Early Diagenesis Research Articles

Insights Into the Terminal Ediacaran Marine Carbonate Record From Shale-Hosted Carbonate Carbon Isotopes

The marine carbon isotope record (δ13C) used for chemostratigraphy and reconstruction of carbon cycle dynamics is commonly assembled using carbonate rocks. There is, however, evidence that carbonate cements hosted within fine-grained clastics (shales and mudstones) in some settings may also express δ13C trends that covary with the record from carbonates. We present new carbon and oxygen isotopic data from shale-hosted carbonate cements (herein termed δ13Ccarb-sh and δ18Ocarb-sh, n = 107, <16 wt% CaCO3) of the terminal Ediacaran Nama Group, Namibia (≥550.5 to <539.6 Million years ago; Ma). These data are compared with the published carbon and oxygen isotopic record from coeval carbonates (δ13Ccarb and δ18Ocarb, n = 1611) and total organic carbon (TOC) concentrations. We show that, in the Nama Group, δ13Ccarb-sh compositions in samples of intermediate to high CaCO3/TOC (>0.4) can approximate contemporaneous δ13Ccarb in open marine mixed carbonate-clastic settings. By contrast, δ13Ccarb-sh values in samples with low CaCO3/TOC (<0.4) that were deposited in clastic settings distant from the locus of carbonate deposition are more negative than contemporaneous δ13Ccarb. These data suggest that δ13Ccarb-sh may approach seawater composition in samples with low TOC when deposited in settings characterized by high CO32- concentration, where carbonate can rapidly precipitate from seawater during early diagenesis. However, the use of δ13Ccarb-sh to infill gaps in the existing δ13Ccarb record remains uncertain, even when these criteria are fulfilled. Intervals of δ13C-δ18O co-variability in the Nama Group succession appear to correlate with units where seawater mixing with meteoric fluids was more likely during early diagenesis, such as clastic-dominated settings, which also show significant decreasing δ18O through time with gradual sub-basin infill. We further consider uncertainties in lithostratigraphic correlation of the upper Urusis Formation of the Nama Group that enable three new possible correlations to be proposed for δ13Ccarb-sh data within the terminal Ediacaran to lower Cambrian (<542.65 Ma to >532 Ma) regional and global δ13Ccarb records.

Fe-(oxyhydr)oxide participation in REE enrichment in early Cambrian phosphorites from South China: Evidence from in-situ geochemical analysis

Marine phosphorites have shown extraordinary REE potential. Enrichment mechanisms have been investigated as yet, however, the roles of Fe/Mn (oxyhydr)oxides in REE enrichment were poorly known. In this paper, we report new data for early Cambrian phosphorites in South China with a particular focus on REE enrichment and Fe-redox cycling. Phosphorites from the Cambrian Gezhongwu occur in the upper and lower units. The phosphorites contain a large number of of Fe (oxyhydr)oxides, especially those in the upper unit. SEM and laser Raman show that Fe (oxyhydr)oxides have a gray rim and a bright core, attributed to transformation from hematite to goethite during the weathering. In-situ elemental analyses show Fe (oxyhydr)oxides have PAAS-normalized REE patterns depleted in LREE and enriched in HREE with negative to positive Eu anomalies and positive Y anomalies. However, two patterns are clearly distinguished according to Ce anomalies and these are related to the origins of Fe-(oxyhydr)oxides: hydrothermal Fe (oxyhydr)oxides with negative Ce anomalies and hydrogenic ones with positive Ce anomalies. The role of Fe (oxyhydr)oxides played in REE enrichment was controlled by Fe-redox pumping. In this process, Fe (oxyhydr)oxides adsorbed REEs in oxic water columns; after precipitating to suboxic/anoxic conditions, the absorbed REEs were released into porewater. Francolites subsequently formed and captured REEs during early diagenesis. Even though additional evidence is required, our investigations were significant in conforming the origins of Fe/Mn (oxyhydr)oxides and their role in REE enrichment in marine phosphorites.

Differences in and factors controlling organic matter enrichment in the Ziliujing Formation shale in the Sichuan Basin

Lacustrine shale oil and gas are important fields for unconventional exploration and development in China, and organic-rich shale deposition lays down the critical foundation for hydrocarbon generation. There are two sets of shale, the Dongyuemiao and Da’anzhai Members, in the Ziliujing Formation in the Sichuan Basin. To identify the differential enrichment characteristics of organic matter and clarify its controlling factors, geochemical analyses of organic and inorganic geochemical analyses were performed. The results showed that the total organic carbon content of the Dongyuemiao shale (1.36%) is slightly higher than that of the Da’anzhai shale (0.95%). The enrichment of organic matter in the two shales resulted from the comprehensive controls of paleoproductivity, paleoenvironment, and terrigenous input, but different factors have different effects. In addition, driven by climate, the change in the sulfate concentration in the bottom water further led to the different intensities of bacterial sulfate reduction in early diagenesis. This made a great difference regarding organic matter accumulation in the two members. In general, climate may have played a dominant role in organic matter enrichment in the two sets of shale.

Postdepositional Behavior of Molybdenum in Deep Sediments and Implications for Paleoredox Reconstruction

AbstractMolybdenum (Mo) is a trace element sensitive to oceanic redox conditions. The fidelity of sedimentary Mo as a paleoredox proxy of coeval seawater depends on the extent of Mo remobilization during postdepositional processes. Here we present the Mo content and isotope profiles for deep sediments from the Nankai Trough, Japan. The Mo signature suggests that these sediments have experienced extensive early diagenesis and hydrothermal alteration at depth. Iron (Fe)‐manganese (Mn) (oxyhydr)oxide alteration combined with Mo thiolation leads to a more than twenty‐fold enrichment of Mo within the sulfate reduction zone. Hydrothermal fluids and Mo adsorption onto Fe‐Mn (oxyhydr)oxides cause extremely negative Mo‐isotope values at the underthrust zone. These postdepositional Mo signals might be misinterpreted as expanded anoxia in the water column. Our findings highlight the importance of constraining postdepositional effects on Mo‐based proxies during paleoredox reconstruction.

Coring tools have an effect on lithification and physical properties of marine carbonate sediments

Abstract. The International Ocean Discovery Program (IODP) JOIDES Resolution Science Operator typically uses an advanced piston corer (APC) in soft ooze and sediments and an extended core barrel (XCB) in firm sediments. The coring tool exchange typically occurs around the same depth in adjacent holes of the same site. However, during IODP Expedition 356, the coring tool switch occurred at different depths: IODP Sites U1463 and U1464 are marked by a stratigraphic interval (> 25 m thick) that was XCB cored in one hole and APC cored in other holes. Shipboard scientists remarked that APC-cored sediments were unlithified or partially lithified, while XCB-cored sediments were fully lithified. This difference in sedimentological description of the same formation seems to be an effect of coring technique. To provide further insight, we assessed the physical properties (bulk density, porosity, and P-wave velocity), downhole wireline logging data, scanning electron microscope (SEM) images, and micro-computed tomography (µCT) scans of those intervals. We find systematic differences between the different coring techniques. XCB cores are characterized by systematically lower bulk density, higher porosity, and higher P-wave velocity than APC cores. Downhole logging data suggest that the original P-wave velocity of the formation is better preserved in XCB cores, despite the typical “biscuit-and-gravy” core disturbance (i.e. well-preserved core fragments surrounded by squelched core material). In conjunction with SEM and µCT images, we conclude that the APC tool destroyed early lithification by breaking cements between individual grains. Moreover, µCT images reveal denser packing and smaller pore volumes in the APC cores. These sedimentary changes likely occur when the APC pressure wave passes through the sediment. The destruction of grain-to-grain cements provides an explanation for the significantly lower P-wave velocities in APC cores. Interestingly, the gravy sections in XCB drilled cores mimic the destruction of early lithification and reduction of pore volume. We conclude that APC remains the tool of choice for recovering soft sediments, especially for paleoclimate purposes. However, for the study of lithification, XCB biscuits provide a more representative image of the formation. For the study of early diagenesis, further studies are required to ascertain the preservation of key sedimentary features using existing and new drilling tools.

Rock physics characteristics and their control factors of carbonate in different sedimentary microfacies of the Yingshan Formation, Gucheng Area, Tarim Basin

Abstract Understanding the influence of geological characteristics on rock physics properties is crucial for accurately recognizing the relationship between rock physics variation and reservoir characteristics. Unlike the conventional rock species, the rock physics properties of the deep carbonate rocks in the third member of Yingshan Formation (Ying-III Member) in Gucheng area, Tarim Basin are relatively more complex. To address this problem, we investigated the rock physics characteristics and controlling factors of different sedimentary microfacies samples, combined with sedimentological analysis and rock physics experiments. The results show that the sedimentary environment affects the lithology and pore structure by controlling the properties of the primitive rock and early diagenesis. Dolomitized shoal microfacies and shoal top dolomitic flat microfacies primarily form crystalline dolomite and siliceous dolomite, with pores consisting of inter-crystalline pores, dissolution pores, and cracks. Inter-shoal dolomitic flat microfacies develops silty dolomite, with only a few inter-crystalline pores and cracks. Middle- to high-energy shoal microfacies and inter-shoal sea microfacies develop tight calcarenite and micritic limestone. Samples with similar mineral composition have relatively consistent density values and acoustic properties. Soft pores, such as micro-cracks, have a significant impact on the effective pressure and acoustic wave velocity, velocity and velocity ratio, and velocity and porosity relationships. The research shows a new approach for the rock physics characteristics of deep carbonate reservoirs under geological background constraints, as well as the rock physics basis for seismic prediction of the Ying-III Member reservoir.

The Impact of Phosphorus Level on Vivianite Precipitation during Microbial Reduction of Ferrihydrite

Iron minerals play a pervasive role in the cycling of phosphorus (P) within both terrestrial and aquatic environments. The behavior of P, especially in oxygen-depleted environments, is frequently regulated by changing redox conditions and the associated phase transformations of Fe (III) (hydr)oxides (Borch and Fendorf 2007). Although the stability of Fe (III) hydroxides under changing redox conditions is well established, the relationship between specific minerals and their influence on the mechanisms of P retention and release remain unclear. In particular, the minerals vivianite (Fe3(PO4)2·8H2O) and ferrihydrite (Fe2O3·0.5H2O) are of interest. Vivianite crystallization in sediments has attracted increasing attention as a major contributor to P retention during early diagenesis (Slomp et al. 2013) and ferrihydrite reduction has been linked to fluctuating P concentrations which can impact vivianite crystallization. Additionally, there exists strong biological controls on these minerals as the reduction of Fe (III) oxides by dissimilatory iron-reducing bacteria may result in the formation of a suite of Fe (II)-bearing secondary minerals (O’Loughlin et al. 2021). To better understand the biogeochemical mechanisms behind these interactions, we examined the effects of fluctuating P concentrations on the reduction of ferrihydrite by Shewanella putrefaciens CN32 and resulting vivianite formation. In this study, bio-reduction experiments were conducted under sterile conditions in serum bottles containing 80 mL of mineral medium, with 80 mM Fe (III) in the form of Ferrihydrite, and various P concentrations (0, 1 mM, and 10 mM). The bottles were placed on a roller drum (180 rpm) and incubated at 30 oC in the dark for 15 days. Dissolved Fe (II), P concentrations, pH, and optical density (OD) values throughout the experiments were also measured. Our results showed that during incubation, Shewanella putrefaciens CN32 accelerated Ferrihydrite reduction as the dissolved iron (Fe2+) concentration increased significantly when compared to other bacterial and ferrihydrite treatments, as well as the control treatment. Additionally, P in both the 1mM and 10mM treatments was depleted after 5 and 7 days, respectively, and resulted in crystalline precipitates. Scanning electron microscopy (SEM) analyses of precipitates showed well-formed and highly crystalline vivianite particles of up to 30 µm in length in high P level. Some crystalline precipitates were confirmed as vivianite through SEM and Raman spectroscopy matching a known vivianite reference (Taylor et al. 2008), but only within the 10mM P incubations. After 15 days of incubation the morphology of the vivianite changed from aggregates of lath-shaped crystals to acicular crystals (Fig. 1). Based on our results, fluctuating P concentrations do indeed have a pronounced effect on ferrihydrite reduction and thus vivianite formation.

Extremely 13C-enriched dolomite records interval of strong methanogenesis following a sulfate decline in the Miocene Ries impact crater lake

In impact crater lakes, the lacustrine sedimentary records may not solely reflect climatic changes but also potential erosional effects from lithologically distinct impactite formations. The hydrochemical and biogeochemical processes during the deposition of the Nördlinger Ries impact crater lake, which fall in the range of the mid-Miocene Climate Transition, were studied by analysing microcrystalline authigenic carbonates in a drill core succession, using stable oxygen and carbon isotopes in conjunction with biomarkers. These investigations revealed an early sulfidic interval characterized by thiophenes, iso- and anteiso-C15:0 acids derived from sulfate reducing bacteria, and dolomites with low to intermediate δ13Ccarb values. The subsequent distinctive interval is characterized by extremely 13C-enriched dolomite (δ13Ccarb up to +20.93 ± 0.05‰ V-PDB), decline of iso- and anteiso-C15:0 acids and is rich in an Archaea-derived archaeol that is 13C-enriched (−14.7‰), indicating extensive methanogenesis in sulfate-depleted lake porewater during early diagenesis. The sulfate decline results from successive sulfate reduction when replenishment by sulfate-bearing inflow water is limited. The carbonates exhibit enriched 18O due to pronounced evaporation in a long-resided water body and enriched 13C by methanogenesis. A change in provenance of water derived from the sulfur-rich suevite (impact melt-bearing breccia) and crystalline source rocks to the sulfur-poor Bunte Breccia (continuous ejecta blanket) is required. Intermittently high Si/Al and Zr/Al at the high δ13C interval suggests sporadic short-term runoff increase, leading to fluctuating physiochemical lake conditions. A subsequent decline in both δ13Ccarb and archaeol indicates a decreasing lake level with intermittent subaerial exposure events, supported by bioturbation and mud cracks. The concomitant lake oxygenation is well supported by increasing Pr/Ph ratios and lipids derived from aerobic methanotrophs (13C-depleted 3-methyl-hopanoids). In the youngest unit, allochthonous lignites and biomarkers from lacustrine/soil sources appear, high total sulphur contents and thiophenes recur, and stable C and O isotope values decrease again. These observations suggest another major provenance change of the inflowing solutes, with increasing influx from weathered pyrite-bearing Jurassic claystones. These findings demonstrate that the climatic record expected from the stable carbon and oxygen isotopes of the Ries carbonates is strongly overprinted by hydrochemical and biogeochemical processes due to changing ion influx from substrate rocks, along the course of the successive ejecta erosion and catchment changes. Such an intrinsic control of lacustrine biogeochemical processes is also expected for other hydrologically closed impact crater lake basins, where catchment rocks with distinctively different lithologies are present.

Bottom-trawling signals lost in sediment: A combined biogeochemical and modeling approach to early diagenesis in a perturbed coastal area of the southern Baltic Sea

Trawl-fishing is broadly considered to be one of the most destructive anthropogenic activities toward benthic ecosystems. In this study, we examine the effects of bottom-contact fishing by otter trawls on the geochemistry and macrofauna in sandy silt sediment in an area of the Baltic Sea where clear spatial patterns in trawling activity were previously identified by acoustic mapping. We calibrated an early diagenetic model to biogeochemical data from various coring locations. Fitting measured mercury profiles allowed for the determination of the sediment mixing and burial velocity. For all sites, independent of the trawl mark density, good fits were obtained by applying the model with the same organic matter loading and parameter values, while iron fluxes scaled linearly with the burial velocity. A sensitivity analysis revealed that the fitted sulfate reduction rate, solid sulfur contents, ammonium concentration, and both the isotopic composition and concentration of dissolved inorganic carbon provided reliable constraints for the total mineralization rate, which exhibited a narrow range of variability (around ±20 % from the mean) across the sites. Also, the trawling intensity did not significantly correlate with total organic carbon contents in surficial sediment, indicating limited loss of organic matter due to trawling. The fits to the reactive iron, acid volatile sulfur, chromium(II) reducible sulfur contents, and porewater composition demonstrate that sediment burial and mixing primarily determine the redox stratification. The mixing depth did not correlate with trawling intensity and is more likely the result of bioturbation, as the analyzed macrofaunal taxonomy and density showed a high potential for sediment reworking. The extraordinarily long-lived Arctica islandica bivalve dominated the infaunal biomass, despite the expectation that trawling leads to the succession from longer-lived to shorter-lived and bigger to smaller macrofauna. Our results further suggest that a clear geochemical footprint of bottom-trawling may not develop in sediments actively reworked by tenacious macrofauna.

Sedimentary phosphorus burial in three contrasting boreal lakes in Finland

AbstractPermanent phosphorus (P) burial in sediment regulates lake trophic state over long timescales, but the controls on P burial are only partially understood. A diversity of biogeochemical settings may be found in lake sediments, which may have a strong impact on the processes controlling P burial from one location to another. Here, we investigate early diagenesis of P in three contrasting lakes in Southwest Finland. Eutrophic Lake Köyliönjärvi and mesotrophic Lake Pyhäjärvi have a history of nutrient loadings from agriculture, while Lake Vähäjärvi is an oligotrophic small forest lake, leading to potentially contrasting sediment biogeochemical dynamics. We combined porewater data and solid-phase sediment geochemical data to identify P phases in each system and investigate the dominant processes controlling P burial. Porewater profiles showed opposite gradients between the oligotrophic and the mesotrophic/eutrophic systems, implying net diffusive fluxes into and out of the sediments, respectively. Furthermore, sediment P data showed contrasting P speciation. Reactive P is buried in all systems, but the role of reducible iron (Fe) oxides in P retention is greater in mesotrophic/eutrophic lakes. In the oligotrophic system, aluminium (Al) oxides controlled P sorption into the sediment after diffusion from lake water. Evidence for vivianite formation was found only in the mesotrophic Lake Pyhäjärvi sediment, where 42–47% of total P was released in a Fe(II)-P specific extraction from the deeper part of the sediment column and vivianite crystals could be isolated from sediment samples.

REE + Y uptake in bioapatite revisited: Facies-controlled variability in coeval conodonts

REE + Y concentrations in bioapatite have been repeatedly shown to be ambiguous carriers of information for paleo-seawater reconstructions. The uptake of REE + Y begins rapidly after deposition, and bioapatite can, therefore, potentially record the pore water conditions during early diagenesis. In this study, we examined the REE + Y composition of conodonts belonging to a single species from three coeval facies across the Ordovician Baltic Paleobasin carbonate shelf. We show that the REE + Y concentrations, as well as Ce/Ce*, bell-shape index (BSI), Y/Ho and LaN/SmN ratios, vary systematically in conodonts from facies with different host rock compositions. Dissimilarly, REE + Y patterns and ratios of conodont dental elements from the same facies are uniform, indicating that the local depositional environment imposes principal control on the REE + Y uptake by bioapatite. Furthermore, our results show that even though REE + Y concentrations in different histological tissues of the same conodont vary significantly, no REE-specific fractionation occurs, resulting in similar REE + Y ratios in all tissue types. Our results illustrate that conodonts can record modified seawater-like characteristics of REE compositions if the input of FeMn oxyhydroxides and particulate organic carbon (POC) is low. In contrast, under elevated POC and FeMn oxyhydroxide loading, the conodonts show increasingly positive Ce anomalies and MREE enrichment. Collectively, these results suggest that, although primary seawater signatures are not captured in bioapatite, the conodont REE + Y patterns are controlled by local depositional environments and can record early diagenetic pore water conditions.

Early marine carbonate cementation in a relict shelf-edge coral reef North of Miami, Florida: Pseudo-coupled depositional and diagenetic modeling

Carbonate depositional and early diagenetic processes overlap in time because of the high chemical reactivity of calcite. Previous studies have modeled early diagenesis in a static depositional framework. They followed a rule-based modeling approach instead of applying rigorous thermodynamics and reaction kinetics.This study has developed a pseudo-coupled approach based on a dynamic depositional framework and reactive transport modeling for diagenesis. It has been tested for the “Pompano” reef north of Miami, Florida. The model includes five reef parasequences deposited over 9.0 kyr with five facies zones. After deposition of the first parasequence, diagenetic processes are modeled. The resulting framework forms the starting condition for depositing the next parasequence. Subsequently, its diagenetic overprint and the continued overprint in the underlying parasequence(s) are modeled. These workflow steps are repeated until all five parasequences are covered.The results show the spatial and temporal development of calcite cementation and porosity distribution. During the reef catch-up phase, cement mainly developed at the depositional surface (up to 10% of the total sediment volume). During the keep-up phase, cement abundance increases to a maximum value of ∼17%. After the reef's demise, cement forms a crust at the sediment-water interface with up to 38% of the total sediment volume. In general, controlling factors of cement distribution include facies-specific flow velocities, reef facies, relative sea-level changes, depositional gaps, and seawater salinity.The modeling results of this study are well aligned with the actual cement and porosity distribution in the Pompano reef and with other sub-recent and ancient examples described in the literature. Pseudo-coupled depositional-diagenetic modeling represents a suitable approach for quantitative porosity prediction in comparable depositional and diagenetic settings. It reduces uncertainties in prediction at inter-well to prospect scale for oil and gas exploration, geothermal exploration, and subsurface storage of CO2.

REE + Y distribution in Tremadocian shelly phosphorites (Toolse, Estonia): Multi-stages enrichment in shallow marine sediments during early diagenesis

Sedimentary phosphorites in Cambrian-Ordovician boundary beds in Estonia are among Europe's most extensive phosphate rock reserves, with a tonnage of approximately three billion tonnes. In addition, they are potential sources of rare earth elements and yttrium (REE + Y). The ore consists of sandstone rich in phosphatic brachiopod fragments deposited in a shallow marine peritidal environment of the Baltic Paleobasin. A detailed geochemical and mineralogical investigation was conducted on the northern part of the Toolse deposit based on three drill core cross-sections. The REE sequestration model was developed based on the correlation of the sedimentary facies of the members of the Kallavere formation. Analyses were conducted on 140 whole-rock samples and 14 discrete black shale samples. Inductively coupled plasma mass spectrometry (ICP-MS), X-ray fluorescence (XRF) and X-ray diffraction (XRD) measurements were performed for each sample. Textural analyses were conducted with field-emission scanning electron microscopy (FE-SEM), and quantitative wavelength dispersive spectrometer (WDS) analysis of major and trace elements was performed with an emission electron probe microanalyser (EPMA).The PAAS normalised REE + Y patterns were relatively homogeneous throughout the deposit except for the cerium and yttrium contents. The REE + Y contents of the phosphorites indicate a distinctive ‘bell-shaped’ pattern enriched with middle-REE (MREE) enriched. In the lower part of the deposit, the ∑REE + Y concentration is enriched up to 12-fold, compared to the average shale, reaching up to 1234 ppm. LREE enrichment is observed in these horizons, with a 9-fold peak at neodymium. In terms of absolute values, the most prevalent REEs are, in order, Ce, Y, Nd, and La, which respectively reach maximum mean values of 296 ppm, 248 ppm, 164 ppm and 135 ppm.The profiles indicate multistage uptake during transport, deposition, and early diagenesis of brachiopod detritus. The sedimentation was associated with a steep redox gradient in the porewater in a coastal environment during the beginning of marine transgression. The near-shore upwelling of oxygen-poor deeper water rich in dissolved nutrients and Mn, combined with the input of lithogenic Fe, helped create favourable conditions for REE sequestration. Positive Y anomalies indicate an initial uptake of REE + Y by hydroxyapatite through Ca substitution in an oxic environment near the sediment–water interface. The sediment burial induced the formation of carbonate fluorapatite (CAF) and a shift to the REE adsorption uptake mechanism, reinforced by the significant intercrystalline porosity of the shells. The transition to a suboxic environment below the surface layers resulted in the reductive dissolution of Mn-(oxyhydr)oxides and the release of LREE in the porewater system, resulting in an overprint of the original REE pattern to the point of inducing positive Ce anomalies. A late enrichment in MREE occurred during early diagenesis as a result of desorption of REE from the Fe-(oxyhydr)oxides and organic-rich particles under relatively anoxic conditions.

Uranium pre-concentration in sandstone-hosted U deposits: A case study from the Hailijin ore field, SW Songliao Basin, NE China

Economic U mineralization in sandstone was commonly attributed to oxygenated fluid infiltration in reduced sandstone, leading to U precipitation in redox front during late diagenetic stage. It was proposed that U can be pre-concentrated before this period based on trace amounts of U in altered products of Fe-Ti oxides, as revealed by electron microprobe analysis (EMPA). However, the obtained trace elements concentrations in altered Fe-Ti oxides (<1 wt%) is generally accompanied with relatively low analytical precision, leading to a skeptical explanation under this situation. In this work, laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) was used to determine trace elements concentrations in hematite from the uraniferous Lower Yaojia Formation (Hailijin ore field, SW Songliao Basin, NE China), aiming to fingerprint this proposed U pre-concentration in details. Some trace elements contents, such as Mo, Ge, Ca, P, Y, Pb, Th, Sn, and W, are positively correlated with U content in hematite. Given that uraniferous oxygenated groundwater with a near-neutral pH is unable to efficiently dissolve detrital phosphate (i.e., apatite and monazite) in the Lower Yaojia Formation, phosphorous must have been released from these P-rich phases before late diagenesis. Such a conclusion is supported by our observations that monazite from the reduced sandstone has a corrosive morphology, and exhibits a corresponding U enrichment in the corrosive rim. The dissolved P and U can be adsorbed by porous titania up to several thousand ppm during early diagenesis, providing a pre-concentrated metal source for remobilization during late diagenesis. It can not only account for a trace-element signature derived from monazite (i.e., Ca, P, Y, Pb, and Th) in hematite geochemistry, but also fingerprint Mo and Ge migrations resulted from oxidation of pyrite and organic matter, respectively. In addition to U, tungsten and Sn would be re-equilibrated with infiltrated oxidized fluid during the recrystallization of porous titania, and partially incorporated into simultaneously formed hematite grains due to high compatibility. The remobilization of pre-concentrated U induced by oxygenated fluid incursion led to U accumulation in a redox front during late diagenesis, forming uraninite nanoaggregates with relatively high concentrations of P, Y and Ti. Our work has verified that trace-element signature in hematite from sandstone is a reliable fingerprint to reveal ore-forming processes, and we propose for the first time that titania and phosphate play a crucial role for U pre-concentration in sandstone-hosted U deposits.

Geology, geochemistry, fluid inclusion microthermometry and depositional environment of the Early Cretaceous Meymeh sub-seafloor replacement sideritic-ankeritic iron deposit, Malayer-Esfahan Metallogenic Belt, Iran

Meymeh sideritic-ankeritic iron deposit in the eastern part of Malayer-Esfahan Metallogenic Belt (MEMB) is formed in the sedimentary-volcanic rocks of the Early Cretaceous sequence. Ore mineralization in the study area based on stratigraphic location and type of host rocks divided in two ore horizons. Lower ore horizon located in the dolomitic-sandy limestone (Kc3) unit, which upper ore horizon is formed in thin-bedded limestone (Km) rocks. Petrographic studies indicates that mineralization comprises three ore facies: stockwork, bedded and massive ore facies. The most important primary minerals are siderite, ankerite, ferroan-dolomite, pyrite, pyrolusite, barite and minor chalcopyrite. The most frequent textures in the ore zones include laminae, replacement, vein-veinlet, massive and banded. Dolomitization and silicification are the main wall rock alteration styles; alteration intensity increases towards the ore zones. Based on relationships between ore minerals and rock forming minerals, ore mineralization (hypogene and supergene) in the Meymeh deposit formed during three main stages: fine-grained Fe-carbonate bands are intricately interlayered with dolomite beds. Sideritic-ankeritic bands exhibit classic sedimentary textures, such as laminations and bedding, indicative of a syn-sedimentary to early diagenesis origin. Coarser-grained stage two siderites and ankerites show breccia and vein-veinlet textures, and are considered to have formed by replacement during burial diagenetic sub-seafloor fluid flow. In stage three, siderite and ankerite were converted to secondary iron oxides such as oxide/hydroxide Fe minerals during meteoric water flow through the inverted normal and thrust faults and uplift. The primary two-phase fluid inclusions in the quartz-2 and ankerite-2 minerals that have been investigated from the ore mineralization section of the Meymeh deposit are homogenized at temperatures between 110.3 to 226.9 °C. Salinities of the primary fluid inclusions range from 3.39 wt.% to 14.77 % NaCl eq. This finding suggests that hydrothermal brine fluid mixing with seawater could be the primary mechanism that prompted ore formation. The similarity of REE patterns between siderite and ankerite in different ore facies and host rock carbonates indicates their derivation from the same ore fluids. The Meymeh deposit is considered a typical case of sedimentary hydrothermal diagenetic sideritic-ankeritic mineralization, in which minerals deposited when hydrothermal fluid was released from anoxic to suboxic water columns.

Carbonate-associated sulfate as an archive of sulfur and oxygen stable isotope composition of seawater sulfate: Evidence from reef carbonate rocks of the southern South China Sea

Carbonate-associated sulfate (CAS) is a useful proxy for the reconstruction of the isotopic composition of ancient seawater sulfate, archiving information on the global sulfur cycle, Earth's surface redox evolution, and biological activity. The CAS proxy is possibly compromised by early diagenetic alteration, which may cause secondary inhomogeneities in the isotopic composition of pore water sulfate achieved in the form of CAS. However, the effects of early diagenesis on the retention of primary δ34S signals of CAS in reef carbonates are not well understood, and it is unknown whether reef carbonate deposits can faithfully record the coeval isotopic composition of marine sulfate. To provide new constraints on the resilience of CAS isotopic signatures of reef carbonate, we have measured and analyzed CAS content and CAS isotopic composition of reef carbonates from the Meiji Atoll (well NK-1) of the South China Sea, representing carbonates affected by meteoric and marine diagenesis including dolomitization. CAS contents were found to decrease rapidly during meteoric and marine diagenesis, with minimum CAS contents caused by prolonged subaerial exposure. Despite low contents, the corresponding δ34SCAS values were found to vary within a narrow range from 21.5 to 24.1‰, compared to values of 20.9 to 22.7‰ of coeval barite, suggesting that the original δ34SCAS values are preserved and largely unaffected by recrystallization. However, δ18OCAS values were found to vary widely, ranging from 7.1‰ to 15.0‰, only recording the same oxygen isotope signal as coeval marine barite for reef carbonates younger than 5 Ma. Our data reveal that reef carbonates can be used as an archive of paleoceanic change, faithfully tracking δ34S changes in coeval seawater regardless of whether reef carbonates underwent recrystallization, including dolomitization. In contrast to sulfur isotopes, the oxygen isotope composition of CAS is more susceptible to alteration during diagenesis. Overall, this study provides new insight for the future reconstruction of the sulfate sulfur and oxygen isotope record of ancient seawater.

Impacts of nepheloid layers and mineralogical compositions of oceanic margin sediments on REE concentrations and Nd isotopic compositions of seawater

Nepheloid layers and turbidity currents transport large quantities of detrital sediments to deep-sea basins. Locally they may control seawater – detrital particle interactions and modify dissolved REE concentrations and εNd distributions of the oceanic margin. However, to date, such processes have been poorly documented, especially in oceanic margins characterized by contrasted mineralogical compositions. Dissolved REEs and εNd from three water stations collected in the northern South China Sea (SCS) close to the Taiwanese deep-sea canyons have been analyzed, with the aim of determining the influence of nepheloid layers and mineralogical composition of lithogenic inputs on dissolved REE and εNd distributions in the ocean. Results indicate that the most radiogenic εNd value (−2.5 ± 0.2), observed at about 400 m, corresponds to the inflow of North Pacific Intermediate Water (NPIW) into the SCS. When compared with previously published Nd isotope, our current study suggests that the εNd value of NPIW which enters the SCS (from 300 to 800 m water depth) is not modified by unradiogenic sediments off the southern margin of Taiwan Island, whereas they become less radiogenic along the margin of eastern China due to Nd exchange with unradiogenic sediments (between −10.2 and −12.6) from the tropical soils of Chinese rivers basins. We have proposed that pedogenetic minerals from Chinese tropical soils modify the Nd isotopic compositions of seawater more efficiently than fresh detrital minerals resulting from very high rates of physical erosion on Taiwan Island. The MREE/MREE*, La/Sm and La/Yb ratios in nepheloid layers indicate that lateral transportation of lithogenic material within the water column can preferentially adsorb LREE compared to MREE and HREE through the scavenging process. However, this process has a negligible impact on seawater εNd of the northern SCS. At the bottom of several water stations, a negative excursion of εNd values, associated with an increase in MREE/MREE* ratios and a decrease in La/Sm ratios, suggests a remineralization flux resulting from the dissolution of Mn oxides during early diagenesis of deep-sea sediments.

Corrigendum: Early diagenesis and benthic fluxes of redox-sensitive metals in eastern China shelf sediments

Corrigendum: Early diagenesis and benthic fluxes of redox-sensitive metals in eastern China shelf sediments

Evidence for the formation of fused aromatic ring structures in an organic soil profile in the early diagenesis

The presence of fused aromatic ring (FAR) structures in soil define the stability of the recalcitrant soil organic matter (RSOM). FAR are important skeletal features in RSOM that contribute to its extended residence time. During the early diagenesis, FAR structures are formed through condensation and polymerization of biomolecules produced during plant residue and microbial product decay. Molecular level characterization of the RSOM extracted from an organic soil profile gives important insights into the formation of FAR. Advanced solid-state 13C nuclear magnetic resonance (NMR) spectroscopy, including recoupled long-range C–H dipolar dephasing experiments on extracted humic acids (HA) showed that they contain diagenetically formed FAR different from charcoal and lignin. Peaks characteristic of FAR are observed at all depths in the soil profile, with a greater prevalence observed in the HA extracts from the clay soil layer at the bottom. In the clay soil layer, 78% of the aromatic carbon was non-protonated, and this was 2.2-fold higher than the topsoil. These data further strengthen our understanding of the humification process that could occur in early diagenesis and help explain the importance of incorporating diagenesis as an important phenomenon for long-term carbon sequestration in soil.

Organic Biosignature Degradation in Hydrothermal and Serpentinizing Environments: Implications for Life Detection on Icy Moons and Mars.

Evidence of liquid water is a primary indicator of habitability on the icy moons in our outer solar system as well as on terrestrial planets such as Mars. If liquid water-containing environments host life, some of its organic remains can be fossilized and preserved as organic biosignatures. However, inorganic materials may also be present and water-assisted organic-inorganic reactions can transform the organic architecture of biological remains. Our understanding of the fate of these organic remains can be assisted by experimental simulations that monitor the chemical changes that occur in microbial organic matter due to the presence of water and minerals. We performed hydrothermal experiments at temperatures between 100°C and 300°C involving lipid-rich microbes and natural serpentinite mineral mixtures generated by the subaqueous hydrothermal alteration of ultramafic rock. The products reveal what the signals of life may look like when subjected to water-organic-inorganic reactions. Straight- and branched-chain lipids in unaltered samples are joined by cyclization and aromatization products in hydrothermally altered samples. Hydrothermal reactions produce distinct products that are not present in the starting materials, including small, single-ring, heteroatomic, and aromatic compounds such as indoles and phenols. Hydrothermal reactions in the presence of serpentinite minerals lead to significant reduction of these organic structures and their replacement by diketopiperazines (DKPs) and dihydropyrazines (DHPs), which may be compounds that are distinct to organic-inorganic reactions. Given that the precursors of DKPs and DHPs are normally lost during early diagenesis, the presence of these compounds can be an indicator of coexisting recent life and hydrothermal processing in the presence of minerals. However, laboratory experiments reveal that the formation and preservation of these compounds can only occur within a distinct temperature window. Our findings are relevant to life detection missions that aim to access hydrothermal and serpentinizing environments in the subsurfaces of icy moons and Mars.