Age· The columns of time

Meghalayan

2,251 BCE → Today

The Meghalayan, the last 4,200 years, is the third age of the Holocene, the briefest unit of the time scale and the only one defined in a stalagmite, opened by a drought.

Third age of the Holocene, the briefest on the geological time scale and the only one defined in a stalagmite. It begins with a drought episode and extends to the present.

What the evidence shows

A stalagmite from Mawmluh cave, in Meghalaya, north-eastern India, carries in its oxygen isotopes the mark of a markedly weakened Asian summer monsoon, and the layer where that mark begins defines the base of the age, at 4,250 years before the year 2000. This is the 4.2 ka event, a drought contemporaneous with cultural disruption in a band from the eastern Mediterranean to the Yangtze. Ratified in 2018, it is by far the shortest stage admitted to the chart and the only one whose reference point lies in a cave; with the two ice cores defining the preceding ages, the Holocene becomes the only epoch none of whose limits rests on a rock section. Tree rings from subarctic Europe place that limit in 2190 BCE, a few decades after the round value of the definition, and show that the anomaly came in two stages, the first being the stronger.

Bounds and conventions

The base is precise and the status of the age contested: ratified in 2018, it was criticised immediately, partly because the 4.2 ka event is a regional monsoon signal used to divide global time, partly because it was defined while the Anthropocene was under examination, a proposal that, had it succeeded, would have shortened the Meghalayan. One study argues for distinguishing the hydroclimatic anomaly at the limit from the longer event usually designated by that name.

GSSP in a speleothem from Mawmluh cave, Meghalaya, 4,250 years b2k.