A PRE-GLACIAL d13C ANOMALY IN SVALBARD: IMPLICATIONS FOR THE NUMBER, CAUSES AND CORRELATION OF NEOPROTEROZOIC GLACIATIONS

A large decline (~11 per mil) in d¹³C occurs in carbonates immediately below the Petrovbreen tillite, the older of two glacial horizons in the Neoproterozoic Polarisbreen Group of northeastern Svalbard. This negative isotope shift appears gradual, spans shallow water sediments, is followed by a slight shift towards more positive values, and is variably truncated by the overlying sub-glacial surface. This d¹³C pattern is analogous to that in the upper Ombaatjie Formation in northern Namibia (Halverson et al., 2002) in all regards but one: in Namibia, it predates the younger (Ghaub) of two glaciations. Insofar as these anomalies record the same event, at least three Neoproterozoic glaciations are implied. Similar anomalies precede glaciation in northern Canada, South Australia, Death Valley, Scotland, and northeastern Norway. These data support the hypothesis that the pre-glacial anomaly records a large and protracted releases of methane into the atmosphere prior to glaciation (Schrag et al., 2002).

Taking into account strikingly similar sedimentological and geochemical features in likely coeval cap dolostones (e.g. Dracoisen, Nyborg, Keilberg, Nuccaleena, Ravensthroat, Noonday), we propose a new correlation scheme for the Neoproterozoic glaciations. We correlate tillite pairs in Svalbard, Greenland and Death Valley with single glacigenic formations in Scotland (Port Askaig), Norway (Smalfjord), Namibia (Ghaub), Australia (Elatina), and Canada (Ice Brook). Deposits associated with the older Chuos-Rapitan-Sturtian glaciation are absent in most North Atlantic basins and Death Valley, but associated cap carbonates can generally be identified. A younger glaciation is represented by tillites in Norway (Mortenses), Scotland (Loch na Cille/Macduff), and the Avalon Terrain (Gaskiers/Squantum). In Norway, this glaciation is preceded by highly ¹³C-depleted carbonates and deep incision into underlying strata (up to 400 m). These features suggest correlation with the paleovalley beneath the Stirling Quartzite in Death Valley, which overlies the isotopically negative Johnnie Rainstorm Member (Corsetti and Kaufman, in press).