Multiple Modes of Orbital-Scale Change in Thermohaline Circulation

Principal component analysis of Late Pleistocene δ13C and δ18O time series from approximately 30 globally distributed marine sites reveals two independent modes of thermohaline circulation change associated with different orbital frequencies. Most paleoclimate studies of deep ocean circulation have either reconstructed only a few points in time (e.g., comparing the Last Glacial Maximum and the Holocene) or have examined the temporal evolution of relatively few (3-6) paleoceanographic records. These techniques have produced a simple model of circulation change in which North Atlantic Deep Water (NADW) shoals during glacial conditions and is replaced at depth by water from the Southern Ocean. Our analysis is consistent with NADW shoaling proportional to ice volume change over the last four glacial cycles. However, our analysis also reveals a second, independent mode of circulation change with more precessional power. Spatial gradients in δ18O also indicate orbital-scale changes in the physical properties of deep water masses.