Project 16 (WP4): Towards a better proxy calibration for improved T°C and pH reconstructions in corals’ paleoclimatic studies: Case study of the impact of seawater acidification on coral larvae and boring microflora.

Project lead: Delphine Dissard, Aline Tribollet

Post-doctoral researcher: Henry Wu

Project Start/End : June 2015 – November 2016 (extension May 2017)

Context: Global climate change and ocean acidification are one of the most pressing issues in Earth Sciences today. This is supported by the growing scientific evidence that changes in ocean chemistry due to anthropogenic input are already occurring and will continue in the next decades to centuries at a rate that will depend on future CO2 emissions. Temperature and carbonate chemistry of the oceans are critical parameters that not only control chemical and physical processes, but also a wide range of biogeochemical processes that are important for the development and survival of marine biota such as corals, which are the main framebuilders of coral reef ecosystems. These organisms are highly threatened by global change and ocean acidification, as well as the associated eco-systemic resources (1/15 of the world population depends on these resources). Recent studies suggest that net reef dissolution will occur by 2100 as corals will calcify less and one of the main agents of carbonate dissolution, the microboring flora, will be stimulated. It is therefore crucial to better understand and quantify the impacts of the rapid decrease in pH and increase in T°C on living corals from the early stages of calcification and the role of boring microflora on coral dissolution in order to predict the fate of reef ecosystems.

Geochemical proxies preserved in the carbonate skeleton of marine calcifying organisms such as corals provide a unique tool to reconstruct changes in seawater environmental parameters since the beginning of the industrial era. Many parameters however might impair high-resolution time-series (decadal to centennial) like vital effects and environmental stressors, which can bias geochemical signatures in corals and therewith paleo-reconstructions. It is thus essential to identify the biogeochemical processes involved in the incorporation of trace elements and isotopic signatures at the early stage of the aragonitic skeleton formation in corals as well as possible diagenesis effects. For this purpose, interactions between corals, microborers, and their environment need to be better assessed to fully constrain the reliability of coral skeletons as natural archives for climatic signals and better understand the impact of ocean acidification on boring microflora, one of the main agents of coral reefs dissolution.

Description of workwork: Here, we propose to develop a mechanistic understanding of elemental and isotopic composition of coral skeleton - with special emphasis on Li/Mg, Sr/Ca and δ11B - at different stages of coral life cycles (i.e., post-larvae, and adults), in stressed and impaired corals (e.g., coral grown under ambient vs pCO2 predicted for 2100) and to study the effects of pH on microboring flora development in living corals. In fine, this project will intend to derive knowledge from these experiments in order to establish simultaneously a temperature and pH record, and microboring flora successions, of a coral core from the western equatorial Pacific Ocean covering the last 600 hundred years. This core will therefore allow studying pre-industrial (low atmospheric CO2) and post-industrial (high atmospheric CO2) environmental impacts on coral skeleton (growth rate, geochemistry, diagenesis) and associated boring microflora (species, abundance, rates of dissolution).

Position offer and results (January 2017 update)