ALKALINITY CHANGES IN THE SARGASSO-SEA - GEOCHEMICAL EVIDENCE OF CALCIFICATION

Strong seasonal patterns in upper ocean total carbon dioxide (TCO2), a lkalinity (TA) and calculated pCO(2) were observed in a time series of water column measurements collected at the US Joint Global Ocean Flux Study (JGOFS) BATS site (31 degrees 50'N, 64 degrees 10'W) in the Sar gasso Sea. TA distribution was a conservative function of salinity. Ho wever, in February 1992, a non-conservative decrease in TA was observe d, with maximum depletion of 25-30 mu moles kg(-1) occuring in the sur face layer and at the depth of the chlorophyll maximum (similar to 80- 100 m). Mixed-layer TCO2 also decreased, while surface pCO(2) increase d by 25-30 mu atm. We suggest these changes in carbon dioxide species resulted from open-ocean calcification by carbonate-secreting organism s rather than physical processes. Coccolithophore calcification is the most likely cause of this event although calcification by foraminifer a or pteropods cannot be ruled out. Due to the transient increase in s urface pCO(2), the net annual transfer of CO2 into the ocean at BATS w as reduced. These observations demonstrate the potential importance of open-ocean calcification and biological community structure in the bi ogeochemical cycling of carbon.