Andrei Dronov

Onset of main Phanerozoic marine radiation sparked by emerging Mid Ordovician icehouse

The Great Ordovician Biodiversification Event (GOBE) was the most rapid and sustained increase in marine Phanerozoic biodiversity. What generated this biotic response across Palaeozoic seascapes is a matter of debate; several intrinsic and extrinsic drivers have been suggested. One is Ordovician climate, which in recent years has undergone a paradigm shift from a text-book example of an extended greenhouse to an interval with transient cooling intervals – at least during the Late Ordovician. Here, we show the first unambiguous evidence for a sudden Mid Ordovician icehouse, comparable in magnitude to the Quaternary glaciations. We further demonstrate the initiation of this icehouse to coincide with the onset of the GOBE. This finding is based on both abiotic and biotic proxies obtained from the most comprehensive geochemical and palaeobiological dataset yet collected through this interval. We argue that the icehouse conditions increased latitudinal and bathymetrical temperature and oxygen gradients initiating an Early Palaeozoic Great Ocean Conveyor Belt. This fuelled the GOBE, as upwelling zones created new ecospace for the primary producers. A subsequent rise in δ13C ratios known as the Middle Darriwilian Isotopic Carbon Excursion (MDICE) may reflect a global response to increased bioproductivity encouraged by the onset of the GOBE.

Shallow-Marine Carbonates

Abstract The ichnology of carbonate sedimentary systems is less well studied compared to that of siliciclastic systems. Shallow-marine carbonates differ from their siliciclastic counterparts by a number of criteria, such as the composition and distribution of trace-fossil associations, broad and very shallow facies belts with intense bioturbation and subaerial exporsure, and a strong impact of diagenetic processes on carbonate sediments. Early cementation typically leads to the occurrence of firmgrounds and hardgrounds with intense bioerosion and preferential trace-fossil preservation. The Psilonichnus, Skolithos, and Cruziana ichnofacies are important for the reconstruction of paleoenvironments on carbonate shelves (or ramps), whereas the substrate-controlled Glossifungites and Trypanites ichnofacies can overprint the former ones during periods with stagnant or low net-sediment accumulation. The value of ichnological analysis in shallow-marine carbonates is demonstrated on Middle Ordovician cool-water carbonates in Russia, Middle Triassic epicontinental carbonates in Germany, the Quaternary rimmed carbonate platform of the Bahamas, and the Quaternary homoclinal carbonate ramp of the southern Arabian Gulf.