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Heath et al., 1998

Early Silurian carbon and oxygen stable-isotope stratigraphy of Estonia: implications for climate change

Heath, R. J., Brenchley, P. J., Marshall, J. D.
BookSilurian cycles - linkages of dynamic stratigraphy with atmospheric, oceanic and tectonic changes. Landing, E. & Johnson, M. E. (Eds)
Editor(s)Landing, E., Johnson, M. E.
JournalNew York State Museum Bulletin
Belongs toLanding & Johnson, 1998 (eds)
Typearticle in book


Early Silurian recovery followed the Ashgill extinction; however, there were smaller episodes of extinction of Llandovery plankton and benthos. Recent studies of Late Ordovician events have shown that the Ashgill extinction was associated with major glaciation, glacio-eustatic changes, and a large positive excursion in d180 and d13C in marine carbonates. It has been suggested that Early Silurian events are also related to changes in ocean state, or sea-level fluctuations associated with climatic change. To test this model, a carbon and oxygen stable-isotope stratigraphy is established for the Llandovery and lowermost Wenlock. Data was obtained from brachiopod calcite from Estonian shelf-carbonate sequences with shallow burial history and low thermal maturity. A gradual trend towards more negative and constant d18O values through the Llandovery and nearly constant values of d13C exists, with two small excursions that are based on limited data. In the early Wenlock, a large positive excursion in oxygen (nearly 2 permil) and carbon (>2 permil) persists for five graptolite zones, and is associated with extinctions of planktic faunas. Gradual decrease in 8180 through the Llandovery is interpreted to reflect a progressive warming, and the d13C data suggest relatively stable oceanic conditions without major changes in carbon cycling. The magnitude of the positive isotopic excursion in the early Wenlock may indicate major cooling, possibly associated with growth of ice caps, and the carbon excursion reflects major changes in carbon cycling. The isotope data suggest relatively stable oceanic conditions that are consistent with progressive recovery of ecosystems after the Ashgill extinction. The isotope data do not confirm proposals of four glacio-eustatic changes, nor do they identify major perturbations of the carbon cycle during planktic extinction events in the Llandovery or show any systematic variation with proposed P and S states.

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