The biogeochemistry of Si in the McMurdo Dry Valley lakes, Antarctica Heather E. Pugh 1 *, Kathleen A. Welch 1 , W. Berry Lyons 1 , John C. Priscu 2 and Diane M. McKnight 3 1 Byrd Polar Research Center, The Ohio State University, Columbus OH, 43210-1002, USA e-mail: lyons.142@osu.edu 2 Dept of Land Resources and Environmental Sciences, Montana State University, Bozeman, MT 59717, USA 3 INSTAAR, University of Colorado, Boulder, Colorado 80309-0450, USA Abstract : The biogeochemical dynamics of Si in temperate lakes is well documented and the role of biological uptake and recycling is well known. In this paper we examine the Si dynamics of a series of ice-covered, closed-basin lakes in the McMurdo Dry Valley region (y78x S) of Antarctica. Our data and calculations indicate that biological uptake of Si is not a major process in these lakes. Mass balance considerations in Lake Hoare, the youngest and the freshest lake, suggest that annual stream input during relatively low-flow years is minor and that Si dynamics is greatly influenced by hydrological variation and hence climatic changes affecting stream flow and lake level. The data imply that the Si input during high-flow years must dominate the system. Subtle changes in climate have a major control on Si input into the lake, and Si dynamics are not controlled by biogeochemical processes as in temperate systems. Accepted 1 October 2002 Key words : Antarctica, lakes, silica, biogeochemistry. Introduction The McMurdo Dry Valleys (MCM) region of Antarctica has often been described as a terrestrial analogue of Mars. The ice-covered lakes in MCM have been proposed as analogues of the paleolakes that might have existed on Mars during its early history (Wharton et al. 1989, 1995; Doran et al. 1998). Doran et al. (2003) have reviewed the exopaleolimnology and have argued that the variations of the aquatic environments that presently exist in the MCM region may provide import- ant clues as to what features to look for in a search of signs of former life on Mars. Therefore, the investigation of biogeo- chemical processes in the dry valley lakes could potentially provide insight into the functioning of ancient lake systems on Mars. In addition, the MCM lakes provide extreme en- vironmental end members, the biogeochemistries of which should be compared with more temperate lake systems that have more complicated dynamics (i.e. organic matter input from the landscape). Although the MCM lakes have been investigated since the International Geophysical Year (1957–1958), there have been few attempts to understand and quantify nutrient dynamics in these lakes (Canfield & Green 1985; Green et al. 1989; Priscu 1995 ; Priscu et al. 1999), and these works have pri- marily dealt with carbon, nitrogen and phosphorus dynamics. The first quantitative evaluation of silicon biogeochemistry in the Taylor Valley lakes, Southern Victoria Land, was con- ducted as part of the McMurdo Dry Valleys, Long-Term Ecological Research (MCM-LTER) programme and is pres- ented in this paper. Silicon is a very important element in that it is the major building block of aluminosilicate minerals, the primary mineral of the Earth’s (and Martian) crust. It is also a major plant nutrient, especially for diatoms, as they use it for the building of frustules. Because diatoms are ubiquitous in Earth’s aquatic environments, the biogeochemistry of sili- con is of biological importance. Study location Taylor Valley (Fig. 1), 77x 40k S, 163x 00k E, is part of the McMurdo Dry Valleys in Southern Victoria Land, Antarc- tica. The valley is 33 km long and 12 km wide (Fig. 1). Taylor Valley is a polar desert with a mean annual temperature of approximately x20 xC (Clow et al. 1988) and a total annual precipitation of less than 10 cm (Keys 1980). The geomorphology of Taylor Valley has been modified by the movement of glaciers, the inflow of ocean waters and the rising and falling of lake levels over the past few million years (Porter & Beget 1981). Because of the movements of glaciers, the valley floor contains a mosaic of tills of differing age and composition (Pe´we´ 1960; Stuvier et al. 1981; Burkins et al. 1999). The ages of the morainal materials in the region date to 2.5 Myr (Brown et al. 1991). The soils and tills are derived from a number of rock types within the Victoria Land region. * Present address: Department of Marine Sciences, University of Connecticut, Avery Pt, Groton, CT, USA. International Journal of Astrobiology 1 (4): 401–413 (2003) Printed in the United Kingdom DOI: 10.1017/S1473550403001332 f 2003 Cambridge University Press 401