Direct Observations of Aluminosilicate Weathering in the Hyporheic Zone of an Antarctic Dry Valley Stream

Home , Asgard Range

Geochimica et Cosmochimica Acta, Vol. 66, No. 8, pp. 1335–1347, 2002 Copyright © 2002 Elsevier Science Ltd Pergamon Printed in the USA. All rights reserved 0016-7037/02 $22.00 ϩ .00 PII S0016-7037(00)00890-0 Direct observations of aluminosilicate weathering in the hyporheic zone of an Antarctic Dry Valley stream

1, 2 3 4 2 PATRICIA A. MAURICE, *DIANE M. MCKNIGHT, LAURA LEFF, JULIA E. FULGHUM, and MICHAEL GOOSEFF 1Department of Civil Engineering and Geological Sciences, University of Notre Dame, Notre Dame, IN 46556, USA 2INSTAAR, 1560 30th Street, Boulder, CO 80309, USA Departments of 3Biological Sciences and 4Chemistry, Kent State University, Kent, OH 44242, USA

(Received January 4, 2001; accepted in revised form July 20, 2001)

Abstract—This study focused on chemical weathering and bacterial ecology in the hyporheic zone of Green Creek, a McMurdo Dry Valley (Antarctica) stream. An in situ microcosm approach was used to observe dissolution features on the basal-plane surface of muscovite mica. Four mica chips were buried in December 1999 and dug up 39 d later. Atomic force microscopy (AFM) of the basal-plane surfaces revealed small, anhedral ϳ10-Å-deep etch pits covering ϳ4% of the surfaces, from which an approximate basal-plane dissolution rate of 8.3 ϫ 10Ϫ18 mol muscovite cmϪ2 sϪ1 was calculated (on the basis of the geometric surface area) for the study period. This is an integrated initial dissolution rate on a fresh surface exposed for a relatively brief period over the austral summer and should not be compared directly to other long-term ﬁeld rates. The observation of weathering features on mica agrees with previous stream- and watershed-scale studies in the Dry Valleys, which have demonstrated that weathering occurs where liquid water is present, despite the cold temperatures. AFM imaging of mica surfaces revealed bioﬁlms including numerous small (Ͻ1 ␮m long), rounded, oblong bacteria. The AFM observations agreed well with X-ray photoelectron microscopy results showing increased organic C and N. Bacteriologic analysis of the hyporheic zone sediments also revealed Ͻ1-␮m-long bacteria. ␣-Proteobacteria were observed, consistent with the oligotrophic conditions of the hyporheic zone. Nitrate- reducing bacteria were found, in agreement with a previous tracer test at Green Creek that suggested nitrate reduction occurs in the hyporheic zone. The results of this study thus provide direct evidence of dynamic geochemical and microbial processes in the hyporheic zone of a Dry Valley stream despite the extreme conditions; such processes were inferred previously from stream-scale hydrogeochemical studies. Copyright © 2002 Elsevier Science Ltd

1. INTRODUCTION Yet a growing body of geochemical evidence, including The McMurdo Dry Valleys are among the coldest and driest isotopic evidence, suggests that chemical weathering does oc- environments on Earth. Climatic conditions in these polar cur in the Dry Valleys, where and when liquid water exists deserts are harsh, with a mean annual temperature of Ϫ20°C (e.g., Jones and Faure, 1978; Green and Canfield, 1984; Green and an annual precipitation of Ͻ10 cm yrϪ1, all of which is et al., 1988; Lyons and Mayewski, 1993; Blum et al., 1997; snow and most of which is quickly lost to sublimation (Brom- Nezat et al., 2001 Gooseff et al., unpublished data). Although ley, 1985). A large body of evidence has suggested that weath- there is little precipitation, glacial meltwater flows through ering rates tend to increase with increasing temperature, pre- stream channels during the austral summer and interacts with cipitation, or both (e.g., Holland, 1978; Velbel, 1993; Bluth and unconsolidated alluvium in the hyporheic zone adjacent to each Kump, 1994; White and Blum, 1995; Berner et al., 1998; White stream. The high permeability of this alluvium permits rapid et al., 1999), although a host of other factors, such as rates of exchange rates with the main channel, as compared with typical physical removal, can also be important in the field (e.g., temperate streams (Runkel et al., 1998). Weathering within the Stallard and Edmond, 1987; Bluth and Kump, 1994; Kump et hyporheic zone provides solutes to the stream. Examination of al., 2000). Considering the cold, dry climate, one would expect the dissolved load suggests that chemical weathering is an minimal weathering rates in the Dry Valleys. Other factors, active process (Lyons et al., 1997; Nezat et al., 2001). Longer such as pH and biologic activity, also suggest minimal weath- streams have higher solute concentrations, and solute concen- ering rates. Dry Valley stream waters tend to be close to neutral trations typically increase longitudinally in streams, suggesting pH (Green et al., 1988; McKnight et al., 1993), and dissolution that exchange with the hyporheic zone introduces ions released rates of aluminosilicates are generally orders of magnitude by weathering to the stream water (Chin, 1993). slower under near-neutral conditions than at either low or high Other lines of evidence also indicate that chemical weather- pH (e.g., Blum and Stillings, 1995). In addition to the lack of ing does indeed occur in the Dry Valleys. First, although terrestrial vegetation in the Dry Valleys, the extremely dry soils Claridge and Campbell (1977) showed that salts are prevalent (with gravimetric moisture contents typically Ͻ1%; Bockheim, in the Dry Valleys, hydrogeochemical modeling by Lyons et al. 1997) have limited microbial activity (Vishniac, 1993). (1997) revealed that the stream chemistry cannot be explained by salt dissolution alone. Second, the ratios of K:Cl and Ca:Cl * Author to whom correspondence should be addressed in lakes suggest a continental source from weathering, rather ([email protected]). than a marine-salt source (Lyons et al., 1998); minor elements 1335 1336 P. A. Maurice et al. also suggest a weathering source. Third, Sr isotopic studies by Jones and Faure (1978) demonstrated crustal rather than marine-salt or aerosol sources for Dry Valley streams and lakes. More recently, W. B. Lyons et al. (pers. commun.) showed that 87Sr/86Sr ratios of Dry Valley streams are consistent with the weathering of various intrusive rocks. Finally, Gibbs plots (Gibbs, 1970) of stream chemistry suggest that chemical weathering and atmospheric precipitation are both contributors to the dissolved loads of the streams (Lyons et al., 1997). Microbiologic processes also may play important roles in weathering despite the cold temperatures, oligotrophic conditions, and limited duration of exposure to liquid water. Many forms of algae, fungi, and bacteria occur in microbial mats on the streambeds of Dry Valley streams (e.g., Broady, 1982; Vincent et al., 1993; McKnight et al., 1998). These persist in a freeze-dried state when there is no stream flow and begin growing as soon as flow begins (Vincent and Howard-Wil- liams, 1986, 1989). The microbial community in hyporheic zone sediments has not been examined previously, and it is not known whether microbial biofilms in the hyporheic zone sediments persist through the winter or are regenerated when Fig. 1. Location of study site and sampling localities along Greek sediments thaw and flow begins in the austral summer. In Creek in Taylor Valley. streams, studies have generally found that abraded biofilms on the streambed are regenerated over 1 to 2 weeks (Allan, 1995). As discussed below, results of this study confirmed that alumi- Thus, development of new biofilms would not be expected to nosilicate weathering occurs in the hyporheic zone in the aus- greatly constrain microbial processes in the hyporheic zone or tral summer and that there is substantial microbial colonization the role that they may play in weathering. of mineral surfaces. The study results also shed light on the role Organic acids may also enhance weathering rates. The bio- of the hyporheic zone in the N cycle in the Dry Valleys. logic activity of the microbial mats on the streambed results in a flux of dissolved organic carbon (DOC) to the stream water. 2. EXPERIMENTAL This biologic activity is reflected in the DOC concentrations of streams. Downes et al. (1986) reported DOC concentrations 2.1. Description of the Study Site Ϫ from2to6mgCL 1 in Canada Stream in the Lake Fryxell The McMurdo Dry Valleys are a group of so-called desert oases basin. Aiken et al. (1996) found that DOC concentrations of clustered near the coast of Antarctica in the general region of McMurdo streams feeding Lake Fryxell varied from 0.2 to 9.7 mg C LϪ1 Sound. In addition to the cold, dry climate, strong katabatic winds produce a desiccating effect, enhance erosion, and are the agents of with higher values in streams with more abundant algal mats. sediment transport. Green et al. (1993) review the local geology as Chemical characterization of this DOC showed that humic related to the lake geochemistry. Basement geology includes schist, substances comprised only ϳ10%. This suggests that most of hornfels, and marble metasediments; granite intrusions; and lampro- the DOC is fresh leachate of the microbial mats and represents phyre and granitic dykes. These basement rocks are unconformably overlain by the Beacon sandstones, with dolerite sills cutting through labile substrates to support biofilms in the hyporheic zone. Both both. studies (Downes et al., 1986; Aiken et al., 1996) showed that Dry Valley streams carry glacial meltwater. Given that permafrost DOC concentrations were higher at the start of summer, when begins only ϳ0.5 to 1 m below the soil, there is no shallow ground- algal mats and the streambed were first wetted. water system available to contribute base flow to the stream. Despite In research reported herein, we applied an in situ microcosm the lack of a shallow groundwater system, stream water interacts with alluvium in the hyporheic zone. This is a zone adjacent to and under- approach to observe dissolution features and to directly mea- neath the stream where water flows in a downstream direction, inter- sure initial weathering rates of muscovite mica in the hyporheic acting with water in the main channel. Runkel et al. (1998) showed that zone of Green Creek, a first-order stream that drains the tongue the high permeability of the alluvium results in rapid exchange rates of the Canada Glacier and discharges into Lake Fryxell in with the main stream channel. Stream waters in the Dry Valleys tend to be relatively dilute, with Taylor Valley. Freshly cleaved pieces of muscovite mica were near-neutral pH (Green et al., 1988). By use of the terminology of buried in reaction vessels for ϳ6 weeks, after which they were Gibbs (1970), Green et al. (1988) showed that Dry Valley stream dug up and returned to the laboratory for surface chemical and composition was consistent with influence by atmospheric precipitation microtopographic analysis by atomic force microscopy (AFM) (marine aerosols) and rock weathering reactions (or salts derived from and X-ray photoelectron spectroscopy (XPS). We chose to use rock weathering) rather than by evaporation and crystallization. Taylor Valley extends from the Polar Plateau to the Ross Sea. The muscovite mica because it can be freshly cleaved to produce valley is bordered on the north by the Asgard Range and on the south relatively flat basal-plane surfaces such that even subtle weath- by the Kukri Hills. The study site (Fig. 1) is located along Green Creek ering features formed over a short reaction time can be dis- (Fig. 2), which drains the Canada Glacier and flows a distance of 1.2 cerned. This was crucial because environmental considerations km along unconsolidated alluvium into Lake Fryxell. Bockheim (1997) characterized this alluvium as mostly sand-sized particles interbedded in the Dry Valleys precluded a long-term (multiyear) experi- with large cobbles and boulders consisting primary of gneiss, diorite, ment. In addition, we characterized the bacterial community and schist. From 1990 to 1995, the annual streamflow of Green Creek within the hyporheic zone by means of a variety of approaches. varied from 28 to 330 ϫ 103 m3, and the estimated contribution to Lake Direct observations of aluminosilicate weathering 1337

quality data from the hyporheic zone, January 2000, are listed in Table 2. As shown in these tables, Green Creek water tends to be dilute, with near-neutral pH. The average pH for Green Creek surface-water samples collected during the 1999 to 2000 sampling period was 7.6 (n ϭ 28, SD ϭ 0.5). The average pH for hyporheic waters samples was 7.7 (n ϭ 28, SD ϭ 0.6).

2.3. In Situ Microcosms

Mineral-containing reaction vessels (i.e., in situ microcosms) were introduced into the hyporheic zone 5 m upstream from the gauging station at Green Creek to monitor changes in mineral surface microtopography and surface composition as related to chemical weathering. A similar approach has been used to monitor weathering at various temperate zone locations (e.g., Bennett et al., 1996; Nugent et al., 1998). Muscovite mica was chosen as the reacting mineral on the bases of several experimental considerations: (1) it can be easily cleaved to Fig. 2. Photograph along Greek Creek showing stream channel and produce fresh basal-plane surfaces; (2) the resulting surfaces are flat alluvium. and hence do not require polishing that could disturb surface structure, microtopography, composition, and hence reactivity; (3) it has long been considered to be a standard for AFM imaging; and (4) previous Fryxell ranged from 7 to 18% of total streamflow into the lake (Cono- laboratory-based AFM and XPS studies of muscovite weathering (e.g., vitz et al., 1998). The primary source of water in the stream is glacial Johnsson et al., 1992) provide background for comparison. Several melt, which begins in late November to mid-December and ends in geochemical considerations must be brought to bear as well: (5) mus- mid-January to early February (Conovitz et al., 1998). Hence, the covite micas are present in the Dry Valley sediments, and Claridge and stream typically only flows from 4 to 10 weeks, at most, during the Campbell (1977) concluded that weathering of muscovite was an austral summer. important source of leachable K, whereas Blum et al. (1997) found that At the headwaters of Green Creek, meltwater runs off the glacier into K increased with Si in the downstream direction in a nearby Dry Valley a series of ponds. Stream water then proceeds through a broad, flat area stream; and (6) muscovite is a common aluminosilicate mineral with strewn with cobbles and small boulders. The stream then follows a surface structure similar to that of many 2:1 clays, although with more distinct channel, with a shallow slope, for ϳ200 m. The creek different surface charge properties. Because we could only bury the then becomes steep for ϳ100 m. The Green Creek stream gauge is reaction vessels for a single austral summer, we needed to use a mineral located in this region. Our experiment was located 5 m upstream from with excellent cleavage so that even subtle weathering features would the stream gauge, on the left-hand bank (Fig. 1). be readily discernible on the exposed surface. Six-millimeter-scale mica samples (chips) of a single ruby mica 2.2. Previous Investigations at Green Creek (Fe-bearing muscovite) specimen from the Kent State University Min- eralogical collection were freshly cleaved as described by Namjesnik- Green Creek has been the subject of ongoing hydrobiogeochemical Dejanovic and Maurice (1997). These six chips were all taken from a investigations by McKnight and colleagues since 1990. In December single section of the specimen (i.e., we cut one small cleavage sheet 1990, a stream gauge was installed in Green Creek with continuous into six pieces). Five samples were then placed inside individual monitoring of streamflow and conductivity during the period of flow. In pouches within plastic containers for transport to the field; the sixth January 1994, as part of the McMurdo Dry Valleys Long-Term Eco- sample was similarly packed and kept at Kent State as a blank.

logical Research Project, a network of 16 stream ecosystem monitoring Reaction vessels consisted of acid-washed (in 10% HNO3) amber sites was established in the McMurdo Dry Valleys, including estab- 15-mL high density polyethylene (HDPE) bottles with ϳ1-mm-diam- lishment of a permanent transect and sampling of algal mats and water eter holes punched through the vessel’s sides and bottom. The reaction quality at Green Creek. The distribution of algal mats was mapped vessels were installed at two sites close to Green Creek sampling (Alger et al., 1997). Most of Greek Creek was found to have a wide (5 transect 3, wells A (GCT3WA, 2 m from the stream) and B (GCT3WB, to 10 m) ﬂat streambed covered with extensive orange algal mats; black 4 m from the stream) (Fig. 1). These sites were known to have possible mats dominated by Nostoc spp. exist along the margins. hyporheic solution exchange with the stream, as shown by a tracer test Average major ion and nutrient concentrations at the outlet of Green conducted the previous summer. On the day of installation, December Creek, based on data from 1993 to 1997, are provided in Table 1. Water 14, 1999, the stream ﬂow was very low (Fig. 3). Two GCT3WA

Table 1. Average major ion and nutrient concentrations in surface water at the outlet of Green Creek from 1993 to 1997.a

␮ ␮ Conductivity ( S) pH Ca (mg/L) Mg (mg/L) Na (mg/L) K (mg/L) Cl (mg/L) SO4 (mg/L) NO3 (mM) PO4 ( M)

47.67 7.57 5.93 0.86 2.99 0.91 4.32 1.55 0.44 0.13

a From McKnight et al. (2002).

Table 2. Major ion concentrations in hyporheic zone water in January 2000.a

␮ Sample Conductivity ( S) pH Ca (mg/L) Mg (mg/L) Na (mg/L) K (mg/L) Cl (mg/L) SO4 (mg/L)

GCT3WA 1/7/00 80 7.68 10.46 1.59 4.64 2.25 5.05 2.46 GCT3WA 1/22/00 218 NM 6.89 1.12 25.61 3.07 42.12 2.54 GCT3WB 1/22/00 208 8.65 19.37 4.21 22.24 4.70 28.61 8.65

aSampling locations shown in Figure 1. NM ϭ not measurable. 1338 P. A. Maurice et al.

Table 3. Summary of mica sample handling.

Burial depth With DI Mica chip Location (ft) sediment? rinsed?

1 GCT3WA 0.55 No Yes 2 GCT3WB 0.65 No No 3 GCT3WA 0.55 Yes Yes 4 GCT3WB 0.65 Yes Yes 5 Controla No

aThe control was transported but not buried.

sample surface, minimizing lateral frictional forces during scanning. This decreases the probability of damage of soft and easily deformable materials. However, TMAFM does not provide atomic-scale resolution. Height, amplitude, and phase-mode TMAFM images were collected simultaneously. Height mode images provide data in three dimensions and were used for all measurements of topographic relief (z-direction) reported here. Amplitude-mode images are similar, but they provide essentially the first derivative of the height—hence accentuating edges in the amplitude mode images. Widths of features were measured on amplitude-mode images. In phase imaging, the phase lag of the canti- Fig. 3. Flow (top) and specific conductance (bottom, in ␮S) mea- lever oscillation, relative to the signal sent to the cantilever’s piezo sured at Green Creek gauge from December 4, 1999, to January 22, driver, is monitored (Magonov et al., 1997; http://www.di.com). The 2000. Bounded lines represent the period of the microcosm experiment. phase lag is highly sensitive to variations in material properties such as adhesion and viscoelasticity. Forsythe et al. (1998) showed that phase imaging can separately highlight organic (bacteria and exopolysaccha- ride attachment features) vs. inorganic (mineralogic) components of vessels, each containing one mica chip, were placed at a depth of 16.5 biofilms. Several AFM images were taken of each sample, after which cm, and two GCT3WB vessels, each containing one mica chip, were they were analyzed by XPS, then again by AFM. AFM images were placed at a depth of 19.5 cm. The fifth mica chip was kept in its original analyzed by the computer program ENVI (Environment for Visualizing pouch as a control. Installation depths were dictated by the depth to Images; Research Systems) to determine the percentages of features at permafrost. One reaction vessel at each location was gently filled with various heights or depths. This proved invaluable for estimation of sediment from the site; the other vessel was not. Because of the fairly dissolution rates. large diameter of the holes, some sediment eventually entered the other vessels. The reaction vessels were tied to a stake before backfilling so 2.5. XPS Analysis that they could be easily retrieved later. Samples of sediment collected while digging the holes at each site were placed in clean plastic bags for After initial AFM analysis of all mica samples, they were analyzed microbiologic and mineralogic analysis, carefully packed, and placed by X-ray photoelectron spectroscopy (XPS). XPS is a surface-sensitive in a refrigerator for transport to McMurdo base station. On December analytical technique used for evaluation of elemental and chemical 21, 1999, two Onset temperature data loggers, which were programmed state information on solid samples. The sampling depth is to ϳ10 nm. to log data at 15-min intervals, were buried near the vessels and close Samples are analyzed in ultrahigh vacuum. Data acquisition times were to piezometers. highly variable; the low concentrations of some components on the The vessels were removed from the ground on January 22, 2000. We mica samples necessitated long acquisition times (several hours per had to remove the in situ reactors before the onset of winter conditions sample). The mica samples were analyzed with a Kratos AXIS ULTRA and to accommodate strict environmental concerns in the Dry Valleys. photoelectron spectrometer with a monochromatic Al K␣ source. Sam- The sediment was obviously wetted but not necessarily fully saturated, ples were mounted on double-sided cellophane tape, maintaining the although the creek flow was again very low. After removing the same “up” side, or sampling side, as used for AFM analysis. Two vessels, portions of surrounding sediment were collected in clean 300 ϫ 700–␮m areas were analyzed for each sample. Both survey plastic bags for microbiologic and mineralogic analysis. All samples scans and high-resolution scans for several specific elements were and vessels were carefully packed and placed in a refrigerator for conducted. further laboratory work in McMurdo. On January 29, the mica chips were carefully taken out of the vessels for shipment back to Kent State 2.6. Bacteriology Methods University. Latex gloves were worn, and the chips were removed with forceps; we were careful only to touch the chip around the edges. Sediment samples for bacterial cultivation (5 to 9 g dry weight) were Because some sediment adhered to the samples, the chips were gently mixed with 6 mL of 0.1% sodium pyrophosphate and sonicated for 5 rinsed with deionized water. After being rinsed, each mica chip was min (Branson 2210) to dislodge bacteria. From each sample, four plates laid to dry on a clean Kimwipe under protective cover, then returned to each of two types of media (standard methods agar [SMA] and R2A) its original pouch and repacked in a protective plastic case. Mica 2 were inoculated. SMA (BBL), also known as plate count agar, is used appeared to have a clean surface, so it was not rinsed. We did not apply for the isolation of bacteria from water and other sources (Atlas, 1993) any sample preservation techniques because it is our experience that and contains casein, yeast extract, and glucose. R2A (BBL) is used to preservatives tend to leave behind residues on mica surfaces at a scale enumerate bacteria in water samples (Atlas 1993) and contains yeast observable by AFM. Sample handling information is summarized in extract, casein, glucose, starch, pyruvate, and other compounds. Sam- Table 3. ple remaining after plate inoculation was preserved with 0.2% forma- lin, and the number of bacteria was determined by epifluorescent Ј 2.4. AFM Imaging microscopy after staining with 4 ,6-diamidino-2-phenylindole (Porter and Feig, 1980). A Digital Instruments Nanoscope III AFM with an extender module Two plates of each type from each sample were incubated at 23°C, was used throughout. We used tapping mode AFM (TMAFM; e.g., and two other plates of each type from each sample were incubated at Magonov and Reneker, 1997), wherein the probe intermittently taps the 4°C. Colony-forming units (CFU) of bacteria were enumerated after Direct observations of aluminosilicate weathering 1339

48 h for the 23°C plates and for the 4°C plates, after 10 and 40 d. From each sample, colonies with different morphologies were streaked for isolation. Isolates were frozen at Ϫ70°C to preserve their features and subjected to various standard tests. Isolates were stained with Gram stain, and cell morphology and Gram stain responses were noted. Isolates were subjected to the oxidase (which detects the presence of cytochrome c) and the catalase (which detects catalase, which breaks down hydrogen peroxide) test. Nitrate reduction was also examined. Organic compound use was examined by testing for starch hydrolysis and protein degradation (based on gelatin liquefaction). Fermentation of sucrose, lactose, and glucose was also determined. Isolates were also identified with the aide of API Rapid NFT test strips (BioMerieux) following the manufacturer’s instruc- tions. Bacterial isolates were also examined by fluorescent in situ hybridization (FISH); in this method, fluorochrome-labeled, taxon-specific oligonucleotide probes are hybridized to rRNA inside intact bacterial cells. Briefly, cultures grown in nutrient broth were mixed with 0.5ϫ phosphate-buffered saline (7.6 g NaCl, 1.9 g Na2HPO4):8% parafor- maldehyde and then subjected to FISH following the method of Lemke et al. (1997). A probe specific for the ␣-Proteobacteria, a major group of aquatic bacteria developed by Manz et al. (1992), was used; the hybridization and washing temperature was 54°C. Positive and negative controls consisted of ␣-proteobacteria and members of other proteobacteria groups from the American Type Culture Collection.

3. RESULTS

3.1. Results of Physical Measurements at Green Creek

Stream water flow rate, water temperature, and specific conductance (sc) were measured continuously at 15-min intervals at the Green Creek stream gauge (Fig. 1) throughout the 6-week flow season. Figure 3 illustrates the hydrograph and sc measurements. The hydrograph shown in Figure 3 is typical for Dry Valley streams, showing intermittent high flows interspersed with more moderate flow and some periods of no measurable flow. 1999 to 2000 was a low to moderate flow year compared with the previous seven flow seasons of data. The sc profile for the season is also typical of Dry Valley streams. High values at Fig. 4. Water temperatures (°C) in the subsurface, near GCT3WA (top); in the subsurface, near GCT3WB (middle); and of the surface the beginning of the flow season mark the flushing out of water (bottom) at the Green Creek stream gauging station. Data pre- atmospherically deposited salts and other material that may sented here begin on December 21, 1999, the deployment date of the have been deposited in the streambed by winter winds or left temperature data loggers. This was 1 week after the deployment of the over from the previous season. Once this flush is completed, in situ microcosms. Temperature data set ends on January 22, 2000. lower sc values prevail throughout the remaining majority of the flow season. Stream water temperatures measured at the gauging station This mica surface is relatively depleted in Al and also enriched were generally in the single digits (°C), reaching as high as in Fe. We do not know the proportion of the Fe that is ϳ10°C in late December 1999 (Fig. 4). Subsurface waters in substituted for Al vs. present in other forms (e.g., adsorbed or the hyporheic zone were considerably colder, never rising to accessory phases such as Fe hydroxide impurities). more than 2°C at either measuring location (Fig. 4). Subsur- After reaction in the hyporheic zone, K:Si decreased, in face-water temperatures in the hyporheic zone fluctuate primar- agreement with a previous study (Johnsson et al., 1992) that ily in response to solar heating of the sediment surfaces. Inter- showed decreased surface K:Si upon muscovite reaction with actions with exchanging stream water also influence deionized water as a result of rapid surface Kϩ-Hϩ exchange. temperature. Subsurface-water temperatures at GCT3WB were C:Si also increased, as is commonly observed for silicates generally higher than at GCT3WA, and the maximum temper- reacted in aqueous solutions, and could be due to so-called ature was also slightly higher (2°C at GCT3WB vs. 1.5°Cat adventitious carbon, to sorbed or precipitated carbonates, to GCT3WA). adsorbed natural organics as postulated from AFM images presented below, or to some combination of these. Peak details 3.2. Results of XPS Analysis provide some evidence for small amounts of C bound to oxygen (C-O, CϭO) or perhaps C-N. Nitrogen bound to C does not Results of XPS analysis of mica samples are presented in produce binding energy shifts that are as pronounced as C-O Table 4 and Figure 5. Results of the surface elemental analysis bonds, however. Although most of the C is in the form of of the blank (mica 5) showed a deviation from the 1:1 Al:Si hydrocarbons; C-O bonds were not observed on the blanks. stoichiometry expected for a pure mica (KAl2(AlSi3O10)(OH)2). Ca:Si increased by at least 10-fold for all of the reacted 1340 P. A. Maurice et al.

Table 4. Elemental concentrations (% atomic) on mica samples determined by XPS.a

Mica sample Al Ca C Cl F Fe N O K Si Na P

1 11.0 0.34 18.3 BD 1.1 1.4 0.39 50.3 3.3 13.5 0.40 BD 11.5 0.26 17.1 BD 1.1 1.3 0.42 51.0 3.2 13.9 0.36 BD 2 11.0 0.73 20.1 0.25 1.1 1.3 0.17 49.2 3.3 12.1 0.62 BD 11.5 0.54 18.0 0.11 1.2 1.3 0.25 50.4 3.5 12.4 0.72 BD 3 11.9 0.34 12.9 BD 1.2 1.5 0.27 54.6 3.5 13.3 0.56 BD 11.6 0.24 13.4 0.10 1.4 1.5 0.22 54.3 3.5 13.4 0.33 0.06 4 9.7 0.27 29.6 BD 1.0 1.3 BD 43.2 3.1 11.3 0.48 BD 10.2 0.35 24.0 BD 1.0 1.4 BD 47.5 3.1 12.1 0.50 BD 5 12.1 0.03 10.4 BD 1.1 1.8 BD 56.4 4.3 13.4 0.50 BD 11.8 0.02 13.6 BD 1.1 1.7 BD 53.9 4.1 13.2 0.52 BD

a Each sample was analyzed in two 300 ϫ 700–␮m regions; results for each of these two regions are presented separately. Micas 1 to 4 were reacted in the hyporheic zone; mica 5 was an unreacted blank. BD ϭ below detection.

micas, whereas Na:Si showed no clear trend. Relatively high Ca is characteristic of many Dry Valley streams, and Green et al. (1988) determined that CaCO3 was the only simple salt that would form in Dry Valley lakes. They further noted that calcite was common in Dry Valley soils. Three of the four mica samples showed little or no change in Al:Si, but mica 1 showed a slight depletion in Al. A number of processes such as mineral dissolution, secondary phase formation, and adhering particles can all contribute to Al:Si ratios on weathered surfaces (e.g., Johnsson et al., 1992; Nugent et al., 1998). Hence, surface Al:Si is not always a clear indicator of any speciﬁc weathering processes. Fe:Si ratios decreased slightly for all of the hyporheic zone–reacted samples, relative to the blanks. This is important because Fe is a nutrient that tends to be immobile in near-neutral pH conditions. Thus, the mica surface could potentially serve as a source of Fe for microorganisms. Kalinowski et al. (2000) and Maurice et al. (2001) showed that some species of bacteria can remove Fe from aluminosilicates. For the Fe and Al XPS data, both of which showed at most only subtle changes, it is possible that accumulation of C at the surface affected the elemental ratios if the bacteria or organic C contain different surface ratios of Fe or Al to Si. For example, Buss et al. (in press) demonstrated that removing bacteria by several different methods either slightly decreased or did not change the surface Fe/Si ratios measured by XPS on bacteria-reacted mineral substrates. We chose not to attempt to remove bacteria from our ﬁeld-reacted samples because of the potential for disturbing surface composition or structure. A trace of P was detected on one reacted mica chip, and N was observed on most of the hyporheic zone–reacted samples. Peak details suggested an organic nitrogen rather than, for example, a nitrate. Neither P nor N were detected on unreacted blanks. We did not conduct any high-resolution scans for Mg.

3.3. Results of AFM Imaging

AFM imaging of unreacted mica blanks showed surfaces typical for cleaved, unreacted mica (e.g., Johnsson et al., 1992): mostly ﬂat (at the nanometer scale) surfaces with occasional Fig. 5. (a) Elemental atomic ratios to Si at the muscovite surface for adhering particles and cleavage steps. The adhering particles abundant elements, as determined by XPS. (b) Elemental atomic ratios were generally in the range of tens to hundreds of nanometers. to Si at the muscovite surface for less abundant elements, as determined We did not observe any major changes in surface microtopog- by XPS. Direct observations of aluminosilicate weathering 1341

Fig. 6. TMAFM height mode image in air of small, anhedral pits that formed on muscovite surfaces reacted in the hyporheic zone. The pits are ϳ10 Å deep and are thus about the depth of one tetrahedral– octahedral–tetrahedral structural layer in muscovite. Several pits are pointed out with arrows. raphy after XPS analysis of the samples, although the density of ultraﬁne particles increased slightly. In addition to cleavage steps and small particulates, several other surface microtopographic features were observed on samples buried in the hyporheic zone; examples of salient features are shown in Figures 6 to 9. (1) Numerous clusters of shallow

Fig. 8. (a) TMAFM amplitude mode image of mica 4 showing a single oblong-shaped bacterium (ϳ300 nm long) on the surface of hyporheic zone–reacted mica. The size and shape of this bacterium matches those observed in hyporheic zone sediments by epifluores- cence microscopy. (b) TMAFM phase mode image of biofilm on muscovite, with several bacteria (ϳ400 nm long) highlighted in the box. The top edge of the film is highly linear, which suggests that it is running along the edge of a step (lower left to upper right) on the mica surface.

(ϳ10 Å deep), ﬂat-bottomed, anhedral pits (Fig. 6) appeared, similar to those described by Johnsson et al. (1992) after mica dissolution in deionized water. (2) In some locations on the Fig. 7. TMAFM height mode image of mica 1 showing blocky mica surfaces, large irregular or blocky particles were ob- particles that are likely either precipitated salts, adhering particles from served. They were likely either mineral precipitates, including the hyporheic zone, or both. salts, or adhering particles from the hyporheic sediments (Fig. 1342 P. A. Maurice et al.

Fig. 9. (a) TMAFM height mode image of mica 1 showing a thin film present along the left side of the image. A portion of this film, to the right of ϳ2.0 mm and above ϳ1.4 mm, was partially eroded by a prior smaller area scan. The film is likely an organic deposit, perhaps of humic substances. The circular structures are likely caused by film rupture on drying, as commonly observed when humic substances dry on a mica surface (Namjesnik-Dejanovic and Maurice, 1997). The full z-range scale from black to white is 20 nm for this image. Horizontal streaks are an imaging artifact. (b) Phase-mode image taken simultaneously with (a). The organic film shows different phase contrast than the underlying mica surface, consistent with different viscoelastic properties—that is, the organic material is softer and “stickier.” This is also consistent with the ease by which it was eroded during small-region scans. (c) Amplitude mode image taken simultaneously with (a) and (b).

7). (3) Surfaces contained extensive films that were easily hundred nanometers long), oblong bacteria were observed in deformed by the AFM tip and that showed strong phase con- many locations on sample surfaces (Figs. 8 and 9). These trast and therefore were likely organic in nature (Figs. 8 and 9). bacteria often were associated with organic films; hence, ex- The structures shown in Figure 9 were similar morphologically tensive biofilms were present in some areas. There were no to adsorbed humic substances imaged previously in our labo- noticeable differences between mica 2, which was not rinsed ratory, with pores indicative of film rupture (Namjesnik-De- with deionized water, and the other three hyporheic zone sam- janovic and Maurice, 1997). However, morphologic similarity ples. The bacteria often were observed lined up along step is not sufficient to identify this material. (4) Small (several edges on the mica surface (images not shown). Direct observations of aluminosilicate weathering 1343

Table 5. Microbiologic characterization of isolates from the Green Creek hyporheic zone.a

Feature GCT3WA 12/14/1999 GCT3WB 12/14/1999 GCT3WA 01/22/2000 GCT3WB 01/22/2000

DAPI counts/g dry wt. (ϫ108) 7.4 3.1 7.0 4.7 CFU (incubated at RT) Counts/g dry wt. 31.2 (SMA) 5.9 (SMA) 1.6 (SMA) 0.2 (SMA) (ϫ105) 29.7 (R2A) 13.2 (R2A) 2.4 (R2A) 1.2 (R2A) CFU (incubated at 4°C) Counts/g dry wt. 26.8 (SMA) 5.7 (SMA) 4.4 (SMA) 0.5 (SMA) (ϫ105) 23.8 (R2A) 13.4 (R2A) 4.4 (R2A) 0.8 (R2A) DNA extractionb Some Lots Some Some

aSMA ϭ standard methods agar; R2A ϭ R2A agar. bExtracted in the manner of Zhou et al. (1996); we encountered some difﬁculties with the extraction, and results are thus not discussed in detail.

3.4. Results of Microbiologic Analysis (GCT3WB) than the near site (GCT3WA) and was also greater in January than in December. In terms of organic compound In agreement with AFM imaging of reacted micas, bacteri- use, starch hydrolysis was uncommon. Sucrose, glucose, and ologic analysis of the hyporheic zone sediments revealed small lactose fermentation occurred for many isolates, but the fer- (Ͻ1 ␮m long) bacteria. Because the samples were not pre- mentation ability was rather weak compared with typical fer- served in the field, the microscopic examination was limited. menters. The total number of bacteria was higher at site GCT3WA (the Examination of bacteria via FISH using an ␣-Proteobacteria site nearest the stream) than at site GCT3WB; however, the probe revealed that ϳ46% of the bacterial isolates hybridized numbers were similar on the two collection dates (Table 5). the probe, suggesting that they are in the ␣-Proteobacteria Because these samples were not preserved in the field but only group. This group of bacteria contains many oligotrophs. Also, on arrival at Kent State University, we do not know how the some bacteria were identified as Pseudomonas or Sphingomo- values changed after sample collection. nas on the basis of API test strip results. The number of CFU was consistently higher at site GCT3WA (near stream) than site GCT3WB (further from stream), regardless of media type or incubation temperature. At 4. DISCUSSION both sites, CFU were considerably higher in December than in 4.1. Chemical Weathering in the Hyporheic Zone January. This is despite the fact that the December sampling was at the beginning of the season, when temperatures were Development of numerous small, shallow, anhedral etch pits just beginning to warm. December daily average air tempera- with a depth appropriate for the unit cell of muscovite (ϳ10 Å) tures measured at a meteorological station located ϳ1 km from indicated that dissolution occurred during weathering in the the experimental site were higher (Ϫ1.66°C) than those for hyporheic zone. The pit morphology was similar to that ob- January (Ϫ2.84°C) during the course of this experiment. We do served by Johnsson et al. (1992) after muscovite dissolution in not have water temperatures for the period in December di- deionized water at room temperature. Because AFM provides rectly leading up to the sampling day (December 14, 1999), so three-dimensional data, we could calculate the dissolution rate we cannot compare specific changes in water temperature. The over the 39-d microcosm experiment. On the basis of measure- differences in CFU likely result from changes in nutrient con- ments from 10 of the highest-quality AFM images, we calcu- centrations. As discussed above, DOC concentrations are gen- lated that etch pits covered an average of ϳ4% of the reacted erally higher at the start of the austral summer and decrease mica surfaces. By using an etch pit depth of 10 Å, we therefore thereafter (Downes et al., 1986; Aiken et al., 1996). estimated a dissolution rate of 8.3 ϫ 10Ϫ18 mol muscovite All of the bacteria were gram negative (Table 6). Optical cmϪ2 sϪ1 on the basis of geometric surface area. This rate microscopy showed rounded, rod-shaped cells several hundred should be considered to be an approximation because of the nanometers (Ͻ1 ␮m) in length. The oxidase test was used to small sampling size. Moreover, it is an integrated rate over the distinguish among groups of bacteria on the basis of cyto- study period (austral summer) and should be considered to be chrome oxidase activity (i.e., the ability to produce oxidase an initial rate because of the relatively short duration of the enzymes). Oxidase-positive organisms include most pseudo- field experiment. If the dissolution is stoichiometric, then the Si monads, which have a complete electron transport chain based dissolution rate would be 2.5 ϫ 10Ϫ17 mol cmϪ2 sϪ1; nons- on the presence of the final cytochrome oxidase. Oxidase- toichiometric dissolution could lead to somewhat slower or negative organisms include some environmental bacteria, such faster Si rates. as Acinetobacter. More oxidase-positive isolates were obtained A number of researchers (Nickel, 1973, in Kalinowski and in December, compared with January, at both sites. As ex- Schweda, 1996; Clemency and Lin, 1981; Lin and Clemency, pected for gram-negative bacteria, the bacteria at both sites 1981a,b; Acker and Bricker, 1992; Kalinowski and Schweda, were 100% catalase positive from both sampling rounds. Ni- 1996; Taylor et al., 2000) have investigated mica (muscovite, trate reduction is a test to determine whether bacteria can use phlogopite, biotite) weathering rates in the laboratory at or nitrate as a terminal electron acceptor. Nitrate reduction was around 22 to 25°C. Experiments most similar to our conditions common for all samples but was greater at the far site in terms of pH and mica mineralogy (i.e., muscovite) were 1344 P. A. Maurice et al.

Table 6. Microbiologic characterization of isolates from the Green Creek hyporheic zone sediments.

Feature GCT3WA 12/14/1999 GCT3WB 12/14/1999 GCT3WA 01/22/2000 GCT3WB 01/22/2000

Total number of isolates 9 (100%) 12 (100%) 7 (100%) 15 (100%) Cell morphology Rods (100%) Rods (100%) Rods (100%) Rods (100%) Approximate cell length Ͻ0.5 ␮m Ͻ0.5 ␮m Ͻ0.5 ␮m Ͻ0.5 ␮m Gram reaction 100% Ϫ 100% Ϫ 100% Ϫ 100% Ϫ Oxidase 78% ϩ 58% ϩ 43% ϩ 47% ϩ Catalase 100% ϩ 100% ϩ 100% ϩ 100% ϩ Nitrate reduction 55% ϩ 67% ϩ 86% ϩ 93% ϩ Starch hydrolysis 100% Ϫ 92% Ϫ 86% Ϫ 93% Ϫ Sucrose fermentation 67% ϩ 83% ϩ 43% ϩ 53% ϩ Lactose fermentation 67% ϩ 50% ϩ 43% ϩ 40% ϩ Glucose fermentation 67% ϩ 50% ϩ 29% ϩ 40% ϩ those of Nickel (1973, in Kalinowski and Schweda, 1996) and The Antarctic dissolution rates can be compared with a Lin and Clemency (1981a). Muscovite dissolution data of previous in situ study of feldspar dissolution at a temperate Nickel (1973, in Kalinowski and Schweda, 1996) provide a zone site. Such a comparison is useful because both studies dissolution rate of 5.0 ϫ 10Ϫ17 mol cmϪ2 sϪ1, assessed on the used AFM to calculate weathering rate. Nugent et al. (1998) basis of Si release and of 1.0 ϫ 10Ϫ19 mol cmϪ2 sϪ1, assessed calculated a feldspar dissolution rate of between 10Ϫ16 and on the basis of Al release, in deionized water at pH 6 at room 10Ϫ18 mol feldspar cmϪ2 sϪ1 on the basis of AFM images of temperature. Lin and Clemency (1981a) measured a dissolution feldspars buried in a State College, Pennsylvania, USA, spo- rate of ruby mica (Fe-bearing muscovite) at pH ϳ5 and room dosol for 3 yr. Considering that feldspar is likely to dissolve temperature of 2.4 ϫ 10Ϫ17 mol muscovite cmϪ2 sϪ1. more rapidly than muscovite mica (e.g., Birkeland, 1974), and Although our field rate appears consistent with these labo- that sites on the basal-plane surfaces of micas are likely less ratory rates, one must be cautious not to compare the rates reactive than are edge sites, our results are consistent with directly. First, the laboratory rates were for entire particles, relatively rapid initial dissolution rates of mica in the hyporheic whereas our rate was for the basal-plane surface alone, which zone during the austral summer. is probably less reactive than edge sites. Second, our surface XPS results showed a decrease in the surface Al:Si ratio for area was calculated on the basis of geometry as determined by one of the reacted mica surfaces, as well as a slight decrease in the relatively newly developed AFM and is not equivalent to the Fe:Si ratio for all of the reacted micas. These results are BET surface area (i.e. surface area by gas adsorption, Brunauer suggestive of dissolution, and they are thus in agreement with et al., 1938). Differences in methods of surface area measure- the observation of etch pits. ment may translate to several orders of magnitude greater The weathering rate calculated in this study represents the dissolution rates when normalized to geometric surface area capacity of Dry Valley stream systems for initial weathering of than when normalized to BET surface area (Brantley et al., an aluminosilicate mineral at two particular points in one 1999). Third, our rate is at temperatures considerably below stream. The hyporheic zone of any stream is inherently heter- typical room temperature, and aluminosilicate dissolution rates ogeneous, and the initial weathering rates calculated in these tend to increase with increasing temperature. Fourth, our rate is experiments may not be directly applicable to other portions of based on direct observation of changes in surface microtopog- the hyporheic zone along Green Creek or other Dry Valley raphy rather than on inference from solution chemistry. streams. However, several previous studies have concluded that Hochella et al. (1999) used AFM to measure dissolution rates primary silicate weathering is an important process in Dry in deionized water of etch pits on a preetched phlogopite basal Valley streams on the basis of analysis of long-term records of plane. They measured a pit dissolution rate of 5.1 Ϯ 2.1 ϫ water chemistry for many Dry Valley streams (Lyons et al., 10Ϫ14 mol cmϪ2 sϪ1. This rate is several orders of magnitude 1997) and on analyses of longitudinal increases in solutes in faster than the rate we measured, but it is specific to highly one Dry Valley stream (Gooseff et al., unpublished data). reactive sites. Gooseff et al. (unpublished data) computed Si denudation rates Several researchers have investigated rates of biotite mica on the order of 10Ϫ15 to 10Ϫ16 mol Si cmϪ2 sϪ1 of sediment weathering in the field. Biotite has long been known to dissolve (geometric) surface area for the hyporheic zone of a Dry Valley faster than muscovite (e.g., Birkeland, 1974). As reported in stream. Our mica dissolution rate is one to two orders of Murphy et al. (1998), determinations of field dissolution rates magnitude slower, most likely because mica is one of the most of biotite range from 10Ϫ19 to 10Ϫ17 mol biotite cmϪ2 sϪ1. unreactive of aluminosilicate phases and because sites on the This range includes geometric field dissolution rates on the basal-plane surface are less reactive than edge sites. Our study order of 10Ϫ18 to 10Ϫ17 mol biotite cmϪ2 sϪ1 (Velbel, 1985; directly demonstrated that chemical weathering may occur in Swoboda-Colberg and Drever, 1993). Our initial basal-plane the hyporheic zone during the limited period of streamflow in geometric muscovite dissolution rate for the hyporheic zone in one austral summer. Gooseff et al. (unpublished data) also the austral summer is within this range. However, the rates are observed that the rates of hyporheic exchange act as another not directly comparable because of the short duration of our major control on the extent to which this weathering is ob- experiment. It must be remembered that our rate is only for the served in the stream. We hypothesize that the reported rela- hyporheic zone and only for the portion of the year when liquid tively high weathering rates in the hyporheic zone of Dry water is present. Valley streams (Lyons et al., 1997; Nezat et al., 2001; Gooseff Direct observations of aluminosilicate weathering 1345 et al., unpublished data, data herein) are likely related to the GCT3WB, which is further removed from the stream. Water abundant supply of fresh, unweathered surfaces exposed to within the hyporheic zone at this site would have been in the dilute water that flows and exchanges rapidly with water in the hyporheic zone longer and the exchange at the surface is less. stream channel. These conditions would be conducive to greater oxygen deple- Research has shown that bacteria can significantly affect the tion in microzones and to increased potential for nitrate reduc- rates of aluminosilicate dissolution, thus influencing chemical tion. weathering processes and elemental cycling (e.g., Ehrlich, 1981, 1996; Hiebert and Bennett, 1992; Vandevivere et al., 5. CONCLUSIONS 1994; Welch and Vandevivere, 1994; Bennett et al., 1996; Ullman et al., 1996; Barker et al., 1998; Barker and Banfield, Our observation of dissolution features on the basal-plane 1998; Rogers et al., 1998; Liermann et al., 2000; Maurice et al. surfaces of mica upon weathering in the hyporheic zone is 2001). Although we observed bacteria and organic films on the consistent with the findings of previous studies, which indi- mica surfaces after reaction in the hyporheic zone, we did not cated that aluminosilicate dissolution occurs in the Dry Valleys notice any increase in dissolution pit density in the vicinity of despite the cold temperatures, scant precipitation, and lack of the bacteria. This may be misleading, though, because some terrestrial vegetation. We suggest that the combination of microorganisms were likely lost from the surface during rinsing largely unweathered and highly reactive hyporheic zone sedi- and transport, and we also could not image directly beneath ments, with relatively dilute glacial meltwaters and high ex- attached bacteria and biofilms. Hence, although the extensive change rates between the stream and the hyporheic zone, likely and quite rapid surface colonization suggests that the bacteria results in fairly rapid initial dissolution rates analogous to the may be utilizing nutrients in or on the muscovite surface, the rapid rates commonly observed (e.g., Rimstidt and Dove, 1986, role or roles of bacteria and biofilms in the dissolution and and references therein) at the beginning of laboratory-based weathering reactions remains to be determined. It is also pos- dissolution experiments. Our observed rates are specifictothe sible that the bacteria merely used the mica surface as a hyporheic zone during the austral summer and cannot be ex- physical substrate for growth. Observation of decreased Fe:Si trapolated to the surrounding (dry) soils or to the full year. ratios on the reacted mica surfaces suggests that the bacteria Analysis of bacterial ecology revealed an abundance of fairly could potentially be depleting Fe from the mica, as Fe is an small bacteria that rapidly formed biofilms on mineral surfaces. important nutrient. Clearly, further studies of microbe–mineral Overall, our results demonstrate that the hyporheic zone of a interactions in Dry Valley hyporheic sediments are needed. Dry Valley stream is a hydrobiogeochemically active location during the austral summer. 4.2. Nitrate-Reducing Bacteria: Implications for the N Cycle Acknowledgments—We thank the Antarctic Support Associates, Jon Mason, and Ethan Chatfield for help with fieldwork. We thank Olabow- The detection of nitrate-reducing bacteria has important im- ale Olapade and Joanneke Talsma for assistance with bacteriologic plications for N cycling in the Dry Valleys and agrees well with analysis and Dana Rosenberg for some of the AFM imaging. W. B. Lyons provided much helpful discussion. We thank S. Brantley, M. results of a prior study at Green Creek. McKnight et al. (2002) Velbel, and A. White for detailed review comments that greatly im- investigated hyporheic zone exchange and inorganic nitrogen proved the manuscript. This research was funded in part by the Na- and phosphorus dynamics at Green Creek in late January 1995. tional Science Foundation, Office of Polar Programs (grant OPP- They injected lithium chloride, sodium nitrate, and potassium 9813061). phosphate into the creek. Variations of Cl over time demonstrated that substantial exchange of surface water with the Associate editor: D. E. Canfield hyporheic zone occurred at low flow conditions of Ͻ3LsϪ1. Microbial communities associated with the algal mats in the REFERENCES streambed were found to have significant nitrate and phosphorus uptake abilities. Moreover, production of nitrite and am- Acker J. G. and Bricker O. P. (1992) The influence of pH on biotite dissolution and alteration kinetics at low temperature. Geochim. monium at sites located several hundred meters downstream Cosmochim. Acta 56, 3073–3092. from the injection site indicated that dissimilatory nitrate re- Aiken G., McKnight D., Harnish R., and Wershaw R. (1996) Geo- duction was occurring. McKnight et al. (2002) estimated that chemistry of aquatic humic substances in the Lake Fryxell Basin, hyporheic nitrate uptake, which represented the loss due to Antarctica. Biogeochemistry 34, 157–188. Alger A. S., McKnight D., Spaulding S. A., Tate C. M., Shupe G. H., assimilatory and dissimilatory nitrate reduction, was 7 to 16% Welch K. A., Edwards R., Andrews E. D., and House H. R. (1997) of the total uptake. They also suggested that microbial pro- Ecological Processes in a Cold Desert Ecosystem: The Abundance cesses of nitrate reduction were likely important in the hypo- and Species of Algal Mats in Glacial Meltwater Streams in Taylor rheic zone. Our XPS-based observation of N accumulation at Valley, Antarctica.Occasional Paper 51. INSTAAR . the mica surface, along with our AFM images showing organic Allan J. D. (1995) Stream Ecology Structure and Function of Running Waters. Chapman and Hall. adsorption, bacteria, and biofilms and our identification of Atlas R. M. (1993) Handbook of Microbiological Media.CRC Press. nitrate-reducing bacteria in hyporheic zone sediments, thus Barker W. W. and Banfield J. F. (1998) Zones of chemical and physical confirm the interpretation of McKnight et al. of the tracer interaction at interfaces between microbial communities and miner- experiment. The activity of these bacteria and the exchange of als: A model. Geomicrobiol. J. 15, 223–244. Barker W. W., Welch S. A., Chu S., and Banfield J. F. (1998) Exper- hyporheic zone water with stream water thus can be expected to imental observations of the effects of bacteria on aluminosilicate play an important role in the N flux to Lake Fryxell. Moreover, weathering. Am. Mineral. 83, 1551–1563. we observed more nitrate-reducing bacterial percentages at site Bennett P. C., Hiebert F. K., and Choi W. J. (1996) Microbial coloni- 1346 P. A. Maurice et al.

zation and weathering of silicates in a petroleum-contaminated Hochella M. F. Jr., Rakovan J. F., Rosso K. M., Bickmore B. R., and groundwater. Chem. Geol. 132, 45–53. Rufe E. (1999) New directions in mineral surface geochemical Berner R. A., Rao J.-L., Chang S., O’Brien R., and Keller C. K. (1998) research using scanning probe microscopes. In Mineral–Water In- Seasonal variability of adsorption and exchange equilibria in soil terfacial Reactions: Kinetics and Mechanisms (eds. D. L. Sparks and waters. Aquat. Geochem. 4, 273–290. T. J. Grundl), pp. 37–56. ACS Symposium Series 715. American Birkeland P. E. (1974) Pedology, Weathering, and Geomorphological Chemical Society. Research. Oxford University Press. Holland H. D. (1978) The Chemistry of the Atmosphere and Oceans. Blum A. E. and Stillings L. L. (1995) Feldspar dissolution kinetics. Wiley. Rev. Miner. 31, 291–351. Johnsson P. A., Blum A. E., Hochella M. F. Jr., Parks G. A., and Blum A. E., McKnight D. M., and Lyons W. B. (1997) Silicate Sposito G. (1992) Direct observation of muscovite basal-plane dis- weathering rates along a stream channel draining into Lake Fryxell, solution and secondary-phase formation: An XPS, LEED, and SFM Taylor Valley, Antarctica. In Seventh Annual V. M. Goldschmidt study. In Water–Rock International VII (eds. G. Arehardt and R. Conference, p. 29. LPI Contribution 921. Lunar and Planetary Insti- Hulston), pp. 159–162. Balkema. tute. Jones L. M. and Faure G. (1978) A study of strontium isotopes in lakes Bluth G. J. S. and Kump L. R. (1994) Lithologic and climatologic and surﬁcial sediments of the ice-free valleys, southern Victoria controls on river chemistry. Geochim. Cosmochim. Acta 58, 2341– Land, Antarctica. Chem. Geol. 22, 107–120. 2359. Kalinowski B. E. and Schweda P. (1996) Kinetics of muscovite, Bockheim J. G. (1997) Properties and classiﬁcation of cold desert soils phlogopite, and biotite dissolution and alteration at pH 1–4, room from Antarctica. Soil Sci. Soc. Am. J. 61, 224–231. temperature. Geochim. Cosmochim. Acta 60, 367–385. Brantley S. L., White A. F., and Hodson M. E. (1999) Surface area of Kalinowski B. E., Liermann L. J., Givens S., and Brantley S. L. (2000) primary silicate minerals. In Growth and Pattern Formation Disso- Rates of bacteria-promoted solubilization of Fe from minerals: A lution in Geosystems (eds. B. Jamtveit and P. Meakin), pp. 291–326. review of problems and approaches. Chem. Geol. 169, 357–370. Kluwer Academic. Kump L. R., Brantley S. L., and Arthur M. A. (2000) Chemical

Broady P. A. (1982) Taxonomy and ecology of algae in freshwater weathering, atmospheric CO2, and climate. Annu. Rev. Earth Planet. stream in Taylor Valley, Victoria Land Antarctica. Arch. Hydrobiol. Sci. 28, 611–667. 32(Suppl. 63.3), 331–349. Lemke M. J., McNamara C. and Leff L. G. (1997) Comparison of Bromley A. M. (1985) Weather observations, Wright Valley, Antarc- methods for the concentration of bacterioplankton for in situ hybrid- tica. In Infor. Publ. 11. New Zealand Meteorological Service. ization. J. Microbiol. Methods 29, 23–29. Brunauer S., Emmett P. H., Teller E. (1938) Adsorption of gases in Liermann L. J., Kalinowski B. E., Brantley S. L., and Ferry J. G. (2000) multimolecular layers. J. Phys. Chem. 60, 309–319. Role of bacterial siderophores in dissolution of hornblende. Buss H. L., Brantley S. L., and Liermann L. J. (in press) Nondestructive Geochim. Cosmochim. Acta 64, 587–602. methods for removal of bacteria from silicate surfaces. Geomicro- Lin F.-C., and Clemency C. V. (1981a) The kinetics of dissolution of

biol. muscovites at 25°C and 1 atm CO2 partial pressure. Geochim. Chin T. J. (1993) Physical hydrology of the Dry Valley Lakes. In Cosmochim. Acta 45, 571–576. Physical and Biogeochemical Processes in Antarctic Lakes (eds. Lin F.-C. and Clemency C. V. (1981b) Dissolution kinetics of phlogo- W. J. Green and E. I. Friedman), pp. 1–51. American Geophysical pite. 1. Closed system. Clays Clay Minerals 29, 101–106. Union. Lyons W. B. and Mayewski P. A. (1993) The geochemical evolution of Claridge G. G. C. and Campbell I. B. (1977) The salts in Antarctic terrestrial waters in the Antarctic: The role of rock–water interac- soils, their distribution and relationship to soil processes. Soil Sci. tions. In Physical and Biogeochemical Processes in Antarctic Lakes, 123, 377–384. pp. 135–143. Antarctic Research Series 59. Clemency C. V. and Lin F.-C. (1981) Dissolution kinetics of phlogo- Lyons W. B., Welch K. A., Nezat C. A., Crick K., Toxey J. K., pite 2: Open system using an ion-exchange resin. Clays Clay Min- Mastrine J. A., and McKnight D. M. (1997) Chemical weathering erals 29, 107–112. rates and reactions in the Lake Fryxell basin, Taylor Valley: Com- Conovitz P. A., McKnight D. M., MacDonald L. H., Fountain A. G., parison to temperate river basins. In Ecosystem Processes in Ant- and House H. R. (1998) Hydrologic processes influencing stream- arctic Ice-Free Landscapes (eds. W. B. Lyons and C. Howard- flow variation in Fryxell Basin, Antarctica. In Ecosystem Dynamics Williams), pp. 147–154. Balkema. in a Polar Desert: The McMurdo Dry Valleys, Antarctica (ed. J. C. Lyons W. B., Welch K. A., Neumann K., Toxey J. K., McArthur R., Priscu), pp. 93–108. American Geophysical Union. Williams C., McKnight D. M., and Moorhead D. (1998) Geochemi- Downes M. T., Howard-Williams C., and Vincent W. F. (1986) cal linkages among glaciers, streams and lakes within the Taylor Sources of nitrogen, phosphorus and carbon in Antarctic streams. Valley, Antarctica. In Ecosystem Dynamics in a Polar Desert: The Hydrobiologia 134, 215–225. McMurdo Dry Valleys, Antarctica (ed. J. C. Priscu), pp. 77–92. Ehrlich H. L. (1981) Geomicrobiology. Marcel Dekker. Antarctic Research Series 72. American Geophysical Union. Ehrlich H. L. (1996) How microbes influence mineral growth and Magonov S. N., Cleveland J., Elings V., Denley D., and Whangbo dissolution. Chem. Geol. 132.5–9. M. H. (1997) Tapping-mode atomic force microscopy study of the Forsythe J. H., Maurice P. A., and Hersman L. E. (1998) Attachment of near-surface composition of a styrene–butadiene–styrene triblock a Pseudomonas sp. to Fe(III)-(hydr)oxide surfaces. Geomicrobiol. J. copolymer film. Surf. Sci. 389, 147–154. 15, 293–304. Magonov S. N. and Reneker D. H. (1997) Characterization of polymer Gibbs R. J. (1970) Mechanisms controlling world water chemistry. surfaces with atomic force microscopy. Ann. Rev. Mater. Sci. 27, Science 170, 1088–1090. 175–222. Green W. J. and Canfield D. E. (1984) Geochemistry of the Onyx River Manz W., Amman R., Ludwig W., Wagner M., and Schleifer K. H. (Wright Valley, Antarctica) and its role in the chemical evolution of (1992) Phylogenetic probes for the major subclasses of proteobac- Lake Vanda. Geochim. Cosmochim. Acta 48, 2457–2467. teria: Problems and solutions. Syst. Appl. Microbiol. 15, 593–600. Green W. J., Angle M. P., and Chave K. E. (1988) The geochemistry Maurice P. A., Vierkorn M. A., Hersman L. E., and Fulghum J. E. of Antarctic streams and their role in the evolution of four lakes of (2001) Effects of an aerobic Pseudomonas bacteria on rates of the McMurdo Dry Valleys. Geochim. Cosmochim. Acta 52, 1256– kaolinite dissolution. Geomicrobiol. J. 18, 21–35. 1274. McKnight D. M., Aiken G. R., Andrews E. D., Bowles E. C., and Green W. J., Canfield D. E., Shengsong Y., Chave K. E., Ferdelman Harnish R. A. (1993) Dissolved organic material in Dry Valley T. G., and Delanois G. (1993) Metal transport and release processes lakes: A comparison of Lake Fryxell, Lake Hoare, and Lake Vanda. in Lake Vanda: The role of oxide phases. In Physical and Biogeo- In Physical and Biogeochemical Processes in Antarctic Lakes,pp. chemical Processes in Antarctic Lakes, pp. 145–163. Antarctic Re- 119–133. Antarctic Research Series 59. American Geophysical search Series 59. American Geophysical Union. Union. Hiebert F. K. and Bennett P. C. (1992) Microbial control of silicate McKnight D. M., Alger A., Tate C. M., Shupe G., and Spaulding S. weathering in organic-rich ground water. Science 253, 278–281. (1998) Longitudinal patterns in algal abundance and species distri- Direct observations of aluminosilicate weathering 1347

bution in meltwater streams in Taylor Valley, Southern Victoria Kinetics of dissolution and Sr release during biotite and phlogopite Land, Antarctica. In Ecosystem Dynamics in a Polar Desert: The weathering. Geochim. Cosmochim. Acta 64, 1191–1208. McMurdo Dry Valleys, Antarctica (ed. J. C. Priscu), pp. 109–128. Ullman W. J., Kirchman D. L., Welch S. A., and Vandevivere P. (1996) American Geophysical Union. Laboratory evidence for microbially mediate silicate dissolution in McKnight D. M., Runkel R. L., Duff J. H., Tate C. M. and Moorhead nature. Chem. Geol. 132, 11–17. D. (2002) Inorganic nitrogen and phosphorus dynamics of Antarctic Vandevivere P., Welch S. A., Ullman W. J., and Kirchman D. L. (1994) glacial meltwater streams as controlled by hyporheic exchange and Enhanced dissolution of silicate minerals by bacteria at near-neutral benthic autotrophic communities. J. N. Am. Benthol. Soc., in press. pH. Microbial Ecol. 27, 241–251. Murphy S. F., Brantley S. L., Blum A. E., White A. F., and Dong H. Velbel M. A. (1985) Geochemical mass balances and weathering rates (1998) Chemical weathering in a tropical watershed, Luquillo Moun- in forested watersheds of the southern blue ridge. Am. J. Sci. 285, tains, Puerto Rico, II: Rate and mechanism of biotite weathering. 904–930. Geochim. Cosmochim. Acta 62, 227–243. Velbel M. A. (1993) Constancy of silicate-mineral weathering-rate Namjesnik-Dejanovic K. and Maurice P. A. (1997) Atomic force ratios between natural and experimental weathering: Implications for microscopy of soil and stream fulvic acids. Coll. Surf. A 120, 77–86. hydrologic control of differences in absolute rates. Chem. Geol. 104, Nezat C. A., Lyons W. B., and Welch K. A. (2001) Chemical weath- 89–99. ering in streams of a polar desert (Taylor Valley, Antarctica) GSA Vincent W. F. and Howard-Williams C. (1986) Antarctic stream eco- systems: Physiological ecology of a blue-green algal epilithon. Bull., 113, 1401–1408. Freshwater Biol. 16, 219–233. Nugent M. A., Brantley S. L., Pantano C. G., and Maurice P. A. (1998) Vincent W. F. and Howard-Williams C. (1989) Microbial communities The influence of natural mineral coatings on feldspar weathering. in Southern Victoria land streams (Antarctica) II: The effects of low Nature 395, 588–591. temperature. Hydrobiology 172, 39–49. Porter K. and Feig Y. (1980) The use of DAPI for identifying and Vincent W. F., Howard-Williams C., and Broady P. A. (1993) Micro- counting aquatic microflora. Limnol. Oceanogr. 25, 943–8. bial communities and processes in Antarctic flowing waters. In Rimstidt J. D. and Dove P. M. (1986) Mineral/solution reaction rates in Antarctic Microbiology (ed. E. I. Friedman), pp. 543–570. Wiley. a mixed flow reactor: Wollastonite hydrolysis. Geochim. Cosmo- Vishniac H. S. (1993) The microbiology of Antarctic soils. In Antarctic chim. Acta 50, 2509–2516. Microbiology. (ed. E. I. Friedman), pp. 297–342. Wiley. Rogers J. R., Bennett P. C., and Choi W. J. (1998) Feldspars as a source Welch S. A. and Vandevivere P. (1994) Effect of microbial and other of nutrients for microorganisms. Am. Mineral. 83, 1532–1540. naturally occurring polymers on mineral dissolution. Geomicrobiol. Runkel R., McKnight D. M., and Andrews E. A. (1998) Analysis of J. 12, 227–238. transient storage subject to unsteady flow: Diel flow variation in an White A. F. and Blum A. E. (1995) Effects of climate on chemical Antarctic stream. J. N. Am. Benthol. Soc. 17, 143–154. weathering in watersheds. Geochim. Cosmochim. Acta 59, 1729– Stallard R. F. and Edmond J. M. (1987) Geochemistry of the Amazon. 1747. 3. Weathering chemistry and limits to dissolved inputs. J. Geophys. White A. F., Blum A. E., Bullen T. D., Vivit D. V., Schulz M., and Res. Oc. Atm. 92, 8293–8302. Fitzpatrick J. (1999) The effect of temperature on experimental and Swoboda-Colberg N. G. and Drever J. I. (1993) Mineral dissolution natural chemical weathering rates of granitoid rocks. Geochim. Cos- rates in plot-scale field and laboratory experiments. Chem. Geol. mochim. Acta 63, 3277–3291. 105, 51–69. Zhou J., Bruns M. A., and Tiedje J. M. (1996) DNA recovery from soils Taylor A. S., Blum J. D., Lasaga A. C., and MacInnis I. N. (2000) of diverse composition. Appl. Environ. Microbiol. 62, 316–322.