The total organic carbon export rate based on 13C and 12C of DIC budgets in the equatorial Pacific region

Abstract

Carbon stable isotopic composition (δ 13C) of total dissolved inorganic carbon (DIC) is independent of DIC as a tracer for net organic carbon production rate in the surface ocean due to the Significantly different equilibration times for CO 2 and 13CO 2 during air-sea exchange. The rates of the organic carbon export and net physical supply of DIC, can therefore, be determined from the mixed layer mass balances for DIC and DIC 13 (DIC· 13C/ 12C), given an air-sea C0 2 flux estimated from wind speed and measured pCO 2. This method does not rely on specifying the absolute advetive and diffusive transports of DIC. The mass balance approach was applied to measurements of concentration and δ 13C of DIC made in the equatorial Pacific ocean in the Spring and Fall of 1991–1992 during WOCE P16C and US JGOFS EqPac cruises. The calculated organic carbon export rate out of mixed layer during Spring 1992 El Nin˜o was 3.2±1.4 mmol C m −2 d −1 (between 2°N to 2°S and 170°W to 110°W). No spatial trend for the total organic carbon export rate was determined despite the large variation in C02 degassing rate from ∼0.1 mmol C m −2 d −1 at 170°W to ∼4.0 mmol Cm −2 d −1 at 110°W over this region. Under the cold tongue conditions in the end of August 1992, a higher organic carbon export rate, about 11.5±6.3 mmol C m -2 d −1 was obtained on the equator at 140°W while the rate for the previous September (in 1991) was 4.9 ± 2.4 mmol C m −2 d −1. Most of the uncertainty in our estimated results from the uncertainty in gas exchange coefficient. The comparison between the estimated organic carbon export rates and the particulate organic carbon (POC) export rates derived from thorium budget and sediment drifting traps at the same stations (Buesseler et al., 1995; Murray et al., 1996) suggests that particulate organic carbon probably accounts for about 70% of the organci export. Since physical, rather than biological, processes primarily control the distribution of DIC, NO 3 O 2 and other nutrient-related properties in this region, it is very difficult to accurately obtain the net organic carbon export rate from advection rates and nutrient gradients, as demonstrated by a 3-D calculation.