Environmental Proxies for Future Mars Sample Return: Novel Terrestrial Analogues in Alkaline Lakes

Introduction: Sample return is an on-going priority for the planetary science community. The Perseverance rover is currently caching samples from Jezero crater for potential return to Earth to be studied in terrestrial laboratories. Samples returned for study in a fully equipped terrestrial laboratory provide unprecedented opportunity to analyse Martian rocks for biosignatures and paleoenvironmental markers in the search and characterization of ancient habitability and life. Before samples are brought back to Earth for laboratory analysis, it is necessary to establish a clear framework for sample selection and analysis. Tremendous strides in proxy development have opened windows to reconstructing past microbial ecology, pH, and alkalinity on Earth. Ultimately, we hope to extend these advances to Martian samples; however, such efforts require a new generation of paleo-proxy validation and calibration in suitable terrestrial analogues. In this study, we combine stable isotope and lipid biomarker analysis within a well constrained basin and climate system in the Eocene Green River Formation (GRF), USA.The Green River Formation: Alkaline lakes are a prime target for astrobiological exploration as they are among the most biologically productive ecosystems on Earth. The paleolakes that formed the GRF may have reached pH values of up to 11 [1]. Further, paleolake evolution can be linked to the rise and decline of the Early Eocene Climatic Optimum (EECO), around 52.6-50.1 Ma [2], thus establishing a link between global climate and regional environments with possible relevance to interpretations of ancient Mars. Post the hyperthermal regime, the lake basin was increasingly restricted and gradually infilled, becoming more hypersaline as the lakes shallowed.Proxies: This project explores the expression of each climatic phase of lake evolution through environmental proxies. We use 1) δ15N values as a proxy for pH, 2) lipid δ2H and carbonate δ13C and δ18O as a proxies for salinity, 3) the gammacerane index to determine the extent of water-column stratification, and 4) the pristane/phytane ratio (Pr/Ph) as a proxy for redox conditions.The relative abundance of ammonium (NH4+) and ammonia (NH3) is highly pH dependent. Ammonium is stable at circumneutral pH, whereas ammonia is unstable at pH < 8 [3]. Lighter 14N preferentially enters volatile NH3, permitting it to be favourably removed from a high-pH aqueous system through degassing, which is accelerated as salinity increases. Therefore, a trend of increasing δ15N enrichment in the rock record is interpreted as a proxy for rising pH, and vice-versa.Methods: The Utah Geological Survey provided core samples from the wells PR 15-7C and Skyline 16. Rock samples were powdered for isotope and biomarker analysis. Carbonate δ18O and δ13C were measured through a CNSOH isotope ratio analyser. δ15N and organic δ13C were measured using isotope ratio mass spectrometry at the University of St. Andrews. Total lipid extracts were extracted from rock powder using an accelerated solvent extractor. The extracts were run through silica gel columns to separate hydrocarbon fractions using solvents of varying polarities. Pristane, phytane and gammacerane were then identified, and their peaks measured through gas chromatograph-mass spectroscopy.Results and Discussion: Before and during the EECO, the GRF in the eastern Uinta Basin was characterized by high energy fluvio-deltaic facies. The isotope record shows two cycles of rising and falling δ15N, interpreted as fluctuating pH levels. The presence of gammacerane implies a stratified water-column, supported by low Pr/Ph values. After the peak EECO hyperthermal regime, Lake Uinta deepened and became more alkaline and reducing, as shown by higher δ15N and lower Pr/Ph.The δ13Ccarb, δ18Ocarb, and δ2H records correlate with one another, strengthening their use as evaporation and salinity proxies; however, principal component analysis shows no correlation with δ15N. This lack of relationship may indicate either a decoupling of salinity from pH during this time of fluctuating lake properties or second-order biogeochemical controls on these proxies. The possibility of decoupling pH from salinity and total alkalinity from samples returned from Martian paleolakes will support more accurate environmental interpretations and specifically climate models that may help us understand early Mars habitability. For example, high pH (suggested by high δ15N values) place upper limits on pCO2, while circumneutral pH coupled with high alkalinity (via rock weathering) would suggest that high atmospheric pCO2 was acting as a buffer [4]. Further work is directed at disentangling these complexities and optimizing the utility of stable isotopes as paleoenvironmental indicators.Conclusions: δ15N can be used as a proxy for pH in ancient rock settings. When used with proxies for salinity — e.g., lipid δ2H, δ13C, and δ18O — the data can be used to coupling/decoupling between pH and alkalinity and salinity during climate-driven change in lake conditions. When applied to returned samples from Mars, more comprehensive interpretations of the poorly known atmospheric pCO2 conditions at the time of lake sedimentation should be possible.Future Work: Our future work will explore boron isotope measurements of GRF samples as a complement to C, O, H, and N. Boron is used as a proxy for past oceanic pH through the isotopic fractionation between boric acid B(OH)3 and borate B(OH)4- and is underexplored as a proxy for modern and ancient saline lacustrine environments.References: [1] Tuttle, M. L., & Goldhaber, M. B. (1993). Sedimentary sulfur geochemistry of the Paleogene Green River Formation, western USA: Implications for interpreting depositional and diagenetic processes in saline alkaline lakes. Geochimica et Cosmochimica Acta, 57, 3023–3039.[2] Birgenheier, L. P., Vanden Berg, M., Plink-Björklund, P., Gall, R. D., Rosencrans, E., Rosenberg, M. J., Toms, L. C., & Morris, J. (2019). Climate impact on fluvial-lake system evolution, Eocene Green River Formation, Uinta Basin, Utah, USA. GSA Bulletin, 132(3–4), 562–587.https://doi.org/10.1130/B31808.1[3] Li, L., Lollar, B. S., Li, H., Wortmann, U. G., & Lacrampe-Couloume, G. (2012). Ammonium stability and nitrogen isotope fractionations for NH4+-NH 3(aq)-NH 3(gas) systems at 20-70°C and pH of 2-13: Applications to habitability and nitrogen cycling in low-temperature hydrothermal systems. Geochimica et Cosmochimica Acta, 84. https://doi.org/10.1016/j.gca.2012.01.040[4] Stüeken, E. E., Tino, C., Arp, G., Jung, D., & Lyons, T. W. (2020). Nitrogen isotope ratios trace high-pH conditions in a terrestrial Mars analog site. Science Advances, 6(9). https://doi.org/10.1126/sciadv.aay344

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