Lower–Middle Ordovician carbon and oxygen isotope chemostratigraphy at Hällekis, Sweden: implications for regional to global correlation and palaeoenvironmental development

Abstract

A high‐resolution chemostratigraphical (coupled δ13Ccarb and δ18Ocarb) study of the topmost Floian through the middle Darriwilian (Ordovician) succession at the Hällekis quarry, Kinnekulle, southern Sweden, shows relatively steady isotopic values with overall minor changes, although some notable short‐ and long‐term shifts are discernible. A pronounced positive shift in δ13C in the uppermost part of the study succession is identified as the Middle Darriwilian Isotopic Carbon Excursion (MDICE), representing the only named global isotopic excursion in the data set. Regional and global comparisons suggest that few details in the different carbon and oxygen isotope curves can be confidently correlated, but longer‐term patterns appear quite consistent. Trends in the isotope data are in agreement with palaeogeographical reconstructions. Differences in stratigraphical patterns of both carbon and oxygen isotopes between localities suggest strong secular development at several spatiotemporal scales; any global signal involving relatively minor isotopic shifts is often masked/subdued by local and regional overprinting and care should be taken not to overinterpret data sets. Collectively, the data suggest rising sea levels and cooling climates through the studied time interval, but detailed interpretations remain problematic.

A Baltic Perspective on the Early to Early Late Ordovician δ13C and δ18O Records and Its Paleoenvironmental Significance

“…Importantly, this trend visible in the brachiopod record at the base of the MDICE interval cannot be confirmed as the East Baltic data set it is recorded in does not extend further upwards stratigraphically (e.g., C. M. Ø. Rasmussen et al., 2016) and is not present in the only other coeval brachiopod record from Baltica (i.e., the full Öland record, Figure 10). Nevertheless, while the reason for this disparity is not immediately clear, what is apparent is that a positive excursion is recorded in both bulk rocks and brachiopods during the middle Darriwilian which corresponds to the MDICE as reported by other workers (e.g., Lindskog et al., 2019; C. M. Ø. Rasmussen et al., 2016; Wu et al., 2017).…”

“…In addition to paleontological evidence, several sedimentological, sequence‐ and cyclostratigraphical studies have argued for global lower Darriwilian cooling that likely transgressed into the Sandbian. In Baltica, detailed lithofacies and microfacies analyses on lowermost Darriwilian carbonates corroborate this view (Lindström, 1984; Lindskog et al., 2019) and so does sequence stratigraphical evidence from Armorica (Dabard et al., 2015). In Siberia, a sudden shift from continuous deposition of warm‐water carbonates to the lower−middle Darriwilian Baykit Sandstone is seen in the Tunguska Basin.…”

“… Comparison of regional and global whole rock carbonate and brachiopod δ 13 C trends. Whole rock δ 13 C data (δ 13 C bulk ) all originate from Baltoscandia (left figure): Öland (Wu et al., 2017); Kinnekulle (Lindskog et al., 2019); Estonia (Kaljo et al., 2007). Brachiopod δ 13 C data (right figure) elucidate Baltoscandia δ 13 C trends using best‐preserved brachiopods (Mn/Ca ≤ 1 mmol/mol) from this study in conjunction with data from C. M. Ø. Rasmussen et al.…”

“…The 0.5‰ offset between δ 13 C brachiopod and δ 13 C bulk in the current study seems to be a consistent pattern throughout the Baltoscandian Paleobasin. Specifically, Floian to Sandbian δ 13 C bulk values from different parts of Baltoscandia (Figure 11) show a range between −1‰ and +2‰ (Kaljo et al., 2007; Lindskog et al., 2019; Wu et al., 2017). For fossil brachiopods, values ranging between −1‰ and +1‰ have been reported for the eastern Baltoscandian Paleobasin (e.g., Bergmann et al., 2018; C. M. Ø. Rasmussen et al., 2016) and these are comparable with our observations (Figure 6).…”

Dating of sedimentary rock intervals using visual comparison of carbon isotope records: a comment on the recent paper by Bergström et al . concerning the age of the Winneshiek Shale

“…; Lindskog et al . in press). The potential for confident age interpretations based on the data from the Winneshiek Shale (Bergström et al .…”