Carbon and conodont apatite oxygen isotope records of Guadalupian–Lopingian boundary sections: Climatic or sea-level signal?

Abstract

The Guadalupian–Lopingian (G–L) boundary (260.4 Ma) is one of the major extinction events in Earth history that coincides with the Emeishan Large Igneous Province and palaeoclimatic changes. Carbon isotopes of whole rock samples were studied in order to document changes in the global carbon cycle. In contrast to earlier studies, we observe no major negative excursion in δ 13C in the middle Capitanian. A positive δ 13C excursion is observed in the latest Capitanian with a 1.5‰ increase registered in the J. xuanhanensis Zone to C. postbitteri hongshuiensis Subzone, followed by a decrease of 1‰ within the C. postbitteri postbitteri Subzone and a 2‰ decrease in the C. dukouensis to C. asymmetrica Zone. Oxygen isotopes of conodonts from two G–L boundary sections were measured in order to reconstruct conodont habitat and potential changes in water temperature. Oxygen isotope ratios of gondolellid conodonts are higher in comparison to oxygen isotope ratios measured on hindeodid conodonts suggesting that gondollelids lived in cooler and thus deeper waters compared with hindeodids. The oxygen isotope record reconstructed from gondollelid conodonts suggests warming of water temperatures of about 4 °C in the late Capitanian ( J. postserrata to J. granti Zone), cooling of about 6 to 8 °C across the G–L boundary and in the earliest Wuchiapingian, and again significant warming in the Wuchiapingian ( C. dukouensis to C. liangshanensis Zone). The temperature increase can be correlated with the main phase of Emeishan volcanism suggesting that climatic warming may have resulted in an intensified hydrological cycle, fertilisation of the oceans and enhanced primary productivity, the latter documented in the positive late Capitanian carbon isotope excursion. However, changes in sea-level seem to parallel reconstructed water temperatures suggesting that changes in water depth in combination with superimposed climatic changes may be responsible for the observed temperature changes. This study documents that oxygen isotope studies on Permian conodonts should be performed on mono-generic conodont samples and that oxygen isotopes not only provide valuable palaeoclimatic information but also may help to constrain the life habitat of conodonts.