Oligocene-Miocene reef coral growth at its northernmost limits (Aquitaine Basin, France)

The globally warm Miocene epoch is considered a model case for the near future climate. In view of the current coral reef crisis, studies on Miocene reefs can contribute to predictions regarding the stress resilience of today's reefs against global warming.

Here, we present stable isotope (δ¹⁸O, δ¹³C) and biomineralization (annual extension rate, skeletal density, annual calcification rate) records of massive reef corals of Upper Oligocene to Middle Miocene age (28 – 15 Ma) from the Aquitanian Basin (SW-France). The Aquitaine Basin formed a large, shallow-marine embayment of the North Atlantic at a paleolatitude of ~40°N, making this one of the northernmost reef coral environments at that time.

We find very low extension rates, typically indicative of low sea surface temperatures (SSTs), which are consistent with a habitat at the outer margin of the Oligocene-Miocene reef belt, but contradicting with measured low bulk densities, typically indicative of warm SSTs. According to clearly expressed annual δ¹⁸O and δ¹³C cycles, maximum skeletal growth and zooxanthellate photosynthesis occurred in winter – not in summer. This is surprising, since modern high-latitude reef corals preferentially grow in summer due to critically small radiation doses for zooxanthellae photosynthesis in winter. As published Miocene TEX₈₆-based SST reconstructions are critically high for reef corals, we currently test the hypothesis that high SSTs in summer were more limiting to reef coral growth than a low dose of photosynthetically available radiation in winter. Our findings tentatively suggest mid-latitudes may indeed represent refugia for reef corals in a warmer world.