primary multiple sulfur isotopic compositions of pyrite in 2 7 ga shales from the joy lake sequence superior province show felsic volcanic array like signature abstract multiple sulfur isotopes provide a powerful tool to study photochemical and biological processes controlling the archean sulfur cycle and infer related atmospheric and marine environments however our understanding of early earth s environment remains limited by the availability of well preserved geological samples as most archean sedimentary rocks have experienced some degree of metamorphic alteration to evaluate sulfur isotopic behavior during post depositional processes and elucidate the sulfur cycle at 2 7 ga we use high resolution in situ analytical techniques epma and sims to determine elemental compositions and multiple sulfur isotopic compositions of large diagenetic pyrite nodules and fine grained secondary pyrite disseminated in quartz veins formed during a lower greenschist metamorphic event in shales from the 2 7 ga joy lake sequence in the southwest superior province results show that trace metals and sulfur in the secondary pyrite were derived from both metamorphic fluid and pre existing diagenetic pyrite diagenetic pyrite nodules could have been partially dissolved by metamorphic fluid but the surviving nodules show elemental and isotopic features different from those of the deduced metamorphic fluid endmember suggesting the nodules were not geochemically altered by metamorphism and thus preserve primary isotopic signatures acquired during diagenesis the sulfur isotopic ratios of pyrite nodules show strong variations with decreasing 34 s values and increasing 33 s values from cores to rims this negative 33 s 34 s relationship is different from the commonly observed archean reference line defined by most archean pyrite data but similar to the felsic volcanic array our observation provides a first possible case from 2 7 ga the age of peak crustal growth in the archean to support the hypothesis that photochemical pathways could be different under conditions of intense volcanic emission this study also shows that high resolution sims and epma analyses are a robust method for distinguishing primary geochemical signatures from post depositional alteration and thus may be used to evaluate the sulfur cycle in archean sedimentary rocks with low grade metamorphic overprints

subglacial deforming bed conditions recorded by late quaternary sediments exposed in vineland quarry ontario canada abstract there has been considerable interest in recent years in the development of theoretical models of subglacial transport and deposition of sediment but relatively few studies report field documentation of the resultant sediment stratigraphies this paper presents detailed sedimentological description and analysis of a succession of late quaternary deposits interpreted to record subglacial overriding and deformation of previously deposited lacustrine sediments exposed in the vineland quarry that sits close to the crest of the niagara escarpment within the lake ontario basin the predominately fine grained sediments record deposition under glaciolacustrine conditions followed by deformation and deposition by overriding glacial ice laminated silt and clay deposits overlie the striated bedrock surface and were deposited within a lake that formed as the ontario lobe of the laurentide ice sheet advanced during the port huron stadial and ponded water against the niagara escarpment the laminated silt and clay facies show increasing amounts of deformation up section passing from planar through ductile to brittle deformation this succession of deformed facies is overlain by a macroscopically massive clay rich diamict that caps the section this pattern of sediment deposition and deformation is consistent with that proposed by current models of subglacial sediment deformation with the disrupted laminated silts and clays representing glacitectonites resulting from downward penetrating stresses imparted by an overriding ice sheet the uppermost massive diamict unit represents full macroscopic homogenization of the overridden sediment and is classified as a subglacial traction till the gradual transition from undisturbed laminated deposits through increasingly deformed sediment to structureless diamict suggests that these deposits record a single episode of ice advance across the region this ice advance was probably the short lived advance of the ontario lobe of the laurentide ice sheet that occurred at approximately 13 000 ybp

environmental perturbations caused by the impacts of asteroids and comets we review the major impact associated mechanisms proposed to cause extinctions at the cretaceous tertiary geological boundary we then discuss how the proposed extinction mechanisms may relate to the environmental consequences of asteroid and comet impacts in general our chief goal is to provide relatively simple prescriptions for evaluating the importance of impacting objects over a range of energies and compositions but we also stress that there are many uncertainties we conclude that impacts with energies less than about 10 mt are a negligible hazard for impacts with energies above 10 mt and below about 104 mt i e impact frequencies less than one in 6 104 years corresponding to comets and asteroids with diameters smaller than about 400 m and 650 m respectively blast damage earthquakes and fires should be important on a scale of 104 or 105 km which corresponds to the area damaged in many natural disasters of recent history however tsunami excited by marine impacts could be more damaging flooding a kilometer of coastal plain over entire ocean basins in the energy range of 104 105 mt intervals up to 3 105 years corresponding to comets and asteroids with diameters up to 850 m and 1 4 km respectively water vapor injections and ozone loss become significant on the global scale in our nominal model such an impact does not inject enough submicrometer dust into the stratosphere to produce major adverse effects but if a higher fraction of pulverized rock than we think likely reaches the stratosphere stratospheric dust causing global cooling would also be important in this energy range thus 105 mt is a lower limit where damage might occur beyond the experience of human history the energy range from 105 to 106 mt intervals up to 2 106 years corresponding to comets and asteroids up to 1 8 and 3 km diameter is transitional between regional and global effects stratospheric dust sulfates released from within impacting asteroids and soot from extensive wild fires sparked by thermal radiation from the impact can produce climatologically significant global optical depths of the order of 10 moreover the ejecta plumes of these impacts may produce enough no from shock heated air to destroy the ozone shield between 106 and 107 mt intervals up to 1 5 107 years corresponding to comets and asteroids up to 4 and 6 5 km diameter dust and sulfate levels would be high enough to reduce light levels below those necessary for photosynthesis ballistic ejecta reentering the atmosphere as shooting stars would set fires over regions exceeding 107 km and the resulting smoke would reduce light levels even further at energies above 107 mt blast and earthquake damage reach the regional scale 106 km tsunami cresting to 100 m and flooding 20 km inland could sweep the coastal zones of one of the world s ocean basins fires would be set globally light levels may drop so low from the smoke dust and sulfate as to make vision impossible at energies approaching 109 mt 108 years the ocean surface waters may be acidified globally by sulfur from the interiors of comets and asteroids the cretaceous tertiary impact in particular struck evaporate substrates that very likely generated a dense widespread sulfate aerosol layer with consequent climatic effects the combination of all of these physical effects would surely represent a devastating stress on the global biosphere

the last british ice sheet a review of the evidence utilised in the compilation of the glacial map of britain this paper reviews the evidence presently available as at december 2003 for the compilation of the glacial map of britain see clark c d evans d j a khatwa a bradwell t jordan c j marsh s h mitchell w a bateman m d 2004 map and gis database of glacial landforms and features related to the last british ice sheet boreas 33 359 375 and http www shef ac uk geography staff clark chris britice html in an effort to stimulate further research on the last british ice sheet and promote a reconstruction of ice sheet behaviour based on glacial geology and geomorphology the wide range of evidence that has been scrutinized for inclusion on the glacial map is assessed with respect to the variability of its quality and quantity and the existing controversies in ice sheet reconstructions landforms interpreted as being of unequivocal ice marginal origin moraines ice contact glacifluvial landforms and lateral meltwater channels and till sheet margins are used in conjunction with available chronological control to locate former glacier and ice sheet margins throughout the last glacial cycle subglacial landforms drumlins flutings and eskers have been used to demarcate former flow patterns within the ice sheet the compilation of evidence in a regional map is crucial to any future reconstructions of palaeo ice sheet dynamics and will provide a clearer understanding of ice sheet configuration ice divide migration and ice thickness and coverage for the british ice sheet as it evolved through the last glacial cycle

reply to comment by o cofaigh dowdeswell king anderson clark dja evans j evans hindmarsh lardner and stokes comments on shaw j pugin a young r 2009 a meltwater origin for antarctic shelf bedforms with special attention to megalineations geomorphology 102 364 375 0169 555x see front matter 2009 elsevier b v a doi 10 1016 j geomorph 2009 12 008 andre pugin s name does not appear on this reply because there was a delay in our receiving the comment and we responded to a tight deadline unfortunately this left insufficient time for an internal review required by the geological survey of canada before andre could be an author we expected that therewould be a reaction to our paper and serious questions after all we applied controversial ideas to a part of the globe for which we have no first hand experience indeed the observations of king et al 2009 on lineations beneath the rutford ice stream do present a serious challenge to the megaflood hypothesis otherwise our proposals for the formation of subglacial bedforms stand up well to the criticism by cofaigh et al they agree with us that meltwater action was significant to seascape evolution in antarctic shelf crossing troughs and in this respect we gave full credit to their research in many citations however in contrast to our megaflood hypothesis minimal meltwater supply and transfer were central to many earlier models of meltwater in troughs since the volumes of running water in minimal models are negligible shaw et al fig 6 meltwater in these models could have contributed little to seascape evolution much of this reply concerns questions that have been asked repeatedly over the last twenty years especially those concerning a meltwater origin for drumlins we respond emphatically to these questions which have become less not more discerning with time this reply follows the order of the comment and in places simply responds to a particular comment why is it more important that shelf crossing troughs would have been the preferred routes for ice streams surely this assertion prejudges the relative importance of ice streams and meltwater in the formation of bedforms

a bistable organic rich atmosphere on the neoarchaean earth it has been hypothesized that before widespread oxygenation about 2 45 billion years ago the earth s atmosphere contained an organic haze similar to that on titan however these theoretical predictions have not been substantiated by geological evidence here we use multiproxy geochemical analyses of sediments from the 2 65 2 5 billion year old ghaap group in south africa to reconstruct ocean and atmospheric chemistry during this time we find evidence for oxygen production in microbial mats and localized oxygenation of surface waters carbon and sulphur isotopes indicate that this oxygen production occurred under a reduced atmosphere that was periodically rich in methane consistent with the prediction of a hydrocarbon haze we use a photochemical model to corroborate our geochemical data our simulations predict transitions between two stable atmospheric states one with organic haze and the other haze free the transitions are presumably governed by variations in the amount of biological methane production during the archaean eon we find that the isotopic signatures we observe are evident in other data sets from this period and conclude that methane was an important component of the atmosphere throughout the archaean before the rise of oxygen the atmosphere of the early earth may have consisted of an organic haze geochemical data and modelling suggest that from 2 65 to 2 5 gyr ago several transitions between hazy and haze free atmospheric conditions occurred potentially linked to variations in biogenic methane production

identifying palaeo ice stream tributaries on hard beds mapping glacial bedforms and erosion zones in nw scotland ice streams are fed by tributaries that can extend deep into the heart of ice sheets these tributaries are born at onset zones the abrupt transitions from slow sheet flow to fast streaming flow that often occur at significant topographic steps on hard beds bedrock dominated beds for this reason tributary onset zones leave only a subtle erosional geomorphic signature in the landscape record that is rarely studied this paper examines in detail the geomorphic signature of ice sheet flow on a hard bed at the head of a palaeo ice stream we use field survey techniques to map glacial bedforms within an 200 km2 area of hard crystalline bedrock in a landscape of areal scour around loch laxford in nw scotland the bedrock bedforms range from plastically moulded p forms and wholly abraded forms to stoss lee forms and plucked surfaces all on an outcrop scale 1 100 m we devise a five zone classification system to map in a gis the presence absence and abundance of glacial erosional forms within 619 500 m square grid cells we go on to use these erosional bedform zones along with known glaciological relationships to interpret the spatial and altitudinal pattern of palaeo ice sheet processes and glacier dynamics in this part of nw scotland our interpretation highlights the strong vertical thermal zonation on mountains and the spatial variations in ice rheology softness ice temperature and by inference ice velocity in troughs intimately associated with the onset of ice streaming in tributaries consequently we define the laxfjord palaeo ice stream tributary a feeder to the minch palaeo ice stream in nw scotland finally we suggest that this new mapping approach could be performed in other deglaciated hard bed terrain to examine more widely the subtle erosional signatures preserved in areas traditionally thought to represent ice sheet areal scour

structural characterization by raman hyperspectral mapping of organic carbon in the 3 46 billion year old apex chert western australia abstract the 3 46 billion years old apex chert pilbara craton western australia is well known for hosting the oldest highly disputed microfossils on earth this rock has a complex history of thermal alteration that includes circulation of hydrothermal fluids lower greenschist facies regional metamorphism and post metamorphic weathering by meteoric fluids carbonaceous material occurs in the sedimentary stratiform part of the chert as well as the underlying intruding hydrothermal black chert veins in order to identify a least altered remnant of early life it is necessary to develop a method that enables rapid evaluation of cm structural order on a small spatial scale here we present the detailed characterization of cm in the apex chert by raman hyperspectral mapping it is shown that this approach gives better estimates of average raman band ratios than individual point analyses and it is demonstrated that significant differences in structure exist between cm in the stratiform part of the apex chert and cm in an underlying black chert vein the large raman map based datasets also reveal that significant mixing took place between these two end members cm s indicating that the apex chert has been thoroughly altered by hydrothermal fluid circulation at the brecciated intersection between the stratiform chert and the intrusive hydrothermal chert vein very poorly ordered cm was found that is not in line with lower greenschist facies regional metamorphism it is speculated here that this cm represents an organic fraction that was introduced or thoroughly altered by late stage meteoric fluids alternatively the ubiquitous presence of hematite in this sample caused a perturbation in the raman spectra of the cm leading to an artifact in the calculated raman based band ratios overall it can be concluded that the best preserved cm occurs in the stratiform parts of the apex chert while earlier discussions on organic microfossils in this rock often focused on parts of the chert that either represented the hydrothermal veins or the brecciated intersection between the veins and the stratiform part

uv induced mass independent sulfur isotope fractionation in stratospheric volcanic sulfate 1 sulfuric acid aerosols produced in the stratosphere following massive volcanic eruptions possess a mass independent sulfur isotopic signature acquired when volcanic so2 experiences uv photooxidation the volcanic data are consistent with laboratory so2 photooxidation experiments using uv light at 248 nm maximum absorption of ozone whereas sulfur isotopic anomalies previously observed in archean samples are consistent with photodissociation at 190 220 nm a mechanism of so2 photooxidation occurring in the early stage of a stratospheric volcanic plume in the range of 220 320 nm weak band absorption of so2 is also proposed since mass independent sulfur isotope anomalies in stratospheric volcanic sulfate appear to depend on the exposure of so2 to uv radiation their measurements might therefore offer the possibility to determine the degree of uv penetration in the ozone absorption window for the present and past atmospheres they can also be used to determine the stratospheric or tropospheric nature of volcanic eruptions preserved in glaciological records offering the possibility to reassess the climatic impact of past volcanic eruptions

first paleomagnetic results from mio pliocene series of the hellenic sedimentary arc abstract paleomagnetic measurements have been conducted on the mio pliocene series along the external sedimentary hellenic arc in order to detect the presence or absence of rotational deformations in different sections of the arc in the central eastern part of the arc crete and rhodes paleomagnetic directions are very closely aligned along the north south direction showing that this portion of the arc has not undergone significant rotation since at least 7 m y for crete and 4 m y for rhodes in the western and northwestern parts results from south peloponnesos zakinthos kephallinia and corfu indicate that this section of the arc has undergone a clockwise rotation detailed analysis of the data from the three ionian islands shows that their rotation began about 5 m y b p and has proceeded since then at roughly 5 m y giving a total amplitude of about 26 it is suggested that the beginning of this rotation is related to the lower pliocene compressive phase which most probably initiated the tectonic activity of the present day ionian margin

palaeoglaciology of the last irish ice sheet reconstructed from striae evidence abstract a database comprising some 5200 individual striation measurements on bedrock surfaces across the island of ireland was used to produce maps of flowsets corresponding to individual ice flow events during the last late devensian glacial cycle these flowsets were identified on the basis of regional scale correspondence between striae orientations which when linked together spatially are able to identify consistent ice flow vectors four main chronological stages are identified on the basis of this evidence i incursion of scottish ice into ireland ii glacial maximum conditions iii ice retreat and dissolution and iv development of localised ice domes striae based reconstructions of the glaciology of the last irish ice sheet are qualitatively different from those based on bedform mainly drumlin and ribbed moraine evidence significant differences are apparent in upland areas which have fewer preserved bedforms and a higher concentration of striae combining bedform and striae datasets will enable a better understanding of the temporal evolution of the ice sheet it is likely that both datasets record a snapshot of ice flow direction and subglacial conditions and environments immediately prior to preservation of this directional evidence

refining the ice flow chronology and subglacial dynamics across the migrating labrador divide of the laurentide ice sheet with age constraints on deglaciation geological survey of canada s geo mapping for energy and minerals ii gem2 program 2013 2020

prolonged magmatism and time constraints for sediment deposition in the early archean barberton greenstone belt evidence from the upper onverwacht and fig tree groups the single zircon evaporation shrimp ion microprobe and conventional dissolution techniques were used to determine 207pb 206pb and u pb ages on samples from the upper onverwacht and fig tree groups of the early archean barberton greenstone belt south africa zircons from dacitic rocks of the upper hooggenoeg formation yield ages of 3445 3452 ma a tuff in the basal kromberg formation has a mean age of 3416 5 ma a tuffaceous band 5 cm thick in the uppermost kromberg formation contains igneous zircons with a mean age of 3334 3 ma the 1700 m section of kromberg formation between these two samples is composed of basaltic lavas minor komatiites and cherty metasediments the overlying mendon formation is composed of interbedded komatiitic lavas and metasediments with a minimum thickness of 600 m a cherty stromatolitic metasediment 300 m above the base contains several thin ash layers with a mean zircon age of 3298 3 ma the basal fig tree group has units as old as 3259 3 ma and upper units in the fig tree are as young as 3225 3 ma xenocrystic zircons in the upper onverwacht and overlying fig tree group samples suggest that successive igneous units inherited zircons from underlying units and that over several hundred million years episodes of intermediate to felsic igneous activity took place at 20 40 ma intervals structural repetition by isoclinal folding and thrust faulting are important components of late greenstone belt evolution but should not obscure the importance of the prolonged interval of magmatic evolution represented by the thick pile of volcanic rocks observed in the barberton greenstone belt

variations in atmospheric sulphur chemistry on early earth linked to volcanic activity the nature of the atmospheric sulphur cycle on the early earth has been difficult to reconstruct an analysis of sulphur isotopes from 3 2 billion year old volcanic rocks suggests that episodic volcanism released pulses of sulphur dioxide that was then broken down by ultraviolet photodissociation

first paleomagnetic results from neocene formations in evia skyros and the volos region and the deformation of central aegea paleomagnetic results from neogene volcanic and sedimentary formations in central aegea indicate that evia and skyros have undergone a 48 and 26 clockwise rotation respectively the volos region does not show any significant rotation in the last 3 ma the rotation of evia is about twice as large as the one measured from coeval formations in the external hellenic arc the results are interpreted in terms of a published model of distributed deformation by faulting which also provides an explanation for the significant differential rotation of evia and skyros

bedrock mega grooves in glaciated terrain a review abstract bedrock mega grooves are assemblages of straight and parallel troughs eroded in bedrock typically over 1000 m in length most sites occur within the limits of the last glacial maximum both on and off shore in this paper we review the current understanding of these important yet enigmatic landforms and propose a framework for their future research mega grooves are important to our understanding of ice sheet dynamics ice bedrock interactions and bedrock landscape evolution in glaciated areas the overall straightness of mega grooves across the landscape their parallel alignment to palaeo ice flow direction and occurrence below the general land surface level has led to their unanimous interpretation as landforms of subglacial erosion scenarios proposed for mega groove formation focus on either glacier ice or subglacial meltwater as the principal agent of erosion yet none offers a comprehensive explanation at locations where mega grooves occur along lines of structural geology their location formation and morphology were largely controlled by the bedrock characteristics where no underlying structural control is apparent mega grooves were likely initiated through glacial abrasion and subsequently modified through a range of erosional processes potentially involving multiple morphogenetic agencies and feedbacks operating between bedrock topography and basal ice flow in the absence of absolute dates morphostratigraphic analyses suggest mega groove survival through multiple glacial cycles no specific ice flow characteristics have been identified as a condition for bedrock grooving but it has been suggested that some bedrock mega grooves are related to ice streaming which deserves further study an initial analysis of bedrock grooves with seemingly similar morphology at a range of scales hints at a bedrock groove landform size continuum which could be a useful framework for exploring process landform relationships future research could usefully focus on quantitative analysis of mega groove morphology augmented with detailed field analysis of landform relationships to bedrock structure and lithology and thereby potentially provide further insight into the age and glaciological significance of these landforms

origin of two distinct multiple sulfur isotope compositions of pyrite in the 2 5ga klein naute formation griqualand west basin south africa abstract quadruple sulfur isotope ratios 32s 33s 34s 36s were measured by a high precision sf6 method for early diagenetic pyrite in the 2 5 ga upper prieska facies of the griqualand west basin from the agouron drill core gkp01 measured 33s 33s 0 515 34s values range between 2 3 and 8 5 with no apparent meter scale stratigraphic trend for 70 m section it was found that early diagenetic pyrites occur in two texturally and isotopically distinct populations such that bulk rock isotope data would reflect mixing of these two sulfur pools nodular and layered pyrite yield mostly negative down to 2 3 and relatively constant 33s values over a few tens of cm scale disseminated pyrites in contrast yield positive 0 0 8 5 33s values that are heterogeneous in sub cm scales the former pyrite sulfur was likely derived from seawater sulfate through microbial sulfate reduction and the latter preferentially sampled aqueous h2s s0 reservoirs in late archean oceans relatively constant 33s values for layered pyrite suggest archean sulfate reservoirs had residence times of a few million years most data yield a well defined 36s 33s ratios of 0 9 suggesting predominance of a single s mif source reaction four analyses of a two cm length sample 306 5 m yield 36s 33s slope of 1 6 this slope is consistent with the relationship expected for a symmetry breaking isotope effect during the s s2 reaction but deviations from the main archean array i e 36s 33s 0 9 may also result from various combinations of so photolysis and so2 oxidation high spatial resolution high precision sf6 measurements of archean pyrite provide significant new insights into the origin of archean sulfur mass independent fractionation s mif and the late archean sulfur cycle

comment on shaw j pugin a and young r 2008 a meltwater origin for antarctic shelf bedforms with special attention to megalineations geomorphology 102 364 375 the submarine glacial geomorphology and sedimentology of the cross shelf troughs and the adjacent continental slope around the antarctic peninsula and west antarctica have been the focus of a series of marine geophysical and geological investigations over the last decade e g shipp et al 1999 canals et al 2000 wellner et al 2001 lowe and anderson 2002 o cofaigh et al 2002 dowdeswell et al 2004 heroy and anderson 2005 evans et al 2005 domack et al 2006 mosola and anderson 2006 wellner et al 2006 dowdeswell et al 2006 o cofaigh et al 2007 o cofaigh et al 2008 c o cofaigh j a dowdeswell j evans and r d larter geological constraints on antarctic palaeo ice stream retreat earth surface processes and landforms 33 2008 pp 513 525 o cofaigh et al 2008 and dowdeswell et al 2008 these studies have interpreted sets of characteristic streamlined glacial bedforms and sediments on the shelf which in numerous cases occur in front of modern ice streams as largely the product of fast flowing palaeo ice streams that drained across the shelf during or following the last glacial maximum a key feature of these studies is the observation of highly attenuated bedforms known as mega scale glacial lineations msgl formed in soft sediment on the outer continental shelf these lineations are regarded as key evidence for streaming flow in their recent paper shaw et al present a radically different interpretation of the glacial geomorphology of antarctic cross shelf troughs in terms of catastrophic discharge of subglacial meltwater floods across the shelf in their interpretation msgl are regarded as the product of erosion by turbulent meltwater flow the following comment discusses a number of the key assertions made in their paper

pyrite in a sulfate poor paleoarchean basin was derived predominantly from elemental sulfur evidence from 3 2 ga sediments in the barberton greenstone belt kaapvaal craton abstract multiple sulfur isotope variability in archean sedimentary rocks provides constraints on the composition of the earth s earliest atmosphere the magnitude and sign of mass independent anomalies reflect not only atmospheric processes but also transformations due to the archean marine sulfur cycle prior to preservation into sedimentary pyrite the processes affecting the archean marine sulfur cycle and the role of microbial or abiotic redox reactions during pyrite formation remain unclear here we combine iron fe and multiple sulfur s isotope data in individual pyrite grains with petrographic information and a one dimensional reactive transport model to investigate the sources of fe and s in pyrite formed in a paleoarchean sedimentary basin pyrites were selected from mudstones sandstones and chert obtained from a drill core in the ca 3 2 ga mapepe and mendon formations of the fig tree and onverwacht groups respectively in the barberton greenstone belt kaapvaal craton south africa pyrite textures and 56 fe distinguish early diagenetic pyrite formed with pore water ferrous iron disseminated grains with average 56 fe pyrite 0 from late diagenetic pyrite formed through sulfidation of iron oxide minerals layered and aggregate forms with average 56 fe pyrite 1 mass dependent s isotope variability in pyrite was small 34 s pyrite ranged from 1 1 to 3 3 with a correspondingly minor spread in 33 s pyrite ranging from 0 3 to 2 1 and 36 s pyrite ranging from 3 08 to 0 27 that indicates a lack of post depositional re working with other distinct sulfur sources our combined fe and s isotope data are most readily explained with pyrite sulfide derived from microbial reworking of solid elemental s iron oxide minerals were necessary to buffer sulfide concentrations and provide favorable conditions for microbial sulfur disproportionation to proceed the lack of a negative 33 s signal indicates that pyrite from relatively deep marine diagenetic environments only partially records the products of atmospheric photolysis consistent with low sulfate concentrations in the paleoarchean ocean

subglacial erosional marks wilton creek ontario a variety of p forms are discussed in terms of the agent producing them a general conclusion based on form and the location of striations relative to p forms is that they are produced by meltwater rather than ice erosional marks in the wilton creek valley ontario are different from earlier described p forms their relationship to striations establishes them as being of subglacial origin and their form similarity with flutes together with the partial or total absence of striations on them suggests a fluvial origin large scale forms are interpreted as spindle shaped flutes associated with flow separation small scale forms are subdivided into straight spindle forms curved spindle forms braided forms and scallops they appear not to have followed the inverse relationship between size and flow velocity reported for some other erosional marks potential but speculative explanations for these small scale forms are that they are related to taylor gortler vortices vortices on mixing layers or trailing

early archean silicate spherules of probable impact origin south africa and western australia layers of sand sized silicate spherules in 3 2 to 3 5 ga archean greenstone belts in south africa and western australia formed by the accumulation of quenched liquid silicate droplets having primary compositions resembling those of immediately underlying rock sequences the layers and particles appear to be unrelated to volcanic activity and we suggest that both occurrences may represent melt droplets formed during meteorite or comet impacts if so these spherules are the oldest known terrestrial impact products if these objects are representative of a larger population of early archean impact features it may be possible to directly estimate the meteorite flux on the early archean earth

new insights into archean sulfur cycle from mass independent sulfur isotope records from the hamersley basin australia we have measured multiple sulfur isotope ratios 34 s 33 s 32 s for sulfide sulfur in shale and carbonate lithofacies from the hamersley basin western australia the v 33 s values v 33 swn 33 s30 515un 34 s shift from 31 9 to 6 9x over a 22 m core section of the lower mount mcrae shale v2 5 ga likewise sulfide sulfur analyses of the jeerinah formation v2 7 ga yield v 33 s values of 30 1 to 8 1x over a 50 m section of core despite wide variations in v 33 s and n 34 s these two shale units yield a similar positive correlation between v 33 s and n 34 s in contrast pyrite sulfur analyses of the carawine dolomite v2 6 ga yield a broad range in n 34 s 3 2 to 16 2x but a relatively small variation and negative values in v 33 s 32 5 to 31 1x the stratigraphic distribution of n 33 s n 34 s and v 33 s in western australia allows us to speculate on the sulfur isotopic composition of archean sulfur reservoirs and to trace pathways in the archean sulfur cycle our data are explained by a combination of massindependent fractionation mif in the atmosphere and biological mass dependent fractionation in the ocean in the archean volcanic sulfur bearing gas species were photolysed by solar ultraviolet uv radiation in an oxygen free atmosphere resulting in mif of sulfur isotopes aerosols of s8 with v 33 ss 0 and sulfuric acid with v 33 s6 0 formed from the products of uv photolysis and carried mass independently fractionated sulfur into the hydrosphere the signatures of atmospheric photolysis were preserved by precipitation of pyrite in sediments pyrite precipitation was mediated by microbial enzymatic catalysis that superimposed mass dependent fractionation on mass independent atmospheric effects multiple sulfur isotope analyses provide new insights into the early evolution of the atmosphere and the evolution and distribution of early sulfur metabolizing organisms a 2003 elsevier science b v all rights reserved

erosion of bedrock by subglacial meltwater georgian bay ontario a regional view a spectacular series of sculpted erosional forms s forms is mapped and described from a 70 km wide area along the shore of georgian bay ontario which except for a scattered boulder lag has been swept clean of sediment a great variety of sculpted forms is described and illustrated and grouped into three classes transverse longitudinal and nondirectional forms transverse forms comprise transverse troughs muschelbruche sichelwannen and comma forms longitudinal forms comprise spindle flutes cavettos and furrows and nondirectional forms consist of undulating surfaces and potholes transverse forms are preferentially located on stoss slopes and longitudinal forms on lee slopes of rock rises undulating nondirectional forms are found on distal slopes and potholes at major breaks in slope this correlation of form and bed topography suggests that relief exerts considerable control on both form and location form geometry is also inferred to be related to coherent flow structures and their int

anomalous sulfur isotope compositions of volcanic sulfate over the last millennium in antarctic ice cores 1 the reconstruction of past volcanism from glaciological archives is based on the measurement of sulfate concentrations in ice this method does not allow a proper evaluation of the climatic impact of an eruption owing to the uncertainty in classifying an event between stratospheric or tropospheric this work develops a new method using anomalous sulfur isotope composition of volcanic sulfate in order to identify stratospheric eruptions over the last millennium the advantages and limits of this new method are established with the examination of the 10 largest volcanic signals in ice cores from dome c and south pole antarctica of the 10 seven are identified as stratospheric eruptions among them three have been known to be stratospheric tambora kuwae the 1259 unknown event and they exhibit anomalous sulfur isotope compositions three unknown events circa 1277 1230 1170 a d and the serua eruption have been identified as stratospheric eruptions which suggests for the first time that they could have had significant climatic impact however the kuwae and the 1259 unknown event stratospheric eruptions exhibit different anomalous sulfur isotope compositions between south pole and dome c samples differences in sulfate deposition and preservation patterns between the two sites can help explain these discrepancies this study shows that the presence of an anomalous sulfur isotope composition of volcanic sulfate in ice core indicates a stratospheric eruption but the absence of such composition does not necessarily lead to the conclusion of a tropospheric process because of differences in the sulfate deposition on the ice sheet

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

aegean paleomagnetic inclination anomalies is there a tectonic explanation abstract more than a decade of paleomagnetic research in the aegean area mainly by french investigators c laj c kissel and their colleagues has provided a valuable data base for the evaluation of cenozoic aegean tectonics systematic rotations are documented and have been interpreted by the original investigators in the light of the evolution of the hellenic arc however less attention has been paid to the significance of the paleomagnetic inclinations which also are anomalous twenty one out of 22 paleomagnetic studies performed to date within the general aegean region have yielded paleolatitudes that are too shallow when compared with reference curves for either africa or eurasia suggesting northward relative transport with respect to both continents sedimentary volcanic and plutonic units are involved ages range from oligocene to pliocene some of these apparent paleolatitude anomalies may merely be the result of inclination flattening in certain kinds of clastic sedimentary rocks however shallow inclinations in igneous rocks cannot be so easily dismissed the explanation for these anomalously shallow inclinations does not appear to lie with the reference curves because several other reference curves calculated in several different ways yield essentially the same result we suggest that the paleomagnetic pattern may reflect real northward displacement of crustal blocks within the aegean region measured with respect to both europe and africa the amount of displacement appears to increase systematically to the west the pattern of local block rotations noticed by earlier investigators argues that the displacement did not involve a single coherent aegean block but rather a collection of smaller blocks moving quasi independently geological consequences of northward relative displacement of the aegean block ought to include mid tertiary and or later compressional structures in the north e g rhodope and extensional structures in the leading edge of the african plate

mineralogy and diagenesis of 3 24ga meteorite impact spherules abstract spherules in the s3 bed of the barberton greenstone belt south africa are distal fallout formed within an impact plume after a large impact event at 3 24 ga since that time diagenesis and lower greenschist grade metamorphism of the spherules has changed the mineralogy though shape and texture are largely preserved alteration of the s3 bed has resulted in spherules composed of quartz phyllosilicates ti and fe oxides and some ni rich chromites initially glassy spherules were palagonitized and silica cementation of the spherules began during low temperature interaction with seawater further alteration by si and k rich fluids resulted in a mineralogical assemblage of quartz feldspar and clays crystalline minerals were replaced by dissolution precipitation processes preserving relict textures further silica cementation resulted in complete lithification of the bed most of this alteration occurred at the seafloor and during shallow burial with continued burial amorphous silica recrystallized to microcrystalline quartz later recrystallization of clays to micas occurred during regional metamorphism at peak temperatures of 300 320 c late stage shearing and mineralization preferentially affected the northern region of the belt samples from different sections record highly variable local conditions water depth the amount of pre depositional transport location within the belt and proximity to igneous dikes all affect the diagenesis of the s3 spherules silica and barite concentrations are lower and carbonate concentrations are higher in the deep water depositional environments element mobility during diagenetic and lower greenschist grade metamorphism can be inferred based on studies of multiple sections throughout the bgb the most immobile elements are al zr ti sc and the high field strength elements and present element ratios can be used to infer original composition of the spherules the large ion lithophile elements are highly mobile as are the light rare earth elements rees which are particularly susceptible to mobilization during carbonate diagenesis and phosphate authigenesis of the rees ce and eu show the largest variability suggesting significant mobilization during diagenesis and low grade metamorphism consistent cr ir ratios particularly in the high concentrations suggest limited mobility and segregation of platinum group elements into ni rich chromite bearing spherules sulfide mineralization has not affected the ir concentration in the s3 layer

quadruple sulfur isotope analysis of ca 3 5 ga dresser formation new evidence for microbial sulfate reduction in the early archean abstract multiple sulfur isotope system is a powerful new tracer for atmospheric volcanic and biological influences on sulfur cycles in the anoxic early earth here we report high precision quadruple sulfur isotope analyses 32s 33s 34s 36s of barite pyrite in barite and sulfides in related hydrothermal and igneous rocks occurring in the ca 3 5 ga dresser formation western australia our results indicate that observed isotopic variations are mainly controlled by mixing of mass dependently md and non mass dependently fractionated non md sulfur reservoirs based on the quadruple sulfur isotope systematics 34s 33s 36s for these minerals four end member sulfur reservoirs have been recognized 1 non md sulfate 34s 5 2 33s 3 1 2 md sulfate 34s 10 3 3 non md sulfur 34s 6 33s 4 and 4 igneous md sulfur 34s 33s 0 the first and third components show a clear non md signatures thus probably represent sulfate and sulfur aerosol inputs the md sulfate component 2 is enriched in 34s 10 3 and may have originated from microbial and or abiotic disproportionation of volcanic s or so2 our results reconfirm that the dresser barites contain small amounts of pyrite depleted in 34s by 15 22 relative to the host barite these barite pyrite pairs exhibit a mass dependent relationship of 33s 34s with slope less than 0 512 which is consistent with that expected for microbial sulfate reduction and is significantly different from that of equilibrium fractionation 0 515 the barite pyrite pairs also show up to 1 difference in 36s values and steep 36s 33s slopes which deviate from the main archean array 36s 33s 0 9 and are comparable to isotope effects exhibited by sulfate reducing microbes 36s 33s 5 to 11 these new lines of evidence support the existence of sulfate reducers at ca 3 5 ga whereas microbial sulfur disproportionation may have been more limited than recently suggested

flow induced mixing in the grip basal ice deduced from the co2 and ch4 records this paper documents a larger degree of mixing in ice near the bottom of an ice sheet than described or suspected previously it shows thanks to favourable circumstances due to co2 and ch4 production underneath the ice that flow induced mixing within the basal ice has taken place at the scale of a few centimeters in the grip core such a mechanism must be considered when interpreting the ice properties in the bottom part of ice sheets and must be taken into account as a potential process of layer disruption in the low levels of the central greenland ice cores

contemporaneous localized basal ice flow variations implications for bedrock erosion and the origin of p forms a detailed study of a proglacial bedrock site and a subglacial cavity of an outlet of oksfjordjokelen norway is presented together with observations from the foreland of konowbreen spitsbergen striation directions and subglacial observations indicate that local ice flow paths were highly variable deviating at angles of approximately 90 from the main ice flow direction stepped bedrock topography appears conducive to the production of highly variable ice flow paths because the high bed roughness creates a locally variable stress regime within the ice including low pressure lee side areas into which ice can flow if ice flow is sustained along a specific path and the ice contains debris then abrasion should produce an erosional bedform models are proposed whereby locally variable ice flow patterns could produce erosional bedforms which would be described as p forms purely through mechanical abrasion

evaluating the role of microbial sulfate reduction in the early archean using quadruple isotope systematics abstract microscopic pyrites with low 34 s 32 s ratios in 3 47 gyr old sedimentary barites from north pole australia have been interpreted as evidence for microbial sulfate reduction and or sulfur disproportionation in the early archean we show that these microscopic sulfides have similar to slightly less negative 33 s and slightly more negative 36 s values compared to the enclosing sulfate crystals this finding is consistent with a primary mass independent signature overprinted by biological sulfate reduction as calibrated by previous experimental laboratory culture studies however it is inconsistent with an overprint by abiological sulfate reduction or sulfur disproportionation as predicted by isotope exchange theory and laboratory culture studies thus our multiple sulfur isotope measurements support the contention that sulfate reducing microbes had evolved by 3 47 billion years ago

basal flow characteristics of a linear medium sliding frictionless over small bedrock undulations the basal deformation of a gravity driven linear creeping flow sliding frictionless over slowly varying bed undulations in two dimensions is analysed analytically using results from second order perturbation theory one of the key results is that close to sinusoidal bedrock undulations up to two different spatial regions of local extrusion flow may arise the offset and onset of extrusion flow is controlled primarily by the amplitude to wavelength ratio above the crest of a sinusoidal bed line a local maximum of the surface parallel velocity develops for e ak 0 138 where a is the amplitude and k is the wave number as e increases from zer to this critical value the vertical position of the velocity maximum moves from kz 1 to kz 1 98 where z is the vertical distance above the mean bed line within and above the trough of a sinusoid a region of local minimum of the surface parallel velocity component develops which shifts from kz 1 towards the bed line as e increases front zero to 1 2 below this velocity minimum and for some distance above the velocity maximum the surface parallel velocity increases with depth this type of extrusion flow will cause a reversal of borehole inclination profiles close to the bedrock

lithofacies control on multiple sulfur isotope records and neoarchean sulfur cycles abstract triple sulfur isotope ratios 32 s 33 s 34 s were measured for 141 bulk rock samples from two agouron scientific drill cores gkp01 and gkf01 that recovered neoarchean successions of the transvaal supergroup south africa these two deep time cores are correlated to each other with 14 tie lines using volcanic and impact spherule layers in a sequence stratigraphic framework allowing us to evaluate both lithofacies and temporal controls over multiple sulfur isotope systematics the ca 2 5 ga giga annum before present basinal klein naute formation and the ca 2 6 ga peritidal boomplaas and vryburg formations yield an array of data characterized by 33 s 1 4 34 s these linear trends are found in both shallow water and deepwater facies but are characteristic to rocks with high iron content suggesting these may reflect isotopic compositions of aqueous sulfide elemental sulfur reservoirs in the neoarchean oceans data that deviate from this linear array are interpreted as resulting from additional inputs of sulfide from microbial sulfate reduction with or without contribution from sulfur disproportionation sulfate derived sulfur evolved to be either enriched or depleted in 34 s depending on the local depositional environment for example the reivilo formation in core gkf01 is characterized by abundant microbialite textures and shows an isotopic signature of closed system sulfate reduction rapid cementation of these carbonate fabrics may have attenuated the supply of sulfate to pore waters resulting in the progressive 34 s enrichments during bacterial sulfate reduction below the sediment water interface in contrast signatures of open system sulfate reduction are associated with slope facies dominated by granular dolostones preserved in the upper nauga formations in gkf01 and gkp01 the two different sulfur isotope patterns interpreted to reflect closed versus open system sulfate reduction are both found in the lower reivilo formation in both cores this lateral variation of isotope signals documents that observed 34 s shifts can be controlled locally and may not have temporal significance this study demonstrates critical importance of recovering sulfur isotope data from stratigraphically correlated drill cores to evaluate both geographical and temporal shifts of sulfur cycles and their links to the great oxidation event at the end of the archean eon

glacial abrasion and sliding their dependence on the debris concentration in basal ice a model of bedrock abrasion by a temperate glacier is developed that takes into account the significant impeding effect of scattered rock fragments at the ice rock interface on glacier sliding an increase in the debris concentration above a relatively low value can reduce the abrasion rate by decreasing both the flux of rock fragments and the effective forces which press fragments against the bed abrasion of a simple wavy bed is shown to be most rapid for a relatively low debris concentration above which the abrasion varies inversely with the debris content a similar concentration dependence of abrasion is expected for beds of arbitrary geometry but low roughness sliding glaciers will tend to abrade until the basal ice is nearly stagnant at which point rocks of all sizes start to lodge against the bed

paleomagnetism of the late cretaceous early tertiary north central montana alkalic province paleomagnetic results have been obtained from the late cretaceous early tertiary igneous complexes of the north central montana alkalic province data from 94 sites in eocene volcanic and intrusive rocks give a paleomagnetic pole located at 82 0 n 170 2 e a95 3 5 k 18 6 while 36 sites in paleocene intrusions yield a paleomagnetic pole at 81 8 n 181 4 e a95 5 4 k 20 2 these poles differ by only 1 6 and are not significantly different statistically the 130 site virtual geomagnetic poles show no significant elongation and suggest no significant apparent polar wander apw during the period of magnetization of the igneous centers postmagnetization structural complications in these rocks are minimal the presence of a single predominant polarity in these intrusive complexes reinforces the radiometric age data that suggest that igneous activity within individual centers was of short duration the northcentral montana data together with other early tertiary cretaceous and mid tertiary paleomagnetic results require modification of our earlier apw chronology diehl et al 1980 it now appears that apw relative to north america since the early cretaceous consists of a polar still stand during much of the cretaceous 120 75 m y b p a period of rapid movement in latest cretaceous time 75 65 m y b p and a period of slow polar movement thereafter the onset of this period of rapid apw correlates well with a major change in plate motions at 80 m y and the beginning of the laramide orogeny

atmospheric sulfur rearrangement 2 7 billion years ago evidence for oxygenic photosynthesis mass independently fractionated sulfur isotopes mif s provide strong evidence for an anoxic atmosphere during the archean moreover the temporal evolution of mif s shows increasing magnitudes between 2 7 and 2 5 ga until the start of the great oxidation event g o e at around 2 4 ga the conclusion of a completely anoxic atmosphere up to the g o e is in contrast to recent studies on redox sensitive elements which suggest slightly oxidizing conditions during continental weathering already several hundred million years prior to the g o e in order to investigate this apparent inconsistency we present multiple sulfur isotopes for 2 71 ga pyritic black shales derived from the kidd creek area ontario canada these samples display high positive 33s values up to 3 8 and the typical late archean slope in 36s 33s of 0 9 in contrast the time period before 3 2 2 73 ga is characterized by greatly attenuated mif s magnitudes and a slope in 36s 33s of 1 5 we attribute the increase in 33s magnitude as well as the contemporaneous change in the slope of 36s 33s to changes in the relative reaction rate of different mif s source reactions and changes in atmospheric sulfur exit channels both of these are dependent on atmospheric ch4 co2 and o2 mixing ratios we propose a distinct change in atmospheric composition at 2 7 ga resulting from increased fluxes of oxygen and methane as the best explanation for the observed neoarchean mif s record our data and modeling results suggest that oxygenic photosynthesis was a major contributor to primary productivity 2 7 billion years ago

evidence for a low o2 archean atmosphere from nickel rich chrome spinels in 3 24 ga impact spherules barberton greenstone belt south africa abstract the composition of spinels in 3 24 billion year old archean impact spherules in the s3 spherule bed in the barberton greenstone belt south africa provides important clues about the environments within which they formed including the redox conditions of the impact plume and the oxygen levels in the early atmosphere despite pervasive diagenetic alteration of the impact spherules and nearly complete alteration of primary mineralogy primary ni rich chrome spinel is preserved the impact spinels are significantly more oxidized than detrital spinels of komatiitic origin that are also present in the spherule bed the average fe 3 fe t atoms value in the ni rich impact spinels is 0 43 whereas the average fe 3 fe t of detrital spinels is 0 17 fe 3 fe t ratios of the impact spinels range from 0 26 to 0 69 atoms and suggest formation at oxygen fugacities below 10 4 bar based on comparisons with experimental results comparison of the s3 impact spinels with similar spinels from the k t boundary layer eocene impact layer and late pliocene impact layer also suggests much lower o 2 levels in the 3 24 ga atmosphere oxidation of spinels present in condensed melt droplets was affected by the oxygen fugacity of the atmospheric component mixed with the ejected plume and the broad range in values may reflect f o 2 heterogeneity both temporally and spatially within the impact produced rock vapor plume compositional and morphological variability of spinels in the spherules suggests temperature and cooling rate were heterogeneous through the plume

sulfur isotope mass independent fractionation in impact deposits of the 3 2 billion year old mapepe formation barberton greenstone belt south africa abstract theoretical and experimental studies have shown that atmospheric so 2 isotopologue self shielding effects in the 190 220 nm region of the solar spectrum are the likely cause for mass independent fractionation of sulfur isotopes s mif the main products of this photochemical reaction so 3 and s 0 typically define a compositional array of ca 33 s 34 s 0 06 0 14 this is at odds with the generally observed trend in archean sulfides which broadly defines an array of ca 33 s 34 s 0 9 various explanations have been proposed including a diminution of 34 s caused by chemical and biogenic mass dependent fractionation of sulfur isotopes s mdf mixing with photolytic products produced during felsic volcanic events or partial blocking of the low wavelength part of the spectrum due to the presence of reduced atmospheric gases or an organic haze early in earth history large meteorite impacts would have ejected dust and gas clouds into the atmosphere that shielded solar radiation and affected global climate it is thus likely that at certain time intervals of high meteorite flux the atmosphere was significantly perturbed having an effect on atmospheric photochemistry and possibly leaving anomalous sulfur isotopic signatures in the rock record here we describe the sulfur isotopic signatures in sulfides of spherule beds s2 s3 and s4 of the barberton greenstone belt south africa in particular in spherule bed s3 and to a lesser extent s4 a trend of ca 33 s 34 s 0 23 is observed that closely follows the expected trend for so 2 photolysis in the 190 220 nm spectral range this suggests that an impact dust cloud deposited as spherule beds which sampled the higher region of the atmosphere specifically incorporated products of so 2 photolysis in the 190 220 nm range and blocked photochemical reactions at higher wavelengths 250 330 nm band by implication the generally observed archean trend appears to be the result of mixing of different mif s sources arising from a variety of photochemical reactions that took place in the lower part of the atmosphere

chronology of early archaean granite greenstone evolution in the barberton mountain land south africa based on precise dating by single zircon evaporation abstract we report precise 207 pb 206 pb single zircon evaporation ages for low grade felsic metavolcanic rocks within the onverwacht and fig tree groups of the barberton greenstone belt bgb south africa and from granitoid plutons bordering the belt dacitic tuffs of the hooggenoeg formation in the upper part of the onverwacht group yield ages between 3445 3 and 3416 5 ma and contain older crustal components represented by a 3504 4 ma old zircon xenocryst fig tree dacitic tuffs and agglomerates have euhedral zircons between 3259 5 and 3225 3 ma in age which we interpret to reflect the time of crystallization a surprisingly complex xenocryst population in one sample documents ages from 3323 4 to 3522 4 ma we suspect that these xenocrysts were inherited during the passage of the felsic melts to the surface from various sources such as greenstones and granitoid rocks now exposed in the form of tonalite trondhjemite plutons along the southern and western margins of the bgb and units predating any of the exposed greenstone or intrusive rocks several of the granitoids along the southern margin of the belt have zircon populations with ages between 3490 and 3440 ma coeval with or slightly older than onverwacht felsic volcanism while the kaap valley pluton along the northwestern margin of the belt is coeval with fig tree dacitic volcanism these results emphasize the comagmatic relationships between greenstone felsic volcanic units and the surrounding plutonic suites some of the volcanic and plutonic units contain zircon xenocrysts older than any exposed rocks these indicate the existence of still older units possibly stratigraphically lower and older portions of the greenstone sequence itself older granitoid intrusive rocks or bodies of older unrelated crustal material our data show that the onverwacht and fig tree felsic units have distinctly different ages and therefore do not represent a single tectonically repeated unit as proposed by others unlike the late archaean abitibi greenstone belt in canada which formed over about 30 ma exposed rocks in the bgb formed over a period of at least 220 ma the complex zircon populations encountered in this study imply that conventional multigrain zircon dating may not accurately identify the time of felsic volcanic activity in ancient greenstones a surprising similarity in rock types tectonic evolution and ages of the bgb in the kaapvaal craton of southern africa and greenstones in the pilbara block of western australia suggests that these two terrains may have been part of a larger crustal unit in early archaean times

subglacial observations from oksfjordjokelen north norway stepped bedrock topography at the snout of a small outlet glacier from oksfjordjokelen north norway produces an extensive subglacial cavity system which stretches some 70m across and 100m up glacier giving access beneath ice 50 m thick inside the cavity regelation ice clean glacier ice and deforming basal ice have been observed samples were taken and basal debris concentrations at the glacier sole were found to vary between 0 005 and 15 38 per cent by volume the basal ice velocity has been determined using a linear variable differential transformer attached to an analogue recorder and also by means of measured displacements of ice crews and clasts embedded in the basal ice velocities were found to differ both spatially and temporally from a maximum of 2 55 mm h1 to a minimum of 0 3 mm h 1 the measurements and observations which have been related to present theory show how spatially averaged values for a number of variables could lead to inaccuracies in predicted erosion values certainly at a local scale on the exposed foreland jointcontrolled lee side faces provide evidence for extensive subglacial plucking here taken to mean the removal of preloosened bed material and or material resulting from bed failure indeed in the cavity the early stages of removal of joint controlled blocks by ice deformation along joints have been observed the importance of debris rich basal ice is shown in the formation of large striations up to 500cm 16cm 2cm present on the foreland

metamorphic core complexes of cordilleran type in the cyclades aegean sea greece the islands of ios and naxos in the cycladic archipelago greece contain elongate domes over which a pervasive north to north northeast stretching lineation is warped these islands may be metamorphic core complexes of cordilleran type involving middle crustal rocks drawn upward and outward from underneath the sedimentary basins now 100 km to the south from ca 25 ma onward during uplift the crystalline rocks now exposed on the islands were deformed noncoaxially in the deeper levels of a major shallow dipping ductile shear zone this shear zone may have connected with low angle normal faults in its upper levels because it is probable that its later history involved the metamorphic complexes being dragged out from under a stretching and fracturing upper plate composed mainly of unmetamorphosed sedimentary and ophiolite suite rocks this model can be extended to explain the exhumation of the high pressure low temperature metamorphic rocks on crete which with the cyclades forms a distended paired metamorphic belt separated by an active sedimentary basin

early archean spherule beds chromium isotopes confirm origin through multiple impacts of projectiles of carbonaceous chondrite type three early archean spherule beds from barberton south africa have anomalous cr isotope compositions in addition to large ir anomalies confirming the presence of meteoritic material with a composition similar to that in carbonaceous chondrites the extra terrestrial components in beds s2 s3 and s4 are estimated to be approx l 50 60 and 15 30 respectively these beds are probably the distal and possibly global ejecta from major large body impacts these impacts were probably much larger than the cretaceous tertiary event and all occurred over an interval of approx 20 m y implying an impactor flux at 3 2 ga that was more than an order of magnitude greater than the present flux

quantifying climate and glacier mass balance in north norway during the younger dryas abstract oksfjordjokelen is located at 70 n on the troms finnmark border in north norway during the younger dryas it was decoupled from and sat just beyond the margin of the scandinavian ice sheet at this time the major fjords in troms and finnmark were ice free with outlet glaciers from the icefield filling a number of smaller side valleys only one outlet from the icefield sorfjorddalen is temporally well constrained by 14 c dating and association with the main shoreline associated with a period of minimal crustal rebound dated to the younger dryas sorfjorddalen is reconstructed using a valley centre line iterative model and assuming a no slip basal boundary condition this assumption of cold based ice is supported by the geomorphological evidence of angular bouldery fronto lateral moraines formed during the younger dryas the equilibrium line altitude for the sorfjorddalen is calculated using both the balance ratio and accumulation area ratio methods and this is used to constrain the snout positions generally to mapped moraines of the other outlets this approach assumes similarity of mass balance gradients and geometries of the outlet glaciers which is supported by present day symmetry of the icefield this method is extremely useful in such environments where dateable material is often difficult if not impossible to find some margins terminated in deep water where bathymetry was lacking making calving quantification problematic with subsequent impacts on equilibrium line altitudes poorly constrained these deep water terminating snouts were discounted from subsequent palaeo climate reconstructions an empirical equilibrium line altitude temperature precipitation relationship was used to define limits of climate change required to sustain the reconstructed icefield palaeo precipitation estimates were refined using a palaeo temperature estimate for the younger dryas from andoya calculations of ice flux through the equilibrium line altitude were used to further constrain the mass balance characteristics of the reconstructed icefield and these suggest similarities with ice masses found in the northern nordaustlandet regions of svalbard

paleomagnetism of middle tertiary volcanic rocks from the western cascade series northern california the western cascade series wcs is a 3 5 km thick crudely homoclinal east dipping calcalkaline volcanic sequence of mid oligocene to early miocene age that crops out near the southern tip of the cascade range in northern california the mean direction of remanent magnetization in the wcs is d 4 9 i 57 6 n 53 k 14 4 95 5 3 when compared to a reference direction for the north american craton the wcs direction indicates that the southern cascade range has rotated 14 0 9 0 since the wcs accumulated a difference in mean direction between the lower and upper halves of the wcs suggests that much of this rotation occurred during the late oligocene six other paleomagnetic studies of rock units of an age roughly comparable to the wcs also are available from western oregon northwestern california and southwestern washington all show statistically significant clockwise rotation inclinations of all but one are concordant comparison of directions indicates that the pacific northwest did not rotate as a rigid body but neither did it behave as a collection of small independently rotating domains the amount of rotation found throughout the area increases to the west or northwest suggesting a driving force for rotation operating at the continental margin