undeRstanding coral thErmal bleaching threSholds during past InterglaciaL extremes: Insight into thermal strEss dyNamics on tropical Coral reef Ecosystems (RESILIENCE)

Tropical and subtropical coral reefs are globally distributed biodiversity hotspots presently threatened by unprecedented environmental forcing. Their ecological functioning and ultimate survival depend on their sensitivity to climatic perturbations and in addition to anthropogenic forcing. One of the main visual signs for thermal stress is bleaching, a predominant stressor on reefs, i.e. the loss of the photosynthetic symbionts that live within their tissues. The increase in magnitude, duration and frequency of coral bleaching events over the last few decades has seriously compromised the resilience of these vulnerable ecosystems. Future bleaching events are predicted to become even more frequent and, therefore, will likely not ensure reef persistence in the Anthropocene. The best approach to gain insights to how global climate change will impact reef ecosystems is to learn from the past. However, in the fossil record, bleaching cannot be observed! The Earth’s paleo-climatic record found in deep sea sediments adjacent to coral reefs can provide us with relevant information on multiple prospective analogues for modern times. In particular, some interglacial periods in the Earth’s history were characterized by climatic conditions similar to modern-day marine environmental conditions, whereas others were warmer. We will investigate deep-sea sedimentary records in basins adjacent to coral reef-bearing carbonate platforms from the Atlantic (IODP Leg 166-Bahamas), Indian (IODP Expedition 359-Maldives) and Pacific Oceans (ODP Leg 130-Ontong Java Plateau; ODP Leg 133 and IODP Leg 194-Marion Plateau; Expedition 320-Tahiti); in the interval spanning the Present to Marine Isotope Stage (MIS) 31 in the Pleistocene (a time span of around 1100 kyr) and compare these with their shallower sedimentary counterparts.

The overarching objective of this research is to evaluate the magnitude and frequency of interglacial periods characterized by anomalously high temperatures that may have produced coral bleaching in the past and to test if nutrients have also contributed to this process. This objective will be achieved by an initial assessment of the modern benthic habitat in the target regions, and evaluating photo-inhibition caused by photo-oxidative stress using the large benthic foraminifer Amphistegina (which similarly to corals can undergo bleaching).

This project contributes to the understanding of climate dynamics in tropical and subtropical regions, and to understand the cause of the survival and or decline of coral reefs, which is presently a serious global concern. An additional contribution of this project is to highlight the importance of the Swiss access to IODP samples recovered during past expeditions and stored in core repositories around the world.