The trace element composition of planktic foraminiferal calcite provides a useful means of determining past surface ocean conditions. We have assembled the results of culture experiments for three species of symbiont-bearing planktic foraminifera, Globigerinoides ruber, Globigerinoides sacculifer, and Orbulina universa, and one symbiont-barren species, Globigerina bulloides, to evaluate their responses to temperature, salinity, pH, carbonate ion, and dissolved inorganic carbon (DIC) growth conditions. Trace element ratios (Li/Ca,

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

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