Stable silicon isotopes uncover a mineralogical control on the benthic silicon cycle in the Arctic Barents Sea
Ward, J.P.J.; Hendry, K.R.; Arndt, S.; Faust, J.C.; Freitas, F.S.; Henley, S.F.; Krause, J.W.; März, C.; Ng, H.C.; Pickering, R.A.; Tessin, A.C. (2022). Stable silicon isotopes uncover a mineralogical control on the benthic silicon cycle in the Arctic Barents Sea. Geochim. Cosmochim. Acta 329: 206-230. https://dx.doi.org/10.1016/j.gca.2022.05.005 In: Geochimica et Cosmochimica Acta. Elsevier: Oxford,New York etc.. ISSN 0016-7037; e-ISSN 1872-9533, more |   |
| Keyword | | | Author keywords | Silicon isotopes; Benthic flux; Pore water; Reactive pools; Sediment nutrient cycling |
| Authors | | Top | - Ward, J.P.J.
- Hendry, K.R.
- Arndt, S., more
- Faust, J.C.
| - Freitas, F.S., more
- Henley, S.F.
- Krause, J.W.
- März, C.
| - Ng, H.C.
- Pickering, R.A.
- Tessin, A.C.
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| Abstract | Biogeochemical cycling of silicon (Si) in the Barents Sea is under considerable pressure from physical and chemical changes, including dramatic warming and sea ice retreat, together with a decline in dissolved silicic acid (DSi) concentrations of Atlantic inflow waters since 1990. Associated changes in the community composition of phytoplankton blooms will alter the material comprising the depositional flux, which will subsequently influence recycling processes at and within the seafloor. In this study we assess the predominant controls on the early diagenetic cycling of Si, a key nutrient in marine ecosystems, by combining stable isotopic analysis (delta Si-30) of pore water DSi and of operationally defined reactive pools of the solid phase. We show that low biogenic silica (BSi) contents (0.26-0.52 wt% or 92-185 mu mol g dry wt(-1)) drive correspondingly low asymptotic concentrations of pore water DSi of -100 mu M, relative to biosiliceous sediments (> 20 wt% BSi) wherein DSi can reach-900 mu M. While Barents Sea surface sediments appear almost devoid of BSi, we present evidence for the rapid recycling of bloom derived BSi that generates striking transient peaks in sediment pore water [DSi] of up to 300 mu M, which is a feature that is subject to future shifts in phytoplankton community compositions. Using a simple isotopic mass balance calculation we show that at two of three stations the pore water DSi pool at 0.5 cm below the seafloor (+0.96 to +1.36 %0) is sourced from the mixing of core top waters (+1.46 to +1.69 %0) with the dissolution of BSi (+0.82 to +1.50 %0), supplemented with a lithogenic Si source (LSi) (-0.89 +/- 0.16%0). Further, our sediment pore water delta Si-30 profiles uncover a coupling of the Si cycle with the redox cycling of metal oxides associated with isotopically light Si (-2.88 +/- 0.17%0). We suggest that a high LSi:BSi ratio and apparent metal oxide influence could lead to a degree of stability in the annual background benthic flux of DSi, despite current pressures on pelagic phytoplankton communities. Coupled with supporting isotopic evidence for the precipitation of authi-genic clays in Barents Sea sediment cores, our observations have implications for the regional Si budget. |
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