Neoproterozoic Snowball Earth oceans

During the Neoproterozoic era, about 1,000 to 541 million years ago, ice may have reached very low latitudes. A long-standing question is whether the oceans were sealed beneath ice or whether regions of open water remained. The reappearance of banded iron formations, layers of iron-rich sediment, has often been interpreted as evidence for a fully ice-covered, or “hard snowball,” ocean.

In my doctoral work, I tested that interpretation by modeling the ocean itself. I developed a biogeochemistry module for the MITgcm and ran near-global simulations under Snowball-like conditions. The simulations showed that nutrient-depleted, partially ice-covered oceans, often called “soft snowball” or “waterbelt” states, can produce iron-deposition patterns similar to those in the geological record.

In these simulations, limited nutrients reduce cyanobacterial productivity and oxygen levels, allowing iron to accumulate and remain mobile. A completely frozen ocean is therefore not required to explain several geochemical signals often associated with a hard snowball climate. Ocean circulation and chemical gradients may have remained active parts of the climate system.

Further work could combine thick marine-ice dynamics with a more complete treatment of ocean biogeochemistry. This would help test how physical circulation and chemistry influenced oxygen and carbon cycles during glacial and post-glacial transitions.