Three global carbon isotope shifts in the Silurian of Podolia (Ukraine): stratigraphical implications

Abstract

Podolia is a classical Silurian area in the southwestern part of the Ukraine. Shallow to open shelf rocks cropping out there are usually, except for a few dolomitic horizons, highly fossiliferous and therefore the biostratigraphy of the region has a long successful history. The Ukrainian meeting of the Subcommission on Silurian Stratigraphy of the International Union of Geological Sciences, held in 1983, became an important milestone. Tsegelnyuk et al. summarized different results and views in a field guide published for the meeting (The Silurian of Podolia. The Guide to Excursion. Naukova Dumka, Kiev, 1983) and introduced a practically new stratigraphical terminology, and in some parts of the section, also new age interpretation of the beds. For our paper carbon isotopes were studied in eight sections on the banks of the Dniester River and its tributaries, covering ca 80% of the Silurian succession (Pridoli excluded). Three positive δ 13 C excursions were identified. The first excursion in the Kitaigorod 30 section reaches its peak value of 4.0‰ in the very bottom of the Demshin Subformation. The second shift reaches the peak value of 4.3‰ in the middle of the Muksha Subformation. The third shift is the most prominent one identified in the Silurian of Podolia -δ 13 C values increase steadily through the Isakovtsy Subformation, reaching 6.9‰ in the top, and a slightly lower value of 6.6‰ occurs in the bottom of the Prigorodok Formation. In the "middle" Silurian of the World three major positive excursions have been identified: in the early and latest Wenlock and in the late Ludlow. The general character of the carbon isotope trend and stratigraphical positions of the excursions established in this paper demonstrate that in Podolia there occurs the same set of global shifts, which can be used for the improvement of regional and global correlation of Wenlock and Ludlow sections of that area. Some refinements are suggested, but some details need additional study.

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

“…The excursion has been confirmed by Kaljo et al (2007Kaljo et al ( , 2012 in the Dniester River valley, where it begins entirely in the Prygorodok Fm. (Isakivtsy-45 locality, max.…”

“…Of potential utility for constraining the stratigraphic position of the interval investigated in the present study is the positive stable CIE recognized globally in the Ludfordian (Wenzel and Joachimski 1996;Bickert et al 1997;Kaljo et al 1997;Azmy et al 1998;Wigforss-Lange 1999;Wenzel et al 2000;Saltzman 2001;Calner 2005b;Martma et al 2005;Kaljo and Martma 2006;Jeppsson et al 2007;Kaljo et al 2007;Lehnert et al 2007;Eriksson and Calner 2008;Barrick et al 2010;Kozłowski and Munnecke 2010;Munnecke et al 2010;Cramer et al 2011;Loydell and Frýda 2011;Kaljo et al 2012;Kozłowski and Sobień 2012;Manda et al 2012). The beginning of the excursion coincides with an abrupt decrease in abundance of Polygnathoides siluricus, shortly before its Last Appearance Datum (LAD) (Martma et al 2005;Kaljo and Martma 2006;Jeppsson et al 2007;Lehnert et al 2007;Eriksson and Calner 2008).…”

“…are isochrones, the return to base-level values is clearly diachronous, e.g., between the Ataki-117 ? Braga-119 section (Kaljo et al 2007; below the C2 metabentonite bed) and the Isakivtsy-45 section (Kaljo et al 2012; below the C3 metabentonite bed). These authors also introduced the ''top Ludfordian twin excursion'' based on the analysis of the Vidukle-61 and Ohesaare cores, where this smaller double peak is developed entirely within the O. crispa zone.…”

“…Some local factors seem to have been involved in the record of the Lau excursion, which is mostly documented by d 13 C carb of limestones from the epicontinental basin developed during the early Paleozoic on Baltica. This Ludfordian CIE has been recorded in Sweden (?11.2 %, Wigforss-Lange 1999; ?8.80 %, Samtleben et al 2000), Lithuania (?8.2 %, Martma et al 2005), Latvia (?5 %, Kaljo and Martma 2006;Kaljo et al 2012), Poland (?8.9 %, Kozłowski and Munnecke 2010;?6.7 %, Kozłowski and Sobień 2012) and Ukraine (?6.9 %, Kaljo et al 2007; ?6.0 %, Kaljo et al 2012). These localities represent different settings across the carbonate platform and enable the detection of local variability in the CIE record and its dependence on local facies development.…”

“…The Ludfordian (Upper Silurian) stage was the time of the most pronounced positive stable carbon isotope excursion (CIE) in the Phanerozoic, known as the Lau excursion (Wenzel and Joachimski 1996;Bickert et al 1997;Wigforss-Lange 1999;Calner et al 2004;Martma et al 2005; Kaljo and Martma 2006;Jeppsson et al 2007;Kaljo et al 2007; Lehnert et al 2007;Eriksson and Calner 2008;Barrick et al 2010;Kozłowski and Munnecke 2010;Munnecke et al 2010;Cramer et al 2011;Loydell and Frýda 2011;Jeppsson et al 2012;Kaljo et al 2012;Kozłowski and Sobień 2012). 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).…”