The shells of a large, invasive snail in India can provide exceptionally fine grained records of past precipitation in the region, a new study finds. By measuring isotope ratios in shells from some of these snails from as long ago as 1918, the researchers have demonstrated that they could reconstruct the subseasonal rainfall rate of past monsoon seasons.
The snails in question are of the giant African land snail species Achatina fulica, which can grow shells up to 20 centimeters long. “We had the opportunity to observe [the snail] growing at extraordinary rates during monsoon time,” said Prosenjit Ghosh of the Indian Institute of Science in Bangalore, India.
According to Ghosh, the fast growth rate of these snails makes them “ideal candidate[s] to record high-frequency changes in the monsoon precipitation.” Ghosh is lead author on the research paper, published on 7 September in Geochemistry, Geophysics, Geosystems, that presents the research.
An Archive of Rain Rates at Fine Scales
Because of the rapid growth rate of the snails, which are a widespread invasive species in India, moisture-sensitive oxygen isotope ratios in growth bands in the animals’ shells preserve week-to-week rainfall rates, the researchers found.
“Very few proxies give you that sort of [time] resolution in reconstructing precipitation,” said coauthor Kaustubh Thirumalai of Brown University in Providence, R. I. The snail shells sample more finely spaced increments of time than do data from bands found in caves, trees, or mollusks, which have been used in prior paleoclimate research, he added.
“We’d like to understand the bigger picture of how the Indian monsoon and the rainfall varied in the past,” Thirumalai said. These snail shells, which are made of calcium carbonate, could fill in gaps in historical rainfall records and help scientists understand the evolution of monsoons on the Indian subcontinent, he explained.
“Snail shells are pretty ubiquitous across the Indian subcontinent,” Thirumalai said. “The idea was that, if we could use snails and snail shells to piece together the history of the Indian monsoon, it could be a powerful archive.”
Slithering into the Past
The researchers began their new work by reconstructing the rainfall record in Kolkata, India, an area at the edge of the core monsoon zone, during the 2009 Indian summer monsoon (ISM). They collected snail shells, water samples, and instrumental records from that season.
To estimate the amount of precipitation, Ghosh’s team measured a common paleoclimate proxy for temperature, humidity, and atmospheric circulation. This proxy, known as the δ18O stable oxygen isotope ratio, varies in proportion to the ambient moisture.
The proxy works like this: Although water can form using either a light or a heavy oxygen isotope, snails more readily draw in water with oxygen-16 than with its heavier counterpart, oxygen-18. The more water available for snails to pick up and incorporate into their shells, the less likely they are to expend extra work to take up water that holds the heavier isotope. In times of drought, however, snails end up using water with the heavy oxygen by necessity and are forced to incorporate heavier molecules into their shells. So dry spells correspond to higher δ18O levels in the shells.
Just how sensitive is this proxy?
Very, the team discovered. The researchers found that variations in the δ18O ratio in growth bands of the 2009 shells directly mirrored week-to-week fluctuations in recorded rainfall. “Because we extracted high-resolution stable isotopic profiles from these shells, we were able to match them to active spells and breaks in the monsoon,” Thirumalai said.
Next they turned to two shells of the same species that had been gathered in Kolkata in 1918 and archived at the Natural History Museum in London in the United Kingdom. The modern shells then served as a Rosetta Stone to translate the 1918 shells, turning the δ18O measurements from the historical shells into rainfall intensity during the lifetime of the shells.
The rainfall patterns reconstructed from the 2009 snail shells have “good consistency with rainfall,” said Anant Parekh of the Indian Institute of Tropical Meteorology, Pune, India, who did not participate in the project. He added that this technique “can be used to identify active and break spell[s] in the past” as was done with the 1918 shells but that repeating the experiment with more shells and comparing them to independent records of rainfall would help verify the method’s accuracy.
A Data Boon from an Invasive Pest
Ghosh and his colleagues report in their paper that by using data from fresh and archived shells, they can independently verify historical human rainfall records. Plus, in areas and time periods for which human records are sparse or inaccurate, snail shell data could fill in gaps and extend the record.
What’s more, the authors suggest that this new pathway for reconstructing subseasonal rainfall of past ISMs may help us understand how climate variables like the El Niño–Southern Oscillation (ENSO) and the Indian Ocean Dipole (IOD) have changed over time.
“ENSO and IOD bring…shift[s] in the seasonal rainfall distribution,” Ghosh said, “which can be easily captured using isotopic fingerprinting of carbonates in growth bands of Achatina fulica snails.”
The team also plans to carry out similar experiments with snail shells collected across the Indian subcontinent. Although these snails tend to have devastating impacts on local ecosystems wherever they invade, their expansive reach could lead to mapping rainfall history across the entire Indian subcontinent.
“We know that there’s a lot of spatial heterogeneity in the Indian monsoon,” Thirumalai said. “Snails could potentially be a really useful tool to fill in the puzzle pieces of how rainfall changed in the past and how it changed spatially.”
—Kimberly M. S. Cartier (@AstroKimCartier), News Writing and Production Intern