persistent scatterer interferometric synthetic aperture radar for crustal deformation analysis with application to volcan alcedo galapagos 1 while conventional interferometric synthetic aperture radar insar is a very effective technique for measuring crustal deformation almost any interferogram includes large areas where the signals decorrelate and no measurement is possible persistent scatterer ps insar overcomes the decorrelation problem by identifying resolution elements whose echo is dominated by a single scatterer in a series of interferograms existing ps methods have been very successful in analysis of urban areas where stable angular structures produce efficient reflectors that dominate background scattering however man made structures are absent from most of the earth s surface furthermore existing methods identify ps pixels based on the similarity of their phase history to an assumed model for how deformation varies with time whereas characterizing the temporal pattern of deformation is commonly one of the aims of any deformation study we describe here a method for ps analysis stamps that uses spatial correlation of interferogram phase to find pixels with low phase variance in all terrains with or without buildings prior knowledge of temporal variations in the deformation rate is not required for their identification we apply stamps to volcan alcedo where conventional insar fails because of dense vegetation on the upper volcano flanks that causes most pixels to decorrelate with time we detect two sources of deformation the first we model as a contracting pipe like body which we interpret to be a crystallizing magma chamber the second is downward and lateral motion on the inner slopes of the caldera which we interpret as landsliding

a multivariate mixing model for identifying sediment source from magnetic measurements a sequential method for quantitative identification of sediment source components based on magnetic measurements has been developed and tested for sediments from the rhode river maryland simulated mixing tests and multiple regression were employed to establish numerical relationships between source component proportions and the magnetic measurements of mixtures on the basis of these multivariate mixing models source components of three estuarine sediment cores were estimated by linear programming the results strongly support the previous studies on this catchment which indicated a dramatic change in sediment source some 150 to 200 yr ago quantitative calculations are more useful and informative than purely qualitative descriptions

coseismic interferograms of two ms 6 6 earthquakes in the south iceland seismic zone june 2000 we present insar observations of deformation due to two ms 6 6 earthquakes in the south iceland seismic zone in june 2000 coseismic deformation predominates in the ers interferograms range change due to mainly right lateral strike slip on n s striking faults amounting to more than 15 cm is observed although displacement is mainly perpendicular to the satellite look direction using elastic dislocation models in a trial and error scheme we find a best fitting model that agrees well with the aftershock locations and moment magnitudes estimated from seismograms the june 17 model has a fault patch 16 km long 10 km deep striking n05 e dipping 86 with a slip maximum of 2 40 m the june 21 model has a vertical patch 15 km long 9 km deep striking n01 w with a slip maximum of 2 15 m

loess palaeosol sequence in kashmir correlation of mineral magnetic stratigraphy with the marine palaeoclimatic record in this paper we use the mineral magnetic data and fd to stratigraphically correlate different loess palaeosol profiles in kashmir valley the two available thermoluminescence tl chronologies on the kashmir loess profiles have been reinterpreted to identify the soil of the last interglacial ca 125 000 yr bp we show that the mineral magnetic data correlates well with the global marine 18o record this correlation has revealed the well known milankovich periodicities of 40 ka and 20 ka in the loess palaeosol record on the basis of this study we conclude that loess deposition in the kashmir valley started around 200 ka and not 350 ka ago as suggested by earlier workers and that magnetic susceptibility variations provide a tool to decipher climatic changes from loess palaeosol sequences

the late pleistocene palaeoclimatic record of north atlantic deep sea sediments revealed by mineral magnetic measurements eight cores of north atlantic deep sea sediment are correlated using simple rapid and non destructive measurements of magnetic susceptibility and isothermal remanence this stratigraphy is demonstrably related to late pleistocene palaeoclimatic variations by comparison with the oxygen isotope and microfossil records of these cores fluctuations in the concentration dependent magnetic properties of the cores largely reflect their lithological composition consisting of climatically modulated cycles of marl glacial horizons and nannofossil chalk interglacial horizons high magnetic concentration characterises glacial horizons which correspond to periods when carbonate productivity was severely restricted and ice rafted detrital inputs were prolific coincident with the migration south of the north atlantic polar front to within 3 of latitude north of the coring site in contrast interglacial horizons are characterised by low magnetic concentration and high carbonate content of the sediment consequent on the retreat of the polar front and the re establishment of coccolith floras in its wake during these periods ratios formed between individual magnetic parameters are independent of magnetic concentration and are thus free of any climatic influences wrought by carbonate oscillations in this study however such ratios exhibit a definite response to palaeoclimatic variations attributable to differences in the composition and grain size of the magnetic mineral assemblage of glacial vis a vis interglacial horizons of the cores these differences in mineral magnetic composition are substantially confirmed by mossbauer effect studies of selected samples representing peak glacial and interglacial horizons of one core such glacial interglacial contrasts in the concentration mineralogy and grain size of the magnetic assemblage most probably result from variations in the source and flux density of detritral inputs induced by changes in climate x ray diffraction studies of the same core indicate that variations in the mineralogy of the detrital fraction of the sediment are consistent with those of the magnetic assemblage together these data point to the conclusion that a subsidiary low latitude possibly aeolian detrital flux supplements high latitude ice rafted input in glacial horizons of the cores the low latitude flux however is also identifiable in interglacial horizons independent evidence suggests that this low latitude aeolian flux may be derived from the sahel region of north africa and transported to the coring site by harmattan winds associated with saharan dust outbreaks

partly pedogenic origin of magnetic variations in chinese loess quaternary sequences of interbedded aeolian silts loess and buried soils palaeosols potentially provide one of the best terrestrial records of past climates1 5 magnetic susceptibility variations in loess and palaeosol sequences from china are strongly correlated with climate induced fluctuations of oxygen isotope ratios in deep sea sediments2 4 6 8 as an explanation of this correlation it has been suggested that the recorded variations in magnetic susceptibility depend primarily on the degree of dilution by non magnetic bulk loess constituents of a uniform magnetic assemblage derived from remote but unknown sources6 8 it has also been suggested that such a model involving changes in the accumulation rate of loess superimposed on a constant flux of magnetic particles provides a relative geological timescale through the control of loess deposition by astronomically modulated climate fluctuations7 here we report a preliminary test of this model using rock magnetic properties especially sensitive to variations in magnetic grain size our results show that palaeosols are characterized by much finer magnetic grain size assemblages than are the intervening loess units this suggests that a simple model based on constant magnetic influx and dilution by variable amounts of non magnetic loess is inadequate our magnetic measurements establish the close comparability of the fine grained magnetic minerals in the palaeosol samples to those in contemporary soils and thus point to a partially pedogenic origin for the magnetic mineral assemblages in the palaeosols

the record of magnetic mineral and heavy metal deposition at regent street bog fredricton new brunswick canada measurements of isothermal remanence and of heavy metal concentrations are presented for two recent moss increment dated cores from hummocks of ombrotrophic peat from regent street bog fredricton new brunswick canada the magnetic record from both profiles appears to be closely related to the documented history of industrial and urban development in the region the heavy metal record displays greater variation between elements and profiles and mirrors less consistently the documentary record some vertical mobility within the peat column is inferred for all the metal elements measured

satellite radar interferometry 1993 1999 suggests deep accumulation of magma near the crust mantle boundary at the krafla volcanic system iceland 1 deep magma accumulation near the crust mantle boundary 21 km depth at the krafla volcanic system is suggested from insar observations a best fit model derived from four interferograms covering 1993 1999 comprises an opening dike representing plate spreading and post rifting deformation and two mogi sources a mogi source deflating at a rate of 0 3 106 m3 yr coincides with the shallow krafla magma chamber while a deeper inflating mogi source further north at 21 km depth inflates at a rate of 26 106 m3 yr the inflating source is at or near the crust mantle boundary as identified by seismic studies and is interpreted as accumulating magma

mineral magnetic analyses as a means of lithostratigraphic correlation and provenance indication of glacial diamicts intra and interunit variation mineral magnetic analyses are applied to a series of diamict samples taken from the quaternary sequence in the isle of man in order to assess the relative magnitude of the intra and inter unit variation of the magnetic properties the results are analysed both qualitatively and quantitatively and show that the level of intra unit variation is significantly less than the level of inter unit variation the intra unit variation detected appears to show some relationship to the basic stratigraphy previous correlations based upon field evidence of diamict units exposed upon the east and west coasts of the isle of man are supported by their mineral magnetic properties the statistically significant differences in the magnetic mineralogy of the stratigraphically lower wyllin and shellag tills and the upper orrisdale ballavarkish and ballaquark tills suggests distinct provenances for these two suites of sediments allowing potential source areas to be suggested these source areas are consistent with published data on ice flow directions in the irish sea basin during the devensian

fault slip distribution of two june 2000 m w 6 5 earthquakes in south iceland estimated from joint inversion of insar and gps measurements abstract we present the first detailed estimates of co seismic slip distribution on faults in the south iceland seismic zone sisz an area of bookshelf tectonics we have estimated source parameters for two m w 6 5 earthquakes in the sisz on june 17 and 21 2000 through a joint inversion of insar and gps measurements our preferred model indicates two simple 15 km long near vertical faults extending from the surface to approximately 10 km depth the geometry is in good agreement with the aftershock distribution the dislocations experienced pure right lateral strike slip reaching maxima of 2 6 m and 2 9 m for the june 17 and 21 events respectively we find that the distribution of slip with depth may be correlated to crustal layering with more than 80 of the total geometric moment release occurring in the uppermost 6 km according to the distributed slip model the middle and upper crust appears to be more apt to generate large displacements than the lower crust the geodetic estimates of seismic moments are 4 4 10 18 nm m w 6 4 and 5 0 10 18 nm m w 6 5 the total moment released by the two events equals that generated by several decades of plate motion in the area but is only a fraction of the moment accumulated in the area since the last major earthquake in 1912

a new greenland deep ice core the polar ice sheets are rich sources of information on past atmospheric conditions including paleoclimates a new deep ice core has been drilled in south greenland comparison of the oxygen isotopic profile with that from camp century and with a deep sea foraminifera record indicates that the new core reaches back to about 90 000 years before present in a continuous sequence the details in the wisconsin part of the ice core records seem to be climatically significant and the general trends reveal all of the relevant emiliani stages recorded in deep sea cores the redated camp century record suggests a dramatic termination of the eem sangamon interglacial

examination of selected microparticles from the sentik glacier core ladakh himalaya india analyse par microscopie electronique et spectrometrie atomique rx dispersive des quatre groupes morphologiques des microparticules de l himalaya aplaties anguleuses biogeniques et anormales

abrupt increase in greenland snow accumulation at the end of the younger dryas event the warming at the end of the last glaciation was characterized by a series of abrupt returns to glacial climate the best known of which is the younger dryas event1 despite much study of the causes of this event and the mechanisms by which it ended many questions remain unresolved1 oxygen isotope data from greenland ice cores2 4 suggest that the younger dryas ended abruptly over a period of about 50 years dust concentrations2 4 in these cores show an even more rapid transition 20 years this extremely short timescale places severe constraints on the mechanisms underlying the transition but dust concentrations can reflect subtle changes in atmospheric circulation which need not be associated with a large change in climate here we present results from a new greenland ice core gisp2 showing that snow accumulation doubled rapidly from the younger dryas event to the subsequent preboreal interval possibly in one to three years we also find that the accumulation rate change from the oldest dryas to the bo11ing allerod warm period was large and abrupt the extreme rapidity of these changes in a variable that directly represents regional climate implies that the events at the end of the last glaciation may have been responses to some kind of threshold or trigger in the north atlantic climate system

the flickering switch of late pleistocene climate change polar ice contains a unique record of past climate variations previous greenland ice cores have documented relatively warm interstadial periods during the last glaciation and short centuryscale returns to colder conditions during the glacial to interglacial warming see for example ref 1 these climate features have also been observed to varying degrees in ocean sediment cores2 4 and terrestrial pollen and insect records5 7 here we report electrical conductivity measurements from a new greenland ice core which confirm these previous observations and also reveal a hitherto unrecognized mode of rapid climate variation fluctuations in ice conductivity on the scales of 5 20 years reflect rapid oscillations in the dust content of the atmosphere this flickering between two preferred states would seem to require extremely rapid reorganizations in atmospheric circulation

pressure sources versus surface loads analyzing volcano deformation signal composition with an application to hekla volcano iceland the load of lava emplaced over periods of decades to centuries induces a gradual viscous response of the earth resulting in measurable deformation this effect should be considered in source model inversions for volcanic areas with large lava production and flow emplacement in small centralized regions if deformation data remain uncorrected constructive load and pressure source interference may result in an overestimate of depth and volume of a magma reservoir whereas destructive signal interference may cause these values to be underestimated in both cases the source geometry preference could be biased the ratio of horizontal and vertical displacements aids the identification of composite signals we provide a method to quantify and remove the lava load deformation signals using deformation at hekla volcano iceland as an example

characterisation of soils by mineral magnetic measurements processes of iron mineral authigenesis diagenesis and dissolution in interaction with primary inputs of iron minerals act to produce vertical differentiation of soil magnetic properties thus magnetic iron oxides in common with other iron forms may both respond to and reflect soil forming processes investigation of the degree of detectable and persistent magnetic differentiation within different soil materials can provide insights into the nature and direction of distinct sets of soil processes evidence of clear association between magnetic variation and discrete types of soil environment also has applied significance in several areas of environmental study for example the observed magnetic variation can be used to discriminate between individual soils and soil horizons for the purposes of soil surveying sediment tagging and tracing and empirical modelling of source sediment linkages this paper reports the use of mineral magnetic techniques to characterise the iron oxide assemblages within soils of varying type and provenance the effect of different pedogenic regimes have been investigated through measurement of a range of magnetic parameters including magnetic susceptibility frequency dependent susceptibility and anhysteretic and saturation remanences contrary to current pedological thought but in accord with other magnetically based studies e g mullins the presence of magnetite maghemite typically of superparamagnetic single domain grain size within soils has been found to be an extremely widespread phenomenon its contemporary neoformation within the soil environment is inferred however the input of artificially generated ferrimagnetic material is indicated for one of the magnetically enhanced soils examined here and the possibility of extraneous sources of magnetite in topsoils should be considered before pedogenic processes of magnetic enhancement are inferred from the data presented discrimination of industrially derived magnetic particles appears possible on the basis of their very low levels of frequency dependent susceptibility fd low anhysteretic remanence arm and dissociation from the finer clay sized not the formation but the active dissolution of ferrimagnetic minerals is indicated for soils affected by the processes of gleying and podsolisation eluvial horizons of podsolised profiles demonstrate almost total loss of detectable magnetic content variations in the form of the iron reprecipitated in the underlying illuvial layers may reflect differences in the pedological characteristics of these horizons within gleyed waterlogged soils processes of magnetite dissolution appear to be grain size specific preferential reduction of those grains of ultrafine superparamagnetic and fine single domain magnetic grain size is indicated where soil forms a major contributor to catchment sediment loads this has significance in the area of source sediment modelling soil derived from permanently wet or highly leached catchment areas is unlikely to contribute to peaks in the magnetic content of deposited sediments further only those erosional processes that operate at a faster rate than those of magnetic depletion will have the capacity to produce peaks in sediment ferrimagnetic concentrations such as have been observed within the historical magnetic record

magnetization of polar ice a measurement of terrestrial dust and extraterrestrial fallout abstract laboratory induced remanent magnetization of polar ice constitutes a measurement of the magnetization carried by the ferromagnetic dust particles in the ice this non destructive technique provides a novel kind of information on the dust deposited on the surface of polar ice sheets measurements made on ice samples from greenland north grip ice core and antarctica vostok and epica dome c ice cores allowed the recognition of a fraction of magnetic minerals in ice whose concentration and magnetic properties are directly related to that of insoluble dust the source of this fraction of magnetic minerals thus appears closely related to terrestrial dust transport and deposition and its magnetic properties are informative of the dust provenance areas the rock magnetic properties of the dust may reflect distinct changes of dust source areas from glacial to interglacial periods in agreement with and adding further information to the isotopic 87 sr 86 sr and 143 nd 144 nd analyses a second magnetic fraction consists of particles of nanometric size which are superparamagnetic at freezer temperature and whose concentration is independent of the mass of aerosol dust found in the ice the source of these nanometric sized magnetic particles is ascribed to fallout of meteoric smoke and their concentration in ice was found to be compatible with the extraterrestrial fallout inferred from ir concentrations the diameter of the smoke particles as inferred from magnetic measurements is in the range of about 7 20 nm

interpretation of the chemical and physical time series retrieved from sentik glacier ladakh himalaya india analyse a partir de mesures de la radioactivite d analyses chimiques chlorures sodium fer silicates phosphates ammonium d 18 o et ph et de donnees physiques des variations cycliques temporelles des glaciers application aux paleoclimats

deep magma storage at hekla volcano iceland revealed by insar time series analysis 1 hekla volcano is one of the most active volcanoes in iceland the most recent eruption occurred from 26 february to 8 march 2000 when about 0 19 km3 of magma was erupted we present deformation data from multitemporal analyses of synthetic aperture radar sar images acquired between 1993 and 2008 focusing on pixels with low phase variance using persistent scatterer and small baseline approaches prior to and after the 2000 eruption we find a broad area of inflation around the volcano radius about 20 km with satellite line of sight los shortening of up to 5 mm yr we interpret this signal as the result of pressure increase in a deep seated magma chamber which we model as a spherical source at 14 20 km depth increasing in volume by 0 003 0 02 km3 yr within a 6 km radius of the summit of the volcano a los lengthening is superimposed on the broad inflation signal which correlates partly with recent lava flows we interpret this signal as the result of thermally contracting lava flows combined with viscoelastic yielding due to the load of the volcano and its lavas coeruptive deflation during the 2000 eruption was similar to the cumulative inflation from 1993 to 2000 and is consistent with a spherical magma chamber at 14 18 km depth that decreases in volume by 0 04 0 08 km3 interferograms spanning the 2000 eruption show a local coeruptive deformation signal near the eruptive fissure this is consistent with a dike opening from the surface to depths up to 5 8 km with a volume of 0 005 0 006 km3

seismic activity related to the 2000 eruption of the hekla volcano iceland the 2000 hekla eruption took place from february 26 to march 8 its seismic expressions were a swarm of numerous small earthquakes related to its onset and low frequency volcanic tremor that continued throughout the eruption a swarm of small earthquakes was observed some 80 min before the onset of the eruption and the size of the events increased with time low frequency volcanic tremor with a characteristic frequency band of 0 5 1 5 hz and dominant spectral peak s at 0 7 0 9 hz became visible at 18 19 gmt on february 26 marking the onset of the eruption the tremor amplitude rose quickly and was very high in the beginning of the eruption however it soon began to decrease after about an hour in general the seismic activity related to the 2000 hekla eruption was very similar to what was observed in the previous eruption in 1991 based on knowledge gained from seismicity and strain observations from 1991 this was the first time that a hekla eruption was predicted

frank oldfield and his contributions to environmental change research frank oldfield s immense contribution to palaeoenvironmental science is summarized in relation to pollen analysis vegetation history and peat stratigraphy palaeolimnology and 210pb dating environmental magnetism catchment lake relationships and soil erosional history multiproxy approaches pages and editing the holocene the tribute includes an introduction to the 14 other papers in this special issue which were inspired in diverse ways by this remarkable scientist

seismic constraints on magma chambers at hekla and torfajokull volcanoes iceland hekla and torfajokull are active volcanoes at a rift transform junction in south iceland despite their location next to each other they are physically and geologically very different hekla is an elongate stratovolcano built mainly of basaltic andesite torfajokull is a prominent rhyolitic centre with a 12 km diameter caldera and extensive geothermal activity the scope of this study is to examine the propagation of body waves of local earthquakes across the hekla torfajokull area and look for volumes of anomalous s wave attenuation which can be evidence of magma chambers so far the magma chamber under hekla has been modelled with various geophysical means and its depth has been estimated to be 5 9 km a data set of 118 local earthquakes providing 663 seismic rays scanning hekla and torfajokull was used in this study the major part 650 seismograms did not show evidence for s wave attenuation under these volcanoes only six seismograms had clear signs of s wave attenuation and seven seismograms were uncertain cases the data set samples hekla well at depths of 8 14 km and south part of it also at 4 8 km and 14 16 km western torfajokull is sampled well at depths of 4 14 km eastern and southern torfajokull at 6 12 km conclusions cannot be drawn regarding the existence of magma beyond these depth ranges also magma volumes of smaller dimensions than about 800 m cannot be detected with this method if a considerable molten volume exists under hekla it must be located either above 4 km or below 14 km the former possibility seems unlikely because hekla lacks geothermal activity and persistent seismicity usually taken as expressions of a shallow magma chamber an aseismic volume with a diameter of 4 km at the depth of 8 km in the west part of torfajokull has been inferred in earlier studies and interpreted as evidence for a cooling magma chamber our results indicate that this volume cannot be molten to a great extent because s waves travelling through it are not attenuated intense geothermal activity and low frequency earthquakes are possibly signs of magma in the south part of torfajokull but a magma chamber was not detected there in the areas sampled by this study

the magnetic record of inorganic fly ash deposition in lake sediments and ombrotrophic peats interest in identifying a geological marker signifying the starting point for the anthropocene has prompted an exploration of the stratigraphic record of inorganic particulates generated by industrial activities magnetic measurements of recent lake sediments and ombrotrophic peats are here used to reconstruct the history of deposition of inorganic fly ash spheres resulting mainly from solid fuel combustion and metal smelting the chronologies used have been based on moss increment counting radioisotope dating and pollen analysis the sites come from the united kingdom scandinavia and north america in several cases where detailed chronologies of both fly ash deposition and local industry can be compared the sequence of concentration linked magnetic measurements appears to capture accurately the record of industrial development despite incontrovertible evidence from other peat based records for some selective dissolution of magnetic minerals the dates at which magnetic concentration increases begin range from the 16th century in the peat profiles around the head of morecambe bay south cumbria in north west england where early iron manufacture using charcoal fuelled bloomery hearths is well documented to the mid 20th century at the remotest sites in arctic scandinavia the lake sediment profiles used here come mainly from the united kingdom and in most cases they date increases to the late 19th century or the first decades of the 20th century any attempt to use the magnetic record of fly ash deposition in lake sediments and or peats to mark the date chosen as the onset of the anthropocene would require careful choice of site location and archive bearing in mind the issue of selective magnetic mineral dissolution

the record of land use change and soil erosion in the late holocene sediments of the petit lac d annecy eastern france pollen chemical and magnetic analysis of a 6 m core from the petit lac d annecy provides a record of land use change and soil erosion spanning the last 2000 years variations in the magnetic properties of the sediments especially above 3 m in the core can be linked to the history of soil inwash over the last 1000 years these changes together with parallel ones in element concentrations and pollen frequencies both concentrations and percentages can be interpreted by reference to the land use history in the catchment during medieval and post medieval times peak deforestation and human impact on vegetation and soils is recorded from the eleventh to fourteenth centuries and again during the eighteenth century the present study suggests that future detailed analyses of the laminated sediments of the lake could provide a precisely datable fine resolution environmental history of the lake and its drainage basin

1983 2003 decaying rate of deflation at askja caldera pressure decrease in an extensive magma plumbing system at a spreading plate boundary new deformation data from the askja volcano iceland show that the volcano s caldera has been deflating continuously for over 20 years and confirm that the rate of subsidence is slowing down the decay in subsidence rate can be fitted with a function of the form e t where is 39 years reanalysis of gps data from 1993 1998 show that these data can be fitted with a model calling for two mogi point sources one shallow and another one much deeper 16 2 km depth pressure decrease occurs in both sources the deeper source is responsible for observed horizontal contraction towards askja at distances that cannot be explained by the shallower source plate spreading of 19 mm year distributed evenly over about 100 km wide zone is also favoured by the data

magnetic measurements of greenland and himalayan ice core samples this paper summarizes isothermal remanence irm300mt measurements from holocene ice core samples from two contrasting glaciological regimes i the greenland ice sheet and ii a himalayan glacier cores had been previously characterized in terms of chronologies geochemistry and for the himalayan core microparticle concentrations six samples from the greenland 20 d ice core from the time period ad 1876 1983 did not yield magnetic measurements above instrumental noise level significant isothermal remanence measurements irm300mt were obtained from some levels in the himalayan ice core scanning electron microscopy and edax analyses indicate weathered crustal material the techniques used require ice core samples with relatively high dust loadings and or larger volume ice samples

the millennium eruption of hekla in february 2000 the 18th historic eruption of hekla started on 26 february 2000 it was a short lived but intense event emitting basaltic andesitic 55 5 wt sio2 pyroclastic fragments and lava during the course of the eruption monitoring was done by both instruments and direct observations together providing unique insight into the current activity of hekla during the 12 day eruption a total of 0 189 km3 dre of magma was emitted the eruptive fissure split into five segments the segments at the highest altitude were active during the first hours while the segments at lower altitude continued throughout the eruption the eruption started in a highly explosive manner giving rise to a subplinian eruptive column and consequent basaltic pyroclastic flows fed by column collapses after the explosive phase reached its maximum the eruption went through three more phases namely fire fountaining strombolian bursts and lava effusion in this paper we describe the course of events of the eruption of hekla and the origin of its magma and then show that the discharge rate can be linked to different style of eruptive activity which are controlled by fissure geometry we also show that the eruption phases observed at hekla can be linked with inferred magma chamber overpressure prior to the eruption

mineral magnetic record of the chinese loess and paleosols variations in the magnetic susceptiblity of the chinese loess and its interbedded paleosols correlate strikingly well with the deep sea oxygen isotope record low susceptibility values are found for the loess layers and high values are found for the soil horizons two interpretative models have been advanced to account for these magnetic variations both of these models discount any significant in situ formation of magnetite during soil forming periods instead they infer relative concentration of detrital or atmospheric magnetite at these times we critically examine the assumptions made in these models and on the basis of some new mineral magnetic data and fromalized magnetic flux calculations offer an alternative explanation of the loess magnetic record our model identifies pedogenic formation of magnetite as the major contributor to the high magnetic susceptibility of the paleosols the formation and preservation of this pedogenic magnetite is dependent on soil froming conditions and hence reflects the regional climate

observed ground deformation at hekla iceland prior to and during the eruptions of 1970 1980 1981 and 1991 abstract ground tilt measurements in the vicinity of hekla have demonstrated a slow rate of deformation during periods of repose and rapid deformation during eruptions an eruption started at hekla on january 17 1991 after a repose period of almost 11 years this eruption was preceded by a slow inflation of the volcano centered 4 to 6 km north or northwest of the summit an uplift rate of 3 to 4 cm yr is indicated with a probable depth to the source of deformation of 5 to 6 km a deflation during the eruption is centered at the same location and the maximum subsidence was 30 40 cm the volume of the subsidence bowl is estimated as 0 05 0 1 km 3 if this subsidence is the result of removal of magma from a shallow reservoir then the volume of escaped magma is approximately twice the volume of the subsidence bowl hekla has erupted four times since the initiation of deformation measurements in 1968 first in may july 1970 then in august 1980 and again in april 1981 and finally in january february 1991 if the vertical component of ground deformation is used to estimate the relative magnitude of these eruptive events then the 1991 event is similar to or smaller than the 1970 event while the 1980 and 1981 events are significantly smaller

rock magnetism of late neogene and pleistocene deep sea sediments relationship to sediment source diagenetic processes and sediment lithology we use bivariate scatter plots to illustrate variations in selected rock magnetic properties low field susceptibility anhysteretic and isothermal remanence of late neogene and pleistocene deep sea sediments from 16 sites in the arctic ocean north atlantic equatorial atlantic and north pacific ocean and the arabian sea our intention is to examine the ability of the rock magnetic properties to differentiate the sediments according to factors such as lithology geographical area and the dominant mode of terrigenous sedimentation which at these sites is via ice rafting via bottom currents or via eolian processes overall correlations between sediment magnetic properties and gross lithology is poor and factors such as the source and transport path of terrigenous sediment and detrital magnetic minerals together with the action of reductive diagenetic processes are the major controls on the magnetic properties on the bivariate scatter plots sites with major ice rafted contributions tend to have high sedimentary ferrimagnetic mineral concentrations relatively coarse ferrimagnetic grain sizes and scattered sample point distributions in contrast sites where we infer significant bottom current supply of terrigenous material have tightly grouped sample point distributions carbonate sediments in which the terrigenous component is supplied by eolian processes tend to have a broad range of magnetic mineral concentration caused by glacial interglacial fluctuations in carbonate accumulation and eolian activity sediments containing significant volcanogenic material have high concentrations of relatively coarse grained ferrimagnetic material reductive diagenesis is a significant determinant of sediment magnetic properties in high productivity areas and has the effect of preferentially removing the fine grained ferrimagnetic fraction causing a coarsening of the ferrimagnetic grain size distribution and a rise in the antiferromagnetic ferrimagnetic ratio

evidence for general instability of past climate from a 250 kyr ice core record recent results1 2 from two ice cores drilled in central greenland have revealed large abrupt climate changes of at least regional extent during the late stages of the last glaciation suggesting that climate in the north atlantic region is able to reorganize itself rapidly perhaps even within a few decades here we present a detailed stable isotope record for the full length of the greenland ice core project summit ice core extending over the past 250 kyr according to a calculated timescale we find that climate instability was not confined to the last glaciation but appears also to have been marked during the last interglacial as explored more fully in a companion paper3 and during the previous saale holstein glacial cycle this is in contrast with the extreme stability of the holocene suggesting that recent climate stability may be the exception rather than the rule the last interglacial seems to have lasted longer than is implied by the deep sea specmap record4 in agreement with other land based observations5 6 we suggest that climate instability in the early part of the last interglacial may have delayed the melting of the saalean ice sheets in america and eurasia perhaps accounting for this discrepancy

seismicity around the hekla and torfajokull volcanoes iceland during a volcanically quiet period 1991 1995 the volcano hekla in south iceland had its latest eruption in january march 1991 the eruption was accompanied and followed by considerable seismic activity this study examines the seismicity in the hekla region 63 42 64 18 n 18 30 20 12 w during a period when the high activity related to the eruption had ceased from july 1991 to october 1995 the aim is to define the level of the normal background seismicity of the area that can be compared to the eruption related activity the hekla volcano proper was generally aseismic during the study period the most prominent earthquake cluster is in the neighbouring torfajokull volcano the epicentres are concentrated in the western part of the caldera and west of it the hypocentres are located at all depths from the surface down to 14 km with highest activity at 5 12 km inside this cluster in the northwest part of the caldera is a spherical volume void of earthquakes approximately 4 km in diameter and centred at 8 km depth this is interpreted as a cooling magma body small low frequency events of volcanic origin were occasionally recorded at torfajokull this activity has mainly occurred in swarms and was most abundant during the first year of the study period presumably reflecting some kind of connection to the 1991 hekla eruption our study area also includes the easternmost section of the south iceland seismic zone a transform zone characterized by bookshelf faulting on transverse faults two lineaments of epicentres were identified roughly corresponding to mapped faults of the south iceland seismic zone the hypocentres are relatively deep mainly at 6 12 km matching the general trend of hypocentral depth increasing toward the east the seismicity is highest in the area of the mapped faults however the epicentres extend beyond them and indicate greater width of the south iceland seismic zone or 20 30 km rather than approximately 10 km as indicated by the length of the surface faults the seismicity in the volcanic systems of hekla and vatnafjoll shows some characteristics of the south iceland seismic zone epicentres are concentrated into two n s lineaments one of which coincides with the location of the 1987 vatnafjoll earthquake mw 5 9 a strike slip event on a n to s trending fault the hypocentres of the hekla vatnafjoll events are mainly at 8 13 km depth which indicates a continuation of the depth trend of the earthquakes of the south iceland seismic zone the events located at hekla proper and immediately north of it are all of low frequency character which can be held as an indication of volcanic origin on the other hand they show clear s arrivals at observing stations like normal high frequency tectonic earthquakes

revel a model for recent plate velocities from space geodesy 1 we present a new global model for recent plate velocities revel describing the relative velocities of 19 plates and continental blocks the model is derived from publicly available space geodetic primarily gps data for the period 1993 2000 we include an independent and rigorous estimate for gps velocity uncertainties to assess plate rigidity and propagate these uncertainties to the velocity estimates the velocity fields for north america eurasia and antarctica clearly show the effects of glacial isostatic adjustment and australia appears to depart from rigid plate behavior in a manner consistent with the mapped intraplate stress field two thirds of tested plate pairs agree with the nuvel 1a geologic 3 myr average velocities within uncertainties three plate pairs caribbean north america caribbean south america and north america pacific exhibit significant differences between the geodetic and geologic model that may reflect systematic errors in nuvel 1a due to the use of seafloor magnetic rate data that do not reflect the full plate rate because of tectonic complexities most other differences probably reflect real velocity changes over the last few million years several plate pairs arabia eurasia arabia nubia eurasia india move more slowly than the 3 myr nuvel 1a average perhaps reflecting long term deceleration associated with continental collision several other plate pairs including nazca pacific nazca south america and nubia south america are experiencing slowing that began 25 ma the beginning of the current phase of andean crustal shortening

isothermal remanent magnetization of greenland ice preliminary results the magnetic mineral content of wind transported dust should reflect atmospheric transport dynamics and conditions in its source areas and could thus be used as an environmental proxy to test the feasibility of determining the magnetic mineral content in polar ice isothermal remanent magnetization irm was measured on a small suite of greenland ice samples of holocene interglacial and last glacial maximum lgm age although the extremely low dust concentrations limit weak field susceptibility measurements all samples contained an easily measurable concentration of magnetic minerals that can be estimated using irm intensity provided that special precautions are used irm experiments at liquid nitrogen temperatures indicate ice magnetic properties which are consistent with that expected from varying concentrations of magnetite or maghemite interestingly the holocene ice samples tend to have higher magnetic concentrations despite having much lower total polar dust contents than the few lgm ice samples tested thus far

magnetization of greenland ice and its relationship with dust content 1 we estimate the concentration of fine magnetic particles in ice samples from the north greenland ice core project core from the central greenland ice sheet using lowtemperature 77k isothermal remanent magnetization irm analysis and compare it with the mass concentration of aerosol dust samples were taken from six climatic intervals spanning the time from the holocene preboreal back to the last glacial dansgaard oeschger cycle 5 the mean irm intensity of the ice varies by a factor of 3 from glacial to interglacial stages being lower during interglacials the irm acquisition curves of the ice do not quite saturate at the maximum available field of 0 8 t and show a relatively broad coercivity which is compatible with a mixture of maghemite or magnetite and hematite comparison of the irm intensity and total dust mass shows a remarkably good correlation but also reveals a large background magnetization which may be essentially constant over the different climatic stages irm suggests that the dust properties are independent of the background signal and that the dust aerosol has a magnetization within about 30 of pristine loess from the chinese loess plateau which is considered to have the same source in the same east asian deserts as dust in greenland ice ice contamination and the flux of extraterrestrial dust particles were considered in order to explain the origin of the background magnetization nevertheless we could not find a convincing explanation for this signal which represents a considerable part of the irm signal and is the dominant component during interglacial intervals without invoking the presence of undetected dust mass the alternative hypothesis of a varying magnetization of the ice dust at different climatic periods would suggest that different sources of aerosol are active during different climatic periods this however has not proven to be the case so far for studies of the provenance of dust in greenland ice index terms 0305 atmospheric composition and structure aerosols and particles 0345 4801 0399 atmospheric composition and structure general or miscellaneous 0394 atmospheric composition and structure instruments and techniques keywords magnetization irm greenland ice core northgrip aerosol