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Stable silicon isotopes uncover a mineralogical control on the benthic silicon cycle in the Arctic Barents Sea

Ward, James P.J. ; Hendry, Katharine R. ; Arndt, Sandra ; Faust, Johan C. ; Freitas, Felipe S. ; Henley, Sian F. ; Krause, Jeffrey W. ; März, Christian ; Ng, Hong Chin and Pickering, Rebecca A. LU orcid , et al. (2022) In Geochimica et Cosmochimica Acta 329. p.206-230
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 (δ30Si) of pore water DSi and of operationally defined reactive pools of the... (More)

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 (δ30Si) 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 μmol g dry wt−1) drive correspondingly low asymptotic concentrations of pore water DSi of ∼100 μM, relative to biosiliceous sediments (>20 wt% BSi) wherein DSi can reach ∼900 μM. 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 μM, 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 ‰) is sourced from the mixing of core top waters (+1.46 to +1.69 ‰) with the dissolution of BSi (+0.82 to +1.50 ‰), supplemented with a lithogenic Si source (LSi) (−0.89 ±0.16‰). Further, our sediment pore water δ30Si profiles uncover a coupling of the Si cycle with the redox cycling of metal oxides associated with isotopically light Si (−2.88 ±0.17‰). 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 authigenic clays in Barents Sea sediment cores, our observations have implications for the regional Si budget.

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type
Contribution to journal
publication status
published
subject
keywords
Benthic flux, Pore water, Reactive pools, Sediment nutrient cycling, Silicon isotopes
in
Geochimica et Cosmochimica Acta
volume
329
pages
206 - 230
publisher
Elsevier
external identifiers
  • scopus:85131807091
ISSN
0016-7037
DOI
10.1016/j.gca.2022.05.005
language
English
LU publication?
yes
id
6f8a482a-dbcd-4dae-b9c3-14554ab5927f
date added to LUP
2022-09-29 09:50:04
date last changed
2022-09-29 09:50:04
@article{6f8a482a-dbcd-4dae-b9c3-14554ab5927f,
  abstract     = {{<p>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 (δ<sup>30</sup>Si) 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 μmol g dry wt<sup>−1</sup>) drive correspondingly low asymptotic concentrations of pore water DSi of ∼100 μM, relative to biosiliceous sediments (&gt;20 wt% BSi) wherein DSi can reach ∼900 μM. 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 μM, 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 ‰) is sourced from the mixing of core top waters (+1.46 to +1.69 ‰) with the dissolution of BSi (+0.82 to +1.50 ‰), supplemented with a lithogenic Si source (LSi) (−0.89 ±0.16‰). Further, our sediment pore water δ<sup>30</sup>Si profiles uncover a coupling of the Si cycle with the redox cycling of metal oxides associated with isotopically light Si (−2.88 ±0.17‰). 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 authigenic clays in Barents Sea sediment cores, our observations have implications for the regional Si budget.</p>}},
  author       = {{Ward, James P.J. and Hendry, Katharine R. and Arndt, Sandra and Faust, Johan C. and Freitas, Felipe S. and Henley, Sian F. and Krause, Jeffrey W. and März, Christian and Ng, Hong Chin and Pickering, Rebecca A. and Tessin, Allyson C.}},
  issn         = {{0016-7037}},
  keywords     = {{Benthic flux; Pore water; Reactive pools; Sediment nutrient cycling; Silicon isotopes}},
  language     = {{eng}},
  month        = {{07}},
  pages        = {{206--230}},
  publisher    = {{Elsevier}},
  series       = {{Geochimica et Cosmochimica Acta}},
  title        = {{Stable silicon isotopes uncover a mineralogical control on the benthic silicon cycle in the Arctic Barents Sea}},
  url          = {{http://dx.doi.org/10.1016/j.gca.2022.05.005}},
  doi          = {{10.1016/j.gca.2022.05.005}},
  volume       = {{329}},
  year         = {{2022}},
}