Germanium–silicon fractionation in a river-influenced continental margin: The Northern Gulf of Mexico

In this study we have sampled the water column and sediments of the Gulf of Mexico to investigate the effects of high riverine terrigenous load and sediment redox conditions on the cycling of Ge and Si. Water column Ge/Si ratios across the Gulf of Mexico continental shelf range from 1.9 to 25 lmol/mol, which is elevated compared to the global ocean value of 0.7 lmol/mol. The Ge enrichment in the Gulf of Mexico seawater is primarily due to anthropogenic contamination of the Mississippi river, which is the main Ge and Si source to the area, and to a smaller extent due to discrimination against Ge during biogenic silica (bSi) production (Ge/Si = 1.2–1.8 lmol/mol), especially by radiolarians and siliceous sponges (Ge/Si = 0.6–1.1 lmol/mol). Most sediment pore waters (Ge/Si = 0.3–4.5 lmol/mol) and sediment incubation experiments (benthic flux Ge/Si = 0.9–1.2 lmol/mol) indicate precipitation of authigenic phases that sequester Ge from pore waters (non-opal sink). This process appears to be independent of oxidation–reduction reactions and suggests that authigenic aluminosilicate formation (reverse weathering) may be the dominant Ge sink in marine sediments. Compilation of previously published data shows that in continental margins, non-opal Ge burial flux is controlled by bSi supply, while in open ocean sediments it is 10–100 times lower and most likely limited by the supply of lithogenic material. We provide a measurementbased estimate of the global non-opal Ge burial flux as 4–32 Mmol yr , encompassing the 2–16 Mmol yr 1 needed to keep the global marine Ge cycle at steady state. 2016 Elsevier Ltd. All rights reserved.