Tropical Pacific signals of suborbital-scale climate variability during the mid-Pleistocene climate transition

Abstract

The mid-Pleistocene climate transition (MPT) marks the shift in dominance from 41- to 100-kyr high latitude climate cyclicity between the early and late Quaternary. The fundamental mechanisms responsible for the MPT, and consequent 100-kyr cycle dominance is much debated, suffering from a paucity of data particularly from the low-mid latitudes. A large proportion of studies have focused on northern hemisphere ice sheet growth associated with the MPT. However, this thesis seeks to explore the hypothesis that not only do the tropical latitudes hold valuable information pertaining to the cause/s and development of the transition, but also the possibility that they play a key role in the initiation of the MPT itself, through powerful feedbacks holding the potential to alter global climate. In this study, both the physical (SST, thermocline depth) and biological (primary productivity) properties of the east Pacific are examined across the MPT.

This study finds that prior to and including MIS 22, the glacial-stage tropical Pacific operates under a La Niña-like state, suggested here to be caused by strengthened meridional temperature gradients which alter the tropical ocean-atmosphere system. MIS 22 culminated in an intense upwelling event, indicating a threshold has been crossed, agreeing with “900-ka event” theories of an extreme glaciation. This research does not find evidence for a pervasive cooling prior to the onset of the MPT in the tropical Pacific, negating the notion of strengthened Walker circulation as a driver for the MPT. Instead, an enhanced glacial biological carbon pump is presented as a more plausible explanation.