Impact of ambient conditions on the Si isotope fractionation in marine pore fluids during early diagenesis, Geilert et al.

Abstract. Benthic fluxes of dissolved silicon (Si) from sediments into the water column are driven by the dissolution of biogenic silica (bSiO2) 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 δ30Si values can help to decipher the complex Si cycle in surface sediments. In this study, the δ30Si signatures of pore fluids and bSiO2 in the Guaymas Basin (Gulf of California) were analyzed, which is characterized by high bSiO2 accumulation and hydrothermal activity. The δ30Si 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 δ30Sipf signatures differ significantly depending on the ambient conditions. Within the basin, δ30Sipf is essentially uniform, averaging +1.2±0.1 ‰ (1 SD). Pore fluid δ30Sipf values from within the OMZ are significantly lower (0.0±0.5 ‰, 1 SD), while pore fluids close to the hydrothermal vent field are higher (+2.0±0.2 ‰, 1SD). Reactive transport modeling results show that the δ30Sipf is mainly controlled by silica dissolution (bSiO2 and terrigenous phases) and Si precipitation (authigenic aluminosilicates). Precipitation processes cause a shift to high pore fluid δ30Sipf 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 (MARterr) is required to promote the low δ30Sipf signatures, while precipitation of authigenic aluminosilicates seems to be hampered by high water ∕ rock ratios. Guaymas OMZ δ30Sipf 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 δ30Sipf signals in OMZs worldwide are not alike and each setting can result in a range of δ30Sipf 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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Impact of ambient conditions on the Si isotope fractionation in marine pore fluids during early diagenesis

10.5194/bg-2019-481 ◽

2020 ◽

Author(s):

Sonja Geilert ◽

Patricia Grasse ◽

Kristin Doering ◽

Klaus Wallmann ◽

Claudia Ehlert ◽

...

Keyword(s):

Hydrothermal Vent ◽

Reactive Transport ◽

Specific Effect ◽

Pore Fluid ◽

High Mass ◽

Pore Fluids ◽

Ambient Conditions ◽

Silica Cycle ◽

Si Isotope ◽

Hydrothermal Vent Field

Abstract. Benthic fluxes of dissolved silica (Si) from sediments into the water column are driven by the dissolution of biogenic silica (bSiO2) 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 silica dissolved in sediment pore waters such that the determination of pore water δ30Si values can help to decipher the complex Si cycle in surface sediments. In this study, the δ30Si signatures of pore fluids and bSiO2 in the Guaymas Basin (Gulf of California) were analyzed, which is characterized by high bSiO2 accumulation and hydrothermal activity. The δ30Si 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 δ30Sipf signatures differ significantly depending on the ambient conditions. Within the basin, δ30Sipf is essentially uniform averaging &plus;1.2 ± 0.1 ‰ (1SD). Pore fluid δ30Sipf values from within the OMZ are significantly lower (0.0 ± 0.5 ‰, 1SD), while pore fluids close to the hydrothermal vent field are higher (&plus;2.0 ± 0.2 ‰, 1SD). Reactive transport modelling results show that the δ30Sipf is mainly controlled by silica dissolution (bSiO2 and terrigenous phases) and Si precipitation (authigenic aluminosilicates). Precipitation processes cause a shift to high pore fluid δ30Sipf 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 (MARterr) is required to promote the low δ30Sipf signatures while precipitation of authigenic aluminosilicates seems to be hampered by high water / rock ratios. Guaymas OMZ δ30Sipf values are markedly different from those of the Peruvian OMZ, the only other marine setting where Si isotopes have been investigated to constrain early diagenetic processes. These differences highlight the fact that δ30Sipf signals in OMZs worldwide are not alike and each setting can result in a range of δ30Sipf values as a function of the environmental conditions. We conclude that the benthic silica 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 silica cycle.

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An easy-to-use, low-cost evaporation protection to collect more reliable stable water isotope data with Teledyne ISCO portable samplers

10.5194/egusphere-egu21-2764 ◽

2021 ◽

Author(s):

Jana von Freyberg ◽

Julia L. A. Knapp ◽

Andrea Rücker ◽

Bjørn Studer ◽

Massimiliano Zappa ◽

...

Keyword(s):

Water Samples ◽

Isotope Fractionation ◽

Field Experiments ◽

Isotope Composition ◽

Ambient Conditions ◽

Storage Duration ◽

Full Size ◽

Isotope Data ◽

Protection Method ◽

Wide Range

<p>Off-the-shelf portable automatic water samplers, such as the 6712 full-size portable sampler (Teledyne ISCO, Lincoln, USA), are often used in remote locations to collect precipitation or streamwater for subsequent analysis of deuterium and oxygen-18. &#160;The bottles inside these automatic samplers remain open during the full duration of sampler deployment and the collected water samples can thus be subjected to evaporation and vapor exchange. &#160;Both processes are known to alter the isotope composition of the water sample, and thus the questions arise as to 1) how credible the isotope measurements from automatically collected water samples are and 2) how can these isotope effects in the automatic water sampler be reduced?</p><p>We evaluated these questions through laboratory and field experiments in which we quantified the change in isotope composition in the water samples with respect to ambient conditions (air temperature and relative humidity), storage duration, and sample volume.&#160; We found that isotope fractionation in the water samples was substantial under very warm and dry condition, when sample volumes are small or when sample storage exceeded 10 days. &#160;To address these problems, we have designed an evaporation protection method which modifies autosampler bottles using a syringe housing and silicone tube.&#160; We performed paired experiments with open vs. evaporation-protected bottles in Teledyne ISCO 6712 full-size portable samplers to evaluate our design. &#160;We could show that the evaporation protection successfully reduced isotope fractionation in the water samples for storage durations of up to 24 days and for a wide range of ambient conditions; e.g., while deuterium concentrations in the water samples in open bottles changed by ca. 3&#8240; under very warm and dry conditions, no isotope effect was measured in the bottles equipped with the evaporation protection. Because our design is very cost efficient it can easily be implemented to upgrade Teledyne ISCO&#8217;s 6712 full-size portable samplers or other similar devices for collecting more reliable isotope data.</p>

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Serpentine alteration as source of high dissolved silicon and elevated δ30Si values to the marine Si cycle

Nature Communications ◽

10.1038/s41467-020-18804-y ◽

2020 ◽

Vol 11 (1) ◽

Author(s):

Sonja Geilert ◽

Patricia Grasse ◽

Klaus Wallmann ◽

Volker Liebetrau ◽

Catriona D. Menzies

Keyword(s):

Closed System ◽

Isotope Fractionation ◽

Mantle Wedge ◽

Important Process ◽

Sr Isotope ◽

Element Cycling ◽

High Fluid ◽

Dissolved Silicon ◽

Si Isotope ◽

Forearc Region

Abstract Serpentine alteration is recognized as an important process for element cycling, however, related silicon fluxes are unknown. Pore fluids from serpentinite seamounts sampled in the Mariana forearc region during IODP Expedition 366 were investigated for their Si, B, and Sr isotope signatures (δ30Si, δ11B, and 87Sr/86Sr, respectively) to study serpentinization in the mantle wedge and shallow serpentine alteration to authigenic clays by seawater. While serpentinization in the mantle wedge caused no significant Si isotope fractionation, implying closed system conditions, serpentine alteration by seawater led to the formation of authigenic phyllosilicates, causing the highest natural fluid δ30Si values measured to date (up to +5.2 ± 0.2‰). Here we show that seafloor alteration of serpentinites is a source of Si to the ocean with extremely high fluid δ30Si values, which can explain anomalies in the marine Si budget like in the Cascadia Basin and which has to be considered in future investigations of the global marine Si cycle.

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