The late Mesozoic, especially the Cretaceous, is the latest example of the Earth's greenhouse intervals, which was terminated by the initial glaciation in the Cenozoic at the Eocene/Oligocene boundary. In the late Mesozoic and early Paleogene, prior to the initial glaciation, both poles were free from continental ice sheets. It has been widely accepted that the high atmospheric carbon dioxide level was responsible for this warming. Sea surface temperatures (SSTs) derived from oxygen isotopic compositions of mixed layer dwelling planktic foraminifers and pelagic vertebrate remains, and TEX86 paleothermometry indicate that the tropical temperature at the middle Cretaceous exceeds 35°C. The mid-Cretaceous equatorial SST is warmer than the modern highest temperature in open ocean by at least 6°C. In addition, although data are scarce in the early Cretaceous, the early Cretaceous (-130 Ma) SST may be much warmer than we previously expected. The other notable character in the Cretaceous SST is distinctively reduced meridional SST gradient. In the mid-Cretaceous, temperature difference between the equator and 60° in latitude was approximately 15°C, which is significantly smaller than the modern value of ~25°C. These reduced meridional temperature gradients are also observed in the early and late Cretaceous as well, indicating that the reduced meridional gradient is the nature of the greenhouse climate. According to recent advancement in coupled climate models, proxy SST records are well explained by those numerical examinations. Preparing a global array of proxy paleotemperature data is eagerly required for the better understanding of the greenhouse climate system.
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