Reconstruction of the End Member Contributions to and Ventilation Rate of Glacial-Age Lower Atlantic Deep Water

ABSTRACT

OCE-0221979

Reorganizations of large-scale ocean circulation very likely triggered the large and abrupt climate changes which punctuated the glacial world. Hence, as part of our preparation to deal with the ongoing greenhouse gas buildup, we must learn as much as possible regarding the nature of the ocean's alternate modes of operation. This project is aimed toward doing just this.

Two paleo observations for the glacial deep Atlantic fail to jibe. One is the difference between the 14 C to C ages for coexisting benthic and planktonic foraminifera, which was greater than that for pre-industrial time but still far less than that for the deep Pacific. This suggests that the conveyor circulation was operating at a somewhat reduced but still significant rate. The other is the tilt of density horizons across the Florida Straits reconstructed from 18O measurements on benthic foraminifera which suggest that the strength of conveyor circulation was far less than today's.

In the absence of a clear way to reconcile these observations with one another and also with the nutrient content reconstructed for glacial age lower deep water, researchers at the Lamont Dougherty Eart Observatory of Columbia University will attempt to improve upon both the paleonutrient reconstruction and the estimates of benthic-planktonic age difference. To do so, they have identified 21 cores in the LDEO collection forming a traverse sampling along the western flank of the Mid Atlantic Ridge extending from 45'N to 45'S (water depth 3.6 + 0.4 km). They will establish the stratigraphy in each core by 14C and 18O measurements on planktonic foraminifera and also attempt to quantify the extent to which Holocene foraminifera have been mixed downward into the late glacial section by analyzing single benthic foraminifera for 18O. They will then do d18O, d13C, Cd, and Zn measurements on benthic foraminifera, and neodymium isotope measurements on dispersed ferromanganese coatings on fine silicate debris, in late glacial sections for all cores. Additionally they will measure benthic-planktonic 14C age differences on late glacial samples from the cores with the highest sedimentation rate.