Friday, January 29, 2010

The bugs and the paleomagician

We are still on site and drilling. A few icebergs have been sited on the horizon and the Captain and the ice observer are watching their movements. This is important, because we are stuck 800 m into the sea floor with a drill pipe and it takes some time to pull all that pipe and to be able to move out of the way. These icebergs are big: they can be several hundreds of meters to miles long and the ship is dwarfed by such a size. In the mean time we are busy with our science.
This is the Paleontology Lab. Three paleontologists are busy looking for microfossils who can give us an age for the cores we are recovering. In Antarctica, the diatoms (microscopic algae) are the most important group for the shallower depths in the core. They evolve fast and there are many different species, so overlapping age ranges produce good time resolution. Unfortunately at greater depth below the sea floor it doesn't work as well. Upon burial below sediment layers several hundreds of meters below the sea floor where the temperature and pressure is greater, the skeletons of these organisms (called frustules) dissolve. The frustules are made of opal, a type of silica, and have characteristic shapes, which are lost through dissolution at burial. Fortunately we have other microfossil groups that we can work with to provide an age and then there is the paleomagician!
The paleomagnetists measure the magnetic properties of the sediments. The sediments acquire the magnetic properties through the workings of the Earth's magnetic field. Through time the Earth's magnetic field has been shifting from pointing into the Earth at the North Pole (normal polarity, like today), to pointing into the Earth at the South Pole (reverse polarity). Minerals in the sediments align themselves with the Earth's magnetic field during deposition and so a polarity record becomes entombed into the sediment layers below the sea floor. The record of the Earth's polarity reversals is known from areas of seafloor spreading, where volcanic rocks can be dated through radiometric dating (using radioactive decay of elements in minerals). This record is known as the geomagnetic polarity time scale. Paleomagnetists can count magnetic reversals in the cores back in time and try to match the cores to the geomagnetic polarity time scale to provide an age.

One interesting aspect of our current location is that we are almost right on top of the magnetic South Pole. It means that if compass needles were freely suspended they would point straight up. The magnetic North and South Pole do not coincide with the geographic North and South Pole. In fact the magnetic poles are always on the move. The magnetic South Pole is now located off-shore Wilkes Land in our area, but about a century ago it was further inland in Antarctica. Douglas Mawson, an Australian Antarctic explorer tried to find it back then, but he barely survived the trip. He almost died of vitamin A poisoning because his sled fell down a crevasse and he and his pals only had the sled dogs to eat, with their livers rich in vitamin A. His two pals (Ninnis and Mertz) died. His journal is a great read, but I am glad our tip is so much more convenient!

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