R. Ramesh

Rayleigh fractionation of stable isotopes from a multicomponent source

A formulation of the Rayleigh equation for the stable isotopic evolution of a multicomponent source reservoir is presented. Its applicability to the carbon and oxygen isotopic evolution of a fluid-rich carbonate magma and the crystallizing calcite carbonatite is demonstrated using data from the Amba Dongar carbonatite complex, Deccan province, India. The initial δ13C of the parent magma for this complex has been estimated to be −5.3 ± 0.2‰ relative to V-PDB.

Evolution of carbonatite complexes of the Deccan flood basalt province: Stable carbon and oxygen isotopic constraints

The stable carbon and oxygen isotopic composition of carbonatites from three carbonatite-alkaline complexes (Amba Dongar, Mundwara and Sarnu-Dandali) of Deccan province, India, can be placed into two groups, primary and secondary. The primary variations indicate the mantle origin of these complexes. The source of these complexes, the Reunion plume head, was largely composed of a mantle having δ18O similar to that of mean upper mantle (5 to 8‰) but higher δ13C (>−5.5‰). The higher δ13C, particularly that of a batch of parent magma for Amba Dongar (−3.4‰), suggests the incorporation of recycled inorganic crustal carbon in these carbonatites. As in the plume model for generation of continental flood basalts and associated carbonatite-alkaline complexes, the fluid-rich rim of the plume head acts as the source for the carbonatites, so the above incorporation of crustal carbon could have been facilitated through the migration of CO2-rich, 13C enriched fluids (derived from ancient subducted carbonates) into the Reunion plume head. The correlated variations of δ13C and δ18O in unaltered calcite carbonatites in all three complexes are consistent with the formation of these rocks by fractional crystallization from CO2-rich carbonate magmas, derived from parent carbonated silicate magmas through liquid immiscibility. The extreme enrichment of δ13C and δ18O in some carbonatites and metasomatic rocks is a result of postcrystallization alteration process caused by CO2-bearing aqueous fluids. In Amba Dongar a pure magmatic fluid and/or a magmatic-hydrothermal fluid could have caused the alteration, whereas rocks of Mundwara and Sarnu-Dandali appear to have been altered by meteoric-hydrothermal fluids.

Environmental significance of carbon and oxygen isotope ratios of banded corals from Lakshadweep, India

Abstract The stable isotope ratios (δ18O and δ13C) of the coral Porites compressa from the Lakshadweep islands, eastern Arabian Sea, preserve environmental parameters related to monsoon-induced upwelling. The oxygen isotopic ratios in P. compressa record the seasonal drop in sea surface temperature (SST) during the monsoon season. An earlier calibration of the coralline δ18O with observed changes in SST is used to estimate past SST changes and to evaluate the relationship with upwelling and rainfall. The seasonal variations in carbon isotope ratios seem to be controlled by photosynthesis of the algal symbiont, which in turn is modulated by monsoon activities; sedimentation, sediment suspension, water current and turbidity. Both δ18O and δ13C in this species do reflect the monsoon clearly.

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.

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

Agricultural production and the availability of fresh water in Indian subcontinent critically depend on the monsoon rains. Therefore it is vital to understand the causal mechanisms underlying the observed changes in the Indian monsoon in the past. Paleomonsoon reconstructions show that the water discharge from the Ganges-Brahmaputra River system to the Bay of Bengal was maximum in the early to mid-Holocene; data from the Western Arabian Sea and Omanian speleothems indicate declining monsoon winds during the Holocene, whereas records from the South West Monsoon (SWM) precipitation dominated eastern Arabian Sea show higher runoff from the Western Ghats indicating gradually increasing monsoon precipitation during the Holocene. Thus there exists considerable spatial variability in the monsoon in addition to the temporal variability that needs to be assessed systematically. Here we discuss the available high resolution marine and terrestrial paleomonsoon records such as speleothems and pollen records of the SWM from important climatic regimes such as Western Arabian Sea, Eastern Arabian Sea, Bay of Bengal to assess what we have learnt from the past and what can be said about the future of water resources of the subcontinent in the context of the observed changes.

Paleoproductivity variations in the equatorial arabian sea : Implications for east african and indian summer rainfalls and the el nino frequency

We analyzed a sediment core from the equatorial Arabian Sea, chronologically constrained by accurate acceleratornnmass spectrometry (AMS) radiocarbon dates on selected planktonic foraminiferal species, for paleoproductivity variationsnncorresponding to the variations in the Indian Ocean Equatorial Westerlies (IEW). The IEW in turn are positivelynncorrelated to the Southern Oscillation Index (SOI), which is a measure of El Nio, Southwest monsoon (SWM), and east Africannnrainfall (EAR). The productivity data show that Indian and east African rainfalls declined from 35,000 calendar yr BP upnnto the last glacial maximum (LGM), with the maximum El Nio frequency during the last glacial period. From ~14,500 tonn~2000 calendar yr BP (i.e. core top), we find strengthening SWM and EAR along with declining El Nio frequency.

Relationship between 18O enrichment in leaf biomass and stomatal conductance

Models that explain the oxygen isotope enrichment in leaf water (and biomass) treat the relationship between the kinetic fractionation that occurs during evapotranspiration and the stomatal conductance in an empirical way. Consequently, the isotopic enrichment is always predicted to decrease with increasing stomatal conductance, regardless of the experimental evidence to the contrary. We explain why and suggest an alternative method to reconcile theory and experiment. We support this with our experimental data on rice and groundnut plants.

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.

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.

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.