Coupling of surface temperatures and atmospheric CO_2 concentrations during the Palaeozoic era

摘要

Atmospheric carbon dioxide concentrations seem to have been several times modern levels during much of the Palaeozoic era (543-248 million years ago), but decreased during the Carboniferous period to concentrations similar to that of today. Given that carbon dioxide is a greenhouse gas, it has been proposed that surface temperatures were significantly higher during the earlier portions of the Palaeozoic era. A reconstruction of tropical sea surface temperatures based on the δ~(18)O of carbonate fossils indicates, however, that the magnitude of temperature variability throughout this period was small, suggesting that global climate may be independent of variations in atmospheric carbon dioxide concentration. Here we present estimates of sea surface temperatures that were obtained from fossil brachiopod and mollusc shells using the 'carbonate clumped isotope' method—an approach that, unlike the δ~(18)O method, does not require independent estimates of the isotopic composition of the Palaeozoic ocean. Our results indicate that tropical sea surface temperatures were significantly higher than today during the Early Silurian period (443-423 Myr ago), when carbon dioxide concentrations are thought to have been relatively high, and were broadly similar to today during the Late Carboniferous period (314-300 Myr ago), when carbon dioxide concentrations are thought to have been similar to the present-day value. Our results are consistent with the proposal that increased atmospheric carbon dioxide concentrations drive or amplify increased global temperatures.%地球大气中高浓度的二氧化碳在遥远的地质年代是否始终与全球变暖 联系在一起?关于这个问题，地质学家有不同看法。对全球变暖持怀 疑态度的人们总是能抓住这一点。但一项新的研究工作为传统观点提 供了进一步支持：大气二氧化碳浓度和地球表面温度在地球的整个历 史上一直是紧密联系在一起的。应用“carbonate clumped isotope" “古温度计”(palaeothermometer)对古生代海洋温度所做的分析表 明，当大气二氧化碳浓度相对于目前水平较高时，地球温度明显较 高；而当大气二氧化碳浓度低到跟今天差不多时，地球温度也跟今天 的相似。