• Supriyo Chakraborty

      Articles written in Journal of Earth System Science

    • Oxygen and carbon stable isotope studies onGloborotalia menardii from Pleistocene DSDP Cores in northern Indian Ocean and their paleoclimatic and paleoceanographic implications

      Ashish Sarkar Asit K Guha Supriyo Chakraborty

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      Stable isotope ratios of oxygen (δ18O) and carbon (δ13C) in tests ofGloborotalia menardii from samples at 25 cm intervals of top 900 cm cores, representing different thicknesses of the Pleistocene, from DSDP Sites 219, 220 and 241 in the northern Indian Ocean have been measured. Based on the δ18O stratigraphy, glacial and interglacial phases during the Pleistocene have been recognized, which are in good agreement with the standard Quaternary planktonic foraminiferal/climatic zones i.e., Ericson zones at these sites, based onG. menardii abundances. The GIA (glacial interglacial amplitude) at Sites 241, 219 and 220 are of the order of 1·2, 1·4 and 1·9‰ respectively. The last glacial and interglacial maxima (18 ka BP and 125 ka BP respectively) could be identified in DSDP Cores 241, and 219 with some precision. ‘Isotopic ages’ could be assigned to the different levels of these core sections based on the correlation of δ18O record from these sites with the SPECMAP record (Imbrieet al 1984). Changes in sediment accumulation rates at different levels of the Pleistocene have been worked out on the basis of changes in oxygen isotopic ratio.

      Oscillations in δ13C stratigraphy at Site 241 indicated southwest monsoon induced increase in upwelling and productivity during warmer periods. At Sites 219 and 220, variations in the δ13C record were due to the mixing of bottom water.

    • Estimating gross primary productivity of a tropical forest ecosystem over north-east India using LAI and meteorological variables

      Pramit Kumar Deb Burman Dipankar Sarma Mathew Williams Anandakumar Karipot Supriyo Chakraborty

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      Tropical forests act as a major sink of atmospheric carbon dioxide, and store large amounts of carbon in biomass. India is a tropical country with regions of dense vegetation and high biodiversity. However due to the paucity of observations, the carbon sequestration potential of these forests could not be assessed in detail so far. To address this gap, several flux towers were erected over different ecosystems in India by Indian Institute of Tropical Meteorology as part of the MetFlux India project funded by MoES (Ministry of Earth Sciences, Government of India). A 50 m tall tower was set up over a semi-evergreen moist deciduous forest named Kaziranga National Park in north-eastern part of India which houses a significant stretch of local forest cover. Climatically this region is identified to be humid sub-tropical. Here we report first generation of the in situ meteorological observations and leaf area index (LAI) measurements from this site. LAI obtained from NASA’s Moderate Resolution Imaging Spectroradiometer (MODIS) is compared with the in situ measured LAI. We use these in situ measurements to calculate the total gross photosynthesis (or gross primary productivity, GPP) of the forest using a calibrated model. LAI and GPP show prominent seasonal variation. LAI ranges between 0.75 in winter to 3.25 in summer. Annual GPP is estimated to be 2.11kg C m−2year−1.

    • Carbon dioxide, water vapour and energy fluxes over a semi-evergreen forest in Assam, Northeast India

      Dipankar Sarma Kushal Kumar Baruah Rulee Baruah Nirmali Gogoi Abhijit Bora Supriyo Chakraborty Anand Karipot

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      The eddy covariance method is a powerful technique for quantification of $\hbox {CO}_{2}$, $\hbox {H}_{2} {\rm O}$ and energy fluxes in natural ecosystems. Leaf area index (LAI) and its changes are significant drivers of $\hbox {CO}_{2}$ and $\hbox {H}_{2}$O exchange in a forest ecosystem due to their role in photosynthesis. The present study reports the seasonal variation of $\hbox {CO}_{2}$ and energy fluxes and their relationship with other meteorological parameters of a semi-evergreen primary forest of Kaziranga National Park, Assam, India during February 2016–January 2017. The diurnal pattern of half hourly average $\hbox {CO}_{2}$ fluxes over the forest was found to be mostly dominated by the incident photosynthetically active radiation. During the period of study, diurnal variations of $\hbox {CO}_{2}$ flux showed a maximum value of $-9.97 \mu mol \hbox {m}^{-2}\hbox {s}^{-1}$ in the month of June during summer which is also the beginning of the monsoon season. The monthly averaged diurnal $\hbox {CO}_{2}$ flux and variation in LAI of the forest canopy closely followed each other. The annual net ecosystem exchange of the forest estimated from the $\hbox {CO}_{2}$ flux data above the canopy is 84.21 g C $\hbox {m}^{-2}\,\hbox {yr}^{-1}$. Further studies are in progress to confirm these findings. The estimated average annual evapotranspiration of the semi-evergreen forest is 2.8 $\pm$ 0.19 mm $\hbox {day}^{-1}$. The study of partitioning of energy fluxes showed the dominance of latent heat fluxes over sensible heat fluxes. The energy balance closure was found to increase with an increase in instability and the highest closure of around 83% was noted under neutral conditions.

    • Seasonal variation of evapotranspiration and its effect on the surface energy budget closure at a tropical forest over north-east India

      Pramit Kumar Deb Burman Dipankar Sarma Ross Morrison Anandakumar Karipot Supriyo Chakraborty

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      This study uses 1 yr of eddy covariance (EC) flux observations to investigate seasonal variations in evapotranspiration (ET) and surface energy budget (SEB) closure at a tropical semi-deciduous forest located in north-east India. The annual cycle is divided into four seasons, namely, pre-monsoon, monsoon, post-monsoon and winter. The highest energy balance closure (76%) is observed during pre-monsoon, whereas the lowest level of closure (62%) is observed during winter. Intermediate closure of 68% and 72% is observed during the monsoon and post-monsoon seasons, respectively. Maximum latent heat flux during winter (150 W m$^{-2}$) is half of the maximum latent heat (300 W m$^{-2}$) flux during the monsoon. ET is a controlling factor of SEB closure, with the highest rates of closure corresponding to the periods of the highest ET. The Bowen ratio ranges from 0.93 in winter to 0.27 during the monsoon. This is the first time the role of ET in the seasonal variation of SEB closure has been reported for any ecosystem in north-east India using EC measurements.

    • Eddy covariance measurements of $\rm{CO_{2}}$ exchange from agro-ecosystems located in subtropical (India) and boreal (Finland) climatic conditions


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      Climate impacts agriculture in various complex ways at different levels and scales depending on the local natural crop growth limitations. Our objective in this study, therefore, is to understand how different is the atmosphere–biosphere exchange of $\rm{CO_{2}}$ under contrasting subtropical and boreal agricultural (an oilseed crop and a bioenergy crop, respectively) climates. The oilseed crop in subtropical climate continued to uptake $\rm{CO_{2}}$ from the atmosphere throughout the year, with maximum uptake occurring in the monsoon season, and drastically reduced uptake during drought. The boreal ecosystem, on the other hand, was a sustained, small source of $\rm{CO_{2}}$ to the atmosphere during the snow-covered winter season. Higher rates of $\rm{CO_{2}}$ uptake were observed owing to greater day-length in the growing season in the boreal ecosystem.The optimal temperature for photosynthesis by the subtropical ecosystem was close to the regional normal mean temperature. An enhanced photosynthetic response to the incident radiation was found for the boreal ecosystem implying the bioenergy crop to be more efficient than the oilseed crop in utilizing the available light. This comparison of the $\rm{CO_{2}}$ exchange patterns will help strategising the carbon management under different climatic conditions.

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