THE SYNCHRONEITY AND DURATION OF THE SHURAM CARBON-ISOTOPE EXCURSION

van der Pluijm and Joseph have used anisotropy of magnetic susceptibility to distinguish between different depositional environments in marine sediments. We have used magnetic susceptibility to monitor magnetic mineral concentration variations in marine sedimentary sequences to track astronomically-forced global climate cycles and thus provide high-resolution chronostratigraphies. In this study our goal was to determine the duration and check the global synchroneity of the Ediacaran-age Shuram carbon-isotope excursion. It is the largest inorganic carbon isotope anomaly in Earth history and occurs just before the explosion of multicellular life in the Cambrian. Global synchroneity of the event supports a primary, rather than diagenetic, origin for the excursion. The duration of the excursion is necessary to test one model of the excursion’s origin, the oxidation of the world ocean. Magnetostratigraphy of two formations that record the Shuram excursion, the Johnnie Formation in Death Valley and the Wonoka Formation of the Flinders Ranges, South Australia, can check the synchroneity of the excursion globally. Both localities demonstrate magnetic reversal stratigraphies that are reproducible at multiple sites within their respective regions and show a reversed to normal polarity transition nearly coincident with the nadir of the Shuram excursion, indicating global synchroneity. Time series analysis of the magnetic susceptibility variations at both localities shows spectral power consistent with the expected ratios of eccentricity, obliquity, and precession. The 60 meter thick portion of the Johnnie Formation that we sampled at 25 cm intervals records 10% of the Shuram excursion. Extrapolation of the rock magnetic cyclostratigraphy to the full thickness of the excursion indicates a 8.2±1.2 Myr duration at Death Valley. The 600 meter thick section of the Wonoka Formation that we sampled at 40 cm intervals records the complete excursion. The rock magnetic cyclostratigraphy provides a similar duration of the Shuram excursion for South Australia of 9.5±1.2 Myr. The 8-9 Myr duration of the Shuram excursion is much shorter than initial estimates by others of ~50 Myr. The shorter duration supports a geochemical model of ocean oxidation that would act as an environmental catalyst for the Cambrian explosion.