The Younger Dryas and the 8200-Year Cold Event in the Southern Hemisphere Subtropics

Understanding the mechanisms driving natural millennial-scale climate change is critical for distinguishing human impacts on the climate system from its inherent variability. We investigated Holocene variations in Southern Hemisphere subtropical sea-surface temperatures using Mg/Ca, stable isotopes, and faunal abundances of planktonic foraminifera from Ocean Drilling Program (ODP) Site 1084B off the coast of Namibia (23.5S, 13.0E, 1992m water depth). Mg/Ca measurements were made at 1-cm intervals by ICP-OES on ~75 individuals of G. bulloides, a species associated with winter upwelling in this region. 13 AMS radiocarbon dates extending to 17,000 calendar years ago constrain the age model. Coretop Mg/Ca values indicate calcification in 13.6°C waters, consistent (within analytical error) with observed winter SSTs for this location. Site 1084B has high deglacial and Holocene sedimentation rates (11-27 cm/kyr) and is located within the Benguela coastal upwelling system. Historical SST variations in this region reflect variations in the intensity and zonality of the trade winds, as well as the northward advection of cooler waters by the subtropical gyre: faunal abundance data help distinguish these two influences on the Mg/Ca temperature record.

The "8200 year cold event" appears in this record as an abrupt 1.5°C cooling at 8100 yr ago, accompanied by shifts in faunal abundances that suggest increased upwelling. A nearby Holocene paleotemperature record from a non-upwelling region does not show this cooling event (Kim et al. 2002), suggesting that in the Southern Hemisphere subtropics, this event was driven more by changes in wind strength than changes in ocean circulation. The 1084B Mg/Ca record also exhibits the Younger Dryas cool period 13-11.5 kyr ago, indicating that subtropical Southern Hemisphere deglacial climate variations were in-phase with the Northern Hemisphere. This is consistent with several other tropical, subtropical, and midlatitude Southern Hemisphere records (Kim et al. 2002, Kirst et al. 1999, Little et al. 1997, and Moreno et al. 2001). High-latitude records, however, show the "Antarctic Cold Reversal" preceding Younger Dryas cooling (Blunier et al. 1998, Charles et al. 1996). Perhaps this spatial pattern is explained by changes in ocean circulation (Stocker et al. 2000). The 1084B record does not show Younger Dryas faunal abundance shifts indicating changes in upwelling, as seen at the 8200 year cold event. This suggests that different mechanisms caused these two cooling events.