Deep-Atlantic Interhemispheric Water Mass Competition since the Mid-Pleistocene Transition (ODP 1063 versus ODP 1094/1090)

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The Mid-Pleistocene Transition (MPT) is marked by a major disruption of the ocean thermohaline circulation (THC) system recorded in Nd isotopes, which subsequently reoccurs during glacial climates, according to studies by Pena and Goldstein (2014), Dausmann et al. (2017), Farmer et al. (2019) and Tachikawa et al. (2021). The subsequent fundamental change in glacial-interglacial periodicity clearly marks a change from minor glacial-interglacial differences in deep ocean εNd (radiogenic 143Nd/144Nd – isotope ratios) at 2082 m and 3702 m depth to large interglacial-glacial εNd contrasts. Other studies in the North Atlantic confirm systematic glacial–interglacial Nd-isotope pattern reflecting the competition between southern and northern interior Atlantic water masses. Thus, accurate knowledge of the north-south gradient of Nd isotopy along the flow path is fundamental to quantitatively estimate water mass mixing as a function of the strength and structure of deep water flow. Here we define Δε10 as the sensitivity of water masses to Nd-isotope mixing per 10° latitude, i.e. the interhemispheric gradient per latitude. Recent studies and our preliminary work show a north-south εNd difference per 10 ° latitude (Δε10) of 0.6-1.1 when Circum Antarctic Current water south of the polar front is included in the balance. Long-term trends and dramatic changes in Nd - sensitivity could be demonstrated for the past 140 ka. Overall, however, the sensitivity is very high due to the increasing influence of Pacific deep water in the South Atlantic during cold climate. First evidence was collected on the influences of reduced Nd supply due to the formation of northern hemispheric ice sheets around 110 ka before present. Known perturbations of the North Atlantic circulation during Heinrich Event 1 are thought to halve the Nd sensitivity (but so far without considering a Pacific contribution to the balance). Thus, in order to fully capture changes in water mass origin and deep circulation strength through combined studies of εNd and δ13C, a high-resolution reconstruction of Δε10 is required over more than 1 million years. The goal is therefore to generate a time series of Nd isotopy spanning more than 1 Ma in the South Atlantic, south of the polar front. We selected ODP 1094 for this study because we can demonstrate perfect positioning between circumpolar water masses for the last 140ka. Similarly, sedimentation rates are high, allowing temporal resolution of millennia. Based on a large number of Nd analyses generated over the project duration of two years, the observations will be refined to assess the impact of changing sensitivity to the decoupling of ocean circulation and global CO2, as well as the behavior of "lukewarm" interglacials and the advance of Pacific waters during glacial periods.