Lower Maeotian Bryozoan Bioherms of Cape Kazantip, Crimea: A New Concept of the Paleoecological Environment of Their Origin

The lower Maeotian carbonate encrustation of bryozoan bioherms and local problematic carbonate buildups at Cape Kazantip (Kerch Peninsula) were studied to elucidate their genesis. Analytical (lithological and mineralogical, X-ray diffractometry, scanning electron microscopy, energy dispersive spectrometry, and isotopy) studies have shown that hardness of the bryozoan framework is related to the syndepositional, biologically induced cement around bryozoans and carbonate encrustation of bioherms. In addition to fossilized traces of products of the microbiotic vital activity (bacteriomorphic structures, mineralized biofilms, glycocalyx—an exopolymer substance (IPS), and framboidal pyrite), the carbonate crusts on bryozoan bioherms and mollusk–polychaete minibioherms contain abundant bitumen, strontianite, barite, celestine, Mn-rich calcite (kutnohorite), Mg-calcite, aragonite, dolomite are widespread. Mineralized biofilms include trace elements Fe, Si, Mg, Al, K, Na, Cl, Ba, S, Ni, and Co. The isotopic composition of different carbonate rock types is marked by wide variations of carbon (–2.76…7.17‰) and oxygen (24.20–33.01‰) and manifested in fluctuations of water salinity (16.67–39.74‰). The chemical composition and mineral specificity of rocks, confinement of carbonate crusts and minibioherms to saline waters, and local pattern of their formation suggest the manifestation of near-bottom cold gas-fluid seeps, probably, of a complex chloride-sodium-sulfate-magnesium composition or various modifications of these components in a shallow sea basin, whereas the bryozoan biohermal complex is most likely a near-hydrothermal oasis.