Paleocene-Eocene Thermal Maximum

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Climate change during the last 65 million years. The Paleocene-Eocene Thermal Maximum is labeled PETM and is likely to be understated by a factor of 2 or more due to coarse sampling and averaging in this data set.
Climate change during the last 65 million years. The Paleocene-Eocene Thermal Maximum is labeled PETM and is likely to be understated by a factor of 2 or more due to coarse sampling and averaging in this data set.

The end of the Paleocene (55.5/54.8 Ma) was marked by one of the most significant periods of global change during the Cenozoic, a sudden global climate change, the Paleocene-Eocene Thermal Maximum (PETM), which upset oceanic and atmospheric circulation and led to the extinction of numerous deep-sea benthic foraminifera and on land, a major turnover in mammals.

In an event marking the start of the Eocene, the planet heated up in one of the most rapid and extreme global warming events recorded in geologic history, currently being identified as the 'Paleocene-Eocene Thermal Maximum' or the 'Initial Eocene Thermal Maximum' (PETM or IETM). Sea surface temperatures rose between 5 and 8°C over a period of a few thousand years, and in the high Arctic, sea surface temperatures rose to a sub-tropical ~23°C/73°F.[1] In 1990, marine scientists James Kennett and Lowell Stott, both then at the University of California, Santa Barbara, reported analysis of marine sediments showing that, not only had the surface of the Arctic ocean heated up about 10 degrees at the beginning of the Eocene, but that the entire depth of the ocean had warmed, and its chemistry changed disastrously. There was severely reduced oxygen in deep sea waters, and 30 to 40% of deep sea foraminifera suddenly went extinct. Geologist Jim Zachos of the University of California, Santa Cruz has connected the Eocene heat wave to drastic changes in ocean chemistry (Zachos et al., 2005) that caused the massive worldwide die-off.

What unleashed the PETM is unclear. Most evidence points to reservoirs of methane gas that were breached by warmer temperatures, or to the thermal combustion of sedimentary organic matter. Alternatively, it has been proposed that extensive peat deposits were burned, thus releasing CO2, and that a comet impact triggered the PETM.

Tracking the ratio of carbon isotopes in marine calcium carbonate sediments, Kennett and Stott (1991) found a sharp decrease in the amount of heavy carbon in 55-million-year-old marine fossils, a decline that caused the relative ratio of 13C to 12C to plunge. One year later a synchronous drop in carbon isotope ratios has been identified in many terrestrial environments by Paul Koch and colleagues (Koch et al., 1992). A gas with very low amounts of heavy 13C must have flooded the atmosphere. In 1995, Gerald (Jerry) Dickens, University of Michigan, argued that only methane gas had enough light carbon to produce the early Eocene plunge. He proposed that a belch of methane escaped from ice in seafloor sediments as the Earth warmed during the latest Paleocene. The methane escaped from submarine clathrates, ice crystals that trap methane hydrate, a form of methane 'ice' that forms in cold bottom water under great pressures and is widely distributed and plentiful in sediments on the outer edges of continental margins. Methane has a global warming potential (GWP) over a 100 year period of 23[1], meaning per kilogram it is estimated to be 23 times as effective as carbon dioxide as a greenhouse gas. The massive sublimation and release of sedimentary methane hydrates into the ocean-atmosphere reservoir upset the global carbon cycle and led to runaway global warming.

Scientists Flavia Nunes and Richard Norris of the Scripps Institution of Oceanography in California explored how these warmer temperatures might have affected ocean currents. They measured carbon-13 isotopes from 14 cores that had been drilled into the deep floor in four different ocean basins, taking samples from sediment layers deposited before, during and after the PETM. These isotopes are considered to be an indicator of the nutrients deposited by the water at the time. The higher the isotope value, the likelier that the source came from the deep ocean, the prime source for nutrients. With a in-depth reconstruction, Nunes and Norris found that the world's ocean current system changed during the PETM ultimately reversing itself. Before the PETM, deep water upwelled in the southern hemisphere; over about 40,000 years, the source of this upwelling shifted to the northern hemisphere; it took another 100,000 years before recovering completely.

In the atmosphere, methane breaks down and releases carbon dioxide. According to Zachos and Dickens, methane combined with oxygen in the air and water, forming carbon dioxide and essentially suffocating marine life. But whether volcanic activity or a methane belch was the culprit, the greenhouse gas locked in the sun's warmth, sending global temperatures soaring.

Dissolved in the oceans, the added carbon dioxide also increased the overall acidity of seawater. This, in turn, would increase the dissolution of calcite shells of microplankton, which are the dominant component of seafloor sediments, leaving behind only nonsoluble clays. A documented change in colors of the sediment, from bright white carbonate to deep red clays, marks the Paleocene-Eocene event. Normal deposition of microscopic carbonate foram shells on the deeper reaches of the seafloor did not resume for at least 50,000 years, and the total recovery time to a "normal state" took as long as 100,000 years (Zachos et al., 2005).

At the start of the Eocene, the Earth remained warm for about 80,000 to 200,000 years. On land, there was a massive turnover of mammals, as most of the primitive mammals that had developed since the end of the Cretaceous were suddenly replaced by the ancestors of most of the surviving modern mammal groups, all of them in small versions, adapted to Eocene heat. Plant life was characterised by the boreotropical flora, with extensive high-latitude forests composed of large, fast-growing trees such as Dawn Redwood as far north as 80°N. In 2005, Dutch scientists studying material from the Arctic Coring Expedition found fossilized algae characteristic of subtropical waters averaging about 20C (the current average: -1.5C) in sediment cores taken on the Lomonosov Ridge between Siberia and Greenland. Atmospheric carbon levels then are thought to have been about 2-3,000 parts per million (ppm), compared with 380 ppm today.

Recently, several other rapid global warming events have been recorded in slightly younger sediments, of which one has been named after sesamestreet character Elmo (Lourens et al., 2005). These events appear to have been similar to the PETM, but of a smaller magnitude.

Sudden release of methane hydrate has also been hypothesized as a cause of the Permian-Triassic extinction event and this is also a serious "humanity killing" concern for contemporary global warming and climate change theorists.

[edit] Notes

  1. ^ The results were obtained by Dutch scientists from Utrecht University and the Royal NIOZ by analyzing the temperature-dependent composition of fatty substances called lipids in the cell membranes of marine archea. The presence of fossils of algae called dinoflagellates, belonging to the genus Apectodinium, a marker for sub-tropical climates, confirmed the findings "North Pole's ancient past holds clues about future global warming". Using this technique, the researchers found that sea surface temperatures at the North Pole had soared to 23 degrees Celsius, or around 73 degrees Fahrenheit, see Sluijs et al., 2006 for the scientific article.
  • Kennett, J.P. & Stott, L.D. 1991. Abrupt deep-sea warming, palaeoceanographic changes and benthic extinctions at the end of the Palaeocene. Nature, 353: 225-229
  • Koch, P.L., Zachos, J.C. & Gingerich, P.D. 1992. Correlation between isotope records in marine and continental carbon reservoirs near the Palaeocene/Eocene boundary. Nature, 358: 319-322
  • Lourens, L.J., Sluijs, A., Kroon, D., Zachos, J.C., Thomas, E., Röhl, U., Bowles, J. & Raffi, I. 2005. Astronomical pacing of late Palaeocene to early Eocene global warming events. Nature, 435(7045): 1083-1087
  • Sluijs, A., Schouten, S., Pagani, M., Woltering, M., Brinkhuis, H., Sinninghe Damsté, J.S., Dickens, G.R., Huber, M., Reichart, G.-J., Stein, R., Matthiessen, J., Lourens, L.J., Pedentchouk, N., Backman, J., Moran, K. & the Expedition, S. 2006. Subtropical Arctic Ocean temperatures during the Palaeocene/Eocene thermal maximum. Nature, 441(7093): 610-613
  • Zachos, J.C., Röhl, U., Schellenberg, S.A., Sluijs, A., Hodell, D.A., Kelly, D.C., Thomas, E., Nicolo, M., Raffi, I., Lourens, L.J., McCarren, H. & Kroon, D. 2005. Rapid Acidification of the Ocean during the Paleocene-Eocene Thermal Maximum. Science, 308(5728): 1611-1615


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