Coupled biostratigraphy and chemostratigraphy at Lanna, Sweden: A key section for the Floian–lower Darriwilian interval (Lower–Middle Ordovician)

The Lanna area in the province of Närke, south-central Sweden, hosts a succession of cool-water carbonate strata that are largely devoid of tectonic and diagenetic alteration, and span the Floian through lower Darriwilian Global stages (Ottenbyan–Kundan Regional stages). In this study we assess the integrated biostratigraphic and chemostratigraphic framework for the Ordovician sequence at Lanna, utilizing trilobites and conodonts coupled with bulk-rock carbon and oxygen isotopes (δ13Ccarb and δ18Ocarb). The local succession is thin but relatively complete; biostratigraphic details are similar to coeval successions in Baltoscandia. The new high-resolution δ13C and δ18O data are broadly consistent with other chemostratigraphic records at the regional scale, but notable differences in δ13C are obvious compared to global records. δ13C trends diverge markedly in the Dapingian and intra- and inter-regional differences become increasingly pronounced through younger strata. This may largely be explained by an overall low eustatic sea level, which led to more widespread oceanic restriction of epeiric seas. As a result, δ13C records became more influenced by local variables, muddling the global δ13C signal. This, in turn, suggests that sea level played a larger role than long-term changes within the global carbon cycle for regulating local–regional δ13C gradients in the Middle Ordovician. Regardless of the underlying cause(s), the documented variability poses significant challenges for using δ13C as a tool for detailed stratigraphic correlations. The limited expression of many named ‘carbon isotope excursions’ (CIEs) in the studied interval indicate that they are artefacts of local overprinting rather than reflective of changes to the global carbon cycle. Thus, these CIEs are of questionable correlative importance and validity. Although the scatter in numeric values is relatively large, long-term δ18O trends are overall consistent at the global scale, tentatively supporting the notion that climate cooled during the studied time interval.