chronology of late wisconsinan glaciation in middle north america abstract we propose a chronology of late wisconsinan glacial fluctuations in middle north america from alberta to wisconsin based on radiocarbon dates derived solely from wood previous chronologies of the southwestern margin of the north american continental ice sheet have depended to a considerable degree on radiocarbon dates from fine grained organic sediment this material is commonly contaminated with older carbon resulting in chronologic confusion by using only dates from wood much of the confusion disappears however because of the scarcity of wood dates only four of the sixteen identified fluctuations are accurately dated an advance into iowa about 14 000 to 13 500 bp an advance into south dakota and iowa about 12 300 bp an advance into the lake michigan basin about 11 700 bp and an advance into the lake superior basin about 9900 bp in addition the beginning of late wisconsinan glaciation before 20 000 bp is fairly well documented none of the fluctuations in the western part of the region are accurately dated

laurentide cordilleran and montane glaciation in the western peace river grande prairie region alberta and british columbia canada abstract geological investigations over the last decade in the western peace river grande prairie district of northwestern alberta and northeastern british columbia have provided important chronostratigraphic information regarding laurentide cordilleran and montane quaternary glacial events numerous 14 c age determinations as well as stratigraphic and sedimentological analyses of numerous exposures indicate that this area was first glaciated by cordilleran ice sometime before the late wisconsinan during the late wisconsinan ca 22 000 14 000 bp laurentide glacier ice covered much of the area between approximately 15 000 and 10 000 bp montane ice affected only the western part of the peace region there is no evidence for synchroneity of the ice advances and therefore no evidence for ice coalescence in this region the resultant ice free corridor was most likely a laterally fluctuating zone which existed during a several thousand year period in the late pleistocene climatic conditions along the corridor during this period were probably unfavorable for human migration but adequate for some plants and small animals numerous isolated areas along the western foothills of the northern rocky mountains west of the laurentide glaciation limit were never glaciated during the late wisconsinan

environmental interpretation and a sequence stratigraphic framework for the terminal proterozoic ediacara member within the rawnsley quartzite south australia abstract fossils of the ediacara biota are confined to preservational windows in the ediacara member of the rawnsley quartzite pound subgroup from the flinders ranges south australia the base of the ediacara member is a type 1 sequence boundary incised into the partly lithified chace quartzite member of the rawnsley quartzite the ediacara member and the upper half of the rawnsley quartzite comprise the rawnsley depositional sequence bounded above by the early cambrian uratanna depositional sequence the rawnsley sequence developed over an erosional surface cut into the chace quartzite member with some 250 m of relief southeasterly directed palaeovalleys are filled with a sequence of massive sandstone and laminated siltstone passing up into well bedded sandstone at lowstand local channels were filled by massive sand the highstand systems tract consists of several parasequence sets of laminated siltstone and channelized amalgamated massive sandstone overlain by interbedded siltstone and sandstone grading up into well bedded clean sandstone impressions of soft bodied ediacaran organisms are preserved above the valley fill facies on sandstone partings within upward shoaling delta front environments between storm and fairweather wavebase the ediacara member is in places conformably overlain by up to 500 m of sandstone of the same lithofacies as those underlying the member distinctive microbial mat bound sand laminae described as petee structures are characteristic of the rawnsley quartzite above and below the ediacara member during deposition of the ediacara member colonization of the deeper substrates by microbial mat communities enabled preservation of the ediacara biota the erosional surface at the base of the ediacara member is one of at least four incision events recorded in the terminal proterozoic and early cambrian succession of the adelaide geosyncline the similarity of incision filling lithofacies of the rawnsley and uratanna sequences has in condensed sections led to confusion between the depositional sequences characteristic trace and body fossil assemblages provide the only clear distinction between depositional cycles spanning the proterozoic cambrian boundary

a quantitative approach to understanding dated dune stratigraphies attempts to reconstruct past changes in climate related forcing of dryland landscapes are hampered by the lack of an adequate quantitative framework for understanding the production and interpretation of dated sedimentary records in drylands as in other environments information on past forcing conditions is progressively modified degraded and removed from the available stratigraphic record by a series of filters involving changes in the primary forcing factors themselves geomorphological processes and the sampling dating procedures in this paper we describe a quantitative model that includes these effects and use the model to examine the nature of preserved dryland sedimentary records and their relationships to primary forcing conditions thicker preserved sedimentary records reflect periods of more intense aeolian activity localized switching between erosion and deposition results in discontinuous and highly variable stratigraphic sequences a preservation bias towards younger deposits is observed potentially leading to a continuum of accumulation that decays approximately in proportion to time periods not represented by deposition can in some cases be interpreted as periods of higher precipitation and or lower wind energy an asymmetry exists between the efficiency with which past drier and wetter episodes can be identified which relates to the time separation of depositional periods and the correct distinction between hiatuses due to forcing conditions and those due to under sampling relevant to this is the effect of random dating errors statistical uncertainty which increasingly with age filter out higher frequency events from the record a new data treatment method termed accumulation rate variability provides an efficient proxy for accumulation rates and therefore the intensity of aeolian activity with significant improvements over existing date frequency methods the filtering problem discussed applies to all attempts at understanding the timing and nature of past events independent of the proxies and dating methods employed further explicit analysis of these issues would be beneficial copyright 2013 john wiley sons ltd

exceptionally preserved microbially induced sedimentary structures from the ediacaran post glacial successions in the kimberley region northwestern australia abstract a set of exceptionally preserved microbially induced sedimentary structures miss is described from the post glacial yurabi formation of late ediacaran age in the kimberley region northwestern australia these miss comprise oriented grains bamboo leaf like cracks polygonal cracks worm like cracks sinuous cracks gas domes pyrite concentrated laminae and micrite laminae most of the yurabi miss are morphologically comparable with their ancient and modern counterparts micrometre or nannometre scale bacteria or biofilm like inorganic objects are detected from most miss and are interpreted as bacterial bodies or remains that have been replaced with inorganic minerals upon fossilization oriented grains indirectly support microbial control on their accumulation both pyrite conformable laminae and micrite laminae suggest degradation and decay of original microbial mats upon diagenesis sinuous cracks were likely generated in a soft sediment deformation process whereas other sand cracks bamboo leaf like cracks worm like cracks polygonal cracks could have formed from sediment surface layer microbial organics shrinkage upon desiccation gas domes may have formed as the result of anoxic redox reactions and being filled with gases and sediments transported from below two proxies bedding plane coverage percentage and accumulative thickness of single miss bearing beds are developed to semi quantitatively evaluate miss abundance on bedding plane and vertical outcrop respectively the yurabi miss have substratum preference to fine sandstone facies and intertidal settings the yurabi formation yields the most abundant and diverse post glacial miss assemblages during the late ediacaran worldwide both kimberley and global fossil records indicate that miss may have proliferated in shallow marine ecosystems soon after the retreat of ediacaran gaskiers glaciation during late neoproterozoic

assemblage palaeoecology of the ediacara biota the unabridged edition fossils of the ediacara biota offer our earliest insight into diverse macroscopic life on this planet in particular given the diversity and range of exquisite soft bodied preservation the potential for unraveling aspects of the paleobiology and paleoecology is great clearly however there can be a taphonomic overprint that dictates how complete the assemblage is new diversity data including richness and evenness from south australia is compared to previously published data from newfoundland and the white sea and is within the range of values for both modern and phanerozoic assemblages however missing from our current understanding of ediacaran ecosystems is a full description and interpretation of the many problematica structures and organic surfaces

textured organic surfaces associated with the ediacara biota in south australia abstract the ediacaran period takes its name from the fossils of the ediacara biota which represent the first appearance of large and diverse assemblages of organisms in the fossil record although the global record of these distinctive body fossils is now well known a previously unrecognized megascopic organic record of textured organic surfaces tos occurs in the ediacara biota however tos is also a feature over a wider range of paleoenvironmental settings where body fossils are unknown in ediacaran siliciclastic successions that have been studied in australia namibia and western north america paleoecological analysis of successive bedding planes of strata from the late ediacaran rawnsley quartzite in the flinders ranges of south australia reveals that tos represent the most common organic features in bedding surface assemblages of the ediacara biota the tos consist of preserved patterned assemblages of textured organic mats fibers and simple tubular body fossils complex ediacara body fossils while striking for their distinctive body plans and dominating some of the beds are relatively minor components of combined overall surface area many elements of tos have previously been miss diagnosed as trace fossils which are in practice limited to two or at most three morphotypes that indicate the presence of bilateria although tos represent a simpler grade of organismic construction than discrete and more complex ediacara body fossils they were preserved in a similar manner marked variability in all components of the biota between successive surfaces suggests that ediacara ecologies fluctuated at short intervals despite an apparently consistent sedimentary regime

sedimentology and development of parabolic dunes grande prairie dune field alberta the grande prairie dune field in northwest central alberta is composed of parabolic and dome dunes presently inactive but which had developed under westerly winds two parabolic dune types are rec

problematic fossils in the late neoproterozoic wonoka formation south australia abstract complex biogenic structures are described from the upper wonoka formation late neoproterozoic of the adelaide geosyncline south australia the structures are preserved within silty limestones and calcareous siltstones as dark bedding plane markings defining branching arrays of parallel elongate curved elements the fossils bear at least a superficial resemblance to the putative trace fossil palaeopascichnus normally preserved in siliciclastic sediments and reported from a number of late neoproterozoic sections worldwide the material from the wonoka formation is not a trace fossil and seems most likely to represent a surface encrusting alga the nature of palaeopascichnus needs to be carefully reassessed in the light of the possibility that the wonoka fossils represent a different preservational window into the same phenomenon the thin ca 2 m but widespread nature of the fossil horizon makes it a useful stratigraphic marker within the wonoka formation with the potential to aid in regional correlation within the adelaide geosyncline

importance of the rossendale site in establishing a deglacial chronology along the southwestern margin of the laurentide ice sheet the timing of deglaciation in the lake agassiz basin is critical in establishing the routing of meltwater and precipitation runoff from a 2 000 000 km2 region of central north america and in evaluating the influence this water had on rivers and oceans into which it drained dates of 12 400 420 and 12 100 160 yr b p for moss at the rossendale site in manitoba have long been a key for those advocating an early deglacial chronology in this region however new dates for wood from this site and paleoecological interpretations of ostracods molluscs and the dated moss all support a young deglacial scenario of particular significance is the fact that the dated moss scorpidium scorpioides is a subaquatic type subject to contamination by old carbon dissolved from bedrock in fact most subaquatic moss may be unreliable for radiocarbon dating for these reasons the 12 400 and 12 100 yr b p dates are rejected new dates of 9600 70 and 9510 90 yr b p for wood from the same organic rich unit containing the dated moss ostracods and molluscs fit well with the young deglacial chronology of the southwestern laurentide ice margin advocated by many in short the ice margin appears to have retreated into the southern lake agassiz basin after 12 000 yr b p and north of the rossendale site by 11 000 yr b p about 10 000 yr b p following the moorhead low water phase lake agassiz rose to the campbell level the dated organic matter at rossendale was deposited in a lagoon behind the campbell beach

enigmatic chambered structures in cryogenian reefs the oldest sponge grade organisms abstract previously undescribed chambered structures are common and widespread in the cryogenian post sturtian glacial carbonates of the oodnaminta reef complex adelaide geosyncline south australia the rasthof berg aukas formation namibia and the gauss formation namibia these carbonate structures have millimetre to centimetre scale chambers separated by well defined and generally thin micritic walls chamber walls now consist of dolomite but were probably originally aragonitic the chambers may have a lobate polygonal or dendritic morphology and are often further divided into smaller chambers chambered structures occur as reefal growth frameworks as cavity fillings in neptunean dykes and growth cavities and as intercolumnar material within stromatolite frameworks in the oodnaminta reef complex these structures are only present in the sub photic deep water framework these structures probably represent the calcified remains of an organism or community of organisms that was globally distributed and widespread for a significant time period following the sturtian glaciation no precisely analogous structures have been previously described from modern or ancient settings but the complexity and degree of organization suggests a significant evolutionary advance over older proterozoic fossils the closest morphological analogues for the structures are a some types of reef dwelling sponges and b some complex microbialites from archaean and paleoproterozoic carbonates the structures lack spicules and ostia found in sponges ruling out a true poriferan origin however it is plausible that they are proto sponges sponge grade organisms or complex microbial precursors to sponge grade organisms whatever their affinity we suggest these structures record a significant evolutionary event on the path towards organic complexity

neoproterozoic stratigraphy of the centralian superbasin australia abstract the basement of central australia includes the arunta block and the 1200 1100 ma musgrave block succeeded by 1075 1000 ma bimodal volcanic rifts and dolerite dykes a regional hiatus at 1000 800 ma interpreted as reflecting a central post volcanic underplated upland was followed at 800 ma by a second swarm of dykes in the musgrave block gawler craton and stuart shelf associated with the crustal sagging that initiated the centralian superbasin the centralian superbasin is defined here to encompass the neoproterozoic fill of the amadeus georgina ngalia officer and savory basins the intra cratonic superbasin was disrupted internally 540 600 ma ago petermann ranges orogeny by a central uplift and associated thrusts and by mid carboniferous 320 ma tectonism alice springs orogeny to form the structural basins listed above the stratigraphy and sedimentology of the relatively well known areas can be used as a basis for predicting those of the poorly known regions new results from acritarch biostratigraphy and isotope chemostratigraphy in conjunction with conventional lithostratigraphy and sequence analysis allow better stratigraphic resolution than previously possible field observations in the recently discovered savory basin in western australia provide a basis for correlations with the adjacent basins

a new modular palaeopascichnid fossil curviacus ediacaranus new genus and species from the ediacaran dengying formation in the yangtze gorges area of south china non biomineralizing ediacaran macrofossils are rare in carbonate facies but they offer valuable information about their three dimensional internal anatomy and can broaden our view about their taphonomy and palaeoecology in this study we report a new ediacaran fossil curviacus ediacaranus new genus and species from bituminous limestone of the shibantan member of the dengying formation in the yangtze gorges area of south china curviacus is reconstructed as a benthic modular organism consisting of serially arranged and crescent shaped chambers the chambers are confined by chamber walls that are replicated by calcispars and are filled by micritic sediments such modular body construction is broadly similar to the co occurring yangtziramulus zhangii and other ediacaran modular fossils such as palaeopascichnus the preservation style of curviacus is similar to yangtziramulus although the phylogenetic affinities of both genera remain unresolved the new fossil adds to the diversity of ediacaran modular organisms

optically stimulated luminescence dating of sediments over the past 200 000 years optical dating of sediment using optically stimulated luminescence has become important for studying earth surface processes and this technique continues to develop rapidly a group of closely linked luminescence methods can be used to estimate the time since grains of quartz and feldspar were last exposed to daylight by detecting their subsequent response to environmental ionizing radiation exposure the technique is well suited to the dating of deposits as young as one year to several hundred thousand years recent technical developments have established a dating protocol with improved precision a high degree of reliability and an in built means to detect incomplete signal removal during deposition this approach has been extended to age estimation for single grains opening up a wider range of potential environments and new possibilities for understanding postdepositional grain movement ongoing research offers the possibility of significant age range extension and novel applications including low t

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

late glacial drainage systems along the northwestern margin of the laurentide ice sheet abstract the evolution of drainage systems along the retreating northwestern laurentide ice sheet was complex the interaction of ice margin configuration topography and glacioisostasy resulted in a network of meltwater rivers that variably overflowed to the arctic and pacific oceans and to the gulf of mexico glacial lakes also changed dramatically in size and location during the period of deglaciation at the last and all time glacial maximum the ice sheet extended into the eastern cordillera blocking northward and eastward drainage to the arctic ocean some meltwater and most non glacial runoff were diverted through the mountains to the yukon river basin into alaska and the pacific ocean retreat from the glacial maximum prior to 21 ka bp allowed proglacial drainage from the western margin of the ice sheet to flow into the beaufort sea arctic ocean deglaciation was rapid after about 13 ka bp with the present route of the lower mackenzie river established between 13 and 11 5 ka bp continued ice retreat led to significant southward expansion of the mackenzie beaufort drainage basin at about 11 5 ka bp through drainage capture of glacial lake peace which previously had drained southeastward into the missouri river and to the gulf of mexico very rapid ice retreat between 10 5 and 10 ka bp allowed glacial lake mcconnell to expand down slope in contact with the ice margin numerous glacial lakes occurred along the northwestern margin of the ice sheet during the maximum and retreat phases these include ice dammed glacial lake old crow which occupied unglaciated terrain of the northern yukon and glacial lake peace which utilized a number of outlets as it migrated eastward with the ice front along the peace valley the largest glacial lakes in the region were the result of glacioisostatic depression reversing the regional drainage the mackenzie phase of glacial lake mcconnell was the second largest pleistocene lake in north america 215 000 km 2 late glacial and post glacial changes in drainage systems were largely in response to isostatic rebound

zircons from the acraman impact melt rock south australia shock metamorphism u pb and 40ar 39ar systematics and implications for the isotopic dating of impact events abstract this study presents the first optical and scanning electron microscopic characterization and u pb shrimp dating results for zircon grains separated from the most likely autochthonous impact melt rock in the central domain of the large 40 90 km eroded ediacaran acraman impact structure in south australia microtextural characteristics define five zircon subtypes corresponding to different levels of progressive shock metamorphism from virtually unshocked monocrystalline zircon grains that exhibit original magmatic zoning in cathodoluminescence images to fully granular zircons that have completely lost their primary zoning pattern and locally contain neocrystallized submicrometer sized spots of zro 2 probably baddeleyite that pseudomorph pre impact zircon the granular zircons correspond to the highest observed level of shock metamorphism and impact induced recrystallization zro 2 bearing granular zircons indicate shock pressures in excess of 65 70 gpa which are considerably higher than previous shock pressure estimates for the acraman impactites u pb systematics of untreated and chemically abraded melt rock zircons indicate that u pb ratios of the acraman zircons were variably reset during impact weakly shocked crystalline grains yield ages on concordia at 1 59 1 60 ga reflecting the magmatic age of the gawler range volcanics only the entirely granular zircon population was apparently impact reset but based on an ediacaran age from stratigraphic constraints on the ejecta layer experienced significant post impact pb loss the microcrystalline nature of granular zircons could have promoted pb diffusion and recoil in post impact time as suggested by grain size dependent diffusion and recoil modeling a positive correlation of u concentration and shock level suggests that granularization might have preferentially occurred in initially u rich probably metamict zircons 40 ar 39 ar dating of a melted yardea dacite clast from the acraman melt rock as well as k feldspar separated from shocked yardea dacite resulted in post impact alteration plateau ages suggestive of hydrothermal events at 500 ma and 450 ma that selectively affected the impactites exposed in the central domain of the acraman impact structure our study demonstrates that the acraman impact is particularly difficult to date in the absence of accurate and precise isotopic ages for acraman the ediacaran ejecta stratigraphic age of 635 541 ma is considered the most reliable age constraint currently available for the timing of the large acraman impact

ubiquity of anomalous fading in k feldspars and the measurement and correction for it in optical dating anomalous fading rates were measured in k feldspars separated from 49 sediment samples mainly from north america the intensity of the optically stimulated luminescence was found to decrease linea

orbisiana linearis from the early ediacaran lantian formation of south china and its taphonomic and ecological implications abstract orbisiana is an ediacaran fossil characterized by linearly arranged circular or cylindrical units the genus was first described from the late ediacaran redkino series in the moscow syncline of russia where it is preserved as pyritic molds in siltstone here we report orbisiana linearis originally described as seirisphaera lineare from black shale of the early ediacaran lantian formation in south china unlike the type species o simplex o linearis is preserved as carbonaceous compressions on the bedding surface with rare specimens preserved three dimensionally allowing morphological characterization using x ray microct techniques morphological paleoecological and taphonomic analysis suggests that o linearis was a chain like modular organism characterized by uniserially arranged millimeter sized and tangentially contacting circular or cylindrical units ontogenetic growth of o linearis was likely achieved through both accretionary addition of new modules and inflational expansion of existing units and it appears that inflational growth was determinate although its phylogenetic affinity remains uncertain o linearis was likely a procumbent epibenthic or semi endobenthic organism that lived on the seawater sediment interface or partly buried in sediments o linearis in the lantian formation was preserved in situ although some specimens are disarticulated and many are tectonically deformed orbisiana joins a growing list of ediacaran genera that have a remarkable range of stratigraphic environmental ecological and taphonomic distribution

meltwater routing and the younger dryas the younger dryas the last major cold episode on earth is generally considered to have been triggered by a meltwater flood into the north atlantic the prevailing hypothesis proposed by broecker et al 1989 nature 341 318 321 more than two decades ago suggests that an abrupt rerouting of lake agassiz overflow through the great lakes and st lawrence valley inhibited deep water formation in the subpolar north atlantic and weakened the strength of the atlantic meridional overturning circulation amoc more recently tarasov and peltier 2005 nature 435 662 665 showed that meltwater could have discharged into the arctic ocean via the mackenzie valley 4 000 km northwest of the st lawrence outlet here we use a sophisticated high resolution ocean sea ice model to study the delivery of meltwater from the two drainage outlets to the deep water formation regions in the north atlantic unlike the hypothesis of broecker et al freshwater from the st lawrence valley advects into the subtropical gyre 3 000 km south of the north atlantic deep water formation regions and weakens the amoc by 30 we conclude that meltwater discharge from the arctic rather than the st lawrence valley was more likely to have triggered the younger dryas cooling

comparative radiocarbon dating of terrestrial plant macrofossils and aquatic moss from the ice free corridor of western canada in order to assess the reliability of aquatic moss for radiocarbon dating sup 14 c analyses were performed on a stratigraphic series of terrestrial plant macrofossils and samples of drepanocladus crassicostatus from a small hard water lake ph 8 2 in the ice free corridor of alberta all sup 14 c dating was done by using accelerator mass spectrometry mazama ash provided an independent chronological control the aquatic bryophyte samples consistently produced sup 14 c ages significantly older than the terrestrial macrofossils the relation between the radiocarbon dates from the macrofossils and the moss was not linear and age differences ranged from approximately 1400 to 6400 yr the sup 14 c content of d crassicostatus growing in the lake at present was less than 85 modern despite the apparent inability to take up sup 14 c deficient carbon by the direct incorporation of bicarbonate the bryophytes clearly do not provide reliable material sup 14 c dating the sup 14 c deficiency of aquatic mosses may be explained by the generation of sup 14 c deficient co sub 2 through isotopic exchange the formation of co sub 2 from bicarbonate by chemical processes and metabolic co sub 2 production these results demonstrate the potential unreliability of sup 14 c dates from aquatic mosses and raise serious concernsmore about the deglaciation dates from the ice free corridor that were obtained from aquatic drepanocladus less

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

interpretations and common challenges of aeolian records from north american dune fields abstract geomorphic and chronologic data for dune fields are evaluated for three contrasting areas of north america 1 the prairie parkland and boreal ecozones of the northern great plains in canada 2 the central great plains of the usa and 3 the deserts of southwestern usa and northern mexico luminescence and radiocarbon ages for periods of dune accumulation and stability are compared with palaeoenvironment proxies to provide an assessment of the boundary conditions of dune system response to changes in sediment supply availability and mobility dune fields in the northern great plains were formed from sediment originating from glaciofluvial or glaciolacustrine sediments deposited during deglaciation 16 11 ka subsequent aeolian deposition occurred in parkland and prairie dune fields as a result of mid holocene 8 5 ka and late holocene despite spatial and temporal gaps in chronometric data as a result of sampling biases the record from north american dune fields indicates the strong influence of sediment supply on dune construction with changes in sediment availability as a result of drought episodes resulting in dune field reactivation and reworking of pre existing sediment

the problems and potential of using animal fossils and trace fossils in terminal proterozoic biostratigraphy abstract despite the discovery of soft bodied megascopic fossils of late neoproterozoic age at numerous localities world wide there has been slow acceptance of their potential for intercontinental correlation the stratigraphic thickness of sediments separating individual occurrences of such fossils from the base of the cambrian is very varied basic questions have also been posed as to the classification of the organisms represented and hence their likely time significance recent geochronological studies support notions of a narrow time distribution well studied neoproterozoic sequences in eurasia eastern and western north america and southern australia show strikingly similar patterns of increase in the numbers and diversity of trace fossils and soft bodied megascopic remains up section though strata immediately preceding the cambrian are relatively barren other than for a sparse palynoflora and compressions of primitive plants trace fossils from australian sections relating to an early part of the ediacaran apparently overlap a time of abundance of large process bearing acritarchs and big leiosphaerids this overlap of early animal traces and distinctive acritarchs may prove to be of significant biostratigraphic importance rare discoveries of forms resembling rangea gurich in the flinders ranges of south australia precede the classical ediacara assemblage and may provide a possible correlation with occurrences of this genus in namibia the distinctive remains and trace fossils at this level constituting the rangeid hiemalora assemblage zone resemble a biota from the podolia region of the ukraine and show a similarity to finds in parts of the windermere supergroup of the northwest territories canada similarly the offshore frondose assemblage of newfoundland is comparable with the biota of charnwood forest england the type ediacara assemblage of the flinders ranges characterizing the dickinsonia costata assemblage zone is a climax biota with diverse taxa and large individual species as well as some 20 ichnogenera it closely correlates with a late redkinian or early kotlinian assemblage in the upper part of the section on the zimny gory coast of the white sea northern russia the trace fossils of these levels show that significant diversification began before the cambrian

coalescence of late wisconsinan cordilleran and laurentide ice sheets east of the rocky mountain foothills in the dawson creek region northeast british columbia canada abstract geomorphic stratigraphic and geochronological evidence from northeast british columbia canada indicates that during the late wisconsinan approximately equivalent to marine oxygen isotope stage mis 2 a major lobe of western sourced ice coalesced with the northeastern sourced laurentide ice sheet lis high resolution digital elevation models reveal a continuous 75 km long field of streamlined landforms that indicate the ice flow direction of a major northeast flowing lobe of the cordilleran ice sheet cis or a montane glacier 200 km wide was deflected to a north northwest trajectory as it coalesced with the retreating lis the streamlined landforms are composed of till containing clasts of eastern provenance that imply that the lis reached its maximum extent before the western sourced ice flow crossed the area since the lis only reached this region in the late wisconsinan the cis montane ice responsible for the streamlined landforms must have occupied the area after the lis withdrew stratigraphy from the murray and pine river valleys supports a late wisconsinan age for the surface landforms and records two glacial events separated by a non glacial interval that was dated to be of middle wisconsinan mis 3 age

trace fossils in the ediacaran cambrian transition behavioral diversification ecological turnover and environmental shift abstract after taxonomic revision trace fossils show a similarly explosive diversification in the ediacaran cambrian transition as metazoan body fossils in shallow marine deposits of ediacaran age trace fossils are horizontal simple and rare and display feeding strategies related to exploitation of microbial matgrounds equally notable is the absence of arthropod tracks and sinusoidal nematode trails this situation changed in the early cambrian when a dramatic increase in the diversity of distinct ichnotaxa is associated was followed by the onset of vertical bioturbation and the disappearance of a matground based ecology agronomic revolution on deep sea bottoms animals have been present already in the ediacaran but ichnofaunas were poorly diverse and dominated by the horizontal burrows of undermat miners as shown by the ichnogenus oldhamia this life style continued to be predominant into the early and to a lesser extent middle cambrian nevertheless there was an explosive radiation of behavioral programs during the early cambrian when exactly the bioturbational revolution arrived in the deep sea is uncertain in any case the nereites ichnofacies was firmly established in the early ordovician the rich ichnofauna in the early cambrian guachos formation of northwest argentina probably marks a first step in this ecological onshore offshore shift

temporal geographical meltwater influences on the north atlantic conveyor implications for the younger dryas the temporal and geographical roles of meltwater discharge from the laurentide ice sheet on north atlantic deep water nadw production are investigated utilizing a global realistic geometry coupled climate model which does not require the use of flux adjustments model results suggest that preconditioning by meltwater discharge to the mississippi prior to the younger dryas yd is capable of pushing nadw beyond the limit of its sustainability the diversion of meltwater to the st lawrence then merely serves to completely inhibit nadw production the modeled change in surface air temperature generally agrees with the global pattern and magnitude of temperature change seen in paleoclimatic reconstructions of the yd and is intimately linked to changes in nadw formation the global thermohaline circulation provides an interhemispheric teleconnection with the southern oceans while changes in the atmospheric heat transport reacting to a global redistribution of oceanic heat transport provide a mechanism for interbasin teleconnection although the primary thermodynamic and hydrological cycle feedback processes are included within the atmospheric model in the absence of additional feedbacks an equilibrium without the presence of nadw is possible the inclusion of the wind stress speed feedback is found to significantly contribute to the resumption of nadw production as suggested by previous studies contrary to these same studies however the coupled model indicates an advective spin up timescale is required for resumption of nadw production and hence the termination of the modeled yd like climate event as opposed to a decadal century timescale the reason for the discrepancy is unclear but may be associated with the use of fixed salt flux fields applied in previous studies or the duration strength and geographical location of the imposed meltwater applied

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

holocene paleohydrology of central alberta testing the general circulation model climate simulations paleoecological investigations of 28 lakes and bogs provide the basis for a holocene paleohydrological record for central alberta shallow basins empty during the early holocene began flooding shortly after 8000 years ago most filled from 6500 to 4500 years ago and none filled later than 3000 years ago pollen of a hypersaline indicator genus ruppia was found in cores of lakes where the plant does not presently grow indicating lower lake levels evaporation stress and increased salinity from 8000 to 3000 years ago moore lake 54 30 n 100 30 w may have dropped 15 m from 9200 to 5900 years ago and lofty lake 54 44 n 112 29 w largely dried up between 8700 and 6300 years ago lower water levels are suggested for several lakes by shifts in fossil diatom populations from planktonic to benthic particularly epipelic species including pioneering taxa these shallow warm lakes were highly productive and sedimentary pigment levels reached maximum values between about 9000 and 4000 years ago the pale

a candidate stratotype for the precambrian cambrian boundary fortune head burin peninsula southeastern newfoundland the burin peninsula exhibits an exceptionally thick and essentially continuous succession of marine strata through the precambrian cambrian transition fossils are abundant and include trace fossils small shelly fossils vendotaenid algae soft bodied megafossils and microfossils the burin peninsula is readily accessible and has long been considered a potential area for a precambrian cambrian boundary stratotype a continuous section through the upper part of member 1 and all of member 2 of the chapel island formation is exposed at fortune head and this section is herein proposed as a global stratotype for the precambrian cambrian boundary the boundary horizon is located 2 4 m above the base of member 2 of the chapel island formation this horizon marks the base of the basal cambrian phycodes pedum ichnofossil zone and immediately overlies the top of the late precambrian harlaniella podolica ichnofossil zone shelly fossils sabelliditids first appear a few metres below the proposed boundary sof

delineating stratigraphic breaks at the bases of postglacial eolian dunes in central alberta canada using a portable osl reader temporal constraints for the late pleistocene deposition of eolian dunes that occur in central and northern alberta canada can be provided by dating sediments from the dune bases using luminescence techniques in places however the postglacial dunes overlie glaciofluvial sands that resemble the eolian deposits in texture such that demarcating the bases of the dunes is often problematic in this study we address the problem by employing a portable optically stimulated luminescence osl reader to construct luminescence profiles that depict luminescence signal variation with depth with the portable osl reader measurement can be performed on bulk sediments negating the need for laborious separation procedures to isolate pure mineral fractions as is required in regular luminescence dating measurements can also be carried out in the field permitting quicker decision making during sample collection results from this study presented as depth variations of feldspar derived infrared stimulated luminescence irsl signals and predominantly quartz generated post ir blue osl signals show that luminescence profiling enables one to distinguish between eolian deposits that make up the dunes from the underlying non eolian sands the identification of such cryptostratigraphic interfaces is made possible by differences in the dosimetric histories of the sediments the delineation of the dune bases allows targeted sampling that yields best luminescence age approximations for the initiation of postglacial eolian deposition in the region luminescence profiling of eolian sequences would also in theory permit the identification of depositional breaks of extended duration copyright 2012 john wiley sons ltd

new constraints on the age of the last ice sheet glaciation in nw england using optically stimulated luminescence dating optically stimulated luminescence osl has been used to date silts interpreted as either primary or colluviated loess deposited in karstic depressions at three sites on the carboniferous limestone of nw england two of the ages presented here constrain the onset of the last late devensian ice in this region to later than ca 27 2 ka and the other two date initial deglaciation prior to ca 19 2 ka in the morecambe bay area and ca 17 2 ka in the karst of the yorkshire dales while these ages are broadly consistent with existing estimates from lacustrine and speleothem evidence they offer the tightest constraints yet published on the presence of the last ice sheet from northern england copyright 2009 john wiley sons ltd

cosmogenic 36cl dating of the foothills erratics train alberta canada the foothills erratics train of alberta was carried into place during coalescence between montane glaciers and the laurentide ice sheet its age has been controversial late wisconsinan and pre wisconsinan ages have been assigned to it cosmogenic 36 cl exposure dating was carried out on erratics over a 130 km segment of the erratics train seven of eight age determinations support emplacement during the late wisconsinan glaciation these ages eliminate the possibility of an ice free corridor during the last glacial maximum they are also consistent with the hypothesis that the laurentide ice sheet was the most extensive of all pleistocene continental ice sheets in western canada

the oldest skeletal macroscopic organism palaeopascichnus linearis abstract the existing problems regarding the phylogenetic resolution of ediacaran macroscopic organisms stem from a failure to factor in the information captured through different taphonomic windows here we integrate the information from sandstone and limestone hosted occurrences of the fossil palaeopascichnus linearis that together show a conspicuously greater range of taxonomic and taphonomic variation the limestone hosted material comes from a new locality discovered on the northwestern slope of the olenek uplift in arctic siberia that characterises the uppermost ediacaran strata of the khatyspyt formation khorbusuonka group detailed taphonomic and morphodynamic studies of the fossil material revealed evidence of possible chamber wall agglutination and growth through addition of new chambers with little sideway growth palaeopascichnus linearis outlived the intra ediacaran 550 myr ago drop in species richness the advent of bilaterians and the increase in complexity of burrowing behaviour in metazoans it remains to be seen whether or not the remarkable ecological success achieved by palaeopascichnus linearis during the times of rich evolutionary experimentation has anything to do with the invention of an agglutinated test palaeopascichnus poses major scientific challenges and it is now clear that such preservation as occurs in siberia may hold the answer to what they were how they lived and what they were related to

late quaternary landscape history and archaeology in the ice free corridor some recent results from alberta abstract multidisciplinary research programs conducted within the alberta portion of the ice free corridor since 1986 as part of the archaeological survey s first albertans project have used studies of palaeogeography and palaeoenvironments to formulate archaeological search strategies this approach has resulted in the discovery of two new important sites saskatoon mountain ghqt 4 and the james pass meadow complex ekpu 3 to 9 that are yielding information on terminal late wisconsinan early holocene human occupation of the corridor both sites are dated by ams or conventional radiocarbon dates to the millennium 10 000 9 000 bp in the fifteen years since the 1978 amqua conference on the corridor other researchers have identified four additional significant paleo indian sites in this region so far none confirm human occupation of the ice free corridor region before about 11 000 bp

optical dating of sediments a new method for dating sediments is proposed which determines the time since the sediment was last exposed to sunlight an argon ion laser is used to excite electrons from thermally stable light sensitive traps and the subsequent luminescence used as a measure of the past radiation dose two sample sequences spanning the periods 0 700 kyr and 0 6 kyr show steadily increasing luminescence with age an age of 62 8 kyr is obtained for a silt radiocarbon dated at 58 8 0 3 kyr problems were found with two much younger samples

basin migration caused by slow lithospheric extension sedimentary basin migration caused by low lithospheric extension rates is investigated using a two dimensional dynamic numerical model of the lithosphere we find that continental breakup will eventually occur when larger extension velocities are used the duration of rifting prior to continental breakup is dependent on the extension velocity stretching the lithosphere with lower velocities does not lead to breakup instead the locus of maximum extension migrates deformation localizes outside the first formed basin that is in turn uplifted this basin then becomes a cold spot in the area in this case syn rift cooling predominates the lithosphere regains its strength during stretching instead of becoming weaker and the lithosphere necking zone becomes stronger than surrounding regions the transition velocity is for the cases studied about 8 mm yr while the locus of maximum thinning migrates after about 50 60 myr a comparison with observations of the mid norwegian galicia and ancient south alpine margins shows a close resemblance of important features fl 2002 elsevier science b v all rights reserved

glacial lake mcconnell paleogeography age duration and associated river deltas mackenzie river basin western canada abstract glacial lake mcconnell lasted from 11 8 to 8 3 ka bp while occupying parts of the great bear great slave and athabasca lake basin the retreating laurentide ice front formed the eastern margin whereas low rolling hills formed the north west and south shorelines three major deltas were deposited at the mouths of the laird peace and athabasca rivers the total extent of all phases of the lake was 240 000 km 2 while the largest extent was 210 000 km 2 at 10 5 ka bp downwarping of the basin by glacial ice was the main cause of the lake whereas sediment blockage between jean marie river and fort simpson was secondary initially glacial lake mcconnell occupied the northwestern corner smith arm of the great bear lake basin and discharged through the hare indian river outlet by 11 5 ka bp the enlarged water body flowed out the great bear river but only for a short period of time the mackenzie river formed the third outlet near jean marie river at 11 ka bp and flow in the great bear river ceased until 9 ka bp at 9 9 ka bp glacial lake mcconnell was impacted by a major flood from glacial lake agassiz with a peak discharge of 2 7 10 6 m 3 sec flood water discharged from glacial lake mcconnell peaking at 0 35 0 57 10 6 m 3 sec and receding flow continued for 30 months the massive influx of floodwater into glacial lake mcconnell caused an abrupt increase of discharge which enlarged the outlet channel to between 6 and 13 km wide between fort simpson and jean marie river at 8 3 ka bp isostatic rebound ended the 3500 year old extensive lake by dividing it into the great slave lake and lake athabasca

storm dominated mixed carbonate siliciclastic shelf sequence displaying cycles of hummocky cross stratification late proterozoic wonoka formation south australia abstract the regressive late proterozoic wonoka formation of the adelaide geosyncline in south australia contains a thick mixed carbonate siliciclastic interval of middle to inner shelf origin the shelf was storm dominated and displays abundant hummocky cross stratification hcs of varying scales and other wave formed sedimentary structures some parts of this sequence are notably cyclic two kinds of cycles are described here the first average cycle 8 m thick consists of the alternation of silty and argillaccous limestone with fine grained hummocky cross stratified calcareous sandstone climatic cyclicity e g milankovitch cycles involving alternate wet and dry periods or minor sealevel fluctuations along a low gradient shoreline are considered the most likely cause of the observed sediment cyclicity carbonate deposition predominated during dry periods or high sealevel followed by influxes of sand from deltaic sources during wet periods or times of low sealevel the prevalence of hcs in the sandstones is most likely due to much slower rates of lithification in comparison to the limestones which give evidence of rapid lithification on the seafloor the second type of cycle average cycle 5 10 m thick is characterised by the cyclic appearence of red hummocky cross stratified glauconitic and phosphatic limestones within a sequence dominated by grey green stylonodular limestone tempestites the most likely origin of the red limestone involves contributions of volcanic ash fall material the resulting sediment did not undergo the rapid seafloor lithification of the associated grey green limestones and thus was involved in considerable storm wave reworking before deposition the micritic shelf carbonates of the wonoka formation are considered to represent direct inorganic precipitation into the middle shelf environment rather than reworking of detrital material from shallow carbonate banks

neoproterozoic marine dolomite hardgrounds and their relationship to cap dolomites abstract neoproterozoic cap dolomites are unusual and distinctive marker units that occur after large cryogenian glaciations several thin dolomite units that have features resembling cap dolomites occur within the cryogenian ediacaran succession of the adelaide geosyncline while each of these dolomite units has distinctive features they also share many similarities with cap dolomites including stratigraphic thickness ranging from several centimetres to several metres impure dolomicrite composition graded beds with a sharp top that become more dolomite rich upwards and evidence of volume expansion in the form of sheet cracks tepees or other soft sediment deformation structures the carbon isotopic compositions for these thin dolomite horizons have values ranging from 5 to 2 per mil vpdb which overlap those commonly found in both sturtian and marinoan cap dolomites the generally negative carbon isotope compositions of these dolomites and cap dolomites is most easily explained by mixing of marine and authigenic carbonate sedimentological evidence indicates the graded beds within cap dolomites and other dolomite units are a form of submarine hardground or firmground where dolomicrite is precipitated authigenically at or near the seafloor during periods of little or no sedimentation dolomite replacement of clay and other silicates is an important process in the formation of these dolomicrite firmgrounds volume expansion structures are a product of this near surface dolomicrite precipitation brought about by the high degrees of dolomite supersaturation with a consequent large force of crystallization and unlimited marine carbonate source for the precipitating dolomite we suggest that this deepwater authigenic dolomicrite precipitation is a result of the dominantly anoxic nature of cryogenian early ediacaran oceans and caused by alkalinity producing anoxic bacterial reactions when sedimentation rates slow sufficiently e g during transgressions thin dolomicrite horizons are produced in this hypothesis cap dolomites may largely be the result of sediment starvation combined with a possible increase in carbonate saturation during and following the post glacial transgressions associated with the sturtian and marinoan glacial events this scenario explains most of the sedimentological features of these unusual carbonates the widespread development of such authigenic carbonate would have profound implications for the interpretation of precambrian marine carbon isotopic compositions

temporal constraints for the late wisconsinan deglaciation of western canada using eolian dune luminescence chronologies from alberta abstract the laurentide ice sheet lis covered most of canada during the last glacial maximum sometime after 20 cal ka bp the lis began to recede from western canada and by 11 cal ka bp it had retreated from most of the province of alberta due to the scarcity of datable contemporaneous organic materials the precise timing of the retreat of the ice sheet from the region remains poorly constrained so that the chronology of the sequential positions of the ice sheet margin between 20 and 11 cal ka bp is largely tentative in this study we use luminescence dating of postglacial eolian deposits sourced primarily from glaciolacustrine and deltaic sediments in central and northern alberta to provide an updated chronology for the retreat of the lis from the region we examine 14 new and 13 previously published quartz optically stimulated luminescence osl ages as well as 19 previously published k feldspar infrared stimulated luminescence ages the chronologies are used to constrain successive regional ice sheet margin positions that are geomorphically recognizable across the region between 16 and 11 ka calendar years these data suggest that the lis may have retreated from central alberta earlier than portrayed in radiocarbon derived deglaciation models we propose that the osl chronology from postglacial dunes constrains the deglaciation of the region more accurately because luminescence dating does not rely on the development of vegetation to produce material for dating implications for the revised chronology include an improved timing of the emergence of an ice free corridor that may have been used for paleoindian migration into the americas as well as enhanced age constraints on when meltwater drainage routes to the arctic became open in relationship to the younger dryas the chronology could also provide improved constraints for geophysical models that reconstruct regional isostatic rebound which followed deglaciation

impact ejecta horizon within late precambrian shales adelaide geosyncline south australia a solitary layer of shattered crustal rock fragments has been traced over a distance of 260 kilometers within folded 600 million year old precambrian marine shales of the adelaide geosyncline south australia the fragments consist entirely of acid to intermediate volcanics approximately 1575 million years old displaying shattered mineral grains shock lamellae in quartz and small shatter cones fragments reach 30 centimeters in diameter and show evidence of vertical fall emplacement available evidence points to derivation of the rock fragments from a distant hypervelocity impact into the gawler range volcanics at lake acraman approximately 300 kilometers west of the adelaide geosyncline

vertebrate paleontology and the alleged ice free corridor the meat of the matter abstract the existence of a full glacial ice free corridor through western alberta has been argued for many years it has been assumed to have existed notwithstanding a near total lack of evidence by many in the north american archeological community as a means of explaining the occurrence of early man south of the ice in early postglacial time in the last 15 years much has been done to fill the knowledge gap including theories on alternate routes but there is still no consensus quaternary vertebrate fossils from alberta accompanied by numerous radiocarbon dates with a clear gap between 21 ka bp and at least 11 6 ka bp now suggest that alberta was overridden by ice during an extensive late wisconsinan glaciation therefore a full glacial ice free corridor could not have existed however much of the province could have served as a migration route northward and southward both before and after late wisconsinan glaciation the preglacial passage could not have been a corridor because numerous megafaunal remains and conifer wood from numerous localities with finite dates from ca 43 ka to 21 ka bp indicate extensive ice free conditions similarly a late pleistocene corridor critically appraised from the standpoints of vertebrate paleontology and archeology is irrelevant because by the time human migrants arrived in southern alberta less than 11 ka bp the ice had been in retreat for well over 1000 years relative to human influx the continuing use of the term corridor with its connotation of a narrow or restricted passage seems inappropriate

mammalian faunal dynamics in late pleistocene alberta canada abstract as a conduit for the passage of faunal and human species into the new world the much abused concept of the so called ice free corridor has served its purpose the corridor concept has had an important bearing on discussions of zoogeography and paved the way for hypotheses about the recolonization of north america by mammalian species from asia alberta situated along the eastern slopes of the western cordillera adjacent to the northern great plains was long considered the ideal route for late pleistocene migrations into the north american heartland yet a temporal hiatus in the large suite of bone and wood dates from central alberta indicates that no corridor existed during the full glacial from about 22 000 to 12 000 bp dna isotopic and other studies of equids bovids cervids ursids mustelids and others confirm that animals flowed through alberta except during full glaciation the mid wisconsinan fauna in alberta indicates movements from both south and north reflecting the absence of ice sheets during that interval migrations during the last glacial maximum were precluded because the cordilleran and laurentide ice masses had coalesced to close the corridor finally with the postglacial reopening of the passage late surviving proboscideans muskoxen horses camels and lions facing imminent extinction may have formed a northwesterly bound megafaunal vanguard simultaneously a thriving population of moose and wapiti drove southward from beringia towards the mid continent

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

quaternary stratigraphy and glacial history of the peace river valley northeast british columbiathis article is one of a selection of papers published in this special issue on the theme geology of northeastern british columbia and northwestern alberta diamonds shallow gas gravel and glaciers two cordilleran and three laurentide glacial advances are recorded in quaternary sediments and landforms in the peace river valley northeast british columbia the advances are inferred from fluvial gravels glaciolacustrine sedi ments and tills within nested paleovalleys excavated during three interglaciations and from the distribution of granitoid clasts derived from the canadian shield till of the last late wisconsinan laurentide glaciation occurs at the surface ex cept where it is overlain by postglacial sediments the advance that deposited this till was the most extensive in the study area and the only advance definitively recognized in western alberta south of the study area late wisconsinan cordil leran till has not been found in the study area but cordilleran and laurentide ice may have coalesced briefly during the last glaciation support for this supposition is provided by the inferred deflection of laurentide flutings to the southeast by cordilleran ice the earliest laurentide advance may have been the least extensive of the three laurentide events recog nized in the study area erratics attributed to this advance occur only east of the halfway river beatton river drainage divide

a late pleistocene human presence at huaca prieta peru and early pacific coastal adaptations abstract archaeological excavations in deep pre mound levels at huaca prieta in northern peru have yielded new evidence of late pleistocene cultural deposits that shed insights into the early human occupation of the pacific coast of south america radiocarbon dates place this occupation between 14 200 and 13 300 cal yr bp the cultural evidence shares certain basic technological and subsistence traits including maritime resources and simple flake tools with previously discovered late pleistocene sites along the pacific coast of peru and chile the results help to expand our knowledge of early maritime societies and human adaption to changing coastal environments

revised wonoka isotopic anomaly in south australia and late ediacaran mass extinction the global late ediacaran shuram wonoka carbon isotope anomaly has been regarded as the largest and longest known isotopic anomaly in the ocean assuming that all ediacaran carbonate is marine disregarding carbonate in south australia shown here to be palaeosol or palaeokarst the synchronous marine organic carbonate excursion is only 8 for 13 c organic and 6 for 13 c carbonate and lasted less than a million years this revised magnitude and duration is comparable with perturbations across the permian triassic boundary and correlative with a global late ediacaran acritarch mass extinction like permian triassic isotopic excursions the revised organic carbonate wonoka excursion may also have been a greenhouse palaeoclimatic warm spike which terminated valley incision and glacioeustatic drawdown during the mid ediacaran fauquier glaciation and preceded chill of the late ediacaran billy springs glaciation supplementary material measured sections and tables of mineral and grain size proportions major element and stable isotope analyses are available at www geolsoc org uk sup18756

particle size distribution analysis of sand sized particles by laser diffraction an experimental investigation of instrument sensitivity and the effects of particle shape laser diffraction is now widely used for particle size distribution analysis of sediments and soils the technique can be very precise and offers advantages of speed and cost over many other methods when used to analyse mixtures of sand silt and clay this study presents the results of an experimental investigation that examined the sensitivity of the beckman coulter ls230 instrument to mixtures of different grain populations and differences in particle shape the instrument was found to have high sensitivity to coarse particles in a finer matrix detection threshold 1 2 but much lower sensitivity to finer particles in a coarser mixture detection threshold 12 17 experiments using near spherical ballotini showed that laser analysis provides very similar values to dry sieving for the mean median and mode but for a range of natural sand samples values for the mean median and mode were offset by 8 21 with an average of ca 15 compared with sieving analysis using a beckman coulter rapidvue instrument which provides both size and shape information provided support for the hypothesis that the differences between laser analysis and sieving are partly attributable to the effects of particle shape however an additional factor is the way in which the laser software interprets the optical diffraction data the software predicts a high degree of log normality in the size distribution such that highly skewed truncated or bimodal samples are poorly represented experiments using sieved fractions of ballotini indicated that even with near perfectly spherical particles the particle size distribution predicted by the laser software includes a relatively large percentage of particles outside the sieve class limits

first finds of problematic ediacaran fossil gaojiashania in siberia and its origin we describe the first occurrence of the problematic fossil gaojiashania outside china in the ediacaran yudoma group of the siberian platform in both areas gaojiashania characterizes the lower upper ediacaran strata and precedes the appearance of cloudina and other skeletal fossils which highlights its significance for the ediacaran subdivision and correlation features of this fossil such as indeterminate length the absence of a distinct growth pattern and self avoiding behaviour indicate its trace fossil origin but do not necessarily imply metazoan affinities for its producers several organisms including stem group social amoebozoans and unicellular protists may have been proterozoic trace fossil producers

human colonization of the americas timing technology and process abstract geological and archeological research indicates that humans first colonized the americas with the use of watercraft along the southern coast of the bering land bridge and the western coast of the americas early dates from a number of archeological sites in the americas indicate human colonization of the americas began prior to ca 13 000 bp a review of archeological sites in eastern beringia identifies several distinctive cultural traditions which had developed by 11 000 10 000 bp geological biological linguistic evidence and dated human skeletal remains all suggest human occupation of the americas prior to ca 11 500 bp glacial geology indicates colonization could have begun ca 14 000 13 000 bp along the western coasts of the americas and ended about 5000 bp with deglaciation of the canadian eastern arctic and coastal greenland the use of watercraft and coastal navigation prior to 11 000 bp are inferentially demonstrated a model for early coastal and subsequent inland colonization of the americas along large ecological zones best fits current geologic and archeological data

microbial and animal evolution in relation to redox fluctuations in a deep water setting of south china during the ediacaran cambrian transition ca 551 523 ma abstract the ediacaran cambrian e c transition witnessed significant ecosystem perturbations of the oceanic environment e g oxygen concentrations of seawater and biological evolution however in spite of abundant ediacara type megafossils trace fossils small shelly fossils ssfs macroscopic algae and metazoans recovered from shallow water e c transition strata throughout south china the manifestation of geological events in deep water settings remains elusive here we present new paleontological and geochemical evidence from the 38 m thick deep water liuchapo formation ca 551 523 ma of the yangtou section northeastern guizhou province china which consists mainly of chert through the e c transition the liuchapo formation can be divided into two members based on biostratigraphy and redox proxies the lower member 0 32 m is characterized by low mo al 0 15 1 82 ppm and u al 0 47 2 51 ppm values that suggest deposition under suboxic conditions these deposits are dominated by horodyskia minor paleopascichnus jiumenensis spherical cyanobacteria myxococcoides and random acritarchs the upper member 32 38 m records a transition to anoxic non sulfidic conditions as suggested by marked increases in mo al 5 47 ppm and u al 8 810 ppm the upper member contains ssfs sponge spicules filamentous cyanobacteria multicellular algae and sheet like fossils the survival of diverse organisms under strongly reducing conditions reflected in the upper member suggests that marine redox fluctuations may limit the growth of late ediacaran organisms and provide the living space and elevated nutrients for early cambrian biota

deglaciation ages and meltwater routing in the fort mcmurray region northeastern alberta and northwestern saskatchewan canada a field based reconstruction of the deglacial paleogeography in the fort mcmurray area permits 1 constraining the timing of meltwater routing to the arctic from the present hudson bay drainage basin and 2 minimum age estimates for ice margin positions that can be used to constrain ice sheet modeling results a downslope recession of the laurentide ice sheet resulted in a series of proglacial lakes forming between the ice margin and higher land to the southwest the paleogeography of these lakes is poorly constrained in part from the masking effect of boreal forest vegetation and map scale issues however recent space shuttle based dems increase the number and spatial extent of moraines identified within the study area resulting in a coherent pattern of ice margin retreat focused on the athabasca river valley an intensive lake coring program resulted in a minimum ten fold increase in the radiocarbon database used to limit moraine ages results indicate that deglaciation in this region was younger than previously reported and it is likely that the meltwater could not drain northward to the arctic ocean from any source southeast of the fort mcmurray area until approximately 9850 9660 14 cbp

laurentide glaciation in west central alberta a single late wisconsinan event the grande prairie region in west central alberta shows evidence of late wisconsinan laurentide glaciation in the form of a widespread till containing abundant erratic clasts derived from the canadian shield two sections located on the smoky river at watino and on the simonette river 80 km to the southwest expose sediment lying stratigraphically below the till a similar sequence is exposed in both localities 5 10 m of coarse quartzite gravel overlain by a thick sequence of sand and silt the gravel contains no laurentide erratics and is thus preglacial the contact between the basal gravel and the overlying sand and silt is sharp and conformable 14c dates from the base of the simonette section and throughout the watino section suggest a middle wisconsinan age thus the sediment exposed is interpreted as a conformable middle wisconsinan sequence the sedimentary sequences were deposited in a moderate to low energy braided stream system flowing towards the north the general fining up trend in the

an optical age chronology of late pleistocene fluvial deposits in the northern lower mississippi valley the lower mississippi valley lmv contains many large braided channel belts that are preserved west of the holocene floodplain previous efforts to establish geochronologic control on channel belt construction have been hindered by the lack of organic material for radiocarbon dating luminescence techniques provide a burial date for the sediment itself and may prove useful in this context samples from three channel belts in the northern lmv were analyzed using the single aliquot regenerative technique on 90 125mm quartz optical ages 19 7 17 8 16 1 15 0 and 12 5 12 1 ka are consistent with geomorphic relationships and indicate that channel belts were formed in the late pleistocene under glacial conditions these optical ages provide the first detailed chronology of lmv channel belt formation and are the first step towards developing a chronology for the entire lmv r 2003 elsevier science ltd all rights reserved

life in near anoxic conditions a case study of the ichnology and infaunal ecology of silurian graptolitic black shales from sardinia italy abstract a trace fossil assemblage from the silurian llandovery black shales of the genna muxerru formation is reported for the first time the ichnofossil record is abundant well preserved and comprises alcyonidiopsis cochlichnus phymatoderma planolites large morphotype planolites rough morphotype and a small chondritid sedimentological and ichnological evidence indicates that the bioturbated black shales were deposited in outer shelf or deeper settings with severe depletion in oxygen the seafloor provided a high food supply for the infauna the palaeoenvironment of the genna muxerru formation is an ancient analogue of modern oxygen minimum zones burrow fills are apparently massive in macroscopic view whereas observations under the petrographic and stereoscopic microscope show that a lifestyle dominated i e that of pellet filling sediment feeders as such the genna muxerru formation provides not only a new ethologic reference for low oxygen settings but also a methodological benchmark for ichnological analysis of black shales bioturbation in black shales may be commoner than expected but its recognition requires microscopic observation

sequence stratigraphy and the interpretation of neoproterozoic earth history abstract the application of sequence stratigraphy to neoproterozoic successions is important for improving the resolution of time correlation within individual sedimentary basins and potentially at a global scale the methodology is illustrated in this paper by reference to two contrasting examples from the flinders ranges adelaide geosyncline of south australia where the younger part of the neoproterozoic to earliest cambrian succession 770 ma to 540 ma has been divided into thirteen unconformity bounded depositional sequences one of the most prominent sequence boundaries at or near the base of the wonoka formation is characterized by a series of buried canyons as much as 1 km deep high resolution sequence stratigraphic studies at umberatana syncline continue to support the view that the canyons were cut subaerially and filled by fluvial and shallow water sediments in contrast to the wonoka canyons sequence boundaries interpreted at the base of the nuccaleena formation seacliff sandstone and near the top of the abc range quartzite are relatively subtle with only limited evidence for erosion and valley incision four sequence boundaries at the level of the sturtian and marinoan varanger glacial deposits and in the vicinity of the precambrian cambrian boundary are thought to correlate with surfaces in the amadeus basin of central australia other prominent sequence boundaries including the wonoka canyons and surfaces within the upper part of the wonoka formation and at the base of the ediacara member of the rawnsley quartzite correspond with a relatively condensed section in the amadeus basin and their lateral persistence beyond the adelaide geosyncline is therefore difficult to evaluate given the lack of precision in biostratigraphy and isotope geochemistry in neoproterozoic rocks and in a marked departure from phanerozoic practice we recommend placement of a terminal proterozoic gssp at a sequence boundary a prime candidate in australia is the sequence boundary at the base of the nuccaleena formation seacliff sandstone immediately above the marinoan glacial rocks in the adelaide geosyncline and its likely correlative at or near the base of the gaylad sandstone in the amadeus basin

late quaternary paleoenvironments of northwestern north america implications for inland versus coastal migration routes abstract although the long held normative view of new world colonization relies on entrance through an interior ice free corridor by terrestrially adapted big game hunters a recently demonstrated human presence in southern chile dating prior to the existence of the corridor route implies different colonization scenarios which must be seriously considered a renewed focus of archaeological and paleoecological research on early post glacial landscapes in northwestern north america has revitalized the ice free vs coastal corridor controversy geological findings from glacial geology and paleo sea levels support the possibility of coastal migration from beringia to the pacific northwest between about 14 000 and 10 000 bp and preliminary paleoecological data suggest that the coastal landscape was in part vegetated and probably able to support a terrestrial fauna including humans the same cannot be said about the ice free corridor

global glacial isostasy and the surface of the ice age earth the ice 5g vm2 model and grace abstract the 100 kyr quasiperiodic variation of continental ice cover which has been a persistent feature of climate system evolution throughout the most recent 900 kyr of earth history has occurred as a consequence of changes in the seasonal insolation regime forced by the influence of gravitational n body effects in the solar system on the geometry of earth s orbit around the sun the impacts of the changing surface ice load upon both earth s shape and gravitational field as well as upon sea level history have come to be measurable using a variety of geological and geophysical techniques these observations are invertible to obtain useful information on both the internal viscoelastic structure of the solid earth and on the detailed spatiotemporal characteristics of glaciation history this review focuses upon the most recent advances that have been achieved in each of these areas advances that have proven to be central to the construction of the refined model of the global process of glacial isos

silicified horodyskia and palaeopascichnus from upper ediacaran cherts in south china tentative phylogenetic interpretation and implications for evolutionary stasis horodyskia is one of the earliest known macroscopic life forms with a fossil record dating from c 1 4 ga palaeopascichnus represents a key ediacaran element with world wide distribution however their body constructions and affinities are poorly understood partly because previously described species are mostly preserved as casts and moulds in siliciclastic rocks silicified specimens from the upper ediacaran liuchapo formation in eastern guizhou south china are described as horodyskia minor sp nov and palaeopascichnus jiumenensis sp nov their taxonomic assignments are based on their uniserial arrangement of spheroidal or discoidal units which are connected by a filament and surrounded by a quartz halo they are unlikely to be brown algae animal traces faecal pellets colonial metazoans or giant sulphide oxidizing bacteria instead we propose that horodyskia and palaeopascichnus may be phylogenetically related and their collective morphologies allow tentative comparison with agglutinated foraminifers the segments can be compared with cytoplasm filled chambers connecting filament with small passage between chambers and quartz haloes with agglutinated tests however their ontogeny appears to be distinct from that of modern foraminifers the occurrences of horodyskia fossils in mesoproterozoic and ediacaran rocks indicate an extremely long range c 900 ma and echoes the proposition of extended evolutionary stasis in the proterozoic

a reassessment of the problematic ediacaran genus orbisiana sokolov 1976 abstract the genus orbisiana was established in 1976 by b s sokolov based on a collection of seven specimens within core material from a borehole drilled through the ediacaran age gavrilov yam formation of the moscow basin russia here we reassess the original material for the type species orbisiana simplex sokolov 1976 which was long considered to be lost fix the holotype of the type species and revise the original diagnosis of the genus and species pyritisation of the fossils which are preserved in finely laminated shales allows three dimensional morphological characterisation of this taxon using x ray microtomography ct morphological and taphonomic analyses of the type material and additional three dimensionally preserved specimens from the verkhovka formation vendian of the white sea area suggest that orbisiana simplex consisted of submillimetric to millimetric globular chambers arranged in compact grape like clusters or forming sinuous to linear aggregates occasionally aggregates can bifurcate with no appreciable change in chamber dimensions or shape the phylogenetic affinity of orbisiana remains uncertain but its chambered construction putative agglutinated structure of the chamber walls and compact occasionally branching chamber arrangement are shared with agglutinated tests of the ediacaran genus palaeopascichnus our reassessment and systematic study of the genus orbisiana sheds new light on one of the least studied members of the late ediacaran macroscopic biota

atlantic meridional overturning and climate response to arctic ocean freshening 1 the global deglaciation event that followed last glacial maximum was punctuated by a sequence of rapid climate changes this began with the laurentide ice sheet derived calving event called heinrich event 1 continued with the bolling allerod warming that occurred synchronously with meltwater pulse 1a and was in turn followed by the younger dryas abrupt return to cold conditions although it has long been believed that these events are consequences of the response of atlantic meridional overturning circulation to freshwater inputs it has always been assumed that these inputs were applied directly to the atlantic ocean itself we address the question of how the atlantic meridional overturning circulation would have responded to the recently hypothesized addition of freshwater into the arctic ocean at the time of onset of the younger dryas and thereby demonstrate that this response is essentially identical to the response to north atlantic freshening

integrated stratigraphy of the upper neoproterozoic succession in yunnan province of south china re evaluation of global correlation and carbon cycle abstract this study provides a chemostratigraphic and sequence stratigraphic framework of a relatively unfossiliferous sub cambrian marine sedimentary succession in yunnan province of south china our results allow its global stratigraphic correlation and provide an improved understanding of marine carbon cycle during the late neoproterozoic the succession has a basal glacial diamictite nantuo formation and comprises two large scale depositional cycles towards the precambrian cambrian pc c boundary each of which changes from siliciclastic to carbonate sediments with environments ranging from shallow subtidal to evaporitic supratidal in the absence of biostratigraphic control attribution of the tillite has remained equivocal between the 710 ma sturtian and 635 ma marinoan glaciations a compilation of known paleomagnetic and other geochronologic constraints suggests that the nantuo glaciation in south china was at least diachronous in which the nantuo tillite in yunnan belongs to the older glaciation stratigraphic patterns of the diagenetically screened 13c variation and depositional sequences during the carbonate dominated interval show a striking similarity with those of the upper cryogenian and entire terminal proterozoic carbonates at widely separated other localities rather than the terminal proterozoic alone supporting the view that deposition of the nantuo tillite in yunnan reflects the sturtian rather than marinoan glaciation further considerations of the 13c results provide three important implications for the late neoproterozoic marine carbon cycle first comparison of different 13c profiles suggests considerable variability in the absolute values of carbonate on both intra and inter basinal scales which differs profoundly from their relative homogeneity in marine phanerozoic counterparts and may therefore reflect lateral isotopic heterogeneity in the oceanic carbonate reservoir during the late neoproterozoic second an empirical correlation between the 13c and relative sea level changes was applied to evaluate hypothesized explanations for 13c temporal variation in surface sea yielding short term inverse and long term positive relationships the short term trend marks periods of the negative 13c excursions and is compatible to most of the existing arguments for the 13c events on the other hand mechanisms causing the long term trend remain enigmatic in the absence of comparable data from other localities our observation however suggests that at least two different mechanisms operated marine carbon cycle during the late neoproterozoic third the negative 13c anomaly characterizing the period following the marinoan glaciation worldwide was not coincident between kerogen and carbonate but occurs as a lagged drop in yunnan where a 7 depletion in kerogen preceded an 3 depletion in carbonate their lagged trends likely reflect different sources as kerogen and carbonate signals are considered to monitor surface and deeper waters respectively the negative 13c event would therefore have occurred in surface water followed by a smaller 13c depletion through the entire water column due to vertical mixing between surface and deeper waters at the studied site this mechanism supports the proposed hypothesis that injection of the 13c depleted atmospheric co2 into the surface sea deriving ultimately from quantitative dissociation of methane hydrate in terrestrial permafrost during the post glacial warming contributed to the negative 13c anomaly

the cambrian revolutions trace fossil record timing links and geobiological impact several concepts pertaining to the dramatic changes that occurred during the cambrian have been proposed namely the agronomic revolution ar the cambrian substrate revolution csr and the cambrian information revolution cir the original concept of the ar refers to the replacement of precambrian type substrates matgrounds by phanerozoic type ones mixgrounds the csr highlights the evolutionary and ecological effects of cambrian substrate changes on epifaunal sessile metazoans the cir involves the distribution of signals in an environment that an organism can potentially respond to underscoring an increased complexity and heterogeneity of marine ecosystems which may have played a major role as a driving force of further evolutionary change during the cambrian a systematic review of the ediacaran cambrian ichnofossil record allows to provide some temporal resolution for these cambrian revolutions and to explore the synergies among these evolutionary breakthroughs and possible geobiological outcomes the ar is not coincident with the ediacaran cambrian boundary but rather it is manifested by cambrian age 2 because the csr focuses on the response of the epifauna to newly evolved mixgrounds and the late cambrian appearance of hardgrounds it may be understood at least in part as a consequence of the ar in contrast the cir involved the appearance of sophisticated innovative feeding strategies that allowed benthic organisms to successfully exploit resources in an increasingly more heterogeneous sea bottom the occurrence in the fortunian of highly patterned grazing trails reflects the establishment of efficient navigational devices although the cir signals the appearance of novel body plans and novel ways of animal substrate interactions strategies involving matground exploitation were still widespread in the fortunian more dramatic changes took place in cambrian age 2 as revealed by the onset of the ar and later the effects on epifaunal sessile organisms and communities during the csr signalling to the transition to shallow marine ecosystems of modern aspect

development of an osl chronology for late pleistocene channel belts in the lower mississippi valley usa abstract the broad alluvial valley of the lower mississippi river contains large channel belts for which age control has been limited by the lack of organic material for radiocarbon dating and deposit ages greater than the limit of radiocarbon in order to produce a detailed chronology of fluvial evolution samples for optical dating were collected from late pleistocene channel belt deposits quartz sand was analyzed using the coarse grained single aliquot regenerative dose sar technique samples showed little evidence of partial bleaching and optical ages are for the most part consistent with the available radiocarbon and relative age constraints optical ages range from 85 to 11 ka and along with geomorphic and stratigraphic relationships indicate that the mississippi river was meandering during the end of the last interglacial 85 83 ka and switched to a braided regime by 64 50 ka during initial marine oxygen isotope stage ois 4 3 glaciation of the upper mississippi drainage basin the mississippi river switched back to a meandering regime after formation of the youngest braid belt at 11 ka the 69 optical ages presented here provide the first detailed chronology of lower mississippi valley braid belt and pleistocene meander belt formation and provide an example of the utility of optical dating in fluvial settings where material for radiocarbon dating is scarce

optical dating studies of postglacial aeolian deposits from the south central interior of british columbia canada abstract tests of the validity of optical dating of the fine grained 4 11 m component of aeolian deposits from six sites in south central british columbia using 1 4 ev infrared excitation of potassium feldspars were made making use of known age tephra beds at one site on the western side of the fraser valley samples bracketing the mazama 7 5 7 6 ka cal yrs and bridge river tephras 2 3 ka cal yrs yielded optical ages in accordance with the known ages when a correction for thermal transfer was used the same method was applied to aeolian deposits on the fraser plateau known to have been deposited between 14 and 2 3 ka cal yrs the ages obtained from five samples from four separate exposures spanning 10 km varied from 30 to 94 ka clearly indicating that this sediment was not exposed to sufficient sunlight prior to burial as a result of a detailed investigation it is suggested that the reason is that some grains had been transported over short distances

late pleistocene northward dispersing bison antiquus from the bighill creek formation gallelli gravel pit alberta canada and the fate of bison occidentalis late pleistocene bison skeletal remains from the gallelli gravel pit in the bighill creek formation at calgary alberta document at least two individuals including the largest postglacial bison r

proliferation of miss forming microbial mats after the late neoproterozoic glaciations evidence from the kimberley region nw australia abstract a total of 20 morphological types of microbially induced sedimentary structures miss are recognized from the late neoproterozoic tillite bearing successions of the kimberley northwestern australia which include alpha petees triradiate cracks multidirectional linear ridges millimeter ripples erosional remnants and pockets and so on of these triradiate cracks multidirectional linear ridges and millimeter ripples are reported for the first time from the neoproterozoic worldwide most kimberley miss are morphologically comparable with their ancient and present day counterparts whose biogenicity have been well demonstrated in the kimberley during the interglacial period miss have colonization preference to relatively coarse sandy substrates ranford formation and their abundance decreased towards the younger strata throssell shale in contrast miss abundance increased from the older strata yurabi formation towards the younger strata flat rock formation during the postglacial period however overall both miss abundance and bedding plane coverage percentage increased from the marinoan younger marinoan gaskiers interglacial successions to younger marinoan gaskiers postglacial successions indicating a proliferation of microbial mats after the younger marinoan gaskiers glaciation the increase of microbial mats could have ameliorated marine biogeochemical conditions and ecosystems and thus laid the foundation for the rise of ediacaran biota in australian basins