Hammertime: THOR’s Cores
text by Linda Welzenbach and Becky Minzoni; images by Linda Welzenbach
“Where’s the mud, people?” asks marine technician Joee Patterson from the doorway of the E-lab the morning of 6 March. If all goes to plan, THOR’s Hammer would deliver 8 cores of mud from the deep sea in front of Thwaites over the next 24 hours. As with most planned science aboard a research ice breaker in Antarctica, there will be successes, there will be set-backs, and there will be adjustments. By the end of the day, however, the THOR team get their mud.
The THOR team will tell the story of Thwaites Glacier from what they find on the seafloor. Until now the focus has been on multibeam mapping of the underwater topography, and the details of Thwaites’ story will be fleshed out through detailed study of the muddy sediments that rest on the seabed. Mud is defined in Webster’s dictionary as a “slimy, sticky mixture of solidmaterial with a liquid (e.g. water).” The mud that THOR seeks is slimy and sticky and at times even soupy, with consistency ranging from Nutella to chunky peanut butter, and other not so complimentary comparisons that are an endless source of amusement for the scientists.
For THOR, the sometimes pungent, green, brown to gray colored mud holds the key to their questions of how Thwaites Glacier behaved recently and back to the distant past. Seafloor mud contains bits and pieces of old Antarctic continent that hasbeen ground into sand, silt and clay by glaciers scraping the land. The ice carries its sediment cargo off the continent and into the sea. The seafloor mud hosts life at its surface, and remains of dead organisms as evidence of past ocean conditions. The sediment similarly traps water as it is deposited on the ocean floor, and records environmental regimes that operated below and in front of the glacier from thousands of years ago to the present day. These are the key clues that will help us tell Thwaites story.
The depth of the seafloor, even as we sail close to the ice shelf, is astounding. At Thwaites, depths range from less than 100 meters (300 feet) to over 1,200 meters (3,600 feet). To access the seafloor in Antarctica you need a ship that can remain “on station” without an anchor, that can battle ice, and that must be reinforced with steel framing and a winch system to deploy coring devices to great depths, where the gear becomes increasingly heavy. The captain and crew work diligently to monitor and avoid icebergs and sea ice during coring operations.
The coring device consists of a large weight on top of a metal barrel that is suspended from a wire and lowered through the ocean to the sea bottom. The barrel penetrates the seafloor mud like a straw, collecting up to maximum of 6 meters of sediment. Imagine trying to drop a skinny straw suspended from a thread through a glass of swirling milk with chocolate syrup on the bottom, and keeping it straight. In depths of a thousand or so meters, dropping the straw onto the ocean floor can take more 30 minutes, and it takes much longer to properly assemble and deploy the core device into the water. All the while the ship has to remain stationary, especially in heavy or icy seas, for the final drop so that the corer doesn’t end up leaning to its side or falling over. Most of the time, the corer successfully collects sediment.
How do we decide where to collect seafloor mud? Despite the astounding depths of the coring operations, site selection is not blind. Selection involves information gathered by the ships’ multibeam mapping and sound imaging of the mud and bedrock just beneath seafloor. Doing it well requires experience and interdisciplinary collaboration. The THOR team on NBP1902 includes three geophysicists who specialize in geomorphology (Rob Larter, Ali Graham, and Kelly Hogan) and a geologist who specializes in sedimentology and paleontology (Becky Totten Minzoni). Each project investigator brings their experience and expert knowledge to the table to target the best places that can create a complete picture of an area. They also have to consider the tools they will use to gather the best samples, within the constraints of time and manpower.
At each shift change the team meets to pore over both the recently collected multibeam bathymetric data and the Knudsen sub-profile imagery as it is captured in real time. For this cruise, the areas selected will tell a discrete story for the now ice-free ocean in front of Thwaites, yet also provide a framework for planning the activities during the THOR cruise next season. When selecting a site, the main characteristics the geoscientists look for are sedimentary layers (reflections in the sub-bottom profiler) from target depths that typically preserve shells, which are used to date the advance and retreat history of the ice sheet and ice shelf.
The team plans to gather eight cores. A suite of cores from three different sites will be taken along a transect from a deep basin to a high point near the modern ice shelf. All of these cores will tell the geoscientists about the recent retreat of the Thwaites ice shelf over the last century.
They pick the Kasten corer to use first. The Kasten is the primary go-to system employed to explore the seafloor because it can sample a variety of sediment types. The Kasten core results are then used to decide the next core type to use. The Kasten has a square barrel that is screwed together with plates to capture up to 3 m of sediment. Attaching weights around the square barrel up to 15 tons, the corer is sent to the seafloor by its wire tether at a particular rate (ranging from 10 to 30 meters per second). The weights and the speed of impact are optimized for extraction of different seafloor sediment densities and strengths. THOR team watches a monitor that displays the tether wire tension, both before and after the corer impacts the seafloor. An increase in the wire tension after it is pulled from the seabed is a first indication that tells the team how well the corer performed and how much material they may have collected.
After a long wait for one of the Kasten cores to resurface, the marine technicians Joee Patterson and Jack Greenberg settle the bottom end of the core into a heavy-duty metal “basket” mounted on the deck, allowing it to pivot gently, while draining some of the seawater from the top. Once the overhead winch is secure, the THOR team springs into action. The first thing is to preserve the integrity of the softer sediments in the top of the core with a perfectly cut foam block they have prepared. Once the core is separated from its top weights, everyone helps to heave the hundred plus pound full core barrel onto a cart. It is then carefully rolled across the deck to the Baltic room, where it will be carried into the lab for a long night of processing, sampling, and describing.
Muddy float coats, hard hats and unwieldy steel toed boots forgotten, we all hasten to unveil the sticky treasure within thebarrel that holds other worlds from the deep ocean beneath our feet. Once we remove the lids of the Kasten core barrel, it becomes apparent that the very top of the seafloor was washed out, a not uncommon result of Kasten coring, especially when the seafloor muds are soupy and soft. Immediately following this assessment, Becky Minzoni and Ali Graham quickly decide that the soupy seafloor surface absolutely must be captured from this site. That’s where the Megacorer comes in.
Unlike the single-strike gravity driven corers, the Megacorer is a set of 12 short precision hole punches. The tubes are mounted to a metal frame, that can be deployed with a wooden foot that can act as a “snow shoe” to keep it from pushing too far down through the soft, soupy sediment. When the tethered frame hits the bottom, a piston pushes the 12 tubes into the seafloor. A guillotine-like door closes off the bottom of the tubes and a lid seals the top of each tube. If the conditions are optimal, all 12 core tubes will be filled with 50 cm of perfectly preserved upper seafloor mud and sea water, and may also include the occasional squiggly Antarctic creature.
Yet, more often than not, some number tubes will be empty. Rocks, thick sticky mud, or tilt of the frame may impact the function of the trigger systems. The redundancy of 12 tubes provides more than enough material, and will capture at least some sediment even in less than optimal conditions.
An alternative to Megacores is the mighty box core. The box corer uses a frame similar to the Megacorer, but instead of 12 individual tubes, it forcefully inserts a metal box into the seafloor and then triggers a scoop that slides underneath to seal and hold the bottom.
The box core preserves the upper seafloor like the Megacore, but the box core does so as a larger and sturdier unit that provides more material in context. The box core is usually used for biological studies, and the sampling methods are flexible, allowing a variety of archiving tubes to be extracted from the sediment section. While the box corer was not used in the Thwaites coring transect, it was tested in an area of the eastern Amundsen Sea and will be employed in the 2020 THOR cruise.
Last but not least is the Jumbo gravity corer. The Jumbo is a metal barrel lined with PVC pipe, and has a heavy weight on its top that can capture up to 6 meters of sediment on this cruise. The PVC liner is extruded from the barrel and archived immediately following extraction. Extrusion happens on the deck right after it is secured at the surface. The core liner is pushed from the coring tube, cut into measured lengths, briefly described, capped and immediately refrigerated. It will later split into halves, with one half held as an archive and the other for sampling. These archives will be useful for years to come.
Within 36 hours, THOR deployed all but the box corer to collect 6 cores out of 8 attempts, but with end result achieving a full sample of the targeted transect, including both seafloor surfaces and long sediment records. As of the 12th of March, the THOR team completed 28 coring events resulting in 26 cores. Each one will provide samples to all the science team members and provide the foundation for a more extended coring strategy for next year’s cruise.