Surface layer carbon budget for the subtropical N. Pacific: [formula omitted] constraints at station ALOHA

Abstract

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 δ 13 C of the DIC in the surface layer between 1994 and 1999. The most consistent feature of the annual DIC and δ 13 C cycles occurs during summer (April–September) when there is a 14 μmol kg −1 decrease in DIC and a 0.1‰ increase in δ 13 C . 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 pCO 2 of the surface ocean during the summer is ∼10 μatm lower than atmospheric pCO 2 yielding a net air–sea CO 2 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 CaCO 3 loss is negligible. Surface layer DIC and DI 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 mmol m −2 d −1 , respectively. Thus the measured DIC and δ 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 2 exchange and vertical diffusion. Over an annual cycle, the DIC and DIC 13 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 .