A reversal of the Earth s magnetic field recorded in mid-miocene lava flows of Gran Canaria: Paleointensities

Transcription

1 JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 107, NO. B11, 2299, doi: /2001jb000949, 2002 A reversal of the Earth s magnetic field recorded in mid-miocene lava flows of Gran Canaria: Paleointensities R. Leonhardt and H. C. Soffel Institut für Allgemeine und Angewandte Geophysik, Ludwig-Maximilians-Universität, Munich, Germany Received 6 August 2001; revised 7 April 2002; accepted 7 May 2002; published 15 November [1] An extensive paleointensity study was carried out on an approximately 14.1 Myr old reverse-to-normal transition of the geomagnetic field. A total of 188 samples from a volcanic sequence on Gran Canaria (Canary Islands) were subjected to Thellier-type paleointensity determinations. The paleodirectional results of this sequence are reported by Leonhardt et al. [2002]. A modified Thellier technique, which facilitates the recognition of MD tails and the formation of new magnetic remanences with higher blocking temperatures than the actual heating step, was used on the majority of the samples. We obtained reliable paleointensity results for 35% of the 87 sampled lava flows. In general, the obtained field intensities are lower than expected for the mid-miocene. This observation is very likely related to a long-term reduction of the field close to transitions. Very low paleointensities with values <5 were obtained during an excursion, preceding the actual transition, and also close to significant changes of the local field directions. These are interpreted as nondipolar components becoming dominant for short periods and provoking a rapid change of field directions. During the transition, 15 successive lava flows recorded similar local field directions corresponding to a cluster of virtual geomagnetic poles close to South America. Chronologically, within this cluster, the paleointensity increases from about 9 to 28 followed by a decrease back to approximately 9. This variation of paleointensity between lava flows with similar direction indicates independent records of the geomagnetic field and gives strong evidence for a stable transitional state that lasted a significant period. INDEX TERMS: 1535 Geomagnetism and Paleomagnetism: Reversals (process, timescale, magnetostratigraphy); 1521 Geomagnetism and Paleomagnetism: Paleointensity; 1513 Geomagnetism and Paleomagnetism: Geomagnetic excursions; KEYWORDS: paleomagnetism, absolute paleointensity, geomagnetic reversals, lava flows, Gran Canaria, Canary Islands Citation: Leonhardt, R., and H. C. Soffel, A reversal of the Earth s magnetic field recorded in mid-miocene lava flows of Gran Canaria: Paleointensities, J. Geophys. Res., 107(B11), 2299, doi: /2001jb000949, Introduction [2] At present it is well established that the source of the geomagnetic field and of the reversal process is in the Earth s liquid outer core. Paleomagnetism is the only known method which provides information about the magnetic field during polarity transitions and, therefore, paleomagnetic records of the variations in direction and intensity during geomagnetic transitions and excursions yield important constraints for models of the geodynamo. The directional analysis of transitional records led to basically two descriptions of the character of the transitional field. A continuous change of the field is often found in sedimentary records [Clement, 1991; Laj et al., 1991] which are possibly biased by a delayed remanence acquisition process [e.g., Langereis et al., 1992]. Data from volcanic rocks show mainly periods of steady directions followed by rapid Copyright 2002 by the American Geophysical Union /02/2001JB000949$09.00 changes of the field [Prévot et al., 1985; Hoffman, 1992, 1996], possibly related to the sporadic nature of volcanic activity. Additionally, sediments allow only the determination of relative intensities whereas volcanic rocks can provide an absolute paleointensity estimate of the past geomagnetic field. However, the determination of absolute paleointensities on volcanic rocks is a difficult and time consuming task, especially when using the Thellier and Thellier [1959] method. Modifications of this method allow checks for alteration during the experiment [Coe, 1967] and the identification of multidomain tails [McClelland and Briden, 1996]. [3] In comparison to directional investigations, far fewer paleointensity determinations have been carried out. So far the available paleointensity results point to a significant reduction of the geomagnetic field intensity during transitions [e.g., Merrill and McFadden, 1999]. However, observations of high intensities during transitions have also been reported. During a Pleistocene Icelandic transition Shaw [1975] found very high paleointensities using the Shaw EPM 5-1

2 EPM 5-2 LEONHARDT AND SOFFEL: PALEOINTENSITIES DURING A FIELD REVERSAL [1974] method. Using the Thellier and Thellier [1959] method for paleointensity determination on the same transition, Goguitchaichvili et al. [1999] did not find any evidence for such high intensities. The detailed record of a mid- Miocene transition from the Steens Mountain (Oregon, USA) exhibits strong variations of intensity with some relatively high intensity values during transitional states of the field [Prévot et al., 1985]. Similar observations are reported from excursions of the Earth s magnetic field recorded in Pliocene lavas from Oahu, Hawaii [Coe et al., 1984] and from mid- Miocene lavas from Gran Canaria, Canary Islands [Leonhardt et al., 2000] which are stratigraphically lower than the volcanic sequence presented here. [4] Another characteristic which is often found in absolute paleointensity records is an unusual state of the geodynamo following polarity transitions. Compared to pretransitional paleointensities, strong posttransitional fields were found in several volcanic sequences covering geomagnetic transitions [Prévot et al., 1985; Bogue and Paul, 1993; Valet et al., 1999; Riisager and Abrahamsen, 2000]. [5] In this paper we present absolute paleointensity results of a mid-miocene volcanic sequence from the shield basalts of Gran Canaria, Canary Islands, determined with a slightly modified Thellier and Thellier [1959] method. Within this sequence of 87 lava flows an approximately 14.1 Myr old reverse-to-normal transition of the Earth s magnetic field is recorded. The directional analysis shows that 34 lava flows record intermediate states of the field [Leonhardt et al., 2002]. During the transition the virtual geomagnetic poles (VGPs) cluster in three consecutive regions: south east of South America, east of India, and in the central Pacific. In addition to the transition, three excursions are recorded in the sequence, two preceding and one following the transition. The data indicate very high eruption rates and, therefore, a high temporal resolution. 2. Paleointensity Determination [6] Samples for this study were collected along two overlapping profiles on the flanks of Montaña del Lechugal close to the El Paso restaurant on Gran Canaria (Canary Islands). Whenever possible, individual samples were taken with a horizontal spacing of 1 2 m in the central part of the lava flow. An accurate description of the locations and sampling procedures is given by Leonhardt et al. [2002] Sample Selection [7] In order to select suitable samples for Thellier-type paleointensity determinations extensive rock magnetic examinations and ore microscopy observations were performed on the samples of the El Paso profiles [Leonhardt et al., 2002]. Only samples yielding the mean characteristic remanence direction of the flow were used for paleointensity determinations. Samples carrying a significant isothermal remanence due to lightning strikes were rejected. Such lighting strike remagnetizations were identified by comparing of magnetic and Sun orientation data as well as by characteristics of alternating field (Af) demagnetizations of sister samples. The viscosity index [Thellier and Thellier, 1944] was determined for 30% of the collection and was below 5% for all successful paleointensity determinations. Strong field thermomagnetic curves were obtained for at Figure 1. ARM demagnetizations at room temperature versus demagnetizations after heating steps to 200 C, 400 C, and 550 C. least one sample of every flow and susceptibility measurements accompanied each thermal demagnetization step. These experiments allow the estimation of the thermal stability of the samples. Samples showing high Curie temperatures were used to assess absolute paleointensities, corresponding to group 1 and group 2 thermomagnetic curve types classified by Leonhardt et al. [2002]. Using ore microscopy, it has been shown by these authors that the magnetization of the samples is usually carried by titanomagnetites, oxidized at high temperatures during cooling of the lavas. Such highly oxidized phases are relatively insensitive to oxidation under laboratory conditions. The relative increase or decrease of the saturation magnetization after heating and cooling cycles was not used as a selection criterion, since such ratios bear no information concerning the temperature interval at which the alteration occurred. Irreversible changes of the magnetic information due to alteration at moderate temperatures [e.g., Kosterov and Prévot, 1998] could lead to spurious interpretations of the Arai plots. These changes normally can not be detected using standard checks during the Thellier experiment, simply because only a small percentage of the NRM is lost below 300 C. In order to quantify possible alteration mechanisms, AF demagnetization of an ARM (150 AC, 50 DC) after several heating steps was performed on specimens from the same drill cores as used for paleointensity determination. In Figure 1 the AF demagnetization of an ARM at room temperature is plotted versus the AF demagnetization of an ARM after 200, 400 and 550 C. If the ARM demagnetizations after different heating steps are linearly related with a slope of one, then no alteration occurred (Figures 1b and 1d). The suitable samples do not show any significant alteration at least until 550 C (Figure 1c). A magnetization increase for the low-coercivity phase and decrease for the high coercivities

3 LEONHARDT AND SOFFEL: PALEOINTENSITIES DURING A FIELD REVERSAL EPM 5-3 after heating to 200 C (Figure 1a) is possibly related to a stress release in pseudo-single-domain (PSD) or multidomain (MD) grains and indicates that such samples are not suited for paleointensity determination. Figure 2. Two representative examples of accepted MT3 paleointensity determinations. The slope is related to the characteristic remanent magnetization (ChRM) and is not affected by MD tails. The alteration during the experiment, determined by the relative error of the checks, is negligible Methods [8] For this study three different modifications, MT1, MT2, and MT3, of the Thellier-Thellier technique [Thellier and Thellier, 1959] have been used to assess absolute paleointensities. These modifications are discussed in detail by Leonhardt et al. [2000]. The first measurements of the absolute paleointensity were performed according to the method of Coe [1967] using alteration checks at every temperature step, further on referred to as MT1-type experiments. However, there are two possible mechanisms that can cause a undetectable failure of the MT1 experiment. These mechanisms are multidomain (MD) remanences and alteration products with higher unblocking temperatures than the actual heating step. In order to detect alteration products with higher unblocking temperatures than the actual heating step, a few samples were subjected to the MT2 method, where the ptrm is acquired before demagnetization at the same temperature. Most of the samples were subjected to the MT3 method. This experiment was developed in order to check whether MD remanence or alteration products with high unblocking temperatures affect the paleointensity determination in addition to normal alteration identified by ptrm checks [Leonhardt et al., 2000]. This method is based on the Coe [1967] experiment with checks every second heating step. At some temperatures (200 C, 400 C, and 550 C) the acquired ptrm is additionally demagnetized at the same temperature. By comparing the results of two demagnetization steps before (NRM(T i )) and after (NRM re (T i )) the ptrm acquisition, a possible bias of the result due to one or both of these mechanisms can be detected. If the direction of the remanence changes between the two demagnetization steps we conclude that alteration products with higher unblocking temperatures are present. A significant difference in magnetization intensity of NRM(T i ) and NRM re (T i ) accompanied by stable directions is an indication of an MD tail [McClelland and Briden, 1996]. Two representative examples of NRM TRM plots are shown in Figure 2. The temperature interval used for paleointensity calculation in Figure 2a covers also a minor overprint of the direction between 200 C and 430 C. The intensity for a five-point segment between 460 C and 580 C would result in the same value as for the used segment. Therefore it is assumed that the slight deviation of the direction below 460 C does not affect the paleointensity determination in this case. For determining absolute paleointensities two different sample sizes were used: standard inch samples and minisamples with a diameter and length of 5 mm [Leonhardt et al., 2000]. The main benefit of the minisamples is the reduced heating and cooling time required during the demagnetization and ptrm acquisition steps. Minisamples were taken from the lower part of the inch-sized drill cores, some of them not oriented relative to the core. In this case the primary character of the magnetization of the minisample was ascertained by stepwise thermal demagnetization of an inch-sample from the same drill core. Up to 35 minisamples were heated in the center of a MMTD20 thermal demagnetizer, which was also used for the inch specimens. The applied field intensity of the ptrm steps was usually between 20 and 38. Minisamples were heated and cooled within a quartz-glass tube in an inert argon atmosphere. Remanence measurements on these samples were performed using a 2G cryogenic magnetometer. The inch samples were measured with a Molspin Fluxgate Spinner magnetometer. Inch samples and minicores yield similar results for all accepted paleointensity determinations, independent of heating in argon or in air Reliability Criteria [9] The analysis of the paleointensity determinations was done using standard NRM TRM plots [Arai, 1963]. For identification of a linear segment in the NRM TRM plot only successive points were used. To get a qualitative measure of the difference between ptrm steps and ptrm checks, the relative CK-error was calculated according to Leonhardt et al. [2000]. The following criteria were used to characterize the quality of the absolute paleointensity values from the NRM TRM plots: 1. The temperature range used for the calculation of a linear fit must be related to the characteristic remanent magnetization of the sample. Therefore the maximum angular deviation (MAD) of the direction of the used segment has to be less than The segments used for the linear fits consist of at least 5 successive points.

4 EPM 5-4 LEONHARDT AND SOFFEL: PALEOINTENSITIES DURING A FIELD REVERSAL Specimens showing values of MAD > 15, F < 0.2, and N < 5 were not accepted. Figure 3. Typical examples of Thellier experiments which were not accepted for calculation of an absolute paleointensity value. Most experiments failed due to alteration during the experiment (a d). No linear segment of at least 5 points related to the ChRM could be identified. A few samples were rejected due to the influence of MD tails (e, f). The first slope of these examples is not related to the ChRM, the second slope is affected by a significant intensity variation of the MD checks. 3. The segments cover a fraction (f) of at least 20% of the NRM. 4. No alteration has occurred during the experiment. Therefore the CK-error must not exceed 5% before and within the linear segment. 5. The scatter of the used data points about the linear fit should be small. The standard deviation of the paleointensity determination has to be 10%. Additional criteria for MT2 and MT3 experiments are: 6. The remanence direction in core coordinates (MT2) or the direction of the repeated demagnetization (MT3) should not move toward the direction of the applied field before and within the linear segment. For MT3 experiments: 7. To exclude any significant bias because of MD tails, the intensity change between NRM(T i ) and NRM re (T i ) must not exceed 5%. Samples which meet all these criteria were termed to be class A determinations (Figure 2a). If only one parameter slightly exceeded the criteria value the result was termed to be a class B determination (Figure 2b). The limiting values for class B determinations are that either the CK-error < 7%, the standard deviation <15% or the intensity change between NRM(T i ) and NRM re (T i ) < 7%. In case of one parameter exceeding the class A criteria by more than these values, the results were classified as C. Class C determinations were only accepted for lava flows where at least one class A or B determination is available and the results are in agreement with the flow mean NRM TRM Diagrams [10] Further representative examples of NRM TRM plots are shown in Figures 3 and 4. NRM TRM plots of paleointensity determinations which were rejected are shown in Figure 3. Most samples were rejected due to alteration during the experiment. Standard alteration checks indicate the formation or destruction of remanence carrying magnetic material (Figures 3a 3d). No linear segment of at least 5 points could be identified within the temperature range of the characteristic remanent direction. In some cases the presence of MD tails was observed (Figures 3e and 3f). These examples show two slopes, one in the lower blocking temperature spectrum up to 400 C and one in the upper temperature range. The MD tail-checks at 400 C and 550 C, represented by the relative change of intensity between NRM(T i ) and NRM re (T i ) in (Figures 3e and 3f), indicate the presence of MD particles. Therefore the higher temperature range can not be used to assess absolute paleointensities. Within the lower temperature range no linear segment of at least 5 successive points, completely related to the ChRM direction, could be found. Some typical and accepted determinations of paleointensity are Figure 4. NRM TRM plots and Zijderveld diagrams in core coordinates of accepted intensity determinations of lava flows with reverse (a c), intermediate (d g), and normal (h) directions.

5 LEONHARDT AND SOFFEL: PALEOINTENSITIES DURING A FIELD REVERSAL EPM 5-5 Figure 5. VGP movement across the El Paso sequence. The lower part of the sequence is characterized by reverse directions of the geomagnetic field interrupted by two intermediate states with VGPs close to Africa and South America. During the transition the VGP clusters first for 15 successive lava flow directions close South America. Then VGPs close to India and in the central Pacific are recorded. From here the VGP moves along the 180 meridian to the North pole and back to the Pacific. Finally, after a movement over North America, normal directions are recorded in 8 successive lavas. shown in Figure 4. Samples of reversed magnetizations (Figures 4a 4c), transitional magnetizations (Figures 4d 4g), and normal magnetizations (Figure 4h) are presented. The paleointensity determinations of Figures 4d 4f are from the South America VGP cluster of the El Paso sequence (Figure 5), the paleointensity of Figure 4g corresponds to a VGP direction near India, directly after the South America cluster. 3. Paleointensity Results 3.1. Statistics [11] One hundred eighty-eight samples were subjected to Thellier-type paleointensity experiments (Table 1). Reliable results were obtained from 35% of the lava flows. Most of these determinations cover a fraction of the NRM between 30% and 60% and show maximum temperatures of the linear segment above 490 C (Gray bars in Figure 6) which further demonstrates the high quality of the results. The hatched bars in Figure 6 show the maximum blocking temperatures of paleointensity determinations with results above 10. This graph emphasizes that relatively high intensity values and low intensity values were both obtained with a similar maximum blocking temperature distribution. Of the 80 accepted paleointensity determinations 47 were of class A, 26 of class B, and 7 of class C (see Table 1). For all but 6 lava flows at least two absolute paleointensity determinations were used to calculate a weighted mean intensity according to Prévot et al. [1985]. The six lava flows with only one successful determination are either of class A or show a quality factor q [Coe et al., 1978] above 5. The results of all samples and the corresponding mean values are summarized in Table Results of the El Paso Sequence [12] The weighted means of the paleointensity determinations for each flow and the corresponding VDMs versus the stratigraphic position are shown in Figure 7. Compared to today s value of the local geomagnetic field intensity (38 ), the obtained paleointensity estimates are generally low. During the reversed period at the beginning (C- TP1 to C-TP23) the average intensity is about 10, corresponding to a VDM of Am 2. A decreasing trend of the intensity is observed toward the excursion to South America, 230 m below the top of the El Paso sequence (C-TP26,27). During this excursion very low values of 3 have been determined. The reversed part of the profile, between C-TP29 and C-TP46 is characterized by large variations of the paleointensity. The mean value of the intensity is 11.8 during this period. During the cluster state of the VGPs near South America a remarkable increase of the paleointensity is observed. Despite the fact that the position of the VGP is nearly constant during this transitional state, the paleointensity increases by a factor of three between C-TP49 and C-TP57. Toward the end of the South America cluster the intensity drops to values of 9, similar to the value at the beginning of the VGP cluster. Immediately after the shift of the VGP from South America to India the intensity is very low with values of 1 and then rises slightly to 6. During the loop of the VGP toward the north pole an intensity (16 ) similar to that of the uppermost normal flows is obtained, corresponding to a VDM of Am 2. The high paleointensities during the South America cluster are recorded in two lava flows overlying three lightning strike remagnetized flows (Figure 7). An influence of an isothermal remanence on the lava flows showing high paleointensities could be excluded due to the similarity of Sun and magnetic compass orientation and the normal decrease of intensity during AF demagnetization of other samples from these lava flows. 4. Discussion 4.1. Characteristics of the El Paso Transition [13] The paleomagnetic data of the El Paso sequence comprises a new and detailed transitional record of direction and absolute intensity which contains information about possible configurations of the geomagnetic field during transitions. Neither a linear relationship between the logarithmic decrease of the VDM and the reversal angle [Lin et al., 1994], nor the hypothesis that the intensity is reduced throughout a transition of the Earth s magnetic field is supported by this record. [14] The paleointensity before and after the El Paso transition is generally lower than the mid-miocene mean value of the geomagnetic field ( Am 2 according to Juárez et al. [1998]). This supports the hypothesis that a directional change of the field is accompanied by a longer period of reduced field intensity [Merrill and McFadden, 1999]. However, it is observed that the mean intensity values are higher after the El Paso transition than before. Additionally, at the beginning of the record a gradual decrease of the paleointensity toward the excursions is observed. Increases of intensity directly after transitions were also found in the relative paleointensity records of sedimentary data of the last 5 Myr [Valet and Meynadier, 1993]. These records show a sharp increase of intensity after a transition followed by a gradual decrease toward the next transition, called an asymmetric sawtooth pattern. Our observations do not contradict a geomagnetic origin of this

6 EPM 5-6 LEONHARDT AND SOFFEL: PALEOINTENSITIES DURING A FIELD REVERSAL Table 1. Paleointensity Results of the El Paso Sequence a Unit Site Depth, m Polarity n/n Sample Size Atmosphere Type Class Temperature Range Np f g q w C-TP1 A R 3/ mini argon MT2 A ± ± mini argon MT1 B ± D inch air MT1 A ± 0.61 C-TP2 A R 0/3 C-TP3 A R 4/ b inch air MT3 B ± ± C inch air MT1 B ± mini argon MT1 B ± mini argon MT3 A ± 2.12 C-TP4 A R 0/1 C-TP5 A R 4/ mini argon MT1 A ± ± mini argon MT3 A ± a inch air MT3 A ± mini argon MT3 B ± 0.44 C-TP6 A R 0/1 C-TP7 A I 0/1 C-TP8 A I 0/2 C-TP9 A I 0/1 C-TP10 A R 0/4 C-TP11 A R 3/ mini argon MT2 B ± ± mini argon MT2 A ± mini argon MT2 A ± 0.29 C-TP12 A R 0/1 C-TP13 A R 0/1 C-TP14 A R 0/2 C-TP15 A R 0/1 C-TP16 A R 0/3 C-TP17 A R 3/ A inch air MT3 A ± ± mini argon MT1 A ± A inch air MT3 B ± 1.38 C-TP18 A R 0/1 C-TP19 A R 0/1 C-TP20 A R 1/ mini argon MT1 A ± C-TP23 A R 2/ mini argon MT1 A ± ± A inch air MT1 B ± 0.28 C-TP24 A R 2/ A inch air MT3 A ± ± mini argon MT1 A ± 0.08 C-TP26 A I 2/ mini argon MT1 A ± ± C inch air MT3 C ± 0.1 C-TP27 A I 2/ D inch air MT3 B ± ± mini argon MT1 A ± 0.25 C-TP29 A R 0/1 C-TP31 A R 3/ C inch air MT3 B ± ± mini argon MT1 A ± mini argon MT3 A ± 1.25 C-TP33 A R 1/ mini argon MT2 A ± C-TP34 A R 1/ mini argon MT1 B ± C-TP35 A R 2/ b inch air MT3 A ± ± mini argon MT3 A ± 0.71 C-TP37 A R 0/4 C-TP39 A R 0/1 H p ± SD, F p ± SD,

7 LEONHARDT AND SOFFEL: PALEOINTENSITIES DURING A FIELD REVERSAL EPM 5-7 Table 1. (continued) Unit Site Depth, m Polarity n/n Sample Size Atmosphere Type Class Temperature Range Np f g q w C-TP40 A R 0/1 C-TP41 A R 3/ mini argon MT1 A ± ± mini argon MT3 A ± A inch air MT3 A ± 0.52 C-TP42 A R 3/ mini argon MT1 A ± ± C inch air MT3 B ± mini argon MT2 A ± 0.22 C-TP43 A R 0/1 C-TP44 A R 0/2 C-TP45 A R 0/1 C-TP46 A R 0/2 C-TP47 A I 0/4 C-TP48 A I 0/3 C-TP49 A I 4/ mini argon MT3 A ± ± mini argon MT1 A ± a inch air MT3 A ± mini argon MT3 A ± 0.18 C-TP50 A I 3/ mini argon MT3 A ± ± mini argon MT3 A ± c inch air MT3 B ± 0.42 C-TP51 B I 1/ C inch air MT3 B ± C-TP52 A I 0/2 C-TP54 A I 4/ mini argon MT1 A ± ± A inch air MT3 B ± mini argon MT3 B ± c inch air MT3 C ± 0.9 C-TP57 B I 3/ mini argon MT3 A ± ± C inch air MT3 A ± B inch air MT3 A ± 0.91 C-TP58 B I 2/ mini argon MT3 A ± ± a inch air MT3 B ± 1.76 C-TP59 B I 4/ mini argon MT2 A ± ± mini argon MT3 A ± A inch air MT3 B ± X mini argon MT3 C ± 1.13 C-TP60 B I 3/ C inch air MT3 B ± ± mini argon MT3 A ± X mini argon MT3 B ± 0.46 C-TP61 B I 1/ B inch air MT3 A ± C-TP62 B I 2/ B inch air MT3 B ± ± mini argon MT3 B ± 0.04 C-TP63 B I 0/4 C-TP66 B I 3/ mini argon MT3 B ± ± A inch air MT3 C ± b inch air MT3 B ± 0.54 C-TP68 B I 0/4 C-TP69 B N 1/ mini argon MT3 A ± Hp ± SD, Fp ± SD,

8 EPM 5-8 LEONHARDT AND SOFFEL: PALEOINTENSITIES DURING A FIELD REVERSAL Table 1. (continued) Unit Site Depth, m Polarity n/n Sample Size Atmosphere Type Class Temperature Range Np f g q w C-TP72 B N 2/ mini argon MT3 A ± ± mini argon MT3 A ± 0.77 C-TP74 B I 0/1 C-TP76 B I 0/1 C-TP78 B I 0/1 C-TP79 B I 0/4 C-TP81 B I 0/1 C-TP83 B N 2/ mini argon MT3 C ± ± A inch air MT3 B ± 0.24 C-TP84 B N 3/ B inch air MT3 C ± ± d inch air MT3 C ± mini argon MT3 B ± 1.5 C-TP86 B N 3/ B inch air MT3 A ± ± a inch air MT3 A ± mini argon MT3 A ± 0.55 C-TP88 B-39 8 N 0/1 a The flows are listed in stratigraphic order from the lowest to the highest. Site indicates the flow number for profiles A and B [Leonhardt et al., 2002]. The success rate n = N is the number of successful determinations per flow versus the number of used specimens. Size describes the type of the specimen, mini-sample or inch sample. Atmosphere denotes whether heating was performed in air or argon, type the used Thellier-type experiment, and class the quality according to the reliability criteria. The temperature range specifies the interval used for paleointensity calculation and Np the number of independent and successive points in this interval. f, g, q represent the fraction of NRM, the gap factor, and the quality factor [Coe et al., 1978]. Hp is the individual paleointensity estimate with standard deviation. Fp is the weighted mean paleointensity for the flow using the weighting factor w of Prévot et al. [1985]. H p ± SD, F p ± SD,

9 LEONHARDT AND SOFFEL: PALEOINTENSITIES DURING A FIELD REVERSAL EPM 5-9 Figure 6. Histograms of (a) the distribution of the NRM fraction and (b) of the upper temperature limit used for all accepted paleointensity determinations, represented by gray bars. Most samples were analyzed with 30% to 60% fraction of the NRM and with an upper temperature of above 490 C. A curvature within the NRM TRM plot is often reported from paleointensity determination. This could lead to high intensity values in a lower temperature range and lower paleointensities in a higher temperature range. To emphasize that high intensity values were obtained in a similar blocking temperature spectrum, the hatched bars show the maximum blocking temperature distribution of accepted paleointensity determinations with values above 10. pattern, but, in order to support or to reject this theory, a long-term record of absolute paleointensity between two transitions is necessary. [15] During the South America cluster of the VGPs, very high intensities are found. The observation of high intensities during transitional states of the geomagnetic field could be explained by a dominantly dipolar field where a significant part of the dipole energy is in equatorial dipole components. However, a field state with localized and strong vertical fluxes, while the geomagnetic field is dominantly nondipolar [Glatzmaier et al., 1999; Coe et al., 2000], could also explain this observation. [16] After the intermediate VGP cluster near South America, a VGP grouping near India occurs about 125 of longitude to the east. While it may be a coincidence that no VGPs were found in between these two intermediate positions, the preferred interpretation here is that a change of the field configuration happened rapidly during this interval. This hypothesis is supported by the observation of very low paleointensities directly after the rapid move from one intermediate VGP cluster to the other. If the intensity of the Earth s magnetic field drops to very low values, which is theoretically possible when the field is strongly nondipolar, an almost instantaneous change in direction could occur. Similar observations of very low intensities close to supposedly rapid changes of field directions were obtained from the Steens Mountain polarity transition [Prévot et al., 1985]. By analyzing systematic variations of remanence direction as a function of demagnetization temperature and vertical position of the sample within the flow, a rate of change for the rapid movements of 6 per day was estimated [Coe and Prévot, 1989; Coe et al., 1995]. However, later examinations did not support these values [Camps et al., 1999]. According to the calculation of the secular variation rate of the El Paso sequence it can be estimated that the time between two successive lava flows is on average approximately 200 years [Leonhardt et al., 2002] assuming a more or less continuous accumulation of lava flows. Within these explicit limitations this value may provide some rough upper boundary for the time interval of rapid changes. [17] In the upper part of the El Paso sequence the VGP moves to the central Pacific position twice, interrupted by four flows which record a movement of the VGP toward the north pole. During this loop the VDM reaches values of Am 2, slightly higher than the posttransitional field. A very similar rebound effect was observed during the approximately 16.2 Myr [Baksi, 1993] reverse-to-normal Steens Mountain reversal [Mankinen et al., 1985; Prévot et al., 1985]. After a few normal directions, marked by pretransitional values of field intensity, the field returns to an intermediate state with VGPs in the central Pacific, exactly at the same spot as the Pacific VGPs of the El Paso transition. Additional similarities between these two records are a VGP accumulation close to South America and intensity increases, while transitional directions are relatively stable. In order to establish or reject a possible concordance of these two reversal records, it is necessary to pin down the absolute age of the El Paso sequence which is currently defined by 40 Ar/ 39 Ar and K/Ar ages obtained in stratigraphically higher and lower sections of the Gran Canaria shield basalts [McDougall and Schmincke, 1976; van den Bogaard and Schmincke, 1998], and through correlation with the Cande and Kent [1995] polarity timescale [Leonhardt et al., 2002]. New radiometric age determinations on lavas from the El Paso sequence are planned Stable Transitional Periods [18] One of the most striking features of the El Paso sequence is the threefold increase in paleointensity toward the middle of the South America VGP cluster and the drop toward the end of this cluster. These variations demonstrate that several independent records of the geomagnetic field are present within the South American cluster. The strong initial increase of the intensity during this cluster period provides strong evidence that the VGP was stable at this transitional location for a significant time. This finding verifies the proposed stable period based on the analysis of directional data [Leonhardt et al., 2002] which show slight variations of local field directions between successive lava flows during the cluster period. These authors estimated the accumulation rate of the lavas during this interval (2.3 cm/yr) using the Holocene secular variation rate according to Mankinen et al. [1985]. This value is similar to the accumulation rate of 1.7 cm/yr of the subjacent reversed part of the sequence, which supports the hypothesis that the secular variation rate does not change significantly during transitions. Therefore the duration of the stable transitional period was estimated by these authors to be approximately 1700 years. [19] The stability of the VGPs, whose calculation is based on the assumption of a dipolar field, accompanied by a great increase of the paleointensity, may give indications for dominantly dipolar or nondipolar transitional field configurations. In a dominantly nondipolar field state, a region of

10 EPM 5-10 LEONHARDT AND SOFFEL: PALEOINTENSITIES DURING A FIELD REVERSAL Figure 7. VGP latitude, absolute paleointensity, and VDM across the El Paso sequence. Open symbols without error bars in the intensity plot and the VDM diagram indicate lava flows with only one successful determination. Also shown is the stratigraphy of profile A (C-TP1 to C-TP56) and profile B (C-TP48 to C-TP89) of the El Paso sequence versus the depth below the uppermost sampled flow. Black units indicate ash layers, white units denote A 0 a lava flows, and dashed units are Pahoehoe flows. The stars mark flows showing a lightning strike remagnetization. The correlation of the two profiles is based on similar altitude and similar mean directions [Leonhardt et al., 2002]. From these correlated lava flows only two sites of profile A yielded successful paleointensity determination wherefore this correlation could not be verified using intensity data. strong magnetic flux must build up to explain the increasing intensity [Coe et al., 2000]. Additionally, this changing nondipole field configuration must sustain relatively constant local field directions on Gran Canaria which lead to the observation of a VGP cluster. [20] Another possible explanation of the observed characteristics of the South America VGP cluster during the El Paso transition could be that the Earth s magnetic field adopts quasi-stable states where dipolar components are strong or getting stronger [Hoffman, 1992, 1996]. Since such a transi-

11 LEONHARDT AND SOFFEL: PALEOINTENSITIES DURING A FIELD REVERSAL EPM 5-11 tional state cannot be stable for a long time, simply due to the influence of the Coriolis force on the fluid motion of the outer core, the quasi-stabilized field configuration vanishes and the field gets strongly nondipolar for a short time, provoking a rapid change of the direction. However, to prove or reject these dipolar and nondipolar models, it is necessary to compare many more detailed records of the same transition from different locations of the world. 5. Conclusions [21] The paleointensity during the pre- and posttransitional field of the El Paso sequence is lower than expected for the mid-miocene. The field intensity before the transition is about half the value of the intensity after the transition which is an indication of an asymmetry between pre- and posttransitional states of the geodynamo. Within 15 successive lava flows which record relatively constant transitional field directions, corresponding to a VGP cluster near South America, high paleointensities were determined showing that geomagnetic high-field states can be obtained during transitions. Within this transitional phase the intensity increases by a factor of three before decreasing again. This feature is interpreted as a stabilization of this transitional field configuration for a significant period of time which can be estimated to be about 2 kyr using the Holocene secular variation rate. This stable transitional period is followed by a large directional move and very low paleointensities. We suppose that this directional change occurred rapidly, possibly within 200 years, during a dominantly nondipolar regime of the transitional Earth s magnetic field. [22] Acknowledgments. We thank H.-U. Schmincke for his support during the field trips to Gran Canaria. F. Heider provided invaluable assistance during this project. We greatly appreciated the help of J. Matzka, F. Hufenbecher, and B. Herr during the field trips and the fruitful discussion. Reviews by P. Camps, one anonymous reviewer and the associate editor T. Evans improved the manuscript. D. Krása, C. Heunemann and J. Tait are acknowledged for their helpful suggestions. The Servicio Medio Ambiente kindly provided permission for sampling. The research was supported through grants from the Deutsche Forschungsgemeinschaft (He1814/9-1 and So72/67-2). References Arai, Y., Secular variation in intensity of the past geomagnetic field, M.S. thesis, Univ. Tokyo, Tokyo, Japan, Baksi, A. K., A geomagnetic polarity time-scale for the period 0 17 Ma, based on Ar-40/Ar-39 plateau ages for selected field reversals, Geophys. Res. Lett., 20, , Bogue, S. W., and H. A. Paul, Distinctive field behavior following geomagnetic reversals, Geophys. Res. Lett., 20, , Camps, P., R. S. Coe, and M. Prévot, Transitional geomagnetic impulse hypothesis: Geomagnetic fact or rock-magnetic artifact, J. Geophys. Res., 104, 17,747 17,758, Cande, S. C., and D. V. Kent, Revised calibration of the geomagnetic polarity timescale for the late Cretaceous and Cenozoic, J. Geophys. Res., 100, , Clement, B., Geographical distribution of transitional VGPs: Evidence for nonzonal equatorial symmetry during the Brunhes Matuyama geomagnetic reversal, Earth Planet. Sci. Lett., 29, 48 58, Coe, R. S., Palaeointensity of the Earth s magnetic field determined from tertiary and quaternary rocks, J. Geophys. Res., 72, , Coe, R. S., and M. Prévot, Evidence suggesting extremly rapid field variation during a geomagnetic reversal, Earth Planet. Sci. Lett., 92, , Coe, R. S., S. Grommé, and E. A. Mankinen, Geomagnetic paleointensities from radiocarbon-dated lava flows on Hawaii and the question of the Pacific nondipol low, J. Geophys. Res., 83, , Coe, R. S., S. Grommé, and E. A. Mankinen, Geomagnetic paleointenities from excursion sequences in lavas on Oahu, Hawaii, J. Geophys. Res., 89, , Coe, R., M. Prévot, and P. Camps, New evidence concerning impulsive field change during a geomagnetic reversal, Nature, 374, , Coe, R. S., L. Hongre, and G. A. Glatzmaier, An examination of simulated geomagnetic reversals from a palaeomagnetic perspective, Philos. Trans. R. Soc. London, 358, , Glatzmaier, G. A., R. S. Coe, L. Hongre, and P. H. Roberts, The role of the Earth s mantle in controlling the frequency of geomagnetic reversals, Nature, 401, , Goguitchaichvili, A. T., M. Prévot, and P. Camps, No evidence for strong fields during the R3 N3 Icelandic geomagnetic reversal, Earth Planet. Sci. Lett., 167, 15 34, Hoffman, K. A., Dipolar reversal states of the geomagnetic field and core mantle dynamics, Nature, 359, , Hoffman, K. A., Transitional paleomagnetic field behavior: Preferred paths or patches?, Surv. Geophys., 17, , Juárez, M. T., L. Tauxe, J. S. Gee, and T. Pick, The intensity of the Earth s magnetic field over the past 160 million years, Nature, 394, , Kosterov, A. A., and M. Prévot, Possible mechanisms causing failure of Thellier palaeointensity experiments in some basalts, Geophys. J. Int., 134, , Laj, C., A. Mazaud, R. Weeks, M. Fuller, and E. Herrero-Bevera, Geomagnetic reversal paths, Nature, 351, 447, Langereis, C., A. A. M. van Hoof, and P. Rochette, Longitudinal confinement of geomagnetic reversal paths as a possible sedimentary artefact, Nature, 358, , Leonhardt, R., F. Hufenbecher, F. Heider, and H. Soffel, High absolute paleointensity during a mid-miocene excursion of the Earth s magnetic field, Earth Planet. Sci. Lett., 184, , Leonhardt R., J. Matzka, F. Hufenbecher, H. C. Soffel, and F. Heider, A reversal of the Earth s magnetic field recorded in mid-miocene lava flows of Gran Canaria: Paleodirections, J. Geophys., 107(B1), 2024, doi: /2001jb000322, Lin, J. L., K. L. Verosub, and A. P. Roberts, Decay of the virtual dipole moment during polarity transitions and geomagnetic excursions, Geophys. Res. Lett., 21, , Mankinen, E. A., M. Prévot,S.Grommé,andR.S.Coe,TheSteens Mountain (Oregon) geomagnetic polarity transition, 1, Directional history, duration of episodes, and rock magnetism, J. Geophys. Res., 90, 10,393 10,416, McClelland, E., and J. C. Briden, An improved methodology for Thelliertype paleointensity determination in igneous rocks and its usefulness for verifying primary thermoremanence, J. Geophys. Res., 101, 21,995 22,013, McDougall, I., and H.-U. Schmincke, Geochronology of Gran Canaria, Canary Islands: Age of shield building volcanism and other magmatic phases, Bull. Volcanol., 40, 57 77, Merrill, R. T., and P. L. McFadden, Geomagnetic polarity transitions, Rev. Geophys., 37, , Prévot, M., E. A. Mankinen, R. S. Coe, and S. Grommé, The Steens Mountain (Oregon) geomagnetic polarity transition, 2, Field intensity variations and discussion of reversal models, J. Geophys. Res., 90, 10,417 10,448, Riisager, P., and N. Abrahamsen, Palaeointensity of West Greenland Palaeocene basalts: Asymmetric intensity around the C27n C26r transition, Phys. Earth Planet. Inter., 118, 53 64, Shaw, J., A new method of determining the magnitude of the palaeomagnetic field: Application to five historic lavas and five archaeological samples, Geophys. J. R. Astron. Soc., 39, , Shaw, J., Strong geomagnetic fields during a single polarity transition, Geophys. J. R. Astron. Soc., 40, , Thellier, E., and O. Thellier, Recherches géomagnétiques sur des coulées volcaniques d Auvergne, Ann. Geophys., 1, 37 52, Thellier, E., and O. Thellier, Sur l intensité du champ magnétique terrestre dans le passé historique et géologique, Ann. Geophys., 15, , Valet, J.-P., and L. Meynadier, Geomagnetic field intensity and reversals during the past four million years, Nature, 366, , Valet, J.-P., J. Brassat, X. Quidelleur, V. Soler, P.-Y. Gillot, and L. Hongre, Paleointensity variations across the last geomagnetic reversal at La Palma, Canary Islands, Spain, J. Geophys. Res., 104, , van den Bogaard, P., and H.-U. Schmincke, Chronostratigraphy of Gran Canaria, Proc. Ocean Drill. Program Sci. Results, 157, , R. Leonhardt and H. C. Soffel, Institut für Allgemeine und Angewandte Geophysik, Ludwig-Maximilians-Universität, Theresienstr. 41, Munich, Germany. (leon@geophysik.uni-muenchen.de)