- Researchers studied the changing ratio of magnesium and calcium of a stalagmite in the Mawmluh Cave in Meghalaya that has earlier helped pinpoint the Meghalayan Age that began 4200 years ago and runs to the present.
- This distant link between land and ocean records could aid in predicting dry season rainfall amounts in northeast India.
- Stalagmite chemistry shows a link between climatic conditions in the Pacific Ocean and winter rainfall amounts in northeast India.
Hidden in interesting and beautiful mineral rock formations inside Meghalaya’s caves is potentially powerful information about annual rainfall variability in northeast India. This information could help predict and prepare for rainfall variations, crucial for the country where over a billion people rely on the monsoons for water.
A study by Jessica Oster and Elli Ronay of Vanderbilt University’s Department of Earth and Environmental Sciences and Sebastian Breitenbach of Ruhr University Bochum, Germany, followed chemical clues within stalagmites found in the Mawmluh Cave of Meghalaya, where such information has gone unnoticed so far.
Stalagmite record in the cave had earlier helped pinpoint the Meghalayan Age, a slice of geological time that began with a megadrought 4200 years ago and runs to the present.
Stalagmite growth in the cave has now also revealed a link between climatic conditions in the Pacific Ocean and winter rainfall amounts in the water shortage-hit region. This distant link between land and ocean records could aid in predicting dry season rainfall amounts in northeast India according to the researchers.
Understanding rainfall variability is crucial given that each year, monsoon rains between June and October provide water for roughly 1.5 billion people in India and beyond. Changes in monsoon strength and the timing of its onset or withdrawal can trigger either drought or flooding, with devastating consequences, highlighting the need for effective ways to predict and prepare for rainfall variations. Winter rainfall following weak monsoon years can offer relief from water stress for farmers in India, according to a Vanderbilt University press release.
The researchers analysed the last 50 years of growth of a stalagmite in the 7.2 km-long famous Mawmluh Cave in the northeast Indian state of Meghalaya, an area that experiences so much summer monsoon rainfall that it is credited as the rainiest place on Earth.
But in the last 36 years (1979-2014), northeast India has experienced a rapid decrease in summer monsoon rainfall (about 355 mm), which has serious implications on the ecosystem and the livelihood of the people of this region.
Elli Ronay told Mongabay-India that the group studied the chemical composition of a stalagmite from Mawmluh Cave that grew from around 1964 to 2013.
“Our record documents relationships between stalagmite chemistry, rainfall in Meghalaya, and other aspects of the climate system such as the Pacific Decadal Oscillation. We believe that further developing these relationships can be useful for understanding how Meghalayan rainfall will change in the future,” Ronay told Mongabay-India.
A stalagmite, a mound of mineral deposits (calcium carbonate) on cave floors, grows when mineral-rich water slowly drips onto the floor. Ronay said as a stalagmite grows, it records the chemistry of the water that has trickled down from the surface, through soil and rock, into the cave, and which ultimately drips on the stalagmite.
As the water makes its way from the surface through the soil and rock layers to the cave, its chemistry changes in a way that is primarily dependent on the amount of water flowing through the cave system.
“One of the chemical indicators in stalagmites we look at is the ratio of magnesium to calcium concentrations,” she said. This ratio is often used as a tool to infer variability in past rainfall amounts.
They examined the ups and downs in the ratios of magnesium to calcium concentrations of the stalagmite (MAW 0201) during summer and winter, which are controlled by rainfall amounts.
Time-tripping through stalagmite chemistry
The summer and winter magnesium to calcium ratios are very different because the amount of water flowing through the ground and into the cave varies at these times.
“During the summer, magnesium to calcium ratios are typically low and during the winter they are typically higher, a pattern which is controlled by high and low rainfall amounts, respectively. We looked at how large the difference between the winter and summer magnesium to calcium ratio is, something we call the seasonal amplitude,” Ronay said.
They used an instrument called a mass spectrometer attached to a laser, which was moved along the centre of the stalagmite to essentially vaporise the stalagmite layers.
“This allows us to measure how the elemental composition of the stalagmite changed through time. We compare the chemical signatures to the instrumental climate record, and with our knowledge from previously published studies, we can make inferences about the relationship between climate and the chemistry of stalagmites,” said Ronay.
They could compare how the chemistry of the stalagmite varied with climate variability as the 22 mm-long stalagmite MAW 0201 grew quickly over a time period for which they had instrumental climate data.
In this study, Ronay and co-authors show that the summer to winter difference in magnesium to calcium ratios is more sensitive to changes in winter rainfall, rather than summer monsoon rainfall.
“This interpretation is rather new in Indian Summer Monsoon (ISM) regions, where stalagmite records are typically interpreted as summer monsoon strength records. We also show that there may be a link between winter rainfall amounts in Northeast India and decadal-scale variability in North Pacific sea surface temperatures,” she said in an email.
Winter rainfall variability and cyclical variations
Rainfall regime changes are known to be influenced by climate change and also several coupled oceanic and atmospheric oscillations around the globe including the Pacific Decadal Oscillation (PDO).
In their stalagmite record and the instrumental rainfall record, the researchers interpreted an increase in winter rainfall that coincided with an observed change in sea surface temperature patterns in the North Pacific Ocean.
“Variation in these sea surface temperature patterns mainly affects climate in the North Pacific, but researchers continue to find its effects in farther reaching places like India,” Ronay said.
“We see that measured winter rainfall in Sohra (Cherrapunji), near the cave, increased during the 1976-1998 warm phase of the Pacific Decadal Oscillation. We see a simultaneous decrease in seasonal amplitude of magnesium to calcium ratios in our stalagmite, linking winter rainfall, our stalagmite record, and climate variability in the Pacific,” Ronay said.
The Pacific Decadal Oscillation (PDO) refers to cyclical variations in sea surface temperatures in the Pacific Ocean and consists of a warm and cool phase with each phase lasting 20 to 30 years.
“We suggest that winter rainfall variability in northeast India may be related to some of these distant sea surface temperature patterns and it’s something we would like to continue investigating with longer stalagmite records that reach farther back than our observational record allows,” Ronay explained.
These new results advocate for caution when interpreting stalagmite records from regions characterised by strong seasonality like the monsoon.
“Some challenges we encounter are related to the variability between cave environments. All caves are slightly different in their depth, the composition of the rock in which the cave is formed, the way their hydrology responds to rainfall events throughout the year, and more,” Ronay observed.
Stalagmite records from monsoon regions such as India are vital to understanding past variability in the global climate system and the underlying reasons for this variability, said Jessica Oster.
“The ultimate goal of our research is to reconstruct past climate change and we can do that by studying stalagmites. You can look at how their chemistry changes through time and it can give us an idea of how climate has changed in the past,” said co-author Oster.
Ashish Sinha, Professor of Earth Science, California State University Dominguez Hills, who was not associated with the study said the research is a good step in the right direction but science is never a done deal.
“I agree with the authors that there is some signal (in the stalagmite record) that is coming from the winter rainfall in that particular area, but, by, and, large, it is dwarfed by the summer monsoon signal,” Sinha told Mongabay-India.
“While oceanic and atmospheric oscillations such as the Pacific Decadal Oscillation may be supporting actors in monsoon regime changes, one has to factor in anthropogenic pressures as well. It’s hard to tease them apart with the data in hand,” he said.
Sinha believes paleoclimate reconstruction will help India prepare for bigger eventualities. “In India, rainfall records have been kept since the late 19th century so we have 150 years of data. But this is not sufficient. To fully understand the complex system of Indian monsoons, we need to extend this record and then we can see the whole spectrum of monsoon variability,” he added.
Ronay, E. R., Breitenbach, S. F., & Oster, J. L. (2019). Sensitivity of speleothem records in the Indian Summer Monsoon region to dry season infiltration. Scientific reports, 9(1), 5091.