Lab Work: Core logs and Magnetic Susceptibility

As I wrote in my last post, when we returned from field work, I had many cores to process before selecting the cores that I am focusing on for my thesis. Each core we collected in the field had to be split, and we selected one half as a “working half” to be sampled and the other as an “archive” which will be stored for future reference. After splitting each core, I completed a core log, describing the visual properties of the sediment in addition to the length of each core section. This first step, describing changes with depth in the core, is used as a reference for sampling and identifying different units (sections of the core with similar sedimentation history) within the core.

The next variable I looked at was magnetic susceptibility, or how magnetic the sediment is. Using a sensor, I measured the magnetic susceptibility every 0.5cm along each core. This is a relatively fast and easy way to record some information about changes in the sediment with depth and magnetic susceptibility often tracks changes in the sediment related to the depositional environment. For example, in one of my cores (Farstadvatnet), the magnetic susceptibility is low throughout the first 45cm, which we think was likely deposited in a freshwater environment. Then at 45cm, the magnetic susceptibility spikes up, which is potentially a transition into sediment deposited in a marine environment. Measuring magnetic susceptibility gives a better idea of what distinct units the sediment may have, and gives a framework for further sampling.

From these two assessments, I chose the sediment cores that I am focusing on for my senior thesis. Both are from lakes on the Lofoten Island of Vestvågøy: Upper Storfjordvatnet (a relatively small, coastal lake) and Farstadvatnet (a larger, deeper lake).