CONSTRAINING WATER MASS TRANSPORT DURING MILLENNIAL-SCALE GLACIAL CLIMATE CHANGES

The use of the magnetic properties of sediments as tracers for past environmental changes plays an increasing role in marine environments. Coupling them with climatically independent variations in the intensity of the earth magnetic field to correlate widely separated oceanic records, allows to determine and quantify leads and lags between climatic events.

We report on the analyses of magnetic properties of sediments deposited during the glacial time and distributed in North Atlantic, a key region to study past changes in the global thermohaline circulation. Large amplitude, short-term fluctuations are observed in the concentration of magnetic particles in North Atlantic sediment cores during marine isotopic stage 3 (27 kyr to 60 kyr B.P.), caused by the changes in the bottom current transport of magnetic particles. The progressive southward decrease in the average concentration of magnetic particles indicates that the signal initiated in the northern basaltic provinces (Iceland-Faeroe Islands) was propagated to the Bermuda Rise by the flow of North Atlantic Deep Water (NADW). Stacking multiple high resolution records, after normalizing to their mean value, filters out possible local differences in flow speed. When placed on the GISP2 age scale using stable isotopic data, the fluctuations in NADW strength match Greenland paleotemperature changes with high fidelity.

The agreement between the northern hemisphere magnetic property-based NADW flow speed and the southern hemisphere Nd isotope NADW export record is remarkable. Warm (cold) periods were accompanied by increased (decreased) flow speed of NADW and relatively more (less) NADW reaching the Southern Ocean.

The difference in the two proxy record resolutions and the time constants on the order of 400-500 years of the fluctuations of the dipolar field used for the time scale transfer, are challenging quantitative estimates of phasing differences. However, the use of geomagnetic-assisted stratigraphy shows that during the most prominent and best defined cycles, the south Atlantic record lags changes in North Atlantic flowspeeds by about 600-800 years. This significant offset may document the needed time for glacial circulation changes in the North Atlantic to propagate to the Southern Ocean.