Fig. 2.

Southern Hemisphere climate system feedbacks and responses, and their relationship to Antarctic climate evolution between 4–2 Ma. Steps in the deterioration of Pliocene climate at ∼3.3 Ma and ∼2.6 Ma are highlighted by blue dashed lines. (A) Atmospheric CO₂ concentration derived from boron-(purple) and alkenone- (green) based marine proxies (26), compared with present (red line) and preindustrial (blue line) concentrations. (B) Benthic foraminiferal δ¹⁸O proxy for ice volume and temperature (δ¹⁸O; 42). (C) Influence of southern sourced bottom waters on deep ocean ventilation at ∼2.8–2.6 Ma shown by comparison of benthic foraminiferal δ¹³C records from South Atlantic/Southern Ocean ODP Sites 704/1090 (56), equatorial Pacific Ocean ODP Site 849 (57), and North Atlantic DSDP site 607 (58). (D) Atmospheric circulation (relative westerly wind strength) from dust mass accumulation rates for ODP Site 1090 (59). (E) Southern Ocean primary productivity based on biogenic opal mass accumulation rates at ODP Site 1091 shows a sharp increase coincident with increased windiness and nutrient supply by Fe-rich dust at ∼2.6 Ma (53, 59). (F) Onset of Antarctic sea ice at ∼2.6 Ma marked by a decline in primary productivity recorded in biogenic opal mass accumulation at Antarctic Peninsula ODP Site 1096 (53). (G) Summary of ocean, sea ice, and ice sheet evolution in Ross Embayment based on the AND-1B record. Note the cooling in SST and the return of periodic grounded ice sheets to western Ross Embayment occurs at ∼3.3 Ma, ending a ∼1.2 Ma period of relatively warm, ice free, open ocean conditions, and the appearance of sea ice and the development Ross Sea polynya between 3.2 and 2.6 Ma. Drill site locations are shown in Fig. 3.