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

Abstract

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, SO 2− 4 and Ca 2+ 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 O 2 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 Ca 2+ 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 CO 2 buffering capacity of these coastal waters. Estuarine sediments should therefore receive more attention in future estimations of global carbon fluxes.

The roles of resuspension, diffusion and biogeochemical processes on oxygen dynamics offshore of the Rhône River, France: a numerical modeling study

“…1). Our study is co-located with the site from Toussaint et al (2014) at the "Mesurho" station (Pairaud et al, 2016) and is only a few kilometers away from Site A in Pastor et al (2011a); both locations are at ∼ 25 m water depth and are characterized by similar biogeochemical characteristics (e.g., Rassmann et al, 2016), and so data from both sites were used for model input, validation, and evaluation. Importantly, data from Toussaint et al (2014) included a time series of oxygen profiles with submillimeter-scale resolution within the seabed and bottom centimeter of the water column.…”

Controlling the diffusive boundary layer thickness above the sediment–water interface in a thermostated rotating‐disk reactor

“…Examples of water-side transport-controlled reaction are the dissolution of calcium carbonate at the deep-seafloor (Schink and Guinasso 1977;Santschi et al 1991;Sulpis et al 2017) and the accretion of manganese at the surface of deep-sea nodules (Boudreau and Scott 1978). Commonly, the diffusion through the DBL of most solutes is ruled by a combination of both regimes, termed mixed-control, e.g., dissolved oxygen (Jørgensen and Revsbech 1985;Hondzo 1998;Lorenzen et al 1998;Rassmann et al 2016) or radon (Homoky 2016;Cook et al 2018), that display concentration gradients extending on both sides of the SWI. For more details on the theoretical framework of solute exchange across the SWI, we refer the reader to the comprehensive review published by Boudreau and Guinasso (1982).…”

Modelling biogeochemical processes in sediments from the north-western Adriatic Sea: response to enhanced particulate organic carbon fluxes

“…Measurements were performed in the laboratory: DIC was analysed in a DIC analyzer (Apollo SciTech ® ) using a 1 mL sample volume, with four to six replicates which provided a standard deviation of 0.5 %. TA concentrations were measured in a potentiometric open-cell titration in a 3 mL sample volume (Rassmann et al, 2016) with an uncertainty of 0.5 %. Ammonium concentration was measured spectrophotometrically following Grasshof et al (1983) with an uncertainty of about 5 %.…”