Understanding short term changes in seawater redox during Cretaceous Oceanic Anoxic Events using combined isotopic tracers

了解能够推动部分海洋从缺氧(含氧)到缺氧(无氧)或正辛酸(无氧且含有有毒硫化物)的过程，对于确定未来地球变暖的海洋可能出现的情景至关重要。白垩纪中期是众所周知的全球高温时期，与此相关的是广泛沉积的黑色页岩(通常在海洋缺氧事件期间沉积的富含有机物质的沉积物)。这些黑色页岩单元的详细化学性质可能会记录地球系统对快速气候变化的驱动因素和反应，因此它们已成为极端气候研究的焦点。最近的短时间尺度(千年或更短)沉积记录表明，白垩纪海洋的特点是在有氧和缺氧沉积条件之间反复快速变化，在黑色页岩沉积期间尤其极端。了解这些短期周期对于增进我们对未来气候快速波动的预测可能如何影响海洋化学以及随后对生态系统稳定性和气候反馈机制的影响的理解至关重要。虽然大多数研究都集中在对缺氧或缺氧条件进行简单区分，但这种方法通过应用一些公认的和新的氧化还原指标，提供了一种不同程度的氧气消耗的测量方法。重点将是使用同位素示踪剂(钼、铬、S)来评估海洋氧气枯竭的空间程度，并认识到地方、区域和全球机制之间的关系，因为据预测，全球变暖的局部和区域影响将在未来造成重大不确定性。拟议的研究使用尖端分析技术的组合，通过将新的同位素示踪剂与成熟的技术相结合来开发高分辨率的地球化学记录。该提案的具体预期成果包括：1.更好地了解化学转化过程中的铬同位素体系。这对于将铬同位素系统作为一个强大的氧化还原替代物应用是至关重要的。该方法包括在有氧、缺氧(非硫化)和硫化条件下，对不同Fe(羟基)氧化物吸附和共沉淀时的铬同位素分馏进行了一系列实验测量。2.利用多种地球化学技术，对几个明确的、短时间尺度的循环的氧耗竭的具体性质(例如，缺氧与缺氧或丁二酸)进行高分辨率记录。3.通过配对高分辨率的钼和铬同位素分析，洞察海洋局部和全球氧化还原状态的短期变化。4.利用C-S-Fe系统学、地球化学和同位素数据的详细分析，对驱动海洋氧化还原水平变化的机制进行了评价。对氧化还原循环和短期氧化还原转变的研究将揭示哪些途径可能导致观察到的氧化还原转变，其中现有数据突出了地球化学特征的时间和相位关系的差异。

Understanding the processes capable of driving parts of the ocean from oxic (oxygen containing) to either anoxic (no oxygen) or euxinic (no oxygen and containing toxic sulfide) conditions is critical for identifying possible scenarios for the future ocean on a warmer Earth. The mid Cretaceous is well-known as a time of high global temperatures, and linked to this was the widespread deposition of black shales (organic-rich sediments commonly deposited during ocean anoxic events). The detailed chemical nature of these black shale units can potentially document a remarkable record of the driving factors and response of the Earth System to rapid climate change, and they have therefore become a focal point for extreme climate research. Recent short time-scale (millenial and shorter) sediment records have demonstrated that the Cretaceous ocean was characterized by repeated rapid changes between oxic and anoxic depositional conditions, which were particularly extreme during black shale deposition. Understanding these short-term cycles is essential for improving our understanding of how predictions of future rapid swings in climate may effect ocean chemistry, with subsequent impacts on ecosystem stability and climate feedback mechanisms. While most studies have focussed on making a simple distinction between oxic or anoxic conditions, this approach offers a measure for different levels of oxygen depletion, by applying a number of well established and novel redox indicators. The focus will be to use the isotope tracers (Mo, Cr, S) to assess the spatial extent of marine oxygen depletion and to recognise relationships between local, regional, and global mechanisms, since it is the local and regional effects of global warming that are predicted to cause major uncertainties in the future. The proposed research uses a combination of cutting-edge analytical techniques to develop high resolution geochemical records by combining novel isotope tracers with well-established techniques. Specific expected outcomes of the proposal include: 1. A better understanding of Cr isotope systematics during chemical transformations. This is essential for applying the Cr isotope system as a robust redox proxy. The approach includes a series of experimental measurements of Cr isotope fractionations upon sorption to and co-precipitation with different Fe (oxyhydr)oxides, under oxic, anoxic (non-sulphidic) and sulphidic conditions. 2. High resolution records of the specific nature of oxygen depletion (e.g. oxic versus anoxic versus euxinic), of several well defined, short time-scale cycles, by using multiple geochemical techniques. 3. Insights into short term changes in the local and global redox state of the ocean by pairing high resolution Mo and Cr isotope analyses. 4. An evaluation of the mechanisms involved in driving changes in ocean redox levels, utilizing detailed analyses of C-S-Fe systematics, geochemical and isotopic data. The investigation of redox cycles and short-term redox transitions, where existing data highlight differences in the timing and phase relationships of geochemical features, will reveal which pathways could have led to the observed redox changes.