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 magnetization (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

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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 intensities in sediments, and Coe (1967) discussed problems in obtaining absolute intensities 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 information from a core. We will not elabo- rate on the first class of errors in this section, except to note that the usual assumption is that the remanence is acquired parallel to the Earth's field at the time of deposition. 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 paleofield direction probably will be obtained. This subject will be discussed at length with regard to our own studies later in this paper. One of the largest sources of experimental errors seems to be associated with the coring procedure. Usually piston coring is used, rather than gravity coring, to obtain longer cores. This method not only commonly disturbs the upper part of the core, but it can occasionally invert the order of flat-lying beds (Burns, 1963). Often, but not always, disturbance of part of the core due to coring or to reworking by organisms Figure 2. Inclination data for the 8 cores shown in Figure 1 after partial AF demagnetization at 150 can be detected either by x-ray radiograph Oe. Cores 1 and 2 are referred to as NF cores, cores 3 through 8 are referred to as F cores. Declination measurements or are visible to the eye as data are consistent with the inclination data. Measurements were made every 10 cm down the core.

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were obtained with a Norelco x-ray diffrac- few million years appear to occur close to tometer with a graphite-focusing mono- reversal boundaries (Hays, 1971). Whether chromator that eliminates iron or not there is a cause and effect relation be- fluorescence. X-ray fluorescence experi- tween reversals and extinctions of some ments were conducted using an energy dis- species is controversial (Hays, 1971; Black, persive x-ray fluorescence system with a 1967). It seems to us that one of the major 1,024-channel analyzer. hurdles still present in demonstrating a significant correlation between reversals PALEOMAGNETIC RESULTS and extinctions is the necessity of showing the absence of hiatuses or significant de- Based on results of stepwise AF demag- creases in sedimentation rates near reversal netization measurements conducted on boundaries. Such changes would have the Figure 3. AF demagnetization of the NRM in every sample from core 2 and selected sam- effect of bringing a reversal and an extinc- the unstably magnetized core 1 compared to AF ples from the other cores, a 150-0e peak al- tion closer together in the core. However, demagnetization of an IRM and ARM given to ternating field was selected to demagnetize estimates of the time taken for reversal the same sample in the laboratory. each sample from every core. Using 0.69 transitions from measurements of deep-sea anhysteretic remanent magnetization m.y. as the age of the Brunhes-Matuyama cores (Opdyke, 1972) are somewhat longer (ARM) experiments were conducted in a boundary, the sedimentation rates for the F (but perhaps not significantly so) than those 0.5-0e constant field and a peak alternating cores (except core 4, which is too short for obtained from igneous rocks and statistical field of 800 Oe. Alternating fields with peak proper analysis) varied from 3 to 11 arguments (Cox and Dalrymple, 1967). values higher than 800 Oe sometimes re- mm/1,000 yr. These rates are consistent Resolution of this controversy appears to sulted in nonreproducible ARMs, for with the silicoflagellate data of Ling (1970) require additional data. reasons we still do not understand but that taken from the same cores and are reason- The remanence in the NF cores is not as may be related to rotation of sediment ably consistent with those rates determined stable as in the F cores, and reversals appear grains in the higher fields. Fortunately, by paleomagnetic studies of different cores unpredictably in them (Fig. 2). Although 800-0e demagnetization always appeared by Opdyke and Foster (1970). The extinc- not shown in Figure 2, relative declination to reduce the remanence substantially tions of three ilicoflagellate species was also measured and shows analogous below measurable values so that virtually (Dictyocha cf. ausonia, subarctios and behavior. Similar results have been previ- no natural remanent magnetization (NRM) Mesoscena cf. elliptica) appear to occur ously found (Opdyke and Foster, 1970). component remains after a sample is given near the Brunhes-Matuyama reversal Core 1, an NF core, appears to be unstably an ARM. boundary (Ling, 1970, and personal com- magnetized throughout (a typical AF de- Strong magnetic field versus temperature mun.), an occurrence that may or may not magnetization curve is shown in Fig. 3). measurements (J-T) and hysteresis mea- be coincidental. The majority of radiolarian Samples from the top of core 2 of the NF surements were made with a Princeton Ap- extinctions that have occurred in the last cores are slightly less stably magnetized plied Research Vibrating Magnetometer. Several techniques were tried to obtain pre- ARM (RELATIVE INTENSITY) cise magnetic ordering temperatures. 0 1.0 2.0 3.0 0 0.5 1.0 1.5 Magnetic separates were given an isother- o T mal remanent magnetization (IRM) in a field of 1,500 Oe and subsequently were heated in a vibrating sample magnetometer in a constant field. Best results were obtained by using a moderate field between 35 200 and 1,000 Oe during heating. This moderate field optimized the Hopkinson effect and minimized the relative contribution from the paramagnetic component. Heat- ing was occasionally done in dry 02 for 400 special experiments; otherwise it was done in a vacuum (10~3 torr) with nitrogen or helium as the residual gas. Great care x t- should be taken in making and interpreting 0. J-T runs, because it is often easy to alter the UJ a 600 mineralogy on heating to produce new magnetic minerals (particularly magnetic spinels, Johnson and others, 1972). Nearly 50 J-T measurements were made in this study using these methods at heating rates 800 of 10°C per minute. Thermal demagnetization and drying remanent magnetization studies were made in a nonmagnetic space (±50 -y), for which the quality was main- tained by an automatic feedback system. 1000 Magnetic separations were made by a procedure similar to that described by Figure 4. Relative intensity of ARM down cores 3 and 2. A similar large increase in the relative Lesvlie and others (1971). X-ray patterns intensity of ARM with depth is found in core 1. No trends were observed in any of the F cores.

than most samples from the F cores, and the in magnetic behavior (Harrison, 1966). INTENSITY stability decreases with depth. The NRM in However, these techniques may overem- this core at a depth of 1 m is reduced by 40 phasize the role of large grain size particles. percent by demagnetization in a peak field Because it is now widely accepted that the of 100 Oe, whereas that at 9 m is reduced most stable remanence resides in small par- by 70 percent after similar AF demagnetiza- ticles, we have used ARM. However, even tion. The NRM direction changes little for though ARM has been reported to have these samples on AF demagnetization to similar stability properties as TRM (as de- 100 Oe, and if AF demagnetization criteria termined by AF demagnetization; Nagata, alone were used to estimate stability, this 1961), there are some significant differences core would be judged suitable for between ARM and TRM. For example, the paleomagnetic studies. Yet the apparent absolute ARM intensity of magnetite sam- chaotic variation of directions of the NRM ples (0.5-0e steady field, 2,000-0e alternat- and demagnetized remanence between ing field) examined by Johnson and Merrill samples (Fig. 2) suggests that something is (1972) was considerably lower than that of amiss. We will show later that chemical TRM acquired in the same field. The prob- changes are apparently responsible for pro- lem is compounded by the fact that we do ducing these unwanted effects. not know a priori the origin of the remanence in deep-sea cores. Surprisingly, we ANHYSTERETIC REMANENT found that AF demagnetizations of ARM MAGNETIZATION and IRM for these cores are often quite similar (Fig. 3). ARM was given to samples taken at Figure 4 shows ARM results down core 2 10-cm intervals down several of the cores to (a NF core) along with results from core 3 detect how the ability of the core to acquire (a F core). The increased magnitude of the remanence varies with depth. Previously, fluctuations of the intensity of the ARM initial susceptibility and isothermal rema- down core 2 probably is a manifestation of nent magnetization (IRM) measurements the decrease in stability with depth. Mea- have been used to distinguish changes surements of the water volume in core sam- peak field of 150 Oe compared to the same inten- ples increased with depth down the NF sity normalized by dividing by 0.10 x ARM intensity for core 6. cores. Therefore, the ARM increase in NF 5+ cores is not a compaction effect. The two- intensity, we divided the demagnetized (at to threefold ARM increase in the mean 150 Oe) NRM of core 3 by the intensity of ARM with depth in core 2 is a manifesta- the ARM multiplied by 10. Results are tion of an increase in the core's ability to shown in Figure 6. This assumes that ARM acquire remanence with depth. Figure 5 is acquired by the same minerals as the shows results for ARM given in different . NRM. Thus, an increase in the intensity of steady fields to several samples from core 2. NRM due to an increase in the amount of Similar results were found for core 1. These magnetic minerals in a sample should be de- data demonstrate that the use of biasing tected and removed by the above procedure fields larger than 0.5 Oe to give ARM will because the ARM will also similarly in- result in magnifying magnetic variations crease. The apparent lack of improvement that exist in the cores. However, possible obtained by using this procedure in the F grain-size effects associated with ARM cores suggests that the intensity fluctuations must be better resolved before an optimum in this core are not primarily due to varia- field can be determined (Johnson, 1972; S. tions in the amount of magnetic material Levi, 1973, personal commun.). These represent. An accurate measure of relative in- sults indicate that either there has been a tensity can only be obtained for a given sec- change in the source of the material, includ- tion of a core if (1) there are no significant ing changes due to variation in sedimenta- trends in chemical effects and magnetic sta- tion rates (that is, an increase in the fraction bility down the section, (2) there has not of lithogenic sedimentary rocks due to a de- been substantial mixing of the sediment in crease in the sedimentation rate of biogenic the section, and (3) there are no significant sediments), or that in situ chemical changes overlaps of normal and reversed polarities that alter the magnetic mineralogy have oc- in a sample. If these conditions are ever de- curred in the NF cores. This last explana- monstrated to exist, then ARM acquisition tion is the most pleasing because it also should be a useful method for obtaining re- provides a possible explanation for differ- lative intensities. ences between F cores and NF cores. Sedimentation accumulation rates in the DRYING REMANENT CONSTANT BIAS FIELD (Oe) F cores are sufficiently slow that each of our MAGNETIZATION samples averages over most major Figure 5. ARM intensity at 3 different levels (60 cm, 300 cm, and 450 cm depth) in core 1, a fluctuations of the Earth's magnetic field. To carry out the thermal demagnetiza- NF core, as a function of the intensity of the con- Nevertheless, to see in principle whether tion experiments described later, samples stant bias field. The peak alternating field used relative intensity measurements could be had to be dried and the magnetic effects as- was 800 Oe. improved upon by normalizing with ARM sociated with drying assessed. It is also im-

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portant to determine these effects because and in some cases the viscous component is a given F core and between F cores. Figure 8 some scientists routinely dry their cores on of similar magnitude to the drying compo- shows a typical example from core 3. This shipboard. The magnetic effects resulting nent. No clear differences were observed curve provides some indication of the accu- from this latter situation probably should for drying effects between F and NF cores. racy of a single Curie temperature determi- also be determined directly by measuring The drying remanent magnetization was nation. The value quoted above represents samples from the core immediately after it generally softer than the NRM. This may a mean of Curie temperatures obtained has been brought up to compare with be typical of most deep-sea cores; Opdyke from our best data (runs made at high fields measurements on samples taken after dry- (1972) argued that if drying effects exist, were not used because a large paramagnetic ing has occurred. In our case, we dried then they must be easily removable by AF component made it impossible to select a samples in a controlled 0.5-0e field for sev- demagnetization because inclinations close Curie temperature within several tens of eral days. The applied field was always in to the Earth's field have been obtained for degrees). Standard deviation from this the opposite hemisphere from the NRM. recent sediments after demagnetization. mean amounted to only a few degrees. If After drying occurred, the samples were in- Nevertheless, Figure 7 illustrates that some- this Curie temperature is associated with a verted and stored in the same field for a times a drying remanence can be acquired pure Fe-Ti oxide phase, then it is consistent time equal to that used for drying to assess that can only be removed by AF demagneti- with a titanomagnetite with roughly 15 viscous magnetization effects. In addition, zation to 200 Oe or higher. Similar results percent ulvospinel in solid solution, a some samples were dried in a nonmagnetic are found for samples that have not been titanomaghemite with slightly less titanium space. previously demagnetized. Because 200 Oe than the above titanomagnetite, or a The effects of drying are (1) reduction in is as high as, or higher than, one usually titanohematite with roughly 15 percent NRM intensity, (2) a rotation of the total demagnetizes deep-sea core samples, drying ilmenite in solid solution (Nagata, 1961). remanence vector, and (3) an acquisition of remanence may account for some spurious However, the list of possible minerals in- a drying remanent magnetization. In addi- magnetic directions. Recently, Lavlie creases dramatically if cations other than tion to direct effects of drying, the cores (1974) independently found a drying rema- iron and titanium are present. commonly acquired a viscous remanent nence with laboratory experiments on a re- J-T curves for core 1, the NF core that is magnetization. Effects (1) and (2) are present deposited deep-sea core. unstably magnetized throughout, are simi- if the sample is dried in a nonmagnetic The acquisition of a drying remanence is lar to those obtained for the F cores. This space and appear to be due, respectively, to probably due to the rotation of magnetic behavior was also found in the top of core partial randomization of the magnetic grains as the core shrinks during the drying. 2. However, samples near the bottom of grains and warping of the entire sample If the sample is isotropic, then the only this core indicated Curie temperatures that during drying. anisotropy present in the situation is the appeared slightly higher than 600°C. Ex- Figure 7 illustrates some of the properties magnetic field. One therefore anticipates an amples of data for the top and bottom of of the drying remanence for a sample from acquisition of a remanence that is statisti- core 2 are also shown in Figure 8b and 8c. core 2. In this case, the viscous component cally parallel to the applied field. There are This Curie temperature is too high for is small relative to the drying remanent several possible reasons why the drying re- titanomagnetite, but it is compatible with a magnetization. This is variable, however, manence usually appears to be softer than cation-deficient spinel (it is now well the NRM in these cores. One possibility is documented that some cation-deficient that the stable remanence is held by shape spinels do not convert on heating to rhom- anisotropy in long, thin particles that may bohedral phases, possibly due to small not rotate as much during drying as do amounts of foreign cations) or a more symmetrical particles with softer titanohematite. This high Curie tempera- x 80 ¿VISCOUS FIELD APPLIED remanence. Another possibility is that the ture phase is apparently not present in core 1. soft remanence is held by smaller particles Hysteresis loops for several samples from (that are close to the superparamagnetic- ¡5 * cores 2 (a NF core) and 3 (a F core) were K \ single-domain size threshold) of different measured at -196°C, 0°C, 200°C and ^ 60-- \ \ mineralogy than that which holds the hard 400°C in an 8 kOe field (Fig. 9). A kink in remanence and that these smaller particles the hysteresis curve for core 3 occurs near * " \ rotate more upon drying. Several other § 40- 0°C and becomes fairly pronounced at N variations of this general theme exist. On — 196°C. This kink is not obvious in any of the other hand, future work may indicate the samples from the NF core but appears 8 •• \ that drying remanence is typically of higher 20 • - ^ ^ ^ to be always present in samples from the F stability than we have found in most of our DEMAGNETIZED RM core. This suggests the possible presence of v* • samples. two phases in the F core. However, this -I 1 1 1 1- does not necessarily imply that there are at 0 25 50 100 200 J-T AND HYSTERESIS least two distinct magnetic mineral compo- PEAK AF DEMAGNETIZATION FIELD (Oe) MEASUREMENTS sitions present — two phases of identical Figure 7. Drying remanent magnetization chemistry but different physical properties given to a sample of core 3 that has undergone Initial J-T runs were carried out in helium could also produce this effect. Even if pres- AF demagnetization to 600 Oe. About 20 per- and showed pronounced irreversible ent, the possible second phase does not ap- cent of the drying remanence appears to be of characteristics, probably due to the pres- pear to contribute much above room temp- viscous origin. Although the remanence is softer ence of water (Johnson and others, 1972). erature, and therefore it would not seri- than the NRM, as indicated by the fact that 100 However, when run in a vacuum (10~4 torr) ously affect the remanence. Oe AF demagnetization is sufficient to reduce the with nitrogen as the residual gas, the J-T Saturation typically occurs below 5 kOe remanence of the sample to that it had after initial demagnetization at 600 Oe, the drying rema- curves for the F cores are essentially rever- at room temperature, and the ratio of sat- nence does have components with stability great- sible and indicate clearly only one Curie uration remanence to saturation magnetiza- er than 50 to 150 Oe, fields that are typically temperature, near 545°C. This 545°C Curie tion is usually an order of magnitude less used to demagnetize deep-sea sediments. temperature appears invariant both within than the 0.5 value expected for single-

domain grains. Therefore, super- versely magnetized) of an F core (core 3) are imity to each other, samples 2.5a and 2.5b paramagnetic and (or) multidomain grains shown in Figure 10. A slight viscous com- of Table 1, have different manganese con- are probably common in both the F and NF ponent is present, as is most apparent in the tents, whereas their magnetic stabilities, di- cores (Neel, 1955). reversed samples, and the remanence ap- rections, and intensities are essentially iden- pears to lie in a phase that has a Curie point tical. Therefore, there does not appear to be THERMAL DEMAGNETIZATION consistent with that obtained from J-T a direct cause-and-effect relation between measurements. manganese content and stability. As is the Numerous stepwise thermal demagneti- Figure 11a and lib shows typical ther- case with igneous rocks, it is doubtful that zation runs of NRM yielded significant dif- mal demagnetization from the top and bot- bulk chemical composition of a sample will ferences in magnetic behavior both between tom of a NF core (core 2). Both NF cores yield much detailed informaton on the rock cores and along some cores. In the case of appeared to behave similarly on thermal magnetics of that sample. The values in the NF cores, for which we believe the demagnetization. Samples from the top of Table 1 are all normalized to those ob- thermal demagnetization data are valuable, the core shown in lib appeared to have tained at the top of the core. runs were made with samples that were remanence that could lie in the mineral phase dried in a nonmagnetic space (±50 y) to with the 545°C Curie temperature indicated X-RAY DIFFRACTION eliminate any drying remanence. by J-T measurements. Samples from the RESULTS Typical data for samples from the top bottom of the core have a distribution of (normally magnetized) and bottom (re- blocking temperatures that are mostly be- The general procedures used to obtain tween 50°C and 150°C. However, J-T x-ray diffraction data were described ear- measurements indicate an increase in the lier. However, it is desirable to comment highest Curie temperature with depth in here on the accuracy of our methods. Al- these same cores. If th« remanence is in the though a few workers claim to have distin- mineral phase with a Curie temperature guished the lattice parameter of a naturally above 600°C, then its blocking tempera- occurring magnetic spinel accurately to tures are atypically hundreds of degrees three decimal points (Leivlie and others, below its Curie temperature. Alternatively, 1971), others, such as Harrison and Peter- it is possible that there is a second mineral son (1965), found the lattice parameter var- phase present with a much lower Curie ied by 0.04 A, depending on the beam angle temperature. A sample from the bottom of used. The problem can be complicated be- core 2 was given an ARM and thermally cause there sometimes appear to be small demagnetized (Fig. 11c). Unlike the thermal chemical variations in the magnetic miner- demagnetization data of NRM, the block- als in adjacent core samples. In any case, ing temperatures appear to lie mostly above 200°C. The phase holding the ARM appears to have a Curie point above 500°C. An increase in the amount of this phase down the NF cores would account for the increase in ARM down the cores. Three adjacent samples from the middle of NF core 2 were thermally demagnetized. These samples showed transitional behavior between those found at the top and bottom of the core. However, large variations existed between these samples, indi- cating that the observed magnetic trends already described are nonuniform. Moderate directional changes were commonly observed on thermal demagnetization of NRM from the NF cores, although no reversals were observed. On the other hand, no significant directional changes were observed on thermal demagnetization of the F cores.

X-RAY FLUORESCENCE ANALYSIS TEMPERATURE °C Figure 8. Strong field versus magnetization Results of x-ray fluorescence analysis measurements for (a) typical F core sample and down core 2 are shown in Table 1. Hag- (b) sample from the top of core 2 (NF core) and gerty (1970) suggested an inverse relation Figure 9. Hysteresis loops for a sample from (c) near the bottom of core 2. The method used between manganese content and magnetic to obtain estimate of the Curie temperature, Tc, core 3 at liquid nitrogen temperature (—196°C), stability. Table 1 shows that in core 2, for was to pass a straight line through the bottom 30°C, 200°C, and 400°C. The scale for relative section of the curve, as graphically indicated. which the stability decreases with depth, intensity at the temperature of liquid nitrogen is Heating and cooling were done in a vacuum with there is the suggestion that the manganese expanded relative to that used at higher tempera- nitrogen as the residual gas. Curves obtained on does increase with depth as predicted. tures to better illustrate the kink in the hysteresis cooling were essentially reversible. However, two samples taken in close prox- loop.

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because we used identical procedures on ad- DISCUSSION OF THE perature than magnetite is present. That jacent samples from the same core, we ROCK-MAGNETIC RESULTS conclusion now requires modification to believe that the values quoted here have a allow for significant numbers of vacancies precision (not accuracy) of roughly ±0.01 Not only does the magnetic mineralogy or cations other than Fe and Ti in the A. Lattice-parameter calculations are based of deep-sea cores appear to vary consider- lattice. on four diffraction peaks, with the excep- ably for different geographical locations, The origin of this spinel is still somewhat tion of core 1 where only three peaks were but it sometimes can vary significantly with puzzling. Even if there are cations present used. depth in a given core. The complexity of the other than Fe and Ti, the apparent low Spinel samples with lattice parameters magnetic minerals in deep-sea sediments titanium content suggested from the high that varied between cores from 8.37 A to has been previously suggested by Haggerty Curie temperatures indicates that this spinel 8.39 A were found in the tops of all cores. (1970), who made polished samples of With the exception of core 2 (a NF core), cores from the Atlantic, Pacific, and Indian there was no observable change in this lat- Oceans. He found that these sediments con- tice parameter with core depth. In core 2, tained numerous opaque minerals, includ- the lattice parameter decreased from 8.38 A ing all the iron-titanium oxides and sulfides at the top of the core to 8.33 A at the bot- that commonly carry remanence in igneous tom. rocks. In addition, he suggested that miner- The only pure iron-titanium oxide that als such as birnessite, todorokite, and ran- has both the lattice parameter of 8.38 A cieite, which are common to manganese and a Curie temperature near 545°C is a nodules but have been little studied for their titanomaghemite (oxidation parameter magnetic properties, might contribute near Z = 0.7), formed by low-temperature significantly to the remanence. Even though oxidation of a titanomagnetite with 15 mol it now appears unlikely that these minerals percent ulvospinel in solid solution carry the remanence in manganese nodules (O'Reilly and Readman, 1971). Any alter- (Carpenter and others, 1972) and may not native to a cation-deficient spinel would re- even be capable of ever carrying remanence quire the presence of cations in addition to (Goodenough, 1966), Haggerty's implicit iron and titanium. In any case, the decrease suggestion that some undetected phase(s) in lattice parameter with depth in core 2 may be responsible for carrying the rema- can be explained if further low-temperature nence in deep-sea cores is still a possibility. oxidation of this spinel to a more In spite of this complexity, it may still be cation-deficient phase has occurred (Prevot, possible to divide the oceans into provinces 1971). Such oxidation could reduce the for which crude estimates of the magnetic NRM intensity and could result in an in- minerals carrying the remanence can be creased ability to hold ARM (Johnson and given. Merrill, 1972, 1973). Unfortunately, small We suggest that the North Pacific cores changes in the lattice parameter and Curie appear to fall into two general classes: F temperature (within our experimental cores and NF cores. As previously found error) can make relatively large changes in (Opdyke and Foster, 1970), the F cores ap- the oxidation state (Z parameter) for our pear to record accurately the paleomagnetic samples that have low titanium contents field, whereas the NF cores do not. How- (O'Reilly and Readman, 1971). ever, one reservation that must be ex- pressed in this regard is that there may be significant changes in sedimentation rate, including hiatuses, in the F cores. The magnetic mineral that chiefly holds the remanence in the F cores appears to be a magnetic spinel with a Curie temperature near 545°C and lattice parameter near 8.38 i§s A. These values are consistent with a S 60 titanomaghemite formed through low- temperature oxidation of a titanomagnetite with 15 mol percent ulvospinel in solid \ solution. This interpretation differs slightly ¡ë from previous interpretations of similar 200 300 400 500 600 20 cores (Lavlie and others, 1971). The x-ray TEMPERATURE °C data of Lavlie and others (1971) indicates Figure 11. Thermal demagnetization data for the presence of pure magnetite (the lattice NRM of a sample from the top (a) and bottom o 100 200 300 400 500 600 parameter reported is 8.396 ± 0.008 A, (b) of core 2, a NF core. Line c shows thermal TEMPERATURE °C an identical value to that given for pure demagnetization data for an ARM from the bot- Figure 10. Typical thermal demagnetization magnetite). The interpretation of their J-T tom of core 2. This figure shows the increase in data for the F core. The sample taken from bot- data, which were obtained by heating in air, blocking temperatures with depth in the core. tom of core was reversed in polarity, and the Figure 11c and Figure 3 also indicate that the is difficult because of probable chemical sample from top was normal. The increase in in- NRM and the ARM do not reside in identical tensity observed for the first two demagnetiza- changes (Johnson and others, 1972; Fig. 2, sites in the bottom of core 2. The question marks tion treatments of the bottom sample is due to Lovlie and others, 1971), but it does sug- indicate intensities of magnetization that are removal of normal viscous component. gest that a spinel with a lower Curie tem- close to the noise level of the magnetometer.

probably did not originate from oceanic (Johnson and Merrill, 1972, 1973). Al- REFERENCES CITED basalts. This follows because the quenched though the cores we examined have mag- surfaces of these basalts, which would be netic spinels with low ulvospinel content, Black, D. I., 1967, Cosmic ray effects and faunal the first to weather, contain titanomagne- there may be quite different titanium con- extinctions at geomagnetic field reversals: tite with Curie temperatures that are typi- tent in other areas of the Pacific where Earth and Planetary Sci. Letters, v. 3, p. cally less than 250° to 300°C (correspond- weathering of oceanic basalt may contrib- 2433-2448. ing to more than 50 mol percent ulvospinel ute significantly to the sediments. It is of in- Burns, Robert E., 1963, A note on some possible in magnetite). Our speculation is that the terest that many of the titanomagnetites in misinformation from cores obtained by piston-type coring devices: Jour. Sed. Pe- oceanic basalt probably contain ulvospinel spinel minerals in the F cores have been trology, v. 33, p. 950-952. wind transported from terrestrial sites, contents that place them in the composition Carpenter, Roy, Johnson, H. P., and Twiss, E. S., which is consistent with the origin of North range where self-reversal may occur on or- 1972, Thermomagnetic behavior of man- Pacific sediments (Windom, 1969). Regard- dering and oxidation (Verhoogen, 1956; ganese nodules: Jour Geophys. Research, v. less of its origin, the remanence appears to O'Reilly and Banerjee, 1966). Unfortu- 77, p. 7163-7174. be acquired slightly below the sea floor in nately, this is not the only possibility where Coe, R., 1967, The determination of paleointen- the F cores, as can be seen from the direc- the specter of self-reversal occurs for deep- sities of the Earth's magnetic field with em- tional information shown in Figure 2. sea sediments. Kobayashi and Nomura phasis on mechanisms which could cause nonideal behavior in Thellier's method: The situation for the NF cores of the (1972) have shown that pyrrhotite, a mineral that sometimes exhibits self-reversing Jour. Geomagnetism and Geoelectricity, v. North Pacific is quite different, and it 19, p. 157-180. properties, carries a CRM in the sedimen- definitely appears that in situ chemical Cox, A., and Dalrymple, G. B., 1967, Statistical tary deposits of the Japan Sea. changes have altered the remanence. Oxi- analysis of geomagnetic reversal data and dation of the magnetic minerals with depth In any case, any CRM that is acquired at the precision of potassium-argon dating: appears to reduce the intensity of NRM and depth will record the same polarity (or if Jour. Geophys. Research, v. 72, p. to increase the NF cores' ability to acquire a self-reversal occurs, the opposite polarity) 2603-2615. new remanence (ARM or IRM). This ox- that is being recorded by DRM near the Goodenough, John B., 1966, Magnetism and the idized spinel is not detectable by the charac- sediment surface. Clearly such an occur- chemical bond: New York, John Wiley & teristic J-T changes so helpful for rence can produce an erroneous magnetic Sons, Inc., p. 98-109, 144-149. Haggerty, S., 1970, Magnetic minerals in pelagic identification of titanomaghemite in record of the Earth's magnetic field. sediments: Ann. Rept. Geophys. Lab., Car- oceanic basalt samples (Ozima and Ozima, In this paper we have shown various negie Inst., p. 332-336. 1971), probably because unmixing is im- ways by which deep-sea sediments can ac- Harrison, C.G.A., 1966, The paleomagnetism of peded by the low titanium content (Read- quire spurious magnetizations, including deep-sea sediments: Jour. Geophys. Re- man and O'Reilly, 1970) or by the presence components acquired during coring, drying search, v. 71, p. 3033-3043. of cations other than iron and titanium. remanent magnetization, CRM, and possi- Harrison, C.G.A., and Peterson, M.N.A., 1965, A CRM that has a low blocking tempera- ble self-reversals. Although the numerous A magnetic mineral from the Indian Ocean: ture distribution is probably acquired by successful magnetic studies of deep-sea Am. Mineralogist, v. 50, p. 704-712. these cores. This CRM either resides in the sediments suggest that such spurious mag- Harrison, C.G.A., and Somayajulu, B.L.K., 1966, Behavior of the Earth's magnetic field oxidized spinel or in a phase(s) not yet de- netizations usually are not dominant, their during a reversal: Nature, v. 212, p. termined. We can only speculate as to why presence in some cores indicates that a great 1193-1195. the oxidation significantly affects the rema- deal of caution should be used before one Hays, J. D., and Opdyke, N. D., 1967, Antarctic nence in the NF cores and not in the F cores attributes apparently anomalous behavior, radiolaria, magnetic reversals and climatic — perhaps small amounts of organic mate- such as "reversal events" and "excursions," change: Science, v. 158, p. 1001-1011. rial remain with the fossils and act as an to the Earth's field. Finally, we suggest that Hays, James D., 1971, Extinctions and reversals oxygen buffer. the following criteria be satisfied before the of the Earth's magnetic field: Geol. Soc. Apparently maghemization is common to presence of a new magnetic field event or America Bull., v. 82, p. 2433-2448. many deep-sea cores (Haggerty, 1970). excursion is advocated: (1) the event or ex- Johnson, H. P., 1972, An irreversible low temperature transition in magnetite: EOS (Am. Moreover, the careful studies of Harrison cursion should be found in a minimum of two cores taken from the same area to re- Geophys. Union Trans.), no. 11, p. 973. and Peterson (1965) indicate that Johnson, H. P., and Merrill, R. T., 1972, Mag- maghemization can result in the production duce the possibility of erroneous results oc- netic and mineralogical changes associated of a stable CRM in deep-sea cores, a result curring due to coring procedures, (2) a with low-temperature oxidation of magne- consistent with laboratory experiments minimum of two samples should show evi- tite: Jour. Geophys. Research, v. 77, p. dence of the event or excursion, and (3) 334-341. rock magnetic studies should demonstrate 1973, Low-temperature oxidation of a that the samples that show the event or ex- titanomagnetite and the implications for TABLE 1. X-RAY FLUORESCENCE ANALYSES OF CORE 2 cursion are not of different mineralogy paleomagnetism: Jour. Geophys. Research, Distance T1 Mn Fe Comments from those of the rest of the core. The latter v. 78, p. 4938-4949. (m from Johnson, H. P., Kinoshita, H., and Merrill, R. T., bottom) criteria will reduce the possibility of chemical change affecting the paleomagnetic re- 1972, Spinel formation by heating haema- tite in air and water vapour: Nature Phys. 11 1 1 1 1 1 sults. 10 0.957 1.02 1.01 0.99 0.99 Sci., v. 239, p. 151-152. 9 1.05 1.10 1.00 1.03 1.03 8 1.01 1.10 0.90 1.01 1.00 Kobayashi, K., and Nomura, M., 1972, Iron 7 1.02 1.11 1.05 1.03 1.03 ACKNOWLEDGMENTS sulfides in the sediment cores from the Sea 6 1.06 1.54 1.06 1.03 1.01 5 1.03 1.47 2.02 1.06 1.04 of Japan and their geophysical implica- 4 1.05 1.32 1.31 1.04 1.03 We thank Shaul Levi for many helpful tions: Earth and Planetary Sci. Letters, v. 3 1.15 1.04 1.19 1.09 1.09 16, p. 200-208. 2.5B 1.07 1.10 0.53 1.08 1.07 Mottled comments and Roger L. Larson for a con- 2.5A 1.07 1.01 1.64 1.05 1.00 Not mottled Kobayashi, Kazuo, Kitazawa, Kazuhiro, Kanaya, 2 1.20 1.43 2.11 1.11 1.08 structive review of an earlier version of this 1 1.14 1.20 2.07 1.12 1.09 Taro, and Sakai, Toyasaburo, 1971, 0 1.09 1.37 2.31 1.05 1.01 paper. Funds for this research from the Na- Magnetic and micropaleontological study Standard tional Science Foundation (Grant of deep-sea sediments from the west central deviation 0.02 0.14 0.01 0.002 0.006 GA-35251) are gratefully acknowledged. equatorial Pacific: Deep-Sea Research, v.

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