Stable Isotopic Constraints on The Paleoclimate of a High- Latitude Cretaceous Terrestrial Basin

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The Cretaceous has long been considered a time of equable climate while the cenozoic is regarded as a time of ice ages. Recent work suggests that the concept of an ice free Cretaceous may need to be re-examined. The best place to look for the extremes of Cretaceous climate is within a high latitude terrestrial sedimentary basin. Such rocks exist within the Otway and Strzelecki groups, southeastern Australia, which were deposited at high latitude (70 degrees S) during the lower Cretaceous. Stable isotopic measurements provide a vehicle for inferring the surface temperature. Previous work on the carbonate concretions recovered from terrestrial sandstones revealed extremely O-depleted values for calcite, and these data indicate precipitation from low O meteoric fluids and annual temperatures on the order of 0 degrees C. In addition, isotopic profiling of individual concretions and cemented beds indicates that secular changes in the O values of calcite cements can be correlated on three different scales: within an individual concretion, within a relative age sequence in a composite concretion, or over meter scales in a vertical section by comparing the isotopic compositions of concretion cores. This type of detailed isotopic work in conjunction with textural studies indicate that it is possible to differentiate in these rocks the near surface primary meteoric water signature from complications arising from subsequent diagenetic events. Interpreted in terms of temperature change, the maximum O change would correspond to about a 5 degree C change in mean annual surface temperature. In this study, we propose to further constrain the carbonate oxygen isotope data with O/ O data on phosphate from fossil bones, and with D/H and O/ O data from paleosols. Because of the high paleolatitude of this site and because of its terrestrial character, any Cretaceous global climatic changes should be amplified and easily detected in sequential samples in stratigraphic sections. The changes already detected in limited stratigraphic sampling suggest that it may be possible to recognize Milankovitch climatic forcing in the isotopic record of the Cretaceous, southeastern Australia.