March 21, 2017
We have finally reached our destination, the Bellingshausen Sea, and we have started collecting some data. In this post, I will explain the types of data we are collecting and how we use it to interpret past events in the geologic record.
As a side note, it is very cold! and I have only seen one penguin in the distance, they seem to dislike big, red boats breaking sea ice near them.
1. Multibeam swath bathymetry
The multibeam data are collected using a Kongsberg echosounder system. It has a transducer installed at the bottom of the ship that produces a fan-shaped pattern of sounds in a swath perpendicular to the ship track. The sounds reach the seafloor and return to a receiver, and we can calculate water depth by measuring the time it takes the sound to travel through the water column, reach the seafloor, and come back to the ship. By putting together all the points produced over a period of time, we can create an image of the seafloor.
By using the multibeam we can get a very accurate image of what the bathymetry (seafloor topography) looks like. When ice retreats or advances over the seafloor, the flowing ice produces a series of landforms that we can use to interpret the behavior of the ice. For example, when ice is flowing very fast, it produces elongated hills parallel to ice flow, and when ice is slower or stops moving, it produces mounts of sediment, perpendicular to ice flow. And so, we want to map the geomorphic features we find in the seafloor to interpret the history of ice flow in the Bellingshausen Sea.
2. Subbottom profiler
The subbottom profiler is also an echosounder, but in this case the system produces a high frequency sound, the sound penetrates the upper layers of the seafloor generating a 2D “seismic” line. We use high frequencies so that the echosounder shows detailed sediment layers but only from the shallow subsurface. Sometimes it can penetrate as deep as 50 meters, but usually we get less than 20 m of penetration.
We use the subbottom profiler to give us an idea of how much sediment there is covering the seafloor. In Antarctica, it is very common to have only a few meters of sediment covering bedrock. This is because as ice advances, it removes or “bulldozes” all the sediment, leaving bare rock. In addition, the subbottom profiler helps us get an idea of the type of sediment forming the seafloor, ex. rocky seafloor vs. muddy seafloor, soft mud vs. indurated/compacted diamicton.
3. Sediment cores
We will be collecting three types of sediment cores in this expedition: long cores, gravity (short) cores, and multicores. We want to collect as much sediment as possible but sampling depends on how much sediment there is in the areas we are visiting, for this we rely on the subbottom profiler. The long core can collect a continuous sediment sample of up to 39 m below the seafloor. The shorter cores vary in length from 1m to 9 m, and the multicorer device samples up to 8 cores, less than 1 m long. The long core works best if we want to sample a long section, the short core works best for sediment that is slightly more compacted than mud, and we use the multicorer when we want to sample the sediment-water interphase, where the sediment is very “soupy”.
After we obtain the core, then we sample it in the geology laboratory in the ship. These samples are used to measure grain size, lithology composition, lithic fragments, diatoms, pollen, foraminifera, shells, organic matter, and water content. The long cores will not be sampled in the ship, instead they will be sent to the Korea Polar Research Institute where sampling will take place in the summer. By looking at the sediment facies we can get an idea about the environment of deposition and how the paleoenvironments have changed (or not) through time.