227Ac in the Deep South Pacific along the Peru-Tahiti GEOTRACES Transect: Mixing and Transport Rates
Douglas E Hammond1, Matthew A Charette2, Willard S Moore3, Paul Henderson2, Virginie Sanial4,5, Lauren Elizabeth Kipp2, Robert F Anderson6 and Francois Primeau7, (1)University of Southern California, Los Angeles, CA, United States, (2)Woods Hole Oceanographic Institution, Woods Hole, MA, United States, (3)University of South Carolina Columbia, Columbia, SC, United States, (4)WHOI, Woods Hole, MA, United States, (5)LEGOS Laboratoire d'Etudes en Geophysique et Océanographie Spatiale, TOULOUSE, France, (6)Columbia University of New York, Palisades, NY, United States, (7)University of California Irvine, Irvine, CA, United States
Abstract:
227Ac (22 yr half life) diffuses from sediment and is mixed vertically and horizontally as it decays, providing a distribution that can be used to infer transport rates for other solutes in the deep ocean. Profiles were collected during the fall of 2013 at 19 stations along the US Peru-Tahiti GEOTRACES transect by pumping water through acrylic cartridges impregnated with MnO2, to trap Ac Th, and Ra. Extraction efficiency has been found to vary in past efforts, so two cartridges were deployed in series to allow more accurate estimation of extraction efficiency for each sample. Supported activity in the water column has been determined through measurement of its 231Pa parent, a data set that will soon be completed.
Based on data in hand, profiles of excess 227Ac indicate several features of transport that are important across this transect: 1) Apparent vertical mixing rates from 1-D fits decrease as the water column stratification becomes more pronounced; 2) Integrated excess 227Ac in the water column increases with increasing distance from the East Pacific Rise (EPR), suggesting that either the source function changes, or lateral diffusion dominates vertical diffusion as the EPR is approached, a result of increasing stratification in the waters near the ridge crest; 3) Most profiles show elevated activities near 2500-2600 m depth (sigma-theta 27.72), just below the 3He anomaly, indicating a source of 227Ac in the hydrothermal plume that can be traced as far as 152°W. The observed decrease can be used to find a 1-D horizontal velocity equivalent to about 0.4 cm/s, requiring about 40 years to travel from the EPR to 152°W. Additional work is in progress to integrate 227Ac results with 228Ra results for these samples, and to quantify relative rates of transport with a 2-D model.
