Distribution of dissolved and particulate 230Th and 232Th in seawater from the Gulf of Mexico and off Cape Hatteras as measured by SIMS
We present vertical and horizontal profiles of dissolved and particulate 230Th and 232Th 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-lived 230Th between particles and seawater with that of short-lived 234Th. Concentrations of dissolved and particulate 230Th and 232Th 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 of 230Th and 232Th in ocean margin regions. Sea water samples collected off Cape Hatteras contained significantly higher dissolved and particulate 230Th and 232Th concentrations than those found in the open Gulf of Mexico. Likely, the distributions of 230Th and 232Th in seawater off Cape Hatteras are, to a large extent, controlled by terrestrial particles. In contrast, in-situ production (for 230Th) and scavenging are the major processes regulating the concentrations of dissolved 230Th and 232Th 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) for 234Th and 230Th xs in both study areas, demonstrated a "particle-concentration effect" down to very low Cp concentrations (∼ 20 μg/l). Based on the 230Th inventory, in-situ production, and the small estimated surface input, the overall residence time of 230Th in the water column at a slope station in the Gulf of Mexico is calculated to be about 5 yr.