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The World in a Grain of Silica

Updated Tuesday 25th March 2008

The study of grass phitoliths can reveal what the weather was like ten thousand years ago.

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It was 11.30 pm at Lucknow railway station and it was still hot – around 30 °C. As I gazed out of the train window I saw crowds milling on the next platform, a sort of human Brownian motion, in the middle of which was a cow placidly oblivious to all the frantic activity. Chewing as only cows chew. Then I realised that that particular platform is only accessible by footbridge….

As I pondered the cow’s presence the train started to move, imperceptibly at first, and then itslowly gathered speed. I was on my way to Agra to see the Taj Mahal. The overnight three tier AC compartment was crowded and I climbed on to my pre-assigned top bunk. There was not quite enough room to sit up below the carriage roof. It was a clear drop of two metres to the floor. Combined with the swaying of the train it was enough to generate the unlikely combination of vertigo and claustrophobia. Amazingly I slept well.

Just after dawn the train rattled over the Yamuna River Bridge and through the open carriage door I peered down to where people were washing clothes on the river banks. By 7 am I had arrived at the Taj Mahal and, as it hung in the soft morning light, it lived up to all that has been said about it, and more.

The rest of the day I travelled all over Agra taking in the sights. There was so much to see within the city but I took the time to travel the 40 km out to Fatehpur Sikri. This magnificent red sandstone complex was built by the Mughal Emperor Akbar who briefly made it his capital. Fortunately the Jama Masjid mosque is in good repair and is one of the finest examples of Indo-Islamic architecture. Much of the rest of the complex is largely in ruins now, the victim of water shortages that became apparent soon after it was completed around 1571. Water, or rather the lack of it, has been a constant issue for the northwestern part of India and is likely to remain so under foreseeable future climate change scenarios.

The 54 metre high Buland Darwaza Gate at the entrance to the Jama Masjid Mosque. Copyrighted image Icon Copyright: Bob Spicer
The 54 metre high Buland Darwaza Gate at the entrance to the Jama Masjid Mosque

The persistence of this problem and the dependency on the monsoon rains was brought home to me recently during a talk given by a PhD student, Vartika Singh, who was reporting on her work as part of the Quaternary Research Group here at BSIP. This group has made detailed studies of climate change in northwestern India over the past ten thousand years using, alongside the more conventional studies of fossil pollen, diatoms and dinoflagellates, magnetostratigraphy and radiocarbon dating techniques, and things called grass phytoliths.

Phytoliths are made of silica (like sand) and are secreted within and between cells. Amongst other things they serve as a deterrent against browsing. To counter this, grass-eating animals like the cow on Lucknow station, have developed teeth that constantly grow as they are worn down by the abrasive action of the phytoliths. Incidentally we see the same plant/animal “arms race” towards the end of the Cretaceous Period of geological history when similar dental replacement systems evolved in herbivorous dinosaurs. Grass-like phytoliths have been found in fossil dinosaur dung. Because there is no evidence in the Cretaceous for extensive grasslands the dinosaurs may well have also been feeding on the ancestors of modern horsetails or scouring rushes (Equisetum). These appear just about everywhere in the Cretaceous. Equisetum also possess phytoliths, which is why they can be used to scour pots and pans.

A dumbell-shaped grass phytolith. 100 placed end-to-end would measure 1 mm Copyrighted image Icon Copyright: Bob Spicer
A dumbell-shaped grass phytolith - 100 placed end-to-end would measure 1 mm

It turns out that phytoliths have shapes that are specific to particular kinds of grasses and, unlike the organic remains of the grasses, they preserve well in dry and semi dry environments. This means they are ideal for recording changes in vegetation in such drought-prone regions.

The work of Vertika Singh and colleagues shows that there have been significant changes over the last ten thousand years in the strength of the monsoon winds coming on to northwestern India from the Arabian Sea. These have affected the distribution of rainfall throughout the year. A weakened monsoon results in lower summer rainfall but wetter winters. A stronger monsoon leads to wetter summers but drier winters.

A weak monsoon and a more even distribution of rainfall coincided with the rise of the world’s earliest . Weakening of the monsoon occurred gradually from around 5,500 years ago and the weakening became more marked between 3,600 and 3,400 years ago. Thereafter, a strengthening monsoon led to winter drying and a decline in the Indus Valley agriculture and society.

Remembering that the Indus Valley agriculture supported the world’s first large scale cities and was the source of many aspects of Indian and global culture today, the influence of the monsoon on human history has been profound. However at present the interpretation of phytolith and other data in terms of monsoon dynamics necessarily remains a little speculative.

The next stage in this fascinating research is to apply the same techniques to the area that drives the monsoon - the Tibetan Plateau. Here aridity and a mainly grass-dominated vegetation favour phytolith studies. If changes in vegetation and climate on the plateau can be accurately characterised and dated, they can be compared to coincidental changes in northwestern India. By understanding better the Tibetan Plateau/monsoon links in this way it should be possible to predict more reliably future rainfall patterns upon which so many people in Asia depend.

 

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