Calcium isotope composition of Morozovella over the late Paleocene-early Eocene

Ocean acidification (OA) during the Paleocene-Eocene thermal maximum (PETM) likely caused a biocalcification crisis. The calcium isotope composition (delta Ca-44/40) of primary car bonate producers may be sensitive to OA. To test this hypothesis, we constructed the first high-resolution, high-precision planktic foraminiferal delta Ca-44/40 records before and across the PETM. The records employ specimens of Morozovella spp. collected from Ocean Drilling Program Sites 1209 (Shatsky Rise, Pacific Ocean) and 1263 (Walvis Ridge, Atlantic Ocean). At Site 1209, delta Ca-44/40 values start at -1.33 parts per thousand during the Upper Paleocene and increase to a peak of -1.15 parts per thousand immediately before the negative carbon isotope excursion (CIE) that marks the PETM onset. Values remain elevated through the PETM interval and decrease into the earliest Eocene. A shorter-term record for Site 1263 shows a similar trend, although delta Ca-44/40 values are on average 0.22% lower and decrease shortly after the CIE onset. The trends support neither diagenetic overprinting, authigenic carbonate additions, nor changes in the delta Ca-44/40 value of seawater. Rather, they are consistent with a kinetic isotope effect, whereby calcite delta Ca-44/40 values inversely correlate with precipitation rate. Geologically rapid Ca isotope shifts appear to reflect the response of Morozovella to globally forced changes in the local carbonate geochemistry of seawater. All data combined suggest that the PETM-OA event occurred near the peak of a gradual reduction in seawater carbonate ion concentrations during a time of elevated atmospheric pCO(2), potentially driven by North Atlantic igneous province emplacement.