“They teach anything in universities today. You can major in mud pies.” – Orson Welles
In the fourth episode of the Core Ideas podcast, Adam and Josh discuss the retrieval of the mud critical to paleolimnology (i.e. sediment cores) from the lake bottom with minimal disturbance. This mudslinging discussion focuses mostly on three different types of sediment corers: freeze corers, gravity corers and piston corers (although we do briefly mention some others).
Sediment cores are usually collected from a platform at the lake surface, such as a canoe, boat, the floats of a helicopter/plane, or even the ice of a frozen lake, and as discussed last time, sediment cores of similar length can differ massively in the amount of time they represent. Therefore, selection of an appropriate sediment corer will depend on the study lake (size, depth, location), study goals (time period of interest, amount of sediment required) and the fieldwork conditions (remoteness, time of year).
Freeze corers are coolant-filled chambers lowered on a rope to the lake bottom. Once in the sediments, a crust of sediment freezes to the surface of the corer, preserving both the sediment profile and the sediment-water interface.
Gravity corers are relatively simple devices, that follow the same principle as placing a finger on a straw to lift liquid out of a cup. Gravity corers are typically small enough to deploy over the side of a canoe, and their penetration into the sediments is limited by their weight of the corer, so they typically only recover the most recent history of the lake. A number of variations exist related to the triggering and sealing mechanisms of gravity corers.
Piston corers are used to obtain long sediment sequences. The “piston” seals the bottom of the coring tube as it is pushed into the sediment using a series of rods. At the desired coring depth, the coring tube is pushed past the piston to collect the sediment core. This process can be repeated to obtain a sequence of cores that can cover the complete post-glacial history of a lake (i.e. >10, 000 years).
Once a sediment core has been collected, it needs to be “extruded” or sectioned into discrete intervals to work with. This can either be done immediately on shore, or after transporting the core back to the lab, and the way this is done will depend on the type of core collected.
For a more detailed introduction to sediment corers check out the chapter written by John Glew in the first volume of the Developments in Paleoenvironmental Research (DPER) textbook series.