The Sturtian glaciation is the first of two ‘Snowball Earth’ glaciations that characterise the Cryogenian Period (ca. 720 – 635 Ma), when ice is predicted to have extended to the equator where it persisted. The forcings that tipped Earth into this unprecedented climate state for more than 58 million years remain unclear as Earth scientists are unable to probe large parts of the late Tonian geological record due to deep glacial scouring of the ~717 Ma Tonian – Cryogenian boundary worldwide.

The Garvellach islands in the Firth of Lorn, Inner Hebrides, Scotland are host to one of the thickest and most continuous records of the Cryogenian ‘Sturtian Snowball Earth’ glaciation. The exposed rocks here are globally unique as they preserve the only known demonstrably transitional record between late Tonian shallow water carbonates and Cryogenian glacial sediments. In 2018, the section was established as the type locality for the pre-glacial Garvellach carbon isotope anomaly and proposed as an ideal candidate for the unratified basal Cryogenian Global Stratotype Section and Point (GSSP). With only ca. 70 m of late Tonian carbonates exposed on land at the type section, and only older parts of the carbonate platform elsewhere in the region (and most other sections worldwide), the potential to produce the highest resolution and most complete record of this critical interval via a scientific drilling project is compelling. Such a record would stimulate research across the earth sciences and help address outstanding questions surrounding how and why global climate descended into a ‘Snowball’ state.

The shallow water depth makes the Firth of Lorn a viable target for oceanic drilling whilst exposure on the Garvellach islands offers the possibility for integration with continental drilling. Bathymetric maps show that the offshore geology is structurally continuous with what is observed on land and likely records a complete Tonian stratigraphic record of >1.5 km. Prospective ocean drilling (<2 m deep) in the Firth of Lorn in 2015 (BGS Rockdrill RD2) returned carbonate samples with geochemical signatures which align with expected trends in the carbon isotope record.

During this ECORD MagellanPlus Workshop we initially want to highlight the importance of the Garvellach islands as a globally unique locality for scientific research into the late Tonian and ‘Sturtian Snowball Earth’ glaciation. Secondly, we anticipate that the workshop will lay the foundations for a new IODP/ICDP proposal for a future drilling project in the Firth of Lorn for the 2025 call.