tectonics of the antarctic scotia plate boundary near elephant and clarence islands west antarctica over 5000 km of new bathymetric data collected from near the northern antarctic peninsula 60 s 63 5 s latitude 53 5 w 63 w longitude show the morphology of an irregular segment of the antarctic scotia plate boundary and nearby shetland microplate the irregular plate boundary is formed by an oblique intersection 70 of the sinistral transpressional shackleton fracture zone sfz and the sinistral transtensional south scotia ridge transform ssr near elephant ei and clarence ci islands mapped boundaries of the shetland microplate include the south shetland trench and the volcanic rift axis of bransfield strait marginal basin bathymetric data single channel seismic reflection profiles and geosat ers 1 free air gravity data show a southeast trending fault zone on the northeast side of a prominent ridge in the sfz the fault zone is defined by scarps that affect ocean floor sediments fault bounded subbasins rotated sedimentary layers angular unconformities linear gravity trends and transtensional followed by contractional deformation southeast of a termination of the sfz ridge at the south shetland trench the fault zone subdivides into segments displaying steep scarps up to 23 and canyons on the northeast margin of the ei platform these features become east west trending nearer to the western ssr south of the islands southwest trending extensional or transtensional fault zones disrupt the bransfield strait volcanic rift axis these data suggest that 1 recent 4 ma changes in the configuration of the antarctic plate near the antarctic peninsula caused a segment of the sfz transform to adjust to a more stable rectilinear geometry with the ssr transform and 2 diffuse transtension resulting from current antarctic scotia relative motion is dissecting the shetland microplate near ei and ci and transferring slivers of the scotia plate onto the antarctic plate

high resolution resistivity imaging of marine gas hydrate structures by combined inversion of csem towed and ocean bottom receiver data we present high resolution resistivity imaging of gas hydrate pipe like structures as derived from marine controlled source electromagnetic csem inversions that combine towed and ocean bottom electric field receiver data acquired from the nyegga region offshore norway two dimensional csem inversions applied to the towed receiver data detected four new prominent vertical resistive features that are likely gas hydrate structures located in proximity to a major gas hydrate pipe like structure known as the cne03 pockmark the resistivity model resulting from the csem data inversion resolved the cne03 hydrate structure in high resolution as inferred by comparison to seismically constrained inversions our results indicate that shallow gas hydrate vertical features can be delineated effectively by inverting both ocean bottom and towed receiver csem data simultaneously the approach applied here can be utilised to map and monitor seafloor mineralisation freshwater reservoirs co2 sequestration sites and near surface geothermal systems

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

effects of ridge crest trench interaction on antarctic phoenix spreading forces on a young subducting plate precise measurements of spreading rates on marine magnetic profiles collected to the west of the antarctic peninsula have enabled some consideration of the forces governing plate motion since antarctic phoenix motion has been controlled by the local rather than global force balance over the last 35 m y the total effective driving force per unit length of trench is calculated to have ranged between 2 6 and 3 6 1012 n m which is much less than is commonly thought necessary to support subduction conventional calculations may overestimate slab pull for old slabs because they neglect the effect of extensional disruption in limiting the contribution to the balance of forces at the trench the low estimate of driving forces obtained here implies that resistive forces are also smaller than is generally assumed driving forces show a strong correlation with observed spreading rates which indicates that resistive forces were largely velocity dependent fluid migration up the subduction zone may elevate temperatures in and around the shear zone reducing resistive forces below the levels required by purely conductive models changes in convergence rate may affect the depths of both the brittle ductile deformation boundary and the basalt eclogite phase change causing a negative feedback which would appear as a velocity dependent resistive force the different driving forces acting on the ne and sw parts of the phoenix plate as a consequence of older oceanic lithosphere at the trench in the ne caused antarctic phoenix spreading to take place about a near pole to the sw since 21 m y ago and ultimately resulted in disruption of the phoenix plate about 9 m y ago spreading rates decreased abruptly about 6 m y ago probably because of e w compression across the long transform faults bounding the phoenix plate however spreading on the last three segments of the antarctic phoenix ridge continued at least until 4 m y ago either spreading stopped progressively from sw to ne or the final stage took place about a very near pole to the sw a magnetic quiet zone extends up to 95 km from the margin between the tula fracture zone and the north anvers fracture zone and is thought to indicate that the ridge crest became buried by terrigenous sediment prior to collision the absence of a magnetic quiet zone associated with the most recent ridge crest trench collisions suggests a change in sedimentary regime during the late miocene anomalously fast apparent spreading rates between 23 and 21 m y ago are thought to indicate an error in this part of the magnetic reversal time scale

fluid methane and energy flux in an active margin gas hydrate province offshore costa rica joint analysis of seafloor temperature data borehole temperature measurements and observations that constrain the position of the top and base of the gas hydrate zone ghz at sites on the non accretionary costa rican margin provide quantitative constraints on the flux of energy fluid and methane through the sediments on the overriding plate a comparison of borehole temperature data obtained in the trench at the toe of the deformed sedimentary wedge and at a midslope location indicates that the thermal regimes are perturbed by appreciable advective flux at 1 km oceanward 19 mm yr 1 and 1 4 km landward 5 7 mm yr 1 of the deformation front this observation is loosely consistent with models that predict the most rapid dewatering and highest rates of fluid expulsion within a few kilometers of the deformation front using data that constrain the thickness of the ghz at three sites on the forearc we estimate methane and fluid flux rates through application of a one dimensional analytical model that constrains the nature of the plumbing system for the gas hydrate reservoir our results indicate a two fold decrease in methane flux between a site located 400 m from the toe of the wedge and a site 12 km from the toe an intermediate site located 1 4 km from the toe of the wedge has a thick ghz extends to at least 340 365 m below seafloor that requires a very high methane supply rate 345 mol m 2 kyr 1 for a mass flux of 5 mm yr 1 we conclude that this site has the most two dimensional fluid flux regime with fluids being fed into the zone laterally by conduits and fractures and vertically by expulsion during compaction of underthrusting sediments

little ice age clearly recorded in northern greenland ice cores four ice cores drilled in the little investigated area of northern and northeastern greenland were evaluated for their isotopic 18o and chemical content from these rather uniform records a stable isotope temperature time series covering the last 500 years has been deduced which reveals distinct climate cooling during the 17th and the first half of the 19th century timing of the preindustrial temperature deviations agrees well with other northern hemisphere temperature reconstructions however their extent 1 c significantly exceeds both continental records as well as previous southern and central greenland ice core time series a 20 30 increase in the sea salt aerosol load during these periods supports accompanying circulation changes over the north atlantic comparison with records of potential natural climate driving forces points to an important role of the long term solar influence but to only episodically relevant cooling during years directly following major volcano eruptions

a double bottom simulating reflector in the western ross sea antarctica 1 we present seismic evidence of the presence of gas hydrates and free gas in victoria land basin western ross sea antarctica the occurrence of gas hydrates is inferred from a bottom simulating reflector bsr the first identified in the ross sea area the gas presence was analyzed through targeted reprocessing of the multichannel seismic reflection data 3000 m streamer 120 channels 60 fold acquired in 1990 by the italian r v ogs explora the seismic data were reprocessed in order to increase the signal to noise ratio by adopting a true amplitude approach allowing a subsequent amplitude variations with offset avo analysis to extract information on both p and s wave reflectivity along a profile from the deeper basin to an intrabasin structural high the bsr evolves into crosscutting reflectors ccrs and enhanced amplitude reflectors ears locally another seismic event is interpreted to be a second bsr bsr0 identified about 150 m below the first bsr and is possibly related to the presence of a mixture of gases comprising methane and hydrocarbons with heavier molecular weight and therefore associated with a deeper hydrate stability zone the presence of faults controls the upward gas migration connecting the free gas zone below the bsr and the mud volcanoes and pockmarks on the seafloor the modeling of the hydrate equilibrium zone shows that the observed bsr bsr0 could be stable at the present pressure temperature conditions

temporal and spatial evolution of a gas hydrate bearing accretionary ridge on the oregon continental margin a seismic reflection survey on the oregon continental margin conducted in 1989 indicates the widespread presence of gas hydrate beneath the middle and lower slope of this accretionary margin the seismic signature of gas hydrate a bottom simulating reflector bsr with negative polarity that locally cuts across stratigraphic horizons is especially well developed beneath hydrate ridge this anomalously shallow accretionary ridge was drilled during ocean drilling program leg 146 to study fluid venting in this paper we focus on the seismic data from the southern part of hydrate ridge where little evidence of active venting has previously been reported but where the seismic data indicate a complicated subsurface plumbing system apparent disruptions of the bsr beneath the western ridge flank suggest dissociation of gas hydrate in response to slumping a double bsr beneath the southern crest suggests hydrate destabilization in response to tectonic uplift and folding on the basis of these and other observations we propose a qualitative model for the evolution of a hydrate bearing ridge in an active accretionary complex in which gas hydrate initially stabilizes the sea floor permitting construction of large ridges that are then eaten away by slumps along their margins the north to south variation in sea floor venting and subsurface seismic structure along hydrate ridge may reflect different stages in the temporal evolution of one of these ridges

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

compressional velocity structure and poisson s ratio in marine sediments with gas hydrate and free gas by inversion of reflected and refracted seismic data south shetland islands antarctica abstract a bottom simulating reflector bsr on the south shetland margin antarctic peninsula was surveyed within the progetto nazionale di ricerche in antartide pnra high resolution multichannel mcs reflection profiles were acquired and a three component ocean bottom seismograph obs was deployed where the bsr was particularly strong the compressional velocity field associated to the bsr was determined by the inversion of travel times of reflected mcs data and refracted obs data events information on shear wave velocities was extracted from the horizontal components of the obs records the analysis indicates the presence of velocity anomalies both above and below the bsr that are not related to the lithologic characteristics of the sedimentary column but can be associated to gas hydrate and free gas presence in pore space amplitude versus offset avo analysis was performed on common depth point gathers which correspond to the obs position the study determined reflection coefficient trends that provided qualitative information about gas hydrate and free gas abundance a theoretical model for elastic porous media was applied to quantify amounts of gas hydrate and free gas the resulting theoretical velocity curves were compared to interval velocity obtained by seismic inversion a quantitative estimation of free gas and gas hydrates trapped in the sediment pore spaces was made by interpreting velocity anomalies with respect to reference velocity curves normally consolidated marine terrigenous sediments the comparison of the results obtained by these independent methods seismic inversion avo analysis and theoretical model gives consistent values of poisson s ratio and gas hydrate and free gas estimation

a geological map of the scotia sea area constrained by bathymetry geological data geophysical data and seismic tomography models from the deep mantle abstract the scotia sea is one of the tectonically most complex and least understood back arc basins on earth which partly results from its remote location making the acquisition of data challenging here we provide a review of current publicly available geophysical and geological data in the scotia sea realm including magnetic bouguer gravity anomaly high resolution bathymetric heat flow and reflection seismic data and rock type data from cored and dredged samples with this inter disciplinary data set we performed an offshore geological mapping exercise that allowed us to identify lithologies in the predominantly submerged scotia sea domain cross sections combining crustal structure and mantle tomography enabled us to address some of the still persisting geological challenges in this tectonically complex area the data review revealed that basalt is the dominant lithology in the scotia sea area occupying most of the west and east scotia sea wss and ess andesitic and more felsic lithologies are identified in the central scotia sea css and the northern east scotia sea ess mesozoic palaeozoic metamorphic sedimentary lithologies are reported from the highs along the north and south scotia ridges nsr and ssr these highs originate from a land bridge that until the late mesozoic connected the south american and antarctic continents scarcely available and contradicting data prevent the age determination of several structural units surrounding and located on the scotia plate but our mapping exercise allowed us to confirm the presence of the early oligocene late miocene ancestral south sandwich arc assa in the east of the css setting the minimum age of the older segment of the css crust to eocene earliest oligocene three cross sections cross cutting the scotia sea reveal two high velocity zones indicating cold mantle material one is situated below the structural highs along the ssr which we interpreted as remnant slab material of the assa and another below south georgia and the css which we interpreted as the front of the slab that currently subducts at the south sandwich trench ssat the neutral velocity discontinuity below the wss implies mantle conditions of a recently extinct spreading centre the upper mantle low velocity anomaly below the css is interpreted as warmer toroidal flows around the slab edges of the subducting plate at the ssat we have demonstrated that the geological and geophysical data publicly available today allows us to create offshore geological maps in remote inaccessible offshore domains this is a less time consuming economically advantageous exercise which re uses existing geological and geophysical data for a new purpose it is the most data inclusive study there is today of the scotia sea region and serves as a guideline for future expeditions targeting the css and the structural features along the nsr to identify their age and origin a georeferenced version of the map is provided in the supplementary material

the cenozoic subduction history of the pacific margin of the antarctic peninsula ridge crest trench interactions new magnetic anomaly identifications w of the shackleton fracture zone show 5 spreading sections separated by fracture zones in the 2 most southerly the ridge crest collided with a trench at the margin of the antarctic peninsula only 6 5 and 4 ma ago the latest of a series of collisions starting at the base of the peninsula so ma ago following each collision spreading and subduction both stopped opposite the south shetland trench and actively extending bransfield strait spreading also stopped 4 ma ago but before the spreading centre reached the trench the tendency of the subducting plate to continue sinking probably initiated the opening of the bransfield strait the parallelism between fracture zone orientation on the descending plate and convergence direction previously thought responsible for the tectonic segmentation of the peninsula was effective for only 10 30 ma before collision before that either the fracture zones did not extend to the trench or the subducting ocean floor formed at the farallon phoenix nazca aluk boundary with fracture zones oblique to the subduction direction the aleutian arc kula ridge model in which arc magmatism virtually ceases when ocean floor younger than 25 30 ma occurs beneath the arc fits the distribution of antarctic peninsula radiometric dates explaining a 50 60 ma gap between collisions and youngest ages in the s and possibly the migration of the youngest activity towards the trench thus gaps in the geologic record of arc magmatism need not imply cessation of subduction the progressive steepening of the peninsular margin towards the 4 ma collision site suggests tectonic erosion of the fore arc as the ridge crest approached

seismic velocity studies of a gas hydrate bottom simulating reflector on the northern cascadia continental margin amplitude modeling and full waveform inversion on the northern cascadia subduction margin the multichannel seismic amplitude versus offset avo behavior of a bottom simulating reflector bsr suggests a p wave velocity change from high velocity hydrate bearing sediment to lower velocity sediment containing a small amount of free gas the observed nonlinear avo behavior constant or slightly decreasing amplitudes at near to mid offsets and a large amplitude increase at far offsets can be reproduced in the models if an s wave velocity enhancement is assumed as expected from hydrate cementation the avo behavior of the hydrate bsr is found not to be as useful as was earlier thought for determining the amount of free gas below the bsr this is because poisson s ratio change below the bsr due to gas is likely very small in high porosity unconsolidated sediments the uncertainty in the models is large as there is no reliable s wave velocity information for the sediments containing hydrate or gas avo modeling alone is not sufficient to distinguish different velocity models across the bsr our interpretation of the bsr amplitude behavior is that a p wave velocity increase above the bsr is the main cause of the bsr reflection amplitude increase at large incidence angles caution must be taken in applying avo analysis as little is known about s wave velocities however subtle differences in bsr amplitude behavior and reflection waveform can provide constraints through very careful full waveform inversion a well defined reference velocity depth profile is also required to represent water saturated sediment unaffected by either hydrate or free gas using full waveform velocity inversion a high resolution velocity model for the hydrate bsr has been derived the best fit model for the seismic data near the ocean drilling program odp sites 889 890 consists of a high velocity zone above the bsr and a thin low velocity layer below in agreement with the odp downhole velocity data

precipitation over greenland retrieved by a dynamic method and its relation to cyclonic activity abstract in order to calculate the vertical motion over some high mountain regions such as greenland an equation without the quasigeostrophic approximation in coordinates has been developed a dynamic method for retrieving precipitation over greenland is based on this equation the retrieved annual mean precipitation distribution for 1987 and 1988 is in very good agreement with the observed annual accumulation pattern over the greenland ice sheet the major weather system producing precipitation over greenland is the frontal cyclone based on the precipitation characteristics greenland can be divided into five subregions precipitation over the north coastal and central interior regions primarily occurs in summer for the three other subregions if the composite monthly mean sea level pressure charts for high and low monthly precipitation amounts are constructed a clear relationship between precipitation and cyclonic activity emerges if a mean cyclone exists in the labrador sea heavy precipita

seismic investigation of a bottom simulating reflector and quantification of gas hydrate in the black sea a bottom simulating reflector bsr which marks the base of the gas hydrate stability zone has been detected for the first time in seismic data of the black sea the survey area is in the northwestern black sea at 44 45 n and 31 5 32 5 e in this paper seismic wide angle ocean bottom hydrophone obh and ocean bottom seismometer obs data are investigated with the goal to quantify the gas hydrate and free gas saturation in the sediment an image of the subsurface is computed from wide angle data by using kirchhoff depth migration the image shows the bsr at 205 270 m depth below the seafloor and six to eight discrete layer boundaries between the seafloor and the bsr the top of the hydrate layer and the bottom of the gas layer cannot be identified by seismic reflection signals an analysis of traveltimes and reflection amplitudes leads to 1 d p wave velocity depth and density depth models an average s wave velocity of 160 m s 1 between the seafloor and the bsr is determined from the traveltime of the p to s converted wave the normal incidence pp reflection coefficient at the bsr is 0 11 where the p wave velocity decreases from 1840 to 1475 m s 1 velocities and density are used to compute the porosity and the system bulk modulus as a function of depth the gassmann equation for porous media is used to derive explicit formulae for the gas hydrate and free gas saturation which depend on porosity and on the bulk moduli of the dry and saturated sediment a gas hydrate saturation depth profile is obtained which shows that there is 38 10 per cent hydrate in the pore space at the bsr depth where the porosity is 57 per cent obs 24 this value is derived for the case that the gas hydrate does not cement the sediment grains a model that is supported by the low s wave velocities there is 0 9 or 0 1 per cent free gas in the sediment below the bsr depending on the model for the gas distribution in the sediment the free gas layer may be more than 100 m thick as a result of a zone of enhanced reflectivity which can be identified in the subsurface image

the gisp2 δ18o climate record of the past 16 500 years and the role of the sun ocean and volcanoes measured 18o 16o ratios from the greenland ice sheet project 2 gisp2 ice core extending back to 16 500 cal yr b p provide a continuous record of climate change since the last glaciation high resolution annual 18o 16o results were obtained for most of the current millennium a d 818 1985 and record the medieval warm period the little ice age and a distinct 11 yr 18o 16o cycle volcanic aerosols depress central greenland annual temperature 1 5 c maximally and annual 18o 16o for about 4 yr after each major eruptive event on a bidecadal to millennial time scale the contribution of solar variability to holocene greenlandic temperature change is 0 4 c the role of thermohaline circulation change on climate problematic during the holocene is more distinct for the 16 500 10 000 cal yr b p interval analogous to 14c age calibration terminology we express time in calibrated cal yr b p a d 1950 0 cal yr b p the oldest dryas bolling older dryas allerod younger dryas sequence appears in great detail bidecadal variance in 18o 16o but not necessarily in temperature is enhanced during the last phase of lateglacial time and the younger dryas interval suggesting switches of air mass transport between jet stream branches the branched system is nearly instantaneously replaced at the beginning of the bolling and holocene at 14 670 and 11 650 cal yr b p respectively by an atmospheric circulation system in which 18o 16o and annual accumulation initially track each other closely thermodynamic considerations of the accumulation rate temperature relationship can be used to evaluate the 18 o 16o temperature relationship the gisp2 ice layer count years of major gisp2 climate transitions also support the use of coral 14c ages for age calibration

forearc tectonic evolution of the south shetland margin antarctic peninsula the main provinces of the south shetland margin antarctic peninsula characterized on the basis of multichannel seismic long range side scan sonar and swath bathymetry data include from northwest to southeast 1 the oceanic crust of the former phoenix plate flexed down toward the margin and affected by normal faulting in the upper crust 2 a narrow trench with a horizontally layered sediment fill which onlaps the thin sedimentary cover on the oceanic crust and shows incipient deformation near its landward edge 3 an accretionary prism with a complex internal structure the toe of which is overthrust above the youngest trench deposits 4 a midslope forearc basin with an asymmetric synformal structure and 5 the continental shelf which includes two distinct tectonic provinces calculations of the late cenozoic convergence history at the trench indicate a rapid decrease in convergence rate after 6 7 ma from about 60 mm yr resulting from the slowing and eventual cessation of spreading on the antarctic phoenix ridge once spreading had completely ceased 3 5 2 4 ma the convergence rate at the trench equalled the rate of extension in bransfield strait which was probably less than 10 mm yr at first but may have increased since 1 3 ma and this in turn would imply a corresponding increase in convergence rate above the basal detachment the toe of the accretionary prism is composed of a stack of thrust fault bounded wedges laterally and vertically segmented by normal faults most of the trench fill sediments are overthrust by the toe of the accretionary prism and subducted beneath it for as far as they can be traced on the seismic profiles this suggests that there may be tectonic erosion of the forearc since a large part of the trench sediment appears to be derived from erosion on the forearc slope the forearc basin records a long history of subsidence during which the depocenter migrated landward paralleling retrogradational erosion of the distal margin of the basin subduction of the hero fracture zone at approximately the same location since 10 ma may have enhanced tectonic erosion of the accretionary prism in the forearc basin a synform is coincident with a bathymetric reentrant but its axis is oblique to the trends of both the basin and the fracture zone the origin of this synform and the final phase of uplift of the blueschist facies terrane on which smith island stands are probably related to their position above the subducting fracture zone ridge the present elevation of smith island is thought to result from uplift which followed a ridge crest trench collision directly southwest of the hero fracture zone at 5 5 ma the northwestern edge of the mesozoic to early cenozoic arc batholith trending northeast southwest on the shelf is proposed to have acted as a backstop to the accretionary prism during cenozoic subduction

assessment of gas hydrate and free gas distribution on the south shetland margin antarctica based on multichannel seismic reflection data summary processing and interpretation of a grid of intermediate resolution multichannel seismic reflection profiles collected on the ne sector of the south shetland continental margin allowed us to map the lateral extent of a bottom simulating reflector bsr the margin an accretionary wedge located off the northern tip of the antarctic peninsula consists of two distinct and superimposed tectonic regimes an older regime is related to mesozoic middle cenozoic subduction related tectonism a younger one is associated with a mainly extensional tectonic phase and related to the oligocene development of the western scotia sea the occurrence of the bsr appears to be controlled by the geological structure of the margin the bsr lacks continuity near basement structures main geological discontinuities and faults on the other hand the amplitude and continuity of the bsr are not affected by the presence of folded structures and undeformed sedimentary layering we found that the bsr is mostly confined to the ne sector of the south shetland margin where propagation of faulting associated with the shackleton fracture zone may have driven migration of natural gas towards the surface and created the conditions for a bsr to appear the application of reflection tomography techniques allowed us to reconstruct the averaged seismic velocity field between the seafloor and bsr in order to map the depth of bsr by averaging the observed velocity structure above and below the bsr and applying a theoretical model of elastic wave propagation in porous media we attempted as rigorous a quantitative assessment as possible of the natural gas present as gas hydrate above the bsr and as free gas between the bsr and the base of gas reflector bgr

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

amplitude versus offset modeling of the bottom simulating reflection associated with submarine gas hydrates abstract a bottom simulating seismic reflection bsr that parallels the sea floor occurs worldwide on seismic profiles from outer continental margins the bsr coincides with the base of the gas hydrate stability field and is commonly used as indicator of natural submarine gas hydrates despite the widespread assumption that the bsr marks the base of gas hydrate bearing sediments the occurrence and importance of low velocity free gas in the sediments beneath the bsr has long been a subject of debate this paper investigates the relative abundance of hydrate and free gas associated with the bsr by modeling the reflection coefficient or amplitude variation with offset avo of the bsr at two separate sites offshore oregon and the beaufort sea the models are based on multichannel seismic profiles seismic velocity data from both sites and downhole log data from oregon odp site 892 avo studies of the bsr can determine whether free gas exists beneath the bsr if the saturation of gas hydrate above the bsr is less than approximately 30 of the pore volume gas hydrate saturation above the bsr can be roughly estimated from avo studies but the saturation of free gas beneath the bsr cannot be constrained from the seismic data alone the avo analyses at the two study locations indicate that the high amplitude bsr results primarily from free gas beneath the bsr hydrate concentrations above the bsr are calculated to be less than 10 of the pore volume for both locations studied

deuterium excess record from central greenland over the last millennium hints of a north atlantic signal during the little ice age a stacked water isotope record for summit central greenland has been established covering approximately the last 900 years measurements of 18o and d in the grip deep ice core and the 230 m core s93 allow the reconstruction of a millennial record of the deuterium excess in a near annual resolution a short period of particularly high values of the deuterium excess at the beginning of the fourteenth century may be associated with the medieval warm period mwp the little ice age lia might be represented by a 100 year period of very low excess values in the sixteenth and seventeenth centuries using a simple isotope model 18o and deuterium excess are interpreted in terms of surface temperature variations over central greenland and over the subtropical north atlantic greenland s principal vapor source region an estimated cooling of 0 7 c of subtropical sea surface temperatures ssts during the little ice age and a warming of 0 6 c during the short warm period in the medieval is in agreement with previous studies over periods of about 100 years an antiphasing between gradually decreasing 18o and increasing deuterium excess is observed interannual to decadal scale variability associated to the north atlantic oscillation may be responsible for this anti phase relationship an alternative explication is a north south oscillation in north atlantic sea surface temperatures associated with short term changes in the thermohaline circulation

a 2000 year tree ring record of annual temperatures in the sierra nevada mountains tree ring data have been used to reconstruct the mean late season june through january temperature at a timberline site in the sierra nevada california for each of the past 2000 years long term trends in the temperature reconstruction are indicative of a 125 year periodicity that may be linked to solar activity as reflected in radiocarbon and auroral records the results indicate that both the warm intervals during the medieval warm epoch a d 800 to 1200 and the cold intervals during the little ice age a d 1200 to 1900 are closely associated with the 125 year period significant changes in the phase of the 125 year temperature variation occur at the onset and termination of the most recent radiocarbon triplet and may indicate chaotic solar behavior

glaciological investigations in the crete area central greenland a search for a new deep drilling site the results of the 1984 85 post gisp campaigns in central greenland are presented eight ice cores were obtained some spanning up to 360 years we present i geographical positions and elevations at the drill sites

fluid venting in the eastern aleutian subduction zone fluid venting has been observed along 800 km of the alaska convergent margin the fluid venting sites are located near the deformation front are controlled by subsurface structures and exhibit the characteristics of cold seeps seen in other convergent margins the more important characteristics include 1 methane plumes in the lower water column with maxima above the seafloor which are traceable to the initial deformation ridges 2 prolific colonies of vent biota aligned and distributed in patches controlled by fault scarps over steepened folds or outcrops of bedding planes 3 calcium carbonate and barite precipitates at the surface and subsurface of vents and 4 carbon isotope evidence from tissue and skeletal hard parts of biota as well as from carbonate precipitates that vents expel either methane or sulfide dominated fluids a biogeochemical approach toward estimating fluid flow rates from individual vents based on oxygen flux measurements and vent fluid analysis indicates a mean value of 5 5 0 7 l m 2 d 1 for tectonics induced water flow wallmann et al 1997b a geophysical estimate of dewatering from the same area von huene et al 1997 based on sediment porosity reduction shows a fluid loss of 0 02 l m 2 d 1 for a 5 5 km wide converged segment near the deformation front our video guided surveys have documented vent biota across a minimum of 0 1 of the area of the convergent segment off kodiak island hence an average rate of 0 006 l m 2 d 1 is estimated from the biogeochemical approach the two estimates for tectonics induced water flow from the accretionary prism are in surprisingly good agreement

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

a method for determining gas hydrate or free gas saturation of porous media from seismic measurements the occurrence of gas hydrate or free gas in a porous medium changes the medium s elastic properties explicit formulas for gas hydrate or free gas saturation of pore space on the basis of the frenkel gassmann equations describe the elastic moduli and seismic velocities of a porous medium for low frequencies a key assumption of the model is that either gas hydrate or free gas is present in the pore space in addition to water under this assumption the method uses measured p and s wave velocities and bulk density along with estimates of the moduli and densities of the solid and fluid phases present to determine whether gas or hydrate is present the method then determines the saturation level of either the gas or the hydrate i apply the method to published velocity and density data from seismic studies at the antarctic shetland margin and at the storegga slide offshore norway and to borehole log and core data from ocean drilling program odp leg 164 at blake ridge offshore south carolina a sensitivity analysis reveals that the standard deviations of the gas hydrate and free gas saturations reach 30 70 of the saturations if the standard deviations of the p and s wave velocities and of the bulk density are 50m s

comparative rheological behaviour of albite and quartz in siliceous schists revealed by the microboudinage of piedmontite abstract this paper presents a detailed comparison of the microboudinage of piedmontite in two different mineralogical hosts a quartz matrix and albite porphyroblasts in a siliceous schist with the aim of clarifying the rheological properties of albite in relation to those of quartz stress and strain analyses of the microboudinage confirm that the boudinage took place in the retrograde stage of metamorphism during decreasing temperature and reveal that albite deformed at the same strain rate as quartz above the plastic brittle transition temperature of albite

the δ18o record along the greenland ice core project deep ice core and the problem of possible eemian climatic instability over 70 000 samples from the 3029 m long greenland ice core project grip ice core drilled on the top of the greenland ice sheet summit have been analyzed for 8o a highly detailed and continuous 8o profile has thus been obtained and is discussed in terms of past temperatures in greenland we also discuss a three core stacked annual 8o profile for the past 917 years the short term 50 years variability of the annual 8o signal is found to be 1 in the holocene and estimates for the coldest parts of the last glacial are 3 or higher these data also provide insights into possible disturbances of the stratigraphic layering in the core which seems to be sound down to the onset of the eemian spectral analysis of highly detailed sequences of the profile helps determine the smoothing of the 8o signal which for the holocene ice is found to be considerably stronger than expected we suggest this is due to a process involving diffusion of water molecules along crystal boundaries in the recrystallizing ice matrix deconvolution techniques were employed for restoring with great confidence the highly attenuated annual 8o signal in the holocene we confirm earlier findings of dramatic temperature changes in greenland during the last glacial cycle abrupt and strong climatic shifts are also found within the eem sangamon interglaciation which is normally recorded as a period of warm and stable climate in lower latitudes the stratigraphic continuity of the eemian layers is consequently discussed in section 3 of this paper in terms of all pertinent data which we are not able to reconcile

gas hydrate destabilization enhanced dewatering benthic material turnover and large methane plumes at the cascadia convergent margin mixed methane sulfide hydrates and carbonates are exposed as a pavement at the seafloor along the crest of one of the accretionary ridges of the cascadia convergent margin vent fields from which methane charged low salinity fluids containing sulfide ammonia 4he and isotopically light co2 escape are associated with these exposures they characterize a newly recognized mechanism of dewatering at convergent margins where freshening of pore waters from hydrate destabilization at depth and free gas drives fluids upward this process augments the convergence generated overpressure and leads to local dewatering rates that are much higher than at other margins in the absence of hydrate discharge of fluids stimulates benthic oxygen consumption which is orders of magnitude higher than is normally found at comparable ocean depths the enhanced turnover results from the oxidation of methane hydrogen sulfide and ammonia by vent biota the injection of hydrate methane from the ridge generates a plume hundreds of meters high and several kilometers wide a large fraction of the methane is oxidized within the water column and generates 13c anomalies of the dissolved inorganic carbon pool

a seismic study of methane hydrate marine bottom simulating reflectors multichannel seismic reflection data have been analyzed from an area of clear bottom simulating reflectors bsrs on the northern cascadia subduction zone margin off vancouver island the reflector at a depth of about 300 m subbottom is interpreted to represent the base of a layer of methane hydrate or clathrate the shallow water depth of 1300 m and the 3600 m long hydrophone array have allowed bsr amplitude versus offset and high resolution velocity analysis as well as modelling of vertical incidence data the results of all three types of analysis can be best explained by a 10 to 30 m thick high velocity layer located immediately above the bsr about 300 m below the seafloor having a sharp base and transitional top in the layer about one third of the sediment pore spaces must be filled with hydrate ice there is no seismically detectable free gas beneath the bsrs these results put important constraints on models for the distribution and formation of bsr hydrate

seismic velocities from the yaquina forearc basin off peru evidence for free gas within the gas hydrate stability zone multichannel seismic mcs data from the yaquina forearc basin off peru reveal a complex distribution of gas and gas hydrate related reflections lateral variations of the reflection pattern at the assumed base of the gas hydrate stability zone in terms of continuity amplitude and signal attenuation underneath are observed as well as the possible occurrence of paleo bottom simulating reflectors bsrs phase reversed reflections above the bottom simulating reflector point to free gas within the gas hydrate stability zone ghsz to constrain the interpretation of the observed reflection pattern we calculated the velocity distribution along the mcs line from high resolution ocean bottom hydrophone recordings with two independent methods heat flux values estimated on the basis of the velocity depth functions increase with decreasing amplitude of the bsr and peak near chemoherms these results suggest a model of the yaquina basin where free gas is trapped under parts of the bsr and within the ghsz particularly under the seafloor and under an erosional unconformity the hypothesis of a paleo bsr that reflects the uplift of the base of the hydrate stability zone caused by the deposition of a particular sediment sequence is supported by the estimated heat flux values

complexity of holocene climate as reconstructed from a greenland ice core glaciochemical time series developed from summit greenland indicate that the chemical composition of the atmosphere was dynamic during the holocene epoch concentrations of sea salt and terrestrial dusts increased in summit snow during the periods 0 to 600 2400 to 3100 5000 to 6100 7800 to 8800 and more than 11 300 years ago the most recent increase and also the most abrupt coincides with the little ice age these changes imply that either the north polar vortex expanded or the meridional air flow intensified during these periods and that temperatures in the mid to high northern latitudes were potentially the coldest since the younger dryas event

aperture compensation tomography1 conventional velocity analysis can handle a horizontally stratified medium well there is no indication though that it will be as successful when applied to a more complicated geological structure in fact a small angle of incidence may transform to a wide angle reflection event for a dipping interface in this case conventional velocity analysis may lead to large errors and thus cannot be applied seismic tomography is attractive as it is virtually free from any restrictions imposed on the velocity distribution in the model space or on the setup of a seismic experiment it is important however to recall that seismic tomography yields results of inferior quality compared to medical tomography this paper investigates the reason for this and how to suppress a significant blurring of seismic tomograms unlike medical tomography one cannot provide full angular coverage of the model space in a typical seismic experiment the sources and the receivers cannot surround an unknown object inside the earth to provide a complete spectrum of view angles incomplete angular coverage may lead to the occurrence of large inaccuracies in the computed tomograms especially when the initial model is poorly chosen we demonstrate a method of suppressing the adverse effects related to an incomplete angular recording this is compensation tomography which can be used efficiently in the case of a limited angular aperture numerical experiments illustrate the theory

methane hydrate and free gas on the blake ridge from vertical seismic profiling seismic velocities measured in three drill holes through a gas hydrate deposit on the blake ridge offshore south carolina indicate that substantial free gas exists to at least 250 meters beneath the bottom simulating reflection bsr both methane hydrate and free gas exist even where a clear bsr is absent the low reflectance or blanking above the bsr is caused by lithologic homogeneity of the sediments rather than by hydrate cementation the average methane hydrate saturation above the bsr is relatively low 5 to 7 percent of porosity which suggests that earlier global estimates of methane in hydrates may be too high by as much as a factor of 3

elastic wave velocity in marine sediments with gas hydrates effective medium modeling we offer a first principle based effective medium model for elastic wave velocity in unconsolidated high porosity ocean bottom sediments containing gas hydrate the dry sediment frame elastic constants depend on porosity elastic moduli of the solid phase and effective pressure elastic moduli of saturated sediment are calculated from those of the dry frame using gassmann s equation to model the effect of gas hydrate on sediment elastic moduli we use two separate assumptions a hydrate modifies the pore fluid elastic properties without affecting the frame b hydrate becomes a component of the solid phase modifying the elasticity of the frame the goal of the modeling is to predict the amount of hydrate in sediments from sonic or seismic velocity data we apply the model to sonic and vsp data from odp hole 995 and obtain hydrate concentration estimates from assumption b consistent with estimates obtained from resistivity chlorinity and evolved gas data

recent greenland accumulation estimated from regional climate model simulations and ice core analysis the accumulation defined as precipitation minus evaporation over greenland has been simulated with the high resolution limited area regional climate model hirham4 applied over an arctic integration domain this simulation is compared with a revised estimate of annual accumulation rate distribution over greenland taking into account information from a new set of ice core analyses based on surface sample collections from the north greenland traverse the region with accumulation rates below 150 mm yr21 in central northwest greenland is much larger than previously assumed and extends about 500 km farther to the south it is demonstrated that good agreement between modeled and observed regional precipitation and accumulation patterns exists particularly concerning the location and the values of very low accumulation in the middle of greenland the accumulation rates in the northern part of greenland are reduced in comparison to previous estimates these minima are connected with a prevailing blocking high over the greenland ice sheet and katabatic wind systems preventing humidity transports to central greenland the model reasonably represents the synoptic situations that lead to precipitation maxima of precipitation and accumulation occur at the southwestern and southeastern coasts of greenland and are connected with cyclonic activity and the main storm tracks around greenland the central region of the greenland ice sheet acts as a blocking barrier on moving weather systems and prohibits cyclones moving from west to east across this region and thus prevents moisture transports

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

structural geology and geochronology of subduction complexes along the margin of gondwanaland new data from the antarctic peninsula and southernmost andes subduction complexes along the andean margin in central and southern chile yield mid paleozoic to lower mesozoic ages yet they crop out within 100 km of the modern trench that shows evidence of active accretion along much of its length the scarcity of uplifted subduction complex rocks younger than mid mesozoic along the chilean margin and in parts of the scotia arc suggests to us that these old crystalline rocks uplifted in the triassic and jurassic represent a boundary in the forearc beyond which tectonic erosion does not easily occur greenschist blueschist and amphibolite facies subduction complex rocks from the scotia arc were originally thought to be a simple continuation of the subduction complexes in chile based on new 40 ar 39 ar ages the scotia arc subduction complexes reveal a complex history related to distinct local tectonic events and are not a simple continuation of the old accretionary prism in chile structural and metamorphic analysis indicates the earliest and most penetrative deformation in the subduction complexes around the scotia arc occurred at some depth in a subduction zone or zones certainly below the brittle ductile transition and in some cases under blueschist facies conditions we believe that the early subduction related deformation and metamorphism in the greenschist and blueschist facies rocks of the scotia arc to be overprinted by mid cretaceous transpression along the south america antarctica plate boundary in the case of elephant island transpression and subsequent localized transtension in the earliest cenozoic in the case of the darwin complex a mid cenozoic spreading rate change in the case of smith island and early neogene initiation of drake passage opening shackleton fracture zone formation in the case of the gibbs island subgroup

the nature and distribution of bottom simulating reflectors at the costa rican convergent margin summary bottom simulating reflectors bsrs at the base of the hydrate stability zone are often observed in marine seismic reflection data the compressional p wave velocity structure of a bsr offshore costa rica was analysed by applying a full waveform inversion technique the resulting velocity profile indicates that the bsr is caused by a thin low velocity layer suggesting the presence of at least a small amount of free gas in the sediment pore space in undisturbed sediment sections bsrs offshore costa rica are observed over much of the continental margin they frequently occur at shallow depths beneath the seafloor usually 100 400 m at some locations bsrs appear to intersect the seafloor this was interpreted as an indication that hydrates in the study area form partly from methane which was produced beneath the hydrate stability zone and migrated upwards in the study area the vertical movement of the hydrate stability zone relative to a given point in the sediment column one of the potential factors leading to bsr formation or suppression is controlled by both sedimentation and vertical tectonism both processes may hence play a role in controlling bsr distribution bsrs are absent in areas which have been affected by slumping except where the sedimentary section above the bsr remained intact during slumping this indicates that slope failure can cause the destruction of bsrs

seismic characterization of a gas hydrate system in the gulf of mexico using wide aperture data summary gas hydrates were discovered in a mud mound in the lease block mississippi canyon 798 gulf of mexico through piston coring subsequently a seismic experiment was carried out to investigate the dynamics behind the hydrate formation during the experiment high resolution multichannel seismic reflection data using a 24 channel 240 m long streamer and wide aperture data using six ocean bottom seismometers were collected along five lines high reflectivity zones hrzs are present in the reflection data along all lines to better constrain the interpretation of the reflection data the traveltimes from the multichannel and wide aperture data sets were jointly inverted to estimate a 2 d p wave layered velocity model for each line a minimum parameter minimum structure modelling approach yielded simple models and a comparison of the models at their intersection points shows they are consistent to within 10 m s 1 in velocity and 20 m in depth in the final p wave velocity models the hrzs are associated with a lowering of velocity in the reflection data the top of the hrzs show a polarity reversal with respect to the seafloor presence of free gas in the hrzs best explains the velocity lowering and polarity reversal it is speculated that the gas has deeper sources and migrates upwards through conduits formed by salt movement in the vicinity the upward migrating gas accumulates in the axis of a channel complex and manifests itself as hrzs in the reflection data the fluids circulating along the conduits push the base of the hydrate stability zone close to the seafloor from the channel axis the free gas migrates further upwards and close to the seafloor and as it comes within the gas hydrate stability zone it forms hydrates

seismic velocities for hydrate bearing sediments using weighted equation a weighted equation based on the three phase time average and wood equations is applied to derive a relationship between the compressional wave p wave velocity and the amount of hydrates filling the pore space the proposed theory predicts accurate p wave velocities of marine sediments in the porosity range of 40 80 and provides a practical means of estimating the amount of in situ hydrate using seismic velocity the shear s wave velocity is derived under the assumption that the p to s wave velocity ratio of the hydrated sediments is proportional to the weighted average of the p to s wave velocity ratios of the constituent components of the sediment in the case that all constituent components are known a weighted equation using multiphase time average and wood equations is possible however this study showed that a three phase equation with modified matrix velocity compensated for the clay content is sufficient to accurately predict the compressional wave velocities for the marine sediments this theory was applied to the laboratory measurements of the p and s wave velocities in permafrost samples to infer the amount of ice in the unconsolidated sediment the results are comparable to the results obtained by repeatedly applying the two phase wave scattering theory the theory predicts that the poisson s ratio of the hydrated sediments decreases as the hydrate concentration increases and the porosity decreases in consequence the amplitude versus offset avo data for the bottom simulating reflections may reveal positive negative or no avo anomalies depending on the concentration of hydrates in the sediments

new precipitation and accumulation maps for greenland annual total precipitation and the annual accumulation on the greenland ice sheet are evaluated and presented in two maps the maps are based on accumulation measurements of 25 1 pits and cores obtained from the upper accumulation zone and precipitation measurements made at 35 meteorological stations in the coastal region to construct the accumulation map the annual precipitation was split into solid and liquid precipitation components annual total precipitation exceeding 2500 mm w e occurs on the southeastern tip of greenland while the minimum precipitation is estimated to occur on the northeastern slope of the ice sheet the mean annual precipitation for all of greenland is 340 mm w e the largest annual accumulation of about 1500 mm w e is found on the glaciers in the southeastern corner of greenland while the smallest accumulation is found on the northeastern slope of the ice sheet west of danmarkshavn the mean accumulation on the greenland ice sheet is estimated at 310 mm w e the regional difference in accumulation is examined with respect to the 850 hpa m bar level circulation the present surface topography is found to play an important role in determining regional accumulation on the ice sheet

reassessment of crete greenland ice core acidity volcanism link to climate change previous comparisons of the crete greenland ice core acidity record with climate variations of the last 1 400 years suggested that changes in volcanism may have significantly influenced climate on this time scale however the ice core acidity record consists of volcanically induced spikes superimposed on a background acidity of nonvolcanic origin that varied significantly in amplitude herein the authors produce a record of crete volcanism based solely on an objective definition of individual volcanic events in the ice core record there are 92 acidity peaks in the 1 420 year record since at least 26 of the post 1 600 eruptions are of local icelandic origin they estimate a mean recurrence interval of 20 8 years for hemispheric scale eruptions rich in sulphate on decadal time scales volcanism has varied by a factor of three four over the last 1 400 years however the climate volcanism correlation is considerably less impressive r minus 0 23 than previously concluded r minus 0 52 the good correspondence between volcanism and climate previously reported are due to background acidity levels that show a significant little ice age increase the background increase may reflect changes in ocean productivity 24 refs 5 figs 1 tab