How cold were the Early Permian glacial tropics? Testing sea surface temperature using the oxygen isotope composition of rigorously screened brachiopod shells

Brachiopod carbonate from Early Permian brachiopod shells from low palaeolatitude north Iran and higher palaeolatitude Pakistan Karakorum were screened for diagenesis and analysed for oxygen isotope ratios to derive seawater alaeotemperatures. Screening techniques employed included SEM ultrastructural analysis, cathodoluminescence (CL), image analysis of CL images, trace-element (Sr, Mn, Fe) determinations, and carbon and oxygen stable-isotope determinations. The Karakorum shells were found to be diagenetically altered, but those from north Iran were judged to be pristine. Using data from pristine material, two distinct time slices were analysed: the early and middle Asselian. Two contrasting 18O values for seawater (0‰ and +1.0‰ V-SMOW) were used to account for different extensions of the Gondwanan ice caps. The 18O data from north Iran indicate a range of seawater temperatures from +24.3 to +30.3 8C (for 18Oseawater ¼ 0‰ V-SMOW) or from +30.3 to +35.4 8C (for 18Oseawater ¼ +1.0‰ V-SMOW) for the early Asselian. Results for 18O from the middle Asselian indicate seawater temperatures of +24.4 to +28.0 8C (for 18Oseawater ¼ 0‰) or +29.2 to +32.8 8C (for 18Oseawater ¼ +1.0‰). The maximum calculated temperatures in the middle Asselian are about 2 8C lower than those for the early Asselian. The average temperature for both time slices is similar to modern tropical sea-surface temperatures, indicating that low-latitude Early Permian ocean waters in Iran did not undergo significant cooling during the final Glacial III episode of Gondwanan glaciation. This confirms other evidence based on biotic provinces, which suggests that during the Permo-Carboniferous glaciation, the low-latitude warm belt became narrower and confined to the western Tethys and Cathaysian provinces, and was not subject to a reduction in temperature, but rather a reduction in size.