Drift pumice, ice sheets and toothpaste
by Graham Cogley, environmentalresearchweb, June 28, 2010
When I was doing field work in Arctic Canada, for my master’s thesis, we were under orders always to travel from A to B along the coast at 22 metres above sea level, with our eyes on the ground. There was method in this madness.
The orders originated with Wes Blake, Jr., a friend of my thesis supervisor. Through us, Wes was hunting for drift pumice. Pumice forms when gas-rich, frothy lava is cooled very rapidly. Because of the gas bubbles, pumice floats. If it gets into the sea, it drifts, for a few years or decades and perhaps a few hundred to a few thousand kilometres, and eventually some of it drifts ashore. Our pumice erupted, possibly from Hekla in Iceland, in about 3,000 BC.
The pumice drifts ashore at sea level. We were following a raised-beach strandline at the elevation at which Wes Blake reckoned the shoreline of 3,000 BC ought to be today. The whole region has rebounded from the weight of the ice that was there up to about 7,000 BC.
Sadly, we never found any drift pumice, but Wes Blake and others did, all around the Canadian Arctic. Its altitude today varies, lower than 10 m in the marginal parts of the archipelago but reaching 25 m and more along a broad axis trending north-eastwards from Bathurst Island to Ellesmere Island. The higher the 5,000-year-old strandline, the thicker the ice used to be. This evidence helped to settle a then-current debate in favour of the idea that the Queen Elizabeth Islands were once covered by an Innuitian Ice Sheet, as opposed to each island having had a smaller ice cap of its own.
Other things wash up on beaches all the time, including whalebone and driftwood that are datable by radiocarbon dating. Sometimes you find datable fossils of shelly organisms that used to live in the beach (or at any rate the nearshore) sediment.
My crowning achievement in this way was to find a bivalve in the “position of death.” Its two shells were still joined and the shell aperture was facing upwards. I forget its age, except that it was older than 22 m, but my bivalve was a small contribution to the relative sea-level curve for the locality.
RSL curves tell you lots of things besides the age and altitude of the marine limit (the highest sea level, reached just after the disappearance of the ice) and the history of emergence. With enough curves, you can reconstruct the former dimensions, including the thickness, of the ice sheet. But you are not limited to where the ice used to be. The land around the ice sheet also emerges when the ice load is taken away. During the ice age, it formed a depressed moat around the ice margin. Go somewhat further and you reach the peripheral bulge.
The peripheral bulge is where most of the toothpaste in the Earth’s mantle went when it got squeezed away by the growing ice sheet. Upon deglaciation, the toothpaste flows back slowly. All right, I know the analogy is breaking down (ever tried getting the toothpaste back in the tube?), but the peripheral bulge subsides, and here you observe not emergence but submergence of old shorelines.
If you go far enough from the ice sheet, you enter what the geophysicists call the “far field,” where relative sea-level change can be complicated, and in any case subtle. But taken together the available RSL curves are an incomparable tool for probing the Earth’s inaccessible deep interior. The subtleties in the curves are best explained by variations in the flexural rigidity of the lithosphere and by subtleties in the depth profile of viscosity, or stiffness, in the deeper mantle. Indeed, the information flows both ways (just like the toothpaste).
Another way in which RSL curves help is by showing that toothpaste takes its time. So drift pumice and dead bivalves help us with the complications of interpreting things like the changing gravity field as monitored by the GRACE satellites. Any redistribution of mass, be it due to modern exchanges between glaciers and the ocean or to the slow flow of toothpaste, shows up in the signal from GRACE. Which just goes to show that it’s an interconnected world.
http://www.iop.org/mt4/mt-tb.cgi/3752
Link: http://environmentalresearchweb.org/blog/2010/06/drift-pumice-ice-sheets-and-to.html
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