Particle sinking dynamics and POC fluxes in the Eastern Tropical South Pacific based on 234Th budgets and sediment trap deployments

Abstract

Assuming steady-state over seasonal to annual timescales, and limited horizontal export of dissolved nutrients, the vertical fluxes of limiting nutrients into the euphotic zone should be balanced by particle export. Sediment traps and 234Th budgets have both been used extensively throughout the oceans as a means to measure this particulate flux from the upper ocean. One main goal of these efforts has been to determine the amount of CO2 fixed by primary producers in the surface ocean that is exported as particulate organic carbon (POC) and conversely, the decrease of particle flux with depth has been used to estimate remineralization rates of nutrients. Although disagreement between trap-derived and 234Th-derived fluxes has often been noted, the possible reasons for the imbalance are numerous, and thus often it is difficult to assign causes. Here, we examine many commonly implicated contributors to the disagreement, allowing us to assess data from a recent 2-year study in the ETSP that shows systematic disagreement between the two methods. Averaging results from both years, sediment traps collected 0.2–1.5mmolCm−2d−1 (mean: 0.74mmolCm−2d−1) of POC, while the thorium-based method estimated an average POC flux of 1.5–14mmolCm−d−1 (mean: 6.2mmolCm−2d−1). The study area spans regions of differing ecological structure, as inferred from trap mineralogy, and the flux disagreement coincides with this ecological range. We interpret the difference as undercollection of poorly ballasted, slowly sinking particles by the sediment traps. Using both methods simultaneously offers insight into ecosystem structure and resulting particle flux dynamics. The thorium deficit-based flux is 5–10% of previously published estimates of primary productivity based on 14C incubations (Pennington et al., 2006), and 8–20% of concurrent estimates based on 14C incubations and oxygen supersaturation (Capone et al., personal communication; Prokopenko et al., personal communication).