Late Eifelian climate perturbations: Effects on tropical coral communities

The Early to Middle Devonian (ca. 418-383 Ma) is known as an interval dominated by global greenhouse conditions with an acme in diversity, size and latitudinal distribution of reefs during the Middle Devonian (Eifelian-Givetian). Nonetheless, also the Middle Devonian climax witnessed several climate perturbations that resulted in more or less severe biotic-events. One of these events is the Late Eifelian Kacak Event, which is represented by a black shale and chert interval documented in sedimentary sequences globally. The polyphase dysoxic/anoxic event-interval is constrained to the kockelianus-ensensis conodont biozones covering about 20010 kyr. It is characterized by distinctive faunal changes concurrent with significant variations in the geochemical and geophysical signals. As far as is documented, biotic response related to the event is observed mainly from benthic organisms of pelagic deposits. Therefore recent comprehensive studies deal mainly with deeper marine deposits providing data related to conodont stratigraphy, stable isotope analysis and magnetic susceptibility. The proposed project focuses on changes among tropical coral communities (especially emphasizing rugose corals) from neritic deposits of the Proto-Alps (Carnic Alps and Graz Paleozoic), the Barrandian and Moravia, contemporaneous with this event. Chosen areas were located in different low latitudes on the continental shelf of northern Gondwana during that time interval. Fossil collections obtained from Eifelian to Givetian strata of the Carnic Alps and the Graz Paleozoic yield abundant and various rugose corals. Apart from a lot of un-described material present in these collections the coral assemblages described from proto-alpine localities include also species which show inconsistency regarding their taxonomy as well as according to their stratigraphic appearance. Additionally, comprehensive rugose coral material is known from Moravia which has not been studied in detail yet. The aim of this project is to reveal an overview of rugose coral assemblages affected by the Kacak Event to clarify questions related to the reaction of climate sensitive organisms to changing environmental conditions. Additional calculation of seawater temperatures across the event interval from different low latitude settings and the application of geochemical and geophysical methods should contribute to uncover the nature of the Kacak Event. Results of this study will provide helpful information on the understanding and severity of recent and future climate perturbations affecting biota globally.

Project FWF P 23775-B17 focused on late Eifelian climate perturbations and effects on climate sensitive organisms from low latitude shallow marine environments in central Europe. Results show that the so called Kacak crisis, which took place short before the Eifelian-Givetian boundary (numerical age: 387.7 million years ago), conforms to a sea-level rise contemporaneous with increasing sea surface temperatures. This is confirmed by specific trends from the magnetic susceptibility of sedimentary rocks and by palaeotemperature calculations from the stable oxygen isotope composition of phosphatic microfossils (conodonts). An important issue of this project was the multidisciplinary study of sections from different bathymetric settings within a well-known platform to basin transect in low latitudes. Respectively, we worked out detailed profiles of five formations in the Carnic Alps, a mountain chain allocated at the Austrian/Italian border (Spinotti Formation, Kellergrat Formation, Freikofel Formation, Hoher Trieb Formation, Valentin Formation). We found out that sea surface temperatures from shallow marine to pelagic deposits record increasing values across the event interval. Similar results are obtained for the neritic Middle Devonian sequence of the Prague Synform (Acanthopyge Limestone, Upper Dark Interval). In order to establish a model on the cause of the Kacak crisis comprehensive geochemical analysis (stable carbon isotopy, XRF analysis and total organic carbon and sulfur contents) and gamma ray spectrometry together with information from microfacies were performed. Effects of the event on shallow marine communities are obvious. At least within carbonate platform deposits in the Carnic Alps, a change, for example in the composition of rugose coral taxa, from a low diversity fauna during and probably until short after the event interval to a rich and diverse fauna in the early Givetian is recognized. However, we could clarify that the rugose coral fauna preceding the Kacak Event is not late Eifelian in age, but older, which concludes that the disappearance of corals was not directly influenced by the Kacak Event, but probably a result of regionally changed palaeoenvironmental conditions. In order to display biodiversity patterns of climate sensitive organisms across the event interval we entered more than 1000 collections of published record on corals and conodonts into the Palaeobiology Database. This gives us the opportunity to model the response of rugose corals to late Eifelian climate perturbations set in a global context. Because the Database is very complex it will take more time until we can conclude that issue.For further comparison of our results from the Carnic Alps and the Czech Republic, we started to investigate Middle Devonian deposits allocated in shallow marine settings of southern Laurussia (Eifel, Germany) and of an volcanic island arc complex in northern hemisphere low latitudes of the Palaeotethys (Baruunhuurai Terrane, Mongolia). Intense cooperation with two other international projects (IGCP 580 and 596) resulted in strong support of the community for dissemination of results.