A First Look at Dissolved Ge Isotopes in Marine Sediments

The removal of chemical species from seawater during the precipitation of authigenic minerals is difficult to constrain but may play a major role in the global biogeochemical cycles of some elements, including silicon (Si) and germanium (Ge). Here, we present Ge/Si, delta Ge-74, and supporting chemical data of pore waters and core incubations at three continental margin sites in California and the Gulf of Mexico. We used these data to partition Ge release and uptake by the various allogenic (delivered via sedimentation) and authigenic (formed in situ) phases in these sediments. About half of the pore water Ge (delta Ge-74(pw) = 1.3-2.4 parts per thousand) is supplied by biogenic silica dissolution (delta Ge-74 similar to 3 parts per thousand), with the other half contributed by lithogenic particulates (delta Ge-74 similar to 0.6%). The highest Ge/Si (similar to 3 mu mol/mol) and lowest delta Ge-74 (1.3-1.9 parts per thousand) are observed at the Fe redox horizon, suggesting a supply from detrital Ge-rich Fe oxides. The precipitation of authigenic phases (most likely aluminosilicate clays) in deeper sediments preferentially incorporates Ge over Si, resulting in low pore water Ge/Si (similar to 0.3 mu mol/mol). The lack of corresponding delta(74)Gepw trend indicates negligible Ge isotope fractionation during this process. Ge fluxes measured via core incubations were variable and appeared strongly controlled by Fe redox behavior near the sediment-water interface. In some cases, reductive Fe oxide dissolution appeared to enhance the benthic Ge flux by over 100% and released fractionated low delta Ge-74 of similar to-0.7 parts per thousand, resulting in overall benthic delta Ge-74(inc) between -0.2 and 3.6%, depending on Fe oxide contribution to Ge flux. We estimate that detrital inputs supply 12-31% of total dissolved Ge to continentalmargin pore fluids globally, resulting in an average pore water and benthic flux delta Ge-74 between 2.2 and 2.7 parts per thousand. Assuming 10-60% of pore water Ge is captured by the authigenic aluminosilicate sink, the dissolved Ge flux to the ocean derived from terrigenous inputs should be roughly 2.5-6.6 Mmol/y, much higher than previously estimated. Our results imply that authigenic Si burial in continental margins should be in the range of 1-8 Tmol/y (best estimate 3.1 Tmol/y), sufficient to close the global marine Si budget.