Mollusk shells reveal new clues about past climate change

Researchers at the Universitat Autònoma de Barcelona have presented a brand new perspective on how early humans adapted to climate change. The evidence was discovered in the remains of ancient marine mollusk shells.

“Current global climate warming is having, and will continue to have, widespread consequences for humans. Looking to the past, multiple climatic and environmental changes have long been thought to have shaped human evolution and behavior,” wrote the researchers.

According to the experts, the “8.2 ka event” was the largest and most abrupt climate change event of the past 11,700 years. This was caused by meltwater from North American lakes flooding into the North Atlantic and stopping ocean circulation systems.

“The influx of cold water into the Atlantic Ocean led to a reduction of sea surface salinity and a decline of the Atlantic meridional overturning circulation (AMOC), provoking a reduction in sea surface temperatures (SST) across the North Atlantic,” explained the researchers. “The cooling effects of this event have been documented in proxies from the Greenland ice cores and across Europe.”

Study lead author Asier García Escárzaga and colleagues used stable carbon isotope analyses to examine marine mollusk shells that were collected in the El Mazo cave along the European Atlantic coast. El Mazo has a long stratigraphic sequence of 1,500 years, with especially high chronological resolution of each archaeological layer.

The results showed that colder ocean water temperatures led to changes in the availability of different shellfish species. For example, one of the most commonly consumed species decreased during the 8.2 ka event, while another commonly harvested species increased.

The results also revealed a major increase in mollusk exploitation by humans. This activity was indicated by a decrease in average mollusk size and evidence of increased harvesting in more dangerous coastal areas. 

“A decrease in shell size was also observed at El Mazo during this colder period, something that has been variously linked to either changes in sea surface temperatures or a higher human pressure on coastal resources, mostly in the form of demographic increase and additional resource pressure,” wrote the study authors.

“At El Mazo, a clear reduction in shell size for the three most dominant mollusc species can be seen during the 8.2 ka event. Given that each of these species have well-documented differences in temperature tolerance, it seems that changes in sea surface temperatures cannot be considered responsible for changes in shell size.”

The researchers propose that humans were flocking to the Atlantic coast as a refuge during the cold event. Despite the influx of people, however, the fossil evidence suggests that humans living around El Mazo managed to avoid over-exploiting their coastal resources. The experts found that the average mollusk size very rarely decreased below 20 millimeters, which has since been established as the minimum size needed to guarantee long-term species survival. 

“Our results suggest an ongoing application of local marine ecological knowledge by some of the last foragers in western Europe, despite major changes to climate and demography,” explained García-Escárzaga.

The techniques used in the study could be extremely useful for other studies seeking to determine the significance of climate change on marine environments. The research also provides new clues to help predict the nature of future climate change and the impacts on human societies. 

The study is published in the journal Scientific Reports.

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By Chrissy Sexton, Earth.com Staff Writer