Ge/Si And Ge Isotope Fractionation During Glacial And Non-Glacial Weathering: Field And Experimental Data From West Greenland

Glacial environments offer the opportunity to study the incipient stages of chemical weathering due to the high availability of finely ground sediments, low water temperatures, and typically short rock-water interaction times. In this study we focused on the geochemical behavior of germanium (Ge) in west Greenland, both during subglacial weathering by investigating glacier-fed streams, as well as during a batch reactor experiment by allowing water-sediment interaction for up to 2 years in the laboratory. Sampled in late August 2014, glacial stream Ge and Si concentrations were low, ranging between 12-55 pmol/L and 7-33 mu mol/L, respectively (Ge/Si = 0.9-2.2 mu mol/mol, similar to parent rock). As reported previously, the dissolved stable Ge isotope ratio (delta Ge-74) of the Watson River was 0.86 +/- 0.24 parts per thousand, the lowest among global rivers and streams measured to date. This value was only slightly heavier than the suspended load (0.48 +/- 0.23 parts per thousand), which is likely representative of the bulk parent rock composition. Despite limited Ge/Si and delta Ge-74 fractionation, both Ge and Si appear depleted relative to Na during subglacial weathering, which we interpret as the relatively congruent uptake of both phases by amorphous silica (aSi). Continued sediment-water interaction over 470-785 days in the lab produced a large increase in dissolved Si concentrations (up to 130-230 mu mol/L), a much smaller increase in dissolved Ge (up to similar to 70 pmol/L), resulting in a Ge/Si decrease (to 0.4-0.5 mu mol/mol) and a significant increase in delta Ge-74 (to 1.9-2.2 parts per thousand). We argue that during the experiment, both Si and Ge are released by the dissolution of previously subglacially formed aSi, and Ge is then incorporated into secondary phases (likely adsorbed to Fe oxyhydroxides), with an associated Delta Ge-74(secondary-dissolved) fractionation factor of -2.15 +/- 0.46 parts per thousand. In summary, we directly demonstrate Ge isotope fractionation during the dissolution-precipitation weathering reactions of natural sediments in the absence of biological Ge and Si uptake, and highlight the significant differences in Ge behavior during subglacial and non-glacial weathering.