A Miocene Climatic Optimum Tropical Barrier Reef: Combining biomarker and facies analysis to understand conditions of reef survival.

One of the key questions is how and if large complex reef systems can survive under future climate change scenarios. It is known that during the Middle Miocene, there were extensive reefs across the Indo-Pacific. Since the Middle Miocene was a time of global temperatures and CO₂ similar to end-of-century predictions, this is an ideal time interval to study. However, there has been a lack of multi-million-year proxy records from these ecosystems for the Miocene. For the first time, we present an SST record from a site (IODP U1464) near the large Miocene Barrier Reef on the NW Shelf of Australia. In this study, we use biomarkers to reconstruct oceanic conditions and look at the changing environments from a carbonate facies perspective. Our research shows that the reef system might have initiated at temperatures that were close to modern summer values in the region. However, during the height of the Miocene Climatic Optimum, when summer temperatures reached 34°C, the reef system was at its maximum extent. Arid conditions, heat-tolerant corals, and a low local subsidence rate allowed the barrier reef to tolerate the hot summer conditions. When the sea level dropped, and sabkhas covered the site during the Middle Miocene climate transition, cooler SSTs allowed for faster reef growth. However, starting at 12 Ma, higher summer SSTs and increased local subsidence led to the end of the barrier system and the continuous collapse of the reef between 11-7 Ma. This suggests that a reduction in stressing conditions is critical for reef survival in a warmer world. Predictions for rapidly changing oceanic conditions coupled with sea-level rise indicate that future climate change will harm modern large complex reefs, much like during the Late Miocene.