Earth's changing environment and life through time - a geochemical perspective

纵观地球历史，许多生物的创新和灭绝都与地球表面环境和构造的重大变化有着错综复杂的联系，比如气候、海平面、火山活动和氧气。其中，氧的作用是研究得最少的。这项发现基金的研究旨在通过系统研究海洋中过去氧含量的新型地球化学示踪剂来解决这一缺陷。铬的四种同位素的微小丰度变化受铬氧化态变化的控制。铬的氧化状态反过来又与环境氧含量的变化有关。因此，海洋中氧含量变化的地质历史可以通过海洋岩石记录中保存的四种铬同位素相对丰度的变化来重建。为了确保我们从铬同位素的地质应用中提取最可靠的信息来解决“氧气问题”，需要做更多的工作来了解当今海洋中控制其同位素变化的因素。因此，本研究的第一部分将对世界上所有海洋中的铬同位素进行全面调查，从河流流向海洋的风化铬输入，以及大洋中脊的热液喷口，以及海水铬同位素在海相碳酸盐岩中的保存程度。接下来，我们将根据白垩纪海洋缺氧事件校准铬同位素代理的行为，这将在海洋钻探计划的海洋缺氧事件1a的海洋碳酸盐岩序列中进行研究。特别是，我们将探索变化的氧气水平（海洋缺氧）和变化的海底火山活动强度与Ontong爪哇高原就位的关系。我们将使用相关部分的文献数据来解决我们的古氧化还原重建与其他全球变化变量的相对时间，如气候（二氧化碳排放和温室变暖），主要生产者群落（如生物标志物和n同位素），以及通过事件记录的生物灭绝（海洋浮游生物）。最后，随着解释框架的建立，我们将把注意力转向约4.44亿年前的希尔南梯冰期和大灭绝事件，这是过去5.4亿年中第二大的冰期和大灭绝事件。我们将使用铬同位素来测试Hirnantian灭绝的两个脉冲是否与海洋缺氧变化的时间和强度有关。这项研究将作为学生博士、硕士和荣誉学位课程的要求之一，在分析化学和质谱方面接受的培训将帮助他们在加拿大竞争日益激烈的技术工作场所获得技术就业。这项研究也具有社会意义。全球变暖的有害影响之一是世界海洋缺氧。通过研究过去地质时期海洋中氧变化的影响（记录在海洋沉积物中），我们可以更好地预测未来海洋缺氧的影响，包括对初级生产者群落、海洋食物网和生物灭绝的影响。

Many biological innovations and extinctions throughout Earth History are intricately linked to major changes in Earth’s surface environment and tectonics through time, such as climate, sea level, volcanism, and oxygen. Among these, the role of oxygen is the least well studied. This Discovery Grant research aims to address this deficiency through systematic study of a novel geochemical tracer of past oxygen levels in the oceans. Small abundance variations in the four isotopes of chromium are controlled by changes in the oxidation state of chromium. The oxidation state of chromium is in turn linked to changing environmental oxygen levels. The geological history of changing oxygen levels in the oceans can therefore be reconstructed from changes in the relative abundance levels of the four chromium isotopes preserved in the marine rock record. To be sure that we extract the most reliable information from geological applications of chromium isotopes to the ‘oxygen problem’, more work is needed to understand the factors controlling its isotopic variability in the present day oceans. The first part of this research will therefore undertake a through investigation of chromium isotopes in all the World’s oceans, the weathering inputs of chromium to the oceans from river flows, and hydrothermal venting at mid-ocean ridges, as well as the degree to which seawater chromium isotopes are preserved in marine carbonate rocks. Next, we will calibrate the behavior of the chromium isotope proxy against an episode of ocean anoxia in the Cretaceous, which will be studied in marine carbonate successions spanning Ocean Anoxic Event 1a cored by the Ocean Drilling Program. In particular, we will explore the relationship between changing oxygen levels (ocean anoxia) and changing intensity of submarine volcanism associated with the emplacement of the Ontong Java Plateau. We will use literature data on correlative sections to address the relative timing of our paleo-redox reconstructions with other global change variables such as climate (carbon dioxide emissions and greenhouse warming), primary producer community (e.g., biomarkers and N-isotopes), and biological extinction (marine plankton) documented through the event. Lastly, with the interpretive framework firmly established, we will turn our attention to the ~ 444 million-year-old Hirnantian glaciation and mass extinction event, the second largest of the past 540 million-years. We will use chromium isotopes to test whether two pulses of Hirnantian extinction were related to the timing and intensity of changes in ocean anoxia. This research will be performed by students as one of the requirements of their doctoral, masters, and honours degree programs, and the training received in analytical chemistry and mass spectrometry will help them to secure skilled employment in Canada’s increasingly competitive and technological workplace. This research also has societal relevance. One of the harmful effects of global warming is de-oxygenation of the World’s oceans. By studying the impacts of oxygen changes in the oceans of the geological past (recorded in marine sediment), we are in a better position to predict future impacts of de-oxygenation of the oceans in the future, including the impact on primary producer communities, ocean food webs, and biological extinction.