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For sediment scientists like Stephen Roberts of the British Antarctic Survey, extracting a long thin column of dirt from the ground is satisfying but routine. , and similar to the rings on a tree, studying the layers of dirt lets scientists figure out what happened in the past. 

But for Roberts, that routine came with a surprising twist in 2007 while doing research on Ardley Island, a small chunk of land tacked onto the edge of the South Shetlands, where Antarctica stretches toward South America. There, the sediment cores he and his colleagues pulled from a lake were different. When the scientists split the cores open, they were unusually smelly. Sections were “gloopy” and “dark green,” unlike a core’s usual brown color, Roberts says, and the samples didn’t match the geochemistry the team had expected for the area. Most odd were the bones dotting the cores—juvenile Gentoo Penguin bones, to be precise. Occasionally small pieces of bone work their way into cores, but these included pieces as large as a chain of five bones from one penguin’s upper back and neck.

It might not qualify for an episode of CSI, but it was a compelling enough mystery to warrant solving for the scientists, whose work on Ardley Island was published this week in .

You've probably already guessed what made the cores so green and smelly: guano. But while finding the penguin poop in the core samples was a surprise to the researchers, they also thought it might help them discover the story of the peculiar cores. 

Thanks to the dedication of another team of scientists, Roberts and his team knew that the average Gentoo Penguin produces about 84.5 grams of guano per day (that’s about the weight of a small lime). They also knew how to calculate the rate at which gunk gathers in Ardley Lake. So the team put all this data together, studying the 8,500 years’ worth of sediment. By analyzing its chemistry, they estimated how many penguins were living near the lake. They found five high-guano periods, some of which matched the ages of the penguin bones embedded in the sediment.

From the original cores and a second round of sampling in 2011, scientists were able to compare Ardley Lake samples with nearby sites, which allowed them to construct a detailed timeline of temperature and sea level. In addition, they took into account that the penguins were within range of Deception Island, a volcano that cataclysmically blew its top long before penguins came to the neighborhood. It has erupted sporadically ever since, leaving sediment cores streaked with dark gray ash.

By combining these timelines, Roberts and his colleagues were able to compare penguin populations with climate patterns and Deception Island eruptions through millennia. They didn’t find much connection between climate changes and penguin populations. But they did find that after three major Deception Island eruptions—perhaps around as large as the Mt. St. Helens’ eruption in 1980—penguin guano became much scarcer, and it took the colony as long as 400 to 800 years to recover its full poo-producing prowess. Researchers have analyzed guano in sediment before, but Roberts says this is the first study looking at the long history of penguins in relationship to volcanoes.

The team can’t be sure precisely what happened to the penguins when the nearby volcano coated their colony with ash and rock particles. “They only really had three options” at a colony level, Roberts says. The least lucky birds might have been killed in place, poisoned, or injured. Adults may have simply swum away, waiting for better time to return and recolonize the site. Or the colony may have fallen on very hard times for a few years followed by a long, slow rebound.

Deception Island has been fairly quiet for as long as scientists have been visiting the area. According to Roberts, the volcano erupted in 1967 and 1969, but those were relatively small. “[] looks quite dramatic, but it’s actually this tiny, tiny eruption compared to the caldera-forming eruption,” he says. The eruptions that decimated the penguin colony are between those magnitudes.

There are still Gentoo Penguins on Ardley Island, although they’ve moved east, away from the lake that preserved their predecessors’ poo for thousands of years. It’s one of the largest Gentoo colonies, at about 5,000 nesting pairs, with a couple hundred Adélie Penguins and a couple dozen Chinstrap Penguins in the mix. If Deception Island grows violent once more, it could take the spiffy birds a few centuries to recover, but as Roberts's research reveals, the penguins—and their poo—will likely endure.