Madhusudan G. Yadava

Is there a persistent control of monsoon winds by precipitation during the late Holocene

Past variations in the south Asian summer monsoon (Southwest Monsoon, SWM) have been inferred earlier mainly using wind proxies in marine sediment cores from the upwelling-dominated western Arabian Sea. Here we report precipitation proxies, i.e., high-resolution stable oxygen isotope variations, of two different species of planktonic foraminifera, namely, Gs. sacculifer and Gs. ruber, in an AMS 14C dated sediment core from the monsoon-runoff-dominated eastern Arabian Sea. The above data sets reveal that during the past ∼1800 years (and perhaps up to ∼2800 years; resolution of the wind record is too low for comparison) reductions in monsoon wind strength in the western Arabian Sea appear to be persistently accompanied by aridity over India. It clarifies that past fluctuations in SWM precipitation over the Indian subcontinent followed the wind intensity records from the western Arabian Sea on centennial time scales. We further provide the first paleoclimatic evidence for the control exerted by SWM precipitation over the SWM wind strength via latent heat release.

Potential of Stable Carbon and Oxygen Isotope Variations of Speleothems from Andaman Islands, India, for Paleomonsoon Reconstruction

The Indian monsoon activity, coinciding with the Inter-Tropical Convective Zone (ITCZ), progresses from the southern Indian Ocean during the boreal summer and withdraws towards the south in winter. Islands situated to the south of India receive, therefore, the first and last showers of the monsoon; speleothems in such islands have not yet been explored for their potential to reconstruct past monsoon rainfall. Here, we present the first measurements of stable carbon and oxygen isotopic compositions (δ13C and δ18O) of a stalagmite collected from the Baratang Island of Andamans, along with new data on δ18O of modern monsoon precipitation (May to July 2010). The aim was to detect (i) whether these samples are amenable to dating using 14C, (ii) whether their oxygen isotopes indicate precipitation under isotopic equilibrium, and (iii) if (i) and (ii) above are true, can we reconstruct monsoon activity during the past few millennia? Our results indicate that while δ18O of speleothem does show evidence for precipitation under isotopic equilibrium; dating by 14C shows inversions due to varying contributions from dead carbon. The present work highlights the problems and prospects of speleothem paleomonsoon research in these islands.

Chronology of major terrace forming events in the Andaman islands during the last 40 kyr

Major earthquakes that trigger tsunamis are great natural hazards. The devastations caused by the December 26, 2004 Sumatran earthquake, and the March 11, 2011 Japan earthquake, and associated tsunamis will remain in our memories for a long time. Such events reaffirm the need for studying the cause and effects of large earthquakes of the past and to prepare the world better for the future. In such an effort, to understand the pattern of earthquakes and their effects on the geomorphic evolution, we have studied deformation history in the Andaman and Nicobar Islands, located in one of the most active convergent margins of the world. Focusing on tectonically formed coastal terraces and determining the timing of their formation from the exposed dead corals, we have been able to reconstruct the history of major earthquakes in these islands for the last 40 kyr. Our results in conjunction with the existing radiocarbon age data from coastal terraces of these islands appear to suggest that the frequency of major earthquakes (M > 7) in the region has increased during the last 9 kyr. In confirmation with some earlier work, we find evidences for a major earthquake and a tsunami between 500-600 cal yr BP and possibly 4 others during 6–9 cal kyr BP. Our results also indicate that there has been a continuous subsidence of the south Andaman Islands.

Evidence of Late Quaternary seismicity from Yunam Tso, Lahaul and Spiti, NW Himalaya, India

A relict fluvio-lacustrine sediment of an 8 m thick section exposed at Kilang Sarai along Yunam river, near Baralacha La shows presence of cycloids or pseudonodules, ball and pillow structures, flame-like and pocket structures, sand dyke injections, bed dislocation/faulting and flow folds. Within this section four deformed levels of soft sediment structures have been identified which were dated ca. 25 ka BP at level 1 (∼0.4 m from the modern river level (mrl), 20.1 ka BP at level 2 (∼1.8 m mrl), 17.7 ka BP at level 3 (∼2.56 m mrl) and 12.2 ka BP at level 4 (∼4.25 m mrl)). Detailed study of these soft sediment structures allow us to demonstrate that deformation level 3 is not related to seismic trigger, but remaining three deformation levels (1, 2 and 4) are ascribed to seismic origin. From compilation of earlier palaeoseismological studies using soft sediment deformational structures (SSDS) in the palaeolake deposits in the adjoining area, suggest that the deformational events identified in the present study are regional in nature and thus tectonic process plays an important role in the evolution of landform in the Spiti region.

Stable carbon isotopes in dissolved inorganic carbon: extraction and implications for quantifying the contributions from silicate and carbonate weathering in the Krishna River system during peak discharge

We present a comparative study of two offline methods, a newly developed method and an existing one, for the measurement of the stable carbon isotopic composition (δ13C) of dissolved inorganic carbon (DIC; δ13CDIC) in natural waters. The measured δ13CDIC values of different water samples, prepared from laboratory Na2CO3, ground and oceanic waters, and a laboratory carbonate isotope standard, are found to be accurate and reproducible to within 0.5 ‰\ (1σ). The extraction of CO2 from water samples by these methods does not require pre-treatment or sample poisoning and can be applied to a variety of natural waters to address carbon cycling in the hydrosphere. In addition, we present a simple method (based on a two-end-member mixing model) to estimate the silicate-weathering contribution to DIC in a river system by using the concentration of DIC and its δ13C. This approach is tested with data from the Krishna River system as a case study, thereby quantifying the contribution of silicate and carbonate weathering to DIC, particularly during peak discharge.

Abrupt climate change at ~2800 yr BP evidenced by a stalagmite record from peninsular India:

This study presents an analysis of the Indian summer monsoon (ISM) rainfall variations for a 1460-year period (1720–3180, Before Present BP: 1950 AD), based on a long record of stable isotopic variations (δ18O) with high temporal resolution (~annual) obtained from a U-Th dated stalagmite from the Kadapa cave in peninsular India. This stalagmite proxy record captures variations associated with wet and dry monsoons on decadal to centennial time-scales, together with a general declining trend in the ISM during the 1460-year period. It is noted that the declining trend of the ISM follows the northern hemispheric summer insolation, which is known to influence the location and strength of the Inter tropical convergence zone (ICTZ). The stalagmite record also indicates an abrupt climate change, characterized by the decline of ISM around 2800 yr BP, as manifested in the enrichment of 18O values. Furthermore, the enriched 18O values around 2800 yr BP are corroborated by changes in the stalagmite growth rate, its trace elemental ratios (Sr/Ca, Ba/Ca and U/Ca) and crystallographic structure. In addition, the decline of ISM around 2800 yr BP coincides with a sudden rise in the atmospheric Δ14C, indicative of reduced solar activity. This period around 2800 yr BP is widely reported as the cold European climate associated with ice debris events in the North Atlantic (also known as the Iron Age Cold Epoch), which were reportedly forced by low solar activity. Syntheses of other available stalagmite records from the Indian region, during the common time-frame, show coherent variations with the Kadapa stalagmite and also the Dongge cave stalagmite (southern China), pointing to synchronous variations of the Indian and the East Asian monsoon systems.

Proxy Climatic Records of Past Monsoons

We summarize the current knowledge about the Indian monsoon variations in the past using only those monsoon proxies which can be dated unambiguously with a high resolution (tree rings and speleothems) and those which are quantifiable with associated uncertainties. The quantification is done based on the amount effect observed in the tropics. The stable oxygen isotopic composition of rain decreases by ~1.5 ‰ for an increase in the monthly rainfall of 100 mm. Monsoon fluctuations have been documented from teak trees in Kerala for the last 553 years and quantified for the last 330 years from a stalagmite that grew in Ulvi, northern Karnataka. Marine sediments from the eastern Arabian Sea and speleothems from Central India clearly show that the monsoon rainfall has been steadily increasing during the Holocene (the last ~10,000 years). The quantified rainfall data are presented with associated uncertainties.

Paleoclimate of Peninsular India

The limited spatial and temporal coverage of instrumental weather records precludes the knowledge of long-term climatic changes. To infer such changes, recourse is taken to natural archives that serve as climate proxies. The prominent proxies that offer annual to seasonal temporal resolution include annual rings of trees (Ramesh et al., 1985; Ramesh et al., 1986a; Ramesh et al., 1986b; Ramesh et al., 1988; Ramesh et al., 1989; Managave et al., 2010a; Managave et al., 2010b; Sano M. et al., 2010; Managave et al., 2010c; Managave et al., 2010d; Managave et al, 2010e), corals (Chakraborty et al., 1992; Chakraborty et al., 1993a; Chakraborty et al., 1993b; Chakraborty et al., 1993c; Chakraborty et al., 1994; Chakraborty et al., 1997), ice cores (Nijampurkar et al., 1986), speleothems (Yadava et al., 2004) in some cases and varved sediments (Von Rad et al., 1999). Among these, tree-rings have specific advantages: they have a wide geographic distribution, are annually resolved, show a continuous record, and are easily dated by ring-counting. Seasonality in the growth rate of trees driven by seasonality in the climatic factors can result in well-defined annual growth rings in trees. Individual tree-rings faithfully record contemporary climatic signatures and hence provide an opportunity to decipher the variation in climatic parameters for a duration equivalent to the life-span of the tree.

Rainfall Variation of Peninsular India Reconstructed From a 331- Year-Old Speleothem

The southwest monsoon is of prime importance for the Indian sub-continent as it contributes the majority of annual rainfall. Any major departure from normal monsoon behavior seriously affects agricultural yield and the economy. To enable successful monsoon forecasting in order to plan agricultural activities, a systematic study of variations in monsoonal rainfall is required. This calls for long-term records of past rainfall. In tropical India, the major growth of speleothems occurs during the southwest monsoon (June to September) when water is most abundant. Therefore, speleothems would seem to show good potential as a rainfall proxy. While δ18O of tropical precipitation shows an inverse correlation with the amount of rainfall, it does not show a correlation with surface air temperature (Dansgaard, 1964). Speleothem δ18O, on the other hand, is related to both the δ18O of meteoric water and the temperature of the cave during carbonate precipitation. Therefore, it should in principle function as a proxy for past variations in the δ18O of meteoric water and the mean annual surface air temperature. Such an argument can be tested using a young speleothem. An active stalagmite (Fig. 1), with distinct annual layers covering the past 331 years (the tip is AD 1996), was found in the Akalagavi cave in the mountainous terrain of the Western Ghats, in the state of Karnataka, peninsular India (Yadava et al., submitted). δ18O measured along the growth axis varies between -2.7 and +1.6‰ (Fig. 2). During the past 300 years, mean annual temperature fl uctuations of more than 1oC (corresponding to only a ~0.22‰ change in the speleothem δ18O) seem unlikely. Hence, past δ18O variations in precipitation may have been largely responsible for the variations observed in speleothem δ18O. If speleothem δ18O is primarily governed by variation in rainfall, then a comparison of the δ18O time series of the most recent part of the speleothem with the instrumental rainfall record of the region should result in a high correlation coeffi cient. Indeed, a comparison of the decadal running mean yields a signifi cant value (r = -0.62, n = 80). This suggests that δ18O in cave deposits is a reasonable proxy for decadal variations in local rainfall. A strong positive correlation (r = 0.62, n = 301) between δ18O and δ13C along the growth axis indicates that on an annual scale, δ13C is also correlated with rainfall.