Ge and Si isotope signatures in rivers: A quantitative multi-proxy approach

Abstract

Solutes derived from the dissolution of silicate minerals play a key role in Earth's climate via the carbon and other biogeochemical cycles. Silicon (Si) is a unique constituent of silicate minerals and a biologically important nutrient, so tracing its behavior in near-surface environments may provide important insights into weathering processes. However, Si released by weathering is variably incorporated into secondary mineral phases and biota, obscuring signals derived from primary weathering processes. Due to chemical similarities, Germanium (Ge) may help better understand the Si cycle and its relationship to chemical weathering. With this aim, we report new measurements of the concentration and isotopic composition of Ge for both the dissolved and particulate phases of a variety of global rivers. These measurements are combined with analyses of concentration and isotopic ratio of Si on the exact same sample set in order to make direct comparisons of the behavior of these two elements in natural river systems. With this dataset, we develop a new modeling framework describing the full elemental and isotopic systems of these solutes in rivers (i.e., Ge/Si, δ74Ge, and δ30Si). This multi-proxy approach allows us to ascertain the relative importance of biological versus mineral uptake in modulating the fluxes of these elements delivered to the modern ocean. Dissolved δ74Ge composition of rivers studied thus far range from 0.9 to 5.5 ‰ with a discharge-weighted global average of 2.6±0.5 ‰. The Ge isotope composition of riverine suspended and bedload sediments is indistinguishable from silicate source rocks, which is consistent with mass balance expectations. The multi-proxy modeling suggests that, among the watersheds studied here, the isotopic fractionation of Si during secondary mineral phase precipitation (∆30Sisec) ranges from-2.7 to-0.2 ‰, which removes between 19-79% of the initial dissolved Si sequestration, while between 12-54% is incorporated by biota. For Ge, modeling indicates that 79-98% of the dissolved load is incorporated into secondary mineral phases with a ∆74Gesec ranging from-4.9 to-0.3 ‰. The fractionation induced by biological uptake is calculated to range from-2.6 to-1.3 ‰ for ∆30Sibio and-0.7±0.7 ‰ for ∆74Gebio. In addition to improving our understanding of the coupled Ge and Si cycles, our study provides a framework for Please note that this is an author-produced PDF of an article accepted for publication following peer review. The definitive publisher-authenticated version is available on the publisher Web site. using multiple isotopic tracers to elucidate the chemical behavior of solutes in natural waters Highlights ► Ge and Si isotope composition of water and sediments in various rivers is presented. ► Dissolved δ 74 Ge range 0.9-5.5‰, river sediment δ 74 Ge range 0.5-0.7‰. ► Dissolved δ 74 Ge is strongly fractionated due to low chemical Ge mobility. ► Ge/Si-isotope multi-proxy provides quantitative constraints on secondary vs biological fractionation.

Barium stable isotopes as a fingerprint of biological cycling in the Amazon River Basin

“…Element fluxes and isotope composition can be used in tandem to estimate the role of various processes on elemental budgets 275 (e.g. Bouchez et al, 2013;Baronas et al, 2018). To this aim, we model the behavior of Ba in the critical zone at the catchment scale under a scenario in which dissolved Ba, originally derived from the partial dissolution of rocks, is partitioned into different net uptake and dissolved export fluxes following the framework developed by Bouchez et al (2013).…”

Ge and Si isotope behavior during intense tropical weathering and ecosystem cycling

“…Different silicate weathering intensity proxies have been investigated, most notably germanium to silicon (Ge/Si) ratios , lithium isotope ratios (e.g., Hathorne & James, 2006;Misra & Froelich, 2012;Dellinger et al, 2015;Pogge von Strandmann & Henderson, 2015), and silicon isotope ratios (e.g., Ziegler et al, 2005;Georg et al, 2006a; in both the dissolved and the solid products of weathering. Recently, germanium isotopes have also been shown to trace chemical weathering of silicates (Baronas et al, 2017(Baronas et al, , 2018. However, each elemental or isotopic proxy reflects a multitude of processes in the critical zone, and -if reconstructions are based on marine records -in the ocean, resulting in a range of possible interpretations of each proxy record (e.g., Hammond et al, 2004;Li & West, 2014;Baronas et al, 2017).…”

“…Germanium is a trace element that is chemically similar to Si, and in the critical zone, both elements are primarily sourced from silicate rocks that exhibit Ge/Si ratios in the range of 1-3 μmol/mol (De Argollo & Schilling, 1978;Bernstein, 1985;Mortlock & Froelich, 1987). During the precipitation of secondary weathering phases, dissolved Ge is removed from solution preferentially over dissolved Si, resulting in natural waters with Ge/Si ratios of 0.1-3 μmol/mol (Froelich et al, 1985;Mortlock & Froelich, 1987;Anders et al, 2003;Lugolobi et al, 2010;Meek et al, 2016;Baronas et al, 2017Baronas et al, , 2018. Similarly, Si isotope ratios (δ 30 Si) are fractionated during the formation of secondary weathering solids which preferentially incorporate lighter Si isotopes (δ 30 Si = -3 to 0‰), leaving the corresponding soil waters and streams isotopically heavy (δ 30 Si = 0 to 3‰) relative to primary silicates (δ 30 Si ≈ -0.3 to 0 ‰) (e.g., Ziegler et al, 2005;Georg et al, 2007;Opfergelt & Delmelle, 2012;.…”

“…In contrast, most plants discriminate against Ge, and as a result the phytoliths exhibit low Ge/Si values (Derry et al, 2005;Blecker et al, 2007;Lugolobi et al, 2010;Sparks et al, 2010;Meek et al, 2016). The combined use of δ 30 Si and Ge/Si therefore allows evaluation of the vegetation influence on the local Si cycle, both on the soil profile scale (Cornelis et al, 2010;Opfergelt et al, 2010;White et al, 2012;Cornelis et al, 2014) and on the catchment scale (Baronas et al, 2018).…”

“…Two recent studies have shown that Ge isotopes behave similarly to Si during weathering, with fluids enriched (δ 74 Ge = 0.9 to 5.5‰; Baronas et al (2018)) and weathered solids depleted in heavy isotopes (δ 74 Ge = -0.1 to 0.6‰; Qi et al (2019)) relative to primary igneous silicates (δ 74 Ge = 0.4 to 0.8‰; Rouxel and Luais (2017)). Over geological timescales, the heavy isotopic signatures of the fluids can only be maintained if the secondary solids are continuously eroded from the continents (Bouchez et al, 2013).…”