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

白垩纪一直被认为是气候稳定的时期，而新生代则被认为是冰河时期。最近的研究表明，白垩纪无冰的概念可能需要重新审视。寻找白垩纪极端气候的最佳地点是高纬度陆相沉积盆地。这种岩石存在于澳大利亚东南部的Otway和Strzelecki群中，它们在白垩纪晚期沉积在高纬度（南纬70度）。稳定同位素测量为推断地表温度提供了一种工具。先前对从陆地砂岩中回收的碳酸盐结块的研究表明，方解石的O值极度耗尽，这些数据表明，低O的大气流体和年温度在0摄氏度左右的降水。单个固结物和胶结层的同位素分析表明，方解石胶结物O值的长期变化可以在三个不同的尺度上进行关联：在单个固结物内，在复合固结物的相对年龄序列内，或通过比较固结物岩心的同位素组成在垂直剖面上的米尺度上。这种详细的同位素工作结合结构研究表明，在这些岩石中有可能区分近地表原始大气水特征和随后成岩事件产生的复杂特征。用温度变化来解释，最大的O变化相当于年平均地表温度大约5摄氏度的变化。在本研究中，我们建议用化石骨骼中磷酸盐的O/ O数据和古土壤中的D/H和O/ O数据进一步约束碳酸盐氧同位素数据。由于该遗址的高古纬度和陆地特征，任何白垩纪全球气候变化都应该被放大，并且很容易在地层剖面的连续样品中检测到。在有限的地层采样中已经检测到的变化表明，在澳大利亚东南部白垩纪的同位素记录中，有可能识别米兰科维奇气候强迫。

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.