Surface-water cooling and salinity decrease during the Middle Miocene climate transition at Southern Ocean ODP Site 747 (Kerguelen Plateau)

Paleoceanographic variability at southern high latitude Ocean Drilling Program (ODP) Site 747 was investigated in this study through the interval which spans the Middle Miocene Climate Transition (MMCT). Between 15.0 and 12.2 million years ago (Ma), foraminiferal delta O-18 records derived from both benthic (Cibicidoides spp.) and planktonic taxa (Globorotalia praescitula and Globigerina bulloides) reveal a history of changes in water column thermal and salinity structure and a strong imprint of seasonality. Prior to the MMCT, in the interval between 14.35 and 13.9 Ma, G. bulloides displays relatively high delta O-18 values similar to those of G. praescitula, interpreted to indicate weakening of the thermocline and/or increased seasonality with cooler early-spring and/or late-fall temperatures. Following this interval, G. bulloides delta O-18 values diverge significantly from benthic and G. praescitula values, with G. bulloides values remaining relatively low for at least 600 kyr following the benthic foraminiferal delta O-18 shift during the MMCT at similar to 13.9 Ma. This divergence in delta O-18 records occurs in direct association with the Mi3 cooling and glaciation event and may suggest: (1) a strengthening of the vertical temperature gradient, with greater cooling of deep waters than surface waters, (2) changes in the depth habitat of G. bulloides. (3) changes in the dominant season of G. bulloides calcification, (4) modification of surface-water delta O-18 values in association with enhanced sea-ice formation, (5) increased surface-water carbonate ion concentration, and/or (6) a significant decrease in surface-water salinity across the MMCT. The first of these possible scenarios is not likely, particularly in light of recent Mg/Ca evidence for significant surface-water cooling in the Southern Ocean associated with the MMCT. Of the remaining possibilities, we favor a change in surface salinity to explain the observed trends in delta O-18 values and hypothesize that surface salinity may have decreased by up to 2 salinity units at similar to 13.9 Ma. In this scenario, the development of a lower-salinity Antarctic surface layer coincided with regional cooling of both surface and deep waters of the Southern Ocean during the Mi3 glaciation of East Antarctica, and contributed into the dominance of Neogloboquadrina spp. between 13.8 and 13.2 Ma. Additionally, the distinct patterns observed in planktonic foraminiferal delta O-18 records spanning the MMCT correspond with changes in the vertical delta C-13 gradient between planktonic and benthic foraminiferal records and major changes in planktonic foraminiferal assemblages at Site 747, providing further evidence of the environmental significance of this climatic transition. (C) 2009 Elsevier B.V. All rights reserved.