Radiochemical Constraints on Ocean Circulation During the Last Glacial Maximum

Y. Ein-Fen Dept. of Earth Sciences, National Taiwan Normal University, Taipei, 117 Taiwan

R. Francois Dept. of Marine Chemistry and Geochemistry, W.H.O.I., Woods Hole, MA 02543, USA

M. P. Bacon Dept. of Marine Chemistry and Geochemistry, W.H.O.I., Woods Hole, MA 02543, USA

231Pa (t1/2=3.2 x 104y) and 230Th (t1/2=7.5 x 104y) are produced in the water column at a constant activity ratio of 0.093 from the decay of dissolved U. Thorium is extremely particle-reactive and has a very short residence time in the water column
(ca. 25 years). Consequently, very little 230Th can be transported laterally prior to scaveng-ing, and the removal of 230Th is
dominated by the vertical flux on settling particles. In comparison, 231Pa is less particle-reactive and has a longer residence time in the Atlantic water column (ca. 110 years). It can thus be transported laterally over longer distance before removal into the underlying sediments. Reflecting this contrasting reactivity with respect to scavenging, modern thermohaline circulation exports half of the 231Pa produced in the Atlantic water column into the southern ocean, but exports only 10% of the 230Th. This export imparts low ex231Pa/ex230Th ratios (<0.093) to the Atlantic sediments and high ex231Pa/ex230Th rations (>0.093) to the
southern ocean sediments.

The contracts in ex231Pa/ex230Th ratios between the two oceans depends on the relative scavenging intensity in both oceans and the rate of thermohaline circulation. If we can estimate past changes in the mean scanvenging rate of 231Pa from past changes in particle flux, the sedimentary record of ex231Pa/ex230Th ratios would constrain past changes in thermohaline
circulation. We have applied this approach to distinguish between the different deep water circulation scenarios which have been proposed, based on paleo-nutrient proxies. We have found that the 231Pa/230Th ratios in glacial Atlantic sediments were as low as during the Holocene, while the scavenging rates of 231Pa were somewhat higher, implying that glacial North Atlantic Deep/Intermediate Water (GNAIW) was exported from the Atlantic at a rate similar to or slightly higher than for the modern North Atlantic Deep Water (NADW). In addition, high ratios in glacial southern ocean sediments indicate that GNAIW reached the Circumpolar Deep Water (CPDW), which thus remained an integral part of the glacial conveyor belt circulation. 231Pa mass balance calculation for the southern ocean also suggests relatively little 231Pa exchange with the Pacific during both periods.