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The oxygenation of the Earth’s ocean-atmosphere system from the late Precambrian to the early Phanerozoic is thought to play a major role in shaping the biosphere of our modern world. However, the timing of the oxygenation of Earth’s surface environments and its link to biological evolution remains contentious. The use of non-traditional stable isotope geochemistry on marine sedimentary archives has recently emerged as a powerful tool in transforming our understanding of the rise of oxygen. Recently, a trend towards the analysis of large data sets has provided exciting new insights, however studies often lack careful consideration of the samples’ sedimentary and diagenetic history. This can have major implications for how results are interpreted. For example, component-specific analyses of carbonates suggest large variability in metal isotope and trace metal signatures between depositional and burial diagenetic phases. Within a single hand sample, uranium isotope composition can span nearly the entire range of measured ?238U in modern systems. These findings necessitate a return to petrographic and sedimentological analysis prior to geochemical studies in order to make robust interpretations about environments in deep time.
As an example of this more holistic approach, well preserved marine carbonates have revealed exciting new details about oxygenation of the Earth’s oceans in the Neoproterozoic and Paleozoic that challenge the traditional view of a unidirectional rise of oxygen through Earth’s history. Investigation of rare earth elements (REEs), specifically Ce anomalies, in petrographically screened carbonate marine cements suggest only transient marine oxic conditions in the Ediacaran, during the rise of animals, followed by a return to large-scale anoxia through much of the Paleozoic. It is not until the late Devonian, co-incident with the rise of large, vascular land plants, that modern-like REE profiles and true negative Ce anomalies develop in shallow marine seawater. This implicates a protracted oxygenation of the Earth, with the establishment of forest ecosystems in the Paleozoic driving the final oxygenation of the ocean-atmosphere system.