May 2008 GEOLOGY media highlights

Uranium-lead dating of zircon is the most accurate and precise way to date igneous rocks, but age precision is rarely better than about plus or minus 1 million years for the Precambrian. Davis describes a new technique that potentially offers a considerable improvement in the accuracy of zircon ages on a wider variety of samples and at less cost than what is generally available from conventional methods. Zircon is first pre-heated in a vacuum to evaporate lead from disturbed crystal domains, leaving lead with isotopes that preserve the primary age of the sample. This refractory lead is removed from the zircon by thermal extraction at higher temperature directly into a silica-melt ionization activator within a thermal ionization mass spectrometer. This produces strong ion beams that allow the daughter lead isotope ratios to be measured to high precision with almost negligible contamination from common lead. Isotope fractionation appears to be more reproducible than with conventional solution analysis, allowing routine age determinations with precision of approximately plus or minus 0.2 million years in the Precambrian. As a test application, zircon from a mafic boundary phase of the Sudbury impact melt gives 1849.53 ± 0.21 million years ago (Ma), while a phase from several hundred meters higher in the impact melt sheet is resolvably younger at 1849.11 ± 0.19 Ma (95% confidence errors). The enhanced precision and ease of application of this method should greatly increase the scientific power and availability of zircon dating.

Reconstructing relative flooding intensities responsible for hurricane-induced deposits from Laguna Playa Grande, Vieques, Puerto Rico
Jonathan D. Woodruff et al., Massachusetts Institute of Technology-Woods Hole Oceanographic Institution Joint Program in Oceanography, Woods Hole, Massachusetts 02543, USA. Pages 391-394.

Woodruff et al. estimate the relative flooding intensities for hurricanes striking eastern Puerto Rico over the past 5000 years. Flooding conditions are inversely modeled based on the grain-size distribution of sediment deposited in a coastal lagoon during each hurricane event. A deposit associated with the infamous A.D. 1928 San Felipe hurricane is used as a modern analogue to test the technique, which produces reasonable estimates for wave heights that exceeded the site’s barrier during this Category 5 event. The lagoon’s sedimentary record exhibits large-scale changes in the frequency of hurricane flooding on centennial-to-millennial time scales; however, Woodruff et al. show that the magnitude of flooding at the site during these events appears to have stayed relatively constant through time. Over the past 5000 years, no evidence exists for an anomalously large hurricane or tsunami event with a competence for sediment transport any greater than historical hurricane events.

Groundwater and climate dynamics derived from noble gas, 14C, and stable isotope data
Stephan Klump et al., EBA Engineering Consultants Ltd., Calcite Business Centre, Unit 6, 151 Industrial Road, Whitehorse, Yukon Y1A 2V3, Canada. Pages 395-398.