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 234Th-based approach along a transect through the mid-Atlantic from 35°S to 10°N during May and June of 1996. The median residence time of dissolved 234Th along the transect was 135 days, with intense scavenging from 5°N to 6°N (τ̄d=86±17 days). Vertical profiles and depth-integrated samples (surface-100 m) of dissolved (<0.7-μm), suspended particulate (0.7–53-μm), and large particulate (>53-μm) 234Th activities were collected to estimate export fluxes of particulate organic carbon (POC). A steady-state model of the 234Th activity balance in the upper 100 m was combined with measurements of the POC/234Th ratio on >53-μm particles. POC export ranged from 1.5 mmol C m−2 d−1 in the central South Atlantic to 31 mmol C m−2 d−1 at 2.5°S. Export production along the Atlantic equator (5°S–5°N, 25°E–5°W) is estimated to be 0.22 Gt C yr−1. The 234Th-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.