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.

Reference-

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.