Sedimentary Characteristics And Geochemical Records Of The Mid-carboniferous Boundary Interval In South China

A positive ?¹³ Ccarb excursion?by?1.5-3.0‰?has been reported across the mid-Carboniferous boundary?Mississippian-Pennsylvanian boundary,MPB?from Euramerican epicontinental seas,implying a coeval pCO₂ decrease and the 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 durations.Therefore,these strata contain significant hiatus and their 813Ccarb values might have been altered during meteoric diagenesis.The South China Block was located in the eastern Paleo-Tethys Ocean near the paleo-equator,which contains well-preserved carbonate slope strata across the MPB.In this study,we investigated four carbonate slope sections in order to understand the glacio-eustatic and ?¹³Ccarb changes across the MPB.Four lithofacies were defined in the studied sections,including thin-bedded lime mudstones,laminated wacke-to packstones,normally graded packstones,and slumped limestones,collectively suggesting a carbonate slope setting.High-resolution conodont biostratigraphic studies show a complete evolutionary lineage of conodonts across the MPB,which,together with facies analysis,suggest a near-continuous deposition.Immediately above the MPB,a bed??75 cm thick?of normally graded packstones occurs in the Naqing section,whereas a bed of slump mass in the Narao??2 m thick?and Dianzishang??5 m thick?sections,probably indicating a significant sea-level fall.All studied sections show an obvious rise in ?¹³Ccarb across the MPB?by?0.3-1‰?,but relatively small-amplitude than that of the Euramerica continent.The relatively small-amplitude rise in 813Ccarb would have resulted from intensified upwelling in the eastern Paleo-Tethys Ocean during glacial peak.Well-preserved slope carbonates as well as well-mixed seawater in the eastern Paleo-Tethys Ocean suggest that the?13Ccarb values recorded in the South China Block may represent a mean ?¹³C of the dissolved inorganic carbon in global ocean water at that time.Correlation between?13Ccarb and previously published conodont ?18O and 87Sr/86Sr from the Naqing section suggests the MPB glaciation was mainly driven by enhanced continental weathering?indicated by rapid rising 87Sr/86Sr?,rather than increased organic carbon burial rate(nearly invariant ?¹³Ccarb).?B value??13Crog-?13Ccarb?of Narao section shows a rise across the MPB??1.0‰?,which is consistent with genrally decrease of pCO₂ reconstructed by other proxies.The ?15N values from the Narao section show a drop across the MPB??2.0‰?perhaps due to the shrinking of oxygen minimum zone?OMZ??enhanced exchange of ocean water at that time?.Futher quantitative simulation is required to better understand the inter-linked processes during the Earth's penultimate icehouse.