Resilient coral reef ecosystems: The case study of turbid-mesophotic coral buildups during the Late Oligocene Warming Event (Tertiary Piedmont Basin, NW Italy)

Nowadays, but also in the geological record, coral communities living in marginal conditions (i.e. low light, high turbidity, extreme temperatures, high nutrients and sediment input) are quite common and there is evidence that some of these reefs, often associated with mesophotic environments, may be more resilient to the current global warming trend, thus serving as possible ecological refugia. As an attempt to understand the response and resilience to past warming events of marine calcifying organisms, such as zooxanthellate corals of marginal reefs, herein we reconstruct the palaeoenvironment of an Oligocene mixed siliciclastic‑carbonate system in the Tertiary Piedmont Basin (NW Italy), where a suite of coral assemblages formed small buildups at the edge of a coarse-grained delta system. Geochronological dating by strontium isotope stratigraphy places the coral buildups in the middle Chattian, thus within the Late Oligocene Warming Event (LOWE): a crucial climatic event for coral reefs as characterised by substantial warming coincident with declining atmospheric CO2. The depositional model that we propose is ascribed to a fan-delta system, deposited within a narrow valley cutting through the metamorphic basement, where small and laterally discontinuous coral buildups grow-up in the prodelta setting, thus suggesting a mesophotic environment. In this context, the coral buildups appear to have been intermittently deactivated or smothered by fine-grained terrigenous input during flood phases or shifts in distributary channels until their final suffocation by fluvial sediments input. Within the coral buildups, the four distinguished coral facies (branching Stylophora floatstone, Acropora-Stylophora floatstone/rudstone, Goniopora/Caulastraea pillarstone, and mixed-coral domestone) are characterised in general by sediment-resistant corals and by specific taphonomic features. These coral facies shifted in time and space during the LOWE, highlighting the noteworthy adaptability and resilience of Oligocene corals to a complex interplay of environmental stressors such as turbidity, hydrodynamic energy, sediment and nutrient supply within a mesophotic setting.