JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 98, NO. Bll, PAGES 19,649-19,665, NOVEMBER 10, 1993

Early Oligocene Geomagnetic Field Behavior From Deep Sea Drilling Projec Sie 522

PAUL HARTL, LISA TAUXE, 1 AND CATHERINE CONSTABLE

Scripps Institution of Oceanography, La Jolla, California

Hydraulic piston coring operations at Deep Sea Drilling Project site 522 in the South Atlantic retrieved an unusually continuous section of late Eocene to late Oligocene pelagic sediments, which we sampled at 3-4 cm intervals (~3-5 kyr). Natural reinanent magnetization demagne- tization studies indicate a well-behaved remanence. Various rock magnetic procedures strongly suggest the magnetic carrier is dominated by pseudo-single domain magnetite appropriate for recoaxiing relative intensity variations of the paleomagnetic field. Nine zones of unusually low relative paleointensity were identified within the 2 my Chron C12R interval. Seven can be typified by a ~20-40 kyr interval of low field intensity accompanied by apparently random, low-amplitude, short-duration directional fluctuations. The other two are of approximately equal duration and intensity but exhibit an orderly progression of directional changes that result in well-defined vir- tual geomagnetic pole (VGP) paths confined along a preferred meridian of ~70-90øW longitude. We propose that both styles occur when the main dipole term diminish• significantly but that the former result when undiminished '•normal" secular variation is continuous during the period of low axial dipole moment. We propose that the other two lows in relative paleointemity, along with one reversal record, reflect a field structure of low axial dipole moment dominated by a low-degree nonzonal spherical harmonic term. Alternatively, the confined V GP paths could be an artifact of heavy remanence smoothing between nonantipodal, semistable transitional geomagnetic pole po- sitions. Geographical control of VGP paths, parti•ly along ~70-90øW longitude, has recently been noted for much younger reversals. The site 522 record may indicate that the underlying cause of this phenomenon was present at 32 Ma. We c•mpax•. our CI2R record of paleolntensity lows with C12R marine magnetic anomaly %iny wiggles". These data appear to indicate that C12R tiny wiggles resulted from periods of low geomagnetic field intensity that were sometimes accompanied by directional excursions.

INTRODUCTION

Since the early 1950s when it was first established that the geomagnetic field has repeatedly reversed itself over geologic time, Earth scientists have sought ever more complete and higher-resolution records of past geomagnetic field behavior. A steadily growing body of geomagnetic and paJeomagnetic literature since that time has clearly demon- strated the restless nature of the Earth's magnetic field over time scales that range from seconds to hundreds of millions of years. On the broadest scale, analyses of marine magnetic anomalies have yielded a continuous history of reversals going back to the age of the oldest ocean crust, > 150 Ma [Heirtzler et al., 1968; Berggren et al., 1985; Cande and Kent, 1992b]. Short duration episodes have come from the interpretation of magnetic remanence in marine sediments and lava flows, and even shorter-duration records have come from Holocene lacustrine sediments and studies

of archeological sites (see Clement and Constable [1991] for a recent review). Additionally, direct measurements over the last 350 years have documented significant changes in the field's intensity a•d direction over historical times [Fraser-Smith, 1987]. Unfortunately, there is generally an inverse relationship between the resolution of a record and its age and duration; marine magnetic a•omaJJes give us

• Also at Fort Hoofddijk Paleomagnetic Lab, Utrecht.

Copyright 1993 by the American Geophysical Union.

Paper number 93JB02019. 0148-0227]93/93JB-02019505.00.

our broaAest scope of paJeomagnetic history but also the most smoothed vision of it, whereas the historical record and varved lake sediments yield detailed but painfully short slices of very recent field behavior. Marine and lacustrine sediments have best bridged the gap between these end members; relatively detailed records of reversals and transitional field behavior during the Brunhes and Matuyama Chrons have recently been published [Clement, 1991; Tric et al., 1991; Clement and Kent, 1987; Clement and Kent, 1984], as well as investigations into secular varia- tion over the last 100 kyr [Levi and Karlin, 1989]. However, generally low sedimentation rates and problems associated with drilling and diagenesis have kept most investigators interested in the details of reversals, excursions, and secular variation from working with records older than ~2 Ma. Thus we have relatively detailed records of Pleistocene field behavior but very little information about the field earlier than the late Neogene.

Recently, analysis of a number of new reversal records has led to the hypothesis that transitional fields may be dominated by nonzonal components of the field's spherical harmonic representation, resulting in reversal paths (expressed as virtual geomagnetic poles or VGPs) longitudinally confined over preferred geographical bands [Clement, 1991; Tric et al., 1991]. It is further hypothesized that this VGP geographical preference is related to high velocity zones in the lower mantle and convection cell

geometry in the outer core [Clement, 1991; Laj et al., 1991]. The oldest of these longitudinally confined VGP paths is late Miocene, but the great majority are Matuyama and Brunhes events. It is not known how far back in time this

phenomenon persists. A record from Deep Sea Drilling

19,649

19,650 HARTL ET AL.: OLIGOCENE GEOMAGNETIC FIELD, DEEP SEA DRILLING PROJECT SITE 522

Project (DSDP) site 522 is presented here, and although it suffers from sedimentary smoothing, we believe it may contain early Oligocene records of this phenomenon.

We compare our record with globally recorded features of contemporaneously formed marine magnetic anomalies. Although the marine magnetic anomaly record is commonly viewed as a sort of global bar code of polarity reversals over the last 150 m.y., there is more complexity to the record than this simplified (but very useful) first order approximation. Actual marine magnetic anomMies, when properly deskewed (see Blakely and Cox [1972a]) commonly contain low-amplitude (40 to 80 gamma), short-wavelength (10 to 20 km) features that can be correlated from record to record. The origin of these "tiny wiggles" has been the subject of several studies [Blakely and Cox, 1972b; Blakely, 1974; Cande and œaBrecque, 1974; Red and Blakely, 1975; Cande and Kent, 1992a ]. Blakely and Cox [1972b] and Blakely [1974] investigated and identified tiny wiggles in anomalies 5 and 21-25 by deskewing and stacking multiple ship track records from the North Pacific, East Pacific Rise, and the Indian-Antarctic ridge. Both studies concluded that tiny wiggles were "missing" (e.g., previously unidentified) short-duration field reversals, although the possibility was raised in both papers that tiny wiggles could Mso result from paleointensity fluctuations. Cande and œaBrecque [1974] examined multiple ship track sets from the North Pacific, Chile Rise, and Southeast Indian Ocean and used both full field reversals and intensity fluctuations to model field behavior for anomalies 5 and 12-13 tiny wiggles, noting that surface magnetic profiles lack the detail to discriminate between the two hypotheses. Recent]y, Cande and Kent [1992a] have studied the record produced by very fast spreading ridge systems in the Indian and southern oceans and conclude that paleointensity fluctuations best exp]aln the tiny wiggles they observe. In this study, the higher- resolution intensity information from site 522 sediments is examined and compared to the marine magnetic anomaly record for C12R tiny wiggles.

SITE LOCATION, SAMPLING AND SEDIMENT ACCUMULATION RATE

Site 522 from DSDP leg 73 is presently located in the South Atlantic, 26øS, 5øW at 4455 m water depth. Hydraulic piston coting retrieved 147 m of late Eocene to re- cent pelagic sediments, largely nannofossil and foraminifera oozes. Shipboard sampling at 20-cm intervals yielded an excellent magnetostratigraphy and indicated an unusually continuous and largely undisturbed 90 m of late Eocene to late Oligocene sediments [Tauxe et al., 1983]. We resam- pied this sequence at high resolution (approximately 3-cm to 4-cm intervals) in order to conduct an environmental magnetism study (in progress) of Milankovitch (orbital) cyclicity throughout the Oligocene. All in all, some 2600 oriented samples, 570 of which span the C12R interval, were retrieved. Hsii ee al. [1984] and Mead ee al. [1986] estimate a sediment accumulation rate of ~0.9 cm/kyr at site 522 for the late Eocene to early Oligocene, yielding a temporal sampling interval of about 3 to 5 kyr across the C12R interval. This estimate is based on magne- tostratigraphy and an assumption of no loss of matetia] at core breaks. Milankovitch cyclicity in susceptibilty and percent carbonate records within individual cores appear to indicate a somewhat higher sediment accumulation rate

of ~1.2 cm/kyr and accordingly a slightly shorter temporal sampling interval. Each individual specimen sampled ~0.9 cm of core length, thereby averaging an approximately 1-kyr signal.

MEASUREMENT OF MAGNETIC PROPERTIES

Our original intent was to investigate Milankovitch cycles in the Oligocene, and because this involved a very large sample suite, the natural remanence was originally retrieved only for magnetostratigraphic purposes. It was not until destructive rock magnetic measurements had already been performed that we became inttigued with what appeared to be a very interesting remanence record, particularly across the C12R interval. The resulting lack of detailed step-wise demagnetization is, we feel, largely mitigated by a very high quality rock magnetic record, as outlined below.

Natural remanence (NRM), anhysteretic remanence (ARM), isothermal remanence (IRM), low field mag- netic susceptibility (X) and dry weight of each sample were measured at the Scripps Institution of Oceanography. ARM was produced in a 0.1-mT bias field and an 85-mT alternating field (AF). IRM was induced with a 0.6-T impulse field. NRM, ARM, and IRM were all measured with a three-axis CTF cryogenic magnetometer housed in a magnetically shielded room at Scripps. Low field magnetic susceptibility was measured on a Kappa Bridge KLY-2.02 susceptibility bridge. The resulting data for the lower 46 m (1300 samples spanning the late Eocene to early mid-Oligocene) are plotted against core depth in Figure 1. Note that because core sections were not retrieved with

azimuthal otientation, the declination of the vector mean of each 1.5-m section was first calculated and then set to

0 ø (180 ø) for normal (reverse) polarity state, based upon inclination data for that section.

Stepwise AF demagnetization of NRM was performed on selected samples. A small viscous component was commonly removed by 5 mT, after which remanence typically decayed linearly to the origin (Figure 2a). Based on these results, "blanket" AF demagnetization at 17.5 mT was applied to remove the small viscous component for the 2600+ samples measured for the larger study. Thermal demagnetization of NRM was performed on samples from offset hole 522A, one of which is depicted in Figure 2b. Median destructive fields (MDF) of ARM and IRM were determined for a test group of samples with widely varying NRM values. MDFs were tightly clustered with values of p = 24mT, a= lmTforARM andp = 20 mT, a= 0.8 mT for IRM, suggesting very stable coercivity spectra for NRM as well.

A typical IRM acquisition result is shown in Figure 2c. Saturation is nearly attained at 0.3 T, with a slight increase occurring up to 0.6 T, the highest value obtainable with our in-house impulse magnetizer. In order to see if the proportion of this more highly coercive phase might change significantly from sample to sample, the inverted ratio of IRM induced by a 0.6-T field to that imparted subsequently in an antiparallel 0.3-T field, -IRM_o.3T]IRMo.(;T, was determined for each sample. The mean of the ratio was found to be /•- 0.954 with a- 0.01. We conclude that the magnetic mineralogy is largely unchanging downcore and is dominated by a moderate coercivity phase with a very small, nearly fixed percentage of a more highly coercive phase.

HAP•TL ET AL.: OLIGOCENE GEOMAGNETIC FIELD, DEEP SEA DPdLLING PP•OJECT SITE 522 19,651

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DSDP Site 522

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- ß ß .... •, 0 ß . •- ½..;• • •z ß ß . C3•.. ' '

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Fig. 1. NRM and rock magnetic record from DSDP site 522 spanning the late Eocene to early mid-Oligocene. Chron C12R is the long positive inclination interval from 108 to 128.5 mbsf (southern hemisphere is positive inclinations for reverse polarity). Because core sections were notoriented, declinations for each ~.5-m section were averaged to a value consistent with the polarityinterpretation for that section (based on inclination data).

19,652 HARTL ET AL.: OLIGOCENE GEOMAGNETIC FIELD, DEEP SEA DRILLING PROJECT SITE 522

AF Dernag, Sample: 35-1 -125 Ticks=5.0e-09 Arn 2

-X

NRM• 15mT

60rot

IRM Acquisition

c 1.0 .o_ o 0.8

._N

0',0.6

"004 • ß

E02 ,L

o z

0.0 0.0 0.2 0.4 0.6

Magnetizing Field, Tesla Fig. 2c. IRM acquisition curve for typical sample. Saturation is 95% attained at 0.3 T.

Stepwise thermal demagnetization was performed on selected samples. Each was subjected to impulse fields of

Fig. 2a. Typical AF demagnetization plot for site 522 samples, indicative of a single-component remanence. Solid dots are the 2.4 T, 0.6 T and 0.12 T oriented along orthogonal axes pr6jections of the remanence vector at eac• step onto the hor- after the method of Lowrie [1990] in order to determine izontal (X,Y) plane and open squares are projections onto the unblocking temperature profiles (Figure 2d). A magnetic vertical (X, Z) plane. The samples were unoriented with respect carrier with an ultimate blocking temperature of ~585øC to north. The coordinate system used follows a right-hand rule with Z measured positive down and X and Y defining the hori- was found to dominate all samples tested. A very small zontal plane. fraction of a more highly coercive phase, as represented

-Y -Z

150

RM

Y, Z

Therrnol Demag, Sample 5,569 Ticks= 1.0e-09 Am 2

Fig. 2b. Thermal demagnetization of offset hole 522A sample.

by the 0.6-2.4 T and 0.12-0.6 T components, exhibits an ultimate unb]ocking temperature very close to ~680øC. These results suggest that the magnetic carrier is dominated by magnetite with a small amount of hematite.

Hysteresis properties of selected samples were measured with a MicroMag model 2900 alternating gradient field magnetometer. The ratios of saturation remanence to saturation magnetization Mrs/Ms were very consistent at ~0.24 as were ratios of the coercivity of remanence to bulk coercivity, Her/He, at ~2.45 (Figure Z). These results suggest pseudo-single domain (PSD) magnetite of a very homogeneous grain size.

Plotting ARM versus X and ARM versus IRM (Figures 4a and 4b) also suggests a consistent composition and grain size of the magnetic carrier downcore, with correlation coefficients of 0.90 and 0.94, respectively. Concentration of the magnetic carrier, as measured by ARM, IRM, or X, varies downcore by ]ess than a factor of 3.

RELATIVE PALEOINTENSITY DETERMINATION

Attempts to determine relative changes in the Earth's ancient geomagnetic field strength from sedimentary records have been and continue to be controversial, largely because little is known about the details of the actual mechanism of

remanence acquisition in sediments. Experiments studying remanence acquisition in sediments redeposited in the laboratory [e.g., Barton et al., 1980; Kent, 1973; Tucker, 1980, 1981; Hamano, 1980; Tauxe and Kent, 1984; Lu et al., 1990] suggest that a host of environmental factors are influential, including but not limited to, water content, degree of compaction, type and concentration of magnetic minerals, and the nature of the sedimentary matrix.

Site 522 was situated in a very quiet water pelagic envi- ronment above the CCD in the South Atlantic gyre. The

HARTL ET AL.: OLIGOCENE GEOMAGNETIC FIELD, DEEP SEA DRILLING PROJECT SITE 522 19,653

1.0 5-Orthogon(•l Axes IRM Therrn(•l Derneg

>,08

c

c

o 0.4

,, (].2

0 - 0.12 T component

0.12 - 0.60 T component

0.6 - 2.40 T component

0.0 0 200 400 600

T Celsius

Fig. 2d. Thermal demagnetization of three orthoganally oriented isothermal remanences [•fter Lowri•, 1990] indicates a magnetic carrier dominated by a moderate coercivity phase with an unblocking temperature of 585•C and a sm•all component of high-coercivity phase with •n ultimate unblocklng temperature of 680•C. Ten samples selected across the 46-m section yielded very similar results.

sediments that accumulated throughout the Oligocene are pelagic nannofossil oozes with carbonate contents ranging from 87% to 94%. X ray diffraction and Fourier transform infrared analyses indicate a noncarbonate fraction, made up of nonopaline quartz aad clay (largely smectite) that also appears to be unchanging throughout this time period. The ratio of IRM to noncarbonate fraction is also quite constant, indicating that the magnetic carrier and the non- carbonate fraction share the same source. The consistency of rock magnetics mirrors these observations, indicating that environmental influences at site 522 offered minimal

perturbations to the remanence acquisition process during this time.

0.6

0.5

0.4

0.2

King et al. [1983] set forth minimum rock magnetic criteria which they felt must be met in order that changes in relative paleointensity be determined from sediments. The carrier must be PSD magnetite and must not vary in concentration by more than a factor of 20 or 30. The rock magnetics of the site 522 sediments easily meet these criteria.

If much of the variability in NRM strength can be attributed to changing concentrations of the magnetic carrier, then an appropriate parameter such as ARM, IRM or X may be used to normalize these fluctuations [e.g., Johnson et al., 1948; Harrison, 1966; Johnson et al., 1975; Levi and Banerjee, 1976]. ARM has far outstripped

0.1

0.0 1.0

Site 522 Hysteresis Results

$D

PSD

- I::] = Early Oligocene + = Mid/Late Oligocer

_

--

SP MD&SP

I I I i , I

1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0

Hrc/Hc

Fig. 3. Hysteresis parameters for 36 samples at site 522 spannin• the late Eocene to the late Ollgocene. All C12P• samples fall within the area depicted by squares.

19,654 HA•.TL ET AL.: OLIGOCENE GEOMAGNETIC FIELD, DEEP SEA DB. ILLING PB. OJECT SITE 522

a b

25-

E <15- I

o 10-

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ARM vs. IRM ARM vs. X 25-

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Ol 0 -

corr. coef. :2 5- = 0.94 <

I I I I I I 0 I I I I I I I 5 10 15 20 25 50 0 4 8 12

IRM (10-4Am2/kg) X (10-8rn3/kg)

. .

ß ., a% -. ß ß ß

..

corr. coef.

-- 0.90 I I I

16 20

Fig. 4. ARM versus X and ARM versus IRM display the largely homogeneous nature and small magnetic grain size range of the magnetic carrier for the C12R interval, with correlation coefficients of 0.90 and 0.94, respectively.

IRM or X in popularity as an appropriate normalizing parameter in recent years [King et ai., 1983; Constable, 1985; Constable and Tauxe, 1987; Tauxe and Valet, 1987; Tric et ai., 1992] because it is generally thought to have a coercivity spectrum that is more likely to mimic that of the natural remanence. IRM and X will both tend to overemphasize the role of multidomain (MD) particles [Levi and Banerjee, 1976] and, in addition, X will record changes in superparamagnetic (SP) particles which do not contribute to the stable remanence. However, the ratios of ARM/x, ARM/IRM, and IRM/x show little variation at site 522, and there is little difference between X, ARM, or IRM in the normalized results. Significant differences that do occur we regard as indicative of changes in grain size or mineralogy. For the following discussion, we define three estimates of geomagnetic field paleointensity as

, NRA4r (1) B• = IRM NRM

- ( )

NRM sx = (3)

x

where the asterisks denote that the value is an estimate of

the actual paleofield strength B. We note that although it has become standard procedure

to AF demagnetize the normalizing remanence at the same intensity as is done for NRM cleaning, the extreme consistency exhibited in MDF behavior of site 522 samples for both ARM and IRM indicate AF demagnetization is unnecessary. We chose to omit this additional step for the IRM and ARM of our suite of 2600 samples. However, at the prompting of review comments, we tested this practice by subjecting the IRM of approximately 1/3 of the samples to a 17.5-mT AF demagnetization. The variance introduced by this step when compared to the undemagnetized data set was found to be less than 4% of the signal. When normalized to the same mean and plotted together against core depth (at the scale of Figures 8, 10, 12, etc.) the two sets of results are indistinguishable.

In Figure 5 we plot B; (circles) and B• (triangles), each normalized to a mean of unity for Chron C12R and its

and B A' vs. core depth, Site 522

2.0

3.0 - C 12N .

2.5 -

0.5

c 12R c 13N

e_'4 e

! , I I I

115 120 125

Meters below seafloor (rnbsf)

0.0 • 110 130

Fig. 5. Downcore variation in paleointensity estimators B I (triangles) and B; (circles), each normalized to its mean value for the Chron 012R interval. Vertical lines join B• and B; for same sample. Note the tendency for B• to yield much higher estimates at highest values.

HARTL ET AL.: OLIGOCENE GEOMAGNETIC FIELD, DEEP SEA DRILLING PROJECT SITE $22 ]9,655

boundaries (indicated by vertical bars), with the pairs of estimates at each core depth joined by straight lines. (For clarity B•, which very is similar to B;, is not shown.) On the whole they are in excellent agreement, although it is clear that scatter in the results increases with B*

magnitude. We note that when B* is large compared with the average, there is a tendency for B; (and B•) to provide lower field estimates than B•.

As a check on the independence of intensity results from rock magnetic influences, we used simple scatter plots of B; and B• versus ARM, IRM, X,-IRM_o.aT/IRMo.•T, ARM/x and ARM/IRM and found no discernible relation. In a more sophisticated treatment of the data, we performed coherency tests [Tauxe and Wu, 1990] between these same data sets and found no significant coherence between any rock magnetic parameter and paleointensity proxy. We did note discrete exceptions, however, in that a small number of samples (7) with excellent remanence directions exhibited anomalously high B• and BA/B; ratios and very high IRM and X- Hysteresis properties of these samples were normal. To check if the anomalous behavior could be due to concentration effects, we incrementally imparted IRM and then demagnetized in both DC and AF fields. No differences were observable in plots of the results. AF demagnetization of ARM likewise provided no

discriminating information. Despite these negative results, because the four samples at 123.4 mbsf and the three at 126.5 m below sea floor (mbsf) exhibit such high B,• and IRM values, we feel that some rock magnetic effect is most probably being expressed in B,• for these samples and to a lesser extent B;. We view the resulting paleointensity determinations from these seven samples as very poorly constrained.

Figure 6 is a histogram of the observed values of B•, B;, and B x. The standard deviation is 42% of the mean for B•, 38% for B;, and 38% for B x. Standard deviations of the normalizers, ARM, IRM, and X, are 16%, 19%, and 21% of their means, respectively. Thus IRM and X normalizations yield lower variance than does ARM normalization, in spite of the fact that IRM and X exhibit greater downcore variance than ARM. This suggests that IRM or X normalization may be more effective in compensating for NRM intensity variations due solely to changes in magnetic concentration. In the absence of other criteria for preferring a particular estimate, we believe that these factors make B; and B• more conservative estimates of relative intensity variations.

It is appropriate to ask whether this variability is reasonable. Direct comparison with other records would be desirable; unfortunately, no such time series of intensity

120

lOO

80

60

40

20

C12R Histogroms for Relotive Intensity Estimotes

t _ _

i

•3i=0.38

•3A=0.41 •3 ---0.38

X

1 .o 2.0 5.0

Field estimotor

Fig. 6. Comparison of distributions for paleointensity est•nators B.•, B;, and B•( for site 522 sediments. B• yields the lowest variance of the three, yielding the most conservative estimate of paleointensity fluctuations.

19,656 HARTL ET AL.: OLIGOCENE GEOMAGNETIC FIELD, DEEP SEA DRILLING PROJECT SITE 522

Core 28..3; C12R / C12N Reversol

3'0 I 2.5 /%

c 2.0

c: ø • 1.5

.>

__o 1.0

0.5

0.0 i 107.7 108.0 108.3

• lOO

c 0

c• -100 I I I I I I I

107.7 108.0 108.3

8O

._o 40 o

._c_ 0 o -40

-80 - i I I I I I I

107.7 108.0 108.3

mbsf

Fig. 7. The C12R-•C12N reversal occurs at 108.2 mbsf. The intensity low has a duration of approximately 20 - 25 kyr which precedes the inclination shallowing and reversal. The rapid direc- tional and intensity changes across the reversal indicate minimal remanence smoothing at our sampling resolution of 3-4 cm. B• is denoted by dots connected by lines, B• by unconnected pluses,

BBi by unconnected squares. A three-point running average of is plotted 1 unit higher, denoted by diamonds connected by lines. Intensity lows are denoted by the large triangle. Asterisks in declination record mark the location of section breaks.

variations are yet available. We note, however, that as we would expect, the record exhibits extreme lows in intensity corresponding to the reversals at both the upper and lower boundaries of Chron C12R. Other intensity lows are also present, and these will be discussed in the next section. There is obviously a great deal of structure in this record, much of which is at finer scales than we can resolve. We

can, however, compare the gross variation in our Chron C12R record with that seen in absolute paleointensity measurements. McFadden and McEIhinny [1982] estimated that over the past 5 m.y. the dipole moment has had a standard deviation of 41.9% of its average value. This is very comparable to the size of variation obtained from the site 522 sediments, indicative that these are not unreasonable relative paleointensity estimates.

THE PALEOMAGNETIC RECORD

Even under ideal drilling conditions with the hydraulic piston corer (HP½), loss of material and/or sediment disturbance is common at core breaks II•udd•man et 1•8•]. Examination of both the physica• condition and the paleoma•netic recor• from t•e six cores t•at contain t•e C13N to C12N interval indicates t•at t•e top 15-50 cm are typically •istur•e• an• t•at some core •ottoms axe disturbed as well. With the exception of a few instructive examples, these disturbed zones have been excluded from the followin• plots and discussions. Missin• material at core breaks should •o be expected, although this is extremely difficult to evaluate without a corroborating core nearby. Because of these problems, we detail the record of each core sepaxately. In addition to core brea•s, each 4.5-m core is broken by two section breaks at 1.5-m intervals, across which •zimuthal continuity is lost. Section breaks typically have a much more benign influence on the paleomagnetic record; the location of each is marked with an asterisk in the decUnation portion of each plot.

We next examine the inclination, declination, and intensity data for each core, stacked one above another in the following plots for easy visual correlation. We also will plot VGP paths for key intervals. The data used to calculate virtual geomagnetic poles (VGP) were subject to two adjustments: (1) the previously discussed declination adjustment and (2) a correction factor to account for the ~ 7 ø northward drift of the African plate since the early Oligocene [Tauxe et al., 1983; Besse and Courtillot, 1991]. VGP paths calculated without the latter correction are very similar to those calculated with it, but are grouped in a slightly Unear-sided n fashion with respect to the South Pole. For the intensity plots, each contains (do• •onnec•ed •y Une•), • (unconnected V]u•e•),

* (unconnected squares), normalized to a unity mean Bx calculated for the whole C12R interval, for easy comparison. Plotting of all three together provides the side benefit of simultaneously indicating any major change in the various ratios of the normalizers, ARM/IRM, ARM/x, and IRM/x, which should mark significant departures from the Site 522 norm for magnetic grain size or mineralogy over the C12R interval. Last, a three-point running (vector) average of B• is offset and plotted I unit higher (/• -- 2) on each intensity graph.

Working from younger to older, we start with core 28, section 3 (Figure 7) in which the C12R -• C12N transition is recorded at 108 mbsf. The reversal is easily recognizable in the inclination and intensity records, but the declination record is much more unusual. No evidence

of core deformation was apparent in this interval, although the initial stage of the transition must missing in the core break at 108.4 mbsf. The intensity drop, of duration ,,,20-25 kyr, preceeds inclination shallowing and reversal, although a prereversal anomalous inclination is apparent. The rapid directional and intensity changes across this reversal indicate minimal remanence smoothing detectable at this saznple resolution of 3-4 cm. Note that the low intensity at the reversal is marked with a large triangle, with which we will likewise mark all low-intensity intervals in the C12R record for which the smoothed B• value falls below 0.6 of mean. The (partial) reversal VGP path is shown in Figure 8.

HARTL ET AL.: OLIGOCENE (lEOMAGNETIC F•S•n, DSSl• SSA DRILLING P•o.•sc'r S•'r• 522 19,657

C12R --• C12N 108.1mbsf

Fig. 8. VGP p•th of the C12R-•C12N reversal. Core breal• has removed beginning pa• of the reversal record. Site 522 loc•tion is denoted by square.

Core 29, the core immediately below the C12R--•C12N reversal, is shown in Figure 9. Visual inspection of the top section, section 29-1 (108.5-110 mbsf), indicated strong disturbance due to coring between 108.5 a•d 109.0 mbsf, less disturbance from 109.0 to 109.5 mbsf, and minor disturbance between 109.5 and 110 mbsf. The rema•ence

record of the 108.5-109 interval is largely chaotic, confirming our visual observations and is therefore omitted from Figure 9. The directional portion of the record from 109 to 110 mbsf is much more sensible a•d clearly indicates a broad, continuous zone of core twist with a few directional

outlyers, again confirming visual impressions. We feel confident, based on visual and magnetic observations, that the remainder of core 29, sections 2 a•d 3 (110-113 mbsf) are well below the zone of disturbance.

Core 29 contains a subtle, serially correlated feature at 110.5-110.8 mbsf. B• values drop to a reversal-like level of 0.2 (relative to mean), declinations swing gently westwaxd a•d back again, and inclinations abruptly shallow and then steepen. VGP mapping of this feature (Figure 10) indicates coherent excursionaxy behavior. Both the directional and B• trends of this feature resemble those of the bounding reversals of C12R. In addition, because the B• record •blends • sensibly into the surrounding record a•d because coring disturbances axe not apparent, we believe that the B• record is a reliable paleointensity proxy for core 29 below ~110 mbsf, a•d possibly even in the 109-110 mbsf

interval. These paleointensities are serially correlated, with wavelengths of the order of 0.4 m (~45 kyr) and amplitudes that vary by up to a factor of 6.

Working down section to the next core, core 30 (Figure 11), we find that the high paleointensity values apparent in the bottom of core 29 are matched at the top of core 30, although 10-20% discrepancies between B•, B•, a•nd B x caution us that rock magnetic changes may be influential between 113 and 114 mbsf. Amplitude vaxiations are similar to those of core 29. Two interesting features axe present in core 30 (maxked by triangles), centered at 114.2 a•d 116.6 mbsf. The feature at 114.2 mbsf is

characterized by a sudden intensity drop (although not to vaJues as low as at reversals a•d thus the question mark), a shift to a westward declination and steepened inclinations followed perhaps by a •relaxation • period in which stability is regained. It occurs over ~0.35 m, or ~40 kyr time. The more draxaatic feature at 116.6 mbsf is also characterized by an abrupt intensity drop, a sudden shift to westerly declinations a•d low inclinations followed by a counterclockwise rotation of declinations (well past 180ø), a resteepening of inclinations, a•d a rebuilding of intensity. A VGP map (Figure 12) indicates an excursion to low latitudes confined to a path along ~ 90øW longitude. This excursion occurs over ~0.3 m, indicating an approximate duration of 35 kyr.

Core 31 directional a•d intensity values axe plotted in

19,658 HAP,.TL ET AL.: OLIGOCENE GEOMAGNETIC FIELD, DEEP SEA DRILLING PROJECT SITE 522

3.0

>, 2.5

• 2.0

1.5

o 1.0

0.5

0.0 109.0

Core 29

I J I i I I I

110.0 111.0 112.0

I

113.0

300

200

100

0 109.0

I - core twist - I /•

I I • I I ' I • I I 110.0 111.0 112.0

I

113.0

80-

.9 40 o

.g o A o -40

-80 109.0 110.0 111.0 112.0

mbsf

I

113.0

Fig. 9. Core 29 record. Interval from 108.5 to 109 mbsf was badly disturbed by drilling and is not shown; the remaining portion of this section from 109 to 110 mbsf appears to have suffered core twist. Intensity low marked by the large triangle at 110.6 mbsf is believed genuine and is accompanied by coherent directional changes. See Figure 7 caption for explanation of symbols.

Figure 13. Superposed over our directional data are the original shipboard results (plotted as open circles) from DSDP leg 73. In the top two sections of core 31, the older data essentially overplot our own. We include the shipboard data, however, to illustrate that one section, section 31-3, suffered degradation after the original measurements were made, apparently during storage. Luckily, no other C12R core section was found to be similarly altered. The altered section exhibits overly steep reverse inclinations and disturbed declinations. It must be surmised that the

intensity information for core 31-3, representing the interval from 120.3 to 121.6 mbsf, is likewise suspect.

The record from the upper two sections of core 31 resemble those of cores 29 and 30 except that no clearcut excursions in the directional data are present. The intensity record features a prominent low at 119 mbsf resembling those previously noted at 110.6, 114.2, and 116.6 mbsf, but one that is not associated with directional changes of any note. Lows at 118 and 120 mbsf, although less clear, are also indicated. The apparent low at 121 mbsf will not be considered here because of the previously mentioned degradation of section 31-3.

Core 32 (Figure 14) yields an intensity record of a lower average value. As was mentioned earlier, the short- duration high at 123.4 mbsf, although associated with well-behaved inclinations and declinations, also exhibits anomalously high IRM and X values and must therefore be considered as suspect. Elsewhere, the low at 125.3

mbsf is accompanied by unstable inclinations and coherent small-scale trends in declination. Similar results were

obtained for shipboard measurements but because visual evidence of core disturbance was evident from 124.8 to

125.3 mbsf, it is likely that core twist is responsible for this signal. In contrast, intensity lows spanning 125.4-125.6 and 123.55-124.1 mbsf are not associated with any visually observable core deformation, nor did we experience unusual sa•npling difficulties in this interval that might explain these features. Therefore, we regard these two features as paleointensity lows, noting that they are accompanied by somewhat unstable inclinations but no coherent directional trends.

The C13N --• C12R reversal occurs at 128.5 mbsf in the

lowest core, core 33 (Figure 15). It could be a product of heavy remanence smoothing, but smoothing between two antipodal states would result in simultaneous shifts in inclination and declination, which clearly is not the case here. Instead, the reversal is initiated by inclination shallowing and an intensity drop, followed by a gradual counterclockwise rotation of declinations. The inclination

change and low-intensity interval occur over ~0.4 m of core, although inclinations do not appear to completely stabilize for another ~ 0.8 m. Declinations complete the reversal in ~0.5 m, slightly higher (later) than the low-intensity interval. The associated VGP path (Figure 16) displays a decided longitudinal confinement along ~ 70øW longitude, quite similar to the Matuya•na-Bruhnes VGP record from

HARTL ET AL.: OLIGOCENE GEOMAGNETIC FIELD, DEEP SEA DRILLING PROJECT SITE 522 19,659

..

..........

Excursion 110.6mbsf

Fig. 10. VGP excursion associated with intensity low st 110.6 mbsf. Note preferential trend along 80øW and serial coherence of feature.

3.0

• 2.5 ._

• 2.0

1.5 ._>

o 1.0

0.5

0.0 11.3.0

Core .30

4-

i •" + +

+ + +

I I I I I I I

14.0 115.0 116.0 117,0

3O0

•00

•00

0 11.3.o 114.o 115.0 116.0 117.0

8o

.9 40

._.c_ 0 u -40

-80

11,3.0

I I I I I I I I

114.0 115,0 116.0 117,0

mbsf

Fig. 11. Core 30 record. Intensity lows at 114.2 and 116.6 mbsf, the latter is accompanied by & VGP excursion. Note similar core length (duration) for these features. See Figure 7 caption for plot explanation.

19,660 [ARTL ET AL.: OLIGOCENE (]lEOMAGNETIC FIELD, DEEP SEA DRILLING PROJECT SITE 522

Excursion 116.6mbsf

Fig. 12. VOP excursion associated with intensity low at 116.6 mbsf. Note the longitudinal confinement to 90øW longitude.

Core 31

3.0

>, 2.5

e 2.0

1..5

.> o 1.0

0.5

0.0 • i • i I , 118.0 119.0 120.0

I I

f2•1.0

.• 300f I - disturbed

2øø I .._

• 100

r• 0 I I I •l I I I I 118.0 119.0 120.0 121.0

u -40• _c -80 •

0

I I I I I I I

8.0 119.0 120.0 121.0 mbsf

Fig. 13. Core 31 record. Large triangles mark intensity lows discussed in the text. Open circles plotted over inclinAtion and declination records are determinations made shipboard at time of core retrieval. Note the high degree of agreement between the shipboard record and this record for the top two sections (117.5-121.3 mbsf), but the scatter of declinations and overly steep inclinations of our record from the lowest section compared. Section 31-3 has apparently suffered alteration since retrieval, and we therefore do not mark the apparent low at 121 mbsf. See Figure 7 caption for more symbol explanation.

ß

H•,•T•, gT •,•,.: O•,ZOOCgl• G•OM•,OI•.TZC Fm•,I), Dg•P Sg•, D•,•,n•o P•oJ•cT S•T•. 522 19,661

Core .32

3.0

2.5 A 2.0

•.5 •.o

0.5 0.0

' 122.0 123.0 124.0 125.0 126.0

C 300f I -disturbed-I o 200• • 100• 0 / i I • i a i - i i a i i i

122.0 123.0 124.0 125.0 126.0

8O

õ 4o '.• 0

• -40

-80 t i i i i i ' I 'i I

122.0 123.0 124.0 125.0 126.0

mbsf

Fig. 14. Core 32 record. High intensity values at 123.4 mbsf are az• søCiated with stable remanence directions, but a larger than normal difference in the ratios ARM/IRM, apparent in disparent intensity •stimates by .B• versus B• (see text). Low at 125.25 mbsf is assoc/ated with core deformation and probable tw/st (not//declination record). Lows centered at 123.8 and 125.5 mbsf appear to be bona fide but are not associated •ith any coherent trends in declination or inclination, although some dire•ti0nai instabilities are apparent (see "discussion and conclusions" in text). See Figure 7 caption for symbol explanation.

3.0

• 2.5

• 2.0

1.5 .> o 1.0

0.5

0.0

Core 33, C13N => C12R Reversal -

_

i i I I I I

126.5 127.0 127.5 128.0 128.5 129.0

I

129.5

300

200

100

0 I I I 4l I I I

126.5 127.0 127.5 128.0 128.5 129.0

I

129.5

8O

.9 40

.• 0 u -40

-80

/ I I I I I I

126.5 127.0 127.5 128.0 128.5 129.0

mbsf

I

129.5

Fig. 15. Inclination, declination, and intensity recorc• for core 33. The C13N--•C12R reversal occurs at 128.5 mbsf. Note that inclination and intensity (B• or B•) abruptly changes at the initiation of the reversal, followed by a more gradual shift in declinations.

19,662 HARTL ET AL.: OLIGOCENE GEOMAGNETIC FIELD, DEEP SEA DRILLING PROJECT SITE 522

C13N => C12R 1283mbsf

Fig. lB. VGP path of C13N--•Cl•R reversal record from site 5•2. VGPs are largely confmed to ?0-90øW longitude.

2.0

• 1.5 ._

• 1.0

n- 0.,5

0.0

C12N [

I

105

Site 522 Poleo•ntensity Record

I C13N

Z• Z•? A Z•?

I I I I I

110 115 120 125 130

mbsf

8O

.40

o

-40

-80

C12N

u I u I I u II I

C12R C13N

Morine Mognetic Anomoly "Tiny Wiggles" (Conde & Kent, 1992) I I I I I I

31.0 31.5 32.0 32.5 33.0 33.5

Mo

Fig. 17. Schematic plot of temporal locations of "tiny wiggles"in C12R from C%r•de sr•d Kent [1992a] (bottom), compared to smoothed B• record (15-cm window) from site 522 (top). Triangles mark locatiotin of intensity lows identified previously in Figures 7-15. Missing section from site 522 core breaks and misspacing due to sedimentation rate changes may account for discrepancies (see "discussion and conclusions"in text). Dashed horizontal line at 0.6 of mean value for horizontal reference.

HARTL ET AL.: OLIGOCENE GEOMAGNETIC FIELD, DEE? SEA DRILLING PROJECT SITE 522 19,663

the nearby V16-58 [see Clement, 1991]. The zone of low-intensity represents approximately 45 kyr of time.

Above 127.5 mbsf, inclinations and declinations are stable, whereas intensity fluctuates in a coherent fashion from ~2 to 10 times the lowest value observed during the reversal. The low-intensity interval at 127.4 mbsf is marked with a question mark because of its short duration.

As discussed previously, the three samples with very high relative intensity values at 126.5 mbsf also exhibit anomalously low ARM/IRM and ARM/x, apparent in the large discrepency between the intensity estimates from NRM/ARM versus NRM/IRM and NRM/x. Estimates from these three points must be considered highly suspect.

DISCUSSION AND CONCLUSIONS

If we have been successful in identifying and accounting for the effects of core disturbances and rock magnetic irregularities, then the remaining signal is that of a time series of geomagnetic field behavior at site 522 from 31 to 33 Ma. The signal from each sample is known to be smoothed over ~1 kyr due to the sample size and is probably also averaged over a longer interval of time due to the unknown duration of remanence lock-in processes. The smoothing, combined with a sampling interval of ~3.5-4.5 kyr, means that if short duration features such as the Mono Lake excursion [Liddicoat and Coe, 1979] or the Laschamp excursion [Roperch et al., 1988] occurred, they would either be missed entirely or largely smoothed beyond detection. Levi and Karlin [1989] have shown that the Mono Lake, Laschamp and perhaps other excursions occurred during a general paleointensity low that occurred from 53-22 ka. An intensity low of similar duration would be readily apparent in the site 522 record, and we propose that many of the low-intensity features marked by triangles in our plots, with their attendant directional instabilities, are best understood within this context.

The features centered at 110.6 mbsf, and particularly at 116.6 mbsf, seem to be of a fundamentally different nature, however. The sequential coherency of directional changes of these features results in smoothly progressing, confined VGP excursion paths along ~70-90øW longitude that are sustained away from the pole for about 10-15 kyr. The pseudo-sinusoidal profile of the accompanying intensity low appears much less random and bears strong resemblence to intensity behavior at reversals. For these reasons, we interpret these features to be either aborted reversals or heavily smoothed short subchrons.

Both types of intensity lows in the site 522 record are of similar duration, and both reach similarly low B• values. Although the feature at 116.6 mbsf and the bounding reversals dip to slightly lower values, the m•in difference between the two groups is found in the directional behavior. If we assume that the general low in both cases occurs in response to a diminished axial dipole field component, then the much more coherent directional structure for the bounding reversals and the features at 110.6 and 116.6 mbsf imply a coherent nondipole field structure, or perhaps a record produced by heavy smoothing between two nonantipodal pole positions. Overprinting or remagnetization of low-intensity intervals can also produce such features, but we reject this model because it predicts similar directional behavior for all low-intensity intervals, whereas we observe it in only 4 of 11 cases. Furthermore, overprinting between antipodal polarity states

should produce VGP paths confined relatively close to the site longitude, which is not observed. On the other hand, the temporal structure of remanence directions, intensities and VGP directions across these intervals bears

similarities to those observed by Clement [1991], Laj et al. [1991] and Tric et al. [1991] for more recent reversals. Clement [1991] modeled such features as resulting from the dominance of a low order nonzonal term (in the spherical harmonic representation of the field) during a period of diminished axial dipole strength. Hoffman [1992] has suggested an alternative model in which the geomagnetic field poles migrate during field transitions to relatively fixed positions some distance from the Earth's rotational axis. Sedimentary (remanence) smoothing between two of these points can also produce a confined VGP path over the Americas as well as the "other" path at ~90øE longitude. We do not feel that our relatively low sedimentation rate record at site 522 permits us to support one of these models over the other, but note that either model can reasonably explain the VGP results. We suggest that the early Oligocene geomagnetic field was at times subject to a geographic control similar to that exhibited during some of the more recent reversals as observed by Clement [1991], Laj et al. [1991], Tric et al. [1991] and/or Hoffman [1992]. If true, this observation should be important to modeling the core and deep mantle processes that produce and influence the geomagnetic field.

As noted above, for the majority of the low-intensity intervals we have identified, no serial trends in the directional data are evident. For these cases, we suggest a different geomagnetic field model in which the axial dipole field is greatly diminished while no other field component predominates. Levi and Karlin [1989] present evidence for such an event for the period 22-55 ka, during which time they postulate that a nearly random, temporaJly unstructured nondipole field produced a record with numerous short-term (<2 kyr) excursions.

It is reasonable to assume that the low dipole moments that we postulate produced the intensity lows and associ- ated behavior in the C12R record at site 522 would also

leave a mark on C12R marine magnetic anomalies. Cande and LaBrecque [1974] concluded that the "tiny wiggles" that they observed in C12R marine magnetic anomalies could have resulted from either paleointensity lows or short-duration field reversals. Cande and Kent [1992a], with higher-resolution data for anomalies 5 and 21-25, concluded that "tiny wiggles" are more likely to be records of paleointensity lows (but had no newer, higher-resolution data on C12R). In Figure 17 we plot and compare the core locations (in meters below sea floor) of the paleoin- tensity lows from the site 522 record with the temporal locations of C12R "tiny wiggles" as deduced by Cande and LaBrecque [1974] and Cande and Kent [1992a, b]. Although the correlation is far from perfect, missing and/or disturbed record at core breaks and changes in sedimentation rate in the site 522 record could account for much of the

discrepancy; without a corroborating record, this is diffi- cult to assess. However, because the number and timing of site 522 intensity lows and C12R "tiny wiggles" are approximately the same, and because the same mechanism, i.e., global geomagnetic field intensity fluctuations to low values, is postulated as a cause for both, we choose to make the correlation. If this is correct, then the site 522 C12R paleointensity record contains a sedimentary record

19,664 HAP. TL ET AL.: OLIGOCENE (]lEOMAGNETIC FIELD, DEEP SEA DRILLING PROJECT SITE 522

of "tiny wiggles •, which result primaxily from geomagnetic intensity lows, but caxt also be accompaxtied by directionM excursions; conclusions that are largely in agreement with those of Cande and œaBrecque [1974] and Cande and Kent [1992a].

Acknowledgments. The authors would like to thank Tim Her- bert for useful and stimulating discussions, as well as Jeff Gee and Peter Woodbury at Scripps and Paula Weiss at the ODP East Coast Repository for technical assistance. We would also like to thax• Deamis Kent and Steve Cande for generously pro- riding a copy of their manuscript while still in review. This work was supported by NSF grant OCE 89-17252. Funds for paleo- magnetic equipment at Scripps were provided by the W.M. Keck Foundation.

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C. Constable, P. Hartl, and L. Tauxe, Scripps Institution of Oceanography, La Jolla, CA, 92093-0208

(Received September 28, 1992; revised June 10, 1993;

accepted July 21, 1993.)