Paleocene/Eocene Thermal Maximum

Past extreme warming events linked to massive carbon release from thawing permafrost

Robert M. DeConto, Simone Galeotti, Mark Pagani, David Tracy, Kevin Schaefer, Tingjun Zhang, David Pollard, David J. Beerling

 

Between about 55.5 and 52 million years ago, Earth experienced a series of sudden and extreme globalwarming events (hyperthermals) superimposed on a long-term warming trend. The first and largest of these events, the Palaeocene–EoceneThermalMaximum (PETM), is characterized by a massive input of carbon, ocean acidification and an increase in global temperature of about 5-6C within a few thousand years3. Although various explanations for the PETM have been proposed, a satisfactory model that accounts for the source, magnitude and timing of carbon release at the PETMand successive hyperthermals remains elusive. Here we use a new astronomically calibrated cyclostratigraphic record from central Italy to show that the Early Eocene hyperthermals occurred during orbits with a combination of high eccentricity and high obliquity. Corresponding climate–ecosystem–soil simulations accounting for rising concentrations of background greenhouse gases and orbital forcing show that the magnitude and timing of the PETM and subsequent hyperthermals can be explained by the orbitally triggered decomposition of soil organic carbon in circum-Arctic and Antarctic terrestrial permafrost. This massive carbon reservoir had the potential to repeatedly release thousands of pentagrams (1015 grams) of carbon to the atmosphere–ocean system, once a long-term warming threshold had been reached just before the PETM. Replenishment of permafrost soil carbon stocks following peak warming probably contributed to the rapid recovery from each event, while providing a sensitive carbon reservoir for the next hyperthermal. As background temperatures continued to rise following the PETM, the areal extent of permafrost steadily declined, resulting in an incrementally smaller available carbon pool and smaller hyperthermals at each successive orbital forcing maximum. A mechanism linking Earth’s orbital properties with release of soil carbon from permafrost provides a unifying model accounting for the salient features of the hyperthermals.


Arctic Hydrology During Global Warming at the Palaeocene-Eocene Thermal Maximum

Mark Pagani, Nikolai Pedentchouk, Matthew Huber, Appy Sluijs, Stefan Schouten, Henk Brinkhuis, Jaap S. Sinninghe Damste, Gerald R. Dickens, and the IODP Expedition 302 Expedition Scientists

 

Sediments were recovered from the central Arctic Ocean, providing the first opportunity to evaluate the environmental response at the North Pole during the PETM. Here we present stable hydrogen and carbon isotope measurements of terrestrial-plant and aquatic-derived n-alkanes that record changes in hydrology, including surface water salinity and precipitation. Records are interpreted as documenting decreased rainout during hemispheric moisture transport and increased moisture delivery to the Arctic at the onset of the PETM, consistent with predictions of poleward storm track migrations during global warming. The terrestrial-plant carbon isotope excursion is substantially larger than those of marine carbonates. Previously, this offset was explained by a plant’s physiological response to increases in surface humidity. However, this mechanism is not an effective explanation in this wet Arctic setting, leading us to hypothesize that the true magnitude of the carbon isotope excursion and associated carbon input was greater than originally surmised. Greater carbon release and strong hydrological cycle feedbacks help explain the maintenance of this unprecedented PETM warmth.


Subtropical Arctic Ocean Temperatures During the Palaeocene/Eocene Thermal Maximum

Appy Sluijs, Stefan Schouten, Mark Pagani, Martijn Woltering, Henk Brinkhuis, Jaap S. Sinninghe Damste, Gerald R. Dickens, Matthew Huber, Gert-Jan Reichart, Ruediger Stein, Jens Matthiessen, Lucas J. Lourens, Nikolai Pedentchouk, Jan Backman, Kathryn Moran and the Expedition 302 Scientists

The Palaeocene/Eocene thermal maximum, 55 million years ago, was a brief period of widespread, extreme climatic warming, that was associated with massive atmospheric greenhouse gas input. Although aspects of the resulting environmental changes are well documented at low latitudes, no data were available to quantify simultaneous changes in the Arctic region. Here we identify the Palaeocene/Eocene thermal maximum in a marine sedimentary sequence obtained during the Arctic Coring Expedition. We show that sea surface temperatures near the North Pole increased from 18˚C to over 23˚C during this event. Such warm values imply the absence of ice and thus exclude the influence of ice-albedo feedbacks on this Arctic warming. At the same time, sea level rose while anoxic and euxinic conditions developed in the ocean’s bottom waters and photic zone, respectively. Increasing temperature and sea level match expectations based on palaeoclimate model simulations, but the absolute polar temperatures that we derive before, during and after the event are more than 10 C warmer than those model-predicted. This suggests that higher-than-modern greenhouse gas concentrations must have operated in conjunction with other feedback mechanisms perhaps polar stratospheric clouds or hurricane-induced ocean mixing to amplify early Palaeogene polar temperatures.


Orbital Chronology of Early Eocene Hyperthermals from the Contessa Road Section, Central Italy

Simone Galeotti, Srinath Krishnan, Mark Pagani, Luca Lanci, Alberto Gaudio, James C. Zachos, Simonetta Monechi, Guia Morelli, Lucas Lourens

High-resolution geochemical analyses of the Lower Eocene Contessa Road section (Italy) reveal orbitally controlled fluctuations in the percent concentration of calcium carbonate (wt.% CaCO3) that include the ETM2 (Elmo) and ETM3 hyperthermal events. Patterns of increased dissolution, negative carbon isotope excursions, and warmer global climates are intimately linked to maxima in insolation, through the global carbon cycle. Extraction of short- and long-eccentricity orbital periodicities of the wt.% CaCO3 record provides a relative cyclochronology for the interval ranging from ca.52 to 55.5 Ma. The Contessa Road section is easily accessible and offers a continuous integrated stratigraphic record (stable isotopes, standard calcareous plankton biostratigraphy, magnetostratigraphy, and cyclostratigraphy), thus providing a potential type succession for the study of Early Eocene hyperthermals.


An Ancient Carbon Mystery

Mark Pagani, Ken Caldeira, David Archer, James C. Zachos

Sudden global warming 55 million years ago provides evidence for high climate sensitivity to atmospheric CO2, but the source of the carbon remains enigmatic.
 


Effects of Rapid Global Warming at the Paleocene-Eocene Boundary on Neotropical Vegetation

Carlos Jaramillo, Diana Ochoa, Lineth Contreras, Mark Pagani, Humberto Carvajal-Ortiz, Lisa M. Pratt, Srinath Krishnan, and others

Temperatures in tropical regions are estimated to have increased by 3 to 5 degrees C, compared with Late Paleocene values, during the Paleocene-Eocene Thermal Maximum (PETM, 56.3 million years ago) event. We investigated the tropical forest response to this rapid warming by evaluating the palynological record of three stratigraphic sections in eastern Colombia and western Venezuela. We observed a rapid and distinct increase in plant diversity and origination rates, with a set of new taxa, mostly angiosperms, added to the existing stock of low-diversity Paleocene flora. There is no evidence for enhanced aridity in the northern Neotropics. The tropical rainforest was able to persist under elevated temperatures and high levels of atmospheric carbon dioxide, in contrast to speculations that tropical ecosystems were severely compromised by heat stress.