volcano ice sea interaction in the cerro santa marta area northwest james ross island antarctic peninsula abstract we present here the results of detailed mapping lithofacies analysis and stratigraphy of the neogene james ross island volcanic group antarctic peninsula in the cerro santa marta area northwest of james ross island in order to give constraints on the evolution of a glaciated volcanic island our field results included recognition and interpretation of seventeen volcanic and glacial lithofacies together with their vertical and lateral arrangements supported by four new unspiked k ar ages this allowed us to conclude that the construction of the volcanic pile in this area took place during two main eruptive stages eruptive stages 1 and 2 separated from the cretaceous bedrock and from each other by two major glacial unconformities u1 and u2 the u1 unconformity is related to antarctic peninsula ice sheet expansion during the late miocene before 6 2 ma and deposition of glacial lithofacies in a glaciomarine setting following this glacial advance eruptive stage 1 6 2 4 6 ma volcanism started with subaerial extrusion of lava flows from an unrecognized vent north of the study area with eruptions later fed from vent s centered at cerro santa marta volcano where cinder cone deposits and a volcanic conduit lava lake are preserved these lava flows fed an extensive 7 km long hyaloclastite delta system that was probably emplaced in a shallow marine environment a second unconformity u2 was related to expansion of a local ice cap centered on james ross island which truncated all the eruptive units of eruptive stage 1 concomitant with glacier advance renewed volcanic activity eruptive stage 2 started after 4 6 ma and volcanic products were fed again by cerro santa marta vents we infer that glaciovolcanic eruptions occurred under a moderately thin 300 m glacier in good agreement with previous estimates of paleo ice thickness for the james ross island area during the pliocene

continental inputs of beryllium to the oceans abstract an understanding of the geochemistry of the cosmogenic radioisotope 10 be t 1 2 1 5 my and the stable isotope 9 be is necessary for the development of their use in deep sea sediments as a geochronometer and as a monitor of variations in cosmogenic nuclide production we examined these isotopes in oceanic regions strongly influenced by input of continental materials the effects of atmospheric aerosols were studied through examination of the water column chemistry of the mediterranean a basin strongly influenced by saharan dust while salinity transects through the amazon and ganges brahmaputra estuaries were used to investigate riverine fluxes to the oceans these results were used to quantify riverine and eolian fluxes they indicate that eolian dust is probably the most significant source of 9 be to the oceans changes in 10 be 9 be in the marine sedimentary record which are not associated with radioactive decay or variability in 10 be production can thus result from local or regional changes in dust inputs

airborne gravity reveals interior of antarctic volcano abstract understanding antarctic volcanoes is important as they provide a window on magmatic and tectonic processes of the antarctic plate and contain datable records of ice sheet changes we present the results from the first detailed airborne radar and gravity surveys across james ross island northern antarctic peninsula which is dominated by mt haddington an ice covered miocene recent alkaline stratovolcano the surveys provide new insights into the subsurface structure of the volcano and hence its development which are unavailable from the surface geology alone we show that mt haddington is associated with a significant negative bouguer gravity anomaly 26 mgal which suggests that there has not been significant pooling and solidification of a dense shallow level mafic magma chamber during the growth of the volcano over at least the past 6 m y which is consistent with independent geochemical evidence simple flexural isostatic models cannot explain the localised negative bouguer anomaly 3d modelling techniques show that the negative anomaly is best explained by a shallow low density intra crustal body with its top close to or at the surface although comparable gravity anomalies are commonly associated with large 20 km ash filled calderas as seen at yellowstone or toba there is no geological evidence on james ross island for a similar structure we therefore propose that the james ross island volcanic edifice subsided into the thick underlying pile of relatively soft jurassic and cretaceous sediments which were displaced by low density hyaloclastite breccia the type of deformation envisaged is similar to that associated with concepciou or iwaki volcanoes in south america although mt haddington is much larger

melt generation during rifting in the north sea in a forthcoming paper mckenzie and o nions1 invert rare earth element concentrations in mid ocean ridge basalts to calculate the variation of melt fraction with depth in the asthenospheric mantle beneath ocean ridges they find that melting persists to greater depths about 80 km than would be predicted by parameterizations of melting experiments2 about 50 km this result has important implications for predicting the volume and composition of melt produced from asthenosphere of normal potential temperature during continental rifting the north sea rift is a good location to test models for melt generation because the physical conditions of rifting are well constrained3 8 using the distribution of melt fraction with depth derived in ref 1 we calculate the volume and composition of melt produced from the asthenosphere in the most stretched part of the north sea the forties region when this melt is mixed in approximately equal proportions with an alkaline melt similar to that produced from metasomatized lithosphere by very small amounts of stretching on the rift flanks it accounts for the volume and the elemental and isotopic composition of the basalts in the most extended region

geochemical and sr nd isotopic constraints on the mantle source of neoproterozoic mafic dikes of the rifted eastern laurentian margin north central appalachians usa abstract abundant and widely distributed unmetamorphosed mafic dikes intrude mesoproterozoic rocks of the new jersey highlands the age of the dikes is imprecisely known but interpreted to fall between 615 and 576 ma which is consistent with the range of ages of mafic dikes from labrador and nova scotia south to pennsylvania that were emplaced along the rifted eastern laurentian margin new jersey highlands dikes are a few cm to 18 m wide and have lengths of as much as several km they have sharp largely discordant contacts against enclosing mesoproterozoic rocks and aphanitic chilled margins that grade into coarser grained interiors columnar joints are present locally and suggest emplacement at a shallow crustal level geochemical compositions of the dikes range from alkalic to less common tholeiitic basalt having generally high tio2 p2o5 zr nb y and la yb and low mgo cr and ni tio2 contents define high ti and low ti dikes that differ in high field strength elements hfse and light rare earth elements lree but overlap in abundances of most other elements dike magma evolved in an ascending mantle plume of oib like asthenosphere from enriched higher tio2 compositions to more depleted lower tio2 compositions subtle differences in the dike compositions are due to variations in the amount of partial melting within the plume and the depth of melt segregation sr nd isotope values of both dike compositions overlap and are characterized by end t of 1 5 to 3 8 and initial 87sr 86sr ranging from 0 7032 to 0 7077 higher sr isotope ratios are interpreted as resulting from local interaction of the dike magma with heterogeneous high 87sr 86sr lithospheric mantle having emi or emii like geochemical characteristics dikes form tabular structures that have long segments striking an average of n44 e and short segments striking about east west their regional geometries form right stepping rhomb shaped patterns due to emplacement into rift related dilational fractures likely formed through a combination of southeast directed extension and strike slip shear stresses geochemical compositions of the dikes are the same regardless of their structural trend or location implying they formed during a single magmatic event they along with other mafic dikes in the north central appalachians were emplaced in a within plate tectonic setting along the rifted margin of eastern laurentia prior to opening of the iapetus ocean

chemical weathering in antarctica an example of igneous rock particles in big lachman lake sediments james ross island jezerni sedimenty velkeho lachmanova jezera ostrov jamese rosse antarktida dominuji jilovite a prachovite castice r zneho petrografickeho slo eni ty jsou sopecneho p vodu a jsou odvozeni p eva n z mendelova souvrstvi intenzita chemickeho zv travani vulkanick ch castic v sedimentech byla vyjad ena chemick m indexem alterace cia kter popisuje p em nu ivc na jilove mineraly pozorovane mineralni asociace montmorillonitu illitu a kaolinitu pravd podobn pochazela z p em ny sopecneho skla a ivc na jilove mineraly mira zv travani vyjad ena pomoci cia z lakustrialnich sediment je podobna jako u spodnojursk ch na ti bohatach vulkanick ch hornin souvrstvi mount poster a st ednojursk ch na ti bohat ch vulkanick ch hornin ze souvrstvi mapple z antarktickeho poloostrova nami studovane sedimenty ukazuji dobrou shodu se slo enim st ednojursk ch ryolit ze souvrstvi mapple a naopak se castecn lisi od slo eni kenozoick ch bazalt vulkanicke skupiny ostrova jamese rosse zdrojov material z jezernich sediment pochazi z antarktickeho poloostrova a byl pravd podobn transportovan p es kanal prince gustava mal mi ledovci v pr b hu neogenu a kvarteru

sensitivity and response of beryllium 10 in marine sediments to rapid production changes geomagnetic events a box model study the response and the sensitivity of the marine beryllium 10 flux to abrupt production changes e g caused by a geomagnetic event are investigated by using a simple box model the effect of mixing processes in the water column on the 10be flux is simulated and the influence of bioturbation on the sedimentary record is studied while the effect of the oceanic residence time is found to be small bioturbation can drastically change the shape and the amplitude of the modeled 10be peak the simulations indicate that 10be production peaks persisting for more than 1500 years are recorded well in deep sea sediments if bioturbation induced integration times are small tbio 1000 years while sediment cores with tbio 1000 years are found to be not well suited for stratigraphic purposes the results further show that the sedimentary 10be peak lags initial signal this time lag is almost independent of the event duration for tbio 1000 years ranging between about 400 and 600 years therefore the model results support the idea that marine 10be peaks corresponding to geomagnetic events can be used as a global time marker to match marine terrestrial and ice core chronologies on the millennial and if the time lag of 500 100 years is considered the submillennial timescale

cretaceous alkaline intra plate magmatism in the ecuadorian oriente basin geochemical geochronological and tectonic evidence abstract small volumes of cretaceous alkaline basaltic magmas have been identified in the sedimentary infill of the ecuadorian oriente foreland basin they are characterized by a restricted range of compositional variation low lile hfse ratios and sr nd isotope values within the range of oceanic island basalts oib reflection seismic data show that a pre existing nne ssw triassic and jurassic rift controls the location and occurrence of these alkaline eruptive sites radiometric ages 40ar 39ar incremental heating method and the biostratigraphic record of their surrounding sediments indicate a nne ssw systematic age variation for the emplacement of this alkaline volcanism from albian 110 5 2 ma in the northern part of the oriente basin to campanian 82 2 2 0 ma in the west central part the geochemical geochronological and tectonic evidences suggest that asthenospheric mantle has upwelled and migrated to the ssw into the region underlying the pre existing triassic and jurassic rift thin spot we propose that subduction was abandoned subsequent to the accretion of allochthonous terranes onto the ecuadorian and colombian margin in the latest jurassic earliest cretaceous causing the relict slab material corresponding to the eastwards directed leading plate to roll back unmodified asthenospheric mantle migrated into the region previously occupied by the slab this resulted in partial melting and the release of magmatic material to the surface in the northern part of the oriente basin since at least aptian times then magmatism migrated along the ssw trending central wrench corridor of the oriente basin during the upper cretaceous probably as a consequence of the lateral propagation of the transpressive inversion of the triassic jurassic rift eventually the late cretaceous east dipping andean subduction system was renewed farther west and the development of the compressional retro foreland oriente basin system halted the cretaceous alkaline magmatic activity

tongan pottery chronology 14c dates and the hardwater effect abstract as with many other pacific iarchaelogical sites the chronology of the lapita sites on tongatapu tonga rests on two bases a seriation of form and ornament attributes of pottery and the absolute chronology as determined by radiocarbon dates both systems were in disagreement casting doubt on the reliability of the pottery seriation the enclosed nature of the lagoon of tongatapu coupled with the dissolution of fossil limestone creates a water reservoir with an apparent age greater than the apparent age of that provided by the open ocean a lagoon specific reservoir correction factor was measured using pre modern reference shells with known dates of collection when the radiometric dates are corrected using this factor both chronologies pottery seriation and radiometric dates are in perfect agreement the paper demonstrates how micro reservoirs impact on ages derived from 14 c determinations and shows the need to determine the magnitude of localised variations in 14 c

effects of ridge crest trench interaction on antarctic phoenix spreading forces on a young subducting plate precise measurements of spreading rates on marine magnetic profiles collected to the west of the antarctic peninsula have enabled some consideration of the forces governing plate motion since antarctic phoenix motion has been controlled by the local rather than global force balance over the last 35 m y the total effective driving force per unit length of trench is calculated to have ranged between 2 6 and 3 6 1012 n m which is much less than is commonly thought necessary to support subduction conventional calculations may overestimate slab pull for old slabs because they neglect the effect of extensional disruption in limiting the contribution to the balance of forces at the trench the low estimate of driving forces obtained here implies that resistive forces are also smaller than is generally assumed driving forces show a strong correlation with observed spreading rates which indicates that resistive forces were largely velocity dependent fluid migration up the subduction zone may elevate temperatures in and around the shear zone reducing resistive forces below the levels required by purely conductive models changes in convergence rate may affect the depths of both the brittle ductile deformation boundary and the basalt eclogite phase change causing a negative feedback which would appear as a velocity dependent resistive force the different driving forces acting on the ne and sw parts of the phoenix plate as a consequence of older oceanic lithosphere at the trench in the ne caused antarctic phoenix spreading to take place about a near pole to the sw since 21 m y ago and ultimately resulted in disruption of the phoenix plate about 9 m y ago spreading rates decreased abruptly about 6 m y ago probably because of e w compression across the long transform faults bounding the phoenix plate however spreading on the last three segments of the antarctic phoenix ridge continued at least until 4 m y ago either spreading stopped progressively from sw to ne or the final stage took place about a very near pole to the sw a magnetic quiet zone extends up to 95 km from the margin between the tula fracture zone and the north anvers fracture zone and is thought to indicate that the ridge crest became buried by terrigenous sediment prior to collision the absence of a magnetic quiet zone associated with the most recent ridge crest trench collisions suggests a change in sedimentary regime during the late miocene anomalously fast apparent spreading rates between 23 and 21 m y ago are thought to indicate an error in this part of the magnetic reversal time scale

large volume silicic volcanism along the proto pacific margin of gondwana lithological and stratigraphical investigations from the antarctic peninsula jurassic magmatism in western gondwana produced the most voluminous episode of continental volcanism in the phanerozoic era during the early to middle jurassic some 2 5 3 million km3 of dominantly basalt and to a lesser extent rhyolite were erupted onto a supercontinent in the early stages of break up the major silicic portion of the gondwana magmatic province is exposed in patagonian south america the volcanic rocks of patagonia have been collectively termed the chon aike province and constitute one of the world s most voluminous silicic provinces the volcanic rocks are predominantly pyroclastic dominated by ignimbrite units of rhyolite composition volcanic rocks crop out sporadically across much of the once contiguous antarctic peninsula and are considered to form an extension of the chon aike province a continuation of the province to include the antarctic peninsula would extend its strike length along the active pacific margin by c 2000 km volcanic rocks exposed along the east coast of the antarctic peninsula defined here as the mapple formation are also dominated by rhyolitic ignimbrite flows with individual units up to 80 m in thickness and a total thickness of c 1 km the ignimbrites vary in degree of welding from high grade rheomorphic ignimbrites with parataxitic textures to unwelded lithic rich ignimbrites rhyolite lava flows air fall horizons debris flow deposits and epiclastic deposits are volumetrically minor occurring as interbedded units within the ignimbrite succession the lithology and stratigraphy of the jurassic volcanic rocks of the mapple formation are presented and comparisons are made to the chon aike province a consistent stratigraphy of permo triassic metasedimentary rocks unconformably overlain by terrestrial mudstone siltstone sequences which are in turn conformably overlain by largely silicic subaerial volcanic rocks is present at several localities along the antarctic peninsula and at localities in the chon aike province precise zircon u pb middle jurassic ages exist for two volcanic formations from the antarctic peninsula and a middle lower jurassic age has been suggested for the underlying sedimentary formations based on fossil flora analysis the antarctic peninsula chronostratigraphy coupled with lithological similarities indicate a close relationship to those sequences of the chon aike province

the benthic cycle of copper evidence from sediment trap experiments in the eastern tropical north pacific ocean abstract the benthic fluxes of 24 elements and components are calculated from sediment trap fluxes and sediment accumulation rates at manop sites h and m in the eastern tropical north pacific ocean in this paper we focus our discussion on our data for copper benthic flux estimates for copper are 0 7 and 1 4 nmol cm 2 yr at h and m respectively these results compare well with estimates from another trap deployment in the galapagos and modeling of pprewater and water column concentration data by other authors partitioning of near bottom cu flux indicates that the two sites receive similar amounts of primary cu flux flux from upper water column and resuspended cu flux only the site located on the east pacific rise site m shows any input of hydrothermal cu at site m 35 of the near bottom flux is due to some source other than primary resuspended or hydrothermal flux we speculate that dissolved cu from the sediments is readsorbed onto particles near the bottom at this site sequential leaching of trap material indicates that the flux of cu associated with organic matter using an operational definition ranges from 56 to 84 of the trap material this fraction of the total cu increases with depth in the water column probably through scavenging

evidence for an increase in cosmogenic 10be during a geomagnetic reversal reversals in the geomagnetic field which occur every few hundred thousand years represent a dramatic change in the earth s environment although there is no satisfactory theory for such reversals it is generally accepted that the dipole field intensity decreases to 20 of its normal value for a few thousand years during the change in direction1 because the galactic and solar cosmic rays which impinge on the earth s atmosphere are charged a significant fraction about half of them are deflected by the geomagnetic field2 at the time of a reversal this magnetic shielding is greatly reduced and it has been suggested that the increased flux of high energy particles could have effects on evolutionary3 or climatic4 processes for example the statistically significant coincidence in levels of some marine faunal extinctions and reversal boundaries in ocean sediments5 could be caused directly or indirectly by the decreased geomagnetic intensity during the reversal we report here evidence in marine sediments for an increase in cosmogenic 10be production in the earth s atmosphere during the brunhes matuyama reversal 730 000 yr ago in addition to confirming an increase in cosmogenic isotope production the results provide information on the magnitude and duration of the geomagnetic intensity decrease during such an event and the depth at which rmanent magnetism is acquired in marine sediments

boundary scavenging in the pacific ocean a comparison of 10be and 231pa concentrations of u th 231pa and 10be were measured in holocene sediments from two cores collected off the west coast of south america two cores from the east pacific rise two from the equatorial pacific and one from the south pacific central gyre our results together with data from 5 cores reported in the literature show that boundary scavenging plays a major role in the removal of 10be from the pacific ocean deposition rates of 10be at three margin sites are more than an order of magnitude greater than at sites of red clay accumulation in the deep central pacific deposition of 231pa is 4 to 5 fold greater at the margin sites the residence time of 10be with respect to chemical scavenging defined as its inventory in the water column divided by its rate of removal to the sediments varies regionally from 1000 years at the red clay sites in the deep central pacific to 100 years at the margin sites different factors control boundary scavenging of pa and be for example scavenging of 231pa is enhanced by metal oxide coatings of particles whereas this seems to have little influence on the scavenging of 10be 1990

on the importance of opal carbonate and lithogenic clays in scavenging and fractionating 230th 231pa and 10be in the ocean abstract the influence of particle composition on the scavenging of 230 th 231 pa and 10 be in the ocean is critically re examined based on the particle solution distribution coefficient k d data reported by chase et al earth planet sci lett 204 2002 215 229 our re examination shows that the k d s for these radionuclides are strongly influenced by the amount of lithogenic material lithogenics in particulates as reported previously by us and that a strong correlation between the lithogenic and carbonate contents in open ocean particulates results in an apparent non linear correlation between caco 3 and k d for each of the three radionuclides using a three end member model we estimate that the respective k d s g g of 230 th 231 pa and 10 be in the open ocean are 2 3 10 8 1 2 10 7 and 3 8 10 6 for lithogenics 1 0 10 6 1 0 10 5 and 3 0 10 4 for carbonate and 2 5 10 5 1 2 10 6 and 7 5 10 5 for opal suggesting that lithogenics have a much stronger affinity for any of the three nuclides than does carbonate or opal lithogenics and carbonate preferentially scavenge 230 th over 231 pa and 10 be whereas opal slightly favors 231 pa and 10 be over 230 th relative to lithogenics carbonate plays a very limited role in fractionating 230 th 231 pa and 10 be due to its orders of magnitude smaller k d values for the three nuclides our results indicate that fractionations between pa and th and between be and th through particle scavenging are mainly determined by the opal to lithogenic ratio in particulates rather than the opal to carbonate ratio suggested by chase et al

the south american retroarc foreland system the development of the bauru basin in the back bulge province abstract the aim of this research is to understand the tectonic setting of the bauru basin this basin in central eastern south america has been classified as intracratonic but the basin fill geometry the involved subsidence mechanisms and the age of the deposits are poorly understood in this work the ranges of the fossil taxa are analyzed and ages are proposed for the lithostratigraphic units isopach maps were used to reconstruct the stratigraphic intervals of the basin fill the stratigraphy of the bauru basin is compared with that of the adjacent basins and the data are integrated with the available information on south american geodynamics the fossil record indicates that sediment accumulated from the cenomanian to early paleocene beginning after the mochica phase of the andean orogeny the basin fill geometry demonstrates migration of the depocenter through time which occurred simultaneously with migration of the andean basin and immediately after the orogenic events of the peruvian phase we propose that the bauru basin is a component of a retroarc foreland system developed during the early stages of the andean evolution and that it was developed in the back bulge province of this system the andean basin constitutes the foredeep depozone of this foreland system including the potosi oriente acre and maranon basins in addition the upper cretaceous of the parecis and solimoes basins were likely also developed in the back bulge province the thickness of the bauru accumulation indicates that other mechanisms might have overlapped the flexural subsidence in this back bulge province

quasi conservative behaviour of 10be in deep waters of the weddell sea and the atlantic sector of the antarctic circumpolar current we present the first transect of dissolved 10be depth profiles across the antarctic circumpolar current acc in the atlantic sector north of the polar front the 10be concentrations increase continuously from very low values at the surface to values of up to 1600 atoms g at depth deep water 10be concentrations of particular water masses are consistent with earlier results obtained further north south of the polar front and in the weddell sea the distribution of 10be is also characterised by low surface concentrations but below 1000 m depth the concentrations are relatively constant and significantly higher up to 2000 atoms g than further north probably as a result of mixing and advection of water masses of pacific origin overall the deep water 10be distribution is obviously not significantly affected by scavenging processes or ice melt and comparison with the density distribution suggests that 10be can be viewed as a quasi conservative tracer this provides a tool for an improved understanding of the behaviour of other more particle reactive trace metals in the southern ocean such as 230th in deep waters north of the acc weddell gyre boundary awb 10be 230th has a relatively constant value 1 7 0 3 109 atoms dpm over a wide density range whereas south of the awb the ratio is significantly lower 1 1 0 2 109 atoms dpm this normalisation to 10be corroborates that 230th is enriched by 50 due to accumulation south of the awb as a consequence of minimal particulate fluxes the quasi conservative behaviour deduced from our results also implies that 10be can only be used as a tracer for southern ocean particle fluxes in the past if ocean circulation patterns and water mass residence times did not change significantly

bimodal back arc alkaline magmatism after ridge subduction pliocene felsic rocks from central patagonia 47 s abstract volumetrically minor microsyenites alkali microgranite and related trachytic dykes intrude early pliocene oib like alkali basaltic and basanitic flows of the meseta del lago buenos aires in central patagonia 47 s 71 30 w and occur together with scarce trachytic lava flows whole rock k ar ages between 3 98 and 3 08 ma indicate that the emplacement of these felsic rocks occurred more or less synchronously with that of the post plateau basaltic sequence that they intrude during a bimodal mafic felsic magmatic episode devoid of intermediate compositions chemically these rocks have a1 type granitoid affinities and are characterized by high silica and alkali contents 60 68 wt sio2 8 7 10 8 wt na2o k2o major and trace elements patterns evidencing evolution by low pressure fractional crystallization and sr and nd isotopic signatures similar to those of coeval basalts 87sr 86sr o 0 70488 0 70571 143nd 144nd o 0 512603 0 512645 nevertheless some of them have the most radiogenic sr values ever reported for a magmatic rock in the meseta and even in the whole neogene patagonian plateau lavas province 87sr 86sr o 0 70556 0 70571 143nd 144nd o 0 512603 0 512608 in addition very high contents of strongly incompatible elements in the most evolved rocks together with sr isotopic ratios higher than those of coeval basalts suggest the occurrence of open system magmatic processes continuous fractional crystallization from a primitive basaltic source similar to post plateau coeval basalts towards alkali granites combined with small rates of assimilation of host jurassic tuffs afc in a shallow magmatic reservoir best explains the geochemical and petrographic features of the felsic rocks therefore a1 type magmatic rocks can be generated by open system crystallization of deep asthenospheric melts in back arc tectonic settings in central patagonia these 3 4 ma old alkaline intrusions occur aligned along a n160 170 trending lineament the zeballos fault zone stacking the morphotectonic front of one segment of the patagonian cordillera intrusion along this fault zone occurred during the onset of a new transtensional or extensional event in the area related to major regional tectonics occurring in possible relation with the collision of one segment of the chile spreading ridge with the trench

antarctic peninsula continental magnesian andesites indicators of ridge trench interaction during gondwana break up subduction of young hot oceanic lithosphere and a major episode of lithospheric extension have been considered as key tectonomagmatic factors in the genesis of setouchi type continental magnesian andesites we present data on a new discovery of such andesites from the northern part of the antarctic peninsula and propose that subduction of newly created oceanic lithosphere occurred along the proto pacific margin of antarctica during the middle jurassic continental magnesian andesite magmatism was contemporaneous with the early stages of gondwana break up and we suggest that subduction of young hot oceanic lithosphere and ridge trench interaction may have contributed to the disintegration of gondwana by thermally weakening the lithosphere and developing a broad linear extensional zone along the proto pacific margin

the global average production rate of10be precipitation collected in continuously open containers for about a year at seven sites around the united states was analyzed for10be 90sr 210pb and238u based on these data and long term precipitation 90sr and210pb delivery patterns the stratospheric tropospheric and recycled10be components in the collections were estimated and the global10be production rate was assessed single station production rate estimates range from 0 52 106 atoms cm 2 yr 1 to 2 64 106 atoms cm 2 yr 1 the mean value is 1 21 106 atoms cm 2 yr 1 with a standard error of 0 26 106 atoms cm 2 yr 1

the cenozoic subduction history of the pacific margin of the antarctic peninsula ridge crest trench interactions new magnetic anomaly identifications w of the shackleton fracture zone show 5 spreading sections separated by fracture zones in the 2 most southerly the ridge crest collided with a trench at the margin of the antarctic peninsula only 6 5 and 4 ma ago the latest of a series of collisions starting at the base of the peninsula so ma ago following each collision spreading and subduction both stopped opposite the south shetland trench and actively extending bransfield strait spreading also stopped 4 ma ago but before the spreading centre reached the trench the tendency of the subducting plate to continue sinking probably initiated the opening of the bransfield strait the parallelism between fracture zone orientation on the descending plate and convergence direction previously thought responsible for the tectonic segmentation of the peninsula was effective for only 10 30 ma before collision before that either the fracture zones did not extend to the trench or the subducting ocean floor formed at the farallon phoenix nazca aluk boundary with fracture zones oblique to the subduction direction the aleutian arc kula ridge model in which arc magmatism virtually ceases when ocean floor younger than 25 30 ma occurs beneath the arc fits the distribution of antarctic peninsula radiometric dates explaining a 50 60 ma gap between collisions and youngest ages in the s and possibly the migration of the youngest activity towards the trench thus gaps in the geologic record of arc magmatism need not imply cessation of subduction the progressive steepening of the peninsular margin towards the 4 ma collision site suggests tectonic erosion of the fore arc as the ridge crest approached

igneous history of the andean cordillera and patagonian plateau around latitude 46 degrees s from the middle jurassic onwards persistent igneous activity in the southern andes around 46 s was controlled by easterly dipping subduction along the pacific margin cogenetic plutonic rocks belonging to the patagonian batholith and calc alkaline volcanics ranging from basaltic andesites to rhyolitic tuffs and ignimbrites are the principal products erosion of the primary volcanics has led at various times to the development of thick volcaniclastic sequences for example in the cretaceous lower tertiary divisadero formation the coyhaique region marks the northerly extension of a narrow back arc basin in which the marine neocomian successions accumulated volcaniclastics from the island arc which presumably lay to the west are intercalated with the sediments although the marine basin was short lived a mildly extensional back arc regime may have existed through much of mesozoic recent times widespread basalt rhyolite volcanism on the eastern side of the cordillera seems to have been associated with this tectonic environment remnants of the patagonian basalt plateau at latitude 45 47 s extend from the argentine chile frontier to lago colhue huapi four principal age and compositional groups have been distinguished in the lavas i the oldest which are about 80 ma occur in sections at senguerr and morro negro they are almost exclusively tholeiitic but show some calc alkaline affinities and resemble in other respects basalts from marginal basins ii the second group 57 43 ma occur in the lower part of the chile chico section with a compositional spread from olivine tholeiites through alkali basalts to one occurrence of a basanite iii the upper part of the main plateau sequence where the flows are in the range 25 9 ma are dominantly of alkali basalt composition iv post plateau flows from small cinder cones on the surface of the plateau range in age from ca 4 ma to 0 2 ma or less they are mostly highly undersaturated basanites with occasional leucite basanites enriched in incompatible elements a few of the earlier tholeiites with calc alkali traits may have been closely associated with subduction or marginal basin processes the younger lavas are more alkalic intraplate types generated in the remote back arc extensional zone

radiocarbon reservoir effects in human bone collagen from northern iceland human bone collagen from a series of icelandic human pagan graves was radiocarbon 14c dated to aid understanding of early settlement landnam chronologies in northern iceland these individuals potentially consumed marine protein the 14c age of samples containing marine carbon requires a correction for the marine 14c reservoir effect the proportion of non terrestrial sample carbon was quantified via measurement of carbon stable isotopes 13c using a simple mixing model based on 13c measurements of archaeofaunal samples non terrestrial carbon was also quantified in six pig bones from the archaeofaunal dataset assuming all non terrestrial carbon in human and pig bone collagen was marine derived calibrated age ranges calculated using a mixed intcal09 marine09 calibration curve were consistent with an early settlement date close to landnam but several samples returned pre landnam age ranges measurements of nitrogen stable isotopes 15n strongly suggest that many of the human bone collagen samples contain freshwater diet derived carbon icelandic freshwater systems frequently display large freshwater 14c reservoir effects of the order of 10 000 14c years and we suggest that the presence of freshwater carbon is responsible for the anomalously early ages within our dataset in pig samples the majority of non terrestrial carbon is freshwater in origin but in human samples the proportion of freshwater carbon is within the error of the marine component 10 this presents a major obstacle to assessing temporal patterns in the ages of human remains from sampled graves although the majority of grave ages are within the same broad calibrated range

beryllium 10 thorium 230 and protactinium 231 in galapagos microplate sediments implications of hydrothermal activity and paleoproductivity changes during the last 100 000 years biogenic particle fluxes from highly productive surface waters boundary scavenging and hydrothermal activity are the main factors influencing the deposition of radionuclides in the area of the galapagos microplate eastern equatorial pacific in order to evaluate the importance of these three processes throughout the last 100 kyr concentrations of the radionuclides 10be 230th and 231pa and of mn and fe were measured at high resolution in sediment samples from two gravity cores klh 068 and klh 093 high biological productivity in the surface waters overlying the investigated area has led to 10be and 231pa fluxes exceeding production during at least the last 30 kyr and probably the last 100 kyr however during periods of high productivity at the up welling centers off peru and extension of the equatorial high productivity zone a relative loss of 10be and 231pa may have occurred in these sediment cores because of boundary scavenging the effects of hydrothermal activity were investigated by comparing the 230thex concentrations to the mn fe ratios and by comparing the fluxes of 230th and 10be which exceed production the results suggest an enhanced hydrothermal influence during isotope stages 4 and 5 and to a lesser extent during isotope stage 1 in core klh 093 during isotope stages 2 and 3 the hydrothermal supply of mn was deposited elsewhere probably because of changes in current regime or deep water oxygenation a strong increase of the mn fe ratio at the beginning of climatic stage 1 which is not accompanied by an increase of the 230thex concentration is interpreted to be an effect of mn remobilization and reprecipitation in the sediment

10 be profiles in seawater a knowledge of the marine geochemical cycle of 10be is basic to the use of this long lived cosmogenic radioisotope half life 1 5 myr in geochronological and geophysical studies1 3 the first attempts at measuring 10be in seawater were made only recently using nuclear accelerator spectrometry and six concentration values from four locations have been reported4 6 this data base must be expanded to include detailed water column profiles so that the behaviour of 10be in the ocean can be more fully described a parallel case in point is the recent measurement7 of a detailed oceanic profile for the stable 9be we report here two water column profiles of 10be one in the drake passage near antarctica and the other in the san nicolas basin about 100 km off the coast of southern california the san nicolas basin profile clearly demonstrates the regulation of 10be distribution by particulate cycling the data confirm the earlier suggestion4 of particle uptake of 10be in the surface ocean they also furnish evidence for regeneration of 10be across the thermocline and near the sea floor and for mid water scavenging

stability in the south pacific surface marine 14c reservoir over the last 750 years evidence from american samoa the southern cook islands and the marquesas abstract although minor climatic and sea level changes have been documented for the south pacific during the late holocene our understanding of the consequent impact of these changes on the marine 14 c reservoir and therefore the 14 c content of shellfish is limited ultimately this has implications for documenting the chronology of human movement and adaptation in this region in this paper we compare marine reservoir r data obtained from tightly controlled archaeological proveniences with known age pre ad 1950 shells from the southern cook islands american samoa and marquesas islands results indicate that there has been no significant change in the near shore marine reservoir in these three locations over the last ca 750 years furthermore known age pre ad 1950 shell samples provide more precise r values for use in sample calibration than archaeological paired shell charcoal samples this is attributed in part to the limitations of assigning provenance and age to material from archaeological sites on the basis of these results we conclude that the known age pre ad 1950 shell derived r values can be used to calibrate shell 14 c results from deposits of late holocene age

short paper the significance of mid palaeozoic basement in graham land antarctic peninsula a mid palaeozoic basement to the antarctic peninsula is confirmed by a geochronological study of the gneissose country rock of the mesozoic magmatic arc of eastern graham land rb sr whole rock ages of 410 15 and 426 12 ma from samples of orthogneiss indicate a previously unrecognized episode of granitic magmatism in the antarctic peninsula during the silurian sm nd data on garnet whole rock pairs indicate that subsequent amphibolite facies metamorphism occurred in late carboniferous times probably as a result of plutonism

southern patagonian plateau basalts and deformation backarc testimony of ridge collisions abstract the distribution and volume of tertiary patagonian plateau basalts and the evolution of the patagonian fold and thrust belt between 46 and 49 s appear to be closely tied with ridge trench interactions along the continental margin to the west eocene plateau basalts south of 43 s are associated with a gap in the eocene arc and can be related to a proposed collision of the aluk farallon ridge against the trench at this time eruptions of late miocene to recent plateau basalts between 46 and 49 s can be related to time transgressive slab windows in the underlying mantle generated by the collision of segments of the chile ridge at 10 14 ma and 6 ma the principal deformation in the patagonian fold and thrust belt predates the eruption of the basalts and also appears to be related with late miocene ridge collision comparison of plateau basalts related to the collision of the three distinct ridge segments suggests that the volume of basalt erupted increases with the length of the collided ridge segment almost all eocene to recent plateau basalts between 46 and 49 s have oib like ocean island basalt trace element signatures showing that the mantle source region changed little with time plateau basalts with more arc like signatures appear to be contaminated by continental crust relative melting percentages inferred from the trace element chemistry of the plateau basalts correlate with volumes of erupted basalt implying spatial and temporal changes in temperatures in the mantle source that can be correlated with slab windows

examination of hydrothermal influences on oceanic beryllium using fluids plume particles and sediments from the tag hydrothermal field we have carried out a comprehensive study of the distributions of9be and the cosmogenic radionuclide10be in fluids nearfield sediments and neutrally buoyant plume particles associated with the tag hydrothermal field these results coupled with previously published uranium decay series radionuclide distributions 1 2 suggest that the level of mno2 in iron oxide rich sediments and particles may play a role in be scavenging in addition the data are used to quantify the net effects of nearfield coprecipitation with fes and of scavenging by iron oxyhydroxide plume particles on the oceanic balance of the be isotopes these calculations suggest that hydrothermal input of9be is nearly balanced by nearfield and plume removal processes and that hydrothermally induced scavenging has a negligible effect on the oceanic10be budget

geochemical and mineralogical evidence for the occurrence of at least three distinct magma types in the famous region bulk rock major and trace element variations in selected basalts from the famous area in conjunction with a detailed study of the chemical compositions of phenocryst minerals and associated melt inclusions are used to place constraints on the genetic relationship among the various lava types the distribution of nio in olivine and cr spinel phenocrysts distinguishes the picritic basalts plagioclase phyric basalts and plagioclase pyroxene basalts from the olivine basalts for a given mg mg fe2 atomic ratio of the mineral the nio content of these phenocrysts in the former three basalt types is low relative to that in the phenocrysts in the olivine basalts the zr nb ratio of the lavas similarly distinguishes the olivine basalts from the plagioclase phyric and plagioclase pyroxene basalts and in addition distinguishes the picritic basalts from the other basalt types these differences indicate that the different magma groups could not have been processed through the same magma chamber and preclude any direct inter relationship via open or closed system fractional crystallization

melting of heterogeneous mantle in a slab window environment examples from the middle tertiary masset basalts queen charlotte islands british columbia voluminous basalts dominate a middle tertiary bimodal volcanic assemblage in the queen charlotte islands british columbia canada these moderately enriched mid ocean ridge basalt morb like magmas were associated with significant crustal extension related to the subduction of the pacific farallon ridge and the initiation of a slab window environment which persisted for 35 ma these asthenospheric melts were derived from a low degree of melting of a heterogeneous peridotite source mainly in the spinel stability field as inferred from fractional melting inversion calculations the basaltic magmas in turn gave rise to both tholeiitic and calc alkaline volcanic strata up to 3 5 km thick in local grabens the tholeiitic basalts resemble morb with rare earth element ree patterns ranging from slightly depleted to enriched in light ree lree the calc alkaline basaltic andesites are enriched in lree but are not strongly depleted in nb the rocks have overlapping nd sr and pb isotopic compositions sim

combined sr nd pb and li isotope geochemistry of alkaline lavas from northern james ross island antarctic peninsula and implications for back arc magma formation abstract we present a comprehensive geochemical data set for a suite of back arc alkaline volcanic rocks from james ross island volcanic group jrivg antarctic peninsula the elemental and isotopic sr nd pb and li composition of these cenozoic basalts emplaced east of the antarctic peninsula is different from the compositions of the fore arc alkaline volcanic rocks in southern shetlands and nearby bransfield strait the variability in elemental and isotopic composition is not consistent with the jrivg derivation from a single mantle source but rather it suggests that the magma was mainly derived from a depleted mantle with subordinate oib like enriched mantle component em ii the isotopic data are consistent with mantle melting during extension and possible roll back of the subducted lithosphere of the antarctic plate magma contamination by triassic early tertiary clastic sediments deposited in the back arc basin was only localized and affected li isotopic composition in two of the samples while most of the basalts show very little variation in 7 li values as anticipated for mantle driven li isotopic composition these variations are difficult to resolve with radiogenic isotope systematics but li isotopes may prove sensitive in tracking complex geochemical processes acting through the oceanic crust pile including hydrothermal leaching and seawater equilibration

a preliminary assessment of the hydrocarbon potential of the larsen basin antarctica abstract the larsen basin on the northwest margin of the weddell sea formed as a mesozoic ensialic basin during gondwana breakup deposition was either in half grabens on the extending weddell sea margin or in a restricted back arc basin at the northern end of this basin 5 6 km of sedimentary rock crop out on james ross island exposing elements of a large potential hydrocarbon system aeromagnetic and outcrop data suggest that the basin structure inferred from james ross island can be recognised at least as far south as 70 s upper jurassic anoxic marine strata deposited prior to the main phase of arc development form a rich potential source t o c up to 3 5 with both marine and terrestrial kerogens arc derived volcaniclastic sediments of barremian oligocene age form a regressive megasequence basal strata represent slope apron and rudaceous submarine fan deposits proximal to the margin fan conglomerates form lenticular bodies hundreds of metres thick and tens of kilometres across enveloped in slope apron mudstones late cretaceous fault reactivation and uplift led to dramatic shallowing of the basin with deposition of shelf facies although there are many potentially attractive reservoir targets there may be problems of pore occlusion due to the abundant labile volcanic grains however there is evidence of more quartzose sandstone towards the top of the section and inferentially toward the basin centre in the larsen basin there is moderate potential for oil generated from upper jurassic source rocks and reservoired in cretaceous and tertiary sandstones and conglomerates in large stratigraphic or structural traps caused by partial basin inversion during deposition

distribution of10be and9be in the pacific ocean abstract the vertical distributions of10be and9be at three locations in the pacific 25 n 170 e 17 n 118 w 3 s 117 w are presented the results show that both isotopes exhibit nutrient like profiles from the surface to the bottom the increase for10be is two to threefold and that for9be is about fivefold while the inter station variations in surface water concentrations may reach a factor of two deep water values tend to be much more uniform averaging about 2000 atoms g for10be and 30 pm for9be a similar situation applies to the10be 9be ratio it varies approximately from 1 to 3 10 7 atom atom at shallow depths but tends toward a value close to 1 1 10 7 in the deep ocean the variation of10be 9be can be viewed as resulting from the fact that10be in a given parcel of water consists of two components recycled and primary the recycled component is that part of10be which has reached tracer equilibrium with9be as opposed to the primary component which upon entering the sea from the atmosphere has yet to equilibrate with9be through particle cycling and mixing processes it is estimated that 70 to nearly 100 of10be at the three stations are being recycled and the recycled beryllium bears an atomic ratio of10be 9be close to 1 10 7 the oceanic residence time of be is of the order of 1000 4000 years comparable to or slightly longer than the ocean mixing time

particle mixing rates in sediments of the eastern equatorial pacific evidence from 210pb 239 240pu and 137cs distributions at manop sites abstract particle mixing rates db calculated from excess 210pb gradients in sediments of the east equatorial pacific range from 0 04 to 0 5 cm2 y with variation of a factor of 3 4 at a single site diffusion of the 236ra daughter 222rn may affect 210pb distributions under conditions of slow mixing and low 210pb flux to the seafloor as shown by a siliceous ooze clay core which contained the fallout radionuclides 239 240pu and 137cs but no excess 210pb relative to 226ra there is no clear relationship between 210pbderived mixing rates and sediment type accumulation rate or organic carbon flux to the sediments comparison of 210pb mixing rates with those calculated from 239 240pu and 137cs distributions reveals better agreement for a pulse input of the fallout radionuclides db 0 03 0 4 cm2 y than for continuous input at a constant rate db 0 1 1 6 cm2 y although the pu and 137cs data are better fit by the latter model the agreement may be fortuitous because 239 240pu and 137cs appear significantly deeper than 210pb in at least one core tracer separation could be caused by particle size selective mixing by the benthic fauna or by chemical mobilization if the fallout radionuclides are scavenged from surface waters by large organic rich particles such as fecal pellets their release and migration may result from decomposition of the carrier in surface sediments either a relatively unreactive form of pu e g oxidized pu has been released by this process or a one dimensional model is inadequate to explain its observed penetration into the sediments activity ratios of 239 240 pu 137 cs in the sediments decrease with increasing north latitude and the trend reflects higher fluxes of 239 240pu near the weapons test site at christmas island 2 n the 239 240 pu 137 cs ratios and fluxes to the sediment assuming constant input at the siliceous ooze red clay site are consistent with published sediment trap data from a nearby site thus if fallout radionuclide fluxes to the sea floor were higher in the past both 239 240pu and 137cs have been released from sinking particles

geochemistry and origins of ueno and on take basaltic to andesitic rocks 3 ma produced by distinct contributions of subduction components central japan abstract ueno basaltic rocks 2 7 1 6 ma and on take volcanic rocks 0 75 ma crop out in a restricted area in central japan and were analyzed for major and trace elements and sr and nd isotopes the ueno rocks were erupted as numerous monogenetic volcanoes largely of basaltic compositions whereas the on take lavas form a composite stratovolcano dominated by andesites the ueno basalts are primarily geochemical mixtures of two components 1 sub alkalic common basalts generated from the lithospheric subcontinental mantle with 87sr 86sr 0 706 end 2 and k nb 1000 and 2 rare within plate basalts wpb of alkali basaltic composition with 87sr 86sr 0 705 end 5 k nb 180 subordinate ueno andesites were formed by assimilation fractional crystallization processes within the lower crust having 87sr 86sr 0 709 end 8 but without appreciable assimilation in the upper crust on the other hand the on take basalts have 87sr 86sr 0 705 0 0005 end 2 1 k nb 1200 and geochemical signatures characteristic of continental arc basalts the on take andesites appear to have been contaminated frequently by upper crustal materials it is proposed that the injection of a wpb type diapir yielded partial melting of the subcontinental mantle forming the common ueno basalt magmas the arc type basaltic magmatism which was probably the result of subduction of the pacific plate occurred at 1 ma after the ueno volcanism addition of slab derived components with 87sr 86sr 2300 modified the subcontinental mantle forming the source of on take basalts

beryllium in the water column of the central north pacific the existence of the cosmogenic radioisotope of beryllium 10be t1 2 1 5 myr was first confirmed1 2 almost 25 yr ago subsequently merrill et al 3 worked out the sedimentary geochemistry of the stable isotope of the element 9be the geochronological potential of 10 be could not be exploited however because of its extremely low abundance global average production rate 1 08 atoms cm 2 min 1 4 and the difficulties associated with the analysis of the stable species 9be particularly in natural waters this situation is changing dramatically with the application of accelerator techniques to the detection of the radioisotope5 it thus becomes important to obtain a detailed understanding of the geochemistry of 9be since the potential use of the stable isotope for normalization purposes would significantly enhance the usefulness of the radioactive species in dating studies we report here the first detailed profile for 9be in the oceanic water column

testing the human factor radiocarbon dating the first peoples of the south pacific archaeologists have long debated the origins and mode of dispersal of the immediate predecessors of all polynesians and many populations in island melanesia such debates are inextricably linked to a chronological framework provided in part by radiocarbon dates human remains have the greatest potential for providing answers to many questions pertinent to these debates unfortunately bone is one of the most complicated materials to date reliably because of bone degradation sample pre treatment and diet this is of particular concern in the pacific where humidity contributes to the rapid decay of bone protein and a combination of marine reef c4 c 3 and freshwater foods complicate the interpretation of 14 c determinations independent advances in bone pre treatment isotope multivariate modelling and radiocarbon calibration techniques provide us for the first time with the tools to obtain reliable calibrated ages for pacific burials here we present research that combines these techniques enabling us to re evaluate the age of burials from key archaeological sites in the pacific

beryllium 10 in continental sediments abstract the concentration of 10 be has been measured in 10 samples taken from a transect of surface sediments beginning in the atchafalaya river and extending across the bay 136 km into the gulf of mexico if corrected for a lower retentivity of sand for be they have a concentration that is constant within 13 this concentration is about an order of magnitude smaller than that of deep ocean sediments for comparison measurements of 10 be in rainwater in a sample of soil and in a deep ocean core were made

role of strike slip faulting in the tectonic evolution of the antarctic peninsula the antarctic peninsula mesozoic magmatic arc has had a long history of dextral strike slip deformation the deformation was initially associated with the development of a wide accretionary complex by the migration of fore arc slivers and the formation and inversion of a thick fore arc basin succession it also formed an important component within major shear zones in the arc and may have controlled the formation of sedimentary basins in the back arc region although some transcurrent motion within the fore arc region was related to a component of oblique subduction the main movement occurred during the breakup of gondwanaland and the formation of a major transtensional rift system a new reconstruction for this part of gondwanaland is presented taking this transcurrent motion into consideration

δd and δ18o variations of the magmatic system beneath deception island volcano antarctica implications for magma ascent and eruption forecasting abstract geochemistry of volatiles in active volcanoes provides insights into the magmatic processes and evolution at depth such as magma evolution and degassing which can be implemented into volcanic hazards assessment deception island is one of the most active volcanoes in antarctica with more than twenty explosive eruptions documented over the past two centuries hydrogen and oxygen isotopic variations in the volatiles trapped in the deception island rocks glass and melt inclusions in phenocrysts provide essential information on the mechanisms controlling the eruptive history in this volcanic suite thus understanding the petrological and related isotopic variations in the island has the potential to foresee the possible occurrence and its main eruptive features of a future eruption the new isotopic results of this study combined with exhaustive petrologic knowledge reveal in deception island i fast ascent and quenching of most magmas preserving pre eruptive magmatic signal of water contents and isotopic ratios with local modification by rehydration due to glass exposition to seawater and by meteoric and fumarolic waters ii a plumbing system s variable with time and currently dominated by closed system degassing leading to explosive eruptions iii control on the interactions of ascending magmas with the surface waters producing hydrovolcanic activity throughout the two main fault systems in deception island and at different magma stagnation depths these results can be considered in further studies of volcanic monitoring to improve the capability to interpret geophysical data and signals recorded during volcanic unrest episodes and hence forecast volcanic eruptions and related hazards

geochemistry of holocene volcanic rocks associated with ridge subduction in baja california mexico although the geochemistry of magmatism associated with the subduction of oceanic lithosphere is well understood1 6 the geochemical signature of lavas produced shortly after subduction of an oceanic spreading centre has not been characterized the baja california peninsula in mexico provides an ideal scenario to study the latter process geophysical models indicate that in response to oblique collision of the ancestral east pacific rise epr with the western seaboard of north america at 29 myr subduction of the farallon plate was succeeded by the progressive southwards development of a transform fault system7 9 at 12 5 myr a considerable length of the epr was simultaneously subducted beneath baja thus terminating subduction processes along this segment10 by 3 5 myr the locus of transcurrent faulting had switched from the west of baja to within the developing gulf of california coupling baja to the pacific plate11 13 we present here data for holocene volcanic rocks from baja california norte which show that post ridge subduction volcanism has a distinctive geochemistry these results have important consequences for the interpretation of ancient orogenic belts

geochemistry and tectonic setting of alkaline volcanic rocks in the antarctic peninsula a review abstract the numerous miocene recent alkaline volcanic outcrops in the antarctic peninsula form a substantial volcanic province the least well known part of a major belt of alkaline volcanism that extends between south america and new zealand the outcrops consists mainly of aa and pahoehoe lavas and hyaloclastites which locally contain accidental nodules of spinel lherzolite and other mantle derived lithologies the province is predominantly basaltic with two major differentiation lineages 1 a sodic series of olivine and alkali basalt hawaiite mugearite trachy phonolite and trachyte and 2 a relatively potassic highly undersaturated series of basanite tephrite and phono tephrite all the lavas show varying effects of fractionation by crystallization of olivine and clinopyroxene joined by plagioclase in the hawaiites to trachytes fractional crystallization can probably explain most of the chemical variation observed within each outcrop but variable partial melting is necessary to account for the differences in incompatible element enrichment between the two series and between the individual outcrops the degree of partial melting may not have exceeded 3 as is the case for many other alkaline magmas the volcanism is an intraplate phenomenon but there is no correlation in timing between the cessation of subduction and the inception of alkaline volcanism the activity cannot be related to the passage of the coupled pacific antarctic plate over a stationary mantle hot spot although the precise causal relationship with tectonic setting is unknown regional extension was a prerequisite for giving the magmas rapid access to the surface

transport of10be and9be in the ocean beryllium isotopes 10be and9be have been measured in suspended particles of 1 mm size collected by mid water sediment traps deployed in the eastern pacific at manop sites h 6 32 n 92 50 w water depth 3600 m and m 8 50 n 104 00 w 3100 m for comparison surface sediments from box cores taken from the two sites were also studied the concentrations of10be and9be in sediment trap particles are about an order of magnitude smaller than those in the bottom sediments which contain about 8 109 and 6 1016 atoms g 1 of10be and9be respectively the sediment trap samples collected from 50 m off the bottom showed significant 26 63 contributions from resuspended bottom sediments the10be 9be ratio in trap samples varies from 3 to 20 10 8 the variation may partly result from varied proportion of authigenic detrital material the fluxes of both isotopes exhibit a very strong seasonality the fluxes of10be into the traps at about 1500 m are estimated as 9 105 and 4 105 atoms cm 2 a 1 at sites h and m respectively these values are to be compared with the fluxes into the sediments of 4 5 105 atoms cm 2 a 1 at both locations good correlations exist between10be 9be and27al indicating that the primary carrier phase s for the beryllium isotopes in the water column may be aluminosilicates

petrogenesis of the eocene and mio pliocene alkaline basaltic magmatism in meseta chile chico southern patagonia chile evidence for the participation of two slab windows the meseta chile chico mcc 46 48s is the westernmost exposure of eocene lower basaltic sequence lbs 55 40 ma k ar ages and mio pliocene upper basaltic sequence ubs 16 4 ma k ar ages flood basalt volcanism in patagonia the mcc is located south of the lago general carrera buenos aires lgcba southeast from the present day chile triple junction ctj east of the actual volcanic gap between southern south volcanic zone and austral volcanic zone ssvz and avz respectively and just above the inferred location of the south chile ridge segment subducted at 6 ma scr 1 erupted products consist of mainly ne normative olivine basalt with minor hy normative tholeiites basalt trachybasalt and basanite mcc lavas are alkaline 42 7 53 1 wt sio2 3 8 wt na2o k2o and relatively primitive ni 133 360 ppm cr 161 193 ppm co 35 72 ppm 4 16 5 mgo wt they have a marked oib like signature as shown by their isotopic compositions 87 sr 86 sro 0 70311 0 70414 and qnd 4 7 5 1 and their incompatible trace elements ratios ba la 10 20 la nb 0 46 1 09 ce pb 15 52 27 5 sr lab25 reflecting deep mantle origin ubs primitive lavas have characteristics similar to those of the eocene lbs basalts while ubs intermediate lavas show geochemical imprints la nbn1 sr lan25 low ce pb nb u compatible with contamination by arc slab derived and or crustal components we propose that the genesis and extrusion of

subduction of diverging plates and the principles of slab window formation consumption of an ocean basin by subduction commonly brings a sea floor spreading ridge toward a deep sea trench if plate divergence and convergence continue after the ridge intersects the subduction zone a slab window forms between the subducted parts of the diverging oceanic plates producing anomalous thermal physical and chemical effects in the surrounding asthenospheric mantle in turn these conditions alter the tectonic and magmatic evolution of the overriding plate usually disturbing ordinary fore arc and arc regimes differential lithospheric stresses on opposite sides of the triple junction contribute to disturbances in the overriding plate anomalous magmatism from fore arc to back arc may be accompanied by fore arc metamorphism strike slip faulting uplift extension and in extreme cases rifting the shape and size of the window are controlled mainly by the pre subduction ridge transform trench configuration slab dip angles and vectors of plate convergence subducted ridge segments expand into windows whose margins approximately parallel the motion vectors between the triple junction and the subducting plates subducted transform faults continue to be active usually as oblique slip faults until the plates separate as transform faults subduct they become longer on the plate which occupies the acute angle between ridge and trench and shorter on the other plate trains of isolated windows produced by subduction of a segmented ridge transform system progressively expand during descent commonly merging together to form a composite slab window oblique subduction of a highly segmented ridge is likely to produce two or more fraternal slab windows one at each site of ridge trench intersection above a slab window arc volcanism diminishes and may be replaced by volcanism of mid ocean ridge or rift affinity the change in chemical character reflects various processes including elevated heat flow decreasing hydration of the upper mantle juxtaposition of supra and sub slab mantle reservoris asthenospheric upwelling and melting of the trailing plate edges if the slab window migrates the anomalous magmatic regime may be replaced by renewed arc volcanism identifying the effects of slab windows in ancient convergent margin assemblages requires an understanding of slab window principles and implications

cordilleran slab windows the geometry and geologic implications of subducted spreading ridges are topics that have bedeviled earth scientists ever since the recognition of plate tectonics as a consequence of subduction of the kula farallon and east pacific rises slab windows formed and migrated beneath the north american cordillera the probable shape and extent of these windows which represent the asthenosphere filled gaps between two separating subducting oceanic plates are depicted from the late cretaceous to the present possible effects of the existence and migration of slab windows on the cordillera at various times include cessation of arc volcanism and replacement by rift or plate edge volcanism lithospheric uplift attenuation and extension and increased intensity of compressional tectonism eocene extensional tectonism and alkaline magmatism in southern british columbia and the northwestern united states were facilitated by slab window development

constancy of oceanic deposition of 10 be as recorded in manganese crusts recent developments in the use of nuclear accelerators as high energy mass spectrometers1 2 has renewed interest in the study of cosmogenic 10be half life 1 5 myr as a geophysical time tracer the accelerator techniques enable us to measure 10be with a sensitivity of 107 atoms or less the potential age dating span with 10be is extended back to at least 15 myr bp providing that the 10be production and deposition rates remain constant or nearly constant over that time period a knowledge of the long term constancy in the 10be deposition rate is thus very important we report here the first detailed measurements of 10be and 9be in two ferromanganese oxide crusts from the sea floor of the equatorial atlantic sample no k 9 21 and the north pacific schw 1d each representing 10 myr of accumulation history we have found in these crusts that the deposition of 10be and 10be 9be during the past 7 9 myr has been constant averaged over time intervals of 1 myr the variation is within 6 before that time both crusts show similar significant deviations the 7 9 myr demarcation may be related to the reported late miocene global abyssal circulation change in the ocean3

mantle flow and dynamic topography associated with slab window opening insights from laboratory models abstract we present dynamically self consistent mantle scale laboratory models that have been conducted to improve our understanding of the influence of slab window opening on subduction dynamics mantle flow and associated dynamic topography over geological time scales the adopted setup consists of a two layer linearly viscous system simulating the subduction of a fixed plate of silicone lithosphere under negative buoyancy in a viscous layer of glucose syrup mantle our experimental setting is also characterized by a constant width rectangular window located at the center of a laterally confined slab modeling the case of the interaction of a trench parallel spreading ridge with a wide subduction zone we found that the opening of a slab window does not produce consistent changes of the geometry and the kinematics of the slab on the contrary slab induced mantle circulation quantified both in the vertical and horizontal sections using the feature tracking image analysis technique is strongly modified in particular rollback subduction and the opening of the slab window generate a complex mantle circulation pattern characterized by the presence of poloidal and toroidal components with the importance of each evolving according to kinematic stages mantle coming from the oceanic domain floods through the slab window indenting the supra slab mantle zone and producing its deformation without any mixing between mantle portions the opening of the slab window and the upwelling of sub slab mantle produce a regional scale non isostatic topographic uplift of the overriding plate that would correspond to values ranging between ca 1 and 5 km in nature assuming that our modeling results can be representative of the natural behavior of subduction zones we compared them to the tectonics and volcanism of the patagonian subduction zone we found that the anomalous backarc volcanism that has been developing since the middle miocene could result from the lateral flow of sub slab mantle and that part of the patagonian uplift could be dynamically supported

subduction products according to experimental prediction knowledge of the thermal structure and processes in subduction zones is so uncertain that our more extensive knowledge of source materials and phase relationships remains insufficient for prediction of magmatic products the phase relationships do provide important constraints for testing geophysical and petrogenetic models especially when considered along with geochemical constraints physical questions such as the behavior of partially molten rocks are intimately related to the compositions of magmas from source to eruption site tentative predictions and conclusions include 1 andesite is not a primary magma from oceanic crust 2 it is unlikely that liquids from subducted oceanic crust yield andesites by fractionation except by melting of amphibolite in an especially warm subducted crust 3 it is more likely that hydrous siliceous magmas from the crust leak into the overlying mantle 4 andesite magmas can be generated in peridotite only under exceptional circumstances 5 h2o undersaturated basic magmas from peridotite modified by aqueous fluids or hydrous magma may yield andesite by fractionation 6 phlogopite peridotite could escape melting and yield alkalic magmas at deeper levels 7 andesite magmas could be produced in continental crust only by extreme heating through underplating of basalts or by mixing of basalt and rhyolite derived from crust most of these conclusions need revision if convection in the asthenosphere wedge raises the temperature of the subducted slab to 1250 c at 100 km as proposed by marsh 1979a this demonstrates the dependence of experimental predictions on geophysical models

radiocarbon in annual coral rings from the eastern tropical pacific ocean sixty radiocarbon measurements were performed on aragonite from annually banded corals collected from three sites in the galapagos islands preanthropogenic 14c values of coral that grew around a d 1930 averaged 70 this is substantially lower than average values previously reported 51 for corals from florida and belize in the western north atlantic ocean a decrease of 6 was noticed in coral that grew from 1930 to 1954 this decrease could be interpreted as a suess effect in surface ocean water the 100 increase in 14c for coral that grew from 1954 to 1973 is the result of bomb produced 14c that was introduced to the surface ocean waters the 14c levels in corals that grew during el nino years were considerably higher than those for normal years these higher values are attributed to the absence of up welling at the equator during el nino events

10be dating of large manganese nodules from world oceans the results of 10be and 9be measurements in several sections of ten large manganese nodules are presented along with u and th isotope data on five of them the 10be and 10be 9be methods yield growth rates covering a period of 5 10 ma which are in excellent agreement with each other in three cases these rates are also in very good agreement with those derived from the 230th methods whereas in the other two the 230th based rates are faster by over a factor of two based on the discrepancy between the inventory of 10be as well as 230th in nodules and the overhead seawater column it is calculated that the former scavenge their be and th isotopes from 2 900 m of the overhead water column the deduced 9be concentrations of ocean water based on its 10be contents and the 10be 9be ratios of nodule surfaces are found to be in good agreement with the direct 9be measurements of seawater implying that 9be in nodules like10be is mostly authigenic

beryllium 10 in deep sea sediments a tracer for the earth s magnetic field intensity during the last 200 000 years abstract we present a new approach for the reconstruction of the earth s magnetic field intensity during the last 200 000 years based on 10 be in deep sea sediments a simple box model that is able to describe boundary scavenging of 10 be in the ocean is used to study the sensitivity of 10 be to changes of water circulation and or varying particle rain rates the model is applied to separate transport and production signal in single 10 be records vertical sediment accumulation rates are used as input values for the model calculations the calculated scavenging factors are applied to correct the 10 be records for boundary scavenging the model corrected 10 be based paleointensity record correlates well with other records from very different archives

trench proximal volcanism following ridge crest trench collision along the antarctic peninsula on the basis of multichannel seismic reflection magnetics and bathymetry data a pleistocene holocene 0 1ma volcanic seamount has been identified at a location very close to the shelf edge off the west coast of the antarctic peninsula the seamount overlies the subducted trace of a transform fracture zone which divides segments of the margin along which collision took place 3 1 and 6 0 ma ago new trace element and isotopic data from dredged samples demonstrate that the seamount appears to be predominantly formed of relatively primitive undersaturated alkali basalt the basalts have geochemical signatures that are indistinguishable from ocean island basalts and some continental alkali basalts e g th ta 1 0 1 5 rb nb 0 5 87sr 86sr 0 70265 0 7028 143nd 144nd approximately 0 5129 and are totally lacking in any evidence for interaction with subduction enriched mantle geochemical similarities with other more trench distal postsubduction alkalic basalts along the antarctic peninsula are striking strongly implying that all the postsubduction basalts were derived from a chemically similar asthenospheric source region the basalts were most likely to have been generated as a result of the formation of a slab window beneath the antarctic peninsula following ridge crest trench collision subduction component free subslab asthenosphere upwelled into the incipient void left by the continued sinking of the leading plate following collision and decompressional melting resulted this type of trench proximal volcanism following ridge crest trench collision differs from that in other locations where calc alkaline volcanism persisted or ophiolite obduction occurred

tectonic setting of eocene boninite magmatism in the izu bonin mariana forearc abstract middle eocene boninites were simultaneously generated over a large region during the early history of the izu bonin mariana ibm arc however widespread boninite magmatism is not recognised in younger subduction zones of similar dimensions this suggests that an additional tectonic or thermal factor influenced the generation of the ibm boninite suite regional uplift ocean island basalt style magmatism and high heat flow also characterised the northern philippine sea plate psp at the start of the middle eocene these features are similar to those observed in large volume basaltic volcanic provinces such as the early tertiary north atlantic and suggest that the ibm boninite suite may have been produced because there was already a thermal anomaly in the mantle beneath the psp the reconstructed middle eocene location of the ibm arc and west philippine basin lies close to the present day manus basin where petrological and geochemical evidence indicate the presence of a mantle plume a calculated plume track linking these locations through time also passes close to the eauripik rise an aseismic ridge on the caroline plate during the oligocene and miocene therefore we propose that a thermal anomaly or mantle plume influenced the magmatic and tectonic development of the western pacific from the middle eocene until the present day

a geochemical traverse across the north chilean andes evidence for crust generation from the mantle wedge major and trace element and sr and nd isotope analyses are presented on 186 0 ma magmatic rocks along an east west traverse across north chile at 22 s esr ranges from 25 to 100 and end from 6 to 9 but the low end and high esr values are in rocks generated in the last 15 ma it is argued that previous discussions of petrogenesis in north chile have been hampered because the changes in magma chemistry in this area of unusually thick crust reflect not one but two processes one results in a progressive shift of esr from 25 to 20 and end from 6 to 6 in jurassic to recent rocks which is accompanied by increasing ta sm and sr and decreasing th ta the second is largely confined to the younger rocks and it is characterised by esr increasing up to 100 with increasing sio2 and decreasing sr and it results in relatively shallow trends on an end esr diagram the preferred interpretation is that trend 1 is due to the mobilisation of old late proterozoic mantle lithosphere as magmatism migrated eastwards and that trend 2 is due to crustal melting and contamination with differentiation in this area of thickened continental crust it follows that the mantle wedge is the principal site of crust generation and it is argued that 20 of the sr in the recent northern chile rocks is derived from the subducted ocean crust

neogene patagonian plateau lavas continental magmas associated with ridge collision at the chile triple junction extensive neogene patagonian plateau lavas 46 5 to 49 5 s southeast of the modern chile triple junction can be related to opening of asthenospheric slab windows associated with collisions of chile rise segments with the chile trench at 12 ma and 6 ma support comes from 26 new total fusion whole rock 40ar 39ar ages and geochemical data from back arc plateau lavas in most localities plateau lava sequences consist of voluminous tholeiitic main plateau flows overlain by less voluminous 2 to 5 million year younger alkalic postplateau flows northeast of where the ridge collided at 12 ma most lavas are syncollisional or postcollisional in age with eruptions of both sequences migrating northeastward at 50 to 70 km ma plateau lavas have ages from 12 to 7 ma in the western back arc and from 5 to 2 ma farther to the northeast trace element and isotopic data indicate main plateau lavas formed as larger percentage melts of a garnet bearing oceanic island basalt oib like mantle than postplateau lavas the highest percentage melts erupted in the western and central plateaus in a migrating slab window model main plateau lavas can be explained as melts that formed as upwelling subslab asthenosphere which flowed around the trailing edge of the descending nazca plate and then interacted with subduction altered asthenospheric wedge and continental lithosphere alkaline postplateau lavas can be explained as melts generated by weaker upwelling of subslab asthenosphere through the open slab window thermal problems of high pressure melt generation of anhydrous mantle can be explained by volatiles h2o and co2 introduced by the subduction process into slab window source region s an oib like rather than a mid ocean ridge basalt morb like source region and the lack of magmatism northeast of where ridge collision occurred at 13 to 14 ma can be explained by entrainment of weak plume s or regional variations in an ambient oib like asthenosphere

10be and th isotopes in manganese nodules and adjacent sediments nodule growth histories and nuclide behavior abstract the concentration profiles of 9 be 10 be 230 th 232 th 231 pa via 227 th and 238 u have been measured in three manganese nodules one each from the north pacific a47 16 4 the south pacific tf 5 and the indian ocean r v vitiaz in addition the 10 be concentration in deep water from the geosecs reoccupation station 500 of the north pacific and in cores raised from the manganese nodule field in the north pacific have been measured the 10 be concentration in nodule and seawater samples was measured by the accelerator masss spectrometric technique employing the yale tandem van de graaff accelerator the concentrations of 10 be 230 th exc and 231 pa exc and ratios of 10 be 9 be and 230 th exc 232 th all decrease with depth in the nodules this decrease interpreted in terms of nodule growth yields average growth rates of a few millimeters per million years for the nodules the growth rates of the nodules exhibit temporal variations both on short time 50 000 years and long time several million years scales of the three nodules studied only in tf 5 is the short term average growth rate based on 230 th exc in the top 0 0 5 mm the same as the long term average rate based on be isotope data for the 0 5 17 mm interval for the other two nodules the recent average growth rates based on 230 th exc data differ significantly from the long term average growth rates based on be isotopes in a47 16 4 the 10 be based rate is less than the 230 th exc rate and in r v vitiaz the 10 be based rate is greater than the 230 th exc rate this observation coupled with measurable changes in growth rates even during the past few hundred thousand years suggests but does not prove that the discordant growth rates deduced from 230 th exc and 10 be profiles document changes in nodule growth rate with time rather than mixing effects on 230 th exc profiles the 10 be concentration in the geosecs north pacific deep water is 6100 1200 atoms g this value coupled with the average surface 10 be 9 be ratio of north pacific nodules predicts a 9 be concentration within the limits of measured values the inventory of 10 be and 230 th exc in the nodules is only 10 of the total the remaining being in sediments the 10 be concentrations in the upper portions of two adjacent cores studied are nearly the same but the deposition fluxes of both 10 be and 230 th based on 230 th dating vary by a factor of two this difference is attributable to local redistribution of sediment at the time of deposition prior to accumulation

26al 10be and u th isotopes in blake outer ridge sediments implications for past changes in boundary scavenging we have investigated the distributions of 26 al 10 be u th isotopes and stable 27 al and 9 be in core ch88 11p 30 40pn 74 41pw 3337 m from the blake outer ridge bor in the western north atlantic an area that is characterized by high terrigenous input results show authigenic 26 al 27 al of 6 2 8u10 314 atom atom authigenic 10 be 9 be of 0 5 2 6u10 38 atom atom 6 0 2 1 8u10 6 atoms g of 26 al and 5 3 15 1u10 8 atoms g of 10 be in the core over the last glacial interglacial cycle these values as well as total 230 thex 232 th and 10 be 9 be ratios were all at their minima during the last glacial maximum lgm reflecting intensified terrestrial influx to the area aluminosilicate material clays in this influx preferentially scavenged 230 th over the two other particle reactive nuclides 26 al and 10 be such that their boundary scavenging during lgm followed the order 230 ths 10 bes 26 al as opposed to 10 bev 230 ths 26 al in the holocene they appear at variance with the order 10 bes 26 alw 230 th as would be predicted from the relative particle reactivities of the three species these scavenging characteristics point to 10 be 26 al as a potentially more suitable proxy for paleoproductivity than 10 be 230 thex mass balance considerations for 26 al and 10 be show a five fold lgm to holocene increase in deep water circulation in the study area with little change in ocean productivity except during the deglaciation when it increased noticeably fl 2001 elsevier science b v all rights reserved

holocene sea level reconstruction using lagoon specific local marine reservoir effect and geophysical modeling in tongatapu kingdom of tonga abstract reconstructing the history of holocene relative sea levels around tonga provides essential constraints on the recent geological evolution of this region and paleoenvironmental context for archaeological studies however few sea level records are currently available in the region and no quantitative paleoenvironmental studies using geochemical or geophysical methods have been reported here we reconstruct the sea level history of tongatapu island using radiocarbon measurements and glacio hydro isostatic adjustment gia modeling our analyses reconstructing the evolution of the lagoon suggest that the average size of gafrarium tumidum decreased synchronously with corresponding changes in the paleoenvironment these changes also correspond to the increasing trend of the lagoon specific local marine reservoir ages rlagoon from 219 78 to 368 69 years between 2 6 and 0 4 ka sea surface salinity sss decline within fanga uta lagoon was also synchronous with these changes caused by a gradual decrease in the exchange of water in and out of the lagoon estimated sss from shell was somewhat higher ca 2 6 cal kyr b p than the present suggesting that the lagoon was relatively open to the ocean at that time our gia modeling predicts mid holocene sea level highstand hhs was less than 1 m above the present sea level in tongatapu suggesting that previously reported observations of an hhs require additional contributions perhaps from crustal uplift furthermore recent gnss observations of vertical uplift rates at tongatapu are an order of magnitude higher than the long term uplift rate obtained from holocene sea level data

among polynesia s first pots x ray fluorescence spectroscopy and petrographic thin section microscopy are applied to a sample of anomalous appearing lapita pottery sherds from nukuleka the earliest archaeological site in polynesia both analyses indicate non local origins for the ceramic vessels suggesting they were brought on founding canoes mineral inclusions in ceramic tempers eliminate sources to the west of tonga in eastern melanesia rather the temper source is most probably a dacitic high island from a distant island arc of central melanesia polynesian origins and settlement are issues more complex than currently recognized by the orthodox progressive settlement model for lapita expansion across oceania

analysis of james ross island volcanic complex and sedimentary basin based on high resolution aeromagnetic data abstract high resolution aeromagnetic surveys provide a geophysical tool to help image the subsurface structure of volcanoes and their tectonic framework here we interpret high resolution aeromagnetic data and models for james ross archipelago and surrounding regions to provide a new perspective on neogene magmatism emplaced along the eastern margin of the antarctic peninsula based on the analysis and modelling of magnetic anomalies we were able to image the subglacial extent of miocene to recent alkaline rocks of the james ross island volcanic group and map tectonic structures that appear to have exerted important controls on neogene magmatism high amplitude linear anomalies detected over mount haddington stratovolcano were modelled as caused by subvertical feeder bodies extending to a depth of at least 3 km these feeder bodies may have been emplaced along a n s oriented volcano tectonic rift zone we also identified several effusive subglacial centres and imaged two concentric magnetic arcs which we related to neogene volcanic and subvolcanic lineaments likely controlled by mid cretaceous strike slip fault belts and associated deformation zones the regional magnetic quiet zone that encompasses james ross island is caused by the thick sedimentary infill of the larsen basin and a low amplitude magnetic high within the basin is inferred to reflect a basement push up structure associated with strike slip faulting along the eastern edge of the antarctic peninsula in the offshore regions magnetic anomalies southwest of tabarin peninsula and west of vega island may reflect recent volcanic structures that have yet to emerge from the sea floor

geology geochronology and tectonic setting of late cenozoic volcanism along the southwestern gulf of mexico the eastern alkaline province revisited a nnw trending belt of alkaline mafic volcanic fields parallels the gulf of mexico from the u s border southward to veracruz state in eastern mexico previous studies grouped this volcanism into the so called beastern alkaline provinceq eap and suggested that it resulted from gulf parallel extensional faulting migrating from north to south from oligocene to present on the basis of new geologic studies forty nine unspiked k ar and two 40 ar 39 ar ages we propose a new geodynamic model for the volcanism along the southwestern gulf of mexico we studied in detail four of the six recognized fields of mafic alkaline volcanism in veracruz state 1 the lavas flows of tlanchinol area 7 3 5 7 ma 2 the alamo monogenetic field and sierra de tantima 7 6 6 6 ma 3 the poza rica and metlatoyuca lava flows 1 6 1 3 ma and 4 the chiconquiaco palma sola area 6 9 3 2 ma other two mafic volcanic fields may represent the continuation of alkaline volcanism to the southeast the middle miocene lavas at anegada high offshore port of veracruz and the middle to late miocene volcanism at the los tuxtlas the existence of major neogene extensional faults parallel to the gulf of mexico i e n s to nnw sse proposed in previous works was not confirmed by our geological studies elongation of volcanic necks vent alignment and faults mapped by subsurface data trend dominantly ne to ene and nw to nnw these directions are parallel to transform and normal faults that formed during the late jurassic opening of the gulf of mexico ascent of mafic magmas was likely facilitated and controlled by the existence of these pre existing basement structures

last glacial to post glacial 10be fluctuations in a sediment core from the academician ridge lake baikal 10be concentrations sediment composition and sediment accumulation rates were determined in a sediment core from the academician ridge of lake baikal the 10be concentrations vary from 5 07 108 atoms g in the last glacial section to 11 25 108 atoms g in the section representing the post glacial climate stage 10be fluxes on the other hand increase toward the last glacial stage coinciding with an increase of the sediment accumulation rate these facts imply that climate related sedimentation is related to both flux and concentration of the nuclide in the sediment core during the last glacial stage arid conditions in the lake s watershed caused a marked increase in flux of clay particles with low 10be content to the lake resulting in an increased flux but lowered concentration of 10be in the sediments it is suggested that the 10be record of the bottom sediments of lake baikal can be used as a proxy indicator of climate change in central asia

the scotia sea gateway no outlet for pacific mantle abstract the scotia sea in the south atlantic holds a prominent position in geodynamics because it has been proposed as a potential outlet of asthenosphere from under the shrinking pacific into the mantle beneath the opening atlantic shear wave splitting and geochemical studies have previously tested this hypothesis here we take a different approach by calculating present day dynamic topography of the region in search for a systematic trend in dynamic topography decreasing from west to east in response to a flow related pressure gradient in the sublithospheric mantle to this end we reconstruct the kinematic history of the scotia sea which is characterized by complex back arc spreading processes active on a range of time scales our plate reconstructions allow us to derive an oceanic age grid and to calculate the associated residual dynamically maintained topography of the scotia sea by comparing present day isostatically corrected topography with that predicted from our reconstruction the results provide no indication for a systematic trend in dynamic topography and we conclude that the material needed to supply the growing subatlantic mantle must be derived from elsewhere