Eastern equatorial Pacific cold tongue during the Last Glacial Maximum as seen from alkenone paleothermometry

摘要

We present new alkenone-based sea surface temperature (SST) estimates from the eastern equatorial Pacific (EEP) for the last 30 kyr. By combining these new results with recently published records from the region, we reconstruct the spatial pattern of changes in SST during the Last Glacial Maximum (LGM). Alkenone-based SST estimates show a greater glacial cooling in the upwelling environment of the cold tongue than in sites located further north in the equatorial front and eastern Pacific Warm Pool. This result agrees with the paradigm of stronger glacial winds, increased upwelling, steeper zonal thermocline tilt, and stronger advection of cold water in the Peru Current. Furthermore, we investigate possible changes in glacial surface hydrography by using the alkenone-based SST reconstructions to correct planktonic foraminifera δ~(18) O for the temperature effect. After additional correction for the global ice volume effect, the residual changes in seawater δ~(18) O show a clear latitudinal pattern that would be consistent with a southward shift of the Intertropical Convergence Zone. We thus suggest that changes in sea surface salinities could explain contrasting SST reconstructions based on planktonic foraminifera δ~(18) O, which implied a weakening of the cold tongue. The controversial LGM dynamics of the EEP reconstructed by different proxies, i.e., a weakening or a strengthening of the cold tongue, highlight the necessity to better assess the influence of various biases on these proxies.