Time wars flare up again

English: A diagram of the geological time scal...
Time's spiral Image via Wikipedia

Last year Earth Pages News reported a rationalisation of the way in which geological time is signified (Rationalising geological time 7 May 2011). A working group set up by the International Union of Pure and Applied Chemistry (IUPAC) and the International Union of Geological Sciences (IUGS) defined the year as the base unit, standardizing it to the time in seconds between one solstice and the next at the equator for year 2000 (3.1556925445 × 107 s) thereby linking it to the Système international d’unités or SI base unit of the second, itself defined in terms of behaviour of the caesium atom. It is to be signified by ‘a’ for annus (year in Latin) and preceded by ‘k’, ‘M’ and ‘G’  for thousands, millions and billion years, complying with the SI progression in steps of 103 for units.

The sticking point for some, mainly in the US (e.g. Science magazine and many geoscientists there) is that the ka, Ma, Ga symbols are to apply not only to times before the present but also to spans of geological time. Since the agreed convention is incorporated into SI it has almost the force of law for scientists , so that the Cretaceous Period will be said to have begun at 145.5±4.0 Ma ago, ended at 65.5±0.3 Ma ago and was 80 Ma long, instead of the latter being in m.y., m.yr., mya  or Myr according to what seem to have been personal quirks or those of scientific journals.

Somewhat florid reaction against the rationalisation (Christie-Blick, N. 2012 Geological time conventions and symbols. GSA Today, v. 22 (February 2012 issue), p. 28) seems to have flowed from a deliberation on the IUPAC-IUGS proposal (in Prague, Spring 2010) by a lesser world body: the International Commission on Stratigraphy’s  (ICS) International Subcommission on Stratigraphic Classification (ISSC). The meeting voted 16 to 2 to reject the proposal – a substantial number of voting members abstained – claiming that it violated SI ‘rules’ regarding base- and derived units. The issue, on reaching the ICS meeting, as the same Prague workshop, seems to have been greeted by a 50:50 split. A closed meeting of the ICS Bureau (now we can begin to see the kind of thinking involved here…) on the workshop’s last day unanimously adopted the motion ‘We neither accept nor reject the IUGS-IUPAC Task Group’s recommendation to apply Ma, generally, as the unit of deep time. We accept the argument for Ma as a single unit for time but would recommend flexibility, allowing for the retention of Ma as specific notation for points in time (i.e., dates) and myr as a unit of time denoting duration. We agree with the spirit of this statement’ [my italics]. ‘Neither accepting nor rejecting’ is something familiar from minutes of the Central Committee of the former USSR, being rumoured to have been Joseph Vissarionovich Stalin’s favoured formulation in moments of uncertainty: a little like the old ‘Belfast Question’, ‘Are you for us or against us’ from someone whose politics is not entirely clear.

An argument proffered by Christie-Blick is, ‘No one objects to the storming of the Bastille on 14 July 1789 (a date) or to the construction of Stonehenge from 2600–1600 BC (an interval specified by two dates). In the case of the latter, we say that the job took 1000 years, not 1000 BC.’ This forgets something quite practical: geochronologist rarely if ever, ‘neither accept nor reject’ AD, BC BCE, or CE but express time in years before present, with the odd convention that ‘the present’ was 1950, before atmospheric testing of thermonuclear devices. What is wrong with the answer to the question, ‘When did the Cretaceous begin?’ being 145.5 Ma ago, or ‘80 Ma’ in answer to, ‘How long did it last?’ Who would prefer the alternative to the second question –  80 (choose your preferred symbol from the following: m.y., m.yr., mya.  Myr., million years or millions of years)?

Rationalising geological time

A diagram of the geological time scale
The Geologic Time Spiral: A Path to the Past. Designed by Joseph Graham, William Newman, and John Stacy. Get it from http://pubs.usgs.gov/gip/2008/58/

The Système International d’Unités (SI) is the agreed arbiter that defines the units in which phenomena are measured. There are 7 SI base units (length, mass, time, electric current, temperature, intensity of radiation and amount of substance) from which others are derived as they become necessary. Geoscientists have striven to comply, though not always happily. For instance the doubly-derived SI unit for pressure, the pascal (Pa) is a newton (derived unit of force) per square metre (N m-2), and in base units 1 kg m-1 s-2. The pascal replaced the long employed arbitrary unit, the kilobar (1 kb = 1000 x surface atmospheric or barometric pressure) one of which represents about 3.5 km depth in the earth. The reluctance to shift units is probably innate conservatism, for 1 kb = 100 MPa: simples!

Another problem has arisen as regards the SI base unit for time – the second. This is unwieldy for geological time, the Earth having formed approximately 1.435 x 1017 seconds ago. It’s not so handy for history either, about 3 x 1010 seconds having elapsed since William of Normandy won the Battle of Hastings.

The year is what we remember, but even that in a historical sense has its problems, for instance the BC/AD division where some scholars even dare to suggest that Christ was born in 4 BC. The more politically correct Common Era (CE) and Before the Common Era (BCE) of course don’t fool anyone. Interestingly, Wikipedia (en.wikipedia.org/wiki/Year) indicates, there are over ten current versions of a ‘year’ depending on context (for instance, astronomers favour the Julian year). Historical and thus geological time has the unnerving habit of continually getting longer, and it is a major problem to measure historical time precisely, either from increasingly vague records as one delves back in historical documents or because of the inherent imprecision in measuring radioactive isotopes and their daughter products that underpins archaeological and geological time. Archaeologists have a very hard time of it, for their workhorse is radiocarbon dating that depends on the production of radioactive 14C in the atmosphere by cosmic ray’s interaction with nitrogen. The rate of 14C production varies over time with the cosmic ray flux from extra-solar sources, and even worse, a very large amount was produced by testing nuclear weapons in the atmosphere in the mid 20th century. Abandoning the BC/AD division that lurks still with historians and archaeologists, geoscientists speak of time ‘before present’ (bp), which doesn’t matter a damn for geological Periods, Eras and Eons which are immensely long whatever the unit. But it does for the Holocene, mainly calibrated by radiocarbon methods: bomb-test production of 14C , which will linger about 50 thousand years before near-complete decay, has forced the ‘present’ to be set at 1950 AD!

So the year is here to stay, even though it is arbitrary and changes all the time, along with kilo, mega and giga prefixes for thousands, millions and billions of years. Yet teeth are now being ground over what the unit’s symbol should be (Biever, C. 2011. Push to define year sparks time war. New Scientist, v. 210 (30 April 2011), p. 10).  A task group of geoscientists and chemists set up by the International Union of Pure and Applied Chemistry, IUPAC, and the International Union of Geological Sciences, IUGS in 2006 have now defined the year – why chemists, you might wonder; they measure the radioactive decay constants of isotopes used in radiometric dating. The link to the SI system through the base unit of one atomic-standard second is to be standardised by the solar year; the time in seconds between one solstice and the next at the equator for year 2000: i.e. 3.1556925445 × 107 s (Holden, N.E. et al. 2011. IUPAC-IUGS common definition and convention on the use of the year as a derived unit of time (IUPAC Recommendations 2011). Pure and Applied Chemistry, v. 83, p. 1159-1162). It is to be called the annus (a), applied in ka, Ma or Ga to two usages of time, the time difference between ‘now’ and an event in the past, and the time difference between two events in the past. This dual usage of the same symbol is the source of the gnashing. Whereas Ma, for instance, was quite acceptably used for the measured age of a rock relative to the present, there are at least three schools of thought for other uses of time. Some have been quite happy to use Ma for measured age, a fixed time datum in the past such as the Precambrian-Cambrian boundary, and a time duration such as that of a geological Period or some major event such as an orogeny (that has been used in Earth Pages News since its outset). Others would distinguish between the first and the other two, as for instance Ma for the first and Myr for the other two. But there are variants, the symbol mya having been used for ‘million years ago’, and the international science journal Nature currently uses Myr for the first but now takes the safe path of using ‘million years’ for the other two. Nicholas Christie-Blick of Columbia University in New York is reported as having opined that the rationalisation to one-symbol-fits-all is a huge step backwards, and he is not alone; Science editorial staff will continue to demand of their authors a distinction between age and time span, since a switch would ‘confuse its readers’, long accustomed to that usage.

Also it is so easy to write, ‘the rock has an Ar-Ar age of 25 Ma’, ‘it took 25 Ma for this trilobite to disappear from the geological record’, and ‘about 25 Ma ago, there is a gap in the fossil record of primates’. I personally welcome the simplification, especially as it will encourage authors to write more nicely.