Dan Hammarlund

Variations in molybdenum isotopic composition, spanning the range of similar to 2.3 parts per thousand in the terms of Mo-97/Mo-95 ratio, have been measured in sediment cores from three lakes in northern Sweden and north-western Russia. These variations have been produced by both isotopically variable input of Mo into the lakes due to Mo isotopic heterogeneity of bedrock in the drainage basins and fractionation in the lake systems due to temporal variations in limnological conditions. Mo isotope abundances of bedrock in the lake drainage basins have been documented by analysis of Mo isotope ratios of a suite of molybdenite occurrences collected in the studied area and of detrital fractions of the lake sediment cores. The median delta Mo-97 value of the investigated molybdenites is 0.26 parts per thousand with standard deviation of 0.43 parts per thousand (n=19), whereas the median delta Mo-97 value of detrital sediment fractions from two lakes is -0.40 parts per thousand with standard deviation of 0.36 parts per thousand (n=15). The isotopic compositiori of Mo in the sediment cores has been found to be dependent on redox conditions of the water columns and the dominant type of scavenging phases. Hydrous Fe oxides have been shown to be an efficient scavenger of Mo from porewater under oxic conditions. Oxidative precipitation of Fe(II) in the sediments resulted in co-precipitation of Mo and significant authigenic enrichment at the redox boundary. In spite of a pronounced increase in Mo concentration associated with Fe oxides at the redox boundary the isotopic composition of Mo in this zone varies insignificantly, suggesting little or no isotope fractionation during scavenging of Mo by hydrous Fe oxides. In a lake with anoxic bottom water a chironomid-inferred reconstruction of O-2 conditions in the bottom water through the Holocene indicates that increased O-2 concentrations are generally associated with low delta Mo-97/Mo-95 values of the sediments, whereas lowered O-2 contents of the bottom water are accompanied by relatively high delta Mo-97/Mo-95 values, thus confirming the potential of Mo isotope data to be a proxy for redox conditions of overlying waters. However, it is pointed out that other processes including input of isotopically heterogeneous Mo and Mn cycling in the redox-stratified water column can be a primary cause of variations in Mo isotopic compositions of lake sediments.