Changes in Deep Circulation and Carbon Storage in the Eastern North Atlantic Ocean Across the Mid-Pleistocene Transition

The duration and magnitude of Earth’s glaciation cycles increased substantially during the course of the Pleistocene without an obvious shift in external forcing. Changes in ocean circulation have been posited as one potential influence on this mid-Pleistocene transition (MPT). New drilling of a depth transect of sites on the Iberian margin during IODP Expedition 397 offers the opportunity to examine the record of deep-ocean circulation changes in the eastern north Atlantic over a range of bottom depths and water masses. Benthic carbon isotopes (d¹³C) and sedimentary characteristics that reflect bottom water conditions at Site U1587 (37°35′N, 10°22′W, 3.5 km) reveal persistent glacial-interglacial changes throughout the Pleistocene, with a shift toward larger and longer glacial cycles evident in benthic oxygen isotopes (d¹⁸O) across the MPT. A comparison with shallower Site U1385 (37°34′N 10°08′W 2.6 km) indicates that the vertical carbon-isotope gradient over this water depth also increased across the transition, particularly within glacial intervals, with far more negative d ¹³C at the deeper Site U1587. Uranium and thorium isotope analyses also indicate intervals of reduced dissolved oxygen and the deposition of authigenic uranium at greater depth. These observations suggest greater stratification and carbon storage in the deep eastern north Atlantic after the MPT, and support a likely role for ocean circulation in this important climate transition.