CALCAREOUS MICROFOSSIL RECORDS OF THE PALEOCENE-EOCENE THERMAL MAXIMUM: A "CRYSTAL BALL" FOR GAUGING FUTURE BIOTIC CHANGE CAUSED BY GLOBAL WARMING AND OCEAN ACIDIFICATION?

Continued input of anthropogenic carbon into the ocean/atmosphere system is fueling global warming and steadily acidifying the surface layer of the modern ocean. Hence, deep-sea records of past global warming events are garnering increased attention as researchers strive to constrain the potential effects of human activities on the ocean. Of particular interest is an ancient (~55 Ma) global warming event known as the Paleocene-Eocene thermal maximum (PETM). On average, the global ocean warmed above background levels by ~5 ºC during the PETM. In deep-sea cores the PETM is recognized by an abrupt (several 10³ years) 3‰ decrease in the d¹³C composition of inorganic carbon and pervasive carbonate dissolution. These two hallmarks signify the rapid release of vast quantities (1,200 to 4,000 Gt) of isotopically light carbon. Oceanic uptake of this excess carbon triggered a pH buffering response that lowered seawater [CO₃^2-], causing a marked shoaling of the carbonate saturation profile. This episode of ocean acidification had profound consequences for the pelagic ecosystem and temporarily (120–200 kyr) altered global patterns of carbonate sedimentation. The sedimentological shift marking the PETM onset likely reflects both intensified dissolution and diminished fine-fraction (<63 μm) carbonate production by calcareous nannoplankton. It is plausible that elevated pCO₂ levels inhibited coccolithophorid calcification during the initial stages of the PETM, and that the reduced flux of inorganic carbon to the deep ocean contributed to the demise of many deep-sea benthic foraminifera. This assertion is corroborated by the presence of weakly calcified “excursion taxa” among some PETM planktic foraminiferal assemblages. A curious paradox is that the extreme magnitude of the PETM did not irrevocably alter calcareous microplankton diversity, although the effects of this transient perturbation would have had significant repercussions on timescales relevant to humanity.