Cambrian marine radial cerebroid ooids: Participatory products of microbial processes

Abstract

The Cambrian Miaolingian Series strata at the Kouquan section, North China, are primarily divided into the Xuzhuang, Zhangxia, and Gushan formations. This particular series comprises oolitic limestone in the middle to upper intervals. Thin section observations of carbonate rock samples from the Zhangxia Formation show that the well‐preserved and most remarkable grains among them are cerebroid ooids. Although these ooids have characteristic serrated margins, they are distinct from ooids reported in previous publications due to inconspicuous concentric laminae and radial structures composed of micrite and microspar. To understand the origin of these ooids, we employed petrographic examination, X‐ray diffraction (XRD), scanning electron microscopy (SEM), electron probe microanalysis (EPMA), and isotope analyses to explore their compositions and conditions of development. In addition, the occurrence of these cerebroid ooids is closely related to that of dark‐coloured micrite: most cerebroid ooids and quasi‐cerebroid ooids float inside dark micrite; dark micrite appears in the interior of cerebroid ooids as a radial micrite structure. EPMA and energy‐dispersive X‐ray (EDX) spectroscopy analyses of micrite reveal that the dark micrite consists of lithomicrobial mats. Fine‐scale characterization of the dark micrite inside these cerebroid ooids shows the presence of filamentous and bowl‐shaped calcified microbial fossils and extracellular polymeric substance (EPS) calcified remnants. Isotopic data indicate a variation of δ13C isotopic values between −4.92‰ and −0.11‰ (VPDB) and −9.89‰ and −4.93‰ for δ18O values (VPDB). The negative values of δ13C in the studied ooid suggest the genesis of carbon from the high organic‐rich zone such as the microbial zone. Also, the 13C‐depleted carbon values in the ooids advocate that the carbon is mainly derived from organic sources associated with the sulphate reduction bacterial activity. Thus, isotopic data of this study also provide consistent clues in favour of microbial participation during the genesis of cerebroid ooids. In conclusion, our new findings strongly confirm that the cerebroid ooids of the North China Platform are genetically related to microbial processes.

Oolitic Limestone Induced by Photosynthetic Biofilms in the Xuzhuang Formation of Nangao Section, North China

“…Previous studies have summarized a series of the important characteristics of cerebroid ooids: (1) the high-salt, shallow-water environment where cerebroid ooids are produced [26]; (2) the special cortices of cerebroid ooids and their production environment, which are related to the biofilm [19]; and (3) the fact that cerebroid ooids have a variety of production forms [37]. The ooids of Nanga have the following additional characteristics: (1) residual calcified biofilm residues between the nuclei of the ooids and the ooids, and (2) a correlation between the ooid cortex and the nucleus of the ooid.…”

“…This biofilm might be similar to the in situ biofilm found by Suarez [30] in the central Pacific atoll, where cerebroid ooids produced within the biofilm were also located. The clotted micrite structure might be a calcified biofilm influenced by organisms, with the following evidence: (1) the cerebroid ooids and other radial ooids produced in dense micrite have been confirmed to be composed of a variety of calcified microbial microfossils and calcified EPS [19]; (2) the irregular shape of the sparry filamentous bodies in the clotted micrite structure of Nangao section makes other interpretations challenging to accept [29]; and (3) the production of calcified biofilms is often similar to the dense micrite of the clotted micrite structure in the Nangao Section [13].…”

“…When biofilm accretion dominates, the internal irregularities will gradually weaken under the influence of surface tension on the biofilm, eventually forming cerebroid ooids [18]. Other evidence includes scanning electron microscope observations of cerebroid ooids, which have found a large number of microbial fossils in the ooid interior and surrounding mud crystal matrix, and that the cortex has an elemental composition similar to that of biofilm calcification products [19].…”