The basal Cambrian carbon isotope excursion revealed in the Central Iberian Zone, Spain

Abstract

The causes of early metazoan diversification during the Ediacaran-Cambrian transition interval are controversial, partly because the global correlation of the Ediacaran-Cambrian boundary remains problematic due to a lack of unambiguous stratigraphic markers. Here we report geochemical data from two fossiliferous sections (Majada de Andaluz and Vía Verde) that straddle the Ediacaran-Cambrian interval in the Central Iberian Zone, Spain. Carbon isotope chemostratigraphy reveals a rise to a low positive δ¹³C_carb plateau at the base of the Majada section, which is associated with the first appearance of Cloudina and a pristine Sr isotope value of 0.708512, suggesting that the lower Villarta Formation correlates with the terminal Ediacaran strata in China and Namibia. A prominent negative δ¹³C_carb excursion is also revealed from the lower member of the Villarta Formation at Majada, likely corresponding to the global basal Cambrian carbon isotope excursion (BACE). Nitrogen isotope data from the correlative Vía Verde section reveal a similar trend to that found in coeval strata of South China, and indicate a change from predominantly anaerobic to aerobic nitrogen cycling in Central Iberia. By correlating sections in which both the BACE excursion and Treptichnus pedum are recognized, it appears that T. pedum is exclusively found above the BACE, which postdates the last appearance of Cloudina and a negative δ¹³C_carb plateau. Further studies of sections with both fossil and isotopic control will help to constrain the timing and causation of Ediacaran-Cambrian bioradiations as well as their relationship to global carbon cycle perturbations.