A new high-resolution palaeotemperature record during the Middle–Late Ordovician transition derived from conodont $δ$¹⁸O palaeothermometry

Climate cooling has been commonly considered as one of the most plausible triggers of the Great Ordovician Biodiversification Event (GOBE). However, the details of coeval sea-surface temperature (SST) variation during and after the biodiversification peak are still unclear owing to lack of continuous, high-resolution palaeotemperature records. Here, we report the first high-resolution in situ secondary ion mass spectrometry conodont apatite oxygen isotope (⁠$δ$¹⁸O_apatite) curve from the Kalpin Shuinichang and Dawangou sections (the auxiliary Global Stratotype Section and Point (GSSP) for the base of the Upper Ordovician Series) to reconstruct the detailed Ordovician palaeotemperature history in the Tarim Basin, northwestern China. The new oxygen isotope data, based on mono-generic specimens, reveal a similar first-order variation trend to published records from other palaeocontinents, which is thus suggestive of a global signal. A −0.7‰ $δ$¹⁸O_apatite shift (c. 3.2°C warming) is identifiable in the Dawangou Formation (early–middle Darriwilian). This is followed by a long-term +2.2‰ positive shift in $δ$¹⁸O_apatite (c. 10°C cooling) in the overlying strata spanning the Saergan and the Kanling formations (late Darriwilian–Sandbian), probably indicating a global climatic signal. Examining the coeval marine ⁸⁷Sr/⁸⁶Sr records, the early–middle Darriwilian minor $δ$¹⁸O_apatite decrease is coincident with the onset of a long-term negative ⁸⁷Sr/⁸⁶Sr excursion, possibly indicating a slight climate warming in response to increasing volcanic degassing in the context of accelerating spreading of mid-oceanic ridges. In contrast, the following long-term positive $δ$¹⁸O_apatite excursion, in parallel with persistent ⁸⁷Sr/⁸⁶Sr drop, probably indicates a major climate cooling initially caused by the rapid tropical continental weathering, which in turn may have reduced the continental radiogenic Sr influx.

Supplementary material: Three supplementary tables (S1, S2, and S3) indicating the in-situ oxygen isotopic results of conodont apatite from the Kalpin Shuinichang-Dawangou composite section, Qinghu standard, and Durango standard, respectively are available at https://doi.org/10.6084/m9.figshare.c.5924175