Tying down the Devonian-Carboniferous boundary

Getting the stratigraphic column properly calibrated from relative to absolute time is all the rage these days (New benchmarks for geological time in EPN June 2004).  On the recent stratigraphic chart published in late 2003 by the International Commission on Stratigraphy, the Devonian-Carboniferous boundary has a “golden spike” global standard section and point (GSSP) dated at 359.2 ± 2.5 Ma.  Already, that is disputed because of new radiometric dating from an “auxiliary” global stratotype section (Trapp, E. et al. 2004.  Numerical calibration of the Devonian-Carboniferous boundary: Two new U-Pb isotope dilution-thermal ionization mass spectrometry single-zircon ages from Hasselbachtal (Sauerland, Germany).  Geology, v. 32, p. 857-860).  As well as holding the record for length of any publication title yet covered by EPN, the paper contains some intriguing points.  That a carefully determined age for the strata at Hasselbachtal has been possible is thanks to about six, centimetre-thick ash beds in richly fossiliferous sediments just above the faunally determined boundary.  Twenty-three single-zircon ages from the two ashes just above the accepted faunal boundary give ages of 360.5 ± 0.8 and 360.2 ± 0.7 Ma.  Now, to you and I and many less pernickety geochronologists, that spells out the well-known phrase or saying, “within error”, as indeed is that of the GSSP.  And, for a convoluted reason based on plotting an age from another tuff with these ages against the palaeontological data, the age presented for D-C itself is 360.7 ± 0.7 Ma.  This may be a better age than that of the GSSP.  But, so what?  The D-C boundary is not associated with any family-crushing catastrophe like the P-T or K-T boundaries, nor even that within the Late Devonian itself.  Are “they” going to move the GSSP from its present location in southern France, ratified in 1990, along with the vast pyramid of precious and intricately carved crystal, which no doubts marks its spot?  An altogether more serious threat to the established order is the stealthy attempt to abolish the last remnant of the great stratigraphic divisions inspired by Giovanni Arduino’s work in the 18th century; the Quaternary is besieged!  One of my spies, not unconnected with this episode of our own emergence on the planet, attended a stormy meeting at the 32nd International Geological Congress in Florence in August 2004, which seemed likely to expunge the Quaternary from the minds of all future geologists.  He gleefully reported that a mighty rearguard action had put off that evil day, at least for a while.  Sadly, the writing is already on the great IUGS/ICS stratigraphic wall chart – its is no longer there!  The last relic in officialdom is in the latest definitive publication (Gradstein, F.M. et al. 2004.  A new geologic time scale with special reference to Precambrian and Neogene.  Episodes, v. 27, p. 83-100).  On page 86, at the very top of the table conferring status on GSSPs, it is written “This composite epoch [the “Quaternary”] is not a formal unit in the chronostratigraphic hierarchy”.  So there you have it; the issue is getting things into proportion.

A record of the Palaeoproterozoic lunar cycle

One of the many natural processes that produce rhythmic sediments is the ebb and flow of the tide, twice a day and with an amplitude that peaks and falls twice each lunar month (today a 28-day cycle) to produce spring (new and full moon) and neap tides (the two half moons).  Tidal rythmites consist of thin laminae whose thicknesses vary regularly for many cycles.  Their occurrence dates back to 3.2 Ga, and along with other sedimentary structures formed by tidal action, such as “herring-bone” cross stratification formed by reversals in tidal currents, prove the presence of the Moon in orbit around the Earth.  Fine rythmites can be analysed to work out the length of the lunar month in the past, and help refine ideas on the evolution of the Earth-Moon system.  Rajat Mazumder of Asutosh College, Kolkata, India has analysed the earliest known tidal rythmites from the Palaeoproterozoic of NE India (Mazumder, R. 2004.  Implications of lunar orbital periodicity from the Chaibasa tidal rhythmite (India) of late Paleoproterozoic age.  Geology, v. 32, p. 841-844).  His work shows that between 2.1 to 1.6 Ga the lunar month was 32-days long.  Remarkably, the record in these sediments is as detailed as found in modern ones from estuarine silts.  As well as rhythms, they record occasional perturbations due to storms.  Using the changes in the lunar month during the last 450 Ma erroneously suggests that the system emerged from a period around 1.5 to 2.0 Ga following a major collision – that of course is ruled out by a total lack of evidence of such a catastrophe.  The new datum suggests instead a steady decrease in the lunar month, that corresponds with the Moon’s gradually receding from the Earth.  Energy apparently lost by tidal action is conserved by an increase in the angular momentum of the Earth-Moon system, and that forces the Moon ever further from us – its orbital velocity increases.

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