CERIUM ANOMALIES IN WELL-PRESERVED CARBONATES REVEAL OCEAN OXYGENATION WITH THE RISE OF LAND PLANTS

The oxygenation of the ocean-atmosphere system in the late Precambrian and early Paleozoic is considered to play a major role in the emergence and diversification of animals. While much research has focused on the Neoproterozoic, a broader focus over the Precambrian-Phanerozoic transition is key to evaluate the links between metazoan diversification and surface oxygenation.

We provide a new perspective on marine oxygenation during this time through the application of rare earth element (REE) and Ce-anomaly proxy work in well-preserved marine cements. REE profiles in carbonates are a commonly used paleo-oceanographic proxy with the presence of a negative Ce anomaly (i.e. the depletion of Ce due to oxidative removal) used to track well-oxidised water masses. REE profiles preserved in marine carbonates can be used to reveal the widespread oxygenation of the oceans. Petrography and cathodoluminescence microscopy are essential first steps in characterising the preservation of REY in marine carbonates prior to geochemical analysis. Laser ablation ICP-MS work on marine and late-stage carbonate components suggests that even small incorporations of burial cements in porosity may significantly alter REE profiles, suggesting care must be taken when sampling for trace metal geochemistry in carbonates. REE patterns in a suite of screened marine cements (preservation determined through petrographic and geochemical criteria) suggest a protracted surface oxygenation. While transient marine oxic conditions (moderate negative Ce anomalies) are recorded in the Ediacaran, a return to large-scale anoxia occurs over 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 data suggests that the establishment of land plants in the Paleozoic may have been essential to fully oxygenate the ocean-atmosphere system and confirms that petrography is essential for robust geochemical studies in carbonates.