self organization in the thermomechanical flow of ice sheets this paper uses a three dimensional thermomechanical ice sheet model to investigate the initiation and evolution of ice streams within ice sheets ice streams are distinguished within general ice sheet flow by their relatively fast of the order of kilometers per year velocity and are thought to be crucial in determining the response of ice sheets to climatic change we show that streaming can arise solely as a consequence of internal feedback between ice sheet flow and temperature inhomogeneities in topography and or geology are not therefore necessary conditions for ice stream development this spatial patterning replaces the large scale surging reported from previous modeling work in one horizontal dimension we suggest that surging is restricted to situations where ice flow is channelized e g valley glaciers and that it is replaced by streaming where horizontal ice flow is unconstrained ice sheets the ice streams in some of our experiments show switching behavior reminiscent of that known from the siple coast ice streams west antarctica the cause appears to be the competition for ice discharge and drainage basin area between adjacent ice streams this may lead to the gradual reduction of a stream s discharge below the threshold necessary to generate enough frictional heat to maintain warm viscous ice at the ice stream bed in this case the stream will stagnate abruptly which then prompts localized ice thickening and the initiation of a new stream in a different location

replacement dolomites and ore sulfides as recorders of multiple fluids and fluid sources in the southeast missouri mississippi valley type district halogen 87sr 86sr δ18o δ34s systematics in the bonneterre dolomite replacement dolomites may continue to recrystallize in the presence of subsequent diagenetic and hydrothermal fluids during recrystallization they can change both their solid geochemistry and that of their included fluids by releasing older fluids and entrapping younger fluids thus it may be possible to decipher a complex fluid history by analyzing both the composition of the dolomite 18o 87sr 86sr and its contained fluids cl br as the former reflects temperature and or fluid rock interactions whereas the latter reflects salinity source regions of the last fluid to affect the dolomite applying this approach to the bonneterre dolomite of southeast missouri which hosts the world class viburnum trend mississippi valley type mvt pb zn district reveals the presence of two geochemically distinct types of brines whose influence varied temporally during the bonneterre s history of dolomitization and ore deposition a plot of cl br molar ratios of included fluids versus 87sr 86sr ratios of the enclosing dolomite indicates distinct salinity sources for early halite dissolution and later extreme evaporation of seawater dolomitizing brines comparison with fluids in ore sulfides may permit us to relate replacement dolomites to ore forming events early stage copper and zinc bearing sulfides 34s 0 have fluids with cl br molar ratios 150 1 050 reflecting a similar importance of halite dissolving brine during ore deposition main stage cuboctahedral galena 34s 15 has fluids with cl br molar ratios 100 400 similar to extremely evaporated seawater brines responsible for late replacement dolomites we suggest that resident brines which gained salinity in part through halite dissolution sulfur and metals from local sedimentary and igneous rocks were important for early replacement dolomitization and early stage ore deposition this fluid type was supplanted by later main stage brines that acquired salinity through seawater evaporation and metals in a more distal basinal setting late stage vug filling cubic galena records a renewed importance of the halite dissolving brine as the influence of the more distal basinal brine waned halogen sulfur isotope systematics of sulfides indicate that fluid mixing likely was a dominant ore depositing mechanism during all stages of ore mineralization in the viburnum trend

the paleohydrology of southeast missouri mississippi valley type deposits interplay of faults fluids and adjoining lithologies observations and geochemical data provide insight into the paleohydrology influencing development of mississippi valley type deposits in southeast missouri and provide clarification and justification of flow paths in light of proposals of fault hydrodynamics versus a continuous aquifer model faults are recognized as pathways for as well as horizontal barriers to fluid flow in a hydrologically compartmentalized region these relations also indicate that at least two geochemically distinct fluids mineralized late cambrian hosts and that these fluids had a similar history of progressive undersaturation with respect to galena an overview of the synthesis indicates that one of the fluids may have originated in the reelfoot rift whereas the other evolved in a recharge area stretching along the ouachita front and exploited the reelfoot rift northwest striking faults in the precambrian basement acted as channels of discharge for flow out of or through the reelfoot rift fluid movement along these pathways was rapid and allowed fluids to reach ore hosts during a migration interval related to the late paleozoic ouachita orogeny vertical circulation along faults and fracture systems introduced fluids into the existing paleozoic section dispersion into the lamotte sandstone resulting from relative permeability differences between discordant subvertical faults and the adjoining quartzarenite was restricted to a northwest trending fault bounded corridor and was concentrated at the base of the bonneterre formation continued vertical movement of fluids maintained fracture networks as vertical pathways during the period that circulation localized mississippi valley type deposits in the late cambrian section

a model study of ocean circulation beneath filchner ronne ice shelf antarctica implications for bottom water formation 1 an isopycnic coordinate ocean circulation model has been applied to the southern weddell sea including the cavity beneath filchner ronne ice shelf with the aim of investigating the buoyancy forced circulation on the continental shelf buoyancy forcing is associated with both the annual growth and decay of sea ice and the interaction between ice shelf and ocean in the model a generally anti cyclonic circulation develops beneath the ice shelf so that new shelf waters entering the cavity in the west emerge colder and fresher in the east the outflow contributes to a dense current that spills off the continental shelf and descends the slope oceanographic observations from the region are consistent with this picture and highlight the overflow as a major source of weddell sea bottom water

fluid migration and associated diagenesis in the greater reelfoot rift region midcontinent united states the sources and pathways of basinal fluid flow on the north american craton have been the subjects of intense debate regional dolomitization and the distribution of petroleum and sulfide mineralization on the southern midcontinent are believed to be related to migration of warm basinal fluids from and within the arkoma and illinois basins because of its hydrologic connection with these two basins the reelfoot rift has been postulated as a potential source and or conduit for one or more of these fluids cathodoluminescence petrography has revealed nine distinct carbonate cement microstratigraphies in the reelfoot rift and on its adjacent carbonate platforms in missouri western platform and tennessee eastern platform temporal geochemical trends within each of the three areas are distinct eastern platform carbonate cements and their fluid inclusions exhibit decreasing 13c 18o homogenization temperatures t h and last ice melting t m values with decreasing age reelfoot rift dolomite and calcite cement and their fluid inclusions define a trend of decreasing 13c and 18o values and increasing t h and t m values western platform dolomite cements including those in the nearby southeast missouri mississippi valley type mineral district display a paragenetic trend of increasing 13c and 18o values followed by a return to lower values during the final stage of cement deposition distinct geochemical patterns indicate that stratigraphically controlled fluid flow through carbonate aquifers in the reelfoot rift was restricted within individual fault blocks that had limited communication with the adjacent carbonate platforms lateral fluid migration between fault blocks and vertical migration within fault blocks in the reelfoot rift likely was restricted by faults stratigraphic offset and impermeable units if significant fluid movement occurred from the reelfoot rift onto the adjacent western carbonate platform it likely occurred along northwest striking strike slip faults

new gmpes for the sagami bay region in japan for moderate magnitude events with emphasis on differences on site amplifications at the seafloor and land seismic stations of k net

modelling circumpolar deep water intrusions on the amundsen sea continental shelf antarctica results are presented from an isopycnic coordinate model of ocean circulation in the amundsen sea focusing on the delivery of circumpolar deep water cdw to the inner continental shelf around pine island bay the warmest waters to reach this region are channeled through a submarine trough accessed via bathymetric irregularities along the shelf break temporal variability in the influx of cdw is related to regional wind forcing easterly winds over the shelf edge change to westerlies when the amundsen sea low migrates west and south in winter spring this drives seasonal on shelf flow while inter annual changes in the wind forcing lead to inflow variability on a decadal timescale a modelled period of warming following low cdw influx in the late 1980 s and early 1990 s coincides with a period of observed thinning and acceleration of pine island glacier

site amplifications for generic rock sites seismic shear wave velocity as a function of depth for generic rock sites has been estimated from borehole data and studies of crustal velocities and these velocities have been used to compute frequency dependent amplifications for zero attenuation for use in simulations of strong ground motion we define a genetic rock site as one whose velocity at shallow depths equals the average of those from the rock sites sampled by the borehole data most of the boreholes are in populated areas for that reason the rock sites sampled are of particular engineering significance we consider two genetic rock sites rock corresponding to the bulk of the borehole data and very hard rock such as is found in glaciated regions in large areas of eastern north america or in portions of western north america the amplifications on rock sites can be in excess of 3 5 at high frequencies in contrast to the amplifications of less than 1 2 on very hard rock sites the consideration of unattenuated amplification alone is computationally convenient but what matters for ground motion estimation is the combined effect of amplification and attenuation for reasonable values of the attenuation parameter k 0 the combined effect of attenuation and amplification for rock sites peaks between about 2 and 5 hz with a maximum level of less than 1 8 the combined effect is about a factor of 1 5 at 1 hz and is less than unity for frequencies in the range of 10 to 20 hz depending on t using these amplifications we find provisional values of about ao 70 bars and k 0 0 035 sec for rock sites in western north america by fitting our empirically determined response spectra for an m 6 5 event to simulated values the borehole data yield shear velocities 033o of 618 and 306 rrdsec for rock and soil sites respectively when averaged over the upper 30 m from this we recommend that v30 equals 620 and 310 m sec for applications requiring the average velocity for rock and soil sites in western north america by combining the amplifications for rock sites and the site factors obtained from our analysis of strong motion data we derive amplifications for sites with 17 3o 520 m sec nehrp class c corresponding to a mix of rock and soil sites and v3o 310 and 255 m sec average soil and nehrp class d sites respectively for the average soil site the combined effect of amplification and attenuation exceeds a factor of 2 0 for frequencies between 0 4 and about 4 hz with a peak site effect of 2 4 the peak of the nehrp class d site effect is 2 7

sensitivity of 21st century sea level to ocean induced thinning of pine island glacier antarctica 1 pine island glacier pig antarctica is rapidly losing mass supporting arguments that it may play a major role in 21st century sea level rise yet this glacier s quantitative contribution to sea level based on theoretical and computational models is poorly known we have developed a basin scale glaciological model to examine the sensitivity of pig to a range of environmental forcings while oceanic melt likely played the leading role in recent thinning and retreat we find that the particular grounding line geometry with an extended ice plain in the 1990s made it susceptible to such forcing our model further indicates that while the rate of grounding line retreat should diminish soon the glacier s mass loss may continue at rates similar to or moderately elevated from the present while substantial our model derived maximum rate of 2 7 cm century is considerably smaller than previous heuristically derived bounds on the sea level contribution

channelized bottom melting and stability of floating ice shelves 1 the floating ice shelf in front of petermann glacier in northwest greenland experiences massive bottom melting that removes 80 of its ice before calving into the arctic ocean detailed surveys of the ice shelf reveal the presence of 1 2 km wide 200 400 m deep sub ice shelf channels aligned with the flow direction and spaced by 5 km we attribute their formation to the bottom melting of ice from warm ocean waters underneath drilling at the center of one of channel only 8 m above sea level confirms the presence of ice shelf melt water in the channel these deep incisions in ice shelf thickness imply a vulnerability to mechanical break up and climate warming of ice shelves that has not been considered previously

constraints from fluid inclusions on sulfide precipitation mechanisms and ore fluid migration in the viburnum trend lead district missouri measurements on fluid inclusions in hydrothermal dolomite cements place constraints on sulfide precipitation mechanisms and on the thermal hydrologic processes which formed the viburnum trend mississippi valley type lead district homogenization temperatures and freezing point depressions were determined for fluid inclusions in bonneterre dolomite hosted dolomite cements in mine samples as well as drill core from up to 13 km outside of the district a well defined cathodoluminescent zonation distinguishes dolomite growth zones in the vi burnurn trend as older or younger than main stage mineralization octahedral galena and facilitates correlation with other dolomites outside the viburnum trend homogenization temperatures and salinities in samples from mines are not systematically different from those of samples outside of the district medians of homogenization temperature distributions differ by not more than 25 degrees c so that a temperature gradient if present should not have exceeded approximately 25 degrees c within the study area these observations are interpreted to indicate that the viburnum trend was not strongly thermally anomalous with respect to surrounding country rock and that fluid flow occurred on a broad scale through not only the lamotte sandstone but through the overlying cambrian carbonates as well the absence of a significant recognizable decrease in temperature either vertically within the section or east west across the district coupled with the minor amount of silica in the district argues against cooling as a primary cause of sulfide precipitation fluids whose primary aquifer was the lamotte sandstone predominantly a quartz arenite should have been in equilibrium with quartz quartz in the viburnum trend occurs as a minor drusy vug lining phase but the district lacks the intense silicification found in other mississippi valley type districts such as tri state oklahoma kansas missouri quartz solubility is strongly temperature dependent and under equilibrium conditions a decrease of 10 degrees c or more should have precipitated at least as many moles of silica as galena assuming a galena solubility of between 1 and 10 ppm clearly this is not the case as galena is far more abundant than quartz in the viburnum trend ice final melting temperatures t m in fluid inclusions generally range from 14 degrees to 27 degrees c for primary dolomite hosted inclusions using these t m values and cation ratios for the inclusion fluids absolute concentrations for the individual cations and chloride were calculated using the thermochemical model of spencer et al 1990 the corresponding high but variable salinities 3 9 to 5 9 chloride molality are evidence for the presence of more than one distinct fluid during mineralization in a reduced sulfur mineralization model with pb carried as chloride complexes dilution is also a possible sulfide precipitation mechanism the difference in pb solubility for an equal quantity of reduced sulfur in the extremes of the chloride concentration range 3 9 vs 5 9 molal reaches 1 ppm only for ph values below approximately 4 5 accepting 1 ppm as a minimum metal concentration for a viable ore forming fluid dilution only appears capable of precipitating sulfides in a fluid with ph near the lower limit of values considered geologically reasonable or attainable dolomite cements hosting warm approximately 105 degrees 125 degrees c saline fluid inclusions are ubiquitous in the porous dolomitic facies of the bonneterre dolomite based on stratigraphic reconstructions however it is unlikely that the bonneterre was buried deeper than 1 5 km the distribution of warm inclusions beyond the viburnum trend district implies that fluid migration was regional in scale fluid inclusion temperatures inconsistent with typical basement heat flow controlled geothermal gradients 25 degrees 35 degrees c km may be explained by long distance migration of warm basin derived brines elevated temperatures observed in fluid inclusions at shallow stratigraphic depths are consistent with a gravity flow hydrologic system characterized by rapid flow rates and the capacity for advective heat transport

basin waves on a seafloor recording of the 1990 upland california earthquake implications for ground motions from a larger earthquake abstract the velocity and displacement time series from a recording on the seafloor at 74 km from the 1990 upland earthquake m 5 6 are dominated by late arriving waves with periods of 6 to 7 sec these waves are probably surface waves traveling across the los angeles basin response spectra for the recording are in agreement with predictions from empirical regression equations and theoretical models for periods less than about 1 sec but are significantly larger than those predictions for longer periods the longer period spectral amplitudes are controlled by the late arriving waves which are not included in the theoretical models and are underrepresented in the data used in the empirical analyses when the motions are scaled to larger magnitude the results are in general agreement with simulations of wave propagation in the los angeles basin by graves 1998

submarine permafrost depth from ambient seismic noise permafrost inundated since the last glacial maximum is degrading potentially releasing trapped or stabilized greenhouse gases but few observations of the depth of ice bonded permafrost ibp below the seafloor exist for most of the arctic continental shelf we use spectral ratios of the ambient vibration seismic wavefield together with estimated shear wave velocity from the dispersion curves of surface waves for estimating the thickness of the sediment overlying the ibp peaks in spectral ratios modeled for three layered 1 d systems correspond with varying thickness of the unfrozen sediment seismic receivers were deployed on the seabed around muostakh island in the central laptev sea siberia we derive depths of the ibp between 3 7 and 20 7 m 15 increasing with distance from the shoreline correspondence between expected permafrost distribution modeled response and observational data suggests that the method is promising for the determination of the thickness of unfrozen sediment

recent changes on greenland outlet glaciers aircraft laser altimeter surveys during the 1990s showed near coastal parts of the greenland ice sheet to be thinning despite slow thickening at higher elevations the ice sheet lost mass to the ocean many outlet glaciers thinned more rapidly than could be explained by increased melting during the recent warmer summers indicating dynamic imbalance between glacier velocity and upstream snow accumulation results from more recent surveys presented here show that thinning rates have increased in most coastal regions for almost half of the surveys these increases might have resulted from increases in summer melting but rapid thinning on others is indicative of dynamic changes that increased with time in particular thinning rates on the three fastest glaciers increased to tens of m a 1 after 2000 and other observations show an approximate doubling in their velocities the deep beds of these glaciers appear to have a strong influence on rates of grounding line retreat and thickness change with periods of glacier acceleration and rapid thinning initiated by flotation and break up of lightly grounded glacier snouts or break up of floating ice tongues near simultaneous thinning of these widely separated glaciers suggests that warming of deeper ocean waters might be a common cause nearby glaciers without deep beds are thinning far more slowly suggesting that basal lubrication as a result of increased surface melting has only a marginal impact on greenland outlet glacier acceleration

sulfur isotope evidence for penetration of mvt fluids into igneous basement rocks southeast missouri usa previous studies of galena and sphalerite from paleozoic mvt deposits in the viburnum trend southeast missouri documented large variations in 34s values throughout the ore forming event the present study of cu fe sulfides reveals a similar 34s variation that reflects two end member sulfur reservoirs whose relative importance varied both temporally and spatially more 34s enriched sulfides 34s approaching 25 indicate introduction of sulfur from basinal sedimentary sources whereas more 32s enriched sulfides 34s 600 m vertically through underlying precambrian igneous rocks such basement involvement has been suggested for other carbonate hosted base metal districts e g irish base metal deposits and should be considered an integral part of the ore forming process in southeast missouri

interpretation of seismic vertical amplification observed at an array site strong ground motions were recorded at port island kobe by a borehole array during the 1995 hyogo ken nanbu kobe earthquake these records indicate that while the horizontal peak accelerations were reduced as seismic waves traveling from the bottom to the surface motions in the vertical direction were significantly amplified at the surface with peak value of 1 5 to 2 times larger than the horizontal components some studies have discussed local site effects on the horizontal motions whereas the available study on the vertical amplification is limited in this article we present a possible mechanism to explain the observed vertical amplification in detail we consider that the observed large vertical amplification is mainly associated with incomplete saturation of near surface soils which causes substantial amplification of p waves but does not affect the propagation of s waves based on the concept of homogeneous pore fluid and biot s theory of two phase media we discuss the characteristics of p wave velocity poisson s ratio and degree of saturation in shallow soil layers and show evidence of incomplete saturation of near surface soils at the array site we analyze a simple model to theoretically investigate the effects of saturation on vertical motion amplification the results show that the degree of saturation may produce substantial influence on the amplification both amplitude and frequency content using a solid fluid coupled finite element procedure we perform a simulation of the observed vertical motions at the array site by including the effects of saturation the results demonstrate the mechanism of large amplification caused by incomplete saturation of near surface layers the present study indicates that vertical component motions may significantly be affected by pore water saturation of soils suggesting that we may need to carefully examine the condition of saturation in the study of vertical site amplification manuscript received 24 may 1999

changes in the velocity structure of the greenland ice sheet using satellite radar interferometry observations of greenland we detected widespread glacier acceleration below 66 north between 1996 and 2000 which rapidly expanded to 70 north in 2005 accelerated ice discharge in the west and particularly in the east doubled the ice sheet mass deficit in the last decade from 90 to 220 cubic kilometers per year as more glaciers accelerate farther north the contribution of greenland to sea level rise will continue to increase

marine ice in larsen ice shelf 1 it is argued that larsen ice shelf contains marine ice formed by oceanic freezing and other mechanisms missing basal returns in airborne radar soundings and observations of a smooth and healed surface coincide downstream of regions where an ocean model predicts freezing visible imagery suggests that marine ice currently stabilizes larsen c ice shelf and implicates failure of marine flow bands in the 2002 larsen b ice shelf collapse ocean modeling indicates that any regime change towards the incursion of warmer modified weddell deep water into the larsen c cavity could curtail basal freezing and its stabilizing influence

mineralogy paragenesis and mineral zoning of the west fork mine virburnum trend southeast missouri asarco9s west fork mine the newest mine in the viburnum trend began production in 1985 and reached full production in 1988 although significant mineralogical and paragenetic studies have been performed on the other ore deposits of the viburnum trend no attempt has been made to correlate these studies with mineral zoning patterns the distinct mineral zoning present at the west fork mine provides a unique opportunity to relate mineral paragenetic sequence to mineral zoning the present study concentrated on the main ore horizon m bed which is composed of four north south trending linear metal zones which together consist of an inner zone of colloform iron and zinc sulfides bordered outward by zones of disseminated iron and zinc sulfides and fringed by outermost zones of galena that also overprint the inner zone principal metallic sulfides in the ores at the west fork mine are pbs galena zns sphalerite and wurtzite and fes 2 pyrite and marcasite paragenetic sequence and metal zoning show that the ore forming fluids migrated initially through the inner zone where zns was deposited and subsequently traveled outward depositing fes 2 followed by pbs the presence of rapidly deposited colloform zns and fes 2 and bladed galena in the central fe zn zone indicates that ore fluids which deposited the sulfides in the central zone were more saturated with respect to metal sulfides outer layers of the colloform zns bodies in the fe zn zone are characterized by an abundant complex intergrowth of sphalerite and wurtzite schalenblende x ray analysis indicates that the wurtzite within the schalenblende inverted to sphalerite thereby producing pseudomorphous sphalerite after wurtzite because wurtzite is the sulfur deficient form of zns it can be postulated that sulfur fugacities were low at the end of the main stage of zinc sulfide deposition subsequent increase in sulfur fugacity during the main stage of iron deposition caused inversion of wurtzite to sphalerite interlayered pyrite and marcasite are evidence that the ore fluids during the main stage of deposition were oscillating above and below a ph of 5 minerals subsequently deposited in the outer zones were precipitated more slowly from fluids that were relatively depleted in metals the distinct differences in ore character between the metal zones may well be a result of multiple fluids separate fluids may have been channelized each fluid representing a zone or may have mixed through time the results of this study are not compatible with ore deposition from a single fluid source

impact of tide topography interactions on basal melting of larsen c ice shelf antarctica 1 basal melting of ice shelves around antarctica contributes to formation of antarctic bottom water and can affect global sea level by altering the offshore flow of grounded ice streams and glaciers tides influence ice shelf basal melt rate wb by contributing to ocean mixing and mean circulation as well as thermohaline exchanges with the ice shelf we use a three dimensional ocean model thermodynamically coupled to a nonevolving ice shelf to investigate the relationship between topography tides andwb for larsen c ice shelf lcis in the northwestern weddell sea antarctica using our best estimates of ice shelf thickness and seabed topography we find that the largest modeled lcis melt rates occur in the northeast where our model predicts strong diurnal tidal currents 0 4 m s 1 this distribution is significantly different from models with no tidal forcing which predict largest melt rates along the deep grounding lines we compare several model runs to explore melt rate sensitivity to geometry initial ocean potential temperature 0 thermodynamic parameterizations of heat and freshwater ice ocean exchange and tidal forcing the resulting range of lcis averagedwb is 0 11 0 44 m a 1 the spatial distribution of wb is very sensitive to model geometry and thermodynamic parameterization while the overall magnitude of wb is influenced by 0 these sensitivities in wbpredictions reinforce a need for high resolution maps of ice draft and sub ice shelf seabed topography together with ocean temperature measurements at the ice shelf front to improve representation of ice shelves in coupled climate system models

numerical modeling of ocean ice interactions under pine island bay s ice shelf 1 a two dimensional numerical model is used to simulate the dynamics of buoyant meltwater rich plumes flowing beneath the ice shelf occupying much of pine island bay west antarctica recent studies have shown that this ice shelf along with all others fringing the amundsen sea is thinning rapidly in the model both the coriolis effect and subshelf topography are important in controlling plume dynamics and the spatial distribution of ice melt melt is concentrated in a narrow zone within 20 km of the grounding line where steep subshelf slopes and access to warm ambient water allow melt rates to exceed 100 m yr 1 the plume generated by entrainment of ambient water into the meltwater in these areas is guided by the topography of the ice shelf underside and exits the ice shelf at three distinct outflow locations melt rates generated along the course of the plume are higher approximately 2 5 than rates elsewhere the model suggests that the observed ice shelf thinning rates could have resulted from a hypothetical instantaneous 0 25 c warming of the ambient water entrained by the plume a context for this value is provided by the 40 year warming trend documented by jacobs et al 2002 for circumpolar deep water in the nearby ross sea

variability of basal melt beneath the pine island glacier ice shelf west antarctica observations from satellite and airborne platforms are combined with model calculations to infer the nature and efficiency of basal melting of the pine island glacier ice shelf west antarctica by ocean waters satellite imagery shows surface features that suggest ice shelf wide changes to the ocean s influence on the ice shelf as the grounding line retreated longitudinal profiles of ice surface and bottom elevations are analyzed to reveal a spatially dependent pattern of basal melt with an annual melt flux of 40 5 gt a 1 one profile captures a persistent set of surface waves that correlates with quasi annual variations of atmospheric forcing of amundsen sea circulation patterns establishing a direct connection between atmospheric variability and sub ice shelf melting ice surface troughs are hydrostatically compensated by ice bottom voids up to 150 m deep voids form dynamically at the grounding line triggered by enhanced melting when warmer than average water arrives subsequent enlargement of the voids is thermally inefficient 4 or less compared with an overall melting efficiency beneath the ice shelf of 22 residual warm water is believed to cause three persistent polynyas at the ice shelf front seen in landsat imagery landsat thermal imagery confirms the occurrence of warm water at the same locations

committed sea level rise for the next century from greenland ice sheet dynamics during the past decade we use a three dimensional higher order ice flow model and a realistic initial condition to simulate dynamic perturbations to the greenland ice sheet during the last decade and to assess their contribution to sea level by 2100 starting from our initial condition we apply a time series of observationally constrained dynamic perturbations at the marine termini of greenland s three largest outlet glaciers jakobshavn isbrae helheim glacier and kangerdlugssuaq glacier the initial and long term diffusive thinning within each glacier catchment is then integrated spatially and temporally to calculate a minimum sea level contribution of approximately 1 0 4 mm from these three glaciers by 2100 based on scaling arguments we extend our modeling to all of greenland and estimate a minimum dynamic sea level contribution of approximately 6 2 mm by 2100 this estimate of committed sea level rise is a minimum because it ignores mass loss due to future changes in ice sheet dynamics or surface mass balance importantly 75 of this value is from the long term diffusive response of the ice sheet suggesting that the majority of sea level rise from greenland dynamics during the past decade is yet to come assuming similar and recurring forcing in future decades and a self similar ice dynamical response we estimate an upper bound of 45 mm of sea level rise from greenland dynamics by 2100 these estimates are constrained by recent observations of dynamic mass loss in greenland and by realistic model behavior that accounts for both the long term cumulative mass loss and its decay following episodic boundary forcing

submarine melting of the 1985 jakobshavn isbrae floating tongue and the triggering of the current retreat 1 photogrammetric reanalysis of 1985 aerial photos has revealed substantial submarine melting of the floating ice tongue of jakobshavn isbrae west greenland the thickness of the floating tongue determined from hydrostatic equilibrium tapers from 940 m near the grounding zone to 600 m near the terminus feature tracking on orthophotos shows speeds on the july 1985 ice tongue to be nearly constant 18 5 m d 1 indicating negligible dynamic thinning the thinning of the ice tongue is mostly due to submarine melting with average rates of 228 49 m yr 1 0 62 0 13 m d 1 between the summers of 1984 and 1985 the cause of the high melt rate is the circulation of warm seawater thermal forcing of up to 4 2 c beneath the tongue with convection driven by the substantial discharge of subglacial freshwater from the grounding zone we believe that this buoyancy driven convection is responsible for a deep channel incised into the sole of the floating tongue a dramatic thinning retreat and speedup began in 1998 and continues today the timing of the change is coincident with a 1 1 c warming of deep ocean waters entering the fjord after 1997 assuming a linear relationship between thermal forcing and submarine melt rate average melt rates should have increased by 25 57 m yr 1 sufficient to destabilize the ice tongue and initiate the ice thinning and the retreat that followed

an ice shelf model test based on the ross ice shelf a standard numerical experiment featuring the ross ice shelf antarctica is presented as a test package for the development and intercomparison of ice shelfmodels the emphasis of this package is solution of stress equilibrium equations for an ice shelf velocity consistent with present observations as ademonstration we compare five independently developed ice shelf models based on finite difference and finite element methods our results suggest thatthere is little difference between finite element and finite difference methods in capturing the basic large scale flow features of the ice shelf we additionallyshow that the fit between model and observed velocity depends strongly on the ice shelf temperature field for which there is presently little observationalcontrol the main differences between model results are due to the equations being solved the boundary conditions at the ice front and the discretisationmethod finite element vs finite difference

an atypical orebody in the brushy creek mine viburnum trend mvt district missouri early cu ni co zn rich ores at the lamotte sandstone bonneterre dolomite contact mississippi valley type mvt deposits of the viburnum trend are typically lead dominant pb zn 5 and occur mainly in the reef grainstone facies of the upper bonneterre dolomite cambrian recent drilling has encountered economic mineralization within the lower bonneterre dolomite of the brushy creek mine more than 30 m below the main ore bearing horizon of the district these ores are more zinc and copper rich than typical deposits zn pb commonly 2 and cu up to 8 wt with notable enrichments in ni co and ag there is pronounced zoning of ni co cu zn and pb with increasing distance above the lamotte sandstone observations of drill cores and samples from active mine faces and mineral textural relationships in thin section indicate that the lower ore mineralization contains dominantly early zn and cu bearing sulfides which are overprinted by main stage pb zn mineralization the lower ores contain multiple generations of chalcopyrite and sphalerite that display frequent breccia textures at the meso and microscale this style of breccia is uncommon for the viburnum trend and is composed dominantly of sulfides supported by clay insoluble residue which are cemented by subsequent generations of sulfides and carbonates resulting in massive high grade ore cathodoluminescence microscopy reveals that ore is associated with two generations of dolomite cement that appear to predate the regional dolomite cement associated with main stage pb zn mineralization in the viburnum trend the stratigraphic position massive character metal contents mineral zoning and textures and gangue dolomite cements of the orebody are unusual for the viburnum trend mineralogically and stratigraphically these ores are more similar to the historically mined ores of the indian creek and fredericktown mine la motte districts to the north and east of the viburnum trend in the brushy creek mine the lower ores are not related to obvious stratigraphic controls such as pinch outs of the lamotte sandstone against precambrian knobs but appear to be localized along early zones of fracture enhanced porosity and permeability within the lamotte sandstone which promoted extreme dissolution of host rocks at the base of the bonneterre dolomite and development of high grade cu ni co zn rich ore the presence of distinct ni and ni co rich areas and the spatial variability of sphalerite types across the orebody indicate that there were likely multiple sites of ore fluid introduction whose influence varied with time during development of the ores

ocean circulation and properties in petermann fjord greenland the floating ice shelf of petermann glacier interacts directly with the ocean and isthought to lose at least 80 of its mass through basal melting based on three opportunisticocean surveys in petermann fjord we describe the basic oceanography the circulationat the fjord mouth the hydrographic structure beneath the ice shelf the oceanic heatdelivered to the under ice cavity and the fate of the resulting melt water the 1100 m deepfjord is separated from neighboring hall basin by a sill between 350 and 450 m deep fjord bottom waters are renewed by episodic spillover at the sill of atlantic water from thearctic glacial melt water appears on the northeast side of the fjord at depths between200 m and that of the glacier s grounding line about 500 m the fjord circulation isfundamentally three dimensional satellite imagery and geostrophic calculations suggest acyclonic gyre within the fjord mouth with outflow on the northeast side tidal flowsare similar in magnitude to the geostrophic flow the oceanic heat flux into the fjordappears more than sufficient to account for the observed rate of basal melting cold low salinity water originating in the surface layer of nares strait in winter intrudes farunder the ice this may limit basal melting to the inland half of the shelf the melt rate andlong term stability of petermann ice shelf may depend on regional sea ice cover andfjord geometry in addition to the supply of oceanic heat entering the fjord

minor and trace element distributions in the bonneterre dolomite cambrian southeast missouri evidence for possible multiple basin fluid sources and pathways during lead zinc mineralization the sources and flow paths of basinal fluids that precipitated the dolomite hosted mississippi valley type orebodies of southeast missouri have historically been a subject of debate this study presents geochemical data for epigenetic dolomites of the bonneterre dolomite directly above the lamotte sandstone aquifer and from the viburnum trend orebodies samples of epigenetic replacement dolomite were collected at the bonneterre dolomite lamotte sandstone contact from 35 drill cores covering an area of more than 25 000 km 2 west of the st francois mountains there are distinct aerial trends in fe mn and sr contents of the dolomite from values of 3 47 wt 0 38 wt and 27 ppm in the south to 0 82 wt 0 12 wt and 54 ppm in the north respectively in and near the viburnum trend lead zinc subdistrict the distributions of fe mn and sr are reversed from values of 0 50 wt 0 10 wt and 105 ppm in the south to 3 15 wt 0 55 wt and 40 ppm in the north respectively fe and mn contents of gangue dolomite cement in the viburnum trend orebodies show a similar south to north enrichment ranging from 1 41 mole feco 3 and 0 11 mole mnco 3 in the south to 2 34 mole and 0 23 mole in the north respectively on the basis of fractionation of minor and trace elements between fluids and dolomites the data are interpreted to indicate a regional south to north flow of water from the arkoma basin through the lamotte sandstone a second fluid flow system with a northern source possibly the illinois basin precipitated the late phases of gangue dolomite cement in the viburnum trend and may have been active during the earlier precipitation of the dolomite at the lamotte bonneterre contact the inferred fluid flow paths suggest a more complex multiple basin fluid involvement than previously suggested for the mississippi valley type pb zn mineralization of southeast missouri

origin characteristics and distribution of fault related and fracture related dolomitization insights from mississippian carbonates isle of man abstract the southwest davis zinc field sdzf is among the westernmost mvt districts associated with the allegheny ouachita orogeny in the midcontinent u s it consists of a series of small zinc sulfide deposits and associated dolomitization covering an area of 16 km2 6 mi2 adjacent to the arbuckle anticline in southern oklahoma mineralization occurs within the royer dolomite upper cambrian and the butterly dolomite lower ordovician of the arbuckle group and is associated with faults and breccia bodies that parallel the trend of the anticline mining activity in the sdzf was restricted to the early part of the 20th century ore mineralization consists chiefly of sphalerite and subordinate smithsonite with minor galena chalcopyrite and pyrite as well as supergene alterations of these minerals homogenization temperatures th of fluid inclusions in gangue dolomite cement range from 88 to 101 c and have salinities of 6 5 to 20 9 wt nacl equivalent rarer fluid inclusions in sphalerite have an average th value of 105 c and average salinity of 19 2 wt nacl equivalent gangue dolomite cements have 18o values of 6 9 to 9 1 vpdb and 13c values of 0 2 to 1 1 vpdb the 87sr 86sr ratios for these dolomites range from 0 7088 to 0 7105 mineralization likely occurred from a flow system in which basement influenced brines entered the host formations through faults and circulated through the brecciated zones as the ore hydrothermal system waned the relative importance of local brines within the carbonate section increased and resulted in precipitation of late dolomite cements

reactive flow models of ore formation in the southeast missouri district two dimensional numerical reactive transport modeling was used to investigate the deposition of pb zn cu ores in the southeast missouri district central united states by two potential mechanisms cooling of a fluid carrying metals and reduced sulfur and mixing of two fluids one enriched in metals but poor in sulfur and the other enriched in reduced sulfur but poor in metals the mechanisms were studied within the context of the topography driven ground water flow regime established in the ozark region by the late paleozoic alleghanian orogeny the models integrated geology fluid flow heat and solute transport and chemical reaction generating the most complete depiction yet of potential ore forming processes in southeast missouri both mechanisms were found capable of generating important characteristics of the mineralization patterns observed in the field but fell short in other respects in models of the cooling hypothesis mineralization was indeed found to be concentrated within the southeast missouri portion of the ozark region this was primarily because of the fact that mineralizing fluid underwent very little temperature change as it traversed the ozark region until it arrived in southeast missouri where its temperature decreased significantly the important association of mineralization with local basement highs within the southeast missouri district was also produced caused by local increases of fluid velocity over the crests of the basement highs smaller scale models however revealed much of the mineralization to be concentrated within the basal lamotte sandstone rather than the overlying bonneterre dolomite where it is actually concentrated in the field this model result stemmed directly from the fact that permeability and therefore fluid velocities were higher in the lamotte than in the bonneterre a further discrepancy with field observations was the high predicted concentrations of quartz which exceeded those of galena by over two orders of magnitude models of the mixing hypothesis also led to the regional concentration of mineralization in southeast missouri provided vertical dispersivities in the lamotte sandstone and bonneterre dolomite the formations that transmitted the two different types of fluids were low when dispersivities were high mineralization was produced more uniformly along the flow path in further conformity with field observations ore was concentrated in the bonneterre dolomite and particularly around the basement highs demonstrating the importance of the lamotte pinch outs in causing the fluids to mix the models reproduced well the observed vertical zonation in metals which was controlled by the relative solubility differences among the primary sulfide ore minerals however though mixing was induced by the configuration of the local geology its intensity was relatively low leading to ore grades that were considerably less than would be expected from geochemical grounds alone because the flow regime is predominantly laminar the two fluids tended to miscibly displace one another rather than vigorously mix with one another while still potentially a viable ore forming mechanism under conditions of high local dispersivity high dissolved metal and sulfur concentrations or high fluid velocity the mixing hypothesis as typically formulated in southeast missouri is nonetheless much less efficient than previously supposed greater efficiency could possibly be generated by reaction of aqueous species with solid phases in the matrix or gas e g h 2 s being generated in the matrix

ice shelf water plume flow beneath filchner ronne ice shelf antarctica 1 a two dimensional plume model is used to study the interaction between filchner ronne ice shelf antarctica and its underlying ocean cavity ice shelf water isw plumes are initiated by the freshwater released from a melting ice shelf and if they rise may become supercooled and deposit marine ice due to the pressure increase in the in situ freezing temperature the aim of this modeling study is to determine the origin of the thick accretions of marine ice at the base of filchner ronne ice shelf and thus improve our understanding of isw flow paths the model domain is defined from measurements of ice shelf draft and from this isw the model is able to predict plumes that exit the cavity in the correct locations the modeled plumes also produce basal freezing rates that account for measured marine ice thicknesses in the western part of ronne ice shelf we find that the freezing rate and plume properties are significantly influenced by the confluence of plumes from different meltwater sources we are less successful in matching observations of marine ice under the rest of filchner ronne ice shelf which we attribute primarily to this model s neglect of circulations in the ocean outside the plume

in situ measurement of seismic shear wave absorption in the san francisco holocene bay mud by the pulse broadening method by the pulse broadening method seismic absorption of san francisco holocene bay mud has been determined with the use of an impulsive shear wave source at the surface two receivers embedded in clay holocene bay mud and digital seismic recording the seismic anelastic quality factor is determined from 20 measurements to be 16 0 with a random error of 0 9 three sources of systematic error are examined the fractional systematic error on the anelastic quality factor is estimated to be 11 details of experimental arrangement and error analysis are given

a geology based 3d velocity model of the los angeles basin sediments abstract seismic hazard studies of the los angeles basin area require a realistic seismic velocity model we use geologic information about depth to crystalline basement depths to sedimentary horizons uplift of sediments and surface geology in a velocity depth age function to construct a three dimensional velocity model in earthquake location tests the model predicts travel times satisfactorily and in earthquake ground motion simulations the model correctly determines the timing and amplitude of late arriving waves

observations beneath pine island glacier in west antarctica and implications for its retreat thinning ice in west antarctica is currently contributing about 10 of the observed rise in global sea level observations obtained from an autonomous underwater vehicle operating beneath pine island glacier west antarctica reveal that the glacier was recently grounded on a transverse ridge in the sea floor but now warm sea water flows through the widening gap above the ridge thinning ice in west antarctica resulting from acceleration in the flow of outlet glaciers is at present contributing about 10 of the observed rise in global sea level1 pine island glacier in particular has shown nearly continuous acceleration2 3 and thinning4 5 throughout the short observational record the floating ice shelf that forms where the glacier reaches the coast has been thinning rapidly6 driven by changes in ocean heat transport beneath it as a result the line that separates grounded and floating ice has retreated inland7 these events have been postulated as the cause for the inland thinning and acceleration8 9 here we report evidence gathered by an autonomous underwater vehicle operating beneath the ice shelf that pine island glacier was recently grounded on a transverse ridge in the sea floor warm sea water now flows through a widening gap above the submarine ridge rapidly melting the thick ice of the newly formed upstream half of the ice shelf the present evolution of pine island glacier is thus part of a longer term trend that has moved the downstream limit of grounded ice inland by 30 km into water that is 300 m deeper than over the ridge crest the pace and ultimate extent of such potentially unstable retreat10 are central to the debate over the possibility of widespread ice sheet collapse triggered by climate change11 12

the effects of rotation and ice shelf topography on frazil laden ice shelf water plumes a model of the dynamics and thermodynamics of a plume of meltwater at the base of an ice shelf is presented such ice shelf water plumes may become supercooled and deposit marine ice if they rise because of the pressure decrease in the in situ freezing temperature so the model incorporates both melting and freezing at the ice shelf base and a multiple size class model of frazil ice dynamics and deposition the plume is considered in two horizontal dimensions so the influence of coriolis forces is incorporated for the first time it is found that rotation is extremely influential with simulated plumes flowing in near geostrophy because of the low friction at a smooth ice shelf base as a result an ice shelf water plume will only rise and become supercooled and thus deposit marine ice if it is constrained to flow upslope by topography this result agrees with the observed distribution of marine ice under filchner ronne ice shelf antarctica in addition it is found that the model only produces reasonable marine ice formation rates when an accurate ice shelf draft is used implying that the characteristics of real ice shelf water plumes can only be captured using models with both rotation and a realistic topography

a new three dimensional higher order thermomechanical ice sheet model basic sensitivity ice stream development and ice flow across subglacial lakes 1 a new three dimensional thermomechanically coupled ice sheet model is developed contrary to the majority of three dimensional ice sheet models shallow ice approximation higher order stress gradients longitudinal and transverse are accounted for in the force balance equations the horizontal velocity field is determined from the force balance equations in their derivative form elliptical equation using the method presented by pattyn 2000 2002a the model is solved on a regular grid in the horizontal and an irregular grid in the vertical and is numerically stable basic experiments include the european ice sheet modeling initiative eismint benchmarks for largescale ice sheet models and a comparison with the saito blatter ice sheet model including higher order stress gradients saito et al 2003 detailed calculations of ice flow over three dimensional bedrock perturbations showed the validity of the higher order solution the model is capable of simulating the evolution of an ice stream within the ice sheet and shows important aspects of observed ice stream features such as the surface flattening and the importance of side drag the simulation of the ice flow over a subglacial lake results in a flattening of the surface a local velocity increase over the lake and a deviation of the ice flow from the main flow direction features which are also observed at lake vostok antarctica index terms 1827 hydrology glaciology 1863 1863 hydrology snow and ice 1827 3230 mathematical geophysics numerical solutions keywords higher order ice sheet model thermomechanical ice stream subglacial lake citation pattyn f a new three dimensional higher order thermomechanical ice sheet model basic sensitivity ice stream development and ice flow across subglacial lakes j geophys res 108 b8 2382 doi 10 1029 2002jb002329 2003

modeling sea ice transport using incremental remapping sea ice models contain transport equations for the area volume and energy of ice and snow in various thickness categories these equations typically are solved with first order accurate upwind schemes which are very diffusive with second order accurate centered schemes which are highly oscillatory or with more sophisticated second order schemes that are computationally costly if many quantities must be transported e g multidimensional positive definite advection transport algorithm mpdata here an incremental remapping scheme originally designed for horizontal transport in ocean models is adapted for sea ice transport this scheme has several desirable features it preserves the monotonicity of both conserved quantities and tracers it is second order accurate except where the accuracy is reduced locally to preserve monotonicity and it efficiently solves the large number of equations in sea ice models with multiple thickness categories and tracers remapping outperforms the first order upwind scheme and basic mpdata scheme in several simple test problems in realistic model runs remapping is less diffusive than the upwind scheme and about twice as fast as mpdata

oceanographic regime shift during 1997 in disko bay western greenland data from a long time series of temperature salinity and nutrient measurements in disko bay west greenland reveal a marked change in the water characteristics during recent years seasonal dynamics in the upper 150 m of the water column were highly affected by the seasonality in meteorological conditions while the deeper water strata were more stable and were primarily influenced by large scale circulation patterns there was a marked increase in the average water temperatures at 200 m depth in spring 1997 with the long term average increasing from 1 30 c to 2 25 c weekly data from 1996 to 1997 show that the sudden change in bottom water occurred in april 1997 due to the inflow of a larger proportion of north atlantic water which propagated north along the coast before entering the bay further support for this inflow was found when tracing the relative proportion of atlantic water in the bay using inorganic nutrients these changes in the oceanography of the bay will not only lead to further glacial retreat but will also affect the local marine ecosystem by changing the relative dominance of major copepod species that overwinter in bottom waters of the bay

velocity and stress fields in grounded glaciers a simple algorithm for including deviatoric stress gradients a new and efficient algorithm for computing the three dimensional stress and velocity fields in grounded glaciers includes the role of deviatoric stress gradients a consistent approximation of first order in the aspect of ratio of the ice mass gives a set of eight field equations for the five stress and three velocity components and the corresponding boundary conditions a coordinate transformation mapping the local ice thickness on to unity and approximating the derivatives in the horizontal direction by centred finite differences yields five ordinary differential and three algebraic equations this allows use of the method of lines starting the integration with prescribed stress and velocity components at the base and a simple iteration procedure converges rapidly the algorithm can lie used for a wide range of stress strain rate relations as long as strain only depends on deviatoric and shear stresses and on temperature sensitivity tests using synthetic and realistic ice geometries show the relevance of normal deviatoric stresses in the solutions for the velocity components even for ice sheets stress and velocity fields may deviate substantially from the widely used shallow ice approximation

acceleration of jakobshavn isbrae triggered by warm subsurface ocean waters observations over past decades show a sudden switch of jakobshavn isbrae a large outlet glacier feeding a deep ocean fjord on greenland s west coast from slow thickening to rapid thinning in 1997 this switch is associated with a doubling in glacier velocity hydrographic data show a concurrent sudden increase in subsurface ocean temperatures along the entire west coast of greenland suggesting that the changes in jakobshavn isbrae were triggered by the arrival of relatively warm water originating from the irminger sea observations over the past decades show a rapid acceleration of several outlet glaciers in greenland and antarctica1 one of the largest changes is a sudden switch of jakobshavn isbrae a large outlet glacier feeding a deep ocean fjord on greenland s west coast from slow thickening to rapid thinning2 in 1997 associated with a doubling in glacier velocity3 suggested explanations for the speed up of jakobshavn isbrae include increased lubrication of the ice bedrock interface as more meltwater has drained to the glacier bed during recent warmer summers4 and weakening and break up of the floating ice tongue that buttressed the glacier5 here we present hydrographic data that show a sudden increase in subsurface ocean temperature in 1997 along the entire west coast of greenland suggesting that the changes in jakobshavn isbrae were instead triggered by the arrival of relatively warm water originating from the irminger sea near iceland we trace these oceanic changes back to changes in the atmospheric circulation in the north atlantic region we conclude that the prediction of future rapid dynamic responses of other outlet glaciers to climate change will require an improved understanding of the effect of changes in regional ocean and atmosphere circulation on the delivery of warm subsurface waters to the periphery of the ice sheets

effects of seawater of tokyo bay on short period strong ground motion the effects of the seawater of tokyo bay on strong ground motions are investigated in a short period range of less than about 2 0 sec for engineering importance a recent study based on observed accelerograms revealed that considerable short period surface waves were included in strong ground motions in the reclaimed zone of tokyo and that those surface waves were mainly love waves however it was not clarified whether short period rayleigh waves were present the present study is aimed at assessing the excitation of short period rayleigh waves on land and at exploring the effects of the shallow approximately 25 m water layer of tokyo bay on rayleigh waves in the reclaimed zone of tokyo we use a theoretical approach in which a 2d p sv wave field is calculated for a multilayered structural model the model is composed of a sea zone and a land zone and includes a water layer and soft surface layers to examine the effects of seawater we use models without and with a water layer as a result it is concluded that in the reclaimed zone of tokyo stripping the water layer off of a typical model for shallow tokyo bay has little influence on the short period ground motions around the theoretical predominant period of the ground of about 1 0 sec it is not essential to incorporate the water layer in modeling shallow tokyo bay for short period ground motion simulations

local site effects in the northern north sea based on single station spectral ratios of obs recordings in recent years there has been a growing awareness of the importance of local site effects in earthquake damage a number of studies of recent destructive earthquakes have illustrated the relative contribution of enhanced ground shaking due to unconsolidated sediment layers among the different methods used to estimate local site response the spectral ratio of shaking at a sedimentary site with respect to a bedrock reference site has been successfuly applied in different geological environments in this study a technique recently proposed by nakamura 1989 is used to evaluate site response using spectral ratios of horizontal vs vertical components of earthquake recordings from a temporary ocean bottom seismograph obs network in the northern north sea and a permanent obs at oseberg oil field comparison with results obtained from the standard spectral ratios indicate that the method is applicable also to subsea conditions and the estimates obtained in this study indicate similar amplification factors to those obtained previously from analytical techniques the ambient noise data on the other hand gave unstable results probably due to different noise characteristics in the marine environment the results obtained on the earthquake data however provide an encouraging alternative to previously used analytical techniques for estimating local site response

stable isotope and fluid inclusion studies of early zn cu ni co rich ores lower ore zone of brushy creek mine viburnum trend mvt district missouri u s a products of multiple sulfur sources and metal specific fluids abstract mississippi valley type mvt deposits of the viburnum trend are typically lead dominant and occur in the upper portion of the cambrian bonneterre dolomite unusual zinc and copper rich ores with notable enrichments in ni co and ag have been discovered within the lower bonneterre dolomite more than 30 m below the district s main ore bearing horizon these ores appear to be localized along fault fracture zones within the lamotte sandstone which promoted extreme dissolution of the base of the overlying bonneterre dolomite and resulted in pronounced vertical zoning of early ni co cu zn and pb ores above the sandstone dolomite contact stable isotope and fluid inclusion studies were undertaken to assess whether the deep ores resulted from a single fluid reservoir that evolved from cu ni co rich to zn rich to pb rich or are instead products of distinct fluids unrelated to the stratigraphically higher main pb rich ores of the district sphalerite hosted fluid inclusions from the lower orebody appear to be geochemically distinct from those that formed the upper orebodies they record the highest k na and mg na ratios in the southeast missouri mvt district suggesting dominance of siliceous lithologies arkose and or felsic igneous rocks over limestones along their ore fluid pathways paragenetic trends indicate that distinct fluid compositions are associated with specific generations of sphalerite which likely reflects the presence of multiple fluids the 34s values of ore minerals early pyrite and chalcopyrite 7 to 5 early sphalerite 6 to 15 main sphalerite 7 to 17 v cdt indicate deposition from fluids that utilized isotopically distinct sulfide reservoirs low 34s sulfur sources for early ores likely included sulfide in local brines within the lamotte sandstone and diagenetic sulfide minerals within the basal units of the bonneterre dolomite a trend of increasing 34s values of ore sulfides from 5 toward 17 with vertical distance above the lamotte sandstone bonneterre dolomite contact indicates that as the deep ore fluid system worked its way upward it breached less permeable units in the lower bonneterre dolomite allowing incorporation of high 34s sulfide from brines present higher in the stratigraphic section a concomitant up section pattern of decreasing 18o values of dolomite cement and recrystallized remnant host rock from 3 toward 9 v pdb is also consistent with a localized deep mineralizing fluid system which interacted progressively with the stratigraphically higher regional scale pb rich fluid system the episodic nature of metal sulfide deposition in the lower ore zone points to a system in which fluid mixing would have been highly variable both temporally and spatially reaction path models require mixing of multiple ore metal specific and sulfide bearing fluids in order to form the orebody the sequence and concentrations of metal sulfides are inconsistent with ore formation from a single evolving metal bearing fluid this study indicates that faults localized early metal and sulfide bearing fluids facilitating mixing of fluids that resulted in the accumulation of high grade ores near the bonneterre dolomite lamotte sandstone contact recognition of fault fracture patterns beneath more typical pb rich orebodies in the viburnum trend may be a viable exploration strategy for similar zn cu co ni rich deposits

mafic and ultramafic plutons associated with the new madrid rift complex a possible major source of the copper cobalt nickel mineralization of southeast missouri the main rift structure of the new madrid rift complex the reelfoot rift strikes northeast ward extending from northeastern arkansas through the missouri boot heel and western kentucky into southern illinois the recently discovered bloomfield lineament zone transects the reelfoot rift in the boot heel area and strikes north northwest passing through the mine la motte fredericktown subdistrict of the southeast missouri lead zinc copper mining district studies suggest a strong relationship between the proximity of the new madrid rift complex to the southeast missouri district and the anomalously high amounts of cu co and ni in the region particularly in the mine la motte fredericktown subdistrict supporting evidence comes from three sources 1 mineral zoning and paragenetic sequence 2 geotectonics of the new madrid rift complex and 3 regional variations of 87 sr 86 sr and 18 o 16 o isotope ratios a detailed microscopic and sem eds study of the pb zn cu ores from the viburnum no 28 brushy creek and goose creek mines in the southeast missouri district indicates a complicated paragenetic sequence characterized by repeated periods of mineral deposition the sequence is composed of 19 minerals including abundant galena sphalerite chalcopyritc pyrite marcasite dolomite calcite and quartz as well as minor siegenite bornite bravoite blaubleibender covellite covellite gersdorffite digenite enargite tennantite dickite and malachite in addition to the mineralogy and paragenetic sequence a number of significant findings resulted from a comprehensive microscopic study of ore textures from the three mines the paragenetic sequence of the main colloform stage of deposition indicates that the ore solutions evolved in the following general order 1 cu rich 2 co ni rich 3 pb zn rich and 4 fe rich the general depositional sequence corresponds with mineral zoning and is similar throughout the entire lead district most importantly the paragenetically early cu co ni sulfides may represent a regional early development of the cu co ni rich mineralization similar to that in the mine la motte fredericktown subdistrict and the southern extension of the viburnum trend this early cu co ni rich zone always lies at the lowest stratigraphic level and in the mine la motte fredericktown subdistrict indicates a southerly source for the ore fluids it is proposed that the source of at least part of the cu co ni and some siderophile elements of the southeast missouri lead deposits may be the result of basinal brine leaching of a series of probably multiaged mafic and ultramafic intrusions or their sediments controlled by the new madrid rift complex one such branch of the complex observed on landsat 2 imagery is expressed as a fracture zone that extends north northwest from the reelfoot rift toward fredericktown missouri namely the bloomfield lineament zone strong 87 sr 86 sr and 18 o 16 o isotope anomalies show respectively a consistent decrease and a consistent increase from south to north in southeast missouri with the contour lines strikingly tracking the bloomfield fracture zone this suggests that the lineament zone was a channel for ore fluids entering southeast missouri via the mississippi embayment the fracture zone may represent a series of transform faults or a portion of a failed continental rifting event the zone is an epicenter trend and a distinct first order lineament which appears to form part of the northeast boundary of the northwest trending missouri gravity low

antarctic ice sheet loss driven by basal melting of ice shelves accurate prediction of global sea level rise requires that we understand the cause of recent widespread and intensifying glacier acceleration along antarctic ice sheet coastal margins atmospheric and oceanic forcing have the potential to reduce the thickness and extent of floating ice shelves potentially limiting their ability to buttress the flow of grounded tributary glaciers indeed recent ice shelf collapse led to retreat and acceleration of several glaciers on the antarctic peninsula but the extent and magnitude of ice shelf thickness change the underlying causes of such change and its link to glacier flow rate are so poorly understood that its future impact on the ice sheets cannot yet be predicted here we use satellite laser altimetry and modelling of the surface firn layer to reveal the circum antarctic pattern of ice shelf thinning through increased basal melt we deduce that this increased melt is the primary control of antarctic ice sheet loss through a reduction in buttressing of the adjacent ice sheet leading to accelerated glacier flow the highest thinning rates occur where warm water at depth can access thick ice shelves via submarine troughs crossing the continental shelf wind forcing could explain the dominant patterns of both basal melting and the surface melting and collapse of antarctic ice shelves through ocean upwelling in the amundsen and bellingshausen seas and atmospheric warming on the antarctic peninsula this implies that climate forcing through changing winds influences antarctic ice sheet mass balance and hence global sea level on annual to decadal timescales

ocean regulation hypothesis for glacier dynamics in southeast greenland and implications for ice sheet mass changes synchronous acceleration and thinning of southeast se greenland glaciers during the early 2000s was the main contributor that resulted in the doubling of annual discharge from the ice sheet we show that this acceleration was followed by a synchronized and widespread slowdown of the same glaciers in many cases associated with a decrease in thinning rates and we propose that ice sheet ocean interactions are the first order regional control on these recent mass changes sea surface temperature and mooring data show that the preceding dynamic thinning coincides with a brief decline in the cold east greenland coastal current egcc and east greenland current we suggest this decline was partly induced by a reduction in ice sheet runoff which allowed warm water from the irminger current to reach the se greenland coast a restrengthening of the cold waters coincides with the glaciers subsequent slowdown we argue that this warming and subsequent cooling of the coastal waters was the cause of the glaciers dynamic changes we further suggest that the restrengthening of the egcc resulted in part from cold water input by increased glacier calving during the speedup and increased ice sheet runoff we hypothesize that the main mechanism for ice sheet mass loss in se greenland is highly sensitive to ocean conditions and is likely subject to negative feedback mechanisms copyright 2010 by the american geophysical union

analysis of earthquake recordings obtained from the seafloor earthquake measurement system sems instruments deployed off the coast of southern california for more than 20 years a program has been underway to obtain records of earthquake shaking on the seafloor at sites offshore of southern california near oil platforms the primary goal of the program is to obtain data that can help deter mine if ground motions at offshore sites are significantly different than those at onshore sites if so caution may be necessary in using onshore motions as the basis for the seismic design of oil platforms we analyze data from eight earthquakes recorded at six offshore sites these are the most important data recorded on these stations to date seven of the earthquakes were recorded at only one offshore station the eighth event was recorded at two sites the earthquakes range in magnitude from 4 7 to 6 1 because of the scarcity of multiple recordings from any one event most of the analysis is based on the ratio of spectra from vertical and horizontal compo nents of motion the results clearly show that the offshore motions have very low vertical motions compared to those from an average onshore site particularly at short periods theoretical calculations find that the water layer has little effect on the horizontal components of motion but that it produces a strong spectral null on the vertical component at the resonant frequency of p waves in the water layer the vertical to horizontal ratios for a few selected onshore sites underlain by relatively low shear wave velocities are similar to the ratios from offshore sites for frequencies less than about one half the water layer p wave resonant frequency suggesting that the shear wave velocities beneath a site are more important than the water layer in determining the character of the ground motions at lower frequencies

stability of the west antarctic ice sheet in a warming world equivalent to 0 28 to 0 56 mm yr 1 sea level rise with the rate growing over at least the past two decades 1 4 these observations intensify concerns about the ice sheet s stability that were first raised more than four decades ago 5

changes in west antarctic ice stream dynamics observed with alos palsar data the advanced land observation system alos phased array synthetic aperture radar palsar is an l band frequency 1 27 ghz radar capable of continental scale interferometric observations of ice sheet motion here we show that palsar data yield excellent measurements of ice motion compared to c band 5 6 ghz radar data because of greater temporal coherence over snow and firn we compare palsar velocities from year 2006 in pine island bay west antarctica with those spanning years 1974 to 2007 between 1996 and 2007 pine island glacier sped up 42 and ungrounded over most of its ice plain smith glacier accelerated 83 and ungrounded as well their largest speed up are recorded in 2007 thwaites glacier is not accelerating but widening with time and its eastern ice shelf doubled its speed total ice discharge from these glaciers increased 30 in 12 yr and the net mass loss increased 170 from 39 15 gt yr to 105 27 gt yr longer term velocity changes suggest only a moderate loss in the 1970s as the glaciers unground into the deeper smoother beds inland the mass loss from this region will grow considerably larger in years to come copyright 2008 by the american geophysical union

adjoint sensitivities of sub ice shelf melt rates to ocean circulation under the pine island ice shelf west antarctica we investigate the sensitivity of sub ice shelf melt rates under pine island ice shelf west antarctica to changes in the oceanic state using an adjoint ocean model that is capable of representing the flow in sub ice shelf cavities the adjoint code is based on algorithmic differentiation ad of the massachusetts institute of technology s ocean general circulation model mitgcm the adjoint model was extended by adding into the ad process the corresponding sub ice shelf cavity code which implements a three equation thermodynamic melt rate parameterization to infer heat and freshwater fluxes at the ice shelf ocean boundary the inferred sensitivities reveal dominant timescales of 30 60 days over which the shelf exit is connected to the deep interior via advective processes they exhibit rich three dimensional time evolving patterns that can be understood in terms of a combination of the buoyancy forcing by inflowing water masses the cavity geometry and the effect of rotation and topography in steering the flow in the presence of prominent features in the bedrock bathymetry dominant sensitivity pathways are found over a sill as well as shadow regions of very low sensitivities to the extent that these transient patterns are robust they carry important information for decision making in observation deployment and monitoring

ice shelf basal channels in a coupled ice ocean model a numerical model for an interacting ice shelf and ocean is presented in which the iceshelf base exhibits a channelized morphology similar to that observed beneath petermann gletscher s greenland floating ice shelf channels are initiated by irregularities in the ice along the grounding line and then enlarged by ocean melting to a first approximation spatially variable basal melting seaward of the grounding line acts as a steel rule die or a stencil imparting a channelized form to the ice base as it passes by ocean circulation in the region of high melt is inertial in the along channel direction and geostrophically balanced in the transverse direction melt rates depend on the wavelength of imposed variations in ice thickness where it enters the shelf with shorter wavelengths reducing overall melting petermann gletscher s narrow basal channels may therefore act to preserve the ice shelf against excessive melting overall melting in the model increases for a warming of the subsurface water the same sensitivity holds for very slight cooling but for cooling of a few tenths of a degree a reorganization of the spatial pattern of melting leads surprisingly to catastrophic thinning of the ice shelf 12 km from the grounding line subglacial discharge of fresh water along the grounding line increases overall melting the eventual steady state depends on when discharge is initiated in the transient history of the ice showing that multiple steady states of the coupled system exist in general