Reconstruction of the silicate weathering feedback in the mid-Cretaceous high-CO2 world

构造和气候过程之间的相互作用和长期关系是我们理解全球碳循环变异性的关键问题。白垩纪中期(130-90 Ma)经历了大陆裂谷和洋壳生产速率的增加，导致大气二氧化碳增加和明显的温室气候条件。反过来，大气中二氧化碳的减少要么通过有机物埋藏实现，要么通过硅酸盐风化强度的变化实现，或者两者兼而有之。海相碳酸盐的锂同位素组成代表了硅酸盐风化反馈的强度，并记录了海水δ7Li的长期变化。海水δ7Li反映了由于粘土形成过程中的同位素分馏作用而引起的陆相和洋相硅酸盐风化变化所控制的源汇平衡。在这个项目中，我们的目标是生成130-90 Ma前的中白垩世海水锂同位素曲线，以评估硅酸盐风化反馈，类似于已有的新生代记录。我们将使用大量碳酸盐和原始保存下来的骨骼方解石(腕足类、贝壳类和双壳类的贝壳)的档案，这些档案有很好的年代标本收藏。新记录包括两次海洋缺氧事件OAE1a和OAE2，以及阿普提亚晚期/阿尔比安早期和西诺马期的间歇性冷期，将为极端温室气候条件下的风化提供参考。该项目的主要预期成果将是1)更好地了解中白垩世海水δ7Li值的长期演化，2)中白垩世背景δ7Li值的信息，以评估其相对于OAE的长期变异性，海水锶同位素记录，以及玄武岩风化量，3)关于相对于间歇性冷期的风化一致性(强度)的新信息，以及4)关于δ7Li相对于不同骨架方解石保存程度的潜在不确定性的估计，以及不同钙质中锂同位素分馏的分类群特有的重要影响。

The interaction and secular relationship between tectonic and climatic processes are key issues for our understanding of global carbon cycle variability. The mid-Cretaceous period (130-90 Ma) witnessed elevated rates of continental rifting and oceanic crust production, resulting in increased atmospheric CO2 and pronounced greenhouse climate conditions. Atmospheric CO2 reduction, in turn, is achieved either by organic matter burial, by changes in the intensity of silicate weathering or a combination of both. The lithium isotopic composition of marine carbonates is a proxy for the intensity of the silicate weathering feedback and records secular changes in seawater δ7Li. The seawater δ7Li reflects the balance of sources and sinks controlled by variations in the weathering of continental and oceanic silicates due to the isotopic fractionation during the formation of clays. With this project we target the generation of a new mid-Cretaceous seawater lithium isotope curve for the interval between 130-90 Ma ago to assess the silicate weathering feedback, analogue to the already existing record for the Cenozoic. We will use the archives of bulk carbonate and pristine preserved skeletal calcites (shells of brachiopods, belemnites and bivalves) for which a well-dated collection of specimens exists. The new record includes the two oceanic anoxic events OAE1a and OAE2 as well as intermittent cold phases in the late Aptian/early Albian and Cenomanian and will serve a reference for weathering during extreme greenhouse climate conditions. The main expected deliveries of this project will be 1) an improved knowledge about the secular evolution of mid-Cretaceous seawater δ7Li values, 2) information about mid-Cretaceous background δ7Li values to assess its long-term variability relative to OAEs, the seawater strontium isotope record, and the amount of basalt weathering, 3) new information about the congruency (intensity) of weathering relative to intermittent cold phases, and 4) estimates about potential uncertainties in δ7Li relative to different degrees of skeletal calcite preservation and taxon-specific vital effects of lithium isotope fractionation among different calcifiers.