HR: 0830h
AN: OS31C-0220    [PDF]
TI: Vertical Fluxes of Particulate Trace Metals From the Upper Ocean Using $^{234}$Th/$^{238}$U Disequilibrium
AU: * Weinstein, S E
EM: sarahw@gso.uri.edu
AF: Graduate School of Oceanography, University of Rhode Island, South Ferry Rd., Narragansett, RI 02882-1197 United States
AU: Moran, S B
EM: moran@gso.uri.edu
AF: Graduate School of Oceanography, University of Rhode Island, South Ferry Rd., Narragansett, RI 02882-1197 United States
AU: Edmonds, H N
EM: edmonds@utmsi.utmsi.utexas.edu
AF: Marine Sciences Institute, University of Texas at Austin, 750 Channel View Drive, Port Aransas, TX 78373-5015 United States
AU: Smith, J N
EM: SmithJN@mar.dfo-mpo.gc.ca
AF: Bedford Institute of Oceanography, P.O. Box 1006, Dartmouth, NS B2Y4A2 Canada
AB: An empirical technique to quantify the vertical flux of particulate trace metals from the upper ocean using simultaneous measurement of $^{234}$Th/$^{238}$U disequilibrium and metal/$^{234}$Th ratios on large, rapidly sinking particles is presented. $^{234}$Th-derived particulate metal fluxes represent a bulk metal flux, which includes both scavenged and refractory metal fractions, and metals derived from external input (e.g. atmospheric input). Study sites were selected to include shelf-slope (Gulf of Maine-Scotian Shelf) and open ocean waters (Labrador Sea). In shelf waters, particulate Al/$^{234}$Th, Fe/$^{234}$Th, and Pb/$^{234}$Th ratios in $>$53-$\mu$m particulate matter increase with depth toward the sediment interface. Sinking fluxes of particulate Al (8.91-197 $\mu$mol m$^{-2}$ d$^{-1}$), Fe (25.5-151 $\mu$mol m$^{-2}$ d$^{-1}$), and Pb (13.4-79.0 nmol m$^{-2}$ d$^{-1}$) in shelf waters were calculated at 50 m, a depth chosen to limit the influence of sediment resuspension on the calculated particulate metal flux. For slope waters, fluxes of particulate Al (5.92-96.1 $\mu$mol m$^{-2}$ d$^{-1}$), Fe (2.20-59.3 $\mu$mol m$^{-2}$ d$^{-1}$), and Pb (2.77-89.7 nmol m$^{-2}$ d$^{-1}$) were calculated at 50 m (mixed layer) and 100 m (total $^{234}$Th deficit). In the Labrador Sea, particle size-fractionated (0.4-10-$\mu$m, 10-53-$\mu$m, and $>$53-$\mu$m) Al/$^{234}$Th, Fe/$^{234}$Th, Ba/$^{234}$Th, and Pb/$^{234}$Th ratios in the upper 250 m exhibit an increase with depth and are essentially invariant with particle size. Sinking fluxes of particulate Al (2.47-95.9 $\mu$mol m$^{-2}$ d$^{-1}$), Fe (2.69-60.8 $\mu$mol m$^{-2}$ d$^{-1}$), Ba (0.13-2.91 $\mu$mol m$^{-2}$ d$^{-1}$), and Pb (2.85-59.5 nmol m$^{-2}$ d$^{-1}$) at this open ocean site were calculated at 50 m (euphotic zone) and 100 m (mixed layer). The $^{234}$Th-derived trace metal fluxes are within the range or approximately two to four times greater than available published sediment trap results compiled for the world ocean.
DE: 4825 Geochemistry
DE: 4860 Radioactivity and radioisotopes
DE: 4875 Trace elements
SC: Ocean Sciences [OS]
MN: 2003 Fall Meeting