Paul J. Hearty1, Michael O’Leary2, Maureen E. Raymo3, Marci M. Robinson4,
Harry J. Dowsett4
1 Dept. of Environmental Studies, University of North Carolina, Wilmington, NC
2 Dept. Environmental & Geographical Sciences, Manchester Metropolitan University, Manchester, UK
3 Dept. of Earth Sciences, Boston University, Boston MA
4Eastern Geology and Climate Science Center, U.S. Geological Survey, Reston, VA
Unraveling the geologic record of past sea-level changes during warm or long interglacials provides a scientific perspective on the “behavior” of sea level under non-anthropogenic conditions. Geological and geochronological datasets are being compiled for interglacial intervals of the Pleistocene (MIS 5e/11; 125 and 400 Kyr) and the mid-late Pliocene (G17, K1 and KM3; 3.3 to 2.9 Myr) seeking to understand the fluctuations of interglacial sea level and the commensurate loss of polar ice volume from Greenland (GIS), West (WAIS), and East Antarctic Ice Sheets (EAIS).
Proxy deep-sea oxygen isotope records suggest that both MIS 5e and MIS 11 experienced higher seas and warmer oceans. Furthermore, the coastal rock record from relatively stable coastlines of Bermuda, Bahamas, and Australia reveals direct physical evidence of extreme sea-level fluctuations during both warm (and in the case of MIS 11) long interglaciations. In each case, TIMS and ICPMS U/Th methods confirm the correlation with these warm periods. Because Fennoscandian and Laurentide ice sheets were probably at their minimum extent, rapid terminal sea-level rises as high as +6 to +9 m (MIS 5e) and +18 to +20 m (MIS 11) would require melting or collapse of some remaining ice sheets. Reasoning that the GIS melts back from the effects of warmer seas and the marine-based WAIS becomes unstable with rising seas, it is suggested that partial GIS melting was a precursor for WAIS collapse during MIS 5e. A similar scenario is envisioned for MIS 11, with melting GIS leading to WAIS collapse, followed by drawdown of adjacent EAIS basins. A recently initiated investigation in S and W Australia (our “PLIOMAX” project) has identified at least 8 potential Pliocene shoreline sites generally +20 to +70 m -- some documented as uplifted -- some lacking any evidence of the same. The Roe Plain along the Southern Ocean appears to be a geomorphic manifestation of high frequency (41 Kyr), low amplitude (±10-30 m) sea-level oscillations over perhaps >0.5 Myr of the Pliocene. The persistence of sea level at this position acted as a buzzsaw that carved out a vast 30 x 250 km platform surface. Upon confirmation of the age of these deposits by a combination of paleomag, Sr-isotopes, and biostratigraphy, we expect to more accurately define sea level and greatly reduce the uncertainty of sea/ice-volume changes during the Pliocene warm period. These examples demonstrate a dynamic Antarctic ice sheet from at least the mid Pliocene onward.