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

seismic detection of the lunar core despite recent insight regarding the history and current state of the moon from satellite sensing and analyses of limited apollo era seismic data deficiencies remain in our understanding of the deep lunar interior we reanalyzed apollo lunar seismograms using array processing methods to search for the presence of reflected and converted seismic energy from the core our results suggest the presence of a solid inner and fluid outer core overlain by a partially molten boundary layer the relative sizes of the inner and outer core suggest that the core is 60 liquid by volume based on phase diagrams of iron alloys and the presence of partial melt the core probably contains less than 6 weight of lighter alloying components which is consistent with a volatile depleted interior

revisiting the age and paleomagnetism of the modipe gabbro of south africa abstract the modipe gabbro is a precambrian intrusive unit hosted by the kaapvaal craton of south africa and provides one of the oldest records of geomagnetic field behavior in the late archean earlier paleomagnetic research into the modipe gabbro provided valuable insight into its magnetic properties and mineralogy but predate the development of paleomagnetic tools that are important in unraveling potentially complex histories of precambrian rocks further a lack of information about the precise age of the intrusion prevented this earlier paleomagnetic study from being used to evaluate the tectonic history of the kaapvaal craton here we present a comprehensive paleomagnetic and geochronologic study of the modipe gabbro that complements and builds upon the existing work the emplacement age of the gabbro 2784 0 1 0 ma established using the u pb id tims method on baddeleyite links the intrusion to nearby igneous bodies in the kaapvaal craton establishing one of the oldest fully differentiated magmatic events in the geologic record the precise age of the modipe gabbro and field observations about its strike allow us to perform a tilt correction using the both the attitude and paleomagnetism of the nearby derdepoort basalts the tilt corrected data allow a comparison of kaapvaal craton s paleolatitude with that of the pilbara craton at 2 78 ga with implications for the existence of the supercraton vaalbara

thermochronometry and palaeomagnetism of the archaean nelshoogte pluton south africa summary 207pb 206pb single grain zircon 40ar 39ar single grain hornblende and biotite and 40ar 39ar bulk sample muscovite and biotite ages from the nelshoogte trondhjemite pluton located in eastern transvaal south africa show that this granitoid had a protracted thermal history spanning 3213 4 ma to about 3000 ma whole rock 40ar 39ar ages from cross cutting dolerite dykes indicate that these were intruded at about 1900 ma there is no evidence of this or other later events significantly avecting the argon systematics of the minerals from the pluton dated by the 40ar 39ar method the pluton has a well defined palaeomagnetic pole which is dated at 3179 18 2s ma by 40ar 39ar dating of hornblende this pole 18 n 310 e a 95 9 yields a palaeolatitude of 0 significantly diverent from other archaean poles from the kaapvaal craton the palaeolatitude diverence implies that there was significant apparent polar wander during the archaean a second overprinting magnetization seen in the pluton is also seen in the lower proterozoic dolerite dykes and is consistent with other lower proterozoic 2150 1950 ma poles for southern africa

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

prospects for an ancient dynamo and modern crustal remanent magnetism on venus abstract venus lacks an internally generated magnetic field today whether one existed in the past is unknown but critical to atmospheric evolution and potential habitability canonical models assume the core of venus is cooling too slowly for convection and thus a magnetic dynamo to occur today core mantle heat flow is suppressed in these models after a putative transition in mantle dynamics associated with widespread volcanic resurfacing however recent studies of impact craters and other surface features support more steady heat loss over geologic time precipitation of mgo and or sio2 from the core can also drive compositional convection even with slow cooling here we reevaluate the likelihood that venus has an earth like at least partially liquid and chemically homogeneous core using numerical simulations of the coupled atmosphere surface mantle core evolution an earth like core is only compatible with the modern lack of a dynamo if the thermal conductivity of core material is towards the higher end of modern estimates i e 100 w m 1 k 1 if lower estimates like 40 50 w m 1 k 1 are actually correct then we favor recent proposals that the core has completely solidified or preserved primordial stratification any simulation initialized with a homogeneous liquid core predicts a global magnetic field with earth like surface strength for 2 3 billion years after accretion consistent with all available observations and also sporadic activity within the surface age while temperatures remain below the curie point of magnetite therefore future spacecraft missions should prioritize the first ever magnetometer measurements below the ionosphere to search for crustal remanent magnetism

the age of the solar system redefined by the oldest pb pb age of a meteoritic inclusion the age of the solar system is defined by the formation of the first solid grains in the solar nebula pb pb age dating of these solids which were later trapped in a meteorite indicates that the solar system is 0 34 1 91 million years older than previously thought the age of the solar system can be defined as the time of formation of the first solid grains in the nebular disc surrounding the proto sun this age is estimated by dating calcium aluminium rich inclusions in meteorites these inclusions are considered as the earliest formed solids in the solar nebula their formation marks the beginning for several long and short lived radiogenic clocks that are used to precisely define the timescales of solar system events such as the formation and evolution of planetary bodies1 2 3 here we present the 207pb 206pb isotope systematics in a calcium aluminium rich inclusion from the northwest africa 2364 cv3 group chondritic meteorite which indicate that the inclusion formed 4 568 2 million years ago this age is between 0 3 refs 4 5 and 1 9 refs 1 6 million years older than previous estimates and is the oldest age obtained for any solar system object so far we also determined the 26al 26mg model age of this inclusion and find that it is identical to its absolute pb pb age implying that the short lived radionuclide 26al was homogeneously distributed in the nebular disc surrounding the proto sun from the consistently old ages in the studied inclusion we conclude that the proto sun and the nebular disc formed earlier than previously thought

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

an early lunar core dynamo driven by thermochemical mantle convection although the moon currently has no internally generated magnetic field palaeomagnetic data combined with radiometric ages of apollo samples provide evidence for such a magnetic field from 3 9 to 3 6 billion years gyr ago1 possibly owing to an ancient lunar dynamo1 2 but the presence of a lunar dynamo during this time period is difficult to explain1 2 3 4 because thermal evolution models for the moon5 yield insufficient core heat flux to power a dynamo after 4 2 gyr ago here we show that a transient increase in core heat flux after an overturn of an initially stratified lunar mantle might explain the existence and timing of an early lunar dynamo using a three dimensional spherical convection model6 we show that a dense layer enriched in radioactive elements a thermal blanket at the base of the lunar mantle can initially prevent core cooling thereby inhibiting core convection and magnetic field generation subsequent radioactive heating progressively increases the buoyancy of the thermal blanket ultimately causing it to rise back into the mantle the removal of the thermal blanket proposed to explain the eruption of thorium and titanium rich lunar mare basalts7 plausibly results in a core heat flux sufficient to power a short lived lunar dynamo

radio echo sounding of svalbard glaciers peculi arities of radio echo sound ing of mountain gl aciers and ice fields between nunataks from terrestria l vehicles and from heli copters are consi dered in this paper the possibility of using compar atively high frequen cies of up to 865 mhz for sounding suc h glaciers is demonst rated on the basis of expe rimental data results of airbo rne radio echo so unding of svalba rd gl aciers of various types dimensions and regime obtained with the help of the 620 m hz airbo rne equipment of high resolution are shown returns from the gla cier bed have been obtained on the majority of glaciers unde r in vestigation the firn regions of ice fields with temp erate regimes flowing between nuna taks are mostly an exception depre ssion in the glacier bed with the ice thic kne ss up to 540 m as well as int ernal reflecting boundaries have been detected in some glaciers ic e volum es of glaciers have been determined from the airborne radio echo sounding data with the use of a parabolic appr oximation to the cross section glacier profiles good correl ational dependence between ice volume and surface area of glaciers has been established and is use d to estimate the ic e and water resources in sva lbard glaciers

a long lived lunar dynamo driven by continuous mechanical stirring lunar rocks contain a record of an ancient magnetic field that seems to have persisted for more than 400 million years and which has been attributed to a lunar dynamo models of conventional dynamos driven by thermal or compositional convection have had difficulty reproducing the existence and apparently long duration of the lunar dynamo here we investigate an alternative mechanism of dynamo generation continuous mechanical stirring arising from the differential motion due to earth driven precession of the lunar spin axis between the solid silicate mantle and the liquid core beneath we show that the fluid motions and the power required to drive a dynamo operating continuously for more than one billion years and generating a magnetic field that had an intensity of more than one microtesla 4 2 billion years ago3 are readily obtained by mechanical stirring the magnetic field is predicted to decrease with time and to shut off naturally when the moon recedes far enough from earth that the dissipated power is insufficient to drive a dynamo in our nominal model this occurred at about 48 earth radii 2 7 billion years ago thus lunar palaeomagnetic measurements may be able to constrain the poorly known early orbital evolution of the moon this mechanism may also be applicable to dynamos in other bodies such as large asteroids

a hadean to paleoarchean geodynamo recorded by single zircon crystals knowing when the geodynamo started is important for understanding the evolution of the core the atmosphere and life on earth we report full vector paleointensity measurements of archean to hadean zircons bearing magnetic inclusions from the jack hills conglomerate western australia to reconstruct the early geodynamo history data from zircons between 3 3 billion and 4 2 billion years old record magnetic fields varying between 1 0 and 0 12 times recent equatorial field strengths a hadean geomagnetic field requires a core mantle heat flow exceeding the adiabatic value and is suggestive of plate tectonics and or advective magmatic heat transport the existence of a terrestrial magnetic field before the late heavy bombardment is supported by terrestrial nitrogen isotopic evidence and implies that early atmospheric evolution on both earth and mars was regulated by dynamo behavior

linear and nonlinear thellier paleointensity behavior of natural minerals 1 we tested single domain sd and multidomain md theories of the acquisition and thermal demagnetization of thermoremanent magnetization trm and partial trm ptrm in thellier paleointensity determination using as samples plagioclase and mafic minerals extracted from matachewan diabase dikes susceptibility hysteresis and coercivity data indicate that the plagioclase contains mainly sd grains of magnetite while the mafic minerals clinopyroxene biotite and actinolite contain mainly md magnetite the extracted minerals were used in simulated thellier experiments with laboratory trm as a proxy for natural remanent magnetization nrm the plagioclase behaved ideally as predicted by neel sd theory giving a linear nrm ptrm arai plot with positive ptrm checks the mafic minerals behaved nonideally with strongly curved arai plots sagging below the ideal sd line in accord with observations for md magnetites and with predictions that in heating to moderate temperatures t more trm is lost than can be regained as ptrm causing convex down arai plots in experiments with ptrm parallel to nrm all ptrm tail checks were zero but ptrm checks increasingly deviated from the arai curve toward the sd line as t increased however in repeat thellier experiments ptrms at all t agreed with the original ptrms the negative ptrm checks were therefore not due to physicochemical alteration but rather to departure from initial states with different magnetic histories than those of the original ptrms the inability of ptrm checks to reliably distinguish between md behavior and irreversible alteration could lead to overestimates of paleointensity from straight line fits to the low t unaltered part of the arai curve or underestimates from fits to high t data alternating field cleaning to 15 mt before each thellier step made the arai plot only marginally less curved low temperature demagnetization ltd zero field cycling through the 120 k verwey transition hardly changed the curvature of the arai plot for ptrm parallel to nrm for ptrm perpendicular to nrm ltd led to an inflected arai curve straddling the ideal sd line data of either form would not give reliable paleointensity values no presently available pretreatment renders the paleointensity data of md grains usable sd and nearly sd grains which behave ideally in the thellier experiment remain the only useful recorders of absolute paleointensity

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

palaeomagnetism of flood basalts in the pilbara craton western australia late archaean continental drift and the oldest known reversal of the geomagnetic field 1 a late archaean circa 2775 2715 ma succession of terrestrial continental flood basalts mafic tuffs felsic volcanic rocks and clastic sedimentary rocks in the nullagine synclinorium and meentheena centrocline of the east pilbara basin western australia has been sampled for a palaeomagnetic study over 500 oriented mostly basalt drill cores were collected from the supracrustal succession and associated dykes thermal and alternating field demagnetization revealed two distinct components positive fold conglomerate and reversal tests confirm that the primary natural remanent magnetization nrm is still preserved the secondary component is interpreted as the record of remagnetization during a major thermal event possibly in the early proterozoic analysis of the primary nrm directions results in a magnetostratigraphy and an apparent polar wander path apwp for the 60 myr interval covered by the sampled succession assuming a geocentric axial dipole during this time interval the apwp shows that the pilbara craton was drifting during the late archaean and that drift rates probably varied significantly in particular a mean 27 2 shift in palaeolatitude is recorded across an unconformity that represents a relatively short time period and that marks a significant change in basalt geochemistry this study suggests that continents moved horizontally during the late archaean and that the rates of movement were significantly faster than in the phanerozoic in addition a reversed polarity interval with a positive reversal test is recorded we argue that it documents the oldest known geomagnetic reversals of the geomagnetic field

lunar paleointensity measurements implications for lunar magnetic evolution abstract we analyze published and new paleointensity data from apollo samples to reexamine the hypothesis of an early 3 9 3 6 ga lunar dynamo our new paleointensity experiments on four samples use modern absolute and relative measurement techniques with ages ranging from 3 3 to 4 3 ga bracketing the putative period of an ancient lunar field samples 60015 anorthosite and 76535 troctolite failed during absolute paleointensity experiments samples 72215 and 62235 impact breccias recorded a complicated multi component magnetic history that includes a low temperature 500 c component associated with a low intensity 2 t similar multi component behavior has been observed in several published absolute intensity experiments on lunar samples additional material from 72215 and 62235 was subjected to a relative paleointensity experiment a saturation isothermal remanent magnetization or sirm experiment neither sample provided unambiguous evidence for a thermal origin of the recorded remanent magnetization we test several magnetization scenarios in an attempt to explain the complex magnetization recorded in lunar samples specifically an overprint from exposure to a small magnetic field an isothermal remanent magnetization results in multi component behavior similar to absolute paleointensity results from which we could not recover the correct magnitude of the original thermal remanent magnetization in light of these new experiments and a thorough re evaluation of existing paleointensity measurements we conclude that although some samples with ages of 3 6 to 3 9 ga are strongly magnetized and sometimes exhibit stable directional behavior it has not been demonstrated that these observations indicate a primary thermal remanence particularly problematic in the interpretation of lunar sample magnetizations are the effects of shock as relative paleointensity measurements for lunar samples are calibrated using absolute paleointensities the lack of acceptable absolute paleointensity measurements renders the interpretation of relative paleointensity measurements unreliable consequently current paleointensity measurements do not support the existence of a 3 9 3 6 ga lunar dynamo with 100 t surface fields a result that is in better agreement with satellite measurements of crustal magnetism and that presents fewer challenges for thermal evolution and dynamo models

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

geomagnetic paleointensity at 2 41 ga as recorded by the widgiemooltha dike swarm western australia abstract absolute geomagnetic paleointensity measurements were conducted on samples from six mafic dikes of the 2 41 ga widgiemooltha swarm western australia rock magnetic analyses indicate that the paleointensity signal is carried by nearly stoichiometric pseudosingle domain magnetite and or low ti titanomagnetite paleointensity values were determined using the thellier double heating method supplemented by low temperature demagnetizations the ltd thellier method in order to reduce the effect of magnetic remanence carried by large pseudosingle domain and multidomain magnetite grains thirty one samples from five dikes yielded successful paleointensity determinations with the mean value of 41 2 3 8 t which corresponds to a virtual dipole moment of 6 65 0 98 a m 2 the mean and range of paleofield strength values are similar to those of the recent earth s magnetic field and are consistent with a compositionally driven geodynamo established by the earliest paleoproterozoic era the existence of a stable dipolar geomagnetic field during the proterozoic indicated by paleointensity and paleodirectional data from this and prior studies seems incompatible with a young age of the inner core suggested by recent models of the earth s thermal evolution acquisition of reliable paleointensity estimates using the novel approaches such as the ltd thellier method is crucial in constraining the development of more realistic earth like models of long term geodynamo behavior

low pco2 in the pore water not in the archean air arising from m t rosing d k bird n h sleep c j bjerrum 744 747 2010 10 1038 nature08955 the solar luminosity during the archean 3 8 to 2 5 billion years ago was 20 25 lower than at present and was probably compensated at least in part by a stronger greenhouse effect1 2 3 rosing et al 4 estimate the archean partial pressure of carbon dioxide to have been about 10 3 bar on the basis of the simultaneous occurrence of magnetite fe3o4 and siderite feco3 in banded iron formations bifs a type of chemical sediment here we question a central assumption by rosing et al 4 that the mineralogy of bifs reflects near thermodynamic equilibrium with the atmosphere ocean system just as the presence of authigenic pyrite in modern sediments does not imply that the modern atmosphere is anoxic the mineralogy of bifs cannot be used to argue for a low archean atmosphere carbon dioxide is therefore still a viable greenhouse gas candidate with which to explain the warm archean climate

flow field in a rectangular basin with a line inlet and a circular outlet an experimental study was conducted to investigate the flow field in a rectangular basin with a line inlet and a circular outlet the basin has a length about four times of the water depth and the outlet level was varied in the study the flow field in the basin can be characterized into three zones the wall jet zone with an induced recirculation eddy the outlet zone in the vicinity of the outlet and the transition zone in between the two zones the spreading of the wall jet was found to significantly exceed that of a classical wall jet in the transition zone the wall jet separated from the basin bed due to the adverse pressure gradient created by the jet impingement on the outlet wall upstream of the outlet a special three dimensional balloon like flow pattern was observed within a distance of three times of the outlet diameter potential flow theory predicts well the measured velocity

the cretaceous superchron geodynamo observations near the tangent cylinder abstract if relationships exist between the frequency of geomagnetic reversals and the morphology secular variation and intensity of earth s magnetic field they should be best expressed during superchrons intervals tens of millions of years long lacking reversals here we report paleomagnetic and paleointensity data from lavas of the cretaceous normal polarity superchron that formed at high latitudes near the tangent cylinder that surrounds the solid inner core the time averaged field recorded by these lavas is remarkably strong and stable when combined with global results available from lower latitudes these data define a time averaged field that is overwhelmingly dominated by the axial dipole octupole components are insignificant these observations suggest that the basic features of the geomagnetic field are intrinsically related superchrons may reflect times when the nature of core mantle boundary heat flux allows the geodynamo to operate at peak efficiency

high geomagnetic intensity during the mid cretaceous from thellier analyses of single plagioclase crystals recent numerical simulations have yielded the most efficient geodynamo having the largest dipole intensity when reversal frequency is low reliable paleointensity data are limited but heretofore have suggested that reversal frequency and paleointensity are decoupled we report data from 56 thellier thellier experiments on plagioclase crystals separated from basalts of the rajmahal traps 113 to 116 million years old of india that formed during the cretaceous normal polarity superchron these data suggest a time averaged paleomagnetic dipole moment of 12 5 1 4 1022 amperes per square meter three times greater than mean cenozoic and early cretaceous late jurassic dipole moments when geomagnetic reversals were frequent this result supports a correlation between intervals of low reversal frequency and high geomagnetic field strength

subterranean clues to the future of our planetary magnetic shield the geomagnetic field generated in the liquid outer core provides a shield from cosmic radiation that can cause damage to man made satellites electrical power grids and the ozone layer 1 in the absence of this shield the solar wind might gradually erode the atmosphere eventually robbing the planet of its water 2 but the dipole magnetic field has decreased in strength over the last 160 y at an alarming rate 3 motivating continued spirited debate 4 6 over whether we are in the early stages of a geomagnetic pole reversal some even believe we are overdue but 160 y is just a blink of the eye in geological terms the record is too short to have great confidence about what these trends might mean about causal processes and the future in pnas trindade et al 7 provide insightful context for these modern changes in the form of a magnetic record from south america that extends 1 500 y into the past we know much about the recent waning geomagnetic field since the time of gauss in 1840 we have had a rich data source in the form of geomagnetic observatory records in the satellite era we have a deluge of data from magnetometers in orbit from analyses of these data we know that the field intensity decrease is concentrated in an area known as the south atlantic anomaly saa a vast swatch stretching from south africa to chile amazingly when one estimates the field structure beneath this region at the core mantle 1 1email john tarduno at rochester edu 1 xref corresp 1 1

toward a robust normalized magnetic paleointensity method applied to meteorites abstract we propose a new paleointensity method based on normalization by isothermal remanent magnetization derivative vs alternating field rem method it provides an estimate of the absolute paleointensity with an uncertainty of about a factor two contrary to methods using normalization by total isothermal remanent magnetization or anhysteretic magnetization it is applicable to multicomponent magnetizations artificial or natural isothermal remanent magnetizations can be recognized and relaxation of natural remanent magnetization can be taken into account it is applicable for magnetizations carried by magnetite feni alloys and pyrrhotite the rem method is of particular interest to estimate paleointensities in meteorites as these materials are often characterized by complex multicomponent magnetization of various origins impact magnetization thermoremanence isothermal remanent magnetization and metastable magnetic minerals not suitable for thellier experiments however like other paleointensity methods the rem method underestimates the true paleointensity for meteorite samples with magnetizations that are heterogeneous on spatial scales below the sampling scale for l ordinary chondrites an upper limit of 1 t is proposed for the paleofield tentative paleofield estimates in the 0 05 0 5 t range are proposed for ll ordinary chondrites which is much lower than previous results the first paleofield estimates for rumuruti chondrites indicate the presence of magnetic fields around 6 t during the last major impact on their parent body aubrites and hed achondrites paleofields of at least 10 t may suggest the existence of a dynamo field during the cooling of their parent body results from martian meteorites are scattered between 1 and 24 t and may represent the crustal magnetic field of the planet after dynamo shutdown carbonaceous chondrites provide contrasting results with possible evidence of strong field mt processes that clearly require additional studies

early lunar magnetism it is uncertain whether the moon ever formed a metallic core or generated a core dynamo the lunar crust and returned samples are magnetized but the source of this magnetization could be meteoroid impacts rather than a dynamo here we report magnetic measurements and 40ar 39ar thermochronological calculations for the oldest known unshocked lunar rock troctolite 76535 these data imply that there was a long lived field on the moon of at least 1 microtesla approximately 4 2 billion years ago the early age substantial intensity and long lifetime of this field support the hypothesis of an ancient lunar core dynamo

palaeointensity of the 1 3 billion yr old gardar basalts southern greenland revisited no evidence for onset of inner core growth

detrital magnetite and chromite in jack hills quartzite cobbles further evidence for the preservation of primary magnetizations and new insights into sediment provenance abstract the magnetization of zircons from sedimentary rocks of the jack hills yilgarn craton western australia provide evidence for a hadean to paleoarchean geodynamo 4 0 to 4 2 billion years old these magnetizations pass a microconglomerate test attesting to the fidelity of jack hills zircons as recorders of these most ancient magnetic signals the lack of pervasive remagnetization of the jack hills is also documented through a positive conglomerate test conducted on cobble sized clasts a key element of the latter test is the preservation of a high unblocking temperature magnetization that can survive peak metamorphic temperatures rock magnetic studies suggest the mineral carrier is magnetite herein we investigate the magnetic mineral carriers in cobble samples through scanning electron microscope and microprobe analyses conduct an inter laboratory paleomagnetic study to evaluate sensitivities required to evaluate the weak magnetizations carried by the jack hills sediments and assess provenance information constrained by the opaque minerals these data confirm magnetite as a detrital phase and the presence of high unblocking temperature magnetizations further supporting the posit that the jack hills sediments can preserve primary magnetic signatures we note that some of these magnetizations are near the measurement resolution of standard cryogenic magnetometers and thus exacting laboratory procedures are required to uncover these signals in addition to magnetite the cobbles contain an assemblage of mg poor cr fe chromites ni sulfides and pyrrhotite that suggest a source in a layered intrusion different from the granitoid source of the zircons any hadean rock fragment in these sediments if present remains elusive

interpretation of radio echo returns from internal water bodies in variegated glacier alaska u s a des sondages radios ont revele la presence de cavites remplies d eau dans le glacier variegated alaska discussion sur l origine et les consequences glaciologiques de ces cavites

the effect of remanence anisotropy on paleointensity estimates a case study from the archean stillwater complex paleomagnetism of archean rocks potentially provides information about the early development of the earth and of the geodynamo precambrian layered intrusive rocks are good candidates for paleomagnetic studies such complexes are commonly relatively unaltered and may contain some single domain magnetite armored by silicate mineral grains however layered intrusives often have a strong petrofabric that may result in a strong remanence anisotropy magnetic anisotropy can have particularly disastrous consequences for paleointensity experiments if the anisotropy is unrecognized and if its effects remain uncorrected here we examine the magnetic anisotropy of an anorthosite sample with a well developed magmatic foliation the effect of the sample s remanence fabric on paleointensity determinations is significant paleointensities estimated by the method of thellier and thellier range from 17 to 55 t for specimens magnetized in a field of 25 t we describe a technique based on the remanence anisotropy tensor to correct paleointensity estimates for the effects of magnetic fabric and use it to estimate a paleointensity for the stillwater complex mt usa of 32 t adjusted for the effects of slow cooling

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

exsolved magnetite inclusions in silicates features determining their remanence behavior submicroscopic needle shaped titanomagnetite inclusions exsolved in silicate minerals commonly occur in mafic intrusive rocks and are protected from alteration by their silicate hosts making them excellent candidates for paleomagnetic studies a suite of samples containing clinopyroxene and plagioclase hosted magnetite inclusions from five geologically diverse sites was examined using magnetic force microscopy to image the inclusions magnetic domain state alternating field demagnetization experiments indicate that some inclusions are more stable recorders than others the two factors controlling the remanence behavior of the inclusions are internal microstructures and inclusion dimensions magnetite ulvospinel unmixing within an inclusion subdivides the original titanomagnetite solid solution into a boxwork structure composed of 103 105 magnetite prisms separated by thin ulvospinel lamellae the conversion of multidomain sized needles into assemblages of interacting single domains increases the coercivity and hence relaxation time of the inclusions and results in a thermochemical magnetic remanence in samples without this exsolution microstructure the inclusions diameters determine coercivity and their magnetization is thermoremanent both styles of high coercivity inclusions successfully record paleomagnetic directions in mesozoic rocks and their ubiquity within silicate minerals clinopyroxene and plagioclase of mafic intrusive rocks indicates their value as chemically and magnetically stable tools for elucidating the ancient magnetic field marine magnetic anomalies and crustal kinematics

thermochemical remanent magnetization in precambrian rocks are we sure the geomagnetic field was weak 1 thellier paleointensity determinations from two dikes of the early proterozoic 2 46 ga matachewan dike swarm canada yield field values of 2 14 0 18 and 9 81 1 65 t corresponding values of virtual axial dipole moment are very low 0 54 0 05 1022 and 2 49 0 42 1022 am2 respectively when compared with the modern field the characteristic remanent magnetization chrm was isolated over a narrow range of high unblocking temperatures 520 580 c detailed rock magnetic analyses indicate that the chrm is carried by nearly stoichiometric pseudo single domain magnetite scanning electron microscopy sem reveals that the magnetite is in the form of fine intergrowths with ilmenite formed by oxyexsolution during cooling the high temperature oxidation defined in the sem images could have continued at temperatures below the curie point of magnetite in this case the chrm would be a thermochemical remanent magnetization tcrm rather than a thermal remanent magnetization trm estimates of the tcrm trm ratio show that the thellier data could underestimate the true field value by a factor of 4 without violating experimental selection criteria this uncertainty in trm fidelity translates into a potential range of field values that spans that defined by the modern field 8 1022 am2 and proposed low precambrian levels 2 1022 am2 therefore understanding further how tcrm is acquired and records the field represents a major challenge if these and many other similar rocks are to be used in thellier studies aimed at defining the strength of the precambrian field

an impact driven dynamo for the early moon recent palaeomagnetic and seismological studies have strengthened the suggestion that the moon once possessed a core dynamo despite its importance as a constraint on lunar evolution there is currently no consensus on how such a dynamo was driven two groups working independently have arrived at the idea that the lunar dynamo was powered by mechanical stirring of the liquid core dwyer et al investigate the mechanism of a dynamo driven by continuous mechanical stirring arising from the differential motion between the solid silicate mantle and the liquid core beneath they show that the fluid motions and the power required to drive a dynamo operating continuously for more than 1 billion years are readily obtained by such mechanical stirring le bars et al propose a model whereby the dynamo action comes from impact induced changes in the moon s rotation rate they show that basin forming impact events are energetic enough to have unlocked the moon from synchronous rotation and that the subsequent large scale fluid flows in the core excited by the tidal distortion of the core mantle boundary could have powered a lunar dynamo the origin of lunar magnetic anomalies1 2 3 4 5 remains unresolved after their discovery more than four decades ago a commonly invoked hypothesis is that the moon might once have possessed a thermally driven core dynamo3 but this theory is problematical given the small size of the core and the required surface magnetic field strengths6 an alternative hypothesis is that impact events might have amplified ambient fields near the antipodes of the largest basins7 but many magnetic anomalies exist that are not associated with basin antipodes here we propose a new model for magnetic field generation in which dynamo action comes from impact induced changes in the moon s rotation rate basin forming impact events are energetic enough to have unlocked the moon from synchronous rotation8 and we demonstrate that the subsequent large scale fluid flows in the core excited by the tidal distortion of the core mantle boundary9 could have powered a lunar dynamo predicted surface magnetic field strengths are on the order of several microteslas consistent with palaeomagnetic measurements5 and the duration of these fields is sufficient to explain the central magnetic anomalies associated with several large impact basins

microfossils of sulphur metabolizing cells in 3 4 billion year old rocks of western australia sulphur isotope data from early archaean rocks suggest that microbes with metabolisms based on sulphur existed almost 3 5 billion years ago leading to suggestions that the earliest microbial ecosystems were sulphur based 1 5 however morphological evidence for these sulphur metabolizing bacteria has been elusive here we report the presence of microstructures from the 3 4 billion year old strelley pool formation in western australia that are associated with micrometre sized pyrite crystals the microstructures we identify exhibit indicators of biological affinity including hollow cell lumens carbonaceous cell walls enriched in nitrogen taphonomic degradation organization into chains and clusters and 13 c values of 33

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

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

does size matter statistical limits of paleomagnetic field reconstruction from small rock specimens as samples of ever decreasing sizes are being studied paleomagnetically care has to be taken that the underlying assumptions of statistical thermodynamics maxwell boltzmann statistics are being met here we determine how many grains and how large a magnetic moment a sample needs to have to be able to accurately record an ambient field it is found that for samples with a thermoremanent magnetic moment larger than 10 11am2 the assumption of a sufficiently large number of grains is usually given standard 25 mm diameter paleomagnetic samples usually contain enough magnetic grains such that statistical errors are negligible but single silicate crystal works on for example zircon plagioclase and olivine crystals are approaching the limits of what is physically possible leading to statistic errors in both the angular deviation and paleointensity that are comparable to other sources of error the reliability of nanopaleomagnetic imaging techniques capable of resolving individual grains used for example to study the cloudy zone in meteorites however is questionable due to the limited area of the material covered

an archaean heavy bombardment from a destabilized extension of the asteroid belt the late heavy bombardment lasted much longer than previously thought up to 1 7 billion years ago on earth with impacts on the moon and earth coming mostly from the e belt survivor hungaria asteroids the late heavy bombardment was a period of time generally put at about 4 1 billion to 3 8 billion years ago when the inner planets of the solar system were subjected to a high frequency barrage of asteroids this left its mark on the moon but on earth the craters quickly disappeared owing to tectonic processes and erosion in the first of two papers on the bombardment brandon johnson and jay melosh determine the properties of the asteroids by looking at spherule beds layers of debris ejected during the impacts the thickness of spherule layers is expected to vary according to the size of the impactor and the speed at which it hit earth this historical record of impacts indicates that the number of projectiles colliding with earth was substantially higher 3 5 billion years ago than it is today with a gradual decline in the number of strikes after the late heavy bombardment bottke et al modelled the evolution of an asteroid belt that extended farther towards mars than the present one they find that most of the impactors traced by the spherule beds probably originated in this e belt which was disrupted during migrations of some of the giant planets the barrage of comets and asteroids that produced many young lunar basins craters over 300 kilometres in diameter has frequently been called the late heavy bombardment1 lhb many assume the lhb ended about 3 7 to 3 8 billion years gyr ago with the formation of orientale basin2 3 evidence for lhb sized blasts on earth however extend into the archaean and early proterozoic eons in the form of impact spherule beds globally distributed ejecta layers created by chicxulub sized or larger cratering events4 at least seven spherule beds have been found that formed between 3 23 and 3 47 gyr ago four between 2 49 and 2 63 gyr ago and one between 1 7 and 2 1 gyr ago5 6 7 8 9 here we report that the lhb lasted much longer than previously thought with most late impactors coming from the e belt an extended and now largely extinct portion of the asteroid belt between 1 7 and 2 1 astronomical units from earth this region was destabilized by late giant planet migration10 11 12 13 e belt survivors now make up the high inclination hungaria asteroids14 15 scaling from the observed hungaria asteroids we find that e belt projectiles made about ten lunar basins between 3 7 and 4 1 gyr ago they also produced about 15 terrestrial basins between 2 5 and 3 7 gyr ago as well as around 70 and four chicxulub sized or larger craters on the earth and moon respectively between 1 7 and 3 7 gyr ago these rates reproduce impact spherule bed and lunar crater constraints

magnetization of the lunar crust 1 magnetic fields measured by the satellite lunar prospector show large scale features resulting from remanently magnetized crust vector data synthesized at satellite altitude from a spherical harmonic model of the lunar crustal field and the radial component of the magnetometer data have been used to produce spatially continuous global magnetization models for the lunar crust the magnetization is expressed in terms of localized basis functions with a magnetization solution selected having the smallest root mean square magnetization for a given fit to the data controlled by a damping parameter suites of magnetization models for layers with thicknesses between 10 and 50 km are able to reproduce much of the input data with global misfits of less than 0 5 nt within the uncertainties of the data and some surface field estimates the magnetization distributions show robust magnitudes for a range of model thicknesses and damping parameters however the magnetization direction is unconstrained these global models suggest that magnetized sources of the lunar crust can be represented by a 30 km thick magnetized layer average magnetization values in magnetized regions are 30 40 ma m similar to the measured magnetizations of the apollo samples and significantly weaker than crustal magnetizations for mars and the earth these are the first global magnetization models for the moon providing lower bounds on the magnitude of lunar crustal magnetization in the absence of multiple sample returns and can be used to predict the crustal contribution to the lunar magnetic field at a particular location

mineralogical constraints on precambrian pco2 arising from m t rosing d k bird n h sleep c j bjerrum 744 747 2010 10 1038 nature08955 rosing and others1 recently proposed a new model for the early earth s atmospheric composition they suggest on the basis of mineral assemblages in sedimentary rocks that atmospheric co2 and ch4 concentrations have been consistently overestimated this proposal is intriguing given that high concentrations of these gases are traditionally considered to be the solution to the faint young sun paradox of why the earth did not freeze over in our sun s youth when solar luminosity was greatly decreased rosing and others1 instead invoke a decreased planetary albedo during the earth s early history new insights into this problem which has long captivated those interested in the earth s early climate and biological history are welcome however we believe that the arguments presented by rosing and others1 for roughly modern values of the partial pressure of co2 are not robust

new constraints on the magnetic history of the cv parent body and the solar nebula from the kaba meteorite abstract recent paleomagnetic studies of allende cv chondrite as well as thermal modeling suggest the existence of partially differentiated asteroids with outer unmelted and variably metamorphosed crusts overlying differentiated interiors to further constrain the magnetic history of the cv parent body we report here paleomagnetic results on kaba cv chondrite this meteorite contains 11 wt pseudo single domain magnetite making it a rock with an excellent paleomagnetic recording capacity kaba appears to carry a stable natural remanent magnetization acquired on its parent body upon cooling in an internally generated magnetic field of about 3 t from temperatures below 150 c during thermal metamorphism about 10 to several tens of myr after solar system formation this strengthens the case for the existence of a molten advecting core in the cv parent body furthermore we show that no significant magnetic field i e lower than 0 3 t was present when aqueous alteration took place on the kaba parent body around 4 to 6 myr after solar system formation suggesting a delay in the onset of the dynamo in the cv parent body and confirming that nebular fields had already decayed at that time

the matachewan dyke swarm canada an early proterozoic magnetic field reversal abstract consistent age relationships between oppositely magnetized dykes of the 2 45 ga matachewan dyke swarm suggest that only a single magnetic field reversal occurred during the period of igneous activity the magnetic field throughout most of this time was characterized by a ssw declination and shallow negative inclination but reversed toward the waning stages of magmatism the new paleomagnetic data provide the oldest known magnetic reversal for which the relative reversal sense is known

analysis of the thrust force on the temperature control curtain in a large stratified reservoir abstracta temperature control curtain tcc is an effective facility to regulate the outflow temperature in reservoirs and its safety and stability are of great concern the pressure difference be

primary pseudo single and single domain magnetite inclusions in quartzite cobbles of the jack hills western australia implications for the hadean geodynamo

pervasive remagnetization of detrital zircon host rocks in the jack hills western australia and implications for records of the early geodynamo it currently is unknown when earth s dynamo magnetic field originated paleomagnetic studies indicate that a field with an intensity similar to that of the present day existed 3 5 billion years ago ga detrital zircon crystals found in the jack hills of western australia are some of the very few samples known to substantially predate this time with crystallization ages ranging from 3 0 4 38 ga these zircons might preserve a record of the missing first billion years of earth s magnetic field history however a key unknown is the age and origin of magnetization in the jack hills zircons the identification of 3 9 ga i e hadean field records requires first establishing that the zircons have avoided remagnetization since being deposited in quartz rich conglomerates at 2 65 3 05 ga to address this issue we have conducted paleomagnetic conglomerate baked contact and fold tests in combination with u pb geochronology to establish the timing of the metamorphic and alteration events and the peak temperatures experienced by the zircon host rocks these tests include the first conglomerate test directly on the hadean zircon bearing conglomerate at erawandoo hill although we observed little evidence for remagnetization by recent lightning strikes we found that the hadean zircon bearing rocks and surrounding region have been pervasively remagnetized with the final major overprinting likely due to thermal and or aqueous effects from the emplacement of the warakurna large igneous province at 1070 million years ago ma although localized regions of the jack hills might have escaped complete remagnetization there currently is no robust evidence for pre depositional 3 0 ga magnetization in the jack hills detrital zircons

flow upstream of orifices and sluice gates potential flow solutions of a point line sink are extended to study the velocity field upstream of a finite size orifice and sluice gate it is found that in the near field zones the iso velocity surfaces appear to be semiellipsoidal while in the far field zones they become hemispheres the shape and size of the orifice sluice gate were found to be of no effect on the flow behavior beyond a certain distance the development of velocity profile away from the orifice and sluice gate is examined and the effects of water depth are studied the results of this study compare very well with other numerical and experimental studies and provide a general understanding of the flow field upstream of orifices and sluice gates

evolving core conditions ca 2 billion years ago detected by paleosecular variation abstract paleomagnetic data provide one of the few probes available to interrogate early evolution of the core here we apply this probe by examining the latitudinal dependence of paleosecular variation psv data derived from high quality paleomagnetic data collected from proterozoic and neoarchean rocks these data define a neoarchean geomagnetic field that was more dipolar than that during proterozoic times indicating a change in core conditions the signals observed may reflect a change in forcing of the dynamo and an early onset of inner core growth we propose a model that links evolution of the core mantle and crust in three principal phases i before approximately 3 5 ga an entirely liquid core may not have hosted a geodynamo if heat transport was sufficient across the core mantle boundary however a geodynamo could have been generated if so sources in the shallow outer core could have been more important for generating the dynamo relative to deeper convection resulting in a field that was less dipolar than that generated in later times ii cooling of the lower mantle between ca 2 and 3 5 billion years ago was promoted by deep subduction and possibly coincided with inner core growth the geodynamo during this episode was deeply seated producing a highly dipolar surface magnetic field iii after ca 2 billion years ago continued subduction led to large scale core mantle boundary compositional and heat flux heterogeneity with these changes shallow core contributions to the geomagnetic field grew in importance resulting in a less dipolar field

continuous production of nanosized magnetite through low grade burial geological processes such as burial can lead to remagnetization in rocks due to neoformed magnetic minerals that have passed a critical volume called blocking volume in this study we designed a heating experiment for claystones obtained from the paris basin france in the 50 130 c temperature range in order to simulate 20 nm and 20 nm minerals respectively this result led us to propose a conceptual model of nucleation and growth process of magnetite during low grade burial from 2 to 4 km depth ultrafine magnetite 20 nm was predominant over single domain magnetite 20 nm for 4 km depth transposed to natural conditions our heating steps experiment suggested that claystone type rocks are remagnetized during burial for temperatures higher than 200 c the extrapolation of our results indicated that burial remagnetization due to the chemical remanent magnetization might be larger than the natural remanent magnetization 2012 american geophysical union all rights reserved

experimental study on selective withdrawal in a two layer reservoir using a temperature control curtain an experimental study was conducted to investigate the use of a temperature control curtain in selective withdrawal from a two layer stratified reservoir this study focused on the case where cool water at a depth was forced to flow under the curtain the evolution of the mean flow the withdrawal water quality and the mean velocity field were studied using particle image velocimetry and laser induced fluorescence practical relationships were developed for predicting the withdrawal water quality and the interface height as a function of time the structures of the flow field in both the upper and lower layers are discussed in detail the flow in the lower layer was dominated by the recirculation eddy induced by the jet flow under the curtain and a relation between the eddy length and the interface height was obtained close to the intake within about 3d where d intake diameter the velocity field can be well described by the potential flow theory beyond 3d however the flow field considerably deviated from the potential flow theory due to the jet expansion and stratification a general discussion of the results and engineering applications are also provided

3d numerical flow model for fish diversion studies at wanapum dam a numerical flow model is developed for evaluating different fish bypass systems at the wanapum dam on the columbia river wash the model solves the three dimensional reynolds averaged navier stokes equations combined with the k e turbulence equations the governing equations are formulated in generalized curvilinear coordinates and the bed roughness is introduced through wall functions the paper discusses the calibration of the numerical model and its application in the design of a bypass system at wanapum dam on the columbia river wash the numerical model is validated with data taken in a 1 16 scale hydraulic model of a powerhouse intake bay the model is found to reproduce accurately all of the flow characteristics that are believed to be important for the evaluation of a fish diversion system

importance of cooling rate dependence of thermoremanence in paleointensity determination 1 the practical effect of cooling rate on the magnitude of thermal remanent magnetization trm is experimentally tested using both natural and synthetic magnetites for synthetic and natural single domain sd magnetites trm intensity increases as the cooling rate decreases because a longer exposure time for slower cooling offers more chances to achieve an equilibrium magnetization multidomain md magnetites have the opposite response trm decreases as the cooling rate increases because slower cooling allows more time to achieve lower magnetization because of self demagnetization for pseudo single domain psd magnetite the effect of cooling rate on the remanence intensity appears to be insignificant intermediate in behavior between sd and md for a sd trm differences are restricted to lower temperatures but diminish near the curie point strongly indicating a serious problem in a conventional treatment of cooling rate correction on the practical side merely applying trm differences as a correction can overcorrect the true paleointensity another important significance of the present study is that a cooling rate difference alone can cause a slight nonlinearity in an arai plot for a sd

the kiaman reversed polarity superchron at kiama toward a field strength estimate based on single silicate crystals abstract we present the first geomagnetic paleointensity measurements on single plagioclase crystals from lavas formed during the 262 318 ma permo carboniferous reverse polarity superchron commonly known as the kiaman rock magnetic experiments suggest that feldspar crystals separated from two lavas of the kiaman superchron type area near kiama australia carry pseudo single domain low ti magnetic inclusions capable of faithfully retaining a primary paleointensity signal thellier paleointensity experiments yield virtual dipole moments of 8 5 0 2 1022 am2 and 9 3 0 3 1022 am2 for these crystals furthermore these results lack the alteration problems that prohibit paleointensity and sometimes paleodirection determination using whole rock lava samples from the type area the derived field strengths are 5 6 times higher than some other virtual dipole moment vdm values of kiaman age derived from whole rock igneous samples this suggests either high paleointensity variability during the kiaman superchron or that unrecognized non ideal magnetic behavior has compromised some prior paleointensity results reported from whole rocks elsewhere

secondary magnetic inclusions in detrital zircons from the jack hills western australia and implications for the origin of the geodynamo the time of origin of earth s dynamo is unknown detrital zircon crystals containing ferromagnetic inclusions from the jack hills of western australia have the potential to contain the oldest records of the geodynamo it has recently been argued that magnetization in these zircons indicates that an active dynamo existed as far back as 4 2 ga however the ages of ferromagnetic inclusions in the zircons are unknown here we present the first detailed characterization of the mineralogy and spatial distribution of ferromagnetic minerals in jack hills detrital zircons we demonstrate that ferromagnetic minerals in most jack hills zircons are commonly located in cracks and on the zircons exteriors hematite is observed to dominate the magnetization of many zircons while other zircons also contain significant quantities of magnetite and goethite this indicates that the magnetization of most zircons is likely to be dominantly carried by secondary minerals that could be hundreds of millions to billions of years younger than the zircons crystallization ages we conclude that the existence of the geodynamo prior to 3 5 ga has yet to be established

enhanced atmospheric oxygen outflow on earth and mars driven by a corotating interaction region 1 solar wind controls nonthermal escape of planetary atmospheric volatiles regardless of the strength of planetary magnetic fields for both earth with a strong dipole and mars with weak remnant fields the oxygen ion o outflow has been separately found to be enhanced during corotating interaction region cir passage here we compared the enhancements of o outflow on earth and mars driven by a cir in january 2008 when sun earth and mars were approximately aligned the cir propagation was recorded by stereo ace cluster and mars express mex during the cir passage cluster observed enhanced flux of upwelling oxygen ions above the earth s polar region while mex detected an increased escape flux of oxygen ions in the martian magnetosphere we found that 1 under a solar wind dynamic pressure increase of 2 3 npa the rate of increase in martian o outflow flux was 1 order higher than those on earth and 2 as a response to the same part of the cir body the rate of increase in martian o outflow flux was on the same order as for earth the comparison results imply that the dipole effectively prevents coupling of solar wind kinetic energy to planetary ions and the distance to the sun is also crucially important for planetary volatile loss in our inner solar system

dipole strength and variation of the time averaged reversing and nonreversing geodynamo based on thellier analyses of single plagioclase crystals 1 single plagioclase crystals separated from lavas can contain minute magnetic inclusions that faithfully record the geomagnetic field while the silicate host limits natural and experimentally induced alteration here we use paleointensity analyses of plagioclase crystals to examine three characteristic geodynamo regimes spanning the last 200 myr an interval of moderate reversal frequency during the paleogene the nonreversing field of the cretaceous normal polarity superchron and a period of very high reversal occurrence during the late jurassic an inverse relationship between reversal rate and intensity is supported furthermore the strength of the time averaged reversing field is more variable than that of the nonreversing field these observations are consistent with an active lower mantle shaping core mantle boundary heat flux and efficiency of the geodynamo on timescales of tens of millions of years

in search of high fidelity geomagnetic paleointensities a comparison of single plagioclase crystal and whole rock thellier thellier analyses the strength of earth s magnetic field on timescales greater than a million years is known primarily from thellier thellier analyses of igneous rocks in the thellier thellier method the natural remanent magnetization nrm is compared with a thermoremanent magnetization trm imparted during heating in a known laboratory field unfortunately the multiple heating steps required by the method often result in alteration of magnetic minerals in clay bearing groundmass of whole rock samples this alteration may result in the creation of new magnetic minerals that could affect trm acquisition analyses of single plagioclase crystals which contain magnetic inclusions that retain paleofield values may offer a way to retrieve paleointensity data that is less affected by alteration we test this hypothesis through a study of whole rocks and single plagioclase crystals from a mid cretaceous 113 115 ma basalt flow of the rajmahal traps northeastern india magnetic hysteresis and thermal demagnetization data from plagioclase crystals and whole rock samples are similar suggesting a common magnetic mineralogy however hysteresis data from heated whole rock samples indicate that a fine grained magnetic phase has formed this effect was either absent or much less pronounced in the single plagioclase crystals tested paleointensity data from these crystals suggest a paleofield value of 65 1 5 3 t n 15 data from whole rock samples overlap these values but are on average lower we attribute the difference in this study to the preferential growth of new magnetic minerals in the whole rock samples resulting in an enhanced acquisition of trm and shallowing of the slope of nrm lost versus trm gained curves these data suggest that subtle alteration effects may bias thellier thellier results from some whole rock samples toward low paleofield values in these cases the single plagioclase crystal approach may yield more reliable estimates of geomagnetic paleointensity

young inner core inferred from ediacaran ultra low geomagnetic field intensity an enduring mystery about earth has been the age of its solid inner core plausible yet contrasting core thermal conductivity values lead to inner core growth initiation ages that span 2 billion years from 0 5 to 2 5 billion years ago palaeomagnetic data provide a direct probe of past core conditions but heretofore field strength data were lacking for the youngest predicted inner core onset ages here we present palaeointensity data from the ediacaran 565 million years old sept iles intrusive suite measured on single plagioclase and clinopyroxene crystals that hosted single domain magnetic inclusions these data indicate a time averaged dipole moment of 0 7 1022 a m2 the lowest value yet reported for the geodynamo from extant rocks and more than ten times smaller than the strength of the present day field palaeomagnetic directional studies of these crystals define two polarities with an unusually high angular dispersion s 26 at a low latitude together with 14 other directional data sets that suggest a hyper reversal frequency these extraordinary low field strengths suggest an anomalous field behaviour consistent with predictions of geodynamo simulations high thermal conductivities and an ediacaran onset age of inner core growth a late onset of inner core growth is inferred from ultra low palaeomagnetic field strengths about 565 million years ago as measured in magnetic inclusions in ediacaran crystals

a long lived lunar core dynamo paleomagnetic measurements indicate that a core dynamo probably existed on the moon 4 2 billion years ago however the subsequent history of the lunar core dynamo is unknown here we report paleomagnetic petrologic and 40ar 39ar thermochronometry measurements on the 3 7 billion year old mare basalt sample 10020 this sample contains a high coercivity magnetization acquired in a stable field of at least 12 microteslas these data extend the known lifetime of the lunar dynamo by 500 million years such a long lived lunar dynamo probably required a power source other than thermochemical convection from secular cooling of the lunar interior the inferred strong intensity of the lunar paleofield presents a challenge to current dynamo theory

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

an assessment of long duration geodynamo simulations using new paleomagnetic modeling criteria qpm abstract long term temporal variations of the magnetic field timescales 10 myr characterized from paleomagnetic data have been hypothesized to reflect the evolution of earth s deep interior and couplings between the core and mantle by tying observed changes in the paleomagnetic record to mechanisms predicted from numerical geodynamo simulations we have a unique tool for assessing changes in the deep interior back in time however numerical simulations are not run in an earth like parameter regime and assessing how well they reproduce the geomagnetic field is difficult criteria have been proposed to determine the level of spatial and temporal agreement between simulations and observations spanning historical and holocene timescales but no such criteria exist for longer timescales here we present a new set of five criteria quality of paleomagnetic modeling criteria q pm that assess the degree of semblance between a simulated dynamo and the temporal and spatial variations of the long term 10 myr paleomagnetic field these criteria measure inclination anomaly virtual geomagnetic pole dispersion at the equator latitudinal variation in virtual geomagnetic pole dispersion normalized width of virtual dipole moment distribution and dipole field reversals we have assessed 46 geodynamo simulations using the q pm criteria the simulations have each been run for the equivalent of at least 300 kyr span reversing and non reversing regimes and include either homogeneous or heterogeneous heat flux boundary conditions we find that none of our simulations reproduce all salient aspects of the long term paleomagnetic field behavior for the past 10 myr nevertheless our simulations bracket earth values suggesting that an earth like simulation is feasible within the available computationally accessible parameter space this new set of criteria can inform future simulations that aim to reproduce all aspects of earth s long term magnetic field behavior

signals from the ancient geodynamo a paleomagnetic field test on the jack hills metaconglomerate abstract the oldest history of earth s magnetic field cannot be directly read from extant bulk rocks because of subsequent metamorphism at temperatures close to or exceeding the curie temperature of common magnetic minerals the jack hills metasediments of western australia which have seen lower peak metamorphic temperatures contain zircons as old as 4 4 billion yr old to assess whether these sediments can retain an ancient signal of the geodynamo we present a paleomagnetic conglomerate test on a cobble bearing jack hills unit thermal demagnetization reveals a distinct magnetic component with high unblocking temperatures between 550 and 580 c that passes the conglomerate test indicating magnetization prior to deposition of the conglomerate this result together with rock magnetic data indicates that the high unblocking temperature component is carried by magnetite which records magnetization in an ambient field and the simplest explanation is that a dynamo was present existing geochronological data imply that the clasts could contain mixtures of minerals extending to ages only slightly older than the maximum depositional age at 3 05 billion yr ago however the positive conglomerate test reported here indicates that the jack hills metasediments have the potential to record paleorchean to hadean magnetic fields on a clast or sub clast mineral scale

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 a dynamo in the main group pallasite parent body understanding the origin of pallasites stony iron meteorites made mainly of olivine crystals and feni metal has been a vexing problem since their discovery here we show that pallasite olivines host minute magnetic inclusions that have favorable magnetic recording properties our paleointensity measurements indicate strong paleomagnetic fields suggesting dynamo action in the pallasite parent body we use these data and thermal modeling to suggest that some pallasites formed when liquid feni from the core of an impactor was injected as dikes into the shallow mantle of a 200 kilometer radius protoplanet the protoplanet remained intact for at least several tens of millions of years after the olivine metal mixing event

persistence and origin of the lunar core dynamo the lifetime of the ancient lunar core dynamo has implications for its power source and the mechanism of field generation here we report analyses of two 3 56 gy old mare basalts demonstrating that they were magnetized in a stable and surprisingly intense dynamo magnetic field of at least 13 t these data extend the known lifetime of the lunar dynamo by 160 my and indicate that the field was likely continuously active until well after the final large basin forming impact this likely excludes impact driven changes in rotation rate as the source of the dynamo at this time in lunar history rather our results require a persistent power source like precession of the lunar mantle or a compositional convection dynamo

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

short pulse radar wavelet recovery and resolution of dielectric contrasts within englacial and basal ice of matanuska glacier alaska u s a wavelets transmitted by short pulse radar are recovered from continuous profiles and used to determine interfacial dielectric contrasts within the englacial and basal ice at the terminus area of matanuska glacier alaska u s a the field studies were in the ablation region where radar horizons could at some point be identified with interfaces between clear ice and air water or basal ice and were performed in early spring before drainage fully developed the profiles usedclosely spaced antennas with bandwidths centered near 50 and 400 mhz transmitted wavelets reflected from interfaces of known dielectric contrasts are used to establish a phase reference for other events from interfaces between unknown contrasts migration and fourier transform filtering are then applied to the profiles and shown to recover these wavelets from diffractions and reflections interfacial dielectric contrasts are determined from the relative phase of the wavelets near the terminus lake some basal ice events above 8 m depth are interpreted as voids further up glacier most englacial events are interpreted as voids but deeper localized reflectors and horizons to 90 m depth and within the basal zone are interpreted as voids water or debris phase cannot be determined for the basal substrate transition reflections recommendations are made for improving the wavelet recovery process and the quality of gpr migration

long lived magnetism on chondrite parent bodies abstract we present evidence for both early and late stage magnetic activity on the cv and l ll parent bodies respectively from chondrules in vigarano and bjurbole using micro ct scans to re orientate chondrules to their in situ positions we present a new micron scale protocol for the paleomagnetic conglomerate test the paleomagnetic conglomerate test determines at 95 confidence whether clasts within a conglomerate were magnetized before or after agglomeration i e for a chondritic meteorite whether the chondrules carry a pre or post accretionary remanent magnetization we found both meteorites passed the conglomerate test i e the chondrules had randomly orientated magnetizations vigarano s heterogeneous magnetization is likely of shock origin due to the 10 to 20 gpa impacts that brecciated its precursor material on the parent body and transported it to re accrete as the vigarano breccia the magnetization was likely acquired during the break up of the original body indicating a cv parent body dynamo was active 9 ma after solar system formation bjurbole s magnetization is due to tetrataenite which transformed from taenite as the parent body cooled to below 320 c when an ambient magnetic field imparted a remanence we argue either the high intrinsic anisotropy of tetrataenite or brecciation on the parent body manifests as a randomly orientated distribution and a l ll parent body dynamo must have been active at least 80 to 140 ma after peak metamorphism primitive chondrites did not originate from entirely primitive never molten and or differentiated parent bodies primitive chondrite parent bodies consisted of a differentiated interior sustaining a long lived magnetic dynamo encrusted by a layer of incrementally accreted primitive meteoritic material the different ages of carbonaceous and ordinary chondrite parent bodies might indicate a general difference between carbonaceous and ordinary chondrite parent bodies and or formation location in the protoplanetary disk

paleointensity results from the late archaean modipe gabbro of botswana 1 rock magnetic and thellier thellier coe paleointensity results are reported for a new collection of samples from the 2 78 ga modipe gabbro of botswana the magnetic properties are very favorable leading to an unusually high success rate and a well constrained result of 36 40 t 95 confidence interval we discuss the long standing problem of allowing for the enormous differences between the natural and laboratory cooling rates and apply a temperature dependent correction derived from first order reversal curve forc data whereas pure single domain sd corrections often lead to quite large decreases in paleointensity estimates sometimes exceeding 50 we find a modest increase of about 10 the earth s magnetic dipole moment in the late archaean is thereby estimated to have been 6 1022 am2

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

paleomagnetism indicates that primary magnetite in zircon records a strong hadean geodynamo determining the age of the geomagnetic field is of paramount importance for understanding the evolution of the planet because the field shields the atmosphere from erosion by the solar wind the absence or presence of the geomagnetic field also provides a unique gauge of early core conditions evidence for a geomagnetic field 4 2 billion year gy old just a few hundred million years after the lunar forming giant impact has come from paleomagnetic analyses of zircons of the jack hills western australia herein we provide new paleomagnetic and electron microscope analyses that attest to the presence of a primary magnetic remanence carried by magnetite in these zircons and new geochemical data indicating that select hadean zircons have escaped magnetic resetting since their formation new paleointensity and pb pb radiometric age data from additional zircons meeting robust selection criteria provide further evidence for the fidelity of the magnetic record and suggest a period of high geomagnetic field strength at 4 1 to 4 0 billion years ago ga that may represent efficient convection related to chemical precipitation in earth s hadean liquid iron core