Ancient penguin eggs tell a tale
A recent study looked at the chemistry of some ancient Adelie penguin eggs to understand where the penguins were getting their water – and thus learn more about their environment. Actually, these eggs are not too ancient by our standards. While many of the fossil penguin species we’ve covered on this blog are tens of millions of years old, the oldest eggs in this study are about 8,000 years old.
The scientists in this study looked at oxygen data from eggshell pieces. There are several “types” of oxygen atom called isotopes, differing by their number of neutrons. “Light” oxygen has 8 protons and 8 neutrons, and is known as oxygen-16 to scientists. “Heavy” oxygen has 8 protons but 10 neutrons, and is known as oxygen-18. One of the interesting things about these isotopes of oxygen is that they are treated differently by natural processes and end up getting sorted, or fractionated. Water of course has one oxygen atom per molecule, hence the formula H2o. Water with light oxygen evaporates more rapidly than heavy oxygen, which ends up leaving a chemical signal of where it came from. Water in the oceans tends to have more heavy oxygen in it, while snow tends to have more light oxygen, because it is made up of evaporated water that has returned from the sky as snowflakes.
Oxygen contributes to other molecular besides water, and one of these is calcium carbonate (CaCO3). Calcium carbonate is an important component of eggshells. Penguins, like all birds, shell their eggs with calcium carbonate. Oxygen from water sipped by the mother penguin gets added to the calcium carbonate of the egg, and thus leaves a trace of their water source behind when they lay the eggs. Scientists can collect fragments of hatched eggs and analyze them to figure out what the ratio of heavy to light oxygen tells us about the penguin’s water source.
Penguins are very good at getting water. There are no water fountains or bottles of sparkling water in Antarctica, but that poses no problem for Adélie penguins. Like other species, they are able to drink directly from the ocean while at sea, removing the salts through specialized glands to make the water usable by their bodies. On land, these little penguins have a habit of eating snow. This behavior is very useful in sub-zero temperatures where liquid water is impossible to find on land. However, it can get penguins in trouble in the wrong environment, as we saw when Happy Feat the stray Emperor Penguin tried to eat some sand thinking it was snow. In the video below, you can see a Chinstrap “drinking” some fresh snow.
In the new study, there was evidence that the older eggshells had relatively more light oxygen atoms. This indicates the penguins were probably munching on more snow or drinking more from freshwater streams in the past. Samples younger than 2000 years show an increase in heavy oxygen, indicating that the more recent generations of penguins at the colony have relied more on seawater and less on freshwater and snow. This makes sense when we look at whats happening on a large scale. Starting around 2000 years ago, there was probably less glacial meltwater available to drink. It’s also possible that changes in climate led to there being a larger distance between the penguin colony and edible snowdrifts than in the deeper past. Without a convenient snow snack nearby, the younger penguins probably waited until the next foraging trip to the sea to replenish their water supply.
Lorenzini, B.S., I. Baneschi, A. E. Fallick, M.C. Salvatore, G. Zanchetta, L. Dallai and C. Baroni. 2012. Insights into the Holocene environmental setting of Terra Nova Bay region (Ross Sea, Antarctica) from oxygen isotope geochemistry of Adélie penguin eggshells. The Holocene 22: 63-69.