The Indian economy is primarily dependent on harvests, which in turn are regulated by monsoon rains. Frequent droughts and floods can not only have severe repercussions on public health, but significantly impair a country’s economy.
Even when the monsoon is regular, one notices significant variations in the intensity, occurrence and duration on annual time scales. On longer timescales (centennial or millennial) the rains are found to be modulated by the interplay between the intensity of the insolation in the northern hemisphere, northward movement of the inter-tropical convergence zone (Gadgil, 2003) and changes in ice extent during glacial-interglacial periods (Zhinseng et al., 2011). These governing factors operate on different timescales and control decadal, centennial and millennial variability of monsoon rains.
The fall of the Indus valley civilization could be partly attributed to the significant receding of monsoon over time (Staubwasser, 2003). Given the importance of rain and water for human survival, we need to understand and mitigate the causative processes behind droughts. Some concerns regarding monsoon are:
- Can natural changes in rainfall patterns be anticipated?
- Are their magnitudes predictable?
- Can the response of the Indian monsoon to anthropogenic disturbances be quantified?
- Can this information be used in future planning of water resources and food security under a changing climatic regime?
The above questions require dealing with a complex earth system that needs continuous instrumental monitoring of meteorological parameters and the evolution of mathematical models to correctly simulate such observations. We also need to investigate the variability and conditions of the earth’s climate in the near and distant past. This involves the study of palaeoclimatology in order to understand the climate that preceded the era of direct measurement.
Instrumental records of monsoon rainfall extends back to a maximum of 150 years. Further back, historical records give qualitative estimates for a few hundred years. For long term reconstruction of the monsoon rain scenario, natural archives that preserve signatures of monsoon rain in the form of faunal and floral fossils, characteristic chemical constituents and isotopic compositions are resorted to.
The most widely used and accepted archives for past monsoon reconstruction are the annual rings on trees, speleothems of cave deposits such as stalagmites, lake and ocean sediments.
The word speleothem is derived from the Greek ‘spelaion’ meaning cave and ‘thema’ meaning deposit. Speleothems are secondary carbonate deposits formed in karstified limestone rocks. The formation of speleothems is explained in Figure 1.
As rain water infiltrates into the soil, carbon dioxide from plants and bacterial decay react with water to form carbonic acid. This mildly acidic water starts corroding the bedrock beneath, forming bicarbonate ions. When such soil water, rich in bicarbonate and carbonate ions, comes in contact with the cave environment, carbon dioxide starts degassing and calcium carbonate is precipitated.
The oxygen isotopes in the precipitating carbonate are used to infer the amount of rainfall using the principle of ‘amount effect’ (Dansgard, 1964). For instance, whenever the amount of rainfall is low, the carbonate has a higher oxygen isotope ratio, 18O/16O. That is there is more 18O as compared to the normal 16O variety. On the other hand, during higher rainfall, there is a relative enrichment of the lighter oxygen isotope, hence the 18O/16O ratio is lower. Thus, heavy rainfall events are marked by lower 18O/16O values and droughts show higher 18O/16O values. For about 100 mm change in rainfall, the isotope ratio changes by 0.15 per cent, and the precision of measurement of isotope ratios is 0.01 per cent. Thus, past changes in rain of the order of <7 millimeter (mm) can be detected using an isotope ratio mass spectrometer.
Speleothems can be of various types—stalactmites, stalagmites, flow stones or soda straws. Of these, stalagmites are widely used for paleoclimate reconstructions because of their simple geometry. Using uranium-thorium dating technique, speleothems as old as 500,000 years can be dated. Since most speleothems have annual to decadal growth layers, they are used to reconstruct past precipitation or temperature records.
Speleothems in India
The use of speleothem as a proxy to reconstruct the Indian monsoon is well established at the Physical Research Laboratory, Ahmedabad. In India, speleothem formations are found wherever there are large outcrops of limestone bedrocks (fig. 2). A cluster of caves is found in Kanger Valley National Park, Chhattisgarh. As this area falls within the core monsoon region of India, speleothems here are likely to have vividly captured changes in monsoon precipitation in the past. Monsoon variability in the 14th and 15th centuries is well recorded in a 900-year old, 600-1500 AD stalagmite from the Dandak cave. Major droughts seem to have lasted from 1396-1409 AD. Floods during the Medieval warm period of 900-1350 AD are also recorded here. Monsoon variability during the Little Ice Age and warming in medieval times are also correlated with change in the solar activity (Sinha et al., 2007).
Based on the 3400 year BP (in palaeoclimatology 1950 AD is considered as present, marking it as 0 year BP) old Gupteshar stalactite sample, it was inferred that high rainfall persisted from 3400-2900 years BP with declining monsoon intensity during 2900-1200 years BP. Since then, an increase in precipitation has been recorded till present.
Speleothems elsewhere in India have also helped in monsoon reconstruction. A 331-year old stalagmite, from the Akalagavi cave of Northern Karnataka, India, revealed distinct annual layers. Variability in the monsoon rain during 1650-1997 AD, with the highest precipitation at 1666 AD and the lowest around 1900 AD (Yadava et al., 2004) was observed. Another stalagmite sample from the Valmiki cave, southern India, covering a time span of 15,700 to 14,700 a BP was used to infer abrupt changes in monsoon rain during the last deglaciation (Lone et al., 2014). Solar forcing and strong ocean-atmospheric circulation were suggested as possible controllers of the Indian summer monsoon dynamics (Allu et al., 2014).
The study of speleothems like stalagmites can give us an idea of the frequency or otherwise of droughts and wet seasons. A growing area of research across India, speleothems are a pathway to a number of climate related studies. Learning and understanding the past can equip us to handle our climate scenario far better and plan ahead in a changing climate scenario.
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Lone, M.A., Ahmad, S.M., Nguyen, D.C., Shen, C.C., Raza, W. & Kumar, A. (2014). Speleothem based 1000-year high resolution record of Indian monsoon variability during the last deglaciation. Palaeogeography Palaeoclimatology Palaeoecology, 395, pp1–8. doi: 10.1016/j.palaeo.2013.12.010.
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Yadava, M.G., Ramesh, R., & Pant, G.B. (2004). Past monsoon rainfall variations in peninsular India recorded in a 331-year-old speleothem. The Holocene, 14(4), pp517-524.
Zhisheng, A., Clemens, S.C., Shen, J., Qiang, X., Jin, Z., Sun, Y., … Lu, F. (2011). Glacial-Interglacial Indian Summer Monsoon Dynamics. Science, 333, pp719–723. doi:10.1126/science.1203752.