Raiders of the lost guano discover ancient penguin devastation

About 7,000 years ago, gentoo penguins first came to Ardley Island in the South Shetlands chain just off the Antarctic Peninsula.

The island is a little over a mile long, almost small enough for a classic castaway cartoon, except that it is the Antarctic. And instead of a lone palm tree, there are now about 5,000 breeding pairs of gentoo penguins, one of the largest colonies in the Antarctic, and a lot of guano (penguin excrement), much of which is washed into the freshwater Ardley Lake, where it accumulates in the sediment.

In that guano, scientists have found the record of a recurring natural historical drama. Three times since the gentoos arrived on Ardley, the colony was devastated by volcanic eruptions. The ash and smoke killed them or drove them away. Penguins gather in colonies to breed, so there may well have been chicks caught in the ash fall even if adults escaped. The landscape the eruptions left cannot have been hospitable, because each time it took 400 to 800 years for a colony of similar size to re-emerge.

That is the story, reported on April 11 in Nature Communications, that Stephen J Roberts of the British Antarctic Survey, Patrick Monien of Bremen University in Germany and other scientists from Poland, Scotland and England teased out of lake sediments that show, in the rise and fall of guano concentration, the rise and fall of the penguin colony.

Wide fluctuations

Stephen said the team of scientists did not set out to study guano. Rather, their interest was in evidence of historical changes in climate and sea level. But something about the sediment samples drilled from the bottom of Ardley Lake prompted them to take a different approach this time. The samples were a bit ripe. “When we opened them up they smelt differently,” he said.

The team could see the ash from volcanic deposits and penguin bones, and began to compile information on the ash layers, biochemical analysis of the guano and similar samples from a lake whose shores did not have a penguin colony nearby. They estimated penguin population by the percentage of guano in a sediment sample, figuring three ounces of guano per day per penguin, and calculating how much of the colony’s output would flow into the lake.

Using a sample to determine the amount of penguin guano flowing into the lake in a given period of time, they could calculate how big the colony was. This showed wide fluctuations in the colony’s size, with the peaks similar to the current numbers. Overall, they did not find any consistent pattern related to climate or sea level.

But three times the population crashed — indicating the near extinction of the colony — coinciding with eruptions from the Deception Island volcano, also in the South Shetlands chain. The events do not have broad implications for climate studies, Stephen said. But they show one case where local events had a far greater effect on the population than global trends.

Deception Island is an Antarctic landmark and a destination for tourists. The island is the rim of the volcano’s caldera. It’s a circle in the ocean, with only one entrance to the protected waters inside. Heat from the volcano makes some spots swimmable, and many passengers on Antarctic cruises bring back a picture of themselves in the waters of the island caldera.

The scientists estimate that three large eruptions of the Deception Island volcano all but wiped out the penguins on Ardley Island — one 5,500 to 5,400 years ago, one 4,500 to 4,200 years ago, and the last 3,200 to 3,000 years ago. In more recent times, there have been smaller eruptions. The last eruption was in 1970.