Anthropogenic acidification of surface waters drives decreased biogenic calcification in the Mediterranean Sea

Anthropogenic carbon dioxide emissions directly or indirectly drive ocean acidification, warming and enhanced stratification. The combined effects of these processes on marine planktic calcifiers at decadal to centennial timescales are poorly understood. Here, we analyze size normalized planktic foraminiferal shell weight, shell geochemistry, and supporting proxies from 3 sediment cores in the Mediterranean Sea spanning several centuries. Our results allow us to investigate the response of surface-dwelling planktic foraminifera to increases in atmospheric carbon dioxide. We find that increased anthropogenic carbon dioxide levels led to basin wide reductions in size normalized weights by modulating foraminiferal calcification. Carbon (δ13C) and boron (δ11B) isotopic compositions also indicate the increasing influence of fossil fuel derived carbon dioxide and decreasing pH, respectively. Alkenone concentrations and test accumulation rates indicate that warming and changes in biological productivity are insufficient to offset acidification effects. We suggest that further increases in atmospheric carbon dioxide will drive ongoing reductions in marine biogenic calcification in the Mediterranean Sea.

We are grateful to Manuel F. G. Weinkauf, Laura Haynes and an anonymous reviewer for their helpful comments and suggestions. We thank the captain and crew of the Spanish R/V Angeles Alvarino, and the researchers as part of the MedSeA cruise for supporting the sampling of this study. We also thank Marta Casado, Yolanda Gonzalez-Quinteiro and Natalia Bravo for laboratory assistance, and Joan Manuel Bruach for his work on the analysis of 210Pb. This work contributes to the ICTA-UAB “Unit of Excellence” (FPI/MDM-2015-0552- 16-2; CEX2019-000940-M) and was funded by the Spanish Ministry of Science and Innovation, BIOCAL Project (PID2020-113526RB-I00), the EU-FP7 “Mediterranean Sea Acidification in a Changing Climate” project (MedSeA; grant agreement 265103), and the Generalitat de Catalunya (MERS, 2021 SGR 00640). J.W.B.R. acknowledges the European Research Council under the European Union’s Horizon 2020 research and innovation program (grant agreement 805246) and B.M the Severo Ochoa grant CEX2018-000794-S and CSIC LINKA20102. G.L. acknowledges funding from the Spanish Ministry of Universities through a Maria Zambrano grant.

Peer reviewed