Rock Magnetism and Paleomagnetism of Some North Pacific Deep-Sea Sediments H. P. JOHNSON* ] H. KINOSHITA* f Department of Oceanography and Geophysics Program, University of Washington, Seattle, Washington 9819S R. T. MERRILL J ABSTRACT INTRODUCTION is primarily depositional remanent mag- netization (DRM), either acquired during Detailed paleomagnetic and rock mag- The study of remanent magnetization in initial settling or on late compaction, al- netic studies have been conducted on eight deep-sea sediments has yielded extremely though the evidence for chemical changes deep-sea cores from the North Pacific. valuable results, particularly with respect to affecting the remanence in some cores has Magnetic studies include alternating field the history of sedimentation and the history been recognized (Harrison and Peterson, demagnetization, thermal demagnetization, of the Earth's magnetic field. Analyses of 1965). anhysteretic remanent magnetization magnetic polarity have provided important This work provides additional studies, magnetic hysteresis measurements data on the reversal chronology (Watkins paleomagnetic data to that already ob- over a variety of different temperatures, and Goodell, 1967; Opdyke and others, tained for the North Pacific (Opdyke and viscous and drying effects, strong field ver- 1966), analyses of magnetic transition Foster, 1970). However, its main purpose is sus temperature measurements, x-ray dif- zones have provided data on the behavior to illustrate the types of problems, particu- fraction, and x-ray fluorescence analyses. of the Earth's magnetic field during rever- larly rock-magnetic problems, that can be Six of the eight cores studied contain an sals (Harrison and Somayajulu, 1966), and encountered in attempts to determine the abundance of fossils, particularly statistical analysis of data from numerous paleomagnetic field from measurements of silicoflagellates, and appear to have ac- cores have provided valuable information deep-sea sediments. Some of these problems quired their remanent magnetization concerning the hypothesis that the Earth's may lead to erroneous conclusions regard- sufficiently close to the surface to reliably main magnetic field averages to a dipole ing reversal events, stratigraphie correla- record the Earth's paleomagnetic field. The field over a few thousands of years (Opdyke tions, age determinations, correlatons be- remaining two cores do not contain fossils and Henry, 1969). Through the use of the tween reversals and fauna extinctions, and and do not appear to accurately record the magnetic reversal chronology, magnetic other phenomena. We will define some of Earth's paleomagnetic field. Low- measurements have been used for dating of temperature oxidation appears to have oc- sediments and have provided valuable esti- curred in situ in these cores. A gamma mates of the sedimentation rates in some phase (cation-deficient spinel) iron- oceanic regions (Ninkovich and others, titanium oxide with lattice parameter of 1966; Opdyke and Foster, 1970) and even 8.38 A and Curie temperature of 545°C given information on possible paleocurrent near the top of the cores changes with depth systems (Watkins and Kennett, 1971). to a gamma phase with lattice parameter of Speculations have linked magnetic field re- 8.33 A and Curie temperature near 600°C versals with climatic changes and (or) close to the bottom of the cores. These faunal changes (Hays and Opdyke, 1967; chemical changes appear to be associated Wollin and others, 1971); to date, such with the production of a chemical remanent speculations have not been supported by magnetization that makes it impossible to theoretical considerations (Black, 1967). use these cores for paleomagnetic studies. In spite of these numerous uses of the This work summarizes many of the problems remanent magnetization in deep-sea sedi- in obtaining reliable paleomagnetic results ments, little is known about its origin, or from deep-sea cores, including possible even minerals in which the remanence re- spurious magnetic directions resulting from sides. Studies, such as those of Harrison chemical changes, drying, and coring ef- and Peterson (1965), Haggerty (1970), fects. Key words: paleomagnetic stratig- Kobayashi and Nomura (1972), and Levlie raphy, sediments, rock magnetics, and others (1971) represent important at- paleomagnetism, magnetic mineralogy. tempts to resolve this difficult problem. Figure 1. Location of piston cores in the These studies also suggest the complexity of North Pacific that are used in this study. Cores 1 * Present address: (Johnson) Cooperative Institute for the problem: remanence may be acquired through 8 respectively correspond to cores Research in Environmental Sciences, Department of by different minerals and in different ways Geology, University of Colorado, Boulder, Colorado TT2803, TT2804, TT2814, TT2817, TT2819, 80302; (Kinoshita) Meteorological College, Asahi-Cho, for different geographic areas of the oceans. TT2822, TT2823, and TT2824 used in a Kashiwa, Japan. Most studies have assumed the remanence paleontological study by Ling (1970). Geological Society of America Bulletin, v. 86, p. 412^20, 11 figs., March 1975, Doc. no. 50317. 412 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/86/3/412/3433772/i0016-7606-86-3-412.pdf by guest on 26 September 2021 MAGNETISM AND PALEOMAGNETISM OF NORTH PACIFIC DEEP-SEA SEDIMENTS 413 these problems and discuss the holes, some types of mottling, or nonparal- not amount to more than a few degrees, but paleomagnetic results from eight North lel beds. These observations alone support they generally do increase with a decrease Pacific cores. Remanence in two of these Opdyke's (1972) conclusion that a "rever- in the intensity of the sample. cores does not appear to accurately record sal event" in a single core must be regarded the Earth's magnetic field, whereas that in with considerable suspicion. Once the cores SAMPLING AND EXPERIMENTAL six of the cores appears to accurately rep- are brought up onto the ship, they are split PROCEDURES resent the Earth's magnetic field. Detailed and either are allowed to dry or are stored rock-magnetic studies will be presented to wet at a few degrees above freezing. Either Eight piston cores containing red clay determine why these two groups of cores method may result in altering the rema- were collected from the North Pacific (Fig. manifest such different magnetic behavior. nence. We will show later that a remanence 1) and stored without drying at a tempera- can be accquired when samples are dried in ture near 3°C. Cores 1 and 2 do not contain PROBLEMS ENCOUNTERED IN an external field. Alternately, depending on fossils and are hereafter referred to as the ATTEMPTS TO DESCRIBE THE the amount of liquid fraction and the grain non-fossil-bearing cores or NF cores. Cores PALEOMAGNETIC FIELD USING size and shape distributions, vibrations of a numbered 3 through 8 contain an abun- DEEP-SEA SEDIMENT CORES wet core during transport may result in dance of fossils (silicoflagellates, diatoms, realignment of some of the magnetic grains. and radiolaria) and are referred to as the F Fortunately, in most paleomagnetic Other experimental procedures usually cores (fossil-bearing cores). Cubical sam- studies one is concerned primarily with de- will not result in large changes in the direc- ples were taken at 10-cm intervals down termining the ancient field direction rather tions of the remanence. Cubical samples each of the cores. Remanent magnetization than determining the ancient field intensity. were taken from the cores and sealed in was measured with a Schönstedt spinner Although it may be possible to obtain reli- plastic boxes with epoxy. This sealing usu- magnetometer. Alternating field (AF) de- able field intensities from a few extrusive ally inhibits drying so that one does not en- magnetization was conducted in a non- igneous rocks by using Thellier's technique counter the problems discussed above. magnetic space with the use of a four-axis or some modified version of that technique Measurement errors themselves usually do tumbler system. Unless otherwise stated, (Coe, 1967), there is no known method for obtaining a reasonable estimate of the ac- CORE I CORE 2 CORE 3 CORE 4 tual paleofield intensity from a deep-sea INCLINATION (DEGREES) INCLINATION (DEGREES) INCLINATION (DEGREES) INCLINATION (DEGREES) core. However, in some cases closely spaced measurements on cores from regions of rapid sedimentation may give an indication of the relative change in the intensity of the Earth's field. Kobayashi and others (1971) and Opdyke (1972) briefly discussed some problems in obtaining relative field inten- sities in sediments, and Coe (1967) dis- cussed problems in obtaining absolute in- tensities in igneous rocks. Two general classes of errors can result in significantly erroneous paleomagnetic field directions: errors in determining how and when the remanence was acquired by a core, and experimental errors associated with accurately retrieving the directional in- formation from a core. We will not elabo- rate on the first class of errors in this sec- tion, except to note that the usual assump- tion is that the remanence is acquired paral- lel to the Earth's field at the time of deposi- tion. If this assumption is incorrect — for CORE 5 CORE 6 CORE 7 CORE 8 example, if a chemical remanent magnetiza- INCLINATION (DEGREES) INCLINATION (DEGREES) INCLINATION (DEGREES) INCLINATION (DEGREES) tion (CRM) is acquired at depth — then an erroneous