Journal of Conference Abstracts

Correlating Early Silurian ¹³C and ¹⁸O Events Within and Between Sedimentary Basins

Rachel Heath (rjheath@badley-ashton.co.uk)¹, James Marshall (isotopes@liverpool.ac.uk)² & Patrick Brenchley²

¹ Badley, Ashton & Associates Ltd, Winceby House, Winceby, Horncastle, Lincs., LN9 5 PB, U.K.

² Dept. Earth Sciences, University of Liverpool, PO Box 147, Brownlow Street, Liverpool, U.K.

An isotopic study carried out for the purpose of investigating the nature and timing of palaeoceanographic events shows that it is possible to correlate isotope shifts between widely separate sedimentary basins. A correlation of published oxygen and carbon stable isotope records from the Baltic identified isotopic variations that occurred independently of facies and lithology. This enabled at least one major event to be identified as potentially of environmental significance on a regional scale. Large shifts (>2 per mil) in ¹³C and ¹⁸O were found near the Llandovery-Wenlock boundary (Early Silurian) in carbonate sequences from locations across the Baltic basin. This isotopic event coincides with extinctions and diversity changes in benthic and planktonic faunas and other indicators of climatic and oceanographic change (eustatic variations and likely glacial sediments).

To test the significance of this isotopic event further, the Baltic isotope record was compared with those available from other palaeocontinents, namely Laurentia and Gondwana. Isotopic records from different source materials and from different depositional environments were compared, specifically; whole-rock carbonate, brachiopod shell calcite and organic matter from an immature carbonate source rock. Similar positive shifts in ¹³C from organic carbon and whole-rock carbonate were found in the Australian and Canadian sections and it was possible to correlate them with the Baltic isotope stratigraphy. The apparent difference in the position of the isotope shift relative to the Llandovery-Wenlock boundary between sedimentary basins is ascribed to the use of different biostratigraphical schemes in contrasting depositional environments and separate geographic areas. From this, it is evident that stable isotope curves can be used in conjunction with biostratigraphical data to increase the accuracy of regional and inter-regional correlations.

The comparisons of isotopic records from the different source materials also show that the whole-rock isotopic record is more variable than that from brachiopods. Whilst both brachiopod ¹³C and whole rock records reflect an environmental signal, the whole-rock ¹⁸O record is much more likely than brachiopod calcite to be significantly altered by diagenesis.

It is assumed that in such widely separate basins as studied here, the sedimentary successions, and materials within those successions (such as biogenic calcite and organic matter), have completely unrelated diagenetic histories and, therefore, that any isotopic events that can be correlated between the two are primarily the result of wide-reaching environmental influences prior to deposition. From this, and the evidence for corresponding extinctions and eustatic changes, it is concluded that changes in ocean chemistry and climatic variations are the likely causes of the isotopic variation observed in the case of the Llandovery-Wenlock isotope shift. An episode of climatic cooling associated with changes in the carbon cycle (involving a combination of variations in carbon burial, pCO₂ and productivity) could account for the observed isotopic changes.

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