GLACIO-EUSTASY AND δ13C ACROSS THE MID-CARBONIFEROUS BOUNDARY RECORDED IN CARBONATE SLOPE SUCCESSIONS IN THE EASTERN PALEO-TETHYS OCEAN (SOUTH CHINA)

A positive carbonate δ¹³C excursion (by ~1.5–3.0‰) has been reported across the mid-Carboniferous boundary (i.e., Mississippian–Pennsylvanian boundary, MPB) from Euramerican epicontinental seas, coincident with climate cooling and Gondwanan glaciation at that time. Most of these studied MPB successions were, however, deposited in shallow-water carbonate platforms and experienced repeated subaerial exposure of variable duration. Therefore, these strata contain significant hiatus and their carbonate δ¹³C values might have been altered during meteoric diagenesis. During the mid-Carboniferous, the South China Block was located in the eastern Paleo-Tethys Ocean near the paleo-equator, which contains well-preserved carbonate slope deposits across the MPB. In this study, we investigated three carbonate slope sections (Naqing, Narao, and Dianzishang) in order to better understand the glacio-eustasy and δ¹³C changes across the MPB.

Four lithofacies are defined in the studied sections, including thin-bedded lime mudstones, laminated wacke-to packstones, normally graded packstones, and slumps, collectively suggesting a carbonate slope setting. High-resolution conodont biostratigraphy shows a complete evolutionary lineage of conodonts across the MPB, which, together with lithofacies analysis, suggests near-continuous deposition. Immediately above the MPB, the occurrence of normally graded packstones (~75 cm thick) in the Naqing section, and slump deposits in the Narao (~2 m thick) and Dianzishang (~5 m thick) sections, indicates a significant sea-level fall. Carbonate δ¹³C records show a consistent value of ~3‰ below the MPB in the Naqing and Narao sections, and both sections have an obviously positive shift in δ¹³C across the MPB (by~0.5–1.0‰), which is comparable to the coeval positive δ¹³C excursions recorded in Euramerica. The relatively small-amplitude shift in δ¹³C would have resulted from intensified upwelling in the South China Block where the studied sections were located during the MPB glacial peak. Well-preserved slope carbonates as well as well-mixed seawater suggest that the carbonate δ¹³C values recorded in the South China Block may represent a mean value of global dissolved inorganic carbon in the ocean during the mid-Carboniferous.