AN INVESTIGATION OF GERMANIUM AND SILICON CYCLES IN GLACIAL AND PERIGLACIAL BASINS: COPPER RIVER BASIN, CENTRAL ALASKA

The germanium to silicon (Ge/Si) ratio observed in biogenic opal cores has been proposed as a record of the glacial to interglacial variations in the rate and intensity of silicate weathering on land and therefore may be an indicator of the impact of chemical weathering on atmospheric carbon dioxide levels on this timescale. However, the germanium and silicon cycles in glacial and periglacial regions have not been well characterized. In this context, we measured the Ge/Si ratio and major element chemistry of the dissolved and suspended sediment loads of over forty streams draining glacial and periglacial basins within the Copper River basin of South-central Alaska. This relatively small area is the dominant source of fresh water and nutrients to Prince William Sound, and is one of the world's largest sediment sources to the oceans. When possible, we sampled pairs of glacial and periglacial streams of similar discharge, running through similar lithology to isolate the influence of distinctly glacial physical and biological processes on the geochemistry in several distinct regions. Ge/Si ratios of the dissolved load of glacial rivers are elevated compared to periglacial and non-glacial rivers in the same region. This would indicate a higher weathering intensity in glacial basins than non-glacial basins and does not help explain the observed low Ge/Si ratios of biogenic opal from the south Atlantic during glacial periods. However, we conclude that the Ge/Si ratio in these temperate glacial settings is not an indicator of weathering intensity, but reflects a more complex germanium and silicon cycle. The preferential weathering of biotite, which has a high Ge/Si ratio, in glacial systems and differences in the sorption behavior of germanium and silicon on suspended sediment are investigated as possibly critical elements of these geochemical cycles.