HR: 10:35h
AN: PP22A-02 INVITED     [Abstracts]
TI: Reconstructing Paleocirculation Using d13C and 231Pa/230Th
AU: * mcmanus, j
EM: jmcmanus@whoi.edu
AF: woods hole oceanographic institution, dept. g&g, woods hole, MA 02543 United States
AU: francois, r
EM: rfrancois@whoi.edu
AF: woods hole oceanographic institution, dept. mc&g, woods hole, ma 02543 United States
AU: major, c o
EM: cmajor@whoi.edu
AF: woods hole oceanographic institution, dept. g&g, woods hole, MA 02543 United States
AU: gherardi, j
EM: jgherardi@whoi.edu
AF: woods hole oceanographic institution, dept. g&g, woods hole, MA 02543 United States
AU: gherardi, j
EM: jgherardi@whoi.edu
AF: woods hole oceanographic institution, dept. mc&g, woods hole, ma 02543 United States
AU: oppo, d w
EM: doppo@whoi.edu
AF: woods hole oceanographic institution, dept. g&g, woods hole, MA 02543 United States
AB: Past changes in ocean circulation may be estimated from reorganization of subsurface water masses as indicated by nutrient proxies such as d13C. These reconstructions are invaluable, yet the changes they reveal are not necessarily related to the climatically significant transport of heat. Better estimates of this transport may be made using dynamical proxies of the rate of overturning. The burial ratio of unsupported 231Pa: 230Th in bulk North Atlantic sediments, age-corrected to the time of deposition, provides such a proxy. Uranium is well mixed in the ocean, and its decay produces 231Pa and 230Th at a constant activity ratio (0.093). Because the residence time in seawater with respect to removal by particles of Th is on the order of decades and that of Pa on the order of centuries, subsurface advection leads to a deficit in buried 231Pa/230Th in the North Atlantic. Changes in this burial ratio thus vary with the residence time of water in the basin, and are inversely proportional to the rate of meridional overturning and export of subsurface waters. The combination of d13C and 231Pa/230Th can therefore provide more complete insight into past circulation changes. Glacial, deglacial, and interglacial paleo-circulation configurations and rates have now been investigated with this paired approach. Large changes in water mass structure characterize the Atlantic Ocean at the last glacial maximum (LGM), yet the overall rate of meridional overturning was reduced by no more than approximately one third. In contrast, briefer millennial-scale reductions were larger in scale and were accompanied by dramatic climate changes in the North Atlantic region. Evidence from d13C suggests similarly dramatic changes during the glacial period and even during the Holocene. Only the glacial and deglacial events appear to have comparable impact on the rate of meridional overturning. Taken together, the results point to the transient nature of the response to forcing by surface buoyancy fluxes and the importance of location in such forcing. The paired results help constrain the effects of additional physical and bio-logical effects on d13C and 231Pa/230Th and confirm the value of these proxies for reconstructing ocean circulation.
DE: 3344 Paleoclimatology
DE: 4267 Paleoceanography
DE: 1836 Hydrologic budget (1655)
DE: 1600 GLOBAL CHANGE (New category)
DE: 1620 Climate dynamics (3309)
SC: Paleoceanography and Paleoclimatology [PP]
MN: 2004 AGU Fall Meeting