Deep ocean circulation changes across the Mid-Pleistocene Transition
Date
2023
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Publisher
University of Delaware
Abstract
The Mid-Pleistocene Transition (MPT), occurring between ~1250 to 700 Ka (1.25 to 0.7 Ma) marks a fundamental change in Earth’s climate system when the glacial cyclicity of Earth changed from 41 to 100 Kyr. This observed change occurred independently of changes in solar insolation suggesting changing internal climate dynamics might have driven the transition. The South Pacific represents the largest fraction of the Southern Ocean and hence an important target region for studying internal climate dynamics during the MPT. Here, we use sediment core U1541 (54˚13’S, 125˚25’W, 3604 m) collected during the International Ocean Discovery Program (IODP) Expedition 383 to investigate changes in deep ocean circulation and the role of these changes played in solidifying the conditions of the MPT. ☐ In this thesis, we present an authigenic Nd isotope (expressed as εNd) time series from Site U1541 from 1500 – 500 ka. εNd shifts can be interpreted as tracking changes in the mixing proportions of two water mass end members, unradiogenic Atlantic (εNd ~-13.5) and radiogenic Pacific sourced waters (PSW, εNd ~-4). In our record, the proportions of water mass end members fluctuate across climate cycles with glacial cycles reflecting more pacific source water, indicative of weakened Meridional Overturning Circulation (MOC). Across the MPT, we have observed three periods of increased PSW influence at ~1250 ka (MIS 38), ~900 ka (MIS 22), and ~650 ka (MIS 16) interpreted as periods of a more complete breakdown in MOC than other glacials. The ~1250 ka event predates previous research suggesting ~900 ka was the first event of such magnitude. Comparison of our results with the ice volume (using δ18Osw, a direct proxy of ice volume or sea level change) record for ODP 1123 (Elderfield et al., 2012) shows there is a strong correlation between the deep ocean circulation changes observed and global ice volume suggesting there is a mechanistic link between the two variables. We propose this link is a shift in the nature of Antarctic glaciers from terrestrial sourced to glaciomarine. The influx of freshwater into the surrounding oceans from melting glaciomarine glaciers near the Sub-Antarctic front (SAF) has the potential to destabilize the water column producing perturbations in deep ocean circulation of the magnitude observed in our record. The MPT ocean circulation perturbations could have promoted increased drawdown in CO₂ and ice growth to produce the 100 kyr cycles of the MPT.
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Keywords
Mid-Pleistocene Transition, Neodymium, Ocean circulation, International Ocean Discovery Program