Planktonic foraminiferal oxygen isotope analysis by ion microprobe technique suggests warm tropical sea surface temperatures during the Early Paleogene

[1] Cool tropical sea surface temperatures (SSTs) are reported for warm Paleogene greenhouse climates based on the dO of planktonic foraminiferal tests. These results are difficult to reconcile with models of greenhouse gas–forced climate. It has been suggested that this “cool tropics paradox” arises from postdepositional alteration of foraminiferal calcite, yielding erroneously high dO values. Recrystallization of foraminiferal tests is cryptic and difficult to quantify, and the compilation of robust dO records from moderately altered material remains challenging. Scanning electron microscopy of planktonic foraminiferal chamber‐wall cross sections reveals that the basal area of muricae, pustular outgrowths on the chamber walls of species belonging to the genus Morozovella, contain no mural pores and may be less susceptible to postdepositional alteration. We analyzed the dO in muricae bases of morozovellids from the central Pacific (Ocean Drilling Program Site 865) by ion microprobe using 10 mm pits with an analytical reproducibility of ±0.34‰ (2 standard deviations). In situ measurements of dO in these domains yield consistently lower values than those published for conventional multispecimen analyses. Assuming that the original dO is largely preserved in the basal areas of muricae, this new dO record indicates Early Paleogene (∼49–56 Ma) tropical SSTs in the central Pacific were 4°–8°C higher than inferred from the previously published dO record and that SSTs reached at least ∼33°C during the Paleocene‐Eocene thermal maximum. This study demonstrates the utility of ion microprobe analysis for generating more reliable paleoclimate records from moderately altered foraminiferal tests preserved in deep‐sea sediments.