New Research Opportunities in the Earth Sciences - New Research Opportunities in the Earth Sciences - page 82

RECENT ADVANCES IN GEOCHRONOLOGY
A common theme running through previous sections of this chapter is the
growing reliance on geochronology to provide quantitative estimates of the age,
duration, and rate of events and processes over many different timescales. As a result
of improvements in analytical methods and in the theoretical underpinnings and
calibrations of a variety of dating methods, the past few years have seen
transformative advances in many approaches to geochronology. Areas of notable
growth include surface exposure dating using rare isotopes produced by cosmic rays,
determination of cooling histories of rocks (thermochronometry), extremely high
precision dating of volcanic ashes, and high-throughput dating of detrital minerals.
These geochemical techniques provide quantitative estimates of time that are an
essential complement to dates and rates established using magnetostratigraphy and
increasingly reliable methods of cyclostratigraphy (counting of orbitally paced
oscillations recorded in sedimentary rocks).
Recent work greatly improving the ability to extract extremely precise and
accurate ages from both the U/Pb and 40 Ar/ 39 Ar methods underscore recent advances
and illustrate likely future directions both in terms of method development and
application.

High Precision-High Accuracy Radiometric Dating

Given the wide applicability of the U/Pb and 40 Ar/ 39 Ar methods, especially to
dating ashfalls in sedimentary sequences, recent improvements have had and will
continue to have a major impact on the Earth sciences. In the case of U/Pb dating, a
remarkable series of discoveries culminating in the work of Mattinson (2005) has
revealed an analytical approach by which the consequences of Pb loss on zircon U/Pb
dates can be almost entirely removed. This new approach permits routine
determination of U/Pb dates with a precision of better than 0.1 percent.
Geochronologists are also continuing to reduce other sources of error, including spike
calibration, instrumental mass fractionation, decay constants, and the magma chamber
residence time of zircon crystals prior to eruption and deposition.
Profound new insights into the rates of geochemical and biological processes
are possible with ages precise to a small fraction of a percent. For example, Maloof et
al. (2010) recently investigated a portion of the early Cambrian period associated with
the appearance of the first calcite biomineralizing organisms and an associated
dramatic change in global carbon cycling, as indicated by a large

13 C shift of marine
carbonate (see Figure 2.26). Dates of multiple ash fall zircons show that the event
occurred at 525.34 ± 0.09 Ma, and the adjustment in global carbon cycling occurred
in 506±126 kyr. The rate of this event suggests that these changes arose from
biological diversification occurring at that time.
The ability to obtain extremely accurate and reliably inter-calibrated ages
allows previously impossible high precision cross-correlation of events recorded in
different localities. For example, Schoene et al. (2010) dated the end-Triassic mass
extinction to 201.32 Ma in sedimentary sections in both Peru and Nevada and
determined that the extinction was complete in <300 kyr. Additional dates from the

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