Ordovician integrative stratigraphy of the HK-1 Borehole in the South Yellow Sea and its geochronological and paleoclimatic implications

The South Yellow Sea Basin, situated at the northeastern margin of the Lower Yangtze Craton, represents a significant prospective area for hydrocarbon exploration. However, insufficient drilling data has impeded the comprehensive study of Ordovician strata, which are crucial for evaluating the characteristics of the petroleum system. The HK-1 Borehole marks the first encounter with Ordovician strata in this region, offering essential stratigraphic records that enhance the understanding of early Paleozoic tectonic, climatic, and environmental evolutions. In this study, carbon isotope stratigraphy and cyclostratigraphy from Ordovician strata at the depths of 4083 m to 4492 m in the HK-1 were studied to establish relationships for stratigraphic correlation on a global scale. Two carbon isotopic shifts through middle Darriwilian to Katian have been recognized, including Middle Darriwilian Isotopic Carbon Excursion (MDICE) and Pagoda Isotopic Carbon Excursion (PICE). The identification and documentation of these anomalies further reinforce both regional and global correlations within the Ordovician succession on the Lower Yangtze Craton. Astronomical calibration of ∼11 m Th (Thorium) cycles to 405-kyr orbital eccentricity cycle period aligns other significant, shorter sedimentary cycles to periods recognizable as short orbital eccentricity (136 kyr and 100 kyr), obliquity (32.4 kyr), and precession (18.4 kyr). Astronomical calibration produces an ∼12-Myr floating time scale using twenty-nine 405-kyr eccentricity cycles, which indicates a presumed age of ∼462.6 Ma for the lowermost sediments of HK-1 Borehole. Our proxy-based study also shows that the PICE was linked to marine anoxia during a global sea-level rise. In contrast, the increased influx of lighter organic carbon during sea-level regression may have disturbed the MDICE.