k ca and rb sr dating of two lunar granites relative chronometer resetting k ca and rb sr age determinations were made for lunar granite 12033 576 this small fragment is a subsample of soil particle 12033 507 which was collected subsurface at the north rim of head crater the k ca mineral isochron is well defined and yields an age of 3 62 0 11 2 ga for 40k 0 5543ga 1 and an initial 40ca44ca of 47 160 0 006 2 normalized to 42ca44ca 0 31221 the rb sr mineral isochron is quite disturbed and yields an imprecise young age of 2 20 0 65 ga for 87rb 0 0139 ga 1 and a high initial 87sr86sr of 0 775 0 022 assuming that the granite crystallized 4 0 ga ago a modeled formation age and was shock heated 0 8 ga ago 39ar 40ar age the intermediate rb sr and k ca ages are interpreted as due to partial resetting by thermal diffusion of strontium and calcium diffusion data determined for argon in 12033 507 give a value of da2 1 10 6 s 1 for a heating temperature of 700 c at this temperature it would require a time on the order of days to totally degas argon the required diffusion coefficients d for calcium 7 9 10 13cm2s 1 and sr 8 10 12cm2s 1 that are predicted from the observed age resetting are in good agreement with those determined experimentally in granitic melts the k ca system was about a factor of ten more resistant to resetting than the rb sr system which in turn was 10 times more resistant to resetting than the k ar system diffusion data indicate that isotopic exchange of calcium and strontium was responsible for the partial resetting of the k ca and rb sr chronometers rather than elemental exchange of potassium and rubidium among mineral phases rb sr isotopic data also were obtained for granite 14303 206 a subsample of breccia clast 14303 204 which was collected at the fra mauro region about 180 km from the apollo 12 site the mineral isochron is quite disturbed and yields an imprecise age of 3 95 0 38 ga for 87rb 0 0139 ga 1 and initial 87sr86sr 0 7046 0 0051 the rb sr age is comparable to the previously determined k ca age of 4 04 0 64 ga the rb sr and k ca ages of this clast are imprecise and are statistically indistinguishable from the well defined 39ar 40ar degassing age of 3 83 0 03 ga or the rb sr and k ca model ages of 4 30 0 06 ga and 4 25 0 12 ga respectively because these model ages are within error of a reported zircon u pb age of 4 31 ga we assume this granitic clast crystallized 4 3 ga ago and was shock heated 3 83 ga ago using the apparent resetting of the k ar rb sr and k ca isotopic systems and literature data for relative strontium and calcium diffusivity we estimate a shock reheating temperature of 860 c would be necessary to partially reset the rb sr and k ca ages to their nominal values this reheating temperature is uncertain because of the large errors on the ages but would be consistent with complete resetting of the k ar age and partial resetting of the rb sr and k ca ages if the hot clast was buried under 1 m of the ejecta blanket for 1 day

strength behavior of methane hydrate bearing sand in undrained triaxial testing abstract gas hydrates represent a potential future energy source as well as a considerable geohazard in order to assess both the benefits and risks that gas hydrate bearing sediments pose fundamental information about their physical properties is required in this study the undrained shear strength of methane hydrate bearing sand was investigated the experimental program required modifications to an existing triaxial apparatus and accurate determination of the hydrate saturation lead to the use of two methods for comparison of the saturation calculations strength results indicated that the presence of gas hydrate will increase the sediment s undrained shear strength and corresponding stiffness the relative contribution of cohesion and friction angle was observed to be a function of the hydrate saturation for this particular hydrate formation methodology

recognition of 3850 ma water lain sediments in west greenland and their significance for the early archaean earth a layered body of amphibolite banded iron formation bif and ultramafic rocks from the island of akilia southern west greenland is cut by a quartz dioritic sheet from which shrimp zircon 206pb 207pb weighted mean ages of 3865 11 ma and 3840 8 ma 2 sigma can be calculated by different approaches three other methods of assessing the zircon data yield ages of 3830 ma the bifs are interpreted as water lain sediments which with a minimum age of approximately 3850 ma are the oldest sediments yet documented these rocks provide proof that by approximately 3850 ma 1 there was a hydrosphere supporting the chemical sedimentation of bif and that not all water was stored in hydrous minerals and 2 that conditions satisfying the stability of liquid water imply surface temperatures were similar to present carbon isotope data of graphitic microdomains in apatite from the akilia island bif are consistent with a bio organic origin mojzsis et al 1996 extending the record of life on earth to 3850 ma life and surface water by approximately 3850 ma provide constraints on either the energetics or termination of the late meteoritic bombardment event suggested from the lunar cratering record on earth

bottom response to a tsunami earthquake submersible observations in the epicenter area of the 1993 earthquake off southwestern hokkaido sea of japan we conducted a research campaign of nine dives using the manned submersible vehicle shinkai 6500 in 1995 beneath the central okushiri ridge along the eastern margin of the sea of japan the main objective was to investigate coseismic response and bottom disturbance to the tsunamigenic 1993 offshore southwestern hokkaido earthquake ms 7 8 42 47 n 139 12 e in the northern aftershock area including the mainshock epicenter the whole aftershock area occupies the western half of source area of the related tsunami between the okushiri ridge and the neighboring shiribeshi trough to the east we found a gentle scarplet of the earthquake fault that bounds the first terrace and a zone of open gashes on the flat terrace a number of sand blows open cracks and other slope failures occupied the slope of the okushiri ridge on the uplifted hanging wall side of the tsunami source area some fractures and holes whose activity had ceased soon after the earthquake began to be buried by the 2 year accumulation of recent sediments in contrast the shiribeshi trough on the footwall of the fault and in the subsided domain of the tsunami source area suffered no remarkable disturbance other than gentle folding our submersible observations elucidated the seafloor response to tsunamigenic uplift of the hanging wall of a seismogenic thrust fault which displays the present time section of back thrusting with an embryonic subduction along the eastern margin of the sea of japan

dating the oldest terrestrial rocks fact and fiction abstract this paper is a review of well documented geochronological and related isotopic evidence for the age and total crustal residence time of the oldest known terrestrial rocks mainly using rb 1bsr sm 1bnd and u 1bpb decay schemes the oldest known stable sialic crust of true continental character predominantly composed of calc alkaline orthogneisses was produced during major mantle differentiation some 3 7 3 5 ga ago and is represented on several continents enclaves of older supracrustal rocks as well as a major remnant of a volcano sedimentary sequence of greenstone belt affinity dated at 3 8 ga are preserved within the surrounding gneisses in southern west greenland recent ionmicroprobe evidence for 4 1 ga old detrital zircon grains in a younger early archaean metasedimentary sequence in western australia is also discussed several cases are reviewed where claims for early archaean 3 4 ga dates are believed to be erroneous and to result from oversimplistic and incorrect interpretation of isotopic data some of the geological and geochemical mechanisms which have given rise to claims for spuriously old sm 1bnd u 1bpb and pb pb dates are critically discussed to supplement published data we present several new sets of whole rock sm 1bnd and pb pb isotopic data from west greenland labrador zimbabwe and eastern india relevant to the overall topic of this review

the influence of vertical cylindrical tetrahydrofuran hydrate veins on fine grained soil behaviour over the last 10 years marine gas hydrate drilling expeditions utilizing advances in pressure coring techniques and imaging have routinely encountered gas hydrates residing in fine grained sediments the hydrate typically occurs as fracture filling near vertical veins that displace the sediment potentially leading to increased sediment strength that may prevent normal consolidation of the sediment thus leading to underconsolidation destabilization of this hydrate through climate change or human activity on the seafloor may cause dramatic loss of strength of the sediment and pose a significant geohazard to assess the impact of hydrate veins on sediment behaviour this paper reports on a series of consolidated cu and unconsolidated undrained uu triaxial tests carried out on fine grained soil specimens hosting simplified vertical cylindrical tetrahydrofuran thf hydrate veins of varying diameters the results show that increasing hydrate vein diameter significantly increases strength and stiff

the structure of methane gas hydrate bearing sediments from the krishna godavari basin as seen from micro ct scanning abstract the indian national gas hydrate program nghp expedition 1 of 2006 cored through several methane gas hydrate deposits on the continental shelf around the coast of india the pressure coring techniques utilized during the expedition hyacinth and pcs enabled recovery of gas hydrate bearing fine grained sediment cores to the surface after initial characterization core sections were rapidly depressurized and submerged in liquid nitrogen preserving the structure and form of the hydrate within the host sediment once on shore high resolution x ray ct scanning was employed to obtain detailed three dimensional images of the internal structure of the gas hydrate using a resolution of 80 m the detailed structure of the hydrate veins present in each core could be observed and allowed for an in depth analysis of orientation width and persistence of each vein hydrate saturation estimates could also be made and saturations of 20 30 were found to be the average across the core section with some portions showing highs of almost 60 saturation the majority of hydrate veins in each core section were found to be orientated between 50 and 80 to the horizontal analysis of the strikes of the veins suggested a slight preferential orientation in individual sample sections although correlation between individual sections was not possible due to the initial orientation of the sections being lost during the sampling stage the preferred vein orientation within sample sections coupled with several geometric features identified in individual veins suggest that hydraulic fracturing by upward advecting pore fluids is the main formation mechanism for the veined hydrate deposits in the k g basin

gas hydrate dissociation in sediments pressure temperature evolution 1 hydrate bearing sediments may destabilize spontaneously as part of geological processes unavoidably during petroleum drilling production operations or intentionally as part of gas extraction from the hydrate itself in all cases high pore fluid pressure generation is anticipated during hydrate dissociation a comprehensive formulation is derived for the prediction of fluid pressure evolution in hydrate bearing sediments subjected to thermal stimulation without mass transfer the formulation considers pressure and temperature dependent volume changes in all phases effective stress controlled sediment compressibility capillarity and the relative solubilities of fluids salient implications are explored through parametric studies the model properly reproduces experimental data including the pt evolution along the phase boundary during dissociation and the effect of capillarity pore fluid pressure generation is proportional to the initial hydrate fraction and the sediment bulk stiffness is inversely proportional to the initial gas fraction and gas solubility and is limited by changes in effective stress that cause the failure of the sediment when the sediment stiffness is high the generated pore pressure reflects thermal and pressure changes in water hydrate and mineral densities comparative analyses for co2 and ch4 highlight the role of gas solubility in excess pore fluid pressure generation dissociation in small pores experiences melting point depression due to changes in water activity and lower pore fluid pressure generation due to the higher gas pressure in small gas bubbles capillarity effects may be disregarded in silts and sands when hydrates are present in nodules and lenses and when the sediment experiences hydraulic fracture

influence of gas hydrate morphology on the seismic velocities of sands this paper reports the results of a series of resonant column tests on specimens where gas hydrate has been formed in sands using an excess water technique in these specimens the amount of hydrate formed is restricted by the amount of gas in the specimen and with an excess of water being present in the pore space results of resonant column tests carried out to determine compressional and shear wave velocities suggest that gas hydrate formed in this way are frame supporting in contrast the behavior observed in sands where the hydrate is formed from finite water where the remaining pore space is saturated with methane gas termed in this paper the excess gas method exhibits a cementing behavior while tetrahydrofuran hydrate sands or where the hydrate is formed from dissolved methane within the pore water exhibit a pore filling behavior for hydrate saturations less than 40 for sands where the hydrate is formed using the excess water method much larger volumes of hydrate are required before a significant increase in shear wave velocity occurs although increases in compressional wave velocity are seen at lower hydrate contents these results suggest that hydrate interaction with the sediment is strongly dependent on morphology and that natural hydrate may exhibit contrasting seismic signatures depending upon the geological environment in which it forms

mechanical properties of sand silt and clay containing tetrahydrofuran hydrate 1 the mechanical behavior of hydrate bearing sediments subjected to large strains has relevance for the stability of the seafloor and submarine slopes drilling and coring operations and the analysis of certain small strain properties of these sediments for example seismic velocities this study reports on the results of comprehensive axial compression triaxial tests conducted at up to 1 mpa confining pressure on sand crushed silt precipitated silt and clay specimens with closely controlled concentrations of synthetic hydrate the results show that the stress strain behavior of hydrate bearing sediments is a complex function of particle size confining pressure and hydrate concentration the mechanical properties of hydrate bearing sediments at low hydrate concentration probably 50 of pore space the behaviorbecomesmoreindependentofstressbecausethehydratescontrolbothstiffnessand strength and possibly the dilative tendency of sediments by effectively increasing interparticle coordination cementing particles together and filling the pore space the cementation contribution to the shear strength of hydrate bearing sediments decreases with increasing specific surface of soil minerals the lower the effective confining stress the greater the impact of hydrate formation on normalized strength

physical properties and rock physics models of sediment containing natural and laboratory formed methane gas hydrate this paper presents results of shear strength and acoustic velocity p wave measurements performed on 1 samples containing natural gas hydrate from the mallik 2l 38 well mackenzie delta northwest territories 2 reconstituted ottawa sand samples containing methane gas hydrate formed in the laboratory and 3 ice bearing sands these measurements show that hydrate increases shear strength and p wave velocity in natural and reconstituted samples the proportion of this increase depends on 1 the amount and distribution of hydrate present 2 differences in sediment properties and 3 differences in test conditions stress strain curves from the mallik samples suggest that natural gas hydrate does not cement sediment grains however stress strain curves from the ottawa sand containing laboratory formed gas hydrate do imply cementation is present acoustically rock physics modeling shows that gas hydrate does not cement grains of natural mackenzie delta sediment natural gas hydrates are best modeled as part of the sediment frame this finding is in contrast with direct observations and results of ottawa sand containing laboratory formed hydrate which was found to cement grains waite et al 2004 it therefore appears that the microscopic distribution of gas hydrates in sediment and hence the effect of gas hydrate on sediment physical properties differs between natural deposits and laboratory formed samples this difference may possibly be caused by the location of water molecules that are available to form hydrate models that use laboratory derived properties to predict behavior of natural gas hydrate must account for these differences

prediction of the mechanical response of hydrate bearing sands in recent years there has been an increasing interest in production of methane gas from hydrate bearing sediments located below the permafrost in arctic regions and offshore within the continental margins in order to simulate the geomechanical response of the hydrate accumulation during gas production comprehensive evaluation of the sediments properties is imperative this paper presents an analysis of the mechanical properties of methane hydrate bearing sediments determined through numerical simulation of drained triaxial compression tests on three different sand types the adjustment of the numerical to the experimental results was performed for the entire stress strain curves and therefore enables a good understanding of the material constitutive relations new constitutive relations are suggested for the hydrate related properties an optimization process was used finding separately the soil skeleton related coefficients and the hydrate related coefficients the values of the obtained coefficients associated with the hydrate were found to have minor deviations from each other for the three examined sand types by separating the soil skeleton and the hydrate related response this paper suggests a prediction method for the mechanical response of hydrate bearing sands

ca isotope fingerprints of early crust mantle evolution abstract the utility of 40 ca 44 ca as a tracer of pre existing crustal contributions in early archaean cratons has been explored to identify traces of hadean crust and to assess the style of continental growth the relatively short half life of 40 k 1 3 gy means that its decay to 40 ca occurs dominantly during early earth history if archaean crust had a significant component derived from a more ancient protolith as anticipated by steady state crustal evolution models this should be clearly reflected in radiogenic 40 ca 44 ca ratios or positive initial e ca in different archaean cratons a high precision thermal ionisation technique has been used to analyse the 40 ca 44 ca ratios of plagioclase separates and associated whole rocks in 3 6 ga early archaean samples from zimbabwe and west greenland three out of four tonalite trondhjemite granodiorite ttg suite samples from zimbabwe display initial 40 ca 44 ca ratios indistinguishable from our measured modern morb value i e e ca 3 6 0 greenland samples however are very diverse ranging from e ca 3 7 0 1 in mafic pillow lavas and felsic sheets from the isua supracrustal belt up to very radiogenic signatures e ca 3 7 2 9 in both mafic rocks of the akilia association and felsic ttg from the coastal amitsoq gneisses at face value these results imply the zimbabwe crust is juvenile whereas most greenland samples include an earlier crustal component yet the west greenland craton as with many archaean localities has experienced a complex geological history and the interpretation of age corrected initial isotope values requires great care both felsic and mafic samples from greenland display e ca 3 7 so radiogenic that they are not readily explained by crustal growth scenarios the presence of such radiogenic 40 ca 44 ca found in low k ca plagioclases requires ca isotope exchange between plagioclase and whole rock during later metamorphic event s in addition the unexpectedly radiogenic ca isotope ratios in some mafic samples reflect anomalous k ca ratios as a result of intense k metasomatism 3 6 ga thus ca isotope measurements are not a robust tracer of crustal growth in the presence of intense tectono metamorphic processes coupled with other isotope data however the degree of overprint can be estimated and the 40 ca 44 ca ratio of a little disturbed sample hints at a small contribution of hadean protocrust in the coastal part of the godthabsfjord area southwest greenland in the majority of zimbabwe ttg samples unradiogenic initial ca isotope ratios point to very little prior crustal history and minor subsequent disturbance we thus infer that the modest initial e nd 0 8 of the zimbabwean samples is representative of the depleted mantle at 3 6 ga furthermore ca isotope systematics provide little support for a steady state model of crustal growth

origin of granitic magma by crustal remobilisation rb sr and pb pb geochronology and isotope geochemistry of the late archaean qorqut granite complex of southern west greenland abstract rb sr and pb pb whole rock isochrons on the qorqut granite complex yield ages of 2530 30 myr initial 87 sr 86 sr 0 7081 0 0008 and 2580 80 myr respectively a model relating initial sr and pb isotopic compositions of the qorqut granites to the sr and pb isotopic compositions of the amitsoq gneisses ca 3700 myr and nuk gneisses ca 2900 myr at 2550 myr ago as well as sr and pb contents of the gneiss units suggests that between 40 and 50 of the qorqut granite magma was generated by partial melting of amitsoq gneisses and the remainder by partial melting of nuk gneisses

hf nd isotope evidence for a transient dynamic regime in the early terrestrial mantle modern basalts have seemingly lost all memory of the primitive earth s mantle except for an ambiguous isotopic signal observed in some rare gases1 2 although the earth is expected to have reached a thermal steady state within several hundred million years refs 3 4 of accretion it is not known how and when the initial chemical fractionations left over from planetary accretion and perhaps a stage involving a magma ocean were overshadowed by fractionations imposed by modern style geodynamics because of the lack of samples older than 4 gyr this early dynamic regime of the earth is poorly understood here we compare published hf nd isotope data on supracrustals from isua greenland with similar data on lunar rocks and the snc martian meteorites and show that about 3 8 gyr ago the geochemical signature of the archaean mantle was partly inherited from the initial differentiation of the earth the observed features seem to indicate that the planet at that time was still losing a substantial amount of primordial heat the survival of remnants from an early layering in the modern deep mantle may account for some unexplained seismological thermal and geochemical characteristics of the earth as observed today

episodic seafloor mud brecciation due to great subduction zone earthquakes the nankai trough off southwest japan has an 1300 yr historical record of great earthquakes including the most recent the a d 1944 tonankai m 8 2 earthquake evaluation of the activity of an individual submarine fault is difficult when only onland observations are available submarine core records can pinpoint individual fault activity here we present integrated ocean drilling program iodp nankai trough seismogenic zone experiment data from the shallow portion iodp expedition 316 drilled and cored several holes in the shallow portion of the offshore tonankai earthquake area including sites in the hanging wall of a margin dominating splay fault that has previously been interpreted to have ruptured coseismically during megathrust earthquakes x ray computed tomography scanning revealed that the uppermost core at one site contains repeated occurrences of mud breccia radioisotope dating of the uppermost mud breccia indicates a deposition time consistent with the 1944 tonankai earthquake suggesting that the mud breccia layers result from episodic brecciation caused by seismic shaking mud brecciation provides a potential new tool to reconstruct ancient earthquake history in subduction zones

calcium isotopes in the global biogeochemical ca cycle implications for development of a ca isotope proxy abstract at the earth s surface calcium ca is a critical element at a variety of scales it is soluble in natural waters a biological nutrient and a major constituent of the dominant mineral sink for carbon in the ocean caco3 there is a 4 variation in the ca isotopic composition 44ca 40ca expressed as 44ca of various ca reservoirs on earth suggesting ca isotopes as a promising tracer of ca cycling in both the present and the past fifteen years of high precision ca isotope measurements has revealed much about the behavior of ca isotopes in the earth surface environment but there remain fundamental questions concerning how ca isotopes are used to elucidate the marine and terrestrial ca cycles the current work presents a data compilation of over 70 published ca isotope studies totaling over 2600 measurements presented on a common delta scale that includes data on rivers and groundwater dust soils and soil pore fluids vegetation rainwater silicate minerals rocks and authigenic marine minerals carbonates sulfates and phosphates both modern and ancient the data compilation suggests that 1 there is a significant difference between carbonate 0 60 and silicate 44ca 0 94 2 riverine 44ca 0 88 does not simply reflect the compiled carbonate 44ca and 3 terrestrial vegetation exhibits the largest range of ca isotopic compositions 3 5 in the terrestrial setting we discuss these observations in the context of the global ca cycle exploring the extent to which seawater 44ca variability is feasible and how we can achieve accurate reconstructions of seawater 44ca over geologic time scales the current study presents simple mass balance models that quantify the leverage of inputs to change the ca isotopic composition of the ocean as this directly impacts the manner in which ca isotopes are interpreted although ca fractionates isotopically in the modern system during continental cycling the 44ca range of riverine inputs to the ocean is considerably smaller than the variability observed in putative seawater proxies such as nannofossil ooze and marine barite in the terrestrial realm plants exhibit a wide 44ca range and there is evidence that ca fluxes via biomass degradation are significant at the catchment scale we therefore assess the ability of the continental biosphere to influence riverine and consequently seawater 44ca a steady state biosphere has little leverage to alter riverine 44ca except in cases where the 44ca of the recycling flux is isotopically distinct from the 44ca of the uptake flux a non steady state biosphere can substantially impact both soil and riverine 44ca driving exchangeable ca either heavier or lighter depending on the magnitude of the recycling flux relative to the uptake flux based on estimates of the size of the global biosphere 1 5 1015 mol ca we suggest a decaying biosphere has the potential to impact riverine 44ca by tenths of a permil over time scales in the marine realm we evaluate the effect of a variable fractionation factor accompanying global removal of ca from the ocean on seawater 44ca and suggest methods by which such a mechanism can be recognized in the rock record experimental data suggest that there is considerable leverage

early history of earth s crust mantle system inferred from hafnium isotopes in chondrites the 176lu to 176hf decay series has been widely used to understand the nature of earth s early crust mantle system1 2 3 4 5 6 the interpretation however of lu hf isotope data requires accurate knowledge of the radioactive decay constant of 176lu as well as bulk earth reference parameters a recent calibration of the value calls for the presence of highly unradiogenic hafnium in terrestrial zircons with ages greater than 3 9 gyr implying widespread continental crust extraction from an isotopically enriched mantle source more than 4 3 gyr ago7 but does not provide evidence for a complementary depleted mantle reservoir here we report lu hf isotope measurements of different solar system objects including chondrites and basaltic eucrites the chondrites define a lu hf isochron with an initial 176hf 177hf ratio of 0 279628 0 000047 corresponding to 1 983 0 033 10 11 yr 1 using an age of 4 56 gyr for the chondrite forming event this value indicates that earth s oldest minerals were derived from melts of a mantle source with a time integrated history of depletion rather than enrichment7 the depletion event must have occurred no later than 320 myr after planetary accretion consistent with timing inferred from extinct radionuclides8 9 10

resetting of smnd systematics during metamorphism of 3 7 ga rocks implications for isotopic models of early earth differentiation abstract new sm nd isotopic results for sixteen samples from the garbenschiefer amphibolite unit of the 3700 ma isua supracrustal belt of west greenland are reported the data illustrate the difficulty in identifying primary nd t values in poly metamorphic and poly deformed rock sequences the investigated samples are interpreted as derived from a mg and al rich igneous protolith which can be argued to have been essentially isotopically homogeneous at least on the outcrop scale at the time of its emplacement however the data do not give uniform nd t values even from single outcrops the wide variation of nd t which we obtain 1 0 to 5 0 demonstrates that the sm nd systematics of the samples were disturbed by one or more metamorphic event s which took place some considerable time after the original crystallization of the isua belt a sm nd isochron age of 2849 116 ma has been obtained on plagioclase and amphibole separates which represent the oldest metamorphic assemblage now preserved at isua this result indicates that the earliest amphbolite facies metamorphism of the isua belt for which pressure and temperature conditions can be obtained p 5 kbar t 500 600 c is late archaean and not early archaean as had been assumed earlier by some authors 2850 ma is also thought to represent the main time of disturbance of the sm nd system in the garbenschiefer amphibolite unit a consideration of available isotopic data suggests that many of the rocks from isua underwent some nd isotopic re equilibration and or fractionation of their 147 sm 144 nd ratio during the 2850 ma metamorphic episode in these rocks the computed nd t are apparent values only and need not represent either the primary isotopic composition of their protoliths or early archaean mantle values only nd rich rocks 20 ppm such as some felsic schists and the grey gneisses adjacent to the belt are arguably sufficiently robust to retain nd t values which are close to their original composition these average 2 0 which may represent a close approximation of the nd value of the earth s mantle at 3800 ma the results presented in this paper show that great care should be exercised in the use of sm nd data from very early archaean rocks as constraints for isotopic models of early earth s evolution particularly in terranes which have been affected by several metamorphic episodes

in situ 40k 40ca double plus sims dating resolves klokken feldspar 40k 40ar paradox abstract the 40k 40ca decay system has not been widely utilized as a geochronometer because quantification of radiogenic daughter is difficult except in old extremely high k ca domains even these environments have not heretofore been exploited by ion microprobe analysis due to the very high mass resolving power mrp of 25 000 required to separate 40k from 40ca we introduce a method that utilizes doubly charged k and ca species which permits isotopic measurements to be made at relatively low mrp 5000 we used this k ca double plus approach to address an enduring controversy in 40ar 39ar thermochronology revolving around exsolved alkali feldspars from the 1166 ma klokken syenite southern greenland ion microprobe 40k 40ca analysis of klokken samples reveal both isochron and pseudoisochron behaviors that reflect episodic isotopic and chemical exchange of coarsely exsolved perthites and a near end member k feldspar until 719 ma and perhaps as late at 400 ma feldspar microtextures in the klokken syenite evolved over a protracted interval by non thermal processes fluid assisted recrystallization and thus this sample makes a poor model from which to address the general validity of 40ar 39ar thermochronological methodologies

alteration and metamorphism of amitsoq gneisses from the isukasia area west greenland recommendations for isotope studies of the early crust early archaean gneisses 3400 3700 ma in the isukasia area of west greenland have been subjected to a series of local and regional metamorphic processes metasomatic alteration accompanied the intrusion of the early archaean white gneisses 3600 ma into the grey gneisses 3700 ma and resulted in local formation of altered rocks pb pb isotope results on whole rock and feldspar from the gneisses reveal a major rearrangement of pb isotopes at about 2600 ma together with some local third stage pb changes at a later time this 2600 ma event is also shown by rb sr data for phengites in the 3400 ma pegmatitic gneiss sheets and can be correlated with metamorphism accompanying the intrusion of a late archaean pegmatite 2600 ma swarm local hydrothermal alteration by fluids emanating from proterozoic faults and fractures is petrographically recognised in and near major faults and lithological boundaries this fault controlled alteration though only locally significant may correlate with low grade thermal metamorphism reflected in a 1600 ma rb sr mineral isochron on biotites from both altered and unaltered gneisses oxygen isotope results show that this alteration probably was associated with influx of a low 18o fluid perhaps meteoric water selection of the least altered amitsoq gneisses can be made on the basis of petrographic and field criteria and furnishes the best material to study original geochemical and isotopic systematics in these early archaean rocks

trampoline effect in extreme ground motion in earthquake hazard assessment studies the focus is usually on horizontal ground motion however records from the 14 june 2008 iwate miyagi earthquake in japan a crustal event with a moment magnitude of 6 9 revealed an unprecedented vertical surface acceleration of nearly four times gravity more than twice its horizontal counterpart the vertical acceleration was distinctly asymmetric the waveform envelope was about 1 6 times as large in the upward direction as in the downward direction which is not explained by existing models of the soil response we present a simple model of a mass bouncing on a trampoline to account for this asymmetry and the large vertical amplitude the finding of a hitherto unknown mode of strong ground motion may prompt major progress in near source shaking assessments

earliest part of earth s stratigraphic record a reappraisal of the 3 7 ga isua greenland supracrustal sequence investigations of the isua supracrustal rocks in west greenland allow us to identify the protoliths and alteration history for most isua rocks the protoliths consisted of alternating basalt and banded iron formation this sequence was invaded by dunitic sills metamorphosed and later intruded by felsic gneisses pervasive carbonation and k metasomatism produced a sequence of lithologies mimicking those found in modern platform deposits however the protoliths could have originated in a purely oceanic environment with no sialic detrital components the isua sequence probably consists of several tectonic panels some of which are cut by 3810 ma felsic intrusive rocks

metasomatic origin of quartz pyroxene rock akilia greenland and implications for earth s earliest life a quartz pyroxene rock interpreted as a banded iron formation bif from the island of akilia southwest greenland contains 13 c depleted graphite that has been claimed as evidence for the oldest 3850 million years ago life on earth field relationships on akilia document multiple intense deformation events that have resulted in parallel transposition of early archean rocks and significant boudinage the tails of which commonly form the banding in the quartz pyroxene rock geochemical data possess distinct characteristics consistent with an ultramafic igneous not bif protolith for this lithology and the adjacent schists later metasomatic silica and iron introduction have merely resulted in a rock that superficially resembles a bif an ultramafic igneous origin invalidates claims that the carbon isotopic composition of graphite inclusions represents evidence for life at the time of crystallization

re os sm nd isotope and ree systematics on ultramafic rocks and pillow basalts from the earth s oldest oceanic crustal fragments isua supracrustal belt and ujaragssuit nunât area w greenland abstract ultramafic rocks and associated pillow basalts from the western northwestern sector of the isua supracrustal belt isb west greenland reveal a complex pattern of trace elements mainly with respect to their rare earth elements ree the heterogeneity of lree patterns in combination with widely scattering end t 3 81 ga values between 3 7 and 12 4 indicate that the ree systematics of the samples were disturbed by one or more metamorphic events which took place some considerable time after the original crystallization of the isua belt furthermore lree rich metasomatic fluids associated with the emplacement of tonalites into the supracrustals may have significantly altered the primary trace element budget extreme variations in whole rock os t 3 81 ga values of these rocks ranging from 8500 to 9 are favorably interpreted to derive from re additions during fluid hydrothermal alterations accompanying the early and late archean metamorphic events in contrast the re os isotope system remained intact in ultramafic hosted chromites from isua and from the ujaragssuit nun t area about 20 km se of the isb an average os t 3 81 ga value of 1 3 2 5 2 for the chromite population covers the entire range defined by the chondritic average the convecting primitive upper mantle pum nature 383 1996 517 geochim cosmochim acta 65 2001 1311 and models involving outer core entrainment at this time given the uncertainty in the origin of the ultramafic rocks and thus the chromites and the very high degree of alteration of these rocks within the isb our data do not conclusively allow to further impinge on the petrogenesis of the isb mantle derived rocks our data are the first though to vaguely hint at an early archean mantle with a possible slightly suprachondritic initial 187os 188os isotopic composition similarities of unaltered pillow basalts with phanerozoic boninites chem geol 184 2002 231 may indicate a formation in an intra oceanic subduction zone like geodynamic process in which extreme early hadean depletion of a mantle source by melt extraction was followed by a second stage of melting induced by enriched subduction components alternatively core mantle interaction as a plausible mechanism for possible weak 187os enrichments in these rocks remains a valid hypothesis

geochronology and isotopic variation of the early archaean amitsoq gneisses of the isukasia area southern west greenland abstract relatively unaltered samples of the amitsoq gneiss complex in the isukasia area were identified and analysed despite the effects of metamorphism and local proterozoic hydrothermal alteration and contamination the parents of the three lithologies that make up the amitsoq gneisses of the area were emplaced over about 320 ma taking a weighted average of dates from u pb on zircons rb sr on wr sm nd on wr and pb pb on wr the most probable ages of intrusion are grey gneisses at 3690 50 ma white gneisses at 3590 40 ma and pegmatitic gneisses at 3370 60 ma the rb sr isochron initial 87 sr 86 sr ratios for the grey white and pegmatitic gneisses are 0 7006 1 0 7016 1 and 0 8248 3 respectively the corresponding average nd t values are 1 0 0 7 and 0 2 since the ree chemistry of the rocks that were the direct parents of the grey gneisses is not known the 1 0 average nd t of the grey gneisses need not imply generation from a depleted mantle the sr initial ratio and nd t values for the white and pegmatitic gneisses are compatible with consecutive derivation at two discrete intervals by partial melting of crust mainly comprised of rocks similar to the grey gneisses the progressive evolution of nd with t for the amitsoq gneisses in the early archaean shows that values of nd t near zero in granitoids do not necessarily indicate derivation from the undepleted mantle chur

neodymium and lead isotope evidence for enriched early archaean crust in north america there has been considerable debate as to whether isotopically enriched continental crust was an important geochemical reservoir in earliest archaean times fundamental questions such as the volume of the early crust its composition and the effect of its formation and or recycling on mantle geochemistry have still to be answered the most direct clues to the nature of the earliest crust and mantle are obtained from the isotope systematics of old rocks1 5 here we report neodymium and lead isotope measurements and uranium lead zircon geochronology from archaean gneisses of the slave province northwest territories canada the gneisses contain zircons with cores older than 3 842 gyr one sample of tonalitic gneiss has a nd chondritic model age of 4 1 gyr and an end 3 7 gyr of 4 8 this is the oldest reported chondritic model age for a terrestrial sample and provides evidence for strongly enriched pre 3 8 gyr crust a reservoir complementary to the depleted mantle already in existence by 3 8 gyr before present2 6

comment on the nd and hf isotopic evolution of the mantle through the archean results from the isua supracrustals west greenland and from the birimian terranes of west africa by blichert toft et al 1999

a comprehensive review on the geomechanical properties of gas hydrate bearing sediments abstract natural gas hydrates ngh that form in sediments beneath the ocean beds are a potential source to meet growing global energy demand gas hydrate bearing sediments ghbs exhibit improved geomechanical properties viz shear strength stiffness compressibility and poisson s ratio that are responsible for their greater stability as compared to the native sediments however dissociation of gas hydrates during extraction of gas instability in the ghbs could trigger submarine slope failures seabed subsidence and failure of the foundations of seafloor installations in order to reduce the risk of occurrence of such geohazards it is imperative to ascertain the stability of ghbs by understanding the variations in their geomechanical properties through field and laboratory investigations which are time consuming and prohibitively expensive for many researchers with this in view the present study attempts to decipher the variation of geomechanical properties with respect to various parameters that affect them viz hydrate saturation sh hydrate morphology and confining pressure temperature conditions by critically synthesizing the existing data in the literature it has been noticed that the shear strength and stiffness are the major parameters influencing the stability of ghbs and both share power law relationship with sh further it is noticed that the cohesion is the predominant component of the shear strength of ghbs and shows significant improvement for sh values greater than 15 50 the variability of cohesion and stiffness with respect to sh have been analysed in tandem with the volumetric deformation during the shearing process to propose particle level mechanism hypothesis that provides an insight in to the stress strain behavior of ghbs the proposed relationships and mechanisms would aid in understanding the behavior and development of improved modeling of geomechanical properties of ghbs

strontium diffusion kinetics in plagioclase feldspars abstract a pronounced systematic variation by a factor of 104 in strontium tracer diffusion kinetics has been measured in four feldspar specimens that span the range of plagioclase compositions the experiments were carried out in air using isotopically enriched 86sr and analyzed with an ion microprobe the arrhenius relations for strontium diffusion are plagioclase ab d 0 m 2 s q kj g atom sr temperature range c albite 98 1 05e 279 550 1080 oligoclase 73 7 4e 7 261 750 1100 labradorite 36 1 1e 6 295 800 1300 anorthite 4 5 7e 9 267 900 1300 no difference was found within the measurement uncertainty between these results and those for albite under hydrothermal conditions at one kbar 100 mpa strontium diffusion rates parallel to and normal to the albite twin lamellae are the same there is no effect on the strontium diffusion rate in albite between an oxygen partial pressure of 0 2 bar air and 10 19 bar graphite buffer a relation is developed that permits calculation of the strontium diffusion arrhenius parameters for any normal plagioclase composition the large variation in diffusion rate with composition has a major effect on the ease of strontium isotope exchange at high temperatures between plagioclases and coexisting phases in rocks which may prove useful in determining thermal histories

paleoseismicity along the southern kuril trench deduced from submarine fan turbidites abstract large m 8 damaging interplate earthquakes occur frequently in the eastern hokkaido region northern japan where the pacific plate is subducting rapidly beneath the okhotsk north american plate at approximately 8 cm yr 1 with the aim of estimating the long term recurrence intervals of earthquakes in this region seven sediment cores were obtained from a submarine fan located on the forearc slope along the southern kuril trench japan the cores contain a number of turbidites some of which can be correlated among the cores on the basis of the analysis of lithology chronology and the composition of sand grains foraminiferal assemblages and the composition of sand grains indicate that the upper middle slope 1000 m water depth is the source of the turbidites the deep sea origin of the turbidites is consistent with the hypothesis that they were derived from slope failures initiated by strong shaking associated with earthquake events the recurrence intervals of turbidite deposition are 113 439 yr for events that occurred over the past 7 kyr the short intervals are recorded in the cores obtained from levees on the middle fan although many large earthquakes 150 cm s 2 of peak ground acceleration at the inferred slump sources occurred during the 19th and 20th centuries the pilot core from the upper fan contains only three turbidites located stratigraphically above layers of 17th century volcanic ash the results of 210pbex and 137cs dating combined with simulations of the ground accelerations of historical earthquakes enable correlation of the three turbidites with known historical earthquakes the 1952 tokachi oki and the 1961 and 1973 nemuro oki earthquakes the turbidites within the sampled cores potentially record about half of the large earthquakes known to have occurred over the interval covered by the cores the fact that any single core records only a portion of the known seismic events suggests that the recurrence interval of earthquakes in this region is less than 113 yr during holocene

3820 ma zircons from a tonalitic armisoq gneiss in the godthab district of southern west greenland abstract zircons from a sample of the tonalitic amitsoq gneisses near godthhb west greenland exhibit several stages of crystal growth measurements of the u th pb isotopic composition of 30 m areas within individual grains using the ion microprobe shrimp reveal that components of magmatic zircon from the original igneous protolith are at least 3822 5 ma l old slightly but significantly older than the age of zircons from a fclsic volcanic unit in the isua supracrustal belt as measured at high precision by the same technique and older than all ages previously determined by mineral and whole rock isotopic techniques for amitsoq gneisses at godthab and at isukasia overgrowths of younger zircon formed at ca 3630 ma coinciding with an episode of major pb loss from the older grains these results indicate that previous whole rock isochron ages refer to later isotopic redistribution and or that other phases of the amitsoq gneisses intruded during this later period and dominate the isotopic systematics comparison of the ion probe results with previous conventional u th pb data for the same zircon sample demonstrates the strong influence of discordant high uranium parts of zircon on the mean isotopic composition of multi grain samples

kca chronology of lunar granites abstract k ca age determinations were undertaken for three lunar granitic samples 14321 1062 14303 206 and 12013 141 k ca isotopic analyses of bulk samples of seven pristine lunar igneous rocks including four mare basalts 10017 15555 14321 1394 and 14305 304 371 one norite 15445 17 one anorthosite 60025 one kreep basalt 72275 543 and seven meteorites juvinas pasamonte moore county stannern y75011 84b zagami and shergotty were also performed the k ca mineral isochron for lunar granitic clast 14321 1062 yields an age of 4 06 0 07 ga for 40k 0 5543 ga 1 and an initial 40 ca 44 ca of 47 141 0 010 normalized to 42 ca 44 ca 0 31221 the k ca age is in excellent agreement with the rb sr and sm nd ages and is slightly older than the average 39ar 40ar and u pb zircon ages low precision k ca ages of 4 04 0 64 ga and 3 76 0 72 ga were obtained for lunar granites 14303 206 and 12013 141 respectively due to their lower k ca ratios initial 40 ca 44 ca values for seven lunar and seven meteoritic bulk samples are similar and yield an average initial 40 ca 44 ca of 47 136 0 002 2 m and 0 006 2 p assuming this value as the lunar initial 40 ca 44 ca a ten to twenty fold fractionation for k ca during the formation of granite 14321 is calculated for a two stage model the large k ca enrichment is consistent with a granite genesis model involving a silicate liquid immiscibility process literature data for lunar granites studied so far exhibit a wide range of ages from 3 9 to 4 4 ga thus lunar granites probably represent samples from about eight different intrusive bodies the positive k and k ca correlations of lunar rocks strongly supports an early global lunar differentiation however most granites were probably not directly produced from this early differentiation event

calcium isotope constraints on the uptake and sources of ca2 in a base poor forest a new concept of combining stable δ44 42ca and radiogenic eca signals abstract plant available reserves of major base cations ca 2 and mg 2 decreased markedly in soils over the past century thus posing a potential threat to forest ecosystem health trees are thought to obtain dissolved ca 2 ions mainly from an easily accessible soil water reservoir also termed the exchangeable cation pool the status of ca reserves in this soil pool is sensitive to anthropogenic perturbations such as soil acidification induced by acid rain and or excessive timber harvesting here we show that in a base poor forest of the northeastern usa i e wachusett mountain massachusetts the exchangeable ca pool of deeper mineral soils has a unique isotope signature that is significantly enriched in the radiogenic 40 ca due to the dissolution of k rich silicate minerals such as biotite using a simple isotope mass balance and assuming that the input of ca from biotite has a e ca signature of 16 the results of our calculation indicate that the weathering of biotite may supply a sizeable fraction up to 25 of ca 2 ions into the exchangeable cation pool of deeper mineral soils importantly samples of local vegetation i e woody tissues of red oak show no detectable excess of the radiogenic 40 ca and based on our model the upper limit of a possible biotite derived ca contribution in vegetation is estimated at 5 we also found no evidence of the radiogenic 40 ca signal in the samples of forest floor and the uppermost organic rich soils 0 15 cm depth which in turn suggest that over the long term development of the forest and its organic matter accumulation the vegetation growth must have also relied primarily on the non radiogenic ca sources based on our experimental data such sources may include i wet atmospheric deposition ii the organically complexed ca in topsoil horizons and iii chemical weathering and or fungal mediated dissolution of apatite and ca rich plagioclase hence our stable and radiogenic ca isotope data indicate that the studied base poor forest is able to bypass the exchangeable cation pool of deeper i e below 15 cm mineral soils and still manages to meet its nutritional requirements with respect to ca another important implication of this study is that the organically complexed ca in the topsoil horizon 0 15 cm depth has to be tightly bound to the ion exchange sites otherwise the large radiogenic 40 ca signatures present in the exchangeable cation pool of deep mineral soils would be swamped by the downward gravitational flux of non radiogenic ca from the decaying organic matter and litterfall hence the limited mobility of the organically complexed ca in soils and its tight biological cycling could explain the lack of a significant impact of vegetation on the ca isotope systematics observed in large rivers

calcium isotopes in deep time potential and limitations abstract calcium is an essential element in the biogeochemical cycles that regulate the long term climate state of earth the removal of co2 from the ocean atmosphere system is controlled by the burial of carbonate sediments caco3 ultimately linking the global calcium and carbon cycles this fundamental link has driven the development of the stable calcium isotope proxy with application to both ancient skeletal and non skeletal bulk carbonate sediments calcium isotope ratios 44 40ca have been used to track long term changes in seawater chemistry e g aragonite vs calcite seas and to elucidate short term climatic perturbations associated with mass extinction events however developments in the calcium isotope proxy have shown that 44 40ca values in carbonate minerals also are sensitive to changes in precipitation rates mineralogy and diagenesis thereby complicating the application of the proxy to the reconstruction of global cycles first inorganic carbonate precipitation experiments have demonstrated that carbonate 44 40ca values are sensitive to precipitation rates with higher rates generally leading to larger fractionation second 44 40ca values are sensitive to carbonate mineralogy with inorganic aragonite and calcite being on average 1 5 and 0 9 depleted relative to contemporaneous seawater respectively the effects of both changes in carbonate mineralogy and precipitation rates affect primary and secondary minerals but are particularly pronounced during carbonate diagenesis where relatively slow rates of recrystallization and neomorphism can lead to significant changes in bulk sediment 44 40ca values third changes in faunal composition expressed in skeletal fossil archives can lead to large changes in carbonate 44 40ca values that are decoupled from changes in global cycles nevertheless when these factors are appropriately considered the application of calcium isotopes in ancient carbonate sediments becomes a powerful tool for understanding biogeochemical processes that operate over many scales from diagenetic changes within the sediment pore space to regional changes across ancient carbonate platforms and to global changes in seawater chemistry through time importantly the processes that contribute to variability in carbonate 44 40ca values are likely to impact other carbonate bound proxies highlighting the potential for calcium isotopes as a tool to better understand the variability of other isotope systems

a rare earth element study of complex zircons from early archaean amitsoq gneisses godthabsfjord south west greenland we present high spatial resolution ion microprobe rare earth element ree data for discrete growth phases of complex polyphase zircons from early archaean amitsoq gneisses outer godthabsfjord sw greenland in matsuda diagrams the two major growth phases 3 8 ga cores and ca 3 65 ga rims have steep positive slopes from la to lu prominent positive ce anomalies and negative eu anomalies that are consistent with growth in a melt exceptions to this are non cathodolurnmescent zircon developed between the cores and rims sometimes truncating zoning in the cores and late archaean prismatic tip overgrowths both of which exhibit flatter light ree lree patterns and have small or no eu anomaly which we interpret as the result of metamorphism and or small degree isolated partial melting our data support previous interpretations that the ca 3 65 ga zircon phase was generated in a melt with the 3 8 ga phase representing either original protolith zircons in a large degree partial melt or inherited zircons in an introduced magma regardless which of these two interpretations is correct for these and similar rocks in the outer godthabsfjord the 3 65 ga event will have profoundly affected isotopic systems and obscured beyond recognition any earlier igneous features such as cross cutting relationships which may only be assigned a minimum 3 65 ga age c 2003 elsevier science b v all rights reserved

ocean warming and gas hydrate stability on the mid norwegian margin at the storegga slide abstract the sensitivity of oceanic gas hydrates and submarine slope stability to the combined forcing of sea level changes and bottom water perturbation is a critical issue for risk assessment in the storegga slide area on the mid norwegian margin evidence for the existence of gas hydrates both inside and outside the storegga slide complex comes from reflection seismic profiles where a bottom simulating reflection marks today s base of the gas hydrate stability zone paleo bottom water temperatures show a relatively fast increase at approximately 12 5 10 ka calendar years following the younger dryas while stable warm water conditions have prevailed since then despite sea level rise this warm water inflow caused a major reduction in the thickness of the gas hydrate stability zone along the upper slope of the mid norwegian margin modelling results indicate that critical hydrate stability conditions and consequently the maximal potential pore pressure build up occur at the location of the storegga slide headwall although the major phase of hydrate melting predates the storegga slide event dated at 8 2 ka calendar years reduced hydrate stability conditions could have facilitated or contributed to sub marine slope failure additionally the bottom simulating reflections within the slide complex seem to have nearly adjusted to new equilibrium conditions highlighting the dynamics of hydrate stability in continental margin sediments under environmental changes climate change geohazards

effect of gas hydrates melting on seafloor slope instability we present a theoretical study of the thermodynamic chemical equilibrium of gas hydrate in soil by taking into account the influence of temperature pressure pore water chemistry and the mean pore size distribution the model uses a new formulation based on the enthalpy form of the law of conservation of energy the developed model shows that due to a temperature and pressure increase hydrates may dissociate at the top of the hydrate occurrence zone to ensure a chemical equilibrium with the surrounding bulk water this original result confirms what has been already shown through experiments the second part of the paper presents an application of the model through a back analysis of the giant storegga slide on the norwegian margin two of the most important changes during and since the last deglaciation hydrostatic pressure due to the change of the sea level and the increase of the sea water temperature were considered in the calculation simulation results show that melting of gas hydrate due to the change of the gas solubility can be at the origin of a retrogressive failure initiated at the lower part of the storegga slope once again the developed model leads to predictions which are supported by laboratory experiment results but contradictory to previous interpretations and beliefs considering that hydrate dissociation occurs only at the bottom of the gas hydrate stability zone

40k 40ca isotopic constraints on the oceanic calcium cycle the contributions of crustal silicates to the oceanic calcium cycle are investigated using high precision 40 k 40 ca measurements in archean and proterozoic carbonates river waters and a series of terrestrial and extraterrestrial mafic ultramafic samples using a multidynamic data collection scheme with strict controls on instrumental mass fractionation we show that a reproducibility of 0 35 e units 2 is obtained for the 40 ca 44 ca ratio using the thermofinnigan triton this represents an improvement by a factor of 3 compared with previous generations of data well defined excesses of 40 ca from 40 k decay were found in river waters draining the himalayas but not in the mississippi and columbia rivers all marine carbonate samples ranging in age from archean to recent show no discernable effects of 40 k decay to within the limit of 0 35 e units 2 in 40 ca 44 ca our results therefore indicate that the ca isotopic composition of seawater has remained constant and indistinguishable from that of the mantle for the past 3 5 ga despite the influx of radiogenic calcium delivered by weathering of the high k ca components of the continental crust thus over most of geologic time the contributions of the high k ca sources from the continents are far below the contributions of hydrothermal sources mass balance constraints indicate that unless the present day contribution of continental silicates is much less than 8 of the river ca flux the total hydrothermal flux of calcium must exceed the input from high temperature vents at ridge axes by at least one order of magnitude the present day oceanic mass balance requires a high input of calcium 4 15 10 12 mol ca yr at low temperature hydrothermal sites

a laboratory investigation into the seismic velocities of methane gas hydrate bearing sand remote seismic methods which measure the compressional wave p wave velocity vp and shear wave s wave velocity vs can be used to assess the distribution and concentration of marine gas hydrates in situ however interpreting seismic data requires an understanding of the seismic properties of hydrate bearing sediments which has proved problematic because of difficulties in recovering intact hydrate bearing sediment samples and in performing valid laboratory tests therefore a dedicated gas hydrate resonant column ghrc was developed to allow pressure and temperature conditions suitable for hydrate formation to be applied to a specimen with subsequent measurement of both vp and vs made at frequencies and strains relevant to marine seismic investigations thirteen sand specimens containing differing amounts of evenly dispersed hydrate were tested the results show a bipartite relationship between velocities and hydrate pore saturation with a marked transition between 3 and 5 hydrate pore saturation for both vp and vs this suggests that methane hydrate initially cements sand grain contacts then infills the pore space these results show in detail for the first time using a resonant column how hydrate cementation affects elastic wave properties in quartz sand this information is valuable for validating theoretical models relating seismic wave propagation in marine sediments to hydrate pore saturation

state of the antarctic and southern ocean climate system this paper reviews developments in our understanding of the state of the antarctic and southern ocean climate and its relation to the global climate system over the last few millennia climate over this and earlier periods has not been stable as evidenced by the occurrence of abrupt changes in atmospheric circulation and temperature recorded in antarctic ice core proxies for past climate two of the most prominent abrupt climate change events are characterized by intensification of the circumpolar westerlies also known as the southern annular mode between 6000 and 5000 years ago and since 1200 1000 years ago following the last of these is a period of major trans antarctic reorganization of atmospheric circulation and temperature between ad1700 and 1850 the two earlier antarctic abrupt climate change events appear linked to but predate by several centuries even more abrupt climate change in the north atlantic and the end of the more recent event is coincident with reorganization of atmospheric circulation in the north pacific improved understanding of such events and of the associations between abrupt climate change events recorded in both hemispheres is critical to predicting the impact and timing of future abrupt climate change events potentially forced by anthropogenic changes in greenhouse gases and aerosols special attention is given to the climate of the past 200 years which was recorded by a network of recently available shallow firn cores and to that of the past 50 years which was monitored by the continuous instrumental record significant regional climate changes have taken place in the antarctic during the past 50 years atmospheric temperatures have increased markedly over the antarctic peninsula linked to nearby ocean warming and intensification of the circumpolar westerlies glaciers are retreating on the peninsula in patagonia on the sub antarctic islands and in west antarctica adjacent to the peninsula the penetration of marine air masses has become more pronounced over parts of west antarctica above the surface the antarctic troposphere has warmed during winter while the stratosphere has cooled year round the upper kilometer of the circumpolar southern ocean has warmed antarctic bottom water across a wide sector off east antarctica has freshened and the densest bottom water in the weddell sea has warmed in contrast to these regional climate changes over most of antarctica near surface temperature and snowfall have not increased significantly during at least the past 50 years and proxy data suggest that the atmospheric circulation over the interior has remained in a similar state for at least the past 200 years furthermore the total sea ice cover around antarctica has exhibited no significant overall change since reliable satellite monitoring began in the late 1970s despite large but compensating regional changes the inhomogeneity of antarctic climate in space and time implies that recent antarctic climate changes are due on the one hand to a combination of strong multi decadal variability and anthropogenic effects and as demonstrated by the paleoclimate record on the other hand to multi decadal to millennial scale and longer natural variability forced through changes in orbital insolation greenhouse gases solar variability ice dynamics and aerosols model projections suggest that over the 21st century the antarctic interior will warm by 3 4 1oc and sea ice extent will decrease by 30 ice sheet models are not yet adequate enough to answer pressing questions about the effect of projected warming on mass balance and sea level considering the potentially major impacts of a warming climate on antarctica vigorous efforts are needed to better understand all aspects of the highly coupled antarctic climate system as well as its influence on the earth s climate and oceans

coastal sedimentation associated with the december 12th 1992 tsunami in flores indonesia this paper presents the result of a detailed granulometric investigation of sediments deposited by a modern tsunami the 1992 tsunami in flores indonesia eyewitness accounts indicate that sediments were deposited upon coastal lowlands over wide areas as a result of the tsunami inundation distinctive vertical and lateral variations in particle size composition are characteristic features of the tsunami deposits and these are intimately related to sedimentary processes associated with flood inundation the geomorphological and sedimentary evidence is used here to establish a preliminary model of tsunami sedimentation this information is believed to be of great value in understanding sedimentary processes associated with tsunami flooding and in the interpretation of palaeo tsunami deposits

controversial pb pb and sm nd isotope results in the early archean isua west greenland oxide iron formation preservation of primary signatures versus secondary disturbances pb stepwise leaching pbsl determinations on two magnetite enriched fractions of a bif sample from the northeastern part of the isua supracrustal belt west greenland yield an isochron of 3691 22 ma mswd 0 4 in combination with previously published geochronological constraints for a minimum deposition age of 3 71 ga for volcanic sequences nutman et al 1997 comprising the studied bifs and supported by microtextural observations this demonstrates an early archean amphibolite facies metamorphic event in the supracrustal pbsl data on magnetite from slightly discordant veins within the same iron stone together with bulk pb isotope values of paragenetically late pyrite yield a 3 63 0 07 ga mswd 8 6 errorchron with significantly different pb isotopic compositions from those obtained from the main magnetite layers and this suggests that the discordant layers of magnetite and sulfides crystallized when externally derived fluids passed through the formation a few tens of million of years later the fluids controlling this redistribution of magnetite can be tentatively correlated with metasomatic alteration features produced during the widespread intrusion of 3 6 ga granitic into 3 7 ga tonalitic amitsoq gneisses enclosing the belt tremolite rich layers in the bif are characterized by sm nd ratios close to chondritic end 3800 of 1 7 and 2 1 within error of published results from adjacent basic units in the supracrustal suite in contrast the magnetite dominated layers yield unrealistically high end 3800 of 14 8 and 14 4 indicative of sm nd ratios resembling ree fractionated continental sources these high end 3800 together with radiogenic sr leached from the magnetite enriched separates is ascribed to secondary hydroxyapatite which predominantly forms as crystal overgrowths in the magnetite rich bands the timing of the hydrothermal event during which apatite was deposited within the bif remains uncertain but a tchur model age of 1 85 ga from the apatite dominated hcl leachate may point to a close genetic relationship with local proterozoic metamorphism and granite formation by crustal remelting in the isukasia area step leaching of magnetite from a similar silicate oxide facies bif from the western part of the isua supracrustal belt yield a pb pb isochron age of 2 84 0 05 ga mswd 1 43 the agreement between the pbsl age and previously published field and isotopic evidence for a major late archean metamorphic event affecting the western area suggests there was widespread equilibration accompanying the intrusion of the 2 75 2 83 ga granodioritic ikkattoq gneisses west and southwest of the western limb of the isua supracrustal belt we argue that the pbsl isochron ages date the peak of amphibolite facies metamorphic events which respectively affected the eastern and western sections of the isua supracrustal belt during the early and late archean our results give additional support to the suggestion that the scatter on published end t values from the isua supracrustal belt and adjoining gneisses can be assigned to post formational hydrothermal processes and underline the need for care in the interpretation of sm nd bulk data from polymetamorphic rocks to constrain isotopic models of early earth s evolution

splay fault branching along the nankai subduction zone seismic reflection profiles reveal steeply landward dipping splay faults in the rupture area of the magnitude m 8 1 tonankai earthquake in the nankai subduction zone these splay faults branch upward from the plate boundary interface that is the subduction zone at a depth of approximately 10 kilometers approximately 50 to 55 kilometers landward of the trough axis breaking through the upper crustal plate slip on the active splay fault may be an important mechanism that accommodates the elastic strain caused by relative plate motion

methane hydrate formation in partially water saturated ottawa sand bulk properties of gas hydrate bearing sediment strongly depend on whether hydrate forms primarily in the pore fluid becomes a load bearing member of the sediment matrix or cements sediment grains our compressional wave speed measurements through partially water saturated methane hydrate bearing ottawa sands suggest hydrate surrounds and cements sediment grains the three ottawa sand packs tested in the gas hydrate and sediment test laboratory instrument ghastli contain 38 1 porosity initially with distilled water saturating 58 31 and 16 of that pore space respectively from the volume of methane gas produced during hydrate dissociation we calculated the hydrate concentration in the pore space to be 70 37 and 20 respectively based on these hydrate concentrations and our measured compressional wave speeds we used a rock physics model to differentiate between potential pore space hydrate distributions model results suggest methane hydrate cements unconsolidated sediment when forming in systems containing an abundant gas phase

the late archaean qorqut granite complex of southern west greenland granites and granite pegmatites composing the 2550 ma qorqut granite complex occur in a ssw nne trending linear belt 150 km long extending through the buksefjorden ameralik godthabsfjord region of southern west greenland the main body of the complex crops out over a distance of 50 km from ameralik to kapisigdlit kangerdluat and reaches a maximum outcrop width of 18 km between storo and qorqut around qorqut the complex comprises three main groups of granites early leucocratic granites various grey biotite granites and late aplogranite granite pegmatites within the 1500 m vertical section available in this area the complex has a tripartite structure comprising a lower zone dominantly of polyphase granite an intermediate zone where country rock occurs as rafts in polyphase granite with a complex sheeted structure and an upper zone dominantly of country rock sheeted by granite fifty two specimens of granite have been analyzed for major minor and some trace elements geochemical variation within the complex is consistent with either fractional crystallization or partial melting but in both cases feldspar biotite must have been involved either as fractionating phases or as residual phases during melting to account for the trace element chemistry two possible models for the generation of the complex are either anatexis of granulite facies rocks in the lower crust following an influx of volatiles and heat from the mantle or melting at intermediate depths of amphibolite facies rocks with volatiles supplied by breakdown of hydrous phases

triaxial compressive properties of artificial methane hydrate bearing sediment 1 knowledge of the mechanical properties of gas hydrate bearing sediments is essential for simulating the geomechanical response to gas extraction from a gas hydrate reservoir in this study drained triaxial compression tests were conducted on artificial methane hydrate bearing sediment samples under hydrate stable temperature pressure conditions toyoura sand average particle size d50 0 230 mm number 7 silica sand d50 0 205 mm and number 8 silica sand d50 0 130 mm were used as the skeleton of each specimen axial loading was conducted at an axial strain rate of 0 1 min 1 at a constant temperature of 278 k the cell and pore pressures were kept constant during axial loading we found that the strength and stiffness of the hydrate sand specimens increased with methane hydrate saturation and with the effective confining pressure and the secant poisson s ratio decreased with the effective confining pressure the stiffness depends on the type of sand forming the skeleton of the specimens although the strength has little dependence on the type of sand according to an earlier work hydrate sand specimens are thought to contract in the early stage of axial loading before starting to expand owing to the dilatancy effect as is the case for many other geological materials the test results in this study are discussed in relation to the deformation mechanism proposed in an earlier work

calcium isotope record of phanerozoic oceans implications for chemical evolution of seawater and its causative mechanisms a total of 280 brachiopods of ordovician to cretaceous age complemented by published data from belemnites and planktonic foraminifera are used to reconstruct the evolution of calcium isotope composition of seawater 44 40casw over the phanerozoic the compiled 44 40casw record shows a general increase from 1 3 nist srm 915a at the beginning of the ordovician to 2 at present superimposed on this trend is a major long term positive excursion from the early carboniferous to early permian as well as several short term mostly negative oscillations a numerical model of the global cycles of calcium carbon magnesium and strontium was used to estimate whether the recorded 44 40casw variations can be explained by varying magnitudes of input and output fluxes of calcium to the oceans the model uses the record of marine 87sr 86sr ratios as proxy for seafloor spreading rates a record of oceanic mg ca ratios to estimate rates of dolomite formation and reconstructed atmospheric co2 discharge and erosion rates to estimate continental weathering fluxes the model results indicate that varying magnitudes of the calcium input and output fluxes cannot explain the observed 44 40casw trends suggesting that the isotope signatures of these fluxes must also have changed as a possible mechanism we suggest variable isotope fractionation in the sedimentary output flux controlled by the dominant mineralogy in marine carbonate deposits i e the oscillating calcite aragonite seas the ultimate control of the calcium isotope budget of the phanerozoic oceans appears to have been tectonic processes specifically variable rates of oceanic crust production that modulated the hydrothermal calcium flux and the oceanic mg ca ratio which in turn controlled the dominant mineralogy of marine carbonates hence the 44 40casw as to the causes of the short term oscillations recorded in the secular 44 40casw trend we tentatively propose that these are related to variable rates of dolomite formation and or to changing chemical composition of the riverine flux in particular and ratios induced by variable proportions of silicate vs carbonate weathering rates on the continents

strain rate dependent hardening softening characteristics of gas hydrate bearing sediments the presence of gas hydrates ghs increases the stiffness and strength of marine sediments in elasto plastic constitutive models it is common to consider gh saturation sh as key internal variable for defining the contribution of ghs to composite soil mechanical behavior however the stress strain behavior of gh bearing sediments ghbs also depends on the microscale distribution of gh and on gh sediment fabrics a thorough analysis of ghbs is difficult because there is no unique relation between sh and gh morphology to improve the understanding of stress strain behavior of ghbs in terms of established soil models this study summarizes results from triaxial compression tests with different sh pore fluids effective confining stresses and strain histories our data indicate that the mechanical behavior of ghbs strongly depends on sh and gh morphology and also on the strain induced alteration of gh sediment fabrics hardening softening characteristics of ghbs are strain rate dependent which suggests that gh sediment fabrics dynamically rearrange during plastic yielding events we hypothesize that rearrangement of gh sediment fabrics through viscous deformation or transient dissociation and reformation of ghs results in kinematic hardening suppressed softening and secondary strength recovery which could potentially mitigate or counteract large strain failure events for constitutive modeling approaches we suggest that strain rate dependent micromechanical effects from alterations of the gh sediment fabrics can be lumped into a nonconstant residual friction parameter we propose simple empirical evolution functions for the mechanical properties and calibrate the model parameters against the experimental data plain language summary gas hydrates ghs are crystalline like solids which are formed from natural gas molecules and water at high pressure and low temperature ghs and particularly methane hydrates are naturally abundant in marine sediments it is known that the presence of gh increases the mechanical stiffness and strength of sediments and there is strong effort in analyzing and quantifying these effects in order to understand potential risks of sediment destabilization or slope failure based on our experimental results from high pressure geotechnical studies we show that not only the initial amount and distribution of gh are important for the increased strength of gh bearing sediments but also the dynamic rearrangement of gh sediment fabrics during deformation characterizes the stress strain response and enables strength recovery after failure we propose that different microstructural mechanisms contribute to this rearrangement and strength recovery of gh sediment however we consider these complicated processes in a simplified manner in an improved numerical model which can be applied for geotechnical risk assessment on larger scales

rupture process of the 1944 tonankai earthquake ms 8 1 from the inversion of teleseismic and regional seismograms 1 we digitized teleseismic and regional records of the 1944 tonankai earthquake we used the multiple time window method to invert these records for the spatial and temporal distribution of slip and rake we assume a 220 140 km fault with a spacing of 20 20 km and a maximum rupture velocity of 2 5 km s the inversion resolved an asperity along the accretionary wedge and under the shima and atsumi peninsulas the rupture propagated from southwest to northeast and there was little slip near the hypocenter consistent with the absence of uplift in the overlying submarine forearc basin the peak slip is 2 3 m and the total seismic moment is 2 4 1028 dyn cm mw 8 2 we compared the predicted and observed vertical geodetic displacements and inferred that the limit of rupture is consistent with the change from subsidence to uplift west of the atsumi peninsula this suggested that the earthquake did not rupture the plate interface in the tokai gap we modeled the tsunami using sea bottom displacements generated from this slip model the tide gauge amplitudes and frequency content are consistent with those predicted resolution tests from inverting synthetic seismograms computed using a simplified slip model indicated that the combination of teleseismic and regional data sets best recovers the asperity locations and peak slips given similar station geometry and degrees of freedom an inversion of the synthetic regional data set indicates that slip artifacts can occur early in the rupture process due to many clipped records while the inversion of synthetic teleseismic data set had less resolution and recovered only 50 of the peak slip

role of critical state framework in understanding geomechanical behavior of methane hydrate bearing sediments a proper understanding of geomechanical behavior of methane hydrate bearing sediments is crucial for sustainable future gas production there are a number of triaxial experiments conducted over synthetic and natural methane hydrate mh bearing sediments and several soil constitutive models have been proposed to describe their behavior however the generality of a sophisticated model is questioned if it is tested only for a limited number of cases furthermore it is difficult to experimentally determine the associated parameters if their physical meanings and significance are not elucidated the objective of this paper is to demonstrate that a simple extension of the critical state framework is sufficient to capture the geomechanical behavior of mh bearing soils from various sources around the world while the significance of each parameter is quantified through variance based global sensitivity analyses our results show that the influence of hydrates can be largely represented by one hydrate dependent parameter pcd which controls the expansion of the initial yield surface this is validated through comparisons with shearing and volumetric response of mh bearing soils tested at various institutes under different confining stresses and with varying degrees of hydrate saturation our study suggests that the behavior of mh bearing soils can be reasonably predicted based on pcd and the conventional critical state parameters of the host sediments that can be obtained through typical geotechnical testing procedures

vestiges of life in the oldest greenland rocks a review of early archean geology in the godthabsfjord region and reappraisal of field evidence for 3850 ma life on akilia abstract the godthabsfjord region of west greenland contains the most extensive best exposed and most intensely studied early archean rocks on earth a geological record has been described of numerous magmatic events between 3 9 and 3 6 ga and evidence of life at 3 85 ga and 3 8 3 7 ga has been proposed from two widely separated localities some of these claims have recently been questioned and the nature of the best preserved remnants of the oldest known terrestrial volcanic and sedimentary rocks in the isua greenstone belt are being reinvestigated and substantially reinterpreted the first part of this article reviews the evolution of geological research and interpretations outlining the techniques by which the geological history has been determined and the ensuing controversies the second part re examines crucial field evidence upon which the antiquity of the oldest terrestrial life is claimed from the island of akilia

precise age determinations and petrogenetic studies using the kca method high precision mass spectrometric determination of calcium isotope ratios allows the 40k 40ca radioactive decay to be used for dating a much broader range of geologic materials than is suggested by previous work 40ca42ca is used to monitor enrichments in 40ca and can be measured to 0 01 2 using an exponential mass discrimination correction russell et al 1978 and large ion currents the earth s mantle has such a low kca 0 01 that it has retained primordial 40ca42ca 151 016 0 011 normalized to 42ca44ca 0 31221 as determined by measurements on two meteorites pyroxene from an ultramafic nodule metabasalt and carbonatite 40ca42ca ratios can be conveniently expressed relative to this value as ca in units of 10 4 to test the method for age dating a mineral isochron has been obtained on a sample of pikes peak granite which has been shown to have concordant k ar rb sr and u pb ages plagioclase k feldspar biotite and whole rock yield an age of 1041 32 m y 2 in agreement with previous age determinations k 0 5543 b y 1 k 0 8952 40k 0 01167 the initial 40ca42ca of 151 024 0 016 ca 0 5 1 0 indicates that assimilation of high k ca crust was insufficient to affect calcium isotopes measurements on two mica granite from eastern nevada indicate that the magma sources had k ca 1 similar to intermediate composition crustal rocks these results show that the k ca system can be used as a precise geochromometer for common felsic igneous and metamorphic rocks and may prove applicable to sedimentary rocks containing authigenic k minerals the relatively short half life of 40k the non volatile daughter and the fact that potassium and calcium are stoichiometric constituents of many minerals make the k ca system complementary to other dating methods and potentially applicable to a variety of geologic problems

corrigendum 146 sm 142 nd evidence from isua metamorphosed sediments for early differentiation of the earth s mantle nature 423 428 432 2003 in this letter the subscripts l and k were accidentally transposed so coefficient a in equation 4 should have read

initial pb of the amitsoq gneiss revisited implications for the timing of early archaean crustal evolution in west greenland reply abstract the rapid change in 207 pb 206 pb ratio during early earth history provides a powerful constraint for temporal resolution of archaean crustal evolution new and published data for amitsoq gneiss feldspars and whole rocks define the earth s least radiogenic silicate pb known until now the most important feature of the data is that independent of the exact slope of the regression in a common pb diagram a very well defined intercept with plausible terrestrial pb evolution lines is obtained between 3 63 and 3 66 ga identical to a combined pb pb regression age of 3654 73 ma mswd 17 6 we conclude that the magmatic precursors of the amitsoq orthogneisses separated from a depleted mantle like source at ca 3 65 ga this contrasts sharply with claims based on ion probe u pb zircon dating that the amitsoq gneisses had a prolonged episodic magmatic evolution extending from ca 3 87 to 3 60 ga pb isotopes offer no evidence that any amitsoq gneiss analysed until now began its evolution in the crust as long ago as 3 85 ga we propose that zircons significantly older than 3 65 ga were inherited from chemically evolved rocks as old as ca 3 85 ga geochemical arguments are presented to support this proposal new feldspar and whole rock pb isotope data are also reported for sheets of quartz dioritic gneiss on akilia island that yield the oldest zircon u pb age clusters in the range 3 84 3 87 ga these sheets are discordant to a metasedimentary enclave containing accessory apatite with graphite inclusions whose c isotope ratios are regarded as biogenic in origin as above pb isotopes in the sheets are consistent with a magmatic age of 3 65 ga which we regard as the true minimum age for these waterlain sediments and their possibly biogenic markers additional pb isotope data as well as limited published sm nd and u pb zircon data on the akilia enclaves themselves suggest an age between ca 3 65 and 3 70 ga for their original deposition some 150 200 ma younger than proposed previously thus the question of overlap of earliest life with a bolide impact scenario terminating 3 80 ga as on the moon becomes irrelevant

paleomagnetism of the santa fe group central brazil implications for the late paleozoic apparent polar wander path for south america 1 paleomagnetic and rockmagnetic data are reported for the floresta formation santa fe group of the sanfranciscana basin central brazil this formation represents the permo carboniferous glacial record of the basin and comprises the brocoto diamictites and flow diamictites brejo do arroz red sandstones and shales with dropstones and invertebrate trails and lavado red sandstones members which crop out near the cities of santa fe de minas and canabrava minas gerais state both brejo do arroz and lavado members were sampled in the vicinities of the two localities alternating field and thermal demagnetizations of 268 samples from 76 sites revealed reversed components of magnetization in all samples in accordance with the permo carboniferous reversed superchron the magnetic carriers are magnetite and hematite with both minerals exhibiting the same magnetization component suggesting a primary origin for the remanence we use the high quality paleomagnetic pole for the santa fe group 330 9 e 65 7 s n 60 95 4 1 k 21 in a revised late carboniferous to early triassic apparent polar wander path for south america on the basis of this result it is shown that an early permian pangea a type fit is possible if better determined paleomagnetic poles become available

sequential closure of k ca and rb sr isotopic systems in archaean micas comparative k ca and rb sr data for extremely radiogenic muscovites eca 2300 to 18 700 87sr 86sr 52 to 991 from two unmetamorphosed archaean pegmatitic rocks from the jack hills in the northwestern part of the yilgarn craton yield model ages ranging from 2580 ma to 2100 ma with the k ca dates being younger than rb sr the muscovites occur as two textural types radial clusters of bladed crystals rosettes in a massive pegmatitic granite and large euhedral books in cross cutting pegmatites there are marked differences between the dates from the two mineral forms as well as between the two isotopic systems and no individual date can be used to provide a point for p t t plots the data can be interpreted collectively in terms of delayed closure of the isotopic systems in a slowly cooled terrane the closure temperature modelling demonstrates that the diffusion coefficient of ca in muscovite is approximately an order of magnitude higher than that of sr a curved assumed exponential cooling trajectory with extremely slow cooling 0 1 c ma 1 at 2100 ma is required to reproduce the youngest k ca dates biotites ca 150 from unmetamorphosed archaean rocks from the southwest of the craton show differences between k ca and rb sr model ages rb sr data from libby and de laeter 1979 that resemble the differences in the jack hills muscovite dates for biotites which have been affected by regional reheating at or uplift until 500 ma the responses of the two isotopic systems are similar but the analysed samples are not sufficiently radiogenic ca 40 to permit identification of any closure sequence muscovites with a common cooling history but differing closure temperatures define empirical cooling curves in 87sr 87rb vs 40ca 40k plots these can be used in conjunction with the k ca rb sr concordia to provide an improved minimum age estimate for the samples when only single samples or very similar samples are available the difference between the k ca and rb sr dates gives a quality control indication of the discrepancy between the rb sr date and crystallisation age

hydrothermal metasomatic and tectono metamorphic processes in the isua supracrustal belt west greenland a multi isotopic investigation of their effects on the earth s oldest oceanic crustal sequence abstract despite superimposed metamorphic overprinting and metasomatic alterations primary volcanic features remain preserved in low strain domains of mafic volcanic sequences in the western isua supracrustal belt isb west greenland these basaltic successions represent the hitherto oldest known fragments of oceanic crust on earth early archean metasomatic fluids rich in light rare earth elements lree th u pb ba and alkalies invaded the supracrustal package and distinctively altered the basaltic sequences field relationships source characteristics traced by pb isotopes and geochronological results provide indications that these fluids were genetically related to the emplacement of tonalite sheets into the isb between 3 81 and 3 74 ga ago subsequent early archean metamorphism homogenized the mixed primary and metasomatic mineral parageneses of these metavolcanic rocks allanite occurs as the most characteristic and critical secondary metasomatic metamorphic phase and is developed in macroscopically discernible zones of increased metsomatic alteration even in domains of low strain because of its high concentration of lree th and u this secondary mineral accounts for much of the disturbances recorded by the sm nd and th u pb isotope systematics of the pillowed metabasalts the supracrustal sequences were tectono metamorphically affected to varying degrees during a late archean 2 6 to 2 8 ga old event also recognized in the adjacent gneiss terranes of the isuakasia area the degree to which bulk rocks were isotopically reequilibrated is directly dependent on the different relative contributions of allanite hosted parent daughter elements to the overall whole rock mass budget of the respective isotope systems although low strained initially only weakly metasomatized pillow basalts remained more or less closed with respect to the u pb and rb sr systems since 3 74 ga the sm nd system appears to have been partially opened on a whole rock scale during the late archean event this diversified behavior of the whole rock isotope systems with respect to late archean overprinting is explained by the combination of mass budget contributions of the respective elements added during metasomatism and the partial opening of metasomatic macroenvironments during late archean recrystallization processes with associated renewed fluid flow in reactivated zones of high strain where primary metasomatic alteration is most prominently developed late archean partial resetting also of the u pb isotope system on a whole rock scale occurred this is consistent with an apparent late archean age of kyanite which initially crystallized during the early archean metamorphism its age is controlled by the u pb systematics of allanite inclusions which have exchanged their isotopic properties during the tectono metamorphic event that overprinted the oceanic crustal sequence at isua more than 1000 myr later these results underline the need for care in the interpretation of whole rock geochemical data from polymetamorphic rocks in general and from the isua oceanic crustal sequences in particular to constrain isotopic models of early earth s evolution likewise this study cautions against the indiscriminate use of geochemical data of metavolcanic rocks from isua to infer models for geotectonic settings relevant for their formation

increased marine sediment suspension and fluxes following an earthquake earthquakes are commonly cited as one possible triggering mechanism for turbidity flows dense sediment water plumes that can transport large volumes of sediment great distances down slope in both marine and lacustrine settings1 2 3 4 5 6 heezen and ewing1 were the first to make such a suggestion attributing breaks in a sea floor telephone cable in the north atlantic ocean to turbidity flows generated by the 1929 grand banks earthquake anumber of workers have consequently used sedimentary turbidite records to reconstruct the earthquake histories of various regions2 7 8 here we present direct observations of a seismically induced turbidity flow measurements of light scattering and sediment fluxes in the cariaco basin indicate that the earthquake that occurred along the coast of northern venezuela on 9 july 1997 resulted in considerable downslope displacement of sediments probably 105 tonnes into the deep part of the basin in such a seismically active region this mechanism of sediment transport may be responsible for a significant component of the long term sediment accumulation in the basin furthermore this process may result in the sequestration in deep sea sediments of large amounts of carbon initially deposited at shallow depths

physical properties of hydrate bearing sediments 1 methane gas hydrates crystalline inclusion compounds formed from methane and water are found in marine continental margin and permafrost sediments worldwide this article reviews the current understanding of phenomena involved in gas hydrate formation and the physical properties of hydrate bearing sediments formation phenomena include pore scale habit solubility spatial variability and host sediment aggregate properties physical properties include thermal properties permeability electrical conductivity and permittivity small strain elastic p and s wave velocities shear strength and volume changes resulting from hydrate dissociation the magnitudes and interdependencies of these properties are critically important for predicting and quantifying macroscale responses of hydrate bearing sediments to changes in mechanical thermal or chemical boundary conditions these predictions are vital for mitigating borehole local and regional slope stability hazards optimizing recovery techniques for extracting methane from hydrate bearing sediments or sequestering carbon dioxide in gas hydrate and evaluating the role of gas hydrate in the global carbon cycle

experimental verification of a prediction model for hydrate bearing sand this paper presents an experimental verification of a prediction model for the mechanical properties of hydrate bearing sand the model is examined using experimental drained triaxial test results of three independent data sets which are associated with different hydrate formations and testing conditions for each data set an optimization process is applied based on numerical modeling of the testing conditions in order to evaluate the pure sand properties based on these properties the model forecasts the stress stain curves for different hydrate saturations based on an advanced hydrate simulator although the model does not show a fair forecast with ice seeding hydrate formation samples and for specimens tested under gas saturation it predicts the mechanical behavior of samples with partly saturated hydrate formation tested under water saturation

detailed coseismic slip distribution of the 1944 tonankai earthquake estimated from tsunami waveforms coseismic slip distribution on the fault plane of the 1944 tonankai earthquake is estimated from inversion of tsunami waveforms three improvements from a previous study satake 1993 are made these are 1 smaller subfaults are used to resolve detailed slip distribution 2 the sub faults fit better to the plate interface geometry and 3 finer and more accurate bathymetry data is used the inversion result shows that a maximum slip of about 3 m occurred on the plate interface off shima peninsula the total seismic moment is estimated to be 2 0 1021 nm mw 8 2 the result confirms that the 1944 tonankai earthquake did not rupture the plate interface beneath the tokai region and supports the existence of the seismic gap in the tokai region suggested by ishibashi 1981 the slip of about 1 5 m on the plate interface beneath atsumi peninsula northeast of the large slip area is necessary to explain the observed tsunami waveforms although no seismic moment release was estimated from strong motion data by kikuchi et al 1999 this may suggest that the rupture beneath atsumi peninsula was slow

holocene mass wasting on upper non polar continental slopes due to post glacial ocean warming and hydrate dissociation 1 incorporating both late post glacial bottom water warming and eustatic sealevel rise into a mh methane hydrate stability model explains why at least some major submarine landslides e g the storegga slide norwegian margin may have been triggered by mh dissociation during the early holocene rising sealevels not during the lowest sealevels of the lgm last glacial maximum 18 20 ka in the case of storegga failure may have been initiated either below the upper slope or under the shelf for fresh pore water especially with admixed ethane at water depths below ca 800m persistent low bottom water temperatures allowed the sealevel rise to thicken the mh stability zone over time ruling out holocene initiation of failure mh release by holocene mass wasting cannot have initiated deglaciation

calcium isotopes in igneous rocks and the origin of granite abstract the k ca radioactive parent daughter system provides a tool for tracing the origins of igneous rocks it is complementary to other isotopic systems because as stoichiometric constituents of major minerals the concentrations of k and ca and the k ca ratio in rocks are simply related to mineralogy in this paper we report the first high precision calcium isotopic analyses of continental granitic rocks island arc rocks and mid ocean ridge basalts these data show that mid ocean ridge basalts have the low 40 ca 42 ca ratios expected for the earth s mantle but that island arc rocks have slightly higher 40 ca 42 ca ratios indicative of crustal calcium in their magma sources many granitic rocks have high initial 40 ca 42 ca ratios and in conjunction with independent evidence for the age of the crustal sources these ratios provide constraints on the k ca ratios and in turn on the silica contents and residual mineralogy of the deep crustal magma sources

formation of gas hydrate from dissolved gas in natural porous media abstract migration of methane bearing fluids from the depth is commonly proposed to explain the formation of marine gas hydrate this formation mechanism is viable only if the gas hydrate can nucleate and grow directly from an aqueous solution while phase equilibrium calculations suggest that the gas hydrate should crystallize from the dissolved gas barriers associated with nucleation may impede or even prevent hydrate formation at present experimental support for the phase equilibrium calculations is insufficient to confirm whether the hydrate can form in the sea floor from the dissolved gas here we report on a set of experiments in which the hydrate is crystallized from dissolved co 2 in natural porous media the experiments demonstrate that the hydrate crystals can nucleate in the absence of free gas providing important experimental justification for models of hydrate formation that depend on fluid migration our results are also relevant for other formation models that depend on in situ methane generation from biogenic sources experimental evidence for hydrate formation at low gas concentrations implies less stringent requirements on the biogenic sources of methane compared with some previous estimates