Alkalinity changes in the Sargasso Sea: Geochemical evidence of calcification?

Strong seasonal patterns in upper ocean total carbon dioxide (TCO2), alkalinity (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 Sargasso Sea. TA distribution was a conservative function of salinity. However, in February 1992, a non-conservative decrease in TA was observed, with maximum depletion of 25-30 mu moles kg(-1) occuring in the surface layer and at the depth of the chlorophyll maximum (similar to 80-100 m). Mixed-layer TCO2 also decreased, while surface pCO(2) increased by 25-30 mu atm. We suggest these changes in carbon dioxide species resulted from open-ocean calcification by carbonate-secreting organisms rather than physical processes. Coccolithophore calcification is the most likely cause of this event although calcification by foraminifera or pteropods cannot be ruled out. Due to the transient increase in surface pCO(2), the net annual transfer of CO2 into the ocean at BATS was reduced. These observations demonstrate the potential importance of open-ocean calcification and biological community structure in the biogeochemical cycling of carbon.