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

abrupt changes in deep atlantic circulation during the transition to full glacial conditions 1 six ocean drilling program odp sites in the northwest atlantic have been used to investigate kinematic and chemical changes in the western boundary undercurrent wbuc during the development of full glacial conditions across the marine isotope stage 5a 4 boundary 70 000 years ago sortable silt mean grain size ss measurements are employed to examine changes in near bottom flow speeds together with carbon isotopes measured in benthic foraminifera and planktic foraminiferal fragmentation as proxies for changes in water mass chemistry a depth transect of cores spanning 1 8 4 6 km depth allows changes in both the strength and depth of the wbuc to be constrained across millennial scale events ss measurements reveal that the flow speed structure of the wbuc during warm intervals interstadials was comparable to modern holocene conditions however significant differences are observed during cold intervals with higher relative flow speeds inferred for the shallow component of the wbuc 2 km depth during all cold stadial intervals including heinrich stadial 6 and a substantial weakening of the deep component 3 4 km during full glacial conditions our results therefore reveal that the onset of full glacial conditions was associated with a regime shift to a shallower mode of circulation involving glacial north atlantic intermediate water that was quantitatively distinct from preceding cold stadial events furthermore our chemical proxy data show that the physical response of the wbuc during the last glacial inception was probably coupled to basin wide changes in the water mass composition of the deep northwest atlantic

sheathfolds in rheomorphic ignimbrites structural reappraisal of several classic rheomorphic ignimbrites in colorado idaho the canary islands and italy has for the first time revealed abundant oblique folds curvilinear folds and sheathfolds which formed during emplacement like their equivalents in tectonic shear zones the sheathfold axes lie sub parallel to a pervasive elongation lineation and appear as eye structures on rock surfaces normal to the transport direction with the recognition of sheathfolds ignimbrites previously inferred to have undergone complex rheomorphic deformation histories are re interpreted as recording a single progressive deformation event in some examples the trends of sheathfolds and related lineations change with height through a single ignimbrite suggesting that rheomorphism did not affect the entire thickness of ignimbrite synchronously instead we infer that in these ignimbrites a thin ductile shear zone rose gradually through the aggrading agglutinating mass whilst the flow direction varied with time this suggests that in some cases both welding and rheomorphism can be extremely rapid with ductile strain rates significantly exceeding rates of ignimbrite aggradation

the deglacial evolution of north atlantic deep convection deepwater formation in the north atlantic by open ocean convection is an essential component of the overturning circulation of the atlantic ocean which helps regulate global climate we use water column radiocarbon reconstructions to examine changes in northeast atlantic convection since the last glacial maximum during cold intervals we infer a reduction in open ocean convection and an associated incursion of an extremely radiocarbon 14c depleted water mass interpreted to be antarctic intermediate water comparing the timing of deep convection changes in the northeast and northwest atlantic we suggest that despite a strong control on greenland temperature by northeast atlantic convection reduced open ocean convection in both the northwest and northeast atlantic is necessary to account for contemporaneous perturbations in atmospheric circulation

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

size sorting in marine muds processes pitfalls and prospects for paleoflow speed proxies the basis for and use of fine grain size parameters for inference of paleoflow speeds is reviewed here the basis resides in data on deposited sediment taken in conjunction with flow speed measurements in the field experimental data on suspended sediment transport and deposition and theoretical treatments of the generation of size distributions of deposits from suspension controlled by particle settling velocity and flow speed in the deep sea sorting events occur under resuspension deposition events in benthic storms at flow speeds below 10 15 cm s 1 size in the noncohesive sortable silt 10 63 m range is controlled by selective deposition whereas above that range removal of finer material by winnowing also plays a role the best particle size instruments to measure a flow speed related grain size employ the settling velocity method while laser diffraction sizers can yield misleading results because of particle shape effects potential problems including source effects downslope supply on continental margins spatial variability of flow over bedforms and influence of ice rafted detritus are examined a number of studies using the sortable silt flow speed proxy are reviewed and inverse modeling of grain size distributions is examined outstanding problems are that corroboration is sparse because almost no studies have yet used the full range of proxies for flow rate and water mass identification and that the sortable silt mean size is not yet properly calibrated in terms of flow speed

rapid changes of glacial climate simulated in a coupled climate model abrupt changes in climate termed dansgaard oeschger and heinrich events have punctuated the last glacial period approximately 100 10 kyr ago but not the holocene the past 10 kyr here we use an intermediate complexity climate model to investigate the stability of glacial climate and we find that only one mode of atlantic ocean circulation is stable a cold mode with deep water formation in the atlantic ocean south of iceland however a warm circulation mode similar to the present day atlantic ocean is only marginally unstable and temporary transitions to this warm mode can easily be triggered this leads to abrupt warm events in the model which share many characteristics of the observed dansgaard oeschger events for a large freshwater input such as a large release of icebergs the model s deep water formation is temporarily switched off causing no strong cooling in greenland but warming in antarctica as is observed for heinrich events our stability analysis provides an explanation why glacial climate is much more variable than holocene climate

deepwater circulation on blake outer ridge western north atlantic during the holocene younger dryas and last glacial maximum three depth transects containing a total of 33 sediment cores were investigated along the blake outer ridge in the western subtropical north atlantic sortable silt mean grain size and stable isotope records were used to assess the position and relative intensity of the western boundary undercurrent wbuc during the holocene the last glacial maximum lgm and the younger dryas yd intervals the holocene reconstruction is consistent with modern physical and chemical hydrographic measurements in the area suggesting a deep position for the fast flowing core of the wbuc 3000 4000 m deepening to 4500 m water depth on the ridge flanks and a water column dominated by north atlantic deep water nadw the lgm and yd reconstructions show that a comparable hydrographic regime was present during both these intervals suggesting a similar mode of circulation that was appreciably different from the holocene reconstruction the wbuc s zone of maximum flow speed during these intervals is suggested to have shifted above 2500 m water depth consistent with nutrient depleted glacial north atlantic intermediate water formation with an increasing influence of southern source water ssw beneath below 4000 m water depth results hint at increased ssw flow vigor during both the lgm and yd with higher flow speeds than during the holocene this study provides a framework for aiding the interpretation of time series records of paleocurrent flow speed changes in the region of the wbuc

volatiles in pantellerite magmas a case study of the green tuff plinian eruption island of pantelleria italy abstract the green tuff gt plinian eruption the largest in magnitude at pantelleria erupted 3 to 7 km3 dre of pantellerite magma and a small volume of trachyte fifty nine anorthoclase hosted melt inclusions from the two basal pumice members were analyzed by ft ir spectroscopy in order to assess the pre eruptive h2o content in the pantellerite melt microanalytical methods were used to determine major element cl f and s contents melt inclusions and glassy groundmasses have a nearly homogeneous pantelleritic composition peralkaline index 1 9 2 2 and variable water contents ranging from 1 4 to as high as 4 2 wt i e much higher than the 1 4 wt of earlier published studies the chlorine content is constant at about 1 wt combined cl and h2o data were used to estimate a confining pressure of about 50 mpa depth around 2 3 km for the gt magma chamber the chamber was characterized by a compositional zoning with a dominant pantellerite overlying a trachyte magma soon after the gt eruption intra caldera volcanism was dominated by the eruption of voluminous trachyte lava flows while pantellerite melt production resumed after about 20 ka with numerous low volume mildly explosive strombolian to effusive eruptions comparison with data from the literature reveals that despite the different explosivity the post caldera strombolian eruptions and the gt plinian eruption were fed by pantelleritic magmas with similar water contents chlorine and co2 contents suggest that the young magma reservoirs feeding the strombolian to effusive activity were deeper h 4 5 km than the much larger based on erupted volumes magma chamber which fed the gt eruption

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

petrology and geochemistry of submarine volcanism in the sicily channel rift abstract submarine magmatism in the sicily channel rift began in the early pliocene and lasted until almost 200 yr ago we present here petrological and geochemical data on volcanic rocks dredged from graham and nameless banks and pantelleria seamounts in the pelagian sector of the sicily channel rift petrological evidence suggests that the ascent of magmas to the surface was relatively rapid probably through channels superimposed over the major tectonic discontinuities of the rift major and trace element data indicate an ocean island basalt affinity for graham and nameless bank alkaline lavas and a depleted tholeiitic signature for one pantelleria seamount which had a shallower mantle source sr nd pb isotopic compositions suggest a heterogeneous mantle source involving both focus zone fozo and high himu components lead isotopic compositions are slightly less radiogenic in the tholeiitic basalt and more radiogenic closer to himu in the alkaline lavas we hypothesize that a decompressional me

emplacement and rheomorphic deformation of a large lava like rhyolitic ignimbrite grey s landing southern idaho the miocene grey9s landing ignimbrite reaches 70 m thick and covers at least 400 km 2 in the central region of the snake river plain it shows particularly intense welding and rheomorphic deformation and although parts are eutaxitic most is lava like with flow banding and no fiamme a near ubiquitous penetrative flow lamination associated with a well developed elongation lineation is folded into small intrafolial tight to isoclinal oblique and sheath folds which are refolded by larger folds in the upper parts structural and kinematic analysis reveals that welding and early deformation occurred rapidly during deposition from a very hot 1000 c high mass flux pyroclastic density current that flowed westward across a graben faulted landscape as hot particles were deposited they rapidly agglutinated and coalesced and underwent noncoaxial shear in a subhorizontal ductile shear zone close to the current deposit interface the shear zone is interpreted to have been less than 2 m thick it produced and deformed the rheomorphic fabric and it migrated upward with the rising current deposit interface during aggradation so that it transiently affected all levels of the resultant thick ignimbrite deformation was progressive and after the density current had dissipated viscous spreading and downslope flow continued and involved an increasingly thick portion of the sheet this folded the flow banding and f 1 intrafolial isoclines into larger sheath folds and into more upright periclines near the top of the ignimbrite we demonstrate that structural and kinematic analysis can elucidate the emplacement history of rheomorphic ignimbrites

the thermal threshold of the atlantic meridional overturning circulation and its control by wind stress forcing during glacial climate 1 paleo proxy data suggest that the atlantic meridional overturning circulation amoc was shallower and weaker at the last glacial maximum lgm than at present in this study we have identified the existence of a thermal threshold of the amoc which may explain why many coupled climate models fail to simulate the weaker amoc during the lgm by using results obtained from a coupled climate model and conducting sensitivity simulations with an ocean general circulation model we found that the sudden transition from the present day amoc to the weaker glacial amoc occurs when we gradually change the degree of surface cooling from present day to glacial conditions this result is related to response of deep convection in the northern north atlantic ocean moderate cooling enhances deep convection whereas sufficient cooling results in total covering of sea ice there and suppression of deep convection the findings indicate the existence of a thermal threshold controlling the amoc where the present day type amoc suddenly shifts to the weaker glacial amoc once the surface cooling exceeds this threshold we also demonstrate that wind stress forcing plays a critical role in controlling the value of the thermal threshold our study suggests that slight differences in the degree of surface cooling or wind stress forcing for lgm simulations could lead to the very different response of the amoc during the lgm as reported in previous lgm simulations

paleomagnetic dating of the most recent silicic eruptive activity at pantelleria strait of sicily we report on the paleomagnetism of ten sites in the products of the most recent silicic eruptive cycle of pantelleria strait of sicily previously radiometrically dated at 5 10 ka our comparison with proxies for geomagnetic field directions allows us to narrow considerably the time window during which these eruptions occurred the strongly peralkaline composition causes the magmas to have low viscosities locally resulting in strong agglutina tion of proximal fall deposits this allows successful extraction of paleomagnetic directions from the explosive phases of eruptions one of our sites was located in the serra della fastuca fall deposit produced by the first explosive event of the eruptive cycle the other nine sites were located in the most recent explosive pumice fall and agglutinate from cuddia del gallo and cuddia randazzo and effusive khaggiar lava products the very similar paleomagnetic directions gathered from eight internally consistent sites were compared to reference geomagnetic field directions of the last 5 10 ka directions from cuddia del gallo agglutinate and khaggiar flows translate into 5 9 to 6 2 ka ages whereas the fastuca pumices yield a slightly older age of 6 2 6 8 ka hence the most recent silicic eruptive cycle lasted at most a millennium and as little as a few centuries around 6 0 ka paleomagnetically inferred ages are in good agreement with published and calibrated by us 14 c dates from paleosols charcoals sampled below the studied volcanic units whereas k ar data are more scattered and yield 30 older ages our data show that the time elapsed since the most recent silicic eruptions at pantelleria is comparable to the quiescence period separat ing the two latest volcanic cycles

phase equilibrium constraints on pre eruptive conditions of recent felsic explosive volcanism at pantelleria island italy we present experimental phase equilibria carried out on a pantelleritic bulk rock composition with a peralkalinity index pi molar na2o k2o al2o3 1 4 which is representative of the most energetic pumice fall eruption of the resurgent post caldera volcanism on pantelleria island for the explored conditions p 25 150 mpa t 680 800 c h2omelt up to 6 wt fo2 nno where nno is nickel nickel oxide buffer clinopyroxene is the liquidus phase followed by alkali feldspar and then quartz the crystallization of amphibole is limited to temperatures below 700 c aenigmatite crystallizes near the liquidus for p 100 mpa when clinopyroxene is the sole liquidus phase the coexisting melt is characterized by a decrease in peralkalinity index to 1 2 if compared with the starting material approaching comendite composition in contrast when alkali feldspar joins the liquidus phase assemblage residual melts become extremely peralkaline pi 3 2 with feo contents in excess of 14 wt comparison of our experiments with natural phase abundances and compositions constrains the depth of the fastuca pantellerite reservoir to be around 5 1 km or p 120 20 mpa at t 730 10 c with h2omelt around 4 wt and fo2 1 5 log unit below nno such conditions correspond to the dominant phase assemblage of feldspar clinopyroxene and aenigmatite amphibole occurrence requires both higher melt water contents and lower temperatures up to 6 5 wt at 680 c and may reflect provenance from a deeper and wetter portion of the tapped reservoir in contrast quartz is more probably related to crystallization during magma ascent which explains the lower melt water content of its melt inclusions overall the results confirm that pantellerites may be both water rich and reduced magmas these characteristics imply that any coexisting fluid phase in the reservoir will be dominated by water and hydrogen species the latter amounting to a few mole per cent magma unrest at pantelleria may thus be heralded by emissions of hydrous and reduced gases i e co2 poor and h2 rich if the present day reservoir is felsic in composition

deep western boundary current variability in the subtropical northwest atlantic ocean during marine isotope stages 12 10 high resolution 125 500 year temporal resolution sortable silt mean grain size data of subtropical northwest atlantic ocean drilling program odp leg 172 site 1061 monitor fluctuations of the deep western boundary current dwbc throughout marine oxygen isotope stages mis 12 10 330 460 kyr in great detail comparison with foraminiferal stable isotope data from the same site and odp site 1063 bermuda rise data of poli et al 2000 shows that the dwbc shoals during glacial times in relation to a reduced north atlantic deep water production and increased advection of southern ocean waters superimposed fluctuations in the flow speed position of the dwbc at a semiprecession scale are persistent throughout mis 10 12 and independent of the glacial state additional millennial scale variations in dwbc flow speed parallel surface water cooling events and ice rafted debris episodes especially at the transition of mis 11 10 indicating the sensitivity of deep current flow variations to changes in the surface water conditions these millennial scale variations are amplified when benthic foraminiferal 18o values exceed 3 8 and may be related to the transition of an ice volume threshold where climate variability is possibly amplified by feedback mechanisms associated with ice sheet growth

phase relationships between millennial scale events 64 000 24 000 years ago a core recovered on the iberian margin off southern portugal can be correlated with greenland ice cores using oxygen isotope variability in planktonic foraminifera which closely matches the ice core records of temperature over greenland our age model identifies the base of every interstadial between 64 000 and 24 000 years ago and uses the greenland ice core project grip timescale the oxygen isotope signal in benthic foraminifera on this grip based timescale is quite different from the planktonic record and resembles the temperature record over antarctica when this is synchronized with greenland using the record of methane in the atmospheric air in the polar ice cores we interpret the benthic record as indicating significant fluctuations in ice volume during millennial events and we suggest that antarctic temperature changed as a function of ice volume

the origin of trachyte and pantellerite from pantelleria italy insights from major element trace element and thermodynamic modelling abstract trachyte and peralkaline rhyolite pantellerite and comendite frequently comprise the felsic end member in bimodal suites in continental rift and oceanic island settings in these settings the relationship between the mafic mildly alkaline or transitional basalt and felsic lavas is ambiguous major and trace element models and isotopic data are often consistent with an origin for felsic lavas from either fractional crystallization of transitional basalt or partial melting of alkali gabbro followed by fractional crystallization in this paper we present representative mineral analyses and whole rock analyses from forty samples of a basalt trachyte pantellerite suite collected at pantelleria italy in the strait of sicily rift zone and compare the results of major and trace element modelling with the results of thermodynamic melts modelling from these results we conclude that metaluminous trachyte formed as a result of 70 to 75 low pressure 0 1 gpa fractional crystallization of an assemblage of plagioclase clinopyroxene olivine magnetite and apatite from a hydrous 1 0 1 5 wt h2o transitional basalt magma at relative oxygen fugacities approximately one log unit below the fayalite magnetite quartz buffer fmq 1 the daly gap a lack of intermediate 49 62 wt sio2 volcanic rocks at pantelleria is concluded to be primarily the result of rapid differentiation through that interval relatively rapid crystallization at low pressure may have effected the partial degassing of water saturated 4 wt h2o metaluminous trachyte magma some metaluminous trachyte lavas have positive eu anomalies high k rb ratios high concentrations of ba and low concentrations of incompatible trace elements these are interpreted to be the result of up to 40 accumulation of alkali feldspar comenditic trachyte pantelleritic trachyte and pantellerite formed after an additional 20 to 80 fractional crystallization of an assemblage dominated by alkali feldspar from metaluminous trachyte magma the most evolved pantellerite lavas and tuffs are the result of a total of 95 fractional crystallization of transitional basalt with phenocrysts that equilibrated at low temperatures 4 wt water contents

confort 15 model of conduit dynamics applications to pantelleria green tuff and etna 122 bc eruptions numerical simulations are useful tools to illustrate how flow parameters and physical processes may affect eruption dynamics of volcanoes in this paper we present an updated version of the conflow model an open source numerical model for flow in eruptive conduits during steady state pyroclastic eruptions mastin and ghiorso in a numerical program for steady state flow of magma gas mixtures through vertical eruptive conduits u s geological survey open file report 00 209 2000 in the modified version called confort 15 the rheological constraints are improved incorporating the most recent constitutive equations of both the liquid viscosity and crystal bearing rheology this allows all natural magma compositions including the peralkaline melts excluded in the original version to be investigated the crystal bearing rheology is improved by computing the effect of strain rate and crystal shape on the rheology of natural magmatic suspensions and expanding the crystal content range in which rheology can be modeled compared to the original version conflow is applicable to magmatic mixtures with up to 30 vol crystal content moreover volcanological studies of the juvenile products crystal and vesicle size distribution of the investigated eruption are directly incorporated into the modeling procedure vesicle number densities derived from textural analyses are used to calculate through toramaru equations maximum decompression rates experienced during ascent finally both degassing under equilibrium and disequilibrium conditions are considered this allows considerations on the effect of different fragmentation criteria on the conduit flow analyses the maximum volume fraction criterion porosity criterion the brittle fragmentation criterion and the overpressure fragmentation criterion simulations of the pantelleritic and trachytic phases of the green tuff pantelleria and of the plinian etna 122 bc eruptions are performed to test the upgrades in the confort 15 modeling conflow and confort 15 numerical results are compared analyzing the effect of viscosity decompression rate temperature fragmentation criteria critical strain rate porosity and overpressure criteria and equilibrium versus disequilibrium degassing in the magma flow along volcanic conduits the equilibrium simulation results indicate that an increase in viscosity a faster decompression rate a decrease in temperature or the application of the porosity criterion in place of the strain rate one produces a deepening in fragmentation depth initial velocity and mass flux of the mixture are directly correlated with each other inversely proportional to an increase in viscosity except for the case in which a faster decompression rate is assumed taking into account up to date viscosity parameterization or input faster decompression rate a much larger decrease in the average pressure along the conduit compared to previous studies is recorded enhancing water exsolution and degassing disequilibrium degassing initiates only at very shallow conditions near the surface brittle fragmentation i e depending on the strain rate criterion in the pantelleritic green tuff eruption simulations is mainly a function of the initial temperature in the case of the etna 122 bc plinian eruption the viscosity strongly affects the magma ascent dynamics along the conduit using confort 15 and therefore incorporating the most recent constitutive rheological parameterizations we could calculate the mixture viscosity increase due to the presence of microlites results show that these seemingly low viscosity magmas can explosively fragment in a brittle manner mass fluxes resulting from simulations which better represent the natural case i e microlite bearing are consistent with values found in the literature for plinian eruptions 106 kg s the disequilibrium simulations both for green tuff and etna 122 bc eruptions indicate that overpressure sufficient for fragmentation if present occurs only at very shallow conditions near the surface

a geostrophic transport estimate for the florida current from the oxygen isotope composition of benthic foraminifera we present a new method for the quantitative reconstruction of upper ocean flows for during times in the past for the warm t 5 c surface ocean density can be accurately reconstructed from calcite precipitated in equilibrium with seawater as both of these properties increase with decreasing temperature and increasing salinity vertical density profiles can be reconstructed from the oxygen isotopic composition of benthic foraminifera the net volume transport between two vertical density profiles can be calculated using the geostrophic method using benthic foraminifera from surface sediment samples from either side of the florida straits florida keys and little bahama bank we reconstruct two vertical density profiles and calculate a volume transport of 32 sv using this method this agrees well with estimates from physical oceanographic methods of 30 32 sv for the mean annual volume transport we explore the sensitivity of this technique to various changes in the relationship between temperature and salinity as well as salinity and the oxygen isotopic composition of seawater

nested calderas and trapdoor uplift at pantelleria strait of sicily pantelleria is dominated by a caldera 6 km in diameter that formed 55 000 yr ago during eruption of a rheomorphic fall deposit that is zoned from pantellerite to trachyte trachytic lavas then built a shield that nearly filled the caldera magma subsequently reinflated the shallow reservoir and raised the northwest half of the trachyte shield 275 m creating montagna grande by trapdoor uplift the lid eventually failed and pantellerite magma erupted as domes flows and pumice cones along the trapdoor faults and their hingeline we have identified a second caldera 93 000 yr old within which the younger caldera is nested a still older structure may encircle them both replenishment of magma reservoirs beneath calderas can promote dilatant reactivation of older subsidence structures resulting in multicyclic collapses that reuse these structures and produce nested calderas

paleomagnetic constraints on a time stratigraphic framework for the evolution of ohachidaira volcano and the summit caldera central hokkaido japan we present a revised stratigraphy of the proximal deposits around ohachidaira volcano and their paleomagnetic directions obtained from 21 sites 193 samples we identify four proximal pyroclastic members from ohachidaira volcano that were produced after the early edifice building effusive volcanism from older to younger 1 vent opening mafic pyroclastic density current pdc deposits kobachidaira ignimbrite 2 maar forming mafic tephra ring deposits mamiyadake tephra ring 3 mafic to silicic caldera forming deposits sounkyo member and 4 mafic pdc deposits kumonotaira ignimbrite well grouped paleomagnetic directions obtained from the kobachidaira ignimbrite and the sounkyo member indicate that each deposit was formed over a short time interval compared to geomagnetic secular variation that is less than a century conversely scattered paleomagnetic directions of the mamiyadake tephra ring record the secular variation of the geomagnetic field that took place over a longer time interval that is at least 750 yr probably more the kumonotaira ignimbrite which has a similar paleomagnetic direction to the underlying sounkyo member may have been produced during the sounkyo eruption from comparison between our data and the previously reported paleomagnetic directions of two petrologically distinct distal ignimbrites we conclude that the sounkyo member is a proximal correlative of the distal pyroxene rich ignimbrite and that the distal hornblende rich ignimbrite may not be of an ohachidaira origin the revised stratigraphy suggests that ohachidaira volcano is a maar caldera complex volcano and that the summit caldera is a flaring funnel formed incrementally by explosive erosion and syn eruptive collapse of the vent walls volume estimates imply that the sounkyo eruption fundamentally enlarged the vent and formed the caldera our data confirm that the comparison of paleomagnetic directions can offer a useful identification and correlation tool for the ohachidaira pyroclastic sequences

downsag and extension at calderas new perspectives on collapse geometries from ice melt mining and volcanic subsidence structures at calderas may form as a result of precursory tumescence subsidence due withdrawal of magmatic support resurgence and regional tectonism structural reactivation and overprinting are common to explore which types of structures may derive directly from subsidence without other factors evidence is reviewed from pits caused by the melting of buried ice blocks mining subsidence scaled subsidence models and from over 50 calderas this review suggests that complex patterns of peripheral deformation with multiple ring and arcuate fractures both inside and outside caldera rims topographic embayments arcuate graben and concentric zones of extension and compression may form as a direct result of subsidence and do not require a complex subsidence and inflation history downsag is a feature of many calderas and it does not indicate subsidence on an inward dipping ring fault as has been inferred previously where magmatic inflation is absent or slight initial arcuate faults formed during collapse are likely to be multiple and dip outwards to vertical associated downsag causes the peripheries of calderas undergo radial centripetal extension and this accounts for some of the complex peripheral fractures arcuate crevasses graben and some topographic moats the structural boundary of a caldera defined here as the outermost limits of subsidence and related deformation including downsag commonly lies outside ring faults and outside the embayed topographic wall it is likely to be funnel shaped i e inward dipping even though ring and arcuate fractures within it may dip outward inward dipping arcuate normal faults at shallow levels and steep inward dipping contacts between a caldera s fill and walls may both occur at a caldera that has initially subsided on outward dipping ring faults they arise due to peripheral surficial extension gravitational spreading and scarp collapse topographic enlargement at some calderas and the formation of embayments may reflect general progressive downsag and localized downsag respectively these processes may occur in addition to surficial degradation of oversteep ring fault scarps

glacial water mass geometry and the distribution of δ13c of σco2 in the western atlantic ocean 1 oxygen and carbon isotopic data were produced on the benthic foraminiferal taxa cibicidoides and planulina from 25 new piston cores gravity cores and multicores from the brazil margin the cores span water depths from about 400 to 3000 m and intersect the major water masses in this region these new data fill a critical gap in the south atlantic ocean and provide the motivation for updating the classic glacial western atlantic 13c transect of duplessy et al 1988 the distribution of 13c of co2 requires the presence of three distinct water masses in the glacial atlantic ocean a shallow 1000 m southern source water mass with an end member 13c value of about 0 3 0 5 vpdb a middepth 1500 m northern source water mass with an end member value of about 1 5 and a deep 2000 m southern source water with an end member value of less than 0 2 and perhaps as low as the 0 9 values observed in the south atlantic sector of the southern ocean ninnemann and charles 2002 the origins of the water masses are supported by the meridional gradients in benthic foraminiferal 18o a revised glacial section of deep water 13c documents the positions and gradients among these end member intermediate and deep water masses the large property gradients in the presence of strong vertical mixing can only be maintained by a vigorous overturning circulation

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

high resolution record of northern hemisphere climate extending into the last interglacial period high resolution record of northern hemisphere climate extending into the last interglacial period

a regional archeomagnetic model for europe for the last 3000 years scha dif 3k applications to archeomagnetic dating 1 the available european database of archeomagnetic field values and instrumental data has been used to produce a regional model for the geomagnetic field in europe for the last 3000 years from 1000 b c to 1900 a d connecting with the epoch covered by the igrf models this new model scha dif 3k constitutes an improvement with respect to the previous regional archeomagnetic model scha di 00 f which used relocated values and was only valid for the last 2000 years the new model has been obtained by least sums of absolute deviation inversion of paleomagnetic data using spherical cap harmonics for the spatial representation of the field and sliding windows in time an algorithm has been developed to jointly model the three archeomagnetic elements declination inclination and intensity the resulting model provides the direction and intensity of the earth s magnetic field over the european continent northern africa and western asia for the last 3000 years the fit to the european archeomagnetic database is more accurate than that provided by global archeomagnetic models in addition this model represents a step forward in archeomagnetic dating studies since the relocation error is avoided and can also be used to study the rapid changes of the geomagnetic field archeomagnetic jerks that have been recently proposed

refining the holocene eruptive activity at tenerife canary islands the contribution of palaeomagnetism abstract the timing of the holocene volcanic activity of tenerife canary islands is poorly constrained and the volcanological framework for this area is still incomplete most of the eruptions are dated only by a single 14c dating or the ages are simply stratigraphically determined we apply palaeomagnetism increasingly used in the last years to date holocene volcanism to improve the knowledge of tenerife volcanic history we report on the palaeomagnetic dating using the sha dif 14k global model of nine holocene eruptions that produced scoria cones and major lava flows and we compare our results with those previously obtained by 14c method four of the studied eruptions were previously dated by 14c four were stratigraphically constrained and one was never dated so far concerning the first group for boca cangrejo and mna reventada eruptions palaeomagnetic and 14c ages agree while for the others we obtained older or younger ages than radiocarbon data for the second group one or more age ranges smaller than stratigraphic intervals were found finally we provided the first dating 790 723 bce of the mna grande eruption we confirm that palaeomagnetism can be considered an excellent complement to the radiocarbon method because it is applicable on volcanics with nearly all compositions and provides higher resolution dating at least where reliable geomagnetic reference curves are available the improved framework of the holocene volcanic activity of tenerife shows alternating periods characterized by low and high eruptive frequencies with the last 3 kyr characterized by high eruptive frequency and dominated by basaltic eruptions

how is the ocean filled 1 the ocean surface rapidly exchanges heat freshwater and gases with the atmosphere but once water sinks into the ocean interior the inherited properties of seawater are closely conserved previous water mass decompositions have described the oceanic interior as being filled by just a few different property combinations or water masses here we apply a new inversion technique to climatological tracer distributions to find the pathways by which the ocean is filled from over 10 000 surface regions based on the discretization of the ocean surface at 2 by 2 resolution the volume of water originating from each surface location is quantified in a global framework and can be summarized by the estimate that 15 of the surface area fills 85 of the ocean interior volume ranked from largest to smallest the volume contributions scaled by surface area follow a power law distribution with an exponent of 1 09 0 03 that appears indicative of the advective diffusive filling characteristics of the ocean circulation as demonstrated using a simple model this work quantifies the connection between the surface and interior ocean allowing insight into ocean composition atmosphere ocean interaction and the transient response of the ocean to a changing climate

intermediate water links to deep western boundary current variability in the subtropical nw atlantic during marine isotope stages 5 and 4 records from ocean drilling program sites 1057 and 1059 2584 m and 2985 m water depth respectively have been used to reconstruct the behavior of the deep western boundary current dwbc on the blake outer ridge bor from 130 to 60 kyr b p marine isotope stage mis 5 and the 5 4 transition site 1057 lies within labrador sea water lsw but close to the present day boundary with lower north atlantic deep water lnadw while site 1059 lies within lnadw high resolution sortable silt mean inline equation grain size and benthic 13c records were obtained and changes in the dwbc intensity and spatial variability were inferred comparisons are made with similar proxy records generated for the holocene from equivalent depth cores on the bor during mis 5e inline equation evidence at site 1057 suggests slower relative flow speeds consistent with a weakening and a possible shoaling of the lsw sourced shallower limb of the dwbc that occupies these depths today in contrast the paleocurrent record from the deeper site suggests that the fast flowing deep core of the dwbc was located close to its modern depth below 3500 m during this interval the benthic 13c suggests little chemical stratification of the water column and the presence of a near uniform lnadw dominated water mass after 111 kyr b p the inline equation record at site 1057 increases to reach values similar to site 1059 for the rest of mis 5 the strengthening of flow speeds at the shallow site may correspond to the initiation of glacial north atlantic intermediate water formation also suggested by a divergence in the benthic 13c records with site 1057 values increasing to 1 2 coupled suborbital oscillations in dwbc flow variability and paleohydrography persisted throughout mis 5 comparison of these data with planktonic 18o records from the sites and alkenone derived sea surface temperature sst estimates from the nearby bermuda rise suggest a hitherto unrecognized degree of linkage between oscillations in subtropical north atlantic sst and dwbc flow

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

constraining age and volume of lava flow invasions of the alcantara valley etna volcano italy new insights from paleomagnetic dating and 3d magnetic modeling abstract the alcantara river marks the northern border of etna volcano and along its ne sector it forms a narrow nw se elongated valley that is carved in the mts peloritani sedimentary sequences the valley has been invaded by several lava flows during the growth of the main bulk of the stratovolcano edifice in the past 60 ky ellittico and mongibello volcanoes in this paper we have constrained the age of the main lava flows forming the valley floor and reconstructed the length and volume of the main lava flow that impacted this area through a multidisciplinary approach integrating stratigraphic and aeromagnetic data analysis together with new paleomagnetic and 14c dating the new age determinations showed that the evolution of the present day geological and hydrographic setting of the valley was mainly conditioned by a few flank eruptions that occurred in the lower n flank of etna during the activity of the mongibello volcano in particular between 13 9 and 9 7 ka bp the valley floor was filled by the so called alcantara lava flow for its entire length up to the river mouth reaching a maximum length of about 24 km later two other flank eruptions producing the so called mt dolce and solicchiata lava flows occurred at 9 1 7 2 and 7 3 7 2 ka bp respectively covering the eruptive fissure and the proximal portion of the alcantara lava field and generated the most recent northward shift of the riverbed finally 3d magnetic modeling allowed us to calculate the total on and off shore volume of the alcantara lava flow as 1 km3 a value comparable to those of the long lasting historical eruptions of etna conversely the 24 km length reached by this lava flow is anomalously high when compared to lava flows of the past 15 ky likely because its emplacement occurred within the valley floor preventing any possibility of lateral expansion

magnetic anisotropy in rhyolitic ignimbrite snake river plain implications for using remanent magnetism of volcanic rocks for correlation paleomagnetic studies and geological reconstructions some of the field and laboratory expenses for this work were covered by the natural environment research council nerc grant ne g005372 1 the salary and school fees for the first author were covered in part by a national science foundation nsf grant ear 443549 81179 awarded to x zhao

carbon dioxide effects of antarctic stratification north atlantic intermediate water formation and subantarctic nutrient drawdown during the last ice age diagnosis and synthesis in a geochemical box model in a box model synthesis of southern ocean and north atlantic mechanisms for lowering co2 during ice ages the co2 changes are parsed into their component geochemical causes including the soft tissue pump the carbonate pump and whole ocean alkalinity when the mechanisms are applied together their interactions greatly modify the net co2 change combining the antarctic mechanisms stratification nutrient drawdown and sea ice cover within bounds set by observations decreases co2 by no more than 36 ppm a drawdown that could be caused by any one of these mechanisms in isolation however these antarctic changes reverse the co2 effect of the observed ice age shoaling of north atlantic overturning in isolation the shoaling raises co2 by 16 ppm but alongside the antarctic changes it lowers co2 by an additional 13 ppm a 29 ppm synergy the total co2 decrease does not reach 80 ppm partly because antarctic stratification antarctic sea ice cover and the shoaling of north atlantic overturning all strengthen the sequestration of alkalinity in the deepest ocean which increases co2 both by itself and by decreasing whole ocean alkalinity increased nutrient consumption in the sub antarctic causes as much as an additional 35 ppm co2 decrease interacting minimally with the other changes with its inclusion the lowest ice age co2 levels are within reach these findings may bear on the two stepped co2 decrease of the last ice age

palaeomagnetic data from the borrowdale volcanic group volcano tectonics and late ordovician palaeolatitudes nineteen palaeomagnetic sampling sites from the borrowdale volcanic group between borrowdale and langdale english lake district yield high unblocking temperature magnetization components which postdate the structural tilt and therefore postdate the formation of the broad block faulted syncline which characterizes this part of the central lake district the mean magnetization direction before tilt correction dec 347 2 inc 61 9 95 6 9 corresponds to a palaeolatitude of 43 s 5 7 the magnetization is associated with pervasive late ordovician hydrothermal alteration and is certainly pre devonian based on comparison with available palaeomagnetic data from southern britain this implies that the block faulted synclinal structure of the borrowdale langdale region is pre acadian in age consistent with recent interpretations of late ordovician volcano tectonics based on mapping and volcanological studies

the pantelleria graben sicily channel central mediterranean an example of intraplate passive rift abstract we present new high resolution swath bathymetric data and multichannel seismic profiles acquired in the pantelleria graben one of the three main tectonic depressions forming the sicily channel rift zone this region experienced a late miocene early pliocene continental extension with the development of nw trending fault bounded troughs later accompanied by widespread volcanic manifestations data support the interpretation that the pantelleria graben evolution was dominated by two tectonic phases a lithospheric scale continental rifting early pliocene in which the whole graben was formed and a successive phase late pliocene pleistocene characterized by a magma assisted extensional mechanism ascending magmas within the graben floor seem to migrate from the s e sector of the depression toward the n w sector which is almost entirely floored by igneous material and where the volcanic edifice of the pantelleria island is emerged the volcanic activity is presently concentrated north of the pantelleria island the tectonic evolution of the pantelleria graben characterized by the chronological sequence of events rifting doming volcanism can be referred to as a passive rifting model crustal stretching and subsequent fault development and rifting within the pelagian block may have been controlled by slab pull forces of the northward subducting african slab

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

changing modes and rates of mafic magma supply at pantelleria sicily channel southern italy new perspectives on the volcano factory drawn upon olivine records

the eruptive history of pantelleria sicily channel in the last 50 ka six silicic eruptive cycles have been recognized in the last 50 ka at pantelleria the products of each cycle exhibit a compositional variation from pantellerite to less peralkaline rhyolite or to trachyte the relationships between the range of chemical variation the erupted volume and the time of eruptions allow us to estimate an average differentiation rate of 5 crystal fractionation per 1000 years and a constant long term rate of magma discharge of 0 1 km3 per 1000 years pressure increase in the magma chamber caused by the addition of new magma accumulation of highly differentiated volatile rich magma in the roof zone and a concomitant build up of a vapour phase is postulated as a possible triggering mechanism for eruptions

explosive eruptive history of pantelleria italy repeated caldera collapse and ignimbrite emplacement at a peralkaline volcano a new pre green tuff 46 ka volcanic stratigraphy is presented for the peralkaline pantelleria volcano italy new 40ar 39ar and paleomagnetic data are combined with detailed field studies to develop a comprehensive stratigraphic reconstruction of the island we find that the pre 46 ka succession is characterised by eight silica rich peralkaline trachyte to pantellerite ignimbrites many of which blanketed the entire island the ignimbrites are typically welded to rheomorphic and are often associated with lithic breccias and or pumice deposits they record sustained radial pyroclastic density currents fed by low pyroclastic fountains the onset of ignimbrite emplacement is typically preceded more rarely followed by pumice fallout with limited dispersal and some eruptions lack any associated pumice fall deposit suggesting the absence of tall eruption columns particular attention is given to the correlation of well developed lithic breccias in the ignimbrites interpreted as probable tracers of caldera collapses they record as many as five caldera collapse events in contrast to the two events reported to date inter ignimbrite periods are characterised by explosive and effusive eruptions with limited dispersal such as small pumice cones as well as pedogenesis these periods have similar characteristics as the current post green tuff activity on the island and while not imminent it is reasonable to postulate the occurrence of another ignimbrite forming eruption sometime in the future

distinguishing and correlating deposits from large ignimbrite eruptions using paleomagnetism the cougar point tuffs mid miocene southern snake river plain idaho usa in this paper we present paleomagnetic geochemical mineralogical and geochronologic evidence for correlation of the mid miocene cougar point tuff cpt in southwest snake river plain srp of idaho the new stratigraphy presented here significantly reduces the frequency and increases the scale of known srp ignimbrite eruptions the cpt section exposed at the black rock escarpment along the bruneau river has been correlated eastward to the brown s bench escarpment six common eruption units and cassia mountains three common eruption units regions of southern idaho the cpt records an unusual pattern of geomagnetic field directions that provides the basis for robust stratigraphic correlations paleomagnetic characterization of eruption units based on geomagnetic field variation has a resolution on the order of a few centuries providing a strong test of whether two deposits could have been emplaced from the same eruption or from temporally separate events to obtain reliable paleomagnetic directions the anisotropy of anhysteretic remanence was measured to correct for magnetic anisotropy and an efficient new method was used to remove gyroremanence acquired during alternating field demagnetization

ignimbrite correlation using whole rock geochemistry an example from the sulcis sw sardinia italy ignimbrites are useful chronological markers in the geological record at local and regional scales they also provide information on the dynamics of the eruption that produced them making their study of great importance in terms of volcanic hazard assessment however their study is usually hampered by their lateral variation and discontinuity when stratigraphic and lithologic criteria are not sufficient for correlation purposes the use of multiple complementary correlation tools may be necessary to correctly determine their areal extension volume and facies variations whole rock geochemistry is considered one of the less reliable correlation techniques due to the pyroclastic nature of these deposits and their emplacement dynamics these may introduce vertical and horizontal geochemical heterogeneity in the final deposit in addition the occurrence of zoned ignimbrites due to magma supply of changing composition is common in this work we show that if appropriately used whole rock geochemistry can be a valid and highly useful tool for ignimbrite correlation we provide an example from the study of an ignimbrite sequence containing 18 units sensu lato in the sulcis region sw sardinia italy a protocol has been developed for unit recognition based on successive simple binary diagrams where the whole rock composition of a problem sample can be plotted immobile trace elements have been preferentially used to minimize effects of element mobilization associated with alteration and weathering the diagrams provided here are designed for the sulcis but the methodology followed to develop them may be applied to other study areas

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

recent volcanic history of pantelleria a new interpretation abstract the island of pantelleria is located in the pantelleria rift between sicily and africa this rift is floored by a 20 km thick continental type crust that is characterized by horsts and grabens formed by nw se tensional faults and ne sw shear faults several faults and fractures that intersect the island follow these regional trends prominent volcano tectonic features on the island include two calderas the younger being related to the eruption of the green ignimbrite and the uplifted montagna grande block pantelleria mostly consists of peralkaline silicic rocks with minor trachytes and basalts volcanism on the island is characterized by both explosive and effusive activity reconstruction of the volcanic history of the island since the eruption of the green ignimbrite is based on geological and geochronological data the eruption of the green ignimbrite 50 000 y b p was followed by collapse of the monastero caldera that has been the site of most of the subsequent acid volcanism the montagna grande volcanic complex 35 000 y b p fills the central part while the lower pantelleritic lava flows 16 000 y b p erupted along its rim very probably the young mt gelfiser and khaggiar domes are also located on the caldera rim the montagna grande block rose within the caldera at an estimated average rate between 1 0 and 1 5 cm y for about 20 000 y subsequent pumice cones 9 000 y b p endogenous domes and lava flows also erupted inside this caldera among the main acid volcanic units younger than the green ignimbrite only the mt gelkhamar endogenous dome is not linked to this caldera in the last 50 000 years basaltic activity only occurred in the northern part of the island the basaltic vents are aligned parallel with the main tensional trends of the rift at least two phases have been recognized the older is about 29 000 y b p and the younger is a few thousands of years old a change occurred in the eruptive style after the eruption of the green ignimbrite and collapse of the monastero caldera explosive activity diminished and extrusion of lava became dominant

simulating the amplification of orbital forcing by ocean feedbacks in the last glaciation according to milankovitch theory the lower summer insolation at high latitudes about 115 000 years ago allowed winter snow to persist throughout summer leading to ice sheet build up and glaciation1 but attempts to simulate the last glaciation using global atmospheric models have failed to produce this outcome when forced by insolation changes only2 3 4 5 these results point towards the importance of feedback effects for example through changes in vegetation or the ocean circulation for the amplification of solar forcing6 7 8 9 here we present a fully coupled ocean atmosphere model of the last glaciation that produces a build up of perennial snow cover at known locations of ice sheets during this period we show that ocean feedbacks lead to a cooling of the high northern latitudes along with an increase in atmospheric moisture transport from the equator to the poles these changes agree with available geological data10 11 12 13 14 15 and together they lead to an increased delivery of snow to high northern latitudes the mechanism we present explains the onset of glaciation which would be amplified by changes in vegetation in response to weak orbital forcing

lowering of glacial atmospheric co2 in response to changes in oceanic circulation and marine biogeochemistry 1 we use an earth system model of intermediate complexity climber 2 to investigate what recent improvements in the representation of the physics and biology of the glacial ocean imply for the atmospheric concentration the coupled atmosphere ocean model under the glacial boundary conditions is able to reproduce the deep salty stagnant water mass inferred from antarctic deep pore water data and the changing temperature of the entire deep ocean when carbonate compensation is included in the model we find a co2 drawdown of 43 ppmv associated mainly with the shoaling of the atlantic thermohaline circulation and an increased fraction of water masses of southern origin in the deep atlantic fertilizing the atlantic and indian sectors of the southern ocean north of the polar front leads to a further drawdown of 37 ppmv other changes to the glacial carbon cycle include a decrease in the amount of carbon stored in the terrestrial biosphere 540 pg c which increases atmospheric co2 by 15 ppmv and a change in ocean salinity resulting from a drop in sea level which elevates co2 by another 12 ppmv a decrease in shallow water caco3 deposition draws down co2 by 12 ppmv in total the model is able to explain more than two thirds 65 ppmv of the glacial to interglacial co2 change based only on mechanisms that are clearly documented in the proxy data a good match between simulated and reconstructed distribution of 13c changes in the deep atlantic suggests that the model captures the mechanisms of reorganization of biogeochemistry in the atlantic ocean reasonably well additional poorly constrained mechanisms to explain the rest of the observed drawdown include changes in the organic carbon caco3 ratio of sediment rain reaching the seafloor iron fertilization in the subantarctic pacific ocean and changes in terrestrial weathering

a revision of the structure and stratigraphy of pre green tuff ignimbrites at pantelleria strait of sicily abstract at pantelleria peralkaline silicic magmas were erupted across a range of eruptive typologies and magnitudes pyroclastic flows plinian to strombolian pumice fallout and lava flows in this paper we focus on the intermediate cycle of eruptive activity which is bracketed by ignimbrite units slightly older than the two caldera collapses which marked the volcanological activity of the island this age interval 180 85 ka was punctuated by six ignimbrite forming eruptions silicic and variably peralkaline for a cumulative erupted magma volume of approximately 6 km 3 dense rock equivalent based on new 40 ar 39 ar na k feldspar ages and petrographic data we propose an updated volcanostratigraphic scheme for these welded and rheomorphic ignimbrites that can be summarised as follows i the age of the old la vecchia caldera collapse is now tightly constrained between 140 and 146 ka and the caldera forming eruption can be traced to a lithic rich welded tuff breccia that outcrops in two opposite sectors of the island south west and north east ii four ignimbrite units previously considered unrelated are now merged in two distinct eruptive paroxysmal events at 107 and 85 ka in particular the 85 ka eruptive event is comparable in magnitude to the younger caldera forming green tuff plinian eruption iii the recurrence patterns of the 107 and 85 ka eruptions compared to the green tuff allow us to qualitatively assess that the climax in production of low temperature silicic and peralkaline melt was focused in the age interval 85 45 ka

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

transport and bottom boundary layerobservations of the north atlantic deep western boundary current at the blake outer ridge abstract the north atlantic deep western boundary current dwbc was surveyed at the blake outer ridge over 14 days in july and august 1992 to determine its volume transport and to investigate its bottom boundary layer bbl this site was chosen because previous investigations showed the dwbc to be strong and bottom intensified on the ridge s flanks and to have a thick bbl the primary instrument used was the absolute velocity profiler a free falling velocity and conductivity temperature depth device in two sections across the width of the dwbc volume transports of 17 1 sv and 18 1 sv were measured for all water flowing equatorward below a potential temperature of 6 c 1 sv 1 10 6 m 3 s 1 transport values were derived using both absolute velocities and avp referenced geostrophic velocities and were the same within experimental uncertainty good agreement was found between our results and historical ones when both were similarly bounded and referenced although this was a short term survey the mean of a 9 day time series of absolute velocity profiles was the same as the means of year long current meter records at three depths in the same location a turbulent planetary bbl was found everywhere under the current the thickness of the bottom mixed layer bml where concentrations of density nutrients and suspended sediments were vertically uniform was asymmetrical across the current and up to 5 times thicker than the bbl there was no velocity shear above the bbl within the thicker bmls and the across slope density gradient was very small the extra thick bml is perhaps maintained by a combination of processes including turbulence downwelling ekman transport a weak up slope return flow above the bbl and buoyant convection from the bbl into the bml the frictional bottom stress was mostly balanced by a down stream change in the current s external potential energy evidenced by a drop in the velocity core of the current

millennial scale propagation of atlantic deep waters to the glacial southern ocean the compilation of changes in the magnetic properties at various sites distributed along the path of the deepwater mass in north atlantic shows large amplitude short term fluctuations reflecting changes in the strength of the north atlantic deep water nadw these changes which suggest a two mode deep glacial circulation dynamic are perfectly concurrent with air temperature changes over greenland they also share a similar pattern with those reported in the nd isotope ratios from the deep cape basin during the same time period greenland interstadials were accompanied by increased flow speed of nadw and relatively more nadw reaching the southern ocean while during greenland stadials and heinrich events both the north atlantic flow speed of nadw and its presence in the south atlantic were reduced it is demonstrated that both proxies are tracing the same water mass and their reliability for monitoring changes in the deepwater circulation is therefore clearly established after using the climatically independent geomagnetic assisted stratigraphy to put the northern and southern hemisphere records on the same age scale the south atlantic record appears to lag changes in north atlantic flow speeds by approximately 860 220 years during the most prominent and best defined cycles interstadials 12 and 8 although future work is needed this significant offset provides a first observation and tentative quantification of the time needed for glacial northern component water to mix downward and to flow from the north to the south atlantic

surface and deep ocean variability in the northern sargasso sea during marine isotope stage 3 new geochemical proxy data from bermuda rise piston cores reveal ocean and climate conditions in the northern sargasso sea during marine isotope stage 3 using 18o on the planktonic foraminifer globigerinoides ruber we can correlate explicitly with every stadial interstadial change in greenland ice between 32 and 58 ka these data suggest repetitive changes of 4 c in the annual average sea surface temperature sst with maximum cooling comparable to or greater than sst during glacial maximum conditions the extent of sst depression is about the same for typical stadial events and for heinrich events 4 and 5 which we have identified on the bermuda rise by traces of ice rafting benthic foraminiferal 13c decreases during every stadial event consistent with reduced production of the deepest component of north atlantic deep water and shoaling of its interface with antarctic bottom water this interpretation is supported by benthic cd ca data from the climate cycle associated with interstadial 8

the first world catalog of earthquake rotated objects eros part ii statistical analyses and related interpretations the article presents the results of the quantitative statistical analyses of the first world catalog of earthquake rotated objects eros presented in part i of the study cucci et al 2016 we searched for possible relations between the epicentral distance of eros occurrence and a number of customary seismological observables such as magnitude intensity focal mechanism etc the reliability of results is quantitatively checked by means of some suitable statistical tests we found strong evidence of a clear log linear dependence of the epicentral distance to which an ero can be observed on the magnitude m w of the source event we note that the probability of observing eros near the epicentral area d 10 km inversely decreases with the earthquake magnitude and that for large earthquakes m w 8 this probability remains significant around 30 beyond 100 km from the epicenter unexpectedly we did not find significant relations between eros occurrence and epicentral intensity possibly because of high dispersion of intensity values the data analyzed in the present study identifies intensity 6 as the lowest intensity for rotation occurrence this is different from the main macroseismic scales which indicate the eros as a diagnostic of larger intensity degrees this outcome could suggest the need for revision and for updating the diagnostics indicated in the intensity scales as for the focal mechanisms of the seismic events we found higher probabilities of observing rotations beyond 10 km distance from the epicenter of a thrust faulting earthquake than for a normal faulting earthquake this probability reverses beyond 50 km distance especially for high magnitude seismic events our results indicate interesting insights to potential end users of the eros catalog in the fields of historical seismology and earthquake engineering

the emplacement history of a remarkable heterogeneous chemically zoned rheomorphic and locally lava like ignimbrite tl on gran canaria abstract ignimbrite tl on gran canaria is a complex compositionally zoned rheomorphic tuff that locally exhibits features previously considered to be diagnostic of lavas it is made up of two locally overlapping lobes of ignimbrite that were emplaced during a single eruptive episode the eastern lobe is high grade with rheomorphic zones and localised patches that are lava like the western lobe is extremely high grade more extensively lava like and welded to its top surface both parts are zoned with a basal comendite rich zone grading up through a mixed zone into an upper trachyte rich zone lithic contents and the relative proportions of comendite and trachyte pyroclasts vary with height each comendite rich zone is vitroclastic whereas each trachyte rich zone is partly lava like with local gradations into vitroclastic ignimbrite mixed zones are intermediate in character and locally show compositional banding gradational zoning in massive ignimbrite best seen in lower strain zones and welding fabrics that are pervasively lineated and oblique to bedding suggest that deposition was sustained agglutination was rapid and rheomorphic deformation began during the sustained deposition the viscosity and porosity of the agglutinate varied with height because successively deposited pyroclast populations varied in grainsize composition and temperature the hot agglutinate continued to compact and shear downslope after the density currents had dissipated causing further rheomorphic folding thrusting attenuation and autobrecciation the western lobe locally overlies the partly welded top of the eastern lobe in part because it advanced rheomorphically across it for at least 300 m hot state loading and auto intrusion occurred due to unstable density layering in the chemically zoned agglutinate deformation behaviour changed during cooling and degassing and because of heat transfer between juxtaposed agglutinates and localised retention of dissolved volatiles where there was an overlying impermeable cap

fission track analysis of apatite and zircon defines a burial depth of 4 to 7 km for lowermost upper devonian catskill mountains new york apatite and zircon grains separated from a sandstone layer of earliest late devonian age catskill mountains have been subjected to fission track analysis a 125 m y age obtained on the apatite grains requires a temperature for the sediment of less than 120 c during the past 125 m y at some time prior to 125 m y ago temperatures were above 120 c long enough to cause complete fading of tracks analysis of zircon grains resulted in a fission track age of 320 m y zircon data indicate that the temperature of the sediment layer enclosing the grains did not exceed 175 to 200 c over a 235 rn y period time between sedimentation and 125 m y ago if one assumes a typical geothermal gradient of 25 c km a burial depth of between 4 and 7 km is indicated for the lowermost upper devonian atskill mountains

magnetic fabric welding texture and strain fabric in the nuraxi tuff sardinia italy anisotropy of magnetic susceptibility ams has been used to interpret flow directions in ignimbrites but no study has demonstrated that the ams fabric corresponds to the flow fabric in this paper we show that the ams and strain fabric coincide in a high grade ignimbrite the nuraxi tuff a miocene rhyolitic ignimbrite displaying a wide variability of rheomorphic features and a well defined magnetic fabric natural remanent magnetization nrm data indicate that the magnetization of the tuff is homogeneous and was acquired at high temperatures by ti magnetite crystals comparison between the magnetic fabric and the deformation features along a representative section shows that ams and anisotropy of isothermal remanent magnetization airm fabric are coaxial with and reproduce the shape of the strain ellipsoid magnetic tests and scanning electron microscopy observations indicate that the fabric is due to trails of micrometer size pseudo single domain magnetically interacting magnetite crystals microlites formed along discontinuities such as shard rims and vesicle walls mimicking the petrofabric of the tuff the fabric was thus acquired after deposition before late rheomorphic processes and accurately mimics homogeneous deformation features of the shards during welding processes and mass flow

glass transition temperatures of natural hydrous melts a relationship with shear viscosity and implications for the welding process abstract glass transition temperatures tg have been determined for natural multicomponent melts using differential scanning calorimetry trachytic dacitic phonolitic and basaltic base compositions have been analysed over a range of water contents up to 3 75 wt for each sample tg has been obtained over a range of cooling heating rates using the extrapolated onset and the peak temperatures in heat capacity temperature curves tg of all compositions are strongly reduced by increasing water content particularly for the first 1 wt added base composition also has an effect with the lowest tg occurring in the peralkaline phonolite suite for all samples a clear dependence on the cooling heating rate has been recorded these results have been compared with rheological investigations on the same samples on the basis of the equivalence of the shear and enthalpic relaxation process timescales we provide a method to predict the shear viscosity at the glass transition for all the melts investigated in this study both dry and hydrous our determinations of tg provide a lower limit for the time temperature envelope that gives rise to densely welded deposits and constraints on their emplacement temperature furthermore by using the viscosity values predicted at the glass transition we suggest that welding processes may occur over timescales on the order of tens of seconds to tens of minutes at tg

early arrival of southern source water in the deep north atlantic prior to heinrich event 2 the atlantic meridional overturning circulation amoc plays an important role in the northern hemisphere climate system significant interest went into the question of how excessive freshwater input through melting of continental ice can affect its overturning vigor and hence heat supply to higher northern latitudes such forcing can be tested by investigating its behavior during extreme iceberg discharge events into the open north atlantic during the last glacial period the so called heinrich events he here we present neodymium nd isotope compositions of past seawater a sensitive chemical water mass tag extracted from sediments of ocean drilling program site 1063 in the western north atlantic bermuda rise covering the period surrounding he 2 the last glacial maximum and the early deglaciation these data are compared with a record of the kinematic circulation tracer 231pa 230th xs extracted from the same sediment core both tracers indicate significant circulation changes preceding intense ice rafting during he 2 by almost 2 kyr moreover the nd isotope record suggests the presence of deeply ventilating north atlantic deep water early during marine isotope stage 2 until it was replaced by southern source water at 27 ka the early switch to high pa th xs and radiogenic nd in relation to intensified ice rafting during he 2 suggests that ice rafting into the open north atlantic during major he 2 was preceded by an early change of the amoc this opens the possibility that variations in amoc contributed to or even triggered the ice sheet instability rather than merely responding to it

a refined age calibrated paleosecular variation and relative paleointensity stack for the nw barents sea implication for geomagnetic field behavior during the holocene abstract reconstruction of paleomagnetic secular variation psv of the geomagnetic field is fundamental both to assess geodynamo models and to obtain age constraints for rocks sediments and archaeological material we present refined age calibrated holocene psv and relative paleointensity rpi stack curves derived from arctic marine sediments northwestern barents sea the holocene sections of four sedimentary cores were correlated on the basis of paleomagnetic trends and age models and stacked the resultant composite psv and rpi holocene records nbs stack and the reconstructed holocene virtual geomagnetic pole vgp path were evaluated in comparison with the most recent paleomagnetic stack curves and geomagnetic field models the data indicate that during the holocene time the vgps moved within the superficial projection of the inner core tangent cylinder with the exception of short time intervals around 5600 and 3200 cal yr bp when vgps extended to lower latitudes these deviations might reflect regional geomagnetic features such as persistent geomagnetic flux lobes at core mantle boundary our data confirm that the large vgp shift observed around 5600 cal yr bp is the result of an increased radial magnetic field at the core mantle boundary over north america whilst the vgp shift around 3200 cal yr bp represents a major swing to middle latitudes toward the middle east and might be associated to a regional high paleointensity peak known as levantine iron age anomaly liaa

geology of the peralkaline volcano at pantelleria strait of sicily situated in a submerged continental rift pantelleria is a volcanic island with a subaerial eruptive history longer than 300 ka its eruptive behavior edifice morphologies and complex multiunit geologic history are representative of strongly peralkaline centers it is dominated by the 6 km wide cinque denti caldera which formed ca 45 ka ago during eruption of the green tuff a strongly rheomorphic unit zoned from pantellerite to trachyte and consisting of falls surges and pyroclastic flows soon after collapse trachyte lava flows from an intracaldera central vent built a broad cone that compensated isostatically for the volume of the caldera and nearly filled it progressive chemical evolution of the chamber between 45 and 18 ka ago is recorded in the increasing peralkalinity of the youngest lava of the intracaldera trachyte cone and the few lavas erupted northwest of the caldera beginning about 18 ka ago inflation of the chamber opened old ring fractures and new radial fractures along which recently differentiated pantellerite constructed more than 25 pumice cones and shields continued uplift raised the northwest half of the intracaldera trachyte cone 275 m creating the island s present summit montagna grande by trapdoor uplift pantellerite erupted along the trapdoor faults and their hingeline forming numerous pumice cones and agglutinate sheets as well as five lava domes degassing and drawdown of the upper pantelleritic part of a compositionally and thermally stratified magma chamber during this 18 3 ka episode led to entrainment of subjacent crystal rich pantelleritic trachyte magma as crenulate inclusions progressive mixing between host and inclusions resulted in a secular decrease in the degree of evolution of the 0 82 km3 of magma erupted during the episode

tephrochronology of the astronomically tuned kc01b deep sea core ionian sea insights into the explosive activity of the central mediterranean area during the last 200 ka abstract we present a major trace and rare earth element characterization of single glass shards from fifteen tephra layers found in the astronomically tuned kc01b deep sea core ionian sea central mediterranean 36 15 25 n 17 34 44 e 3642 m water depth kc01b is considered a reference core for the mediterranean area since it provides a new reliable astronomical tuned timescale for the last 1 112 my lourens 2004 the studied deposits punctuate the marine record in a time span ranging from ca 16 ka to ca 191 ka b p encompassing the climatic zones y x w and v they are related to volcanic activity of the campania plain aeolian islands mt etna and pantelleria island along with major main markers of the central mediterranean area such as y 1 y 5 and x 6 astronomically dated at 16 7 ka 39 1 ka and 110 5 ka respectively the succession contains a number of well preserved deposits which can represent useful inter archive instruments of correlation the campania plain in particular sourced at least nine compositionally homogeneous tephra layers prior to the campanian ignimbrite event the data obtained in this work provide a new detailed analytical reference database for ionian sea tephrochronology and for proximal distal correlation studies they may help to unravel from the marine record the history of poorly known or unknown explosive activity on land since the middle pleistocene along with chemical composition size and dispersal of the products