Thorium-234 is increasingly used as a tracer of ocean particle flux, primarily as a means to estimate particulate organic carbon export from the surface ocean. This requires determination of both the 234 Th activity distribution (in order to calculate 234 Th fluxes) and an estimate of the C / 234 Th ratio on sinking particles, to empirically derive C fluxes. In reviewing C / 234 Th variability, results obtained using a single sampling method show the most predictable behavior. For example, in most studies that employ in situ pumps to collect size fractionated particles, C / 234 Th either increases or is relatively invariant with increasing particle size (size classes N 1 to 100s Am). Observations also suggest that C / 234 Th decreases with depth and can vary significantly between regions (highest in blooms of large diatoms and highly productive coastal settings). Comparisons of C fluxes derived from 234 Th show good agreement with independent estimates of C flux, including mass balances of C and nutrients over appropriate space and time scales (within factors of 2-3). We recommend sampling for C / 234 Th from a standard depth of 100 m, or at least one depth below www.elsevier.com/locate/marchem the mixed layer using either large volume size fractionated filtration to capture the rarer large particles, or a sediment trap or other device to collect sinking particles. We also recommend collection of multiple 234 Th profiles and C / 234 Th samples during the course of longer observation periods to better sample temporal variations in both 234 Th flux and the characteristic of sinking particles. We are encouraged by new technologies which are optimized to more reliably sample truly settling particles, and expect the utility of this tracer to increase, not just for upper ocean C fluxes but for other elements and processes deeper in the water column. D
Sinking particulate organic matter (POM) is the major vehicle for exporting carbon from the sea surface to the ocean interior. Collection of sinking material in a manner that enables identification of the underlying mechanistic controls on settling and decomposition is fundamental for quantifying the vertical flux and changing composition of particles. Observations made during the Joint Global Ocean Flux Study (JGOFS) indicate that 50% to 80% of the vertical flux of carbon occurs due to gravitational sinking (e.g., Gardner 2000; Baliño et al. 2001;Fasham et al. 2001). During its transit toward the sea floor, most (usually >90%) particulate organic carbon (POC) is returned to inorganic form and redistributed in the water column. This redistribution determines the depth profile of dissolved CO 2 , including its concentration in the surface mixed layer, and hence the rate at which the ocean can absorb CO 2 from the atmosphere and sequester it in the deep ocean. A quantitative and mechanistic understanding of water column POC remineralization is critical to predicting the response of the global carbon cycle to environmental change.Sediment traps are widely used to quantify vertical fluxes of material and to collect sinking POM for chemical analyses. They have provided proof of spatial, episodic, seasonal and interannual fluxes of POM through the water column, have linked variability in surface water processes to fluxes in the interior of the ocean and to the sea floor, and have shown the intense remineralization and alteration that affects POM in the water column (see papers in Ittekkot et al. 1996; JGOFS Deep-Sea Research volumes, see http://usjgofs.whoi.edu/ publications/deepsearesearch.html). Trap designs have been highly variable, ranging from small cylinders (e.g., Vertex MultiPits, Knauer et al. 1979) to large cones (Parflux traps, Honjo and Doherty 1988); both free-drifting/surface-tethered and bottom-moored traps have been used.Considerable effort has focused on quantifying the material collected in traps. Questions about trap accuracy are related to trap geometry, hydrodynamic biases, trapping efficiencies and flux calibration, use of poisons/preservatives, and zooplankton "swimmers." Accuracy of traps is especially questionable in surface waters and regimens where current-driven shear is high or where swimmers are abundant (e.g., Lee et al. 1988;Ittekkot et al. 1996;Gardner 2000
AbstractThree new approaches for collecting and processing sinking particles for biogeochemical studies are presented. They include (1) a modification of our existing indented rotating sphere carousel (IRSC) sediment trap design from its conventional time-series function to one that collects particles based on discrete particle settling-velocity ranges; (2) development of a large, free-floating NetTrap based on the design of a closing plankton net capable of collecting large amounts (~1 g) of very fresh sinking particulate material in short time periods (24-36 h) to facilitate microbial decomposition experiments; and (3) an elut...
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