CLIMATE × LIFE

The Fern That Covered the Arctic

~49 to 48 million years ago

Phanerozoic · Cenozoic · Paleogene · Eocene

For about a million years the surface of the Arctic Ocean was fresh enough to carry a floating fern that doubles its mass in a few days, among the fastest-growing plants alive. It grew, died and sank into water with no oxygen in it, where nothing could rot it. Grown today under high carbon dioxide, the same fern grows markedly faster, so the Eocene air would have pushed the bloom rather than held it back.

Why it matters

It falls at the point where the greenhouse world shows its first signs of sliding towards the icehouse we live in, which makes a floating pond weed a suspect in the shift that produced the climate we know. Whether it was a cause or a footnote turns on a figure nobody can measure directly, and the published estimates of it differ by almost a factor of ten.

Dating & uncertainty

The fern layers in the Arctic sea-floor cores are unambiguous. How much carbon dioxide their burial removed is a flux calculation rather than a measurement: the estimates span 55 to 470 parts per million, and what the blooms did to the global carbon cycle remains largely unknown.

Sources

1. Speelman et al. (2009), Geobiology · The Eocene Arctic Azolla bloom: environmental conditions, productivity and carbon drawdown
2. Barke et al. (2012), Palaeogeography, Palaeoclimatology, Palaeoecology · Coeval Eocene blooms of the freshwater fern Azolla in and around Arctic and Nordic seas
3. Hutchinson et al. (2021), Climate of the Past · The Eocene-Oligocene transition: a review of proxy data, models and comparisons

Earth history as one day

23:44:2715 min 33 s before midnight

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