EARLY CRETACEOUS ISOTOPE VARIATION IN AUSTRALIAN SEAS

Oxygen-isotope data are presented for a suite of belemnite rostra 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. Screened for diagenesis by means of CL and trace element geochemistry, the oxygen isotope record for the Carnarvon Basin (palaeolatitude ~40°S) is interpreted to show a long-term warming trend with maximum temperatures in the Cenomanian. Data from the Great Artesian Basin (palaeolatitude 40°-55°S) show a wider range which is attributed to differing water mass characteristics. Nevertheless, Aptian-Albian 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 the coolest temperatures within the Aptian, our 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 are poorly constrained with respect to our isotope data. To conform with greenhouse warmth and equability, and to achieve parity with other palaeotemperature records (from TEX86 estimates), the generally used estimate for Cretaceous oceans in an ice-free world needs to be adjusted to a higher value.