MORPHOLOGICAL RESPONSE OF PLANKTIC FORAMINIFERA TO A TRANSIENT RISE IN PCO₂ DURING THE PALEOCENE-EOCENE THERMAL MAXIMUM

Study of modern coccolithophorids indicates that calcification in some bloom-forming taxa is inhibited by increasing pCO₂; hence, continued input of anthropogenic carbon into the ocean-atmosphere system may prove detrimental to the pelagic ecosystem. To constrain the effects of fossil fuel emissions on marine calcification, researchers are examining calcareous microfossil assemblages associated with deep-sea records of an ancient (ca. 55 Ma) global warming event referred to as the Paleocene-Eocene Thermal Maximum (PETM). In marine cores, the PETM is signified by a sudden decrease in the carbon isotopic composition of biogenic calcite and pervasive carbonate dissolution. These two hallmarks signal the rapid release of vast quantities (1,200 - 5,000 Gt) of carbon into the ocean-atmosphere system. Uptake of this carbon acidified the oceans, fueling pH-buffering reactions that led to carbonate undersaturation and shoaling of the lysocline. Thus, the sharp decrease in carbonate content at the base of many deep-sea PETM sections is attributed to intensified dissolution. Still, it seems plausible that dissolution and lysocline shoaling may have been exacerbated by reduced carbonate production. Unfortunately, differentiating between increased dissolution and decreased carbonate production is made difficult by the intimate inverse relationship between these two processes. Here it is noted that the presence of weakly calcified “excursion taxa” among planktic foraminiferal assemblages in some PETM records is consistent with the view that calcification was impeded. Specifically, heavily calcified forms of Morozovella velascoensis temporarily “morph” into weakly calcified forms referred to as M. allisonensis. The ephemeral nature of M. allisonensis, its restriction to the PETM interval and lack of descendants all corroborate the notion that this form was an ecophenotype that developed in response to transient environmental conditions. This interpretation warrants further study since reduced calcification would decrease the levels of CO₂ generated by calcite precipitation, temporarily enhancing the capacity of the surface ocean to absorb carbon from the atmosphere.