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The first ice in Antarctica

Palaeogeography is a seductive subject. The appeal of conjuring a vanished landscape out of a few strands of evidence and a good deal of restrained conjecture is irresistible. We know that the reconstructed world is imaginary, but with right treatment of the evidence, under the right constraints, we also know that it must represent a realistic approximation to the way things were.

That is what Douglas Wilson and Bruce Luyendyk appear to have done for West Antarctica at the end of the Eocene, about 34 Ma (million years ago). Records of oxygen isotope ratios in the microfossils preserved in deep-sea sediment suggest that glacier ice began to accumulate in Antarctica at about that time. But models of the palaeoclimate have trouble simulating as much ice on East Antarctica as the isotopes suggest. East Antarctica, palaeoclimatically interesting in itself but not our focus here, is the larger, higher part of the continent.

West Antarctica is not big enough to house the missing ice. Most of it is below sea level. The prevailing wisdom is that you can’t grow a marine ice sheet — that is, one with its bed below sea level — from scratch, at least not quickly. So, assuming we are reading the isotopes aright, where was the missing ice?

Enter Wilson and Luyendyk . Their geography of West Antarctica as it may have been at 34 Ma offers a persuasive answer: most of it wasn’t below sea level back then.

The first step in the palaeogeographic reconstruction, starting from a map of the modern ice thickness, is to remove the ice and allow the underlying lithosphere to recover from the removed load, rebounding and flexing. You have to juggle with the calculated new surface elevations because in some parts, where the new surface is below sea level, the place of the ice load is taken by a new load of ocean water. This requires care, but it is not a deep problem. On the other hand the result isn’t much help. Most of West Antarctica remains under water.

The second step is to account for thermal contraction of the lithosphere. As constrained by palaeomagnetic and other measurements, the main feature of the tectonic-plate system around here until about 28 Ma, the West Antarctic Rift System, was the surface expression of the rising limb of a convection cell in the Earth’s mantle. The convection stretched the overlying lithosphere while causing the two sides of the rift to spread apart. Now too high for the fluid mantle rock supporting it, the lithosphere subsided gradually. Wilson and Luyendyk estimate that the subsidence since 34 Ma has varied from 200 to 500 m across West Antarctica.

Apart from correcting for the subsidence, you also have to undo the stretching, moving the Pacific side of the rift some tens of kilometres back towards the East Antarctic side.

A lot of erosion can happen in 34 million years. How do you restore a landscape that has ceased to exist? The answer is that we know, first of all, that the erosive products go downhill, and second that they have to end up somewhere. Most of them end up as sediment offshore, and not too far away. Wilson and Luyendyk rely on sediment thickness estimates for this, their third step. Not all of the sediment is due to erosion, a good deal of it being the fossils of marine microorganisms, and the eroded rock would have been more dense than the deposited sediment. Nevertheless, their approach is very conservative. The volume they restore is only 13% of the volume of offshore sediment.

This step also requires corrections for rebound and subsidence. Shifting loads of sediment are just like shifting loads of water and ice when it comes to the response of the underlying lithosphere.

In the end, Wilson and Luyendyk found another 1.5 million km² of land, turning the West Antarctica of 34 Ma from an archipelago into a landmass. This plausible landmass, imaginary as it is, is consistent with all the evidence and is constrained by basic principles of physics. In turn it makes what the isotope records imply, that there was quite a lot of ice at 34 Ma, more plausible.

Getting beyond plausibility is a challenge, but one that would be worth rising to because it would allow us to move on to the next questions. Why so much ice? And why then and not earlier or later?

Posted by Graham Cogley on March 1, 2010 3:00 PM | Permalink