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My First Lake Coring Trip
Since I arrived at Harvard Forest, I've heard my mentor, Wyatt Oswald, use some variation of the phrase "when you go down into the mud, you go back in time" on various occasions. This concept, officially called "the law of superposition", hinges on the simple observation that younger layers of sediment are deposited over older layers. By looking through a microscope at samples of sediment taken from different depths of mud in a lake, my mentor and other paleoecologists can see what tree species dominated a specific area at different points in time. They do this by counting the different types of pollen grains they find and graphing the data. Through this method, paleoecologists have documented, for example, the changes in historic forests brought on by European settlement. But before this type of analysis is possible, paleoecologists must first acquire sediment to analyze. A lot of the work that I've done so far in the Harvard Forest REU has been just that.
Even though my mentor tried to explain the process of acquiring sediment cores from lakes before we ever left the Harvard Forest grounds, and completed the lesson with pictures, I still puzzled to get an idea of what exactly I had gotten myself into. As we loaded two canoes, a plywood board, copious amounts of rope, and various types of tubes and tools into the Harvard Forest van, my imagination was painting a very bizarre picture of how exactly the process would go. The reality was not less strange, but it was a lot muddier. My first lake coring trip took place in Ware Pond in Marblehead, Massachusetts. After assembling the raft by tying the plywood board on top of the two canoes, we set out to find the deepest part of the lake. I rowed the raft as my mentor dipped an object that looks exactly like a flashlight and somehow gives a depth measurement into the water.
After finding the ideal spot, we were lucky enough to get a beautiful first core, and witness a band of redwing blackbirds chase away a hawk from their territory. The process my mentor uses for collecting the first meter of sediment is a little different than for the subsequent meters and this first 'sample' is called the surface core. We collected the first meter using a transparent plastic cylinder with a piston inside. First, the piston is kept in place by lowing the cylinder with the piston's cable. Then, once the desired depth is reached, the cylinder is pushed down with metal rods, and as it slides past the piston, the cylinder collects sediment. The rest of the cores are collected with a smaller diameter metal cylinder called a Livingstone core.
The process is almost identical to the one described above except that the mud is not kept in the core but instead extruded, and wrapped in Saran wrap and aluminum foil. The process of collecting sediment is repeated various times, and every time, sediment is collected from deeper in the lake mud. Usually, my mentor told me, he stops collecting when they reach glacial clay, which shows up as a very abrupt transition to gray in the sediment. However, on my first trip, we never got to the glacial clay, and instead had to stop because we reached a point in which we could not push the rods farther down. After this first experience I went on four more coring trips, and unfortunately, that is all for the summer. I am excited, however, to start processing the cores we collected, and to start working on my project, which will be to analyze the Ware Pond surface core.
My mentor boasted to me before my first trip, that the Harvard forest Paleo Lab only cores on beautiful days, and his word was mostly golden. So, if I have any wisdom to pass down to anyone planning on going on their first coring trip, it's don't forget the sunscreen. I've got the farmer's tan to prove how important sunscreen can be when you spend the day on the middle of a lake, and by tan I mean sunburn.