Authigenic Carbonate Research Articles

The role of anaerobic methane oxidation on the carbonate authigenesis in sediments of the subtropical Beibu Gulf, South China Sea: A reactive–transport modelling approach

The formation and burial of authigenic carbonate in marine sediment significantly affect the sedimentary carbon cycle and its isotopic mass balance in geological history. Anaerobic oxidation of methane (AOM) is the primary driver of authigenic carbonate precipitation within the sulfate-methane transition zone (SMTZ). Quantitative estimations of the role of AOM on the authigenic carbonate precipitation and its carbon isotope under non-steady-state processes (e.g., changes in methane fluxes at the bottom sediment, sedimentation rates or organic fluxes in the surface sediment), however, are still limited. In this study, we use geochemical data from porewater (e.g., the concentration of sulfate, calcium, magnesium, strontium, dissolved inorganic carbon, total alkalinity) and solid sediment (e.g., organic matter content, and carbonate content) in different depositional environments of the subtropical Beibu Gulf, South China Sea, combined with a diagenetic reactive-transport modelling approach, to estimate the mineralogy of authigenic carbonate, the relationship between AOM and authigenic carbonate precipitation, and the impact of AOM rate on carbon isotope of sediment carbonate (δ13CCar). The results show that high-Mg carbonates (high-Mg calcite and dolomite) are the main type of authigenic carbonate (∼80%) formed in the methane-bearing sediments, leading to higher porewater Sr2+/Ca2+ (>0.02) and Mg2+/Ca2+ (>20) within the SMTZ. Our modelling analysis highlights that the non-steady-state induced by increased methane flux from the underlying sediments can significantly accelerate the authigenic carbonates formation within the SMTZ. Using parametric sensitivity analysis, we observed that even a 1% increase in the authigenic carbonate fraction of sediment carbonates results in significant changes in δ13CCar within the SMTZ (from −1‰ to −2‰), mainly due to lighter carbon isotopes produced by more intensive AOM processes. Noteworthily, the terrestrial-to-marine transition was identified by the sediment and porewater geochemical profiles at site SO-8. Although lower authigenic carbonate precipitation occurs in terrestrial sedimentary environments, the proportion of authigenic carbonate in terrestrial environments (11%) is much higher than that in marine environments (1%), resulting in carbon isotopes of carbonate in terrestrial sediments becoming more negative (−5‰).

Volcanogenic aluminosilicate alteration drives formation of authigenic phases at the northern Hikurangi margin: Implications for subseafloor geochemical cycles

Alteration of volcanogenic aluminosilicates (VAs) in marine sediments is recognized as critical in regulating geochemical cycles and sustaining the oceanic deep biosphere, but rates of VA alteration and its associated authigenic mineral formation are not commonly reported. Here we present results on analyses of sediments and pore water recovered from the upper 150 mbsf of four sites drilled on the northern Hikurangi margin during IODP Expeditions 372 and 375. Petrographic analyses show that volcanogenic materials (glass shards, feldspar, volcanic lithoclasts) constitute important components (15–45 wt%) of the hemipelagic mud, and reveal ongoing glass alteration with accompanying authigenic phase formation. A reaction-transport model constrained by pore water Sr, 87Sr/86Sr, Ca, Mg, and Si was applied to simulate VA diagenetic reactions. Our model results yield VA alteration rates of 0.047–0.64 mmol Sr m−2 yr−1, with substantially higher values at Sites U1517 and U1520 that experienced rapid sediment emplacement. In addition, our simulations show that >99% of the dissolved Si generated by VA alteration is fixed in silica cement and authigenic clay, and that ∼50% of Ca incorporated in the authigenic carbonate is supplied by VA alteration. First-order estimates suggest that, in addition to authigenic carbonate precipitation, authigenic clay formation may represent an important sink for dissolved Mg. This study quantitatively examines the linkage between VA alteration and formation of authigenic phases, highlights its role in subsurface geochemical cycles, and indicates that slope instability may play an important role in promoting VA diagenesis.

ПРОСТРАНСТВЕННЫЕ ЗАКОНОМЕРНОСТИ НАКОПЛЕНИЯ УГЛЕРОДА В ОЗ. ПЕСЧАНОЕ (ЮГ ЗАПАДНОЙ СИБИРИ) В ГОЛОЦЕНЕ

The paper is devoted to the study of the processes of Lake Peshchanoye, located on the territory of the Baraba Lowland (south of West Siberia). By the means of a set of analytical methods, the structure and composition of sapropelic accumulation were characterized and sources of sedimentary matter were identified. The main material source is an autochthonous sedimentary material (organic matter, authigenic minerals). Organic matter has a predominantly planktonic origin with a predominance of phytoplankton. Authigenic minerals are represented by framboid aggregates of pyrite, curdled precipitates of low-magnesian calcite intergrown with remains of biota mortmass and biogenic amorphous silica, the content of which is equal to 60% of the total silica in sapropel. The allochthonous part is represented by terrigenous particles of quartzfeldspar composition. The predominance of allochthonous material over autochthonous one was revealed in the northern part of the lake, it is associated with an input of terrigenous particles by the river. The main forms of carbon burial are undecomposed organic matter and authigenic carbonates. The estimated total carbon reserves in the lake are 113.21 thousand tons.

ПРОЯВЛЕНИЯ АУТИГЕННОЙ МИНЕРАЛИЗАЦИИ ВДОЛЬ КОНТИНЕНТАЛЬНОГО СКЛОНА ЯПОНСКОГО МОРЯ И В ТАТАРСКОМ ПРОЛИВЕ (МАТЕРИАЛЫ 61 РЕЙСА НИС «АКАДЕМИК ОПАРИН»)

During the complex scientific research expedition in the Sea of Japan aboard the research vessel Akademik Oparin (Cruise 61), carbonate nodules were recovered by gravity coring, which were subsequently identified as pseudomorphs after ikaite (glendonite). The article presents the results of electron-probe and tomography studies of authigenic carbonates and gas-geochemical characteristics of host sediments occurring along the continental slope of the Sea of Japan and in the Tatar Strait.

APPLICATION OF MUD GAS DATA AND LEAKAGE PHENOMENA TO EVALUATE SEAL INTEGRITY OF POTENTIAL CO2STORAGE SITES: A STUDY OF CHALK STRUCTURES IN THE DANISH CENTRAL GRABEN, NORTH SEA

Depleted chalk oilfields and chalk structures in the Danish Central Graben, North Sea, are potential CO2storage sites. In most of these fields, the main reservoir is the Upper Cretaceous – Danian Chalk Group and the Eocene – Miocene mudstones of the Horda and Lark Formations constitute the primary seal. In a few fields, the reservoir is composed of the Lower Cretaceous Tuxen and Sola Formations. Here the main seal is assumed to be the Chalk Group which however has poor gas sealing characteristics; the Horda and Lark Formations constitute an efficient secondary seal although they are quite high in the section. This study documents a workflow that may help to evaluate the seal integrity of the structures from an integration of mud gas data from wells with seismic data. Mud gas data provide detailed information about the distribution and types of gas (biogenic or thermogenic) throughout the seal section and overburden. The presence of higher carbon number gases (C3–C5, propane to pentane) in the seal indicates migration of thermogenic gas into the thermally immature sealing mudstones; whereas the dominance of C1(methane) and partly C2(ethane) likely reflects the presence of in situ generated biogenic gas in the mudstones, thus indicating that there are no seal integrity issues. The vertical thermogenic gas migration front has been determined, and a “traffic light” indicator system has been used for seal integrity evaluation. Where no or minor migration of thermogenic gas into the primary seal has occurred and a primary seal >30 m thick is present, the seal is considered to have good matrix seal integrity (green). If some significant thermogenic gas migration has occurred into the primary seal but more than 30 m of primary seal is present above the thermogenic gas migration front, the seal integrity is reduced (yellow). In structures where thermogenic gas migration is recorded through the primary seal and into the overburden, seal integrity is considered to be poor (red). In areas where significant leakage of thermogenic gas has occurred into the seal, high density, low porosity carbonate beds frequently occur encapsulated within the sealing mudstones and are interpreted to be composed of methane‐derived authigenic carbonates (MDACs). Seismic data show that there is a convincing correlation between leakage as indicated from mud gas data and the presence of vertical wipe‐out zones (gas chimneys), bright zones (gas‐charged sediments or MDACs), and depressions (pockmarks). In general, potential CO2storage sites in the study area in tectonically inverted structures show good seal integrity, but this may locally be reduced and require additional analyses. Storage sites associated with salt diapirs generally show poor seal integrity and are likely to be poor candidates for CO2storage. In combination, mud gas and seismic data are therefore powerful tools to investigate (palaeo‐) leakage phenomena and provide support for seal integrity evaluation at local to regional scales.

Paleocene–Miocene topographic and tectonic evolution of the northern central Andean plateau, southern Peru

The Paleocene–Miocene sedimentary successions along the Western Cordillera – Altiplano Plateau margin in southern Peru record the growth history of the northern Altiplano Plateau and the development of the foreland basin system. We evaluate the paleoelevation history of the northern Altiplano Plateau for the period of primary upper crustal shortening (i.e., Paleocene–Miocene) to determine the amount of shortening-related surface uplift. We provide new apatite fission-track and tuff zircon U–Pb ages to add to the existing age models of the strata. We further calculate the first-order approximation of paleoelevations of these formations from paleo-meteoric water δ18Omw values reconstructed from authigenic carbonate δ18Oc. The calculations show that surface elevations increased by more than 1 km along with the basin development from backbulge and foredeep to wedge-top and hinterland basins. The calculated paleoelevation of 1.8–2.4 km at the end of contractional deformation during the late Miocene indicates that a rapid surface uplift of more than 1.5 km is required to attain the modern high elevations. It is further inferred that this post-late Miocene surface uplift was probably associated with the lower crustal flow and ensuing convective removal of lower lithosphere materials.

Sedimentological and geochemical properties of authigenic carbonates in Kyushu, Japan: Implications for the transition from semi-arid to humid climate during the Eocene

The timing of the East Asian monsoon system has attracted attention because of the possible relationship between the tectonic uplift of the Himalaya and Tibetan Plateau and climatic evolution in East Asia. Although the existence of the East Asian monsoon in the late Eocene has recently been suggested, few paleoclimatic records have been reported from the mid-latitude coastal area in East Asia where the climate might have changed dramatically when the East Asian monsoon was established. In this study, we examined the climatic conditions in the early and late Eocene at two localities in the northern part of Kyushu, Japan, focusing on the depositional environments of freshwater carbonate nodules. The lower Eocene Akasaki Formation yields dolomite and calcite nodules in the red mudstone layers, which record low δ13C and variable δ18O. The geochemical and textural features of the nodules suggest a pedogenic origin, which we interpreted as indicating formation on a floodplain with well-drained soils under a semi-arid climate with seasonal precipitation. In contrast, the upper Eocene Yoshinotani Formation yielded nodules composed of siderite with distinctively positive δ13C and stable δ18O values, suggesting that they developed on wetlands covered by poorly drained soils in a temperate climate with stable rainfall throughout the year. These two types of nodule-bearing formations imply that the climate became wet in the middle Eocene. This climate change in Kyushu predated the rapid global cooling at the Eocene-Oligocene transition and may be related to the appearance of the monsoon system in East Asia, which was caused by the uplift of the Himalaya and Tibetan Plateau.

Responses of sedimentary proxy indicators to lake-level fluctuations on the central Tibetan Plateau since the last deglaciation

Lake sediments are valuable archives to investigate past changes in climate and environment on various timescales. Closed-basin lakes can be regarded as precipitation gauges, which are ideal for studying past climate changes. However, the climatic implications of various sedimentary proxy indicators from closed-basin lakes are often confounded by lake-level fluctuations, which are affected by lake watershed processes, besides climate change. Here, we present records of multi-proxy indicators since the last deglaciation at Lake Cuoe, on the central Tibetan Plateau, which was once connected with Selin Co and became separated during the middle Holocene, as evidenced by a series of paleo-shorelines. The sedimentary proxies are categorized into three groups based on their origins, with differences in their environmental significance. The proxy indicators derived from within-lake processes effectively revealed past lake-level fluctuations, including the Ca/(Al, Ti, Fe) ratio, Fe/Mn ratio, carbonate minerals, stable carbon and oxygen isotopes of authigenic carbonate, and the percentage of aquatic inputs (Paq) derived from n-alkanoic acids. These proxy indicators showed obvious changes at ∼7–6 ka, when Lake Cuoe became hydrologically separated from Selin Co. The stable oxygen isotope record of Lake Cuoe, apparently modulated by lake-level changes, differs significantly from that from Selin Co. The proxy indicators reflecting watershed processes, including terrigenous elements, Rb/Sr ratio, grain size, and the chain length proxy based on n-alkanoic acids, directly reflected changes in the regional climate and environment. The proxy indicators derived from both allochthonous and authigenic sources, including total organic carbon (TOC) and total nitrogen (TN), reflected the effects of climate change as well as lake-level fluctuations. Overall, our results suggest that great caution should be applied in interpreting proxy records of climatic and environmental changes from closed-basin lakes on the Tibetan Plateau. They also highlight the potential of using sedimentary proxies to reconstruct continuous records of lake-level fluctuations.

Climatic and hydrological variations on the southwestern Tibetan Plateau during the last 30,000 years inferred from a sediment core of Lake Zabuye

Climate change on the Tibetan Plateau (TP) is governed by the mid-latitude westerlies and Indian summer monsoon (ISM). However, the past hydrological changes on the TP induced by interactions between the westerlies and the ISM remain unclear. A sediment core from Lake Zabuye on the southwestern TP was analyzed to reconstruct climatic and hydrological variability over the last 30 kyr. Stable isotopic compositions (δ13C and δ18O) of authigenic carbonate act as an indicator of hydrological status for closed-basin lakes on the TP. Four major climatic and hydrological stages were identified: (1) last glacial maximum (29.8–18.9 cal kyr BP), the influence of the westerlies dominated, and a cold-wet climate prevailed; (2) last deglaciation (18.9–11.8 cal kyr BP), the influence of the westerlies receded, and a general trend of decreased wetness was interrupted by two pronounced glacial meltwater events recorded at 17.2 and 15.2 cal kyr BP, respectively; (3) early and middle Holocene (11.8–3.8 cal kyr BP), the Holocene climate optimum was attained and sustained under the dominance of ISM circulation; (4) late Holocene (from 3.8 cal kyr BP to the present), the ISM collapsed during the middle to late Holocene, and a hypersaline lacustrine environment was ultimately formed, as inferred from mirabilite deposition and isotopic signals. Our lake record demonstrates an anti-phase relationships between the westerlies and the ISM on the glacial-interglacial timescale over the southwestern TP. We suggest that both North Atlantic climate and tropical ocean temperature are major drivers of climatic variations on the TP.

Trace and rare earth element distribution in hyperalkaline serpentinite-hosted spring waters and associated authigenic carbonates from the Ronda peridotite

The Ronda peridotite in southern Spain is subject to low-temperature serpentinization by circulating groundwaters that produces hyperalkaline fluids of high pH and low dissolved inorganic carbon content (DIC). When these waters resurface as hyperalkaline springs they take up atmospheric carbon dioxide (CO2), which triggers the precipitation of travertine carbonates. Spring waters at Baños del Duque, Fuente Romana, and Fuente Amargosa show typical chemical features of hyperalkaline springs such as high pH and low DIC, yet exhibit an unusual distribution of rare earth elements (REEs) enriched in lanthanum (La) and cerium (Ce). In order to constrain changes in spring water chemistry and carbonate precipitation during atmospheric CO2 uptake, a reaction path model was set up to illustrate that the distribution of REE species in the high pH and low DIC water is strongly dependent on REE complexation by hydroxide and carbonate species. Reaction path modeling results show that during spring fluid emission and CO2 uptake, REE speciation in the fluids shifts from a dominance of hydroxide to carbonate complexes. The complexation of rare earth elements in the high pH, low DIC fluids strongly depends on the DIC content of the respective fluid. The REE distribution of travertine deposits associated with the hyperalkaline springs deviates from that in the fluids, suggesting that travertines precipitated from fluids different to those currently emitted from the springs.

Formation of authigenic carbonates contributed by sulfate- and metal-driven anaerobic oxidation of methane in the northern Okinawa Trough, East China Sea

Authigenic carbonates of the northern Okinawa Trough have been shown to be contributed by both sulfate- and Fe-driven anaerobic oxidation of methane (SD-AOM and Fe-AOM, respectively); however, the microorganisms involved remain poorly constrained. To better understand the types and roles of microorganisms in specific biological methane oxidation processes, authigenic carbonates collected from the northern Okinawa Trough at water depths between ∼540 m and 700 m were studied using petrographic, mineralogical, elemental, and lipid biomarker analyses. The carbonate minerals were primarily aragonite, high-magnesium calcite, low-magnesium calcite, or dolomite. The extremely low δ13C values of bulk carbonates (−59.2‰), molecular fossils of anaerobic methane oxidizing archaea (ANME; −131‰), and sulfate-reducing bacteria (SRB; −77‰) indicated that the authigenic carbonates formed as products of AOM. Biogenic methane was confirmed to be the main carbon source for all carbonates, whereas bio-degraded oil and hydrothermal input complemented carbon sources, as revealed by the occurrence of unresolved complex mixture, strongly enriched As and Sb, and their correlations with Fe/Al. Combined with enriched δ18O values (as high as +5‰), the methane fluids were suggested to derive from the dissociation of gas hydrates. ANME-2 and a mixture of ANME-1 and ANME-2 were identified for the two calcitic carbonates, respectively, as revealed by their specific biomarker patterns. The relatively higher contents of 13C-depeleted isoprenoids and SRB−related fatty acids, strong Mo enrichment, and biological debris in both calcite- and aragonite-dominated carbonates reflect precipitation driven by SD-AOM in a shallow sulfate-methane transition. For other seep carbonates, low amounts of 13C-depeleted isoprenoids and the near absence of crocetane suggest dominance of the ANME-1 assemblage. The extremely low amounts and non-13C-depleted iso-/anteiso-C15:0 fatty acids (−27‰) in the dolomite-containing carbonates indicate the decoupling of ANME and SRB. These carbonates were most likely formed below or at the bottom of the sulfate-methane transition and included contributions from Fe-AOM, as indicated by the low Mo content, occurrence of dolomite and siderite, and decreased δ56Fe values with increasing Fe/Al ratios.

Terrestrial carbonate oxygen isotopes constraints on the interplay between westerlies and monsoonal rains modulating the Cenozoic climate on the northeastern Tibetan Plateau

The relative contribution of the westerlies and the Asian monsoon has played a critical role in regulating the evolution of the Cenozoic Asian interior environment in response to regional tectonic uplift and global climate change. Here, we reconstruct a long-term record of atmospheric moisture oxygen isotopes in the Asian interior using the oxygen isotopic composition of authigenic carbonates in terrestrial sediments from the Xining Basin on the northeastern Tibetan Plateau spanning the last 52 Myrs. On the VPDB scale, the average carbonate δ18O is characterised by a long-term decrease from −1.5‰ at ca. 43 Ma to −7.5‰ at 33 Ma and smaller variations from −7‰ to −8.5‰ after 33 Ma, with the exception of one period that contains samples with higher δ18O values at −6‰ at 17–14 Ma. At the same time, there is a corresponding long-term rise in average carbon isotope δ13C values from −9.5‰ at ca. 43 Ma to −3.5‰ at 33 Ma, followed by a less variable but similar variation in δ18O values after 33 Ma. The oxygen and carbonate isotopic compositions are negatively correlated before 33 Ma and positively after, suggesting a transition at ∼33 Ma in the context of atmospheric circulation coupled with regional aridity. The δ18O record can be explained by a simple binary mixing model of the atmospheric moisture between the recycling moisture with a positive δ18O value in the westerlies-dominated region and the monsoon rainfall or related moisture with a negative δ18O value from the Indo-Pacific Ocean. In that case, the first-order carbonate δ18O decrease from ca. 43 Ma to 33 Ma was primarily controlled by the retreat of the Para-Tethys Sea, which induced a remarkable decline in westerly rainfall relative to the monsoon. After ∼33 Ma, the contribution from the westerlies could be constant and the lower limits of the carbonate δ18O values are modelled by episodic increases in monsoon rainfall corresponding to enhanced Asian monsoon periods at 26–24 Ma, 17–14 Ma, and 9–8 Ma induced by the tectonic uplift of the Tibetan Plateau and the global climate. Our study, also, suggests that the Oligocene was a transitional stage of moisture sources from the dominance of Eocene westerlies to the Miocene-Quaternary monsoon in continental Asian environments.

Metalloenzyme signatures in authigenic carbonates from the Chukchi Borderlands in the western Arctic Ocean

Migration of methane-rich fluids at submarine cold seeps drives intense microbial activity and precipitation of authigenic carbonates. In this study, we analyzed microbially derived authigenic carbonate samples recently recovered from active gas hydrate mounds on the southwestern slope of the Chukchi Borderlands (CB), western Arctic Ocean. Our main aim was to characterize the distribution patterns of trace elements in carbonate-hosted lipid fractions to assess metalloenzyme requirements of microbes involved in anaerobic oxidation of methane (AOM). We measured stable isotopes, trace elements, lipid biomarkers, and genomic DNA, and results indicate the dominance of AOM-related lipid biomarkers in studied carbonate samples, as well as a predominant occurrence of the anaerobic methanotrophic archaea (ANME)-1. We also report evidence for significant preferential enrichments of various trace elements (Li, Ni, Co, Cu, Zn, and Mo) in the total lipid fractions of CB carbonates, relative to elemental compositions determined for corresponding carbonate fractions, which differ from those previously reported for other seep sites. We hypothesize that trace element enrichments in carbonate-hosted lipid fractions could vary depending on the type of AOM microbial assemblage. Additional work is required to further investigate the mechanisms of lipid-bound trace elements in cold seep carbonates as potential metalloenzymes in AOM.

Development and deployment of lander-based multi-channel Raman spectroscopy for in-situ long-term experiments in extreme deep-sea environment

Fluid seepages in deep-sea cold seep and hydrothermal vent areas transport substances and energy from the lithosphere to the hydrosphere, thereby sustaining the seafloor biosphere. The strong inhomogeneity of cold seep and hydrothermal vent systems in terms of time and spatial scales requires in-situ, long-term, and multi-target detection. However, commercial sensors and the developed deep ocean Raman spectrometers have limitations in obtaining a more comprehensive understanding of cold seep and hydrothermal vent systems. Based on a previously developed deep-sea hybrid Raman insertion Probe (RiP), we developed a novel multi-channel Raman insertion probes system (Multi-RiPs) that can simultaneously detect four targets in situ. Multi-RiPs can be used to analyze the geochemistry of fluid seepage, rocks, gas hydrates, and sediment pore water deployed by deep-sea long-term ocean observation platforms (LOOP). The Multi-RiPs was landed with LOOP near an active cold seep vent in the South China Sea and used for in-situ detection of fluids at different depths in the chemosynthetic communities, methane hydrate quickly formed by the cold seep fluids and authigenic carbonates that were exposed to the ambient seawater from June 2, 2020 to June 10, 2020 (8 days). The in-situ long-term Raman spectra acquired by Multi-RiPs indicated a possible novel sulfur transition pathway existing in the chemosynthetic communities, and dynamic evolution of gas hydrate. Overall, the Multi-RiPs has significant potential for long-term multi-target detection and geochemistry analysis in the deep sea.

Messinian seep-carbonates marking the transition to the evaporite deposits in the Romagna sector of the northern Apennines (Italy)

Numerous outcrops of Messinian authigenic carbonates mark the passage between euxinic shales of the Ghioli di letto Fm at the closure stage of the Miocene foredeep, and the evaporites of the Gessoso-solfifera Fm in the Romagna sector of the northern Apennines. Carbonates have a lenticular to stratiform shape up to 20 m thick and are aligned parallel and concordant to the contact with the evaporites, indicating their primary position. The study of the carbonate succession has evidenced 12 different chemofacies (vuggy and mottled fabric, vein network, conduits, breccias) with chemosymbiotic taxa (lucinids, bathymodiolins, vesicomyids) and negative 13C isotopic signatures (δ13C from -36,92‰ up to -54,25), indicating that they are the product of hydrocarbon rich- fluids seepage at the sea-floor. Facies of the upper succession, close to the contact with evaporites, are typically laminated, with millimetric alternances of peloidal and dark micritic levels. They still preserve seepage imprint testified by 13C depletion (-22, 37‰) and occurrence of small lucinids and serpulids, and are here interpreted as microbial mats related to vigorous fluid release. The occurrence of seep-carbonates at direct contact with selenitic gypsum offers new data on the transition from marine conditions to evaporitic environment in the framework of the Messinian Salinity Crisis.

Organic matter diagenesis and precipitation of Mg-rich carbonate and dolomite in modern hypersaline lagoons linked to climate changes

Lipid-biomarkers have been used to reconstruct environmental changes in lacustrine systems on a range of time scales. Lake sediments are excellent archives to apply these tools due to their rapid and amplified response to environmental pressures. For the past thirty years, the hypersaline lagoons of the Rio de Janeiro coastal plain have been studied as natural laboratories for the observation of the biogeochemical processes involved in modern dolomite precipitation. Here, we apply a multiproxy approach to characterize two depositional stages during the Holocene that may have triggered primary dolomite formation in these lagoonal environments. A first stage, with two sub-stages (1A − 6.1 to 4.2 kyr. BP; 1B − 4.2 to ∼3.6 kyr. BP) was deposited during the sea-level rise, with sediments containing an abundance of long-chain n-alkanes with 2H-depleted (δ2Hn-alk) signatures indicating riverine inputs of terrestrial organic carbon during prevailing wet conditions. A second stage (<∼3.6 kyr. BP), comprising lacustrine facies, was characterized by high amounts of authigenic carbonate precipitates (calcite, Mg-calcite, Ca-dolomite, and dolomite). The carbonates are the result of physico-chemical changes in the water after the isolation of the lagoons from both the Atlantic Ocean and the neighboring Lagoa de Araruama due to a fall in sea level and aridification associated with intensification of the coastal upwelling after 2.2 kyr. BP. The n-alkanes deposited during this phase contain variable proportions of long and short-chain homologues indicating a mixed source of organic matter (terrestrial higher plants and microorganisms), as well as changes in vegetation associated with the driest conditions, inferred from the 2H-enriched n-alkane homologous. These results clearly demonstrate a climatic influence on dolomite formation in coastal hypersaline environments linked to sea-level change and coastal upwelling phenomena. With these observations, we hypothesize that the existence of similar palaeoceanographic and environmental conditions in the geologic past may have triggered the formation of extensive microbial dolomite deposits. This study provides new elements to interpret the formation of massive dolomite deposits in the geological record, for example, along the Late Triassic Tethys margin.

Cold-seep carbonates of the Laptev Sea continental slope: Constraints from fluid sources and environment of formation

This study presents results of the petrographic, mineralogical, stable isotopes of oxygen and carbon, and trace element investigation of authigenic carbonates collected at newly discovered active cold seeps on the Laptev Sea continental slope at ∼300 m water depth. These carbonates are mainly represented by Mg-calcite with MgCO3 content from 9.1 mol% to 14.0 mol%. The low δ13C values of carbonates ranging from −50.6 ‰ to −32.4 ‰ (V-PDB) indicate that they were formed from anaerobic oxidation of biogenic methane and minor participation of other carbon sources. The difference between measured (from 4.7 ‰ to 5.5 ‰) and calculated (4.0 ‰) δ18Ocarb values might be inherited from fluids enriched in 18O due to dissociation of gas hydrates, which could be the source of methane. The carbonates exhibit weak enrichment in Co, moderate and strong enrichments in As, Mo, and Sb, and strong enrichment in U. Interestingly, As, Sb, and Co correlate with the pyrite content. This indicates that authigenic iron sulfides promote the immobilization of these redox-sensitive elements in seep sediments. The (Mo/U)EF values and anomalies of concentration of Mo and U probably indicate variations in the redox conditions during carbonate formation due to episodically seepage activity changes. Ascending methane-bearing fluids were the main contributor to the enrichment of cold-seep carbonates in As, Mo, Sb, and U at the Laptev Sea continental slope. However additional input from the particulate shuttle process can not be ruled out.

Organoclastic sulfate reduction in deep-buried sediments: Evidence from authigenic carbonates of the Gulf of Mexico

The fate of organic matter in deep-buried marine sediments determines the formation of hydrocarbon reservoirs, shapes the deep biosphere, and affects the global carbon cycle. Several geochemical processes such as silicate weathering, iron and manganese (hydr)oxide reduction, and sulfate reduction influence the remineralization of organic matter in buried sediments. However, little information on deep-seated organoclastic sulfate reduction is available and the involved geochemical processes are largely unknown. Here, authigenic carbonates recovered from Ship Shoal Block 296 (SS296) of the Gulf of Mexico, all dominated by low-magnesium calcite, were investigated to constrain organoclastic sulfate reduction in deep-buried sediments. The SS296 carbonates had been previously suggested to be of a deep origin and transported upward by salt diapirism. The presence of cone-in-cone textures in samples 3110R1 and 3110R2 as well as vitrinite reflectance values of 0.48% to 0.58% indicate carbonate formation at a relatively deep burial. Likewise, clumped isotope thermometry yielded formation temperatures from 45 °C to 53 °C for sample 3110R1, 60 °C for sample 3110R2, and 11 °C to 37 °C for sample 3110R3. In spite of formation at greater depth, the temperatures of samples 3110R1 and 3110R2 are still within the temperature range of microbial sulfate reduction, reflecting formation depths between 1 km and 2.7 km. The presence of framboidal pyrite and its relatively low δ34S values (−27.8‰ to +11.5‰) agree with the occurrence of microbial sulfate reduction during carbonate precipitation. The carbon isotopic composition of the carbonates (δ13C: −16.6‰ to −8.9‰) and enclosed organic matter (δ13Corg: −26.8‰ to −20.5‰) suggests that organic matter is the dominant carbon source of carbonate minerals. This study confirms that microbial sulfate reduction occurs in deep-buried sediments, driving the degradation of organic matter. Our findings suggest that organoclastic sulfate reduction might be common in deep-seated sedimentary environments, controlling the fate of organic matter in deep subsurface environments and affecting the global carbon cycle.

Anaerobic oxidation of methane and trace-element geochemistry in microbial mat-covered sediments related to methane seepage, northeastern Japan Sea

We evaluated the spatial variability of geochemical features, including trace-element cycles, associated with methane seepage in a knoll off Sakata (called Sakata Knoll) in the eastern Japan Sea. Seafloor observations from a remotely operated vehicle and environmental visualization around Sakata Knoll by an optical mapping device revealed large patches of white and gray microbial mats covering sediment in the depression on the top of the knoll. Push core samples were collected inside and a few meters outside microbial mats, and outside the gas hydrate-bearing areas. For these cores, the interstitial sulfate ion (SO42−) concentrations, sedimentary total organic carbon, total sulfur (TS), and trace-element contents, and bulk carbonate minerals δ13C were analyzed. In the entire core samples below the microbial mats and in sediments below 10 cm depth outside microbial mats, TS contents were substantially higher than those at the reference site, and bulk carbonate minerals δ13C was strongly negative (−20‰ to −40‰). These results imply that the formations of sulfide minerals and authigenic carbonates occur in the anoxic sediments via microbial anaerobic oxidation of methane (AOM) coupled with sulfate reduction. Although a rapid and linear decrease of interstitial water SO42− concentration in the sediments collected inside microbial mats supports the occurrence of AOM, there is no fall in SO42− concentration outside microbial mats, which may be explained by the higher upward flow of methane gas inside than outside the mats. The AOM-impacted sediments at Sakata Knoll were characterized by enrichments in Mo and As that were well correlated with TS contents and the authigenic iron fraction, and by strong enrichment of authigenic Mo relative to U, indicated by higher Mo/U ratios than those of seawater. These observations suggest that Mo and As are effectively transferred from the water column to AOM-impacted sulfidic sediments at Sakata Knoll via precipitation of particulate iron–manganese oxyhydroxides in the water column that have adsorbed Mo and As.

Factors Controlling Brittleness of the Wufeng–Longmaxi Shale in the Yangtze Platform, South China: Insights from Geochemistry and Shale Composition

Brittleness index is an important parameter used to select favorable fracturing intervals during shale gas exploitation. Mineral compositions exert a significant influence on rock brittleness, and various grain assemblages can be deposited under different paleo-environmental conditions. The Wufeng–Longmaxi marine shale is a hot spot of shale gas exploitation in China. However, for the Wufeng–Longmaxi shale, the relationship between sedimentary environments, grain assemblages, and shale brittleness are rarely discussed. To address these problems, a series of experiments including petrographic observations, mineral compositions, rock mechanics, and major (trace) element analyses were carried out on shale samples from the Wufeng–Longmaxi formations. The Wufeng–Longmaxi formations are divided into upper Longmaxi formation (ULMX), lower Longmaxi formation (LLMX), and Wufeng formation (WF). The results show that WF shale samples deposited under a sedimentary environment with low terrigenous detrital influx, suboxic depositional water, and high paleoproductivity are characterized by the highest energy-based BI, compared to ULMX and LLMX shale samples. Dolomite in the samples grew in situ during early diagenesis. Redox conditions, terrigenous detrital flux control authigenic carbonate contents, and the oxic depositional water of the ULMX formation restrain carbonate mineral formation. In contrast, the suboxic sedimentary environment of the LLMX and WF allowed the generation of carbonate minerals. Compared with the WF, the larger terrigenous detrital flux of LLMX formation do favor more authigenic carbonate formation. The paleoproductivity affects biogenic quartz generation, and abundant biogenic quartz, primarily occurring in the form of micron-size microcrystalline quartz in the shale matrix, and can connect the silt-size brittle minerals to form a mechanical framework, which makes a strong positive contribution to shale brittleness. Terrigenous detrital influx decreases brittleness because the weakening effect of terrigenous clay minerals on shale brittleness is greater than the strengthening effect of terrigenous brittle minerals on shale brittleness.