US Geological Survey


Date of this Version



Muhs, D. R., 1992, The last interglacial-glacial transition in North America: Evidence from uranium-series dating of coastal deposits, in Clark, P. U., and Lea, P. D., eds.. The Last Interglacial-Glacial Transition in North America: Boulder, Colorado, Geological Society of America Special Paper 270.


U.S. government work


Considerable uncertainty exists as to whether the last interglacial was relatively "short" (~10 ka) or "long" (-20-60 ka), although most investigators generally agree that the last interglacial correlates with all or part of deep-sea oxygen-isotope stage 5. A compilation of reliable U-series ages of marine terrace corals from deposits that have been correlated with isotope stage 5 indicates that there were three relatively high sea-level stands at ca 125-120 ka, ca. 105 ka, and ca. 85-80 ka, and these ages agree with the times of high sea level predicted by the Milankovitch orbital-forcing theory. At a number of localities, however, there are apparently reliable coral ages of ca. 145-135 ka and ca. 70 ka, and the Milankovitch theory would not predict high sea levels at these times. These ages are at present unexplained and require further study.

The issue of whether the last interglacial was "short" or "long"can be addressed by examining the evidence for how high sea level was during the stands at ca. 125 ka, ca. 105 ka, and ca. 80 ka, because sea level is inversely proportional to global ice volume. In technically stable areas such as Bermuda, the Bahamas, the Yucatan peninsula, and Florida, there is clear evidence that sea level at ca. 125 ka was +3 to +10 m higher than present. During the ca. 105 ka and ca. 80 ka high sea-level stands, there is conflicting evidence for how high sea levels were. Studies of uplifted terraces on Barbados and Haiti and most studies of terraces on New Guinea indicate sea levels considerably lower than present. Studies of the terraces and deposits on the east and west coasts of North America, Bermuda, and the Bahamas, however, indicate sea levels close to, or only slightly below, the present at these times. Thus, data from Barbados, Haiti, and New Guinea indicate a "short" last interglacial centering ca. 125 ka, but data from the other localities indicate that sea level was high during much of the period from 125 to 80 ka, and that there were two minor ice advances in that period.

If it is accepted that the last interglacial period was relatively "long" and ended sometime after ca. 80 ka, then coastal deposits on the California Channel Islands record a shift in the nature of sedimentation at the interglacial/glacial transition. Marine terraces that are ca. 80 ka are overlain by two eolianite units separated by paleosols. U-series ages of the terrace corals and carbonate rhizoliths indicate that eolian sedimentation occurred between ca. 80 and 49 ka, and again between ca. 27 and 14 ka. Eolian sands were apparently derived from carbonate-rich shelf sediments during glaciallylowered sea levels, because there are not sufficient beach sources for calcareous sediment at present. The times of eolian sedimentation agree well with times of glaciation predicted by the Milankovitch model of climatic change.