BENTHIC FORAMINIFERAL RESPONSE TO OCEAN ACIDIFICATION DURING THE PALEOCENE-EOCENE THERMAL MAXIMUM

Calcifying marine organisms commonly respond to lowered carbonate saturation associated with higher atmospheric pCO₂ levels through reduced calcification. We used a novel technique (Synchrotron Radiation X-ray tomographic microscopy, SRXTM) to assess the response of non-symbiont bearing, deep-sea benthic foraminifera to ocean acidification during the Paleocene Eocene Thermal Maximum (PETM, ~55 Ma), arguably the best analog for future global warming in the geological record, and the less severe Eocene Thermal Maximum 2 (ETM-2 ~53.2 Ma). In contrast to expectation and very limited experimental data on living foraminifera, calcification in Oridorsalis umbonatus and Nuttallides truempyi (an infaunal and an epifaunal species), as measured in volume-normalized amount of calcite deposited, increased during the PETM. There were no significant changes in calcification during the less severe ETM2. It is difficult to explain the increased calcification during the PETM in non-symbiont bearing calcifyers. It might be due to the increased metabolic rates due to warming of the oceans, or there might be variation in carbonate saturation in pore waters due to dissolution of carbonate sediment at times when the lysocline moves upwards through the water column (i.e. acidification). We thus document that the response of biota to ocean acidification may be complex, and the geological record is important for the understanding of potential future responses to ocean acidification, as well as estimating the levels of carbon-emission which will affect biota and ecosystems significantly.