Schopf et al.—Berkuta and Chulaktau microbiotas
703 Like Raman spectra, fluorescence spectra (Fig. 5.2, 5.3)
document the composition of the Chulaktau fossils and their associated matrix. The fluorescence spectra show also that the fossil-permineralizing and -infilling apatite of the deposit contains a mixture of Sm+3-replaced Ca I and Ca II lattice sites. Although the analytical uncertainty in measurement of the position of fluorescence bands is typically ≤2 nm, spectral differences between these two varieties of apatite are firmly evidenced by the position and intensity of their fluorescence bands: (1) the upper spectrum in Figure 5.2 exhibits a prominent Sm+3 fluorescence band at ~597 nm, corresponding to the ~598nm band (Gaft et al., 2005) and ~599nm bands (Reisfeld et al., 1996; Gaft et al., 1997a) reported for Sm+3 replacing the Ca I site of apatite under vacuum conditions; (2) the lower spectrum includes a prominent Sm+3 fluorescence band at ~605nm that corresponds to the ~607nm band reported for Sm+3-replacement of the Ca-II site of apatite exposed to air (Reisfeld et al., 1996; Gaft et al., 1997a; Gaft et al., 2005), a band in such fossil-associated apatite that is virtually impercep- tible in the upper spectrum; and (3) the assignment of these bands toSm+3 is supported by the presence of a band at ~643nm (Fig. 5.2) that corresponds in position and relative intensity to a secondary Sm+3 fluorescence band reported to be situated at ~645nm (Reisfeld et al., 1996; Gaft et al., 1997a; Gaft et al., 2005).
permineralizing and infilling the fossils shown in fluorescence images in Fig. 3.5, 3.10, 3.15, and 3.20) exhibits a mixture of Sm+3-replaced Ca I and Ca II sites. Unmixed varieties of Sm+3-replaced apatite also occur. In contrast with the fossil- permineralizing and -infilling apatite (Fig. 3.1–3.20), crystals of fossil-encrusting apatite (Fig. 3.21–3.24) are mostly composed entirely of Ca I site-substituted apatite with some exhibiting peripheral zones of Ca II site-substitution (Figs. 3.24, 5.2), their euhedral form indicating that these apatite crystals were precipitated before consolidation of the surrounding sediment. Both the mode of occurrence of these encrusting apatite crystals and their unmixed rather than intermixed pattern of Sm+3-substitution indicate that they represent a generation of apatite-formation different from that permineralizing and infilling the fossils. Coupled with the paleoenvironmental setting of the
Much of the Chulaktau fossil-associated apatite (e.g., that
Chulaktau fossil-bearing cherts—and assuming that local oxygen concentrations were determinant in Sm+3-replacement of the
calcium sites of apatite, as discussed above—the spectroscopic fluorescence data seem readily explicable. Initially, before microbial decay and disintegration of the fossils, permineralizing and infilling apatite was emplaced in the low-oxygen (dysoxic)
Figure 5. Raman (1) and fluorescence spectra (2, 3) of quartz- and apatite- permineralized organic-walled Early Cambrian Maly Karatau Range microfossils. (1) Raman spectra of kerogen comprising permineralized fossils of the Kyrshabakta (Berkuta Member) and Chulaktau formations, in both units having an RIP value of ~7.5 (see text). The relatively prominent D band shoulder of the Chulaktau kerogen, derived largely from hydrogen situated on the periphery of the platy polycyclic aromatic hydrocarbons (PAHS) of which it is dominantly composed, indicates that that it has a somewhat higher H:C composition than the Berkuta kerogen (Schopf et al., 2005). (2) Fluorescence spectra acquired from differing areas of the Siphonophycus solidum-encrusting apatite crystal denoted by the green arrow in Fig. 3.24 in which Sm+3 replaced Ca II and Ca I sites are spatially distinct. (3) Complete fluorescence spectrum of this apatite-encrusted Chulaktau specimen of S. solidum.
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