The surface of Garbh Eileach holds a record that does not match the long accepted picture of the Cryogenian world. The island exposes a five and a half meter sequence of thin laminations that formed during the Sturtian glaciation, a period often described as a global freeze with little or no interaction between the ocean and the atmosphere. This interpretation assumes a sealed ocean, a collapsed hydrological cycle and temperatures far below anything seen in later climate states. The laminations in the Port Askaig Formation do not support that picture. They show a rhythm that survived through ice, darkness and cold conditions, and they preserve patterns that track short and long climate cycles with a precision that should not exist in a silent frozen world.
The sequence contains 2640 individual layers. Each represents one cycle of deposition inside a quiet water basin influenced by ice cover. The structure repeats without interruption. A lower light layer with graded sand and silt marks melt season input. A darker upper layer of fine mud marks the winter months when water remained still. Under a microscope the contacts are sharp and undisturbed. Larger grains appear at the base of many light layers. These grains do not match the background sediment. Their size and distribution show that they were carried by floating ice and dropped during partial thaw. The pattern remains stable over the entire section, which rules out pulses from floods, tides or marine currents. Nothing in the laminae shows signs of chaotic disturbance. Nothing shows the thick thin bundles expected in tidal rhythmites. The environment was protected, cold and seasonally active.
The wider stratigraphy reinforces this setting. Below the laminations lies an incised valley fill with sandstone, conglomerate and a laterally inconsistent diamictite that formed from a mass flow rather than direct ice advance. Periglacial wedge structures cut through the member. Everything points to a basin held in freezing conditions but not locked under permanent ice. A melt season occurred every year. Sediment reached the basin every year. Ice transported debris and released it only when temperatures allowed partial breakup. The rhythm never lost its structure.
To test whether these couplets represent annual varves, the researchers applied spectral analysis to the full thickness record. If the laminations formed yearly, then thickness would reflect variations in melt intensity. Variations in melt intensity would follow climate patterns. The analysis revealed strong cycles at several scales. The most stable show up near nine years and near the 130 to 150 year range. These match solar cycles that continue into the present. They do not appear as random fluctuations. In the upper half of the record they grow more pronounced because thickness becomes more stable and background noise drops. The dataset spans more than two thousand years, long enough to capture repeated centennial patterns that rarely survive in ancient sequences. The record has enough continuity to lock these cycles in place.
Shorter oscillations appear as well. Many fall between two and six years. These cycles have been found in varved sediments across many intervals of Earth history. They often correspond to internal climate oscillations that influence global temperatures. The presence of these cycles in Precambrian sediments challenges the idea that the Sturtian world lacked the physical processes required to produce such variability. The key question is whether these patterns came from the climate system itself or from irregular behavior inside the basin. To test this, the team ran climate simulations under three scenarios. One represented a hard Snowball Earth. One allowed a narrow equatorial oasis. One represented a waterbelt world with open water wrapped around the equator.
The simulations used a fully coupled ocean and atmosphere model with paleogeography set to 720 million years ago. Solar output was reduced. Continental positions were adjusted. Each run extended more than one thousand years after equilibrium. Annual surface temperatures were extracted both from the location of the Garvellach Islands and from a tropical region that held open water in two of the scenarios. These temperature records were subjected to the same spectral methods used for the laminations.
Every scenario produced strong peaks at two to three years. These originate from atmospheric dynamics and can persist even if sea ice covers nearly the entire planet. Beyond this, the scenarios diverged. The hard Snowball scenario produced narrow multiannual peaks with weak strength. It failed to produce a consistent four year signal. The equatorial oasis scenario produced a wider set of peaks. The waterbelt scenario produced the strongest and broadest multiannual signals, including pronounced peaks near four and five years at the highest confidence levels. These multiannual cycles closely match the persistent four to four and a half year signal in the laminations.
The centennial and decadal scale patterns in the laminations also match long known solar behavior. The simulations did not include solar variability and therefore did not generate these cycles. Their presence in the sediment reflects real forcing that reached the basin through seasonal melt intensity. The persistence of these signals confirms a climate system responding to external variations rather than a basin acting in isolation.
The match between the laminations and the simulations draws attention to a specific condition. The strongest agreement appears in scenarios that contain open water. Fully frozen oceans mute climate signals beyond the shortest atmospheric cycles. When the model contains even a narrow band of exposed seawater, the internal oscillations strengthen and broaden. The four year signal becomes clear. This corresponds to the four year pattern in the laminations, which remains visible despite the age of the deposit and the stability of the environment in which it formed. Such alignment is difficult to achieve unless the Cryogenian environment possessed regions where the ocean and atmosphere could interact.
The laminations also carry a record of the centennial cycle. This cycle matches the Gleissberg range and tracks long solar output variations. The data span more than two thousand years, enough to show repeated centennial patterns. The presence of this cycle in a Sturtian record indicates that even under low solar luminosity, solar forcing remained active and had measurable impact on meltwater delivery into this basin. The sensitivity of the environment to these variations shows that the hydrological system did not collapse entirely. Melt seasons intensified or weakened according to changes beyond the local setting. The basin served as a recorder for climate behavior during one of the most extreme periods in Earth history.
Other lines of evidence point in the same direction. Field observations from other continents show features that require liquid water near low latitudes during parts of the Cryogenian. Numerical studies have shown that even in a hard Snowball state, geothermal heat and surface melting could generate circulation within the ocean, but those signals would not travel upward through kilometers of ice. The laminations hold patterns that cannot come from deep subglacial currents alone. Their frequencies align with processes that require some degree of contact between atmosphere and surface water. The regularity of the record captures this contact through the sediment delivered each year.
The sequence at Garbh Eileach stands out because it survived intact. Most Cryogenian deposits are chaotic. They formed in high energy settings or under conditions that did not allow fine structure to accumulate. The Port Askaig Formation recorded a rare episode in which a basin remained stable long enough to capture the climate rhythm year after year. The two halves of the record show different variance patterns. The lower section shows thicker laminae with more variation. The upper section becomes more uniform. This shift likely reflects a settling of environmental conditions rather than a break in deposition. Throughout the entire sequence the rhythmic pattern continues without losing integrity.
The upper half provides the clearest climate signals because its variance is low and sediment supply stable. In this section the nine year and centennial cycles stand out. The four year cycle repeats with enough consistency to indicate a real climatic force. Even the shorter two and three year cycles appear frequently enough to rise above noise. When compared directly to the simulated conditions of the waterbelt and equatorial oasis scenarios, the frequencies align closely. This alignment once again supports the view that the basin received climate signals through open water pathways.
The system that produced these laminations operated under prolonged ice cover but not permanent sealing. Ice fractured. Melt seasons occurred. Sediment moved through water columns that remained quiet but not frozen solid. Each year left a signature of melt strength, current intensity and seasonal duration. The signatures accumulated layer by layer until more than two thousand years of climate rhythm were locked into the rock. The result is a detailed record of solar linked cycles and multiannual oscillations during a period often described as one of the coldest and most static in Earth history.
The laminations from the Port Askaig Formation provide a direct record of climate variability inside the Sturtian glaciation. They show that solar cycles persisted. They show that multiannual oscillations operated. They show that the hydrological system remained active. They show that melt seasons continued to respond to external forcing. The record extracted from this single Scottish outcrop challenges the idea of a motionless Cryogenian world and replaces it with evidence of a climate system that continued to shift, pulse and vary on timescales from a few years to more than a century.
Source:
Griffin, C. et al., 2026. Interannual to multidecadal climate oscillations occurred during Cryogenian glaciation. Earth and Planetary Science Letters, 679, 119891.
https://doi.org/10.1016/j.epsl.2026.119891
