Abstract:
ODP Leg 172 Site 1060, located at 74 degrees W, 31 degrees N, 3500 meters water depth on the Blake Bahamas Outer Ridge (western edge of the subtropical Atlantic gyre) records the influences of the Gulf Stream and of North Atlantic Deep Water. At this site, the planktonic foraminiferal fauna demonstrate clear millennial-scale variability during MIS3. This fauna is dominated by warm species during interstadials, with 60% indicating the presence of warm surface water. In contrast, this group only makes up about 30% of the fauna during stadials. An age model has been developed by correlating abrupt shifts in the relative abundance of this faunal group with abrupt changes in the isotopic composition recorded in the Greenland ice (time scale of Johnsen et al. 2001). The percentage of carbonate, which shows high values during interstadials, obtained from the same samples, indicates the same variability. During each Greenland Stadial, Ice Rafted Debris (ird) monitored here by counting quartz grains in the 90-150 micrometer size fraction reached this subtropical location. Between stadials and interstadials SSTs as estimated by foraminiferal faunal analysis using SIMMAX varied by about 3 degrees C. This value is supported by UK 37 SST estimates. During MIS3, both the benthic delta 13C values (which are consistently lighter) and the increased fragmentation of foraminifera (indicative of carbonate dissolution) during each stadial indicate that at 3500 meters water depth the present NADW was replaced by the southerly sourced AABW at millennial time scales. Finally, by using the same proxies we compare these results to those obtained from the more coastal Site 1056 76 degrees W, 32 degrees N, 2200 meters water depth at present located on the Gulf Stream path. During MIS3 the summer SST for this Site are warmer and more stable those at Site 1060 (between 25 and 28 degrees C). Percent carbonate provides a clear correlation to Site 1060 and to Greenland ice cores. There the pattern of variability in the planktonic delta 18O parallels the fluctuations of the warm surface species abundances but these records do not mimic Greenland temperature. The cooling associated with Heinrich event is still evident but otherwise the surface records more closely resemble Antarctic records.