Extreme warming of tropical waters during the Paleocene-Eocene Thermal Maximum

The Paleocene–Eocene Thermal Maximum (PETM), ca. 56 Ma, was a major global environmental perturbation attributed to a rapid rise in the concentration of greenhouse gases in the atmosphere. Geochemical records of tropical sea-surface temperatures (SSTs) from the PETM are rare and are typically affected by post-depositional diagenesis. To circumvent this issue, we have analyzed oxygen isotope ratios (d18O) of single specimens of exceptionally well-preserved planktonic foraminifera from the PETM in Tanzania (~19°S paleolatitude), which yield extremely low d18O, down to <–5‰. After accounting for changes in seawater chemistry and pH, we estimate from the foraminifer d18O that tropical SSTs rose by >3 °C during the PETM and may have exceeded 40 °C. Calcareous plankton are absent from a large part of the Tanzania PETM record; extreme environmental change may have temporarily caused foraminiferal exclusion. GEOLOGY, September 2014; v. 42; no. 9; p. 739–742; Data Repository item 2014272 | doi:10.1130/G35637.1 | Published online 25 July 2014 © 2014 eological Soci ty of America. Gold Open Access: This paper is published under the terms of the CC-BY license. 740 www.gsapubs.org | September 2014 | GEOLOGY organic matter (Fig. 1). No mass transport fea­ tures were observed in the core (Nicholas et al., 2006), but reworking, switching sources, or sediment mixing may account for this complex­ ity. The pre­excursion sediments and reworked packets contain an abundant open­ocean plank­ tonic foraminifer assemblage comprising more than 40 species (see the Data Repository). Stratigraphic horizons containing excursion taxa commonly have low abundance (commonly <1 foraminifer/g) and diversity of planktonic fora­ minifera. Between ~24 and 18 mbs, planktonic foraminifer abundances fluctuate between pre­ excursion levels and very low abundances, with a near complete absence of calcareous microfos­ sils between ~18 and 13 mbs. Where planktonic foraminifer abundances are lowest, the CaCO3 content drops to near zero (Fig. 1). Between 24 and 19 mbs (“mixed zone” in Fig. 1), levels with higher microfossil abundances and dCforam are interpreted as dominated by transported pre­ PETM sediments. The top of the PETM sec­ tion is truncated by a hiatus (~13 mbs), and a rich and diverse microfossil assemblage returns higher in the core (13–10 mbs). Geochemical Records Single­specimen pre­PETM dCforam values for mixed layer– and thermocline­dwelling spe­ cies are typically ~4.9‰ (± 1.94‰, 2 standard deviations [SD]) and 1.5‰ (± 1.11‰, 2 SD), respectively (Figs. 1 and 2). Mean pre­PETM dOforam values are –3.3‰ for mixed layer– and –2.7‰ for thermocline­dwelling species (Figs. 1 and 2). Two Morozovella specimens from within the CIE exhibit dOforam lower than –5‰. How­ ever, not all specimens that exhibit the lowest dCforam also record the lowest dOforam (see the Data Repository). Due to the complex stratigra­ phy at TDP­14, PETM and pre­PETM specimens occur in the same stratigraphic intervals, which makes an estimation of the true magnitude of the CIE from foraminiferal calcite problematic.