Meridional Shifts of the Southern Hemisphere Westerlies During the Early Cenozoic.

Despite the crucial role of the Southern Hemisphere (SH) westerlies in modulating modern and past climate evolution, little is known about their behavior and possible forcing mechanisms during the early Cenozoic. We probe changes in the hydroclimate of southwest Australia during 62–51 Ma, based on sedimentary proxy records from the International Ocean Discovery Program Site U1514 in the Mentelle Basin. Our results reveal a transition from a less humid climate to wetter conditions at mid–high latitudes starting from the early Eocene, which suggests poleward migration of the SH westerlies. This long‐term trend is punctuated by short‐lived events of aridification during the Mid‐Paleocene Biotic Event and wetter intervals during the Paleocene‐Eocene Thermal Maximum, indicating additional short‐term meridional shifting of the westerlies. We propose that the evolution of SH westerlies was driven by the equator‐to‐pole temperature gradient regulated by global warming and ephemeral growth of the Antarctic ice sheet. Plain Language Summary: The Southern Hemisphere (SH) westerlies, which are the dominant atmospheric circulation patterns in the middle latitudes, play a key role in regulating global and regional climate. Currently, the knowledge of past changes in SH westerlies relies mainly on the late Quaternary. Its dynamics over longer timescales, especially under early Cenozoic greenhouse climate states, remain poorly understood. The Mentelle Basin off southwest Australia was located at a more southerly location than at the present day, and was potentially under the influence of SH westerlies. To examine the long‐term hydroclimate changes in southwest Australia during the mid‐late Paleocene to the early Eocene (62–51 Ma), we measured neodymium and hafnium isotopic compositions of fine‐grained detrital sediments from a borehole (IODP Site U1514) drilled in the Mentelle Basin, in addition to clay mineralogy, and elemental abundances. Our results suggest a gradual wetting of southwest Australia since the early Eocene, which we relate to the poleward migration of SH westerlies in response to a reduced latitudinal temperature gradient. An abrupt northward shift of SH westerlies was observed during the short‐lived Middle Paleocene Biotic Event (∼59 Ma), possibly driven by ephemeral growth of the Antarctica ice sheet. Key Points: We reconstruct the hydroclimate evolution of southwest Australia during 62–51 MaThe Southern Hemisphere (SH) westerlies remained relatively stable during the mid‐late Paleocene and shifted poleward since the early EoceneProcesses related to variations of pole‐equator temperature gradients drove meridional migration of the SH westerlies [ABSTRACT FROM AUTHOR]