Submitted by Anonymous on Thu, 08/09/2012 - 12:43
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High Arctic Change 2012

Hi Dan- I've really enjoyed reading your daily entries, and learned a lot from them. Could you please elaborate a bit on the design of the "giant" sediment traps? From what I've read, there's "normal" sized tubes inside the giant one- what is the purpose of having the trap so large, if the sediments are still being collected in a smaller-diamter tube? Thanks in advance!

Sat, 08/11/2012 - 03:56

Dan Frost

Hello and thanks for the question! The sediment traps have a bit of interesting geometry that I will try and describe a bit more in detail. It may be helpful to check out one or two of the trap pictures after reading this...
So, the purpose of the sediment trap is to catch sediment from the water column as it would normally be deposited on the lake floor. It is very helpful to be able to have a trap in a known area of sedimentation for a known time so that you can verify sedimentation rates but also the annual nature of the varve couplets. That is, make sure that the light and dark layers happen on annual basis in the sediment cores we take (that being useful for a number of different reasons). The traps are positioned at differing heights in the water column and different lake locations to answer location specific questions.
As for their design, the funnel feeds into the receiving tube below to amplify the sediment signal. This means that we thicken the layers in the receiving tubes as compared to the lake bottom giving us a higher resolution on what happens in a particular sedimentation year. Background info done... : ).
The Big Traps: These 'big' traps are not actually any larger in the funnel size than most of our other traps. That means they don't gather any more sediment than the others and it all goes into the same size tube. The difference in the ones called "6:1" is that on top of the funnel and before the baffel, there is a tall pvc cylinder. This cylinder gives the trap its name because it is 6 times longer in height than its diameter. This extra length serves the trap in that it ensures any sediment reaching the top of the cylinder makes its way down through the funnel to the receiving tube i.e. anything that passes the top circular cross section is ours! On some traps and in some circumstances underwater currents can run over the tops of the trap, create turbulence, and thus potentially scour sediment out of the trap that should have rightfully made its way into our receiving tube record. Some studies have shown that these types of traps should in fact be the standard but they are cumbersome to work with and build and in Linnevatnet they haven't shown much difference as compared to the normal traps we have deployed.
Great question and please let me know if I can help anymore with clarifications!
-Dan

Mon, 08/13/2012 - 04:10

Anonymous

This is Julia from the Polartrec course. I didn't write that question, but I'm glad it was asked. Thanks for the great explanation!