A detailed palaeointensity and inclination record from drill core...

Physics of the Earth and Planetary Interiors 131 (2002) 101–140

A detailed palaeointensity and inclination recordfrom drill core SOH1 on Hawaii

Nicholas Teanbya,b,∗, Carlo Lajb, David Gubbinsa, Malcolm Pringleca School of Earth Sciences, University of Leeds, Leeds LS2 9JT, UK

b Laboratoire des Sciences du Climat et de l’Environnement, Unité Mixte CEA-CNRS,Avenue de la Terrasse, Batiment 12, 91198 Gif-sur-Yvette, France

c Scottish Universities Environmental Research Centre, Scottish Enterprise Technology Park,Rankine Avenue, East Kilbride G75 0QF, Scotland, UK

Received 22 January 2002; received in revised form 29 April 2002; accepted 29 April 2002

Abstract

A new record of absolute palaeointensity was obtained from drill core Scientific Observation Hole 1 (SOH1) on Kilaueavolcano, Hawaii. Kilauea’s high eruption rate resulted in a relatively continuous record and stratigraphic constraints preservedthe chronological order. Three hundred and sixty samples were studied with the Thellier–Thellier technique, which gave 195successful palaeointensity and 271 successful inclination determinations. Three geomagnetic excursions were observed,which exhibited intensity reductions of about 50%. Initial age control from K–Ar and Ar/Ar dating only constrained the totalage between 20 and 120 ka. The final age model was obtained by stretching the SOH1 record relative to other Hawaiianpalaeomagnetic data. This gave an age range of 0–45 ka for the flows and identified the excursions as the Hilina Pali, MonoLake and Laschamp events. The SOH1 record of the Hilina Pali event is the most detailed ever, incorporating data from around40 flows. This age model suggests that Kilauea had a burst of activity at the SOH1 site around 20 ka. All available data wascombined to form a composite record of palaeointensity and inclination on Hawaii for 0–45 ka.© 2002 Elsevier Science B.V. All rights reserved.

Keywords:Palaeointensity; Inclination; Geomagnetic excursion; Kilauea; Hawaii

1. Introduction

Drill cores provide a unique opportunity to studylong continuous sequences of lava flows and gain valu-able information on the magnetic field. The advantageof using lavas over the more readily available sedi-mentary cores is that the acquisition process in lavasis well understood and absolute palaeointensities canbe recovered, unlike sediments where only relativeintensities can be obtained. However, sediments give

∗ Corresponding author.E-mail address:[email protected] (N. Teanby).

a continuous record of the field, compared to lavaswhich are less continuous because of sporadic eruptionrates. Dating of lava sequences rely on linear interpola-tions between radiogenic tie points, whereas sedimentscan be dated more precisely by using techniques, suchas astronomically calibratedδ 18O. Drilling in lava isalso much more costly than in sediments and cores arethus relatively rare.

In Hawaii we are fortunate in having the opportunityto study a number of lava cores, both from the HawaiiScientific Drilling Project (HSDP) and the ScientificObservation Hole (SOH) projects. The SOH projectwas drilled to investigate the potential for geothermal

0031-9201/02/$ – see front matter © 2002 Elsevier Science B.V. All rights reserved.PII: S0031-9201(02)00032-8

102 N. Teanby et al. / Physics of the Earth and Planetary Interiors 131 (2002) 101–140

energy production on Kilauea. Three holes SOH1,SOH2 and SOH4 were drilled along the Kilauea EastRift Zone (KERZ) near to the rift axis. Temperaturesin SOH1 were found to be much too low for geother-mal energy production, providing us with an excellentopportunity for studying the magnetic field. The proxi-mity of SOH1 to other lava cores also allows us tocompare the SOH1 data with palaeomagnetic studieson SOH4 (Laj et al., 2002) and the HSDP1 pilot hole(Laj and Kissel, 1999).

SOH1 comprises 351 volcanic units with a totallength of 1685 m. This study focuses on the upper241 flows (0–787 m), which consists of subaeriallyerupted lavas and a few intrusives. Palaeointensity andinclination records were obtained from the core, butdue to rotation of the core during drilling, declinationinformation was lost.

Results already obtained from SOH4 and HSDP1provide detailed records of palaeointensity and incli-nation for the last 400 kyr over hundreds of flows.SOH1 covers a much shorter time span with a compa-rable number of flows and, therefore, provides a verydetailed record of recent palaeosecular variation onHawaii.

2. Stratigraphy and sampling

SOH1 is located on Kilauea volcano, Hawaii, atlatitude 19◦29′N and longitude 154◦54′W, about 2 kmnorth of the axis of the KERZ and close to Lava TreesState Park. The locations of the SOH and HSDP coresare shown inFig. 1.

All of the 351 units (1685 m) sampled by SOH1 arefrom Kilauea, they have relatively low alkali (Na2O+K2O) content compared to silica (SiO2), which placesthem in the tholeiite field, as defined byMacdonaldand Katsura (1964). The stratigraphy used for SOH1is based on an in-depth description of core lithologyprovided byQuane (1999)andQuane et al. (2000). Wecan roughly describe SOH1 as consisting of subaerialand submarine sections, although at the transition thereis some interleaving of subaerial and submarine flowsdue to the combined effects of subsidence and sea levelchange.

The aerial/marine transition in SOH1 occurs at740 m, although subaerial flows are common until787 m. The section of core from 0 to 787 m consists of

241 flows which are mainly a’a (66%) and pahoehoe(22%) lavas; a small number of dikes are also present(11%). The thickness of the flows ranges from 0.6 to15.2 m (mean 3.9 m) for a’a and from 0.3 to 17.4 m(mean 2.0 m) for pahoehoe. The remaining 898 m(110 units) of core contains occasional subaerialflows, but comprises mainly submarine hyaloclastite(25%), breccia (4%) and dikes (66%). The level ofdike intrusion increases with depth, so much so thatbelow 1200 m dikes dominate the lithology.

Previous palaeomagnetic studies of subaerialHawaiian lavas have been very successful, making theupper 787 m of subaerial a’a and pahoehoe flows ofSOH1 a suitable target for this palaeointensity study.The hyaloclastite deeper in the core is not suitable,being made of fragments for which it is unknownwhether the magnetisation was acquired before or afterdeposition. Therefore, the geomagnetic significanceof any remanent magnetisation is dubious. Althoughsome massive submarine flows were considered suit-able, increased levels of alteration deeper in the core(Quane, 1999) make successful measurements lesslikely. Therefore, sampling was concentrated on thesubaerial section from 0 to 787 m.

SOH1 is stored in the University of Hawaii corerepository at Honolulu harbour. The core is split into aworking half, stored in boxes which each hold 5 m ofcore and an archive half, which is shrink wrapped inPVC tubing. The core slabs are marked with a red/blueline pair for orientation purposes, red on the right indi-cating the up core direction. Samples were taken fromthe working half of the core using a 25 mm diame-ter, water-cooled diamond barrel corer powered by anelectric drill. The diameter of SOH1 decreased withdepth due to the limitations of the drilling rig, hencesample lengths decreased with depth from around 40to 20 mm. Before samples were drilled, the slab facewas marked with an orientation arrow indicating theup core direction based on the red/blue orientationlines.

Two sets of samples were used for this study. Thefirst set was taken in 1994 by Herrero-Bervera. Thesesamples were numbered 1–697 and were taken with-out the use of a stratigraphic log. Inevitably, over andunder sampling of flows occurred. A second set ofsamples was taken, to supplement the initial sampling,by Teanby and Laj in 1998 using the stratigraphic logof Quane (1999). The gaps in the previous sampling

N. Teanby et al. / Physics of the Earth and Planetary Interiors 131 (2002) 101–140 103

Fig. 1. (a) Location map of the Hawaiian Islands; (b) position of SOH1 on the KERZ relative to other deep lava cores in the area. Theheavy solid line represents the rift axis and dotted lines mark the boundaries between volcanoes.

were amended so that, in most cases, a total of threeor more samples existed for each flow.

3. Chronology

Tholeiitic Kilauean basalts have a low potassiumcontent; typically around 0.5 wt.%. Also SOH1 lavasare very young compared to the half life of40K. There-

fore, very little radiogenic40Ar is produced, makingradiogenic dating of young SOH1 basalt very difficult.Unfortunately no organic material was found in thecore which could be used for radiocarbon dating.

Dating of SOH1 was first attempted using the K–Artechnique, which gave reliable ages for the submarinesection, but was less successful at dating the subaerialsection. After completion of the palaeomagnetic study,


the dating of the subaerial section was revisited usingthe Ar/Ar technique.

3.1. K–Ar dating

Quane et al. (2000)carried out K–Ar analyses on14 groundmass samples from SOH1. Samples wereselected from subaerial (11 samples) and submarine(3 samples) sections of the core based on low vesicu-larity and freshness. Analyses were done on ground-mass samples prepared according toGuillou et al.(1997a,b)using the unspiked K–Ar or Cassignol tech-nique (Cassignol and Gillot, 1982; Gillot and Cornette,1986), which had already proved successful in datingyoung Hawaiian basalts, such as those from the SOH4core (Guillou et al., 1997a,b).

Of the 14 samples studied, 4 gave meaningfulresults, the other 10 samples gave negative or zeroages. The four successful results fromQuane (1999)are reproduced inTable 1. The most reliable resultswere from the two deepest samples which gave agesof 174± 8 ka at 940.5 m and 351± 6 ka at 1550.5 m,these constrain the age of the submarine section ofSOH1 very well, giving an average accumulation rateof 3.4 mm per year. The two younger ages of 5± 3 kaat 409.0 m and 28± 3 ka at 755.9 m in the subaerial/transitional section of the core are considered lessreliable than the older dates by the authors (Guillou,personal communication). This is unfortunate as thesection of the core most suitable for palaeointensityand inclination measurements has the most poorlyconstrained chronology.

3.2. Ar/Ar dating

Because of the uncertainty in the K–Ar ages,Ar/Ar dating was used in order to provide addi-tional constraints on the age of the subaerial section.We re-examined the six most promising ground-

Table 1Successful K–Ar dates for SOH1 obtained byQuane et al. (2000)

Sample Depth (m) Age (kyr) 1σ (kyr)

5059 1550.5 351 6SQ-01 940.5 174 8SQ-02 755.9 28 31343.5 409.0 5 3

mass samples from the K–Ar analyses using theAr/Ar technique. In addition, eight new samples wereselected from the core by Teanby and Garcia based onlow vesicularity, freshness and strategic importancein relation to features observed in the palaeomagneticrecord. Six of these new samples were analysed by theAr/Ar technique. In total, 12 pairs of samples, 6 wholerock and 6 groundmass, from the subaerial section ofSOH1 were analysed at SURRC in East Kilbride, UKusing a mass spectrometer and gas line devoted to lowradiogenic argon content rocks (Teanby et al., 2000).

Analytical procedures were similar to those used bySinger et al. (1999). Whole rock samples were mini-cores taken from slabs of core material using a 5 mmdiameter drill press. The groundmass samples, previ-ously prepared byQuane et al. (2000)for the K–Arexperiments, were enclosed in 8 mm diameter 99.99%copper foil packets. The masses for both ground-mass and solid rock samples were around 650 mg.These samples along with 27.92 Ma sanadine standardTcr-2a were placed in quartz vials and irradiated at theOregon State University Triga reactor in the cadmium-lined in-core irradiation tube (CLICIT).

Prior to the analyses, samples were degassed in avacuum for approximately 2 weeks to remove anyabsorbed air. To release the radiogenic and potassium-derived argon, individual samples were heated to pro-gressively higher temperatures in a double vacuumfurnace from around 500 to 1200◦C over 9–12 steps.Reactive gases were removed using a two-stage gasclean-up, incorporating hot (450◦C) and cold (roomtemperature) ZrAl getters. A zeolite cold finger wasused to trap hydrocarbons. Relative amounts of the dif-ferent argon isotopes in the cleaned gas fraction weremeasured with a MAP-215 spectrometer.

To show the age of each sample we plot the agespectra inFigs. 2 and 3, which are the calculatedage for each incremental heating step as a functionof the total fraction of39Ar released. The40Ar∗/ 39Arratios used to calculate the age for each step werecorrected for Ca-derived39Ar, air-derived40Ar, theequipment blank and mass spectrometer mass discrim-ination. The analytical errors of all these quantitieswere included in the standard deviation.

Criteria for selecting reliable results have beenestablished bySinger and Pringle (1996). We followthese criteria here, except we prefer the plateau agesover the isochrons. Samples were considered reliable


Fig. 2. Ar/Ar age spectra of groundmass samples, previously studied using K–Ar.


Fig. 3. Ar/Ar age spectra of whole rock minicores taken from new core samples.


if: there was an age plateau with ages concordant atthe 95% confidence level, containing at least 50% ofthe 39Ar released; isochron analysis and plateau ageagreed at the 95% level; and the40Ar/36Ar intercepton the isochron diagram agreed with the atmosphericvalue of 295.5 at the 95% confidence level. The bestestimate of the crystallisation age is given by the errorweighted mean of the plateau ages. Steps included inthe plateau age are shown in dark grey and rejectedsteps are shown in light grey.

A summary of the Ar/Ar dating results is shown inTable 2(complete dataset is available in the electro-nic archive at,http://www.elsevier.com/locate/pepi).From the 12 samples studied, 3 gave successful results:D2 at 296 m, 43.2±4.2 ka; D6 at 506 m, 65.0±11.4 ka;and SQ-02 at 756 m, 17.9 ± 4.6 ka. The Ar/Ar age of17.9±4.6 ka for sample SQ-02 corroborates the K–Arage of 28± 3 ka for the same sample. However, thesetwo ages are in disagreement with ages from sam-ples D2 and D6 further up the core, which were datedat ca. 50 ka. Although these dates are not in strati-graphic order they can be used to estimate lower andupper bounds for the age of the subaerial section of20–120 ka.

Anomalously high or low ages can result if the40Ar∗/40K ratio has been affected.Dalrymple andLanphere (1969)discuss two basic mechanisms: onewhere the system is not closed and potassium and/orargon can be lost or gained; the other where contam-ination occurs during formation. Ages which are tooyoung can result if argon is lost by diffusion. Possi-ble causes include post-formation heating, alterationof the mineral lattice, or unsuitable sample material.Potassium gain can also cause a low age. Any processcausing potassium gain is also likely to result in argonloss and amplify the problem. Anomalously high agescan be caused by excess40Ar∗ incorporated into thelattice at the time of formation,40Ar∗ contaminationfrom older inclusions, or loss of potassium which isusually accompanied by argon loss.

Samples that gave low or negative apparent ageswhen studied with K–Ar had higher K/Ca, higher ab-solute amounts of potassium, higher absolute amountsof atmospheric contamination and significantly greatertotal gas release below 700◦C, when studied withAr/Ar experiments. This suggests that K-addition andhydration rather than Ar-loss is the prime mechanismaffecting the K–Ar radiometric clock of these samples.

There is no direct evidence for K-addition or hydra-tion in either D2, D6 or SQ-02. However, because D2and D6 are from different flows, we consider thesedates more reliable than the dates for the single sam-ple SQ-02. Because there is no experimental evidencefor alteration in SQ-02, it is entirely possible that allthe samples could have been affected by alteration.Therefore, these dates must be used cautiously.

3.3. Comparison with SOH4 and KERZsurface flows

We can supplement the radiogenic dates for SOH1by using information on other lava cores from Hawaiiand the age of surface flows in the KERZ area.

Core SOH4 is located only 8 km from SOH1 andis a similar distance from the rift axis. Although localaccumulation rates on Kilauea can vary substantiallydepending on the configuration of vents and topogra-phy, we would expect the two cores to have similaraccumulation rates over reasonably long timescales.The K–Ar dating for SOH4 (Guillou et al., 1997a,b)was more successful and is considered more reliablethan the SOH1 K–Ar dates. Therefore, using the SOH4depth–age profile as a proxy for SOH1 may provideus with a better idea of the SOH1 timescale.

The K–Ar dates for SOH4 are shown inFig. 5along with the K–Ar and Ar/Ar results from SOH1. Ifwe assume SOH1 and SOH4 accumulation rates aresimilar, the older Ar/Ar dates appear more plausiblethan the younger Ar/Ar and K–Ar dates at the bottomof the subaerial section. The ages for the submarinesection of SOH1 agree with the age of SOH4 at sim-ilar depths, which lends weight to the theory that thetwo cores have similar accumulation rates.

Some insight into the recent behaviour of Kilaueacan be gleaned from the age distribution of the sur-face flows in the KERZ area. These lavas range inage from 7 to 2360 years. To calculate flow areas, theflow boundaries in the region of SOH1 were digitisedfrom the 1:100,000 scale map compiled byWolfeand Morris (1996). Fig. 4 shows the flows and studyarea.

From the area and age of each flow it was possibleto determine the mean surface age of the region aroundSOH1. The mean surface age of the entire study areawas calculated to be 496 years with an error on themean of 34 years. If we assume that the behaviour

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Fig. 4. Surface flows and mean surface ages in the region of SOH1. Plots show: (a) all flows in study area; (b) flows within 2 km ofKERZ axis; (c) flows >2 km from KERZ axis; (d) the study area with the KERZ axis marked with a solid line. Flow boundaries digitisedfrom 1:100,000 scale map compiled byWolfe and Morris (1996).

over the last 2360 years is representative of the last100 kyr or so of activity on KERZ, and that a newlava flow would be equally likely to occur anywherein this region, then the mean time difference between

flows would be 496±34 years. Implying that the totalage of the 241 flows in the SOH1 subaerial section is119±8 kyr, corresponding to an accumulation rate of6.2 ± 0.4 mm per year.


However, the view that a new flow is equally likelyanywhere on the KERZ is too simplistic. In realityflows are more likely to occur nearer to the rift axis.SOH1 is about 2 km from the rift axis. Therefore, inorder to get upper and lower bounds for the meantime difference between flows we repeat the analysisonly including flows either less than or greater than2 km from the rift axis, as shown inFig. 4b and c.With this restriction the mean surface age for the nearand far axis fields are 343± 21 and 535± 37 years,respectively. These bounds imply the total subaerialsection is between 82± 5 and 128± 9 kyr old, corre-sponding to accumulation rates between 9.0±0.6 and5.8 ± 0.4 mm per year.

These estimated accumulation rates are consistentwith rates based on K–Ar and Ar/Ar dating of lavasfrom deep drilling. Guillou et al. (1997a,b)reportaccumulation rates of 3.8–11 mm per year for Kilaueabased on lavas from SOH4, whileSharp et al. (1996)report rates of 7.8 ± 3.2 mm per year for Mauna Keaduring its shield building stage.

3.4. Discussion of age scale of SOH1

Fig. 5 shows all relevant information on the ageof the SOH1 rocks. To recap, the K–Ar study of 14

Fig. 5. All available information on the age of SOH1. Circles (squares) represent Ar/Ar (K–Ar) age determinations of SOH1, whereastriangles show the K–Ar ages of nearby SOH4. The shaded area represents an estimate of accumulation rate based on the age of KERZsurface flows. The solid line shows the preferred radiogenic SOH1 age model.

subaerial and submarine samples byQuane et al.(2000)yielded 4 meaningful ages: 5± 3 ka at 409 m;28± 3 ka at 756 m; 174± 8 ka at 940 m; 315± 6 kaat 1550 m. The ages of 174± 8 and 315± 6 ka areconsidered reliable by the authors. Our Ar/Ar studyof 12 subaerial samples yielded 3 meaningful ages:43.2 ± 4.2 ka at 296 m; 65.0 ± 11.4 ka at 506 m;17.9 ± 4.6 ka at 756 m. The ages of 43.2 ± 4.2 and65.0 ± 11.4 ka are considered more reliable than theage of 17.9 ± 4.6 ka because two samples are pre-ferred over one sample. Also the older dates are moreconsistent with accumulation rate estimates based onsurface flows and SOH4.

The preferred radiogenic age model is formed bycombining the Ar/Ar dates of 43.2 ± 4.2 and 65.0 ±11.4 ka (whole rock minicore samples D2b and D6a)with apparently more reliable K–Ar ages of 174± 8and 315±6 ka (groundmass samples SQ-01 and 5059)from lower in the core. This model, shown as a solidline in Fig. 5, suggests that the lava accumulation rateat SOH1 has averaged ca. 5 mm per year, which isconsistent with eruption rates for the same time periodobserved at the SOH4 drill site 8 km to the south-west,and also with estimates of accumulation rate based onthe age of surface flows. This implies the total ageof the subaerial section is ca. 120 kyr. However, this


figure is disputed by dates from SQ-02 of 28± 3 ka(K–Ar) and 17.9 ± 4.6 ka (Ar/Ar).

The majority of available evidence supports thepreferred radiogenic age model. However, it should beregarded with caution. A more conservative conclu-sion is that the whole subaerial section is aged between20 and 120 ka. The timescale of SOH1 is developedfurther inSection 6using palaeomagnetic data.

4. Palaeomagnetic methods

4.1. Thermomagnetic properties

The suitability of samples for the time consum-ing Thellier–Thellier experiments was determinedby using a horizontal Curie balance to measure thesaturation magnetisation (Js) over a heating/coolingcycle with a maximum temperature of ca. 700◦C.Small amounts of powdered sample were heated by awater-cooled furnace, while a gentle stream of argonwas passed over the sample to minimise oxidation.

The evolution ofJs with temperature gave impor-tant information on how the sample would behaveduring a Thellier–Thellier experiment. If no alterationhad occurred, the heating and cooling curves shouldbe identical. Samples with large differences betweenheating and coolingJs curves were rejected and notstudied further. The Curie temperature was identifiedas whereJs decreased to negligible levels (ca. 0.1of room temperatureJs) and samples exhibiting lowCurie temperatures were rejected from further study.Fig. 6 shows examples of accepted and rejected sam-ples. This initial screening helped improve the successrate of the Thellier–Thellier experiments. From 191samples studied, 63 were rejected on the basis of badthermomagnetic behaviour. The average Curie temper-ature was around 560◦C, implying low-Ti magnetiteas the main magnetic carrier mineral.

4.2. Intensity and inclination measurements

Palaeointensity and inclination measurements weremade using the original Thellier–Thellier technique(Thellier and Thellier, 1959). This involved a series ofdouble heatings made in forward and reverse magneticfields in order to demagnetise the natural remanentmagnetisation (NRM) of the sample while simulta-

neously imparting a thermal remanent magnetisation(TRM). The ratio of NRM to TRM was used to deter-mine the palaeointensity.

All experiments took place at LSCE in Gif-sur-Yvette, France using the medium and large cylindri-cal furnaces, which allowed simultaneous study of upto 70 and 110 samples, respectively. The 25 mm×12.5 mm samples were stored in zero magnetic fieldprior to the experiments to allow short term viscouscomponents to decay. Heatings were performed whileapplying an axial magnetic field of 40�T (similar tothe present field in Hawaii). To minimise oxidation,heating steps were carried out in an argon atmosphere.Activated charcoal was also added to prevent alterationof magnetite during heating (Laj and Kissel, 1999).Most samples appeared unaltered even at high tem-peratures.

Samples were heated from 120 to 560◦C over about15 double heating steps. This completely demagne-tised the NRM to within 5% of its initial value in mostcases. The majority of demagnetisation occurred athigh temperatures, so at low temperatures large stepsof 40–50◦C were used, these increments got progres-sively smaller as the temperature increased so thatat 500◦C steps were 10–15◦C. Temperature stepswere also modified according to sample behaviour.Each temperature was maintained for 1–2 hours andsamples were left to cool overnight.

Samples were monitored for alteration by perform-ing partial TRM (pTRM) checks every few steps. Thisinvolved repeating a step at lower temperature in orderto check that the TRM had not changed. At high tem-perature each double heating was critical so checkswere performed every step. In addition to checking afew steps back it was also useful to check over largetemperature differences of a few hundred degrees,these large pTRM checks were more sensitive andgave a better indication of any alteration. Magnetisa-tion after each heating was measured using a 2G cryo-genic magnetometer in a magnetically shielded room.

NRM direction was also measured using stepwisealternating field (AF) demagnetisation. This was doneto act as a check on the thermal demagnetisation re-sults. Samples were demagnetised by an AF from 50to 900 mT over 9–12 steps, using a Molspin spinnerdemagnetiser. The NRM remaining after each stepwas measured in the same way as the Thellier–Thellierexperiments, using a 2G cryogenic magnetometer.


Fig. 6. Examples of Curie balance measurements ofJs as a function of temperature. The left-hand column shows samples with goodproperties; they have a steady decrease inJs with temperature, high Curie temperature (ca. 560◦C) and a small difference between heatingand cooling curves. The right-hand column shows samples which would be rejected from further analysis; they have large differencesbetween heating and cooling curves or low Curie temperature. An increase inJs with temperature shows that magnetic minerals have beencreated during heating, for example, SOH1-N14A.


5. Results

In total, 360 samples from 162 flows were stud-ied using the Thellier–Thellier technique (Teanby,2001). Table 3reports results from 354 samples (155flows) from the subaerial section (dataset is availablein the electronic archive at,http://www.elsevier.com/locate/pepi). The six samples (from six flows) fromthe submarine section all gave poor results and arenot reported. The results from 24 samples studiedusing AF demagnetisation are given inTable 4.

Inclination was found by plotting the decay of theNRM vector on orthogonal axes, or Zijderveld plot(Zijderveld, 1967), as a function of temperature orAF field strength. From this plot, the points belon-ging to the principal component of magnetisationwere selected. Inclination was then calculated usingprincipal component analysis (Kirschvink, 1980). Astereo plot was also useful to check that the directionof the NRM vector was stable and did not drift towardsthe direction of the applied field during heating.Example Zijderveld diagrams for the AF demagneti-sations are shown inFig. 7 .

To determine the palaeointensity we construct anArai plot (Nagata et al., 1963) of TRM acquiredagainst NRM remaining. The ratio between palaeo

Fig. 7. Example Zijderveld plots for AF demagnetisations. Labels are demagnetising field strength in mT. Solid (open) circles are theprojection of the NRM vector onto the horizontal (vertical) plane.

and laboratory fields is given by the slope of the leastsquares line of best fit. Example diagrams are shownin Figs. 8–10. Solid symbols represent accepted steps,which correspond to the primary remanence. Opensymbols represent rejected steps, which were not usedin the intensity calculations. The pTRM checks areplotted on the Arai diagram with triangles and a linejoining them to the step corresponding to the highesttemperature reached so far.

Selection of reliable inclination and palaeointen-sity determinations was based on examination of Arai,Zijderveld and stereo plots. The selection criteria usedhere are similar to those ofSelkin and Tauxe (2000)andLaj and Kissel (1999).

(1) NRM must decay linearly towards the origin,indicating that the primary remanence is beingdemagnetised. The direction of magnetisationmust not trend toward the direction of the appliedfield, as this indicates the growth of magneticminerals during the step heatings.

(2) The fraction of NRM (f ) used to define the prin-cipal component is >0.3. The number of pointsdefining the line of best fit must be≥4. To excludeviscous components, the highest temperature usedmust exceed 300◦C.



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Table 4Inclination data from 24 AF demagnetisation experiments

Flow Depth (m) N I (◦) ±1σ (◦)

80 295.0 1 15.3 –88 334.4 2 −2.2 1.697 355.3 1 4.1 –

101 360.0 2 −6.2 0.6107 370.6 1 7.8 –124 404.2 2 −19.2 3.0125 408.6 1 −6.4 –126 411.5 2 −13.2 2.0141 453.2 2 −34.6 1.3143 457.8 1 −29.3 –147 466.0 1 −23.1 –151 475.3 2 −10.0 3.7152 476.4 2 −7.3 1.5156 487.1 1 −5.4 –158 491.6 2 29.1 2.3

N is the number of samples per flow.

Palaeointensity determinations have the additionalconstraints.

(3) The section of the Arai plot used to define thepalaeointensity is linear, indicating single domainor pseudo single domain grains, with the leastsquares fit having a regression coefficient (r) withan absolute value >0.97.

(4) There are three or more pTRM checks, up toand including the highest temperature used, whichagree to within 5% of the initial NRM (NRM0).For small gradients, i.e. low palaeointensities, thepTRM checks must agree to within 5% of NRM0/gradient.

Samples which fail conditions (3) or (4) can stillgive reliable inclinations, provided conditions (1) and(2) are satisfied. This is because failure of (3) or (4)indicates a problem with the TRM spectrum; only theNRM spectrum is required for inclination determina-tions. However, these samples were only consideredreliable if f > 0.5.

The reliability of each sample was graded from1 to 5. Reliability 1 samples gave the best resultsand intensities and inclinations from these samplesare considered very reliable, examples are shown inFig. 8. Reliability 2 samples gave reliable intensi-ties and inclinations with examples shown inFig. 9.Fig. 10 shows examples of reliability 3 samples,which gave reliable inclinations but had a suspect

intensity. Reliability 4 samples had suspect intensitiesand inclinations, whereas reliability 5 samples wererejected. We selected intensities from samples withreliability 1 or 2 and inclinations from samples withreliability 1, 2 or 3. In total these selection criteriagave 195 successful palaeointensity determinationsfrom 97 flows. Less stringent criteria for inclina-tion gave 271 successful determinations from 118flows.

For the accepted determinations, the mean valueof f was 0.54 for the palaeointensities and 0.56 forthe inclinations. These are well above our minimumacceptable value off > 0.3. The absolute value ofr is also well above our minimum of 0.97, havinga mean of 0.994. The SOH1 record has an averageof around two palaeointensity and three inclinationdeterminations per flow. The overall success ratesare 54% for palaeointensity and 75% for inclina-tion, which is comparable to success rates from otherstudies on Hawaiian lava cores. These high successrates, which compare well with success rates fromoutcrop data of around 20%, may be because eachflow is only exposed to surface weathering for a shorttime because of the high eruption frequency. Successrates may also be improved by our lab techniqueof using activated charcoal and argon atmosphereto limit alteration upon heating. Similar successrates have been obtained from other long basalticcores on Hawaii (HSDP1 and SOH4) using thismethod.

Successful palaeointensity and inclination determi-nations are shown inFig. 11 as a function of depth.The AF demagnetisation results are also shown. Errorbars are one standard deviation (1σ ) and flows withonly one determination are shown with open symbols.Other studies of Hawaiian lava cores (SOH4) showlimited intra-flow scatter. Hence, as our single sampleresults agree with multiple sample results, they aredeemed acceptable.

Restricting the dataset to flows which had at leastthree reliability 1 or 2 palaeointensity and inclinationdeterminations per flow gives the same pattern as thetotal dataset. This provides further evidence that ourdeterminations are reliable.

Some dikes are present in the subaerial section,which may have affected the magnetisation of nearbyflows. The record was tested for possible remag-netisation by intrusives by modelling the thermal


Fig. 8. Examples of reliability 1 samples. These samples gave exceptionally reliable palaeointensities and inclinations: (a–c) represent someof the best diagrams in this grade, (d) is about the average, whereas (e and f ) are below average. Arai plots are defined by at least five pointsandf > 0.4. Twenty-one percent of samples studied gave these type of diagrams. Arai diagrams (top-left in each panel): circles show theNRM/TRM point calculated for each double step heating; point labels are temperature of the step; solid (open) symbols represent pointswhich are included (excluded) from the palaeointensity determination; triangles represent pTRM checks. Zijderveld diagrams (top-right ineach panel): solid (open) circles are the projection of the NRM vector onto the horizontal (vertical) plane. Stereo plot (bottom-right ineach panel): solid (open) circle represent positive (negative) inclination; open circle in west position is the direction of the applied field.


Fig. 9. Examples of reliability 2 samples. These samples give reliable palaeointensities and inclinations, but are less ideal than reliability1 samples. Thef values are generally lower and there may be some minor problems with pTRM checks: (a, e and f ) havef values below0.4, whereas (c and d) have borderline checks; (b) has failed pTRM checks at 460, 485 and 500◦C, but the check from 515 to 435◦C issuccessful. This means that the extra points from 485 to 515◦C can be accepted, giving a more reliable diagram. Note these extra pointsdo not alter the slope of the best fitting line. Thirty-five percent of samples studied gave these type of diagrams.


Fig. 10. Examples of reliability 3 samples. These samples give reliable inclination determinations. However, if a palaeointensity is determinedthere is something to suggest that the intensity value may not be entirely reliable: (a, c and e) only give inclination determinations. It canbe seen from the Zijderveld plots that these inclinations are reliable; (b, d and f ) give a palaeointensity estimate in addition to reliableinclination determinations. These palaeointensities are suspect because: (b) low temp checks just fail, and checks at 460, 485 and 500◦Cfail, even though the check from 515 to 435◦C is just acceptable; (d) hasf < 0.3; and (f) has smallf and a failed check at 450◦C.Twenty-two percent of samples studied gave these type of diagrams.


Fig. 11. Records of palaeointensity and inclinations versus depth from SOH1. Solid symbols represent the mean value from flows withmore than one successful determination. Error bars are 1σ . Open symbols represent flows with a single successful determination. Dottedlines at 36�T and 37◦ are the present day values on Hawaii.

envelope of a planar dike injected at 1000◦C andallowed to cool by conduction using a thermal dif-fusivity, κ = 0.78 × 10−6 m2 s−1 (Goranson, 1942;Carmichael, 1989; Horai, 1991). Samples whichwere calculated to have been heated to over 250◦Cwere rejected and are marked with a asterisk inTable 3.

6. Discussion

In Section 3, an age model for the SOH1 recordbased on radiogenic Ar/Ar and K–Ar dating was dis-cussed along with a number of problems associatedwith this model. Therefore, features that are indepen-dent of the exact age are discussed first, followed by


the development of an improved age model based oncorrelation with previous Hawaiian palaeomagneticdatasets. The new age model is then used to determinesecular variation on Hawaii and to compare SOH1with global records.

6.1. Features of the SOH1 secular variation record

The final inclination and palaeointensity recordsfrom SOH1 are shown inFig. 12 as a function of

Fig. 12. Final palaeointensity and inclination records from SOH1. Samples calculated to have been heated to above 250◦C by dikeintrusions have been rejected. Features A, B and C (shaded) are the excursions referred to inSection 6.1.1. Dotted lines represent currentfield values on Hawaii.

flow. Data from flows which are believed to havebeen affected by remagnetisation caused by dike in-trusion have been rejected. This mainly affects flows206–220 (depths 600–680 m), where there are severalthick dikes.

The most striking features of the record are threeevents where negative inclinations are observed, label-led A, B and C inFig. 12. Another obvious feature,most apparent in the inclination record, is the discon-tinuous nature of the record (e.g. at flows 88 and 186).


6.1.1. ExcursionsEvents A, B and C possibly represent records of

geomagnetic excursions. In the literature, secularvariation is considered excursional when the VGPlatitude drops below some arbitrary value. In the ab-sence of declination information, which would allowthe calculation of VGP latitude, we define negativeinclination as excursional behaviour.

Excursion A spans flows 88–157 (340–490 m) andexhibits an inclination drop from around 40 to−35◦.This inclination drop is accompanied by a reductionin palaeointensity from around 40 to 20�T. The evo-lution of this feature is documented in detail by suc-cessful inclination and palaeointensity determinationsfrom around 40 flows and represents the most detailedrecord of absolute palaeointensity and inclinationavailable for any excursion. Excursion B occurs atflow 186 (550 m). Inclination drops to a minimum of−25◦, accompanied by low intensities of 20–30�T.Interestingly, the lowest intensities occur where theinclination is around 0◦, not at the inclination mini-mum. Excursion C occurs at flow 227 (715 m). Thelowest inclination recorded is around−10◦, whichcorresponds to a low intensity of about 20�T.

The low intensities observed during A, B and Cagree with previous work that suggests field intensity

Fig. 13. Depth as a function of flow number for the subaerial section of SOH1. The grey line shows the flow thickness averaged over fiveflows to remove scatter. Obvious changes in thickness occur around flows 88 and 210, possibly indicating changes in eruptive behaviourand/or pauses in accumulation.

drops to at least half its normal value during an ex-cursion (Levi et al., 1990; Lin et al., 1994; Langereis,1999). This provides additional evidence that A, B,and C are indeed records of geomagnetic excursions,as well as corroborating previous observations.

It has also been suggested that during an excursionthe intensity reduces before the field direction startsto change (Lin et al., 1994). This effect is most ob-vious near the end of the record from flows 225–241(710–780 m), where during excursion C the intensitylow of 20�T is observed before the inclination dropfrom 40 to −10◦. This low intensity persists duringthe event. However, in contrast to this, when the fieldbegins to recover the intensity increases at the sametime as the inclination.

6.1.2. Discontinuous nature of the recordThe discontinuities in the SOH1 geomagnetic

record may indicate hiatuses in eruptive activity,where the geomagnetic field has continued to changebut has not been recorded. Independent evidence forthese hiatuses is present in the flow versus depthprofile shown inFig. 13. Changes in flow thicknessindicate changing eruptive behaviour, which mayprovide evidence for hiatuses in accumulation. Thesemay also represent changes in accumulation rate.


For example, the reduced flow thickness could indicatean increased accumulation rate, which would explainthe highly detailed record of excursion A.

Two obvious changes in flow thickness occur ataround 340 m (flow 88) and 610 m (flow 210), whichsplit the subaerial section into three main regimes withaverage flow thicknesses of 3.8 m (flows 1–88), 2.2 m(flows 88–210) and 4.5 m (flows 215–241 in order toexclude dikes at flows 211, 212 and 214). The discon-tinuity at flow 88 corresponds to the discontinuity ininclination and provides evidence that the abruptnessof the inclination change may be due to a pause ineruptions, not a feature of excursion A.

6.1.3. Inclination anomalyThe inclination anomaly (I ) at a particular site

is defined as the difference between the predictedinclination from a geocentric axial dipole (GAD) andthe observed inclination. Previous studies byLin et al.(1994), Camps and Prévot (1996)and Love (2000)have found that palaeointensity decreases rapidly asI increases.

SOH1 yielded 86I–palaeointensity pairs, usinga value for the GAD inclination at Hawaii (latitude

Fig. 14. Palaeointensity as a function of absolute inclination anomaly for SOH1 and other data from Hawaii.

19.5◦) of 35◦. Data from other palaeointensity stud-ies covering the last 420 kyr was compiled fromMankinen and Champion (1993a), Coe et al. (1978),Valet et al. (1998), Brassart et al. (1997), Tanaka andKono (1991), Laj and Kissel (1999)and Laj et al.(2002). This compilation gave 373I–palaeointensitypairs. TheI–palaeointensity pairs from each datasetwere binned into 5◦ I intervals and the arithmeticmean and standard error of the palaeointensities werecalculated, as done byLaj and Kissel (1999). Thepalaeointensity of the SOH1 and Hawaiian data areplotted as a function of |I| in Fig. 14.

Previously published Hawaiian data only have sevenflows which haveI > 40◦. The SOH1 data has muchbetter coverage of the highI region of the plot, hav-ing 22 flows withI > 40◦. This allows more detailedanalysis of theI–palaeointensity data from Hawaii.

The main observation fromFig. 14 is that at lowI, the palaeointensity has a wide range of values(10–70�T), as indicated by the scatter in the data,while at highI, the palaeointensity is almost alwayslow (20–25�T) and is much less scattered. This isconsistent with the observation byLin et al. (1994)thatlarge reductions in palaeointensity can occur without


necessarily causing significant deviation in the fielddirection. The trend also agrees withLove (2000)who analysed 353 direction–palaeointensity pairs andfound that the deviation of the field direction fromGAD increases as palaeointensity decreases.

6.2. SOH1 age models

SOH1 provides a very detailed record of recentsecular variation on Hawaii. Its main problem is poorchronological control. We consider two possible agemodels for SOH1. The first is the radiometric modelfrom Section 3. The second is a new model, whichwill be developed by comparing SOH1 with otherpalaeomagnetic records from Hawaii.

6.2.1. Radiogenic modelThe radiogenic model is based on Ar/Ar and K–Ar

dates. This model suffered from a number of prob-lems; the biggest of which was that the ages fromthe three apparently most reliably dated flows from

Fig. 15. SOH1 on the radiogenic age model, compared to the SOH4 record (Laj et al., 2002). Agreement between the two records is poor,possibly indicating deficiencies in this age model. Dotted lines represent current field values on Hawaii.

the subaerial section were not in stratigraphic order.This model assumes that the Ar/Ar and K–Ar agesof the sample from 756 m are too low and have beensimilarly affected by low temperature alteration pro-cesses, whereas the Ar/Ar ages from 296 and 506 mare reliable. This rejection was consistent with accu-mulation rate estimates from SOH4 and surface flows.However, the resulting age model should be regardedwith caution, as apparently reliable ages have been re-jected based on inconsistencies only, not experimentalevidence.

Fig. 15shows the SOH1 record on this age model,along with data from SOH4 (Laj et al., 2002). Theagreement of the two datasets is poor. The negativeinclination events in SOH1 and SOH4 occur at totallydifferent times. In the palaeointensity profiles, bothrecords display a gradual reduction from≈60�Tat 0 ka to ≈35�T with increasing age. However,the reduction occurs at 20 ka in SOH4 compared to40 ka in SOH1. The use of this age model raisesanother major problem. Excursion A apparently lasts

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around 18 ka. This is much longer than the 5 ka max-imum duration proposed byGubbins (1999)and alsodisagrees with observed excursion lengths in recentsedimentary records (Langereis et al., 1997; Laj et al.,2000). Obviously, this age scale is not satisfactoryand needs serious modification. At the very least itneeds adjustment so that excursion A has a reasonablelength.

6.2.2. Correlated palaeomagnetic modelHere we attempt to improve the SOH1 chronology

by correlating common features between palaeomag-netic records from SOH1 and two independent sets ofdated subaerial flows from Hawaii. The first dataset isa compilation of Hawaiian outcrop data for the last 30kyr from Coe et al. (1978), Tanaka and Kono (1991),Mankinen and Champion (1993a,b)and Valet et al.(1998), which we refer to as “Hawaii30”. Each flowhas been dated using either radiocarbon or histori-cal information and, therefore, has good age control.The second dataset is from the SOH4 lava core (Lajet al., 2002). Age control for SOH4 is from five K–Ardates and the basic radiometric chronology has beenslightly adjusted by the authors after comparison withMankinen and Champion (1993b), Laj et al. (2000)andWagner et al. (2000).

Fig. 16shows the correlations between inclinationand palaeointensity records from SOH1, the Hawaii30data and SOH4. Most of the correlations were firstmade between the inclination records because the fea-tures in inclination are more obvious and have greatervariability, thus providing a sounder basis for com-parison. The palaeointensity record was used to checkthese inclination correlations.

The correlated age model was fine tuned by usinga stretching algorithm which minimised the misfit be-tween SOH1 and SOH4 (Teanby, 2001; Teanby andGubbins, 2002). The resulting correlated age modelis shown inFig. 17, which implies accumulation ratesof 15 mm per year for 0–20 ka, 300 mm per year for20 ka, 3 mm per year for 20–35 ka and 20 mm per yearfor 35–45 ka. The coincidence of high accumulationrate with excursion A at 20 ka is very fortunate and re-sults in a very detailed record.Fig. 18shows the SOH1inclination and palaeointensity data on the stretchedage model and compares these to the SOH4 record.The agreement between records has been vastlyimproved.

Fig. 17. Stretched SOH1 age model (solid line) and age correlations(small lighter circles) along with Ar/Ar (black circles) and K–Ardate (square). Dashed lines show estimated upper and lower agebounds based on the Ar/Ar dates.

6.2.3. Which model is better?The advantages of the radiogenic model are that

it is based on independent evidence from Ar/Ar andK–Ar dating, and accumulation rate estimates. Thedisadvantages are the inconsistency of the dates andpoor correlation between SOH1 and other Hawaiiandata when using this age model. Also, the apparentlong duration of excursion A.

The advantages of the correlated age model are theinherent good correlation with SOH4 and Hawaii30datasets, and the reasonable excursion duration. Ac-cumulation rates are slightly higher than those basedon radiogenic dating of SOH4 and estimates using theage of surface flows, except during excursion A wherethe accumulation rate is much higher. This age modelis younger than the Ar/Ar dates at 296 and 506 m, andolder than the K–Ar and Ar/Ar dates at 756 m. The dis-advantages of this model are that the correlations areinevitably subjective and the extremely high accumu-lation rate implied by the age scale around 300–500 min excursion A. The ages of the correlations assumethat the SOH4 age model is correct. However, even ifthe SOH4 age model contains discrepancies, it is use-ful to correlate the SOH1 and SOH4 records so thatany further age information can then be incorporatedinto both records simultaneously.

Based on this evidence, the correlated and stretchedage model is considered the closest representation ofthe true age scale. The problems with this age scale arenot as severe as the those of the radiogenic age model


Fig. 18. Comparison of SOH4 and SOH1 inclination and palaeointensity profiles on the stretched age model. Dotted lines represent currentfield values on Hawaii.

and, most importantly for a palaeomagnetic study, theagreement with previous datasets is good.

6.2.4. Implications of the SOH1 age modelThere are two main implications of the SOH1 depth

versus age model shown inFig. 17. First, for the depthranges 0–200 m (0–20 ka) and 550–750 m (35–45 ka),the average accumulation rates are 15 and 20 mmper year, respectively. These are slightly higher thanestimates of 5.7–8.9 mm per year based on analysisof the ages of surface flows from the last 3 kyr inSection 3. So the SOH1 age model implies that theeruptive activity of KERZ was slightly increased inthe past. This is consistent with Kilauea moving offthe Hawaiian hotspot. These accumulation rates arealso consistent with the highest estimates from otherstudies.Guillou et al. (1997a,b)report accumulationrates of 3.8–11 mm per year for Kilauea based onlavas from SOH4, whileSharp et al. (1996)reportrates of 7.8± 3.2 mm per year for Manna Kea duringits shield building stage.

Secondly, around 20 ka there was a massive increasein accumulation rate around the SOH1 site. Increasedaccumulation rates of 35–40 mm per year are alsoobserved in SOH4 around 20 ka, implying increasedactivity of Kilauea in the KERZ area. The SOH1 agemodel implies that the average accumulation rate was300 mm per year. However, this is probably muchtoo high and is more likely due to matching the 40excursional data points for event A from SOH1 withonly a few excursional data points in SOH4. A smallstretching of the SOH4 timescale in this region wouldnot affect the SOH4 record significantly, but woulddramatically reduce the accumulation rate inferred atSOH1. In any case, the accumulation rate would stillbe very large during this depth interval because themaximum excursion length is about 5 kyr, imply-ing a minimum accumulation rate of (500− 300 m)/5 kyr = 40 mm per year.

A model of Hawaiian summit volcanoes byDePaoloand Stolper (1996), based on the HSDP1 pilot holeon Manna Kea/Loa, indicates accumulation rates


of 25–35 mm per year for subaerial eruptions. Thisfigure is more consistent with our lower limit of40 mm per year and suggests that excursion A shouldhave a longer duration.

6.3. Comparisons with other records

In order to allow comparisons with other datasetsthe stretched age model of SOH1 is assumed to becorrect.

6.3.1. Hawaiian dataFig. 19shows the SOH1 data along with data from

Coe et al. (1978), Tanaka and Kono (1991), Mankinenand Champion (1993a,b), Valet et al. (1998)andLajet al. (2002)for the last 50 kyr. The consistency ofboth palaeointensity and inclination data is very good.

For palaeointensity, all the data show a high of50–60�T around 0–5 ka, followed by a period from10 to 15 ka where the average field intensity is closeto the current value of 36�T. From 35 to 45 ka, thepalaeointensity ranges from around 20 to 40�T andis very scattered. This is consistent with excursionalbehaviour, which occurs around 35 and 40 ka. Allinclination data display lows around 2, 13 and 16 ka,and excursions A, B and C are both coincident andsimilar in magnitude.

Smooth cubic B-spline (CBS) fits (Lancaster andŠalkauskas, 1986)to the palaeointensity and incli-nation data are also shown inFig. 19, along withtheir error envelopes. The fits were constrained topass through the current field values at 0 ka. The knotspacing used was 2 kyr for inclination and 4 kyr forpalaeointensity. These CBS curves fit the data verywell and, assuming the SOH1 and SOH4 age modelsare correct, can be used as a reference curve of secularvariation on Hawaii for 0–50 ka.

Using the combined dataset for 0–50 ka gives amean palaeointensity for Hawaii of 31.2 ± 9.3�Tand a mean inclination of 25.8±13.0◦. Excluding theexcursions, by only considering the 0–18 ka period,changes the means to 40.2 ± 8.7�T and 29.3± 6.9◦.The inclination is consistent with the GAD inclinationof 35◦ and the value of palaeointensity is consistentwith the current value on Hawaii of 36�T.

The interpretation of excursions A, B and C inrelation to previously observed excursions is entirelydependent on the validity of the SOH1 and SOH4 age

models. Assuming the age models are correct allowsthe excursions to be correlated with other excursionfrom around the globe.

Excursion A, at 20 ka, is not well established,other than in the SOH cores. However, some possibleexcursions have been observed around this time:Coeet al. (1978)report a low inclination of 8.5◦ in theHilina Pali, also on Kilauea;Nowaczyk and Knies(2000) found a negative inclination around 22 ka ina sediment core from the eastern Arctic Ocean; andZhu et al. (1998)found a possible excursion around14 ka in a freshwater sediment sequence in China. It ispossible that these all represent the same geomagneticevent, but further evidence is required.

Excursion B, at 35 ka, is interpreted as the MonoLake excursion (Denham and Cox, 1971; Liddicoat,1992), which is well established in the northern hemi-sphere. Low inclinations observed in the Hilina basaltson Kilauea (Holt, 1994; Riley, 1996; Riley et al., 1999;their event A) between 28.6 and 43.0 ka could also bedue to the Mono Lake event.

Excursion C, at 40 ka, is interpreted as the Lasch-amp event (Bonhommet and Babkine, 1967), which isa globally established excursion. The Laschamp hasbeen observed as negative inclinations in cores HSDP1(Laj and Kissel, 1999) and SOH4 (Laj et al., 2002),and also by low inclinations at the Hilina basalts onKilanea (Holt, 1994; Riley, 1996; Riley et al., 1999;their event B) between 28.6 and 43.0 ka.

For all the excursions observed in this core we ob-serve intensities of around 20�T. Although this is lowcompared to the current intensity on Hawaii, it is ratherhigh for during an excursion. However, our intensityof 20–25�T for event A is consistent with obser-vations from SOH4 and a single intensity found byCoeet al. (1978)of 15.6± 6.7�T at 18 ka. A single inten-sity during the Laschamp in the HSDP1 core also has ahigh intensity of around 30�T, which agrees with ourhigh values around 35 ka. However, these correspondto inclinations of around 0◦, which indicates that thiswas probably not the minimum intensity reached.

High intensities during an excursion could be ex-plained by either: a weak dipole and strong quad-rupole; or a strong reversed dipole. It is not possibleto distinguish between these two possibilities basedon inclination and intensity data from just one site.However, because excursion A has so far only beenobserved on Hawaii, this may suggest that the strong


Fig. 19. Palaeointensity and inclination data from SOH1, SOH4 and other studies on Hawaii (seeSection 6.3.1). The CBS fit to eachdataset is also shown: knot spacing for fit is 2 kyr for inclination and 4 kyr for palaeointensity.

quadrupole explanation is the most likely, as a strongreversed dipole would have global effects. However,more observations are required to confirm this. There-fore, the high intensities observed for excursions inthis core before 18 ka provide evidence against an ex-tended period of low non-dipole component at Hawaii.

This corroborates studies byMankinen and Champion(1993b)andMcElhinny et al. (1996).

6.3.2. Global recordsA compilation of lava data from the Univer-

sity of Montpellier (ftp://ftp.dstu.univmontp2.fr/pub/

ftp://ftp.dstu.univmontp2.fr/pub/paleointdb/


Fig. 20. Comparison of SOH1 VADM profile with: (a) database of global absolute palaeointensities fromLaj et al. (2002); (b) archaeomagneticdata fromYang et al. (2000)(0–12 ka) and NAPIS-75 stack (Laj et al., 2000) (12–50 ka); (c) Sint-200 relative intensity stack (Guyodoand Valet, 1996); (d) relative intensity derived from10Be stack ofFrank et al. (1997).

paleointdb/) was recently updated (Kissel, personalcommunication) for inclusion inLaj et al. (2002). Thefirst 50 kyr of this compilation are compared withSOH1 data inFig. 20a. To allow for differences inlatitude of sampling sites, all palaeointensity valueshave been converted to virtual axial dipole moment(VADM).

Large scatter is expected in the data because ofnon-dipole components. However, bearing this inmind, the agreement is generally very good, show-ing a high VADM of around 10–12× 1022 A m2 for0–10 ka, followed by a reduction to 6–8× 1022 A m2

from 10 to 20 ka. During the Laschamp at 40 ka, SOH1records intensities which are consistent with the high-est of the previous observations (≈5 × 1022 A m2). Afew low VADMs (≈5× 1022 A m2) in the global datadisagree with SOH1 for 0–10 ka, although these alsodisagree with the bulk of the other data.

Fig. 20bis a comparison with archaeomagnetic dataand the NAPIS-75 sedimentary stack. Archaeomag-

netic data covers 0–12 ka and is taken fromYanget al. (2000)as supplemented byTeanby and Gubbins(2000). The NAPIS-75 stack (Laj et al., 2000) is usedfor 12–50 ka and is the slightly modified version fromWagner et al. (2000), which stacks only the four high-est sedimentation rate cores to give increased reso-lution. Both archaeomagnetic and SOH1 data have ahigh at around 2.5 ka. The agreement of SOH1 withNAPIS-75 is very good except around 40 ka, just afterthe Laschamp.

Fig. 20c shows the comparison with the Sint-200sedimentary stack ofGuyodo and Valet (1996).Sint-200 is a record of relative intensity, so the first20 ka have been normalised to have the same meanVADM as the first 20 ka of the Montpellier global lavadatabase above. The agreement is again very good,with both records displaying the characteristic high for0–10 ka, followed by a weakening of the field. Again,the only discrepancy occurs around 40 ka. Most ofthe SOH1 data lie within the Sint-200 error envelope.

ftp://ftp.dstu.univmontp2.fr/pub/paleointdb/


Frank et al. (1997)produced a stack of10Be iso-tope records, which should be inversely proportionalto the dipole field intensity. They used this stack tocreate a record of relative dipole moment for the last200 kyr, which is shown inFig. 20d. This stack hasbeen normalised in the same way as the Sint-200record, so that comparisons with SOH1 can be made.Again the agreement with SOH1 is good, except thatSOH1 records higher intensities from around 35 to40 ka, although intensities during the Laschamp areconsistent.

The SOH1 core records intensities during theLaschamp excursion which are consistent with all ofthe global datasets above. However, the SOH1 inten-sities lie at the high end of previous absolute intensityobservations. This could be due to the minimumintensity not being recorded by any of the SOH1flows, as the minimum inclination reached is only−10◦. However, we think that the high intensitiesare more likely due to the non-dipole nature of ex-cursions and that strong non-dipole components werepresent on Hawaii during the Laschamp. These strongnon-dipole components would also explain why theintensity is slightly high during excursion A, eventhough the field has almost fully reversed direction,with a minimum inclination of around−40◦.

7. Conclusions

A Thellier–Thellier study of 360 samples from lavacore SOH1 on Kilauea, Hawaii, yielded around 200new palaeointensity and inclination determinations.The SOH1 secular variation record contains three geo-magnetic excursions, which display reductions in fieldintensities of around 50%. This agrees with previousstudies, although intensities are on the high side ofprevious observations. This may indicate the presenceof strong non-dipole components during excursionsA, B and C. Field intensity reductions were observedto occur before significant directional deviations,both for normal and excursional secular variationregimes.

Three new Ar/Ar dates were obtained for SOH1.These, along with previously published K–Ar dates,formed the basis of the SOH1 age model, which wasthen improved by correlation and stretching relativeto previous Hawaiian palaeomagnetic data. Assuming

that the correlated age scale is correct gives a total agerange of 0–45 ka and implies that the three observedexcursions correspond to the Hilina Pali, Mono Lakeand Laschamp events. The record of the Hilina Palievent is the most detailed record of intensity duringany excursion.

The correlated age model implies that Kilauea hasbeen more active in the past, and that around 20 kathere was a burst of vigorous activity at the SOH1 site.A similar burst in activity at the SOH4 site impliesincreased activity on the KERZ at 20 ka.

Mean palaeointensity and inclination values fromSOH1 are consistent with the current intensity andGAD predicted inclination. However, before 18 kaintensities are generally about 25% lower than thepresent value, and after 18 ka are generally higher.

Agreement between SOH1 and global records ofrelative and absolute intensity is very good, exceptaround 40 ka, when higher intensities are observed inSOH1, possibly due to non-dipole effects during theLaschamp event.

The SOH1 record is a significant contribution tothe dataset of secular variation on Hawaii, providing35% of the intensity data for the last 45 kyr, and mostof the excursional data. It also makes a significantcontribution to the global absolute intensity record.

Acknowledgements

The authors would like to thank Steve Quane forthe use of his detailed stratigraphic log for SOH1 andalso for helping during the second sampling sessionin Honolulu; Mike Garcia for allowing us access tothe core and for help in selecting samples for Ar/Aranalyses; Donald Thomas for permission to sampleSOH1; Catherine Kissel and Alain Mazaud for theirvaluable assistance during the Thellier–Thellier ex-periments; Brian Davidson and Jim Imlach for help inobtaining the Ar/Ar dates at SURRC, East Kilbride;Joe Cann for insightful discussions. We also thanktwo anonymous reviewers whose comments helpedimprove the manuscript. This work was supported byNERC Ph.D. studentship GT04/97/136/ES (N.T.) andNERC Argon Isotope Facility proposal IP/627/0999.Financial assistance was provided by the CEA/CNRSduring experiments undertaken at LSCE (N.T.). Thisis LSCE contribution number 0767.


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