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. A recent study has evaluated the Ant- arctic paleolakes as potential models for regions of extinct life on Mars (Doran et al., 1998). Carbon isotopic trends are re- ported for several lakes in the McMurdo Dry Valleys region, and sedimentation processes are investigated. Lacustrine sand mounds were found to contain abundant authigenic carbonate and organic matter and have good conditions for paleolimno- logical records. Doran et al. (1998) recommended evaluating deltas on Mars through surface images and searching for smaller lacustrine sand mounds for potential evidence of exo- biological activity.

1.3. Applications to Future Mars Missions

The Mars exploration rovers currently planned to launch in 2003 are based on the Athena rovers that were planned for the Mars surveyor missions (Squyres et al., 1998). These rovers will use a combination of instruments, including a thermal infrared emission spectrometer (known as a MINI-TES), an ␣-proton–X-ray spectrometer, and a Mo¨ssbauer spectrometer (Squyres et al., 2001) to characterize the surface and subsurface at two sites on Mars.

2. MATERIALS AND METHODS

2.1. Sample Collection and Preparation Fig. 2. Sediment stratigraphy for the sample cores showing relative The samples studied here are sediments from lake-bottom cores that particle size and the presence of organic material. Core E is from dive were collected through dive holes at DH-2 and DH-4 of Lake Hoare, as hole DH-2, which is located under an oxic region of Lake Hoare, and described in Nedell et al. (1987) and Wharton et al. (1993). A map of core H is from dive hole DH-4, located under an anoxic region of the the Dry Valleys region and Lake Hoare, indicating the locations of lake. these dive holes, is shown in Figure 1. Dive hole DH-2 is located ϳ190 m from the lake shore. The bottom of the lake is ϳ15 m below the surface of the ice near this dive hole, and the lake-bottom sediments here are in contact with oxic water. Dive hole DH-4 is farther from samples is given, whereas the range or central point for each sediment shore (ϳ300 m), where the lake is deeper (ϳ30 m) and extends into the layer is given for the other measurements. anoxic zone. Several cores have been retrieved from a distance of at The samples were alternately ground in a mechanical ball-mill least 5 m from each dive hole. The sediment cores were kept frozen grinder in air and dry-sieved to Ͻ125-␮m particle size to avoid excess until sampled in the laboratory. Two cores are evaluated in this study: grinding of the softer minerals. Each sample aliquot was ground com- core E is number 49 from DH-2 (oxic) and core H is number 13 from pletely until all of the material passed through the Ͻ125-␮m sieve to DH-4 (anoxic). ensure that the initial composition is represented in the homogenized The sediment cores were thawed in an argon atmosphere to select powders. samples for the Mo¨ssbauer measurements without oxidizing the iron- bearing minerals. Individual samples were removed from the core at 2.2. Analytical Techniques—Chemistry and Mineralogy one place for each layer and kept frozen until iron transmission Mo¨ss- bauer measurements were performed at 4.2 and 77 K. The cores were The chemical analyses were carried out on pressed powders with a then thawed to room temperature, and all other measurements were Philips PW2400 wavelength-dispersive X-ray spectrometer and follow- performed on these air-dried samples. The sediment layers in each core ing typical procedures for X-ray fluorescence (XRF). All chemical data were separated according to texture and color and are shown in Figure are based on 105°C dried samples, and the loss on ignition was 2. The layers shown in Figure 2 were separated according to approx- determined at 850°C. imate median grain size: fine sand (less than ϳ250-␮m particle size), Measurement of organic and inorganic C and N concentrations in the medium sand (ϳ300–500-␮m particle size), and coarse sand (ϳ500– samples was performed with an LECO RC-412 multiphase determina- ␮ 1000- m particle size). The samples measured by Mo¨ssbauer were tor, which enables oxidation of the sample in an O2 atmosphere at selected from one specific site for each layer but are assumed to be elevated temperatures. The organic C-H bonds are oxidized at temper- fairly representative of that layer. In figures where the Mo¨ssbauer data atures Ͻ500°C, whereas the inorganic C bonds are oxidized at higher are compared with the other results, the exact position of the Mo¨ssbauer temperatures (ϳϾ700°C). The sum of organic and inorganic C values 2878 J. L. Bishop et al.

Fig. 3. Relative abundances of major minerals in the oxic region sediments (core E, DH-2-49) and the anoxic region sediments (core H, DH-4-13). Mineral abundances were determined by XRD. Pyroxene was identified in these samples from the reflectance spectra and the fits of the high-resolution Mo¨ssbauer spectra; however, the plagioclase XRD peaks mask those of pyroxene, making it difficult to uniquely identify pyroxene in these samples by XRD. The mica is probably biotite. The detection limit of these minerals is ϳ3 wt%.

measured by this technique compare favorably with similar C values The S isotope compositions were determined with a continuous determined from the loss on ignition data. helium flow Carlo Erba elemental analyzer coupled with isotope ratio– X-ray diffraction (XRD) measurements were performed on pressed mass spectrometry (EA-IRMS) with a Micromass Optima instrument, powders with a Philips PW 1710 diffractometer. These measurements as in other studies (Giesemann et al., 1994; Grassineau et al., 1998). were made by CuK␣ radiation, a 0.1-mm receiving slit, and a graphite The samples were weighed into a 5 ϫ 8–mm tin capsule along with a monochromator. few mg of V2O5 to facilitate complete combustion. This mixture was then compacted and loaded into an autosampler, which introduces the samples into the elemental analyzer. 2.3. Analytical Techniques—Isotopes Shortly after introduction of a sample into the combustion furnace, ϳ C and N isotope measurements (␦13C and ␦15N) were performed by oxygen is admitted, producing flash combustion at 1800°C. The Dumas combustion techniques on the bulk organic material in each combustion gases are swept through a column of WO3 granules and sample from cores E (DH-2, oxic region) and H (DH-4, anoxic region) then reduced copper, which ensures that the S-containing gases are pure as in previous studies (DesMarais et al., 1989; Wharton et al., 1993; SO2, and that any surplus of oxygen is trapped. Water is removed in a Doran et al., 1998). Samples were pretreated for 48 h in a mild acetic subsequent trap containing magnesium perchlorate, which is then acid solution to separate and purify the organic material. This technique passed through a chromatographic column where it is separated over removes the calcite without destroying labile organic matter in the time from CO2 and N2. microbial mats. After this pretreatment, samples were rinsed three A small percentage of the gas chromatography (GC) column effluent times, dried at 105°C for 24 h, and cooled in a desiccator. Duplicates is passed into the ion source of the mass spectrometer. The S compo- were run for all samples. Samples were prepared and measured fol- nent of the sample is compared with an SO2 reference gas that is lowing normal procedures (e.g., Wong et al., 1992). Briefly, samples delivered via the inlet. Ion currents of masses 64 and 66 were recorded and the areas under these peaks were integrated. Silver sulfide stan- were combusted at 1000°C along with a pulse of pure O2, transported via a carrier gas through a series of chemical scrubbers, and finally dards approved by the International Atomic Energy Agency, Vienna, transported through a second furnace held at 650°C (contains elemental were routinely analyzed with the samples (approximately one standard copper to remove any excess oxygen and reduce nitrogen oxides to per eight samples) to ensure that there is no fractionation of the nitrogen). Water vapor and the remaining pure N2 and CO2 are sepa- reference gas. The isotope ratios were normalized to Can˜on Diablo rated by chemical or cryogenic traps and are moved directly into the ion troilite (V-CDT) and are given in terms of ␦34S as per mil relative to source of a Micromass Sira Series 2 mass spectrometer for isotopic V-CDT. measurement. EA-IRMS provides major advantages over the classical techniques All carbon isotopic results are reported as per mil (‰) values, of reduced sample weights and much reduced analysis time. The main relative to the Peedee belemnite carbonate standard from the Vienna limitation of this technique is that the S content must be at least 300 conference (V-PDB) according to normal procedures. The standard ppm, due to a maximum sample weight of 20 mg. For samples with S analyzed with our samples had a ␦13CofϪ9.0‰ with respect to concentrations less than 300 ppm, a chemical means of concentrating V-PDB. The 15N/14N value of each sample was reported as the relative the sulfur is necessary (e.g., Sasaki et al., 1979). In this study, we ran per mil difference between sample and atmospheric N2, and the stan- residues as indicated above after HCl digestion of the samples. This dard analyzed with our samples had a ␦15N value of 6.7‰ with respect removes soluble sulfates and allows for measurement of pyrite abun- to atmospheric N2. dance (and also other insoluble S phases, if present). Mineralogy and geochemistry of lake sediments from Antarctica 2879

2.4. Mo¨ssbauer Spectroscopy All samples were kept frozen at 77 K after sampling to prevent oxidation and preserve the natural Fe(II)/Fe(III) ratios. The samples were prepared ϳ1 mm thick in 10-mm-diameter Delrin cups. Thickness effects for these sediments are assumed to be negligible as in previous sediment studies (Drodt et al., 1997, 1998; Ko¨nig et al., 1997) on the basis of calculations for materials of related composition (Long et al., 1983; Rancourt et al., 1993). Initial Mo¨ssbauer spectra were measured on the samples kept at 77 K in an Oxford Instruments refrigerator. The source was 57Co diffused in a Rh foil and retained at room temperature. Isomer shifts are given relative to ␣-Fe at room temperature. Spectra were recorded for samples from the oxic (core E, DH-2) and anoxic (core H, DH-4) regions of the lake in transmission geometry with a 512-channel analyzer over the velocity range Ϫ11.8 to ϩ11.8 mm/s (termed “normal resolution”), as in previous studies (Drodt et al., 1997, 1998; Ko¨nig et al., 1997). Additional “high-resolution” Mo¨ssbauer spectra were measured at 77 K for selected anoxic region samples over the velocity range Ϫ4toϩ4 mm/s. Because pyrite has nearly the same Mo¨ssbauer parameters as high-spin Fe(III) (isomer shift and quadru- pole splitting), identification of diamagnetic pyrite in mixtures that also contain paramagnetic minerals is facilitated by low-temperature mea- surements in the presence of applied external fields (Montano and Seehra, 1976; Gu¨tlich et al., 1978). Mo¨ssbauer spectra were therefore also measured at 4.2 K (normal resolution) in a field of 1 T (applied perpendicular to the ␥-beam) for the anoxic region samples.

2.5. Reflectance Spectroscopy Bidirectional reflectance spectra were measured from 0.3 to 3.6 ␮m under ambient conditions with the standard viewing geometry of 0° incidence and 30° emergence angles and from 1 to 25 ␮m with a Nicolet Fourier transform interferometer (FTIR) spectrometer in a

H2O- and CO2-purged environment, as in previous experiments (e.g., Bishop et al., 1996). Sample preparation for the spectral measurements involved pouring the particulate material into a sample dish 12 mm in diameter and 3 mm in depth. For the Ͻ125-␮m samples, the dish was tapped gently on a hard surface to settle the particles without smooth- ing, so that a natural surface texture was present. The bidirectional data were calibrated to absolute reflectance using Halon (e.g., Pieters, 1983), and the FTIR data were scaled to the bidirectional data near 1.2 ␮m.

3. RESULTS

3.1. Mineralogical Composition 3.1.1. X-ray Diffraction Semiquantitative mineralogical analyses were performed by XRD techniques on pressed powders and are summarized in Figure 3. The dominant minerals include plagioclase, K-feld- spar, quartz, and pyroxene. This is similar to the mineralogy Fig. 4. Mo¨ssbauer spectra of selected samples from the oxic region observed for surface sediments from the Antarctic Dry Valleys (core E) measured at 77 K under high resolution. The spectral character region (Gibson et al., 1983). Pyroxene was difficult to uniquely of samples E-2, E-6 and E-10 is fairly representative of the samples identify with XRD alone because plagioclase is abundant and studied from this core. The spectrum of sample E-3 is shown as well, because it differs from the others. has XRD peaks that mask those of pyroxene. The samples from the anoxic region (core H) contain pyrite, but there is no evidence of pyrite in the oxic region (core E) samples. Second- pyrite, and additional Fe(II) species. The nonpyrite Fe(II) con- ary amounts of mica, amphibole, and chlorite are present tribution is present as two forms of pyroxene, Fe rich and Mg throughout both cores. Calcite is present at high abundance in rich, and very likely as one other phase, which is probably a sample E-3 and in much lower quantities elsewhere. silicate mineral such as chlorite, mica, or amphibole (McCam- mon, 1995). In a first attempt, we have analyzed the spectra recorded at 77 K with only two subspectra for Fe(II) species 3.1.2. Mo¨ssbauer Spectroscopy and one subspectrum for Fe(III) ϩ pyrite species (note that Mo¨ssbauer spectra are shown in Figure 4 for selected oxic pyrite is inseparable from Fe(III) at 77 K). The obtained isomer ␦ ⌬ region (core E) samples at 77 K and in Figure 5 for selected shift ( ) and quadrupole splitting ( EQ) values and the relative anoxic region (core H) samples measured at 77 K and addi- abundances of the Fe phases at 77 K are given in Table 1 for the tionally at 4.2 K in an applied field of 1 T. The spectra were oxic and anoxic region samples. The Mo¨ssbauer parameters for analyzed by performing mathematical fits to separate Fe(III), two pyroxene compositions (FeSiO3 and Mg0.85Fe0.15SiO3) 2880 J. L. Bishop et al.

Fig. 5. Mo¨ssbauer spectra of selected anoxic region (core H) samples. (a) Measured at 77 K under high resolution. (b) Measured at 4.2 K with an applied field of 1 Tesla normal to the ␥-ray direction. These Mo¨ssbauer measurements were necessary for the anoxic region samples to separate out the contributions from pyrite and Fe(III). Spectra are shown here for samples H-3, H-4, and H-9, which exhibit the spectral range observed for the anoxic region samples. Mineralogy and geochemistry of lake sediments from Antarctica 2881 from (McCammon, 1995) are also shown in Table 1 for com- Table 1. (Continued). parison. Sediment depth Iron ␦ ⌬E Relative amount of This comparison indicates that none of the obtained Fe(II)- Q (cm) species (mm/s) (mm/s) Fe species (%) a/Fe(II)-b ratios in Table 1 resembles either FeSiO3 (50/50 ratio) or Mg0.85Fe0.15SiO3 (80/20 ratio). We therefore repeated 1 (H-1) Fe(II)-a 1.3 2.21 54 our analysis of the anoxic region (core H) samples with three Fe(II)-b 1.3 2.94 31 subspectra for Fe(II) species instead of only two. The results of Fe(III) 0.45 0.63 15 3 (H-2) Fe(II)-a 1.28 2.29 25 these analyses are shown in Table 2 and Figure 5a. Estimates of Fe(II)-b 1.29 2.95 37 the FeSiO3 and Mg0.85Fe0.15SiO3 were obtained by comparing Fe(III) 0.45 0.65 38 the relative amounts of the Fe(II)-1 and Fe(II)-2 sites (in Table 9 (H-3) Fe(II)-a 1.29 2.21 32 2). Mg-rich pyroxenes are more prevalent, except near 15 cm, Fe(II)-b 1.28 2.96 24 in core H. Fe(III) 0.42 0.65 44 15 (H-4) Fe(II)-a 1.28 2.32 25 The spectra recorded at 4.2 K in an applied field of 1 T (Fig. Fe(II)-b 1.29 2.95 30 5b) allow for separation of paramagnetic Fe(III) from diamag- Fe(III) 0.46 0.65 45 netic pyrite contributions. In our analysis, we have approxi- 20 (H-5) Fe(II)-a 1.3 2.19 51 mated the subspectrum representing pyrite by a quadrupole Fe(II)-b 1.29 2.96 30 ␦ ϭ Fe(III) 0.42 0.68 19 doublet with fixed parameters—that is, 0.41 mm/s and 21 (H-6) Fe(II)-a 1.29 2.17 45 ⌬ ϭ EQ 0.63 mm/s. These values are results from investigations Fe(II)-b 1.28 2.92 31 on pure pyrite and pyrite in mixtures (Lougear, 2000). This Fe(III) 0.42 0.66 24 approximation is justified because the small field of 1 T causes 24.5 (H-7) Fe(II)-a 1.29 2.18 52 only a slight broadening of the absorption lines of the diamag- Fe(II)-b 1.29 2.94 33 Fe(III) 0.44 0.64 15 netic pyrite. It is, however, strong enough to cause magnetic 29 (H-8) Fe(II)-a 1.29 2.17 58 hyperfine splitting in the Mo¨ssbauer spectrum of paramagnetic Fe(II)-b 1.28 2.94 30 Fe(III) species. This contribution is represented by a hyperfine Fe(III) 0.43 0.67 12 field distribution with a fixed isomer shift of 0.48 mm/s and a 34 (H-9) Fe(II)-a 1.29 2.15 60 Fe(II)-b 1.29 2.92 32 quadrupole splitting of zero. These values are typical for ferric Fe(III) 0.46 0.69 8 iron in clay minerals found in sediments (Drodt et al., 1997, 36 (H-10) Fe(II)-a 1.3 2.17 54 Fe(II)-b 1.29 2.95 33 Fe(III) 0.44 0.75 12

Table 1. Mo¨ssbauer parameters for core E (collected from the oxic 77 K spectra of mineral standards for comparison from McCammon region of the lake) and core H (collected from the anoxic region of the (1995) lake) samples measured at 77 K. ͭpyroxeneͮ Fe(II)-1 1.26 2.00 50 FeSiO Fe(II)-2 1.30 3.13 50 ␦ ⌬ 3 Sediment depth Iron EQ Relative amount of (cm) species (mm/s) (mm/s) Fe species (%) ͭ pyroxene ͮFe(II)-1 1.28 2.16 80 Mg0.85Fe0.15SiO3 Fe(II)-2 1.29 3.06 20 0 (E-1) Fe(II)-a 1.28 2.16 44 ␦ ␣ ⌬ Fe(II)-b 1.27 2.91 30 Note: refers to isomer shift vs. -Fe and EQ refers to quadrupole ␦ ⌬ Fe(III) 0.47 0.79 26 splitting. Typical error margins are 0.02 mm/s for and EQ,2%for 2 (E-2) Fe(II)-a 1.29 2.17 49 Fe(II)-a, and 1% for Fe(II)-b and Fe(III). The relative abundance of Fe(II)-b 1.28 2.93 31 Fe(III) in this analysis includes pyrite contributions. Fe(III) 0.47 0.81 20 4 (E-3) Fe(II)-a 1.29 2.19 20 Fe(II)-b 1.28 2.88 28 Fe(III) 0.47 0.72 52 11 (E-4) Fe(II)-a 1.28 2.18 50 1998). The hyperfine field distribution in this analysis takes Fe(II)-b 1.29 2.94 31 care of the fact that Fe(III) is inhomogeneously distributed, Fe(III) 0.51 0.86 19 causing variations in exchange interactions. 18 (E-5) Fe(II)-a 1.29 2.16 50 The Fe(II) contributions in the spectra recorded at 4.2 K in a Fe(II)-b 1.29 2.95 34 Fe(III) 0.48 0.81 16 field of 1 T are divided into two subspectra, as described in 22 (E-6) Fe(II)-a 1.28 2.16 48 corresponding Mo¨ssbauer studies of sediments from the Peru Fe(II)-b 1.26 2.93 35 Basin (Drodt et al., 1997, 1998). The paramagnetic Fe(II) part Fe(III) 0.46 0.83 17 is assigned by a broad quadrupole doublet because an applied 25 (E-7) Fe(II)-a 1.28 2.15 50 field of1Tisnotstrong enough to cause appreciable spin Fe(II)-b 1.27 2.92 31 Fe(III) 0.47 0.85 19 expectation values of the ferrous high-spin iron (Trautwein et 33 (E-8) Fe(II)-a 1.28 2.14 49 al., 1991) and therefore prevents resolved magnetic hyperfine Fe(II)-b 1.27 2.91 30 splitting. The second Fe(II) part in iron oxide species exhibits Fe(III) 0.46 0.87 21 magnetic ordering and therefore resolved magnetic hyperfine 35 (E-9) Fe(II)-a 1.28 2.15 54 Fe(II)-b 1.27 2.91 31 splitting. We have approximated this by a sextet with line Fe(III) 0.48 0.81 15 intensities of 3:4:1, as is expected for a ferromagnetic powder 39 (E-10) Fe(II)-a 1.26 2.24 36 sample with an external field applied perpendicular to the Fe(II)-b 1.27 2.94 38 ␥-beam; additionally, we have used a fixed isomer shift of 1.28 Fe(III) 0.49 0.77 26 mm/s, which is typical for ferrous high-spin iron, and a fixed 2882 J. L. Bishop et al.

Table 2. Mo¨ssbauer parameters of Fe(II) species for the anoxic bands are observed near 0.93 ␮m and ϳ2 ␮m in all samples. region (core H) samples at 77 K obtained from a fit with four subspec- These features are characteristic of low-Ca (high-Fe) pyroxene. tra (Fe(III) contributions are not listed). Broadening of the ϳ1-␮m band for the anoxic region samples Relative Relative H-1,2,3 may be due to pyrite. Also observed for all samples is amount of amount of a strong water band near 3 ␮m; this feature is stronger and ␦ ⌬ Sample Fe (II) EQ Fe species Fe(II)-1,2 rounded for spectra containing organic (ϳ3.4 ␮m) or calcite (cm) doublet (mm/s) (mm/s) (%) (%) (ϳ3.4 and ϳ4 ␮m) features. The spectra of samples E-2 and 9 (H-3) Fe(II)-1 1.28 2.14 35 83 E-3 exhibit particularly strong calcite bands. Weak features due Fe(II)-2 1.26 3.06 7 17 to mica, clay minerals, or both are observed in some samples Fe(II)-3 1.25 2.77 13 — near 1.4, 1.9, and 2.2 ␮m. Samples H-1,2,3,4,5 exhibit sharp 15 (H-4) Fe(II)-1 1.28 2.22 23 51 chlorophyll bands at 0.67 ␮m. These spectra also show organic Fe(II)-2 1.26 2.95 22 49 C-H stretching bands near 3.4 ␮m. (For detailed visible/NIR Fe(II)-3 1.28 2.63 13 — 20 (H-5) Fe(II)-1 1.28 2.15 49 77 spectra of minerals, see Burns, 1993, and Gaffey et al., 1993.) Fe(II)-2 1.28 3.05 15 23 In the oxic region core (E), organics and calcite are evident in Fe(II)-3 1.26 2.70 17 — the surface layers, and calcite is observed deeper in the core for 21 (H-6) Fe(II)-1 1.27 2.11 46 78 samples E-6 and E-8 on the basis of the reflectance spectra in Fe(II)-2 1.27 3.04 13 22 Fe(II)-3 1.28 2.68 18 — Figure 6a. The upper segments of the anoxic region core (H) 24.5 (H-7) Fe(II)-1 1.27 2.14 54 83 contain organic C-H and chlorophyll spectral features, whereas Fe(II)-2 1.28 3.10 11 17 the lower layers contain calcite features. It is interesting that the Fe(II)-3 1.28 2.77 21 — anoxic region sediments have strong chlorophyll bands 29 (H-8) Fe(II)-1 1.27 2.15 61 85 throughout the upper ϳ20 cm of sediment. In a previous study Fe(II)-2 1.28 3.10 11 15 Fe(II)-3 1.29 2.78 17 — of lake Hoare sediments, chlorophyll absorptions are also ob- 34 (H-9) Fe(II)-1 1.28 2.14 58 77 served in the surface layers of sediments in both the oxic and Fe(II)-2 1.28 3.07 17 23 anoxic regions, and again at a ϳ15-cm depth in the anoxic Fe(II)-3 1.29 2.69 17 — region core (Bishop et al., 1996). 36 (H-10) Fe(II)-1 1.27 2.13 58 78 Fe(II)-2 1.27 3.04 16 22 The mid-infrared region spectra are shown in Figure 6b as a Fe(II)-3 1.27 2.67 14 — function of wavenumber (inverse centimeters) to better display the features. Quartz and pyroxene peaks are observed for most ␦ ␣ ⌬ Ϫ1 Note: refers to isomer shift vs. -Fe, and EQ refers to quadrupole samples. The quartz features occur near 1200 cm (ϳ8–9 splitting. Mo¨ssbauer spectra of selected samples are shown in Figure ␮m), and the pyroxene features occur near 800 to 1100 cmϪ1 5a. (ϳ9–12 ␮m) and near 500 to 550 cmϪ1 (ϳ18–20 ␮m). Spec- tral evidence of carbonates is strongest for the oxic region quadrupole splitting of Ϫ2.5 mm/s, with 2.5 mm/s representing samples E-2 and E-3. Carbonate bands are observed near 1800 cmϪ1 (sharp) and 1470 cmϪ1 (broad) as volume-scattering, the average of all quadrupole splittings obtained for the ferrous Ϫ1 high-spin species of the anoxic region (core H) samples (Table absorption features (troughs) and near 880 cm as surface- 2). The line width as well as the hyperfine field were free scattering features (peaks). The mid-infrared spectral properties parameters. The angle between the main axis of the electric of minerals depend on the particle size and texture of the field gradient and the hyperfine field was kept at 90° (Drodt et samples, as shown for quartz, pyroxene, and calcite (Salisbury al., 1997). The overall contributions of the high-spin Fe(II) and Wald, 1992; Moersch and Christensen, 1995; Wenrich and subspectra were held constant (to within Ϯ2%) to those ob- Christensen, 1996; Mustard and Hays, 1997; Lane, 1999), tained from the 77 K measurements; this is a reasonable ap- making it difficult to identify minerals in mixtures. For this proximation because the Lamb-Mo¨ssbauer factor (f factor) reason, the confidence of spectral mineral identification is in- does not change significantly between 4.2 K and 77 K and from creased by use of the entire spectral region (visible, NIR, one Fe(II) containing oxide to another. mid-infrared). The parameters resulting from this analysis of the anoxic Because grain size plays an important role in the reflectance region (core H) samples, recorded at 4.2 K in an applied field spectra of minerals, spectra were measured of the original Ͻ ␮ of 1 T, are summarized in Table 3. This analysis provides samples as well as the 125- m powders. Shown in Figure 7 quantitative information about the composition of the iron- are mid-infrared reflectance spectra of both the original mate- Ͻ ␮ bearing species in these sediments. The iron-bearing species rial and the 125- m powders for three sediments and the include pyrite, paramagnetic and magnetically ordered Fe(II) minerals quartz, pyrite, and calcite. Spectral features due to species, and Fe(III) species of which the hyperfine field distri- quartz can be seen throughout the spectra of samples E-5 and bution covers paramagnetic as well as magnetically ordered H-9 in Figure 7 and features due to calcite dominate the spectra Fe(III) sites. of sample E-3 in Figure 7. Pyrite exhibits a sharp band near 440 cmϪ1 and a broad doublet near 1500 and 1630 cmϪ1 (Fig. 7). The spectra of calcite and quartz both have features in the range 3.1.3. Reflectance Spectroscopy 1500 to 2000 cmϪ1, which complicates identification of other Visible to infrared reflectance spectra of the Ͻ125-␮m par- minerals in this spectral region. Neither of these pyrite features ticle size fractions of all samples are shown in Figure 6. The can be uniquely identified in spectra of any of the anoxic region visible to near-infrared (NIR) region spectra are shown in samples; however, a broad band centered near 1650 cmϪ1 in Figure 6a as a function of wavelength in microns. Pyroxene samples H-1,2,3 (Fig. 6) is consistent with the presence of both Mineralogy and geochemistry of lake sediments from Antarctica 2883

Table 3. Mo¨ssbauer parameters for anoxic region (core H) samples from spectra measured at 4.2 K spectra with an applied field of 1 Tesla perpendicular to the ␥-beam.

⌫ ␦ ⌬ Sample EQ Bhf Relative amount of (cm) Subspectrum (mm/s) (mm/s) (mm/s) (Tesla) Fe species (%)

9 (H-3) pyrite 0.47 0.41 Ϫ0.63 — 32 Fe(II)-doublet 0.66 1.33 2.48 — 13 Fe(II)-sextet 2.16 1.28 Ϫ2.50 11 43 Fe(III)-distribution 0.60 0.48 0.0 — 12 15 (H-4) pyrite 0.52 0.41 Ϫ0.63 — 11 Fe(II)-doublet 0.58 1.29 2.58 — 8 Fe(II)-sextet 2.13 1.28 Ϫ2.50 11 47 Fe(III)-distribution 0.60 0.48 0.0 — 34 20 (H-5) pyrite 0.44 0.41 Ϫ0.63 — 5 Fe(II)-doublet 0.70 1.37 2.50 — 17 Fe(II)-sextet 2.05 1.28 Ϫ2.50 11 64 Fe(III)-distribution 0.60 0.48 0.0 — 14 21 (H-6) pyrite 0.44 0.41 Ϫ0.63 — 11 Fe(II)-doublet 0.68 1.32 2.37 — 29 Fe(II)-sextet 2.16 1.28 Ϫ2.50 11 47 Fe(III)-distribution 0.60 0.48 0.0 — 13 24.5 (H-7) pyrite 0.40 0.41 Ϫ0.63 — 7 Fe(II)-doublet 0.71 1.33 2.35 — 32 Fe(II)-sextet 2.24 1.28 Ϫ2.50 11 53 Fe(III)-distribution 0.60 0.48 0.0 — 8 29 (H-8) pyrite 0.50 0.41 Ϫ0.63 — 4 Fe(II)-doublet 0.65 1.34 2.33 — 27 Fe(II)-sextet 2.36 1.28 Ϫ2.50 11 61 Fe(III)-distribution 0.60 0.48 0.0 — 8 34 (H-9) pyrite 0.30 0.41 Ϫ0.63 — 2 Fe(II)-doublet 0.88 1.34 2.45 — 21 Fe(II)-sextet 2.08 1.28 Ϫ2.50 11 68 Fe(III)-distribution 0.60 0.48 0.0 — 9 36 (H-10) pyrite 0.41 0.41 Ϫ0.63 — 6 Fe(II)-doublet 1.03 1.36 2.39 — 25 Fe(II)-sextet 2.58 1.28 Ϫ2.50 11 63 Fe(III)-distribution 0.60 0.48 0.0 — 6

⌫ ␦ ␣ ⌬ Note: refers to line width, refers to isomer shift vs. -Fe, EQ refers to quadrupole splitting, and Bhf refers to the magnetic hyperfine field measured at the 57Fe nucleus. Mo¨ssbauer spectra of selected samples are shown in Figure 5b. Typical error margins for the relative amount of Fe species are 2% for the largest values of pyrite and 3% for the other values of pyrite, 2% for the Fe(II)-doublet, and 3% for the Fe(II)-sextet and for the Fe(III)-distribution. calcite and pyrite and a weak feature at 440 cmϪ1 in the E-3 3.2. Chemical Composition spectrum might be due to pyrite as well. Additional mid- infrared spectra of minerals can be found in Farmer (1974) and Major element abundances for samples from the oxic region Salisbury et al. (1991). Recent reflectance spectroscopy studies (core E, DH-2) and the anoxic region (core H, DH-4) were of Martian meteorite ALH 84001 also show reflectance spectra determined by XRF and are given in Table 5. The iron has been of multiple grain sizes of pyroxenes (Bishop et al., 1998a,b). divided into Fe(II) and Fe(III) on the basis of the Mo¨ssbauer In addition to band centers that are characteristic of specific measurements. Samples E-1, H-1, and H-2 are the small, or- minerals, another spectral parameter, called the Christiansen ganic-rich regions from the tops of the cores, and insufficient feature, is diagnostic for silicate minerals (Salisbury, 1993). material was available for chemical analyses. The oxic and The Christiansen feature is the reflectance minimum or emis- anoxic region samples exhibit similar major element chemistry sion maximum that occurs on the short-wavelength flank of the overall. Sample E-3 is dominated by calcite and therefore strong band due to the silicate stretching vibration. The Chris- contains elevated Ca abundance and reduced levels of the tiansen features are given in Table 4 for the samples in this remaining major elements compared with the other samples. In study. Samples E-2 and E-3 do not have strong silicate features general, the Fe content is higher and the Al and Si contents are in this region but do have reflectance minima at somewhat lower for the anoxic region samples (core H) than for the oxic shorter wavelengths (longer wavenumbers) and are listed in region samples (core E). The anoxic region samples also have Table 4 in brackets. Although the mid-infrared spectral features significantly higher S abundances than those from the oxic (Figs. 6 and 7) are dominated by quartz bands for many region. The minor elements Cr, Ni, and V have higher abun- samples, the Christiansen feature for most samples is charac- dances in the anoxic core samples, and the Sr content is higher teristic of K-feldspar. The Christiansen feature is influenced by in the oxic core samples. The Al2O3 to TiO2 ratio is lower and all the components in the mixture and has been shown to be the Th/Cu, Th/Ni, and Th/Y ratios are higher for the anoxic correlated to the silica content of rocks and soils (Salisbury, region samples compared with those from the oxic region. The 1993). relative abundances of Fe, Al, and Si coupled with the differ- 2884 J. L. Bishop et al.

Fig. 6. Reflectance spectra from 0.3 to 25 ␮m of sediments from the oxic and anoxic regions of Lake Hoare. The visible to near-infrared region is shown in terms of wavelength (A) and the mid-infrared region is shown in terms of wavenumber (B) to facilitate visualization of the spectral features. Mineralogy and geochemistry of lake sediments from Antarctica 2885

Table 4. Position of Christiansen features (CF) for the oxic (core E) and anoxic (core H) region samples.

Sample CF Sample CF Mineral CF

E-1 1275 cmϪ1 H-1 1235 cmϪ1 quartz 1350 cmϪ1 E-2 (1400 cmϪ1) H-2 1270 cmϪ1 plagioclase 1300 cmϪ1 E-3 (1390 cmϪ1) H-3 1275 cmϪ1 K-feldspar 1280 cmϪ1 E-4 1280 cmϪ1 H-4 1275 cmϪ1 high-Ca 1190 cmϪ1 pyroxene E-5 1275 cmϪ1 H-5 1280 cmϪ1 low-Ca 1170 cmϪ1 pyroxene E-6 1280 cmϪ1 H-6 1275 cmϪ1 E-7 1280 cmϪ1 H-7 1280 cmϪ1 E-8 1275 cmϪ1 H-8 1275 cmϪ1 E-9 1280 cmϪ1 H-9 1275 cmϪ1 E-10 1280 cmϪ1 H-10 1275 cmϪ1

Note: The Christiansen feature values for the sediment samples shown here were determined from the spectra in Figure 6 and those for the minerals shown here are from Salisbury (1993).

samples after dissolution of soluble sulfates is shown in Table 8. These insoluble S values are less than the total S measured by XRF (Table 5) for most samples.

4. DETECTION OF CARBONATES AND ORGANIC MATTER Relatively strong spectral bands due to carbonates occur near 3.4 and 4 ␮m, and those due to aliphatic hydrocarbons (organ- ics) occur near 3.4 ␮m; these bands are frequently used in remote sensing for identification of carbonates and organics in geologic samples. Difficulty arises in semiquantitative analysis if both carbonates and organic materials are present because both of these species exhibit multiple bands in the 3.3- to Ϫ Fig. 7. Reflectance spectra from 400 to 2700 cm 1 (ϳ3.7–25 ␮m) of 3.5-␮m region. To test correlations of the spectroscopic fea- Ͻ ␮ the original material and the 125 m powders for three sediment tures with the organic and calcite abundances for remote de- samples and Ͻ75 and 75 to 250 ␮m size separates for quartz, pyrite, and calcite (from Salisbury et al., 1991). tection, a reflectance continuum was removed from the infrared spectra in the range 3.2 to 3.7 and 3.7 to 4.2 ␮m. A line is fitted to the spectrum at two points, and the spectrum is divided by the line, giving normalized spectra for the region of interest. ences in the ratio of Al2O3 to TiO2 and the ratios of the mobile element Th with the immobile elements Cu, Ni, and Y are Band depths and band ratios are frequently used in remote consistent with a more mafic source for the anoxic region sensing for mineral identification when absolute reflectance is samples, or that these samples have experienced less weather- not possible. These spectra are shown in Figure 8, along with ing, or both (Nesbitt and Markovics, 1997). spectra of ethanol on a glass slide and fine-grained calcite from The sediments from the oxic and anoxic region cores contain a previous study (Bishop et al., 1998a). C-H stretching features both organic C and calcite, as observed for related samples in due to hydrocarbons in organic material are observed at 3.38 ␮ ϳ Ϫ1 a previous study (Bishop et al., 1996). The results of the C m( 2960 cm ) for asymmetric CH3– vibrations, at 3.42 ␮ ϳ Ϫ1 analyses and inorganic and organic C determinations are given m( 2925 cm ) for asymmetric –CH2– vibrations, and at ␮ ϳ Ϫ1 in Table 6. The volatile component is also listed for each 3.505 m( 2850 cm ) for symmetric –CH2– vibrations. ␮ ϳ sample and includes carbonates, organic materials, and water. Strong calcite bands are observed at 3.87 and 3.98 m( 2590 and 2510 cmϪ1), and medium-strength calcite bands occur at 3.36, 3.42, and 3.48 ␮m(ϳ2975, 2925, and 2875 cmϪ1). Band 3.3. Isotope Compositions depths were measured (e.g., Clark and Roush, 1984) at 3.36, The results of C and N isotopic analyses are given in Table 3.38, 3.42, 3.48, 3.505, and 3.98 ␮m, and band areas were 7 for the oxic region (core E, DH-2) and anoxic region (core H, determined by integrating the area of overlapping bands for the DH-4) samples. Sufficient sulfur for isotope determinations is calcite features near 4 ␮m (3.8–4.1 ␮m) and the multiple present only in the anoxic region samples and the results of organic and carbonate bands near 3.4 ␮m (3.3–3.6 ␮m). Many these ␦34S measurements are given in Table 8. The low S of these values are given in Table 9. abundance for the oxic region samples is consistent with the The band depths for organic features at 3.38, 3.42, and 3.505 XRD and Mo¨ssbauer data that show evidence of pyrite only in ␮m and the spectral area from 3.3 to 3.6 ␮m should correlate the anoxic region core. The quantity of S present in the anoxic well with the abundance of organic carbon if no carbonates are 2886 J. L. Bishop et al.

Table 5. Major (wt%) and minor (mg/kg) elements for Antarctic sediments from the oxic (core E) and anoxic (core H) regions of Lake Hoare.

a Sample SiO2 TiO2 Al2O3 Fe2O3 FeO MnO MgO CaO Na2O K2O P2O5 LOI Total

E-2 55.4 0.43 13.2 0.9 3.2 0.073 3.2 12.3 2.5 2.4 0.14 5.6 99.7 E-3 26.4 0.31 5.5 1.2 1.0 0.026 1.8 32.8 0.9 1.1 0.17 29.1 100.3 E-4 59.4 0.55 15.1 1.0 3.7 0.085 3.9 7.8 2.8 2.6 0.15 2.2 99.6 E-5 60.8 0.53 15.3 0.8 3.9 0.093 4.1 6.4 3.0 2.6 0.14 1.3 99.5 E-6 64.1 0.44 14.1 0.8 3.4 0.079 3.4 5.4 3.4 2.4 0.13 1.3 99.3 E-7 64.4 0.58 14.2 1.0 3.7 0.090 3.9 4.9 3.1 2.5 0.16 0.7 99.6 E-8 60.0 0.61 15.2 1.1 3.8 0.090 4.2 6.4 2.9 2.7 0.17 1.5 99.3 E-9 61.5 0.62 15.3 0.9 4.5 0.100 3.9 6.0 2.9 3.0 0.13 0.6 99.9 E-10 62.3 0.50 15.8 1.2 3.1 0.079 3.4 5.5 3.4 2.7 0.15 1.1 99.5

H-3 59.7 0.51 14.3 0.9 5.5 0.085 3.8 4.6 2.8 2.4 0.18 4.0 99.3 H-4 60.7 0.61 14.9 2.1 3.7 0.098 4.3 5.5 2.8 2.5 0.17 1.5 99.3 H-5 63.9 0.50 13.8 0.8 4.2 0.085 4.4 4.9 2.6 2.2 0.16 1.5 99.5 H-6 62.3 0.48 14.6 0.7 4.1 0.093 4.0 6.1 3.0 2.4 0.13 1.4 99.8 H-7 58.8 0.75 13.0 0.6 6.4 0.130 5.9 7.3 2.8 2.1 0.18 1.5 100.1 H-8 59.4 0.73 13.6 3.4 6.2 0.130 5.9 6.7 2.5 2.1 0.16 1.1 99.7 H-9 59.5 0.73 12.9 0.8 6.2 0.130 6.4 7.2 2.5 2.0 0.17 0.9 99.9 H-10 60.1 0.68 13.1 0.4 6.1 0.115 5.9 6.8 2.7 2.1 0.18 1.1 99.9 SO-1b 55.0 0.87 17.6 (8.6) 0.110 3.8 2.5 2.7 3.2 0.15 4.5 99.0 SO-1c 55.9 0.89 17.4 (8.8) 0.121 4.0 2.4 2.7 3.1 0.14 4.7 100.1

a Loss on ignition at 850°C. b Certified values of the soil standard SO-1 from Potts et al. (1992). c Measured values. present. However, calcite is present in many samples, espe- 6.8 and 11.5 ␮m (1470 and 850 cmϪ1) are detected for calcite cially in the oxic region core. As seen in Figure 8, if organics abundances as low as 0.3 wt% CO2; however, detection of dominate this spectral region, then the 3.42 ␮m band will be the these features is dependent on which other minerals are present strongest feature, and if carbonates dominate, then the 3.48 ␮m and on the relative particle size of the mineral grains. band will be the strongest. Ratios of the band depths at 3.42 ␮m and 3.48 ␮m were used to characterize the relative abundance 5. CORRELATION OF SEDIMENT VARIATIONS AND of hydrocarbons and calcite. Shown in Figure 9a are the spec- BIOLOGICAL ACTIVITY tral band depths associated with organic C (3.38, 3.42, and 5.1. Depth Profiles for Major Elements and Mineralogy 3.505 ␮m) and the spectral area from 3.3 to 3.6 ␮m vs. the organic C content for most samples in this study; samples Variations in the major element compositions from Table 5 having high carbonate abundances were removed from consid- and trends in Fe(II) species from the Mo¨ssbauer analyses eration by requiring that the 3.48 ␮m band depth/3.42 ␮m band (Tables 1 and 3) are shown in Figure 10 for the oxic region depth ratio be Ͻ1.2. The best correlation here is observed for samples (core E, DH-2) and Figure 11 for the anoxic region the 3.42 ␮m band depth, although the other features exhibit samples (core H, DH-4). For the sediments in the shallow, oxic good correlation as well. The correlations with organic C part of the lake (Fig. 10), there is a significant change in all content are decreased when samples containing high calcite elements near 5 cm (E-3) where calcite is extremely abundant. abundance are included. Otherwise, the Al, Ca, and K contents tend to follow trends in The correlation between the spectral area from 3.3 to 3.6 ␮m the Fe content for the oxic region samples, with slight decreases vs. the inorganic C content for all samples in this study is poor near 22 cm (E-6) and slight increases near 35 cm (E-9). Except because overlapping organic and calcite features are present for the dip due to calcite, Si abundance is inversely related to and because the carbonate abundance is very low in some these elements. Na and Mg exhibit trends inversely related to samples. For samples where the 3.48 ␮m band depth/3.42 ␮m each other. band depth ratio is Ͼ1, calcite dominates this spectral region. For the sediments in the deep, anoxic part of the lake (Fig. Shown in Figure 9b are band depths at 3.48 and 3.98 ␮m vs. 11), changes in the chemistry are observed from ϳ15 to 21 cm, inorganic C content for the calcite-bearing samples. The cor- near 32 cm, and near 36 cm. The amounts of Si and Al drop, relation for both band depths is very good. If both features are whereas the Mg and Ca levels increase below ϳ25 cm. A available, the 3.98-␮m band depth would be better to use relative change in the abundance of Fe(II) and Fe(III) occurs because this band is stronger and there are no organic features near 15 cm (Table 3 and Fig. 11) and is accompanied by here (see Fig. 8); however, applying this technique of pre- smaller changes in the other elements. The elevated Fe(III) screening samples by comparing the 3.48-␮m/3.42-␮m ratio level near 15 cm could be due to redox changes during sedi- enables improved spectral analysis in the region of carbonate mentation and/or biologic activity. Samples H-7 (24.5–29 cm) and hydrocarbon overlap. and H-8 (29–34 cm) have fairly similar chemical compositions Similarly, the 3.8- to 4.1-␮m band area compares well with except for the change in iron oxidation state. The increased the inorganic C abundance. The calcite band near 4 ␮m(ϳ2500 ferric oxide abundance near 15 cm is accompanied by higher Ϫ1 cm ) is detected for 0.1 wt% CO2. Mid-infrared features near Fe-rich pyroxene levels compared with other sediment layers, Mineralogy and geochemistry of lake sediments from Antarctica 2887

Table 5. Continued

Sample Cl F S Ba Co Cr Cu Ga Mo Nb Ni Pb Rb Sr Th U V Y Zn Zr

E-2 510 440 320 460 18 103 14 14 Ͻ4 Ͻ4 46 10 62 420 7 Ͻ5 67 11 28 112 E-3 910 1640 1120 200 7 52 22 3 Ͻ4 6 21 13 32 860 Ͻ5 Ͻ5 45 6 15 71 E-4 860 240 420 570 18 100 16 15 Ͻ4 13 46 17 81 340 10 Ͻ5 94 16 31 123 E-5 1120 Ͻ150 400 570 17 114 15 14 Ͻ4 19 52 10 84 340 15 Ͻ5 84 14 37 121 E-6 Ͻ100 Ͻ150 110 580 19 154 14 14 Ͻ4 9 60 22 77 380 11 Ͻ5 85 11 47 104 E-7 160 450 160 600 22 147 14 15 Ͻ4 12 55 24 75 340 13 Ͻ5 94 13 51 120 E-8 150 660 490 660 21 112 15 16 Ͻ4 11 52 18 92 390 Ͻ5 Ͻ5 91 18 36 134 E-9 140 Ͻ150 400 640 17 117 14 16 Ͻ4 8 55 12 87 350 10 5 94 17 35 122 E-10 110 Ͻ150 450 650 16 109 14 16 Ͻ4 9 50 19 98 430 11 Ͻ5 74 18 30 142 H-3 200 970 20,200 580 23 123 26 14 6 13 46 15 76 280 2 5 113 10 27 90 H-4 310 540 9200 560 22 144 20 14 Ͻ4 7 60 23 78 300 11 Ͻ5 115 14 34 117 H-5 230 490 9000 470 19 135 21 13 Ͻ4 15 57 13 76 300 Ͻ5 Ͻ5 108 12 28 105 H-6 300 220 5200 550 19 142 16 14 Ͻ4 13 63 14 78 340 6 Ͻ5 96 14 30 111 H-7 580 1220 8100 410 27 185 17 13 Ͻ4 12 71 13 67 270 8 Ͻ5 159 14 44 137 H-8 620 1010 4400 450 27 195 18 12 4 9 71 16 66 250 14 Ͻ5 152 14 44 132 H-9 520 790 2600 410 28 209 13 15 Ͻ4 10 79 9 65 260 6 Ͻ5 165 13 44 140 H-10 600 940 4200 470 27 194 17 13 Ͻ4 7 77 13 67 260 10 Ͻ5 137 13 41 122 SO-1b 150 850 103 870 29 170 61 24 2 12 92 20 141 330 12 2 133 25 144 84 SO-1c 180 1050 Ͻ100 880 34 166 56 24 Ͻ4 13 92 24 134 330 12 Ͻ5 128 28 138 91

which suggests that variations occurred over time in the lake (Table 8, Figs. 3 and 12). There is a rapid increase in pyrite water iron concentrations. content to a maximum of 2.8 wt% S near the 10-cm depth, The trends for pyrite abundance as determined from Mo¨ss- followed by a partial decrease in insoluble S content for the top bauer, XRD, and the insoluble S data are similar overall, two layers. The weight percent S as pyrite was calculated from indicating that most of the insoluble S is due to pyrite. Both the the relative abundance of Fe as pyrite from the Mo¨ssbauer insoluble S and pyrite abundance are lower and less variable measurements (Table 3) and the FeO contents from XRF (Ta- from ϳ20 cm downward in the anoxic region sample core ble 5) for the anoxic region sediments. The abundances of this S due to pyrite and the insoluble S are shown in Figure 12. A small increase in pyrite abundance at 36 cm (H-10) is observed Table 6. Abundance of volatile components in Antarctic sediments along with increases in Si, Al, K, and Na abundances and from Lake Hoare. decreases in Fe(III), Mg, and Ca abundances from H-9 to H-10 (see Fig. 1). The presence of pyrite in the lake-bottom sedi- C CO2 C ⌺ ϩ ments from the anoxic region in our study is one of the most inorganic carbonate organic H2O (CO2 Corg ϩ Sample (wt%) (wt%) (wt%) (wt%) H2O) striking differences between these sediments and the surface sediments studied by Gibson et al. (1983). A ratio of Corg to the E-1 0.10 0.37 0.23 1.28 1.88 insoluble S content has been determined and is given in Table E-2 1.04 3.81 0.38 1.03 5.22 E-3 6.33 23.2 1.80 3.61 28.60 E-4 0.34 1.24 0.14 0.44 1.82 E-5 0.12 0.43 0.19 0.28 0.90 Table 7. ␦13C and ␦15N isotope data from the oxic (core E) and E-6 0.14 0.52 0.16 — 0.68 anoxic (core H) regions of Lake Hoare. E-7 0.01 0.04 0.08 — 0.12 E-8 0.20 0.72 0.10 0.31 1.13 Core E (oxic region), DH2-49 Core H (anoxic region), DH4-13 E-9 0.01 0.04 0.06 0.61 0.71 E-10 0.09 0.34 0.11 0.82 1.27 Sample ␦13C ␴␦15N ␴ Sample ␦13C ␴␦15N ␴

H-1 0.06 0.23 3.49 5.75 9.47 E-1 Ϫ20.3 0.04 3.3 0.22 H-1 Ϫ29.9 0.07 1.2 0.44 H-2 0.06 0.22 3.38 5.48 9.08 E-2 Ϫ20.4 0.08 3.2 0.20 H-2 Ϫ30.1 0.07 Ϫ4.1 0.30 H-3 0.03 0.11 1.02 1.52 2.65 E-3 Ϫ20.0 0.09 2.6 0.32 H-3 Ϫ26.3 0.03 Ϫ5.6 0.04 H-4 0.03 0.10 0.38 1.16 1.64 E-4 Ϫ19.5 0.44 3.0 0.10 H-4 Ϫ26.0 0.10 Ϫ3.8 0.28 H-5 0.01 0.04 0.41 0.98 1.43 E-5 Ϫ19.6 0.12 2.7 0.22 H-5 Ϫ22.5 0.08 Ϫ5.6 0.28 H-6 0.10 0.36 0.34 0.05 0.75 E-6 Ϫ20.5 0.16 3.4 0.06 H-6 Ϫ22.9 0.02 Ϫ5.2 0.30 H-7 0.12 0.45 0.27 0.74 1.46 E-7 Ϫ20.8 0.06 3.4 0.29 H-7 Ϫ25.7 0.05 Ϫ2.6 0.28 H-8 0.07 0.26 0.11 0.42 0.79 E-8 Ϫ19.9 0.05 3.2 0.21 H-8 Ϫ22.9 0.10 Ϫ3.3 0.31 H-9 0.10 0.35 0.10 — 0.45 E-9 Ϫ20.2 0.03 3.2 0.24 H-9 Ϫ25.8 0.02 0.1 0.38 H-10 0.10 0.35 0.21 0.23 0.79 E-10 Ϫ20.7 0.08 3.9 0.26 H-10 Ϫ27.9 0.03 1.3 0.20

␦13 ␦15 Note: Some H2O values were slightly negative due to changes in Note: C in ‰ relative to V-PDB; N in ‰ relative to atmo- relative humidity; these are indicated with a dash. spheric nitrogen. 2888 J. L. Bishop et al.

Table 8. S abundance and ␦34S isotope data from the anoxic regions (core H) of Lake Hoare.

Corg/Sinsol ␦34 ␴ ␴ Sample S Sinsol, wt% ratio

H-1 Ϫ16.4 1.45 1.51 0.31 2.3 H-2 Ϫ20.9 0.81 2.27 0.12 1.5 H-3 Ϫ18.6 0.92 2.56 0.20 0.4 H-4 Ϫ4.45 0.30 0.47 0.11 0.8 H-5 Ϫ11.2 0.66 0.28 0.17 1.4 H-6 Ϫ2.32 0.09 0.13 0.01 2.5 H-7 Ϫ0.47 0.37 0.16 0.07 1.7 H-8 Ϫ4.27 0.42 0.09 0.00 1.2 H-9 Ϫ6.56 NA 0.07 NA 1.4 H-10 Ϫ14.3 0.75 0.09 0.01 2.2

␦34 Note: S in ‰ relative to V-CDT; Sinsol is S due to pyrite and acid insoluble sulfates, and Corg is wt% organic C from Table 6. NA, not available.

8 and is highest in layers H-1 (0–3 cm), H-5 (20–21 cm), and H-10 (36–38 cm). These layers may indicate locations of higher bacterial reduction in the sediments.

5.2. Depth Profiles for Isotopic Compositions Depth profiles are shown in Figure 12 for the ␦13C and ␦15N patterns in the oxic (core E, DH-2) and anoxic (core H, DH-4) region samples and for the ␦34S pattern in the anoxic region core. For the sediments collected in the oxic region of Lake Hoare, the ␦13C values fall near Ϫ20‰ and the ␦15N near 3‰, and there is little variation in either isotope with depth in the sediment core. For the sediments collected in the anoxic region of the lake the ␦13C values are about Ϫ30‰ near the surface and tend to become more positive with depth. The ␦15N values Fig. 8. Reflectance spectra from 3.25 to 4.15 ␮m for calcite and are ϳ1‰ for the surface sediments in the anoxic region and ethanol (A), continuum-removed reflectance spectra for the oxic region become more negative with depth, whereas the ␦34S values are (core E) samples (B), and anoxic region (core H) samples (C). These Ϫ Ϫ spectra are normalized to more readily compare the band strengths of in the range 16 to 20‰ near the surface and more positive the organic and calcite components. Multiple organic C-H stretching at depth. Variations in the isotopic trends are much more features occur in the range 3.3 to 3.5 ␮m. A strong carbonate band significant in the anoxic sediments than in the oxic sediments, occurs near 4 ␮m and weaker bands occur near 3.4 and 3.8 ␮m. and changes in the ␦13C, ␦15N, and ␦34S tend to parallel each other with depth in the sediment core. This suggests that biologic activity is primarily responsible for these changes— (O’Leary, 1981; Rau et al., 1989; Fogel and Cifuentes, 1993; and further, that this activity is reflected in all three isotopes. Seewald et al., 1994; Laws et al., 1995). The ␦13C values The ␦13C values observed here for the oxic region sediments observed in the anoxic sediments are more negative than the are consistent with the C isotopic composition of organic values often found for dissolved organic carbon and can be matter in sediments and are typically due to the activity of explained by respiration (e.g., Quay et al., 1986). photoautotrophes (Quay et al., 1986; Fogel and Cifuentes, Other processes, such as methanogenesis, may also be influ- 1993; Wharton et al., 1993; Seewald et al., 1994). The isotopic encing the isotopic composition of the organic carbon in the variation observed for N parallels the trends in C isotopes for anoxic region of the lake. Andersen et al. (1998) observed the oxic region sediments. Nitrogen fixation, ammonia uptake, methane just above the sediment surface in the deeper, anoxic and nitrification all potentially contribute to the ␦15N values in regions of Lake Hoare. Galchenko (1994) measured methano- the oxic region sediments. There is no evidence for sulfate gensis in another Antarctic lake, and preliminary data suggest reduction in the oxic regions of Lake Hoare as no pyrite is that methanogenesis is taking place in Lake Hoare as well formed even though Fe(III) is present. (Galchenko, personal communication). Methanogens are nor- The higher abundance of organic material in the sediments mally active below the sulfate zone in sediments, as observed from the anoxic region, coupled with the larger isotopic vari- in a study of anoxic sediments in Aarhus Bay (Jørgensen, ations observed for these sediments, may indicate that addi- 1996). In that study, methane bubbled up through the sediments tional microbial processes are more active here than in the to the water. If methanogens are present in the anoxic region sediments from the oxic region. Typically, in sediments, ␦13C sediments of Lake Hoare, this could explain the methane ob- values for dissolved organic carbon are near Ϫ15 to Ϫ18‰ and served. Methanogenesis (and also respiration by methanotro- for dissolved inorganic carbon are within Ϫ5toϩ7‰ phes) may also be contributing to the depletions in 13C near 26 Mineralogy and geochemistry of lake sediments from Antarctica 2889

Table 9. Band depths for infrared spectral features due to organics and calcite in the Lake Hoare sediments.

Band depth Band depth Area (107) Band depth Area (107) ␴ (spectra) Band depth Band depth Sample 3.42 ␮m 3.48 ␮m 3.3–3.6 ␮m 3.98 ␮m 3.8–4.1 ␮m ϳ4 ␮m ϳ11.5 ␮m ϳ14 ␮m

E-1 4.3 2.7 4.9 2.7 2.2 4 ϫ 10Ϫ4 —5 E-2 10.8 15.6 20.9 31.4 37.8 3 ϫ 10Ϫ4 24 7 E-3 29.7 36.3 56.5 59.0 94.1 4 ϫ 10Ϫ4 41 51 E-4 2.1 4.0 4.3 11.7 12.0 4 ϫ 10Ϫ4 —7 E-5 2.2 2.5 3.6 5.7 5.3 4 ϫ 10Ϫ4 —— E-6 2.1 2.7 3.6 6.3 6.2 2 ϫ 10Ϫ4 4— E-7 0.8 0.7 0.9 — 0.8 4 ϫ 10Ϫ4 —7 E-8 2.0 3.3 3.7 8.5 9.0 4 ϫ 10Ϫ4 32 E-9 0.6 0.7 1.0 — 0.7 3 ϫ 10Ϫ4 —— E-10 1.3 1.8 2.2 4.8 4.2 2 ϫ 10Ϫ4 33

H-1 28.4 14.9 32.8 1.0 0.7 3 ϫ 10Ϫ4 —— H-2 26.0 13.4 30.0 — 0.2 1 ϫ 10Ϫ4 —— H-3 15.6 6.6 15.6 — 0.0 2 ϫ 10Ϫ4 —— H-4 6.4 2.7 6.1 0.7 0.6 2 ϫ 10Ϫ4 —— H-5 9.3 3.4 8.5 — 0.4 2 ϫ 10Ϫ4 —— H-6 5.0 3.7 6.2 4.4 4.2 3 ϫ 10Ϫ4 3— H-7 2.4 2.1 3.5 4.0 4.9 3 ϫ 10Ϫ4 —2 H-8 — 1.4 1.6 3.7 3.7 2 ϫ 10Ϫ4 2— H-9 — 1.4 1.3 3.7 3.8 2 ϫ 10Ϫ4 —— H-10 2.9 2.7 3.9 4.8 5.6 3 ϫ 10Ϫ4 —3

Note: The error margins for the reflectance spectra are negligible based on the standard deviations measured for at least 10 data points near 4 ␮m. Typical error margins for the band depth and area measurements are on the order of 1 to 5%. and 36 cm depth in these sediments. Dissolved methane in the water column of a Norwegian fjord was also thought to have influenced the depleted ␦13C values in the anoxic region sedi- ments in that study (Velinsky and Fogel, 1999). Large variations have been observed in N isotopic fraction- ations in natural environments, due in part to fluctuations in the N abundance, and isotopic discrimination during uptake of ϩ ␦15 NH4 can result in depletions in the N of particulate N by 0 to 27‰ (Fogel and Cifuentes, 1993). The ␦15N values for the sediments in the anoxic region of our study varied from ϳ1‰ near the surface to Ϫ3toϪ5‰ at many places along the core. These more negative ␦15N values are probably due to the ϩ presence of higher NH4 levels at depth and more denitrification occurring in these sediments. A recent study of ␦13C and ␦15N trends in both sediments and water above and below the oxic– anoxic transition from a Norwegian fjord found that both isotopes were more depleted in the anoxic region relative to the oxic region (Velinsky and Fogel, 1999), as we observed in this study. They observed ␦15N levels of ϳ4 to 6‰ in oxic regions as compared with ϳ1 to 4‰ in anoxic regions, whereas we observed ϳ3 to 4‰ in the oxic region sediments vs. ϩ1to Ϫ6‰ for the anoxic region sediments. Velinsky and Fogel (1999) attribute the nitrogen fractionation between the oxic and anoxic sediments to a balance between inorganic N utilization during photosynthesis, the reduction of nitrate to form ammo- nia, ammonia uptake by bacteria, and denitrification as oxygen levels are reduced with depth. A combination of these factors is Fig. 9. Band depths and integrated areas of spectral features are ␦15 probably influencing the N values observed in our sediments correlated with organic and inorganic C abundances. Good correlation as well. Through biogeochemical analysis of the water column is observed between spectral band depths at 3.42 ␮m and measured and sediments, Velinsky and Fogel (1999) show that the de- organic C abundance, and between spectral band depths at 3.48 and pleted N isotope values due to bacterial primary production in 3.98 ␮m and measured inorganic C abundance. Ratios of band depths for carbonate and organic features were used to identify samples that the water column are incorporated into the sediments, whereas are spectrally dominated by organics or carbonates in this region. The the higher isotope values associated with algal production are error bars are typically smaller than the plot symbols and are therefore not. If this is occurring in our anoxic region sediments as well, not included. 2890 J. L. Bishop et al.

Fig. 10. Major element composition of the oxic region samples (core E). Variations in two Fe(II) components are shown

for comparison with Fe2O3, FeO, SiO2,Al2O3, CaO, MgO, Na2O, and K2O abundances. then the depleted ␦15N values in the anoxic sediments can be (Habicht and Canfield, 1997). However, because 90% of the explained by bacterial activity. sulfide produced by this process is reoxidized by both bacterial The S isotope values vary at several points along the anoxic and abiotic pathways (Jørgensen, 1982), the large 34S deple- core H from ϳ0toϪ21‰ relative to V-CDT weighted for tions observed in many marine sulfides can be attained via a insoluble S content). The ␦34S profiles indicate a marked dif- repeated cycle of sulfide oxidation by iron oxides to elemental ference between the upper layers (top 13 cm) and the lower sulfur and subsequent disproportionation. portion of the core. The ␦34S values are depleted for these According to geochemical budgets of Lake Hoare, calculated upper sediment layers (Ϫ6.4, Ϫ20.9 and Ϫ18.6‰) and are from comparing stream inputs of sulfate over the “chloride enriched for samples H-6 (21–24.5 cm, Ϫ2.3‰) and H-7 age” (moles Cl in the lake divided by the annual Cl load in (24.5–29 cm, Ϫ0.5‰). Relatively depleted ␦34S values are also moles per year) of the lake with the total sulfate concentration observed for sample H-5 at 20 to 21 cm depth (Ϫ11.2‰) and (Green et al., 1988), it is estimated that 60% of the total sulfate for the lowest portion of the sediment core, where the S isotopic has been removed over recent geological time. An obvious compositions increase again to Ϫ14.3‰. The isotopic compo- primary cause of sulfate removal is BSR. As shown in Figure sition of S in pyrite formed through bacterial reduction of 12, the minimum ␦34S occurs at a similar depth to the maxi- ϳ Ϫ sulfate has been measured from 0to 40‰ (Schidlowski et mum concentration of sulfides, which are indicated by Sinsol al., 1983). Because microbial sulfate reduction kinetically fa- and pyrite. The ␦34S values of the uppermost sediments in the vors cleavage of the lower energy of 32S-16O bonds relative to anoxic region (Fig. 12) show that the top layers are enriched in those of the heavier S and O isotopes (Harrison and Thode, the lighter isotope. Such profiles are similar to those in other 1958), the resulting pyrite has ␦34S values that are fractionated lakes where BSR is taking place—that is, sulfate is removed towards the lighter isotope. from the lake water and deposited in the sediment as iron Jørgensen (1982) showed the important role of bacterial sulfides, mainly pyrite (White et al., 1989; Fry et al., 1995). sulfate reduction (BSR) in degradation of organic material in sediments, and Sagemann et al. (1998) further showed that 5.3. Summary of Sediment Depth Profiles sulfate reduction can constitute an important metabolic process in cold sediments. The sediments on the surface of the anoxic Lake Hoare contains primarily oxic water with a small region exhibit ␦34S values that are ϳ10‰ lighter than those region that falls below the oxic–anoxic transition at 27 m depth deeper in this core. Laboratory cultures produce enrichments of down to 34 m. The sediments in oxic lake regions contain 32S significantly less than BSR in the natural environment larger amounts of calcite than the sediments in anoxic lake Mineralogy and geochemistry of lake sediments from Antarctica 2891

Fig. 11. Major element composition of the anoxic lake samples (core H). Variations in two Fe(II) components are shown

for comparison with Fe2O3, FeO, SiO2,Al2O3, CaO, MgO, Na2O, and K2O abundances.

regions. Elevated levels of organic carbon were found in the at the maximum rate at which it can diffuse into the mat, almost surface layers of sediments from both regions, and these are eliminating isotopic fractionation. This is observed in the oxic particularly high for the anoxic region sediments. For the region sediments in the current study where the isotopic frac- sediments in the anoxic part of the lake, a transition occurs near tionation is lowered. 10 to 15 cm depth, dividing the surface sediments that are The photosynthetic production rate is dependent on the avail- active in BSR from the deeper sediments. A transition is ability of solar radiation and nutrients (e.g., Fogel and Cifu- observed at this depth in the pyrite abundance and the isotopic entes, 1993). Variations in ␦13C down the core may reflect ratios of C, N, and S for the anoxic region sediments (Fig. 12). changes in productivity, lake level, or both over time. In other The S abundance in the anoxic region sediments is a factor of words, ␦13C in the sediment core should be inversely propor- 10 higher than in the oxic region sediments and the changes in tional to the thickness of the overlying water/ice column, rates C and N isotope ratios with depth are significantly greater for of production at the time of deposition, or both. The ice the anoxic than for the oxic region sediments. thickness influences the penetration depth of visible and infra- Doran et al. (1998) proposed three “photosynthetic zones” in red light necessary for photosynthesis. Thinner ice would en- Lake Hoare to account for the varied isotopic fractionation. In able more photosynthetic organisms to live in the lake, thus the deep zone of the lake, there is unlimited CO2 supply and depleting the lighter C and producing more O2. As the CO2 relatively low rates of photosynthesis, so that uptake of the supply becomes limited, the organisms must use the less de-

plentiful CO2 is slow and the autotrophs can discriminate sirable heavier C, which may be what is occurring in the water 12 between the carbon isotopes and preferentially utilize C. column when the ice is thin. If the O2 produced through These results, together with the presence of bacteria such as photosynthesis were allowed to accumulate, then the oxic– methanogens and methanotrophes, can explain the highly 13C- anoxic boundary would be expected to lower. However, when

depleted organic C observed in the anoxic region sediments in O2 becomes available in natural systems, an increase in the our study. In the shallow zone, CO2 becomes undersaturated, abundance of O2-utilizing organisms is typically observed. 13 creating a diffusion-limited situation in which more C must These organisms then deposit more Corg, and the result is that be incorporated into the production of photosynthate. In the the oxic–anoxic boundary is elevated in the lake. Conversely, clear, shallow moat waters, photosynthetic rates must be high thicker ice would reduce the number of photosynthetic organ-

during the period of constant sunlight in summer. Doran et al. isms, reduce the number of O2-utilizing organisms, and lower (1998) proposed that dissolved inorganic carbon is assimilated the oxic–anoxic boundary in the lake. This scenario is compli- 2892 J. L. Bishop et al.

Fig. 12. Depth profiles for isotopes. ␦13C and ␦15N trends for the oxic and anoxic region cores; ␦34S trends for the anoxic core; and wt% S due to pyrite from the Mo¨ssbauer measurements and due to insoluble S (mostly pyrite) from the EA measurements.

cated by the relative balance of O2 produced by photosynthetic opposite trend is observed at 36 to 38 cm. This suggests that organisms and used up by others. there may have been changes in nutrient availability over time. Possibly, then, parts of the current anoxic region were in an The differences between the S content profiles of sediments oxic zone at the time of sediment deposition. Changes in the ice from Lake Hoare in this study and those of nearby Lake Fryxell thickness on top of Lake Hoare have been observed and are (Howes and Smith, 1990) are not easily explained. In the Lake attributed to climate variations (e.g., Doran et al., 1994a). Fryxell sediment profiles, the chromium-reducible S (equiva- Another feature of Lake Hoare that could contribute to shifts in lent to pyrite with minor elemental S) abundance is again the oxic–anoxic boundary is the topography of the lake bottom. highest in the top 5 cm, but is fairly constant between depths of Rather than one large anoxic basin, there are several small 5 to 20 cm. Pyrite levels in Lake Hoare are elevated deeper pools of anoxia. As these individual anoxic pools fill up and below the surface than in Lake Fryxell, and more variability in overflow into surrounding regions of the lake bottom, mixing of pyrite abundance was observed with depth in the sediments. oxic and anoxic water would occur. If this is happening in Lake Green et al. (1988) found that Lake Fryxell contains signifi- Hoare, then formerly oxic regions could have become anoxic cantly higher sulfate losses and has a much larger anoxic zone rather quickly, which could explain the changes in C, N, and S than Lake Hoare, which suggests that BSR is taking place to a isotope fractionation observed in the anoxic sediment core greater extent in the former lake. Possibly, Lake Hoare was studied here. Velinsky and Fogel (1999) also described a shift subjected to changes in sulfate input, or climate variations in in the oxic–anoxic boundary over time as one explanation for Antarctica caused shifts in the oxic–anoxic boundary that in- the changes in ␦15N with depth in their sediments. fluenced the biologic activity in the lake. Additional samples For the anoxic region core, the highest levels of organic C from the anoxic regions of these lakes should be analyzed to

(Corg) are observed in the top layers (0–13 cm depth), which confirm the trends characteristic of anaerobic microbes ob- are correlated with increases in pyrite levels and depleted ␦34S served here. values. It is assumed that surface sediments in the anoxic region core encompass the active BSR layer. The two lower layers, 6. APPLICATIONS TO ASTROBIOLOGY AND REMOTE around 20 and 35 cm in depth, may have been times of SENSING ON MARS heightened BSR activity, relative to ϳ15- and ϳ25-cm depth, and are associated with slightly elevated pyrite levels. Marked Characterizing the relationships between organisms, water changes also occurred in the C and N isotopes at these layers, chemistry, and putative sediment composition in the lakes from which indicates that other processes may have also occurred. the Antarctic Dry Valleys provides important information for Perhaps most interesting is that an enrichment in ␦13C near 20 interpreting possible sedimentary processes on Mars. Gibson et to 21 cm is accompanied by a depletion in ␦15N and that the al. (1983) showed the importance of chemical and physical Mineralogy and geochemistry of lake sediments from Antarctica 2893 alteration processes on sediments from cold desert environ- ments. Chemical analyses of the rocks and soils on Mars indicate complex mixing (McLennan, 2000). Enrichment of Fe and depletion of Ti in this study raise the possibility of sedi- ment transport. Mobile sedimentary units with distinct spectral properties have been identified on Mars near the Pathfinder landing site (Greeley et al., 1999; Bell et al., 2000). Biogeochemical interactions play an important role in ter- restrial sediment alteration when microorganisms are present, as shown in this study. In other studies of organisms in cold sediments, Rivkina et al. (1998) found that anaerobic microor- ganisms are viable in sediments in near-freezing environments. Sagemann et al. (1998) further observed that organisms living in near-freezing arctic sediments actually have an ideal growth temperature of ϳ25 to 30°C and have apparently adapted to the cold environment. Biogeochemical analyses of sediments from the ice above Lake Vostok, Antarctica, imply that this lake may support a microbial population, although it has an ice layer more than 3600 m thick that isolates the lake from the atmo- sphere (Karl et al., 1999; Priscu et al., 1999). These results suggest that there is a possibility, however remote, of microbial life in permafrost and paleolakes on Mars and other planets. We are not speculating here on how extreme this possibility might be at any given time on Mars or elsewhere. Unique identifica- Fig. 13. Viking image of Palos Crater Lake located in the ancient tion of Martian biosignatures, if present, is likely to be ex- highlands of northern Hesperia Planum, Mars. This 55-km-diameter crater is centered near 2.5° S and 249.5° W (Zimbelman and Rice, tremely difficult and will probably only be possible for returned 1999). samples. Remote sensing on Mars, however, will be an impor- tant tool for selection of which samples to bring back. Identi- fication of minerals and other materials by use of visible, NIR, Carr and Malin (2000) have shown evidence for erosional thermal infrared, and Raman spectroscopy will be important on activity attributed to liquid water that may have occurred in landed missions to Mars in determining the locations for other recent times on Mars. Crystalline hematite has now been iden- in situ studies for astrobiology and for collection of samples to tified at Sinus Meridiani with the Thermal Emission Spectrom- return to Earth for additional study. eter (TES) on the Mars global surveyor (MGS) (Christensen et Sedimentary features on Mars such as paleolakes (Wharton al., 2000a); this is further evidence that sedimentary processes et al., 1995) and lacustrine basins and sand mounds (Scott et al., may have occurred on Mars. The presence of potential paleo- 1991; Doran et al., 1998) have been suggested as potential sites lakes, oceans, and other sedimentary processes on Mars are of interest for exobiology. Paleolakes and lacustrine features on supported by the recent high-resolution imaging and topogra- Mars have also been studied to explain the surface processes on phy data measured by other instruments on MGS (e.g., Clifford Mars (Baker et al., 1991; Goldspiel and Squyres, 1991; Scott et and Parker, 1999; Edgett and Malin, 1999; Head et al., 1999). al., 1991, 1992, 1995; Rice, 1992). An example of a potential These sites would be ideal locations to look for biomarker Martian paleolake is Palos Crater Lake, which is shown in minerals and other evidence of organisms or habitats capable of Figure 13. Palos Crater Lake is one of many sites nominated as supporting life on early Mars. Some images of polar regions a landing site on Mars (e.g., Forsythe and Blackwelder, 1999; taken by the Mars observer camera on MGS show observations Zimbelman and Rice, 1999). As described by Zimbelman and of ice layering (see http://mars.jpl.nasa.gov/mgs/msss/camera/ Rice (1999), this crater, located in the ancient highlands of images/grl_99_shorelines/index.html), where freeze–thaw cy- northern Hesperia Planum, has a smooth crater floor, and its cles may be transporting fresh sediment to the surface. These crater rim is dissected by three valley networks. The chemical would also be areas of interest for evidence of former life, if and mineralogical correlations with biologic activity observed present, on Mars. in this study for the Lake Hoare sediments are further support Biologic processes are generally characterized by depletions for the importance of chemical and mineralogical investiga- in the heavy isotopes and obvious biomarker trends in terres- tions of potential sedimentary sites on Mars. Understanding the trial sediments are lighter ␦13C, ␦15N, and ␦34S values for evolution of Martian sedimentary systems (whether they are organic matter relative to inorganic material, as observed in this biologic or nonbiologic) will help us better understand early and other studies. Because atmospheric sulfur on Mars can sedimentary processes on the Earth. mimic a biologic signature (Farquhar et al., 2000), interpreta- Evidence of fluvial features, such as valley networks and tion of isotopic trends for isolated samples would be difficult. outflow channels, have been characterized in detail on Mars Variations in Fe(II)/Fe(III) levels would be easier to identify (Carr, 1981). Analysis of the topography and geologic units in remotely but could be attributed to multiple factors, not just western Arabia and Sinus Meridiani on Mars led Edgett and biologic activity. Observation of the mineral pyrite in potential Parker (1997) to suggest that sedimentary processes may be sedimentary regions or paleolakes on Mars could be used to occurring there. More recently, Malin and Edgett (2000) and identify regions of possible biologic activity. Unfortunately, 2894 J. L. Bishop et al. pyrite on the surface would likely be reoxized to sulfates over exposed surfaces, could be more successful in detecting organ- Martian time scales, and biogenic pyrite would likely be fine ics on Mars, if present. grained, which would have reduced infrared spectral features Sediment cores from the bottom of Lake Hoare, Antarctica, and be more difficult to detect by means of infrared spectro- were analyzed in this study for their mineralogical and geo- scopy, but still possible with Raman spectroscopy, which is less chemical compositions. Comparison of the sediment chemistry sensitive to particle size. Better methods of finding evidence of and mineralogy with the water chemistry and biologic activity biologic activity on Mars or other planetary surfaces, if present, led to explanations of the biogeochemical cycling in this lake would involve combining multiple measurements either in situ environment and provided implications for astrobiology on on a rover or in the laboratory on Earth after sample return. Mars. Observation of correlations in abundance of pyrite, ferric ox- ides/oxyhydroxides, organic C, and lighter isotopes of C, N, O, 6.1. Oxic vs. Anoxic Lake Regions or S relative to these isotopes occurring in clearly inorganic The lake water is supersaturated in O in the shallower regions elsewhere on the planet would be an indicator that 2 regions and anoxic below a 27-m depth, as measured down biologic processes may have taken place. Such observations from the ice surface (Craig et al., 1992; Wharton et al., 1993). may be used for sample selection. The sediments collected from DH-2 at a depth of ϳ15 m below Carbonates are also frequently associated with organisms, as the ice surface (core E) measured in this study contain plagio- in the oxic region sediments in this study. Remote detection of clase, K-feldspar, calcite, quartz, pyroxene, mica, amphibole, carbonates through NIR or thermal infrared imaging on Mars or and chlorite. These lake-bottom sediments are mineralogically other planetary surfaces would be an indicator of a site with similar to surface sediments from the Dry Valleys region (Gib- potential for life. Identification of carbonate-rich sediments, if son et al., 1983). The sediments collected from DH-4 at a lake present on Mars, would be of interest to astrobiology whether depth of ϳ30 m below the ice surface (core H) contain plagio- or not life evolved on that planet. Carbonate bands near 4 and clase, K-feldspar, quartz, pyroxene, pyrite, mica, amphibole, ␮ ϳ Ϫ1 6.8 m( 2500 and 1500 cm ) were detected in this study for and chlorite. The calcite abundance is minimal in these anoxic carbonate abundances as low as 0.1 wt% CO2. Carbonates can Ͻ region sediments, where 0.5 wt% inorganic CO2 was de- form under a range of temperatures and do not necessarily tected. The difference in calcite abundance for the two cores is imply an environment favorable to life. Carbonates have not attributed to calcite supersaturation in the shallower waters been detected on Mars by TES (Christensen et al., 2000b), (Green et al., 1988). A recent study of mats from Lake Hoare which suggests that they are not pervasive on the surface, found calcite present at ϳ50 wt% abundance in mats collected although Pollack et al. (1990) did observe a band at 6.7 ␮min at depths from 5 to 10 m, whereas only ϳ5 to 10 wt% calcite the atmospheric dust that was attributed to carbonates. was found in mats collected at lower regions of the lake (Hawes Organic hydrocarbons are another species frequently associ- and Schwarz, 1999). ated with living systems. Raman spectra, infrared spectra, or both obtained from a rover would be capable of identifying 6.2. Correlation of Chemistry and Mineralogy with organic material in rocks or sediments if the organics are Sediment Depth present near the surface. Organic features at 3.38, 3.42, and 3.505 ␮m (2960, 2925, and 2850 cmϪ1) were detected for Variations in chemistry and mineralogy were fairly insignif- organic C abundances as low as 0.06 wt% C; high spectral icant for the sediments collected from the shallower, oxic resolution would be required to separate these hydrocarbon region of the lake, with the exception of a particularly calcite- features from those of carbonates that also occur in this region. rich mat layer at 3 to 5 cm depth. In contrast, the sediments Raman spectra in the range 100 to 3500 cmϪ1 have been used collected from the deeper, anoxic lake region exhibited larger to identify endolithic microbial communities in Antarctic rocks variations in mineralogy, elemental trends, and isotopic pat- (Russell et al., 1998). Spectral features from 1200 to 1600 terns. Organisms were associated with calcite mat layers in the cmϪ1 and near 3000 cmϪ1 are particularly useful for identifi- oxic region sediments, whereas organisms were correlated with increased pyrite abundance and depleted ␦34S in the anoxic cation of organic species in infrared and Raman spectra. A region sediments. Raman spectrometer on a rover would be useful in identifying minerals (and also microorganisms if present) in possible pa- leolake environments on Mars. Raman, Mo¨ssbauer, and ther- 6.3. Summary of C, N, and S Isotope Analyses mal infrared spectrometers were components of the Athena The isotope trends in the sediments studied here depend on Rover (Klingelho¨fer, 1998; Squyres et al., 1998; Jolliff et al., the abundance of organically derived hydrocarbons and how 1999), a modified version of which is currently scheduled to fly microorganisms interact with their environment. The ␦13C on the Mars exploration rovers in 2003. Unfortunately, the depth profile for the oxic region sediment core in our study Raman spectrometer is no longer planned for the 2003 Mars shows little variation, with values near Ϫ20‰ throughout the exploration rovers, but we hope it will be added in future core. In contrast, the ␦13C depth profile for the anoxic region landed missions to Mars. Although the Viking biology tests did sediment core varies from Ϫ22 to Ϫ30‰. These ␦13C values not find any evidence of organics in the dust on Mars (e.g., are all consistent with organically derived hydrocarbons in Klein, 1978), some Martian meteorites have been found to sediments (Fogel and Cifuentes, 1993). The ␦15N measure- contain organics (e.g., McKay et al., 1996). A lander sent to ments for the oxic region core are relatively constant at ϳ3‰, one of the paleolakes on Mars with the capability of measuring whereas ␦15N varies from about ϩ1toϪ6‰ for the anoxic the infrared and Raman spectra of rocks, particularly the un- region core. 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