Export flux of carbon at the equator during the EqPac time-series cruises estimated from 234Th measurements

Abstract

Distributions of 234Th were determined in three particle-size classes ( > 53, 1–53 and 0.7–1.0 μm) and in filtered seawater during each of the two time-series cruises of the U.S. JGOFS Process Study in the equatorial Pacific. Four vertical profiles were measured on the equator at 140°W from the sea surface to 400 m depth between 24 March and 9 April 1992 (Time-series I) and again between 3 and 18 October 1992 (Time-series II). In addition, both organic and inorganic carbon were measured in each of the particle fractions. The results were used with a one-dimensional model, which includes the equatorial upwelling, to estimate the flux of particulate carbon sinking out of the surface layer. The flux of particulate organic carbon (POC) at the base of the euphotic zone (0.1 % light level, 120 m depth) was estimated to average 1.9 mmol m −2 day −1 during El Niño (Time-series I) and 2.4 mmol m −2 day −1 during the cold period that followed (Time-series II). These values amount to only ∼ 2% of the primary production measured during each of the same periods and are insufficient to balance the new production, estimated previously to be ∼ 17% of primary production. These results are consistent with the hypothesis that the major part of the new production is removed from the region by advection in the form of dissolved organic matter. The POC flux profile indicates a net remineralization below the 1 % light level (80 m depth) such that the flux reaching 200 m depth has been reduced by ∼ 55%, giving a remineralization length scale of ∼ 155 m. For particulate inorganic (carbonate) carbon the flux at 200 m averaged 0.54 mmol m −2 day −1 during Time-series I and 0.71 mmol m −2 day −1 during Time-series II, very similar to the fluxes reported in deep sediment traps deployed at the same time. Estimates of the average large-particle sinking velocity give values < 10 m day −1 in the upper part of the euphotic zone, show a sharp increase near the base of the euphotic zone and level off to values of 30–60 m day −1 at 200 m depth.