GERMANIUM AND SILICON CYCLES IN GLACIAL AND PERIGLACIAL CATCHMENTS: COPPER RIVER BASIN, ALASKA

Glacial to interglacial variations in the germanium to silicon (Ge/Si) ratio observed in biogenic opal cores have been proposed as records of variations in silicate weathering intensity on land, and yet substantial data connecting Ge/Si ratios to weathering in cold regions are lacking. To characterize the Ge/Si ratios of the dissolved loads of rivers draining modern glacial and periglacial regions, we studied the Ge/Si ratio and major element chemistry of over forty streams in the Copper River Basin of south Alaska. Ge/Si ratios of streams with more than 15% glacial cover in their catchments are higher than those of streams draining regions with less glacial cover. We conclude that, rather than reflecting weathering intensity, the Ge/Si ratio in these temperate glacial settings reflects the disproportionately large contribution of biotite, a mineral with a high Ge/Si ratio relative to bulk continental crust, to the dissolved load. Biotite weathering has been cited as the cause of other unique features of the chemistry of glacial waters including high K/Na and ⁸⁷Sr/⁸⁶Sr ratios. In basins with less than 15% glacial cover lower Ge/Si ratios reflect the weathering of a wider variety of silicate minerals and a process partitioning Ge into the solid, rather than dissolved load. We suggest that the increasing abundance of iron oxides in very young periglacial soils partitions Ge well before significant clay mineral formation occurs because our laboratory experiments indicate that Ge is sorbed 7 times more efficiently than Si on hydrous iron oxides. Thus, glacial to interglacial changes in ocean Ge/Si do not directly and simply reflect changes in glacial cover on land, as the mineralogy of silicates weathered and a variety of processes partitioning Ge in soils strongly influence the fluvial Ge and Si inputs to the ocean.