dune bedforms produced by dilute pyroclastic density currents from the august 2006 eruption of tungurahua volcano ecuador a series of pyroclastic density currents were generated at tungurahua volcano ecuador during a period of heightened activity in august 2006 dense pyroclastic flows were confined to valleys of the drainage network while dilute pyroclastic density currents overflowed on interfluves where they deposited isolated bodies comprising dune bedforms of cross stratified ash exposed on the surface here the description measurement and classification of more than 300 dune bedforms are presented four types of dune bedforms are identified with respect to their shape internal structure and geometry length width thickness stoss and lee face angles and stoss face length 1 elongate dune bedforms have smooth shapes and are longer in the flow direction than wide or thick internal stratification consists of stoss constructional thick lensoidal layers of massive and coarse grained material alternating with bedsets of fine laminae that deposit continuously on both stoss and lee sides forming aggrading structures with upstream migration of the crests 2 transverse dune bedforms show linear crests perpendicular to the flow direction with equivalent lengths and widths internally these bedforms exhibit finely stratified bedsets of aggrading ash laminae with upstream crest migration steep truncations of the bedsets are visible on the stoss side only 3 lunate dune bedforms display a barchanoidal shape and have stratification patterns similar to those of the transverse ones finally 4 two dimensional dune bedforms are much wider than long exhibit linear crests and are organized into trains elongate dune bedforms are found exclusively in proximal deposition zones transverse lunate and two dimensional dune bedforms are found in distal ash bodies the type of dune bedform developed varies spatially within an ash body transverse dune bedforms occurring primarily at the onset of deposition zones transitioning to lunate dune bedforms in intermediate zones and two dimensional dune bedforms exclusively on the lateral and distal edges of the deposits the latter are also found where flows moved upslope elongate dune bedforms were deposited from flows with both granular based and tractional flow boundaries that possessed high capacity and competence they may have formed in a subcritical context by the blocking of material on the stoss side we do not interpret them as antidune or chute and pool structures the dimensions and cross stratification patterns of transverse dune bedforms are interpreted as resulting from low competence currents with a significant deposition rate but we rule out their interpretation as antidunes a similar conclusion holds for lunate dune bedforms whose curved shape results from a sedimentation rate dependent on the thickness of the bedform finally two dimensional dune bedforms were formed where lateral transport exceeds longitudinal transport i e in areas where currents were able to spread laterally in low velocity zones we suggest that the aggrading ash bedsets with upstream crest migration were formed under subcritical flow conditions where the tractional bedload transport was less important than the simultaneous fallout from suspension this produced differential draping with no further reworking we propose the name regressive climbing dunes for structures produced by this process a rapid decrease in current velocity possibly triggered by hydraulic jumps affecting the entire parent flows is inferred to explain their deposition this process can in principle hold for any kind of particulate density current

emplacement of the 18 may 1980 lateral blast deposit ene of mount st helens washington facies variations east northeast of mount st helens preserve a record of depositional processes in the 18 may 1980 lateral blast cloud this paper reports new field grain size and component data from the ene sector of the timber blowdown zone and presents a model for blast flow and sedimentation the first erupted ejecta was rich in juvenile components and extends to the distal blowdown limit the last erupted ejecta was rich in accidental lithics and reached no further than a few kilometres from the mountain due to waning discharge the blast cloud was a turbulent stratified flow which transported and deposited sediment in the manner of a high density turbidity current the possibility that the blast was emplaced as a giant shearing fluidised bed is not favoured by compositional zoning patterns depositional conditions were strongly influenced by the rate of suspended load fallout from the blast within about 8 km from vent rapid sedimentation caused deposition under moderate to high concentration conditions and formation of a basal hindered settling zone able to detach gravitationally and drain into local depressions the resulting proximal facies resembles a low aspect ratio ignimbrite fines depletion in the proximal facies is attributed to a combination of residual turbulence and rapid gas escape during particle settling and compaction through the hindered settling zone component data suggest that the blast head played no significant role in the generation of fines depletion in the blast deposit as suggested by previous workers with increasing distance from vent the rate of particle fallout declined and sedimentation took place under increasingly dilute and tractional conditions building up antidune like bedforms wavelengths of these bedforms range from 20 to 1 m and decrease away from vent there is a systematic relationship between antidune migration direction and depositional slope the transition from proximal ignimbrite like to distal surge like facies suggests a possible gradation in transport and deposition processes between conventional pyroclastic surges and high velocity pyroclastic flows

insolation values for the climate of the last 10 million years new values for the astronomical parameters of the earth s orbit and rotation eccentricity obliquity and precession are proposed for paleoclimatic research related to the late miocene the pliocene and the quaternary they have been obtained from a numerical solution of the lagrangian system of the planetary point masses and from an analytical solution of the poisson equations of the earth moon system the analytical expansion allows the direct determination of the main frequencies with their phase and amplitude numerical and analytical comparisons with the former astronomical solution ber78 are performed so that the accuracy and the interval of time over which the new solution is valid can be estimated the corresponding insolation values have also been computed this analysis leads to the conclusion that the new values are expected to be reliable over the last 5 ma in the time domain and at least over the last 10 ma in the frequency domain

the architecture eruptive history and evolution of the table rock complex oregon from a surtseyan to an energetic maar eruption abstract the table rock complex trc pliocene pleistocene first documented and described by heiken heiken g h 1971 tuff rings examples from the fort rock christmas lake valley basin south central oregon j geophy res 76 5615 5626 is a large and well exposed mafic phreatomagmatic complex in the fort rock christmas lake valley basin south central oregon it spans an area of approximately 40 km2 and consists of a large tuff cone in the south trc1 and a large tuff ring in the northeast trc2 at least seven additional smaller explosion craters were formed along the flanks of the complex in the time between the two main eruptions the first period of activity trc1 initiated with a surtseyan style eruption through a 60 70 m deep lake the trc1 deposits are dominated by multiple 1 2 m thick fining upward sequences of massive to diffusely stratified lapilli tuff with intermittent zones of reverse grading followed by a finely laminated cap of fine grained sediment the massive deposits are interpreted as the result of eruption fed subaqueous turbidity current deposits whereas the finely laminated cap likely resulted from fallout of suspended fine grained material through a water column other common features are erosive channel scour and fill deposits massive tuff breccias and abundant soft sediment deformation due to rapid sediment loading subaerial trc1 deposits are exposed only proximal to the edifice and consist of cross stratified base surge deposits the eruption built a large tuff cone above the lake surface ending with an effusive stage which produced a lava lake in the crater 365 m above the lake floor a significant repose period occurred between the trc1 and trc2 eruptions evidenced by up to 50 cm of diatomitic lake sediments at the contact between the two tuff sequences the trc2 eruption was the last and most energetic in the complex general edifice morphology and a high percentage of accidental material suggest eruption through saturated trc1 deposits and or playa lake sediments trc2 deposits are dominated by three dimensional dune features with wavelengths 200 500 m perpendicular to the flow and 20 200 m parallel to the direction of flow depending on distance from source large u shaped channels 10 32 m deep run up features over obstacles tens of meters high and a large 13 m chute and pool feature are also identified the trc2 deposits are interpreted as the products of multiple erosive highly inflated pyroclastic surges resulting from collapse of an unusually high eruption column relative to previously documented mafic phreatomagmatic eruptions

mass partition during collapsing and transitional columns by using numerical simulations pyroclast dispersal and mass partition between convective and collapsing transport systems represent a major issue in the understanding of the dynamics of quasi steady explosive eruptions and the interpretation of their deposits the spatial and temporal dispersal of pyroclasts during the initial 15 min of collapsing and transitional columns was investigated by using a transient two dimensional and three phase one gas phase and two solid phases representative of pyroclasts of varying size and density flow model the different behaviors of the column were simulated by using different water contents of the eruptive mixture and assuming both pressure balanced and overpressured conditions at the vent for each simulation results allowed the quantification of the mass of pyroclasts of different size forming the pyroclastic density current the co ignimbrite column and the convective plumes rising from the proximal area and above the fountain for collapsing columns the total mass transported in the convective system composed of the plume above the fountain and the co ignimbrite column ranges from about 10 to 30 wt of the total erupted mass according to the specific water content of the mixture at the vent fine particles are dominant in this convective system with the increase of the water content the character of the column shows a collapsing buoyant transitional behavior such a regime is characterized by greater collapse height generation of more dilute density currents shorter flow runout less steady behavior of the column and intermittent feeding of the flows for these columns the total mass forming the convective portions reaches values of up to 50 wt of the erupted pyroclasts with a significant amount of coarse particles entrained in the convective plume above the fountain at higher water contents the column becomes fully buoyant with the whole mass feeding the convective column simulations assuming overpressured conditions at the vent are characterized by more unsteady dynamics of the dispersal process with significant variations in the collapse height of the column mass flow rate of the flow and amount of mass elutriated in the convective system simulation results suggest that plinian columns that undergo a transition from convective to collapsing regimes as a consequence in our case of a water content decrease in the magma but not necessarily limited to this case would be characterized by a gradual decrease of the carrying capacity of the convective column above the fountain and by the emplacement of pyroclastic density currents which evolve from dilute to dense gradually increasing their runout

the emplacement of intermediate volume ignimbrites a case study from roccamonfina volcano southern italy a model is presented for the emplacement of intermediate volume ignimbrites based on a study of two 6 km3 volume ignimbrites on roccamonfina volcano italy the model considers that the flows were slow moving and quickly deflated from turbulent to non turbulent conditions yield strength and density increased whereas fluidisation decreased with time and runout of the pyroclastic flows in proximal locations on the caldera rim heterogeneous exposures including discontinuous lithic breccias stratified and cross stratified units interbedded with massive ignimbrite suggest deposition from turbulent flows in medial locations thick massive ignimbrite occurs associated with three types of co ignimbrite lithic breccia which we interpret as being emplaced by non turbulent flows multiple grading of different breccia lithic concentration types within single flow units indicates that internal shear occurred producing overriding or overlapping of the rear of the flow onto the slower moving front part this overriding of different parts of non turbulent pyroclastic flows could be caused by at least two different mechanisms 1 changes in flow regime such as hydraulic jumps that may occur at breaks in slope and 2 periods of increased discharge rate possibly associated with caldera collapse producing fresh pulses of lithic rich material that sheared onto the slower moving part of the flow in front

environmental implications of the magnetic record in pleistocene lacustrine sediments of the qaidam basin ne tibetan plateau abstract the application of magnetic proxies in paleoclimate studies has the advantage of fast and non destructive measurement procedures which is an important precondition for obtaining high resolution datasets within a manageable time frame here we demonstrate the advantage of using magnetic proxies for a 940 m long drill core comprising lacustrine sediments from the qaidam basin ne tibetan plateau which was previously dated by magnetostratigraphy and optical stimulated luminescence osl dating comparison of three independent mass specific magnetic susceptibility datasets confirms the horizontal homogeneity of the drill core at scales of several cm 3 the potential linkage of to astronomical forcing is suggested by spectral analysis magnetic grain size and mineralogy dependent parameters were used to evaluate possible scenarios explaining the variation moreover using additional palynological results we demonstrate the correlation of with climatic constraints thereby showing the climate sensitivity of higher and lower values are related to dry and more humid conditions respectively our results lead us to conclude that variations are caused by the interference of two driving mechanisms low temperature oxidation in the catchment area and a changing catchment area in summary this study confirms the informative value of magnetic proxies for detecting paleoenvironmental change in high resolution

geology of proximal small volume trachyte trachyandesite pyroclastic flows and associated surge deposits roccamonfina volcano italy abstract this paper describes the 232 ka b p mttt trachyte trachyandesite pyroclastic succession of roccamonfina volcano this small volume proximal sequence crops out along mulino di sotto paratone and pisciariello ravines in the southwest sector of the central caldera and covers a minimum extent of 3 5 km2 area it is made up of seven pyroclastic flows and pyroclastic surge units consisting of trachytic ash matrix containing juvenile trachyandesitic scoria and dense lava fragments pumice clasts of uncertain trachyandesite and a foreign trachyandesitic lithic facies two stratigraphic markers allow correlation of the units no paleosoils and plinian fallout have been observed at the base and within the succession some lateral grading of scoria and lithic clasts suggests that mttt derived from three distinct source vents the sequence consists of a basal ash flow passing laterally to laminated surge deposits unit a this is overlain by a reversely graded scoria and pumice lapilli flow unit b which is in turn overlain by a thinly cross stratified scoria lapilli surge unit c unit c is capped by a prominent ash and scoria flow unit d a ground layer marker mk1 divides unit d from a massive ignimbrite which grades upcurrent to sand wave surge deposits unit e another ground layer marker mk2 separates unit e from unit f this unit consists of a basal ignimbrite passing laterally to bedded surge deposits with convolute structures subunit fl and grading upcurrent to a subhorizontally plane laminated ash cloud subunit f2 containing near the top a layer of millimetric lithic clasts embedded in fine ash the succession is closed by the pyroclastic flow unit g surge unit c can be interpreted in terms of vertical gradients in turbulence particle concentration and velocity during flowage whereas the bedded surge parts present in the massive deposits of units a and e f1 can be related to abrupt changes of velocity down the steep slopes of ravines reverse grading in unit b is probably due to grain dispersive pressures the convolute structures within fl are related to zones of diagenetic cementation associated with groundwater finally the laminated fine grained nature of subunit f2 is interpreted as due to ash clouds elutriated from the basal part of unit f stratigraphic markers mk1 mk2 are ground layer breccias formed by settling of lithic and scoria clasts from overlying units e and f respectively vesiculation and morphologies of glass shards of the mttt succession suggest that eruptions were essentially driven by magmatic explosions which had an appreciable hydromagmatic component

the chicxulub asteroid impact and mass extinction at the cretaceous paleogene boundary the cretaceous paleogene boundary similar to 65 5 million years ago marks one of the three largest mass extinctions in the past 500 million years the extinction event coincided with a large asteroid impact at chicxulub mexico and occurred within the time of deccan flood basalt volcanism in india here we synthesize records of the global stratigraphy across this boundary to assess the proposed causes of the mass extinction notably a single ejecta rich deposit compositionally linked to the chicxulub impact is globally distributed at the cretaceous paleogene boundary the temporal match between the ejecta layer and the onset of the extinctions and the agreement of ecological patterns in the fossil record with modeled environmental perturbations for example darkness and cooling lead us to conclude that the chicxulub impact triggered the mass extinction

proximal record of the 273 ka poris caldera forming eruption las canadas tenerife lithofacies architecture analysis is applied to the most proximal exposures of the 273 ka poris formation in the diego hernandez caldera wall of las canadas here the succession records an opening plinian phase followed by generation of a pyroclastic density current pdc that was partly blocked by near vent topography the upper part of the current surmounted the barrier and deposited an ignimbrite veneer during a hiatus in energetic flow ash and various ash aggregates were deposited from a hybrid cloud comprising parts of both the plinian and the co ignimbrite plumes phreatomagmatic explosions then generated currents rich in fine grained lithics current waxing with local erosion was followed by pulsing unsteady waning recorded in repeated sets of graded tuff reversion to magmatic activity was marked by hybrid deposition as coarse pumice fallout and juvenile ballistics entered pyroclastic currents to form stratified pumice block tuff increasingly energetic flow is recorded in 9 m of mainly massive lapilli tuff that includes evidence of widespread scour and bypass lithic rich layers record incremental onset of edifice disruption due to magma withdrawal laterally variable stratification and load structures show that the pdc was unsteady and highly non uniform at times with locally rapid aggradation of gas rich ignimbrite climactic edifice disruption is marked by a lithic block layer in erosional contact with the underlying ignimbrite recording intense pyroclastic current activity and widespread bypass of material to distal volcano slopes pumice block breccia 10 m thick and with blocks up to 1 2 m in diameter forms the top of the succession and registers modification of the magmatic plumbing the timing of collapse to form a major summit depression is uncertain however the abrupt termination of the proximal aggradation of coarse breccia with no record there of any waning stage pyroclastic currents suggests subsidence at that time conceivably with formation of a caldera that contained late erupted material

goethite concentration variations in the red clay sequence on the chinese loess plateau the goethite is a common magnetic mineral in soils and is important for understanding past climatic and environmental change although goethite concentration records are available for the quaternary loess paleosol sequences from the chinese loess plateau clp no goethite concentration record is available for the underlying red clay sequence here we generate a record of goethite concentration from the chaona section on the clp and investigate the relationship between goethite and ferrimagnetic mineral concentration and climate parameters on the red clay sequence deposited from 6 2 7 ma we found that increasing goethite concentration generally corresponds to decreasing magnetic susceptibility for the red clay sequence with the exception of an anomalous section from 4 3 3 7 ma where both records show increasing trends precipitation seasonality could explain this opposite variation pattern between magnetic susceptibility and goethite concentration in the red clay sequence from 3 6 2 7 ma goethite concentration has a decreasing trend while magnetic susceptibility increased which we interpret as more seasonally distributed precipitation associated with intensification of the east asian summer monsoon easm similar pattern was observed from 0 65 0 ma from the lingtai section taken together the goethite record reveals that the easm experienced two stages of rapid intensification with apparent seasonally distributed precipitation since 6 ma

on the structure and origin of major glaciation cycles 1 linear responses to milankovitch forcing time series of ocean properties provide a measure of global ice volume and monitor key features of the wind driven and density driven circulations over the past 400 000 years cycles with periods near 23 000 41 000 and 100 000 years dominate this climatic narrative when the narrative is examined in a geographic array of time series the phase of each climatic oscillation is seen to progress through the system in essentially the same geographic sequence in all three cycles we argue that the 23 000 and 41 000 year cycles of glaciation are continuous linear responses to orbitally driven changes in the arctic radiation budget and we use the phase progression in each climatic cycle to identify the main pathways along which the initial local responses to radiation are propagated by the atmosphere and ocean early in this progression deep waters of the southern ocean appear to act as a carbon trap to stimulate new observations and modeling efforts we offer a process model that gives a synoptic view of climate at the four end member states needed to describe the system s evolution and we propose a dynamic system model that explains the phase progression along causal pathways by specifying inertial constants in a chain of four subsystems solutions to problems involving systems of such complexity are not born full grown like athena from the head of zeus rather they evolve slowly in stages each of which requires a pause to examine data at great lengths in order to guarantee a sure footing and to properly choose the next step victor p starr

eruptive conditions and depositional processes of narbona pass maar volcano navajo volcanic field navajo nation new mexico usa phreatomagmatic deposits at narbona pass a mid tertiary maar in the navajo volcanic field nvf new mexico usa were characterized in order to reconstruct the evolution and dynamic conditions of the eruption our findings shed light on the temporal evolution of the eruption dominant depositional mechanisms influence of liquid water on deposit characteristics geometry and evolution of the vent efficiency of fragmentation and the relative importance of magmatic and external volatiles the basal deposits form a thick 5 20 m massive lapilli tuff to tuff breccia deposit this is overlain by alternating bedded sequences of symmetrical to antidune cross stratified tuff and lapilli tuff and diffusely stratified clast supported reversely graded lapilli tuffs that pinch and swell laterally this sequence is interpreted to reflect an initial vent clearing phase that produced concentrated pyroclastic density currents followed by a pulsating eruption that produced multiple density currents with varying particle concentrations and flow conditions to yield the well stratified deposits only minor localized soft sediment deformation was observed no accretionary lapilli were found and grain accretion occurs on the lee side of dunes this suggests that little to no liquid water existed in the density currents during deposition juvenile material is dominantly present as blocky fine ash and finely vesiculated fine to coarse lapilli pumice this indicates that phreatomagmatic fragmentation was predominant but also that the magma was volatile rich and vesiculating at the time of eruption this is the first study to document a significant magmatic volatile component in an nvf maar diatreme eruption the top of the phreatomagmatic sequence abruptly contacts the overlying minette lava flows indicating no gradual drying out period between the explosive and effusive phases the lithology of the accidental clasts is consistent throughout the vertical pyroclastic stratigraphy suggesting that the diatreme eruption did not penetrate below the base of the uppermost country rock unit a sandstone aquifer 360 m thick by comparison other nvf diatremes several tens of kilometers away were excavated to depths of 1 000 m beneath the paleosurface e g delaney pt ship rock new mexico the vent of a violent volcanic eruption in beus ss ed geological society of america centennial field guide rocky mountain section 2 411 415 1987 this can be accounted for by structurally controlled variations in aquifer thickness beneath different regions of the volcanic field variations in accidental clast composition and bedding style around the edifice are indicative of a laterally migrating or widening vent that encountered lateral variations in subsurface geology we offer reasonable evidence that this subsurface lithology controlled the availability of external water to the magma which in turn controlled characteristics of deposits and their distribution around the vent

temperature proxy data and their significance for the understanding of pyroclastic density currents a major dome collapse of the soufriere hills volcano montserrat on 26 december 1997 generated a devastating pyroclastic density current that destroyed vegetation and left a distinc tive tar like deposit on the surface the deposit included a range of charred mainly herbaceous angiosperm axes and roots studies of refl ectance of these charcoalifi ed plants indicated mean refl ectances of 0 72 1 16 for the three samples with a maximum refl ectance of 1 77 recorded for all readings these data provide minimum temperatures of the fl ow of 300 425 c con sistent with organic geochemical data obtained by 13 c solid state nuclear magnetic resonance and gas chromatography mass spectrometry these temperatures have been used to calculate characteristics of the pyroclastic density current we estimate fl ow front current densities near the ground of 1 8 3 kg m 3 using constraints from a mean fl ow speed of 90 m s estimated from seismic data the mean temperature of the ash component is estimated as 400 610 c

post collapse volcanic history of calderas on a composite volcano an example from roccamonfina southern italy roccamonfina part of the roman potassic volcanic province is an example of a composite volcano with a complex history of caldera development the main caldera truncates a cone constructed predominantly of this caldera may have been associated with one of the ignimbritic eruptions of the brown leucitic tuff blt around 385 000 yr bp the campagnola tuff the youngest ignimbrite of the blt however drapes the caldera margin and must postdate at least the initial stages of collapse during the subsequent history of the caldera there were several major explosive eruptions the largest of these was that of the galluccio tuff at about 300 000 yr bp it is likely that there was further collapse within the main caldera associated with these eruptions it is of note that despite these subsequent major explosive eruptions later collapse occurred within the confines of the main caldera between eruptions caldera lakes developed producing numerous lacustrine beds within the caldera fill extensive phases of phreatomagmatic activity generated thick sequences of pyroclastic surge and fall deposits activity within the main caldera ended with the growth of a large complex of basaltic trachyandestite lava domes around 150 000 yr bp early in the history of roccamonfina sector collapse on the northern flank of the volcano formed the northern caldera one of the youngest major events on roccamonfina occurred at the head of this northern caldera with explosive activity producing the conca ignimbrite and associated caldera there is no evidence that there was any linkage in the plumbing systems that fed eruptions in the main and northern calderas

modeling 100 000 year climate fluctuations in pre pleistocene time series a number of pre pleistocene climate records exhibit significant fluctuations at the 100 000 year 100 ky eccentricity period before the time of such fluctuations in global ice volume the origin of these fluctuations has been obscure results reported here from a modeling study suggest that such a response can occur over low latitude land areas involved in monsoon fluctuations the twice yearly passage of the sun across the equator and the seasonal timing of perihelion interact to increase both 100 ky and 400 ky power in the modeled temperature field the magnitude of the temperature response is sufficiently large to leave an imprint on the geologic record and simulated fluctuations resemble those found in records of triassic lake levels

reconstruction of the morasko meteoroid impact insight from numerical modeling the morasko strewn field located near poznan poland comprises seven impact craters with diameters ranging from 20 to 90 m all of which were formed in glacial sediments around 5000 yr ago numerous iron meteorites have been recovered in the area and their distribution suggests a projectile with the trajectory from ne to sw similar impact events producing crater strewn fields on average happen every 500 yr and pose a serious risk for modern civilization which is why it is of utmost importance to study terrestrial strewn fields in detail in this work we investigate the morasko meteoroid passage through the atmosphere the distribution of its fragments on the ground and the process of forming individual craters by means of numerical modeling by combining atmospheric entry modeling pi group scaling of transient crater size and hydrocode simulations of impact processes we constructed a comprehensive model of the morasko strewn field formation we determined the preatmospheric parameters of the morasko meteoroid the entry mass is between 600 and 1100 tons the velocity range is between 16 and 18 km s 1 and the trajectory angle is 30 40 such entry velocities and trajectory angles do not deviate from typical values for near earth asteroids although the initial mass we determined can be considered as small our studies on velocities and masses of crater forming fragments showed that the biggest morasko crater was formed by a projectile about 1 5 m in diameter with the impact velocity 10 km s 1 environmental consequences of the morasko impact event are very localized

event stratigraphy and emplacement mechanisms of the last major caldera eruption on sete cidades volcano sao miguel azores the 16 ka santa barbara formation sete cidades is an active yet dormant central volcano located in the westernmost part of sao miguel island azores archipelago characterized by a 5 km wide multi stage caldera formed in the last 36 ka in this work we present new stratigraphic grain size petrographic and geochemical data of the 16 ka santa barbara formation related to the last stage of caldera formation we define the lithostratigraphy of this pyroclastic sequence into three members based on stratigraphic position lithofacies and distribution of the deposits the preserved deposits outcrop in a narrow area over 5 km between the caldera rim and the coastal cliffs and include ash beds with dispersed lapilli lower member coarse grained pumice and lithic rich lapilli tuffs middle member and pumice fall deposits upper member the minimum bulk volume preserved on land is about 0 03 km3 however the majority of the pyroclastic products were deposited in the ocean the presence of magmatic mafic enclaves 48 wt sio2 within the juvenile pumice clasts 63 wt sio2 is testimony to the injection of trachybasaltic magma into the trachytic magma reservoir shortly prior to eruption the mingling of these two magmas acted as the eruption trigger the eruption history can be divided into three main phases each characterized by different eruption styles and deposit emplacement mechanisms accompanied by enlargement of the caldera the eruption started with a sequence of short lived explosions generating unstable columns that produced ash fallout and generated dilute pyroclastic density currents mostly deposited in the sw sector of the volcano the climax of the eruption was marked by successive low fountaining events that generated high particle concentration pyroclastic density currents emplaced on the sw and ne flanks the final phase of the eruption was characterized by establishment of a sub plinian eruption column that produced pumice fall deposits on the ne flank of the volcano our study allows delineation of eruptive scenarios for future major explosive eruptions at sete cidades a volcano which is located on the most populated island of the azores sao miguel island and can provide a framework for the assessment of volcanic hazards on oceanic islands elsewhere

astronomic timescale for the pliocene atlantic δ18o and dust flux records of ocean drilling program site 659 high resolution benthic oxygen isotope and dust flux records from ocean drilling program site 659 have been analyzed to extend the astronomically calibrated isotope timescale for the atlantic from 2 85 ma back to 5 ma spectral analysis of the 18o record indicates that the 41 kyr period of earth s orbital obliquity dominates the pliocene record this is shown to be true regardless of fundamental changes in the earth s climate during the pliocene however the cycles of sahelian aridity fluctuations indicate a shift in spectral character near 3 ma from the early pliocene to 3 ma the periodicities were dominantly precessional 19 and 23 kyr and remained strong until 1 5 ma subsequent to 3 ma the variance at the obliquity period 41 kyr increased the timescale tuned to precession suggests that the pliocene was longer than previously estimated by more than 0 5 m y

astronomical timescale and palaeoclimatic implication of stacked 3 6 myr monsoon records from the chinese loess plateau abstract magnetic susceptibility and grain size records from two continuous red clay and loess palaeosol sequences on the chinese loess plateau have been generated to investigate the evolution and variability of the east asian monsoon eam during the late pliocene and pleistocene tuning the grain size records to orbital obliquity and precession yields an improved astronomical timescale for the loess palaeosol sequence 0 2 6 ma and an extended age model for the upper red clay formation 2 6 3 6 ma this timescale indicates older ages for a number of the magnetic polarity boundaries consistent with lock in depth offsets in loess sequences good site to site correlation enables generation of stacked grain size and susceptibility records spanning the last 3 6 myr these records indicate that monsoon evolution since the late pliocene can be subdivided into three phases 0 1 25 1 25 2 72 and 2 72 3 4 ma each phase is characterized by unique amplitude and frequency characteristics for both summer monsoon magnetic susceptibility and winter monsoon grain size spectral analyses of the stacked monsoon proxies indicate that characteristics of both the summer and winter monsoons are dominated mainly by variance in the eccentricity 410 and 100 kyr obliquity 41 kyr and precession 23 and 19 kyr bands over the past 3 4 myr implying a non linear response of the long term eam evolution to orbital and glacial forcing

event stratigraphy of a caldera forming ignimbrite eruption on tenerife the 273 ka poris formation the 273 ka poris formation in the bandas del sur group records a complex compositionally zoned explosive eruption at las canadas caldera on tenerife canary islands the eruption produced widespread pyroclastic density currents that devastated much of the se of tenerife and deposited one of the most extensive ignimbrite sheets on the island the sheet reaches 40 m thick and includes plinian pumice fall layers massive and diffuse stratified pumiceous ignimbrite widespread lithic breccias and co ignimbrite ashfall deposits several facies are fines rich and contain ash pellets and accretionary lapilli eight brief eruptive phases are represented within its lithostratigraphy phase 1 comprised a fluctuating plinian eruption in which column height increased and then stabilized with time and dispersed tephra over much of the southeastern part of the island phase 2 emplaced three geographically restricted ignimbrite flow units and associated extensive thin co ignimbrite ashfall layers which contain abundant accretionary lapilli from moist co ignimbrite ash plumes a brief plinian phase phase 3 again dispersing pumice lapilli over southeastern tenerife marked the onset of a large sustained pyroclastic density current phase 4 which then waxed phase 5 covering increasingly larger areas of the island as vents widened and or migrated along opening caldera faults the climax of the poris eruption phase 6 was marked by widespread emplacement of coarse lithic breccias thought to record caldera subsidence this is inferred to have disturbed the magma chamber causing mingling and eruption of tephriphonolite magma and it changed the proximal topography diverting the pyroclastic density current s down the guimar valley phase 7 phase 8 involved post eruption erosion and sedimentary reworking accompanied by minor down slope sliding of ignimbrite this was followed by slope stabilization and pedogenesis the fines rich lithofacies with abundant ash pellets and accretionary lapilli record agglomeration of ash in moist ash plumes they resemble phreatomagmatic deposits but a phreatomagmatic origin is difficult to establish because shards are of bubble wall type and the moisture may have arisen by condensation within ascending thermal co ignimbrite ash plumes that contained atmospheric moisture enhanced by that derived from the evaporation of seawater where the hot pyroclastic currents crossed the coast ash pellets formed in co ignimbrite ash clouds and then fell through turbulent pyroclastic density currents where they accreted rims and evolved into accretionary lapilli

sedimentology and geomorphology of the deposits from the august 2006 pyroclastic density currents at tungurahua volcano ecuador the deposits of the pyroclastic density currents from the august 2006 eruption of tungurahua show three facies associations depending on the topographic setting the massive proximal cross stratified and distal cross stratified facies 1 the massive facies is confined to valleys on the slopes of the volcano it contains clasts of 1 m diameter to fine ash material is massive and interpreted as deposited from dense pyroclastic flows its surface can exhibit lobes and levees covered with disk shaped and vesicular large clasts these fragile large clasts must have rafted at the surface of the flows all along the path in order to be preserved and thus imply a sharp density boundary near the surface of these flows 2 the proximal cross stratified facies is exposed on valley overbanks on the upper part of the volcano and contains both massive coarse grained layers and cross stratified ash and lapilli bedsets it is interpreted as deposited from a dense pyroclastic flows that overflowed the gentle ridges of valleys of the upper part of the volcano and b dilute pyroclastic density currents created from the dense flows by the entrainment of air on the steep upper flanks 3 the distal cross stratified facies outcrops as spatially limited isolated and wedge shaped bodies of cross stratified ash deposits located downstream of cliffs on valleys overbanks it contains numerous aggrading dune bedforms whose crest orientations reveal parental flow directions a downstream decrease in the size of the dune bedforms together with a downstream fining trend in the grain size distribution are observed on a 100 m scale this facies is interpreted to have been deposited from dilute pyroclastic density currents with basal tractional boundary layers we suggest that the parental flows were produced from the dense flows by entrainment of air at cliffs and that these diluted currents might rapidly deposit through pneumatic jumps three modes are present in the grain size distribution of all samples independently of the facies which further supports the interpretation that all three facies derive from the same initial flows this study emphasizes the influence of topography on small volume pyroclastic density currents and the importance of flow transformation and flow stripping processes

ecological catastrophe in connection with the impact of the kaali meteorite about 800 400 b c on the island of saaremaa estonia ecological catastrophe in connection with the impact of the kaali meteorite about 800 400 b c on the island of saaremaa estonia

contemporaneous convective and collapsing eruptive dynamics the transitional regime of explosive eruptions 1 two contrasting eruptive regimes have been classically postulated to describe the behavior of explosive eruptions convective and collapsing early studies have recognized that many eruptions evolve from the first to the second regime and have assumed the existence of a sharp boundary between them consequently the dynamics of such transition has never been investigated in detail here we present results of integrated deposit analyses and numerical simulations which demonstrate univocally that both dynamics can and often do coexist for hours in a new intermediate transitional regime of explosive eruptions the study elucidates the features of the new regime in terms of mass partitioning pyroclastic current dynamics and interplay among convective and collapsing styles

climate evolution in the pliocene and pleistocene from marine sediment records and simulations internal variability versus orbital forcing three deep sea 18o records are compared in the temporal and spectral domain with each other a simple ice sheet and bedrock climate model simulation and insolation anomalies at 65 n the model is one dimensional it represents the accumulation and ablation processes of ice its viscoplastic flow and the isostatic adjustment of bedrock under it being forced solely by orbital insolation anomalies the objective of the comparison is to determine the importance of internal variability arising from feedback between the ice sheets and bedrock through the late pliocene and pleistocene in the simulations and in the climate record it is found that a considerable part of the spectral variability at suborbital frequencies i e below the precessional and obliquity frequencies as seen in the sediment cores is simulated by the model simulated variability near 100 kyr has its period determined by the difference between two precessional frequencies the model s internal variability mechanism has a period near 30 50 kyr but is highly damped still it enhances suborbital variability near 100 kyr and 300 kyr and appears to induce nonstationarity of the orbital and suborbital peaks on even longer time scales since the insolation forcing exhibits no noticeable peaks at suborbital or lower frequencies it appears that the nonstationarity in the model simulations as well as the suborbital peaks themselves are due to coupling between model nonlinearities and the orbital forcing since the variability at 100 kyr in the deep sea records is correlated with the simulated variability such a control by the orbital forcing may be also operating in the climate system the absence from the simulation of a trend toward larger ice sheets and colder climate seen throughout the pleistocene in the sediment record is possibly due to slow tectonics and co2 forcing changes not included in the model

hydrogeochemistry of roccamonfina volcano southern italy this is the first hydro geochemical investigation carried out on the roccamonfina volcanic complex groundwaters the chemistry of roccamonfina waters is defined by water rock and water rock gas interactions in fact interactions between rocks of the first eruptive high k formations and circulating groundwaters are recognized by high k concentrations on the other hand inverse concentration of calcium versus alkali metals is related to two different rock interactions occurring in different areas of the volcano a within the caldera where groundwaters flow within latite and pyroclastic formations releasing calcium and b similarly at the base of the volcano where groundwaters flowing from surrounding carbonates got strongly enriched in ca these geochemical processes are also associated with k se of caldera and mg ca in sites located at the ne base of the volcano decrease completely different dynamics occurs at riardo groundwaters se here waters are the result of a mix between the roccamonfina deep aquifer and the carbonate aquifer of the riardo plain rich co2 emissions make these waters strongly mineralized minor elements show a similar geochemical behavior of major ions and are crucial defining interactions processes the evolution of roccamonfina groundwaters is also evident along the simultaneous enrichment of ba sr and ca ba increase is the result of deep local carbonate alteration enhanced by co2 emissions and the lower sr ca ratio from 10 to 2 ppb ppm is also due to the same process in the light of our results the roccamonfina aquifer can be schematically divided into two main reservoirs a a superficial aquifer which basically follows the volcanic structure morphology and tectonics and b a deeper reservoir originating within the oldest roccamonfina volcano ultra potassic lavas and then flowing into the carbonate aquifers of the neighboring plain eventually the chemistry of the roccamonfina aquifer does not show any specific and visible pollution contrary to what happens in the volcano surrounding plains in fact only 14 of the samples we collected 206 show a no3 content 30 mg l these sites are all located at the base of the volcano near the plain

accommodation and sedimentary architecture of isolated icehouse carbonate platforms insights from forward modeling with carb3d abstract a fundamental control on the evolution of carbonate platforms is accommodation the space available for sediment deposition here we adopt a sensitivity analysis approach using the process based forward model carb3d to quantitatively investigate the sedimentary response of isolated carbonate platforms to individual accommodation drivers of eustasy subsidence and surface dissolution sediment production the mechanism invoked to fill accommodation is also investigated fourth order milankovich driven sea level cycles dominate icehouse carbonate sedimentology with 400 ky cycles modulating the effect of 100 ky cycles of comparable magnitude when both cycles are present both overwhelm higher frequency lower amplitude fifth order cycles which are rarely preserved on the platform top subsidence is a fundamental control on sedimentary stacking patterns sequence boundary frequency sbf and platform geometry with rapid subsidence increasing the preservation of platform top sequences most icehouse platforms are subject to extended periods of platform top exposure missed beats when the platform top remains emergent at a sea level maximum are common and create non milankovich sbfs furthermore during platform emergence surface lowering via surface dissolution also creates accommodation for the subsequent platform flooding event with surface dissolution rates in humid climates comparable to moderate rates of subsidence however accommodation generation by surface dissolution is self limiting in as much as accommodation generated by dissolution is rapidly refilled on flooding sediment production is a key control on unfilled accommodation which our simulations suggest is considerable on icehouse platforms increased shallow water 3 m depth sediment production enhances intra platform bathymetric relief between the margin and interior this is driven in large part by a higher sedimentation rate at the margin than in the interior platforms can be flooded to depths up to 45 m during marine transgressions and accommodation often remains unfilled there are two distinct phases of platform top sedimentation during the transgression and early highstand platform margins accumulate at a greater rate than interior sediments generating differential intra platform bathymetry subsequently accommodation in the interior fills by forced regression and progradation from the margins into the platform interior numerical forward models can augment our understanding of icehouse carbonate platform evolution by identifying the nature of the sedimentary response to individual controls they can thus contribute to improving characterization of carbonate reservoirs in icehouse platforms such as the large hydrocarbon fields in the north caspian and s e asia

middle triassic orbital signature recorded in the shallow marine latemar carbonate buildup dolomites italy a new time frequency analysis of sea level controlled carbonate platform cycles in the middle triassic latemar massif dolomites italy reveals a strong depositional signature with characteristics of dominant forcing by climatic precession modes corresponding to long and short precession components at 1 21 7 k y and 1 17 6 k y underwent amplitude modulations matching earth9s orbital eccentricity with major frequency components at 1 400 k y 1 125 k y and 1 98 k y obliquity appears as a minor component at 1 35 4 k y the latemar signature thus constitutes the oldest pristine milankovitch signature yet observed in the geologic record its fidelity rivals that of the pliocene pleistocene record originally used to confirm the theory of orbitally forced climates this evidence deepens a widely noted disagreement between radiometric and cyclostratigraphic time scales for the latemar buildup the latemar cycles indicate that orbitally forced sea level oscillations were operative in the ice free middle triassic hothouse world

x ray fluorescence core scanning records of chemical weathering and monsoon evolution over the past 5 myr in the southern south china sea with benthic foraminiferal d 18 o reveal that phases of the east asian summer monsoon abruptly changed by more than 90 at 4 0 ma 2 75 ma 1 0 ma and 0 6 ma relative to global ice volume at the obliquity and the precession bands over the past 5 myr strong 400 kyr and 100 kyr cycles in the k al and ti al records consistently exist over the past 5 myr particularly these cycles are highly coherent with the long and short eccentricity cycles in the truncated insolation at 65 n indicating an eccentricity forcing of the east asian summer monsoon the chemical weathering recorded in the elemental records of odp site 1143 also shows highly coherent relationship with the ocean carbon reservoir at the eccentricity the obliquity and the precession bands over the late pliocene and pleistocene

dating a small impact crater an age of kaali crater estonia based on charcoal emplaced within proximal ejecta the estimates of the age of the kaali impact structure saaremaa island estonia provided by different authors vary by as much as 6000years ranging from similar to 6400 to similar to 400 before current era bce in this study a new age is obtained based on c 14 dating charred plant material within the proximal ejecta blanket which makes it directly related to the impact structure and not susceptible to potential reservoir effects our results show that the kaali crater was most probably formed shortly after 1530 1450 bce 3237 10 c 14 yr bp saaremaa was already inhabited when the bolide hit the earth thus the crater forming event was probably witnessed by humans there is however no evidence that this event caused significant change in the material culture e g known archeological artifacts or patterns of human habitation on saaremaa

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

stratigraphy chronology and sedimentology of ignimbrites from the white trachytic tuff roccamonfina volcano italy abstract we describe the stratigraphy chronology and grain size characteristics of the white trachytic tuff wtt of roccamonfina volcano italy the pyroclastic rock was emplaced between 317 and 230 ma bp during seven major eruptive events units a to g and three minor events units bc cd and de these units are separated by paleosol layers and compositionally well differentiated pyroclastic successions stratigraphic control is favored by the occurrence at the base of major units of marker layers four wtt units 1 to 4 occur within the central caldera these are not positively correlated with specific extracaldera units the source of most of the wtt units was the central caldera units b and c were controlled by the western wall of the caldera whereas units d and e were able to overcome this barrier spreading symmetrically along the flanks of mc the maximum pumice size mp of units increases with distance from the caldera whereas the maximum lithic size ml decreases mp and ml of the marker layer of unit d mkda mkdp do not show any systematic variations with respect to the central caldera in contrast the thickness of surge mkda decreases with distance from the source and mkdp accumulates to the north of mc probably controlled respectively by mobility transport power and by wind blowing northwards the grain size characteristics of the wtt deposits are used for classifying the units there is no systematic variation of the grain size as a function of stratigraphic height either among units or within single units large variation of components in subunit e1 with repetitive alternation of pyroclastic flow to surge through fallout vs surge deposits suggests that the process of eruption took place in a complex or piecemeal fashion pumice concentration zones pcz occur at all wtt levels on the volcano but they are much thicker and pumice clasts are much larger within the central caldera these were probably originated by the disruption of lava flow or dome to pumice fragments and fine ash due to sudden depressurization and interaction with lake waters of the molten lava local basal pcz are in some cases similar to the lapilli rich layer 1p that has been described elsewhere and may have been deposited from currents transitional between pyroclastic surge and flow other basal pcz formed in response to small undulations in the substrate or can be originated by fallout lenticular pcz within ignimbrite interiors and tops are interpreted to record marginal pumice levees and pumice rafts some of which were buried by subsequant pyroclastic flows lithic concentration zones lcz also occur at various stratigraphic height within the extracaldera ignimbrites whereas intracaldera lcz are absent probably due to the fact that ignimbrite currents are strongly energetic and erosive near vent lcz at the top of basal inversely graded layers are formed by mechanical sieving or dispersive pressure in response to variable velocity gradients and particle concentration gradients a segregation process coarse lcz and coarse lithic breccias lb that reside in the interior or tops of pyroclastic flows and that occur in medial to distal areas are interpreted to be the result of slugs of lithic rich debris introduced by vent collapse or rockslides into the moving pyroclastic flows along their flow paths these lcz become mixed to varying degrees due to differential densities and velocities relative to the pyroclastic flows desegregation processes

single pyroclastic beds deposited by simultaneous fallout and surge processes roccamonfina volcano italy abstract a sequence of fine grained hydrovolcanically generated pyroclastic surge deposits in the white trachytic tuff roccamonfina volcano italy contains two layers of coarse pumice clasts pumice fragments in these layers show no evidence for abrasion during lateral transport unlike pumice in the enclosing deposits and are well sorted in most locations they have textural and grain size characteristics typical of plinian fall deposits bedding of these pumice layers ranges from planar with gradual thickness variation over a few meters distance to highly variable with strong thickening in depressions the layers are interpreted to record deposition of pumice falling from a plinian eruption column through laterally moving pyroclastic surges depending on the local vigor of the surges during the fallout event bedding characteristics range from those approaching pure fallout to those approaching pure surge the simultaneous plinian and hydrovolcanic pyroclastic surge producing activity may have come from two or more separate vents or from a single vent that experienced rapid variations in magma water interaction

modeling the time dependent response of the asian summer monsoon to obliquity forcing in a coupled gcm a phasemap sensitivity experiment to study the time dependent response of the asian summer monsoon to obliquity forcing we analyze a 284 000 year long transient simulation produced by a fully coupled global climate model gcm using a new phase mapping phasemap approach here we focus on understanding the phase response of monsoonal circulation to insolation forcing at the earth orbital obliquity band 41 kyr our results show that the east asian summer monsoon easm can be divided into two geographic regions the north east asian summer monsoon neasm and the south east asian summer monsoon seasm the indian summer monsoon ism and the seasm are in phase at the obliquity band strengthened with an increase in obliquity from obliquity minima omin to obliquity maxima omax the neasm is out of phase with the ism and seasm weakened with an increase in obliquity from omin to omax we hypothesize that the inverse phase between the neasm and the ism at the obliquity band results from an ism neasm teleconnection linked to the formation mechanism of the bonin high

volcanostratigraphy and petrogenesis of the nemrut stratovolcano east anatolian high plateau the most recent post collisional volcanism in turkey abstract the nemrut stratovolcano with its well defined summit caldera is the most recent volcanic eruption center 1441 a d in the east anatolian high plateau one of the best examples of an active colllision zone widespread volcanism has been active in this region for the last 10 ma producing large volumes of lavas and pyroclastics covering a broad belt across the bitlis suture zone from the anatolian plate in the north to the arabian plate in the south three major evolutionary stages have been identified in the evolution of the nemrut stratovolcano pre caldera post caldera and late stages the pre caldera stage is further differentiated into effusive extrusive and explosive phases the latter being responsible for the caldera formation the products of this stage cover a broad compositional range from basalts to rhyolites and consist of lava flows domes large volumes of ignimbrites and associated pyroclastics the post caldera stage consists of three phases comprising phreatomagmatic eruptions vitrophyric rhyolitic lava flows and dykes the late stage consists of the explosive and the following effusive phases with vitrophyric rhyolitic and basaltic lava flows respectively the post caldera and late stages show a compositional gap between the younger mafic and the older felsic members the products of all three stages are silica saturated and display a gradual transition in nature from sodic in the mafic to potassic in the felsic members the overlaps in the compositional spectra and the geochemical trends on bivariate plots suggest the derivation of the products of all three stages from a single parental magma or similar parental magmas yet the stratigraphic position of the mafic and felsic members points to a periodically replenished magma chamber the major and trace element systematics along with petrographic evidences suggest clinopyroxene plagioclase olivine amphibole as the major fractionating phases governing the liquid evolutionary paths with a progressive increase in the proportions of plagioclase and amphibole joined by k feldspar biotite and apatite in the fractionating assemblages during the course of magmatic differentiation the volcanics of all three stages exhibit enrichment in light rare earth elements lree over heavy rare earth elements hree and large ion lithophile elements lile over high field strength elements hfse the multi element patterns and low th y and nb y ratios imply a within plate type mantle asthenospheric source for this volcanics trace element modelling suggests derivation of parental magma s from 10 to 30 partial melting of a spinel lherzolite source assimilation fractional crystallization afc modelling reveals significant but variable degrees of crustal contamination in the evolution of the volcanics with a range of r values assimilation rate crystallization rate from 0 2 to 0 8 these interpretations are in conformity with the recent findings from seismic studies showing that lithospheric mantle is absent in eastern anatolia the crust in the region is about 45 km thick and directly underlain by the asthenosphere possibly due to slab breakoff following the collision of the arabian and the anatolian plates along the bitlis suture zone

anatomy of a young impact event in central alberta canada prospects for the missing holocene impact record small impact events recorded on the surface of earth are signifi cantly underrepresented based on expected magnitude frequency relations we report the discovery of a 36 m diameter late holocene impact crater located in a forested area near the town of whitecourt alberta canada although undetectable using visible imagery the presence of the crater is revealed using a bare earth digital elevation model obtained through airborne light detection and ranging lidar the target material comprises deglacial quaternary sediments with impact ejecta burying a late holocene soil dated to ca 1100 14 c yr b p most of the 74 iron meteorites 0 1 1196 g recovered have an angular exterior morphology these meteorites were buried at depths 25 cm and are interpreted to result from fragmentation of the original projectile mass either at low altitude or during the impact event impact of the main mass formed the simple bowl shaped impact structure associated with an ejecta blanket and crater fi ll the increasing availability of lidar data for many terrestrial surfaces will serve as a useful tool in the discovery of additional small impact features

filtering of milankovitch cycles by earth s geography abstract earth s land sea distribution modifies the temperature response to orbitally induced perturbations of the seasonal insolation we examine this modification in the frequency domain by generating 800 000 yr time series of maximum summer temperature in selected regions with a linear two dimensional seasonal energy balance climate model previous studies have demonstrated that this model has a sensitivity comparable to general circulation models for the seasonal temperature response to orbital forcing on land although the observed response in the geologic record is sometimes significantly different than modeled here differences attributable to model limitations and feedbacks involving the ocean atmosphere cryosphere system there are several results of significance 1 in mid latitude land areas the orbital signal is translated linearly into a large 10 c seasonal temperature response 2 although the modeled seasonal response to orbital forcing on antarctica is 6 c the annual mean temperature effect

the basal ash deposit of the sovana eruption vulsini volcanoes central italy the product of a dilute pyroclastic density current a low aspect ratio decimeter thick ash deposit axisymmetrically distributed around the latera caldera western vulsini volcanoes central italy has been studied by means of field and laboratory investigations field studies comprise facies analysis at centimeter scale and maximum clast size and deposit thickness measurements grain size and component distribution chemical composition and particle morphoscopic features have been determined on selected samples we discuss the co ignimbrite ash fall vs pyroclastic surge origin of the deposit and the hydrovolcanic vs magmatic eruption nature complex facies association textural features and grain size data rule out an ash fall origin for the whole deposit the hydrovolcanic nature of the eruption has been discarded on the grounds of componentry and morphoscopic features of vitric fragments we propose that the main body of the ash deposit formed from a radially expanding dilute turbulent pyroclastic density current originated by a continuous collapse of a low altitude a few kilometers eruptive column with a possible radial jet component

complementary roles of orbital insolation and north atlantic deep water during late pleistocene interglacials a long standing dilemma of the milankovitch hypothesis involves the observation that the holocene deglaciation was comparable in extent to previous late pleistocene interglacials even though insolation forcing was less in this paper we propose an explanation that is based on an observed complementary relationship between eccentricity and north atlantic deep water nadw production rates a 13c record from the south atlantic indicates that nadw production rates are high when eccentricity is low stages 1 11 and vice versa for the holocene the inferred stronger thermohaline circulation injects approximately the same heat into the high latitude north atlantic 9 w m as the insolation differences between the early holocene and last interglacial peak we utilize an energy balance model to calculate that the excess heat from the holocene thermohaline increase can explain at least 50 65 and 100 of the energy budget shortfall from milankovitch forcing over the laurentide and fennoscandian ice sheets respectively we suggest that the observed complementary relationship between milankovitch and nadw reflects a coupling due to a reverse monsoonal effect over the north atlantic during times of high eccentricity and strong land sea contrast enhanced heating over land results in increased subsidence over the ocean which causes circulation and precipitation changes these processes could lower ocean salinities and decrease thermohaline circulation thus deglaciation can progress through either mechanism with the total heat balance for each interglacial being approximately the same however the combined effect of increased insolation and reduced thermohaline circulation for the last interglacial yields slightly warmer 1 0 c temperatures in mid and high latitude northern hemisphere land areas than in the holocene this result is approximately in line with geologic data from these regions

eccentricity forcing of pliocene early pleistocene climate revealed in a marine oxygen isotope record milankovitch theory that climate is controlled by variations in the earth s orbital parameters has gained wide acceptance for its ability to account for two climate cycles a 23 kyr cycle that is phase locked to the precession driven insolation cycle and a 41 kyr cycle that is phase locked to the obliquity driven insolation cycle1 6 but explaining the observed 100 kyr climate cycle in terms of milankovitch theory especially for the late pleistocene ice age cycle remains controversial in spite of a strong correlation with the 100 kyr cycle in the earth s orbital eccentricity5 one problem is that eccentricity affects insolation mainly by modulating the precession cycle its direct contribution to radiation change is too small 0 1 to effect the observed climate change directly5 7 another is the absence of a late pleistocene ice volume cycle in oxygen isotope records to match the 404 kyr component of the eccentricity cycle5 8 here we examine an oxygen isotope record spanning the interval 1 2 to 5 2 million years ago before the late pleistocene ice age regime we find 404 kyr and 100 kyr climate cycles which are coherent with eccentricity and which have amplitudes that are similar to the coexisting 23 kyr cycle analysis of these low frequency cycles suggests that they originate through an asymmetrical response mechanism that preferentially introduces variance into the climate system from the warmer portions of the eccentricity modulated precession cycle our data thus support eccentricity s role in the origin of low frequency oxygen isotope cycles before the late pleistocene ice age

charcoal reflectance as a proxy for the emplacement temperature of pyroclastic flow deposits hot pyroclastic flow deposits often entomb and preserve vegetation as charcoal when studied in polished section this charcoal is highly reflective novel data from experimentally charred woods demonstrate that reflectance increases with both temperature and time at temperatures above 250 c reflectance rises rapidly for the first hour effectively stabilizing after 4 h for temperatures below 400 c however for higher temperatures reflectance only stabilizes after 24 h charcoalified woods from block and ash flow deposits generated by the soufriere hills volcano in montserrat have yielded interpreted deposit temperatures based on reflectance data of 325 525 c these temperatures compare favorably with directly measured data 365 640 c from the same deposits indicating that charcoal reflectance is useful as a temperature proxy for ancient pyroclastic flow deposits particularly where magnetic or mineral data are absent for this purpose however caution should be exercised as only a minimum temperature can be inferred where the heating duration of the deposit is unknown unlike magnetic or mineral data temperature data from charcoalified woods can be obtained from reworked deposits providing a valuable means of validating observations made about the style of eruption of volcanoes in ancient settings

a late holocene explosive mafic eruption of villarrica volcano southern andes the chaimilla deposit abstract villarrica chile is one of the most active volcanoes in south america having erupted about 60 times in the last 460 years although its historical eruptive activity has been mainly effusive and weakly explosive it had strong explosive behaviour in postglacial times chaimilla

motion of a fragmented meteoroid through the planetary atmosphere meteor observations and crater field investigations support the view that some meteoroids undergo fragmentation into a finite number of splinters which move a long distance without further disruption the motion of these fragments cannot be described in the frame of liquid like models the primary purpose of this study is to achieve some success in improving the alternative model of separate fragments which is based on studying the motion of a finite number of fragments interacting with each other through the air pressure the three dimensional 3 d numerical technique is elaborated for the direct modeling of the flow around several arbitrarily arranged fragments the model allows us to calculate the ablation mass rate and aerodynamic loading for each fragment the new approach hybrid model based on the simultaneous solution of the 3 d hydrodynamic equations for the airflow and the differential equations for the motion of discrete particles is proposed to study the debris cloud evolution taking into account successive fragmentation the simplified version of this model was applied to the sikhote alin meteorite shower benesov bolide and small meteoroids impacts against the martian surface

basal layered deposits of the peach springs tuff northwestern arizona usa basal layered deposits of the large volume peach springs tuff occur beneath the main pyroclastic flow deposit over a minimum lateral distance of 70 km in northwestern arizona usa the basal deposits are interpreted to record initial blasting and pyroclastic surge events at the beginning of the eruption the pyroclastic surges traveled a minimum of 100 km from the as yet unknown source changes in bedding structures with increasing flow distance are related to the decreasing sediment load of the surges some bed forms in the most proximal part of the study area kingman arizona can be interpreted as being shock induced reflecting a blast origin for the surges component analyses support a hydrovolcanic origin for some of the blasting and subsequent pyroclastic surges the eruption apparently began with magmatic blasts which were replaced by hydrovolcanic blasts hydrovolcanic activity may be partially related to failure of the conduit walls that temporarily plugged the vent a single large volume pyroclastic flow immediately followed the blast phase and no evidence has been observed for a plinian eruption column the stratigraphic sequence indicates that powerful hydrovolcanic blasting rapidly widened the vent thus bypassing a plinian fallout phase and causing rapid evolution to a collapsing eruption column similar processes may occur in other large volume ignimbrite eruptions which commonly lack significant plinian fallout deposits

using hydraulic equivalences to discriminate transport processes of volcanic flows we characterized stratified deposits from the upper toluca pumice at toluca volcano mexico to distinguish the various modes of transport at play in their genesis using the concept of hydraulic equivalence we determined that deposits resulted from a combination of suspended load fallout saltation and rolling in particular some well sorted coarse stratified beds have a single pumice mode most likely indicative of clasts having traveled through both the transport system and the traction bed such beds are likely remnants of the sorting operated within the large scale transport system other coarse beds have pumice and lithic modes suggesting rolling in the traction bed we propose that boundary layer processes control the sorting of those beds and all finer beds by helping to discriminate between transport mechanisms hydraulic equivalences have a general applicability in geophysical flows involving clasts of contrasted densities

past hydrographic and climatic changes in the subantarctic zone of the south atlantic the pleistocene record from odp site 1090 abstract pleistocene summer sea surface temperatures ssst have been reconstructed on a composite core section recovered in the subantarctic zone of the southern ocean from planktonic foraminifers applying the modern analog technique the composite consists of core ps2489 2 and the sections recovered at odp site 1090 and documents the last 1 83 ma three distinct climatic periods can be identified that mirror the pleistocene development of the southern ocean hydrography cold climatic conditions prevailed at 43 s during glacial as well as during interglacial periods during the early pleistocene 1 83 0 87 ma indicating a northward shift of isotherms that characterize the present day polar front zone by about 7 of latitude evidence shows a strong linkage between southern ocean and low latitude climate during that interval time between the mid pleistocene revolution ca 0 9 ma and the mid brunhes event ca 0 4 ma we observe higher amplitude fluctuations in the ssst between glacial and interglacial periods corresponding to the temperature range between the present polar front and subantarctic front these climatic variations have been related to changes in the northern hemisphere ice sheets the past 0 4 ma are characterized by strong ssst variations of up to 8 c between glacials and interglacials only during the climatic optima stages 11 3 9 3 7 5 7 1 5 5 and the early holocene ssst exceeded present ssst at the core locality 10 2 c although the carbonate dissolution record exhibits high variability during the pleistocene it can be shown that ssst estimates were not significantly biased the mid brunhes dissolution cycle as well as the mid pleistocene enhanced carbonate preservation appear to belong to a global long term variability in carbonate preservation

lake baikal record of continental climate response to orbital insolation during the past 5 million years the sedimentary record of biogenic silica from lake baikal in south central siberia suggests that this region of central asia was impacted by two major cooling episodes at 2 8 to 2 6 and 1 8 to 1 6 million years ago the spectral evolution of this continental interior site parallels the evolutionary frequency spectra for various marine oxygen isotope records in the baikal record the 41 000 year obliquity cycle is particularly strong from 1 8 to 0 8 million years ago variance in the 100 000 year eccentricity band increases during the past 0 8 million years the expected precession frequency of 23 000 years is highest during the past 400 000 years the modulation of the predicted 23 000 and 41 000 year insolation forcing by the 100 000 and 400 000 year eccentricity bands indicates that the transfer of variance from the precession and obliquity frequencies to the eccentricity part of the spectrum occurred in the eurasian continental interior as well as in tropical and high latitude ocean sites

transport and sedimentation dynamics of transitional explosive eruption columns the example of the 800 bp quilotoa plinian eruption ecuador abstract impact of large scale explosive eruptions largely depends on the dynamics of transport dispersal and deposition of ash by the convective system in fully convective eruptive columns ejected gases and particles emitted at the vent are vertically injected into the atmosphere by a narrow buoyant column and then dispersed by atmosphere dynamics on a regional scale in fully collapsing explosive eruptions ash partly generated by secondary fragmentation is carried and dispersed by broad co ignimbrite columns ascending above pyroclastic currents in this paper we investigate the transport and dispersion dynamics of ash and lapillis during a transitional plinian eruption in which both plinian and co ignimbrite columns coexisted and interacted the 800 bp eruptive cycle of quilotoa volcano ecuador produced a well exposed tephra sequence our study shows that the sequence was accumulated by a variety of eruptive dynamics ranging from early small phreatic explosions to sustained magmatic plinian eruptions to late phreatomagmatic explosive pulses the eruptive style of the main 800 bp plinian eruption u1 progressively evolved from an early fully convective column plinian fall bed to a late fully collapsing fountain dense density currents passing through an intermediate transitional eruptive phase fall syn plinian dilute density currents in the transitional u1 regime height of the convective plinian column and volume and runout of the contemporaneous pyroclastic density currents generated by partial collapses were inversely correlated the convective system originated from merging of co plinian and co surge contributions this hybrid column dispersed a bimodal lapilli and ash fall bed whose grain size markedly differs from that of classic fall deposits accumulated by fully convective plinian columns sedimentological analysis suggests that ash dispersion during transitional eruptions is affected by early aggregation of dry particle clusters

the age of the kaalijarv meteorite craters abstract precise radiometric age determination of the kaalijarv meteorite craters on the island of saaremaa in estonia have so far proved inconclusive here we present trace element analyses of peat cores taken several kilometers away from the kaalijarv craters that reveal a distinct ir enriched layer produced by the meteorite impact by radiocarbon dating the peat cores we have determined for the first time the precise age of the impact that generated the kaalijarv craters the calibrated date of the impact is 400 370 b c at 1

reconstruction of glaciation over the past 6 myr from ice borne deposits in the norwegian sea it is well known that a significant intensification of northern hemisphere glaciation occurred 2 5 myr ago in contrast to the much earlier onset 35 myr of glaciation in antarctica1 3 much less is known about the behaviour of the climate before 2 5 myr and it has remained unclear when sizeable glaciers first started to develop in the northern hemisphere here we deduce the history of high northern latitude glaciation over the past 6 myr from records of ice borne deposits in deep sea sediments of the norwegian sea we find that glaciers large enough to reach sea level were present in the norwegian sea area as early as 5 5 myr three million years before the intensification of glaciation at 2 5 myr fluctuations in ice volume can be deduced from the oxygen isotope record but this provides only a global average which may not reflect the history of ice sheet growth in specific regions from a comparison of the oxygen isotope records with the record of ice rafted material we derive an estimate of the relative contributions of southern and northern hemisphere glaciation to global variations in ice volume

numerical ages for plio pleistocene glacial sediment sequences by 26al 10be dating of quartz in buried paleosols abstract we describe a method for dating plio pleistocene sediments by analysing 26 al and 10 be in quartz from buried paleosols the method amounts to a generalization of the technique of burial dating which has previously been used to date river sands deposited in caves as well as fluvial terraces we a measure nuclide concentrations at multiple depths in the paleosol b use the geologic context of the sample to construct an exposure burial history an exposure model that can be used to predict the nuclide concentrations in the samples as a function of whatever unknown parameters are of geological interest e g the ages of geologic units and then c find the parameters in the exposure model that result in the best fit between predicted and observed nuclide concentrations we apply the method to a paleosol developed on the uppermost till in eastern nebraska which is buried by several middle to late pleistocene loess units the ages of some of which are known independently the lowest loess unit whose age was not previously known was deposited 0 58 0 12 ma and the till was emplaced 0 65 0 14 ma which agrees with existing age constraints from paleomagnetic measurements and the position of the lava creek ash in the regional stratigraphy the most important source of uncertainty in these age determinations is the analytical uncertainty in 26 al and 10 be measurements uncertainties in either the nuclide production rates or the geologic model are comparatively unimportant

on the structure and origin of major glaciation cycles 2 the 100 000 year cycle climate over the past million years has been dominated by glaciation cycles with periods near 23 000 41 000 and 100 000 years in a linear version of the milankovitch theory the two shorter cycles can be explained as responses to insolation cycles driven by precession and obliquity but the 100 000 year radiation cycle arising from eccentricity variation is much too small in amplitude and too late in phase to produce the corresponding climate cycle by direct forcing we present phase observations showing that the geographic progression of local responses over the 100 000 year cycle is similar to the progression in the other two cycles implying that a similar set of internal climatic mechanisms operates in all three but the phase sequence in the 100 000 year cycle requires a source of climatic inertia having a time constant similar to 15 000 years much larger than the other cycles similar to 5 000 years our conceptual model identifies massive northern hemisphere ice sheets as this larger inertial source when these ice sheets forced by precession and obliquity exceed a critical size they cease responding as linear milankovitch slaves and drive atmospheric and oceanic responses that mimic the externally forced responses in our model the coupled system acts as a nonlinear amplifier that is particularly sensitive to eccentricity driven modulations in the 23 000 year sea level cycle during an interval when sea level is forced upward from a major low stand by a milankovitch response acting either alone or in combination with an internally driven higher frequency process ice sheets grounded on continental shelves become unstable mass wasting accelerates and the resulting deglaciation sets the phase of one wave in the train of 100 000 year oscillations whether a glacier or ice sheet influences the climate depends very much on the scale the interesting aspect is that an effect on the local climate can still make an ice mass grow larger and larger thereby gradually increasing its radius of influence

charring of woods by volcanic processes an example from the taupo ignimbrite new zealand abstract one of the most violent and explosive eruptions in historic times was that of the taupo volcano in new zealand 1800 years ago this culminated in emplacement of the taupo ignimbrite laid down by a single vent generated pyroclastic density current that entombed and preserved abundant plant material the ignimbrite consists of two layers generated by processes within the concentrated part of the density current flow sensu stricto and where the current interacted with the atmosphere and the pre eruptive vegetative cover eighty four wood samples were collected from eight localities representing both layers at varying distances from the vent including the previously described pureora fossil forest samples were studied by light scanning electron and reflectance microscopy in order to determine their systematic affinity angiosperm or conifer degree of charring and reflectance values most samples 59 were conifer probably derived from podocarps that are known to have been common in the pre eruptive vegetation samples ranged from uncharred through incompletely charred to fully charred woods the variation in degrees of charring and reflectance values implies that the entrained charcoal did not equilibrate with the flow temperature minimum charring temperatures interpreted from the mean of the three highest maximum reflectance values showed no major differences between ignimbrite layers and were generally low ranging from 269 to 398 c both ignimbrite layers also show a trend of increasing deposit temperature away from the vent consistent with estimates from palaeomagnetic techniques

in situ10be 26al exposure ages at meteor crater arizona a new method of dating the surface exposure of rocks from in situ production of 10be and 26al has been applied to determine the age of meteor crater arizona a lower bound on the crater age of 49 200 1 700 years has been obtained by this method