submarine landforms and shallow acoustic stratigraphy of a 400 km long fjord shelf slope transect kangerlussuaq margin east greenland kangerlussuaq fjord is a relatively uniform steep walled basin whose floor has an almost smooth surface debris is supplied mainly from icebergs from the fast flowing kangerlussuaq glacier sedimentation after iceberg release from multi year sea ice is mainly by rain out of fine grained englacial debris streamlined glacial lineations and drumlins were produced at the sedimentary bed of an ice sheet that expanded into kangerlussuaq trough at the last glacial maximum lgm bedrock channels and crescentic overdeepenings indicate warm based ice and free water beneath parts of the former ice sheet cross cutting iceberg scour marks which characterise outer kangerlussuaq shelf were produced not only during deglaciation but also occasionally through the holocene by deep keeled icebergs from further north in east greenland the outward convex contours of the shelf edge and slope beyond kangerlussuaq trough and debris flows on the slope suggest a glacier influenced high latitude fan the distribution of streamlined subglacial landforms demonstrates that the greenland ice sheet extended throughout kangerlussuaq fjord and reached at least 200 km across the shelf in kangerlussuaq trough at the lgm streamlined landform orientation indicates ice flow from the interior of greenland down the axis of kangerlussuaq trough there is little evidence for discrete sedimentary depocentres in the trough implying that ice probably retreated rapidly from the outer and mid shelf during deglaciation

late quaternary glacial interglacial changes in sediment composition at the east greenland continental margin and their paleoceanographic implications stable isotope records and sedimentological and organic geochemical investigations of marine sediments from the east greenland sea at 70 n provide important information about glacial interglacial variations of paleoenvironments through the last 225 kyr the oxygen isotope records established on the planktonic foraminifer n pachyderma sin show some excursions from the global climate pattern probably due to local regional overprint by meltwater supply the cold low saline east greenland current and fluctuations in sea ice covering were a crucial element controlling the carbonate production in the subsurface surface water column in the east greenland sea over the last 225 kyr the beginning of termination ia is ams 14c dated at about 15 8 kyr b p and interpreted as a greenland ice sheet meltwater signal the stage 2 3 boundary is dated at about 25 kyr b p the timing of the onset of the last deglacial meltwater event is about 800 years earlier than that of the barents shelf ice sheet meltwater signal recorded in the fram strait several major pulses of increased supply of coarse grained terrigenous material by glacio marine processes occurred during the last 225 kyr the supply of coarse grained ice rafted debris at the east greenland continental slope reached maximum values during the last glacial maximum stage 2 weichselian 15 19 kyr b p the drastic climatic change and the gradual retreat of continental ice masses glaciers during the last deglaciation termination i are clearly documented in the marine sedimentary sequences from shelf and upper slope environments this process resulted in distinctly decreased supply and deposition of ice rafted debris in the open shelf upper slope environments during termination i the sea ice cover also decreased causing an increase in surface water productivity indicated by increased organic carbon and biogenic opal deposition

thickness and extent of the subglacial till layer beneath an antarctic paleo ice stream fast flowing ice streams and outlet glaciers currently account for as much as 90 of the discharge from the antarctic and greenland ice sheets although the deformation of subglacial material has been proposed as the mechanism for this rapid motion such sediment is usually hidden under several kilometers of ice marine geophysical records have allowed reconstruction of the three dimensional thickness of the sedimentary bed beneath a large antarctic paleo ice stream for the first time fast flow is indicated by streamlined seafloor lineations that form the surface of a layer of low shear strength unsorted sediment averaging 4 6 m thick rapid motion of the paleo ice stream was a result of subglacial deformation within this layer

the last glaciation and deglaciation of the northeast greenland continental shelf revealed by hydro acoustic data about 16 of the greenland ice sheet drains in the area of the northeast greenland shelf between 76 n and 80 5 n via marine terminating glaciers most of it is via the northeast greenland ice stream the largest ice stream of greenland during ice ages the ice sheet extended onto the continental shelf and modern day cross shelf troughs were filled by ice streams in this study high resolution hydro acoustic data acquired during three decades of research were jointly investigated to reveal the past glacial conditions our data shows that westwind trough and norske trough were filled by fast flowing ice streams that extended to the shelf edge during the last glacial maximum in between the cross shelf troughs ice domes resided on shallow banks that may have contributed about a decimetre to global sea level most probably these ice domes initiated fast ice flow through sinks in the inter trough area in westwind trough ice sheet retreat to the inner shelf after the last glacial maximum was intermittent in contrast in norske trough the ice sheet retreat appears relatively rapid with no evidences for phases of grounding line stabilization probably during the younger dryas the ice sheet readvanced to a mid shelf position in both troughs documented by grounding zone wedges during this time a thick ice shelf was present in norske trough releasing tabular icebergs ice sheet retreat from the mid shelf to the coastline during holocene deglaciation was rapid

why did the 2011 2012 la nina cause a severe drought in the brazilian northeast the brazilian northeast ne is strongly affected by el nino southern oscillation enso during la nina events the precipitation over the ne is generally above average however during the last la nina event in 2011 2012 the ne went through its worst drought in the last 30 years in this study observations and numerical simulations are used to determine what made the 2011 2012 event different from other events we find that eastern pacific canonical la nina events cause a cooling of the tropical north atlantic and warming of the tropical south atlantic that lead to a southward migration of the intertropical convergence zone which in turn brings rain to the ne on the other hand la nina events with the cooling concentrated in the central pacific cause the opposite meridional sea surface temperature sst gradient in the tropical atlantic leading to droughts over the ne the 2011 2012 event was of the latter type this study also shows that it is possible to predict the sign of the ne rainfall anomaly during enso events using a simple sst index

hydroclimate in africa during the medieval climate anomaly the medieval climate anomaly mca is a recognized period of distinct pre industrial climate change with a core period of 1000 1200 ce the field of palaeoclimatology has made major progress over the past 15 years during which a great number of high and medium resolution case studies were published reconstructing climate change of the past millennia in many parts of the world regional data coverage has now reached a point which allows compiling palaeoclimate maps for well defined time intervals here we present hydroclimatic trend maps for the mca in africa based on 99 published study locations key hydroclimatic proxy curves are visualized and compared in a series of 16 correlation panels proxy types are described and possible issues discussed based on the combined mca dataset temporal and spatial trends are interpreted and mapped out three areas have been identified in africa in which rainfall seems to have increased during the mca namely tunisia western sahel and the majority of southern africa at the same time a reduction in precipitation occurred in the rest of africa comprising of nw and ne africa west africa eastern africa and the winter rainfall zone of south africa mca hydroclimate change in africa appears to have been associated with characteristic phases of ocean cycles as also supported by modern climate observations aridity in morocco typically coincides with the positive phase of the north atlantic oscillation nao whilst increased rainfall in the western sahel is often coupled to the positive phase of the atlantic multidecadal oscillation amo reduction in rainfall in the region gulf of aden southern red sea to eastern africa could be linked to a negative indian ocean dipole iod or a derived long term equivalent indian ocean cycle parameter the intertropical convergence zone itcz appears to have been shifted pole wards during the mca for both the january and july positions mca hydroclimate mapping revealed major data gaps in the sahara south sudan somalia central african republic democratic republic of congo angola northern mozambique zambia and zimbabwe special efforts are needed to fill these gaps e g through a dedicated structured research program in which new multiproxy datasets are created based on the learnings from previous african mca studies

low gradient outlet glaciers ice streams drained the laurentide ice sheet observations of geomorphic features dated with cosmogenic nuclides are used to determine the configuration of glaciation in cumberland sound baffin island during the last glacial maximum cumberland sound which was a major drainage of the laurentide ice sheet at its northeastern arctic canada margin contained low gradient ice with a surface slope of 0 038 and a driving stress between 2 and 7 kpa these values are similar to those observed for the ice streams that dominate the dynamics of the west antarctic ice sheet today the presence of low gradient ice in cumberland sound and other major outlets may similarly have influenced laurentide ice sheet behavior and would account for the iceberg discharge episodes that produced the heinrich event debris layers observed in north atlantic ocean sediments

evolution of the relationship between near global and atlantic sst modes and the rainy season in west africa statistical analyses and sensitivity experiments monthly sea surface temperature anomalies ssta at near global scale 60 n 40 s and may to october rainfall amounts in west africa 16 n 5 n 16 w 16 e are first used to investigate the seasonal and interannual evolutions of their relationship it is shown that west african rainfall variability is associated with two types of oceanic changes 1 a large scale evolution involving the two largest ssta leading eigenmodes 16 of the total variance with stronger loadings in the equatorial and southern oceans related to the long term multiannual component of rainfall variability mainly expressed in the sudan sahel region and 2 a regional and seasonally coupled evolution of the meridional thermal gradient in the tropical atlantic due to the linear combination of the two largest ssta modes in the atlantic 11 with strong inverse loadings over the northern and southern tropics which is associated with the interannual and quasi decadal components of regional rainfall in west africa linear regression and discriminant analyses provide evidence that the main july september rainfall anomalies in sudan sahel can be detected with rather good skills using the leading april june or synchronous july september values of the four main oceanic modes in particular the driest conditions over sahel more marked since the beginning of the 1970s are specifically linked to the warm phases of the two global modes and to cold warm anomalies in the northern southern tropical atlantic idealized but realistic ssta patterns obtained from some basic linear combinations of the four main oceanic modes appear sufficient to generate quickly from mid july to the end of august significant west african rainfall anomalies in model experiments consistent with the statistical results the recent negative impact on west african rainfall exerted by the global oceanic forcing is primarily due to the generation of subsidence anomalies in the mid troposphere over west africa when an idealized north to south ssta gradient is added in the tropical atlantic strong north to south height gradients in the middle levels appear these limit the northward excursion of the rainbelt in west africa the sahelian area experiences drier conditions due to the additive effect subsidence anomalies latitudinal blocking while over the guinea regions wet conditions do not significantly increase since the subsidence anomalies and the blocking effect act here in opposite ways

is the boreal spring tropical atlantic variability a precursor of the equatorial mode the equatorial mode em governs the tropical atlantic inter annual variability during boreal summer it has profound impacts on the climate of adjacent and remote areas however predicting the em is one of the most challenging and intriguing issues for the scientific community recent studies have suggested a possible connection between the boreal spring meridional mode mm and the em through ocean wave propagation here we use a set of sensitivity experiments with a medium resolution ocean model to determine the precursor role of a mm to create equatorial sst variability our results demonstrate that boreal summer equatorial ssts following a mm are subject to two counteracting effects the local wind forcing and remotely excited oceanic waves for a positive mm the anomalous easterly winds blowing along the equator shallow the thermocline cooling the sea surface via vertical diffusion and meridional advection anomalous wind curl excites a downwelling rossby wave north of equator which is reflected at the western boundary becoming an equatorial kelvin wave kw this downwelling kw propagates eastward deepening the thermocline and activating the thermocline feedbacks responsible for the equatorial warming moreover the local wind forcing and rw reflected mechanism have a significant and comparable impact on the equatorial sst variability changes in the intensity and persistence of these distinct forcings will determine the equatorial sst response during boreal summer our results give a step forward to the improvement of the em predictability

on the atlantic pacific ninos connection a multidecadal modulated mode atlantic and pacific el nino are the leading tropical oceanic variability phenomena at interannual timescales recent studies have demonstrated how the atlantic nino is able to influence on the dynamical processes triggering the development of the pacific la nina and vice versa however the stationarity of this interbasin connection is still controversial here we show for the first time that the atlantic pacific ninos connection takes place at particular decades coinciding with negative phases of the atlantic multidecadal oscillation amo during these decades the atlantic pacific connection appears as the leading coupled covariability mode between tropical atlantic and pacific interannual variability the mode is defined by a predictor field the summer atlantic sea surface temperature sst and a set of predictand fields which represent a chain of atmospheric and oceanic mechanisms to generate the pacific el nino phenomenon alteration of the walker circulation surface winds in western pacific oceanic kelvin wave propagating eastward and impacting on the eastern thermocline and changes in the pacific sst by internal bjerknes feedback we suggest that the multidecadal component of the atlantic acts as a switch for el nino prediction during certain decades putting forward the amo as the modulator acting through changes in the equatorial atlantic convection and the equatorial pacific sst variability these results could have a major relevance for the decadal prediction systems

processes in the pacific la nina onset triggered by the atlantic nino previous observational and model studies have shown that a warm cold event in the equatorial atlantic during the boreal summer are related to the development of a pacific la nina el nino event that is fully developed in the following winter although the connection takes place via atmospheric bridge the processes at work have not been clarified for such a remote and lagged relationship the present paper uses a partially coupled atmosphere ocean model to infer a mechanism by which a pacific el nino event can be developed in this way enhanced equatorial convection in the equatorial atlantic during a warm event results in enhanced subsidence and surface wind divergence over the equatorial pacific around the dateline this wind anomaly contributes to pile up water in the western equatorial pacific triggering a perturbation in the depth of the oceanic thermocline which propagates eastward as an equatorial kelvin wave from autumn to winter the thermocline shallowing as the wave propagates allows for cooling of the oceanic mixed layer through anomalous temperature advection by anomalous zonal currents and by mean vertical entrainment velocity zonal advective and thermocline feedbacks reinforce the surface winds anomalies over the central eastern equatorial pacific setting up the conditions for the development of a cold event in this ocean the sequence during an atlantic cold event is similar with the appropriate change in signs these findings are relevant to enso predictability at seasonal timescales

the physiography of high arctic cross shelf troughs abstract a comprehensive inventory of high arctic cross shelf troughs is compiled from international bathymetric chart of the arctic ocean ibcao bathymetric data v 3 0 the location of 75 cross shelf troughs is presented alongside a synthesis of their key physiographic characteristics and available glacial geological evidence the troughs are interpreted to have been occupied intermittently and eroded by marine terminating ice streams that traversed the shelf during at least one and often many quaternary full glacial periods considerable variation in cross shelf trough physiography exists in the high arctic trough lengths range between 35 and 1400 km widths from 12 to 260 km and maximum depths from 200 to 1000 m the longest cross shelf troughs extend through inter island channels on the beaufort sea queen elizabeth islands and barents kara sea margins the gradient of the upper slope beyond high arctic glacial troughs which ranges between 0 3 and 13 is shown to have a negative relationship with palaeo ice stream drainage basin area and trough length glacial sedimentary depocentres or trough mouth fans tmfs are inferred on the basis of bathymetric and where available seismic evidence to exist beyond the majority of high arctic cross shelf troughs evidence of shelf progradation or tmf development is absent from the slope beyond several high arctic troughs probably as a result of limited sediment supply to the margin and or sediment by passing of the upper slope on the south greenland continental margin it is likely that steep slopes prevented the development of significant glacial sedimentary depocentres beyond cross shelf troughs or that glacigenic debris has been removed from the upper slope by submarine slope failure

bedform signature of a west antarctic palaeo ice stream reveals a multi temporal record of flow and substrate control abstract the presence of a complex bedform arrangement on the sea floor of the continental shelf in the western amundsen sea embayment west antarctica indicates a multi temporal record of flow related to the activity of one or more ice streams in the past mapping and division of the bedforms into distinct landform assemblages reveals their time transgressive history which implies that bedforms can neither be considered part of a single downflow continuum nor a direct proxy for palaeo ice velocity as suggested previously a main control on the bedform imprint is the geology of the shelf which is divided broadly between rough bedrock on the inner shelf and smooth dipping sedimentary strata on the middle to outer shelf inner shelf bedform variability is well preserved revealing information about local complex basal ice conditions meltwater flow and ice dynamics over time these details which are not apparent at the scale of regional morphological studies indicate that past ice streams flowed across the entire shelf at times and often had onset zones that lay within the interior of the antarctic ice sheet today in contrast highly elongated subglacial bedforms on sedimentary strata of the middle to outer shelf represent a timeslice snapshot of the last activity of ice stream flow and may be a truer representation of fast palaeo ice flow in these locations a revised model for ice streams on the shelf captures complicated multi temporal bedform patterns associated with an antarctic palaeo ice stream for the first time and confirms a strong substrate control on a major ice stream system that drained the west antarctic ice sheet during the late quaternary

fracture detection using p wave and s wave vertical seismic profiling at the geysers in a pilot vertical seismic profiling study p wave and cross polarized s wave vibrators were used to investigate the potential utility of shear wave anisotropy measurements in characterizing a fractured rock mass the caprock at the geysers geothermal field was found to exhibit about an 11 percent velocity variation between sh waves and sv waves generated by rotating the s wave vibrator orientation to two orthogonal polarizations for each survey level in the well the effect is generally consistent with the equivalent anisotropy expected from the known fracture geometry

evaluation of anisotropy by shear wave splitting the polarizations of three component shear wavetrains carry unique information about the internal structure of the rock through which they pass specifically commonly observed shear wave splitting may contain information about the orientation of crack distributions this information cannot usually be recovered from shear waves recorded at the free surface because of interference with the interaction of the shear wave with the surface even for nearly vertical incidence however shear wave splitting in synthetic three component vertical seismic profiles in some cases may be interpreted directly in terms of the direction of strike of vertical cracks because the human eye is not skilled at identifying the phase relationships between three component seismograms played out conventionally as parallel time series the polarizations are displayed in orthogonal sections of the particle displacements to facilitate recognition and evaluation of the shear wave splitting estimating the orientations of cracks an

spatial and temporal patterns of iceberg rafting ird along the east greenland margin ca 68 n over the last 14 cal ka twelve 1 2 m 10 cm diameter gravity cores collected in 1988 and 1991 from the continental shelf and fjords of east greenland near kangerlussuaq fjord trough ca 68 n 32 w have distinct changes in lithofacies and in the quantity of iceberg rafted ird sediments these changes are readily observed in x radiographs of the split cores we quantify the ird contribution through grain size analyses and counting the number of clasts 2 mm from the x radiographs chronological control is provided by acclerated mass spectroscopy 14c dates on foraminifera during deglaciation after 14 cal ka there was one interval of ird accumulation ca 12 13 cal ka followed by a brief return to ird conditions centred at 9 cal ka thereafter a prominent feature of most cores on the shelf is an increase in ird accumulation that started ca 5 6 cal ka and which has increased toward the present indicators of iceberg rafting such as the net sand flux and numbers of clasts 2 mm ka 1 follow a power law distribution when graphed against distance from the present east greenland coast a measure of the position of the glacier margins the form of the relationship indicates that there is a dramatic decrease in the supply of sediment from the fjords to the shelf these relationships are used to estimate changes in the location of the ice margin during the late quaternary based on a site on the east greenland slope denmark strait and to discuss factors that can negate such a simple transfer function 1997 by john wiley sons ltd

late quaternary sedimentation in kejser franz joseph fjord and the continental margin of east greenland the marine sedimentary record in kejser franz joseph fjord and on the east greenland continental margin contains a history of late quaternary glaciation and sedimentation evidence suggests that a middle shelf moraine represents the maximum shelfward extent of the greenland ice sheet during the last glacial maximum on the upper slope coarse grained sediments are derived from the release of significant quantities of iceberg rafted debris ird and subsequent remobilization by subaqueous mass flows the middle lower slope is characterized by hemipelagic sedimentation with lower quantities of ird dropstone mud and sandy mud punctuated episodically by deposition of diamicton and graded sand gravel facies by subaqueous debris flows and turbidity currents derived from the mass failure of upper slope sediments the downslope decrease of ird reflects either the action of the east greenland current egc confining icebergs to the upper slope or to the more ice proximal setting of the upper slope relative to the lgm ice margin sediment gravity flows on the slope are likely to have fed into the east greenland channel system contributing to its formation in conjunction with the cascade of dense brines down the slope following sea ice formation across the shelf deglaciation commenced after 15 300 14c years as indicated by meltwater derived light oxygen isotope ratios an abrupt decrease in both ird deposition and delivery of coarse grained debris to the slope at this time supports ice recession with icebergs confined to the shelf by the egc glacier ice had abandoned the middle shelf before 13 000 14c years with ice loss through iceberg calving and deposition of diamicton continued retreat of glacier ice from the inner shelf and through the fjord is marked by a transition from subglacial till bedrock in acoustic records to ice proximal meltwater derived laminated mud to ice distal bioturbated mud ice abandoned the inner shelf before 9100 14c years and probably stabilized in fosters bugt at 10 000 14c years distinct oxygen isotope minima on the inner shelf indicate meltwater production during ice retreat the outer fjord was free of ice before 7440 14c years glacier retreat through the mid outer fjord was punctuated by topographically controlled stillstands where ice proximal sediment was fed into fjord basins the dominance of fine grained commonly laminated facies during deglaciation supports ablation controlled ice mass loss glacimarine sedimentation within the holocene middle outer fjord system is dominated by sediment gravity flow and suspension settling from meltwater plumes suspension sediments comprise mainly mud facies indicating significant meltwater deposition that overwhelms debris release from icebergs in this east greenland fjord system the relatively widespread occurrence of fine grained lithofacies in east greenland fjords suggests that meltwater sedimentation can be significant in polar glacimarine environments the ice distal continental margin is characterized by meltwater sedimentation in the inner shelf deep iceberg scouring over shallow shelf regions winnowing and erosion by the east greenland current on the middle outer shelf and hemipelagic sedimentation on the continental slope

ice stream retreat following the lgm and onset of the west greenland current in uummannaq trough west greenland the deglacial history and oceanography of uummannaq trough central west greenland continental shelf was investigated using foraminiferal sedimentological and bathymetric records together with a radiocarbon chronology providing a timeline for the retreat of glacial ice after the last glacial maximum lgm to map ice stream retreat data were collected from cores from the outer jr175 vc45 and jr175 vc43 and inner jr175 vc42 uummannaq trough a large ice stream fed by confluent glaciers draining the interior of the greenland ice sheet extended across the outer shelf during the lgm and was in retreat by 15 0 cal kyr bp foraminiferal data indicate that the warm west greenland current wgc was established prior to 14 0 cal kyr bp which is the hitherto earliest record of atlantic water found on the west greenland shelf for each of the cores foraminifera indicate that ice sheet retreat was followed quickly by incursion of the wgc suggesting that the warm water may have enhanced ice retreat prior to the younger dryas cold event the radiocarbon chronology indicates that the ice sheet retreated to the mid shelf where it subsequently stabilised and formed a large grounding zone wedge gzw after the younger dryas around 11 5 cal kyr bp the ice retreated rapidly from the gzw and into the fjords

analysis of p and s wave vertical seismic profile data from the salton sea scientific drilling project as part of the salton sea scientific drilling project at california state well 2 14 vertical seismic profile vsp data were collected from p and s wave sources at two distances from the well use of a three component geophone along with rotation of the recorded data traces into a wave front based coordinate system allows analysis of many aspects of seismic wave propagation properties around the well standard vsp analysis techniques were used to measure interval p and s wave velocities and to identify reflecting horizons both within and below the survey interval from 455 to 1735 m a reflection from below the survey interval seen with both p and s sources seems to be associated with a fractured reservoir near 2100 m indications of fracturing were observed including vertical scattering of p waves from a zone near 915 m orthogonally polarized shear waves were generated at each offset to study anisotropy by travel time measurement and particle motion analysis of the shear wave arrivals three component particle motion analysis of shear wave arrivals was found to be effective for characterizing the subtleties in the s wave splitting throughout the various zones in the well the sht source horizontal and transverse to the well produced complicated elliptical particle motion while the sv source in line with the well produced linear particle motion the difference in linearity of particle motion from orthogonally polarized shear wave sources was unexpected and may be related to regional tectonics anomalous zones may be related to transition depths in the salton sea geothermal field travel time difference between sv and sh waves while clearly observable indicates only about 1 average anisotropy

greenland ice sheet retreat history in the northeast baffin bay based on high resolution bathymetry new swath bathymetric data acquired in 2010 and 2015 indicate a variety of glacial landforms in cross shelf troughs of the melville bay northeast baffin bay these landforms reveal that at their maximum extent ice streams in the troughs crossed the shelf all the way to the shelf edge moraines grounding zone wedges gzws and subglacial till lobes on the continental shelf define a pattern of variable ice stream retreat in the individual troughs on the outer shelf in the northern cross shelf trough ice stream retreat was slow compared to more episodic retreat in the central at least one stabilization on the outer shelf and southern cross shelf trough re advances at the shelf edge and fast retreat thereafter large gzws on the mid to inner shelf of the troughs indicate periods of grounding zone stabilization according to glacial landforms the final retreat across the inner shelf before 8 41 ka bp was episodic to slow furthermore evidence has been found for localized ice domes with minor ice streams on inter trough banks the glacial landforms in melville bay thus indicate the varying and discontinuous ice sheet retreat history across the northwest greenland continental shelf

bedmachine v3 complete bed topography and ocean bathymetry mapping of greenland from multibeam echo sounding combined with mass conservation greenland s bed topography is a primary control on ice flow grounding line migration calving dynamics and subglacial drainage moreover fjord bathymetry regulates the penetration of warm atlantic water aw that rapidly melts and undercuts greenland s marine terminating glaciers here we present a new compilation of greenland bed topography that assimilates seafloor bathymetry and ice thickness data through a mass conservation approach a new 150 m horizontal resolution bed topography bathymetric map of greenland is constructed with seamless transitions at the ice ocean interface yielding major improvements over previous data sets particularly in the marine terminating sectors of northwest and southeast greenland our map reveals that the total sea level potential of the greenland ice sheet is 7 42 0 05 m which is 7 cm greater than previous estimates furthermore it explains recent calving front response of numerous outlet glaciers and reveals new pathways by which aw can access glaciers with marine based basins thereby highlighting sectors of greenland that are most vulnerable to future oceanic forcing

african hydroclimatic variability during the last 2000 years abstract the african continent is characterised by a wide range of hydroclimate regimes ranging from humid equatorial west africa to the arid deserts in the northern and southern subtropics the livelihoods of much of its population are also vulnerable to future climate change mainly through variability in rainfall affecting water resource availability a growing number of data sources indicate that such hydroclimatic variability is an intrinsic component of africa s natural environment this paper co authored by members of the pages africa 2k working group presents an extensive assessment and discussion of proxy historical and instrumental evidence for hydroclimatic variability across the african continent spanning the last two millennia while the african palaeoenvironmental record is characterised by spatially disjunctive datasets with often less than optimal temporal resolution and chronological control the available evidence allows the assessment of prominent spatial patterns of palaeomoisture variability through time in this study we focus sequentially on data for six major time windows the first millennium ce the medieval climate anomaly 900 1250 ce the little ice age 1250 1750 ce the end of the lia 1750 1850 ce the early modern period 1850 1950 and the period of recent warming 1950 onwards this results in a continent wide synthesis of regional moisture balance trends through history allowing consideration of possible driving mechanisms and suggestions for future research

tropical response to the atlantic equatorial mode agcm multimodel approach on the frame of the amma eu project sensitivity experiments for an atlantic equatorial mode aem which origin development and damping resembles the observed one during the last decades of the 20th century has been analysed in order to investigate the influence on the anomalous summer west african rainfall recent studies raise the matter of the aem influence on the next pacific enso episodes and also on the indian monsoon this paper evaluates the response of four different atmospheric global circulation models using the above mentioned aem sensitivity experiments to study the tropical forcing associated with the atlantic nino mode the results show a remote signal in both the pacific and indian basins for a warm phase of the aem the associated southward location of the itcz with rising motions over the equatorial atlantic leads to a global subsidence over the rest of the tropics weakening the asian monsoon and favouring the la nina conditions in the central pacific although ocean atmosphere coupled experiments are required to test the latter hypothesis the present studies shows how the aem is able to influence the rest of the tropics a result with important implications on enso seasonal predictability

tropical atmospheric response to decadal changes in the atlantic equatorial mode it has been shown that the atmospheric response to the atlantic equatorial mode is non stationary after the 1970s sea surface temperature sst anomalies in the tropical atlantic are able to alter the atmosphere in the tropical pacific via modifications of the walker circulation such changes could be related to the differences in the background state of the global ssts before and after the 1970s but also to changes in the interannual equatorial mode itself in this work we first describe the differences in the interannual equatorial mode before and after the 1970s then we use two agcms to perform different sensitivity experiments changing the spatial structure of the equatorial mode and we explore the differences in the atmospheric response over the tropical pacific region to each of the sst patterns considered it is shown that the changes in the walker atlantic pacific cell produced by the em are stronger after the 1970s and are reinforced by the change in the impact of the em over the indian ocean and the maritime continent it is also shown that although the atlantic pacific connection is established by the aforementioned changes in the walker circulation between the two basins the modulation of the indian sector is crucial for a realistic simulation of such connection by climate models

formation of mega scale glacial lineations observed beneath a west antarctic ice stream most discharge from large ice sheets takes place through fast flowing ice streams and their speed is strongly modulated by interactions between the ice and the underlying sediments seismic surveys and investigations through boreholes have revealed a spatial association between fast ice flow and saturated deformable sediments nevertheless our knowledge of the morphology of the interface between ice and sediments is still limited resulting in only rudimentary understanding of the basal boundary conditions beneath ice streams and the generation of subglacial bedforms here we present radar data from the bed of a west antarctic ice stream that reveal the presence of mega scale glacial lineations we combine these data with previously published seismic data and show that these lineations develop in areas of dilatant deforming till and are part of a dynamic sedimentary system that undergoes significant change by erosion and deposition on decadal timescales we find that the mega scale glacial lineations are indistinguishable from those found on beds of palaeo ice streams providing conclusive evidence for the hypothesis that highly elongate bedforms are a characteristic of fast flow regions in ice sheets

tropical atlantic variability modes 1979 2002 part i time evolving sst modes related to west african rainfall abstract this work presents a description of the 1979 2002 tropical atlantic ta sst variability modes coupled to the anomalous west african wa rainfall during the monsoon season the time evolving sst patterns with an impact on wa rainfall variability are analyzed using a new methodology based on maximum covariance analysis the enhanced climate prediction center cpc merged analysis of precipitation cmap dataset which includes measures over the ocean gives a complete picture of the interannual wa rainfall patterns for the sahel dry period the leading ta sst pattern related to the atlantic el nino is coupled to anomalous precipitation over the coast of the gulf of guinea which corresponds to the second wa rainfall principal component the thermodynamics and dynamics involved in the generation development and damping of this mode are studied and compared with previous works the sst mode starts at the angola benguela region and is caused by alongshore wind anomalies it then propagates wes

seismological studies at parkfield ii search for temporal variations in wave propagation using vibroseis abstract for more than 3 years the propagation characteristics of shear waves have been monitored for paths near the 1966 hypocenter at parkfield the presumed nucleation site for the expected next m 6 earthquake there data have been collected repeatedly 33 sets as of april 1991 from eight s wave vibroseis source positions into the 10 borehole installed three component seismometers of the local high sensitivity digital network twenty second correlated records from a 6 to 24 hz sweep are acquired and the entire seismogram is viewed for analysis as the elastic response of the local crustal structure which includes the san andreas fault zone amplitudes travel times spectra and particle motions of the p and s waves are monitored for indications of any changes in these properties that may be attributed to processes associated with nucleation the horizontal vibrator at each source point is positioned at three surface orientations to study anisotropy unorthodox methods have been developed to display the waveform properties in time in order to visualize the resulting massive data sets the first order variations seen in some of the parameters are attributed to changes from dry to wet conditions in the shallow subsurface due to the seasonal rainfall which affects the source function of the vibrator corrections have been devised for these source specific variations secular variations not obviously coupled to seasonal near surface changes are also seen in some localized time intervals within the 20 sec records the most striking of such changes is a progressive travel time decrease at rates of 3 to 7 msec year seen for late arrivals 7 to 11 sec travel time on at least five paths into station vca which sample the region southeast of the anticipated epicenter at middle mountain this anomaly appears to be genuine and is now the subject of intensified study in the same general area along the fault in the southwestern block the direct s wave is clearly split with the faster of the split phases polarized parallel to the fault zone a result in agreement with that from the vsp survey in the varian well on the northeast side of the fault

relative sea level change kjove land scoresby sund east greenland implications for seasonality in younger dryas time the age of the milne land stade mls moraines in the scoresby sund region of east greenland is key for testing the hypothesis that strong seasonality marked abrupt cooling events such as the younger dryas registered in greenland ice cores the relevant chronology is based on 69 radiocarbon dates of shells from raised beaches and deltas related to marine inundation and isostatic rebound that accompanied glacier retreat from the moraines taken together these dates form the basis for a relative sea level curve that shows very high rates of emergence typical of recently deglaciated regions upward extrapolation of this curve suggests that the marine limit 134 m a s l dates to about 12 400 cal yr b p the shorelines that mark the marine limit lie within areas that were ice covered when the glaciers were at the outer limit of the mls advance hence the moraines that form the outer limit must antedate 12 400 cal yr b p and probably are from earliest younger dryas or allerod time at holger danskes briller an inner moraine grades to a massive ice contact delta now at 101 m a s l which is dated to 11 000 11 300 cal yr b p the age of the outer mls moraines along with constraints on the maximum possible younger dryas ice extent and snowline lowering are consistent with the idea that east greenland climate experienced strong seasonality during younger dryas time

recurrence models and parkfield california earthquakes main shocks of the earthquake sequences that occurred on the parkfield section of the san andreas fault in central california in 1922 1934 and 1966 are characterized by southeast rupture expansion over the same 20 to 30 km long section of the fault whereas the seismic moments for the 1922 and 1934 events are identical to within a precision of 10 the seismic moment for 1966 is 20 greater than for the earlier events to within a precision of 20 the parkfield area seismicity in general seems well described by a recurring moderate size characteristic earthquake repeating the same epicenter magnitude seismic moment rupture area and southeast direction of rupture expansion an unexplained 10 year advance of the 1934 event is the only discrepancy in the hypothesis that the parkfield earthquakes in 1857 1881 1901 1922 1934 and 1966 represent a strictly periodic process assuming the strictly periodic model and the absence since 1966 of the perturbations hypothesized for the 1922 to 1934 period the next characteristic parkfield earthquake should occur between 1983 and 1993

be 10 ages from central east greenland constrain the extent of the greenland ice sheet during the last glacial maximum traditional ice sheet reconstructions have suggested two distinctly different ice sheet regimes along the east greenland continental margin during the last glacial maximum lgm ice to the shelf break south of scoresby sund and ice extending no further than to the inner shelf at and north of scoresby sund we report new 10be ages from erratic boulders perched at 250 m a s l on the kap brewster peninsula at the mouth of scoresby sund the average 10be ages calculated with an assumed maximum erosion rate of 1 cm ka and no erosion respectively 17 3 2 3 ka and 15 1 1 7 ka overlap with a period of increased sediment input to the scoresby sund fan 19 15 ka the results presented here suggest that ice reached at least 250 m a s l at the mouth of scoresby sund during the lgm and add to a growing body of evidence indicating that lgm ice extended onto the outer shelf in northeast greenland

multiply polarized shear wave vsps from the cajon pass drillhole a 3 component vsp data set was acquired in the cajon pass drillhole with a multiply polarized shear wave source and a conventional p wave source producing a 9 component data set the vsp data show effects on the full wavefield from the sediment granite interface mode converted waves are identified with significant scattering of s wave energy from the leading p wavefront hodographs are used to display s wave particle motions in two dimensions shear wave splitting is observed in the hodographs the shear wave particle motion is used to infer anisotropy orientation with respect to the local strike of the san andreas fault stress induced fracturing is assumed to be a possible cause of the shear wave splitting a variation of particle motion orientation with depth is observed implying a variation in anisotropy orientation as a function of depth

holocene evolution of the western greenland ice sheet assessing geophysical ice sheet models with geological reconstructions of ice margin change abstract geophysical ice sheet models are used to predict future ice sheet dimensions and in turn these projections help estimate the magnitude of eustatic sea level rise before models can confidently predict ice sheet behavior they must be validated by being able to duplicate the geological record of ice sheet change here we review geological records of greenland ice sheet gris change with emphasis on the warmer than present middle holocene and compare these records to published studies that numerically simulate gris behavior through the holocene geological records are concentrated in west and southwest greenland which are also the regions where the gris margin likely experienced the greatest distance of inland retreat during the middle holocene several records spanning from melville bugt to jakobshavn isfjord in western greenland indicate the gris achieved its minimum extent between 5 and 3 ka and farther south in the kangerlussuaq region new data presented here indicate the ice margin reached its minimum extent between 4 2 and 1 8 ka in the narsarsuaq region in southern greenland the gris likely achieved its minimum configuration between 7 and 4 ka we highlight key similarities and discrepancies between these reconstructions and model results and finally we suggest that despite some degree of inland retreat the west and southwest gris margin remained relatively stable and close to its current position through the holocene thermal maximum

seismic quiescence at parkfield an independent indication of an imminent earthquake the parkfield segment of the san andreas fault midway between san francisco and los angeles has experienced six moderate sized earthquakes since 1850 with an average recurrence time of about 22 years based on this regular behaviour and the occurrence of the most recent earthquake in 1966 a shock of local magnitude ml 5 7 is expected to occur by 19931 2 here we suggest that a current lack of small earthquakes on this segment could be a precursory anomaly to the next characteristic parkfield earthquake only four earthquakes of m 2 5 have occurred on the segment since january 1986 whereas twenty would have been expected extrapolating from the mean background rate comparison with other cases of precursory quiescence suggests that the m 5 7 earthquake should occur in the next two years rupturing the same 35 km fault segment that ruptured in 19663

simulated impacts of the south atlantic ocean dipole on summer precipitation at the guinea coast an intermediate complexity atmospheric general circulation model has been used to investigate the influence of the south atlantic ocean sao dipole saod on summer precipitation over the guinea coast of west africa two ensemble integrations in which idealized but realistic saod type sea surface temperature sst anomaly is prescribed only in the sao and then globally are performed and inter compared consistently above below the average precipitation is simulated over the guinea coast during the positive negative phase of the saod comparison of the two set of experiments reveal that in its active years the saod is a dominant mechanism that shapes the spatial character of summer precipitation at the guinea coast the global sst variability merely slightly moderate its effects during the saod cool sst anomaly in the extra tropical sao off the brazil uruguay argentina coast gives rise to suppressed convection and mass divergence in turn the subsidence tends to amplify the sub tropical arm of anomalous hadley type circulation and consequently large scale convection and mass flux convergence in the equatorial atlantic ocean gulf of guinea region bordering on the coastal fringes of west africa precipitation is therefore increased at the guinea coast

ice sheet grounding line dynamics steady states stability and hysteresis 1 the ice sheet ice shelf transition zone plays an important role in controlling marine ice sheet dynamics as it determines the rate at which ice flows out of the grounded part of the ice sheet together with accumulation this outflow is the main control on the mass balance of the grounded sheet in this paper we verify the results of a boundary layer theory for ice flux in the transition zone against numerical solutions that are able to resolve the transition zone very close agreement is obtained and grid refinement in the transition zone is identified as a critical component in obtaining reliable numerical results the boundary layer theory confirms that ice flux through the grounding line in a two dimensional sheet shelf system increases sharply with ice thickness at the grounding line this result is then applied to the large scale dynamics of a marine ice sheet our principal results are that 1 marine ice sheets do not exhibit neutral equilibrium but have well defined discrete equilibrium profiles 2 steady grounding lines cannot be stable on reverse bed slopes and 3 marine ice sheets with overdeepened beds can undergo hysteresis under variations in sea level accumulation rate basal slipperiness and ice viscosity this hysteretic behavior can in principle explain the retreat of the west antarctic ice sheet following the last glacial maximum and may play a role in the dynamics of heinrich events

chronology of the last recession of the greenland ice sheet a new deglaciation chronology for the ice free parts of greenland the continental shelf and eastern ellesmere island canada is proposed the chronology is based on a new compilation of all published radiocarbon dates from greenland and includes crucial new material from southern northeastern and northwestern greenland although each date provides only a minimum age for the local deglaciation some of the dates come from species that indicate ice proximal glaciomarine conditions and thus may be connected with the actual ice recession in addition to shell dates dates from marine algae lake sediments peat terrestrial plants and driftwood also are included only offshore and in the far south have secure late glacial sediments been found other previous reports of late glacial sediments older than 11 5 cal kyr bp from onshore parts of greenland need to be confirmed most of the present ice free parts of greenland and nares strait between greenland and ellesmere island were not deglaciated until the early holocene copyright c 2002 john wiley sons ltd less

freshwater forcing from the greenland ice sheet during the younger dryas evidence from southeastern greenland shelf cores abstract we present evidence for freshwater forcing from the greenland ice sheet during the last deglaciation explore its potential involvement in the younger dryas cooling and investigate the role of atlantic intermediate water in the retreat of the greenland ice sheet paleoceanographic conditions and ice sheet retreat history are interpreted from ird stable isotopes and foraminiferal assemblages in sediment cores retrieved along the length of the kangerlussuaq trough a cross shelf trough on the southeast greenland shelf the vedde ash 11 980 cal yr bp which is a marker bed within the younger dryas and the saksunarvatn tephra 10 180 cal yr bp an early holocene marker horizon have been located in these cores the tephra markers and calibrated 14c dates on foraminifers and molluscs provide the basis for the age models in the cores the stable isotope records indicate a 0 5 depletion of the 18o between 13 4 and 12 8 cal ka bp a pronounced light isotope event exceeding 1 5 on the inner shelf and 0 8 on the outer shelf occurs at the beginning of the younger dryas cold period 12 8 cal ka bp and ends by 12 cal ka bp immediately before deposition of the vedde ash the light isotope spikes are interpreted as glacial meltwater spikes associated with retreat of the adjacent greenland ice sheet associated with the light isotope events are episodic high percentages of the atlantic intermediate water benthic foraminiferal species cassidulina neoteretis which continue from the bolling allerod into the younger dryas high percentages of this species suggest that atlantic intermediate water entered the kangerlussuaq trough during the allerod and the younger dryas via the irminger current as it does today we conclude that advection of atlantic intermediate water into the kangerlussuaq trough south of the denmark strait promoted melting and retreat of the marine portions of the greenland ice sheet explaining the observed glacial meltwater spikes

antarctic palaeo ice streams we review the geomorphological sedimentological and chronological evidence for palaeo ice streams on the continental shelf of antarctica and use this information to investigate basal conditions and processes and to identify factors controlling grounding line retreat a comprehensive circum antarctic inventory of known palaeo ice streams their basal characteristics and minimum ages for their retreat following the last glacial maximum lgm is also provided antarctic palaeo ice streams are identified by a set of diagnostic landforms that nonetheless display considerable spatial variability due to the influence of substrate flow velocity and subglacial processes during the lgm palaeo ice streams extended via bathymetric troughs to the shelf edge of the antarctic peninsula and west antarctica and typically to the mid outer shelf of east antarctica the retreat history of the antarctic ice sheet since the lgm is characterised by considerable asynchroneity with individual ice streams exhibiting different retreat histories this variability allows antarctic palaeo ice streams to be classified into discrete retreat styles and the controls on grounding line retreat to be investigated such analysis highlights the important impact of internal factors on ice stream dynamics such as bed characteristics and slope and drainage basin size whilst grounding line retreat may be triggered and to some extent paced by external atmospheric and oceanic forcing the individual characteristics of each ice stream will modulate the precise timing and rate of retreat through time

an extensive and dynamic ice sheet on the west greenland shelf during the last glacial cycle considerable uncertainty surrounds the extent and timing of the advance and retreat of the greenland ice sheet gis on the continental shelf bordering baffin bay during the last glacial cycle here we use marine geophysical and geological data to show that fast flowing ice sheet outlets including the ancestral jakobshavn isbrae expanded several hundred kilometers to the shelf edge during the last glaciation ca 20 ka retreat of these outlets was asynchronous initial retreat from the shelf edge was underway by 14 880 calibrated cal yr b p in uummannaq trough radiocarbon dates from the adjacent disko trough and adjoining trough mouth fan imply later deglaciation of jakobshavn isbrae and significantly an extensive readvance and rapid retreat of this outlet during the younger dryas stadial yd this is notable because it is the first evidence of a major advance of the gis during the yd on the west greenland shelf although the short duration suggests that it may have been out of phase with yd temperatures

tropical sst and sahel rainfall a non stationary relationship 1 sea surface temperature sst anomalies in the tropical atlantic have been associated with precipitation anomalies in west africa that form a dipole pattern with centers over the sahel and the gulf of guinea whilst this was clear before the 1970 s the dipole pattern almost disappeared after that date as the anti correlation between rainfall anomalies in the sahel and guinea dropped abruptly simultaneously the anti correlations between sahel rainfall and tropical pacific ssts strengthened it has been posited that these changes after the 1970 s developed as rainfall over west africa started to co vary with ssts in the global tropics in this co variability enhanced summer rainfall over west africa with a monopole pattern corresponds to warmer ssts in the tropical atlantic and maritime continent and colder ssts in the tropical pacific and western indian oceans the present paper describes the hitherto unexplored seasonal evolution of this co variability and the physical mechanisms at work sensitivity experiments with two atmospheric general circulation models demonstrate that after the 1970 s the impacts of sst anomalies in the indo pacific counteract those in the atlantic in terms of generating rainfall anomalies over the sahel and that this superposition of effects is primarily linear therefore at interannual timescales the change in the patterns of co variability between west african rainfall and tropical ssts can explain the non stationary relationship between the anomalies in these two fields

the origin of massive diamicton facies by iceberg rafting and scouring scoresby sund east greenland almost 90 of 39 m of core material recovered from scoresby sund and the adjacent east greenland shelf is massive diamicton interpreted to be formed predominantly by the release of iceberg rafted debris and reworking by iceberg scouring there is also likely to be a contribution from suspension settling of fines derived from glaciofluvial sources model calculations suggest that the 14c derived holocene sedimentation rate of 0 1 0 3 m 1000 yr 1 in scoresby sund can be accounted for mainly by iceberg rafting of debris a further 4 of core material is of gravel or coarse sand lenses interpreted to reflect iceberg dumping of debris intensive iceberg scouring which reworks sea floor sediments is observed on acoustic records from over 30 000 km2 of the scoresby sund fiord system and the adjacent east greenland shelf 69 72 n and 75 n the rate of iceberg production from greenland ice sheet outlet glaciers and iceberg drift tracks on the shelf suggests that iceberg rafting and scouring may be important over a significant proportion of the 500 000 km2 area above the shelf break the relatively extensive modern occurrence of massive diamicton formed by iceberg rafting and scouring together with suspension settling of fines suggests that it may also be a significant facies in the glacier influenced geological record the recognition in the geological record of the massive diamicton facies described above may also indicate the former presence of fast flowing ice sheet outlet glaciers

history of a stable ice margin east greenland during the middle and upper pleistocene abstract the east greenland record of glaciation and environmental change is based on a combination of onshore and offshore studies glaciological work on contemporary outlet glaciers provides background for interpreting the glaciation styles of the past the scoresby sund fjord system the single largest outlet of the inland ice in east greenland and the adjacent continental shelf and slope were the main working areas the regionality of these results was tested with onshore and offshore work on sites further to the north in the east greenland fjord zone the record spans the period since isotope stage 7 ca 240 ka the most extensive glaciation phases were the lollandselv and scoresby sund glaciations isotope stages 7 and 6 the langelandselv interglaciation isotope substage 5e comprises shallow marine sediments at more than 20 sites onshore and cibicides wuellerstorfii carrying sediments on the continental slope summer temperatures were 2 3 warmer than the holocene optimum and there was vigorous advection of warm atlantic water into the fjords the warm period seems to be accompanied by a eustatic sea level rise of ca 20 m during the weichselian there were three major glacier advances the aucellaelv jyllandselv and flakkerhuk stades isotope substages 5d 5b and stages 3 2 all three advances mark a shift from one topographically determined stable position to another in each phase the glaciers occupied almost the same space in the fjord basins and had their fronts on the inner shelf but the glacial regime and duration was different the aucellaelv stade lasted ca 10 ka and the regime resembled that of present day outlet glaciers 600 km further to the north the flakkerhuk stade lasted for ca 50 ka but left only thin and sporadic till as well as glaciolacustrine sediments in ice dammed river valleys the regime was polar similar to present day glaciers in antarctica its culmination the last glacial maximum in the period between 22 and 14 ka is shown by a maximum in ird deposition on the continental slope after 12 ka and before 10 ka the outer fjord basins in the southern fjord zone had been abandoned by the glaciers and at ca 9 5 ka the last resurgence the milne land stade had ended

a multi model approach to the atlantic equatorial mode impact on the west african monsoon this paper is focused on the west african anomalous precipitation response to an atlantic equatorial mode whose origin development and damping resembles the observed one during the last decades of the xxth century in the framework of the amma eu project this paper analyses the atmospheric response to the equatorial mode using a multimodel approach with an ensemble of integrations from 4 agcms under a time varying equatorial sst mode the guinean gulf precipitation which together with the sahelian mode accounts for most of the summer west african rainfall variability is highly coupled to this equatorial atlantic sst mode or atlantic nino in a previous study done with the same models under 1958 1997 observed prescribed ssts most of the models identify the equatorial atlantic sst mode as the one most related to the guinean gulf precipitation the models response to the positive phase of equatorial atlantic mode warm ssts depicts a direct impact in the equatorial atlantic leading to a decrease of the local surface temperature gradient weakening the west african monsoon flow and the surface convergence over the sahel

timing and significance of glacially influenced mass wasting in the submarine channels of the greenland basin the most extensive systems of submarine channels on the continental margins of the norwegian greenland sea occur in the greenland basin 72 75jn geophysical records show that these channels are up to 100 m deep 4 km wide and extend for about 300 km from the middle continental slope off northeast greenland to the abyssal depths of the basin mass wasting deposits in the form of debris flows and turbidity currents as well as hemipelagic sediments dominate sediment cores recovered from the channels radiocarbon dates indicate that debris flow and turbidity current activity in the channels had ceased by 13 000 years bp and sedimentation rates show a corresponding order of magnitude decrease after this time masswasting in the submarine channels of the greenland basin therefore relates to full glacial and deglacial conditions this implies a more extensive late weichselian ice sheet in northeast greenland than traditionally thought the ice sheet extended onto the outer continental shelf and may have reached the shelf edge delivering debris and sediment laden meltwater onto the upper slope channel formation is most likely the product of turbidity current activity during successive glaciations of the northeast greenland continental shelf holocene sediments from the channel systems are predominantly of hemipelagic origin and formed in a low energy ice distal environment the low gradient 1j channels of the greenland basin are therefore a glacially influenced sedimentary system contrasts in slope morphology and sediment architecture between the greenland basin and trough mouth fans elsewhere in the norwegian greenland sea are not the result of variations in slope gradient rather these contrasts reflect the interplay between bedrock lithology of the continental shelf rates of sediment delivery sediment texture meltwater volume and relative size of ice sheet drainage basins

irregular glacial interstadials recorded in a new greenland ice core the greenland ice sheet offers the most favourable conditions in the northern hemisphere for obtaining high resolution continuous time series of climate related parameters profiles of 18o 16o ratio along three previous deep greenland ice cores1 3 seemed to reveal irregular but well defined episodes of relatively mild climate conditions interstadials during the mid and late parts of the last glaciation but there has been some doubt as to whether the shifts in oxygen isotope ratio were genuine representations of changes in climate rather than artefacts due to disturbed stratification here we present results from a new deep ice core drilled at the summit of the greenland ice sheet where the depositional environ ment and the flow pattern of the ice are close to ideal for core recovery and analysis the results reproduce the previous findings to such a degree that the existence of the interstadial episodes can no longer be in doubt according to a preliminary timescale based on stratigraphic studies the interstadials lasted from 500 to 2 000 years and their irregular occurrence suggests complexity in the behaviour of the north atlantic ocean circulation

iceberg scouring in scoresby sund and on the east greenland continental shelf modern and relict scours produced by iceberg keels have been reported from a number of high latitude continental shelves in the northern hemisphere and antarctica but observations are largely absent from the east greenland shelf about 18 km3 yr 1 of icebergs are calved into the scoresby sund fjord system east greenland these icebergs have submarine keels with drafts up to 550 m modern iceberg contacts with the sea floor are inferred from calculated keel depths and stationary icebergs identified on sequential landsat imagery acoustic profiles parasound 3 5 khz of the sea floor show that scouring is a significant process in water depths 550 m iceberg scouring intensity varies inversely with water depth the most intense scouring occurs at depths of 300 400 m iceberg scours were observed on over 25 000 km2 of the east greenland shelf at 69 72 n and 75 n scours are being actively produced on the modern shelf but relict scours probably dating back to the late weichselian are also present the rate of iceberg production from greenland ice sheet outlet glaciers and iceberg drift tracks on the shelf suggests that iceberg scouring is important over a significant proportion of the 500 000 km2 area above the shelf break around greenland

microearthquake imaging of the parkfield asperity microearthquake data from a downhole seismometer network on the san andreas fault appear to outline two aseismic asperities that may correspond to the locations of the foreshocks and main shocks of the parkfield characteristic earthquakes the source parameters of the microearthquakes show that a few of the earthquakes have significantly higher stress drops than most furthermore the magnitude frequency statistics suggest that at local magnitude 0 6 the cumulative number of small events begins to fall off the usual gutenberg richter b 1 relation in which the number of events increases exponentially with decreasing magnitude the downhole seismometer data establish a baseline from which the evolution of the earthquake process at parkfield can be monitored and suggest that different mechanical conditions than those that lead to the typical gutenberg richter relation may be operating for the smallest of parkfield microearthquakes

a 10be chronology of lateglacial and holocene mountain glaciation in the scoresby sund region east greenland implications for seasonality during lateglacial time abstract thirty eight new cosmogenic 10 be exposure ages from the scoresby sund region of east greenland indicate that prominent moraine sets deposited by mountain glaciers date from 780 to 310 yr approximately during the little ice age from 11 660 to 10 630 yr at the end of the younger dryas cold interval or during preboreal time and from 13 010 to 11 630 yr during lateglacial time equilibrium line altitudes elas interpreted from lateglacial to early holocene moraines indicate summertime cooling between 3 9 and 6 6 c relative to today s value much less than the extreme younger dryas cooling registered by greenland ice cores mean annual temperatures of 15 c colder than today s value this apparent discrepancy between paleotemperature records supports the contention that younger dryas cooling was primarily a wintertime phenomenon 10 be ages of lateglacial and holocene moraines show that mountain glaciers during the little ice age were more extensive than at any other time since the early holocene epoch in addition 10 be ages of lateglacial moraines show extensive reworking of boulders with cosmogenic nuclides inherited from prior periods of exposure consistent with our geomorphic observations and cosmogenic exposure dating studies in other arctic regions

ice sheet grounding zone wedges gzws on high latitude continental margins abstract grounding zone wedges gzws are asymmetric sedimentary depocentres which form through the rapid accumulation of glacigenic debris along a line source at the grounding zone of marine terminating ice sheets during still stands in ice sheet retreat gzws form largely through the delivery of deforming subglacial sediments the presence of gzws in the geological record indicates an episodic style of ice retreat punctuated by still stands in grounding zone position moraine ridges and ice proximal fans may also build up at the grounding zone during still stands of the ice margin but these require either considerable vertical accommodation space or sediment derived from point sourced subglacial meltwater streams by contrast gzws form mainly where floating ice shelves constrain vertical accommodation space immediately beyond the grounding zone an inventory of gzws is compiled from available studies of bathymetric and acoustic data from high latitude continental margins the locations and dimensions of gzws from the arctic and antarctic alongside a synthesis of their key architectural and geomorphic characteristics are presented gzws are only observed within cross shelf troughs and major fjord systems which are the former locations of ice streams and fast flowing outlet glaciers typical high latitude gzws are less than 15 km in along flow direction and 15 to 100 m thick gzws possess a transparent to chaotic acoustic character which reflects the delivery of diamictic subglacial debris many gzws contain seaward dipping reflections which indicate sediment progradation and wedge growth through continued delivery of basal sediments gzw formation is inferred to require high rates of sediment delivery to a fast flowing ice margin that is relatively stable for probably decades to centuries although the long term stability of the grounding zone is controlled by ice sheet mass balance the precise location of any still stands is influenced strongly by the geometry of the continental shelf the majority of high latitude gzws occur at vertical or lateral pinning points which encourage grounding zone stabilisation through increasing basal and lateral drag and reducing mass flow across the grounding zone

deglaciation chronology sea level changes and environmental changes from holocene lake sediments of germania havn so sabine o northeast greenland abstract germania havn so is located at the outermost coast of northeastern greenland according to radiocarbon dating the lake basin was deglaciated in the early holocene around 11 000 cal yr bp at that time the lake was a marine bay but the lake was isolated soon after deglaciation at 10 600 cal yr bp the marine fauna was species poor indicating harsh conditions with a high sedimentation rate and lowered salinity due to glacial meltwater supply the pioneer vegetation around the lake was dominated by mosses and herbs deposition of relatively coarse sediments during the early holocene indicates erosion of the newly deglaciated terrain remains of the first woody plant salix herbacea appear at 7600 cal yr bp and remains of other woody plants salix arctica dryas octopetala cassiope tetragona and empetrum nigrum appear around one millennium later declining concentrations of d octopetala and the caddis fly apatania zonella in the late holocene probably imply falling summer temperatures only moderate changes in the granulometric and geochemical record during the holocene indicate relatively stable environmental settings in the lake which can probably be explained by its location at the outer coast and the buffering effect of the neighboring ocean

teleconnections between the tropical pacific and the sahel a greater understanding of equatorial teleconnections is a key aspect of research into seasonal prediction and future climate change for tropical regions here the impact of pacific sea surface temperature sst anomalies on rainfall fluctuations over the semi arid sahel is explored using a combination of observed and model data the first key topic of this study is the identification of those aspects of anomalous pacific sst variability that are most strongly linked to the sahel one of these also illuminated by earlier studies is similar to the classic el nino southern oscillation pattern in the central and east pacific which in its el nino phase increases the likelihood of sahel drought it is shown here that although a part of this link is indirect operating via atlantic ssts its main effect appears to be through a direct atmospheric teleconnection the other critical pattern of equal importance and revealed here by a novel analysis technique is the large scale zonal gradient of ssts from the west pacific to the east indian ocean if weakened this too enhances the likelihood of sahel drought atmospheric general circulation model experiments forced either by observed or idealized ssts are used to confirm these two influences on the sahel crucially their sahelian impact is substantially reinforced when both are present and additionally further empirical analysis shows them to be largely independent the second key topic is an investigation of the mechanisms for this pacific sahel teleconnection these appear to involve anomalous stationary equatorial waves with communication occurring in both the eastward and westward directions in el nino years for example a kelvin wave emanates across the atlantic from east pacific convective heating anomalies and an equatorial rossby wave appears over the indian ocean in response to the anomalous west pacific indian ocean sst gradients via convective heating anomalies over the indian ocean these interact over africa to enhance large scale subsidence over the sahel thus reducing seasonal rainfall totals interannual changes in propagating equatorial waves or in the residence of subseasonal regimes appear not to play a substantial role

is there evidence of changes in tropical atlantic variability modes under amo phases in the observational record abstractthe atlantic multidecadal oscillation amo is the leading mode of atlantic sea surface temperature sst variability at multidecadal time scales previous studies have shown that the amo could modulate el nino southern oscillation enso variance however the role played by the amo in the tropical atlantic variability tav is still uncertain here it is demonstrated that during negative amo phases associated with a shallower thermocline the eastern equatorial atlantic sst variability is enhanced by more than 150 in boreal summer consequently the interannual tav modes are modified during negative amo the atlantic nino displays larger amplitude and a westward extension and it is preceded by a simultaneous weakening of both subtropical highs in winter and spring in contrast a meridional seesaw slp pattern evolving into a zonal gradient leads the atlantic nino during positive amo the north tropical atlantic nta mode is related to a scandinavian blocking pattern during winter and spring

holocene glacimarine sedimentation inner scoresby sund east greenland the influence of fast flowing ice sheet outlet glaciers abstract holocene glacimarine sedimentation associated with fast flowing outlet glaciers draining the greenland ice sheet is investigated using sedimentary and acoustic data from inner scoresby sund east greenland sedimentation in inner scoresby sund is dominated by three processes which are influenced by differences in proximity to fast flowing outlet glaciers extent of glacier ice cover and fjord bathymetry 1 sediment gravity flow principally in the form of turbidity currents and debris flows 2 suspension sedimentation from turbid meltwater plumes and 3 iceberg rafting these processes result in texturally and sedimentologically heterogeneous lithofacies proportionally fine grained muds laminated stratified and massive facies dominate cores recovered from inner scoresby sund accounting for 80 of the total whereas diamict facies account for only 15 abundant fine grained muds demonstrate that meltwater flux and sedimentation is significant in this high arctic glacimarine environment in settings proximal to fast flowing outlet glaciers with increasing distance from these glacier termini muds are replaced progressively by iceberg rafted coarse grained sediment the dominance of this iceberg rafted sediment in outer scoresby sund reflects both its more distal location from fast flowing glacier termini and the high calving flux associated with these ice masses laminated muds deposited by turbidity currents and suspension settling from overflow plumes in inner scoresby sund are similar to lithofacies produced in temperate and subpolar glacimarine systems this implies a similarity in sedimentation processes and resulting facies across a wide spectrum of climatically glaciologically fast flowing and non fast flowing ice masses and oceanographically variable glacimarine settings recognition of laminated fine grained facies in the geological record therefore does not necessarily indicate a temperate palaeo glacial setting however the predominance of iceberg rafted diamict facies in ice distal sedimentary records suggests the former presence of relatively cold environmental conditions in which iceberg sedimentation played a dominant role

improved extended reconstruction of sst 1854 1997 abstract an improved sst reconstruction for the 1854 1997 period is developed compared to the version 1 analysis in the western tropical pacific the tropical atlantic and indian oceans more variance is resolved in the new analysis this improved analysis also uses sea ice concentrations to improve the high latitude sst analysis and a modified historical bias correction for the 1939 41 period in addition the new analysis includes an improved error estimate analysis uncertainty is largest in the nineteenth century and during the two world wars due to sparse sampling the near global average sst in the new analysis is consistent with the version 1 reconstruction the 95 confidence uncertainty for the near global average is 0 4 c or more in the nineteenth century near 0 2 c for the first half of the twentieth century and 0 1 c or less after 1950

deglaciation of a major palaeo ice stream in disko trough west greenland recent work has confirmed that grounded ice reached the shelf break in central west greenland during the last glacial maximum lgm here we use a combination of marine sediment core data including glacimarine lithofacies and ird proxy records and geomorphological and acoustic facies evidence to examine the nature of and controls on the retreat of a major outlet of the western sector of the greenland ice sheet gris across the shelf retreat of this outlet which contained the ancestral jakobshavns isbrae ice stream from the outer shelf in disko trough was rapid and progressed predominantly through iceberg calving however minor pauses in retreat tens of years occurred on the middle shelf at a trough narrowing forming subtle grounding zone wedges by 12 1 cal kyr bp ice had retreated to a basalt escarpment and shallow banks on the inner continental shelf where it was pinned and stabilised for at least 100 years during this time the ice margin appears to have formed a calving bay over the trough and melting became an important mechanism of ice mass loss fine grained sediments muds were deposited alternately with ird rich sediments diamictons forming a characteristic deglacial lithofacies that may be related to seasonal climatic cycles high influxes of subglacial meltwater emanating from the nearby ice margins deposited muddy sediments during the warmer summer months whereas winters were dominated by iceberg calving leading to the deposition of the diamictons this is the first example of this glacimarine lithofacies from a continental shelf setting and we suggest that the calving bay configuration of the ice margin plus the switching between calving and melting as ablation mechanisms facilitated its deposition by channelling meltwater and icebergs through the inner trough the occurrence of a major stillstand on the inner shelf in disko trough demonstrates that the ice dynamical response to local topography was a crucial control on the behaviour of a major outlet in this sector of the gris during retreat

earthquake deformation cycle on the san andreas fault near parkfield california six moderate earthquakes are thought to have ruptured the parkfield segment of the san andreas fault since 1857 the similar characteristics of the three seismically recorded events together with the quasi regular 22 year recurrence interval have led to the forecast of a similar event in 1988 5 years in this study the potential for the hypothesized earthquake is assessed by estimating the time required for interseismic straining to recover the strain energy released by the most recent june 1966 parkfield earthquake for a simple model of the earthquake cycle this interval is proportional to the ratio of the 1966 seismic moment to the interseismic moment deficit rate baseline lengths measured before and after the 1966 earthquake are inverted to estimate the magnitude and distribution of the combined coseismic and transient postseismic slip although the geodetic data determine only broad spatial averages of the fault slip they do require that slip at depth exceeded the surface offsets measured in 1966 inversions with extreme prior models are calculated to assess the range of seismic moment and moment deficit rates consistent with the available geodetic data inversions assuming only that the slip distributions are to some degree smooth yield strain energy recovery intervals of 14 25 years extreme values range from as little as 6 years to as much as 29 years in comparison historic recurrence intervals of parkfield earthquakes range from 12 to 32 years these results are consistent with the expectation of a magnitude 5 6 earthquake near parkfield by 1993

warm water pathways toward nioghalvfjerdsfjorden glacier northeast greenland nioghalvfjerdsfjorden glacier 79ng is the largest of three marine terminating outlet glaciers draining the northeast greenland ice stream to understand how atlantic waters supply waters in the cavity beneath the floating 79ng we analyze historic and recent bathymetric hydrographic and velocity observations obtained on the northeast greenland continental shelf the bathymetry is characterized by a trough system consisting of the westwind trough and the norske trough in the northern and southern part of the continental shelf respectively atlantic waters recirculating in fram strait cross the shelf break and enter the trough system at its southeastern inlet toward the inner shelf warm atlantic intermediate water aiw present below 200 m in the norske trough shows large contributions of the recirculating atlantic water we found that the bathymetry is sufficiently deep to provide a direct subsurface pathway for warm aiw between the shelf break and the 79ng cavity via the norske trough likewise based on the hydrographic data we show that the norske trough supplies aiw warmer than 1 c to the 79ng which is not present in the westwind trough our moored and lowered velocity measurements indicate that a boundary current carries warm aiw along the northeastern slope of norske trough toward the 79ng we suggest that anomalies in atlantic water temperatures in fram strait could reach 79ng within less than 1 5 years thereby modifying the glacier s basal melt rates

west african monsoon dynamics and eastern equatorial atlantic and pacific sst anomalies 1970 88 abstract the laboratoire de meteorologie dynamique atmospheric gcm is used to investigate relationships between west african monsoon dynamics and sst anomalies in the eastern equatorial atlantic and pacific for the period 1970 88 positive sst anomalies in the eastern equatorial pacific mainly associated with a larger east west divergent circulation over the tropical atlantic are found to coincide with negative rainfall anomalies over west africa this is the case for the composite enso warm episodes of 1972 1976 1982 and 1983 by contrast positive sst anomalies in the eastern equatorial atlantic are accompanied by a southward shift of the intertropical convergence zone along with negative rainfall anomalies in the sahel and positive rainfall anomalies in the guinean region this was the case in 1987 the enso warm event during this year had apparently no significant impact on west african monsoon dynamics a zonal atmospheric coupling associated with differences of sst anomalies between the eastern

what determines the position and intensity of the south atlantic anticyclone in austral winter an agcm study abstract the south atlantic anticyclone is a major feature of the austral winter climatology an atmospheric general circulation model agcm is used to study the dynamics of the south atlantic anticyclone by means of control simulations and experiments to investigate sensitivity to prescribed orography sea surface temperatures and soil wetness the south atlantic anticyclone in the first control simulation is unrealistically zonally elongated and centered too far west errors typical of coupled ocean atmosphere gcms results of the sensitivity experiments suggest that these deficiencies are associated with another family of systematic model errors the overprediction of convection over the tropical land surfaces particularly over eastern tropical africa and india and the concurrent large scale westward shift in the divergence center at upper levels and the convergence center at lower levels the results also confirm the important role of south american and african orography in localizing the south atl

the role of seasonality in abrupt climate change abstract a case is made that seasonality switches dominated by wintertime were instrumental in abrupt climate changes in the north atlantic region during the last glaciation and into the holocene the primary evidence comes from mismatches between mean annual temperatures from greenland ice cores in comparison with snowline changes in east greenland northern europe and north america the most likely explanation is a shutdown or reduction in strength of the conveyor this allows the spread of winter sea ice across the north atlantic thus causing the northern region to experience much colder winters because they mimic the greenland temperature rather than the snowline signal changes in the atlantic intertropical convergence zone and the asian monsoon may also share a winter linkage with greenland thus the paleoclimate record is consistent with the notion that a huge continental sector of the northern hemisphere stretching from greenland to asia was close to an extreme winter threshold during much of the last glaciation winter climate crossed this threshold repeatedly with marked changes in seasonality that may well have amplified and propagated a signal of abrupt change throughout the hemisphere and into the tropics

african monsoon teleconnections with tropical ssts validation and evolution in a set of ipcc4 simulations a set of 12 state of the art coupled ocean atmosphere general circulation models oagcms is explored to assess their ability to simulate the main teleconnections between the west african monsoon wam and the tropical sea surface temperatures ssts at the interannual to multi decadal time scales such teleconnections are indeed responsible for the main modes of precipitation variability observed over west africa and represent an interesting benchmark for the models that have contributed to the fourth assessment report of the intergovernmental panel on climate change ipcc4 the evaluation is based on a maximum covariance analysis mca applied on tropical ssts and wam rainfall to distinguish between interannual and multi decadal variability all datasets are partitioned into low frequency lf and high frequency hf components prior to analysis first applied to hf observations the mca reveals two major teleconnections the first mode highlights the strong influence of the el nino southern oscillation enso the second mode reveals a relationship between the sst in the gulf of guinea and the northward migration of the monsoon rainbelt over the west african continent when applied to hf outputs of the twentieth century ipcc4 simulations the mca provides heterogeneous results most simulations show a single dominant pacific teleconnection which is however of the wrong sign for half of the models only one model shows a significant second mode emphasizing the oagcms difficulty in simulating the response of the african rainbelt to atlantic sst anomalies that are not synchronous with pacific anomalies the lf modulation of these hf teleconnections is then explored through running correlations between expansion coefficients ecs for ssts and precipitation the observed time series indicate that both pacific and atlantic teleconnections get stronger during the twentieth century the ipcc4 simulations of the twentieth and twenty first centuries do not show any significant change in the pattern of the teleconnections but the dominant enso teleconnection also exhibits a significant strengthening thereby suggesting that the observed trend could be partly a response to the anthropogenic forcing finally the mca is also applied to the lf data the first observed mode reveals a well known inter hemispheric sst pattern that is strongly related to the multi decadal variability of the wam rainfall dominated by the severe drying trend from the 1950s to the 1980s whereas recent studies suggest that this drying could be partly caused by anthropogenic forcings only 5 among the 12 ipcc4 models capture some features of this lf coupled mode this result suggests the need for a more detailed validation of the wam variability including a dynamical interpretation of the sst rainfall relationships

mega scale glacial lineations and cross cutting ice flow landforms landsat images reveal a previously unsuspected large scale pattern of streamlining within drift that is assumed to reflect former phases of ice flow such a glacial grain can be regarded as a landform assemblage comprised of a number of components drumlins and megaflutes form part of the pattern but in addition there are two previously undocumented ice moulded landform elements streamlined lineations of much greater proportions referred to as mega scale glacial lineations and a distinctive cross cutting topology within the grain the ice moulded landform assemblage is described and illustrated with reference to examples from canada possible modes of genesis of such landforms are discussed and their glaciological implications outlined the discovery of this pattern indicates the pervasive nature of subglacial deformation of sediment and demands a radical re interpretation of ice sheet dynamics

evidence for the asynchronous retreat of large outlet glaciers in southeast greenland at the end of the last glaciation recent rapid changes in the marine terminating sectors of the greenland ice sheet gris have prompted concerns about the future stability of the ice sheet long term records of ice sheet behaviour provide valuable context to assess the magnitude of current change and may help resolve the mechanisms driving deglaciation we report 23 new 10be exposure ages which constrain the deglacial history of two large fjord systems in southeast se greenland we compare our chronologies with existing data from the centre of the sector to examine the timing and style of deglaciation at a regional scale glacial erratic 10be exposure ages demonstrate that kangerdlugssuaq fjord deglaciated at 11 8 ka at the end of the younger dryas yd 12 8 11 6 ka retreat at kangerdlugssuaq fjord coincided with known incursion of the warm irminger current ic onto the continental shelf this is inferred to have initiated retreat comparison with recently published results from sermilik fjord and new 10be ages from bernstorffs fjord indicates deglaciation occurred 1 ka later in the south of the se region sermilik fjord 10 9 ka and bernstorffs fjord 10 4 ka deglaciated later retreat likely occurred in response to dramatic climatic amelioration at the termination of the yd stadial we suggest that the disparate timing of deglaciation across the se region may be primarily explained by the varying influence of the warm ic glaciers in southern se greenland were isolated from warm atlantic waters during the yd by complex shelf bathymetry in all fjord settings ice retreat was rapid and persistent consistent with the absence of geomorphological evidence for stillstand or readvance events ice retreat was accompanied by rapid thinning and likely continued to well within present day ice sheet margins glacial erratic 10be age determinations and geomorphological observations show no evidence for holocene readvance events prior to the little ice age lia

substantial export of suspended sediment to the global oceans from glacial erosion in greenland approximately 8 of the fluvial suspended sediment exported to the world s oceans comes from rivers draining the greenland ice sheet according to an analysis of satellite imagery furthermore the export is dominated by areas where subglacial erosion is high

seismological studies at parkfield i simultaneous inversion for velocity structure and hypocenters using cubic b splines parameterization a model for the 3d p and s wave velocity structure at parkfield has been developed by the systematic inversion of arrival times for microearthquakes and surface vibroseis sources recorded by nearby calnet stations and by the local borehole installed 10 station high resolution seismic network there a modification of the linear b splines model parameterization method of thurber 1983 to a cubic b splines formulation through tests inverting synthetic data sets for the parkfield configuration is found to produce smooth velocity distributions that still retain some small scale features application of the method to 953 p wave and 288 s wave arrivals from 111 microearthquakes and 8 vibroseis locations throughout the study area yields a velocity structure having variable resolution a low velocity zone appears to coincide with the creeping segment of the san andreas fault zone northwest of the 1966 parkfield earthquake epicenter an increase of about 15 per cent in seismic velocities is seen from the northeast to southwest across this segment of the fault zone the higher velocities to the southwest apparently correspond to the granitic basement the contrast across the fault is subdued to the southeast along the 1966 rupture zone a high vp vs ratio of 1 9 is seen in a 2 km wide volume near the location of the 1966 main shock hypocenter presumably the nucleation zone for the expected m 6 parkfield earthquake