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© Author(s) 2019. This work is distributed under
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© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.

Research article 04 Feb 2019

Research article | 04 Feb 2019

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This discussion paper is a preprint. A revision of the manuscript is under review for the journal Biogeosciences (BG).

Benthic alkalinity and DIC fluxes in the Rhône River prodelta generated by decoupled aerobic and anaerobic processes

Jens Rassmann1,*, Eryn M. Eitel2,*, Cécile Cathalot3, Christophe Brandily3, Bruno Lansard1, Martial Taillefert2, and Chrsitophe Rabouille1 Jens Rassmann et al.
  • 1Laboratoire des Sciences du Climat et de l’Environnement, LSCE/IPSL,CEA-CNRS-UVSQUniversité Paris Saclay, 91198 Gif-sur-Yvette, France
  • 2School of Earth and Atmospheric Sciences; Georgia Institute of Technology, GA 30332-0340 Atlanta, USA
  • 3IFREMER, Laboratoire Environnement Profond, 29280 Plouzané, France
  • *These authors contributed equally to this article.

Abstract. Estuarine regions are generally considered a net source of atmospheric CO2 as a result of the high organic carbon (OC) mineralization rates in the water column and their sediments. Yet, the intensity of anaerobic respiration processes in the sediments tempered by the reoxidation of reduced metabolites controls the net production of alkalinity from sediments that may partially buffer the metabolic CO2 generated by OC respiration. In this study, a benthic chamber was deployed in the Rhône River prodelta and the adjacent continental shelf (Gulf of Lions, NW Mediterranean) to assess the fluxes of total alkalinity (TA) and dissolved inorganic carbon (DIC) from the sediment. Concurrently, in situ O2 and pH microprofiles, electrochemical profiles, pore water and solid composition were measured in surface sediments to identify the main biogeochemical processes controlling the net production of alkalinity in these sediments. The benthic fluxes of TA and DIC, ranging between 14 and 74 mmol m−2 d−1 and 18 and 78 mmol m−2 d−1, respectively, were up to 8 times higher than the DOU fluxes (10.4 ± 0.9 mmol m−2 d−1) close to the river mouth, but their intensity decreased offshore, as a result of the decline in OC inputs. Low nitrate concentrations and strong pore water sulfate gradients indicated that the majority of the TA and DIC was produced by sulfate and iron reduction. Despite the complete removal of sulfate from the pore waters, dissolved sulfide concentrations were low due to the precipitation and burial of iron sulfide minerals (12.5 mmol m−2 d−1 near the river mouth), while soluble organic-Fe(III) complexes were concurrently found throughout the sediment column. The presence of organic-Fe(III) complexes together with low sulfide concentrations and high sulfate consumption suggests a dynamic system driven by the variability of the organic and inorganic particulate input originating from the river. By preventing reduced substances from being reoxidized, the precipitation and burial of iron sulfide decouples the iron and sulfur cycles from oxygen, therefore allowing a flux of alkalinity out of the sediments. In these conditions, the sediment provides a source of alkalinity to the bottom waters which mitigates the effect of the benthic DIC flux on the carbonate chemistry of coastal waters.

Jens Rassmann et al.
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Status: final response (author comments only)
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Jens Rassmann et al.
Jens Rassmann et al.
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