Application of the apatite oxygen paleobarometer (AOP) across the Neoproterozoic-Cambrian transition

Abstract

Fluorescence signatures of samarium3+ substituted into apatite for calcium2+ have been shown experimentally to correlate with oxygen availability, such substituted apatite being a principal diagenetic mineral emplaced in shallow-water environments during the Neoproterozoic-Cambrian global phosphogenic event coeval with the “Cambrian Explosion of Life.” The occurrence of Sm3+-substituted apatite, encrusting, infilling and permineralizing components of shallow water microbial communities suggested that quantification of these fluorescence signatures might provide a widely applicable mineral-based paleobarometer by which to measure oxygen concentrations in the environment in which such apatite precipitated. To test this hypothesis, fluorescence spectroscopic analyses have been carried out on the Sm3+-substituted apatite of photic-zone microscopic fossils and fossil-associated ooids of near-shore facies of the Ediacaran Doushantuo (South China) and Early Cambrian Chulaktau (South Kazakhstan) Formations, and of the open ocean apatite-biomineralized phytoplanktonic “scale fossils” (Characodictyon) of the mid-Neoproterozoic (Tonian) Fifteenmile Group (Yukon Canada). Results obtained – quantified by the newly devised apatite oxygen paleobarometer metric – indicate a low (<5% O2) oxygen concentration in the depositional environment of the Ediacaran Doushantuo, a similarly low but slightly higher O2 level in the Early Cambrian Chulaktau, and are consistent with a low oxygen concentration in the scale fossil-evidenced mid-Neoproterozoic photic zone of the open marine environment. By documenting the rise of oxygen across the Neoproterozoic-Cambrian transition these results confirm the efficacy of this newly devised technique and suggest its value for future global application.