Glacial conditions during the latter part of the Neoproterozoic Era extended to tropical latitudes, probably as far as the Equator, thereby giving rise to the concept of Snowball Earth events. They left evidence in the form of sedimentary strata known as diamictites, whose large range of particle size from clay to boulders has a range of environmental explanations, the most widely assumed being glacial conditions. Many of those from the Cryogenian Period are littered with dropstones that puncture bedding, which suggest that they were deposited from floating ice similar to that forming present-day Antarctic ice shelves or extensions of onshore glaciers. Oceans on which vast shelves of glacial ice floated would have posed major threats to marine life by cutting off photosynthesis and reducing the oxygen content of seawater. That marine life was severely set back is signalled by a series of perturbations in the carbon-isotope composition of seawater. Its relative proportion of 13C to 12C (δ13C) fell sharply during the two main Snowball events and at other times between 850 to 550 Ma. The Cryogenian was a time of repeated major stress to Precambrian life, which may well have speeded up evolution, sediments of the succeeding Ediacaran Period famously containing the first large, abundant and diverse eukaryote fossils.
For eukaryotes to survive each prolonged cryogenic stress required that oxygen was indeed present in the oceans. But evidence for oxygenated marine habitats during Snowball Earth events has been elusive since these global phenomena were discovered. Geoscientists from Australia, Canada, China and the US have applied novel geochemical approaches to occasional iron-rich strata within Cryogenian diamictite sequences from Namibia, Australia and the south-western US in an attempt to resolve the paradox (Lechte, M.A. and 8 others 2019. Subglacial meltwater supported aerobic marine habitats during Snowball Earth. Proceedings of the National Academy of Sciences, 2019; 201909165 DOI: 10.1073/pnas.1909165116). Iron isotopes in iron-rich minerals, specifically the proportion of 56Fe relative to that of 54Fe (δ56Fe), help to assess the redox conditions when they formed. This is backed up by cerium geochemistry and the manganese to iron ratio in ironstones.
In the geological settings that the researchers chose to study there are sedimentological features that reveal where ice shelves were in direct contact with the sea bed, i.e. where they were ‘grounded’. Grounding is signified by a much greater proportion of large fragments in diamictites, many of which are striated through being dragged over underlying rock. Far beyond the grounding line diamictites tend to be mainly fine grained with only a few dropstones. The redox indicators show clear changes from the grounding lines through nearby environments to those of deep water beneath the ice. Each of them shows evidence of greater oxidation of seawater at the grounding line and a falling off further into deep water. The explanation given by the authors is fresh meltwater flowing through sub-glacial channels at the base of the grounded ice fed by melting at the glacier surface, as occurs today during summer on the Greenland ice cap and close to the edge of Antarctica. Since cold water is able to dissolve gas efficiently the sub-glacial channels were also transporting atmospheric oxygen to enrich the near shore sub-glacial environment of the sea bed. In iron-rich water this may have sustained bacterial chemo-autotrophic life to set up a fringing food chain that, together with oxygen, sustained eukaryotic heterotrophs. In such a case, photosynthesis would have been impossible, yet unnecessary. Moreover, bacteria that use the oxidation of dissolved iron as an energy source would have caused Fe-3 oxides to precipitate, thereby forming the ironstones on which the study centred. Interestingly, the hypothesis resembles the recently discovered ecosystems beneath Antarctic ice shelves.
Small and probably unconnected ecosystems of this kind would have been conducive to accelerated evolution among isolated eukaryote communities. That is a prerequisite for the sudden appearance of the rich Ediacaran faunas that colonised sea floors globally once the Cryogenian ended. Perhaps these ironstone-bearing diamictite occurrences where the biological action seems to have taken place might, one day, reveal evidence of the precursors to the largely bag-like Ediacaran animals