CHANGING ZONAL AND LATITUDINAL SEA SURFACE TEMPERATURE GRADIENTS THROUGH THE INTENSIFICATION OF NORTHERN HEMISPHERE GLACIATION

Global warm periods and climate transitions are noteworthy in light of recent predictions for future global climate change. The Pliocene represents both circumstances: the early Pliocene (5.3-3 Ma) was a period of prolonged warmth and the Plio-Pleistocene transition was marked by the growth of large ice sheets in the northern hemisphere. Using the alkenone organic proxy, we tracked the evolution of sea surface temperature (SST) from the early Pliocene (4 Ma) to the present at Ocean Drilling Program Sites 982 in the North Atlantic (58°N, 16°W), 662 in the Eastern Equatorial Atlantic (1°S, 11°W), and 846 in the Eastern Equatorial Pacific (3°S, 90°W). Our high resolution records indicate that SST cooled over the past 4 Myr in the tropics as well as high latitudes and that the rate of cooling varied by site. The North Atlantic cooled by approximately 1.5°C/Myr, though most cooling took place between 3.5 and 2.5 Ma. Both tropical sites cooled more gradually; the Eastern Equatorial Atlantic (EEA) cooled ~0.6°C/Myr and the Eastern Equatorial Pacific (EEP) cooled ~1°C/Myr. The latitudinal gradient between the two Atlantic sites increased from 9.5°C during the early Pliocene (~4 Ma), reaching its modern value (~12° C) at around 2.5 Ma. The zonal gradient between the EEA and the EEP steadily increased from 1°C to 3°C from 4 Ma to the present. Additionally, data from our equatorial sites show remarkably similar long-term trends and orbital-scale structure. Previous studies have proposed that increased heat transport and/or an increase in atmospheric greenhouse gases were responsible for early Pliocene warmth. Our SST records, which indicate warmer early Pliocene conditions in both low and high latitude regions as well as remarkably similar overall structure suggests a global mechanism like a change in atmospheric greenhouse gases likely played an important role in the Plio-Pleistocene transition.