geochemical evidence for a catastrophic biotic event at the frasnian famennian boundary in south china a strong delta sup 13 c anomaly coincides with a weak ir anomaly at the frasnian famennian f f boundary exposed at xiangtian guangxi south china the maximum whole rock ir abundance is 0 23 ppb 0 35 ppb on a carbonate free basis compared with averages of 0 016 and 0 044 ppb above and below the boundary interval the delta sup 13 c in carbonate abruptly shifts from a late frasnian level of about 1 per thousand to minus 2 49 per thousand in the boundary interval and then abruptly returns to pre boundary levels suggesting a temporary reduction of surface water biomass a widely distributed over at least several hundred kilometers limestone breccia unit of possible wave deposit origin is present directly below the boundary in south china enrichments of al v cr as and u and a dip in the mn abundance in the boundary interval indicate that reducing conditions might have been associated with the element enrichment however an alternative hypothesis is that oceanic impact s at the f f boundary near south china provided the excess ir and caused the biotic crisis and the formation of breccia deposits observed in the late devonian south china sea

brazilian structural provinces an introduction abstract the territory of brazil coincides almost entirely with the south american platform the crystalline core of the continent its basement is composed of ancient metamorphic and igneous rocks and it has not suffered any tectonic regeneration since the beginning of the phanerozoic sedimentary rocks with almost horizontal bedding cover this crystalline basement this latter shows ages as old as early precambrian although ages between 500 and 1000 m y are conspicuously frequent the cratonic areas became consolidated more than 1700 m y ago whereas the fold belts formed essentially between 500 and 1700 m y ago the sedimentary cover accumulated from the early silurian in three large intracratonic basins until the platform became completely stabilized a late jurassic to early cretaceous reactivation caused by the break up of the ancient gondwana continent created another basin sequence chiefly along the atlantic continental margin based on the nature of the crystalline basement rocks and the sedimentary cover ten structural provinces can be distinguished 1 rio branco province in the north of the country occupied by the guyana shield and still only poorly known with an important fold belt developed during the so called transamazonian cycle 2000 200m y with high grade metamorphic rocks and only slight influence of later events 2 tapajos province in central brazil corresponding to the amazonas or guaporecraton showing chiefly precambrian crystalline rocks and behaving as an cratonic area during the phanerozoic 3 sa o francisco province located on the atlantic shield with its basement covered by rocks of different ages chiefly affected by the brasiliano cycle between 1000 and 500 m y and constituting another cratonic area 4 tocantins province between the amazonas and sa o francisco cratons with the oldest rocks in its centre ages over 2600 m y and at the eastern and western borders metamorphic sequences of various fold belts and almost no phanerozoic deposits 5 mantiqueira province located along the southern part of the atlantic coast affected chiefly by the brasiliano folding cycle 6 borborema province in the northeast brazilian fold belt affected by the brasiliano cycle in a very complex way and with important faulted zones it was reactivated in phanerozoic times when also sedimentary covers accumulated 7 amazonas province represented by the amazonas sedimentary basin syneclise and subdivided into four parts by three important arcs 8 parnaiba province coinciding with the piauf maranha o syneclise and filled with a rather thick sedimentary sequence 9 paranaprovince the sedimentary basin of southern brazil in which the well known late paleozoic glaciation features are found and which possesses an extensive cover of basaltic rocks of late jurassic to early cretaceous age 10 coastal province and continental margin the youngest structural unit developed during the separation of the continent and represented by rift valleys and coastal basins filled with mesozoic cenozoic deposits of various kinds

gravity derived moho for south america crustal structure in south america is one of the least understood among the earth s continental areas variations in crustal thickness are still poorly constrained over large portions of the continent because of scarce or unevenly distributed crustal thickness estimates throughout south america to address this scarce and inhomogeneous data cover we explore the possibility to derive crustal thickness from satellite gravity data in this study we utilize the combined gravity model eigen 6c which is composed of goce and other gravity data the gravity field and steady state ocean circulation explorer goce satellite has a much more uniform spatial resolution than any land based gravity or seismic survey in south america the gravity data inversion is for a simple two layer model with fixed density contrast over the interface the moho the method is not relying on point constraint data and assumes that all of the signal is related to topography of the moho model quality can therefore be assessed by a comparison with point observations on crustal thickness we show that for the stable part of the continent 90 of our estimates are similar within error bounds to seismic observations variations occur in active orogenic zones or regions with suspected non standard moho density contrasts a comparison with seismological models shows a high correlation with the most recent model especially in areas where continental and global models of crustal structure have limitations in terms of wave paths or point constraints the gravity based model provides a unique continuity of crustal structure providing new insights on structure and tectonics and increase our understanding of the earth s structure underneath south america

observations at terrestrial impact structures their utility in constraining crater formation hypervelocity impact involves the near instantaneous transfer of considerable energy from the impactor to a spatially limited near surface volume of the target body local geology of the target area tends to be of secondary importance and the net result is that impacts of similar size on a given planetary body produce similar results this is the essence of the utility of observations at impact craters particularly terrestrial craters in constraining impact processes unfortunately there are few well documented results from systematic contemporaneous campaigns to characterize specific terrestrial impact structures with the full spectrum of geoscientific tools available at the time nevertheless observations of the terrestrial impact record have contributed substantially to fundamental properties of impact there is a beginning of convergence and mutual testing of observations at terrestrial impact structures and the results of modeling in particular from recent hydrocode models the terrestrial impact record provides few constraints on models of ejecta processes beyond a confirmation of the involvement of the local substrate in ejecta lithologies and shows that z models are at best first order approximations observational evidence to date suggests that the formation of interior rings is an extension of the structural uplift process that occurs at smaller complex impact structures there are however major observational gaps and cases e g vredefort where current observations and hydrocode models are apparently inconsistent it is perhaps time that the impact community as a whole considers documenting the existing observational and modeling knowledge gaps that are required to be filled to make the intellectual breakthroughs equivalent to those of the 1970s and 1980s which were fueled by observations at terrestrial impact structures filling these knowledge gaps would likely be centered on the later stages of formation of complex and ring structures and on ejecta

microtektite like impact glass associated with the frasnian famennian boundary mass extinction abstract glass spherules of probable impact origin occur in the late devonian hony section in belgium they are found at the stratigraphic level of the frasnian famennian boundary which is marked by one of the largest mass extinctions in the phanerozoic the hony glass spherules are chemically identical to those previously described from the stratigraphically equivalent senzeille section located 90 km to the west southwest in contrast to the senzeille layer the age of the hony spherule layer is well constrained by conodont biostratigraphy the spherules have morphological and chemical characteristics similar to tektites and microtektites they have a broad range of chemical composition with generally high k2o na2o and al2o3 na2o k2o their water content is in the range measured for tektites and impact glasses small inclusions of lechatelierite may be present in the spherules these glass spherules are likely to be the product of an impact event that occurred at or very near the frasnian famennian boundary the target rock was probably a clay rich pelitic sediment intermixed with carbonates the 368 1 ma siljan ring crater sweden is the most likely impact site candidate the hony and senzeille glass spherules represent the first substantial evidence that an impact event may be linked to the frasnian famennian mass extinction

tightening up ne brazil and nw africa connections new u pb lu hf zircon data of a complete plate tectonic cycle in the dahomey belt of the west gondwana orogen in togo and benin the dahomey belt in togo and benin is an important segment of the larger west gondwana orogen here we review the geodynamic evolution of the dahomey belt and discuss new u pb and lu hf zircon data in light of similar data previously acquired on the geologically related northern borborema province in ne brazil eighteen samples from different tectonic settings and regions within the belt were collected for zircon isotopic investigation passive margin deposits of the atacora structural unit and lower units of the volta basin have detrital zircon signatures compatible with the flanking west africa craton the arc related magmatism resulted from the east dipping subduction of the goias pharusian oceanic lithosphere and is represented by a variety of granitoids emplaced in the benino nigerian shield between 670 and 610 ma these granitoids were mainly sourced from crustal reservoirs with subordinate juvenile input detrital zircon ages from syn orogenic deposits in benino nigerian shield suggest that arc development could have started as early as 780 ma the main period of melting in the internal part of the belt the benino nigerian shield is related to crustal thickening and occurs only ca 30 m y after initiation of the continental collision marked by the ca 610 ma ultra high pressure uhp metamorphism recorded at lato hills foreland development represented by the upper units of the volta basin developed soon after continental collision and persisted with the development of the west verging thrust front synchronously with the main period of crustal melting due to collision at ca 580 ma the subvertical transbrasiliano lineament in south america that corresponds to the kandi lineament in africa provides a present day fit between nw africa and ne brazil restoration of the movement of the transbrasiliano kandi lineament strike slip plate boundary places the dahomey belt and borborema province ne brazil along the same section of the west gondwana orogen this configuration would explain some of the misfits previously discussed in the literature and aligns the uhp eclogites in togo and ne brazil

one dimensional evolution of the upper water column in the atlantic sector of the arctic ocean in winter a one dimensional model is employed to reproduce the observed time evolution of hydrographic properties in the upper water column during winter between 26 january and 11 march 2015 in a region north of svalbard in the nansen basin of the arctic ocean from an observed initial state vertical diffusion equations for temperature and salinity give the hydrographic conditions at a later stage observations of microstructure are used to synthesize profiles of vertical diffusivity k representative of varying wind forcing conditions the ice ocean heat and salt fluxes at the ice ocean interface are implemented as external source terms estimated from the salt and enthalpy budgets using friction velocity from the rossby similarity drag relation and the ice core temperature profiles we are able to reproduce the temporal evolution of hydrography satisfactorily for two pairs of measured profiles suggesting that the vertical processes dominated the observed changes sensitivity tests reveal a significant dependence on k variation in other variables such as the temperature gradient of the sea ice the fraction of heat going to ice melt and the turbulent exchange coefficient for heat are relatively less important the increase in salinity as a result of freezing and brine release is approximately 10 significantly less than that due to entrainment 90 from beneath the mixed layer entrainment was elevated during episodic storm events leading to melting the results highlight the contribution of storms to mixing in the upper arctic ocean and its impact on ice melt and mixed layer salt and nutrient budgets

greenland sea convection in the winter of 1987 1988 during february 1988 a layer of almost uniform temperature and salinity between 100 and 1250 m depth was observed in the central greenland sea this feature indicative of previous deep convection is explained in terms of a conceptual penetrative convection model

on the parameterisation of oceanic sensible heat loss to the atmosphere and to ice in an ice covered mixed layer in winter abstract in high latitude oceans with seasonal ice cover the ice and the low salinity mixed layer form an interacting barrier for the heat flux from the ocean to the atmosphere the presence of a less dense surface layer allows ice to form and the ice cover reduces the heat loss to the atmosphere the ice formation weakens the stability at the base of the mixed layer leading to stronger entrainment and larger heat flux from below this heat transport retards and perhaps stops the growth of the ice cover as much heat is then entrained from below as is lost to the atmosphere this heat loss further reduces the stability and unless a net ice melt occurs the mixed layer convects two possibilities exist 1 a net ice melt sufficient to retain the stability will always occur and convection will not take place until all ice is removed the deep convection will then be thermal deepening the mixed layer 2 the ice remains until the stability at the base of the mixed layer disappears the mixed layer then convects through haline convection into the deep ocean warm water rises towards the surface and the ice starts to melt and a new mixed layer is reformed the present work discusses the interactions between ice cover and entrainment during winter when heat loss to the atmosphere is present one crucial hypothesis is introduced when ice is present and the ocean loses sensible heat to the atmosphere and to ice melt the buoyancy input at the sea surface due to ice melt is at a minimum using a one dimensional energy balance model applied to the artificial situation where ice melts directly on warmer water it is found that this corresponds to a constant fraction of the heat loss going to ice melt it is postulated that this partitioning holds for the ice cover and the mixed layer in the high latitude ocean when a constant fraction of heat goes to ice melt at least one deep convection event occurs before the ice cover can be removed by heat entrained from below after one or several convection events the ice normally disappears and a deep reaching thermal convection is established conditions appropriate for the weddell sea and the greenland sea are examined and compared with field observations with realistic initial conditions no convection occurs in the warm regime of the weddell sea a balance between entrained heat and atmospheric heat loss is established and the ice cover remains throughout the winter at maud rise convection may occur but late in winter and normally no polynya can form before the summer ice melt in the central greenland sea the mixed layer generally convects early in winter and the ice is removed by melting from below as early as february or march this is in agreement with existing observations

mineral prospectivity analysis for bif iron deposits a case study in the anshan benxi area liaoning province north east china abstract banded iron formation bif deposits are the most important source of iron in china and they are dominantly distributed in the north china craton ncc the anshan benxi area is situated in the northeastern part of the ncc and is considered to be the most fertile and productive iron metallogenic province in china the recent successive discoveries of large bif deposits in this area especially the world class dataigou bif deposit indicates that there is great potential for iron resources in this area the bif deposits are well defined by magnetic and gravity anomalies and thereby extensive petrophysical and geophysical studies are critical for bif prospective mapping a data driven weights of evidence wofe analysis was used for bif prospectivity modeling the results indicated that prospective areas are generally confined to the precambrian greenstone belt basin and high prospectivity zones class a are mainly distributed in the middle and northeastern parts of the study area moderate prospectivity zones class b are mainly distributed in southwestern and northeastern parts of the study area finally we employed selective human intervention to redefine priority targets to be more reliable the results suggest the method is successful as 82 target areas were narrowed to class a 20 and 11 class b targets the total of class a and class b targets was just over one third of the total number of targets offering a much smaller but effective selection of targets for consideration by mineral explorers

importance of convective mixing in seasonal ice margin simulations a two dimensional ice ocean model applicable to the greenland sea is constructed and used to examine the role of penetrative convection in the seasonal advance and retreat of the sea ice margin the standard version of the model is based on the full dynamic and thermodynamic equations of sea ice and on the primitive equations discretized on a vertical slab for the ocean it includes a special treatment of upper ocean vertical mixing based on a parameterization of entrainment at the bottom of the ocean mixed layer the simulation results with this standard version of the model show that when ice advection is included with ice moving outward over open water penetrative convection delays and greatly reduces the ice edge advance the resulting seasonal ice distribution yields better agreement with observation than a simulation that does not allow mixed layer entrainment when lateral heat transport is neglected in the ocean the ice margin again moves too a great extent outward this result demonstrates that in addition to vertical mixing a heat source must exist in the ocean mixed layer in order to resist and ultimately stop the advection of ice finally in the absence of ice motion penetrative convection has a less pronounced effect on the ice edge location

the neoproterozoic cambrian paraguay belt central brazil part i new structural data and a new approach on the regional implications abstract together with the araguaia and brasilia belts the paraguay belt forms in central brazil the tocantins province that is one of the largest orogens of western gondwana the corumba area occupies the site where the northern and southern parts of the paraguay belt form together with the chiquitos tucavaca aulacogen stretching e w in the adjacent bolivian territory an r r r basin system opened filled in the 700 650 540 ma interval within the amazon rio apa paleo continent the sedimentary volcanic rocks of the jacadigo and corumba groups found around the corumba city record part of the neoproterozoic cambrian passive margin precursor of the paraguay belt our pioneer structural analysis reveals that these rocks experienced progressive deformation phases d 1 d 2 d 3 and low grade metamorphism during the brasiliano cycle 540 513 ma the crystalline basement was also involved according to structural data and k ar ages in the literature the paleo passive margin was thickened during the d 1 d 2 deformation and was lately shortened d 3 in two orthogonal directions se nw d 3p and sw ne d 3t developed co axially and verging to nw d 1 d 2 d 3p structures record the closure of the basin precursor of the paraguay belt whereas d 3t structures seem related to the inversion of the aulacogen although the tectonic transport to nw as observed in the corumba area matches the reported transport of paraguay belt s supracrustal rocks towards the eastern margin of the rio apa block and araguaia belt s rocks towards the amazon craton the transport direction is opposite in other parts of the paraguay belt our comprehensive discussion of these facts brings to light profound regional implications

causes of intraplate seismicity in central brazil from travel time seismic tomography abstract new results of travel time seismic tomography in central brazil provide evidence that the relatively high seismicity in this region is related to the thinner lithosphere at the limit between the amazonian and sao francisco paleocontinents the transition between these paleocontinents is marked by low velocity anomalies spatially well correlated with the high seismicity region which are interpreted as related to the lithospheric thinning and consequent rise of the asthenosphere which have increased the temperature in this region the low velocity anomalies suggest a weakness region favorable to the build up of stress the effective elastic thickness and the strain stress regime for the study area are in agreement with tomographic results a high velocity trend is observed beneath the parnaiba basin where low seismicity is observed indicating the presence of a cratonic core our results support the idea that the intraplate seismicity in central brazil is related to the thin lithosphere underlying parts of the tocantins province between the neighboring large cratonic blocks

rapakivi granites from brazil and adjacent areas abstract a review of the rapakivi magmatism in brazil and adjacent areas demonstrates that the dominantly felsic anorogenic complexes of the amazonian craton can be considered as rapakivi suites they are found in almost all provinces of the craton and are concentrated in the timespan covering the end of the palaeoproterozoic until the beginning of the neoproterozoic the mesoproterozoic tin granites of the goias province correlated with the anorogenic series of the amazonian craton are also included in the rapakivi series both are correlated with the laurentia baltica rapakivi suites neoproterozoic rapakivi granites and mangerite charnockite series have been identified in post collisional tectonic settings related to the brasiliano event in southeastern brazil most of the rapakivi suites of the amazonian craton and those of goias are considered as anorogenic but probably there are also rapakivi suites associated with post collisional settings such as those formed during the brasiliano event the latter are possibly related to anatexis of the lower crust associated with crustal thickening the precambrian rapakivi granites of south america display a type and within plate geochemical signature they are generally metaluminous to slightly peraluminous but peralkaline rocks are also found basic rocks and charnockitic series coeval with the rapakivi granites are apparently less frequent than in north america massif type anorthosite is very rare amcg suites have been found only in the roraima state of brazil mangerites and charnockites associated and contemporaneous with the rapakivi granites have been described in rondonia but anorthositic complexes are lacking typical wiborgite and pyterlite are found in the rondonia and roraima regions in brazil and in venezuela parguaza granite in other regions medium even grained granites are largely dominant the general picture is thus similar to that observed in the amcg suites of north america differing in some degree from the classic finnish rapakivi granites which display wiborgitic and pyterlitic textures more frequently the most striking difference between the rapakivi granites of south america and their counterparts of laurentia baltica is that the former are very rich in tin deposits with associated y ree th f cryolite zr and in while the latter are relatively poor in them the study of rapakivi series in south america and more particularly in the amazonian craton will be extremely useful for a better understanding of their petrogenesis and tectonic evolution the rapakivi distribution in the craton demonstrates that the suggested tectonic provinces should be re evaluated furthermore the tectonic evolution of the amazonian craton seems to differ significantly from that of the sao francisco craton of brazil and the congo kasai craton in africa

arctic ocean circulation processes and water masses a description of observations and ideas with focus on the period prior to the international polar year 2007 2009 abstract the evolving knowledge of the arctic ocean its hydrography and its water masses and their transformations and circulation is reviewed starting with the observations made on fram 1893 1896 and extending to the international polar year ipy 2007 2009 the expeditions and observations after fram to the mid 20th century as well as the more extensive and systematic studies of water masses and circulation made from ice stations and airborne expeditions from the late 1940s to the late 1970s are briefly described the early concepts of the connections and exchanges between the arctic ocean and the world ocean are also discussed in the 1980s scientific icebreakers were beginning to enter the inner parts of the arctic ocean and large international programmes were launched culminating in the ipy the changes in the arctic ocean first noted in the atlantic layer in 1990 and shortly after in the upper layers are described the exchanges between the arctic ocean and the surrounding seas through the four main openings fram strait barents sea bering strait and the canadian arctic archipelago as well the volume and freshwater balances of the arctic ocean are examined

arctic ocean stability the effects of local cooling oceanic heat transport freshwater input and sea ice melt with special emphasis on the nansen basin the arctic loses energy to space and heat is transported northward in the atmosphere and ocean the largest transport occurs in the atmosphere the oceanic heat flux is significantly smaller and the warm water that enters the arctic ocean becomes covered by a low salinity surface layer which reduces the heat transfer to the sea surface this upper layer has two distinct regimes in most of the deep basins it is due to the input of low salinity shelf water ultimately conditioned by net precipitation and river runoff the nansen basin is different here warm atlantic water is initially in direct contact with and melts sea ice its upper part being transformed into less dense surface water the characteristics and depth of this layer are determined as functions of the temperature of the atlantic water and for different energy losses using a one dimensional energy balance model the amount of transformed atlantic water is estimated for two different sea ice melt rates and the assumption of a buoyant boundary outflow to create the upper layer sea ice formed elsewhere has to drift to the nansen basin with reduced ice cover this ice drift might weaken and the ice could disappear by the end of winter the surface buoyancy input would disappear and the upper layer might eventually convect back into the atlantic water reducing the formation of less dense polar water the created ice free areas would release more heat to the atmosphere and affect the atmospheric circulation

moho undulations beneath tibet from grace integrated gravity data summary knowledge of the variation of crustal thickness is essential in many applications such as forward dynamic modelling numerical heat flow calculations seismologic applications and geohistory reconstructions we present a 3 d model of the moho undulations over the entire tibetan plateau derived from gravity inversion the gravity field has been obtained by using the gravity recovery and climate experiment grace potential field development which has been integrated with terrestrial data and is presently the best available in the studied area for the effective use of the global geopotential model that has no height information of observation stations upward continuation is applied the moho model is characterized by a sequence of troughs and ridges with a semi regular pattern which could reflect the continent continent collision between the indian and eurasian plates the three deep moho belts troughs and shallow moho belts ridges between them are clearly found to have an e w directional trend parallel to the border of the plateau and tectonic lines while variation of the directionality is observed in central to southeast tibet to describe the distinctive shape of the moho troughs beneath tibet we introduce the term moho ranges the most interesting aspects of the moho ranges are 1 that they run in parallel with the border and tectonic sutures of the plateau 2 that the distances between ranges are found at regular distances of about 330 km except in northeast tibet and 3 that the splitting of the ranges into two branches is found as the distance between them is increasing from our study we conclude that the distinctive undulations of the tibetan moho have been formed by buckling in a compressional environment superimposed on the regional increase in crustal thickness according to our analysis the grace satelliteonly data turns out to have good enough resolution for being used to determine the very deep moho beneath tibet our moho model is the first one that covers the entire plateau

comment on k ar evidence from illitic clays of a late devonian age for the 120 km diameter woodleigh impact structure southern carnarvon basin western australia by i t uysal s d golding a y glikson a j mory and m glikson earth planet sci lett 192 2001 218 289 abstract k ar isotopic ages presented by uysal et al for illitic clay minerals from drill core samples were interpreted to date the woodleigh impact event at 359 4 ma allegedly implicating woodleigh in the late devonian mass extinction however only very equivocal evidence is presented by uysal et al to support a link between clay mineral paragenesis and impact related features and the k ar ages reveal a distribution that is essentially a continuum between 308 and 364 ma the age computed by uysal et al is based on an average of the five oldest ages within this group which has no geological or statistical basis the stratigraphic age constraints considered by uysal et al to be consistent with this age are much weaker than acknowledged and the impact could have been much older than mid devonian the size of the woodleigh crater is poorly constrained and the subject of an ongoing controversy uysal et al s suggestion of 120 km diameter is probably overestimated by a factor of two in which case a link to any mass extinction is unlikely

long wavelength undulations of the seismic moho beneath the strongly stretched western anatolia summary western turkey provides spectacular examples of the two end member models of deformation of the continental lithosphere with strain localization along the north anatolian fault and distributed north south extension along the aegean coast to provide constraints on the mechanisms of continental deformation we present a new high resolution image of lithospheric structurealonga 650kmtransectcrossingwesternanatoliaat28 elongitudefromtheblack sea to the mediterranean more than 2600 receiver functions are computed from records of teleseismic earthquakes at 40 broadband seismic stations with an average spacing of 15km lateral variations of crustal thickness and vp vs ratio are inferred from both h k and common conversion point stacks we observe long wavelength variations of moho depth from 31km in the thrace basin to 25km beneath the marmara sea 32km beneath the izmir ankara suture 25km beneath the menderes massif and 20km on the coast of the mediterranean no mid or lower crustal interface is visible in the migrated depth section and seismic discontinuities are confined to the shallow crust the small amplitude and long wavelength lateral variations of the moho topography suggest that viscous flow in a hot lower crust has smoothed out the lateral variations of crustal thickness induced by cenozoic continent continent collision thecrust mantleboundaryisflatbeneaththecentralandsouthernmenderesmassif the rougher moho topography and more heterogeneous crust of the marmara region likely result from transtension in the north anatolian fault zone superimposed onto aegean extension the moho of the subducted african lithosphere is observed dipping northward between 40 and 60km depth at the southern end of the profile the abrupt termination of the subducted slab only 50km to the north of the mediterranean coast confirms the slab tear inferred from previous tomographic studies

mechanisms of argon retention in clays revealed by laser 40ar 39ar dating a method for dating clays is important for studies of weathering diagenesis hydrocarbon migration and the formation of major metalliferous deposits however many attempts have produced imprecise or inaccurate results data from shales show that contrary to expectations the 40ar 39ar dating technique can be successfully used to determine the diagenetic age of ancient sediments because 39ar losses during irradiation are controlled by release from low retentivity sites in illite equivalent to those that have lost radiogenic 40ar in nature rather than by direct recoil as is generally assumed

a new understanding of the provinces of the amazon craton based on integration of field mapping and u pb and sm nd geochronology abstract new conventional and sensitive high resolution ion microprobe zircon u pb dating has led to a new understanding of the subdivision and evolution of the amazon craton during precambrian time with major improvements and changes made to the previous rb sr based model the interpretation of u pb and sm nd isotopic data identifies eight main precambrian tectonic provinces in the craton with ages ranging from 3 1 to 0 99 ga some of the provinces were generated by accretional arc related processes carajas transamazonic tapajos parima and rondonia juruena and others by recycling of continental crust central amazon rio negro and sunsas the exposed archean crust is restricted to the east carajas and south amapa in brazil and north imataca in venezuela of the craton indicating that the amazon craton is largely a proterozoic crust the carajas imataca 3 10 2 53 ga and transamazonian 2 25 2 00 ga provinces are composed predominantly of granite greenstone terranes the tapajos parima 2 10 1 87 ga and rondonia juruena 1 75 1 47 ga provinces represent new crust added as orogenic belts while the rio negro 1 86 1 52 ga and sunsas 1 33 0 99 ga provinces originated mainly by magmatic tectonic recycling of the above two orogenic belts the only zone with a prominent northeast trend is the poorly known k mudku shear belt characterized by a 1 20 ga shear zone which deforms the rocks of at least three different provinces rio negro tapajos parima and transamazonic the central amazon province comprises mostly orosirian volcano plutonic rocks uatuma magmatism and is a terrane in which the exposed crustal structure and deformation are pluton related the sm nd tdm model ages and nd suggest that the central amazon province was generated by the partial melting of archean continental crust carajas province perhaps related to underplating that began at the end of the tapajos parima orogeny 1 88 1 86 ga

main stages of the development of the sedimentary basins of south america and their relationship with the tectonics of supercontinents abstract twelve main stages of sedimentary basins of the south american platform have been identified as follows one neoarchean seven proterozoic and four phanerozoic basin forming tectonics are related to the accretion fusion and fission of the major continental landmasses of the earth s history such as atlantica nena rodinia gondwana pannotia and pangea the first proposed stage of basin development precedes the formation of these major landmasses and was developed on mesoarchean microcontinents the last stage succeeds the fission of pangea and is still in progress

a convective model for the weddell polynya abstract mechanisms are considered which may induct the large over 105 km2 area of open water or polynya which frequently occurs within the weddell sea winter sea ice we propose that when surface cooling and ice formation decrease the temperature and increase the salinity of the surface water the latter by salt rejection during ice formation in a preconditioned area static instability with intense vertical mixing can occur the upwelled warm salty deep water can then supply enough heat to melt the ice or prohibit its formation even in the middle of winter a simple two level model is derived to test this theory and is found to agree well with observations the process is found to be irregular due to different times of ice onset from one year to the next and to a lesser extent from variations in surface heating and cooling further it is shown that unless the freshwater input exactly balances the increased salinity from the overturn each year the system will either gain or lose salt yearly an

quantifying the influence of atlantic heat on barents sea ice variability and retreat abstractthe recent arctic winter sea ice retreat is most pronounced in the barents sea using available observations of the atlantic inflow to the barents sea and results from a regional ice ocean model the authors assess and quantify the role of inflowing heat anomalies on sea ice variability the interannual variability and longer term decrease in sea ice area reflect the variability of the atlantic inflow both in observations and model simulations during the last decade 1998 2008 the reduction in annual july june sea ice area was 218 103 km2 or close to 50 this reduction has occurred concurrent with an increase in observed atlantic heat transport due to both strengthening and warming of the inflow modeled interannual variations in sea ice area between 1948 and 2007 are associated with anomalous heat transport r 0 63 with a 70 103 km2 decrease per 10 tw input of heat based on the simulated ocean heat budget it is found that the heat transport into the western barents sea sets the bou

response to critical comment on a j mory et al 2000 woodleigh carnarvon basin western australia a new 120 km diameter impact structure by w u reimold and c koeberl

crustal framework of the northwest parana basin brazil insights from joint modeling of magnetic and gravity data abstract the transbrasiliano lineament tbl is a major proterozoic strike slip shear zone crossing the parana basin brazil yet the relationship between the tbl and basin structures is poorly understood magnetic and gravity data are available from the whole northwestern region of the parana basin and provide insights into crustal partitioning and basement geometry joint 2d modeling of these data was performed in three strategic sections of the study area two smaller sections were constrained by available borehole data and receiver function analysis modeling of these sections facilitated modeling of the third larger nw se section which crosses the basin in an area with significant lack of data these three sections together form the basis for a preliminary tectonic model of the northern parana basin the tbl includes strong n30 e trending gravity and magnetic gradients represented by the serra negra fault zone this structure separates two crustal tectonic domains the paranapanema block to the east and the amazon paleocontinent to the west the tectonic domain northeast of the fault is represented by the arenopolis magmatic arc the n60 70 e trending general carneiro fault also reflected in gravity anomalies forms another important magnetic contact our tectonic model shows a region of crustal thinning between the two fault zones where structures associated with possible reactivation form small cretaceous basins the baliza fault may accommodate significant vertical displacements of devonian and carboniferous sections of the parana basin the main structures revealed in the sections are associated with grabens and horsts interpreted as reactivated structures that controlled parana basin sedimentation the northwestern portion of the basin exhibits basement asymmetry caused by dip and strike slip faulting along the tbl

a model of sea ice and the upper ocean mixed layer off labrador an existing dynamic thermodynamic ice model is coupled to an existing bulk mixed layer model of the upper ocean the model is applied to the seasonal evolution of ice in the marginal ice zone off labrador ocean currents are prescribed daily atmospheric observations are used as forcing the coupled model simulates the growth phase of ice and the evolution of the upper ocean temperature and salinity reasonably well the results show that mixed layer deepening can account for the offshore ice extent the southern extent of ice is influenced more by the meridional gradients in atmospheric heat flux and in ocean heat content lateral advection within the ocean is important in determining the southern ice extent

thermal history of potential gas reservoir rocks in the eastern parnaiba basin brazil the parnaiba basin is a major intracratonic sedimentary basin in brazil with unconventional petroleum systems as a potential natural resource formed by the influence of igneous intrusions to constrain the thermal history of unexplored potential reservoir rocks in the eastern part of the parnaiba basin sedimentary rocks near intrusions were analyzed by petrography and thermochronology apatite fission track aft and zircon fission track zft dating petrography shows grain dissolution and carbonate pore filling generated by thermal destabilization of feldspars the aft results indicate partial annealing by the last magmatic event in the basin and zft results show ages of maximum paleotemperature compatible with the sardinha magmatic event 120 130 ma in thermal history models rocks close to intrusions experienced maximum paleotemperatures above 300 c which is higher than what is considered favorable for reservoir rocks hydrothermal fluids modified the diagenetic evolution of the succession by dissolving and precipitating carbonate cement in pore spaces in the studied area at distances greater than 50 m 164 ft from the intrusion the rocks were not substantially altered and in terms of paleotemperature they can be considered potentially viable reservoirs the thermal history analysis of potential tight gas sandstone reservoirs affected by intrusive bodies is valuable for characterizing the thermal enhancement or destruction of the reservoir s qualities

proterozoic links between the borborema province ne brazil and the central african fold belt the congo cc and the sao francisco sfc cratons were joined at about 2 05 ga northern parts of palaeoproterozoic basement subsequently underwent extension at about 1 ga forming intracratonic basins neoproterozoic metasedimentary rocks in these basins yield detrital zircons as young as 630 ma the brasiliano and pan african c 620 580 ma assembly of west gondwana extensively altered this system the sergipano domain occurs north of the sfc and the comparable yaoundedomain occurs north of the cc crust north of the sergipano domain com prises the pernambuco alagoas peal domain the ne sw striking tchollire banyo fault in cameroon may extend southwestwards between the peal and sergipano domains defining northern limits of abundant sfc cc basement the adamawa yadedomain in africa does not appear to extend into brazil the transverse domain of brazil is a collage of palaeoproterozoic crustal blocks the 1 0 ga cariris velhos orogen cvo late neoproterozoic basins and brasi liano granites the cvo extends ene for more than 700 km in brazil but eastern continuation into africa has not been identified north of the transverse domain contiguous c 2 15 ga gneisses comprise basement of rio grande do norte and cearadomains which continue eastwards into western nigeria and western sahara

the silurian glaciation in the eastern parnaiba basin brazil paleoenvironment sequence stratigraphy and insights for the evolution and paleogeography of west gondwana abstract glacial strata are excellent stratigraphic markers in paleozoic basins worldwide the eastern parnaiba basin in northern brazil is a highly favorable setting for studying episodes related to the ordovician silurian glaciation previous studies in this region led to the recognition of unequivocally glaciogenic sediments associated with pre and postglacial deposits favoring the understanding of lower paleozoic events that occurred in west gondwana outcrop based facies and stratigraphic analyses of the 170 m thick siliciclastic succession of the serra grande group indicate fluvial glacial and coastal depositional systems seventeen sedimentary facies are grouped into seven facies associations fas fa1 interpreted as intermediary sheet braided plain deposits that consist of tabular cross bedded coarse grained sandstones and conglomerates fa2 fa4 which correspond to subaqueous glacial deposits comprising massive to stratified diamictites cross laminated sandstones diamictites and organic matter rich black shales with dumpstone and dropstone structures fa5 fa6 constituting organic matter rich black shales fine to medium grained sandstones with sigmoidal cross bedding and occurrences of the ichnotaxon arthrophycus interpreted as delta front deposits and fa7 composed of medium to coarse grained pebbly sandstones interpreted as channelized braided plain deposits this interpretation partially confirms the previous paleoenvironmental interpretation discounting the presence of subaerial alluvial fan and outwash plain deposits for this unit the sequence stratigraphy of the serra grande group is refined mainly using the coherent interpretation of the key surfaces and comparing the systems tracts with the global sea level curve providing a more robust third order sequential evolutionary model that includes three depositional sequences the middle ordovician sheet braided deposits are the lowstand sediments of sequence 1 truncated by an extensive unconformity that possibly removed the transgressive and highstand strata this unconformity generated by glacial dynamics represents 23 myr during the late ordovician to early silurian the second sequence started with the advance of ice sheets in marginal areas of the basin causing the deposition of ablation diamicton during the lowstand the postglacial transgression is marked by ice rafted debris from icebergs deposited onto organic matter rich mud in the shoreface offshore zone the maximum flooding during the middle to late silurian was succeeded by the highstand phase marked by the progradation of deltaic deposits biological activity is marked by the presence of polychaetes that reworked the marine seafloor the early devonian featured extensive sea level fall and renewed continental drainage with the formation of a braided plain marking a lowstand phase of sequence 3 these new interpretations allow the global correlation of the serra grande group and provide an improved understanding of the role of ice sheets and postglacial transgressions that affected west gondwana during the ordovician silurian periods

lake cyclicity as response to thermal subsidence a post camp scenario in the parnaiba basin ne brazil abstract the jurassic cretaceous transition was marked by the fragmentation of the west gondwana supercontinent and consequent opening of the atlantic ocean this event resulted in extensive lava flows in the central portion of west gondwana composing the central atlantic magmatic province in the parnaiba basin ne brazil a large lacustrine system succeeded this event nevertheless the post camp scenario and the influence of the decreasing isotherms in the mesozoic sedimentation are poorly understood in this sense cyclostratigraphy and outcrop based facies analysis were carried out in the lacustrine stratigraphic record allowing the recognition of four paleoenvironments central lake sheet like delta front lakeshore and ephemeral fluvial channels mainly organized in shallowing upward cycles the upsection transition of thin black shales and limestones to thick oxidized mudstones and stratified sandstones reflects the evolution of underfilled to overfilled lake settings the lacustrine succession is organized in four major depositional cycles characterized by millimeter to centimeter scale cycles bounded by unconformities or flooding surfaces the cycles define a retrogradational progradational retrogradational stacking pattern with random and frequent changes in lithology and cycle thickness cyclostratigraphic data suggest that the upward increasing accommodation space was controlled by the post camp thermal subsidence as well as shifts in sedimentary supply and water inflow outflow the time span of this subsidence phase lasted from even before the camp magmatism 200 ma affecting the whole deposition of the lacustrine succession and was interrupted by the cretaceous magmatic event 126 ma subsidence was also enhanced by crustal loading associated with thick subsurface jurassic intrusions and probably influenced the whole basin in consequence of flexural effects

a coupled sea ice mixed layer pycnocline model for the weddell sea a dynamic thermodynamic sea ice model is coupled to a one dimensional model of the oceanic mixed layer and pycnocline and is applied to the weddell sea this model prognostically determines the vertical oceanic heat flux from the mixed layer dynamics in contrast to earlier sea ice modeling where the oceanic heat flux was prescribed in addition to the standard simulation polynya and paleoclimate experiments were performed to investigate the effects of sea ice dynamics furthermore the mixed layer pycnocline model is compared to the original kraus turner approach

k ar evidence from illitic clays of a late devonian age for the 120 km diameter woodleigh impact structure southern carnarvon basin western australia abstract woodleigh is a recently discovered impact structure with a diameter of 120 km and thereby represents the third largest proven phanerozoic impact structure known after morocweng and chicxulub k ar isotopic studies of fine grained authigenic illitic clay minerals

geochemistry and zircon geochronology of paleoproterozoic granitoids further evidence on the magmatic and crustal evolution of the sao luis cratonic fragment brazil abstract the tromai intrusive suite is the predominant exposed unit of the sao luis cratonic fragment in northern brazil the suite forms batholiths and stocks of granitoids that were emplaced between 2168 4 ma and 2149 4 ma and intruded a 2240 5 ma old metavolcano sedimentary sequence the batholiths are composed of a variety of petrographic types that have been grouped in three sub units based on the predominant petrographic type and named cavala tonalite bom jesus granodiorite and areal granite from the more primitive to the more evolved phases in addition to subordinate shallow felsic intrusions the tromai suite is an expanded magmatic association comprising minor mafic rocks to predominantly intermediate and felsic low to high k and metaluminous to weakly peraluminous granitoids that follow a na enriched calc alkaline trend combined rock association geochronology nd isotopes and geochemical signature indicate that the tromai suite formed from magmas derived from juvenile protoliths modified by fractional crystallization the juvenile protoliths included ocean plate mantle wedge and minor sediments the data also indicate an intra oceanic arc setting that possibly transitioned to a continental margin and that the tromai intrusive suite records the main accretionary stage of the rhyacian orogen ca 2 24 2 15 ma that culminated with a collision stage at about 2 1 ga and gave rise to the present day sao luis cratonic fragment this time interval is coincident with the main period of crustal growth in the south american platform and in the paleoproterozoic terranes of the west african craton the beginning of this period is also coincident with the end of a period in which only minor amounts of juvenile crust is found worldwide the negra velha granite is a distinct unit that forms a few stocks that intruded the granitoids of the tromai suite between 2076 and 2056 ma ago negra velha is an association of monzogranite and subordinate quartz monzonite and syenogranite with an alkaline signature that shows high rb sr ba enrichments resembling shoshonitic associations this granite represents the post orogenic phase of the rhyacian orogenesis

palaeogeographical and palaeoclimatic evolution of the intracratonic parnaiba basin ne brazil using gplates plate tectonic reconstructions and chemostratigraphic tools the intracratonic paleozoic parnaiba basin ne brazil covers c 600 000 km 2 lithostratigraphic mineralogical and geochemical approaches were used to deduce the changing environments of deposition of the parnaiba basin based on 150 mostly argillite samples silurian earliest triassic the results indicate that brackish conditions and mixed kerogen sources were important and that the palaeowater depth and sources of meteoric water varied from west to east the chemical index of alteration results together with other elemental and mineralogical data indicate that the western margin was humid until the end of the carboniferous and then became arid whereas the eastern margin showed the reverse pattern this supports observations of major stratigraphic differences between the two margins there were also major variations in the palaeoredox and palaeosalinity conditions oxic and fresh to brackish water systems in the east v suboxic to dysoxic and saline water systems in the west the clay mineral assemblages are characteristically dominated by kaolinite which is considered to be inherited from weathering horizons developed in silicic granitic terranes regional palaeoclimate reconstructions were carried out in the parnaiba basin by linking gplates software with the south america plate motion model northwards drift during the paleozoic coupled with a latest paleozoic rotation phase and with the regional database of information on the location formation name and original references for each locality

laser argon dating of melt breccias from the siljan impact structure sweden implications for a possible relationship to late devonian extinction events in earlier studies the 6575 km diameter siljan impact structure in sweden has been linked to the late devonian mass extinction event the siljan impact event has previously been dated by kar and ar ar chronology at 342368 ma with the commonly quoted age being 362 7 2 2 ma 2 recalculated using currently accepted decay constants until recently the accepted age for the frasnian famennian boundary and associated extinction event was 364 ma which is within error limits of this earlier siljan age here we report new ar ar ages extracted by laser spot and laser step heating techniques for several melt breccia samples from siljan interpreted to be impact melt breccia the analytical results show some scatter which is greater in samples with more extensive alteration these samples generally yield younger ages the two samples with the least alteration yield the most reproducible weighted mean ages one yielded a laser spot age of 377 2 2 5 ma 95 confidence limits and the other yielded both a laser spot age of 376 1 2 8 ma 95 confidence limits and a laser stepped heating plateau age over 70 6 39ar release of 377 5 2 4 ma 2 our conservative estimate for the age of siljan is 377 2 ma 95 confidence limits which is significantly different from both the previously accepted age for the frasnian famennian f f boundary and the previously quoted age of siljan however the age of the f f boundary has recently been revised to 374 5 2 6 ma by the international commission for stratigraphy which is within error the same as our new age however the currently available age data are not proof that there was a connection between the siljan impact event and the f f boundary extinction this new result highlights the dual problems of dating meteorite impacts where fine grained melt rocks are often all that can be isotopically dated and constraining the absolute age of biostratigraphic boundaries which can only be constrained by age extrapolation further work is required to develop and improve the terrestrial impact age record and test whether or not the terrestrial impact flux increased significantly at certain times perhaps resulting in major extinction events in earths biostratigraphic record

characterization and significance of shocked quartz from the woodleigh impact structure western australia we re examined the buried woodleigh structure in western australia which has been inferred to be a multi ringed 120 km diameter impact crater because the proposed size and possible synchronicity with one of the pre mesozoic mass extinction events has attracted controversy we undertook a detailed study of the petrology and mineralogy of a number of samples of core from the woodleigh 1 borehole that was drilled into the central uplift of the structure crystalline proterozoic basement rocks comprising granites and gneisses in the woodleigh 1 core contain minor brecciation in discrete veins and reveal clear evidence of shock metamorphism over the full extent of the core imaging of laboratory etched quartz showed that a large number of grains contain shock deformation lamellae microstructural and crystallographic analysis of these lamellae by tem showed that they are planar deformation features pdfs that have subsequently undergone annealing and water assisted recrystallization the available geological petrographic and mineralogical evidence suggest that woodleigh is an eroded impact crater that is nearer to 60 km than 120 km in diameter future drilling projects should better constrain the level of erosion and may reveal any preserved impact lithologies

a coupled one dimensional sea ice ocean model a prognostic one dimensional mixed layer pycnocline model describing the vertical structure of the upper ocean is coupled to a thermodynamic sea ice model the coupled prognostic model is compared with the more usual sea ice model overlying a fixed mixed layer with constant oceanic heat flux and is then applied to investigate the effect of temporal and regional variations of the entrained oceanic heat flux on the sea ice cover in both polar regions in the southern ocean a standard simulation is compared with two perturbation experiments which both describe the occurrence of a polynya in the arctic ocean the effect of possible soviet river diversions is investigated finally the response of the coupled model to paleoclimatic forcing and boundary conditions is presented

pancake ice formation in the weddell sea 1 the ice formation resulting from two low temperature events at the weddell sea ice edge during april 2000 is presented pancake and frazil ice were sampled at seven stations at varying distances from the ice edge the ice cover was further characterized from above using helicopter aerial photography and from below using a remotely operated vehicle previously undescribed two layer pancake types were observed and classified a novel pancake growth mechanism is introduced to account for these involving the washing of frazil ice over the pancake top surface and its subsequent freezing the process was directly observed in ice tank experiments layer thicknesses seen in the field were compared to the ice growth that would occur both under calm conditions and from free surface frazil ice growth classical bottom accretion pancake growth was found to proceed at a rate similar to that of thin congelation ice top layer growth was more rapid at approximately double the congelation rate overall ice volume production was similar to congelation ice for the thin pancakes considered 20 cm though subsequent thickening was expected to be faster as the rapid top layer process continued and the equivalent congelation growth slowed it is suggested that parameterization of this new process is important for models that aim to simulate the rapid advance and thickening of wave influenced ice covers

stress variability in the parnaiba basin brazil during cretaceous rifting the cretaceous section of the parnaiba basin designated as grajau basin represents an intracontinental half graben formed during the early cretaceous due to the separation between the south american and african continents during the final dispersal of western gondwana here through a synergetic approach between fluid inclusion planes fips outcropping geologic structures borehole breakouts and remote sensing data we elucidate in different scales the main structural features and their kinematic indicators normal faults strike mainly nne ssw and wnw ese while deformation bands and extension joints trend to nw se and ne sw and fips to ne sw and wnw ese in addition normal fault generated scarps border geomorphological units and constitute dense zones of deformation bands and fluidization microthermometric fip analyses suggest this fracturing event occurred at shallow basin levels at temperatures below 50 c furthermore joints bands and fips present mutually cross cutting relationships thus indicating contemporaneity the numerical inversions applied to striated faults non striated faults joints deformation bands and fips suggest the occurrence of an extensional event characterized by variable direction of extension 3 trending from nw se to wnw ese or ne se to nne ssw this event probably started in the albian during the final gondwana fragmentation stages the quasi perpendicular 3 trend could be caused by one or all of the following phenomena a stress ratio r values obtained 0 4 indicate the possibility of radial extension 2 3 perhaps in response to the deformation partitioning due to kinematic simple shearing constraints along the equatorial atlantic transform margin b along the wnw oriented release normal faults the local extension nne ssw switch positions in relation to the regional extension wnw ese by rotating 90 c influence of the pre existing structures on the regional stress field thus the spatial and temporal relationships between cretaceous fault activity stress field and the development of the geomorphological features in the grajau basin contribute to understanding of the brazilian equatorial margin geodynamics

arctic ocean heat impact on regional ice decay a suggested positive feedback abstractbroad long living ice free areas in midwinter northeast of svalbard between 2011 and 2014 are investigated the formation of these persistent and reemerging anomalies is linked hypothetically with the increased seasonality of arctic sea ice cover enabling an enhanced influence of oceanic heat on sea ice and in particular heat transported by atlantic water the memory of ice depleted conditions in summer is transferred to the fall season through excess heat content in the upper mixed layer which in turn transfers to midwinter via thinner and younger ice this thinner ice is more fragile and mobile thus facilitating the formation of polynyas and leads when openings in ice cover form along the atlantic water pathway weak density stratification at the mixed layer base supports the development of thermohaline convection which further entrains warm and salty water from deeper layers convection induced upward heat flux from the atlantic layer retards ice formation either keeping ice thick

brasiliano crustal structure and the tectonic setting of the parnaiba basin of ne brazil results of a deep seismic reflection profile a 1430 km deep crustal seismic reflection profile of the parnaiba basin shows an asymmetric structured western margin and a gently dipping eastern margin the 3 km thick phanerozoic sedimentary section overlies a pronounced planar regional unconformity that crosses three precambrian blocks with differing seismic facies the amazonian araguaia block the parnaiba block and the borborema block the blocks are separated by steep crustal scale boundaries across which seismic facies change abruptly in the west the ophiolitic metasedimentary rocks of the araguaia group overlie the amazonian craton both craton and metasediments terminate eastward against a steep east dipping fault zone defining the amazonian araguaia block and parnaiba block boundary reactivation of this neoproterozoic margin in the late triassic and late jurassic early cretaceous folded and elevated basement and basin over 2 km a second crustal boundary defines the eastern margin of the parnaiba block with the neoproterozoic borborema block this boundary is interpreted as the extension of the transbrasiliano shear zone these data demonstrate that the basement of the parnaiba basin was formed during brasiliano orogenesis by west directed collision related thrusting succeeded by lateral accretion along steep crustal scale boundaries after formation of a post brasiliano peneplain the parnaiba basin developed seamlessly across three very different crustal blocks and appears to have been significantly larger than its present outline no extensive underlying rift system is evident suggesting that basement structure had little to do with basin formation but that episodic reactivation of the boundary zones and basement fabric has controlled the structuring and preservation of the basin

comment on impact ejecta layer from the mid devonian possible connection to global mass extinctions the interesting report by ellwood et al 1 1 demonstrated the applicability of magnetic susceptibility field methods to the identification of distal impact deposits however the evidence that they have presented for a bolide impact during the middle devonian immediately preceding the

glassy microspherules microtektites from an upper devonian limestone the properties of microspherules recovered from an upper devonian marine limestone immediately overlain by a geochemical anomaly of siderophile and chalcophile elements are similar to those of impact derived microtektites these microspherules are glass have splash form shapes contain spherical vesicles and lechatelierite inclusions and show oxide compositional variations similar to those in known microtektites these characteristics suggest that these upper devonian microspherules have an impact origin a bolide impact may have occurred about 365 million years ago on the south china plate and caused a faunal extinction on eastern gondwana

numerical modeling of flexural stresses in the upper crust of ne brazil implications for stress regimes in intraplate settings abstract we model the contribution of the stresses generated by the topography and density heterogeneities in the stress field of ne brazil and the interplay between these and regional stresses our model consists of a horizontally layered elastic slab the lithosphere that floats above an inviscid fluid the asthenosphere where the surface load is generated by the topography and bathymetry and the internal loads are calculated from the bouguer anomalies the deviatoric flexural stresses are calculated in the fourier domain our results showed local flexural stresses with magnitudes comparable to those of the plate wide stresses and mainly controlled by the long wavelength bouguer anomalies scenarios for the total stress field were calculated by superposing different values for an e w oriented regional compression we predict the stress axes orientations and stress regimes for the equatorial margin and the pernambuco shear zone pesz region however we highlight the importance of spreading stresses not considered here in rotating the maximum horizontal compression shmax predictions the prediction of stress regime and stress axes orientation in pesz using low to zero regional compression suggest that a decoupling of the shallow stresses from the plate wide stresses might occur in this region another possible explanation found was that the spreading stresses might counterbalance the regional stresses we conclude that the superposition of principally flexural and plate wide stresses explains reasonably the observed stress regimes and shmax orientations in a great part of ne brazil furthermore flexural stresses are playing an important role in the reactivation of the pesz possibly controlling the stress field

critical comment on a j mory et al woodleigh carnarvon basin western australia a new 120 km diameter impact structure mory et al 1 claim to report a newly discovered basement cored multi ring impact structure and hint at a possible permian triassic boundary age for the impact they purport that this structure could be as large as 120 km in diameter this comment emphasizes that these authors indeed identi ed a potentially interesting structure but that the evidence presented is inconclusive and not well constrained it does not prove that woodleigh is of impact origin mory et al s 1 claims especially regarding the alleged size of this structure and correlation with the p tr boundary mass extinction have generated extensive public interest presentation of such an alleged discovery must adhere stringently to scienti c principles namely that convincing evidence is obtained before interpretation which itself should not go beyond the limits set by the data at hand this comment demonstrates that mory et al did not conform to this principle issue will be taken with the arguments of mory et al in the sequence according to the structure of their paper 1 regional geological setting

circulation and transformation of atlantic water in the eurasian basin and the contribution of the fram strait inflow branch to the arctic ocean heat budget abstract the inflows of atlantic water from the nordic seas to the arctic ocean through fram strait and over the barents sea are examined and their pathways and transformations in the eurasian basin are described based primarily on hydrographic observations from two cruises with rv polarstern in 2007 and 2011 the contrast between the high salinity atlantic water core in the nansen basin and the lower atlantic water salinity in the amundsen basin combined with a salinity minimum below the atlantic layer suggest that the fram strait branch mainly remains in the nansen basin while the barents sea branch continues along the continental slope enters the amundsen basin and provides most of the atlantic water in the makarov and canada basins to examine the consequences of such a circulation pattern recently published estimates of the in and outflows of volume and liquid freshwater through the barents sea bering strait and the canadian arctic archipelago as well as river runoff net precipitation and ice export are reviewed and added together to achieve volume and freshwater balances net outflows through fram strait of 2 3 sv and 100 msv respectively are required this is reasonably close to the recently reported fram strait transport estimates the net outflows are separated into two parts upper layer transports less dense than the entering atlantic water and lower layer transports as dense as or denser than the atlantic water the largest net volume outflow occurs in the lower layer 1 65 sv while the liquid freshwater is almost exclusively exported in the 0 64 sv net outflow in upper layer leading to unrealistically low salinities in the upper layer compared with observations atlantic water is north of svalbard transformed into less saline surface water through melting of sea ice which can supply the necessary higher salinity halocline water to the upper layer 1 23 sv of halocline water of salinity 34 2 must be added to obtain a more realistic salinity of 33 1 leading to a total export of 1 84 sv upper layer water in the east greenland current existing observations of the net transports into and out of the arctic ocean through fram strait in different temperature classes are used to estimate the heat loss of the atlantic water entering the arctic ocean through fram strait about 10 tw are supplied to the atmosphere in addition to the loss of 24 tw occurring in connection with the formation of the halocline water if the hypothesis that the fram strait branch remains in the nansen basin is true it implies a heat loss between 5 and 15 w m 2 in the nansen basin depending upon how much heat is temporarily stored in the water column this is high compared to the 2 w m 2 commonly cited as the ocean heat flux to the atmosphere in the arctic ocean

neoproterozoic crustal accretion in central brazil recent geochronological studies have demonstrated the existence of several neoproterozoic orthogneiss and metavolcanic are suites ca 900 to 600 ma in the southern part of the tocantins province a major neoproterozoic orogenic area in central brazil exposed between the amazon and sao francisco cratons nd and sr isotopic characteristics of these metamorphic suites are primitive initial 87 sr 87 sr ratios vary from 0 7024 to 0 7042 and initial nd values are positive varying between 0 2 and 6 9 the most primitive initial sr and nd isotopic compositions are very similar to model depleted mantle compositions at the time of formation of the original magmas the mantle like characteristics of these are suites in western goias contradict previous ensialic evolution models for the tocantins province and reveal that crustal accretion processes were important in large areas of central brazil during neoproterozoic time

coeval 40ar 39ar ages of 65 0 million years ago from chicxulub crater melt rock and cretaceous tertiary boundary tektites 40 ar 39 ar dating of drill core samples of a glassy melt rock recovered from beneath a massive impact breccia contained within the 180 kilometer subsurface chicxulub crater in yucatan mexico has yielded well behaved incremental heating spectra with a mean plateau age of 64 98 0 05 million years ago ma the glassy melt rock of andesitic composition was obtained from core 9 1390 to 1393 meters in the chicxulub 1 well the age of the melt rock is virtually indistinguishable from 40 ar 39 ar ages obtained on tektite glass from beloc haiti and arroyo el mimbral northeastern mexico of 65 01 0 08 ma mean plateau age for beloc and 65 07 0 10 ma mean total fusion age for both sites the 40 ar 39 ar ages in conjunction with geochemical and petrological similarities strengthen the recent suggestion that the chicxulub structure is the source for the haitian and mexican tektites and is a viable candidate for the cretaceous tertiary boundary impact site

observations of vertical currents and convection in the central greenland sea during the winter of 1988 1989 during the winter of 1988 1989 five acoustic doppler current profilers adcps were moored in the central greenland sea to measure vertical currents that might occur in conjunction with deep mixing and convection two adcps were looking up from about 300 m and combined with thermistor strings in the depth range 60 260 m two were looking downward from 200 m and one was looking upward from 1400 m first maxima of vertical velocity variance occurred at two events of strong cold winds in october and november when cooling and turbulence in the shallow mixed layer generated internal waves in the thermocline beginning in late november the marginal ice zone expanded eastward over the central greenland sea reaching its maximum extent in late december in mid january a bay of ice free water opened over the central greenland sea leaving a wedge of ice the is odden curled around it along the axis of the jan mayen current and then northeastward and existing well into april 1989 below the ice a mixed layer at freezing temperatures developed that increased in thickness from 60 to 120 m during the period of ice cover corresponding to an average heat loss of about 40 w m 2 through brine rejection mixed layer salinity increased steadily reducing stability to underlying weakly stratified layers roach et al 1993 during the ice cover period vertical currents were at a minimum after the opening of the ice free bay successive mixed layer deepening to 350 m occurred in conjunction with cooling events around february 1 and 15 accompanied by strong small scale vertical velocity variations upward mixing of more saline waters of atlantic origin during this phase reduced the stability further generating a pool of homogeneous water of 50 km horizontal extent in the central greenland sea preconditioned for subsequent convection to greater depths individual convection events were observed during march 6 16 associated with downward velocities at the 1400 m level of about 3 cm s l one event was identified as a plume of about 300 m horizontal scale in agreement with recently advanced scaling arguments and model results and with earlier similar observations in the gulf of lions western mediterranean the deep convection occurred in the center of the ice free bay hence brine rejection did not seem necessary for its generation plume temperatures at 1400 m were generally higher than that of the homogeneous surface pool suggesting entrainment of surrounding warmer waters on the way down mean vertical velocity over a period of convection events was indistinguishable from zero suggesting that plumes served as a mixing agent rather than causing mean downward transport of water masses however different from the surface pool that was governed by mixed layer physics the water between 400 and 1400 m was not horizontally homogenized in a large patch by the sporadic plumes overall and compared to results from the gulf of lions convection activity in the central greenland sea was weak and limited to intermediate depths in winter 1988 1989

early to middle devonian ironstone and phosphorite in the northwestern gondwana parnaiba basin brazil a record of an epeiric margin paleoceanographic changes abstract pragian to givetian successions in the caninde group parnaiba basin brazil present phosphorites ironstone and organic rich sediments that globally coincide with greenhouse conditions and an increase in continental weathering ocean oxygen levels and black shale deposition events these facies are associated with the evolution of the northern gondwana epeiric sea margin of the rheic ocean and can tell us how ocean chemistry changed and the paleoceanographic factors involved in this particular transition of the ocean record we conducted an integrated study based on sedimentology sequence stratigraphy petrography scanning electron microscopy and x ray diffraction to understand the paleoenvironmental stratigraphic and diagenetic controls lithofacies stacking patterns indicate that deposition occurred during marine transgressions resulting in four depositional sequences late eifelian and givetian maximum flooding surfaces are represented by phosphogenesis and francolite precipitation associated with condensed zones pragian emsian to givetian ironstones were represented by authigenic coated phosphatic iron silicate odinite chamosite clinochlore francolite and siderite precipitation these are related to normal regressions under lowstand and highstand systems tract oxic waters prevailed and episodic upwelling is interpreted as the source of nutrients that maintained fe si and p supply on a storm influenced to storm dominated shelf with an additional source of iron also associated with river discharges phosphorite was formed in a stratified ocean near storm wave base under anoxic sulfide reducing bacteria microbial communities and suboxic conditions productivity fueled by recycled p shallower anoxic conditions near the fair weather wave base existed possible related to a shift in the locus of upwelling and then resumption to oxic conditions indicate that water column stratification was not a stable feature due to variations in base level ventilation terrigenous or marine contribution and progressive microbial degradation at the beginning of late eifelian transgression an event of increased delivery of continental flux and eutrophication can also be proposed the link between organic rich intervals and third order black shale events daleje chot c and kacak suggests that extinction events can be tracked by analyzing sections in south america global correlation with ironstone precipitation and organic rich sediments shows that stepped periods of c sink conditions probably contributed to the ocean oxygenation

ocean circulation and freshwater pathways in the arctic mediterranean based on a combined nd isotope ree and oxygen isotope section across fram strait the water masses passing the fram strait are mainly responsible for the exchange of heat and freshwater between the nordic seas and the arctic ocean the arctic mediterranean am disentangling their exact sources distribution and mixing however is complex this work provides new insights based on a detailed geochemical tracer inventory including dissolved nd isotope end rare earth element ree and stable oxygen isotope 18o data along a full water depth section across fram strait we find that nd isotope and ree distributions in the open am primarily reflect lateral advection of water masses and their mixing seawater particle interactions exert important control only above the shelf regions as observed above the ne greenland shelf advection of northward flowing warm atlantic water aw is clearly reflected by an end signature of 11 7 and a nd concentration nd of 16 pmol kg in the upper 500 m of the eastern and central fram strait freshening and cooling of the aw on its way trough the am are accompanied by a continuous change towards more radiogenic end signatures e g 10 4 of dense arctic atlantic water this mainly reflects mixing with intermediate waters but also admixture of dense kara sea waters and pacific derived waters the more radiogenic end signatures of the intermediate and deep waters reaching 9 5 are mainly acquired in the sw nordic seas through exchange with basaltic formations of iceland and se greenland inputs of nd from svalbard are not observed and surface waters and nd on the svalbard shelf originate from the barents sea shallow southward flowing arctic derived waters 200 m form the core of the east greenland current above the greenland slope and can be traced by their relatively radiogenic end reaching 8 8 and elevated nd 21 to 29 pmol kg these properties are used together with 18o and standard hydrographic tracers to define the proportions of pacific derived 30 based on nd isotopes and atlantic derived waters as well as of river waters 8 shallow waters 150 m on the ne greenland shelf share some characteristics of arctic derived waters but exhibit less radiogenic end values reaching 12 4 and higher nd up to 38 pmol kg in the upper 100 m this suggests local addition of greenland freshwater of up to 6 in addition to these observations this study shows that the pronounced gradients in end signatures and ree characteristics in the upper water column provide a reliable basis for assessments of shallow hydrological changes within the am

testing a coupled ice mixed layer model under subarctic conditions abstract a one dimensional oceanic mixed layer model has been coupled with a thermodynamic sea ice model in order to study the seasonal cycle of ice ocean interaction in the subarctic ocean the ice thickness is assumed constant and only variations of ice compactness under the effect of the atmospheric and oceanic heat fluxes are considered the mixed layer model allows prediction of the rate of entrainment mainly as the result of the penetrative buoyancy driven convection both heat and salt conservation equations are included so that the respective efficiency with regard to vertical mixing of brine rejection below the ice and intense surface cooling in open water can be compared the model has been run for ten years on a zonal vertical section across the greenland sea the annual entrainment retreat cycle of the mixed layer is well reproduced with maximum depths occurring in late winter even down to the bottom at some locations near the ice edge surface cooling is found to be a more efficient mecha

low temperature phanerozoic history of the northern yilgarn craton western australia abstract the phanerozoic cooling history of the western australian shield has been investigated using apatite fission track aft thermochronology aft ages from the northern part of the archaean yilgarn craton western australia primarily range between 200 and 280 ma with mean confined horizontal track lengths varying between 11 5 and 14 3 m time temperature modelling of the aft data together with geological information suggest the onset of a regional cooling episode in the late carboniferous early permian which continued into late jurassic early cretaceous time present day heat flow measurements on the western australian shield fall in the range of 40 50 mw m 2 if the present day geothermal gradient of 18 2 c km 1 is representative of average phanerozoic gradients then this implies a minimum of 50 c of late palaeozoic to mesozoic cooling assuming that cooling resulted from denudation the data suggest the removal of at least 3 km of rock section from the northern yilgarn craton over this interval the perth basin located west of the yilgarn craton contains up to 15 km of mostly permian to lower cretaceous clastic sediment however published u pb data of detrital zircons from permian and lower triassic basin strata show relatively few or no grains of archaean age this suggests that the recorded cooling can probably be attributed to the removal of a sedimentary cover rather than by denudation of material from the underlying craton itself the onset of cooling is linked to tectonism related to either the waning stages of the alice springs orogeny or to the early stages of gondwana breakup

reply to comment on k ar evidence from illitic clays of a late devonian age for the 120 km diameter woodleigh impact structure southern carnarvon basin western australia

the position of the amazonian craton in supercontinents abstract this paper examines the extensive regions of proterozoic accretionary belts that either formed most of the amazonian craton or are marginal to its southeastern border their overall geodynamic significance is considered taking into account the paleogeographic reconstruction of columbia rodinia and gondwana amazonia would be part of columbia together with laurentia north china and baltica forming a continuous continental landmass linked by the paleo to mesoproterozoic mobile belts that constitute large portions of it the rodinia supercontinent was formed in the mesoproterozoic by the agglutination of the existing cratonic fragments such as laurentia and amazonia during contemporary continental collisions worldwide the available paleomagnetic data suggest that laurentia and amazonia remained attached until at least 600 ma since all other cratonic units surrounding laurentia have already rifted away by that time the separation between amazonia and laurentia marks the final break up of rodinia with the opening of the lapetus ocean

jaibaras trough an important element in the early tectonic evolution of the parnaiba interior sag basin northern brazil abstract the jaibaras trough jt is a late precambrian to early paleozoic basin that is related to the subsequent formation of the paleozoic mesozoic parnaiba intracratonic basin in northern brazil based on available geological and geophysical data we propose that the jt is a prototypical rift basin formed by brittle reactivation processes along ancient shear zones associated with the ne sw transbrasiliano lineament of the brazilian panafrican orogeny the jt evolution is partially constrained by the stratigraphic sequences outcropping at the border of the basin or drilled by exploratory boreholes the preserved sedimentary column records in a broad sense the sedimentary and tectonic controls on the early subsidence in the parnaiba basin these sequences are constituted by basal fault scarp related paraconglomerates followed laterally and vertically by fluvial lacustrine sandstones interbedded with reddish shale the jt geological evolution is also constrained by petrology of four magmatic events coreau dike swarm mucambo and meruoca plutons and mafic parapui suite related to continental rifting the late precambrian early paleozoic age of jt is perfectly compatible with continental fragmentation during the processes of lithospheric stretching that affected western gondwana at that time the contemporaneous development of a magmatic sedimentary system dated as cambrian to ordovician and confined to the rift trend raised the thermal gradient and produced a mantle softening discontinuity giving rise to a broad uplifted region this might explain the close temporal and spatial association between the transbrasiliano lineament and the early paleozoic depositional axis of the parnaiba basin an integrated interpretation of the jt and the early tectonostratigraphic behaviour of the parnaiba basin indicates that the jt might constitute a precursory stage that was slightly inverted before the initiation of sag deposition in the intracratonic basin

permian paleogeography of west central pangea reconstruction using sabkha type gypsum bearing deposits of parnaiba basin northern brazil abstract extreme aridity during late permian early triassic period was the main factor for resetting the entire paleoclimate of the planet permian evaporite basins and lacustrine red beds were widely distributed along the supercontinent of pangea sulphate deposits in western pangea particularly in northern brazil accumulated in an extensive playa lake system outcrop based facies and stratigraphic analysis of up to 20 m thick evaporite siliciclastic deposits reveal the predominance of laminated reddish mudstone with subordinate limestone marl and lenses of gypsum the succession was deposited in shallow lacustrine and inland sabkha environments associated with saline pans and mudflats gypsum deposits comprise six lithofacies 1 bottom growth gypsum 2 nodular micronodular gypsum 3 mosaic gypsum 4 fibrous prismatic gypsum 5 alabastrine gypsum and 6 rosettes of gypsum gypsum types 1 and 2 are interpreted as primary deposition in saline pans bottom growth gypsum forms grass like crusts while nodular micronodular gypsum indicates displacive precipitation of the crust in shallow water and the groundwater capillary zone types 3 and 4 are early diagenetic precipitates abundant inclusions of tiny lath like anhydrite crystals suggest a primary origin of anhydrite alabastrine gypsum fibrous gypsum satinspar and rosettes of gypsum probably derived from near surface hydration of anhydrite the gypsum bearing deposits in the parnaiba basin contribute towards understanding paleogeographic changes in western pangea a progressive uplift of east pangea culminated in the forced regression and retreat of epicontinental seas to the west restricted seas or large lakes were formed before the definitive onset of desert conditions in pangea leading to the development of extensive ergs

implications of preliminary subsidence analyses for the parnaiba cratonic basin cratonic sedimentary basins are underlain by thick lithosphere and subside over hundreds of millions of years the driving mechanism for these basins remains enigmatic due to the paucity of crustal scale constraints to address this shortcoming an integrative study of the parnaiba cratonic basin of ne brazil is underway we summarize field observations well log information seismic reflection profiles and subsidence analyses with the objective of shedding light on possible basin forming mechanisms lithological descriptions and borehole logs reveal a stratigraphic succession that consists of interbedded shallow marine and continental deposits on seismic reflection profiles this succession is divided into packages of relatively undisturbed reflections bounded by basin wide disconformities these disconformities are manifest as bright rugose reflections that probably represent buried palaeo landscapes backstripped and water loaded subsidence curves calculated from boreholes distributed throughout the basin show that tectonic subsidence decreases exponentially over c 350 ma a simple model suggests that this subsidence pattern agrees with a thermal time constant of 70 100 ma placing a significant constraint on the likely basin forming mechanism this background trend is punctuated by minor increases in subsidence that follow basin wide erosional disconformities these departures could be indicative of transient epeirogenic uplift events caused by changing patterns of dynamic topography

upper devonian iridium anomalies conodont zonation and the frasnian famennian boundary in the canning basin western australia abstract two iridium anomalies have been identified near the frasnian famennian boundary upper devonian in the canning basin of western australia both anomalies are associated with the cyanobacterium frutexites and are located in marginal slope facies the first was identified in the virgin hills formation on the west flank of mcwhae ridge near the southeastern end of the devonian outcrop belt this anomaly initially identified as being from the famennian upper palmatolepis triangularis zone is now known to be from the early palmatolepis crepida zone on the basis of the presence of palmatolepsis crepida in the bed the second anomaly found in drill core from the napier formation just south of the napier range is older than the first and is from the frasnian montagne noire conodont zone 12 or 13 of klapper 1989 the palmatolepis rhenana zone of ziegler and sandberg 1990 these two iridium anomalies are thus significantly below and above the frasnian famennian boundary and are not associated with the extinction event in the palmatolepis linguiformis zone in the canning basin the association of two iridium anomalies with beds containing abundant frutexites microstromatolites indicates that the concentrations of iridium are probably associated with a process of organic concentration there is no evidence that either example is directly associated with an impact event

woodleigh impact structure australia shock petrography and geochemical studies the large complex woodleigh structure in the carnarvon basin of western australia has recently been added to the terrestrial impact crater record many aspects of this structure are however still uncertain this work provides a detailed petrographic assessment of a suite of representative drill core samples from the borehole woodleigh 1 that penetrated uplifted basement rocks of the central part of this structure fundamental rock and mineral deformation data and high precision chemical data including results of pge and oxygen isotopic analysis are presented the sampled interval displays likely impact produced macrodeformation in the form of fracturing and breccia veining at the microscopic scale contrary to earlier reports that these breccias represent pseudotachylite friction melt or even shock shear produced pseudotachylitic melt breccia cannot be confirmed due to pervasive post impact alteration abundant planar deformation features pdfs in quartz in addition to diaplectic glass and partial isotropization are the main shock deformation effects observed confirming that woodleigh is of impact origin over the investigated depth interval the statistics of quartz grains with a variable number of sets of pdfs does not change significantly and the patterns of crystallographic orientations of pdfs in randomly selected quartz grains does not indicate a change in absolute shock pressure with depth either the value of oxygen isotopes for the recognition of meteoritic contamination as proposed by earlier woodleigh workers is critically assessed neither ina nor pge analyses of our samples support the presence of a meteoritic component within this basement section as had been claimed in earlier work

southern ocean winter mixed layer austral winter 1986 observations from the polarstern along the greenwich meridian from the ice edge to the antarctic margin show the mixed layer beneath the winter sea ice cover to be significantly depressed in oxygen saturation incorporation of weddell deep water wdw into the winter mixed layer responsible for this undersaturation also introduces heat and salinity into the surface layer which strongly influences the mixed layer sea air exchanges and sea ice formation processes the total wdw transfer into the mixed layer averages 45 myr 1 implying a residence time for the surface water of 2 5 years the associated winter heat flux is 41 w m 2 which limits ice thickness to about 0 55 m agreeing quite well with observations the air temperatures during the cruise are just sufficient to remove the wdw heat input in the presence of observed ice thickness and concentration this suggests that the sea ice cover and wdw heat input into the mixed layer are in approximate balance by midwinter the annual heat flux from wdw to the surface layer and hence into the atmosphere is estimated as 16 w m 2 extrapolation of the greenwich meridian wdw entrainment value to the full circumpolar 60 70 s belt yields total up welling of 24 106 m3 s 1 similar extrapolation of the heat flux value gives a circumpolar total of 2 8 1014 w as a consequence of circulation topography interaction the maud rise water column stands out as an anomaly relative to the surrounding region with a significantly more saline and dense mixed layer below the mixed layer the water column over the crest of the rise is identical to that over the flanks if the latter water column is upwelled by 400 m this uplifting is believed to be a response of the upstream flow encountering the rise increased upstream flow would be expected to increase maud rise upwelling and the dependent salinity density of the mixed layer slight increases in the mixed layer density could trigger a convective mode and generation of a polynya it is hypothesized that spin up of the weddell gyre s barotropic circulation induced by an increase of the regional wind stress curl would enhance the probability of polynya development over maud rise

seismic characteristics of central brazil crust and upper mantle a deep seismic refraction study 1 a two dimensional model of the brazilian central crust and upper mantle was obtained from the traveltime interpretation of deep seismic refraction data from the porangatu and cavalcante lines each approximately 300 km long when the lines were deployed they overlapped by 50 km forming an e w transect approximately 530 km long across the tocantins province and western sao francisco craton the tocantins province formed during the neoproterozoic when the sao francisco the paranapanema and the amazon cratons collided following the subduction of the former goias ocean basin average crustal vp and vp vs ratios moho topography and lateral discontinuities within crustal layers suggest that the crust beneath central brazil can be associated with major geological domains recognized at the surface the moho is an irregular interface between 36 and 44 km deep that shows evidences of first order tectonic structures the 8 05 and 8 23 km s 1 p wave velocities identify the upper mantle beneath the porangatu and cavalcante lines respectively the observed seismic features allow for the identification of 1 the crust has largely felsic composition in the studied region 2 the absence of the mafic ultramafic root beneath the goias magmatic arc and 3 block tectonics in the foreland fold and thrust belt of the northern brasilia belt during the neoproterozoic seismic data also suggested that the bouguer gravimetric discontinuities are mainly compensated by differences in mass distribution within the lithospheric mantle finally the goias tocantins seismic belt can be interpreted as a natural seismic alignment related to the neoproterozoic mantle domain

national geophysical data center candidate for the world digital magnetic anomaly map marine and airborne magnetic anomaly data have been collected for more than half a century providing global coverage of the earth furthermore the german champ satellite is providing increasingly accurate information on large scale magnetic anomalies the world digital magnetic anomaly map project is an international effort to integrate all available near surface and satellite magnetic anomaly data into a global map database teams of researchers were invited to produce candidate maps using a common pool of data sets here we present the national geophysical data center ngdc candidate to produce a homogeneous map the near surface data were first line leveled and then merged by least squares collocation long wavelengths were found to agree surprisingly well with independent satellite information this validates our final processing step of merging the short wavelength part of the near surface data with long wavelength satellite magnetic anomalies

evolution of the southern ocean winter mixed layer and sea ice open ocean deepwater formation and ventilation an analytical model has been developed to describe the nature of the winter ocean sea ice interaction in the southern ocean to determine the principal processes responsible for maintaining stability in this marginally stable region and to predict the response to changes in the external forcing and initial conditions the model accurately describes the sparse winter observations and the general temporal evolution of the ice mixed layer depth and salinity for the period following the elimination of the seasonal pycnocline by rapid fall ice growth a scale analysis and sensitivity study reveal the system parameter dependence this indicates that the magnitude of entrainment is proportional to fst h0 s where fs is the external salt flux h0 the depth of the winter mixed layer and s the salinity gradient through the pycnocline thermodynamic ice growth reflects predominantly the competition between the oceanic diffusive heat gain across the base of the mixed layer and the loss to the atmosphere through the ice and leads a strong thermal gradient across the pycnocline leads to a significant diffusive flux which balances most of the heat lost to the atmosphere this limits the amount of ice growth possible and plays the principal role in maintaining the system stability since the ice growth is required for destabilization through haline rejection during the growth process the system is also sensitive to the large ratio of heat to salt in the pycnocline this introduces a negative feedback where increasing ice growth induces entrainment by the associated salt flux which in turn drives an entrainment heat flux which compensates for heat lost to the atmosphere this moderates the ability to grow ice and drive further entrainment the absolute strength of the salinity gradient on the other hand dictates the effectiveness which a same magnitude external salt flux will have in driving entrainment the effectiveness with which changes in these parameters as well as several other external parameters can influence the southern ocean stability is explicitly evaluated through the sensitivity study and analysis

crustal structure beneath the paleozoic parnaiba basin revealed by airborne gravity and magnetic data brazil abstract the parnaiba basin is a large paleozoic syneclise in northeastern brazil underlain by precambrian crystalline basement which comprises a complex lithostructural and tectonic framework formed during the neoproterozoic eopaleozoic brasiliano pan african orogenic collage a sag basin up to 3 5 km thick and 1000 km long formed after the collage the lithologic composition structure and role in the basin evolution of the underlying basement are the focus of this study airborne gravity and magnetic data were modeled to reveal the general crustal structure underneath the parnaiba basin results indicate that gravity and magnetic signatures delineate the main boundaries and structural trends of three cratonic areas and surrounding neoproterozoic fold belts in the basement triangular shaped basement inliers are geophysically defined in the central region of this continental scale neoproterozoic convergence zone a 3 d gravity inversion constrained by seismological data reveals that basement inliers exhibit a 36 40 5 km deep crustal root with borders defined by a high density and thinner crust forward modeling of gravity and magnetic data indicates that lateral boundaries between crustal units are limited by brasiliano shear zones representing lithospheric sutures of the amazonian and sao francisco cratons tocantins province and parnaiba block in addition coincident residual gravity residual magnetic and pseudo gravity lows indicate two complex systems of eopaleozoic rifts related to the initial phase of the sag deposition which follow basement trends in several directions