modulation of equatorial turbulence by a tropical instability wave 1 strong modulation of turbulent mixing by a westward propagating tropical instability wave tiw was observed in the stratified shear layer between the equatorial undercurrent euc and the surface mixed layer during october and november 2008 at 0 n 140 w the unique deep diurnal cycle mixing in the stratified layer beneath the equatorial cold tongue was observed where nighttime turbulent mixing was a factor of 10 greater than during daytime the turbulent kinetic energy dissipation rate e was 10 6 w kg 1 and the turbulent heat flux was 500 w m 2 at least 5 10 times greater than observed previously in the central equatorial pacific turbulence mixing varied significantly during the four distinct phases of the meridional flow associated with the tiw observations during the northward to southward transition recorded the largest values of reduced shear squared the thickest nighttime surface mixed layer the deepest penetration of the deep cycle turbulence and the largest turbulent heat flux and largest integrated e in the deep cycle layer dcl during steady southward flow the depth of the bases of the nighttime surface mixed layer and of the dcl were the shallowest a 50 m thick layer of strong turbulence was observed immediately above the euc core during the northward to southward and steady southward phases here the average e exceeded 10 6 w kg 1 the eddy diffusivity exceeded 10 3 m2 s 1 and the turbulent heat flux was 500 w m 2 to parameterize mixing in the central equatorial pacific accurately numerical models must simulate the enhancement of mixing associated with tiws and also the variability of mixing in different tiw phases

ice sheet sources of sea level rise and freshwater discharge during the last deglaciation we review and synthesize the geologic record that constrains the sources of sea level rise and freshwater discharge to the global oceans associated with retreat of ice sheets during the last deglaciation the last glacial maximum 26 19 ka was terminated by a rapid 5 10 m sea level rise at 19 0 19 5 ka sourced largely from northern hemisphere ice sheet retreat in response to high northern latitude insolation forcing sea level rise of 8 20 m from 19 to 14 5 ka can be attributed to continued retreat of the laurentide and eurasian ice sheets with an additional freshwater forcing of uncertain amount delivered by heinrich event 1 the source of the abrupt acceleration in sea level rise at 14 6 ka meltwater pulse 1a 14 15 m includes contributions of 6 5 10 m from northern hemisphere ice sheets of which 2 7 m represents an excess contribution above that derived from ongoing ice sheet retreat widespread retreat of antarctic ice sheets began at 14 0 15 0 ka which together with geophysical modeling of far field sea level records suggests an antarctic contribution to this meltwater pulse as well the cause of the subsequent younger dryas cold event can be attributed to eastward freshwater runoff from the lake agassiz basin to the st lawrence estuary that agrees with existing lake agassiz outlet radiocarbon dates much of the early holocene sea level rise can be explained by laurentide and scandinavian ice sheet retreat with collapse of laurentide ice over hudson bay and drainage of lake agassiz basin runoff at 8 4 8 2 ka to the labrador sea causing the 8 2 ka event

lake agassiz during the younger dryas abstract lake agassiz was ponded on the northward sloping surface of the hudson bay and arctic ocean basins as the laurentide ice sheet retreated the level of lake agassiz abruptly fell 12 9 cal 11 14 c ka bp exposing the lake floor over a large region for 1000 yr the routing of overflow during this moorhead low water period is uncertain and there is evidence on the continent and in ocean basins for both an easterly route through the great lakes st lawrence to the north atlantic and for a northwesterly route through the clearwater athabasca mackenzie system to the arctic ocean the moorhead low water phase coincides with the younger dryas cooling and a cause effect relationship has been proposed by attributing a change in ocean thermohaline circulation to the re routing of lake agassiz freshwaters from the gulf of mexico to more northern oceans paleoclimatic interpretations from ecosystems in lake sediments in the region and a simple calculation of the paleohydrological budget of lake agassiz indicate that the climate remained wet and cool throughout the yd in this region and was not warm nor dry enough to allow evaporative loss to offset the influx of meltwater and precipitation thus the moorhead phase resulted from changes in the outlet that carried overflow

formation of large beaches in an area of rapid differential isostatic rebound the three outlet control of lake agassiz abstract shorelines in basins undergoing differential isostatic rebound do not remain geographically fixed for long periods and are either transgressive or regressive during regression a relatively uniform blanket of shoreface sediment may be deposited but unless regression is slow or punctuated by periods of lake level equilibrium large beaches do not form in contrast along transgressive shorelines which migrate upslope large morphostratigraphic features commonly are maintained along the lake s margin these may subsequently be stranded when lake level declines beaches around isostatically rebounding proglacial lakes are commonly considered to have formed during a period of lake regression punctuated by abrupt drops in lake level however in basins that are differentially rebounding large beaches cannot form this way and only periods of transgressing waters allow enough time for large beaches to form and define a continuous shoreline around the basin with this in mind some of the largest and most extensive beaches in the glacial lake agassiz basin the norcross tintah and upper campbell constructed between 10 4 and 9 4 ka 14 c yr bp are reinterpreted as strandlines that were formed by transgression each of these three transgressions followed lower lake phases and resulted from in sequence differential isostatic rebound of the eastern outlet rebound and ice damming of the northwestern outlet and rebound of the northwestern outlet each of these transgressions ended when lake level reached the southern outlet stranding the norcross tintah and upper campbell beaches about 10 2 9 9 and 9 4 ka 14 c yr bp respectively

constraints on lake agassiz discharge through the late glacial champlain sea st lawrence lowlands canada using salinity proxies and an estuarine circulation model during the last deglaciation abrupt freshwater discharge events from proglacial lakes in north america such as glacial lake agassiz are believed to have drained into the north atlantic ocean causing large shifts in climate by weakening the formation of north atlantic deep water and decreasing ocean heat transport to high northern latitudes these discharges were caused by changes in lake drainage outlets but the duration magnitude and routing of discharge events factors which govern the climatic response to freshwater forcing are poorly known abrupt discharges called floods are typically assumed to last months to a year whereas more gradual discharges called routing events occur over centuries here we use estuarine modeling to evaluate freshwater discharge from lake agassiz and other north american proglacial lakes into the north atlantic ocean through the st lawrence estuary around 11 5 ka bp the onset of the preboreal oscillation pbo faunal and isotopic proxy data from the champlain sea a semi isolated marine brackish water body that occupied the st lawrence and champlain valleys from 13 to 9 ka indicate salinity fell about 7 8 range of 4 11 around 11 5 ka model results suggest that minimum 1600 km3 and maximum 9500 km3 estimates of plausible flood volumes determined from lake agassiz paleoshorelines would produce the proxy reconstructed salinity decrease if the floods lasted 1 day to 5 months and 1 month to 2 years respectively in addition champlain sea salinity responds very quickly to the initiation within days and cessation within weeks of flooding events these results support the hypothesis that a glacial lake flood rather than a sustained routing event discharged through the st lawrence estuary during the pbo

sedimentary structures preserved in greater himalayan gneiss phenomenon or fantasy abstractunder favorable conditions sedimentary structures may be preserved in high grade metamorphic rocks to yield valuable clues about the depositional and paleogeographic setting of the protolith two examples from gneisses of the greater himalayan sequence of bhutan demonstrate this potential laminated channeled dolostone within gneiss of upper amphibolite facies indicates tidal flat deposition and thus proximity of a paleoshoreline the type takhtsang gneiss within granulite facies consists of alternating millimeter bands rich in quartz and biotite interpreted as silt and clay interlaminations respectively in the protolith typical of tidal deposits or varves the gneissic banding drapes around isolated lenses of contrasting and varied compositions interpreted as dropstones this combination suggests deposition in a varved glacial lake published comparisons of takhtsang lithologies indicate a minimum dimension of 315 km for the postulated lake these examples demonstrate the potential of r

evidence for an eolian origin for the silt enriched soil mantles on the glaciated uplands of eastern upper michigan usa abstract we provide textural geochemical and mineralogical data on a thin silty deposit that unconformably mantles glaciated uplands in the eastern upper peninsula of michigan previous research on this deposit which we hypothesize to be loess is nonexistent the uplands were islands or narrow peninsulas within one or more glacial lakes we compare the distribution likely source and nature of the 20 60 cm thick silty mantle by using the loess formation model of mason et al mason j a nater e a zanner c w bell j c 1999 a new model of topographic effects on the distribution of loess geomorphology 28 223 236 which focuses on the generation of eolian silt by saltating sand across upwind barren surfaces parabolic dunes with arms open to the nw are common on former lake floors upwind of the silt mantled uplands attesting to the strength and direction of paleowinds the abrupt termination of the dunes at the footslopes of the uplands associated with silt deposition on upland soil surfaces in downwind locations are both consistent with the model of mason et al mason j a nater e a zanner c w bell j c 1999 a new model of topographic effects on the distribution of loess geomorphology 28 223 236 sediments on former lake floors contain abundant strata of fine medium sand and silt and thus are likely sources for the silt and dune sand the cap dune and lake sediments are similar along many different geochemical axes whereas the substrate sediment i e the drift below the cap is unique cap sediments normally containing roughly 30 silt are enriched in quartz and depleted in ti and zr relative to dune sediment the dune sediment a more residual eolian deposit is enriched in ti and zr relative to the cap probably due to its greater abundance of heavy minerals therefore we conclude that the silty cap is loess that was deflated from abandoned lake floors after nearby glacial lakes drained probably contemporaneously with dune migration across the former lake floors

a high resolution record of climate variability and landscape response from kettle lake northern great plains north america abstract a decadal scale multiproxy record of minerals pollen and charcoal from kettle lake north dakota provides a high resolution record of climate and vegetation change spanning the entire holocene from the northern great plains ngp in north america the chronology is established by over 50 ams radiocarbon dates this record exhibits millennial scale trends evident in other lower resolution studies but with much more detail on short term climate variability and on the rapidity and timing of major climatic shifts as a proxy for precipitation we utilize the rate of endogenic carbonate sedimentation which depends on groundwater inflow which in turn depends on precipitation independent cluster analyses of mineral and pollen data reveal major holocene mode shifts at 10 73 ka ka cal yr bp 9 25 ka and 4 44 ka the early holocene 11 7 9 25 ka was generally wet with perhaps a trend to higher evaporation associated with warming temperatures a switch from calcite to aragonite deposition associated with a severe but brief drought occurred at 10 73 ka from 10 73 ka to 9 25 ka climate was generally humid but punctuated at 100 300 yr intervals by brief droughts including the most severe drought of the entire holocene at 9 25 ka this event was coeval with the 9 3 9 2 ka event in the greenland ice cores and observed at a number of sites worldwide in contrast the prominent 8 2 ka event in greenland is not remarkable at kettle lake the prominence of the 9 25 event locally in the ngp may be due to a major drawdown and northward retreat of lake agassiz at this time reducing its mesoclimatic effect on the ngp and thrusting the region into an insolation controlled regime the mid holocene 9 25 4 44 ka was characterized by great variability in moisture on a multi decadal scale with severe droughts alternating with more humid periods the high abundance of the weedy but drought intolerant ambrosia generally during the mid holocene and specifically during the multi decadal drought periods is seemingly paradoxical but can be explained by high interannual variability of moisture overlaid on the multi decadal variability the late holocene 4 44 ka present was also characterized by multi decadal variability in moisture but was generally wetter than the mid holocene and the magnitude of variability was less the trends in wet dry mineral pollen and charcoal proxies were similar to the mid holocene but late holocene mineral pollen assemblages are distinct from mid holocene the shift to wetter climate in the late holocene was more gradual than the abrupt shift to arid conditions 9 25 ka which may explain the asymmetric retreat and readvance of forest along the eastern margin of the ngp precipitation variations in the ngp have been linked with pacific and atlantic sea surface temperatures and mid holocene drought in the ngp has been linked with sustained la nina like conditions in the pacific these linkages may explain the decadal and millennial scale variations seen in the ngp but cause of the prominent century scale variations remains elusive

an analysis of the late glacial lake levels within the western lake superior basin based on digital elevation models abstract this study establishes a detailed lake level history for the lake superior basin by mapping strandlines from 10 m and 3 m digital elevation models there are 24 levels above the mid holocene nipissing level and elevations increase along a direction of 23 1 due to post glacial rebound the highest level the epi duluth is steeper than subsequent levels and may pre date the lake view ice advance into the western lake superior basin at the end of the younger dryas stade the most prominent level is the duluth ca 10 800 cal yr bp ice retreat exposed successively lower outlets routing overflow to the lake michigan and huron basins by 10 600 cal yr bp lake levels in the western superior basin had dropped almost 200 m this transformative period is complicated by multiple basin wide events the influx of glacial lake agassiz overflow the creation of three sub aqueous moraines and a red to gray color transition in basin sediments a later drawdown event has been hypothesized to have initiated the 9300 cal yr bp cooling event but this flood was much smaller than estimated previously if freshwater triggered the 9300 cal yr bp event the source of the water must have been lake agassiz not lake superior

mapping and measuring lake agassiz strandlines in north dakota and manitoba using lidar dem data comparing techniques revising correlations and interpreting anomalous isostatic rebound gradients based on light detection and ranging lidar digital elevation model dem data at 1 m spatial resolution lake agassiz strandlines were mapped and their elevations determined in northeastern north dakota and southern manitoba two different techniques for determining strandline elevations from lidar imagery are compared one based on measuring the crests of constructional beaches and the other using the elevation of slope breaks along the shorelines defined by shadows on the lidar hillshade map our measurements on lake agassiz strandlines show that previous correlations of younger strandlines around the pembina delta accepted for more than a century are incorrect the gladstone ojata and emerado strandlines south of the delta are offset by 10 m from their namesakes to the north and so are equivalent to the ojata emerado and hillsboro strandlines respectively furthermore anomalously low gradients for the ojata and emerado strandlines around the pembina delta are interpreted as resulting from postdepositional compaction of underlying fine sediment elevations on the isostatically rebounded strandlines confirm that older higher ones have steeper northward rising elevations and that gradients on those strandlines can be used as proxies for relative age

alternative routing of lake agassiz overflow during the younger dryas new dates paleotopography and a re evaluation abstract overflow from glacial lake agassiz has been implicated in affecting late glacial ocean circulation and climate the timing of the diversion of lake agassiz overflow away from its southern routing to the gulf of mexico correlates closely with the younger dryas ca 11 10 14 c ka new models of paleotopography in the eastern outlet region of lake agassiz adjusted for differential isostatic rebound show that overflow to the superior basin would have occurred when the lake formed its herman norcross and tintah beaches just after 10 9 14 c ka if the laurentide ice sheet lis had retreated from the thunder bay ontario region by that time however the absence of coarse flood deposits normally associated with large overflows plus the morphology of possible overflow channels in the thunder bay region suggest that agassiz overflow may not have been routed into the great lakes during the younger dryas as previously believed in addition non carbonate radiocarbon dates in the thunder bay area are all younger than 10 14 c ka suggesting that this area may not have been deglaciated until after the younger dryas thus preventing lake agassiz overflow through this area an alternative route of overflow from lake agassiz during the younger dryas could have been through the northwestern clearwater outlet to the arctic ocean although all research to date shows that this region was covered by ice until 10 14 c ka on the other hand it is possible that older and coarser overflow sediment may now lie buried beneath finer deposits associated with the 10 14 c ka marquette readvance of the lis which ponded waters in valleys between lake agassiz and lake superior or that eastward overflow in this area was not catastrophic

the lake superior varve stratigraphy and implications for eastern lake agassiz outflow from 10 700 to 8900 cal ybp 9 5 8 0 14c ka abstract glaciolacustrine rhythmites within sediment cores from lake superior record the regional recession of the laurentide ice sheet lis from 10 700 to 8900 cal ybp ca 9 5 8 0 14 c ka lis retreat from superior opened eastern lake agassiz outlets so that the rhythmites reflect the combined impacts of sediment laden meltwater and lake agassiz discharge multiple rhythmite stratigraphies a time series analysis of the thickness measurements and high resolution inorganic carbonate data demonstrate that this is an annual record varved the varve thickness records primarily document regional ice margin dynamics correlative thick varve sequences at 9100 cal ybp 8 1 14 c ka and 10 400 10 200 cal ybp 9 2 9 0 14 c ka record two periods of enhanced glaciofluvial discharge most likely moraine formation the nakina and nipigon general varve cessation is associated with the circumvention of lake agassiz and glacial meltwater into lake ojibway at 9040 cal ybp 8 1 14 c ka although adjacent to the inlets from lake nipigon rhythmic sedimentation persisted for 200 years positively identifying lake agassiz catastrophic discharge events remains speculative but seems feasible following retreat of marquette ice that had re advanced to fill the basin the initial influx of lake agassiz water is expected at around 10 600 cal ybp 9 4 14 c ka but at this time most of northeastern lake superior was covered by ice three sets of thick thin varves in western lake superior perhaps record influxes of lake agassiz at around 10 630 10 600 and 10 570 cal ybp 9 4 14 c ka varve formation in superior coincides with high lake levels in lake huron suggesting that high lake levels in huron correspond to periods of high agassiz and or meltwater flow into lake superior

comment on alternative routing of lake agassiz overflow during the younger dryas new dates paleotopography and a re evaluation by teller et al 2005

a review and analysis of varve thickness records from glacial lake ojibway ontario and quebec canada abstract varve thickness records from glacial lake ojibway provide a detailed picture of ice margin dynamics and paleohydrology for the laurentide ice sheet during the early holocene the demise of lake ojibway occurred when an ice dam of the shrinking laurentide ice sheet failed perhaps releasing enough freshwater to impede atlantic meridional overturning circulation and precipitate the 8200 cal bp climatic cooling event previously published records and new information from lacustrine sediment cores are combined to reconstruct a varve record over 2100 years long between varves numbered 1 to 1500 average rates of ice margin retreat increased from 140 m y to as high as 400 m y beginning prior to varve year 1800 and lasting to around year 2000 ice readvanced along a front at least 300 km wide during an episode called the cochrane the total distance of the advance was at least 75 km and averaged around 200 m y during the advance a series of 50 anomalously thick varves were deposited the cause of which is uncertain a widespread unconformity occurred during the end of the cochrane advance which was followed at around varve year 2060 by the return of varves which thicken abruptly unlike previous varves these varves include ice rafted pellets of lake ojibway sediment these varves may have resulted from rapid melting and calving rather than a second advance and may relate to basin re filling following a lowstand there are no direct dates on the lake ojibway varves but a sudden increase in varve thickness at year 1528 is a regional stratigraphic marker that may have resulted from glacial sediment and meltwater including lake agassiz overflow diverted from routes to the lake superior basin into the lake ojibway basin dates from the lake superior basin suggest that the diversion of this water occurred at around 9040 cal bp applying this date to varve 1528 constrains the age of the lake ojibway varves numbered 1 to 2100 to between 10 570 and 8470 200 cal bp

volume and routing of late glacial runoff from the southern laurentide ice sheet melting of the laurentide ice sheet during the last deglaciation added large volumes of water to many rivers and lakes of north america and to the world s oceans the volume and routing of this meltwater not only helped shape the land s surface but also played a role in the evolution of late glacial climate a computerized model was prepared to quantify meltwater generation from seven drainage areas along the southern side of the laurentide ice sheet at 500 yr time slices between 14 000 and 8000 yr b p nearly all waters reaching the oceans flowed through the st lawrence hudson or mississippi river valleys discharge through the mississippi river to the gulf of mexico during late glacial time varied by more than a factor of 5 ranging between 17 400 m3 sec 1 550 km3 yr 1 and 98 200 m3 sec 1 3200 km3 yr 1 discharge entering the north atlantic ocean through the st lawrence and hudson valleys ranged between 20 300 m3 sec 1 640 km3 yr 1 and 65 300 m3 sec 1 2060 km3 yr 1 with very abrupt twofold changes at about 11 000 10 000 and 9500 yr b p as a result of the rerouting of water from the lake agassiz basin as the areal extent and mass of the laurentide ice sheet diminished the total volume of meltwater plus runoff due to precipitation from its southern side declined from 3800 km3 yr 1 at about 14 000 yr b p to 2100 to 2600 km3 yr 1 between 11 500 and 8000 yr b p no meltwater entered the gulf of mexico after 9500 yr b p after the demise of the ice sheet over hudson bay about 8000 yr b p the modern continental drainage network was established and flows through the st lawrence declined to modern values of about 320 km3 yr 1

sediment sequences and palynology of outer south bay manitoulin island ontario connections to lake huron paleohydrologic phases and upstream lake agassiz events abstract south bay on the southern coast of manitoulin island is a fjord like embayment connected to lake huron by a natural narrow gap in the bay s outer sill 6 5 14 m above the lake a seismic profile pollen plant macrofossil grain size analyses and other sediment properties of two piston cores from a shallow outer basin of the bay document a 9 m thick sediment section comprising rhythmically laminated clay under silty clay containing zones with small molluscan shells and marsh detritus a sandy pebbly layer under soft silty clay mud overlies these sediments this stratigraphy represents inundation by deep glacial lake algonquin followed by the shallowing post algonquin series of lakes and exposure in the early holocene by 5 lake stanley lowstands in the lake huron basin separated by 4 lake mattawa highstands overflow from south bay in the first lowstand is thought to have eroded the outer sill gap marsh environments are inferred to have formed in the bay during subsequent lowstands the lake mattawa highstands are attributed to outburst floods mainly from glacial lake agassiz palynological evidence of increased spruce occurrence an apparent regional climate reversal during the dry pine period is attributed to cold northwest winds from the lake superior basin and a lake effect from the mattawa highstands in the lake huron basin lake waters transgressed south bay following the pine period to form the nipissing shore on manitoulin island transfer of lake huron basin drainage to southern outlets and continued glacioisostatic uplift of the region led to the present configuration of south bay and lake huron

internal gravity waves in the upper eastern equatorial pacific observations and numerical solutions on the basis of data from a towed thermistor chain collected near 140 w on the equator during april 1987 the zonal wavenumber and vertical structure of internal gravity waves were observed to vary significantly between wave events our hypothesis is that this variability is due to changes in the vertical structure of mean horizontal velocity and density assuming that the observed waves were the fastest growing modes for shear instability we solve the taylor goldstein equation using different analytical basic states including a zonal and meridional flow to simulate the different conditions during 4 nights of intense internal wave activity we find that while the observed waves are of finite amplitude linear shear instability is sufficient to explain the wavelength and vertical structure of vertical displacement for most of the waves the fastest growing unstable mode one solutions have e folding growth times of less than 10 min these solutions show wave phase speeds and vertical structures to be highly dependent upon the velocity structure of the uppermost 40 m near the base of the mixed layer at a flow inflection point the kinetic energy of the mean flow is extracted for wave growth wave vertical displacement is maximum near this inflection point zonal phase speeds range from 0 8 to 0 1 m s the propagation direction of waves with growth rates of 75 of the maximum growth rate can range from about 45 north to 45 south of the zonal direction the vertical wave induced reynolds stress divergence could explain a discrepancy in zonal momentum budgets of the upper 90 m of this region estimates of this stress divergence show that only about 100 days of intense internal wave activity is needed per year for these internal waves to explain estimated residuals of the mean zonal momentum budgets of this region at 50 to 100 m depth

the tintah campbell gap and implications for glacial lake agassiz drainage during the younger dryas cold interval abstract reconstructions of glacial lake agassiz paleogeography and drainage have been an important contribution to formulating a hypothesis in which glacial lake agassiz drainage to the atlantic ocean initiated the younger dryas cold interval this study evaluates the lake level and outlet history of lake agassiz as recorded by strandlines visible on lidar digital elevation models from north dakota and minnesota the former lake levels are warped due to glacial isostatic adjustment older levels have experienced more uplift and therefore have more curvature the strandline data establish that the moorhead lowstand of lake agassiz was bracketed by the strongly diverging campbell and tintah lake levels which creates a vertical gap between the former lake levels this gap exists due to a lake level drop of 90 m when the laurentide ice sheet retreat opened a lower outlet which must have been a northwest outlet to the arctic ocean by applying an exponential decay rebound model this event dates to 12 180 480 cal yr bp post dating the beginning of the younger dryas at 12 900 cal yr bp eastern drainage outlets to the atlantic ocean through the laurentian great lakes that were contemporaneous with the onset of the younger dryas cannot be ruled out but if these outlets existed their duration of occupation was short lived and not characterized by significant drawdown events within glacial lake agassiz

lakewater paleothermometry from deep lake minnesota during the deglacial holocene transition from combined δ18o analyses of authigenic carbonate and aquatic cellulose abstract carbonate cellulose and bulk organic isotope data from varved sediments of deep lake minnesota provide a record of paleohydrologic and paleoclimatic change during the deglacial holocene transition from 12 000 to 7500 cal bp deep lake became enriched in 18 o in response to increased influence of maritime tropical moisture from the gulf of mexico between 12 000 and 9500 cal bp culminating in regional expansion of pinus and quercus and then again as a result of increased aridity with the establishment of more zonal circulation and prairie vegetation by 8500 cal bp summer lake water temperatures simultaneously increased during the deglacial holocene transition as suggested by decoupling of carbonate 18 o and cellulose inferred lake water 18 o values between 11 000 and 9500 cal bp and then declined briefly around 8600 cal bp possibly in response to the cooling event associated with the final drainage of glacial lakes agassiz and ojibway into the labrador sea although the carbonate cellulose paleothermometer generated unrealistically high values these can readily be explained by imposing kinetic effects that occur during rapid carbonate precipitation and conservative estimates of a seasonal lag effect in the synthesis of cellulose with respect to carbonate these new results suggest that climatic variations during the deglacial holocene transition in northwestern minnesota were driven mainly by changes in large scale atmospheric circulation and that lake agassiz played a comparatively minor role except during its final drainage

chronology of glacial lake agassiz meltwater routed to the gulf of mexico abstract sediment cores with new radiocarbon dates from the southern outlet of glacial lake agassiz indicate that meltwater delivery to the mississippi valley was disrupted at 10 800 14 c yr b p and the outlet was abandoned by 9400 14 c yr b p these findings confirm the timing of generally accepted terminations of the lockhart and emerson phases of lake agassiz additionally the radiocarbon chronology indicates that the spillway was fully formed by 10 800 14 c yr b p and that the occupancy in late emerson time was likely short lived with minimal spillway erosion

isotopic evolution of glacial lake agassiz new insights from cellulose and porewater isotopic archives abstract significantly differing estimates of the oxygen isotope composition of lake agassiz have been obtained from two co existing isotopic archives within a sediment core originating from montcalm manitoba in the southern basin of the ancient proglacial lake oxygen isotope analysis of cellulose extracted from the sediments which originated during the lockhart phase 11 700 11 000 14 c yr bp suggests that phytoplankton lived in surface waters having 18 o around 18 1 vsmow substantially enriched relative to connate porewaters in the same core which indicate bottom waters had much lower values of around 24 5 0 5 vsmow this difference may be attributable to seasonal isotopic stratification of the upper part of the water column in the 250 m deep lake modern observations from analogous environments in northern canada suggest that inflow of evaporatively enriched runoff from the large area of deglaciated terrain contributing to lake agassiz possibly enhanced by evaporation from the surface of the lake itself could readily account for sufficient seasonal 18 o enrichment in the epilimnion sediment porewaters in contrast have preserved the isotopic signature of hypolimnion waters supplied by a mixture of glacial meltwater and precipitation derived runoff from the laurentide ice sheet and lack discernable isotopic alteration by evaporation these new estimates are combined with inferred lake water compositions from other isotopic archives to develop a speculative framework for the isotopic evolution of the lake providing improved constraints on the probable isotopic composition of lake agassiz outflow over time which has important implications for efforts to trace and model its changing discharge

paleotopographic reconstructions of the eastern outlets of glacial lake agassiz paleotopographic reconstructions of the eastern outlets of glacial lake agassiz provide a foundation for understanding the complex manner in which terrain morphology controlled the routing of overflow through the eastern outlets during the lake s nipigon phase ca 9400 8000 14 c years bp and for understanding the causes of outlet driven declines in lake level during that period although flow paths across the divide between the agassiz and nipigon basins were numerous eastward releases from lake agassiz to glacial lake kelvin modern lake nipigon were channeled downslope into a relatively small number of major channels that included the valleys of modern kopka river ottertooth creek vale creek whitesand river pikitigushi river and little jackfish river from lake kelvin these waters overflowed into the superior basin the numerous lowerings in lake level between stages of the nipigon phase controlled by topography and the position of the retreating southern margin of the laurentide ice sheet had magnitudes of between 8 and 58 m although some of these drawdowns may have occurred as multiple individual events that could have been as small as several metres the total volumes of water released in association with these drops were as great as 8100 km 3 and for all nipigon stages were probably between about 140 and 250 km 3 per metre of lowering the topographic reconstructions demonstrate that lake agassiz occupied the area of glacial lake nakina located northeast of modern lake nipigon by the the pas stage ca 8000 14 c years bp and that lake agassiz drainage through the nipigon

changes in the bathymetry and volume of glacial lake agassiz between 11 000 and 9300 14c yr b p abstract computer reconstructions of the bathymetry of the lake were used to quantify variations in the size and form of lake agassiz during its final two phases the nipigon and ojibway phases between about 9200 and 7700 14c yr b p ca 10 300 8400 cal yr b p new bathymetric models for four nipigon phase stages corresponding to the mccauleyville hillsboro burnside and the pas strandlines indicate that lake agassiz ranged between about 19 200 and 4600 km3 in volume and 254 000 and 151 000 km2 in areal extent at those times a bathymetric model of the last ponton stage of the lake corresponding to the period in which lake agassiz was combined with glacial lake ojbway to the east shows that lake agassiz ojibway was about 163 000 km3 in volume and 841 000 km2 in areal extent prior to the final release of lake waters into the tyrrell sea during the nipigon phase a number of catastrophic releases of water from lake agassiz occurred as more northerly lower outlets were made available by the retreating southern margin of the laurentide ice sheet we estimate that each of the four newly investigated nipigon phase releases involved water volumes of between 1600 and 2300 km3 the final release of lake agassiz waters into the tyrrell sea at about 7700 14c yr b p is estimated to have been about 163 000 km3 in volume

megaflooding associated with glacial lake agassiz abstract significant baseline and episodic megaflooding from glacial lake agassiz was routed to the south east and north coasts of north america over the lake s nearly 6000 year history the five phases of lake level change were controlled by which outlet was active which in turn was controlled by ice margin position and glacioisostatic adjustment the southern outlet is the oldest and best understood outlet while successively younger outlets are progressively less understood eastern drainage synchronous with the younger dryas chronozone had been assumed by most studies but a spillway to accommodate the 90 m drop in lake level is yet to be described with the south and east outlets unable to accommodate the necessary lake level drawdown the northwest outlet has become the default outlet with arctic ocean core data supporting it however extant terrestrial data from the continent only provides data for a large flood from the fort mcmurray area closer to the end of the younger dryas coinciding with the timing of the preboreal oscillation the challenge remains to find agreement between the marine and terrestrial records uncontroversial is the geomorphic and sedimentologic evidence for flooding into and out of the lake consisting of large spillways large boulders and in places giant current ripples composed of boulders flood discharge estimates into and out of the lake range from 0 04 to 1 sv with final subglacial drainage into the tyrell sea estimated at 5 sv sufficient to raise global sea level by 0 18 m and initiate the 8 2 ka stadial

northwest outlet channels of lake agassiz isostatic tilting and a migrating continental drainage divide saskatchewan canada lake cores obtained from the northwest outlet of glacial lake agassiz in northwest saskatchewan canada provide a minimum date for the cessation of the flood from the northwest outlet and a chronology for abandonment of mid holocene channels that presently straddle the mackenzie and churchill drainage divide the stratigraphy of a vibracore taken from long lake consists of a lower pebble gravel fining to massive sand silty clay and then fibrous peat wood fragments from the base of the clay yielded an accelerator mass spectrometry ams date of 9120 bp because the lake is scour in origin and is in the head of the spillway the date is considered to be a minimum estimate for cessation of the flood from the northwest outlet at the beginning of the emerson phase a vibracore taken at haas lake in an abandoned channel surrounded by muskeg with no influent streams consists of 0 8 m of stratified pebble gravel containing abundant shell and wood fragments overlain by 1 62 m of gyttja with a sharp conformable lower contact ams dates range from 5590 bp from the topmost gravel to 3080 bp within the gyttja the gravel is interpreted as fluvial recording a river draining wasekamio lake north into the clearwater river across the present day drainage divide today a drop of 2 m occurs from wasekamio lake southeast to lac ile a la crosse along 150 km of lake basins parallel to the cree lake moraine the dates from haas lake suggest that before 5200 bp the drainage divide was about 100 km further southeast implying that during the emerson phase lake level was controlled by a sill near flatstone lake at about 430 m instead of between wasekamio lake and the clearwater river as was previously proposed holocene differential isostatic uplift caused the flow reversal in the upper churchill basin anastomosed channels at the mouth of rivers flowing north into lakes indicate that uplift is still active in the area today

glacial lake agassiz its northwest maximum extent and outlet in saskatchewan emerson phase abstract six different lines of evidence support the hypothesis that glacial lake agassiz expanded an additional 70 000 km2 over that previously mapped in northwestern saskatchewan and that the lake discharged out the northwestern clearwater outlet then through glacial lake mcconnell and mackenzie river to the arctic ocean elevations of formerly unmapped 1 strandlines and 2 glaciolacustrine sediments between the previously mapped northwest limit of lake agassiz and the clearwater lower athabasca spillway indicate that water extended 170 km farther northwest recently mapped strandlines at elevations up to 60 m above the previously mapped extent of lake agassiz can be traced along 3 isobases to the mouth of the spillway based upon 4 six radiocarbon dates recovered from spillway flood deposits in the athabasca river valley and its late pleistocene delta the 5 clearwater spillway was cut at 9 9 ka bp this date is synchronous with the initiation of the emerson phase 9 9 ka bp in the southern lake agassiz basin and 6 coincides with the position of the laurentide ice sheet at the cree lake moraine 10 ka bp along the northern margin of the lake following closure of the eastern outlets at the onset of the emerson phase lake agassiz transgressed toward the northwest into the deglaciated and isostatically depressed glacial foreland in the churchill river valley to an elevation of 490 m the pre flood elevation of the churchill mackenzie drainage divide at the head of the clearwater lower athabasca spillway the beaver river moraine an earthen drainage divide was breached resulting in lowering lake agassiz 52 m to a stable elevation at 438 m the lake discharged 21 000 km3 of water into the arctic ocean that raised global sea level by 6 cm

10be ages of flood deposits west of lake nipigon ontario evidence for eastward meltwater drainage during the early holocene epoch the nipigon channels located to the west and northwest of lake nipigon ontario are thought to have enabled the eastward drainage of meltwater from glacial lake agassiz during the last deglaciation here we present the first direct ages of flood deposits in two of these channels using 10be surface exposure dating five 10be ages of a coarse grained deposit near the roaring river in the kaiashk channel complex indicate deglaciation and cessation of water flow by 11 070 430 years to test for inherited nuclides in boulder samples we also measured the 10be concentrations of the undersides of two boulders at the roaring river site five 10be ages of boulders atop a large bedform near mundell lake in the pillar channel complex indicate deglaciation and cessation of water flow by 10 770 240 years two 10be ages of nearby bedrock are slightly younger 10 340 260 and 9860 270 years the 10be ages from the two sites are statistically indistinguishable and indicate that laurentide ice sheet recession

a tectonic model for ridge transform ridge plate boundaries implications for the structure of oceanic lithosphere abstract the first order geologic and morphologic relationships at along and proximal to ridge transform ridge plate boundaries are used to construct an empirical and speculative tectonic model the distinctive but variable morphotectonic fabric and crustal structure of the transform domain are the product of a tectonic continuum that ranges from very slow rates of strain associated with very thick edges of lithosphere slowly slipping ridge transfrom ridge plate boundaries to extremely high rates of strain associated with very thin edges of lithosphere fast slipping ridge transform ridge plate boundaries the geometry of a ridge transform intersection necessitates the juxtaposition of a relatively cold thick edge of lithosphere against the truncated end of an accreting plate boundary the cold face of lithosphere cools the adjacent wedge of asthenosphere rising beneath the axis of accretion and restricts the amount of partial melting thus attenuating the volume of basaltic melt segregated from the asthenosphere per unit time the shallow level manifestation of this cold edge effect is a thinner oceanic crust at depth the evolving lithosphere thickens rapidly as the cold edge is approached creating changes in the properties of the young lithosphere and forming a weld of upper mantle material against the cold edge of truncating lithosphere the ridge transform weld of upper mantle material creates a shear couple in the lithosphere underlying the intersection resulting in the progressive reorientation of the maximum tensile stress from normal to the ridge axis at some distance from the ridge transform intersection to an oblique angle near the boundary the brittle carapace of oceanic crust that overlies the mantle weld will deform accordingly with the development of oblique dip slip faults the model predicts that the geologic expressions of this cold boundary effect will become more dramatic with increasing thickness of the truncating edge at very slow rates of accretion 100 km juxtapose thick 30 50 km edges of lithosphere against the accreting plate boundary all but nullifying the processes that lead to the emplacement of normal oceanic lithosphere in this environment a relatively strong mantle weld will form at the ridge transform intersection and the thick edges of opposing lithosphere along the length of the transform will tend to confine the strike slip tectonism to a relatively narrow and temporally stable principal transform displacement zone in contrast at very fast rates of accretion large offset transforms 100 km place relatively thin edges of lithosphere

routing of meltwater from the laurentide ice sheet during the younger dryas cold episode rooth1 proposed that the younger dryas cold episode which chilled the north atlantic region from 11 000 to 10 000 yr bp was initiated by a diversion of meltwater from the mississippi drainage to the st lawrence drainage system the link between these events is postulated to be a turnoff during the younger dryas cold episode of the north atlantic s conveyor belt circulation system which currently supplies an enormous amount of heat to the atmosphere over the north atlantic region2 this turnoff is attributed to a reduction in surface water salinity and hence also in density of the waters in the region where north atlantic deep water nadw now forms here we present oxygen isotope and accelerator radiocarbon measurements on planktonic foraminifera from orca basin core en32 pc4 which reveal a significant reduction in meltwater flow through the mississippi river to the gulf of mexico from about 11 200 to 10 000 radiocarbon years ago this finding is consistent with the record for lake agassiz which indicates that the meltwater from the southwestern margin of the laurentide ice sheet was diverted to the northern atlantic ocean through the st lawrence valley during the interval from 11 000 to 10 000 years before present yr bp

correcting distorted paleosecular variation in late glacial lacustrine clay abstract an undisturbed horizontal chronostratigraphic marker horizon of laminated red glacio lacustrine clay crops out over 25 000 km 2 in northern ontario canada the primary clastic hematite laminae possess two stable vector components of magnetization we sampled a 1 6 m vertical section at overlapping 2 cm intervals in cubic specimens 8 cm 3 n 106 precisely oriented in geographic coordinates which permitted measurement of inclination and declination alternating field demagnetization 12 17 steps per specimen isolated a characteristic chrm primary component coercivity 40 mt approximately 30 shallower than the mean geomagnetic field inclination at this latitude with distorted paleosecular secular variation psv a lower coercivity overprint 20 40 mt is 6 shallower than the present geomagnetic field and similarly inclined to the depositional remanence deflection when the clay was re sedimented in the laboratory we believe this angle to be representative of the inclination of the deflected primary remanence inclination during deposition caused by the magnetic anisotropy of the clay using this as a proxy for the initial inclination the chrm hard component inclinations were restored to their original values assuming vertical compaction which averages to 51 the restored inclinations are compatible with site latitude and the restored secular variation loops centre reasonably on the geographic north pole the structural correction based on homogeneous vertical shortening over simplifies the reality of heterogeneous particulate flow in which grain rotations depend on shape size and packing and effectively combines influences of compaction and depositional settlement nevertheless this correction makes the psv data more useful and interpretable in terms of paleopole migration the more logical correction technique using anisotropy of anhysteretic remanence aarm jackson m j banerjee s k marvin j a lu r gruber w 1991 detrital remanence inclination errors and anhysteretic remanence anisotropy quantitative model and experimental results geophys j int 104 95 103 is foiled here due to the high coercivity of the remanence bearing grains and their unknown orientation distribution and shape distribution

freshwater outbursts to the oceans from glacial lake agassiz and their role in climate change during the last deglaciation abstract lake agassiz was the largest lake in north america during the last deglaciation as the laurentide ice sheet lis retreated large volumes of water stored in this proglacial lake were episodically released into the oceans these waters were variably routed to the gulf of mexico arctic ocean north atlantic ocean and hudson bay during this period the three largest cooling events in the northern hemisphere closely followed 4 of the 5 largest outbursts from lake agassiz 1 the younger dryas which was preceded by a release of 9500 km 3 2 the preboreal oscillation preceded by releases of 9300 km 3 and 5900 km 3 and 3 the 8 2 ka cold event preceded by a 163 000 km 3 outburst these are respectively fluxes of 0 30 sv 0 29 sv 0 19 sv and 5 2 sv if released in 1 year because the influx of freshwater reaching the north atlantic ocean can inhibit thermohaline circulation partly depending on whether the ocean was in a glacial interglacial or transitional mode of circulation we believe that at least these large outbursts from lake agassiz may have provided the triggers for changes in ocean circulation and in turn for widespread climate change

history of sedimentation in the northwestern lake superior basin and its relation to lake agassiz overflow following the marquette glacial advance which blocked the eastern outlets of lake agassiz and reached northern michigan about 10 000 years bp the ice margin wasted back toward the northeast eventually allowing lake agassiz to overflow into the lake superior basin through a series of channels sediments deposited east of thunder bay near the mouth of the wolf wolfpup shillabeer and black sturgeon channels reflect three phases in the history of lake superior and provide the basis for reconstructing the early postglacial history of the region the lower part of the sedimentary sequence in the northwestern superior basin consists of a distinctive red stoney sandy till deposited during the marquette glacial advance and is overlain by pink rhythmites deposited in lake superior when it was a deep proglacial lake at the minong level the nearly 300 rhythmites deposited at this time typically consist of 4 cm thick silt clay couplets which are punctuated by silt laminae and sandy turbidites that probably

glacial lake agassiz the northwestern outlet and paleoflood valley morphology and sediment in the fort mcmurray region of alberta indicate that a catastrophic flood discharged down the lower clearwater and athabasca river valleys 9900 yr b p geomorphic and chronologic evidence suggests that glacial lake agassiz emerson phase was the probable water source as the flood incised a drainage divide located near the alberta saskatchewan border the level of glacial lake agassiz decreased by 46 m discharged 2 4 x 106 m3 s for at least 78 days and stabilized at 438 m above sea level in the lake wasekamio area at that time water entered the arctic ocean via glacial lake mcconnell and the mackenzie river rather than the gulf of mexico via the mississippi river as previously thought such a large influx of fresh water 8 6 km3 h into the arctic at the close of the last glaciation may have had an abrupt major influence on northern climate

a new procedure for gridding elevation and stream line data with automatic removal of spurious pits abstract a morphological approach to the interpolation of regular grid digital elevation models dems from surface specific elevation data points and selected stream lines is described the approach has given rise to a computationally efficient interpolation procedure which couples the minimization of a terrain specific roughness penalty with an automatic drainage enforcement algorithm the drainage enforcement algorithm removes spurious sinks or pits yielding dems which may be used to advantage in hydrological process studies the drainage enforcement algorithm has also been found to significantly increase the accuracy of dems interpolated from sparse but well chosen surface specific elevation data moreover it facilitates the detection of errors in elevation data that would not be detected by more conventional statistical means and forms a sound physical basis for cartographic generalization

meltwater and precipitation runoff to the north atlantic arctic and gulf of mexico from the laurentide ice sheet and adjacent regions during the younger dryas runoff from north america may have played a significant role in ocean circulation and climate change during the last deglaciation because the driving force behind such changes may have been related to salinity of the north flowing atlantic ocean conveyor circulation it is critical to know the volume timing and location of fresh water entering the north atlantic from the melting laurentide ice sheet during the younger dryas cold episode 11 000 10 000 years b p there was a two fold increase in the volume of meltwater plus precipitation runoff to more than 1700 km yr 1 flowing through the st lawrence valley to the north atlantic mainly because retreating ice allowed the glacial lake agassiz basin to drain eastward into the great lakes at this time there was a corresponding decline in discharge from lake agassiz through the mississippi river to the gulf of mexico runoff to the arctic ocean also increased at about the beginning of the younger dryas from 740 to 900 km yr 1 because of the capture of what is now the headwater region of the mackenzie river watershed this in combination with rising sea level and warming climate may have increased the amount of pack ice reaching the north atlantic through the norwegian sea from the arctic ocean at 10 000 years b p eastward overflow from the western interior of north america was blocked by advancing ice again forcing overflow to the gulf of mexico and possibly to the northwest into the arctic ocean although total runoff to the oceans from all regions draining from the laurentide ice sheet did not vary substantially between 12 000 and 9000 years b p if discharge to the gulf of mexico is excluded fresh water reaching the north atlantic averaged 4000 km yr 1 during the younger dryas in contrast to 2870 km yr 1 just before this cold episode and 3440 km yr 1 just after it

the timing of regional lateglacial events and post glacial sedimentation rates from lake superior abstract we analyze both new and previously published paleomagnetic records of secular variation psv from lake superior sediment cores and compare these records to correlated rhythmite varve thickness records to determine post glacial sedimentation rates and to reassess the termination of glaciolacustrine varves in the basin the results suggest that offshore sedimentation rates have exhibited considerable spatial variation over the past 8000 years particularly during the mid holocene we attribute offshore mid holocene sedimentation changes to alterations in whole basin circulation perhaps precipitated by a greater dominance of the gulf of mexico air mass during the summer season nearshore bays are characterized by high sedimentation rates for at least 1000 years after varve cessation and during a period between around 4500 and 2000 cal bp after 2000 cal bp sedimentation rates subsided to earlier rates the increases between 4500 and 2000 cal bp are probably due to lake level fall after the nipissing ii highstand the older glaciolacustrine varve thickness records suggest that the influx of glacially derived sediment ended abruptly everywhere in the lake except near the lake nipigon inlets multiple sediment cores reveal 36 anomalously thick varves previously ascribed to the formation of the nakina moraine which were deposited just prior to varve cessation in the open lake the psv records support the observation that the cessation of these thick varves is a temporally correlative event occurring at 9035 170 cal bp calibrated years before 1950 ca 7950 8250 14 c bp this date would correlate to the eastern diversion of lake agassiz and glacial meltwater into lake ojibway

lakes of the huron basin their record of runoff from the laurentide ice sheet the 189 000 km2 hur n basin is central in the catchment area f the present q s r lanrentian great lakes that now drain via the st lawrence river to the north atlantic ocean during deglaciation from 21 7 5 ka bp and owing to the interactions of ice margin positions crustal rebound and regional topography this basin was much more widely connected hydrologi cally draining by various routes to the gulf of mexico and atlantic ocean and receiving over flows from lakes impounded north and west of the great lakes hudson bay drainage divide early ice marginal lakes formed by impoundment between the laurentide ice sheet and the southern margin of the basin during recessions to interstadial positions at 15 5 and 13 2 ka be in i each of these recessions lake drainage was initially southward to the mississippi river and gulf of mexico in the first recession drainage subsequently switched eastward along the ice margin to the north atlantic ocean in the second recession drainage continued southward through the michigan basin and later eastward via the ontario basin and mohawk river valley to the north atlantic ocean during the final retreat of ice in the huron basin from 13 to 10 ka be proglacial lake drainage switched twice from the michigan basin and the mississippi river system to the i north atlantic via the ontario basin and mohawk river valley finally diverting to the champlain sea in the st lawrence river valley at about 11 6 ka be new seismo and litbo stratigraphic information with ostracode data from the offshore lacustrine sediments were integrated with the traditional data of shorelines uplift histories of outlets and radio carbon dated shallow water evidence of transgressions and regressions to reconstruct the water level history and paleolimnological record for the northern huron basin for the 11 7 ka bp period negative excursions in the 8 80 isotopic composition of ostracodes and bivalves in southern lake michigan southwestern lake huron and eastern lake erie indicate an influx of water from ice marginal lake agassiz in central north america about 11 ka be a major decline in water levels of the huron basin after 10 5 ka bp followed the high level main lake algonquin phase as ice receded and drainage was established through the north bay area to ottawa river valley during the subsequent mattawa stanley phase the lake level history was dominated by fluctuations of tens of meters highstands of the earliest oscillations whose origin is not clear might be related to some of the well known post algonquin shorelines after 9 6 ka be it is suggested that large inflows from lake agassiz and hydraulic damming in downstream outlets were the likely cause of the lake mattawa highstands a lowstand at 9 3 9 1 ka bp occurred when these inflows were diverted or impeded by an ice advance in the nipigon basin area while undiluted meltwater continued to enter the huron basin assemblages and isotopic composition of the ostracode fauna indicate very dilute meltwater during the lowstands as late as 7 5 ka be and precipitation runoff with comparatively higher dissolved solids dur ing the highstands we speculate that the water composition of the lake mattawa highstands was dom inated by the agassiz inflows by that time much of lake agassiz was remote from ice marginal envi ronments and the inflows were drawn from surface water of the southern sector of the lake which was largely supplied by runoff and dissolved solids from the exposed land area of western canada major inflows apparently ended about 8 ka be but northern proglacial lakes apparently continued as melt water persisted in the huron basin until about 7 5 ka bp the cessation of major inflows initiated the final lowstand in the huron basin and the present hydrological regime of local runoff

cooling in the gulf of st lawrence and estuary region at 9 7 to 7 2 14c ka 11 2 8 0 cal ka palynological response to the pbo and 8 2 cal ka cold events laurentide ice sheet air mass circulation and enhanced freshwater runoff abstract an examination of post younger dryas yd pollen stratigraphies in the gulf of st lawrence and st lawrence estuary region reveals features in the pollen records that represent breaks in the normal vegetation succession widespread vegetation suppression and a delay in migration of plant taxa between 9 7 and 7 2 14 c ka 11 2 and 8 0 cal ka the domination of alnus crispa at sites bordering the st lawrence estuary gulf region in gaspesie and northern new brunswick within this timeframe represents a diversion from the typical vegetation progression from picea and or populus or picea betula to pinus and or betula and signifies a shift to a cooler drier climate coinciding with the a crispa expansion and domination in that region was the contraction of picea populations in other areas in southwestern new brunswick and eastern and southeastern nova scotia picea was replaced by the first appearance of tree birch b papyrifera whereas in western and southwestern newfoundland picea gave way to a resurgence of shrub birch betula glandulosa the picea contraction and immediate resurgence of betula represents cooling and is reliably dated at 9720 110 14 c bp 10 800 11 240 cal bp in southwest newfoundland this first post yd episode of widespread cooling is correlated with the north atlantic preboreal oscillation pbo centered around 9650 14 c bp 10 900 11 180 cal bp in the adjacent great lakes region sites exposed to winds from the gulf of st lawrence in eastern new brunswick prince edward island and northern nova scotia show a lingering persistence of picea and delay in arrival of pinus to 8 0 and 7 7 14 c ka 9 0 and 8 4 cal ka yet pinus was dominant as early as 9 4 14 c ka 10 6 cal ka in southwestern new brunswick at the same time tundra vegetation persisted at high elevations in western and southwestern newfoundland only to be replaced by upslope migration of shrub birch heath by 8 14 c ka prolonged broad scale cooling to 8 14 c ka and to as late as 7 7 14 c ka extended up to 200 km inland in areas exposed to the st lawrence estuary and gulf region and was in response to strong cold dry anticyclonic winds coming off the retreating laurentide ice sheet in combination with enhanced freshwater runoff through the gulf of st lawrence the end of the period of prolonged cooling and onset of regional warming coincided with the diversion of western canada runoff and agassiz ojibway drainage to hudson bay and reduced effect or final break up of the laurentide ice sheet several sites document a subsequent cold shift that interrupted regional warming at 7650 to 7200 14 c bp 8400 to 8000 cal bp and which is variously represented by the suppression of pinus and resurgence of picea sometimes with a crispa quebec maritime region or by an abrupt decrease of picea and resurgence of betula western newfoundland this second post yd cool interval is equated with the 8200 cal bp cold event registered in the greenland ice isotopic record

evolution and paleohydrology of glacial lakes barlow and ojibway abstract the evolution of lakes barlow and ojibway two large ice contact glacial lakes that covered parts of northern ontario and western quebec from about 10 1 8 0 ka bp during the retreat of the laurentide ice sheet was reconstructed using deglaciation landforms ice flow indicators inscribed on bedrock 14 c dates of early postglacial vegetation accumulated in lake basins the distribution of mysis relicta a biological indicator of lacustrine submergence varve chronology and a detailed photogrammetric survey of the lake limit although the rate of ice retreat varied greatly throughout the basin the average rate in lake barlow ojibway was 450 m year rates of ice retreat deduced from varve chronology were assessed using predictable relationships between the lake limit the shoreline gradient and the rate of relative uplift over a 590 km long profile the gradient on the maximum elevation of the lake defines a parabola that is attributed to restrained rebound although in part it could be due to the position of the profile with respect to the overall uplift pattern generated by the ice sheet the life span of the lake determined from varve chronology 2110 years shows excellent agreement with the age difference obtained between that of marine shells overlying lake ojibway sediments at its northern end in hudson bay and the oldest radiocarbon ages on basal organics in lakes in the southern part of the barlow ojibway basin both lakes drained eastward through the ottawa river between 10 1 and 8 0 ka bp and the average yearly discharge to the north atlantic ocean during the last 1000 1500 years is estimated at about 946 km 3 at break up of the ice sheet in southern hudson bay at about 8 0 ka bp lake ojibway drained abruptly releasing an estimated 114 396 km 3 of freshwater into the tyrrell sea and raising sea level by about 30 cm

an assessment of the minimum timing of ice free conditions of the western laurentide ice sheet abstract this study presents an investigation of the minimum timing of retreat of the western laurentide ice sheet which covered parts of canada and the northern united states during the wisconsin glaciation the retreat of the ice sheet is poorly constrained due to low spatial resolution of chronological data that indicate ice free conditions a limitation of radiocarbon and luminescence dating methods is that it is only possible to determine the time when it is certain that ice is absent from a region which may differ substantially from the true date of ice margin retreat the minimum timing of ice free conditions is determined using a model of ice sheet retreat inferred from landforms that formed during the late stages of glaciation and extrapolating the calibrated probability distribution of chronological data in the opposite direction the analysis excludes radiocarbon samples from materials that have been shown to give artificially old ages such as bulk sediments the results of the analysis identify 55 dates that have the most impact on the model the retreat of the western laurentide ice sheet commenced by at least 15 000 cal yr bp while the minimum timing of ice free conditions between the laurentide ice sheet and the cordillera is after 11 000 cal yr bp a lack of data in the keewatin sector prevents the determination of a precise timing of ice free conditions in the last vestiges of the laurentide ice sheet the model provides a guide to where additional samples could improve chronological control on ice margin location

glacial lake agassiz a 5000 yr history of change and its relationship to the δ18o record of greenland lake agassiz was the largest lake in north america during the last period of deglaciation the lake extended over a total of 1 5 10 6 km 2 before it drained at ca 7 7 14 c ka 8 4 cal calendar ka new computer reconstructions controlled by beaches isostatic rebound data the margin of the laurentide ice sheet outlet elevations and a digital elevation model dem of modern topographic data show how variable the size and depth of this lake were during its 4000 14 c yr 5000 cal yr history abrupt reductions in lake level ranging from 8 to 110 m occurred on at least 18 occasions when new outlets were opened reducing the extent of the lake and sending large outbursts of water to the oceans three of the largest outbursts correlate closely in time with the start of large 18 o excursions in the isotopic records of the greenland ice cap suggesting that those freshwaters may have had an impact on thermohaline circulation and in turn on climate

testing northwest drainage from lake agassiz using extant ice margin and strandline data extant data on shorelines and ice margins in the glacial lake agassiz basin defines the limits of lake agassiz through deglacial time the ice sheet margin had not retreated far enough northwards by the beginning or end of the younger dryas chronozone for the lake to drain northwest however once lake agassiz reached the campbell beach stage at 10 800 10 300 cal bp then projections of lake agassiz strandlines were higher than the topography allowing for the first possible pathway to the northwest this relationship is used to reject the recent assertion that there was northwest drainage from lake agassiz at the beginning and end of the younger dryas chronozone

strandline analysis in the southern basin of glacial lake agassiz minnesota and north and south dakota usa glacial lake agassiz a large pleistocene lake left behind strandlines at multiple elevations within its basin prior researchers have used these strandlines to identify four relatively stable water level stages but tracing them over long distances is often problematic in this study the elevation of the tintah strandline is returned to its original designation and a newly interpreted stable lake level at 1020 1030 ft 311 314 m is referred to as the upham level based on a concentration of ridges and a spit at that elevation cores from strandline and lagoon complexes from the milnor to tintah beaches reveal a decreasing thickness of strandline sediment associated with shorter aggregate length strandlines this relationship is most likely controlled by sediment starvation greater water level fluctuations and shallower water beaches near the head of the southern outlet spillway indicate that shoreline occupation at the lower campbell level was either very short lived or strandline development was hampered by shallow water the modern day sill elevation between the southern outlet spillway and the lake basin is found to be 3 km south of white rock minnesota at an elevation of 974 ft 297 m all but the campbell strandlines at the southern outlet are interpreted as the result of a regression driven by incision of the southern outlet spillway

paleohydrology of the upper laurentian great lakes from the late glacial to early holocene abstract between 10 500 and 9000 cal yr bp 18o values of benthic ostracodes within glaciolacustrine varves from lake superior range from 18 to 22 pdb in contrast coeval ostracode and bivalve records from the lake huron and lake michigan basins are characterized by extreme 18o variations ranging from values that reflect a source that is primarily glacial 20 pdb to much higher values characteristic of a regional meteoric source 5 pdb re evaluated age models for the huron and michigan records yield a more consistent 18o stratigraphy the striking feature of these records is a sharp drop in 18o values between 9400 and 9000 cal yr bp in the huron basin this low 18o excursion was ascribed to the late stanley lowstand and in the lake michigan basin to lake agassiz flooding catastrophic flooding from lake agassiz is likely but a second possibility is that the low 18o excursion records the switching of overflow from the lake superior basin from an undocumented northern outlet back into the great lakes basin quantifying freshwater fluxes for this system remains difficult because the benthic ostracodes in the glaciolacustrine varves of lake superior and lake agassiz may not record the average 18o value of surface water

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

shoreline reconstructions for the persian gulf since the last glacial maximum abstract sea level change in the persian gulf since the time of the last maximum glaciation at about 18 000 yr bp is predicted to exhibit considerable spatial variability because of the response of the earth to glacial unloading of the distant ice sheets and to the meltwater loading of the gulf itself and the adjacent ocean models for these glacio hydro isostatic effects have been compared with observations of sea level change and palaeoshoreline reconstructions of the gulf have been made from the peak of the glaciation until about 14 000 yr bp the gulf is free of marine influence out to the edge of the biaban shelf by 14 000 yr bp the strait of hormuz had opened up as a narrow waterway and by about 12 500 years ago the marine incursion into the central basin had started the western basin flooded about 1000 years later momentary stillstands may have occurred during the gulf flooding phase at about 11 300 and 10 500 yr bp the present shorelines was reached shortly before 6000 yr ago and exceeded as relative sea level rose 1 2 m above its present level inundating the low lying areas of lower mesopotamia these reconstructions have implications for models of the evolution of the euphrates tigris karun delta as well as for the movements of people and the timing of the earliest settlements in lower mesopotamia for example the early gulf floor would have provided a natural route for people moving westwards from regions to the east of iran from the late palaeolithic to early neolithic

superlakes megafloods and abrupt climate change about 8200 years ago the climate of much of the northern hemisphere cooled abruptly for a period of about 200 years in their perspective clarke et al examine the most likely culprit for this cooling an outburst of fresh water from a vast ice dammed glacial lake in north america the superlake had formed when the kilometers thick ice sheet covering much of north america disintegrated when the ice dam became unstable fresh water flooded from the lake into the north atlantic it remains unclear how this fresh water affected ocean circulation or whether the outburst occurred in more than one stage but the timing points strongly to the outburst flood as the trigger of the 8200 year climate event

preboreal oscillation caused by a glacial lake agassiz flood the preboreal oscillation pbo has been attributed to increased meltwater but the source of the meltwater and causative mechanism of the pbo has remained elusive here we attribute the source to a massive meltwater discharge event from an abrupt drainage of glacial lake agassiz canada via the mackenzie river into the arctic ocean a maximum volume of 21 000 km 3 was discharged over a 1 5 3 yr period with a peakdischarge of 0 500 sverdrups sv equivalent to a 6 m rise in the arctic ocean or 0 062 m rise in global sea level the flood occurred at about 11 335 cal yr bp and was followed by a b0 042 sv flow until 10 750 cal yr bp when the southern outlet of lake agassiz reopened and diverted drainage to the mississippi river system we estimate that only 2 4 of the flood water would have frozen into sea ice within the beaufort region but coupled with increased river ice production during winter and thicker pack ice growth throughout the arctic ocean a thicker longer lasting and more extensive packice may have been flushed through fram strait the thicker and more extensive packice and freshened sea surface may have triggered the pbo by increasing albedo and generating a low salinity anomaly upon melting in the north atlantic thus decreasing the formation of north atlantic deep water r 2002 elsevier science ltd all rights reserved

impacts of post glacial lake drainage events and revised chronology of the champlain sea episode 13 9 ka abstract lithologic chirp compressed high intensity radar pulse sonar paleomagnetic stable isotopic and micropaleontological analyses of sediment cores from lake champlain new york vermont were used to determine the age of the post glacial champlain sea marine episode the timing of salinity changes and their relationship to freshwater discharge from mid continent glacial lakes calibrated radiocarbon ages on plant material provide an improved post glacial chronology overcoming problems from shell ages caused by carbon reservoir effects up to 1500 yr the final drainage of glacial lake vermont and the inception of marine conditions occurred 13 1 12 8 ka kiloannum calendar years and a sharp decrease in champlain sea salinity from 25 to 7 8 psu practical salinity units occurred approximately 11 4 11 2 ka reduced salinity was most likely caused by rapid freshwater inflow eastward from glacial lake algonquin into the champlain basin the timing of inferred freshwater event coincides with the widespread climatic cooling called the preboreal oscillation

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