A reassessment of post-depositional remanent magnetism:preliminary experiments with natural sediments

K. Katari a, L. Tauxe b;*, John King c

a Harvard University, Department of Earth and Planetary Sciences, 20 Oxford St., Cambridge, MA 02138, USAb Scripps Institution of Oceanography, La Jolla, CA 92093-0220, USA

c University of Rhode Island, Graduate School of Oceanography, South Ferry Rd., Narragansett, RI 02882-1197, USA

Received 8 May 2000; received in revised form 28 August 2000; accepted 28 August 2000

Abstract

Realignment of magnetic grains below the sediment^water interface is thought to impart a post-depositionalremanent magnetization (pDRM). However, there is little convincing evidence in the published literature that pDRM isthe dominant mechanism by which sediments become magnetized. We report here preliminary results from two kinds oflaboratory experiments designed to investigate whether post-depositional reorientation of magnetic particles is likely tooccur in nature. In the first experiment, we monitored changes in the magnetization of natural sediments in response tochanging laboratory fields. Our results are inconsistent with post-depositional reorientation of magnetic particles. In asecond experiment, we put live worms in a multi-core tube with the original sediment^water interface intact.Remagnetization was only observed in samples taken from a mound of fecal pellets formed at the surface. These pelletshad been suspended by a worm, and redeposited in the laboratory field. The other samples, which were notresuspended, but nonetheless bioturbated by the worms, showed no change in magnetization. Our preliminary resultsdo not support the hypothesis that post-depositional reorientation occurs in natural, undisturbed sediments below thesediment^water interface. ß 2000 Elsevier Science B.V. All rights reserved.

Keywords: paleomagnetism; depositional remanent magnetization; remagnetization; £occulation; bioturbation

1. Introduction

Sedimentary deposits act as continuous record-ers of the geomagnetic ¢eld, but the derivation ofthe magnetic record from sediments depends onour understanding of the process by which thesediments acquire a remanence magnetization.While most paleomagnetists have accepted a basic

mechanism for the acquisition process, i.e. thatthe Earth's magnetic ¢eld imposes a torque onmagnetic particles which tends to align themwith the ¢eld, an important detail that is stillnot clear is the timing of the mechanism, i.e.whether the magnetization is syn- or post-deposi-tional. Several redeposition experiments have re-ported a post-depositional remanent magnetiza-tion (pDRM), and the physical explanation forthe phenomenon is that magnetite grains arefree to rotate in pore-spaces after deposition[1^4]. At some depth, due to dewatering and com-paction, the pore-spaces are reduced and the mag-

0012-821X / 00 / $ ^ see front matter ß 2000 Elsevier Science B.V. All rights reserved.PII: S 0 0 1 2 - 8 2 1 X ( 0 0 ) 0 0 2 5 5 - 7

* Corresponding author. E-mail: ltauxe@ucsd.edu

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netite grains are immobilized, thereby recording astable magnetic signal.

A crucial aspect of sedimentation that has notreceived adequate attention in the theory of sedi-mentary magnetism is £occulation or aggregationof sediments, although the e¡ects of £occulationon magnetization are well documented [5^10].Flocculation is ubiquitous in natural sediments[11], and it is highly unlikely that magnetic grainswill settle as individual particles and not as partof a larger £oc. As a result, they will be part ofthe sedimentary matrix, and will not be placed inpore-spaces, as previously envisioned. The energyof a remanence bearing grain in the geomagnetic¢eld is roughly 1 kT [12], whereas inter-particlebonds due to van der Waals forces are estimatedto be between 10 kT and 100 kT [13]. Therefore,once attached to other particles, magnetic par-ticles cannot be realigned by the geomagnetic¢eld, and the post-depositional movement ofgrains in pore-spaces will not be possible. Thereis, however, the possibility that some inter-particlebonds will be su¤ciently weak that a fraction ofthe magnetic particles will be reoriented, as wasobserved in experiments by Stober and Thompson[14]. The e¡ects of post-depositional rotation,however, did not signi¢cantly a¡ect the naturalremanent magnetization (NRM) [14].

We start with a review of the pDRM experi-ments, and ¢nd that most of them are not rele-vant to the natural sedimentary environment forthe following reasons: the original sedimentaryfabric was destroyed during the experiment and/or the sediment was dried during the experiment.We test the pDRM hypothesis on natural undis-turbed marine sediments, with water contentgreater than 75%, and ¢nd that these sedimentsare not remagnetized in a laboratory ¢eld which isdi¡erent from the ¢eld they were deposited in.

2. Prior investigations of pDRM

Early theoretical work by Nagata [15] proposedthat magnetic grains were aligned during deposi-tion and the magnetization was acquired beforethe grains came to rest at the sediment^water in-

terface. The magnetization of sediments wastherefore termed `depositional remanent magnet-ization' or DRM. Subsequent laboratory experi-ments [1^3] showed that sediments can acquire amagnetization after deposition under certain lab-oratory conditions, and this type of magnetizationwas termed pDRM.

A model of pDRM acquisition in which thesediment was considered to be a £uid whose vis-cosity increases with depth was proposed by Den-ham and Chave [16]. In this model, the character-istic time of alignment of a magnetite grainincreases exponentially with depth. But, naturalsediments are not well modeled as a £uid of grad-ually increasing viscosity. During deposition,there is a zone in which the sediment acts as athick suspension, that is a Newtonian £uid ofhigh viscosity (referred to as the homogeneouslayer [17]). With depth, the behavior of sedimentsabruptly becomes non-Newtonian [18], and thesediment is better modeled as a Bingham £uidwhich deforms plastically when a yield shearstress is exceeded.

Shcherbakov and Shcherbakova [19] took intoaccount the appropriate rheological properties ofa sediment, and considered two models of pDRMbased on the deformation of the sedimentary ma-trix: slow elastic deformation and plastic strain.According the their models, slow elastic deforma-tion of the sedimentary matrix could producepost-depositional realignments on the time scaleof about 1000 s, similar to the ¢ndings of Tucker[20]. They also concluded that plastic deformationof the matrix by magnetic grains is not possibleand will not contribute to pDRM in sediments,since the magnetic torque is not su¤cient to over-come the Bingham yield stress of most naturalsediments.

If sediments are indeed most often magnetizedby post-depositional reorientation of magneticphases, pDRM should be observable in laborato-ry experiments that duplicate the natural environ-mental conditions. In the following, we review theliterature for experimental evidence of pDRM,and critically examine the conditions of the ex-periments and their relevance to the natural mag-netization process.

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2.1. Redeposition experiments

The idea of pDRM was ¢rst proposed by Irvingand Major [1] who observed that a mixture ofquartz sand and magnetite accurately recorded amagnetic ¢eld when the mixture was £ooded withwater and then dried in the presence of the ¢eld.Noting the prevalence of bioturbation in naturalsediments, Kent [2] used a thick slurry made ofdeep-sea sediments, which when stirred and driedin the presence of a magnetic ¢eld recorded the¢eld accurately. The process of stirring was sup-posed to be an analog for bioturbation, whichwould aid pDRM acquisition in natural sedi-ments. Lovlie [3] performed a redeposition experi-ment in which he deposited a suspension of deep-sea sediment in the presence of a magnetic ¢eld,and changed the declination of the ¢eld in themiddle of the experiment. The depth at whichthe magnetic ¢eld change occurred was markedin the sediments by a layer of calcium carbonate.The redeposited sediment was dried in the ¢eldwith reversed declination, and subsequent mea-surements showed that sediments from about 10cm below the marker horizon also recorded thereversed declination.

A major problem with all these pDRM experi-ments is that the samples were dried before mea-surements were made. The e¡ects of drying on thesediment fabric and the magnetic signal recordedin the sediment can be signi¢cant. Experiments byHenshaw and Merrill [21] showed that sedimentswith more than 70% initial water content (as in allof the experiments described above) acquired astrong remanence when dried in the presence ofa magnetic ¢eld. According to Henshaw and Mer-rill [21], the process of drying could disturb thefabric such that mobility of particles is facilitateddue to the formation of cracks, shear zones andvoids in the sedimentary fabric. The presence of amagnetic ¢eld causes the magnetic moments toalign along the ¢eld during drying, and this `dry-ing' remanence was stable against alternating¢elds greater than 30 mT.

Irving and Major [1] observed that the mag-netic signal was not stable until the sampleswere completely dry. Natural sedimentary sam-ples, however, retain a stable signal even at water

content greater than 90% [14] and the PDRMobserved by Irving and Major [1] is not represen-tative of a natural remanence. The slurry used byKent [2] was obtained from the top of a core andprobably had a water content greater than 70%which would make it susceptible to a drying rem-anence as described by Henshaw and Merrill[21]. Similarly, the results of Lovlie [3] can beexplained by assuming that the water content inhis redeposited sedimentary column was 70% ormore to a depth of about 10 cm below the markerhorizon when the drying process began. As a re-sult, sediments to this critical depth recorded thedeclination of the ¢eld they were dried in, as op-posed to the ¢eld that they were deposited in.

Careful not to dry his samples, Tucker [20] re-investigated the problem of pDRM. He used amix of silica and magnetite, and found that inthe presence of magnetic ¢eld of strengths 0.02^0.72 mT, the intensity of magnetization in thesamples increased on a time scale of 103 s subse-quent to deposition. The pDRM intensity was lessthan 10% of the DRM intensity, and it wasstrongly dependent on water content of the sam-ple. Samples with water content greater than 70%showed pDRM, but samples with 68% showed nopDRM. Magnetization was enhanced by using a£uid of lower surface tension, and non-magneticgrains of larger size that would create larger pore-spaces. Magnetization was inhibited by increasingcompaction. There was a di¡erence in the acqui-sition and dissipation of the magnetization. Afraction of the acquired remanence decayed ontime scales of about 100 s. Thereafter, the decaywas very gradual to time scales of 100 h. Thisbehavior would rule out a viscous remanent mag-netization (VRM) for the acquired magnetization,which would dissipate on the same time scale asmagnetic acquisition.

In a similar experiment using natural river sedi-ments, Tauxe and Kent [22] also showed thatpDRM could be acquired in a laboratory ¢eld,after deposition of sediment in a null ¢eld. Theyused natural sediments and did not dry theirsamples before measurement. However, the origi-nal sedimentary fabric had been destroyedthrough agitation, and chemical de£occulantswere used to prevent particle clumping. Even so,

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the pDRM that they observed was only about10% of a DRM acquired in the same ¢eld.

In a very careful pDRM experiment, Stoberand Thompson [14] remagnetized pairs of freshsamples from two di¡erent lakes, where one sam-ple was impregnated with wax to immobilize thegrains without disturbing the sediment fabric. Itwas observed that the impregnated samples ac-quired remanences that were 40^70% less thantheir non-impregnated counterparts. This experi-ment demonstrates that in some sediments, a frac-tion of the magnetic grains are susceptible to re-orientation, probably by the elastic deformationof the sediment matrix [19]. The pDRM thus ac-quired, however, did not alter the NRM of thesamples, and the coercivity of pDRM was almostan order of magnitude lower than the coercivityof NRM. Furthermore, Stober and Thompson[14] observed that surface samples from lakesVuokonjarvi and Pielinen, with water contentsgreater than 90%, carried a stable NRM. Theypropose that in these sediments, the NRM is ac-quired by rotation of grains `shortly after deposi-tion', and is stabilized by the formation of organicgels which hold the grains in place. They do not,however, give a numerical value for the lock-indepth.

In a set of pDRM experiments, Hamano [23]observed that pDRM in a synthetic mixture oftalc and magnetite is an order of magnitude largerthan pDRM in redeposited natural red clay sedi-ments. Inter-particle forces in natural sediments,especially clay-bearing sediments, are strongerthan the same forces in arti¢cial sediments, andundisturbed sediments show stronger shearstrength than disturbed or remolded sediments.

The original idea of pDRM, whereby all grainsare considered free to rotate in pore-spaces, was¢rst questioned by Verosub [24] who proposedthat only a small fraction of grains remained mo-bile after deposition. In a critical review, Payneand Verosub [25] questioned the relevance ofmany of the pDRM experiments to the naturaldepositional remanence. They performed a num-ber of experiments with a wide variety of naturalsediments and concluded that for sediments withless than 60% sand, the original magnetizationwas preserved regardless of the water content.

They suggested that the inter-granular forces be-tween sediment particles could be a critical factorin understanding pDRM acquisition. As men-tioned earlier, van der Waals forces can be 1^2orders of magnitude larger than the magneticforces in natural sediments, supporting the notionof Payne and Verosub [25] that inter-granularforces dominate the mobility of particles afterdeposition.

3. Preliminary results of laboratory experiments

We turn now to new laboratory studies in thesearch for pDRM. To acquire a better under-standing of the acquisition of remanence in sedi-ments, we present preliminary results from a setof experiments using natural sediments in ordi-nary sea-water. These experiments are of twokinds: observing the changes in magnetizationover time in response to changing magnetic ¢eldconditions and observing changes in magnetiza-tion as a result of bioturbation.

3.1. Changes in magnetization in response tochanging ¢elds

3.1.1. Materials and methodsThe sediments for pDRM experiments were

collected from the Gulf of Mexico (see Table 1).The cores we used came from four successful de-ployments of the multi-core collector. Multi-cor-ing preserves the sediment^water interface anddoes not disturb the delicate sur¢cial sediments.This is achieved by lowering tubes, six at a time,which have both ends open. These have lids thatare held open by springs on each end. The tubesare 70 cm long and 10 cm wide. Once the coringdevice hits the bottom, the tubes penetrate thesur¢cial sediments to a depth of 50^60 cm, andthe springs on the ends of the tubes shut the lids.On raising the multi-corer on board, we obtainedsix cores with the sediment^water interface intact.Burrow marks were visible on the core-top, andthe top of the tube was ¢lled with water. Most ofthe water was drained and an o-ring was placedon the top of the sediment which slightly dis-turbed the top 2^3 mm of the sediment. The

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draining of the water assured that there would beno disturbance of the delicate sediments duringtransportation.

The sites of the cores and the length of corerecovery for the ¢rst set of experiments are shownin Table 1. Two cores from each site were mea-sured in a 2-G Whole Core Cryogenic Magneto-meter at the paleomagnetic laboratory in theGraduate School of Oceanography (GSO), Uni-versity of Rhode Island. The top 15 cm of theMC02 cores consisted of a dark brown clay withforaminifera, which changed to a lighter color ata depth of 25^30 cm. There was evidence of bio-turbation throughout this zone. Below 45 cm, theclay was brownish gray. Measurements at depthsbetween 20 an 34 cm showed that the total organ-ic content in the sediment is about 3.4% with totalcarbonate content of about 27%. Water content

and shear strength of the undisturbed and re-molded sediments were measured by Prof. Silva'sgroup at the GSO, and these are shown in Fig. 1.The water content at the top of the core is about75% and decreases to about 55% at the bottom.The undisturbed sediments show a stronger shearstrength than remolded sediments probably be-cause inter-particle bonds are preserved in the for-mer.

The access port diameter of the magnetometeris 12 cm, and the top 12 cm of the magnetic mea-surements represent a convolution with the mag-netization of air, and is therefore more di¤cult tointerpret. Below 12 cm, the inclination in all thecores was between 50³ and 60³ (Fig. 2). For alatitude of 27³N (the latitude of the deploymentsites), a geocentric axial dipole would predict aninclination of 45.5³. The high inclinations, how-

Fig. 1. Geotechnical properties of core MC02. (a) Water content versus depth. (b) Shear strength of two sets of undisturbed sam-ples, and one set of remolded samples. Note the loss of shear strength when the original fabric is destroyed by remolding.

Table 1Site information for multi-cores from Gulf of Mexico

Core Site lat. Site long. Water depth Recovery Comments(m) (m)

MC01 26.88³N 92.39³W 1590 ^ No recoveryMC02 27.14³N 92.38³W 1355 3.17 Six tubesMC03 26.33³N 91.93³W 2030 3.40 Six tubes (E, F disturbed)MC04 26.53³N 92.27³W 2580 3.29 Six tubesMC05 26.23³N 91.85³W 2325 3.48 Six tubes (D disturbed)

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ever, are in agreement with the inclination anom-aly in the region for the last 100 yr, over whichthe inclination has been approximately 57³. Thesur¢cial sediments were therefore capable of re-cording a paleomagnetic signal. Hysteresis mea-surements on sediments from the top 2 cm ofthe sediment column show that the principal mag-netic carrier is pseudo-single-domain magnetite.

A pair of Helmholtz coils was used to create avertical ¢eld for the remagnetization experiments.The coils were 1 m in diameter, and were placedabout 1 m apart. These generated a ¢eld ofþ 86.5³ inclination and 0.1 mT intensity. We per-formed experiments with both the `remolded'sediments whose original sedimentary fabric hadbeen destroyed, and fresh sediments with the orig-inal sedimentary fabric intact. The remolded sedi-ments came from the top 5 cm of core MC02Awhich was extruded on board. These were trans-ported in a plastic bag to preserve the initial watercontent. Thus there was no loss of water, butbonds and cements holding sedimentary particlestogether had been broken (as indicated by themuch lower shear strength as shown in Fig. 1b).Two samples were made from this sediment, la-beled MC02A.1 and MC02A.2. Undisturbed sedi-ments came from the top 2 cm of MC02C, and

the samples are labeled MC02C.1a andMC02C.1b.

Samples of the sediment in cubic plastic boxeswere placed in the Helmholtz coil set up. Verti-cally downward (`normal') and upward (`re-versed') magnetic ¢elds were used to remagnetizethe samples. The samples were also placed in azero magnetic ¢eld in order to test the stabilityof the (re)magnetization. Magnetic measurementswere made periodically with the single-samplemagnetometer at GSO.

3.2. Results

For the `remolded' sediments, we ¢rst put sam-ple MC02A.1 in the normal ¢eld for 1 h, thenobserved the changes in inclination and intensityover a period of several days in zero ¢eld. Theinclination increased from 33³ to 55³ in the nor-mal ¢eld. In the zero ¢eld, the inclination thendecreased rapidly to a steady value of about 42³(see Fig. 3a). The intensity, which was initiallyabout 9 nAm2, increased to 13 nAm2 after expo-sure to the ¢eld, then decreased to about 6.5nAm2 which is lower than its initial value (Fig.3b). Then, we put sample MC02A.2 in a normal¢eld for 72 h after which the ¢eld was set to zerofor 1 day. The inclination increased from 13³ toabout 58³ in the normal ¢eld and then decreasedto 44³ in the zero ¢eld (Fig. 3c). The initial inten-sity of the sample was roughly 13 nAm2. It in-creased to 22 nAm2 in the normal ¢eld, and de-creased to 15 nAm2 after 24 h in zero ¢eld (Fig.3d).

For the `undisturbed' samples, we placed thesample MC02C.1a in the reversed ¢eld for 1 day,and then switched the ¢eld o¡ for an additional36 h. The inclination at the beginning of the ex-periment was 76³, which decreased to 56³ in thereversed ¢eld, but rebounded to 73³ in the zero¢eld (see Fig. 4a). The intensity, initially 15 nAm2,reduced to 7 nAm2 after 24 h in the reverse ¢eld.This too rebounded to 13 nAm2 in the zero ¢eld(Fig. 4b). Sample MC02C.1b was initially placedin a zero ¢eld for 24 h to check the stability of theremanence signal. It was then put in a reverse ¢eldfor 26 h, and in a zero ¢eld for 10 h. The incli-nation was initially 66³ and increased slightly to

Fig. 2. Inclination versus depth for eight multi-cores, twofrom four di¡erent sites. The top 12 cm of the data are di¤-cult to interpret because of convolution problems. The dot-ted line shows the approximate current inclination in the vi-cinity of the sites.

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69³ in the zero ¢eld (Fig. 4c). In the reverse ¢eld,it decreased to 44³, but achieved 68³ after 10 h inthe zero ¢eld (Fig. 4d).

3.3. Interpretation of pDRM experiments

Samples with intact fabrics (MC02C.1a andMC02C.1b) showed changes in direction and in-tensity when placed in a reverse ¢eld, but thesechanges appear to be largely reversible. The incli-nation and intensity return to essentially the ini-tial values when placed in a zero ¢eld. Thechanges in inclination persisted in the remoldedsamples, and in the case of MC02A.1, the inten-sity in the zero ¢eld decreased below its initialintensity. The water contents of both the dis-turbed and undisturbed samples were the same,but the shear strength of remolded sedimentswas signi¢cantly lower, as seen in Fig. 1. There-fore, it is reasonable to view the changes inthe remolded samples as a result of realignment

of magnetite grains in a relatively weak matrix,in other words, a genuine pDRM. It is impor-tant to note, however, that the magnetization ofthis pDRM is a small fraction of the originalDRM.

Remagnetization may result from mechanicalreorientation of magnetite grains, which wouldbe a pDRM in the conventional sense, or it couldresult from movement of moments and domainsin magnetically soft, low-coercivity mineralgrains, in which case it would be a VRM. In orderto assess the role of pDRM, we must distinguishbetween the two. One possible test would be acomparison of the time dependent acquisitionand decay of remagnetization in the samples. Inthe case of VRM, we would expect a similaritybetween acquisition and decay of magnetization.The time dependent viscous magnetization M(t)of a sample may be described as:

M�t� �Meq � �M03Meq�e3t=d ; �1�

Fig. 3. (a) Change of inclination versus time for MC02A.1. The sample was exposed to a normal ¢eld for 1 h, and then kept ina zero ¢eld. (b) Change in intensity in MC02A.1. The solid line is the ¢t of a VRM model to the data. (c) Change in inclinationduring remagnetization of MC02A.2. The sample was kept in a normal ¢eld for 72 h, and then placed in a zero ¢eld. (d) Changein intensity in MC02A.2. The two solid lines are the VRM models of acquisition and dissipation.

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where M0 is the initial magnetization of the sam-ple. The magnetization grows or decays exponen-tially to the equilibrium magnetization Meq astCr, and d is the relaxation time, dependenton a variety of factors such as saturation magnet-ization, grain volume, coercivity and temperature.Given a natural sample with a wide variety ofmagnetic minerals of di¡erent sizes, we expect arange of d. The VRM behavior can therefore bemodeled as [26]:

M�t� �Meq �M03Meq

N

XN

i�1

e3t=d i ; �2�

where the values of di follow some distributionf(d). In laboratory experiments, relaxation timesof the order of days are relevant. The changesin the magnetic vector on remagnetization inour experiments are reasonably ¢t by assuming

a VRM model with two relaxation times:

M�t� �Meq � �M03Meq��e�3t=d 1� � e�3t=d 2��:�3�

Using the value of the initial intensity of mag-netization as M0, we ¢t Meq, d1, d2 (Table 2).

The distribution f(d) in a sample does notchange from experiment to experiment. Therefore,we expect the ¢tted parameters d1, d2 to be similarfor a given sample, throughout the entire VRMacquisition and decay experiment. (Because of ap-proximation in using only two values of d insteadof ¢nding the best-¢tting distribution f(d), we donot expect perfect agreement, however.) For me-chanical realignment of the grains, as in pDRM,we expect the decay of magnetization to be muchslower. However, the estimated d values for a giv-en sample for acquisition and decay of magnet-

Fig. 4. (a) Change of inclination versus time for MC02A.1a. The sample was exposed to a reverse ¢eld for 24 h, and then keptin a zero ¢eld. (b) Change in intensity in MC02A.1a. The two segments of solid line are VRM model ¢ts for magnetic acquisi-tion and dissipation. (c) Change in inclination during remagnetization of MC02C.1b. The sample was kept in a zero ¢eld for 24h, then in a reverse ¢eld for 28 h, and then back in a zero ¢eld. (d) Change in intensity in MC02C.1b. The two solid line seg-ments are VRM models of acquisition and dissipation.

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ization in Table 2 are similar and VRM is there-fore a more likely explanation for the observedchanges in magnetization. A better test ofpDRM versus VRM would be the method pro-posed by Stober and Thompson [14], where a lowviscosity wax was used to immobilize sedimentarygrains without disturbing the sedimentary fabric,and it was noted that samples impregnated withwax recorded a smaller change in intensity uponremagnetization. These changes are probably dueto slow elastic deformation of the sediment matrix[19], and a part of the remagnetization in oursamples could result from such a process. How-ever, post-depositional rotations in non-impreg-nated samples did not a¡ect the NRM of thesamples, showing that the e¡ect of pDRM is in-deed small.

In summary, we ¢nd weak evidence for pDRMin the sediment whose natural fabric was de-stroyed but none in those whose original fabricwas preserved. In the former, the pDRM was asmall fraction of the original DRM. These resultscon¢rm our interpretation of the published datathat suggest that pDRM is rare in natural sedi-ments.

4. Changing magnetization with bioturbation

A mechanism for pDRM proposed by Kent [2]and Tucker [20] is that the magnetite grains canbe stirred by the movements of worms below thesedimentary surface, which would realign thegrains and create a pDRM di¡erent from the dep-ositional remanence. Stirring involves the suspen-sion of sediment grains above the sediment^water

interface, and redeposition of the grains. Biotur-bation, on the other hand, may or may not resus-pend sediment grains, and the e¡ects of bioturba-tion on NRM acquisition can be very di¡erentfrom the e¡ects of mechanical stirring. Ellwood[27] set up an elaborate experiment to study thee¡ects of bioturbation in a natural environment.A meter long trench, 30 cm wide and 3^5 cmthick, was cleared in a high biological productiv-ity tidal £at on Sapelo Island, Georgia. Thetrench was ¢lled with a layer of white kaolin,followed by a layer of magnetite, and coveredwith another layer of kaolin, all layers less than1 mm thick. The experiment was started in De-cember of 1982, and the magnetite layer wassampled periodically for the next 8 months andthe intensity and inclination were monitored. Theinclination started out with high variability, butafter 50 days, settled to a constant value whichwas roughly 20³ shallower than the site inclina-tion. With the advent of summer (about 160 daysinto the experiment), the inclination values in-creased, and 211 days into the experiment, theinclination was statistically similar to the site in-clination. Ellwood [27] attributes the reduction ofinclination shallowing to high bioturbation in thesummer which enhances pDRM and therefore, amore accurate recording of the inclination. How-ever, he notes that the bioturbation rates on thecoast are much higher than rates in the deepocean, and bioturbation is therefore not a signi¢-cant factor in deep oceans. In order to test di-rectly the e¡ects of bioturbation on the remagne-tization of sediments, we performed the followingexperiment. We placed a sediment core that wasdeposited in the current normal polarity in a re-

Table 2Best ¢t parameters for a VRM model with two relaxation times

Sample Field M0 Meq d1 d2

(Am2) (Am2) (h) (h)

2A Zero 1.30W1038 0.65W1038 1.78 47.172C Normal 1.30W1038 2.32W1038 1.49 38.352C Zero 2.30W1038 1.30W1038 0.96 34.20MC02C.1 Reverse 1.52W1038 0.47W1038 1.1 33.46MC02C.1 Zero 0.70W1038 1.59W1038 1.0 49.72MC02C.1A Reverse 1.42W1038 0.64W1038 0.68 11.72MC02C.1A Zero 0.68W1038 1.35W1038 0.71 8.34

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versed ¢eld, and put live worms in the sediment.If there was indeed post-depositional alignmentbelow the sediment surface, we expected to ¢ndreversed inclination below the sediment surface.

4.1. Materials and methods

We conducted the bioturbation experiment inthe paleomagnetic laboratory at Scripps Institu-tion of Oceanography (SIO) of the University ofCalifornia, San Diego, CA, USA. The sedimentsamples were two multi-core tubes that were col-lected in the Gulf of Mexico (23.29³N, and108.19³W) at a water depth of 2655 m, andwere kindly provided by Barbara Ransom ofSIO. The length of the multi-core tubes was 70cm, of which about 46 cm was ¢lled with sedimentwith the rest of the cores ¢lled with sea-water.

To obtain paleomagnetic samples from the sedi-ment, we used a glass vial (1 cm diameter, and5 cm length). The closed end of the vial had beenpenetrated with a blow torch to give an apertureof about 0.5 cm diameter. We attached a plastictube to the blow-torched end. The other end ofthe plastic tube was attached to a hypodermicsyringe. We inserted the glass tube gently intothe sediment. In order to prevent deformation ofthe water-saturated sediments, we used the syringeto provide a slight vacuum, drawing the sedimentinto the glass tube without deforming it. We re-moved the tube from the mud and sealed the openend with para¢lm. Then we removed the rubbertube from the other end of the glass tube, andmeasured the sample in the cryogenic magneto-meter.

Based on multiple measurements and replicatesamples, we estimate our reproducibility to be afew degrees for repeat measurements of the samespecimen and within 10 degrees for replicate sam-ples from the same horizon. This level of repro-ducibility is su¤cient for our experiment, in whichwe seek changes in the sign of the inclination dueto remagnetization.

We placed the second multi-core tube in a sol-enoid, which produced a vertically upward ¢eld of0.05 mT. The solenoid has a diameter of 30 cm,and was about 75 cm long, which encompassedthe multi-core tube completely. We obtained live

worms from the inter-tidal zone which was ex-posed during low tide of Mission Bay in San Die-go. These sediments support a variety of organ-isms such as polychaetes, oligochaetes, andphronoids, with total macrofaunal density sur-passing 200 000 individuals per square meter[28]. We brought a bucket of mud into the lab,sieved it with sea-water, picked out individualworms, and placed them in the multi-core tube.About 50 such worms were placed in the multi-core tube, which gives a population density of3000 individuals per square meter. The wormswere identi¢ed by Prof. Lisa Levin as polychaetes.They had body lengths of 1^3 cm and were clearlyvisible in their burrows when the burrows wereagainst the clear surface of the multi-core tube.The worms formed burrows within 1 h of beingplaced in the multi-core tube, and after 24 h thetop 5^6 cm of the sediment was visibly riddledwith worm burrows. The spacing of the burrows,from the side of the tube, appeared to be 1^2 mm,indicating a high level of biological activity. Theworms were also observed in motion, sometimesemerging at the surface. No additional food wasadded for the sustenance of the worms, but thewater was aerated with a pump during the courseof the experiment.

4.2. Results

The experiment was run for 3 weeks and at theend of this period, the worms were still active.The top 4 cm was extensively bioturbated, withburrows throughout the layer. There were twoworms whose tails were visible above the mudsurface surrounded by mounds of fecal pellets.We selected two sites on the sediment surfacefor sampling: Site 1 was over one mound of fecal

Table 3E¡ects of bioturbation on remagnetization of sediments

Site Sample depth Inclination Unnormalized intensity(cm) (A/m2)

1 0^0.5 333³ 7.429W1037

1 0.5^1 88.1³ 7.029W1037

1 1.0^2.5 77.2³ 5.050W1036

1 2.5^4.0 69.2³ 6.016W1036

2 0^1.0 67.9³ 1.500W1036

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pellets, and Site 2 was removed from the moundsof pellets. The data are shown in Table 3. Onlythe top sample from Site 1 exhibited a negativeinclination. All the rest retained the original mag-netic direction.

The results of our experiment clearly demon-strate that realignment of magnetite grains oc-curred when the sediment was resuspended. Re-suspended fecal pellets at Site 1 at the sedimentsurface recorded a negative inclination, but thesediments just 1 cm below the mound of pelletsdid not. Therefore, bioturbation had a¡ected themagnetization of the sediment within the mound,but apparently not below it. Bioturbation belowthe sediment surface, and away from the mound,did not produce realignment of magnetite grains.It is possible, however, that bioturbation belowthe surface can have a randomizing e¡ect on thesedimentary fabric, reducing the net intensity ofmagnetization. To understand the e¡ects of bio-turbation on sedimentary magnetism, we proposea model of sedimentation that includes bioturba-tion as a component.

5. A new model of sedimentary magnetism

In an excellent review article, Rhoads [29] sum-marizes the e¡ect of bioturbation on muddy sedi-ments on the sea £oor. All organisms modify theenvironment in which they live, and the most sig-ni¢cant e¡ect results from the feeding habits ofthe organisms. Movements of organisms throughthe surface sediments, and the feeding and depo-sition of fecal pellets increase the water contentand reduce the shear strength of the sedimentsin the top 2^3 cm [30,31]. The movements of or-ganisms are not restricted to the top couple ofcentimeters, and burrows can reach 10^20 cm be-low the sediment surface, but the shear penetra-tion of bottom currents is restricted to the zone ofreduced shear strength. This weak layer is easilyresuspended by bottom currents and is completelyhomogenized.

The idea of a well layer above the permanentsedimentary layer was also used by Berger andHeath [17] to explain the appearance and extinc-tion of species in sedimentary records. They as-

sume a homogeneous layer of thickness L, the topof which constitutes the sediment^water interface.This layer corresponds to the thoroughly mixedlayer due to shear penetration in the model ofRhoads [29]. A small fraction dL of this layer Lis gradually incorporated into the historical sedi-mentary layer, where sediment particles remainstatic over time. A model of sedimentation, com-bining the ideas of Berger and Heath [17] andRhoads [29] is shown in Fig. 5. We can use thismodel to understand the e¡ects of sediment cy-cling on the acquisition of sedimentary magnet-ism. Rates of sediment reworking by depositfeeders vary from 100 to 400W1036 m3 of wetmud/individual/year [29]. The number of organ-isms varies between 102 and 104 individuals/m2

[29]. Therefore, if we assume a homogeneousmixed layer of 2 cm, then the volume of sedimentover an area of a square meter is 0.02 m3. Assum-ing 103 organisms per square meter, with eachindividual processing 1034 m3 of sediment peryear, the total amount of sediment that gets pro-

Fig. 5. A simple conceptual model of detrital deep-sea sedi-mentation. Detrital particles are deposited at the sediment^water interface, and then incorporated into the homogeneouslayer of thickness L. The rate of deposition determines therate at which a thin basal slice of the homogeneous layer ofthickness dL gets incorporated into the historical layer. Mag-netic particles in £ocs or fecal matter will be reoriented inthe homogeneous layer many times before they enter the per-manent deposit. Once in the historical layer, no more re-alignment to changing magnetic ¢eld is possible.

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cessed per year is about 0.1 m3, which is ¢ve timesthe volume of sediment in the homogeneous layer.Therefore, the homogeneous layer gets thoroughlymixed several times during a year. Although bio-turbation will penetrate below the homogeneouslymixed layer, it will not cause the shear strength toweaken su¤ciently, and material from below thehomogeneous layer will not be resuspended due tobottom currents. As our experiments show, re-magnetization cannot happen until there is resus-pension of sediments. Therefore, sediments belowthe homogeneous layer cannot be remagnetized.

The results of Ellwood [27] can be understoodin the light of this model. The initial inclinationshallowing during the time of low bioturbationprobably resulted from the magnetite grainsbonding to the kaolin clay particles, which areplatey [32] and therefore have a horizontal bias.With increasing bioturbation, the clay and mag-netite layers were homogenized, and resuspendedin the form of fecal pellets. Pellets do not have thehorizontal bias of kaolinite grains, and on set-tling, they did not record a shallow inclination.Although Ellwood [27] predicted that the e¡ectsof bioturbation would not be important in deep-sea sediments, our order of magnitude calcula-tions of volume of sediments processed by organ-isms, and direct observations of deep-sea sedi-ments suggest that almost all sediments havebeen processed by organisms before deposition[33]. Therefore, according to Fig. 5, the magnetitegrains (in £ocs and fecal pellets) have been resus-pended and redeposited several times in a yearbefore they have been permanently incorporatedin the historical layer. Changes in the Earth'smagnetic ¢eld occur on time scales much longerthan 1 yr. As a result, any change in the ¢eldwould be recorded virtually instantaneously byall the magnetite in the homogeneous layer. Thee¡ect of bioturbation on the recording of themagnetic ¢eld is therefore not one of smoothing,but simply a lag which is equal to the thickness ofthe homogeneous layer.

6. Discussion

The magnetic signature in sediments has long

been considered to be post-depositional in origin.While numerous experiments have been designedto test this hypothesis, most of them have ne-glected the e¡ects of inter-granular forces thatmake sediments cohesive. The attractive van derWaals forces are not dependent on sedimentchemistry, and therefore, are present in all typesof sediments. In order for magnetic grains to re-orient after deposition, they would have to breakthe inter-granular bonds, resulting in either anelastic deformation or plastic deformation of thematrix. The latter is not feasible because the mag-netic torque is not large enough to overcome theBingham yield strength of most natural sediments[19], and elastic deformations will be mostly re-versible, and therefore will not contribute to asigni¢cant pDRM. Bioturbation can break inter-particle bonds, but as our experiment and modeldemonstrate, bioturbation enhances remagnetiza-tion only through resuspension of sediments, andnot by reworking sediments below the surface.

Post-depositional reorientation of magneticphases would lead to observable features in thepaleomagnetic records such as a signi¢cant depthlag for the lock-in of magnetic remanence orsmoothing of the paleomagnetic record. deMeno-cal et al. [34] reported data which were interpretedas evidence for a 16 cm lock-in depth for thepaleomagnetic signal. In a re-examination of thedeMenocal et al. [34] data set, including more re-cords with better magnetic data, the lock-in depthwas shown by Tauxe et al. [35] to be less than2 cm, or an order of magnitude less than thatpreviously reported. The e¡ects of pDRM wouldalso be seen in the smoothing of magnetic rec-ords in sediments, with more smoothing expectedin records with low sedimentation rates. Suchsmoothing has been observed in two paleomag-netic studies [36,37], but a similar study by Hartland Tauxe [38] with an augmented data base sug-gests that there is little evidence of smoothing.

While we do not ¢nd compelling evidence for apDRM based on our experiments and a survey ofthe literature, we do not claim that there are nochanges in magnetization subsequent to deposi-tion. We emphasize that mechanical reorientationof magnetic grains subsequent to deposition ishighly unlikely. This does not rule out post-dep-

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ositional changes in magnetization due to forma-tion of authigenic magnetic grains at depth as aresult of chemical diagenesis or biochemical reac-tions, which have been observed in numerous pa-leomagnetic studies [39^41].

7. Conclusion

We show that pDRM is most likely a rare phe-nomenon as opposed to the dominant mechanismby which sediments get magnetized with experi-ments on natural undisturbed sediments. We per-formed two experiments, one that observedchanges in magnetization of the sediments in re-sponse to changes in the magnetic ¢eld conditionsand one that observed changes in magnetizationin response to bioturbation. The former showedlittle evidence for permanent changes whichwould result from physical reorientation of themagnetic phases. The latter showed remagnetiza-tion only when the sediment had actually beenresuspended.

Acknowledgements

We thank Dr. Armand Silva and his group forproviding us the multi-cores and the data on thephysical properties of the sediment. Thanks to J.Bloxham for his support and useful discussions.We are grateful to Barbara Ransom who pro-vided two of the multi-cores used in the experi-ments and Lisa Levin who guided us in the art ofworm collection and identi¢cation. Cathy Consta-ble and Je¡rey Love provided useful critical com-ments. Finally, many thanks to the reviewers, oneanonymous, Jim Channell and Peter deMenocal,for their insightful comments. Financial supportfrom the National Science Foundation(EAR9706019) partially defrayed the cost of theseinvestigations.[RV]

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