2015 Annual Science Report
University of Wisconsin Reporting | JAN 2015 – DEC 2015
Project 3A: Apatitic Latest Precambrian and Early Cambrian Fossils Provide Direct Evidence of Concentrations of Environmental Oxygen
Means are not currently available to asses either quantitatively or semi-quantitatively the concentration of oxygen in Earth’s atmosphere over geological time. Despite this, the environmental availability of O2 has been repeatedly postulated to be a cause of major changes in Earth’s biota, most particularly at the Precambrian-Cambrian boundary-defining “Cambrian Explosion of Life,” a time in Earth history when large deposits of phosphate-rich apatite were deposited in shallow basins worldwide. This study shows that substitution of Sm+3 in the Ca I and Ca II sites of fossil-permineralizing, -infilling, and -encrusting apatite can differentiate between oxic, dysoxic, an anoxic settings of apatite formation. Further studies are underway to date such apatite and establish its REE-substitution as a quantitative O2 paleobarometer.
Coupled with geologic evidence of the paleoenvironmental setting of the fossil-bearing cherts of the units studied, spectroscopic fluorescence data indicate that phosphorites deposited during the worldwide episode of apatite fossil-permineralization near the Precambrian to Cambrian boundary can evidence oxygen concentrations in the local environment of formation. In particular, and apatite are typically emplaced in the low-oxygen (dysoxic) environment of basinal waters at and near the sediment-water interface (resulting in Sm+3-replacement of a mixture of the Ca I and Ca II lattice sites); after burial, in unconsolidated anoxic mud, permeating waters may carry in phosphate that emplaces fossil- apatite crystals (and Sm+3-replacement of their Ca I lattice site). Representative specimens are illustrated in Figures 1-9; Figure 10 compares the Florescence spectra of apatite formed under anoxic and oxic conditions (Schopf et al., In Press).
Such data hold promise for deciphering the concentration of dissolved oxygen present in a local apatite-forming environment, data of particular importance to understanding the Precambrian-Cambrian boundary-defining “Cambrian Explosion of Life,” an event widely postulated to have coincided with a global increase in environmental oxygen and a time when economically important phosphate-rich and commonly fossiliferous strata were deposited in shallow basins worldwide.
Our work focuses on three aspects of these studies:
(1) Paleobiologic studies of the fossil permineralizing-, infilling-, and encrusting-apatite grains syngenetic with the enclosing sediment.
(2) Use of the Raman-analyzed apatite grains for geochronology (based initially on a large number of specimens in thin sections of units from China and Kazakhstan, with the geochronometric studies to be conducted by our colleagues in Tainan, Taiwan);
(3) On-going experimental studies at UCLA to establish that Raman fluorescence spectra can provide (at 1%, 0.1%, and 0.01% oxygen concentrations) a quantitative O2 paleobarometer.