REDUCED BIOCALCIFICATION DUE TO ELEVATED pCO2 AND ANCILLARY EFFECTS ON OXYGEN ISOTOPE RATIOS IN PLANKTIC FORAMINIFERAL TESTS DURING THE PALEOCENE-EOCENE THERMAL MAXIMUM

Model simulations for oceanic uptake of anthropogenic carbon predict that surface-ocean pH will drop by 0.7 units in less than 500 years. A concomitant effect of this acidification will be a marked decline in the carbonate ion concentration of seawater; hence, there is concern that carbonate undersaturation might inhibit biocalcification in some marine organisms. We address this issue by examining the response of planktic foraminifera to an ancient (~55 Ma) analog for future climate change, the Paleocene-Eocene thermal maximum (PETM). The PETM was a transient (170 ka) global warming event associated with massive carbon input (4500-6800 Pg C) signaled by a carbon isotope excursion (CIE) of -4‰ in inorganic carbon and pervasive carbonate dissolution. Study of PETM planktic foraminiferal assemblages from the central, equatorial Pacific Ocean (ODP Site 865) reveals that Morozovella velascoensis temporarily “morphs” into the weakly calcified form, M. allisonensis. The tests of short-lived M. allisonensis invariably yield anomalously low δ¹³C, confirming they lived only during the CIE. We attribute this reduction in test calcification to elevated pCO₂ levels. The frosty appearance of the tests indicates they have undergone a modest degree of post-depositional diagenesis. To assess the effect of diagenesis on the δ¹⁸O of these tests, ion microprobe analyses of unaltered subdomains were performed (10 µm spot size) with an analytical precision and accuracy of 0.3‰ (2 SD). The in situ measurements yield an average δ¹⁸O value (-4.1‰ PDB) that is 2.6‰ lower than the average whole test δ¹⁸O value (-1.5‰) published for M. allisonensis from the same core depths. In contrast, the δ¹⁸O difference between unaltered subdomains (-3.3‰) and conventional whole test values (-1.9‰) is ~1.4‰ in M. velascoensis. These results suggest (1) biocalcification in some planktic foraminifera was inhibited by elevated pCO₂ during the PETM, (2) sea-surface temperatures were much warmer (~33ºC) than published records for this site indicate, (3) the δ¹⁸O signature of diagenesis is more strongly expressed in thin, weakly calcified tests and (4) the extreme conditions of the PETM did not cause a mass extinction among tropical calcareous plankton, although its transient affects would have had repercussions on timescales relevant to humanity.