Physics of the Earth and Planetary Interiors 145 (2004) 219–238

Paleointensity studies on Holocene–Pleistocene sedimentsfrom lake Escondido, Argentina

C.S.G. Gogorzaa,b,∗, J.M. Lirioc, H. Nuñezc, M. Chaparroa,b,H.R. Bertorellob,d, A.M. Sinitoa,b

a IFAS, Universidad Nacional del Centro, Pinto 399, 7000 Tandil, Argentinab CONICET, Rivadavia 1917, 1033 Buenos Aires, Argentina

c Instituto Antártico Argentino, Cerrito 1248, Buenos Aires, Argentinad Facultad de Matemática Astronom´ıa y Fısica, Universidad Nacional de Córdoba, H. de la Torre y M. Allende,

Ciudad Universitaria, 5000 Córdoba, Argentina

Received 1 September 2003; received in revised form 29 March 2004; accepted 30 March 2004

Abstract

Relative changes in geomagnetic field intensity over the last 16,000 years BP were recovered from the study of four coresobtained from Lake Escondido (South Argentina). Rock magnetic analysis shows that the main magnetic mineral is magnetite,the concentrations being mainly between 0.01 and 0.1%, and the grain size between 1 and 8�m. In order to fulfil the criteriafor assessing the reliability of paleointensity data derived from sediments [Rev. Geophys. 31 (1993) 319], the samples whoseconcentrations and size are beyond this range were rejected. The remanent magnetisation at 20 mT (NRM20 mT) was normalisedusing the anhysteric remanent magnetisation (ARM100 mT), the saturation of the isothermal remanent magnetisation (SIRM)and the low field magnetic susceptibility (k). Coherence function analysis indicates that the records are not significantlyaffected by local environmental conditions. This suggests that the variations in normalised remanence are mostly likely dueto geomagnetic paleointensity fluctuations. ARM100 mT is the best normaliser for the Lake Escondido sediments. The recordof relative paleointensity (NRM20 mT/ARM100 mT) shows peaks and troughs whose amplitudes are similar to those in the St.Lawrence Estuary, Lake Baikal, Lake Pepin and Larsen-A Ice shelf data set and a stack of archeomagnetic data given asvirtual axial dipole moment (VADM).© 2004 Elsevier B.V. All rights reserved.

Keywords:Relative paleointensity; Holocene; South America; Lake Escondido; Coherence

1. Introduction

One of the first motivations for investigating themagnetism of rocks was to study the behaviour ofEarth’s magnetic field in the past. The magnetic fieldis a vector field, having both direction and intensity,

∗ Corresponding author. Tel.:+54-2293444432;fax: +54-2293444433.E-mail address:cgogorza@exa.unicen.edu.ar (C.S.G. Gogorza).

and a complete understanding of it requires study ofthe full vector properties. However, paleointensity de-terminations are much more difficult than directionalones, and the majority of paleomagnetic studies areconcerned only with directional variability of the field.

Absolute values of paleomagnetic field intensity re-quire the paleomagnetic analysis of materials such aslavas and baked archaeological artefacts that containthermally acquired natural remanent magnetisation(NRM). However, such materials are notoriously poor

0031-9201/$ – see front matter © 2004 Elsevier B.V. All rights reserved.doi:10.1016/j.pepi.2004.03.010

220 C.S.G. Gogorza et al. / Physics of the Earth and Planetary Interiors 145 (2004) 219–238

in their spatial and temporal coverage, require labori-ous measurement, and are often difficult to be datedprecisely (Tauxe, 1993). On the other hand, sedimen-tary sequences offer the advantages of continuoushigh-resolution records of the NRM. Relative paleoin-tensity variations can be estimated by normalising theNRM in rapidly deposited sediment sequences. It isimportant to bear in mind that the paleomagnetic in-tensity is controlled not only by the Earth’s magneticfield but also by the magnetic mineral assemblagescontained in the samples. Moreover, the mechanismfor the acquisition of the remanent magnetisationplays another important role. Typically, SIRM, ARMand k are used as normalisers of NRM to correct ornormalise the variation in the density of remanencecarriers in the sediment. The interpretation of thegeomagnetic intensity record for sediments is usuallydifficult, because the records can be biased by envi-ronmental effects (Lund and Schwartz, 1999; Kruiveret al., 1999).

There are a series of papers that establish a set ofcriteria for reliability of sediments for relative pa-leointensity (e.g.Banerjee et al., 1981; King et al.,1982, 1983; Thouveny, 1987; Tric et al., 1992). Tauxe(1993) reviewed the experimental and theoreticalconsiderations for assessing the reliability of pale-ointensity data derived from sediments. The basic re-quirements which a sediment should fulfil for reliablepaleointensity determinations are that the NRM mustbe carried by stable magnetite, preferably in the grainsize range of about 1–15�m, the detrital remanencemust be an excellent recorder of the geomagnetic field,concentration variations must be lower than aboutan order of magnitude, and the normalisation shouldbe preferably done by several methods, all yieldingconsistent results. Besides these tests for magneticuniformity, several other criteria should be applied,including: (1) a stable, well defined, single compo-nent of magnetisation should characterise the NRMat the demagnetisation level used for paleointensitynormalisation; (2) the detrital remanent magnetisationmust be a reliable recorder of the geomagnetic fieldorientation, with no evidence of inclination error; (3)records obtained from multiple normalisation param-eters should agree; (4) coherence between paleointen-sity determinations and the magnetic parameters usedfor normalisation should be minimal and (5) dupli-cate or multiple records from a given region should

agree within the limits of the common time scales(Tauxe, 1993).

There are numerous studies that have recovered therelative paleointensity from marine cores (Guyodoand Valet, 1996; Cisowski and Hall, 1997; Robertset al., 1997; Channell, 1999; Channell et al., 2000;Laj et al., 2000; Stoner et al., 2002). However, fewstudies have been made on terrestrial sediments,probably due to their more complex sedimentologicalcharacteristics (Levi and Banerjee, 1976; Peck et al.,1996; Brachfeld and Banerjee, 2000; Nowaczyk et al.,2001; St-Onge et al., 2003). Comparatively, there arenot paleointensity results from South America, ex-cept for preliminary studies carried out on one corefrom El Trébol Lake (Sinito and Nuñez, 1997). Thispaper describes relative paleointensity results fromcores obtained from Lake Escondido. The directionalparameters corresponding to these cores have alreadybeen presented (Gogorza et al., 2002).

2. Site description

Lake Escondido is a small closed basin of about130,000 m2 and a depth up to 8.3 m, related to se-ries of large glacier-carved lakes. It is situated on theeast side of the Andean Patagónica Cordillera; in theLlao Llao area, 20 km NW of San Carlos de BarilocheCity (about 41◦S, 71◦30′W), at an altitude of 800 mabove sea level (Fig. 1). Strandlines surrounding lakesat higher levels indicate the former presence of largepaleolakes in many areas along the Andes and thePatagonian Steppe in South America. The network ofLate Pleistocene glacial paleolakes was extended overthe chain of recent Great Lakes (Lake Nahuel Huapi,Lake Gutiérrez, and Lake Mascardi) and smaller lakeslike Lake Escondido and Lake El Trébol (Tatur et al.,2002).

Volcanic rocks of Tertiary (Eocene) age assigned toVentana Formation outcrop in the Lake Escondido area(González Dıaz and Nullo, 1980). These rocks showevidence of glacial erosion by Andean glaciers dur-ing Late Pleistocene. The Ventana Formation is par-tially covered by Late Pleistocene–Holocene deposits,consisting principally of glacial related sediments andabundant volcanic ash layers (tephra). These layersthicken westwards, in the direction of the main ef-fusive centres.Feruglio (1941), Gonzalez Bonorino

C.S.G. Gogorza et al. / Physics of the Earth and Planetary Interiors 145 (2004) 219–238 221

Fig. 1. (a) Geographical location of lake Escondido. (b) Location of coring sites in the lake.

(1973, 1979)and González Dıaz and Nullo (1980),among others, have described the regional geologicalfeatures of the Andean Patagónica Cordillera and stud-ied the geology and stratigraphy of the southern zoneof Nahuel Huapi Lake area. A brief report of theirconclusions was summarised byGogorza et al. (1999,2002).

3. Field work, sampling, sedimentology andradiocarbon dating

A detailed description of these items can be found inGogorza et al. (2002). A brief summary follows. Fourcores (labelled les97-1, les97-2, les97-3 and les97-4,respectively) were collected in water depths of about

222 C.S.G. Gogorza et al. / Physics of the Earth and Planetary Interiors 145 (2004) 219–238

7 m using a push-corer installed on a raft with a centralhole. We successfully cored to basement (or erraticblocks), recovering four sediment columns of 6 cmdiameter and up to 12.3 m thick. The locations of thecore sites are shown inFig. 1. The recovered sedimentsequences consist of several separate and successivepush-core segments with a maximum length of 2 m,with a common internal orientation, but not orientatedrelative to magnetic north. There is no overlap betweenthe 2 m long segments, and no section gets lost. Thecompaction is minimal. The sediments were extrudedusing the included piston.

The cores were subsampled with plastic cubic boxesof 8 cm3. In total, 1347 subsamples were taken. Sub-sampling for rock magnetic studies and for14C andδ13C analyses was also carried out.

The four cores are sedimentologically similar, andrepresent the most complete sedimentary record fromLake Escondido to date. Three principal lithologiesare present in the Lake Escondido sediment column.From bottom to top: light reddish clayey silt (A), lightgrey clay (B) and dark brown organic-rich clay withhigh water content (C).

About 100 tephra layers are interbedded in litholo-gies A, B and C. They can be easily identified, allow-ing good lithological correlation between cores. Thefrequency and thickness of tephra increases upward.Up to 30 conspicuous tephra layers along the wholesedimentary column were recognised.

Table 1The δ13C, radiocarbon and14C years BP

Core Material Depth (cm) Shorteneddepth (cm)

Date14C yearsBP ± 2σ

δ13C (‰) Calibrated(age± 2σ, years)

References

es3 Clam 0 0 123.3± 0.7%a b 0 ± 150 Gogorza et al. (1999)es3 Sediment 74 48 1325± 110 b 1272± 110 Gogorza et al. (1999)les97-1 Cuticle 91 84 1870± 80 −27.8 1815± 175 Bianchi (2000)les2 Sediment 95 90.5 2300± 130 b 2319± 130 Gogorza et al. (1999)les97-1 Wood 218 168 4210± 90 −25.4 4743± 287 Bianchi (2000)les2 Leaf 265 199 5235± 130 b 5937± 130 Gogorza et al. (1999)les2 Sediment 330 235.5 7950± 150 b 8641± 150 Gogorza et al. (1999)les97-4 Sediment 574.5–582 280 9840± 80 −24.3 10995± 224 This workles97-1 Leaf 374 301 11150± 280 −28.5 13059± 600 Bianchi (2000)les97-4 Sediment 677–682.5 322 11620± 80 −23.5 13549± 400 This workles2 Sediment 391.5 355 12050± 190 b 14005± 380 Gogorza et al. (1999)les97-2 Sediment 878.5–883.5 492 14450± 100 −25.9 17313± 405 Gogorza et al. (2002)les97-4 Sediment 1058–1063 724 15680± 120 −27.1 18576± 413 Gogorza et al. (2002)

a 14C activity of 5-year-old clam (genusDiplodon) was 123± 0.7% of the modern atmospheric activity of14C.b Dates are based onδ13C standardised to−25‰ with respect to the PDB standard.

Two different facies can be recognised on LakeEscondido: a basal glacio-lacustrine facies (“LakeElpalafquen” facies), that includes lithologies A, Band tephras, and a younger organic-rich lacustrine fa-cies (lithologies C and tephras) or “Lake Escondido”facies. “Lake Elpalafquen” facies have been recog-nised within cores from other lakes of the area and onoutcrops (del Valle et al., 2000), and represent sedi-mentation during Late Pleistocene times when a largepro-glacial lake named Elpalafquen (del Valle et al.,2000) existed in the region. The “Lake Escondido”facies represent lacustrine sedimentation under nodirect influence of glaciers. These sediments were de-posited mainly during the Holocene, after the collapseof Lake Elpalafquen, which gave rise to the formationof the present lakes of the region. The palynologicalstudies ofJackson (1996)andBianchi (2000)show aclear climatic improvement (warming trend) recordedby the “Lake Escondido” facies.

The age scale for this study was provided byAMS radiocarbon dates described in previous works(Gogorza et al., 1999, 2002; Bianchi, 2000). AMS14C ages are listed inTable 1. During the field work,made in order to obtain the cores studied byGogorzaet al. (1999), a small clam was found living at thesediment–water interface. This clam was identifiedto the genusDiplodon (Castellanos, 1959, 1960),and was determined to be 5-year-old based on an-nual growth lines. The clam was dated to assess the

C.S.G. Gogorza et al. / Physics of the Earth and Planetary Interiors 145 (2004) 219–238 223

potential for ancient dead carbon within the basin. Ifthe age of the clam (which was living and samplingthe carbon at the sediment water interface) came backolder than modern, it could be inferred that the or-ganism was sampling dead carbon. The final resultsindicate the basin is not receiving dead carbon; the14C activity of the clam measured 123.3 ± 0.7% ofthe 1950 pre-bomb14C activity, consistent with themodern atmospheric14C concentration for the region(Jackson, 1996).

4. Magnetic studies

In order to characterise these lake sediments andto assess the reliability of paleointensity data derivedfrom them and to carry out the normalisation, a set oflaboratory experiments performed in a former work(Gogorza et al., 2002) were analysed.

The following measurements were performed:NRM; magnetic susceptibility at low frequency (spe-cific, X and volumetric, k); isothermal remanentmagnetisation (IRM) in increasing steps up to 1.2 T,reaching the SIRM; back-field, in growing steps untilcancelling the magnetic remanence; anhysteric rema-nent magnetisation (ARM100 mT), with a direct fieldof 0.1 mT and a peak alternating field of 100 mT.Associated parameters calculated byGogorza et al.(2002) were also used: (IRM−300 mT/SIRM), rema-nent coercitive field (HCR), SIRM/k; ARM100 mT/kand SIRM/ARM100 mT.

Additional studies were carried out in this work:magnetic demagnetisation from NRM to determinemedian destructive field (MDFNRM), partial anhys-teric remanence (pARM), hysteresis curves and tem-perature dependence of SIRM. NRM,X, k, SIRM andARM100 mT were obtained for all samples; the rest ofthe magnetic parameters were measured for a groupof selected pilot samples from different lithologies.

The studies of pARM were carried out using a peakAF of 100 mT (the rate of decrease was 17�T percycle), and applying a DC field of 50�T over specificAF windows. A width of 5 mT for the window (itwas moved into a range between 5 and 100 mT) waschosen.

The hysteresis parameters were obtained using aVSM Lake Shore 7300 with a maximum applied fieldof 1.5 T.

Thermal demagnetisation was made by the Ther-mal Specimen Demagnetiser, model TD-48 ASCScientific. Stepwise thermal demagnetisation curveswere represented and critical temperatures (TC) wereestimated.

5. Reliability of sediments as paleointensityrecorders

New analyses of the data were carried out to com-plete the study of reliability of paleointensity results.These results and those obtained byGogorza et al.(2002)are briefly summarised to demonstrate that thesediments meet the established criteria for relative pa-leointensity determinations.

Since the profiles of the different parameters arepresented as function of “shortened depth” a brief de-scription of the method used to obtain this depth isgiven below.

As explained in former papers (Gogorza et al.,1999, 2001, 2002), k logs can be used to define tielines, which describe the lithostratigraphic correla-tion of cores of the same lake and are, in general,consistent with the lithology. The depth scales of allthe cores were adjusted to the depth scale of a chosenmaster core (les97-2) using lithology andX tie linesfor correlation. One of the most important problemsin lake sediments of volcanic areas, is the presence ofabundant tephra layers along the sequence. On the onehand, the tephra layers represent rapid instantaneousdeposition of thick layers, whereas the rest of thesediments represent slow accumulation. On the otherhand, tephra is not a very good magnetic recorderof directions (Peng and King, 1992; Gogorza et al.,1999). For these reasons, after the identification of thetephra layers, they were removed from the sequenceand the gaps that were produced along the profilesby their removal were closed, obtaining a “shorteneddepth” scale. This method was described in detail byGogorza et al. (1999).

5.1. Mineral-magnetic characteristics

Reliable paleointensity estimates require uniformityof concentration of magnetic grains and magnetic min-eralogy (Tauxe, 1993). Stepwise acquisition of theisothermal remanence in fields up to 1.2 T shows that

224 C.S.G. Gogorza et al. / Physics of the Earth and Planetary Interiors 145 (2004) 219–238

Fig. 2. Thermal demagnetisation curves of SIRM for two samples from the upper part of the sequence (a) and two from the lowerone (b).

between 82 and 95% of the SIRM is acquired at anapplied field of≤300 mT. Progressive removal of thisSIRM by back-field demagnetisation indicates thatHCR varies between 61 and 86 mT.Fig. 2shows ther-mal demagnetisation curves of SIRM for four sam-ples, two from the upper part of the sequence (“LakeEscondido” facies) and two from the lower part (“LakeElpalafquen” facies). Although Curie temperature isabout 580◦C for all samples, the curves ofFig. 2ashow a minor inflection, which suggests the presenceof more than one magnetic mineral. These results in-dicate that magnetite is the dominant magnetic carrierof remanence in these cores, but the presence of a very

low proportion of titanomagnetite in younger sectioncannot be excluded.

The variation in the concentration of magnetic min-erals can typically be monitored by the measurementsof k and SIRM. The latter is usually regarded as a bet-ter parameter to use for this purpose because it hasno systematic grain size dependence, while the for-mer can be affected by super-paramagnetic (SP) grains(Pan et al., 2001). FromFig. 3, which showsk versusSIRM for all samples, we can estimate that the con-centration varies between 0.01 and 0.1% (Thompsonand Oldfield, 1986). In order to meet the criteria foruniformity in terms of concentration (Tauxe, 1993),

C.S.G. Gogorza et al. / Physics of the Earth and Planetary Interiors 145 (2004) 219–238 225

Fig. 3. k vs. SIRM for all samples in order to estimate concentration and grain size according toThompson and Oldfield (1986).

those samples whose concentrations are beyond thisrange were rejected.

5.2. Magnetic granulometry

Down-core homogeneity in grain size is an im-portant factor for reliable estimation of the relativechanges of paleointensity of the geomagnetic field(King et al., 1983; Tauxe, 1993; Lehman et al., 1996).Here we used several different approaches to esti-mate grain size. We determined the MDFNRM, theARM100 mT/k ratio, the ARM100 mT/SIRM ratio, andkversus SIRM, pARM, and hysteresis parameters.

MDFNRM values are between 26 and 35 mT, withinthe range of those generally found for other Late

Pleistocene–Holocene lake sediments from the area(Gogorza et al., 2002). These values are consistentwith the predominance of magnetite as magneticcarrier in the samples.

From Fig. 3, which showsk versus SIRM forall samples (Thompson and Oldfield, 1986) wecan estimate that the grain size varies between1 and 8�m. In order to meet the criteria forreliability of paleointensity data (Tauxe, 1993),those samples which are beyond this range wererejected.

Fig. 4a shows normalised pARM versus appliedfield (H). The position of the peak pARM acquiredindicates an estimated grain size of about 1 and 4�m(Jackson et al., 1988).

226 C.S.G. Gogorza et al. / Physics of the Earth and Planetary Interiors 145 (2004) 219–238

0 20 40 60 80 1000.0

0.2

0.4

0.6

0.8

1.0

pAR

M/p

AR

Mm

ax

H (mT)1 2 3 4 5

0.0

0.2

0.4

0.6

Lake Escondido Lake Elpalafquen

(a) (b)

MS

R/M

S

HCR

/HC

Fig. 4. (a) Normalised pARM vs.H for pilot samples and (b) hysteresis parameter ratios,HCR/HC vs. MRS/MS for estimation of grain size.

The hysteresis parameter ratios,HCR/HC andMRS/MS vary between 2.5 and 3.7 (except one sam-ple, 1.6) and 0.17–0.3 (except one sample, 0.4),respectively. These results indicate that the mag-netite grains are pseudo-single-domain (PSD) size(Fig. 4b).

Changes in the ratios of ARM100 mT/k andARM100 mT/SIRM imply changes in grain size, higherratios indicating smaller grain size and a higherproportion of single-domain (SD) grains providedthat chemical composition of the grains is constant(Hunt et al., 1995). Fig. 5 shows that the variationof ARM100 mT/k and ARM100 mT/SIRM with the“shortened depth” of these sediments is lower than 3,which indicates a rather uniform grain size along thesequence.

After these analyses we can confirm that themineral-magnetic characteristics described above in-dicate that the sediments of the Lake Escondido meet

stringent criteria for relative paleointensity determi-nation.

5.3. Paleomagnetic stability

Stability of the magnetisation was analysed by al-ternating field (AF) demagnetisation (Gogorza et al.,2002). For core les-97, every fourth sample waschosen as a pilot sample. Pilot samples were demag-netised successively at 2.5, 5, 10, 15, 20, 25, 30, 40,50 and 60 mT peak field. Most of the samples showno systematic change in the direction of their rema-nent magnetisation, which could easily be removedat AF demagnetisation at 10 or 15 mT, therefore, theymostly have a single component. We have chosen thevalue of the NRM after the 20 mT step as represen-tative of the stable characteristic remanent magneti-sation (NRM20 mT) and we have used this value forconstructing the normalised intensity records.

C.S.G. Gogorza et al. / Physics of the Earth and Planetary Interiors 145 (2004) 219–238 227

0 100 200 300 400 500 600 700 800 9000

4 les97-4

0

4

AR

M10

0mT/S

IRM

(10

-2)

AR

M10

0mT/k

(10

2 A/m

)

Shortened Depth (cm)

les97-3

0

4 les97-2

0

4 les97-1

0

4

8 les97-4

0

4

8 les97-3

0

4

8

Lake Escondido Lake Elpalafquen

les97-2

0

4

8 les97-1

Fig. 5. ARM100 mT/SIRM and ARM100 mT/k records vs. shortened depth from cores les97-1–4.

228C

.S.G

.G

ogo

rzae

ta

l./Ph

ysicso

fth

eE

arth

an

dP

lan

eta

ryIn

terio

rs1

45

(20

04

)2

19

–2

38

1

10

100

1000

NR

M20

mT (

mA

/m)

1

10

100

1000

1

10

100

1000

1

10

100

1000 les97-4

10

100

1000

10000

AR

M10

0mT (

mA

/m)

10

100

1000

10000

10

100

1000

10000

10

100

1000

10000

1000

10000

100000

1000000

SIR

M (

mA

/m)

1000

10000

100000

1000000

1000

10000

100000

1000000

1000

10000

100000

1000000

0 200 400 600 80010

100

1000

k (1

0-5 S

I)

Shortened Depth (cm)

0 200 400 600 80010

100

1000

Shortened Depth (cm)0 200 400 600 800

10

100

1000

Shortened Depth (cm)0 200 400 600 800

10

100

1000

les97-3les97-2les97-1

Shortened Depth (cm)

Fig. 6. k, SIRM, ARM100 mT/SIRM and NRM20 mT records vs. shortened depth from cores les97-1–4.

C.S.G. Gogorza et al. / Physics of the Earth and Planetary Interiors 145 (2004) 219–238 229

6. Normalisation methods

Fig. 6 shows NRM20 mT, ARM100 mT, SIRM andkversus shortened depth for cores les97-1 to les97-4.We observed that there is good agreement betweenARM100 mT, SIRM andk, it means that NRM intensityis mainly managed by changes in the geomagneticfield rather than by environmental factors. ARM100 mT,SIRM andk vary by a factor of about 10, within therange considered to be appropriate for paleointensitystudies (Tauxe, 1993).

For each core, we have obtained three estimatesof normalised field intensity using ARM100 mT, k andSIRM as normalising parameters. For each core, thethree normalised records are similar and support thehypothesis that they represent a geomagnetic signalrather than an artefact of sedimentary magnetisationprocesses. However, a different behaviour is clear inthe upper and lower sections. In the upper section,belonging to the “Lake Escondido” facies, the meanvalues are lower than in the “Lake Elpalafquen”facies. In order to compare the records they areboth normalised by their respective mean values(Fig. 7).

The results of the four cores were then integratedinto a single curve by a stacking process, consist-ing of an arithmetical average. In order to carry outthis average, it is necessary to have data at the samedepth for each core, for this reason a interpolationwas performed, getting data every 20 cm (Fig. 8). Theerror interval, consisting of the standard deviation,is also shown. The three methods of normalisationgive similar results, which indicates that environmen-tal effects are small, as expected from the unifor-mity of the magnetic properties in the cores derivedfrom the stringent constraint that we asked to thesediments.

It is worthwhile to discuss the different amplitude ofthe peak observed at about 100 cm depth (Fig. 8). Thispeak is lower for the ratio NRM20 mT/ARM100 mT, i.e.the values of ARM100 mT are higher than the values ofSIRM andk. Since ARM is better recorded by finergrains andFig. 5shows at the same depth a change thatreflects the presence of finer grain in this zone, theseresults are consistent, and suggest that ARM100 mT isthe more appropriate normaliser parameter.

In order to analyse the different parameters as func-tion of time and to compare our results with previously

published paleomagnetic records, mostly derived fromuncalibrated14C dates, a transfer function from Radio-carbon Years Before Present (14C years BP) to short-ened depth was defined using the data ofTable 1. Rel-ative paleointensity profiles as a function of14C yearsBP are shown inFig. 9.

7. Power spectral and coherence function analysis

To confirm that the normalised records (NRM20 mT/ARM100 mT, NRM20 mT/k and NRM20 mT/SIRM) arefree of environmental influences, spectral analyses ofnormalised remanences, normalisation parameters andcoherence test were carried out following the methodof Tauxe and Wu (1990). Their argument is that ifthe normaliser and the intensity record exhibit sig-nificant coherence, then the remanence may not havebeen adequately corrected for environmental effects.This test has been advocated as a means of assess-ing relative paleointensity data quality (Tauxe, 1993).Spectral analysis and calculation of coherence spectrawere performed using MATLAB 6.1 software.

7.1. Lake Escondido facies

It is shown inFig. 10 that at the 95% confidencelevel, NRM20 mT/ARM100 mT is generally incoherentwith its normaliser. On the other hand, NRM20 mT/kand NRM20 mT/SIRM are not coherent with theirnormaliser for most frequencies except at periods ofabout 140 and 207–211 years. There is a widespreadgeographic occurrence of 200-year cycles in a vari-ety of terrestrial records such14C anomalies in treerings (Stuiver and Braziunas, 1993) and 10Be con-centrations in ice cores (Raisbeck et al., 1990). The200-year cycles are well-documented in Minnesotaduring the Holocene in varves thickness records fromseveral lakes (Anderson, 1992; Slawinksi, 1998), inthe Antarctic Peninsula in multiproxy records from asediment core retrieved from a deep basin (Leventeret al., 1996), in the Maya Lowlands in drought fre-quencies (Hodell et al., 2001) and in Africa throughpaleolimnological proxies (Verschuren et al., 2000).The spectral features at 200 years in the normalisedintensity record is likely a climatic signal that hasnot been satisfactorily removed by the normalisationprocess.

230 C.S.G. Gogorza et al. / Physics of the Earth and Planetary Interiors 145 (2004) 219–238

0 100 200 300 400 500 600 700 800 9000

2 les97-40

2

NR

M20

mT/k

nor

mal

ised

(un

itles

s)N

RM

20m

T/S

IRM

nor

mal

ised

(un

itles

s)N

RM

20m

T/A

RM

100m

Tnorm

alis

ed (

unitl

ess)

Shortened Depth (cm)

les97-30

2 les97-20

2 les97-10

2 les97-40

2 les97-30

2 les97-20

2 les97-10

2 les97-40

2 les97-30

2 les97-20

2

Lake Escondido Lake Elpalafquen

les97-1

Fig. 7. Normalised NRM20 mT/k, NRM20 mT/SIRM and NRM20 mT/ARM100 mT records vs. shortened depth from cores les97-1–4.

C.S.G. Gogorza et al. / Physics of the Earth and Planetary Interiors 145 (2004) 219–238 231

0 50 100 150 200 250 300 350 400 450 500 550 600 650 700 750 800 850 9000

1

2

3Nor

mal

ised

Pal

eoin

tens

ities

(un

itles

s)

NRM20mT

/SIRM

NRM20mT

/k

NRM20mT

/ARM100mT

Shortened Depth (cm)

0

1

2

3

0

1

2

3

Fig. 8. Stacked NRM20 mT/ARM100 mT, NRM20 mT/SIRM and NRM20 mT/k records vs. shortened depth.

7.2. Lake Elpalafquen facies

The analysis suggests that the normalised curvesare not coherent with the parameters of normalisation(Fig. 10). This analysis indicates that the parameterARM100 mTis the more appropriate normaliser in thesesediments (not coherent frequencies above the 95%confidence level) and that the NRM20 mT/ARM100 mTrecord is not affected by climatic or lithologic factorsbut represents a true geomagnetic signal. This result isconsistent with the results suggested by the grain sizeanalysis (item 6).

8. Comparison with relative paleointensity records

We have compared our normalised intensity record(NRM20 mT/ARM100 mT) with relative paleointensityrecords from Southern and Northern Hemisphere(Fig. 11a–c).

Fig. 11a shows our record and the results froma sedimentary sequence collected from beneaththe former Larsen-A Ice shelf, Antarctic Peninsula(Brachfeld et al., 2003). Relative paleointensitiesare represented versus radiocarbon ages. Our recordshows some similarities to Larsen record, displayinga high at about 3000 RCYBP, preceded by a relativelow from 5500 to 7500 RCYBP. Although this low issmoother in Larsen results, the trend of both recordsis comparable.Fig. 11bshows our record and the re-sults from South Atlantic geomagnetic paleointensitystack, SAPIS (Stoner et al., 2002) and from three cores(SEDANO) collected from Antarctic late pleistocenesediments (Sagnotti et al., 2001). In these cases rela-tive paleointensities are represented versus calibratedages. The lack of correlation between our record andthe results from SAPIS may be attributed to the per-turbations introduced by the presence of ultra-finemagnetite in the upper part of some of the cores usedfor the SAPIS stack. This reason is mentioned by

232 C.S.G. Gogorza et al. / Physics of the Earth and Planetary Interiors 145 (2004) 219–238

0 2000 4000 6000 8000 10000 12000 14000 16000 180000

1

2

3

NRM20mT /SIRM

NRM20mT

/k

NRM20mT

/ARM100mT

14C Years BP

0

1

2

3

0

1

2

3

Nor

mal

ised

Pal

eoin

tens

ities

(un

itles

s)

Fig. 9. Stacked NRM20 mT/ARM100 mT, NRM20 mT/SIRM and NRM20 mT/k records vs.14C years BP.

Stoner et al. (2002)to explain the differences existingin the 0–25 ka interval between SAPIS and NAPISstack. The comparison from our record and thosefrom Sagnotti et al. (2001)shows some similaritieswith SED-06 and SED-04 results, a low and a high inthe lower part of the sequences are observed in bothrecords, although a lag between them is observed.

Fig. 11c shows the comparison of the post-glacialsection [0–10,000 RCYBP] of our normalised inten-sity record (NRM20 mT/ARM100 mT) with the relativepaleointensity record from Lake Pepin (Brachfeld andBanerjee, 2000), St. Lawrence Estuary (St-Onge et al.,2003), Lake Baikal (Peck et al., 1996) and a com-pilation of archeomagnetic data (Yang et al., 2000).The five records are similar in shape from∼1700to 9600 years. BP with values above and below themean, which is consistent with global absolute pale-ointensity estimates derived from archaeological ma-terial (Yang et al., 2000). Assuming that not all the

record age models are necessarily correct, it is interest-ing to note that in some intervals the Lake Escondidorecord is in better agreement with the record from St.Lawrence Estuary. At other intervals the Lake Escon-dido and Lake Pepin record shows the closest agree-ment (e.g. the long trend between∼3400 and 7350years BP). Surprising agreement is observed betweenthe record of Lake Escondido and the record of LakeBaikal, taking into account that both lakes are thou-sands of kilometres away. A peak around 2300 years.BP and an oscillating behaviour superimposing on along trend below the mean between∼3400 and 8000years. BP is observed in both records. Values abovethe mean between∼1700 and 3400 years and valuesbelow the mean between∼3400 and 7350 years. BPare observed in both Lake Escondido and Lake Pepinrecords. Such behaviour is also observed in Holoceneabsolute paleointensity estimates (Yang et al., 2000),supporting Lake Escondido and Lake Pepin records in

C.S

.G.

Gog

orza

et

al./P

hysics

of

the

Ea

rtha

nd

Pla

ne

tary

Inte

riors

14

5(2

00

4)

21

9–

23

8233

0.000 0.002 0.004 0.006 0.008 0.0101E-6

1E-5

1E-4

1E-3

0.01

0.1

1 ARM

100mT Power

NRM20mT

/ARM100mT

Power

Pow

er

Frequency (yr-1)

0.000 0.002 0.004 0.006 0.008 0.0101E-6

1E-5

1E-4

1E-3

0.01

0.1

1

95%

ARM100mT

Power NRM

20mT/ARM

100mT Power

Pow

er

Frequency (yr -1)

0.000 0.002 0.004 0.006 0.008 0.0101E-6

1E-5

1E-4

1E-3

0.01

0.1

1

95%

211 yrs

145 yrs SIRM Power NRM

20/SIRM Power

Pow

er

Frequency (yr-1)

0.000 0.002 0.004 0.006 0.008 0.0101E-6

1E-5

1E-4

1E-3

0.01

0.1

1

95%

Squared C

oherence

Squared C

oherence

SIRM Power NRM

20mT/SIRM Power

Squared C

oherenceS

quared Coherence

Squared C

oherenceS

quared Coherence

Pow

er

Frequency (yr -1)

0.000 0.002 0.004 0.006 0.008 0.0101E-6

1E-5

1E-4

1E-3

0.01

0.1

1

95%

139 yrs

211 yrs

k Power NRM20mT/k Power

Pow

er

Frequency (yr -1)

0.000 0.002 0.004 0.006 0.008 0.0101E-6

1E-5

1E-4

1E-3

0.01

0.1

1

95%

k Power NRM

20mT/k Power

Pow

er

Frequency (yr -1)

0.0

0.2

0.4

0.6

0.8

1.0

95%

207 yrs 145 yrs

Squared Coherence

0.0

0.2

0.4

0.6

0.8

1.0Lake ElpalafquenLake Escondido

Squared Coherence

0.0

0.2

0.4

0.6

0.8

1.0

Squared Coherence

0.0

0.2

0.4

0.6

0.8

1.0

Squared Coherence

0.0

0.2

0.4

0.6

0.8

1.0

Squared Coherence

0.0

0.2

0.4

0.6

0.8

1.0

Squared Coherence

Fig. 10. Spectral analysis of three normalisation parameters (ARM100 mT, SIRM and k) and three normalised remanences (NRM20 mT/ARM100 mT, NRM20 mT/SIRM andNRM20 mT/k). Coherence tests results are shown separately for samples from facies Escondido and Elpalafquen. The 95% confidence level is denoted by the horizontal line.

234 C.S.G. Gogorza et al. / Physics of the Earth and Planetary Interiors 145 (2004) 219–238

that time interval. The relative paleointensity peak at∼2900 years BP in Lake Escondido is not observed inLake Pepin and St. Lawrence Estuary records but it ispresent in Lake Baikal and the absolute paleointensityestimates (Fig. 11c), supporting this feature at thattime. In brief, clear common features are found inall the sedimentary records (Fig. 11c), suggesting apossible common geomagnetic origin.

We compared our pre-glacial record with the recordof Lake Baikal (Fig. 11c) and the results were mixed.The comparison with records of the same age obtainedfrom marine cores is more difficult. Marine recordsare usually characterised by lower sedimentation ratesthan lacustrine records. Consequently, short-lived in-

0 2000 4000 6000 8000 10000 12000 14000 16000 18000 200000

1

2

Southern Hemisphere

Rel

ativ

e In

tens

ity

(

Uni

tless

)

SA

PIS

NR

M20m

T /AR

M100m

T

Escondido Lake

NR

M20

mT/A

RM

100m

T

Esc

ondi

do L

ake

NR

M20m

T/k

SE

D-06

NR

M20m

T/k

SE

D-02

NR

M20

mT

/k

SE

D-0

4

Calibrated Ages (years)

0

1

20

1

2 0

1

2

1

2 0

1

2

(b)

(a)

0

1

2

0 2000 4000 6000 8000 10000 12000 14000 16000 18000 20000

14 C Years BP

Rel

ativ

e In

tens

ity(U

nitle

ss)

Lars

en-A

Fig. 11. Comparison of normalised intensity record from the Lake Escondido–Elpalafquen stack with relative paleointensity records fromLarsen-A Ice Shelf (a), SAPIS stack and SEDANO cores (b) Lake Pepin, St. Lawrence Estuary, Lake Baikal and a global compilation ofarcheomagnetic data (c).

tensity fluctuations may have been recorded with dif-ferent resolution, precluding their unambiguous recog-nition (Lehman et al., 1996). There are additionalproblems because different methods of dating are usedto analyse marine records. In brief, it would be nec-essary further comparative analyses to draw a reason-able conclusion about the pre-glacial paleointensitiesobtained from Lake Escondido.

9. Discussion

Inter-parametric ratios reflecting magnetite grainsize variations are shown inFig. 5. ARM100 mT/SIRM

C.S.G. Gogorza et al. / Physics of the Earth and Planetary Interiors 145 (2004) 219–238 235

0 2000 4000 6000 8000 10000 12000 14000 16000 180000.6

1.2

VA

DM

(1022 A

m2)

Lake Baikal

(

NR

M/IR

M) 20

mT

St L

awre

nce

Est

uary

NR

M/A

RM

Lake Pepin

VA

DM

(10

22 A

m2 )

NR

M20

mT/A

RM

100m

T

Lak

e E

scon

dido

14C Years BP

0.0

0.3

0.6

0

1

2

0

8

16

6

12

(c)

Northern Hemisphere

Fig. 11. (Continued).

is purely a ratio of two remanence dependent parame-ters; thus, it reflects the ferrimagnetic grain size betterthan ratios that include susceptibility. The behaviourof ARM100 mT/SIRM and ARM100 mT/k suggests thanthe magnetic grain size was coarser in sediments fromElpalafquen facies than sediments from Escondidofacies. Furthermore, it is possible to observe inFig. 3that the mean value of grain size of the samples ofEscondido facies are lower than the mean value ofgrain size of the samples of Elpalafquen facies.

The down-core profiles patterns of the differentmagnetic parameters (ARM100 mT and SIRM intensi-ties andk) look very similar (Fig. 6). A high dispersionis present in the upper section of the sequence (LakeEscondido facies), while the lower part (Elpalafquenfacies) is more uniform, although the mean values aresimilar for both facies (except les97-1 where slightdifferences between lower and upper section are ob-

served) (Fig. 6) (Gogorza et al., 2002). On the otherhand, NRM20 mT values from the upper part is anorder of magnitude lower that the values of the lowerpart. This is probably a consequence of a combinationof factors like differences in concentration of mag-netite (observed inFig. 3) and less efficient recordingdue to grain size and/or lithology effects (high watercontent, clay content, etc.).

Fig. 7shows good agreement between cores for thesame ratio as well as between different ratios for agiven core. The differences in the behaviour of the nor-malised NRM20 mT in the four cores, may be explainedas a consequence of slight differences in the amount ofmagnetite or in the mean of the grain size in the fourcores (Lehman et al., 1996). Although the upper sec-tion shows more scattered results, a trend to a slightincrease of the ratios between 50 and 250 cm depthand a decrease between 250 and 400 cm is observed.

236 C.S.G. Gogorza et al. / Physics of the Earth and Planetary Interiors 145 (2004) 219–238

In the lower part a significant peak between 425 and475 cm and uniform values around 1 from 475 cm tothe bottom are observed. The section between 75 and180 cm shows lower values in NRM20 mT/ARM100 mTthan NRM20 mT/SIRM and NRM20 mT/k, which indi-cate that the magnetic mineral assemblage are domi-nated by finer grains. These grains are more efficientcarriers of detrital remanent magnetisation and ARMthan of SIRM andk (Brachfeld and Banerjee, 2000).The behaviour of the records of ARM100 mT/SIRM andARM100 mT/k (Fig. 5) supports this statement.

The stacked ratios (Fig. 8) show more sim-ilar behaviour than the individual results, butsmoother. NRM20 mT/k and NRM20 mT/SIRM showmore scattered results in the upper section thanNRM20 mT/ARM100 mT. It is necessary to keep in mindthat the upper 300 cm spans 7500 radiocarbon yearswhile the lower 400 cm spans 4000 radiocarbon years.For this reason, the behaviour of the upper sectionmay be explained by larger environmental changes.

The most significant feature is the peak (a two-foldincrease in normalised intensity) at 475 cm “shorteneddepth”. There are not distinctive changes in the con-centration or grain size that account for this behaviour,which may genuinely reflect changes in the geomag-netic signal. However, this feature is not observedwhen the records are compared with the records ofLake Baikal, and further studies would be necessaryto draw a conclusion about it.

Although there are different opinions about thenormalisation process, several authors have indicatedthat ARM is the best normalisation factor (Johnsonet al., 1975; Levi and Banerjee, 1976; King et al.,1983). Levi and Banerjee (1976)pointed out thatsusceptibility, similar to saturation magnetisation, ismeasured in the presence of a field and hence is hardto relate to remanence, which is measured in zerofield. Johnson et al. (1975)advocated the use of an-hysteric remanence because it is particularly sensitiveto single-domain particles. Part of the reason for thepreference of anhysteric remanence is the argumentthat the detrital magnetic grains in a sediment presum-ably acquired their individual moments as a thermalremanence prior to deposition (Tauxe, 1993). Anotherargument is that the magnitude of ARM is a few tensto hundreds of times that of NRM, whereas SIRM istypically thousands of times larger (Tauxe, 1993). Inour paper, the main arguments that favour the choice

of ARM100 mT are the results of the coherence test andthe analysis of grain size. At the 95% confidence levelthe normalised intensity is not coherent with the nor-maliser in Escondido and Elpalafquen records. Sig-nificant coherence above the 95% confidence limit isobserved for bothk and SIRM normalisation in LakeEscondido records. Besides, ARM is better recordedby fine grains and the more significant differencesamong the three ratios are just observed in the upperpart, where the grains are finer. On the other hand,when the records of the three normalised paleointen-sities are compared with records of other parts in theworld, NRM20 mT/ARM100 mT shows the best agree-ment. Taking into account our results and the opinionof numerous authors, who advocated the use of ARM,we set NRM20 mT/ARM100 mT as our paleointensityrecords.

The relative paleointensity records that were anal-ysed are located in distinct sedimentary environmentsrepresenting four different places situated thousandsof kilometres away from Lake Escondido. The goodagreement is suggestive of a dominant global (i.e.dipolar) character of these records. They also re-flect the general trend of the Earth’s dipole moment(Fig. 11). The discrepancy between the pre-glacialrecords of Lake Escondido could be explained as alocal characteristic due to a non-dipolar source butmore data would be necessary to draw a conclusionabout this behaviour.

10. Conclusions

We have shown that the sediments from Lake Es-condido meet the criteria required for the constructionof a reliable paleomagnetic record. The NRM20 mTis carried by magnetite of grain size 1–8�m (PSD)and the mineral concentration varies between 0.01 and0.1%. ARM100 mT is the best normaliser for the LakeEscondido sediments. NRM20 mT/ARM100 mT showsno coherence with its normaliser. Our normalised in-tensity record (NRM20 mT/ARM100 mT) shows goodagreement with Lake Pepin, St. Lawrence Estuary,Lake Baikal, Larsen-A Ice shelf records and an abso-lute paleointensity global record. There is a relativeagreement of our records with two cores (SEDANO)collected from Antarctic late pleistocene sediments,and no agreement with SAPIS record.

C.S.G. Gogorza et al. / Physics of the Earth and Planetary Interiors 145 (2004) 219–238 237

Acknowledgements

The authors wish to thank Universidad Nacionaldel Centro de la Provincia de Buenos Aires, InstitutoAntártico Argentino, CONICET and Universidad deBuenos Aires and Martın Santiago and Nestor Villa-corta for their help on the field. They wish to thank sin-cerely S. Brachfeld, J. Peck, J. Stoner and G. St-Ongefor generously sharing their data. We thank L. Sagnottifor generously sharing his data and also for his helpfulcomments. Thanks are due to Alan Chave for his helpabout coherence test. The authors are also indebted tothe anonymous reviewers for their useful suggestions.

References

Anderson, R.Y., 1992. Possible connection between surface winds,solar activity and the Earth’s magnetic field. Nature 358, 51–53.

Banerjee, S.K., King, J., Marvin, J., 1981. A rapid methodfor magnetic granulometry with applications to environmentalstudies. Geophys. Res. Lett. 8, 333–336.

Bianchi, M.M., 2000. Historia de fuego en Patagonia: Registrode carbón vegetal sedimentario durante el Post-glacial y elHoloceno en Lago Escondido (41◦S–72◦O). Revista Cuaternarioy Ciencias Ambientales. Publicación Especial No 4, pp. 23–29.

Brachfeld, S.A., Banerjee, S.A., 2000. A new high-resolutiongeomagnetic relative paleointensity record for the NorthAmerican Holocene: a comparison of sedimentary and absoluteintensity data. J. Geophys. Res. 105 (B1), 821–834.

Brachfeld, S.A., Domack, E., Kisel, C., Laj, C., Leventer, A.,Ishman, S., Gilbert, R., Camerlenghi, A., Eglinton, L.B., 2003.Holocene history of the Larsen-A Ice Shelf constrained bygeomagnetic paleointensity dating. Geology 31 (9), 749–752.

Castellanos, Z.A., 1959. Las especies del género Diplodon en laArgentina. Actas y Trabajos Primer Congr. Sudam. Zool. 2 (3),85–94.

Castellanos, Z.A., 1960. Almejas nacarıferas de la RepúblicaArgentina del género Diplodon (Mol. Mutelidos). Secret. Agric.De la Nación, Publ. Misc. 421, pp. 1–40.

Channell, J.E.T., 1999. Geomagnetic paleointensity and directionalsecular variation at Ocean Drilling Program (ODP) Site 984(Bjorn Drift) since 500 ka: comparisons with ODP Site 983(Gardar Drift). J. Geophys. Res. 104 (B10), 22,937–22,951.

Channell, J.E.T., Stoner, J.S., Hodell, D.A., Charles, C.D.,2000. Geomagnetic paleointensity for the last 100 kyr fromthe sub-Antartic South Atlantic: a tool for inter-hemisphericcorrelation. Earth Planet. Sci. Lett. 175 (1–2), 145–160.

Cisowski, S.M., Hall, F.R., 1997. 12. An examination of thepaleointensity record and geomagnetic excursions recordedin leg 155 cores. In: Flood, R.D., Piper, D.J.W., Klaus,A., Peterson, L.C. (Eds.), Proceedings of the Ocean DrillingProgramme, Scientific Results, p. 15.

del Valle, R.A., Lirio, J.M., Nuñez, H.J., Tatur, A., Rinaldi, C.A.,2000. Sedimentary cores from Mascardi Lake, Argentina: a keysite to study Elpalafquen paleolake. In: Smolka, P., Volkheimer,W. (Eds.), Southern Hemisphere Paleo and Neoclimates.Springer-Verlag, 381 pp.

Feruglio, E., 1941. Nota preliminar sobre la hoja 40b, San Carlosde Bariloche. Boletın Informaciones Petroleras 18 (200), 26–64.

Gogorza, C.S.G., Sinito, A.M., Di Tomasso, I., Vilas, J.F., Creer,K.M., Nuñez, H., 1999. Holocene Secular variation recordedby sediments from Lake Escondido (South Argentina). EarthPlanets Space 51, 93–106.

Gogorza, C.S.G., Sinito, A.M., Di Tomasso, I., Vilas, J.F.,Creer, K.M., Nuñez, H., 2001. Geomagnetic Secular Variations0–12000 yr. as recorded by sediments from Moreno Lake (SouthArgentina). J. S. Am. Earth Sci. 13 (7), 627–645.

Gogorza, C.S.G., Sinito, A.M., Lirio, J.M., Nuñez, H., Chaparro,M., Vilas, J.F., 2002. Paleosecular variations 0–19,000 yearsrecorded by sediments from Escondido Lake (Argentina). Phys.Earth Planet. Inter. 133, 35–55.

González Dıaz, E., Nullo, F., 1980. Cordillera Neuquina. In:Geologıa Regional Argentina. Academia Nacional de Cienciasde Córdoba, vol. II, pp. 1099–1147.

Gonzalez Bonorino, F., 1973. Geologıa del área entre San Carlosde Bariloche y Llao-Llao. Departamento Recursos Naturales yEnergıa, Fundación Bariloche, Public., p. 16.

González Bonorino, F., 1979. Esquema de la evolución geológicade la Cordillera Nordpatagónica. Asoc. Geol. Arg. Rev. 34 (3),184–202.

Guyodo, Y., Valet, J.P., 1996. Relative variations in geomagneticintensity from sedimentary records: the last 200,000 years. EarthPlanet. Sci. Lett. 143, 23–36.

Hodell, D.A., Brenner, M., Curtis, J.H., Guilderso, T., 2001. Solarforcing of drought frequency in the Maya Lowlands. Science292, 1367–1370.

Hunt, C.P., Banerjee, S.K., Han, J., Solheid, P.A., Oches, E., Sun,W., Liu, T., 1995. Rock-magnetic proxies of climate change inthe loess-paleosol sequences of the western Loess Plateau ofChina. Geophys. J. Int. 123, 232–244.

Jackson, B.M., 1996. Paleoenvironmental record from LakeEscondido, Rıo Negro Province, Argentina. Thesis. Universityof Wisconsin, 136 pp.

Jackson, M., Marvin, J., Banerjee, S.K., 1988. Partial anhystericremanence and its anisotropy: applications and grain-sizedependence. Geophys. Res. Lett. 15, 440–443.

Johnson, H.P., Kinoshita, H., Merrill, R.T., 1975. Rock magnetismand paleomagnetism of some North Pacific deep-sea sediments.Geol. Soc. Am. Bull. 86, 412–420.

King, J., Banerjee, S.K., Marvin, J., Özdemir, Ö., 1982. Acomparison of different magnetic methods for determining therelative grain size of magnetite in natural materials: some resultsfrom lake sediments. Earth Planet. Sci. Lett. 59, 404–419.

King, J.W., Banerjee, S.K., Marvin, J., 1983. A new rock magneticapproach to selecting sediments for geomagnetic paleointensitystudies: application to paleointensity for the last 4000 years. J.Geophys. Res. 88, 5911–5921.

Kruiver, P.P., Kok, Y.S., Dekkers, M.J., Langereis, C.G., Laj, C.,1999. A pseudo-Thellier relative paleointensity record, and rock

238 C.S.G. Gogorza et al. / Physics of the Earth and Planetary Interiors 145 (2004) 219–238

magnetic and geochemical parameters in relation to climateduring the last 276 kyr. in the Azores region. Geophys. J. Int.136, 757–770.

Laj, C., Kissel, C., Mazaud, A., Channell, J.E.T., Beer, J., 2000.North Atlantic palaeointensity stack since 75 ka (NAPIS-75)and the duration of the Laschamp event. Phil. Trans. R. Soc.London, Ser. A 358, 1009–1025.

Lehman, B., Laj, C., Kissel, C., Mazaud, A., Paterne, M., Labeyrie,L., 1996. Relative changes of the geomagnetic field intensityduring the last 280 kyear from piston cores in the Azores area.Phys. Earth Planet. Inter. 93, 269–284.

Leventer, A., Domack, E.W., Ishman, S.E., Brachfeld, S., McClennen, C.E., Manley, P., 1996. Productivity cycles 200–300years in the Antarctic Peninsula region: understanding linkagesamong the Sun, atmosphere, oceans, sea-ice and biota. GSABull. 108, 1626–1644.

Levi, S., Banerjee, S.K., 1976. On the possibility of obtainingrelative paleointensities from lake sediments. Earth Planet. Sci.Lett. 29, 219–226.

Lund, S.P., Schwartz, M., 1999. Environmental factors affectinggeomagnetic field paleointensity estimates from sediments.In: Maher, B., Thompson, R. (Eds.), Quaternary Climate,Environments and Magnetism. Cambridge University Press,New York, pp. 323–351.

Nowaczyk, N.R., Harwart, S., Melles, M., 2001. Impact of earlydiagenesis and bulk particle grain size distribution on estimatesof relative geomagnetic palaeointensity variations in sedimentsfrom Lama Lake, northern Central Siberia. Geophys. J. Int.145, 300–306.

Pan, Y., Zhu, R., Shaw, J., Liu, Q., Guo, B., 2001. Canrelative paleointensities be determined from the normalisedmagnetisation of the wind-blown loess of China? J. Geophys.Res. 106 (B9), 19,221–19,232.

Peck, J.A., King, J.W., Colman, S.M., Kravchinsky, V.A., 1996.An 84-kyr. paleomagnetic record from the sediments of LakeBaikal, Siberia. J. Geophys. Res. 101 (B5), 11,365–11,385.

Peng, L., King, J.W., 1992. A late quaternary geomagnetic secularvariation record from Lake Waiau, Hawaii, and the questionof the Pacific nondipole low. J. Geophys. Res. 97 (B4), 4407–4424.

Raisbeck, G.M., Yiou, F., Jouzel, J., Petit, J.R., 1990.10Be andδ2Hin polar ice cores as a probe of the solar variability’s influenceon climate. Phil. Trans. R. Soc. London, Ser. A 330, 463–470.

Roberts, A.P., Lehman, B., Weeks, R., Verosub, K., Laj, C., 1997.Relative paleointensity of the geomagnetic field over the last200,000 years from ODP Sites 883 and 884, North PacificOcean. Earth Planet. Sci. Lett. 152, 11–23.

Sagnotti, L., Macri, P., Camerlenghi, A., Rebesco, M., 2001.Environmental magnetism of Antarctic Late Pleistocenesediments and interhemispheric correlation of climatic events.Earth Planet. Sci. Lett. 192, 65–80.

Sinito, A.M., Nuñez, H.J., 1997. Paleosecular variation recordedon lake sediments from South Argentina. J. Geomag. Geoelectr.49, 473–483.

Slawinksi, D., 1998. Varved lake sediments of midwestern NorthAmerica, indicators of cyclic climate change, M.S. DissertationThesis, University of Minn., Minneapolis.

Stoner, J.S., Laj, C., Channell, J.E.T., Kissel, C., 2002. SouthAtlantic and North Atlantic geomagnetic paleointensity stacks(0–80 ka): implications for inter-hemispheric correlation. Quart.Sci. Rev. 21, 1141–1151.

St-Onge, G., Stoner, J.S., Hillaire-Marcel, C., 2003. Holocenepaleomagnetic records from St. Lawrence Estuary, easternCanada: centennial- to millennial-scale geomagnetic modulationof cosmogenic isotopes. Earth Planet. Sci. Lett. 209, 113–130.

Stuiver, M., Braziunas, T.F., 1993. Sun, ocean, climate andatmospheric 14CO2: an evaluation of causal and spectralrelationships. Holocene 3, 289–305.

Tatur, A., del Valle, R., Bianchi, M.M., Outes, V., Villarosa, G.,Niegodzisz, J., Debaene, G., 2002. Late Pleistocene palaeolakesin the Andean and Extra-Andean Patagonia at mid-latitudes ofSouth America. Quart. Int. 89, 135–150.

Tauxe, L., 1993. Sedimentary records of relative paleointensitiesof the geomagnetic field: theory and practice. Rev. Geophys.31, 319–354.

Tauxe, L., Wu, G., 1990. Normalised remanence in sedimentsof the western equatorial Pacific: relatively intensity of thegeomagnetic field? J. Geophys. Res. 95, 12,337–12,350.

Thompson, R., Oldfield, F., 1986. Environmental Magnetism. Allen& Unwin Ltd., 225 pp.

Thouveny, N., 1987. Variations of the relative paleointensity ofthe geomagnetic field in western Europe in the interval 25–10kyr BP as deduced from analyses of lake sediments. Geophys.J. R. Astronom. Soc. 91, 123–142.

Tric, E., Valet, J.P., Tucholka, P., Paterne, M., LaBeyric, L.,Guichard, F., Tauxe, L., Fontugne, M., 1992. Paleointensity ofthe geomagnetic field during the last 80,000 years. J. Geophys.Res. 97, 9337–9351.

Verschuren, D., Laird, K., Cumming, B., 2000. Rainfall anddrought in equatorial east Africa during the past 1100 years.Nature 403, 410–414.

Yang, S., Odah, H., Shaw, J., 2000. Variations in the geomagneticdipole moment over the last 12,000 years. Geophys. J. Int. 140,158–162.