Slow-growing reef corals as climate archives: A case study of the Middle Eocene Climatic Optimum 40 Ma ago

Abstract

The skeletons of stony corals on tropical shallow-water reefs are high-resolution climate archives. However, their systematic use for unlocking climate dynamics of the geologic past is limited by the susceptibility of the porous aragonite skeleton to diagenetic alterations. Here, we present oxygen and carbon isotope time series (monthly resolution) from reef corals with an unusual unaltered preservation from the Middle Eocene Climatic Optimum (MECO) “hyperthermal” (40 million years ago). Annual extension of the corals at the studied midlatitude site (France) was remarkably low (0.2 cm). Nonetheless, isotope signatures display no evidence for kinetic disequilibria that discredit their use as climate archive, but growth rate–dependent annual signal amplitude attenuations need corrections using an innovative sampling approach. Thereafter, we present evidence of symbiotic zooxanthellae in reef corals of the Paleogene and subdued sea surface temperature seasonality of only 7° to 8°C during the MECO, consistent with the globally equant climate of the hothouse.

Astronomical Pacing of Middle Eocene Sea‐Level Fluctuations: Inferences From Shallow‐Water Carbonate Ramp Deposits

“…Our study provides evidence for small amplitudes of third order sea-level changes <10 m in concert with orbital insolation, but a strong, eustatic sea-level pulse of ≥30 m during the MECO unrelated to insolation forcing. In the light of a drop in surface ocean pH of ∼0.2 during the MECO (Henehan et al, 2020) and spectacular skeletal preservation of many PB fossils (Brachert et al, 2022;Merle, 2008), our findings suggest the PB record to be highly suited as a natural laboratory for future studies on the effects of ocean warming and acidification on skeletal calcification patterns of shallow marine biota. 3).…”

“…Thus, the alleged misfit of the age model presented with the 2.4 Myr astronomical pattern is compatible with a telluric climatic forcing dominating the climate system of the MECO in the short term and overriding the effect of the astronomically controlled insolation changes. Given a drop in surface ocean pH of ∼0.2 during the MECO (Henehan et al., 2020), the spectacular skeletal preservation of many PB fossils (Brachert et al., 2022; Merle, 2008) may therefore allow us to examine the impact on epicontinental shelf ecosystems. In particular, currently it is unclear whether the MECO surface ocean was buffered at constant carbonate saturation state by silicate weathering (Henehan et al., 2020), or whether a reduced weathering feedback (Krause et al., 2023; van der Ploeg et al., 2018) meant that this drop in ocean pH also coincided with a drop in ocean carbonate saturation, and thereby “true” ocean acidification (Hönisch et al., 2012).…”

“…A highly diverse fossil fauna with very abundant small and larger benthic foraminifers, bryozoans, mollusks, echinoids, crustaceans, together with calcareous green algae (Dasycladaceae) documents a shallow‐water environment with extensive seagrass beds with ≤10 m depth (Gall, 1983). A diverse reef coral association (≥9 genera, e.g., Astreopora , Goniopora , Acropora ), again without evidence of reefal frameworks, documents a new phase of colonization of the gulf by reef corals (Brachert et al., 2022; Gall, 1983; Mégnien & Mégnien, 1980; Wallace & Rosen, 2006; White, 2013).…”