Biochemostratigraphy of the Devonian−Carboniferous boundary global stratotype section and point, Griotte Formation, La Serre, Montagne Noire, France

Abstract

The Griotte Formation and the base of Bed 89 exposed at La Serre, Montagne Noire, France represent the Global Stratotype Section and Point (GSSP) for the Devonian–Carboniferous (D–C) boundary. This study represents the geochemical work on brachiopods and matrix covering the Siphonodella praesulcata to Siphonodella sulcata Zones at the GSSP. Values of 87Sr/ 86Sr of unaltered brachiopods provide the most powerful biochemostratigraphic tool in recognizing and correlating the Devonian–Carboniferous boundary on a global scale. The frequency and variations of the latest Devonian 87Sr/ 86Sr trend (0.708167±0.000042) are invariant but the population means are similar ( p=1.000) to the essentially pronounced trend for the post-boundary Carboniferous (0.708165±0.000054) brachiopods from the GSSP and supplementary sections. The Devonian–Carboniferous boundary 87Sr/ 86Sr value based on unaltered brachiopod samples from within 0.2 m of either side of the boundary is 0.708231±0.000021 (NBS 987=0.710240). Oxygen isotope values of unaltered brachiopods from the Devonian side of the boundary are −1.99±2.29‰ (PDB) and those from the Carboniferous side are −3.08±0.51‰ (PDB, p=0.229), which are from within to outside the range recorded in Recent low-latitude counterparts. Carbon isotope values for the Devonian brachiopods with +4.37±1.69‰ (PDB) are dissimilar to those from the Carboniferous side of the boundary with +1.93±0.49‰ (PDB; p=0.0001), but both mostly overlap with the range observed in Recent low-latitude brachiopods. The large positive excursions in δ 13C (Δ5.5‰) and δ 18O (Δ6.9‰) of the unaltered brachiopods reflect changing oceanographic conditions of the La Serre Sea and possibly of the global oceans during the uppermost Middle and Upper S. praesulcata Subzones. Oxygen isotope trend may reflect, in part, a climatic response of a rapid, short and distinct glacial event during the upper Middle S. praesulcata Subzone. This cryogenic event of a larger oceanographic/climatic shift correlates, in part, to the Hangenberg Event sensu lato. Excursions in the seawater– 87Sr bracket the onset and offset of the glacial event, reflecting changes in continental weathering patterns and processes and with it changes in riverine fluxes. In contrast, the carbon isotope shift although related to the cooling trend is probably associated with a drawdown of atmospheric CO 2 and/or burial of organic matter. The biochemostratigraphic values/contents and trends presented in this study, in conjunction with biostratigraphic observations, facilitate the correlation of sequences at Hasselbachtal and Wocklum (Germany), Nanbiancun (China) and Starks Road and Hannibal (Missouri, USA).