Spatial and temporal variability of total organic carbon along 140°W in the equatorial Pacific Ocean in 1992

Abstract

Total organic carbon (TOC) was analyzed on four transects along 140°W in 1992 using a high temperature combustion/discrete injection (HTC/DI) analyzer. For two of the transects, the analyses were conducted on-board ship. Mixed-layer concentrations of organic carbon varied from about 80 μM C at either end of the transect (12°N and 12°S) to about 60 μM C at the equator. Total organic carbon concentrations decreased rapidly below the mixed-layer to about 38–40 μM C at 1000 m across the transect. Little variation was observed below this depth; deep water concentrations below 2000m were virtually monotonic at about 36 μM C. Repeat measurements made on subsequent cruises consistently found the same concentrations at 1000 m or deeper, but substantial variations were observed in the mixed-layer and the upper water column above 400 m depth. Linear mixing models of total organic carbon versus σ θ exhibited zones of organic carbon formation and consumption. TOC was found to be inversely correlated with apparent oxygen utilization (AOU) in the region between the mixed-layer and the oxygen minimum. In the mixed-layer, TOC concentrations varied seasonally. Part of the variations in TOC at the equator was driven by changes in the upwelling rate in response to variations in physical forcing related to an El Niño and to the passage of tropical instability waves. TOC export fluxes, calculated from simple box models, averaged 8±4 mmol C m −2day −1 at the equator and also varied seasonally. These export fluxes account for 50–75% of the total carbon deficit and are consistent with other estimates and model predictions.