doi:10.1016/j.gca.2004.08.001 Geochemical processes in the Onyx River, Wright Valley, Antarctica: Major ions, nutrients, trace metals WILLIAM J. GREEN,* BRIAN R. STAGE,ADAM PRESTON,SHANNON WAGERS,JOSEPH SHACAT, and SILVIA NEWELL School of Interdisciplinary Studies, Miami University, Oxford, OH 45056, USA (Received October 24, 2002; accepted in revised form June 17, 2004) Abstract—We present data on major ions, nutrients and trace metals in an Antarctic stream. The Onyx River is located in Wright Valley (77–32 S; 161–34 E), one of a group of ancient river and glacier-carved landforms that comprise the McMurdo Dry Valleys of Antarctica. The river is more than 30 km long and is the largest of the glacial meltwater streams that characterize this relatively ice-free region near the Ross Sea. The complete absence of rainfall in the region and the usually small contributions of glacially derived tributaries to the main channel make this a comparatively simple system for geochemical investigation. Moreover, the lack of human impacts, past or present, provides an increasingly rare window onto a pristine aquatic system. For all major ions and silica, we observe increasing concentrations with distance from Lake Brownworth down to the recording weir near Lake Vanda. Chemical weathering rates are unexpectedly high and may be related to the rapid dissolution of ancient carbonate deposits and to the severe physical weathering associated with the harsh Antarctic winter. Of the nutrients, nitrate and dissolved reactive phosphate appear to have quite different sources. Nitrate is enriched in waters near the Lower Wright Glacier and may ultimately be derived from stratospheric sources; while phosphate is likely to be the product of chemical weathering of valley rocks and soils. We confirm the work of earlier investigations regarding the importance of the Boulder Pavement as a nutrient sink. Dissolved Mn, Fe, Ni, Cu, and Cd are present at nanomolar levels and, in all cases, the concentrations of these metals are lower than in average world river water. We hypothesize that metal uptake and exchange with particulate phases along the course of the river may serve as a buffer for the dissolved load. Concurrent study of these three solute classes points out significant differences in the mechanisms and sites of their removal from the Onyx River. Copyright © 2005 Elsevier Ltd 1. INTRODUCTION Insight into chemical weathering processes, including the comparative behavior of major solute classes along the length of river channels, can often be obtained in those rare natural settings which offer a somewhat restricted list of physical, chemical and biologic variables. Even the rates of chemical weathering and, consequently, of carbon dioxide drawdown, can be more easily, though roughly, estimated in those streams which are of smaller scale and which are not significantly complicated by rainfall or by the usual extraneous inputs, whether natural or human. The present study of the Onyx River was undertaken with a view toward capitalizing on these pos- sible advantages in scale and complexity to compare, through simultaneous measurement, the behavior of major ions, nutri- ents and trace metals in a remote and relatively simple stream and to examine chemical weathering rates and carbon dioxide uptake in a cold-climate system. We note that this is the first study to report on dissolved and particulate trace metals over the entire length of an Antarctic river and the first to establish preliminary weathering rates for the Onyx River itself. Al- though the exotic setting of this river, and, indeed, other streams of the McMurdo Dry Valleys, expands the range of flowing water environments generally considered by ecolo- gists, the evolving principles of solute acquisition and trans- formation apply. 1.1. McMurdo Dry Valleys and the Onyx River A small number of ice-free desert oases occur along the coastline of an otherwise ice-burdened Antarctic Continent. The largest of these, the McMurdo Dry Valleys, cuts inland from the Ross Sea—in an east-west direction—toward the vast ice cap of the Polar Plateau. Extreme polar deserts, where precipitation falls only as dry snow, the valleys are character- ized by a mean annual temperature of -20°C and by fierce desiccating winds that act as agents of extreme physical weath- ering and material transport. The lithology of the area has been described in some detail by Vocke and Hanson (1981), and Claridge and Campbell (1977) and Keys and Williams (1981) have discussed soil development in the region. Briefly, base- ment rocks consist of pre-Ordovician schists, hornfels and marbles; a younger set of lower Paleozoic granites and granite gneiss intrusions; overlying Beacon sandstones intruded by Ferrer Dolerites; and thin Quaternary dimictics and McMurdo volcanics. Soils are low in organic content but contain salts of either marine origin or derived from chemical weathering. Jones and Faure (1978), Green et al. (1988) and Lyons et al. (1998) have examined weathering and transport of ions by streams and have discussed the evolution of closed-basin lakes in Wright and Taylor Valleys. The Onyx River is the longest of the many glacial meltwater streams that flow through the McMurdo Dry Valleys during the warmer (six to eight) weeks of the austral summer. Over 30 km in length, the Onyx flows westward from the Lower Wright Glacier into Lake Vanda, a highly stratified, closed-basin lake * Author to whom correspondence should be addressed (greenwj@ muohio.edu). Geochimica et Cosmochimica Acta, Vol. 69, No. 4, pp. 839 – 850, 2005 Copyright © 2005 Elsevier Ltd Printed in the USA. All rights reserved 0016-7037/05 $30.00 + .00 839