Obtaining long cores of lake sediments
By: K. Gajewski
Collecting a core of the entire sediment sequence from a lake is not an easy matter. There can be up to 10-20 meters of sediment in the bottom of small lakes in northern North America. You need to remain stationary long enough to collect the sequence.
Here we are discussing small lakes typically cored in Holocene paleoecological studies. Coring in large lakes (greater than a few km) or deep lakes (greater than 20 or so meters) is another matter. The corer used almost exclusively for small lakes is the so-called Livingstone square rod (or modified) sampler.
In the discussion, there are a number of options. You can core in winter or summer (i.e. on or off ice); you can core in gyttja or in clayey sediment.
You need to decide where to core, usually the deepest spot of the lake. If you can map the water depth and sediment thickness, then you can find the ideal coring spot - the thickest sediment in the deepest water. There is a lot of recent work showing that the deepest water is not necessarily associated with the thickest sediment. It is not clear what are the implications for the core.
(B) Setting up
a: Coring from ice
If you can core from an ice surface, then you can stay stationary long enough to take the core. In this case, you will need an ice auger. It is best to work at the end of the winter, when there is still ice, but the temperature is above zero. You can core in temperatures below zero, but the corer freezes and work becomes more difficult the colder it gets. In the arctic, we have found the early summer an ideal time for work. At this point the snow has melted from the land (although you need to wait long enough after snow melt for the ground to harden allowing the Twin Otter to land), but there is still ice on the lake. Even if there is a moat on the lake, you can use small boats to bring your equipment to the ice surface. At this point, the ice may be starting to candle. Since the ice is typically >2 meters in thickness, it is still safe to walk. It is more difficult to drill the hole in the ice, but quite possible. Don't try this down south, however, where the ice is a few cm in winter, and when it starts to candle, it becomes too weak to support your weight.
Drill a hole through the ice for the casing. If you are using the drive rig, you will need 4 other holes to hold the rig. Measure out the distance, and drill two holes 30-40 cm into the ice, one on either side of the hole you will core from. Then drill two others, and angle these inwards inwards to meet the other two holes. See Figure (ASK KG). Alternatively, if the ice is shallow, you can use a "deadperson" to support the drive rig. Use the chains to attach the drive rig to the ice.
b: Coring from open water
First, you need a calm day. Rain is not a big problem (contrary to the advice in Faegri and Iversen); it makes a mess, but if you work quickly, it does not affect the core. But if it is too windy, the platform will not remain in one position, and you cannot get the core.
Here you will need a coring platform. The type of platform varies according to where you are coring, the nature of the sediment how much time and money you want to spend on the platform. If you can drive to the lake, and money and weight is not an object, you can make an elaborate one: I have seen photos of a large wooden platform supported by 4 50 gallon drums; large pre-made platforms supported by two canoes, etc. The description below assumes you have to bring your equipment to the lake on your back, perhaps a long distance.
We use three rubber inflatable dinghys (which are, unfortunately no longer available). Two are used to support the platform, and the third is for equipment. The platform is attached to the dinghys with rope. See Figure (ASK KG).
A sufficient, yet cozy platform can be made from a sheet of plywood. Cut the plywood into 4 pieces of 2' x 4'. We put hinges on the two pairs of pieces for easier transport, and notched the central portion. A number of holes are drilled for the bolts to attach to the 2x4s. A few other holes are drilled along the outside to attach ropes. The plywood is bolted onto the 3 2"x4"x12' pieces of wood using 5" bolts and wingnuts. Guy Lortie and I made this platform in a few minutes, and I used it for 5 seasons in northern Quebec. You need to be careful walking on it, due to the wingnuts, and it is small. A couple of clamps are attached at one end for extruding.
The attached dinghys are paddled to the coring spot. We have three coils of rope (1/4" or 3/8" yellow plastic rope sold in any hardware store is relatively cheap and works fine) of a couple of hundred meters each. Make sure to wind them onto a wooden spool, as the carboard ones will be destroyed at the first lake. You can pick these up wooden coils at construction sites, or the hardware store may save some for you. If the lake is small, you can attach yourself to three points on the shore successively, unwinding the rope as you go. As much as possible, it is easier if you leave the coils on the shore, that is, attach the end of the coil to the platform anchored in the center, and row the third dingy toward the shore, unrolling the coil as you go. Attach the rope to a tree or rock, and leave the rest of the coil there. If the lake is too big, you will need anchors. Boat anchors are rarely strong enough, unless they are too big to carry. One trick that works well is to get some burlap bags. You can fill these with rocks at the shore, and attach the rope to this anchor. This works well, since you can get it just the right weight by filling with small rocks (basically as heavy as you can haul up and down in the water). The rope attaches easily to the irregular shape if the bag. And when you are done, you can haul the anchor up and dump the rocks into the lake, saving hauling it back. Getting the anchors into position is not easy, but they need to be a fair distance from the coring site in order to hold the boat. Be neat. After you have tightened all the ropes, take all the loose rope, coil it and leave it in the water. When you are in position, you can begin coring.
(C) Taking the core
First, you need to take the sediment/water interface. This is done with the clear plastic tube, and this is called the Drive 0 (zero). Run the piston through the tube and adjust it. Tape the plastic tube to the drive rods using electrical tape. Make sure to cover the threads of the rod with tape as well, to keep them clean. (see Figure **). Tape the upper end of the tube as well, to keep the piston from pushing through should you push too far.
You first need an accurate measure of the depth at the coring site. Lower the tube to just above the sediment water interface; leaving about 10-20 cm of water. Fix the piston (by tradition, it is wrapped around your foot, but more elaborate mechanisms can be imagined. Then push the tube into the sediment; you should have measured the length of the plastic tube to know how deep to push it; mark this on the drive rod. When you are done, bring up the drive rods and the piston cable together. When the bottom of the plastic tube is just under the surface of the water, take your rubber stopper and plug the bottom. Tape it on, cut the tabe holding the plastic tube to the drive rods. Then drill a hole just below the piston to release the water. After you have used the tubes for several times, there will be holes already there, plug them with tape before taking the cores; you can now simply take the tape off. Secure the tube to the raft; keeping it upright.
Note, if you measured the water depth poorly, you may not get a good interface. There should be a little water above the sediment and below the piston. Keep this in the tube until you get back to the place you will be extruding it (see below)
Now you need to lower the casing. You need to put on enough to reach the sediment, and go in a few cm, but not as deep as the bottom of the drive one depth. Remember to account for the distance between the water surface and the raft. Lower the casing, fixing it in place with the casing holder.
Now take the Livingstone. Determine the depth of the bottom of Drive 0, and lower the corer and piston to this depth. Fix the piston as above, raise the square rod and lock it into place. Drive downwards one meter. Bring up the the corer and piston, keeping the piston cable taut. The suction of the piston is usually enough to keep the core in the tube as you lift it above the water, but put your hand over it to keep the sediment in, just in case.
Now you need to extrude the core. This is the hard part. John Glew (Queen's University, Department of Biology) has invented a nice extruder, or you can push the sediment out by hand. Lay down a layer of aluminum foil and plastic wrap. You need to keep this from blowing away in the wind that is always present. Good luck! With the piston unlocked, the Livingstone barrel must be pushed back upwards, while someone is holding the drive rods. Hold the Livingstone barrel next to the foil/wrap, so that the sediment core does not break apart. Note, you push the barrel back up the square rod, you do not push the square rod down the barrel (which would destroy the core). It helps to have a clamp to attach to the end of the drive rod, and push against this.
The above two paragraphs need to be repeated until you have obtained all of the sediment, always lowering the bottom of the corer to the depth of the previous section.
It is good practice to take a second overlapping core a few meters away from this site.
After you have all the core sections collected, wrap up everything, go to shore, wind up the ropes, etc.
(D) Back in the lab or base camp
Unless you are going directly back to the University lab, you will need to extrude the drive 0. The soft sediment can be put into labeled plastic bags, at 0.5 or 1 cm intervals. The core must be kept vertical for this procedure. You can use a small rod and a tape measure to it. Push the plastic tube downwards over this rod and catch the sediment as it falls out the top. You can use an extruder for this purpose.
Finally, pack the cores for shipment. They can be sent in black sewerpipe which has been previously split in half. Tape these around your cores, making sure all space at the ends is plugged with something. Tape 4-6 of these tubes together and they ship quite well.
Key references: See the LPC library page
Acknowledgements: A Swain taught me how to core and R Spear and J Ritchie taught me how to core in the arctic. My field assistants over the past decade also helped refine the methods.
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