Ice cores, sediment samples, and tree rings are commonly consulted for the history of the planet’s climate. But the Antarctic, it turns out, has another climate record.
This record is remote: Researchers braved the icy waters of the Antarctic to reach islands where they dug up valuable data. This buried treasure can help them understand changes in climate and penguin populations in the Antarctic.
Their data sources? Mounds of ancient penguin guano. That’s right: penguin poop.
Nesting sites for Adélie penguins on these remote islands are older than any others previously found in the Antarctic Peninsula, scientists report in a poster at AGU’s Fall Meeting 2018. Because the penguins, year after year, continue to return to the same nests, some of these islands have layers of guano spanning the past 2,900 years.
“Penguins not only tell us about what’s going on now, but they have this amazing ability to tell us about what has happened in the past,” said Michael Polito, an ecologist at Louisiana State University who conducted this research. “And they can supply some insights that are not possible through more traditional paleoclimatic archives.”
Layers of Guano…and Data
Adélie penguins (Pygoscelis adeliae) mate for life. Like a suburban couple looking for a home near a good school, their jobs, places to buy groceries, and nice parks, penguins can get quite picky. They tend to look for open-water access to the coast, ice-free terrain to build their nest on, and their equivalent of a good grocery store—a good supply of marine prey.
At nesting sites, penguins use pebbles to build and rebuild nests year after year, spanning generations. But since penguins don’t have indoor plumbing or garbage men, the nests get buried in their poop and other waste, such as feathers and eggshells. These materials form a distinct “ornithogenic” soil, a term that reflects its avian origins. Megan Balks, a soil scientist who has studied ornithogenic soils in the Ross Sea region and was not affiliated with this research, describes ornithogenic soil as “some of the most unique and often odiferous soils” she has ever worked with.
The cold, dry Antarctic environment helps to preserve these soils, which build up year after year, and any samples hidden in the muck. Layers of soil can thus represent hundreds or even tens of thousands of years of penguin occupation.
These soils provide a stratigraphic tell. As scientists dig into the ground, pebbly ornithogenic soils have a color and characteristic distinct from the beach gravel and sand they are built on. These soils are a reliable indication of a time when penguin nesting needs were met.
And because samples like feathers in these soils hold DNA, these soils can reveal penguins’ genetic history. Stable isotope analysis of nitrogen and carbon in guano and eggshells can shed light on whether penguins dined more on fish or krill at the time.
Researchers can also collect small, hard pieces of penguins’ prey from the layers of poop to more directly observe their diets. Fish bones, squid beaks, and otoliths—small calcium carbonate structures—from the inner ear of penguin prey can paint a picture of what penguins were eating when layers were deposited, which can indicate changes in fish and squid species sizes and abundances in the Antarctic.
Where’s the Ancient Guano on the Antarctic Peninsula?
Polito and his colleagues are using the presence of ornithogenic soil and radiocarbon dating of penguin tissue from bones, feathers, and egg membranes recovered from the soil to help establish the history of deglaciation on the eastern side of the Antarctic Peninsula. After all, penguins can nest only where land is exposed. The data also can shed light on how penguin populations respond to changes in the environment.
Ornithogenic soils have been a rich source of information in the Ross Sea area of the Antarctic but have raised some mysteries on the Antarctic Peninsula. On the Antarctic Peninsula it has been harder to find abandoned nesting sites, and those that have been found have all been newer, occupied less than a thousand years.
“And this has always begged the question, are we not finding evidence of older colonies in the Antarctic Peninsula the way we have in places like the Ross Sea because penguins have only recently inhabited this area or because we are missing something?” said Polito. Sampling is useful but limited to the sites you have on hand, sites you’ve discovered. “You don’t know if you missed something around the corner,” he explained.
Another possible answer is that something destroyed nests on the Antarctic Peninsula, but that leaves the mystery of why they were destroyed in one part of the Antarctic and not the rest.
Polito and his colleagues finally found the right corner to check behind. They extended their search to sites farther north along the peninsula, which might have been made accessible by deglaciation earlier, as the massive ice sheets retreated south more than 3,000 years ago.
The Search for Poop-Covered Rocks
Their new data come from samples from both occupied and abandoned Adélie penguin colonies at three points along the northern Antarctic Peninsula: the Danger Islands, Paulet Island, and Brown Bluff on the Tabarin Peninsula. The Danger Islands and Paulet Island can be difficult to reach since they are in the northern extent of the Weddell Gyre, a large circulating system of ocean currents swirling with sea ice.
“It’s really common that ice will be in those areas, and it makes it hard to access them,” said Polito. “Just getting there was really exciting.”
At the sites they dug up the nest “in the same way you would dig up an archeological or paleontological dig, layer by layer pulling out all the bones and egg shells,” and “would come back with these bags of basically guano—penguin poop—covered rocks,” Polito explained.
The expedition that collected the Danger Islands samples also performed the first full census of the islands to determine how many penguins lived there. They determined there were over 750,000 nests.
They had to charter a small ship to get to the Danger Islands, but to get to Paulet Island and Brown Bluff, they were on a commercial tour ship, which made the experience very different.
Most of the tourists were fascinated, he said, but half were also horrified at the idea of them digging up penguin poop. “We would leave the ship perfectly clean, and we’d come back kind of smelly and stinky,” he said. “And being able to share that with all the guests on the ship was very exciting.”
After the excavations, they sent remains recovered during the excavation off to a lab for radiocarbon dating. Then they waited until results came back. “It’s sort of like Christmas when you open the file and can see how old the colony is,” he said.
They found that Adélie penguins began occupying the Danger Islands and Paulet Island around 2,900 years ago, earlier than the dates of any other ornithogenic soil tested on the Antarctic Peninsula. By contrast, Brown Bluff, which is lower on the peninsula, appears to have been occupied for less than 400 years. These dates, when combined with previous data, help paint a picture of Adélie penguins starting to make homes on the northern end of the Antarctic Peninsula around 2,900 years ago before traveling farther south along its eastern side.
“This is the first time we have been able to see evidence of older occupation” in the Antarctic Peninsula, said Polito. “And it actually lines up better with the other paleoclimate proxies—the sediment cores and things like that tell us.”
Such alignments are key, added Polito. “These are two, sort of independent records that are saying that, especially in the northeastern part of the Antarctic Peninsula by around 3,000 years ago, climate conditions were such that it was suitable to penguins,” he explained. “That gives a very specific window of climate conditions and that then confirms some of the sediment cores and paleorecords from that area.”
Balks said these results give “important new insight into the timing of arrival of penguins in different areas” but suggested that the lags in penguin colonization may be the result of the time required “to build populations to the point where some birds are forced to seek new locations as generally penguins return to their home colony to breed.”
Polito acknowledges this is a possibility. “But there is exciting evidence to show that when conditions change [penguins] can respond rapidly,” he said. He thinks penguins probably took advantage of cleared land over the geological timescales involved.
There are still enigmas surrounding the occupation of penguins on the Antarctic Peninsula, Polito explained. Some old penguin bones have been found in beach and lake deposits without correspondingly old penguin nests, and there is biochemical evidence of penguins from sediment cores taken from ponds that indicate the presence of penguins on islands at older dates than can be demonstrated using nesting sites.
He noted that these observations require additional explanation, but without ornithogenic soil it can be difficult to tell whether penguins bred at a location or only visited it.
Changes in the Future
The group also plans to perform stable isotope analysis of samples collected from these locations to help identify how the penguins’ diets have changed over time with the environment.
This research is part of a larger ongoing National Science Foundation–funded project “to understand penguin distributions and penguin diets in the past and how that has changed relative to past environmental conditions,” Polito explained. “The goal is to understand or generalize how these penguins respond to environmental change, and that will help us better understand the environmental changes that are occurring now and help us predict potential change in the future.”
These data will hopefully help answer the question of how much of recent penguin population decline is related to changes in ice conditions and how much is related to changes in food conditions.
“We hope to really increase the number of sites where we can sample, so we can get a much more holistic view of the past occupation history of penguins relative to change of the climate throughout the entire peninsula,” said Polito. “Because the more sites we can get, the clearer the picture comes into focus.”