Impact of ambient conditions on the Si isotope fractionation in marine pore fluids during early diagenesis

Geilert, Sonja; Grasse, Patricia; Doering, Kristin; Wallmann, Klaus; Ehlert, Claudia; Scholz, Florian; Frank, Martin; Schmidt, Mark; Hensen, Christian

Benthic fluxes of dissolved silicon (Si) from sediments into the water column are driven by the dissolution of biogenic silica (inline-formulabSiO2) and terrigenous Si minerals and modulated by the precipitation of authigenic Si phases. Each of these processes has a specific effect on the isotopic composition of silicon dissolved in sediment pore fluids, such that the determination of pore fluid inline-formulaδ30Si values can help to decipher the complex Si cycle in surface sediments. In this study, the inline-formulaδ30Si signatures of pore fluids and inline-formulabSiO2 in the Guaymas Basin (Gulf of California) were analyzed, which is characterized by high inline-formulabSiO2 accumulation and hydrothermal activity. The inline-formulaδ30Si signatures were investigated in the deep basin, in the vicinity of a hydrothermal vent field, and at an anoxic site located within the pronounced oxygen minimum zone (OMZ). The pore fluid inline-formulaδ30Siinline-formulapf signatures differ significantly depending on the ambient conditions. Within the basin, inline-formulaδ30Siinline-formulapf is essentially uniform, averaging inline-formula M11inlinescrollmathml + normal 1.2 ± normal 0.1 52pt10ptsvg-formulamathimgca38ecc4ca5ca8e0ad111d2d99737f8b bg-17-1745-2020-ie00001.svg52pt10ptbg-17-1745-2020-ie00001.png  ‰ (1 SD). Pore fluid inline-formulaδ30Siinline-formulapf values from within the OMZ are significantly lower (inline-formula0.0±0.5 ‰, 1 SD), while pore fluids close to the hydrothermal vent field are higher (inline-formula M15inlinescrollmathml + normal 2.0 ± normal 0.2 52pt10ptsvg-formulamathimg09a7def3a55ea0f403188d2166f2a85f bg-17-1745-2020-ie00002.svg52pt10ptbg-17-1745-2020-ie00002.png  ‰, 1SD). Reactive transport modeling results show that the inline-formulaδ30Siinline-formulapf is mainly controlled by silica dissolution (inline-formulabSiO2 and terrigenous phases) and Si precipitation (authigenic aluminosilicates). Precipitation processes cause a shift to high pore fluid inline-formulaδ30Siinline-formulapf signatures, most pronounced at the hydrothermal site. Within the OMZ, however, additional dissolution of isotopically depleted Si minerals (e.g., clays) facilitated by high mass accumulation rates of terrigenous material (MARinline-formulaterr) is required to promote the low inline-formulaδ30Siinline-formulapf signatures, while precipitation of authigenic aluminosilicates seems to be hampered by high water inline-formula∕ rock ratios. Guaymas OMZ inline-formulaδ30Siinline-formulapf values are markedly different from those of the Peruvian OMZ, the only other marine OMZ setting where Si isotopes have been investigated to constrain early diagenetic processes. These differences highlight the fact that inline-formulaδ30Siinline-formulapf signals in OMZs worldwide are not alike and each setting can result in a range of inline-formulaδ30Siinline-formulapf values as a function of the environmental conditions. We conclude that the benthic silicon cycle is more complex than previously thought and that additional Si isotope studies are needed to decipher the controls on Si turnover in marine sediment and the role of sediments in the marine silicon cycle.

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Geilert, Sonja / Grasse, Patricia / Doering, Kristin / et al: Impact of ambient conditions on the Si isotope fractionation in marine pore fluids during early diagenesis. 2020. Copernicus Publications.

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