A Lost World
'The ice covered Antarctic continent, bathed by seas of uniform coldness, is in the grip of the polar anticyclone. High winds blow from the land and repel any warming influence that might seek to penetrate it. The mean temperature of this bitter world is never above the freezing point. On exposed rocks the lichens grow, covering the barrenness of cliffs with their gray or orange growths and here and there over the snow is the red dust of the hardier algae. Mosses hide in the valleys and crevices less exposed to the winds, but of the higher plants only a few impoverished strands of grasses have managed to invade this land. There are no land mammals; the fauna of the Antarctic continent consists only of birds, wingless mosquitoes, a few flies, and microscopic mites.'
- Rachel Carson, The Sea Around Us
Artist's reconstruction of Crylophosaurus whose fossilized remains were recovered in Antarctica. Image courtesy of William Stout
In 1994, two researchers from Augustana College made a strange discovery in the mountains of Antarctica. While excavating a siltstone formation 650 km from the South Pole, William Hammer and William Hickerson uncovered the skull and bones of a large carnivorous dinosaur. Found alongside the gnawed ends of the bones were the teeth of other smaller scavenging dinosaurs and fossilized tree trunks. What could explain the presence of these plants and animals in a location that today is so cold and lifeless? It seems obvious that Antarctica's climate must have been quite different during Jurassic when the organisms were alive. But how do scientists fill in all the gaps and piece together Earth's climate history?
Earth's history is a long story going back 4.6 billion years. So great, in fact, are the lengths of time that the current geologic era we live in (the Cenozoic) stretches all the way back to the extinction of the dinosaurs 65 million years ago. But if we took our era and compared it to all of Earth's history, the Cenozoic would represent less than 2% of total time! To help you visualize the long spiral of geologic time consider the image pictured below.
Discovering the Climate of the Past
Scientists study past climates using a variety of techniques. Instrument records of temperature, rainfall, and other factors can be used to study only a very small segment of climate history. For instance, direct measurement of air temperatures only extends back to the 1860s and much of the early data is subject to error. Written records such as diaries, ships logs, and harvest information go back further but usually give us only qualitative data about past conditions.
Scientists can use a wide variety of indirect methods, known as proxy measurements, to understand Antarctic's past climate. One example of a proxy measurement for temperature is found in the chemical data sampled from ice cores. These ice cores, from Antarctica's ice sheet, contain valuable temperature and atmospheric composition information from the past 740,000 years. An ice core is essentially a long narrow cylinder of ice that is drilled from the ice sheet and studied by scientists. The deepest parts of the cylinder are made up of snowfall from hundreds of thousands of years ago. When researchers sample these ancient layers they have several methods of determining the climate at the time the layer formed. Isotopic ratios from several chemical elements can be used to infer temperature and precipitation. In addition, gas bubbles trapped in the annual layers of ice allow researchers to sample the composition of the ancient atmosphere.
Drilling for Sediments
The RV/IB Nathaniel B. Palmer in Charlotte Bay, Antarctic Peninsula in 2005. Note the drill rig near the stern of the ship. Photo courtesy of Julia Smith Wellner.
Sediments found in ocean basins around Antarctica contain micro-fossils that are used to reconstruct past atmospheric and sea-surface conditions. Sediment, you recall, is naturally occurring material transported by wind or water. Sometimes sediment found in the ocean contains fossils and other material from nearby continents along with fossils from the ocean environment. Sediment cores are studied in ways similar to ice cores. For example, changing distributions of phytoplankton different layers of sediment can be tied to changing water properties such as temperature and dissolved gases. Sedimentary rates are also important indicators of wet / dry cycles. Sediment sequences from deep undisturbed locations give us information going back millions of years. Typically, scientists will remove small portions of a sediment core to look for these small fossil remains.
Examining a sediment core. Note the foam blocks where material has been removed to process for microfossils. Photo courtesy of Sophie Warny.
Some cores, such as those recovered near land, can also tell scientists about terrestrial conditions. Pollen, which is very resistant to decay, is produced by plants in large quantities. It can be transported by wind or water and buried with sediment. Many types of pollen have such characteristic shape and size that researchers using microscopes can use to identify the plants that produced them. This allows them to isolate vegetation type and distribution in both time and space. Past climate in turn can then be inferred by understanding the temperature and rainfall where similar species are found today.
The pollen grains are tiny! The following images are at approximately 600X magnification. Their diameters are about 45 microns for the Nothofagus (top image) and about 65 microns for the gymnosperm (bottom image).
Nothofagus fusca pollen recovered from Antarctic sediment core. Photo credit: Sophie Warny.
Conifer pollen recovered from Antarctic sediment core. Photo credit: Sophie Warny.
Climate records archived in sediment are made more valuable by techniques which allow them to be age dated. This is done by combining and correlating observations from various techniques such as paleomagnetic data with radioactive decay of materials contained in the sample and identifying characteristic biological remains called biostratigraphic markers, and in some cases simply counting the number depositional layers.
Together, all these lines of indirect evidence can be assembled to help fill in the details that explain just how dinosaurs fit into Antarctic's ancient landscape.
Key questions addressed by this chapter are:
- How do we know what Earth's climate was like thousands or even millions of years ago?
- What does the study of past environments have to do with our world today?