Reconstruction of sea-surface temperatures from asseblages of planktonic foraminifera

We present a conceptual framework for a new approach to environmental calibration of planktonic foraminifer census counts. This approach is based on simultaneous application of a variety of transfer function techniques, which are trained on geographically constrained calibration data sets. It serves to minimise bias associated with the presence of cryptic species of planktonic foraminifera and provides an objective tool for assessing reliability of environmental estimates in fossil samples, allowing identification of adverse effects of no-analog faunas and technique-specific bias. We have compiled new calibration data sets for the North (N=862) and South (N=321) Atlantic and the Pacific Ocean (N=1111). We show evidence that these data sets offer adequate coverage of the Sea-Surface Temperature (SST) and faunal variation range and that they are not affected by the presence of pre-Holocene samples and/or calcite dissolution. We have applied four transfer function techniques, including Artificial Neural Networks, Revised Analog Method and SIMMAX (with and without distance weighting) on faunal counts in a Last Glacial Maximum (LGM) data set for the Atlantic Ocean (748 samples in 167 cores; based on the GLAMAP-2000 compilation) and a new data set for the Pacific Ocean (265 samples in 82 cores) and show that three of these techniques provide adequate degree of independence for the advantage of a multi-technique approach to be realised. The application of our new approach to the glacial Pacific lends support to the contraction and perhaps even a cooling of the Western Pacific Warm Pool and a substantial (>3 °C) cooling of the eastern equatorial Pacific and the eastern boundary currents. Our results do not provide conclusive evidence for LGM warming anywhere in the Pacific. The Atlantic reconstruction shows a number of robust patterns, including substantial cooling of eastern boundary currents with considerable advection of subpolar waters into the Benguela Current, a cooling of the equatorial Atlantic by ~5 °C, and steep SST gradients in the mid-latitude North Atlantic. The transfer function techniques generally agree that subtropical gyre areas in both hemispheres did not change significantly since the LGM, although the ANN technique produced glacial SST in the southern gyre 1-2 °C warmer than today. We have revisited the issue of sea-ice occurrence in the Nordic Seas and using the distribution of subpolar species of planktonic foraminifera in glacial samples, we conclude that the Norwegian Sea must have been ice-free during the summer.