Wednesday, February 5, 2014

Swedish varves

Examining Swedish varves is like eating Dutch Gouda cheese or Russian Kaviar: it is the real thing, not some cheap knock-off! The word varve (varv) has its origin in Sweden and in geology it is used to indicate an annual layer of sediment. In our cores we have been logging what appear to be many different types of varves, but the examples in this picture struck me as the textbook type. Our Swedish co-chief scientist Thomas Andren, a varve specialist, said that the hardcore varve scientists sit and wait for hours next to the core for the sediment to dry out, so that it brings out the contrasting seasonal layers.Varves are the result of seasonal changes in the discharge of meltwater into a lake from an ice sheet or glacier. As the Scandinavian ice sheet retreated it dammed the drainage of water and large proglacial lakes were formed in the Baltic Basin. You can see a picture of an ice-marginal lake here in this photo from a field work in Greenland years ago. Note the dark muddy water: this is the sediment that will slowly sink to the bottom of the lake and form these seasonal layers. In these ice lakes bottom life is very minimal and the sediments hence are preserved due to the absence of bioturbation (animal burrowing). The light-colored somewhat coarser layers are the "summer" layers deposited during peak meltwater discharge bringing sediment of many sizes into the basin. The darker layers are the "winter" layers of fine-grained clay, particles that sink much slower. In the photo to the right, again from Greenland, you can see the brown sediment-laden meltwater emerging from a tunnel in the ice during the summer. During the winter there is not much meltwater and only what is already in the water column of the lakes will slowly contribute sediment to the bottom and form the winter layer. During the winter, lakes are also frozen over, so the water is very quiet and is not stirred by wind-driven waves or currents. What a story, huh, for such a small piece of core! See if you can find the winter and summer layers in our core picture above....

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