Paper No. 19-3
Presentation Time: 8:30 AM
THE SEARCH FOR ROECE IN NORTHWEST SCOTLAND
The Cambrian Series 2/ Series 3 (early-middle Cambrian) transition is marked by major shifts in carbon isotopes values, the eruption of the Kalkarindji large igneous province in Australia, substantial sea-level oscillations and two extinction crises. However, the relative timing of all these phenomena is poorly understood – no single section or location records all these features – making cause-and-effect correlations difficult to evaluate. Here we show that both the Redlichiid-Olenellid Extinction Carbon isotope Excursion (ROECE) and a major regressive-transgressive couplet can be seen in the Cambrian strata of NW Scotland that was deposited on the Laurentian passive margin. ROECE is a -4‰ negative δ13C excursion known from North America and North and South China (Zhu et al., 2004). A negative spike is seen in the Salterella Grit Member of the An t-Sròn Formation, a shallow marine clastic unit. Heavier δ13C values occur in the overlying peritidal, dolomitic carbonates of the Durness Group. A sequence boundary occurs between these units and we therefore link the ROECE δ13C excursion with the Sauk I/Sauk II sequence boundary in Scotland. This transition also coincides with a fundamental, long-term change in depositional style in the region, likely to reflect a climate change to intense aridity, with the clastics of the lower Cambrian replaced by middle Cambrian-Ordovician carbonates. However, the ROECE excursion predates the extinction of olenellids in Scotland, as it does elsewhere in the world, making the link between these two phenomena moot.
Zhu, M. Y., Zhang, J. M., Li, G. X., & Yang, A. H. (2004). Evolution of C isotopes in the Cambrian of China: implications for Cambrian subdivision and trilobite mass extinctions. Geobios, 37(2), 287-301.