Expansion and connection of Early Jurassic oceanic anoxia: A complementary approach based on coupled Mo-U isotopes of black shales and U isotope signatures of carbonates

下侏罗统具有多段富有机质沉积的特征，可能与大气中大量的碳（CO2和/或CH4）注入以及相关的环境变化（如全球变暖、海平面变化、水文循环加速和大陆风化）有关。这些变化导致了碳循环的扰动，记录为黑色页岩或碳酸盐档案中有机（Corg）和/或无机（Ccarb）碳的碳同位素偏移（CIE）。尽管在世界范围内的盆地中都有这样的CIE记录，但对于区域或全球驱动力在多大程度上导致了有机质的增强沉积，以及这是否（或在多大程度上）与全球尺度下海底缺氧的面积扩大相关，人们仍然知之甚少。本文拟研究欧洲陆表海道（EES）三个不同盆地的黑色页岩的耦合Mo-U同位素（+微量元素）特征，即最近由Prees-2钻井（早期由Mochras Farm钻探）相交的柴郡盆地和德国西北部盆地（由Schandelah钻探岩心取样）。我们重点研究了三叠纪-侏罗纪（T-J）边界（包括下河塘期）和Pliensbachian-Toarcian （Pl-To）边界（包括T-OAE），这两个边界分别具有显著的负CIEs，全球变暖和一阶或二阶大灭绝的特征。对于后一个时间间隔，我们还考虑了德国西南部盆地（由SEPIA Metzingen钻探岩心取样）。黑色页岩的Mo和U同位素特征都取决于当地的沉积条件，如水团混合时间尺度、水柱H2S和金属来源。然而，将它们与氧化还原敏感的微量元素和TOC的浓度和比例结合使用，为表征不同研究盆地内部和之间的联系的水循环时间尺度提供了极其有力的手段。作为补充，我们建议进一步研究摩洛哥和葡萄牙的下侏罗统浅海相碳酸盐岩的U同位素特征。我们想在这里特别关注Pl-To和T-OAE，并考虑早先关于T-J边界的发现。海洋碳酸盐是古海水U同位素组成的理想档案，可用于估算全球尺度下海底缺氧的面积扩张。通过这些补充研究，我们希望揭示下侏罗统CIE和EES中增强的黑色页岩形成在多大程度上是由全球或区域环境变化驱动的。这些发现将为后续关于碳循环扰动与深海海底缺氧扩张之间联系的研究以及对未来的预测提供开创性的帮助。

The Lower Jurassic is characterized by multiple intervals of organic-rich sediment deposition likely related to high carbon injection into the atmosphere (CO2 and/or CH4) and associated environmental changes, such as global warming, sea level changes, accelerated hydrological cycles and continental weathering. These changes resulted in perturbations in the carbon cycle which are recorded as C isotope excursions (CIE) of organic (Corg) and/or inorganic (Ccarb) carbon in black shale or carbonate archives. Even though such CIE are recorded in basins worldwide, it is still poorly understood to what extent regional or global driving forces resulted in enhanced deposition of organic matter and if (or to which degree) this correlates with an areal expansion of seafloor anoxia at global scales. Here we propose to investigate the coupled Mo-U isotope (+ trace element) signatures of black shales from three different basins of the European epicontinental seaways (EES), i.e. the Cheshire basin, that was recently intersected by the Prees-2 drilling (and earlier by Mochras Farm) and the Northwest German Basin (sampled by the Schandelah drill core). We focus on selected time intervals, such as e.g. the Triassic-Jurassic- (T-J) boundary incl. the lower Hettangian and the Pliensbachian-Toarcian (Pl-To) boundary incl. the T-OAE), both of which were characterized by significantly negative CIEs, global warming and a first- or second order mass extinction, respectively. For the latter time interval we also consider the Southwest German Basin (sampled by SEPIA Metzingen drill cores). The Mo and U isotope signatures in black shales are both depending on local depositional conditions, such as water mass mixing time scales, water column H2S and metal sources. Their combined use, however, together with concentrations and ratios of redox-sensitive trace element and TOC, provides extremely powerful means to characterize water-circulation timescales within and connection between the different investigated basins. In a complementary approach, we furthermore propose to investigate the U isotope signatures of selected Lower Jurassic shallow-marine carbonates from Morocco and Portugal. We want to focus here in particular on the Pl-To and T-OAE and consider earlier findings for the T-J boundary. Marine carbonates are ideal archives for the U isotope composition of paleo-seawater that can be used to estimate the areal expansion of seafloor anoxia at global scales. With these complementary investigations, we expect to unravel to what extent the observed Lower Jurassic CIE and enhanced black shale formation in the EES, were driven by global or regional environmental changes. These findings will be ground-breaking for subsequent investigations about the linkage of carbon cycle perturbations and seafloor anoxia expansion in deep time and for projections to the future.