An abrupt episode of global warming marked the end of the Paleocene epoch. Oxygen and carbon isotope records from two widely separated sites support the notion that degassing of biogenic methane hydrate may have been an important factor in altering Earth's climate. The data show evidence for multiple injections of methane, separated by intervals in which the carbon cycle was in stasis. Correlations between the two sites suggest that even these small-scale events were global in nature.

The onset of the Palaeocene/Eocene thermal maximum (about 55 Myr ago) was marked by global surface temperatures warming by 5-7 degrees C over approximately 30,000 yr (ref. 1), probably because of enhanced mantle outgassing and the pulsed release of approximately 1,500 gigatonnes of methane carbon from decomposing gas-hydrate reservoirs. The aftermath of this rapid, intense and global warming event may be the best example in the geological record of the response of the Earth to high atmospheric carbon dioxide concentrations and high temperatures. This response has been suggested to include an intensified flux of organic carbon from the ocean surface to the deep ocean and its subsequent burial through biogeochemical feedback mechanisms. Here we present firm evidence for this view from two ocean drilling cores, which record the largest accumulation rates of biogenic barium--indicative of export palaeoproductivity--at times of maximum global temperatures and peak excursion values of delta13C. The unusually rapid return of delta13C to values similar to those before the methane release and the apparent coupling of the accumulation rates of biogenic barium to temperature, suggests that the enhanced deposition of organic matter to the deep sea may have efficiently cooled this greenhouse climate by the rapid removal of excess carbon dioxide from the atmosphere.

[1] A major perturbation of the global carbon cycle $55 million years ago, believed to result from release of 1000-2000 Gt of C from methane hydrates, correlates with an intense but transient greenhouse warming event known as the Paleocene Eocene Thermal Maximum (PETM). The rapid (10 5 years) recovery of global temperatures reflects important negative feedbacks in the climate system and carbon cycle. Enhanced marine productivity may be one important feedback, but indicators for productivity changes have yielded conflicting results. Here we use a new independent indicator, Sr/Ca in coccolith carbonate, which covaries with the productivity of coccolithophorid algae, to investigate the biotic response in the most complete PETM deep sea record which was recovered at ODP Site 690 in the Weddell Sea. In the dominant coccolithophorid genus Toweius a large (40%) Sr/Ca increase immediately after the gas hydrate release signals a dramatic productivity increase. Productivity levels remained high for 60,000 years but decreased to pre-event levels by 120,000 years after the gas hydrate release. Productivity levels during the PETM are higher than observed at any other time in our $400,000 year record. Other coccolithophorid genera Chiasmolithus and Discoaster show a brief modest (25% Sr/Ca increase) increase in productivity that lags behind the methane event by 50,000 years and is within the range of productivity variation elsewhere in the record. The timing of the Toweius productivity increase agrees well with Os isotope records of globally increased weathering intensity, which may have provided higher nutrient fluxes to stimulate algal productivity. If this type of productivity response occurred globally, it would also be consistent with the timing of C drawdown that may have returned temperatures to near pre-event levels.

A fossil-bearing continental sequence that spans the Paleocene-Eocene boundary (ca. 55 Ma) can now be accurately correlated to expanded deep-sea oceanic sediments at an extremely high resolution (∼ ∼10 k.y.), thus facilitating detailed investigations into abrupt global climate change and its influence on the evolution of terrestrial organisms. Here we show that the onset of this extremely warm interval is associated with a stepped terrestrial carbon isotope (δ δ 13 C) excursion. This suggests that a pulsed sublimation of submarine gas hydrate accumulations at this time may have caused a rapid venting of significant quantities of light carbon through the ocean/atmosphere interface. Major mammalian turnover occurred near the onset of the ensuing greenhouse event, and this also appears to have occurred in a sequential fashion, although the changes we see in population composition and morphology lag the major features of the global δ δ 13 C record by some ∼ ∼10-20 k.y., which could represent the duration required for evolutionary mechanisms to occur due to greenhouse-associated stresses. Additionally, we have evidence that increased soil respiration rates occurred in response to the core episode of global warmth. Paleocene-Eocene boundary carbon cycle perturbations were apparently as remarkable in the atmospheric and terrestrial reservoirs as they were in the oceans, and these changes had a dramatic effect on terrestrial biota.

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