The Palaeoproterozoic global carbon cycle: insights from the Loch Maree Group, NW Scotland

摘要

Two Palaeoproterozoic events have particularly interested Earth scientists. These are the global LomagundiJatuli Event, the greatest magnitude positive carbonate carbon isotope excursion in Earth history, and the Shunga Event, the world's largest organic carbon burial event. Analysis of newly acquired high-resolution C-O isotope data and U-Pb zircon geochronology refine understanding of carbon isotope characteristics and timing of deposition of the Palaeoproterozoic Loch Maree Group of NW Scotland. Petrographic examination reveals a basal unconformity between the Loch Maree Group and Archaean basement, permitting a stratigraphy and younging direction to be assigned. Detrital zircon ages from immediately above the unconformity are dated at c. 2.3 Ga. delta C-13(carbonate) data on two temporally discrete carbonate packages range from c. + 15 to 2 parts per thousand in the older unit and c. 2 to -5 parts per thousand in the younger carbonate unit. Current age constraints indicate that the Loch Maree Group is too young to be fully coeval with the Lomagundi-Jatuli Event but is within the age range of the Shunga Event. This revives consideration of a straightforward mass-balance process involving burial of organic carbon as an explanation for at least some of the C-cycle perturbations of Palaeoproterozoic time.