On the PETM and ETM2 global warming events: New evidence for a tectonic-magmatic trigger mechanism (Invited)

The climate record covering the past 70 my shows that global temperatures peaked during the Early Eocene around ∼54 to 52 Ma; the Early Eocene Climatic Optimum (EECO) (1). Preceding and during the EECO were three ephemeral global warming events, each with approximately half the intensity of its predecessor: the Paleocene Eocene Thermal Maximum (PETM), the Eocene Thermal Maximum 2 (ETM2) and the Eocene Thermal Maximum 3 (ETM3). Each is marked by an abrupt decrease in the δ13C of sedimentary carbon, consistent with the rapid addition of massive amounts of 13C-depleted C, in the form of carbon dioxide and/or methane, into the hydrosphere/atmosphere. These events are regarded as the best deep-time analogues to future climate scenarios through higher CO2 concentration in the atmosphere. There is, however, no consensus on the source of the carbon for these ancient warming events or the triggering mechanism/s for the release. Two ideas are (i) breakdown of marine methane hydrate (2), possibly related to orbital cycles forcing and/or (ii) for the PETM, thermogenic release of methane by massive sill intrusion into C-rich sedimentary basins, with the gas being released via a complex of thousands of hydrothermal vents (3). These two potential carbon sources have quite different isotopic ratios, which in mass balance calculations determine the amount of carbon required to account for the carbon isotope excursions. Better knowledge of the carbon sources/s is essential if these events are to be factored in to models of climate sensitivity to atmospheric CO2 concentration. The PETM has been shown to be synchronous with the onset of peak, continental-breakup related, magmatic activity during the formation of the East Greenland and the Faeroes flood basalt province (4), supporting the sill intrusion model for the source of the methane/CO2 (3). One perceived weakness of this explanation, however, is that it apparently fails to explain the younger ETM2 and ETM3 events. Here we combine relative astronomical ages for the PETM, ETM2 and Danish Ash-17 with a new, more precise, 40Ar/39Ar age for Ash-17 and U-Pb zircon and 40Ar/39Ar ages for a massive sill complex exposed in the Trail Ø/Jameson Land sedimentary basin on the East Greenland margin. This igneous-sedimentary complex is conjugate to the sill-bearing Røst basin on the Norwegian margin, north of the Vøring basin. The combined age data demonstrate synchronicity between ETM2 and intrusion of the sill complex, implicating an identical mechanism as the one proposed for the PETM (3). The PETM and ETM2 can be understood in the context of 2-stage separation of Greenland and Europe, first with the opening of the NE Atlantic, followed 1. 8 my later by the opening of the Norwegian-Greenland Sea, north of the Jan Meyen Fracture Zone, with each rifting event being accompanied by sill intrusion into C-rich sedimentary basins, triggering the release of massive amounts of methane. 1. J. Zachos et al., Science, 292, 686, (2001); 2, G. R. Dickens et al., Paleoceanography, 10, 965 (1995); 3. H. Svensen et al., Nature 429, 542 (2004).0; 4. M. Storey et al., 316, 587, Science (2005).