Summary

• High-resolution varved sediments from the Garvellach Islands are the main evidence base, with about 2,600 annual layers used to reconstruct Cryogenian climate variability.
• Multiple studies converge on the idea that small ice-free ocean areas, especially in the tropics, could preserve atmosphere-ocean coupling during a largely frozen Earth.
• The reconstructed variability includes annual, decadal, and centennial oscillations, sometimes compared to ENSO-like or solar-linked patterns, suggesting Snowball Earth was dynamically active rather than uniform.
• Modeling repeatedly points to a threshold near 15% open ocean for restoring recognizable climate oscillations under extreme glaciation.
• A separate modeling thread argues that seafloor weathering, by drawing down atmospheric CO2, may have strongly influenced how long Snowball Earth episodes lasted.
• The topic is internally coherent around Cryogenian climate mechanics, but it is split between sediment-based reconstruction, coupled climate simulation, and deep-time carbon-cycle explanation.
• The current signal is dense but narrow: most items reinforce the same scientific frame rather than introducing unrelated subtopics.