laser step heating 40ar39ar age spectra from early archean 3 5 ga barberton greenstone belt sediments a technique for detecting cryptic tectono thermal events abstract samples from sediments of the fig tree group located in the central part of the circa 3 2 to 3 5 ga barberton greenstone belt bgb have been analyzed by the 40 ar 39 ar laser step heating technique this technique has enabled previously cryptic thermal overprints to be detected in various sedimentary units which include reworked chemical sediments barite clastic sediments sandstone shale and one stromatolite in most cases the apparent age plateaux can be identified by corresponding ca k and cl k ratio plots as belonging to separate mineral phases most of the samples exhibit simple ar diffusion loss during the low temperature part of the experiments while occasionally showing excess ar or possible 39ar recoil effects the various sediments analyzed show evidence for distinct overprinting between 2025 and 2090 ma one barite sample gave t0 original blocking age 2673 3 ma 1 which is close to a calculated model ar diffusion loss age for the same sample of 2688 3 ma 1 this age is interpreted as representing final granitoid activity adjacent to the bgb and or craton scale tectonism associated with the limpopo orogeny two samples gave t0 ages of 2350 2400 ma which may reflect increased thermal gradients associated with the formation of the thick transvaal sedimentary basin that may once have covered the bgb the dominant apparent age plateaux together with modelled diffusive ar loss ages of 2025 2090 ma are thought to represent regional thermal anomalies related to large scale tectono thermal activity in the kaapvaal craton of which the bushveld complex which covers a surface area of 60 000 km2 and formation of the vredefort structure are obvious manifestations the strong thermal overprinting recorded by the sediments has effectively removed any ancient atmosphere i e 40 ar 36 ar ratios

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

records of an ancient martian magnetic field in alh84001 although mars does not presently appear to have a global dynamo magnetic field strong crustal fields have recently been detected by the mars global surveyor above surfaces formed 3 or more ga we present magnetic and textural studies of martian meteorite alh84001 demonstrating that 4 ga carbonates containing magnetite and pyrrhotite carry a stable natural remanent magnetization because 40 ar 39 ar thermochronology demonstrates that most alh84001 carbonates have probably been well below the curie point of magnetite since near the time of their formation weiss et al earth planet sci lett 2002 this issue their magnetization originated at 3 9 4 1 ga on mars this magnetization is at least 500 million years myr older than that known in any other planetary rock and its strong intensity suggests that mars had generated a geodynamo and global magnetic field within 450 650 myr of its formation the intensity of this field was roughly within an order of magnitude of that at the surface of the present day earth sufficient for magnetotaxis by the bacteria whose magnetofossils have been reported in alh84001 and possibly for the production of the strong crustal anomalies chromite in alh84001 may retain even older records of martian magnetic fields possibly extending back to near the time of planetary formation

geodynamo solar wind and magnetopause 3 4 to 3 45 billion years ago stellar wind standoff by a planetary magnetic field prevents atmospheric erosion and water loss although the early earth retained its water and atmosphere and thus evolved as a habitable planet little is known about earth s magnetic field strength during that time we report paleointensity results from single silicate crystals bearing magnetic inclusions that record a geodynamo 3 4 to 3 45 billion years ago the measured field strength is 50 to 70 that of the present day field when combined with a greater paleoarchean solar wind pressure the paleofield strength data suggest steady state magnetopause standoff distances of 5 earth radii similar to values observed during recent coronal mass ejection events the data also suggest lower latitude aurora and increases in polar cap area as well as heating expansion and volatile loss from the exosphere that would have affected long term atmospheric composition

geomagnetic paleointensity derived from single plagioclase crystals we suggest a new approach for the determination of absolute geomagnetic paleointensity that uses single plagioclase crystals to avoid problems caused by alteration during thellier thellier analyses transmission electron microscope imaging and rock magnetic data indicate that plagioclase crystals from a recent basalt flow in hawaii 1955 kilauea eruption contain pseudo single to single domain titanomagnetite inclusions these feldspars yield paleointensity values that agree within error with magnetic observatory data and paleointensity values reported from whole rock samples these results suggest that single plagioclase crystals from older igneous rocks may provide a viable source of paleointensity data

the paleomagnetism of single silicate crystals recording geomagnetic field strength during mixed polarity intervals superchrons and inner core growth 1 the basic features of the geomagnetic reversal chronology of the last 160 million years are well established the relationship between this history and other features of the field however has been elusive the determination of past field strength paleointensity is especially challenging commonly accepted results have come from analyses of bulk samples of lava historic lavas have been shown to faithfully record the past field strength when analyzed using the thellier double heating method data from older lavas however tend to show effects of in situ and laboratory induced alteration here we review an alternative approach single plagioclase crystals can contain minute magnetic inclusions 50 350 nm in size that are potential high fidelity field recorders thellier experiments using plagioclase feldspars from an historic lava on hawaii provide a benchmark for the method rock magnetic data from older lavas indicate that the feldspars are less susceptible to experimental alteration than bulk samples this resistance is likely related to the lack of clays in addition magnetic minerals are sheltered by the encasing silicate matrix from natural alteration that can otherwise transform the well defined thermoremanent magnetization into an irresolute chemical remanent magnetization if there is a relationship between geomagnetic reversal frequency and paleointensity it should be best expressed during superchrons intervals with few or no reversals thellier data sets based on single plagioclase crystals from lavas erupted during the cretaceous normal polarity superchron 83 120 million years ago suggest a strong 12 1022 am2 stable field consistent with an inverse relationship between reversal frequency and paleointensity superchrons may represent times when the pattern of core mantle boundary heat flux allows the geodynamo to operate at peak efficiency as suggested in some numerical models thellier data from single plagioclase crystals formed during times of moderate 10 reversals million years reversal occurrence suggest a weaker and more variable field these paleointensity data together with a consideration of paleomagnetic directions suggest that geomagnetic reversals field morphology secular variation and intensity are related the linkages over tens of millions of years imply a lower mantle control on the geodynamo on even longer timescales the magnetization held by plagioclase and other silicate crystals can be used to investigate the proterozoic and archean geomagnetic field during the onset of growth of the solid inner core data from plagioclase crystals separated from mafic dikes together with directional data from whole rocks indicate a dipole dominated field similar to that of the modern 2 5 2 7 billion years ago older archean rocks are of great interest for paleomagnetic and paleointensity investigations because they may record a time when the compositionally driven convection of the modern dynamo may not have been operating and a solid inner core did not play its current role in controlling the geometry of outer core flow most rocks of this age have been affected by low grade metamorphism investigations using single silicate grains provide arguably our best hope of seeing through secondary geologic events and reading the early history of the geodynamo absolute paleointensity measurements of the oldest rocks on the planet will require the further development of methods to investigate silicate crystals with exsolved magnetic minerals that address the uncertainties posed by thermocrystallization remanent magnetization anisotropy and slow cooling fortunately prior work in rock magnetism together with advances in analytical equipment and techniques provides a solid foundation from which these frontier issues can be approached

plant and insect fossils at norwood in south central minnesota a record of late glacial succession abstract the norwood site in sibley co minnesota contains 1 6 m of silt resting on till and overlain by peat the base of the peat has been radiocarbon dated at 12 400 60 and the top at 11 200 250 yr b p the pollen plant macrofossils and insect remains in the basal silt consist of boreal species inhabiting open environments but not tundra no modern analogue exists for the insect assemblage which includes elements of boreal forest tundra forest and western affinities the transition from an unstable open environment to a stable coniferous forest is reflected by both plant and insect fossils and is interpreted as a successional rather than a climatic event during this time of significant biologic change the climate is inferred to have been relatively uniform with temperatures similar to those presently existing in the boreal forest south of the tundra forest transition zone the geologic and ecologic succession at norwood is generally similar to that presently associated with ice stagnation of the klutlan glacier in the yukon territory localized successional sequences similar to those at norwood are conceived to have occurred repeatedly during the melting of the laurentide ice and thus the proposed model has potentially broad application to the interpretation of late glacial sequences

potbellies wasp waists and superparamagnetism in magnetic hysteresis because the response of a magnetic substance to an applied field depends strongly on the physical properties of the material much can be learned by monitoring that response through what is known as a magnetic hysteresis loop the measurements are rapid and quickly becoming part of the standard set of tools supporting paleomagnetic research yet the interpretation of hysteresis loops is not simple it has become apparent that although classic single domain pseudo single domain and multidomain loops described in textbooks occur in natural samples loops are frequently distorted having constricted middles wasp waisted loops or spreading middles and slouching shoulders potbellies such complicated loops are often interpreted in oversimplified ways leading to erroneous conclusions the physics of the problem have been understood for nearly half a century yet numerical simulations appropriate to geological materials are almost unavailable in this paper we discuss results of numerical simulations using the simplest of systems the single domain superparamagnetic sd sp system examination of the synthetic hysteresis loops leads to the following observations 1 wasp waisting and potbellies can easily be generated from populations of sd and sp grains 2 wasp waisting requires an sp contribution that saturates quickly resulting in a steep initial slope and potbellies require low initial slopes the sp contribution approaching saturation at higher fields the approach to saturation is dependent on volume hence the cube of grain diameter therefore there is a very strong dependence of hysteresis loop shape on the assumed threshold size 3 we were unable to generate potbellies using an sp sd threshold size as large as 30 nm and wasp waists cannot be generated using a threshold size as small as 8 nm the occurrence of both potbellies and wasp waists in natural samples is consistent with a room temperature threshold size of some 15 nm 5 4 simulations using a threshold size of 15 20 nm with populations dominated by sp grain sizes that is with a small number of sd grains produce synthetic hysteresis loops consistent with measured hysteresis loops and transmission electron microscopic observations from submarine basaltic glass 5 simulations and measurements using two populations with distinct coercivity spectra can also generate wasp waisted loops a relatively straightforward analysis of the resulting loops can distinguish the latter case from wasp waisting resulting from sp sd behavior

granite greenstone terranes in the pilbara block australia as coeval volcano plutonic complexes evidence from u pb zircon dating of the mount edgar batholith the mount edgar batholith in the pilbara block western australia consists in part of a gneiss complex that resembles other archean high grade gneiss terranes the complex is extensively intruded by variably foliated granitoids which also intrude the surrounding greenstone succession the warrawoona group the contact between the greenstone belt and the gneiss complex is tectonic field evidence indicates that the banded gneisses are the oldest part of the batholith zircon u pb ages for three samples are 3429 13ma 3443 10ma and3448 8ma all very similar to the age of zircons in the felsic volcanic rocks near the base of the greenstone succession pidgeon 1978 1 two granitoids representing pre and post tectonic phases respectively give zircon ages of 3304 10ma and3314 13ma thereby constraining the major structural events to be contemporary with granite magmatism consistent with a diapiric model for the tectonic evolution of the batholith collins 1989 2 these ages are similar to the zircon age for terminal volcanism in the warrawoona group pidgeon 1984 3 but are 100ma older than the precisely defined rb sr whole rock ages obtained throughout the batholith collins and gray 1989 4 batholith wide resetting or late closure of rb sr isotopic systems is implied possibly associated with late fluid circulation the temporal coincidence between plutonism and volcanism in the mount edgar region strongly suggests that the greenstone succession contains the extrusive equivalents of both the 3300ma granites and the 3440ma gneisses in the batholith the isotopic evidence combined with the structural data indicates that the granites cannot be derived from the surrounding greenstone belt further there is no compelling evidence for a sialic basement to the greenstone succession

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

modern sangamon and yarmouth soil development in loess of unglaciated southwestern illinois abstract the thebes section in unglaciated southwestern illinois contains a well preserved 500 kyr loess paleosol sequence with four loesses and three interglacial soils various magnetic mineralogical and elemental properties were analyzed and compared over the thickness of soil sola these proxies for soil development intensity have the following trend yarmouth geosol sangamon geosol modern soil quartz plagioclase zr sr and tio 2 na 2 o ratios were most sensitive to weathering frequency dependent magnetic susceptibility and anhysteretic remanent magnetization greatest in a horizons also correspond well with soil development intensity neoformed mixed layered kaolinite expandables suggestive of a warm humid climate were detected in the sangamon and yarmouth soil sola clay illuviation in soils was among the least sensitive indicators of soil development differences in properties among interglacial soils are interpreted to primarily reflect soil development duration with climatic effects being secondary assuming logarithmic decreases in weathering rates the observed weathering in the sangamon geosol is consistent with 50 kyr of interglacial weathering oxygen isotope stage 5 compared to 10 kyr for the modern soil oxygen isotope stage 1 we propose that the yarmouth geosol in the central midwest formed over 180 kyr of interglacial weathering including oxygen isotope stages 7 9 and 11

single zircon dating constraining the maximum age of the barberton greenstone belt southern africa one of the central issues in early precambrian crustal evolution is the age and genetic relationship between greenstone belt supracrustal assemblages and nearby high grade gneiss terranes the crucial question in most cases is which of the two is older we report zircon ages using the new single grain evaporation technique from a metaquartzite and a felsic volcanic flow of the onverwacht group in the barberton greenstone belt bgb south africa and swaziland that constrain the age of this basal part of the greenstone sequence between 3451 15 and 3438 6 ma this is almost 100 ma younger than previous age estimates assigned to the onverwacht and agrees well with recent sm nd pb pb and zircon ion microprobe dating tonalitic orthogneisses of the ancient gneiss complex agc adjacent to the bgb have ages up to 3644 4 ma and are thus demonstrably older than the greenstones also they cannot be derived from them by partial melting as has been proposed previously this age relationship as well as the presence of metaquartzites and thrust slices of pregreenstone tonalite in the onverwacht tectonic sequence rule out an entirely oceanic evolution for the early history of the bgb and suggest that continental crust was either below or nearby

palaeomagnetism of archaean rocks of the onverwacht group barberton greenstone belt southern africa evidence for a stable and potentially reversing geomagnetic field at ca 3 5 ga abstract palaeomagnetic data from the palaeoarchaean era 3 2 3 6 ga have the potential to provide us with a great deal of information about early conditions within and processes affecting the earth s core mantle and surface environment here we present new data obtained from some of the oldest palaeomagnetic recorders in the world igneous and sedimentary rocks from the onverwacht group of the barberton greenstone belt kaapvaal craton southern africa our palaeomagnetic measurements strengthen a recently published positive conglomerate test y usui j a tarduno m watkeys a hofmann and r d cottrell 2009 and our new u pb date constrains the conglomerate to older than 3455 8 ma the new palaeomagnetic data from other units are nontrivial to interpret and are of uncertain reliability when taken individually similar we argue to all other published palaeomagnetic data of this age nonetheless four poles two new two derived from published data produced from high temperature components of magnetisation recorded in the komati noisy and hooggenoeg formations exhibit considerably improved clustering when their directions are corrected for differences in attitude resulting from a large fold structure dated at 3 23 ga on the basis of this enhanced consistency in stratigraphic coordinates the positive conglomerate test and the absence of any clear indications of their remagnetisation from comparison with younger poles we argue that these are the most trustworthy palaeomagnetic results yet produced from any rocks of palaeoarchaean age when taken in conjunction with published data the new results present the most compelling evidence to date that the earth had a stable geomagnetic field at ca 3 5 ga in addition to presenting tentative evidence that it was undergoing polarity reversals the data do not appear to support a claim made previously from palaeoarchean palaeomagnetic data from the pilbara craton y suganuma y hamano s niitsuma m hoashi t hisamitsu n niitsuma k kodama and m nedachi 2006 of extremely rapid latitudinal plate motion during this period finally when compared with similarly aged data from the pilbara craton western australia the new data do not rule out the hypothesis that the two cratons were conjoined at this point in their history in the supercraton vaalbara

a primary magnetization fingerprint from the cretaceous laytonville limestone further evidence for rapid oceanic plate velocities paleomagnetic data from the cretaceous 101 88 ma laytonville limestone of northern california are of geodynamic interest because they suggest a rate of absolute plate motion 15 cm yr that exceeds those of all present day plates such findings assume that a primary magnetization is preserved in these accreted pelagic limestone outcrops this assumption is supported by a trend in the paleomagnetic inclination data that matches in magnitude and sign the expected sense of motion predicted by comparing the mean paleolatitude value with the north american apparent polar wander path the pervasiveness of remagnetizations seen in shallow water carbonates of north america and elsewhere however raises questions as to whether the laytonville limestone is remagnetized regardless of tests indicating a primary magnetization the remagnetized carbonates have a distinctive wasp waisted magnetic hysteresis signature thought to reflect a bimodal distribution of magnetite grain sizes this signature can be quantified by the relationship between saturation remanence mr saturation magnetization ms coercivity hc and the coercivity of remanence hcr mr ms 0 89 hcr hc 06 this power law is compared with reference data from pacific deep sea carbonates deep sea drilling project sites 167 317 and 463 ocean drilling program site 806 and tethyan pelagic carbonates valle del mis section southern alps which together define a different curve mr ms 0 53 hcr hc 1 0 the laytonville limestone falls in two categories on the basis of unblocking temperature spectra resistance to alternating field demagnetization and behavior during the acquisition of isothermal remanent magnetization red laytonville limestone is thought to contain pigmentary hematite in addition to magnetite hysteresis curves from red laytonville limestone are wasp waisted but because such curves also typify magnetite hematite mixtures these data do not provide an unambiguous test for remagnetization some white laytonville limestone however appears to contain negligible amounts of hematite and can be directly compared with the remagnetized carbonates hysteresis parameters derived from white samples are indistinguishable from those of the reference pelagic limestones such samples have a stratigraphic significance occurring where paleolatitude data indicate an equatorial crossing these samples may have depleted pigmentary hematite due to increased magnetic dissolution as the limestone entered the equatorial belt of high productivity the lack of a remagnetization fingerprint in these white samples together with paleomagnetic lithologic and paleontologic data form a consistent data set supporting the primary nature of the laytonville limestone magnetization and rapid oceanic plate velocities a small plate size coupled with the unusually vigorous mantle plume volcanism of the cretaceous pacific basin could have combined to reduce the effectiveness of asthenospheric drag accounting for the rapid motion

flow mechanism of the des moines lobe of the laurentide ice sheet rapid flow of the des moines lobe of the laurentide ice sheet may have been related to its unlithified substrate new reconstructions of the lobe based on moraine elevations sediment subsidence during moraine deposition and flow direction indicators indicate that the lobe may have been 3 times thicker than in previous reconstructions nevertheless implied basal shear stresses are 15 kpa so internal ice deformation was not significant instead the lobe likely moved by a combination of sliding plowing of particles through the bed surface and bed shear consolidation tests on basal till yield preconsolidation stresses of 125 300 kpa so effective normal stresses on the bed were small a model of sliding and plowing indicates that at such stresses most particles gripped by the ice may have plowed easily through the till bed resulting in too small a shear traction on the bed to shear it at depth consistent with this prediction measurements of orientations of clasts in basal till yield a weak fabric implying pervasive bed shear strain less than 2 although some stronger fabrics have been reported by others we infer tentatively that movement was principally at the bed surface by plowing

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

evidence for a 3 45 billion year old magnetic remanence hints of an ancient geodynamo from conglomerates of south africa 1 paleomagnetic and rock magnetic analyses of 3445 million year old dacite conglomerate clasts and parent body rocks from the barberton greenstone belt south africa define two dominant components of magnetization one component unblocked at low temperature is an overprint acquired 180 million years ago the other component is unblocked at high temperatures and passes a conglomerate test indicating that this component is older than the depositional age of the conglomerate 3416 ma the high unblocking temperature component shows scatter in the parent body rocks that can be explained by the effects of modern lightning strikes archean overprinting and the presence of multidomain magnetic grains that are conducive to carrying secondary magnetizations alternatively this scatter can be explained by exotic magnetization scenarios in the absence of a dynamo including magnetization by an external field related to solar wind interaction with the atmosphere such exotic mechanisms can be tested with the acquisition of paleointensity data while more scattered than paleomagnetic data recording the more recent geomagnetic field the high unblocking temperature component in the dacite parent body shows some consistency and the simplest explanation of the data is that they reflect a geodynamo 3445 million years ago

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

denitrification in the recharge area and discharge area of a transient agricultural nitrate plume in a glacial outwash sand aquifer minnesota 1 recharge rates of nitrate no3 to groundwater beneath agricultural land commonly are greater than discharge rates of no3 in nearby streams but local controls of no3 distribution in the subsurface generally are poorly known groundwater dating cfc 3h was combined with chemical ions and gases and stable isotope n s and c analyses to resolve the effects of land use changes flow patterns and water aquifer reactions on the distributions of o2 no3 so4 and other constituents in a two dimensional vertical section leading from upland cultivated fields to a riparian wetland and stream in a glacial outwash sand aquifer near princeton minnesota within this section a plume of oxic no3 rich groundwater was present at shallow depths beneath the fields and part of the wetland but terminated before reaching the stream or the wetland surface groundwater dating and hydraulic measurements indicate travel times in the local flow system of 0 to 40 years with stratified recharge beneath the fields downward diversion of the shallow no3 bearing plume by semiconfining organic rich valley filling sediments under the wetland and upward discharge across the valley and stream bottom the concentrations and 15n values of no3 and n2 indicate that the no3 plume section was bounded in three directions by a curvilinear zone of active denitrification that limited its progress however when recalculated to remove the effects of denitrification the data also indicate changes in both the concentrations and 15n values of no3 that was recharged in the past isotope data and mass balance calculations indicate that fes2 and other ferrous fe phases were the major electron donors for denitrification in at least two settings 1 within the glacial fluvial aquifer sediments beneath the recharge and discharge areas and 2 along the bottom of the valley filling sediments in the discharge area combined results indicate that the shape and progress of the oxic no3 plume termination were controlled by a combination of 1 historical and spatial variations in land use practices 2 contrast in groundwater flow patterns between the agricultural recharge area and riparian wetland discharge area and 3 distribution and abundance of electron donors in both the sand aquifer and valley filling sediments the data are consistent with slow migration of redox zones through the aquifer in response to recharging oxic groundwater during holocene time then an order of magnitude increase in the flux of electron acceptors as a result of agricultural no3 contamination in the late twentieth century to which the redox zone configuration still may be adjusting the importance of denitrification for no3 movement through formerly glaciated terrains should depend on the source areas and depositional environments of the glacial sediments as well as geomorphology and recent stream valley sediment history

paleointensity of the early geodynamo 2 45 ga as recorded in karelia a single crystal approach we report thellier paleointensity results from single plagioclase crystals derived from ca 2 45 ga border dikes of the burakovka layered intrusion karelia russia rock magnetic and transmission electron microscope analyses indicate that these plagioclase crystals contain pseudosingle to single domain magnetite inclusions fifteen determinations from four dikes meet reliability criteria and yield an average paleointensity of 43 2 10 8 t which corresponds to a virtual dipole moment of 8 43 2 11 x 10 2 2 am 2 although our result is unlikely to adequately represent the time averaged field the mean and range of values are similar to those of the present day field these values suggest that the inner core which may stabilize the geodynamo had started to grow by early proterozoic time

identification of paleomagnetic remanence carriers in ca 3 47 ga dacite from the duffer formation the pilbara craton abstract the ca 3 47 ga duffer formation has been considered to carry one of the oldest paleomagnetic records yet the lack of rock magnetic data limits the interpretation of the nature of the remanence we conducted a rock magnetic and paleomagnetic investigation on columnar dacite of the duffer formation the main magnetic minerals are phenocrysts of titanomagnetite and magnetite and secondary hematite in groundmass detailed thermal demagnetization revealed more complex natural remanence than previously estimated consisting of four components with typical unblocking temperature of 200 350 200 500 590 and 690 c combined with alternating field demagnetization and rock magnetic data they are attributed to titanomagnetite coarse grained magnetite fine grained magnetite and hematite respectively the comparison of unblocking temperature and coercivity suggests that the previously proposed secondary component is carried by fine grained magnetite as well as hematite while the putative primary component is carried by coarse grained magnetite and titanomagnetite microscopic observations showed that coarse grained magnetite and titanomagnetite are primary crystals although this does not necessarily indicate they preserve primary remanence the remanence directions of all components revealed higher scatter than the previous studies suggesting the need for caution in interpretation the low unblocking temperature of titanomagnetite suggests that if their remanence is truly primary the rocks must have kept below 250 c for 3 47 billion years

simulating 2 ga of geodynamo history the paleomagnetic record indicates the geodynamo has been active over much of earth history with surprisingly little trend in paleointensity variability however is expected from models that predict a sharp increase in intensity following inner core nucleation icn and implied by neoproterozoic anomalies that hint at a highly variable field over several hundred million years here we demonstrate with a suite of numerical dynamos driven by a new thermal evolution model that the geodynamo could have transitioned from a multipolar to dipolar regime around 1 7 ga then to a weak field dynamo around 1 0 ga and finally to a strong field dipole following icn around 650 ma that is maintained to the present day the occurrence of a weak field geodynamo in the neoproterozoic may be consistent with the observed anomalous apparent polar wander paths and reversal behavior recovery to a dipolar geodynamo in the phanerozoic could be a signature of inner core nucleation index terms 1507 1560 and 1521

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

the geochemistry of loess asian and north american deposits compared abstract loess is widely distributed over asia and north america and constitutes one of the most important surficial deposits that serve as terrestrial records of the quaternary the oldest pleistocene loess in china is likely 2 6 ma thus spanning much or all of the pleistocene in north america most loess is no older than the penultimate glacial period with the exception of alaska where the record may go back to 3 0 ma on both continents loess deposits date primarily to glacial periods and interglacial or interstadial periods are represented by paleosols both glacial and non glacial sources of silts that comprise the bulk of loess deposits are found on both continents although loess has been considered to be representative of the average upper continental crust there are regionally distinctive compositions of loess in both asia and north america loess deposits in asia from yakutia tajikistan and china have compositionally distinct major element compositions due to varying abundances of silicate minerals carbonate minerals and clay minerals in north america loess in the mississippi river valley the great plains and alaska are also distinguishable with regard to major element composition that reflects highly diverse source sediments trace element geochemistry sc th zr and the rare earth elements also shows regional diversity of loess bodies in both asia and north america on both continents most loess bodies show significant contributions from later cycle altered sedimentary rocks as opposed to direct derivation from igneous rocks further some loess bodies have detectable contributions from mafic igneous rocks as well as major contributions from average upper crustal felsic rocks intercalated paleosols in loess sections show geochemical compositions that differ significantly from the underlying loess parent materials ratios of soluble to insoluble elements show depletions in paleosols due to chemical weathering losses of calcite dolomite plagioclase mica apatite and smectite in asia and north america the last interglacial paleosol is more weathered than equivalent modern soils which could be due either to a climate that was warmer and more humid a longer period of pedogenesis or both in asia early pleistocene loess and paleosols are both more weathered than those from the middle and late pleistocene forming prior to a mid pleistocene aridification of asia from uplift of the tibetan plateau understanding the geochemistry of loess and paleosols can tell us much about past atmospheric circulation past temperature and moisture regimes and even tectonic processes

growth fault structure and stratigraphic architecture of the buck ridge volcano sedimentary complex upper hooggenoeg formation barberton greenstone belt south africa abstract the 3 45 3 42 ga buck ridge volcano sedimentary complex br vsc forms the uppermost part of the hooggenoeg formation in the early archaean onverwacht group of the barberton greenstone belt south africa the complex consists mainly of massive and pillowed basalts and felsic quartz plagioclase porphyritic rocks which are capped by pervasively silicified sedimentary rocks deposition of the felsic extrusive and volcani clastic sedimentary rocks occurred in shallow water to emersion during an extensional tectonic regime the extensional regime is expressed by a number of listric normal faults that transect the entire 2 km thick complex the syndepositional i e growth fault character of the faults is indicated by variations in thickness and facies of rock units across the faults and by systematic rotation of the fault blocks broadly contemporaneous with the faulting felsic rocks intruded the complex into the highest levels the geometry of these intrusive bodies was controlled by the major growth faults extension also created space for the intrusion of approximately bedding perpendicular felsic and mafic dykes and black chert veins the new field observations and u pb zircon shrimp data have pinned the timing of events during and after deposition of the br vsc the growth fault model accounts well for previously unexplained structural observations that include both extensional and compressional features recognition of an early extensional phase of deformation demonstrates further similarities between the tectonic histories of the barberton greenstone belt and the greenstone belts of the east pilbara in australia

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

palaeomagnetism of the neoarchaean pongola and ventersdorp supergroups and an appraisal of the 3 0 1 9 ga apparent polar wander path of the kaapvaal craton southern africa the style of tectonic processes on the young earth is a topic of intense debate palaeomagnetism provides the only robust tests with which to settle theoretical disputes based on the rock record that infer large scale horizontal motions between palaeocontinents a possible apparent polar wander path apwp of the kaapvaal craton shows significant distances between adjacent palaeomagnetic poles that can be interpreted as evidence for such horizontal displacements during the archaean palaeoproterozoic there are however substantial uncertainties build into this apwp currently the published apwp for the kaapvaal craton comprises eight palaeopoles between 3 0 and 1 9 ga including several between 2 8 and 2 7 ma that indicate possible horizontal motions exceeding 6500 km the aim of this contribution is to increase the pole density within this interval and in doing so to test the robustness of the kaapvaal apwp within this time frame more than 250 samples of basalt diamictite and dolerite of the neoarchaean pongola ca 2 95 2 85 ga and ventersdorp ca 2 71 2 70 ga supergroups of the kaapvaal craton were sampled both at the surface and in an underground mine thermal demagnetisation revealed two magnetic components a high temperature component is present in three pongola formations and in two ventersdorp formations these may be recorders of the primary natural remanent magnetisation because of lack of precise geochronology and positive field tests this magnetisation can only be constrained to be older than ca 2 05 ga a medium temperature overprint in many samples probably records remagnetisation at ca 180 ma during the emplacement of basalts and dolerites of the mesozoic karoo large igneous province this remagnetisation appears to have affected a vast area of the kaapvaal craton our new palaeopoles together with those published previously demonstrate that the neoarchaean apwp of the kaapvaal craton remains poorly defined direct comparisons between the apwp of the kaapvaal craton with apwps of other archaean cratons to determine relative motions between archaean continents therefore need more reliable data

geomagnetic field strength 3 2 billion years ago recorded by single silicate crystals analysis of silicate rocks from the archaean kaapvaal craton is used to obtain palaeodirections and intensities of the earth s magnetic field some 3 2 billion years ago and finds that the field strength was within 50 per cent of the present day value indicating that a viable magnetosphere sheltered the early earth s atmosphere from solar wind erosion

discriminating texturally similar tills in central minnesota by graphical and multivariate techniques abstract the sequence of glaciation in the midwestern united states has been defined mainly by till stratigraphy correlation of tills over long distances however is complicated by spatial variation of sediment characteristics in east central minnesota three till units were previously defined two were considered to be lithologically identical and were correlated with the late wisconsin advance of the rainy lobe whereas a third till was assigned to the contemporaneous advance of the superior lobe this interpretation was inconsistent with subsequent glaciological reconstructions of the rainy and superior lobes in an effort to define the parent lobe of the deposits in central minnesota the tills were sampled analyzed and compared by graphical and multivariate techniques graphical comparisons of sedimentological parameters were inadequate to distinguish important differences among the groups however principal component analysis of the analytical data indicates that the original interpretation is untenable and that one of the tills formerly assigned to the rainlylobe should now be attributed to the superior lobe problems encountered with correlation of tills exposed at the surface illustrate the complexity and problems associated with subsurface stratigraphic correlation

isotope evidence of paleo el nino southern oscillation cycles in loess paleosol record in the central united states abstract the 13 c of soil carbonate in rhizoconcretions collected from aloess paleosol sequence in the central united states indicates that grow ing season c 3 c 4 plant ratio oscillated by 35 on a 900 200 yr timescale during the late wisconsinan glaciation the pattern appears inphase with advance and retreat of the southern margin of the lauren tide ice sheet suggesting influence by paleo el nino southern oscilla tion cycles the 13 c of soil organic matter indicates that the annual av erage c 3 c 4 plant ratio oscillated only by 18 with a periodicity of450 100 yr and closely matched the cyclic pattern of loess paleosollayers it suggests a periodic enhancement of the penetration of thegulf of mexico air over the region during this time keywords carbon isotope loess paleosol glacial phase paleo enso cycle introduction the el nino southern oscillation enso causes global scale climateanomalies the el nino and la nina are two counterparts of enso whichare associated with warming and cooling in surface water in the eastern trop ical pacific on the basis of modeling studies cane 1998 cane andclement 1999 and clement and cane 1999 proposed that frequency in tensity and duration of long term el nino or la nina intervals have oscil lated on a millennial time scale during the past 150 k y they proposed thatduring glacial periods these long term enso cycles might alter the globalclimate via atmospheric teleconnections in which el nino dominant inter vals may cause retreat and la nina dominant intervals may cause advanceof the southern margin of the laurentide ice sheet the enso sensitive re gions currently in north america partially overlap the area that was coveredby the laurentide ice sheet during the last glaciation fig 1 modernla nina conditions cause heavy snowfall whereas el nino conditions re duce winter precipitation in these regions http www ogp noaa gov enso it is reasonable to assume that la nina or el nino dominant intervals mighthave controlled advance and retreat of the southern margin of the laurentideice sheet if these intervals oscillate on a long term time scale in this paper we use stable isotope ratios from a loess paleosol exposure in the centralunited states to seek evidence for millennial scale paleo enso cycles

origin and paleoclimatic significance of late quaternary loess in nebraska evidence from stratigraphy chronology sedimentology and geochemistry muhs d r bettis iii e a aleinikoff j n mcgeehin j p beann j skipp g marshall b d roberts h m johnson w c benton r 2008 origin and paleoclimatic significance of late quaternary loess in nebraska evidence from stratigraphy chronology sedimentology and geochemistry gsa bulletin 120 11 12 1378 1407

evidence for a very long term trend in geomagnetic secular variation reconstructions of palaeosecular variation suggest that the earth s magnetic field reversed less frequently 2 82 to 2 45 billion years ago relative to the cenozoic era this suggests a long term trend of decreasing geodynamo stability since the archaean eon the earth s inner core is believed to inhibit rapid fluctuations in the geomagnetic field from developing into full polarity reversals1 2 consequently during the precambrian the smaller size of the inner core might suggest that polarity reversals could occur more readily it is therefore surprising that there are indications that reversals were rare during this period3 4 here we use new and existing palaeomagnetic data from three continents to examine the stability of the earth s magnetic field from 2 82 to 2 45 billion years ago we show that on average geomagnetic secular variation the field variations produced by normal geodynamo action during the late archaean and early proterozoic was different from that of the past 200 million years specifically the apparent variability of the geomagnetic pole as viewed at low and mid latitudes was reduced relative to the past 200 million years according to both dynamo simulations4 and more recent palaeomagnetic field observations5 the observed pattern of secular variation suggests a lower frequency of polarity reversals 2 5 billion years ago this may imply that the geodynamo is becoming progressively less stable over long timescales consistent with some numerical simulations6 7 possibly as a result of changing outer core geometry that has accompanied inner core growth

ice streams in the laurentide ice sheet identification characteristics and comparison to modern ice sheets this paper presents a comprehensive review and synthesis of ice streams in the laurentide ice sheet lis based on a new mapping inventory that includes previously hypothesised ice streams and includes a concerted effort to search for others from across the entire ice sheet bed the inventory includes 117 ice streams which have been identified based on a variety of evidence including their bedform imprint large scale geomorphology topography till properties and ice rafted debris in ocean sediment records despite uncertainty in identifying ice streams in hard bedrock areas it is unlikely that any major ice streams have been missed during the last glacial maximum laurentide ice streams formed a drainage pattern that bears close resemblance to the present day velocity patterns in modern ice sheets large ice streams had extensive onset zones and were fed by multiple tributaries and where ice drained through regions of high relief the spacing of ice streams shows a degree of spatial self organisation which has hitherto not been recognised topography exerted a primary control on the location of ice streams but there were large areas along the western and southern margin of the ice sheet where the bed was composed of weaker sedimentary bedrock and where networks of ice streams switched direction repeatedly and probably over short time scales as the ice sheet retreated onto its low relief interior several ice streams show no correspondence with topography or underlying geology perhaps facilitated by localised build up of pressurised subglacial meltwater they differed from most other ice stream tracks in having much lower length to width ratios and have no modern analogues there have been very few attempts to date the initiation and cessation of ice streams but it is clear that ice streams switched on and off during deglaciation rather than maintaining the same trajectory as the ice margin retreated we provide a first order estimate of changes in ice stream activity during deglaciation and show that around 30 of the margin was drained by ice streams at the lgm similar to that for present day antarctic ice sheets but this decreases to 15 and 12 at 12 cal ka bp and 10 cal ka bp respectively the extent to which these changes in the ice stream drainage network represent a simple and predictable readjustment to a changing mass balance driven by climate or internal ice dynamical feedbacks unrelated to climate or both is largely unknown and represents a key area for future work to address

estimates of heat flow in the deep mantle based on the power requirements for the geodynamo 1 regeneration of the magnetic field by convection in the core places demands on heat flow into the base of the mantle if the heat flow is too low thermal convection is shut off and the rate of generation of compositional buoyancy by the solidification of the core becomes too low to sustain the geodynamo conversely a large heat flow causes rapid growth of the inner core so that convection prior to the appearance of the inner core must be sustained by thermal buoyancy alone the attendant requirements on primordial heat become more severe as the age of the inner core decreases we show that estimates of the present day heat flow satisfy the power requirements for the geodynamo however the resulting thermal history is incompatible with estimates of mantle temperatures prior to 3 ga this discrepancy can be resolved by accumulating radioactive isotopes in d or adding heat sources to the core

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

detecting the oldest geodynamo and attendant shielding from the solar wind implications for habitability abstract the onset and nature of the earliest geomagnetic field is important for understanding the evolution of the core atmosphere and life on earth a record of the early geodynamo is preserved in ancient silicate crystals containing minute magnetic inclusions these data indicate the presence of a geodynamo during the paleoarchean between 3 4 and 3 45 billion years ago while the magnetic field sheltered earth s atmosphere from erosion at this time standoff of the solar wind was greatly reduced and similar to that during modern extreme solar storms these conditions suggest that intense radiation from the young sun may have modified the atmosphere of the young earth by promoting loss of volatiles including water such effects would have been more pronounced if the field were absent or very weak prior to 3 45 billion years ago as suggested by some models of lower mantle evolution the frontier is thus trying to obtain geomagnetic field records that are 3 45 billion years old as well as constraining solar wind pressure for these times in this review we suggest pathways for constraining these parameters and the attendant history of earth s deep interior hydrosphere and atmosphere in particular we discuss new estimates for solar wind pressure for the first 700 million years of earth history the competing effects of magnetic shielding versus solar ion collection and bounds on the detection level of a geodynamo imposed by the presence of external fields we also discuss the prospects for constraining hadean paleoarchean magnetic field strength using paleointensity analyses of zircons

mantle regulation of core cooling a geodynamo without core radioactivity abstract the case for radioactivity in the core based on the power requirements of the geodynamo is re evaluated previous calculations of mantle regulation of core thermal evolution have used an inappropriate formula new calculations with a more appropriate formula yield lower core heat loss in the past thus mitigating the implication of unreasonably high past core and mantle temperatures multiple thermal evolutions leading to present heat flows are also demonstrated depending on the efficiency of mantle removal of core heat some with moderately high past core heat loss and some with low and steady core heat loss the latter would permit a low or moderate power dynamo without core radioactivity key uncertainties are the efficiency of core cooling by the mantle the thermal conductivity of the core and the energy or entropy flow required to maintain the dynamo the present rate of heat loss from the core is argued to be still rather uncertain and a commonly used estimate of the thermal conductivity of the core is shown plausibly to be too high and in any case to be uncertain by perhaps a factor of 2 the geochemical difficulties associated with postulating radioactive heat sources in the core are stressed

an updated radiocarbon based ice margin chronology for the last deglaciation of the north american ice sheet complex abstract the north american ice sheet complex naisc consisting of the laurentide cordilleran and innuitian ice sheets was the largest ice mass to repeatedly grow and decay in the northern hemisphere during the quaternary understanding its pattern of retreat following the last glacial maximum is critical for studying many facets of the late quaternary including ice sheet behaviour the evolution of holocene landscapes sea level atmospheric circulation and the peopling of the americas currently the most up to date and authoritative margin chronology for the entire ice sheet complex is featured in two publications geological survey of canada open file 1574 dyke et al 2003 quaternary glaciations extent and chronology part ii dyke 2004 these often cited datasets track ice margin recession in 36 time slices spanning 18 ka to 1 ka all ages in uncalibrated radiocarbon years using a combination of geomorphology stratigraphy and radiocarbon dating however by virtue of being over 15 years old the ice margin chronology requires updating to reflect new work and important revisions this paper updates the aforementioned 36 ice margin maps to reflect new data from regional studies we also update the original radiocarbon dataset from the 2003 2004 papers with 1541 new ages to reflect work up to and including 2018 a major revision is made to the 18 ka ice margin where banks and eglinton islands once considered to be glacial refugia are now shown to be fully glaciated our updated 18 ka ice sheet increased in areal extent from 17 81 to 18 37 million km2 which is an increase of 3 1 in spatial coverage of the naisc at that time elsewhere we also summarize region by region significant changes to the deglaciation sequence this paper integrates new information provided by regional experts and radiocarbon data into the deglaciation sequence while maintaining consistency with the original ice margin positions of dyke et al 2003 and dyke 2004 where new information is lacking this is a pragmatic solution to satisfy the needs of a quaternary research community that requires up to date knowledge of the pattern of ice margin recession of what was once the world s largest ice mass the 36 updated isochrones are available in pdf and shapefile format together with a spreadsheet of the expanded radiocarbon dataset n 5195 ages and estimates of uncertainty for each interval

geomorphology and till architecture of terrestrial palaeo ice streams of the southwest laurentide ice sheet a borehole stratigraphic approach abstract a multidimensional study utilising geomorphological mapping and the analysis of regional borehole stratigraphy is employed to elucidate the regional till architecture of terrestrial palaeo ice streams relating to the late wisconsinan southwest laurentide ice sheet detailed mapping over a 57 400 km2 area of southwestern saskatchewan confirms previous reconstructions of a former southerly flowing ice stream demarcated by a 800 km long corridor of megaflutes and mega scale glacial lineations ice stream 1 and cross cut by three formerly southeast flowing ice streams ice streams 2a b and c analysis of the lithologic and geophysical characteristics of 197 borehole samples within these corridors reveals 17 stratigraphic units comprising multiple tills and associated stratified sediments overlying preglacial deposits the till thicknesses varying with both topography and distance down corridor reconciling this regional till architecture with the surficial geomorphology reveals that surficial units are spatially consistent with a dynamic switch in flow direction recorded by the cross cutting corridors of ice streams 1 2a b and c the general thickening of tills towards lobate ice stream margins is consistent with subglacial deformation theory and variations in this pattern on a more localised scale are attributed to influences of subglacial topography including thickening at buried valley margins thinning over uplands and thickening in overridden ice marginal landforms

sm nd rb sr pb pb dating of carbonates from the early archaean barberton greenstone belt south africa evidence for post depositional isotopic resetting at low temperature abstract two silicic carbonate sequences occurring in the onverwacht group og and the overlying fig tree group ftg of the early archaean barberton greenstone belt south africa contain silicified dolomites with primary structures and textures they occur with volcanic rocks dated by single zircons the associated onverwacht felsic volcanics have an age of 3440 3416 ma whereas the fig tree volcanics are 3225 ma old therefore the depositional age of both sequences is between 3440 and 3225 ma pb isotopic data for different parts of a single og whole rock sample form a linear array corresponding to a pb pb age of 2632 99 ma sm nd and rb sr isotopes were analysed in four whole rock samples of the ftg silicified carbonates and form an sm nd array corresponding to an age of 2612 84 ma with initial end 8 1 0 5 and an rb sr array of 2743 36 ma with initial 87 sr 86 sr 0 7013 0 0003 these whole rock samples have rb sr ratios of 1 and unusually high rb concentrations for carbonates 100 ppm which derive from processes accompanying silicification all three isotopic systems of whole rocks indicate ages of 2 7 ga much younger than the depositional age of the successions the agreement provides strong evidence that an alteration event reset all of the isotopic systems sm nd as well as rb sr and u pb we suggest that the ages represent the timing of silicification and sericitization of the carbonates by large scale fluid movement at an apparent temperature of 200 c

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

paleomagnetic evidence of a low temperature origin of carbonate in the martian meteorite alh84001 indirect evidence for life on mars has been reported from the study of meteorite alh84001 the formation temperature of the carbonates is controversial some estimates suggest 20 to 80 c whereas others exceed 650 c paleomagnetism can be used to distinguish between these possibilities because heating can remagnetize ferrimagnetic minerals study of two adjacent pyroxene grains from the crushed zone of alh84001 shows that each possesses a stable natural remanent magnetization nrm implying that mars had a substantial magnetic field when the grains cooled however nrm directions from these particles differ implying that the meteorite has not been heated significantly since the formation of the internal crushed zone about 4 billion years ago the carbonate globules postdate this brecciation and thus formed at low temperatures

the intensity of the geomagnetic field at 3 5 ga paleointensity results from the komati formation barberton mountain land south africa abstract paleointensity measurements have been carried out on 3 5 ga samples from the komati formation type locality using both the thellier and van zijl methods these samples contain a single steeply directed negative trm component acquired during metamorphism of the komati lavas thellier experiments yielded values ranging from 12 to 37 t but an average paleofield intensity for the four best determinations is 20 3 t a slightly lower average paleointensity of 15 3 t was obtained using van zijl experiments preheating was used to chemically stabilize seven samples used in van zijl determinations and these produced nearly ideal plots with an average paleointensity of 13 2 t a single basaltic komatiite sample gave a nearly ideal van zijl plot indicating about 21 t nearly the same paleointensity as the peridotitic komatiite samples even though its nrm intensity was several orders of magnitude lower since the komati characteristic remanence was acquired during a slow cooling the data must be reduced by a factor of 1 55 to account for the difference between laboratory and natural cooling rates calculation of an equivalent equatorial paleointensity using the paleolatitude implied by the steep komati characteristic remanence then gives value of 5 t for the intensity of the geomagnetic field at 3 5 ga lower than the present value of about 30 t

paleomagnetism of the marble bar chert member western australia implications for apparent polar wander path for pilbara craton during archean time abstract the archean biosphere drilling project abdp drilled a continuous 270 m long oriented core from the towers formation which includes the marble bar chert member 3456 1 3476 0 ma in the pilbara craton northwestern australia a paleomagnetic study of 261 discrete specimens collected from a 158 5 to 182 0 m section of the marble bar chert member revealed two distinct magnetic components lt and mt the mt component yields seven different mean paleomagnetic directions clustered as mb1 to mb7 these together with the published paleomagnetic poles of early archean rocks from the pilbara craton draw a continuous paleomagnetic pole path which likely to be regarded as the early to late archean apparent polar wander path apwp for the pilbara craton the apwp implies that the pilbara craton underwent a latitudinal drift of about 21 during the interval when the magnetization of the marble bar chert member was acquired the estimated speed of the lateral drift is 12 112 cm yr 120 1120 km myr which is large compared with current plate motion velocities suggesting that continents might have moved during the archean faster than in the phanerozoic

rock magnetism of remagnetized paleozoic carbonates low temperature behavior and susceptibility characteristics we have conducted a new set of rock magnetic experiments on samples of remagnetized paleozoic carbonates of eastern north america these experiments were designed to investigate the origin of the unusual hysteresis behavior of these rocks by evaluating 1 the importance of ferrimagnetic pyrrhotite as a remanence carrier and 2 the sources of low field susceptibility low temperature measurements of saturation isothermal remanent magnetization sirm indicate that the pyrrhotite magnetic transition at 32 k is absent in the trenton and onondaga limestones of new york this transition is observed but poorly expressed in the knox group of east tennessee room temperature sirms cooled to 10 k in zero field lose a small fraction of their intensity between 30 and 35 k samples from all three formations show a broad peak in irm intensity at about 200 k which is typical of pyrrhotite in the trenton and onondaga samples the ferrimagnetic component of low field susceptibility is significantly larger than the ratio mrs hcr and is thus probably due dominantly to magnetite in some of the knox samples the reverse is true suggesting an important pyrrhotite contribution for all of the samples the ferrimagnetic susceptibility normalized by the saturation magnetization is anomalously high about a factor of 5 or 10 higher than the typical value for magnetite we believe that this indicates a very substantial contribution from superparamagnetic particles strong frequency dependence of susceptibility and very high ratios of anhysteretic to saturation remanence confirm the importance of ultrafine particles spanning the superparamagnetic single domain boundary all three of these chemically remagnetized carbonates units exhibit the following properties which have not previously been found together for any rock or synthetic analog and which therefore appear to constitute a diagnostic set of rockmagnetic criteria for recognizing chemically remagnetized rocks mrs ms 0 89 hcr hc 0 6 k ms 50 m a arm sirm 20 ka mrs hcr 50

iib trioctahedral chlorite from the barberton greenstone belt crystal structure and rock composition constraints with implications to geothermometry iib trioctahedral chlorite in the barberton greenstone belt bgb metavolcanic rocks was formed during pervasive greenschist metamorphism the chem ical composition of the chlorite is highly variable with the fe fe mg ratio ranging from 0 12 to 0 8 among 53 samples the chemical variation of the chlorite results from the chemical diversity of the host rock especially the mgo content of the rock but major details of the variation pattern of the chlorite are due to the crystal structure of the chlorite all major cation abundances in the chlorite are strongly correlated with each other sil icon increases with mg and decreases with fe while aliv and alvi decrease with mg and increase with fe2 a complex exchange vector explains over 90 of the chlorite compositional variation mg4sife2 3alvi 1 aliv 1 which has 3 parts fe mg substitution coupled with one part tschermakite substitution this ratio is required to maintain the charge and site balances and the dimensional fit between the tetrahedral and octahedral sheets the subtle change in al substitution in chlorite implies that alvi is preferentially ordered in the m 4 site and about 84 of the alvi present is in the m 4 sites when they are nearly filled with alvi based on 47 analyzed chlorite bearing rock samples chlorite chl composition is strongly correlated with the mgo content of the host rock calculated correlation coefficients are 0 91 for sio2chl mgorock 0 87 for al2o3chl mgorock 0 89 for mgochl mgorock and 0 85 for feochl mgorock only weak correlations have been found between chlorite oxides and other oxides of rock between same oxides in chlorite and rock sio2 0 67 al2o3 0 59 feo 0 41 however mgochl is saturated at about 36 wt in rocks that have mgo above 22 wt the mgochl is about 5 wt when the host rock approaches 0 wt of mgo this implies that mg substituting into the chlorite is approximately limited to 1 5 9 2 mg atoms per formula unit and 1 0 3 2 aliv chlorite geothermometers can not be applied to all bgb samples however the empirical chlorite geothermometer based on aliv of chlorite may be applicable to chlorites formed under metamorphic conditions because it can predict the chemical composition of the chlorite from basaltic and dacitic samples in this study an estimated temperature of about 320 c for the greenschist metamorphism of the greenstone belt through this geothermometer is consistent with that obtained by other geothermometers

the age of the inner core abstract the energy conservation law when applied to the earth s core and integrated between the onset of the crystallization of the inner core and the present time gives an equation for the age of the inner core in this equation all the terms can be expressed theoretically and given values and uncertainties of all relevant physical parameters the age of the inner core can be obtained as a function of the heat flux at the core mantle boundary and the concentrations in radioactive elements it is found that in absence of radioactive elements in the core the age of the inner core cannot exceed 2 5 ga and is most likely around 1 ga in addition to have an inner core as old as the earth concentrations in radioactive elements needed in the core are too high to be acceptable on geochemical grounds

evidence at lomax illinois for mid wisconsin 40 000 yr b p position of the des moines lobe and for diversion of the mississippi river by the lake michigan lobe 20 350 yr b p abstract an abrupt change in environment from a wetland to a deeper slackwater lake at 20 345 85 yr b p occurred in a second order valley tributary to the mississippi river near lomax illinois the age of this shift is associated with the overflow of glacial lake milan and diversion of the mississippi river from the princeton valley rock island to peoria to its present course rock island to st louis the diversion occurred due to blockage of the princeton valley segment of the ancient mississippi river by the lake michigan lobe impoundment of glacial lake milan and eventual overflow of the lake southwest of rock island across a former drainage divide near andalusia illinois fossil ostracode assemblages in the slackwater sediment at lomax indicate changes in the post diversion full glacial paleohydrology and based on multivariate analysis hint at the area s paleoclimate an older part of the succession at lomax is consistent with glaciation in the upper iowa river basin about 40 000 yr b p aggradation of sediment rich in coarse silt is attributed to a response of the ancient iowa river basin associated with deposition of the glacigenic sheldon creek formation by the des moines lobe

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

reassessment of archean crustal development in the barberton mountain land south africa based on u pb dating this paper presents u pb ages for 23 samples from the barberton mountain land an archean granite greenstone terrain in the kaapvaal craton of south africa a reexamination of the geology of this area in the light of previous u pb dating and the new ages indicates that much of its deformation can probably be accounted for by a single short lived compressional event within the time span 3230 3215 ma which was synchronous with deposition of the moodies group and the upper part of the fig tree group sediments this event may have been responsible for thrusting of much older sequences in the onverwacht group including slivers of 3 54 ga old gneiss circa 3 47 ga old komatiitic and tholeiitic sequences and circa 3 45 ga old felsic sequences key constraints on the timing of deformation are previously determined 3227 3225 ma ages for detrital zircons in thrusted units of the fig tree group a 3229 4 3 ma age for a deformed porphyry which is pre or synthrusting a previously determined 3227 3 ma age for an undeformed porphyry in an inferred thrust fault 3227 1 ma ages for both the kaap valley pluton and an ignimbrite in the stolzburg syncline both of which predate regional upright folding and a 3216 2 ma age for the dalmein pluton which appears to cut regional upright ne sw trending folds in the kromberg syncline part of the final phase of deformation associated with regional compression areally extensive granitoid sheets and syenogranitic plutons were subsequently emplaced into the barberton region at about 3105 ma the linear configuration of the syenogranitic plutons which are also aligned with the gabbroic usushwana complex as well as the geometry of the granitoid sheets suggests that the circa 3105 ma magmatism was caused by incipient rifting around a nw se trending crustal fracture this episode was probably coeval with a late regional transtensional phase of deformation recorded in the greenstone belt some of the above interpretations are in conflict with previous models for early development of the barberton greenstone belt a 3352 6 ma age from a metagabbro which is comagmatic with a proposed ultramafic sheeted dike complex is over 100 m y younger than the probable age of volcanic rocks which host the dikes this conflicts with the suggestion that much of the onverwacht group comprises an ophiolitic section obducted in an intraoceanic setting at circa 3 45 ga that partially melted to form trondhjemitic magmas which intruded along active thrust faults present data suggest that despite its antiquity and the diversity of ages and lithologies found within it the barberton greenstone belt can generally be understood in terms of actualistic tectonic processes at convergent margins

felsic igneous rocks within the 3 3 to 3 5 ga barberton greenstone belt high crustal level equivalents of the surrounding tonalite trondhjemite terrain emplaced during thrusting felsic igneous rocks within the circa 3 5 ga ophiolitic onverwacht group of the barberton greenstone belt have traditionally been mapped as recurring volcanic units within a continuous stratigraphic succession these felsic units have been interpreted as part of several mafic to felsic volcanic cycles making up this succession our field and geochemical data show that the felsic igneous rocks are not closely related to the mafic ultramafic rocks of the onverwacht group rather the data indicate that these felsic units are high level equivalents of the widespread gneissic to isotropic trondhjemite tonalite plutons which surround and intrude the greenstone belt the felsic igneous rocks are predominantly shallow level intrusions and subsurface felsic domes associated with minor volcanics and volcanoclastics and postdate the onverwacht group by 40 50 ma the felsic intrusions were preferentially emplaced along thrust zones cutting the ophiolite basement and its overlying sedimentary cover structural and chemical data imply that the hydrated ophiolitic rocks of the barberton greenstone belt are allochthonous and were thrust over similar simatic rocks from which the felsic igneous rocks were syntectonically derived by partial melting such structural imbrication of simatic crust can be best incorporated in a model advocating archean intraoceanic obduction associated with shallow angle subduction similar to that documented for the neotectonics of the western aleutians

thermal history of the 3 5 3 2 ga onverwacht and fig tree groups barberton greenstone belt south africa inferred by raman microspectroscopy of carbonaceous material raman spectra of carbonaceous material were collected in situ from samples of cherts of the onverwacht and fig tree groups in the central barberton greenstone belt the spectra feature two dominant peaks characteristic of disordered carbon the d peak at 1310 cm 21 and the o peak at 1580 1600 cm 21 d peak positions and relative peak intensities and areas indicate that all samples have been altered to lower greenschist facies or above no correlation was observed between maximum temperature and stratigraphic position or degree of hydrothermal alteration implying that metamorphism in the central barberton greenstone belt was regional and unaccompanied by the flow of large quantities of hydrothermal fluids samples from the marble bar chert of the pilbara block western australia have been heated to the same extent as samples from barberton this study demonstrates the use of raman spectra of carbonaceous material as a sensitive geother mometer for low temperature metamorphic facies this application could also be used to establish the antiquity of putative microfossils from metamorphic terranes

the miller volcanic spark discharge experiment miller9s 1950s experiments used besides the apparatus known in textbooks one that generated a hot water mist in the spark flask simulating a water vapor rich volcanic eruption we found the original extracts of this experiment in miller9s material and reanalyzed them the volcanic apparatus produced a wider variety of amino acids than the classic one release of reduced gases in volcanic eruptions accompanied by lightning could have been common on the early earth prebotic compounds synthesized in these environments could have locally accumulated where they could have undergone further processing

importance of the regelation process to certain properties of basal tills deposited by the laurentide ice sheet in iowa and illinois u s a

are post seismic effects of the m 8 4 bolnay earthquake 1905 july 23 still influencing gps velocities in the mongolia baikal area summary gps measurement campaigns in the mongolia baikal area show that 1 the baikal rift zone is currently opening at 4 5 mm yr 1 in a nw se direction and 2 the western part of the amurian north china block is moving eastward relative to eurasia at 5 7 mm yr 1 these results are consistent with recent geodetic studies in asia and indicate that crustal motions in mongolia and north china derived from geodetic measurements are significantly faster than those proposed by most deformation models of asia using a numerical model we test whether post seismic deformation caused by viscoelastic relaxation in the lower crust following the tsetserleg bolnay earthquake sequence mw 7 9 and 8 4 1905 july can explain this discrepancy we find that surface velocities at the gps sites presented here for the 1997 2000 time period can reach 4 mm yr 1 depending on the earthquake source parameters and the rheology used in the models using a model based on our best current knowledge of lithospheric structure and rheology in the mongolia baikal area we find that the contribution of post seismic effects to the gps velocities presented here is less than 2 mm yr 1 for the mongolian sites and less than 1 mm yr 1 for the baikal sites we therefore conclude that the post seismic effects of the mw 8 4 1905 bolnay earthquake cannot explain why the gps derived crustal motions in mongolia and the baikal rift zone are significantly larger than those proposed by deformation models of asia this discrepancy must therefore be sought in processes not accounted for in most of these deformation models such as the farfield contribution of the pacific subduction zones and or the effect of gravitational forces on intracontinental deformation

paleomagnetic constraints on the archean geomagnetic field intensity obtained from komatiites of the barberton and belingwe greenstone belts south africa and zimbabwe abstract the intensity of the geomagnetic field during archean is an important source of information about the evolution of the earth s core hale earth planet sci lett 86 1987b 354 reported a very low equatorial paleointensity of 5 t at ca 3 5 ga obtained from komatiites of the barberton greenstone belt south africa and regarded their remanences as thermal overprints due to a metamorphic event however this result has a major drawback since remanences of the barberton komatiites are carried by magnetites formed during serpentinization that typically results in acquisition of chemical remanent magnetization crm we report the results of detailed paleomagnetic rock magnetic and paleointensity investigations using komatiite samples from the barberton ca 3 5 ga and belingwe ca 2 7 ga greenstone belts in south africa and zimbabwe respectively paleodirectional data for barberton are consistent with results of previous work and the characteristic remanent magnetization chrm of the belingwe samples reveals a positive fold test suggesting that samples from both areas record the archean geomagnetic field the main carriers of chrm are ti free magnetites presumed to be akin to single domain grains and their rock magnetic properties are very stable when subjected to laboratory heating up to 600 700 c our microscopic observations provide support that these are secondary minerals formed during serpentinization thellier thellier paleointensity experiments for the barberton and belingwe samples yielded very low mean virtual dipole moment vdm estimates of 1 8 1 3 1022 a m2 and 1 1 0 9 1022 a m2 respectively these correspond to about 24 and 15 of the present day value considering the low metamorphic grade for the sampling areas however it seems to be somewhat difficult to regard the entire ranges of komatiite chrm as thermal overprints acquired by reheating during metamorphic events we hence propose as the most likely scenario the possibility that most fractions of chrm are survivors of primary grain growth crm taking the theoretical and experimental estimates of the ratio of crm to thermoremanent magnetization trm into account this suggests that these low mean vdm values can provide constraints on the lower limits of the geomagnetic field intensities in both greenstone belts at ca 3 5 and 2 7 ga

evidence for an early archean geomagnetic field a paleomagnetic study of the komati formation barberton greenstone belt south africa 3 5 ga old peridotitic komatiites of the komati formation barberton greenstone belt south africa have a characteristic nrm natural remanent magnetization that is stable during alternating field and thermal demagnetization the thermal nature of this remanence is demonstrated by successful thellier type paleo intensity determinations on some samples in the temperature range 350 to 575 c the characteristic nrm is an almost total thermal overprint acquired during unroofing and cooling following pervasive greenschist metamorphism at 3470 20 ma the mean direction of magnetization after cleaning is d 326 i 81 k 60 95 8 n 7 sites with a paleopole at 11 s 21 e p m 15 the paleopole falls in southern africa ancestral southern africa was therefore in polar latitudes around 3 5 ga the same position it occupied in the late archean and early proterozoic paleofield intensities are about one half present day intensities for polar latitudes a vigorous earth s magnetic field was already in existence in the early archean implying that the core had a suitable size composition and convective flow to support dynamo action

new constraints on the variation of the geomagnetic field during the late neolithic period archaeointensity results from sichuan southwestern china we have carried out an archaeomagnetic study on a late neolithic locality liujiazhai in sichuan southwestern china we pull together various dating techniques including radiocarbon analysis optically stimulated luminescence dating stratigraphic information as well as archaeological and archaeomagnetic estimations to constrain the age of the studied samples rock magnetic results indicate thermally stable fine grained magnetite or titanomagnetite as the dominant magnetic carriers more than half of the specimens 141 246 in the paleointensity experiment pass the selection criteria and are considered to record robust intensity values the virtual axial dipole moments range from approximately 2 8 to 7 8 1022 am2 with an average of 5 9 1022 am2 indicating that the geomagnetic intensity around 3000 before the common era b c e is overall lower than the present field intensity 9 8 1022 am2 of this area the new results from liujiazhai are generally consistent with the published data of similar age but deviate from the only available model of cals10k 1b at certain time periods making them important for future improvements of the model those data are significant for constraining the variation of geomagnetic field intensity between 3100 and 2600 b c e and improving the regional model of eastern asia

40ar 39ar age constraints on tectonothermal events in the shaw area of the eastern pilbara granite greenstone terrain w australia 700 ma of archean tectonic evolution abstract the shaw area in the eastern pilbara craton consists of granitoids with intrusion ages ranging from 3470 ma to 2850 ma and supracrustal rocks with ages ranging from 3515 ma to 3100 ma as determined by u pb and pb pb dating methods in this terrain several major deformation events have recently been recognized including late strike slip deformation compression and early extension in order to provide time constraints for these events twenty four samples from the shaw area have been used for 40ar 39ar step heating experiments twenty two hornblendes one muscovite one biotite resulting in spectra defining ages between 3520 and 2800 ma the late strike slip event occurred prior to 2944 9 ma the argon cooling ages provide time constraints for the compressional event between 3325 ma and 3200 ma cooling ages between 3300 and 3200 ma cannot be related to granitoid intrusion in this area and are interpreted as cooling after regional metamorphism related to the compressional event the best time constraint from 40ar 39ar ages for the split rock shear zone a major ductile detachment in the shaw batholith associated with early extension is that it was last active prior to 3222 13 ma a 3341 13 ma plateau age of a metabasalt sampled in contact with granites of the northwestern corunna downs batholith provides a minimum age for granitoid intrusion and a minimum age for the warrery shear zone hornblendes from the black range dolerite dyke gave a plateau age of 2796 9 ma providing a constraint on the time of cratonization of the pilbara granitoid greenstone terrain most of the cooling ages including a 3466 13 ma age for the contact zone of the north shaw granodiorite are related to intrusion of granitoids in the shaw batholith a large number but significantly not all of the sample population has been reset during the intrusion of late to post tectonic adamellites between 3050 ma and 2850 ma preservation of such a large range 700 ma in argon isotopic ages in a relatively small area is unusual the resetting of argon ages in the granitoid domain is best explained by a combination of preferential intrusion of younger granites and the difference in heat conductivity of granite versus basalt in the greenstone domain the resetting effect is more localized into a network of fluid flow channels mostly situated in shear zones

secular variation of the late archean early proterozoic geodynamo 1 we report new paleomagnetic data from 2 5 ga dikes of the superior province and derive estimates of paleosecular variation when combined with data from coeval dikes of the karelia craton russia and older 2 7 ga volcanics of the pilbara craton these estimates suggest a latitudinal variation of secular variation similar to that of the last 5 million years but with potentially smaller quadrupole family contributions the observations are consistent with some numerical models of the field that suggest a more dipolar field when the earth had a much smaller solid inner core

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

observation of shrinkage cracks in ocean floor titanomagnetites abstract a characteristic of low temperature ocean floor titanomagnetite oxidation is the formation of irregular curved cracks which are caused by shrinkage of the spinel lattice the spatial distribution of the shrinkage cracks within individual titanomagnetite grains magnetically extracted from different ocean floor basalts was studied by scanning electron microscopy the cracks do not develop in grains smaller than approximately 5 microns in diameter under ocean floor conditions the critical titanomagnetite oxidation state z for the onset of cracking is between z 0 23 and z 0 37 shortly before the onset of cracking internal stress reaches a maximum which if reflected in the magnetic coercivity of the material

rock magnetism of tiny exsolved magnetite in plagioclase from a paleoarchean granitoid in the pilbara craton granitoids are widespread in precambrian terranes as well as the phanerozoic orogenic belts but they have garnered little attention in paleomagnetic studies because granitoids often contain abundant coarse grained magnetically unstable oxides in this study the first example of tiny needle shaped exsolved oxides in plagioclase in a paleoarchean granitoid is reported the magnetic properties of single plagioclase crystals with the exsolved oxide inclusions have been studied to determine their paleomagnetic recording fidelity demagnetization experiments and hysteresis parameters indicate that the oxide inclusions are near stoichiometric magnetite and magnetically very stable first order reversal curve forc diagrams reveal negligible magnetostatic interactions minimal interactions are also reflected by very efficient acquisition of anhysteretic remanent magnetization single plagioclase crystals exhibit strong magnetic remanence anisotropies which require corrections to their paleodirectional and paleointensity data nonetheless quantitative consideration of anisotropy tensors of the single plagioclase crystals indicates that the bias can be mitigated by properly averaging data from a few tens of single crystals from the nonlinear thermoremanence acquisition of the plagioclase crystals we estimate that the plagioclase crystals can reconstruct paleointensity up to 50 t local metamorphic condition suggests that those magnetite may carry remanence of 3 2 to 3 3 ga we suggest that exsolved magnetite in granitoids is potentially a suitable target for the study of the early history of the geomagnetic field and prompt detailed microscopic investigations as well as paleomagnetic tests to constrain the age of remanence

the stratigraphy of the 3 5 3 2 ga barberton greenstone belt revisited a single zircon ion microprobe study abstract recent field and geochemical studies indicate a need to test the stratigraphy of the ca 3 5 ga barberton greenstone belt as it is presently adopted 1 2 this work uses the ion microprobe shrimp to attempt such a test results show that 1 volcaniclastic sediments of the theespruit formation