Warmer tropics during the mid-Pliocene? Evidence from alkenone paleothermometry and a fully coupled ocean-atmosphere GCM

Traditional reconstructions of sea-surface temperatures (SSTs) produced by the Pliocene Research Interpretations and Synoptic Mapping (PRISM) Group indicate that mid-Pliocene surface ocean temperatures were unchanged or slightly cooler than modern at the tropics and low latitudes and significantly warmer at higher latitudes, particularly in the North Atlantic. This change in the latitudinal pattern of SSTs has been attributed to enhanced meridional ocean heat transport generated by more vigorous surface ocean gyres and/or thermohaline circulation (THC). This study derives new SST estimates using the alkenone paleothermometer in the Pacific Ocean. These estimates, combined with published alkenone SST data from two Atlantic sites, are located in tropical and subtropical regions where the distribution of PRISM sites is sparse. A fully coupled ocean-atmosphere general circulation model (OAGCM) running for the mid-Pliocene was also used to predict SST. Significant differences are noted between absolute PRISM SSTs interpolated to our core locations and alkenone-derived SSTs. These differences may be related to errors in the PRISM and alkenone paleothermometry estimates or to regional differences between localities where data was collected. However, the available alkenone and model-based SST estimates are consistent in the sign of temperature change they predict and provide the first indication of warmer SSTs during the mid-Pliocene in the tropics and subtropics. This contrasts with PRISM's estimates of unchanged or slightly cooler SSTs for the same geographical regions. The pattern of SSTs, produced by alkenone estimates and modeling, is not characteristic of that produced by enhanced meridional ocean heat transport or THC. Instead, the pattern is similar to that which might be expected as a result of higher concentrations of atmospheric CO2, which would act to warm the oceans at the tropics and other latitudes. Furthermore, model diagnostics indicate that reduced sea ice and terrestrial ice sheet extents and a strong ice-albedo feedback played a major role in forcing mid-Pliocene warmth. Further work aimed at validating these conclusions should concentrate on expanding the mid-Pliocene SST data set, particularly at low latitudes, as well as undertaking model/model comparisons to assess model dependency of the results.

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