Increase in strength and orbital variability of the South Pacific Antarctic Circumpolar Current across the Mid-Pleistocene Transition

The Antarctic Circumpolar Current (ACC) represents the world’s largest ocean current system and impacts global ocean circulation, climate, and Antarctic ice sheet stability. Today, ACC dynamics are controlled by atmospheric forcing, Southern Ocean density gradients, and mesoscale eddy activity in the southern high latitudes. Yet, its role in driving the lengthening and intensification of glacial cycles is insufficiently studied. Here, we present a 1.5 Ma-record of changes in ACC strength based on bottom water flow reconstructions and sedimentary opal contents at IODP Sites U1540 and U1541 drilled in the Subantarctic Zone (SAZ) of the Pacific Southern Ocean. Our new data indicate that glacial and interglacial ACC strength gradually increased between ~1.3 and ~ 1 Ma coinciding with the early part of the MPT. This interval culminates in a pronounced ACC maximum during Marine Isotope Stage (MIS) 31 at ~1 Ma reaching ~160 % of the mean Holocene values. The increase in subantarctic ACC strength during the initial part of the MPT is paralleled by the emergence of stronger orbital-scale fluctuations in opal contents at both Sites U1540 and U1541 after MIS 31, suggesting a link between the onset of consistently higher amplitude glacial-interglacial fluctuations of ACC changes and latitudinal shifts of the ‘opal belt’ in the Southern Ocean. Specifically, higher opal contents correlate to decreased ACC strength, suggesting that the opal belt extended northward into the SAZ during glacials. We argue that the early change in ACC dynamics at the beginning of the MPT might be linked with sea surface temperature changes in the eastern subtropical and tropical Pacific, because surface cooling by ~2-3 °C at ODP Site 1237 off Peru between ~1.05 Ma and ~0.8 Ma parallels the reconstructed ACC strengthening at IODP Sites U1540 and U1541. This may result from enhanced advection of subantarctic water masses northward along the Humboldt Current system as a response to the intensification of the ACC starting during MIS 31. Our findings emphasize a contribution of Southern Ocean processes to the climate events causing intensification of glacial-interglacial climate variability during the MPT.