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Geological Magazine; September 2009; v. 146; no. 5; p. 674-689; DOI: 10.1017/S0016756809006451
© 2009 Cambridge University Press (CUP)
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Carbon isotope stratigraphy across the Silurian–Devonian transition in Podolia, Ukraine: evidence for a global biogeochemical perturbation

K. MALKOWSKI*, G. RACKI*,{dagger}, D. DRYGANT{ddagger}, and H. SZANIAWSKI*

* Institute of Paleobiology, Polish Academy of Sciences, Twarda 51/55, 00-818 Warszawa, Poland
{ddagger} State Museum of Natural History, National Academy of Sciences of Ukraine, Teatralna 18, Lviv 79008, Ukraine

{dagger} Author for correspondence: racki{at}twarda.pan.pl

The carbon and oxygen isotope composition of marine carbonates ({delta}13C and {delta}18O, respectively) are studied in the fossiliferous, stratigraphically well-constrained and remarkably expanded successions of Podolia, SW Ukraine, spanning the Silurian–Devonian transition. Significant isotopic shifts are directly comparable to previously published global secular trends in well-preserved brachiopod calcite isotopic ratios from this region, and therefore may be taken as a reliable primary record of seawater {delta}13C changes. The sections reveal a major positive {delta}13C excursion, with an amplitude above 6 {per thousand}, beginning in the upper Pridoli and reaching peak values as heavy as +4.2 {per thousand} in the lowermost Lochkovian. This turnover in carbon cycling is followed by a general trend toward more negative {delta}13C values in the upper Lochkovian. The Podolian isotopic signals provide strong support for the previously inferred global biogeochemical perturbation across the Silurian–Devonian transition, reflecting a complex combination of palaeogeographical, biogeochemical and evolutionary processes in the late Caledonian geodynamic setting, with a likely undervalued role of the expanding vegetation in vast near-coastal shallows and deltas.

Key Words: Silurian • Devonian • Podolia • carbon isotopes • chemostratigraphy






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