R. Lawrence Edwards and Hai Cheng
Department of Geology and Geophysics
University of Minnesota
We have improved the accuracy and precision of 230Th dating of carbonates by increasing sample size (about an order of magnitude higher than our typical sample sizes) to about 1 mg of 238U and taking advantage of the 1 to 2% ionization/transmission efficiency (about an order of magnitude increase over previous values reported by several laboratories) of the Thermo-Finnigan Neptune. The method developed around this two-order-of-magnitude increase in detected ions improves the precision of 230Th ages by about an order of magnitude over typical errors previously reported from our laboratory. We applied our method to materials that are expected to be in secular equilibrium. For most of these materials, we obtained consistent results within the precision of the measurements. We used the analyses (for the samples that gave consistent results) to make new estimates for the half-lives of 230Th and 234U. The new values are within error of the old values, but have substantially smaller errors.
We also applied our methods to the dating of stalagmites from the region of China currently affected by the Asian Monsoon. We correlated the stalagmite records to ice core and marine records using correlation strategies verified for (more recent) times when all records are independently well-dated. Using these strategies we have determined the timing of the major events during each of the last 4 terminations. Of particular interest, in the context of this workshop, are the timing of the marine terminations, all four of which are consistent with a classic northern hemisphere summer insolation trigger for initial breakup of northern ice sheets, predisposed to collapse. Meltwater entering the north Atlantic alters oceanic and atmospheric circulation patterns and associated fluxes of heat and carbon, causing increases in atmospheric CO2 and Antarctic temperatures. Increasing CO2 and insolation drive the termination, with likely positive feedbacks between sea level and CO2. This scenario explains the rapidity of terminations, clear from their very first characterizations (Emiliani, 1955; Broecker and von Donk (1970), as well as aspects of the “100 ky” problem, evident in the first spectral analyses of ice age cycles (Hays et al., 1976).
Emiliani, C. (1955), Pleistocene temperatures, J. Geol., 63, 538-578.
Broecker, W. S., and von Donk J. (1970), Insolation changes, ice volumes, and the d18O record in deep-sea cores, Rev. Geophys. Space Phys., 8, 169-198.
Hays, J. D., Imbrie, J., and Shackleton, N. J. (1976), Variations in the Earth’s orbit: Pacemaker of the ice ages, Science, 194, 1121-1132.