Thorium-234 as a tracer of spatial, temporal and vertical variability in particle flux in the North Pacific

An extensive Th-234 data set was collected at two sites in the North Pacific: ALOHA, an oligotrophic site near Hawaii, and K2, a mesotrophic HNLC site in the NW Pacific as part of the VERTIGO (VERtical Transport In the Global Ocean) study. Total Th-234:U-238 activity ratios near 1.0 indicated low particle fluxes at ALOHA, while Th-234:U-238 similar to 0.6 in the euphotic zone at K2 indicated higher particle export. However, spatial variability was large at both sites-even greater than seasonal variability as reported in prior studies. This variability in space and time confounds the use of single profiles of Th-234 for sediment trap calibration purposes. At K2, there was a decrease in export flux and increase in Th-234 activities over time associated with the declining phase of a summer diatom bloom, which required the use of non-steady state models for flux predictions. This variability in space and time confounds the use of single profiles of Th-234 for sediment trap calibration purposes. High vertical resolution profiles show narrow layers (20-30 m) of excess Th-234 below the deep chlorophyll maximum at K2 associated with particle remineralization resulting in a decrease in flux at depth that may be missed with standard sampling for Th-234 and/or with sediment traps. Also, the application of Th-234 as POC flux tracer relies on accurate sampling of particulate POC/Th-234 ratios and here the ratio is similar on sinking particles and mid-sized particles collected by in-situ filtration (>10-50 mu m at ALOHA and >5-350 mu m at K2). To further address variability in particle fluxes at K2, a simple model of the drawdown of Th-234 and nutrients is used to demonstrate that while coupled during export, their ratios in the water column will vary with time and depth after export. Overall these Th-234 data provide a detailed view into particle flux and remineralization in the North Pacific over time and space scales that are varying over days to weeks, and 10's-100's km at a resolution that is difficult to obtain with other methods. (C) 2009 Elsevier Ltd. All rights reserved.