By Graham Cogley
How long have the Gamburtsev Mountains been there, deep in the interior of Antarctica? In a paper just published in Geophysical Research letters, S E Cox and co-authors explain how they think have the answer, which is a bit surprising.
Apatite is an interesting mineral. It contains most of the phosphorus in the Earth’s crust, is familiar to many as the mineral that defines a hardness of 5 on Mohs’ scale of hardness, and is unfamiliar to just about everybody as a basic constituent of tooth enamel. Its name comes from Greek apatao, “I cheat” — allegedly because of the variety of its forms, although all the chunks of apatite I have ever seen are a pleasing shade of light green with a hint of lemon.
One curious attribute of apatite is that uranium quite likes it, snuggling in happily, in trace amounts, into the basic structure of the crystal lattice. Every so often, an atom of uranium-238 splits into two fragments that set off at high speed, crashing through the molecules in their neighbourhood. The collisions slow the fragments down and eventually they stop, but not before having done a good deal of damage. The trail of wreckage is a fission track, and it can be brought to light under the microscope.
Here comes one of the more fascinating twists in the tale: the damaged crystal lattice gets better. It can heal itself by restoring the disordered array of molecules to something like its original tidy state, a process called annealing.
The payoff for the drudgery of counting fission tracks in apatite crystals is that annealing reduces the number of tracks in a way that depends principally on temperature and time. Above about 120°C, the so-called closure temperature, annealing erases the tracks as fast as they form. Below about 90°C, annealing is so slow that the number of tracks depends on the time elapsed since cooling through the closure temperature.
The temperature decreases as the apatite crystal travels upwards through the geothermal gradient, which is about —30°C for every kilometre nearer to the surface. The fission tracks tell us when the crystal was last at a depth greater than 3 to 4 km. (Very roughly. The geothermal gradient had to be guessed in this study.)
In other words, fission-track dating is a way to estimate long-term erosion rates.
How do you estimate the erosion rate of a mountain range buried beneath several kilometres of ice? You go to the sediments deposited offshore as a result of the erosion. Cox and co-authors went to Prydz Bay, offshore from Lambert Glacier, the largest outlet of the Antarctic Ice Sheet. It drains the northern part of the Gamburtsev Mountains. They sampled Eocene sediments, about 35 Ma (million years) old, and found fission-track erosion rates of the order of 10 to 20 m Ma-1 that must have been sustained for at least 250 Ma.
Such rates are extraordinarily low. The Alps are shedding sediment at 400 to 700 m Ma-1, and while the Appalachians are suffering rates of only about 30 m Ma-1 they are much less rugged than the Gamburtsevs. The Gamburtsev rates are more typical of very low-relief terrains like the Canadian Shield. Incidentally, they are upper limits. The crystals sampled in this study are likely to have come from whichever part of the Lambert basin has been shedding sediment fastest.
The geomorphologists, then, have the problem of explaining why the Gamburtsev Mountains have been rugged without yielding significant detritus for several hundred Ma. One possibility is aridity. If the Gamburtsevs and their surroundings were a desert for most of the required time span, that would account for their not evolving very rapidly. It doesn’t seem probable. They have been far from the desert belts for at least 100 Ma.
Burial beneath glacier ice seems like a better bet, according to the Cox paper. It also seems harder to swallow. Before, we glaciologists had the problem of the survival of alpine relief in the heart of Antarctica for tens of Ma, and the related problem of the apparent non-glaciation of the polar continent for tens of Ma before that. If Cox and co-authors are on the right track, the problem metamorphoses into trying to explain a protective ice cover on the Gamburtsevs even though they were not near a pole, and even though the rest of the world was warm. They are holding up what Winston Churchill called the flickering lamp of history, and the scene it reveals is decidedly murky at present.