Late Pleistocene Paleoclimates of North America as Inferred from Stable Isotope Studies of Speleothems1

Some speleothems (CaCO 3 cave deposits) can be demonstrated to have been formed in oxygen isotopic equilibrium with their parent seepage waters and thus a record of relative fluctuations in depositional temperature can be obtained from the δ 18 O variations in successive growth layers of such deposits. These temperature fluctuations reflect variations in the average annual air temperature at the surface above the cave, and therefore permit inference of past continental climate changes. Equilibrium deposits have been obtained from caves in San Luis Potosi, Bermuda, Kentucky, West Virginia, Iowa, and Alberta, ranging in age from 200,000 years BP to the present, as determined by 230 Th/ 234 U dating of the speleothems. The δ 18 O time curves for the six sites show the following synchronous climatic fluctuations: warm periods from 190 to 165 and from 120 to 100 Ka, at 60 and 10 Ka, and cold periods from 95 to 65 and from 55 to 20 Ka. The periods of thermal maxima correspond in time to the interglacial periods of the marine foraminiferal isotopic and faunal temperature records and to periods of high sea stand as observed from radiometric dating of raised coral reefs. Maxima and minima in insolation appear to be synchronous with this record as well.