Shipboard Laboratories INTEGRATED OCEAN DRILLING PROGRAM

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1 Riserless Vessel Shipboard Laboratories INTEGRATED OCEAN DRILLING PROGRAM JOIDES Resolution Paleomagnetism Laboratory February, 2005

2 What is the Paleomagnetism Laboratory? The oceanic crust below marine sediments is made up of igneous rocks. As the rising magma that forms these rocks cools, small magnetic iron-bearing minerals crystallize within them (Figure 1). These minerals become magnetized in the direction of the Earth s magnetic field and become a frozen record of the location of ancient magnetic poles think of them as tiny compasses locked into the crystal lattices of the magnetic grains (Figure 2, next page). Magnetic grains in terrestrial environments also can be liberated from their host rocks by erosion and transported to the ocean by rivers, wind, and glaciers. Once in the ocean, these particles slowly settle to the bottom of the ocean, where they become aligned with the Earth s magnetic field. As the sediments become buried, the magnetic direction is locked in. The goal of paleomagnetism studies is to read magnetic field directions that are locked into igneous rocks and sedimentary layers. Magnetic directions change through time because of magnetic field reversals and the motions of plates on the Earth s surface (Figure 3, next page). To imagine how this works, think of the following: You set up a camera on a tripod in your room and rig it to Figure 1. According to the theory of plate tectonics, spreading between two tectonic plates takes place at mid-oceanic ridges. As the two plates move apart, magma rises and forms the ocean crust, consisting of pillow basalts, basaltic dikes, and gabbro. When these igneous rocks cool, small iron-bearing minerals crystallize and align themselves with the orientation of the Earth s magnetic field (illustration modified from Thurman, H.V., and Burton, E.A., 2000).

3 2 take a picture every hour, while you were out of the room, someone moved the camera several times. Later, you look at the pictures and you can tell where the camera was by what it was pointing at, and you can tell whether it was day or night by the view out the window. Where the camera was pointing is akin to the magnetic direction and the day/ night cycle is akin to magnetic reversals. The paleomagnetism laboratory facilities available on scientific ocean drilling expeditions have made it possible for scientists to analyze igneous and sedimentary rocks for paleomagnetic directions (Figure 4, next page) that can be used to conduct a variety of scientific research activities, such as unraveling the movement of tectonic plates through geologic history. Figure 2. Cross-section of Earth showing magnetic field inclination (dip). The arrows represent the direction of the Earth s magnetic field relative to the Earth s surface. Magnetic minerals that crystallize at a point on the Earth s surface would be magnetized in the direction of the arrow (modified from Cox, A., and Hart, R.B., 1986). (Note that the field inclination at the poles is ± 90, horizontal at the equator, and some intermediate value in between). Figure 3. This is the geomagnetic polarity timescale for middle Miocene to Holocene time. In the polarity column, black represents periods in the Earth s history when the magnetic field was normal (as it is today); white represents times of reversed polarity (north and south magnetic poles opposite from present day). The ages of the polarity boundaries are well-established in the literature (Shipboard Scientific Party, 2001; from Cande and Kent, 1995).

4 Figure 4. Will Sager, Paleomagnetist (USA), and Lisa Hawkins, Undergraduate Student Trainee (USA), are using the Schonstedt alternating field demagnetizer to remove the secondary magnetization from basaltic rock samples so that they can be analyzed using the cryogenic magnetometer. The samples were cored from the east flank of the Juan de Fuca Ridge in the northeastern Pacific Ocean during IODP Expedition 301. Where is the Paleomagnetism Laboratory Located? The paleomagnetism laboratory is located in the labstack on Level 6 (Bridge Deck) of the JOIDES Resolution. You can find a virtual tour of the paleomagnetism laboratory on the following page (use your arrow keys to pan around the room): publicinfo/tour1/index.html. Who Works in the Paleomagnetism Laboratory? Paleomagnetists from more than 15 countries participate in the Integrated Ocean Drilling Program (IODP) expeditions. Typically, two paleomagnetists sail on each expedition and work alternate 12-hour shifts. The selection of the scientists is based on a match between their research goals and the scientific objectives of each expedition. Sailing paleomagnetists spend much of their time in this lab preparing and analyzing samples that are taken from cores retrieved during their expedition (Figure 5, next page). A marine laboratory specialist, who is an expert at using all of the scientific equipment in the paleomagnetism laboratory (Figure 6, next page), sails on every IODP-USIO expedition and collects data for the participating scientists.

5 Figure 5. Will Sager, Paleomagnetist (USA), and Lisa Hawkins, Undergraduate Student Trainee (USA), are examining basalt samples recovered from Hole 1301B, drilled on the eastern flank of the Juan de Fuca Ridge during IODP Expedition 301. Selected samples will be cut into one-inch cubes using the rock saws behind the scientists and then placed in the paleomagnetism laboratory s superconducting cryogenic magnetometer to measure the paleomagnetic signature of small, iron-bearing minerals in the basalt. Figure 6. Trevor Cobine, Marine Laboratory Specialist (USA), checks the computers used to record the magnetic data from the paleomagnetism laboratory s superconducting cryogenic magnetometer. The data, ultimately accessible from any computer on the ship, are then used by shipboard scientists to interpret the magnetic properties of the rocks recovered during the expedition. What Types of Research Questions are Investigated in the Paleomagnetism Laboratory? Magnetic directions that are preserved in the rock record have proven to be one of the cornerstones of the plate tectonics theory, which states that the upper part of the Earth s crust is made up of several independently-moving lithospheric plates (Cox and Hart, 1986). In 1963, one of the most fundamental discoveries that involved the theory of plate tectonics and seafloor spreading was based on the use of symmetric patterns of normal and reversed polarity magnetic anomalies--that are recorded in the rocks and sediments--to determine that new oceanic crust was being formed at midocean ridges (Vine and Mathews, 1963). Paleomagnetists also found that the magnetic directions that are preserved in marine sediments and igneous rocks provide invaluable information that can be used to answer a variety of geologic questions that are related to plate tectonics, polar wandering (Figure 7, next page), and marine sedimentation (McElhinny and McFadden, 2000; Thurman and Burton, 2000). For example, magnetic information from cores that were recovered during IODP expeditions may be used by paleomagnetists to research such questions as: 1) How have continents (and tectonic plates) moved through time (Figure 7)? 2) What are the spreading rates along a given mid-oceanic ridge (Figure 8)? 3) What are the rates of sedimentation in a study area? 4) How do stratigraphic sequences within a basin and in different basins correlate to each other?

6 5 Figure 7. Paleomagnetic data can be used to reconstruct polar wander paths. These allow scientists to reconstruct the motion of tectonic plates through geologic time. Diagram (a) shows the polar wander paths that were calculated for Europe and North America from 300 million years ago to present. In diagram (b), North America and Europe have been reconstructed to their relative positions million years ago by closing the Atlantic Ocean. This brings the two polar wander paths into coincidence and demonstrates that differences in polar paths can be used to study plate motion (illustration from Thurman, H.V., and Burton, E.A., 2000). Figure 8. Magnetic stripes on the seafloor off the coast of the northwestern United States and southwestern Canada have been used to determine spreading rates along the Juan de Fuca Ridge. Because the ages of the polarity events are well known (see Figure 3), distances between magnetic stripes on either side of the spreading can be divided by the number of years that an event lasted to determine the spreading rate of the plate. In this illustration, the normal polarity stripes (same pole orientation as today) are colored and the reversed polarity stripes (opposite pole orientation as today north and south poles are reversed) are white (illustration from Thurman, H.V., and Burton, E.A., 2000).

7 What are Some Examples of Analyses Performed in the Paleomagnetism Laboratory? Two main types of experiments are carried out in the shipboard paleomagnetism laboratory on samples that are recovered from the oceanic crust. The primary objective of the lab is the determination of stable magnetic field orientations.. This is accomplished by passing core samples through a superconducting cryogenic magnetometer (Figure 9) that measures each sample s magnetic remanence (the permanent magnetic field properties of the rock). Paleomagnetists then use a mathematical relationship between magnetic dip (Figure 10) and latitude to determine the paleolatitude of the rock or sediment at the time during which it was formed or deposited. By comparing this information with the current orientation of the Earth s magnetic field, paleomagnetists can determine how the plate on which the sediment was deposited has moved through time. A challenge that paleomagnetists have to overcome when measuring remanence is the magnetic overprint. Core samples can be contaminated in several ways. For example, the drilling crew on the JOIDES Resolution magnetizes the drill pipe to detect flaws that may cause the loss of equipment during drilling operations. This magnetism can be transferred to the samples while they are being cored and cause a measurable magnetic overprint. The paleomagnetics laboratory also is used to demagnetize the samples by heating them or exposing them to an alternating magnetic field that randomizes or cancels out the overprint signal. Figure 9. The paleomagnetism laboratory aboard the JOIDES Resolution has a 2G Enterprises 750-R superconducting cryogenic magnetometer. This instrument is used to measure the orientation of the magnetic field preserved in rock or sediment core samples. The samples are placed on the track in the foreground and repeatedly passed through the hole into the central chamber. The magnetic field sensors are in this central chamber (the large cylinder) next to Trevor Cobine, Marine Laboratory Specialist (USA). The magnetic shielding around the cryogenic magnetometer virtually eliminates the effects of the Earth s current magnetic field; this allows the instrument to measure the weak magnetic field that is preserved within the sample. Figure 10. Earth s magnetic field (F) generally points at an angle downward towards magnetic north (MN) in the northern hemisphere, and at an angle upwards away from magnetic south in the southern hemisphere (it is parallel to Earth s surface at the equator). This angle (D), called the magnetic dip or inclination, increases from 0 at the equator to 90 at the poles.

8 For more information, go to: f_pal.htm. What Types of Scientific Equipment are Used in the Paleomagnetism Laboratory? The centerpiece of the paleomagnetism laboratory is the 2G Enterprises 750-R superconducting cryogenic magnetometer (Figure 9) that is used to measure the orientation of the magnetic field preserved in rock or sediment core samples. Once a core sample is placed on the magnetometer s track, it is passed repeatedly through the central chamber where the magnetic field sensors reside. While operating, shields around the cryogenic magnetometer virtually eliminate the effects of the Earth s present magnetic field. The cryogenic magnetometer can measure and record the weak magnetic field preserved within the rock or sediment sample. The paleomagnetism laboratory contains other pieces of equipment, such as a Schonstedt alternating field demagnetizer for demagnetizing small samples of rock or sediment, and a Schonstedt thermal demagnetizer that is used for thermal demagnetization of dry rock samples over a temperature range of 0 C to 800 C. For a more detailed list of equipment that is used in the paleomagnetism laboratory, you can check out the following IODP web page: paleomagnetism. What Do the Paleomagnetism Laboratory Data Look Like, and Can Teachers and Students Use Them? Data collected during Ocean Drilling Program (ODP) and IODP expeditions are made available to the public one year after the completion of each cruise. The raw paleomagnetic data generated during ODP and IODP are stored in an Oracle-based database (Janus) on the following website: Paleomagnetic data archived in the Janus database consist mostly of measurements made on the archive halves of whole cores or on discrete samples with the shipboard cryogenic magnetometer (see cryomagnetometer data query). Some of these data are magnetic field intensity and magnetic directions, such as inclination (dip) and declination (azimuth) to the magnetic poles. Such directions can be used to determine original paleomagnetic pole positions or magnetic reversal sequences. The Janus database also contains orientation tool data for some sediment cores (see tensor tool and core orientation data queries) as well as data used to analyze the direction of stable magnetic orientation within a sample (see zplot data query). Paleomagnetism data were collected during most ODP cruises. Teachers and students interested in finding out more about the research conducted in the JOIDES Resolution s

9 paleomagnetism laboratory during ODP and IODP can access the Proceedings of the Ocean Drilling Program at (click on Initial Reports or Scientific Results ) and the Proceedings of the Integrated Ocean Drilling Program at (click on Scientific Publications ). Lists of peer-reviewed scientific papers organized by expedition that describe the results of shore-based paleomagnetism studies can be found for IODP at: and for ODP at: In addition, a database of over 20,000 citations related to DSDP, ODP, and IODP research can be accessed at: Where Can Students and Teachers Find More Resources About Paleomagnetism? Several interesting Web sites are designed for students and teachers who are interested in paleomagnetism and plate tectonics. Two examples are included for your consideration. The Dynamic Earth: The Story of Plate Tectonics, by W.J. Kious and R.I. Tilling, United States Geological Survey (USGS), is located at: dynamic.html. The site provides a comprehensive overview of the plate tectonics theory, including sections on the historical perspective, developing the theory, understanding plate motions, hotspots, mantle thermal plumes, some unanswered questions, and plate tectonics and people. A Teacher s Guide to the Geology of Hawaii Volcanoes National Park, at und.nodak.edu/vwdocs/vwlessons/plate_tectonics/introduction.html, is a site sponsored by the University of North Dakota. This web site contains an overview of plate tectonics; a teacher s guide, including lessons for students of different grade levels from K-12 with key concepts; lesson outcomes; and a list of references about plate tectonics. References: Cande, S.C., and Kent, D.V., Revised calibration of the geomagnetic polarity timescale for the Late Cretaceous and Cenozoic. Journal of Geophysical Research 100: Cox, A., and Hart, R.B., Plate Tectonics: How it Works: Boston, MA (Blackwell Scientific Publications). Heirtzler, J.R., Le Pichon, X., and Barron, J.C., Magnetic anomalies over Reykjanes Ridge. Deep Sea Research 13:

10 McElhinny, M.W., and McFadden, P.L., Paleomagnetism: Continents and Oceans: San Diego, CA (International Geophysical Series, Volume 73, Academic Press) Shipboard Scientific Party, Leg 191 summary. In Kanazawa, T., Sager, W.W., Escutia, C., et al., Proc. ODP, Init. Repts., 191: College Station TX (Ocean Drilling Program, Thurman, H.V., and Burton, E.A., Introductory Oceanography (9 th ed.): Upper Saddle River, NJ (Prentice Hall). Vine, F.J., and Matthews, D.H., Magnetic anomalies over oceanic ridges. Nature 199: Created by 2004 Teacher-At-Sea Jonathan A. Rice, Ph.D., Teacher At Sea: Expedition 301 (USA), in collaboration with the following IODP Expedition 301 scientists and staff: Will Sager, Ph.D., Paleomagnetist (USA); Trevor Cobine, Marine Laboratory Specialist (Australia); and Gerardo Iturrino, Ph.D., Logging Staff Scientist. Document design by Carl Brenner and Jim Murray, USIO LDEO. Cover photograph by William Crawford, Imaging Specialist, USIO TAMU.