Contrasting meridional displacement of the western and eastern North Pacific Subtropical Gyre occurred during ∼4.5–3.5 Ma: A response to strengthened SST gradient

Abstract

The causes of Northern Hemisphere Glaciation (NHG) during the Pliocene remain controversial. The western North Pacific Subtropical Gyre (wNPSG), the dominant conduit for poleward heat and moisture transport, may have modulated NHG development, yet its Pliocene evolution remain unconstrained. In this study, we analyzed diatom assemblage changes along the northern boundary of the wNPSG to investigate its meridional displacement. We compared these changes with those of the eastern NPSG (eNPSG) reconstructed using compiled published sea surface temperature data, and examined their linkages to the NHG. The diatom assemblages indicate a northward expansion of the wNPSG from ∼5.8 Ma to ∼4.5 Ma. This wNPSG expansion coincided with an expansion of the eNPSG, both likely related to global warming. Subsequently, a continued northward expansion of the wNPSG occurred between ∼4.5 Ma and ∼ 3.7 Ma, corresponding to an expansion of the Western Pacific Warm Pool but an eNPSG contraction. These contrasting changes probably linked to stronger easterly trade winds driven by an increased SST gradient between low- to mid-latitudes. We propose that the northward expansion of wNPSG during this period likely suppressed NHG formation until ∼3.7–3.5 Ma by transporting more heat to high latitudes via oceanic and atmospheric circulations. From ∼3.7 Ma to ∼2.5 Ma, diatom assemblages indicate that wNPSG retreated southward, coinciding with the onset of NHG. This study highlights the different behavior of the wNPSG versus the eNPSG during the early Pliocene and its potential role in suppressing the onset of NHG until ∼3.7 Ma.