Fizz in the Soil Carries Mysteries of Past Climates

The effervescence in the aerated soft drinks is because of carbonates in them. The soil and sub-soil too have their share of carbonates. We all know that water is the best solvent. Therefore the river waters since times immemorial have been carrying dissolved carbonate with them. These get deposited in the floodplain and river channel from time to time. Apart from rivers, soils, groundwater, lake and pond deposits also have carbonates. These deposits are known as calcrete and are found all over the globe. Wide occurrence has given them local names, such as kankar, rimrock, chebi-chebi, giglin, dhanda, mbuga etc. 

Calcrete are formed as water containing carbonates of calcium and magnesium passes through rock or soil through the pores and capillaries and leaches out the carbonates. On flat alluvial surface flowing water forms slight depressions which are shallow and wide. Water often collects in these depressions called 'pans'. When the temperature is high, humidity is low and annual rainfall is low, the water evaporates rapidly and the carbonates become more concentrated until the solution is saturated. These carbonates are precipitated out of solution on to the bed of depression. Over a period of time as the precipitation thickens carbonate rich crystals are formed and the resultant material is the calcrete.

Such carbonates are common in areas that have a dry climate, as well as in sub-humid areas with a marked dry season. They are useful palaeo-environmental indicators for alluvial succession as proxies. The proxies are measurable descriptors for desired but unobservable variables.

Sediments of the flood plains have the history of climatic changes preserved in them. Proxies are used to reconstruct the flood plain history. Calcretes from Ganga Plains have been studied by R.Sinha of IIT, Kanpur and his associates from Delhi University, IIT Kharagpur and from Dalhousie University, Halifax, Canada. Similarly D.A. Sant of M.S. University, Baroda and his associates from Indian Institute of Science, Bangalore and Indian Institute of Geomagnetism, Mumbai have studied the proxy records from Mahi River basin in Gujarat. Both studies are published in the proceedings of the Diamond Jubilee International Conference held in November 2006 at Birbal Sahni Institute of Palaeobotany, Lucknow and the Journal of the Indian Geophysical Union, January 2004.

Quaternary or the last chapter of the earth's history which started about 1.75 million years ago and still continues has most of the mysteries of the climate that affect today's human society. We have contributed to accelerate the warming phase of the present day earth, but nature had started it much earlier. Around 18, 000 years ago after the glaciation reached its maximum state and the glaciers started to melt due to warming, the climate entered a warmer phase. 

In the later part of the Quaternary the Ganga plains are abundant with calcrete deposits say Sinha and his associates. For their study they chose the interfluve or the part of the alluvial plain between the Ganga and the Yamuna Rivers at Kalpi. This area is part of southern Ganga Plains and has a large variety of carbonates present. In alluvial plains it is difficult to study the layers of river borne sediments in a vertical section. The river brings sediments in its different 'moods'. Sometimes furious and flooded like a naughty or angry child and at others calm and placid like an infant asleep. Energies of the river vary in various 'moods'. Naturally the load it carries also changes. Layer after layer the varieties of sediments are deposited. These scientists chose Kalpi because river cliffs there are high and extend for several kilometres, constituting one of the best exposed sections in the plains. They found the succession of varieties of carbonates deposited here quite reliable and free from contamination. 

The soil generated carbonates at Kalpi were aggradational floodplain deposits. Meaning they were deposited year after year when the rivers were in an angry 'mood' –in spate. Such deposits correspond to a relatively high monsoonal precipitation and consequently increased river discharge. On the other hand the carbonates associated with groundwater at Kalpi have degradational surfaces or discontinuities. Such carbonates correspond to relatively low precipitation.

Within the interfluves of the Ganga and the Yamuna Rivers channels have developed as in any normal river system. In these channels Sinha and his colleagues found calcrete derived both from groundwater and soil. These are a mixed type of calcrete. 

It is the dating of the cement of these different types of calcretes that helps the palaeo-climatologists to find out if the area was experiencing droughts or excess precipitation in the past. The cement of the mixed type of calcrete at Kalpi shows 'alpha fabric' which is indicative of dry conditions. The isotopes of carbon and oxygen (δ13C and δ18O) suggest very little variation in precipitation and vegetation. The floodplains of the Ganga-Yamuna interfluves have two types of plant remains, viz. the C4 and C3 types. The former are the plants that 'fix' atmospheric carbon through photosynthesis and the later use a more primitive way of fixing carbon with the help of an enzyme. In the evolutionary tree of plants first the C3 type appeared on the land then the C4 type. It was found that there was a higher proportion of C4 type of plants in the floodplains. The C3 type being restricted to the channel deposits only. 

When it comes to proxies of past climate record many cross checks are used. One of them is the use of Marine Isotopic Stages deduced from alternate warm and cool period of the earth's past climate. Based on the study of the isotopes of carbon and oxygen from the marine sediments/organisms these stages are chronologically arranged as MIS1, 2, 3, 4 etc. What Sinha and his colleagues found was that the calcretes of Kalpi represented a succession of 60000 years in terms of MIS 3-5, yet the vegetation remained constantly the same! During this period, Asia experienced radical fluctuations in monsoon intensity and precipitation. Sinha credits the anomaly to preferential preservation of aggradational strata that represent relatively active monsoonal periods, as well as by the mixing of drier floodplain (C4) and riparian (C3) vegetation. 

Similarly Sant and his colleagues tried to reconstruct the climatic history of Mahi River floodplains in Gujarat. They found that sediments of Mahi floodplain fall within the time span of 30000 to 10000 years. Calcrete and other proxies have identified strong and weak monsoon events. The sediments of the Mahi floodplain cover a considerable part of western India. Hence the alternating strong and weak monsoon of the past suggests impact of glaciation and glacier melting on the regional and global climate. 

Though a lot of studies are still required to develop proxy records of the past climates throughout the country, yet it is certain that it was the rise of Himalayas that controlled the climate. And again the ice melting in the Himalayas is going to affect the future climate alas it is beyond our control. We have already damaged the eco-system by hacking the trees and now our children have to face the consequences. 


More by :  V. K. Joshi (Bijji)

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