|Guest commentary by Sarah Feakins
Our recent study in Nature Geoscience reconstructed conditions at the Antarctic coast during a warm period of Earth’s history. Today the Ross Sea has an ice shelf and the continent is ice covered; but we found the Antarctic coast was covered with tundra vegetation for some periods between 20 million and 15.5 million years ago. These findings are based on the isotopic composition of plant leaf waxes in marine sediments.
That temperatures were warm at that time was not a huge surprise; surprising, was how much warmer things were – up to 11ºC (20ºF) warmer at the Antarctic coast! We expected to see polar amplification, i.e. greater changes towards the poles as the planet warms. This study found those coastal temperatures to be as warm as 7ºC or 45ºF during the summer months. This is a surprise because conventional wisdom has tended to think of Antarctica being getting progressively colder since ice sheets first appeared on Antarctica 34 million years ago (but see Ruddiman (2010) for a good discussion of some of the puzzles).
Where did this record come from?
The ANDRILL program is a multinational collaboration involving scientists from Germany, Italy, New Zealand and the United States to drill through ocean sediments around Antarctica. The drilling effort in the austral summer of 2007 involved a rig perched upon the Ross Ice Shelf, drilling down through the ice, 400m of water below that and then grinding down 1km into the sediments. The sediments are bagged and then transported back to the storage facility in Florida from where they are parcelled out to analysis laboratories across the world.
It can take years to process all this sediment and perform all the compositional, elemental and isotopic analyses that need to be done. Numerous scientists work on getting the most information possible out of the core. One of the early findings was the unexpected discovery of abundant pollen in the Miocene part of the core by Sophie Warny (Warny et al, 2009). The pollen came from types of tundra vegetation and indicated summer temperatures above freezing, which was also inferred from the presence of freshwater algae.
After Sophie found the pollen, I began to search for molecular fossils of those same plants. The waxy coating of plant leaves is remarkable for its resilience in sediments. In addition those leaf wax molecules capture an isotopic record of past rainfall. It is these isotopic signatures that allow quantitative insights into temperature and rainfall.
To extract the leaf waxes we don’t look for visual fossils, instead we use organic solvents to dissolve and extract the leaf waxes out from the sediments. Those organic molecules are then purified by passing through a series of filtering steps in the lab. Ultimately we wind up with a pure concentration of the leaf waxes which can be analyzed by mass spectrometry (see photo).
How are the results interpreted?
The leaf wax hydrogen isotope evidence was interpreted in comparison to model experiments. Jung-Eun Lee (JPL) conducted experiments, after adding water isotopes into a model dubbed GRAM (Frierson et al, 2006) because it requires a gram of computational effort rather than a ton in a full general circulation model. With the aid of the isotope-enabled model version, iGRAM, we can simulate the movement of water around the planet and track the water isotopic signatures. The goal was to see if modern relationships between different points in space that have different isotopes in precipitation and temperature are valid when we instead consider changes at the same point over time. Model experiments suggested a small upwards tweak in the temperature reconstructions for the Miocene from 2ºC to 7ºC. These experiments also reveal the dynamics behind the isotopic values: more evaporation from the warmer high latitude oceans and increased rainfall at high latitudes. (Ed. In similar experiments for Greenland (Werner et al, 2000), the changes in the seasonal cycle were important in understanding the isotope paleo-thermometer).
The iGRAM model is however an idealised aquaplanet, (i.e. no continents at all) so it isn’t useful for the interior of Antarctica, but deep sea records suggest that glacial ice volume was about 50% of modern volume at that time. It is however difficult to do full general circulation model experiments for this period because of the difficulty of constraining boundary conditions in the Miocene – what the land surface looked like, what greenhouse gas levels were, etc. An aquaplanet is perhaps good enough for these tests as conditions at the coast are really set by the oceans.
In terms of figuring out how the climate system operates, temperature is one of the simpler variables to reconstruct (not that any of this is really simple). Figuring out how precipitation changes is harder, largely because models can’t capture the scale of clouds let alone raindrops. What the leaf waxes provide is an archive of the isotopic composition of precipitation – much as the ice cores do for the past million years. Of course an ice core is not as simple as a rain gauge, and a plant has biology that an ice core doesn’t, but crucially if plants are growing, leaf waxes are probably preserved in sediments allowing us to push these isotopic records back beyond the ice core records to address questions about what climate was like further back in time.
How robust are these results?
What is reassuring here is that all the lines of evidence presented, from various microfossils, molecular fossils, isotopes and model experiments, all point to temperatures at the coast of Antarctica reaching above freezing point in summer months, probably around 7ºC (45ºF).
It is in those warm, periods further back in time, that might help us understand a little more about how warmer climate systems operate, and that information might just be important as we contemplate our future.