Interpreting phosphatic microspheres in dysoxic clays: an example from the Jurassic (Upper Oxfordian) of the Central Russian Sea

Minute phosphatic microspheres occur within anoxic black shales of the Upper Oxfordian of the Central Russian Sea deposited in relatively deep and calm water. The chemical composition of the examined microspheres is most consistent with carbonate-substituted fluorapatite (commonly referred to as francolite or dahllite) rather than with stoichiometric, end-member fluorapatite. These microspheres (0.23-0.30 mm in diameter) exhibit an exceptionally smooth and lustrous outer surface. They are nearly perfectly spherical, with only a few specimens displaying slight indentations or irregularities. Small depressions resemble plastic deformation features. Considering the Lagerstätte-type preservation conditions, they are most likely phosphatic steinkerns of the chlorophycean Tasmanites phycomata, less likely egg capsules, phos-phatized embryos, or exceptionally large acritarchs. The abiotic origin of phosphatic microspheres is unlikely, given their near-uniform size and especially microstructure, suggesting that their external crust formed on the inner surface of an organic cyst.