LARGE VERTICAL SEPARATION IN THE ONSET OF IRON AND MANGANESE REDUCTION IN FIVE ANTARCTIC LAKES: IMPLICATIONS FOR TRACE METAL CYCLING AND MODELING OF MN OXIDE PHASES

We present water chemistry data from five lakes in the McMurdo Dry Valleys, Antarctica: Lakes Fryxell, Hoare, Joyce and Miers in comparison with previously published data from Lake Vanda. With the exception of an intermittent outflow from Lake Miers, these lakes are all closed-basin, evaporite systems. The lakes are covered by a thick, permanent layer of ice. The ice cover allows for stable stratification of the lakes, with fresh, oxic, cold water above the deeper, warm, saline, anoxic waters. Due to their remote location and geochemical simplicity, these lakes provide a natural laboratory in which to study redox-driven trace metal cycling and scavenging.

We are investigating the role of Mn and Fe oxides in the transport and fate of trace metals. Trace metals have been shown to adhere to the surface of Mn and Fe oxides, hydroxides and oxyhydroxides. However, it is difficult to ascertain the specific particulate phases of Mn and Fe that are involved in this process. Due to the stable stratification regimes found in these lakes, reduction of oxides follows thermodynamic constraints based on free energies of formation allowing for large vertical separation in the onset of Mn and Fe reduction.

Samples were analyzed for Al, Cd, Co, Cu, Fe, Mn, and Ni. Analyses of trace metals in both filtered and unfiltered samples were performed using GFAAS. For analyses of metals with very low concentrations, such as Cd (<0.3 ppb), an organic solvent extraction/concentration procedure using MIBK or Freon TF was performed on the filtered (<0.45 µm) samples. Analyses on the particulate fraction show that most metals seem to be either dissolved or in particulate phases small enough to pass through the 0.45 µm pore filter. There is a vertical separation between the release of Mn and Fe in the water column of all the lakes. Both Mn and Fe concentrations are notably high in the anoxic zone (up to 25 µM). In Lakes Joyce and Vanda trace metals seem to be released simultaneously with Mn as opposed to Fe. This implies the relative importance of Mn oxide particles in trace metal scavenging, transport and recycling. Thermodynamic analysis suggests that in deeper, lower pH waters, the mineral hausmannite (Mn₃O₄) may be undergoing reductive dissolution. Model calculations, using this mineral phase, generate Mn depth profiles that closely resemble those observed in the lakes.