Carbon isotope stratigraphy of the Lower Ordovician Dumugol Formation, Korea: first identification of a global Early Ordovician carbon isotopic excursion in the Sino-Korean Block

초록

Carbon isotopic excursions, reflect environmental changes and biotic events of varying scales and have been widely used for relative chronological correlations. The numerous larger-scale excursions of the Middle and Late Ordovician, which coincide with major environmental and biological changes, have been well studied. In contrast, the potential global correlations of Early Ordovician delta C-13 excursions remain unclear due to their smaller magnitude and relatively limited study. In this paper, we report C isotope data for the Lower Ordovician Dumugol Formation in the Cheoram section, South Korea and discuss its global correlations and geological significance. In the Cheoram section, a continuous exposure from the uppermost Dongjeom Formation to the middle of the Dumugol Formation was investigated, which mainly represents a mid- to outer ramp succession. The depositional age of this interval is the middle part of the Tremadocian, based on conodont and trilobite biozones. Two positive delta C-13 excursions were recognized. The first is an excursion of similar to 1.9 parts per thousand from the baseline, which occurs in the Chosonodina herfurthi-Rossodus manitouensis conodont Biozone, and the second is an excursion of similar to 1.5 parts per thousand in the upper Glyptoconus quadraplicatus to lower Scalpellodus tersus conodont zones. These excursions can be correlated to the Top Skullrockian positive Isotopic Carbon Excursion (TSICE) and the Late Stairsian positive Isotopic Carbon Excursion (LSICE), which is their first identification in the Sino-Korean Block. Our results confirm that the TSICE, which has suggested a signal of marine anoxic event, can be used for global chemostratigraphic correlations. Further interdisciplinary studies on the drivers of the TSICE are needed to understand how the early phase of the Great Ordovician Biodiversification Event occurred.

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