Fossilised spit reveals 3600 years of snow petrel colonisation

Fossilised layers of spit, or ‘mumijo’, at snow petrel breeding sites, show the birds colonised the southern Prince Charles Mountains in East Antarctica at least 3680 years ago.

The research, led by Dr Sonja Berg of the University of Koln, Germany, and involving Australian Antarctic Division seabird ecologist Dr Louise Emmerson, provides the first dating of snow petrel mumijo deposits in the region.

“Snow petrels regurgitate their stomach oil as a defence mechanism, and over time it solidifies in Antarctica’s cold, dry climate to form a waxy deposit called mumijo,” Dr Emmerson said.

“By radiocarbon dating these deposits we’ve shown the birds colonised the southern Prince Charles Mountains several thousand years after the Antarctic ice sheet retreated. This suggests that environmental factors, other than the availability of ice-free nesting sites, are important for snow petrel colonisation.”

Most snow petrels (Pagodroma nivea) are found within 100 kilometres from the coast, where they nest in cavities created by large boulders, on nunataks, rocky hills or mountains. In the Prince Charles Mountains, however, snow petrels nest up to 440 kilometres from the coastline — the furthest inland of any known populations.

Because the birds only nest in ice-free areas, radiocarbon dating (14C) of the fossilised spit can be used to infer local deglaciation history and the location of ice-free areas during the last glacial period. As the birds usually nest within about one day’s flight from feeding grounds, mumijo also provides information on changes in the coastal food web.

The research team collected samples of mumijo from Greenall Glacier and Pagodroma Gorge, 450 kilometres and 260 kilometres inland of Prydz Bay, respectively, near Davis research station.

Radiocarbon dating of three mumijo deposits from Greenall Glacier, one of which was six centimetres thick, provided ages of 3680, 2400 and 895 years. The oldest deposit from Pagodroma Gorge was dated at 2675 years.

“Our results suggest that snow petrels occupied Greenall Glacier at least 3680 years ago and that they have probably inhabited the region continuously since,” Dr Emmerson said.

“A comparison of our results to mumijo ages reported at more coastal sites, indicates that colonisation was not directly linked to de-glaciation, but rather enabled by changing environmental conditions, either at the birds’ nesting sites or in their foraging range.

“Improved conditions in their foraging range would make it energetically feasible to live in these inland areas.”

As part of a broader study looking at the movement and changes in snow petrel populations over geological time scales, this past summer, field biologists Marcus Salton and Dr Anna Lashko collected mumijo samples from rocks outside snow petrel nests in the Masson Range near Mawson research station. The samples were collected while the birds were out foraging at sea, to prevent disturbance when they returned to lay their eggs.

Mr Salton said the mumijo was so hard it has to be chipped off with a hammer and chisel.

“The spit gets a lot of bird foot traffic and is exposed to high winds, so it gets amalgamated with bird poo, dirt and grit, and becomes very hard,” he said.

“Once we collected a sample we wrapped it in kitchen foil and stored it in a freezer on station, before returning it to Australia.”

A team at Casey research station also searched for mumijo, however they are yet to find layers of any substantial depth.

“The absence of thick mumijo layers may mean the birds haven’t been around for as long in this area of Antarctica, and this is consistent with recent genetics studies that suggest that the birds are likely to have moved in from the Davis and Mawson populations,” Dr Emmerson said.

Wendy Pyper
Australian Antarctic Division