Uranium-Series Tracers of Mudwave Migration in the Argentine Basin

Large wave-like sediment structures are formed in the Argentine Basin where active bottom currents carry a heavy load of suspended sediments. A model previously developed to explain the formation and migration of these mudwaves predicts that sediment deposition occurs on the upstream face of the wave while the downstream face experiences reduced deposition, or erosion, depending on the current velocity. Two mudwaves on the Zapiola sediment drift in the Argentine Basin were cored to evaluate the relationship between present-day currents and the migration of these particular structures. Pb-210 and Th-230 were measured in sediments from upstream and downstream faces of each mudwave. Pb-210 inventories ranged from 33 to 118 dpm cm-2 among the various locations cored. The general tendency is for higher inventories of Pb-210 to occur on upstream faces, indicating higher rates of sediment deposition there, as predicted by the model. Over the time scale reflected by the Pb-210 inventories, sediments have accumulated on the downstream faces at rates at least one third to one half as high as on the upstream faces, which rules out complete non-deposition on the downstream faces. Unsupported Th-230 activities of upstream-face sediments are a factor of 3-4 higher than activities of sediments on downstream faces, indicating that late Pleistocene sediments are being exposed by erosion on the downstream faces. Taken together, these results suggest a situation on the downstream face of each mudwave where fresh particulate matter deposited from the water column is being worked into mixed layer sediments under conditions of net sediment erosion.