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Isotopic tracer for quantifying CO2 and moisture sources over peninsular Indian region

Did you know, it is possible to find the location of where a bottle water was packed, by analysing small volume of water? Similarly, we can trace seasonal variation in rainfall looking at the growth bands in the land snails. The technique that makes these seemingly improbable things possible is called 'isotope analysis'. Prof. Prosenjit Ghosh at the Centre for Earth Sciences, Indian Institute of Science finds answers to such and many other questions through his work in the OASIS laboratory (Operation and Application of Stable Isotope Systems).

Isotopes are variants of the same element having different atomic mass numbers. The core of the atom, called the nucleus, contains protons and neutrons. An element always has a fixed number of protons, but the number of neutrons varies. . Elements may have stable as well as radioactive isotopes. For example, the three commonly observed isotopes of Carbon are C-12, C-13 and C-14 with 6, 7, and 8 neutrons respectively. C-14 is radioactive, and is commonly known for its usage in assignment of age -- for finding out how old something is. The other isotopes, C-12 and C-13 are stable which are useful in understanding the biogeochemical processes like photosynthesis, respiration, evaporation and condensation etc. Similar to carbon, stable isotopes of other elements like nitrogen, hydrogen and oxygen also exist in nature. Water vapour originating from the Arabian Sea has a different isotopic signature than water from the Bay of Bengal. Water vapour from these water bodies replenishes the ground water supply via rainfall at various places in India during the southwest and north east monsoon. Hence, water from each location in India has a unique ratios. When we have enough information, it is possible to map the ratio to the latitude and longitude of the source popularly known as “isoscape”. This methodology made it possible to trace the source of bottled water.

The instrument used to identify the composition of various isotopes in substances is called Isotope Ratio Mass Spectrometer. The stable isotopes method has large number of applications. For example isotopic analysis of carbon dioxide can help us identify the source of CO2in the urban region. In a paper he wrote in 2013, Prof. Ghosh described how carbon dioxide (CO2) emission from cement is affecting the isotopic composition of CO2 for the city like Bangalore.

He said, “This is a major concern. As cities develop, consumption of cement will increase, and so would the polluting gases”. The cement production and their contribution to the carbon budget in the atmosphere can be estimated to better manage the changing emission scenarios.

In another study, Ghosh and his team identified region over Indian Ocean region where CO2 is oozing out and it is a consequence of low productivity and rise in temperature of ocean water.

Expertise in multiple disciplines is needed to carry out this sort of work. Prof Ghosh collaborates extensively with people from other fields like chemistry, bioscience, atmospheric science and ocean physics.

Some experiments need samples taken over longer time duration. Samples from ocean and sea are typically collected once in a year whereas samples from land can be collected more frequently or in a continuous mode

This research helps us gather information about various aspects like rainfall variation, impact of CO2 emission, sea level rise over coastal region etc. Ongoing work in Ghosh’s lab is oceanic CO2, rainwater isotope characterization, isotope based mapping of rice variety over Indian region, seasonality of water near estuarine region and monsoon during periods of high atmospheric CO2 level in the past.

About the lab

Prosenjit Ghost is an Associate Professor affiliated to three departments at the Indian Institute of Science: Centre for Earth Sciences, Centre for Atmospheric and Oceanic Science and the Divecha Centre for Climate Change.