The Life of a Core

by Andrea Johnson, JOI

At any time of day or night, operations aboard a scientific drill ship may be punctuated by the welcome cry, "Core on deck!" With that summons, crew, technicians, and scientists rush to the drilling deck to welcome a fresh core from the ocean's depths. And so, the life of a core begins.

That exciting moment is typically preceded by years of planning and meticulous preparation. Even once a scientific ocean drilling expedition is under way, there may be a long transit before the drill site is reached. Then it may take hours for the drill string to be assembled and lowered through the ship's moonpool. Once the drill string reaches the seafloor, the crew lowers core barrels through the drill pipe. Drilling engineers orchestrate how drilling or coring proceeds, and nature of the substrate determines which drill bit or "corer" is used. These core barrels collect one 9.5-meter section of core every 90 minutes on average--and 24 hours a day when the ship is on site.

Once the core arrives on the rig floor, the drill crew passes the plastic-sheathed cylinder to technicians who carry it to the catwalk. While the core rests in a rack, the technicians label and cut it into 1.5-meter sections before moving it to the core receiving station. Here the core officially enters a database and more permanent labels are printed to record each section's location and depth below the seafloor.

Next, scientists perform various whole-core analyses. First, a Multi-Sensor Track System analyzes various properties, including density, porosity, and magnetic properties. In addition, scientists spend time measuring other physical properties of the core.

At the next station, the core is split lengthwise into two halves, the “working half,” which scientists sample and use aboard the drill ship, and the “archive half,” which is kept in pristine condition for future reference.

Scientists describe the split core by documenting its composition, grain size, and color. Other aspects, such as interesting inclusions or disturbances due to, are noted. In addition, the shipboard photographer takes several pictures of the core for research purposes.

While some of the scientific party describe the archive half of the core, the working half is prepared for sampling. Scientists identify sections of the core they want to sample with colored flags. These samples are then removed form the core and are either analyzed on the ship or stored for postcruise research. As drilling continues, it will be guided to a certain extent by the preliminary shipboard analyses. There are twelve different laboratories aboard the current drill ship, JOIDES Resolution , including a chemistry lab, a downhole measurements lab, a paleomagnetism lab, a microscopy laboratory, a paleontology lab, and a microbiology lab.

When scientists finish describing the core, both halves are stored in refrigerated space in the ship's hold. Following the expedition, the cores are sent to an onshore repository. Scientific ocean drilling program cores are stored at four onshore repositories: one at the University of Bremen, Germany; the East Coast Repository located at the Lamont-Doherty Earth Observatory of Columbia University in New York; the Gulf Coast Repository located at Texas A&M University; and the West Coast Repository located at Scripps Institution of Oceanography in California. Among these four repositories, more than 235 kilometers of core is housed and made available for future research.

As the life of a core begins, so does the life of a borehole, which becomes a laboratory of sorts once the core is removed. In a process called "downhole logging," instruments are lowered into the hole to record the physical and chemical properties of the surrounding rock or sediment. Afterwards, some boreholes may be sealed off to become sites of long-term observatories. Instruments to measure temperature, pressure, and other properties may be left behind for future retrieval by remotely operated vehicles or submersibles.