paleodrainage on the chukchi shelf reveals sea level history and meltwater discharge abstract chirp subbottom data collected across the chukchi shelf offshore nw alaska imaged numerous paleochannels and valleys that appear to have been downcut and incised during sea level falls associated with glacial intervals in contrast the two most recent incisions appear to have been formed during the period of sea level rise following the last glacial maximum lgm the architecture and infill associated with these two incisions suggests that they were formed by an increase in discharge these events appear to be unrelated to sea level fluctuations but rather triggered by climatic variations during the most recent deglaciation i e meltwater discharge radiocarbon dates from sediment cores within the southern incised valley suggest that the two episodes of meltwater discharge may correlate with meltwater pulse 1 a 14 000 cal yr bp and evidence of iceberg scouring on outer chukchi shelf 12 000 to 13 000 cal yr bp respectively regional transgression across the interfluves on the middle chukchi shelf appears to postdate the second meltwater discharge and may correlate with meltwater pulse 1 b 11 500 cal yr bp this evidence suggests that in glacially dominated landscapes episodes of large discharge to the shelf might be out of phase with the sea level cycle in addition the presence of glacial meltwater drainage on the shelf implies a greater volume of continental glaciation during the lgm than previously recognized

sediment provenance changes in the western arctic ocean in response to ice rafting sea level and oceanic circulation variations since the last deglaciation two sediment piston cores were recovered from the chukchi alaskan 05jpc and canadian beaufort 02pc margins to investigate grain size geochemical and mineralogical compositions this allowed the reconstruction of changes in detrital sediment provenance and transport related to climate variability since the last deglaciation the end member modeling analyses of grain size indicate that sea ice and nepheloid transport and the mackenzie river sediment plume are major factors influencing sedimentation in the chukchi alaskan and canadian beaufort margins respectively unmixing of the sediment composition indicates that detrital sediments in core 02pc are derived mainly from the mackenzie river whereas sediments from core 05jpc are derived mainly from the mackenzie river during the deglaciation and include a mixture of holocene sediments from the bering strait mackenzie river and eurasian margin the dolomite rich ice rafted debris recorded in both cores could be related to the different phases of iceberg discharges from the amundsen gulf ice stream quartz and feldspar rich ice rafted debris dated at 13 and 10 6 ka cal bp before present are related to the lake agassiz outburst in core 02pc and meltwater discharge from the brooks range glaciers in core 05jpc detrital proxies in core 02pc support the hypothesis that large meltwater and iceberg discharges from the lake agassiz outburst to the arctic ocean and amundsen gulf ice stream may have triggered the younger dryas finally similar trends observed between sea level curves and our detrital proxy suggest that the sea level changes in the western arctic ocean have an important influence on the sediment dynamic during the early to middle holocene keywords bulk and clay mineralogy elemental geochemistry grain size distribution sediment provenance canadian beaufort sea chukchi sea

enriched seismic imaging by using multiple scattering abstract the migration of full wavefields primaries multiples by using the concept of secondary sources at each subsurface grid point is summarized these secondary sources are two way and contain the grid point reflection as well as the grid point transmission properties they assist the primary sources in illumination of the subsurface full wavefield migration is implemented as a closed loop process in which the estimated reflectivity properties are used to generate all higher order scattering multiples and transmission effects such that the modeled full wavefield response matches the observed measurements in addition velocities can be updated simultaneously theoretical considerations and examples lead to the principal conclusion that multiples should be used not removed

origin and consequences of cyclic ice rafting in the northeast atlantic ocean during the past 130 000 years deep sea sediment cores recovered from the northeast atlantic ocean were examined in order to elucidate the influence of the earth s orbital parameters on major ice rafting analyses of coarse grained ice rafted debris and planktonic foraminifers revealed a strong reaction to the precession signal since 130 000 yr b p dropstone layers have been deposited each half period of a precessional cycle 11 000 1000 yr ice rafting occurs during times of winter minimum summer maximum insolation and summer minimum winter maximum insolation in the first case high summer insolation forces meltwater discharge from the ice sheets into the polar seas which subsequently enhances formation of sea ice during the winter in the second case growth of continental ice enhances iceberg production which also leads to a salinity reduction of surface seawater both situations result in a southward penetration of polar water thus the marine record of dropstones documents ice rafting not only during weichselian stades but also during cold events within interstades the regularity of ice rafting yields a useful framework to calibrate and elucidate climatic changes not only in the region of the north atlantic ocean but also in remote areas such as the pacific ocean and the antarctic

wave equation migration velocity analysis i theory we present a migration velocity analysis mva method based on wavefield extrapolation similarly to conventional mva our method aims at iteratively improving the quality of the migrated image as measured by the flatness of angle domain common image gathers adcigs over the aperture angle axis however instead of inverting the depth errors measured in adcigs using ray based tomography we invert image perturbations using a linearized wave equation operator this operator relates perturbations of the migrated image to perturbations of the migration velocity we use prestack stolt residual migration to define the image perturbations that maximize the focusing and flatness of adcigs our linearized operator relates slowness perturbations to image perturbations based on a truncation of the born scattering series to the first order term to avoid divergence of the inversion procedure when the velocity perturbations are too large for born linearization of the wave equation we do not invert directly the image perturbations obtained by residual migration but a linearized version of the image perturbations the linearized image perturbations are computed by a linearized prestack residual migration operator applied to the background image we use numerical examples to illustrate how the backprojection of the linearized image perturbations i e the gradient of our objective function is well behaved even in cases when backprojection of the original image perturbations would mislead the inversion and take it in the wrong direction we demonstrate with simple synthetic examples that our method converges even when the initial velocity model is far from correct in a companion paper we illustrate the full potential of our method for estimating velocity anomalies under complex salt bodies

stratigraphic constraints on late pleistocene glacial erosion and deglaciation of the chukchi margin arctic ocean at least two episodes of glacial erosion of the chukchi margin at water depths to 450 m and 750 m have been indicated by geophysical seafloor data we examine sediment stratigraphy in these areas to verify the inferred erosion and to understand its nature and timing our data within the eroded areas show the presence of glaciogenic diamictons composed mostly of reworked local bedrock the diamictons are estimated to form during the last glacial maximum lgm and an earlier glacial event possibly between ois 4 to 5d both erosional events were presumably caused by the grounding of ice shelves originating from the laurentide ice sheet broader glaciological settings differed between these events as indicated by different orientations of flutes on eroded seafloor postglacial sedimentation evolved from iceberg dominated environments to those controlled by sea ice rafting and marine processes in the holocene a prominent minimum in planktonic foraminiferal 18 o is identified in deglacial sediments at an estimated age near 13 000 cal yr bp this 18 o minimum also reported elsewhere in the amerasia basin is probably related to a major laurentide meltwater pulse at the younger dryas onset the bering strait opening is also marked in the composition of late deglacial chukchi sediments

holocene variability in sea ice cover primary production and pacific water inflow and climate change in the chukchi and east siberian seas arctic ocean in this study we present new detailed biomarker based sea ice records from two sediment cores recovered in the chukchi sea and the east siberian sea these new biomarker data may provide new insights on processes controlling recent and past sea ice changes the biomarker proxy records show i minimum sea ice extent during the early holocene ii a prominent mid holocene short term high amplitude variability in sea ice primary production and pacific water inflow and iii significantly increased sea ice extent during the last ca 4 5k cal a bp this late holocene trend in sea ice change in the chukchi and east siberian seas seems to be contemporaneous with similar changes in sea ice extent recorded from other arctic marginal seas the main factors controlling the millennial variability in sea ice and surface water productivity are probably changes in surface water and heat flow from the pacific into the arctic ocean as well as the long term decrease in summer insolation the short term centennial variability observed in the high resolution middle holocene record is probably related to solar forcing our new data on holocene sea ice variability may contribute to synoptic reconstructions of regional to global holocene climate change based on terrestrial and marine archives

the laurentide and innuitian ice sheets during the last glacial maximum the late wisconsinan advance of the laurentide ice sheet started from a middle wisconsinan interstadial minimum 27 30 14 c ka bp when the ice margin approximately followed the boundary of the canadian shield ice extent in the cordillera and in the high arctic at that time was probably similar to present ice advanced to its late wisconsinan stage 2 limit in the northwest south and northeast about 23 24 14 c ka bp and in the southwest and far north about 20 21 14 c ka bp in comparison to some previous reconstructions of ice extent our current reconstruction has substantially more late wisconsinan ice in the high arctic where an innuitian ice sheet is generally acknowledged to have existed in the atlantic provinces where ice is now thought to have extended to the continental shelf edge in most places and on eastern baffin island where ice probably extended to the fiord mouths rather than to the fiord heads around most of the ice margin the late wisconsinan maximum ice extent either exceeded the extent of earlier wisconsinan advances or it was similar to the early wisconsinan advance ice marginal recession prior to 14 14 ck a bp occurred mainly in deep water and along the southern terrestrial fringe however heinrich event 1 probably drew down the entire central ice surface at 14 5 14 c ka bp sufficiently to displace the labrador sector outflow centre 900 km eastward from the coast of hudson bay the onset of substantial ice marginal recession occurred about 14 14 c ka bp in the northwest southwest and south but not until about 10 11 14 c ka bp in the northeast and in the high arctic thus the period of maximum ice extent in north america generally encompasses the interval from b24 21 to 14 14 c ka bp or considerably longer than the duration of the lgm defined as occurring during a period of low global sea level as well as during a time of relative climate stability b18 14 c ka bp the interval of advance of much of the laurentide ice sheet to its maximum extent between b27 14 c ka bp and b24 14 c ka bp coincides with a suggested interval of rapid fall in global sea level to near lgmlevels r 2001 elsevier science ltd all rights reserved

sensitivity of the northern hemisphere climate system to extreme changes in holocene arctic sea ice the extent of seasonal and perennial sea ice changed dramatically through the late quaternary and these changes influenced both the ocean and atmosphere by controlling the exchange of energy moisture and gases between them and by altering the planetary albedo reconstructing the changing patterns of sea ice distribution in the recent past remains one of the outstanding challenges to the paleo community to evaluate the importance of these reconstructions we performed sensitivity tests using ncar s community climate model ccm3 and a series of prescribed sea ice extents designed to capture the full range of arctic sea ice variability under interglacial holocene and full glacial last glacial maximum boundary conditions our simulations indicate that surface temperatures and sea level pressures in winter djf are most sensitive to changes in sea ice and that these changes are propagated over the surrounding land masses in the north atlantic but that equivalent changes in sea ice produce smaller corresponding changes in temperature or sea level pressure in the north pacific region a comparison between climap map chart series mc 36 geological society of america boulder co 1981 and a more realistic assessment of lgm sea ice yields dramatic changes in winter temperatures and precipitation patterns across eurasia these differences forced only by changed sea ice conditions reinforce the need to develop accurate maps of past sea ice to correctly simulate late quaternary environments such reconstructions also will be essential to validate the next generation of sea ice models

review paper an outlook on the future of seismic imaging part ii full wavefield migration the next generation seismic imaging algorithms will consider multiple scattering as indispensable information being referred to as full wavefieldmigration in addition these algorithms will also include autonomous velocity updating in the migration process being referred to as joint migration inversion full wavefield migration and joint migration inversion address the industrial needs to improve images of very complex reservoirs as well as the industrial ambition to produce these images in a more automatic manner automation in seismic processing in this vision paper on seismic imaging full wavefield migration and joint migration inversion are formulated in terms of a closed loop estimation algorithm that can be physically explained by an iterative double focusing process full wavefield common focus point technology a critical module in this formulation is forward modelling allowing feedback from migrated output to unmigrated input closing the loop for this purpose a full wavefield modelling module has been developed which utilizes an operator description of complex geology the full wavefield modelling module is pre eminently suited to function in the feedback path of a closed loop migration algorithm the future of seismic imaging is presented as a coherent trilogy proposing the migration framework of the future in three consecutive parts in part i it was shown that the proposed full wavefield modelling module algorithm differs fundamentally from finite difference modelling because velocities and densities need not be provided instead an operator description of the subsurface is used in addition the concept of reverse modelling was introduced in part ii it is shown how the theory of primary wavefield migration can be extended to full wavefield migration by correcting for angle dependent transmission effects and by utilizing multiple scattering the potential of the full wavefield migration algorithm is illustrated with numerical examples a multidirectional migration strategy is proposed that navigates the full wavefield migration algorithm through the seismic data cube in different directions

responses of an arctic landscape to lateglacial and early holocene climatic changes the importance of moisture many of the physical and biological processes that characterize arctic ecosystems are unique to high latitudes and their sensitivities to climate change are poorly understood stratigraphic records of land surface processes and vegetation change in the arctic foothills of northern alaska reveal how tundra landscapes responded to climatic changes between 13 000 and 8000 14 c yr bp peat deposition began and shrub vegetation became widespread ca 12 500 14 c yr bp probably in response to the advent of warmer and wetter climate increased slope erosion caused rapid alluviation in valleys and populus trees spread northward along braided floodplains before 11 000 14 c yr bp lake levels fell and streams incised their floodplains during the younger dryas yd 11 000 10 000 14 c yr bp a hiatus in records of populus suggest that its geographic range contracted and pollen records of other species suggest a cooler and drier climate during this interval basal peats dating to the yd are rare suggesting that rates of paludification slowed immediately after 10 000 14 c yr bp lake levels rose streams aggraded rapidly again intense solifluction occurred and populus re invaded the area moist acidic tundra vegetation was widespread by 8500 14 c yr bp along with wet organic rich soils most of these landscape scale effects of climatic change involved changes in moisture although low temperature is the most conspicuous feature of arctic climate shifts in effective moisture may be the proximate cause for many of the impacts that climate change has in arctic regions r 2002 elsevier science ltd all rights reserved

multibeam bathymetric and sediment profiler evidence for ice grounding on the chukchi borderland arctic ocean multibeam bathymetry and 3 5 khz sub bottom profiler data collected from the us icebreaker healy in 2003 provide convincing evidence for grounded ice on the chukchi borderland off the northern alaskan margin arctic ocean the data show parallel glacially induced seafloor scours or grooves and intervening ridges that reach widths of 1000 m rim to rim and as much as 40 m relief following previous authors we refer to these features as bmegascale glacial lineations msgls q additional support for ice grounding is apparent from stratigraphic unconformities interpreted to have been caused by ice induced erosion most likely the observed sea floor features represent evidence for massive ice shelf grounding the general ese wnw direction of the msgls together with sediment evidently bulldozed off the chukchi plateau that is mapped on the western siberian side of the plateau suggests ice flow from the canada basin side of chukchi borderland two separate generations of glacially derived msgls are identified on the chukchi borderland from the healy geophysical data the deepest and oldest extensive msgls appear to be draped by sediments less than 5 m thick whereas no sediment drape can be distinguished within the resolution of the sub bottom profiles on the younger generation d 2005 university of washington all rights reserved

the role of sea ice and other fresh water in the arctic circulation salinity stratification is critical to the vertical circulation of the high latitude ocean we here examine the control of the vertical circulation in the northern seas and the potential for altering it by considering the budgets and storage of fresh water in the arctic ocean and in the convective regions to the south we find that the present day greenland and iceland seas and probably also the labrador sea are rather delicately poised with respect to their ability to sustain convection small variations in the fresh water supplied to the convective gyres from the arctic ocean via the east greenland current can alter or stop the convection in what may be a modern analog to the halocline catastrophes proposed for the distant past the north atlantic salinity anomaly of the 1960s and 1970s is a recent example it must have had its origin in an increased fresh water discharge from the arctic ocean similarly the freshening and cooling of the deep north atlantic in recent years is a likely manifestation of the increased transfer of fresh water from the arctic ocean into the convective gyres finally we note that because of the temperature dependence of compressibility a slight salinity stratification in the convective gyres is required to efficiently ventilate the deep ocean

angle domain common image gathers for migration velocity analysis by wavefield continuation imaging we analyze the kinematic properties of offset domain common image gathers cigs and angle domain cigs adcigs computed by wavefield continuation migration our results are valid regardless of whether the cigs were obtained by using the correct migration velocity they thus can be used as a theoretical basis for developing migration velocity analysis mva methods that exploit the velocity information contained in adcigs we demonstrate that in an adcig cube the image point lies on the normal to the apparent reflector dip that passes through the point where the source ray intersects the receiver ray the image point position on the normal depends on the velocity error when the velocity is correct the image point coincides with the point where the source ray intersects the receiver ray starting from this geometric result we derive an analytical expression for the expected movements of the image points in adcigs as functions of the traveltime perturbation caused by velocity errors by applying this ana

variable ti content and grain size of titanomagnetite as a function of cooling rate in very young morb transmission electron microscopy observations and rock magnetic measurements on a pillow from the new flow extruded in 1993 at the juan de fuca ridge demonstrate that ti content in large up to 40 m titanomagnetite grains varies as a function of the cooling rate large grains in the interior of the pillow have a narrow composition range of approximately x 0 6 where x represents the ulvospinel content in the solid solution series with magnetite in contrast the titanomagnetite grains near the pillow rim are progressively smaller and have a broad composition range average x 0 45 within 0 5 cm of the rim a significant portion of the grains is single domain sd to superparamagnetic sp and appears to have little or no ti this lower ti content provides a ready explanation for the higher curie temperature near the glassy margin of the pillow moreover determination of the oxidation state of the titanomagnetite does not show low temperature oxidation anywhere within the pillow indicating no rapid alteration to titanomaghemite as previously suggested for this very young mid ocean ridge basalt morb in order to explain the curie temperatures submicrometer titanomagnetite in interstitial glass in the interior of the pillow also shows variable x values these grains make contributions to susceptibility that show up as high up to 580 c curie temperatures in thermomagnetic analyses and they appear to be sd to sp the variation of x values for the titanomagnetite in the morb is consistent with thermodynamic and kinetic considerations because crystallization of titanomagnetite in the morb is controlled by processes depending on cooling rate and crystal melt fractionation

surface water productivity and sediment transport by bering strait throughflow in the chukchi shelf the western arctic ocean during the holocene diverse paleoceanographic proxies from three sediment cores gc12ex jpc35 and jpc30 collected from the chukchi shelf north of the bering strait elucidate the holocene paleoceanographic changes surface water productivity and sediment transport caused by the bering strait throughflow from the bering sea into the chukchi sea lithology of three sediment cores identified the same three units based on comparison and correlation to adjacent age dated cores as well as ams 14c dates of core gc12ex the boundary between unit 1 and unit 2a is dated about 8500 cal yr bp and the boundary between unit 2a and unit 2b is also dated about 4500 cal yr bp consistent down core profiles of the geochemical and isotopic properties among the three cores differentiate the paleoceanographic conditions corresponding to lithologic units based on the biogenic opal total organic carbon and 13c values unit 1 is characterized by low surface water marine productivity under relatively shallow water with weak transport of b

trace element geochemistry of magnetite implications for ore genesis of the talate skarn pb zn fe deposit altay nw china abstract the talate skarn pb zn fe deposit is hosted in the kangbutiebao formation of the abagong polymetallic belt altay nw china the talate magnetite comprises the magmatic hydrothermal hydrothermal disseminated and massive magnetite types and formed during the early skarn and quartz magnetite stage the magmatic hydrothermal disseminated magnetite is nb ta ti depleted whereas the hydrothermal disseminated magnetite associated with skarn alteration is geochemically similar to the ca skarn with slight ta mg ni depletions the massive magnetite ores have similar ni concentrations with the ca skarn but contain minor depletions in sn sc ta nb mg and co probably attributed to magma host rock interactions the talate hydrothermal disseminated magnetite contains lower sn than magnetite from typical skarn deposits and likely resulted from cassiterite crystallization as it is found co precipitated with disseminated magnetite both magmatic hydrothermal and hydrothermal disseminated magnetite are characterized by narrow ti concentration range and variable co ni ratios which likely reflect rapid cooling rate and significant fluid rock interaction during multiple successive stages of skarn alteration as the co has a higher solubility over the ni in the ore forming fluid whereas the talate massive magnetite contains wider ti concentration range and in some cases with 120 triple junctions the co ni ratios mostly

a review of the geological characteristics and mineralization history of iron deposits in the altay orogenic belt of the xinjiang northwest china abstract in this review we describe the geological characteristics and metallogenic tectonic origin of fe deposits in the altay orogenic belt within the xinjiang region of northwestern china the fe deposits are found mainly within three regions ordered from northwest to southeast the ashele kelan and maizi basins the principal host rocks for the fe deposits of the altay orogenic belt are the early devonian kangbutiebao formation the middle to late devonian altay formation with minor occurrences of lower carboniferous and early paleozoic metamorphosed volcano sedimentary rocks the principal mineral forming element groups of the deposits are fe fe cu fe mn fe p fe pb zn fe au and fe v ti the fe deposits are associated with distinct formations such as volcanic rocks skarn deposits pegmatites granite related hydrothermal vein mineralization and mafic pluton related v ti magnetite deposits the fe deposits are most commonly associated with volcanic rocks in the upper kangbutiebao formation in the volcano sedimentary kelan basin and in skarn deposits at several localities including the lower kangbutiebao formation in the volcano sedimentary maizi basin and the altay formation at jiaerbasidao kekebulake region homogenization temperatures of fluid inclusions in the prograde retrograde and sulfide stages of the skarn type deposit are mainly medium to high temperature cluster between 200 and 500 c medium temperature cluster between 200 and 340 c and low to medium temperature cluster between 160 and 300 c respectively ore fluids in the sedimentation period in the volcano sedimentary type deposit are characterized by low to medium temperature with a peak around 190 c low to moderate salinity 3 23 to 22 71 wt nacl equiv ore fluids in the pegmatite type deposit are characterized by low to medium temperature with a peak at 240 c low salinity with a peak around 9 wt nacl equiv an analysis of the isotopic data for fe deposits from the altay orogenic belt indicates that the sulfur was derived from several sources including volcanic rocks and granite as well as bacterial reduction of sulfate from seawater the present results indicate that different deposit types were derived from various sources the ree geochemistry of rocks and ores from the fe deposits in the altay orogenic belt suggests that the ore forming materials were derived from mafic volcanic rocks based on isotopic age data the timing of the mineralization can be divided into four broad intervals early devonian 410 384 ma middle devonian 377 ma early permian 287 274 ma and early triassic c 244 ma the ore forming processes of the fe deposits are closely related to volcanic activity and the emplacement of intermediate and felsic intrusions we conclude that fe deposits within the altay orogenic belt developed in a range of tectonic settings including continental arc post collisional extensional settings and intracontinental settings

sea ice scouring on the inner shelf of the southeastern canadian beaufort sea abstract about 2200 ice scours were observed and analyzed over a distance of approximately 500 km on the inner shelf of the southeastern canadian beaufort sea ice scours were divided into two types based on their morphology multiple scours consisting of a series of parallel scours and ridges and single scours single scours are the dominant type representing more than 85 of the total observations the mean scour depth and width are 0 3 m and 11 m respectively but scour depths of more than 2 m and a scour width up to 345 m were documented the magnitude of ice scouring processes increases with water depth in water depths less than 10 m less than 25 of the seafloor surface is reworked by ice scours this percentage increases significantly seaward of the 10 m isobath being more than 75 in water depths in excess of 12 m a break in the seabed slope at about 10 to 12 m water depth marks a boundary between a nearshore zone moderately influenced by ice processes and an outer zone affected by intense ice scouring this morphological boundary could be due to intense erosion by the keels of pressure ice ridges at the inner edge of a zone of grounded ice ridges most of the observed ice scours appear to be reworked especially in water depths of less than 10 m and represent small scale sediment sinks inshore of the 10 m isobath scour reworking is believed to be mainly due to frequent bottom disturbance by wave orbital currents and mean near bottom flows during the open water season scour orientations show that the dominant motion of ice during scouring events is east or west which is subparallel to the bathymetric contours and coastline scour terminal push mounds however suggest a dominant east to southeast movement that may contribute to onshore sediment transport during ice push events

multiple meltwater discharge and ice rafting events recorded in the deglacial sediments along the beaufort margin arctic ocean abstract the first regional model for the deglacial history of the beaufort margin slope in the western arctic is presented the conceptual model was developed using new high resolution chirp seismic reflection multibeam bathymetry and sediment core data acquired along the margin this synthesis provides important constraints on sediment source and dispersal patterns for the last deglaciation the continental slope from barrow canyon to the mackenzie trough is characterized by thick holocene sediments mostly sourced from barrow canyon and continental discharge this acoustically transparent fine grained unit overlies acoustically laminated sediments sourced from the mackenzie river and ice rafting the deglacial history of the margin from mackenzie trough to the amundsen gulf is characterized by many ice rafting and meltwater discharge events ice rafted debris layers were deposited around 14 6 ka and 14 1 ka and likely document enhanced ice shedding events from the amundsen and m clure ice streams as they retreated there are three inferred meltwater discharge events all sourced from the mackenzie region the oldest discharge event had two phases one sometime between 14 5 ka and 14 2 ka and another between 14 0 ka and 13 0 ka this discharge deposited finely laminated sediments more than 7 m thick sourced from proglacial lakes in the area following this was a flood event starting at 12 94 ka which generated high amplitude reflectors deposited coarse debris and caused a lowering in the 18o record this is possibly a major outburst flood from glacial lake agassiz with an additional component of agassiz meltwater that was diverted from its southern drainage pathway down the mississippi to the gulf of mexico finally a third discharge event was initiated by 11 3 ka which deposited coarse laminated sediments focused in the mackenzie trough timing of these second and third events correlates with the onset of the younger dryas cold period and the preboreal oscillation event respectively suggesting that fresh water discharge may have reached a tipping point leading to those climate events

iceberg discharge to the chukchi shelf during the younger dryas abstract the extent of glaciation in northwestern alaska the source of sediment supply to the chukchi shelf and slope and the movement of sea ice and icebergs across the shelf during the last glacial maximum lgm remain poorly constrained here we present geophysical and geological data from the outer chukchi margin that reveal a regionally extensive heavily ice scoured surface 5 8 m below the modern seafloor radiocarbon dating of this discrete event yields age estimates between 10 600 and 11 900 14c yr bp indicating the discharge event occurred during the younger dryas based on mineralogy of the ice rafted debris the icebergs appear to be sourced from the northwestern alaskan margin which places important constraints on the ice extent in northern alaska during the lgm as well as existing circulation models for the region

new evidence for high discharge to the chukchi shelf since the last glacial maximum using chirp subbottom profiling across the chukchi shelf offshore nw alaska we observed a large incised valley that measures tens of kilometers in width the valley appears to have been repeatedly excavated during sea level lowering however the two most recent incisions appear to have been downcut during the last sea level rise suggesting an increase in the volume of discharge modern drainage from the northwestern alaskan margin is dominated by small low discharge rivers that do not appear to be large enough to have carved the offshore drainage the renewed downcutting and incision during the deglaciation and consequent base level rise implies there must have been an additional source of discharge paleoprecipitation during deglaciation is predicted to be at least 10 less than modern precipitation and thus cannot account for the higher discharge to the shelf glacial meltwater is the most likely source for the increased discharge

arctic ocean glacial history while there are numerous hypotheses concerning glacial interglacial environmental and climatic regime shifts in the arctic ocean a holistic view on the northern hemisphere s late quaternary ice sh

widespread multi source glacial erosion on the chukchi margin arctic ocean abstract multibeam bathymetry and sub bottom profiler data acquired in 2011 from r v marcus langseth in a broad grid over the chukchi sea margin reveal multiple glacigenic features on the top and slopes of the outer chukchi shelf rise and adjacent borderland glacial lineations record a complex pattern of erosion likely formed by both local glaciation and far traveled ice shelves streams sourced from the laurentide and possibly east siberian ice sheets multiple till units and stacked debris flows indicate recurrent glacial grounding events composite till wedges of several hundred meters thick extend the shelf edge by 10 20 km in places distribution of ice marginal features on the chukchi rise suggests stepwise deglacial retreat towards the shelf backing up the broad bathymetric trough at the eastern side of the rise glacigenic features other than extensive iceberg scouring cannot be identified above 350 m depth and no glacigenic bedforms are present on the current swept shallow shelf despite the resulting uncertainty with the southern extent of the glaciation the data suggest a widespread grounded ice presence on the northern chukchi shelf which makes it an important previously underestimated component of the arctic paleo glacial system

rapid sea level rise and holocene climate in the chukchi sea three new sediment cores from the chukchi sea preserve a record of local paleoenvironment sedimentation and flooding of the chukchi shelf 50 m by glacial eustatic sea level rise radiocarbon dates on foraminifera provide the first marine evidence that the sea invaded hope valley southern chukchi sea 53 m as early as 12 ka the lack of significant sediment accumulation since ca 7 ka in hope valley southeastern chukchi shelf is consistent with decreased sediment supply and fluvial discharge to the shelf as deglaciation of alaska concluded abundant benthic foraminifera from a site west of barrow canyon indicate that surface waters were more productive 4 6 ka and this productivity varied on centennial time scales an offshore companion to this core contains a 20 m record of the holocene these results show that carefully selected core sites from the western arctic ocean can have a temporal resolution equal to the best cores from other regions and that these sites can be exploited for high resolution studies of the paleoenvironment

least squares migration of incomplete reflection data a least squares migration algorithm is presented that reduces the migration artifacts i e recording footprint noise arising from incomplete data instead of migrating data with the adjoint of the forward modeling operator the normal equations are inverted by using a preconditioned linear conjugate gradient scheme that employs regularization the modeling operator is constructed from an asymptotic acoustic integral equation and its adjoint is the kirchhoff migration operator we tested the performance of the least squares migration on synthetic and field data in the cases of limited recording aperture coarse sampling and acquisition gaps in the data numerical results show that the least squares migrated sections are typically more focused than are the corresponding kirchhoff migrated sections and their reflectivity frequency distributions are closer to those of the true model frequency distribution regularization helps attenuate migration artifacts and provides a sharper better frequency distribution of estimated reflectivity the least squares migrated sections can be used to predict the missing data traces and interpolate and extrapolate them according to the governing modeling equations several field data examples are presented a ground penetrating radar data example demonstrates the suppression of the recording footprint noise due to a limited aperture a large gap and an undersampled receiver line in addition better fault resolution was achieved after applying least squares migration to a poststack marine data set and a reverse vertical seismic profiling example shows that the recording footprint noise due to a coarse receiver interval can be suppressed by least squares migration

freshwater forcing of abrupt climate change during the last glaciation large millennial scale fluctuations of the southern margin of the north american laurentide ice sheet occurred during the last deglaciation when the margin was located between about 43 and 49 n fluctuations of the ice margin triggered episodic increases in the flux of freshwater to the north atlantic by rerouting continental runoff from the mississippi river drainage to the hudson or st lawrence rivers we found that periods of increased freshwater flow to the north atlantic occurred at the same time as reductions in the formation of north atlantic deep water thus providing a mechanism for observed climate variability that may be generally characteristic of times of intermediate global ice volume