Sedimentology, organic geochemistry, and stable isotopes of core PS2138-1

We studied variations in terrigenous (TOM) and marine organic matter (MOM) input in a sediment core on the northern Barents Sea margin over the last 30 ka. Using a multiproxy approach, we reconstructed processes controlling organic carbon deposition and investigated their paleoceanographic significance in the North Atlantic-Arctic Gateways. Variations in paleo-surface-water productivity are not documented in amount and composition of organic carbon. The highest level of MOM was deposited during 25-23 ka as a result of scavenging on fine-grained, reworked, and TOM-rich material released by the retreating Svalbard/Barents Sea ice sheet during the late Weichselian. A second peak of MOM is preserved because of sorptive protection by detrital and terrigenous organic matter, higher surface-water productivity due to permanent intrusion of Atlantic water, and high suspension load release by melting sea ice during 15.9-11.2 ka.

Stable oxygen isotope and Mg/Ca ratios of planktonic foraminifera from the Caribbean and North Atlantic

Variations in the strength of the North Atlantic Ocean thermohaline circulation have been linked to rapid climate changes during the last glacial cycle through oscillations in North Atlantic Deep Water formation and northward oceanic heat flux. The strength of the thermohaline circulation depends on the supply of warm, salty water to the North Atlantic, which, after losing heat to the atmosphere, produces the dense water masses that sink to great depths and circulate back south. Here we analyse two Caribbean Sea sediment cores, combining Mg/Ca palaeothermometry with measurements of oxygen isotopes in foraminiferal calcite in order to reconstruct tropical Atlantic surface salinity during the last glacial cycle. We find that Caribbean salinity oscillated between saltier conditions during the cold oxygen isotope stages 2, 4 and 6, and lower salinities during the warm stages 3 and 5, covarying with the strength of North Atlantic Deep Water formation. At the initiation of the Bølling/Allerød warm interval, Caribbean surface salinity decreased abruptly, suggesting that the advection of salty tropical waters into the North Atlantic amplified thermohaline circulation and contributed to high-latitude warming.

Grain size analysis of ODP Site 162-984

Past changes in the freshwater balance of the surface North Atlantic Ocean are thought to have influenced the rate of deep-water formation, and consequently climate (Broecker and Denton, 1989, doi:10.1016/0016-7037(89)90123-3; Manabe and Stouffer, 1996; doi:10.1038/378165a0). Although water-mass proxies are generally consistent with an impact of freshwater input on meridional overturning circulation (Boyle and Keigwin, 1987, doi:10.1038/330035a0), there has been little dynamic evidence to support this linkage. Here we present a 25,000 year record of variations in sediment grain size from south of Iceland, which indicates vigorous bottom-water currents during both the last glacial maximum and the Holocene period. Together with reconstructions of North Atlantic water-mass distribution, vigorous bottom currents suggest a shorter residence time of northern-source waters during the last glacial maximum, relative to the Holocene period. The most significant reductions in flow strength occur during periods that have been associated with freshening of the surface North Atlantic. The short-term deglacial oscillations in bottom current strength are closely coupled to changes in Greenland air temperature, with a minimum during the Younger Dryas cold reversal and a maximum at the time of rapid warming at the onset of the Holocene. Our results support a strong connection between ocean circulation and rapid climate change.