The geochemistry of Lake Joyce, McMurdo Dry Valleys, Antarctica

Lake Joyce is one of the least studied lakes of the McMurdo Dry Valleys. Similar to other lakes in this region, Lake Joyce is a closed-basin, permanently ice-covered, meromictic lake. We present here a detailed investigation of major ions, nutrients, and dissolved trace elements for Lake Joyce. Specifically, we investigate the role of iron and manganese oxides and hydrous oxides in trace metal cycling. Lake Joyce is characterized by fresh, oxic waters overlying an anoxic brine, primarily Na - Cl. Surface waters have a maximum nitrate concentration of 26 muM with a molar dissolved inorganic nitrogen to phosphorus ratio of 477. The supply of nitrogen is attributed to atmospheric deposition, possibly from polar stratospheric clouds. Dissolved phosphorus is scavenged by hydrous iron oxides. The pH is highest (10.15) just beneath the 7-m thick ice cover and decreases to a minimum of 7.29 in the redox transition zone. Dissolved Al exceeds 8 muM in surface waters, and appears to be controlled by equilibrium with gibbsite. In contrast, concentrations of other trace elements in surface waters are quite low ( e. g., 5.4 nM Cu, 0.19 nM Co, < 20 pM La). Dissolved Fe, Mn, Ni and Cd were below our detection limits of 13 nM, 1.8 nM, 4.7 nM and 15 pM( respectively) in surface waters. There was a 6-m vertical separation in the onset of Mn and Fe reduction, with dissolved Mn appearing higher in the water column than Fe. Based on thermodynamic calculations, dissolved Mn appears to be controlled by equilibrium with hausmannite (Mn3O4). Co tracks the Mn pro. le closely, suggesting Co(III) is bound in the lattice of Mn oxides, whereas the Ce pro. le is similar, yet the Ce anomaly suggests oxidative scavenging of Ce. Release of Cu, Ni, Cd and trivalent REE appears to be controlled by pH-induced desorption from Fe and Mn oxides, although Cu ( and perhaps Ni) may be scavenged by organic matter in surface waters.