Paleoclimate

Deputy Project Director: Dr. Supriyo Chakraborty

• To understand past climatic and monsoon rainfall variations by reconstructing responsive climate parameters for the last few thousand years using high resolution proxies such as tree-ring, speleothems, corals etc. over different parts of India and Asian Monsoon region.
• Use of isotopic tracers to study the hydro-geological processes.
• Study of the moisture transport processes using the precipitation isotopes.
• Understanding the role of cloud microphysical process in isotope amount effect.
• Identification of extreme climate events from the proxy records.
• To understand the nature of biogeochemical interactions and their response to environmental changes using the isotopic tracers.

Information on past climate, extending back to thousands of years and even longer, is vital to understand the various external and internal forcing on the climate and thereby make reliable future projections. In this time frame, the tree-rings, corals and speleothem with their interannual to sub-annual resolution, have been recognized to be reliable sources of proxy climate data. Past climate information, for example, high resolution monsoon rainfall data prior to the instrumental records for a few thousand years is crucial to understand the natural modes of monsoon variability. If such kind of records are retrieved from a wide geographical area of the Asian monsoon region it may be possible to determine whether the current declining trend in Indian monsoon rainfall is part of a natural climate variability, or it is driven by human induced climate change. We are involved in analyzing various natural archives that provide proxy records of climate state parameters, such as, monsoon rainfall, temperature,  relative humidity, drought index, atmospheric pCO₂ etc. in order to address such kind of scientific issues. We mainly rely on the analysis of tree ring records (i.e. ring , ring density) and the isotopic analysis of tree rings, sediments, such as, carbonate deposits etc. Since quantification of past rainfall variability using the isotopic records of tree ring, speleothem requires the knowledge of isotopic variability of modern rainwater, a significant amount of work consists of the collection and analysis of modern rainwater from different locations across India. Isotopic analysis of rainwater also helps study the hydro-meteorological processes and some aspects of short term monsoon variability.

• Reconstructed the drought index using the ring data from a southern Indian location for the past 523 years.
• Reconstructed pre monsoon temperature variation of the Western Himalayan region using the tree ring analysis.
• Reconstructed Indian summer monsoon rainfall on near annual scale for the period of 300AD to 1300BC using the oxygen isotopic record of speleothem from the south central Indian region.
• Study of the moisture dynamical processes and cloud microphysical processes using the precipitation isotopes from the Andaman Islands and the Western Ghats mountain region respectively.
• Reconstructed atmospheric pCO₂ variability for the last one hundred year from the equatorial and temperate region using a tree ring fractionation model.
• Reconstructed annual variation of the summer monsoon rainfall variability for the seventeenth century using the isotopic records of corals from the equatorial Pacific.
• Reconstructed oxygen isotopic composition of soil moisture using oxygen isotopic composition of tree rings in the central Himalayan region for the last one hundred years.
• Constraining the Bay of Bengal circulation and identification of deep water masses using the isotopic tracers.

Paleoclimate Poster

Facilities

• Isotope Ratio Mass Spectrometer- Thermo Scientific- Delta V Plus
• Water and Water Vapor Isotope Analyzer - LGR TIWA-45-EP
• CO₂ Isotope Analyzer - LGR CCIA-45-EP
• Green House Gas Analyzer (CO₂, CH₄, H₂O) - LGR FGGA24r-EP
• Ring density measurement equipment- Walesch Electronics Dendro 2003
• Olympus Microscope- Model  SZ-STB2
• Nikon Microscope- Model SMZ 645
• Microscope- Model Bausch and Lomb
• Micromill Sampling System: New Wave Research Inc. Model: 90-1539C
• Branson Digital Sonifier-Model 1800
• Water Purifier- TKW Pacific
• Microbalance Sartorius Model: CPA26P

• Isotopic analysis of rainwater at the Andaman Islands:

  Isotopic analysis of precipitation over the Andaman Islands in the Bay of Bengal was carried out for the year 2012 and 2013 in order to study the atmospheric controls on rainwater isotopic variations. Figure 1 shows the oxygen isotopic records (d¹⁸O) of Port Blair rain water for the years 2012 and 2013. The oxygen and hydrogen isotopic compositions are typical of the tropical marine sites but show significant variations depending on the ocean-atmosphere conditions; maximum depletion was observed during the tropical cyclones. The isotopic composition of rainwater seems to be controlled by the dynamical nature of moisture rather than the individual rain events. Precipitation isotopes undergo systematic depletions in response to the organized convection occurring over a large area and are modulated by the integrated effect of convective activities. Precipitation isotopes appear to be linked to the 10-20 day mode of monsoon intraseasonal variability in addition to synoptic scale fluctuations. During the early to mid monsoon, the amount effect arose primarily due to rain re-evaporation but in the later phase, it was driven by moisture convergence rather than evaporation. Amount effect had distinct characteristics in these two years, which appears to be modulated by the intraseasonal variability of monsoon. It is shown that the variable nature of amount effect limits the ability to reconstruct the past-monsoon rainfall variability on annual to sub-annual time scale. [Chakraborty S., Sinha N., Chattopadhyay R., Sengupta S., Mohan P.M., Datye A., Atmospheric controls on the precipitation isotopes over the Andaman Islands, Bay of Bengal, Scientific Reports, 6:19555, January 2016, doi:10.1038/srep19555] 

Figure 1. Oxygen isotopic records of rainfall at the Andaman Island for the summer monsoon period. Solid line represents the year 2012 and the broken line is for 2013.

• Heat index reconstruction based on tree–ring records of western Himalaya in India

  Tree ring- index chronology based on a well replicated tree core samples (Cedrus deodara and Pinus roxburghii) of the western Himalaya have been carried out in relation to study climate variability / change. The first principal component (PC1) prepared by using multiple-sites tree ring- chronologies of western Himalaya is seen to be negatively correlated with heat index and positively with the Palmer drought Severity Index (PDSI) and moisture index during February to May as representative of regional climate. The correlation coefficient of PC1 with heat index, PDSI and moisture index for the period 1901-1988 is found to be -0.60, 0.37 and 0.59 respectively which are highly significant at 0.1% level. The result shows that increasing heat index may enhance transpiration and evaporation over western Himalaya; which may cause insufficient moisture at root zone of the trees. Based on the tree ring data, heat index of spring season (February to May) has been reconstructed back to AD 1839 (Figure 2). The reconstructed heat index showed the widest warm periods during 1952-1963 and 1966-1976, in 20^th century. [Somaru Ram and H.P. Borghaonkar, 2016, Reconstruction of heat index based on tree-ring records of western Himalaya in India Dendrochronologia, 40, 64-71].

Figure-2 a: Comparison between actual heat index for the period 1901-2014 (dashed line) and estimated heat index (solid line) for the period 1901-1988 during February to May. The A.D. 1901-1988 regression model was used for calibration; CC is the correlation coefficient in upper panel for the period 1901-1988. b: A 31- year sliding correlation coefficients between actual and reconstructed heat index, correlation coefficients are plotted against the central year of the 31 –yr period, with the  significant at 5% level. c: February to May heat index reconstruction  (A.D. 1839-1988). The smoothed line is a 10 year cubic spline fit in lower panel.

• Bose. T., Sengupta, S., Chakraborty, S., Borgaonkar, H.P. 2015 Reconstruction of soil water oxygen isotope values from tree ring cellulose and its implications for paleoclimate, Quat. Int. http://dx.doi.org/10.1016/j/quaint.07.052
• Chakraborty, S., Sinha, N., Chattopadhyay, R., Sengupta, S., Mohan, P.M., Datye, A. 2016 Atmospheric controls on the precipitation isotopes over the Andaman Islands, Bay of Bengal. Sci. Rep. 6, 19555; doi:10.1038/srep19555
• Somaru Ram, Borgaonkar H.P. 2016 Reconstruction of heat index based on tree-ring records of western Himalaya in India Dendrochronologia, 40, December 2016, DOI:10.1016/j.dendro.2016.06.003, 64-71
• Mukherjee, P., Sinha, N., Chakraborty, S. 2016 Investigating the dynamical behavior of the Inter-tropical Convergence Zone since the last glacial maximum based on terrestrial and marine sedimentary records. Quat. Int. http://dx.doi.org/10.1016/j.quaint.2016.08.030
• Naveen Gandhi and R. Ramesh (2016) Natural Isotopic Composition of Particulate Organic Nitrogen in the Eastern and Central Arabian Sea, Proceedings of Indian National Science Academy 82 (4) 1283-1291, doi: 10.16943/ptinsa/2016/48574.

Deputy Project Director: Dr. Supriyo Chakraborty, Scientist-E