Carbon isotope stratigraphy across the Silurian–Devonian transition in Podolia, Ukraine: evidence for a global biogeochemical perturbation

Abstract

The carbon and oxygen isotope composition of marine carbonates (δ 13 C and δ 18 O, 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 δ 13 C changes. The sections reveal a major positive δ 13 C excursion, with an amplitude above 6 ‰, beginning in the upper Pridoli and reaching peak values as heavy as +4.2 ‰ in the lowermost Lochkovian. This turnover in carbon cycling is followed by a general trend toward more negative δ 13 C 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.

The Early Devonian Armoured Agnathans of Podolia, Ukraine

“…The upper part of the Lower Devonian succession in Podolia is in the Old Red Sandstone units of the area, earlier known as the "Terebovlia beds", "Podolian Old Red", "Babyn sandstones", has been included into the Dniester Formation (Narbutas 1984). It is up to 1800 m thick near the Trans−European Suture Zone, elsewhere representing coastal−continental facies with fossil remains limited to numerous agnathan fish (Drygant 2003;Małkowski et al 2009).…”

“…It is characterised by platy limestone and variably interbedded dark grey argillaceous shale, marl, and marly to micritic limestone, intercalated with rare bioclastic limestone beds (mostly encrinite) in the higher part. The upper part of the formation is a thick succession of dark grey argillaceous shale with rare and thin intercalations and lenticles (up to 1-6 cm) of grey marly or biodetrital limestone, frequently represented by brachiopod coquinas (Małkowski et al 2009;Drygant 2010). Well−preserved fossils of diverse exclusively marine invertebrates, including graptolites, chitinozoans, THE EARLY DEVONIAN ARMOURED AGNATHANS OF PODOLIA, UKRAINE 13 Fig.…”

“…2. Lithologic units of the Upper Silurian-Lower Devonian strata on the southwestern margin of the East European Platform (after Drygant 2003Drygant , 2010Małkowski et al 2009). corals, bivalves, nautiloids, bryozoans, brachiopods, ostracods, trilobites, and crinoids, as well as conodonts, rare agnathan and fish microremains have been described from the Borshchiv deposits (Karatajãt −Talimaa 1978;Paris and Grahn 1996;Karatajãt−Talimaa and Märss 2004;Drygant 2010). The Chortkiv Formation, more than 200 m thick, is represented by alternating dark grey argillaceous shale and thin−bedded (1-10 cm, rarely up to 20 cm) fine−grained limestone.…”

“…The fossil assemblages are very diverse in the limestone and not so in the shale. In addition to abundant marine invertebrate fossils similar to those of the Borshchiv Formation, numerous agnathan remains occur (Obruchev and Karatajãt−Talimaa 1968;Karatajãt−Talimaa 1978;Karatajãt−Talimaa and Märss 2004;Małkowski et al 2009;Drygant 2010). Ostracodes, bivalves and tentaculites often form coquinas.…”

Facies development and sequence stratigraphy of the Ludfordian (Upper Silurian) deposits in the Zbruch River Valley, Podolia, western Ukraine: local facies overprint on the δ13Ccarb record of a global stable carbon isotope excursion

“…Very good geochemical, biostratigraphical, and paleoecological documentation exists for the Lau excursion, and it is often used as a model to study positive CIEs, which are widely used as chemostratigraphic markers and as indicators of perturbations of the global carbon cycle (Jeppsson 1990;Aldridge et al 1993;Bickert et al 1997;Jeppsson and Aldridge 2000;Munnecke et al 2003;Calner 2005a;Melchin and Holmden 2006;Cramer and Saltzman 2007a, b;Fanton and Holmden 2007;Calner et al 2008;Małkowski et al 2009;Kozłowski and Sobień 2012). Studies of recent and sub-fossil carbonate sediments indicate that sea-level forcing, restricted circulation and laterally variable sediment composition can significantly influence the d 13 C carb values, leading to amplification of positive excursions in carbonate platform interiors during flooding intervals (Immenhauser et al 2003;Swart and Eberli 2005;Swart 2008;Gischler et al 2009;Oehlert et al 2012).…”