Holocene Variations in Upper North Atlantic Deep Water Circulation

The Holocene was punctuated by a series of millennial-scale cooling events which have been documented in subpolar and subtropical settings in several ocean basins. The Little Ice Age (LIA; ca.1300-1850 AD) and the Medieval Warm Period (MWP; ca. 900-1300 AD) comprise the most recent oscillation of Holocene climate variability. Although the timing and succession of these Holocene cooling events are now better known, the corresponding history of deep ocean circulation changes remain poorly understood. The upper component of North Atlantic thermohaline circulation is represented by Labrador Sea Water (LSW), which is exported from formation sites in the Labrador Sea into the North Atlantic at depths between 1-2 km. Hydrographic studies show that LSW is very sensitive to changes in climate at the formation region, and LSW compositional changes are transmitted at depth across the deep Atlantic basin within 4-6 years. We have reconstructed the late Holocene history of LSW export temperature using a multicore/gravity core pair taken from the Laurentian Slope (10-MC/GGC-09; 1850 m) within the modern core of LSW. A temporal framework is provided by AMS radiocarbon dates and counts of hematite-stained ice-rafted grains which reveal millennial-scale cooling events 1-3 and the LIA. Specimens of the benthic foraminifer Cibicides pachyderma were picked and cleaned for Mg/Ca analysis by ICP-AES and for stable isotopic analyses. Mg/Ca data show a striking correspondence to the ice-rafted record, indicating dramatic coolings of LSW (by approximately 3° C) during cold events 2, 1, and the LIA. Benthic δ ¹⁸O shows less variability, suggesting that cooling was largely offset by freshening. By analogy with modern measurements, these cold events imply vigorous convection within the Labrador Sea.