- The Ka'ena Highstand of O'ahu, Hawai'i:Further Evidence of Antarctic Ice Collapse during the Middle Pleistocene1
Marine isotope stage (MIS) 11 may well represent one of the most significant interglacial highstand events of the past million years. Ocean volume changes charted from coastal exposures imply partial or complete melting of some of the world's major ice caps during a middle Pleistocene interglacial. The coastal geology of both Bermuda and the Bahamas yields evidence of an MIS 11 highstand 20 m higher than present. Further support for this catastrophic episode in sea-level history is revealed in subtidal and intertidal deposits at +28±2 m in O'ahu, Hawai'i. The stratigraphy, petrology, and uplift history of the Hawaiian deposits strongly suggest a correlation with MIS 11, and a compilation of amino acid racemization, uranium/thorium (alpha and mass spectrometry), and electron spin resonance ages shows a scatter between 300 and 550 kyr. When corrected for uplift, the Ka'ena Highstand succession at Wai'anae Health Center (OWH1) reveals a "stepping up" of sea level through the interglaciation, similar to that described in the Bahamas. Previous studies on O'ahu attributed all 28 m elevation of the Ka'ena Highstand to uplift since 0.5 Ma, but now it appears that only 8 m of that was caused by uplift, and the remaining 20 m by eustatic sea-level rise. These findings from O'ahu strengthen evidence for the complete disintegration of the Greenland and West Antarctic ice sheets and partial melting of the East Antarctic ice sheet during the middle Pleistocene. If the instability of polar ice sheets can be linked to prolonged warm interglaciations as the data suggest, then existing conservative predictions for the magnitude of sea-level change by future "greenhouse" warming are seriously underestimated.
A key to evaluating the potential threat of abrupt sea-level rise may be in understanding the behavior of ice sheets during past very warm and/or prolonged interglaciations. Of particular interest in assessing the Holocene climate, because of similar insolation values at that time, is marine isotopic stage (MIS) 11, which the deep-sea oxygen isotope records of ODP Site 677 (Shackleton et al. 1990) and SPECMAP (Imbrie et al. 1984) suggest occurred between 423 and 362 kyr (Figure 1) and lasted for 60 kyr. The deep-sea oceanographic evidence is compelling, yet remains at best an indirect or circumstantial measure of sea-level change during MIS 11. Other than direct studies at or beneath the ice sheet (Sherer et al. 1998), ocean volume changes in response to ice melting are best measured along stable shorelines or at locations where the tectonic element is well understood. Direct dating of MIS 11 tectonically elevated deposits near Rome, Italy, indicate that the interglacial highstand persisted for 15-37 kyr (Karner et al. 1999) and ended around 400 kyr ago (Karner and Renne 1998). A sequence of coral reefs and shoreline deposits of middle Pleistocene age between modern sea level and +28±2 m are well exposed along the west and northwest coasts between Ka'ena Point and Wai'anae town (Figure 2) and at lower [End Page 65] elevations (e.g., Black Point) elsewhere on O'ahu.
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Reviews concerning the sea-level, oceanographic, and ice history of MIS 11 are available (Burckle 1993, Oppenheimer 1998, Poore et al. 1999). On the North Slope of Alaska, Kaufman and Brigham-Grette (1993) and Brigham-Grette (1999) identified a +22 to +23 m middle Pleistoceneage shoreline and interpreted it to be associated with the Wainwrightian transgression. Similarly, in southern Britain, Bowen (1999) estimated an uplift-corrected sea-level highstand of +23 m and correlated this with MIS 11 on the basis of amino acid racemization (AAR) age estimates. Outcrops in Bermuda and the Bahamas reveal strong evidence of an important interglacial highstand with a rise of sea level to approximately +20 m during the middle Pleistocene. This...