Germanium/silicon ratios in the Copper River Basin, Alaska: Weathering and partitioning in periglacial versus glacial environments

Glacial to interglacial variation in the germanium to silicon ratio (Ge/Si) of biogenic opal in oceanic sediment cores has been proposed to record terrestrial silicate weathering intensity. However, data connecting Ge/Si ratios to weathering are sparse, especially for cold regions. To characterize Ge/Si of the dissolved loads of rivers draining modern glacial and periglacial regions, we measured Ge and major element chemistry of more than 30 streams in the Copper River Basin of south Alaska. We measured significantly higher Ge/Si in rivers with greater than 15% glacial cover in their basins than in rivers draining areas with less glacial cover, which we attribute to preferencial subglacial weathering of biotite. We also ran laboratory experiments to investigate the role of differential sorption of Ge and Si on hydrous iron oxides in moderating the Ge/Si ratio of rivers in recently deglaciated basins. These experiments indicate that Ge is preferentially adsorbed over Si by hydrous iron oxides over a wide range of pH values and support differential sorption as a potentially significant fractionation process. Our measured high Ge/Si in glaciated basins is in striking contrast to the low Ge/Si in the oceanic record during glacial periods. The glacial to interglacial changes in marine Ge/Si cannot simply reflect changes in glacier cover on land, as the mineralogy of silicates weathered and processes partitioning Ge in soils must also strongly influence the fluvial Ge and Si inputs to the ocean.