Interannual to multidecadal climate oscillations occurred during Cryogenian glaciation

During the two Cryogenian snowball Earth glaciations, the Sturtian (ca. 717–658 Ma) and Marinoan (ca. 639–635 Ma), ice persisted in the tropics for millions of years. Previous analyses of varves deposited before and after these glaciations have revealed climate variability linked to solar, oceanic, and atmospheric dynamics. However, to our knowledge, no evidence of sub-Milankovitch scale climatic variability has been documented during the glaciations themselves. The proposition of reduced solar luminosity in the Cryogenian, an attenuated hydrological cycle, and an expected hiatus in atmosphere-ocean interactions due to ocean freezing, raises questions regarding whether solar-ocean-atmospheric interactions continued during these glaciations. We analyze a unit of 2,640 laminites within the Sturtian Port Askaig Formation on the Garvellach Islands, Scotland, to better understand climate variability during a discrete interval of the Sturtian. Our study indicates the laminites most likely represent annual varves, reflecting seasonal freeze-thaw cycles in a deep, quiescent waterbody. Spectral analysis of laminae thickness reveals decadal and centennial periodicities consistent with present-day Schwabe and Gleissberg solar cycles, alongside interannual periodicities, likely tied to ocean-atmospheric climate modes, resembling the modern El-Niño Southern Oscillation. Our coupled Cryogenian climate simulations under varying degrees of ice coverage produce similar interannual periodicities in surface temperatures near the paleo-coordinates of the Garvellach Islands and in the tropics. This evidence reveals that solar-ocean-atmospheric interactions generated a wider range of climatic variability than expected during snowball Earth and hints at the possibility of transient unfrozen tropical waters during the Sturtian, or other yet unexplored modes of internal climate variability.