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Induced and remanent magnetic properties of marine sediments as indicators of depositional processes
Marine Geology, 38 (1980) 233--244 Elsevier Scientific Publishing Company, Amsterdam -- Printed in The Netherlands
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INDUCED AND REMANENT MAGNETIC PROPERTIES OF MARINE SEDIMENTS AS INDICATORS OF DEPOSITIONAL PROCESSES
BROOKS B. ELLWOOD Department of Geology, University of Georgia, Athens, GA 30602 (U.S.A.) (Received and accepted April 23, 1980) ABSTRACT Ellwood, B.B., 1980. Induced and remanent magnetic properties of marine sediments as indicators of depositional processes. Mar. Geol., 38: 233--244. Modern, unconsolidated deep-sea sediments acquire a characteristic magnetization during and after deposition. Using results obtained with remanent and induction magnetometers, it is possible in some cases to determine and evaluate: (1) bottomcurrent flow direction; (2) relative bottom-current flow magnitude; (3) post-sedimentation disturbances such as compaction, biogenic activity, slumping, etc.; (4) factors controlling sediment movement, such as flocculation, and therefore mode of transport, bedload versus sediment suspension;and (5) magnetic grain sizes and therefore possible sediment sources. The magnetization of such sediments, then, provides an indirect indicator of deep-sea depositional processes. The methods are quick, easy, and indirectly testable. INTRODUCTION It has proven necessary, when studying deep~ea sedimentary processes, to employ indirect observational techniques. This is due to the obvious inaccessibility o f depositional sites. Measurement of the magnetic properties of sediments recovered through piston coring operations is one such remote technique which has been employed in investigations aimed at a better understanding of sedimentary processes. While the remanent magnetic (RM) properties of such sediments have proven more useful, in conjunction with paleontological methods, as stratigraphic tools to estimate relative age and sedimentation rates in piston cores, such work, has led to the delineation of widespread zones o f bottom-current scour in the southern oceans (e.g. Watkins and Kennett, 1971). It has been inferred t h a t these zones, defined by obvious hiatuses in the magnetostratigraphic and hiostratigraphic records, are due to increases in b o t t o m ~ u r r e n t velocity yielding zero or negative sedimentation during the time span o f the hiatus. Another useful magnetic property easily determined for sediments is the magnetic susceptibility and its anisotropy. Such analyses are performed using induction magnetometers and results give an indication o f the bulk sediment magnetite content, and in the case of the anisotropy of magnetic susceptibility 0025-3227/80/0000--0000/$02.25 © 1980 Elsevier Scientific Publishing Company
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(AMS) an indication of the sedimentary magnetic fabric (e.g. Taira and Lienert, 1979). Therefore, the physical processes which produce that fabric can be inferred and experiments can be designed to test such inferences. The use of AMS as an indicator of sedimentary magnetic fabric is well documented (e.g. Rees, 1965; Kent and Lowrie, 1975; Marino and Ellwood, 1978). Susceptibility {×, in mass SI units) relates an inducing magnetic field to an induced magnetization and is a tensor of the second rank with a representation quadric which is an ellipsoid (magnitude ellipse of Nye, 1969). When magnetite is present in sedimentary samples the whole-sample susceptibility is anisotropic in low applied fields and represents the combined shape contribution of all magnetite grains, each acting as an individual susceptibility ellipsoid. The resulting magnitude and direction of three mutually orthogonal principal susceptibility axes determines the AMS. The final results for each sample are maximum Xa, intermediate Xb, and minimum ×~ susceptibility axial magnitudes and corresponding directions in mass SI units (or Ka, Kb, and K~ susceptibilities in volume SI units). MAGNETIC FABRIC
There are two major grain fabrics which are developed as a result of slow, particle by particle deposition. One is a planar fabric, due to particle settling, and the other is a linear fabric, due to forces tangential to the bottom such as the drag of depositing currents (Rees, 1965).
Factors affecting magnetic fabric in sediments Magnetic fabric analyses of sediments have been performed since the early 1940's (Ising, 1942) using the anisotropy of magnetic susceptibility (AMS) of sediments determined with a torque meter. Such methods were very time-consuming, however, since the result had to be hand~alculated. With the advent of computers, the AMS method became much less timeconsuming. Today, with the many commercially available AMS magnetometers, the method, potentially, provides a routine tool for oceanographic exploration. An adequate understanding of the acquisition of magnetic fabric in sediments must precede such routine analyses. There are many factors which tend to affect the alignment of magnetic particles settling onto the substrate. These include:
(1) Bottom currents. Long axes of intermediate sized grains tend to align in response to current flow, stronger alignment due to stronger flow velocities. Two types of alignment have been reported, current-parallel (the most common alignment), and long axes aligning normal to flow directions (examples found in Rusnak, 1957; Hand, 1961; Rees, 1965; Ellwood and Ledbetter, 1977, 1979). It has been suggested that the latter result from traction transport processes due to grain interactions with the bottom at intermediate to high relative current velocities; and the former
235 to alignment within the water column, or as a result of low to intermediate current-velocity alignment on the bottom (Ellwood, 1975; Ellwood and Ledbetter, 1979). A simple AMS parameter, F, has been used (Ellwood, 1975): (a) to enhance long-axis alignment of magnetic grains, (b) to diminish the effect of varying between-sample initial susceptibilities and (c) because it is dimensionless, where:
t and ×'a, ×b, and ×ct are the principal AMS magnitudes recalculated using a standard initial susceptibility, ×x x. The parameter, F,, as applied to sediments, was discussed by Ellwood and Ledbetter (1977, 1979). Since F, is designed to be sensitive to the efficiency of long-axis alignment of magnetic grains, as magnetic~rain alignment improves due to an increase in bottomwater velocity, F, increases. The AMS parameter F,, then, emphasizes the linear fabric elements in the sediment sample, while it de-emphasizes variations in sample-to-sample magnetic grain content. High F, values represent the development o f a strong linear fabric and we have suggested that in the deep-sea this represents high relative-bottom-current velocities. The orientation of this linear fabric can be measured using the AMS. method, and we have suggested that the alignment is indicative of bottomcurrent flow direction. Since most sediment cores are unoriented during recovery, we have utilized the remanent magnetism of these cores for reorientation purposes. Our results, in cores taken where we have bathymetric control and current meter data, indicate that using the AMS method in conjunction with an RM realignment of cores provides a good indication of paleo-current flow direction (Ellwood and Ledbetter, 1979). In the Vema Channel, located in the South Atlantic Ocean, current velocities are high, and magnetic grains have become aligned normal to the current flow direction. Similar results have been suggested for grains from other sediments (for example, Hamilton and Rees, 1970}. In most grain alignment studies, however, an alignment parallel to the current is observed (Rees, 1965). Resolution of the problem of interpreting the alignment tendency of magnetic grains (current-parallel versus current-normal) lies in several factors, one of which is the manner of particle sedimentation. I¢a particle is aligned within the water column and is directly deposited without further movement along the bottom then current-parallel alignment will result. It has been argued that for higher current velocities, such as those expected in the Vema Channel, grain alignments may become current-normal due to movement along the substrate in a traction transport mode of deposition, and I have suggested a flocculation mechanism for this process (EUwood, 1979a). An evaluation of the long-axis parallel alignment hypothesis, for lowcurrent-velocity depositional sites, was performed by measuring the AMS for piston-core samples recovered from a southeastern Indian Ocean low-
236 velocity bottom-water contour current (Ellwood, 1979b). Fig.1 gives the location of the core (E 48-03, taken during cruise 48 of the USNS "Eltanin") relative to the contour current which has previously described by Corliss (1979). The long-axis AMS azimuths, when realigned using the RM mean directions, either from the entire core (B), or just from the top 100 cm of the core (A), where the RM directions may better represent the modern geomagnetic field direction, probably directly represent the azimuth of bottom-current flow at the site (dashed arrows in Fig.1 ). This suggests that grains are oriented current-parallel and were deposited directly from the contour current without further flow-controlled realignment. This alignment pattern is in agreement with the flow pattern for the contour-following current predicted by Corliss (1979).
(2) Gravity. The center of mass of the particle is acted upon by the force of gravity. The greater the particle size, the greater is the control of gravity on the initial orientation, causing long axes of longer grains to lie flat (Hamilton and Rees, 1970). The magnetic grains in deep-sea sediments 80E
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237
are essentially represented by two basic size ranges of grains (>5/~m! which generally are the major contributor to the AMS signal, and the smaller grains (<5/lm), in which range the RM resides (Ellwood and Ledbetter, 1977; Ellwood, 1979a). Therefore, measurement of the AMS in sediments, represents the fairly large magnetic grains, usually non-symmetrical, which are more readily controlled by forces such as gravity, producing a planar, near-horizontal or settling fabric (Fig.2,a). Such grains are only partly affected by the geomagnetic field, and this effect diminishes with increasing grain size. L
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(3) Geomagnetic field. The RM in sediments (after magnetic cleaning), is controlled by the small magnetic grains which are readily aligned or realigned during or after deposition. Such very small magnetic particles in the water column tend to act as simple dipoles and align with the geomagnetic field. Intermediate grain sizes (> 5 ~m and < 50 ~m) are only partly controlled by the geomagnetic field during deposition. (4) Biogenic reworking. It has been suggested that the reworking of sediments by borrowing organisms is random with respect to the orientations of magnetic grains and the effect of this activity is random, reducing the overall AMS magnitude but leaving net directions in sediment samples intact (Rees et al., 1968). This is analogous to a.f. demagnetization of weak components of the RM of samples. (5) Magnetic particle size and sample shape. It has been experimentally shown that the AMS reported for sediments is dependent on the magneticgrain size, and if that size is small (<5 pm), on the shape of the sample measured (Ellwood, 1979b). Fig.3 shows the effect of a changing length to diameter ratio (L/D} on the AMS of synthetic cylindrical samples with varying magnetic grain sizes. It is apparent (Fig.3) that as the magnetic grain size decreases, the measured AMS becomes more dependent on changes in sample shape. Selection of the proper sample shape for AMS samples, then, is an important factor which must be considered before sediments are sampled. Some anomalous AMS results which have previously been reported (for example, L~vlie and others, 1971) may, in part, be due to the presence of very fine magnetic grains, the lack of larger magnetic-grain sizes, and a less than optimum sample shape. 1,2
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(6) Flocculation.
The process of grain aggregation (or flocculation) is common in marine sediments (Kranck, 1975). It has been proposed as an important control on the development of sedimentary magnetic fabric in areas where grain concentrations are high, such as in association with nepheloid layers (Ellwood, 1979a). It is suggested that this process, in areas of high bottom-current activity, results in an initial RM inclination error and in a traction transport grain alignment, and therefore, current-normal AMS long-axis orientation. INSTRUMENTS There are three basic types of magnetometers which can precisely measure the RM and/or the AMS of rocks. (1) Spinner magnetometers (for example, Gough, 1964) have been commercially available since the middle 1960's and have the capability of measuring both the RM and AMS of 5--15-cc rock or sediment samples. They are relatively sensitive, fast, precise, and can be readily modified (or directly purchased ) for utilization with an on-line micro-processor or main computer system. (2) Cryogenic magnetometers (operating at superconducting temperatures) became commercially available in the 1970's (Goree and Fuller, 1976) and can measure samples of much larger volume and much lower magnetic intensity than can conventional spinner magnetometers. They are extremely fast, sensitive, and precise, can be operated in conjunction with a computer system, and are capable of measuring both the RM and AMS of rock and sediment samples. (3) Torsion fiber magnetometers (torque meters) are used primarily for AMS-type measurements (for example, King and Rees, 1962), are generally not commercially available, and are relatively slow. They are, however, extremely precise, can be used in conjunction with on-line computers, and can provide extremely high sensitivity.
Instrumen tal comparisons Paleomagnetic field and laboratory tests (such as those discussed in McElhinny, 1973) and inter-magnetometer calibration with known synthetic and natural samples are normally performed in evaluating the accuracy and sensitivity of RM and AMS magnetometers. One of the first reports using a cryogenic magnetometer to measure the AMS of low-sensitivity rock samples also compared the instrumental precision of that instrument with a commercially available computerized spinner magnetometer (Scriba and Heller, 1978). The spinner magnetometer was found to be more precise but total measurement time was longer (~ 1 rain as opposed to ~ 5 rain). A similar comparison was made between this type of spinner magnetometer and a torque meter (Ellwood, 1978) and the torque meter was found to be more
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precise b u t more time~onsuming per measurement (~ 5 min as opposed to ~ 30 min). While it appears from these results that the torque meter provides superior instrumental precision, much larger numbers of samples can be run in an equivalent length o f time on either a cryogenic or spinner magnetometer, an obvious trade off which must be evaluated by the investigator in terms of the project. DISCUSSION
Since the remanent and induced magnetic properties of sediments vary as a result of changes in magnetic-particle size and concentration and in response to the forces present during and after deposition, it is possible to use these magnetic properties as an indicator of depositional processes. Typical deep-sea sediments exhibit either RM stability or instability, and in some cases an error in magnetic inclination. Very stable magnetic results, characterized b y RM directional consistency u p o n demagnetization and by downcore sample-to-sample directional consistency, are expected for relatively undisturbed sediment containing small magnetic grains (>0.5/~m or so and <~5/~m). Sample-to-sample inconsistency or instability after demagnetization: (a) m a y be the result of the intrinsic R M properties of the sediment, such as extremely small magnetic grains and superparamagnetic behavior or large grain sizes,or (b) may be due to sediment disturbances such as slumping, deposition by turbidity currents, secondary mineralization or increases in biogenic activity.W h e n unusual or anomalous R M results are exhibited in a core, the A M S for these same samples may yield distorted or anomalous magnetic fabric, possibly indicating sediment disturbance. A n example of such anomalous A M S results,for samples exhibiting unusual R M directions, is given in Fig.2,b. In comparison, data in Fig.2,a are typical undisturbed A M S results from the same core. An R M inclination error which disappears with depth in the core is one piece of evidence which has been used in suggesting a flocculation mechanism of deposition for sediment at site C H 61 in the V e m a Channel (Ellwood, 1979a). An inclination error, with R M directions depressed toward the horizontal, is expected from experimental evidence (for example, King, 1955), b u t has n o t been generally reported for deep-sea sediment. It was proposed that the downcore steepening and eventual disappearance of the inclination error in core CH 61 (Fig.4) resulted from dewatering and physical or chemical breakdown of aggregates within the accumulating sediment. Less readily bound smaller magnetic grains are then free to realign with the geomagnetic field. Independent evidence for the flocculation mechanism of deposition comes from the A M S results in the core. Fig.5 shows that the long-axis inclination decreases with depth in C H 61. This can be explained by a depression of A M S long axes toward the horizontal causing slightcompression, compaction and aggregate breakup and dewatering. Other effects, such as changes in p H within the accumulating sediment cause
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242 further aggregate breakup. Since the magnetic fabric is being developed during deposition from a high-velocity bottom-current, the flocculated "fluff" is rolled, pushed, and tumbled along the "bottom", developing a magnetic long-axis alignment normal to the flow direction with a short-axis girdle resulting from tumbling or rolling (Fig.6). While long axes of magnetic grains are being affected by forces causing the various alignments observed, the RM, residing in smaller grains, still effectively represents the geomagnetic field direction as a result of realignment of these grains after sediment emplacement (see above). Bulk magnetic susceptibility is also a useful magnetic property of deepsea sediment, since it can give an indication of magnetite content (for example, Nettleton, 1976). Fig.7 illustrates this relationship, measured for a cross-section of Vema Channel core-top samples, where, as carbonate content increases, susceptibility decreases. In downcore analyses such measurements can detect volcanic-ash concentrations, and give an indication of ash-layer continuity. SUMMARY
The induced and remanent magnetic properties of deep-sea sediment samples provide a useful and indirect indicator of depositional p r o c e s s e s . The induced magnetic fabric developed in s a m p l e s c a n b e used to delineate zones and magnitude of sediment disturbances. The c h a n g e i n t h a t f a b r i c
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VEMA CHANNEL Fig.7. a. Susceptibility (×) ' 10 -s in mass cgs units and % CaCO 2 in surface samples plotted relative to core location across the eastern flank of the Vema Channel (Fig.7,b). b. Pistonand trigger-core numbers and locations plotted versus water depth along a topographic cross-section of the eastern flank of the Vema Channel (west to the left). Dashed line divides northward-flowing (®) Antarctic Bottom Water (AABW) from southward-flowing (~) North Atlantic Deep Water (NADW). (After Ellwood and Ledbetter, 1977.) w i t h i n individual cores yields useful i n f o r m a t i o n c o n c e r n i n g m o d e o f transp o r t , relative d i s t u r b a n c e , o r b o t t o m - w a t e r v e l o c i t y variability. T h e r e m a n e n t m a g n e t i s m , as well as p r o v i d i n g m a g n e t o s t r a t i g r a p h i c i n f o r m a t i o n , can be used f o r c o r e - r e a l i g n m e n t p u r p o s e s ( f o r u n o r i e n t e d p i s t o n o r gravity cores) a n d as an i n d i c a t o r o f s e d i m e n t m o d e o f f o r m a t i o n ( c o m p a c t i o n , biogenicaUy r e w o r k e d , etc.). REFERENCES Banerjee, S.K. and Stacey, F.D., 1967. The high-yield torque meter method of measuring magnetic anisotropy of rocks. In: D.W. Collison, K.M. Creer and S.D. Runcorn (Editors), Methods in Paleomagnetism (Developments in Solid Earth Geophysics, 3.) Elsevier,Amsterdam, 470 pp. Corliss, B.H., 1979. Recent deep-sea benthonic foraminiferal distributionsin the southeast Indian Ocean: inferred bottom-water routes and ecological implications. Mar. Geol., 31: 115--138. Ellwood, B.B., 1975. Analysis of emplacement mode in basalt from Deep-Sea Drilling Project holes 319A and 321 using anisotropy of magnetic susceptibility.J. Geophys. Res., 80: 4805--4808. Ellwood, B.B., 1978. Measurement of anisotropy of magnetic susceptibility~A comparison
244 of the precision of torque versus spinner magnetometer systems. J. Phys. Earth, 11: 71--75. Ellwood, B.B., 1979a. Particle flocculation: one possible control on the magnetization of deep-sea sediments. Geophys. Res. Lett., 6: 237--240. Ellwood, B.B., 1979b. Sample shape and magnetic-grain sizes: two possible controls on the anisotropy of magnetic susceptibilityvariabilityin deep-sea sediments. Earth Planet. Sci. Lett., 43: 309--314. Ellwood, B.B., 1980. Application of the anisotropy of magnetic susceptibility method as an indicator of bottom-water flow direction. Mar. Geol., 34: M83--M90. Ellwood, B.B. and Ledbetter, M.T., 1977. Antarctic b o t t o m water fluctuations in the Vema Channel: effects of velocity changes on particle alignment and size. Earth Planet. Sci. Lett., 35: 189--198. Ellwood, B.B. and Ledbetter, M.T., 1979. Paleocurrent indicators in deep-sea sediment. Science, 203: 1335--1337. Goree, W.S. and Fuller, M., 1976. Magnetometers using R F driven squids and their application in rock magnetism and paleomagnetism. Rev. Geophys. Space Phys., 14: 591--608. Gough, D.I., 1964. A spinner magnetometer. J. Geophys. Res., 69: 2455--2463. Hamilton, N. and Rees, A.I., 1970. The use of magnetic fabric in paleocurrent estimation. In: S.K. Runcorn (Editor), Paleogeophysics., Academic Press, New York, N.Y., pp. 445--464. Hand, B.M., 1961. Grain orientation in turbidites. Compass, 38: 133--144. Ising, G., 1942. On the magnetic properties of varied clays. Ark. Mat. Astron. Fys., A29: 1--37. Kent, D.V. and Lowrie, W., 1975. On the magnetic susceptibility anisotropy of deepsea sediment. Earth Planet. Sci. Lett., 28: 1--12. King, R.F., 1955. Remanent magnetism of artificially deposited sediments. Mort. Not. R. Astron. Soc., Geophys. Suppl., 7: 115--134. King, R.F. and Rees, A.I,, 1966. Detrital magnetism in sediments: an examination of some theoretical models. J. Geophys. Res., 71: 561--578. Kranck, K., 1975. Sediment deposition from flocculated suspensions. Sedimentology, 22: 11--123. L¢vlie, R., Lowrie, W. and Jacobs, M., 1971. Magnetic properties and mineralogy of four deep-sea cores. Earth Planet. Sci. Lett., 15: 157--168. Marino, R.J. and Ellwood, B.B., 1978. Anomalous magnetic fabric in sediments which record an apparent geomagnetic field excursion. Nature, 274: 581--582. McElhinny, M.W., 1973. Palaeomagnetisrn and Plate Tectonics. Cambridge Univ. Press, Cambridge, 358 pp. Nettleton, L.L., 1976. Elementary gravity and magnetics for geologists and seismologists. Monogr. Soc. Explor. Geophys., 1 : 1 2 1 pp. Nye, J.F., 1969. Physical Properties of Crystals. Oxford Univ. Press, London, 322 pp. Rees, A.I., 1965. The use of anisotropy of magnetic susceptibility in the estimation of sedimentary fabric. Sedimentology, 4: 257--271. Rees, A.I., Von Rad, U. and Shepard, F.P., 1968. Magnetic fabric of sediments from the La Jolla submarine canyon and fan, California. Mar. Geol., 6: 145--178. Rusnak, G.A., 1957. Orientation of sand grains under conditions of undirectional flow. J. Geol., 65: 384--409. Scriba, H. and Heller, F., 1978. Measurements of anisotropy o f magnetic susceptibility using inductive magnetometers. J. Geophys., 44: 341--352. Taira, A. and Lienert, B.R., 1979. The comparative reliability of magnetic, photometric and microscopic methods of determining the orientations of sedimentary grains. J. Sediment. Petrol., 49: 759--771. Watkins, N.D. and Kennett, J.P., 1971. Antarctic Bottom Water: major change in velocity during the late Cenozoic between Australia and Antarctica. Science, 173: 813--818.
233
INDUCED AND REMANENT MAGNETIC PROPERTIES OF MARINE SEDIMENTS AS INDICATORS OF DEPOSITIONAL PROCESSES
BROOKS B. ELLWOOD Department of Geology, University of Georgia, Athens, GA 30602 (U.S.A.) (Received and accepted April 23, 1980) ABSTRACT Ellwood, B.B., 1980. Induced and remanent magnetic properties of marine sediments as indicators of depositional processes. Mar. Geol., 38: 233--244. Modern, unconsolidated deep-sea sediments acquire a characteristic magnetization during and after deposition. Using results obtained with remanent and induction magnetometers, it is possible in some cases to determine and evaluate: (1) bottomcurrent flow direction; (2) relative bottom-current flow magnitude; (3) post-sedimentation disturbances such as compaction, biogenic activity, slumping, etc.; (4) factors controlling sediment movement, such as flocculation, and therefore mode of transport, bedload versus sediment suspension;and (5) magnetic grain sizes and therefore possible sediment sources. The magnetization of such sediments, then, provides an indirect indicator of deep-sea depositional processes. The methods are quick, easy, and indirectly testable. INTRODUCTION It has proven necessary, when studying deep~ea sedimentary processes, to employ indirect observational techniques. This is due to the obvious inaccessibility o f depositional sites. Measurement of the magnetic properties of sediments recovered through piston coring operations is one such remote technique which has been employed in investigations aimed at a better understanding of sedimentary processes. While the remanent magnetic (RM) properties of such sediments have proven more useful, in conjunction with paleontological methods, as stratigraphic tools to estimate relative age and sedimentation rates in piston cores, such work, has led to the delineation of widespread zones o f bottom-current scour in the southern oceans (e.g. Watkins and Kennett, 1971). It has been inferred t h a t these zones, defined by obvious hiatuses in the magnetostratigraphic and hiostratigraphic records, are due to increases in b o t t o m ~ u r r e n t velocity yielding zero or negative sedimentation during the time span o f the hiatus. Another useful magnetic property easily determined for sediments is the magnetic susceptibility and its anisotropy. Such analyses are performed using induction magnetometers and results give an indication o f the bulk sediment magnetite content, and in the case of the anisotropy of magnetic susceptibility 0025-3227/80/0000--0000/$02.25 © 1980 Elsevier Scientific Publishing Company
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(AMS) an indication of the sedimentary magnetic fabric (e.g. Taira and Lienert, 1979). Therefore, the physical processes which produce that fabric can be inferred and experiments can be designed to test such inferences. The use of AMS as an indicator of sedimentary magnetic fabric is well documented (e.g. Rees, 1965; Kent and Lowrie, 1975; Marino and Ellwood, 1978). Susceptibility {×, in mass SI units) relates an inducing magnetic field to an induced magnetization and is a tensor of the second rank with a representation quadric which is an ellipsoid (magnitude ellipse of Nye, 1969). When magnetite is present in sedimentary samples the whole-sample susceptibility is anisotropic in low applied fields and represents the combined shape contribution of all magnetite grains, each acting as an individual susceptibility ellipsoid. The resulting magnitude and direction of three mutually orthogonal principal susceptibility axes determines the AMS. The final results for each sample are maximum Xa, intermediate Xb, and minimum ×~ susceptibility axial magnitudes and corresponding directions in mass SI units (or Ka, Kb, and K~ susceptibilities in volume SI units). MAGNETIC FABRIC
There are two major grain fabrics which are developed as a result of slow, particle by particle deposition. One is a planar fabric, due to particle settling, and the other is a linear fabric, due to forces tangential to the bottom such as the drag of depositing currents (Rees, 1965).
Factors affecting magnetic fabric in sediments Magnetic fabric analyses of sediments have been performed since the early 1940's (Ising, 1942) using the anisotropy of magnetic susceptibility (AMS) of sediments determined with a torque meter. Such methods were very time-consuming, however, since the result had to be hand~alculated. With the advent of computers, the AMS method became much less timeconsuming. Today, with the many commercially available AMS magnetometers, the method, potentially, provides a routine tool for oceanographic exploration. An adequate understanding of the acquisition of magnetic fabric in sediments must precede such routine analyses. There are many factors which tend to affect the alignment of magnetic particles settling onto the substrate. These include:
(1) Bottom currents. Long axes of intermediate sized grains tend to align in response to current flow, stronger alignment due to stronger flow velocities. Two types of alignment have been reported, current-parallel (the most common alignment), and long axes aligning normal to flow directions (examples found in Rusnak, 1957; Hand, 1961; Rees, 1965; Ellwood and Ledbetter, 1977, 1979). It has been suggested that the latter result from traction transport processes due to grain interactions with the bottom at intermediate to high relative current velocities; and the former
235 to alignment within the water column, or as a result of low to intermediate current-velocity alignment on the bottom (Ellwood, 1975; Ellwood and Ledbetter, 1979). A simple AMS parameter, F, has been used (Ellwood, 1975): (a) to enhance long-axis alignment of magnetic grains, (b) to diminish the effect of varying between-sample initial susceptibilities and (c) because it is dimensionless, where:
t and ×'a, ×b, and ×ct are the principal AMS magnitudes recalculated using a standard initial susceptibility, ×x x. The parameter, F,, as applied to sediments, was discussed by Ellwood and Ledbetter (1977, 1979). Since F, is designed to be sensitive to the efficiency of long-axis alignment of magnetic grains, as magnetic~rain alignment improves due to an increase in bottomwater velocity, F, increases. The AMS parameter F,, then, emphasizes the linear fabric elements in the sediment sample, while it de-emphasizes variations in sample-to-sample magnetic grain content. High F, values represent the development o f a strong linear fabric and we have suggested that in the deep-sea this represents high relative-bottom-current velocities. The orientation of this linear fabric can be measured using the AMS. method, and we have suggested that the alignment is indicative of bottomcurrent flow direction. Since most sediment cores are unoriented during recovery, we have utilized the remanent magnetism of these cores for reorientation purposes. Our results, in cores taken where we have bathymetric control and current meter data, indicate that using the AMS method in conjunction with an RM realignment of cores provides a good indication of paleo-current flow direction (Ellwood and Ledbetter, 1979). In the Vema Channel, located in the South Atlantic Ocean, current velocities are high, and magnetic grains have become aligned normal to the current flow direction. Similar results have been suggested for grains from other sediments (for example, Hamilton and Rees, 1970}. In most grain alignment studies, however, an alignment parallel to the current is observed (Rees, 1965). Resolution of the problem of interpreting the alignment tendency of magnetic grains (current-parallel versus current-normal) lies in several factors, one of which is the manner of particle sedimentation. I¢a particle is aligned within the water column and is directly deposited without further movement along the bottom then current-parallel alignment will result. It has been argued that for higher current velocities, such as those expected in the Vema Channel, grain alignments may become current-normal due to movement along the substrate in a traction transport mode of deposition, and I have suggested a flocculation mechanism for this process (EUwood, 1979a). An evaluation of the long-axis parallel alignment hypothesis, for lowcurrent-velocity depositional sites, was performed by measuring the AMS for piston-core samples recovered from a southeastern Indian Ocean low-
236 velocity bottom-water contour current (Ellwood, 1979b). Fig.1 gives the location of the core (E 48-03, taken during cruise 48 of the USNS "Eltanin") relative to the contour current which has previously described by Corliss (1979). The long-axis AMS azimuths, when realigned using the RM mean directions, either from the entire core (B), or just from the top 100 cm of the core (A), where the RM directions may better represent the modern geomagnetic field direction, probably directly represent the azimuth of bottom-current flow at the site (dashed arrows in Fig.1 ). This suggests that grains are oriented current-parallel and were deposited directly from the contour current without further flow-controlled realignment. This alignment pattern is in agreement with the flow pattern for the contour-following current predicted by Corliss (1979).
(2) Gravity. The center of mass of the particle is acted upon by the force of gravity. The greater the particle size, the greater is the control of gravity on the initial orientation, causing long axes of longer grains to lie flat (Hamilton and Rees, 1970). The magnetic grains in deep-sea sediments 80E
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237
are essentially represented by two basic size ranges of grains (>5/~m! which generally are the major contributor to the AMS signal, and the smaller grains (<5/lm), in which range the RM resides (Ellwood and Ledbetter, 1977; Ellwood, 1979a). Therefore, measurement of the AMS in sediments, represents the fairly large magnetic grains, usually non-symmetrical, which are more readily controlled by forces such as gravity, producing a planar, near-horizontal or settling fabric (Fig.2,a). Such grains are only partly affected by the geomagnetic field, and this effect diminishes with increasing grain size. L
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238
(3) Geomagnetic field. The RM in sediments (after magnetic cleaning), is controlled by the small magnetic grains which are readily aligned or realigned during or after deposition. Such very small magnetic particles in the water column tend to act as simple dipoles and align with the geomagnetic field. Intermediate grain sizes (> 5 ~m and < 50 ~m) are only partly controlled by the geomagnetic field during deposition. (4) Biogenic reworking. It has been suggested that the reworking of sediments by borrowing organisms is random with respect to the orientations of magnetic grains and the effect of this activity is random, reducing the overall AMS magnitude but leaving net directions in sediment samples intact (Rees et al., 1968). This is analogous to a.f. demagnetization of weak components of the RM of samples. (5) Magnetic particle size and sample shape. It has been experimentally shown that the AMS reported for sediments is dependent on the magneticgrain size, and if that size is small (<5 pm), on the shape of the sample measured (Ellwood, 1979b). Fig.3 shows the effect of a changing length to diameter ratio (L/D} on the AMS of synthetic cylindrical samples with varying magnetic grain sizes. It is apparent (Fig.3) that as the magnetic grain size decreases, the measured AMS becomes more dependent on changes in sample shape. Selection of the proper sample shape for AMS samples, then, is an important factor which must be considered before sediments are sampled. Some anomalous AMS results which have previously been reported (for example, L~vlie and others, 1971) may, in part, be due to the presence of very fine magnetic grains, the lack of larger magnetic-grain sizes, and a less than optimum sample shape. 1,2
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239
(6) Flocculation.
The process of grain aggregation (or flocculation) is common in marine sediments (Kranck, 1975). It has been proposed as an important control on the development of sedimentary magnetic fabric in areas where grain concentrations are high, such as in association with nepheloid layers (Ellwood, 1979a). It is suggested that this process, in areas of high bottom-current activity, results in an initial RM inclination error and in a traction transport grain alignment, and therefore, current-normal AMS long-axis orientation. INSTRUMENTS There are three basic types of magnetometers which can precisely measure the RM and/or the AMS of rocks. (1) Spinner magnetometers (for example, Gough, 1964) have been commercially available since the middle 1960's and have the capability of measuring both the RM and AMS of 5--15-cc rock or sediment samples. They are relatively sensitive, fast, precise, and can be readily modified (or directly purchased ) for utilization with an on-line micro-processor or main computer system. (2) Cryogenic magnetometers (operating at superconducting temperatures) became commercially available in the 1970's (Goree and Fuller, 1976) and can measure samples of much larger volume and much lower magnetic intensity than can conventional spinner magnetometers. They are extremely fast, sensitive, and precise, can be operated in conjunction with a computer system, and are capable of measuring both the RM and AMS of rock and sediment samples. (3) Torsion fiber magnetometers (torque meters) are used primarily for AMS-type measurements (for example, King and Rees, 1962), are generally not commercially available, and are relatively slow. They are, however, extremely precise, can be used in conjunction with on-line computers, and can provide extremely high sensitivity.
Instrumen tal comparisons Paleomagnetic field and laboratory tests (such as those discussed in McElhinny, 1973) and inter-magnetometer calibration with known synthetic and natural samples are normally performed in evaluating the accuracy and sensitivity of RM and AMS magnetometers. One of the first reports using a cryogenic magnetometer to measure the AMS of low-sensitivity rock samples also compared the instrumental precision of that instrument with a commercially available computerized spinner magnetometer (Scriba and Heller, 1978). The spinner magnetometer was found to be more precise but total measurement time was longer (~ 1 rain as opposed to ~ 5 rain). A similar comparison was made between this type of spinner magnetometer and a torque meter (Ellwood, 1978) and the torque meter was found to be more
240
precise b u t more time~onsuming per measurement (~ 5 min as opposed to ~ 30 min). While it appears from these results that the torque meter provides superior instrumental precision, much larger numbers of samples can be run in an equivalent length o f time on either a cryogenic or spinner magnetometer, an obvious trade off which must be evaluated by the investigator in terms of the project. DISCUSSION
Since the remanent and induced magnetic properties of sediments vary as a result of changes in magnetic-particle size and concentration and in response to the forces present during and after deposition, it is possible to use these magnetic properties as an indicator of depositional processes. Typical deep-sea sediments exhibit either RM stability or instability, and in some cases an error in magnetic inclination. Very stable magnetic results, characterized b y RM directional consistency u p o n demagnetization and by downcore sample-to-sample directional consistency, are expected for relatively undisturbed sediment containing small magnetic grains (>0.5/~m or so and <~5/~m). Sample-to-sample inconsistency or instability after demagnetization: (a) m a y be the result of the intrinsic R M properties of the sediment, such as extremely small magnetic grains and superparamagnetic behavior or large grain sizes,or (b) may be due to sediment disturbances such as slumping, deposition by turbidity currents, secondary mineralization or increases in biogenic activity.W h e n unusual or anomalous R M results are exhibited in a core, the A M S for these same samples may yield distorted or anomalous magnetic fabric, possibly indicating sediment disturbance. A n example of such anomalous A M S results,for samples exhibiting unusual R M directions, is given in Fig.2,b. In comparison, data in Fig.2,a are typical undisturbed A M S results from the same core. An R M inclination error which disappears with depth in the core is one piece of evidence which has been used in suggesting a flocculation mechanism of deposition for sediment at site C H 61 in the V e m a Channel (Ellwood, 1979a). An inclination error, with R M directions depressed toward the horizontal, is expected from experimental evidence (for example, King, 1955), b u t has n o t been generally reported for deep-sea sediment. It was proposed that the downcore steepening and eventual disappearance of the inclination error in core CH 61 (Fig.4) resulted from dewatering and physical or chemical breakdown of aggregates within the accumulating sediment. Less readily bound smaller magnetic grains are then free to realign with the geomagnetic field. Independent evidence for the flocculation mechanism of deposition comes from the A M S results in the core. Fig.5 shows that the long-axis inclination decreases with depth in C H 61. This can be explained by a depression of A M S long axes toward the horizontal causing slightcompression, compaction and aggregate breakup and dewatering. Other effects, such as changes in p H within the accumulating sediment cause
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242 further aggregate breakup. Since the magnetic fabric is being developed during deposition from a high-velocity bottom-current, the flocculated "fluff" is rolled, pushed, and tumbled along the "bottom", developing a magnetic long-axis alignment normal to the flow direction with a short-axis girdle resulting from tumbling or rolling (Fig.6). While long axes of magnetic grains are being affected by forces causing the various alignments observed, the RM, residing in smaller grains, still effectively represents the geomagnetic field direction as a result of realignment of these grains after sediment emplacement (see above). Bulk magnetic susceptibility is also a useful magnetic property of deepsea sediment, since it can give an indication of magnetite content (for example, Nettleton, 1976). Fig.7 illustrates this relationship, measured for a cross-section of Vema Channel core-top samples, where, as carbonate content increases, susceptibility decreases. In downcore analyses such measurements can detect volcanic-ash concentrations, and give an indication of ash-layer continuity. SUMMARY
The induced and remanent magnetic properties of deep-sea sediment samples provide a useful and indirect indicator of depositional p r o c e s s e s . The induced magnetic fabric developed in s a m p l e s c a n b e used to delineate zones and magnitude of sediment disturbances. The c h a n g e i n t h a t f a b r i c
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VEMA CHANNEL Fig.7. a. Susceptibility (×) ' 10 -s in mass cgs units and % CaCO 2 in surface samples plotted relative to core location across the eastern flank of the Vema Channel (Fig.7,b). b. Pistonand trigger-core numbers and locations plotted versus water depth along a topographic cross-section of the eastern flank of the Vema Channel (west to the left). Dashed line divides northward-flowing (®) Antarctic Bottom Water (AABW) from southward-flowing (~) North Atlantic Deep Water (NADW). (After Ellwood and Ledbetter, 1977.) w i t h i n individual cores yields useful i n f o r m a t i o n c o n c e r n i n g m o d e o f transp o r t , relative d i s t u r b a n c e , o r b o t t o m - w a t e r v e l o c i t y variability. T h e r e m a n e n t m a g n e t i s m , as well as p r o v i d i n g m a g n e t o s t r a t i g r a p h i c i n f o r m a t i o n , can be used f o r c o r e - r e a l i g n m e n t p u r p o s e s ( f o r u n o r i e n t e d p i s t o n o r gravity cores) a n d as an i n d i c a t o r o f s e d i m e n t m o d e o f f o r m a t i o n ( c o m p a c t i o n , biogenicaUy r e w o r k e d , etc.). REFERENCES Banerjee, S.K. and Stacey, F.D., 1967. The high-yield torque meter method of measuring magnetic anisotropy of rocks. In: D.W. Collison, K.M. Creer and S.D. Runcorn (Editors), Methods in Paleomagnetism (Developments in Solid Earth Geophysics, 3.) Elsevier,Amsterdam, 470 pp. Corliss, B.H., 1979. Recent deep-sea benthonic foraminiferal distributionsin the southeast Indian Ocean: inferred bottom-water routes and ecological implications. Mar. Geol., 31: 115--138. Ellwood, B.B., 1975. Analysis of emplacement mode in basalt from Deep-Sea Drilling Project holes 319A and 321 using anisotropy of magnetic susceptibility.J. Geophys. Res., 80: 4805--4808. Ellwood, B.B., 1978. Measurement of anisotropy of magnetic susceptibility~A comparison
244 of the precision of torque versus spinner magnetometer systems. J. Phys. Earth, 11: 71--75. Ellwood, B.B., 1979a. Particle flocculation: one possible control on the magnetization of deep-sea sediments. Geophys. Res. Lett., 6: 237--240. Ellwood, B.B., 1979b. Sample shape and magnetic-grain sizes: two possible controls on the anisotropy of magnetic susceptibilityvariabilityin deep-sea sediments. Earth Planet. Sci. Lett., 43: 309--314. Ellwood, B.B., 1980. Application of the anisotropy of magnetic susceptibility method as an indicator of bottom-water flow direction. Mar. Geol., 34: M83--M90. Ellwood, B.B. and Ledbetter, M.T., 1977. Antarctic b o t t o m water fluctuations in the Vema Channel: effects of velocity changes on particle alignment and size. Earth Planet. Sci. Lett., 35: 189--198. Ellwood, B.B. and Ledbetter, M.T., 1979. Paleocurrent indicators in deep-sea sediment. Science, 203: 1335--1337. Goree, W.S. and Fuller, M., 1976. Magnetometers using R F driven squids and their application in rock magnetism and paleomagnetism. Rev. Geophys. Space Phys., 14: 591--608. Gough, D.I., 1964. A spinner magnetometer. J. Geophys. Res., 69: 2455--2463. Hamilton, N. and Rees, A.I., 1970. The use of magnetic fabric in paleocurrent estimation. In: S.K. Runcorn (Editor), Paleogeophysics., Academic Press, New York, N.Y., pp. 445--464. Hand, B.M., 1961. Grain orientation in turbidites. Compass, 38: 133--144. Ising, G., 1942. On the magnetic properties of varied clays. Ark. Mat. Astron. Fys., A29: 1--37. Kent, D.V. and Lowrie, W., 1975. On the magnetic susceptibility anisotropy of deepsea sediment. Earth Planet. Sci. Lett., 28: 1--12. King, R.F., 1955. Remanent magnetism of artificially deposited sediments. Mort. Not. R. Astron. Soc., Geophys. Suppl., 7: 115--134. King, R.F. and Rees, A.I,, 1966. Detrital magnetism in sediments: an examination of some theoretical models. J. Geophys. Res., 71: 561--578. Kranck, K., 1975. Sediment deposition from flocculated suspensions. Sedimentology, 22: 11--123. L¢vlie, R., Lowrie, W. and Jacobs, M., 1971. Magnetic properties and mineralogy of four deep-sea cores. Earth Planet. Sci. Lett., 15: 157--168. Marino, R.J. and Ellwood, B.B., 1978. Anomalous magnetic fabric in sediments which record an apparent geomagnetic field excursion. Nature, 274: 581--582. McElhinny, M.W., 1973. Palaeomagnetisrn and Plate Tectonics. Cambridge Univ. Press, Cambridge, 358 pp. Nettleton, L.L., 1976. Elementary gravity and magnetics for geologists and seismologists. Monogr. Soc. Explor. Geophys., 1 : 1 2 1 pp. Nye, J.F., 1969. Physical Properties of Crystals. Oxford Univ. Press, London, 322 pp. Rees, A.I., 1965. The use of anisotropy of magnetic susceptibility in the estimation of sedimentary fabric. Sedimentology, 4: 257--271. Rees, A.I., Von Rad, U. and Shepard, F.P., 1968. Magnetic fabric of sediments from the La Jolla submarine canyon and fan, California. Mar. Geol., 6: 145--178. Rusnak, G.A., 1957. Orientation of sand grains under conditions of undirectional flow. J. Geol., 65: 384--409. Scriba, H. and Heller, F., 1978. Measurements of anisotropy o f magnetic susceptibility using inductive magnetometers. J. Geophys., 44: 341--352. Taira, A. and Lienert, B.R., 1979. The comparative reliability of magnetic, photometric and microscopic methods of determining the orientations of sedimentary grains. J. Sediment. Petrol., 49: 759--771. Watkins, N.D. and Kennett, J.P., 1971. Antarctic Bottom Water: major change in velocity during the late Cenozoic between Australia and Antarctica. Science, 173: 813--818.