Turnover of corals and reef ecosystems during the Early Cenozoic hyperthermal events, with a focus on the Paleocene-Eocene thermal maximum (~56 Ma)

The early Cenozoic was a time of major environmental and evolutionary change, especially for shallow water carbonate ecosystems. Of particular importance is the major global carbon-cycle perturbation known as the Paleocene-Eocene Thermal Maximum (PETM) that occurred approximately 56 million years ago. The release of greenhouse gasses in the late Paleocene and early Eocene led to increased temperatures, ocean acidification, as well as increased weathering in coastal environments. On carbonate platforms globally, Paleocene coral patch-reefs were replaced by non-metazoan reef-builders by the early Eocene. In spite of apparently inhospitable conditions and a phase-shift in reef-building taxa, there was no major mass extinction of shallow benthic taxa (e.g. corals and large benthic foraminifera) on carbonate platforms. The goals of this research are to understand how climate and environmental change in the early Cenozoic impacted diversity and structure of reefs, and how reef-dwellers were able to survive. This research shows that during the late Paleocene, coral reefs on the Adriatic Carbonate Platform (Slovenia) were replaced by microbial reefs, likely due to increased temperatures and turbidity. The microbial reef communities are statistically similar to those they replace, showing that losing a keystone species (i.e., corals) did not impact overall community structure. The largest change in community composition happens at the Paleocene/Eocene boundary. Petrography, sedimentology, and geochemistry are used to analyze a truncated surface at the Paleocene/Eocene boundary at a site in Slovenia, hypothesized to be caused by localized ocean acidification during the PETM. Additionally, paleontological databases are analyzed to show that corals with particular physiological traits or behaviors were associated with survival or low extinction rates during the PETM. Analysis of databases also confirms that coral reefs with more integration of coralline algae over their 156 million year evolutionary history have higher diversity and stronger framework. This research is important from a paleontological perspective because it reports quantitative data from poorly studied environments during an important interval of Earth history. It is also significant for reef conservation because it provides clear examples of corals and reefs that have been resilient through past crises, either through physiology and life history or community associations.