Timing and pattern of the Devonian Nehden event: Implication for the biotic recovery following the F–F extinction event

Abstract

The biotic recovery process following the Frasnian–Famennian (F–F) mass extinction played an important role in reestablishing the late Palaeozoic marine ecosystem. The Nehden event is assumed as the global biotic recovery process in the aftermath of the F–F mass extinction, but the timing and pattern of this event is still in dispute due to lack of consistent geological records. In order to recognize the Nehden event and elucidate its timing and pattern, a newly refined conodont biostratigraphic framework spanning the Palmatolepis minuta minuta Zone to the Pa. rhomboidea Zone (370.8–367 Ma) has been established in the Yangdi section, Guangxi, South China. Based on a detailed analysis of microfacies and carbon isotope of carbonates (δ13Ccarb), the start of the Nehden event has been placed at the upper part of the Pa. m. minuta Zone, marked by deposition of calcareous shales and proliferation of palmatolepid conodonts and ostracods. Two episodes of the Nehden event are recognized in this study according to integrated palaeontological, sedimentological, and geochemical data from South China and other continents. The first phase of the Nehden event is assigned at the upper part of the Pa. m. minuta Zone, characterized by radiation of conodonts, ostracods, and brachiopods, which is coincident with the minimum value of negative δ13Ccarb excursion and climate warming. The second phase is placed at the Pa. termini to Pa. glabra pectinata zones, documented by biodiversification of cheiloceratid ammonoids and phacopid trilobites, which is concurrent with climate warming and the maximum transgression during the early Famennian. The extensive development of microbial bindstone in the lower Famennian succession at Yangdi demonstrates that microbial communities occupied the vacant ecological niches left behind by metazoans after the F–F biotic crisis. The expansion of ecological niches of microbial communities and depauperation of metazoan reefs implies the bioconstructor turnover from metazoan to microbial communities following the F–F extinction. The time equivalence of biotic radiation, climate change and sea-level fluctuation in two episodes of the Nehden event indicates the early Famennian biotic recovery was favored by climate warming and transgression. The results in this study not only offer important data to constrain the timing of the Nehden event, but also provide an insight on the process of biotic recovery following the F–F mass extinction event.