Abstract:
We present a detailed study of Leg 172 Site 1056 (32 degrees N, 76 degrees W, 2200 meters water depth) from the Gulf Stream path, and of core MD01-2443 (37 degrees N, 10 degrees W 2900 meters water depth) taken off the Iberian Margin. Our results are based of stable isotope analyses on both planktonic and benthic species. These data were used to put the two cores onto a comparable time scale. Determination of the planktonic foraminifera faunal associations yields crucial information about the surface and sub-surface water dynamics at the two sites (stratification, upwelling, and deep-winter mixing) in relation to surface changes. These counts are also used to estimate Sea Surface Temperatures (SSTs) using the SIMMAX modern analogue approach. In addition, the lithic content (IRD) provides records of the iceberg-melting inputs at our two locations. At Site 1056, the SSTs calculated for August indicate that the Gulf Stream surface water at 27.5 degrees C in MIS11 was no warmer than at present time, although SSTs tend to increase toward the end of MIS11. Curiously, February SSTs display a initial phase warmer than the present day by 3 degrees C, which later cool to about 21.5 degrees C, close to their present value. The Summer SSTs at Site MD 01-2443 (post upwelling SST) increase during the first part of MIS11 but are only about 1 degrees C higher than during the Holocene. During this early phase the upwelling indicators are only slightly reduced, while Subtropical fauna records a maximum. However, these indicators increase and decrease respectively toward the end of MIS11, exhibiting several fluctuations. We also found evidence for two substantial main IRD events occurring during the MIS12 glacial inception. The observed contrast between MIS11 winter/summer SSTs at the Gulf Stream Site suggests that surface water was not warmer than at present, but that the warm stream was apparently extending its influence longer during the year, resulting in the winter warming. As a consequence, the overall heat transfer to the high latitude might have been higher than at present. The SST evolution in core MD 01-2443 may confirm this finding. Since, the seasonal upwelling occurs off Portugal during summer, if the SSTs were mainly higher in winter it would be possible to reconcile evidence from early MIS11 of both maintained upwelling activity and intense post upwelling warming.