Surface layer carbon budget for the subtropical N. Pacific: delta super(13)C constraints at station ALOHA

The rate of biological organic carbon export at the time series station ALOHA in the subtropical N. Pacific (23 degree N 158 degree W) has been estimated from monthly measurements of dissolved inorganic carbon (DIC) and the delta super(13)C of the DIC in the surface layer between 1994 and 1999. The most consistent feature of the annual DIC and delta super(13)C cycles occurs during summer (April- September) when there is a 14 mu molkg super(-1) decrease in DIC and a 0.1ppm increase in delta super(13)C. During this period, when the mixed layer depth is similar to 50 m, the DIC decrease corresponds to a loss of 4.9 mmolm super(-2) d super(-1). The mean pCO sub(2) of the surface ocean during the summer is similar to 10 mu atm lower than atmospheric pCO sub(2) yielding a net air-sea CO sub(2) invasion of 1.6+/- 1.0 mmolm super(-2) d super(-1). Horizontal advection, as a result of Ekman and geostrophic velocities, yields a DIC loss of 1.1+/-0.7 mmolm super(-2) d super(-1). Insignificant alkalinity changes indicate that CaCO sub(3) loss is negligible. Surface layer DIC and DI super(13)C 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 mmolm super(-2) d super(-1), respectively. Thus the measured DIC and delta super(13)C changes during the summer at ALOHA indicate that the DIC draw down is primarily the result of NCP exceeding DIC supplied via air-sea CO sub(2) exchange and vertical diffusion. Over an annual cycle, the DIC and DIC super(13) budgets indicate that NCP at 6.8+/-3.4 mmolm super(-2) d super(-1) is approximately balanced by the upward DIC flux of 6.5+/-4.8 mmolm super(-2) d super(-1) resulting from vertical mixing. These NCP rates agree well with previously published estimates at ALOHA of 6-7.4 mmolm super(-2) d super(-1). Extrapolating a NCP rate of 7 mmolm super(-2) d super(-1) at ALOHA to the global subtropical ocean yields a rate of 6.3 GtCyr super(- 1) that represents more than half of the estimated global ocean organic carbon export rate of similar to 11 GtCyr super(-1).