Thermocline Structure and ENSO Variability in the eastern equatorial Pacific during the Last Glacial Maximum

Abstract:

The mean state and variability of the eastern equatorial Pacific (EEP) during the Last Glacial Maximum (LGM) are of great interest because of the region's role in the El Niño–Southern Oscillation (ENSO) and global climate. We investigated changes in thermocline structure between the Late Holocene (LH) and LGM with stable isotopes of planktonic foraminifera in sediment cores from the Galápagos. We measured δ¹⁸O in two species—Globigerinoides ruber, inhabiting the surface mixed layer, and Neogloboquadrina dutertrei, inhabiting the deep thermocline—in order to evaluate the vertical temperature contrast between the two species. We also measured δ¹⁸O of individual N. dutertrei from modern (late 20^th century) and LGM sediments in order to assess thermocline temperature variability related to ENSO activity.

Our data indicate a reduced vertical contrast in the upper ocean during the LGM, which is most consistent with a deeper thermocline and thicker mixed layer. Additionally, δ¹⁸O of individual N. dutertrei shells shows 2.5 times greater population variance in the LGM than in the modern sample. This large variance indicates that thermocline temperatures were more variable during the LGM than today, consistent with more active ENSO. Together, these results imply that the mean state of the EEP was characterized by a deeper thermocline and greater ENSO variability. The results further show the potential for reconstructing ENSO variability from deep-sea sediments of the EEP, where other geological archives of ENSO are currently extremely limited.