DATUM AND DURATION: A CLARIFICATION

Contention has recently arisen within the earth-historical community with regard to the proper notation for expressing values in geological time. The most widely used is a parallel system in which age before present is indicated by the symbols Ma (Mega-annus), Ga (Giga-annus) and ka (kilo-annus), compared to simple quantities abbreviated as Myr (millions of years), Gyr (billions of years) and kyr (thousands of years), among several schemes. An effort has been made by some geochronologists to restrict this to a single set for both age and quantity, with the result that some professional societies and journals have imposed Ma, Ga, and ka in all cases. This may meet the needs of geochronologists, for whom all time measurements are ages before present, but it is not relevant to the earth sciences as a whole. The dual system reflects the fact that time, in geology as in history, is measured both in the age of a given point in time, and in the duration of an interval between two points, neither of which need be the present. The points themselves -- called dates in historical calendars and datums in geological time scales -- have no duration, aside from the underlying assumption that the stated value is the quantity of years before the present. Each point is unique among an infinity of values in an ordinal progression; in contrast, there can be an infinity of durations with the same value, defined by different points. As for the validity of the Ma/Myr notation system, it must be pointed out that in the International System of Units (SI) there is no approved symbol for year. As a result of orbital variations, the quantity “year” can only be precisely related to the second (the SI base unit for time) if a single reference year is specified. Thus, with no logical or conventional objection, the various abbreviations and symbols for year values that earth scientists have developed out of practical necessity can now be standardized with the Ma convention for datums and the Myr convention for duration, to express the nuances of geological time.