Controls on carbon isotopes during the upwelling system development in shallow marine environments: Regional evolution in the Roadian (Mid-Permian) of South China

Abstract

Throughout geological time, upwelling systems have been widespread in shallow marine environments. However, the regional carbon cycle in shallow marine environments during the development of upwelling systems remains unclear. During the middle Permian, astronomically forced upwelling systems were widespread along the west coast of Pangaea and the paleoequator. This study focuses on the astronomical-forced middle Permian (Roadian) rhythmites from the South China continental shelf and carbonate platform to explore the evolution of carbon isotope control factors in shallow marine environments during upwelling system development. Based on geochemical characteristics, time series analyses, and wavelet coherence analyses, the Roadian stage was subdivided into three phases in upwelling system evolution. As the system developed, the continental shelf became oxygen-depleted, warm–humid, and highly weathered, whereas the carbonate platform remained relatively stable. First, the initial development stage. The δ¹³C_org on the continental shelf and the δ¹³C_carb in the carbonate platform averaged −25.67 ± 0.57 ‰ and 3.61 ± 0.25 ‰, respectively. Carbon isotopes in shallow marine environments were primarily controlled by paleoclimate fluctuations (and the upwelling effect). The relatively cool–dry climate weakened continental weathering, upwelling, and organic carbon burial on the continental shelf. However, these conditions also lowered sea levels and restricted water exchange in the carbonate platform, leading to ¹²C depletion. Next, with the intensification of upwelling, its influence on intra-platform carbon isotopes surpassed paleoclimates, with δ¹³C_carb remaining stable at 3.86 ± 0.39 ‰. The influx of upwelling during sea-level rise promoted organic carbon burial and a positive shift in δ¹³C_carb. On the continental shelf, nevertheless, δ¹³C_org values decreased slightly to −25.98 ± 0.61 ‰. The rapid expansion of the oxygen minimum zone reduced sulfate concentrations, facilitating reverse reactions of the anaerobic oxidation of methane and thereby highlighting the control of sulfate reduction reactions on δ¹³C. Simultaneously, TS and Fe_py contents declined, along with siderite and pyrite deposition. Finally, during the complete development stage, the δ¹³C_carb values in the carbonate platform stabilized at 3.77 ± 0.45 ‰, while the δ¹³C_org declined to −26.93 ± 0.39 ‰. The upwelling intensity and redox conditions gained dominance over δ¹³C by promoting organic carbon burial in shallow marine environments.