EVOLUTION OF PACIFIC MERIDIONAL AND ZONAL TEMPERATURE GRADIENTS OVER THE PLIO-PLEISTOCENE

The Pliocene (2.7-5.0 Ma) is the most recent sustained warm period in Earth history and represents a potential analog for future climate conditions, however many aspects of Pliocene climate remain poorly understood. Two major questions are how higher temperatures (+3°C relative to pre-industrial global average) were maintained with atmospheric CO₂ concentrations around or below 400 ppm, as well as what caused the transition from the Pliocene to the colder, more variable conditions of the Pleistocene (0-2.7 Ma). Additionally, the Pliocene epoch was characterized by permanent El Niño-like conditions, most notably defined by a weakened east to west sea surface temperature (SST) gradient across the tropical Pacific. Even though the El Niño phenomenon is primarily a feature of the equatorial Pacific, extra-tropical climate conditions may have played a key role in maintaining the Pliocene climate state. However, the lack of continuous climate records from the southern extra-tropical Pacific has made it difficult to test the relationship between tropical and extra-tropical dynamics in both hemispheres during the Pliocene and throughout the transition to the Pleistocene. Here we present the first continuous record of SST change in the southern extra-tropical Pacific, extending from 4.4 Ma to the present. When compared with northern and equatorial Pacific SST records, our alkenone-based SST record from ODP Site 1125 reveals strong hemispheric symmetry in the evolution of Pacific meridional temperature gradients throughout the entire Plio-Pleistocene. This result is somewhat surprising, since polar ice sheet growth over this time interval occurred primarily in the northern hemisphere. Furthermore, temperature gradients between Site 1125 and the western equatorial Pacific and between the western and eastern equatorial Pacific are very tightly coupled throughout the entirety of the last 4.4 Myr. Since computer simulations of Pliocene climate cannot reproduce the observed SST patterns in the tropics and extra-tropics assuming changes in CO₂ alone, additional climate system feedbacks, potentially related to cloud albedo, must be at play in order to maintain the tight coupling between meridional and zonal SST gradients throughout the entire Plio-Pleistocene.