Cooling Subsurface Temperatures in the Eastern Equatorial Pacific during the Pliocene and Linkages to Global Cooling

Today in the Eastern Equatorial Pacific, a shallow thermocline and rigorous wind-driven upwelling brings cool, nutrient-rich water to the surface. In contrast, during the warm Pliocene (~3-5 Ma), sea surface temperatures in the Eastern Equatorial Pacific were much warmer than modern. Warm sea surface temperatures in upwelling regions, which have global effects on atmospheric circulation and the heat budget, could be caused by changes in wind-driven upwelling and/or changes in thermocline depth. As changes in wind-driven upwelling cannot entirely explain warm sea surface temperatures in upwelling regions, we investigate changes in subsurface temperature to monitor thermocline depth. We measured subsurface temperature records from Eastern Equatorial Pacific ODP transect Sites 848, 849, and 853 using Mg/Ca records from Globorotalia tumida, which has a depth habitat of ~100 m. Prior to closure of the Isthmus of Panama (5.2-4.8 Ma), subsurface temperatures were ~6-8°C warmer than modern, suggesting the thermocline was deeper during the Pliocene. From 4.8 to 4.0 Ma, subsurface temperatures rapidly cooled and then from 4.0 to present subsurface temperatures continued to cool, indicating the thermocline shoaled from the Pliocene to present. Rapid subsurface cooling between 4.8 to 4.0 Ma is likely a regional signal related to closure of the Isthmus of Panama, but continued cooling from 4.0 Ma to present is likely related to global processes including changes in global thermocline structure. Presently, thermocline waters that upwell in the Eastern Equatorial Pacific are sourced from subtropical surface waters in the South and North Pacific. Changes in Pliocene thermocline conditions could therefore be related to changing density/temperature gradients in these extratropical subduction regions, and could have significant implications on the global heat budget. Since the end of the early Pliocene warm period at ~3.5 Ma is marked by initial high latitude cooling and Northern Hemisphere Glaciation and by gradual low latitude subsurface cooling, large-scale oceanographic circulation changes may have played an important role in the transition from the warm Pliocene to the cold Pleistocene.