ABSTRACT
The behavior of the ocean carbon cycle is continually modifi ed by the increase in atmospheric CO2 due to fossil fuel combustion and land-use emissions of CO2 (Andersson et al. 2005). The consequences of a high-CO2 world and increasing riverine transport of inorganic matter and nutrients arising from human activities were simulated by models between the years 1700 and (projected) 2300. The models show that the global coastal ocean changes from a net source to a net sink of atmospheric CO2 over time. In the 1700s and the 1800s the direction of the CO2 fl ux was from coastal surface waters to the atmosphere, whereas today the net CO2 fl ux is into coastal surface waters. These results agree well with recent syntheses of measurements of air-sea CO2 exchange fl uxes from various coastal environments. These models predict that coastal ocean surface water carbonate saturation state would decrease 46% by the year 2100 and 73% by 2300. Observational evidence from the Atlantic Ocean and the Pacifi c Ocean show that the carbonate saturation state of surface ocean waters has declined during recent decades. For Atolls and other semi-enclosed carbonate systems, the rate of decline is dependent on the residence time of the water in the system. Biogenic production of CaCO3-as based on the positive relation between saturation state and calcifi cation-may decrease as much as 42% by the year 2100 and up to 90% by 2300. If the predicted change in carbonate production were to occur along with rising temperatures, it would threaten the existence of coral reefs and other carbonate systems for some centuries. Cold water carbonate systems are more vulnerable to rising atmospheric
predict that carbonate saturation state of coastal sediment pore water will decrease owing to a decrease in pore water pH and increasing CO2 concentrations attributable to greater deposition and remineralization of organic matter in sediments. In the future, the average composition of carbonate sediments and cements may change as the more soluble magnesium 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 (Andersson et al. 2005).
