New U-Pb zircon age and carbon isotope records from the Lower Silurian Longmaxi Formation on the Yangtze Platform, South China: Implications for stratigraphic correlation and environmental change

Abstract

The Ordovician–Silurian (O–S) transition was a period of dramatic climatic, environmental, and biological change. Numerous studies have documented the Late Ordovician glacial or mass extinction interval, but the Early Silurian has received significantly less attention. This study presents a U-Pb zircon age data and organic carbon isotope (δ13Corg) records from the Upper Ordovician to Lower Silurian succession of South China to better constrain carbon isotope perturbations and to evaluate their possible causes and relationships to paleoenvironmental change during Early Silurian time. A volcanic ash bed located at the top of the organic-rich Lower Silurian Longmaxi Formation yields a 206Pb/238U zircon age of 440 ± 3 Ma (MSWD = 1.4), which is consistent with the Rhuddanian–Aeronian (R–A) boundary age. The newly acquired age is similar to that documented an ash bed in Scotland (439.57 ± 1.13 Ma) perhaps indicating synchronous volcanism. The δ13Corg values show a negative excursion in the organic-rich lower Longmaxi Formation that is interrupted by a positive excursion in the organic-lean upper part of the unit. The positive excursion begins immediately above the dated ash bed thus assigning it to the R–A boundary. The documented δ13Corg trend is comparable to those of other sections throughout the world, and is suggestive of a global perturbation of carbon cycle. The negative δ13Corg excursion of the Rhuddanian stage was coeval with deposition of organic-rich shale under anoxic/euxinic conditions, whereas the subsequent positive δ13Corg excursion coincided with a change to suboxic conditions that could have diminished organic matter preservation. We propose that the negative δ13Corg excursion could be considered as the defining isotopic characteristic of the Rhuddanian oceanic anoxic event (Rhuddanian OAE). Results of the present study in tandem with published age data from the Guanyinqiao Bed immediately below the base of the Longmaxi Formation suggest that the negative δ13Corg excursion and associated Rhuddanian OAE persisted for approximately 3 Myr. This relatively long-lived event reflects a sustained addition of isotopically light carbon into the atmosphere-ocean system, possibly related to the release of volcanic CO2. The enhanced burial of organic carbon in association with the Rhuddanian OAE could have removed the substantial 12C from the carbon reservoir and lowered atmospheric pCO2 thus resulting in a subsequent positive δ13C excursion and global cooling at the R-A boundary.