Upper-ocean temperature and upwelling variability across the Pacific during the Plio–Pleistocene transition: Insights from UK′37 and TEX86

The Plio–Pleistocene transition represents the shift from a warmer Pliocene to a cooler Pleistocene, offering key insights into climate sensitivity to CO₂ forcing and ice-volume changes. However, the upper-ocean thermal response of the Pacific mid-latitudes and upwelling regions remains poorly constrained, despite their critical role in global climate and ocean circulation. Here, we present paired U^K′₃₇–TEX₈₆ upper-ocean temperature records spanning 3.4–2.4 Ma from the subtropical and equatorial Pacific at ODP Site 1012 (California Margin), IODP Site U1338 (Eastern Equatorial Pacific, EEP), and DSDP Site 593 (Tasman Sea). Surface sediment data in these regions indicate that U^K′₃₇ reflects annual mean sea surface temperatures. In contrast, high GDGT [2/3] ratios (>7) observed in surface sediments and downcore records suggest that TEX₈₆ records shallow subsurface temperatures, likely near the nitrite maximum, as inferred from matching TEX₈₆ temperatures to climatological annual mean temperature profiles, regardless of the calibration used. In upwelling regions, this depth correlates strongly with thermocline depth, indicating deeper (shallower) TEX₈₆ recording depths during weakened (intensified) upwelling. Downcore U^K′₃₇ SST records from both subtropical sites indicate warmer-than-present Pliocene conditions, ~5 ºC cooling during the M2 glaciation, warming at KM5c, and a long-term cooling after the Mid-Pliocene Warm Period (MPWP), whereas SSTs at the equatorial Pacific site show no significant Pliocene–Pleistocene cooling trend. However, TEX₈₆-derived subsurface temperatures exhibit significant Pliocene-Pleistocene cooling across all three regions. Based on the U^K′₃₇–TEX₈₆ temperature difference (ΔT), we infer weaker upwelling along the California Margin during the MPWP, possibly linked to an intensified North American Monsoon (NAM) that weakened upwelling-favorable winds, followed by stronger upwelling under a weakened NAM during the early Pleistocene. In contrast, there is no change in upwelling in the EEP from the Pliocene to the Pleistocene. In the Tasman Sea, U^K′₃₇ likely records northward-sourced surface waters transported by the East Australian Current, whereas TEX₈₆ reflects subsurface South Antarctic Mode Water (SAMW). The convergence of U^K′₃₇ and TEX₈₆ temperatures between 3.1 and 2.6 Ma likely indicates northward migration of the Subtropical Front, allowing SAMW to shoal and influence surface conditions at the site. Overall, during the Plio–Pleistocene transition, the mid-latitude Pacific experienced an expansion of cold high-latitude waters. This study highlights the usefulness of paired U^K′₃₇–TEX₈₆ analyses in advancing our understanding of upper-ocean thermal evolution across diverse Pacific hydrological settings during past key climate transitions.