Barremian–Cenomanian palaeotemperatures for Australian seas based on new oxygen-isotope data from belemnite rostra
Oxygen‐isotope ratio (δ18O) data are presented for 55 belemnite rostra specimens of late Aptian–Albian age from the Great Artesian Basin of eastern Australia and of late Barremian–early Cenomanian age from the Carnarvon Basin of Western Australia. The belemnites were carefully screened for diagenesis by means of catholuminescence and trace element geochemistry. The δ18O record for the Carnarvon Basin (palaeolatitude ~ 40°S) is interpreted to show a long‐term warming trend from ~ 10 °C in the Aptian to a maximum of ~ 15 °C in the Cenomanian. Shorter-term fluctuations in the mid-Albian are also apparent. Data from the Great Artesian Basin (palaeolatitude 40°–55°S) show a wider δ18O range which is attributed to the epeiric sea context of this sample set. The larger range in δ18O is interpreted to reflect differing water mass characteristics, due possibly to a combination of variable freshwater dilution, seasonal temperature fluctuations and isotopic exchange between seawater and pyroclastic sediment within the epeiric sea. Nevertheless, the Aptian–Albian δ18O trends are still comparable to those obtained for the Carnarvon Basin. The inferred palaeotemperature trends for the Australian sedimentary basins are consistent with records from other regions and demonstrate that broadly synchronous palaeotemperature variations can be recognised on a global scale during the Cretaceous. Although our data do indeed show cool temperatures within the Aptian (and the Albian) reaching a low of ~ 10 °C, these estimates are somewhat warmer than those based on glendonite genesis. However, the exact position of the glendonite-bearing and dropstone-rich parts of the Bulldog Shale is poorly constrained with respect to our isotope data, and the warmest temperatures were of late Albian and early Cenomanian age. However, to conform to greenhouse warmth and equability, and to achieve parity with other palaeotemperature records, the δ18Oseawater estimate of − 1.0‰ that is generally used for Cretaceous oceans in an ice-free world needs to be adjusted to a slightly higher value.