large sea ice outflow into the nares strait in 2007 1 sea ice flux through the nares strait is most active during the fall and early winter ceases in mid to late winter after the formation of ice arches along the strait and re commences after breakup in summer in 2007 ice arches failed to form this resulted in the highest outflow of arctic sea ice in the 13 year record between 1997 and 2009 the 2007 area and volume outflows of 87 103 km2 and 254 km3 are more than twice their 13 year means this contributes to the recent loss of the thick multiyear arctic sea ice and represents 10 of our estimates of the mean ice export at fram strait clearly the ice arches control arctic sea ice outflow the duration of unobstructed flow explains more than 84 of the variance in the annual area flux in our record seasonal stoppages are always associated with the formation of an arch near the same location in the southern kane basin additionally close to half the time another ice arch forms just north of robeson channel prior to the formation of the kane basin arch here we examine the ice export with satellite derived thickness data and the timing of the formation of these ice arches

upper mantle reflector structure and origin beneath the scottish caledonides the mantle reflector pattern beneath the scottish and irish caledonides is perhaps the most widely studied individual structure in the uppermost continental mantle previous investigations interpreted the reflector as a thrust related to the caledonian orogeny silurian early devonian or as a normal fault or shear zone active during post caledonian e g devonian or permo triassic extension this study uses all available deep seismic reflection data to map mantle reflector structure along the caledonian orogen and to constrain better its geometrical and tectonic relation to regional geology the distribution of the mantle reflector shows that it is not spatially related to either caledonian compressional or subsequent basin extensional structures in a simple way a structural contour map shows two distinct discontinuous surfaces consisting of a steeply plunging antiform synform pair beneath the west orkney basin north of the scottish mainland and the faeroe shetland basin west of shetland respectively separated by an inferred zone of structural disruption a working hypothesis for this study is that these two surfaces may have once been part of a continuous more or less planar surface that has been offset and deformed by either or perhaps a combination of two processes 1 differential extension of crust that was at least partially coupled to the uppermost mantle beneath the west orkney and faeroe shetland basins 2 a broad approximately orthogonal i e east west trending zone of simple shear corresponding to the disrupted zone that was largely confined to the upper mantle but reactivated to deform the crust our results also suggest that the mantle reflector structure may be inherited from neoproterozoic events associated with rifting of the laurentia baltica supercontinent

late cenozoic lacustrine and climatic environments at tule lake northern great basin usa cores of lake sediment to a depth of 334 m in the town of tulelake siskiyou county northern california document the late cenozoic paleolimnologic and paleoclimatic history of the northwestern edge of the great basin the cores have been dated by radiometric tephrochronologic and paleomagnetic analyses lacustrine diatoms are abundant throughout the record and document a nearly continuous paleolimnologic history of the tule lake basin for the last 3 myr during most of this time this basin tule lake was a relatively deep extensive lake except for a drier and cooler interval recorded by fragilaria species about 2 4 ma the pliocene is characterized by a dominance of planktonic aulacoseira solida implying a warm monomictic lake under a climatic regime of low seasonality much of the pleistocene is dominated by stephanodiscus and fragilaria species suggesting a cooler often drier and highly variable climate benthic diatoms typical of alkaline enriched saline waters commonly appear after 1 0 ma and tephrochronology indicates slow deposition and possible hiatuses between about 0 6 and 0 2 ma the chronology of even numbered oxygen isotope stages approximately matches fluctuations in the abundance of fragilaria since 800 ka indicating that glacial periods were expressed as drier environments at tule lake glacial and interglacial environments since 150 ka were distinct from and more variable than those occurring earlier the last full glacial period was very dry but shortly thereafter tule lake became a deep cool lacustrine system indicating a substantial increase in precipitation aulacoseira ambigua characterized the latest glacial and holocene record of tule lake its distribution indicates that warmer and wetter climates began about 15 ka in this part of the great basin diatom concentration fluctuates at 41 000 year intervals between 3 0 and 2 5 ma and at approximately 100 000 year intervals after 1 0 ma in the late pliocene and early pleistocene aulacoseira solida percentages wax and wane in an approximately 400 000 year cycle the possible response of tule lake diatom communities to orbitally induced insolation cycles underscores the importance of this record for the study of late cenozoic paleoclimate change

was there a pliocene pleistocene fluvial lacustrine connection between death valley and the colorado river since the turn of the century a pliocene pleistocene connection between the death valley owens river pluvial system and the colorado river drainage basin has been frequently postulated the two most commonly proposed routes involve 1 a southward overflow from the death valley lake system or 2 southward migration of the mojave river between its present course and a more southerly route under the present topographic regime a death valley lake capable of overflowing the bedrock saddle at ludlow california and discharging southward into the bristol cadiz and danby lake basins and eventually the colorado river would be over 12 000 km2 in size few surface and subsurface indicators exist to support either a fluvial or lacustrine connection evidence from deep cores and boreholes drilled in soda bristol cadiz and danby dry lake basins indicate that a hydrologic connection has not occurred during the past 4 myr no well documented paleoshoreline features have been located at elevations corresponding to the above hydrologic systems in death valley silver soda bristol or danby lake basins in the cadiz silurian and broadwell basins these features have not been found at all therefore we conclude that a hydrologic connection between the death valley owens river system and the colorado river has not occurred along either of these routes since the middle pliocene

the history of the walls boundary fault shetland the northward continuation of the great glen fault from scotland interpretations of the walls boundary fault usually take into account only a small part of the previously published evidence and some even present unsupported speculative interpretations in particular geological accounts rarely mention the geophysical evidence available and geophysical papers take little account of the geology of the area this paper reviews all published evidence geological and marine geophysical evidence show that the fault passes from close to fair isle through shetland and continues to the northeast that a 65 km dextral offset occurred in jurassic times on a gouge filled fracture and that prior to this a net sinistral displacement of the order of 100 km had occurred including a mylonite producing offset of unknown sense and distance in carboniferous times

a model for the deep structure of the moine thrust zone seismic refraction and electrical conductivity profiles across northern scotland together with geological and other geophysical data are interpreted in terms of the deep structure of this part of the orthotectonic caledonides our preferred interpretation is that the moine thrust zone extends to the base of the crust and the caledonian front is steep this contrasts with recently proposed ultra thin skinned solutions the results of the recently completed moist deep seismic reflection experiment should inter alia test our hypothesis in order to test the geometrical feasibility of our interpretation balanced sections have been constructed to illustrate the crustal evolution of the region during the caledonian orogeny these sections permit estimates to be made of crustal shortening thickening and erosion our model requires that the n highlands were once overlain by major exotic nappes which by analogy with nw newfoundland may have been ophiolitic the proposed regional structural pattern comprises a duplex of crustal dimensions we suggest that crustal duplex development is an important mechanism in the orogenic deformation of previously thinned and extended passive continental margins

magnetostratigraphy and tephrochronology of an upper pliocene to holocene record in lake sediments at tulelake northern california combined paleomagnetic and tephra chronologies of one of the most complete middle pliocene through holocene stratigraphic records yet recovered in western north america provide a reference section for much of northwestern north america and adjacent pacific ocean five long drill cores of lacustrine sediments at tulelake northern california recovered a nearly continuous 331 m thick record spanning the past 3 m y the brunhes normal polarity matuyama reversed polarity and gauss normal polarity chronozones are recognized within these the jaramillo olduvai reunion and kaena subchronozones are present six short stratigraphic intervals exhibit anomalous remanent inclinations that may record excursions and brief subchrons within the brunhes and matuyama chronozones age estimates suggest correlation of five of the anomalous intervals with 1 one of the biwa excursions at about 18 000 yr b p 2 the mono lake excursion at about 27 000 yr b p 3 the blake reversed polarity subchron at about 114 000 yr b p 4 the kamikatsura normal polarity subchron at about 850 000 yr b p and 5 the cobb mountain normal polarity subchron at about 1 10 ma age of the sixth interval of anomalous inclination is broadly constrained between 117 000 and 180 000 yr b p sixty three individual tephra layers were characterized by electron microprobe and x ray fluorescence analyses of volcanic glass shards identified tephra of relatively well known age include 1 the basal airfall pumice at llao rock 7015 yr b p 2 the trego hot springs bed 23 400 yr b p 3 the olema ash bed between 55 000 and 75 000 yr b p 4 the airfall pumice at cloudcap road pumice castle like tephra 2 about 120 000 yr b p 5 the rockland ash bed about 410 000 yr b p 6 the lava creek b ash bed 620 000 yr b p 7 the rio dell ash bed about 1 45 ma and 8 the bear gulch ash bed about 1 9 ma a sedimentation rate curve based on independently dated tephra and polarity reversals is used to infer age estimates of undated or previously unidentified ash beds some of these ash beds are found over large areas of the western united states and eastern pacific ocean basin and provide widespread horizons for correlation most of the tephra at tulelake records eruptions from the nearby southern and central cascade range of oregon and northern california and the medicine lake highland of northern california deposition took place during most of the past 3 m y within the tulelake basin notable periods of slow or sporadic accumulation or erosion occurred between about 620 000 and 200 000 yr b p and between about 2 5 and 2 1 ma rapid deposition occurred during marine oxygen isotope stage 6 between about 170 and 125 ka regional volcanism during the past 3 m y was markedly episodic with notable volcanic activity from about 2 1 to 1 9 ma and from 0 4 ma to the present

atmospheric circulation during holocene lake stands in the mojave desert evidence of regional climate change it is commonly thought that the climate conditions that supported lakes over a period of years in the mojave desert in southern california only existed before 8 000 yr bp and that the environment has been arid since1 2 here we look at a drill core in the silver lake playa at the terminus of the mojave river and find holocene lake deposits which indicate that shallow lakes existed for at least a few decades these deposits were radiocarbon dated at 3620 70 and 390 90 yr bp corresponding to the early neo glacial and the little ice age respectively3 to identify the conditions necessary to produce these holocene lake events we have examined the modern climate and hydrological patterns that produce ephemeral lakes in this usually arid watershed available data indicate that there is a link between anomalous winter atmospheric conditions over the north pacific and mojave river floods that produced ephemeral lakes in the silver lake playa and that the mojave river filters out small to medium floods and allows only the extreme floods to reach the terminal playa and leave a record of the anomalous conditions we suggest that the late holocene lakes may have resulted from persistent similar atmospheric circulation patterns and winter floods

500 000 year stable carbon isotopic record from devils hole nevada the record of carbon 13 delta 13 c variations in dh 11 vein calcite core from devils hole nevada shows four prominent minima near glacial terminations glacial interglacial transitions v to ii the delta 13 c time series is inversely correlated with the dh 11 oxygen isotope ratio time series and leads it by as much as 7000 years the delta 13 c variations likely record fluctuations in the delta 13 c of dissolved inorganic carbon of water recharging the aquifer how such variations are transported 80 kilometers to devils hole without obliteration by water rock reaction remains an enigma the record may reflect i global variations in the delta 13 c of atmospheric co 2 and hence the delta 13 c of continental biomass or ii variations in extent and density of vegetation in the southern great basin in the latter case delta 13 c minima at 414 334 246 and 133 thousand years ago mark times of maximum vegetation

a 250 000 year climatic record from great basin vein calcite implications for milankovitch theory a continuous record of oxygen 18 18o variations in the continental hydrosphere during the middle to late pleistocene has been obtained from a uranium series dated calcitic vein in the southern great basin the vein was deposited from ground water that moved through devils hole an open fault zone at ash meadows nevada between 50 and 310 ka thousand years ago the configuration of the 18o versus time curve closely resembles the marine and antarctic ice core vostok 18o curves however the u th dates indicate that the last interglacial stage marine oxygen isotope stage 5 began before 147 3 ka at least 17 000 years earlier than indicated by the marine 18o record and 7 000 years earlier than indicated by the less well dated antarctic 18o record this discrepancy and other differences in the timing of key climatic events suggest that the indirectly dated marine 18o chronology may need revision and that orbital forcing may not be the principal cause of the pleistocene ice ages

a dynamical model of kara sea land fast ice this paper introduces modifications to the traditional viscous plastic sea ice dynamical model which are necessary to model land fast ice in the kara sea in a realistic manner the most important modifications are an increase in the maximum viscosity from the standard value of max 2 5 108s p to max 1013s p and to use a solver for the momentum equation capable of correctly solving for small ice velocities the limit here is set to 10 4 m s given these modifications a necessary condition for a realistic fast ice simulation is that the yield curve give sufficient uniaxial compressive strength this is consistent with the idea that land fast ice in the kara sea forms primarily via static arching the modified model is tested and tuned using forcing data and observations from 1997 and 1998 the results show that it is possible to model land fast ice using this model with the modifications mentioned above the model performs well in terms of modeled fast ice extent but suffers from unrealistic break ups during the start and end of the fast ice season the main results are that fast ice in the kara sea is supported by arching of the ice the arches footers resting on a chain of islands off shore

structure of a continental strike slip fault from deep seismic reflection walls boundary fault northern british caledonides reprocessing of four offshore deep reflection profiles across the walls boundary strike slip fault postulated as the northward continuation of the great glen fault over the shetland platform provides useful constraints on the lithospheric structure of a major continental transform and some insight into the manner in which strike slip faults deform the reprocessing was aimed at delineating complex structure and reducing noise contamination and consisted of analyses of diffractions and amplitude variations and experimentation with seismic migration the results demonstrate a highly variable crustal and uppermost mantle structure for the walls boundary fault along more than 150 km of its length six key observations can be made from this study 1 as middle and lower crustal reflections approach the fault they are truncated by and do not continue across the downward projection of the fault 2 trends in reflection structure of the crust and or uppermost mantle differ markedly from one side of the fault to the other 3 diffractions typically are concentrated at the level of the moho discontinuity directly beneath the fault 4 after seismic migration steeply dipping reflectors are imaged in places directly beneath the fault in the lower crust 5 in one location the moho reflection is deflected across the fault into a narrow synform or keel structure which is exactly bisected by the downward projection of the fault and 6 in a separate location a prominent upper mantle reflector is truncated across the fault a vertical or steeply dipping structure is deduced for the fault in much of the crust and perhaps parts of the uppermost mantle however more diverse reflecting structures are interpreted for the lower crust such as the moho deflection and steeply dipping reflectors directly beneath the fault that may have been related to a localized component of transpression and resultant crustal thickening the presence of diffractions at the level of the moho discontinuity where the downward projection of the fault intersects it suggests points of structural roughness that may be related to deformation of the moho by late motion along the fault integrating the interpretation of the reflection data with previous geological studies implies that the age of the moho deflection is post early cretaceous but that the dipping structure adjacent to and cut off by the fault is probably caledonian silurian early devonian although the disparate structure of the crust on opposite sides of the fault supports geologic interpretations of large scale displacement a major step in the moho discontinuity directly beneath the walls boundary fault cannot be substantiated from the seismic data

radiocarbon and cation radio ages for rock varnish on tioga and tahoe marainal boulders of pine creek eastern sierra nevada california and their paleoclimatic implications abstract accelerator mass spectrometry 14 c analyses of organic matter extracted from rock varnishes on morainal boulders yield limiting minimum ages for three crests of the tioga glaciation at pine creek in the eastern sierra nevada varnish started to form on boulders of the outermost tioga moraine before 19 000 yr b p and varnish originated on the innermost tioga moraine before 13 200 yr b p comparisons with lake level paleohydrological paleoecological colluvial and rock varnish micromorphological data indicate that central eastern california and western nevada experienced a moisture effective period during the late pleistocene but after the tioga maximum and perhaps as tioga glaciers receded from the mouth of pine creek canyon varnishes on tahoeage morainal boulders at pine creek have cation ratio ages of about 143 000 156 000 yr b p suggesting that the tahoe glaciation should not be correlated with oxygen isotope stage 4 in the early wisconsin but rather with stage 6 varnishes on morainal boulders of an older glaciation at pine creek are dated by cation ratio at about 182 000 187 000 yr b p

the moine thrust in the birps data set a second generation of birps profiles combine with older data to form a loose 10 by 40 km spacing grid over the offshore projection of the moine thrust these profiles show clear reflections that arguably represent fault plane reflections from the moine and related caledonian thrust structures in the location suggested by some earlier workers reflectors predominate at two structural levels 8 10 km 2 5 3 5 stwt and 13 16 km 4 0 5 5 stwt depths the deeper set appears to define an antiformal stack of thrusts or duplex with possible continuation to deeper levels by analogy with onshore sections and after subsequent extension is restored this deeper set correlates with the moine thrust onshore the shallower set aligns with normal faults that bound half grabens filled with devonian and permo triassic rocks this set correlates with the naver thrust after similar restoration normal faulting appears to have reactivated some of the thrusts increasing their reflectivity detailed stacking and interval velocity studies indicate that an increase of about 1 2km s 1 occurs across the reflections here associated with the two fault zones the overall reflection pattern is consistent with a complex stack of thrust sheets but lack of resolution prevents a complete structural analysis

seismic exploration of continental strike slip zones abstract seismic exploration of both palaeozoic and active strike slip zones shows strike slip faults that penetrate all or most of the crust offsets on the moho are evident particularly at young and active zones with a component of compression such as the alpine fault of new zealand where a change in crustal thickness of about 20 km is observed moho offsets for the old palaeozoic strike slip zones are usually much less prominent careful migration of crustal seismic reflection data from some of these zones shows that instead of sharp offsets the moho structure consists of a localized keel type crustal thickening of a few kilometres in amplitude and occurs over a zone approximately 10 km wide the large moho offsets of young strike slip zones may in some cases partially decay with time active strike slip zones are becoming an important focus of study but seismic exploration is hampered by complex near surface geology 3d structure and the difficulty of imaging steeply dipping structure in the subsurface in order to meet this challenge a wide range of seismic techniques is now being deployed these include wide angle seismic reflection refraction p wave delays and the study of guided s waves results from california give geophysical images of vertical strike slip faults that penetrate to the moho in contrast the alpine fault of new zealand appears to be a surface manifestation of an inclined 40 ramp extending down to the lower crust and along which uplift and exhumation of the continental crust and possibly strike slip motion is taking place

devonian extensional tectonics versus carboniferous inversion in the northern orcadian basin the old red sandstone middle devonian orcadian basin was formed as a consequence of extensional collapse of the caledonian orogen onshore study of these collapse basins in orkney and shetland provides directions of extension during basin development the origin of folding of old red sandstone sediments that has generally been related to a carboniferous inversion phase is discussed syndepositional deformation supports a devonian age and consequently some of the folds are related to basin formation large scale folding of devonian strata results from extensional and left lateral transcurrent faulting of the underlying basement spatial variation of extension direction and distribution of extensional and transcurrent tectonics fit with a model of regional releasing overstep within a left lateral megashear in nw europe during late caledonian extensional collapse later inversion probably during the upper carboniferous is characterized by e w to ne sw contraction it induced reactivation of extensional faults as thrusts development of small scale folds and thrusts and right lateral transcurrent movement of the major faults such as the great glen and walls boundary faults

multiyear sea ice export through the bering strait during winter 2011 2012 1 six ice beacons deployed in the beaufort sea during august 2011 tracked the anomalous export of multiyear sea ice from the chukchi sea through the bering strait to the bering sea between november 2011 and may 2012 these are the first observations in 34 years of ice beacon export through the bering strait using 34 years of passive microwave derived ice motion fields we find that during 2011 2012 southward ice motion in the chukchi sea persisted for a record six of seven months and that sea ice speeds were significantly faster than the long term mean the combination of increased ice speeds and reduced likelihood of ice arch development through the strait culminated in the record export of 13 5 103 km2 of sea ice through the bering strait monthly sea level pressure fields dominated by an aleutian low and siberian high show anomalies in december and january played a role in initiating this event and forced multiyear ice into the southern chukchi sea however these variations were small and typical of this area yet we find no evidence of a similar export event in the last 34 years even though the forcing was similar to the climatology this leads us to attribute this event to a change in the responsiveness of the arctic ice pack to typical forcing mechanisms

geological constraints structural evolution and deep geology of the northwest scottish caledonides the front of the caledonian orogenic belt in nw scotland is marked by a zone of foreland thrusting which contains imbricates of lewisian basement proterozoic cover and cambrian shelf sediments faults within this moine thrust belt in most localities propagated in a foreland directed sequence therefore the earliest of these thrusts was the moine thrust which carried a basement assemblage of moine and lewisian rocks over the cambrian shelf the development of large duplex and imbricate structures in the footwall postadted movement on this thrust hence a restoration of these imbricates provides a minimum estimate of the eastern extent of the cambrian shelf no synsedimentary or prethrust extensional features are recognised within the cambrian succession which displays a remarkable stratigraphic and thickness consistency this implies a corresponding consistency in crustal structure beneath these cover rocks immediately prior to caledonian thrusting a balanced cross section constructed across the northern part of the moine thrust belt has given a restored width for the cambrian shelf of 54 km geophysical data suggest that the present foreland crustal thickness is 28 km if this is taken to be the crustal thickness beneath the cambrian shelf during caledonian thrusting a minimum cross sectional area of 1512 km of lewisian rocks must remain beneath the present outcrop of the moine thrust sheet this foreland basement wedge may have been imbricated subsequent to the development of the cambrian imbricates caledonian structures within the moine thrust sheet are interpreted as forming an imbricate stack with the moine thrust acting as a floor thrust the ductile imbricate thrusts possibly roofed into the naver slide late open folds which warp foliation slides and metamorphic isograds within the moine sheet may represent culminations in the foreland lewisian imbricates beneath the moine thrust these postulated culminations may have been sufficient to initiate gravity driven structures this may explain late movements on high level faults within the moine thrust belt

a diatom record of climate and hydrology for the past 200 ka from owens lake california with comparison to other great basin records abstract diatoms from lake sediments beneath owens lake playa inyo county california document a nearly continuous paleolimnological record of climate and hydrologic change since the penultimate glacial interglacial cycle based on a chronology established by radiocarbon tephrochronology and paleomagnetic control freshwater planktic diatoms especially species of stephanodiscus plagioclase feldspar rich sediments with high magnetic susceptibility and juniperus type pollen characterized the penultimate glaciation at owens lake saline diatoms dominated in the following interglacial period and there are several episodes during which freshwater planktic diatoms became abundant between 100 and 50 ka that may represent interstadial climatic conditions saline diatoms fell to low values after 50 ka but warm season aulacoseira species indicate episodes of significant summer precipitation in the hydrologic balance of owens lake prior to the last glacial maximum by 25 ka glacial environments were again characterized by abundant juniperus plagioclase feldspar and stephanodiscus species generally arid holocene climates were recorded in owens lake by short term fluctuations of saline and freshwater diatoms desiccation and oolitic sediments barren of diatoms comparison to paleoclimate records both north and south of owens lake suggest a southerly displacement of storm tracks originating from the aleutian low during glacial episodes

processes controlling surface bottom and lateral melt of arctic sea ice in a state of the art sea ice model we present a modelling study of processes controlling the summer melt of the arctic sea ice cover we perform a sensitivity study and focus our interest on the thermodynamics at the ice atmosphere and ice ocean interfaces we use the los alamos community sea ice model cice and additionally implement and test three new parametrization schemes i a prognostic mixed layer ii a three equation boundary condition for the salt and heat flux at the ice ocean interface and iii a new lateral melt parametrization recent additions to the cice model are also tested including explicit melt ponds a form drag parametrization and a halodynamic brine drainage scheme the various sea ice parametrizations tested in this sensitivity study introduce a wide spread in the simulated sea ice characteristics for each simulation the total melt is decomposed into its surface bottom and lateral melt components to assess the processes driving melt and how this varies regionally and temporally because this study quantifies the relative importance of several processes in driving the summer melt of sea ice this work can serve as a guide for future research priorities

on the arctic ocean ice thickness response to changes in the external forcing submarine and satellite observations show that the arctic ocean ice cover has undergone a large thickness reduction and a decrease in the areal extent during the last decades here the response of the arctic ocean ice cover to changes in the poleward atmospheric energy transport fwall is investigated using coupled atmosphere ice ocean column models two models with highly different complexity are used in order to illustrate the importance of different internal processes and the results highlight the dramatic effects of the negative ice thickness ice volume export feedback and the positive surface albedo feedback the steady state ice thickness as a function of fwall is determined for various model setups and defines what we call ice thickness response curves when a variable surface albedo and snow precipitation is included a complex response curve appears with two distinct regimes a perennial ice cover regime with a fairly linear response and a less responsive seasonal ice cover regime the two regimes are separated by a steep transition associated with surface albedo feedback the associated hysteresis is however small indicating that the arctic climate system does not have an irreversible tipping point behaviour related to the surface albedo feedback the results are discussed in the context of the recent reduction of the arctic sea ice cover a new mechanism related to regional and temporal variations of the ice divergence within the arctic ocean is presented as an explanation for the observed regional variation of the ice thickness reduction our results further suggest that the recent reduction in areal ice extent and loss of multiyear ice is related to the albedo dependent transition between seasonal and perennial ice i e large areas of the arctic ocean that has previously been dominated by multiyear ice might have been pushed below a critical mean ice thickness corresponding to the above mentioned transition and into a state dominated by seasonal ice

arctic ocean perennial sea ice breakdown during the early holocene insolation maximum arctic ocean sea ice proxies generally suggest a reduction in sea ice during parts of the early and middle holocene similar to 6000 10 000 years bp compared to present day conditions this sea ic

are crustal thickness variations in old mountain belts like the appalachians a consequence of lithospheric delamination comparison of moho geometry and reflection character of active collision zones like the alps with those of fossil collision zones like the appalachians lends support for the view that lithospheric delamination is both a common and fundamentally important component of collisional orogeny in particular the relative crustal thinning commonly observed beneath the internides of old collisional orogens is an expected consequence of delamination neither precollisional crustal thickness variations nor an unrelated postcollision extensional event is required to produce this feature acceptance of the delamination hypothesis in turn fundamentally alters one s view of the tectonic evolution of old collisional orogens in the case of the appalachians delamination has direct implications for the interpretation of alleghanian granites the direction of subduction leading to final closure between laurentia and gondwana and the thermal state of the lithosphere at the onset of atlantic rifting a corollary of the delamination hypothesis is that lateral reflectivity variations cannot be used to define paleogeographic terranes in the deep crust of old orogens conversely the observation that delamination leaves a distinctive geophysical imprint on the crust itself provides an important additional clue for deciphering the tectonic evolution of such regions

chronology of expansion and contraction of four great basin lake systems during the past 35 000 years abstract during the past 35 000 years lake bonneville lake russell and lake searles underwent a major period of lake level change the lakes were at moderate levels or dry at the beginning of the period and seem to have achieved highstands between about 15 000 and 13 500 yr b p the rise of lake lahontan was gradual but not continuous in part because of topographic constraints intrabasin spill lake lahontan also had an oscillation in lake level at 15 500 yr b p radiocarbon age estimations for materials that were deposited in the lake basins indicate that lake bonneville rose more or less gradually from 32 000 yr b p and had major oscillations in level between 23 000 and 21 000 yr b p and between 15 250 and 14 500 yr b p lake russell and lake searles had several major oscillations in lake level between 35 000 and 14 000 yr b p the timing and exact magnitude of the oscillations are difficult to decipher but both lakes may have achieved multiple highstand states all four lakes may have had nearly synchronous recessions between about 14 000 and 13 500 yr b p after the recessions the lakes seem to have temporarily stabilized or experienced a minor increase in size between about 11 500 and 10 000 yr b p these data provide circumstantial evidence that the younger dryas event affected climate on at least a hemispheric scale during the holocene the four lakes remained at low levels and small oscillations in lake level occurred an important aspect of the lake level data is the accompanying expansion of lake surface area at the time of the last highstand lake bonneville and lake lahontan had surface areas about 10 times larger than their mean historical reconstructed areas whereas lake russell and lake searles had surface areas about 5 times larger than their mean historical reconstructed areas differences in the records of effective wetness may have been due to the locations of the basins relative to the position of the jetstream or they may have resulted from lake atmosphere feedback processes

deep seismic reflection structure of the caledonian orogenic front west of shetland although the moine thrust has been well studied on land in northwestern scotland the subject of its deep crustal structure and regional continuation offshore remains controversial here we use post stack migration and filtering of deep seismic reflection profiles crossing the projection of the moine thrust and coincident caledonian orogenic front around the western margin of the shetland platform in order to characterize the crustal structure of the thrust in an area not substantially affected by basin formation and extension and where onshore geological constraints can be more readily applied this study provides for the first time seismic reflection constraints on middle and lower crustal structure down to the level of the moho discontinuity underlying the thrust the seismic data reveal a thick wedge of east dipping reflections in the lower crust capped by a highly coherent reflection which can be correlated to the postulated moine thrust as previously mapped on shetland the continuation of this wedge from the middle crust down to the moho against which individual reflections are abruptly truncated suggests a thick skinned interpretation for the caledonian orogen beneath the western flank of the shetland platform the available seismic data from around shetland imply that the moine thrust caledonian front forms a boundary between an upper plate which is relatively featureless structurally and a lower plate composed of interpreted northwest verging thrusts possibly a crustal duplex structurally below the level of the moine nappe our observations are thus in contrast with previous interpretations made from seismic data in the west orkney basin north of the northwest highlands that describe a highly reflective moinian upper plate

a reassessment of the proposed lairg impact structure and its potential implications for the deep structure of northern scotland the lairg gravity low may represent a buried impact crater c 40 km across that was the source of the 1 2 ga stac fada member ejecta deposit but the gravity anomaly is too large to represent a simple crater and there is no evidence of a central peak reanalysis of the point bouguer gravity data reveals a ring of positive anomalies around the central low suggesting that it might represent the eroded central part of a larger complex crater the inner or peak rings of complex craters show a broadly consistent 2 1 relationship between ring diameter and total crater diameter implying that the putative lairg crater may be as much as 100 km across this would place the crater rim within a few kilometres of the stac fada member outcrop a location inconsistent with the thickness and clast size of the ejecta deposit we propose that the putative impact crater originally lay further east substantially further from the stac fada member than today and was translocated westwards to its present location beneath lairg during the caledonian orogeny this model requires that a deep seated thrust fault analogous to the flannan and outer isles thrusts exists beneath the moine thrust in north central scotland

formation of sea ice bridges in narrow straits in response to wind and water stresses ice bridges are rigid structures composed of sea ice that form seasonally in the many straits and channels of the canadian arctic archipelago driven primarily by atmospheric stresses these ice bridges are formed when sufficiently thick ice jams during the course of its flow between land masses resulting in a region of stationary compacted ice that is separated from a region of flowing open water a polynya by a static arch using a continuum description of sea ice that is widely used in climate modeling we present an asymptotic theory of the process of formation of such bridges in slender channels when the motion of the ice is driven by external wind and water stresses we show that for an arbitrary channel shape ice bridges can only form within a range of ice properties that is determined by the channel geometry and the external stress we then compare the results of our theory with direct numerical simulations and observational evidence finally we provide simple analytical expressions for the mean velocity of the ice flow as a function of the channel shape the properties of the ice and the wind and water stresses along the channel

geological inheritance and crystal dynamics of the northwest scottish continental shelf abstract deep seismic reflection profiles have been combined with conventional seismic surveys across the northwest scottish continental shelf to interpret basin development in the region post caledonian basins are built upon partially reactivated older structures which have strongly influenced their internal structure and stratigraphy we show that the minch basin along with several others developed in the hangingwall of the partially reactivated outer isles fault a planar fault dipping at about 30 ese that reaches nearly to the moho the internal structure of these hangingwall basins is compared with appropriate scaled analogue models to show that they have developed by passive hangingwall collapse above a footwall block that moved towards the wnw this geometry is incorporated into a lithospheric dynamic model with northwest extension towards the rockall trough from a fixed lithosphere beneath the scottish highlands this model requires non linear lithosphere extension which is accommodated by movements on faults and block rotation in the upper crust extension and thinning of the lower crust along a network of anastomosing ductile shears extensional movement of the flannan fault in the upper mantle and horizontal displacement along the moho beneath the outer hebrides this dynamic model has wider application for other extensional regions

seasonal control of petermann gletscher ice shelf melt by the ocean s response to sea ice cover in nares strait petermann gletscher drains 4 of the greenland ice sheet gris area with 80 of its mass loss occurring by basal melting of its ice shelf we use a high resolution coupled ocean and sea ice model with a thermodynamic glacial ice shelf to diagnose ocean controlled seasonality in basal melting of the petermann ice shelf basal melt rates increase by 20 in summer due to a seasonal shift in ocean circulation within nares strait that is associated with the transition from landfast sea ice to mobile sea ice under landfast ice cold near surface waters are maintained on the eastern side of the strait and within petermann fjord reducing basal melt and insulating the ice shelf under mobile sea ice warm waters are upwelled on the eastern side of the strait and mediated by local instabilities and eddies enter petermann fjord enhancing basal melt down to depths of 200 m the transition between these states occurs rapidly and seasonal changes within nares strait are conveyed into the fjord within the same season these results suggest that long term changes in the length of the landfast sea ice season will substantially alter the structure of petermann ice shelf and its contribution to gris mass loss

reflection seismic evidence for a moho offset beneath the walls boundary strike slip fault birps has acquired two deep seismic reflection profiles across the walls boundary fault a major late caledonian strike slip fault that is probably the northern continuation of the great glen fault of scotland where crossed by the shet deep seismic survey north of the shetlands the walls boundary fault appears to be a narrow vertical structure which penetrates the entire crustal thickness and juxtaposes crusts of different thicknesses the seismic data show a moho offset in travel time of 1 0 1 5 seconds and two high amplitude diffractions originating at moho travel times these diffractions are as yet unique among seismic data across strike slip faults and can be used to argue that the moho is offset in depth by 2 3 km over a lateral distance of less than 6 km seismic modelling is used to constrain this interpretation the preservation of this moho topography suggests that the structure and rheology of the lower crust of the shetland platform was not significantly modified during post devonian extension of the north sea and atlantic margin

mediterranean vegetation lake levels and palaeoclimate at the last glacial maximum the apparent conflict between pollen evidence for widespread artemisia steppe1 6 implying semi arid conditions and geomorphological evidence for high lake levels4 7 11 has produced controversy about the ice age palaeoclimate of the mediterranean region here we use a water balance model12 to predict catchment runoff and a biome model13 to predict vegetation type to reconstruct the palaeoenvironment around lake loannina a type locality for the northern mediterranean region we show that both sets of evidence are compatible with a summer dry winter wet regime with seasonal temperature anomalies similar to those predicted by atmospheric model simulations of the last glacial maximum14 18 the drying effect of the cold north atlantic ocean may have been counteracted in winter by increased storm frequency under a southward shifted jet stream as shown by several atmospheric models16 18

structural evolution in the moine of northwest scotland a caledonian linked thrust system a model is proposed whereby the caledonian metamorphic basement cover complex of northwest scotland the moine is considered as a linked thrust system this system lies between the moine thrust at its base and the naver sgurr beag slide at its top ductile fold and thrust zones which developed at mid crustal levels at metamorphic grades from greenschist to amphibolite facies are interpreted as decoupling from a detachment presently situated at relatively shallow depths this model is illustrated by two preliminary balanced cross sections these imply shortening across the northwest scottish caledonides in excess of 130 km and probably over 200 km when these structures are restored onto a crustal template a considerable quantity of lower crust is found to be required at depth the most likely location for the lower crustal wedge is beneath the grampian highlands

a comparison of the iberian and ebro basins during the permian and triassic eastern spain a quantitative subsidence modelling approach the permian triassic sediments of the iberian plate are a well studied case of classical buntsandstein muschelkalk keuper facies with good sedimentological interpretations and precise datings based on pollen and spore assemblages ammonoids and foraminifera synrift postrift cycles are recorded in these facies but there are only a few studies of quantitative subsidence analysis backstripping method and only a previous one using forward modelling for the quantification of synrift postrift phases of this period here we present the results obtained by the quantitative analysis of fourteen field sections and oil well electric log records in the iberian and ebro basins spain backstripping analysis showed five synrift phases of 1 to 3 million years duration followed by postrift periods for the permian triassic interval the duration however shows lateral variations and some of them are absent in the ebro basin the forward modelling analysis assuming local isostatic compensation has been applied to each observation point using one layer and two layer lithospheric configurations the second one shows a better fit between observation and model prediction in a systematic way so a two layer configuration is assumed for the sedimentary basin filling analysis lithospheric stretching factors and obtained in the forward modelling analysis are never higher than 1 2 but sometimes 1 in the same section if surficial extension is compensated by deep compression either at the roots of the rift basins or in far away zones is not yet clear but this anomaly can be explained using a simple shear extensional model for the iberian and ebro basins 2008 elsevier b v all rights reserved

modeling m2 tidal variability in arctic sea ice drift and deformation semi diurnal oscillations are a ubiquitous feature of polar sea ice motion over much of the arctic basin inertial and semi diurnal tidal variability have similar frequencies so that periodicity alone is inadequate to determine the source of oscillations we investigate the relative roles of tidal and inertial variability in arctic sea ice using a barotropic ice ocean model with sea ice embedded in an upper boundary layer results from this model are compared with levitated ice ocean coupling used in many models in levitated models the mechanical buoyancy effect of sea ice is neglected so that convergence of ice for example does not affect the oceanic ekman flux we use rotary spectral analysis to compare simulated and observed results this helps to interpret the rotation sense of sea ice drift and deformation at the semi diurnal period and is a useful discriminator between tidal and inertial effects results indicate that the levitated model generates an artificial inertial resonance in the presence of tidal and wind forcing contrary to the embedded sea ice model we conclude that sea ice mechanics can cause the rotational response of ice motion to change sign even in the presence of strong and opposing local tidal forcing when a physically consistent dynamic ice ocean coupling is employed

moist and the continuity of crustal reflector geometry along the caledonian appalachian orogen new aspects of the deep structure of the scottish caledonides are revealed by the moine and outer isles deep seismic reflection traverse moist the caledonian foreland is underlain by an easterly dipping strongly reflecting surface cutting through the moho and traceable to more than 45 km depth thrusts within the foreland basement and caledonian orogen have been reactivated as normal faults bounding half grabens filled with sediments of late palaeozoic to mesozoic age the moine thrust which carries rocks of the orogen over lewisian foreland dips at 20 25 to the e on moist and is either a the westernmost of a series of easterly dipping reflections thrusts which flatten or terminate at 17 20 km depth or b a thrust lying further e which structurally overlies these easterly dipping reflectors by comparison with easterly dipping reflections on cocorp lines in the northern and southern appalachians the latter interpretation is preferred implying that many of the easterly dipping reflections on moist correspond to off shelf metasedimentary rocks with slivers of basement stacked and imbricated against the lewisian basement edge regional seismic refraction and conductivity data from northern scotland suggest that regions of the caledonian orogen have lower mid crustal velocity gradients and higher conductivities than the foreland which might be explained by this interpretation of the easterly dipping reflections despite similarities in crustal reflector geometries there are important differences between the moist and cocorp lines including 1 a remarkably continuous relatively horizontal moho seen on moist data at about 25 km depth and 2 the sedimentary basins offshore from northern scotland which have formed by reactivation under crustal extension of the easterly dipping thrusts furthermore it is probable that the easterly dipping reflectors in these areas of the moist and cocorp lines were formed at different times in the early palaeozoic

investigating the crustal structure of a strike slip step over zone along the great glen fault strike slip step overs and bends are regions of anomalous deformation that may yield clues to the mechanical behavior of the earth s crust experimental reprocessing of a marine deep seismic reflection profile cutting across an 35 km wide right stepping step over developed on the great glen fault system north of the inner moray firth basin east of northwest highlands scotland reveals a restricted zone of prominent reflections and diffractions beginning at 7 8 km depth continuing as deep as the interpreted moho discontinuity at 26 27 km geological interpretation of this zone suggests a concentration of possible diffractors marking sharp structural disruption underlain by a 12 km thick layer of subhorizontal and moderately dipping reflectors that dominates the lower crust in the step over region the known kinematic history of the great glen fault system together with the observed direction of the step over implies that a zone of contraction would have formed within the step over during early caledonian sinistral strike slip the seismic reflection structure can be interpreted as developing by deformation associated with contraction and possible block rotation between the two fault segments such that the diffractive zone represents steep structure in a near vertical zone of strike slip which passes deeper into a layer of low angle dipping thrusts or shear zones the results of this study are consistent with a concept of mechanical detachment in the middle crust as documented for areas of contraction along active strike slip faults

cosmogenic chlorine 36 chronology for glacial deposits at bloody canyon eastern sierra nevada deposits from mountain glaciers provide an important record of quaternary climatic fluctuations but have proved difficult to date directly a chronology has been obtained for glacial deposits at bloody canyon california by measurement ofthe accumulation of chlorine 36 produced by cosmic rays in boulders exposed on moraine crests the accumulation ofchlorine 36 indicates that episodes of glaciation occurred at about 21 24 65 115 145 and 200 ka thousand years ago although the timing of the glaciations correlates well with peaks of global ice volume inferred from the marine oxygen isotope record the relative magnitudes differ markedly the lengths of the moraines dating from 115 ka and 65 ka show that the early glacial episodes were more extensive than those during the later wisconsin and indicate that the transition from interglacial to full glacial conditions was rapid

on modeling the anisotropic failure and flow of flawed sea ice the failure and flow of sea ice on scales small and large is characterized by the propagation of oriented leads and cracks in this paper a theory for the dynamical treatment of anisotropic oriented flaws in sea ice is developed and used to examine the interaction of these oriented flaws under idealized stress forcing the essential idea of the theory is to take one or more oriented weak leads imbedded in thick ice a constitutive law for both the thin and thick ice is taken to be similar to laboratory observations which are consistent with mohr coulomb like failure application of normal continuum mechanics boundary conditions leads to anisotropic flow and failure characteristics of this anisotropic composite under both near and far field forcing the local failure characteristics show that there is a preferred orientation for failure with the intersection between leads increasing as confinement increases for spatially separated interacting flaws the theory predicts a preference for the weakening of flaws in a narrow range of angles of 10 20 relative to the principal far field stress imbedding isolated flaws leads to simulated damage directions consistent with local failure characteristics in the case where the ice is considered to have available flaws in all directions fracture propagation is found to proceed by picking out oriented flaws alternating in direction along large scale damage strikes with individual flaw alignments of 20 relative to far field principal stresses the mechanisms responsible for these results and how this anisotropic sea ice rheology might be implemented in current dynamic thermodynamic sea ice models are discussed