Revisiting the dissolution of biogenic Si in marine sediments: a key term in the ocean Si budget

Acta Geochimica, Jun 2017

Of the ~240 × 1012 mol year−1 of biogenic silica (bSi) produced by diatoms and other silicifying organisms, only roughly 3%–4% escapes dissolution to be permanently buried. At the global scale, how, where and why bSi is preserved in sediment is not well understood. To help address this, I compile 6245 porewater dissolved Si concentrations from 453 sediment cores, to derive the concentration gradient at the sediment–water interface and thus diffusive fluxes out of the sediment. These range from <0.002 to 3.4 mol m−2 year−1, and are independent of temperature, depth and latitude. When classified by sediment lithology, predominantly siliceous sediments unsurprisingly have higher mean diffusive fluxes than predominantly calcareous or clay-rich sediment. Combined with the areal extent of these lithologies, the ‘best-guess’ global sedimentary bSi recycling flux is 69 × 1012 mol year−1.

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Revisiting the dissolution of biogenic Si in marine sediments: a key term in the ocean Si budget

Acta Geochim ( Revisiting the dissolution of biogenic Si in marine sediments: a key term in the ocean Si budget Patrick Frings 0 1 2 3 4 Diatoms 0 1 2 3 4 0 & Patrick Frings 1 11th International Symposium on Geochemistry of the Earth's Surface 2 Ocean Si cycle 3 Earth Surface Geochemistry, GFZ German Research Centre for Geosciences , 14473 Telegrafenberg, Potsdam , Germany 4 Department of Geoscience, Swedish Museum of Natural History , 10405 Stockholm , Sweden Of the *240 9 1012 mol year-1 of biogenic silica (bSi) produced by diatoms and other silicifying organisms, only roughly 3%-4% escapes dissolution to be permanently buried. At the global scale, how, where and why bSi is preserved in sediment is not well understood. To help address this, I compile 6245 porewater dissolved Si concentrations from 453 sediment cores, to derive the concentration gradient at the sediment-water interface and thus diffusive fluxes out of the sediment. These range from \0.002 to 3.4 mol m-2 year-1, and are independent of temperature, depth and latitude. When classified by sediment lithology, predominantly siliceous sediments unsurprisingly have higher mean diffusive fluxes than predominantly calcareous or clay-rich sediment. Combined with the areal extent of these lithologies, the 'best-guess' global sedimentary bSi recycling flux is 69 9 1012 mol year-1. Biogenic silica; Dissolution 1 Introduction Ocean dissolved Si concentrations (hereafter [Si]) span two orders of magnitude, from\1 lmol L-1 in surface gyres to almost 200 lmol L-1 in parts of the deep ocean. Yet even these regions are highly undersaturated—amorphous Si solubility is ca. 1800 lmol L-1 at 20 C (Gunnarsson and Arno´rsson 2000) . The ubiquitously high degree of undersaturation attests to the efficiency with which biosilicifying organisms, in particular the diatoms, can take dissolved Si and precipitate it in their biogenic silica (bSi) skeletons. This high degree of undersaturation also means that much of the bSi produced dissolves after the organism’s death. Dissolution begins in the euphotic zone, and continues throughout the water column and into the upper sediment, until the bSi is exhausted (leading to large parts of the ocean floor devoid of siliceous remains) or the bSi reaches equilibrium with sediment porewaters and dissolution ceases. The canonical figure for annual bSi production is 240 Tmol (Tre´guer and De La Rocha 2013) , approximately 259 annual inputs of Si to the ocean (Frings et al. 2016) . Various approaches consistently indicate a large fraction— around 50%–60%—is remineralised in the upper 100 m. This leaves around 100 Tmol year-1 bSi as export production (Tre´guer and De La Rocha 2013) . The fate of this bSi is less well understood, though ca. 10 Tmol year-1 must be preserved to balance the inputs at steady-state. In particular, the relative fractions of dissolution occurring in the water column and in the sediments are not well constrained, but has implications for how we quantify the past and present Si cycle. Diffusive fluxes of Si across the sediment–water interface can be used to estimate bSi dissolution in the sediment; previous global scale estimates include 8–38 Tmol year-1 (Treguer et al. 1995) , 25.5 Tmol year-1 (Laruelle et al. 2009) and 33–159 Tmol year-1 (Tre´guer and De La Rocha 2013) . Clearly, there is scope to improve these estimates. Here, I attempt this by using a compilation of porewater [Si] profiles from marine sediments to calculate diffusive Si fluxes out of sediment. 2 Methods I compiled [450 published porewater [Si] records from the upper 40 cm of marine sediment cores (Fig. 1), assuming that the different methods of coring and sample collection yield comparable results. Most studies kept cores at in-situ temperatures to avoid chemical changes, though no attempt was made to identify and account for those that did not. Both theoretical and empirical studies have shown that porewater [Si] profiles can be described as an exponential function (e.g. McManus et al. 1995; Fig. 1) : ½Si z¼ Casym Casym C0 e bz where Casym is an asymptotic concentration, C0 is the overlying water concentration, b an exponential constant (cm-1) and z the depth below sediment surface (cm). Values of Casym and b were obtained for each profile by searching for the combination that minimised the RMSE between predicted and measured values. The 5% of poorest model fits were discarded. Differentiation of Eq. (1) at z = 0 provides the gradient across the sediment–water interface: d½Si dz z¼0 ¼ b Casym C0 which can then be used in Fick’s first law to calculate the flux J (mol m-2 year-1) across the sediment–water interface, assuming steady-state porewater DSi: ð1Þ ð2Þ ð3Þ J ¼ /D d½Si dz where D is dissolved Si diffusion coefficient which takes a value of ca. 0.03 m2 year-1 at 25 C (Wollast and Garrels 1971) , and is corrected for sediment tortuosity and temperature after Boudreau (1996) . In-situ temperature was extrac (...truncated)


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Patrick Frings. Revisiting the dissolution of biogenic Si in marine sediments: a key term in the ocean Si budget, Acta Geochimica, 2017, pp. 429-432, Volume 36, Issue 3, DOI: 10.1007/s11631-017-0183-1