Carbonate spherulites in a Lower Cretaceous hydrocarbon seep (Outer Carpathians, Poland)

Carbonate spherulites are spherical to sub-spherical grains showing a radial internal structure. Their origin represents an intriguing topic in sedimentology, with significant ramifications in petroleum geology and industrial processes. Carbonate spherulites not only represent a sedimentological conundrum, but they may hold vital information in deciphering depositional settings and early diagenetic transformations with important implications in basin analysis and hydrocarbon exploration. Spherulites are known from diverse sedimentary settings, but they have not yet been characterized in detail from cold seep environments. Spherulites occurring within thrombolites from a Lower Cretaceous hydrocarbon seep in the Outer Carpathians (Poland) were investigated for their mineralogy, petrography and geochemistry. The spherulites, sub-millimetric to 10 mm in diameter, consist of radially-arranged fibrous calcite crystals with sweeping extinction which replaced a sediment originally consisting of microspar-cemented peloidal micrite grains. The original peloidal texture is preserved as organic relics, with the peloids showing pseudo-pleochroism. A clear fascicular-optic calcite rim forms the outer part of the spherulites, in optical continuity with the radiating crystals replacing the original peloidal micrite grains. Carbonate precipitation of both the microspar-cemented peloidal micrite and the fascicular-optic calcite rim is related to anaerobic oxidation of methane (AOM), as confirmed by their ¹³ C-depleted values (as low as -36.7 ‰ relative to the Vienna PeeDee Belemnite). Whilst carbonate precipitation via AOM was microbially-mediated, abiotic processes such as physical mobilization and/or chemical corrosion were also involved in the formation of the spherulites. This work provides a novel example of carbonate spherulites from a Lower Cretaceous hydrocarbon seep, and proposes a model for their formation potentially useful to shed new light on spherulite growth in other sedimentary environments.