Deltaic and Coastal Sediments as Recorders of Mediterranean Regional Climate and Human Impact Over the Past Three Millennia

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Biomineralizations induced by sulfur disproportionating bacteria from marine hydrothermal vents

International audienceSulfur is an important component of the marine hydrothermal vent ecosystem1, 2. Sulfur species are abundant in the hydrothermal fluid (H2S), the surrounding seawater (SO4 2-) and the sulfide minerals (pyrite, chalcopyrite) that form a large part of the structure of mature hydrothermal vents3. Due to its different oxidation states, sulfur species can be used as electron donors or terminal electron acceptor in microbial catabolisms. Sulfur disproportionation, has been shown to accelerate the sulfide mineral formation up to 105-fold with marine and freshwater bacteria4. In this work, we implemented an autecological approach to investigate the characteristics and kinetics of sulfide mineral formation in the presence of thermophilic sulfur-disproportionating bacteria from hydrothermal vents, grown under different culture conditions, in comparison with purely abiotic conditions. Incubations were carried out in batch and gas lift-reactors using a wide range of controls, and pH, temperature, cell density, catabolic end products, and minerals formed were monitored over time. In particular, we characterized the minerals formed by using X-ray diffraction, Raman spectroscopy and SEM-EDX and investigated the processes involved in microbially mediated mineral formation. Our results show that the rate of sulfidemineral formation is much faster in the presence of these thermophilic sulfur-disproportionating bacteriafrom hydrothermal vents than under abiotic conditions, and provide insights into the physiology of thesulfur-disproportionating bacteria studied

Biomineralizations induced by sulfur disproportionating bacteria from marine hydrothermal vents

International audienceSulfur is an important component of the marine hydrothermal vent ecosystem1, 2. Sulfur species are abundant in the hydrothermal fluid (H2S), the surrounding seawater (SO4 2-) and the sulfide minerals (pyrite, chalcopyrite) that form a large part of the structure of mature hydrothermal vents3. Due to its different oxidation states, sulfur species can be used as electron donors or terminal electron acceptor in microbial catabolisms. Sulfur disproportionation, has been shown to accelerate the sulfide mineral formation up to 105-fold with marine and freshwater bacteria4. In this work, we implemented an autecological approach to investigate the characteristics and kinetics of sulfide mineral formation in the presence of thermophilic sulfur-disproportionating bacteria from hydrothermal vents, grown under different culture conditions, in comparison with purely abiotic conditions. Incubations were carried out in batch and gas lift-reactors using a wide range of controls, and pH, temperature, cell density, catabolic end products, and minerals formed were monitored over time. In particular, we characterized the minerals formed by using X-ray diffraction, Raman spectroscopy and SEM-EDX and investigated the processes involved in microbially mediated mineral formation. Our results show that the rate of sulfidemineral formation is much faster in the presence of these thermophilic sulfur-disproportionating bacteriafrom hydrothermal vents than under abiotic conditions, and provide insights into the physiology of thesulfur-disproportionating bacteria studied

A Late Quaternary record of highstand shedding from an isolated carbonate platform (Juan de Nova, southern Indian Ocean)

International audienceA 27 m core collected on the sea floor near Juan de Nova island at 1,909 m depth in the SW Indian Ocean preserves a high-resolution record of carbonate sediment export to the deep sea over the past 1 Myr. Core chronology was established using calcareous nannofossil biostratigraphy and benthic foraminiferal δ 18 O. Throughout the core, preserved highstand intervals (MIS 1, 5, 7, 9, 11, 13, 15, 23 and 25) are marked by an increase in the aragonite content within the sediment. Aragonite is likely sourced from the nearby Juan de Nova carbonate platform ca 10 km to the south, and is interpreted as resulting from flooding of the platform top. Platform inundation allows carbonate muds to be winnowed from their original shallow-water environment of deposition, suspended in the water column, and redeposited onto the proximal slopes and within the basin. Sharp increases in aragonite content at the beginning of each highstand interval can be used to estimate the approximate sea-level range when platform flooding occurred; results show that the depth of the platform top has likely changed little over the past 1 Myr due to balanced aggradation and subsidence. Previously hypothesized large-scale aragonite dissolution cycles are evidenced by a disproportionally low aragonite increase during MIS 11. This study provides a new, exceptionally long record of highstand shedding, expanding the known occurrences of the process to the southern Indian Ocean and supporting its importance as a globally significant depositional mechanism that impacts deep-sea stratigraphic records

Influence of Clay-Containing Sediments on Methane Hydrate Formation: Impacts on Kinetic Behavior and Gas Storage Capacity

On Earth, natural hydrates are mostly encountered in clay‐rich sediments. Yet their formation processes in such matrices remain poorly understood. Achieving an in‐depth understanding of how methane hydrates accumulate on continental margins is key to accurately assess (1) their role in sustaining the development of some chemosynthetic communities at cold seeps, (2) their potential in terms of energy resources and geohazards, and (3) the fate of the methane releases, a powerful greenhouse gas, in this changing climate. This study investigated the formation of methane hydrates and their gas storage capacity in clay‐rich sediments. A set of hydrate experiments were performed in matrices composed of sand, illite‐rich clay and montmorillonite‐rich clay at different proportions aiming to determine the role of mineralogy on hydrate formation processes. The experiments demonstrate that a clay content of 10% in a partially water saturated sand/clay mixture increases the induction time by ∼60%, irrespective of the nature of the clay used. The increase in water saturation in the two matrices promotes hydrate formation. Micro‐Raman spectroscopic analyses reveal that increasing the clay content leads to a decrease in the hydrate small‐cage occupancy, with an impact on the storage capacity. Finally, the analyses of collected natural samples from the Black Sea (off Romania) enable us to estimate the gas storage capacity of the deposit. Our estimates is different from previous ones, and supports the importance of coupling multiscale properties, from the microscale to the geological scale, to accurately assess the total amount of methane hosts in hydrate deposits worldwide

Publisher Correction: Formation of carbonate chimneys in the Mediterranean Sea linked to deep-water oxygen depletion

International audienc

Biogeochemical insights into microbe–mineral–fluid interactions in hydrothermal chimneys using enrichment culture

International audienceActive hydrothermal chimneys host diverse microbial communities exhibiting various metabolisms including those involved in various biogeochemical cycles. To investigate microbe-mineral-fluid interactions in hydrothermal chimney and the driver of microbial diversity, a cultural approach using a gas-lift bioreactor was chosen. An enrichment culture was performed using crushed active chimney sample as inoculum and diluted hydrothermal fluid from the same vent as culture medium. Daily sampling provided time-series access to active microbial diversity and medium composition. Active archaeal and bacterial communities consisted mainly of sulfur, sulfate and iron reducers and hydrogen oxidizers with the detection of Thermococcus, Archaeoglobus, Geoglobus, Sulfurimonas and Thermotoga sequences. The simultaneous presence of active Geoglobus sp. and Archaeoglobus sp. argues against competition for available carbon sources and electron donors between sulfate and iron reducers at high temperature. This approach allowed the cultivation of microbial populations that were under-represented in the initial environmental sample. The microbial communities are heterogeneously distributed within the gas-lift bioreactor; it is unlikely that bulk mineralogy or fluid chemistry is the drivers of microbial community structure. Instead, we propose that micro-environmental niche characteristics, created by the interaction between the mineral grains and the fluid chemistry, are the main drivers of microbial diversity in natural systems

Coupled molybdenum, iron and uranium stable isotopes as oceanic paleoredox proxies during the Paleoproterozoic Shunga Event

The Paleoproterozoic Era was a time of remarkable importance in the redox evolution of Earth's atmosphere and oceans. Here, we present a multi-proxy study of Mo, U and Fe isotopes together with Fe speciation of black shales and siltstones from the upper Zaonega Formation of the Onega Basin in Karelia. We attempt to better understand oceanic redox conditions during the 2.05 Ga Shunga Event as the next step following the Great Oxidation Event (GOE) and the Lomagundi carbon isotope excursion Event. A cautious examination of the Fe speciation data shows that the studied section was deposited under dominantly euxinic conditions (anoxic and sulfidic) and that the lower part of the section experienced metamorphism through which pyrite was altered to pyrrhotite. During this episode, the system was closed with respect to Fe but not sulfur. The Mo and U isotopic compositions (corrected for detrital input) were not affected by the metamorphism and loss of S and are fairly uniform throughout the entire section. The Fe isotope compositions are exceptionally heavy in the lower part of the section (up to δ^(56)Fe_(IRMM-14) = 0.83‰) and become lighter towards the upper intervals, which also show significant [Mo] and [U] enrichments. We suggest that this pattern reflects changes in the position of the deposition site relative to the redox structure of the water column. The upper part was deposited within a locally euxinic portion of the basin where H_2S availability was highest, removal of Mo and U was more efficient and precipitated pyrite captured relatively non-fractionated dissolved Fe. In other words, quantitative uptake of Fe was favored. In contrast, the lower interval was deposited on the lower margin of a euxinic wedge where H_2S availability was lower, and removal of Mo and U was less efficient. Pyrite precipitation in this part of the water column reflected a more fractionated dissolved Fe reservoir due to more protracted, non-quantitative Fe uptake because of less efficient pyrite formation under lower sulfide conditions and greater access to the large oceanic pool of Fe. The U isotopic signal was corrected for detrital contribution giving compositions similar to the riverine input and suggesting that co-precipitation into carbonates was the main process of U removal at this time. We estimate the Mo isotope composition of the contemporaneous ocean to be δ^(98)Mo_(SW) = 0.85 ± 0.21‰. This is the lowest value yet reported for the Proterozoic ocean, suggesting that the oceanic Mo cycle was dominated by euxinic and anoxic sinks with negligible Mo removal into oxic environments. Recent studies have proposed a sharp increase in ocean–atmosphere oxygen levels during the Lomagundi Event followed by a dramatic crash. Our results from black shales of the 2.05 Ga Shunga Event are consistent with a post-Lomagundi decrease in biospheric oxygen levels

Effects of postglacial seawater intrusion on sediment geochemical characteristics in the Romanian sector of the Black Sea

Highlights • Geochemical analyses highlight multiple diagenesis processes occurring in the sediment. • Intense methane seepages and organic matter degradation contribute to the sulfate reduction. • Chemical of dissolved and mineral iron species indicate that iron is associated with clay minerals. • In response to seawater intrusion, ion exchange, dissolution and reverse weathering reactions change the composition of clay constituting the sediment. Abstract Pore water and sediment geochemistry in the western Black Sea were investigated on long Calypso piston core samples. Using this type of coring device facilitates the recovery of the thick sediment record necessary to analyze transport-reaction processes in response to the postglacial sea-level rise and intrusion of Mediterranean salt water 9 ka ago, and thus, to better characterize key biogeochemical processes and process changes in response to the shift from lacustrine to marine bottom water composition. Complementary data indicate that organic matter degradation occurs in the upper 15 m of the sediment column. However, sulfate reduction coupled with Anaerobic Methane Oxidation (AOM) is the dominant electron-accepting process and characterized by a shallow Sulfate Methane Transition Zone (SMTZ). Net silica dissolution, total alkalinity (TA) maxima and carbonate peaks are found at shallow depths. Pore water profiles clearly show the uptake of K+, Mg2+ and Na + by, and release of Ca2+ and Sr2+ from the heterogeneous lacustrine sediments, which is likely controlled by chemical reactions of silicate minerals and changes in clay mineral composition. Iron (Fe2+) and manganese (Mn2+) maxima largely coincide with Ca2+ peaks and suggest a close link between Fe2+, Mn2+ and Ca2+ release. We hypothesize that the Fe2+ maxima below the SMTZ result from deep Fe3+ reduction linked to organic matter degradation, either driven by DOC escaping from the shallow sulfate reduction zone or slow degradation of recalcitrant POC. The chemical analysis of dissolved and solid iron species indicates that iron is essentially associated with clay minerals, which suggests that microbial iron reduction is influenced by clay mineral composition and bioavailability of clay mineral-bound Fe(III). Overall, our study suggests that postglacial seawater intrusion plays a major role in shaping redox zonation and geochemical profiles in the lacustrine sediments of the Late Quaternary