How to utilize fossil corals for paleoenvironmental reconstruction? – Concepts, challenges and perspectives –

Scleractinian reef corals are excellent climate archives, frequently used for paleoclimate and paleoenvironment reconstructions. Reef corals capture environmental signals during the growth of their aragonite skeleton and can therefore record local variations in sea surface temperature (SST), light availability and water chemistry over decades. Thus, fossil corals hold a unique potential to provide insights into past climate. However, the application of ancient corals requires a careful assessment of the preservation state of their aragonite skeleton to identify diagenetic effects in proxy data.

Here, we present sub-annually resolved sclerochronologies of coral calcification and geochemical proxy data of zooxanthellate corals (Astreopora sp.) from the Eocene Greenhouse (Bartonian, 40 Ma) in northwestern France (Paris Basin) to investigate variable calcification responses to growth site specific environmental conditions. During the Bartonian, the Paris Basin embayment was located at a paleolatitude of about 45°N, which marks the northernmost range limit for reef coral growth in the Cenozoic era. Our results indicate that the corals were subjected to increased environmental stress, which is reflected in overall low skeletal growth rates (1.3 ± 0.6 mm/year, n = 15), the frequent occurrence of lesions and high density stress bands in the coral skeletal structure as well as ambiguous cycles of temperature sensitive geochemical proxy data.

This study provides implications on the adaption of calcifying organisms to unfavorable growth conditions including high seasonal contrasts (SST variability and light availability) and critical ocean water chemistry (freshwater inputs, evaporation, and seawater carbonate chemistry affecting Ω_arag).