About 2 decades ago, scientists coined the term "snowball Earth" to describe a period about 700 million years ago when glaciers apparently smothered the planet even at latitudes near the equator. However, new evidence from ancient rocks bolsters the notion that some of the world's seas remained unfrozen during this global deep freeze, striking a blow against the controversial idea that the planet was completely swaddled in ice at the time.
In some versions of the snowball Earth scenario, even the surface of the oceans was completely frozen, blocking the release of oxygen into the atmosphere by ocean-dwelling phytoplankton, thereby potentially affecting the timing and pace of evolution .Yet some climate models have hinted that even in this death grip of global cold, substantial parts of the ocean remained open water. And the geological record contains scattered inklings of glacial flow during this era, a sign that the ice wasn't so cold that it was locked in place.
Now Daniel Le Heron, a sedimentary geologist at Royal Holloway, University of London, and his colleagues report rock-solid evidence of open water during this global cold spell. They examined rocks gathered in southern Australia that were deposited as marine sediments about 700 million years ago during the heart of the 220-million-year-long Cryogenian period, as the snowball Earth era is also known. And some layers contain dome-shaped structures that once were heaps of sea-floor material similar to miniature sand dunes. The shape of these tiny hummocks, as well as the layering of material within them, indicates that the structures were formed by currents that repeatedly alternated direction. Le Heron says that such oscillating flows are produced only by storm-generated waves in ocean depths of less than 200 meters or so—a sure sign that the seas in this region at this time were open and, therefore, unfrozen, he and his colleagues reported online 3 December in Geology.
These wave-generated structures are the first recognized in rocks of the Cryogenian period and suggest that geologists should look for similar features in rock formations of that age in other regions, says Le Heron. The presence of wave-sculpted sediments in other locations would help scientists determine the extent of ice-free regions during the Cryogenian, he adds.
The hummocky structures confirm that the ancient sediments were sculpted by storm waves in relatively shallow waters, says Emmanuelle Arnaud, a sedimentary geologist at the University of Guelph in Canada who was not involved in the work. The team's new report "is one of several studies that suggest Earth wasn't fully encased in ice during this part of the Cryogenian," she says.
However, the results don't mean that the oceans were always ice-free during the Cryogenian, the researchers say. Sitting immediately above the layers that contained storm-sculpted sediments was a thick layer where such structures were absent. This, the researchers say, is a sign that the open seas may have frozen over for an extended interval.
Atop this chronicle of calm seas are several layers of fine-grained sediments that contain pebbles, large stones, and cobbles—some of them more than twice the diameter of a softball. Such out-of-place rocks dropped to the sea floor from ancient icebergs as they melted. Because these dropstones chronicle repeated episodes of glacial advance and retreat that occurred as Earth was warming near the end of the Cryogenian period, it's clear that the period when open seas reigned fell well within the Cryogenian, the researchers contend.