As Michelle’s most recent blog stated, our past couple shifts have been hectic while we processed our first five cores. The jumbo piston core (JPC) and jumbo gravity core (JGC) were both 20-foot long barrels, each returning approximately 10 feet of sediment excluding the 1-foot trigger core associated with the JPC. The JGC is similar to the kasten core in that it is allowed to “free-spool” on the winch as it nears the ocean floor during its descent and then uses its own weight to penetrate the sediment.
The JPC is rigged a little differently, with a counter-weight (the aforementioned short trigger core) suspended below a triggering mechanism that holds the main JPC barrel, the weight (known as “the bomb”), and a coil of slack line. When the trigger core hits bottom, the main JPC barrel is released and free-falls the remaining distance into the sea floor. This method generates more momentum than free-spooling a core on the winch line, thereby increasing the depth to which the core may penetrate. The coring system on the Palmer can support JPCs up to approximately 25 meters long, but other vessels have successfully deployed JPCs that recovered as much as 80 meters of sediment!
The piston that gives the JPC its name ends up at the sediment-water interface if the core is successful and is designed to improve recovery. The way this works is analogous to putting a drinking straw into a glass of water and covering the top of the straw with your finger to take water out of the glass using suction. Even though our cores have been relatively short so far, recovery has generally been good. The recovered sediment is contained in the 4-inch diameter PVC liner that fits in the JPC barrel. When the core is retrieved, we extrude the liner and cut it into 10-foot sections.
Unfortunately, we have to patiently wait to do most of the processing on these cores because if we cut them open now, we won’t be able to securely ship them to the USAP core repository in Florida State University for final processing. Therefore, the only sediment we get to see on-board now is the sediment in the cutter nose and core fingers (the very bottom of the core), the sediment between the sections of PVC liner we cut, and whatever mud is on the coring device.
The only shipboard analysis we are doing for the JPCs on this particular cruise is for magnetic susceptibility (MS). MS gives us clues as to the geologic provenance of the sediments and entrained coarse material, which serves as a proxy for the depositional environment: a high MS indicates the presence of terrigenous material likely to have fallen out of the bottom of a glacier, whereas a low MS indicates a more open-water depositional environment and the absence of an overlying glacier. By running MS on the JPC and JGCs (and JTC, the trigger core), we can see what sediment we collected and start to loosely define different sedimentary and climatic events.
To run the MS on the JPC/JGC/JTC, we use a circular sensor and record a measurement every 2.5 centimeters. We then graph the MS variations and look at the different peaks to analyze the sediment in the core. We also compare MS graphs between cores to assess the persistence of individual climatic events throughout our study area. We run MS on kasten cores by using a point sensor device and we measure the MS every 0.5 centimeters. If we have a U-channel, which is a smaller archive tube taken from a kasten core, we can run that in a smaller circular sensor and compare that with the point sensor data of that kasten core. MS data is a very useful tool for both coring types and helps everyone more easily see the definable units within a core, especially those that we can’t open yet.
Yesterday we analyzed our most recent seismic lines as nightshift and dayshift teams. We had some guidance from the UT-Austin team, our resident seismic experts. We found out during our shift change that both teams managed to point out similar seismic reflectors, which represent different lithologic units beneath the seafloor. As mentioned before, analyzing these lines is crucial in choosing our core and dredge site locations. Most of our shift involved running a multibeam survey of the area in order to fill in some of the gaps in our seafloor data. Now we are off to take another kasten core; it is never a boring day (or night) around here!