Surface layer carbon budget for the subtropical N. Pacific: δ13C constraints at station ALOHA

The rate of biological organic carbon export at the time series station ALOHA in the subtropical N. Pacific (23°N 158°W) has been estimated from monthly measurements of dissolved inorganic carbon (DIC) and the δ13C of the DIC in the surface layer between 1994 and 1999. The most consistent feature of the annual DIC and δ13C cycles occurs during summer (April–September) when there is a 14 μmol kg−1 decrease in DIC and a 0.1‰ increase in δ13C. During this period, when the mixed layer depth is ∼50 m, the DIC decrease corresponds to a loss of 4.9 mmol m−2 d−1. The mean pCO2 of the surface ocean during the summer is ∼10 μatm lower than atmospheric pCO2 yielding a net air–sea CO2 invasion of 1.6±1.0 mmol m−2 d−1. Horizontal advection, as a result of Ekman and geostrophic velocities, yields a DIC loss of 1.1±0.7 mmol m−2 d−1. Insignificant alkalinity changes indicate that CaCO3 loss is negligible. Surface layer DIC and DI13C budgets were used to solve for two carbon fluxes, the net community production (NCP) and the upward supply of DIC resulting from vertical mixing at the base of the mixed layer. During the summer, the calculated rates of NCP and upward DIC supply are 7.2±2.9 and 1.8±3.9 mmol m−2 d−1, respectively. Thus the measured DIC and δ13C changes during the summer at ALOHA indicate that the DIC draw down is primarily the result of NCP exceeding DIC supplied via air–sea CO2 exchange and vertical diffusion. Over an annual cycle, the DIC and DIC13 budgets indicate that NCP at 6.8±3.4 mmol m−2 d−1 is approximately balanced by the upward DIC flux of 6.5±4.8 mmol m−2 d−1 resulting from vertical mixing. These NCP rates agree well with previously published estimates at ALOHA of 6–7.4 mmol m−2 d−1. Extrapolating a NCP rate of 7 mmol m−2 d−1 at ALOHA to the global subtropical ocean yields a rate of 6.3 Gt C yr−1 that represents more than half of the estimated global ocean organic carbon export rate of ∼11 Gt C yr−1.