TRACE METAL CYCLING IN MEROMICTIC MAHONEY LAKE, BRITISH COLUMBIA: IMPLICATIONS FOR MOLYBDENUM BURIAL IN SULFIDIC ENVIRONMENTS

The behavior of redox-sensitive trace metals in modern sedimentary environments provides an essential window to interpreting the chemical structure of ancient water columns. Molybdenum is particularly useful in this regard, and significant solid-phase enrichments (i.e., 10s to 100s of ppm) are typically interpreted to reflect deposition under anoxic/euxinic conditions on the basis of (1) very low crustal concentrations of molybdenum (~1-2 ppm), (2) conservative behavior of molybdenum in the presence of oxygen, and (3) the experimentally and theoretically validated premise that the conservative molybdate anion (MoO42-) is converted to particle-reactive (oxy)thiomolybdates (MoOxS4-x2-) in the presence of free hydrogen sulfide. These ions are then sequestered and buried by iron sulfide minerals and/or organic matter, facilitating significant enrichment above crustal values.

Preliminary results from a meromictic and sulfidic lake in southern British Columbia (Mahoney Lake) suggest anomalous behavior of molybdenum in this environment. In the water column, dissolved Mo concentrations drop sharply across the chemocline (from ~250 nM to ~50 nM on the scale of centimeters). Solid-phase Mo enrichments in sediments overlain by oxygenated water are within the range of typical suboxic sediments, consistent with MnOx-limited diffusion of Mo across the sediment-water interface. However, Mo/TOC ratios and the magnitude of solid-phase Mo enrichments in sediments overlain by sulfidic water suggest molybdenum limitation, despite unusually high Mo concentrations in the pore waters. In addition, dissolved Mo increases across the sediment-water interface despite the presence of >30 mM interstitial H2S. This unusual relationship may suggest that Fe-sulfide minerals play an important role in Mo sequestration under certain conditions, and that the low concentrations of pyrite and iron monosulfide in this environment are inhibiting strong Mo enrichments. Alternatively, the predominant organic matter source (bacterial production at the chemocline) may not produce the type of organic compounds necessary to enhance thiomolybdate trapping. Further examination of these patterns should provide additional constraints on the primary pathways of Mo burial.