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
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We present vertical and horizontal profiles of dissolved and particulate230Th and232Th concentrations in seawater from shelf and slope areas of the Gulf of Mexico and off Cape Hatteras in the Middle Atlantic Bight, as measured by secondary ion mass spectrometry (SIMS), and compare the partitioning of long-lived230Th between particles and seawater with that of short-lived234Th. Concentrations of dissolved and particulate230Th and232Th in surface seawater decreased from near-shore to off-shore in the Gulf of Mexico, demonstrating terrestrial input to the surface waters. Vertical profiles indicate that lateral transport, as well as surface inputs (e.g., fluvial and aeolian) and bottom sediment resuspension, could be an important source of230Th and232Th in ocean margin regions. Sea water samples collected off Cape Hatteras contained significantly higher dissolved and particulate230Th and232Th concentrations than those found in the open Gulf of Mexico. Likely, the distributions of230Th and232Th in seawater off Cape Hatteras are, to a large extent, controlled by terrestrial particles. In contrast, in-situ production (for230Th) and scavenging are the major processes regulating the concentrations of dissolved230Th and232Th in the open Gulf of Mexico. The observed negative correlation between log Kd (particle-solution distribution coefficient) and log Cp (concentration of suspended particulate matter) for234Th and230Thxs in both study areas, demonstrated a "particle-concentration effect" down to very low Cp concentrations ( 20 g/l). Based on the230Th inventory, in-situ production, and the small estimated surface input, the overall residence time of230Th in the water column at a slope station in the Gulf of Mexico is calculated to be about 5 yr. 1995 Elsevier Science B.V. All rights reserved.
