The Cambrian–Ordovician transition in dysoxic facies in Baltica — diverse faunas and carbon isotope anomalies

The Cambrian–Ordovician boundary interval in Scandinavia is characterized by largely endemic trilobite species and fossil-meager intervals within the Alum Shale Formation. Previous investigations of this interval in Scandinavia, based on drill cores, are rather sketchy. In order to characterize the faunal signature in a largely dysoxic setting during this time interval, as well as providing biostratigraphic and chemostratigraphic data valuable for intercontinental correlation, a small strip in the outskirts of the village Södra Sandby in Scania, southern Sweden, was excavated. Nearly 5 m of the Cambrian–Ordovician boundary strata, largely represented by alum shale, were exposed and the profile was meticulously investigated for fossil content and lithological characteristics and sampled for δ13Corg analyses. The uppermost Cambrian in Sweden has previously been described as barren of fossils; however, the present study reveals a rather diverse fauna, including lingulid brachiopods, trilobites, protoconodonts, paraconodonts and euconodonts in the uppermost 1.6 m of the Furongian. The first appearance datum of planktic graptolites is represented by a single shale surface covered by specimens of Rhabdinopora flabelliforme parabola at 1.74 m above the base of the section and roughly corresponds to the Cambrian–Ordovician boundary. The conodont fauna includes several cordylodids important for intercontinental correlation. The Södra Sandby section δ13Corg data were coupled with isotope data from two Scanian drill cores, Håslöv-1 and Tosterup-2, in order to compile a composite isotope curve spanning the uppermost Ctenopyge linnarssoni Trilobite Zone in the Furongian to the upper R. flabelliforme parabola Graptolite Zone in the Lower Ordovician (Lower Tremadocian). Two isotope shifts from baseline values, observed at the base of the Peltura paradoxa Trilobite Zone and the lower part of the Peltura transiens Trilobite Zone, respectively, can be correlated with contemporaneous shifts in other parts of the world. The former, a negative shift of approximately 0.4–0.7‰, corresponds to the widely documented Top of Cambrian carbon isotope Excursion (TOCE) and the latter, a positive shift of approximately 1‰, corresponds to an as-yet-unnamed excursion at the base of the Cordylodus proavus Conodont Zone. In terms of faunal content and isotopic signals, the present study represents the first detailed description of the Cambrian–Ordovician boundary interval in Baltica. The relatively diverse fauna recorded suggests that the dysoxic environment was not a serious inhibitor for marine life. Globally, no isotope values significantly different from background values have been reported at the Cambrian–Ordovician boundary. This is confirmed by the present study; however, as the base of the Ordovician GSSP lacks meaningful isotope data, global correlation of this important boundary is problematic.