Did the Mid-Pliocene warmth bring the Northern Hemisphere Chill?

Abstract:

The relatively fast transition from the warm Pliocene to the Northern Hemisphere Glaciation (NHG) is puzzling. We have previously suggested that expansion of Antarctic glaciation following the mid-Pliocene warm period altered the oceanic circulation and inter-hemispheric transfer of heat and salt thereby providing a dynamic trigger for the intensification of the NHG at ~2.75 Ma and the ensuing glacial cycles (Woodard et al., 2014). Here we explore the hypothesis that enhanced chemical weathering under the warm Pliocene conditions contributed to the gradual cooling leading to the dynamic shift in ocean circulation. Using foraminiferal core-top and culture calibrations we have developed a new multi-elemental proxy approach for reconstructing changes in ocean calcium ([Ca]) and other major ion concentrations throughout the past ~3 Myr. Foraminiferal records from several drill sites in the Atlantic and Pacific Oceans suggest that seawater [Ca] was ~20±5% higher during the mid-Pliocene period (~2.7-3.2 Ma) than at present, and gradually reaching modern seawater concentration by the early Pleistocene. Other seawater ion concentrations (e.g., Sr, Li, B) were also significantly higher at that time than at present. Correction for the estimated change in seawater Mg/Ca yields mid-Pliocene sea surface temperatures in the western equatorial Pacific ~1-2° warmer than today. We suggest that the higher seawater major-ion concentrations, reconstructed here, reflect enhanced chemical weathering, likely due to more intense tropical hydrologic cycle at that time. The implied increase in seawater alkalinity under the mid-Pliocene warm conditions could have acted to sequester atmospheric CO₂ thus providing a negative feedback that possibly contributed to global cooling.

References: Woodard, S.C., Rosenthal, Y., Miller, K.G., Wright, J.V., Chiu, B.K. and K.T. Lawrence. (2014). Antarctic role in Northern Hemisphere Glaciation. Science, 346:847-850.