calibrating the end permian mass extinction the end permian mass extinction was the most severe biodiversity crisis in earth history to better constrain the timing and ultimately the causes of this event we collected a suite of geochronologic isotopic and biostratigraphic data on several well preserved sedimentary sections in south china high precision u pb dating reveals that the extinction peak occurred just before 252 28 0 08 million years ago after a decline of 2 per mil in 13c over 90 000 years and coincided with a 13c excursion of 5 that is estimated to have lasted 20 000 years the extinction interval was less than 200 000 years and synchronous in marine and terrestrial realms associated charcoal rich and soot bearing layers indicate widespread wildfires on land a massive release of thermogenic carbon dioxide and or methane may have caused the catastrophic extinction

intercalibration of 40ar39ar dating standards a number of standards used for k ar and 40ar39ar dating have been intercalibrated multiple splits 15 40 mg each of mmhb 1 sb 3 biotite lp 6 biotite 40 60 ga1550 biotite fish canyon tuff 3 biotite taylor creek rhyolite sanidine bern 4 muscovite bern 4 biotite and pennsylvania state university orthoclase 1a were irradiated in capsules fused and analyzed in replicate analyses most of these standards proved to be homogeneous in 40ar 39ark ratio age k ca and 36ar contents mmhb 1 is inhomogeneous in age at the 15 mg level indicating it is unsuitable for use as an interlaboratory standard using material taken straight from the bottle the orthoclase sample is also inhomogeneous at the 30 mg level quoted relative to an age of 162 9 ma for the sb 3 biotite standard total fusion ages are lp 6 biotite 40 60 128 1 ma ga1550 biotite 97 8 ma fish canyon tuff 3 biotite 27 95 ma taylor creek rhyolite sanidine 28 0 ma bern 4 muscovite 18 51 ma bern 4 biotite 17 21 ma these ages are in good agreement with the absolute k ar ages of these minerals different preparations of biotite and sanidine standards from a welded tuff such as the fish canyon tuff may not be of identical ages tests on submilligram splits reveals that sb 3 biotite ga1550 biotite fish canyon tuff 3 biotite and taylor creek rhyolite sanidine are suitable for use as monitors in laser work the sanidine specimen appears to be the most suitable for use as the primary monitor for 40ar39ar dating

a new calibration point for the late miocene section of the geomagnetic polarity time scale 40ar 39ar dating of lava flows from akaroa volcano new zealand recent dating of geomagnetic field reversals over the past 2 ma by both the radiometric and astrochronologic techniques indicates that the k ar derived ages for these reversals are too young we report herein on 40ar 39ar incremental heating study of two geomagnetic field reversals trapped in lavas on akaroa volcano new zealand that demonstrate that the k ar derived ages for field reversals at 10 9 ma are too young by about 7 the new age for the termination of chron 5 a critical tie point on the geomagnetic polarity time scale for late miocene time is 9 67 0 11 ma

excess 40ar in biotite and hornblende from the noril sk 1 intrusion siberia implications for the age of the siberian traps the siberian traps represent the largest known phanerozoic manifestation of continental flood volcanism the general agreement is now that the volcanism was of short duration 1 m y and was synchronous with the permian triassic boundary discrepancies between different geochronologic results prevent rigorous evaluation of the age and duration of magmatism several alkalic mafic intrusions associated with the siberian traps represent subvolcanic feeders for the trap lavas the noril sk 1 intrusion is intruded into the earliest flows and is chemically and isotopically related to stratigraphically overlying flows biotites from the noril sk 1 intrusion have previously yielded 40ar39ar plateau dates 254 251 ma that are significantly older than seemingly reliable 40ar39ar plateau dates 250 247 ma on plagioclase and whole rock samples from trap basalts that are geologically constrained to be older than the intrusion possible explanations of this discrepancy include 1 plagioclase and whole rock samples are anomalously young due to 40ar loss via alteration or episodic outgassing and or 2 the biotites contain excess 40ar detailed laser heating 40ar39ar analysis of six individual grains of biotite in a total of 214 steps yield data that permit characterization of multiple components of ar through isotope correlation patterns an isochron age of 250 1 1 5 ma 2 and an excess ar component with 40ar36ar 565 39 are obtained by regression of the 115 highest temperature steps step heating of a bulk hornblende sample also reveals excess ar with 40ar36ar 327 10 and an isochron age of 249 3 1 6 ma neglecting uncertainties in the ages of neutron fluence monitors the two dates just overlap at 2 but are distinctly younger than the previous mean plateau date 253 9 1 6 ma for biotite the different initial 40ar36ar for the two phases is interpreted as reflecting the evolution of an initial lower mantle composition towards increasing crustal contamination

mesozoic igneous activity in the maranha o province northern brazil 40ar 39ar evidence for separate episodes of basaltic magmatism low and high ti basaltic rocks of the maranha o province in northern brazil appear to be related to the opening of the atlantic ocean 40ar 39ar dating was carried out on powdered whole rock basalts both washed and unwashed in dilute nitric acid to document this relationship acid treatment does not alter the argon retention properties of retentive phases but is effective in removing alteration phases high ca k degassed at low laboratory temperatures our results identify that at least two periods of volcanic activity formed the maranha o province low ti basalts from the western sector are 200 190 ma and are related to the opening of the north atlantic high ti basalts from the eastern sector are 129 124 ma slightly younger than compositionally similar rocks in the serra geral flood basalt province of southern brazil and are related to the opening of the south atlantic other flood basalts such as the columbia river deccan rajmahal ferrar and siberian provinces were each predominantly formed in 1 2 ma the maranha o basalts remnants of separate igneous events spanning 70 ma should not be classed as a single flood basalt province the temporal complexities noted in the maranha o basaltic rocks appear to be related to the tracks of hotspots that led to the rifting of the atlantic ocean two sets of mesozoic basaltic rocks occur in southern africa india and south america resulting from events leading to the fragmentation and dispersal of sections of gondwanaland in each sub continent the two provinces were formed by events separated by 50 70 ma of these pairs only the maranha o and serra geral show geochemical similarities indicating compositionally distinct upper mantle source regions under sections of brazil separated by 3000 km

a minimum upb age for siberian flood basalt volcanism abstract establishing an accurate and precise age for siberian flood basalt volcanism is of great importance in evaluating causes for the unequaled mass extinction of flora and fauna at the permian triassic boundary we report a new minimum u pb age obtained from zircon and baddeleyite from the mineralized noril sk i intrusion that cuts the lower third of this rapidly deposited 3500 m thick volcanic sequence near noril sk this 251 2 0 3 2 ma age is within analytical error of the shrimp u pb age for zircon from the permian triassic boundary at meishan south china 251 1 3 6 ma 2 and confirms siberian basaltic volcanism as a possible contributor to the mass extinction

fine tuning the radiometrically derived geomagnetic polarity time scale gpts for 0 10 ma new 40ar 39ar step heating ages for geomagnetic field reversals at 3 2 and 4 5 ma along with previous data at 1 2 10 and 16 ma are used to derive a geomagnetic polarity time scale for 0 10 ma these ages for chron boundaries are fitted to the relative widths of the polarity intervals on the ocean floor using a cubic spline technique the interpolated ages of chrons for 0 6 ma are in good agreement with the astronomical time scale and differ considerably from earlier attempts based in part on k ar dates significant differences between the interpolated ages and the astronomical time scale are noted for the onset of the olduvai and reunion sub chrons there is better agreement if 40ar 39ar data obtained from direct dating of these chrons is utilized this time scale is estimated to be accurate to 1 for 0 6 ma for 6 10 ma the lack of a tie point limits the accuracy to about 2

new 40ar 39ar dating of the grande ronde lavas columbia river basalts usa implications for duration of flood basalt eruption episodes by discussion abstract barry et al 2010 presented 40 ar 39 ar ages to argue for a very limited duration 0 4 m y for the eruption of the grande ronde lavas critical examination of their data sets shows 1 a number of their plateau ages are statistically invalid and 2 many of their groundmass matrix samples were altered and did not yield valid estimates of the time of crystallization in general their ages err on the young side reflecting partial 40 ar loss due to alteration and there is no compelling evidence therein to argue for very limited duration for the grande ronde basalt the imnaha grande ronde and wanapum basalt erupted within 0 7 m y starting at 16 4 ma by contrast the ages of barry et al 2010 span the interval 16 6 to 14 7 ma and in some instances are out of stratigraphic sequence

new u pb zircon ages and the duration and division of devonian time newly determined u pb zircon ages of volcanic ashes closely tied to biostratigraphic zones are used to revise the devonian time scale they are 1 417 6 1 0 ma for an ash within the conodont zone of icriodus woschmidti i w hesperius early lochkovian 2 408 3 1 9 ma for an ash of early emsian age correlated with the conodont zones of po dehiscens lower po inversus 3 391 4 1 8 ma for an ash within the po c costatus zone and probably within the upper half of the zone eifelian and 4 381 1 1 3 ma for an ash within the range of the frasnian conodont palmatolepis punctata pa punctata zone to upper pa hassi zone u pb zircon ages for two rhyolites bracketing a palyniferous bed of the pusillites lepidophyta spore zone are dated at 363 8 2 2 ma and 363 4 1 8 ma respectively suggesting an age of363 ma for a level within the late famennian pa g expansa zone these data together with other published zircon ages suggest that the base and top of the devonian lie close to 418 ma and 362 ma respectively thus lengthening the period by20 over current estimates we suggest that the duration of the middle devonian eifelian and givetian is rather brief perhaps no longer than 11 5 myr 394 ma 382 5 ma and that the emsian and famennian are the longest stages in the period with estimated durations of15 5 myr and 14 5 myr respectively 1998 elsevier science b v all rights reserved

40ar 39ar dating of the brunhes matuyama geomagnetic field reversal magnetostratigraphic studies are widely used in conjunction with the geomagnetic polarity time scale gpts to date events in the range 0 to 5 million years ago a critical tie point on the gpts is the potassium argon age of the most recent brunhes matuyama geomagnetic field reversal astronomical values for the forcing frequencies observed in the oxygen isotope record in ocean drilling project site 677 suggest that the age of this last reversal is 780 ka thousand years ago whereas the potassium argon based estimate is 730 ka results from 4039 ar incremental heating studies on a series of lavas from maui that straddle the brunhes matuyama reversal give an age of 783 11 ka in agreement with the astronomically derived value the astronomically based technique appears to be a viable tool for dating young sedimentary sequences

the timing and extent of the eruption of the siberian traps large igneous province implications for the end permian environmental crisis we present new high precision 40ar 39ar ages on feldspar and biotite separates to establish the age duration and extent of the larger siberian traps volcanic province samples include basalts and gabbros from noril sk the lower tunguska area on the siberian craton the taimyr peninsula the kuznetsk basin vorkuta in the polar urals and from chelyabinsk in the southern urals most of the ages except for those from chelyabinsk are indistinguishable from those found at noril sk cessation of activity at noril sk is constrained by a 40ar 39ar age of 250 3 1 1 ma for the uppermost kumginsky suite the new 40ar 39ar data confirm that the bulk of siberian volcanism occurred at 250 ma during a period of less than 2 ma extending over an area of up to 5 million km2 the resolution of the data allows us to confidently conclude that the main stage of volcanism either immediately predates or is synchronous with the end permian mass extinction further strengthening an association between volcanism and the end permian crisis a sanidine age of 249 25 0 14 ma from bed 28 tuff at the global section and stratotype at meishan china allows us to bracket the p tr boundary to 0 58 0 21 myr and enables a direct comparison between the 40ar 39ar age of the traps and the permo triassic boundary section younger ages 243 ma obtained for basalts from chelyabinsk indicate that volcanism in at least the southern part of the province continued into the triassic

ndsr isotopes petrochemistry and origin of the siberian flood basalts ussr the siberian flood basalt province sfbp of permo triassic age is one of the largest flood basalt provinces with an estimated area of exposure of 337 000 km2 average thickness of 1 km and a magma volume of 337 000 km3 forty seven basaltic rocks from two main subprovinces norilsk 5 10 of area thickness up to 3 km and putorana 90 95 of area thickness of more than 2 km were selected on the basis of petrography and volcano stratigraphic relation for major element analysis twentysix of these basalts twelve from norilsk and fourteen from putorana were analyzed for nd and srisotopic compositions the norilsk and putorana basalts show some contrasting behavior in terms of the ratios of the highly incompatible elements of ti p and k as a function of their mg the norilsk basalts are more variable suggesting the role of fractional crystallization assimilation in their evolution in contrast the putorana basalts show remarkable uniformity in their bulk chemical compositions there is no overall apparent correlation between the mg and the initial nd for the basalts most of the putorana and some norilsk lavas show a narrow range of initial nd between 0 and 2 5 at mg 0 53 suggesting that the bulk of the sfbp was derived from a mantle source with near chondritic ndisotopic signature our data and six previously published analyses define a hyperbolic array near the mantle correlation line in 87sr86sr t nd t space however the majority 70 of the volumetrically significant putorana basalts fall in the ranges of 0 to 2 5 and 0 7046 to 0 7052 for the initial nd and 87sr86sr respectively in nd and sr isotopic space most of the siberian basalts also fall within the field defined by the ocean island basalts data implying common mantle sources it is concluded that the sfbp originated by hotspot volcanism due to the rise of a large and relatively primitive lower mantle derived plume beneath the siberian continent

age of the emeishan flood magmatism and relations to permian triassic boundary events the permian triassic p t mass extinction the greatest biological mortality event in the earth s history was probably caused by dramatic and global forcing mechanisms such as the siberian flood volcanism here we present the first set of high precision 40 ar 39 ar dating results of volcanic and intrusive rocks from the emeishan traps south china which define a main stage of the flood magmatism at v251 253 ma and a subordinate precursory activity at v255 ma this time span is generally coeval with or slightly older than the age of the p t boundary estimated by the ash beds in the meishan stratotype section and the main eruption of the siberian traps our data reinforces the notion that the eruption of the emeishan traps rather than eruption of the siberian traps accounted for the formation of the p t boundary ash beds in south china the emeishan flood magmatism which occurred in the continental margin comprising thick marine limestone formations moreover may have triggered rapid release of large volumes of methane and carbon dioxide that could have been responsible for the global n 13 c excursion and associated environmental crisis leading to the mass extinction at the p t boundary fl 2002 elsevier science b v all rights reserved

distribution and geochronology of oregon plateau u s a flood basalt volcanism the steens basalt revisited the timing and petrogenesis of mid miocene flood basalt volcanism in the northwest united states has been extensively addressed yet the chemical characteristics and temporal details of the steens basalt exposed on the oregon plateau are poorly defined steens basalt volcanism has generally been accepted to have occurred at 16 6 ma coeval and or just prior to the onset of columbia river basalt group volcanism to the north new major and trace element analyses and nine 40 ar 39 ar ages ranging from 15 51 0 28 to 16 58 0 18 ma were obtained on oregon plateau flood basalt lava flows from stratigraphic sections in close proximity to steens mountain additionally new 40 ar 39 ar ages were obtained on the uppermost and thirty first lava flow down from the top of the 1 km section of steens basalt exposed at steens mountain and yield eruption ages of 16 59 0 10 and 16 55 0 10 ma respectively field relations between these basalt sections suggest that multiple eruptive centers were present in the vicinity of steens mountain the chemical and chronologic data presented in this study illustrate that flood basalts with the steens basalt chemical signature erupted across the southern oregon plateau over a much greater timespan than what is typically quoted for the steens mountain type section these data suggest that the main volume of steens basalt volcanism erupted over at least an 1 m y duration from eruptive loci in the vicinity of steens mountain while likely much less voluminous volcanism and lithospheric input of mafic magma appears to have occurred for n2 ma across the oregon plateau these new geochemical and geochronological constraints verify a common temporal link between steens malheur gorge region and columbia river basalt group crbg volcanism this direct temporal link requires that petrogenetic and tectonic models of mid miocene northwestern u s flood basalt volcanism recognize that the northern columbia plateau and southern oregon plateau portions of this mid miocene basalt province were erupting simultaneously during portions of the regional event 2007 elsevier b v all rights reserved

the age of the permian triassic boundary abstract the 5 cm boundary clay bed in the chinese stratotype section through the permian triassic boundary has been recognised as a bentonite shrimp ion microprobe dating of zircons in the bentonite indicates a magmatic age of 251 2 3 4 ma 2 this is the first direct constraint on the numerical age of the permian triassic boundary future refinements of ages at this important but poorly constrained level of the phanerozoic timescale may depend on re analysis of this uniquely placed volcanic horizon and other bentonites in the fossiliferous chinese upper permian and lower triassic the utility of defining the permian triassic boundary in the chinese stratotype section in the vicinity of known dateable horizons should be considered

calibration of ga1550 biotite standard for k ar and 40ar 39ar dating abstract the mineral separate ga1550 biotite has become an international standard for k ar and 40 ar 39 ar dating studies although it was prepared as an intralaboratory standard at anu to monitor tracer depletion from a gas pipette it is one of a small number of samples that has been calibrated against 38 ar tracers some of which had been mixed with known amounts of atmospheric argon so that a so called primary calibration has been performed by measuring ga1550 biotite against additional tracers from the same batch we have determined the radiogenic argon content of this sample as 1 342 0 007 10 9 mol g and together with the measured k content of 7 645 0 050 weight percent we derive a best estimate for the k ar age as 98 5 0 5 ma where the error is derived from averaging the ages determined relative to the 38 ar tracer

high 3he plume origin and temporal spatial evolution of the siberian flood basalts an olivine nephelinite from the lower part of a thick alkalic ultrabasic and mafic sequence of volcanic rocks of the northeastern part of the siberian flood basalt province sfbp yielded a 40ar 39ar plateau age of 253 3 2 6 million years distinctly older than the main tholeiitic pulse of the sfbp at 250 0 million years olivine phenocrysts of this rock showed 3he 4he ratios up to 12 7 times the atmospheric ratio these values suggest a lower mantle plume origin the neodymium and strontium isotopes rare earth element concentration patterns and cerium lead ratios of the associated rocks were also consistent with their derivation from a near chondritic primitive plume geochemical data from the 250 million year old volcanic rocks higher up in the sequence indicate interaction of this high 3he sfbp plume with a suboceanic type upper mantle beneath siberia

comment to distribution and geochronology of the oregon plateau u s a flood basalt volcanism the steens basalt revisited by m e brueseke et al j volcanol geotherm res 161 2007 187 214 abstract the 40 ar 39 ar stepheating results of brueseke et al 2007 are critically evaluated firstly for possible alteration of the samples dated steps passing the alteration index test baksi 2007a can then be used to derive plateau ages which must satisfy rigorous statistical tests of the eleven samples studied by brueseke et al 2007 only four yield plateau values that reflect eruption ages these suggest steens basalt was erupted within a relatively short period of time 0 2 m y considerably less than the 1 m y suggested by the authors the seeming difference between the plateau ages for the steens mountain basalts obtained by these authors and two earlier attempts is resolved by a proper adjustment of ages at various laboratories for interlaboratory calibration differences and by rejecting ages from altered samples the age of the important steens mountain geomagnetic polarity transition is 16 59 0 03 ma

geothermometry from 40ar39ar dating experiments abstract we have selected a variety of minerals from three basic intrusions within the high grade metamorphic terrane near haliburton ontario for k ar dating by application of 40 ar 39 ar incremental heating experiments we conclude that the discordant k ar mineral dates can best be explained by a slow cooling history from the stepwise heating experiments we calculated isotopic closure temperatures using dodson s theory for slow cooling and from these constructed a cooling curve this curve was then compared to independent constraints to test the overall validity of our procedures and assumptions the results are geologically reasonable insofar as 1 a the maximum calculated temperatures from hornblendes of 700 c are consistent with independent approximate metamorphic grid arguments 2 b the ar closure temperatures for biotites 350 400 c are consistent with independent evidence and 3 c minerals with older k ar dates have higher calculated closure temperatures the resultant geological cooling curve implies that a maximum temperature of 700 c was reached before 1000 ma ago and thereafter the temperature decreased via changes in uplift and cooling rates to below 200 c by 700 ma ago we have discovered that plutonic plagioclases are very sensitive indicators comparable to k felds pars of ar loss and therefore can usefully extend the range of application of the 40 ar 39 ar stepwise heating method we also describe experiments in which we have tested the effects on the ar systematics of acid leaching of plagioclases

the age of the steens reversal and the columbia river basalt group abstract the columbia river basalt group crbg eruptions have a well defined relative magnetostratigraphy but have not been definitively correlated to the geomagnetic polarity time scale 40 ar 39 ar ages are presented from lavas erupted in the r 0 through n 1 magnetozones of the crbg and in the transition between r 0 and n 0 four ages from transitionally magnetized lava flows at steens mountain catlow peak and poker jim ridge with a weighted mean age 16 58 0 10 ma 1 and the more precise age 16 654 0 025 ma of the normally magnetized oregon canyon tuff at the top of the catlow peak section show that the oldest crbg magnetozone r 0 correlates with the c5cr chron bayesian statistical analysis applied to data from four flows at catlow peak using the mean age of the steens reversal gives a best and preferred age of the steens reversal of 16 73 0 13 0 08 ma 95 confidence depending on the geomagnetic polarity time scale model the eruption rate from n 0 through r 2 0 34 0 45 ma in the middle and the bulk of the crbg emplacement averaged 0 30 0 41 km 3 a and peaked at a rate 1 1 2 to 4 1 2 times higher during r 2

laser probe 40 ar 39 ar ages of early and late stage alteration assemblages rosario porphyry copper molybdenum deposit collahuasi district i region chile sensibly identical laser probe 40 ar 39 ar dates of 32 9 or 0 5 2sigma and 32 9 or 0 6 ma are determined for respectively early stage hydrothermal biotite and late stage hydrothermal muscovite from the giant rosario porphyry copper deposit in the collahuasi district of northern chile the age determinations were carried out on ca 1 cm 2 1 to 0 5 mm thick wafers of altered rock or on smaller chips broken therefrom obviating the requirement for mineral separation and permitting direct petrographic control of the analytical procedure the age data although not unambiguous imply that this extremely large hydrothermal center was emplaced over a short interval hydrothermal activity in this central andean transect migrated eastward over a period of ca 3 to 4 m y from copaquire through quebrada blanca to rosario the intensity of mineralization increasing with time rosario chuquicamata and la escondida the largest of the west fissure domeyko fault system array were apparently emplaced coevally in the terminal stage of the late eocene early oligocene metallogenic episode

40ar 39ar ages of flood basalt provinces in russia and china and their possible link to global faunal extinction events a cautionary tale regarding alteration and loss of 40ar abstract the hypothesis that the permo triassic boundary ptb mass extinctions were caused by flood basalt volcanism in russia siberian traps and or china the emeishan traps is investigated from the point of view of time of occurrence 40 ar 39 ar ages numerous published ages in the literature are rejected as good estimates of the time of crystallization the filters applied in this respect are a statistical reliability of plateau isochron sections of stepheating data and b the alteration state of the material that was dated alteration appears to be ubiquitous unsurprising since most of the material dated was used without acid leaching a procedure that is effective in yielding fresh er samples of 70 ages in the literature for the main pulse of siberian trap volcanism less than ten prove to be reliable ages similar techniques applied to 40 ar 39 ar for the emeishan traps leaves only a single reliable age for the magmatic episode these ages are compared to both published and new 40 ar 39 ar ages for the ptb as based on analysis of minerals from critical ash beds in china there is good overlap in the ages ptb 250 0 0 1 ma siberian trap lavas 250 1 0 4 ma lending credence to a genetic link between the formation of the siberian traps and the faunal extinction event at the ptb a similar link for the formation of the viluy traps russia and the late devonian extinction event is investigated only a single reliable 40 ar 39 ar age is available for the viluy traps and falls close to the interpolated age for the frasnian fammenian boundary the use of the unspiked k ar technique to yield accurate ages for such altered samples is questioned a review of u pb data pertinent to these problems suggests a close temporal link between the formation of the siberian traps and the ptb comparison of u pb and 40 ar 39 ar ages for the ptb raises questions about the accuracy of high precision sanidine ages possibly resulting from very slow leakage of 40 ar from this mineral

a reconnaissance 40ar39ar geochronologic study of ore bearing and related rocks siberian russia abstract 40 ar 39 ar age spectra of biotite from a mineralized vein in the ore bearing noril sk i intrusion and from picritic like gabbrodolerite from the weakly mineralized lower talnakh intrusion show that these bodies were emplaced at 249 2 ma which is not significantly different from the age of the permian triassic boundary the ore bearing intrusions postdate the lower third of the flood basalt sequence in the noril sk area and on the basis of geochemistry can best be correlated with lavas slightly younger than those which they cut thus flood basalt was erupted at the time of the permian triassic mass extinction event although its role in this event is as yet ill defined additional new 40 ar 39 ar age data for a group of intrusive and extrusive rocks on the western margin of the siberian craton indicate that mafic magmatism extended over a period of several tens of million years whereas paleomagnetic data suggest that the bulk of the siberian flood basalt sequence near noril sk has been erupted in only a million years or so 40 ar 39 ar ages of plagioclase from early flood basalt flows are about 2 younger than those obtained for biotite from the crosscutting noril sk i intrusion probably because of slight alteration and argon loss from the plagioclase

long term effects of restoration on soil hydraulic properties in revegetation stabilized desert ecosystems 1 improving structure of soils found at 0 5 cm depth and increasing the thickness of biological soil crusts are both associated with sand dune revegetation stabilization in arid northwestern china since 1956 research on sand dune stabilization has included the use of straw chequer boards to facilitate development of soil structure one method to gauge the degree of stabilization is to compare undisturbed soil hydraulic properties including water retention h and hydraulic conductivity k h functions with properties from stabilized sites of different ages this study examined properties at five experimental sites of different ages since revegetation 51 42 34 20 and 0 years saturated hydraulic conductivity ks was determined in situ from all five sites and the h curve was determined from samples collected from three sites a significant negative correlation existed between ks and the clay silt and organic matter contents differences in most van genuchten parameters for h were observed between the revegetated plots and the migrating sand dune area results of this long term study show that changes in soil hydraulic properties and improvement in soil structure were associated with migrating dune stabilization

mechanical behaviour of gas charged marine plastic sediments understanding how free gas modifies the mechanical behaviour of marine plastic sediments remains a challenging issue special triaxial tests were carried out on natural sediments recovered from the gulf of guinea special attention was devoted to the laboratory preparation procedure involving saturation of natural marine sediment samples with carbonated water and generation of free gas following undrained unloading experimental data show that soil destructuration and damage generated by gas expansion and exsolution are at the origin of a the increase in the isotropic compressibility and decrease in the preconsolidation pressure and b the decrease in the peak undrained strength and abrupt increase of the pore pressure during undrained shearing the experimental data also show that flooding of gas bubbles and dissolution of free gas do not imply a complete strength recovery of natural sediments

40ar 39ar ages from the rhyolite of alder creek california age of the cobb mountain normal polarity subchron revisited new 40 ar 39 ar age determinations on sanidine from the rhyolite of alder creek california indicate a 1 186 0 006 ma age for the cobb mountain normal polarity subchron the new age is statistically older 0 05 than the previously reported k ar age 1 12 0 02 ma and agrees with the age suggested by the astronomical polarity time scale incomplete extraction of radiogenic 40 ar 40 ar from the sanidine is the most likely reason for the disparity between the 40 ar 39 ar and k ar ages because the cobb mountain subchron is a worldwide short duration event and because no widely used interlaboratory 40 ar 39 ar standard younger than 27 ma exists we propose that sanidine from the rhyolite of alder creek be considered for use as a new quaternary 40 ar 39 ar mineral standard

synchrony and causal relations between permian triassic boundary crises and siberian flood volcanism the permian triassic boundary records the most severe mass extinctions in earth9s history siberian flood volcanism the most profuse known such subaerial event produced 2 million to 3 million cubic kilometers of volcanic ejecta in approximately 1 million years or less analysis of 40 ar 39 ar data from two tuffs in southern china yielded a date of 250 0 0 2 million years ago for the permian triassic boundary which is comparable to the inception of main stage siberian flood volcanism at 250 0 0 3 million years ago volcanogenic sulfate aerosols and the dynamic effects of the siberian plume likely contributed to environmental extrema that led to the mass extinctions

the deccan trap cretaceous paleogene boundary connection new 40ar 39ar ages and critical assessment of existing argon data pertinent to this hypothesis abstract the cretaceous paleogene boundary kpgb was dated by the 40ar 39ar method herein from the western interior of north america at 65 48 0 12 ma 1 in good agreement with other recent published estimates for the deccan traps india new argon ages as well as others available in the literature are assessed for reliability based on a statistical reliability of plateau isochron sections and b freshness of material dated utilizing the alteration index method from tholeiitic lavas from the composite western ghats section cwgs only six ages are found to be reliable estimates of the time of crystallization these ages along with the magnetic polarity of the lavas agree with the geomagnetic polarity time scale gpts at 67 64 ma alkaline rocks from the anjar area of kutch provide three reliable ages that suggest a hiatus in lava extrusion around kpgb for the rajahmundry basalts the upper flow s age defines its formation during chron 29n a single age from the lower reversed polarity flow appears somewhat dichotomous when plotted against the gpts the reliable lava ages indicate the most voluminous reversed polarity sections of the cwgs were extruded at a time statistically indistinguishable from that of the kpgb the deccan trap kpgb faunal extinction hypothesis remains plausible but must compete with the latest report favoring a very close temporal connection 0 03 m y between the chixculub impact crater and the kpgb

siberian gas venting and the end permian environmental crisis abstract the end of the permian period is marked by global warming and the biggest known mass extinction on earth the crisis is commonly attributed to the formation of the siberian traps large igneous province although the causal mechanisms remain disputed we show that heating of tunguska basin sediments by the ascending magma played a key role in triggering the crisis our conclusions are based on extensive field work in siberia in 2004 and 2006 heating of organic rich shale and petroleum bearing evaporites around sill intrusions led to greenhouse gas and halocarbon generation in sufficient volumes to cause global warming and atmospheric ozone depletion basin scale gas production potential estimates show that metamorphism of organic matter and petroleum could have generated 100 000 gt co2 the gases were released to the end permian atmosphere partly through spectacular pipe structures with kilometre sized craters dating of a sill intrusion by the u pb method shows that the gas release occurred at 252 0 0 4 million years ago overlapping in time with the end permian global warming and mass extinction heating experiments to 275 c on petroleum bearing rock salt from siberia suggests that methyl chloride and methyl bromide were significant components of the erupted gases the results indicate that global warming and ozone depletion were the two main drivers for the end permian environmental crisis we demonstrate that the composition of the heated sedimentary rocks below the flood basalts is the most important factor in controlling whether a large igneous provinces causes an environmental crisis or not we propose that a similar mechanism could have been responsible for the triassic jurassic 200 ma global warming and mass extinction based on the presence of thick sill intrusions in the evaporite deposits of the amazon basin in brazil

preliminary dating of the viluy traps eastern siberia eruption at the time of late devonian extinction events article i nfo ar 39 ar dating in this short note we report new age determinations from four samples of the middle paleozoic viluy traps in siberia east of the more famous permo triassic siberian traps these samples which were collected from three drill cores have been analyzed in parallel and independently in the orsay france and berkeley geochronology center bgc usa geochronology laboratories using respectively the cassignol gillot k ar and the 40 ar 39 ar techniques dating these samples as a concerted effort in two independent laboratories working jointly on their interpretation is a rather rare yet very valuable exercise with the k ar technique ages ranging from 338 to 367 ma with uncertainties on the order of 5 ma were obtained with the 40

reevaluation of plate motion models based on hotspot tracks in the atlantic and indian oceans abstract plate motion models based on hotspot tracks in the atlantic and indian oceans predict minimal movement less than a few millimeters per year between these hotspots and their counterparts in the pacific ocean for the past 100 m yr whereas plate circuit exercises indicate relative motions of 20 mm yr hotspot based models also suggest that the rajmahal traps india were located 1000 km away from the kerguelen hotspot at 115 ma and the deccan traps india were located a similar distance from the reunion hotspot at 65 ma this is at odds with conclusions derived from paleomagnetism plate circuits and geochemical parameters that suggest a genetic link between flood basalt provinces in india and hotspots in the indian ocean these divergent views may be explained by plume action 1000 km from its center or errors in the hotspot motion models the latter hypothesis is scrutinized in this article by examination of the radiometric ages for hotspot tracks in the atlantic and indian oceans the