Distinguishing Snowball Earth climate modes using field data and climate simulations

The degree to which Earth’s climate retained seasonality and ocean-atmospheric coupling during the two Cryogenian snowball Earth glaciations, the Sturtian (~717-658 Ma) and Marinoan (~654-635 Ma), is unknown. The classic hypothesis envisions ice at equatorial latitudes with a largely quiescent hydrological cycle. However, other observations imply the persistence of open water in the tropics, permitting ocean-atmospheric coupling and reconciling photosynthetic survival with low-latitude glacial activity. Consequently, open questions remain as to whether internal climate cycles could operate during snowball Earth, and if so, what their expression reveals about the extent of open ocean and the dynamics of the Cryogenian climate system; important climate questions that carry key biological implications. Varve-like laminites provide high resolution records of climatic variability as far back as the Proterozoic. However, varved sediments that retain climatic information are rare in the Cryogenian. Here, we analyse field data from rhythmic laminites from the Port Askaig Formation (Scotland). Petrographic and spectral analysis indicates that the laminites represent glacio-lacustrine annual varves, which reveal statistically significant centennial to interannual periodicities strongly similar to solar phenomena and modern ocean-atmospheric climate patterns. We interpret these signals with fully coupled Cryogenian climate simulations using the Community Earth System Model (CESM) under varying degrees of ice coverage to reconstruct climate variability during this interval of the Sturtian glaciation. These simulations suggest that open water is present to some degree in the tropics. Our study reveals a wider range of climatic variability than previously envisaged under snowball Earth conditions, and hints at the possibility of unfrozen tropical waters during this discrete interval of the Sturtian glaciation, or yet unexplored mechanisms of interannual variability on icy worlds.