Biosignatures in modern sulfates: texture, composition and depositional environments of gypsum deposits at Guerrero Negro, Baja, Mexico

Details Biosignatures in modern sulfates: texture, composition and depositional environments of gypsum deposits at Guerrero Negro, Baja, Mexico Biosignatures in modern sulfates: texture, composition and depositional environments of gypsum deposits at Guerrero Negro, Baja, Mexico

Dec 2007

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2007agufm.b31d0622v&link_type=abstract

American Geophysical Union, Fall Meeting 2007, abstract #B31D-0622

Biology

0406 Astrobiology And Extraterrestrial Materials, 0419 Biomineralization, 0424 Biosignatures And Proxies

Scientific paper

Gypsum (CaSO4·H2O) is an important phase in biogeochemistry and sedimentology as a mineral sink for sulfur, a paleoclimatic indicator, and an endolithic niche for phototrophic and chemotrophic bacteria. Sulfate deposits are also important targets of exploration for evidence of habitable environments and life on Mars. Gypsum deposits from a range of sedimentary environments at the Guerrero Negro crystallizer ponds and sabkha settings were investigated for microscale structure and composition to differentiate fabrics formed under microbial influence from those formed under abiogenic conditions. Sub-sedimentary gypsum forms in sabkha environments as mm to cm scale selenite discs (termed bird beak gypsum; Warren, 2006) and selenite disc aggregates. Selenite discs and other sub-sedimentary gypsum are characterized by a sinuous axial microtexture and poikilitically enclosed detrital particles. Sub-aqueous gypsum forms as cements, granules (termed gypsooids), and massive botryoidal crusts that line the sediment water interface and margins of managed crystallizer ponds and natural anchialine pools. Sub-aqueous gypsum exhibits a wide range of textures and mineral/biofilm associations that include amorphous to euhedral, tabular, needle and lensoidal morphologies. Elemental sulfur forms rinds on prismatic, growth aligned gypsum twins and reticulate magnesian carbonate is interspersed with both twinned crystals and rosette aggregates in stratified sub-aqueous environments. Intracrystalline biofilms and cell material was observed in association with nearly all sub-aqueous morphologies but only scarce evidence has been found for intercrystalline microbial communities. Columnar microbial communities living in anchialine pools were found to host precipitation of mm scale gypsum granules in their EPS matrix. Fine scale gypsum textures are unlikely to persist through diagenetic alteration, but understanding their primary associations with sulfur and carbonates is necessary for interpreting sulfates or their replacement phases in the ancient record.

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