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Volume 13, issue 18
Biogeosciences, 13, 5379–5394, 2016
© Author(s) 2016. This work is distributed under
the Creative Commons Attribution 3.0 License.
Biogeosciences, 13, 5379–5394, 2016
© Author(s) 2016. This work is distributed under
the Creative Commons Attribution 3.0 License.

Research article 27 Sep 2016

Research article | 27 Sep 2016

Carbonate chemistry in sediment porewaters of the Rhône River delta driven by early diagenesis (northwestern Mediterranean)

Jens Rassmann1, Bruno Lansard1, Lara Pozzato1,2, and Christophe Rabouille1 Jens Rassmann et al.
  • 1Laboratoire des Sciences du Climat et de l'Environnement, LSCE/IPSL, CEA-CNRS-UVSQ-Université Paris Saclay, 91198 Gif-sur-Yvette, France
  • 2Institut Méditerranéen d'Océanologie, CNRS-IRD-Université de Toulon-Aix Marseille, 13288 Marseille, France

Abstract. The Rhône River is the largest source of terrestrial organic and inorganic carbon for the Mediterranean Sea. A large fraction of this terrestrial carbon is either buried or mineralized in the sediments close to the river mouth. This mineralization follows aerobic and anaerobic pathways, with a range of impacts on calcium carbonate precipitation and dissolution in the sediment near the sediment–water interface. This study focuses on the production of dissolved inorganic carbon (DIC) and total alkalinity (TA) by early diagenesis, consequential pH variations and the effect on calcium carbonate precipitation or dissolution. The sediment porewater chemistry was investigated along a transect from the Rhône River outlet to the continental shelf. TA and concentrations of DIC, SO42− and Ca2+ were analyzed on bottom waters and extracted sediment porewaters, whereas pH and oxygen concentrations were measured in situ using microelectrodes. The average oxygen penetration depth into the sediment was 1.7 ± 0.4 mm close to the river mouth and 8.2 ± 2.6  mm in the continental shelf sediments, indicating intense respiration rates. Diffusive oxygen fluxes through the sediment–water interface ranged between 3 and 13 mmol O2 m−2 d−1. In the first 30 cm of the sediment, TA and DIC porewater concentrations increased with depth up to 48 mmol L−1 near the river outlet and up to 7 mmol L−1 on the shelf as a result of aerobic and anaerobic mineralization processes. Due to aerobic processes, at all stations pH decreased by 0.6 pH units in the oxic layer of the sediment accompanied by a decrease of the saturation state regarding calcium carbonate. In the anoxic layer of the sediments, sulfate reduction was the dominant mineralization process and was associated with an increase of porewater saturation state regarding calcium carbonate. Ultimately anoxic mineralization of organic matter caused calcium carbonate precipitation demonstrated by a large decrease in Ca2+ concentration with depth in the sediment. Carbonate precipitation decreased in the offshore direction, together with the carbon turnover and sulfate consumption in the sediments. The large production of porewater alkalinity characterizes these sediments as an alkalinity source to the water column, which may increase the CO2 buffering capacity of these coastal waters. Estuarine sediments should therefore receive more attention in future estimations of global carbon fluxes.

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Short summary
In situ O2 and pH measurements as well as determination of porewater concentrations of dissolved inorganic carbon, total alkalinity, sulfate and calcium have been measured in the sediments of the Rhône prodelta. Biogeochemical activity decreased with distance from the river mouth. Oxic processes decreased the carbonate saturation state (Ω) by lowering pH, whereas anaerobic organic matter degradation, dominated by sulfate reduction, was accompanied by increasing Ω and carbonate precipitation.
In situ O2 and pH measurements as well as determination of porewater concentrations of dissolved...
Final-revised paper