by Andrea Johnson, JOI
Have you ever put together a 500-piece puzzle without the box-top picture? It's a bit like trying to understand the Earth, but our planet is three-dimensional and the pieces are countless—with many buried or lost. Intimidating? Yes! However, many minds over many years have made progress fitting some of the pieces together. In search of clues, scientists obtain cores—long cylinders of sediment and rock—from beneath the Earth's surface in strategic locations.
Extracting cores from the seafloor, oftentimes beneath several kilometers of water, is particularly challenging but especially rewarding. Seafloor sediments and rocks are immune to many of the erosional forces that scour and redistribute deposits on land, making their records of environmental history and Earth processes unique and exceptional. Yet, it was only in the middle of the last century when scientists both recognized the value of seafloor deposits and learned to obtain cores containing relatively undisturbed sediments. A major breakthrough was "piston coring" where a core tube is pushed into the seabed as a piston is pulled out–with the piston's displacement sucking sediment into the tube.
Methods for deep subseafloor access (100 meters and greater) required many additional technological advances that the complementary efforts of the science community and industry eventually made possible. Today, hydraulic piston corers use compressed seawater to drive a steel barrel through soft sediment and mud. And to penetrate rock and harder sediment, drill bits with cutting heads are used. The rotating heads, which grind rock at the base of the hole, are spaced so as to preserve a cylinder of rock that is pushed up into the core tube.
The JOIDES Resolution
In 1968, the Deep Sea Drilling Project began with its special drill ship, the Glomar Challenger , giving scientists unprecedented opportunities to study seafloor rocks and sediments–and opening up entirely new fields of study such as paleoceanography. This program and its successors–the Ocean Drilling Program and the Integrated Ocean Drilling Program–have drilled and cored nearly 2000 holes. These holes, which have provided countless samples, are in themselves a vast resource for studying subseafloor formations and fluid flow using instruments lowered on the drill string. The achievements of scientific ocean drilling and the work of thousands of people–including scientists, engineers, editors, technicians, drillers, and ships' crews–can be found in a database with more than 18,000 citations. (This database is available online at http://odp.georef.org/dbtw-wpd/qbeodp.htm).
The Chikyu (courtesy JAMSTEC)
Although previous scientific drilling can boast core recovery in water depths up to 8.2 kilometers and substrate penetration up to 2 kilometers, the multiplatform structure of the Integrated Ocean Drilling Program, launched in 2003, will explore new frontiers. Japan, one of the three major partners in this international program, is conducting sea trials for the Chikyu , a state-of-the-art drilling vessel that will be able to drill to greater depths and in deeper waters than ever before. Its special capabilities will allow it to probe high-pressure environments such as subduction zones, which are typically the source of major earthquakes. With funding from the National Science Foundation, the U.S. is in the process of procuring and converting a drill ship to succeed the JOIDES Resolution , the workhorse of the Ocean Drilling Program. The Integrated Ocean Drilling Program's third member, the European Consortium for Ocean Research Drilling, is providing platforms for expeditions with special requirements such as operation in Arctic pack ice and extremely shallow waters. International collaboration has–and will continue to be–a cornerstone of scientific ocean drilling, which has unified geoscientists worldwide in their goals to understand the Earth.
A view of the Vidar Viking and the ice breaker, Sovetskiy Soyuz, from the Oden during IODP Expedition 302 in the Arctic Ocean (Martin Jakobsson, courtesy of IODP)
Although many exciting discoveries made by scientific ocean drilling have been serendipitous, each expedition seeks to test scientific hypotheses put forth by the global scientific community. At any one time, dozens of proposals are being carefully evaluated and reviewed by a Science Advisory Structure. Numerous panels composed of international experts consider each proposal for safety, feasibility, and scientific merit. Although seagoing drilling expeditions are typically about two months in length, they are planned to optimize nearly every minute of a drilling platform's time at sea.
One core at a time, scientific ocean drilling is deciphering the history of the Earth and its interconnected systems, but it often takes years for the relevance of some findings to become clear. And, when some questions are answered, new ones may emerge–so the adventure continues.