Reconstructing paleoenvironments of the Late Cretaceous Western Interior Seaway, USA, using paired triple oxygen and carbonate clumped isotope measurements

J. Wostbrock, J. Witts, Yang Gao, Catherine Peshek, Corinne E. Myers, Gregory Henkes, Zachary D. Sharp
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Abstract

Fossiliferous carbonate concretions are commonly found in sediments deposited in the Late Cretaceous Western Interior Seaway. Although concretions are diagenetic features, well-preserved fossils from within them have been instrumental in reconstructing the temperature and δ18O value of Western Interior Seaway seawater, which is essential for accurate reconstruction of Late Cretaceous climate. Here, we constrain formation conditions of Late Campanian and early Maastrichtian carbonate concretions by combining triple oxygen isotope measurements with carbonate clumped isotope paleothermometry on different carbonate phases within the concretions. We measured both fossil skeletal aragonite and sparry calcite infill from cracks and within macrofossil voids to evaluate differences between “primary” and “altered” geochemical signals. Based on the two temperature-sensitive isotope systems of the primary fossil shell aragonite, the temperature of the Western Interior Seaway was between 20 °C and 40 °C and was likely thermally stratified during the Campanian. The reconstructed δ18Oseawater values of ∼−1‰ for Campanian Western Interior Seaway waters are similar to those expected for the open ocean during greenhouse climates, while the Maastrichtian Western Interior Seaway may have been more restricted, with a δ18Oseawater value of ∼2‰, which reflects more evaporative conditions. We reconstructed the diagenetic history of the sparry infill and altered fossils using a fluid-rock mixing model. Alteration temperature, alteration fluid δ18O value, and the initial formation temperature were calculated by applying the fluid-rock mixing model to a particle swarm optimization algorithm. We found a different range of initial formation temperatures between the Campanian (25−38 °C) and Maastrichtian (9−28 °C). We also found that alteration in the presence of light meteoric fluids (δ18O ≈ −10‰) is required to explain both the sparry infill and the altered fossil isotopic values. Based on our results, both lithification and alteration of the carbonates occurred soon after burial, and light meteoric fluids support prior findings that high-topographic relief existed on the western margin of the Western Interior Seaway during the Late Cretaceous. As one of the first studies to apply these techniques in concert and across multiple mineralogical phases within samples, our results provide important constraints on paleoenvironmental conditions in an enigmatic ocean system and will improve interpretations of the overall health of ecosystems leading into the end-Cretaceous mass extinction.
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利用成对的三重氧和碳酸盐团块同位素测量重建美国晚白垩世西部内陆海道的古环境
白垩纪晚期西内航道沉积物中通常会发现碳酸盐化石凝块。虽然凝灰岩是成岩特征,但其中保存完好的化石有助于重建西内海道海水的温度和δ18O值,这对于准确重建晚白垩世气候至关重要。在这里,我们通过对凝结体内不同碳酸盐相的三重氧同位素测量和碳酸盐团块同位素古温度测定相结合的方法,对晚钟乳期和早马斯特里赫特期碳酸盐凝结体的形成条件进行了约束。我们测量了来自裂缝和大化石空隙中的化石骨架文石和疏松方解石填充物,以评估 "原生 "和 "改变 "地球化学信号之间的差异。根据原生化石外壳文石的两个温度敏感同位素系统,西内海道的温度介于20 °C和40 °C之间,在康潘世期间很可能处于热分层状态。重建的δ18Oseawater值为∼-1‰,与温室气候下开阔洋的预期值相似,而马斯特里赫特期的西内海道可能受到更多限制,δ18Oseawater值为∼2‰,反映了更多的蒸发条件。我们利用流体-岩石混合模型重建了岩浆岩填充物和蚀变化石的成岩历史。将流体-岩石混合模型应用于粒子群优化算法,计算出了蚀变温度、蚀变流体δ18O值和初始形成温度。我们发现,坎帕期(25-38 °C)和马斯特里赫特期(9-28 °C)的初始形成温度范围不同。我们还发现,轻质陨石流体(δ18O ≈ -10‰)作用下的蚀变是解释岩浆岩填充和化石同位素值改变的必要条件。根据我们的研究结果,碳酸盐岩的岩化和蚀变都发生在埋藏后不久,轻陨流体支持了之前的研究结果,即在晚白垩世,西内海道西缘存在高地形地貌。作为首次在样品中的多个矿物学阶段协同应用这些技术的研究之一,我们的研究结果为一个神秘海洋系统的古环境条件提供了重要的制约因素,并将改进对白垩纪末大灭绝前生态系统整体健康状况的解释。
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