Post-glacial adjustment

Happy Christmas Eve!!! I've got an unwanted sequel, or perhaps the Christmas special, of my melting ice mini-series - post glacial isostatic adjustment - or - what happens when all the ice leaves.



(BGS)

The isostasy concept is really simple, imagine that the lithosphere, and all the tectonic plates are floating on the asthenosphere. The plates would really like to be in isostatic equilibrium with the asthenosphere and are always making little adjustments to make sure this is the case (Karner et al, 1984). So when a 4km thick ice sheet grows over North America, the lithosphere responds by sinking a little deeper into the asthenosphere. Sea level in these areas will appear to rise. Adjacent land not covered by the ice sheet also responds by rising slightly higher - this is the peripheral forebulge (Fjeldskaar, 1994). Sea level here will appear to fall.


Click here for a great interactive demonstration of isostasy from Cornell University. I just hope that what happens when you increase the block density to 3.4, is not representative of the real world...


But then, when the ice melts, the land beneath it is still depressed - it has to start making isostatic adjustments again to put it back into equilibrium with the asthenosphere - this can be a really slow process over thousands of years. At the same time, any land that was pushed up during glaciation starts to subside (Fjeldskaar, 1994). It isn't just ice formation and melting that can upset the isostatic equilibrium, volcanoes, erosion, and mountain building can also do it too (Watts, 2001). 


Post-glacial isostatic adjustment is particularly interesting for modern sea level changes because we are still experiencing isostatic adjustments from the last ice age all over the world. In fact it's hard to go anywhere that isn't still responding to the effects of the ice sheets. Locations which were furthest from the ice - generally in the tropics - are called farfield sites - these are also some of the best places to go if you are trying to reconstruct sea level for the past (Bassett et al, 2005). A good excuse to do research in a pleasant climate I think.  


Isostatic adjustment affects only affects regional sea level - by directly raising the land, or causing it to subside. Take the UK for example - 18000 years ago during the Last Glacial Maximim (LGM), Scotland and all of England apart from the south-west, was covered by the British Isles Ice sheet (Massey et al, 2008). The increased weight of the ice caused the land to sink down, with the exception of the south-west which rose up. But now the ice has melted Scotland and the North of England are rising out of the asthenosphere and gaining elevation - they are experiencing a relative sea level fall. Meanwhile, the south is experiencing subsidence and a relative sea level rise throughout the Holocene (Massey et al, 2008, Shennan et al, 2000).


So essentially, post glacial isostatic adjustment is like the secret effects of melting all the ice. It's something that we really can't control, and will still be affecting sea level long after the ice all melts.