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

Abstract

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.

Planktic foraminifera assemblage composition and flux dynamics inferred from an annual sediment trap record in the Central Mediterranean Sea

“…Firstly, the Mediterranean Sea is supersaturated with respect to calcite (Álvarez et al, 2014;Millero et al, 1979) and the depth of the studied material is substantially shallower than the calcite saturation horizon (Álvarez et al, 2014). Secondly, recent work suggests that calcite experiences little to negligible changes in the water column and burial in recent sediments (Béjard et al, 2023;Pallacks et al, 2023). All this evidence suggests that dissolution played a negligible role in the preservation of planktonic foraminifera preserved in the sediment record in the study region.…”

“…As marine calcifying organisms, they are considered particularly vulnerable to the ongoing ocean warming and acidification (Bijma et al, 2002;Fox et al, 2020). Shell calcification of several foraminifera species has been showed to decrease in response to ocean acidification, and therefore, changes in the weight of their shells are considered an indicator of the ocean acidification impact on different timescales (Béjard et al, 2023;de Moel et al, 2009;Fox et al, 2020;Kroeker et al, 2013;Moy et al, 2009;Pallacks et al, 2023). In contrast, ocean warming has been proposed to produce an opposite effect on foraminifera calcification, as some studies have documented that an increase in water temperature results in larger shells and enhanced growth rates (Lombard et al, 2011(Lombard et al, , 2009Schmidt et al, 2006).…”