‘Snowball Earth’ repeatedly thawed during a 56-million-year ice age

For 56 million years, Earth appears to have lurched between two extremes that are hard to picture together: a planet sealed in global ice, then a world hot enough to strip carbon dioxide from the sky and set the stage for the next freeze.

That is the case put forward by Earth scientists at Harvard’s John A. Paulson School of Engineering and Applied Sciences, who argue that the Sturtian glaciation was not one endless deep freeze. Instead, they say, the planet likely flipped back and forth between full Snowball Earth conditions and warm, ice-free intervals during the Cryogenian period, roughly 717 million to 660 million years ago.

The idea offers a way through one of ancient climate science’s most stubborn problems. Standard physical models have long struggled to explain how the Sturtian glaciation could have lasted about 56 million years when a fully frozen Earth should, in theory, thaw much sooner as volcanic carbon dioxide builds up in the atmosphere.

Graduate student Charlotte Minsky led the research with Robin Wordsworth, David T. Johnston and Andrew H. Knoll. Their analysis, published in Proceedings of the National Academy of Sciences, uses a coupled model of ancient climate and the global carbon cycle to test a different possibility: maybe the Sturtian was long not because one frozen state endured, but because freezing kept happening again.

In the classic Snowball Earth picture, ice shuts down silicate weathering, which normally removes carbon dioxide from the atmosphere. Volcanoes keep releasing carbon dioxide, so over time the gas accumulates, warms the planet and eventually melts the ice. That logic works reasonably well for the Marinoan glaciation, a later Cryogenian event that lasted about 4 million years.

The Sturtian is another matter. Geochronology places it at more than ten times longer, from about 717 million to 661 million years ago. In a hard Snowball scenario, the Harvard team notes, deglaciation should happen after roughly 3.8 million years. Slushier versions of the event, with open water or patchy ice near the equator, are even less stable over such long spans because they require less carbon dioxide to thaw.

The oxygen problem is just as serious.

If Earth stayed frozen for the full Sturtian interval, the researchers write, oxygen in the atmosphere and ocean should have been exhausted within a few million years as reduced volcanic gases kept reacting away the remaining supply and marine productivity collapsed. Yet the geologic record does not fit a world plunged into more than 50 million years of near-total anoxia. Some isotopic evidence points to oxygen availability after the Sturtian, and many microbial metabolisms and obligately aerobic eukaryotic groups made it through the interval.

That mismatch pushed the team toward a different framework, one in which short Snowball phases and short warm intervals alternate in a repeating rhythm.

At the center of that rhythm is the Franklin Large Igneous Province, a huge volcanic region in what is now northern Canada. It erupted at about 717 million years ago, essentially right as the Sturtian began.

The team argues that weathering of basalt from that province could have drawn down atmospheric carbon dioxide strongly enough to trigger global glaciation. Once the planet froze, continental weathering would have slowed or stopped, allowing volcanic outgassing to rebuild carbon dioxide until deglaciation began.

Then the system would have turned on itself again.

As ice retreated, fresh basalt would have been exposed to the atmosphere. That renewed weathering would have pulled carbon dioxide back down, driving the planet into another Snowball phase. The process could repeat as long as the volcanic province still had enough weatherable rock left to keep upsetting the carbon cycle.

In the model, this produces a “limit cycle” climate regime, with self-terminating Snowball episodes alternating with hot interglacials. One example configuration highlighted by the researchers uses an initially 4.5 million square kilometer large igneous province drawing down carbon dioxide at a rate similar to modern volcanic fields such as Réunion. In that setup, a Sturtian-length interval of about 56 million years requires an 18 million year decay timescale for the weathering perturbation, which corresponds to an initial basalt thickness of about 2 kilometers, consistent with Neoproterozoic estimates of 1 to 4 kilometers.

The authors found that much of the plausible parameter space for the Franklin province could support this cycling. They also estimate that, under Neoproterozoic background conditions, as little as 0.4 teramoles of extra carbon weathering per year could sustain a limit cycle, while weathering of the Franklin basalt would exceed that threshold.

This version of the Sturtian changes the biological picture too.

In a single unbroken Snowball, life would have had to endure extreme cold, little light in the surface ocean, weak nutrient supply and a steadily collapsing oxygen reservoir for tens of millions of years. In the Harvard scenario, those frozen intervals still last millions of years, but not long enough to fully drain atmospheric oxygen each time.

Warm intervals matter here for another reason. Weathering of the Franklin basalt would not only remove carbon dioxide, it would also release phosphate to the surface environment. That nutrient supply could boost marine productivity during interglacial periods, helping replenish oxygen in the atmosphere and ocean before the next freeze arrived.

“This could help explain how aerobic life persisted through such an extreme interval,” Minsky said.

The argument also lines up with some puzzling features of the sedimentary record. Intermittent ice-rafted debris and non-glacial deposits, including mudstone, sandstone and occasional carbonates, have been interpreted by some researchers as hints of cyclical ice retreat or periods of open water during the Sturtian. The new framework gives those observations a broader climate mechanism.

It may also help explain why atmospheric oxygen did not collapse in a way that would leave sulfur mass-independent fractionation in immediately post-glacial sediments.

The clearest test, the team writes, would be evidence that discrete glacial and interglacial cycles occurred globally and synchronously throughout the Sturtian’s 56 million years. That would require targeted sequence stratigraphic work, along with high-precision geochronology where rocks are well enough preserved.

Records from higher paleolatitudes would be especially useful. Open water at low latitudes might fit so-called Waterbelt scenarios, but ice-free conditions far from the equator would point to true global deglaciation between Snowball phases.

The study leaves open another big question: whether the younger Marinoan glaciation could somehow belong to the tail end of the same limit cycle regime, with the 22 million year gap between the two events representing one last interglacial. In the team’s current parameterization, that would require the silicate weathering feedback to be effectively shut off, a possibility they say is not yet well constrained.

The main impact of the work is not about tomorrow’s weather, but about how scientists think Earth systems behave when pushed hard enough. The study offers a physically grounded way to explain a glaciation that has long looked too long for standard Snowball models and too oxygen-poor for the rock record.

It also suggests that climate extremes do not always come as single, stable states. Under the right conditions, Earth may cycle between opposites for millions of years if the carbon cycle remains out of balance.

That has implications for reading ancient sedimentary records, for understanding how early life survived severe environmental disruption, and even for thinking about repeated Snowball episodes on Earth-like exoplanets.

Research findings are available online in the journal PNAS.

The original story "'Snowball Earth' repeatedly thawed during a 56-million-year ice age" is published in The Brighter Side of News.

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