Carbonate dissolution and alkalinity production in low-carbonate, permeable sediments

Carbonate dissolution is thought to contribute approximately half of shelf-derived alkalinity to the global ocean. However, measurements of sediment carbonate dissolution are lacking and rates are poorly constrained. In particular, low-carbonate detrital sands, which constitute a large fraction of the global shelf and slope area, are under-studied. Better quantification of carbonate chemistry in these sands is important to understand global change-related perturbations to the ocean inorganic carbon cycle, including the effects of ocean acidification. We report on research quantifying carbonate dissolution and alkalinity fluxes from permeable, low-carbonate sediments off the southern California coast. We used several complementary methods, including: flow-through sand columns, in situ benthic chambers, and 13C-labeled carbonate additions, with high-precision measurements of 13C (for isotope mass balance calculations) and alkalinity. Our results show measurable carbonate dissolution at current and near-future seawater saturation states (Ω), but sediments only became net buffering (i.e., alkalinity/DIC>1) when inflowing water Ω fell below Ω(aragonite) = 1. Alkalinity/DIC modeling for the flow through sand columns revealed that: a) most alkalinity was produced by carbonate dissolution, not anaerobic processes; b) in current Ω conditions, carbonate dissolution primarily occurred after (i.e., “downstream” from) zones of aerobic respiration; and c) higher rates of dissolution were observed with slower flushing rates. These findings help predict the effect of ocean acidification on sediment carbonate dissolution, and in turn alkalinity flux to the nearshore ocean.