The role of ocean circulation and regolith removal in triggering the Mid-Pleistocene Transition: Insights from authigenic Nd isotopes

Approximately 1,250,000 to 700,000 years ago, the pacing of glacial-interglacial cycles changed from 41,000 years to ∼100,000 years, a shift known as the ‘Mid-Pleistocene Transition’ (MPT). The cause – or causes – of this shift remain uncertain. However, changes in ocean circulation and removal of northern hemisphere regolith have both been proposed as potential triggers. Here, we present continuous, orbitally resolved reconstructions of deep ocean neodymium isotopes from three locations in the equatorial Atlantic, Indian and southwest Pacific oceans, spanning 1.7 million years from the Holocene to before the MPT, to test these two hypotheses. We find that global seawater neodymium isotope variability over glacial-interglacial cycles is controlled by changes in both neodymium input to the North Atlantic and deep ocean mixing. Using this neodymium isotope data, we show that enhanced northern hemisphere regolith removal began approximately 1.45 million years ago, ∼200,000 years prior to the onset of the MPT and ∼500,000 years prior to a major expansion in northern hemisphere ice sheets between ∼900,000 and 870,000 years ago. This ice sheet expansion was immediately preceded by an interval of reduced mixing of Atlantic-sourced waters into the deep southwest Pacific Ocean. We hypothesize that this circulation reorganization reflected increased stratification of the deep Southern Ocean interior, possibly in response to cooling and Antarctic sea ice expansion at this time. Taken together, these data suggest an expansion and/or thickening of northern hemisphere ice sheets during the MPT was facilitated by a combination of reduced northern hemisphere regolith cover alongside Southern Ocean circulation changes. Together, these shifts allowed the build up of larger northern hemisphere ice sheets that were more resistant to deglaciation, facilitating the longer glacial cycles of the post-MPT world.