ABSTRACT The late early to middle Miocene period (18–12.7 Ma) was marked by profound environmental change, as Earth entered into the warmest climate phase of the Neogene (Miocene climate optimum) and then transitioned to a much colder mode with development of permanent ice sheets on Antarctica. Integration of high-resolution benthic foraminiferal isotope records in well-preserved sedimentary successions from the Pacific, Southern, and Indian Oceans provides a long-term perspective with which to assess relationships among climate change, ocean circulation, and carbon cycle dynamics during these successive climate reversals. Fundamentally different modes of ocean circulation and carbon cycling prevailed on an almost ice-free Earth during the Miocene climate optimum (ca. 16.9–14.7 Ma). Comparison of δ 13 C profiles revealed a marked decrease in ocean stratification and in the strength of the meridional overturning circulation during the Miocene climate optimum. We speculate that labile polar ice sheets, weaker Southern Hemisphere westerlies, higher sea level, and more acidic, oxygen-depleted oceans promoted shelf-basin partitioning of carbonate deposition and a weaker meridional overturning circulation, reducing the sequestration efficiency of the biological pump. X-ray fluorescence scanning data additionally revealed that 100 k.y. eccentricity-paced transient hyperthermal events coincided with intense episodes of deep-water acidification and deoxygenation. The in-phase coherence of δ 18 O and δ 13 C at the eccentricity band further suggests that orbitally paced processes such as remineralization of organic carbon from the deep-ocean dissolved organic carbon pool and/or weathering-induced carbon and nutrient fluxes from tropical monsoonal regions to the ocean contributed to the high amplitude variability of the marine carbon cycle. Stepwise global cooling and ice-sheet expansion during the middle Miocene climate transition (ca. 14.7–13.8 Ma) were associated with dampening of astronomically driven climate cycles and progressive steepening of the δ 13 C gradient between intermediate and deep waters, indicating intensification and vertical expansion of ocean meridional overturning circulation following the end of the Miocene climate optimum. Together, these results underline the crucial role of the marine carbon cycle and low-latitude processes in driving climate dynamics on an almost ice-free Earth.

Three time lines through the neritic stratigraphic record distributed around the northern margin of the Australo-Antarctic Gulf (AAG) mark three fundamental shifts in global environments collectively comprising the Auversian facies shift. The three lines are: (1) the beginning: the Khirthar transgression and the onset of neritic carbonate accumulation in the Bartonian Age (preceding onset of the Middle Eocene climatic optimum [MECO]); (2) the midlife change (Bartonian-Priabonian transition): the shift from carbonate-rich to carbonate-poor, higher-nutrient environments under estuarine circulation, causing widespread dysaerobia culminating in opaline silicas; and (3) the Eocene-Oligocene = Priabonian-Rupelian boundary and glaciation during oxygen isotope event Oi-1, with return of improved ventilation in neritic environments and resumption of carbonate accumulation. Meanwhile, it was warm and very wet at ~60°S. In developing a scenario for the death of the AAG, the birth of the Southern Ocean, and the transition from Paleogene greenhouse Earth to Neogene icehouse Earth, the neritic record of the northern margin is more in accord with the “Dinocyst biogeographic hypothesis” than with the “Tasman gateway hypothesis.”