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Geochimica et Cosmochimica Acta, Vol. 60, No. 5, pp. 765-785, 1996 Copyright 0 1996 Elsevier Science Ltd Printed in the USA. All rights reserved

0016.7037/96 $15.00 + .OO

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Reflectance spectroscopy and geochemical analyses of Lake Hoare sediments, Antarctica: Implications for remote sensing of the Earth and Mars

JANICE L. BISHOP,’ CHRISTIAN KOEBERL,’ CLAUDIA KRALIK,* HEINZ FROSCHL,’ PETER A. J. ENGLERT, DAVID W. ANDERSEN,’ CARLI? M. PIETERS,~ and ROBERT A. WHARTON JR.’

‘DLR, Institute for Planetary Exploration, Rudower Chausee 5, D-l 2489 Berlin, Germany *Institute of Geochemistry, University of Vienna, Althanstrasse 14, A-1090 Vienna, Austria

‘Department of Applied Geology, Universitlt fur Bodenkultur, A- 1180 Vienna, Austria 4The Institute of Geological and Nuclear Sciences, 30 Gracefield Road, P.O. Box 31312, Lower Hutt, New Zealand

‘Department of Geology, San Jose State University, San Jose, CA 95192-0102, USA 6Department of Geological Sciences, Brown University, Providence, RI 02912, USA

‘Desert Research Institute, Biological Sciences Center, Reno, NV 89506, USA

(Received May 12, 1995; accepted in revised form November 21, 1995)

Abstract-Visible to infrared reflectance spectroscopic analyses (0.3-25 pm) have been performed on sediments from the Dry Valleys region of Antarctica. Sample characterization for these sediments includes extensive geochemical analyses and X-ray diffraction (XRD). The reflectance spectra and XRD indicate major amounts of quartz, feldspar, and pyroxene in these samples and lesser amounts of carbonate, mica, chlorite, amphibole, illite, smectite, and organic matter. Calcite is the primary form of carbonate present in these Lake Hoare sediments based on the elemental abundances and spectroscopic features. The particle size distribution of the major and secondary components influences their detection in mixtures and this sensitivity to particle size is manifested differently in the “volume scattering” and “surface scattering” infrared regions. The Christiansen feature lies between these two spectral regimes and is influenced by the spectral properties of both regions. For these mixtures the Christiansen feature was found to be dependent on physical parameters, such as particle size and sample texture, as well as the mineralogy. Semiquantitative spectroscopic detection of calcite and organic material has been tested in these quartz- and feldspar-rich sediments. The relative spectral band depths due to organics and calcite correlate in general with the wt% C from organic matter and carbonate. The amounts of organic matter and carbonate present correlate with high Br and U abundances and high Ca and Sr abundances, respectively. Variation in the elemental abundances was overall minimal, which is consistent with a common sedimentary origin for the forty-two samples studied here from Lake Hoare.

1. INTRODUCTION

The McMurdo Dry Valleys in Antarctica provide a unique opportunity for studying soil weathering processes in a relatively pristine ecosystem. Vegetation and human contact have been minimal in this region, enabling scientists to ob- serve the influence of chemical and physical weathering on soils, separated from soil modification resulting from exposure to vegetation and anthropogenic factors. The distributions of metals and trace elements in lake sediments, lake waters, and inflow streams are interrelated and play a role in the weathering processes in the Antarctic Dry Valleys regions. Understanding these geochemical patterns is also important for environmental applications and climatology studies (Wharton, 1993) and may provide clues to weathering processes on Mars (Gibson et al., 1983; Wharton et al., 1989).

Chemical weathering in the cold, arid environment of the Dry Valleys occurs at a rate that is slower than in other locations on Earth. However, analyses of sediments from this Dry Valley region have provided evidence for complex chemical weathering in such low-temperature desert environments (Gibson et al., 1983). The results of geologic and chemical studies on soils and lake sediments in the Dry Valleys region have been presented by Gibson et al. (1983), Nedell et al. (1987) Bishop et al. (1990, 1993) Squyres et

al. (199 1 ), and Andersen et al. (1993). Geochemical analyses of the lake and meltstream waters in this region have been presented by Jones and Faure (1978), Matsubaya et al. (1979) Masuda et al. (1982), and Green et al. (1986a,b, 1988). These studies indicate that the major geochemical processes controlling the chemical evolution of the lakes and chemical weathering of the soils are not yet well understood. Analyses of several Lake Hoare cores for sediment structure and stratigraphy produced neither evidence of consistent vertical sequences nor signs of chemical changes in the lake water during the period of sedimentation (Schackman, 1994).

Spectroscopy in the visible and infrared regions is an important method for mineralogical analyses of planetary surfaces via remote sensing. Successful interpretation of such remote spectral analyses relies heavily on laboratory reflectance spectroscopy studies. Remote spectroscopic analyses of natural soils have proven to be particularly difficult, as the spectroscopic properties of such soils are highly dependent on the particle size and nature of mixing of the soil components (Pieters et al., 1993; Mustard et al., 1993). These laboratory spectroscopic analyses have shown that the particle size and mineralogy each contribute to the spectral character of mixtures, and further that the influence of the spectral properties of one mineral component on the spectral properties of the mixture depends on the particle size of that component. Spectroscopic analyses of well-characterized

166 J. L. Bishop et al

soils are essential for remote sensing missions on the Earth and other planets. The surface material covering planetary surfaces is generally a mixture of multiple mineral or rock phases and composed of variable particle sizes and grain shapes. In addition, the terrestrial and martian weathering processes, as well as the current atmospheric environment, influence these surface materials and their spectral properties. In order to interpret spectroscopic remote sensing data from Mars and arid regions on the Earth, laboratory spectroscopic analyses of samples from low-temperature, arid environments are especially important.

The work presented here includes spectroscopic and geochemical analyses of samples from four sediment cores from Lake Hoare, Antarctica. Each core was divided stratigraph- ically into several distinguishable sediment units, based on sample color and texture and the presence of organic matter. This study involved measuring visible and infrared reflectance spectra of each sample and documenting changes in chemical composition and mineralogy among the samples. One focus of this research was on the relationship between chemical and mineralogical parameters of the sediments and spectroscopic features. Differences in the infrared spectral properties of variable particle size distributions of these samples were also studied. The results presented here contribute to the understanding of the major geochemical processes governing the Dry Valleys region in Antarctica and improve the understanding of the spectroscopic properties of soils and sediments weathered in low-temperature and arid environments.

2. BACKGROUND

2.1. Lake Hoare, Taylor Valley, Antarctica

The McMurdo Dry Valleys along the western coast of the Ross Sea in Antarctica remain relatively ice-free, because the local abla- tion rates exceed the annual snowfall. These dry valleys and their ice-covered lakes have been the focus of previous studies as analogs for dry valleys on Mars (Gibson et al., 1983; Squyres et al., 1991). An interesting feature of Lake Hoare is the presence of microbial mats and organic material in the lake bottom sediments (Nedell et al., 1987; Squyres et al., 1991; Wharton et al., 1993). Lake Hoare is located at 77”33’S, 162”53’E in Taylor Valley (Fig. 1).

2. I. 1. Lacustrine environment

Lake Hoare is approximately 1 km wide and 4 km long and has a maximum depth of 34 m (Nedell et al., 1987). It is perennially covered by over 3 m of ice and snow (Wharton et al., 1992). The lake water is supersaturated in 0, and Nz (Wharton et al., 1987; Craig et al., 1992) and has a temperature of -0.3 ? l.O”C (Wharton et al., 1993). The lake water is stratified into an oxygen-rich or oxic region below the ice cover down to -27 m deep and an oxygen- poor or anoxic zone below 27 m (Wharton et al., 1986, 1987; Craig et al., 1992). The primary means of deposition of lake bottom sediment in Lake Hoare is transport through the surface ice (Nedell et al., 1987; Squyres et al., 1991). Long-term sedimentation rates are estimated at about 0.01 cm/y; however, local sedimentation rates can vary by an order of magnitude over short time periods (Squyres et al., 1991). Sedimentation rates of 3-4 mg/cm*y were observed in several sediment traps from 1982 to 1985 (Squyres et al., 1991) yielding an average sediment deposition thickness of 0.002-0.003 cm/y (Nedell et al., 1987). Settling velocities have been estimated at 0.2 cm/s for very fine sand and 25 cm/s for very coarse sand based on observed sediment grading patterns (Schackman, 1994). Regional reviews of the geochemical and geophysical processes gov-

162” 1630

~~ n”4o . . ,: :: ‘.:.:.:.. ,... :.__.

FIG. 1. The McMurdo Dry Valleys region of Antarctica. Lake Hoare is located in Taylor Valley, west of the McMurdo Sound. In this figure lakes are shown in black and glaciers in a stippled pattern (after Andersen et al., 1993).

eming the lakes of the Antarctic Dry Valleys are given in Wilson (1981) and Torii and Yamagata (1981).

2.1.2. Geochemistry and mineralogy

Analyses of sediment cores from Lake Hoare (Nedell et al., 1987) indicated that the lake bottom sediments are dominated by moder- ately sorted, medium-grained sand, with compositions ranging from lithic arkose to feldspathic litharenite according to the classification scheme by Folk (1980). Samples collected from the ice cover and lake bottom were found to be mineralogically indistinguishable, although the grain-size tended to be smaller for the lake bottom samples Analyses of thin sections of these Lake Hoare sediments found quartz and feldspar contents near 30-40 ~01% (Nedell et al., 1987).

Major cation and anion concentrations in the melt streams and lake water have been measured for Lake Hoare by Green et al. (1988). The ratio of bicarbonate to Ca is higher in the lake water than in the meltstreams, which supports the idea that carbonates are transported into the lake through the ice cover. Green et al. (1988) also found that the water near the surface of Lake Hoare is supersaturated with respect to calcite, while the water below about 20 m is undersaturated, leading them to believe that CaC03 precipitates in the shallower regions of the lake and dissolves where the water is deeper. This would tend to result in spatial variations in carbonate concentrations in the lake bottom sediments (i.e., higher concentrations are expected near shore). Gardner et al. (1984) found that the nitrate/phosphate ratios varied with depth in Lake Hoare, where N concentrations were relatively low near the surface and below the anoxic zone (about 27 m deep), and relatively high at medium depth. Green et al. (1986b) measured trace metal abundances as a function of depth in Lake Hoare and found that the Fe, Mn, and Co levels increased substantially from 24 to 27 m deep. This coincides with the transition from the oxic to anoxic water zones in the lake.

2.1.3. Microbial mats and organic species

Microbial mats are typically present in the regions of the Antarctic Dry Valleys that are saturated with water during at least the austral summer (Wharton, 1993). The matrix of these mats is composed of the filamentous cyanobacterium Phormidium frigidurn and the primary algal species present are diatoms (Wharton et al., 1993). The sediment samples studied here are also under analysis for the character and abundance of carbonates, organic matter, and siliceous algal

Reflectance spectroscopy of Antarctic sediment 767

LAKE HOARE

Taylor Valley all depths in meters

FIG. 2. Location of the dive holes in Lake Hoare. Sediment core D was extracted from DH-1, cores A and B from DH-2, and core C from DH-4 (after Andersen et al., 1993).

remains. Preliminary results indicate that a mean value of 5.4%0 613C for carbonates and -17%0 613C for organic matter were observed for thirty-one samples from DH-1 and DH-2 cores from Lake Hoare (Doran et al., 1994). The concentration of diatoms was found to vary extremely for Lake Hoare DH-2 samples, where up to 117,000 diatom values per mg dry weight sediment were measured (Doran et al., 1994).

2.2. Reflectance Spectroscopy as Remote Sensing Technique

Reflectance spectroscopy is commonly applied to geological and geochemical studies of the Earth and other planets (e.g., Pieters and Englert, 1993). Remote identification of soils using reflectance spectroscopy benefits from comparison with diverse laboratory analyses; the spectroscopic properties of soils are far more complex than those of minerals or rocks. Reflectance spectra of mineral powders have shown spectral brightening and decreasing band strength with decreasing particle size in the visible to near-infrared region (Pieters, 1983; Gaffey et al., 1993) and spectral darkening and decreasing band contrast with decreasing particle size in the mid- infrared region (Salisbury et al., 1991). Spectroscopic analyses of laboratory mixtures of olivines, feldspars, and pyroxenes have shown that the spectroscopic properties of mineral mixtures are not simply a linear combination of their spectral properties, and that the fine- grained components (<25 pm particle size) contribute dispropor- tionately more to the spectral properties of soils (Crown and Pieters, 1987; Clark, 1983; Mustard et al., 1993). The particle shape in fine-grained soils or mineral mixtures also influences the form and position of the spectroscopic features (Hiroi and Pieters, 1992; Oehler and Hanowski, 1995; Oehler et al., 1995).

Laboratory spectroscopic analyses of mixtures as analogs for the soils on Mars (Morris and Neeley, 1982; Singer and Roush, 1983; Clark, 1983; Fischer and Pieters, 1993; Bishop et al., 1993) have shown that particle size, mineralogy, albedo, and the nature of mixing (i.e., coatings) all influence the spectroscopic properties of soils. Reflectance measurements of montmorillonite-carbon mixtures under moist conditions from 0.4 to 3 pm (Clark, 1983) and under dry conditions from 0.4 to 12 pm (Bishop et al., 1993) have illustrated the significant influence of a minor opaque component (<2 wt%) on the spectral properties of a mixture. Mixtures of Wyoming (SWy) montmorillonite from the Clay Minerals Society (l-2 pm p.s.) and carbon lampblack (<0.23 pm p.s.) exhibited 20 rel.% darkening of the visible to near-infrared reflectance and a change in the near infrared continuum slope from positive to negative for as little as 0.1 wt% C, with this effect increasing with the amount of C in the mixture (Bishop et al., 1993). Reflectance spectra of hematite (6

pm p.s.)-magnetite (13 pm p.s.) mixtures showed -10% spectral darkening in the visible to near-infrared region for 5 wt% magnetite, while similar spectra of hematite (0.1 pm p.s.)-magnetite (0.5 pm p.s.) mixtures showed -80% spectral darkening in this region and a negative near-infrared continuum slope for 5 wt% magnetite (Mor- ris and Neeley, 1982). Reflectance experiments with fine-grained ferric coatings on basaltic substrates have produced negative continuum slopes, as seen in spectra of the surface of Mars and have shown spectral dependence on the surface texture, viewing geometry, and wavelength (Singer and Roush, 1983; Fischer and Pieters, 1993).

3. METHODS

3.1. Sample Collection and Preparation

Cores were collected at dive holes DH-1, DH-2, and DH-4 from Lake Hoare as described in Nedell et al. (1987) and Wharton et al. (1993). A map of Lake Hoare, indicating the locations of these dive holes, is shown in Fig. 2. Dive holes DH-1 and DH-2 are located at about 130 and 190 m, respectively, from the lake shore. The bottom of the lake is - 10 and -15 m, respectively, below the surface of the ice near these dive holes and the lake bottom sediments here are in contact with oxic water. Dive hole DH-4 is farther from shore (- 300 m), where the lake is deeper (- 30 m) and extends into the anoxic zone. Several cores have been retrieved from a distance of about 5-10 m away from each dive hole and are numbered sequentially. Cores A (DH-2), B (DH-2), and D (DH-1) were collected from oxic zones of the lake, while core C (DH-4) was collected from the deeper, anoxic zone. The sediment cores were packed in dry ice for transportation and kept frozen in storage until needed. For sampling, the plastic pipes were slit lengthwise and the cores were thawed at room temperature. The sediment layers in each core were separated according to texture and color, presumably due to compositional differences, and were labeled beginning with 1 at the surface and increasing in number with depth. Some sections contained organic layers as well, which were separated and sublabeled with letters (a, b, c .).

Schematic drawings of the cores indicating sample identification are displayed in Fig. 3. The sand layers shown in Fig. 3 are separated according to approximate median grain size into fine sand (less than -250 pm particle size), medium sand (-300-500 pm particle size), and coarse sand (-500- 1000 pm particle size). Sand collected from the ice surface by R. A. Wharton during the 1992- 1993 summer is also included in this study, and is labeled surface sediment or “SS”.

Each sample was homogenized by hand-grinding with an agate mortar and pestle. Particle size distributions for these crushed sam-

J. L. Bishop et al. 768

Core A: DH-2 Core B: DH-2-37 Core C: DH-4-1 Core D: DH-l-l

5a

6

12.7 cm depth

LEGEND

@ coarse sand

0 medium sand

OOQ organic-rich

24 cm depth

32.5 cm depth -

6

44 cm depth

FIG. 3. Sketches of the sediment cores, indicating sample stratigraphy. The samples in each core were numbered, beginning with 1 for the sediment on the lake bottom and continuing with depth. The cores were divided into sample layers according to changes in the sample texture and color. Core B contains a large amount of organic material; these organic-rich samples are identified with the letters a, b, etc.

ples (prepared in the laboratory) were estimated using a vertical- sieving apparatus with 125, 63, and 45 pm sieves. In general, about one third by volume of the particles were between 63 and 125 pm, one third ranged from 45 to 63 pm, and about one third were less than 45 pm in size. The organic-rich samples contained a higher proportion of smaller particles. The initial grinding of the samples to approximately <I25 pm particle size was necessary to ensure that the sample aliquots measured in different laboratories were as similar as possible for each of the analyses. This may have altered the natural form of the particles and may have illitized some of the smectites; however, for this study it was more important to ensure that the multiple analyses were consistent for each sample.

When sufficient quantities were available after the initial measurements of these samples, the sample was ground further, resulting in -90% of the sample with less than 45 pm particle size. This was not possible for a few of the samples high in C. Reflectance spectra of these finer-grained, ground samples were measured as well. These samples were not sieved because that might have altered the composition of the samples by removing the minerals that are harder and present as larger particles.

3.2. Analytical Techniques

Bulk major element compositions were determined by standard X-ray fluorescence (XRF) procedures on powdered aliquots (about 1 g) of the samples. Precision and accuracy (as determined by standard and duplicate sample analysis) are similar at (in wt%) about 0.4 for SiOz, 0.3 for Na*O, 0.2 for A1203 and MgO, 0.1 for FezOx, CaO, and K20, 0.03 for TiOz, and 0.01 for MnO and P205; see also Reimold et al. (1994).

Trace elements were analyzed by instrumental neutron activation analysis (INAA). About 200-350 mg of the powdered rock samples were weighed into clean polyethylene vials that were then heat sealed. The samples were packed together with natural rock stan-

dards (from the USGS and other sources) and synthetic multi-element standards (adsorbed on high-purity quartz powder) into larger irradiation rabbits. Irradiations were performed at the Triga Mark II reactor of the Atominstitut der ijsterreichischen Universitaten at a neutron flux of 2.10” n/cm’ s. The procedures were checked and corrected by analyzing international geological reference materials (Govindaraju, 1987); precision and accuracy are below 10 rel.% for almost all elements, and in most cases between 0.5-5 rel.%. More details on the analytical procedures are given in Koeberl et al. (1987) and Koeberl (1993).

Measurement of C and S concentrations in the samples was performed with an elemental analyzer by Carlo Erba Instruments (NA 1500, series 2). The samples were dried by heating to 105°C to remove adsorbed water, and placed in tin capsules for measurement. The analytical standard 5-chloro-4-hydroxy-3-methoxy-benzyliso- thiourea phosphate (C,H,,ClN,O,PS) was used for these analyses. Selected samples were treated with 4 N HCl in silver capsules to dissolve any carbonates, then dried at 40°C for 5 h. These samples were remeasured for organic C, and the amount of inorganic C was determined by difference.

The mineralogical composition of the samples was measured by X-ray diffraction (XRD) of pressed powders of the samples using a Philips PW 1710 diffractometer.

3.3. Statistical Treatment of Chemical Data

Multivariate statistical analyses were applied to the chemical data in order to search for natural divisions of the samples based on chemical composition. These statistical analyses were performed using the software package, SPSS” (release 4.0). An initial run was performed on a dataset of thirty-four samples, each with thirty- four trace element values. Some samples were excluded from the statistical analyses because incomplete elemental data were available. The Hg values were also excluded because they were below


the detection limit for many samples. Major element data were included in a second statistical analysis, where twenty-four samples, each with thirty-nine major and trace element values, were used.

Natural logarithms of the elemental concentrations were calculated for all data included in the analysis to reduce the possible influence of outliers on the average values used for the standardiza- tion of the data. In order to give equal statistical weight to differences in the elemental abundances, the data were modified to give an average of 0 and a standard deviation of 1 for the distribution of each element in the dataset (z-transformation). This is important, as the abundances range from wt% to sub-ppb levels for the elements measured.

Principal components analysis, PCA, was carried out on these modified data to reduce the number of variables in the analysis, as well as to eliminate interelement correlations, which might bias the group assignments. These principal components were taken as input for the cluster analyses. Several clustering algorithms were used to extract stable clusters within the dataset. The groups found by cluster analysis were subjected to discriminant analysis to test for spatial separation and to select the most important variables for cluster discrimination.

3.4. Reflectance Spectroscopy

Spectra were measured relative to Halon from 0.3 to 3.6 pm under ambient conditions with the RELAB (reflectance experiment laboratory) spectrometer at Brown University. The Halon spectrum is multiplied by wavelength- and angle-dependent correction factors, giving absolute reflectance values for the samples. A photomultiplier tube is used for the visible to near-infrared region and an InSb detector is used for the infrared region. This instrument has been described in detail previously (Pieters, 1983; Mustard and Pieters. 1989). The RELAB spectrometer allows for variable illumination (i) and emergence (e) angles. Spectra included here were measured at i = 30” and e = 0”. The sample dish is rotated during the measurement to eliminate orientation effects.

Infrared reflectance spectra were measured relative to a rough Au surface, using a Nicolet 740 Fourier transform interferometer (FTIR), in a H,O- and CO,-purged environment. A PbSe detector was used in the range 0.9 pm-3.2 pm and a DTGS (deuterated triglycine sulfate) detector between 1.6 and 25 pm. The sample chamber, including a remotely controlled sample train, was manufactured by SpectraTech and uses a biconical arrangement. Composite spectra were prepared by splicing RELAB data at 1.2 pm with Nicolet data (1.2-25 pm). The Nicolet spectra were scaled to those measured using the RELAB spectrometer.

Sample preparation for the spectral measurements involved pour- ing the particulate material into a sample dish 12 mm in diameter and 3 mm in depth. The dish was tapped gently on a hard surface to settle the particles, then refilled. This procedure was continued until the dish remained full. The sample surface was not smoothed with a spatula. For measurement of some of the more finely ground samples, where very small amounts of material were available, it was necessary to use smaller sample dishes.

4. RESULTS AND DISCUSSION

4.1. Mineralogical Composition

4.1.1. X-ray diffraction and elemental analyses

Qualitative mineralogical analysis were performed using XRD techniques on pressed powders. The major mineralogy was found to be very similar throughout each core. The primary composition of all sediments in this study consists of quartz, pyroxene, and feldspar. Both orthoclase and pla- gioclase forms of feldspar were identified in these samples. Quartz and feldspar contents in the range 30-40 ~01% are likely based on previous analyses of similar samples (Nedell et al., 1987). Many of the core B samples (B-2a, 3a, 3b, 3c, 3d, 3e, 3f, 5a, 5b, 7a) contain major amounts of carbonate

in addition to the other major minerals. These carbonate compositions were especially notable in the inorganic C measurements. This carbonate is primarily present in the form of calcite and is discussed in detail in a later section. Although calcite peaks were identified in the XRD spectra, the presence of other carbonates, such as dolomite, can not be ruled out from the XRD data alone. Quantitative mineralogical measurements were not possible in this study because of the small amount of sample available, the complex nature of these multicomponent sediments, and the destructive nature of quantitative XRD techniques.

Smaller quantities of several other minerals were also identified by XRD. Most samples contain secondary amounts of mica, chlorite, and amphibole. For the samples B-la, B- I, B-2, B-4, B-5, B-IOa, B-10, and D-l carbonate is also a secondary component. The low clay fraction content of the samples made determination of clay silicates in these samples difficult. However, minor amounts (on the order of a few ~01% or less) of illite, smectite, and perhaps vermiculite were found in many samples.

4.1.2. Visible to near-infrared spectroscopy

Reflectance spectra from 0.3 to 5 ym, measured using the RELAB and Nicolet spectrometers, are shown in Fig. 4, panel A. Broad spectral bands near 1 and 2 pm indicate the presence of pyroxene in most of the samples (B-2, B-3d, B- 4, B-5, B-6, B-7, B-8, B-9, B-IOa, B-10, B-II, C-all, D-5, and the surface sand). A narrow spectral feature near 0.7 pm is due to pigmenting agents such as chlorophyll or caro- tene in organic materials and is very similar to a spectral feature observed for green algae (Gaffey et al., 1993). It is readily observable in samples B- 1, C- 1, C-2c, and C-6c and barely observable in samples B-2a, B-3 (a-f), B-7a, B-S, C- 3, C-4b, C-5, and D-l. The relatively broad features near 1.4, 1.9, and 2.5 ym are due to bound or adsorbed water, and are best observed for samples B- 1 a, B-l, B-2a, B-3 (a- f), B-5a, B-5b, B-5, B-7a, D-l, D-2, and D-4. The features near 2.2 pm found in the spectra of D-l, D-2, and D-4 are characteristic of aluminosilicates, and the features near 2.3 pm are probably due to carbonates or smectites. A summary of the spectroscopic features observed for these samples is given in Table 1.

The wavelength of the pyroxene band near 1 pm can provide information about the variety of pyroxene present. As Ca and Fe substitute for Mg in the pyroxene structure, the position of this band center shifts toward longer wavelengths (Adams, 1974; Hazen et al., 1978). Modified Gaussian analyses of spectra of intimate mixtures of orthopyroxene and clinopyroxene have shown that band centers at 0.9-0.93 pm indicate the presence of orthopyroxene or low Ca-clinopy-

roxene, and band centers at 0.98- 1.0 pm indicate the presence of high-Ca clinopyroxene. The composite band centers for mixtures of two pyroxenes fall between these values and MGM fitting approaches are required to separate individual components (Sunshine et al., 1990; Sunshine and Pieters, 1993). Band centers in spectra of the Antarctic sediments presented here were estimated visually using the method of Clark and Roush (1984) and values clustered around 0.93 pm, indicating the presence of orthopyroxene and low-Ca

770 J. L. Bishop et al.

Panel A Panel B 0.6

0.4

0.2

0

0.4

0.2

0- w- .-. -.-. - .-. c -.-. , .---.--. , :

0.4 l,"'.' i , -T

.,’ ‘\ -

0.2 ; ', . a 5

o-,""""""""""',- 1 2

Wavelength Lrn) 4 5

-.-.-B-3a

- - -B-3b

-B-3d

FIG. 4. Reflectance spectra from 0.3 to 25 pm for Lake Hoare samples from cores B, C and D ground to approx. <I25 pm particle size. The spectra are offset in groups and labeled in panel B. This figure is subdivided to allow better illustration of the spectra: panel A includes spectra in the range 0.3-5 pm and panel B includes spectra in the range 5-25 pm. These reflectance values were measured relative to Halon and corrected to give absolute reflectance.

Reflectance spectroscopy of Antarctic sediment

Table 1 Spectral Features Observed in Reflectance Spectra of Lake Hoare Sediments.

Wavelength Wavenumber Vibration/ w cm-’ Transition

0.9-l .05 -10.000 Fez+ electronic

1.8-2.3 -5000

0.7 -14,000

1.46 -6850

1.97 -5080

2.5 4000

2.2 4545

2.3-2.4 -42004300

2.75 -3640

-3 -3000-3500

3.5 -2860

4.0 -2000

-6 -1600

-7 -1400

-7.5 -1330

-7.9 -1270

8-9 1200-l 100

-11.5 -880

-12 -850

-12.5 -800

-14 -730

-18-19 -550

Mineral Samples

(crystal field theory) Fez+ electronic (crystal field theory) organic pigments

Hz0 combination band



AlM-OH comb. band

CO3 comb. band M;?-OH comb. band OH stretching vibration

Ha0 stretching vibrations & Ha0 bending overtone

C-H stretching vibrations

CO3 comb. band

Ha0 bending vibration

Christiansen feature

pyroxenel au

pyroxenel B-2, 3d, 4, 5, 6, 7, 8, 9, B-10, lOa, 11; C-all, D-5; SS

organic2 *B-l , C-l, 2,6c

bound H203 B-la, 1, 2a, 3a-f, 5a, 5, 7a, D-l, 2,4

bound H203 same as 1.46

bound Hz03 same as 1.46

alutninosilicates D- 1.2, 4

carbonates4 or smectites3 aluminosilicate3 B-la, 1, 2a, 2, Sa, 5b, 7, 8,

B-9, lOa, 10, 11; C-all; D-all; SS bound Hz03

organicsz

carbonates4

Hz03

carbonates5

CpltZ6

feldspd

quartz6

carbonates5

feldspar6

quartz6

carbonatess

quartz”

Christiansen feature

Chrlstiansen feature

Reststrahlen features

C@ bending vibration



CO3 bending vibration


*B-la, 3a-f, 5a, 5b, 7a, 10a; C-l

*B-3a to d, 3f, 5a, 5b. 7a, 10a

au

B-3al1, 5a, 5b, 5,7a, 7, 10a

B-2, 4, 7, 8, 9, lOa, 10, 11; C-2c, 3,4b, 5,7a, 10; D-l, 4, 5; SS B-4,6, 9, 10, 11; C-l, 4b, 5,7a, 10; D-l, 2,4; SS B-2, 3d, 4, 6, 7, 8, 9, lOa, 10, 11; C-all; D-all; SS B-3a to d, 5a, 5b

B-la, 1, 2a, 4, 5, 6, 9; C-l B-7, lOa, 10; C-2c, 3,4b, 7a; D-all: SS B-3a to d, 5a, 5b

B-2, 4, 6, 7a, 7, 9, lOa, 10, 11; C-all; D-all; SS

771

*note: samples listed only where strong bands are observed; M refers to metal cations (generally Al, Fe, or Mg). ‘BURNS (1993). kg., GAFEY et al. (1993). 3e.g., BISHOP (1994). 4e.g., CALVIN et al. (1994). 5Farmer (1974). ~SALISBURY et al. (1991).

clinopyroxene in most of the pyroxene-bearing samples. This band occurs at a slightly longer wavelength in spectra of samples B-2, B-4, and D-5, indicating the presence of some high-Ca-pyroxene as well.

4.1.3. Infrared spectroscopy

Reflectance spectra from 5 to 25 pm, measured using the Nicolet FUR spectrometer, are shown in Fig. 4, panel B, and at shorter wavelengths in panel A. A strong, broad band centered near 3 pm is observed in each sample and is due to bound or adsorbed water. The feature near 2.75 ,um in

samples B-la, B-l, B-2a, B-2, B-5a, B-5b, B-7, B-8, B-9, B-lOa, B-10, B-l 1, C-all, D-all, and the surface sand is characteristic of structural OH in clays and other aluminosilicates. The spectral features near 3.5 pm are consistent with both carbonates and organic components and are observed strongly (band depth >15%) in samples B-la, B-3 (a-f), B- 5a, B-5b, B-7a, B-lOa, and C-l. Intense carbonate features (band depth >15%) near 4.0 pm are observed in samples B-3 (a-d, f), B-5a, B-5b, B-7a, and B-1Oa. Weaker features (band depth 5-10%) are observed for samples B-l, B-2a, B-3e, B-5, B-7, and B-10 and very weak features (band depth <5%) for B-la, B-4, C-6c, D-l, D-2, and D-4. Band depths of the organic and carbonate features were measured


according to the method of Clark and Roush (1984) and are listed in Table 7.

The carbonate features imply the presence of calcite in many samples, and possibly smaller amounts of magnesite in some samples. Calcite spectra exhibit a triplet at 2980 cm-’ (-3.36 pm), 2923 cm-’ (-3.42 ym), and 2867 cm-’ (-3.49 pm) and a doublet at 2607 cm-’ (-3.84 pm) and 2508 cm-’ (3.99 pm) (S. A. Sandford, 1995, pers. commun.; Calvin et al., 1994). Spectra of magnesite exhibit less distinct features near 2860 cm-’ (-3.5 pm) and a doublet at 2620 cm-’ (-3.82 pm) and 2550 cm-’ (-3.92 pm) (Sandford, 1995, pers. commun.; Calvin et al., 1994). C-H stretching vibrations, indicative of the presence of organic material, occur in the range 3.35-3.51 pm (-3000-2800 cm-‘) (e.g., Gaffey et al., 1993). Spectral features due to the symmetric (v,) and asymmetric (z+) stretching vibrations occur at 3.48 and 3.41 pm (2874 and 2933 cm-‘) for CH3 and at 3.51 and 3.41 pm (2849 and 2933 cm-‘) for CH,.

A relatively strong feature near 6.1 pm in each spectrum is due to water. Strong absorption bands are also observed near 7 pm for the samples containing carbonates. The Chris- tiansen feature is the reflectance minimum that occurs near 8 pm in these spectra. The doublet feature occurring as a reflectance maximum near 8-9 pm is due to quartz and is present in every sample.

The Christiansen frequency is defined as the frequency at which the real part of the refractive index of individual particles equals that of the surrounding medium (Hem-y, 1948; Conel, 1969). This feature occurs where the real index of refraction, n, is near unity and the extinction coefficient, k, is decreasing strongly. For powdered silicates the Christiansen frequency occurs at the frequency of maximum transmission and minimum reflectance (Salisbury, 1993). The Christian- sen feature can be used for sample identification and has been measured for numerous rocks and minerals (Salisbury et al., 1991; Salisbury, 1993). The Christiansen feature occurs near 7.4 pm for quartz, near 7.9 pm for feldspar, and near 8.4 pm for pyroxene (Salisbury, 1993).

For most of the samples studied here the position of the Christiansen feature in the spectra is consistent with the presence of quartz and feldspar. Based on the Christiansen feature, quartz is spectrally dominant for samples C-2c, C- 3, C-4b, C-7a, C-10, D-l, D-2, and D-3, while feldspar is spectrally dominant for samples C-l, C-5, C-6c, D-4, and the surface sand. Reststrahlen features are observed as reflectance peaks at longer wavelength and are highly sensitive to particle size. These features are discussed in more detail below.

Spectra of the organic-rich samples exhibit a strong, broad feature near 7 pm that hinders identification of the Christian- sen feature in these samples. This is readily observable in samples B-3 a-f, B-5a, B-5b, B-5, B-7a, and B-1Oa. Also in this region is a narrower, carbonate asymmetric stretching band that is centered near 7 pm (1410-1435 cm-‘) for calcite, near 6.95 pm (1425- 1440 cm-‘) for dolomite, and near 6.9 pm (1450 cm-‘) for magnesite (Farmer, 1974). Additional carbonate out-of-plane, and in-plane, bending features are observed, respectively, near 11.4- 11.5 (870- 880 cm-‘) and 14.0-14.1 (710-715 cm-‘) for calcite, near 11.4 (880 cm-‘) and 13.7 (730 cm-‘) for dolomite, and near

11.3 (880-890 cm-‘) and 13.3 (750 cm-‘) for magnesite (Farmer, 1974; S. A. Sandford, 1995, pers. commun.). Based on the CO3 fundamental stretching and bending modes, calcite is detected in samples B-la, B-2a, B-2, B-3 all, B- 5a, B-5b, B-5, B-7a, and B-7.

4.2. Elemental Composition

The elemental abundance values are given in Table 2 for core A, Table 3 for core B, Table 4 for core C, and Table 5 for core D and the sample collected from the ice surface. The spread in chemical composition of these samples is not very broad, indicating similar sediment origins. The abundance of S in these samples was extremely low. Sulfur abundances of 0.3, 0.2, and 0.1 wt% were measured for samples C-l, C-2c, and C-3, respectively. For all other samples, S abundances were below the detection limit (-0.05 wt%).

Depth profiles of selected elements are shown for core B in Fig. 5. Variations in the abundances of these elements are generally observed in conjunction with the presence of organic material and carbonates in the samples. In general, Br, Rb, Cs, Th, U, and Zn abundances are elevated in the organic-rich layers and SC and Cr abundances are reduced. For the samples containing carbonates, Rb, SC, Th, Zr, Hf, Cr, Zn, and Co abundances are lower than average, while Sr and U abundances are higher than average. Many samples contain both organic material and carbonates, which creates complex patterns for the elements with competing trends.

Some trends in the elemental abundances of the chemical data for the core C and D samples may be directly related to the mineralogical composition of the samples. For example, the Cr and Mg values appear to be correlated with each other and probably indicate the presence of pyroxene. For the core C samples, C-10 has the highest amount of both Cr and Mg and C-5 has the lowest amount of both elements. A similar pattern can be observed for the sediments in core D, where D-5 has the highest Cr and Mg levels and D-4 has the lowest Cr and Mg levels. The presence of additional Cr and Mg suggest an increased pyroxene content in these samples. Examination of the reflectance spectra shows stronger pyroxene bands near 1 and 2 pm for C-10 and D- 5 than for C-5 and D-4. The visible to near-infrared spectral reflectance is also darker for samples C-10 and D-5 than for others, which is consistent with additional pyroxene in these samples. These results indicate that for quartz- and feldspar- rich sediments, general trends in the pyroxene content can be observed through the Cr and Mg abundances.

Shown in Fig. 6 are chondrite-normalized abundances of the rare earth elements (REEs). In general, the REE abundances are higher for the B-core sediments than for the others. The REE abundances also tend to be higher for the organic-rich samples.

Statistical analyses were performed on the elemental abundances to search for clusters of samples with similar abundance patterns. Data reduction by PCA yielded six significant principal components, which accounted for more than 86% of the overall variation in the dataset. Certain groups of elements exhibited good correlation of their contents with each other; for example, the abundances of the light REEs correlate well with the Th and Cs contents and the heavy


TABLE 2: L,AKE HOARE SEDIMENTS - CORE A

Sampk A-l A-2 A-3 A4 A-5 A-Se A-6 -pth c-1 0.0-1.0 0.0-3.2 3.2-4.5 4.5-7.0 7.0-10.0 10.0-12.7 12.0-12.7

Na WW K WtW SC Cr Mn WW Fe WW CO zn As Rb Sr zr Sb CS Ba La Ce Nd Sm Eu Tb

2 LU Hf Ta W Au (ppb) Tb u

1.25 1.60 1.95 1.89

14.3 14.2 92.0 120 0.093 0.064 5.81 4.29

14.5 11.8 38 7 1.09 1.42

123 111 1400 1700

1.75 1.63

12.7 110

0.054 3.10 9.1 3 0.80

87 1200

1.98 1.66

17.4 150

0.061 3.68

11.8 3 1.59

170 200 89 250 160 150 160 0.86 0.50 0.48 0.69 0.69 0.56 0.49 3.5

300 34 71.8 30 5.18 1.4 0.8 4.4 1.9 0.25 4.32 2.06

<2 5 7.08 1.44

2 300

24 49.8 20 4.19 1.3 0.6 3.4 1.8 0.25 3.47 1.22

Cl 5 5.38 1.16

1.3 1.3 280 240

18 24 36.2 46.5 16 21 2.81 3.11

0.5 2.7 1.2 0.18 2.44 0.69

<l Cl.5 0.3 2 3.82 5.64 0.57 0.62

0.6 3.6 1.4 0.22 2.35 0.55

13542 16293 28596 26774 4.92 4.64 6.70 9.10

39.35 57.64 36.48 106.38 12.09 9.01 10.14 11.58

1.85 2.10 1.56 1.66 1.82 1.53

12.5 15.0 16.5 130 130 130

0.057 0.054 0.067 3.51 3.45 4.07

10.7 10.6 12.1 35 8 9

1.10 1.16 0.74 87 90 88

1200 1200 1600

1.3 1.4 230 260

15 23 30.2 45.5 19 16 2.79 3.10 0.9 1 0.5 0.5 2.6 3.1

1.9 250

21 40.4 16 3.58 1 0.5 3.2

1.4 1.4 1.7 0.22 0.20 0.26 2.25 2.64 2.79 0.63 0.68 0.89

Cl.5 1.13 Cl.5 2 1 3 3.10 5.78 3.93 0.92 1.01 0.62

18043 18020 24677 3.37 5.72 6.34

71.11 56.82 57.35 7.24 11.10 8.35

Eu/Eu* 0.913 1.090 1.124 0.976 1.012 1.069 0.992

All data in ppm, except as noted.

REE abundances are well correlated with those of Fe, Co, Cr, and SC. These highly correlated elements (r > 0.75) were combined to form the individual principal components. These were then used as input for hierarchical cluster analysis. The best classification results were obtained using Ward’s clustering algorithm (Ward, 1963).

Sample B-7a exhibits enhanced abundances of REEs and appeared as a distinct group after the initial statistical treatment. In order to better distinguish among the remaining samples, sample B-7a was excluded from the dataset for statistical analysis. The elemental abundance averages were recalculated, leaving four discernible clusters in the PCA dataset. A list of the samples in these four groups is shown in Table 6-A.

The majority of the core B and D samples are grouped into two clusters (Groups Al, A2). The samples of Groups Al and A2 are distinguished primarily by their organic matter content and grain size, such that the samples of Group Al are richer in organic matter than those of Group A2. Four samples from core B that are high in Sr and low in all other elements, including REEs, form another group (Group A3) that is separated markedly from the other groups. These samples are also very fine-grained sediments that contain larger amounts of carbonate and organic matter. The samples

from core C form a cluster of their own (Group A4), with the exception of two samples that have higher organic matter contents. The surface sand sample falls into this group as well.

Discriminant analysis confirmed the classification results of cluster analysis. The best discriminating variables for the fine-grained core B sediments, rich in carbonate and organic matter (Group A3), are Sr, Fe, Co, Cr, SC, Zn, and the heavy REEs. For separation of core C samples, plus surface sand, from the remaining B and D core samples, the abundances of Na, K, Rb, Ba, Zr, Hf, and Eu are most important. The two clusters containing B- and D-core samples are best separated by their abundances in As, Se, Sb, and U contents, where samples richer in organic matter (Group Al) have lower abundances in the former elements, but are higher in U. The separation of these four clusters by three discriminant functions is shown in Fig. 7.

Results of the statistical analyses of trace element data were confirmed when including the major element data in the analyses. Correlation analysis exhibited significant correlations of Mg with Co, Cr, Fe, Ni, and SC in the dataset. PCA yielded only five significant principal components due to the lower number of samples processed. These principal components accounted for about 87% of the overall variance

TABL

E 3:

LA

KE

HOAR

E SE

DIM

ENTS

-

CORE

B

Sam

ple

Bla

El

B-za

n-

2 B-

3a

B3b

B-3c

~3

d E3

e B-

3f

B-4

B-sa

BS

b B-

S 8.

6 57

a E-

7 B-

8 B-

9 El

Oa

El0

B-11

De

pth

(cm

) 00

.5 0.

5-1.

5 1.

5-2.

0 2.

0-6.

0 6.0

625

6..7

5-6.

5 6.

56.7

5 X5

.75-7

.0 7.

e7.2

5 7.

25-7

.5

758.5

as

-&,5

8.

75-9

.0

9.Q1

0.0

lO.o

-12.

0 12

.cbt2

.5

12.5

-13.

0 x3

.0-1

7.0

17.c-

18.0

10

519.0

19

.cL20

.0

m.x

z,.c

l

SiO

Ti

Oi

&O,

F%Q

M

nO

MgO

CilO

Na

,O

%O

PZQ

LO

.1.

53.93

52

.98

62.68

63

.41

41.25

27

.18

61.55

63

.64

63.50

63

.51

64.00

61

.12

64.15

0.8

4 0.5

8 0.4

8 0.4

2 0.5

6 0.3

7 0.4

5 0.4

4 0.4

3 0.4

6 0.4

8 0.4

8 0.4

2 12

.45

11.50

12

.37

12.12

8.9

6 6.3

4 12

.13

12.76

12

.99

11.87

13

.89

12.46

13

.65

6.99

6.76

5.87

5.66

5.57

3.66

2.71

2.92

5.27

5.42

5.60

4.63

3.08

5.05

5.31

5.26

4.98

5.99

4.99

4.90

5.49

4.88

0.092

0.0

94

0.097

0.0

88

0.095

0.0

64

0.084

0.0

84

0.083

0.0

9 0.0

85

0.085

0.0

82

4.95

5.02

6.31

6.21

3.81

2.82

5.31

5.33

5.15

6.59

4.81

5.66

4.98

8.01

9.63

6.33

6.20

14.88

26

.85

6.99

5.39

5.79

5.82

5.27

6.42

5.21

1.71

2.27

2.25

2.63

1.16

1.00

0.54

1.08

1.53

2.07

2.76

1.71

1.21

2.53

3.02

1.71

2.95

2.53

3.04

2.07

2.47

3.05

2.16

2.23

2.03

1.89

1.68

1.32

1.08

1.08

1.92

2.16

2.02

1.58

1.05

2.08

2.17

1.80

2.24

1.93

2.27

2.16

2.07

2.34

0.241

0.1

82

0.15

0.139

0.1

86

0.17

0.148

0.1

5 0.1

46

0.142

0.1

63

0.15

0.14

7.95

9.10

1.06

0.96

20.98

30

.22

3.92

0.72

1.52

0.58

1.29

2.66

1.26

TOta

l 99

.72

99.23

99

.66

98.64

10

0.25

100.1

6 99

.93

99.35

99

.01

99.78

99

.51

100.2

9 99

.06

c c-

1 3.5

0 1.6

8 2.2

0 0.2

5 6.7

4 8.5

7 11

.98

7.30

6.49

4.47

0.48

4.89

8.35

0.83

0.09

5.66

0.28

0.12

0.18

1.45

0.49

0.20

SC

14.9

15.3

17.1

20.8

12.8

9.07

5.21

9.60

16.0

14.9

23.4

13.8

8.17

20.9

21.5

20.0

20.1

23.4

16.6

17.3

20.3

17.2

Cr

107

116

138

182

97

22

43

82

135

135

222

104

63

175

189

141

174

221

148

157

189

168

CO

29.1

25.0

24.5

26.7

23.3

16.6

13.1

14.3

27.1

25.3

29.5

21.4

13.0

26.7

28.1

25.8

26.1

32.1

24.2

24.8

29.2

23.3

Ni

75

56

71

94

63

79

<38

21

61

51

61

51

31

94

70

159

88

150

105

55

69

142

Zll

101

84

74

57

82

52

45

42

85

82

72

65

48

68

69

87

58

77

66

63

76

85

Ga

10

11

14

21

19

7 6

9 22

18

63

33

9

39

n.d.

16

15

22

16

73

14

15

As

2.2

2 0.8

9 0.7

5 co

.4

1.07

0.94

0.91

0.33

1.00

1.06

0.30

0.89

0.82

0.34

n.d.

1.0

2 0.3

7 0.2

9 0.4

6 0.4

5 0.4

0 0.2

4 Se

7.

8 1.

4 1.

2 0.

6 1.

8 1.

4 1.

7 0.

6 1.

6 1.

7 0.

8 2.

0 1.

4 1.

0 1.

0 1.

9 1.

0 0.

9 1.

1 1.

1 0.

8 0.

8 Br

41

.4 15

.6 13

.4 0.

7 60

.9 71

.3 96

.8 32

.6 75

.1 33

.2 1.

1 25

.0 22

.9 4.

7 Cl

0 41

.9 1.

3 0.

4 1.

5 5.

3 2.

9 0.

8 Rb

95

.6 78

.3 71

.3 68

.2 70

.7 53

.0 40

.9 42

.8 74

.7 92

.5 80

.6 81

.0 50

.7 80

.9 29

.5 82

.1 98

.8 71

.8 90

.6 72

.5 84

.7 86

.7 SK

40

6 35

7 36

0 25

9 53

5 54

5 66

3 58

4 36

8 46

2 31

8 56

5 72

9 33

1 34

2 52

2 36

7 27

6 37

2 36

6 38

0 32

6

5 10

2 0.12

2 0.3

9 13

1 0.02

0.0

5 12

6 0.02

0.0

8 76

0.06

0.09

134 co.1

4 0.0

8 84

0.14

0.07

<0.1

9 39

0.04

45 0.0

4 0.0

4 co

.2

co.2

5 81

12

8 x0.1

0.0

8 82

0.01

0.06

co.1

73

0.07

~0.1

18

67 0.0

4 94

0.03

0.13

-co.

09

71 0.1

4 12

1 0.04

0.1

4 10

0 0.07

0.0

2 81

0.08

0.06

101 0

.04

0.02

95 0.0

5 0.0

5 14

5 0.05

0.0

7 90

0.01

0.04

CS

2.51

1.64

1.54

0.81

1.85

1.32

1.32

0.81

2.09

1.99

1.02

1.62

1.12

1.03

0.99

1.40

1.09

0.98

1.30

1.21

1.25

1.09

Ba

300

272

271

235

218

179

118

151

247

291

332

238

157

289

333

268

343

165

368

282

345

341

la 31

.4 31

.3 24

.5 16

.0 31

.0 22

.7 24

.0 13

.7 29

.8 27

.2 22

.1 28

.5 22

.2 26

.2 25

.6 57

.6 21

.3 18

.3 22

.6 24

.6 20

.8 17

.2 Ce

64

.6 59

.6 44

.3 32

.0 49

.4 37

.3 36

.7 24

.2 55

.4 50

.1 41

.9 57

.3 35

.8 48

.1 42

.8 58

.1 39

.7 34

.9 42

.1 44

.5 39

.2 35

.4 Nd

27

.4 24

.8 19

.7 14

.4 22

.9 17

.0 17

.9 10

.3 21

.4 19

.8 17

.6 22

.3 15

.1 20

.3 18

.9 57

.6 17

.8 16

.0 18

.1 20

.4 17

.1 17

.3 Sm

4.7

2 4.3

1 3.7

3 2.9

2 4.5

9 2.9

4 2.4

9 2.1

1 3.8

5 3.8

7 3.3

2 4.0

7 2.6

2 3.7

6 3.4

8 9.7

4 3.5

6 3.3

1 3.5

0 3.8

2 3.5

7 3.2

2 EU

1.1

8 1.1

0 1.0

0 0.8

9 0.9

8 0.6

4 0.4

7 0.5

2 0.8

8 0.9

8 0.9

3 0.8

7 0.6

0 0.9

7 1.0

5 2.0

1 1.0

4 0.8

8 1.0

4 0.9

2 1.0

1 1.0

2 Tb

0.5

3 0.4

9 0.5

4 0.4

2 0.4

9 0.3

8 0.2

6 0.3

0 0.4

0 0.4

3 0.5

1 0.4

4 0.3

5 0.4

9 0.3

3 0.5

3 0.5

5 0.5

2 0.5

1 0.5

5 0.5

9 0.9

5 T

m

0.2

0.2

0.2

0.2

0.2

0.2

0.1

0.1

0.2

0.2

0.2

0.2

0.2

0.2

0.2

0.4

0.2

0.3

0.2

0.2

0.2

0.3

Yb

1.32

1.39

1.37

1.39

1.55

1.08

0.80

0.89

1.32

1.39

1.44

1.25

0.89

1.42

1.63

2.33

1.48

1.60

1.44

1.46

1.57

1.74

LU

0.17

0.20

0.20

0.20

0.23

0.16

0.12

0.13

0.19

0.20

0.21

0.19

0.12

0.21

0.24

0.32

0.22

0.22

0.19

0.20

0.21

0.24

Hf

2.85

2.76

2.40

2.42

2.51

1.69

0.94

1.36

1.84

2.54

2.11

1.97

1.46

2.36

2.38

3.21

2.69

2.32

2.58

2.48

2.72

2.59

Ta

1.18

0.95

0.74

0.35

0.95

0.54

0.40

0.32

0.59

0.87

0.48

0.85

0.42

0.45

0.44

0.56

0.54

0.47

0.65

0.59

0.65

0.91

W

1.5

1.0

0.9

1.1

1.8

0.8

0.6

0.5

2.8

1.2

1.3

1.5

0.9

0.8

n.d.

0.

2 1.

1 1.

2 0.

5 0.

9 1.

1 0.

8 Au

(iv

b)

2.3

0.2

0.1

0.2

0.3

0.6

1.7

0.1

0.7

0.7

0.4

0.7

0.7

0.7

n.d.

2.

3 40

.1 40

.1 40

.1 0.

3 0.

2 2.

6 Hg

co

.1

co.1

0.1

7 co

.1

0.25

0.34

0.53

0.51

0.81

<O.l

<O.l

GO

.1

0.62

~0.1

co

.1

0.86

<O.l

<O.l

0.05

co.1

0.1

0 0.5

0 Th

6.8

0 6.4

2 4.9

9 3.5

8 5.7

4 4.0

9 4.1

9 2.3

7 7.1

9 5.7

3 5.2

1 6.5

0 4.1

8 5.7

6 6.5

0 5.1

4 4.8

3 4.6

6 5.4

5 5.1

5 5.0

3 4.2

0 U

1.06

0.85

0.79

0.38

0.85

1.33

1.42

1.06

1.38

1.08

0.48

1.20

1.07

0.60

0.13

0.42

0.67

0.59

0.90

0.83

0.67

0.75

WU

1688

0 21

860

2141

0 41

450

1668

2 84

45

6141

84

95

1121

7 16

427

3507

0 10

970

9810

28

890

1391

00

3598

7 27

860

2756

0 21

020

2203

0 25

750

2600

0 Th

/U

6.39

7.58

6.31

9.38

6.74

3.08

2.94

2.24

5.20

5.29

10.80

5.4

2 3.9

0 9.6

5 SO

.38

12.17

7.1

9 7.9

1 6.0

4 6.2

4 7.5

4 5.6

3 Zr

/Hf

35.74

47

.44

52.54

31

.36

53.43

49

.74

41.61

33

.10

44.00

50

.36

38.84

37

.10

45.86

39

.76

29.82

37

.71

37.12

34

.88

39.09

38

.27

53.33

34

.74

Qm

N 16

.08

15.29

12

.09

7.79

13.54

14

.17

20.20

10

.39

15.22

13

.22

10.36

15

.43

16.95

12

.47

10.63

16

.72

9.72

7.72

10.58

11

.42

8.99

6.70

WEu

** 0.9

9 1.0

1 0.9

3 1.0

7 0.8

7 0.8

0 0.7

7 0.8

8 0.9

4 1.0

1 0.9

6 0.8

7 0.8

4 0.9

5 1.0

6 0.8

8 1.0

0 0.8

7 1.0

3 0.8

8 0.9

6 0.9

9

Maj

or

elem

ent

dam

in

wt%

, tra

ce

elem

ent

data

in pp

m,

excq

t as

no

ted;

Eu

/Eu”

=Eu,

/

v’(S

m,*T

b,,);

n.

d.

= no

t de

term

ined;

bla

nk

spac

es

in m

ajor

ele

men

t se

ction

: no

t en

ough

sa

mple

fo

r XR

F an

alysis

.


TABLE 4: LAKE HOARE &DIMENTS - CORE C

-pie c-l cat G3 c4b G5 c& G7a G10

SiO, TiO, w3 WA MnO MgO CaO Na20 W w, L.O.I.

59.99 62.47 63.22 63.35 63.% 61.43 63.25 60.78 0.60 0.45 0.43 0.41 0.45 0.44 0.49 0.52

12.75 12.08 12.29 12.09 14.00 12.62 11.50 10.97 6.17 5.71 5.76 6.01 4.84 5.38 6.38 7.78 0.087 0.09 0.086 0.088 0.083 0.08s 0.093 0.102 5.49 6.06 5.59 5.94 4.90 5.44 6.99 7.82 5.46 5.57 5.41 5.49 4.90 5.33 5.55 6.57 2.75 2.53 2.69 2.74 3.47 2.97 2.68 2.47 2.12 2.02 2.17 2.01 2.36 2.13 1.98 1.72 0.186 0.135 0.149 0.137 0.164 0.168 0.15 0.131 4.20 2.08 1.28 1.64 0.85 3.31

Total 99.80 99.20 99.07 99.90 99.98 99.31

0.84 0.76

99.90 99.63

c (wt46) 1.01 0.45 0.23 0.47 0.14 1.13 0.13 0.13 SC 20.9 21.9 22.6 24.4 16.0 21.9 23.8 31.2 cr 187 208 215 228 161 198 238 289 co 31.2 28.8 29.2 33.5 23.9 29.1 32.2 42.7 Ni 110 55 120 127 128 64 99 118 Ztl 91 81 77 87 68 86 101 105 Ga 23 9 9 12 18 12 5 15 AS 1.42 0.95 0.64 0.19 0.72 0.38 0.92 0.54 Se 1.6 1.1 0.6 0.4 0.7 1.0 0.8 0.7 Br 2.5 1.4 1.9 5.1 0.8 11.1 0.9 0.4 Rb 88.1 70.8 75.5 75.1 89.2 108.2 76.8 58.5 SC 408 296 307 331 386 402 2% 263 Zr 103 99 66 74 85 85 92 92 Ag 0.14 0.32 0.09 co.14 0.09 co.14 0.08 <0.16 Sb 0.08 0.08 0.05 0.03 0.12 0.06 0.07 0.07 cs 1.35 1.02 0.91 1.01 1.11 1.27 1.01 0.68 Ba 350 301 312 316 338 358 279 248 La 21.4 22.0 24.2 28.7 21.5 18.6 18.9 22.4 ce 44.5 42.6 46.0 54.7 39.6 36.9 36.7 43.8 Nd 20.6 16.7 18.8 18.7 16.7 15.2 16.7 17.6 Sm 3.27 2.98 3.11 3.45 2.97 3.51 3.16 3.51 EU 1.06 0.92 0.92 0.98 1.05 1.02 0.92 0.86 Tb 0.62 0.44 0.47 0.59 0.43 0.51 0.48 0.54 Tm 0.3 0.2 0.2 0.3 0.2 0.3 0.2 0.3 Yb 1.67 1.45 1.59 1.77 1.41 1.66 1.68 1.83 LU 0.25 0.21 0.22 0.24 0.21 0.25 0.25 0.28 Hf 2.94 1.98 1.91 2.62 2.74 2.53 2.86 3.06 Ta 1.04 0.62 0.56 0.65 0.63 0.69 0.62 0.71 w 0.8 0.7 0.6 0.5 0.9 0.5 0.6 0.8 Au (ppb) c2 0.9 0.6 <4 <2 0.2 2.0 <2 Hg 1.81 0.09 0.29 0.29 0.04 0.92 0.56 0.39 Th 5.27 5.60 4.94 8.15 4.60 5.41 4.66 5.53 u 1.88 1.02 0.55 0.63 0.84 1.96 1.52 1.33

WU Th/U

9397 16503 32880 26590 23410 9056 10860 10780 2.81 5.51 8.95 12.98 5.50 2.76 3.08 4.15

35.11 49.89 34.40 28.28 31.13 33.50 32.06 30.04 8.69 10.26 10.26 10.94 10.26 7.61 7.57 8.25

ZdHf bf% Eu/Eu" 0.99 1.08 1.01 0.91 1.24 1.02 0.99 0.83

in the dataset. Highly correlated elements were combined to form one principal component. These principal components were again used as input data for hierarchical cluster analysis. Cluster analysis using Ward’s Algorithm yielded four distinct groups within the reduced dataset including the major element data. These groups are listed in Table 6-B and are congruent with the groups obtained from the trace element dataset (Table 6A).

Interpretation of the classification results was facilitated by including major element data because these can be more easily related to mineralogy. Samples from the B and D cores form two clusters (Groups Bl, B2) that are distinguished by their grain size and organic matter content. Group Bl sam-

ples exhibit higher LOIS and C-contents and also higher amounts of Ti02, CaO, P205, Cs, LREEs, Ta, and Zr than other samples. This indicates that the fine-grained, organic- rich sediments are, in general, enriched in carbonate and a heavy mineral component, probably apatite and/or rutile. The Group B2 samples (mainly from core B) that are made up of samples low in organics (low C-content) and low in carbonate (low LOI) show higher concentrations of Na, K, Si, and Al than the samples in other groups. This may be explained by the higher abundance of some feldspathic-component in these samples. Group B3 contains only one sample (B-5b), and it is rich in Ca and Sr, thus pointing to calcite as the dominant form of carbonate present. Group B4 is

176

TABLE 5: LAKE HOARE SEDIMENTS - CORE D

J. L. Bishop et al.

(4 100

D-l D-2 D4 D-s sumce o-1 5-6 1416 21-23 Saud

SiO, 58.97 TiO, 0.69

.%03 14.05

bO3 6.09 MtlO 0.091

Ml70 4.46 CaO 6.34

NG 2.04

W 2.46 p2os 0.179 L.O.I. 3.76

63.92 64.58 62.84 0.47 0.47 0.44

13.18 15.39 11.66 5.13 3.82 6.54 0.089 0.08 0.094 5.01 3.20 7.19 5.32 4.42 6.03 2.89 3.71 2.69 2.17 2.69 1.76 0.162 0.178 0.137 1.49 0.89 0.47

64.39 0.40

13.63 4.76 0.083 5.10 4.91 3.35 2.35 0.153 0.63

Total 99.93 99.84 99.43 99.85 99.70

c (wt96) 0.70 0.25 0.12 0.076 0.088 SC 16.1 18.4 9.8 27.7 17.3 cr 120 174 93 254 169 CO 24.8 24.6 16.0 35.6 24.5 Ni 45 79 46 117 100 Ztl 91 76 52 75 68 Ga 16 16 15 13 15 As 0.54 0.56 0.40 0.44 0.28 Se 1.3 0.9 0.9 1.1 0.8 Br 3.8 1.6 0.7 0.6 0.4 Rb 107.0 74.9 100.7 71.5 92.8 Sr 392 354 498 334 383 Zr 108 72 113 115 81

As 0.07 0.02 -co.09 co.13 co.12 Sb 0.10 0.12 0.20 0.07 0.05 0 1.84 1.11 1.33 0.94 1.17 Ba 404 338 389 286 358 La 34.0 17.3 18.3 23.8 24.3 Ce 64.7 34.6 30.5 46.3 46.1 Nd 27.0 13.1 15.2 19.9 19.9 Sltl 4.78 2.88 2.77 3.80 3.03 EU 1.27 0.98 1.14 1.02 1.05 Tb 0.64 0.53 0.38 0.53 0.47 T m 0.3 0.2 0.2 0.3 0.2 Yb 1.74 1.50 1.19 1.78 1.30 LU 0.26 0.22 0.17 0.28 0.20 Hf 2.89 2.24 3.06 2.57 2.80 Ta 1.12 0.60 0.68 0.70 0.52 W 1.2 0.7 0.3 0.4 0.4 Au Cwb) 0.3 1.0 0.4 <2 1.6

Hg 0.19 0.05 co.1 0.32 0.60 Tb 8.04 4.15 3.79 5.24 6.36 U 1.10 0.57 0.94 0.73 0.72

K/u l-b/u Zr/Hf

18340 31730 23850 20%‘0 27200 7.28 7.35 4.06 7.22 8.89

37.28 31.91 37.08 44.69 28.98 13.17 7.81 10.42 9.05 12.68 hf%

Eu,‘Eu” U.Y7 1.05 1.4-S 0.95 1.17

Major element data in wt%, tmlce element data in ppm, except ll~ noted; EWEu”=Eu,, / v’(Sm,,*TbJ; alI Fe as Fe,O,.

identical to Group A4 determined from the trace elements- only analyses. This group contains most of the samples of core C, plus one core D sample and the surface sand sample. The common feature of these samples is a low organic content and low LO1 (i.e., low carbonate) value combined with high Mg, Co, Cr, Ni, and SC abundances. These elevated element abundances point to the presence of a mafic component in these sediment samples.

Classification results of cluster analysis were confirmed by subsequent discriminant analysis. Using this analysis Co, Mg, SC, Cr, Fe, and Ni were obtained as the best discriminating variables for separating the carbonate-poor core C samples (Group B2) from the carbonate-poor core B (plus two core C and D) samples of Group B4.

0 2 4 6 8 10 12 14 16 18 20

Depth in Core B (cm)

W 1000

<

0 2 4 6 8 10 12 14 16 18 20


(a 1oooj

1’ 0 2 4 6 8 10 12 14 16 18 20


FIG. 5. Depth profiles for selected elements listed in Table 3 for the Lake Hoare core B samples. (a) SC, Rb, Cs, and Th, (b) Sr, Zr, Hf, and U, (c) Cr. Zn, and Co.


I I I I I I V I I I La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu

La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu

W 10003

:diment B-7a

(d)

I’, I I I I 8 9 I I 8 13 1 8 La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu

La Ce PI Nd

FIG. 6. Chondrite-normalized abundances of the rare earth elements. (a) core B, (b) core C, and (c) core D, plus

the surface sand sample. Examples were selected for each core to exhibit the range of values for the entire core. Chondrite normalization factors from Taylor and McLennan (1985) were used.

4.3. Compositional Trends with Sediment Depth and Core Location in the Lake

The concentrations of several metals were measured at 3 m intervals in Lake Hoare (Green et al., 1986b). They observed significant increases (by a factor of 10-100) in the abundances of Fe, Mn, and Co across the oxic-anoxic transition (24-27 m) in the lake water. The abundances of Fe, Mn, and Co (Tables 2-5) in the sediments studied here are fairly constant with respect to location (depth) in the lake. The sediments of core C (from the deepest location in the lake) appear to exhibit a slight increase in Co abundances relative to the sediments from other cores; however, the Fe and Mn abundances cannot be differentiated with location in the lake. Green et al. (1986b) suggested removal mechanisms for these excess metals, including uptake by minerals, organic matter, or algal mats in the lake. In general, the organic-rich sediment samples exhibit much lower levels of Fe, Mn, and

Co than the organic-poor samples, indicating that the organic matter does not participate in the removal of these metals from the lake water.

Geochemical studies of the lake water in the Dry Valleys region of Antarctica have shown that calcite is more soluble in deeper regions of the lake and precipitates more readily in shallower regions (Green et al., 1988). For the lake bottom sediments studied here, variations have been observed in the carbonate compositions. The wt% C as carbonate is listed in Table 7 for the samples in this study. The core C sediments (from DH-4) contain no measurable carbonates and this core is located in the deepest water of those studied here. The core B sediments from DH-2 contain significant amounts of carbonate, up to 40 wt% or more as calcite in some layers. Dive hole DH-2 is located in a shallower region of the lake relative to dive DH-4, thus the carbonate abundances of the sediments from these two cores are consistent with the observations of Green et al. (1988) that calcite precipitates

.I. L. Bishop et al

Table 6 Sample grouping based on hierarchical cluster analysis of the elemental abundances.

A. Trace Elements:

A4 Group Al Group A2 Group A3 Group

B-11 B-5 B-5b C-4b B-9 C-6C B-10 B-l B-2a B-l D-l B-5a B-3f B-3a

B-8 B-2 B-3e B-la c-5 D2 DA

B-3b B-3c B-3d

ss c-2c C-7a C-l c-3 D-5 c-10

B. Trace and Major Elements:

B4 Group Bl Group B2 Group B3 Group

B-l D-l B-2a B-5a C-l

B-11

E B-10 B-9 c-5 C-6.2

ii?4 B-5 B-8

B-5b C4b ss c-2c C-7a c-3 c-10 D-5

more readily in shallower regions. However, the core D sediments from DH-1 lie closest to the shore and contain lower carbonate concentrations than the core B (DH-2) sediments.

For core C, the S abundances are highest near the surface (-0.3 wt%) and decrease with depth. The S-enriched sediments of core C were in contact with anoxic water at the lake bottom, which may have contributed to the enhanced S- contents of these samples. Sulfur was not detected in any of the other samples studied here. Hydrogen sulfide was first detected at 27 m depth in the water of Lake Hoare and the concentration of H2S was found to increase with depth (Green et al., 1986b). The presence of S in the core C samples is also probably responsible for the darker sediment color.

4.4. Spectral Properties of Lake Hoare Sediments

4.4.1. Injuence of particle size

Reflectance spectra for samples B-5, B-8, C-3, and D-2 are shown as a function of wavenumber from 2000 to 700 cm-’ (5-14 pm) in Fig. 8 as examples of the influence of particle size on the spectral properties of soils. The spectra shown in thick lines are of the crushed samples with particle sizes primarily c12.5 pm (also named larger particle size samples), and are also displayed in Fig. 4, panel B. The spectra shown in thin lines are for the same samples after having been ground further such that the particle sizes are primarily <45 pm (also named smaller particle size samples). In general, the effects of decreasing particle size on

these spectra include a relatively higher reflectance in the region 2OOC-1300 cm-’ (5 to -8 pm), an increase in the spectral band depths in the volume scattering region, a shift in the Christiansen feature near 1300 cm-’ (-8 pm) to longer wavelengths and a weakening of the reststrahlen features at smaller wavenumbers (longer wavelengths). Although not shown here, spectra of the other samples exhibited similar changes. The trend of increasing relative strength of the absorption features from 5 to 8 km in Fig. 8 is especially noticeable in the spectra of samples C-3 and D-2. The type, proportion, and relative particle sizes of the components in mixtures play interdependent roles in determining band strengths.

Spectral experiments (Logan et al., 1973) have shown shifts in the Christiansen feature in emittance spectra of quartz and other minerals as a function of particle size, sample texture, and other factors. In a more recent study involving reflectance spectra of fine-grained olivine, the Christian- sen feature was observed to broaden and shift toward shorter wavelengths (longer wavenumbers) with decreasing particle size (Hayes and Mustard, 1994). For the spectra shown in Fig. 8, however, the Christiansen feature is shifted toward smaller wavenumbers (longer wavelengths), rather than larger wavenumbers, and the magnitude of the shift is much greater. This implies that the Christiansen feature in these spectra is influenced by more than one mineral and is highly sensitive to particle size for samples containing multiple components.

The spectral features at longer wavelengths, such as the


DF

3

-1

1

-5

-9 3.8

FIG. 7. Results of the statistical treatment of the chemical data for the three discriminant functions (DFl, DF2, DF3). Four groups were found by cluster analysis of the trace elements in the Lake Hoare samples from cores B, C, and D.

reststrahlen features, follow surface scattering properties and thus behave differently with respect to particle size than spectral features in the volume scattering regime at wavelengths shorter than the Christiansen feature. Reststrahlen features occur most strongly for smooth surfaces, where “first surface reflection” or “surface scattering” is preva- lent (e.g., Salisbury, 1993). For rougher surfaces or smaller particle sizes, multiple reflections, as well as volume scattering and absorption, take place, enabling absorption of more energy at vibrational frequencies, and leaving fewer “resid- ual rays” or “reststrahlen” bands (e.g., Salisbury, 1993). As expected, the spectra in Fig. 8 exhibit reduced intensity of the reststrahlen bands as the particle size of the samples was decreased. A reststrahlen doublet characteristic of quartz is observed at 1200 cm-’ (8.3 pm) and 1100 cm-’ (9.1 pm) in all of the spectra, but is substantially reduced in intensity and exhibits changes in shape for the smaller particle size samples. Additional features characteristic of quartz are seen near 800 cm-’ in the larger particle size spectra only. Other minor components, such as carbonates and clays or phyllosil- icates, exhibit spectral features in the surface scattering region for the larger particle size spectra that disappear in spectra of the finely ground samples (Fig. 8).

4.4.2. Analysis of the sediments as mixtures

The Christiansen feature and the volume scattering features are influenced by the components in mixtures, includ-

ing the relative particle size distributions as well as the amounts of these components. Several spectral bands between 1900 cm-’ and 1500 cm-’ can be assigned to quartz, pyroxene, feldspar, and calcite for samples B-5, B-8, C-3, and D-2. Near 1600 cm-’ is also a broad band due to HzO.

Careful examination of the larger particle size spectra for samples B-5, B-8, C-3, and C-2 in Fig. 8 shows a Christian- sen feature near 1330 cm-’ (7.5 pm), characteristic of quartz, with a shoulder near 1270 cm-’ (7.9 pm), more characteristic of feldspar. In the smaller particle size spectra for each of these samples the location of the Christiansen feature has shifted towards 1270 cm-’ (7.9 pm). These changes in position of the Christiansen feature parallel changes in the intensity of the reststrahlen bands and can be explained by the change in slope of the shorter-wavelength flank of the reststrahlen bands. This implies that the spectral character of the Christiansen feature for mixtures is dependent not only on the minerals present in the mixture, but also on the particle size distribution, which influences the spectral character of the scattering regime. The Christiansen feature occurs between the volume scattering and surface scattering spectral regions and is influenced by the factors that effect both of these spectral regions. This shows that for mixtures containing multiple minerals the Christiansen feature is not as predictable as previously thought. Future studies of the Christiansen feature and other mid-infrared spectral features

Table

7

Rela

tive

Perc

ent

Band

De

pths

(A

R/R)

of

Org

anic

and

Carb

onat

e Fe

atur

es

in Re

flect

ance

Sp

ectra

an

d W

eigh

t Pe

rcen

t Ca

rbon

.

-325

,u

m p

aticl

e siz

e <4

5 pm

pa

ticle

siz

e W

eigh

t %

C

Sam

ple

3.42

km

3.48

pm

3.51

pm

3.86

pm

3.98

pm

3.42

pm

3.48

pm

3.51

pm

3.86

pm

3.98

pm

as

as

(-292

0 cm

-‘)

(-287

0 cm

’) (26

00

cm-‘)

(25

15

cm.‘)

(-292

0 cm

.‘) (-2

870

cm.‘)

(2600

cm

.‘) (25

15

cm-‘)

ca

rbona

te org

anic

B-la

B-l

B-2a

B-

2 B-

3a

22

I1

16

29

B-3b

B-

3c

B-3d

B-

3e

B-3f

B-

4 B-

5a

21

;z 3 26

B-5b

32

B-

5 16

B-

6 B-

7a

31

B-7

6 B-

8 B-

9 6

B-1O

a 16

B-

10

6 B-

11

7

10

193

23

22

z ii

29

26

21

23

19

17

:2

ll9

31

27

14

11

A 16

16

5 12

19

3

36

I; 4 i:, 18

21

23

3 23

6

4 2

10

:s

6 3

11

6 11 1 14 ;

2 3 8 3 26

10

3

16

8 14

3 20

22 * * 25

17

4 * i 2 : 1 ; 4 2

10

; 2 14

17

* * 16

12

3 * 3 2

59

9 3 14

16

* * 17

10

3 * * 4 1 : 1 1 z 1

4 1

2 8 13 24

5 30

* 2; 19

6 * * 11 1 * 4 1 2 19 5 3

0.22

3.

28

0.77

0.

91

1.07

1.

13

0.10

0.

15

2.40

4.

34

3.41

5.

16

4.83

7.

15

4.97

2.

33

1.03

5.

46

1.54

2.

93

0.27

0.

21

2.49

2.

40

5.35

3.

00

0.29

0.

54

1.41

4.

25

__

__

__

_-

0.86

0.

59

0.21

0.

28

__

--

C-l

27

13

18

19

6 12

0

1.01

c-

2c

10

:. f

* c-

3 6

‘i ;.

-0

0.41

-0

0.

19

C-4b

5

7 7

-0

0.38

c-

5 3

c-6c

13

10

:,

3 ;

: ; 4

1 -i

0.14

1.

03

C-7a

5

7 :

_-

__

c-10

1

__

__

D-l

8 4

2 4

2 2

6 0.

29

0.41

Ei

z

2 1 ;

1 2 1

-- 0

0.19

__

D-

5 1

1 --

__

ss

All

band

de

pths

we

re

dete

rmine

d by

th

e m

etho

d of

CL

ARK

and

ROUS

H (1

984)

.

8 2

5 1

0 0.

09

* in

dica

tes

insu

fficie

nt

amou

nt

of

sam

ple

for

spec

tral

mea

sure

men

t; --

tota

l wt

.%

C <

0.3,

no

or

ganic

an

d ino

rgan

ic C

mea

sure

men

ts pe

rform

ed.


0.40

0.30

0.20

0.10

0.40

0.30

0.20

0.10

0.0

0.30

0.20

0.10

0.0

0.30

0.20

0.10

0.0

___ B5 (~45 pm) ___ B5 (~45 pm) - 85 (cl 25 km) - 85 (cl 25 km)

- 88 (x45 pm) - 88 (x45 pm) - B8 (cl 25 pm) - B8 (cl 25 pm)

Wavelength (pm)

hL

2ooo 1500 1000 500 Wavenumber (cm-’ )

FIG. 8. Reflectance spectra from 2000 to 700 cm-’ (5-14 pm) for selected Lake Hoare samples to illustrate the effects of particle size on spectral properties. The spectra shown in thick lines with symbols were measured initially for samples with particle sizes primarily <125 pm. Following additional grinding that gave particle sizes primarily <45 pm, the same samples were measured again and are shown using a thinner line-type.

3.98 ym have been measured in the larger particle size and smaller particle size spectra and are shown in Table 7. The sediments from core B exhibit strong features near 3.42, 3.48, and 3.98 pm, which are characteristic of calcite (Calvin et al., 1994). The strength of the asymmetric C-H stretching features are stronger than the symmetric features (e.g., Gaffey et al., 1993), which is consistent with the spectra shown in Fig. 9. The spectral band depths for the five bands listed in Table 7 were plotted against wt% C to see how well these trends hold for the sediments containing both organic material and calcite. The best correlations are shown in Fig. 10. Displayed in Fig. 10a are the correlations for the spectral band depths at 3.42 and 3.51 pm and the organic wt% C. The stronger correlation at 3.51 pm than at 3.48 pm with wt% organic C suggests that more of the organic material is in the form of CH2 than CH,. Displayed in Fig. lob are the correlations for the spectral band depths at 3.48, 3.86, and 3.98 pm with the carbonate wt% C.

The near-infrared spectral features of carbonates are slightly different depending on the cations present and the mineral structure. The features observed near 3.42, 3.48, and 3.98 pm for the carbonate-rich sediments in Fig. 9 and the good correlation observed in Fig. lob with the band depths of these features and wt% C from carbonate suggest that calcite is the primary carbonate component in these materials. For samples B-5a and B-5b the wt% C from carbonate is equivalent to -21 and -45 wt% calcite, respectively, assuming that calcite is the only carbonate present. From these calcite values the amount of CaO originating from carbonate can be calculated as well. For samples B-5a and B-5b the CaO from carbonate is -12 and -25 wt%. This leaves 2-3 wt% CaO for each sample for feldspar and other minerals in the sediments. The ratio of SiOa to remaining CaO for samples B-5a and B-Sb is consistent for SiOJCaO ratios for other core B sediments with low carbonate contents. These calculations confirm that calcite is the dominant, if not the only, form of carbonate present in these sediments.

for mineral mixtures are essential for remote sensing of planetary surfaces.

4.4.3. Detection of carbonates and organic matter

As the sediments in this study contain C in the form of organic material and carbonate, both forms of C were measured via chemical and spectroscopic techniques. The wt% C values for carbonate and organic matter are given in Table 7. Reflectance spectra from 3.3-4.1 pm are shown in Fig. 9 for selected samples that contain either a large amount of carbonate or a large amount of organic matter. Samples B- 3d and B-5b contain about 41 and 45 carbonate wt%, respectively. The spectra of these two samples exhibit a strong band near 3.98 pm with a shoulder near 3.86 pm and a triplet with features at 3.36, 3.42, and 3.48 pm. Samples B- 3e and C-l contain significantly more organic C than inorganic C and exhibit relatively weaker features near 4 pm than the carbonate-rich samples. The spectra of these organic-rich samples contain strong features near 3.42 and 3.51 pm. For other samples that contain less wt% C or mixtures of carbonates and organic matter these features are less distinct.

Band depths of the features at 3.42, 3.48, 3.51, 3.86, and

g 0.40 5

5 5 0.30 K

3.3 3.4 3.5 3.6 3.7 3.8 3.9 4.0 4.1 Wavelength (pm)

FIG. 9. Reflectance spectra from 3.3 to 4.1 pm for selected Lake Hoare samples (approx. < 125 pm particle size) to show the spectral features due to carbonates and organic material in these sediments. Samples B-3d and B-5b contain over 40 wt% calcite and significantly less organic material; samples B-3e and C-l contain at least five times more C from organic material than from calcite.


2 40

2 9

30 c

5 p 20

z

i+i 10

0 0 1 2 3 4 5 6 7 6

wt % C (organic material)

+ 3.96 km 1 l

.

0 1 2 3 4 5 6 7 6 wt % C (carbonate)

FIG. 10. Relative percent band depths (AR/R) for the spectral features due to carbonates and organic species vs. wt% C. The spectral band depths exhibiting the best correlations with C are shown here. Along the X axis are plotted the concentrations of C (a) from organic material and (b) from carbonate. Band depths are plotted on the Y axis of the spectral features (a) at 3.42 and 3.51 pm and (b) at 3.48, 3.86, and 3.98 pm. The band depths were determined by the method of Clark and Roush (1984) and are given in Table 7.

The presence of calcite in these samples was also confirmed in the XRD measurements and via the fundamental infrared spectral features, described in an earlier section.

Carbonates are thought to be present in the soils on Mars because of the COZ ice covering some areas of the surface and CO*(g) in the atmosphere (e.g., Carr, 1981). Weak features near 6.7 pm (Pollack et al., 1990) have been attributed to carbonates on Mars; however, carbonate features near 4 pm have not yet been spectroscopically observed in the surface material. Spectroscopic experiments involving mixtures of carbonates and other materials have attempted to develop methods of determining carbonate percentages in mixtures from the spectroscopic properties (Blaney and McCord, 1989; Calvin et al., 1994).

For these quartz- and feldspar-rich sediments detection of carbonates is strongest for the 3.98 pm feature. Assuming that calcite is the only carbonate present, wt% calcite was calculated and is plotted against the 3.98 pm band depths in Fig. Il. Band depths are shown in Fig. 11 for spectra of the larger and smaller particle size samples. Band depths of 2- 3 rel.% were determined using the method of Clark and Roush (1984) for some samples having as little as 2 wt% or less calcite. Spectra of other samples containing -2-3 wt% calcite exhibit band depths of up to 18 rel.%. This shows the sensitivity of the spectral character of minerals in mixtures to the particle size distribution and relative composition of the mixture. Band depths in mixtures have been found to be significantly suppressed by fine-grained, highly absorbing components (Morris and Neeley, 1982; Clark, 1983; Bishop et al., 1993). Continued spectral analyses of carbonate-bearing mixtures is important for Mars, where the presence of carbonates has been predicted, but is difficult to confirm spectrally. In particular, the detectability of carbonate features should be examined as a function of the particle size distribution and mineralogy in mixtures.

5. SUMMARY AND CONCLUSIONS

Sediment cores from the bottom of Lake Hoare, Antarc- tica, were analyzed in this study for their mineralogical and

geochemical compositions. Cores from three locations in the lake (dive holes DH-1, DH-2, DH-4) were collected and separated into sediment layers, giving a total of 41 lake bottom samples, plus one surface sand sample in these analyses. One purpose of this study was to look for consistent geochemical trends in the sediments deposited at different depths in the lake, as such trends have been observed in the lake water (Green et al., 1986a,b, 1988). Another reason for this study was to document trends in the geochemistry of sediments from a perennially ice-covered, closed-system lake, as analogs for dry valley sediments on Mars and low- temperature, arid regions on Earth. Coupled to this is the goal of developing methods to identify and distinguish organic material and carbonates in such sediments. Spectroscopic measurements were performed on these samples to test spectral identification procedures for individual components in natural sediments that are multicomponent mixtures. Such

5o t . -

2 40 .

4 . z E 3o & I .o

. 0

B20 . 0 2 I

0 70 20 30 40 50 wt % Calcite

FIG. 1 I. Relative percent band depth (AR/R) at 3.98 pm vs. wt% calcite. The wt% calcite values were calculated from the wt% C from carbonate measurements, assuming that calcite is the only carbonate present. The band depths for samples with particle sizes primarily <125 pm and primarily <45 pm were determined by the method of Clark and Roush (1984) and are given in Table 7.


identification techniques are important for spectral remote sensing of planetary surfaces.

The mineralogical composition of these sediments is dominated by quartz and feldspar. Based on the elemental abundances and spectroscopic features for these sediments, calcite is the primary (or only) form of carbonate present. Selected samples from core B (DH-2) contain 40 wt% or more calcite and elevated organic matter abundances. The core C and core D sediments contain very little calcite, up to 1 wt% C as organic matter, and exhibit variable pyroxene contents.

Geochemical analyses of these sediment cores from Lake Hoare, Antarctica, indicate minimal variation in the elemental abundances both within each core, and among the four cores from three dive holes. This suggests a common sediment origin for the DH-1, DH-2, and DH-4 lake bottom sediments of Lake Hoare. Small variations in the elemental abundances were observed as a function of depth or distance from shore; however, these are less significant than the variations in the chemical composition with depth of the lake water. The deeper, core C sediments exhibited slightly elevated Co abundances, which may be related to higher Co concentrations in the deeper, anoxic lake water than in the shallower, oxic lake water. The organic-rich layers tend to have lower than average Fe, Mn, and Co abundances, which indicates that the organic matter is not facilitating their removal from the lake water. The increased calcite content of the core B samples, relative to the core C samples, are consistent with the higher calcite precipitation levels in the shallower regions; however, the low calcite contents in core D are not consistent with this trend.

For some samples, relatively high Cr and Mg levels were observed that correlate with the spectral observation of pyroxenes. This suggests that for such quartz- and feldspar- rich sediments, Cr and Mg are good indicators of pyroxene abundance. The results of the statistical analyses gave significant correlations among the elemental abundances of Mg, Fe, and Cr. The occurrence of organic-rich sediments is correlated with high Br and U abundances and carbonate occurrences are correlated with high Ca and Sr abundances.

Reflectance spectroscopy of these samples has shown that the relative particle-size distributions of the components in a mixture can have a significant influence on the spectral properties of the mixture. As well-characterized, natural mixtures, these sediment samples have provided an opportunity to test spectroscopic mixture analyses. The spectral regions measured here (visible, volume-scattering infrared, surface- scattering infrared) are sensitive to different factors, which resulted in different mineral components dominating the spectral properties of each region. For these samples, pyroxene features dominated the spectral character of the visible to near-infrared regions, while quartz and feldspar features dominated the spectral character of the longer wavelength infrared regions. Changes were observed in the position of the Christiansen feature between the larger particle size spectra and the smaller (additionally ground) particle size spectra for these sediment samples. The Christiansen feature of the smaller particle size samples is characteristic of feldspar, while the Christiansen feature of the larger particle size samples is characteristic of quartz. This apparent shift in the

position of the Christiansen feature is attributed here to the slope and intensity of the reststrahlen bands, which are dependent on particle size and sample density. Spectral analyses of these mixtures show that the position of the Christian- sen feature depends on the mineralogic composition and particle size distribution of the sample, and that the wavelength of the Christiansen feature is difficult to predict for mineral mixtures.

The spectroscopic measurements of these sediments also provide a good comparison for spectroscopic analyses of soils in arid regions on the Earth and perhaps on Mars, as the samples have had limited exposure to weathering and anthropogenic factors. Various amounts of carbonate are present in the samples studied here, as may be the case for the surface material on Mars. The presence of organic matter in these samples is correlated well with the band depths of the spectral features at 3.42 and 3.51 pm, while the presence of calcite in these samples is correlated well with the band depths of the spectral features at 3.48, 3.86, and 3.98 pm. These spectral experiments showed that the spectral albedo of the mixture, the relative amounts of the mixture components, and the particle-size distributions of both the carbonates and the matrix material influence spectroscopic detection of carbonates in mixtures.

Acknowledgments-Support through the ZONTA Foundation, the NASA Graduate Student Researchers Program at NASA-ARC, Mof- fett Field, and the Alexander von Humboldt Foundation, Germany, is greatly appreciated by J. Bishop. Infrared spectra of several carbonates were generously contributed by Scott Sandford of NASA- Ames Research Center, Moffett Field, CA. Jack Salisbury and an anonymous reviewer are thanked for their critical reading of the manuscript and helpful suggestions. NASA support for this research under grant NAGW-28 is gratefully acknowledged by C. Pieters. RELAB is a multiuser facility supported by NASA under grant NAGW-748. The Nicolet system was funded under a grant from the Keck Foundation. Technical assistance from T. Hiroi and S. Pratt at Brown University is much appreciated. This project was in part made possible by funding from the University of Vienna to P. Englert and from NSF (OPP-9211773) and NASA (NCA2-799, NAGW- 1947) to R. Wharton.

Editorial handling: S. M. McLennan

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