Geochemistry of the Gohan Formation in central eastern Korea : implication for paleoenvironmental change in the Late Permian

Abstract

Abstract

The Late Permian is a time of significant environmental change, thus it is one of the most important period in the Earth history. In Korea, the Gohan Formation in the Pyeongan Supergroup was deposited in the Late Permian. It distributed in central eastern Korea and was deposited in non-marine setting. Plant fossil, detrital zircon U-Pb age and paleomagnetic studies suggest that depositional age of the Gohan Formation is about 260 Ma. The Gohan Formation is 450m thick and comprises the alternation of medium-grained sandstone and gray to greenish-gray shale. A detailed δ13Corg profile along with some geochemical proxies are presented for the Gohan Formation at Danyang site to investigate the paleoenvironmental change in the Late Permian. Excursion of δ13Corg represents disturbance of carbon cycle. Organic carbon isotope values show a 4‰ negative excursion in the lower part of the studied section. This negative excursion can be interpreted as carbon cycle disturbance from the Capitanian extinction event. Mercury concentration and Hg/TOC in sediments is a proxy for volcanic activity. The horizon of a mercury peak near the bottom of the section is consistent with that of negative carbon isotope excursion, suggestive of the volcanic event and carbon cycle disturbance are closely related to each other and this mercury peak probably indicate the Emeishan volcanism. One more mercury peak noted in the middle part of the section is not related to carbon cycle disturbance, thus this peak may represent local volcanism. Weathering proxies decrease from lower to the upper part of the section. As provenance is constant throughout the measured section, decrease of weathering proxies suggest cooling and drying climate condition in the Gohan Formation. Clay mineral composition shift from kaolinite dominate in the lower part of the section to illite and chlorite dominate in the upper part of the section suggest climate change toward cool and arid climate as well. Paleoclimate study of North China have suggested warm and arid climate condition in the Late Permian. Therefore, the whole of the Sino-Korea block was drying in the Late Permian. The main cause of the arid climate might be topographic barrier generated by subduction on the east side of the Sino-Korean block. C/S ratio indicates the Gohan Formation might deposit in the non-marine setting. C/N ratio of the organic matter reveals the presence of both terrestrial plants and algae. Redox proxies show the Gohan Formation was deposited in the oxic condition. TOC content shows the change of productivity in the Late Permian. Together with C/N ratio, it seems that local terrestrial plant is the main factor of the productivity increase. δ13CBC show significantly low value, indicating influence of canopy effect in the far eastern part of the Sino-Korean block.