Near-bottom 230Th and 231Pa Scavenging and Anomalous 230Th Burial Rates in the South Pacific

Martin Q Fleisher1, Frank J Pavia2, Robert F Anderson3, Sebastian M Vivancos4, Jordan T Abell5 and Gisela Winckler4, (1)Lamont-Doherty Earth Observatory, Columbia University, Palisades, NY, United States, (2)Lamont-Doherty Earth Obs, Palisades, United States, (3)Lamont-Doherty Earth Obs, Palisades, NY, United States, (4)Columbia University, Department of Earth and Environmental Sciences, New York, NY, United States, (5)Lamont-Doherty Earth Observatory, Palisades, NY, United States
Abstract:
Radiogenic thorium (230Th) and protactinium (231Pa) are commonly applied as paleoceanographic tracers of particle flux, sedimentation, and circulation, owing to their insolubility and uniform production rates by uranium decay. The burial rate of 230Th in seafloor sediments is assumed to be constant in space and time, equal to its vertically-integrated water column production rate, allowing for its use as a “constant flux proxy” to reconstruct sediment mass accumulation rates. However, the assumption that 230Th in seawater does not experience net lateral redistribution by advection and eddy diffusion is infrequently tested.

We document depletions of dissolved 230Th and 231Pa relative to concentrations expected from reversible scavenging in the deep southeast Pacific extending thousands of meters above the seafloor, within the semi-enclosed Peru Basin and Bauer Basin, using measurements from the GEOTRACES GP16 transect and the UltraPac transect (SO245, GEOTRACES process study GPpr09). The removal signatures of 230Th and 231Pa reflect distal scavenging by hydrothermal particles and resuspended bottom sediments as waters flow into these basins. The confluence of weak near-bottom stratification with anomalous scavenging suggests that 230Th and 231Pa may find use as tracers of recent seafloor contact and incrop areas of water masses. By measuring 230Th burial rates in co-located coretop sediments, we find that these benthic scavenging processes drive anomalously low sedimentary 230Th fluxes, and discuss the implications for the use of 230Th as a constant flux proxy in the southeast Pacific.