07 Apr 2022
07 Apr 2022
Status: this preprint is open for discussion.

Benthic Alkalinity fluxes from coastal sediments of the Baltic and North Seas: Comparing approaches and identifying knowledge gaps

Bryce Van Dam1, Nele Lehmann1,4,7, Mary Zeller3, Andreas Neumann1, Daniel Pröfrock2, Marko Lipka3, Helmuth Thomas1,7, and Michael E. Böttcher3,5,6 Bryce Van Dam et al.
  • 1Helmholtz-Zentrum Hereon, Institute of Carbon Cycles, Geesthacht, Germany
  • 2Helmholtz-Zentrum Hereon, Institute of Coastal Environmental Chemistry, Geesthacht, Germany
  • 3Leibniz Institute for Baltic Sea Research (IOW), Warnemünde, Germany
  • 4Alfred Wegener Institute Helmholtz Centre for Polar and Marine Research, Potsdam, Germany
  • 5Marine Geochemistry, University of Greifswald, Germany
  • 6Interdisciplinary Faculty, University of Rostock, Germany
  • 7University of Oldenburg, Oldenburg, Germany

Abstract. Benthic alkalinity production is often suggested as a major driver of net carbon sequestration in continental shelf ecosystems. However, information and direct measurements of benthic alkalinity fluxes are limited and are especially challenging when biological and dynamic physical forcing causes surficial sediments to be vigorously irrigated. To address this shortcoming, we quantified net sediment-water exchange of alkalinity using a suite of complementary methods, including 1) 224Ra budgeting, 2) incubations with 224Ra and Bromide as tracers, and 3) numerical modelling of porewater profiles. We choose a set of sites in the shallow southern North Sea and western Baltic Sea, allowing us to incorporate frequently occurring sediment classes ranging from coarse sands to muds, and sediment-water interfaces ranging from biologically irrigated and advective to diffusive into the investigations. Sediment-water irrigation rates in the southern North Sea were approximately twice as high as previously estimated for the region, in part due to measured porewater 224Ra activities higher than previously assumed. Net alkalinity fluxes in the Baltic Sea were relatively low, ranging from an uptake of -35 µmol m-2 hr-1 to a release of 53 µmol m-2 hr-1, and in the North Sea from 1 to 33.6 µmol m-2 hr-1. Lower than expected apparent nitrate consumption (potential denitrification), across all sites, is one explanation for our small measured net alkalinity fluxes. Carbonate mineral precipitation and sulfide re-oxidation also appear to play important roles shaping net sediment-water fluxes in the North Sea and Baltic Sea sites, respectively.

Bryce Van Dam et al.

Status: open (until 19 May 2022)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on egusphere-2022-161', Xinping Hu, 17 Apr 2022 reply
  • RC2: 'Comment on egusphere-2022-161', Anonymous Referee #2, 22 Apr 2022 reply

Bryce Van Dam et al.


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Short summary
We quantified sediment-water exchange at shallow sites in the North and Baltic Seas. We found that porewater irrigation rates in the former were approximately twice as high as previously estimated, likely driven by relatively high bio-irrigative activity. In contrast, we found small net fluxes of alkalinity, ranging from -35 µmol m-2 hr-1 (uptake) to 53 µmol m-2 hr-1 (release). We attribute this to low net denitrification, carbonate mineral (re)precipitation and sulfide (re)oxidation.