Period· The columns of time

Rhyacian

2.3 Ga → 2.05 Ga

The Rhyacian, from 2.3 to 2.05 billion years ago, holds a major swing of the carbon cycle, the largest known magmatic intrusion and the first macroscopic fossils, whose biological origin is debated.

Period marked by a major excursion of the carbon cycle, by the largest known magmatic intrusion and by the first macroscopic fossils whose biological origin is debated.

What the evidence shows

In what is now South Africa, nearly a million cubic kilometres of magma entered the crust between 2.06 and 2.055 billion years, in successive pulses, and cooled slowly enough for its minerals to settle by density, forming the layered Bushveld complex, which holds some of the richest platinum deposits in the world. The period takes its name from lava flows, from the Greek rhyax. Ocean chemistry varies comparably: marine carbonates record the strongest positive excursion in the whole carbon isotope record, the Lomagundi event, attributed on the received reading to unprecedented burial of organic carbon, each carbon atom withheld from decay leaving a molecule of oxygen behind. The excursion then ceases, oxygen falls back and remains low for more than a billion years. In Gabon, rocks 2.1 billion years old yield flattened discs twelve centimetres across whose biological nature remains disputed.

Bounds and conventions

Both limits are conventional round numbers. The contents of the period are better dated than its boundaries: the Bushveld intrusion to within five million years, the Yarrabubba impact at 2.229 billion years. The biological interpretation of the Gabonese fossils is genuinely contested, whereas the rocks carrying them are not. Nor is the Lomagundi excursion read by all as a global signal: a recent compilation finds its highest values confined to coastal settings, which would make it a record of where the carbonate formed rather than of the carbon cycle as a whole.