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

1.46亿至6600万年前的白垩纪，经历了大气中高浓度的二氧化碳，在过去300万年里，气候最温暖，全球海平面最高。有几次，海洋突然缺氧，即所谓的海洋缺氧事件（oae），当有机物在海洋中积聚时，产生了广泛分布的黑色页岩，这些页岩现在充当了石油源岩的角色。然而，引起这些事件的机制仍然存在激烈的争论。本研究旨在利用一种新的多动态方法来更好地理解导致晚白垩纪全球碳埋藏事件发生、持续和停止的机制。有机质的埋藏导致海洋中同位素轻碳优先被去除，提高了海水的13C/12C比率，并通过大气CO2储层提高了整个地表系统的13C/12C比率。风化作用将12C释放回地表碳循环。岩石中的碳酸盐和有机质保存了c同位素比值随时间的变化，为c同位素地层学提供了依据。主要变化是同步的和全球性的，我们提出晚白垩世的c同位素变化可以作为全球海平面变化的一个代理；这还有待检验。锇是铂族元素，在海洋停留时间短（<40 kyr），在海水中也表现出同位素随时间的变化，主要受地壳风化和火山活动（地幔）输入两个端元组分的控制。它们的比值差别很大，因此，在古环境变化时期，例如风化作用增强或火山喷发的时期，Os同位素可能提供高分辨率的地层控制。现代海洋表现出统一的Os-同位素比值，但我们通过~94 Ma的OAE获得的Os同位素新数据表明，大西洋的Os同位素表现出历时性的变化。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，这个OAE可能是海洋o的一个独特事件；需要对更多地区和oae进行测试。该项目将使用来自有机质的c同位素地层学来关联来自不同环境、古纬度和海洋背景的全球演替。在101 ~ 83 Ma的时间区间内，两个oae和其他碳循环变化显著。我们的目标是回答以下问题：(1)长期C-和os -同位素曲线是否与海平面变化有关？(2) os同位素地层学是否可以用于化学地层学：是同步的还是历时的？(3) oae与os -同位素漂移是否一致？海洋的稳定状态是什么？(4)海平面变化、气候与海洋缺氧的关系；我们能否最终确定大范围海洋停滞的关键强迫机制？加拿大、法国、捷克共和国、俄罗斯远东地区、厄瓜多尔、南大西洋和澳大利亚近海的地点将被研究。每个盆地的相对海平面历史，使用c同位素进行对比，将用于测试c同位素地层学与海平面变化之间的关系。主要的地层时间间隔将描述Os同位素和痕量金属，以确定晚白垩世海洋Os同位素的演化；评估海水os同位素组成可能的区域差异；确定相关水团的海水含氧水平；并找出大范围海洋停滞的原因。我们的白垩纪极端温室研究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，以及无冰盖地球上全球碳循环的扰动提供独特的约束条件；我们星球在不久的将来越来越有可能。拟议中的研究将有助于了解海洋停滞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.