Dalradian cover of the Grampian Terrane
Within Shetland's Dalradian lie four thick bands of calcite marble; Shetland's limestones. These (and in particular the Whiteness and Girlsta Limestones) record evidence of the end of episodes of 'Snowball Earth' global scale glaciations during the Neoproterozoic (1000-543 Ma). These glaciations were followed by short-lived ultra-greenhouse events during which the average global surface temperature may have been in the order of 50C. Just above the Neoproterozoic glacial deposits world-wide there is a sharp transition into chemically precipitated limestones known as 'Cap Carbonates' because they lie directly on top of (they 'cap') the glacial deposits. Cap carbonates have unusual chemical composition interpreted as recording a global oceanic alkalinity 'dump' following the 'melting' of a Snowball Earth under the extremely elevated atmospheric CO2 of a ultra greenhouse state. Acid rain would weather exposed silicate and carbonate rock, especially glacial debris, releasing large amounts of calcium which, when washed into the ocean, would form distinctively textured layers of carbonate sedimentary rock. Recent research measuring 13C/12C ratios has shown that the Whiteness and Girlsta Limestones are marked by C-isotopic values characteristic of two of the most significant perturbations of the global C cycle. These are associated with the formation of cap carbonates following the demise of the world-wide 635 Ma Marinoan glaciation (Whiteness) and the unique c.600 - 550 Ma Shuram-Wonoka warming event (Girlsta). This research is of major importance and correlates the Whiteness Limestone and the Girlsta Limestone with dated rocks in Africa and China but not so far with any in the British Isles.
(Paper: A.R. Prave, R.A. Strachan, and A.E. Fallick. Global C cycle perturbations recorded in marbles: a record of Neoproterozoic Earth history within the Dalradian succession of the Shetland Islands, Scotland. Journal of the Geological Society, January 1, 2009; 166(1): 129 - 135.)