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. Their plates have been raining down on the deep seafloor since the Jurassic; the Cretaceous is when they piled up thickly enough to make cliffs.

Why it matters

Each calcite plate that sinks takes carbon out of the surface water and locks it into the seafloor, which eventually carries it down into the mantle. Over the last hundred million years these plankton have been the planet's most important long-term sink for carbon dioxide. What becomes of them in a changing ocean is therefore not a question for specialists alone.

Dating & uncertainty

Coccolithophores appear in the Jurassic, and the thick chalk accumulates mainly between 100 and 66 Ma. The chalk itself is a deep-water deposit, but recent work finds rich coccolithophore assemblages in nearshore sediments under forty metres of water as well, which revises the old view of these algae as open-ocean dwellers alone.

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