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

海洋沉积岩是地质过去全球变化的宝贵档案，记录了气候、海平面、营养循环和海洋氧合的信息。这些现象被认为是地球生命历史上许多生物灭绝和起源的基础。我的研究重点是开发和测试可以用来追踪过去地质历史的全球变化的新型地球化学指标。*主题1：海洋铬循环*在现代环境中进行地球化学指标的开发和测试，以建立将指导其地质应用的解释框架。新出现的铬同位素替代物需要进一步的研究，以检验氧化还原敏感的铬(VI)在海洋中表现出类似营养物的行为的假设，并确定伴随着海水铬(VI)的还原和铬(III)的氧化的铬同位素分馏系数。利用一个具有真实海洋环流的逆模拟工具箱，我们将探索海洋全球铬阵列的起源和意义，并量化进出海洋的输入/输出铬通量。然后，我们将使用铬同位素对与过去5.4亿年来第二大物种灭绝相关的全球变化进行多代理研究。*主题2：与赫南特大灭绝有关的全球变化的原因和后果*最近发表的利用汞代理的证据表明，尽管缺乏增加火山沉积的地质证据，但赫南提大灭绝发生在火山活动加剧的时期。火山释放出的二氧化碳使气候变暖，而火山含硫气体的排放，以及大陆风化的加速，则使气候降温。事实上，赫南提斯冰盖的两次灭绝似乎与大陆冰盖的两次进退有关，因此表明与主要的气候变化有关。我和我的学生、同事和我将使用一系列传统和新型的地球化学指标，重建加拿大北极地区一系列特殊沉积沉积物中围绕两次物种灭绝的环境变化的详细历史。为了避免沉积地层全球对比的不精确性，将在相同的地层剖面上测量所有地球化学指标，以便准确地确定全球变化标志的相对时间(地层位置)。我们将检验这一假设，即物种灭绝与火山活动引发的全球变化事件有关。*影响：拟议的研究将填补有关对氧化还原反应敏感的铬及其同位素在现代海洋中的行为的重要知识空白，同时所获得的知识将用于建立一个更有力的解释框架，指导地质应用。对赫南提斯大灭绝事件期间火山活动增加对环境的影响进行多代理研究，将有助于预测未来化石燃料燃烧带来的全球变化。

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.********