paleomagnetic secular variation at vulcano (aeolian islands) during the last 135 kyr

Paleomagnetic secular variation at Vulcano (Aeolian Islands)during the last 135 kyr

Roberto Lanza, Elena Zanella �

Dipartimento di Scienze della Terra, Universita' di Torino, Via Valperga Caluso 35, 10125 Torino, Italy

Received 17 February 2003; received in revised form 27 May 2003; accepted 3 June 2003

Abstract

Paleosecular variation (PSV) of the Earth’s magnetic field during the last 135 kyr has been investigated in lavas,scoriae and pyroclastic rocks of Vulcano (Aeolian Islands). About 1000 samples have been collected at 77 sites from25 distinct volcanic units, whose age is either known from published isotopical data or constrained on the grounds ofstatigraphical relationships. Magnetic mineralogy investigation shows that Ti-magnetite is the main ferromagneticmineral. At most sites, secondary magnetization components are either absent or easily removed by stepwise thermalor alternating field demagnetization. The mean site direction of the characteristic remanent magnetization is usuallywell-defined, since the semi-angle of confidence is greater than 5‡ at only four sites. The mean paleomagnetic directionover the last 135 kyr (D= 9.4‡, I= 53.2‡, K95 = 3.5‡) differs from the geocentric axial dipole (GAD) at Vulcano (D= 0‡,I= 57.8‡) and might be interpreted as the effect of a long-term, non-axial-dipolar component. The PSV record fromVulcano agrees well with those from the lacustrine sediments of Lago Grande di Monticchio (100 kyr BP) and Lagodi Mezzano (30 kyr BP), located in the Italian peninsula [Brandt et al., Quat. Sci. Rev. 18 (1999) 961^976]. Theinclination anomaly vI found at Vulcano corresponds to about half of the shallowing observed in the sediments of thetwo lakes and the declination anomaly vD may be used to tie the declination values, derived from azimuthallyunoriented cores, to the geographical reference system. In order to find the optimum site to be used as reference forPSV studies in Italy, the angular values of the Earth’s magnetic field measured at the 113 repeat stations of the ItalianGeomagnetic Network [Coticchia et al., Boll. Geod. Sci. Aff. 40 (2001) 277^291] have been analyzed with therelocation via pole method [Noel and Batt, Geophys. J. Int. 102 (1990) 753^756]. The Viterbo station (lat. 42‡27PN,long. 12‡02PE) proved the best, since the mean error is 0.3‡ for both declination and inclination, wherever the originalPSV site is located in Italy. A preliminary, composite PSV curve for the last 30 kyr BP is thus proposed, merging andrelocating to Viterbo the data from Vulcano and the curve from Lago di Mezzano, corrected for the GAD deviationfound at Vulcano.B 2003 Elsevier B.V. All rights reserved.

Keywords: Earth’s magnetic ¢eld; Italy; paleosecular variation; volcanic rock

1. Introduction

The time variations at the scale of 103^105 yrwhich characterize the Earth’s magnetic ¢eld dur-ing stable polarity periods are known as paleosec-

0012-821X / 03 / $ ^ see front matter B 2003 Elsevier B.V. All rights reserved.doi:10.1016/S0012-821X(03)00326-1

* Corresponding author. Tel. : +39-011-670-7191;Fax: +39-011-670-7155.E-mail addresses: [email protected] (R. Lanza),

[email protected] (E. Zanella).

EPSL 6725 5-8-03

Earth and Planetary Science Letters 213 (2003) 321^336

R

Available online at www.sciencedirect.com

www.elsevier.com/locate/epsl

mailto:[email protected]

mailto:[email protected]

http://www.elsevier.com/locate/epsl

ular variations (PSVs). PSV records may be ob-tained by investigating the magnetic remanenceacquired by rocks and, limited to historical times,archaeological artifacts. A high-quality PSV rec-ord needs a sure identi¢cation and reliable datingof primary remanence, should be as continuous aspossible and have a high time resolution. Theseoptimum features seldom occur. Records derivedfrom sedimentary rocks may be continuous, if thedeposition rate is constant and no hiatuses occur,but su¡er from uncertainties about the remanenceacquisition process [4,5]. Depositional remanentmagnetization (DRM) is often a¡ected by inclina-tion shallowing and post-depositional magnetiza-tion (pDRM) results in a time lag between thesediment and remanence ages that depends onthe sedimentary environment (lock-in time, see[6]). Moreover, the PSV record is smoothedwhen the sedimentation rate is low and stackingtechniques may be required when the remanenceintensity is low [7^9]. Another problem concernsthe cores from sea- and lake-bottom sediments,mostly sampled with no azimuthal orientation.The reliability of the declination curves they pro-vide has often been questioned and most paleo-magnetic databases do not include core records.Lacustrine sediments especially from maar lakeshave proved to be suitable for PSV studies up to100^120 ka [1,8,10]. The steady depositional envi-ronment and high sedimentation rate providegood continuity and high time resolution, anddating is possible using various methodologies,such as varve counting, tephrochronology, 14Cand pollen analyses. Accuracy of the declinationand inclination curves may, however, be reducedby the problems related to the DRM acquisitionand lack of core orientation.

The thermal remanent magnetization (TRM) ofvolcanic rocks is acquired immediately after theeruption and usually characterized by high inten-sity and stability. The declination and inclinationrecord is well-de¢ned but lacks continuity, sinceeach individual eruption provides an instantane-ous record of the past geomagnetic ¢eld. High-precision dating of volcanic rocks is a complexmatter and the error range of many isotopicmethods is high when compared to the PSV var-iability in time. However, high-quality PSV rec-

ords have been obtained from such regions asHawaii and Iceland, where long sequences oflava £ows occur [11^13].

The best approach to build a PSV curve is thusto integrate data from di¡erent lithologies, whichprovide independent constraints on the same phe-nomenon. Vulcano, one of the two active volca-noes of the Aelioan Islands in the southern Tyr-rhenian Sea, has been the subject of detailedgeological, volcanological and geochronologicalinvestigations ([14,15] and references therein),which greatly improved the knowledge on theage of the various volcanic units and their strati-graphical relationships. A ¢rm framework wasthus ready to support paleomagnetic data andbuild a PSV curve from about 135 ka to thePresent. This paper reports on the paleomagneticstudies performed to derive the curve, compares itwith those obtained from lacustrine sediments inthe Italian peninsula [1] and discusses their con-sistency in order to propose a PSV reference siteand a preliminary curve for the Italian region.

2. Geological setting and sampling

Vulcano is the southernmost of the seven Aeo-lian Islands, located in the southern TyrrhenianSea (Fig. 1). It is formed of volcanic rocks whosecomposition ranges from basalt to rhyolite. Theoldest dated rocks show a K^Ar age of about 135ka [16] ; the volcanic activity continued up to1888^90 in La Fossa cone [17]. Four main erup-tive centers (Vulcano Primordiale, Lentia Com-plex, La Fossa cone and Vulcanello) and two cal-deras (Il Piano and La Fossa) have beendistinguished. The oldest activity is related to Vul-cano Primordiale (V135^100 ka), which consti-tutes the southernmost part of the island. It is acomposite cone of trachybasaltic^trachyandesiticproducts, whose top is truncated by Il Piano cal-dera. Activity continued with the Il Piano caldera¢ll-in products : initial lava £ows (99.5^78.5 ka)were followed by scoriae and tu¡s (78^48 ka)[18]. According to [16,17], intracaldera volcano-tectonic activity migrated from SW to NE andformed the southeastern sectors of La Fossa cal-dera between 50 and 20 ka. As indicated by iso-

EPSL 6725 5-8-03

R. Lanza, E. Zanella / Earth and Planetary Science Letters 213 (2003) 321^336322

topic data, volcanic activity developed between 25and 20 ka in the southern and western £anks ofVulcano Primordiale. The Lentia complex wasformed between around 28 and 18 ka [15] in thenorthwestern sector of Vulcano. It mainly com-prises rhyolitic domes and lava £ows. The col-lapse of the eastern part of the Lentia complexgenerated the western sector of La Fossa caldera[19], whose deposits erupted between 15 and 8 kaand consist of several pyroclastic and e¡usive

products [14] partly cropping out on Il Piano. Inthe center of La Fossa caldera is located La Fossacone, an active composite edi¢ce formed in thelast 6 kyr by pyroclastic products and minorlava £ows from di¡erent vents. Finally, the north-ern part of Vulcano is constituted by Vulcanello,a composite lava platform with three nested vol-canic cones.

Almost every volcanic unit of the island hasbeen dated ([14^16] and references therein),

Fig. 1. Geological sketch map of Vulcano island (simpli¢ed after [14]) and sampling sites.

EPSL 6725 5-8-03

R. Lanza, E. Zanella / Earth and Planetary Science Letters 213 (2003) 321^336 323

mainly by the K^Ar and the uranium-series dis-equilibria methods. They have also been sampledfor paleomagnetic investigations and some resultshave already been published [16,20,21]. Most ofthe units dated by [15] have been sampled at thevery same sites during two co-ordinated ¢eldtrips. Other sites where in past years rocks werecollected for dating have been indicated by DeAstis, who also helped in the stratigraphical re-construction. Lava £ows, scoriae and pyroclasticproducts have been sampled in 121 sites using agasoline- or battery-powered drill and at each sitethe samples have been oriented by both magneticand solar compass. Magnetic variations varied inthe range 5‡W to 5‡E between sites and less thanS 3‡ within each site. The present paper only con-siders the sites from volcanic units whose age isde¢ned by either isotopic dating or undoubtedstratigraphic constraints. Moreover, the sites af-

fected by either isothermal remanent magnetiza-tion (IRM) due to lighting or substantial miner-alogical alteration or possible post-emplacementmovements were discarded as well as those char-acterized by poor values of the Fisher statisticalparameters (K95 s 10‡ or k6 30). The location ofthe 77 sites that passed the selection criteria isreported in Fig. 1.

3. Paleomagnetism

Magnetic properties were investigated by IRMacquisition, thermal demagnetization of IRMcomponents [22] and susceptibility vs. tempera-ture measurements on at least one specimen persite. IRM investigations (Fig. 2) show that inmost specimens saturation is reached for applied¢elds of 100^200 mT and the remanent coercive

Fig. 2. IRM acquisition curves and back-¢eld removing curves. Specimens from Monte Saraceno lava (a) and Mastro Minicolatite (b).

EPSL 6725 5-8-03


force varies in the range 20^40 mT. The only ex-ception are the specimens from the Lentia Com-plex, where saturation is approached but notreached at the maximum available ¢eld of 1.5 Tand the remanent coercive force may be as high as80 mT. Thermal demagnetization shows that thelow- and intermediate-coercivity IRM compo-nents (Fig. 3) are removed at temperature valuesaround 540^560‡C, together with a negligiblehigh-coercivity component. The Lentia rockshave again a distinctive feature, as the three com-ponents persist up to around 640‡C and the high-coercivity component carries a substantial part ofthe IRM. The susceptibility vs. temperature mea-surements (Fig. 4) yield Curie points in the sametemperature range. These results show that Ti-magnetite is the main ferromagnetic mineral inall Vulcano rocks, together with an oxidizedphase in those of the Lentia Complex. Hysteresisloops measured on about 30 representative speci-

mens (Fig. 5) show that the Ti-magnetite mainlyoccurs as pseudo-single domain (PSD) grains [23].

Natural remanent magnetization (NRM) andsusceptibility were measured using a JR-5 spinnermagnetometer and a KLY-2 bridge. Their values

Fig. 3. Thermal demagnetization of the IRM components.Symbols: dot = low-; square = intermediate-; triangle = high-coercivity component. Specimens from Vulcano Primordialelava (a) and Cala del Formaggio rhyolite (b).

Fig. 4. Magnetic susceptibility as a function of temperature.Symbols: full line = heating, dashed line = cooling; K0 = roomtemperature susceptibility. Specimens from Punte Nere lava£ow (a) and Lentia rhyolitic domes (b).

Fig. 5. Magnetization ratio (Mrs/Ms) as a function of coer-civity ratio (Hcr/Hc). The diagram is divided according to thestate of magnetic grains: single-domain (SD), pseudo-singledomain (PSD) and multi-domain (MD) (after [23]).

EPSL 6725 5-8-03


range from 1 to 14 A/m and 4140U1036 to68 500U1036 SI units, respectively, and mainlydepend on the di¡erent amounts of Ti-magnetitein the various lithotypes. The highest values char-acterize the leucite-tephrite at Vulcanello, the low-est the rhyolitic products of the Lentia Complex[24]. Two pilot specimens per site were demagne-tized, one thermally and one in alternating ¢eld(AF), at 10^15 steps and the results of the two

techniques were comparable. Typical demagneti-zation patterns are shown in Fig. 6. At most sites,irrespective of lithology, secondary componentsare either absent or negligible and the declinationand inclination curves point straight to the originof the Zijderveld diagram (Fig. 6a,e). Substantialsecondary components occur at about 20 sites.They are usually removed in the ¢rst steps andthe characteristic component (characteristic rema-

Fig. 6. Zijderveld diagrams for AF and thermal demagnetizations. Symbols: full dot = declination, open dot = apparent inclina-tion. Specimens from Vulcano Primordiale lava (a), Passo Piano lava (b), Gelso^Petrulla lava (c), Monte Molineddu pyroclastics(d), Monte Saraceno scoriae (e) and Lentia rhyolitic dome (f).

EPSL 6725 5-8-03


Table 1Paleomagnetic directions from Vulcano (lat. 38‡24PN, long. 14‡58PE)

Eruptive unit Volcanic unit Site N/n Age ChRM VGP Relocated values

(ka) D I k K95 PLat. PLong. Dr Ir

Vulcanello lava platform VUL 4/43 1.9 S 0.1 15.6 51.5 154 3.0 76 126 16 55La Fossa cone Palizzi lava PZL 1/8 1.6 S 1.0 16.4 44.5 514 2.4 72 142 16 49

1.5 S 0.2Punte Nere lava PNL 1/12 5.5 S 1.3 17.9 51.0 164 3.4 74 124 18 54

3.8 S 0.8Punte Nere surge (I cycle) PNS 2/29 stratigr.

constraint1.0 51.4 81 3.0 84 187 1 56

Punta Roja lava PRL 1/8 14.0 S 6.0 354.6 52.2 148 5.5 83 235 354 5713.0 S 3.0

Lentia complex rhyolitic lavas and domes RD 8/104 15.5 S 1.5 353.3 48.1 42 2.1 79 228 352 5313.0 S 3.0

Cala del Formaggiorhyolite

CFR 1/16 25.8 S 2.5 2.2 45.9 631 1.5 79 185 2 51

Mastro Minico trachyte MMT 1/16 stratigr.constraint

4.0 52.7 93 3.8 84 161 4 57

Mastro Minico latite MML 2/29 26.5 S 2.0 9.2 64.4 324 1.5 80 53 11 6727.9 S 6.6

La Fossa caldera Monte Saraceno scoria MSS 6/56 8.3 S 1.6 7.3 50.6 65 2.4 81 153 7 55Monte Saraceno lava MSL 5/50 stratigr.

constraint359.7 52.9 67 2.5 85 198 359 57

Grotte dei Rossi tu¡s TGR 11/140 stratigr.constraint

358.1 52.6 50 6.9 85 212 358 57

Il Piano caldera Quadrara scoria QS 7/106 21.3 S 3.4 2.4 52.2 111 1.3 84 175 2 56Spiaggia Lunga scoria SLS 6/91 24.0 S 5.0 1.8 44.4 129 1.3 78 187 1 49Passo Piano lava PPL 1/16 48.5 S 2.4 17.3 58.8 259 2.4 77 95 18 62Monte Molineddupyroclastics

MMP 1/17 stratigr.constraint

21.7 57.8 30 6.6 73 98 23 61

Timpone del Corvo lava TCL 1/12 77.5 S 2.1 22.3 55.0 221 3.5 72 107 23 58Vallone di Rio Grande lava RGL 4/38 78.5 S 4.5 32.6 39.2 38 4.4 58 126 32 42

Gelso^Petrulla Complex Gelso^Petrulla lava GPL 2/12 22.0 S 4.0 4.7 56.6 232 2.9 86 122 5 60Monte Aria Monte Aria lava MAL 3/30 100^78 14.9 55.7 77 3.1 78 11 16 59Vulcano Primordiale trachybasaltic lava VPL 2/32 stratigr.

constraint11.9 59.7 42 3.9 81 88 13 63

2/38 110.0 S 5.5 1.7 49.4 84 2.6 82 185 1 542/34 stratigr.

constraint346.4 49.2 230 1.6 76 252 345 54

2/46 135.0 S 3.5 35.8 52.7 249 1.3 61 104 37 55Spiaggia Lunga lava SLL 1/10 113.0 S 3.0 8.3 65.1 70 6.7 79 47 10 68

Abbreviations: N/n= number of sites/number of specimens; D, I= declination and inclination; k= Fisher’s precision parameter; K95 = semi-angle of con¢dence;Plat., Plong. = latitude and longitude of the VGP; Dr, Ir = declination and inclination relocated via pole to Viterbo (lat. 42‡27PN, long. 12‡02PE). Ages from [14^16]and references therein.

EP

SL6725

5-8-03

R.Lanza,

E.Zanella

/Earth

andPlanetary

Science

Letters

213(2003)

321^336327

nent magnetization, ChRM) is then clearly iso-lated (Fig. 6b^d). The AF method was chosenfor systematic demagnetization at most sites; atleast three steps (20, 40, 60 mT) were done andthe ChRM calculated using the stable endpointanalysis. In few cases (Fig. 6f) the coercivity orunblocking temperature spectra of the secondaryand characteristic components do overlap andsystematic demagnetization was done at up to10 steps. The ChRM was then derived using theconverging remagnetization circles method [25].

The paleomagnetic results are summarized inTable 1, which reports the ChRM mean direction,the k and K95 parameters of the Fisher statisticsand the virtual geomagnetic pole (VGP) of eachvolcanic unit. The mean directions are calculatedfrom the specimen values, since the number ofsampling sites (N) varies according to the unit’sextent and traceability in the ¢eld.

4. PSV at Vulcano and comparison withsedimentary records

Most of the ChRM directions in Table 1 arecoupled to an isotopic age taken from the litera-ture. Seven units are given a time position on thebasis of stratigraphical constraints clearly recog-nizable in the ¢eld. In some cases, they are verystrict, as for example for the MMT unit, whichcrops out in a lava £ow succession and intervenesbetween the MML and CFR units, both isotopi-cally dated. On the other hand, the constraints arerather poor for the MMP unit, which was em-placed some time between 78.5 and 53 ka [18].

Inspection of Table 1 shows that various erup-tions occurred at the same time within the datingerror. In these cases, the McFadden^Lowes test[26] was applied to check the possibility that thecorresponding volcanic units share a commontrue mean direction. Only the PRL and RD units,both with a U^Th age of 13.0 S 3.0 ka, pass thetest. To remove the bias introduced by multiplesampling of the same direction, a mean ChRMdirection (D= 354.0‡, I= 49.0‡, K95 = 2.9‡) wascomputed from all the specimens of the two units.Twenty-four time-independent records of the pastgeomagnetic ¢eld were thus obtained and used to

calculate the mean paleomagnetic direction atVulcano for the last 135 kyr: D= 9.4‡, I= 53.2‡with k= 73 and K95 = 3.5‡. It di¡ers at the 95%statistical level from the geocentric axial dipole(GAD) direction DGAD = 0‡, IGAD = 57.8‡. Thisdeviation is clearly shown by the VGP distribu-tion, which is centered on a mean pole (lat. 82‡N,long. 130‡E, K95 = 4.4‡) displaced by 8‡ from thegeographic pole (Fig. 7).

The Vulcano mean direction refers to a timeinterval of the order of 105 yr that is consideredlong enough to average out the PSV e¡ects [27].The VGP total angular dispersion around thegeographic pole is ST = 14.4‡ and decreases toSW = 14.0‡ after correction for within-site disper-sion [28], with lower and upper limits of 11.7‡ and17.4‡ respectively, as expected at the latitude ofVulcano [29]. This result suggests that the Vul-cano data set is not biased by inadequate sam-pling and that the deviation from the GAD ofthe Vulcano mean direction (vD= 9.4‡,vI=34.6‡) could be ascribed to a long-term,non-axial-dipolar magnetic ¢eld component.

Two PSV records from sedimentary rocks areavailable for the Italian peninsula [1] and theirtime span is comparable to that of Vulcano:Lago di Mezzano (30 kyr BP) and Lago Grande

Fig. 7. Equal-area projection of VGPs. Symbols: diamond =mean paleomagnetic pole with K95 con¢dence circle.

EPSL 6725 5-8-03


Fig. 8. PSV directions at Vulcano (last 30 kyr). Symbols: dot = mean-unit ChRM with associated isotopic age (full dot) or posi-tioned by stratigraphical criteria (open dot); diamond = mean-unit ChRM from volcanic units of Lipari [30]. The vertical bar isequal to the age uncertainty, the horizontal bar to the vD or vI value. Vertical axes correspond to the GAD direction at Lagodi Mezzano. Full line shows the PSV recorded at Lago di Mezzano [1].

EPSL 6725 5-8-03


Fig. 9. PSV directions at Vulcano (last 135 kyr). Symbols as in Fig. 8. Vertical axes correspond to the GAD direction at Lago diMonticchio. Full line shows the PSV recorded at Lago di Monticchio [1].

EPSL 6725 5-8-03


di Monticchio (100 kyr BP) (Fig. 1). Comparisonof paleomagnetic directions obtained at di¡erentsites requires prior elimination of their depen-dence on the site’s geographical location. This isusually done assuming a central dipole ¢eld andcalculating the position of the magnetic pole,which is independent of the location of individualsites. In PSV studies, however, direct comparisonbetween declination and inclination is preferred.It can be achieved by converting the paleomag-netic directions from one site to another. Noeland Batt [3] proposed the conversion via polemethod: the direction observed at one site is ¢rstconverted to a VGP and the VGP position is thenused to calculate the corresponding direction atthe location of the second site. The PSV datafrom Vulcano (lat. 38‡24PN, long. 14‡58PE) werethus converted via pole to Lago di Mezzano (lat.42‡37PN, long. 11‡46PE) and Lago Grande diMonticchio (lat. 40‡56PN, long. 15‡36PE) and theconverted declination and inclination valuessuperposed on the PSV curves of both localities(Figs. 8 and 9), redrawn from [1]. The verticalaxes of the curves correspond to the GAD localvalues: DGAD = 0‡, IGAD = 61.5‡ at Mezzano,IGAD = 59.7‡ at Monticchio. The two directionsfrom the trachybasaltic lavas of Vulcano Primor-diale (VPL) not coupled to an isotopic age (Table1) are not shown in the ¢gures. Even if the strati-graphical position is well constrained by the geo-metrical succession of the £ows, they can hardlybe assigned a de¢nite age because there is no wayto estimate the time elapsed between individual£ows. On the other hand, four data from the is-land of Lipari, immediately to the north of Vul-cano, have been converted and plotted, as theyare associated to K^Ar ages [30].

Figs. 8 and 9 show that the PSV data fromVulcano, plotted as dots, match the trend of thecurves from the two lakes and the agreement isgood in the case of Lago di Mezzano. Consider-ing that volcanic and sedimentary rocks acquiretheir remanent magnetization through completelydi¡erent physical processes and the complexity indating lacustrine deposits, the consistency of theresults is remarkable and shows that each of thethree data sets is a reliable record of the past geo-magnetic ¢eld. They substantiate each other, since

the volcanic record is improved by the continuityin time typical of sediments, whose directionaldata can in turn be referred to the absolute geo-metrical frame supplied by the volcanic rocks.The analysis of the declination and inclinationcurves provides interesting results. The curvesfrom both lakes clearly show that the PSV incli-nation is nearly always lower than the GAD val-ue. Brandt et al. [1] found a mean inclination ofI= 50.7‡ for Lago di Mezzano and I= 52.3‡ forLago Grande di Monticchio, whose expected val-ues according to the GAD hypothesis are IGAD =61.5‡ and IGAD = 59.7‡, respectively. The di¡er-ence of 7‡^10‡ is fully compatible with the rangeof the inclination error which often occurs in thesediments [31] as a consequence of the DRM ac-quisition process. On the other hand, the inclina-tion values from Vulcano too are lower than theGAD and no systematic inclination error a¡ectsthe TRM direction, whose deviation from the am-bient magnetic ¢eld during cooling, if any, maydepend on di¡erent causes, such as emplacementmechanism (lava dome, lava £ow, pyroclastic fall,T) and magnetic anisotropy. At the beginning ofthis section, it was shown that the mean paleo-magnetic direction at Vulcano has deviated fromthe GAD and vI=34.6‡. Therefore, about half ofthe inclination shallowing observed in the lacus-trine sediments at Mezzano and Monticchio cantentatively be ascribed to a non-axial-dipolarcomponent occurring over the last 135 kyr.

The declination values for the two lakes are nottied to the geographical reference system, becausethe cores had no azimuthal orientation. As usualin this case, the mean declination value has beenset to zero [1] on the implicit assumptions that thetime span covered by the core is enough to aver-age out PSV and the average value corresponds tothe GAD one. The declinations from Vulcano aregiven with respect to the geographical north andtheir deviation from the GAD is vD= 9.4‡. It istherefore reasonable to assume that even at thetwo lakes the average declination over the last135 kyr has deviated from the GAD and theiraverage declination is 9.4‡.

In conclusion, the results of the present studytogether with those of [1] concur to de¢ne thePSV trend in the Italian region: the lacustrine

EPSL 6725 5-8-03


Fig. 10. PSV composite curve for Italy. Vulcano and Lipari ChRMs have been relocated via pole to Viterbo. Symbols as inFig. 8. Dotted axes are positioned according to the GAD deviation found at Vulcano, i.e. at 9.4‡ for declination and at 56.9‡for inclination.

EPSL 6725 5-8-03


sediments supply continuity in time, the volcanicrocks the tie to the geographical system and theindication of a deviation from the GAD.

5. A PSV reference site for the Italian region

Historical measurements have shown that geo-magnetic secular variation changes from onepoint to another along the Earth’s surface. Theangular change is very small over distances ofseveral hundreds of kilometers and a basic as-sumption underlying any archaeo- and paleo-magnetic study is that PSV is su⁄ciently coherentto be considered as the same within such a re-stricted region. The directions from various sitesof the region can thus be referred to a single PSVmaster curve. Construction and use of the mastercurve require taking into account the dependenceof the Earth’s ¢eld direction on geographical po-sition. Usually, a reference site is chosen and theD and I values are relocated to it. Noel and Batt[3] have proposed the conversion via pole method,brie£y described in the previous section, andtested it using the IGRF (International Geomag-netic Reference Field) as a model. The resultingangular error depends on the latitude and radiusof the ‘archaeomagnetic’ region. For example, themean angular error is of the order of 1.1‡ for aregion located astride 42‡N and with radius 800km like Italy. The IGRF smoothes the higherharmonics related to the lithospheric ¢eld, whicha¡ects the Earth’s ¢eld direction on a local scale.Evaluation of the litospheric e¡ects helps in theoptimum choice of a reference site and to reducethe relocating error, and may be done using to-day’s actually measured values, as the lithospheric¢eld may be assumed as constant on the PSV timescale.

The Italian Magnetic Network consists of morethan 100 repeat stations uniformly distributedover all Italy with a mean spacing of about 55km. Intensity (F), declination (D) and inclination(I) of the Earth’s ¢eld are regularly measured ateach station and then reduced to the same date,using continuous records at geomagnetic obser-vatories. The angular values are de¢ned to S 0.1P,well above the precision of PSV data. We used the

data of the 1999^2000 survey, reduced at year2000.0 [2], to test the conversion via pole methodand look for an optimum PSV reference site forItaly. Each of the 113 repeat stations was consid-ered as reference site and the D and I values of allthe other stations were relocated to it using theconversion via pole method. The relocated values(Dr, Ir) were then compared to the values actuallymeasured in the reference site (D, I) and the dif-ferences vD=D3Dr and vI= I3Ir considered asexperimental errors. Eight stations yielded poorresults when used as reference site ; the maximumvalues of vD and vI were as high as 2.4‡ and themean vD and vI values were higher than 0.9‡. Atleast ¢ve of them are a¡ected by high lithospheric¢eld: two are close to the huge masses of serpen-tinites in the Western Alps and three to a volcanicdistrict (Mt. Vulture and Mts. Iblei). These sta-tions were thus discarded and calculations re-peated for the remaining 105 stations. Thirty sta-tions, mainly located in central Italy, werecharacterized by very low vD and vI mean values(0.3‡^0.4‡) and maximum vD and vI values lowerthan 1.0‡. Among them, station no. 59 stood out:when used as a reference site, the errors weresymmetrically distributed (30.8‡6vD6 0.8‡,30.7‡6vI6 0.7‡) and both their mean absolutevalues were 0.3‡. The suitability of station no. 59as PSV reference site was further tested using theHistorical Italian Geomagnetic Data Catalogue[32]. The D and I values measured between 1820and 1850 in 25 di¡erent Italian localities wererelocated via pole to station no. 59. Their preci-sion may be reasonably assumed as S 1‡, thus ofthe same order of magnitude as PSV data derivedfrom measurements on rocks and archaeologicalartifacts. The time interval is like those used tostudy secular variation (SV) in archaeomagnetismand negligible with respect to PSV. The relocatedvalues vary by 3‡^4‡ (341.79Dr 9 345.8,59.29 Ir 9 62.8) and the mean direction at thereference site (Dr = 343.8, Ir = 60.7) is de¢nedwith a Fisher semi-angle of con¢dence K95 = 0.4‡.

In conclusion, the conversion via pole method[3] proved e¡ective to relocate PSV data from theItalian region. According to our calculations, thebest reference site is the station no. 59 (lat.42‡27PN, long. 12‡02PE) of the Italian Geomag-

EPSL 6725 5-8-03


netic Network [2], which we rename Viterbo fromthe nearest town easily traceable in ordinary geo-graphic maps. The relocation error is less than 1‡for both D and I, wherever the original PSV site islocated in Italy.

The Viterbo station was thus used to draw apreliminary PSV reference curve for Italy for thelast 30 kyr (Fig. 10), which shows:1. The D and I values from Vulcano relocated to

Viterbo (last two columns in Table 1).2. The Lago di Mezzano curves shifted according

to the GAD deviation found at Vulcano. Thesecurves do not need relocation, because the lakeis some 35 km to the NW of Viterbo and thedi¡erence between original and relocated val-ues is of the order of 0.1‡. The vertical refer-ence axes in Fig. 10 take into account theGAD deviation: they are located at D=DGAD+vD= 0‡+9.4‡ = 9.4‡, and I= IGAD+vI=61.5‡34.6‡ = 56.9‡. With respect to the originalcurves as in Fig. 8, the declination curve isshifted 9.4‡ eastwards, the inclination curve6.2‡ upwards. This last correction correspondsto the di¡erence between the reference value of56.9‡ and the mean experimental value of 50.7‡[1], and could correspond to the inclinationshallowing.Extension of the reference curve back to 135^

100 kyr needs, in our opinion, more data fromdated volcanic rocks, in order to provide a moredetailed time control of the Lago Grande di Mon-ticchio curves.

6. Conclusions

A PSV record has been derived from the vol-canic rocks of Vulcano. It suggests that a long-term, non-axial-dipolar component characterizedthe geomagnetic ¢eld in the southern Tyrrhenianregion during the last 135 kyr. The mean devia-tion from the GAD model is vD= 9.4‡,vI=34.6‡. Comparison with the sedimentary re-cords from Lago di Monticchio and Lago Grandedi Mezzano [1] shows that the punctual data fromVulcano match the trend of the curves from thetwo lakes. The inclination shallowing observed atMezzano and Monticchio can be partly ascribed

to the inclination anomaly vI found at Vulcano,and thus less than originally supposed. The decli-nation anomaly vD may be used to constrain tothe geographical reference system the declinationderived from cores without azimuthal orientation.Analysis of the repeat stations of the Italian Geo-magnetic Network [2] shows that the relocationvia pole method [3] works very well in the Italianregion and the station close to Viterbo gives thebest results.

Therefore, we propose the Viterbo station (lat.42‡27PN, long. 12‡02PE) as the reference site forthe Italian PSV curves and use of the relocationvia pole method to refer PSV data to it. TheGAD deviation found at Vulcano suggests thatthe inclined dipole might be more suitable thanthe axial dipole as a model for evaluation of themean direction of the Earth’s ¢eld in the southernTyrrhenian region during the last 105 kyr. It needsto be substantiated by more PSV data from theother volcanic rocks from the region. Lastly, theLago di Mezzano curves, integrated by the Vul-cano data and corrected for the GAD anomaly,may be used as preliminary, composite PSV refer-ence curve for Italy for the last 30 kyr BP. Thegood agreement between Mezzano and Vulcanobears witness to the robustness of the data andthe Mezzano values do not need to be relocatedto Viterbo because of the closeness of the twolocalities. Further steps toward a master curverequire more data from isotopically dated vol-canic rocks to con¢rm the three minima shownby the inclination curve between 15 and 23 ka,and detailed investigation of materials agedaround 2^5 ka, in order to connect the PSV curveto the SV data for the Italian region [33,34].

Acknowledgements

This work has been supported by the ItalianNational Group of Volcanology (GNV) and theUniversity of Torino. The authors are greatly in-debted to G. De Astis, whose deep knowledge ofVulcano was instrumental during the ¢eld work,to P. Dellino, L. La Volpe, A. Sbrana, P. Tucci-mei for discussions, suggestions and many pro¢t-able and pleasant days in the ¢eld, to the re-

EPSL 6725 5-8-03


viewers N. Abrahamsen and J.C. Tanguy for con-structive criticism and helpful suggestions.[VC]

References

[1] U. Brandt, N.R. Nowaczyk, A. Ramrath, A. Brauer, J.Mingram, S. Wulf, J.F.W. Negendank, Palaeomagnetismof Holocene and Late Pleistocene sediments from Lago diMezzano and Lago Grande di Monticchio (Italy): initialresults, Quat. Sci. Rev. 18 (1999) 961^976.

[2] A. Coticchia, A. De Santis, A. Di Ponzio, G. Dominici,A. Meloni, M. Pierozzi, M. Sperti, Italian magnetic net-work and geomagnetic ¢eld maps at year 2000.0, Boll.Geod. Sci. A¡. 40 (2001) 277^291.

[3] M. Noel, C.M. Batt, A method for correcting geograph-ically separated remanence directions for the purpose ofarchaeomagnetic dating, Geophys. J. Int. 102 (1990) 753^756.

[4] S.P. Lund, S.K. Banerjee, Paleosecular variations fromlake sediments, Rev. Geophys. Space Phys. 17 (1979)244^249.

[5] K.L. Verosub, Depositional and post-depositional pro-cesses in the magnetization of sediments, Rev. Geophys.Space Phys. 15 (1977) 129^145.

[6] R.F. Butler, Magnetic Domains to Geologic Terranes,Blackwell Scienti¢c, Boston, MA, 1992, 312 pp.

[7] N. Thouveny, Variations of the relative palaeointensity ofthe geomagnetic ¢eld in western Europe in the interval 25-10 ka as deduced from analyses of lake sediments, Geo-phys. J. R. Astron. Soc. 91 (1987) 123^142.

[8] H. Stockhausen, Geomagnetic palaeosecular variation (0-13000 yr BP) as recorded in sediments from three maarlakes from the western Eifel (Germany), Geophys. J. Int.135 (1998) 898^910.

[9] D.G. McMillan, C.G. Constable, R.L. Parker, Limita-tions on stratigraphic analyses due to incomplete age con-trol and their relevance to sedimentary paleomagnetism,Earth Planet. Sci. Lett. 201 (2002) 509^523.

[10] N. Thouveny, K.M. Creer, I. Blunk, Extension of the Lacdu Bouchet palaeomagnetic record over the last 120,000years, Earth Planet. Sci. Lett. 97 (1990) 140^161.

[11] K. Saemundsson, L. Kristjansson, I. McDougall, N.D.Watkins, K-Ar dating, geological and paleomagneticstudy of a 5-km lava succession in norhern Iceland,J. Geophys. Res. 85 (1980) 3628^3646.

[12] S.W. Bogue, R. Coe, Successive palaeomagnetic reversalrecords from Kauai, Nature 295 (1982) 399^401.

[13] J.W. Holt, J.L. Kirschvink, F. Garnier, Geomagnetic ¢eldinclinations for the past 400 kyr from the 1-km core of theHawaii scienti¢c drilling project, J. Geophys. Res. 101(1996) 11655^11663.

[14] G. De Astis, L. La Volpe, A. Peccerillo, L. Civetta, Vol-canological and petrological evolution of Vulcano island(Aeolian Arc, southern Tyrrhenian Sea), J. Geophys. Res.102 (1997) 8021^8050.

[15] M. Soligo, G. De Astis, M.C. Delitala, L. La Volpe, A.Taddeucci, P. Tuccimei, Uranium-series disequilibria fromVulcano Island (Sicily, Italy): isotopic chronology andmagmatological implications, Acta Vulcanol. 12 (2000)49^59.

[16] C. Laj, A. Rais, J. Surmont, P.Y. Gillot, H. Guillou, C.Kissel, E. Zanella, Changes of the geomagnetic ¢eld vec-tor obtained from lava sequences on the island of Vulcano(Aeolian Islands, Sicily), Phys. Earth Planet. Inter. 99(1997) 161^177.

[17] J. Keller, The island of Vulcano, Rend. Soc. It. Mineral.Petrogr. 36 (1980) 369^414.

[18] G. Frazzetta, P.Y. Gillot, L. La Volpe, The island ofVulcano, in: IAVCEI Scienti¢c Association ExcursionBook, Rome, 1985, pp. 125^140.

[19] A. Gioncada, A. Sbrana, ‘La Fossa Caldera’, Vulcano:inferences from deep drillings, Acta Volcanol. 1 (1991)115^126.

[20] E. Zanella, G. De Astis, P. Dellino, R. Lanza, L. LaVolpe, Magnetic fabric and remanent magnetization ofpyroclastic surge deposits from Vulcano (Aeolian Islands,Italy), J. Volcanol. Geotherm. Res. 93 (1999) 217^236.

[21] E. Zanella, G. De Astis, R. Lanza, Palaeomagnetism ofwelded, pyroclastic-fall scoriae at Vulcano, Aeolian Archi-pelago, J. Volcanol. Geotherm. Res. 107 (2001) 71^86.

[22] W. Lowrie, Identi¢cation of ferromagnetic minerals in arock by coercitivity and unblocking properties, Geophys.Res. Lett. 17 (1990) 159^162.

[23] R. Day, M. Fuller, V.A. Schmidt, Hysteresis properties oftitanomagnetites: grain-size and compositional depen-dence, Phys. Earth Planet. Inter. 13 (1977) 260^267.

[24] E. Zanella, R. Lanza, Remanent and induced magnetiza-tion in the volcanites of Lipari and Vulcano (AeolianIslands), Ann. Geo¢s. 37 (1994) 1149^1156.

[25] P.L. McFadden, M.W. McElhinny, The combined analy-sis of remagnetization circles and direct observations inpalaeomagnetism, Earth Planet. Sci. Lett. 87 (1988) 161^172.

[26] P.L. McFadden, F.J. Lowes, The discrimination of meandirections drawn from Fisher distributions, Geophys. J.R. Astron. Soc. 67 (1981) 19^33.

[27] C.L. Johnson, C.G. Constable, The time-averaged geo-magnetic ¢eld as recorded by lava £ows over the past 5Myr, Geophys. J. Int. 122 (1995) 489^519.

[28] R.R. Doell, Paleomagnetic secular variation study oflavas from the Massif Central, France, Earth Planet.Sci. Lett. 8 (1970) 352^362.

[29] P.L. McFadden, R.T. Merrill, M.W. McElhinny, Dipole/quadrupole family modeling of paleosecular variation,J. Geophys. Res. 93 (1988) 11583^11588.

[30] R. Lanza, E. Zanella, Palaeomagnetic directions (223-1.4ka) recorded in the volcanites of Lipari, Aeolian Islands,Geophys. J. Int. 107 (1991) 191^196.

[31] J.W. King, J.E.T. Channel, Sedimentary magnetism, en-vironmental magnetism and magnetostratigraphy, Rev.Geophys. Suppl. 29 (1991) 358^370.

[32] L. Cafarella, A. De Santis, A. Meloni, The Historical

EPSL 6725 5-8-03


Italian Geomagnetic Data Catalog, ING, Roma, 1992,160 pp.

[33] J.-C. Tanguy, M. Le Go¡, V. Chillemi, A. Paiotti, C.Principe, S. La Delfa, G. Patane', Variation se¤culaire dela direction du champ ge¤omagne¤tique enregistre¤e par leslaves de l’Etna et du Ve¤suve pendant les deux derniersmille¤naires, C.R. Acad. Sci. Paris 329 (1999) 557^564.

[34] A. Incoronato, A. Angelino, R. Romano, A. Ferrante, R.Sauna, G. Vanacore, C. Vecchione, Retrieving geomag-netic secular variations from lava £ows: evidence fromMounts Arso, Etna and Vesuvius (southern Italy), Geo-phys. J. Int. 149 (2002) 724^730.

EPSL 6725 5-8-03


paleomagnetic secular variation at vulcano (aeolian islands) during the last 135 kyr

Documents