Carbon Isotope Variations in the Early Devonian Salair Paleobasin

Northeastern Salair is the stratotype area for the Lower and Middle Devonian of the western part of the Altai-Sayan Folded Area. The regional stratigraphic units of this area include the Lower Devonian Tomsky Zavod, Peetz, Krekov, Maly Bachat and Salairka formations, having diverse fossils with wide geographic distribution, including conodonts. Tomsky Zavod Fm yielded the Lochkovian conodont complex Caudicriodus woschmidti transiens (Carls et Gandl), Pedavis breviramus Murphy et Matti, Ozarkodina excavata (Branson et Mehl), Pelekysgnathus serratus Jentzsch, Pandorinellina optima (Moskalenko), Pandor. philipi (Klapper). In the Peetz, Krekov and Maly Bachat formations, conodonts are less diverse and represented by Pandorinellina philipi (Klapper) and Panderodus unicoststus (Branson et Mehl). Lower and Middle parts of Salairka Fm are characterized by Pandorinellina philipi (Klapper) and Panderodus unicostatus (Branson et Mehl). The Emsian conodont complex Caudicriodus amplus (Izokh), Caudicriodus vicinus (Izokh) and Polygnathus kitabicus Yolkin et al. appears at the upper part of Salairka Fm. The isotopic composition of the Lower Devonian carbonate rocks of Salair was investigated. The established δ13C curve is characterized by significant variations. For the lower part of Tomsky Zavod Fm, it was found that δ13C are not lower 2.1‰. At the middle part a significant decrease in δ13C to 1.1‰ is observed with following enhance to 2.3–2.7‰. In the upper part of the Formation δ13C decrease to 1.9‰, and in the lower part of the overlying Peetz Fm, they drop sharply to -1‰ with following gradually increase to 2‰, and then to 2.8‰. The Krekov and Maly Bachat limestones are characterized by δ13C values in the range of 1.5–2.7‰. Within the Salairka Fm, two large positive excursions with an amplitude of about 2‰ are observed – at the base of the Formation, an increase occurred from almost 1 to 2.9‰ with a subsequent decrease to almost 0‰, and in the upper part of the Formation, an increase in values up to 3.5‰ is recorded. The composite carbon isotope curve for the Lower Devonian of Salair was constructed, which is characterized by the following C isotopic markers: 1.At the boundary interval of the Tomsky Zavod and Peetz formations that coincide with the middle part of the Lochkovian, a negative excursion has been identified, that is also observed in the other regions of the world and aligned with the base of the Lanea omoalpha Zone. 2.At the lower part of the Salairka Formation, a negative excursion was revealed, that is compared with a negative shift in the typical Pragian deposits in the Kitab State Geological Reserve (KSGR) (Uzbekistan), where it is confined to the middle part of the Polygnathus pireneae Zone. 3.At the upper part of the Salairka Formation, a short-term negative and subsequent positive excursion was revealed, which can be compared with δ13C variations within the lower part of the Emsian in the Zinzilban section of the KSGR. All the obtained excursions can be traced in other regions of the world, that allows them to be used for regional, interregional and global correlation of the Lower Devonian.