Photo: Ian Waldie (Getty Images)

Many prehistoric mass graves located along coastlines around the world may be linked to ancient tsunamis, new research suggests.

Mass graves are common in the archaeological record. There’s the Viking-age Ridgeway Hill Burial Pit in the UK, for example, which contains 54 skeletons and 51 dismembered heads, or the Early Neolithic mass grave of Schöneck-Kilianstädten in Germany, a likely massacre that resulted in the deaths of at least 26 people. In these and similar cases, archaeologists attribute the burials to warfare or pillaging, as evidenced by wounds such as blunt-force trauma, injuries caused by weapons, or decapitations. But in some cases, where the cause of death isn’t obvious, and where no written or oral history exists to explain the presence of a mass grave, archaeologists can only speculate as to the cause.

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New research published in the Journal of Archaeological Method and Theory suggests scientists have overlooked a possible cause of some ambiguous mass graves located along oceanic coastlines: ancient tsunamis. Study co-author James Goff from the PANGEA Research Centre at UNSW Sydney says tsunamis are rarely, if ever, considered as explanations for burial sites, which may explain why no prehistoric mass graves have been identified as being tsunami-related.

“Proving that a site is related to a past tsunami could lead to a fundamental rewrite of how we interpret coastal human settlement in prehistory, and change what we thought we knew about the culture and people living in the region at the time,’ said Goff in a statement. “It could also have dramatic implications for how archaeologists analyze a site.”

Mass graves analyzed in the new study, some of which may be linked to tsunamis.
Image: Genevieve Cain et al., 2018

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For the study, Goff and his colleagues considered mass burial sites located around the world, including the Solomon Islands and Vanuatu (both located in the Pacific Ocean and in areas prone to tsunamis), and the Orkney and Shetland islands (located north of Scotland in a relatively inactive tectonic region where a link to tsunamis hasn’t been considered). These graves ranged in age from 5,000 to 500 years ago.

“We already know that tsunamis have occurred in all the areas that we explored in this study, and the ages for mass burials match those of the geological evidence for past events,” said Goff. “Looking at the evidence from the burial sites considering the potential of tsunamis, we were able to make a strong case for many of these sites being related to major tsunamis, as opposed to more standard explanations like warfare or an epidemic.”

Take a mass grave found in the Solomon Islands. Archaeologists have documented over 200 individual burials, in which the bodies were placed next to each other and covered in coral gravel, requiring virtually no digging. The bodies were laid on their backs, with their arms placed straight down the body. Some individuals exhibited signs of exposure, such as gnawed limbs (likely from rats, dogs, or pigs). Many of the victims died when they were fairly young, averaging about 26 years in age. Virtually no signs were present to indicate differences in social status or wealth. In sum, the burials exhibited “a general sense of disorganization,” as the researchers noted in their study. Taken together, the unorthodox method of the burial, along with the uncharacteristically young age of the victims, is consistent with what would be expected after a tsunami.

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This suggestion is hardly outlandish. Coastal communities are common in the archaeological record, with early humans subsisting off of marine food resources. Also, recent tsunamis in the Indian Ocean and Japan have demonstrated the power of tsunamis to produce deaths en masse. After the Indian Ocean tsunami, for example, as many as 60,000 to 70,000 victims were buried in Banda Aceh alone. Both now and in the past, survivors had to create mass graves and bury the dead without engaging in the usual funeral customs; leaving the bodies in the open to rot, or to picked away by scavengers, is not and was not an option. Finally, given the prevalence of ancient coastal communities and the frequency of tsunamis, it seems odd that archaeologists haven’t identified a single tsunami-related mass grave. As Goff and his colleagues argue, that’s because the cause of these mass graves are being overlooked.

“Our archaeological research in conjunction with geological evidence recognises that exactly what has happened in modern events must have happened in the past,” said Goff. “This line of analysis will hopefully start being at the forefront of researchers’ minds when excavating a coastal mass burial.”

To be clear, the researchers aren’t saying that every prehistoric mass burial along the coast is related to a tsunami. Rather, they’re simply asking archaeologists to reconsider and re-evaluate ancient mass graves found in coastal contexts. The authors present various methods for doing so. Geological, chemical, and biological analyses can be performed to identify the location and timing of ancient tsunami events, such as the identification of inland sedimentary layers consistent with a tsunami, signs of erosion, saltwater signatures found inland, and microfossil evidence, among many other signatures.

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But there’s another technique, one proposed in the new study.

“When people die in a tsunami, they inhale saltwater that contains small marine microorganisms called diatoms, which means they suffocate and then drown,” Genevieve Cain, the lead author of the new study and a researcher at the University of Oxford, said in a statement. “These diatoms travel through the bloodstream and are deposited and preserved within the bone marrow of larger bones. If we can find marine diatoms, this may indicate that the body is a tsunami victim.”

Looking ahead, this team of researchers is hoping to do exactly that, and analyze the skeletal remains of victims found in coastal mass burial sites around the world.

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[Journal of Archaeological Method and Theory]