Skeletal assemblage fluctuations within persistent Mediterranean coral-reef framework during Cenozoic warming events

Abstract

Coral reefs face severe threats from global warming and pollution, yet predicting their long-term response is hindered by the limited duration of ecological observations. This study investigates the geological record to examine the sedimentary and biotic composition of three Cenozoic reef systems developed during major warming events: the Early Eocene Climatic Optimum (EECO), the Late Oligocene Warming Event (LOWE), and the Middle Miocene Climatic Optimum (MMCO). We performed quantitative analysis of 247 thin sections from Monte Postale (NE Italy, EECO), Castro Limestone (SE Italy, LOWE), and Dağpazarı (S Türkiye, MMCO), focusing on shallow-water facies and organic buildups. Results show that while all intervals supported framework reefs with moderate-to-high coral diversity, the carbonate factory compositions varied significantly. The EECO reef system is characterized by abundant foraminifera and reduced coral contribution, reflecting a crisis in coral production likely driven by extreme temperatures. The LOWE reef system displays a peak in coral abundance, favored by less extreme temperatures, low nutrient levels, and optimal Mg/Ca ratio. The MMCO reef system shows a slight decline in corals compared to the Oligocene, with a concurrent increase in red algae and heterotrophs potentially linked to carbon cycle perturbations. These results are backed by data on the stratigraphic distribution of coral-dominated facies, an independent proxy of coral carbonate production. Overall, this highlights the resilience of the reef factory, which, as testified by the textural characteristics of the investigated limestones and by the persistence of the coral dominance in the buildups, maintains its overall structural integrity under diverse warming scenarios, being compromised only by extreme thermal anomalies or by a combined effect of multiple stressors.