Early Hettangian benthic–planktonic coupling at Doniford (SW England): Palaeoenvironmental implications for the aftermath of the end-Triassic crisis

This paper presents a high-resolution micropalaeontological study, integrated with geochemical and sedimentological analyses of the Triassic–Jurassic (Tr–J) transition at the Doniford section (SW England). Our analyses indicate the renewal of surface carbonate production in the Blue Lias Formation (early Hettangian) after the end-Triassic mass extinction. Based on the high abundance of surface carbonate producers (schizosphaerellids), which occurred in phase with development at sea-floor of calcareous hyaline benthic foraminifers, we deduce that a benthic–pelagic coupling resulted in both the initial restoration of the biological carbonate pump, and also a more efficient export production. The presence of monospecific phytoplankton assemblages and benthic foraminiferal assemblages typical of poorly oxygenated environments suggest that the environmental stress continued after the end-Triassic maximal perturbation. Complete environmental recovery was prevented by a succession of alternating dysaerobic and anoxic phases that favoured the deposition of black-shale sediments.The hypothesis of a methane-release mechanism as a trigger for the global carbon cycle perturbation and greenhouse conditions during the early Jurassic is discounted, based on evidence from the decoupled carbon isotope records of carbonate and organic matter. Apart from local control of a high amplitude sea-level change across the Tr–J boundary, the succession and nature of events at Doniford are consistent with a scenario of global environmental and biotic changes triggered by the physical and chemical effects of the Central Atlantic Magmatic Province (CAMP) volcanism. A super greenhouse climate in the aftermath of the end-Triassic mass extinction appears to be the results of the combination of the long-term CO2 accumulation in the atmosphere system due to the CAMP volcanic activity, as well as the inefficiency of the biological pump due to reduced export production.