218-13 Life in the Brine: In-situ modification of fluid inclusion gases by microbial activity

Abstract:

Fluid inclusions are excellent archives for understanding ancient aqueous environments and constraining Earth’s evolving atmospheric composition. These microscopic volumes of trapped air and parent brine preserve the chemical signature of the mineral’s formational environment. Primary inclusions in lab-grown halites accurately record atmospheric gases after accounting for gas partitioning between aqueous and gaseous phases. However, these systems are simplified and do not account for the chemical and biological complexity of natural evaporitic environments where microbial processes modify dissolved gases, carbon isotopes, and the parent brine before mineral precipitation. 

We hypothesize that microbial activity changes seawater chemistry to the point that measurable differences will be observed between gases in fluid inclusions grown from natural seawater (with its full complement of microbial life) and those grown in synthetic solutions. To test this, seawater collected from the Atlantic Shelf off the coast of Delaware was divided into filtered and unfiltered aliquots. Halite was precipitated through controlled evaporation experiments, and the resulting crystals were examined petrographically to characterize growth and identify primary inclusions. Gas compositions of the parent solutions as well as those liberated from crushed halite were measured using quadrupole mass spectrometry to evaluate potential differences between solution, atmospheric, and inclusions gases. Raman spectroscopy is used to characterize fluid inclusion contents and assess if microbes or biological signatures are visible or preserved.

We will discuss the observed and expected shifts in dissolved and entrapped gases as a result of different microbial metabolisms, especially those associated with respiration, photosynthesis, methanogenesis, and related geochemical processes. Microbial activity is also expected to fractionate carbon isotope ratios, potentially altering the isotopic signatures within inclusions. This study will improve the fidelity of halite as a paleoenvironmental archive and tool for atmospheric reconstruction by quantifying the extent to which biological activity modifies fluid inclusion gas compositions. These results will provide an important framework for interpreting fluid inclusion gas records from natural evaporite minerals, and for distinguishing atmospheric signals from biosignatures. 

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