(Table 2) Trace metal ratios of various benthic foraminifera shells

Benthic foraminiferal d13C and Cd/Ca studies suggest that deep Atlantic circulation during the Last Glacial Maximum was very different from today, with high-nutrient (low d13C, high Cd) deep Southern Ocean Water (SOW) penetrating far into the North Atlantic. However, if some glacial d13C values are biased by productivity artifacts and/or air-sea exchange processes, then the existing d13C data may be consistent with the continual dominance of North Atlantic Deep Water (NADW). Cibicidoides wuellerstorfi Cd/Ca results presented here indicate that the glacial North Atlantic was strongly enriched in dissolved Cd below ~2500 m depth. If NADW formation was still vigorous relative to SOW formation, these data could be explained by either increased preformed nutrient levels in the high-latitude North Atlantic or by increased organic matter remineralization within lower NADW. High glacial Zn/Ca values in the same samples, however, are best explained by a substantially increased mixing with Zn-rich SOW. The cause was most likely a partial replacement of NADW by less dense Glacial North Atlantic Intermediate Water. This reorganization also lowered deep North Atlantic [CO3]2- concentrations by perhaps 10 to 15 µmol/kg.

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.

Sea surface temperature reconstruction for the Southwest Pacific Ocean

Paleoceanographic archives derived from 17 marine sediment cores reconstruct the response of the Southwest Pacific Ocean to the peak interglacial, Marine Isotope Stage (MIS) 5e (ca. 125?ka). Paleo-Sea Surface Temperature (SST) estimates were obtained from the Random Forest model-an ensemble decision tree tool-applied to core-top planktonic foraminiferal faunas calibrated to modern SSTs. The reconstructed geographic pattern of the SST anomaly (maximum SST between 120 and 132?ka minus mean modern SST) seems to indicate how MIS 5e conditions were generally warmer in the Southwest Pacific, especially in the western Tasman Sea where a strengthened East Australian Current (EAC) likely extended subtropical influence to ca. 45°S off Tasmania. In contrast, the eastern Tasman Sea may have had a modest cooling except around 45°S. The observed pattern resembles that developing under the present warming trend in the region. An increase in wind stress curl over the modern South Pacific is hypothesized to have spun-up the South Pacific Subtropical Gyre, with concurrent increase in subtropical flow in the western boundary currents that include the EAC. However, warmer temperatures along the Subtropical Front and Campbell Plateau to the south suggest that the relative influence of the boundary inflows to eastern New Zealand may have differed in MIS 5e, and these currents may have followed different paths compared to today.