The endless sheet of rippled white sand of a desert with dunes dotted here and there makes one feel as if it was a sea minus water! The deserts have always fascinated the explorers. One gets glimpses of past civilizations buried in sands of time. Were deserts always like this? Or were they green lands, home of mankind and home to a variety of fauna and flora? Well if the latest researches are to be believed the deserts were not that bad as they are today.
They became deserts over a period of time because of the climate change. Even today world?s 37% population resides in deserts only. Human population means a stress on the deprived land. The dragon of global warming or climate change has been threatening the world society. Will our future be like present day deserts? Is a question that haunts the mind of the environmentalists. That is why a study of past climates of deserts has assumed significance today. Did these areas turn in to a desert because of human pressures in the past along with rapid climate change is also another question whose answer is being sought by the experts.
A.K. Singhvi of Physical Research Laboratory (PRL), Ahmedabad and Naomi Porat of Geological Survey Of Israel, currently working at PRL are two renowned authorities on desert studies. Using Luminescence techniques on desert sands they have been able to work out the ages and climates of those ancient periods of the deserts of both northern and southern hemispheres. They published their researches in the famous journal of Quaternary Research-BOREAS in July 2008. Here is a report.
While reconstructing the past climates one needs clues to proceed further. Because of paucity of plant or animal life in the deserts very often enough organic matter is not available to the researchers to establish dates of past events. Thus in case of desert sands, they realized that if somehow they could be dated the problem could be solved. Luminescence techniques tell the scientists how long ago the sample of sediment was exposed to day light. With the state of art instruments now available, this technique has gained lots of significance amongst the researchers looking for the clues on past climates.
The semi-arid and hyper-arid regions of northern and southern hemispheres where evapo-transpiration is more than twice precipitation drew special attention of Singhvi and Porat. Moreover deserts are least disturbed by living beings, specially the human beings. Therefore deserts are ideal locales to study the interplay between climate and geology. Desert regions are not only affected by climate but can influence climate in a significant manner. The albedo (reflectance) of desert sands is only next to that of ice. The expansion and contraction of deserts leads to large scale albedo changes, which in turn affects the climate of a much wider region, say Singhvi and Porat. Desert dust carried by winds to far away place plays an important role in modulating climate. Dust can cut off the sun light for months together. Similarly records of past climates from regions covered by wind born sand and or silt (loess) provide useful information about various parameters of climatic changes of the past.
Quoting K. Munyikwa and M. Smith and P. Hesse, Singhvi and Porat say that Aeolian mobilization (that is sand blown by wind) in the Deserts of Southern Hemisphere at its peak between and 65000 and 45000 years before present. However, in Australia this mobilization took place between 35000 to 9000 years before present. These phases were synchronous of similar event between 60000 and 57000 years and between 36000 and 8000 years in southern Africa. Munyikwa et al had collected an additional phase of mass movement of sand from 46000 to 41000 years in South Africa. Sands of Australia on the other hand showed more moisture content in the period 45000 to 35000 years. South American deserts perhaps did not yield much information, but these workers could establish an unstable landscape between 30000 to 8000 years ago.
Quoting Bubenzer and other researchers, Singhvi and Porat have given a detailed analysis of the deserts of the Northern Hemisphere. The sands of Sahara were subjected to luminescence dating and it was found that accumulation of sand dunes began some 20000 years ago and till 13000 years it continued unabated. While the Sahara faced active dune building in the period cited, in Sahel region, immediately south of Sahara dune building was low. Perhaps due to limited sediment supply!
The period between nine to five thousand years before present was a period of reduced dune building and during this period soil was formed over the dune. It is yet to be established if feature was uniform all over the Sahara. But it is known that even during this period there were several dry spells. Wind regimes changed several times in Western Sahara during the past 25,000 years.
Luminescence dating of sand grains attached with the stone implements indicated that the stone-age man was busy making his tools in this region between 130000 to 70000 years and 50000 to 40000 years ago.
Wahiba dunes in Arabian Desert at Rub Al Khali have sand which are approximately 75000 years old. These sands are found within the layers of rocks and with the help of Optical Luminescence dating and direction of the layers of rocks, Singhvi and his colleague were able to establish that southwestern monsoon and the northwestern winds (Shamal system) were at play.
Part of the Wahiba information matches with that of the Dune record of Thar Desert in India. The oldest dune construction took place between 165000 and 143000 years. After a lull dune construction activity started again 77000 to 63000 years ago. And again during 50000 years to present there have been several episodes of dune accretion. Southeastern Arabia got more moisture between 35000 years and 250000 years and between 10000 to 6000 years. Aridity prevailed between 25000 to 10000 years and since 6000 years.
Thar Desert of India is on the edge of mid-latitude tropical belt records of past sands are better preserved here. Evidence of deserts being there 165000 years ago removes doubts regarding enhancements of desertification process due to anthropogenic activities. Dune building activity in Thar has been of episodic nature. Major phases took place 115 to 100000 years, around 75000, 55000, 30000 years and 11000 to 13000 years before present.
Singhvi and his colleagues quoting others (Wasson) say that dune building activity in Thar is associated with winds preceding the SW monsoon. During last glacial maxima That is around 18000 years ago, winds became weak, consequently there was less of dune building activity. About four thousand years later winds picked up speed as the monsoon began picking up. But an increased monsoon again reduced the sediment supply for dune building. Therefore in the core of the desert a peak in accumulation of wind born sand at around 12000 to 13000 years ago is deciphered from luminescence dates of sediments and again around 10000 years ago. Both times it was the re-establishment of monsoon winds that enhanced sand dune building.
Based on the data available Singhvi and Porat infer that the sand dune building activity was not due to human related factors, but yes the dune migrated rates changed tenfold due to human interference. About 2000 years ago dune migration rate was 0.9 cm/yr. This was reduced to 0.25 cm/year between 500 and 200 years ago. But since past 200 years it has been about 1.5 to 9cm/year.
Luminescence dating of sediments from Luni River in Thar showed a complete fluvial regime with high floods occurring one thousand years ago. In other words the desert of today was a greenery of the yore.
Nature changes and no one can stop that. But yes we should be able to check our misdeeds. Fiddling with vegetation cover can lead to increased rate of dune movement and it can enhance desertification of areas which are still green. For better future we have to restrain hacking of trees.