Global C cycle perturbations recorded in marbles: a record of Neoproterozoic Earth history within the Shetland Islands, Scotland

The Shetland Islands in Scotland contain a c. 12 km thick, mostly siliciclastic Neoproterozoic succession that experienced deformation and greenschist- to amphibolite-facies metamorphism during the Ordovician–Silurian Caledonian Orogeny. Correlating these rocks with Neoproterozoic successions elsewhere has remained problematic. However, four marble units punctuate the succession; these are, in stratigraphic order, the Weisdale, Whiteness, Girlsta and Laxfirth Limestones, two of which record hallmark C-isotopic excursions diagnostic of Neoproterozoic time. The Whiteness Limestone is lithologically similar to the 635 Ma Marinoan post-glacial cap carbonate sequence and displays a 13Ccarbonate profile in which values decline from a range of 0 to +2 to a nadir of –5 and then rise towards and oscillate around 0. The Girlsta Limestone is a 700–900 m thick marble unit with values of –9 to –11 and is interpreted as recording the late Neoproterozoic Shuram–Wonoka event. These correlations permit linking the Shetland succession to Neoproterozoic sections elsewhere and also show that rift-related rocks in Shetland post-date the Shuram–Wonoka event (post c. 600 Ma–pre c. 550 Ma) thereby supporting the concept of a diachronous phase of extensional tectonism, from c. 700 Ma in east–central Laurentia to near the close of the Proterozoic in northeastern Laurentia, prior to the opening of the Iapetus Ocean.