PhD: The longest delay: the slow recovery from the Hangenberg mass extinction

The longest delay: the slow recovery from the Hangenberg mass extinction

The end-Devonian (Hangenberg) mass extinction is one of the least studied and thus least understood major crises in Earth history. There were major environmental changes including a short but intense glaciation, big sea-level changes and the extensive spread of black shales. Some or all of these may be potential kill mechanisms but the key causes of the losses are unclear. Unlike most other extinction events, the Hangenberg crisis was also unusually selective in terms of its victims. Thus, free-swimming forms, such as fish and ammonoids, were devastated (this was by far the worst crisis ever experienced by fish) whilst seabed communities showed more variable loss rates. Reefs, and their resident corals, disappeared along with most trilobites, whilst other groups, including brachiopods, crinoids and foraminifers, survived virtually unscathed. The subsequent recovery and radiation of marine life was also exceptionally and curiously slow: marine biodiversity remained at low levels for >20 myrs – this is the longest aftermath of any crisis in earth history. This delay compares to recovery intervals typically measured in hundreds of thousands of years after most mass extinction events. In detail the recovery varied according to the setting. Fish and ammonoids bounced back quickly and produced diverse new groups within a few million years whereas seafloor recovery was much more protracted: coral did not diversify until near the end of the Early Carboniferous.

The reasons for the ultra-prolonged, post-Hangenberg delay are little studied and unknown. It may relate to the repeat of environmental stresses and a succession of biotic crises Potentially the recovery from the Hangenberg crisis may have been nipped in the bud by a further crisis. Thus, black shales are exceptionally widespread at this time (they are known as the Lower Alum Shales) and they coincide with a major global carbon isotope perturbation, known as TICE. Similar magnitude black shale and isotopic excursions coincide with extinction events throughout the Palaeozoic and Mesozoic but the TICE episode has hitherto not been linked to an extinction event. However, this lack of correspondence may simply reflect a lack of study. Alternatively, global climatic conditions in the earliest Carboniferous are considered to have been cool and the marine realms well-connected, both factors could potentially be a cause of low diversity.

This project will examine the Hangenberg mass extinction and the subsequent failure to recover during the Early Carboniferous (Tournaisian) by combining a study of select fossil groups, especially the foraminifers and brachiopods, with palaeoenvironmental study, especially of seafloor oxygen levels. Field studies will take place in the type sections of this interval in Belgium, plus sections in southern Ireland, and South Wales and will examine both carbonate and clastic marine strata that record a broad range of palaeoenvironments from shallow-to-deep settings. Details of diversity trends (origination and extinction rates) in marine communities will be established. The student will be trained in a range of both palaeontological and sedimentological disciplines. Foraminifer identification in carbonates will primarily rely on petrographic analysis and it is intended that a detailed evolutionary history of this group will be produced based on taxonomic analysis of samples, supplemented with published records in the palaeontological literature. The records of other common groups (e.g. crinoids, brachiopods and rugose corals) will also allow comparisons to be drawn to see if recovery was initiated in specific environmental settings. Environmental context for Tournaisian biodiversity will come from sedimentary logging, microfacies of carbonates and pyrite petrography using scanning electron microscope analysis of polished blocks to help constrain redox levels. The coverage of diversity trends in distant regions will allow potential regional scale variations to be examined.

This project addresses a large-scale question: what controls global biodiversity? Following a mass extinction, vacated ecospace offers the opportunity for radiation and innovation amongst the surviving groups which is (eventually) seen in the Early Carboniferous. Factors such as temperature, provincialism and habitat area (itself controlled by factors such as sea-level and oxygen levels) are all possible determinants of diversity levels which can be studied during this interval.

The student will join the large and supportive research group at Leeds and receive training in both the palaeontological and fieldwork skills required to assess late Palaeozoic marine sediments. The great strength of the training will be its integration of fieldwork, palaeontological analysis and meta-analysis of large literature-based datasets to address a key issue in macroevolution. Both supervisors are experts in their field and have extensive records of successful research supervision going back over many years, which have fostered many high-profile research publications from their students.

References

Aretz, M., Nardin, E. & Vachard, D. 2014. Diversity patterns and palaeobiogeographical relationships of latest Devonian-Lower Carboniferous foraminifers from South China: What is global, what is local? Journal of Palaeogeography 3, 35-59.

Kaiser, S.I., Aretz, M. & Becker, R.T. 2016. The global Hangenberg crisis (Devonian-Carboniferous transition): a review of a first order mass extinction. In Becker, R.T. et al. (eds) Devonian climate, sea level and evolutionary events. Geological Society of London Special Publications 423, 387-437.

Yao, L., Aretz, M., Wignall, P.B., Chen, JT., Vachard, D., Qi, Y.P., Shen, S.Z. & Wang, X.D. 2020. The longest delay: Re-emergence of coral reef ecosystems after the Late Devonian extinctions. Earth-Science Reviews 203, 103060.

Yao, L., Qie, W.K., Luo, G.M., Liu, J.S., Algeo, T.J., Bai, X., Yang, B. and Wang, X.D. 2015. The TICE event: Perturbation of carbon-nitrogen cycles during the mid-Tournaisian (Early Carboniferous) greenhouse-icehouse transition. Chemical Geology 401, 1-14.