Paper No. 1
Presentation Time: 1:00 PM
EARLY TRIASSIC OCEANIC ANOXIC EVENTS AND THE RECOVERY OF PELAGIC ECOSYSTEMS
WIGNALL, Paul B., School of Earth and Environment, University of Leeds, Woodhouse Lane, Leeds, LS2 9JT, United Kingdom, BOND, David P.G., School of Geography, Environment & Earth Sciences, University of Hull, Hull, HU6 7RX, United Kingdom and SONG, Haijun, State Key Laboratory of Biogeology and Environmental Geology, China University of Geosciences, Wuhan, Lumo Road, No 388, Wuhan, 430074, China, p.wignall@see.leeds.ac.uk
The two phase Permo-Triassic mass extinction coincides with two anoxic events, the first was brief and the second was a much more prolonged phase that lasted well into the Early Triassic. Higher latitude shelf sections indicate that anoxia was already becoming widespread before ocean anoxia developed and may account for the earlier demise of benthic faunas in these regions. Evidence from Japanese accreted terranes indicates that the oceanic realm remained poorly ventilated (often euxinic) for much of the ensuing Early Triassic before a rapid return to full oxygenation in the Middle Triassic. However, the Early Triassic “anoxia” shows considerable fluctuations of intensity as revealed by petrographic and geochemical markers. Thus, intervals around the Smithian/ Spathian stage boundary and within the later Spathian both saw an intensification of anoxia whilst the Dienerian Stage saw a modest improvement of ventilation.
The fate of the world’s pelagic fauna closely corresponds to the onset of the anoxia in the latest Permian when radiolarian productivity crashed and ammonoids were nearly annihilated. However, this link between the fate of pelagic groups and ocean redox changes is not seen again in the Triassic with the possible exception of the Smithian/Spathian anoxic event, when many ammonoids and conodonts were lost. Many pelagic groups diversified rapidly at some point in the Early Triassic (e.g. ammonoids in the Griesbachian, radiolarians in the latest Spathian and many fish groups throughout the interval). This non-coincidence is especially stark in the later Spathian Stage when anoxia intensified in both oceanic and shelf settings and yet fish and ammonoids were recovering rapidly at this time and radiolarian productivity and diversity was beginning to recover from an end-Permian “blackout phase”. Other factors, especially ocean temperature which is in clear lock-step with diversity, are probably much more important controls on pelagic radiations.
