development of the baltic ice lake in the eastern baltic abstract a gis based palaeogeographic reconstruction of the development of the baltic ice lake bil in the eastern baltic during the deglaciation of the scandinavian ice sheet is presented a late glacial shoreline database containing more than 1000 sites from finland nw russia estonia latvia and modern digital terrain models were used for palaeoreconstructions the bil occupied five different levels represented by 492 shoreline features the study shows that at about 13 3 cal ka bp the bil extended to the ice free areas of latvia estonia and nw russia represented by the highest shoreline in this region reconstructions demonstrate that bil initially had the same water level as the glacial lakes peipsi and vortsjarv because these water bodies were connected via strait systems in central estonia these strait systems were closed at about 12 8 11 7 cal ka bp prior to the final drainage of the bil due to isostatic uplift glacial lake vortsjarv was isolated from the bil at about 12 4 12 0 cal ka bp exact timing of glacial lake peipsi isolation is not clear but according to the altitude of the threshold in northeast estonia and shore displacement data it was completed at about 12 4 11 7 cal ka bp

ice age paleotopography a gravitationally self consistent theory of postglacial relative sea level change is used to infer the variation of surface ice and water cover since the last glacial maximum lgm the results show that lgm ice volume was approximately 35 percent lower than suggested by the climap reconstruction and the maximum heights of the main laurentian and fennoscandian ice complexes are inferred to have been commensurately lower with respect to sea level use of these ice age boundary conditions in atmospheric general circulation models will yield climates that differ significantly from those previously inferred on the basis of the climap data set

ice dammed lakes and rerouting of the drainage of northern eurasia during the last glaciation during the quaternary period ice sheets centred over the barents and kara seas expanded several times onto mainland russia and blocked northflowing rivers such as the yenissei ob pechora and mezen large ice dammed lakes with reversed outlets e g toward the caspian sea formed south of these ice sheets some lakes are reconstructed from shorelines and lacustrine sediments others mainly from ice sheet configuration ice dammed lakes considerably larger than any lake on earth today are reconstructed for the periods 90 80 and 60 50 ka the ages are based on numerous optically stimulated luminescence osl dates during the global last glacial maximum lgm about 20 ka the barents kara ice sheet was too small to block these eastern rivers although in contrast to the 90 80 and 60 50 ka maxima the scandinavian ice sheet grew large enough to divert rivers and meltwater across the drainage divide from the baltic basin to the river volga and that way to the caspian sea climate modelling shows that the lakes caused lower summer temperatures on the continent and on the lower parts of the ice sheet the final drainage of the best mapped lake is modelled and it is concluded that it probably emptied within few months we predict that this catastrophic outburst had considerable impact on sea ice formation in the arctic ocean and on the climate of a much larger area c 2003 elsevier ltd all rights reserved less

deglaciation of a soft bedded laurentide ice sheet we present a series of numerical reconstructions of the laurentide ice sheet during the last deglaciation 18 7 14c ka that evaluates the sensitivity of ice sheet geometry to subglacial sediment deformation these reconstructions assume that the laurentide ice sheet flowed over extensive areas of water saturated deforming sediment soft beds corresponding to the st lawrence lowland the great lakes region the western prairies of the u s and canada and the hudson bay and hudson strait regions sediment rheology is based on a constitutive law that incorporates experimental results from late wisconsin till deposited by the laurentide ice sheet which suggest only mildly nonlinear viscoplastic behavior by varying the effective viscosity of till we produced four reconstructions for the ice sheet during the last glacial maximum 18 14c ka and two reconstructions each of the ice sheet at 14 13 12 11 and 10 14c ka we also produced one reconstruction for 9 8 4 8 and 7 14c ka reconstructions that assume a low effective viscosity for all areas of deforming sediment show a multidomed ice sheet with a large bowl shaped depression over hudson bay and thin ice 1000 m above modern sea level over the western and southern margins those reconstructions that assume a higher effective viscosity of till in the hudson bay region than for the western and southern margins also show a multidomed ice sheet but with considerably thicker ice over hudson bay and a more northerly position of the central ice divide these two different geometries may represent ice sheet orographic changes associated with a heinrich event further increases in effective viscosity of till approaching the effective viscosity of ice would result in a high monolithic ice dome centered over hudson bay reinforcing the notion that a multidomed ice sheet reflects the distribution of substrate geology modeled ice surface geometry at the last glacial maximum shows many of the same general features as previous reconstructions that incorporate deformable beds our reconstructions with higher effective till viscosities in hudson bay also agree with the ice 4g reconstructions peltier 1994 which are based on inversion of relative sea level data for the early part of the last deglaciation 18 13 14c ka but then depart significantly from ice 4g beginning at about 12 14c ka due to differing assumptions of the history of deglaciation modeled ice volume for the last glacial maximum suggests a glacioeustatic change of 50 55 m by a soft bedded laurentide ice sheet this would increase as the effective viscosity of till increases subsequent ice volume changes through the last deglaciation generally parallel the trend of eustatic rise recorded at barbados new guinea and tahiti but suggest that the laurentide ice sheet was not the source of meltwater pulse 1a

the evolution of the mobile zones of gondwana and laurasia in the late precambrian abstract the tectonic evolutions of gondwana and laurasia in the late precambrian show some differences which become more striking after the late riphean 1000 700 m y when gondwana separated from pangea these differences include distributions of aulocogens and intrageosynclines as well as zones of non geosyncline tectono thermal reworking of basement geodynamics of interiors in gondwana and laurasia were quite different during the vendian early cambrian the fragmentation of laurasia and the opening of the iapetus and the paleo asian ocean coincide in time with the pan african orogeny and equivalent orogenies in gondwana

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

rapid time transgressive and variable responses to early holocene midcontinental drying in north america the end pleistocene to early holocene drying of the north american midcontinent is an important case study for understanding regional drought its drivers and its impacts this paper synthesizes pollen 13c eolian and paleohydrological proxies to document the spatial and temporal patterns in the onset and rate of drying the timing of onset ranged from 14 to 6 ka with most sites drying between 10 and 8 ka calendar years ago the dominant spatial pattern is time transgressive with interior great plains sites beginning to dry before peripheral sites this time transgressive trend can be explained as a region wide drying that caused ecotones to shift eastward or could indicate a progressive shift in atmospheric drying from west to east one third of sites responded rapidly to drying a cluster of rapid responses occurred ca 8 ka the combination of onsets between 14 and 6 ka and rapid responses at 8 ka strongly suggests that 1 the combination of high summer insolation laurentide ice sheet retreat and lake agassiz drainage drove midcontinental drying and 2 drying accelerated with laurentide ice sheet collapse at 8 4 ka local factors strongly mediate the timing and rate of site responses to regional drying local responses to future drying in the great plains also should be highly variable which challenges impact assessments of future climate changes

mapping eastern north american vegetation change of the past 18 ka no analogs and the future the method of modern analogs and an extensive data base of modern and fossil pollen data were used to generate a new series of paleovegetation maps for eastern north america spanning the past 18 ka the maps illustrate the continuous nature of climate induced vegetation change and the development after about 10 ka of modern regional vegetation patterns before the holocene vegetation biomes without modern analogs were widespread in response to climate conditions without modern analogs and to a lesser extent to the rapidity of climate change over the last glacial interglacial transition this geological perspective suggests that possible future climate changes could force similarly complex changes in natural vegetation including the development of biomes without modern analogs

exceptionally steep north south gradients in lake temperatures during the last deglaciation during the transition from the last glacial to the present interglacial climate late glacial period high summer insolation1 combined with the presence of the laurentide ice sheet is thought to have promoted the development of strong summer thermal gradients in north america south of the ice margin2 here we use palaeoecological methods to obtain quantitative evidence for the existence of these gradients our palaeoclimate reconstructions are based on water temperatures inferred from fossil assemblages of aquatic midge larvae in five lakes along a 240 km transect extending from central new brunswick canada to southeastern maine usa we show that the temperature gradient shifted during the killarney oscillation ko and younger dryas yd cooling events and that water temperature differences of 9 and 11 c existed over distances of 55 and 240 km respectively during parts of the late glacial period these gradients are much stronger than the current north south trends of lake summer surface water temperatures for these five lakes and for lakes across the northern tree line the rapid motion of such steep temperature gradients may have affected the progressive development of aquatic and terrestrial ecosystems in the region

age verification of the lake gribben forest bed and the younger dryas advance of the laurentide ice sheet analysis of nine wood samples from the lake gribben forest bed near lake gribben michigan yielded a combined age of 10 025 100 14c years bp which confirms and refines prior age estimates for the bed the stratigraphic position of these samples below a prograding ice contact fan indicates the time that a glacial margin reached the southern edge of the lake superior basin geomorphic tracing and correlation of associated deposits indicate that a contemporaneous margin extended almost 1000 km from duluth minnesota across the lake superior basin to north bay ontario along the southern shore of lake superior ice margin expansion began during and ended at the close of the younger dryas a surging glacier system would not produce a nearly linear moraine system across both a major basin lake superior and a major upland abitibi upland therefore we attribute this advance of the laurentide ice sheet to climatic forcing of the younger dryas event

paleoclimate simulations for north america over the past 21 000 years features of the simulated climate and comparisons with paleoenvironmental data maps of upper level and surface winds and of surface temperature and precipitation illustrate the results of a sequence of global paleoclimatic simulations spanning the past 21 000 years for north america we review a the large scale features of circulation temperature and precipitation that appear in the simulations from the ncar community climate model version 1 ccm 1 b the implications of the simulated climate for the past continental scale distributions of three plant taxa picea spp pseudotsuga menziesii and artemisia tridentata which are broadly representative of the vegetation across the continent and c the potential explanations in terms of atmospheric circulation or surface energy and water balance processes for mismatches between the simulations and observations most of the broad scale features of previous paleoclimatic simulations with the ncar ccm 0 for north america are present in the current simulations many of the elements of a conceptual model based on previous climate simulations that describes the controls of paleoclimatic variations across north america during the past 21 000 years are found in simulations reviewed here these include 1 displacement of the jet stream by the laurentide ice sheet to the south of its present position in both winter and summer 2 generation of a glacial anticyclone over the ice sheet at the lgm and the consequent induction of large scale sinking motions induced over eastern north america 3 changes in the strength of surface atmospheric circulation features through time including weakening of the aleutian low in winter and strengthening of the eastern pacific and bermuda high pressure systems in summer as the ice sheet decreased in size 4 development of a heat low at the surface and a strengthened ridge in the upper atmosphere over the continent at the time of the maximum summer insolation anomaly 5 increases in summer temperature earlier in regions remote from the ice sheet these increases appear earlier in the present ccm 1 simulations than in the previous ccm 0 ones however and 6 continuation of negative winter temperature anomalies into the middle holocene in general simulated surface conditions that are discordant with paleoenvironmental observations can be attributed to the simulation of particular atmospheric circulation patterns e g those that suppress precipitation or advect warm air into a region with these mismatches amplified in beringia and the southeastern united states by surface energy and water balance processes

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

the timing and nature of late quaternary vegetation changes in the northern great plains usa and canada a re assessment of the spruce phase abstract this paper revises the chronology for the northward migration of picea glauca white spruce across the northern great plains following the recession of the laurentide ice sheet and reinterprets the species composition and structure of the late glacial vegetation on the basis of pollen and plant macrofossil analysis the timing of spruce migration is based on 26 14 c ages obtained from picea macrofossils the date for the appearance of white spruce in southern south dakota usa remains unchanged 12 600 14 c yr bp ca 15 000 cal yr bp but its arrival in southern saskatchewan canada by 10 300 14 c yr bp ca 12 100 cal yr bp is about 1500 years later than previously estimated based on an organic sediment date picea glauca thus migrated northwards at an average rate of 0 38 km 14 c year 0 30 km calendar year significantly slower than the previously published rate of 2 km 14 c year white spruce trees probably inhabited lake shorelines whereas prairie parkland and boreal plants occupied both lowlands and uplands forming an open white spruce parkland this interpretation differs from a previous reconstruction of a boreal type spruce forest and thus offers another paleoclimatic interpretation precipitation was probably low and summer temperatures relatively mild averaging about 19 c

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

paleochemistry of lakes agassiz and manitoba based on ostracodes the ionic composition and salinity of lake manitoba and its late glacial precursor lake agassiz changed significantly over the past 11 000 years the paleochemical record reported here is based on modern analog environments of ostracodes identified in a new 14 5 m core from southern lake manitoba the ionic composition of lake manitoba today is dominated by na cl and hc03 with much less ca2 mg2 and k evaporative concentration of modern lake manitoba water would lead to greater salinity and the near depletion of ca2 due to continued precipitation of calcite during periods of highest salinity in the holocene however lake manitoba supported limnocythere staplini today this species inhabits waters in which ca2 hco3 including springs associated with groundwater in paleozoic bedrock discharging into lake winnipegosis and eventually after much dilution into lake manitoba further complicating the holocene record are intervals containing limnocythere friabilis that suggest periodic

orbital suppression of wind driven upwelling in the california current at 6 ka 1 marine records clearly show activity in the california current cc system to have been altered during the mid holocene however the specific nature of cc activity during this period remains ambiguous as proxies that suggest cooler than present sea surface temperatures co occur in mid holocene marine sediments with multiple proxies that suggest decreased coastal upwelling to test the sensitivity of wind driven upwelling in the cc to mid holocene orbital forcing we have employed a high resolution regional climate model rcm the seasonality of modern cc wind driven coastal upwelling simulated by the rcm is in strong agreement with observational data further we show that changes in the seasonality of solar insolation induced by mid holocene milankovitch forcing decrease early and peak season coastal upwelling in the cc along with increasing late season coastal upwelling these results are in agreement with several marine and terrestrial proxies suggesting that relative to present cc activity during the mid holocene was characterized by a longer and less vigorous upwelling season with decreased seasonal contrast

climatic effects of glacial lake agassiz in the midwestern united states during the last deglaciation stable isotope and pollen analyses of a sediment core from deep lake minnesota provide new information on the climatic effects of glacial lake agassiz in minnesota and insights into the cause of the prominent picea recurrence during the younger dryas in the southern great lakes region bulk carbonate 18o exhibited large fluctuations between 12 0 and 9 1 ka dates are in calendar years throughout this paper unless indicated otherwise probably reflecting the effects of glacial lake agassiz superimposed on climatic warming related to large scale climatic controls in particular a 3 decrease in 18o 11 2 10 2 ka interrupted the 18o enrichment of 1 from 12 0 to 11 2 ka and 3 5 from 10 2 to 9 1 ka this 18o decrease coincided with the expansion of lake agassiz we interpret this decrease as a result of decreased summer temperature and increased precipitation derived from the cold and isotopically light meltwaters of lake agassiz during this 18o decline pinus pollen continued to increase at the expense of picea pollen at deep lake as at other minnesota sites providing evidence that climatic cooling induced by lake agassiz did not cause a reversal to a picea dominated vegetation the absence of such a vegetational response implies that the prominent picea recurrence during the younger dryas in the southern great lakes region was not caused solely by climatic cooling due to increased flux of meltwater from lake agassiz into the great lakes instead the picea recurrence might have been driven primarily by the westward penetration of the younger dryas cooling

enhanced ice sheet growth in eurasia owing to adjacent ice dammed lakes large proglacial lakes cool regional summer climate because of their large heat capacity and have been shown to modify precipitation through mesoscale atmospheric feedbacks as in the case of lake agassiz 1 several large ice dammed lakes with a combined area twice that of the caspian sea were formed in northern eurasia about 90 000 years ago during the last glacial period when an ice sheet centred over the barents and kara seas 2 blocked the large northbound russian rivers 3 here we present high resolution simulations with an atmospheric general circulation model that explicitly simulates the surface mass balance of the ice sheet we show that the main influence of the eurasian proglacial lakes was a significant reduction of ice sheet melting at the southern margin of the barents kara ice sheet through strong regional summer cooling over large parts of russia in our simulations the summer melt reduction clearly outweighs lake induced decreases in moisture and hence snowfall such as has been reported earlier for lake agassiz1 we conclude that the summer cooling mechanism from proglacial lakes accelerated ice sheet growth and delayed ice sheet decay in eurasia and probably also in north america

century scale paleoclimatic reconstruction from moon lake a closed basin lake in the northern great plains estimates of past lake water salinity from fossil diatom assemblages were used to infer past climatic conditions at moon lake a climatically sensitive site in the northern great plains a good correspondence between diatom inferred salinity and historical records of mean annual precipitation minus evapotranspiration p et strongly suggests that the sedimentary record from moon lake can be used to reconstruct past climatic conditions century scale analysis of the holocene diatom record indicates four major hydrological periods an early holocene transition from an open freshwater system to a closed saline system by 7300 b p which corresponds with a transition from spruce forest to deciduous parkland to prairie and indicates a major shift from wet to dry climate a mid holocene period of high salinity from 7300 to 4700 b p indicating low effective moisture p et a transitional period of high salinity from 4700 to 2200 b p characterized by poor diatom preservation and a late holocene period of variable lower salinity during the past 2 200 yr indicating fluctuations in effective moisture

simulated influences of lake agassiz on the climate of central north america 11 000 years ago eleven thousand years ago large lakes existed in central and eastern north america along the margin of the laurentide ice sheet the large scale north american climate at this time has been simulated with atmospheric general circulation models1 2 but these relatively coarse global models do not resolve potentially important features of the mesoscale circulation that arise from interactions among the atmosphere ice sheet and proglacial lakes here we present simulations of the climate of central and eastern north america 11 000 years ago with a high resolution regional climate model nested within a general circulation model the simulated climate is in general agreement with that inferred from palaeoecological evidence our experiments indicate that through mesoscale atmospheric feedbacks the annual delivery of moisture to the laurentide ice sheet was diminished at times of a large cold lake agassiz relative to periods of lower lake stands the resulting changes in the mass balance of the ice sheet may have contributed to fluctuations of the ice margin thus affecting the routing of fresh water to the north atlantic ocean a retreating ice margin during periods of high lake level may have opened an outlet for discharge of lake agassiz into the north atlantic a subsequent advance of the ice margin due to greater moisture delivery associated with a low lake level could have dammed the outlet thereby reducing discharge to the north atlantic these variations may have been decisive in causing the younger dryas cold event3 4

atmospheric scaling of cosmogenic nuclide production climate effect 1 absorption of cosmic rays by atmospheric mass varies temporally due to a redistribution of atmospheric pressure by ice sheets during glaciations the compression and expansion of the atmosphere due to cooling and warming and changes in katabatic winds near large ice masses these atmospheric processes can result in changes in production rates of cosmogenic nuclides which when integrated over long exposure durations may result in 0 to 5 adjustments in site production rates depending on location combining a ccm3 model with imbedded ice sheets for 20 ka we show that production rates changes relative to today are greatest at high elevations 6 7 at 5 km altitude due to atmospheric compression from decreased temperature production rates at specific times for sites near ice sheet margins can be reduced more than 10 due to a combination of katabatic winds draining off the ice sheet margins and atmospheric cooling nunatak settings may be significantly affected by the climate effect due to persistent glacial atmospheric conditions atmospheric variability may explain some of the disparities among cosmogenic nuclide production rate calibrations