" No heredamos la Tierra de nuestros padres, sino que la hemos pedido prestada a nuestros hijos"

Chief Seattle (1788-1866)

viernes, 16 de abril de 2010

Abrupt Climate change: extinctions 1st part

Palaeocene-Eocene thermal maximum (PETM)
Dublin 13th April 2010


Climatologists and Palaeoclimate experts know that the key to predict future climate is to look at the past. The accuracy of climate models depends on the degree to which they can reproduce past events so then we can use them to project future climate scenarios.
It is precisely by looking back to the geological history of the Earth and past climatic events that we scientists find a major source of concern regarding present and future changes in our climate.


http://thedragonstales.blogspot.com/

Fossil-hunters Mary Dawson and Robert West found in 1975 a big fossil bone in the Artic Ellesmere Island (Canada). That was an alligator dated 55 millions ago during the geological epoch of early Eocene. A reptile that today lives in warmer latitudes together with other subtropical species (Lynas, 2007).

How did these bones end up there? According to geologists tectonic plate’s movement could not explain it as this area of Canada was then close to its current position. Then how could this place have had such warm temperatures if this Artic area remains several months a year submerged in polar darkness?. Well maybe the Earth’s tilt (the obliquity of the ecliptic that causes the Earth to be further from the sun) could have been different reducing thus the seasonal difference. Nevertheless, evidences from fossil tree rings ruled out this possibility.

Not only that, dozens of other mammal species during the Eocene and the Palaeocene boundary (the epoch that preceded it), were found to have disappeared in North America around that time.

The one plausible explanation for those mammals emigrating from Asia to end up at these cold Artic places is that the Artic was a much warmer place long time ago. That is not all, something happened in the Sea as well. Two geologists James Kennett and Lowel Scott soon realised that the 10 cm layer of undisturbed mud they obtained at an ocean core at the Weddell Sea off Antarctica in 1991 coincided with this Paelocene-Eocene boundary and that this mud meant that almost everything that lived on the floor on that epoch died and deposited on the sea floor. They were witnessing the evidences of one the of the major deep-sea extinction events.

We are almost certain that the climate originated a sudden warming of the Antarctic waters deriving on a lack of oxygen that the dwelling organisms needed to live, so the deep ocean turned up to be an anoxic and poisonous place. Therefore at the time not only the Artic was warmer but also Antarctica was too!. But why? What could have possibly caused this? .

We know that at the earlier Cretaceous-Tertiary boundary (KTB) a big asteroid collided with the Earth and caused the extinction of the Dinosaurs, which have so far dominated the Earth for over 165 million years, among other species. There not such as evidences found now, neither super volcano nor crater impacts.



Figure 2 http://www.lachlanhunter.deadsetfreestuff.com


Much of The Cenozoic era, (last 65 million years of Earth’s history, see Figure 2) was characterized by higher concentrations of greenhouse gases, much warmer temperatures (than today) and barely no ice if at all. We are certain that 55 ma ago there was interval of extreme global warming (See Figure 3) when temperature rose by about 7 degrees Celsius in less than 10,000. This coincided with negative carbon and oxygen isotope excursions detected in marine and terrestrial sediments all over the world connecting this event to a big and rapid (10,000 yr or less) input of isotopically depleted carbon, implying large amounts of C12-enriched carbon being added to the ocean/atmosphere (Kennett & Stott 1991; Schmitz & Pujalte, 2003). More than 2,000 Gt of Carbon as CO2 was released (to find out more about how the isotopic concentration in atmosphere and oceans see Zachos et al., 2008).

That relevant event is known as the PETM (Paleocene-Eocene Thermal Maximum (See Figure 3). In fact the scientific community think that this particular event can serve as a ancient geological analogue for present-day Earth (Dickens, 1999) in terms of a record to the current trends in atmospheric CO2 rising levels.

Over the last 34 ma global temperature have been more or less cool, even allowing ice formation in polar areas partly due to volcanic erosions and changes in physical weathering of silicate rocks*.

*Note than on long timescales, the negative weathering feedback loop maintains Earth’s climate within a habitable range. During this weathering process sequesters CO2, preventing concentrations from rising too high or from falling too low. As the atmospheric CO2 concentration rises, temperature and precipitation increase and thereby enhance chemical weathering and the other way around.

Sources of carbon during the Cenozoic hyperthermals still remain uncertain. Carbon might have come from buried rocks, liberated as methane and CO2 during intrusive volcanism…

Gerald Dickens, an important palaeoceonographer at the University of Michigan and currently chief executive of the American Geophysical Union, focused his work on methane hydrate resources. He came across this substance during a drilling oil exploration campaign. These methane hydrates form at the deep floors of the ocean by the combination of high pressures, and the mixture of methane and water. It was while drilling that a big blast blew up the tube when the equipment disestablished into the methane hydrate plume causing an explosive chain reaction.



Figure 3: Evolution of atmospheric CO2 levels and global climate over the past 65 ma and Global temperature change (right scale) as inferred from oxygen-18 isotope measurements (left scale) from fossil ocean microorganisms (Zachos et al., 2001).

It is interesting to note that methane gas is a much more powerful green house gas than the CO2, and will remain longer time into the atmosphere. Dickens speculated that 55 million years ago melting and escape of methane hydrates buried in marine sediments into the atmosphere represented the carbon source (Dickens et al., 1995, 1997) and that has caused the global climate change. This heated up the oceans, which in turn caused more releases, so on so forth, a positive feed back.

But an evidence of a big submarine avalanche of methane hydrates at the possible time escape was needed and Katz founded at 512 m down an ocean core in the Florida’s coasts (Katz et al., 1999).

In the North Atlantic evidences of volcanism that lifted the water pressure over the hydrates facilitating the escape of possibly 2,800 billions tones of carbon was also found. That was enough carbon to interfere with the climate when was dissolved into the atmosphere. Evidences of the triggering mechanism needed for explosive release of methane during the Palaeocene/Eocene boundary comes from the sea floor off Norwa : an intrusion of voluminous mantle-derived melts in carbon-rich sedimentary strata in the northeast Atlantic, in the Norwegian Sea (Svensen, H. et al.,2004; Dickens, 2004) and latter at the Siberian traps 255 ma (Svensen, H. et al.,2004). There are also evidences from the South Atlantic deep sea oceans (Zachos et al., 2005). Of course some of the consequences of methane submarine explosions are the subsequent marine landslides and the associated tsunami as detected in the UK coasts associated to avalanches in Norway during the PETM.

What ever the cause, it was a global event; palm mangroves grew up as far as England, rainforests of dawn redwood metasequoia in the High Artic creating their own green house effect by releasing water vapour that insulated them during the cold polar winter. Only 200 km from the North Pole the sea water temperature was around 20 degrees. There was not ice in the Polar Artic, nor there wasn’t going to be for another 15 million years. First the ocean temperature was rising at the surface from 5-10 degrees and deep around 5 degrees. Waters became acidic and oxygen depleted, and global currents altered.

The amount of carbon dioxide in the atmosphere was 1,000 per million. Although this CO2 alone does not explain the total rise in temperature during the PETM. That is, theoretical models cannot explain what we observe in the geological record, somehow the link between temperature and CO2 in our models is not totally right (Zeebe et al., 2009). Richard Zeebe of the University of Hawaii and co-authors, Dickens and James Zachos of the University of California-Santa Cruz determined that the level of carbon dioxide in the atmosphere increased by about 70 percent during the PETM and concluded that models could only explain about half of the warming. They believed that we are leaving something out that should be included in climate models, some important feedbacks. Today's climate models include accepted values for the climate's sensitivity (increase of temperature with CO2 doubling) to doubling CO2 that it could be wrong.

Therefore all this could have been combined with massive amounts of peat burning, pumping more carbon into the atmosphere contributing to the positive feedback or even releases of seabed carbon as continental self’s became isolated and shallower.

There are evidences than NO2 and CH4 should be higher under wetter and warmer conditions. Such feedbacks would enhance the sensitivity of climate to changes in CO2 and might explain the unusual polar warmth of the early Cenozoic (Zachos et al., 2009). Paleao-obervations also revealed hydrological cycle alterations in response to extreme changes in atmospheric C O2 and temperature.

Humans have not yet experienced such a high levels of carbon dioxide in the atmosphere. The rate at which we add carbon into the atmosphere is faster than any other time since life is on the Earth (Wolff, EPICA project, 2008), perhaps 30 times faster than during the PETM. If we compare isotope ratios from the Palaeocene-Eocene we are only half way through to reach the heat wave climate experienced then.

As the weather warms many changes might occur: the warmth of the ocean can violently release more methane into the atmosphere accompanied by permafrost melting at high altitudes that will release more. This could derive in to an unstoppable chain of feedbacks.

The Artic is the place to watch as there are areas currently covered by ice that could soon be left exposed without ice. We know that the warming acceleration the Artic is higher than other places due to its position related to sun. What is scary is that only 5 years ago the projections for the Artic ice to disappear were expected by the end of the century now some experts argue that it could go free of ice in summer by sooner than 2015-2020.

We don’t know exactly how fast this happened in the past, it could have happened across thousands of years but now the warming occurs at a faster rate so the warming of the sea floors could accelerate considerably. We don’t even know how much reservoir is out there (Lynas, 2007).

We have seen that there are some uncertainties about how the future is going to be. ‘The past greenhouse events provide only glimpses of the future, so until dynamical models can replicate the extreme features of these events (i.e: the global patterns of carbonate deposition or the extreme polar warmth, long future forecasts of climate beyond the next century (that is, under extreme greenhouse gas levels) should be viewed with caution’ (Zachos et al., 2008) but never free of concern. We should never forget that large amounts of greenhouse gases being pumped every year into the atmosphere and what it means in physical terms (warming). If relevant associated changes are not yet happening they will indeed, pure physics.

Silvia Caloca

References

Dickens, G. R., O'Neil, J. R., Rea, D. K. & Owen, R. M. 1995 Dissociation of oceanic methane hydrate as a cause of the carbon isotope excursion at the end of the Paleocene. Paleoceanography 10, 965−971 (1995) Article ISI .
Dickens, G. R., Castillo, M. M. & Walker, J. C. G.1997. A blast from the latest Paleocene: simulating first-order effects of massive dissociation of oceanic methane hydrate. Geology 25, 259−262 (1997).
Dickens, G R. 1999. ‘Blast in the past’. Nature, 401, 752-5.
Dickens, G R. 2004 .Hydrocarbon-driven warming. NATURE VOL 429 3 JUNE 2004.
Katz et al.,1999. The Source and Fate of Massive Carbon Input During the Latest Paleocene Thermal Maximum. Science 286, 1531 91999); DOI: 10.1126/SCIENCE. 286.5444. 1531Mark Lynas 2007. "Six Degrees: Our Future on a Hotter Planet". Fourth Estate March 2007 ISBN 000720904.
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 (1991) Article ISI
Schmitz, B. & Pujalte, V. 2004 Sea-level, humidity, and land-erosion records across the initial Eocene thermal maximum from a continental-marine transect in northern Spain. Geology 31, 689−692 (2003)
Svensen, H. et al. 2004 Nature 429, 542−545 (2004).
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.; et al. (2005). "Rapid Acidification of the Ocean During the Paleocene-Eocene Thermal Maximum" (PDF). Science 308 (5728): 1611–1615. doi:10.1126/science.1109004. PMID 15947184.
Zachos J.C, Dickens. G R., & .Zeebe R.E, 2008. An early Cenozoic perspective on greenhouse warming and carbon-cycle dynamics NATURE Vol 45117 January 2008doi:10.1038/nature06588.
Zeebe .R E., Zachos. J. C., Dickens. G.R., 2009 Carbon dioxide forcing alone insufficient to explain Palaeocene-Eocene Thermal Maximum warming. Nature Geoscience, 2009; DOI:10.1038/ngeo578.
http://news.bbc.co.uk/2/hi/science/nature/5314592.stm
http://www.epa.ie/downloads/pubs/other/events/oclr/ccvideo/#d.en.24330



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