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

• Main Authors: S. Geilert, P. Grasse, K. Doering, K. Wallmann, C. Ehlert, F. Scholz, M. Frank, M. Schmidt, C. Hensen
• Format: Article
• Language: English
• Published: Copernicus Publications 2020-04-01
• Series: Biogeosciences
• Online Access: https://www.biogeosciences.net/17/1745/2020/bg-17-1745-2020.pdf

<p>Benthic fluxes of dissolved silicon (Si) from sediments into the water column are driven by the dissolution of biogenic silica (<span class="inline-formula">bSiO₂</span>) 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 <span class="inline-formula"><i>δ</i>³⁰Si</span> values can help to decipher the complex Si cycle in surface sediments. In this study, the <span class="inline-formula"><i>δ</i>³⁰Si</span> signatures of pore fluids and <span class="inline-formula">bSiO₂</span> in the Guaymas Basin (Gulf of California) were analyzed, which is characterized by high <span class="inline-formula">bSiO₂</span> accumulation and hydrothermal activity. The <span class="inline-formula"><i>δ</i>³⁰Si</span> 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 <span class="inline-formula"><i>δ</i>³⁰Si</span><span class="inline-formula">_pf</span> signatures differ significantly depending on the ambient conditions. Within the basin, <span class="inline-formula"><i>δ</i>³⁰Si</span><span class="inline-formula">_pf</span> is essentially uniform, averaging <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M11" display="inline" overflow="scroll" dspmath="mathml"><mrow><mo>+</mo><mn mathvariant="normal">1.2</mn><mo>±</mo><mn mathvariant="normal">0.1</mn></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="52pt" height="10pt" class="svg-formula" dspmath="mathimg" md5hash="ca38ecc4ca5ca8e0ad111d2d99737f8b"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="bg-17-1745-2020-ie00001.svg" width="52pt" height="10pt" src="bg-17-1745-2020-ie00001.png"/></svg:svg></span></span>&thinsp;‰ (1&thinsp;SD). Pore fluid <span class="inline-formula"><i>δ</i>³⁰Si</span><span class="inline-formula">_pf</span> values from within the OMZ are significantly lower (<span class="inline-formula">0.0±0.5</span>&thinsp;‰, 1&thinsp;SD), while pore fluids close to the hydrothermal vent field are higher (<span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M15" display="inline" overflow="scroll" dspmath="mathml"><mrow><mo>+</mo><mn mathvariant="normal">2.0</mn><mo>±</mo><mn mathvariant="normal">0.2</mn></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="52pt" height="10pt" class="svg-formula" dspmath="mathimg" md5hash="09a7def3a55ea0f403188d2166f2a85f"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="bg-17-1745-2020-ie00002.svg" width="52pt" height="10pt" src="bg-17-1745-2020-ie00002.png"/></svg:svg></span></span>&thinsp;‰, 1SD). Reactive transport modeling results show that the <span class="inline-formula"><i>δ</i>³⁰Si</span><span class="inline-formula">_pf</span> is mainly controlled by silica dissolution (<span class="inline-formula">bSiO₂</span> and terrigenous phases) and Si precipitation (authigenic aluminosilicates). Precipitation processes cause a shift to high pore fluid <span class="inline-formula"><i>δ</i>³⁰Si</span><span class="inline-formula">_pf</span> 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 (MAR<span class="inline-formula">_terr</span>) is required to promote the low <span class="inline-formula"><i>δ</i>³⁰Si</span><span class="inline-formula">_pf</span> signatures, while precipitation of authigenic aluminosilicates seems to be hampered by high water&thinsp;<span class="inline-formula">∕</span>&thinsp;rock ratios. Guaymas OMZ <span class="inline-formula"><i>δ</i>³⁰Si</span><span class="inline-formula">_pf</span> 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 <span class="inline-formula"><i>δ</i>³⁰Si</span><span class="inline-formula">_pf</span> signals in OMZs worldwide are not alike and each setting can result in a range of <span class="inline-formula"><i>δ</i>³⁰Si</span><span class="inline-formula">_pf</span> 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.</p>