LIFE × CLIMATE

The Chalk Seas

~100 to 66 million years ago

Phanerozoic · Mesozoic · Cretaceous

Single-celled algae wearing armour of calcite plates became numerous enough that their remains build rock. The white cliffs of northern Europe, and beds like them on three continents, are plankton corpses settling for tens of millions of years. They had existed since the Jurassic on shelves and in inland seas; the Cretaceous is when they took the open ocean.

Why it matters

That move changed where the planet stores its carbonate. Before, it piled up on shallow shelves; after, it rains into deep water, where the seafloor eventually carries it down into the mantle. These plankton have been the largest long-term sink for carbon dioxide ever since.

Dating & uncertainty

Coccolithophores appear in the Jurassic but stay on shelves and in inland seas; they spread into the open ocean during the Cretaceous, and the thick chalk accumulates mainly between 100 and 66 Ma. Recent work finds them abundant in water shallower than forty metres too, which undercuts the long-standing reading of chalk as a purely deep-water deposit.

Sources

1. Hay (2004), Coccolithophores · Carbonate fluxes and calcareous nannoplankton
2. Püttmann et al. (2021), Paleoceanography and Paleoclimatology · Paleoecology of Late Cretaceous coccolithophores: insights from the shallow-marine record
3. Bolton et al. (2025), Annual Review of Earth and Planetary Sciences · Coccoliths as recorders of paleoceanography and paleoclimate over the past 66 million years

Earth history as one day

23:28:1631 min 43 s before midnight

if Earth’s 4.54 billion years were compressed into a single day