New benchmarks for geological time

In the December 2003 issue of EPN, I mentioned a programme aimed at sorting out the calibration of the stratigraphic column to an absolute or radiometric timescale (Recalibrating the stratigraphic column).  The other side of this task is deciding on where to place the “golden spikes”, otherwise known as global standard stratotype-section and points (GSSPs).  They are locations where the best exposures of world-wide events can be found.  The first, defining the disappearance of graptolites at the Silurian-Devonian boundary (no-one knows why that happened), was placed in 1972 near the wonderfully named town of Klonk in the Czech Republic.  GSSPs are essential in defining events, no matter if their ages change as dating methods and results advance.  Until 1999 the problem was that only 15 of the 91 stage boundaries of the Phanerozoic had been defined agreeably by such “golden spikes”.  That year the International Union of Geological Sciences (IUGS) spurred a crash programme of GSSP definition, but there have been political as well as geological disagreements.  The most important “spike” is at the Permian-Triassic boundary – the end of the Palaeozoic Era, and the time of the largest ever mass extinction – and there have been heated discussions over whether to have it in Iran, Kashmir or China.  Zhejiang Province in China won, and it now has a 6 metre high monument at the boundary!  This and Klonk should be on every geologists’ future tourist itineraries.  There are now 50 stage-boundary GSSPs, and together with a revision of currently accepted dates, the revised stratigraphic column can be downloaded as a (rather large) PDF from http://www.stratigraphy.org/.  All is not so well with Precambrian time, for the obvious reason that it contains no tangible fossils, and it is still arbitrarily split by round-number dates.  But there is some hope for a similar system of “golden spikes” that use probably global events such as glacial epochs, and perhaps shifts in the d13C of carbonate sediments that should record global changes in ocean composition.

Source:  Whitfield, J. 2004.  Time lords.  Nature, v. 429, p. 124-125

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