Direct evidence of sediment carbonate dissolution in response to bottom-water acidification in the Gulf of St. Lawrence, Canada

Abstract

Over the past century, dissolved oxygen concentrations have decreased and metabolic CO2 has accumulated in the bottom waters of the Gulf of St. Lawrence (GSL) and Lower St. Lawrence Estuary (LSLE). Oxygen depletion has been attributed primarily to changes in ocean circulation in the northwest Atlantic Ocean, as well as an increase in the flux of organic matter at or near the seafloor and its accompanying biological oxygen demand. The accumulation of metabolic CO2 in these waters has led to their progressive acidification and a decrease in pH (0.3–0.4 pH unit) commensurate to the variation expected for global oceanic surface waters by the end of this century, albeit by a different mechanism (anthropogenic CO2 uptake from the atmosphere). The decrease in bottom-water pH of the GSL and LSLE is accompanied by a decrease in the carbonate ion concentration and the saturation state of the waters with respect to both calcite and aragonite (ΩC and ΩA). Although the Laurentian Trough sediments are mostly devoid of modern calcium carbonate fossils, detrital (Ordovician/Silurian) carbonates, eroded from Anticosti Island, accumulate on the seafloor. Evidence of carbonate mineral dissolution in the sediments of the Laurentian Trough is examined and supported by pore-water data and vertical variations of their inorganic carbon content. Historical, solid-phase profile data are used to estimate temporal variations of the sedimentary calcite dissolution rates and document the anthropogenic modification of the sediment record.