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

Project Summary

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.

4 Institutions
3 Teams
1 Publication
0 Field Sites
Field Sites

Project Progress

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, permineralizing and infilling 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-encrusting 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).

Figure 1-10. Quartz- and apatite-permineralized organic-walled microfossils (1-5, Obruchevella; 6-9, Siphonophycus) from the ~550 Ma Chulaktau Formation of South Kazakhstan, shown in optical photomicrographs (1 and 6); a confocal laser scanning micrograph (2); two-dimensional Raman images documenting the spatial distribution of kerogen (3 and 7; blue, acquired at its ~1605 cm-1 major Raman band) and of apatite (4 and 8; red, acquired at its ~965 cm-1 major Raman band); and two-dimensional spectroscopic fluorescence images showing the spatial distribution of fossil-permineralizing apatite (5 and 9; orange, acquired in the spectral range centered at ~603 nm that includes its major fluorescence bands) — fluorescent because of the presence of Sm+3 replacing both Ca I and Ca II sites of the apatite — and of later-emplaced fossil-encrusting apatite in which Sm+3-replaced Ca I sites (orange, acquired at ~597 nm) and Ca II sites (green, acquired at ~605 nm) are spatially distinct. The red rectangle in 1 denotes the part of the specimen shown in 3 through 5, and that in 6, in 7-9. 10, Fluorescence spectra acquired from differing areas of the Siphonophycus-encrusting apatite crystal denoted by the green arrow in 9 in which Sm+3-replaced Ca II and Ca I sites are spatially distinct. Like that in other earliest Cambrian apatitic fossiliferous deposits, much of the Chulaktau fossil-associated apatite (viz., that permineralizing and infilling fossils such as that shown in 1 through 5) exhibits a mixture of Sm+3-replaced Ca I and Ca II sites, indicating a dysoxic environment characterized by very low but measurable concentrations of environmental oxygen. However, crystals of fossil-encrusting apatite (6, 8 and 9) are mostly composed entirely of Ca I (anoxic) site-substituted apatite with some exhibiting peripheral zones of Ca II (oxic) site-substitution (9), their euhedral form indicating that these apatite crystals were precipitated before consolidation of the surrounding watery sediment.

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.