seismicity related to the glacier of cotopaxi volcano ecuador 1 significant seismic activity is generally recorded on volcanoes covered by an icecap this work was carried out in order to quantify the role of the glaciers in the generation of seismicity for cotopaxi volcano we compared the seismic activity registered on the glacier and on the rock near the snout of the north flank glacier we focused on the analysis of low frequency events 5 hz similar to volcanic lp events when recorded on rock base the particle motion analysis helps to estimate source locations which are distributed in crevasses areas high incident angles suggest a superficial origin these events are interpreted as icequakes for which we propose as source mechanism a fluid driven crack model triggered by ice cracking or hydraulic transients the low quality factor values estimated are consistent with the resonance of an ice crack filled with water this work shows that low frequency icequakes can be confusingly taken as volcanic lp events

melt under cutting and buoyancy driven calving from tidewater glaciers new insights from discrete element and continuum model simulations the simple calving laws currently used in ice sheet models do not adequately reflect the complexity and diversity of calving processes to be effective calving laws must be grounded in a sound understanding of how calving actually works here we develop a new strategy for formulating calving laws using a the helsinki discrete element model hidem to explicitly model fracture and calving processes and b the continuum model elmer ice to identify critical stress states associated with hidem calving events a range of observed calving processes emerges spontaneously from hidem in response to variations in ice front buoyancy and the size of subaqueous undercuts calving driven by buoyancy and melt under cutting is under predicted by existing calving laws but we show that the location and magnitude of hidem calving events can be predicted in elmer ice from characteristic stress patterns our results open the way to developing calving laws that properly reflect the diversity of calving processes and provide a framework for a unified theory of the calving process continuum

high resolution magnetostratigraphy of the upper nacimiento formation san juan basin new mexico usa implications for basin evolution and mammalian turnover lower paleocene deposits in the san juan basin document one of the best records of mammalian change and turnover following the cretaceous paleogene mass extinction and is the type area for the puercan pu and torrejonian to north america land mammal age nalma one of the largest mammalian turnover events in the early paleocene occurs between the torrejonian 2 to2 and torrejonian 3 to3 nalma interval zones the nacimiento formation contains the only deposits in north america where the to2 to3 mammalian turnover can be constrained however the precise age and duration of the turnover is poorly understood due to the lack of a precise chronostratigraphic framework we analyzed paleomagnetic samples produced a 40ar 39ar detrital sanidine age and developed a detailed lithostratigraphy for four sections of the upper nacimiento formation in the san juan basin new mexico kutz canyon escavada wash torreon west and east to constrain the age and duration of the deposits and the to2 to3 turnover the polarity stratigraphy for the four sections can be correlated to chrons c27r c26r of the geomagnetic polarity time scale gpts using the local polarity stratigraphy for each section we calculated a mean sediment accumulation rate and developed a precise age model which allows us to determine the age of important late torrejonian mammalian localities using the assigned ages we estimate the to2 to3 turnover was relatively rapid and occurred over 120 kyr 60 50 kyr between 62 59 and 62 47 ma this rapid duration of mammalian turnover suggests that it was driven by external forcing factors such as environmental change driven by the progradation of the distributive fluvial system across the basin and or changes in regional or global climate additionally comparisons of the mean sediment accumulation rates among the sections that span from the basin margin to the basin center indicate that sediment accumulation rates equalized across the basin from the end of c27r through the start of c26r suggesting an accommodation minimum in the basin associated with the progradation of a distributive fluvial system into the basin this accommodation minimum also likely led to the long hiatus of deposition between the paleocene nacimiento formation and the overlying eocene san jose formation

a previously unreported type of seismic source in the firn layer of the east antarctic ice sheet we identify a unique type of seismic source in the uppermost part of the east antarctic ice sheet recorded by temporary broadband seismic arrays in east antarctica these sources termed firnquakes are characterized by dispersed surface wave trains with frequencies of 1 10 hz detectable at distances up to 1000 km events show strong dispersed rayleigh wave trains and an absence of observable body wave arrivals most events also show weaker love waves initial events were discovered by standard detection schemes additional events were then detected with a correlation scanner using the initial arrivals as templates we locate sources by determining the l2 misfit for a grid of potential source locations using rayleigh wave arrival times and polarization directions we then perform a multiple filter analysis to calculate the rayleigh wave group velocity dispersion and invert the group velocity for shear velocity structure the resulting velocity structure is used as an input model to calculate synthetic seismograms inverting the dispersion curves yields ice velocity structures consistent with a low velocity firn layer 100 m thick and show that velocity structure is laterally variable the absence of observable body wave phases and the relative amplitudes of rayleigh waves and noise constrain the source depth to be less than 20 m the presence of love waves for most events suggests the source is not isotropic we propose the events are linked to the formation of small crevasses in the firn and several events correlate with shallow crevasse fields mapped in satellite imagery

magnetostratigraphy of the ludlow member of the fort union formation lower paleocene in the williston basin north dakota to determine the geomagnetic polarity stratigraphy and the duration and age of the ludlow member of the fort union formation lower paleocene we constructed a 325 m composite lithostratigraphic section of the upper cretaceous hell creek formation and the lower paleocene ludlow and tongue river members of the fort union formation in the little missouri river valley of north dakota usa we analyzed paleomagnetic samples from nine of the logged sections the principal magnetic carrier in the ludlow member sediments is likely titanomaghemite as indicated by predominantly irreversible thermomagnetic curves measured from sandstone siltstone and carbonaceous shale samples the analyzed paleomagnetic samples document a series of polarity zones that can be correlated from c29n to c27r on the geomagnetic polarity time scale gpts we infer that the magnetization of the samples is primary because the characteristic directions are consistent with those of the paleocene of north america and the reversal stratigraphy from this section corresponds to the gpts with reasonable sediment accumulation rates by extrapolating the measured sediment accumulation rate from the cretaceous tertiary k t boundary to the top of c28n and then to the top of the ludlow member we estimate the duration of the member to range from 2 31 to 2 61 m y this is the fi rst estimate for the duration and age of the ludlow member and it can be used as an important tool for interpreting rates of biotic recovery after the k t extinction

acceleration of surface lowering on the tidewater glaciers of icy bay alaska u s a from insar dems and icesat altimetry abstract much of the increasing rate of glacier wastage observed during the late 20th century is attributed to retreat and thinning of tidewater glaciers grounded below sea level in fiords we estimate the area average i e the integrated volume change over glacier area elevation changes on guyot yahtse and tyndall glaciers icy bay alaska our results indicate that from 1948 to 1999 the accumulation area of guyot glacier above 1220 m elevation lowered at an area average rate of 0 7 0 1 m yr the accumulation area of yahtse glacier above 1220 m elevation lowered from 1972 to 2000 at an area average rate of 0 9 0 1 m yr on a same area basis tyndall glacier lowered from 1972 to 1999 at an area average rate of 1 4 0 2 m yr then accelerated substantially to 2 8 0 2 m yr from 1999 to 2002 from 2000 to 2003 the accumulation area of yahtse glacier lowered at 1 5 0 3 m yr on average the drawdown of these accumulation areas have occurred while snow accumulation at 5000 m on mt logan canada has shown a strong increase from 1976 to 2000 concurrently coastal winter mean temperatures at cordova and yakutat south central alaska have increased to above freezing since about 1979 retreat and surface lowering of the icy bay glaciers is attributed to tidewater glacier dynamics with climate warming effects superimposed

episodic propagation of a rift on the amery ice shelf east antarctica 1 we investigate ice shelf rift propagation using a combination of gps and seismic measurements near the tip of an active rift in the amery ice shelf these measurements reveal that propagation occurs in episodic bursts which were identified based on swarms of seismicity accompanied by rapid rift widening the bursts last approximately 4 hours and are separated by 10 24 days in between bursts the rift widens at a rate comparable to that of ice shelf spreading comparison of automatic weather station data and tidal amplitudes show that the propagation bursts are not directly triggered by winds or tides suggesting that rift propagation is driven by the background glaciological stress in the ice shelf we show that the ice debris that partly fills the rift may play a role in controlling the rate of propagation

ice front variation and tidewater behavior on helheim and kangerdlugssuaq glaciers greenland we used satellite images to examine the calving behavior of helheim and kangerdlugssuaq glaciers greenland from 2001 to 2006 a period in which they retreated and sped up these data show that many large iceberg calving episodes coincided with teleseismically detected glacial earthquakes suggesting that calving related processes are the source of the seismicity for each of several events for which we have observations the ice front calved back to a large pre existing rift these rifts form where the ice has thinned to near flotation as the ice front retreats down the back side of a bathymetric high which agrees well with earlier theoretical predictions in addition to the recent retreat in a period of higher temperatures analysis of several images shows that helheim retreated in the 20th century during a warmer period and then re advanced during a subsequent cooler period this apparent sensitivity to warming suggests that higher temperatures may promote an initial retreat off a bathymetric high that is then sustained by tidewater dynamics as the ice front retreats into deeper water the cycle of frontal advance and retreat in less than a century indicates that tidewater glaciers in greenland can advance rapidly greenland s larger reservoir of inland ice and conditions that favor the formation of ice shelves likely contribute to the rapid rates of advance

dynamics of stick slip motion whillans ice stream antarctica article i nfo the stick slip motion and associated seismic emissions of whillans ice stream wis west antarctica are two of the many recent observations of unexpected ice sheet behavior that are challenging traditional models of rapid glacier motion here we find that the wis slip events repeatedly nucleate from a sticky spot located in the middle of the ice stream acting similar to an asperity in traditional models of earthquake physics this region shows less motion than surrounding areas during the inter slip periods thus concentrating stress and producing a pulse of seismic energy at the onset of slip the propagating rupture breaks through an additional asperity in the northern part of the ice stream producing another pulse of seismic energy 6 12 min after initiation both asperities are regions of higher hydraulic potential than surrounding regions suggesting they may have greater bed friction due to reduced water lubrication tidal pacing of the stress accumulation combined with fault healing controls the applied stress at failure with higher stress giving faster propagation of the rupture front and higher slip velocities these differences are reflected in the timing of the teleseismic arrivals our results highlight both the great sensitivity of large ice streams to small changes in external forcing and the importance of limited regions of the subglacial bed in controlling their motion as well as providing insights to the mechanics of repeating earthquakes

facies analysis of a probable large fluvial fan depositional system the upper triassic chinle formation at petrified forest national park arizona u s a abstract the upper triassic blue mesa member and the overlying sonsela member of the chinle formation consist of paleosol bearing alluvial strata whose characteristics vary markedly strata exposed within a 4 km2 study area at petrified forest national park were evaluated using sedimentologic stratigraphic paleopedologic and petrographic criteria to formulate potential depositional models that may account for varying alluvial deposition the study succession records a progressive up section increase in grain size increase in channel depth and width increase in lateral and vertical connectivity of channel deposits decrease in overbank preservation and crevasse splay and or sheetflood deposition and increase in overbank drainage mean annual precipitation from the calmag paleosol weathering proxy suggests precipitation averages approximately 1300 mm yr 1 3 m yr throughout the succession despite changing paleosol drainage these features are consistent with deposition in a progradational fluvial system such as a large fan or splay complex sediment accumulation rates in the study interval suggest decreased subsidence in the upper blue mesa member reduced rates of accommodation gain may have promoted progradation of the fluvial system an up section decrease in sandstone mineralogical maturity may be related to increasing sediment bypass in proximal fan positions or increased rates of erosion and sedimentation increasingly better drained paleosols are attributed to deposition in more upland positions during fan or avulsion complex progradation rather than changing climate

river incision into bedrock mechanics and relative efficacy of plucking abrasion and cavitation improved formulation of bedrock erosion laws requires knowledge of the actual processes operative at the bed we present qualitative field evidence from a wide range of settings that the relative efficacy of the various processes of fluvial erosion e g plucking abrasion cavitation solution is a strong function of substrate lithology and that joint spacing fractures and bedding planes exert the most direct control the relative importance of the various processes and the nature of the interplay between them are inferred from detailed observations of the morphology of erosional forms on channel bed and banks and their spatial distributions we find that plucking dominates wherever rocks are well jointed on a submeter scale hydraulic wedging of small clasts into cracks bashing and abrasion by bedload and chemical and physical weathering all contribute to the loosening and removal of joint blocks in more massive rocks abrasion by suspended sand appears to be rate limiting in the systems studied here concentration of erosion on downstream sides of obstacles and tight coupling between fluid flow patterns and fine scale morphology of erosion forms testify to the importance of abrasion by suspended load rather than bedload particles mechanical analyses indicate that erosion by suspended load abrasion is considerably more nonlinear in shear stress than erosion by plucking in addition a new analysis indicates that cavitation is more likely to occur in natural systems than previously argued cavitation must be considered a viable process in many actively incising bedrock channels and may contribute to the fluting and potholing of massive unjointed rocks that is otherwise attributed to suspended load abrasion direct field evidence of cavitation erosion is however lacking in terms of the well known shear stress or stream power erosion law erosion by plucking is consistent with a slope exponent n of 2 3 to 1 whereas erosion by suspended load abrasion is more consistent with a slope exponent of 5 3 given that substrate lithology appears to dictate the dominant erosion process this finding has important implications for long term landscape evolution and the models used to study it

climate and landscape reconstruction of the arroyo chijuillita member of the nacimiento formation san juan basin new mexico providing environmental context to early paleocene mammal evolution abstract the sedimentologic stratigraphic and paleopedologic attributes of a laterally continuous outcrop exposure of the basal portion of the lower paleocene arroyo chijuillita member of the nacimiento formation located in the bisti de na zin wilderness area was examined to provide paleoenvironmental context for the puercan north american land mammal age nalma terrestrial ecosystems of the san juan basin new mexico the studied outcrop spans the middle and late puercan pu2 pu3 nalma and thus is an ideal location to assess the potential causes of faunal turnover between pu2 and pu3 the stratal succession consists of fluvial channel and laterally continuous sandstone and associated fine grained floodplain deposits including paleosols that document variability in drainage and maturity quantitative and qualitative reconstructions of paleoclimate from paleosols supplemented by analyses of fossil floras indicate no change in climate through the studied interval further there is no evidence that regional tectonics relate to changes in depositional environments given the lack of evidence for climate change or regional tectonics and the characteristics of the fluvial facies and the paleosols across the study locality we interpret the spatial and temporal variability in paleopedologic and sedimentologic characteristics to be the result of autocyclic channel avulsion and migration additionally the stable climatic conditions and lack of tectonic influence between the pu2 and pu3 biozones suggest that mammalian turnover is unlikely to have been caused by extrinsic environmental changes i e climate or tectonics and is instead probably the result of rapid evolution following the k pg boundary and or competition from the migration of new mammalian species into the basin

subglacial discharge at tidewater glaciers revealed by seismic tremor subglacial discharge influences glacier basal motion and erodes and redeposits sediment at tidewater glacier termini discharge drives submarine terminus melting affects fjord circulation and is a central component of proglacial marine ecosystems however our present inability to track subglacial discharge and its variability significantly hinders our understanding of these processes here we report observations of hourly to seasonal variations in 1 5 10 hz seismic tremor that strongly correlate with subglacial discharge but not with basal motion weather or discrete icequakes our data demonstrate that vigorous discharge occurs from tidewater glaciers during summer in spite of fast basal motion that could limit the formation of subglacial conduits and then abates during winter furthermore tremor observations and a melt model demonstrate that drainage efficiency of tidewater glaciers evolves seasonally glaciohydraulic tremor provides a means by which to quantify subglacial discharge variations and offers a promising window into otherwise obscured glacierized environments

ice melange dynamics and implications for terminus stability jakobshavn isbrae greenland 1 we used time lapse imagery seismic and audio recordings iceberg and glacier velocities ocean wave measurements and simple theoretical considerations to investigate the interactions between jakobshavn isbrae and its proglacial ice melange the melange behaves as a weak granular ice shelf whose rheology varies seasonally sea ice growth in winter stiffens the melange matrix by binding iceberg clasts together ultimately preventing the calving of full glacier thickness icebergs the dominant style of calving and enabling a several kilometer terminus advance each summer the melange weakens and the terminus retreats the melange remains strong enough however to be largely unaffected by ocean currents except during calving events and to influence the timing and sequence of calving events furthermore motion of the melange is highly episodic between calving events including the entire winter it is pushed down fjord by the advancing terminus at 40 m d 1 whereas during calving events it can move in excess of 50 103 m d 1 for more than 10 min by influencing the timing of calving events the melange contributes to the glacier s several kilometer seasonal advance and retreat the associated geometric changes of the terminus area affect glacier flow furthermore a force balance analysis shows that large scale calving is only possible from a terminus that is near floatation especially in the presence of a resistive ice melange the net annual retreat of the glacier is therefore limited by its proximity to floatation potentially providing a physical mechanism for a previously described near floatation criterion for calving

seismicity within a propagating ice shelf rift the relationship between icequake locations and ice shelf structure iceberg calving is a dominant mass loss mechanism for antarctic ice shelves second only to basal melting an important process involved in calving is the initiation and propagation of through penetrating fractures called rifts however the mechanisms controlling rift propagation remain poorly understood to investigate the mechanics of ice shelf rifting we analyzed seismicity associated with a propagating rift tip on the amery ice shelf using data collected during the austral summers of 2004 2007 we apply a suite of passive seismological techniques including icequake locations back projection and moment tensor inversion we confirm previous results that show ice shelf rifting is characterized by periods of relative quiescence punctuated by swarms of intense seismicity of 1 to 3 h even during periods of quiescence we find significant deformation around the rift tip moment tensors calculated for a subset of the largest icequakes mw 2 0 located near the rift tip show steeply dipping fault planes horizontal or shallowly plunging stress orientations and often have a significant volumetric component they also reveal that much of the observed seismicity is limited to the upper 50 m of the ice shelf this suggests a complex system of deformation that involves the propagating rift the region behind the rift tip and a system of rift transverse crevasses small scale variations in the mechanical structure of the ice shelf especially rift transverse crevasses and accreted marine ice play an important role in modulating the rate and location of seismicity associated with the propagating ice shelf rifts

shocking rocks by cavitation and bubble implosion cavitation is proposed as a mechanism for the discrete generation of high pressure high temperature polymorphs in veins within meteorites and hypervelocity impact structures cavitation involves the inception isothermal growth and adiabatic implosion of bubbles their collapse has the potential of generating ultrahigh temperatures and pressures the formation of pseudotachylytic melt veins within the shocked target is critical to the cavitation process these veins facilitate bubble growth on decompression and subsequent bubble collapse on relaxation within the liquid asymmetric bubble implosion can lead to the rapid inrush of liquid to form a supersonic jet if the liquid jet strikes a solid surface i e melt vein walls or suspended clasts at high velocities then temperatures of several thousand kelvin and pressures of 10 gpa or more can be realized in the area of jet impact these conditions are capable of generating high pressure phases such as ringwoodite the spinel structured polymorph of olivine which requires formation pressures of 50 gpa these extreme effects are transient and localized being restricted to microscopic regions of liquid solid impact within the cavitating melt vein the presence of high pressure polymorphs in meteorites and impact structures does not necessarily imply that the primary shock event attained the conditions necessary for polymorph formation instead the presence of polymorphs may be the result of extreme pressure temperature excursions due to cavitation

importance of titanohematite in detrital remanent magnetizations of strata spanning the cretaceous paleogene boundary hell creek region montana intermediate composition titanohematite fe2 ytiyo3 with 0 5 y 0 7 is seldom the focus of paleomagnetic study and is commonly believed to be rare in nature while largely overlooked in magnetostratigraphic studies intermediate titanohematite has been identified as the dominant ferrimagnetic mineral in an array of late mesozoic and early cenozoic laramide clastic deposits throughout the central united states intermediate titanohematite is ferrimagnetic and has similar magnetic properties to titanomagnetite except its unique self reversing property due to these similarities and with detrital remanent magnetizations masking its self reversing nature intermediate titanohematite is often misidentified in sedimentary deposits past studies relied upon nonmagnetic techniques including x ray diffraction and electron microprobe analysis while these techniques can identify the presence of intermediate titanohematite they fail to test whether the mineral is the primary recorder to facilitate the identification of intermediate titanohematite in sedimentary deposits we characterize this mineral using low temperature magnetometry and high temperature susceptibility experiments and present a new identification technique based on titanohematite s self reversing property for sediments that span the cretaceous paleogene boundary hell creek region montana results from the self reversal test indicate that the majority of remanence is held by minerals that become magnetized parallel to an applied field but that intermediate self reversing titanohematite y 0 53 0 63 is an important ancillary carrier of remanence while earlier literature suggests that intermediate titanohematite is rare in nature reanalysis using specialized rock magnetic techniques may reveal that it is more abundant in the rock record particularly within depositional basins adjacent to calc alkaline volcanics than previously thought

glacier generated earthquakes from prince william sound alaska analysis of seismic records from stations near the prince william sound supports the association of monochromatic low frequency signals with tidewater glaciers in the area thirty three of the larger better recorded signals with corresponding earthquake magnitudes of ml 1 5 to 2 5 were located using routine seismological techniques epicenters for these glacierquakes clustered in the vicinity of the harvard columbia yale and barry glaciers monthly counts from one station for 1975 1979 and part of 1976 range from less than 4 to 248 events a sudden decrease in the number of events per month in mid 1979 raises the possibility of a single source or a common source mechanism for these glacier related signals the monochromatic nondispersive waveforms which characterize these 1 to 2 hz signals may result from harmonic resonance of the glaciers in the epicentral region we speculate that a variety of source mechanisms within or near the glaciers may provide the initial impulse for these events however the characteristic observed waveform is most likely the result of seismic energy which is internally reflected within the ice at fundamental resonant frequencies boundary conditions associated with tidewater glaciers may also allow the ice body to act as a wave guide for shear or surface waves the signal we observe therefore appears to be a source site phenomenon

does calving matter evidence for significant submarine melt abstract during the summer in the northeast pacific ocean the alaska coastal current sweeps water with temperatures in excess of 12 c past the mouths of glacierized fjords and bays the extent to which these warm waters affect the mass balance of alaskan tidewater glaciers is uncertain here we report hydrographic measurements made within icy bay alaska and calculate rates of submarine melt at yahtse glacier a tidewater glacier terminating in icy bay we find strongly stratified water properties consistent with estuarine circulation and evidence that warm gulf of alaska water reaches the head of 40 km long icy bay largely unaltered a 10 20 m layer of cold fresh glacially modified water overlies warm saline water the saline water is observed to reach up to 10 4 c within 1 5 km of the terminus of yahtse glacier by quantifying the heat and salt deficit within the glacially modified water we place bounds on the rate of submarine melt the submarine melt rate is estimated at 9 m d 1 at least half the rate at which ice flows into the terminus region and can plausibly account for all of the submarine terminus mass loss our measurements suggest that summer and fall subaerial calving is a direct response to thermal undercutting of the terminus further demonstrating the critical role of the ocean in modulating tidewater glacier dynamics

source mechanics for monochromatic icequakes produced during iceberg calving at columbia glacier ak 1 seismograms recorded during iceberg calving contain information pertaining to source processes during calving events however locally variable material properties may cause signal distortions known as site and path effects which must be eliminated prior to commenting on source mechanics we applied the technique of horizontal vertical spectral ratios to passive seismic data collected at columbia glacier ak and found no dominant site or path effects rather monochromatic waveforms generated by calving appear to result from source processes we hypothesize that a fluid filled crack source model offers a potential mechanism for observed seismograms produced by calving and fracture processes preceding calving

iceberg calving as a primary source of regional scale glacier generated seismicity in the st elias mountains alaska 1 since the installation of the alaska regional seismic network in the 1970s data analysts have noted nontectonic seismic events thought to be related to glacier dynamics while loose associations with the glaciers of the st elias mountains have been made no detailed study of the source locations has been undertaken we performed a two step investigation surrounding these events beginning with manual locations that guided an automated detection and event sifting routine results from the manual investigation highlight characteristics of the seismic waveforms including single peaked narrowband spectra emergent onsets lack of distinct phase arrivals and a predominant cluster of locations near the calving termini of several neighboring tidewater glaciers through these locations comparison with previous work analyses of waveform characteristics frequency magnitude statistics and temporal patterns in seismicity we suggest calving as a source for the seismicity statistical properties and time series analysis of the event catalog suggest a scale invariant process that has no single or simple forcing these results support the idea that calving is often a response to short lived or localized stress perturbations our results demonstrate the utility of passive seismic instrumentation to monitor relative changes in the rate and magnitude of iceberg calving at tidewater glaciers that may be volatile or susceptible to ensuing rapid retreat especially when existing seismic infrastructure can be used

observing calving generated ocean waves with coastal broadband seismometers jakobshavn isbrae greenland we use time lapse photography modis satellite imagery ocean wave measurements and regional broadband seismic data to demonstrate that icebergs that calve from jakobshavn isbrae greenland can generate ocean waves that are detectable over 150 km from their source the waves which are recorded seismically have distinct spectral peaks are not dispersive and persist for several hours on the basis of these observations we suggest that calving events at jakobshavn isbrae can stimulate seiches or basin eigenmodes in both ilulissat icefjord and disko bay our observations furthermore indicate that coastal land based seismometers located near calving termini e g as part of the new greenland ice sheet monitoring network glisn can aid investigations into the largely unexplored oceanographic consequences of iceberg calving

possible mechanisms for glacial earthquakes the large glacial earthquakes reported on by ekstrom et al 2003 2006 and tsai and ekstrom 2007 have previously been evaluated in terms of their seismic characteristics in this paper we attempt to take constraints such as known glacial ice properties outlet glacier size calving style and meltwater variability to construct a self consistent physical model of the glacial earthquake process since many glaciological parameters are poorly constrained we parameterize a number of important processes and estimate a wide range of possible values for some properties the range of model outputs is thus fairly large but it is still difficult to match observational constraints under most conditions we find that only a small class of models is able to satisfy the major observational constraints these models are characterized by 1 lost basal resistance coupled to viscoelastic deformation with extensive internal crevassing or with low effective elastic modulus and possibly low effective viscosity or 2 by nonequilibrium calving such as having large icebergs capsize into the glacier front although observational constraints cannot definitively rule out any of the proposed classes of mechanisms the calving model has much stronger support fortunately the various models make different predictions regarding observables that can potentially be measured in the near future

magnetic polarity stratigraphy and biostratigraphy of middle late paleocene continental deposits of south central montana exposures of the fort union formation on the divide between hunt creek and cub creek in the northern clark s fork basin carbon county montana were selected for magnetostratigraphic study of the transition between the torrejonian and tiffanian land mammal ages paleomagnetic samples were collected from 25 sites within a 160 m thick section of the fort union formation at that location rock magnetic analyses indicate that alternating field demagnetization to peak fields in the 10 to 40 mt interval successfully removed secondary components of natural remanent magnetism nrm characteristic nrm directions define three polarity zones a 50 m thick normal polarity zone bracketed by two reversed polarity zones the cub creek local faunule cc 2 to 3 or ti 1 occurs within the upper portion of the normal polarity zone cub creek local faunules cc 1 cc 3 and eagle quarry all ti 1 occur in the upper reversed polarity zone these data along with faunal and magnetostratigraphic data from the san

a successful rock magnetic technique for correctng paleomagnetic inclination shallowing case study of the nacimiento formation new mexico the accuracy of an inclination shallowing correction technique was tested on the remanence of the paleocene nacimiento formation the correction assumes that changes in paleomagnetic inclination during deformation will be directly related to changes in remanence anisotropy the remanence anisotropy is also a function of the magnetic anisotropy of the individual magnetic grains the nacimiento formation was selected for this test because its inclination is shallow by 7 to 8 and previous paleomagnetic studies indicate that it has a primary remanence with a well constrained age and that magnetic overprints are minimal tectonics are an unlikely explanation for its shallow inclination these conditions allow the inclination corrected remanence to be compared to a well defined paleomagnetic pole to determine the accuracy of the technique the characteristic remanence from 20 nacimiento sites was isolated by alternating field and thermal demagnetization and was corrected using the anisotropy of anhysteretic remanence measured for three to five samples per site individual particle anisotropy was determined by drying a magnetic separate epoxy mixture in a 35 50 mt magnetic field laboratory compaction of disaggregated nacimiento material was also used to yield an effective individual magnetic particle anisotropy the two techniques resulted in similar values for individual particle anisotropy and similar inclination shallowing corrections both corrections were successful and the inclination corrected formation mean direction was indistinguishable from the direction predicted by north america s paleocene paleopole the inclination shallowing correction technique reported here should be applied routinely in paleomagnetic studies provided that the remanence is a primary depositional remanence that has been affected only by syndepositional or early postdepositional processes e g compaction

characterization of the rhyolite of bodie hills and 40ar 39ar intercalibration with ar mineral standards abstract the rhyolite of bodie hills california is characterized compositionally and the geochronology of selected phases is studied sanidine bhs from the rhyolite is well suited as a 40ar 39ar reference material with high k ca and radiogenic yield intercalibration with ga1550 biotite from the dromedary igneous complex new south wales australia yields an age of 9 7946 0 0031 ma for bhs relative to an age of 98 79 0 54 ma for ga1550 and a calibration factor rbhs ga1550 of 0 096719 0 000032 bhs is also intercalibrated with sanidines of the taylor creek rhyolite tcs rtcs bhs of 2 90874 0 00067 fish canyon tuff fcs rfcs bhs of 2 88339 0 00088 and rhyolite of alder creek acs racs bhs of 0 12028 0 00024 these calibration factors yield ages of 28 344 0 011 ma 28 099 0 013 ma and 1 1809 0 0024 ma for tcs fcs and acs respectively relative to ga1550 full propagation of errors increases these uncertainties and that of bhs to 0 9 of their ages calibration of bhs using the astronomically tuned age of fcs determined by kuiper et al 2008 yields an age of 9 8295 0 0036 ma stepwise heating of bhs reveals the same small progressive increase in age across the age spectrum reported for fcs acs and other potassium feldspars e g foland and xu 1990 phillips et al 2017 this increase is consistent with mass fractionation of argon during step heating and favors use of single step fusion ages of all sanidines for monitor and calibration purposes zircons from the rhyolite of bodie hills are strongly zoned in u and th and u pb geochronologic analyses suggest multiple generations of zircon growth the youngest ages indicate a crystallization age of 9 97 0 08 ma 2 arguably similar to the 40ar 39ar results when residence time is considered but previous episodes of zircon growth began at least 400 ka prior to eruption precise u th he thermochronology of these zircons is difficult because crystal to crystal variations in the magnitude and complexity of their u th zoning complicates accurate alpha ejection calculations replicate conventional single crystal u th he ages are more widely dispersed than predicted by analytical uncertainties but laser ablation u th he ages are more reproducible with an inverse variance weighted mean of 9 71 0 58 ma

high resolution chronostratigraphy of the terrestrial cretaceous paleogene transition and recovery interval in the hell creek region montana detailed understanding of ecosystem decline and recovery attending the cretaceous paleogene boundary kpb mass extinctions is hindered by limited constraints on the pace and tempo of environmental events near the boundary to mitigate this shortcoming high resolution 40ar 39ar geochronology was performed on tephras intercalated between fossiliferous terrestrial sediments of the upper hell creek and lower fort union formations in the western williston basin of northeastern montana usa tephra samples were collected from 10 stratigraphic sections spanning an area of 5000 km2 several distinctive tephras can be correlated between sections separated spatially by as much as 60 km the tephras are thin distal deposits generally preserved only in lignite beds which are interbedded with clastic deposits yielding vertebrate faunas of lancian late maastrichtian to torrejonian early danian north american land mammal ages sanidine from 15 tephra samples was analyzed in 1649 total fusion experiments 1597 on single crystals and 12 incremental heating analyses of multigrain aliquots ages were determined for 13 distinct tephras ranging from 66 289 0 051 to 64 866 0 023 ma including only analytical uncertainties this level of precision is sufficient to resolve the ages of all of the coal beds that have served as a basis for a regional stratigraphic framework the data confirm that the hell creek fort union formational contact is diachronous and further support the age of the kpb impact layer at 66 043 0 010 ma or 0 043 ma considering systematic uncertainties application of the new results to previous magnetostratigraphic data indicates an appreciably compressed time interval between the base of chron c29r and the top of chron c28r with a maximum duration estimate of 1 421 0 066 ma most notable is the implied brevity of chron c29r with a maximum estimate of 457 54 ka and possibly as brief as 345 38 ka compared to the 710 ka estimate from the geologic time scale 2012 gts2012 further application of new results to terrestrial biostratigraphy adds higher precision to the timing and tempo of biotic change before and after the kpb our results indicate that the timing of pre kpb ecological decline is constrained to the last 200 ka of the cretaceous adding further support to the press pulse extinction hypothesis additionally the duration of the depauperate basal paleogene puercan 1 disaster fauna is confined to a 70 ka interval faunal recovery in this region indicated by the appearance of primitive members of the placental mammal radiation and the restoration of taxonomic richness and evenness occurred within 900 ka after the kpb these results show that biotic recovery after the mass extinction in the terrestrial realm was more rapid than in the marine

magnetic ilmenite hematite detritus in mesozoic tertiary placer and sandstone hosted uranium deposits of the rocky mountains mesozoic and tertiary sandstones throughout the rocky mountains region of the united states contain a de trital fe ti oxide mineral that is generally regarded as unusual and is preserved wherever these sandstones have escaped destructive diagenesis this mineral a medial ilmenite hematite solid solution is highly magnetic and has sometimes been mistaken for magnetite we have encountered this mineral in paleomagnetic studies and in investigations of sandstone hosted uranium deposits and cretaceous shoreline placer deposits it is known to occur elsewhere in intermediate volcanic rocks consistent with quenching from hypersolvus high temperature conditions the detrital occurrences we describe are of economic importance in uranium deposits where they are among the reductants that precipitate uranium and in cretaceous placer deposits whose exploitation would require a separation procedure tailored to this mineral the distribution of detrital hematite ilmenite sandstones in cretaceous rocks suggests a contemporaneous volcanic source in the northern rocky mountains

glacier calving observed with time lapse imagery and tsunami waves at glaciar perito moreno patagonia calving plays a key role in the recent rapid retreat of glaciers around the world however many processes related to calving are poorly understood since direct observations are scarce and challenging to obtain when calving occurs at a glacier front surface water waves arise over the ocean or a lake in front of glaciers to study calving processes from these surface waves we performed field observations at glaciar perito moreno patagonia we synchronized time lapse photography and surface waves record to confirm that glacier calving produces distinct waves compared with local noise a total of 1074 calving events were observed over the course of 39 d during austral summer calving occurred twice more frequently than in spring the cumulative distribution of calving interevent time interval followed exponential model implying random occurrence of events in time we further investigated wave properties and found that source to sensor distance can be estimated from wave dispersion within 20 error we also found that waves produced by different calving types showed similar spectra in the same frequency range between 0 05 0 2 hz and that the amplitude of surface waves increased with the size of calving this study demonstrates the potential of surface wave monitoring for understanding calving processes

avalanching glacier instabilities review on processes and early warning perspectives avalanching glacier instabilities are gravity driven rupture phenomena that might cause major disasters especially when they are at the origin of a chain of processes reliably forecasting such events combined with a timely evacuation of endangered inhabited areas often constitute the most efficient action recently considerable efforts in monitoring analyzing and modeling such phenomena have led to significant advances in destabilization process understanding improving early warning perspectives the purpose of this paper is to review the recent progress in this domain three different types of instabilities can be identified depending on the thermal properties of the ice bed interface if cold 1 the maturation of the rupture is associated with a typical time evolution of surface velocities and passive seismic activity a prediction of the final break off is possible using these precursory signs for the two other types water plays a key role in the development of the instability if the ice bed interface is partly temperate 2 the presence of meltwater may reduce the basal resistance which promotes the instability no clear and easily detectable precursory signs are known in this case and the only way to infer any potential instability is to monitor the temporal evolution of the thermal regime the last type of instability 3 concerns steep temperate glacier tongues switching for several days weeks during the melting season into a so called active phase followed in rare cases by a major break off event although the prediction of such events is still far from being achievable critical conditions promoting the final instability can be identified

evidence for deep icequakes in an alpine glacier to obtain more reliable information about the focal depth distribution of icequakes in april 1997 we operated an array of seven portable digital seismographs on unteraargletscher central swiss alps over 5000 events were detected by at least two instruments during the 9 day recording period p wave velocities 3770 m f were determined from several calibration shots detonated at the glacier surface as well as in a 49 m deep borehole whereas s wave velocities 1860 ms 1 were derived from a simultaneous inversion for vp vs6 applied to 169 icequakes so far hypocentral locations have been calculated for over 300 icequakes besides confirming the occurrence of shallow events associated with the opening of crevasses our results show that a small but significant fraction of the hypocenters are located at or near the glacier bed one event was found at an intermediate depth of about 120 m three dimensional particle motion diagrams of both explosions and icequakes clearly demonstrate that all vertical component seismograms from shallow sources are dominated by the rayleigh wave on the other hand for events occurring at depths greater than about 40 m the rayleigh wave disappears almost entirely therefore a qualitative analysis of the signal character provides direct information on the focal depth of an event and was used as an independent check of the locations obtained from traditional arrival time inversions thus our results demonstrate that deep icequakes do occur and that simple rheological models according to which brittle deformation is restricted to the uppermost part of a glacier may need revision

iceberg calving during transition from grounded to floating ice columbia glacier alaska 1 the terminus of columbia glacier alaska unexpectedly became ungrounded in 2007 during its prolonged retreat visual observations showed that calving changed from a steady release of low volume bergs to episodic flow perpendicular rifting propagation and release of very large icebergs a style reminiscent of calving from ice shelves here we compare passive seismic and photographic observations through this transition to examine changes in calving mechanical changes accompany the visible changes in calving style post flotation generation of seismic energy during calving is substantially reduced we propose this is partly due to changes in source processes

analysis of glacial earthquakes in 2003 ekstrom et al reported on the detection of a new class of earthquakes that occur in glaciated regions with the vast majority being in greenland the events have a characteristic radiation pattern and lack the high frequency content typical of tectonic earthquakes it was proposed that the events correspond to large and sudden sliding motion of glaciers here we present an analysis of all 184 such events detected in greenland between 1993 and 2005 fitting the teleseismic long period surface waves to a landslide model of the source we obtain improved locations timing force amplitudes and force directions after relocation the events cluster into seven regions all of which correspond to regions of very high ice flow and most of which are named outlet glaciers these regions are daugaard jensen glacier kangerdlugssuaq glacier helheim glacier the southeast greenland glaciers the northwest greenland glaciers rinks isbrae and jakobshavn isbrae event amplitudes range from 0 1 to 2 0 10 14 kg m force directions are consistent with sliding in the direction of glacial flow over a period of about 50 s each region has a different temporal distribution of events all glaciers are more productive in the summer but have their peak activity in different months over the study period kangerdlugssuaq has had a constant number of events each year whereas jakobshavn had most events in 1998 1999 and the number of events in helheim and the northwest greenland glaciers has increased substantially between 1993 and 2005 the size distribution of events in kangerdlugssuaq is peaked above the detection threshold suggesting that glacial earthquakes have a characteristic size

basal icequakes during changing subglacial water pressures beneath gornergletscher switzerland using dense networks of three component seismometers installed in direct contact with the ice the seismic activity of gornergletscher switzerland was investigated during the summers of 2004 and 2006 as subglacial water pressures varied drastically these pressure variations are due to the diurnal cycle of meltwater input as well as the subglacial drainage of gornersee a nearby marginal ice dammed lake up to several thousand seismic signals per day were recorded whereas most icequakes are due to surface crevasse openings about 200 events have been reliably located close to the glacier bed these basal events tend to occur in clusters and have signals with impulsive first arrivals at the same time basal water pressures and ice surface velocities were measured to capture the impact of the lake drainage on the subglacial hydrological system and the ice flow dynamics contrary to our expectations we did not observe an increase of basal icequake activity as the lake emptied thereby raising the subglacial water pressures close to the flotation level for several days in fact the basal icequakes were usually recorded during the morning hours when the basal water pressure was either low or decreasing during the high pressure period caused by the drainage of the lake no basal icequakes were observed furthermore gps measurements showed that the glacier surface was lowering during the basal seismic activity these observations lead us to conclude that such icequakes are connected to the diurnal variation in glacier sliding across the glacier bed

seismic detection and analysis of icequakes at columbia glacier alaska 1 contributions to sea level rise from rapidly retreating marine terminating glaciers are large and increasing strong increases in iceberg calving occur during retreat which allows mass transfer to the ocean at a much higher rate than possible through surface melt alone to study this process we deployed an 11 sensor passive seismic network at columbia glacier alaska during 2004 2005 we show that calving events generate narrow band seismic signals allowing frequency domain detections detection parameters were determined using direct observations of calving and validated using three statistical methods and hypocenter locations the 1 3 hz detections provide a good measure of the temporal distribution and size of calving events possible source mechanisms for the unique waveforms are discussed and we analyze potential forcings for the observed seismicity

response to the comment by w h schwarz et al on joint determination of 40k decay constants and 40ar 40k for the fish canyon sanidine standard and improved accuracy for 40ar 39ar geochronology by p r renne et al 2010

analysis of low frequency seismic signals generated during a multiple iceberg calving event at jakobshavn isbrae greenland 1 we investigated seismic signals generated during a large scale multiple iceberg calving event that occurred at jakobshavn isbrae greenland on 21 august 2009 the event was recorded by a high rate time lapse camera and five broadband seismic stations located within a few hundred kilometers of the terminus during the event two full glacier thickness icebergs calved from the grounded or nearly grounded terminus and immediately capsized the second iceberg to calve was two to three times smaller than the first the individual calving and capsize events were well correlated with the radiation of low frequency seismic signals 0 1 hz dominated by love and rayleigh waves in agreement with regional records from previously published glacial earthquakes these low frequency seismic signals had maximum power and or signal to noise ratios in the 0 05 0 1 hz band similarly full waveform inversions indicate that these signals were also generated by horizontal single forces acting at the glacier terminus the signals therefore appear to be local manifestations of glacial earthquakes although the magnitudes of the signals twice time integrated force histories were considerably smaller than previously reported glacial earthquakes we thus speculate that such earthquakes may be a common if not pervasive feature of all full glacier thickness calving events from grounded termini finally a key result from our study is that waveform inversions performed on low frequency calving generated seismic signals may have only limited ability to quantitatively estimate mass losses from calving in particular the choice of source time function has little impact on the inversion but dramatically changes the earthquake magnitude accordingly in our analysis it is unclear whether the smaller or larger of the two calving icebergs generated a larger seismic signal

calving event detection by observation of seiche effects on the greenland fjords with mass loss from the greenland ice sheet accelerating and spreading to higher latitudes the quantification of mass discharge in the form of icebergs has recently received much scientific attention here we make use of very low frequency 0 001 0 01 hz seismic data from three permanent broadband stations installed in the summers of 2009 10 in northwest greenland in order to monitor local calving activity at these frequencies calving seismograms are dominated by a tilt signal produced by local ground flexure in response to fjord seiching generated by major iceberg calving events a simple triggering algorithm is proposed to detect calving events from large calving fronts with potentially no user interaction our calving catalogue identifies spatial and temporal differences in calving activity between jakobshavn isbrae and glaciers in the uummannaq district similar to 200 km to the north the uummannaq glaciers show clear seasonal fluctuations in seiche based calving detections as well as seiche amplitudes in contrast the detections at jakobshavn isbrae show little seasonal variation which may be evidence for an ongoing transition to winter calving activity the results offer further evidence that seismometers can provide efficient and inexpensive monitoring of calving fronts

cameras as clocks consumer grade digital cameras have become ubiquitous accessories of science particularly in glaciology the recognized importance of short term variability has motivated their deployment for increasingly time critical observations however such devices were never intended for precise timekeeping and their use as such needs to be accompanied by appropriate management of systematic rounding and random errors in reported image times this study describes clock drift subsecond reporting resolution and timestamp precision as the major obstacles to precise camera timekeeping and documents the subsecond capability of camera models from 17 leading manufacturers we present a complete and accessible methodology to calibrate cameras for absolute timing and provide a suite of supporting scripts two glaciological case studies serve to illustrate how the methods relate to contemporary investigations 1 georeferencing aerial photogrammetric surveys with camera positions time interpolated from gps tracklogs and 2 coupling videos of glacier calving events to synchronous seismic waveforms

early paleocene magnetostratigraphy and revised biostratigraphy of the ojo alamo sandstone and lower nacimiento formation san juan basin new mexico usa

calving of fuerza aerea glacier greenwich island antarctica observed with terrestrial laser scanning and continuous video monitoring a short term series of quantitative observations of calving activity of fuerza aerea glacier greenwich island the south shetland islands antarctica was conducted in order to test new methods of monitoring calving the volume of single calving events was quantified by combining terrestrial laser scanning tls surveys with continuous video recording of the ice front an empirical formula for area volume scaling of the calved ice block was proposed based on the tls measured calved ice volume and the calved ice front area obtained by manual delineation on the images acquired with the video camera this combination of methods proves to be a valuable tool for glacier monitoring providing both high temporal resolution and precise quantitative measurements of the calving volume the size distribution of calving events is best approximated by a power law and within the short period of observations 14 d calving was found to be an intrinsic process not dependent on environmental forcings over the period of 21 january 04 february 2013 the ice flow velocity at the terminus of fuerza aerea glacier was 0 26 0 07 m d 1 and the calving rate was 0 41 0 07 m d 1

direct u pb dating of cretaceous and paleocene dinosaur bones san juan basin new mexico vertebrate fossils have been important for relative dating of terrestrial rocks for decades but direct dating of these fossils has heretofore been unsuccessful in this study we employ recent advances in laser ablation in situ u pb dating techniques to directly date two dinosaur fossils from the san juan basin of northwestern new mexico and southwestern colorado united states a cretaceous dinosaur bone collected from just below the cretaceous paleogene interface yielded a u pb date of 73 6 0 9 ma in excellent agreement with a previously determined 40 ar 39 ar date of 73 04 0 25 ma for an ash bed near this site the second dinosaur bone sample from paleocene strata just above the cretaceous paleogene interface yielded a paleocene u pb date of 64 8 0 9 ma consistent with palynologic paleomagnetic and fossil mammal biochronologic data this first successful direct dating of fossil vertebrate bone provides a new methodology with the potential to directly obtain accurate dates for any vertebrate fossil

astronomical calibration of 40ar 39ar reference minerals using high precision multi collector argusvi mass spectrometry the new generation of multi collector mass spectrometers e g argusvi permit ultra high precision 1 in 40k decay constants and the ages of natural reference minerals that form the basis of the technique for example reported ages for widely used 40ar 39ar reference materials such as the ca 28 ma fish canyon tuff sanidine fcts and the ca 1 2 ma alder creek rhyolite sanidine acrs vary by 1 recent attempts to independently calibrate these reference minerals have focused on k ar analyses of the same minerals and inter comparisons with astronomically tuned tephras in sedimentary sequences and u pb zircon ages from volcanic rocks most of these studies used older generation effectively single collector mass spectrometers that employed peak jumping analytical methods to acquire 40ar 39ar data in this study we reassess the inter calibration and ages of commonly used 40ar 39ar reference minerals fish canyon tuff sanidine fcts alder creek rhyolite sanidine acrs and mount dromedary biotite md2b equivalent to ga 1550 biotite relative to the astronomically tuned age of a1 tephra sanidine a1ts faneromeni section crete rivera et al 2011 using a multi collector argusvi mass spectrometer these analyses confirm the exceptional precision capability 0 1 in the 40ar 39ar ages of reference minerals without consideration of recoil artefacts thus limiting the benefits of high precision multi collector analyses significant improvement to the accuracy of the 40ar 39ar method 0 1 will require further inter laboratory 40ar 39ar studies utilizing multi collector mass spectrometry additional constraints on recoil 39ark loss from reference minerals further resolution of discrepancies between astronomically tuned sedimentary successions and refinement of the 238u 206pb zircon age cross calibration approach

global plate motion frames toward a unified model 1 plate tectonics constitutes our primary framework for understanding how the earth works over geological timescales high resolution mapping of relative plate motions based on marine geophysical data has followed the discovery of geomagnetic reversals mid ocean ridges transform faults and seafloor spreading cementing the plate tectonic paradigm however so called absolute plate motions describing how the fragments of the outer shell of the earth have moved relative to a reference system such as the earth s mantle are still poorly understood accurate absolute plate motion models are essential surface boundary conditions for mantle convection models as well as for understanding past ocean circulation and climate as continent ocean distributions change with time a fundamental problem with deciphering absolute plate motions is that the earth s rotation axis and the averaged magnetic dipole axis are not necessarily fixed to the mantle reference system absolute plate motion models based on volcanic hot spot tracks are largely confined to the last 130 ma and ideally would require knowledge about the motions within the convecting mantle in contrast models based on paleomagnetic data reflect plate motion relative to the magnetic dipole axis for most of earth s history but cannot provide paleolongitudes because of the axial symmetry of the earth s magnetic dipole field we analyze four different reference frames paleomagnetic african fixed hot spot african moving hot spot and global moving hot spot discuss their uncertainties and develop a unifying approach for connecting a hot spot track system and a paleomagnetic absolute plate reference system into a hybrid model for the time period from the assembly of pangea 320 ma to the present for the last 100 ma we use a moving hot spot reference frame that takes mantle convection into account and we connect this to a pre 100 ma global paleomagnetic frame adjusted 5 in longitude to smooth the reference frame transition using plate driving force arguments and the mapping of reconstructed large igneous provinces to core mantle boundary topography we argue that continental paleolongitudes can be constrained with reasonable confidence

mineralogy of magnetic minerals and revised magnetic polarity stratigraphy of continental sediments san juan basin new mexico detailed paleomagnetic and rock magnetic analyses have been performed on samples from multiple sections through the cretaceous tertiary k t boundary and throughout a 750 m thick sequence of late cretaceous through middle paleocene continental deposits in the san juan basin new mexico curie temperatures have been determined for magnetic separates from 52 levels and with only two exceptions they range from 180 to 300 c along with microprobe and x ray analyses these data indicate that the detrital ferrimagnetic mineral is titanohematite with composition 0 45 x 0 60 this magnetic mineralogy indicates derivation of the continental san juan basin sediments from a volcanic probably dacitic or andesitic source these mineralogical data along with other geological data and the pattern of magnetic polarity zonation in multiple sections across the basin argue strongly for deposition of the late cretaceous and paleocene continental deposits in the san juan basin as a clastic wedge derived from a sourc

calving icebergs a source of low frequency seismic signals from columbia glacier alaska low frequency icequakes produced by columbia glacier alaska are caused by large icebergs which break off the glacier terminus into prince william sound occasionally many ice seracs fall in rapid succession producing complex low frequency icequake signals with durations sometimes exceeding 10 min an empirical relationship v 4000 t between the volume v of an iceberg in cubic meters and the duration t of the seismic signal in seconds can be used to estimate the calving rate of columbia glacier from seismological data alone during 1984 and 1985 a period of rapid retreat of the glacier the calculated number of icequakes per month showed seasonal fluctuations that corresponded to variations in calving rate determined by photogrammetry it is likely that most previously reported low frequency icequakes in southern alaska are produced by calving tidewater glaciers

calving seismicity from iceberg sea surface interactions 1 iceberg calving is known to release substantial seismic energy but little is known about the specific mechanisms that produce calving icequakes at yahtse glacier a tidewater glacier on the gulf of alaska we draw upon a local network of seismometers and focus on 80 hours of concurrent direct observation of the terminus to show that calving is the dominant source of seismicity to elucidate seismogenic mechanisms we synchronized video and seismograms to reveal that the majority of seismic energy is produced during iceberg interactions with the sea surface icequake peak amplitudes coincide with the emergence of high velocity jets of water and ice from the fjord after the complete submergence of falling icebergs below sea level these icequakes have dominant frequencies between 1 and 3 hz detachment of an iceberg from the terminus produces comparatively weak seismic waves at frequencies between 5 and 20 hz our observations allow us to suggest that the most powerful sources of calving icequakes at yahtse glacier include iceberg sea surface impact deceleration under the influence of drag and buoyancy and cavitation numerical simulations of seismogenesis during iceberg sea surface interactions support our observational evidence our new understanding of iceberg sea surface interactions allows us to reattribute the sources of calving seismicity identified in earlier studies and offer guidance for the future use of seismology in monitoring iceberg calving

fast tidewater glaciers some iceberg calving outlet glaciers flow continuously at speeds normally associated with surging glaciers arid exhibit dramatic instability scenarios related to those suggested for marine ice sheets no temperate tidewater glaciers are known to have floating termini but many polar and subpolar tidewater glaciers do the fast flow of temperate calving glaciers is almost entirely due to basal sliding and appears to be a function of the effective pressure on the bed which may approach zero and the longitudinal back stress on the terminus the terminus boundary condition the calving relation is imperfectly known yet is vital to the dynamics of these glaciers calving relations for grounded tidewater glaciers have been suggested on empirical grounds but have not been rigorously tested the calving relations for floating termini are virtually unknown this together with the imperfect understanding of basal sliding inhibits confidence in our understanding of the stability of these glaciers columbia glacier alaska is an instructive example because observations have been made on the major changes in its geometry calving rate and dynamics that have occurred in less than 10 years the calving flux has increased more rapidly than the glacier flux causing thinning and retreat as a result the ice velocity has increased markedly the short term velocity changes relate to changes in back pressure ice recession tidal changes and the flux of water injected to the bed these results have relevance to the mechanisms of basal sliding glacier surges and the stability of marine ice streams

contrasts in the response of adjacent fjords and glaciers to ice sheet surface melt in west greenland neighboring tidewater glaciers often exhibit asynchronous dynamic behavior despite relatively uniform regional atmospheric and oceanic forcings this variability may be controlled by a combination of local factors including glacier and fjord geometry fjord heat content and circulation and glacier surface melt in order to characterize and understand contrasts in adjacent tidewater glacier and fjord dynamics we made coincident ice ocean atmosphere observations at high temporal resolution minutes to weeks within a 10 000 km 2 area near uummannaq greenland water column velocity temperature and salinity measurements reveal systematic differences in neighboring fjords that imply contrasting circulation patterns the observed ocean velocity and hydrography combined with numerical modeling suggest that subglacial discharge plays a major role in setting fjord conditions in addition satellite remote sensing of seasonal ice flow speed and terminus position reveal both speedup and slow down in response to melt as well as differences in calving style among the neighboring glaciers glacier force budgets and modeling also point toward subglacial discharge as a key factor in glacier behavior for the studied region individual glacier and fjord geometry modulate subglacial discharge which leads to contrasts in both fjord and glacier dynamics

glacier fjord and seismic response to recent large calving events jakobshavn isbrae greenland 1 the recent loss of jakobshavn isbrae s extensive floating ice tongue has been accompanied by a change in near terminus behavior calving currently occurs primarily in summer from a grounded terminus involves the detachment and overturning of several icebergs within 30 60 min and produces long lasting and far reaching ocean waves and seismic signals including glacial earthquakes calving also increases near terminus glacier velocities by 3 but does not cause episodic rapid glacier slip thereby contradicting the originally proposed glacial earthquake mechanism we propose that the earthquakes are instead caused by icebergs scraping the fjord bottom during calving

seismic observations of transient subglacial water flow beneath macayeal ice stream west antarctica 1 new seismic observations of harmonic tremors beneath macayeal ice stream west antarctica are reported each of the two tremor events that we recorded during a six week period had sustained arrival of 3 hz energy for approximately 10 minutes during that time the source location migrated a few kilometers the harmonic nature of the tremors is interpreted as the result of resonance in subglacial water filled cracks and conduits the duration monochromatic nature and movement of the tremor indicate that the source mechanism is likely flow in the subglacial water system resulting from the discharge from a small subglacial lake our results suggest that the subglacial water system produces repeated small outburst floods with possible implications for ice stream dynamics

magnetic properties of sedimentary greigite fe3s4 new data are presented here that document the previously poorly known magnetic characteristics of sedimentary greigite fe3s4 several diagnostic magnetic properties can be used in conjunction to rapidly screen sediments to detect the presence of greigite such discriminants are necessary because greigite displays a similar or higher range of coercivities compared to most ferrimagnetic minerals and the commonly used method of alternating field demagnetization cannot be used to distinguish between palaeomagnetic remanence components due to greigite and other ferrimagnetic minerals sedimentary greigite has high ratios of the saturation isothermal remanent magnetization to magnetic susceptibility mrs which are accompanied by hysteresis ratios of mrsms 0 5 and bcr bc 1 5 ms is the saturation magnetization bc is the coercive force and bcr is the coercivity of remanence greigite has no low temperature phase transition whereas the ferrimagnetic iron sulphide pyrrhotite undergoes a major phase transition at about 34 k and unoxidized magnetite displays the verwey transition at 118 k low temperature measurements may therefore prove valuable in distinguishing between magnetite pyrrhotite and greigite although the absence of a low temperature transition cannot be taken as definitive evidence for the presence of greigite greigite also displays characteristic high temperature behaviour with a major drop in magnetization between 270 and 350 c determination of the mrs ratio is an excellent means of rapidly screening sediments to identify stratigraphic intervals that may contain greigite subsequent low and high temperature analyses can then provide unambiguous identification of greigite and enable the evaluation of the presence of other magnetic minerals

seismic multiplet response triggered by melt at blood falls taylor glacier antarctica 1 meltwater input often triggers a seismic response from glaciers and ice sheets it is difficult however to measure melt production on glaciers directly while subglacial water storage is not directly observable therefore we document temporal changes in seismicity from a dry based polar glacier taylor glacier antarctica during a melt season using a synthesis of seismic observation and melt modeling we record icequakes using a dense six receiver network of three component geophones and compare this with melt input generated from a calibrated surface energy balance model in the absence of modeled surface melt we find that seismicity is well described by a diurnal signal composed of microseismic events in lake and glacial ice during melt events the diurnal signal is suppressed and seismicity is instead characterized by large glacial icequakes we perform network based correlation and clustering analyses of seismic record sections and determine that 18 of melt season icequakes are repetitive multiplets the epicentral locations for these multiplets suggest that they are triggered by meltwater produced near a brine seep known as blood falls our observations of the correspondingp wave first motions are consistent with volumetric source mechanisms we suggest that surface melt enables a persistent pathway through this cold ice to an englacial fracture system that is responsible for brine release episodes from the blood falls seep the scalar moments for these events suggest that the volumetric increase at the source region can be explained by melt input

glacial earthquakes in greenland and antarctica glacial earthquakes are a new class of seismic events first discovered as signals in long period seismograms recorded on the global seismographic network most of these events occur along the coasts of greenland where they are spatially related to large outlet glaciers glacial earthquakes show a strong seasonality with most events occurring during the late summer the rateofglacial earthquakeoccurrenceincreasedbetween2000and2005 with a stabilization of earthquake frequency at 2003 2004 levels in 2006 2008 recent observations establish a strong temporal correlation between the distinct seismic signals of glacial earthquakes and large ice loss events in which icebergs of cubic kilometer scale collapse against the calving face linking the seismogenic process to the force exerted by these icebergs on the glacier andtheunderlyingsolidearth asuddenchangeinglacierspeedresultsfrom these glacial earthquake calving events seasonal and interannual variations in glacier terminus position account for general characteristics of the temporal variation in earthquake occurrence glacial earthquakes in antarctica are less well studied but they exhibit several characteristics similar to glacial earthquakes in greenland