“MA” MEANS “MILLIONS OF YEARS BEFORE PRESENT”

Several decades ago, stratigraphers recognized the need for a shorthand way of expressing time before the present. In 1978, for example, a single publication used the following for “million years” in relation to the geologic time scale: Ma, MA, m.a., Ma with a minus sign, m.y., MY, M.Y., My, my, My ago, M.Y. old, m.y.B.P., 10⁶ y. In contrast, only y and yr were used for “year” when part of an expression for decay constant or half-life.

From among several choices in the literature by the early 1980s, the North American Code of Stratigraphic Nomenclature [1983, Article 13, remark (c)] selected the recommended abbreviations: ka, Ma, and Ga. The Code states that these abbreviations mean, respectively, 10³, 10⁶, and 10⁹ years before the present, where the unit of time is the modern year and the present refers to AD 1950. This shorthand has become widely used in the geological literature and was incorporated into the 1994 (second edition) International Stratigraphic Guide. The widespread use is largely due to the need to distinguish clearly the concepts of a specific point in time (e.g., 10 Ma, a time in the Miocene 10 million years before present) from a duration of time (10 million years, anytime). The shorthand is unambiguous because there is no confusion as to the direction of measurement (an event at 100 Ma occurred before an event at 10 Ma) and no confusion as to the reference date (AD 1950).

Recent debate on precise means of expressing time concepts may be a consequence of the distinction, or lack thereof, between units of measure and the coordinate system to which the measurement is related. The goal of unambiguous communication cannot be achieved when one group of scientists uses the specific terms ka, Ma, and Ga to mean simply 10³ years, 10⁶ years, and 10⁹ years; and another group of scientists uses the identical terms to mean 10³ years before AD 1950, 10⁶ years before AD 1950, and 10⁹ years before AD 1950.

The use of the single letter “a” as a symbol or shorthand notation for “annus” where “annus” means a specified number of seconds is undesirable for a variety of historical, procedural, and practical reasons. The abbreviation for petayears would be Pa (international symbol for the unit of pressure Pascal). In English, “1 to 2 a ago” is awkward; in French it’s even more awkward (il y a 1 à 2 a).