When the Earth Warmed Up!

Global warming is very much in the news and we read about the possible consequences everyday. One has just to turn a few pages of the Earth's history to realize that the past has been worst. Of course we were not around, but several genera of animals and plants were not able to bear the heat and became extinct. Many of them changed their shapes and sizes; many migrated to far off lands/oceans, never to return. Climate history of the earth some 55 million years ago (m a) is a pointer towards how gory can be the global warming!

The present era of the earth's history is known as the Cenozoic Era and it started some 65 m a. The first two chapters of the Era are called the Paleocene and Eocene respectively. It was towards the end of the Paleocene, that is, 55.5 to 54.8 m a sudden global climate change upset oceanic and atmospheric circulation causing extinction of numerous deep sea marine forms and compelled the mammals to evolve on lines which continue till present. This sudden warming is called as Paleocene-Eocene Thermal Maximum (PETM).

Those were the days when humans had not even evolved, as such anthropogenic activities like hacking of forests or smoke belching industries were impossible. Then why such a thing happened? 

Right now we observe the climate change in terms of months or years, but we forget that real changes take eons. During the PETM the sea surface temperature rose by 5 to 80C in a few thousand years. Thus the change was there and is considered to be quite rapid; unlike the belief 'this winter is warmer than the last one'! 

By now the readers must be wondering, 'was it real or just a figment of imagination of the geologists'? 

Professor Prabha Kalia an eminent teacher and Marine Micropalaeontologist from the University of Delhi, elaborated about the rapid climatic changes in the Paleocene and Early Eocene in her Key Note Address at the Indian Colloquium of Micropalaeontology and Stratigraphy (ICMS) recently organized at the Birbal Sahni Institute of Palaeobotany (BSIP) at Lucknow.

She says that the episode of PETM towards the end of Paleocene and the beginning of the Eocene has many parallels with the contemporary climate crisis. This very fact has attracted s wide range of interest amongst the palaeontologists, geochemists, sedimentologists and earth system modelers. Though there is plenty of evidence from the deep sea drill cores of sediments about greenhouse forcing for the PETM that warmed all the latitudes she says. However, there is little evidence from the tropics of that era and hence it is poorly defined.

It seems that the level of green house gases during the PETM had suddenly increased. Who was belching Carbon Dioxide or methane in those good old days, no one really knows. But the prolonged global warming for about 50 to 200 Ka of the PETM was probably driven by the concentrations of atmospheric Carbon Dioxide. Was it some external event, like impact of a comet and exhalation of heat generating methane or was it from within the earth, like sudden release of marine clathrates, oxidation of terrestrial organic carbon or oxidation of organic matter due to uplift of epi-continental seas! 

While searching the clues for PETM, geologists stumbled upon mass extinction of benthic foraminifers. Benthos makes the deepest part of the ocean or a lake. The tiny unicellular organisms, foraminifers (of benthic type) reside in swarms in the deepest parts of the seas. They are used to extreme cold and great pressure of the water columns above.

As the PETM set in sea surface temperature (SST) rose by 50C in the tropics and as much as 8-100C at high latitudes, whereas bottom water temperatures increased by 4 to 50C. At that time globally there was a carbon isotope excursion (CIE) of about 3.0 per mil as revealed in the deep sea cores. Those researching on PETM found mass extinction of benthic foraminifera at the onset of CIE. At that time the calcium carbonate content of the sea water also diminished. Geochemical changes in the ocean were such that for few thousand years seafloor carbonates were dissolved. The swarms of foraminifera occur as 'carbonate ooze'. Instead of Carbonate there was a black mud. Oxygen content of the ocean water was drastically reduced. Marine life was just stifled to death.

In the absence of calcium carbonate the foraminifera could not be calcified and preserved and lost in the abyss.

During PETM event even flora faced the brunt of the warming. Jathang and Cherrapunji areas in Khasi hills of Meghalaya have rock formations that represent the transition from Palaeocene to Eocene say Vandana Prasad, Rahul Garg and Khowaja-Ateequzzaman in their paper presented at the ICMS organized recently by the BSIP at Lucknow. Their study revealed considerably low diversity of pollen and spores of land plants prior to the PETM. Once the PETM was set the diversity pattern was enhanced appreciably.

Excessive warm and humid climate of PETM in low equatorial zone (that was the position of the subcontinent those days!) made the sea level rise. There was much greater influx of sediments as the water flow from land to sea too was enhanced. Fresh water marshes and brackish lagoons were formed where the remains of land flora were deposited.

In a nut shell the global warming of the yore had completely changed the chemistry of the sea and the reproductive growth of the plant landforms. They had proliferated. As a consequence pollen rich zones were formed in the lagoons, which form the present rocks of parts of Meghalaya.

The impact of PETM continued in the Eocene for about 80,000 to 200,000 years. The giant mammals of earlier days gradually adopted and adapted to the global warming and reduced their sizes. Fast growing trees and extensive forests came up. It is amazing that the atmospheric carbon was thought to have been 2,000-3,000 parts per million (ppm) compared with 380 ppm today. A study of cores from the Atlantic indicated that between 2000 and 5000 gigatons of carbon was released in the atmosphere during the period.

Biotic data from all over during the Palaeocene-Eocene period indicates prolific life forms. Except areas where the warmth was unbearable as in the case of benthic foraminifera, they had no chance to survive.

Compared to those days our situation is much better. It is not possible to stop the natural processes. But it is definitely possible to check pollution of rivers, surface and ground water and atmosphere. It is certainly possible to increase the green cover and stop hacking of the forests. The forests of developing countries like India face the maximum brunt of human axe. It is time for the society to come forward and put a stop to ravaging of forests.

One must remember that the Nature never sleeps. The natural forces remain active always. Therefore instead of challenging the nature we should accept the challenge of developing an eco-friendly society. Else face something like PETM, proliferate and perish.


More by :  V. K. Joshi (Bijji)

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