THE OCCURRENCE OF THE MIDDLE DARRIWILIAN CARBON ISOTOPE EXCURSION IN KOREAN LOWER PALEOZOIC SUCCESSION AND ITS PALEOCEANOGRAPHICS IMPLICATIONS

In general, Middle Ordovician carbon and sulfur isotope records have shown a large positive and negative excursion, respectively, and the timing of these excursions coincides with the Middle Darriwilian Carbon Isotope Excursion (MDICE) and fast expansion of the Paleo-Tethys Ocean. The Middle Ordovician deposit (the Jigunsan Shale and Duwibong Limestone) in the Taebaeksan Basin, central eastern Korea, includes the MDICE interval biostratigraphically, and is interpreted to have been deposited in an epeiric sea in northern Gondwana. The epeiric sea might have seawater circulation connected with the Paleo-Tethys Ocean. The MDICE occurs in the topmost part of the Jigunsan Shale and ends at the bounday with the overlying Duwibong Limestone. For understanding the paleoceanographic conditions during the Middle Ordovician we analyzed both δ¹³C and δ¹⁵N values of the deposit. During the MDICE, both δ¹³C and δ¹⁵N values show a positive excursion, consistent with other MDICE records in the literature. At the onset of the MDICE, the δ¹⁵N in the studied deposit shows a prominent positive excursion from 2‰ to 9‰, and then remained high with some value fluctuations between 4‰ and 9‰ to the end of the MDICE, is likely resulted from denitrification under anoxic epeiric-sea conditions. After the excursion, the δ¹⁵N value returned to the pre-excursion level with small fluctuations, thus the nitrogen fixation process became enhances and stable again. Changes in total N content, organic C content and C/N ratios representing changes in organic matter production support the the changes in nitrogen fixation and denitrification processes. It has been assumed that during the MDICE global oxygenation, may have triggered the Great Ordovician Biodiversification Event, the positive δ¹⁵N excursion during the MDICE in the studied deposit, on the contrary, suggests that anoxic conditions prevailed in the studied basin. The disparity in paleoenvironmental signatures between the δ¹³C and δ¹⁵N data during the MDICE suggests that δ¹³C shows a global MDICE signature, whereas δ¹⁵N may represent the local conditions.