Abstract In this study, we investigated Early Cretaceous (Valanginian, ca. 135 million years ago) climate from subtropical to boreal palaeolatitudes. Combined carbonate clumped isotope and oxygen isotope data derived from sub-arctic, boreal, and sub-tropical fossil belemnite rostra (Mollusca: Cephalopoda) provide new palaeotemperature estimates as well as a constraint on the oxygen isotope composition of seawater. Our belemnite data reveal balmy high-latitude marine temperatures (ca. 22 °C) and warm sub-tropical temperatures (ca. 31 °C). Supplementing our clumped isotope-based temperature estimates with published TEX86 data results in a conservative reconstruction of a latitudinal temperature gradient that is reduced compared to modern conditions. We find that modelling efforts are close to reproducing tropical temperatures when high pCO2 levels are considered. Warm polar temperatures imply, however, that data-model discrepancies remain. Early Cretaceous seawater oxygen isotope values show a modern profile and are much more positive (up to 1.5‰ SMOW) than typically assumed. Based on our findings, if the positive Cretaceous seawater δ18O values are not considered, carbonate δ18O thermometry would underestimate temperatures, most acute at middle and tropical latitudes.

中文翻译：

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早白垩世海洋纬度海水温度梯度和氧同位素组成的碳酸盐团块同位素证据

摘要 在这项研究中，我们调查了从亚热带到北方古纬度的早白垩世（瓦兰吉阶，约 1.35 亿年前）气候。来自亚北极、北方和亚热带化石箭石（软体动物：Cephalopoda）的碳酸盐团块同位素和氧同位素数据组合提供了新的古温度估计以及对海水氧同位素组成的限制。我们的箭石数据显示温和的高纬度海洋温度（约 22 °C）和温暖的亚热带温度（约 31 °C）。用已发布的 TEX86 数据补充我们基于成簇同位素的温度估计，可以对纬度温度梯度进行保守的重建，与现代条件相比，该梯度降低了。我们发现，当考虑高 pCO2 水平时，建模工作接近于再现热带温度。然而，温暖的极地温度意味着数据模型的差异仍然存在。早白垩世海水氧同位素值显示出现代特征，并且比通常假设的更正（高达 1.5‰ SMOW）。根据我们的研究结果，如果不考虑白垩纪海水 δ18O 的正值，碳酸盐 δ18O 温度测量将低估温度，在中纬度和热带纬度最严重。