Researchers have analyzed 20th-century polar explorer Jørgen Brønlund's last diary entry, including a curious black spot, written before he died alone in Northeast Greenland, shedding light on his last few hours.

In 1906, a Danish expedition set off to map out the last undocumented regions of Greenland. These last sections of the maps remained unfilled for a reason: conditions were harsh in the far northeast, and to attempt it you would have to rely on hunting your own food for sustenance.

The team of six set up base camp in a small weather station in Dove Bay and spent the autumn and winter laying out supply depots along the route they would take, before departing for lands and ice uncharted in the spring. Brønlund, as an expert interpreter, would keep the group's travel diary.

In March, they headed north along the icy coast, and were at once confronted with harsh conditions and rough ice, making their already difficult trip even harder, as they had to regularly stop to repair the sleds. The team grew concerned when they realized the coastline was taking them further to the northeast than they were expecting, so the trip would take longer – and, crucially, use more resources – than they had planned for. The mistake was likely due to sketches drawn by earlier explorer Robert Peary, who had done so through guesswork.

Still committed to their task, the team split in two, with one team going northwest across the sea ice, where they would map the coast and eventually return to safety. The other, comprised of Ludvig Mylius-Erichsen, Niels Peter Høeg Hagen and Brønlund, headed westward, and would eventually end their journey on the ice.

The three, led by Mylius-Erichsen, continued to map the area, losing precious time and energy when they entered a fjord, which would later go on to be named the Hagen Fjord after cartographer Hagen, who mapped it during this very trip. They completed their task of filling in the map and headed back the way they came but found that summer had melted the ice, making their previous route impassible, and forcing them to go inland.

Their situation became dire. The weather turned colder, they were down to four dogs and a single sled, and their shoes had become worn by the rocks beneath their feet.

"No food, no footgear, and several hundred miles to the ship," Brønlund wrote in his diary. "Our prospects are very bad indeed."

They put off eating the dogs as they would need them to get back to their ship, but by the time they reached the ice the dogs were as emaciated as the crew. They put in one final bid for survival, making it an impressive 260 kilometers (160 miles) on bare, frozen feet over the course of 26 days. All this was done in the the dark of the Greenland winter, where daylight only occurs for three or four hours a day.

However, it wasn't enough.

"I perished in 79° N. lat., under the hardships of the return journey over the inland ice in November," Jørgen Brønlund wrote in his final entry, aware he would not survive the night. "I come under a waning moon, and cannot go on because of my frozen feet and the darkness. The bodies of the others are in the middle of the fjord. Hagen died on November 15, Mylius Erichsen some ten days later."

His was the only body to be found, and with him, the maps the team had taken on their journey. Though all three died, their maps – and the purpose of their journey – were complete. When Brønlund was recovered four months later, his diary was found documenting his final days.

Within the last entry, there was a black spot on the final page, just below his signature, which has now been put to chemical analysis by a team at the University of Southern Denmark (SDU). The analysis, published in the journal Archaeometry shows the black spot contained burnt rubber, oils, petroleum, and feces. As the last survivor of the team, Brønlund had reached the last depot and had access to a Lux petroleum burner, matches, and petroleum. The team believes he tried to use oils available to him, which may have been from vegetables, animals, fish, or wax candles, in order to try to get the burner going without any metabolized alcohol to preheat it.

"I see for me, how he, weakened and with dirty, shaking hands, fumbled in an attempt to light the burner," Professor Kaare Lund Rasmussen of SDU said. "But failed."

Exhausted, Brønlund wrapped himself up in his fur and died. His diary is now kept at the Royal Library in Copenhagen, and his Lux burner, found in 1973, was donated to the Arctic Institute in Copenhagen.