09-28-05: What did we accomplish during HOTRAX 05?
09-28-05: What did we accomplish during HOTRAX 05?
Sep 29 2005, 10:06 PM
Group: TREC Team
Joined: 27-April 05
Member No.: 10
What did we accomplish during HOTRAX 05?
Coring Synopsis HOTRAX 05
Chief scientist for coring, Dr. Dennis Darby, told me that we have recovered more than 470 m high resolution core sediments and have taken about 28 JPC and multicores during the HOTRAX 2005 cruise. “This is a great achievement considering the harsh conditions in the Arctic. We are very pleased with the expedition results”, he said.
He gave me the following summary:
A synopsis of the HOTRAX cruise follows with some interesting highlights Over 470 meter of sediment core was collected, more than any coring expedition to the central Arctic Ocean.
1) Cores were collected from areas rarely visited and where few cores longer than a few meters are available, like the Mendeleev and Alpha Ridge. The cores from these areas are between 9.5 and 13.5 meters in length.
2) Two unsurveyed lows in the major ridge systems separating the Arctic Ocean into separate basins were covered by multibeam and chirp sub-bottom profiles that revealed the true nature of these "gaps" in the ridges and that have important implications for deep water exchange between the basins.
3) For the first time, sandy mud waves were mapped on the Arctic seafloor and these also have important implications for deep currents.
4) HOTRAX surveying of the seafloor also discovered that the extent of glacial ice erosion on the Chukchi Borderland was greater and deeper than expected.
How do we recover sediments from the ocean floor? What is coring?
Dale Hubbard, our coring technician is answering the questions. During our cruise he is responsible for assembling, maintaining, rigging, deploying, & recovering coring apparatus. He's also providing technical advice on how to fine-tune the equipment to match geological conditions and how to process, curate, and store core samples. Dale earned two bachelor degrees from TX A&M Universtity at Galveston, one in marine science & one in marine biology. He earned his masters from Oregon State University in marine chemistry. Dale loves surfing, hiking, fishing, mountain biking, snowboarding, skateboarding, and playing his guitar. An interesting fact about Dale: He is a certified marine diver, and had the opportunity to dive with Alvin 3000 m deep to research hydrothermal vents on the seafloor near Easter Island.
Coring in 1100 words or less by Dale Hubbard
We're using two different tools for coring on the HOTRAX expedition, the multicore and the piston core. Think of the multicore as an instrument which gets gently pressed a short distance into the bottom, while the piston core is much longer and gets hammered in. The function of the multicore is to recover replicate samples (up to eight total) of the surficial sediments, with particular emphasis on retrieving an undisturbed sediment/water interface. The piston core is used for retrieving much longer cores (up to 70-80') useful for uncovering longer term historical records.
The multicore resembles a teepee in shape, with a weighted carousel supported in the middle of its framework. This carousel is mounted upon a pair of vertical pipes and has a big piston/cylinder assembly in the middle. When the multicore is deployed over the side, the cylinder fills with seawater. When the multicore lands on the seafloor and the steel cable from which it is suspended goes slack, the water inside of this cylinder/piston assembly (several liters) is pushed out through a hole only a few mm in diameter, making this a hydraulically-dampened piston. The hydraulic-dampening action results in the carousel lowering very slowly yet with sufficient force to penetrate 8 sample tubes aproximately 0.5 m into the sediment. When the multicore is retrieved and the tubes are pulled out of the sediment, a shovel-like mechanism closes over each tube to keep the sample from falling out. One may change the amount of weight on the multicore carousel to adapt to different sediment types, but other aspects of its operation are mostly constant.
The piston core is a much more dynamic beast. It is essentially comprised of a long steel pipe with a heavy weight on top. The way in which a piston core is set up can be adapted to accomodate different geological environments--different sediment depths, different sediment types & textures (soft, compacted, sandy, silty, oozes, etc.), different water depths, etc. Variables which can be adjusted include the length of the core (from 30-80'), the amount of weight atop the core (from 1.5 to 3 tons), and the amount of free fall (read on).
Friction is the major force which must be overcome when coring; it affects not only (1) how far a sampling device can be driven into the sediment , but it also limits (2) how much sediment we can force inside of a sampling tube before the frictional force between the sediment and the wall of the tube exceeds the downward force of the sampling device, resulting in the sediment inside the tube being pushed downward into the seafloor. We overcome #1 by adding more weight to the corer and by allowing it to freefall into the sediment and we overcome #2 by using a material with a smooth inner surface (PVC plastic tubing inserted in the steel pipe as a liner) to collect cores into and by applying a vacuum to dissuade the sediment inside the sampling device from being pushed downward as the core is driven into the seafloor. We apply this vacuum by using a piston inside the core. It functions just like a syringe--imagine holding a syringe by the plunger and pushing the barrel downwards into a cup of water so that the plunger stays stationary while the barrel fills with water. A piston core functions in the same manner.
Piston cores utilize freefall to maximize their downward force. The core is deployed so that the piston (much like a depressed syringe) is located in the distal tip of the core, connected to the steel cable upon which the core is deployed. It is also rigged so that a portion (approximately 22-24 feet) of the steel cable upon which it is deployed is coiled up slack & affixed to a trigger mechanism. This slack portion of steel cable is called the "scope." The trigger mechanism is actuated using a smaller coring device (called, appropriately enough, a "trigger core") alongside and beneath the piston core. When the trigger core touches the bottom, it releases the trigger mechanism, which in turn releases the piston core, which freefalls through the water column. The distance that the core is allowed to fall through the water column (equal to the scope) just happens to be the distance which an object must fall to achieve terminal velocity in water--the velocity beyond which no further acceleration will occur in a given medium if the object were to continue falling indefinetely. As the piston core falls, the scope is consumed, and when the tip of the piston core is just above the sediment surface the entire weight of the piston core comes against the piston which, since it is connected to the end of the steel cable, begins to slide up inside the core (just like the plunger inside of a syringe). The piston core continues to fall past the piston, creating a vacuum inside the core barrel, which keeps the sediment inside from getting pushed down into the seafloor. The piston core, if it's rigged correctly, will eventually come to a stop right around the point at which the piston reaches the top of the core. If it's rigged with too heavy a weight, the piston core bottoms out forcibly on the piston, if it's too light the piston only goes partway up the barrel and the remainder of the core fills with water (disturbing the sediment layers) once it is pulled out of the seafloor. If the core is appreciably longer than the thickness of the sediment cover (or hits a rock) it may get bent. And if the core penetrates too deeply into the sediment, it may get stuck, temporarily anchoring the ship. Sometimes the only way to get un-anchored is to break the steel cable, which has a breaking strength of 32,000 lbs! We prefer to avoid this scenario by rigging the core appropriately to suit the geological conditions.
After we bring a core back aboard, we push the PVC liner out of the steel core pipe and cut it into sections for storage, analysis, and transportation. Depending upon how well the core worked, we make small adjustments to the core weight or amount of freefall. Our goal is to fill the core barrel with as much mud as possible without overpenetrating into the sediments, to recover the longest and best quality historical record possible.
The stratigraphy of yellow and brown sediment bands is clearly visible on the split multicore material
|NSF Acknowledgment & Disclaimer||Time is now: 28th August 2016 - 08:52 AM|