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Detailed chemical mapping of a dead man’s fingernail has illuminated the fate of a sailor in an ill-fated expedition to the Arctic. In one of the biggest unsolved mysteries of the north, a 129-member crew, led by English explorer John Franklin, disappeared in 1845 during its search for the Northwest passage.
Some researchers have blamed the deaths on lead poisoning — caused by the metal leaching from food tins or the pipes that routed drinking water around the expedition's ships. Lead exposure can trigger symptoms including neurological problems, which could have ultimately doomed the exhausted explorers. But scientists have now discovered that one of the few sailors whose bodies have been found had a zinc deficiency — and suggest that it was this, not lead exposure, that led to his death.
“There’s increasing evidence that lead was not the issue,” says Jennie Christensen, formerly a toxicologist at Stantec Consulting in Sidney, Canada, who is now at TrichAnalytics in North Saanich, Canada. She and her colleagues measured levels of copper, zinc and lead at different places along a toenail and a thumbnail from crewmember John Hartnell. He and two other sailors died during the expedition’s first overwintering on Beechey Island in 1845–46, and were buried there.
By studying the accumulated growth of Hartnell's nails, Christensen’s team could pick out week-by-week changes in his body. They reported the findings on 6 December in the Journal of Archaeological Science: Reports1.
“You can actually see what the levels of lead were with John Hartnell, over the period of time he was present on the expedition,” says Keith Millar, a clinical psychologist at the University of Glasgow who has studied the expedition members’ health2. “That is a pretty unique insight.”
Christensen had been working on ways to study metal exposure over time, such as mercury accumulating in grizzly bear hairs from the salmon in the animals' diet3. She drew on those techniques to track Hartnell's levels of lead as well as copper and zinc, low amounts of which can reflect a lack of meat or seafood in a person's diet.
Her team used the Canadian Light Source, a synchrotron particle accelerator in Saskatoon, and other instruments to build a picture of the metals inside Hartnell over time. They found that he suffered from a severe zinc deficiency, which could have suppressed his immune system. The researchers propose that this made him more vulnerable to tuberculosis and pneumonia, diseases that are thought to have ultimately killed him. Hartnell had relatively high levels of lead only during his last few weeks, when his dying body probably broke down and released long-stored lead from his bones into his blood and nails, Christensen says.
“The lead theory is pretty much dismantled by this point,” says Ron Martin, an analytical chemist at Western University in London, Canada. In 2013, he analysed bone fragments from several crewmembers including Hartnell, and concluded that they had experienced consistent lead exposure throughout their lives, with no spike during the expedition4.
Further evidence pointing away from lead poisoning comes from the expedition’s two ships, the HMS Erebus and the HMS Terror, which sank about 100 kilometres apart. Archaeologists with Parks Canada located the two ships in 2014 and 2016 respectively, after years of searching. Exploration of the Terror showed that items had been carefully stowed away, arguing that the crew had not been experiencing the hallucinations or delirium that often come with lead exposure, says Millar.
No nail samples are available for any of the expedition members other than Hartnell, so the work cannot be extended beyond his particular death. But zinc deficiency could help explain why other Arctic expeditions also suffered from poor health, Christensen says.
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