Warm Pool Temperatures and the Zonal Gradient in the Tropical Pacific during the Early Pliocene Warm Period

Abstract:

The absolute temperature of the western Pacific warm pool (WPWP) and the zonal tropical Pacific temperature gradient provide fundamental information regarding the causes of Pliocene warmth and regarding earth system sensitivity when atmospheric CO₂ was similar to today (~350-400 ppm). Temperature estimates, derived from foraminiferal Mg/Ca, Uk’37 and TEX86, differ in detail from each other, but overall indicate that, within error, WPWP temperatures were similar to today while the zonal gradient was reduced relative to today. WPWP TEX86 data (Zhang et al., 2014; O’Brien et al., 2014) indicate a long-term cooling from early Pliocene average values similar to today, to approximately 2C cooler during the Late Pleistocene (although calibration errors of ±2.5C exceed the amplitude of the trend). Notably, Late Pleistocene orbital-scale data indicate a strong coupling between WPWP temperature and CO2, which was as much as ~30% lower than pre-industrial values; however, during the early Pliocene, WPWP temperatures were not warmer even though CO2 was as much as ~30% higher that pre-industrial values. Hence, WPWP temperature sensitivity to CO2 forcing was non-linear over myr-long time scales. Orbital scale sensitivity cannot be evaluated in the early Pliocene until more CO2 and temperature records are generated. By compiling data from the eastern tropical Pacific (ETP) it is possible to resolve changes in the spatial pattern of temperature, with the largest anomalies in the heart of the cold tongue. In all, the cold tongue was warmer than today resulting in a reduced (but existent) zonal gradient of about 1-2C, referred to as a ‘permanent El Niño-like’ or ‘El Padre’ mean state. Although many temperature proxy records agree with each other, some do not. These apparent disagreements can be explained by considering the depth ecology of organisms that produce the fossil shells and molecules that are analyzed, whole-ocean changes in chemistry, and post-depositional processes.