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Mineralogical and Geochemical Analyses of Antarctic Lake Sediments

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Mineralogical and Geochemical Analyses of Antarctic Lake Sediments Geochimica et Cosmochimica Acta, Vol. 65, No. 17, pp. 2875–2897, 2001 Copyright © 2001 Elsevier Science Ltd Pergamon Printed in the USA. All rights reserved 0016-7037/01 $20.00 ϩ .00 PII S0016-7037(00)00651-2 Mineralogical and geochemical analyses of Antarctic lake sediments: A study of reflectance and Mo¨ssbauer spectroscopy and C, N, and S isotopes with applications for remote sensing on Mars 1, 2 3 4 5 2 JANICE L. BISHOP, *ANDRE´ LOUGEAR, JASON NEWTON, PETER T. DORAN, HEINZ FROESCHL, ALFRED X. TRAUTWEIN, 6 6 WILFRIED KORNER¨ , and CHRISTIAN KOEBERL 1SETI Institute/NASA-Ames Research Center, MS-239-4, Moffett Field, CA 94035, USA 2Institute of Physics, Medical University of Luebeck, Ratzeburger Allee 160, D-23538 Luebeck, Germany 3Earth and Marine Sciences C514, University of California, Santa Cruz, CA 95064, USA 4Earth and Environmental Sciences, University of Illinois at Chicago, 845 West Taylor Street, Chicago, IL 60607, USA 5Austrian Research Centers Seibersdorf, Chemical Analytics, Arsenal, Object 214, Faradaygasse 3, A-1030 Vienna, Austria 6Institute of Geochemistry, University of Vienna, Althanstrasse 14, A-1090 Vienna, Austria (Received March 7, 2000; accepted in revised form April 4, 2001) Abstract—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 ␮m), Mo¨ssbauer 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 ␮ ϳ Ϫ1 near 4 and 6.8 m( 2500 and 1500 cm ) were detected for carbonate abundances as low as 0.1 wt% CO2. Organic features at 3.38, 3.42, and 3.51 ␮m (2960, 2925, and 2850 cmϪ1) were detected for organic C abundances as low as 0.06 wt% C. The ␦13C and ␦15N 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 ␦34S values and high organic C levels and could be used as a potential biomarker mineral for paleolakes on Mars. Copyright © 2001 Elsevier Science Ltd 1. INTRODUCTION major element and trace element composition have been ob- served in the water and sediments from these oxic and The McMurdo Dry Valleys in Antarctica provide a unique anoxic regions (Green et al., 1986a,b, 1988; Bishop et al., opportunity for studying mineral formation and alteration pro- 1996). cesses in a closed and relatively pristine ecosystem. Water and Sediment cores from Lake Hoare are analyzed here to build sediments from the Antarctic Dry Valleys region have been on the results of previous studies (Green et al., 1986a,b, 1988; studied previously to gain information about relationships be- Bishop et al., 1996). Because of the differences in water chem- tween microorganisms and water chemistry (Green et al., istry and sediment composition found in these previous studies 1986b, 1988; Craig et al., 1992; Wharton et al., 1993), paleo- for regions of the lake above and below the oxic–anoxic bound- limnology (Doran et al., 1994a), chemical alteration, and sed- ary, the current study focused on the differences in chemistry imentation processes in perennially ice-covered lakes and cold and mineralogy between sediments from these two regions. deserts (Gibson et al., 1983; Nedell et al., 1987; Squyres et al., Visible to infrared reflectance spectra were measured of the 1991) and spectroscopic detection of minerals and organics in bulk and Ͻ125-␮m size fractions of these sediments. Chemical natural sediments (Bishop et al., 1996). Lake Hoare is partic- and mineralogical analyses were performed on the homoge- ularly interesting for biogeochemical study because of the algal nized Ͻ125-␮m material to ensure consistency among mea- mats at the bottom of the lake and the oxygen-rich (oxic) zone surements. A core particularly enriched in carbonate and or- above ϳ27 m and the oxygen-poor (anoxic) zone below ϳ27 m ganic material from the oxic region of the lake was selected for (Wharton et al., 1993; Andersen et al., 1998). Differences in study here to evaluate the spectral features for these species across a range of compositions. The current study includes *Author to whom correspondence should be addressed (jbishop@ Mo¨ssbauer spectroscopy to better characterize the iron-bearing mail.arc.nasa.gov). minerals and C, N, and S isotopic ratios to evaluate the influ- 2875 2876 J. L. Bishop et al. Fig. 1. Location of Lake Hoare in the Dry Valleys region of Antarctica. The sample cores were collected from the lake bottom at dive holes DH-2 and DH-4 in Lake Hoare. (Upper right inset) Study location with respect to the Antarctic continent. (Upper left inset) Taylor Valley, which contains Lake Hoare. ence of the water chemistry and biologic activity on the sedi- spectra of samples from three sediment cores from Lake Hoare ments. and found distinct compositional differences between sedi- Low-temperature Mo¨ssbauer spectroscopy has been effec- ments collected under the oxic and anoxic zones of the lake. tive in measuring Fe(II)/Fe(III) ratios and identifying iron- Higher calcite abundances were found in the sediments from bearing minerals in sediment cores (Drodt et al., 1997, 1998; the most shallow dive hole (DH-2, 15-m depth) (Bishop et al., Ko¨nig et al., 1997) and in hydrothermal vent systems (Wade et 1996), which is consistent with the findings of Green et al. al., 1999). This technique is applied here to look for differences (1988) that the lake water from the surface down to ϳ20min in the mineralogy and Fe oxidation state in sediments from the depth is supersaturated in calcite. Related trends in dissolved oxic and anoxic zones of the lake. CO2 and O2 have been observed in the Lake Hoare water column: the decreases in O2 level and the increases in CO2 1.1. Lake Hoare, Taylor Valley, Antarctica level with depth are thought to be due to sedimentation of organic material (Andersen et al., 1998). The McMurdo Dry Valleys along the western coast of the Ross Sea in Antarctica remain relatively ice-free because the local ablation rates exceed the annual snowfall. These dry 1.2. Importance of Sediment Analysis for Exobiology valleys and their ice-covered lakes have been the focus of previous studies as analogs for dry valleys on Mars (Gibson et Characterizing the relationships between microorganisms, al., 1983; Agresti et al., 1986; Squyres et al., 1991). An inter- water chemistry, and sediment composition in the Antarctic esting feature of Lake Hoare is the presence of microbial mats Dry Valleys will provide important information for interpreta- and organic material in the lake-bottom sediments (Nedell et tion of sediment processes on Mars. The vertical nutrient pro- al., 1987). Previous radiocarbon dates for Lake Hoare sedi- files observed in lake water and sediments are directly related ments indicate a sedimentation rate of 0.15 mm per year for to microbial activities (Nealson, 1997). Therefore, biologic sediments near dive hole 2 (oxic region) (Doran et al., 1999). activity may greatly influence the chemical environment of Green et al. (1988) studied the geochemistry of Lake Hoare sediments. Isotopic trends for C, N, and S are included in the and noted trends in cation and anion concentrations and se- current sediment study to facilitate identification of biologic lected major and minor elements in the lake water. Bishop et al. activity and geochemical cycles in the lake ecosystem. Methane (1996) analyzed the geochemistry, mineralogy, and reflectance was found in the lake water just above the surface sediments in Mineralogy and geochemistry of lake sediments from Antarctica 2877 the anoxic zone, suggesting the presence of methane-producing bacteria there (Andersen et al., 1998). Previous studies have shown that isotope ratios for C and N in lake water can provide information about biologic vs. abio- logic activity in the ice-covered lakes of the Antarctic Dry Valleys region (Doran et al., 1998; Lawrence and Hendy, 1989; Wharton et al., 1993). Analysis of ␦13C for carbonate and organic separates of Lake Hoare sediments in a study by Doran et al. (1994b) gave a range of 3.2 to 7.5‰ for the carbonates throughout the core (0–32 cm) and a range of Ϫ1.9 to Ϫ25.7‰ for the organic carbon.
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