The Lower-Middle Ordovician δ18O Record and Its Relation to Diversification Pulses of the GOBE
1 CAS Key Laboratory of Economic Stratigraphy and Palaeogeography (NIGP, CAS), Nanjing 210008, China; rcwu@nigpas.ac.cn, rbzhan@nigpas.ac.cn;
2 Faculty of Environmental Sciences, Department of Environmental Geosciences, Czech University of Life Sciences Prague, Kamýcká 1176, 165 21 Praha 6 - Suchdol, Czech Republic;
3 Institute of Geology at Tallinn University of Technology, 19086 Tallinn, Estonia; Peep.mannik@ttu.ee;
4 GeoZentrum Nordbayern, University of Erlangen-N甃rnberg, Schlossgarten 5, D-91054, Erlangen, Germany;oliver.lehnert@fau.de, michael.joachimski@fau.de;
5 U.S. Geological Survey. 926A National Center. Reston, VA 20192, USA; jrepetski@usgs.gov;
6 Department of Geology, Lund University, Sölvegatan 12, SE-223 62 Lund, Sweden; mikael.calner@geol.lu.se;
7 Finnish Museum of Natural History, PO Box 44, Fi-00014 University of Helsinki, Finland; bjorn.kroger@helsinki.fi;
8 Univ. Lille, CNRS, UMR 8198 - Evo-Eco-Paleo, F-59000 Lille, France; thomas.servais@univ-lille1.fr;
9 Department of Earth Sciences, Durham University, Durham DH1 3LE, UK; david.harper@durham.ac.uk.
Over the last two decades, extensive information has accumulated on biological radiation patterns during the Great Ordovician Biodiversification Event (GOBE), which included multiple evolutionary pulses and major ecosystem changes. The term GOBE was introduced by Webby about 20 years ago to describe the tremendous explosion of life during the Ordovician after the Cambrian innovations (e.g., Webby et al., 2004). The event has been widely discussed (see for example Servais et al., 2010; Harper et al., 2015) and it became obvious that different marine faunal groups exhibit diversity peaks at different stratigraphic levels and major shifts in biodiversity are observed at different times on different palaeoplates depending on the climatic conditions and ocean currents at different palaeolatitudes (Servais et al., 2016a). Therefore, the concept of the GOBE having begun in the late Cambrian in China and peaking in other areas during later intervals (e.g. Baltica during the Middle Ordovician) has to be refined (Servais et al., 2016a).
In addition to long-term plate tectonic processes, changes in global climate conditions caused by factors such as volcanic events certainly had some major influences on the diversity patterns in the Ordovician world. In this presentation, we will elucidate the climate story prior to the culmination of the Ordovician-Silurian icehouse interval during several major glaciations from the mid-Katian onwards and focus mainly on the Middle Ordovician record on Baltica.
The simple δ18O trend based on the analysis of biogenic apatite (conodonts) presented by Trotter et al. (2008) for the Ordovician can only indicate some overall climate cooling during the Ordovician. Several attempts at different time slices suggest that alternation of glacial and interglacial intervals started during the Middle Ordovician (e.g., Rasmussen et al., 2016).
Our sparse δ18O data from Laurentia show a cooler period across the Cambrian-Ordovician transition. Climatic cooling at this time may have triggered the plankton revolution that had already begun in the latest Cambrian (Servais et al., 2016b). This cooler period was followed by a temperature increase and a warm period during the mid-Tremadocian with a new shift to much cooler climates in the latest Tremadocian and Floian.
There are some fluctuations in the Baltoscandian Floian and Dapingian record, but a really obvious shift to colder conditions is observed in the upper part of the ‘Täljsten interval'in Sweden (Yangtzeplacognathus crassus conodont Biozone) with an overall cooling trend across the well-known Mid-Darriwilian Isotopic Carbon Excursion (MDICE). The δ18O dataset reflects a dramatic cooling in this interval, which may indicate the onset of a larger ice cap in the polar regions of the vast Gondwana supercontinent. Bio- and chemostratigraphic (δ13C) comparisons of MDICE intervals from different palaeoplates reveals that a succession in southeasternmost Sweden (the Tingskullen core from Öland) has the most complete MDICE record in the world (Wu et al., 2017)
There is a notable coincidence between the rise in δ18O values during the early Darriwilian and the Middle Ordovician Brachiopod Event (MOBE), an extreme diversification of brachiopods at the family level during the Darriwilian (Rasmussen in Harper et al., 2013), a trend that is also shown by the main apex predators in the Ordovician food chain, i.e., the cephalopods. Interestingly, the first peak in cephalopod diversity on Baltica and South China occurred during the same interval in the early Darriwilian (Y. crassus conodont Biozone;Kröger&Rasmussen,2014).
The increased nutrient availability in the water column during the Darriwilian caused by rapid cooling is probably reflected in the rising limb of the well-known MDICE. This may also have caused a major diversification event in brachiopods, which resulted in the Mid-Ordovician faunal ‘hotspot' on Baltica (Rasmussen et al., 2007). The later establishment of diverse reef systems on Baltica (Kröger et al., 2017) falls presumably into the time interval when the continent had reached climatically suitable palaeolatitudes for such complex ecosystems: the distribution, demise and rise of these communities, as well as many Late Ordovician bioevents, were probably related to glacially-driven major eustatic sea-level changes.
References
Harper, D. A. T., Rasmussen, C. M. Ø., Liljeroth, M., Blodgett, R. B., Candela, Y., Jin, J. S., Percival, I. G., Rong, J. Y., Villas, E., Zhan, R. B. 2013. Chapter 11 Biodiversity, biogeography and phytogeography of Ordovician rhynchonelliform brachiopods. Geological Society, London, Memoirs, 38: 127—144.
Harper, D. A. T., Zhan, R. B., Jin, J. S. 2015. The Great Ordovician Biodiversification Event (GOBE): reviewing two decades of research on diversity's big bang by mainly illustrated brachiopod data. Palaeoworld, 24: 75—85.
Kröger, B., Rasmussen, J. A. 2014. Middle Ordovician cephalopod biofacies and palaeo-environments of Baltoscandia. Lethaia, 47: 275—295.
Kröger, B., Hints, L., Lehnert, O. 2017. Ordovician reef and mound evolution: the Baltoscandian picture. Geological Magazine, 24: 683—706.
Rasmussen, C. M. Ø., Hansen, J., Harper, D. A. T. 2007. Baltica: a mid Ordovician diversity hotspot. Historical Biology, 19: 255—261.
Rasmussen, C. M. Ø., Ullmann, C. V., Jakobsen, K. G., Lindskog, A., Hansen, J., Hansen, T., Eriksson, M. E., Dronov, A., Frei, F., Korte, C., Nielsen, A. T., Harper, D. A. T. 2016. Onset of main Phanerozoic marine radiation sparked by an emerging Mid-Ordovician icehouse. Scientific Reports, 6: 1-9.
Servais, T., Owen, A. W., Harper, D. A. T., Kröger, B., Munnecke, A. 2010. The Great Ordovician Biodiversification Event (GOBE): the palaeoecological dimension. Palaeogeography, Palaeoclimatology, Palaeoecology, 294: 99-119.
Servais, T., Owen, A. W., Harper, D. A. T. 2016a. The Great Ordovician Biodiversification Event (GOBE): palaeoecological dimension. IGCP 653 (The onset of the Great Ordovician Biodiversification Event) — Opening Meeting, September 25-October 1, 2016, Durham, UK. Abstracts, pp. 32-33.
Servais, T., Perrier, V, Danelian, T., Klug, C., Martin, R., Munnecke, A., Nowak, H., Nützel, A., Vandenbroucke, T. R. A., Williams, M., Rasmussen, C. M. Ø. 2016b. The onset of the ‘Ordovician Plankton Revolution' in the late Cambrian. Palaeogeography, Palaeoclimatology, Palaeoecology, 458: 12—28.
Trotter, J. A., Williams, I. S., Barnes, C. R., Lécuyer, C., Nicoll, R. S., 2008. Did cooling oceans trigger Ordovician biodiversification? Evidence from conodont thermometry. Science, 321:550—554.
Webby, B. D., Cooper, R. A., Bergström, S. M., Paris, F. 2004. Stratigraphic framework and time slices. In: Webby, B. D., Paris, F., Droser, M. L., Percival, I. G. (eds.), The Great Ordovician Biodiversification Event. New York: Columbia University Press, pp. 41-47.
Wu, R. C., Calner, M., Lehnert, O. 2017. Integrated conodont biostratigraphy and carbon isotope chemostratigraphy in the Lower-Middle Ordovician of southern Sweden. Geological Magazine, 154: 334-353.