Clamshells in Peru offer clues on El Niño ocean impacts

Scientists who study clamshells found along the coastline of Peru say they can track the changes in ocean temperatures over time by “reading” the tiniest of changes in the structure of the shells. They’re hoping that it leads to a better understanding of the El Niño Southern Oscillation (ENSO), a climate pattern that affects ocean warmth and often brings more rainfall and tropical storms to parts of the world.

The hard shells of bivalve species like clams and oysters tend to grow in layers across their lifespans. When archaeologists and paleoclimatologists study these tiny layers of shell, they can see clues into the past climate conditions. The idea is much the same as studying the growth rings of a tree to understand its history.

“The equipment available now, compared to the past, is precise and powerful enough to be able to reveal the sea surface temperature and the overall climate at a specific location when the clam was building its shell,” explains Jacob Warner, a doctoral candidate at Louisiana State University in the United States and lead author of a new research paper published in the journal Chemical Geology.

“As we know today, climate can influence all kinds of practices and behaviors, which may have been the case in ancient civilizations as well,” Warner adds.

The team studied the shells of the Donax obesulus clam, which has never been used in this type of research before. Similar work has been done successfully in another short-lived species, the Mesodesma donacium, but those clams are no longer found in the parts of northern Peru where the research is focused.

The team used precision drilling to take samples from the clam shells, marking the intervals of time that contribute to an overall portrait of the ocean temperatures as the shell grew.

“Using the relationship between the chemistry of the shell and the ocean temperature, we found Donax obesulus can record sea surface temperature pretty well,” Warner said. One of the things they look at, for example, is the ratio of specific oxygen isotopes found in the composition of the shells.

“With this information, we can push this back in time and reconstruct what the temperature and climate was in the past,” he said.

But since these types of clams only live for about three to five years, they can’t provide long trend timelines for the ocean temperatures. Warner is collaborating with other research teams to make comparisons with shells that are thousands of years old. He’s also working on past climate records himself at Caylán in Peru’s Nepeña Valley.

Among these other scientists is Aleksa Alaica, a postdoctoral researcher at the University of Alberta who has analyzed Donax obesulus surf clams found at an archaeological site in Peru’s Jequetepeque Valley. They were part of the diets of ancient people who lived there thousands of years ago, and the shells remain in the ancient settlements. Alaica is finding that the shell sizes were larger during warmer ENSO events.

“This collaborative study showcases the power of simple methods to examine complex questions. Our work highlights the multiple factors affecting shell growth and the resilience of communities in the past to respond to climate and social transformations,” Alaica said.