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.