Carbon Isotope, Nitrogen Isotope, and Clay Mineral Compositions of the Korean Middle Ordovician Successions and Their Paleoceanographic Implications

Abstract

The middle Ordovician successions from the Taebaek and Yeongwol sections, Gangwon Province, Korea, were studied with carbon isotope, nitrogen isotope, and clay-mineral compositions to observe the middle Darriwilian carbon isotope excursion (MDICE) event and its paleoceanographic conditions. The carbon isotope value of the Taebaek section shows a gradual shift in the upper Maggol Formation, overlain by a large negative shift of -6.79‰ until the middle part of the Jigunsan Formation, and then to a heavier trend with broad positive peaks that are continued to the upper Jigunsan Formation and the middle Duwibong Formation as recognized to be MDICE. The Yeongwol sections of the Middle Ordovician Yeongheung Formation show higher carbon isotope values and the lower part of the Yeongwol1 and the whole Yeongwol2 section are correlated to MDICE with three positive peaks. The nitrogen isotope compositions are heavier during the early stage of MDICE event in both the Taebaek and the Yeongwol1 sections. The clay minerals shift to kaolinite-enriched compositions concurrently with a stronger signal in the Taebaek section. The nitrogen isotope and clay mineral compositions are interpreted as a result of epeiric-sea denitrification enhanced by seawater stratification due to heavy precipitation on the nearby land, during the early MDICE interval. The documentation of the MDICE event in the study suggests that global MDICE records have a close temporal background and a common point of carbon isotope peaks. This study also proposes regional paleoceanographic conditions prevailed in middle Ordovician epeiric sea during the MDICE event.