Increased seasonality during the intensification of Northern Hemisphere glaciation at the Pliocene-Pleistocene boundary ~2.6 Ma

Near the Pliocene-Pleistocene (Neogene-Quaternary) boundary during Marine Isotope Stage (MIS) 104 at around 2.6 Ma, the intensification of Northern Hemisphere glaciation (iNHG) was marked by a southward shift of the North Atlantic Current and Arctic Front and a concurrent drop in sea surface temperature (SST) in the mid-latitudes of the North Atlantic. Here we evaluate two Mg/Ca-based sea-surface temperature reconstructions for the 2.78-2.52 Ma interval using the planktonic foraminifera Globigerina bulloides and Globigerinoides ruber (white) sensu stricto as the biotic carriers (SSTbul and SSTrub), and a reconstruction based on the alkenone saturation index (SSTalk). The Mg/Ca-based SST estimates begin to diverge from one another significantly during MIS G3 (~2.66 Ma), culminating during MIS 104 (~2.60 Ma), and remaining distinctly different for the remaining 80 kyr of the studied interval. The magnitude of this divergence (δSSTrub-bul) is caused mainly by a decrease in SSTbul, with SSTrub remaining relatively constant. We attribute this difference to a drop in SST during the early-spring blooming season of G. bulloides that was less pronounced during the late-summer blooming season of G. ruber. Using δSSTrub-bul as a seasonality indicator, we show that seasonality increased significantly owing to a drop in early spring temperatures from MIS 104 onwards, and peaked during glacial episodes. Maximum seasonality occurred during MIS 104 in coincidence with a major expansion of the circum-Atlantic ice sheets, particularly the North American ice sheet. This expansion appears to have had a critical influence on global climate and especially seasonality. © 2015 Elsevier Ltd.