paleomagnetic secular variation at the azores during the last 3 ka 1 we report on 33 new paleomagnetic directions obtained from 16 lava flows emplaced in the last 3 ka on sao miguel the largest island of the azores the data provide 27 well dated directions from historical or 14c dated flows which together with 6 directions previously gathered from the same flows by johnson et al 1998 yield the first paleomagnetic directional record of the last 3 ka from the atlantic ocean within flow directions are consistent suggesting that inclination swings from 60 to 25 and declination changes between 10 to 20 reflect variations in the geomagnetic field over the last 3 ka to a first approximation the declination record is consistent with predictions from cals3k 4 and gufm1 global field models conversely inclination values are lower than model predictions at two different ages 1 four sites from the 1652 ad flow yield i 48 instead of i 63 predicted by gufm1 2 data from several flows nicely mimic the inclination minimum of 800 1400 ad but inclination values are lower by 10 than cals3k 4 model predictions by interpolating a cubic spline fit on declination inclination versus age data we tentatively infer the directional evolution of the geomagnetic field at the azores from 1000 bc to 1600 ad the obtained curve shows three tracks in virtual overlap during the 1000 800 bc 800 500 bc and 400 700 ad time spans

osseous skeletal material and fish scales in marine sediments under the oxygen minimum zone off northern and central chile abstract the significance of whale falls for the study of the biogeography evolution and biodiversity of deep sea biota has been recently recognized by international programs since large carcasses are known to give rise to biogenic chemosynthetic ecosystems however the plain accumulation of smaller bone material in the shallower settings of the continental shelf and upper slope under the hypoxic conditions of the oxygen minimum zone omz has received much less attention here we describe new findings of skeletal material and fish scales in marine sediments under the omz off northern and central chile which combined with previous reports for the study area lead us to suggest the existence of a band in the benthos of accumulation of bones and scales extending at least twenty degrees in latitude 18 38 s future studies should focus on the characterization of biotic communities living upon these resources in order to elucidate their peculiarities and importance in the eastern south pacific

paleomagnetic correlation of late quaternary lava flows in the lower east rift zone of kilauea volcano hawaii paleomagnetic data reflecting paleosecular variation psv of the geomagnetic field are used to correlate individual lava flows in the lower east rift zone of kilauea volcano this area has been recently mapped based on petrography stratigraphy soil development and degree of weathering of the flows eight 14c dates also provide a framework for the relative age assignments site mean directions of remanent magnetization for historic flows in this region indicate that local magnetic anomalies do not mask the psv signal that within flow angular differences are generally 5 and that temporal resolution of the paleomagnetic directions is of the order of 100 years the paleomagnetic correlations do not conflict with the observed stratigraphic relationships and psv reference curves from dated lava flows and lake sediments have also been used to help determine the sequence of directional groups paleomagnetic grouping of flows into eruptive events indicates a different and perhaps more refined eruptive history for the lower east rift zone than implied by the geologic mapping alone the number of individual flows based on petrographic distinctions is likely the same but the frequency of events including petrographically distinct flows is apparently lower than previously thought flows from ridge crest vents bifurcated by the rift indicate formation of the central graben in this region after 300 years ago possibly related to the 1790 eruptions along dual fractures

on the persistence of geomagnetic flux lobes in global holocene field models abstract to investigate the longevity and robustness of high latitude flux patches in the geomagnetic field at the core mantle boundary we present time dependent models of the geomagnetic field for the past 7000 years our models use the same data set as previously used for time dependent archaeomagnetic field modelling but constrained with additional priori models from time averages of field models covering the last 150 400 and 3000 years we find that the data are consistent with flux patches existing in both north and south hemispheres for the past 7000 years and that the northern hemisphere patches at least have highly dynamic behaviour simple averaging of the historical field may not be appropriate to obtain a characteristic time averaged model of the field for comparison with other geophysical observables our results should inform geodynamo studies of thermal core mantle coupling and of possible long term structure in the geomagnetic field

paleomagnetism of large igneous provinces case study from west greenland north atlantic igneous province abstract we present new paleomagnetic and multi model stereo photogrammetry data from lava sequences in the west greenland part of the north atlantic igneous province naip the joint analyses of paleomagnetic and photogrammetric data yield a well defined paleomagnetic pole located at lat 73 6 n long 160 5 e n 44 95 6 2 k 13 1 age 61 55 ma which is statistically indistinguishable from a pole recently obtained for the eurasian part of the naip on faroe islands riisager et al earth planet sci lett 201 2002 261 276 combining the two datasets we obtain a joint naip paleomagnetic pole in greenland coordinates lat 71 1 n long 161 1 e n 87 95 4 3 k 13 6 age 61 54 ma the results presented here represent the first study in which photogrammetry profiles were photographed at the exact same locations where paleomagnetic fieldwork was carried out and a direct flow to flow comparison of the two datasets is possible photogrammetry is shown to be particularly useful because of i highly precise dip strike measurements and ii detailed field observations that can be made in the laboratory highly precise determination of the structural attitude of well exposed kanisut mb lava sequences demonstrates that their apparently reliable in field dip strike measurements typically are up to 6 wrong erroneous dip strike readings are particularly problematic as they offset paleomagnetic poles without affecting their confidence limits perhaps more important for large igneous provinces is the recognition of a variable temporal relationship between consecutive lava flows we demonstrate how correct interpretation of paleosecular variation facilitated by the detailed photogrammetry analysis is crucial for the rapidly emplaced vaigat formation lavas inaccurate tectonic correction non averaged paleosecular variation and unrecognized excursional directions may perhaps explain why coeval paleomagnetic poles from large igneous provinces are often discordant the difference between the joint naip paleomagnetitc pole and apparent polar wander path poles suggests that they may be less reliable than suggested by their confidence limits

new and revised 14c dates for hawaiian surface lava flows paleomagnetic and geomagnetic implications radiocarbon dates have been obtained for 30 charcoal samples corresponding to 27 surface lava flows from the mauna loa and kilauea volcanoes on the island of hawaii the submitted charcoal was a mixture of fresh and archived material preparation and analysis was undertaken at the nerc radiocarbon laboratory in glasgow scotland and the associated suerc accelerator mass spectrometry facility the resulting dates range from 390 years b p to 12 910 years b p with corresponding error bars an order of magnitude smaller than previously obtained using the gas counting method the new and revised 14c data set can aid hazard and risk assessment on the island the data presented here also have implications for geomagnetic modelling which at present is limited by large dating errors

icelandic holocene palaeolimnomagnetism abstract sediments spanning the last 9000 y from two sites in lake vatnsdalsvatn lat 66 n long 23 e in northwest iceland hold repeatable palaeomagnetic direction records the vatnsdalsvatn sediments have mean palaeomagnetic inclinations of 76 close to that expected for a geocentric axial dipole field and direction fluctuations of around 20 from the mean the palaeomagnetic directions are stable under alternating field partial demagnetization experiments a time scale for the vatnsdalsvatn sediments has been estimated from 14 c dating the pattern of palaeomagnetic secular change shows few similarities with british records 2000 km distant and a central north american record 5000 km distant

deflection of paleomagnetic directions due to magnetization of the underlying terrain in the summers of 1989 and 1991 we made 344 near ground level measurements of the ambient geomagnetic field above recent basalts on the island of hawaii using a three component fluxgate magnetometer we studied 12 surface features including a lava pond lava channels long tilted blocks smooth sloping surfaces two fissures and a deep u shaped road cut we observed substantial differences up to 20 between the observed and expected international geomagnetic reference field igrf magnetic field directions over these features except those composed of shelly pahoehoe and a flat horizontally thin lava pond we also observed inclinations that were systematically shallower than the igrf field by up to 5 we show that these shallower inclinations can be explained by the magnetization of the regionally sloping surface of the southern side of the island we found that all of the observed inclination deflections can be explained by simple two dimensional models which assume uniform induced and remanent magnetization parameters in the local terrain our observations imply that the inclination deflections cannot be corrected without a complete knowledge of the preexisting terrain and the remanence in the underlying flows upon which the lavas cooled since this information is rarely available it is difficult or impossible to discriminate between dispersion of paleomagnetic directions caused by the magnetic terrain effect and dispersion due to other factors such as paleosecular variation psv we therefore conclude that psv dispersion parameters cannot be accurately determined from paleomagnetic measurements on highly magnetic volcanic flows we also suggest that some of the geomagnetic excursions inferred from measurements on volcanic rocks may be at least in part due to the magnetic terrain effect it is unnecessary to invoke ad hoc mechanisms such as clastic block or crustal rotations distortion of the top crust or flow deformation to explain the large between site dispersions or inclination anomalies observed in many of the paleomagnetic data from volcanic rocks our observations also bring into question the general reliability of paleomagnetic pole positions inferred from volcanic rocks as a systematic inclination deflection due to local and regional slopes and irregular terrain such as those we observed would lead to a corresponding error in the inferred paleolatitude the magnetic terrain effects also offer alternative explanations for anomalous paleomagnetically inferred plate motions

shallow paleomagnetic directions from historic lava flows hawaii a study designed to examine the basic theory of thermoremanent magnetization acquisition parallel to the earth s ambient magnetic field upon cooling and to evaluate intra flow variations of very young basalt flows has yielded some startling initial results two extensively sampled flows the 1950 flow of mauna loa and the 1972 flow from mauna ulu kilauea have been found to have paleomagnetic directions statistically different from the present geomagnetic field direction pfd in hawaii the 1950 flow shows very consistent but shallow directions throughout the flow with a mean inclination 6 shallower than the pfd directions from the 1972 flow vary from site to site both along the length of the unit and within vertical sections relative between site consistency is low with all sites but one having paleomagnetic directions distinct from the pfd however the mean inclination for this flow 33 4 is only about 3 shallower than expected the source of such anomalous behavior in the recording of ambient field directions is yet unknown although several possible causes are examined

a paleomagnetic approach toward refining holocene radiocarbon based chronologies paleoceanographic records from the north iceland md99 2269 and east greenland md99 2322 margins 1 we report the intercalibration of paleomagnetic secular variation psv and radiocarbon dates of two expanded postglacial sediment cores from geographically proximal but oceanographically and sedimentologically contrasting settings the objective is to improve relative correlation and chronology over what can be achieved with either method alone core md99 2269 was taken from the hunafloaall trough on the north iceland shelf core md99 2322 was collected from the kangerlussuaq trough on the east greenland margin both cores are well dated with 27 and 20 accelerator mass spectrometry 14c dates for cores 2269 and 2322 respectively paleomagnetic measurements made on u channel samples document a strong stable single component magnetization the temporal similarities of paleomagnetic inclination and declination records are shown using each core s independent calibrated radiocarbon age model comparison of the psv records reveals that the relative correlation between the two cores could be further improved starting in the depth domain tie points initially based on calibrated 14c dates are either adjusted or added to maximize psv correlations radiocarbon dates from both cores are then combined on a common depth scale resulting from the psv correlation support for the correlation comes from the consistent interweaving of dates correct alignment of the saksunarvatn tephra and the improved correlation of paleoceanographic proxy data percent carbonate these results demonstrate that psv correlation used in conjunction with 14c dates can improve relative correlation and also regional chronologies by allowing dates from various stratigraphic sequences to be combined into a single higher dating density age to depth model

a paleomagnetic study of the pohue bay flow and its associated coastal cones mauna loa volcano hawaii constraints on their origin and temporal relationships abstract detailed paleomagnetic and rock magnetic studies of the pohue bay flow and associated cones on its coastal flat were made to determine if the origin of the cones was due to primary volcanic or secondary littoral processes we used paleomagnetism to determine the temporal relationships between the flow and cones if the flows and cones are from the same eruption the littoral origin of the cones is strongly favored a total of 530 specimens from 232 core samples were collected and studied from the pohue bay flow and from lava ponded within the cones remanent magnetizations are very stable to stepwise alternating field demagnetization and show small angular dispersion and well defined characteristic magnetizations magnetic carriers correspond to members of the titanomagnetite series with single or pseudo single domain states the overall mean directions for the pohue bay flow dec 10 8 inc 23 6 k 287 4 and 95 2 3 and the lavas ponded within the cones dec 12 8 inc 25 2 k 353 and 95 4 9 are statistically indistinguishable and have been drawn from a common fisherian distribution supporting a close age relationship additionally the angular dispersion for the combination of the main flow and lavas ponded in cones is small with a paleosecular variation psv estimate of sf 4 2 this small psv value supports the temporal association between the pohue bay flow and associated cones they can thus be assigned to the same eruption timeframe because the source vent of the pohue bay flow is far upslope the cones must therefore be littoral in origin formed when the pohue bay flow entered the ocean from the secular variation curve and comparison with age dated flows with similar paleomagnetic directions we estimate that the pohue bay flow was erupted approximately 1300 years ago we were also able to distinguish both a possibly younger lava flow that later utilized the main tube of the pohue bay flow and an earlier sub set of cones that were possibly formed before the pohue bay eruption

paleomagnetic directions and k ar dating of 0 to 1 ma lava flows from la guadeloupe island french west indies implications for time averaged field models twenty six lava flows spanning the last million years were sampled in la guadeloupe french west indies because of the lack of continuous volcano stratigraphic sections in la guadeloupe dating is necessary in order to describe the temporal evolution of the geomagnetic field in this time interval new k ar ages ranging from 50 ka to 1 ma have been obtained on andesites using the cassignol gillot technique at the universite paris sud institut de physique du globe de paris orsay laboratory additional flows with k ar ages obtained using the same dating technique were also sampled for paleomagnetic investigations more than 200 samples were analyzed using both alternating field af and thermal stepwise demagnetization techniques duplicate samplings of two flows at three different sites demonstrate that within flow dispersion is negligible for the andesitic lava flows sampled in this study direct comparison with an earlier paleomagnetic study performed on the island indicates that for the three investigated flows modern demagnetization techniques yield much better defined paleomagnetic directions the matuyama brunhes transition was recorded in a three flow section and is dated at 781 18 ka in good agreement with other recent radiometric age determinations the mean paleomagnetic pole calculated from the 23 normal polarity flows is indistinguishable from geographic north which implies that no significant persistent axial quadrupole term can be identified at this site for the last million years this result contradicts earlier results and has important implications for models of the time averaged field taf an occam algorithm was used to construct a taf model from the global volcanic database from the last 5 ma substitution of the mean direction calculated from the earlier study for the lesser antilles by our new mean value reduces the quadrupole term by more than 30 this effect which was produced by changing data from a single site demonstrates that older paleomagnetic sites may need to be reinvestigated furthermore it also highlights the limitations of taf models that can be inferred from paleomagnetic databases in their present state

continuous geomagnetic field models for the past 7 millennia 2 cals7k 1 we present two continuous global geomagnetic field models for recent millennia cals3k 2 covering the past 3000 years and cals7k 2 covering 7000 years from 5000 bc to 1950 ad the models were determined by regularized least squares inversion of archeomagnetic and paleomagnetic data using spherical harmonics in space and cubic b splines in time they are derived from a greatly increased number of paleomagnetic directional data compared to previous efforts and for the first time a significant amount of archeointensity data is used in this kind of global model allowing the determination of evolution of geomagnetic dipole strength while data accuracy and dating uncertainties remain a limitation reliable low resolution global models can be obtained the results agree well with previous results from virtual axial dipole moment vadm studies from archeomagnetic intensity data apart from a systematic offset in strength a comparison of model predictions with the previous 3000 year model cals3k 1 gives general agreement but also some significant differences particularly for the early epochs the new models suggest that the prominent two northern hemisphere flux lobes are more stationary than cals3k 1 implied extending considerably the time span of stationary flux lobes observed in historical models between 5000 bc and 2000 bc there are time intervals of weak dipole moment where dipole power is exceeded by low degree nondipole power at the core mantle boundary model coefficients and evaluation code can be obtained from the earthref digital archive erda together with animations and snapshots plots for every 100 years at http www earthref org detailed urls for the different material are listed in appendix a

40ar 39ar ages and paleomagnetism of sao miguel lavas azores we present new 40 ar 39 ar ages and paleomagnetic data for sao miguel island azores paleomagnetic samples were obtained for 34 flows and one dike successful mean paleomagnetic directions were obtained for 28 of these 35 sites 40 ar 39 ar age determinations on 12 flows from the nordeste complex were attempted successfully ages obtained are between 0 78 ma and 0 88 ma in contrast to published k ar ages of 1 ma to 4 ma our radiometric ages are consistent with the reverse polarity paleomagnetic field directions and indicate that the entire exposed part of the nordeste complex is of a late matuyama age the duration of volcanism across sao miguel is significantly less than previously believed which has important implications for regional melt generation processes and temporal sampling of the geomagnetic field observed stable isotope and trace element trends across the island can be explained at least in part by communication between different magma source regions at depth the 40 ar 39 ar ages indicate that our normal polarity paleomagnetic data sample at least 0 1 myr 0 0 1 ma and up to 0 78 myr 0 0 78 ma of paleosecular variation and our reverse polarity data sample approximately 0 1 myr 0 78 0 88 ma of paleosecular variation our results demonstrate that precise radiometric dating of numerous flows sampled is essential to accurate inferences of long term geomagnetic field behavior negative inclination anomalies are observed for both the normal and reverse polarity time averaged field within the data uncertainties normal and reverse polarity field directions are antipodal but the reverse polarity field shows a significant deviation from a geocentric axial dipole direction 1998 elsevier science b v all rights reserved

palaeomagnetic rock magnetic and microscopy studies of historic lava flows from the paricutin volcano mexico implications for the deflection of palaeomagnetic directions summary we report the results of a detailed rock magnetic study of basaltic andesites for one of the best documented cases of the first occurrence of a new volcano the 1943 1952 eruption of paricutin in central mexico 102 oriented standard palaeomagnetic cores corresponding to 12 sites were collected from seven different lava effusion episodes which cover the interval between 1943 september december and 1946 march august thermomagnetic investigations reveal that remanence is carried in most cases by ti poor titanomagnetite resulting from oxy exsolution that probably occurred during the initial flow cooling unblocking temperature and coercivity point to small pseudo single domain magnetic grains for these titano magnetites thermal and alternating field demagnetization indicates that the lavas are characterized by univectorial or two component magnetizations site mean declinations and inclinations range from 331 9 to 32 3 and from 17 6 to 51 3 respectively the overall mean direction for 12 sites is dec 1 8 inc 37 5 k 26 and 95 8 7 23 samples were pre selected for thellier palaeointensity experiments because of their stable remanent magnetization and relatively low within site dispersion thellier palaeointensity experiments have yielded widely dispersed palaeointensity values from the historically observed geomagnetic intensity although individual specimens yielded technically high quality palaeointensity results the values obtained are significantly different from the expected value of approximately 45 t the palaeomagnetic record is dominantly characterized by shallower than expected sitemean inclinations as compared with data from the teoloyucan geomagnetic observatory and estimates based on the international geomagnetic reference field 1945 and 1950 models several alternative interpretations are examined but no single factor resulting in abnormal shallow inclinations can be isolated direct field observations suggest that shallow inclinations and angular scatter can be associated with lava emplacement in terms of internal viscous deformation and or movement of parts of lava after cooling below the blocking temperatures during lava advance other factors analysed that could contribute to the shallow inclinations and angular scatter are high magnetic refraction effects and outcrop scale to regional magnetic field anomalies

reconstructing the holocene geomagnetic field abstract knowledge of the holocene evolution of earth s magnetic field is important for understanding geodynamo processes in the core is necessary for studying long term solar terrestrial relationships and can provide useful age constraints for archeologicaland stratigraphic applications continuous time varying global field models based on archeo and paleomagnetic data are useful tools in this regard we use a comprehensive data compilation and recently refined modelling strategies to produce cals10k 1b the first time varying spherical harmonic geomagnetic field model spanning 10 ky the model is an average obtained from bootstrap sampling to take account of uncertainties in magnetic components and ages in the data and hence has version number 1b instead of 1 this model shows less spatial and temporal resolution than earlier versions for 0 3 ka and particularly aims to provide a robust representation of the large scale field at the core mantle boundary cmb we discuss the geomagnetic dipole evolution and changes in holocene magnetic field morphology at the cmb as shown by the new reconstruction the results are compatible with earlier models cals3k 3 and cals3k 4 for 0 3 ka but reveal some clear deficiencies in the 0 7 ka cals7k 2 model prior to 3 ka cals10k 1b is able to resolve mobile and structurally evolving high latitude radial field flux lobes at the cmb in both hemispheres as well as persistent non zonal structure in the 10 ky average contributions to the average field from time varying structures in the equatorial indonesian australian region are particularly striking

vulcanicity of historic times in the middle atlantic islands of the five middle atlantic archipelagos three azores canaries and cape verde have been volcanically active probably from the mid 14th century onwards though fogo is the only island in the cape verde group to show such activity of all these islands it has erupted most frequently historic submarine vulcanism is associated only with the azores as per the imperfect data available it is possible that the sixty five eruptions here recorded have resulted in the production of some 24 km3 of lavas and ejecta the matter of correlating vulcanism in these islands with sunspot activity and or earth minima tides is open to question average peridiocity values for vulcanism are 8 1 years for sunspot activity 11 1 years 1700 1965 and 18 6 years for earth minima tides 1400 1978 such extra terrestrial agencies are to be considered more as trigger actions rather than basic causes of vulcanicity

measurement of the degree of smoothing in sediment paleomagnetic secular variation records an example from late quaternary deep sea sediments of the bermuda rise western north atlantic ocean abstract replicate paleomagnetic secular variation psv records for the last 15 000 yr have been recovered from two deep sea sediment gravity cores on the bermuda rise western north atlantic ocean the records have been correlated using several different parameters and dated by correlation to a nearby piston core which has a high resolution 14 c chronology the psv records are systematically subdued with respect to the expected local magnetic field variability during the holocene when sedimentation rates were about 10 cm kyr during the late pleistocene when sedimentation rates were a factor of 2 3 higher the psv variability was significantly larger we attribute this to smoothing of the psv records by a depositional post depositional remanence drm pdrm acquisition process with a 10 20 cm lock in interval the degree of psv smoothing was evaluated by comparing the vgp angular dispersion and vector spectral content of the bermuda rise cores with high resolution psv records from north america and great britain these results suggest a 50 reduction in the drm pdrm recording of vector variability during the holocene on the bermuda rise with most of the psv reduction occurring for features of less than 2000 yr in duration we can reproduce the subdued psv pattern with a mathematical model that simulates the site drm pdrm remanence acquisition process finally the replicate gravity cores document that psv records may be reproducible on a local scale even though they only retain a low pass filtered record of true magnetic field variability

magnetic anomalies of lava fields in the canary islands possible consequences for paleomagnetic records abstract measurements of the total magnetic field have been performed above the surface of 12 lava flows in the canary islands of la palma and tenerife directions can differ by up to 15 and intensity changes reach 20 the mean flow directions do not coincide and the mean flow virtual dipole moments vdms differ by more than 10 the deviations are linked to topographic features at the surface of the flows and should persist during emplacement of the next overlying flow such effects can thus limit the resolution of paleomagnetic records of detailed geomagnetic features and the field intensity variations above the flows and between the mean flow values could explain a significant part of the scatter inherent to many paleointensity studies historical field variations have been recorded from the same flows and are in agreement with the european curve for the same period however there is also some correlation with the present field which suggest that the underlying magnetized bodies could have affected the paleomagnetic records biases induced by the crustal field would increase considerably during periods of reduced field intensity like during reversals and induce erroneous records samplings performed over large distances within each lava flow would provide the best way to average out the contribution of the anomalies in any case such effects should not be neglected in paleomagnetic studies

the last 5000 years of activity at sete cidades volcano sao miguel island azores implications for hazard assessment abstract sete cidades is a central volcano with a summit caldera at the western end of sao miguel island azores its stratigraphy comprises two main geological groups the inferior group the units of which date from more than 200 000 years ago through to 36 000 years before present consisting of thick lava flows and subaerial volcaniclastic deposits that built the base of the central volcano and the superior group which comprises all the activity from the last 36 000 years including pumice and scoria fallout and pdc deposits with minor lava flows the volcanostratigraphy is divided into six main formations risco ajuda bretanha lombas santa barbara and lagoas each defined by different activity phases in the volcano s evolution the present caldera developed in three phases associated with massive paroxysmal eruptions which occurred approximately 36 000 29 000 and 16 000 years before present since the last caldera forming event eruptive activity has been predominantly explosive and a variety of deposits have been produced by different eruptive styles associated with three specific settings 1 pumice fall deposits pyroclastic density currents and hydromagmatic events from eruptions inside the caldera 2 lava domes cinder cones and basaltic lava flows from eruptions on the volcano flanks 3 tuff ring and tuff deposits from surtseyan eruptions offshore the more recent history of the volcano is marked by a change in the intracaldera activity from dominantly magmatic behaviour to a hydromagmatic character which happened about 5000 years ago since then at least 15 basaltic subaerial and submarine eruptions have occurred on the volcano flanks and 17 trachytic s l explosive eruptions have occurred within the caldera the most recent of which took place about 700 years ago this eruptive frequency makes sete cidades probably the most active volcanic centre in the azores in this paper we present the stratigraphy and a description of the deposits which originate from the last 5000 years in order to delineate sete cidades recent eruptive history for the purpose of estimating the hazard associated with this volcano an event tree has been designed to systematize possible future eruptive scenarios and to aid the assessment of relative probabilities of occurrence of different potential eruptive styles the probabilities for all nodes on the event tree together with their associated uncertainties were obtained by expert elicitation providing results that accord with the geologic record these probability estimates indicate that 1 basaltic strombolian and submarine eruptions are the most likely to occur 2 trachytic activity from the central volcano magmatic system is more probable in the intracaldera environment than on the flanks and 3 explosive activity is more likely than effusive for the trachytic system worst case scenario and maximum expected events are identified according to the eruptive scenarios and their probabilities of occurrence

geomagnetic field for 0 3 ka 1 new data sets for global modeling 1 paleomagnetic and archeomagnetic records are used in both regional and global studies of earth s magnetic field we present a description and assessment of five newly compiled data sets also used in the companion paper by korte et al 2009 to produce a series of time varying spherical harmonic models of the geomagnetic field for the last 3000 years data are drawn from our compilation of lake sediment records and from the online database geomagia50v2 the five selections are available from the earthref digital archive at http earthref org cgi bin erda cgi n 944 data are grouped according to the source of material and we conducted separate assessments of reliability for archeomagnetic artifacts and lava flows the arch3k dat data set and for sediments sed3k dat the overall number of data is 55 greater than in previous compilations constrained data sets were selected using different criteria for each group winnowing of archeological data was based on uncertainties supplied by the original data providers the lake sediment data assessment relied on preassigned age uncertainties and one or more of the following comparisons with archeomagnetic data from the same region regional consistency among several lakes and consistency with global archeomagnetic models we discuss relative merits of a larger unconstrained data set or a smaller possibly more reliable one the constrained data sets eliminate a priori up to 35 of the available data in each case and rely on potentially subjective assessments of data quality given the limited data available our analyses indicate that iterative rejection of a small number 1 1 5 of outlying data during global field modeling is a preferable approach specific regional comparisons among the models and data support the conclusion that korte et al s outlier free cals3k 3 model based on all available measurements from sediments and archeological artifacts currently provides the best global representation of the 0 3 ka field the arch3k 1 model provides a better fit to the denser european archeomagnetic data and may be better in that region

paleomagnetism of pleistocene volcanic rocks from pantelleria island sicily channel italy abstract a paleomagnetic investigation has been carried out on pleistocene volcanic rocks from pantelleria in the sicily channel this island is characterised by a bimodal volcanism of peralkaline rhyolite to trachyte and basalt composition the radiometric data indicate that volcanic activity started around 324 ka bp samples have been collected from 30 sites in 16 volcanic units spanning activity during the last 150 ka magnetic properties vary systematically with lithotype and indicate high ti titanomagnetite to magnetite as the main carriers of magnetisation stable characteristic remanent magnetisation chrm directions isolated by alternating fields af demagnetisation show normal polarity consistent with emplacement during the brunhes chron and vary within the paleosecular variation psv range however the inclinations are low as compared with the geocentric axial dipole gad inclination at pantelleria especially for volcanic units younger than 50 ka the mean chrm direction computed from 27 sites is d 358 8 i 46 9 k 30 95 5 2 and the inclination anomaly is i 8 1 both tectonic movements and the presence of large magnetic anomalies around the island of pantelleria fail to explain this low inclination which may therefore be related to a significant long lived non dipolar field contribution in the area over the past 150 kyr

improving geomagnetic field reconstructions for 0 3 ka global geomagnetic field reconstructions on millennial time scales can be based on comprehensive paleomagnetic data compilations but especially for older data these still suffer from limitations in data quality and age controls as well as poor temporal and spatial coverage here we present updated global models for the time interval 0 3 ka where additions to the data basis mainly impact the south east asian alaskan and siberian regions we summarize recent progress in millennial scale modelling documenting the cumulative results from incremental modifications to the standard algorithms used to produce regularized time varying spherical harmonic models spanning 1000 bc to 1990 ad from 1590 to 1990 ad gauss coefficients from the historical gufm1 model supplement the paleomagnetic information in addition to absolute paleointensities calibrated relative paleointensity data from sediments are now routinely included iterative data rejection and recalibration of relative intensity records from sediments ensure stable results bootstrap experiments to generate uncertainty estimates for the model take account of uncertainties in both age and magnetic elements and additionally assess the impact of sampling in both time and space based on averaged results from bootstrap experiments taking account of data and age uncertainties we distinguish more conservative model estimates cals3k nb representing robust field structure at the core mantle boundary from relatively high resolution models cals3k n for model versions n 3 and 4 we assess the impact of newly available data and modifications to the modelling method by comparing the previous cals3k 3 the new cals3k 4 and the conservative new model cals3k 4b we conclude that with presently available data it is not feasible to produce a model that is equally suitable for relatively high resolution field predictions at earth s surface and robust reconstruction of field evolution avoiding spurious structure at the core mantle boundary cmb we presently consider cals3k 4 the best high resolution model and recommend the more conservative lower resolution version for studies of field evolution at the cmb

a method for correcting geographically separated remanence directions for the purpose of archaeomagnetic dating summary spatial variations in the geomagnetic field must be taken into account if secular variation master curves and directional magnetic dates are to be optimized two methods for relocating remanence vectors have been proposed and in this paper their relative accuracies are compared using a numerical model based on the present day field a method which converts archaeomagnetic directions via a virtual geomagnetic pole is shown to be the more efficient transformation for an archaeomagnetic region the size of the british isles 900 km radius the maximum error in relocating vectors to a central location is predicted to be of the order of 1 2

paleomagnetic secular variation at vulcano aeolian islands during the last 135 kyr abstract paleosecular variation psv of the earth s magnetic field during the last 135 kyr has been investigated in lavas scoriae and pyroclastic rocks of vulcano aeolian islands about 1000 samples have been collected at 77 sites from 25 distinct volcanic units whose age is either known from published isotopical data or constrained on the grounds of statigraphical relationships magnetic mineralogy investigation shows that ti magnetite is the main ferromagnetic mineral at most sites secondary magnetization components are either absent or easily removed by stepwise thermal or alternating field demagnetization the mean site direction of the characteristic remanent magnetization is usually well defined since the semi angle of confidence is greater than 5 at only four sites the mean paleomagnetic direction over the last 135 kyr d 9 4 i 53 2 95 3 5 differs from the geocentric axial dipole gad at vulcano d 0 i 57 8 and might be interpreted as the effect of a long term non axial dipolar component the psv record from vulcano agrees well with those from the lacustrine sediments of lago grande di monticchio 100 kyr bp and lago di mezzano 30 kyr bp located in the italian peninsula brandt et al quat sci rev 18 1999 961 976 the inclination anomaly i found at vulcano corresponds to about half of the shallowing observed in the sediments of the two lakes and the declination anomaly d may be used to tie the declination values derived from azimuthally unoriented cores to the geographical reference system in order to find the optimum site to be used as reference for psv studies in italy the angular values of the earth s magnetic field measured at the 113 repeat stations of the italian geomagnetic network coticchia et al boll geod sci aff 40 2001 277 291 have been analyzed with the relocation via pole method noel and batt geophys j int 102 1990 753 756 the viterbo station lat 42 27 n long 12 02 e proved the best since the mean error is 0 3 for both declination and inclination wherever the original psv site is located in italy a preliminary composite psv curve for the last 30 kyr bp is thus proposed merging and relocating to viterbo the data from vulcano and the curve from lago di mezzano corrected for the gad deviation found at vulcano

geomagnetic paleointensity and environmental record from labrador sea core md95 2024 global marine sediment and ice core chronostratigraphy for the last 110 kyr piston core md95 2024 from the labrador rise provides a continuous record of rapidly deposited detrital layers denoting laurentide ice sheet lis instability the core also provides a high resolution record of geomagnetic paleointensity that is consistent with but at higher temporal resolution than previous labrador sea records correlation to the greenland summit ice cores grip gisp2 is achieved by assuming that labrador sea detrital layers correspond to cold stadials in the ice cores this allows a gisp2 official chronology to be placed on md95 2024 which is consistent with the inverse correlation between paleointensity and the flux of cosmogenic isotopes 10 be and 36 cl in greenland ice cores synchronous millennial scale variability observed from the md95 2024 paleointensity

three millennia of directional variation of the earth s magnetic field in western europe as revealed by archeological artefacts directional secular variation of the geomagnetic field over the last 2000 years has been defined in western europe from numerous archeomagnetic studies however the number of archeomagnetic results for older periods is much more limited for this reason we present new data obtained from fired archeological structures found in two french sites loupiac and aspiran dated within the first millennium b c latest bronze earliest iron age transition 850 700 b c and iron age 525 475 b c respectively from a compilation of archeomagnetic results from western europe great britain italy and france and northern africa tunisia we propose a directional secular variation curve for western europe that covers the entire first millennium b c this curve exhibits a large clockwise motion with rapid changes during the first half of the millennium while the last four centuries b c are characterized by weak variations

modes and times of caldera resurgence the 10 ka evolution of ischia caldera italy from high precision archaeomagnetic dating abstract ischia is a well exposed and densely populated late quaternary caldera in the campanian magmatic province of italy ischia caldera experienced an average uplift rate of 3 3 cm year in the last ca 30 ka and is still actively resurging during the last 10 ka coeval with the resurgence a volcanic field of alkali trachytic to trachyandesitic lava domes lava flows tuff and scoria rings and pumice cones developed mainly on the eastern sector of the caldera along both resurgence related faults and regional nnw and ne striking faults in order to improve both our understanding of the recent volcanic history and the evaluation of future risks on ischia island a high precision archaeomagnetic dating method was applied to the products of 12 volcanic centres with probable age 14 c ages archaeomagnetic and other geochronological data as well as stratigraphic constraints show that during the studied time interval ischia volcanism occurred in five periods separated by phases of quiescence and coeval with earthquake and landslide events this fact suggests a pulsating mode of uplifting and deformations of the ischia resurgence during the two oldest periods of activity 7200 6800 bc and 4100 2300 bc resurgence probably produced a dome shaped structure location and geometry of vents suggest the occurrence of magma uprise along the fractures produced by bending of the overburden crustal block most of magma was emplaced as intrusions at the interior of the resurgent block whereas volcanism was represented by very viscous differentiated and crystallized lavas that emplaced as domes and high aspect ratio flows the resurgent dome caused recurrent lateral collapses that removed about 2 5 km 3 of rocks during the three youngest periods of activity 1800 1000 bc 650 bc 355 ad and 1302 ad resurgence affected a fault bounded asymmetric block this resulted from both a hydrostatic rebound of the crustal block after removal of material involved into huge slope instability triggering an increase in uplift rate and b new influxes of less evolved magma batches into the shallow reservoir that in turn favoured the intense volcanism of the last 4000 years

relative geomagnetic paleointensity and δ18o at odp site 983 gardar drift north atlantic since 350 ka abstract at odp site 983 relative geomagnetic paleointensity and planktic and benthic 18o records have been acquired for the last 350 kyr the mean sedimentation rate in this interval is 11 3 cm kyr magnetic properties and hysteresis ratios indicate that pseudo single domain magnetite is the remanence carrier volume susceptibility k anhysteretic arm and isothermal irm remanence values vary by a factor of 3 4 well within the criteria usually cited for paleointensity studies natural remanent magnetization nrm is normalized by arm and irm to acquire the paleointensity proxy arithmetic means of nrm arm and nrm irm calculated for five demagnetization steps in the 25 45 mt range constitute the relative paleointensity estimates some paleointensity lows particularly those at 40 120 and 188 ka are associated with directional excursions of the field especially the event at 188 ka referred to here as the iceland basin event that constitutes a short lived polarity reversal for the last 200 kyr the records can be correlated with other high resolution paleointensity records such as those from the labrador sea mediterranean somali basin and sulu sea implying that the millennial scale features are globally synchronous a labeling system for paleointensity features is proposed that ties prominent highs and lows to oxygen isotope stages

eruptions from zoned magma chambers summary an appreciation of the positions within a magma chamber from which magma will be erupted can be gained from an understanding of the pattern of magma removal upwardly converging streamlines followed by a fluid escaping from a flat topped reservoir into a cylindrical conduit cause magma from widely separated horizontal and vertical locations in a pre eruption chamber to be simultaneously erupted the volume erupted v is related to the maximum depth tapped r by the equation v 2 9 r 3 large ash flow sheets 100 km 3 may therefore sample magma up to 5 km below the chamber roof analysis of the flow field suggests that about one half of a given sample of erupted magma represents material from the deepest part of the chamber being tapped at the time of eruption many petrographic features of chemically zoned ignimbrites can best be explained by the operation of such a flow field during eruption where a series of sub spherical sampling shells intersect the horizontal layers in the magma chamber to produce heterogeneous ejecta mixed magmas erupted late in some zoned ash flow sequences can originate by tapping across the boundary between silicic and underplating basaltic magmas which coexist in the source chamber rather than by convective mixing prior to eruption flow banding in rhyolites can result from the same fluid dynamic processes

geomagnetic secular variation in historical lavas from the canary islands summary the age of historical lava flows on the canary islands can be traced back at least to 1585 ad during the last 400 years eruptions occurred at fairly regular intervals the palaeomagnetic directions of these lavas record the ambient field direction with high precision so that an historical secular variation curve svc can be constructed for the canaries which closely resembles the curve registered at paris the svc can be used as a dating tool for historical eruptions of unknown age and can probably be extended into the last 4000 5000 yr of recent volcanic activity on the canaries

paleomagnetism of spatter lavas from stromboli volcano aeolian islands italy implications for the age of paroxysmal eruptions 1 we report on 270 paleomagnetic directions retrieved from 17 different spatter deposits spread over the northern and western flanks of the stromboli volcano aeolian islands italy the spatter were emplaced during previously undated high energy strombolian eruptions occurring after the third seventh century ad our paleomagnetic directions were superimposed over the paleosecular variation curve of the geomagnetic field for the last two millennia obtained by reducing to the coordinates of stromboli the archeomagnetic and magnetic observatory data from france and italy this comparison suggests that most of the spatter exposed in the northern flank of the volcano were emplaced during several eruptions occurring between ca 1400 and 1600 ad while the western flank of stromboli was diffusely spread by spatter during the powerful xx century eruptions conversely there is no evidence for paroxysmal fire fountain activity occurring before 1400 ad and between 1600 and 1907 ad

archeoint an upgraded compilation of geomagnetic field intensity data for the past ten millennia and its application to the recovery of the past dipole moment 1 this paper presents a compilation of intensity data covering the past 10 millennia archeoint this compilation which upgrades the one of korte et al 2005 contains 3648 data and incorporates additional intensity and directional data sets a large majority of these data 87 were acquired on archeological artifacts and the remaining 13 correspond to data obtained from volcanic products the present compilation also includes important metadata for evaluating the intensity data quality and providing a foundation to guide improved selection criteria we show that 50 of the data set fulfill reasonable reliability standards which take into account the anisotropic nature of most studied objects potsherds the stability of the magnetization and the data dispersion the temporal and geographical distributions of this sub data set are similar to those of the main data set with 72 of the data dated from the past three millennia and 76 obtained from western eurasia approximately half of the selected intensity data are associated with at least an inclination value to constrain the axial and full dipole evolution over the past three millennia requires that we avoid any overrepresentation of the western eurasian data we introduce a first order regional weighting scheme based on the definition of eight widely distributed regions of 30 width within which the selected data are numerous enough the regional curves of virtual axial dipole moments vadm and of mixed vadm virtual dipole moments vdm averaged over sliding windows of 200 years and 500 years testify for strong contributions from either equatorial dipole or nondipole components the computation of global vadm and mixed vadm vdm variation curves assuming an equal weight for each region yields a dipole evolution marked by a distinct minimum around 0 b c a d followed by a maximum around the third fourth century a d a second minimum is present around the eighth century a d this variation pattern is compatible with the one deduced from earlier more

geomagnetic field variations as a dating tool application to sicilian lavas abstract over 40 historically dated mt etna lava flows have been carefully sampled for the purpose of palaeomagnetic analysis the natural remanent magnetization nrm of each lava flow contains a record of both the magnitude and direction of the earth s magnetic field at the time of its eruption our analysis has thus enabled us to determine the change of the magnetic field in sicily through historic times and to construct curves which show this secular variation of the field in both direction and magnitude in this paper the curves are used to reconsider the historical dates attributed to a number of etna lavas dates which had since been questioned by recent palaeomagnetic and geological studies in a similar manner these secular variation curves can be used as a powerful archaeological dating tool by comparison with the nrm of fired artifacts

evaluation of the multispecimen parallel differential ptrm method a test on historical lavas from iceland and mexico summary obtaining reliable geomagnetic palaeointensity pi estimates is still a difficult objective most common techniques in pi studies are based on various modifications of the thellier method but high failure rates and large uncertainties are common furthermore only rocks which obey stringent criteria are assumed to yield faithful pi estimates in general magnetic particles must be of single domain size the magnetization must be a thermoremanent magnetization and chemical alteration of magnetic carriers must be avoided during the laboratory experiment recently a new method for obtaining pis was proposed which on the grounds of results from two historic lavas and three simulated experiments is assumed to be independent of magnetic domain state this method uses multiple samples subjected to only one heating step at moderate temperatures here we present a test of the multispecimen parallel differential ptrm method on 11 historical lavas from iceland and mexico from which the actual field intensity is known either from magnetic observatory data or from magnetic field models our results show that the majority of pi estimates after the multispecimen parallel differential ptrm method yielded results that are very close or indistinguishable within the range of error from the expected intensity and thus largely confirm the findings from the initial study however there is in general a small overestimate which we show is associated with multidomain material when compared to a thellier type pi experiment carried out on sister samples from five icelandic lava flows the average multispecimen pi is closer to the actual field intensity samples from one lava characterized by low curie temperatures failed the thellier approach due to laboratory induced alteration but here the multispecimen method returned a reasonable pi even though there is evidence that the method is not entirely independent of magnetic domain state as previously proposed we regard this method as a valuable alternative in future pi studies

correlation of welded ignimbrites on pantelleria strait of sicily using paleomagnetism although the oldest volcanic rocks exposed at pantelleria strait of sicily are older than 300 ka most of the island is covered by the 45 50 ka green tuff ignimbrite thought to be related to the cinque denti caldera and younger lavas and scoria cones pre 50 ka rocks predominantly rheomorphic ignimbrites are exposed at isolated sea cliffs and their stratigraphy and chronology are not completely resolved based on volcanic stratigraphy and k ar dating it has been proposed that the older la vecchia caldera is related to ignimbrite q 114 ka and that ignimbrites f d and z 106 94 and 79 ka respectively were erupted after caldera formation we report here the paleomagnetic directions obtained from 23 sites in ignimbrite p 133 ka and four younger ignimbrites and from an uncorrelated and loosely dated welded lithic breccia thought to record a caldera forming eruption the paleosecular variation of the geomagnetic field recorded by ignimbrites is used as correlative tool with an estimated time resolution in the order of 100 years we find that ignimbrites d and z correspond in good agreement with recent ar ar ages constraining the d z eruption to 87 ka the welded lithic breccia correlates with a thinner breccia lying just below ignimbrite p at another locality implying that collapse of the la vecchia caldera took place at 130 160 ka this caldera was subsequently buried by ignimbrites p q f and d z paleomagnetic data also show that the northern caldera margin underwent a 10 west northwest outwards tilting after emplacement of ignimbrite p possibly recording magma resurgence in the crust

volcanic geology and eruption frequency sao miguel azores six volcanic zones comprise sao miguel the largest island in the azores all are quaternary in age except the last which is partly pliocene from west to east the zones are 1 the trachyte stratovolcano of sete cidades 2 a field of alkali basalt cinder cones and lava flows with minor trachyte 3 the trachyte stratovolcano of agua de pau 4 a field of alkali basalt cinder cones and lava flows with minor trachyte and tristanite 5 the trachyte stratovolcano of furnas and 6 the nordeste shield which includes the povoacao caldera and consists of alkali basalt tristanite and trachyte new radiocarbon and k ar ages augment stratigraphic data obtained during recent geologic mapping of the entire island and provide improved data to interpret eruption frequency average dormant intervals for the past approximately 3000 years in the areas active during that time are about 400 years for sete cidades 145 for zone 2 1150 for agua de pau and 370 for furnas however the average dormant interval at sete cidades increased from 400 to about 680 years before each of the past two eruptions and the interval at furnas decreased from 370 to about 195 years before each of the past four eruptions eruptions in zone 4 occurred about once every 1000 years during latest pleistocene and early holocene time none has occurred for about 3000 years the povoacao caldera truncates part of the nordeste shield and probably formed during the middle to late pleistocene calderas formed during latest pleistocene time at the three younger stratovolcanoes in the sequence outer agua de pau between 46 and 26 5 ka sete cidades about 22 ka inner agua de pau 15 2 ka and furnas about 12 ka normal faults are common but many are buried by holocene trachyte pumice most faults trend northwest or west northwest and are related to the terceira rift whose most active segment on sao miguel passes through sete cidades and zone 2 a major normal fault displaces nordeste lavas 150 250 m and may mark the location of an ancestral terceira rift recent seismicity e g in the 1980s generally has been scattered but some small earthquake swarms have occurred beneath the north eastern flank of agua de pau

new palaeointensity data from holocene icelandic lavas we present rock magnetic and palaeointensity data from holocene icelandic lava flows including historical and pre historical lavas susceptibility versus temperature measurements indicate that the magnetic mineralogy is dominated by primary titanomagnetite that have experienced variable degrees of high temperature oxidisation the analyses also demonstrate that virtually all studied samples are thermally stable up to their curie temperatures thellier palaeointensity experiments were carried out on 89 samples from fourteen different lava flows spanning the age of 167 to 9450 cal years bp microwave thellier experiments were used for one third of the samples while conventional thermal thellier was used for the remainder the two methods produced similar palaeointensities altogether reliable absolute palaeointensity estimates were obtained from all of the lava flows with values ranging from 38 2 5 0 mu t to 117 9 8 3 mu t the new results indicate that the holocene geomagnetic field intensity was more dynamic than suggested by the current field models possibly including brief intervals of anomalously high field strength c 2011 elsevier b v all rights reserved less