Paleointensity in Hawaiian Scientific Drilling Project Hole (HSDP2): Results from submarine basaltic...

Paleointensity in Hawaiian Scientific Drilling Project Hole(HSDP2): Results from submarine basaltic glass

Lisa TauxeScripps Institution of Oceanography, University of California, San Diego, La Jolla, California 92093, USA([email protected])

Jeffrey J. LoveU. S. Geological Survey, Golden, Box 25046, MS966, DFC, Denver, Colorado 80225, USA ( [email protected])

[1] Paleointensity estimates based on the high quality Thellier-Thellier data from the early Brunhes (420–

780 ka) are rare (only 30 in the published literature). The Second Hawaiian Scientific Drilling Project

(HSDP2) drill hole recovered submarine volcanics spanning the approximate time period of 420–550 ka.

These are of particular interest for absolute paleointensity studies owing to the abundance of fresh

submarine basaltic glass, which can preserve an excellent record of ancient geomagnetic field intensity. We

present here new results of Thellier-Thellier paleointensity experiments that nearly double the number of

reliable paleointensity data available for the early Brunhes. We also show that the magnetizations of the

associated submarine basalts are dominated by viscous magnetizations and therefore do not reflect the true

ancient geomagnetic field intensity at the time of extrusion. The viscous contamination is particularly

severe because of a combination of low blocking temperatures in the basalts and relatively high

temperatures in the deeper parts of the drill core. Our new data, when placed on the approximate timescale

available for HSDP and HSDP2, are at odds with other contemporaneous paleointensity data. The

discrepancy can be reconciled by adjusting the HSDP timescales to be younger by about 35 kyr.

Components: 7257 words, 13 figures, 5 Auxiliary material.

Keywords: Paleointensity; thellier technique; submarine basaltic glass.

Index Terms: 1521 Geomagnetism and Paleomagnetism: Paleointensity; 1560 Geomagnetism and Paleomagnetism: Time

variations—secular and long term; 1594 Geomagnetism and Paleomagnetism: Instruments and techniques; 1540

Geomagnetism and Paleomagnetism: Rock and mineral magnetism.

Received 15 November 2001; Revised 12 June 2002; Accepted 13 September 2002; Published 15 February 2003.

Tauxe, L., and J. J. Love, Paleointensity in Hawaiian Scientific Drilling Project Hole (HSDP2): Results from submarine

basaltic glass, Geochem. Geophys. Geosyst., 4(2), 8702, doi:10.1029/2001GC000276, 2003.

————————————

Theme: Hawaii Scientific Drilling Project Guest Editors: Don DePaolo, Ed Stolper, and Don Thomas

1. Introduction

[2] Information about past variations in the strength

of the geomagnetic field can be derived from

records of two fundamentally different types: abso-

lute paleointensity estimates based on thermally

blocked remanences and relative paleointensity

estimates based on normalized records of sediments

G3G3GeochemistryGeophysics

Geosystems

Published by AGU and the Geochemical Society

AN ELECTRONIC JOURNAL OF THE EARTH SCIENCES

GeochemistryGeophysics

Geosystems

Article

Volume 4, Number 2

15 February 2003

8702, doi:10.1029/2001GC000276

ISSN: 1525-2027

Copyright 2003 by the American Geophysical Union 1 of 18

[see, e.g., Tauxe, 1993] or marine magnetic anoma-

lies [Gee et al., 1996]. Despite major effort, the

average geomagnetic field intensity during the early

Brunhes Chron remains poorly constrained. The

Hawaiian Scientific Drilling Project (HSDP) pene-

trated several kilometers of submarine extrusives

and hyaloclastites which are thought to be of early

Brunhes age. Of particular interest is the recovery

of abundant fresh submarine basaltic glass, a mate-

rial that often retains an excellent record of paleo-

field strength [see, e.g., Pick and Tauxe, 1993]. In

this paper, we review the available data for the

Brunhes Chron and present the results of paleoin-

tensity experiments on submarine basaltic glass

samples obtained from the HSDP2 drill core.

2. Published Brunhes Paleointensity

[3] A compilation of absolute paleointensity data is

described by Perrin et al. [1998] and is available on

line: http://www.ngdc.noaa.gov/seg/potfld/paleo.

html. We have updated the so-called PINT00 data-

base with absolute paleointensities based on the

Thellier-Thellier technique [Thellier and Thellier,

1959] that have been published since 1996. A list of

all data from the Brunhes Chron (<780 kyr) and the

references are listed in the auxiliary material (appen-

dix 1 and appendix 2). We have checked the data in

the data base against the original publication and

corrected information as appropriate. The ages used

here correspond to ages implied or stated in the

original references. In many cases, these are nomi-

nal ages based on interpolation or correlation. In

such cases, we have assigned uncertainty to zero.

We have (re)calculated the virtual axial dipole

moment (VADM) based on the latitudes and paleo-

field information included in the data table.

[4] Databases are intended to be inclusive so each

investigator must decide on a set of criteria that

will exclude data likely to be erroneous. In

Figure 1a we plot the VADMs from the data in

auxiliary material (appendix 1) that were based on

at least three specimens per cooling unit with a

standard deviation of less than 15% of the mean

value. (Please note that we use units of ZAm2

where one Zetta (Z) is 1021. These are therefore a

factor of ten larger than the usual units of

1022Am2). Of the 295 paleointensity estimates

published for the Brunhes that meet these minimal

reliability criteria, only 30 are older than 420 kyr.

In Figure 1b we plot the record of relative paleo-

intensity obtained by Guyodo and Valet [1999] by

stacking many individual sedimentary paleointen-

sity records placed on the astronomical timescale

(SINT800). These data, while likely to represent

variations in paleofield strength, are inherently

relative. They were calibrated to approximate

VADM by comparison with absolute data over

the recent past.

[5] We re-normalized the stack derived from deep-

towed magnetic anomaly profiles Gee et al. [2000]

to have the same mean as the calibrated SINT800

record and plot them in Figure 1b for comparison.

The correspondence between the independent two

relative paleointensity records is remarkably good.

We have numbered several prominant paleointen-

sity lows (PLs) and lettered several paleointensity

highs (PHs) for easy reference. PLs are often

referred to by the names of geomagnetic excursions

defined by directional aberrations observed else-

where [see, e.g., Valet and Meynadier, 1993]. For

example, PL1 is thought to be correlative to the

Laschamp Excursion. Given the difficulty of the

absolute age assignments in many of the excur-

sional records [see, e.g., Kent et al., 2002], it is

premature to make a definitive correlation of each

PL with an associated excursion.

[6] The absolute paleointensity data are less con-

tinuous than the relative paleointensity data and are

likely to have larger uncertainties in age assign-

ment. It appears that in time periods of dense

smapling, however, the absolute paleointensity

data record some of the prominant features in the

relative paleointensity records (e.g., PHa and PL1).

Nonetheless a direct comparison between the abso-

lute and relative paleointensity records for the

entire Brunhes must await more and better dated

absolute paleointensity data.

[7] There are very few absolute paleointensity data

points prior to 420 ka, hence more data from the

early Brunhes are especially needed. The second

Hawaii Scientific Drilling Project has recently

completed drilling a hole (HSDP2) at 19�420N

GeochemistryGeophysicsGeosystems G3G3

tauxe and love: paleointensity in hsdp2 10.1029/2001GC000276

2 of 18

and 155�30W. HSDP2 recovered several thousand

meters of submarine basaltic lavas and hyaloclas-

tites [DePaolo et al., 2001] including significant

amounts of fresh submarine basaltic glass that

range in age from about 420 to 550 kyr [DePaolo

and Stolper, 1996; W. Sharp, personal communi-

cation, 2002].

[8] Submarine basaltic glass is well suited for pa-

leointensity measurements [see, e.g., Selkin and

Tauxe, 2000] for a recent compilation). The mag-

netic remanence is carried by single-domain (tita-

no)magnetite often with excellent rock magnetic

behavior during the paleointensity experiment.

The NRM was acquired during initial quenching

of the glass, hence the cooling rate can be approxi-

mated to within an order of magnitude in the

laboratory. Finally, magnetite encased in glass can

avoid pervasive alteration for very long periods of

time [see, e.g., Zhou et al., 1997, 1999a, 1999b; Xu

et al., 1997a, 1997b]. In this study we seized the

opportunity to augment the early Brunhes geomag-

netic field intensity variations by exploiting the

unaltered submarine basaltic glasses recovered from

HSDP2.

3. Thellier-Thellier Experiment

[9] The most common method for obtaining esti-

mates of paleofield strength from thermally blocked

a)

b)

SINT 800

Gee et al. (2000)

1 23

45

a

b c d ef

Figure 1. (a) Previously published data from Thellier-Thellier experiments (with pTRM checks) meeting minimumcriteria for reliability (see text). Open circles with dots are from HSDP1 [Laj and Kissel, 1999]. The solid line is thepresent field dipole moment and the dashed line is the 5 Myr average of Selkin and Tauxe [2000]. (b) Relativepaleointensity stacks for the Brunhes Chron. Thin line is an inversion from a deep-tow magnetic anomaly stack ofGee et al. [2000] from the East Pacific Rise. Heavy line is the SINT800 stack from relative paleointensity records ofGuyodo and Valet [1999]. The anomaly inversion was normalized to have a common mean to the SINT800 stack.Prominent paleointensity highs are lettered a–f and lows are numbered 1–5.

GeochemistryGeophysicsGeosystems G3G3 10.1029/2001GC000276tauxe and love: paleointensity in hsdp2 10.1029/2001GC000276

3 of 18

remanences is the so-called ‘‘Thellier-Thellier tech-

nique’’ [Thellier and Thellier, 1959]. Thellier-Thel-

lier experiments rely on several implicit assumptions

including:

1. The thermal remanence (TRM) acquired is

linearly related to the field in which the rock cools.

2. The pTRM acquired during laboratory heating

is equivalent to the pTRM acquired during initial

cooling.

3. A partial thermal remanence (pTRM) acquired

by cooling in an applied field from a given

temperature is entirely removed by reheating to

the same temperature in a null field.

[10] There is a great variety in approaches to pa-

leointensity determination. This derives to some

extent from the fact that the original Thellier and

Thellier [1959] article contained a description of

several different experimental protocols. The pro-

tocol used in this study is based on the ‘‘Coe

variant’’ described by Coe [1967]. We heat a

specimen to a temperature Ti and cool it in a null

magnetic field (the ‘‘first zero field step’’). After

measuring the natural magnetic remanence (NRM)

the specimen is re-heated to the same temperature

and cooled in a controlled magnetic field (the ‘‘first

in-field step’’). After the first zero field step we can

repeat an in-field step at a lower temperature to

determine if the capacity to acquire pTRM has

changed (the ‘‘pTRM check’’). In addition to the

standard pTRM check, we can conduct a second

test. After the first in-field step, the specimen is

heated again to Ti and cooled in zero magnetic

field. This measurement checks whether all the

pTRM acquired in the intervening in-field step

was removed by re-heating to the same temperature

and cooling in zero field. This procedure is briefly

described by Dunlop and Ozdemir [1994] and in

more detail by Riisager and Riisager [2001].

Because a difference in blocking and unblocking

temperature (the latter being higher) is character-

istic of multi-domain pTRM, we call this the ‘‘MD

check’’.

[11] Ideally, the assumptions inherent in the Thel-

lier-Thellier technique would be verified by the

experimental protocol. The assumption that the

natural remanence is a linear function of the ancient

field strength is almost certainly true for thermal

remanence magnetizations in fields less than about

100 mT. The validity of this assumption in a par-

ticular paleomagnetic specimen would be compro-

mised if the magnetization was not in fact an

original TRM, but some other magnetization

acquired later in the history of the sample (for

example a viscous remanence, VRM, or chemical

remanence, CRM). To verify the origin of rema-

nence, some investigators demand that the portion

of the NRM used in the Thellier-Thellier paleofield

calculation trend to the origin. This ‘‘origin test’’ is

necessary but not sufficient to establish the applic-

ability of assumption 1. We calculate the scatter of

the NRM measurements about the best fit line

(MAD ofKirschvink [1980]) and the deviation from

the origin of this direction (a of Selkin and Tauxe

[2000]).

[12] The assumption that a given laboratory pTRM

accurately reproduces the original pTRM acquired

between two temperatures can be erroneous for

several reasons. First, if the cooling rate in the

laboratory is different from the original cooling

rate this assumption is demonstrably false [e.g.,

Aitken et al., 1981; Dodson and McClelland-

Brown, 1980; Halgedahl et al., 1980] Furthermore,

if the rock undergoes chemical alteration during

laboratory heating the laboratory pTRM will also

differ from the original pTRM. Compensating for

differences in cooling rate is relatively straight

forward if the original cooling rate is well known

and the samples behave according to single domain

theory (see, e.g., Selkin and Tauxe [2000] for a

simple graphical correction method). Recognizing

changes in mineralogy can be accomplished using

the pTRM checks, monitoring susceptibility, etc.

We use the difference ratio or DRAT parameter

which is the ratio of the difference between repeat

in field steps normalized by the length of selected

line used in the paleofield calculation [see Selkin

and Tauxe, 2000]. The pTRM checks are quite

powerful, but also must be considered necessary

but not sufficient tests for the validity of assump-

tion 2 (see below).

[13] The assumption that the blocking and un-

blocking temperatures for a given pTRM are equi-

valent may not always be true for multi-domain



4 of 18

(MD) remanences [e.g., Levi, 1977; Bol’shakov and

Shcherbakova, 1979; Dunlop and Xu, 1994].

Because the blocking temperature Tb of MD rema-

nence is lower than the unblocking temperature

Tub, the plots of remanence remaining versus

pTRM gained (‘‘Arai diagrams’’ [Nagata et al.,

1963]) are not linear, but are concave upward.

Removal of a viscous remanence also leads to

non-linear Arai diagrams. For these reasons we

use two criteria. The b parameter (b = s/jbj where sis the standard error of the slope and jbj is its

absolute value) ensures linearity of the NRM/

pTRM data. The MD check tests the equivalence

of the blocking and unblocking temperatures in the

specimen.

[14] Our experimental protocol will detect most

errors caused by secondary remanences, MD rema-

nences and alteration of the magnetic mineralogy

during the experiment. There are several patholog-

ical cases, however, that will generate erroneous

results that are not explicitly anticipated by the

protocol. Let us consider three scenarios of alter-

ation and the consequences to the Thellier-Thellier

experiment. The first scenario is that a magnetic

mineral produced has a discrete blocking temper-

ature Tb > Ta (sketched in Figure 2a). The second is

that a magnetic mineral is produced with a distri-

bution of blocking temperatures all above Ta(sketched in Figure 2b). The third scenario is the

more usual one in which a magnetic mineral is

produced with a broad distribution of blocking

temperatures that includes Ta and the lower temper-

ature of the pTRM check Tp (sketched in Figure

2c). In all of these scenarios we assume that Tb =

Tub rendering the MD test blind to the alteration.

[15] A flow chart of an experiment in which the

first scenario occurs is sketched in Figure 3.

Several double heating steps occur, including one

at Tp at which we plan to do a pTRM check

eventually. At temperature Ta a secondary magnetic

mineral is produced with a blocking temperature

Tb > Ta. No erroneous pTRM will be acquired until

the temperature Ta is reached for the first in-field

step. The first zero field step is always done at a

higher temperature than the previous in-field step,

hence the NRM will never be affected. The erro-

neous pTRM will appear in the Arai plot as a

discrete offset at Ta, followed by ‘‘normal’’ NRM/

pTRM behavior as shown in Figure 2d. Such a plot

would fail the b criterion if the temperature steps

included Ta. Otherwise, the estimated paleofield

would in fact be correct.

[16] In the second scenario depicted in Figure 2b,

there are a range of blocking temperatures in the

new magnetic phase, all above Ta. In such a case,

there would be a continuous increase in the erro-

neous pTRM acquired, yet both pTRM (triangles)

and MD (squares) checks would pass. Nonetheless,

these data would fail the b test for linearity.

[17] The third, less worrisome scenario, is depicted

in Figure 2c whereby the blocking temperature

spectrum of the new mineral is quite broad and

includes Tp. In this case the pTRM checks would

fail as well as the test for linearity.

[18] In summary, while not infallible, the basic Coe

[1967] experimental protocol, with the addition of

the MAD, origin and MD checks that have been

added over the years, is quite robust. Most sources

of error can be detected. Other sources of error, for

example, large and variable local magnetic anoma-

lies near the specimen as it cools [see Carlut and

Kent, 2002], can be detected by also demanding

that replicate measurements be made on several

specimens from the same cooling unit.

[19] In the following section, we describe our ex-

perimental efforts on samples of submarine basaltic

glass recovered from the Hawaiian Scientific Drill-

ing Project core, HSDP2. We start with a descrip-

tion of the lithologic log, including downhole

temperature and an age model. We will then dis-

cuss the effect of elevated temperature on the

paleomagnetic behavior of the specimens. Finally,

we will present results from our Thellier-Thellier

experiments and discuss the implications of our

data on the age model and for the paleointensity

behavior of the early Brunhes.

4. Paleomagnetic Data From HSDP2

[20] Figure 4 shows the lithologic log of HSDP2,

sampling levels for the present study, temperature

versus depth and an age model. A list of the sample

names used here and the Box/Run/Unit informa-



5 of 18

tion for each sample is in appendix 3 of the

auxiliary material. The lithologic log and temper-

ature profile are redrawn from DePaolo et al.

[2001] and the age model is from DePaolo and

Stolper [1996]. This age model is consistent with

age information from HSDP2 itself (W. Sharp,

personal communication, 2002). The dashed line

is the best-fitting quadratic relating age and depth

by the equation Age (ka) = 367 + 0.0746D �0.00000462D2 where D is depth in meters below

sea level (mbsl). We sampled submarine basaltic

glass within the lower part of the section (see

Figure 4). Sample depths have been converted to

nominal ages using the age model in Figure 4 of

DePaolo and Stolper [1996] for consistency with

other research efforts on HSDP2. According to this

Ta

Tb

Tp

pTRM

NRM

Ta

Tb1

Tb2

Tp

Tp

NRM

Blocking Temperature Spectrum

Tb

TaTp

TaTp

Temperature

Scenario1

Scenario2

Tb1 Tb2

Ta

Tb1

NRM

TaTp

Scenario3

Tb1 Tb2

a)

b)

c)

d)

e)

f)

Figure 2. (a–c) Three scenarios for hypothetical distribution of blocking termperatures of a new magnetic mineralproduced at Ta (see text). (d–f ) Arai plots for the three scenarios shown in a–c. The filled symbols are the NRM/p-TRM pairs, triangles are ‘‘p-TRM checks’’ and the squares are the ‘‘MD checks’’. While only Scenario 3 generatesfailures in the pTRM checks, all three fail the linearity constraint imposed by the b criterion.



6 of 18

age model, our samples range from approximately

420 ka to 550 ka.

4.1. Rock Magnetic Considerations

4.1.1. Viscous Remanence

[21] Because the samples from HSDP2 have been

at elevated temperature for an extended period of

time (see Figure 4), it is necessary to consider what

viscous remanence (VRM) may have been

acquired and what laboratory blocking temperature

would be expected for such a component. At the

bottom of the hole, the samples are some 550 kawith

an in situ temperature of�40�C.According to singledomain theory developed byPullaiah et al. [1975], a

VRM acquired over a half a million years at 40�Cwould not be removed in the laboratory until approx-

imately 220�C. Therefore, the maximum blocking

temperature of theNRMcomponent used to estimate

paleofield strength must be higher than that. Above

about 1500 mbsl, the temperatures in the drill hole

have been lower, so the blocking temperature

1st zero field step @ Tp

NRM remaining New mineralwith Tb >Ta

1st infield step@Tp

1st infield step@ T > Tb

pTRM gained

2nd zero field step@ Tp

2nd zero field step@ T > Tb

2nd infield step@ Tp

(pTRM check)

erroneous pTRMin new material

MD - Check passesbecause pTRM is lost

pTRM check passes

MD check passes

1st zero field step@ T > Tb

continue several steps until suddenly material is made at Ta

et cetera

Primary minerals

(MD check)

perform several double heating steps

Figure 3. Flow chart of the Thellier-Thellier experiment under the scenario that a mineral is made by heating thesample to temperature Ta which has a blocking temperature Tb > Ta. All pTRM and MD checks pass, but there is anerroneous pTRM acquired at temperature Ta.



7 of 18

required to remove a VRMwould be approximately

125�C.

4.1.2. Magnetic Mineralogy and GrainSize: Transects From Pillow MarginToward Interior

[22] Another rock magnetic concern is the variabil-

ity in grain size and magnetic mineralogy as a

function of distance from the glassy margin. Carlut

and Kent [2002] have examined this issue by

performing Thellier-Thellier experiments on trans-

ects of samples from the glassy margins of submar-

ine basaltic pillows inward toward the increasingly

coarse grained basaltic interiors. They demonstrated

a profound dependence of the estimated paleointen-

sity on the position of the sample with respect to the

glassy margin. As this was done on very recent

MORB pillow fragments, the actual paleofield was

Figure 4. (a) Lithologic cross section of the HSDP2 hole. The upper 1100 meters are the subaerial sequence. Belowthat is the submarine section. Hyaloclastites are orange, massive flows are in green and pillow lavas are in grey.Sampling levels for the present study are shown as dots to the right of the lithologic log. (b) The temperature profileof DePaolo et al. [2001]. (c)The age model in the inset is that of DePaolo and Stolper [1996].



8 of 18

known. The glassy sub-samples reproduced this

field very well, while data from the pillow interiors

overestimated the paleofield. To understand the

origin of this overestimate, they performed a test

whereby the pTRM acquired at a given temperature

step was demagnetized in several successive zero

field steps with increasing temperature. They dem-

onstrated that pTRMs in their pillow interiors failed

to be demagnetized at the temperatures in which the

magnetization was acquired. This is a characteristic

of MD remanences as discussed previously. The

cause of the overestimate was therefore attributed to

both differences in cooling rate and the failure of

pTRM independence in the coarser grained material

(an MD effect).

[23] The methodology Carlut and Kent [2002]

provides a useful means for interpreting paleointen-

sity data from basalts. In the case of the HSDP2

material, we have performed a similar experiment

on a pillow margin. We chose a fragment near the

bottom of HSDP2 as a ‘‘worst case’’ scenario. We

sliced the pillow fragment shown in Figure 5 into 5

mm sub-samples from the glass rim toward the

center into several transects. One transect was sub-

jected to hysteresis analysis; another was subjected

to Thellier-Thellier paleointensity experiments.

[24] We consider first the results of the Thellier-

Thellier experiments. Specimens from the pillow

margin transect were fixed into non-magnetic glass

tubes. The sample tubes were then subjected to the

Thellier-Thellier experiment as described earlier.

Our results are in some respects similar to those of

Carlut and Kent [2002] but differ in significant

ways owing to the elevated temperatures in the

Hawaiian crust. Thellier-Thellier experiments on

specimens a–d from the HSDP2 pillow margin

transect specimens are shown in Figure 6. The

results of specimens ‘‘e–j’’ are virtually identical

to specimen ‘‘d’’. Experimental results were eval-

uated using the criteria of Selkin and Tauxe [2000]

and assigned a grade of ‘‘A’’ through ‘‘F’’ depend-

ing on the number of criteria failed. Grade ‘‘A’’

specimens met all of the criteria (DRAT � 10%,

b � 0.1, a � 15�, MAD � 15�, and the Coe [1967]‘‘quality index’’ Q > 1). All of the interpretations

shown in Figure 6 are grade ‘‘A’’. However, speci-

mens ‘‘c–e’’ have no blocking temperatures above

about 200�C and the remanence removed during

the Thellier-Thellier experiment is therefore entirely

viscous in origin.

4.1.3. Hysteresis Behavior

[25] In order to investigate the cause(s) of the

differences in the Thellier-Thellier experiment

between the glassy margin and the pillow interior,

we performed hysteresis measurements on speci-

mens from the parallel transect. Representative

hysteresis behavior is plotted in Figure 7. Speci-

men ‘‘b’’ (Figure 7a) demonstrates a slightly

‘‘wasp-waisted’’ effect characteristic of many glass

samples. This is thought to be the result of a strong

superparamagnetic (SP) contribution combined

with an essentially single domain population domi-

nated by uniaxial anisotropy [Pick and Tauxe 1993,

1994; Tauxe et al., 1996]. The hysteresis behavior

of the specimen just inside the basalt glass margin

a

b

c

d

e

f

g

h

i

j

glassy margin

Figure 5. Transect from glassy margin of pillow.Specimens a–j taken at 5 mm intervals from margininward. Specimen ‘‘a’’ is glass, ‘‘b’’ has a glassy portionand the rest (c–j) are fine grained basalt with increasinggrain size.



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(specimen ‘‘c’’ in Figure 7b) exhibits the enhanced

saturation remanence to saturation magnetization

rato (here called squareness) and high coercive

fields noted by Gee and Kent [1995]. Moving

away from the glassy margin, the hysteresis loops

of the basalt chips have the lower squareness and

coercivity characteristic of so-called ‘‘pseudo-sin-

gle domain’’ type behavior [e.g., Day et al., 1977].

[26] To summarize the hysteresis behavior of the

specimens from the pillow margin transect, we plot

the squareness versus coercive field data in Figure

8a for the specimens in the transect. These data trace

out a large loop with the largest squareness-coercive

field behavior belonging to specimen ‘‘c’’, just

inside the glassy margin. In order to help understand

these data, we employ the squareness-coercive field

diagram of Tauxe et al. [2002] in Figure 8b. This

diagram shows the interpretations of squareness-

coercive field space based on micromagnetic mod-

elling of magnetite assemblages of grains with

various sizes and shapes. The heavy solid line with

an arrow pointing to the right (labelled ‘‘uniaxial

single domain’’) is the trend expected for uniaxial

particles with increasing aspect ratios [Stoner and

Wohlfarth, 1948] starting with an aspect ratio of

approximately 1.5:1 and increasing to the right (to a

maximum of about 300 mT at infinite aspect ratio).

The point labelled ‘‘cubic single domain’’ is based

on the theoretical predictions of Joffe and Heu-

berger [1974]. The solid arrow originating in the

uniaxial field and trending toward the origin

(labelled ‘‘uniaxial + SP’’) is based on the numerical

simulations of Tauxe et al. [1996]. The dotted curve

originating in the cubic single domain field and

trending toward the origin (labelled ‘‘cubic + SP’’)

a)

NRM(nAm2)

c) d)

NRM(100nA

m2)

NRM(100nA

m2)

pTRM (100 nAm2)pTRM (100 nAm2)

pTRM (nAm2)

NRM(10nA

m2)

pTRM (10 nAm2)

b)

Figure 6. Arai plots for specimens through transect from pillow margin to interior. (a) is outermost specimen; (b–d)is 0.5 cm intervals inward. The solid symbols are those used in the ‘‘paleointensity’’ determination. Triangles arepTRM checks and squares are MD checks (see text). The insets are orthogonal projections of the zero fieldremanence directions during demagnetization. Solid (open) symbols are the ‘‘unoriented’’ X,Y (X,Z) specimencoordinates.



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is based on the numerical simulations of Walker et

al. [1993]. The trends labelled ‘‘PSD’’ are based on

simulations of random assemblages of magnetite

particles of cubes and 2:1 parallelepipeds of sizes

ranging from 20 nm up to 115 nm [Tauxe et al.,

2002]. PSD behavior in hysteresis loops has been

tied to grains whose remanent magnetization is in

the ‘‘vortex’’ state and were found to be in the size

range of approximately 100 to 140 nm. The dash-

dot line is predicted for single domain particles that

are slightly elongate (aspect ratios of less than

�1.5:1). These had elevated squareness values

(above the 0.5 expected for strictly uniaxial behav-

ior) and coercivities higher than those allowed by

strictly cubic (about 10 mT) behavior. Tauxe et al.

[2002] also found that more complex shapes than

cubes and parallelepipeds (for example three inter-

secting rods) could explain the elevated squareness

values with exceptionally high coercive fields (indi-

cated by the gold cross labelled ‘‘intersecting rods’’

in Figure 8b).

[27] The glassy specimens ‘‘a’’ and ‘‘b’’ plot di-

rectly on top of the uniaxial plus superparamag-

a) b)

c) d)

B (mT)B (mT)

M/Ms

M/Ms

Figure 7. Representative hysteresis loops showing magnetization normalized by saturation (M/Ms) versus appliedfield (moH ). (a) is specimen ‘‘b’’ from the transect (a–j) from glassy margin inward, (b) is specimen ‘‘c’’, (c) isspecimen ‘‘d’’ and (d) is specimen ‘‘j’’.



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Squarene

ss

a)

MD

cubic singledomain

uniaxialsingle domain

increasingaspect ratio

b)

uniaxial+SP

cubic

+SP

c

d

j

intersectingrods

a,b

PSD

Coercive field (mT) Coercive field (mT)

increasi

ngwid

th

Figure 8. (a) Squareness-coercive field plot from hysteresis loops for specimens ‘‘a–j’’. Squareness is the ratio ofsaturation remanence and saturation magnetization. Track toward pillow interior shown by arrows. (b) Results ofnumerical simulations redrawn from Tauxe et al. [2002]. The pillow transect data are consistent with increasing grainsize of complex shapes (e.g., skeletal grains) up to a size of about 100 nm.

a

bj

M/M

o

Temperature (oC)

a b c d e f g h i j

gla

ssy

crys

talli

ne

"Banc"

(µT)

a) b)

specimen

Figure 9. (a) Blocking temperature spectra for specimens ‘‘a–j’’ from transect through the glassy margin into thepillow interior. ‘‘a’’ is the outermost specimen. Data are the intensities of the NRM remaining after the first zero fieldstep in the Thellier-Thellier experiment. (b) Estimated ‘‘Banc’’ from the Thellier-Thellier experiments as a function ofdistance from the margin.



12 of 18

netic (SP) trend, consistent with the slightly wasp-

waisted hysteresis loops (Figure 7a). Specimens

‘‘c–j’’ fall on a trajectory starting with quite high

squareness (above the 0.5 expected for uniaxial

behavior) and high coercivities (much higher than

those allowed for cubic anisotropy) and trending in

a curve through the field labelled ‘‘PSD’’ toward

the expected MD point (essentially, the origin).

Thus the trend from specimens ‘‘c’’ to ‘‘j’’ is

consistent with increasing grain sizes of popula-

tions of either slightly elongate particles or more

complex shapes (e.g., skeletal titanomagnetites). A

thin section of the pillow margin revealed no

visible grains under an optical microscope, so the

magnetic crystals in all of these specimens are

probably sub-micron in size.

[28] Blocking temperature spectra from the zero

field steps in the Thellier-Thellier experiments for

the specimens from the pillow margin transect are

plotted in Figure 9a. The blocking temperatures

change dramatically from a maximum of over

400�C in the glassy part (a,b) to less than 200� inthe pillow interior (d–j). This behavior is most

likely the result of a change in titanium substitution

from quite low in the glassy portion to TM60 in the

pillow interior [see, e.g., Zhou et al., 1997].

Because of the relatively high ambient temperature

in the lower part of the drill core (see Figure 4) and

the relatively low blocking temperatures in the

basalts, the ancient geomagnetic field estimates

Banc derived from the pillow margin transect are

a strong function of distance from the pillow

margin. We plot the estimated ‘‘Banc’’ for the

pillow transect in Figure 9b and show the strong

over-estimate of the paleofield from the pillow

interiors.

[29] As already mentioned, paleofield estimates

can be in error for a variety of reasons, including

differences in cooling rate, multi-domain behav-

ior, alteration during the experiment, and over-

printing by viscous (or chemical) remanences.

The pillow interior specimens (‘‘c–j’’) studied

2765 m 3084 m

NRM

(nAm2)

NRM

(nAm2)

pTRM (nAm2) pTRM (nAm2)

c) d)

a) 1118.9 m b) 1127.8 m

Figure 10. Representative results from class ‘‘A’’ Thellier-Thellier experiments. Symbols same as in Figure 6.



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here are from within a few centimeters of the

pillow margin, so differences in cooling rate

cannot account for the factor of two change in

paleofield estimate. The specimens do not exhibit

any indication for MD behavior, either in the MD-

checks (shown as squares in Figure 6) or in the

hysteresis loops (see Figure 7), so multi-domain

behavior is unlikely to be responsible for the

behavior shown in Figure 9b. Instead, because

of the very low blocking temperatures of the

pillow interiors and the high ambient temperatures

of the drill core at that depth, we suspect that the

pillow interiors in our transect are dominated by

viscous remanence and have no relationship to the

geomagnetic field at the time of formation. This is

a different mechanism than that inferred for the

zero age pillow studied by Carlut and Kent

[2002] who found a significant MD signature in

their results.

[30] In the following section we will present results

from the rest of the samples of submarine basaltic

glasses from HSDP2. These were obtained from

pillow rinds and from hyaloclastites from the

sampling levels shown in Figure 4.

4.2. Results

[31] Three to five specimens from each sampling

horizon from HSDP2 (see Figure 4) were pre-

pared for a Thellier-Thellier experiment. Small

glassy chips were broken from the sample, soaked

in dilute HCl and examined under the microscope

for evidence of alteration. Unaltered specimens

with magnetic moments of at least 0.1 nAm2 were

placed in non-magnetic glass tubes. These then

were subjected to the Thellier-Thellier experiment

as described before with the exception that not all

the experiments on glassy specimens included the

"High" Tmean = 30.4

σ = 7.7 "Low" Tmean = 41.1

σ = 8.5

"Paleofield strength" (mT)

Cum

ulativeDistributionFunction

Figure 11. Cumulative distribution function of low temperature (maximum unblocking temperature �200�C)estimates (thin line) and high temperature (maximum unblocking temperature �300�) estimates. The probability thatthe two distributions were drawn from the same population is �2 � 10�19.



14 of 18

MD-checks. Examples of representative grade

‘‘A’’ results are shown in Figure 10. We list all

of the results from this study in appendix 4 of the

auxiliary material.

[32] Many specimens yielded Grade ‘‘A’’ results

with maximum blocking temperatures of 225�C or

less. We have argued that these paleointensity

estimates, while appearing to be of ‘‘high quality’’,

are actually dominated by viscous contamination.

In order to underscore the effect these low temper-

ature estimates would have on estimates of the

average geomagnetic field, we have calculated

paleointensity estimates for the components iso-

lated between 100 and 225�C as well. The grade

‘‘A’’ estimates calculated in this way have the

cumulative distribution function shown as the thin

line in Figure 11. The cumulative distribution

function of the grade ‘‘A’’ data having a maximum

blocking temperature of at least 300�C is shown as

the heavy line. These two data sets are significantly

different, based on a Kolmogorov-Smirnov test, at

well above the 99.9% level of confidence. There-

fore, in addition to the usual criteria, we have added

the criterion that the maximum blocking temper-

ature of acceptable results had to be at least 300�C.

a)

b)

Figure 12. Published data are as in Figure 1. New data meeting the minimum acceptance criteria are shown astriangles. (a) Data plotted on the age model shown in Figure 4. (b) A portion of the data from (a). Data from HSDP[Laj et al., 1999] and HSDP2 (this paper) plotted with an age model that is 35 kyr younger.



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Finally, we have calculated average paleointensity

data for sampling horizons that had at least three

specimens meeting our minimum criteria. These are

listed in appendix 5 in the auxiliary material. Those

samples having standard deviations of less than or

equal to 15% are plotted as triangles in Figure 12.

5. Discussion

[33] The new data shown in Figure 12 nearly

double the absolute paleointensity data available

for the early Brunhes that meet minimum selection

criteria. The magnetic anomaly inversion of Gee et

al. [2000] is shown as the thin line. It appears that

the new data are quite a bit higher than other data

from the same age, in particular, both the data from

HSDP2 (triangles) and HSDP (dots within circles

[Laj et al., 1999]) appear to be offset from the

relative paleointensity stack. In Figure 12b we

replot the HSDP and HSDP2 data using an age

model that is offset to younger ages by 35 kyr. This

offset brings the data into better agreement with the

relative paleointensity pattern, yet is consistent with

the age constraints available for HSDP2 (W. Sharp,

personal communication, 2002). Moreover, the cal-

ibration of the relative paleointensity data to

approximate VADM appears to be consistent with

our data. The data from the basalts from the sub-

aerial section (HSDP [Laj et al., 1999]) are much

higher than the calibrated anomaly inversion stack.

[34] Juarez and Tauxe [2000] compiled paleointen-

sity data spanning the last 5 million years and

concluded that the average dipole moment was

approximately 55 ZAm2. They excluded the period

of time 0–0.3 Ma from the average because they

suspected that the geomagnetic field was unusually

strong during that interval and the overwhelming

early Brunhesmean = 65.2 ZAm2

σ = 20.9N = 82

late Brunhesmean = 80.2 ZAm2

σ = 26.7N = 232

VADM (ZAm2)

Cum

ulativeDistributionFunction

Figure 13. Cumulative distribution function of data from the early Brunhes (heavy line) and the late Brunhes (thinline). The early Brunhes data are those from 0.4–0.78 Ma and the late Brunhes are from 0–0.4 Ma). The probabilitythat the two distributions were drawn from the same population is �4 � 10�8.



16 of 18

majority of data points come from that period;

hence the average value would be heavily biassed

toward a potentially unusual field state. We see

from Figure 12a that, while there is no abrupt jump

from low to high values of the field during the

Brunhes, the latter half does appear to be stronger

on average than the early half. To quantify this, we

divide the data into ‘‘early Brunhes’’ (0.4–0.78 Ma)

and ‘‘late Brunhes’’ (0–0.4 Ma). Cumulative dis-

tribution functions for both data sets are shown in

Figure 13 illustrating that the two are indeed differ-

ent at above the 99.9% level of confidence. The late

Brunhes average is virtually identical to the present

field (�80 ZAm2) and the early Brunhes (65 ± 21

ZAm2) is compatible with the five million year

average of Juarez and Tauxe [2000].

6. Conclusions

[35] The data presented in this paper support the

following conclusions:

1. Submarine basaltic glass appears to give high

quality estimates of the ancient geomagnetic field

strength in the Hawaiian submarine section of the

HSDP2 drill core.

2. Submarine basalts from the hole appear to

give estimates that are too high and are contami-

nated by a strong viscous remanence acquired by

low blocking temperature magnetic phases sub-

jected to elevated temperatures over a significant

period of time. The submarine basalts from HSDP2

cannot be used for paleointensity studies.

3. The average paleointensity for Hawaii over

the time interval 420 to 550 ka is in broad

agreement with the calibrated relative paleointen-

sity records, but agreement improves significantly

by adjusting the age model to be younger by 35 kyr.

4. In order to estimate the average geomagnetic

field intensity, one must avoid sampling bias of

anomalous states of the geomagnetic field. The

preponderance of data from the latter half of the

Brunhes is a likely source of bias in average

paleofield estimates.

Acknowledgments

[36] We are indebted to Carlo Laj for original inspiration for

this work and help in sampling. We also would like to thank

Joe Kirschvink for help at CalTech, Jeff Gee, Julie Bowles,

Peter Selkin and Agnes Genevey for helpful discussions, and

Steve Didonna and Winter Miller for making most of the

measurements. Julie Carlut and two anonymous reviewers

made numerous constructive suggestions. This work was

partially supported by an NSF grant to LT.

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