Over the past three million years, Earth's climate oscillated between warmer interglacials with reduced terrestrial ice volume and cooler glacials with expanded polar ice sheets. These climate cycles, as reflected in benthic foraminiferal oxygen isotopes, transitioned from dominantly 41-kyr to 100-kyr periodicities during the mid-Pleistocene (1,250 to 700 ka). Because orbital forcing did not shift at this time, the ultimate cause of this mid-Pleistocene transition (MPT) remains enigmatic. Here we present foraminiferal trace element (B/Ca, Cd/Ca) and Nd isotope data that demonstrate a tight linkage between Atlantic Ocean meridional overturning circulation and deep-ocean carbon storage across the MPT. Specifically, between 950 and 900 ka, carbonate ion saturation decreased by 30 µmol/kg and phosphate concentration increased by 0.5 µmol/kg coincident with a 20% reduction of North Atlantic Deep Water contribution to the abyssal South Atlantic. These results demonstrate that the glacial deep Atlantic carbon inventory increased by approximately 50 gigatons during the transition to 100-kyr glacial cycles. We suggest that the coincidence of our observations with evidence for increased terrestrial ice volume reflects how weaker overturning circulation and Southern Ocean biogeochemical feedbacks facilitated deep ocean carbon storage, which lowered atmospheric pCO 2 and thereby enabled expanded terrestrial ice volume at the MPT. Cyclic glaciations are the primary feature of Earth's climate since the late Pliocene and occur at periodicities linked to variations in solar insolation 1. However, the dominant periodicity of glaciations transitioned from 41-kyr to 100-kyr during the mid-Pleistocene without concomitant changes in external insolation forcing 2-5. It has been

The causes of the Mid-Pleistocene Transition, the shift from ∼41-ky to 100-ky interglacial–glacial cycles and more intense ice ages, remain intensely debated, as this fundamental change occurred between ∼1,250 and 650 ka without substantial changes in astronomical climate forcings. Recent studies disagree about the relative importance of events and processes in the Northern and Southern Hemispheres, as well as whether the shift occurred gradually over several interglacial–glacial cycles or abruptly at ∼900 ka. We address these issues using a north-to-south reconstruction of the Atlantic arm of the global meridional overturning ocean circulation, a primary means for distributing heat around the globe, using neodymium (Nd) isotopes. Results reveal a period of intense erosion affecting the cratonic shields surrounding the North Atlantic between Marine Isotope Stages (MIS) 27 and 25 (∼980 and 950 ka), reflected by unusually low Nd isotope ratios in deep North Atlantic seawater. This episode preceded a major ocean circulation weakening between MIS 25 and 21 (950 and 860 ka) that coincided with the first ∼100-ky-long interglacial–glacial onset of Northern Hemisphere glaciation at around 2.4 to 2.8 Ma. The data point to a Northern Hemisphere–sourced initiation for the transition, possibly induced through regolith loss and increased exposure of the crystalline bedrock, which would lead to increased friction, enabling larger ice sheets that are characteristic of the 100-ky interglacial–glacial cycles.