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

tonian cryogenian boundary sections of argyll scotland abstract the tonian cryogenian system boundary is to be defined at a gssp global boundary stratigraphic section and point beneath the first evidence of widespread glaciation a candidate lies within the dalradian supergroup of scotland and ireland which is least deformed and metamorphosed in argyll western scotland we present new stratigraphic profiles and interpretations from the isle of islay and the garvellach islands update the chemostratigraphy of the appin group tonian carbonates underlying the thick ca 1 km glacigenic port askaig formation paf and demonstrate an environmental transition at the contact the appin group forms a regionally extensive 4 km thick succession of limestones shales and sandstones deposited on a marine shelf on islay the upper part of the lithostratigraphy has been clarified by measuring and correlating two sections containing distinctive stratigraphic levels including molar tooth structure oolite stromatolitic dolomite and intraclastic microbial mounds significantly deeper erosion at the unconformity at the base of the overlying paf is demonstrated in the southern section carbonate facies show a gradual decline in 13 c vpdb from 5 to 2 upwards in ne garbh eileach garvellach islands a continuously exposed section of appin group carbonates 70 m thick here designated the garbh eileach formation gef lies conformably beneath the paf the gef and the gef paf boundary relationships are re described with new sedimentological logs petrological and stable isotope data interstratified limestone and dolomicrosparite with 13 c of 4 to 7 a feature named the garvellach anomaly replacing the term islay anomaly are overlain by dolomite in which the isotope signature becomes weakly positive up to 1 upwards shallow subtidal conditions become peritidal upwards with evidence of wave and storm activity gypsum pseudomorphs and subaerial exposure surfaces are common near the top of the gef the basal diamictite d1 of the paf is rich in carbonate clasts similar to slightly deeper water parts of the underlying succession d1 is typically several metres thick with interstratified sandstone and conglomerate but dies out laterally scattered siliciclastic coarse sandstone to pebble conglomerate with dropstones associated with soft sediment deformation is interbedded with carbonate below and above d1 dolomite beds with derived intraclasts and gypsum pseudomorphs are found above d1 or equivalent position where d1 is absent published and new sr isotope studies including successive leach data demonstrate primary tonian 87 sr 86 sr values of 0 7066 0 7069 on islay decreasing to 0 7064 0 7066 in the younger gef limestones on the garvellachs with 1700 2700 ppm sr other typically tonian characteristics of the carbonates are the sr rich nature of limestones molar tooth structure and dolomitized peritidal facies with evidence of aridity seabed surveys suggesting uniformly dipping strata and shallow borehole core material illustrate the potential for extending the tonian record offshore of the garvellachs a candidate tonian cryogenian gssp is proposed on garbh eileach within the smooth 13 c profile at the cross over to positive 13 c signatures 4 m below the first occurrence of ice rafted sediment and 9 m below the first diamictite although lacking radiometric constraints or stratigraphically significant biotas or biomarkers the scottish succession has a thick and relatively complete sedimentary record of glaciation coherent carbon and strontium chemostratigraphy lateral continuity of outcrops and 100 exposure at the proposed boundary interval

oceanography bathymetry and syndepositional tectonics of a precambrian intracratonic basin integrating sediments storms earthquakes and tsunamis in the belt supergroup helena formation ca 1 45 ga western north america abstract the carbonate dominated helena formation of the mesoproterozoic belt supergroup of western north america provides an instructive example of how a range of regional depositional and environmental characteristics of an ancient sea can be deduced on the basis of micron to metre scale features particularly revealing is the window opened by the presence of abundant molar tooth structure onto the paleoceanography paleobathymetry paleoclimate and tectonic regime of this intracratonic precambrian basin the facies hosting molar tooth structure is composed dominantly of lime mud with substantial subangular quartz and feldspar silt and clay derived from the western and southwestern side of the basin these are low energy tempestites deposited on a remarkably flat sea bottom at the limit of storm wave base at about 50 m sporadic domical stromatolite patch reefs confirm that the sea bottom was normally within the photic zone the ubiquity of molar tooth structure suggests frequent near field seismic activity during subsidence which generated ground motion sufficient to liquefy granular lime mud and terrigenous silt sporadic tsunamis from major submarine faults far to the west pounded the shallow water platform to the east tsunami off surge swept ooids and rounded coarse grained feldspathic quartz sand westward into deeper water and created strongly erosive currents that left gutter casts composed of lags of preferentially cemented molar tooth structure in otherwise relatively low energy facies mineralogical and geochemical evidence confirms that the belt basin was marine organic matter was essentially fully oxidized in the water column original high mg composition and cementation of lime mud in molar tooth structure indicate that calcite precipitated above the thermocline in supersaturated seawater under tropical conditions scattered bimineralic ooids in allochthonous grainstones indicate that shoals on the platform to the east were intermittently above a shallow aragonite compensation depth in warm water rare vestiges of halite and gypsum demonstrate occasional temporary salinity stratification for periods of up to a few years unaltered feldspar and mica grains point to a generally arid climate but the large volume of clay is suggestive of protracted weathering and transport

changing influence of antarctic and greenlandic temperature records on sea level over the last glacial cycle we use a simple model to analyse the relationship between ice core temperature proxy data and global ice volume eustatic sea level data over the last glacial cycle lgc by allowing the temperature forcing to be a mix of greenlandic and antarctic signals we optimise the proportion of this mixing to fit sea level data we find that sea level forcing is best represented by a mix of antarctic and greenlandic temperature signals through the whole glacial cycle we suggest that a distinct bipolar switch occurs which links eustatic sea level more closely with the antarctic like variability during the glacial period mis 4 3 and 2 and more closely to the greenland like variability during the last termination ti and the interglacial periods holocene and mis 5 this switch may be caused by the spatio temporal distribution of ice sheet collapse perhaps linked to glacial to interglacial changes in deep water distribution in the ocean which in turn drive changes in pole ward heat and moisture transport

microbially induced sedimentary structures from the 1 64 ga chuanlinggou formation jixian north china abstract the present study documents abundant exceptionally preserved microbially induced sedimentary structures miss from the 1 64 ga chuanlinggou formation of the jixian area north china craton the chuanlinggou formation is dominated by black silty shale mudstone with fine sandstone of the lower part and was likely deposited in a relatively restricted shallow marine environment varying from fair weather wave action zone to below storm wave action zone both wrinkle structures and sand veins are very abundant in the lower and middle parts of the formation wrinkled layers are dark colored densely laminated and contain floating quartz grains and abundant mica grains both of which are aligned parallel to bedding plane such an arrangement pattern is attributed to the microbe s binding and trapping of miss sem imaging also shows that sticky networks interwoven by filamentous extracellular polymer substances eps and biofilms with distinct pores are very pronounced in clay matrix of wrinkled layers providing direct evidence of microbial activities during the formation of wrinkle structures in addition the circularly arranged quartz grains forming hollow cavities and vertical channels of quartz grains beneath the laminated layers are unique to the wrinkle structures showing initial forming stage of the sand veins sand veins similar to fills of sand cracks are usually preserved in silty shales and occasionally occur beneath the winkled surfaces they were formed due to methane degassing of decay of microbial mats and biofilms abundant nano particles of various epss isolated nanoglobules paired nanoglobules dumbbell shaped nanoglobules and polyhedron aggregates are detected in clay matrix close to sand veins indicating microbial participation in the formation of sand veins wide spatial distribution broadened environmental inhabitation and extremely high abundance and bed surface density of the misss indicate proliferation of miss forming microbial mats in mesoproterozoic times

the role of ocean atmosphere reorganizations in glacial cycles abstract a case is made that glacial to interglacial transitions involve major reorganizations of the ocean atmosphere system such reorganizations constitute jumps between stable modes of operation which cause changes in the greenhouse gas content and albedo of the atmosphere only in this way can the rapidity of glacial terminations the hemispheric synchroneity and symmetry of mountain glaciation and the large polar air temperature and dustiness variations be accounted for if these reorganizations are driven in some fashion by orbitally induced seasonal insolation changes then the connection between insolation and climate is most likely through impacts of fresh water transport on the ocean s salinity distribution

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

seasonal variability from a model of the tropical atlantic ocean abstract a numerical model incorporating a single baroclinic mode and realistic coastline geometry is used to analyze the linear dynamic response to estimates of the seasonal wind field over the tropical atlantic ocean the forced periodic response consists of a spatially dependent combination of a locally forced response kelvin waves rossby waves and multiple wave reflections the seasonal displacements of the model pycnocline are compared with observed dynamic height annual and semiannual fluctuations dominate the seasonal signal throughout the basin in general the distribution of amplitude and phase are similar for annual changes in dynamic height and pycnocline depth major features of the seasonal response are reproduced e g east west changes in pycnocline depth about a nodal point at the equator the seasonal pycnocline movement along the northern and southern coast of the guinea gulf and a significant changes of phase in the ocean variability north and south of the itcz the relative impo

relationship between zonal and meridional modes in the tropical atlantic the tropical atlantic displays two main modes of variability the meridional or dipole mode and the equatorial zonal mode it was proposed recently that these two modes are correlated and are both forced within the tropics by latitudinal displacements of the itcz this modeling study shows that while the two modes are correlated for limited record lengths prior to and after 1976 the correlation falls apart when longer time series from 1949 to 2000 are considered the 1976 climate shift also occurred in the tropical atlantic seen as a thermocline shift similar to the pacific forced dynamically within the tropics the first eof of the simulated thermocline depth captures the inter decadal mode with the 1976 shift the first eof of sst anomalies prior to after 1976 represents the meridional zonal mode consistent with the previous finding that the relation between the eastern pacific and atlantic itcz is stronger in the 1980 90 s

molar tooth structures of the neoarchean monteville formation transvaal supergroup south africa ii a wave induced fluid flow model sedimentological morphological and geochemical characteristics of molar tooth mt structures in the ca 2 6 ga monteville formation suggest a new fluid flow model for mt formation i intercalated shales and carbonate sands were deposited near to above storm wave base ii sediments cracked forming an interconnected network of mt cracks that were also open to pores in sand lenses iii storm waves pumped sea water into open mt crack networks causing rapid microcrystalline carbonate nucleation ostwald ripening of nuclei and growth of granular carbonate cores some of these cores were transported by water flowing through the cracks iv unfilled mt cracks collapsed and filled mt ribbons deformed plastically as host sediments compacted and dewatered v carbonate cores were overgrown by polygonal rims and vi mt structures deformed brittlely with additional compaction and produced pebbly lags if reworked mt cracks may have formed by multiple mechanisms however expansion of gas from organic decay and sediment heaving due to wave loading best explain mt crack morphology and are most consistent with the fluid flow model for mt caco3 presented here

an analysis of tropical instability waves in a numerical model of the pacific ocean 2 generation and energetics of the waves this work was supported by the department of com merce noaa under grant na56gp0026 one of the au thors sm was partially supported by a nasa global change fellowship ngt 30288 another author ab was supported by a ucar postdoctoral fellowship

the magnitude of error in conventional bulk sediment radiocarbon dates from central north america abstract although the carbon reservoir problem with bulk sediment radiocarbon dates from lakes has long been recognized many synoptic studies continue to use chronologies derived from such dates for four sites in central north america we evaluate chronologies based on conventional radiocarbon dates from bulk sediment versus chronologies based on accelerator mass spectrometry ams radiocarbon dates from terrestrial plant macrofossils the carbon reservoir error varies among sites and temporally at individual sites from 0 to 8000 yr an error of 500 2000 yr is common this error has important implications for the resolution of precise event chronologies

molar tooth structures of the neoarchean monteville formation transvaal supergroup south africa i constraints on microcrystalline caco3 precipitation molar tooth mt structures are enigmatic contorted millimetre to decimetre long veins and spheroids of microcrystalline calcite that formed during very early diagenesis in precambrian sediments mt structures in the ca 2 6 ga monteville formation are 600 800 myr older than previously reported occurrences and establish that conditions necessary for mt genesis were met locally throughout much of the precambrian in the monteville formation mt structures were formed shallow subtidally extending to depths near storm wave base in shale host sediments intercalated with storm generated carbonate sand lenses they are filled with microcrystalline calcite and rare pyrite microcrystalline calcite identical to that in mt structures fills other pore space including porosity between grains in carbonate sand lenses moldic porosity in sand grains sheet cracks in columnar stromatolites and shallow cracks on sandy bedding planes relationships in the monteville formation demonstrate that microcrystalline caco3 precipitated in fluid filled cracks and pores microcrystalline calcite characteristics as well as the paucity of carbonate mud in host rocks are inconsistent with injection of lime mud as the origin of mt structures locally mt cracks were filled by detrital sediment before or during precipitation precipitation occurred in stages and mt caco3 evolved from granular cores to a rigid mass of cores with overgrowths allowing both plastic and brittle deformation of mt structures as well as reworking of eroded mt structures as rigid clasts and lime mud crystal size distributions and morphology suggest that cores precipitated through nucleation ostwald ripening and size dependent crystal growth whereas overgrowths formed during size independent crystal growth

on the formation of a subsurface weakly sheared laminar layer and an upper thermocline strongly sheared turbulent layer in the eastern equatorial pacific interplays of multiple time scale equatorial waves

the application of radiocarbon age estimates to the dating of glacial sequences an example from the miami sublobe ohio u s a abstract despite the importance of dating glacial sequences little evidence has been available to highlight the stratigraphic issues associated with the interpretation of age analysis a compilation of radiocarbon data from the miami sublobe of the laurentide ice sheet provides a large enough data set more than 133 dates to identify at least two sources of error sample and unit variability and transportation problems analysis of different samples from the same stratigraphic level commonly differ by more than the reported one sigma error and 2 000 year differences do occur glacial entrainment and transportation emplace older organic material over younger in situ materials and this effect may also give age errors in the data set of up to 2 000 years at a minimum multiple radiocarbon ages from a sequence of units are necessary to assign a reliable age to events within a glacial sequence however representing multiple ages poses its own problems a graphical representation that combines the probability of each sample appears to be one solution

variation in glacial erosion near the southern margin of the laurentide ice sheet south central wisconsin usa implications for cosmogenic dating of glacial terrains we measured the abundance of cosmo genic 10 be and 26 al in 22 samples collected from five striated granite metarhyolite and quartzite outcrops in south central wiscon sin that were covered by the late wisconsin laurentide ice sheet in two outcrops mea sured nuclide abundances are consistent with the existing radiocarbon chronology of ice retreat in three outcrops nuclide abun dances were up to eight times higher than predicted by the radiocarbon chronology at these three sites several thousand years of ice flow eroded only centimeters to deci meters of rock allowing a significant quan tity of nuclides 10 5 two samples carry the equivalent of 150 000 yr of surface exposure even though they were covered by ice during the last glacial maximum advance samples highest on the landscape or in plucked ar eas have less inheritance than those from the lee sides of large hills or lower in the landscape three quartzite samples collect ed 10 km up ice from the margin contain three to four times the expected nuclide abundance 10 5 to 10 6 atoms per gram of quartz in contrast eight other quartzite and granite samples from two outcrops 50 km up ice from the former margin contain only 10 5 atoms of 10 be per gram of quartz consistent with late pleistocene ex posure and little if any nuclide inheri tance this relationship between glacial erosion and distance from the former ter minus is consistent with a marginal zone of minimal subglacial erosion the ice was ei ther frozen to its bed there or the ice thick ness and duration of ice cover were less near the terminus these data together with simple model ing of nuclide production by deeply pene trating muons suggest that many meters of rock must be removed to reduce inheri tance to negligible levels 1000 yr in con tinental terrains with low long term erosion rates our results indicate that cosmogenic dating of exposed bedrock surfaces near former ice margins or in areas where ice was frozen to the bed may be uncertain and in some cases impossible because nu clides are inherited from prior periods of cosmic ray exposure unfortunately striat ed glacial outcrops that can be used to dem onstrate most easily the assumption of no postglacial erosion are also the most likely to have undergone little glacial erosion this finding suggests that cosmogenic nuclide production rates based on glacially striated surfaces may include cosmogenic nuclides inherited from prior exposure

numeric control on the late glacial chronology of the southern laurentide ice sheet derived from ice proximal lacustrine deposits abstract we used a combination of radiocarbon and osl dating in ice proximal lacustrine silt and clay and outwash sand to estimate when ice of the green bay lobe of the laurentide ice sheet began retreating from its maximum position in south central wisconsin the radiocarbon ages indicate that lakes had formed in the two tributary valleys by 17 2 and 20 1 ka respectively the osl ages indicate that the green bay lobe was at its maximum position from about 26 4 5 1 ka to 21 4 3 3 ka these data provide entirely new chronologic control on late wisconsin marine isotope stage 2 glacial event in the upper midwest as well as the opportunity to directly compare radiocarbon and osl ages in this setting

earthquake supercycles in central italy inferred from 36cl exposure dating we use 36cl surface exposure dating to determine the slip release pattern over the last 15 kyr for the velino magnola fault a major active normal fault in central italy we sampled the fault at five well separated sites along its length and modeled the 36cl concentrations measured in the 376 samples we find that the fault broke in at least 9 large earthquakes that occurred in two 5 6 ka long supercycles each cycle included a 4 5 ka long phase of relative quiescence followed by a cluster of at least 3 large earthquakes or earthquake sequences that released most of the accumulated strain in 1 ka all 9 identified events broke the entire fault and produced maximum surface slips of 2 3 m though the velino magnola fault seems presently in a stage of relative quiescence it may re enter a phase of paroxysmal seismic activity in a few hundred of years

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

gas bubble cavities in deltaic muds lake powell delta glen canyon national recreation area hite utah abstract cavities produced by migrating gas mainly methane emanating from the near subsurface are a rarely reported sedimentary structure from either modern or ancient clastic sediments the delta of the colorado river in lake powell glen canyon national recreation area provides an excellent recently accessible locality for examining soft sediment deformation structures generated by escaping gases and fluids sub meter scale to tens of meter scale structures including craters salses and sand and mud volcanoes are present on the modern lake powell delta abundant sub centimeter scale cavities are found in excavations through crater sediments and exposed along cut bank exposures of the colorado river in cross section the cavities are subvertical irregular to sigmoidal to lenticular shaped 1 3 mm short axes up to 3 0 cm long axes and 1 cm intermediate axes cavities are developed in cm scale laminated silty clay and clay graded beds the tops of the cavities are wider than their bases the cavities observed in the lake powell delta are interpreted as mode 1 fractures and probably represent a clastic morphologic analog to molar tooth carbonate structures the characteristics of the cavities and the geometries and deformation of the surrounding delta mud host are consistent with bubbles and fracturing generated by gas migration experiments and theoretical calculations but vary because of inherent heterogeneity of the sediments fractures developed as gas migrated into pre existing cavities causing overpressure in cavities and leading to subvertical mode 1 fracture propagation recent sudden decompression related to rapid lake level drops of lake powell may have enhanced the release of gas primarily bacterially produced leading to the development of these soft sediment deformation structures

changes in the ventilation of the oxygen minimum zone of the tropical north atlantic changes in the ventilation of the oxygen minimum zone omz of the tropical north atlantic are studied using oceanographic data from 18 research cruises carried out between 28 5 and 23 w during 1999 2008 as well as historical data referring to the period 1972 85 in the core of the omz at about 400 m depth a highly significant oxygen decrease of about 15 mol kg 1 is found between the two periods during the same time interval the salinity at the oxygen minimum increased by about 0 1 above the core of the omz within the central water layer oxygen decreased too but salinity changed only slightly or even decreased the scatter in the local oxygen salinity relations decreased from the earlier to the later period suggesting a reduced filamentation due to mesoscale eddies and or zonal jets acting on the background gradients here it is suggested that latitudinally alternating zonal jets with observed amplitudes of a few centimeters per second in the depth range of the omz contribute to the ventilation of the omz a conceptual model of the ventilation of the omz is used to corroborate the hypothesis that changes in the strength of zonal jets affect mean oxygen levels in the omz according to the model a weakening of zonal jets which is in general agreement with observed hydrographic evidences is associated with a reduction of the mean oxygen levels that could significantly contribute to the observed deoxygenation of the north atlantic omz

deciphering the sedimentological imprint of paleoseismic events an example from the aptian codo formation northern brazil abstract a stratigraphic horizon bearing several styles of soft sediment structures occurs sandwiched within entirely undisturbed deposits in the lacustrine codo formation aptian located in the eastern part of the grajau basin northern brazil these deposits show a variety of sedimentary features distinguished according to their deformation style as follows from bottom to top a zone 1 consisting of widespread molar tooth structures i e fine grained spar filled cracks that are interconnected with small scale faults fissures and stylolites inclined at a high angle to bedding b zone 2 represented by complex convolute folds with lateral gradation of smooth synclines and anticlines into isoclinic folds and internal layers that retain the normal bedding features recumbent folds nappe folds with associated thrust faults pseudonodules and mound and sag structures c zone 3 consisting of normal faults and fissures that are vertical to near vertical show ragged morphology with small delicate peaks taper both downward and upward after a few centimeters and are associated with large up to 2 5 m long collapsed boulders and d zone 4 characterized by shales with irregular convolute folds these deformation zones are interpreted to reflect sediment rheology and alternating periods of sediment accumulation and deformation under the influence of synsedimentary seismic events as suggested mostly by 1 occurrence between undeformed deposits 2 lateral correlation for several kilometers of distance 3 similarity with other features formed in areas affected by seismic activity 4 presence of structures attesting to sediment deformation in an unconsolidated to semi consolidated state and 5 complex association of brittle and ductile deformation structures hence zone 1 records small scale cracking and faulting of early cemented cohesive deposits zone 2 is dominated by ductile deformation structures formed on water saturated unconsolidated sediments probably as a reflex of compressional stress zone 3 records a change in tectonic style to dominantly distension as evidenced by the horizon with normal faults finally zone 4 shows structures attributed to ductile deformation of water saturated muds which was probably formed during the same episode of deformation responsible for the faults and fissures in zone 3 this succession of deformation events appears to be consistent with transtensional and compressional shear stresses during the sedimentary evolution of the grajau basin

possible seismic origin of molar tooth structures in neoproterozoic carbonate ramp deposits north china the xinmincun formation forms the uppermost unit of a thick neoproterozoic section which accumulated near the east margin of the north china block and is overlain by two thin nearshore to continental formations below fossiliferous lower cambrian sediments although tectonically deformed sedimentary structures are preserved undeformed on cleavage parallel surfaces and an 80 m section has been reconstructed by correlation across minor folds and faults in the golden stone beach area 50 km ne of the city of dalian southern liaoning province the measured section shows 65 m of storm dominated deposits consisting of alternations of micrites and sharp based graded intraclastic grainstone beds tempestites some with rudaceous commonly erosional or guttered bases the top 15 m of section shows three alternations of similar subtidal lithofacies with partly to completely dolomitized peritidal deposits laminated sometimes fenestral and desiccated micrite beds and intraclastic rudite and grainstone beds tempestite beds become thinner and less abundant upwards towards a muddy upper shoreface zone this environment was characterized in part by the occurrence of micrite with thin or streaky lamination probable storm resuspended sediment interbedded tempestites numerous erosion surfaces and evidence of liquidization and sediment slumping into hollows molar tooth structures are widespread in micrite beds of the xinmincun formation and are present in lesser abundance in tempestites and liquefied channel fills originally the structures were sub vertical cracks 1 20 cm long tapering upwards and downwards subsequently they became filled with microspar cement and buckled rigidly during compaction of surrounding sediment during burial evidence of repeated episodes of cracking presence of brecciated cracks and localization of cracking within beds together with variable degree of development and variation of preferential alignment in plan indicates a mechanical origin crack generation may have been by seismic surface waves generated by movement along faults defining either the basin s margins or its internal structure or possibly by wave action during storms a seismic origin for molar tooth structure is consistent with other neoproterozoic occurrences their preferential occurrence in precambrian deposits arises from the relative rigidity of micritic sediment at this time related to lack of bioturbation incipient cementation and possibly microbial binding

eolian sand and loess deposits indicate west northwest paleowinds during the late pleistocene in western wisconsin usa our study adds to the quaternary history of eolian systems and deposits in western wisconsin usa primarily within the lower chippewa river valley thickness and textural patterns of loess deposits in the region indicate transport by west northwesterly and westerly winds loess is thickest and coarsest on the southeastern flanks of large bedrock ridges and uplands similar in some ways to shadow dunes in many areas sand was transported up and onto the western flanks of bedrock ridges as sand ramps presumably as loess was deposited in their lee long linear dunes common on the sandy lowlands of the chippewa valley also trend to the east southeast small depressional blowouts are widespread here as well and often lie immediately upwind of small parabolic dunes finally in areas where sediment was being exposed by erosion along cutbanks of the chippewa river sand appears to have been transported up and onto the terrace treads forming cliff top dunes luminescence data indicate that this activity has continued throughout the latest pleistocene and into the mid holocene together these landforms and sediments paint a picture of a locally destabilized landscape with widespread eolian activity throughout much of the postglacial period

anatomy of a cyclically packaged mesoproterozoic carbonate ramp in northern canada abstract carbonates in the upper member of the mesoproterozoic victor bay formation are dominated by lime mud and packaged in cycles of 20 50 m these thicknesses exceed those of classic shallowing upward cycles by almost a factor of 10 stratigraphic and sedimentological evidence suggests high amplitude high frequency glacio eustatic cyclicity and thus a cool global climate ca 1 2 ga the victor bay ramp is one of several late proterozoic carbonate platforms where the proportions of lime mud carbonate grains and microbialites are more typical of younger phanerozoic successions which followed the global waning of stromatolites facies distribution in the study area is compatible with deposition on a low energy microtidal distally steepened ramp outer ramp facies are hemipelagic lime mudstone shale carbonaceous rhythmite and debrites mid ramp facies are molar tooth limestone tempestite with microspar intraclast lags in a marine environment where stromatolitic and oolitic facies were otherwise rare large stromatolitic reefs developed at the mid ramp coeval with inner ramp facies of microspar grainstone intertidal dolomitic microbial laminite and supratidal evaporitic red shale deep subtidal outer ramp cycles occur in the southwestern part of the study area black dolomitic shale at the base is overlain by ribbon nodular and carbonaceous carbonate facies all of which exhibit signs of synsedimentary disruption cycles in the northeast are shallow subtidal and peritidal in character shallow subtidal cycles consist of basal deep water facies and an upper layer of subtidal molar tooth limestone tempestite interbedded with microspar calcarenite facies peritidal cycles are identical to shallow subtidal cycles except that they contain a cap of dolomitic tidal flat microbial laminite and rarely of red shale sabkha facies or of sandy polymictic conglomerate a transect along the wall of a valley extending 8 5 km perpendicular to depositional strike reveals progradation of inner ramp tidal flats over outer and mid ramp facies during shoaling the maximum basinward progradation of peritidal facies coincides with a zone of slope failure that may have promoted the development of the stromatolitic reefs the sea level history of the victor bay formation is represented by three hectometre scale sequences an initial flooding event resulted in deposition of the lower victor bay shale member upper member carbonate cycles were then deposited during highstand mid ramp slumping was followed by late highstand reef development the second sequence began with development of an inner ramp lowstand unconformity and a thick mid ramp lowstand wedge a second transgression promoted a more modest phase of reef development at the mid ramp and shallow water deposition continued inboard a third and final transgressive episode eventually led to flooding of the backstepping ramp overall consistent cycle thickness and absence of truncated cycles as well as the high rate and amount of creation of accommodation space suggest that the periodicity and amplitude of sea level fluctuation were relatively uniform and point to a eustatic rather than tectonic mechanism of relative sea level change high amplitude high frequency eustatic sea level change is characteristic of icehouse worlds in which short term large scale sea level fluctuations accompany rapidly changing ice volumes affected by milankovitch orbital forcing packaging of cyclic upper victor bay carbonates therefore supports the hypothesis of a late mesoproterozoic glacial period as proposed by previous workers

optical ages on loess derived from outwash surfaces constrain the advance of the laurentide ice sheet out of the lake superior basin usa abstract we present textural and thickness data on loess from 125 upland sites in west central wisconsin which confirm that most of this loess was derived from the sandy outwash surfaces of the chippewa river and its tributaries which drained the chippewa lobe of the laurentide front during the wisconsin glaciation mis 2 on bedrock uplands southeast of the widest outwash surfaces in the chippewa river valley this loess attains thicknesses 5 m osl ages on this loess constrain the advance of the laurentide ice from the lake superior basin and into west central wisconsin at which time its meltwater started flowing down the chippewa drainage the oldest mar osl age 23 8 ka from basal loess on bedrock agrees with the established but otherwise weakly constrained regional glacial chronology basal ages from four other sites range from 13 2 to 18 5 ka pointing to the likelihood that these sites remained geomorphically unstable and did not accumulate loess until considerably later in the loess depositional interval other osl ages from this loess taken higher in the stratigraphic column but below the depth of pedoturbation range to nearly 13 ka suggesting that the chippewa river valley may have remained a loess source for several millennia

sequence stratigraphy of a tidally dominated carbonate siliciclastic ramp the tithonian early berriasian of the southern neuquen basin argentina the tithonian berriasian vaca muerta carrin cura and picun leufu formations in the southern neuquen basin were deposited on a tidally dominated mixed carbonate siliciclastic ramp basinal outer middle shallow and back ramp facies associations are recognized and a sequence stratigraphic analysis reveals that the ramp record consists of three shallowing upwards sequences ti1 ti2 and ti3 set within a lower order progradational cycle a higher order of cyclicity is superimposed on to the middle ti2 sequence the majority of the ramp facies belong to the transgressive and highstand systems tracts however at the base of ti2 a lowstand systems tract is identified characterized by a basal unconformity and an abrupt basinward shift of the shallow marine lithofacies transgressive systems tracts were characterized by slow sedimentation rates and rapid sea level rises that affected carbonate productivity highstand systems tracts show the greatest carbonate productivity and an increased progradation rate on account of a reduction in accommodation space generation palaeogeography played a major role in the development of the depositional systems partial isolation from the pacific ocean reflecting the growth of the andean magmatic arc and geographic restriction due to tectonic inversion in the central part of the basin resulted in a meso macrotidal regime that produced a tidally dominated sedimentary record in the shallow and back ramp environments coeval anoxic conditions in the central part of the neuquen basin favoured distal ramp and basinal black shale deposition during episodes of relative sea level rise

decadal and interannual sst variability in the tropical atlantic ocean abstract the mechanisms regulating interannual and decadal variations of sea surface temperature sst in the tropical atlantic are examined observed variations of sea surface temperature are typically in the range of 0 3 0 5 c and are linked to fluctuations in rainfall on both the african and south american continents the authors use a numerical model to simulate the observed time series of sea surface temperature for the period 1960 1989 based on the results experiments are conducted to determine the relative importance of heat flux and momentum forcing two dominant timescales for variability of sst are identified a decadal timescale that is controlled by latent heat flux anomalies and is primarily responsible for sst anomalies off the equator and an equatorial mode with a timescale of 2 5 years that is dominated by dynamical processes the interhemispheric gradient of anomalous sst the sst dipole is primarily linked to the former process and thus results from the gradual strengthening and weak

molar tooth carbonates shallow subtidal facies of the mid to late proterozoic discussion from field work in northern canada and many other examples documented in the literature james et al 1998 highlighted the potential use of molar tooth structures i e a series of spar filled cracks as a stratigraphic indicator for the mesoproterozoic and neoproterozoic these authors also stated that all known occurrences where molar tooth structures are volumetrically significant are from shallow water carbonates although these are comprehensive statements considering that most molar tooth structures reported so far are from shallow water mesoproterozoic and neoproterozoic deposits i am concerned about such proposed temporal and environmental constrictions which are not supported by the field data summarized in the following detailed field work in the grajau basin northern brazil revealed that structures similar to the ones reported by james et al 1998 are volumetrically widespread in the codo formation this is a lacustrine lithostratigraphic succession formed during the early stages of the south atlantic rifting in the aptian e g petri and fulfaro 1983 the sedimentary features described here occur in lime mudstone to grainstone ostracodal grainstone and crystalline limestone interbedded with shales strongly affected by penecontemporaneous deformation as indicated by a variety of complex convolute folds small faults fissures and bed collapses the molar tooth structures occur throughout these beds although they substantially increase in volume downward where evidence for soft sediment deformation diminishes these

ages for the big stone moraine and the oldest beaches of glacial lake agassiz implications for deglaciation chronology glacial lake agassiz has been implicated as the trigger for numerous episodes of abrupt climate change at the close of the last ice age yet the beginning age of the lake has never been determined here we report the fi rst numerical age data on the big stone moraine and the oldest beaches of glacial lake agassiz organic remains from lakes bogs and channels distal to and inset to the big stone moraine require that glacial activity at this moraine ceased prior to 12 000 14 c yr b p 13 950 cal calendar yr a site near new effi ng ton south dakota united states implies full glacial recession north of the topographic divide prior to 11 810 14 c yr b p 13 670 cal yr synchronous with the beginning of glacial lake agassiz lake agassiz shorelines inset to the moraine yield optically stimulated luminescence osl ages from 14 200 12 600 yr cal lower strandlines are younger but the similarity of ages suggests that initial lake lowering was faster than osl ages can currently resolve nevertheless the osl ages represent the fi rst numerical age assignments for the herman norcross and upham beach ridges setting the stage for future numerical age assignments within the lake agassiz basin these two dating methods yield strongly consistent results within stated uncertainties the age of the big stone moraine implies an interval of rapid retreat for the des moines lobe of the laurentide ice sheet during the bolling allerod warm interval the overlapping ages for the uppermost beach levels and abandonment of the highest lake agassiz spillway indicate a rapidly evolving lake until at least 13 500 yr cal

magnetic fabrics of drumlins of the green bay lobe southeastern wisconsin abstract to study if the relief of drumlins of the green bay lobe of the laurentide ice sheet is dominantly the result of depositional or erosional processes we measured the anisotropy of magnetic susceptibility of 2500 samples of the horicon till from five drumlins in southeastern wisconsin these drumlins contain no macroscopic heterogeneities that might otherwise help guide interpretation of deformation kinematics ring shear experiments on this till provided the calibration necessary to relate fabrics based on principal susceptibilities of multiple intact samples to patterns of subglacial till deformation fabric strengths based on orientations of maximum susceptibility and measured from samples collected below the zone of pedogenesis weaken abruptly at depths shallower than 2 m the till lacks evidence of systematically divergent or convergent shear related to drumlin shape most till has been sheared in directions 6 24 to the east of drumlin long axes with shear plane orientations that are unrelated to drumlin morphology an exception is till at shallow depths 2 m where shearing azimuths and shear plane orientations agree more consistently with drumlin trends and morphologies the horicon till was likely deformed in shear during advance of the lobe drumlins formed by erosion of this basal till after glacier flow had shifted to the west when the johnstown moraine was being built a supraglacial till 2 m thick was draped over some of the drumlins during retreat of the lobe these results do not support models of drumlin formation in which sediment deposition helps amplify drumlin relief

optical ages indicate the southwestern margin of the green bay lobe in wisconsin usa was at its maximum extent until about 18 500 years ago abstract samples for optical dating were collected to estimate the time of sediment deposition in small ice marginal lakes in the baraboo hills of wisconsin these lakes formed high in the baraboo hills when drainage was blocked by the green bay lobe when it was at or very near its maximum extent therefore these optical ages provide control for the timing of the thinning and recession of the green bay lobe from its maximum position sediment that accumulated in four small ice marginal lakes was sampled and dated difficulties with field sampling and estimating dose rates made the interpretation of optical ages derived from samples from two of the lake basins problematic samples from the other two lake basins south bluff and feltz basins responded well during laboratory analysis and showed reasonably good agreement between the multiple ages produced at each site these ages averaged 18 2 ka n 6 and 18 6 ka n 6 respectively the optical ages from these two lake basins where we could carefully select sediment samples provide firm evidence that the green bay lobe stood at or very near its maximum extent until about 18 5 ka the persistence of ice marginal lakes in these basins high in the baraboo hills indicates that the ice of the green bay lobe had not experienced significant thinning near its margin prior to about 18 5 ka these ages are the first to directly constrain the timing of the maximum extent of the green bay lobe and the onset of deglaciation in the area for which the wisconsin glaciation was named

radiocarbon analyses from cincinnati ohio and their implications for glacial stratigraphic interpretations abstract detailed analysis of a site near cincinnati ohio shows that 14c ages of samples in a single geologic unit can have a range of several thousand years and ages from different stratigraphic units can overlap at the sharonville site four 14c samples from organic silt below glaciogenic deposits have an inverted chronologic sequence suggesting contamination but nevertheless they indicate the silt was deposited before 27 000 yr b p a stump cluster in growth position wood fragments and moss from the upper surface of the silt may differ by as much as 2300 14c yr five ages from the stump cluster constrain the timing of a glacier advance of the laurentide ice sheet to its southern limit in the cincinnati area at 19 670 68 yr b p overlying glaciogenic sediments contain transported wood that may be as much as 3200 yr older than the advance this range of ages points out that for a given site several age measurements are required to determine when a glacier advance occurred because the measured ages in this study span the entire interval suggested for a twofold sequence of advance retreat and readvance of the margin of the miami sublobe we suggest a single advance to its terminal position in the cincinnati area as an alternate hypothesis for testing

the bjerknes feedback in the tropical atlantic in cmip5 models coupled state of the art general circulation models still perform relatively poorly in simulating tropical atlantic ta climate to investigate whether lack of air sea interaction might be responsible for their biases we investigate the bjerknes feedback bf in the ta the driver of the dominant interannual variability in that region first we analyse this mechanism from reanalysis data then we compare our findings to model output from the coupled model intercomparison project phase 5 the feedback is subdivided into three components the first one consists of the influence of eastern equatorial sea surface temperature anomalies sst on zonal wind stress anomalies tau u in the western basin the second component is the influence of wind stress anomalies in the western ta on eastern equatorial oceanic heat content anomalies hc the third component is the local response of overlying sst to hc in the eastern ta all three components are shown to be present in era interim and oras4 reanalysis by correlating the two variables of each component with each other the obtained patterns are compared to the ones from model output via pattern correlation per component while the models display errors in the annual cycles of sst tau u and hc as well as in the seasonality of the feedback the impact of sst on wind stress and the impact of wind stress on hc are simulated relatively well by most of the models this is especially the case when correcting for the error in seasonality the third component of the bf the impact of hc on sst in the eastern part of the basin deviates from what we find in reanalysis we find an influence of hc anomalies on overlying ssts in the eastern equatorial ta but it is weaker than in the reanalysis and it is not strongly confined to the equator longitude depth cross sections of equatorial temperature variance and correlation between subsurface temperature anomalies and sst in the cold tongue region show that flawed simulation and slow adjustment of the subsurface ocean are responsible for this

geochemistry and mineralogy of late quaternary loess in the upper mississippi river valley usa provenance and correlation with laurentide ice sheet history abstract the midcontinent of north america contains some of the thickest and most extensive last glacial loess deposits in the world known as peoria loess peoria loess of the upper mississippi river valley region is thought to have had temporally varying glaciogenic sources resulting from inputs of sediment to the mississippi river from different lobes of the laurentide ice sheet here we explore a new method of determining loess provenance using k rb and k ba values in k feldspars and micas in loess from a number of different regions in north america results indicate that k rb and k ba values can distinguish loess originating from diverse geologic terrains in north america further different loess bodies that are known to have had the same source sediments using other criteria have similar k rb and k ba values we also studied three thick loess sections in the upper mississippi river valley region at each site the primary composition of the loess changed over the course of the last glacial period and k rb and k ba values parallel changes in carbonate mineral content and clay mineralogy we thus confirm conclusions of earlier investigators that loess composition changed as a result of the shifting dominance of different lobes of the laurentide ice sheet and the changing course of the mississippi river we conclude that k rb and k ba values are effective robust and rapid indicators of loess provenance that can be applied to many regions of the world

3d_fault_offsets a matlab code to automatically measure lateral and vertical fault offsets in topographic data application to san andreas owens valley and hope faults measuring fault offsets preserved at the ground surface is of primary importance to recover earthquake and long term slip distributions and understand fault mechanics the recent explosion of high resolution topographic data such as lidar and photogrammetric digital elevation models offers an unprecedented opportunity to measure dense collections of fault offsets we have developed a new matlab code 3d fault offsets to automate these measurements in topographic data 3d fault offsets mathematically identifies and represents nine of the most prominent geometric characteristics of common sublinear markers along faults especially strike slip in 3 d such as the streambed minimum elevation top free face and base of channel banks or scarps minimum laplacian maximum gradient and maximum laplacian and ridges maximum elevation by calculating best fit lines through the nine point clouds on either side of the fault the code computes the lateral and vertical offsets between the piercing points of these lines onto the fault plane providing nine lateral and nine vertical offset measures per marker through a monte carlo approach the code calculates the total uncertainty on each offset it then provides tools to statistically analyze the dense collection of measures and to reconstruct the prefaulted marker geometry in the horizontal and vertical planes we applied 3d fault offsets to remeasure previously published offsets across 88 markers on the san andreas owens valley and hope faults we obtained 5 454 lateral and vertical offset measures these automatic measures compare well to prior ones field and remote while their rich record provides new insights on the preservation of fault displacements in the morphology

autogenic incision and terrace formation resulting from abrupt late glacial base level fall lower chippewa river wisconsin usa abstract a paucity of research exists regarding the millennial scale response of inland alluvial streams to abrupt base level fall studies of modern systems indicate that over short time scales the response is a diffusion like process of upstream propagating incision in contrast evidence from the lower chippewa river lcr located in the upper midwest of the usa suggests that autogenic controls operating over time scales of several millennia can overwhelm diffusion resulting in incision that is prolonged and episodic during the last glacial maximum the lcr drained the chippewa lobe of the laurentide ice sheet to the glacial upper mississippi river umr as a meltwater stream it aggraded and filled its valley with glacial outwash as did its largest tributaries which were also meltwater streams its nonglacial tributaries aggraded too filling their valleys with locally derived sediment during deglaciation the umr incised at least twice abruptly lowering the lcr s base level 15 m at 16 ka or earlier and an additional 40 m at ca 13 4 ka each of these base level falls initiated incision of the lcr led by upstream migrating knickpoints the propagation of incision has however been a lengthy process the optically stimulated luminescence osl ages of terrace alluvium indicate that by 13 5 ka incision had advanced up the lcr only 15 km and by 9 ka only 55 km the process has also been episodic resulting in the formation of fill cut terraces inferred from gpr surveys and exposures of terrace alluvium that are younger and more numerous in the upstream direction autogenic increases in sediment load and autogenic bed armoring the result of periodic tributary stream rejuvenation and preferential winnowing of fines by the incising river may have periodically caused knickpoint migration and incision to slow and possibly stop allowing lateral erosion and floodplain formation to dominate a decline in sediment flux from stabilizing incised tributary stream systems would have led to renewed knickpoint migration and incision when floods of sufficient magnitude to breach the channel armor occurred minimal floodplain development along the upper section of the present day lcr along with the channel morphology of an unstable wandering gravel bed river immediately downstream from it suggest that the river is still responding to the base level falls that happened many millennia ago the autogenic controls on the lcr s response to umr incision are a direct consequence of the thick fills of noncohesive sediment that accumulated in its valley and the valleys of its tributary streams during the late wisconsinan making the lcr a prime example of a former proglacial river that remains a paraglacial fluvial system

global ocean surface velocities from drifters mean variance el nino southern oscillation response and seasonal cycle 1 global near surface currents are calculated from satellite tracked drogued drifter velocities on a 0 5 0 5 latitude longitude grid using a new methodology data used at each grid point lie within a centered bin of set area with a shape defined by the variance ellipse of current fluctuations within that bin the time mean current its annual harmonic semiannual harmonic correlation with the southern oscillation index soi spatial gradients and residuals are estimated along with formal error bars for each component the time mean field resolves the major surface current systems of the world the magnitude of the variance reveals enhanced eddy kinetic energy in the western boundary current systems in equatorial regions and along the antarctic circumpolar current as well as three large eddy deserts two in the pacific and one in the atlantic the soi component is largest in the western and central tropical pacific but can also be seen in the indian ocean seasonal variations reveal details such as the gyre scale shifts in the convergence centers of the subtropical gyres and the seasonal evolution of tropical currents and eddies in the western tropical pacific ocean the results of this study are available as a monthly climatology

palynological and radiocarbon evidence for deglaciation events in the green bay lobe wisconsin abstract a new and significant site of organic silty sand has been found beneath the valders till at valders quarry in northeastern wisconsin this is now the earliest known late glacial site associated with red till ice advances in the western great lakes area leaves of terrestrial plants washed into a small depression provide a date of 12 965 200 yr b p wis 2293 which is significantly older than the two creeks forest bed ca 11 800 yr b p percentage and concentration pollen diagrams suggest that the site was open and distant from a closed picea forest no wood or picea needles have been found this date is statistically indistinguishable from 12 550 233 yr b p the mean of three dates for the end of inorganic varve sedimentation at devils lake 160 km southwest at the terminus of the green bay lobe assuming that the green bay lobe vacated its outermost moraine in the interval from 13 000 to 12 500 yr b p only a short time was available for retreat of the ice margin over 350 km drainage of red sediment from lake superior into the lake michigan basin readvance of over 250 km retreat of at least 80 km and advance to this site the time for these events appears to have been too short to resolve by current radiocarbon technique this extremely rapid collapse of the green bay lobe has a calibrated age of about 15 000 cal yr b p about that of the dramatic warming seen in the greenland ice cores

ecosystem development following deglaciation a new sedimentary record from devils lake wisconsin usa abstract processes and rates of ecosystem development can be reconstructed using lacustrine sedimentary sequences but this approach often requires records that contain the start of primary succession most lakes in the upper midwestern u s were formed by glaciers at the end of the last ice age approximately 11 700 cal yr bp devils lake wisconsin is a rare example of a lake from this region whose sediments extend into the pleistocene and may include the last glacial maximum sediment magnetic geochemical pollen and charcoal records were generated from a 10 m core whose basal sediments may be 28 000 years old together with a previously published pollen record these proxies combine to reveal a history of long term climatic vegetative and geologic change during the late pleistocene to holocene we identify six sedimentary units that indicate a series of consecutive events rather than a predictable trajectory of ecosystem development at the site productivity in the lake was low during the late pleistocene and increased during the holocene as reflected by the sediment lithology which shows a sudden shift from glacial vivianite rich and organic poor clastic dominated sediments to holocene diatomaceous sapropels several important processes initiated around 17 000 cal yr bp including the onset of organic matter accumulation and fire in the terrestrial ecosystem however the post glacial landscape was not devoid of vegetation because pollen assemblages indicate that terrestrial vegetation likely a spruce tundra survived near the site a switch to a hardwood forest period during the holocene also led to a change in the fire regime with increased frequency of burning aquatic ecosystem productivity lagged terrestrial ecosystem productivity throughout the record nutrient cycling as recorded by sedimentary 15n was variable but not directional and appeared to be correlated with climate conditions early in the record and terrestrial ecosystem processes later in the record throughout the holocene magnetic mineral concentration decreased as productivity increased and the source of magnetic material shifted from almost exclusively lithogenic to approximately 50 derived from soil or biogenic sources magnetic grain size ambrosia pollen percentages and charcoal concentration increased and 15n decreased in the most recent part of the record due to anthropogenic influence in the region including agricultural activities

vertical heat flux in the ocean estimates from observations and from a coupled general circulation model the net heat uptake by the ocean in a changing climate involves small imbalances between the advective and diffusive processes that transport heat vertically generally it is necessary to rely on global climate models to study these processes in detail in the present study it is shown that a key component of the vertical heat flux namely that associated with the large scale mean vertical circulation can be diagnosed over extra tropical regions from global observational data sets this component is estimated based on the vertical velocity obtained from the geostrophic vorticity balance combined with estimates of absolute geostrophic flow results are compared with the output of a non eddy resolving coupled atmosphere ocean general circulation model reasonable agreement is found in the latitudinal distribution of the vertical heat flux as well as in the area integrated flux below about 250 m depth the correspondence with the coupled model deteriorates sharply at depths shallower than 250 m due to the omission of equatorial regions from the calculation the vertical heat flux due to the mean circulation is found to be dominated globally by the downward contribution from the southern hemisphere in particular the southern ocean this is driven by the ekman vertical velocity which induces an upward transport of seawater that is cold relative to the horizontal average at a given depth the results indicate that the dominant characteristics of the vertical transport of heat due to the mean circulation can be inferred from simple linear vorticity dynamics over much of the ocean

recovering paleoearthquake slip record in a highly dynamic alluvial and tectonic region hope fault new zealand from airborne lidar knowing the slip amplitudes that large earthquakes produced in prehistorical times is one key to anticipate the magnitude of large forthcoming events it is long known that the morphology is preserving remnants of paleoearthquake slips in the form of fault offset landforms however the measured offsets that can be attributed to the most recent paleoearthquakes are generally few along a fault so that they rarely allow recovering the slip distributions and largest slips of these earthquakes we acquired 1 m resolution airborne lidar data on a 30 km stretch of a fast slipping strike slip fault eastern hope fault new zealand located in a region of high alluvial dynamics where landforms are rapidly evolving data analysis reveals 200 offset landforms only 30 allow a good to moderate quality offset measurement from these good to moderate quality measures we recover the slip length distributions and largest slips of the four most recent large paleoearthquakes and find evidence of 4 6 prior events the record suggests that large earthquake slip recurred in multiples of about 4 m along the 30 km stretch although they have larger uncertainties the more numerous lower quality offsets that we also measured reveal a similar earthquake slip record this shows that although offset landforms are partly degraded in dynamically active landscapes they store valuable information on paleoearthquake slips this information might be recovered provided that the morphology is analyzed at high resolution and continuously over a significant fault length remote lidar data are powerful to perform such analyses

stratigraphy and correlation of glacial deposits from indiana to new york and new jersey the quaternary glacial history of part of the region from indiana to new york and new jersey fig 1 was outlined in three summaries prepared for the vii congress of the international union for quaternary research wayne and zumberge 1965 goldthwait et al 1965 muller 1965 subsequent regional summaries by white 1969 1982 white et al 1969 ray 1974 and coates 1976 reviewed the entire succession of tills in parts of kentucky northeastern ohio pennsylvania and new york other syntheses and discussions by dreimanis and goldthwait 1973 evenson and dreimanis 1976 dreimanis 1977 fullerton 1980 mayewski etal 1981 and mickelson et al 1983 were concerned only with late wisconsin tills the glacial history of the region fig 2 and chart 1 is much more complex than was assumed in 1965 much of the new information is in unpublished theses and dissertations and other publications of limited distribution and it has not been summarized correlations in chart 1 are based primarily on stratigraphic relationships of tills and paleosols and secondarily on radiometric ages paleomagnetic polarity data and relativeage data this summary serves as an expanded explanation of chart 1 glacial events are discussed within a framework of time divisions from older to younger and by glacial lobes as shown in chart 1 paleomagnetic polarity data in chart 1 are from harrison and terasmae 1961 bleuer 1976 and pers commun johnson 1976 this volume and pers commun hoyer 1977 1983 and pers commun bonnett et al 1978 and teller and last 1981 sources of data related to named tills in indiana ohio pennsylvania new york and new jersey are listed in appendix 1 the term glacial lobe here is applied both to a regional physiographically controlled pattern of ice dispersal and to the geographic region influenced by such movement huron erie lobe glaciation includes ice movement from the georgian bay region ontario into the lake erie basin as well as ice movement through the lake huron basin into the lake erie basin fig 3 illinoian and pre illinoian tills throughout the region have not been well studied and they have no radio

stratigraphy and correlation of the glacial deposits of the des moines and james lobes and adjacent areas in north dakota south dakota minnesota and iowa the glacial deposits of the des moines and james lobes record the extension of late wisconsin laurentide ice from canada into minnesota and iowa and into north dakota and south dakota respectively fig 1 chart 1 d e in this review we also discuss wisconsin age deposits of adjacent parts of north dakota and pre wisconsin deposits in north dakota south dakota and southwestern minnesota chart 1 b c e

loess of the upper mississippi valley driftless area abstract loess of the driftless area includes four distinct late quaternary lithostratigraphic units the wyalusing new loveland roxana and peoria formations erosion has removed parts or all of the pre late wisconsian loess at many sites these formations consist largely of loess and retransported loess that typically occurs on uplands terraces and valley margins in the region the oldest widespread formation probably 125 000 yr b p is here formally named the wyalusing formation the three younger formations correlate with the loveland probably 125 000 yr b p roxana 55 000 25 000 yr b p and peoria 25 000 12 000 yr b p loesses recognized elsewhere in the midcontinental united states the soil stratigraphic morphology of the wyalusing loveland and roxana peoria couplets appears to represent two distinct loess sedimentation sequences related to major expansion and contraction of the laurentide ice sheet during oxygen isotope stages 6 and 3 2 respectively the occurrence frequency of loess units is inversely related to age illustrating the erosional nature of the driftless area landscape the occurrence of four loess units at some sites in driftless area stratigraphic sections corroborates loess stratigraphy along the length of the mississippi valley where typically not more than four or five loess units are found and they represent only the late and middle pleistocene

the chronology climate and confusion of the moorhead phase of glacial lake agassiz new results from the ojata beach north dakota usa rapid drainage of glacial lake agassiz to the north atlantic ocean has been implicated as a triggering mechanism for the younger dryas yd cold event 13 11 6 ka cal a key component to this hypothesis is the interpretation that the ojata beach of lake agassiz in north dakota usa formed during a regression of the lake at the beginning of the yd this paper reviews the chronological data for the lowwater moorhead phase presents new radiocarbon and optically stimulated luminescence osl ages for the ojata beach and utilizes plant and insect macrofossils to reconstruct the paleoenvironmental conditions during the moorhead phase in eastern north dakota the integrated analysis of the geochronologic data emphasizes the need to distinguish between in situ and reworked plant macrofossils new ages obtained stratigraphically below the ojata beach sediments and reinterpreted chronologic data for the moorhead phase suggest that contrary to previous interpretations the ojata beach is a record of transgression of the lake in the later part of the yd and the oldest in situ minimum age for the moorhead phase is 10 47775 ka 14 c bp paleoenvironmental analysis indicates that a

late wisconsin periglacial environments of the southern margin of the laurentide ice sheet reconstructed from pollen analyses full glacial pollen assemblages from four radiocarbon dated interstadial deposits in southwestern ohio and southeastern indiana imply the presence of herbaceous vegetation tundra or muskeg with subarctic indicator selaginella selaginoides on the southern margin of the miami lobe of the laurentide ice sheet ca 20 000 14c yr bp scattered picea spruce and possibly pinus pine may have developed regionally ca 19 000 14c yr bp and ca 18 000 14c yr bp respectively spruce stumps in growth position support a local source of pollen prior to the ca 14 000 14c yr bp glacial advance small amounts of quercus oak and other deciduous pollen suggest development of regional boreal conifer hardwood forests copyright 2002 john wiley sons ltd

ice wedge casts and relict patterned ground in central illinois and their environmental significance abstract sediment features and soil patterns indicate that permafrost developed beyond the late wisconsinan woodfordian ice margin in illinois vertical wedge shaped bodies in woodfordian glacigenic deposits and buried by woodfordian loess are interpreted to be ice wedge casts large scale patterned ground common on the late wisconsinan drift plain and on the illinoian drift plain to the south and west also is interpreted to be the result of ice wedge formation permafrost extended to about 38 30 n latitude and formed during the interval from about 21 000 to 16 000 yr b p formation and degradation of permafrost was time transgressive directly related to the ice marginal climate and probably restricted temporally to less than 1000 years at a site the following environmental conditions are suggested near the ice margin mean annual ground temperature 3 c mean annual air temperature 6 2 c less precipitation than modern particularly in winter strong and likely dry wind systems and relatively open vegetation with few woody plants

morphology of molar tooth structures in precambrian carbonates influence of substrate rheology and implications for genesis abstract molar tooth mt is an enigmatic carbonate fabric composed of variously shaped cracks and voids filled with a characteristically uniform equant microspar mt is both abundant and widespread in mesoproterozoic and early neoproterozoic strata where void filling microspar comprises up to 90 of individual beds and 5 25 of preserved carbonate the temporal restriction of this fabric suggests a potential link between mt formation and the biogeochemical evolution of marine environments detailed petrographic relationships among mt crack morphology distribution of mt microspar and composition of the surrounding substrate suggest that crack formation and microspar precipitation are intimately linked to the decomposition of sedimentary organic matter in the presence of supersaturated proterozoic seawater laboratory experiments have shown that gas generated within unconsolidated mud can reproduce a variety of mt crack morphologies yet current gas expansion and migration models do not explicitly consider the role of substrate variability in determining morphologies of mt cracks a detailed petrographic examination of mt structures from the mesoproterozoic belt supergroup montana permits interpretation of the microscale relationship between crack morphology and lithologic and potentially rheologic variability of the surrounding substrate by tracing the distribution of petrographically distinctive mt microspar observations of lateral offset of mt cracks at bedding planes or within coarser grained siltstone or sandstone layers termination of cracks beneath clay or organic rich horizons grain collapse into underlying mt cracks and the presence of mt microspar as a pore filling precipitate suggest that grain size substrate lithology and substrate cohesion all play critical roles in the development of mt cracks by contrast the presence of a wide range of mt crack morphologies within petrographically homogeneous substrates and an apparent relationship between crack diameter and sinuosity suggest that the void forming process itself also played a role in determining the final morphology of mt cracks together these petrographic observations are used to define a model of microscale gas sediment interactions that can be used to interpret crack morphology in terms of gas pressure and the strength of sedimentary substrates the presence of characteristic void filling microspar is integral to preservation of mt structures cathodoluminescence cl identification of this characteristic microspar within mt voids in pore space of coarse grained facies and interstitially within fine grained facies adjacent to mt voids suggests that mt voids and cement share a common genesis because microspar cores are similar in size and morphology to vaterite precipitated experimentally in the presence of a variety of dissolved organic molecules we suggest that precipitation of mt microspar was intimately linked with gas production during organic decomposition within the host substrate in this scenario gas production would result in pore fluids with elevated concentrations of dissolved organic molecules which would initiate precipitation of mt microspar when the pore fluids come in contact with supersaturated proterozoic seawater restriction of mt largely to mesoproterozoic and early neoproterozoic strata likely reflects a critical level of carbonate saturation that limited early substrate lithification thereby allowing void production but remained high enough that organic catalysts were able to initiate precipitation of mt microspar

the origin of the western lake erie grooves ohio implications for reconstructing the subglacial hydrology of the great lakes sector of the laurentide ice sheet abstract streamlined grooves and s forms cover the western lake erie islands ohio as well as the channels between the islands and the adjacent mainland although previous interpretations advocate direct glacial ice erosion we offer the alternative interpretation that forms were eroded by meltwater specifically by plucking sediment abrasion and cavitation in high velocity sediment laden flows we base our arguments on detailed descriptions and interpretations of the glacial grooves state memorial site ggsm on kelleys island the site consists of a linear furrow that is adorned by s forms in its base and on its walls these include linear forms cavettos sichelwannen comma forms multi troughed forms and plunging forms all s form sites provide a snapshot of the last phase of erosion after evolution of the local landscape through multiple generations of s form formation as s forms are found on topographic highs and lows we believe they represent a regional flow that overtopped the majority of the islands flow was initially as a sheet that quickly collapsed into channelized flow that was diverged around the islands we also believe that this flow had a major impact on the laurentide ice sheet in this region resulting in rapid glacial advance and extremely low ice sheet profiles

subaqueous shrinkage cracks in the sheepbed mudstone implications for early fluid diagenesis gale crater mars the sheepbed mudstone yellowknife bay formation gale crater represents an ancient lakebed now exhumed and exposed on the martian surface the mudstone has four diagenetic textures including a suite of early diagenetic nodules hollow nodules and raised ridges and later diagenetic light toned veins that crosscut those features in this study we describe the distribution and characteristics of the raised ridges a network of short spindle shaped cracks that crosscut bedding do not form polygonal networks and contain two to four layers of isopachous erosion resistant cement the cracks have a clustered distribution within the sheepbed member and transition laterally into concentrations of nodules and hollow nodules suggesting that these features formed penecontemporaneously because of the erosion resistant nature of the crack fills their three dimensional structure can be observed cracks that transition from subvertical to subhorizontal orientations suggest that the cracks formed within the sediment rather than at the surface this observation and comparison to terrestrial analogs indicate that these are syneresis cracks cracks that formed subaqueously syneresis cracks form by salinity changes that cause sediment contraction mechanical shaking of sediment or gas production within the sediment examination of diagenetic features within the sheepbed mudstone favors a gas production mechanism which has been shown to create a variety of diagenetic morphologies comparable to the raised ridges and hollow nodules the crack morphology and the isopachous layered cement fill show that the cracks were filled in the phreatic zone and that the sheepbed mudstone remained fluid saturated after deposition and through early burial and lithification

molar tooth structures in calcareous nodules early neoproterozoic burovaya formation turukhansk region siberia abstract molar tooth structures are abundant in large 1 2 m diameter carbonate nodules within fine grained subtidal carbonates of the early neoproterozoic lower upper riphean burovaya formation along the sukhaya tunguska river turukhansk uplift northwestern siberia although molar tooth structures are regionally abundant in this unit here they occur only within the nodules stable isotopic compositions of molar tooth filling dolomicrospar cements and of thinly bedded dolomicrite within and surrounding the nodules are indistinguishable from one another the carbon isotopic compositions mean 13c 2 8 pdb 0 4 reflect mean average oceanic surface water composition during their formation the light oxygen isotopic compositions mean 18o 6 4 pdb 2 2 are generally similar to those of other little altered meso to neoproterozoic limestones and dolostones these molar tooth structures have no features that would support a tectonic origin they more likely formed through bacterial processes carbonate cement filling of these voids occurred soon after their formation but the mechanism responsible for this carbonate precipitation is currently uncertain local restriction of molar tooth structures to early diagenetic nodules suggests that penecontemporaneous lithification was required for the formation or at least preservation of these widespread mesoproterozoic to neoproterozoic features

seismic slip history of the pizzalto fault central apennines italy using in situ produced 36cl cosmic ray exposure dating and rare earth element concentrations morphological and geological observations reveal that most apenninic faults are highly segmented and that the majority of the fault segments are less than 10 km long although these faults have undergone numerous paleoseismological investigations quantitative data remain crucially lacking for a large number of fault segments because such data are essential to understanding how these faults have ruptured and interacted in the past and how they might behave in the future we investigated the holocene seismic history of the pizzalto normal fault a 13 km long fault segment belonging to the pizzalto rotella aremogna fault system in the apennines we collected 44 samples from the pizzalto fault plane exhumed during the holocene and analyzed the 36cl and rare earth element ree contents together the 36cl and ree concentrations show that at least six events have exhumed 4 4 m of the fault scarp between 3 and 1 ka with slip per event values ranging from 0 3 to 1 2 m no major events have been detected over the last 1 kyr the rotella aremogna pizzalto fault system has a clustered earthquake behavior with a mean recurrence time of 1 2 kyr and a low to moderate probability ranging from 4 to 26 of earthquake occurrence over the next 50 years

sedimentation patterns during the precambrian a unique record abstract although the similarities between depositional processes and products as well as the analogous controls on basin filling and evolution appear to have enjoyed great uniformity throughout the sedimentary rock record a noticeable distinction exists in the rates and intensities of a broad range of geological processes in the precambrian epoch this paper searches for distinctiveness in the precambrian sedimentary record both siliciclastic and carbonate through an extensive though not exhaustive review of the relevant literature augmented by new observations while differences in precambrian deltaic aeolian glacial and possibly also lacustrine deposits and settings appear to have been small their large scale development was controlled largely by a combination of temporal and geodynamic influences essentially of global compass in this regard the onset of the supercontinent cycle and major perturbations in palaeo atmospheric composition appear to have been significant marine environments provide a poor platform for precambrian phanerozoic comparisons of sedimentation patterns as those from the former period are preserved almost exclusively in epeiric settings an environment essentially lacking on modern earth for the shallow marine carbonates biological mediation of chemical sediment deposition changed radically from dominance by microbial biota in the precambrian to a combination of metazons protozoans and algae for the skeletal carbonates of the phanerozoic despite it being widely recognized that precambrian channel systems were braided in all environments deltaic tidal alluvial fluvial as a consequence of the lack of vegetation and poor development of soils the fluvial setting has some enigmatic aspects amongst these is evidence for ponding of muddy detritus in apparently sandstone bed load dominated braided systems with effects on local palaeoslopes which have resulted in unusual palaeohydraulic parameters for precambrian fluvial systems this is perhaps a field of research which holds greater promise when investigating sedimentation patterns prior to the phanerozoic

directly dated mis 3 lake level record from lake manix mojave desert california usa abstract an outcrop based lake level curve constrained by 70 calibrated 14 c ages on anodonta shells indicates at least 8 highstands between 45 and 25 cal ka bp within 10 m of the 543 m upper threshold of lake manix in the mojave desert of southern california correlations of manix highstands with ice marine and speleothem records suggest that at least the youngest three highstands coincide with dansgaard oeschger d o stadials and heinrich events 3 and 4 the lake level record is consistent with results from speleothem studies in the southwest that indicate cool wet conditions during d o stadials notably highstands between 43 and 25 ka apparently occurred at times of generally low levels of pluvial lakes farther north as interpreted from core based proxies mojave lakes may have been supported by tropical moisture sources during oxygen isotope stage 3 perhaps controlled by southerly deflection of pacific storm tracks due to weakening of the sea surface temperature gradient in response to north atlantic climate perturbations

correlation of late wisconsin glacial phases in the western great lakes area late wisconsin glacial phases of the superior lobe in wisconsin and adjacent minnesota are tentatively correlated across northern wisconsin and adjacent michigan to phases of the green bay and lake michigan lobes spanning the period from about 26 000 yr b p to about 9500 yr b p 7 phases of the late wisconsin laurentide ice sheet are discussed but only 2 of these are precisely dated ice advanced out of the basins of lakes superior and michigan and stabilized during the st croix hancock phase the most extensive late wisconsin phase in most of the area between about 18 000 and 15 000 yr ago following the st croix hancock phase the general wasting of the ice was interrupted by stillstands or readvances of the ice margin during the intermediate and mountain phases the ice then wasted into the superior basin before readvancing over a landscape containing ice masses from earlier phases during the winegar early athelstane phase this phase is believed to have occurred just prior to growth of the two creeks forest in eastern wisconsin the ice then wasted back and advanced three more times during the marenisco late athelstane porcupine about 11 000 yr b p and marquette about 9900 yr b p phases

early local last glacial maximum in the tropical andes the local last glacial maximum in the tropical andes was earlier and less extensive than previously thought based on 106 cosmogenic ages from beryllium 10 dating from moraines in peru and bolivia glaciers reached their greatest extent in the last glacial cycle 34 000 years before the present and were retreating by 21 000 years before the present implying that tropical controls on ice volumes were asynchronous with those in the northern hemisphere our estimates of snowline depression reflect about half the temperature change indicated by previous widely cited figures which helps resolve the discrepancy between estimates of terrestrial and marine temperature depression during the last glacial cycle

age stratigraphic position and significance of the lemont drift northeastern illinois the lemont drift was recognized early as a distinctive glacigenic unit in northeastern illinois leached clay enriched zones in the upper part and below the calcareous wadsworth till are not remnants of a paleosol they result from modern weathering processes extending into the subsurface below the solum of the modern soil the lemont is late wisconsinan woodfordian not illinoian or early wisconsinan altonian as had been suggested it correlates in part with the haeger till member in illinois and other units in wisconsin and michigan and is the result of a significant fluctuation of the lake michigan lobe ice margin during woodfordian deglaciation diamicton in the lower part of the unit is the result of earlier pre haeger glacial sedimentation lacustrine sediment between tills within the lemont drift records the first phase of ancestral lake michigan during woodfordian deglaciation the latter event occurred during ice margin retreat following deposition of yorkville till and the interval is na

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

cosmogenic radionuclides from fiord landscapes support differential erosion by overriding ice sheets the interpretation of differentially weathered mountainous areas along the fringes of pleistocene ice sheets is fundamental for determining ice sheet behavior and thickness during the last glaciation two existing interpretations are either that highly weathered uplands remained as nunataks while freshly eroded troughs held outlet glaciers during the last glaciation or that uplands and lowlands were equally covered by ice but that it was differentially erosive as a function of its spatially variable basal thermal regime cosmogenic radionuclide measurements from 33 bedrock samples and 27 upland erratics from differentially weathered fi ord landscapes on northeastern baffi n island shed light on laurentide ice sheet lis dynamics and thickness tors on weathered upland surfaces have minimum 10 be ages between ca 50 and ca 170 ka n 12 whereas the majority of erratics perched in the uplands range from ca 10 to ca 13 ka n 14 indicating that the whole landscape was glaciated during the last glacial maximum lgm glacially sculpted bedrock n 7 near sea level refl ects the age of deglaciation and increasing amounts of isotopic inheritance in highelevation sculpted bedrock n 3 higher elevation intermediately weathered bedrock n 11 and highest elevation intensely weathered bedrock refl ect the weakening of erosive power as the ice sheet transitioned from fi ords to interfi ord plateaus cold based glaciation of uplands was contemporaneous with warm based glaciation in fi ords suggesting the presence of ice streams 26 al and 10 be concentrations measured in 10 bedrock samples indicate that tors have experienced a complex history of alternating periods of exposure burial and limited glacial erosion the paired isotope data reveal a minimum duration of burial of 80 500 k y presumably by cold based ice and indicate that the tors have an age of at least 150 580 ka these data together with other reconstructions of ice streams in the eastern canadian arctic suggest that ice streams were a large infl uence on northeastern lis dynamics throughout the late quaternary thus the northeastern lis was sensitively tied with north atlantic thermohaline circulation and abrupt climate change

tomographic study of paleoproterozoic carbonates as key to understanding the formation of molar tooth structure abstract x ray computed tomographic studies of relatively pure paleoproterozoic limestones from the george formation muskwa assemblage northern british columbia canada indicate that molar tooth structures developed along linked fractures in gel like semi plastic carbonate mud with a high organic content where pore fluid and or gas pressures matched confining loads mt blobs developed where pressure exceeded loads cracks propagated into adjacent semi elastic sediment and were rapidly filled by clusters of uniform equant microcrystalline carbonate where abundant carbonate was not precipitated incipient cracks and sheets collapsed leaving residual trains of microcrystalline carbonate with similar density to the molar tooth carbonate tomographic studies show that the density of calcite domains within petrographically uniform sheets of mt void filling calcite is uneven suggesting that precipitation was not instantaneous but was propagated from discrete centres it is here suggested that carbonate production and sediment rheology were both strongly influenced by organic matter during early sea floor diagenesis microcrystalline carbonate precipitated within organic rich sediment with high water content possibly within decomposing mats of microbial extracellular polymeric substances eps when pore pressures in the host sediment increased in response to cyclic loading by long period waves pore fluids containing eps were injected into newly created fractures allowing rapid precipitation of molar tooth carbonate because tomographic studies allow detailed resolution of minor density differences they provide a useful method of evaluating structures in relatively uniform carbonate rocks of any age

annual and semiannual cycle of equatorial atlantic circulation associated with basin mode resonance seasonal variability of the tropical atlantic circulation is dominated by the annual cycle but semi annual variability is also pronounced despite weak forcing at that period here we use multi year full depth velocity measurements from the central equatorial atlantic to analyze the vertical structure of annual and semi annual variations of zonal velocity a baroclinic modal decomposition finds that the annual cycle is dominated by the 4th mode and the semi annual cycle by the 2nd mode similar local behavior is found in a high resolution general circulation model this simulation reveals that the annual and semi annual cycles of the respective dominant baroclinic modes are associated with characteristic basin wide structures using an idealized linear reduced gravity model to simulate the dynamics of individual baroclinic modes it is shown that the observed circulation variability can be explained by resonant equatorial basin modes corollary simulations of the reduced gravity model with varying basin geometry i e square basin versus realistic coastlines or forcing i e spatially uniform versus spatially variable wind show a structural robustness of the simulated basin modes a main focus of this study is the seasonal variability of the equatorial undercurrent euc as identified in recent observational studies main characteristics of the observed euc including seasonal variability of transport core depth and maximum core velocity can be explained by the linear superposition of the dominant equatorial basin modes as obtained from the reduced gravity model

paleoenvironments and growth of early neoproterozoic calcimicrobial reefs platformal little dal group northwestern canada abstract although the calcimicrobial reefs of the basinal little dal group 780 ma are well described little is known about related shallow water facies to the southeast study of these platformal carbonates shows that they represent inner mid and shallow outer ramp zones of a homoclinal storm influenced ramp seven depositional sequences are recognized and correlated with reef bearing strata in the little dal basin and in the shaler supergroup of victoria island reef distribution is distinctly depth partitioned across the little dal ramp inner and mid ramp carbonates contain meter scale mounds patch reefs and biostromes composed of fine grained stromatolites including tungussia baicalia and jurusania these reefs are strikingly similar to those reported from coeval shallow water strata in the shaler supergroup of victoria island and the bitter springs formation of australia multi storey stromatolitic biostrome bioherm complexes greater than 100 m in height seem to be restricted to the mid ramp zone of the little dal ramp and probably reflect high rates of subsidence in the mackenzie basin calcimicrobial reefs occur only in the deepest water strata of the study area associated with outer ramp sediments these reefs contain calcified cyanobacterial filaments in varying degrees of preservation and are petrographically indistinguishable from the kilometre scale basinal calcimicrobial reefs to the northwest the association of calcimicrobial reefs with deep low energy environments suggests initial establishment of reef building calcimicrobes in deep water benthic environments

moraine preservation and boulder erosion in the tropical andes interpreting old surface exposure ages in glaciated valleys cosmogenic dating provides a long awaited means of directly dating glacial deposits that pre date the last glacial cycle although the potential benefits of longer chronologies are obvious the greater uncertainty associated with older cosmogenic ages may be less readily apparent we illus trate the challenges of developing and interpreting a long chronology using our data from the peruvian andes we used surface exposure dating with cosmogenic radionuclides crns 10 be and 26 al to date 140 boulders on moraines in valleys bordering the junin plain 11 s 76 w in central peru our chronology spans multiple glacial cycles and includes exposure ages greater than 1 million years which indicate that long term rates of boulder erosion have been very low interpreting the chronology of moraines for glaciations that predate the last glacial cycle is complicated by the need to consider boulder erosion and exhumation surface uplift and inheritance of crns from pre vious exposure intervals as an example we recalculate exposure ages using our boulder erosion rates 0 3 0 5 metres per million years and estimated surface uplift rates to emphasise both the chal lenges involved in interpreting old surface exposure ages and the value of chronological data even with large uncertainties when reconstructing the palaeoclimate of a region copyright 2005 john wiley sons ltd