A Ferk

R Leonhardt

The Laschamp geomagnetic field excursion recorded in Icelandic lavas

Keywords: Geomagnetism, Palaeomagnetism, Excursion, Laschamp, Iceland

We sampled 28 lava flows and a tephra layer, dated at about 40 kyr, at the Reykjanes Peninsula, Iceland. 10 flows and the tephra recorded what has been originally referred to as the Skalamaelifell geomagnetic field excursion. The age of this excursion (42.9±7.8 ka) is statistically indistinguishable from the Laschamp excursion (40.4±2.0 ka). Rock magnetic investigations show that the main remanence carriers are (titano-) magnetites with different degrees of oxidation. One excursional flow exhibits partial self-reversal behavior; however, it could be shown by continuous thermal demagnetization that its paleodirection is unaffected. We subjected 52 samples from 16 flows to Thellier-type paleointensity determinations. Reliable paleointensity data were obtained for 10 of the 29 sites. In the beginning of the excursion virtual geomagnetic poles (VGPs) in the Southeast Pacific are recorded. These sites are characterized by paleointensities of 4 to 5 µT, about 1/10 of the intensity of the normal polarity flows, which ranges from 27.4 µT to 59.3 µT. Towards the end of the excursion, VGPs are found in North Africa. At these sites paleointensity has already regained about half of its original value (19.9±2.4 µT).

The actual normal polarity chron (Brunhes) has lasted already 0.78 Myr, which is much longer than the average duration of any other polarity epoch during the last 5 Ma. In this interval, however, several global geomagnetic field excursions like Big Lost (0.6 Ma) and the Emperor (0.5 Ma) are present (e.g. [START_REF] Laj | Treatise in Geophysics[END_REF]. Excursions are defined as short-duration directional changes, which exceed normal secular variation. They may either denote anomalous high secular variation or reflect an aborted reversal [START_REF] Lund | A summary of Brunhes paleomagnetic field variability recorded in Ocean Drilling Program cores[END_REF]Valet et al., 2008). Further excursions have been reported for the last 0.1 Ma, e.g. the Mono Lake excursion (≈32,4 ka) [START_REF] Benson | Age of the Mono Lake excursion and associated tephra[END_REF] and the Laschamp excursion (≈40 ka) [START_REF] Bonhommet | Sur la présence d'aimentations inversées dans la Chaine des Puys[END_REF]. A detailed Email address: ferk@geophysik.uni-muenchen.de (A. Ferk).

2

A c c e p t e d M a n u s c r i p t paleomagnetic investigation is essential to enlarge our knowledge of the geomagnetic field generation during such disturbances. It has been suggested that the Earth's inner core is responsible for the difference between excursions and reversals [START_REF] Gubbins | The distinction between geomagnetic excursions and reversals[END_REF]. One possible process could be that during excursions the field reverses only in the outer core (overturn time 500 years) and then switches back. During reversals (timescales of about 3000 years) the inner core field reverses as well. Comparing paleomagnetic results from different global locations could shed light on the global morphology of the geomagnetic field during such disturbances [START_REF] Leonhardt | 28 solute paleointensity determinations: Acceptance criteria and the software ThellierTool4.0[END_REF]. The majority of excursions have so far been recorded in sediments. However, 40 years ago [START_REF] Bonhommet | Sur la présence d'aimentations inversées dans la Chaine des Puys[END_REF] found evidence for an excursion in Quaternary lava flows of the French Massif Central: The Laschamp event. It was later estimated to have occurred about 40 ka ago [START_REF] Guillou | On the age of the Laschamp geomagnetic excursion[END_REF]. The same excursion was reported in marine sediments at e.g. Bermuda Rise and Blake Outer Ridge [START_REF] Lund | Deep-sea sediment records of the Laschamp geomagnetic field excursion (similar to 41,000 calendar years before present)[END_REF], the Gulf of California [START_REF] Levi | A sixty thousand year paleomagnetic record from Gulf of California sediments: secular variation, late Quaternary excursions and geomagnetic implications[END_REF] and in the North Atlantic [START_REF] Channell | Late Brunhes polarity excursions (Mono Lake, Laschamp, Iceland Basin and Pringle Falls) recorded at ODP Site 919 (Irminger Basin)[END_REF]. In 1980, [START_REF] Kristjánsson | Geomagnetic excursion in lateglacial basalt outcrops in South-Western Iceland[END_REF] observed an excursion, the Skalamaelifell excursion, in three hills on the Reykjanes Peninsula, Iceland. At 95% confidence level this excursions age (42.9±7.8 ka) [START_REF] Levi | Late Pleistocene geomagnetic excursion in Icelandic lavs: confirmation of the Laschamp excursion[END_REF] is indistinguishable from the one of the Laschamp excursion (40.4±2.0 ka) [START_REF] Guillou | On the age of the Laschamp geomagnetic excursion[END_REF]. Age determinations were done by [START_REF] Levi | Late Pleistocene geomagnetic excursion in Icelandic lavs: confirmation of the Laschamp excursion[END_REF] on 19 icelandic samples of which 6 are from sites that correspond to our sites at Bleikshóll. [START_REF] Levi | Late Pleistocene geomagnetic excursion in Icelandic lavs: confirmation of the Laschamp excursion[END_REF], as well as [START_REF] Kristjánsson | Paleomagnetic observation on Late Quaternary basalts around Reykjavik and on the Reykjanes peninsula, SW-Iceland[END_REF], also performed further field work, and some studies were done on paleointensities [START_REF] Levi | Late Pleistocene geomagnetic excursion in Icelandic lavs: confirmation of the Laschamp excursion[END_REF][START_REF] Marshall | Preliminary paleointensity measurements and detailed magnetic analysis of basalts from the Skalamaelifell excursion, southwest Iceland[END_REF], showing that throughout the excursion the intensity was just about 1/10 of the intensity of normal polarity sites. As some of the former studies at Skalamaelifell were mainly designed to look for excursional lava flows, just one or two samples A c c e p t e d M a n u s c r i p t were taken at each site and often the specimens were not fully demagnetized, but retained still 90 to 95% of the natural remanent magnetization (NRM), when the paleodirections were determined. A further aspect is that so far, no stratigraphy could be established as the sampled volcanic structures are isolated and their products do seldom overlap. Examining different marine sedimentary records of the Laschamp excursion, [START_REF] Laj | Geomagnetic field behavior during the Iceland Basin and Laschamp geomagnetic excursions: A simple transitional field geometry?[END_REF] found strikingly similar VGP paths with clockwise loops. They suggested that the simple structure of the paths indicates a simple geometry of the intermediate field and concluded that the field is dominated by a dipolar field. Thus, due to low intensities, both dipolar and non-dipolar fields must have been reduced.

Recently obtained southern hemipshere paleomagnetic data for the Laschamp excursion is not consistent with such simple loops [START_REF] Cassata | Laschamp and Mono Lake geomagnetic excursions recorded in New Zealand[END_REF] and a global field model of the Laschamp excursion, which utilizes the here presented data, also indicates a dominance of non-dipolar components for a brief time interval [START_REF] Leonhardt | 28 solute paleointensity determinations: Acceptance criteria and the software ThellierTool4.0[END_REF]. In order to either support or reject these differing ideas, further data on paleointensities and -directions are needed. In particular, accurate studies of igneous records are required to compare them to the results of the marine sediments.

For this study some of the volcanic structures in the area were sampled and measurements of paleodirection, rock magnetic parameters and paleointensity were carried out. For one sampled volcanic structure previous full vector paleomagnetic data existed, which could be confirmed by this study. Our results are compared with other excursion data of Laschamp and different sedimentary records, and combined in a stratigraphic correlation of the different sites.

Finally, implications for the geomagnetic field are discussed.

A c c e p t e d M a n u s c r i p t 2 Geological setting and sampling Samples were taken on Reykjanes Peninsula in Southwest Iceland. The Reykjanes Peninsula, which is the onshore continuation of the Mid-Atlantic Ridge (Reykjanes Ridge), includes five volcanic systems/fissure swarms: Reykjanes, Grindavik, Krysuvik, Blafjoll and the easternmost Hengill system, which lies at a triple junction between Reykjanes Peninsula, Western Rift Zone and South Iceland Fracture zone. The Peninsula is an arid plain with a median altitude of 200 m in the east, 50 m in the west, and numerous volcanic structures that rise above the plain. Most of the lava flows and volcanic detritus are quite recent, but some of the volcanic structures erupted during the last glacial period. The products of the subglacial eruptions of these older hills are hyaloclastites, breccia and pillows as well as some lava flows [START_REF] Levi | Late Pleistocene geomagnetic excursion in Icelandic lavs: confirmation of the Laschamp excursion[END_REF]. Some of these older volcanic structures were sampled in an area northeast of Grindavik. Fig. 1 shows a map of the sampling area (sampling locations are marked by abbreviations). As mentioned before, [START_REF] Kristjánsson | Geomagnetic excursion in lateglacial basalt outcrops in South-Western Iceland[END_REF] detected shallow negative inclinations and westerly declinations on three different hills on Reykjanes Peninsula. Later, [START_REF] Levi | Late Pleistocene geomagnetic excursion in Icelandic lavs: confirmation of the Laschamp excursion[END_REF] also found other flows with intermediate directions. For this study, suitable locations with excursional rocks were chosen by examining the work of [START_REF] Kristjánsson | Paleomagnetic observation on Late Quaternary basalts around Reykjavik and on the Reykjanes peninsula, SW-Iceland[END_REF] and [START_REF] Levi | Late Pleistocene geomagnetic excursion in Icelandic lavs: confirmation of the Laschamp excursion[END_REF], and by oral communication with Kristjánsson in 2005. Altogether 181 samples were taken from 28 different lava flows and one tephra layer. Whenever possible six or more samples were taken at each flow and were oriented using both magnetic and sun compass, which were later found to show perfect accordance. For later comparison with previous works, it is important to mention, that [START_REF] Kristjánsson | Paleomagnetic observation on Late Quaternary basalts around Reykjavik and on the Reykjanes peninsula, SW-Iceland[END_REF] 1990) used a different map, in which the hills are sometimes labeled differently. The Siglubergsháls on their map is north of the road, and thus, belongs to the area around Bleikshóll on the map used for this study. These sites were also sampled by [START_REF] Marshall | Preliminary paleointensity measurements and detailed magnetic analysis of basalts from the Skalamaelifell excursion, southwest Iceland[END_REF]. The only further overlap of our sampling area with those of previous full-vector studies is at Festarfjall and Siglubergsháls on our map, where [START_REF] Levi | Late Pleistocene geomagnetic excursion in Icelandic lavs: confirmation of the Laschamp excursion[END_REF] took some samples as well. The other sites of [START_REF] Marshall | Preliminary paleointensity measurements and detailed magnetic analysis of basalts from the Skalamaelifell excursion, southwest Iceland[END_REF] and [START_REF] Levi | Late Pleistocene geomagnetic excursion in Icelandic lavs: confirmation of the Laschamp excursion[END_REF] lie east of our sampling area. Further, for future sampling in the area, it is important to note that some years ago a new road, north of the old one was built, which is not on the map. However, the old one is there still and orientation in the area should therefore be quite easy. The drilled flows consist mostly of grey, olivine rich basalts with many vesicles. Occasionally, feldspar phenocrysts are seen.

The flows at Fagradalsfjall (FAG) are very thin, about 20 cm to 50 cm. Those at Nátthagakriki (N), Einbúi (E), Siglubergsháls (SI) and Festarfjall (FS) are about 1 to 2 m, sometimes even 3 m. At the base (B) we sampled the side wall of a lava tube. The roughly 50 cm thick tephra (TE) is baked in some parts, and has some lapilli layers inside. Identification of stratigraphic relationships between the different volcanic structures is very difficult as they are heterogeneous and isolated. Only the flank of Nátthagakriki and Fagradalsfjall are right next to each other. Field observation indicate that lavas from Fagradalsfjall overflowed onto part of the flank, suggesting that the flank is older than Fagradalsfjall. Our interpretation is that the base is the oldest section and that the various hills rose through it. For additional correlations, further information on paleodirections and paleointensities will be used later in this study.

Whenever there was more than one flow at each location, they were numbered from bottom to top. It is important to emphasize, that sometimes the different flows on one hill are far away from each other. In that case, they have Although not all flows are consecutive, their stratigraphic relationship is clear with FAG1 being the lowest and FAG13 the highest. At Nátthagakriki, the sampled flows are spatially apart. Only N3 is on top of N2 and N5 on top of N4. However, it is not clear, if for example N1 is the same flow as N6. Using similarities of paleomagnetic mean directions and intensities, some sites will be treated as contemporaneous later.

Dating

The age of the Skalamaelifell excursion according to [START_REF] Levi | Late Pleistocene geomagnetic excursion in Icelandic lavs: confirmation of the Laschamp excursion[END_REF] is 42.9±7.8 ka. It was projected to improve this, to eventually also obtain constraints on stratigraphy and on the length of the excursion. Based on macroscopic observations and K 2 O contents, two samples, from BL and FAG, were selected for age determinations, as these flows were probably extruded before and after, respectively, the excursion (section 7). The technique used in this study is the unspiked K-Ar technique described in [START_REF] Charbit | Cross calibration of K-Ar standard minerals using an unspiked Ar measurement technique[END_REF] and used to date other Laschamp samples from the type locality [START_REF] Guillou | On the age of the Laschamp geomagnetic excursion[END_REF]. Unfortunately, all attempts to obtain reliable K-Ar ages failed. This is related both to the very young age and to the very low K contents in the samples, 0.191 and 0.183%, respectively in FAG and BL. As a consequence, the amount of radiogenic 40 Ar, in these samples, is below the limit of detection, which was previously calculated to be 0.15% [START_REF] Guillou | On the age of the Laschamp geomagnetic excursion[END_REF]. Rock magnetic measurements aimed to identify the magnetic mineralogy and domain state. Isothermal remanent magnetization (IRM) acquisition, isothermal backfield curves at room temperature, hysteresis loops, and thermomagnetic curves (T max = 600 • C) were measured in that order. All measurements were conducted using a Variable Field Translation Balance (VFTB) and analyzed using the RockMag Analyzer software by [START_REF] Leonhardt | Analyzing rock magnetic measurements: The RockMag-Analyzer 1.0 software[END_REF].

All basalts have similar rock magnetic properties. The tephra showed, however, a very different behavior. Thus, in the following it will be considered separate from the lavas. All (normalized) IRM acquisition curves of the basalts are similar. At fields between 100 and 200 mT, 90% of the saturation remanence M rs is reached and in almost all cases the remanence is already saturated below 300 mT. This shows that most of the magnetization is carried by low coercivity minerals like (titano-)magnetite. The same similiarity is observed for the normalized backfield curves, though values for the coercivity of remanence B cr lie between 9.6 and 46 mT. The S 300 -parameters [START_REF] Bloemendal | Rock magnetism of late Neogene and Pleistocene deep-sea sediments: Relationship of sediment source, diagenetic processes and sediment lithology[END_REF] are close to 1 (between 0.96 and 0.99; average 0.98) for all basaltic samples, which again reflects that the magnetization is carried by low coercive minerals. Hysteresis parameters plotted in a Day plot [START_REF] Day | Hysteresis properties of titanomagnetites: Grain size and composition dependence[END_REF], with domain state related boundaries according to [START_REF] Dunlop | Theory and application of the Day plot (M rs /M s versus H cr /H c ) 1. Theoretical curves and tests using titanomagnetite data[END_REF], indicate a predominat distribution along the SD-MD mixing curve.

Examining the thermomagnetic curves lead to four groups with different characteristic Curie temperatures (Fig. 2), which were determined using the second derivative method [START_REF] Tauxe | Paleomagnetic principles and practice[END_REF]. This method relies on the identification of 8 A c c e p t e d M a n u s c r i p t maximum concave curvature within the measured data. The temperature at which this maximum occurred was used as an approximation for the Curie temperature (T C ). For ore microscopy (reflected light), two specimens of each thermomagnetic group were chosen (one of group 2a and one of 2b) and a Leitz Orthoplan pol microscope with a Nikon DXM 1200 digital camera, and air as well as immersion oil objectives with different magnification (20x and 50x) were used. To emphasize strong magnetic regions samples were covered with Ferrofluid. The specimens from the different thermomagnetic types also have particular appearances. The samples of group 1 have a single Curie temperature between 80 and 170 • C at the heating cycle and one at about 580 • C at the cooling curve (Fig. 2a). The low Curie temperature, and the fact, that only a few exsolution lamellae can be seen, suggest that these samples were not, or only to a small extent, oxidized. Additionally, there are also optical anisotropic grains, indicating the presence of ilmenites. The fact, that the cooling curve is much higher than the heating curve, shows that this titanomagnetite (e.g. TM60) was oxidized during heating in the laboratory. After heating it contains Ti-poor titanomagnetites or even magnetite. The second group has two Curie temperatures in the heating cycle, one at about 160 to 250 • C and one between 500 and 580 • C. This group splits again in two subgroups: The cooling curve of five samples is significantly higher than the heating curve (group 2a; Fig. 2b). The cooling curves of the five samples belonging to group 2b are close to the heating curves (Fig. 2c). The grains of FAG13-7C (group 2a) are very small (25 to 50 µm) and often have skeleton shape. FS2-6D (group 2b) contains many skeleton formed grains, but additionally also large ones (up to 130 µm). Some of the grains in both samples show exsolution lamellae of magnetite and ilmenite. The presence of grains with and without lamellae explains the existence of two different Curie temperatures. The different cooling A c c e p t e d M a n u s c r i p t curves of group 2a and 2b, and the easier high temperature oxidation in group 2a compared to group 2b, may be due to the smaller particles in group 2a and their higher surface/volume ratio. The two samples of group 3 have three Curie temperatures at the heating curve and only one at the cooling curve (Fig. 2d). Ore microscopy from group 3 specimens yielded different results: FAG5-2D shows many skeleton structures, while another sample, N5-5E, has mainly large ore minerals. In both cases some of the minerals show exsolution lamellae and some do not. Like in the case of group 2, this characteristic accounts for at least two of the three Curie temperatures. The third Curie temperature might be related to maghemitization or to only partial oxidation of some grains. However, shrinking cracks or other traces of maghemitization, could not be identified. Furthermore, the pristine olivine found in most samples indicates that no secondary alteration took place. Therefore, it is concluded that partial oxidation is responsible fot the third Curie temperature in this group. Only a single Curie temperature at about 500 to 580 • C and almost reversible heating/cooling cycles are typical for the samples of group 4 (Fig. 2e).

Almost all grains of specimens from Group 4 have exsolution lamellae. The grain sizes strongly vary from about 20 µm to about 150 µm. In all cases the grains again have the typical oktahedral angles, and skeletons are common.

Additionally ilmenite is present in both samples. The high Curie temperature is due to the highly oxidized grains.

In all experiments, the tephra behaves completely different than the basalts.

IRM and backfield measurements indicate the presence of a high coercive fraction (S 300 = 0.88), but as the remanence reaches saturation at about 500 mT, hematite or goethite cannot be the remanence carriers. The hysteresis loop of TE1-7D shows that the tephra has only a very small content of ferro(i)magnetic particles and is dominated by paramagnetism. Stepwise ther- This behavior could be explained by inversion of maghemite to magnetite, which is a quasi-continuous process above 300 • C [START_REF] Krása | Inversion of titanomaghemite in oceanic basalt during heating[END_REF].

The transformation of the iron sulphide pyrhotite to magnetite would show similar effects, but its inversion starts not until temperatures above 500 • C [START_REF] Bina | Mineralogical change and self-reversed magnetizations in pyrrhotite resulting from partial oxidation; geophysical implications[END_REF]. Thus, the inversion of maghemite to magnetite is considered more probable. Nevertheless, due to the good fitting and stable paleodirectional data (subsection 7.1), it is very likely that the remanence was acquired during the excursion. C in up to 17 heating steps. In addition to the NRM measurement, the susceptibility was measured after each heating step with a KLT-3 Minikappa bridge to detect changes of the mineral character. For AF demagnetization, a 2G Enterprises degausser system control was used and up to 13 steps from 0 to 200 mT were carried out to demagnetize the specimens.

Paleodirections for the 178 specimens were calculated using principle component analysis (PCA; Tab. 1). In almost all cases the directions are very well defined by a best-fit line through more than five points (Fig. 3). If the maximum angular deviation (MAD) was more than 5 • , the data was not further used. A viscous component (normal field direction) was usually removed at the 100 • C to 150 • C TH-step or the 10 mT AF-step. Only at some sites a secondary component beside this viscous component was observed. The site mean directions were determined using Fisher statistics [START_REF] Fisher | Dispersion on a sphere[END_REF]. The directional independence of flows, that are close to each other or which likely belong to one eruption, is tested using F-Distribution tests (e.g. at Nátthagakriki). If two sites are not different at a 95% significance level, it is assumed that they have recorded the same field and represent a single record. Therefore, they are combined and compared with the next flow following these. Using this technique repeatedly, groupings of sites are developed assuming that flows with similar directions are identical or contemporaneous (Tab. 1). When available, paleointensity results are considered for the similarity analysis (Tab. 2: FAG11 and FAG13).

All specimens from Fagradallsfjall show normal polarity and have somewhat right-handed declinations (e.g. Fig. 3a). A similar characteristic was observed by [START_REF] Kristjánsson | Paleomagnetic observation on Late Quaternary basalts around Reykjavik and on the Reykjanes peninsula, SW-Iceland[END_REF]. However, as all lavas were likely extruded in a very short time interval, they do not represent sufficient time to average out secular A c c e p t e d M a n u s c r i p t variation and it is not clear if this right-handedness is at all due to long-term behavior of the geomagnetic field in the Brunhes chron [START_REF] Levi | Late Pleistocene geomagnetic excursion in Icelandic lavs: confirmation of the Laschamp excursion[END_REF].

The sites at Festarfjall and Siglubergsháls (SI1, SI2, FS1) also show normal polarity, but with a strongly right-handed declination of 68.3 • and inclination of 79.2 • (Fig. 3e). FS2 even has a declination of 86.1 • (I = 75.7 • ). These values correspond to those that [START_REF] Kristjánsson | Paleomagnetic observation on Late Quaternary basalts around Reykjavik and on the Reykjanes peninsula, SW-Iceland[END_REF] found at Borgarfjall (D = 85 • , I = 76 • ). The mean direction of the base is between those of Fagradallsfjall, Festarfjall and Siglubergsháls, with a declination of 41.3 • . The tephra has a declination of 317 • and an inclination of 56.6 • (Fig. 3b). As will be explained later (subsection 7.1), the onset of the excursion is represented by this paleodirection.

Excursional directions, defined by virtual geomagnetic pole (VGP) latitudes more than 41.3 • away from the geographic pole [START_REF] Mcelhinny | Palaeosecular variation over the past 5 Myr based on a new generalized database[END_REF], were found in ten flows. The sampled flow at Einbúi exhibits a declination of 280.1 • and an inclination of 13.8 • (Fig. 3c,d). At this site many specimens had to be analyzed using great circles (Fig. 3d). The previously reported excursional values (D = 263.3 • , I = -22.1 • , k = 151.8, α 95 = 3.1 • , [START_REF] Kristjánsson | Paleomagnetic observation on Late Quaternary basalts around Reykjavik and on the Reykjanes peninsula, SW-Iceland[END_REF][START_REF] Levi | Late Pleistocene geomagnetic excursion in Icelandic lavs: confirmation of the Laschamp excursion[END_REF])) are almost identically observed at Bleikshóll (Fig. 3f). The specimens contain a relatively strong viscous overprint up to ca. 300 • C respectively 100 mT related to the weak NRM acquired during the excursional low-field state. The directions of the flows on Nátthagakriki fit some sites, whose geographical location [START_REF] Kristjánsson | Paleomagnetic observation on Late Quaternary basalts around Reykjavik and on the Reykjanes peninsula, SW-Iceland[END_REF] Directions that deviate from the normal/inverse polarity state are not necessarily due to reversals or excursions of the geomagnetic field, but may also be due to (partial) self-reversal as has been suggested by [START_REF] Néel | L'inversion de l'aimantation permanente des roches[END_REF], who presented different theoretical mechanisms that could lead to this phenomenon.

As mentioned before, [START_REF] Bonhommet | Sur la présence d'aimentations inversées dans la Chaine des Puys[END_REF] found intermediate paleomagnetic directions in samples from Laschamp and Olby. However, [START_REF] Heller | Self-reversal of natural remanent magnetization in the Olby Laschamp lavas[END_REF], [START_REF] Heller | Self-reversal explanation for the Laschamp/Olby geomagnetic field excursion[END_REF] and [START_REF] Krása | Self-reversal of remanent magnetization in basalts due to partially oxidized titanomagnetites[END_REF] demonstrated that some remanence carriers in samples from Olby exhibit complete or partial self-reversal. Hence, samples from the excursional and normal polarity sites at Iceland were tested for this phenomenon. The samples were continuously thermally demagnetized in up to five subsequent measurement steps with increasing temperature in an high-temperature spinner magnetometer (HOT-SPIN) [START_REF] Matzka | Besondere magnetische Eigenschaften der Ozeanbasalte im 29[END_REF] at the Niederlippach paleomagnetic laboratory. This method allows for monitoring reversibility of heating/cooling cycles. In addition to NRM(T) intensity plots (Fig. 4a,c), directional plots of the core coordinates x and z, where the start of the respective heating cycles is labeled, can be drawn (Fig. 4b,d). FAG11-4F is a typical non self-reversing specimen. There is no maximum in the NRM(T) intensity plot (Fig. 4a) and, besides a small viscous overprint, only one linear directional trend to the origin can be observed (Fig. 4b). (Partial) self-reversing samples show a reversible peak at the same temperature of the HOTSPIN heating and cooling cycles (Fig. 4c). If self-reversal behavior also affects the paleodirection, two different directional trends are observed (one that could be observed also at stepwise demagnetization, i.e. measuring at 20 • C, and another one for the peak in the NRM(T) plot). It was found, that none of the excursional sites from Bleikshóll and Einbúi shows self-reversal. At Nátthagakriki only N6-4D shows partial self-reversal behavior, but it still has a stable direction with only one linear A c c e p t e d M a n u s c r i p t trend to the origin (Fig. 4c,d). Additionally, both samples from the Siglubergsháls flows and also specimen FS2-6C exhibit partial self-reversal with, however, again, stable directions. All specimens showing partial self-reversal are of thermomagnetic group 2b. The decrease of M r in the characteristic manner of partial self-reversal is not observed in the respective M s (T ) curves, and thus is not controlled by the temperature dependence of saturation magnetization. As the effect occurs during heating and cooling, it is probably caused by the blocking of the thermal remanent magnetization (TRM). The data for the different samples show that the remanences of the two carriers are antiparallel, i.e., that the angular difference is 180 • . Hence, the remanence of the low-temperature component is coupled to the high-temperature component.

It has been observed that the temperature of the maximum in the NRM(T) plot is always below the respective first Curie temperature of the sample; thus, it is probable that the low-temperature reversed remanence is carried by the magnetic phase with the lower T c , which is the primary unoxidized titanomagnetite. Both, the mostly non self-reversing samples and the stable direction of N6-4D (Fig. 4c,d), which exhibits partial self-reversal, suggest that the Skalamaelifell excursion is a real feature of the geomagnetic field.

Paleointensities

All paleointensity determinations were conducted in a MMTD20 thermal demagnetizer. Specimens for the Thellier experiments [START_REF] Thellier | Sur l'intensité du champ magnétique terrestre dans le passé historique et géologique[END_REF] were chosen due to high coercivity (AF demagnetization: medium destructive field > 20 mT) and small changes of susceptibility at thermal demagnetizations. Laboratory fields of 30 µT with a field accuracy of 0.1 µT A c c e p t e d M a n u s c r i p t

were used for all measurements and applied during heating and cooling. Intensity determinations were performed with the modified Thellier-technique MT4 (Leonhardt et al., 2004a), which is a zero-field first method that includes the commonly used pTRM* (partial thermoremanence) check [START_REF] Coe | Palaeointensity of the Earth's magnetic field determined from tertiary and quaternary rocks[END_REF], additivity checks [START_REF] Krása | Experimental iments[END_REF], and pTRM*-tail checks [START_REF] Riisager | Detecting multidomain magnetic grains in Thellier paleointensity experiments[END_REF]. For Thellier-type experiments pTRM acquisition is different to the traditional pTRM definition, as the maximum temperature at which the laboratory field is applied is not reached from T C . Therefore, pTRM* is used as an abbreviation. Directional differences between the applied field and the NRM of the pTRM*-tail check are taken into account according to Leonhardt et al. (2004b). The pTRM* checks were conducted "in-field" after the demagnetization step with a laboratory field applied during heating and cooling. All determinations were analyzed using the ThellierTool4.11 software (Leonhardt et al., 2004a). This software allows full vector analysis as well as the application of a check correction [START_REF] Valet | Absolute paleointensity and magnetomineralogical changes[END_REF][START_REF] Leonhardt | Absolute paleointensities and paleodirections from Fernando de Noronha, Brazil[END_REF] and provides a default set of acceptance criteria, which is as follows: For the linear fit at least 5 subsequent data points have to be used, comprising at least a 30% fraction of the NRM (f), its standard deviation has to be less than 0.15 and the MAD less than 15. Only directions with α 95 smaller than 15 are accepted. As no alteration should occur, pTRM checks have to be within 7% of the original TRM value (δ(CK)) and the cumulative check difference (δ(pal)) has to be less than 10%. Further only relative errors of the additivity checks (δ(AC)) up to 10% are accepted, while the relative intensity difference between the remaining NRM and the respective pTRM*tail-check (δ(T R)) has to be within 15% and the true pTRM*-tail (δ(t * )) below 5%. Additionally paleointensity determinations that failed some of the acceptance criteria, but matched other data from the same site which passed the criteria, were used.

Page 17 of 42 A c c e p t e d M a n u s c r i p t For some specimen check correction was applied to analyze the data, because of deviating pTRM-checks, which are probably connected with formation of new magnetic particles. By using the cumulative pTRM check difference the contribution of this new phase could be subtracted. After check correction additivity checks fall on the corresponding pTRM value suggesting that TRM properties of the original magnetic content are preserved and check correction is successful. To exclude biasing effects, check corrected data were compared with other data from the same site. Samples from FAG13 could be analyzed only using check corrected plots. However, the two results are consistent and for the check corrected Arai plot of FAG13-6E, it was possible to analyze the whole temperature interval. Thus, the results are considered reliable. The Thellier experiments of most samples from BL2, FAG7, FAG8, FAG12, N1, N2, N5, SI1 and SI2 yielded no results due to uncorrectable alteration and MD effects. The paleointensity results of each site were averaged and also a standard deviation was calculated, using the quality factor q [START_REF] Coe | Geomagnetic paleointensities from radiocarbon-dated lava flows on Hawaii and the question of the Pacific nondipol low[END_REF] for weighting. Furthermore, a mean paleointensity and a standard deviation (arithmetic mean of different sites) were determined for the directional groups.

All results and the different quality parameters are listed in Tab. 2.

To check for a possible influence of magnetic anisotropy, anisotropy of the magnetic susceptibility was measured using a KLY-2 Kappa bridge, indicating a negligible effect as the overall values for the anisotropy factor P < 1.01 are small. The normal sites yield paleointensities similar to the present-day field value of Iceland, which is 52.3 µT. The base (B1) recorded a weighted mean paleointensity of 59.3±4.1 µT (Fig. 5a). FAG11 shows a weighted mean intensity of 47.8±1.1 µT. This paleointensity determinaton is mainly based on check corrected results, which, however, are absolutely consistent with the The intensities of the excursional sites BL1 and BL3 (Fig. 5b), 5.2±0.2 µT and 4.0±0.4 µT respectively, are only about 1/10 the intensity of the normal sites. The data within these sites are were well defined and match each other.

The flows at Nátthagakriki have also a low paleointensity, which is only about 1/3 of the intensity prior to the excursion, as can be seen by the mean value of the VGP group (N1, N2, N6): 19.9±2.4 µT. Taken together, specimens from unit N1 and N2 yielded just two reliable results. The weighted mean intensity of N6, 18.4±0.4 µT, is the only one that is defined by four specimens (Fig. 5c), but the results of the two specimens at the sites N1 and N2 are consistent with the data at N6. Paleointensity of these flows is significantly higher than the one recorded by the flows at Bleikshóll, but still less than half the paleointensity at sites with normal directions.

Discussion

It has been outlined before, that the paleodirectional data is very consistent and reliable. In almost all cases the PCA-analysis resulted in a small MAD, and the comparison of sun and magnetic compass yielded no difference. After removal of minor VRMs most site mean directions are very well defined with α 95 < 10 • and k > 100. Although the remanence carriers of the tephra could not be unambiguously identified, the direction is still considered reliable due to its good agreement with other data (subsection 7.1). Self-reversal experi-A c c e p t e d M a n u s c r i p t ments led to the conclusion that the observed excursion is not due to partial self-reversal, but a real feature of the geomagnetic field. Only one excursional specimen exhibits partial self-reversal, and continuous thermal demagnetization shows that its direction is not affected. The paleointensity data, tested with various checks, meet strict classification criteria, and show consistent within-site results underlining their high quality.

Comparison with igneous sections and marine records

Fig. 6a shows the Skalamaelifell geomagnetic field excursion VGPs together with those found by previous studies on Iceland and at Laschamp. It is obvious that the Bleikshóll data fits the results of [START_REF] Kristjánsson | Paleomagnetic observation on Late Quaternary basalts around Reykjavik and on the Reykjanes peninsula, SW-Iceland[END_REF] and [START_REF] Levi | Late Pleistocene geomagnetic excursion in Icelandic lavs: confirmation of the Laschamp excursion[END_REF] perfectly. The VGPs of the flows at Laschamp and Olby lay further south at latitudes of -46 • and -56 • [START_REF] Roperch | Paleointensity of the Earth's magnetic field during the Laschamp excursion and its geomagnetic implications[END_REF]. However, this difference has already been recognized by [START_REF] Kristjánsson | Paleomagnetic observation on Late Quaternary basalts around Reykjavik and on the Reykjanes peninsula, SW-Iceland[END_REF] and [START_REF] Levi | Late Pleistocene geomagnetic excursion in Icelandic lavs: confirmation of the Laschamp excursion[END_REF], who suggested that it might be due to non-dipole contributions to the geomagnetic field, small age differences, or local/regional crustal magnetic anomalies. The VGP of the Louchadière lava flow as found by [START_REF] Chauvin | Paleointensity of the Earth's magnetic field and K-Ar dating of the Louchadière volcanic flow (Central France). New evidence for the Laschamp excursion[END_REF] is closer to the VGPs of Nátthagakriki. As mentioned before, the stratigraphy of the Reykjanes area is quite difficult. Only a few sites and directional groups, whose flows are consecutive, can be connected a priori (Fig. 6a).

Comparing the VGPs with sedimentary records helps to establish a chronology between some of the other flows. First, the VGPs are plotted together with data from the ODP (Ocean drilling program) site 919 in the Irminger Basin (62.67 • N, 37.46 • W) [START_REF] Channell | Late Brunhes polarity excursions (Mono Lake, Laschamp, Iceland Basin and Pringle Falls) recorded at ODP Site 919 (Irminger Basin)[END_REF] (Fig. 6b). This site was chosen due to its vicinity to Iceland. A new and even closer record (PS2644-5) shows a very A c c e p t e d M a n u s c r i p t similar VGP path [START_REF] Laj | Geomagnetic field behavior during the Iceland Basin and Laschamp geomagnetic excursions: A simple transitional field geometry?[END_REF] (Fig. 6c). The tephra and Einbúi VGPs plot almost perfectly on the southward VGP path of the ODP 919 record along the eastern coast of America, and the data from Bleikshóll fits in well, too (south-east Pacific). The small deflection may be due to the geographical distance between ODP site 919 and the sampled sites. [START_REF] Laj | Geomagnetic field behavior during the Iceland Basin and Laschamp geomagnetic excursions: A simple transitional field geometry?[END_REF] studied some new records of the Laschamp excursion and compared them also to other records by [START_REF] Lund | Deep-sea sediment records of the Laschamp geomagnetic field excursion (similar to 41,000 calendar years before present)[END_REF]. The records show clockwise loops, which can also be seen at ODP site 919. Many VGP paths go south over the Pacific, then west to Africa, and back north over Europe (Fig. 6c). Thus, one might suggest that the VGPs in North Africa (Nátthagakriki) follow those in the Eastern Pacific (Bleikshóll). As mentioned before, field observations suggest that the base (B1) is the oldest part of the area while younger flows of Fagradallsfjall overlie the older ones of Nátthagakriki. The relationship of the flows from Siglubergsháls and Festarfjall to the other flows, however, cannot be solved with the available paleodirectional and -intensity data. The VGP loops of [START_REF] Laj | Geomagnetic field behavior during the Iceland Basin and Laschamp geomagnetic excursions: A simple transitional field geometry?[END_REF], as well as the paleointensity values between Nátthagakriki and Fagradallsfjall, might suggest that the flows at Siglubergsháls and Festarfjall follow those at Nátthagakriki. However, more data is needed to prove this suggestion. Therefore, the sites FS1, FS2, SI1 and SI2 are not included in the following stratigraphy. Fig. 6d shows the suggested VGP path. The time span between the individual flows is unclear due to the sporadic nature of volcanic eruptions. There might have been changes of the geomagnetic field between different events that are not recorded and for example the base could also be much older. Nevertheless, it is at least likely that the excursion units represent only a few hundred years. [START_REF] Levi | Late Pleistocene geomagnetic excursion in Icelandic lavs: confirmation of the Laschamp excursion[END_REF] suggested that their observed excursional lavas, which all show transitional VGPs close to South America, were extruded during a short time interval, due to their almost identical paleo-20 A c c e p t e d M a n u s c r i p t magnetic directions and their similar chemical and petrologic features. As the excursion units are mostly elevated in the order of 100 m, which has been observed to occur in southwestern Iceland in less than 100 years, [START_REF] Levi | Late Pleistocene geomagnetic excursion in Icelandic lavs: confirmation of the Laschamp excursion[END_REF] further suggested that the lavas were extruded in no more than several hundred years. This opinion is also supported by [START_REF] Kristjánsson | Paleomagnetic observation on Late Quaternary basalts around Reykjavik and on the Reykjanes peninsula, SW-Iceland[END_REF].

The previous intermediate paleointensity values [START_REF] Marshall | Preliminary paleointensity measurements and detailed magnetic analysis of basalts from the Skalamaelifell excursion, southwest Iceland[END_REF] and [START_REF] Levi | Late Pleistocene geomagnetic excursion in Icelandic lavs: confirmation of the Laschamp excursion[END_REF] are quite similar to the paleointensities at Bleikshóll. [START_REF] Marshall | Preliminary paleointensity measurements and detailed magnetic analysis of basalts from the Skalamaelifell excursion, southwest Iceland[END_REF], who could analyze only six excursional samples, of which four yielded reliable results, found paleointensities in a range from 3.4 to 4.6 µT (mean 4.3±0.6 µT). From their five reliable results of normal samples, they got a wider range between 22 µT and 47 µT (mean 30 µT). [START_REF] Levi | Late Pleistocene geomagnetic excursion in Icelandic lavs: confirmation of the Laschamp excursion[END_REF] selected eight excursion specimens for paleointensity studies, which yielded a weighted mean paleointensity of 4.2±0.2 µT. They determined no normal paleointensity values. Our paleointensity at BL3 (4.0±0.4 µT) is statistically indistinguishable from the earlier data and thus confirms the former results, while the one at BL1 (5.2±0.2 µT) is slightly higher, probably due to a slight difference in age. The paleointensity determinations of the Nátthagakriki flows (arithmetic mean: 19.9±2.4 µT) give new results for an excursional unit that have not been reported before. Our paleointensity values of the normal flows range between 27.4 µT and 59.3±4.1 µT and are slightly larger than those reported by [START_REF] Marshall | Preliminary paleointensity measurements and detailed magnetic analysis of basalts from the Skalamaelifell excursion, southwest Iceland[END_REF]. [START_REF] Roperch | Paleointensity of the Earth's magnetic field during the Laschamp excursion and its geomagnetic implications[END_REF] performed paleointensity measurements at the Laschamp and Olby flows in France. Their measurements gave an intensity of 7.7±1.6 µT, while the measurements of [START_REF] Chauvin | Paleointensity of the Earth's magnetic field and K-Ar dating of the Louchadière volcanic flow (Central France). New evidence for the Laschamp excursion[END_REF] yielded a paleointensity of 12.9±3.3 µT for the excursional flow at Louchadière. Sedimentary records reflect the low paleointensities during the Laschamp excursion, too [START_REF] Channell | Late Brunhes polarity excursions (Mono Lake, Laschamp, Iceland Basin and Pringle Falls) recorded at ODP Site 919 (Irminger Basin)[END_REF][START_REF] Lund | Deep-sea sediment records of the Laschamp geomagnetic field excursion (similar to 41,000 calendar years before present)[END_REF].

A c c e p t e d M a n u s c r i p t

Local and global field state during the excursion

In Fig. 7a, declination and inclination as well as paleointensity are plotted versus the established stratigraphy. It is important that no absolute time can be assigned and that the periods between the records may differ significantly.

During the excursion a massive change of declination and inclination as well as a decrease of intensity is observed. The start of the excursion is recorded by the tephra, which shows a strong change of declination, but not in inclination. This is followed by shallow positive and negative inclination values and declination values of about -90 • (E1, DG1: BL1 and BL2, BL3). An intensity measurement of site E1 was not possible. However, the flows at Bleikshóll show that the intensity decreases to only 1/10 of its original value. Towards the end of the excursion, southerly declination values are observed, while inclination already is positive and steep again (DG2: N1, N2, N3, N4, N5, N6).

At this time intensity regained about 1/3 of its value before the excursion.

The time after the excursion is recorded by the flows at Fagradallsfjall. Their declination seems to be quite stable, inclination, however, changes by more than 15 • . Intensity after the excursion is some 10 µT smaller than before it, an effect that has already been observed by [START_REF] Marshall | Preliminary paleointensity measurements and detailed magnetic analysis of basalts from the Skalamaelifell excursion, southwest Iceland[END_REF]. Between FAG11 and FAG13 paleointensity again decreases by almost 15 µT. Fig. 7b shows the variation of declination, inclination and a paleointensity proxy for ODP site 919. Data from Iceland and ODP site 919 agree very well, which confirms the established stratigraphic order. Although no absolute paleointensity values can be assigned for the PI proxy, a qualitative comparison with the icelandic data 919 [START_REF] Channell | Late Brunhes polarity excursions (Mono Lake, Laschamp, Iceland Basin and Pringle Falls) recorded at ODP Site 919 (Irminger Basin)[END_REF] shows that the marine sedimentary record has a higher intensity towards the end of the excursion, too (Fig. 7b).

A c c e p t e d M a n u s c r i p t

Different models were developed in the past to describe excursions. Generally the idea is held, that the noticeable intensity minima, which coincide with directional excursions, imply an emergence of non-dipole geomagnetic components [START_REF] Merrill | Geomagnetic field stability: Reversal events and excursions[END_REF]. Based on the 40 • difference between the VGPs of Laschamp/Olby and Skalamaelifell and the assumption of simultaneous records, [START_REF] Levi | Late Pleistocene geomagnetic excursion in Icelandic lavs: confirmation of the Laschamp excursion[END_REF] supported this hypothesis, arguing that the low latitude VGPs are related to a decrease of the axial dipole and a substantial contribution by higher order multipole terms. The same idea was proposed earlier by [START_REF] Marshall | Preliminary paleointensity measurements and detailed magnetic analysis of basalts from the Skalamaelifell excursion, southwest Iceland[END_REF], who suggested that the non-dipole fields were of about historical intensity while the axial dipole was unusually weak. In the various studies of sedimentary Laschamp records diverse theories were developed to explain a decrease of the axial dipole. A different idea than the one by [START_REF] Merrill | Geomagnetic field stability: Reversal events and excursions[END_REF] was postulated by [START_REF] Laj | Geomagnetic field behavior during the Iceland Basin and Laschamp geomagnetic excursions: A simple transitional field geometry?[END_REF]. As mentioned before, they observed strikingly similar excursion loops with a simple structure for five new records of the Laschamp excursion. Thus, they suggested that the geomagnetic field during the excursion predominately had a relatively simple geometry. The excursion loops would have to be much more different, if non-dipole fields were dominating. In order to account for the reduced intensity, they proposed a decrease of the axial dipole, a substantial equatorial dipole and a simultaneously reduced non-dipole field relative to the axial dipole. Our data, which originates from the Northern hemisphere close to site locations used by [START_REF] Laj | Geomagnetic field behavior during the Iceland Basin and Laschamp geomagnetic excursions: A simple transitional field geometry?[END_REF] agree well to this earlier interpretaion. Neveretheless, a contribution of multipolar terms, at least for a brief time interval, is necessary to explain the directional deviations between Laschamp/Olby [START_REF] Roperch | Paleointensity of the Earth's magnetic field during the Laschamp excursion and its geomagnetic implications[END_REF], New Zealand [START_REF] Cassata | Laschamp and Mono Lake geomagnetic excursions recorded in New Zealand[END_REF] and Skalamaelifell. These observations are supported by a global field model of the Laschamp excursion [START_REF] Leonhardt | 28 solute paleointensity determinations: Acceptance criteria and the software ThellierTool4.0[END_REF], in which the Bayesian VGPs from four normal polarity flows could not be matched unambiguously to those of the marine records. Taken together, our results support the theory of [START_REF] Laj | Geomagnetic field behavior during the Iceland Basin and Laschamp geomagnetic excursions: A simple transitional field geometry?[END_REF], that the geomagnetic field had a simple transitional field geometry at least during the onset of the Laschamp excursion. The data are best explained by a decrease of the axial dipole relative to the equatorial dipole that was accompanied by a considerably reduced non-dipole field, which is further supported by geomagnetic field modelling [START_REF] Leonhardt | 28 solute paleointensity determinations: Acceptance criteria and the software ThellierTool4.0[END_REF]. Fig. 6. a) VGPs as determined from our data (black) and VGPs from volcanic rocks of previous studies of the Laschamp excursion (grey). sc in the site name marks secondary components. Stratigraphic progression due to field evidence is emphasized. b) VGPs of Skalamaelifell (black) together with those of ODP site 919 [START_REF] Channell | Late Brunhes polarity excursions (Mono Lake, Laschamp, Iceland Basin and Pringle Falls) recorded at ODP Site 919 (Irminger Basin)[END_REF]) (grey). c) Excursional VGP loops of different sedimentary records [START_REF] Lund | Deep-sea sediment records of the Laschamp geomagnetic field excursion (similar to 41,000 calendar years before present)[END_REF][START_REF] Laj | Geomagnetic field behavior during the Iceland Basin and Laschamp geomagnetic excursions: A simple transitional field geometry?[END_REF]

[ • ] [ • ] [ • ] [ • ] [ • ] [ • ] [ • ]
Geographic coordinates were determined by GPS (WGS84). n corresponds to the amount of treated samples, N(N gc ) is the number of samples used for calculation of mean direction and in parenthesis those analyzed using great circle. Also shown are declination, inclination, k, α 95 as well as virtual geomagnetic pole (VGP)

coordinates.

  s c r i p t A comparison of the paleointensity data with the results of previous studies gives a very consistent picture, as all records show almost identical intensity values during the Skalamaelifell excursion. A tentative stratigraphic relationship between 25 sites prior to, during and after the Skalamaelifell excursion was established by comparing them with virtual geomagnetic poles (VGPs) from different marine sedimentary records. Only VGPs from four flows could not be matched unambiguously to those of the marine records. Our results support the theory that the geomagnetic field during the Laschamp excursion likely had a simple transitional field geometry at least during the onset of the excursion. The data are best explained by a decrease 1 Introduction

  6

  s c r i p t different GPS-coordinates (see Tab. 1) and each position is marked in Fig.1. At Fagradallsfjall, the flows have the same coordinates, but different altitudes.

  s c r i p t mal demagnetization of tephra specimens revealed a blocking temperature between 150 and 250 • C. Thus, the tephra consists mainly of paramagnetic material, but also contains highly coercive minerals with a low blocking temperature. Unfortunately, ore microscopy could not be performed due to the granular character of the samples. Yet, blocking temperature and coercivity distribution indicate that either maghemite or an iron sulfide are remanence carriers. To obtain further insight, the coercivity variation in dependency of the heating step was investigated. Backfield measurements were conducted after heating to 20 • C, 200 • C, 420 • C, 550 • C, and 700 • C. B cr decreases after heating to 420 • C. It is then reduced to only about 50% of the original value.

  were carried out in the magnetically shielded room at the Niederlippach paleomagnetic laboratory of the University of Munich using a Molspin spinner magnetometer and a 2G Enterprises cryogen magnetometer. Half of the specimen were treated by stepwise thermal demagnetization (hereinafter referred to as TH), the other half by stepwise alternating field A c c e p t e d M a n u s c r i p t (AF) demagnetization. TH demagnetization was done in a Schoenstedt furnace from 20 • C to 640

  described as 'southwestern parts of Fagradallsfjall'. They all have similar directions with southerly declinations: VGP group N1, N2, N3, N5, N6: D = 160.0 • and I = 64.1 • (Fig. 3g) and N4: D = 148.8 • and I = 60.0 • . It is not exactly clear, whether Kristjánsson measured samples of the same flows. Nevertheless, his results of D = 167 • and I = 64 • are very similar.

  result of FAG11-1C. The two check corrected determinations of site FAG13 result in a mean paleointensity of 33.8±0.2 µT, which is considerably smaller than the paleointensity at FAG11. The flows at Siglubergsháls and Festarfjall yield a paleointensity of about 30 µT (27.4 µT; 28.5±2.2 µT).

  s c r i p t inversion method of[START_REF] Leonhardt | Paleomagnetic reconstruction of the global geomagnetic field evolution during the Matuyama/Brunhes transition: Iterative Bayesian inversion and independent verification[END_REF] is used to establish a model of the geomagnetic field during the Laschamp excursion, by incorporating the results of the Skalamaelifell data. It shows decreasing dipolar and non-dipolar terms with equatorial dipolar terms dominating the onset of the excursion and a non-dipolar dominance for a brief time interval during the excursion.8 ConclusionThe Laschamp excursion was sampled at 29 sites on Reykjanes Peninsula, Iceland and a sequence of absolute paleointensities was firstly developed during this study. Paleodirections were determined using stepwise demagnetization methods. Overall, the directional determinations are characterized by very small uncertainties. Ten of the 28 sampled lava flows and a tephra layer recorded the Skalamaelifell excursion. VGPs are found in the East Pacific: One north and three south of the equator. Six other intermediate VGPs are located in West Africa. Rock magnetic investigations show that the main remanence carriers of the basalts are (titano-)magnetites with different degrees of high temperature oxidation. The rock magnetic features of the tephra could not be solved unambiguously. Partial self-reversal was observed for four specimens that show two Curie temperatures and reversible heating/cooling cycles. Only one excursional specimen belongs to this thermomagnetic group and like the other three specimens its paleodirection was unaffected by the partial selfreversal. 52 samples from 16 flows were subjected to Thellier-type paleointensity determinations, which include tests to check for alteration and multidomain effects. Determinations of anisotropy of magnetic susceptibility indicate negligible magnetic anisotropy. Reliable paleointensity data were obtained for A c c e p t e d M a n u s c r i p t ten of the 29 sites. The paleointensities of the excursional sites have values of about 4 to 5 µT, about 1/10 of the intensity of the normal polarity flows (27.4 to 59.3±4.1 µT). Towards the end of the excursion, but still during an intermediate directional state with VGPs over Africa, paleointensity regained about 1/3 of its original value (19.9±2.4 µT). A comparison of the paleointensity data with the results of previous studies at the same sampling location gives a very consistent picture, as all records show almost identical intensity values during the Skalamaelifell excursion. Stratigraphic relationships on Reykjanes Peninsula, Iceland are quite difficult as the different volcanic structures are isolated. However, a tentative stratigraphic relationship between 25 sites prior to, during and after the Skalamaelifell excursion was established by comparing them with paleomagnetic data from nearby marine sedimentary records. Only

  s c r i p t iments were unfortunately unsuccessful. We further want to thank Jim Channell, Carlo Laj and Shaul Levi who provided their sedimentary data. Funding was provided by DFG grants Wi1828/3-1 and So72/67-4.

Fig. 1 .

 1 Fig. 1. Topographic map showing the sampling area. Sampled sites are marked. Inlet: The black square on the map of Iceland marks the location of the sampling area. Topographic map: Stadfraedikort, map sheet 1512-I, 1:50,000, Landmaelingar Islands and American DMA, 1990.

Fig. 3 .

 3 Fig.3. Orthogonal projections of some representative specimens of a, e) normal and c, f, g) excursional polarity. b) shows results from the tephra and d) a stereographic projection of a great circle analysis. Open symbols correspond to the vertical component, closed symbols to the horizontal component.

Fig. 4 .

 4 Fig. 4. Continuous thermal demagnetization curves and directional plots (x, z are core coordinates) of a, b) FAG11-4F which shows no partial self-reversal and c, d) N6-4D which is partial self-reversing. Different heating cycles are labeled.

Fig. 5 .

 5 Fig. 5. Three representative examples of accepted MT4 paleointensity determinations. Arai plots (Circles: pTRM/NRM values; Triangles: Alteration checks; squares: Additivity checks) and orthogonal projections (Open symbols: Vertical component; closed symbols: horizontal component) are shown and the paleointensity as determined from the slope of the straight line is given.

  Fig.6. a) VGPs as determined from our data (black) and VGPs from volcanic rocks of previous studies of the Laschamp excursion (grey). sc in the site name marks secondary components. Stratigraphic progression due to field evidence is emphasized. b) VGPs of Skalamaelifell (black) together with those of ODP site 919[START_REF] Channell | Late Brunhes polarity excursions (Mono Lake, Laschamp, Iceland Basin and Pringle Falls) recorded at ODP Site 919 (Irminger Basin)[END_REF]) (grey). c) Excursional VGP loops of different sedimentary records[START_REF] Lund | Deep-sea sediment records of the Laschamp geomagnetic field excursion (similar to 41,000 calendar years before present)[END_REF][START_REF] Laj | Geomagnetic field behavior during the Iceland Basin and Laschamp geomagnetic excursions: A simple transitional field geometry?[END_REF]. d) Established VGP path (by correlation with b and c) at Skalamaelifell. Locations are always indicated by bigger symbols: a) black: Skalamaelifell, grey: Laschamp; b) black: Skalamaelifell, grey: ODP 919, c) same symbol as VGPs but different colour (grey), d) star: Iceland.

Fig. 7 .

 7 Fig. 7. Declination, inclination and paleointensity trend for a) the suggested stratigraphical sequence at Iceland (vertical axis, which is valid for all three plots, shows the sequence of the flows) and b) ODP site 919. The grey background shows respective time intervals.

  Figure 6

Table 1 :

 1 Paleodirectional results.

	Site	Long	Lat	n	N(N gc )	Dec	Inc	k	α 95	VGP Long	VGP Lat
		[ • ]	[ • ]			[ • ]	[ • ]		[ • ]	[ • ]	[ • ]
	Excursion
	Bleikshóll
	BL1	-22.3367706 63.8650725	6	4 (1)	261.4 -20.2	249	6.2	250.7	-13.1
	BL2	-22.3377301 63.8667618	6	6	263.3 -18.9	322	3.7	249.4	-11.7
	BL3	-22.3373079 63.8670291	5	5	264.9 -27.3	143	6.4	245.7	-15.2
	Einbúi
	E1	-22.3128764 63.8786999	6	6 (5)	280.1	13.8	70	9.6	241.7	10.7
	Nátthagakriki
	N1	-22.2897031 63.8762336	6	6	158.5	64.9	112	6.4	353.4	22
	N2	-22.2905105 63.8789943	6	5	162.6	65.8	164	6	350.2	22.8
	N3	-22.2905105 63.8789943	6	6	154.8	64	304	3.8	356.3	21.4
	N4	-22.2943194 63.8823834	6	6	148.8	60	1466	1.8	1.97	17.7
	N5	-22.2943194 63.8823834	6	6	162.9	61.4	392	3.4	350.8	17.3
	N6	-22.3039511 63.8854599	6	6	161.3	64.3	397	3.4	351.5	21
	Tephra
	TE1	-22.3295086 63.8629993	6	4	317	56.6	110	8.8	222.4	53.1
	Normal polarity
	Base
	B1	-22.3182179 63.8763219 12	10	41.3	71.4	583	2	69.4	68.4
	Fagradalsfjall
	FAG1	-22.3141554 63.8829489	6	5	12.2	70.4	2104	1.7	118.5	78.8
	FAG2	-22.3141554 63.8829489	4	4	4.7	71	93	9.6	139.9	81.2
	FAG3	-22.3141554 63.8829489	5	4	3.2	68.1	109	8.9	148.6	77.2
	FAG4	-22.3141554 63.8829489	6	6	14.5	58.2	499	3	131.7	63.6
	FAG5	-22.3141554 63.8829489	3	3	12.2	76.1	140	10.5	64.5	84.6
	FAG6	-22.3141554 63.8829489	6	5	4.7	72.2	188	5.6	136.3	83
	FAG7	-22.3141554 63.8829489	5	5	19.7	71.3	325	4.3	97.7	77.4
	FAG8	-22.3141554 63.8829489	4	4	26.5	74.9	803	3.2	66.1	77.8
	FAG9	-22.3141554 63.8829489	6	6	13.4	67.9	339	3.6	122.7	75.2
	FAG10 -22.3141554 63.8829489	3	3	35.3	72.6	80	13.9	70.7	72.1
	FAG11 -22.3141554 63.8829489	6	5	11.7	75.1	146	6.3	82.1	84.4
	FAG12 -22.3141554 63.8829489	5	5 (1)	14.7	75.1	293	4.6	76.9	83.1
	FAG13 -22.3141554 63.8829489	6	6 (2)	23.5	73	219	4.7	82.2	77.6
	Festarfjall
	FS1	-22.3341365 63.8578412	6	6	68.7	80.8	292	3.9	19.7	64.6
	FS2	-22.3362097 63.8580434	6	6	86.1	75.7	154	5.4	28.8	54.4
	Siglubergsháls
	SI1	-22.3289044 63.8610773	6	5	73.3	79.5	173	5.8	23.5	62.4
	SI2	-22.3289044 63.8610773	6	6	64.6	77.1	233	4.4	35.1	63.5
	Directional groups DG
	DG1: BL1,BL2		12	10 (1)	262.5 -19.4	328	2.7	250	-12.3
	DG2: N1,N2,N3,N5,N6		30	29	160.0	64.1	218	1.8	352.5	20.8
	DG3: FAG1,FAG2,FAG3		15	13	7	69.9	175	3.1	134.8	79.3
	DG4: FAG5,FAG6,FAG7,FAG8	18	17	15.4	73.4	212	2.5	93.2	81.4
	DG5: FAG10,FAG11,FAG12	14	13(1)	18.9	74.7	150	3.4	75.6	81.0
	DG6: SI1,SI2,FS1		18	17	68.3	79.2	231	2.4	26.3	63.7

Acknowledgements

We gratefully acknowledge Leo Kristjánsson for sharing his fundamental knowledge about geology and paleomagnetism on Iceland. We also thank Hervé Guillou and Sebastien Nomade whose time consuming dating exper-

T min to Tmax gives the temperature range used for paleointensity calculation and Np the number of independent and successive points in this interval. f, g, q, w represent the fraction of NRM, the gap factor, the quality factor [START_REF] Coe | Geomagnetic paleointensities from radiocarbon-dated lava flows on Hawaii and the question of the Pacific nondipol low[END_REF] and the weighting factor [START_REF] Prévot | The Steens Mountain (Oregon) geomagnetic polarity transition 2. Field intensity variations and discussion of reversal models[END_REF]. F, σ are individual paleointensity and standard deviation, F w and σ w are weighted mean paleointensity and weighted standard deviation (using q) for each flow and Fa and σa are arithmetic mean paleointensity and standard deviation for the directional groups.

Table 2