Paleoclimate evolution and origin of organic carbon isotope variations during the Ordovician–Silurian transition in the Upper Yangtze area

The Ordovician–Silurian transition is an important period in Earth’s history, marked by drastic changes in paleoclimate and the well-known Hirnantian isotope carbon excursion (HICE) in the Late Ordovician. However, the causes of paleoclimate changes and carbon isotope excursions, as well as their correlations, are still unclear. Based on the analysis of the total organic carbon (TOC) contents, organic carbon isotopes (δ¹³C_org), as well as major and trace elements, the chemical index of alteration (CIA) values of the Wufeng-Longmaxi Formation black shales of Well JY4 in Jiaoshiba area in the upper Yangtze region were calculated. Combined with biostratigraphy, it is found that the paleoclimate conditions of WF2–WF4 members gradually changed from warm and humid to cold and arid. The paleoclimate of LM1–LM4 members remained cold and arid, with an increasing trend upward in chemical weathering. The LM5 Member marked a gradual transition back to warm and humid climate environment, while the paleocimate conditions of LM6–LM7 and their above members showed an episodic fluctuation to cold and arid. The elemental geochemical proxies indicate that the sedimentary environment of the WF2–WF4 members shifted from oxic to anoxic conditions, with a rapid increase in oxygen content in Guanyinqiao Formation. The shales of LM1–LM3 members were deposited in an extremely anoxic and sulfidic environment, with oxygen content gradually increasing upward in the LM4 Member, transitioning from anoxic to suboxic, hypoxic, and finally oxidized conditions. The "positive drift" of δ¹³C_org in Guanyinqiao Formation is likely related to the burial and oxidation of organic carbon, with enhanced nutrient input from chemical weathering also playing a role. The subsequent "negative drift" in the black shale of Longmaxi Formation may be associated with the return of ¹²C to the marine carbon pool due to large-scale transgression.