High-resolution ice-volume estimates for the early Miocene: Evidence for a dynamic ice sheet in Antarctica

Ice-volume estimates for the early Miocene (23–16 Ma ATS) were determined by applying dO to sea-level calibrations to high-resolution dO records from ODP Sites 1090 and 1218. These calibrated records indicate that ice-volume ranged between 50% and 125% of the present day East Antarctic Ice Sheet (EAIS) during most of the early Miocene (23–17 Ma). Maximum icevolume occurred at each of the early Miocene isotopic events (i.e., Mi-events) concomitant with bottom water temperatures generally between ~1 and 2 8C. Rapid (bb1 myr) and high amplitude ice-volume changes also occurred intermittently during this period, with some fluctuations ranging from a fully glaciated East Antarctic continent to a partial collapse of the ice sheet (equivalent to a 50–70% reduction in the EAIS). These large-scale ice-volume changes often occurred at b100 kyr time scales suggesting an orbitally driven dynamic EAIS existed during the early Miocene. In contrast, the calibrated dO record from Site 1090 indicates significantly less ice-volume (25–70% of the present-day EAIS) was present between 17 and 16 Ma. These results are supported by numerical climate–ice sheet modeling studies that show increased orbitally driven ice-volume variability with elevated levels of atmospheric CO2. The distribution of bottom water masses during the Miocene was likely significantly different from today, owing to variability in the production of deep-water near the Antarctic continent (i.e., proto-Antarctic Bottom Water, proto-AABW). Weakening of proto-AABW is suggested to have occurred during the early Miocene especially during glacial minima, resulting in it becoming more spatially restricted to the Southern Ocean and becoming entrained into a warmer deep-water mass. Increased production of a warmer bottom water mass is also considered to have been important in the lighter isotopic values observed in the Atlantic and Indian Oceans. These changes are supported by evidence of a large dynamic EAIS based on strata cored on the Antarctic margin, and require a reevaluation of the view that Antarctic ice-volume was significantly reduced for most of the early Miocene. Previous estimates have been constrained by the lack of deep-water isotopic records proximal to Antarctica, and unduly influenced by the expansion of warmer deep-water into most of the ocean basins at this time. D 2005 Elsevier B.V. All rights reserved.