Sedimentary Ce anomalies: Secular change and implications for paleoenvironmental evolution

Although Ce anomalies are commonly used to reconstruct past seawater redox conditions, published interpretations are based on various proxy materials and analytical approaches, while no relatively complete compilation of sedimentary Ce anomaly data has yet been made. Here, we report a new compilation comprising >6000 carbonate, iron formation, phosphorite, and chert samples of all ages, of which 1127 passed screening for near primary Ce anomaly values. The Ce anomalies of 592 mudstones were also investigated and found to contain in some cases unambiguous traces of a primary negative Ce anomaly, which was likely inherited during early diagenetic organic degradation or directly from authigenic phases. Here we show that seawater-like rare earth element (REE) patterns may be retained in siliceous mudstones in cases where the detrital REE signature has been diluted by authigenic silica. Mudstone weathering is shown to have a negligible effect on the magnitude of negative Ce anomalies in moderately weathered samples. A critical appraisal of negative Ce anomalies from the published literature, yet to be confirmed through La-Ce isotopic dating, implies that oxygenic photosynthesis likely evolved during 3.0–2.5 Ga. Although our compilation does not capture the onset of the Great Oxidation Episode, a decreasing trend in mean negative Ce anomalies from 2.5 to 2.1 Ga is in accordance with a rise in oxygen levels during the early Paleoproterozoic. Sporadic Ce anomalies might identify transient oxygenation events during 1.8–1.2 Ga with minimum atmospheric concentrations up to 1–6% present atmospheric level. Statistical analysis of Ce anomaly data is consistent with progressive oxygenation of the surface ocean through the late Tonian-early Cambrian interval. While pronounced negative Ce anomalies indicate that shallow marine settings were broadly well oxygenated during the Ediacaran-Cambrian transition, the deeper marine environment remained largely anoxic, consistent with the persistence of a dissolved organic carbon redox buffer. The Ce anomaly compilation also reveals expanded ocean anoxia in the early Paleozoic, followed by more pervasive oxygenation by middle Devonian times, coincident with the emergence and radiation of secondary woody tissues and forests. The redox evolution and estimated pO2 revealed by Ce anomalies are generally compatible with other proxies and modelling results, and therefore we concur that the marine sedimentary Ce anomaly record responds sensitively to oceanic redox changes and can be a useful additional proxy to track Earth system evolution.