Age· The columns of time

Meghalayan

2,251 BCE → Today

The briefest unit on the whole geological time scale, and the only one defined in a stalagmite: it begins with a drought and runs to today.

What the evidence shows

A stalagmite in Mawmluh Cave, in Meghalaya in north-east India, carries in its oxygen isotopes the mark of a sharply weakened Asian summer monsoon, and the layer where that mark begins is the base of the age, 4,250 years before the year 2000. That is the 4.2 ka event, a drought that coincides with upheaval in the cultures of a belt reaching from the eastern Mediterranean to the Yangtze. Ratified in 2018, it is by a wide margin the shortest stage ever admitted to the chart, and the only one whose golden spike sits in a cave. With the two ice cores that fix the ages before it, that leaves the Holocene as the one epoch with no rock section anywhere in its definition. Tree rings in subarctic Europe put the boundary at 2190 BCE, a few decades later than the round figure of the definition, and show the anomaly came in two stages, the earlier the stronger.

Bounds and conventions

Precise at its base and contested in its status: ratified in 2018, it was criticised at once, partly because the 4.2 ka event is a regional monsoon signal used to divide global time, and partly because it was defined while the Anthropocene was under discussion, a proposal that would have cut the Meghalayan short had it succeeded. One study argues for separating the hydroclimatic anomaly at the boundary from the longer event usually cited under that name.

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

Sources

1. Cohen, K. M., Finney, S. C., Gibbard, P. L., Fan, J.-X. (2013). The ICS International Chronostratigraphic Chart. Episodes 36(3), 199–204, mise à jour continue, voir Cohen et al. (2025), Episodes.
2. Walker, M., Head, M. J., Berkelhammer, M., et al. (2018). Formal ratification of the subdivision of the Holocene Series/Epoch: two new GSSPs and three new stages/subseries. Episodes 41(4), 213–223.
3. Helama et al. (2019), Quaternary Science Reviews · Exact dating of the Meghalayan lower boundary based on high-latitude tree-ring isotope chronology
4. Head, M. J. (2019). Formal subdivision of the Quaternary System/Period: Present status and future directions. Quaternary International 500, 32–51.