sediments and sedimentary rocks is a widely used proxy for the chemistry and oxidation state of ancient water bodies. Specifically, the fraction of reactive iron out of the total iron (Fe HR /Fe T) and the fraction of pyrite iron out of the reactive iron pool (Fe PYR /Fe HR) are thought to constrain the oxidation state and the presence of sulfide in the water column, respectively. This approach was developed and tested against modern core-top sediments, but application to sedimentary rocks requires consideration of the effects of diagenesis and lithification on iron speciation. Furthermore, the effects of deep burial, metamorphism, and late-stage alteration during exhumation or sampling (e.g., oxidative weathering) have not been systematically explored. To bridge this gap, we combined new data from four sediment cores (n = 54) with an extensive literature compilation of modern sediments (2936 measurements from 316 cores) and ancient sedimentary rocks (12,173 measurements spanning the Neoarchean to Quaternary). The modern data include both surface and buried sediments, allowing an investigation of the effects of diagenesis on iron speciation. Depending on the thresholds used to distinguish oxic from anoxic environments and ferruginous from euxinic environments, interpretation of the modern sedimentary iron speciation data within the existing framework yields incorrect environmental classifications up to ≈70% of the time. In modern sediments, diagenesis is the main reason that iron speciation does not represent the chemistry and oxidation state of the water column. We find that iron speciation correlates with porewater chemistry and that it changes with progressive burial along three distinctive Fe HR /Fe T –Fe PYR /Fe HR arrays, each of which represents a different set of diagenetic processes. We suggest that similarly to modern sediments, stratigraphic variation in iron speciation in sedimentary rocks primarily reflects progressive burial diagenesis or variation in depositional conditions rather than temporal variation in water-column chemistry and oxidation state. Indeed, analysis of the geologic iron speciation data reveals no statistically significant trends in either Fe HR /Fe T or Fe PYR /Fe HR from the Archean to the present day. The diagenetic Fe HR /Fe T –Fe PYR /Fe HR arrays that we identified in modern marine sediments suggest that under certain conditions, iron speciation analyses may be used to constrain Fe HR /Fe T in the local sediment source(s). Hence, we suggest that iron speciation data, together with complementary petrographic, mineralogical and geochemical constraints, may be used to constrain the local iron source(s) and early and late diagenetic processes, but rarely the chemistry or oxidation state of ancient water columns. [ABSTRACT FROM AUTHOR]

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