EARTH × LIFE
The Shuram Excursion
~574 to 567 million years ago
Proterozoic · Neoproterozoic · Ediacaran
The Shuram excursion is the largest negative carbon isotope excursion in the geological record, a steady fall in the share of heavy carbon in seawater between 574 and 567 million years ago.
The Shuram excursion is a negative carbon isotope excursion, a temporary fall in the share of heavy carbon in seawater. Isotopes are forms of one element with different masses. The Shuram excursion is the largest in the geological record. It falls between the Gaskiers glaciation, an ice age, and the arrival of large fossils of the Ediacaran period. Carbonate rock, such as limestone, records it on four continents: the carbon of seawater grows progressively lighter, down to about minus twelve parts per thousand relative to the standard, a reference sample. That departure is deeper than any in the later rocks that contain animal fossils. After thirty years of argument, researchers have not settled whether the excursion records the ocean of the time or was produced later during diagenesis, the chemical alteration a sediment undergoes as it turns into rock.
Why it matters
If the excursion is primary, that is, produced by the ocean of the time, the carbon and oxygen budgets of the ocean, the balance between what enters it and what leaves it, were being rebuilt immediately before animals attained large size. The explanation that links that rebuilding to the large size of animals then holds. If the excursion was produced in the rock long after burial, that explanation fails.
Dating & uncertaintyDisputed
Researchers date the excursion to between about 574 and 567 million years ago in Oman and Death Valley, and to between 571 and 562 million years ago where it is correlated with the Chinese and Tarim sections. A section is a succession of rock layers at one place, and correlating sections means matching their layers of the same age. Three explanations remain open: a genuine upheaval in the ocean carbon cycle, alteration of the rock long after burial, or volcanic ash formed from ancient marine limestone recycled by subduction, the sinking of an ocean plate into the Earth's interior. The most recent work in Oman, by Bergmann and colleagues (2025), finds alteration of four kinds, real but insufficient on its own to produce an excursion of this amplitude.
Earth history as one day
20:57:563 h 2 min before midnight
if Earth’s 4.54 billion years were compressed into a single day