mineralogical and geochemical analyses of Antarctic lake sediments:: A study of reflectance and Mossbauer spectroscopy and C, N, and S isotopes with applications for remote sensing on Mars

We analyzed lake-bottom sediments from the Dry Valleys region of Antarctica to study the influence of water chemistry on the mineralogy and geochemistry of these sediments, as well as to evaluate techniques for remote spectral identification of potential biomarker minerals on Mars. Lakes from the Dry Valleys region of Antarctica have been investigated as possible analogs for extinct lake environments on early Mars. Sediment cores were collected in the present study from perennially ice-covered Lake Hoare in the Taylor Valley. These sediments were taken from a core in an oxic region of the lake and another core in an anoxic zone. Differences between the two cores were observed in the sediment color, Fe(II)/Fe(III) ratio, the presence of pyrite, the abundance of Fe, S, and some trace elements, and the C, N, and S isotope fractionation patterns. The results of visible-infrared reflectance spectroscopy (0.3-25 mum), Mossbauer spectroscopy (77 and 4 K), and X-ray diffraction are combined to determine the mineralogy and composition of these samples. The sediments are dominated by plagioclase, K-feldspar, quartz, and pyroxene. Algal mats grow on the bottom of the lake and organic material has been found throughout the cores. Calcite is abundant in some layers of the sediment core from the shallow, oxic region, and pyrite is abundant in the upper sediment layers of the core from the deep, anoxic region of Lake Hoare. Analysis of the spectroscopic features due to organics and carbonates with respect to the abundance of organic C and carbonate contents was performed in order to select optimal spectral bands for remote identification of these components in planetary regoliths. Carbonate bands near 4 and 6.8 mum (similar to 2500 and 1500 cm(-1)) were detected for carbonate abundances as low as 0.1 Wt% CO2. Organic features at 3.38, 3.42, and 3.51 mum (2960, 2925, and 2850 cm(-1)) were detected for organic C abundances as low as 0.06 wt% C. The delta C-3 and delta N-15 trends show a more complex organic history for the anoxic region sediments than for the oxic region sediments. The biogenic pyrite found in the core from the anoxic zone is associated with depleted delta S-34 values and high organic C levels and could be used as a potential biomarker mineral for paleolakes on Mars. Copyright (C) 2001 Elsevier Science Ltd.