wetlands at the last glacial maximum distribution and methane emissions the global distribution of potential wetlands and their methane ch4 emissions at the present day and the last glacial maximum lgm are estimated using a gcm simulation of lgm climate a vegetation model and simple algorithms for determining wetland area based on topography and soil moisture and ch4 emissions based on ecosystem carbon turnover in wet soils lgm wetland area was 15 larger than present but ch4 emissions were 24 less extensive wetlands were simulated on the exposed continental shelves the soil ch4 sink was simulated as 14 tg now but 0 5 tg at the lgm ch4 emissions at lgm were limited by substrate availability in turn due to low atmospheric co2 the glacial interglacial change in atmospheric ch4 concentration cannot be completely attributed to changes in the wetland source

temperatures at the base of the laurentide ice sheet inferred from borehole temperature data 1 we use temperature profiles from 4 deep 1600 m boreholes across canada to determine ground surface temperature histories gsth s through and after the last glacial maximum lgm inversion yields the temperature history at the base of the glacier and the surface temperature evolution after the glacial retreat the results indicate geographic differences in basal temperature history across the ice sheet during the last glacial maximum temperatures at the base of the ice sheet were lower in eastern canada at the southeastern edge of the glacier than in central canada southwest of the glacier center at all sites basal temperatures were above the melting point of ice during and after the lgm which may explain the highly unstable character of the ice sheet the gsth s are consistent with information on the history of the laurentide ice sheet and provide quantitative constraints on glacier flow dynamics

influence of the great lakes on the dynamics of the southern laurentide ice sheet numerical experiments a time dependent flow line ice sheet model is used to explore interactions between last glacial climate the laurentide ice sheet and the great lakes we seek to understand perturbations in the ice flow field that caused three neighboring lobes to leave different geomorphic and sedimentary records driven by reconstructed air temperature variations from 65 to 18 ka the simulated lobe dynamics are consistent with constraints from loess and till chronologies till stratigraphy and ice surface profiles contrasts in lobe dynamics are best explained by the geometry of lakes along each flow line as ice entered these lakes calving influenced glacier mass balance in particular deep water in lake superior likely delayed ice advance into northern wisconsin by promoting large calving losses whereas lobes to the east that encountered shallower water were less affected by calving the driftless area of wisconsin may thus owe its existence at least in part to the presence of lake superior our results suggest that morainal bank evolution should be treated in ice sheet models in lacustrine and shallow marine settings determining the sediment flux to the morainal bank remains a difficult task for example ice advance across central lake superior was probably sediment flux limited otherwise the driftless area would have been glaciated

palaeomagnetism of devonian ring complexes from the bayuda desert sudan new constraints on the apparent polar wander path for gondwanaland summary the approach of gondwana and or microplates derived from its northern margins towards laurentia and the closure of the intervening palaeo tethys is a controlling factor in the geodynamic evolution of the atlantic bordering continents during the early and mid palaeozoic however the currently available palaeomagnetic data for gondwana especially for the geodynamically critical silurian to devonian interval are still rather sparse and are compatible with a number of palaeogeographic scenarios in order to address this issue 114 samples 18 sites have been collected from a series of high level acidic intrusive rocks rhyolitic lavas and tuffs of mid devonian age 377 5 ma rb sr along the southern escarpment of the gilif hills sudan 17 83 n 32 67 e detailed demagnetization experiments revealed the presence of three directionally distinct components of magnetization labelled a b and c the overall mean direction of component a dec 0 9 inc 29 8 95 4 6 k 91 1 n 12 sites is indistinguishable from either the direction of the present field or the geocentric axial dipole field in northern sudan mixed polarities suggest a recent or pleistocene age of this component component b also displays mixed polarities the site mean directions for this component average to 6 5 declination 40 2 inclination with 95 7 0 and k 48 1 based on 10 sites and yield a palaeomagnetic pole at 48 8 s 23 5 e dp 5 1 1 dm 8 4 maghaemite and haematite have been identified to be the predominant magnetic phases in samples which carry component a and or b both a and b are the probable results of low temperature oxidation of magnetite although the age of component b can not be directly determined it is inferred by correlation with published data to reflect a late carboniferous overprint component c identified in 11 sites 39 samples is characterized by a very steep downward magnetization dec 296 2 inc 79 3 95 10 8 k 18 7 n 11 sites which corresponds to a palaeomagnetic pole position in the southwestern corner of libya 25 9 n 11 6 e dp 19 6 dm 20 6 maximum blocking temperatures of component c generally below 580 c are indicative of magnetite as carrier of this remanence the results of thermal remanent magnetization experiments demonstrate that component c is a thermo remanent magnetization probably primary and suggest this component to be representative for the direction of the earth s magnetic field during emplacement of the gilif hills volcanic rocks in mid devonian times the position of the resulting palaeomagnetic pole lends new support to geodynamic scenarios advocating the existence of a wide approx 5000 km pre hercynian ocean during devonian times the simplest reconciliation of component b with the reference apparent polar wander path is achieved by assuming it to be of carboniferous age and by invoking a westward cusp in the carboniferous part of the apparent polar wander path taken at face value the age of this cusp coincides with large scale post orogenic right lateral strike slip movements in hercynian europe and north africa this interpretation does not require the apparent polar wander apw loop and very high rates of plate motion implied by some alternative models however if it is valid it will require reinterpretation of the remanence age of one west african intrusive suite and of some eastern australian palaeozoic rocks

early palaeozoic cooling events peri gondwana and beyond the short lived end ordovician hirnantian glaciation allied to marine mass extinction is variously considered as a short lived event or as the peak of long drawn out climatic cooling through at least late ordovician early silurian times evidence from early palaeozoic facies faunas and stable isotope excursions used to interpret climatic cooling events ranges farther from late mid cambrian to late silurian times glacigenic sediments structures and geomorphology provide direct evidence of glacial episodes cool water carbonate deposition which is particularly widespread during the late ordovician boda event in high latitude peri gondwana gondwana and beyond into mid low palaeolatitudes is interpreted as indicating global cooling not warming as has been proposed such carbonates also characterize mid latitude continents widely at horizons earlier in the ordovician and more locally in the mid silurian in high latitude gondwana cool water carbonate mounds have distinctive facies controlled mound faunas across palaeocontinents other facies evidence for palaeoclimates includes black shale deposition including deglacial organic rich hot shales which indicate transgression in epeiric seas and sea level curves interpreted from facies and faunal successions correlation is shown between facies evidence and positive c isotope excursions from which cyclicities are apparent the possible interface of orbitally controlled rhythms is considered against evolving palaeobiogeography and changes in global sea level and in pco2 facies and faunal evidence from peri gondwanan terranes armorica central europe alborz is assessed with that from gondwana mostly north africa south america and correlatives in avalonia baltica and laurentia to establish a wider picture of early palaeozoic cooling events

early silurian glaciation along the western margin of gondwana peru bolivia and northern argentina palaeogeographic and geodynamic setting the record of the ordovician silurian transition within the southern peru bolivia palaeozoic basin extending across peru bolivia and northern argentina is characterized by a diamictite bearing unit which overlies several different ordovician units and underlies mid silurian shales we review the lithostratigraphy sedimentology and biostratigraphy of this unit revising stratigraphic relationships and several ideas and interpretations previously proposed analysis of facies and lithofacies associations allow for a reinterpretation of sedimentary processes and environments involved during its deposition turbidite and shale interbeds within the diamictites indicate a deep marine environment with interbedded mud flows debris flows slumps and large displaced slabs providing evidence for sediment instability and resedimentation chitinozoan biostratigraphy indicates a llandovery age in contrast with previous proposals of a hirnantian age and suggests the need for more detailed sedimentologic and biostratigraphic studies and a reassessment of this diamictite unit in relation to adjacent regions in south america the resedimented character of the deposit explains some of the ordovician fauna previously described which must be considered as recycled from underlying units in situ fauna needs to be reassessed as it may be endemic diachronic and or indicative of migrations within gondwana glacially faceted and striated clasts as well as large granitoid boulders within the resedimented materials provide evidence for glaciation of the source area and are interpreted as recycled from former glacigenic deposits the evidence found in the central andes indicates a glaciomarine origin for the diamictites and corroborates glaciation of a western source area prior to the late llandovery telychian the precise age of glaciation in this part of gondwana cannot be confirmed do to a lack of true tillites tectonic deformation and the resulting relief are respectively identified as the origin for sediment instability and for local glaciation along the active margin of western gondwana during the late ordovician and early silurian the new data imply that the ordovician silurian boundary was not preserved in some areas of the basin and that an erosional hiatus disconformity with variable regional significance is present between different ordovician units and the diamictite unit late ordovician late caradoc early ashgill euxinic black shales locally present in the central altiplano and eastern cordillera of bolivia may represent the youngest ordovician unit in the region and more research should be focused on them paleogeographic reconstruction of the early silurian peru bolivia basin depicts the extension of the diamictite unit southwards into northern argentina and western paraguay connecting with the parana basin through the asuncion arch to the north it continued into northern peru and ecuador connecting with the silurian record in colombia and venezuela and to the northeast into brazil solimoes and amazonas basins the sedimentary record of glaciation in the peru bolivia basin was due to the development of local ice fields to the west of the basin related with tectonism and resulting reliefs along the active margin of gondwana and probably did not coincide with the development of the north african late ashgillian ice cap

atmospheric methane during the last four glacial interglacial cycles rapid changes and their link with antarctic temperature 1 atmospheric methane ch4 recorded in antarctic ice cores represents the closest ice proxy available for greenland temperature changes beyond times when greenland climate records are available the record over four climatic cycles from the vostok ice core offers the opportunity to study the phase relationship between greenland and antarctic climate changes through detailed ch4 profiles combining american and french analytical efforts we have improved the time resolution of the existing ch4 record from petit et al 1999 spectral analyses reveal high and low frequency variability including a strong precessional component the phase relationship between ch4 and the antarctic temperature proxy d shows a systematic lag of ch4 versus temperature by 1100 200 years on long timescales 50 400 kyr and a more complex behavior over shorter timescales i e 25 kyr suggesting that dansgaard oeschger type of climatic variability and associated interhemispheric linkage are robust features of late quaternary climate

on the origin and timing of rapid changes in atmospheric methane during the last glacial period we present high resolution records of atmospheric methane from the gisp2 greenland ice sheet project 2 ice core for four rapid climate transitions that occurred during the past 50 ka the end of the younger dryas at 11 8 ka the beginning of the bolling allerod period at 14 8 ka the beginning of interstadial 8 at 38 2 ka and the beginning of interstadial 12 at 45 5 ka during these events atmospheric methane concentrations increased by 200 300 ppb over time periods of 100 300 years significantly more slowly than associated temperature and snow accumulation changes recorded in the ice core record we suggest that the slower rise in methane concentration may reflect the timescale of terrestrial ecosystem response to rapid climate change we find no evidence for rapid massive methane emissions that might be associated with large scale decomposition of methane hydrates in sediments with additional results from the taylor dome ice core antarctica we also reconstruct changes in the interpolar methane gradient an indicator of the geographical distribution of methane sources associated with some of the rapid changes in atmospheric methane the results indicate that the rise in methane at the beginning of the bolling allerod period and the later rise at the end of the younger dryas were driven by increases in both tropical and boreal methane sources during the younger dryas a 1 3 ka cold period during the last deglaciation the relative contribution from boreal sources was reduced relative to the early and middle holocene periods

evidence of microbial consortia metabolizing within a low latitude mountain glacier carbon dioxide co 2 nitrous oxide n 2 o and methane ch 4 concentrations of ice samples from a south american mountain glacier are larger than those in coeval polar ice cores of the six possible mechanisms that may cause the composition of air measured in ice cores to depart from that of the overlying troposphere gravitational fractionation thermal fractionation gases trapped in refrozen meltwater atmospheric heterogeneity abiotic chemical reactions and biologically mediated chemical reactions only metabolism of a consortium of microorganisms within the ice can explain our data

organic remains in finnish subglacial sediments abstract many sites in fennoscandia contain pre late weichselian beds of organic matter located mostly in the flanks of eskers it is a matter of debate whether these fragmentary beds were deposited in situ or whether they were deposited elsewhere and then picked up and moved by glacial ice the till mantled esker of harrinkangas includes a shallow depression filled with sand and silt containing for example several tightly packed laminar sheets of brown moss bryales remains it is argued that these thin peat sheets were transported at the base of the ice sheet or englacially and were deposited together with the silt and sand on the side of a subglacial meltwater tunnel subglacial meltout till subsequently covered the flanks of the esker near the receding ice margin information about the depositional and climatic environments was obtained from biostratigraphic analysis of the organic matter pollen spectra for the peat represent an open birch forest close to the tundra zone a thin diamicton beneath the peat contains charred pine wood recording the former presence of pine forests in western finland the unhumified extremely well preserved peat evidently originated during the final phase of an ice free period most probably the end of the eemian interglaciation it was redeposited in the esker by the last ice sheet reconstructions of the pleistocene chronology and stratigraphy of central fennoscandia that rely on such redeposited organic matter should be viewed with caution

numerical modeling of advective transport of saturated deforming sediment beneath the lake michigan lobe laurentide ice sheet abstract we investigated the dynamics of advective transport of saturated fine grained deforming sediment beneath the late wisconsinan lake michigan lobe lml of the laurentide ice sheet using a numerical flowline model sediment rheology is modeled using a general constitutive law for mass movement of fine grained material geotechnical tests on selected tills deposited by the lml in northern illinois suggest an effective viscosity of about 5 109 pa s for fully saturated sediment and mild nonlinearity in the stress strain relation numerical calculations of sediment flux based on conservative parameters and assumptions yield estimates on the order of 100 m3 y meter width of the lobe suggesting that advective transport could account directly for at least 25 of the total sediment flux estimated on the basis of field evidence less restrictive parameters and assumptions would increase sediment flux attributable to both advective transport as well as other ice velocity dependent sediment transport processes modeling results also suggest that maximum advective transport rates might be associated with viscosities up to an order of magnitude higher than the viscosity exhibited by fully saturated sediment these results thus suggest that subglacial sediment deformation could account for a substantial portion of total sediment flux in areas with fine grained glacial till and that transport rates are strongly sensitive to the subglacial hydrologic processes that control basal water pressure and sediment viscosity

subglacial methanogenesis a potential climatic amplifier 1 subglacial environments are a previously neglected component of the earth s global carbon cycle a reflection of the view held until recently that they are dominated by abiotic and oxic conditions here we provide a realistic assessment of the theory that the basal regions of the ice sheets that formed over north america and europe during glaciations were host to significant populations of anaerobic microorganisms including methanogens able to metabolize organic carbon sequestered during interglacials and overridden during quaternary glacials in doing so we review the current evidence for subglacial methane release during deglaciation estimate the size of the subglacial reservoir of organic carbon soc and assess the amount of soc available to subglacial microbes and the likely pathways and rates of carbon turnover we then discuss the fate of subglacial methane and the potential impact of its release on atmospheric methane concentrations we calculate that the soc equates to 418 610 pg c and includes carbon from terrestrial soils vegetation peatlands lake and marine sediments the soc that is potentially available for microbial conversion to methane is smaller than this estimate due to 1 glacial erosion 2 accumulation of recalcitrant organic carbon compounds over time 3 conversion to carbon dioxide by aerobic anaerobic respiration and 4 incomplete conversion of labile organic matter to methane we estimate that the total soc available for conversion to methane is 63 pg c our estimates of methane production potentials span a wide range because of the current uncertainty surrounding subglacial metabolic rates we believe however that there is a strong likelihood that subglacial microbes could convert 63 pg of soc to methane during a glacial cycle if this were the case release of this methane from the ice sheet margins during retreat would need to be episodic in order to significantly impact atmospheric methane concentrations our findings suggest that it may well be important to consider subglacial environments as active components of the earth s carbon cycle conclusive determination of the potential impact of subglacial methane production on atmospheric methane concentrations during deglaciation however awaits more precise determination of the ability of subglacial microbes to degrade organic carbon components and their associated rates of metabolism under in situ conditions

subglacial sediment dispersal and till composition dispersal of subglacial sediment down ice from its bedrock source assumes the form of a negative exponential decay curve dispersal decay curves can be quantified by their decay constants half distances half lives and total transport distances mapped dispersal decay curves associated with pleistocene ice sheets in north america and europe identify several scales of dispersal ranging from local kilometers to continental 100 s to 1000 s km scales till composition reflects the various scales of dispersal local scales short transport distances result in tills that reflect underlying bedrock whereas continental scales long transport distances result in tills derived largely from sources far up ice a number of parameters proposed as influencing subglacial sediment dispersal and transport distances do not explain the range of scales observed although they may explain minor variations kilometers in transport distances topography and basal ice velocity sliding velocity are interpreted as

formation of immature asphalt from organic rich carbonate rocks i geochemical correlation abstract the formation of low maturity asphaltic oils immature asphalts from carbonate source rocks was investigated the senonian bituminous rocks sbr in israel are organic rich carbonate rocks that were deposited in a high productivity environment and they are part of a sedimentary sequence that also contains cherts and phosphorites the sbr were studied in different basins and the samples are from outcrops and three deep drillholes the bitumen contents of the rocks are exceptionally high up to 700 mg g c org and immature asphalts are found to be associated with them geochemical analysis indicates that the bitumens are indigenous to the host sbr and the associated asphalts are bitumens that have migrated a short distance with negligible chemical fractionation the high heteroatom content of the kerogens up to 30 in the sbr is suggested as a possible cause of the generation of asphaltic oils in an early stage of maturation

calculating the balance between atmospheric co2 drawdown and organic carbon oxidation in subglacial hydrochemical systems in order to constrain co2 fluxes from biogeochemical processes in subglacial environments we model the evolution of ph and alkalinity over a range of subglacial weathering conditions we show that subglacial waters reach or exceed atmospheric pco2 levels when atmospheric gases are able to partially access the subglacial environment subsequently closed system oxidation of sulfides is capable of producing pco2 levels well in excess of atmosphere levels without any input from the decay of organic matter we compared this model to published ph and alkalinity measurements from 21 glaciers and ice sheets most subglacial waters are near atmospheric pco2 values the assumption of an initial period of open system weathering requires substantial organic carbon oxidation in only 4 of the 21 analyzed ice bodies if the subglacial environment is assumed to be closed from any input of atmospheric gas large organic carbon inputs are required in nearly all cases these closed system assumptions imply that order of 10 g m 2 y 1 of organic carbon are removed from a typical subglacial environment a rate too high to represent soil carbon built up over previous interglacial periods and far in excess of fluxes of surface deposited organic carbon partial open system input of atmospheric gases is therefore likely in most subglacial environments the decay of organic carbon is still important to subglacial inorganic chemistry where substantial reserves of ancient organic carbon are found in bedrock in glaciers and ice sheets on silicate bedrock substantial long term drawdown of atmospheric co2 occurs

paleozoic and mesozoic geological history of northeastern africa based upon new interpretation of nubian strata 1 the nubia terrain in southern egypt and northern sudan is the type area for strata of mainly cretaceous age because nubian sandstone or nubian formation or similar terms were used for strata in other areas and of other stratigraphic positions the name nubian has become meaningless this paper summarizes results of field work done in egypt and sudan since 1976 reconstruction of the tectonics and paleogeography of northeast africa since the early paleozoic permits division of all strata related broadly to the nubian into three cycles the paleozoic cycle cambrian to early carboniferous is mainly marine this cycle was interrupted by the collision of gondwana with northern continents the resultant karoo cycle representing purely continental sedimentation exte ded from the late cretaceous until the early jurassic the disintegration of pangea led to a third cycle that began in the late jurassic and resulted in the marginal marine to continental strata of the nubian cycle that ranges in age from late jurassic to latest cretaceous

glacier motion dominated by processes deep in underlying till black rapids glacier is a 40 km long surge type glacier in the central alaska range in spring 1997 a wireline drill rig was set up at a location where the measured surface velocities are high and seasonal and annual velocity variations are large the drilling revealed a layer of subglacial till up to 7 m thick that is believed to be water saturated at one location a string of instruments containing three dual axis tiltmeters and one piezometer was successfully introduced into the till the tiltmeters monitored the inclination of the borehole at the ice till interface and at 1 and 2 m into the till for 410 days they showed that no significant deformation occurred in the upper 2 m of the till layer and no significant amount of the basal motion was due to sliding of the ice over the till the measured surface velocity at the drill site is about 60 m a 1 of which 20 30 m a can be accounted for by ice deformation almost the entire amount of basal motion 30 40 m a 1 was taken up at a depth of 2 m in the till possibly in discrete shear layers or as sliding of till over the underlying bedrock we propose that the large scale mobilization of such till layers is a key factor in initiating glacier surges

detrital sediment fluxes from continents to oceans abstract detrital sediment is carried from land to the sea by three agents rivers glaciers and winds the shoreline is an arbitrary boundary within the detrital sediment transport system which extends from a site of origin across areas of temporary storage to a site of long term deposition the most important of the agents moving sediment across the land is river transport estimated to be in the order of 20 10 12 kg of sediment annually at present analysis of drainage basins indicates that relief and runoff are the most important factors in determining the sediment load of rivers the competence of rivers to transport sediment is governed by the volume flow gradient and the sediment load itself today most large rivers are fed by snowmelt in highland areas runoff from rainfall in the drainage basin and groundwater inflow along the river course water is lost to evaporation and groundwater infiltration river courses can often be divided into two segments a degradational section in which the gradient is relatively steep and little temporary storage of sediment takes place and an aggradational section where the gradient is sharply reduced through meandering and where large scale temporary sediment storage forms a flood plain lakes trap sediment inland and prevent its transport to the sea today many high and mid latitude rivers are interrupted by lakes of glacial origin there are also some large areas of internal drainage that deliver no sediment to the sea the load carried by rivers has been markedly altered by human activity and may have doubled over the past few thousand years only to be reduced in the past century by the widespread construction of dams the ancient use of fire in hunting and its subsequent use in clearing land has increased erosion extensive deforestation and cultivation processes have also increased the sediment supply dam construction is a relatively new factor and affects the sediment transport system by trapping sediment before it can reach the sea the resulting lower sediment supply from rivers is at least in part compensated by increased coastal erosion glacial erosion is difficult to estimate there is an ongoing controversy whether ice sheets are effective erosive agents or not estimates of the present global flux of glacial detritus range from 0 8 50 10 12 kg annually with the lower value most probable the dust flux is in the order of 0 5 to 0 9 10 12 kg annually but may vary greatly with time

fresh arguments against the shaw megaflood hypothesis a reply to comments by david sharpe on paleohydraulics of the last outburst flood from glacial lake agassiz and the 8200 bp cold event

stratigraphy and ages of four early silurian through late devonian early and middle mississippian glaciation events in the parnaiba basin and adjacent areas ne brazil abstract turbidity currents tectonism debris flows glaciation and other geologic processes are considered to be responsible for the generation of silurian mississippian diamictites in northern brazil processes that best account for the features observed in these strata are glaciation deglaciation and sedimentary deformation four main glacial ages are identified to have occurred in the parnaiba basin during the paleozoic glacially influenced deposits are found in the lower silurian upper devonian lower mississippian and middle mississippian strata the parnaiba basin is characterized by outcrops providing robust lithological evidence for three glacial ages identified by lithological features and dated by palynological studies a fourth glaciation as old as tournaisian was recently recognized only in the subsurface of the basin the glacial features were illustrated and the rocks accurately dated with palynomorphs based on very accurate palynological studies additional palynological information derives from outcrops and shallow boreholes drilled by a private company themag and the national state mineral production department departamento nacional de producao mineral dnpm and the brazilian geological survey company companhia de pesquisa de recursos minerais cprm the purpose of this reexamination including many new illustrations interpretations and conclusions is to evaluate primary sedimentological and paleontological data related to the above mentioned glaciations some authors remain skeptical about the record of early to early late paleozoic glaciations in western gondwana especially in brazil mainly because evidence provided in the literature is relatively scanty the parnaiba basin provides special conditions for the study of these glaciations although evidence for each of them varies considerably as discussed in the text

n2o and ch4 variations during the last glacial epoch insight into global processes 1 greenhouse gas measurements along polar ice cores provide important insight into the former composition of the atmosphere its natural variations and the responses to fast climatic changes in the past we present high resolution nitrous oxide n2o and methane ch4 records measured along two ice cores from central greenland covering part of marine isotope stages 3 and 4 in the last glacial epoch the n2o data confirm the hypothesis that n2o shows variations in phase to fast climatic changes observed in the northern hemisphere the so called dansgaard oeschger d o events variations exist not only for events with a long duration 1500 years and more but also for the shorter ones the comparison with ch4 unveils interesting differences between the response of ch4 and n2o to d o events while the average amplitudes of ch4 oscillations associated with d o events are similar to those of the northern hemisphere summer insolation the magnitude of the n2o concentration change instead correlates with the duration of the d o events the records give further insight into the timing of concentration changes at the beginning of d o events they show that for long lasting events the n2o concentration starts to increase before both the sharp increase in the ch4 concentration and the temperature reconstructed for greenland

geochemical characteristics of organic rich shale upper yangtze basin implications for the late ordovician early silurian orogeny in south china abstract the tectonic sedimentary environment of the upper yangtze basin in china underwent significant changes from the late ordovician to early silurian the widely distributed shale formed during this interval has recorded key information about paleo ocean conditions analysis of major trace and rare earth elements was conducted on 135 shale samples collected from five wells in the study area the covariation of the mo u enrichment factors indicates that the upper yangtze basin in the wufeng stage was characterized by sedimentary differentiation the southern basin was dominated by a reducing bottom water environment under euxinic conditions the northern and central basins were dominated by a suboxic anoxic environment up to the early longmaxi stage the upper yangtze basin formed a relatively uniform sedimentary environment with gradually increasing oxygen content in the bottom water the paleoproductivity index indicates that marine organisms flourished in the wufeng to early longmaxi stages making an important contribution to the siliceous content in organic rich shale subsequently terrestrial siliciclastics gradually dominated analysis of paleogeography rare earth elements and weathering indicates that chuanzhong and qianzhong uplifts in the wufeng stage were the provenance area of the upper yangtze basin with a relatively high weathering degree weathering decreased in the beginning of the longmaxi stage and then gradually increased with the xuefeng uplift becoming the main provenance areas after the early longmaxi stage controlled by stress transfer from the continuous syn caledonian kwangsian orogeny in eastern south china the upper yangtze basin and xiangzhong basin were squeezed and pushed up successively the uplift separated the open shallow sea and formed a relatively limited deep water shelf environment resulting in a relatively euxinic bottom water environment which was favorable for organic matter accumulation the tectonic sedimentary evolution of the upper yangtze basin in the late ordovician to early silurian proposed in this study indicates that the formation of organic enriched shale was not only related to glacio eustatic change but also strongly controlled by tectonic activities in south china

the paleozoic petroleum geology of central arabia exploratory drilling in central saudi arabia has established significant paleozoic petroleum potential in this region in recent discoveries the permian interval of the unayzah formation has been identified as the primary oil and gas reservoir the unayzah formation consists of alluvial and fluvial to shallow marine siliciclastic sandstones and siltstones that were deposited on a major pre unayzah unconformity saudi arabian paleozoic oil is a high gravity 43 53 degrees api crude with low sulfur content usually less than 0 07 that is classified as arabian super light oil gas with condensate has also been discovered in the unayzah formation the identified paleozoic traps are moderate relief fault generated structures with generally 30 100 m of closure unayzah structural traps developed primarily during the triassic to early jurassic however an earlier phase of significant pre unayzah structural development is also indicated between the devonian and late carboniferous and is related to a hercynian structural event post triassic structure in central arabia is mainly regional and results principally from late cretaceous to tertiary development of the central arabian arch the established source rock is the basal qusaiba shale a widespread organic rich shale that was deposited following regional deglaciation in the early silurian migration occurred regionally updip through unayzah formation reservoirs from the underlying qusaiba shale subcrop and or vertically along fault bounded zones on the flanks of structures from the underlying qusaiba shale interbedded upper permian shales and evaporites provide a top seal for the unayzah and form the basal sequence of the overlying transgressive khuff carbonate pre qusaiba reservoir targets include the cambrian ordovician saq sandstone the ordovician qasim formation and upper ordovician to lower silurian periglacial clastics of the zarqa and sarah formations high gravity low sulfur oil has also been discovered in this pre qusaiba clastic section the reservoirs of the pre qusaiba structures are fault bounded and sources laterally by the down faulted qusaiba shale

how glaciers entrain and transport basal sediment physical constraints abstract simple insights from the physics of ice water and sediment place constraints on the possible sediment transport behavior of glaciers and ice sheets because glaciers concentrate runoff streams generated by glaciers transport much sediment and may erode bedrock rapidly deforming glacier beds also can transport much sediment particularly in marginal regions rapid sediment entrainment producing thick debris rich basal zones may occur by regelation into subglacial materials and by freeze on from rising supercooled waters numerous other mechanisms may be important but primarily near ice margins especially those of advancing or fluctuating glaciers several sediment entrainment mechanisms may be active beneath a single glacier but one process is likely to be dominant at any place and time

hydrogeology and redox geochemistry of ch4 in a late wisconsinan till and loess sequence in central iowa the hydrogeology and redox geochemistry of ch4 in two hydrogeological settings within a late wisconsinan till and loess sequence in central iowa were investigated using ch4 eh dissolved o2 no3 n nh4 n dissolved fe so4 h2s dissolved organic carbon doc particulate organic carbon poc 13c ch4 2h ch4 13c of dissolved inorganic carbon dic and 14c ch4 depleted 13c ch4 and 2h ch4 values enriched 13c dic values and the lack of higher hydrocarbons suggest that the ch4 is microbial and is formed by co2 reduction carbon 14 dates of ch4 indicate that 14 000 to 17 000 year old poc and doc in the 1oess and till compose the substrate for methanogenesis production of ch4 probably began after deglaciation at about 12 000 years b p and has continued to the present groundwater recharge particularly in the upland topographic position at the site has decreased ch4 concentrations in the upper part of the till

stable isotope evidence for microbial sulphate reduction at the bed of a polythermal high arctic glacier glacier beds may be host to a range of microbial communities which drive oxic waters towards anoxia along certain hydrological flowpaths chemical and isotopic signatures in meltwaters from finsterwalderbreen a polythermal glacier on sedimentary bedrock in svalbard show clear evidence for anoxia at the glacier bed increases in 34s and 18o of sulphate indicate that microbial sulphate reduction has resulted in significant decreases in sulphate concentration the 13c of the dissolved inorganic carbon dic is isotopically light 13c 8 which is consistent with the use of bedrock kerogen and or the necromass of sulphide oxidising bacteria as organic substrates for the sulphate reducing bacteria calculated rates of organic carbon mineralisation correspond to 10 of the total annual dic flux of the glacial meltwaters this microbial ecosystem is chemoautotrophically based ultimately being sustained by the kerogen and or bacterial necromass and sulphides in the bedrock this work suggests that glacier beds can be refugia for life when climatic and or atmospheric conditions are otherwise inclement and also supports the contention that microbial life is present in subglacial lake vostok

molecular characterization of dissolved organic matter associated with the greenland ice sheet abstract subsurface microbial oxidation of overridden soils and vegetation beneath glaciers and ice sheets may affect global carbon budgets on glacial interglacial timescales the likelihood and magnitude of this process depends on the chemical nature and reactivity of the subglacial organic carbon stores we examined the composition of carbon pools associated with different regions of the greenland ice sheet subglacial supraglacial proglacial in order to elucidate the type of dissolved organic matter dom present in the subglacial discharge over a melt season electrospray ionization esi fourier transform ion cyclotron resonance ft icr mass spectrometry coupled to multivariate statistics permitted unprecedented molecular level characterization of this material and revealed that carbon pools associated with discrete glacial regions are comprised of different compound classes specifically a larger proportion of protein like compounds were observed in the supraglacial samples and in the early melt season spring subglacial discharge in contrast the late melt season summer subglacial discharge contained a greater fraction of lignin like and other material presumably derived from underlying vegetation and soil these results suggest 1 that the majority of supraglacial dom originates from autochthonous microbial processes on the ice sheet surface 2 that the subglacial dom contains allochthonous carbon derived from overridden soils and vegetation as well as autochthonous carbon derived from in situ microbial metabolism and 3 that the relative contribution of allochthonous and autochthonous material in subglacial discharge varies during the melt season these conclusions are consistent with the hypothesis that given sufficient time e g overwinter storage resident subglacial microbial communities may oxidize terrestrial material beneath the greenland ice sheet

what is the deepest part of the vostok ice core telling us abstract this review paper is mainly concerned with a geochemical investigation of the deepest part of the vostok ice core between 3310 m the depth at which the palaeoenvironmental record present in the ice above is lost and the bottom of the core about 130 m above subglacial lake vostok two sections constitute this part of the core the upper section 3310 3539 m depth still consists of ice of meteoric origin but subjected to widespread complex deformation this deformation is analysed in light of a d deuterium excess diagram and information on microparticles crystal sizes and chemical elements distributions in that part of the core such ice deformation occurred when the ice was still grounded upstream from vostok station in a region with subfreezing temperatures the lower section from 3539 m to the bottom of the core at 3623 m depth is lake ice formed by freezing of subglacial lake vostok waters this is indicated by the isotopic properties d 18o and deuterium excess by electrical conductivity measurements ecm crystallography and gas content of the ice these ice core data together with data on ionic chemistry favour an origin of the lake ice by frazil ice generation in a supercooled below pressure melting point water plume existing in the lake followed by accretion and consolidation by subsequent freezing of the host water the helium profile of this deepest part of the vostok core is quite unusual and surprising it has important implications for the interactions between the ice sheet and the lake two constrasting scenarios can be satisfactorily constructed so that the lake residence time is not well constrained

microbial methane in the shallow paleozoic sediments and glacial deposits of illinois u s a abstract methane formed by the microbial decomposition of buried organic matter is virtually ubiquitous in the groundwaters of illinois chemical and carbon isotopic compositions are reported for gas samples collected from over 200 private and municipal water wells and from 39 small gas wells completed in glacial deposits drift gas wells carbon and hydrogen isotopic data for methane carbon dioxide and water show that these gases were formed by the carbon dioxide reduction pathway the same mechanism which has been previously shown to be responsible for microbial methane formation in the marine environment the isotopic composition of methane in these samples can be closely correlated with the chemical composition of the gas and with water chemistry the data are interpreted as indicating that isotopically very light methane is found in waters where the residence time of groundwater in the methanogenesis zone was very short relative to the methane production rate

the silurian glaciation in the eastern parnaiba basin brazil paleoenvironment sequence stratigraphy and insights for the evolution and paleogeography of west gondwana abstract glacial strata are excellent stratigraphic markers in paleozoic basins worldwide the eastern parnaiba basin in northern brazil is a highly favorable setting for studying episodes related to the ordovician silurian glaciation previous studies in this region led to the recognition of unequivocally glaciogenic sediments associated with pre and postglacial deposits favoring the understanding of lower paleozoic events that occurred in west gondwana outcrop based facies and stratigraphic analyses of the 170 m thick siliciclastic succession of the serra grande group indicate fluvial glacial and coastal depositional systems seventeen sedimentary facies are grouped into seven facies associations fas fa1 interpreted as intermediary sheet braided plain deposits that consist of tabular cross bedded coarse grained sandstones and conglomerates fa2 fa4 which correspond to subaqueous glacial deposits comprising massive to stratified diamictites cross laminated sandstones diamictites and organic matter rich black shales with dumpstone and dropstone structures fa5 fa6 constituting organic matter rich black shales fine to medium grained sandstones with sigmoidal cross bedding and occurrences of the ichnotaxon arthrophycus interpreted as delta front deposits and fa7 composed of medium to coarse grained pebbly sandstones interpreted as channelized braided plain deposits this interpretation partially confirms the previous paleoenvironmental interpretation discounting the presence of subaerial alluvial fan and outwash plain deposits for this unit the sequence stratigraphy of the serra grande group is refined mainly using the coherent interpretation of the key surfaces and comparing the systems tracts with the global sea level curve providing a more robust third order sequential evolutionary model that includes three depositional sequences the middle ordovician sheet braided deposits are the lowstand sediments of sequence 1 truncated by an extensive unconformity that possibly removed the transgressive and highstand strata this unconformity generated by glacial dynamics represents 23 myr during the late ordovician to early silurian the second sequence started with the advance of ice sheets in marginal areas of the basin causing the deposition of ablation diamicton during the lowstand the postglacial transgression is marked by ice rafted debris from icebergs deposited onto organic matter rich mud in the shoreface offshore zone the maximum flooding during the middle to late silurian was succeeded by the highstand phase marked by the progradation of deltaic deposits biological activity is marked by the presence of polychaetes that reworked the marine seafloor the early devonian featured extensive sea level fall and renewed continental drainage with the formation of a braided plain marking a lowstand phase of sequence 3 these new interpretations allow the global correlation of the serra grande group and provide an improved understanding of the role of ice sheets and postglacial transgressions that affected west gondwana during the ordovician silurian periods

a pre hirnantian late ordovician interval of global cooling the boda event re assessed facies analysis suggests that the early mid ashgill late ordovician boda event of fortey and cocks fortey r a cocks l r m 2005 late ordovician global warming the boda event geology 33 5 405 408 was an episode of global cooling not of warming as they proposed limestones are widely developed at this level from mid latitude to high latitude commonly interrupting thick ordovician successions of fine clastic sediments bryozoan carbonate mud mounds in a laterally extensive cool water carbonate belt along the high palaeolatitude margin of w gondwana are associated with glacial diamictites and grew during glacial lowstand with sea ice prior to the hirnantian glaciation from high to mid palaeolatitudes contemporaneous cool water carbonates with mud mounds including stromatactis both in gondwana and in baltica developed in mid outer ramp settings during regression in the dominantly clastic welsh basin of avalonia phosphatic facies on an iapetus facing carbonate ramp indicate upwelling cold waters down ramp yet up ramp coralliferous algal limestones suggest warm surface waters we propose that the late ordovician cool water carbonates developed in response to global cooling and expansion of gondwanan terrestrial and sea ice that lowered eustatic sea level and on the w gondwanan shelf reduced or shut down fluvial clastic input previously poorly circulated salinity stratified oceans that had restricted benthic dispersal in mid low latitudes were replaced by thermohaline ocean circulation that oxygenated bottom waters and dispersed benthic faunas an earlier caradoc carbonate interval in low mid palaeolatitudes that shares many facies characteristics with the ashgill cool water limestones and correlates with a widely recognised isotope excursion is interpreted as indicating an earlier global cooling episode

limnological conditions in subglacial lake vostok antarctica subglacial lake vostok is located 4 km beneath the surface of the east antarctic ice sheet and has been isolated from the atmosphere for 15 million yr concerns for environmental protection have prevented direct sampling of the lake water thus far however an ice core has been retrieved from above the lake in which the bottom 85 m represents lake water that has accreted i e frozen to the bottom of the ice sheet we measured selected constituents within the accretion ice core to predict geomicrobiological conditions within the surface

high resolution record of northern hemisphere climate extending into the last interglacial period high resolution record of northern hemisphere climate extending into the last interglacial period

frozen bed fennoscandian and laurentide ice sheets during the last glacial maximum the areal extents of the laurentide and fennoscandian ice sheets during the last glacial maximum about 20 000 years ago are well known1 but thickness estimates range widely from high domed2 to thin3 with large implications for our reconstruction of the climate system regarding for example northern hemisphere atmospheric circulation and global sea levels this uncertainty stems from difficulties in determining the basal temperatures of the ice sheets and the shear strength of subglacial materials4 a knowledge of which would better constrain reconstructions of ice sheet thickness here we show that in the absence of direct data the occurrence of ribbed moraines in modern landscapes can be used to determine the former spatial distribution of frozen and thawed bed conditions we argue that ribbed moraines were formed by brittle fracture of subglacial sediments induced by the excessive stress at the boundary between frozen and thawed bed conditions resulting from the across boundary difference in basal ice velocity maps of glacial landforms from aerial photographs of canada and scandinavia reveal a concentration of ribbed moraines around the ice sheet retreat centres of quebec keewatin newfoundland and west central fennoscandia together with the evidence from relict landscapes that mark glacial areas with frozen bed conditions the distribution of ribbed moraines on both continents suggest that a large area of the laurentide and fennoscandian ice sheets was frozen based and therefore high domed and stable during the last glacial maximum

nitrate in greenland and antarctic ice cores a detailed description of post depositional processes a compilation of nitrate no 3 data from greenland has shown that recent no 3 concentrations reveal a temperature dependence similar to that seen in antarctica except for sites with very low accumulation rates lower temperatures tend to lead to higher no 3 concentrations preserved in the ice accumulation rate which is closely linked to temperature might influence the concentrations preserved in snow as well but its effect cannot be separated from the temperature imprint processes involved in no 3 deposition are discussed and shown to be temperature and or accumulation rate dependent apart from scavenging of nitric acid hno 3 during formation of precipitation uptake of hno 3 onto the ice crystal s surface during and after precipitation seems to contribute further to the no 3 concentrations found in surface snow post depositional loss of no 3 from the top snow layers is caused by release of hno 3 and by photolysis of no 3 it is suggested that photolysis accounts for considerable losses at sites with very low accumulation rates depending on the site characteristic and given that the temperature and accumulation rate dependence is quantified it should be possible to infer changes in atmospheric hno 3 concentrations

lower silurian hot shales in north africa and arabia regional distribution and depositional model abstract lowermost silurian organic rich hot shales are the origin of 80 90 of palaeozoic sourced hydrocarbons in north africa and also played a major role in petroleum generation on the arabian peninsula in most cases the shales were deposited directly above upper ordovician peri glacial sandstones during the initial early silurian transgression that was a result of the melting of the late ordovician icecap deposition of the main organic rich shale unit in the north african arabian region was restricted to the earliest silurian rhuddanian stage acuminatus atavus and probably early cyphus graptolite biozones during this short period 1 2 m y a favourable combination of factors existed which led to the development of exceptionally strong oxygen deficiency in the area in most countries of the study area the post rhuddanian silurian shales are organically lean and have not contributed to petroleum generation the distribution and thickness of the basal silurian hot shales have been mapped in detail for the whole north african region using logs from some 300 exploration wells in libya tunisia algeria and morocco in addition all relevant accessible published and unpublished surface and subsurface data of the lower silurian shales in north africa and arabia have been reviewed including sedimentological biostratigraphic and organic geochemical data the lowermost silurian hot shales of northern gondwana are laterally discontinuous and their distribution and thickness were controlled by the early silurian palaeorelief which was shaped mainly by glacial processes of the late ordovician ice age and by pan african and infracambrian compressional and extensional tectonism the thickest and areally most extensive basal silurian organic rich shales in north africa occur in algeria tunisia and western libya while on the arabian peninsula they are most prolific in saudi arabia oman jordan and iraq the hot shales were not deposited in egypt which was a large palaeohigh at that time the depositional model presented may help in better understanding the source potential of the basal silurian shales in less explored regions of north africa and arabia including morocco northern niger and the kufra basin in southeast libya

ice sheet numerical modeling and marine geophysical measurements of glacier derived sedimentation on the eurasian arctic continental margins long range side scan sonar images of the barents sea continental margin have been analyzed in conjunction with results from previous geophysical investigations to determine a qualitative model for sedimentation over the bear island and storfjorden trough mouth fans these data indicate that gravity driven debris flows are major processes in the downslope transport of glacial material delivered to the shelf break when ice sheets advanced across the continental shelf during late weichselian time 4000 km 3 of sediments were deposited over the bear island fan 280 000 km 2 while 700 km 3 of sediments were deposited over the storfjorden fan 40 000 km 2 a numerical ice sheet model including sediment deformation and transport beneath ice streams reconstructs the glacial conditions required to transport large volumes of sediment to the late weichselian eurasian continental margin model results indicate that glaciation of the eurasian high arctic occurred after 28 ka and that ice streams within bathymetric troughs were active by ca 25 ka the maximum ice sheet thickness over the barents sea was about 1400 m there was a secondary dome 1 0 13 cm yr 1 averaged over the fan the rate was 6 cm yr 1 equivalent to 0 6 cm yr 1 over the fan over the storfjorden trough mouth the modeled sediment volume at the continental margin of the bear island and storfjorden troughs agrees well with the volumes of late weichselian sediment inferred from seismic records from these large prograding submarine fans sensitivity experiments show that adjustments to model environmental inputs do not significantly affect the results

magnitude and significance of carbon burial in lakes reservoirs and peatlands globally lakes are currently accumulating organic carbon oc at an estimated annual rate of about 42 tg yr 1 most of the oc in all but the most oligotrophic of these lakes is autochthonous produced by primary production in the lakes the sediments of reservoirs accumulate an additional 160 tg annually and peatlands contribute 96 tg annually these three carbon pools collectively cover less than 2 of the earth9s surface and constitute a carbon sink of about 300 tg yr 1 although the oceans cover 71 of the earth9s surface they accumulate oc at a rate of only about 100 tg yr 1

subglacial erosion and englacial sediment transport modelled for north american ice sheets abstract reconstructions of north american glacial history have either been based on interpretation of the geologic record or on numerical models of continental ice dynamics because such models make no explicit reference to the geological products of glaciation the full potential of a combined approach has not been realized we present the results of a process based model of sediment production entrainment transport and deposition that represents a first step toward uniting these two lines of investigation in this model basal ice processes are dependent on ice sheet geometry hydrology and dynamics a three dimensional thermomechanical ice sheet model yields the ice geometry flow and thermal conditions that are used by a model of subglacial hydrology together these drive the basal process model comparisons of simulation results with mapped debris distributions of the hudson bay paleozoic carbonates and of the distinctive dubawnt group are used to test the validity of the transport model dispersion of debris radially out of hudson bay is greatly overestimated by the model and dispersion toward the bay equally underestimated this is because the modelled ice sheet has a single dome over hudson bay at glacial maxima including last glacial maximum rather than a saddle separating ice domes over quebec labrador and keewatin estimated rates of glacial erosion for the cordilleran and laurentide ice sheets are compared against those predicted by the model the model underestimates these rates because it underestimates the vigour of the quarrying process

a long term perspective on glacial erosion a rough estimate of glacial erosion in the precambrian basement is related to the total denudation of bedrock since its formation in the proterozoic the basis for the calculations is reference to surfaces of different age glacial erosion is responsible for merely polishing the northern shields major denudation occurred during the proterozoic and amounted to tens of kilometres during long periods the basement was protected by palaeozoic cover rocks it was re exposed successively during the mesozoic and tertiary with denudation amounting to 600 m at the most glacial erosion can generally be counted in tens of metres though with great variations 1997 by john wiley sons ltd

tree ring dated little ice age histories of maritime glaciers from western prince william sound alaska tree ring studies at 13 glacier forefields in western prince william sound show little ice age glacial fluctuations were strongly synchronous on decadal timescales cross dated glacially overrun trees at eight sites indicate ice margins advanced in the early late twelfth through thirteenth centuries and middle seventeenth to early eighteenth centuries little ice age tree ring dates of 22 moraines at 13 glaciers show two main periods of stabilization the earlier of these in the first decades of the eighteenth century overlaps with the second period of glaciers overrunning trees and marks culmination of this middle little ice age expansion stabilization of moraines on nine of the study forefields in the latter part of the nineteenth century delineates a third interval of little ice age glacial advance the detailed little ice age record from land terminating glaciers in western prince william sound is consistent on a timescale of decades with four other tree ring dated glacial histories from across the northern gulf of alaska this coastal northeastern pacific glacial record reveals the structure of the little ice age in the region and provides a strong basis for comparison with other proxy climate records spanning the past 1000 years

some indication for late quaternary pechora river discharge results of vibrocore studies in the southeastern pechora sea the study of upper quaternary sedimentary sequences in the pechora sea by means of vibrocoring revealed three sediment units the youngest sandy unit accumulated in marine environments later than 5000 years b p the clayey unit belongs to a pro deltaic facies probably formed during middle holocene times the diamict unit formed during middle early weichselian times probably has a glacigenic origin and is separated from the clayey unit by a hiatus the changes in lithology are partly related to changes in pechora river discharge during late weichselian holocene times

is a periglacial biota responsible for enhanced dielectric response in basal ice from the greenland ice core project ice core a detailed dielectric profiling dep conductivity profile measured in the 6 m of the basal silty ice sequence from the greenland ice core project grip ice core summit central greenland is presented and compared to previous multi parametric studies dep conductivities span the whole glacial interglacial range observed higher up in the grip core 9 25 s m 1 values in the bottom meter of the sequence reach the level of some of the highest peaks from holocene volcanic layers in the core 33 s m 1 on a steady increase of the values down the sequence are superimposed large fluctuations inphase with other variables measured in the core such as 18o debris content or gas compositions in co2 and ch4 analysis of the type and strength of intercorrelations shows that the controlling variable for the dep signal must be closely related to the gas content and composition of the ice plausible candidates for this causality link are investigated enhancing of the conductivity by co2 and ch4 encaged in the ice lattice as gas hydrates is ruled out since these are nonpolar clathrates of structure i known as having negligible impact on the orientational stability of the water molecules under ac currents nh4 is proposed as the best candidate since it has been shown to enhance dep conductivities by introducing bjerrum defects in the ice lattice and since it could have been initially present partly as gaseous nh3 in the ice this proposition is supported by the nh4 profile in the basal ice sequence using calibration curves from higher up in the core it is shown that is in fact fully explained by intracrystalline conductivity of pure ice solely disrupted by ammonium impurities in the ice lattice the origin of the nh4 signal is discussed in the light of organic acid profiles formate acetate and oxalate it appears that the most likely source is local degradation of biological residues which supports the hypothesis that part of the basal ice was formed locally in the absence of the present day ice sheet

abrupt deglaciation events and holocene palaeoceanography from high resolution cores cartwright saddle labrador shelf canada the uppermost quaternary sediments in cartwright saddle labrador shelf are acoustically laminated with reflectors that can be traced over long distances two piston cores from the saddle record changes in sediment and meltwater delivery from the northeast margin of the laurentide ice sheet lis during deglaciation variations in sediment properties indicate a similar history of sediment accumulation over the last 12 kyr the temporal sampling interval reaches decadal resolution in the last deglacial period 7 9 ka analyses of total carbonate content sediment magnetic variables foraminiferal species and stable isotope measurements on planktic foraminifers show that abrupt changes occurred ca 10 9 9 2 8 8 7 9 and 7 ka with 450 yr correction there was no distinct change in sediment character during much of the younger dryas chronozone in the 18o record the 8 8 ka event is a dramatic 1 shift toward lower values which we associate with the noble inlet glacial event within hudson strait we do not see the pronounced low 18o event at 7 1 ka reported off nova scotia but surprisingly neither the nova scotia records nor other records in the labrador sea capture the impressive 8 8 ka change serious consideration must be given to the final collapse of the lis as the cause of the 8 2 cal ka cold event recorded in greenland and northwest europe copyright 1999 john wiley sons ltd

the end of the ice age methane gas hydrates may have contributed to the rapid rise in atmospheric ch4 co2 and global temperatures at the end of the last major glaciation about 13 500 years ago given suitable orbital conditions and insolation at high latitude a small triggering event such as the release of one or more arctic gas pools may have initiated massive release of methane from hydrate under ice and in permafrost the consequent greenhouse warming would have provided strong positive feedback amplifying emission this warming driven by ch4 may have induced the release of co2 from the oceans to the biosphere stabilizing the interglacial carbon cycle

some northern sources of atmospheric methane production history and future implications northern sources including wetlands and perhaps gas hydrates contribute significantly to the ch4 content of the atmosphere methane production from northern wetlands including bogs swamps and ponds is probably very seasonal being most important in late summer with significant evasion in autumn as lakes overturn the strong recovery of beaver populations in canada from near extinction 50 years ago to present abundance may also be important both in creating new wetlands and in the alteration of them wetlands that have been altered by beaver activity produce orders of magnitude more methane than beaver free wetlands in the arctic methane gas hydrates represent a significant source of methane which may become more important if arctic warming occurs as part of global climate change the danger of a thermal runway caused by ch4 release from permafrost is minor but real other high latitude sources of ch4 include arctic peat bogs and losses from natural gas production especially in the soviet u

the middle rhuddanian lower silurian hot shale of north africa and arabia an atypical hydrocarbon source rock an important hydrocarbon source rock a hot shale has been identified from the middle rhuddanian llandovery lower silurian of a number of localities in north africa and arabia it is unusual in having been deposited during a eustatic fall in sea level evidence for this sea level fall in libya and jordan is manifested in a wide variety of palynological proxy data including reduced acritarch and chitinozoan total abundance reduced acritarch diversity and changes in the relative abundances of various environmentally sensitive acritarch morphogroups and from the presence of a minor positive 13corg excursion at the same stratigraphical level as the hot shale the environment of deposition was anoxic before during and after deposition of the hot shale except for some very brief incursions of more oxygenated water that enabled the development of a very limited burrowing benthos and graptolite preservation as three dimensional pyrite internal moulds these incursions occurred just before and during deposition of the hot shale and are interpreted as resulting from the influx of sediment laden oxygenated currents from shallower water environments enhanced organic matter preservation within the hot shale is attributed to greater productivity resulting from increased nutrient input particularly from wind blown dust more rapid descent through the water column of organic matter in post bloom marine snow macroaggregates development of geochemically sealing microbial mats at the sea floor and more rapid burial of organic material than had occurred earlier in the rhuddanian

lower palaeozoic unconformities in an intracratonic platform setting glacial erosion versus tectonics in the eastern murzuq basin southern libya the stratigraphic record of the eastern murzuq basin has been importantly influenced by deformation resulting in angular and or deeply erosional unconformities though the overall context is intracratonic major transgressive events and the ordovician glaciation are nevertheless documented allowing the delineation of tectonic eustasy or climate driven unconformities lower palaeozoic key events and related unconformities that characterize the north gondwana platform have therefore a signature in the eastern murzuq basin the basement cover unconformity also known as the infra tassilian surface truncates all the deformed and metamorphosed lower cambrian and older rocks above is a middle cambrian to lower ordovician megasequence murizidie and hasawnah fms which is in turn truncated by an intra ordovician angular unconformity this megasequence is unconformably overlain by a middle ordovician hawaz fm to silurian tanzzuft and akakus fms megasequence which includes the upper ordovician glaciogenic unit mamuniyat fm bounded at the base by a polygenic glacial erosion surface showing corrugated glacial lineations tillites and glaciotectonic structures the middle ordovician to silurian megasequence is finally truncated by a base devonian angular unconformity overlain by fluvial sandstones regarding the possibility that those fluvial deposits may be as younger as late devonian in the eastern murzuq basin based on palaeoflora the so called caledonian unconformity might be here a much younger mid eifelian surface and the occurrence of the lower devonian tadrart fm is questioned the upper ordovician glacial erosion surface which is sometimes referred to as the taconic unconformity usually truncates middle ordovician strata in the murzuq basin but reaches significantly deeper stratigraphic levels in places that have been previously involved in the intra ordovician deformation event in the murizidie southeastern murzuq basin the infra tassilian surface the intra ordovician unconformity and the upper ordovician glacial erosion surface amalgamate together here an estimate of the glacial erosion depth cannot be derived from the stratigraphic hiatus beneath the glacial incision the main part of which relate to the intra ordovician tectonic event the upper ordovician climate related glacial erosion surface is not a valid unconformity for a sequence hierarchy framework of the lower palaeozoic although it presents most of the physical attributes of tectonic driven unconformities

did lingering ice sheets moderate anoxia in the early palaeozoic of libya the hirnantian glaciation of west gondwana produced a glacially sculpted topography which is draped by organic rich latest ordovician and early silurian hot shales although these are the most important early palaeozoic source rock in north africa organic enrichment is distributed unevenly for example in al kufrah basin libya hot shales are elusive but outcrop analysis at the western basin demonstrates why this is the case the topmost mamuniyat formation of hirnantian age comprises glaciogenic sandstones passing upward into mixed facies of the tanezzuft formation which has a latest ordovician early silurian age the basal tanezzuft formation contains a shelly carbonate cool water deposits accumulated under oxygenating conditions and bioturbated sandstone succession above hummocky cross bedded and graded sandstone intervals are intercalated with shale and siltstone storm influx onto a muddy shelf these are interrupted by several lonestone bearing intervals ice rafted debris a striated pavement of subglacial origin and manganese oxide crusts and concretions the concretions and bioturbation imply oxygenation of the sea floor during transgression these putative glacial deposits were deposited following the main phase of the hirnantian glaciation at the same stratigraphic level as hot shales elsewhere in northern gondwana lingering ice caps may have produced well oxygenated marine waters precluding hot shale deposition

the jebel hadid structure al kufrah basin se libya a possible impact structure and potential hydrocarbon trap abstract the jebel hadid structure that formed in the nubian sandstone of the southern al kufrah basin is a 4 7 km circular feature with a set of multiple concentric annular ridges the age of the structure is probably post early cretaceous to pre pliocene as the prevalent geological processes that usually account for the formation of circular structures magmatism diapirism sand volcanism karst dissolution glacial erosion can be widely excluded in this region we suggest that the jebel hadid structure might represent an eroded complex impact structure in particular the jebel hadid structure bears a strong morphological resemblance to the impact structure of tin bider in algeria general host rock properties may be compared to those at the upheaval dome impact structure usa numerous terrestrial impact structures are hosts to hydrocarbon reservoirs this investigation provides a first prospective approach based on remote sensing discussing the economic potential of a circular structure of suspected impact origin the still underexplored al kufrah basin is considered as a high potential area for future hydrocarbon exploration if we consider a possible impact origin of the jebel hadid structure this feature might provide local secondary pathways and traps for hydrocarbon migration accumulation in the nubian series and thus ought to be considered as a preferential site for oil and gas exploration

balancing the deglacial global carbon budget the hydrate factor abstract the discovery that methane from dissociation of gas hydrates could be an important factor in the global carbon cycle resolves the major discrepancy in estimates of the increase of terrestrial biomass from the last glacial period to the present carbon isotope budgeting using the marine carbon isotopic record results in an estimate centered around 500 gtc whereas palaeovegetation reconstruction with biosphere models gives averages around 1000 gtc the discrepancy may be resolved by considering release of isotopically light methane through destabilization of gas hydrates this provides a unique means of estimating the contribution of gas hydrates to the deglacial rise in atmospheric methane a release of 120 gtc methane makes a biospheric carbon transfer of 1000 gtc compatible with the marine carbon isotope data this however represents less than 30 of the enhanced atmospheric methane production between 18 and 8 ka observed in ice cores supporting the theory that glacial interglacial variations in atmospheric methane were driven primarily by changes in the extent of tropical and temperate wetlands and not by methane release from clathrates by balancing the deglacial carbon budget we demonstrate that global carbon models will have to incorporate glacial interglacial vegetation shifts of at least 1000 gtc which many currently find difficult

laurentide ice streaming on the canadian shield a conflict with the soft bedded ice stream paradigm elucidating the controls on the location and vigor of ice streams is crucial to understanding the processes that lead to fast disintegration of ice flows and ice sheets in the former north american laurentide ice sheet ice stream occurrence appears to have been governed by topographic troughs or areas of soft sediment geology this paper reports robust evidence of a major paleo ice stream over the northwestern canadian shield an area previously assumed to be incompatible with fast ice flow because of the low relief and relatively hard bedrock a coherent pattern of subglacial bedforms drumlins and megascale glacial lineations demarcates the ice stream flow set which exhibits a convergent onset zone a narrow main trunk with abrupt lateral margins and a lobate terminus variations in bedform elongation ratio within the flow set match theoretical expectations of ice velocity in the center of the ice stream extremely parallel megascale glacial lineations tens of kilometers long with elongation ratios in excess of 40 1 attest to a single episode of rapid ice flow we conclude that while bed properties are likely to be influential in determining the occurrence and vigor of ice streams contrary to established views widespread soft bed geology is not an essential requirement for those ice streams without topographic control we speculate that the ice stream acted as a release valve on ice sheet mass balance and was initiated by the presence of a proglacial lake that destabilized the ice sheet margin and propagated fast ice flow through a series of thermomechanical feedbacks involving ice flow and temperature

origin and formation of methane in groundwater of glacial origin from the cambrian vendian aquifer system in estonia abstract groundwater in the cambrian vendian aquifer system in estonia is characterised by the most depleted isotopic composition known in europe 18o down to 23 the water most likely originates from glacial meltwater recharge from the fennoscandian ice sheet in the pleistocene the aquifer system is characterised by relative high methane concentrations up to 50 of dissolved gases estimated absolute concentration up to 1600 mol l 1 the origin of which has so far remained unclear in this paper we focus on the origin of methane the factors controlling its spatial distribution and its isotope geochemistry in the aquifer system the data reveal a large spatial variability in methane concentration 13cch4 and 2hch4 values from 6 to 105 and from 220 to 420 respectively we show that local oxidation processes rather than different pathways of methane formation have affected the initial isotopic composition of methane using the least modified 13cch4 values from 85 to 105 we conclude that methane most likely originates from the organic material overridden by the fennoscandian ice shield during the late weichselian glaciation that was carried into the aquifer system with infiltrating glacial meltwater the estimated 18o values of the water where the methane was formed are 17 1 5 supporting the inference that the methane was formed during the middle weichselian interstadial the study shows that groundwater of glacial origin in the cambrian vendian aquifer system can serve as an alternative palaeoenvironmental archive to be used for studying the variations in climatic and environmental conditions in northern europe during glacial interglacial cycles of the pleistocene

diagenetic changes of lignin compounds in a more than 0 6 million year old lacustrine sediment lake biwa japan a vertical profile of lignin in the upper 700 m layer of a 1400 m sediment core of lake biwa an oligotrophic freshwater lake in japan was determined using a cuo oxidative degradation method the results indicated that lignin is found throughout the core demonstrating lignin to be very stable for over 0 6 million years moreover the upper 250 m approximately 0 6 million years old segment of the sediment core was investigated to determine the apparent long term degradation rate of lignin a downward lignin concentration decrease is observed over the upper 250 m of the core which corresponds to a calculated half life of at least approximately 40 104 years assuming that lignin decrease is due to its in situ degradation diagenesis

carbon and oxygen isotopic composition of silurian brachiopods gotland sweden palaeoceanographic implications abstract one hundred and seventy two non luminescent brachiopod shells from the silurian sequence of gotland were investigated for their carbon and oxygen isotopic composition diagenetic alteration of the shell calcite was excluded by means of petrographic trace element and cathodoluminescence analysis oxygen and carbon isotopic signals vary considerably within the uppermost llandovery to the ludlow average 18o and 13c values of well defined stratigraphic units range from 2 4 to 5 9 and 1 to 8 respectively we observed parallel trends in the carbon and oxygen isotope curves with the most prominent shifts occurring in the lower wenlock upper wenlock and middle ludlow average 18o values 2 4 to 2 9 recorded in the middle ludlow are among the highest 18o values reported for the early palaeozoic the recorded 18o variations cannot solely be explained by changes in sea surface temperature and salinity the variations in the isotopic signals correlate negatively with changes in sea level heavy carbon and oxygen signals coincide with sea level lowstands whereas light signals are recorded during sea level highstands sea level highstands were probably characterized by the formation of warm saline waters on subtropical epicontinental shelves inducing oxygen deficiency in the basins alumshale facies the low 18o values are attributed to 18o depletion in surface waters owing to the storage of 18o enriched saline waters in the basins low 13c values are explained by a low surface water productivity since nutrients were not supplied to the photic zone but rather stored in the deeper ocean during sea level lowstands colder waters that formed in higher latitudes may have represented the main intermediate water source a better ventilation of the basins is documented by the deposition of well aerated sediments whereas the high 18o values are explained by the change in oceanic circulation the high 13c values are attributed to an enhanced primary productivity and possibly to an enhanced burial of corg the model discussed here may provide an attractive scenario for accounting for the recorded short term variations in the isotopic signals without the necessity of introducing an isotopic correction factor for silurian sea water

calendar dated early little ice age glacier advance at robson glacier british columbia canada dendrochronological studies at robson and bennington glaciers have provided the first calendar dating of an early little ice age glacier advance in north america dates derived from in situ stumps indicate that robson glacier began over riding forest between c ad 1142 and 1150 and continued until at least ad 1350 the highest rates of glacier advance c 3 8 m yr 1 occurred between ad 1214 and 1261 based on a series of kill dates from in situ stumps between c ad 1142 1214 and 1261 1350 rates were somewhat lower at c 2 2 5 m yr 1 this glacier advance terminated upvalley of the little ice age maximum position of robson glacier and there is no evidence at this site for glacial events between ad 1350 and 1783 however two smaller glaciers in the robson area and the main tongue of small river glacier have outermost little ice age moraines that date from the first decades of the ad 1700s all glaciers examined in the mount robson area have several well developed moraines dating from the period c

fennoscandian palaeoglaciology reconstructed using a glacial geological inversion model the evolution of ice sheet configuration and flow pattern in fennoscandia through the last glacial cycle was reconstructed using a glacial geological inversion model i e a theoretical model that formalises the procedure of using the landform record to reconstruct ice sheets the model uses mapped flow traces and deglacial melt water landforms as well as relative chronologies derived from cross cutting striae and till lineations as input data flow trace systems were classified into four types i time transgressive wet bed deglacial fans ii time transgressive frozen bed deglacial fans iii surge fans and iv synchronous non deglacial event fans using relative chronologies and aggregation of fans into glaciologically plausible patterns a series of ice sheet configurations at different time slices was erected a chronology was constructed through correlation with dated stratigraphical records and proxy data reflecting global ice volume geological evidence exists for several discrete ice sheet configurations centred over the scandinavian mountain range during the early weichselian the build up of the main weichselian fennoscandian ice sheet started at approximately 70 ka and our results indicate that it was characterised by an ice sheet with a centre of mass located over southern norway this configuration had a flow pattern which is poorly reproduced by current numerical models of the fennoscandian ice sheet at the last glacial maximum the main ice divide was located overthe gulf of bothnia a major bend in the ice divide was caused by outflow of ice to the northwest over the lowest part of the scandinavian mountain chain widespread areas of preserved pre late weichselian landscapes indicate that the ice sheet had a frozen bed core area which was only partly diminished in size by inward transgressive wet bed zones during the decay phase

widespread bacterial populations at glacier beds and their relationship to rock weathering and carbon cycling bacterial populations found in subglacial meltwaters and basal ice are comparable to those in the active layer of permafrost and orders of magnitude larger than those found in ice cores from large ice sheets populations increase with sediment concentration and 5 24 of the bacteria are dividing or have just divided suggesting that the populations are active these findings 1 support inferences from recent studies of basal ice and meltwater chemistry that microbially mediated redox reactions may be important at glacier beds 2 challenge the view that chemical weathering in glacial environments arises from purely inorganic reactions and 3 raise the possibilities that redox reactions are a major source of protons consumed in subglacial weathering and that these reactions may be the dominant proton source beneath ice sheets where meltwaters are isolated from an atmospheric source of co 2 microbial mediation may increase the rate of sulfide oxidation under subglacial conditions a suggestion supported by the results of simple weathering experiments if subglacial bacterial populations can oxidize and ferment organic carbon it is important to reconsider the fate of soil organic carbon accumulated under interglacial conditions in areas subsequently overridden by pleistocene ice sheets

modeling the subglacial hydrology of the late pleistocene lake michigan lobe laurentide ice sheet the fast ice flow characteristic of the low gradient lake michigan lobe of the southern laurentide ice sheet during the last glaciation likely resulted from some combination of subglacial sediment deformation and sliding at the ice bed interface both mechanisms require high basal water pressure relative to overlying ice pressure to evaluate the basal water pressures of the lake michigan lobe we used the groundwater model modflow to simulate groundwater flow along a 1040 km long flow line corresponding to the flow path taken by the ice lobe our simulations show that groundwater flow directions and velocities were substantially different than modern conditions simulations also indicate that subglacial aquifers were not capable of evacuating the estimated basal meltwater but that excess water draining through a basal drainage system such as one that may underlie ice stream b west antarctic ice sheet would prevent basal water pressure from exceeding the ice overburden pressure the buried mahomet bedrock valley system near the distal end of the lake michigan lobe could have drained enough subglacial water to substantially lower basal water pressure and thus affect the dynamics of the lobe by draining basal meltwater away from the lake michigan lobe the buried mahomet bedrock valley system may have attenuated the effect of permafrost on basal water pressure during the last glacial maximum groundwater flow patterns and velocities were altered or reversed relative to present conditions

early paleozoic paleogeography of the northern gondwana margin new evidence for ordovician silurian glaciation during the early paleozoic transgressions and the distribution of sedimentary facies on the northern gondwana margin were controlled by a regional nnw sse to almost north south striking structural relief in early silurian times a eustatic highstand enabled the sea to reach its maximum southward extent

carbon stocks and isotopic budgets of the terrestrial biosphere at mid holocene and last glacial maximum times abstract the carbon fluxes stocks and isotopic budgets of the land biosphere at mid holocene 6 ka bp and last glacial maximum 21 ka bp times are reconstructed with the carbon assimilation in the biosphere caraib model forced with two different sets of climates simulated by the european centre hamburg echam and lmd general circulation models it is found that the trends predicted on the basis of both sets of gcm climatic fields are generally consistent with each other although substantial discrepancies in the magnitude of the changes may be observed actually these discrepancies in the biospheric results associated with the use of different gcm climatic fields are usually smaller than the differences between biospheric runs performed while considering or neglecting the co2 fertilization effect which might however be overestimated by the model due to uncertainties concerning changes in nutrient availability the calculated changes with respect to the present of the biosphere carbon stock range from 132 to 92 gt c for the mid holocene and from 710 to 70 gt c for the last glacial maximum it is also shown that the relative contribution of the material synthesized by c4 plants to the total biomass of vegetation litter and soils was substantially larger at mid holocene and last glacial maximum times than today this change in the relative importance of the c3 and c4 photosynthetic pathways induced changes in the 13 c fractionation of the land biosphere these changes in the average biospheric fractionation resulting from the redistribution of c3 and c4 plants were partly compensated for by changes of opposite sign in the fractionation of c3 plants due to the modification of the intercellular co2 pressure within their leaves with respect to present times the combination of both processes reduced the 13 c discrimination i e less negative fractionation of the land biosphere by 0 03 to 0 32 during the mid holocene and by 0 30 to 1 86 at the last glacial maximum

gas isotopes in ice reveal a vegetated central greenland during ice sheet invasion 1 environmental conditions prevailing during build up of the greenland ice sheet gis are not yet established here we use results from gas isotopes from basal ice of the grip ice core to show that central greenland was vegetated during invasion by the ice sheet the 13cch4 and 18o of o2 values obtained were never encountered before in ice core studies such values are indeed the signature of a significant biological activity

very low oxygen concentration in basal ice from summit central greenland oxygen concentration as low as 3 has been detected in the basal silty ice of the grip core such values were never observed in ice from ice sheets they are most probably the consequence of organic matter oxidation in ice developed in a peaty deposit when the greenland ice sheet was not present at the site flow induced mixing has further incorporated this ice into glacier ice during the ice sheet build up the part of the local ice component in the mixing process diminishes with the distance from the bed this is the process which explains the oxygen profile

the lower silurian qalibah formation of saudi arabia an important hydrocarbon source rock 1 the lower silurian qalibah formation represents a distinct lithostratigraphic time transgressive sequence it is disconformably bounded below by the upper ordovician to possibly lower silurian sarah formation and above by the continental to marginal marine lower devonian and possibly upper silurian tawil sandstone formation this paper redefines the qalibah formation within these boundaries in its reference section near the town of al qalibah in northwest saudi arabia the qalibah is 499 m thick whereas in a subsurface composite section it reaches 955 m the qalibah formation is a coarsening upward progradational sequence consisting of a lower qusaiba shale member and an upper sharawra sandstone member the basal hot shale of the qusaiba is a black euxinic bottomset shale up to 70 m thick which was deposited during the early silurian sea level rise following the deglaciation of gondwana this paleozoic shale is organic rich and the most likely source rock of the low sulfur high gravity oil condensate and gas discovered in recent years in the paleozoic rocks of central and eastern saudi arabia lithologically similar silurian sequences were deposited over most of the broad stable shelf of gondwana from the middle east to the african sahara

the ice age methane budget 1 accounting for the natural 40 reduction in methane during the last glacial maximum lgm represents one of the most challenging problems posed by polar ice core studies inverse modelling constrained by observations suggest it is driven by a decline in wetland sources but this is not supported by forward modelling using a comprehensive earth system model we show that closure of the methane budget requires reduced emissions and an amplified sink caused by reduction in emissions of volatile organic compounds vocs from lgm forests our study emphasizes a key role for the terrestrial biosphere in the methane budget during the quaternary

neoglacial glacier fluctuations in the canadian rockies abstract the little ice age was the most extensive neoglacial glacier advance in the canadian rockies evidence of earlier less extensive neoglacial glacier advances is based on wood recovered from several glacier forefields wood flushed out of athabasca glacier 7550 8230 yr b p three dates and dome glacier 6120 6380 yr b p two dates indicates that forests occurred upvalley of present glacier termini during the hypsithermal detrital logs from peyto 14 dates saskatchewan 3 dates robson 3 dates and yoho 1 date glaciers plus in situ slumps at peyto and robson glaciers have yielded 14 c dates between 2490 and 3300 yr b p 12 dates between 2800 and 2990 this wood is derived from sources at or upvalley from present glacier termini and represents forests overridden by glaciers between ca 3100 and 2500 yr b p treeline was higher than present immediately prior to this advance this advance which did not extend beyond the little ice age maximum position is designated the peyto advance and correlated with the tiedemann advance in western british columbia earliest little ice age advances at peyto and robson glaciers are dated ca 800 600 yr b p at positions ca 500 m upvalley from little ice age limits

timing of millennial scale climate change in antarctica and greenland during the last glacial period a precise relative chronology for greenland and west antarctic paleotemperature is extended to 90 000 years ago based on correlation of atmospheric methane records from the greenland ice sheet project 2 and byrd ice cores over this period the onset of seven major millennial scale warmings in antarctica preceded the onset of greenland warmings by 1500 to 3000 years in general antarctic temperatures increased gradually while greenland temperatures were decreasing or constant and the termination of antarctic warming was apparently coincident with the onset of rapid warming in greenland this pattern provides further evidence for the operation of a bipolar see saw in air temperatures and an oceanic teleconnection between the hemispheres on millennial time scales

basal temperature evolution of north american ice sheets and implications for the 100 kyr cycle 1 we simulate three dimensional ice temperature fields to examine spatial temporal history of the subglacial thermal environment during the last glacial cycle model results suggest that 60 80 of the laurentide ice sheet was cold based frozen to the bed at the lgm and therefore unable to undergo large scale basal flow the fraction of warm based ice increases significantly through the ensuing deglaciation with only 10 20 of the laurentide ice sheet frozen to the bed by 8 kyr bp this basal thermal evolution a function of both climatic and ice sheet history could enable a dynamical switch to widespread basal flow through the deglacial period because basal flow has the capacity to evacuate large amounts of ice from the interior of continental ice sheets creating thin and climatically vulnerable ice masses this switch in flow regime may have played a significant role in glacial terminations and the 100 kyr glacial cycle

taku glacier alaska u s a on the move again active deformation of proglacial sediments taku glacier is one of the few glaciers in alaska u s a that has advanced over the last century 7 km since 1890 this advance slowed substantially during the past decade but in summer 2001 the glacier terminus began to readvance at a rate of 30 cm d 1 the advance produced dramatic proglacial sediment deformation up to 200 m in from of the terminus two to three large bulges and several secondary bulges developed in the proglacial sediments as a result of glacial compression along a 1 km wide portion of the terminus the bulge nearest the terminus was 10 m high and 65 m wide the middle bulge 7 m high advanced at 15 cm d 1 and the distal bulge 3 m high and 50 m wide at 9 cm d 1 crenulations and prominent fractures developed in the overlying vegetation layer the frontal lobes of the bulges were steep and overlaid a shear zonc where sediments were being thrust up and over the ground surface ice proximal push moraines 1 10 m high formed along much of the 9 km wide terminus although deformation was minimal at some locations

pleistocene hydrology of north america the role of ice sheets in reorganizing groundwater flow systems 1 while the geomorphic consequences of pleistocene megafloods have been known for some time it has been only in the past 2 decades that hydrogeologists and glaciologists alike have begun to appreciate the important impact that ice sheet aquifer interactions have had in controlling subsurface flow patterns recharge rates and the distribution of fresh water in confined aquifer systems across north america in this paper we document the numerous lines of geochemical isotopic and geomechanical evidence of ice sheet hydrogeology across north america we also review the mechanical thermal and hydrologic processes that control subsurface fluid migration beneath ice sheets finite element models of subsurface fluid flow permafrost formation and ice sheet loading are presented to investigate the coupled nature of transport processes during glaciation deglaciation these indicate that recharge rates as high as 10 times modern values occurred as the laurentide ice sheet overran the margins of sedimentary basins the effects of ice sheet loading and permafrost formation result in complex transient flow patterns within aquifers and confining units alike using geochemical and environmental isotopic data we estimate that the volume of glacial meltwater emplaced at the margins of sedimentary basins overrun by the laurentide ice sheet totals about 3 7 104 km3 which is about 0 2 of the volume of the laurentide ice sheet subglacial infiltration estimates based on continental scale hydrologic models are even higher 5 10 of meltwater generated these studies in sum call into question the widely held notion that groundwater flow patterns within confined aquifer systems are controlled primarily by the water table configuration during the pleistocene rather groundwater flow patterns were likely much more complex and transient in nature than has previously been thought because pleistocene recharge rates are believed to be highly variable these studies have profound implications for water resource managers charged with determining sustainable pumping rates from confined aquifers that host ice sheet meltwater

carbon dating reveals a seasonal progression in the source of particulate organic carbon exported from the greenland ice sheet surface melt from the greenland ice sheet gris collects particulate organic carbon poc as it drains into subglacial environments and transports it downstream where it serves as a microbial substrate we hypothesized that older poc is entrained by meltwaters as the subglacial drainage network expands upglacier over the summer to test this poc samples were collected from a meltwater river exiting the gris over an ablation season and 14c dated resulting values were compared with meltwater hydrochemistry and satellite observations of the catchment area we found that poc ages increased from 5000 to 9000 years b p until peak discharge and catchment size afterward significant fluctuations in poc age were observed interpreted to result from periods of high and low subglacial hydrological pressure and sediment supply and subsequent exhaustion these observations suggest a seasonal progression in the source of poc exported from the gris and provide evidence for a seasonally evolving subglacial drainage system

a stacked mixing sequence at the base of the dye 3 core greenland this paper documents by a detailed isotopic and gas composition study of the basal ice from the dye 3 core a large degree of flow induced mixing at the ice sheet base this mixing results most probably from circular motion in troughs of bedrock undulations and flow separations with some entrainment of the underlying ice the measured distribution of isotopes and gases in the basal ice is the consequence of successive mixing events having occurred upglacier from the site in an area of very rough bedrock topography the deformation produced makes it possible for the basal part of the ice sheet to cross the subglacial valley existing in the dye 3 area