Exobiology and Evolutionary Biology

Distinguishing Biotic and Abiotic Nitrous Oxide Sources in Mars Analog Brines, Sediments, and Soils in the McMurdo Dry Valleys, Antarctica.

PI: Samantha Joye

Nitrous oxide is an atmospheric trace gas. On Earth, biological processes, mainly nitrification and denitrification, are thought to dominate nitrous oxide production in the environment. We recently measured large areal fluxes of nitrous oxide to the atmosphere in a cold, hypersaline environment, Don Juan Pond, Wright Valley, Antarctica. Stable isotopic measurements showed that this nitrous oxide had a unique depletion of heavy nitrogen (15N) in both the internal and external positions of the linear nitrous oxide molecule. This pattern of 15N distribution resulted in a site preference that was low and sometimes negative (down to -45 per mil). In contrast, biological processes generate a positive site preference in nitrous oxide. Laboratory experiments of Don Juan Pond samples revealed an abiotic production mechanism, whereby nitrous oxide was produced during reaction of nitrate-rich brine with a variety of Fe(II)-containing minerals. The process of chemodenitrification we documented represents a novel mechanism of nitrous oxide production that could be important in other Earth habitats and on Mars. In this project, we will study the distribution of chemodenitrification in nitrate-rich, cold, desert habitats in Antarctica to evaluate whether this process is widespread and environmentally relevant. We will describe, in detail, the kinetics of chemodenitrification by evaluating rates of nitrate reduction/nitrous oxide production fueled by Fe(II) derived from pure mineral phases (in the laboratory) or by Fe(II) contained in geologic field samples from Antarctic sites. We will characterize the isotopic signature of chemodenitrification-derived nitrous oxide. At selected field sites, we will compare the magnitude of nitrous oxide fluxes and the isotopic signatures of nitrous oxide at sites where biological vs. abiotic production mechanisms dominate. We will quantify the importance of biological and abiotic processes explicitly. By determining the isotopic composition of reactants (nitrate, nitrite) and product (nitrous oxide), the production mechanism (biotic or abiotic), and the environmental factors that influence the production rate (e.g. temperature), we will develop an index to describe the isotopic composition of nitrous oxide. This index can be used to evaluate whether nitrous oxide in a given environment derived from biological or abiotic processes. The brine-rock reactions we propose to study could return nitrogen oxides in Martian soils and brines to the atmosphere, providing a potentially dynamic and unexpected link between the geosphere and atmosphere. The isotopic composition of this nitrous oxide may provide a new and robust way to distinguish nitrous oxide arising from biological processes from that arising from chemodenitrification.

This work is relevant to the study of Astrobiology, Exobiology, and Evolutionary Biology as it will advance our understanding of nitrogen biogeochemical cycling on both Earth and Mars. Our proposed work will: 1) identify novel sources of an atmospheric trace gas, nitrous oxide; 2) advance the knowledge of environmental cycling of a key elemental building block of life, nitrogen; 3) identify novel geosphere-brine reactions that facilitate nitrogen recycling between the lithosphere and atmosphere; and, 4) evaluate the utility of nitrogen stable isotopes in nitrous oxide as an unambiguous biosignature for life.