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Hoare and Lake Fryxell Basins, Taylor Valley, Antarctica

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Hoare and Lake Fryxell Basins, Taylor Valley, Antarctica Annals qfGlaciology 27 1998 © International Glaciological Society Inorganic carbon-isotope distribution and budget in the Lake Hoare and Lake Fryxell basins, Taylor Valley, Antarctica KLAUS N EUMANN, I W. BERRY LYONS,I D AVID J. D ES M AR A IS2 1 University qf Alabama, De/Jart71lent qfGeology, nlscaloosa, A L 35487, US.A. 2 NASA Ames Research Centel; J1Ioffitt Field, CA 94035, US.A . ABSTRACT. One of the unusua l features of Lakes Fryxell and Hoare in Taylor Vall ey, so uthern Victori a Land, Antarcti ca, is their perennial ice cover. This ice cover limits gas excha nge between the atmosphere and the la ke water, and causes a ve ry stabl e stratifica­ tion of the lakes. We ~ n a l yzed a series of water samples from profil es of these la kes and their tributaries for 813 C of the dissolved inorganic carbon (DIC) in order to qualify the carbon flu x fr om the streams into the lakes, and to i\1Vestigate the carbon cycling within the lakes. Isotopic values in the uppermost waters (813C = + 1.3%0 to 5.3%0 in Lake H oare, +0.4%0 to +3.0%0 in Lake Fryxell ) are close to the carbon-isotope values encountered in the streams feeding Lake Fryxell , but distinctively heavier than in streams feeding Lake H oare (813C = - 2.3%0 to 1.4%0). T hese ratios a re much heavier than ratios found in the moat that forms around the lakes inJanuary- February (813C = - 10.1 %0). In the oxic photic zones of the lakes, photosynthesis clearly influences the isotopic c<? mposition, with layers of high productivity having enriched carbon-isotope signatures (813 C = +2.7%0 to +6.1% 0). In both lakes, the iso topic values become li ghter wi th depth, reaching minima of - 3.2%0 a nd - 4.0%0 in Lakes Fryxell and H oare, res pective ly. These minima are caused by the microbia l reminera li zati on of iso topically li ght organic carbon. \ Ve present DIC flu x cal­ cul ati ons that hel p to interpret the isotopic distribution. For example, in Lake Hoare the higher utilizati on of C0 2a(1' and a subs ta ntia ll y smaller inflow of CO 2 from streams cause the heavi er observed iso topic rati os. Differences in the hydrology and stream morpholo­ gies of the tributari es a lso greatl y influence the carbon budgets of the basins. INTRODUCTION more moderate cl imates (Doran a nd others, 1994). Because they a re located in the d ri es t desert of the Ea rth, and are Closed-basin lakes are excellent indicators of changes in recha rged onl y by glacial meltwater, their water levels are cl imate. Due to their nature, water levels can vary dramati­ ,·ery sensitive to climate changes (Clow and others, 1988; cally, as precipitation and evaporati on change over time. Evi­ Whar to n and others, 1992). The amount of recharge can dence of these changes can be fo und in the form of perched or vary d rastically from year to year, a nd is controll ed by the fl ooded shorelines (Gilbert, 1890; J ones and others, 197 1; Cut­ temperature (Chinn, 1985). These lakes are also covered fi eld, 1974), and salt deposits in lake sediments (H ardie and with a perennial ice cover. This "lid" a ffects the lakes by re­ others, 1978; Eugs ter, 1980). Changes in lake level and water ducing gas exchange, excluding wind mixing and abso rbing volume ofte n correspond wi th changes in the water chemi s­ 97- 99% of the incoming li ght (Vincent, 1987; Lizotte and tr y. During water-level ri ses, solutes are diluted, while during Prisc u, 1992; Wharton and others, 1992). The balance water depletion solu tes can be concentrated. Minerals, such between ice-cover gain (by fr eezing of water to the bottom as calcite, can be precipitated if their saturati ons are reached. of the ice) and loss (via ablati on from the surface) itself is T hese chemical changes, in turn, affect the bi ology of the affected by climate variation, and as a res ult the thickness lakes, as hi gher nutrient concentrations cause higher produc­ changes over time (\Vharton and others, 1992). This in turn ti vit y, and hi gher solutes generall y limit the number of spe­ directly influences the transmissivity, and the productivity cies present in a lake (Melack, 1983). Bi ological acti vity in in these partia ll y light-limited lake systems. these types of la kes, as anywhere on Earth, is closely con­ nected to the carbon cycle. Photosy nthesis and respi ration de­ SITE DESCRIPTION plete and enrich inorganic carbon in lakes. T hese processes can change the stable carbon isotopic composition (813 C) of Lakes Hoare and Fryxell are located in Taylor Valley, so uth­ the lake water. Shifts in rates of productivity and mineraliza­ ern Victoria La nd, Anta rctica (Fig. 1). In 1993 a U. S. ti on can cause the organic contents oflake sediments to vary. National Science Foundation- (NSF-)supported long-term These changes in the quality (isotopic) and quantity (organi c ecological research (LTER ) site was established in Taylor vs inorgani c matter) of the carbon have been used widely to Va lley, a nd vari ous chemical, physical and biological pa­ deduce paleo-conditions from lake sedi ments (M cKenzie, rameters have been monito red by the LT ER team on a rou­ 1985). In order to inte rpret sedimentary records it is essential tine basis. The valley is located "-' 100 km northwest of to qualify and quantify present closed-basin lake systems. M cMurdo Stati on, at 76- 78° S, 160- 164° E. The vall ey is Closed-basin lakes in Antarctica respond like lakes in arid, with a precipitation of :::; 100 mm a 1 (Clow and others, Downloaded from https://www.cambridge.org/core. 01 Oct 2021 at 05:30:19, subject to the Cambridge Core terms of use. 685 Ne umann and others: Carbon-isotope distribution in Tay lor Valley lakes METHODS o 1 2 3 4 5 6 7 Kilometers Water samples were collected at the location of the greatest water depth. Holes were drilled a nd melted at the beginning of each season. The holes could be used during the whole season ( October-J anuary). Five-liter Niskin bottles were lowered through the holes, and samples collected 3- 4 times per season. pH was measured electrometrically (Beck man portable pH meters with vari ous silver- silver-chloride glass electrodes; accuracy ± 0.05) in the fi eld within hours aft er sample coll ection. Temperature was measured with a Sea­ Bird GTD probe. Dissolved inorganic carbon (DIG) samples from the lakes were stabilized with chloroform, ~ Streams. _ Watershed!lilllJ Lakes 0 Glaciers stored in cooler boxes at ,..,A°e. The samples were inj ected Creeks - Boundary into 6 N H 2S04, sparged with nitrogen gas and measured with an MSA Lira infrared gas analyzer (personal commu­ Fig. 1. M ap if the eastern part if Tay lor Valley, Antarctica. ni cation fr omJ e. Priscu, 1997), generally within 2 weeks of Glaciers are light gray, lakes da rk gray; white areas consist if sampling. For streams, alkalinity was determined by titra­ morainal material and bedrock. All streams feeding Lake ti on and Gran plot (Drever, 1988). The relative standard de­ Hoareflow along Canada and Suess Glaciers. The lake levels viations for the DIG and alkalinity analyses were ± 3%. are afew meters above sea level. T he Ross Sea is about 5 km to The sampl es for major cations/ani ons were filtered through the east. Whatman 0.4 /1m filters in the fi eld, and analyzed in M cMurdo Station by ion chromatography (Welch and 1988). The average annual temperature is less than - 20o G, others, 1996), using a Dionex DX-300 chromatograph. but temperatures ri se above freezing during a few days of Those analyses were fini shed within "-'2 months after the summer. No vascular plants grow in the valley. There sampling, a nd the error, expressed as percentage errors in are algae and lichens growing in the soil (Campbell and the charge balance between cati ons and anions, was 3.4% Cla ridge, 1987), but most pla nts are located in algal mats in for the streams and 3.0- 1.4% for the lakes. The speciation the lakes and the streams (Wharton and others, 1983). As a models PHREEQE and PHRqJ'ITZ (Parkhurst and res ult, terrestrial input of organic matter into the la kes is others, 1980; Plummer and others, 1988) were used to cal­ ve ry limited (M cKnight and others, 1991, 1993). Lake H oare culate CO2 concentrations in the water. Samples [or bl3 C is a fresh-water lake (total dissolved solids (TDS) = 300- a nalysis were filtered into pre-evacuated 60 ml serum bot­ 750 mg I I) of 30 m depth, that is oxygenated all the way to tles, using ""hatman 0.4 1£m GF-F filters, a nd stabilized with the bottom, except for some small anoxic pockets in its dee­ 0.2 ml concentrated mercuric chloride solution. They were pest parts. Lake Fryxell is 18 m deep, is brackish analyzed at the NASA Ames Research Genter on a modi­ (TDS = 300- 8000 mg I- I) and has a steep chemo- and oxy­ fi ed Nuclide 6-60RMS mass spectrometer (Hayes and cline at 9 m depth.
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