wind driven submesoscale subduction at the north pacific subtropical front 1 upper ocean observations from the north pacific subtropical front during late winter demonstrate the generation of submesoscale intrusions by buoyancy loss prior to generation a sharp thermohaline front was intensified by confluent flow of 1 2 10 5 s 1 relative vertical vorticity across a surface intensified along front jet on the warm side of a frontal trough was 0 5 f during the storm buoyancy loss arose due to cooling of 650 w m 2 and down front wind stress 0 5 n m 2 that generated a southward cross front ekman transport of dense water over light the resulting wind driven buoyancy flux was concentrated at the front where it exceeded that due to convection by an order of magnitude the intrusions appeared immediately following the storm both within the surface mixed layer and beneath the seasonal pycnocline they were approximately 20 m thick and horizontally elongated in the cross frontal direction the near surface intrusions had cool and fresh properties characteristic of the water underlying the seasonal pycnocline whereas the subsurface intrusions were composed of warm and saline water from the surface the apparent vertical exchange was constrained within a thin filament of 2 km zonal extent that was characterized by o 1 rossby and richardson numbers pronounced cyclonic veering in the horizontal velocity throughout the surface mixed layer and sloping isopycnals the intrusion properties background environmental context and forcing history are consistent with prior numerical modeling results for the generation of ageostrophic vertical circulations by frontogenesis intensified by buoyancy loss possibly resulting in symmetric instability

millennial scale fluctuations of the european ice sheet at the end of the last glacial and their potential impact on global climate abstract reconstructing northern hemisphere ice sheet oscillations and meltwater routing to the ocean is important to better understand the mechanisms behind abrupt climate changes to date research efforts have mainly focused on the north american laurentide ice sheets lis leaving the potential role of the european ice sheet eis and of the scandinavian ice sheet sis in particular largely unexplored using neodymium isotopes in detrital sediments deposited off the channel river we provide a continuous and well dated record for the evolution of the eis southern margin through the end of the last glacial period and during the deglaciation our results reveal that the evolution of eis margins was accompanied with substantial ice recession especially of the sis and simultaneous release of meltwater to the north atlantic these events occurred both in the course of the eis to its lgm position i e during heinrich stadial hs 3 and hs2 31 29 ka and 26 23 ka respectively and during the deglaciation i e at 22 ka 20 19 ka and from 18 2 0 2 to 16 7 0 2 ka that corresponds to the first part of hs1 the deglaciation was discontinuous in character and similar in timing to that of the southern lis margin with moderate ice sheet retreat from 22 5 0 2 ka in the baltic lowlands as soon as the northern summer insolation increase from 23 ka and an acceleration of the margin retreat thereafter from 20 ka importantly our results show that eis retreat events and release of meltwater to the north atlantic during the deglaciation coincide with amoc destabilisation and interhemispheric climate changes they thus suggest that the eis together with the lis could have played a critical role in the climatic reorganization that accompanied the last deglaciation finally our data suggest that meltwater discharges to the north atlantic produced by large scale recession of continental parts of northern hemisphere ice sheets during hs could have been a possible source for the oceanic perturbations i e amoc shutdown responsible for the marine based ice stream purge cycle or so called he s that punctuate the last glacial period

characterizing thermohaline intrusions in the north pacific subtropical frontal zone abstract a monthlong field survey in july 2007 focused on the north pacific subtropical frontal zone stfz near 30 n 158 w combined towed depth cycling conductivity temperature depth ctd profiling with shipboard current observations measurements were used to investigate the distribution and structure of thermohaline intrusions the study revealed that local extrema of vertical salinity profiles often used as intrusion indicators were only a subset of a wider class of distortions in thermohaline fields due to interleaving processes a new method to investigate interleaving based on diapycnal spiciness curvature was used to describe an expanded class of laterally coherent intrusions stfz intrusions were characterized by their overall statistics and by a number of case studies thermohaline interleaving was particularly intense within 5 km of two partially compensated fronts where intrusions with both positive and negative salinity anomalies were widespread the vertical and cross frontal scales o

complex kame belt morphology stratigraphy and architecture the development of glacier karst at the margins of melting ice sheets produces complex glaciofluvial sediment landform assemblages that provide information on ice sheet downwasting processes we present the first combined geomorphological sedimentological and geophysical investigation of the brampton kame belt an important glaciofluvial depositional zone at the centre of the last british irish ice sheet ground penetrating radar gpr data allow the broad scale internal architecture of ridges eskers and flat topped hills ice walled lake plains to be determined at four sites in combination with sediment exposures these provide information on lateral and vertical variations in accretion styles depositional boundaries and grain size changes building on existing work on the subject we propose a refined model for the formation of ice walled lake plains resulting from the evolution and collapse of major drainage axes into lakes as stable glacier karst develops during deglaciation the internal structure of esker ridges demonstrates variations in sedimentation that can be linked to differences in ridge morphologies across the kame belt this includes low energy flow conditions and multiple accretion phases identified within large s n oriented esker ridges and fluctuating water pressures hyperconcentrated flows and significant deformation within a fragmented sw ne oriented esker ridge in combination with updated geomorphological mapping this work allows us to identify two main styles of drainage within the kame belt 1 major drainage axes aligned broadly s n that extend through the entire kame belt and collapsed into a chain of icewalled lakes and 2 a series of smaller fragmented sw ne aligned esker ridges that represent ice marginal drainage as the ice sheet receded south eastwards up the vale of eden our study demonstrates the importance of integrated geomorphological sedimentological and geophysical investigations in order to understand complex and polyphase glaciofluvial sediment landform assemblages

observations of energetic high wavenumber internal waves in the kuroshio abstract intense and spatially coherent shear layers were detected in and under the kuroshio in an april 2000 survey in the east china sea the sloping layers revealed by shipboard doppler sonars on the r v roger revelle appeared to cross isopycnal surfaces except in a small region near the kuroshio shelfbreak front the rms finescale shear associated with the layers significantly exceeded the geostrophic shear an april 2002 follow on cruise was organized to establish whether these motions were propagating internal waves shipboard and lowered adcps were both operated from the r v melville in addition to ctd sonar transects a 30 h time series of currents and shear was obtained in the core of the kuroshio near the island of kyushu japan the shear structures were indeed found to be propagating with both up and down going internal wave motions present in comparison with the nearby open ocean the finescale 160 m vertical shear variance is increased by a factor of 3 in the kuroshio and by a fact

southern laurentide ice sheet retreat synchronous with rising boreal summer insolation establishing the precise timing for the onset of ice sheet retreat at the end of the last glacial maximum lgm is critical for delineating mechanisms that drive deglaciations uncertainties in the timing of ice margin retreat and global ice volume change allow a variety of plausible deglaciation triggers using boulder 10 be surface exposure ages we date initial southern laurentide ice sheet lis retreat from lgm moraines in wisconsin usa to 23 0 0 6 ka coincident with retreat elsewhere along the southern lis and synchronous with the initial rise in boreal summer insolation 24 23 ka we show with climate surface mass balance simulations that this small increase in boreal summer insolation alone is potentially sufficient to drive enhanced southern lis surface ablation we also date increased southern lis retreat after ca 20 5 ka likely driven by an acceleration in rising isolation this near instantaneous southern lis response to boreal summer insolation before any rise in atmospheric co 2 supports the milankovic hypothesis of orbital forcing of deglaciations

putting the younger dryas cold event into context abstract the younger dryas event is by far the best studied of the millennial scale cold snaps of glacial time yet its origin remains a subject of debate the long held scenario that the younger dryas was a one time outlier triggered by a flood of water stored in proglacial lake agassiz has fallen from favor due to lack of a clear geomorphic signature at the correct time and place on the landscape the recent suggestion that the younger dryas was triggered by the impact of a comet has not gained traction instead evidence from chinese stalagmites suggests that rather than being a freak occurrence the younger dryas is an integral part of the deglacial sequence of events that produced the last termination on a global scale

mixing in the west spitsbergen current abstract twenty eight profiles of temperature and salinity down to 1 km depth have been measured in the area north from the coast of svalbard to latitude 84 40 n between 0 and 32 e longitude they show atlantic waters of the west spitsbergen current entering the arctic ocean in a bifilimentary mode one filament following the coastline and the other 300 km farther north apparently following the continental shelf break the profiles show these filaments mixing into the ambient water in a series of intrusive layers which are remarkably uniform over distances of hundreds of kilometers a triple peak structure at the atlantic water maximum is the dominant oceanographic characteristic of the area with salt fingering occurring on the upper surface of the cold intrusions and indications of double diffusive exchanges through the lower surface a system or eddies or meanders is postulated as the mechanism for the production of multiple fronts allowing the intrusive layers to extend to distances much greater t

characteristics of interleaving in the western equatorial pacific 1 the characteristics of intrusive thermohaline features in the thermocline of the western equatorial pacific are examined during two different climatic states the vertical and meridional structure of the features is determined from a total of 12 high resolution meridional sections although there is considerable variability of the broader structure of the flow and temperature salinity fields the spatial scale of the interleaving is found to be persistent over all sections the turner angle and correlations between the spice anomaly and density anomaly suggest that double diffusive processes may be important in the vertical flux of properties the characteristics of the interleaving are compared with the results from the theories of double diffusive interleaving and inertial instability it is found that the observations are consistent with both it is difficult to differentiate between the two theories in explaining the presence of the layers indeed both mechanisms may be playing a role the magnitude of the cross equatorial fluxes of heat and salt due to the features is estimated and shown to be comparable with estimates due to other processes there is a strong potential for the interleaving to impinge on the large scale dynamics of the equatorial pacific ocean

absence of altonian glaciation in illinois abstract illinois was free of glacier ice from about 130 000 to 25 000 yr b p deposits of this time interval in northeastern illinois represented by core samples from test hole isgs s 30 are composed of leached nonglacigenic stratified or pedoturbated silty sediments that typically are organic rich and pedogenically modified these deposits overlie the sangamon soil which is identified by its stratigraphic position soil morphology and distinct alteration products in the clay mineral fraction including a variably swelling vermiculite like phase and a randomly interstratified kaolinite 10 a phase the regional extent of these deposits suggests that if altonian ice existed in the lake michigan basin it did not extend westward beyond the cuesta of silurian dolomite that rims the southwestern shore of lake michigan

is interleaving in the agulhas current driven by near inertial velocity perturbations abstract recent observations taken at a number of latitudes in the agulhas current reveal that the water mass structure on either side of its dynamical core is distinctly different moreover interleaving of these distinct water masses is observed at over 80 of the stations occupied in the current particularly within the subsurface density layer between tropical surface water and subtropical surface water masses and within the intermediate layer between the antarctic intermediate water and red sea water masses direct velocity measurements allow for a comparison between the characteristic vertical length scales of the agulhas intrusions and those of velocity perturbations found throughout the current it is found that the interleaving scales match those of the velocity perturbations which are manifest as high wavenumber vertical shear layers and are identified as near inertial oscillations furthermore the properties of the intrusions indicate that double diffusion is not an important process in thei

formation of interleaving layers in the bay of bengal abstract observations of the upper 200 m of the strongly stratified bay of bengal reveal a host of interleaving layers of watermass properties the features appear as isopycnal spice anomalies and form multiple parallel 10 m thin layers that are coherent over horizontal scales of 30 80 km they are coincident with anomalies in density stratification and in some cases with the vertical shear of horizontal velocity but are not aligned with isopycnals the cross isopycnal slopes of these prominent features are o 1 0 4 and are on average 5 to 10 times smaller than the expected slopes of passive tracer anomalies due to mesoscale stirring we present two alternate mechanisms by which such interleaving layers can be created from existing lateral spice gradients i stirring by vertically sheared submesoscale eddies and ii near inertial wave shear a numerical simulation of a density and spice front based on observations of a narrow and fast jet develops a submesoscale vortex that produces spice layering similar to our observations the layering suggests that even a weak vertical diffusivity would greatly enhance the horizontal mixing of spice at submesoscales the scales of the proposed processes through shear dispersion the analysis of the observations draws attention to the prevalence of submesoscale dynamics and niws with unusually high vertical wavenumbers in the bay of bengal

anomalous characteristics of equatorial thermohaline finestructure abstract finestructure observations gathered in the equatorial pacific are discussed the measurements consist of a 48 h ctd time series within the salinity front at the northern edge of the equatorial undercurrent at 110 w the water column at depths shallower than 150 m at this site was dominated by intense inter leaving the vertical temperature and salinity gradient spectra from this depth range are strongly peaked at a wavelength of 20 m and the temperature salinity anomalies are coherent and in phase at this scale yet unlike the finestructure observed in fronts at higher latitudes the finestructure is not density compensating on any scale on account of an excess of temperature variance rather than interpret this equatorial finestructure as resulting from nearly isopycnal differential advection it is suggested that the features may be the signature of low frequency internal waves where lateral displacements are reflected in the salinity profiles and lateral and vertical displacements induce the

patterns of shear and turbulence across the florida current abstract measurements by the multi scale profiler msp at seven stations spanning the straits of florida characterize levels and patterns of internal wave activity and mixing in this vertically sheared environment contrasting properties suggest five mixing regimes the largest and most vast is the interior regime where the background flow has an inverse richardson number ri 1 ranging up to 0 55 shear is dominated by fluctuations that are 1 4 times stronger than in the open ocean and turbulent diffusivities are similarly moderate at 1 4 10 5 m2 s 1 the high velocity core of the current near the surface at midchannel is associated with weak mixing to its west is a zone of high mean shear where strong stratification results in background ri 1 of only 0 4 fluctuations are weak and diffusivity is moderate intermittent shear features beneath the core have mean ri 1 1 and strong turbulence two regimes are related to channel topography adjacent to the steep eastern slope finescale shear is

effect of baroclinicity on double diffusive interleaving although ocean fronts are often baroclinic existing models of double diffusive interleaving have ignored such baroclinic effects as velocity shear and horizontal density gradients to determine the importance of these effects the authors have formulated a linear instability analysis applicable to baroclinic fronts two limiting cases are considered one for fronts with strong vertical and or horizontal shear the other for fronts with weak shear in both limits double diffusive interleaving can be enhanced or suppressed by baroclinicity interleaving motion is enhanced if isopycnals rise toward the fresh side of the front conversely interleaving is suppressed if isopycnals slope downward across the front a significant result is that the salinity gradient along isopycnals is not a good indicator of interleaving strength as an example the model is applied to a mediterranean salt lens the effect of baroclinicity is significant the predicted growth rates are increased by 35 90 the large scale velocity and hydrographic fields indicate that meddy sharon lies somewhere between the high and low shear limits nevertheless the model predictions agree reasonably well with the observed interleaving characteristics

pattern of southern laurentide ice sheet margin position changes during heinrich stadials 2 and 1 abstract with concerns over rising global temperatures ice sheet stability and implications for sea level rise the responses of former ice sheets to past climate change provide useful insights into linkages between the atmosphere cryosphere and oceans one example is the behavior of the laurentide ice sheet lis during heinrich stadials times of cooler sea surface temperature sst in the north atlantic using existing cosmogenic surface exposure ages varve chronologies radiocarbon ages and optically stimulated luminescence ages from the southern lis margin this report tests the hypothesis that the southern side of the lis advanced during times of surface cooling of the north atlantic this test reconstructs the timing of local glacial maxima and the times immediately pro and preceding them from multiple areas and compares them to the timing of heinrich stadial 2 hs2 26 6 23 6 ka and to the timing of heinrich stadial 1 hs1 19 3 15 3 ka during hs2 southern sector lobes new england region huron erie lobe lake michigan lobe green bay lobe and the chippewa lobe advanced towards their maximum position except for the des moines lobe for which there is no applicable data during hs1 these same lobes experienced substantial retreat but the des moines lobe advanced thus the southern laurentide margin behaved differently during the two heinrich stadials this pattern may be attributed to differing east west temperature gradients across north america and associated changes in atmospheric circulation

spatial variability of turbulent mixing in the abyssal ocean ocean microstructure data show that turbulent mixing in the deep brazil basin of the south atlantic ocean is weak at all depths above smooth abyssal plains and the south american continental rise the diapycnal diffusivity there was estimated to be less than or approximately equal to 0 1 x 10 4 meters squared per second in contrast mixing rates are large throughout the water column above the rough mid atlantic ridge and the diffusivity deduced for the bottom most 150 meters exceeds 5 x 10 4 meters squared per second such patterns in vertical mixing imply that abyssal circulations have complex spatial structures that are linked to the underlying bathymetry

magnetic susceptibility and mineral zonations controlled by provenance in loess along the illinois and central mississippi river valleys abstract magnetic susceptibility ms patterns have proven useful for regional stratigraphic correlations of zones within thick oxidized peoria and roxana silts along the illinois and central mississippi river valleys for more than 350 km variations in ms of c horizon loess are controlled by silt sized magnetite content and are interpreted to reflect changes in sediment provenance due to fluctuations of the superior and lake michigan glacier lobes and the diversion of the mississippi river to its present course grain size distributions and scanning electron microscopic observations indicate that stratigraphic changes in ms are not significantly influenced by eolian sorting or diagenetic dissolution respectively three compositional zones lower middle and upper are delineated within peoria silt which usually can be traced in the field by ms the occurrence of clay beds interstadial soils and or subtle color changes these zones can be correlated with but are generally of more practical use than previously studied dolomite zones mckay 1977 or clay mineral zones frye et al 1968 however mineralogical analyses can help to substantiate zone boundaries when in question ms and compositional zones may indirectly record a climatic signal primarily through the effect that global cooling has had on ice lobe fluctuations in the upper mississippi drainage basin

provenance of freshwater pulses in the gulf of mexico during the last deglaciation during the last deglaciation the decaying laurentide ice sheet lis delivered huge volumes of meltwater toward the gulf of mexico the present investigation of clay mineralogy and grain size characteristics of terrigenous sediments deposited in the orca basin gulf of mexico offers a unique opportunity to link the marine record of these meltwater floods with the reconstructed continental glacial history and the modeled drainage patterns five peculiar sedimentary levels characterized by high smectite content and low caco3 content were identified and occurred simultaneously with major meltwater floods according to recently published clay mineral distribution maps for north america these results help to pinpoint the southwestern margin of the lis as a main contributor to most of the meltwater discharges in addition the peculiar mineralogical composition illite and chlorite rich of the sediments characterizing the meltwater episode associated with heinrich event 1 suggests a provenance from the great lakes area supporting the interpretation of destabilization of the lis southeastern margin during this event decreased terrigenous contribution associated with changing provenance of sediments after 12 9 cal ka bp suggests strong modifications of the continental hydrography in relation to lake agassiz history and changes in the morphology of mississippi delta due to rising sea level

pseudo gamma ray volume from extended elastic impedance inversion for gas exploration the unayzah formation has been an exploration target in saudi arabia for more than a decade with oil and gas production from the unit now well established however exploration in the unayzah in central saudi arabia remains technically challenging this is due in large part to its complex interplay of continental clastic depositional environments that include braided stream wadi fill playa lake interfluve alluvial fan and eolian dune systems

double diffusive interleaving part i linear stability analysis abstract quasi horizontal interleaving between water masses is frequently observed in the frontal regions between different water masses where there are significant compensating isopycnal gradients of temperature and salinity it is believed that these intrusions are driven by double diffusive convection which transports salt heat and density across sharp quasi horizontal interfaces i e across regions of large vertical property gradients stern and other investigators have parameterized the vertical flux of salt in these intrusions by an eddy diffusivity multiplied by the vertical gradient of salinity whereas laboratory experiments show that sharp interfaces tend to occur by double diffusive convection and that the vertical fluxes of properties depend on the property contrasts across the interfaces in this paper it is assumed that the vertical fluxes of double diffusive convection are proportional to the salinity contrast across the sharp interfaces that occur between the quasi horizontal intrusio

evidence of enhanced double diffusive convection below the main stream of the kuroshio extension in this study a navis microrider microstructure float and an em apex float were deployed along the kuroshio extension front the observations deeper than 150 m reveal widespread interleaving thermohaline structures for at least 900 km along the front presumably generated through mesoscale stirring and near inertial oscillations in these interleaving structures microscale thermal dissipation rates are very high o 10 7 k2s 1 while turbulent kinetic energy dissipation rates are relatively low o 10 10 10 9 wkg 1 with effective thermal diffusivity k of o 10 3 m2s 1 consistent with the previous parameterizations for double diffusion and k is two orders of magnitude larger than the turbulent eddy diffusivity for density k the average observed dissipation ratio in salt finger and diffusive convection favorable conditions are 1 2 and 4 0 respectively and are larger than that for turbulence our results suggest that mesoscale subduction obduction and near inertial motions could catalyze double diffusive favorable conditions and thereby enhancing the diapycnal tracer fluxes below the kuroshio extension front

the last glacial maximum at 44 s documented by a 10be moraine chronology at lake ohau southern alps of new zealand abstract determining whether glaciers registered the classic last glacial maximum lgm 26 500 19 000 yrs ago coevally between the hemispheres can help to discriminate among hypothesized drivers of ice age climate here we present a record of glacier behavior from the southern alps of new zealand during the local lgm llgm we used 10 be surface exposure dating methods and detailed glacial geomorphologic mapping to produce a robust chronology of well preserved terminal moraines deposited during the llgm near lake ohau on central south island we then used a glaciological model to estimate a llgm glacier snowline and atmospheric temperature from the ohau glacier record seventy three 10 be surface exposure ages place culminations of terminal moraine construction and hence completions of glacier advances to positions outboard of present day lake ohau at 138 600 10 600 yrs 32 520 970 yrs ago 27 400 1300 yrs ago 22 510 660 yrs ago and 18 220 500 yrs ago recessional moraines document glacier recession into the lake ohau trough by 17 690 350 yrs ago exposure of an ice molded bedrock bench located inboard of the innermost llgm moraines by 17 380 510 yrs ago indicates that the ice tongue had receded about 40 of its overall length by that time comparing our chronology with distances of retreat suggests that the ohau glacier terminus receded at a mean net rate of about 77 m yr 1 and its surface lowered by 200 m between 17 690 and 17 380 yrs ago a long term continuation of ice retreat in the ohau glacier catchment is implied by moraine records at the head of irishman stream valley a tributary of the ohau glacier valley the irishman stream cirque glacier advanced to produce a set of lateglacial moraines at 13 000 500 yrs ago implying that the cirque glacier was less extensive prior to that advance we employed a glaciological model fit to these mapped and dated llgm moraines to derive snowline elevations and temperature parameters from the ohau glacier record the modeling experiments indicate that a snowline lowering of 920 50 m and temperature depression of 6 25 0 5 c below modern values allows for the ohau glacier to grow to an equilibrium position within the lgm moraine belt taken together with a glaciological simulation reported from the irishman stream valley snowlines and temperatures increased by at least 520 m and 3 6 c respectively between 18 000 and 13 000 yrs ago climate parameters derived from the ohau glacier reconstruction are similar to those derived from glacier records from patagonia to air temperature indicators from antarctica as well as to sea surface temperature and stratification signatures of the southern ocean we think that the best explanation for the observed southern llgm is that southern winter duration modulated southern ocean sea ice which in turn influenced southern ocean stratification and made the surface ocean cooler orbitally induced cooling of the southern ocean provides an explanation for the llgm in the southern alps having been coincident with the northern lgm we argue further that the global effect of north atlantic stadials led to disturbance of southern ocean stratification southward shifts of the subtropical front and retreat of southern alps glaciers collapse of southern ocean stratification during heinrich stadial 1 along with attendant sea surface warming triggered the onset of the last glacial termination in the southern alps of new zealand

absence of glaciation in illinois during marine isotope stages 3 through 5 a10be inventory and14c ages of material from a core from northernmost illinois support previous interpretations that this area was ice free from ca 155 000 to 25 000 yr ago during much of this period from about 155 000 to 55 000 yr ago 10be accumulated in the argillic horizon of the sangamon geosol wisconsinan loess containing inherited10be was deposited above the sangamon geosol from ca 55 000 to 25 000 yr ago and was subsequently buried by late wisconsinan till deposited by the lake michigan lobe of the laurentide ice sheet the sangamonian interglacial stage has been correlated narrowly to marine oxygen isotope substage 5e our data indicate instead that the sangamon geosol developed during late stage 6 all of stages 5 and 4 and early stage 3

the velocity and vorticity structure of the agulhas current at 32 s the full depth velocity field of the agulhas current the western boundary current of the southwest indian ocean has been directly measured using a lowered acoustic doppler current profiler ladcp fifteen combined conductivity temperature depth ctd and ladcp stations were occupied across the current at 32 s off the east coast of south africa in february march 1995 as part of the world ocean circulation experiment indian ocean program the deep velocity structure of the agulhas current was found to be very different than previously described using geostrophic estimates in particular ladcp results reveal a v shaped pattern for the level of no motion across the agulhas current and an agulhas undercurrent is observed flowing equatorward below 800 m depth directly beneath the surface core of the poleward flowing agulhas current comparisons of direct and geostrophic velocities suggest that the velocity structure of the agulhas current is essentially geostrophic below about 200 m depth where differences are generally less than estimated errors above these depths the ladcp measured shears and geostrophic shears exhibit differences and velocities can diverge shipboard adcp data support ladcp results near the surface indicating that both instruments are measuring real signals the geostrophic and ladcp measured shears are well matched away from the surface and a depth averaged fit below 200 m between them exhibits small standard deviations the geostrophic volume transport of the agulhas current as referenced to ladcp is 73 sv and is just 3 different from the direct ladcp transport estimate the most recent full depth estimate of agulhas current transport in the literature is given by toole and warren 1993 who estimate a geostrophic volume transport of 85 sv the baroclinic velocity structures from their section and from this work are similar indicating that the difference in the transport estimates is due to the choice of geostrophic reference level and ultimately to the presence of the previously unobserved agulhas undercurrent combined ctd ladcp data are very useful for potential vorticity analysis since velocity and density measurements are simultaneous and coincident the vorticity structure of the agulhas current suggests the presence of a mixing boundary at middepths similar to features observed in the gulf stream the feature appears to be associated with a sharp upturn in isopycnals coincident with the foot of the continental slope the boundary inhibits cross stream mixing of intermediate water masses and may help explain the appearance of a discrete filament of red sea water inshore of antarctic intermediate waters in the same density layer despite intense cross stream shears

an updated radiocarbon based ice margin chronology for the last deglaciation of the north american ice sheet complex abstract the north american ice sheet complex naisc consisting of the laurentide cordilleran and innuitian ice sheets was the largest ice mass to repeatedly grow and decay in the northern hemisphere during the quaternary understanding its pattern of retreat following the last glacial maximum is critical for studying many facets of the late quaternary including ice sheet behaviour the evolution of holocene landscapes sea level atmospheric circulation and the peopling of the americas currently the most up to date and authoritative margin chronology for the entire ice sheet complex is featured in two publications geological survey of canada open file 1574 dyke et al 2003 quaternary glaciations extent and chronology part ii dyke 2004 these often cited datasets track ice margin recession in 36 time slices spanning 18 ka to 1 ka all ages in uncalibrated radiocarbon years using a combination of geomorphology stratigraphy and radiocarbon dating however by virtue of being over 15 years old the ice margin chronology requires updating to reflect new work and important revisions this paper updates the aforementioned 36 ice margin maps to reflect new data from regional studies we also update the original radiocarbon dataset from the 2003 2004 papers with 1541 new ages to reflect work up to and including 2018 a major revision is made to the 18 ka ice margin where banks and eglinton islands once considered to be glacial refugia are now shown to be fully glaciated our updated 18 ka ice sheet increased in areal extent from 17 81 to 18 37 million km2 which is an increase of 3 1 in spatial coverage of the naisc at that time elsewhere we also summarize region by region significant changes to the deglaciation sequence this paper integrates new information provided by regional experts and radiocarbon data into the deglaciation sequence while maintaining consistency with the original ice margin positions of dyke et al 2003 and dyke 2004 where new information is lacking this is a pragmatic solution to satisfy the needs of a quaternary research community that requires up to date knowledge of the pattern of ice margin recession of what was once the world s largest ice mass the 36 updated isochrones are available in pdf and shapefile format together with a spreadsheet of the expanded radiocarbon dataset n 5195 ages and estimates of uncertainty for each interval

the dekalb mounds of northeastern illinois as archives of deglacial history and postglacial environments article i nfo article history received 28 august 2009 available online 12 june 2010 the type dekalb mounds of northeastern illinois usa 42 0 n 88 7 w are formed of basal sand and gravel overlain by rhythmically bedded fines and weathered sand and gravel generally from 2 to 7 m thick the fines include abundant fossils of ostracodes and uncommon leaves and stems of tundra plants rare

reconciling records of ice streaming and ice margin retreat to produce a palaeogeographic reconstruction of the deglaciation of the laurentide ice sheet abstract this paper reconstructs the deglaciation of the laurentide ice sheet lis including the innuitian ice sheet from the last glacial maximum lgm with a particular focus on the spatial and temporal variations in ice streaming and the associated changes in flow patterns and ice divides we build on a recent inventory of laurentide ice streams and use an existing ice margin chronology to produce the first detailed transient reconstruction of the ice stream drainage network in the lis which we depict in a series of palaeogeographic maps results show that the drainage network at the lgm was similar to modern day antarctica the majority of the ice streams were marine terminating and topographically controlled and many of these continued to function late into the deglaciation until the ice sheet lost its marine margin ice streams with a terrestrial ice margin in the west and south were more transient and ice flow directions changed with the build up peak phase and collapse of the cordilleran laurentide ice saddle the south eastern marine margin in atlantic canada started to retreat relatively early and some of the ice streams in this region switched off at or shortly after the lgm in contrast the ice streams draining towards the north western and north eastern marine margins in the beaufort sea and in baffin bay appear to have remained stable throughout most of the late glacial and some of them continued to function until after the younger dryas yd the yd influenced the dynamics of the deglaciation but there remains uncertainty about the response of the ice sheet in several sectors we tentatively ascribe the switching on of some major ice streams during this period e g m clintock channel ice stream at the north west margin but for other large ice streams whose timing partially overlaps with the yd the drivers are less clear and ice dynamical processes rather than effects of climate and surface mass balance are viewed as more likely drivers retreat rates markedly increased after the yd and the ice sheet became limited to the canadian shield this hard bed substrate brought a change in the character of ice streaming which became less frequent but generated much broader terrestrial ice streams the final collapse of the ice sheet saw a series of small ephemeral ice streams that resulted from the rapidly changing ice sheet geometry in and around hudson bay our reconstruction indicates that the lis underwent a transition from a topographically controlled ice drainage network at the lgm to an ice drainage network characterised by less frequent broad ice streams during the later stages of deglaciation these deglacial ice streams are mostly interpreted as a reaction to localised ice dynamical forcing flotation and calving of the ice front in glacial lakes and transgressing sea basal de coupling due to large amount of meltwater reaching the bed debuttressing due to rapid changes in ice sheet geometry rather than as conveyors of excess mass from the accumulation area of the ice sheet at an ice sheet scale the ice stream drainage network became less widespread and less efficient with the decreasing size of the deglaciating ice sheet the final elimination of which was mostly driven by surface melt

intrusions what drives them abstract the driving mechanism for the observed interleaving of water masses is generally assumed to be double diffusive mixing however some observations of intrusions have been made in regions where the mean stratification is stable to double diffusive mixing it has been hypothesized that a finite amplitude disturbance must occur to produce regions where the stratification allows double diffusive mixing or that an instability due to differences in the molecular diffusivity of salinity and temperature produces the desired stratification for double diffusive mixing to start there is also the possibility of a differential vertical flux of salt and heat due to incomplete mixing by turbulence the basis of this idea is described in this paper growth rates vertical scales and cross frontal slopes of intrusions predicted by this process are compared to those predicted by double diffusive mixing

intrusive mixing in a mediterranean salt lens intrusion slopes and dynamical mechanisms abstract thermohaline intrusions found in meddy sharon were laterally coherent the slope of these intrusions across density surfaces was in opposite senses in the upper and lower part of the meddy consistent with i the mcintyre instability for prandtl number less than one in which case mass flux assumed equal in heat and salt diffusivity dominates over viscosity or ii double diffusive intrusions in which diffusive fluxes dominate in the upper diffusively unstable portion of the meddy and finger fluxes dominate in the lower fingering unstable part the magnitudes of the intrusion slopes are outside the range of angles for which the mcintyre mechanism can provide energy to the intrusive motions but within the range of angles for which double diffusive mixing can drive the intrusions thus the energy source for the intrusions is not the kinetic or potential energy of the geostrophic shear flow but is the potential energy associated with the lateral salinity and temperature gradients furthe

chronological framework for the deglaciation of the lake michigan lobe of the laurentide ice sheet from ice walled lake deposits a revised chronological framework for the deglaciation of the lake michigan lobe of the south central laurentide ice sheet is presented based on radiocarbon ages of plant macrofossils archived in the sediments of low relief ice walled lakes we analyze the precision and accuracy of 15 ams 14 c ages of plant macrofossils obtained from a single ice walled lake deposit the semi circular basin is about 0 72 km wide and formed of a 4 to 16 m thick succession of loess and lacustrine sediment inset into till the assayed material was leaves buds and stems of salix herbacea snowbed willow the pooled mean of three ages from the basal lag facies was 18 270 50 14 c a bp 21 810 cal a bp an age that approximates the switch from active ice to stagnating conditions the pooled mean of four ages for the youngest fossil bearing horizon was 17 770 40 14 c a bp 21 180 cal a bp material yielding the oldest and youngest ages may be obtained from sediment cores located at any place within the landform based on the estimated settling times of overlying barren rhythmically bedded sand and silt the lacustrine environment persisted for about 50 more years at a 67 confidence level the dated part of the ice walled lake succession persisted for between 210 and 860 cal a modal value 610 cal a the deglacial age of five moraines or morainal complexes formed by the fluctuating margin of the lake michigan lobe have been assessed using this method there is no overlap of time intervals documenting when ice walled lakes persisted on these landforms the rapid readvances of the lobe during deglaciation after the last glacial maximum probably occurred at some point between the periods of ice walled lake sedimentation copyright 2011 john wiley sons ltd

episodic deposition of illinois valley peoria silt in association with lake michigan lobe fluctuations during the last glacial maximum the chronology and cause of millennial depositional oscillations within last glacial loess of the central lowlands of the united states are uncertain here we present a new age model that indicates the peoria silt along the illinois river valley accumulated episodically from 28 500 to 16 000 cal yr bp as the lake michigan lobe margin fluctuated within northeastern illinois the age model indicates accelerated loess deposition coincident with regional glacial advances during the local last glacial maximum a weakly developed paleosol the jules geosol represents a period of significantly slower deposition from 23 700 to 22 000 cal yr bp a gastropod assemblage based reconstruction of mean july temperature shows temperatures 6 10 c cooler than modern during peoria silt deposition stable oxygen and carbon isotope values 18 o and 13 c of gastropod carbonate do not vary significantly across the pedostratigraphic boundary of the jules geosol suggesting slower loess accumulation was a result of reduced glacial sediment supply rather than direct climatic factors however a decrease in 18 o values occurred between 26 000 and 24 000 cal yr bp synchronous with the lake michigan lobe s southernmost advance this 18 o decrease suggests a coupling of regional summer hydroclimate and ice lobe position during the late glacial period

fine structure and mixing at the edge of a warm core ring observations of temperature and salinity fine structure near the edge of a newly formed gulf stream warm core ring are presented a thermohaline front and a cold tongue of shelf water were tracked for about 16 km in a series of crossings by a depth cycling ctd the core was observed to become warmer saltier and more dense in the downstream direction leading to estimates of eddy diffusivities larger than 10 3 m2 s it is concluded that fluxes this large cannot be due to double diffusive processes either lateral or vertical shear driven turbulence can account satisfactorily for the mixing of the core but it would require that the richardson number be near 1 4 which is much smaller than that estimated from thermal wind

provenance age and environment of mid wisconsinan slackwater lake sediment in the st louis metro east area usa abstract valleys tributary to the mississippi river contain fossiliferous slackwater lake sediment equality formation deposited in response to aggradation of the mississippi river valley during the last glaciation in the st louis metro east area the lower part of the equality formation is primarily laminated fossiliferous silt and clay deposited from about 44 150 to 24 310 14c yr b p the upper equality formation is primarily very fine sand to silt deposited from about 21 200 to 17 000 14c yr b p among the four cores that sample this succession in the st louis metro east area core mnk 3 38 64en 90 01ew was selected for detailed study three sources are distinguished by the following characteristics 1 gray smectite quartz se rich feldspar poor material of the des moines wadena and james lobes 2 reddish brown kaolinite cu fe rich sediment of the superior and rainy lobes and 3 brown illite dolomite sr rich sediment of the lake michigan and green bay lobes the earliest sediments 44 150 to 41 700 14c yr b p were derived from the central and western provenances and are chronocorrelative with the lower roxana silt a hiatus occurred from about 41 700 to 29 030 14c yr b p when much of the middle roxana silt meadow member was deposited on adjacent uplands the youngest sediment includes evidence of heightened activity of the superior lobe at about 29 000 14c yr b p the lake michigan and green bay lobes from about 25 000 to 24 000 14c yr b p and the wadena des moines james lobes at about 21 000 14c yr b p

geotechnical model for great plains hummocky moraine formed by till deformation below stagnant ice abstract hummocky or disintegration moraine is a widespread late wisconsin glacial terrain type across the glaciated great plains of mid continent north america this terrain consists of chaotic hummocks and enclosed kettle hollows that are widely underlain by clay rich till the prevailing interpretation of this terrain is one of supraglacial deposition from debris covered stagnant ice and is based largely on geomorphologic comparisons with modern valley glacier margins recently in contrast it has been argued that hummocky moraine can form subglacially by the pressing of fine grained deformable till under the overburden pressure of stagnant ice in this paper we test the postulated subglacial origin of hummocky moraine using a geotechnical model of a deforming soil till the model confirms that hummocky topography can be the result of failure and deformation of fine grained till by differential loading below stagnant ice differential pressures at the ice bed contact arise from the complete or partial perforation of thin stagnant ice sheet lobes by numerous water filled doline like depressions the growth and decay of ponds as ice down wastes result in repeated cycles of subglacial till failure and landform development resulting in the observed random linear circular elongate and other patterns of hummocks ridges and hollows formerly streamlined preferentially oriented subglacial landforms such as drumlins and rogen moraines can also be overprinted forming washboard moraine

the role of double diffusion in a gulf stream frontal intrusion double diffusive convection is possible where large vertical gradients in temperature and salinity tend to compensate in density frontal intrusions have these large gradients and can provide the possibility for a salt finger interface at one boundary and a diffusive interface at the other but large vertical gradients of velocity are present at the boundaries of intrusions which cause mechanical stirring and turbulent mixing as well in the exceptionally active intrusions at the gulf stream front near 38 n 69 w a convective process resulting from incomplete mechanical mixing was observed photographically on both intrusive boundaries

calibration of radiocarbon ages and the interpretation of paleoenvironmental records abstract calibration of the radiocarbon timescale of paleoecological records is essential if they are to be explained correctly in terms of their governing ecological or climatological controls the differences between calendar ages and radiocarbon ages that arise from variations in 14c production through time can distort the chronologies of individual records and the interpretations based on them misleading impressions of synchrony or diachrony of events among multiple records can result and estimates of the apparent duration of episodes and rates of sedimentation and local population changes can be biased displays of the temporal patterns of migration or extinction may also be affected spurious correlations may arise between records with radiocarbon controlled chronologies and time series of potential controls that are expressed on a calendar time scale support for particular explanations of features in a paleoecological record may vary depending on whether radiocarbon ages are calibrated or not this situation is illustrated using the eastern beringian populus subzone as an example when the radiocarbon ages that control the timing of the populus subzone are calibrated the contemporaneous decrease in ice volume and increase in summer insolation are implicated as the ultimate controls of the occurrence of the subzone

an event stratigraphy for the last termination in the north atlantic region based on the greenland ice core record a proposal by the intimate group it is suggested that the grip greenland ice core should constitute the stratotype for the last termination based on the oxygen isotope signal in that core a new event stratigraphy spanning the time interval from ca 22 0 to 11 5 k grip yr bp ca 19 0 10 0 k 14 c yr bp is proposed for the north atlantic region this covers the period from the last glacial maximum through termination 1 of the deep ocean record to the pleistocene holocene boundary and encompasses the last glacial late glacial of the traditional northwest european stratigraphy the isotopic record for this period is divided into two stadial episodes greenland stadials 1 gs 1 and 2 gs 2 and two interstadial events greenland interstadials 1 gi 1 and 2 gi 2 in addition gi 1 and gs 2 are further subdivided into shorter episodes the event stratigraphy is equally applicable to ice core marine and terrestrial records and is considered to be a more appropriate classificatory scheme than the terrestrially based radiocarbon dated chronostratigraphy that has been used hitherto 1998 john wiley sons ltd

the sources and mixing characteristics of the agulhas current recent observations taken at four principal latitudes in the agulhas current show that the watermass properties on either side of its dynamical core are significantly different inshore of its velocity core are found waters of predominantly arabian sea red sea and equatorial indian ocean origin while offshore waters are generally from the atlantic ocean the southern ocean and the southeast indian ocean for the most part the inshore waters approach the agulhas current through the mozambique channel while those offshore are circulated within the southern indian ocean subtropical gyre before joining the current these disparate water masses remain distinct during their 1000 km journeys along the south african continental slope despite the convergence extreme velocity shears and high eddy kinetic energies found within the agulhas current both potential vorticity conservation and kinematic arguments are discussed as potential inhibitors of along isopycnal mixing it is concluded that a high cross stream gradient of potential vorticity is the dominant mechanism for watermass separation near the surface while the kinematic steering of water particles by the current is dominant at intermediate depths where cross stream potential vorticity is homogeneous hence three lateral mixing regimes for the agulhas current are suggested the surface and thermocline waters are always inhibited from mixing by the presence of both a strong cross frontal potential vorticity gradient and kinematic steering at intermediate depths mixing is inhibited by steering alone and thus in this regime periodic mixing is expected during meander events such as natal pulses when the steering level will rise and allow cross frontal exchange below the steering level in the deep waters there is a regime of free lateral mixing the deep waters of the agulhas current are homogeneous in the cross stream sense being from the same north atlantic source and their salinity steadily and rather rapidly decreases to the north here it is suggested that mixing must be dominated by vertical processes and a large vertical mixing coefficient of order 10 cm 2 s 1 is estimated

up in the refrigerator geomorphic response to periglacial environments in the upper mississippi river basin usa abstract james c knox was best known for his work on holocene and historical changes in fluvial systems but he also had a long standing interest in the effects of late pleistocene periglacial environments on landscape evolution in parts of the upper mississippi river basin that were just outside the laurentide ice sheet margin or as knox put it up in the refrigerator knox and others in the quaternary community of the midwestern u s often suggested that hillslope erosion was accelerated under periglacial conditions so that glacial periods have had a dominant effect on the landscape we see today this paper reviews the evidence and reasoning supporting that view in a study area of the upper mississippi basin bordered on three sides by ice margins of the last glaciation including the wisconsin driftless area and adjacent landscapes sparse but compelling paleoecological data and relict ice or sand wedge polygons provide clear evidence for a cold climate and widespread permafrost around the peak of the last glaciation in highly dissected relatively high relief parts of the study area the loess and soil stratigraphy on ridgetops and the colluvial mantles on steeper slopes are best explained by highly effective hillslope erosion including solifluction during and just after the last glacial maximum knox used the post depositional truncation of a loess unit to quantify contrasting late pleistocene and holocene sediment yields from a small driftless area watershed while the late pleistocene yield indicates accelerated erosion it is still lower than modern sediment yields in many tectonically active or semiarid landscapes and it may reflect deposition of highly erodible loess as well as effects of periglacial conditions the views of knox and other midwestern geomorphologists on landscape evolution through glacial interglacial cycles were highly influenced by the work of robert v ruhe ruhe proposed that an episode of widespread erosion during and just after the last glacial maximum can explain enigmatic aspects of quaternary stratigraphy and the soil landscape on the iowan erosion surface a very low relief landscape of the study area ruhe s key evidence is still valid though it needs to be separated from an implausible model of landscape evolution interpretation of the iowan erosion surface and other low relief landscapes just outside the ice margin also requires recognition of the profound effect of eolian processes on those landscapes under periglacial conditions many new insights on landscape evolution in the study area could result from wider application of cosmogenic nuclide based methods to assess glacial interglacial changes in basinwide rates of erosion and residence time of soils just as important a need exists for much more field based characterization of hillslope fluvial and eolian sediments for comparison with those of modern permafrost regions and past periglacial environments in europe

evolution of a mediterranean salt lens scalar properties abstract the evolution of a mediterranean salt lens meddy over a two year period is examined several nondimensional numbers can be used to describe the overall decay in the structure of the moddy two rossby numbers one using the central relative vorticity and another using the radius and velocity of the azimuthal velocity maximum did not change over the two year period however the burger number n2h2 f2l2 increased as the meddy decayed another burger number the ratio of total kinetic energy to total available potential energy decreased from 1 1 to 0 6 over a one year period the rates at which the meddy lost salt and heat are consistent with estimates of horizontal fluxes by intrusions a horizontal diffusivity of o 5 m2 s 1 is needed if this flux by intrusions is parameterized by an eddy coefficient simple models of the evolution of an isolated eddy by horizontal and vertical mixing of mass and momentum are examined these simple attempts to explain the evolution of the meddy suggest more c

the production and dissipation of compensated thermohaline variance by mesoscale stirring abstract temperature salinity profiles from the region studied in the north atlantic tracer release experiment natre show large isopycnal excursions at depths just below the thermocline it is proposed here that these thermohaline filaments result from the mesoscale stirring of large scale temperature and salinity gradients by geostrophic turbulence resulting in a direct cascade of thermohaline variance to small scales this hypothesis is investigated as follows measurements from natre are used to generate mean temperature salinity and shear profiles the mean stratification and shear are used as the background state in a high resolution horizontally homogeneous quasigeostrophic model the mean state is baroclinically unstable and the model produces a vigorous eddy field temperature and salinity are stirred laterally in each density layer by the geostrophic velocity and vertical advection is by the ageostrophic velocity the simulated temperature salinity diagram exhibits fluctuations at depths ju

ice walled lake plains implications for the origin of hummocky glacial topography in middle north america abstract ice walled lake plains are prominent in many areas of hummocky till topography left behind as the laurentide ice sheet melted from middle north america the formation of the hummocky till topography has been explained by 1 erosion by subglacial floods 2 squeezing of subglacial till up into holes in stagnant glacial ice or 3 slumping of supraglacial till the geomorphology and stratigraphy of ice walled lake plains provide evidence that neither the lake plains nor the adjacent hummocks are of subglacial origin these flat lake plains up to a few kilometers in diameter are perched as much as a few tens of meters above surrounding depressions they typically are underlain by laminated fine grained suspended load lake sediment many ice walled lake plains are surrounded by a low rim ridge of coarser grained shore sediment or by a steeper rim ridge of debris that slumped off the surrounding ice slopes the ice walled lakes persisted for hundreds to thousands of years following glacial stagnation shells of aquatic molluscs from several deposits of ice walled lake sediment in south central north dakota have been dated from about 13 500 to 10 500 b p calibrated radiocarbon ages indicating a climate only slightly cooler than present this is confirmed by recent palaeoecological studies in nearby non glacial sites to survive so long the stagnant glacial ice had to be well insulated by a thick cover of supraglacial sediment and the associated till hummocks must be composed primarily of collapsed supraglacial till

stochastic inversion by matching to large numbers of pseudo wells abstractwe have developed a 1d stochastic algorithm for estimating reservoir properties based on matching large numbers of pseudo wells to seismic angle stacks the pseudo wells are part deterministic and part stochastic 1d stratigraphic profiles with consistent elastic and reservoir properties pseudo wells are sampled from a prior distribution defined by the geological interpretation a rock physics model and a model for the vertical statistics that provides close control of the lithofacies proportions a new set of pseudo wells typically 103 104 tied to the local stratigraphy is constructed for each seismic trace synthetics derived from the pseudo wells using extended elastic impedance are matched to either one or two seismic angle stacks and the best matches are selected and averaged to provide a joint estimate of reservoir properties and impedances and the associated uncertainties the algorithm has been tested on a number of data sets and validated by blind well ties the algorithm is 1d with