May geologists now synchronise their watches?

Calibrating the stratigraphic column to absolute time depends, of course, on radiometrically dating geochemically suitable rocks or minerals. Yet there is a range of available methods based on decay of unstable isotopes, such as 14C, 40K, 87Rb, 147Sm, uranium and thorium. All depend on a variety of assumptions, of which that of a constant, well-established half-life is common to all. If all were perfect, several methods applied to the same materials should give the same results. The trouble is, each parent isotope favours different minerals and different compositions of igneous rocks, so that discrepancies in the dates assigned by different methods to the same stratigraphic unit may either be due to disturbance of one isotopic system relative to the other or to the half-life of one (or both) parent isotope being inaccurate. Currently, the two most widely used and best-regarded methods are U-Pb and Ar-Ar, the latter depending on 40K being converted to 40Ar by neutron bombardment. The first often uses zircons, the second various potassium minerals such as alkali feldspar. Both minerals are magmatic in origin and so the same igneous rock may sometimes be dated by either method or both. It is becoming increasingly clear that the two approaches do not give the same age, which is worrisome at the detailed level permitted by the high precision of each of the methods.

A means of checking the timing parameters for radiometric dating is to compare its results with absolute age determined by a non-radiometric method. The best-calibrated and most widely possible method that does not rely on radioactive decay is based on the astronomical pacing of climate, with its 100, 41, 23 and 19 ka cycles. Analysis of cyclicity in repetitive sedimentary sequences reveals patterns of frequencies that match the astronomical signals. So, within such a sequence it is possible to chart time differences to within a few thousand years. If there are igneous rocks interlayered with the cyclical sediments it should be possible to check their radiometric age differences against the difference determined independently. A Miocene sequence in Morocco has many intercalations of igneous tephras, and therefore provides a crucial test for radiometric approaches (Kuiper, K.F. et al. 2008. Synchronizing rock clocks of Earth history. Science, v. 320, p. 500-504). The team from the University of Utrecht, the Free University of Amsterdam in the Netherlands, and the University of California, dated sanidine (K-feldspar) from the tephras using the Ar-Ar method. This involved using a standard age determined for sanidines from a similar rock type at Fish Canyon in Colorado USA. By turning the approach on its head, i.e. by using astronomically calibrated ages for the samples, they recalculated the age of the Fish Canyon standard. It seems to be 0.65% older than previously thought (from rather dodgy U-Pb dating of  zircons in the Fish Canyon Tuff).

All Ar-Ar ages involve the Fish Canyon standard. So, an underestimate of its age would imply revision of quite a lot of geological events dated by Ar-Ar, especially those that happened abruptly, such as mass extinctions, impacts and magnetic reversals. Using the new standard age puts the K/T boundary event back to 66 Ma from 65.5 Ma. The formerly 251.0 Ma mass extinction at the end of the Permian becomes 252.5 Ma, which coincides better with the outpouring of the Siberian Traps. Similarly, the once 200 Ma end-Triassic extinction, but now possibly 201.6 Ma, links better to the Central Atlantic Magmatic Province outpourings. Quite a stir may be on the horizon, if Kuiper and colleagues’ recalibration is confirmed by similar independent measures.

That radiocarbon dates need to be used with caution is well known, as the amount of 14C produced by cosmic ray bombardment of atmospheric nitrogen varies markedly over time. Again, the ‘work-around’ involves using non-radiometric ages to calibrate the fluctuating relationship between radiocarbon ages and real time. The data of choice are those from tree-ring analysis, but ice cores also preserve ages with a 1-year precision from their annual layering. The Younger Dryas cold period that interrupted the global deglaciation began when atmospheric 14C production was high. It was also a tremendously important event in the progress of human migration and perhaps even genetics – population crashes in hard times can have a ‘bottleneck’ effect on evolution. A multinational team has addressed the interrelations between radiocarbon dating, ice-core climate proxy records and tree-ring analysis for this crucial episode (Muscheler, R. et al. 2008. Tree rings and ice cores reveal calibration uncertainties during the Younger Dryas. Nature Geoscience, v. 1, p. 263-267). They combined measures of varying 14C in tree rings and 10Be in ice cores, both of which are cosmogenic. Rather than resolving the issue, they discovered that the best marine record of the carbon-cycle during the YD, in the Cariaco basin off Venezuela, has a bias caused by anomalous concentration of 14C in shallow seawater as the YD began. Their study open the possibility of resolving such changes in the marine C-cycle.

See also: Kerr, R.A. 2008. Two geological clocks finally keeping the same time. Science, v. 320, p.434-435.


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