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Microbe-mineral interactions in travertines and tufas: unraveling paleoenvironmental and microbiological signatures through isotope and trace element geochemistry

Rodolfo Agustín Mors

University of Southern California

The recent discovery of putative hot spring deposits on Mars and in the early rock record on Earth has reinvigorated the study of the possible role of life in travertine and tufa formation—that is, can we discern whether life was involved with the formation of such rocks that were traditionally interpreted as simple physiochemical deposits? Differentiating morphological biosignatures from abiogenic mineral assemblages is fundamental in the interpretation of the evolution of life in the geologic rock record and for the search for life on other planets. Furthermore, the lack of sedimentary models that can be useful for the exploration of world-class oil resources related to travertine-like carbonate systems (e.g., the Pre-Salt of the offshore basins of Brazil) also highlights the economic importance of understanding the formation of these deposits. The terms travertine and tufa usually refer to non-marine carbonate deposits developed in or near hot spring systems, rivers, lakes and caves. Their mineral composition, mainly calcite and/or aragonite (CaCO3), constitute a record of the water chemistry, hydrology and climate. Furthermore, the diverse range of fabrics and textures they present, can be related to a set of physio-chemical and biological processes that are still difficult to discern based on field work and traditional optical petrographic microscopy. Integrating high-resolution textural and geochemical analysis can help to differentiate the physicochemical and microbiological processes that control carbonate precipitation and the environmental signatures recorded in the carbonate minerals. Here we propose an unprecedented opportunity to study an active hydrothermal system called Terma Los Hornos, where active hydrothermal deposits can be compared to nearby fossil examples from the same system, effectively providing a time series of rock units with which to investigate the formation and preservation of biosignatures over time. Preliminary work has focused on providing a good characterization of both the active and the fossil travertine-tufaceous system. In this project we want to complement these studies by providing a precise high-resolution tracking of similar petrographic, geochemical (e.g. stable isotopes, major and trace elements), and biogechemical (C-O-S) parameters of specific textures and fabrics from the recent carbonate precipitates to the ancient ones. In addition, paleotemperature studies using new isotope techniques such as carbonate clumped isotope thermometry will allow to reconstruct past temperature conditions during carbonate deposition and thus better constrain the original fluid chemistry. Through integrating new information with previous findings, we will have a deeper understanding of the environmental and microbial contributions in carbonate precipitation that can shed light into understanding the petrographic, textural and petrophysical properties of travertines and tufas. Given that hot springs are considered as potential astrobiological targets on Mars and other planets, understanding biosignature preservation in these systems is extremely important to understand the microbial biosphere sedimentary record.

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