precise ages of the reunion event and huckleberry ridge excursion episodic clustering of geomagnetic instabilities and the dynamics of flow within the outer core the reunion event is one of the earliest recognized periods of normal polarity within the reversed matuyama chron named for the site at which it was first discovered on reunion island it has since purportedly been found globally in both volcanic rocks and sediments and thus has become a key chronostratigraphic marker however geochronologic results from several locations thought to have recorded this event have caused considerable confusion regarding not only its age and duration but also the number of reunion events new 40ar 39ar ages from eight reunion island lavas in three distinct sections are indistinguishable from one another thereby placing the event at 2 200 0 007 0 010 ma2 200 0 007 0 010 ma 2 2 analytical total uncertainty note this format is used throughout the paper the paleomagnetic behavior recorded at two of the island sites shows that the opposite normal polarity was reached and sustained for a period during which several lava flows were erupted whether this can be classified as a very short subchron bounded by a rapid set of back to back reversals or as a special case of a geomagnetic excursion is unclear hence we choose to continue labeling the dynamo activity recorded by these reunion island lavas as an event this event preceded a 38 kyr 38 kyr period of normal polarity that we name the feni subchron after its locality of discovery at odp site 981 the feni subchron was succeeded by the huckleberry ridge excursion for which 40ar 39ar sanidine and u pb zircon ages of 2 077 0 001 0 003 ma2 077 0 001 0 003 ma and 2 084 0 012 0 013 ma2 084 0 012 0 013 ma respectively from member b of the huckleberry ridge tuff in idaho are in agreement these findings suggest that the full normal polarity recorded on reunion island is a singular brief period of unstable field behavior at the onset of a 125 kyr 125 kyr bundling of dynamo instabilities from 2 20 to 2 07 ma disturbances to the axial dipole component of earth s magnetic field during this period and by analogy similar periods of temporally clustered excursions during the early and late portions of the brunhes chron may reflect disruptions to convective flow arising from parcels of material introduced into the outer core from either the inner core or core mantle boundaries a proposition that might be tested by future numerical dynamo simulations

why are geomagnetic excursions not always recorded in sediments constraints from post depositional remanent magnetization lock in modelling abstract many geomagnetic excursions have been documented during the brunhes chron however high resolution paleomagnetic studies of sediments often provide evidence of few if any excursions we have investigated the reasons for this observation by modelling the post depositional remanent magnetization pdrm lock in process a cubic lock in function produces more rapid lock in compared to linear and exponential functions proposed in the literature we therefore used a cubic function to model best case scenarios for the quality of the paleomagnetic record when a high frequency geomagnetic input signal is convolved with the sediment lock in function for a wide range of sedimentation rates even for a lock in depth of 10 cm where 95 of the pdrm is locked in 5 cm below the surface mixed layer an input signal containing abundant excursions with 1 kyr duration is smoothed so drastically at low sedimentation rates 1 3 cm kyr that no excursions are recorded excursions are further attenuated by increasing the lock in depth as well as by sampling at discrete stratigraphic intervals the pdrm process acts as a low pass filter for high frequency secular variation signals even at moderate sedimentation rates 10 cm kyr we have also modelled the effects of modulation of sedimentation rate by a climatic forcing function e g insolation even though the average sedimentation rate might be substantial pdrm recording becomes much less reliable in intervals where sedimentation rates are lowest in these cases some excursions are reliably recorded in detail while others may not be recorded at all our modelling suggests that in order to consistently detect the presence of geomagnetic excursions it is ideal to work with sediments that maintain minimum sedimentation rates above 10 cm kyr if the normal spectrum of geomagnetic field behaviour contains abundant excursions failure to document excursions in many apparently high resolution analyses probably results from pdrm lock in within relatively slowly deposited sediments or to unrecognized intervals of slow sedimentation in environments with higher average sedimentation rates

neogene ignimbrites of the nevsehir plateau central turkey stratigraphy distribution and source constraints abstract in anatolia turkey extensive calc alkaline volcanism has developed along discontinuous provinces from neogene to quaternary times as a consequence of plate convergence and continental collision in the nevsehir plateau which is located in the central anatolian volcanic province volcanism consists of numerous monogenetic centres several large stratovolcanoes and an extensive mainly neogene rhyolitic ignimbrite field vent and caldera locations for the neogene ignimbrites were not well known based on previous studies in the neogene ignimbrite sequence of the nevsehir plateau we have identified an old group of ignimbrites kavak ignimbrites followed by five major ignimbrite units zelve sarimaden tepe cemilkoy gordeles kizilkaya and two smaller less extensive ones tahar sofular other ignimbrite units at the margin of the plateau occur as outliers of larger ignimbrites whose main distributions are beyond the plateau excellent exposure and physical continuity of the units over large areas have allowed establishment of the stratigraphic succession of the ignimbrites as from bottom to top kavak zelve sarimaden tepe cemilkoy tahar gordeles sofular kizilkaya our stratigraphic scheme refines previous ones by the identification of the zelve ignimbrite and the correlation of the previously defined akkoy ignimbrite with the sarimaden tepe ignimbrite correlations of distant ignimbrite remnants have been achieved by using a combination a field criteria 1 sedimentological characterisitics 2 phenocryst assemblage 3 pumice vesiculation texture 4 presence and characteristics of associated plinian fallout deposits and 5 lithic types the correlations significantly enlarge the estimates of the original extent and volume of most ignimbrites volumes range between 80 km 3 and 300 km 3 for the major ignimbrites corresponding to 2500 10 000 km 3 in areal extent the major ignimbrites of the nevsehir plateau have an inferred source area in the derinkuyu tectonic basin which extends mainly between nevsehir and the melendiz dag volcanic complex the kavak ignimbrites and the zelve ignimbrite have inferred sources located between nevsehir and derinkuyu coincident with a negative gravity anomaly the younger ignimbrites sarimaden tepe cemilkoy gordeles kizilkaya have inferred sources clustered to the south between the erdas dag and the melendiz dag volcanic complex we found evidence of collapse structures on the northern and southern flanks of the erdas dag volcanic massif and of a large updoming structure in the sahinkalesi tepe massif the present day derinkuyu tectonic basin is mostly covered with quaternary sediments and volcanics the fault system which bounds the basin to the east provides evidence that the ignimbrite volcanism and inferred caldera formation took place in a locally extensional environment while the basin was already subsiding drilling and geophysical prospecting are necessary to decipher in detail the presently unknown internal structure of the basin and the inferred probably coalesced or nested calderas within it

stratigraphy and age of the cappadocia ignimbrites turkey reconciling field constraints with paleontologic radiochronologic geochemical and paleomagnetic data the stratigraphy and age of the neogene cappadocia ignimbrites central turkey have been inferred in previous studies from fieldwork and k ar age determinations the resulting stratigraphic schemes however differed from each other suggesting that further studies were required to produce a reliable succession in this paper we examine the chronostratigraphy of mammalian remains recovered in the continental sediments interbedded with the cappadocia ignimbrites using recent advances in mammalian chronostratigraphy we evaluate selected taxa and faunal associations to place new and independent constraints on the ignimbrite ages the biostratigraphically bracketed ages concur with some published radiometric dates but they disagree with others principally at localities where major stratigraphic discrepancies have arisen in the literature in order to reconcile these apparent inconsistencies we combine at selected sites our field observations with the biostratigraphic and radiometric age limits and we compare these with the available geochemical and magnetic data this allows us to present revised age estimates and a revised stratigraphy which includes the correlation of the local sofular deposits with the large volume gordeles ignimbrite the issues faced in this study apply to other ignimbrite provinces in the world for instance ignimbrite eruption frequency in cappadocia is higher than the resolution of many published k ar ages furthermore different k ar ages have led to the description of individual and distinct ignimbrites that fieldwork and geochemical data allow to merge into a single ignimbrite argon loss from pumice samples leading to radiometric rejuvenation provides a likely explanation for most stratigraphic discrepancies cappadocia is the only documented ignimbrite field in which the chronostratigraphy of vertebrate remains provides better constraints on some ignimbrite ages than scattered k ar dates we further argue that k ar dates from the cappadocia area are too imprecise to establish a reliable magnetostratigraphic scheme for the ignimbrite succession with the exception of the not vert similar2 8 ma valibaba tepe ignimbrite

a miocene phreatoplinian eruption in the north eastern pannonian basin hungary the jato member abstract a middle miocene 8 m thick pyroclastic succession reported from the bukk foreland volcanic area bfva in northern hungary central europe specified here as the jato member was produced by silicic phreatomagmatism phreatoplinian sensu lato two well preserved outcrops 8 km apart and inferred to be within 10 50 km from source represent the discontinuously exposed layered paleosol bounded phreatomagmatic jato member they show an identical phenocrystal assemblage of feldspar biotite and amphibole without weathered zones or signs of erosion that suggest deposition in one eruption phase lasting hours to months the succession contains three subunits 1 subunit a 1 8 m thick a series of well sorted fine to coarse ash or lapilli tuff layers with constant thickness 2 subunit b 2 1 m thick a series of normal graded layers with an upper fine grained zone containing abundant ash aggregates with a coarser grained core and distinctively finer grained outer rim 3 subunit c 4 5 m thick a massive poorly to well sorted coarse ash with gas escape structures and ash aggregates at its base the upward change of these lithofacies implies an initially sustained dry fallout dominated deposition of ash and pumice lapilli resulting in subunit a subsequently multiple wet and dilute pyroclastic density currents pdcs dispersed subunits b and c the general abundance of pdc related ash aggregates in the middle upper part of the succession particularly in subunit b and the transformation of a fall dominated to a collapsing depositional regime producing wet dilute pdcs imply the increasing influence of water during the eruption phreatoplinian sensu lato the presence of water is related to an epicontinental sea during middle to late miocene in the carpatho pannonian region the transition from an initial dry magmatic phase generated fallout activity followed by the emplacement of wet pdcs rich in ash aggregates when external water infiltrated from a surrounding lake or sea water entered the vent

mechanisms and timescales of generating eruptible rhyolitic magmas at yellowstone caldera from zircon and sanidine geochronology and geochemistry we constrain the physical nature of the magma reservoir and the mechanisms of rhyolite generation at yellowstone caldera via detailed characterization of zircon and sanidine crystals hosted in three rhyolites erupted during the c 170 70 ka central plateau member eruptive episode the most recent post caldera magmatism at yellowstone we present 238 u 230 th crystallization ages and trace element compositions of the interiors and surfaces i e unpolished rims of single zircon crystals from each rhyolite we compare these zircon data with 238 u 230 th crystallization ages of bulk sanidine separates coupled with chemical and isotopic data from single sanidine crystals zircon age and trace element data demonstrate that the magma reservoir that sourced the central plateau member rhyolites was long lived 150 250 kyr and genetically related to the preceding episode of magmatism which occurred c 256 ka the interiors of most zircons in each rhyolite were inherited from unerupted material related to older stages of central plateau member magmatism or the preceding late upper basin member magmatism i e are antecrysts conversely most zircon surfaces crystallized near the time of eruption from their host liquids i e are autocrystic the repeated recycling of zircon interiors from older stages of magmatism demonstrates that sequentially erupted central plateau member rhyolites are genetically related sanidine separates from each rhyolite yield 238 u 230 th crystallization ages at or near the eruption age of their host magmas coeval with the coexisting zircon surfaces but are younger than the coexisting zircon interiors chemical and isotopic data from single sanidine crystals demonstrate that the sanidines in each rhyolite are in equilibrium with their host melts which considered along with their near eruption crystallization ages suggests that nearly all central plateau member sanidines are autocrystic the paucity of antecrystic sanidine crystals relative to antecrystic zircons requires a model in which eruptible rhyolites are generated by extracting melt and zircons from a long lived mush of immobile crystal rich magma in this process the larger sanidine crystals remain trapped in the locked crystal network the extracted melts plus antecrystic zircon amalgamate into a liquid dominated i e eruptible magma body that is maintained as a physically distinct entity relative to the bulk of the long lived crystal mush zircon surfaces and sanidines in each rhyolite crystallize after melt extraction and amalgamation and their ages constrain the residence time of eruptible magmas at yellowstone residence times of the large volume rhyolites 40 70 km 3 are 1 kyr conservatively 6 kyr which suggests that large volumes of rhyolite can be generated rapidly by extracting melt from a crystal mush because the lifespan of the crystal mush that sourced the central plateau member rhyolites is two orders of magnitude longer than the residence time of eruptible magma

40ar 39ar dating of a tephra layer in the pliocene seneze maar lacustrine sequence french massif central constraint on the age of the reunion matuyama transition and implications on paleoenvironmental archives the sedimentary sequence cored in the seneze maar velay france is very favourable for magnetostratigraphy of the late pliocene and for studying the paleoenvironmental changes that occurred during that period the magnetostratigraphic studies revealed that the sediments from this lacustrine sequence were deposited during the matuyama reverse period except for the upper part where a short normal polarity episode was recorded a tephra layer interbedded in the sediments contemporaneous with this normal polarity event is dated at 2 10 0 01 ma by the 40ar 39ar method using this chronological marker it can be inferred that the normal polarity episode recorded in the seneze sequence corresponds to the reunion event and that the age of the normal reverse transition closing this event is ca 2 09 ma these chronological constraints also permit to 1 demonstrate that the villafranchian mammal fauna found in the seneze maar is younger than 2 09 ma and 2 correlate the climatic events recorded in the seneze sequence to the marine d18 o records this suggests that the seneze pollen sequence 5 120 m depth ranges from isotopic stage 85 to 76

palynostratigraphy and vegetation characteristics of neogene continental deposits interbedded with the cappadocia ignimbrites central anatolia turkey a palynological analysis of the late miocene and early pliocene successions of the cappadocian volcanic province central anatolia turkey has been carried out with the aim of reconstructing the palaeovegetation and palaeoclimatic conditions the samples are collected from outcrop sections from three different localities a akdag b bayramhacili and g guzeloz the pollen spectra reveals the existence of a steppe vegetation dominated by amaranthaceae chenopodiaceae and artemisia typical of present day arid and semi arid continental areas i e middle east and asteraceae asteroideae in the a section late miocene developed in arid climatic conditions the flora of the g early pliocene and lower part of the b late miocene sections reflects steppes on which asteraceae asteroideae was dominant this flora also indicates arid conditions the development of trees in the upper part of the b section was possibly related to local paleoenvironmental conditions rather than a climatic change this study provides the first palynological data to reveal the presence of an amaranthaceae chenopodiaceae and artemisia steppe already during the late miocene in turkey

a refined astronomically calibrated 40ar 39ar age for fish canyon sanidine abstract intercalibration between the astronomical and radio isotopic dating methods provides a means to improving accuracy and reducing uncertainty of an integrated multi chronometer geologic timescale here we report a high precision 40 ar 39 ar age for the fish canyon sanidine fcs neutron fluence monitor by multi collector noble gas mass spectrometry through cross calibration with a1 tephra sanidines a1ts of the direct astronomically tuned faneromeni section crete the astronomically intercalibrated 40 ar 39 ar age of fcs of 28 172 0 028 ma 2 external errors is within the uncertainty of but more precise 0 10 than the previous 40 ar 39 ar age determined by intercalibration with astronomically tuned tephras from the melilla basin morocco using this proposed age for fcs combined with measurements using the a1ts as the neutron fluence monitor a weighted mean bishop tuff 40 ar 39 ar sanidine age of 0 7674 0 0022 ma 2 external errors is indistinguishable from the id tims u pb zircon age 0 7671 0 0019 ma the consistency between the astronomically calibrated 40 ar 39 ar sanidine age and u pb zircon age for this quaternary unit suggests that agreement between these two radio isotopic dating techniques is now achievable at better than 0 3 2 in the youngest part of geologic time

the distinction between geomagnetic excursions and reversals summary two recent studies of the geomagnetic eld in the last 1 myr have found 14 excursions large changes in direction lasting 5 10 kyr each six of which are established as global phenomena by correlation between diierent sites the older picture of the geomagnetic eld enjoying long periods of stable polarity may not therefore be correct instead the eld appears to suier many dramatic changes in direction and concomitant reduction in intensity for 10 20 per cent of the time during excursions the eld may reverse in the liquid outer core which has timescales of 500 yr or less but not in the solid inner core where the eld must change by diiusion with a timescale of 3 kyr this timescale is consistent with the remarkably uniform duration of well dated excursions the disparity of dynamical timescales between the inner and outer cores a factor of 10 is consistent with the 10 excursions between full reversals

the paleocene of antarctica dinoflagellate cyst biostratigraphy chronostratigraphy and implications for the palaeo pacific margin of gondwana abstract the paleocene 66 56 ma was a critical time interval for understanding recovery from mass extinction in high palaeolatitudes when global climate was warmer than today a unique sedimentary succession from seymour island antarctic peninsula provides key reference material from this important phase of the early cenozoic dinoflagellate cyst data from a 376 m thick stratigraphical section including the cretaceous paleogene boundary is correlated with biozones from new zealand the east tasman plateau and southeastern australia a detailed age model is suggested for the lopez de bertodano ldbf and sobral sf formations based on dinoflagellate cyst biostratigraphy and u pb dating of zircons supported by correlated magnetostratigraphy and strontium isotope values from macrofossils the top of the ldbf is confirmed as latest maastrichtian to earliest danian 66 2 65 65 ma in age the overlying sf is mostly danian in age with an inferred hiatus near the top overlain by sediments dated as late thanetian rare apectodinium homomorphum first appear in the uppermost sf the earliest in situ record from antarctica the distribution of marine and terrestrial fossils from uppermost cretaceous to eocene sediments in patagonia antarctica new zealand and australia required both sea and land connections between these fragments of gondwana fossil evidence and reconstructions of antarctic palaeogeography and palaeotopography reveal evidence for persistent embayments in the proto weddell and ross sea regions at this time we conclude that a coastal dispersal route along the palaeo pacific margin of gondwana could explain the fossil distribution without requiring a transantarctic strait or closely spaced archipelago a region in the west to east antarctic boundary zone elevated until the early paleogene perhaps acted as a site for high elevation ice caps this supports fossil geochemical and sedimentological evidence for cold climate intervals and significant sea level falls during the maastrichtian and paleocene

correlation of ignimbrites in the central anatolian volcanic province using zircon and plagioclase ages and zircon compositions abstract episodes of high eruptive fluxes 10 3 km 3 year in continental environments are associated with magmatism related to subduction post orogenic collapse intra plate hot spots or rifting during such episodes voluminous ignimbrite deposits are produced which cover landscapes over 10 4 10 5 km 2 in such sequences brief eruptive recurrence and chemical similarity limit the applicability of geochronological and geochemical correlation methods here we present complementary geochronological data 40 ar 39 ar plagioclase eruption and 206 pb 238 u zircon crystallization ages for ignimbrites from the miocene holocene central anatolian volcanic province cavp in addition we successfully employed zircon geochemistry trace elements oxygen isotopes as an alteration resistant indicator to correlate rhyodacitic to rhyolitic ignimbrites whose eruption age differences are too brief to be resolved by 40 ar 39 ar geochronology by applying this method we dismiss previous correlations between stratigraphic members i e sofular and gordeles sofular and sarimadentepe but demonstrate close relationships for other cavp ignimbrites i e kavak units 1 to 4 cemilkoy ignimbrite and overlying fallout deposits our chronostratigraphy reveals two previously unrecognized eruptive pulses at 9 8 ma and 7 5 ma which are characterized by increasing magmatic temperatures 75 100 c within each cycle despite a long term 10 ma eruptive productivity that is about one order of magnitude smaller than in other magmatically active continental plateaus the cavp achieved high eruptive fluxes during brief 1 2 ma intervals

weak compositional zonation in a silicic magmatic system incesu ignimbrite central anatolian volcanic province kayseri turkey abstract the central anatolian volcanic province cavp one of four major volcanic provinces in turkey plays a significant role in the interpretation of the tectonic evolution of central anatolia the cavp developed within a complex collisional system involving the african arabian and eurasian plates during the miocene the volcanism exhibits complicated variations in mineralogical petrological and geochemical compositions resulting from post collisional lithospheric dynamics the incesu ignimbrite has 5 20 m thick and covers an area of 7800 km2 it is composed of three stratigraphic levels the lower level ll shows blackish brown and glassy welded structure the middle level ml is a well welded reddish pink in color and has large amounts of fiamme the upper level ul is grayish pink weakly welded and has rock fragments of different compositions the incesu ignimbrite is composed of plagioclase oligoclase andesine pyroxene augite clinoenstatite opaque minerals and low amount of amphibole biotite and quartz eutaxitic texture is dominant in ml and ll samples these levels are more strongly and contain more flattened pumice fragments and volcanic glass shards than in the ul a sharp color contrast defines the contact between ll and ml major trace and rare earth element of the incesu ignimbrite characterized by their rhyolite rhyodacite dacite composition medium high k calcalkaline and peraluminous nature show fractional crystallization primarily controlled by plagioclase clinopyroxene magnetite ilmenite and titanomagnetite sr and nd isotopic ratios of incesu ignimbrite display isotopic variations between the ignimbrite levels they exhibit a limited range in 87sr 86sr 0 7043 0 7049 and 143nd 144nd 0 512716 0 512760 the sr nd isotopic ratio of incesu ignimbrite reveals an age of 3 ma the ignimbrite evolved through fractional crystallization and crystal contamination of the parent magma derived from ocean island basalt oib like magma this suggestion is supported by the afc modeling based on the trace elements and sr isotope data variation of several major oxide concentrations fe2o3 tio2 cao and k2o trace element concetrations v sr cs and rb and trace element ratios ba rb sr k sr k nb rb sr rb y and rb nb versus sio2 concentration show the magma chamber that generated the incesu ignimbrite was compositionally zoned all geochemical and sr nd isotpic datas can be interepreted to be the result of a subduction related source

composition effects on pore structure of transitional shale a case study of the upper carboniferous taiyuan formation in the eastern uplift of the liaohe depression china abstract transitional shale is very different from marine shale with respect to mineral composition and pore structure insights into the effects of rock composition on pore structure can aid understanding of the storage and migration mechanisms of shale gas in a reservoir to investigate the compositional effects on pore structure a combination of geochemical and petrophysical experiments were conducted on four bulk shale samples and their corresponding extracted organic matter om and clay mineral samples the experimental results show that the removal of om using h2o2 during clay mineral extraction oxidizes and removes pyrite and siderite from the samples while other inorganic phases remain unaffected om hosted more abundant micropores compared with clay minerals while mesopores and fine macropores are dominant in clay minerals effects of the om and clay minerals on pore volume and surface area are quantitatively evaluated on the basis of weight normalized pore size distribution psd curves of extracted om and clay mineral samples in terms of the co2 adsorption contribution from om and clay minerals to micropore volume miv are 19 43 32 92 and 18 75 39 05 respectively and to micropore surface area mis are 19 30 32 28 and 16 42 35 72 other minerals other than organic matter and clay minerals account for 34 29 48 93 of the miv and 39 75 52 98 of the mis n2 adsorption results reveal that the pore volume and surface area are mainly attributable to the clay minerals providing 54 37 80 75 of the bjh pore volume bjhv average 65 83 and 70 68 87 04 of the bet surface area bets average 78 71 the primary medium for pore development in marine shale is om whereas pores in the transitional shale are mostly found in the clay and other inorganic minerals therefore the hydraulic fracturing design and drilling related operations involved in the exploitation of transitional shale gas should be appropriately adjusted and improved in order to accommodate intrinsic characteristics of the transitional shale gas reservoir

lake level changes and hominin occupations in the arid turkana basin during volcanic closure of the omo river outflows to the indian ocean in the east african rift the western margin of lake turkana northern kenya exposes mio plio pleistocene lake sediments with dated volcanic horizons constraining basin dynamics at the astronomical time scale since the late pliocene coastal archaeological sites have formed within the lacustrine dynamics here lake levels are reconstructed from 2 4 to 1 7 ma using sedimentary facies and water depth controlled sediment association the lacustrine stratigraphy is measured with a total station and cyclostratigraphy is derived from tephrochronology the water depths are evaluated from paleochemical properties of lake sediments analyzed by inductively coupled plasma optical emission spectrometry and inductively coupled plasma mass spectrometry our reconstruction highlights that lake turkana rose during 100 ka insolation eccentricity maxima periods in response to higher monsoonal inputs of the omo river however lake turkana also expanded through an insolation minimum at 2 17 1 95 ma this asynchronous lake phase coincides with volcanic closure of the omo river and lake turkana outflow sill to the east and the indian ocean an archaeological hiatus occurs during this endorheic lake phase and alkalinity increases at the beginning of the hiatus the lake rose again during insolation eccentricity maxima at 1 9 1 7 ma and a new outflow sill opened to the west and the nile basin hominin coastal occupations return during this exorheic freshwater lake phase

geomagnetic excursions and paleointensities in the matuyama chron at ocean drilling program sites 983 and 984 iceland basin 1 we report natural remanent magnetization nrm directions and geomagnetic paleointensity proxies for part of the matuyama chron 0 9 2 2 ma interval from two sites located on sediment drifts in the iceland basin at ocean drilling program sites 983 and 984 mean sedimentation rates in the late matuyama chron are 15 9 and 11 5 cm kyr 1 respectively for the older part of the record 1 2 ma oxygen isotope data are too sparse to provide the sole basis for age model construction the resemblance of the volume susceptibility record and a reference 18o record led us to match the two records to derive the age models this match based on site 983 984 susceptibility is consistent with available site 983 984 benthic 18o data paleointensity proxies were derived from the slope of the nrm versus anhysteretic remanent magnetization plot for alternating field demagnetization in the 30 60 mt peak field range paleointensity lows correspond to polarity reversals at the limits of the jaramillo olduvai cobb mountain and reunion subchrons and to seven excursions in nrm component directions magnetic excursions defined here by virtual geomagnetic polar latitudes crossing the virtual geomagnetic equator are observed at 932 1048 1115 1190 1215 cobb mountain subchron 1255 1472 1480 1567 1575 gilsa subchron and 1977 ka the results indicate that geomagnetic directional excursions associated with paleointensity minima are a characteristic of the matuyama chron and probably of polarity chrons in general

volcanism at 1 45 ma within the yellowstone volcanic field united states abstract rhyolitic volcanism in the yellowstone volcanic field has spanned over two million years and consisted of both explosive caldera forming eruptions and smaller effusive flows and domes effusive eruptions have been documented preceding and following caldera forming eruptions however the temporal and petrogenetic relationships of these magmas to the caldera forming eruptions are relatively unknown here we present new 40ar 39ar dates for four small volume eruptions located on the western rim of the second cycle caldera the source of the 1 300 0 001 ma mesa falls tuff we supplement our new eruption ages with whole rock major and trace element chemistry pb isotopic ratios of feldspar and paleomagnetic and rock magnetic analyses eruption ages for the effusive green canyon flow 1 299 0 002 ma and moonshine mountain dome 1 302 0 003 ma are in close temporal proximity to the eruption age of the mesa falls tuff in contrast our results indicate a period of volcanism at ca 1 45 ma within the yellowstone volcanic field including the eruption of the bishop mountain flow 1 458 0 002 ma and tuff of lyle spring 1 450 0 003 ma these high silica rhyolites are chemically and isotopically distinct from the mesa falls tuff and related 1 3 ma effusive eruptions the 40ar 39ar data from the tuff of lyle spring demonstrate significant antecrystic inheritance prevalent within the upper welded ash flow tuff matrix and minimal within individual pumice antecrysts are up to 20 kyr older than the eruption with subpopulations of grains occurring every few thousand years we interpret these results as an indicator for the timing of magmatic pulses into a growing magmatic system that would ultimately erupt the tuff of lyle spring and which we more broadly interpret as the tempo of crustal accumulation associated with bimodal magmatism we propose a system whereby chemically isotopically and temporally distinct isolated small volume magma batches are periodically generated and erupted in a low magmatic flux state which is punctuated by larger volume caldera forming eruptions

dipolar reversal states of the geomagnetic field and core mantle dynamics palaeomagnetic records from lavas suggest that at least two specific inclined dipolar field configurations have dominated the reversal process for the past ten million years these long lived states provide a way to explain directional rebounds aborted reversals and the recording in sediments of what appear to be preferred longitudinal paths of the virtual geomagnetic pole the polar orientations correlate with near radial flux concentrations recognizable when today s field is stripped of its axial dipole and with lower mantle seismic anomalies suggesting a tie to deep earth dynamics

a buoyancy profile for the earth s core summary we investigate the thermal and chemical buoyancy forces that drive convection in the earth s liquid outer core and derive a radial buoyancy profile that can be used in geodynamo models we assume the core is well mixed adiabatic and cools as a result of mantle convection the buoyancy profile is developed for a boussinesq fluid and incorporates secular cooling latent heat release at the inner core boundary radiogenic heating the effect of the adiabat and compositional buoyancy due to inner core freezing surprisingly these complex effects can be modelled accurately by a simple combination of bottom heating and near uniform heat sinks which is implemented using a cotemperature formulation that converts compositional effects into effective thermal effects the relative importance of internal and bottom heating is then defined by just two parameters the cooling rate at the core mantle boundary cmb and the uniform rate of internal radiogenic heat production both of which can be obtained from core evolution calculations we vary these parameters in geodynamo models and compare basic features of the generated fields with the geomagnetic field in this manner we link core evolution models geodynamo simulations and geomagnetic observations we consider three end member scenarios for core evolution 1 rapid cooling and a young inner core 2 moderate cooling and neutral stability at the cmb 3 slow cooling and enough radiogenic heating to allow the inner core to be 3 5 gyr old we find that compositional buoyancy dominates thermal buoyancy everywhere except near the cmb even with large amounts of radiogenic heating and buoyancy forces are far larger at depth than higher up reducing the cooling rate and increasing radiogenic heating reduces the drop in the superadiabatic gradient between the inner and outer boundaries for rapid cooling the drop is by a factor 50 for slow cooling it is a factor of 5 we demonstrate the effects of these different buoyancy profiles in numerical simulations as a function of the rayleigh number at low rayleigh number the internal velocity and magnetic fields vary between the core evolution scenarios but these differences do not affect the surface field significant differences in the surface field emerge when the rayleigh number is sufficiently large for rapid cooling we find dipolar magnetic fields in the time average that reverse and are dominated by large scale features at high latitudes moderate cooling results in magnetic fields that are stable and dominantly dipolar slow cooling produces multipolar magnetic fields that reverse very frequently this preliminary study suggests that the generated fields are sufficiently different that geodynamo simulations together with geomagnetic observations could be used to discriminate between different core evolution scenarios

rhyolitic fallout and pyroclastic density current deposits from a phreatoplinian eruption in the eastern aegean sea greece abstract the kos plateau tuff consists of pyroclastic deposits from a major quaternary explosive rhyolitic eruption centred about 10 km south of the island of kos in the eastern aegean greece five main units are present the first two units a and b were the product of a phreatoplinian eruption the eruption style then changed to dry explosive style as the eruption intensity increased forming a sequence of ignimbrites and initiating caldera collapse the final waning phase returned to phreatomagmatic eruptive conditions unit f the phreatomagmatic units are fine grained poorly sorted and dominated by blocky vitric ash thickly ash coated lapilli and accretionary lapilli they are non welded and were probably deposited at temperatures below 100 c all existing exposures occur at distances between 10 km and 40 km from the inferred source unit a is a widespread 42 km from source thin upwind on kos to very thick downwind internally laminated dominantly ash bed with mantling sheet like form upwind unit a and the lower and middle part of downwind unit a are ash rich ash rich facies whereas the upper part of downwind unit a includes thin beds of well sorted fine pumice lapilli pumice rich facies unit a is interpreted to be a phreatoplinian fall deposit although locally the bedforms were influenced by wind surface water and topography the nature and position of the pumice rich facies suggests that the eruption style alternated between wet phreatoplinian and dry plinian during the final stages of unit a deposition unit b is exposed 10 19 km north of the inferred source on kos overlying unit a it is a thick to very thick internally stratified bed dominated by ash coated medium and fine pumice lapilli in an ash matrix unit b shows a decrease in thickness and grain size and variations in bedforms downcurrent that allow definition of several different facies and laterally equivalent facies associations unit b ranges from being very thick coarse and massive or wavy bedded in the closest outcrops to source to being partly massive and partly diffusely stratified or cross bedded in medial locations pinch and swell clast supported pumice layers are also present in medial locations in the most distal sections unit b is stratified or massive and thinner and finer grained than elsewhere and dominated by thickly armoured lapilli unit b is interpreted to have been deposited from an unsteady density stratified pyroclastic density current which decelerated and progressively decreased its particle load with distance from source condensation of steam during outflow of the current promoted the early deposition of ash and resulted in the coarser pyroclasts being thickly ash coated the distribution texture and stratigraphic position of unit b suggest that the pyroclastic density current was generated from collapse of the phreatoplinian column following a period of fluctuating discharge when the eruptive activity alternated between wet and dry the pyroclastic density current was transitional in particle concentration between a dilute pyroclastic surge and a high particle concentration pyroclastic flow unidirectional bedforms in unit b suggest that the depositional boundary was commonly turbulent and in this respect did not resemble conventional pyroclastic flows however unit b is relatively thick and poorly sorted and was deposited more than 19 km from source implying that the current comprised a relatively high particle concentration and in this respect did not resemble a typical pyroclastic surge

the pre ignimbrite phreato plinian and phreatomagmatic phases of the akdag zelve ignimbrite eruption in central anatolia turkey abstract the eruptive sequence of the 7 6 ma old akdag zelve ignimbrite azi eruption comprises five units totalling up to 50 m on akdag mountain they are in stratigraphic order a the main pumice fall deposit up to 10 m b the lower surge series up to 4 m c the upper pumice beds up to 0 8 m d the upper surge series up to 5 m and e the akdag zelve ignimbrite up to 40 m the main pumice fall deposit shows a second distal thickness maximum about 20 km from the source whereas the upper pumice beds form a simple nne extending fan the source area about 6 km north northeast of kaymakli is inferred from fallout thickness size and amount of lithic clasts depositional facies and flow directions of the surge deposits the main pumice fall deposit encloses accretionary lapilli up to 6 cm in size indicating a steady phreatomagmatic influence on the eruptive style from the very beginning the major surge sequence beneath the ignimbrite is similar to that of the 1400 yr b c minoan eruption on santorini greece and resembles the minoan b stratigraphic unit attributed to phreatomagmatic eruptions due to ingression of seawater into the conduit similarly the depositional environment of marshes and marginal flats of a lacustrine basin in cappadocia accounts for the external water supply and the generation of surges during the azi eruption estimates of eruption column height of 45 km indicate that the azi eruption ranks in the order of the 1956 bezymianny 1815 tambora and the waimihia plinian pumice eruption of new zealand volume estimates of the erupted tephra are on the order of 41 5 km 3 for the main pumice and 0 35 km 3 of the upper pumice

timing of quaternary geomagnetic reversals and excursions in volcanic and sedimentary archives abstract we review the ages of quaternary magnetic polarity reversals and magnetic excursions improvements in radioisotopic 40ar 39ar dating of reversals and excursions reflect the use of multi collector mass spectrometers along with ultra low backgrounds laser based incremental heating methods and adoption of 28 201 ma for the fish canyon sanidine fcs standard age kuiper et al 2008 these improvements have brought 40ar 39ar ages of reversals and excursions into close correspondence with astrochronological ages based on the correlation of 18o data to lr04 lisiecki and raymo 2005 and in a few cases correlation to ice core chronologies ages of two polarity reversals the matuyama brunhes boundary and base olduvai are at odds with ages given in popular geomagnetic polarity timescales sedimentary records of the laschamp and iceland basin excursion are numerous and widespread and attest to global manifestation and global synchronicity of these excursions records of other quaternary excursions are less numerous the blake excursion 115 ka has been recorded in north atlantic sediments chinese loess indian ocean volcanics and spanish speleothems the santa rosa excursion 932 ka has been recorded in north atlantic and phillipine basin sediments as well as in volcanic rocks from new mexico and galapagos the punaruu excursion 1115 ka has been observed in sediments from the north atlantic and california margin as well as in volcanic rocks on tahiti uniformity of individual excursion ages using astrochronological 40ar 39ar and u th methods argues for synchronous global manifestation of excursions strengthening the usefulness of magnetic excursions and reversals in quaternary stratigraphy

rapid magma evolution constrained by zircon petrochronology and 40ar 39ar sanidine ages for the huckleberry ridge tuff yellowstone usa understanding the time scales of magmatic differentiation storage and eruption of large volume silicic magmas is a primary goal of igneous petrology within the huckleberry ridge tuff hrt idaho usa representing the earliest and largest caldera forming eruption associated with yellowstone volcanic activity zircon morphological zoning patterns coupled to strongly correlated changes in ti in zircon thermometry and trace element indicators of progressive differentiation provide a proxy record for the evolution of the hrt member b magma body tandem in situ and isotope dilution u pb dating of single zircon crystals demonstrates an absence of pre pleistocene xenocrysts but reveals the presence of antecrysts recycled from pre caldera rhyolites in the hrt magma the petrochronologic interpretation of autocrystic zircon thermal chemical and temporal characteristics suggests that hrt member b differentiated over 10 k y prior to eruption at 2 0794 0 0046 ma as defined by new astronomically calibrated single crystal total fusion 40 ar 39 ar sanidine analyses this refined eruption age demonstrates that the transitional polarity preserved by hrt member b does not record the reunion subchron but rather a separate younger geomagnetic event our novel approach places the thermal and chemical regime of silicic magmas within a temporal context and demonstrates the rapid evolution of a large volume of silicic magma

multiple brunhes chron excursions recorded in the west eifel germany volcanics support for long held mantle control over the non axial dipole field volcanic records of excursional geomagnetic field behavior in particular paleointensity estimates are fragmentary for the pleistocene the west eifel volcanic field is unique in that 12 of 66 measured lava flow sites record virtual geomagnetic poles vgps clustered between 34 45 n latitude and 30 50 e

link between geomagnetic reversal paths and secular variation of the field over the past 5 myr palaeomagnetic records provide information about the behaviour of the geomagnetic field during reversals1 2 existing records are incompatible with transitional field configurations that are either entirely dipolar or entirely zonal dependent only on latitude 3 4 recent compilations5 8 have indicated that the transi tional paths of virtual geomagnetic poles vgps for the past few reversals are located preferentially within two antipodal longitudinal bands suggesting that simple but non zonal field configurations dominate during reversals here i point out that one of the longitudinal bands coincides with that expected from the reversal of a non axial dipole field exactly like that present today the other requires only a sign change in the non axial dipole terms of today s field evidence for persistent non zonal contributions to the field has generally9 13 but not always14 13 been regarded as not statistically significant in the light of poor data distributions i show here that a non zonal bias similar to that observed in reversal data is evident in data on secular variation of the field over the past 5 myr even after normalization according to site locations these results suggest that the time averaged field does indeed contain persistent but not constant non zonal contributions

split grain 40ar 39ar dating integrating temporal and geochemical data from crystal cargoes nerc is acknowledged for continued funding of aif at suerc east kilbride this work was supported by an eth research grant eth 05 13 2 and funds from swiss national science foundation research grants snsf 200021 146268 and snsf 200021 155923 1 and us national science grant 1425491

the reunion subchronozone at odp site 981 feni drift north atlantic abstract at ocean drilling program site 981 the reunion subchron is recorded from the marine isotopic stage mis 81 82 boundary to mis 79 as a single normal polarity zone spanning the 2 115 2 153 ma interval according to the age model produced by matching the site 981 18 o record to an orbitally tuned reference record the duration of the reunion subchron 38 kyr is almost four times that assumed in current geomagnetic polarity timescales high interval sedimentation rates 18 cm kyr during mis 81 at site 981 have resulted in an unusually detailed record of the subchron virtual geomagnetic poles during the onset and demise of the reunion subchron trace a series of mainly anticlockwise loops over eastern asia

latest miocene and pleistocene ages of faulting determined by 40ar 39ar single crystal dating of airfall tuff and silicic extrusives of the erciyes basin central turkey evidence for intraplate deformation related to the tectonic escape of anatolia the ericiyes basin is a trans tensional basin situated 20 km north of the regional ecemis fault zone recently it has been hypothesized that faulting within the erciyes basin links with the ecemis fault zone further south as part of a regional central anatolian fault zone new 40ar 39ar dating of volcanic and volcaniclastic rocks adjacent to faults both along the margins and in the centre of the erciyes basin constrains the timing of basin inception and later faulting extensional faulting occurred along the eastern and western margins of the basin during the early messinian latest miocene sinistral and minor normal faulting were active along the axis of the basin during the early pleistocene these fault timings are similar to those inferred for the ecemis fault zone further south and support the hypothesis that faulting within the erciyes basin and the ecemis fault zone are indeed linked

the upper olduvai geomagnetic field reversal from death valley california a fold test of transitional directions high resolution records of the upper olduvai geomagnetic field reversal were obtained from two localities within steeply dipping lacustrine sediments of the confidence hills southern death valley california the difference in bedding attitude between the two localities allowed us to perform a fold test of both transitional and non transitional paleomagnetic directions this is the first positive fold test obtained for transitional directions from any geomagnetic field reversal in addition variations in lithology allowed a comparison of transitional records from different sedimentary environments alternating field and thermal demagnetization reveals that most samples have either single or simple two component magnetizations the low coercivity low blocking temperature component is most likely held by large multidomain grains of magnetite and aligns with the present day field when uncorrected for bedding orientation the high coercivity component is held primarily by single and pseudo single domain titanomagnetite as determined from standard magnetic studies and electron microscopy this component is interpreted as a primary magnetic signature based on the presence of stratigraphically bound reversals in the tilt corrected data and positive reversal tests short term field variations can be correlated between the two records and differences are readily explained by the recording process inherent to the two sedimentary environments the sampling site is located within one of the preferred bands of transitional virtual geomagnetic poles vgp s 1 3 yet the transitional vgp paths from both localities are clustered within two primary longitudinal bands which are approximately 90 away from the sampling site longitude one lies over the atlantic ocean 10 50 w and the other antipodal to this 130 170 e these bands are also inconsistent with eight previous studies of the upper olduvai transition 4 9 3 10 when all reported studies of this reversal are examined however no global consistency exists instead the transitional vgp s are better grouped in a sampling site reference frame

analyzing nitrogen in natural and synthetic silicate glasses by secondary ion mass spectrometry abstract volatile releases through volcanic eruptions are one of the major processes contributing to the global nitrogen cycle past studies have often estimated the magnitude of this flux using volcanic emission measurements which are limited to currently active systems and sensitive to atmospheric contamination another possible approach is the measurement of nitrogen in melt inclusions which are parcels of magmatic melt trapped prior to eruption this methodology requires appropriate analytical parameters for nitrogen analysis in silicate glasses by secondary ion mass spectrometry sims which have not yet been established to this end calibrations for nitrogen were obtained using ion implanted basaltic and rhyolitic glasses we demonstrate that water content significantly affects the ion yields of 14n and 14n16o as well as the background intensity of 14n and 12c application of implant derived calibrations to natural samples provide the first reported concentrations of nitrogen in melt inclusions these measurements were made on samples from the bishop tuff in california the huckleberry ridge tuff of the yellowstone volcanic center and material from the okaia and oruanui eruptions in the taupo volcanic center in studied material we often find maximum nitrogen contents of less than 45 ppm and that nitrogen concentration varies positively with co2 concentration which reflects a partial degassing trend using the maximum measured nitrogen contents for each eruption we find that the bishop released 3 4 1013 the huckleberry ridge released 1 4 1014 the okaia released 1 0 1011 and the oruanui released 4 5 1013 g of nitrogen simple calculations suggest that with concentrations such as these rhyolitic eruptions may ephemerally increase the nitrogen flux to the atmosphere but are insignificant compared to the 4 1021 g of nitrogen stored in the atmosphere

reply to the comment on 40ar 39ar dating of the tephra layer in the pliocene seneze maar lacustrine sequence french massif central constraint on the age of the reunion matuyama transition and implications on paleoenvironmental archives

the nature origins and distribution of ash aggregates in a large scale wet eruption deposit oruanui new zealand abstract this study documents the processes and products of volcanic ash aggregation in phreatomagmatic phases of the 25 4 ka oruanui supereruption from taupo volcano new zealand detailed textural and stratigraphic relationships of aggregates are examined in six of the ten erupted units which range from relatively dry styles of eruption and deposition units 2 5 to mixed units 6 7 8 and dominantly wet unit 3 aggregate structures and grain size distributions shift abruptly over vertical scales of cm to dm providing diagnostic features to identify deposits emplaced primarily as vertical fallout or pyroclastic density currents pdcs the six categories of ash aggregates documented here are used to infer distinct volcanic and meteorological interactions in the eruption cloud related to dispersal characteristics and mode of emplacement our field observations support the notion of brown et al 2010 origin of accretionary lapilli within ground hugging density currents evidence from pyroclastic couplets on tenerife geol soc am bull 122 305 320 that deposits bearing matrix supported accretionary lapilli with concentric internal structure and abundant rim fragments are associated with emplacement of pdcs however on the basis of grain size distributions and field relationships it is inferred that these types of ash aggregates formed their ultrafine ash dominantly 100 km from vent it is inferred that ambient meteorological conditions play a more important role in ash fallout in these cases entrainment of moist air and distal subsidence and melting of ice carried by the plume are both likely to have contributed to the observed features of late stage aggregation in the drier phases of eruption these observations suggest that proximal column influenced aggregation processes which weaken with distance from source are overprinted by secondary later stage aggregation mechanisms farther downwind

palaeomagnetism of the cappadocian volcanic succession central turkey major ignimbrite emplacement during two short miocene episodes and neogene tectonics of the anatolian collage abstract the central anatolian volcanic province in cappadocia includes 13 high volume calc alkaline ignimbrite sheets emplaced by plinian eruptions within a succession the urgup formation after 10 ma recording the last phase of neotethyan subduction and accompanying emplacement of the tauride orogen in southern turkey to evaluate magnetostratigraphy in the context of recent revisions of the chronostratigraphy we have extended palaeomagnetic investigation to 32 new sites yielding significant chrm directions integrated rock magnetic and palaeomagnetic investigations identify magnetic remanence residing predominantly in ti poor titanomagnetites although secondary processes within the ignimbrite sheets notably post emplacement oxidation have locally produced hematisation expressed by composite irm spectra and variable reduction in intensity of magnetisation the ignimbrite sheets possess weak anisotropies of magnetic susceptibility ams mostly 3 of pyroclastic deposits during polarity chron c4r 1n between 9 31 and 9 43 ma subsequent activity from the acigol centre further to the south west cemilkoy gelveri gordeles and kizilkaya produced in excess of 620 km 3 of pyroclastic deposits during polarity chrons between 5 3 and 7 1 ma the younger incesu ignimbrite was sourced in the sultansazligi pull apart basin to the east during the gauss chron 2 58 3 60 ma all pre incesu ignimbrites are rotated uniformly anticlockwise and the overall reversed group mean direction of magnetisation is d i 170 8 52 4 n 9 r 8 91 95 5 4 k 91 the implied tectonic rotation in this sector of central anatolia 16 4 relative to eurasia is young and postdates the 5 3 7 1 ma acigol centre whilst largely predating emplacement of the incesu ignimbrite whilst rotational deformation within anatolia is young it proves to be distributed with a temporal variation from block to block it is embraced by a complex post miocene tectonic regime of strike slip and extension during tectonic escape and suction towards the hellenic arc to the west

a geomagnetic paleointensity stack between 0 8 and 3 0 ma from equatorial pacific sediment cores 1 we have conducted a paleomagnetic study of six sediment cores taken from the equatorial pacific three from the west caroline basin and three from the manihiki plateau in order to make a relative paleointensity stack during the matuyama and late gauss chrons the age of the bottom of the cores ranges from 1 2 to 3 0 ma the sediments show little downcore changes in the proxies of magnetic grain size and mineralogy and are hence suitable for estimating relative paleointensity the age of the cores is controlled by correlating variations in magnetic concentration magnetic susceptibility and or anhysteretic remanent magnetization arm to target oxygen isotope 18o curves all cores from the manihiki plateau show an upward decrease of the natural remanent magnetization nrm intensity normalized by arm and isothermal remanent magnetization irm with a decrease in sedimentation rate such a trend was removed before being converted to relative paleointensity this observation implies that a sedimentation rate change can affect relative paleointensity estimation relative paleointensity records from the six cores coincide well with each other within uncertainty of age we constructed a stacked curve between 0 8 and 3 0 ma the equatorial pacific paleointensity stack epapis 3ma after adjusting age the number of cores stacked is three to four throughout the record quasiperiodic paleointensity lows occur in the epapis 3ma and corresponding paleointensity lows can be found in previously published records with some shifts in age at least parts of the paleointensity minima seem to be accompanied by geomagnetic excursions the asymmetric sawtooth pattern of paleointensity variations is not observed at the matuyama gauss boundary and thereafter in our record a spectral analysis shows that 100 kyr orbital eccentricity frequency may exist in paleointensity variations during the matuyama and late gauss chrons

post depositional fracturing and subsidence of pumice flow deposits lascar volcano chile unconsolidated pyroclastic flow deposits of the 1993 eruption of lascar volcano chile have with time become increasingly dissected by a network of deeply penetrating fractures the fracture network comprises orthogonal sets of decimeter wide linear voids that form a pseudo polygonal grid visible on the deposit surface in this work we combine shallow surface geophysical imaging tools with remote sensing observations and direct field measurements of the deposit to investigate these fractures and their underlying causal mechanisms based on ground penetrating radar images the fractures are observed to have propagated to depths of up to 10 m in addition orbiting radar interferometry shows that deposit subsidence of up to 1 cm year 1 occurred between 1993 and 1996 with continued subsidence occurring at a slower rate thereafter in situ measurements show that 1 m below the surface the 1993 deposits remain 5 c to 15 c hotter 18 years after emplacement than adjacent deposits based on the observed subsidence as well as estimated cooling rates the fractures are inferred to be the combined result of deaeration thermal contraction and sedimentary compaction in the months to years following deposition significant environmental factors including regional earthquakes in 1995 and 2007 accelerated settling at punctuated moments in time the spatially variable fracture pattern relates to surface slope and lithofacies variations as well as substrate lithology similar fractures have been reported in other ignimbrites but are generally exposed only in cross section and are often attributed to formation by external forces here we suggest that such interpretations should be invoked with caution and deformation including post emplacement subsidence and fracturing of loosely packed ash rich deposits in the months to years post emplacement is a process inherent in the settling of pyroclastic material

dating transitionally magnetized lavas of the late matuyama chron toward a new 40ar 39ar timescale of reversals and events the k ar based geomagnetic polarity timescale was constructed using data from lavas and tuffs that bracketed but rarely dated the transitions between polarity intervals subsequent 40ar 39ar dating indicated that the ages of some polarity transitions had been underestimated by about 6 although the accepted ages of the polarity chron boundaries have increased their precise temporal definition remained uncertain we have taken a different approach and used incremental heating techniques to obtain 18 new 40ar 39ar ages from basaltic lavas within flow sequences at punaruu valley tahiti and haleakala volcano hawaii these lavas record transitional paleomagnetic directions corresponding to four mid pleistocene polarity reversals or events three lavas from punaruu valley previously thought to record the cobb mountain normal polarity subchron cmns gave a mean age of 1 105 0 005 ma indicating that they were erupted about 76 kyr after the cmns this period of transitional field behavior is designated the punaruu event in addition seven new 40ar 39ar ages from the punaruu valley indicate that the jaramillo normal polarity subchron jns lasted about 67 kyr starting at 1 053 0 006 ma and ending 0 986 0 005 ma this agrees with astronomical estimates but conflicts with jns ages proposed by spell and mcdougall 1992 and izett and obradovich 1994 on the basis of 40ar 39ar dating of rhyolite domes in the valles caldera indistinguishable 40ar 39ar ages of seven lavas including one from punaruu valley and six from haleakala that record broadly similar intermediate paleodirections suggest that the kamikatsura event occurred at 0 886 0 003 ma moreover these data indicate that the kamikatsura event occurred 20 40 kyr after another geomagnetic event most probably taking place at 0 92 ma we designate this earlier field behavior the santa rosa event adopting its name from that of a transitionally magnetized rhyolite dome which happened to figure prominently in the original definition of the end of the jns in the 1968 study of doell et al 1968 the discovery of these new short lived polarity events during the matuyama reversed chron suggests that the 400 kyr period between 1 18 and 0 78 ma experienced no less than 7 and perhaps more than 11 attempts by the geodynamo to reverse this newly determined higher frequency of geomagnetic activity illustrates vividly the importance of obtaining precise age control directly from transitionally magnetized rocks

comment on 40ar 39ar dating of the tephra layer in the pliocene seneze maar lacustrine sequence french massif central constraint on the age of the reunion matuyama transition and implications on paleoenvironmental archives by roger et al

geomagnetic paleointensity in historical pyroclastic density currents testing the effects of emplacement temperature and postemplacement alteration thellier type paleointensity experiments were conducted on welded ash matrix or pumice from the 1912 novarupta nv and 1980 mt st helens msh pyroclastic density currents pdcs with the intention of evaluating their suitability for geomagnetic paleointensity studies pdcs are common worldwide but can have complicated thermal and alteration histories we attempt to address the role that emplacement temperature and postemplacement hydrothermal alteration may play in nonideal paleointensity behavior of pdcs results demonstrate two types of nonideal behavior unstable remanence in multidomain md titanomagnetite and nonideal behavior linked to fumarolic and vapor phase alteration emplacement temperature indirectly influences msh results by controlling the fraction of homogenous md versus oxyexsolved pseudo single domain titanomagnetite nv samples are more directly influenced by vapor phase alteration the majority of nv samples show distinct two slope behavior in the natural remanent magnetization partial thermal remanent magnetization plots we interpret this to arise from a thermo chemical remanent magnetization associated with vapor phase alteration and samples with high water content 0 75 loss on ignition generate paleointensities that deviate most strongly from the true value we find that pdcs can be productively used for paleointensity but that as with all paleointensity studies care should be taken in identifying potential postemplacement alteration below the curie temperature and that large welded flows may be more alteration prone one advantage in using pdcs is that they typically have greater areal spatial exposure than a basalt flow allowing for more extensive sampling and better assessment of errors and uncertainty

on the age and morphology of the reunion event data pertinent to the age of the normal polarity reunion event or subchron and whether it is a split feature are critically examined new 40 ar 39 ar age determinations on lavas from two sequences on reunion island along with available potassium argon k ar 40 ar 39 ar and astrochronological analyses indicate an age of 2 15 2 13 ma although the possibility cannot be completely ruled out at present there is no convincing evidence supporting the contention of a short duration of normal polarity some 70 90 kyr later

a model for the development of stable isotopic water signatures of tephra deposited on ice following subglacial caldera collapse abstract large scale silicic volcanism has occurred frequently in regions mantled by thin to very thick glacial ice cover with several notable examples during the pleistocene yellowstone long valley iceland including high altitude activity during the most recent glacial e g cascades kamchatka andes more ancient caldera forming events must have occurred during episodes of long lasting ca 10 50 ma snowball earth glaciation in the neo and paleo proterozoic and many extraterrestrial eruptions on other ice covered planets and moons are also of this general form recent work suggests that the process of caldera collapse typically lasts hours to weeks during which the caldera floor drops by several hundreds to thousands of meters and is covered by hot tephra with this scenario in mind we investigate glaciovolcanic interactions immediately following the deposition of thick hot ash layers on ice and consider the destiny of buried tephra and melting ice inside calderas our focus is drawn in particular to the post emplacement hydration of volcanic glasses with the goal of assessing whether the 18o 17o and d signatures in ancient deposits might be used to infer the syn eruptive climate state scaling arguments augmented by an idealized 1d model suggest that ice should often survive for several decades or centuries as an active meltwater source to the overlying cooling intracaldera tuff as liberated glacial water both liquid and vapor infiltrates and interacts with the tephra layer volcanic glasses can become fully hydrated to water saturations of several wt our theoretical treatment is motivated in part by our recent measurements of lower than modern d values in products of several pleistocene eruptions in the western u s occupying regions that were likely glaciated immediately prior to the emplacement of volcanic products we discuss how 18o d and 18o 17o systematics can be used to recognize syn glacially hydrated intracaldera tephra potentially including samples that have been buried altered and subsequently exposed either by fault uplift and erosional exposure or by drilling operations such as those being performed currently in the central snake river plain near yellowstone

contemporary flowstone development links early hominin bearing cave deposits in south africa abstract the cradle of humankind cave sites in south africa preserve fossil evidence of four early hominin taxa australopithecus africanus australopithecus sediba paranthropus robustus and early homo in order to integrate this record into a pan african scenario of human evolutionary history it is critical to have reliable dates and temporal ranges for the southern african hominins in the past a lack of precise and accurate chronological data has prevented the evaluation of the temporal relationships between the various sites here we report new uranium lead u pb radiometric ages obtained from sheets of calcium carbonate flowstone inter bedded between clastic cave sediments at the site of swartkrans providing bracketing ages for the fossiliferous deposits the fossil bearing units of swartkrans specifically the hanging remnant and lower bank of member 1 are underlain by flowstone layers dated to 2 25 0 05 ma and 2 25 0 08 ma and capped by layers of 1 8 0 01 ma and 1 7 0 07 ma the age bracket of the member 1 deposits is therefore between 2 31 and 1 64 ma however by combining the u pb with biostratigraphic data we suggest that this can be narrowed down to between 1 9 and 1 8 ma these data can be compared with other recently dated sites and a radiometrically dated u pb age sequence formed sterkfontein member 4 swartkrans member 1 malapa and cooper s d from this new u pb dataset a pattern of contemporary flowstone development emerges with different caves recording the same flowstone forming event specifically overlapping flowstone formation takes place at swartkrans and sterkfontein at 2 29 ma and 1 77 ma and at sterkfontein and malapa at 2 02 ma this suggests a regional control over the nature and timing of speleothem development in cave deposits and these flowstone layers could assist in future correlation both internal to specific deposits and regionally between sites

the eruptive center of the late quaternary laacher see tephra about 5 km3 of phonolite magma were erupted 11 000 years ago in the laacher see area in plinian and vulcanian eruptions the eruptive vents for all tephra in dispute for over 200 years must have been located entirely within the laacher see basin because 1 all deposits are thickest in the tuffring surrounding the basin with one exception pyroclastic flow deposits are thickest 2 to 6 km away from the rim of the basin having ponded in radial valleys three plinian fallout layers show minor secondary thickness maxima 12 to 16 km east of laacher see 2 axes of all depositional fans and single tephra sheets converge in the laacher see basin 3 diameters of both lithic and pumice clasts reach their maxima in outcrops close to laacher see volcano 4 directions of asymmetric ballistic impact sags indicate the laacher see basin as source area 5 the dimension of laacher see crater roughly corresponds to the erupted volume of phonolitic tephra 5 3 km3 magma dre and lithic clasts 0 7 km3 6 xenolith types are consistent with an eruptive center within the laacher see basin 7 the systematic chemical and mineralogical zonation of the tephra deposits from highly differentiated aphyric phonolite at the base to highly phyric mafic phonolite at the top strongly suggests eruption from a single compositionally zoned magma column 8 several lines of evidence indicate migration of the main eruptive focus from the south to the north during the later part of the eruption both vents however being confined within the laacher see basin

intercalibration of radioisotopic and astrochronologic time scales for the cenomanian turonian boundary interval western interior basin usa we develop an intercalibrated astrochronologic and radioisotopic time scale for the cenomanian turonian boundary ctb interval near the global stratotype section and point in colorado usa where orbitally influenced rhythmic strata host bentonites that contain sanidine and zircon suitable for 40ar 39ar and u pb dating paired 40ar 39ar and u pb ages are determined from four bentonites that span the vascoceras diartianum to pseudaspidoceras flexuosum ammonite biozones utilizing both newly collected material and legacy sanidine samples of j obradovich comparison of the 40ar 39ar and u pb results underscores the strengths and limitations of each system and supports an astronomically calibrated fish canyon sanidine standard age of 28 201 ma the radioisotopic data and published astrochronology are employed to develop a new ctb time scale using two statistical approaches 1 a simple integration that yields a ctb age of 93 89 0 14 ma 2 total radioisotopic uncertainty and 2 a bayesian intercalibration that explicitly accounts for orbital time scale uncertainty and yields a ctb age of 93 90 0 15 ma 95 credible interval total radioisotopic and orbital time scale uncertainty both approaches firmly anchor the floating orbital time scale and the bayesian technique yields astronomically recalibrated radioisotopic ages for individual bentonites with analytical uncertainties at the permil level of resolution and total uncertainties below 2 using our new results the duration between the cenomanian turonian and the cretaceous paleogene boundaries is 27 94 0 16 ma with an uncertainty of less than one half of a long eccentricity cycle

incremental heating of bishop tuff sanidine reveals preeruptive radiogenic ar and rapid remobilization from cold storage abstract accurate and precise ages of large silicic eruptions are critical to calibrating the geologic timescale and gauging the tempo of changes in climate biologic evolution and magmatic processes throughout earth history the conventional approach to dating these eruptive products using the 40ar 39ar method is to fuse dozens of individual feldspar crystals however dispersion of fusion dates is common and interpretation is complicated by increasingly precise data obtained via multicollector mass spectrometry incremental heating of 49 individual bishop tuff bt sanidine crystals produces 40ar 39ar dates with reduced dispersion yet we find a 16 ky range of plateau dates that is not attributable to excess ar we interpret this dispersion to reflect cooling of the magma reservoir margins below 475 c accumulation of radiogenic ar and rapid preeruption remobilization accordingly these data elucidate the recycling of subsolidus material into voluminous rhyolite magma reservoirs and the effect of preeruptive magmatic processes on the 40ar 39ar system the youngest sanidine dates likely the most representative of the bt eruption age yield a weighted mean of 764 8 0 3 0 6 ka 2 analytical full uncertainty indicating eruption only 7 ky following the matuyama brunhes magnetic polarity reversal single crystal incremental heating provides leverage with which to interpret complex populations of 40ar 39ar sanidine and u pb zircon dates and a substantially improved capability to resolve the timing and causal relationship of events in the geologic record

fall out and deposition of volcanic ash during the 1979 explosive eruption of the soufriere of st vincent abstract the 1979 explosive activity of soufriere volcano st vincent commenced on april 13th and continued for two weeks producing eleven eruption columns the activity produced air fall ash with abundant accretionary lapilli plus minor pyroclastic flows and base surge deposits the dispersal of the columns was observed by an sms 1 geostationary satellite all but two of the plumes were dispersed entirely to the east the last event on april 26th formed a minor lobe to the east and a major lobe to the south southeast the april 26th column was observed to rise to 8 km within five minutes and ascended to a maximum of 14 km in the next hour the magma discharge rate was greater than 3000 m3 s the plume expanded at a rate of 4000 km2 hr and was dispersed at a mean velocity of 15 m s the ash deposits show bimodal grain size distribution with 95 of the ash collected being sub 1 mm diameter modelling of the ash fall out has allowed interpretation of grainsize parameters in a quantitative manner the fall velocities of ash particles appear to have been reduced by turbulence and downwind ascent of the plume which is attributed to fallout of ash reducing the plume density the coarse population of the april 26th deposit displayed a systematic change in md and away from source the fine mode displays uniform md and everywhere and is interpreted as having been deposited in the form of accretionary lapilli

testing the accuracy of the geomagnetic polarity time scale gpts at 2 5 ma utilizing40ar 39ar incremental heating data on whole rock basalts abstract recent40ar 39ar dating and astrochronologic calculations indicate that the k ar derived gpts underestimates the ages of geomagnetic field reversals in the interval 2 0 ma we report tests to check the accuracy of k ar data utilized to date the reunion event 2 1 ma and part of the early pliocene portion 4 6 ma of the gpts 40ar 39ar incremental heating studies on whole rock basalts from the island of reunion yield an age of2 14 0 03 ma for the reunion event somewhat older than the earlier k ar value of2 07 0 02 ma on the same set of lava flows study of a single specimen from the newer volcanics australia indicates that the earlier k ar date was accurate but the new40ar 39ar results yield a plateau age 4 56 ma with a standard error of 0 6 as compared to the k ar value 3 the improved precision and accuracy resulting from such40ar 39ar incremental heating studies will play a key role in calibrating more precisely the gpts for the interval 10 0 ma

a reappraisal of the geomagnetic polarity time scale to 4 ma using data from the turkana basin east africa recalibration of the pliocene and early pleistocene geomagnetic time scale using the k ar dated fluvial sequence of the turkana basin in east africa agrees with calibrations based on astronomical calculations ages estimated here are olduvai subchron 1 78 1 96 ma reunion subchrons 2 11 2 15 ma and 2 19 2 27 ma matuyama gauss boundary 2 60 ma kaena subchron 3 02 3 09 ma mammoth subchron 3 21 3 29 ma gauss gilbert boundary 3 57 ma

40ar 39ar dating links albuquerque volcanoes to the pringle falls excursion and the geomagnetic instability time scale abstract incremental heating of basaltic lava groundmass from the albuquerque volcanoes new mexico and plagioclase from ash d at pringle falls oregon yield 40ar 39ar isochron ages of 218 14 and 211 13 ka respectively sediments in which ash d was deposited and the eight lava flows of the albuquerque volcanoes display excursional paleomagnetic data with virtual geomagnetic poles vgps in the southern hemisphere and together with the statistically indistinguishable 40ar 39ar dates establish that both sites record the pringle falls excursion this excursion is also recorded by the 227 8 ka mamaku ignimbrite new zealand and by high deposition rate sediments at odp site 919 in the north atlantic ocean that are dated astrochronologically at 209 207 ka we propose that the names albuquerque and jamaica excursion be abandoned and that a radioisotopic age of 211 13 ka be adopted for the pringle falls excursion which is one of five globally expressed well documented excursions in marine sediment cores or dated by 40ar 39ar methods that took place from 220 to 30 ka together with at least five other well dated excursions between 730 and 520 ka some ten excursions define the geomagnetic instability time scale gits for the bruhnes chron eight of these excursions have been dated using 40ar 39ar methods if the temporal clustering of excursions in the gits is not a sampling artefact this suggests that the geodynamo is not intrinsically unstable but rather that instability in the flow pattern of the outer core fluid flow causes the main dipole field to be considerably weakened such that it becomes unstable with a 200 to 300 ka recurrence interval excursions are an important part of dynamo behavior over geologic time scales that in addition to reversals of the main dipole field need to be fully considered when assessing whether theoretical and numerical simulations of the dynamo produce earth like results

mineral compositional and textural evidence for magma mingling in the saraykent volcanics abstract upper cretaceous paleocene calc alkaline dacitic and rhyodacitic lava flows and domes are exposed in the saraykent yozgat region within central anatolian crystalline complex cacc saraykent volcanics are generally plagioclase hornblende biotite titanomagnetite quartz augite phyric subrounded vesicular magmatic enclaves ranging from a few millimeters to a few centimeters in size are present in the saraykent dacites the enclaves are mainly holocrystalline and have similar mineral assemblage to the host dacites almost all phases in saraykent volcanics exhibit varying degrees of disequilibrium features indicative of magma mingling plagioclase phenocrysts display sieved and normal type populations a wide range in rim compositions and oscillatory zoning maximum core to rim compositional change in sieved rhyodacite plagioclases is up to 24 mol increase in an some plagioclase core compositions are unusually ca rich they range from an27 to an75 for the saraykent dacites and from an30 to an66 for the saraykent rhyodacites hornblende and biotite phenocrysts have reaction rims indicating disequilibrium crystallization with magma both hornblende and biotite crystals show reverse zoning in terms of decreasing mg mg fe quartz phenocrysts are corroded and embayed presence of normally and reversely zoned augites in the same sample and high ti high al augites more comparable with basaltic compositions are other lines of evidence for disequilibrium the enclave hornblendes are both edenitic and pargasitic the latter being more typical of mafic compositions the enclave biotite crystals have higher mg mg fe2 ratios than the host dacite biotite crystals and display normal reverse and oscillatory zoning plagioclase core compositions vary between an44 and an55 in the enclaves although the development of sieve texture in plagioclase and reaction rims on mafic minerals may be attributed to decompression this cannot explain the observed compositional change and normal type of plagioclase in the same sample similarly a cognate i e fractionation origin for the enclaves can be suggested due to their mineralogical similarity to the host dacite however other criteria such as oscillatory zoning and unusually calcic cores of plagioclases heterogeneity of plagioclase rims resorbed and embayed phenocrysts and augite compositions indicating a more mafic magma are enough to support the suggestion that magma mingling was a viable process in the generation of the saraykent volcanics

the timing and origin of pre and post caldera volcanism associated with the mesa falls tuff yellowstone plateau volcanic field abstract we present new sanidine 40ar 39ar ages and paleomagnetic data for pre and post caldera rhyolites from the second volcanic cycle of the yellowstone plateau volcanic field which culminated in the caldera forming eruption of the mesa falls tuff at ca 1 3 ma these data allow for a detailed reconstruction of the eruptive history of the second volcanic cycle and provide new insights into the petrogenesis of rhyolite domes and flows erupted during this time period 40ar 39ar age data for the biotite bearing bishop mountain flow demonstrate that it erupted approximately 150 kyr prior to the mesa falls tuff integrating 40ar 39ar ages and paleomagnetic data for the post caldera island park rhyolite domes suggests that these five crystal rich rhyolites erupted over a centuries long time interval at 1 2905 0 0020 ma 2 the biotite bearing moonshine mountain rhyolite dome was originally thought to be the downfaulted vent dome for the pre caldera bishop mountain flow due to their similar petrographic and oxygen isotope characteristics but new 40ar 39ar dating suggest that it erupted near contemporaneously with the island park rhyolite domes at 1 2931 0 0018 ma 2 and is a post caldera eruption despite their similar eruption ages the island park rhyolite domes and the moonshine mountain dome are chemically and petrographically distinct and are not derived from the same source integrating these new data with field relations and existing geochemical data we present a petrogenetic model for the formation of the post mesa falls tuff rhyolites renewed influx of basaltic and or silicic recharge magma into the crust at 1 2905 0 0020 ma led to 1 the formation of the island park rhyolite domes from the source region that earlier produced the mesa falls tuff and 2 the formation of moonshine mountain dome from the source region that earlier produced the biotite bearing bishop mountain flow these magmas were stored in the crust for less than a few thousand years before being erupted contemporaneously along a 30 km long structurally controlled vent zone related to extracaldera basin and range faults these data highlight the rapidity with which magma can be generated and erupted over large distances at yellowstone

revised ages for tuffs of the yellowstone plateau volcanic field assignment of the huckleberry ridge tuff to a new geomagnetic polarity event 40 ar 39 ar ages were determined on the three major ash flow tuffs of the yellowstone plateau volcanic field in the region of yellowstone national park in order to improve the precision of previously determined ages total fusion and incremental heating ages of sanidine yielded the following mean ages huckleberry ridge tuff 2 059 0 004 ma mesa falls tuff 1 285 0 004 ma and lava creek tuff 0 639 0 002 ma the huckleberry ridge tuff has a transitional magnetic direction and has previously been related to the reunion normal polarity subchron dating of the reunion event has been reviewed and its ages have been normalized to a common value for mineral standards the age of the huckleberry ridge tuff is significantly younger than lava flows of the reunion event on re union island supporting other evidence for a normal polarity event younger than the reunion event

zircon petrochronology and 40ar 39ar sanidine dates for the mesa falls tuff crystal scale records of magmatic evolution and the short lifespan of a large yellowstone magma chamber

paleomagnetic and geochronological identification of the reunion subchron in ethiopian afar this paper reports the paleomagnetic and geochronological analysis of a suite of samples from a basaltic fault related scarp of pliocene age in the central part of the afar depression ethiopia paleomagnetic work uncovers a clear characteristic remanent magnetization carried by titano magnetites with high unblocking temperatures close to the curie temperature of pure magnetite the characteristic direction based on 27 non transitional flows is d 8 1 o i 10 1 o 5 4 1 o this is consistent with earlier determinations of the clockwise tectonic rotation due to rift propagation and overlap of the block to which the section belongs magnetic stratigraphy consists of a succession from bottom to top of 16 reversed 7 normal and 4 reversed flows one flow at the upper normal to reverse transition demonstrates complex behavior upon thermal and alternating field af demagnetization which is due to remagnetization by the overlying flow k ar dating of five samples provides consistent determinations averaging 2 07 0 05 ma all ages agree with this average given their rather large individual uncertainties from 0 04 to 0 08 ma at the lc level comparison with recent reference geomagnetic polarity timescales gpts indicates that the normal subchron recorded in the gamarri section must be linked with one of the r6union event s the normal subchron and an earlier episode of large secular variation could provide the best volcanic record of a double r6union event although uncertainties in ages are too large to allow unequivocal inferences our age determination for the r6union event is in better agreement with earlier determinations of mcdougall and watkins 1973 than with a more recent value proposed by baksi et al 1993 and used in the gpts

40ar 39ar k ar and 230th 238u dating of the laschamp excursion a radioisotopic tie point for ice core and climate chronologies abstract a brief period of enhanced 10 be flux that straddles the interstadial warm period known as dansgaard oeschger event 10 in greenland and its counterpart in antarctica the antarctic isotope maximum 10 is but one consequence of the weakening of earth s magnetic field associated with the laschamp excursion this 10 be peak measured in the grip ice core is dated at 41 250 y b2k before year 2000 ad in the most recent gicc05 age model obtained from the northgrip core via multi parameter counting of annual layers uncertainty in the age of the 10 be peak is however no better than 1630 y at the 95 confidence level reflecting accumulated error in identifying annual layers the age of the laschamp excursion guillou h singer b s laj c kissel c scaillet s jicha b 2004 on the age of the laschamp geomagnetic excursion earth planet sci lett 227 331 343 is revised on the basis of new 40 ar 39 ar unspiked k ar and 238 u 230 th data from three lava flows in the massif central france together with the 40 ar 39 ar age of a transitionally magnetized lava flow at auckland new zealand combined these data yield an age of 40 700 950 y b2k where the uncertainty includes both analytical and systematic 40 k and 230 th decay constant errors taking the radioisotopic age as a calibration tie point suggests that the layer counting chronologies for the northgrip and gisp2 ice cores are more accurate and precise than previously thought at depths corresponding to the laschamp excursion

the duration of a yellowstone super eruption cycle and implications for the age of the olduvai subchron abstract small volume rhyolitic eruptions preceding and following a caldera forming eruption can provide insights into the tempo of eruption cycles and timing of magmatic recharge in this contribution high precision 40 ar 39 ar eruption ages were obtained on the three effusive eruptions bracketing the huckleberry ridge tuff which comprise yellowstone s first volcanic cycle these dates are supplemented with detailed paleomagnetic and rock magnetic analyses to resolve discrepancies with previous reported stratigraphy the huckleberry ridge tuff 2 08 ma was preceded by an eruption at 2 14 ma and followed by eruptions at 1 98 and 1 95 ma all of which occurred during four distinct periods of geomagnetic instability within the matuyama chron the first volcanic cycle of yellowstone has now been constrained to within a 200 kyr timespan or half of the previously proposed duration and similar to the duration of volcanic activity for caldera forming systems in the jemez volcanic field the maximum duration for magmatic recharge for the first yellowstone volcanic cycle is no greater than 100 kyr and likely closer to 40 kyr furthermore the combined 40 ar 39 ar eruption ages and paleomagnetic results provide polarity anchors for the pre olduvai excursion and olduvai subchron which are often used as tie points in studies of early pleistocene hominin evolution

geodynamo energy source constraints from palaeomagnetic data abstract analyses of palaeo geomagnetic data can provide surprisingly strong constraints upon models of the geodynamo in particular it is shown here from an analysis of the geomagnetic reversal sequence that long term variation in reversal rate does not occur as a result of change in any inherent inhibition or encouragement in the reversal process instead there must be some physical process that actually triggers reversals and this process must derive from an energy source effectively independent of the energy source powering the main geomagnetic field from this two very simple generalised models referred to as the hot blob and cold blob models are suggested data from individual reversal transitions are used to discriminate between these two models the preferred model is that the geodynamo is driven primarily by convection related to freezing of the outer core at the inner core outer core boundary cold blobs descending from the core mantle interface are identified as the instabilities that trigger reversals

an evidence based approach to accurate interpretation of 40ar 39ar ages from basaltic rocks abstract since its inception in the mid 1960s the 40 ar 39 ar dating technique has been the premier method for determining the eruption ages of basaltic rocks providing valuable insights into a plethora of terrestrial and planetary processes advances in multi collector mass spectrometry and improved sample preparation procedures are enabling ever improving analytical precision and clearer evaluation of the isotopic disturbances that affect many basaltic samples and cause discordant 40 ar 39 ar age spectra here we present 40 ar 39 ar step heating data for multiple samples from two quaternary basalt flows 0 8038 0 0017 and 2 309 0 009 ma of the intraplate newer volcanic province southeast australia a small proportion of these samples give concordant 40 ar 39 ar results but most are variably discordant the factors controlling these disturbances and implications for accurate age determination are examined and modelled in both step heating spectra and inverse isochron space we demonstrate that the proportion of radiogenic 40 ar 40 ar present in these samples strongly influences the nature of the discordance reflected in 40 ar 39 ar data mass dependent fractionation appears to have a major influence on low 40 ar samples whereas 39 ar recoil loss redistribution effects are evident in samples with higher 40 ar proportions the impact of mass fractionation is quantified via step heating analyses of unirradiated basalt whereby a 4 difference in 38 ar 36 ar ratios is observed between low and high temperature heating steps on an inverse isochron plot 39 ar 40 ar vs 36 ar 40 ar isotopic disturbance for groundmass samples primarily manifests as isochron rotation leading to a negative correlation between initial 40 ar 36 ar 40 ar 36 ar i values and associated 40 ar 39 ar ages we propose a new framework for the interpretation of 40 ar 39 ar step heating data for basaltic samples through judicious evaluation of inverse isochron data 40 ar 36 ar i ratios and inverse isochron ages results from this study suggest that only samples exhibiting both flat 40 ar 39 ar age spectra and atmospheric 40 ar 36 ar i ratios yield accurate eruption ages in the case of more discordant age spectra intermediate temperature steps with atmospheric 40 ar 36 ar i ratios may provide the closest approximation of the eruption age

palaeomagnetism and magnetic properties of the cappadocian ignimbrite succession central turkey and neogene tectonics of the anatolian collage abstract the cappadocian ignimbrite succession of central southern anatolia comprises at least nine major and two minor calc alkaline rhyolitic sheets emplaced at 1 2 ma intervals between 11 2 and 1 1 ma it records the last phase of neotethyan subduction during final emplacement of the tauride orogen in southern turkey this study reports magnetostratigraphy and describes associated rock magnetic properties remanence resides in ti poor titanomagnetites haematisation is locally produced by post emplacement oxidation but does not contribute significantly to the palaeomagnetic signature although secondary processes within the ignimbrite sheets have produced composite isothermal remanent magnetisation spectra and variable intensities of magnetisation weak anisotropy of magnetic susceptibility describes tensors with maximum axes close to bedding and minimum axes perpendicular to this plane directions of kmax with weak imbrication mostly suggest flow away from centres of eruption located by gravity and remote sensing older ignimbrites upper and lower goreme akdag zelve from the cardak centre are all of normal polarity later ignimbrites partly erupted from the derinkuyu centre comprise the sarimaden r cemilkoy r tahar r kizilkaya r incesu n and valibaba sofular r ignimbrites the overall reversed group mean is d i 174 51 n 10 units r 9 84 95 6 6 k 55 and all magnetisation directions from the upper goreme 9 0 ma onwards are rotated anticlockwise with respect to eurasian and african palaeofields this sense of rotation characterises most of central anatolia and averages 9 5 in this sector the rotation rate from 8 to 1 ma bp was 1 25 ma but it appears to have accelerated during the latter part of the quaternary to about an order higher than rates determined from gps rotation has resulted from extrusion of fault blocks during tectonic escape of the anatolian collage to the southwest and followed crustal thickening as the afro arabia plate has continued to impinge differentially into eurasia cenozoic magnetic inclinations are systematically shallower across anatolia than inclinations expected from the geocentric dipole model and the apparent polar wander of bordering major plates only part of this difference 400 km can be accommodated by northward movement and crustal thickening in central anatolia since mid miocene times shallow inclinations observed in the aegean region extend into turkey they are observed in young volcanics and appear to reflect a regional geomagnetic anomaly the pattern of neotectonic declinations across anatolia shows rotations that are strongly anticlockwise rotated in the east near the arabian pincer but diminish towards the west to become zero or slightly clockwise at the western extremity of the collage rotations also seem to become generally younger towards the south crustal deformation has therefore been distributed and the net effect of terrane extrusion to the west and south has been to expand the curvature of the tauride arc and by inference the cyprus arc

assessments on the stability of natural slopes prone to toe erosion and man made historical semi underground openings carved in soft tuffs at zelve open air museum cappadocia turkey abstract the zelve open air museum consisting of three valleys is one of the oldest historical semi underground settlements in the cappadocia region of turkey which was designated as a world heritage site in 1985 by unesco particularly in the first valley there are a lot of underground openings of different sizes carved in a soft tuff next to the valley cliffs used for cliff settlement in the past and several antique churches with some geo engineering problems in addition steep natural slopes in this valley are prone to toe erosion resulting in stability problems in this study it is aimed to investigate engineering characteristics of the surrounding tuff and factors affecting them and to assess the effect of rate of toe erosion on the stability of steep valley slopes and structural stability of some selected typical underground openings with large spans subjected to spalling for these purposes some observational experimental and numerical modelling studies were conducted experimental results indicate that strength and deformability properties of the weak tuff surrounding the openings drastically reduce when it becomes wet or saturated this situation suggests that possibility of occurrence of failures is expected to be higher in rainy seasons due to increase in water content of the tuff experimental results also suggest that spalling commonly observed in the valley is related with freezing thawing and wetting drying cycles in nature and that the process of freezing and thawing accelerates further the degradation of the rock the long term measurements at selected locations suggest that erosion in the tuff is important which increases particularly in winter and spring seasons and results in the loss of support accelerating the occurrence of further slope failures the 2 d numerical solutions indicate that no significant change in shear and tensile stresses in steep valley slopes prone to toe erosion would be expected after 100 years however the solutions obtained from the 500 and 1000 year scenarios suggest that failure zones due to the tensile stresses would develop in the form of slabs with thicknesses of 50 60 cm and 90 100 cm respectively which would break off from the slope and fall down the results from 3 d numerical solutions indicate that failure yield zone resulting from tensile stresses around the semi underground openings will develop parallel to the sidewalls these results confirm the observed spalling of the tuff in the form of slabs parallel to the walls of the openings

40ar 39ar dating on tephra of the velay maars france implications for the late pleistocene proxy climatic record abstract pollen contents and magnetic records of the pleistocene sedimentary sequences cored in the praclaux and lac du bouchet maars velay region french massif central demonstrated that they have high potential as detailed climatic and environmental archives tephra layers interbedded in these sequences have been studied in order to perform isotopic dating first petrographic studies and 40 ar 39 ar ages have shown that these tephra layers are the witness of the same eruption from the sancy volcano centre located 100 km to the north according to fourteen 40 ar 39 ar ages carried out on sanidine and biotite grains this eruption occurred 275 5 ka ago this result allows to construct a time scale for a composite high resolution record of the climate covering the last 300 000 years and to establish an absolute chronology for the palaeoclimatic events recorded in these sequences these tephra layers are interbedded in organic layers deposited during an interstadial stage called the amargiers interstadial which according to its age can be considered as equivalent to isotopic stage 9a moreover the geographical extension of the tephra and its well constrained age give to this tephra the interest of a chronostratigraphic marker for upper quaternary sequences in southeastern france

the role of the earth s mantle in controlling the frequency of geomagnetic reversals a series of computer simulations of the earth s dynamo illustrates how the thermal structure of the lowermost mantle might affect convection and magnetic field generation in the fluid core eight different patterns of heat flux from the core to the mantle are imposed over the core mantle boundary spontaneous magnetic dipole reversals and excursions occur in seven of these cases although sometimes the field only reverses in the outer part of the core and then quickly reverses back the results suggest correlations among the frequency of reversals the duration over which the reversals occur the magnetic field intensity and the secular variation the case with uniform heat flux at the core mantle boundary appears most earth like this result suggests that variations in heat flux at the core mantle boundary of the earth are smaller than previously thought possibly because seismic velocity anomalies in the lowermost mantle might have more of a compositional rather than thermal origin or because of enhanced heat flux in the mantle s zones of ultra low seismic velocity

a detailed paleomagnetic record between 2 1 and 2 75 ma at iodp site u1314 in the north atlantic geomagnetic excursions and the gauss matuyama transition 1 this study investigated the detailed geomagnetic field variation between 2 1 and 2 75 ma from a sediment core iodp site u1314 with high sedimentation rate 10 cm kyr and good age control characteristic remanent magnetization directions were well resolved by stepwise alternating field demagnetization as a proxy of relative paleointensity natural remanent magnetization nrm normalized by anhysteretic remanent magnetization arm was used after testing that the influence of magnetic interaction in arm is negligible as a result the following features of the geomagnetic field in the studied period have been revealed during the transition of the gauss matuyama g m reversal and the reunion subchron the paleointensity decreased to the value lower than 20 of the average intensity in the whole studied interval in addition to these lows eight paleointensity lows were found associated with large directional changes that satisfy the definition of a geomagnetic excursion four of these have ages close to ages reported for geomagnetic excursions in prior studies whereas the other four excursions have not previously been observed in our results we confirm that the g m transition occurred in marine isotope stage 103 even if we consider the shift in depth due to the lock in process of magnetic particles the temporal variation in paleointensity showed asymmetric behavior associated with the g m transition with a gradual decrease prior to the transition and a rapid recovery after the transition

40ar 39ar geochronology of submarine mauna loa volcano hawaii 1 new geochronologic constraints refine the growth history of mauna loa volcano and enhance interpretations of the petrologic geochemical and isotopic evolution of hawaiian magmatism we report results of 40ar 39ar incremental heating experiments on low k tholeiitic lavas from the 1 6 km high kahuku landslide scarp cutting mauna loa s submarine southwest rift zone and from lavas in a deeper section of the rift obtaining precise40ar 39ar ages from young tholeiitic lavas containing only 0 2 0 3 wt k2o is challenging due to their extremely low radiogenic 40ar contents analyses of groundmass from 45 lavas yield 14 new age determinations 31 success rate with plateau and isochron ages that agree with stratigraphic constraints lavas collected from a 1250 m thick section in the landslide scarp headwall were all erupted around 470 10 ka implying an extraordinary period of accumulation of 25 mm yr possibly correlating with the peak of the shield building stage this rate is three times higher than the estimated vertical lava accumulation rate for shield building at mauna kea 8 6 3 1 mm yr based on results from the hawaii scientific drilling project between 470 and 273 ka the lava accumulation rate along the southwest rift zone decreased dramatically to 1 mm yr we propose that the marked reduction in lava accumulation rate does not mark the onset of post shield volcanism as previously suggested but rather indicates the upward migration of the magma system as mauna loa evolved from a submarine stage of growth to one that is predominantly subaerial thereby cutting off supply to the distal rift zone prior to 250 ka lavas with loihi like isotopic signatures were erupted along with lavas having typical mauna loa values implying greater heterogeneity in the plume source earlier in mauna loa s growth in addition to refining accumulation rates and the isotopic evolution of the lavas erupted along the southwest rift zone our new40ar 39ar results constrain the eruption of the ninole basalts from 227 to 108 ka and provide maximum estimates on the timing of the ka lae and south kona landslides

an algorithm for u pb isotope dilution data reduction and uncertainty propagation this is the publisher s version also available electronically from http onlinelibrary wiley com