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Organic Carbon Isotopes and Silurian Chronostratigraphy in Eastern Europe

Abstract

The Late Ordovician and Silurian periods are punctuated by disturbances of the global climate and oceanographic system; phenomena identified by significant, yet stratigraphically narrow positive shifts in stable carbon isotope ratios (δ13C). Given the widespread distribution and highly isochronous nature of these signals, they have been widely employed for chronostratigraphy in sequences where biostratigraphic data are limited. Globally recognized patterns have recently been used to create generalized δ13C curves that have been integrated with the geologic timescale, which permits the calibration of global events and correlations with unprecedented resolution. However, much of this prior work is limited to inorganic δ13Ccarb data derived from carbonates, largely limiting these studies to calcareous sequences. This study presents new δ13Corg data from biostratigraphically well-constrained intervals through the Hirnantian to Ludfordian at localities in Poland. The resulting δ13Corg curves show distinct zones of heavy values that appear to be coincident with the positive excursions of the Hirnantian (HICE) and lower Sheinwoodian (“Ireviken”). Although less conclusive, Homerian (“Mulde”) and lower Ludfordian excursions may also be recognized, along with several intermediate features of the global composite curve, such as a sharp negative shift in values above the HICE, steadily rising-upward values through the upper Telychian, and gently declining-upward values that follow the “Ireviken” excursion.