Abrupt changes in deep Atlantic circulation during the transition to full glacial conditions

Six Ocean Drilling Program (ODP) sites, in the Northwest Atlantic have been used to investigate kinematic and chemical changes in the Western Boundary Undercurrent (WBUC) during the development of full glacial conditions across the Marine Isotope Stage 5a/4 boundary (similar to 70,000years ago). Sortable silt mean grain size ((S) over barS) measurements are employed to examine changes in near bottom flow speeds, together with carbon isotopes measured in benthic foraminifera and % planktic foraminiferal fragmentation as proxies for changes in water-mass chemistry. A depth transect of cores, spanning 1.8-4.6km depth, allows changes in both the strength and depth of the WBUC to be constrained across millennial scale events. (S) over barS) measurements reveal that the flow speed structure of the WBUC during warm intervals (interstadials) was comparable to modern (Holocene) conditions. However, significant differences are observed during cold intervals, with higher relative flow speeds inferred for the shallow component of the WBUC (similar to 2km depth) during all cold stadial intervals (including Heinrich Stadial 6), and a substantial weakening of the deep component (similar to 3-4km) during full glacial conditions. Our results therefore reveal that the onset of full glacial conditions was associated with a regime shift to a shallower mode of circulation (involving Glacial North Atlantic Intermediate Water) that was quantitatively distinct from preceding cold stadial events. Furthermore, our chemical proxy data show that the physical response of the WBUC during the last glacial inception was probably coupled to basin-wide changes in the water-mass composition of the deep Northwest Atlantic.