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Journal of Paleontology 89(5):695–729
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 carried in phosphate that emplaced fossil-encrusting apatite crystals (and Sm+3-replacement of their Ca I lattice site). At some later time, presumably by an influx of oxygen-containing waters, the peripheries of some encrusting crystals became oxidized (resulting in replacement at the Ca II lattice site). This scenario is consistent with what is now is known
regarding both samarium-replacement of calcium in apatite and the paleoecology of the Chulaktau basin. Nevertheless, the use of such substitution in apatite-permineralized fossils to establish the relative oxygen-concentrations of their preservational history is a concept new to paleobiology. Confirmation of this novel interpretation will depend on additional investigations. In summary, as with virtually all comparable perminer-
alized microbiotas, the Early Cambrian Berkuta and Chulaktau assemblages inhabited a carbonate-precipitating shallow photic- zone environment. The localized relatively low pH produced by microbial metabolism in these benthic communities resulted in dissolution of associated carbonate and its replacement by colloidal silica that infused microbial cell walls and mucilagi- nous envelopes and sheaths prior to their decay and disintegra- tion. Upwelling of phosphate-laden deep marine waters into the restricted Chulaktau basin resulted in the deposition of phosphorites and, in the benthic dysoxic parts of near-shore facies, the infusion of phosphate into partially silicified microbes and its intracellular precipitation to infill cells. After near-surface burial but before consolidation of the anoxic microbe-enclosing mud, extracellular precipitation of a later insurge of phosphate produced swaths of microbe-encrusting euhedral apatite crystals, the surfaces of some of which were subsequently oxidized by interaction with oxygen-bearing percolating waters. It has been suggested that microbial physiology may have
played an active role in the concentration and precipitation of phosphate in apatite-mineralized fossil microbes (e.g., Gerasimenko et al., 1996, 1999; Zhegallo et al., 2000). Although it is plausible that an influx of dissolved phosphate into the Chulaktau basin may have promoted the proliferation of cyanobacteria, as it does in similarly shallow water settings today, the evidence presented here indicates that the infusion of silica and phosphate into the microbes that resulted in their quartz- and apatite-permineralization and -infilling were post-mortem, not under biological control. An analogous occurrence of apatite-permineralization, of essentially the same age as the Chulaktau fossils and also studied by CLSM and Raman, has been documented for a ctenophore (“comb jelly”) embryo from the ~540 million-year-old Kuanchuanpu phosphorite of China (Chen et al., 2007).
Materials and methods
Fossiliferous localities.—As shown in Figures 1 and 2, the Berkuta and Chulaktau microfossils studied here occur in chert samples from the Maly Karatau Range of South Kazakhstan collected from stratigraphic sections at seven outcrops: K-27, K-28, K-29, K-30, K-32, K-33 (designations used also in
Sergeev, 1992), and K-40. Listed below are the geographic localities of these outcrops of Early Cambrian fossiliferous chert (which for outcrops K-27, K-30, and K-33 differ slightly from those previously noted for microfossiliferous strata of the underling Neoproterozoic Chichkan Formation; Sergeev and Schopf, 2010). Outcrop K-27: to the northwest of Zhanatass town in the
basin of Koksu River (Google Map Coordinates, decimal degrees latitude and longitude, 43.6208 N lat., 69.6094 E long., samples 4681/115-119, 220). Outcrop K-28: along the middle reaches of the Kyrshabakta
River, north of Baikadam (43.5859N, 69.9642E, samples 4681/ 294, 295).
Outcrop K-29: to the southeast of Zhanatass town in the basin
of Berkuta River about 1km east from the Berkuta settlement (43.5941N lat., 69.7365E long., samples 4681/237-241). Outcrop K-30: north of outcrop K-33, in the Au-Sakan
region where the Shabakta River valley widens (43.5226N lat., 69.8739E long., samples 4681/97-104). Outcrop K-32: near the Zhanaaryk settlement along the
Zhanaaryk Creek valley and adjacent to the road between Karatau and Zhanatass towns (43.5146N lat., 69.79072E long., samples 4681/244-247). Outcrop K-33: along the lower reaches of the Shabakta
River near the Aktogai settlement, north of Baijansai (43.4744N, 69.8610E, samples 4681/14-16). Outcrop K-40: near and south of outcrop K-27 in the basin
of Koksu River along the Kurtlybulak Creek valley (decimal degree coordinates not available, samples 4681/273, 278). The best preserved and most fossilferrous samples studied
are from the cherty-phosphorite Aksai Member of the Chulaktau Formation. Abundant though less well-preserved microfossils are also here reported from the Berkuta Member of the underlying Kyrshabakta Formation (outcrops K-27 and K-32).
Repository of illustrated specimens.—The specimens illustrated here are reposited in the Paleontological Collection of the Geological Institute (GIN), Russian Academy of Sciences, Moscow.
Location of specimens within thin sections.—All illustrated fossils are from cherts of GIN field collection 4681. The figure caption for each illustrated specimen indicates its catalogue
number in the GIN paleontological collection (GINPC); the field collection number of the fossil-bearing rock; the Kyrshabakta Formation (Berkuta Member) or Chulaktau For- mation horizon from which the studied rock sample was obtained; the identifying number of the specimen containing petrographic thin section; and the location of the specimen within the fossiliferous thin section (indicated both by its England Finder Slide coordinates and by a “p”, the point within the section where the specimen occurs and a number indicating the position of this point in an overlay map attached to the section).
Optical microscopy.—At UCLA, photomicrographs were obtained by use of Leitz Orthoplan 2 (#0026635) and Orthoplan (#654016659) microscopes (Leitz, Wetzlar, Germany) equipped, respectively, with a Nikon Digital Sight DS-Fi1 Camera System
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