Establishing causes of sea-level change and oceanic anoxia in the Late Cretaceous: regional versus global patterns

1.46亿-6600万年前的白垩纪经历了高水平的大气二氧化碳，在过去的300 Ma经历了最温暖的气候和最高的全球海平面。有几次，海洋突然耗尽氧气，即所谓的海洋缺氧事件(OAEs)，当时有机物在海洋中积累，产生了广泛的黑色页岩，现在这些页岩充当了油源岩。然而，引发这些事件的机制仍存在激烈争论。这项提议旨在使用一种新的多动态方法来更好地理解导致晚白垩世全球碳埋事件开始、持续和停止的机制。有机物质的埋藏导致优先从海洋中清除同位素轻碳，增加海水的13C/12C比率，并通过大气二氧化碳储存库增加整个地球表面系统。风化作用会释放12C回到表面的碳循环。岩石中的碳酸盐和有机质保存了碳同位素比值随时间的变化，为碳同位素地层学提供了基础。主要变化具有同步性和全球性，我们提出了晚白垩世碳同位素变化可以作为全球海平面变化的代用指标，这一点还有待检验。Os是一种铂族元素，在海洋中的停留时间很短，约为40Kyr，在海水中也显示出同位素随时间的变化，主要受两个端元组分的控制：地壳的风化和火山活动(地幔)的输入。它们的比例截然不同，因此，在古环境变化时期，例如风化或火山活动增加的时期，Os同位素可能提供高分辨率的地层控制。现代海洋显示出均匀的Os同位素比值，但我们在~94 Ma通过OAE获得的Os同位素的新数据表明，大西洋的Os同位素显示出历时移位。这为研究古海洋混合提供了一个令人兴奋的潜在新工具。然而，这次大洋海洋事件可能是一个关于海洋海洋事件的独特事件；更多的区域和大洋事件还需要测试。该项目将使用有机物质的碳同位素地层学来关联来自不同环境、古纬度和海洋环境的全球序列。要研究的时间间隔为101-83 Ma，其特征是两个OAEs和碳循环中的其他重大变化。我们的目标是回答以下问题：(1)长期的碳和Os同位素曲线与海平面变化有关吗？(2)Os同位素地层学是否可以用于化学地层学：它是同步的还是历时的？(3)OAEs是否与Os同位素漂移重合，海洋的稳定状态是什么？(4)海平面变化、气候和海洋缺氧之间有什么关系；我们最终能否确定造成大范围海洋停滞的关键强迫机制？加拿大、法国、捷克共和国、远东俄罗斯、厄瓜多尔、南大西洋和澳大利亚近海的地点将被研究。使用碳同位素关联的每个盆地的相对海平面历史将被用来检验碳同位素地层学和海平面变化之间的关系。将确定Os同位素和痕量金属的关键地层时间间隔，以确定晚白垩世海洋中Os同位素的演化；评估海水Os同位素组成的可能区域变化；确定相关水团中海水的含氧量；以及查明大范围海洋停滞的原因。我们白垩纪极端温室研究的结果将为模拟海平面和气候变化之间的相互作用及其影响，以及全球碳循环的扰动提供独特的约束条件，以实现无冰盖的地球；地球越来越有可能在不久的将来出现这种情况。这项拟议的研究将有助于理解海洋停滞时期是否可能是当今全球变暖的未来后果。

The Cretaceous, 146-66 million years ago, experienced high levels of atmospheric CO2, and the warmest climates and highest global sea-levels in the last 300 Ma. On several occasions, the oceans became abruptly depleted in oxygen, so-called oceanic anoxic events (OAEs), when organic matter accumulated in the oceans, producing widespread black shales that now act as oil source rocks. However, the mechanisms that caused these events remain hotly debated. This proposal aims to use a new multi-dynamic approach to better understand the mechanisms that caused the onset, duration and cessation of global carbon burial events during the Late Cretaceous. Burial of organic matter leads to the preferential removal of isotopically light carbon from the oceans, increasing the 13C/12C ratio of seawater and, via the atmospheric CO2 reservoir, the entire Earth surface system. Weathering releases 12C back to the surface carbon cycle. Carbonates and organic matter in rocks preserve changes in C-isotope ratios through time, providing a basis for C-isotope stratigraphy. Major changes are synchronous and global in extent, and we have proposed that C-isotope variation in the Late Cretaceous may be used as a proxy for global sea-level change; this remains to be tested. Osmium, a platinum group element with a short ocean residence time of <40 kyr, also shows isotopic variation in seawater through time, being controlled predominantly by two end-member components: weathering of crust and input from volcanic activity (mantle). These have drastically different ratios, so Os isotopes potentially may provide high-resolution stratigraphic control during times of palaeoenvironmental change, such as episodes of increased weathering or volcanic input. The modern oceans display a uniform Os-isotope ratio, but our new data for Os isotopes through an OAE at ~94 Ma indicate that the Atlantic displayed diachronous shifts in Os isotopes. This offers an exciting potential new tool for studying palaeocean-mixing. However, this OAE may be a unique event with regard to oceanic Os; further regions and OAEs need to be tested. This project will use C-isotope stratigraphy from organic matter to correlate global successions from diverse environments, palaeolatitudes and oceanic settings. The time interval to be investigated, 101 - 83 Ma, was characterized by two OAEs and other significant changes in the carbon cycle. We aim to answer the following: (1) Are secular C- and Os-isotope curves related to sea-level change? (2) Can Os-isotope stratigraphy be used for chemostratigraphy: is it synchronous or diachronous? (3) Do OAEs coincide with Os-isotope excursions, and what was the steady state of the oceans? (4) What are the relationships between sea-level change, climate and ocean anoxia; can we finally identify the key forcing mechanism for widespread ocean stagnation? Sites in Canada, France, Czech Republic, Far East Russia, Ecuador, South Atlantic, and offshore Australia will be studied. The relative sea-level histories for each basin, correlated using C-isotopes, will be used to test relationships between C-isotope stratigraphy and sea-level change. Key stratigraphic time intervals will be characterised for Os isotopes and trace-metals to: establish the evolution of Os isotopes in the Late Cretaceous oceans; evaluate possible regional variation in the Os-isotope composition of seawater; establish levels of seawater oxygenation in the associated water masses; and identify the causes of widespread ocean stagnation. Results from our Cretaceous extreme-greenhouse study will provide unique constraints for modelling interactions between, and the impacts of, sea-level and climate change, and perturbations of the global carbon cycle for an icecap-free Earth; the increasingly likely near-future for our planet. The proposed research will aid in understanding whether periods of ocean stagnation are a likely future consequence of present-day global warming.