THE IMPACT OF OCEAN ACIDIFICATION ON FORAMINIFERA IN MODERN SEAS AND ITS RECOGNITION IN THE GEOLOGICAL RECORD

With increasing atmospheric CO₂ the oceans are becoming more acidic, with the fall in carbonate ions beginning to cause dissolution in numerous invertebrate groups, including foraminifers, pteropods, heteropods and calcareous nannoplankton. Our work in the Mediterranean Sea, Gulf of California and Caribbean Sea has shown how modern assemblages are responding to acidification. Around Ischia (Italy) shallow sea floor CO₂ vents create a low carbonate environment; at a pH 7.8 the assemblage of benthic foraminifera is less diverse than at ambient pH 8.2 and below pH 7.6 there are no calcite-secreting benthic foraminifera, only agglutinated taxa. In the Gulf of California foraminifera are also affected by deeper CO₂ vents, although high carbonate saturation ensures that calcite-secreting foraminiferids are able to live and reproduce in relatively low pH environments, only becoming impacted by dissolution effects once dead.

Using data from the Triassic/Jurassic, Cretaceous/Paleogene and Paleocene/Eocene boundaries, it is possible to consider the effects of previous acidification events, both in near-surface and in deeper-water environments caused by the migration of the Carbonate Compensation Depth (CCD) as a result of reduced productivity. In the Lias Group sediments of the Wessex Basin, in the relatively high CO₂ environment created by the Central Atlantic Magmatic Province (CAMP) volcanism, both ostracods and aragonitic foraminifera dominate shallow water marine assemblages, with any acidification limited to the surface waters. In Texas and Alabama, the Cretaceous/Paleogene boundary successions, which were deposited in a relatively shallow water setting, contain benthic foraminifera that record no direct evidence of ocean acidification despite the proximity to the Chicxulub impact site and the proposed source of some of the CO₂ (in addition to that from the Deccan Volcanic Centre in India) required to cause acidification. The planktic foraminifera, calcareous nannofossils and juvenile ammonites were, however, decimated. Interpretation of biotic changes during global bioevents is complicated by the changing nature of the oceans through time, which switched from being aragonitic to calcitic a number of times during the Phanerozoic.