Abstract We report stable isotope ratios (δ13C, δ18O), minor and trace elements (Mn, Fe, Sr, Mg) together with Ca concentrations from bivalve shells and belemnites from the Middle-Upper Jurassic Sundance Seaway (western United States), we compare them with coeval open-ocean Tethyan data, and reconstruct the palaeo-circulation of seaway waters. The Sundance Seaway was a 2000 km long epicontinental sea with a single entrance at mid latitudes (55–60°N), which would have fostered substantial evolution of seawater chemistry relative to its open-ocean source. Samples are distributed across the 13-million-year marine history of the seaway, and across a 540 km east-west transect spanning Wyoming. Delta13C values are in the same range as Tethyan data, and this suggests that they might record global changes in the carbon cycle, with one exception in the Oxfordian. Delta18O values from the seaway are in contrast highly depleted compared with Tethyan data (−2 to −6‰), and they indicate unrealistically high palaeotemperatures (20–40 °C), assuming an isotopic composition of seawater of −1‰, as generally used for the Jurassic. Given more realistic temperature estimates from Mg/Ca ratios of bivalve shells (10–25 °C), we explain such negative δ18O values by the southward inflow of normal-salinity, isotopically depleted (−3, −4‰), Arctic water into the seaway. Such water would become progressively more saline and denser as it flowed towards the southernmost portion of the seaway. In the Late Jurassic, characterised by wetter climate conditions, less dense Sundance waters may have instead exhibited a northward flow, reducing the southward surface flow from the Arctic. The observed partial geochemical decoupling of Sundance Seaway water masses from the open ocean strongly recommends caution in interpreting the geochemical record of ancient shallow seas, where local, regional and global drivers of change all need to be considered.

中文翻译：

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侏罗纪陆上海和公海之间部分地球化学解耦的同位素证据

摘要 我们报告了稳定同位素比率（δ13C、δ18O）、微量元素和微量元素（Mn、Fe、Sr、Mg）以及来自中上侏罗统圣丹斯海道（美国西部）的双壳类贝壳和箭石的 Ca 浓度，我们比较了用同时代的公海特提斯数据进行分析，并重建航道水域的古环流。圣丹斯海道是一个 2000 公里长的大陆表海，在中纬度（55-60°N）有一个入口，这将促进海水化学相对于其开放海洋源的实质性演变。样本分布在 1300 万年的航道海洋历史中，以及横跨怀俄明州的 540 公里东西向横断面。Delta13C 值与 Tethyan 数据在同一范围内，这表明它们可能记录了碳循环的全球变化，除了牛津人的一个例外。相比之下，来自海道的 Delta18O 值与特提斯数据（-2 到 -6‰）相比高度枯竭，并且它们表明古温度高得不切实际（20-40 °C），假设海水的同位素组成为 -1‰，通常情况下用于侏罗纪。根据双壳贝壳 (10–25 °C) 的 Mg/Ca 比率进行更现实的温度估计，我们通过正常盐度向南流入、同位素耗尽 (-3, -4‰)、北极水流入来解释这种负的 δ18O 值。海道。当这些水流向海道的最南端时，它会变得越来越咸，越来越稠密。在以湿润气候条件为特征的晚侏罗纪，密度较低的圣丹斯水域可能反而表现出向北流动，从而减少了从北极向南的表面流动。