Authigenic calcium carbonate precipitation in the "bathtub ring" around the anoxic Alum Shale Basin during the Furongian SPICE event (Baltic Basin, northern Poland)

The precipitation of both biotic and abiotic calcium carbonate is of great importance in modern and ancient global biogeochemical cycles. In the present-day oceans, the widespread precipitation of inorganic CaCO3 on the seafloor or in the water column is possible only under extraordinary circumstances. By contrast, in the geological record, authigenic seafloor carbonate cements were widespread in the supersaturated, anoxic oceans of the Precambrian. Widespread authigenic carbonate precipitation ceased by the end of the Neoproterozoic as a consequence of global oceanic oxygenation: in the Phanerozoic, it occurred only during major anoxic events (for example, at the Permian/Triassic boundary) or in restricted, stagnant basins. Here, we present an anomalous record of CaCO3 precipitation from the Cambrian Alum Shale Basin of the Baltica palaeocontinent with Precambrian-like authigenic, seafloor encrusting, crystalline carbonates. The depositional environment of this well-recognized, cool-water, stagnant anoxic basin favoured local carbonate precipitation via the surplus generation of alkalinity in anoxic bottom waters. However, the correlation of the acme of authigenic carbonate formation with the onset and peak of the Steptoean Positive Carbon Isotope Excursion (SPICE) suggests a driver-trigger relation between the two phenomena. On a smaller scale, carbonate authigenic precipitation is manifested by a cement-supported texture of the limestone intercalations in the Alum Shale facies. Instantaneous calcification of the faunal remains, termed here the Snedronningen phenomenon, must have been of great importance in the formation of Orsten-type Konservat Lagerstatten.