Cryogenian glaciations on the southern tropical paleomargin of Laurentia (NE Svalbard and East Greenland), and a primary origin for the upper Russøya (Islay) carbon isotope excursion
Neoproterozoic successions in NE Svalbard and East Greenland host a pair of glacigenic formations, the younger of which are correlated with the terminal Cryogenian (Marinoan) glaciation based on their lithologically and isotopically diagnostic cap dolostones. A deep negative carbon isotope excursion (CIE) occurs stratigraphically beneath the older glacigenic formations in both areas, but no CIE is preserved beneath the younger glacial horizon in either area. This led to uncertainty over the age of the CIE and the paleoenvironmental significance of the units separating the glacigenic formations. 87Sr/86Sr ratios in Sr-rich limestone associated with the CIEs are ≤0.7064 in East Greenland and ≤0.7068 in NE Svalbard, consistent with early Cryogenian values globally and inconsistent with late Cryogenian ratios, which are exclusively ≥0.7071. The CIEs are tentatively correlated with the Islay excursion in the Scottish and Irish Caledonides, and potentially correlative subglacial CIEs in northwestern Canada, northwestern Tasmania and the southwestern United States. In NE Svalbard, newly-acquired δ13Corg data covary with δ13Ccarb across the CIE, but the organic excursion is roughly half the amplitude of the inorganic excursion. δ13Ccarb and δ18Ocarb do not covary, nor does δ13Corg vary as a function of total organic carbon content. A primary origin for the CIE is supported and the accompanying proliferation of stromatolites suggests rising carbonate saturation (falling pCO2) prior to glaciation. New data suggest that the hiatus responsible for the missing Trezona CIE occurs at the top, not the bottom, of the Bråvika member sandstone in NE Svalbard. A dramatic decline in regional average thickness of older Cryogenian glacial deposits from the paleomagnetically-determined subtropics (British Isles) to the deep tropics (Svalbard) is consistent with meteoric net accumulation and sublimation zones on a snowball Earth, and inconsistent with the Jormungand climate zonation.