Cr and U isotope perspectives on Earth's oxygenation history informed by studies of their modern cycles

海洋沉积岩是过去地质学中全球变化的宝贵档案，记录了气候，海平面，营养循环和海洋氧化的信息。这些现象被认为是地球生命历史上许多生物进化和起源的基础。我的研究中心围绕开发和测试新的地球化学代理，可用于跟踪地质过去的全球变化。 主题一：在现代环境中开展地球化学代用指标的开发和测试，以建立指导其地质应用的解释框架。新出现的铬同位素代用品需要进一步的研究来检验这一假设，即氧化还原敏感的铬（VI）在海洋中表现出类似营养物的行为，并确定伴随海水铬（VI）还原和铬（III）氧化的铬同位素分馏因素。使用具有现实海洋环流的逆建模工具箱，我们将探索海洋“全球铬阵列”的起源和意义，并量化输入/输出铬通量和海洋。然后，我们将使用铬同位素对与过去5.4亿年来第二大灭绝相关的全球变化进行多代理研究。主题二：最近发表的利用汞替代物的证据表明，赫南阶大灭绝发生在火山活动增加的时期，尽管缺乏火山沉积物增加的地质证据。火山释放的二氧化碳使气候变暖，而火山硫气体的排放和加速的大陆风化使气候变冷。事实上，赫南灭绝的两个脉冲似乎与大陆冰盖的两次前进和后退有关，从而表明与重大气候变化有关。使用传统的和新颖的地球化学代理的选择，我的学生，同事和我将重建环境变化的详细历史，在加拿大北极地区的沉积矿床的特殊序列的两个选区周围。为了避免沉积地层全球对比的不精确性，将在同一地层剖面上测量所有地球化学代用指标，以便准确确定全球变化指标的相对时间（地层位置）。我们将检验这一假设，即火山喷发与火山活动引发的全球变化事件有关。 影响力：拟议的研究将填补有关氧化还原敏感铬及其同位素在现代海洋中行为的知识空白，而所获得的知识将用于建立一个更强大的解释框架，指导地质应用。对赫南阶大规模灭绝事件期间火山活动增加的环境影响进行的多代理研究将有助于预测当今燃烧化石燃料造成的未来全球变化。

Marine sedimentary rocks are valuable archives of global-change in the geological past, recording information on climate, sea level, nutrient cycling, and ocean oxygenation. These phenomena are believed to underpin many biological extinctions and originations in the history of life on earth. My research centers around development and testing of novel geochemical proxies that can be used to track global-change in the geological past. Theme 1: Oceanic Cr cycle  Development and testing of geochemical proxies is undertaken in modern environments to build the interpretive frameworks that will guide their geological applications. The emerging Cr isotope proxy requires additional study to test the hypothesis that redox-sensitive Cr(VI) exhibits nutrient-like behavior in the oceans, and to determine Cr isotope fractionation factors accompanying reduction of seawater Cr(VI) and oxidation of Cr(III). Using an inverse modeling toolbox with realistic ocean circulation, we will explore the origin and significance of the oceanic `global Cr array' and quantify input/output Cr-fluxes to and from the oceans. We will then use Cr isotopes in a multiproxy study of global-change associated with the second largest extinction of the past 540 million-years. Theme 2: Causes and consequences of global-change related to the Hirnantian mass extinction  Recently published evidence utilizing the Hg proxy suggests that the Hirnantian mass extinction occurred during a period of increased volcanism, despite the lack of geological evidence for increased volcanic deposits. Volcanic outgassing of carbon dioxide warms the climate, while emissions of volcanic sulfur gases, and accelerated continental weathering, cools the climate. Indeed, two pulses of Hirnantian extinction appear to correlate with two advances and retreats of continental ice sheets, thus indicating a link with major climate change. Using a selection of traditional and novel geochemical proxies, my students, colleagues and I will reconstruct the detailed history of environmental changes surrounding the two extinctions in an exceptional sequence of sedimentary deposits in the Canadian Arctic. To avoid imprecisions in global correlation of sedimentary strata, all geochemical proxies will be measured on the same stratigraphic sections in order to accurately determine the relative timing (stratigraphic position) of global-change indicators. We will test the hypothesis that the extinctions are linked to global-change events triggered by volcanism. Impact: The proposed research will fill important gaps in knowledge regarding the behaviour of redox-sensitive Cr and its isotopes in the modern oceans, while the knowledge gained will be used to build a more robust interpretive framework guiding geological applications. A multiproxy study of the environmental effects of increased volcanism during in the Hirnantian mass extinction event will help to predict future global-change from present-day burning of fossil fuels.