It came from the Lake

Another item I worked on today was to collect a sediment core from the "lake" near campus. Created in the late 1800's as a holding pond for logs, today it's often choked with cyanobacteria (blue-green algae) in the late summer. This renders the lake a smelly, green mess.



The "device" was a 1.5 m length of plastic tube, attached to about 12 ft. of iron pipe that we used to push the plastic tube into the muck below the ice. The tennis-ball piston visible inside one of the tubes is connected to a cable that holds these tennis balls just above the sediment/water interface. As the core is pushed into the sediment, the piston creates a vacuum and holds the muck inside the tube.


Our first job was to find a good spot. We used a soil auger to check the bottom material. Our first spot was too shallow, and there wasn't much sediment at all. Just a few inches of grey-green clay (often called "gley").


Of course, no field expedition is perfect - I hadn't done a good job securing an ice auger, so I showed up to the lake empty-handed for getting through the ice. Fortunately, one of my students had an ice chisel in his truck (go wisconsin ice-fishing!) and we found a nice spot - about 8' of water and a little over a foot of muck.



Pushing the corer into the sediment.


The homebuilt rig was a little wobbly, so we didn't push too hard - as we started pulling the core out, my heart kind of sank: the water inside the tube was very clear. Too clear, I thought, to have pulled up any sediment. Fortunately, though, we managed to capture the sediment-water interface intact. The water was clear inside the tube because it wasn't disturbed. Yay!


There's the bottom of the lake, now above the lake. WIth the temperature dropping rapidly, I figured we weren't going to improve on our efforts, so we took the cores back home.


The first core, on the right, was tipped over after being pulled out - so the top part was mixed and folded over a little bit. The second core shows about 30 cm of lovely, dead cyanobacteria and poorly decomposed organic material. Below this is a 3 cm-thick layer of peat and then about 7 cm of sand and gravel and scattered organic material.

This is exciting to me because we managed to sample part of what is probably the old floodplain (more specifically one of the flooded intermediate terrace surfaces). If we can get into a deeper part of the lake, perhaps this summer (yer gonna need a longer pipe) the muck layer could provide a high-resolution record of sediment input over the course of 140 years.

I'll be posting more info as time goes on - right now, I've split the core and sectioned up one of the halves to look at variation in organic matter, mineral material and other sedimentary characteristics with my students.