Significant change in the flow regime in the deep Southern Ocean through the MPT

Abstract

The deep Southern Ocean (SO) circulation is of major significance for the understanding of the ocean´s impact on Earth’s climate as uptake and release of CO2 strongly depend on the redistribution of well and poorly ventilated water masses. Neodymium isotopes (εNd) preserved in deep sea sediment have proven useful to study the Deep Ocean Circulation and water mass provenance and are of special interest over major climate changes as the Mid Pleistocene Transition (MPT). The MPT marks the change from a 41 ka to a 100 ka glacial-interglacial cyclicity and goes along with a significant intrusion of southern sourced waters (SSW) in the deep North Atlantic. Here, we present the first millennial resolved authigenic εNd data in the Southern Atlantic spanning across  the MPT of a deep sea sediment core positioned at the polar front. The pre-MPT εNd values of ODP 1093 show a small variability of approx. 2 ε-units around the modern AABW signature of -8. In contrast, the post-MPT εNd variability increases to 6 ε-units with glacial extremes of around -3 – εNd values that can not be found in any Atlantic sourced water mass today! This supports not only the exsiting hypotesis of stonger SSW export to the North, but rather advocates for a radiogenic  watermass influencing the flow regime in the Atlantic south of the polar front. Increasing ice volume during post-MPT glacials has been argued to lead to a reduced AABW production. Due to continuity of flow, this opens up the possiblity of glacial intrusion through the Drake passage of a water masses likely originating in the Pacific, which would generate  the strongly radiogenic glacial εNd values. At present Pacific deep waters are enriched with respired carbon. Assuming this to hold true in the past, the intrusion of such carbon rich water masses into the deep South Atlantic could further reinforce the strong glacials and the overall global cooling trend after the MPT as suggested previously. Hence, the SO south of the polar front played a leading role in  reinjecting respired CO2 into the deep Atlantic Ocean and the Atmosphere during climate transitions.