The first jumbo gravity core (JGC-1) from Hugo Island trough was a great success. The marine technicians and the night shift helped secure and extrude the sediment at approximately 2 a.m. on Oct. 22. Overall, the process involved everyone on the ship; the engineers, the winch operators, the marine techs, the science crew and the captain and his mates. Each group’s communication with the other groups was the backbone for success. For example, the marine techs on the aft deck must give hand signals to the winch operator in the aft control room. The scientific investigators must ensure the captain has the ship in the right spot which involves the crucial step of confirming the reading from the Knudsen 3.5kHz. Its seismic reflection shows ideal depths to target. Obtaining the jumbo gravity core was a highly organized and well-documented process on a multitude of levels.
After extruding the core, the night shift (Amelia and I) ran the magnetic susceptibility of the entire 4.5 meter core length. This measurement takes place in the port side wet lab where we have set up an elaborate system. The core sits on a series of four rollers that span the length of the lab. As the core is pushed towards the bow, it passes through a magnetic susceptibility sensor that is connected to a laptop with a software interface for recording the data. We take magnetic readings every 5 centimeters and then plot the magnetic susceptibility versus core depth. Magnetic susceptibility measures how much terrigenous sedimentTerrigenous – land derived; as opposed to marine sediment is in the core. Meanwhile the LMG is already well on its way to the next core site in the northern reaches of Hugo Island Trough.
Just after lunch, we arrived at our second core site. The day shift took over and helped extrude this six- meter jumbo gravity core (JGC-2). Magnetic susceptibility promptly followed. The wind had picked up to around 30-40 knots out of the northeast, and it was snowing rather heavily. A storm was brewing, and the LMG faced headwinds that brought up some major swells. Rough seas put a damper on any further coring activity for the day.
Because of the change in weather, we will sail along the western side of Anvers Island and back through the Schollaert Channel to the Gerlache Strait. This course between Anvers Island and Brabant Island will help us avoid the ice pack that initially left us stranded at Palmer Station. It will also allow us to circumnavigate Anvers Island.
Once back in the Gerlache, the plan is to fire off some XBTs. An XBT is essentially a shot of metal that transmits a signal back to the ship. It is used to collect data about the water column. We have picked this location for several XBTs because it is along a frontal boundary of two water masses with very different physical properties. One water mass is derived from Circumpolar Deep Water (CDW), a warm water mass. The other water mass is derived from Weddell Sea Water, a cold water mass.
The location of the boundary between the two will provide a general idea of where warm water interacts with ice. At or near our XBT site, warm CDW is upwelling into shallower areas and coming into contact with glaciers and icebergs. This melting, as a result of CDW, is an extensively studied mechanism in the Antarctic Peninsula that directly relates to the melting of large ice shelves.
And we’re off again! This morning, the triumvirate of leadership on the LMG deemed the pack ice passable. After three days stuck at the Palmer Station peer, we departed at 13:00 for an offshore coring location called Hugo Island Trough. This trough is a bathymetric low that trends north-south to the east of Hugo Island. It is the result of a fault and a paleo ice streampaleo ice stream – a distinct flow path along which ice mass was purged from an interior concentration in the past that gouged out the seafloor.
Excited about our destination, the science crew jumped back to the 12-hour shifts originally planned before the pack ice delay. It is time for us to begin three days of round-the-clock science. The day shift promptly returned to watch, keeping an accurate navigation log and plotting the LMG’s position every 15 minutes and collecting seismic data.
Once we arrived at the Hugo Island Trough, we provided the bridge with the exact coordinates of Core Station 1. The coring technique used at Core Station 1 is called a jumbo gravity core (JGC) which will hopefully allow us to break into the glacial till. We are attempting to capture the contact between a coarse-grained deglaciation layer and the overlying fine-grained layer of marine sediment. The depth of this contact will allow us to constrain (limit) the timing of ice loss in this area after the Last Glacial Maximum.
The coarse-grained layer is a diamictite of gray mud with inclusions of ice-rafted debris (IRD). Diamictites include a variety of grain sizes from clay particles to pebbles. Such pebbles, and sometimes larger inclusions, are deposited on the seafloor as IRD carried by calved icebergs. The age of deglacial diamictite facies tells us when in Earth’s climate history icebergs were calving, melting and subsequently dropping their debris onto the seafloor. On a cruise in 2010 (NBP 10-01), a core was taken in the Hugo Island Trough area, but it only reached depths with sediment dating back to the early Holocene, and did not penetrate deep enough to sample the glacial diamictite facies.
We returned to the trough to take a deeper, six meter core. We will also compare this core to a similar core (Core 1099) from the Palmer Deep area. The timing and duration of deglaciation may differ between the two spots offshore localities. Additionally, this comparison will help us explain the mechanism of deglaciationdeglaciation/deglacial – the loss of glacial ice masses brought on by a climatic warming in the area. Deglaciation may have occurred through a calving frontcalving front (in context) - the terminus of a glacier at which ice falls into the ocean at the head of a fjord or as a large scale retreat of an inner ice shelf.
The night shift took over after an all-ship meeting in the bridge. Spirits were high as we were finally about to grab some “Antarctic mud” that will unveil an important part of Earth’s climate history. This is what we came here for. After a few words about logistics and safety, the crew set out to the aft deck to core.