Astronomical pacing of late Palaeocene to early Eocene global warming events

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(1-4). 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 gradually(7), comet impact(8), explosive volcanism(9,10) or ocean current reorganization and erosion at continental slopes(11), whereas orbital forcing has been excluded(12). Here we report a distinct carbonate-poor red clay layer in deep-sea cores from Walvis ridge(13), 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 similar to 405-kyr and similar to 100-kyr eccentricity cycles that post-date prolonged minima in the 2.25-Myr eccentricity cycle, implying that they are indeed astronomically paced.