Paleoecology of upper Triassic reef ecosystems and their demise at the Triassic-Jurassic extinction, a potential ocean acidification event

The Triassic, 251-201.3 million years ago (Ma), is considered to be the period in Earth history when modern-style reefs, constructed by scleractinian corals (modern stony corals) evolved. Modern corals began calcifying (building a stony skeleton) in the Middle Triassic (245--241 Ma) and reefs built by these newly evolved corals and hypercalcified sponges began to appear in the fossil record, becoming large, extensive structures through the Late Triassic (235--201.3 Ma). This dissertation focuses on the ecology of these new reef ecosystems in the Late Triassic, particularly the zonation of reef constructors, organisms, and ecosystems, as well as how Upper Triassic reefs varied across the globe and through time. Five Upper Triassic reef localities were investigated (three from North America and two from Europe) and these reefs show distinct differences between regions. The southernmost North American reef (Nevada) is built by plate-like stony corals and other coral forms similar to modern reef corals. The reefs from British Columbia and Oregon are very different in structure and composition and were constructed by microbial encrusters with corals, sponges, and algae being only minor reef builders. This difference is likely due to the latitudinal gradient of the reef localities, with the northern temperate waters (British Columbia and Oregon) being too cool to support robust coral assemblages. The paleotropical Austrian reefs are both coral-sponge reefs. The pinnacle reefs of the Tennengebirge are built by tall, narrow phaceloid corals that exhibit phototaxis, which implies that the pinnacle corals had zooxanthellae (symbionts). The Gosausee reef has a complete cross section of the barrier reef preserved, making Gosausee one of the few outcrops that allows a transect from the deep sediments below the reef, up through the reef complex, and into the back reef and lagoon.;This dissertation also addresses the reef demise and extinction at the Triassic--Jurassic (T--J) boundary (∼200 Ma), which has been hypothesized as an ocean acidification event driven by the eruption of the Central Atlantic Magmatic Province (CAMP) flood basalts. The evidence for ocean acidification at the T--J boundary and the feasibility of an acidification event are discussed, and a novel proxy that utilizes the acid sensitivity of corals and coral reefs to detect paleocean acidification is proposed. The eruption of CAMP released massive amounts of carbon dioxide extremely rapidly (>5000 Gt of C as carbon dioxide in <600,000 years) and there is evidence for a gap in carbonate sedimentation across the T--J boundary consistent with ocean acidification (dissolution of shelf sediments due to low seawater carbonate saturation states). Additionally, the T--J extinction is particularly selective against pH-sensitive organisms, suggesting that ocean acidification, if not the key cause was certainly an important extinction factor. The new proxy proposed here combines the extreme sensitivity of corals to carbonate saturation (a measure of ocean acidity), with carbon dioxide concentrations to calculate the sensitivity of the Late Triassic Ocean to an acidification event. The sudden carbon dioxide injection at the T--J boundary could have caused the disappearance of coral reefs by acidifying the ocean to intolerable levels for these organisms.