Interpreting the paleoecosystems of ancient microbialites relies on our understanding of how mode... more Interpreting the paleoecosystems of ancient microbialites relies on our understanding of how modern microbialites form in relation with the bio-physico-chemical conditions of their environment. In this study, we investigated the formation of modern carbonate microbialites in the hydrothermal system of La Salsa in Laguna Pastos Grandes (Bolivia), which spans a wide range of physicochemical conditions and associated microbial communities. By combining dissolved inorganic carbon (DIC) isotope mass balance modeling, analysis of carbonates solubility diagram, and imaging of the microorganisms–mineral assemblages within microbial mats, we found that several modes of carbonate precipitation dominate in distinct portions of the hydrothermal system. (1) In high-[DIC] waters, undersaturated to slightly saturated with respect to calcite, cyanobacterial calcification is promoted by CO2 degassing and photosynthetic activity within the microbial mats. (2) In alkaline waters undergoing sustained e...
Sedimentary records of superheavy pyrites in Phanerozoic and Proterozoic successions (i.e., extre... more Sedimentary records of superheavy pyrites in Phanerozoic and Proterozoic successions (i.e., extremely positive δ34 Spyrite values together with higher δ34 Spyrite than coeval δ34 SCAS ) are mostly interpreted as resulting either from secondary postdepositional processes or from multiple redox reactions between sulfate and sulfide in stratified sulfate-poor environments. We report here the first observation of strongly positive δ34 S values for both dissolved sulfate and sulfide (average δ34 Sdiss.sulfate value of 34.6‰ and δ34 Sdiss.sulfide values of 36.7‰) compared to the present-day seawater δ34 Sdiss .sulfate (~21‰), with a negative apparent fractionation between sulfate and sulfide (∆34 Sdiss.sulfate-diss.sulfide ~ -2.1 ± 1.4‰), in the sulfate-poor (<3 mm) modern thalassohaline lacustrine system Dziani Dzaha (Mayotte, Indian Ocean). Overall, surface sediments faithfully record the water column isotopic signatures including a mainly negative ∆34 Ssed.sulfate-sed.sulfide (-4.98 ± 4.5‰), corresponding to the definition of superheavy pyrite documented in the rock record. We propose that in the Dziani Dzaha this superheavy pyrite signature results from a two-stage evolution of the sulfur biogeochemical cycle. In a first stage, the sulfur cycle would have been dominated by sulfate from initially sulfate-rich marine waters. Overtime, Raleigh distillation by microbial sulfate reduction coupled with sulfide burial in the sediment would have progressively enriched in 34 S the water column residual sulfate. In a second still active stage, quantitative sulfate reduction not only occurs below the halocline during stratified periods but also in the whole water column during fully anoxic episodes. Sulfates are then regenerated by partial oxidation of sulfides as the oxic-anoxic interface moves downward. These results demonstrate that the atypical superheavy pyrite isotope signature does not necessarily require postdepositional or secondary oxidative processes and can result from primary processes in restricted sulfate-poor and highly productive environments analogous to the Dziani Dzaha.
Le rapport n/c et le 1 5n de la matiere organique ont ete mesures sur des series naturelles de ma... more Le rapport n/c et le 1 5n de la matiere organique ont ete mesures sur des series naturelles de maturation du kerogene comprises entre les stades charbon et graphite. Pour cela, un protocole analytique a ete developpe pour permettre l'analyse de l'azote des kerogenes de n/c inferieur a 0,5%. Les resultats montrent que les graphites bien cristallises ne contiennent pas plus de 3 ppm d'azote, que ce dernier est elimine de la matiere organique sans variation du 1 5n au cours de la maturation, et que le 1 5n organique est donc un indicateur paleo-environnemental. La fixation de l'azote, produit par la matiere organique, sous forme de nh 4 + substitue au k + dans l'illite, a ete etudie dans le cas des meta-anthracites de pennsylvanie. L'ammonium fixe ( 1 5n-nh 4 + compris entre 3 et 13&) est d'autant plus enrichi par rapport a l'azote organique ( 1 5n organique compris entre 3 et 5&), que le taux de substitution du nh 4 + au k + dans l'illite est import...
In continental volcanic settings, abundant carbonate precipitation can occur with atypical facies... more In continental volcanic settings, abundant carbonate precipitation can occur with atypical facies compared with those of marine settings. The (bio-)chemical processes responsible for their development and early diagenesis are typically complex and not fully understood. In the Bolivian Altiplano, Laguna Pastos Grandes hosts a 40-km2 carbonate platform composed of a great diversity of carbonate facies mainly fed by thermal springs. It provides for a spectacular natural laboratory for studying carbonate precipitation in a continental province dominated by volcanism. As a first step toward understanding the processes responsible for the precipitation of carbonates in Laguna Pastos Grandes, we will focus on characterizing the fluids (liquid and gas) that feed the laguna. The content of major elements and stable isotope compositions (δ2H-δ18O, δ37Cl, δ7Li, δ11B and 87Sr/86Sr) of brine and freshwater inputs were investigated to trace the origin of both water and solutes in the laguna. The ...
Interpreting the paleoecosystems of ancient microbialites relies on our understanding of how mode... more Interpreting the paleoecosystems of ancient microbialites relies on our understanding of how modern microbialites form in relation with the bio-physico-chemical conditions of their environment. In this study, we investigated the formation of modern carbonate microbialites in the hydrothermal system of La Salsa in Laguna Pastos Grandes (Bolivia), which spans a wide range of physicochemical conditions and associated microbial communities. By combining dissolved inorganic carbon (DIC) isotope mass balance modeling, analysis of carbonates solubility diagram, and imaging of the microorganisms–mineral assemblages within microbial mats, we found that several modes of carbonate precipitation dominate in distinct portions of the hydrothermal system. (1) In high-[DIC] waters, undersaturated to slightly saturated with respect to calcite, cyanobacterial calcification is promoted by CO2 degassing and photosynthetic activity within the microbial mats. (2) In alkaline waters undergoing sustained e...
Sedimentary records of superheavy pyrites in Phanerozoic and Proterozoic successions (i.e., extre... more Sedimentary records of superheavy pyrites in Phanerozoic and Proterozoic successions (i.e., extremely positive δ34 Spyrite values together with higher δ34 Spyrite than coeval δ34 SCAS ) are mostly interpreted as resulting either from secondary postdepositional processes or from multiple redox reactions between sulfate and sulfide in stratified sulfate-poor environments. We report here the first observation of strongly positive δ34 S values for both dissolved sulfate and sulfide (average δ34 Sdiss.sulfate value of 34.6‰ and δ34 Sdiss.sulfide values of 36.7‰) compared to the present-day seawater δ34 Sdiss .sulfate (~21‰), with a negative apparent fractionation between sulfate and sulfide (∆34 Sdiss.sulfate-diss.sulfide ~ -2.1 ± 1.4‰), in the sulfate-poor (<3 mm) modern thalassohaline lacustrine system Dziani Dzaha (Mayotte, Indian Ocean). Overall, surface sediments faithfully record the water column isotopic signatures including a mainly negative ∆34 Ssed.sulfate-sed.sulfide (-4.98 ± 4.5‰), corresponding to the definition of superheavy pyrite documented in the rock record. We propose that in the Dziani Dzaha this superheavy pyrite signature results from a two-stage evolution of the sulfur biogeochemical cycle. In a first stage, the sulfur cycle would have been dominated by sulfate from initially sulfate-rich marine waters. Overtime, Raleigh distillation by microbial sulfate reduction coupled with sulfide burial in the sediment would have progressively enriched in 34 S the water column residual sulfate. In a second still active stage, quantitative sulfate reduction not only occurs below the halocline during stratified periods but also in the whole water column during fully anoxic episodes. Sulfates are then regenerated by partial oxidation of sulfides as the oxic-anoxic interface moves downward. These results demonstrate that the atypical superheavy pyrite isotope signature does not necessarily require postdepositional or secondary oxidative processes and can result from primary processes in restricted sulfate-poor and highly productive environments analogous to the Dziani Dzaha.
Le rapport n/c et le 1 5n de la matiere organique ont ete mesures sur des series naturelles de ma... more Le rapport n/c et le 1 5n de la matiere organique ont ete mesures sur des series naturelles de maturation du kerogene comprises entre les stades charbon et graphite. Pour cela, un protocole analytique a ete developpe pour permettre l'analyse de l'azote des kerogenes de n/c inferieur a 0,5%. Les resultats montrent que les graphites bien cristallises ne contiennent pas plus de 3 ppm d'azote, que ce dernier est elimine de la matiere organique sans variation du 1 5n au cours de la maturation, et que le 1 5n organique est donc un indicateur paleo-environnemental. La fixation de l'azote, produit par la matiere organique, sous forme de nh 4 + substitue au k + dans l'illite, a ete etudie dans le cas des meta-anthracites de pennsylvanie. L'ammonium fixe ( 1 5n-nh 4 + compris entre 3 et 13&) est d'autant plus enrichi par rapport a l'azote organique ( 1 5n organique compris entre 3 et 5&), que le taux de substitution du nh 4 + au k + dans l'illite est import...
In continental volcanic settings, abundant carbonate precipitation can occur with atypical facies... more In continental volcanic settings, abundant carbonate precipitation can occur with atypical facies compared with those of marine settings. The (bio-)chemical processes responsible for their development and early diagenesis are typically complex and not fully understood. In the Bolivian Altiplano, Laguna Pastos Grandes hosts a 40-km2 carbonate platform composed of a great diversity of carbonate facies mainly fed by thermal springs. It provides for a spectacular natural laboratory for studying carbonate precipitation in a continental province dominated by volcanism. As a first step toward understanding the processes responsible for the precipitation of carbonates in Laguna Pastos Grandes, we will focus on characterizing the fluids (liquid and gas) that feed the laguna. The content of major elements and stable isotope compositions (δ2H-δ18O, δ37Cl, δ7Li, δ11B and 87Sr/86Sr) of brine and freshwater inputs were investigated to trace the origin of both water and solutes in the laguna. The ...
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