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

贯穿地球历史的许多生物创新和灭绝都与地球表面环境和构造随时间发生的重大变化密切相关，如气候、海平面、火山活动和氧气。其中，氧的作用是研究最少的。探索基金的这项研究旨在通过系统地研究一种新的海洋中过去氧气水平的地球化学示踪剂来解决这一不足。铬的四种同位素的微小丰度变化由铬的氧化态的变化控制。铬的氧化状态反过来又与环境氧水平的变化有关。因此，可以根据保存在海洋岩石记录中的四种铬同位素的相对丰度水平的变化来重建海洋中氧水平变化的地质历史。为了确保我们从铬同位素在地质上的应用中提取到最可靠的信息，我们需要更多的工作来了解控制其在当今海洋中的同位素变异性的因素。因此，这项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，调查河流流向海洋的铬的风化输入，大洋中脊的热液喷发，以及海水铬同位素在海相碳酸盐岩中的保存程度。接下来，我们将根据白垩纪的一次海洋缺氧事件来校准铬同位素代用品的行为，这将在海洋钻探计划为核心的跨越海洋缺氧事件1a的海相碳酸盐序列中进行研究。特别是，我们将探索与安通爪哇高原侵位有关的氧水平变化(海洋缺氧)和海底火山活动强度变化之间的关系。我们将使用相关章节的文献数据来说明我们的古氧化还原重建与其他全球变化变量的相对时间，这些变量包括气候(二氧化碳排放和温室气体变暖)、初级生产者群落(例如，生物标志物和氮同位素)以及通过该事件记录的生物灭绝(海洋浮游生物)。最后，随着解释框架的确立，我们将把注意力转向~4.44亿年前的赫南提斯冰川和大灭绝事件，这是过去5.4亿年来第二大的冰川和大灭绝事件。我们将使用铬同位素来测试两次赫南提斯物种灭绝是否与海洋缺氧变化的时间和强度有关。这项研究将由学生进行，作为其博士、硕士和荣誉学位课程的要求之一，所接受的分析化学和质谱学培训将帮助他们在加拿大竞争日益激烈和技术日益发达的工作场所获得熟练的就业机会。这项研究也具有社会相关性。全球变暖的有害影响之一是世界海洋的脱氧。通过研究地质过去(记录在海洋沉积物中)海洋中氧气变化的影响，我们可以更好地预测未来海洋脱氧的影响，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.