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Article Biogeosciences, 17, 1745–1763, 2020 https://doi.org/10.5194/bg-17-1745-2020 © Author(s) 2020. This work is distributed under the Creative Commons Attribution 4.0 License. Impact of ambient conditions on the Si isotope fractionation in marine pore fluids during early diagenesis Sonja Geilert1, Patricia Grasse1, Kristin Doering1,2, Klaus Wallmann1, Claudia Ehlert3, Florian Scholz1, Martin Frank1, Mark Schmidt1, and Christian Hensen1 1GEOMAR Helmholtz Centre for Ocean Research Kiel, Wischhofstr. 1–3, 24148 Kiel, Germany 2Department of Oceanography, Dalhousie University, Halifax, Canada 3Marine Isotope Geochemistry, ICBM, University of Oldenburg, 26129 Oldenburg, Germany Correspondence: Sonja Geilert ([email protected]) Received: 4 December 2019 – Discussion started: 8 January 2020 Revised: 27 February 2020 – Accepted: 4 March 2020 – Published: 3 April 2020 Abstract. Benthic fluxes of dissolved silicon (Si) from sed- seems to be hampered by high water = rock ratios. Guaymas 30 iments into the water column are driven by the dissolution OMZ δ Sipf values are markedly different from those of the of biogenic silica (bSiO2) and terrigenous Si minerals and Peruvian OMZ, the only other marine OMZ setting where Si modulated by the precipitation of authigenic Si phases. Each isotopes have been investigated to constrain early diagenetic 30 of these processes has a specific effect on the isotopic com- processes. These differences highlight the fact that δ Sipf position of silicon dissolved in sediment pore fluids, such signals in OMZs worldwide are not alike and each setting 30 30 that the determination of pore fluid δ Si values can help can result in a range of δ Sipf values as a function of the en- to decipher the complex Si cycle in surface sediments. In vironmental conditions. We conclude that the benthic silicon 30 this study, the δ Si signatures of pore fluids and bSiO2 cycle is more complex than previously thought and that addi- in the Guaymas Basin (Gulf of California) were analyzed, tional Si isotope studies are needed to decipher the controls which is characterized by high bSiO2 accumulation and hy- on Si turnover in marine sediment and the role of sediments drothermal activity. The δ30Si signatures were investigated in the marine silicon cycle. in the deep basin, in the vicinity of a hydrothermal vent field, and at an anoxic site located within the pronounced 30 oxygen minimum zone (OMZ). The pore fluid δ Sipf sig- natures differ significantly depending on the ambient condi- 1 Introduction 30 tions. Within the basin, δ Sipf is essentially uniform, aver- 30 aging C1:2 ± 0:1 ‰ (1 SD). Pore fluid δ Sipf values from Silicon (Si) is one of the key macronutrients in the ocean, within the OMZ are significantly lower (0:0 ± 0:5 ‰, 1 SD), mainly utilized by siliceous organisms, such as diatoms, ra- while pore fluids close to the hydrothermal vent field are diolarians, or sponges (see the recent review by Sutton et higher (C2:0±0:2 ‰, 1SD). Reactive transport modeling re- al., 2018). The marine Si cycle is closely linked to the car- 30 sults show that the δ Sipf is mainly controlled by silica dis- bon (C) cycle by marine siliceous organisms, which transport solution (bSiO2 and terrigenous phases) and Si precipitation C to the sediment and thus exert a strong control on C ex- (authigenic aluminosilicates). Precipitation processes cause a port from the atmosphere impacting present and past climate 30 shift to high pore fluid δ Sipf signatures, most pronounced at (e.g., Lewin, 1961; Tréguer and Pondaven, 2000; Tréguer the hydrothermal site. Within the OMZ, however, additional and De La Rocha, 2013, and recent reviews by Frings et al., 30 dissolution of isotopically depleted Si minerals (e.g., clays) 2016, and Sutton et al., 2018). Studies of Si isotopes (δ Si) facilitated by high mass accumulation rates of terrigenous have revealed complex uptake and dissolution processes of 30 material (MARterr) is required to promote the low δ Sipf siliceous organisms, which have a dominant control on the 30 signatures, while precipitation of authigenic aluminosilicates δ Si distribution in ocean waters (e.g., De La Rocha et al., 1997; Varela et al., 2004; Cardinal et al., 2005; Beucher et Published by Copernicus Publications on behalf of the European Geosciences Union. 1746 S. Geilert et al.: Impact of ambient conditions on Si isotope fractionation al., 2008; Fripiat et al., 2011; Ehlert et al., 2012; Grasse et 2016). The few modeling and experimental studies address- al., 2013; Sutton et al., 2013; de Souza et al., 2014, 2015). ing Si isotope fractionation during formation of secondary Diatoms constitute the largest part of the Si cycling fluxes in phases, report isotope fractionation factors between −1:6 ‰ the ocean (Ragueneau et al., 2000) and discriminate between and −2 ‰ (Ziegler et al., 2005; Méheut et al., 2007; Dupuis its isotopes during Si uptake, whereby the light isotopes are et al., 2015; Ehlert et al., 2016). preferentially incorporated into the diatom frustules (e.g., De The δ30Si data for marine pore fluids from the Peruvian La Rocha et al., 1997). Si isotope fractionation during Si up- margin upwelling region, which is characterized by very high take is dependent on, e.g., the diatom species, the availabil- diatom productivity and bSiO2-rich sediments (Abrantes et ity of Fe, and the degree of Si utilization. Fractionation fac- al., 2007; Bruland et al., 2005), agree with these findings and tors between −0:5 ‰ and −2:1 ‰ have been derived from clearly indicate Si isotope fractionation as a consequence of regional water mass mixing and laboratory studies (De La authigenic aluminosilicate precipitation accompanied by a Si Rocha et al., 1997; Varela et al., 2004; Cardinal et al., 2005; isotope fractionation factor of −2:0 ‰ (Ehlert et al., 2016). Beucher et al., 2008; Sutton et al., 2013; Meyerink et al., Pore fluids from the Greenland margin and Labrador Sea 2017). also reflect early diagenetic reactions detected by pore fluid After a planktonic bloom, whereupon the nutrients are δ30Si values (between C0:76 ‰ and C2:08 ‰), and the δ30Si exhausted, biogenic silica (bSiO2, mainly diatoms) sinks values were interpreted as the product of reverse weather- through the ocean, partially dissolves, and accumulates on ing reactions (Ng et al., 2020). In this study, the Guaymas the seafloor, where its preservation and recycling is con- Basin in the Gulf of California was chosen as study area, as trolled by dissolution and Si re-precipitation processes. The it is characterized by a relatively high diatom productivity dissolution of bSiO2 mainly controls the accumulation of and sediments that are predominantly composed of diatoma- silicic acid (Si.OH/4) in pore fluids, although the in situ con- ceous muds (up to 50 % diatoms; Kastner and Siever, 1983). centration remains below the equilibrium concentration of Moreover, the Guaymas Basin in the Gulf of California is in- dissolved bSiO2, which has been explained by simultaneous fluenced by hydrothermal activity (e.g., Von Damm, 1990). formation of authigenic silicates (e.g., McManus et al., 1995; These bSiO2-rich sediments are thus ideal for dedicated stud- Van Cappellen and Qiu, 1997a, b; Rickert et al., 2002). This ies of early diagenesis under the influence of different ther- process is termed reverse weathering, given that the authi- mal and redox conditions. We investigated the processes con- genic precipitates are rich in seawater-derived cations, like trolling Si isotope fractionation during early diagenesis based Na, K, or Mg (Mackenzie et al., 1981; Michalopoulos and on pore fluid and bSiO2 data from three fundamentally dif- Aller, 1995). Experimental studies of bSiO2 dissolution ki- ferent environmental settings within the Guaymas Basin, in- netics revealed a dependence of the bSiO2 reactivity on sed- cluding the deep basin, a hydrothermal site, and a site within iment depth as well as the ratio between terrigenous material the oxygen minimum zone (OMZ) on the slope of the basin and bSiO2 (Michalopoulos and Aller, 1995; Van Cappellen (Fig. 1). In addition, the Si isotope composition of the water and Qiu, 1997a, b; Dixit et al., 2001; Rickert et al., 2002). column, bottom waters, and hydrothermal fluids was deter- Marine weathering of terrigenous material (primary silicates mined. A numerical transport–reaction model was applied to like feldspars and secondary silicates like clays) was found the OMZ setting to constrain marine weathering processes to release cations, such as aluminum (Al) or iron (Fe), which and to compare the results to the Peruvian margin. The aim reduce the solubility and dissolution rate of bSiO2 and in- of this study was to constrain the factors controlling Si iso- duce aluminosilicate precipitation (Michalopoulos and Aller, tope fractionation during early diagenesis and to identify pro- 1995; Van Cappellen and Qiu, 1997a, b; Michalopoulos et cesses influencing bSiO2 dissolution and authigenic silicate al., 2000; Dixit et al., 2001; Rickert et al., 2002; Loucaides precipitation. et al., 2010). The accumulation of Si in pore fluids and the Si reflux into bottom waters are controlled by three interdepen- dent processes, namely opal dissolution, dissolution of ter- 2 Geological setting, sampling, and methods rigenous solids, and precipitation of authigenic minerals. Early silica diagenesis has been shown to fractionate Si 2.1 Geological setting isotopes as a function of the crystallization state, seawater Si concentration, sedimentation rate, and terrigenous min- The Guaymas Basin in the Gulf of California is a currently eral content (Tatzel et al., 2015; Geilert et al., 2016). Since opening continental rifting environment with two graben sys- the light 28Si isotope is more reactive compared to the heav- tems (northern and southern trough), which are offset by a ier isotopes (29Si, 30Si), processes, such as reverse weath- transform fault and reaches spreading rates of up to 6 cm yr−1 ering, adsorption, and direct Si precipitation from saturated (Calvert, 1966).
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