Germanium as a Critical Zone proxy: δ74Ge and Ge/Si in waters from the Peruvian Andes and Amazon

Abstract:

Germanium (Ge) is an element with a chemical behavior very similar to that of silicon (Si), albeit with some important differences. As a result, Ge/Si ratios in the environment can be used to investigate the Si cycle. For example, glacial-interglacial variations of the global marine Ge/Si ratio¹ could be reflecting either changes in terrestrial weathering (river water Ge/Si has been proposed as a proxy for silicate weathering intensity¹) or the degree of decoupling between Ge and Si during marine sediment diagenesis and burial².

The investigation of Ge stable isotopes (δ⁷⁴Ge) may help to better constrain the dynamics of the Ge cycle and aid in the interpretation of Ge/Si paleorecords. It also has the potential to provide useful information on the weathering environment, complementing other isotopic critical zone proxies, such as δ⁷Li, δ³⁰Si, ⁸⁷Sr^/86Sr, etc..

We present the first δ⁷⁴Ge measurements of river waters. We analyzed δ⁷⁴Ge as well as Ge/Si ratios and several other major and trace element concentrations across a mountain-to-floodplain gradient reaching from the Peruvian Andes to the Amazon. In contrast with other areas and the global trend¹, there is no correlation between Ge/Si and Si concentration among different tributaries, indicating either the heterogeneity of Ge sources or the dominance of different secondary weathering reactions within different tributary catchments. We are currently investigating if Ge/Si varies with seasonal changes in runoff, which may help constrain the factors controlling Ge/Si.

River water δ⁷⁴Ge ranges from +3.4 to +6.5‰ among the different tributaries and is heavy relative to Bulk Silicate Earth (δ⁷⁴Ge = +0.6‰³). Significant negative correlation of δ⁷⁴Ge and Ge/Si suggests that both ratios may be fractionated during Ge uptake with secondary mineral precipitation. Similar (but weaker) correlation is observed for an extended dataset, including multiple U.S. rivers draining various lithologies.

Overall, our data show that δ⁷⁴Ge and Ge/Si have the potential to provide useful information on continental weathering processes and further investigation of these proxies in the critical zone is warranted.

1. Froelich et al. (1992). Paleocean., 7(6), 739–767
2. Hammond et al. (2000). GCA, 64(14), 2453–2465
3. Escoube et al. (2011). Geostand. Geoanal. Res., 36(2), 149–159