Rates of particle scavenging and particulate organic carbon export estimated using 234Th as a tracer in the subtropical and equatorial Atlantic Ocean

The export flux of particulate organic carbon and particle scavenging rates were calculated using a Th-234-based approach along a transect through the mid-Atlantic from 35 degrees S to 10 degrees N during May and June of 1996. The median residence time of dissolved Th-234 along the transect was 135 days, with intense scavenging from 5 degrees N to 6 degrees N (<(tau)over bar>(d) = 86 +/- 17 days). Vertical profiles and depth-integrated samples (surface-100 m) of dissolved ( < 0.7-mu m), suspended particulate (0.7-53-mu m), and large particulate( > 53-mu m) Th-234 activities were collected to estimate export fluxes of particulate organic carbon (POC). A steady-state model of the Th-234 activity balance in the upper 100 m was combined with measurements of the POC/Th-234 ratio on > 53-mu m particles. POC export ranged from 3.2 mmol C m(-2) d(-1) in the central South Atlantic to 43 mmol C m(-2) d(-1) at 2.5 degrees S. Export production along the Atlantic equator (5 degrees S-5 degrees N, 25 degrees E-5 degrees W) is estimated to be 0.29 Gt C yr(-1). The Th-234-derived POC export estimates reported here and from other studies are comparable with model simulations of export production from the Princeton General. Circulation Model (GCM) for the Atlantic Ocean. (C) 1999 Published by Elsevier Science Ltd. All rights reserved.