Rates of particle scavenging and particulate organic carbon export estimated using Th-234 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 b ar>(d) = 86 +/- 17 days). Vertical profiles and depth-integrated samples (s urface-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 e stimate export fluxes of particulate organic carbon (POC). A steady-state m odel of the Th-234 activity balance in the upper 100 m was combined with me asurements of the POC/Th-234 ratio on > 53-mu m particles. POC export range d 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 G t C yr(-1). The Th-234-derived POC export estimates reported here and from other studies are comparable with model simulations of export production fr om the Princeton General. Circulation Model (GCM) for the Atlantic Ocean. ( C) 1999 Published by Elsevier Science Ltd. All rights reserved.