COSMOGENIC-NUCLIDE AND VARVE CHRONOLOGIES FOR THE DEGLACIATION OF SOUTHERN NEW ENGLAND
G. Balco, J.M. Schaefer
Quaternary Geochronology 1, 15-28 (2006)
Abstract
We report new cosmogenic-nuclide exposure ages from the Ledyard and Old Saybrook Moraines in eastern Connecticut, summarize previously published exposure ages from elsewhere in southern New England, and compare the resulting deglaciation chronology with that derived from the New England varve chronology. The geomorphic context of southern New England moraine boulders indicates that postdepositional disturbance of boulders, and consequent scatter in boulder exposure ages, should be negligible. Exposure ages of these boulders reinforce this conclusion: the scatter among boulder ages from each moraine is no more than that expected from the measurement uncertainty of the ages. We therefore conclude that geologic uncertainties in the exposure histories of the boulders are relatively unimportant, and that the precision of the exposure-age chronology for deglaciation of southern New England is limited only by the measurement uncertainty of each exposure age and the number of exposure ages. However, exposure ages for deglaciation are nominally at least 1700 yr younger than deglaciation ages inferred from the New England varve chronology and its associated calibration to the absolute calendar year time scale, which is a significant discrepancy relative to the internal precision of each chronology. This discrepancy is similar in size to the uncertainties in the two independently determined parameters that link the two chronologies to the absolute calendar year time scale, that is, the Be-10 production rate and the varve year-calendar year offset. Taking into account the uncertainty in these two parameters, the two chronologies essentially agree, and present the opportunity to more accurately determine these parameters by enforcing internal consistency between the two chronologies. The combined deglaciation chronology that results from this exercise indicates that southeastern Connecticut was deglaciated 18,500-19,000 yr BP. It suggests that the varve year-calendar year offset has been overestimated by several hundred years and that the local Be-10 production rate has been overestimated by a few percent, and it is consistent with: (a) independent measurements of the varve year-calendar year offset; (b) independent measurements of the Be-10 production rate; (c) relevant limiting radiocarbon ages; and (d) the present understanding of the most likely relationship between the exposure age of major moraines and North Atlantic climate changes. The internal consistency of the two chronologies could be further improved by additional exposure dating of ice-marginal landforms that have direct stratigraphic links to the varve chronology. This, in turn, would also result in improved estimates of both the varve year-calendar year offset and cosmogenic-nuclide production rates.
