Schopf et al.—Berkuta and Chulaktau microbiotas
and L. tenuissima), acanthomorphs seem not to be represented (with the possible exception of rare specimens of the enigmatic taxa Vandalosphaeridium koksuikum and Cymatiosphaera sp.). Why are acanthomorphic acritarchs, typical of Early
Cambrian microbiotas in other locales, not abundant and perhaps not present in the Berkuta and Chulaktau assemblages? This absence may simply reflect the early Early Cambrian age of these assemblages, dating from the immediate aftermath of the phytoplankton extinction event of the latest Proterozoic (e.g., Vidal and Knoll, 1982; Schopf, 1992b; Knoll, 1994; Vidal and Moczydłowska-Vidal, 1997; Knoll et al., 2006), with available data indicating that the upsurge in diversity of acanthomorphs during the Early Ediacaran (Grey, 2005; Vorob’eva et al., 2009) was followed by a major decrease until the mid-Early Cambrian when there was a second sharp increase near the beginning of the Atdabanian (as evidenced, for example, in the Lükati [Talsy] Horizon of the East European Platform; Volkova et al., 1979; Sergeev, 1992). A second possible explanation is environmental, a product
of the fossil-preserving facies. In particular, it seems plausible that phytoplanktonic acanthomorphs may have been prevalent only in open marine settings and that their absence from the Berkuta microbiota is a result of its preservation in a shallow post-Baykonurian glacial basin of the Early Cambrian seas. Similarly, in consonance with the environmental model for deposition of the Karatau Member of the Chulaktau Formation proposed by Kholodov and Paul (1993a, 1993b; 1994), we envision the immediately underlying acanthomorph-lacking Aksai Chulaktau cherts to represent an extremely shallow environment of a restricted marine basin. Additional studies of distal offshore deposits of Berkuta- and Askai-age will be needed to establish the role that habitat may have played in excluding acanthomorphs from sediments in which they might otherwise have been expected. The Berkuta microbiota is appreciably less diverse than
that of the younger Chulaktau microbiota, lacking, for example, such taxa as Leiosphaeridia minutissima, L. tenuissima, Myxococcoides minor, M. inornata, Archaeophycus yunnaensis, Siphonophycus kestron,and Oscillatoriopsis sp. (of which the last two filamentous taxa are not illustrated in this paper). Although we interpret the relative lack of diversity of the Berkutamicrobiota as most likely reflecting its preservation in a restricted shallow post-glacial basin, such differences may be due to vagaries of preservation: even in the comparatively better preserved andmore diverse Chulaktau assemblage, Siphonophycus kestron and Oscillatoriopsis sp. are known only from sample 244 (outcrop K-32) whereas in the underlying Berkuta, Berkutaphycus elongatus, unknown fromthe Chulaktau, is present only in sample 115a (outcrop K-27). By the facies-based interpretation suggested here, the
compositions of the Berkuta assemblage and Chulaktau micro- biota reflect their local environments, settings that are a product of the Early Cambrian global environment. The Neoproterozoic Ediacaran and Phanerozoic Cambrian Periods span a distinctive time in Earth history when the ecosystem changed radically with the rise of metazoans. During this time, huge accumulations of economically important phosphatic ores were deposited in basins worldwide—an event of particular paleontological significance because of the capability of such phosphate to
709
permineralize microorganisms, as shown in Fig. 3, as well as soft animal tissues (e.g., Chen et al., 2007). Of the many such deposits known, that of the economically important Maly Karatau Range Lower Cambrian Chulaktau Formation has been investigated in particular detail. Deposition of the Chulaktau phosphorites has typically
Kholodov and Paul (1993a, 1993b, 1994) and an influx of dissolved phosphate spurring the proliferation of cyanobacteria would have been ideal for the thriving “Proterozoic-like” community preserved in the Chulaktau cherts and might help to explain the apparent absence from the assemblage of more open ocean-inhabiting acanthomorphic acritarchs, while also suggesting that unornamented sphaeromorph leiosphaerids may have been relatively more abundant in near shore habitats. Similarly, this model is consistent with the Raman and fluores- cence spectroscopic data presented here for the apatite- permineralization, -infilling and -encrustation of the Chulaktau fossils (Figs. 3 and 5). Among the various taxa identified in the Chulaktau
been modeled as resulting from the upwelling of phosphate- saturated deep marine waters into shallow settings where phosphatic nodules, granules and oolitic sediments were pre- cipitated (see Baturin, 1978, for additional references and dis- cussion). This scenario and many variants have been suggested for the deposition of Karatau Member phosphorites (e.g., Cook and Shergold, 2005). Although a full discussion of such models is beyond the scope of this paper, because the mode of formation of the Chulaktau phosphorites bears on the fossilization, and, thus, the composition of the preserved microbiota, it merits consideration. Our favored model is that of Kholodov and Paul (1993a, 1993b, 1994) according to which the phosphorites were deposited in exceedingly shallow waters of a restricted basin that had a complicated shore geography composed of lagoons, inlets, evaporitic pools and diverse other near-shore facies. We envision this Early Cambrian restricted shallow basinal setting to have served as a trap for phosphate deposition that resulted in apatite permineralization of the Chulaktau microbiota (e.g., Fig. 3). The diverse shallow near shore settings hypothesized by
assemblage, entophysalidacean cyanobacteria occur today in extremely shallow intertidal or peritidal environments. That they also inhabited such settings in the Chulaktau basin is shown by the occurrence in the fossiliferous succession of such shallow-water indicators as desiccation cracks, intraclastic grainstones (flat-pebble conglomerates) and oolitic grainstones. To explain the oolitic texture of most of the Chulaktau phos- phatic ores, the Kholodov and Paul (1993a, 1993b, 1994) model suggests that the shallow phosphate-depositing environment was highly energetic. This, in turn is consistent with our findings of the absence of coherent cyanobacterial mats in the Chulaktau cherts and the predominant occurrence of entophysalidaceans as loose clusters of gloeocapsoid cells and spheroidal aggregations rather than as lamina-defining crustose colonies such as those reported from Proterozoic cherts by Hofmann (1976) and Sergeev et al. (1995, 2012). In sum, the Berkuta and Chulaktau strata were deposited
in evidently very shallow waters, evidenced both by their sedimentological characteristics and by the compositions of their permineralized microbial assemblages. Subsequent to
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