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

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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.