GEOCHEMICAL PROCESSES IN LAKE MIERS

Geochemical Processes In Lake Miers

Lake Miers has been one of the least investigated of the McMurdo Dry Valley lakes. It is situated in the vicinity of the Koettlitz Glacier, in the foothills of the Royal Society Range. During summer, the Miers and Adams Glaciers discharge meltwaters into this relatively fresh, weakly stratified, water body. Among the lakes of the region, Miers is unusual in that it has an intermittent outflow, which empties into McMurdo Sound.

We report recent water column data for major ions, nutrients and trace metals. Calcium and bicarbonate are the major cation and anion, respectively, and their profiles reveal a concentration gradient which begins at 12 meters and extends down to the sediment-water interface at 18 meters. Calculations indicate that sedimentary calcite may be dissolving at depth, in waters whose pH is circumneutral. There is a strong gradient in dissolved oxygen, with values decreasing rapidly below 12 meters. Just below 16 meters, the lake becomes anoxic.

The sediments act as a sink for both sulfate, whose concentration decreases from 60 to 10 micromolar through the chemocline, and for the metals Co, Ni, Cu and Cd, each of which exhibits a negative concentration gradient with depth. Sulfate reduction, sulfide generation and metal sulfide precipitation likely play a significant role in the fate of metals in this system. Dissolved Al, by contrast, appears to be regulated by gibbsite solubility,

Manganese oxide dissolution begins to occur below 15 meters in Lake Miers, but there is no evidence that this process results in the release of surface or lattice-bound trace metals, as has been observed in Lake Vanda and Lake Joyce. A possible explanation for this difference, and for trace metal scavenging in the lake, generally, are discussed. Throughout the watercolumn, dissolved metals are well in excess of metals associated with suspended particles.

Nitrate to phosphate ratios are low at all depths, suggesting nitrogen limitation on biological production. The nitrate profile is characterized by a pronounced mid-depth maximum near 14 meters and phosphate and ammonia concentrations reach their highest levels in the anoxic zone. In regard to nutrients, comparisons between this system and other lakes in the region will be briefly discussed. Bell’s original observations, from nearly forty years ago, will also be considered.