A New, Nauseating Climate Archive

Claire Penny, PhD Candidate in Organic Geochemistry, Durham University, explores a novel technique for reconstructing historic variations in Antarctic sea ice using dietary biomarkers from snow petrel stomach oil deposits.

Antarctica, a wilderness known for its unforgiving landscape and intrepid exploration, has long piqued the scientific curiosity of polar researchers and wider society alike. Despite its isolated location, Antarctica plays a crucial role in the global climate system: the vast Southern Ocean surrounding it forms deep-water currents circulating the world’s oceans and functions as the largest sink for heat and carbon ⁽¹⁾. With sea-ice loss predicted to accelerate, seasonal sea ice fluctuations lower the amount of solar radiation reflected into space (planetary albedo effect), resulting in the intensification of solar absorption in a warming world. However, the extent to which Antarctic sea ice will respond to future environmental change have many unknowns. First, reconstructions of past sea-ice extents disagree with each other and struggle to capture its spatial changes through time. Second, the ice and marine sediment cores that scientists normally use to understand sea ice are sparse, leaving large gaps in our understanding of sea-ice behaviour through the geological record ⁽²⁾. When these conventional techniques cannot find the answers we need, we must find new routes to understanding future sea-ice change, to uphold the UK’s limit of a 1.5°C temperature rise ⁽³⁾.

This is where the snow petrel comes in. A small seabird endemic to Antarctica, it can be found nesting in the crevices of Antarctica’s exposed rock faces (Figure 1a) or as small specks of white flying along the sea-ice edge, twittering distinctively (Figure 1b). However, they are not as endearing as they first appear. These unassuming seabirds have a stomach-churning habit: when hunting, they partially digest their prey and store the liquid remains in a small organ located just below the oesophagus, the proventriculus (Figure 2). This bright orange substance, termed ‘stomach oil’, is primarily used as an additional source of food for their chicks ⁽⁴⁾. However, as well as an appetising breakfast, this stomach oil can be voluntarily regurgitated to fend off predatory birds (think spitting llamas). This smelly, sticky liquid splashes onto rocks below their nest, accumulating and solidifying into stratified grey-yellow layers resembling the surrounding bedrock (Figure 1a). Snow petrel colonies return to the same inland nesting sites each summer, and so the cold, dry climate allows these deposits to remain in place for many thousands of years with little disturbance, some for over 50,000 years.

But what do these distant (and nauseating) remains have to do with Antarctic sea-ice? The layers reveal an ancient story: from the oldest bottom layers to the youngest top layers, these slices record inter-annual to millennial periods of time when a snow petrel was actively foraging, returning to its nest and feeding its young. Modern studies show that fish, krill or squid make up the primary components of a snow petrel’s diet ⁽⁵⁾, and this depends on how close it forages to the continental shelf (Figure 3).  

During the summer breeding season, the shallower, colder waters closer to the continental shelf edge encourage sea ice to form, concealing the abundant fish a snow petrel typically hunts close to the coastline. The expanding seasonal sea ice forces the birds to forage further offshore, where ample open water provides a more varied diet of fish, krill and cephalopods (squid). Although a snow petrel can fly over 500 km in a single foraging trip ⁽⁶⁾, if the sea-ice edge is too far from their nest, they will take advantage of transient openings in the sea-ice pack, called polynyas (Figure 3). Polynyas occur when high-speed winds from higher altitudes blow towards the coastline, pushing the newly formed sea ice oceanward to create pools teeming with fish and krill ⁽⁷⁾. A snow petrel’s diet, whether it consumes more fish, krill, or both, can therefore be used as an indirect proxy for the latitudinal extent of summer sea ice. The preserved stomach oil layers retain a signal of these dietary changes through time in the form of biomarkers. Biomarkers are organic molecules present in all living tissues, including the partially digested prey remains, as building blocks of fat called fatty acids (Figure 4). These fatty acids encode information pertaining to the sea-ice conditions at the time, reflected in the proportions in which they are found ⁽⁸⁾. When present in different layers of the stomach oils, fatty acids can help to build a picture of sea ice evolution.

To understand how Antarctic sea-ice might change in the future, we must understand its past behaviour. This novel biological record provides a past perspective on a turbulent and vital aspect of global climate mechanisms. As sea-ice decline is projected to become significantly more pronounced under all climate change scenarios, the scientific research generated from stomach oil deposits can help solidify future model predictions and allow governments to act in securing Antarctica’s future.

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