Limited range of interspecific vital effects in coccolith stable isotopic records during the Paleocene‐Eocene thermal maximum

Abstract

Small but significant differences exist among stable carbon and oxygen isotopic excursions measured in coccolith‐dominated bulk carbonate and planktic foraminifera during the Paleocene‐Eocene thermal maximum (PETM). One hypothesis suggests that the bulk carbonate isotopic record is compromised by changing nannofossil assemblages, since modern nannofossils show a large (5 permil) range of interspecific vital effects. New techniques are employed here to separate different size fractions of coccoliths from PETM sediments at ODP Site 690 for isotopic analysis, removing a major portion of the variation in nannofossil assemblages. Isotopic compositions of coarse and fine coccolith fractions dominated by coccoliths of genus Chiasmolithus and Toweius, respectively, differ by less than 0.5 permil for both oxygen and carbon. The near‐monogeneric Toweius record closely parallels the main trends in the bulk carbonate isotope records, including multiple steps in the negative carbon isotopic excursion, suggesting that the trends in the bulk carbonate record are not artifacts of changing species assemblages. Because both coccolithophorids and symbiont‐bearing foraminifera like Acarinina must inhabit the photic zone, it is unlikely that the 103 year lags in isotope event onset between coccoliths and Acarinina reflect true time‐transgressive invasion of isotopically depleted CO2 into the water column. The small range of vital effects among Paleocene coccoliths is unlikely to result from diagenetic homogenization, and instead may reflect more similar carbon acquisition strategies of Paleocene coccolithophorid algae due to larger and/or more similar cell sizes and higher atmospheric carbon dioxide. The small range of vital effects suggests that bulk carbonate records are likely reliable for other early and pre‐Cenozoic sediments where foraminifera are often scarce.