RE-VISITING THE REDOX-SENSITIVE TRACE METAL RECORD OF THE NEOPROTEROZOIC OXYGENATION EVENT (Invited Presentation)
Analyses accounting for spatial-temporal sampling density re-capitulate the rise in Mo (in euxinic shale) and U (in anoxic shale) through the Ediacaran-Cambrian seen in previous publications. However, the new analyses show either constant levels (Mo) or lower levels (U) in the Ordovician-early Devonian, followed by a second rise to even higher levels in the mid-Devonian. When normalized to TOC, the Ediacaran-Cambrian RSM increase disappears: metal/TOC ratios show no long-term trend until the middle and late Paleozoic. Machine learning analyses that simultaneously account for TOC, detrital input, ‘degree of pyritization,’ basin type, latitude/longitude, lithology, site type, depositional environment and metamorphic grade confirm these results.
Overall, our increased data density and ability to account for sampling/geological biases suggest that the visually dramatic increase between the Proterozoic and Phanerozoic in plots of raw trace metal data was due to 1) over-emphasis of outlier points, and 2) the large increase in TOC contents of sampled Phanerozoic shale compared to the Neoproterozoic. The analyses do not rule out some degree of oxygenation in the Ediacaran-Cambrian, or re-organization of the seafloor redox landscape. However, to the extent that metal/TOC ratios are a proxy for the area of reducing seafloor, the major long-term inflection point is not Neoproterozoic but rather Devonian.