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

Type Article
Date 2018-12
Language English
Author(s) Baronas J. Jotautas1, Torres Mark A.1, West A. Joshua1, Rouxel OlivierORCID2, Georg Bastian3, Bouchez Julien4, Gaillardet Jerome4, Hammond Douglas E.1
Affiliation(s) 1 : Univ Southern Calif, Dept Earth Sci, Los Angeles, CA 90089 USA.
2 : Ctr Brest, Inst Francais Rech Exploitat Mer, Technopole Brest Iroise, F-29280 Plouzane, France.
3 : Trent Univ, Water Qual Ctr, Peterborough, ON K9L 1Z8, Canada.
4 : Univ Paris Diderot, Sorbonne Paris Cite, IPGP, CNRS, F-75231 Paris, France.
Source Earth And Planetary Science Letters (0012-821X) (Elsevier Science Bv), 2018-12 , Vol. 503 , P. 194-215
DOI 10.1016/j.epsl.2018.09.022
WOS© Times Cited 3
Keyword(s) germanium, silicon, isotopes, rivers, weathering, fractionation
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 using multiple isotopic tracers to elucidate the chemical behavior of solutes in natural waters

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Baronas J. Jotautas, Torres Mark A., West A. Joshua, Rouxel Olivier, Georg Bastian, Bouchez Julien, Gaillardet Jerome, Hammond Douglas E. (2018). Ge and Si isotope signatures in rivers: A quantitative multi-proxy approach. Earth And Planetary Science Letters, 503, 194-215. Publisher's official version : https://doi.org/10.1016/j.epsl.2018.09.022 , Open Access version : https://archimer.ifremer.fr/doc/00460/57196/