Biotic and stable-isotope characterization of the Toarcian Ocean Anoxic Event through a carbonate–clastic sequence from Somerset, UK

Ian Boomer*, Philip Copestake, Kevin Page, John Huxtable, Tony Loy, Paul Bown, Tom Dunkley Jones, Matt O'Callaghan, Sarah Hawkes, David Halfacree, Henry Reay, Natalie Caughtry

*Corresponding author for this work

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Abstract

This study focuses on a condensed sequence of alternating carbonate–clastic sediments of the Barrington Member, Beacon Limestone Formation (latest Pliensbachian to early Toarcian) from Somerset (SW England). Abundant ammonites confirm (apart from the absence of the Clevelandicum and Tenuicostatum ammonite subchronozones) the presence of Hawskerense Subchronozone to Fallaciosum–Bingmanni subchronozones. Well-preserved, sometimes diverse assemblages of ostracods, foraminifera, nannofossils and low-diversity dinoflagellate assemblages support the chronostratigraphic framework. Stable-isotope analyses demonstrate the presence of a carbon isotope excursion, relating to the Toarcian Oceanic Anoxic Event, within the early Toarcian. Faunal, geochemical and sedimentological evidence suggest that deposition largely took place in a relatively deep-water (subwave base), mid-outer shelf environment under a well-mixed water column. However, reduced benthic diversity, the presence of weakly laminated sediments and changes in microplankton assemblage composition within the Toarcian Oceanic Anoxic Event indicates dysoxic, but probably never anoxic, bottom-water conditions during this event. The onset of the carbon isotope excursion coincides with extinction in the nannofossils and benthos, including the disappearance of the ostracod suborder Metacopina. Faunal evidence indicates connectivity with the Mediterranean region, not previously recorded for the UK during the early Toarcian.
Original languageEnglish
Pages (from-to)239-268
Number of pages30
JournalGeological Society Special Publication
Volume514
Issue number1
Early online date13 May 2021
DOIs
Publication statusPublished - 5 Jul 2021

Bibliographical note

Funding: Kevin Page received support from the Geological Society of London, Gloyne Outdoor Geological Research Fund.

Acknowledgments: The authors thank the landowners Mr John Hallett and Mr Mathew Hallett for permission to access the site and collect samples. JH thanks Dr Mike Howarth for support and guidance. KP thanks the Geological Society of London for the award of a Gloyne Outdoor Geological Research Fund, including Stephanie Jones for assistance; KP also thanks Zöe Hughes, Department of Palaeontology, NHM London, Tracey Gallagher and Paul Shepherd, BGS Keyworth, for assistance with access to institutional collections and Dr Andy King (Geck-oella Ltd) and Matthew Williams (Bath Royal Literary and Scientific Institute) for access to the Strawberry Hill research excavations in 2019. PC thanks Merlin Energy Resources Ltd for permission to publish. Alan Lord, Emma Cieslak-Jones and Thomas Fogerty provided additional assistance. The authors acknowledge the contributions of our guest editor Matias Reolid and reviewers M. Luisa Canales and Annachiara Bartolini for their diligence and support.

Publisher Copyright:
© 2021 The Author(s).

ASJC Scopus subject areas

  • Water Science and Technology
  • Ocean Engineering
  • Geology

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