OCEAN ACIDIFICATION: INSIGHTS FROM THE PALEOCENE-EOCENE THERMAL MAXIMUM (PETM)

Throughout the Anthropocene, CO₂ has been released to the atmosphere as a by-product of human activities, mostly through fossil fuel burning. A fraction (~30%) of this anthropogenic CO₂ has been absorbed by the oceans and decreased the pH, carbonate ion concentration and the saturation state of marine waters with respect to carbonate minerals, a phenomenon called ocean acidification (OA). OA events are recurrent in Earth’s history, including at the Paleocene-Eocene Thermal Maximum (PETM) and the Cretaceous-Paleogene boundary.

A 100m section of well-preserved, shallow-water carbonates that crosses the PETM was sampled at Campo, Spain. The stratigraphy, sedimentology and geochemistry of these rocks display changes that can be attributed to OA. The stable carbon isotope (δ¹³C) signature of these rocks reveals a large negative excursion. This excursion is interpreted as resulting from the massive release of depleted carbon to the atmosphere during the formation of these rocks and the potential cause of an OA event. Thin sections of these rocks show a variety of dissolution patterns around and in bioclasts composed of high-magnesium calcites and aragonite, metastable carbonate mineral phases. The bulk geochemistry of the rocks reveals the loss of Mg²⁺ and Sr²⁺ across the boundary, which may be interpreted as the dissolution of high-magnesium calcites and aragonite and the preferential preservation and/or precipitation of low magnesium calcites. These observations appear to document the impact of an OA event on the sediment record and provide insights into the response of modern carbonate shelf sediments to the current, anthropogenically-driven OA event. Additional analyses, including stable Sr isotope measurements and scanning electron microscopy will be carried out and presented to better quantify the extent of the OA event at Campo.