Benthic alkalinity and dissolved inorganic carbon fluxes in the Rhône River prodelta generated by decoupled aerobic and anaerobic processes

Abstract

Abstract. Estuarine regions are generally considered a major source of atmospheric CO2, as a result of the high organic carbon (OC) mineralization rates in their water column and sediments. Despite this, the intensity of anaerobic respiration processes in the sediments tempered by the reoxidation of reduced metabolites near the sediment–water interface controls the flux of benthic alkalinity. This alkalinity may partially buffer metabolic CO2 generated by benthic OC respiration in sediments. Thus, sediments with high anaerobic respiration rates could contribute less to local acidification than previously thought. In this study, a benthic chamber was deployed in the Rhône River prodelta and the adjacent continental shelf (Gulf of Lion, northwestern Mediterranean) in late summer to assess the fluxes of total alkalinity (TA) and dissolved inorganic carbon (DIC) from the sediment. Concurrently, in situ O2 and pH micro-profiles, voltammetric profiles and pore water composition were measured in surface sediments to identify the main biogeochemical processes controlling the net production of alkalinity in these sediments. Benthic TA and DIC fluxes to the water column, ranging between 14 and 74 and 18 and 78 mmol m−2 d−1, respectively, were up to 8 times higher than dissolved oxygen uptake (DOU) rates (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. In the zone close to the river mouth, pore water redox species indicated that TA and DIC were mainly produced by microbial sulfate and iron reduction. Despite the complete removal of sulfate from pore waters, dissolved sulfide concentrations were low and significant concentrations of FeS were found, indicating the precipitation and burial of iron sulfide minerals with an estimated burial flux of 12.5 mmol m−2 d−1 near the river mouth. By preventing reduced iron and sulfide reoxidation, the precipitation and burial of iron sulfide increases the alkalinity release from the sediments during the spring and summer months. Under these conditions, the sediment provides a net source of alkalinity to the bottom waters which mitigates the effect of the benthic DIC flux on the carbonate chemistry of coastal waters and weakens the partial pressure of CO2 increase in the bottom waters that would occur if only DIC was produced.

Early Diagenesis in Sediments of the Venice Lagoon (Italy) and Its Relationship to Hypoxia

“…Sulfate reduction was the most important diagenetic pathway at stations ALG1 and TRES, where the largest SO 4 2− gradients and integrated consumptions, as well as the largest increases of dissolved sulfide were observed (Figure 4). The difference between the TS increase, about 0.5 mM, and the decrease in porewater sulfate concentration, ranging from 2 to 20 mM, can be explained by the precipitation of iron sulfides (Metzger et al, 2007;Rassmann et al, 2020).…”

“…This is consistent with the increase in pH below the oxic zone at all station (Figures 2, 3), which can be due to iron and probably manganese oxide reduction. The low Fe 2+ concentration at depth below 5 cm can be explained by the precipitation of FeS or FeS 2 (Rassmann et al, 2020). Sulfate reduction was the most important diagenetic pathway at stations ALG1 and TRES, where the largest SO 4 2− gradients and integrated consumptions, as well as the largest increases of dissolved sulfide were observed (Figure 4).…”

“…The relative uncertainty for DIC and TA was ±0.5% of the final value. Sulfate (SO 4 2− ) concentration was quantified after dilution by a factor of 100 by ion chromatography on an ICS 1000 chromatograph (Dionex) with a relative uncertainty of ±1.6% (Rassmann et al, 2020). To measure ammonium (NH 4 + ) concentrations, samples were diluted and analyzed using the indophenol blue method (Grasshof et al, 1983).…”

The impact of intertidal areas on the carbonate system of the southern North Sea

“…In comparison to the central and northern part of the North Sea, TA levels in the southern part are significantly elevated during summer (Salt et al, 2013;Thomas et al, 2009;Brenner et al, 2016;Burt et al, 2016). The observed high TA levels have been attributed to an impact from the adjacent tidal areas (Hoppema, 1990;Kempe and Pegler, 1991;Reimer et al, 1999;Thomas et al, 2009;Winde et al, 2014), but this impact has not been rigorously quantified. Using several assumptions, Thomas et al (2009) calculated an annual TA export from the Wadden Sea and Southern Bight of 73 Gmol TA yr −1 to close the TA budget for the southern North Sea.…”

“…Some studies have investigated regional carbon cycles in detail (e.g. Kempe and Pegler, 1991;Brasse et al, 1999;Reimer et al, 1999;Thomas et al, 2004Thomas et al, , 2009Artioli et al, 2012;Lorkowski et al, 2012;Burt et al, 2016;Shadwick et al, 2011;Laruelle et al, 2014;Carvalho et al, 2017) and pointed out sources of uncertainties specifically for coastal settings.…”