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

influence of tidal effects on the periodicity of earthquake activity in diverse geological settings with particular emphasis on submarine hydrothermal systems abstract hydrothermal earthquakes may be defined as earthquakes caused by the hydrofracturing of the oceanic crust by submarine hydrothermal fluids it is suggested that they correspond to the cracking of the ocean crust at the brittle ductile transition that marks the downward limit of seawater circulation into the ocean crust semi diurnal variations in submarine hydrothermal discharge in backarc settings and at the crests of mid ocean ridges have been detected by hydrophone but no influence of tidal effects on the periodicity of microearthquakes around submarine hydrothermal vents has been observed so far although tidal stresses are too small to initiate earthquakes themselves they may trigger earthquake activity when imposed on slowly increasing tectonic stress these effects therefore tend to be observed in geologically active regions where stress levels in the earth s crust are high the influence of tides on the periodicity of microearthquakes tends to be complex because the effects of ocean and earth tides are not uniform at the surface of the earth and because the microearthquakes occur in a wide variety of geological settings earth tides cause compression and dilatation of the oceanic crust which may be the most significant factor in initiating changes in fluid flow rate and microseismicity in submarine hydrothermal systems much of the information now available on the tidal periodicity of microearthquakes is based on relatively short term monitoring which lacks statistical validity the development and installation of deep sea monitoring stations will permit rigorous long term monitoring programmes to be carried out which will lead to a better understanding of the nature and periodicity of microearthquakes around submarine hydrothermal vents

a first order global model of late cenozoic climatic change ii further analysis based on a simplification of co2 dynamics a model developed recently for the long term variations of global ice mass carbon dioxide and mean ocean temperature through the late cenozoic is simplified by hypothesizing a new equation for the co2 variations containing one less adjustable parameter but retaining the essential physical content of the previous equation including nonlinearity and the potential for instability by assuming plausible time constants for the glacial ice mass and global mean ocean temperature and setting the values of six adjustable parameters rate constants a solution for the last 5 my is obtained displaying many of the features observed over this period including the transition to the near 100 ky major ice age oscillations of the late pleistocene in obtaining this solution it is also assumed that variations in tectonic forcing lead to a reduction of the equilibrium co2 concentration perhaps due to increased weathering of rapidly uplifted mountain ranges over this period as a consequence of this co2 reduction the model dynamical system can bifurcate to a free oscillatory ice age regime that is under the pacemaker influence of earthorbital milankovitch forcing expanded discussions are given of the surface temperature variations accompanying the evolution of ice co2 and ocean temperature and of the bifurcation properties of the model from both mathematical and physical viewpoints

extending the vostok ice core record of palaeoclimate to the penultimate glacial period the ice core record of local temperature dust accumulation and air composition at vostok station antarctica now extends back to the penultimate glacial period 140 200 kyr ago and the end of the preceding interglacial this yields a new glaciological timescale for the whole record which is consistent with ocean records temperatures at vostok appear to have been more uniformly cold in the penultimate glacial period than in the most recent one concentrations of co2 and ch4 correlate well with temperature throughout the record

tidal height and frequency dependence of acoustic velocity and attenuation in shallow gassy marine sediments remote prediction of gassy marine sediment properties is important for geohazard assessment gas bubble resonance theory suggests that gassy sediments exhibit acoustic wave velocity frequency and attenuation frequency relationships that depend on gas bubble size gas content and sediment elastic properties an acoustic monitoring experiment to investigate gas bubble resonance effects was undertaken at an intertidal site at dibden bay southampton united kingdom a vertical hydrophone array was positioned to straddle the top of the gassy zone identified on acoustic reflection profiles at about 1 m below the seabed a miniboomer in the seabed above the array was used to generate broadband 600 hz to 3000 hz acoustic signals every 10 min during a 24 hour period with water depths varying between 0 m subaerial exposure at low tide and 2 35 m at high tide the calculated frequency spectra of compressional wave attenuation coefficient show an attenuation maximum over 200 db m that shifts in frequency from 1050 hz at low tide to 1250 hz at high tide thus for the first time providing direct evidence of in situ gas bubble resonance in marine sediments modeling suggests that effective gas bubble radii of 11 mm to 13 mm are responsible for the attenuation maximum supported by x ray computed tomography scan observations on a pressure core which also indicate that bubble shape depends on sediment type modeling of bubble size fluctuations due to pressure equilibration cannot reproduce the observed frequency shift of the attenuation maximum implying that gas diffusion and nonspherical bubbles are significant

velocity structure of a gas hydrate reflector seismic reflection profiles across many continental margins have imaged bottom simulating reflectors bsrs parallel to the seabed these are often interpreted as the base of a zone in which methane hydrate ice is stable waveform inversion of seismic reflection data can be used to estimate from seismic data worldwide the velocity structure of a bsr and its thickness a test of this method at a drill site of the ocean drilling program predicts that sediment pores beneath the bsr contain free methane for approximately 30 meters the hydrate and underlying gas represent a large global reservoir of methane which may have economic importance and may influence global climate

palaeomagnetism and 40ar 39ar geochronology of upper palaeogene volcanic rocks from central tibet implications for the central asia inclination anomaly the palaeolatitude of tibet and post 50 ma shortening within asia summary palaeomagnetic results from upper palaeogene volcanics from three localities in the qiangtang terrane of central tibet record a maximum likelihood palaeolatitude of 28 7 3 7 n these are the first volcanic based palaeomagnetic results from the plateau interior that appear to average secular variation well enough to yield a reliable palaeolatitude estimate this palaeolatitude estimate is significantly lower than the 36 3 n predicted for central tibet by late palaeogene eurasia reference poles consistent with the so called central asia inclination anomaly but it is consistent with values predicted from upper palaeogene reference poles for mongolia the mean inclination of coeval sedimentary rocks from central tibet is 15 5 shallower than our volcanic results similar to the magnitude of sedimentary inclination shallowing estimated from elongation inclination analysis of remanent directions in clastic rocks from the same region we conclude that most of the inclination anomaly observed in volcanic rocks and that which remains after removal of sedimentary flattening in clastic rocks probably is due to the combined effect of reference poles that are not representative of stable asia and by cenozoic intracontinental shortening within central asia on the order of several hundred not more than 1000 kilometers our compilation of palaeomagnetic data from the qiangtang terrane further suggest that the southern margin of the lhasa terrane the southern margin of pre india asia was located as far south as 20 n throughout eocene time these palaeolatitudes in conjunction with the upper crustal shortening record for asia indicate that only approximately one third of the post 50 ma india asia convergence is partitioned in asian lithosphere low latitudes also imply that much of the proto tibetan plateau occupied southerly tropical to subtropical latitudes which has important implications for the weathering and erosion of the tibetan plateau palaeoaltimetry proxies and early cenozoic climate dynamics

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

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

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

re evaluation of the ages of 40ar 39ar sanidine standards and supereruptions in the western u s using a noblesse multi collector mass spectrometer abstract the rewards from resolving the tempo of magmatic paleobiologic paleoclimatic and tectonic processes at better than the per mil level of precision have spurred a quest within the geochronology community to improve the precision accuracy and calibration of several chronometers including the 40 ar 39 ar variant of the k ar clock a new generation of multi collector mass spectrometers for 40 ar 39 ar geochronology provides the means to address these issues as well as to evaluate the ages of 40 ar 39 ar standards and key marker horizons deposited by large volume caldera forming supereruptions in the western u s at unprecedented temporal resolution we highlight the utility of a 5 collector noblesse mass spectrometer that has a sufficiently high mass resolving power to distinguish 36 ar h 35 cl from a 36 ar h 35 cl 12 c 3 sum peak at m e 36 maintains a remarkably low and stable background and thus yields 40 ar 39 ar dates of exceptionally high precision we have calibrated a reference gas and developed a simple analytical routine involving one peak hop both of which are used to evaluate detector bias correction factors because the 40 ar 39 ar chronometer is a relative dating technique that relies on calibration against mineral standards of known age precise and accurate determination of the ages of these standards and verification of isotopic composition and homogeneity is essential we present results from both total fusion and incremental heating experiments on single crystals of the widely used alder creek acs and fish canyon fcs sanidine standards several recent independent measurements of acs using multi collector mass spectrometers including those presented herein favor an age of 1 1864 0 0003 0 0012 ma 95 confidence analytical full external uncertainty that is 1 younger than the widely used value obtained by nomade et al 2005 using a single collector mass spectrometer incremental heating experiments on single crystals of fcs most commonly do not yield plateau ages instead many crystals appear to be affected by ar loss or excess ar our findings obtained using ion counters independently confirm those recently obtained using a multiple faraday detectors or a combination of faraday and ion counting detectors thus careful consideration must be given to using fcs in future high precision 40 ar 39 ar calibrations of the geologic timescale notwithstanding 40 ar 39 ar dating of sanidine using the 5 collector noblesse can now provide leverage critical to interpreting the large range of u pb zircon dates commonly found in rhyolitic lavas and tuffs

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

climate an exceptionally long interglacial ahead today9s comparatively warm climate has been the exception more than the rule during the last 500 000 years or more if recent warm periods or interglacials are a guide then we may soon slip into another glacial period but berger and loutre argue in their perspective that with or without human perturbations the current warm climate may last another 50 000 years the reason is a minimum in the eccentricity of earth9s orbit around the sun

secular variation of the geomagnetic dipole during the past 2000 years we have constructed a very simple model of a time varying geocentric dipole based on the archeomagnetic records obtained at four widely separated sites on the globe for the past 2 ka the predictions of the model in terms of directional variations have been tested against actual archeomagnetic data from 12 sites distributed over the globe they were also compared with the hongre et al 1998 time varying spherical harmonic model and with the cals7k 2 model by korte et al 2005 we find that the misfits between the predictions of our simple dipole model and the data are equivalent to those of the spherical harmonic models for the european sites and not strikingly larger for the rest of the world many discrepancies can be accounted for by uncertainties inherent to the archeomagnetic records which along with the small number and poor geographical distribution of sites leads us to conclude that the present state of the database does not allow the extraction of secular variations described by terms going beyond degree 2 it appears also that dipole tilt could be responsible for the main part of the secular variation associated with time constants exceeding 102 years in a second step we used the paleointensity records contained in the same database to construct the curve depicting the variations of the true dipole moment the present decrease of the dipole did not begin prior to 1000 years ago and the dipole was actually increasing from 0 until a d 500 the dipole moment of cals7k is lower than the present estimate probably due to large repartition of energy to higher harmonics to minimize the misfit between the inversion and the data the tilt and strength of the dipole can predict the dipole field at any site and were used to derive the contribution of the nondipole field to values of paleointensity at paris during the past 2 ka the results show that the archeomagnetic jerks are associated with various configurations depending on the phase relationship between the nondipolar and dipolar parts of the field

milankovitch forcing of the last interglacial sea level during the last interglacial sea level was as high as present 4000 to 6000 years before peak northern hemisphere insolation receipt 126 000 years ago the sea level results are shown to be consistent with climate models which simulate a 3 to 4 c july temperature increase from 140 000 to 130 000 years ago in high latitudes with all northern hemisphere land areas being warmer than present by 130 000 years ago the early warming occurs because obliquity peaked earlier than precession and because precession values were greater than present before peak precessional forcing occurred these results indicate that a fuller understanding of the milankovitch climate connection requires consideration of fields other than just insolation forcing at 65 n

a one dimensional model of continental ice volume fluctuations through the pleistocene implications for the origin of the mid pleistocene climate transition abstract a new one dimensional zonally averaged high resolution spherical model of the accumulation and flow of continental ice sheets in response to variations in insolation forcing has been constructed the model includes a detailed representation of the influence of the isostatic adjustment of the earth under the weight of the ice at a rate determined by the viscosity of the planetary mantle with this model we address the issue as to whether the combination of physical processes that it includes is sufficient to allow us to reconcile the variations of continental ice volume that are inferred to have dominated the pleistocene epoch on the basis of the analysis of 18o data from deep sea sedimentary cores we use high resolution data from the newly analyzed odp site 677 and from dsdp site 607 as principal observational constraints these datasets reveal a marked transition in climate to have occurred at mid pleistocene time about 900 000 years bp in order to understand this transition we find that we

low δ18o rhyolites from yellowstone magmatic evolution based on analyses of zircons and individual phenocrysts the yellowstone plateau volcanic field is one of the largest centers exchange time to form zoned zircons is between a few hundred and a few thousand years which reflects the residence time of lowo of rhyolitic magmatism on earth major caldera forming eruptions are followed by unusual lowo rhyolites new oxygen isotope magmas after formation and before eruption petrologic and geochemical data from rhyolites belonging to the 2 0 my eruptive history of yellowstone are presented with emphasis on the genesis of lowo magmas erupted after the huckleberry ridge tuff 2 0 ma 2500 km and lava creek tuff 0 6 ma

response of submarine hydrologic monitoring instruments to formation pressure changes theory and application to nankai advanced corks 1 we describe the response of a compressible submarine hydrologic monitoring instrument to formation pressure changes in low diffusivity rock the measured pressure depends on the frequency of the pressure signal the hydraulic diffusivity and the wellbore storage the nankai advanced circulation obviation retrofit kits acorks offshore japan record tide induced formation pressure changes with small amplitudes 25 the pressure measurements occur in thick homogeneous compressible low permeability sediment where in situ tidal pressure responses should approximate the seafloor tidal signal a wellbore storage of 2 10 8 m3 pa 1 can explain many of the observed tidal responses given the hydraulic diffusivities of the monitored intervals a reduced permeability around the wellbore of 1000 fold and a wellbore storage of 10 11 m3 pa 1 can also reconcile the data our analysis suggests that acork screens in the lower shikoku basin facies have a critical frequency on the order of 5 10 8 hz equivalent to a period of 250 days higher frequency formation pressure signals will be distorted in the pressure record within the lower shikoku basin facies the time for this monitoring system to record 90 of an instantaneous pressure change is on the order of 10 d we suggest that the acork instrument compliance contributes to but does not fully explain the small tidal amplitudes and large phase shifts recorded at the least permeable monitoring intervals

velocity structure of a bottom simulating reflector offshore peru results from full waveform inversion abstract much of our knowledge of the worldwide distribution of submarine gas hydrates comes from seismic observations of bottom simulating reflectors bsrs full waveform inversion has proven to be a reliable technique for studying the fine structure of bsrs using the compressional wave velocity we applied a non linear full waveform inversion technique to a bsr at a location offshore peru we first determined the large scale features of seismic velocity variations using a statistical inversion technique to maximise coherent energy along travel time curves these velocities were used for a starting velocity model for the full waveform inversion which yielded a detailed velocity depth model in the vicinity of the bsr we found that the data are best fit by a model in which the bsr consists of a thin low velocity layer the compressional wave velocity drops from 2 15 km s down to an average of 1 70 km s in an 18 m thick interval with a minimum velocity of 1 62 km s in a 6 m interval the resulting compressional wave velocity was used to estimate gas content in the sediments our results suggest that the low velocity layer is a 6 18 m thick zone containing a few percent of free gas in the pore space the presence of the bsr coincides with a region of vertical uplift therefore we suggest that gas at this bsr is formed by a dissociation of hydrates at the base of the hydrate stability zone due to uplift and subsequently a decrease in pressure

oceanic mechanisms for amplification of the 23 000 year ice volume cycle situated adjacent to the largest northern hemispher ice sheets of the ice ages the mid latitude north atlantic ocean has an important role in the earth s climate history it provides a significant local source of moisture for the atmosphere and adjacent continents forms a corridor that guides moisture bearing storms northward from low latitudes and at times makes direct contact along its shorelines with continental ice masses evidence of major ice ocean air interactions involving the north atlantic during the last 250 000 years is summarized outflow of icebergs and meltwater initially driven by summer insolation over the ice sheets affects midlatitude ocean temperatures summer heat storage winter sea ice extent and global sea level these oceanic responses in turn influence the winter moisture flux back to the ice sheets as well as ablation of land ice by calving spectral data indicate that the oceanic moisture and sea level feedbacks in part controlled by glacial melt products amplify milankovitch insolation forcing of the volumetrically dominant mid latitude ice sheets at the 23 000 year precessional cycle

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

annual fluctuations of groundwater levels as a result of loading by surface moisture abstract long term hydrographs for deep confined aquifers in southern saskatchewan canada show annual fluctuations characterized by a gradual rise in head from october to may june and a rapid drop from may june to october these fluctuations are distinctly different from the seasonal fluctuations observed in surficial and shallow semi confined aquifers which reflect the response of these aquifers to recharge derived from snow melt and early spring rains on the basis of field data and theoretical considerations it is demonstrated that seasonal fluctuations of the water table are not transmitted down to the deep aquifers provided the confining layers are thick and have sufficiently low hydraulic diffusivity the annual fluctuations in the deep aquifers thus do not reflect seasonal recharge to these aquifers plots of moisture accumulation i e precipitation minus calculated areal evapotranspiration are strikingly similar to the observed hydrographs for the deep aquifers in timing and magnitude of the annual fluctuations it is postulated that the annual head fluctuations in the deep confined aquifers reflect changes of the mechanical load on the formations caused by seasonal changes of the soil moisture snow and storage at the water table the deep confined aquifers thus act as large scale inexact weighing lysimeters

origin of bottom simulating reflectors geophysical evidence from the cascadia accretionary prism vertical seismic profile vsp data from two drill sites on the cascadia margin show low velocity zones indicative of free gas be neath a bottom simulating reflector bsr offshore oregon at ocean drilling program odp site 892 velocities drop from an average of 1750 m s above the bsr to less than 1250 m s below it sonic logs confirm that seismic velocity in the sediments adjacent to the borehole is less than that of water for at least 50 m beneath the depth of the bsr at this site similarly at odp site 889 offshore vancouver velocities range from 1700 to 1900 m s in the 100 m above the bsr and drop abruptly to 1520 m s in the 15 m just beneath it the low velocities observed beneath the bsr are strong evidence for the presence of 1 5 free gas by volume the bsr at these two sites results from the contact between gas free sediments containing a small quantity of hydrate above the bsr and a low velocity free gas zone beneath it although the bsr is associated with the base of the hydrate stability field hydrate appears to account for relatively little of the velocity contrast that produces the bsr velocity above the bsr at site 889 is only about 100 m s greater than that expected for sediments of similar porosity sediments above the bsr at site 892 appear to have normal velocity for their porosity and may contain little hydrate

quantitative detection of methane hydrate through high resolution seismic velocity analysis a laterally extensive high resolution travel time velocity analysis and acoustic wave form inversion were used to quantitatively determine methane hydrate content in deep water sediments of the blake ridge off the southeast u s coast the interval acoustic velocity vp analyses were performed in the p domain by interactively picking the p trajectories of prominent reflections in each of 50 plane wave decomposed common midpoint gathers the reflections correspond to seismic stratigraphic boundaries so that lateral vp changes due to lithology changes are mitigated and vp changes due to changing hydrate content are enhanced two separate interval vp analyses were performed one with thick 0 4 km layers which yielded lower uncertainty but also lower resolution and one with thinner layers 0 1 km yielding higher resolution but slightly larger uncertainties results show no correlation between low sediment reflectivity and vp however in the areas exhibiting a bottom simulating reflector bsr a high vp interval 2 0 km s and 0 15 km thick is seen immediately above the bsr where the bsr is strongest a 256 layer least squares acoustic wave form inversion reveals the bsr to be caused by a vp decrease from 2 0 to 1 5 km s with little or no change in density the inversion also reveals a thin 0 025 km layer of anomalously low vp lying immediately below the bsr two models of methane hydrate distribution are tested each indicating that the volume of methane hydrate in the intervals of elevated vp is up to 25 of the total volume

clues from mis 11 to predict the future climate a modelling point of view simulations performed with the lln two dimensional northern hemisphere climate model have confirmed that climate is largely triggered by changes in insolation forcing although atmospheric co concentration also plays an important role in particular in the amplitude of the simulated variations marine isotope stage 11 mis 11 some 400 kyr ago and the future share a common feature related to climate forcing i e the insolation at these times displays small similar variations mis 11 can be considered an analogue for future natural climate changes different simulations were performed to identify the conditions constraining the length of the mis 11 simulated interglacial clearly its length strongly depends on the phase relationship between insolation and co2 variations it is only when insolation and co act together towards a cooling i e they both decrease together that the climate enters quickly into glaciation and that the interglacial may be short otherwise each forcing alone is not able to drive the system into glaciation and the climate remains in an interglacial state the same situation applies for the future however we already know that co2 and insolation do not play together indeed insolation has been decreasing since 11 kyr bp and co2 concentration remains above 260 ppmv with a general increasing trend over the last 8000 yr therefore we conclude that the long interglacial simulated for the future is a robust feature and the earth will not enter naturally into glaciation before 50 kyr ap c 2003 elsevier science b v all rights reserved

recycling of crystal mush derived melts and short magma residence times revealed by u series disequilibria at stromboli volcano abstract the presence of crystal mushes in the feeding system of active volcanoes is generally revealed by antecrysts representing the crystalline portion of old magmas recycled in the juvenile material but very little is known about the fate of interstitial liquid hosted within the crystal rich mush i e antemelt u series disequilibria measured in magmas erupted in the past 18 years at stromboli volcano provide the first geochemical evidence of the involvement of antemelt and help constraining the timescales of the processes occurring in the plumbing system of the volcano despite almost constant major and trace element composition significant variations in isotope ratios are observed 230 th 232 th decreases with time whilst 238 u 232 th 226 ra 230 th and 87 sr 86 sr are different in the two types of magma erupted magma with low phenocryst content lp is erupted as pumices during paroxysm and is thought to belong to a deep reservoir highly porphyritic magma hp is erupted during the normal strombolian activity as scoria and during the effusive events as lavas and it is considered to derive from the former one within a shallow reservoir through degassing driven crystallisation mixing and incorporation of antecrysts the distinct 238 u 232 th of lp and hp magma requires the involvement of a component with high 87 sr 86 sr and 238 u 232 th deriving from older magmas erupted earlier in the volcano history up to 2 5 ka the incompatibility of u and th in major mineral phases limits the possible effect of antecrysts hence requiring the involvement of a u and th rich antemelt the decrease of 226 ra excess from lp to hp magmas provides further and independent evidence for the involvement of a few thousands years old antemelt the variation with time of 230 th 232 th within lp and hp magmas is exploited to constrain the residence time of magmas in the deep and shallow reservoir of the volcano to 55 yrs inferred reservoir volume 0 5 km 3 and 2 10 yrs inferred reservoir volume 0 02 0 09 km 3 respectively our results show the occurrence of magmatic processes operating at different timescale within the feeding system of a so called steady state volcano such as stromboli we show that while most of the magma is erupted within few years a portion of it made of both crystals antecrysts and residual liquid antemelt can be stored in the plumbing system for thousands of years to be eventually rejuvenated and mixed back into the shallow reservoir the presence of antemelts may also affect the eruptive mechanism by promoting heat transfer in locked crystal mushes hence favouring their remobilisation

analysis and application of in situ pore pressure measurements in marine sediments recent measurements of in situ subseafloor pore pressure in sediment ponds on the flank of the mid atlantic ridge using a pop up pore pressure instrument have provided important information regarding fluid exchange processes at the seafloor as well as in situ estimates of physical properties of marine sediments the in situ pore pressure recordings of 4 5 days duration contain three distinct components the early part of the record is dominated by the decay of a pressure pulse associated with probe penetration the rate of decay of this pulse is used to estimate in situ permeabilities which range from 3 1 10 16 m2 to 6 5 10 15 m2 the recording also exhibits a low amplitude oscillation that is associated with tidal pressure variation on the seafloor a study based on biot s theory shows that amplitudes and phases of tidally induced pore pressure oscillations are determined by both the frame compressibility and the permeability of the sediments and thus the oscillation components are used to estimate these parameters the frame compressibilities and the permeabilities estimated from tidal oscillations range from 1 2 10 8 pa 1 to 4 1 10 8 pa 1 and from 1 1 10 15 m2 to 4 8 10 15 m2 respectively laboratory measured permeabilities range from 7 6 10 16 m1 to 1 2 10 15 m2 permeabilities determined by these three methods are comparable the third component is the ambient equilibrium pore pressure negative pore pressure gradients were identified on all deployments when combined with the permeabilities of the sediments the estimated average rates of drawdown of water through the sedimentary layers in the ponds are 2 7 mm yr the pore pressure gradient near the seafloor can be used to construct a pore pressure profile through the sediment extrapolated pore pressure at the top of the basement is about 80 kpa 0 8 bar below the hydrostatic pressure

carbon cycle instability as a cause of the late pleistocene ice age oscillations modeling the asymmetric response a dynamical model of the pleistocene ice ages incorporating many of the qualitative ideas advanced recently regarding the possible role of ocean circulation chemistry temperature and productivity in regulating long term atmospheric carbon dioxide variations has been constructed this model involves one additional term and free parameter beyond that included in a previous model b saltzman and a sutera 1987 providing the capacity for an asymmetic for example saw toothed response it is shown that many of the main features exhibited by the 18o derived ice record and the vostok core 13c derived carbon dioxide record in the late pleistocene can be deduced as a free oscillatory solution of the model including a rapid deglaciation during which a spike of high co2 and a rapid surge in north atlantic deep water production occurs it is expected that the addition of reasonable levels of external for example earth orbital forcing will enable the model to account for a significant amount of the remaining observed variance and covariance of the slow response climatic variables over the full pleistocene including the mid pleistocene transition

age intercalibration of 40ar 39ar sanidine and chemically distinct u pb zircon populations from the alder creek rhyolite quaternary geochronology standard abstract we report results from a 40 ar 39 ar sanidine and ca tims 238 u 206 pb zircon dating study of eruption and crystal residence timescales of the alder creek rhyolite acr california extruded during the cobb mountain normal polarity subchron c1r 2n a 40 ar 39 ar acr sanidine date of 1 1850 0 0016 ma 2 external uncertainty determined relative to the astronomically dated a1 tephra sanidine is interpreted as the acr eruption age this age is supported by ca tims u pb zircon dating guided by la icpms trace element analyses titanium in zircon tizr thermometry and cathodoluminescence cl imaging using these data two compositionally distinct zircon populations were revealed pre acr group b zircon exhibit oscillatory zoning large positive ce and negative eu anomalies high incompatible trace element contents tizr temperatures of 650 c 750 c and th u disequilibrium corrected 238 u 206 pb dates of 1 38 1 24 ma these crystals are interpreted as antecrysts inherited from earlier intrusives in the geysers cobb mountain magma source region acr related group a zircon present as discrete grains and overgrowths on group b zircon display less intense cl with diffuse zoning and have less pronounced positive ce and negative eu anomalies lower incompatible trace element contents higher tizr temperatures that range up to 840 c and significantly younger dates the youngest group a dates yield a weighted mean of 1 1978 0 0046 ma 2 including systematic uncertainties that is interpreted as the mean age of zircon crystallization prior to eruption the 13 5 ka offset between the 40 ar 39 ar and 238 u 206 pb dates can be attributed to zircon magma residence time recognition of a young population of acr zircon is consistent with the 40 ar 39 ar eruption age which coincides with the astronomical age estimate for the cobb mountain subchron determined by correlating the oxygen isotope record of the giant piston core md972143 to the la93 1 1 orbital solution on the basis of independent radio isotopic and orbital forcing results we propose the refined age of 1 1850 0 0016 ma 2 external uncertainty for the quaternary acr 40 ar 39 ar sanidine standard

velocity structure of a gas hydrate reflector at ocean drilling program site 889 from a global seismic waveform inversion strong subhorizontal reflectors in the upper few hundred meters of oceanic sediments have been imaged by seismic reflection profiling adjacent to many continental margins these reflectors are known as bottom simulating reflectors bsrs as they run roughly parallel to the seabed bsrs generally have been interpreted as representing the base of a methane gas hydrate stability field a series of boreholes was drilled in october november 1992 during leg 146 of the ocean drilling program to examine fluid discharge and its relationship to the bsr in the cascadia accretionary wedge independently we performed a global waveform inversion of seismic reflection data collected near site 889 we use a multistep strategy for global waveform inversion of seismic reflection data the strategy is based on the bayesian inference theory in which the a posteriori probability distribution is determined by the a priori probability distribution and the likelihood function relating the data with the model three different likelihood functions have been defined relating different parts of the data with different wavelengths of model parameter variation first global nonlinear search algorithms are used to estimate the large scale features of one dimensional velocity variations then a nonlinear iterative search is used to retrieve the velocity structure in fine detail at each stage bounds on the model parameters are estimated the strategy has been implemented in the slowness and intercept time domain we find that the bsr corresponds to a 15 20 m zone where the velocity drops from about 1 8 km s to a minimum of 1 4 km s suggesting the presence of free methane gas in the low velocity zone and a 15 20 m transition zone where the velocity gradually increases to a velocity of the surrounding sediments thus the total thickness of the low velocity zone is about 30 40 m a small or negligible velocity increase just above the bsr suggests that the hydrate fills less than 10 of the pore space in the sediments our result was confirmed by the drilling the velocity derived using a vertical seismic profile and sonic log from the drill hole agree closely with our result waveform inversion can recover detailed and well constrained velocity variations from a high amplitude target such as a bsr combined with the detailed interpretation of these velocity variations which the results from site 889 will provide the inversion approach is a powerful tool for analysis of bsrs elsewhere

summary of marine sedimentary shear modulus and acoustic speed profile results using a gravity wave inversion technique experimental techniques and inverse methods have been established to extract marine sediment shear modulus with depth profiles from measurements of wave induced seabed motion on the shallow continental shelf seabed sediments are modeled as a quasistatic incompressible layered elastic medium in response to wave induced pressures an iterative eigenvalue expansion technique is used to extract the inverse under typical continental shelf conditions this inversion method is found to have a depth resolution limit of approximately 3 m with a maximum penetration of 200 m using semiempirical sediment models it is possible to deduce sediment porosity bulk density and shear and compressional wave speeds from the shear modulus profile one representative result from the new jersey shelf is examined in detail and favorably compared with independent sediment profiling methods a summary of experimentally determined sediment profiles from four other sites on the eastern u s continental shelf is given

months between rejuvenation and volcanic eruption at yellowstone caldera wyoming rejuvenation of previously intruded silicic magma is an important process leading to effusive rhyolite which is the most common product of volcanism at calderas with protracted histories of eruption and unrest such as yellowstone caldera wyoming long valley caldera california and valles caldera new mexico in the united states although orders of magnitude smaller in volume than rare caldera forming supereruptions these relatively frequent effusions of rhyolite are comparable to the largest eruptions of the 20th century and pose a considerable volcanic hazard however the physical pathway from rejuvenation to eruption of silicic magma is unclear particularly because the time between reheating of a subvolcanic intrusion and eruption is poorly quantified this study uses nanometer scale trace element diffusion in sanidine crystals to reveal that rejuvenation of a near solidus or subsolidus silicic intrusion occurred in 10 mo or less following a protracted period 220 k y of volcanic repose and resulted in effusion of 3 km3 of high silica rhyolite lava at the onset of yellowstone s last volcanic interval the future renewal of effusive silicic volcanism at yellowstone will likely require a comparable energetic intrusion of magma that remelts the shallow subvolcanic reservoir and generates eruptible rhyolite on month to annual time scales

the δ18o of atmospheric o2 from air inclusions in the vostok ice core timing of co2 and ice volume changes during the penultimate deglaciation one important question about the co2 climate connection is how increasing atmospheric pco2 levels affect climate and thereby the mass balance of continental ice sheets a record of atmospheric co2 variations over the last 160 000 years has recently been constructed by analyzing the trapped gas in the vostok ice core barnola et al 1987 the relationship between changes in atmospheric co2 and the size of the continental ice sheets has been difficult to ascertain because the co2 record is obtained from ice cores while the ice volume record has been constructed from the stable isotopic composition of biogenic caco3 in deep sea sediment cores in order to compare these two records in a more precise manner we present a record of the isotopic composition 18o of atmospheric o2 trapped in the vostok ice core and propose that it may be considered a proxy for the 18o of seawater and hence ice volume having a record of atmospheric co2 along with a continental ice volume proxy in trapped air in the same ice core allows us to compare the timing of changes in these two parameters with little uncertainty in the relative ages of important events our results suggest that during the penultimate glacial termination atmospheric co2 began to increase at least 3 kyr before the initial introduction of meltwater to the oceans possible errors in the relative age depth curve of the vostok ice core and uncertainties in the influence of biological and hydrologic processes on the 18o of atmospheric o2 introduce some uncertainty into our conclusions however our results are in general agreement with the observed phase relationship between atmospheric co2 and ice volume imbrie et al 1984 imbrie et al 1989 inferred from changes in the 13c difference between contemporaneous planktonic and benthic forams deposited in deep sea cores and the specmap ice volume record

on the effects of thermal properties structure and water bottom temperature variation on temperature gradients in lake sediments in heat flow determinations it is customary to treat the surface temperature variation as a finite sum of fourier components the medium is assumed to be homogeneous or horizontally stratified wit

slow physics of large continental ice sheets and underlying bedrock and its relation to the pleistocene ice ages very simple two component models of the earth s cryosphere lithosphere system are analyzed the models consist of a continental ice sheet coupled to a responding earth model because of relatively long response times of the bedrock at least over shorter wavelengths long period of the order of 50 000 years internal or free oscillations in the system occur over a wide range of parameters and parameterizations the oscillatory mechanism operates primarily in the ablation zone and in particular at the firn line these free oscillations could play a dynamic role in the response of the climate system to eccentricity forcing a simple analysis presented strongly suggests that for the simplest rheological models of the earth response to pleistocence ice sheets elastic response is confined to a layer considerably less than the thermal thickness of the lithosphere furthermore because of the striking lack of geological evidence for large depressions of bedrock at the leading edge of growing continental ice sheets predicted by viscoelastic models of the earth more sophisticated rheological models of the earth s uppermost layer appear necessary

crustal scale recycling in caldera complexes and rift zones along the yellowstone hotspot track o and hf isotopic evidence in diverse zircons from voluminous rhyolites of the picabo volcanic field idaho abstract rhyolites of the picabo volcanic field 10 4 6 6 ma in eastern idaho are preserved as thick ignimbrites and lavas along the margins of the snake river plain srp and within a deep 3 km borehole near the central axis of the yellowstone hotspot track in this study we present new o and hf isotope data and u pb geochronology for individual zircons o isotope data for major phenocrysts quartz plagioclase and pyroxene whole rock sr and nd isotope ratios and whole rock geochemistry for a suite of picabo rhyolites we synthesize our new datasets with published ar ar geochronology to establish the eruptive framework of the picabo volcanic field and interpret its petrogenetic history in the context of other well studied caldera complexes in the srp caldera complex evolution at picabo began with eruption of the 10 44 0 27 ma u pb tuff of arbon valley tav a chemically zoned and normal 18o 18o magma 7 9 unit with high zoned 87sr 86sri 0 71488 0 72520 and low e nd 0 18 and e hf 0 28 the tav and an associated post caldera lava flow possess the lowest end 0 23 indicating 40 60 derivation from the archean upper crust normal 18o rhyolites were followed by a series of lower 18o eruptions with more typical lower crustal sr nd hf isotope ratios and whole rock chemistry the voluminous 8 25 0 26 ma west pocatello rhyolite has the lowest 18o value 18omelt 3 3 and we correlate it to a 1 000 m thick intracaldera tuff present in the inel 1 borehole with published zircon ages 8 04 8 35 ma and similarly low 18o zircon values the significant 4 5 decrease in magmatic 18o values in picabo rhyolites is accompanied by an increase in zircon 18o heterogeneity from 1 variation in the tav to 5 variation in the late stage low 18o rhyolites a trend similar to what is characteristic of heise and yellowstone and which indicates remelting of variably hydrothermally altered tuffs followed by rapid batch assembly prior to eruption however due to the greater abundance of low 18o rhyolites at picabo the eruptive framework may reflect an intertwined history of caldera collapse and coeval basin and range rifting and hydrothermal alteration we speculate that the source rocks with pre existing low 18o alteration may be related to 1 deeply buried and unexposed older deposits of picabo age or twin falls age low 18o volcanics and or 2 regionally abundant late eocene challis volcanics which were hydrothermally altered near the surface prior to or during peak picabo magmatism basin and range extension specifically the formation of metamorphic core complexes exposed in the region could have facilitated the generation of low 18o magmas by exhuming heated rocks and creating the large water rock ratios necessary for shallow hydrothermal alteration of tectonically rift zones and volcanically calderas buried volcanic rocks these interpretations highlight the major processes by which supereruptive volumes of magma are generated in the srp mechanisms applicable to producing rhyolites worldwide that are facilitated by plume driven volcanism and extensional tectonics

estimating aquifer parameters from analysis of forced fluctuations in well level an example from the nubian formation near aswan egypt 3 diffusivity estimates for saturated and unsaturated zones atmospheric pressure variations provide a broadband signal that may force a sympathetic response in well water levels in this paper time series analysis techniques are used to estimate the response as a frequency dependent admittance function which is then modeled to provide estimates of the fluid transport properties of strata the data derive from five cased piezometer wells sampling aquifers in the nubian formation southwest of aswan egypt three shallow wells 100 140 m deep sample a water table aquifer a fourth w3 400 m deep samples a basal aquifer in the same area that behaves in a confined manner up to a period of several years the fifth well samples another basal aquifer and shows evidence of partial blockage nontidal water level variations in the shallow wells are due almost entirely to barometrically driven flow of air and water using a simple model to fit the observed barometric admittance spectra we obtain estimates of horizontal and vertical permeabilities for water in the saturated zone local horizontal permeability is constrained by modeling the effects of flow induced pressure gradients near the screen for the w3 deep well sampling the basal aquifer the resulting values 0 15 0 3 2 are marginally lower than the large scale 5 km estimates 0 32 0 43 2 derived in a previous paper however the values for the three wells sampling the water table aquifer although consistent among themselves 0 2 0 5 2 are significantly lower than the large scale estimate 1 0 1 5 2 this is contrary to what might be expected given that the wells are preferentially screened in clean sandstones vertical permeability estimated by modeling partial confinement effects is constrained for only one well a low value was obtained because of the presence of claystone beds in the diffusion path between screen and water table at this well the effects of air wave diffusion are clearly manifest in the spectra of one well where the water table lay at a depth of about 40 m the form of the spectra was well fit by ascribing a uniform pneumatic diffusivity of 1 75 10 m 3m2 s to the unsaturated zone however it was also necessary to include an apparent attenuation of the air wave at the capillary fringe of about 0 5 we propose that the effect is due not to attenuation but that it reflects compression of the phreatic surface arising from the presence of trapped air pockets in the underlying saturated zone a 40 m rise in the water table at the site during the decade prior to the measurements might explain the presence of significant quantities of trapped air this rise in water table together with the arid climate might be taken to suggest that the moisture content of the unsaturated zone is negligible except within a meter or so of the water table however calculation of the intrinsic rock permeability from pneumatic diffusivity assuming zero moisture content yields an estimate which is considered to be too low the likely explanation is that the assumption of zero moisture content is in error despite conditions which are as favorable as are ever likely to be realized under field conditions

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

a simple ice sheet model yields realistic 100 kyr glacial cycles records of global ice volume for the past 700 kyr based on oxygen isotopic data from deep sea cores and reflecting mainly the changing northern hemispheric ice sheets show a dominant cycle of roughly 100 kyr period the records also show smaller amplitude oscillations with spectral peaks at roughly 40 and 20 kyr periods which are well correlated with the milankovich insolation variations due to perturbations in the earth s orbital parameters however no model has accurately simulated the 100 kyr glacial cycle recently birchfield et al 1 and oerlemans2 have obtained encouraging agreement with some features of the glacial cycle by using a simple ice sheet model with a realistic time lag in the response of the bedrock to the ice load this study extends their basic model first by including topography to represent high ground in the north improved results can then be obtained but only with unrealistic parameter values and for some aspects of the record further improvements areobtained by crudely parameterizing possible calving at the equatorward ice sheet tip during deglaciation by proglacial lakes and or marine incursions from the atlantic as emphasized by andrews3 the resulting ice volume curves agree fairly well with the observed records and their power spectra over the past 700 kyr

phase relations and adiabats in boiling seafloor geothermal systems abstract observations of large salinity variations and vent temperatures in the range of 380 400 c suggest that boiling or two phase separation may be occurring in some seafloor geothermal systems consideration of flow rates and the relatively small differences in density between vapors and liquids at the supercritical pressures at depth in these systems suggests that boiling is occurring under closed system conditions salinity and temperature of boiling vents can be used to estimate the pressure temperature point in the subsurface at which liquid seawater first reached the two phase boundary data are reviewed to construct phase diagrams of coexisting brines and vapors in the two phase region at pressures corresponding to those of the seafloor geothermal systems a method is developed for calculating the enthalpy and entropy of the coexisting mixtures and results are used to construct adiabats from the seafloor to the p t two phase boundary results for seafloor vents discharging at 2300 m below sea level indicate that a 385 c vent is composed of a brine 7 nacl equivalent in equilibrium with a vapor 0 1 nacl brine constitutes 45 by weight of the mixture and the fluid first boiled at approximately 1 km below the seafloor at 415 c 330 bar a 400 c vent is primarily vapor 88 wt 0 044 nacl with a small amount of brine 26 nacl and first boiled at 2 9 km below the seafloor at 500 c 520 bar these results show that adiabatic decompression in the two phase region results in dramatic cooling of the fluid mixture when there is a large fraction of vapor

development of a new sea ice growth and lead parameterization a thermodynamic sea ice model that has been numerically structured to take time steps on the order of a week has been shown to be sensitive to time step size this sensitivity was caused by the extrapolation of initial ice growth rates over the long time step a new parameterization of new sea ice growth on open ocean and in leads that can be used over a large range of time step sizes at least from 0 3 to 12 days is described here in this parameterization new sea ice growth is computed as a power law function of the initial energy deficit in the ocean this power law takes into account the rapid reduction of the ice growth rate as the sea ice gets thicker and therefore reduces sensitivity to time step size tests of this parameterization show that this method does a good job of simulating the rate of new ice growth when compared to data from mawson antarctica and is relatively insensitive to the length of the time step

meridional sea ice transport and its impact on climate sea ice is an important factor in controlling the ocean atmosphere energy exchange in polar regions and has an important impact on climate this is because sea ice insulates the relatively warm ocean from the cold winter atmosphere and has a higher surface albedo than the ocean in this study the effect of sea ice transport on the energy exchange between the atmosphere and ocean and thus on climate is examined using a coupled energy balance climate thermodynamic sea ice model transport of sea ice produces a thinning of the zonally annually average d sea ice in the poleward most zones and an extension of the ice edge equatorward thinning of the poleward most ice produces an increase in the annual sensible heat flux from the ocean to the atmosphere which produces an increase in the mean annual atmospheric temperatures sensible heat flux is decreased in the zones that contain the ice edge however warming in the poleward most zones produces an increased meridional energy convergence at the ice edge that offsets the decrease in the sensible heat flux to the atmosphere resulting in a net warming

seismic studies of a bottom simulating reflection related to gas hydrate beneath the continental margin of the beaufort sea the upper continental margin of the beaufort sea north of alaska is underlain by a strong bottom simulating reflection bsr that lies 300 to 700 m beneath the seafloor and corresponds to the phase boundary between interstitial water and natural gas below and solid gas hydrate above bsrs of similar origin are common worldwide where they are usually interpreted to mark the base of gas hydrate bearing clastic sediment with or without underlying free gas in the sediment surprisingly little is known about the origin of these strong reflections in this paper we analyze the contrasting physical properties which produce bsrs by comparing synthetic bsr amplitudes and waveforms for varying source receiver offsets with multichannel seismic reflection data across the well developed bsr of the beaufort sea in order to discriminate whether free gas is present under the bsr or not it was necessary to supplement near vertical incidence data with prestack offset data the amplitude versus offset avo analysis indicates that the bsr is produced mainly by the existence of free gas in the clastic sediments beneath the bsr the zone of free gas is based on vertical incidence synthetics estimated to be thinner than 11 16 m it is possibly thicker than 16 m if the gas concentration decreases with depth saturation of gas hydrate in the sediment above the bsr is tentatively estimated from the avo modeling to be less than 10 of the pore volume

high resolution la icp ms trace element mapping of igneous minerals in search of magma histories we report experiments on optimisation of la icp ms mapping as a tool for visualising and quantifying internal structure of trace element concentration in igneous minerals the experimental design was refined with maps on clinopyroxene and amphibole macrocrysts mainly antecrysts from a porphyritic lamprophyre in ne spain as well as on a high precision metal wire grid in terms of spatial resolution we demonstrate with scanning electron microscope and white light interferometry that a full ablation removes between 0 4 and 0 7 mu m of material depending on ablation parameters maps were produced with square laser beam spots of 12 and 24 mu m it was found that complexities can be resolved in the sample even though they are smaller than the beam diameter e g 7 10 mu m discontinuities using 12 mu m laser beam resolution in x and y was found to be identical probably reflecting the fast washout of the two volume ablation cell and the short total dwell time of the analyte menu selected due to the excellent stage reproducibility and the limited ablation depth it is feasible to re ablate the identical map area many times employing different instrument parameters or analyte menus on the magmatic crystals la icp ms maps define very sharp compositional zoning in trace elements highlighting complex crystallisation histories where normal magmatic fractionation is not the only process events of mafic recharge are easily recognised as zones enriched in compatible metals such as cr ni or sc further trace element maps reveal complexities in mineral zoning previously undetectable with petrography or major element data these include resorbed primitive cores and oscillatory zoning within apparently homogeneous mineral zones therefore la icp ms mapping opens a new window of opportunity for analysis of magmatic histories the wide combination of instrumental parameters such as laser beam size scan speed and repetition rate make it possible to carry out experiments at different levels of detail we recommend a two step approach to mapping the initial step involves rapid maps to gain an overview of potential complexities in the sample this enhances representativeness of the analysed materials as a large number of crystals and trace elements can be tested in little time subsequently detailed maps can be carried out on areas of interest an additional functionality is to create 1d profiles from 2d maps the potential of the technique to unveil compositional complexities efficiently and at greater detail than traditional microanalysis will help to improve our understanding of processes in the magmatic environment and beyond c 2015 elsevier b v all rights reserved

theory for the effects of free gas in subsea formations on tidal pore pressure variations and seafloor displacements loading of the seafloor by regional scale pressure variations such as those imposed by ocean tides is supported by both the rock matrix and interstitial fluid the nature of the partitioning of the support between the two depends primarily on the compressibility of the fluid and the compressibility and fluid transport properties of the rock matrix in this paper we examine theoretically the influence of free gas on pore fluid compressibility on the nature of time dependent load partitioning and on the consequent vertical rock deformation and seafloor displacement an example is the gas trapped below deep sea gas hydrate we have derived an expression for the steady state compressibility of pore fluid considering the influence of gas solubility in water the effect of gas solubility is seen to be important at low such as tidal loading frequencies and thus must be included when observations of tidally induced pore fluid pressure variations or seafloor displacements are used to constrain gas content for very low gas concentrations ng much less than 0 1 the steady state fluid compressibility can be enhanced by gas solution dissolution over the loading cycle by several factors at high ambient pressure and more than an order of magnitude at low ambient pressures 2 the fluid compressibility increases sensitively with ng and greatly affects the tidal response of the pore fluid pressure regardless of the solubility thus with careful experimental design tidally induced pore pressure variations may be used to detect very small amounts of free gas and constrain the quantity if ng 2 this method is complementary to using acoustic velocity to constrain the quantity of free gas which works well in the ng 0 2 2 range we have also given an expression for the vertical deformation of subsea formations and hence of the seafloor displacement under tidal loading the presence of free gas enhances tidally induced seafloor displacement but the maximum effect is limited by the compressibility of the matrix frame given relatively low frame compressibility tidally induced seafloor displacement is small of the order of 1 mm which is presently difficult to detect at tidal frequencies

near field observations of an offshore mw 6 0 earthquake from an integrated seafloor and subseafloor monitoring network at the nankai trough southwest japan an mw 6 0 earthquake struck 50 km offshore the kii peninsula of southwest honshu japan on 1 april 2016 this earthquake occurred directly beneath a cabled offshore monitoring network at the nankai trough subduction zone and within 25 35 km of two borehole observatories installed as part of the international ocean discovery program s nantroseize project the earthquake s location close to the seafloor and subseafloor network offers a unique opportunity to evaluate dense seafloor geodetic and seismological data in the near field of a moderate sized offshore earthquake we use the offshore seismic network to locate the main shock and aftershocks seafloor pressure sensors and borehole observatory data to determine the detailed distribution of seafloor and subseafloor deformation and seafloor pressure observations to model the resulting tsunami contractional strain estimated from formation pore pressure records in the borehole observatories equivalent to 0 37 to 0 15 strain provides a key to narrowing the possible range of fault plane solutions together these data show that the rupture occurred on a landward dipping thrust fault at 9 10 km below the seafloor most likely on the plate interface pore pressure changes recorded in one of the observatories also provide evidence for significant afterslip for at least a few days following the main shock the earthquake and its aftershocks are located within the coseismic slip region of the 1944 tonankai earthquake mw 8 0 and immediately downdip of swarms of very low frequency earthquakes in this region illustrating the complex distribution of megathrust slip behavior at a dominantly locked seismogenic zone

determination of fluid flow properties from the response of water levels in wells to atmospheric loading the water level in a well that taps a partially confined aquifer is often sensitive to atmospheric loading the magnitude and character of this response is partly governed by the well radius the lateral hydraulic diffusivity of the aquifer the thickness and vertical pneumatic diffusivity of the unsaturated zone and the thickness and vertical hydraulic diffusivity of the saturated zone overlying the aquifer these key elements can be combined into five dimensionless parameters that partly govern the phase and attenuation of the response in many cases the response of a well to atmospheric loading can be broken up into a high intermediate and low frequency response the high frequency response is governed largely by the well radius and lateral diffusivity of the aquifer the intermediate frequency response is governed by the loading efficiency of the aquifer the low frequency response is governed by the vertical pneumatic diffusivity and thickness of the unsaturated zone and the vertical hydraulic diffusivity and thickness of the saturated material above the aquifer cross spectral estimation is used to fit the response to atmospheric loading of three water wells to the theoretical curves in order to yield estimates of three of the key dimensionless parameters these estimates then are used to make estimates or place bounds on the vertical pneumatic diffusivity of the unsaturated zone the lateral permeability of the aquifer and the composite vertical hydraulic diffusivity of the overlying saturated materials

vostok ice core provides 160 000 year record of atmospheric co2 direct evidence of past atmospheric co2 changes has been extended to the past 160 000 years from the vostok ice core these changes are most notably an inherent phenomenon of change between glacial and interglacial periods besides this major 100 000 year cycle the co2 record seems to exhibit a cyclic change with a period of some 21 000 years

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

linking rapid magma reservoir assembly and eruption trigger mechanisms at evolved yellowstone type supervolcanoes the geological record contains evidence of volcanic eruptions that were as much as two orders of magnitude larger than the most voluminous eruption experienced by modern civilizations the a d 1815 tambora indonesia eruption perhaps nowhere on earth are deposits of such supereruptions more prominent than in the snake river plain yellowstone plateau srp yp volcanic province northwest united states while magmatic activity at yellowstone is still ongoing the heise volcanic field in eastern idaho represents the youngest complete caldera cycle in the srp yp and thus is particularly instructive for current and future volcanic activity at yellowstone the heise caldera cycle culminated 4 5 ma ago in the eruption of the 1800 km3 kilgore tuff accessory zircons in the kilgore tuff display significant intercrystalline and intracrystalline oxygen isotopic heterogeneity and the vast majority are 18o depleted this suggests that zircons crystallized from isotopically distinct magma batches that were generated by remelting of subcaldera silicic rocks previously altered by low 18o meteoric hydrothermal fluids prior to eruption these magma batches were assembled and homogenized into a single voluminous reservoir u pb geochronology of isotopically diverse zircons using chemical abrasion isotope dilution thermal ionization mass spectrometry yielded indistinguishable crystallization ages with a weighted mean 206pb 238u date of 4 4876 0 0023 ma mswd 1 5 n 24 these zircon crystallization ages are also indistinguishable from the sanidine 40ar 39ar dates and thus zircons crystallized close to eruption this requires that shallow crustal melting assembly of isolated batches into a supervolcanic magma reservoir homogenization and eruption occurred extremely rapidly within the resolution of our geochronology 103 104 yr the crystal scale image of the reservoir configuration with several isolated magma batches is very similar to the reservoir configurations inferred from seismic data at active supervolcanoes the connection of magma batches vertically distributed over several kilometers in the upper crust would cause a substantial increase of buoyancy overpressure providing an eruption trigger mechanism that is the direct consequence of the reservoir assembly process

continuous geomagnetic field models for the past 7 millennia 1 a new global data compilation 1 a global data set of archeomagnetic and paleomagnetic data covering the past 7000 years has been compiled it consists of 16 085 results of inclination 13 080 of declination and 3188 of intensity for the time span 5000 bc to 1950 ad declination and inclination data come partly from existing databases and partly from original literature a new global compilation of intensity data for the millennial scale is included data and dating uncertainties are discussed as we attempted to obtain an internally coherent data set the global distribution of the data is very inhomogeneous in both time and space all the data are compared to predictions from the previous 3000 year global model cals3k 1 this collection of data will be useful for global secular variation studies and geomagnetic field modeling although southern hemisphere data are still underrepresented in particular we will use it in a further study to update and extend the existing global model cals3k 1 the huge increase in data compared to the previous compilation will result in significant changes from current models as we might have missed some suitable data we encourage the reader to notify us about any data that have not been included yet and might fit in as improving our global millennial scale models remains our aim for the future the data files described in this paper are available from the earthref digital archive erda at http earthref org cgi bin erda cgi n 331

physical volcanology of the pavagadh rhyolites northern deccan traps stratigraphic structural and textural record of explosive and effusive eruptions abstract mount pavagadh 840 m in the northern deccan traps india is mainly composed of mafic lava flows picrites ankaramites mugearites basalts and capped by rhyolites rhyolites also found at lower elevations on and around the mountain are thought to represent blocks of the eroded upper rhyolites or local eruptions in situ although the pavagadh sequence has been studied for 150 years the stratigraphy structures textures and physical volcanology of the pavagadh rhyolites have been poorly documented and are discussed here at length the upper rhyolite sequence contains tens of non welded crystal bearing glass and pumice rich ash beds of 47 m total thickness these represent a primary fallout ash succession derived from distant plinian eruptions possibly corresponding with massive ignimbrite deposits recently recognised in saurashtra several hundred kilometers to the southwest the fallout ash succession is overlain by a 50 m thick lava flow showing well developed flow banding and folding basal and upper breccias and a local basal vitrophyre the lava flow is overlain by an ignimbrite with a basal vitrophyre both these units showing excellent vitroclastic textures the upper rhyolites thus represent alternating explosive effusive explosive eruptions the lower level rhyolites in the area showing intense rheomorphic deformation in outcrop but lacking vitroclastic textures are probably also lavas however they are not blocks of the eroded upper rhyolite lava flow but from separate eruptions the pavagadh rhyolites were probably far more laterally extensive than their present outcrops with a minimum original volume of 9 km3 their source areas remain unknown geochemical and geochronological work on the pavagadh rhyolites is needed in the stratigraphic and volcanological context provided here to understand their petrogenesis and potential relationship with the cretaceous palaeogene k pg boundary mass extinction

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

summer solstice solar radiation the 100 kyr ice age cycle and the next ice age modeling studies suggest that the summer solstice solar radiation is more important than the caloric half year solar radiation in producing glacial interglacial cycles because it is more representative of the energy available to melt ice during the short melt season here it is shown that the correlation between the summer solstice solar radiation and the rate of change of the oxygen isotope record is generally greater than that between the caloric half year radiation and the rate of change of the oxygen isotope record these results also suggest that the sawtoothed nature of the 100 kyr cycle may be produced by periods of relatively slow changes in ice volume punctuated by periods of rapid growth that are initiated at times of extremely low summer solstice radiation and that it is unlikely that an ice age will begin in the next 70 kyr

carbon isotope records from pacific surface waters and atmospheric carbon dioxide we have stacked planktonic carbon isotope data from three cores in the western equatorial pacific in order to generate a new reconstruction of atmospheric carbon dioxide over the past 450 000 years our new reconstruction resembles that of shackleton et al 1983 based on data from east pacific core v19 30 which successfully predicted features that were subsequently verified by barnola et al 1987 in the record from the vostock ice core in addition the new data confirm the discovery of shackleton and pisias 1985 that changes in atmospheric co2 lead changes in ice volume and hence probably contributed to the glacial interglacial cycles our new reconstruction avoids some of the deficiencies of the previous reconstruction in particular the planktonic species neogloboquadrina dutertrei on which the earlier reconstruction depends does not calcify in truly nutrient free surface water as the model assumes whereas our new reconstruction uses globigerinoides sacculifer which is expected to be more reliable in addition the surface waters in the west pacific are closer to the nutrient free ideal on which the model broecker 1982 depends on the other hand the amplitude of the new reconstruction is significantly smaller than the amplitude observed by barnola et al 1987 it is not clear whether this smaller range is a better estimate of the amplitude of the biological pump effect or whether the true amplitude is reduced by bioturbation in the west pacific cores that we studied

origin and evolution of silicic magmatism at yellowstone based on ion microprobe analysis of isotopically zoned zircons the origin of large volume yellowstone ignimbrites and smallervolume intra caldera lavas requires shallow remelting of enormous volumes of variably o depleted volcanic and sub volcanic rocks altered by hydrothermal activity zircons provide probes of these processes as they preserve older ages and inherited do values this study presents a high resolution oxygen isotope examination of volcanism at yellowstone using ion microprobe analysis with an average precision of 0 2o and a 10 mm spot size we report 357 analyses of cores and rims of zircons and isotope profiles of 142 single zircons in 11 units that represent majoryellowstone ignimbrites and post caldera lavas many zircons from these samples were previously dated in the same spots by sensitive high resolution ion microprobe shrimp and all zircons were analyzed for oxygen isotope ratios in bulk as a function of grain size by laser fluorination we additionally report oxygen isotope analyses of quartz crystals in three units the results of this work provide the following new observations 1 most zircons from post caldera low do lavas are zoned with higher do values and highly variable u pb ages in the cores that suggest inheritance from pre caldera rocks exposed on the surface 2 many of the higher do zircon cores in these lavas have u pb zircon crystallization ages that postdate caldera formation but pre date the eruption age by 10 20 kyr and represent inheritance of unexposed post caldera sub volcanic units that have do similar to the lava creek tuff 3 young and voluminous 0 25 0 1ma intra caldera lavas which represent the latest volcanic activity atyellowstone contain zircons with both high do and lowdo cores surrounded by an intermediate do rim this implies inheritance of a variety of rocks from high do pre caldera and low do post caldera units followed by residence in a common intermediate do melt prior to eruption 4 major ignimbrites of huckleberry ridge and to a lesser extent the lava creek and mesa falls tuffs contain zoned zircons with lower do zircon cores suggesting that melting and zircon inheritance from the lowdo hydrothermally altered carapace was an important process during formation of these large magma bodies prior to caldera collapse 5 the do zoning in the majority of zircon core rim interfaces is step like rather than smoothly inflected suggesting that processes of solution reprecipitation were more important than intracrystalline oxygen diffusion concave downward zircon crystal size distributions support dissolution of the smaller crystals and growth of rims on larger crystals this study suggests that silicic magmatism atyellowstone proceeded via rapid shallow level remelting of earlier erupted and hydrothermally alteredyellowstone source rocks and that pulses of basaltic magma provided the heat for melting each postcaldera yellowstone lava represents an independent homogenized magma batch that was generated rapidly by remelting of source rocks of various ages and do values the commonly held model of a single large volume super solidus mushy state magma chamber that is periodically reactivated and produces rhyolitic offspring is not supported by our data rather the source rocks for theyellowstone volcanism were cooled below the solidus hydrothermally altered by heated meteoric waters that caused low do values and then remelted in distinct pockets by intrusion of basic magmas each packet of new melt inherited zircons that retained older age and do values this interpretation may have significance for interpreting seismic data for crustal low velocity zones in which magma mush and solidified areas experiencing hydrothermal circulation occur side by side new basalt intrusions into this solidifying batholith are required to form the youngest volcanic rocks that erupted as independent rhyolitic magmas we also suggest that the lava creek tuff

tidally controlled gas bubble emissions a comprehensive study using long term monitoring data from the neptune cabled observatory offshore vancouver island long term monitoring over one year revealed high temporal variability of gas emissions at a cold seep in 1250 m water depth offshore vancouver island british columbia data from the north east pacific time series underwater networked experiment observatory operated by ocean networks canada were used the site is equipped with a 260 khz imagenex sonar collecting hourly data conductivity temperature depth sensors bottom pressure recorders current meter and an ocean bottom seismograph this enables correlation of the data and analyzing trigger mechanisms and regulating criteria of gas discharge activity three periods of gas emission activity were observed a short activity phases of few hours lasting several months b alternating activity and inactivity of up to several day long phases each and c a period of several weeks of permanent activity these periods can neither be explained by oceanographic conditions nor initiated by earthquakes however we found a clear correlation of gas emission with bottom pressure changes controlled by tides gas bubbles start emanating during decreasing tidal pressure tidally induced pressure changes also influence the subbottom fluid system by shifting the methane solubility resulting in exsolution of gas during falling tides these pressure changes affect the equilibrium of forces allowing free gas in sediments to emanate into the water column at decreased hydrostatic load we propose a model for the fluid system at the seep fueled by a constant sub surface methane flux and a frequent tidally controlled discharge of gas bubbles into the ocean transferable to other gas emission sites in the world s oceans

the impact of milankovitch solar radiation variations on sea ice and air temperature in a coupled energy balance climate sea ice model the sensitivity of thermodynamicall y varying sea ice and surface air temperature to variations in solar radiation on the 104 to 105 time scales is examined in this study model simulation results show the mean annual sea ice thickness is very sensitive to the magnitude of mid summer solar radiation during periods of high mid summer solar radiation between 115 ka b p and the present

an analysis of a seismic reflection from the base of a gas hydrate zone offshore peru seismic reflection data recorded near odp site 688 offshore peru exhibit a persistent bottom simulating reflector bsr from a depth corresponding to the theoretical base of the gas hydrate stability field although gas hydrate has been recovered in cores from scientific drilling in both the pacific and the atlantic drilling in areas exhibiting a bsr has usually been avoided because of the perceived risk of releasing free gas trapped beneath the hydrated sediments to carry out a quantitative analysis of the bsr the seismic data were reprocessed using signature deconvolution and true amplitude recovery techniques synthetic seismograms that were created using acoustic parameters extracted from the seismic data and physical properties from site 688 borehole measurement were compared to the observed seismic data to estimate the thickness of the free gas zone results indicate the bsr is discontinuous laterally where the bsr is of high amplitude free gas in a zone 5 5 17 m thick beneath the hydrated sediments provides the observed waveform where the bsr amplitude is low the free gas zone is much thinner than 5 5 m or is entirely absent

crystal scale anatomy of a dying supervolcano an isotope and geochronology study of individual phenocrysts from voluminous rhyolites of the yellowstone caldera a voluminous 600 km3 and long lived 520 75 ka phase of rhyolitic eruptions followed collapse of the yellowstone caldera 640 ka whether these eruptions represent a dying cycle or the growth of a new magma chamber remains an important question we use new u th zircon ages and 18o values determined by ion microprobe and sanidine pb isotope ratios determined by laser ablation to investigate the genesis of voluminous post caldera rhyolites the oldest post caldera rhyolites erupted between 520 and 470 ka exhibit extreme age and oxygen isotopic heterogeneity requiring derivation from individual parcels of low 18o melts we find a progressive increase in zircon homogeneity for rhyolite eruptions from 260 to 75 ka with homogeneous low 18o zircon values of 2 7 2 8 that are in equilibrium with low 18o host melts for the majority of the youngest eruptions new sanidine pb isotope data define separate arrays for post caldera rhyolites and the caldera forming tuffs that preceded them indicating that they were not sourced from a mushy lava creek tuff batholith that remained after caldera collapse rather our new age and isotopic data indicate that the post caldera rhyolites were generated by remelting of a variety of intracaldera source rocks consisting of pre lava creek tuff volcanic and plutonic rocks and earlier erupted post lava creek tuff rhyolites batch assembly of low 18o melts starting at 260 ka resulted in progressive homogenization followed by differentiation and cooling up until the last rhyolite eruption 75 ka a trend that we interpret to be characteristic of a dying magma reservoir beneath the yellowstone caldera

magma mixing and the generation of isotopically juvenile silicic magma at yellowstone caldera inferred from coupling 238 u 230 th ages with trace elements and hf and o isotopes in zircon and pb isotopes in sanidine the nature of compositional heterogeneity within large silicic magma bodies has important implications for how silicic reservoirs are assembled and evolve through time we examine compositional heterogeneity in the youngest 170 to 70 ka post caldera volcanism at yellowstone caldera the central plateau member cpm rhyolites as a case study we compare 238u 230th age trace element and hf isotopic data from zircons and major element ba and pb isotopic data from sanidines hosted in two cpm rhyolites hayden valley and solfatara plateau flows and one extracaldera rhyolite gibbon river flow all of which erupted near the caldera margin ca 100 ka the hayden valley flow hosts two zircon populations and one sanidine population that are consistent with residence in the cpm reservoir the gibbon river flow hosts one zircon population that is compositionally distinct from hayden valley flow zircons the solfatara plateau flow contains multiple sanidine populations and all three zircon populations found in the hayden valley and gibbon river flows demonstrating that the solfatara plateau flow formed by mixing extracaldera magma with the margin of the cpm reservoir this process highlights the dynamic nature of magmatic interactions at the margins of large silicic reservoirs more generally hf isotopic data from the cpm zircons provide the first direct evidence for isotopically juvenile magmas contributing mass to the youngest post caldera magmatic system and demonstrate that the sources contributing magma to the cpm reservoir were heterogeneous in 176hf 177hf at ca 100 ka thus the limited compositional variability of cpm glasses reflects homogenization occurring within the cpm reservoir not a homogeneous source

the initiation of ice sheet growth milankovitch solar radiation variations and the 100 ky ice age cycle much work has gone into deciphering the causes of the large scale glacial interglacial variations in the climate system over the last 900 000 years while variations on the 41 thousand year ky and 23 ky time scales seem to be linearly linked to the variations in the distribution of solar radiation at the top of the atmosphere milankovitch solar radiation variations the causes of the dominant 100 ky cycle in the geologic record are still unknown one of the aspects of this cycle that is not well understood is how large scale ice sheet growth is initiated here we describe the mechanisms by which large scale ice sheet growth may have been initiated by the changes in the seasonal and latitudinal distribution of solar radiation over the past 160 ky this is done through the use of a coupled energy balance climate thermodynamic sea ice model that includes a hydrologic cycle which computes precipitation and a land surface energy balance which determines the net accumulation of snow and ice results indicate that the initiation of ice sheet growth is possible during times of extremely low summer solstice solar radiation as a result of a large decrease in ablation during the critical melt season

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

the hysteresis properties of multidomain magnetite and titanomagnetite titanomaghemite in mid ocean ridge basalts abstract hysteresis measurements have been carried out on a suite of ocean floor basalts with ages ranging from quaternary to cretaceous approximately linear yet separate relationships between coercivity bc and the ratio of saturation remanence saturation magnetization mrs ms are observed for massive doleritic basalts with low ti magnetite and for pillow basalts with multi domain titanomagnetites with x 0 6 even when the morb has undergone low temperature oxidation resulting in titanomaghemite the parameters are still distinguishable although offset from the trend for unoxidized multidomain titanomagnetite the parameters for these iron oxides with different titanium content reveal contrasting trends that can be explained by the different saturation magnetizations of the mineral types this plot provides a previously underutilized and non destructive method to detect the presence of low titanium magnetite in igneous rocks notably morb

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

the oxygen isotope composition of karoo and etendeka picrites high δ18o mantle or crustal contamination oxygen isotope compositions of karoo and etendeka large igneous province lip picrites and picrite basalts are presented to constrain the effects of crustal contamination versus mantle source variation olivine and orthopyroxene phenocrysts from lavas and dykes mg 64 80 from the tuli and mwenezi nuanetsi regions of the ca 180 ma karoo lip have 18o values that range from 6 0 to 6 7 they appear to have crystallized from magmas having 18o values about 1 1 5 higher than expected in an entirely mantle derived magma olivines from picrite and picrite basalt dykes from the ca 135 ma etendeka lip of namibia and karoo age picrite dykes from dronning maud land antarctica do not have such elevated 18o values a range of 18o values from 4 9 to 6 0 and good correlations between 18o value and sr nd and pb isotope ratios for the etendeka picrites are consistent with previously proposed models of crustal contamination explanations for the high 18o values in tuli mwenezi picrites are limited to 1 alteration 2 crustal contamination and 3 derivation from mantle with an abnormally high 18o previously a variety of models that range from crustal contamination to derivation from the enriched mantle lithosphere have been suggested to explain high concentrations of incompatible elements such as k and average end and esr values of 8 and 16 in mwenezi nuanetsi picrites however the primitive character of the magmas mg 73 combined with the lack of correlation between 18o values and radiogenic isotopic compositions mgo content or mg is inconsistent with crustal contamination thus an 18o enriched mantle source having high incompatible trace element concentration and enriched radiogenic isotope composition is indicated high 18o values are accompanied by negative nb and ta anomalies consistent with the involvement of the mantle lithosphere whereas the high 18o themselves are consistent with an eclogitic source magma 18o values about 1 higher than expected for mantle derived magma are also a feature of the bushveld mafic and ultramafic magmas and the possibility exists that a long lived 18o enriched mantle source has existed beneath southern africa a mixed eclogite peridotite source could have developed by emplacement of oceanic lithosphere into the cratonic keel during archaean subduction

recent investigations of the 0 5 ma geomagnetic field recorded by lava flows we present a synthesis of 0 5 ma paleomagnetic directional data collected from 17 different locations under the collaborative time averaged geomagnetic field initiative tafi when combined with regional compilations from the northwest united states the southwest united states japan new zealand hawaii mexico south pacific and the indian ocean a data set of over 2000 sites with high quality stable polarity and declination and inclination measurements is obtained this is a more than sevenfold increase over similar quality data in the existing paleosecular variation of recent lavas psvrl data set and has greatly improved spatial sampling the new data set spans 78 s to 53 n and has sufficient temporal and spatial sampling to allow characterization of latitudinal variations in the time averaged field taf and paleosecular variation psv for the brunhes and matuyama chrons and for the 0 5 ma interval combined the brunhes and matuyama chrons exhibit different taf geometries notably smaller departures from a geocentric axial dipole field during the brunhes consistent with higher dipole strength observed from paleointensity data geographical variations in psv are also different for the brunhes and matuyama given the high quality of our data set polarity asymmetries in psv and the taf cannot be attributed to viscous overprints but suggest different underlying field behavior perhaps related to the influence of long lived core mantle boundary conditions on core flow psv as measured by dispersion of virtual geomagnetic poles shows less latitudinal variation than predicted by current statistical psv models or by previous data sets in particular the brunhes data reported here are compatible with a wide range of models from those that predict constant dispersion as a function of latitude to those that predict an increase in dispersion with latitude discriminating among such models could be helped by increased numbers of low latitude data and new high northern latitude sites tests with other data sets and with simulations indicate that some of the latitudinal signature previously observed in vgp dispersion can be attributed to the inclusion of low quality insufficiently cleaned data with too few samples per site our matuyama data show a stronger dependence of dispersion on latitude than the brunhes data the taf is examined using the variation of inclination anomaly with latitude best fit two parameter models have axial quadrupole contributions of 2 4 of the axial dipole term and axial octupole contributions of 1 5 approximately 2 of the octupole signature is likely the result of bias incurred by averaging unit vectors

age dating and the orbital theory of the ice ages development of a high resolution 0 to 300 000 year chronostratigraphy using the concept of orbital tuning a continuous high resolution deep sea chronostratigraphy has been developed spanning the last 300 000 yr the chronology is developed using a stacked oxygen isotope stratigraphy and four different orbital tuning approaches each of which is based upon a different assumption concerning the response of the orbital signal recorded in the data each approach yields a separate chronology the error measured by the standard deviation about the average of these four results which represents the best chronology has an average magnitude of only 2500 yr this small value indicates that the chronology produced is insensitive to the specific orbital tuning technique used excellent convergence between chronologies developed using each of five different paleoclimatological indicators from a single core is also obtained the resultant chronology is also insensitive to the specific indicator used the error associated with each tuning approach is estimated independently and propagated through to the average result the resulting error estimate is independent of that associated with the degree of convergence and has an average magnitude of 3500 yr in excellent agreement with the 2500 yr estimate transfer of the final chronology to the stacked record leads to an estimated error of 1500 yr thus the final chronology has an average error of 5000 yr