At the boundary between the Palaeocene and Eocene epochs, about 55 million years ago, the Earth experienced a strong global warming event, the Palaeocene−Eocene thermal maximum^{1, 2, 3, 4}. The leading hypothesis to explain the extreme greenhouse conditions prevalent during this period is the dissociation of 1,400 to 2,800 gigatonnes of methane from ocean clathrates^{5, 6}, resulting in a large negative carbon isotope excursion and severe carbonate dissolution in marine sediments. Possible triggering mechanisms for this event include crossing a threshold temperature as the Earth warmed gradually⁷, comet impact⁸, explosive volcanism^{9, 10} or ocean current reorganization and erosion at continental slopes¹¹, whereas orbital forcing has been excluded¹². Here we report a distinct carbonate-poor red clay layer in deep-sea cores from Walvis ridge¹³, which we term the Elmo horizon. Using orbital tuning, we estimate deposition of the Elmo horizon at about 2 million years after the Palaeocene−Eocene thermal maximum. The Elmo horizon has similar geochemical and biotic characteristics as the Palaeocene−Eocene thermal maximum, but of smaller magnitude. It is coincident with carbon isotope depletion events in other ocean basins, suggesting that it represents a second global thermal maximum. We show that both events correspond to maxima in the 405-kyr and 100-kyr eccentricity cycles that post-date prolonged minima in the 2.25-Myr eccentricity cycle, implying that they are indeed astronomically paced.

1. Faculty of Geosciences, Department of Earth Sciences, and
2. Laboratory of Palaeobotany and Palynology, Department of Palaeoecology, Utrecht University, Budapestlaan 4, 3584 CD Utrecht, The Netherlands
3. Faculty of Earth and Life Sciences, Vrije Universiteit, De Boelelaan 1085, 1081 HV Amsterdam, The Netherlands
4. Earth Science Department, University of California, Santa Cruz, Earth and Marine Sciences Building, Santa Cruz, California 95064, USA
5. Department of Earth and Environmental Sciences, Wesleyan University, 265 Church Street, Middletown, Connecticut 06459-0139, USA, and Center for the Study of Global Change, Department of Geology and Geophysics, Yale University, PO Box 208109, New Haven, Connecticut 06520-8109, USA
6. DFG Research Center for Ocean Margins (RCOM), University of Bremen, Leobener Strasse, 28359 Bremen, Germany
7. Scripps Institution of Oceanography, University of California, San Diego, 9500 Gilman Drive, MC0208, La Jolla, California 92093, USA
8. Facoltà di Scienze, Dipartimento Scienze della Terra., Università "G. d'Annunzio" di Chieti, Campus Universitario Madonna delle Piane, Via dei Vestini 31, 66013 Chieti Scalo, Italy

Correspondence to: Lucas J. Lourens¹ Correspondence and requests for materials should be addressed to L.J.L. (Email: llourens@geo.uu.nl).

Received 17 November 2004; Accepted 10 May 2005

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