Preprints
https://doi.org/10.5194/bg-2021-112
https://doi.org/10.5194/bg-2021-112

  30 Apr 2021

30 Apr 2021

Review status: a revised version of this preprint was accepted for the journal BG and is expected to appear here in due course.

Geochemical consequences of oxygen diffusion from the oceanic crust into overlying sediments and its significance for biogeochemical cycles based on sediments of the NE Pacific

Gerard J. M. Versteegh1,2,3, Andrea Koschinsky2,3, Thomas Kuhn4, Inken Preuss2,a, and Sabine Kasten1,3,5 Gerard J. M. Versteegh et al.
  • 1Alfred Wegener Institute, Helmholtz Centre for Polar and Marine Research, Bremerhaven, 27570, Germany
  • 2Jacobs University Bremen, Department of Physics and Earth Sciences, Bremen, 28759, Germany
  • 3MARUM – Zentrum für Marine Umweltwissenschaften, Universität Bremen, Bremen, 28359, Germany
  • 4Bundesanstalt für Geowissenschaften und Rohstoffe. Hannover, 30655, Germany
  • 5University of Bremen, Faculty of Geosciences, Bremen, 28359, Germany
  • apresent address: GEOMAR Helmholtz Centre for Ocean Research, Kiel, 24148, Germany

Abstract. Exchange of dissolved substances at the sediment–water interface provides an important link between the short–term and long–term geochemical cycles in the ocean. A second, as yet poorly understood sediment–water exchange is supported by low–temperature circulation of seawater through the oceanic basement underneath the sediments. From the basement, upwards diffusing oxygen and other dissolved species modify the sediment whereas reaction products diffuse from the sediment down into the basement, where they are transported by the basement fluid and released to the ocean. Here, we investigate the impact of this “second” route with respect to transport, release and consumption of oxygen, nitrate, manganese, nickel, and cobalt on the basis of sediment cores retrieved from the Clarion Clipperton Zone (CCZ) in the equatorial Pacific Ocean. We show that in this abyssal ocean region characterised by low organic–carbon burial and sedimentation rates vast areas exist where the downward and upward directed diffusive fluxes of oxygen meet so that the sediments are oxic throughout. This is especially the case where sediments are thin or in the proximity of faults. Oxygen diffusing upward from the basaltic crust into the sediment contributes to the degradation of sedimentary organic matter. Where the oxygen profiles do not meet, they are separated by a suboxic sediment interval characterised by Mn2+ in the pore–water. Where the sediments are entirely oxic, nitrate produced in the upper sediment by nitrification is lost both by upward diffusion into the bottom water and by downward diffusion into the fluids circulating within the basement. Where pore–water manganese in the suboxic zones remains low, nitrate consumption is low and the sediment continues to deliver nitrate to the ocean bottom waters and basement fluid. We observe that at elevated pore–water manganese concentrations, nitrate consumption exceeds production and the basement becomes a nitrate source. Within the suboxic zone, not only manganese but also cobalt and nickel are released into the pore–water by reduction of oxides, diffuse towards the oxic/suboxic fronts above and below where they precipitate, effectively removing these metals from the suboxic zone and concentrating them at the oxic/suboxic redox boundaries. We show that not only diffusive fluxes in the top part of deep–sea sediments modify the geochemical composition over time, but also diffusive fluxes of dissolved constituents from the basement into the bottom layers of the sediment. Hence, paleoceanographic interpretation of sedimentary layers should carefully consider such deep secondary modifications in order to prevent misinterpretation as primary signatures.

Gerard J. M. Versteegh et al.

Status: closed

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on bg-2021-112', Charles Wheat, 19 May 2021
    • AC1: 'Reply on RC1', Gerard J.M. Versteegh, 23 Jul 2021
  • RC2: 'Comment on bg-2021-112', Beth Orcutt, 21 May 2021
    • AC2: 'Reply on RC2', Gerard J.M. Versteegh, 23 Jul 2021

Status: closed

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on bg-2021-112', Charles Wheat, 19 May 2021
    • AC1: 'Reply on RC1', Gerard J.M. Versteegh, 23 Jul 2021
  • RC2: 'Comment on bg-2021-112', Beth Orcutt, 21 May 2021
    • AC2: 'Reply on RC2', Gerard J.M. Versteegh, 23 Jul 2021

Gerard J. M. Versteegh et al.

Gerard J. M. Versteegh et al.

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Short summary
Oxygen not only penetrates sediments from the ocean bottom waters but also from the basement. The impact of the latter is poorly understood. We show that this basement oxygen has a clear impact on the nitrogen cycle, the redox state and the distribution of manganese, nickel cobalt and organic matter in the sediments. This is important for (1) global biogeochemical cycles (2) understanding sedimentary life (3) the interpretation of the sediment record to reconstruct the past.
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