The response of Cretaceous Calcareous nannoplankton to pCO2 and ocean acidification

Recent environmental change and instability pose urgent questions regarding biota ability to keep pace with excess CO2, increase in acidity and drop in pH of surface waters. Calcareous nannoplankton cannot efficiently calcify if the medium acidity passes a critical level, but laboratory studies are not conclusive as to assess crises, adaptations, evolutionary innovation and/or extinctions. The geological record indicates that ocean biota can adapt to higher acidity, but while the current decrease in pH is compressed in a very short period of time, past changes in oceanic acidity occurred over tens of thousands of years, giving time for life to adjust or even take advantage.
Within the oceanic biosphere, calcareous nannoplankton play a key-role as is abundant from coastal to open ocean settings, has a 220 My-long evolutionary history, is one of the most effective calcite producers of the planet, and is extremely sensitive to environmental variations. We explored cases of excess CO2 derived from major volcanic episodes and marked by calcareous nannoplankton changes in evolutionary rates, species richness, abundance, and calcite production.
Available stratigraphy and cyclochronology allow highresolution dating of biotic and environmental fluctuations, providing the precision necessary for understanding the role of pCO2 and ocean acidification on nannoplankton biocalcification and evolution. Selected case histories are the Aptian and the latest Cenomanian, corresponding to the onset and climax of the mid-Cretaceous SuperGreenhouse, respectively. During formation of the Ontong Java-Maniniki and Caribbean Plateaus, rapidly increasing pCO2 reached values as high as 2000 ppm, causing a calcification crisis and accelerated evolutionary rates. Maximum ocean acidification, was preceded by a 0.5-1 myñlong period of biocalcification decline, dwarfism and production of ìdeformed-malformedî coccoliths. During the Aptian, the acidification climax lasted some 200 ky and is marked by short-lived abundance peaks of peculiar nannoliths, perhaps representing alkalinity recovery following CaCO3 dissolution.
As far as evolution is concerned, rising pCO2 triggered false extinctions (Lazarus effect) among calcareous nannoplankton; conversely, a major origination episode perhaps represents a biocalcification strategy to overcome ocean acidification.

Erba, E. & Bottini, C., 2009. The response of Cretaceous Calcareous nannoplankton to pCO2 and ocean acidification. Awards Ceremony Speeches and Abstracts of the 19th Annual V.M. Goldschmidt Conference, V.M. Goldschmidt Conference. Geochimica et Cosmochimica Acta 73(13)-1: A334. Supplement.

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