Age· The columns of time
Northgrippian
6,237 BCE → 2,251 BCE
An age that opens on the sharpest cold spell of the Holocene, and in which some adults keep the ability to digest milk.
What the evidence shows
Its base is defined in the NGRIP1 Greenland ice core at 8,236 years before the year 2000, on the sharpest climatic swing of the last ten thousand years: about a century and a half of cold, one to three degrees across the northern hemisphere. The cause was long put down to glacial lakes bursting out into the sea, but no model could hold the cold that long from so brief a flood; the melt now proposed is slower and larger, a century of it, from the collapse of the ice saddle joining the domes over Hudson Bay. Within the age, a change to a single letter of DNA leaves the enzyme that breaks down milk sugar switched on for life. Milk was already in use long before that change spread, so the mutation did not open milk to adults so much as protect those who drank it when times turned hard: around seven thousand pottery residues and the trajectories read in ancient genomes make famine and disease a better explanation of its spread than the sheer availability of milk.
Bounds and conventions
Here too the layer count gives a boundary good to the year, but the event under it is a regional signal read as global: how far the 8.2 ka cooling reached beyond the North Atlantic is reconstructed from proxies that do not all agree. Estimates for the spread of lactase persistence vary with the method, and the selective pressure behind it was long assumed rather than demonstrated.
NGRIP1 GSSP, 8,236 years b2k.
Sources
- 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.
- 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.
- Matero et al. (2017), Earth and Planetary Science Letters · The 8.2 ka cooling event caused by Laurentide ice saddle collapse
- Evershed et al. (2022), Nature · Dairying, diseases and the evolution of lactase persistence in Europe