Erratic rainfall, extreme heat waves, rising sea levels—the effects of climate change are unfurling in front of our eyes. One wonders what the coming years have in store in terms of drastic climatic events. The answer to this might lie in the thousands of caves scattered across India. Paleo-climatologists are studying the stalactites and stalagmites in these subterranean ecosystems to understand the transformations in climate over time, and what those portend for the future. Dr Jaishri Sanwal Bhatt, DST Women Scientist at the Geodynamics Unit of the Jawaharlal Nehru Centre for Advanced Scientific Research, Bengaluru, is one such expert. She has been studying caves in the Himalayan region for signs of climate change, history of old earthquakes and applications of these studies in the field of nuclear energy. In an interview with Lounge, Bhatt elaborates on what makes caves ideal ecosystems for forecasting changes in climate:
What does the climate of the past tell us about the climate of the future?
One has seen climate changing through geological time, and there is a significant link between the past, present and future. The role of paleo-climatologists is to make these connections by reconstructing the past climatic variations and their cause by using proxy data. Geological and archeological records suggest that a prolonged drought led to the collapse of many old civilisations, including the advanced Indus valley. There are three major causes for climatic changes: atmospheric composition, solar irradiance and aerosols.
What led you to base your research in the Himalayas?
It is challenging to reconstruct climate in a heterogeneous land use, and variable topography and terrain like the Himalayas. For the last two years, I have limited my research to the past few thousand years. I am setting up a template for this. This mountain belt influences the overall rainfall pattern in India and obstructs the path of the north mid-latitude cold winds owing to its altitude and location. The region mainly experiences two seasons tied to the summer and winter rains. Under such dynamic conditions, the Himalayas offer an excellent natural laboratory to capture the pattern of extreme climate events. And yet, it is equally difficult to capture these climatic fluctuations in such complex terrain without employing the most reliable and chronologically-controlled geological proxy such as the speleothems.
What are speleothems?
Speleothems are secondary crystalline deposits, which consist of calcite and aragonite minerals. They develop inside the caves due to the reaction of calcium carbonate rocks. When you look at caves, the role of water can’t be ignored—the entire story of this subterranean ecosystem starts from there. One can find stalactites, which hang downwards from the ceiling. As each drop of water falls, it loses carbon dioxide and deposits a fragile film of calcite. Stalactites are very fragile in nature, with a diameter just like that of a water droplet.
Then there are stalagmites, which grow upwards from the cave floor. They are formed when water drips from stalactites. If you cut a stalagmite vertically, you can see how the growth bands of calcite and aragonite have developed. The oxygen isotope (δ18O) records of speleothem provide vital information about deviations in precipitation, temperature, and atmospheric circulation. The δ13C values of speleothem are locally controlled by biogenic soil productivity associated with the vegetation type and density, which regulates the soil CO2 content.
How do speleothems shed light on climate change?
The stalagmite growth depends on the amount of water that drops seasonally or regularly from the cave ceiling. The slightly acidic rainwater percolates through the limestone bedrocks, dissolving the carbonate. These gradually get saturated with calcium carbonate. This process is accompanied by the dissolution of various elements present in the bedrock, such as magnesium, which eventually merge as trace elements in the growing stalagmites. The concentration of these elements depends on multiple factors such as the nature of the host rock, regional climatic conditions, acidity of the rainwater, nature of topsoil, cave temperature, and humidity. The vigorous chronologies are crucial to the accuracy of climatic research.
What has led to a rising interest in stalagmites in recent years?
This is due to their amenability to U-series dating. We use the uranium-thorium ratio in the speleothem to accurately date them back to several thousand years. Understanding the past climatic patterns is crucial to simulating the future climatic trend. As stated earlier, speleothems are one of the most potential proxies to reconstruct past climatic patterns; even though they grow only a few inches each year, they serve as record keepers of past environmental and hydroclimatic conditions.
Could you also elaborate on the rising focus of caves within the scientific community?
Caves are our natural geological heritage, and extremely useful to researchers from diverse fields. The cavernous limestone caves are among the most productive aquifers (water-bearing beds) and important water sources. Caves are home to incredibly diverse life forms, but some human activities are now threatening the cave ecosystem and the life within. Being a life member of the Speleological Society of India (SAI), I can't stress enough on the need for public awareness about cave and karst conservation as many are still unaware of the gradual degradation of cave resources.