A MAJOR REVISION OF U/TH SPELEOTHEM DATES FROM COLD WATER CAVE, IOWA

Previously reported alpha counting U/Th ages from eight speleothems collected in Cold Water Cave, Iowa were interpreted to indicate continuous speleothem growth from the present back through the last glacial maximum to 81 ka (R.S. Harmon, 1974 PhD thesis and subsequent publications). That interpretation, widely published and quoted, has become a common boundary condition on the geomorphic and paleoclimatic history of the Upper Mississippi Valley. R.S. Lively's (1983, Geology, v. 11, p. 259-262) more extensive data set found a hiatus in speleothem growth between about 13 ka and 35 ka in caves near Cold Water - but did not include any speleothems from Cold Water Cave.

The development of TIMS and magnetic sector ICP-MS techniques has improved the precision of the dates and reduced sample sizes by several orders of magnitude. A re-dating of the surviving Cold Water speleothem samples is currently underway. Nine preliminary ICP-MS ages from five stalagmites do not replicate Harmon's ages. The new ages are younger by factors of 2 to over 4. The results are preliminary because these samples have low 230Th/232Th ratios and the optimal correction factors for initial 230Th remain to be determined. However, our ages place strict upper limits on the true ages. A clear example is 74015, a 132 mm tall brown, crystalline stalagmite. Harmon (1974) reported ages of 20 ± 2.5 ka (115-132 mm, measured from base of stalagmite), 27 ± 1 ka (65-80 mm), and 34 ± 5.5 ka (0-15 mm). The ICP-MS upper limit ages, as yet uncorrected for initial 230Th, are 9.25 ± 0.26 ka (131-132 mm), 9.89 ± 0.21 ka (26-28 mm), and 11.38 ± 0.34 ka (4-5 mm). All nine ages are < 11.4 ka.

Based on these data we recommend that 1974 Cold Water Cave dates be abandoned and models and conclusions based on them reexamined as necessary.