The Middle Miocene climatic transition in the Southern Ocean: Evidence of paleoclimatic and hydrographic changes at Kerguelen plateau from planktonic foraminifers and stable isotopes

Middle Miocene (14.8–11.9 Ma) deep-sea sediments from ODP Hole 747A (Kerguelen Plateau, southern Indian Ocean) contain abundant, well-preserved and diverse planktonic foraminiferal assemblages. A detailed study of the climatic and hydrographic changes that occurred in this region during the Middle Miocene Climatic Transition led to the identification of an intense cooling phase (the Middle Miocene Shift). Abundance fluctuations of planktonic foraminiferal species with different paleoclimatic affinities, and oxygen and carbon stable isotopes have been integrated in a multi-proxy approach. Reconstruction of changes in foraminiferal faunal composition and diversity through time were the basis for identification of three foraminiferal biofacies. The most prominent faunal change took place at 13.8 Ma, when a fauna with warmwater affinity (marked by high abundance of Globorotalia miozea group and Globoturborotalita woodi plexus) was replaced by an oligotypic, opportunistic fauna with typical polar characters and dominated by neogloboquadrinids. This faunal change is interpreted as the result of foraminiferal migration from adjacent bioprovinces, caused by modifications in climate and hydrography. A positive 2.0‰ shift in δ18O (interpreted as the Mi3 event) and a related positive 1.0‰ shift in δ13C (corresponding to the CM6 event) accompanied this faunal turnover. These are interpreted to reflect substantial reorganization of Southern Ocean waters, the northward migration of the Polar Front and a strong increase in primary productivity. The second faunal change took place at 12.9 Ma and was characterized by the gradual decrease in abundance of the neogloboquadrinids and the recovery of Globorotalia praescitula/scitula group and Globigerinita glutinata. A positive 1.5‰ shift in δ18O (interpreted as the Mi4 event) and a concurrent gradual negative shift in δ13C accompanied this faunal change, witnessing further modifications of the climate/ocean system. Variations in sea surface temperature, considered as the main factor causing changes of surface hydrography at the Kerguelen Plateau, seem to have been driven by obliquity and long-term eccentricity, thus suggesting a key role played by the astronomical forcing on the evolution of Southern Ocean dynamics during the Middle Miocene. Also an evident 1.2 Myr modulation of the δ13C record suggests a main control of the long-term obliquity cycles on the carbon cycle dynamics. Particularly, the Mi3/CM6 events exactly fit with a node of the 1.2 Myr modulation cycles. This confirms the key role played by orbital parameters on high-latitude temperatures and Antarctic ice volume, and indirectly on global carbon burial and/or productivity. This climatic transition was marked also by changes in surface hydrography. From 14.8 to 13.8 Ma an intermediate-strength thermocline controlled by seasonality developed just below the photic zone. Weaker seasonality characterized the interval from 13.8 to 12.9 Ma, when the thermocline became shallower and sharper and favored intermediate-water foraminifers. From 12.9 Ma, seasonality increased again and an intermediate-strength thermocline re-developed.