The Sturtian glaciation, the longest Snowball Earth event, lasted about 57 million years — almost as long as the age of mammals after the dinosaurs vanished — yet new research from Scottish rocks shows that during at least one brief “slushy” interval, seasons, solar cycles and even El Niño-like rhythms were still pulsing through the frozen planet.

The Sturtian glaciation is difficult to hold in the mind because its scale is so out of proportion to human history. It began roughly 717 million years ago and lasted tens of millions of years, making it the longest of the Snowball Earth episodes. A commonly cited duration is about 57 million years, almost as long as the entire age of mammals since the asteroid impact ended the reign of the non-avian dinosaurs 66 million years ago.

For that span of deep time, Earth is thought to have been locked in one of its most extreme climate states. Ice reached low latitudes. The oceans may have been largely sealed from the atmosphere. The planet’s surface became so reflective that the cold reinforced itself, a runaway ice-albedo trap in which more ice meant more sunlight bounced back into space.

That image has often carried a second assumption: if the world was frozen hard enough, then ordinary climate rhythms should have been muted or stopped. Seasons might still have existed astronomically, because Earth’s orbit and tilt did not vanish, but the surface climate would have been insulated under ice. Ocean-atmosphere exchanges should have been greatly reduced. A Snowball Earth, in the strictest version, should have been climatically numb.

New research from ancient Scottish rocks makes that picture less simple. In a 2026 study in Earth and Planetary Science Letters, Chloe Griffin, Thomas M. Gernon and colleagues analysed 2,640 fine layers in the Sturtian Port Askaig Formation on the Garvellach Islands off Scotland’s west coast. Those layers appear to record annual varves: repeated seasonal deposits laid down in a deep, quiet waterbody during a discrete interval of the Sturtian glaciation.

When the team measured the thickness of the layers and looked for repeating patterns, they found signals on interannual, decadal and centennial timescales. Some were consistent with solar cycles such as the roughly 11-year Schwabe cycle and the longer Gleissberg cycle. Others resembled ocean-atmosphere modes such as the modern El Nino-Southern Oscillation. In other words, even inside one of Earth’s great freezes, climate rhythms may still have been pulsing.

A frozen planet that still had a beat

The key evidence comes from laminites, rocks made of thin, repeated layers. In the right setting, such layers can behave like pages in a climate diary. A darker or lighter band, a thicker or thinner couplet, can reflect changes in sediment supply, meltwater, freezing, thawing, storms or seasonal productivity. If the layers are annual, then thousands of them can preserve thousands of years of climate variability.

Griffin and colleagues argue that the Port Askaig laminites most likely represent annual varves formed by seasonal freeze-thaw cycles. That alone is important. A seasonal signal during Snowball Earth implies that, at least locally and temporarily, the surface environment was capable of responding to the year. It was not merely a sealed world of static ice.

The spectral analysis pushed the result further. Layer thicknesses showed periodicities comparable to known modern climate rhythms. The study does not claim that a 700-million-year-old Earth had an El Nino system identical to today’s Pacific Ocean. The continents were arranged differently, the Sun was fainter, the atmosphere was different and the ocean basins were not modern. The point is more cautious and more interesting: the ancient record contains variability with timescales analogous to modern ocean-atmosphere oscillations.

That suggests some kind of coupled climate machinery was still operating. Solar forcing, ocean response and atmospheric behaviour may not have been entirely shut down. The planet was frozen, but not dead.

The meaning of a slushy interval

The phrase “slushy” can be misleading if it sounds like Earth had simply thawed back into a mild climate. The Sturtian remained an extraordinary glaciation. The new study concerns a discrete interval recorded at one place, not a wholesale rejection of Snowball Earth.

But the Scottish rocks may preserve a brief window when part of the climate system became active enough for seasonal deposition and higher-frequency oscillations to leave a record. Public reporting on the study described it as a slushy interlude lasting a few thousand years, when a small fraction of ocean may have thawed. The paper itself says the findings hint at transient unfrozen tropical waters during the Sturtian, or at other unexplored modes of internal climate variability.

That caution matters. There are several possible interpretations. The Garvellach Islands were not at their present latitude during the Cryogenian. Climate simulations in the paper found similar interannual periodicities in annual surface temperatures near the reconstructed paleo-coordinates of the Garvellachs and in the tropics under varying degrees of ice coverage. That modelling helps connect the rock record to plausible climate dynamics, but it does not turn the frozen Earth into an ordinary ice-age world.

Instead, it narrows the question. If Snowball Earth had intervals or regions where water, atmosphere and sunlight interacted strongly enough to generate cycles, then the frozen planet was more dynamic than a simple white sphere. Its climate may have flickered, pulsed and reorganised even under extreme ice cover.

Why the Sturtian was so extreme

The Sturtian glaciation belongs to the Cryogenian Period, an interval that also includes the later Marinoan glaciation. Geological evidence from multiple continents shows glacial deposits at low latitudes, one of the clues that forced scientists to consider the Snowball Earth hypothesis in the first place.

The basic mechanism is powerful. If ice expands far enough toward the equator, Earth’s albedo rises sharply. More incoming sunlight is reflected, less heat is absorbed, and cooling accelerates. Once the system crosses a threshold, the planet can become trapped in global or near-global ice.

Escaping such a state is also extreme. Volcanic carbon dioxide can keep accumulating because weathering reactions that normally remove CO2 are suppressed under ice. Eventually, greenhouse forcing may build high enough to trigger rapid deglaciation. The aftermath of Snowball Earth events is often associated with dramatic chemical and environmental changes, including cap carbonates deposited as the frozen world broke open.

That broad picture remains. What the Scottish laminites add is texture inside the freeze. They suggest the Sturtian was not necessarily one uninterrupted climate silence. Even if most of the planet was ice-bound for millions of years, at least one interval may have been sensitive enough to record annual seasons, solar cycles and ocean-atmosphere variability.

Why tiny layers matter

The discovery is striking because the evidence is small. The Sturtian lasted for millions upon millions of years. The laminites studied in Scotland represent only a slice of that time. Yet small slices are often how deep time becomes legible.

A 57-million-year glaciation is too long to imagine as weather. It becomes a label. But 2,640 annual layers bring it back toward experience: winter and thaw, sediment pulse and quiet water, repeated change written thinly into rock. If the interpretation is right, those layers caught a few thousand years when Snowball Earth’s climate was not simply frozen in place.

That is also why the result matters beyond the Cryogenian. Climate systems can retain variability under conditions that appear hostile to variability. They can respond to small forcings even when the background state is severe. They can behave as coupled systems rather than as simple on-off switches.

For modern climate science, the analogy is not direct. Today’s warming world is not becoming Snowball Earth; it is being pushed in the opposite direction. But deep-time climate records are useful because they show how Earth’s system behaves near extremes. The Sturtian may have been one of the planet’s hardest freezes, yet the Scottish rocks suggest that rhythms of season, Sun and ocean-like oscillation were still capable of leaving a signal.

The frozen planet, in other words, may not have been silent. For at least one brief interval in Scotland’s ancient rock record, it was still keeping time.

Sources

Griffin et al., Interannual to multidecadal climate oscillations occurred during Cryogenian glaciation, Earth and Planetary Science Letters, 2026
ScienceDirect article page for the Griffin et al. study
The Guardian report on the Garvellach Islands Snowball Earth research
Hoffman et al., Snowball Earth climate dynamics and Cryogenian geology-geobiology, Science Advances, 2017
Macdonald et al., Calibrating the Cryogenian, Nature Geoscience, 2010