Climate Records: Sampling
Sections of ocean sediments have been sampled extensively by a suite of coring operations dating back to the 1940s. These operations include a variety of different drill ships as well as rigs like the ones that are used in oil exploration. In all cases, the retrieval of continuous cores from the seafloor requires a drill string, a continuous line of pipe that is assembled to extend from the ship or platform to the seafloor.
Once the drill string reaches the seafloor, a metal core barrel is lowered through the pipe to recover sediment. Typically, each core that is extracted is between 2 and 10 m long. At the head of the core barrel, is the bottom hole assembly, a device that is positioned to cut the core. A variety of different techniques are used to cut sediment. Where the sediment is young and soft, as occurs in younger layers close to the seafloor, cores can be cut using a piston-coring device (see figure below). The piston is situated inside the core, and it is triggered when the bottom hole assembly is close to the ocean bottom. When it hits the bottom, the piston rapidly moves up, so the mud fills the empty pipe as it sinks. At the end of the bottom hole assembly is a drill bit that actually helps cut through the core. In the case of the piston core, the bit is a sharp, knife-like device that helps the rapidly advancing core cut readily through the soupy sediment. If the formation is harder, as occurs deeper in the sediment column where the overlying burial has led to compaction and cementation, cores can only be retrieved using a hard bit that cuts the rock by rotating at high speeds assisted by water, and, in some cases, drilling mud (this is known as rotary coring). Compared to the piston core which advances in seconds or less, this rotary coring can take a very long time, often up to an hour for each meter of core; moreover, some of the sediment can be lost during coring, as pressure from water or mud must be applied to cut through the rock. Once the coring mechanism is fully extended, a core catcher slips into place underneath the coring device and a wire line is used to retrieve the core to the ship or drilling platform.
Probably the best-known and longest-running drilling operation, the Integrated Ocean Drilling Program (IODP) has been in operation for more than 40 years. The program began as the Deep Sea Drilling Project in 1969 and is a collaborative scientific operation between a number of different countries. The ship chartered by the program is the JOIDES Resolution, a highly sophisticated drilling platform that has the ability to take cores in locations where the ocean is 6000 meters deep. The ship is about 150 meters long, with a drilling derrick that is about 50 meters high. The deepest hole drilled by IODP exceeds 2000 meters. The ship can operate in stormy seas with large waves and strong currents as it is held in position by powerful turbines, and the drill pipe is stabilized by a device called a heave compensator that keeps the drill bit on target even when the ship is listing. The Resolution has berths for about 120 engineers, scientists, drilling “rough necks,” and caterers, and typically stays out to sea for six- to eight-week expeditions.
As we will see below, valuable information about Earth’s climate has been obtained by drilling in the ocean basins. However, rocks that were deposited in continental environments and marine sediments that have been uplifted onto the continents via plate tectonics also provide important information about past climates. These rocks are much more readily accessible and can be sampled in road cuts, stream beds, and many other places. In addition, sedimentary rocks on land can be sampled via coring with much less expense than their oceanic counterparts.
Samples of land materials are often taken by digging a trench to obtain fresher material beneath the weathered surface zone. If the material is indurated, samples of core are removed using a saw and samples of outcrop are removed using a hammer. Soft material usually in cores can be sampled by inserting plastic tubes into the sediment. Once the samples are taken, the material is ready for analysis.
Over the long term, cores of sediment are usually kept in refrigerators to keep them from drying out. Cores of ice are housed in warehouse-sized refrigerators that are cooled to -30oC. The longest ice core sampled is the three-kilometer long Dome C core from Antarctica that extends back some 750,000 years.
The following video provides an excellent overview of how cores are collected by the Integrated Ocean Drilling Program. Click the play button in the center of the video to watch it.