Past monsoon rainfall variations in peninsular India recorded in a 331-year-old speleothem

Abstract

An actively growing stalagmite collected from a cave located in the hills of the Western Ghats in the Uttar Kannada District of Karnataka, India, has been studied for stable isotope ratios of oxygen and carbon, width of growth layers and grey-level changes. Distinct carbonate layers, alternate coarse and compact, are seen in cross-section. Each couplet of compact and coarse layer is found to represent a single year. A total of 331 such couplets has been counted, indicating that the stalagmite started growing in AD 1666 and continued until it was sampled. Stable isotope ratios of oxygen (δ18O) and carbon (δ13C) show variations ranging from-13.6 to-7.9%° and from-2.7 to 1.6%°, respectively. We have reconstructed past rainfall changes of the cave site using the‘amount effect’ in &δ18O of rain. Speleothemδ18O and instrumental rainfall data from the associated climate subdivision show a significant correlation (r =-0.62, decadal average). Several sharp spikes of enrichment and depletion in 18O are indicative of the past deficiency and excess in rainfall. Most of the severe drought years recorded independently by meteorological observations are found registered in the stalagmite layers. During the 331-year-period, rainfall was highest at Añ 1666 and lowest around AD 1900. The stalagmite-generated past rainfall record can serve as a reasonable proxy for testing monsoon models.

Modification and preservation of environmental signals in speleothems

“…The speleothem data e.g. in southern and eastern China, Oman and India (Wang et al, 2001;Fleitmann et al, 2004;Yadava et al, 2004) have helped to demonstrate the sensitivity of monsoonal delivery of rainfall to subtle external drivers, as well as to the amplification by large-scale reorganization of the climate system that accompanied glacial-interglacial fluctuations. More generally, in tropical to sub-tropical areas, the long-term migration of the inter-tropical convergence zone (ITCZ) can be sensitively recorded by episodes of speleothem growth (e.g.…”

“…A number of authors (e.g. Hellstrom et al, 1998;Plagnes et al, 2002;Genty et al, 2003;Fleitmann et al, 2004;Yadava et al, 2004;Lachniet et al, 2004) have also drawn attention to climatic controls of covariation of the isotope species over time. For example, in regions where the amount effect is strong, reduced rainfall leads to heavier δ 18 O values which may be further enhanced by evaporation within the cave in correlation with δ 13 C (Fleitmann et al, 2004).…”

High-Resolution Monsoon Records Since Last Glacial Maximum: A Comparison of Marine and Terrestrial Paleoarchives from South Asia

“…The data thus generated has helped document the fluctuations in the past monsoon strength both in space and time. Different proxies such as planktic/benthic foraminiferal abundances, their stable oxygen and carbon isotope ratios [18,19,21,23,24,76,77], varved sediments [41,42], speleothems [2,[29][30][31][32], tree-rings [79], δ 13 C and δ 15 N of lacustrine and marine organic matter [10,73,[80][81][82][83], and pollen records [84,85] have been used to obtain paleomonsoon records with different resolutions. A few of the important proxies and their significance are shown in Table 2.…”

“…The surface productivity that manifests itself in many forms such as organic, calcareous, and siliceous productivity, also affects the carbon isotopic composition of the seawater, which is preserved in the calcitic ; RC27-23 [73]; 723A [19,20]; 63KA [44]; 39KG/56KA [41]; 136KL [40]; 3268G5 [21]; SK145-9 [23]; GC-5 [22]; SS3827G [24]; 126KL [36]; 31/11 [37]. Circles represent terrestrial speleothem records: Qunf [32]; Akalagavi [31]; Gupteswar, Dandak [30]; Sota [86]; Timta [87].…”

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“…The strong relationship between stalagmite growth rate and grey level indicates that increasing stalagmite growth rates may decrease detrital particle contents per unit volume of carbonate deposition. Furthermore, the high drip rate does not provide sufficient time for the detrital particles to become trapped, these particles are removed by the splashing of the next drip, which, in turn, results in high grey levels (whitish in colour) 19 . The grey level of stalagmite NJ1 closely correlates with the October rainfall amount (Table 2 and Fig.…”