227Ac along the GEOTRACES Meridional Pacific Transect: Preliminary Results

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 along US GEOTRACES meridional transect in the Pacific (Fall 2018; Alaska-Tahiti) and at a limited number of stations on a CDISK cruise (late Summer, 2017; Honolulu to Alaska) by pumping water through filter cartridges impregnated with MnO₂, to trap Ac Th, and Ra. Two cartridges were deployed in series to estimate extraction efficiency for each sample. While analysis of the 2018 samples is still in progress, results to date are comparable to the 2017 samples, and several features of the measurements in the N. Pacific are apparent.

Stations between Hawaii and Alaska typically have a bottom layer ~500 m thick, with modest density stratification and relatively uniform 227Ac, consistent with vertical diffusivities > 20-30 cm2/s. Above this layer, density stratification increases and vertical eddy diffusivity appears to be <1 cm2/s, although accurate estimates cannot be made until 231Pa measurements have been completed. Analyses of 231Pa and 227Ac profiles in sediments are underway to quantify the benthic flux of 227Ac; this should help to establish the significance of horizontal transport of 227Ac. Preliminary results from comparing water column inventories to benthic fluxes indicate horizontal transport is significant, particularly north of 40° N.