Development of an apatite oxygen paleobarometer: Experimental characterization of Sm3+-substituted apatite fluorescence as a function of oxygen availability

Abstract

Global, apatite-bearing phosphorites represent unique biogeochemical periods coincident with major transitions in biological evolution and, particularly, marine oxygenation. However, current understanding of such oxygenation is limited by qualitative and not uncommonly contrasting interpretations of the marine redox conditions evidenced by sedimentary isotope and trace element abundances. A potentially novel measure of ancient marine oxygenation has previously been indicated by fluorescence signatures detected in Sm3+-substituted microfossil-preserving apatite from Early Cambrian phosphorites, consistent with experimental studies demonstrating an oxygen-dependent mechanism for Sm3+ incorporation. Quantitative calibration of these signatures would enable the interpretation of oxygen concentrations during sedimentary apatite formation. In the experiments described, heat-promoted substitution of Sm3+ in apatite powder pellets has been conducted under gaseous oxygen concentrations ranging from 0 to 20.9%. The resulting fluorescence signals provide a comparison to those observed in naturally occurring phosphorites that should reflect the levels of oxygen in the immediate precipitating environment. These analyses suggest that a metric describing the relative spectral characteristics derived from end-member oxic and anoxic experiments, here described as an apatite oxygen paleobarometer (AOP), robustly correlates with gaseous oxygen concentrations. The application of this quantifiable metric to phosphorite apatite is expected to provide a fundamentally new dataset by which to probe ancient marine redox conditions, particularly during the Neoproterozoic-Cambrian phosphogenic event and diversification of early metazoans.