What is under the Ice? How do we know?
How Do We Know What's Underneath the Ice in Antarctica?
After I sent my last postcard, someone asked me "How do you know what is underneath the ice in Antarctica?" I got the question just before I had the incredible opportunity to fly to Mario Zucchelli Station (Italian station at Terra Nova Bay); the flight crossed the Ross Sea, which was largely covered with sea ice. The sea ice is where the surface water of the ocean is frozen. The ice covering Antarctica forms two enormous Ice Sheets, with numerous outlet glaciers. Some of the outlet glaciers extend across and float on seawater, and are termed Ice Shelves. Before we even start considering the rocks that make up the continent underneath the East and West Antarctic Ice Sheets, let us start with the question: "When did people realize Antarctica was a continent?"
Early Greek philosophers including Aristotle had proposed a southern continent "Terra Australis Incognita". Early explorers including Diaz in the late 1400s and Magellan in the 1500s recognized the presence of Antarctic Land(s), and seafaring expeditions between 1600-1900 indicated the presence of southern land that was in a permanent cold condition. Subsequent expeditions led by Bellinghausen, Bransfield and Palmer (all in 1820) sighted the Antarctic continent. Following these confirmed sightings, further expeditions landed at various points on the Antarctic Peninsula. It was Charles Wilkes (1840 South Seas Expedition) that finally recognized that Antarctica was indeed a continent.
Secondly, before we get to the details of the rocks, let us answer "How do people know how thick the ice is?"
The East Antarctic Ice Sheet has a maximum thickness of about 4000 meters. Two of the geologists instrumental in developing our knowledge of the thickness of the ice sheets are Sir Vivian Fuchs and Ed Thiel. Fuchs was the leader of the Commonwealth Trans-Antarctic Expedition (1957/1958), which completed the first overland crossing of Antarctica. The data collected during the expedition included seismic soundings and gravity and magnetic measurements, which allowed them to establish the thickness of ice at the pole, and the existence of a land mass beneath the ice. The seismic survey basically 'bounced' sound off the rock under the ice, and the time it took the sound to travel back to the receivers allowed the geoscientists to calculate how far beneath the surface of the ice the rock actually is. The gravity survey allowed the scientists to figure out where the gravitational pull was greater. Rocks have more gravitational pull than ice, so this also helped them calculate where the higher areas of the continent (beneath the ice!) were. The data were analyzed and interpreted by Ed Thiel and John Behrendt to produce the first map showing the shape and topography of the land beneath the ice as well as the ice thickness.
Finally, let's get back to the original question "How Do We Know What's Underneath the Ice in Antarctica?"
First of all, we are lucky to have some exposed rocks (rocks that are sticking up above the ice in mountain ranges). Rocks are exposed in the Transantarctic Mountains, and geologists can correlate the rock units from one part of the range to another, just like piecing together a jigsaw or a torn piece of newspaper. There are also small isolated exposures in Nunataks (don't confuse with the band that performed at the Live Earth Concert 2007!) in East and West Antarctica. These rock outcrops are the only DIRECT pieces of evidence we have for the geological makeup of the continent.
The rest of our information is indirect - in other words we infer the presence of certain rock types based on other observations. For example we look at the composition of the sediments that have been carried by glaciers and deposited in the basins on the margin of the Antarctic Continent, and that tells us that those rocks came from somewhere under the ice on the continent; we use geological detective work to figure out where they most likely moved from. In the ANDRILL project we are using the presence of certain rock types in the sediment core to interpret which area of the continent the ice moved across, because some rock types are only known from certain localities in the Transantarctic Mountains.
We also use information derived from relatively limited gravity and magnetic surveys that tell us about the properties of rocks underneath the ice, which allows us to make reasoned speculations about rock types beneath the ice. The data via is collected via planes with specialized equipment that fly in a grid pattern above the ice – there is understandably relatively limited aeromagnetic and gravity data for the region beneath the East Antarctic Ice Sheet – with the exception of specific areas such as Lake Vostok, which was first discovered in 1996.
More recently researchers at Lake Vostok (located beneath Vostok Station, for which it is named) have used laser altimeter, ice penetrating radar, and gravity measurements collected by aircraft flying over the lake. They have learned that the lake is actually made up of two 'basins'. A joint Russian, French and US team has drilled one of the world’s longest ice cores (more than 3500 meters long) in the area above the lake. The ice is as old as 420,000 years, which tells us that the lake has probably been 'locked away' beneath the ice for 500,000 years. The hole has not penetrated the lake because of concerns and debate over the issues surrounding consequences of 'punching through' into this ancient lake environment. More recently, glaciologists have identified numerous subglacial lakes (over 140 of them), and realized that there is much more 'activity' beneath the ice than they had realized.
A seismic survey was undertaken this field season in the Ross Sea region in the vicinity of the Mackay Sea Valley; the aim was to determine whether there are enough young sediments there to obtain a core, and thus provide information for the time period between the older part of the Vostok Ice core and the younger sediments already cored from the Ross Sea region, and thus obtain a more complete record of climate change in Antarctica for the time between 35 million years ago and present.
Postcards from the Field: ANDRILL