Carbonate parameters in high and low productivity areas of the Sulu Sea, Philippines

To further understand dissolved carbon dioxide (CO2) dynamics and the link between surface productivity and carbon content in the sediments of the Sulu Sea, seawater samples were obtained from high and low productivity areas during the northeast monsoon of December 2007/January 2008, which also corresponded to a period of weak La Niña. CO2 concentrations in surface waters of the Sulu Sea are mostly governed by productivity, while sub-surface CO2 concentrations are influenced by physical processes including vertical mixing and bottom-intensified flow southwest of Mindoro and Panay Straits (central Philippines), and upwelling off the west coast of Zamboanga in Mindanao (south Philippines). These physical processes are manifested in the downward sloping isolines of carbonate parameters near the straits and weak upwelling of waters saturated with respect to atmospheric CO2. Deep waters ventilated from the Sulu Sea have lower dissolved inorganic carbon (DIC) and total alkalinity (TA), and higher pHT (pH in the total scale) and carbonate ion concentrations (CO32 −) than the deep waters of the South China Sea. Nevertheless, TA in the deep waters of the Sulu Sea increases below 2000 m at almost twice the rate as DIC, suggesting possible dissolution of sedimentary CaCO3. A small positive “excess” alkalinity signal is observed in the South China Sea but negative values in the Sulu Sea may reflect the possible contribution of organic acids. This study suggests that the upwelling area within the Sulu Sea, albeit limited in size, could be a source of CO2 to the atmosphere especially during the stronger northeast monsoon periods. Other processes such as dissolution of CaCO3 in the sediments and possible organic acidity should be considered in understanding the Sulu Sea's CO2 sink capacity for the coming years.