Preprints
https://doi.org/10.5194/bg-2022-51
https://doi.org/10.5194/bg-2022-51
 
23 Feb 2022
23 Feb 2022
Status: this preprint is currently under review for the journal BG.

Benthic Silicon Cycling in the Arctic Barents Sea: a Reaction-Transport Model Study

James P. J. Ward1, Katherine R. Hendry1, Sandra Arndt2, Johan C. Faust3,4, Felipe S. Freitas1, Sian F. Henley5, Jeffrey W. Krause6,7, Christian März4, Allyson C. Tessin8, and Ruth L. Airs9 James P. J. Ward et al.
  • 1School of Earth Sciences, University of Bristol, Bristol, BS8 1QE, UK
  • 2BGeosys, Department of Geosciences, Université libre de Bruxelles, Brussels, CP160/03 1050, Belgium
  • 3MARUM - Center for Marine Environmental Sciences, University of Bremen, Bremen, 28359, Germany
  • 4School of Earth and Environment, University of Leeds, Leeds, LS2 9JT, UK
  • 5School of GeoSciences, The University of Edinburgh, Edinburgh, EH9 3FE, UK
  • 6Dauphin Island Sea Lab, Dauphin Island, AL, USA
  • 7School of Marine and Environmental Sciences, University of South Alabama, Mobile, AL, USA
  • 8Department of Geology, Kent State University, Kent, OH, USA
  • 9Plymouth Marine Laboratory, Prospect Place, Plymouth, PL1 3DH, UK

Abstract. Over recent decades the highest rates of water column warming and sea ice loss across the Arctic Ocean have been observed in the Barents Sea. These physical changes have resulted in rapid ecosystem adjustments, manifesting as a northward migration of temperate phytoplankton species at the expense of silica-based diatoms. These changes will potentially alter the composition of phytodetritus deposited at the seafloor, which acts as a biogeochemical reactor, pivotal in the recycling of key nutrients, such as silicon (Si). To appreciate the sensitivity of the Barents Sea benthic system to the observed changes in surface primary production, there is a need to better understand this benthic-pelagic coupling. Stable Si isotopic compositions of sediment pore waters and the solid phase from three stations in the Barents Sea reveal a coupling of the iron (Fe) and Si cycles, the contemporaneous dissolution of lithogenic silicate minerals (LSi) alongside biogenic silica (BSi) and the potential for the reprecipitation of dissolved silicic acid (DSi) as authigenic clay minerals (AuSi). However, as reaction rates cannot be quantified from observational data alone, a mechanistic understanding of which factors control these processes is missing. Here, we employ reaction-transport modelling together with observational data to disentangle the reaction pathways controlling the cycling of Si within the seafloor. Processes such as the dissolution of BSi are active on multiple timescales, ranging from weeks to hundreds of years, which we are able to examine through steady state and transient model runs.

Steady state simulations show that 60 to 98 % of the sediment pore water DSi pool may be sourced from the dissolution of LSi, while the isotopic composition is also strongly influenced by the desorption of Si from metal oxides, most likely Fe (oxyhydr)oxides (FeSi), as they reductively dissolve. Further, our model simulations indicate that between 2.9 and 37 % of the DSi released into sediment pore waters is taken up with a fractionation factor of approximately −2 ‰, most likely representing reprecipitation as AuSi. These observations are significant, as the dissolution of LSi represents a source of new Si to the ocean DSi pool and precipitation of AuSi an additional sink, which could address imbalances in the current regional ocean Si budget. Lastly, transient modelling suggests that at least one-third of the total annual benthic DSi flux could be sourced from the dissolution of more reactive, diatom-derived BSi deposited after the surface water bloom at the marginal ice zone. This benthic-pelagic coupling will be subject to change with the continued northward migration of Atlantic phytoplankton species, northward retreat of the marginal ice zone and the observed decline in DSi inventory of the subpolar North Atlantic Ocean over the last three decades.

James P. J. Ward et al.

Status: final response (author comments only)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on bg-2022-51', Anonymous Referee #1, 16 Mar 2022
  • RC2: 'Comment on bg-2022-51', Anonymous Referee #2, 20 May 2022

James P. J. Ward et al.

Data sets

Benthic silica flux magnitudes and silicon isotopic composition of marine sediment pore waters and solid phase leachates for the Barents Sea (summer 2017-2019) James P. J. Ward; Sian F. Henley; Johan C. Faust; Felipe S. Freitas https://doi.org/10.5285/8933AF23-E051-4166-B63E-2155330A21D8

Model code and software

Barents_Sea_Si_BRNS_Ward_etal James P. J. Ward; Felipe S. Freitas; Sandra Arndt https://doi.org/10.5281/zenodo.6023767

James P. J. Ward et al.

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Short summary
The seafloor plays an important role in the cycling of silicon (Si), a key nutrient that promotes marine primary productivity. In our model study, we disentangle the major controls on the seafloor Si cycle to better anticipate the impacts of continued warming and sea ice melt in the Barents Sea. We uncover a coupling of the iron redox and Si cycles, dissolution of lithogenic silicates and authigenic clay formation, comprising a Si sink that could have implications for the Arctic Ocean Si budget.
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