The Palaeocene-Eocene Thermal Maximum Super Greenhouse: Biotic and Geochemical Signatures, age Models and Mechanisms of Climate Change

The Palaeocene–Eocene Thermal Maximum (PETM), a geologically brief episode of global warming associated with the Palaeocene–Eocene boundary, has been studied extensively since its discovery in 1991. The PETM is characterized by a globally quasi-uniform 5–8 8C warming and large changes in ocean chemistry and biotic response. The warming is associated with a negative carbon isotope excursion (CIE), reflecting geologically rapid input of large amounts of isotopically light CO2 and/or CH4 into the exogenic (ocean–atmosphere) carbon pool. The biotic response on land and in the oceans was heterogeneous in nature and severity, including radiations, extinctions and migrations. Recently, several events that appear similar to the PETM in nature, but of smaller magnitude, were identified to have occurred in the late Palaeocene through early Eocene, with their timing possibly modulated by orbital forcing. Although debate continues on the carbon source, the mechanisms that caused the input, the mechanisms of carbon sequestration, and the duration and pacing of the event, the research carried out over the last 15 years has provided new constraints and spawned new research directions that will lead to improved understanding of PETM carbon cycle and climate change. A distinct period of extreme global warmth was initiated close to the boundary between the Palaeocene and Eocene epochs, approximately 55.5 Ma ago (Gradstein et al. 2004). This event, termed the Palaeocene–Eocene Thermal Maximum (PETM), occurred during a time of generally warm, ‘greenhouse’ climate conditions, but stands out against the background warmth as an abrupt and short-lived spike in global temperatures. Evidence for global warming is preserved by the TEX86 0 temperature proxy (Sluijs et al. 2006; Zachos et al. 2006), oxygen isotope (dO) excursions in marine foraminiferal calcite (Fig. 1) (Kennett & Stott 1991; Thomas et al. 2002) and terrestrial carbonates (Koch et al. 1995), increased Mg/Ca values in planktonic and benthic foraminifera (Zachos et al. 2003; Tripati & Elderfield 2005), poleward migrations of (sub)tropical marine plankton (Kelly et al. 1996; Crouch et al. 2001) and terrestrial plant species (Wing et al. 2005), and mammal migrations across high northern latitudes (Bowen et al. 2002, 2006; Smith et al. 2006). Associated with the warming is a negative 2.5–6‰ carbon isotope (dC) excursion (CIE) (Kennett & Stott 1991; Koch et al. 1992; Thomas et al. 2002; Pagani et al. 2006), generally accepted to reflect the geologically rapid injection of C-depleted carbon, in the form of CO2 and/or CH4, into the global exogenic carbon pool (Fig. 1). The apparent conjunction between carbon input and warming has fuelled the hypothesis that increased greenhouse gas concentrations resulted in greenhouse warming during the PETM. The total amount of carbon input during the PETM, which is known to within an order of magnitude (Dickens et al. 1997; Zachos et al. 2005; Pagani et al. 2006), was about 4–8 times the anthropogenic carbon release from the start of the industrial era up From: WILLIAMS, M., HAYWOOD, A. M., GREGORY, F. J. & SCHMIDT, D. N. (eds)Deep-Time Perspectives on Climate Change: Marrying the Signal from Computer Models and Biological Proxies. The Micropalaeontological Society, Special Publications. The Geological Society, London, 323–349. 1747-602X/07/$15.00 # The Micropalaeontological Society 2007. to today (Marland et al. 2005), and comparable to that expected from gross anthropogenic emissions through the end of the 21st century (Intergovernmental Panel on Climate Change 2001). In association with carbon cycle and climatic change, the PETM also stands out as a time of major biotic restructuring. Given the probable ties between releases of near-modern levels of carbon-based greenhouse gases and PETM climatic and biotic change, the PETM has developed as a provocative geological case study in global change, and many of the event’s characteristics and mechanisms are under intensive study. A large volume of research on the PETM has appeared over the past decade (Fig. 2), and in this paper we aim to review and synthesize this material, including the duration and magnitude of carbon cycle perturbation, magnitude of warming, changes in ocean chemistry and marine and terrestrial biotic response. 167