Abstract

During the Mid-Pleistocene Transition (MPT; 1200–800 thousand years ago, kyrs) Earth’s orbitally paced ice-age cycles intensified, lengthened from _{40 to} 100 kyrs, and became distinctly asymmetrical. Testing hypotheses that implicate changing atmospheric CO2 levels as a driver of the MPT has proven difficult with available observations. Here we use orbitally resolved, boron-isotope CO2 data to demonstrate that the glacial-to-interglacial CO2 difference increased from _{43 to} 75 µatm across the MPT , mainly due to lower CO2 levels during glacials. Through carbon-cycle modelling, we attribute this decline primarily to the initiation of substantive dust-borne iron fertilization of the Southern Ocean during peak glacial stages. We also observe a two-fold steepening of the relationship between sea level and CO2 -related climate forcing that is suggestive of a change in the dynamics that govern ice-sheet stability, such as that expected from the removal of subglacial regolith. We argue that neither ice-sheet dynamics nor CO2 change in isolation can explain the MPT . Instead, we infer that the MPT initiated by a change in ice-sheet dynamics, and that longer and deeper post-MPT ice ages were sustained by carbon-cycle feedbacks related to dust fertilization of the Southern Ocean as a consequence of larger ice sheets.

This talk is part of the Quaternary Discussion Group (QDG).