Ocean-floor drilling programs

Ocean-floor drilling programs have allowed geologists and oceanographers to extend their knowledge of the earth's history by analyzing long marine sediment cores and basement rock cores recovered from the sea floor. Data from ocean-floor drilling have provided evidence supporting the theories of seafloor spreading and plate tectonics and have permitted the investigation of the paleoclimatic and paleoceanographic history of the earth.

Origins of Technology

Most of our knowledge of the history of the earth comes from the study of sedimentary rocks, as sediments contain the preserved fossil remains of ancient plants and animals, while sedimentary structures record the processes of deposition. Sedimentary rocks exposed on land tend to have an incomplete record because they may be deformed by folding and faulting, which may destroy both the fossils and the sedimentary structures, and because they may be eroded by wind, water, and ice moving across the surface. In contrast, marine sedimentary sequences contain a more complete record of accumulation because they accumulate in a lower-energy environment, which is not as affected by folding, faulting, erosion, and post-depositional alteration as are terrestrial sediments. As a result, the deep-sea sediments tend to preserve a continuous record of sediment deposition in the ocean basins.

Ocean drilling techniques were originally developed in the 1950s by petroleum exploration companies searching for shallow-water hydrocarbon and petroleum deposits located on the continental shelves. These industrial exploration methods were adapted in the 1960s to obtain long sediment cores from the sea floor in deep-water areas on the continental slopes and abyssal plains. By drilling through the entire sediment record into the harder basement rocks below marine sediments, geologists hoped to acquire the complete history of sediment deposition within an ocean basin from the time that sediments were first deposited atop volcanic basalts.

Glomar Challenger

Preliminary attempts to drill the ocean floor included engineering tests for Project Mohole by the drilling barge Cuss I, which in 1961 drilled marine sediments off La Jolla, California, and at the deep-water Experimental Mohole Site east of Guadalupe Island, off Baja California, Mexico, in a water depth of 3,566 meters. Although further Project Mohole development was not undertaken because of a combination of political conflicts and increasing cost estimates for the project, in 1964 four American universities formed a consortium to initiate a program of scientific deep-sea drilling. JOIDES, the Joint Oceanographic Institutions for Deep Earth Sampling, successfully operated a drilling program in April-May 1965, using the vessel Caldrill I to drill six holes on the Blake Plateau off Florida to sub-bottom depths of more than 1,000 meters, with continuous core recovery.

Following these successful trials, JOIDES proposed an eighteen-month program of scientific drilling in the Atlantic and Pacific oceans, to be called the Deep Sea Drilling Project (DSDP), operated by the Scripps Institution of Oceanography of La Jolla, California, using the drilling vessel Glomar Challenger. Glomar Challenger left Orange, Texas, on July 20, 1968, on Leg 1 of the Deep Sea Drilling Project. The results of DSDP drilling on the first nine cruises in the Atlantic and Pacific oceans caused the National Science Foundation to extend the drilling program beyond the initial eighteen-month period, with further drilling in the Indian Ocean and in the seas surrounding Antarctica.

When DSDP began operations, many other American oceanographic institutions joined JOIDES in support of the drilling program, and the success of DSDP also attracted scientific participation and financial support from foreign countries. The International Program of Ocean Drilling (IPOD) started in 1975 when the Soviet Union, the Federal Republic of Germany, France, the United Kingdom, and Japan joined JOIDES, with each country providing $1 million yearly to support drilling programs. DSDP/IPOD drilling activities continued through the early 1980s, leading to international scientific exchange of information between oceanographers.

Sedco/BP 471

Because the initial JOIDES proposal was only for eighteen months, it was never expected that ocean drilling would continue for fifteen years. Because of demands for ocean drilling in deeper waters and in high-latitude polar areas, JOIDES proposed in the early 1980s that a larger drilling vessel be acquired for continued drilling. The last cruise of the Glomar Challenger, DSDP Leg 96, ended in Mobile, Alabama, on November 8, 1983, with the retirement of the drilling vessel from service. In 1983, responsibility for scientific supervision of the international project, now called the Ocean Drilling Program (ODP), passed from the Scripps Institution to Texas A&M University, and the drilling vessel Sedco/BP 471 replaced Glomar Challenger. The first cruise of a ten-year ODP drilling program began on March 20, 1985, when the Sedco/BP 471, informally called the JOIDES Resolution, left port to begin drilling on ODP Leg 101. Leg 186 was scheduled for completion in the year 2000, and planning continued for cruises up to Leg 201 in 2002.

The results of each cruise, or leg, have been published in a series of books, entitled Initial Reports of the Deep Sea Drilling Project, which are published by the U.S. Government Printing Office. The cores recovered from the DSDP and ODP holes represent an invaluable record of the history of ocean sediment deposition around the globe. These recovered sediment cores are studied by a variety of scientists, who are interested in the sediment type, fossil content, geochemistry, magnetic orientation and strength, shear strength, and other sedimentary properties of the samples.

Drilling Procedures

In shallow water, drilling is accomplished either by building a drilling platform directly atop the sea floor or by firmly anchoring a drilling vessel to the bottom. In deep-water ocean drilling, however, it is not possible to anchor the drilling vessel to the bottom, so the technique of dynamic positioning is used to maintain the position of the vessel above the hole being drilled. In dynamic positioning, an acoustic beacon emitting sounds at either 12.5 kilohertz or 16 kilohertz is dropped to the sea floor. Four hydrophones, located at different points on the hull of the drillship, receive the signal from the acoustic beacon at slightly different times, depending on the position of the hull relative to the beacon. The position of the ship is maintained by a shipboard computer, which interprets the information from the hull hydrophones and controls the position of the ship by driving both the main propellers and two laterally oriented propellers, or hull thrusters. If the vessel is pushed off location by waves or surface currents, the shipboard computer attempts to compensate by using the propellers and hull thrusters to maintain the ship's location relative to the seafloor beacon.

To drill sediment and rock samples from the sea floor, a drill bit is attached to the bottom of a 9.5-meter-long piece of hollow cylindrical stainless steel drill pipe. More individual lengths of pipe are connected on the rig floor of the drillship to make a “drill string,” which extends from the vessel through the water down to the sea floor, where coring may begin. Usually, about 450-510 lengths of drill pipe are required simply to reach the bottom, and the assembly of this drill string may take twelve hours before bottom drilling may be started. Once the string reaches the sea floor, the drill pipe is rotated by hydraulic motors on the rig floor, and the rotary action combined with the weight of the drill string causes the drill bit to spiral down into the sea floor. Sharp iron carbide or diamond cutting teeth on the drill bit assist the penetration of the drill string into the sediment and the rock on the ocean bottom.

Samples of sediment and rock are retrieved from the sea floor by drilling a hole about 25 centimeters in diameter, using a drill bit with a 7.5-centimeter hole in its center. In effect, sediment is cored by “drilling the doughnut and saving the doughnut hole”: Rotating the drill string grinds the outer ring of sediment to small pieces against the diamond teeth of the drill bit, while the material in the center of the drill hole is saved as a core of drilled sediment 6.6 centimeters in diameter. As the drill string is lowered deeper into the drilled hole, the core is pushed up into a plastic core liner in a steel “core barrel” within the lowest stand of drill pipe. After 9.5 meters of the sea floor has been drilled, the core barrel is pulled up to the rig floor by a cable lowered through the drill string. By the time ODP ended in 2004, Joides Resolution completed 110 expeditions, collecting about 2,000 deep sea cores from major geological features in the oceans of the world.

Atlantic Ocean Drilling

The first three cruises of the Glomar Challenger provided information proving that seafloor spreading had occurred in the Atlantic Ocean. A series of DSDP holes across the Mid-Atlantic Ridge showed that the age of bottom sediments increased with distance from the ridge crest and indicated that the ages of sediments with depth correlated from one hole to the next. The total thickness of sediment atop the basaltic basement also increased with greater distance from the ridge crest, on both the east and west sides of the Mid-Atlantic Ridge. Further DSDP and ODP drilling has provided evidence that seafloor spreading has occurred in all the earth's ocean basins. In addition, ocean drilling has confirmed the relative youth of the ocean basins, as predicted by plate tectonics; the oldest sea floor yet discovered is Early Jurassic in age (160 million years old), compared to continental rocks, which may be as old as 4.5 billion years.

Pacific Ocean Drilling

Glomar Challenger and JOIDES Resolution have operated from the Norwegian Sea to the Ross Sea off Antarctica and have drilled holes in water depths from 193 meters on the Oregoncontinental shelf to 7,050 meters in the Mariana Trench off Guam, in the western Pacific Ocean. The deepest hole through seafloor sediment deposits is more than 1,750 meters below the sea floor, and one site in the equatorial Pacific Ocean west of South America (DSDP Hole 504B) has been drilled through 300 meters of sediment and 1,500 meters of volcanic basement rock.

Global Stratigraphic Correlation

Seafloor drilling has indicated that deep-sea sediments contain long sequences of well-preserved microfossils, which may be used for global stratigraphic correlation, in contrast to the fragmentary record preserved on land, where structural deformation of sediment deposits may complicate the problem of correlating different sedimentary sequences. Analysis of these sediments has revealed the history of deposition in the different ocean basins and has provided information on ancient climates and oceanographic conditions (such as the position, strength, and temperature of past ocean currents).

Sediment cores have indicated the presence of great shifts in oceanic climate conditions during the geologic past and have demonstrated that the Antarctic continent has been covered by glacial ice caps for at least 40-50 million years, rather than the 5 million years accepted prior to DSDP drilling near Antarctica. Another startling result of ocean drilling has been the discovery that the Mediterranean Sea dried up between 12 and 5 million years ago. Massive salt and evaporite mineral deposits below the Mediterranean basin indicate that the Strait of Gibraltar connection to the Atlantic Ocean was blocked during this time. Blockage of the Gibraltar connection allowed the water in the basin to evaporate, causing the deposition of vast salt and evaporite mineral deposits as the Mediterranean dried up.

Study of Seafloor Basement Rocks

Not all the information provided by ocean drilling has been concerned with the sediment column. Drilling into basement rocks has allowed geophysicists to compare the structure of seafloor basement to that of layered igneous-rock deposits that have been uplifted above sea level on the edges of certain continents. Similarly, direct drilling through these basalt and gabbro layers has allowed a comparison of the rock type to sound velocities measured by marine geophysicists. Some other results of hard-rock seafloor drilling have been the investigation of sediment and mineral deposition by hydrothermal processes at rapidly opening mid-ocean ridge segments, such as the sulfides deposited by high-temperature fluids emitted by “black smoker” and “white smoker” structures near the Galápagos Islands west of South America. Drilling of bare basement rocks along mid-ocean ridges in the Atlantic, Pacific, and Indian oceans has enabled geochemists and igneous petrologists to study the frequency at which seafloor volcanic rocks are produced at individual ridge segments and to determine whether temporal changes occur in the chemistry of basalts erupted from one location on the ridge. These studies of seafloor basement rocks may be applied to mineral exploration of marine rocks that have been uplifted above sea level and exposed on continents.

Improvements in Drilling Technology

In addition to their scientific results, DSDP and ODP operations have resulted in improvements in drilling technology by developing the ability to reenter sea floor boreholes, by devising techniques for “bare-rock” drilling on the sea floor, and through the development of new coring bits. During DSDP Leg 1, it was discovered that existing drill bits could not penetrate hard chert beds; thus, they also would not be able to penetrate through deeper igneous rocks below seafloor sediments. The drag bits were solid, consisting of a central opening and radial curved ridges of steel or tungsten carbide, capped with industrial diamonds and designed to churn through soft sediments. As a result, DSDP began a drill-bit design program, which led to the development of roller bits capable of penetrating both chert layers and seafloor basalts. These bits consist of four conical cutting heads studded with tungsten carbide or diamond cutting teeth, situated around the central core opening in the bit.

Another important technical development of DSDP, first successfully accomplished on Leg 15, was the ability to reenter a drilled borehole on the sea floor. Even with roller bit designs, drill bits wear out from the stresses of rotary coring through seafloor sediments and rocks. When a bit fails, the entire drill string has to be “tripped,” or pulled up to the vessel to replace the bit at the lower end of the string, which in most deep-ocean drilling sites requires pulling the string up not only several hundred meters from below the sea floor but also through 2,000-5,000 meters of seawater. During early DSDP legs, bit failure forced the abandonment of a hole because after the fatigued bit was replaced, it was impossible to reenter the original borehole. Successful reentry techniques were facilitated by the development of a steel reentry cone 6 meters in diameter, topped with three sonar reflectors and a rotating sonar scanner that can be lowered through the drill string. In areas where hardened sediment layers are anticipated, requiring bit replacement to complete drilling, the reentry cone is placed on the sea floor prior to drilling the initial borehole. As bits become worn, they may be replaced and the hole reentered by using the sonar scanner to locate the reentry cone (and thus the original hole).

DSDP and ODP drilling specialists have also devised methods to enable drilling in hard seafloor areas, such as mid-ocean ridges, which were not previously drillable by existing techniques. Development of a seafloor “guide base” for drilling has allowed successful drilling and reentry of boreholes in these areas and has permitted the implantation of seafloor sensing devices, such as earthquake-measuring seismometers, in these holes.

Role in Development of Paleoceanography

Ocean-floor drilling programs have enabled scientists to correlate apparently unconnected phenomena through the theory of plate tectonics, a global synthesis of geology and oceanography. Ocean drilling has provided verification of the seafloor spreading hypothesis as it applies to plate tectonics and has indicated that seafloor spreading has occurred in all the earth's ocean basins.

Before long sediment cores could be acquired from the sea floor, scientists' knowledge of seafloor geology was sparse, based on limited samples available from dredging and shallow coring of the sea floor by oceanographic vessels. Prior to DSDP, global stratigraphic correlation was based on a fragmentary record preserved on land, where structural deformation of sediment deposits may complicate the problem of correlating different sedimentary sequences; DSDP drilling, however, has revealed that deep-sea sediments contain long sequences of well-preserved microfossils. Furthermore, seafloor sediment cores have revealed the history of the ocean basins and have provided information on ancient climates and oceanographic conditions (such as the position, strength, and temperature of past ocean currents). A new science, paleoceanography, has been developed based on this information from DSDP and ODP drilling. Analysis of the earth's ancient climates may provide information to predict future shifts in the biosphere.

Industrial Applications

Ocean drilling has also provided evidence for the existence of deep-water hydrocarbon accumulations, which has enabled petroleum exploration companies to drill petroleum deposits on the continental slopes and may eventually lead to the discovery of significant hydrocarbon deposits in ocean basins. If future technology is developed, humans may be able to exploit these deep-water petroleum resources. Furthermore, scientific ocean drilling has enabled geologists to understand the processes controlling the deposition of “black shale” deposits and other high-productivity seafloor sediments, which may be altered by burial into source beds for the generation of petroleum hydrocarbons. Understanding of the processes affecting the formation and distribution of these sediments may assist in future exploration for fossil fuel resources. In addition, studies of seafloor basement rocks may lead to a more complete understanding of the nature of mineral deposition at mid-ocean ridges, which may be applied to mineral exploration of similar marine rocks that have been uplifted above sea level and exposed on continents.

Finally, deep-ocean drilling has led to technological innovations in the tools and techniques used to sample the sea floor. These methods have been adapted by industrial companies exploring for hydrocarbons buried beneath marine sediments and for mineral deposits on the sea floor. In 2004 the Ocean Drilling Program was transformed into the Integrated Ocean Drilling Program (IODP). The final report of ODP was published as the “Ocean Drilling Program Final Technical Report 1983-2007.” The IODP is an international program utilizing a variety of drilling platforms to investigate the earth deep below the sea floor. Scientists from a variety of backgrounds collaborate on a long-term scientific plan—The IODP Science Plan for 2013-2023—concerning scientific ocean drilling. The plan is four pronged, concerning the biosphere, earth connections, hazards of human impact, and climate and ocean change.

Principal Terms

abyssal plains: flat-lying areas of the sea floor, located in the ocean areas far from continents; they cover more than half the total surface area of the earth

basalt: a dark-colored, fine-grained rock erupted by volcanoes, which tends to be the basement rock underneath sediments in the abyssal plains

chert: a hard, well-cemented sedimentary rock that is produced by recrystallization of siliceous marine sediments buried in the sea floor

correlation: the demonstration that two rocks in different areas were deposited at the same time in the geologic past

deposition: the process by which loose sediment grains fall out of seawater to accumulate as layers of sediment on the sea floor

mid-ocean ridge: a continuous mountain range of underwater volcanoes located along the center of most ocean basins; volcanic eruptions along these ridges drive seafloor spreading

paleoceanography: the study of the history of the oceans of the earth, ancient sediment deposition patterns, and ocean current positions compared to ancient climates

plate tectonics: a theory that the earth's crust consists of individual, shifting plates that are formed at oceanic ridges and destroyed along ocean trenches

seafloor spreading: a theory that the continents of the earth move apart from each other by splitting of continental blocks, driven by the eruption of new ocean floor in the rift

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