formation and age of sphalerite mineralization in carbonate rocks of bajocian age in the swiss jura mountains evidence of mesozoic hydrothermal activity a combination of petrographic and geochemical techniques was applied to better constrain the origin and evolution of the fluid systems responsible for the formation of disseminated cd rich up to 0 6 wt sphalerite zns mineralization in the northeastern part of the jura mountains switzerland the rb sr ages of sphalerite samples indicate that a main phase of sphalerite formation occurred near the boundary between the late middle and early late jurassic at around 162 ma the negative 34s values 22 3 to 5 3 suggest that biogenic sulfide sulfur was involved in zns precipitation the strontium isotope composition is more radiogenic than that of contemporaneous seawater reflecting the interaction of mineralizing fluids with silicate rocks lead isotope signatures are very uniform 206pb 204pb 18 63 18 67 207pb 204pb 15 63 15 64 208pb 204pb 38 51 38 63 indicating an isotopically well homogenized fluid system the basement rocks underlying the jurassic strata are considered to be the main source of metals for the sphalerite mineralization the migration of deep sourced hydrothermal saline metal bearing fluids into the bajocian host carbonates containing sedimentary reduced sulfur resulted in the precipitation of sulfides the period of sphalerite formation near the middle late jurassic boundary is characterized by enhanced tectonic and hydrothermal activity in europe related to the opening of the central atlantic and tectonic thermal subsidence during spreading of the alpine tethys our study provides evidence that the bajocian carbonate rocks in the jura mountains area were affected by the circulation of deep sourced metal bearing hydrothermal fluids in response to these continent wide tectonothermal events the presence of sphalerite mineralization and associated geochemical anomalies in zn and cd contents in carbonate rocks may also be used to trace basement features

mean age of oceanic lithosphere drives eustatic sea level change since pangea breakup abstract the atlantic and indian oceans and the oceanic part of the antarctic plate have formed at the expense of panthalassa as a result of pangea breakup over the last 180 myr this major plate reorganization has changed the age vs surface distribution of oceanic lithosphere and has been a likely driver of sea level change assuming that the age surface structure of panthalassa has remained similar to the present day global distribution from 180 ma to present and using the isochron patterns preserved in the newly formed oceans we model resulting relative sea level change we find a first slower phase of sea level rise by 90 to 110 m culminating between 120 and 50 ma followed by a faster phase of sea level drop we show that this result is not strongly sensitive to our hypothesis of constant mean age of panthalassa for which much of the information is now erased due to subduction when the effects of oceanic plateau formation and ice cap development are added the predicted sea level curve fits remarkably well the first order variations of observed sea level change we conclude that the changes in mean age of the oceanic lithosphere varying between 56 and 62 0 2 myr which are simply the expression of the wilson cycle following pangea breakup are the main control accounting for 70 of first order changes in sea level

the tectonically controlled emplacement of a vertically sheeted gabbro pyroxenite intrusion feeder zone of an ocean island volcano fuerteventura canary islands abstract the miocene px1 gabbro pyroxenite pluton fuerteventura canary islands is a 3 5 5 5 km shallow level intrusion 0 15 0 2 gpa and 1100 1120 c interpreted as the feeder zone to an ocean island volcano it displays a vertical magmatic banding expressed in five 50 to 100 metre wide nne ssw trending alkaline gabbro sequences alternating with pyroxenites this emplacement geometry was controlled by brittle to ductile shear zones generated by a regional e w extensional tectonic setting that affected fuerteventura during the miocene at a smaller scale the px1 gabbro and pyroxenite bands consist of metre thick differentiation units which suggest emplacement by periodic injection of magma pulses as vertical dykes that amalgamated similarly to a sub volcanic sheeted dyke complex individual dykes underwent internal differentiation following a solidification front parallel to the dyke edges this solidification front may have been favoured by a significant lateral horizontal thermal gradient expressed by the vertical banding in the gabbros the fractionation asymmetry within individual dykes and the migmatisation of the wall rocks pyroxenitic layers result from the fractionation and accumulation of clinopyroxene olivine plagioclase crystals from a mildly alkaline basaltic liquid they are interpreted as truncated differentiation sequences from which residual melts were extracted at various stages of their chemical evolution by subsequent dyke intrusions either next to or within the crystallising unit compaction and squeezing of the crystal mush is ascribed to the incoming and inflating magma pulses the expelled interstitial liquid was likely collected and erupted along with the magma flowing through the newly injected dykes clinopyroxene mineral orientation as evidenced by ebsd and micro x ray tomography investigations displays a marked pure shear component supporting the interpretation of the role of compaction in the generation of the pyroxenites conversely gabbro sequences underwent minor melt extraction and are believed to represent crystallised coalesced magma batches emplaced at lower rates at the end of eruptive cycles clinopyroxene orientations in gabbros record a simple shear component suggesting syn magmatic deformation parallel to observed nne ssw trending shear zones induced by the regional tensional stress field this emplacement model implies a crystallisation time of 1 to 5 years for individual dykes consistent with px1 emplacement over less than 0 5 my a minimum amount of approximately 150 km3 of magma is needed to generate the pluton part of it having been erupted through the central volcanic centre of fuerteventura if the regional extensional tectonic regime controls the px1 feeder zone initiation and overall geometry rates and volumes of magma depend on other source related factors high injection rates are likely to induce intrusion growth rates larger than could be accommodated by the regional extension in this case dyke intrusion by propagation of a weak tip combined with the inability of magma to circulate through previously emplaced and crystallised dykes could result in an increase of non lithostatic pressure on previously emplaced mushy dyke walls thus generating strong pure shear compaction within the pluton feeder zone and interstitial melt expulsion these compaction dominated processes are recorded by the cumulitic pyroxenite bands

oceanwide imprint of large tectonic and oceanic events on seawater nd isotope composition in the indian ocean from 90 to 40 ma 1 we have analyzed four sediment cores from the southern indian ocean odp sites 757 758 1135 and 762 with high carbonate content in order to reconstruct the neodymium isotopic composition end of ancient intermediate south indian seawater from late cretaceous 90 ma to early eocene 40 ma the endvariations are highly consistent and exhibit reproducible patterns over a very large geographic area confirming the seawater origin of the signal combining geochemical constraints with paleogeographic reconstructions we highlight the respective roles of 1 large scale tectonic events 2 continental weathering from surrounding precambrian terrains 90 65 ma 3 oceanic circulation changes 50 40 ma and possibly 4 local volcanism of the ultra fast spreading south east indian ridge seir 60 50 ma on the nd isotopic composition of south indian seawater between 60 ma and 50 ma the regional nd isotopic variations closely mimic changes in seir spreading rate we suggest that the nd isotopic composition of seawater could be influenced by nd of volcanic origin in the vicinity of ultra fast spreading ridges 13 cm yr the india asia collision closed the equatorial seaway between asia and india and drastically changed oceanic circulation patterns in the indian ocean warm and more radiogenic pacific equatorial seawater was diverted to the south by the east indian coast a stronger mixing of this pacific seawater with south indian seawater would explain the rapid shift ofend from 50 ma 11 to 40 ma 8

the rates of plate creation and consumption summary an alternative vector derivation of the expression for the rates of plate creation and consumption is given from which it can be seen that the rates are proportional to the scalar product of the edges of the geohedron and plate polyhedron corresponding to the same boundary the plate polyhedron is defined by the set of triple junction vectors this is used together with a recent determination of the angular velocity vectors and a number of auxiliary plate boundary vectors to revise the rates of plate creation and consumption along sections of plate boundaries all plates which have a substantial proportion of their boundary connected to a downgoing slab are found to be decreasing in area the total rate of plate generation is used in estimates of the area of the ocean floor and the total heat loss from the oceans the rate of consumption within a relatively small area around new guinea implies a large mass flux away from this region knowledge of the rates of plate creation and consumption is used in a number of different areas calculating the contribution of plate creation to the total heat loss of the earth calculating geochemical fluxes out of the mantle such as for the rare gases and other volatiles and return fluxes back into the mantle resulting from reactions between sea water and the ocean crust during hydrothermal circulation calculating the area of the ocean floor as a function of age mass fluxes related to the distribution of the rates of creation and destruction along the plate boundaries are important boundary conditions on the flow pattern in the mantle previous estimates of these rates deffeyes 1970 chase 1972 garfunkel 1975 have applied a simple expression derived for spreading ridges deffeyes 1970 using angular distances from the poles of rotation measured graphically on a globe an alternative derivation of the expression for rates of plate creation and consumption which is independent of assumptions about the nature of the boundary is given below in terms of the framework used to describe plate motions by mckenzie parker 1967 1974 this derivation emphasizes that the rates depend on two sets of vectors one the instantaneous rotation vectors defining the relative plate motions the other a set of triple junction and auxiliary vectors defining the plate boundaries

crustal structure of the french guiana margin west equatorial atlantic summary geophysical data from the amazon cone experiment are used to determine the structure and evolution of the french guiana and northeast brazil continental margin and to better understand the origin and development of along margin segmentation a 427 km long combined multichannel reflection and wide angle refraction seismic profile acquired across the southern french guiana margin is interpreted where plate reconstructions suggest a rift type setting the resulting model shows a crustal structure in which 35 37 km thick pre rift continental crust is thinned by a factor of 6 4 over a distance of 70 km associated with continental breakup and the initiation and establishment of seafloor spreading the ocean continent boundary is a transition zone up to 45 km in width in which the two layered oceanic type crustal structure develops although relatively thin at 3 5 5 0 km such thin oceanic crust appears characteristic of the margin as a whole there is no evidence of rift related magmatism either as seaward dipping sequences in the reflection data or as a high velocity region in the lower crust in the p wave velocity model and as a such the margin is identified as non volcanic in type however there is also no evidence of the rotated fault block and graben structures characteristic of rifted margins consequently the thin oceanic crust the rapidity of continental crustal thinning and the absence of characteristic riftrelated structures leads to the conclusion that the southern french guiana margin has instead developed in an oblique rift setting in which transform motion also played a significant role in the evolution of the resulting crustal structure and along margin segmentation in structural style

high mantle temperature during cretaceous avalanche abstract a new sequence of axisymmetrical spherical mantle convection simulations has been computed to monitor the influence of mantle avalanches on the mantle thermal state during an avalanche that is characterized by a high rate of mixing between upper mantle and lower mantle a complex perturbation of the thermal field occurs due to the advection of cold upper mantle into the lower mantle and the hot return flows from the lower mantle the computation shows that the avalanches trigger global upper mantle warming of several tens of degrees and make it possible to explain both core mantle boundary and transition zone origins for mantle plumes recently the chemical compositions of dsdp odp drill holes on crust older than 80 myr have been interpreted to reflect a higher mean mantle temperature during the mesozoic humler et al earth planet sci lett 173 1999 7 23 here we have used and extended the data set for oceanic crust younger than 80 myr combining the mean chemistry for old oceanic crust and the mean depth of the ocean basin versus the square root of age we propose a continuous curve for the temperature evolution of the upper mantle for the last 130 myr it shows a mantle thermal high around 125 ma with amplitude 50 k and duration 30 myr comparable with those obtained from theoretical convection models it is proposed that such a high mantle temperature may be due to the thermal effects of an avalanche which may have started 180 myr ago

eustasy as a test of a cretaceous superplume hypothesis the mantle plume model has been used to explain a variety of geological and geodynamic events for instance a large plume or superplume under the pacific basin has been proposed to account for a number of cretaceous events such as a global sealevel rise an increase in global average temperature and a pause in the reversals of the earth s magnetic field 1 2 the primary evidence for this hypothesis is a model for the rate of production of ocean ridge material and ocean plateaus with a high from about 120 80 ma mostly due to activity in the pacific basin which is claimed to have been driven by the upwelling of a large plume from the core mantle boundary the hypothesis requires that a large part of the primary evidence for this high productivity has disappeared by subduction here we test this hypothesis by comparing the cretaceous eustatic sealevel highstand which would result from the superplume model with generally accepted values our model includes estimates of eustatic sealevel change resulting from phenomena explicitly specified for the proposed plume such as the volume of ocean crust produced and the extent of lithospheric swelling associated with the plume head as well as other events known to affect eustatic sealevel such as glaciation and continental collision the estimated cretaceous highstand resulting from all the modeled effects is 220 470 m compared to the observed value of 120 200 m this discrepancy indicates a probable overestimate of the rate of seafloor and plateau creation and of the size of plume that could have existed the breakup of the pangean supercontinent is a more viable explanation of the cretaceous sealevel rise

crustal volumes of the continents and of oceanic and continental submarine plateaus global topographic data and the assumption of airy isostasy have been used to estimate the crustal volumes of the continents and the oceanic and continental submarine plateaus the calculated crustal volumes are 7182 x lo6 km3 for the continents 242 x lo6 km3 for continental submarine plateaus and 369 x lo6 km3 for oceanic plateaus the falkland plateau and the lord howe rise are the two largest continental submarine plateaus with volumes of 48 x lo6 km3 and 47 x lo6 km3 respectively total continental crustal volume is 7581 x lo6 km3 including the volume of continental sediments on the ocean floor 160 x lo6 km3 in good agreement with previous estimates continental submarine plateaus on the seafloor comprise 3 2 of the total continental crustal volume the largest oceanic plateaus in order of decreasing size are the ontong java plateau the kerguelen plateau the caribbean the chagos laccadive ridge the ninetyeast ridge and the mid pacific mountains together they comprise 54 of the total anomalous crustal volume in oceanic plateaus an upper bound to the continental crust addition rate by the accretion of oceanic plateaus is 3 7 km3 yr a value that assumes accretion of all oceanic plateaus with a total volume of 4 9 of the continental crustal volume on a 100 myr time scale even if a substantial fraction of the crustal volume in oceanic plateaus is subducted accretion of oceanic plateaus could make a contribution to continental growth since the upper bound to the addition rate exceeds recent estimates of the island arc addition rate subduction of continental submarine plateaus with the oceanic lithosphere on a 100 myr time scale gives an upper bound to the continental crustal subtraction rate of 2 4 km3 yr much larger than recent estimates of crustal subtraction by subduction of seafloor sediments effective subduction of all oceanic plateaus implies equally effective subduction of continental submarine plateaus a potentially important way to recycle continental crust back into the mantle may be the break off of small fragments from the continents entrapment of the continental fragments in the seafloor and subduction of the fragments with the oceanic lithosphere this process may be occurring in the mediterranean for corsica and sardinia

re os results from odp site 801 evidence for extensive re uptake during alteration of oceanic crust abstract re and os concentrations and os isotopic ratios were determined for composite samples prepared from volcanoclastics vcl and basaltic flows flo from jurassic oceanic crust ocean drilling program leg 185 site 801 in the western pacific with the aim of determining the effect of seafloor weathering on the re os budget a supercomposite sample prepared from a proportionate mixture of the various composite powders served to represent the average composition of the altered oceanic crust kelley k a plank t ludden j and staudigel h 2003 composition of altered oceanic crust at odp sites 801 and 1149 geochem geophys geosyst 4 6 8910 doi 10 1029 2002gc000435 re contents vary from 0 2 to 1 3 ng g 1 and from 2 2 to 3 1 ng g 1 in the vcl and flo composites respectively os contents vary from 0 005 to 0 047 ng g 1 in the vcl and from 0 008 to 0 027 ng g 1 in the flo composites the flo composites have much higher re os ratios and thus have more radiogenic os compositions 187os 188os 1 38 to 8 48 than the vcl composites 187os 188os 0 32 to 4 40 the vcl composite from the upper section of the crust shows evidence for substantial re loss and os uptake consistent with oxidative weathering processes however re uptake during weathering processes under more reducing conditions evident in the flo samples from throughout the section and to a lesser extent in the lower vcl samples more than compensates for this re loss in the upper vcl os concentrations were essentially unchanged by these reductive processes model age calculations suggest that re uptake continued for tens of millions of years after crust formation abundant secondary pyrite is found throughout the altered hole 801c crust in zones of restricted seawater flow and this may have accommodated an important part of the input re the re content of the supercomposite 2 2 ng g 1 is about 1 ng g 1 higher than would be expected on the basis of its yb content if the results from hole 801c are typical they suggest that the re concentration of at least the upper part of the oceanic crust may be nearly doubled during seafloor alteration such large extents of re uptake would have a significant effect on the oceanic re budget furthermore assuming that they survive passage through the subduction zone these elevated re contents would greatly decrease the proportion of subducted oceanic crust required in the source region to explain the radiogenic os compositions of many ocean island basalts

digital isochrons of the world s ocean floor we have created a digital age grid of the ocean floor with a grid node interval of 6 arc min using a self consistent set of global isochrons and associated plate reconstruction poles the age at each grid node was determined by linear interpolation between adjacent isochrons in the direction of spreading ages for ocean floor between the oldest identified magnetic anomalies and continental crust were interpolated by estimating the ages of passive continental margin segments from geological data and published plate models we have constructed an age grid with error estimates for each grid cell as a function of 1 the error of ocean floor ages identified from magnetic anomalies along ship tracks and the age of the corresponding grid cells in our age grid 2 the distance of a given grid cell to the nearest magnetic anomaly identification and 3 the gradient of the age grid i e larger errors are associated with high age gradients at fracture zones or other age discontinuities future applications of this digital grid include studies of the thermal and elastic structure of the lithosphere the heat loss of the earth ridge push forces through time asymmetry of spreading and providing constraints for seismic tomography and mantle convection models

the impact of the central atlantic magmatic province on climate and on the sr and os isotope evolution of seawater abstract there is a wide range of geochemical sedimentological and palaeontological evidence suggesting that the interval at the end of the triassic and the beginning of the jurassic some 200 ma ago was a time of major environmental change the supercontinent pangaea began to rift eustatic sea level rose and there was a mass extinction in marine and terrestrial biota the eruption of at least 3 10 6 km 3 of basaltic lavas that comprise the central atlantic magmatic province camp also took place at that time and was intimately associated with the onset of rifting of pangaea emplacement of this large igneous province occurred within a relatively very short period of perhaps no more than 1 ma although relatively little of camp now remains the original province was extremely large extending from present day nova scotia to bolivia and from central southern usa to morocco of major relevance to the environmental impact of the camp was its relatively rapid emplacement and its original location across the equator recent advances in our understanding of the weathering of silicate rocks and in particular of fresh basaltic rocks have highlighted the importance of the weathering process in influencing environmental change in this contribution we examine the details of the seawater sr and os isotope records that span the triassic jurassic boundary in order to understand the nature and course of environmental change at that time correlation of sr isotope data from mainland europe and the uk shows that the rapid rate of decrease in the 87 sr 86 sr ratio of seawater which had started in the early rhaetian late triassic came to an end in the late rhaetian close to the triassic jurassic boundary at that time the 187 os 188 os ratio of seawater had also decreased to particularly low values in the late rhaetian the seawater 87 sr 86 sr ratio rapidly increased from 0 70765 to 0 70775 and then it and the seawater 187 os 188 os ratio both remained approximately constant for the duration of the hettangian the first 3 ma of the jurassic apart from a transient excursion to radiogenic os isotope values i e high 187 os 188 os ratios near the triassic jurassic boundary the unradiogenic 187 os 188 os ratio of seawater during the hettangian is interpreted as reflecting a major perturbation to global weathering patterns that was caused by the rapid weathering and erosion of the camp immediately after its emplacement we suggest that the rapid increase of seawater 87 sr 86 sr ratios in the late rhaetian and the approximately constant 87 sr 86 sr ratios throughout the hettangian were caused by the input of a large amount of additional radiogenic sr to the oceans which was likely to have been derived from a readily weathered lithology such as carbonate or evaporite at the start of the sinemurian the 87 sr 86 sr ratio of seawater began to decrease while at the same time the 187 os 188 os ratio of seawater increased to more radiogenic values we interpret the trends in the seawater sr and os isotope compositions at the start of the sinemurian as evidence that much of the camp had been removed by chemical weathering by which time the camp had ceased to have a major influence on earth s environment the patterns of change in the seawater sr and os isotope records that occurred in the latest triassic and earliest jurassic show many similarities with those that took place in the pliensbachian toarcian 183 ma ago overlapping the eruption of the karoo ferrar igneous province in both cases the seawater sr and os isotope records indicate that there was a relatively very brief interval during which rates of continental weathering increased greatly the similarities in the responses of the seawater sr and os isotope records at those times strongly suggest that the eruption and subsequent erosion of basaltic large igneous provinces have played a major role in defining the course of environmental change in the past

a mesozoic time scale we present an integrated geomagnetic polarity and stratigraphic time scale for the triassic jurassic and cretaceous periods of the mesozoic era with age estimates and uncertainty limits for stage boundaries the time scale uses a suite of 324 radiomenc dates including high resolution 40 ar 39 ar age estimates this framework involves the observed ties between 1 radiometric dates biozones and stage boundaries and 2 between biozones and magnetic reversals on the seafloor and in sediments interpolation techniques include maximum likelihood estimation smoothing cubic spline fitting and magnetochronology

depth versus age new perspectives from the chemical compositions of ancient crust petrological data provide a new approach to an evaluation of the depth age problem for ancient seafloor the correlations among basalt chemical composition axial depth and mantle temperature at current ocean ridges allow the determination of initial depth and mantle temperature for any portion of ancient seafloor that was created at a spreading center provided the chemical composition of the ancient crust is determined it is then possible to calculate a petrologically constrained depth at any age which can be compared to observed depths and depths from the classical half space models we evaluate data from dsdp and odp drill holes on crust older than 80 ma considering chemical composition back tracked depth and crustal thickness the data are complex and interpretation of their chemical composition requires consideration of alteration absence of glass compositions data quality and the influence of off axis volcanism and near ridge hot spots to check and expand the data set we develop and use trace element proxies for major element compositions since many trace element ratios are less influenced by alteration and by variable proportions of phenocrysts the twenty drill holes for which reliable data can be obtained are well distributed around the globe and include multiple sites on old crust in the atlantic pacific and indian ocean basins comparison of the chemical and crustal distributions between ancient and current n morb show that the oceanic crust older than 80 ma has significantly lower na 8 0 z r y sm ybn and higher cao al2o3 f e 8 0 and crustal thickness quantitative modeling of these results suggests that the mantle was hotter in this time period by about 50oc that the crust was several hundred meters shallower and 1 2 km thicker these observations show that half to two thirds of the observed flattening relative to a half space model is due to the change in mantle temperature and crustal composition thus only a few hundred meters of flattening by plate reheating by hot spots or by other mechanisms is required these results are consistent with the existence of abundant oceanic plateaus even at fast spreading rates in the mesozoic and with the apparent thickening of ocean crust with time 1999 elsevier science b v all rights reserved

mammoth mammuthus sp excavation on a college campus in western illinois usa abstract the discovery of the remains of a mammoth mammuthus sp on the principia college campus in elsah illinois has allowed for careful excavation by students over several years the mammoth is relatively well preserved in late wisconsin peoria silt on the top of the bluffs of the mississippi river the mammoth was buried by loess perhaps as a result of persistent dust storms approximately 17 500 14 c years ago this age estimate is based on the mammoth s stratigraphic position within the peoria silt and is loosely supported by an ams radiocarbon age on poorly preserved collagen from tooth dentine 17 810 4300 14 c years cams 104172 over one hundred bones or bone pieces have been excavated to date the skull containing the tusks and maxillary teeth is complete and in close proximity to associated postcranial remains indicating that the mammoth died where it was found the size and characteristics of skeletal elements have allowed us to determine that this was a mature male of about 41 years of age taxonomic attribution either to mammuthus primigenius or mammuthus jeffersonii is pending based on the outcome of thorough analysis once the skull with dentition has been prepared from enclosing matrix

detailed record of the mid oxfordian late jurassic positive carbon isotope excursion in two hemipelagic sections france and switzerland a plate tectonic trigger the oxfordian late jurassic was a time of widespread change in jurassic marine carbonate sedimentation patterns a marked positive excursion in 13 c is dated as middle oxfordian in age in this study we investigate if changes in carbonate sedimentation coincided with altered carbon cycling and climate we use c isotope records as a proxy for the evolution of the carbon cycle and compare 13 c trends with the evolution of sedimentation in a segment of the opening tethys seaway one of the studied sections is located in the subalpine basin of france trescleoux and oze the other in the swiss jura mountains liesberg carbon isotope stratigraphy of carbonate carbon locates the peak values of the middle oxfordian positive excursion to theantecedens andparandieri subzonesoftheplicatilisandtransversariumammonitezones respectively causesofthe excursionremain enigmatic theexcursion seems not to coincide with a global oceanic anoxic event but regionally enhanced organic carbon accumulation during the early and early middle oxfordian may be the cause of the increase in 13 c the excursion occurs during a time of progressive climate warming anditspeakvaluescoincidewiththefirstcalcareoussedimentsrecurringafteraperiodofreducedcarbonateaccumulationintheearly and early middle oxfordian the excursion is also time equivalent to a major reorganisation of global ocean currents the opening atlantic and tethys oceans combined with rising sea level led to the formation of an efficient circumglobal seaway we conclude that this favoured the widespread recurrence of carbonate producers by providing new habitats as a result ccarb burial increased leading to a lower corg ccarb burial rate and therefore to stabilisation and decrease in 13 c in the middle oxfordian thus the mid oxfordian positive excursion in carbonate c isotopes may reflect changes in the carbon cycle that were triggered by a copious reorganisation of the ocean current system caused by major plate tectonic movements 2007 elsevier b v all rights reserved

constraints on the structure of the east pacific rise from seismic refraction data university of washington seismic data taken during rose phase 1 have been used to study the structure of the east pacific rise in the vicinity of 12 n off axis but within 50 km of the axis we find that the oceanic crust can be modeled with a rapid change in velocity in the top 2 5 km a zone of low but uniform velocity gradient from 2 5 to about 5 5 km and a transitional zone about 1 km thick to mantle material there is also evidence for substantial anisotropy in mantle velocity gradients with a positive velocity gradient in the spreading direction and a negative velocity gradient parallel to the rise under the rise axis the data allow at most a very narrow zone of low velocity less than about 2 km wide in the crust

temporal variation of oceanic spreading and crustal production rates during the last 180 my we present a re evaluation of seafloor spreading and generation rates mainly based on a direct measurement of the remaining surfaces of oceanic crust and isochron lengths defined in the most recent isochron maps j y royer r d muller l m gahagan l a lawyer c l mayes d nurnberg j g sclater a global isochron chart tech rep 117 austin univ of tex inst for geophys 1992 r d muller w r roest j y royer l m gahagan j g sclater digital isochrons of the world s ocean floor j geophys res 102 1997 3211 3214 our evaluation of the amount of oceanic crust per unit age da dt as a function of age which can be expressed as da dt co 1 t tm is in fairly good agreement with previous determinations j g sclater b parsons c jaupart oceans and continents similarities and differences in the mechanisms of heat loss j geophys res 86 1981 11 535 11 552 d b rowley rate of plate creation and destruction 180 ma to present geol soc amer bull 114 2002 927 933 with co 2 850 0 119 km2 year 1 and tm 180 2 9 7 ma dividing these da dt by the ridge lengths l defined as the isochron length at each epoch allowed us to compute the evolution of global half spreading rates these have been roughly constant at 25 9 3 3 mm year 1 for at least the last 150 ma we propose that the global seafloor surface generation rate is roughly constant as well with a mean half value of 1 298 0 284 km2 year 1 and varying 20 with time this study corroborates the recent conclusion of rowley d b rowley rate of plate creation and destruction 180 ma to present geol soc amer bull 114 2002 927 933 of a constant generation rate since 180 ma and completely contradicts the commonly accepted idea of high seafloor spreading and surface generation rates during a large part of the cretaceous combining the oceanic surface generation rates derived here with crustal thicknesses deduced from the chemical composition of old oceanic crusts and seismic measurements e humler c h langmuir v daux depth versus age new perspectives from the chemical compositions of ancient crust earth planet sci lett 173 1999 7 23 the magmatic flux at young 0 80 ma oceanic ridges appears to be about 18 1 3 4 km3 year 1 and was possibly 15 to 30 higher during the mesozoic we propose that mantle temperature variation provides an alternative mechanism to spreading rate for the cretaceous highstand in sea level and atmospheric co2 generation

seismic structure of the upper crust from 0 75 ma in the equatorial atlantic ocean on the african plate using ultralong offset seismic data

is the middle cretaceous pulse of rapid sea floor spreading real or necessary it is commonly accepted that the major control on long term eustasy is variation in sea floor spreading rate the middle cretaceous sea level highstand in particular has been correlated with a postulated pulse of rapid spreading in the pacific basin this inferred event took place during the cretaceous normal superchron cns and has also been used as evidence for the existence of superplumes from the deep mantle giving a causative link between core mantle interactions and the record of sea level change there are reasons however to question the foundation upon which this linkage is based recent studies of marine geochemistry show no evidence for large hydrothermal fluxes during middle cretaceous time that require major increases in oceanic spreading rates plate reorganizations in the pacific during the cns show evidence of ridge jumps indicating that more of the sea floor of this age may be preserved than has been presumed improvements in the mesozoic time scale indicate that the cns was longer than previously assumed reducing the need for rapid spreading rates in the pacific basin middle cretaceous rapid plate generation rates are not needed to explain the history of sea level change long term eustasy can easily be accounted for by supercontinent breakup variations in the distribution of crust consumed at subduction zones and other effects because the inferred pulse of spreading was in part used to argue for the existence of a superplume the basis for this hypothesis is diminished the phenomena attributed to deep mantle plumes can be accounted for by plate tectonic forces and plate reorganization hence we question the basis and need for the cretaceous pulse of rapid sea floor spreading

a case for late archaean continental emergence from thermal evolution models and hypsometry abstract the secular cooling of the earth s mantle and the growth of the continental crust together imply changes in the isostatic balance between continents and oceans in the oceanic bathymetry and in the area of emerged continental crust the evolution of these variables is of fundamental importance to the geochemical coupling of mantle continental crust atmosphere and ocean to explore this further we developed a model that evaluates the area of emerged continental crust as a function of mantle temperature continental area and hypsometry in this paper we investigate the continental freeboard predicted using different models for the cooling of the earth we show that constancy of the continental freeboard 200 m is possible throughout the history of the planet as long as the potential temperature of the upper mantle was never more than 110 210 c hotter than present such numbers imply either a very limited cooling of the planet or most likely a change in continental freeboard since the archaean during the archaean a greater radiogenic crustal heat production and a greater mantle heat flow would have reduced the strength of the continental lithosphere thus limiting crustal thickening due to mountain building processes and the maximum elevation in the earth s topography rey p f coltice n neoarchean strengthening of the lithosphere and the coupling of the earth s geochemical reservoirs geology 36 635 638 2008 taking this into account we show that the continents were mostly flooded until the end of the archaean and that only 2 3 of the earth s area consisted of emerged continental crust by around 2 5 ga these results are consistent with widespread archaean submarine continental flood basalts and with the appearance and strengthening of the geochemical fingerprint of felsic sources in the sedimentary record from 2 5 ga the progressive emergence of the continents as shown by our models from the late archaean onward had major implications for the earth s environment particularly by contributing to the rise of atmospheric oxygen and to the geochemical coupling between the earth s deep and surface reservoirs

pore scale imaging of geological carbon dioxide storage under in situ conditions 1 while geological carbon dioxide co2 storage could contribute to reducing global emissions it must be designed such that the co2 cannot escape from the porous rock into which it is injected an important mechanism to immobilize the co2 preventing escape is capillary trapping where co2 is stranded as disconnected pore scale droplets ganglia in the rock surrounded by water we used x ray microtomography to image at a resolution of 6 4 m the pore scale arrangement and distribution of trapped co2 clusters in a limestone we applied high pressures and temperatures typical of a storage formation while maintaining chemical equilibrium between the co2 brine and rock substantial amounts of co2 were trapped with an average saturation of 0 18 the cluster sizes obeyed a power law distribution with an exponent of approximately 2 1 consistent with predictions from percolation theory this work confirms that residual trapping could aid storage security in carbonate aquifers

geological evolution of seawater boron isotopic composition recorded in evaporites the abundance of boron isotopes in ancient marine carbonates can be used to estimate oceanic ph that reflects atmospheric co 2 levels this proxy requires that the boron isotopic composition of seawater at the time the carbonate has formed is known and thus the past changes in seawater chemistry here we report the boron isotopic composition of selected ancient marine halites and modern sea salts the signal interpreted as marine reveals a clear evolution of the boron isotopic composition of seawater to 8 variations over the cenozoic comparison between this reconstructed curve and the mg ca ratio reveals a high level of consistency that will help to better define oceanic geochemical cycles

causes and consequences of the relation between area and age of the ocean floor the distribution of area of the ocean floor with age t is approximately described by da dt c0 1 t tm where c0 is the rate of crustal generation and tm the maximum age a linear differential area versus age relation can be explained by a balance between crustal generation and consumption where consumption is uniformly distributed with age the present distribution of consumption with age was estimated from the isochron map used to derive the area age relation and a recently published set of angular velocity vectors describing present plate motions the rate of consumption per unit age shows considerable scatter about a mean value equal to c0 tm 1 7 10 8 km2 yr2 but consumption is sufficiently close to being uniformly distributed with age to explain the observed degree of approximation to a linear differential area age relation the trenches appear to be distributed randomly with respect to age provinces in the oceans it was not possible to discern any trend of mean subduction velocities with age apart from the smaller velocities observed for the two youngest age provinces 0 9 ma changes in the rate of plate generation and the distribution of consumption with age result in shifts in the area age distribution in turn these shifts produce changes in the plate driving forces which act to restore the rate of plate generation and distribution of consumption to their initial states the ridge pushing force for example decreases as the rate of crustal generation increases this coupling between driving forces and the area age distribution provides a feedback mechanism limiting the extent of any changes a measure of the magnitude of shifts in the area age distribution is given by global changes in sea level if long term changes in sea level are less than 150 m as suggested by recent estimates then only small changes 12 in the rate of plate generation and distribution of consumption with age are required sea level changes as large as 300 m require about a 50 change in the rate of crustal generation the above area age relation can be combined with simple expressions for depth and heat flow versus age to obtain an empirical hypsometric distribution parameterized in terms of age and exact expressions for the heat loss from the ocean floor the latter is estimated to be 2 9 1013 w with 25 of this heat being lost by seafloor younger than 5 ma of the total heat loss from the mantle 26 is estimated to be transported by small scale convection

modeling the early paleozoic long term climatic trend the early paleozoic climate has been described as warm and equable however recent data based on conodont oxygen isotopic composition reveal a large long cooling trend through the ordovician followed by an abrupt cooling during the late ordovician glaciation this long term climate change is associated with a major radiation in the earth life history nonetheless the driving mechanisms for this cooling trend remain unknown carbon dioxide consumption by the weathering of fresh rocks from volcanic arcs has recently been suggested as a possible driver for this climate change however the impact of the plate motion context has not been explored yet although it might have a major impact on atmospheric co 2 levels simulations with a climate model coupled to a biogeochemical model geoclim show that the atmospheric co 2 decreased from more than 20 pal 5600 ppmv in the furongian down to approximately 10 pal 2800 ppmv in the llandovery before rising again in the early devonian we suggest that changes in geography and exposure of fresh volcanic rocks on continents are required to explain the large co 2 drawdown that led to the onset of cooler to glacial conditions from the middle ordovician to the llandovery the weathering of fresh volcanic rocks is itself responsible for 33 of the late ordovician atmospheric co 2 decrease the rest being related to the continent motion through the intertropical convergence zone itcz mean annual continental temperature falls by 3 c in the early ordovician reaching 13 5 c during the glacial interval and rises to 16 c in the early devonian

reheating of old oceanic lithosphere deductions from observations deep wide oceanic basins are the only regions of old seafloor where depth is truly representative of thermal isostasy when the depths of these basins are corrected for the effect of sediment accumulation and variation in crustal thickness the principal non thermal factors have been eliminated we collect the most precise and reliable values of heat flow for the same basins from multi penetration measurements with in situ thermal conductivity or deep sea drilling thermal gradients backed up by surface surveys the 9 data points that result from this selection process have been plotted on a depth versus heat flow graph and compared to published thermal models of lithosphere when considered without regard to age all the points fall at greater depths than predicted by the plate models with constant temperature lower boundaries and remarkably close to boundary layer cooling with parameters determined from the pre 80 ma depth and heat flow history of the ocean floor they are differentiated by their heat flows not much by their depths and the order they plot in along the heat flow axis is random with respect to crustal age modeling of discrete reheating events shows that near boundary layer conditions are re established after about 40 myr but corresponding to a younger than real age the data therefore favor discrete reheating events rather than a continuously hot basal boundary as implicitly assumed by the plate model lithospheric reheating appears to start only on ocean floor 100 ma the data alone cannot discriminate between a few discrete reheating events due to convective peel off at the base of the lithosphere or one or more catastrophic events however the distribution of points from the blake bahama basin is more consistent with distal reheating associated with the bermuda hotspot than local convective peel off

plate tectonic trigger of changes in pco2 and climate in the oxfordian late jurassic carbon isotope and modeling evidence abstract the transition from the middle to the late jurassic was characterized by significant changes in oceanography and climate and by changes in global carbon cycle as shown in the c isotope record a prominent mid oxfordian positive excursion in bulk carbonate carbon isotope values 13 c carb with an amplitude of more than 1 has been documented from many sections in the northern tethys realm in this study we present new bulk organic matter c isotope data 13 c org from northwestern tethys that do not record the mid oxfordian positive excursion in carbonate carbon on the contrary 13 c org decreases during the interval of the most rapid increase in 13 c carb we demonstrate that this decrease is not due to a changing marine terrestrial organic carbon partitioning but that the contrasting isotope trends record peculiar environmental and climate changes which occurred near the beginning of the late jurassic using a simple carbon cycle model we show that an increase in atmospheric p co 2 starting at modern levels could be the cause of contrasting trends in 13 c carb and 13 c org we suggest that a reorganisation of ocean currents related to the opening and or widening of the tethys atlantic pacific seaway and a massive spread of shallow sea carbonate production led to higher p co 2 model simulations indicate that this increase in p co 2 may have triggered changes in the biological carbon pump and in organic carbon burial that can explain the middle oxfordian c isotope record