OOLITES AS MARKERS FOR GLOBAL CHANGE IN THE OCEAN-ATMOSPHERE SYSTEM

Geographically widespread oolite formations are excellent marker beds for global environmental changes in the ocean-atmosphere system. They are often associated with ‘pre-Cambrian-style’ facies such as wrinkle structures, flat-pebble conglomerates and microbialites underlining their importance as indicators for unusual depositional environments, biodiversity crises, and demise of shallow-marine ecosystems. The ultimate control on oolite formation, however, remains unresolved. Nonetheless, there is a number of oceanic and atmospheric changes that recurrently are closely associated with oolite formation and which may be useful for interpreting the significance of these intriguing deposits on a broader environmental scale: 1. Continental weathering and marine biogeochemistry: Presence of oolites signal an overturn in the Ca²⁺ and CO₃^2- ion concentrations (weathering rate), a regime shift in the marine CaCO₃ cycle, and a short-term supersaturation pulse in the ocean. 2. Ecology and biodiversity: Thin but widespread oolites cap strata yielding reefs and high-diversity skeletal grain associations in carbonate platforms of various ages. In this context, the oolites are ‘dead sediments’ and closely associated with the time period of the lowest marine biodiversity during extinction events (Strangelove Oceans). Such occurrences are a conspicuous feature of for example the end-Ordovician mass-extinction, Silurian bioevents, and the end-Permian mass extinction 3. Sea-level change: Sequence stratigraphical principles and literature data suggest that most oolites are of transgressive origin and, thus, must form when skeletal carbonate production is suppressed or clastic source areas are drowned. 4. Global carbon cycle: Many Palaeozoic oolites are associated with perturbations in the short-term carbon cycle, and thereby with the exchange of carbon between the oceanic and atmospheric carbon sinks. In conclusion, the close relationship between widespread oolites and oceanic overturn, biodiversity decline, sea-level change, and carbon cycle anomalies make them excellent as markers for complex and often interrelated global environmental changes in the ocean- atmosphere system. In this presentation we will highlight the significance of Hirnantian oolites from Baltoscandia, Laurentia and South China.