Obtaining short cores and surface samples
By: K. Gajewski
"Short cores" is the common name given to cores of the uppermost few cm to a meter of the sediment of a lake. These are used for several purposes. In the first place, you can use the uppermost cm to 1/2 cm of these cores as the surface sample. "Surface sample" (actually, it should be surface sediment sample) is the most recently deposited sample and is used for calibration. That is, it, the microfossils from the surface sample can be compared to the living organisms in the lake or on the surrounding landscape to better understand the process of fossilisation. You can determine if some organisms don't make it into the sediment (they may be decomposed before arriving in the sediment), or if they are over- or under represented in relationship to their abundance in the ecosystem. Or these samples can be directly compared to a climatic or environmental variable to derive a transfer function of some sort (see the research section of this web site for many examples of this kind of work).
A second reason for collecting short cores is for studies of the recent past. For example, if you are interested in the impact of European settlement on the landscape, or the potential deposition of pollutants during the past century, or the effects of the Little Ice Age on the regional ecosystems, you can frequently determine this using only the uppermost few cm of the core.
In any event, there are a series of potential corers to use when collecting the uppermost lake sediments, which are designed to ensure that you collect the sediment-water interface with a minimum amount of disturbance. This is of critical importance in paleoenvironmental studies, since this is one time horizon that you can know with certainty. There are dozens of designs of short corers; here I discuss the types that we have available. We will discuss how to use three of them: a "Glew mini-corer", a "Davis-Doyle" corer and a "freezer" corer. These are the ones that I have used (the ones that actually work). See also the long-core section of this web for another way to collect the top metre. These corers have in common that they are relatively easy to use and reliable, they collect an undisturbed core - especially the sediment-water interface, they are lowered to the sediment by a rope, thus avoiding heavy drive rods, and they collect a metre or less of sediment. Finally, I will describe the use of the Ekman Dredge, which can give larger volumes of surface sediment, if used properly.
A general reference is the Berglund volume (see the library page)
Reference: Glew, J.R. 1991. Miniature gravity corer for recovering short sediment cores. Journal of Paleolimnology 5:285-287.
Glew corer, with lead weight, cord and messenger
Description of use
Attach the weight and the plastic tube onto the corer. Make sure the core tube is flush against the rubber seal at the top and add some grease to get a tight seal. Verify that the rubber band is attached correctly. Trigger the corer. Lower the corer to the bottom, holding the messenger in your left hand and letting the rope slide through it. When you feel it hit the sediment, lift it, and slowly lower it into the sediment. Don't lower it too much, as it may tip over. Drop the messenger. Lift the corer, and when it is near the water surface, put your hand with a rubber stopper in the water. Before the bottom of the core tube lifts above the water surface, plug the bottom. Lift up the piston sealer, and unscrew the band clamp that holds the core tube onto the corer and remove it. Make sure to store it upright! To extrude, use the extruding device to push the sediment through the top of the core tube and into labeled bags.
Reference: R.B. Davis and R.W. Doyle. 1969. A piston corer for upper sediment in lakes. Limnology and Oceanography 14:643-648.
Davis corer, including corer, wire, battery, buzzer, bottom
feeler with rope, lead weight
Description of use
I haven't used this in years. If you feel you want to try our model, see me.
Reference: I was shown how to use this corer by Al Swain, who used it to great effect in his project on varved-sediment lakes. I think the proper reference is Wright, H.E. 1980. Cores of soft lake sediments. Boreas 9:107-114. The flat model was described in Huttunen, P and J. Merilainen. 1978. New freezing device providing large unmixed sediment samples from lakes. Ann Bot Fennici 15:128-130.
Freezer corer, with top cap or plug and rope
Description of use
There are two general models - the slab and round tube. The slab is a nicer design, but more difficult to make, the tube design is very simple. Both work the same way.
The corer head is attached to a rope. The corer is filled with dry ice crushed into pieces (small enough to fit into the tube). Some alcohol and lead weight are added periodically as you fill it. The lead weights make the corer penetrate deeper (not needed with the slab model) and the alcohol ensures that there are no gaps of warmer core; the whole barrel is kept at the freezing temperature. You can crush the ice with a hammer in the pail and dump it in the tube. USE HEAVY GLOVES, as the dry ice will burn your hand. When the tube is filled, place the cap on. MAKE SURE that the release hole is not plugged, as you don't want to build up pressure in the tube.
Drop the corer into the sediment. We have let it simply freefall; surprisingly, the surface sediment is not disturbed. I am not sure if you can lower it into the sediment less violently. Let it sit there for 5 minutes or so. You will need to calibrate this for your particular machine.
Then, simply pull the corer to the surface and remove the cap. You need to warm the corer just enough so that the core slides off the tube. We carried a long narrow aluminum tube that fit inside the corer and was long enough to reach near the bottom. One end was stuck in the truck exhaust and the other in the corer, the exhaust warmed the tube. Be careful not to breathe the fumes. Alternatively, you can pour water in the tube (in summer) to warm it.
Once the core is off, you simply wrap it in aluminum foil and keep it frozen in the insulated box until you return to the lab. Here also, it must be kept frozen. You can work with it in an upright freezer.
Reference: This collecting device is so common, that I suspect the reference is lost in the depths of time.
Description of use
Lower the dredge to the bottom of the lake; try to keep the boat as stable as possible, as it doesn't work well if the rope is not vertical. It works best if you hold the messenger in your left hand (over the side of the boat) and let out the rope with your right hand. When you feel it reach the bottom, lift it slightly, and get the rope vertical. Lower it until you feel it just enter the sediment, then release the messenger. You want the fill the dredge, but still have some water on top of the sediment when you lift it to the surface, that is, you don't want it too deep in the sediment. It may take several tries. Lift the corer, and at the surface let it slowly drain. If done correctly, you should not disturb the sediment. scoop off the uppermost sediment. You should be able to control the depth of sediment that you collect, and the amount depends on your research question.
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