Characterization of dust and productivity fluxes in the North Pacific indicate shifts in the westerly winds during the Pliocene

Abstract:

The Pliocene epoch is considered the optimal analogue for predicted warming over the next 50-100 years: temperatures were 2-4 °C higher than today and atmospheric carbon dioxide (CO₂) was ~400-450 ppm. While certain aspects of the climate system are well quantified during the Pliocene, large uncertainties related to major components of atmospheric circulation, such as the position/path? Of the westerly winds, still exist. Because the westerlies play a key role in the transport of aerosols, steer mid-latitude cyclones, and drive surface ocean circulation, understanding how they functioned in a warmer world is critical to accurately modeling the future climate system. To reconstruct the position and strength of northern hemisphere westerlies during the Pliocene, we present two high resolution North Pacific dust flux records (ODP 1208A and ODP 885/886) spanning the period of ~2.5-4.5 Ma. We also measure fluxes of Ba_xs, opal, and C_{37 Total} at site ODP 1208A to provide additional proxies relevant to evaluating changes in the westerly winds. All fluxes are calculated using the constant flux proxy (CFP) extraterrestrial ³He, the first such application of a CFP in the Pliocene North Pacific. Our results provide evidence for weakened and poleward-shifted northern hemisphere westerly winds during the Pliocene. Subsequently, during the intensification of Northern Hemisphere Glaciation at ~2.7 Ma, the westerlies moved equatorward and strengthened, producing more dust and enhancing productivity in the core of the westerly wind belt over the North Pacific. The timing of these changes’ points to meridional thermal gradients and extent of high-latitude ice sheets acting as the dominant controls on the westerly’s position and strength. Comparison of our North Pacific dust fluxes to those from other major ocean basins shows that the Plio-Pleistocene variability in the westerly winds is a global phenomenon. We posit that the coherency of proxy dust records suggests that as the planet continues to warm in the future, the westerlies in both hemispheres will move poleward and likely weaken.