Biogenic Carbonate Dissolution in Shallow Marine Environments

Ocean acidification (OA), the decrease in surface ocean pH and seawater saturation state with respect to carbonate minerals (Ω), is expected to increase carbonate mineral dissolution. However, the influence of OA on carbonate dissolution has been largely neglected despite evidence that it is more sensitive to OA than calcification. Increases in the rate of carbonate dissolution could have severe impacts for ecosystems such as coral reefs, which rely on the accumulation of carbonate structures and substrates to exist. At present, dissolution rates of bulk shallow biogenic carbonate sediments are largely unknown and laboratory dissolution rates exceed in situ rates by orders of magnitude. The goal of this study was to develop a better understanding of the drivers and controls of bulk carbonate sediment dissolution in coral reef environments. Based on results from in situ benthic chambers and laboratory free-drift experiments of bulk biogenic carbonate sediments from global locations, dissolution rates were found to be primarily controlled by organic matter decomposition, but significantly influenced by the overlying seawater carbonate chemistry and the solubility of the most soluble mineral phase in the sediments. Shallow carbonate dissolution will therefore be enhanced via ocean acidification, increased respiration, or a combination of these processes. The sensitivity of bulk sediment dissolution rates to changes in Ω was not related to median grain size or mineralogy, but may be attributed to organic coatings on sediment grains. Dissolution rates in bulk sediments increased ~2-3-fold when these coatings were removed, suggesting that they act as a protective barrier that limits direct interaction of seawater with the mineral surface, thus inhibiting dissolution. On the ecosystem scale, carbonate dissolution was inferred from calcium anomalies measured using a novel spectrophotometric titration system and confirms seasonal and inter-annual trends in reef biogeochemical processes based on parallel alkalinity measurements. However, calcium measurements may be best employed in environments where multiple processes significantly influence alkalinity or Mg-calcites are precipitating and dissolving. Although many questions remain, this work has elucidated certain key drivers and controls of shallow carbonate sediment dissolution and how they may respond to a rapidly changing ocean