A revised sedimentary pyrite proxy for atmospheric oxygen in the Paleozoic: Evaluation for the Silurian-Devonian-Carboniferous period and the relationship of the results to the observed biosphere record

Several past attempts to model the variation in atmospheric oxygen through the Paleozoic give generally conflicting results, but all agree that pO₂ rose to a peak in the late Paleozoic between 320 and 260 Ma. The Pyrite Proxy, based on the concentrations of Se and Co in sedimentary pyrite, provides an alternative to the modelling approach. Here we reformulate the Pyrite Proxy for the Paleozoic with 'best fit' to measured oxygen levels in fluid inclusion gases in sedimentary halite over the Phanerozoic. The proxy exhibits systematic variations in pO₂ through the Silurian to Carboniferous period that are shown to closely correlate with evolutionary changes in the biosphere. In particular the proxy demonstrates how oxygen levels increased through the Silurian and early Devonian along with the development of vascular plants, the first hexapods and giant fish, followed by a sharp decline through the late Devonian coinciding with a period of high frequency magnetic reversals, the hexapod and charcoal gaps and the Devonian mass extinction events. Oxygen rises dramatically in the mid to late Mississippian reaching a peak in the Pennsylvania accompanied by development of tall tropical arborescent lycopsids and a period of giant volant insects. The difference between traditional box models based on O₂ (PAL) and the actual composition of gas from halite fluid inclusions and the Pyrite Proxy, in turn, also offers a new and interesting methodology for estimating trends in atmospheric density over time.