Coastal ocean and carbonate systems in the high CO2 world of the Anthropocene

Abstract

The behavior of the ocean carbon cycle has been, and will continue to be, modified by the increase in atmospheric CO~2~ due to fossil fuel combustion and land-use emissions of this gas. The consequences of a high-CO~2~ world and increasing riverine transport of organic matter and nutrients arising from human activities were investigated by means of two biogeochemical box models. Model numerical simulations ranging from the year 1700 to 2300 show that the global coastal ocean changes from a net source to a net sink of atmospheric CO~2~ over time; in the 18th and 19th centuries, the direction of the CO~2~ flux was from coastal surface waters to the atmosphere, whereas at present or in the near future the net CO~2~ flux is into coastal surface waters. These results agree well with recent syntheses of measurements of air-sea CO~2~ exchange fluxes from various coastal ocean environments. The model calculations also show that coastal ocean surface water carbonate saturation state would decrease 46 percent by the year 2100 and 73 percent by 2300. Observational evidence from the Pacific and Atlantic Oceans shows that the carbonate saturation state of surface ocean waters has already declined during recent decades. For atolls and other semi-enclosed carbonate systems, the rate of decline depends strongly on the residence time of the water in the system. Based on the experimentally observed positive relationship between saturation state and calcification rate for many calcifying organisms, biogenic production of CaCO~3~ may decrease by 42 percent by the year 2100 and by 85 to 90 percent by 2300 relative to its value of about 24 × 10^12^ moles C/yr in the year 2000. If the predicted change in carbonate production were to occur along with rising temperatures, it would make it difficult for coral reef and other carbonate systems, to exist as we know them now into future centuries. Because high-latitude, cold-water carbonates presently occur in waters closer to saturation with respect to carbonate minerals than the more strongly supersaturated waters of the lower latitudes, it might be anticipated that the cool-water carbonate systems might feel the effects of rising atmospheric CO~2~ (and temperature) before those at lower latitudes. In addition, modeling results show that the carbonate saturation state of coastal sediment pore water will decrease in the future owing to a decreasing pore water pH and increasing CO~2~ concentrations attributable to greater deposition and remineralization of land-derived and *in situ* produced organic matter in sediments. The lowered carbonate saturation state drives selective dissolution of metastable carbonate minerals while a metastable equilibrium is maintained between the pore water and the most soluble carbonate phase present in the sediments. In the future, the average composition of carbonate sediments and cements may change as the more soluble Mg-calcites and aragonite are preferentially dissolved and phases of lower solubility, such as calcites with lower magnesium content, increase in percentage abundance in the sediments.