Thermocline state change in the eastern equatorial Pacific during the late Pliocene/early Pleistocene intensification of Northern Hemisphere glaciation

The late Pliocene/early Pleistocene intensification of Northern Hemisphere Glaciation (iNHG) ~ 2.5 million years ago (Marine Isotope Stages [MIS] 100–96) stands out as the most recent major tipping point in Earth's climate history. It strongly influenced oceanographic and climatic patterns including trade-wind and upwelling strength in the Eastern Equatorial Pacific (EEP). The thermocline depth in the EEP, in turn, plays a pivotal role in the evolution of short-term climate phenomena such the El Nino-Southern Oscillation, and thus bears important consequences for the Earth's climate system. However, thermocline dynamics in the EEP during to the iNHG have yet remained unclear. While numerous studies have suggested a link between a thermocline shoaling in the EEP and Northern Hemisphere ice growth, other studies have indicated a stable thermocline depth during iNHG, thereby excluding a causal relationship between thermocline dynamics and ice-sheet growth. In light of these contradictory views, we have generated geochemical (planktic foraminiferal δ 18 O, δ 13 C and Mg/Ca), sedimentological (sand-accumulation rates) and faunal (abundance data of thermocline-dwelling foraminifera) records for Ocean Drilling Program Site 849 located in the central part of the EEP. Our records span the interval from ~ 2.75 to 2.4 Ma (MIS G7–95), which is critical for understanding thermocline dynamics during the final phase of the iNHG. They document a thermocline shoaling from ~ 2.64 to 2.55 Ma (MIS G2–101) and a relatively shallow thermocline from ~ 2.55 Ma onwards (MIS 101–95). This indicates a state change in EEP thermocline depth shortly before the final phase of iNHG. Ultimately, our data support the hypothesis that (sub-)tropical thermocline shoaling may have contributed to the development of large Northern Hemisphere ice sheets.