Particle fluxes associated with mesoscale eddies in the Sargasso Sea

We examined the impact of a cyclonic eddy and mode-water eddy on particle flux in the Sargasso Sea. The primary method used to quantify flux was based on measurements of the natural radionuclide, Th-234, and these flux estimates were compared to results from sediment traps in both eddies, and a Po-210/Pb-210 flux method in the mode-water eddy. Particulate organic carbon (POC) fluxes at 150m ranged 1-4 mmol Cm-2 d(-1) and were comparable between methods, especially considering differences in integration times scales of each approach. Our main conclusion is that relative to summer mean conditions at the Bermuda Atlantic Time-series Study (BATS) site, eddy-driven changes in biogeochemistry did not enhance local POC fluxes during this later, more mature stage of the eddy life cycle (> 6 months old). The absence of an enhancement in POC flux puts a constraint on the timing of higher POC flux events, which are thought to have caused the local O-2 minima below each eddy, and must have taken place > 2 months prior to our arrival. The mode-water eddy did enhance preferentially diatom biomass in its center, where we estimated a factor of three times higher biogenic Si flux than the BATS summer average. An unexpected finding in the highly depth-resolved Th-234 data sets is narrow layers of particle export and remineralization within the eddy. In particular, a strong excess Th-234 signal is seen below the deep chlorophyll maxima, which we attribute to remineralization of Th-234-bearing particles. At this depth below the euphotic zone, de novo particle production in the euphotic zone has stopped, yet particle remineralization continues via consumption of labile sinking material by bacteria and/or zooplankton. These data suggest that further study of processes in ocean layers is warranted not only within, but below the euphotic zone. (C) 2008 Elsevier Ltd. All rights reserved.