DISTRIBUTION OF DISSOLVED AND PARTICULATE TH-230 AND TH-232 IN SEAWATER FROM THE GULF-OF-MEXICO AND OFF CAPE-HATTERAS AS MEASURED BY SIMS

We present vertical and horizontal profiles of dissolved and particula te Th-230 and Th-232 concentrations in seawater from shelf and slope a reas of the Gulf of Mexico and off Cape Hatteras in the Middle Atlanti c Eight, as measured by secondary ion mass spectrometry (SIMS), and co mpare the partitioning of long-lived Th-230 between particles and seaw ater with that of short-lived Th-234. Concentrations of dissolved and particulate Th-230 and Th-232 in surface seawater decreased from near- shore to off-shore in the Gulf of Mexico, demonstrating terrestrial in put to the surface waters. Vertical profiles indicate that lateral tra nsport, as well as surface inputs (e.g., fluvial and aeolian) and bott om sediment resuspension, could be an important source of Th-230 and T h-232 in ocean margin regions. Sea water samples collected off Cape Ha tteras contained significantly higher dissolved and particulate Th-230 and Th-232 concentrations than those found in the open Gulf of Mexico . Likely, the distributions of Th-230 and Th-232 in seawater off Cape Hatteras are, to a large extent, controlled by terrestrial particles. In contrast, in-situ production (for Th-230) and scavenging are the ma jor processes regulating the concentrations of dissolved Th-230 and Th -232 in the open Gulf of Mexico. The observed negative correlation bet ween log K-d (particle-solution distribution coefficient) and log C-p (concentration of suspended particulate matter) for Th-234 and Th-230( xs) in both study areas, demonstrated a ''particle-concentration effec t'' down to very low C-p concentrations (similar to 20 mu g/l). Based on the Th-230 inventory, in-situ production, and the small estimated s urface input, the overall residence time of Th-230 in the water column at a slope station in the Gulf of Mexico is calculated to be about 5 yr.