Facies-dependent δ 13 C variation and diagenetic overprinting at the onset of the Sturtian glaciation in North-East Greenland

Abstract Time-significant surfaces are used to construct a stratigraphic test of the intrabasinal reproducibility of a 16‰ Neoproterozoic carbon-isotope (δ 13 C) anomaly in the NE Greenland Caledonides. The δ 13 C excursion from +6‰ to −10‰ occurs in the carbonate-dominated Andree Land Group and lies below glacial diamictites of the Tillite Group commonly correlated as Sturtian in age (∼720 Ma) and has been widely interpreted to record a global isotopic event reflecting a perturbation in the carbon cycle preceding the initiation of the snowball ice age. δ 13 C stratigraphic patterns were determined in two strike sections with relatively shallow platform deposits at Kap Weber and slope deposits on Ella O preserved directly below the first evidence for glaciation. The top and bottom of the δ 13 C profile was bounded by contiguous chronostratigraphic surfaces including an unconformity (sequence boundary) at the base recognisable in both sections and a subaerial exposure surface with a basinal correlative conformity at the top. The δ 13 C profile at Kap Weber shows δ 13 C values of +6‰ that drop to −8‰ in fine-grained slope deposits before returning to +6‰ in platform carbonates at the base of the glacial diamictite. The slope section on Ella O shows similar values for platformal carbonates of +6‰ with a similar drop to ∼−10‰ in slope deposits, but this section lacks the return to platform deposits evident at Kap Weber beneath the diamictite as well as a return to positive δ 13 C values. The absence of the platformal sediments and positive δ 13 C values on Ella O cannot be attributed to erosional truncation because the contact in this section shows a conformable, interbedded transition into the glacial deposits indicating that a continuous record is preserved. This disparity in δ 13 C values below the base of the Tillite Group suggests that the most negative δ 13 C values recorded in slope mudstone facies (−10‰) occur near synchronously with values of +6‰ on the platform. Elemental mapping of the mudstone facies indicates that carbonate is largely pore-filling and authigenic, representing a secondary phase that is unlikely to record a seawater value. Elevated δ 13 C values in shallow inner-ramp carbonate intervals may record locally modified seawater enriched in 13 C by photosynthesis and evaporation. The excursion in δ 13 C values in the upper Andree Land Group is therefore interpreted to result from a shift in facies from platform carbonates to carbonate cemented and diagenetically overprinted mudstone and does not record the secular change in seawater δ 13 C used for correlation or interpretation of biogeochemical events preceding the snowball ice age.