When we come to the near past, signifying time that has elapsed becomes unclear. Most christians divide the last four thousand years into AD and BC (with some confusion as to whether the division is at 1 or 0 AD), yet muslims place their starting year differently, and so might many other faiths, if they so chose. The adoption of ‘Before the Common Era’ and ‘the Common Era’ (BCE and CE, which are the same as BC and AD) really doesn’t help politically, being based on a now obvious fact: that the dominantly christian US and EU dominate the planet. The only foolproof way to judge elapsed time in years is to have some continual and irrefutably annual events to count. Now, it is not always convenient to use the annual growth rings in a collection of enormous logs of a variety of ages to tell time, and the same goes for snow layers in polar ice caps and layered stalagmites. Using the decay of radioactive 14C in preserved carbon-containing materials revolutionised archaeology and the science of recent climate change. But it has a snag, for 14C, unlike many other geochronometers, is continually being formed, by cosmic ray bombardment of nitrogen in the upper atmosphere. Cosmic ray flux is not constant, so the proportion of 14C to stable carbon was different at any time in the past. Until recently nobody knew how that proportion had varied. Radiocarbon ages have to be calibrated in some way, so that they record events in a truly absolute time-frame. Without calibration, even the most precise age determinations give a warped view of history (see Rationalising radiocarbon dating in the February 2004 issue of EPN). For instance, the date when the Younger Dryas glacial pulse began was a thousand calendar years before its calibrated 14C age. Despite heroic efforts to establish a link between radiocarbon ages and the true passage of years from long annual records in dateable materials, calibration gaps in the ~50 ka period achievable by using the quite short half-life of 14C have caused a problem. Many published and even some new dates are given without calibration, while others are in ‘years before present (BP)’, i.e. before the start of above-ground atomic bomb tests in 1950, which uniformly contaminated all later atmospheric carbon with 14C produced by nuclear transformation. The confusion should soon be resolved as the effort to match productivity of 14C to real time nears completion (Balter, M. 2006. Radiocarbon dating’s final frontier. Science, v. 313, p. 1560-1563). But some workers are impatient to give real ages using calibration curves for difficult periods, which have not yet been verified and are controversial. An interesting case relates to the possible overlap period, roughly around 35 to 30 ka ago, between fully modern humans and Neanderthals in Europe. That awkward era may soon be clarified with the unearthing of monstrous logs from New Zealand swamps, which may contain annual rings back to the 50 ka limit.
Is the idea of Hadean continental crust bunkum?
As these monthly jottings have noted several times, the geological record of the Hadean (before 4 Ga ago) could easily be lost through an ill-timed sneeze: it consists of a few minute zircon grains extracted from common or garden Archaean meta-sandstones in Western Australia. Milligram for milligram, these have become the heaviest punchers in the world of geochemical debate. They undoubtedly crystallized as long ago as 4.4 Ga. More controversially their detailed chemistry has been suggested to indicate that their crystallization was from granitic magma formed by partial melting of materials that interacted with water at around 700°C; materials that were not primarily of mantle composition (see Zircons and early continents no longer to be sneezed at in EPN February 2006 issue). If true, that would suggest low-density crust that found difficulty in being recycled into the mantle only a few tens of Ma after the Earth’s formation. Either that crust was too thin to resist subduction by some kind of tectonic slicing and has gone for ever, or some of it is still out there waiting to be found…by those who become very excited by extremely aged rocks. There is a simple way of putting the early-granite hypothesis to the test — by seeing if zircons in basalts are any different from them (Coogan, L.A. & Hinton, R.W. 2006. Do the trace element compositions of detrital zircons require Hadean continental crust? Geology, v. 34, p. 633-636).
Coogan and Hinton, of the University of Waterloo, Canada and Edinburgh University respectively, show that Hadean zircons cannot be distinguished chemically from those found in gabbros that have differentiated from basaltic magmas at modern mid-ocean ridges. As if that were not sufficiently deflating, they also made crystallization-temperature estimates of the gabbro-derived zircons, using a geothermometer that uses the titanium content of zircon in equilibrium with rutile. Despite the fact that the real temperature of gabbro crystallization is well over 1000°C, these estimates came in at between 700 and 800°C. That is, about the same as those proposed as evidence for the crystallization temperature of Hadean zircons from a granitic magma. Coogan and Hinton were not content, and go on to offer an alternative explanation for the zircon’s oxygen isotopes, used by others as evidence for the influence of water at shallow depths back to 4.4 Ga. The seemingly water-derived 18O excess in the zircons could well have come from carbonates recycled from surface weathering of basalt, to be assimilated by deep basaltic magma chambers.