zircon hf isotopic constraints on the sources of the indus molasse ladakh himalaya india 1 india asia collision has resulted in the largest orogenic event on earth since at least 500 ma and this has had a profound influence on the drainage patterns of asia on the asian monsoon climate on global oceanography as well as on faunal extinctions despite this importance understanding of the paleogene evolution of this system is limited u pb and lu hf isotopic analyses of detrital zircons from the clastic sedimentary rocks of the indus molasse located within the indus suture of the ladakh himalaya india show that most have mesozoic cenozoic ages and juvenile hf isotopic compositions this suggests that the main source of these sediments is the transhimalayan batholith with minor flux from the lhasa karakoram terrane and indian plate the lower indus molasse chogdo formation is inferred to have been deposited at 60 49 ma whereas the upper choksti formation was deposited later than 45 41 ma and contains a greater contribution from the lhasa terrane the transhimalayan batholith was eroded throughout indus molasse sedimentation but increasing exhumation of lhasa karakoram terrane indicates its progressive uplift after initial collision and requires formation of apaleo indus river shortly after collision

summary of meteorological conditions over the north atlantic ocean during gce case watox during the summer of 1988 a team of scientists aboard the noaa ship mt mitchell and the noaa king air aircraft investigated the spatial distributions of sulfur nitrogen and related species and their interactions over the north atlantic ocean in support of these measurements meteorological data from the national meteorological center and from rawinsonde data obtained from the ship were archived and back trajectories were calculated a summary of the meteorological conditions during the cruise is presented using synoptic maps soundings cross sections and isobaric and isentropic back trajectories since day to day variability of the synoptic situation was generally small one representative day was chosen to illustrate the overall meteorology during the cruise three synoptic regimes were encountered 1 north of the polar front 2 under the bermuda azores high and 3 under the intertropical convergence zone itcz soundings from three different days illustrate these regimes boundary layer depth and cloud layers were also estimated from all the soundings cross sections of temperature wind and relative humidity describing the vertical structure of the atmosphere along the cruise show the general day to day uniformity except near the polar front and near the itcz boundary the back trajectories show general air flow patterns and the land mass source regions of air reaching the ship within three days for parts of the cruise air reached the ship from north america iceland or greenland africa and south america

trace elements in the north atlantic troposphere shipboard results of precipitation and aerosols samples for trace metals in precipitation rain snow and fog and total aerosols were collected at sea as part of the gce case watox expedition during august and september 1988 in the greater north atlantic between bermuda iceland the azores and barbados the samples were collected using ultra clean equipment and methods for the analysis of the trace elements al fe mn zn cu ni cd and pb including stable isotopes the samples were analyzed by graphite furnace atomic absorption spectrometry and thermal ionization mass spectrometry based on stable lead isotopes and retrospective isentropic air mass analyses four air mass sources were evidenced including 1 temperate u s and 2 boreal canadian westerlies and 3 boreal and northern europe easterlies associated with 4 mediterranean and saharan easterlies lead isotopic signatures clearly distinguished emission source regions over the north atlantic with 206pb 207pb ratios of 1 19 1 22 for north american westerlies 1 10 1 12 for european boreal easterlies and 1 15 1 16 for saharan trade easterlies also the contrasting ratios between precipitation and aerosol demonstrate a vertical mixing of chemical sources in precipitation from contrasting air masses of different origin the air mass trajectories were independently calculated and are in general agreement with the lead isotopic tracers the isotopic tracers allowed more confident conclusions on air mass sources particularly after a few days of oceanic transport also the trajectories appeared to be very useful in explaining the strong variability of the trace metal concentrations during restricted periods of sampling time aboard a moving ship to a lesser extent the atmospheric emission sources were characterized by trace element concentrations the occurrence of large dust loadings in the saharan trade easterlies contributed abundant crustal elements and were also associated with enriched anthropogenic elements from southern europe scavenging ratios were calculated for lead and cadmium during select periods when aerosol and precipitation were collected under the same meteorological regimes the cadmium ratio is much greater than that of lead indicating that cadmium and lead are each transported or scavenged on distinct populations of aerosols for lead the ratio is less in the north atlantic than in the pacific suggesting an aerosol concentration dependence like the pacific the ratios increase for the lead dust and sea salt one may thus conclude that marine scavenging ratios cannot yet be used effectively until factors such as concentration dependency and local conditions of tropospheric processing are better defined

pb isotopes in surficial pelagic sediments from the north atlantic we measured pb isotopic composition and concentration in sediment samples close to the sea water interface in 6 box cores from the ne atlantic 2 box cores from the sargasso sea and one from the us continental shelf the anthropogenic pb input to marine sediments due to the increase of pb contamination over the ocean during the last century can be identified in all these cores in the eastern part of the atlantic i e in regions under aeolian influence from europe pb pollution can be recognized using its distinctive unradiogenic composition clearly different from the upper crustal values commonly found in pre holocene sediments in contrast pb pollution in regions influenced by north american sources can be identified only in detailed concentration profiles because the american pb pollution has an isotopic composition much closer to that of the natural detrital pb input coming from weathering of the continental crust pb excess inventories are in good agreement with fluxes estimated from sedimenttrap data and with the time record of pb contamination increase given by analyses in coral growth bands inventories of pb contamination to the sediments of the mud patch american shelf are tenfold higher 84 g cm2 than those to hatteras and bermuda abyssal plains 4 3 and 2 8 g cm2

fish taphonomy and environmental inference in paleolimnology abstract the contribution of fish studies to palaeoecology generally takes the form of 1 inference from analogies in modern fish faunas and 2 fish taphonomy the pattern of death and dispersal of bones 1 modern fish faunas and associated organisms provide taxonomic ecological or functional analogues for interpretation of ancient limiting factors and behaviors these inferences presume taxonomic conservatism they also presume functional relationships between morphological form and feeding mode or habitat they become weak with increased geologic age or phyletic distance between ancient subject and modern anaogue 2 fish taphonomy may contribute information about limnology community composition life history mortality depositional environment and preservation taphonomic reconstruction of ecology and preservation depends on the applicability of analogous processes in modern ecology and l limnology in aquatic taphonomy temperature is the most important factor in determining the fate of a carcass above about 16 c depending on depth and pressure most carcasses are made buoyant by bacterial decay gases and are transported to the surface where they may decay further and fall piecemeal into deepwater environments or drift to beach environments where wave energy disarticulates abrades and scatters the bones below about 16 c most carcasses remain on the bottom until buried they may be disturbed by scavengers depending on oxygen concentration in the hypolimnion

ultrahigh pressure minerals and metamorphic terranes the view from china abstract ultrahigh pressure uhp metamorphism refers to mineralogical modifications of continental and oceanic crustal protoliths associated mafic ultramafic rocks initially formed or emplaced in shallow levels of the lithosphere but which subsequently have experienced p t conditions within or above the coesite stability field 2 7 gpa 700 c typical products include eclogite garnet peridotite and uhp varieties of metapelite quartzite marble paragneiss and orthogneiss uhp metamorphic assemblages require relatively cold lithospheric subduction to mantle depths some recrystallization even occurs under forbidden p t conditions characterized by a geotherm of the current explosion of research on continental uhp terranes reflects their significance for mantle dynamics and the tectonics of continental subduction collision exhumation mantle slab interactions and geochemical recycling a further characterization of uhp phases and positive identification of uhp minerals requires new experimental studies coupled with state of the art analyses for example the very rare occurrence of microdiamond inclusions in zircons from dabie sulu uhp rocks may reflect high f o 2 attending recrystallization inasmuch as epidote is rather common rutile needles within garnets from sulu uhp eclogitic rocks may not be the result of exsolution so in such cases the apparent uhp pressure may have been over estimated hadean igneous microdiamond inclusions in jack hills detrital zircons could have originated from mantle xenoliths whereas abundant detrital phanerozoic diamonds containing inclusions of coesite and other eclogitic minerals from new south wales might have been derived from unexposed uhp metamorphic terranes micro mineral intergrowth and nano size minerals may hold important key to deciphering the actual p t paths of subduction and mantle return flow although most exhumed terranes have returned surfaceward relatively rapidly after short time of uhp condition the long duration of storage at great depth and slow exhumation for the largest uhp terranes remain as major problems

carbonate concretions from the london clay ypresian eocene of southern england and the exceptional preservation of wood boring communities marine driftwood bored by teredinid bivalves shipworms is common in the london clay formation ypresian eocene of southern england the london clay of sheppey kent has yielded six genera of teredinids and is the most diverse fossil assemblage of the family known the presence of several genera that are known exclusively from mangroves at the present day demonstrates the potential value of teredinids in the recognition of ancient mangroves the rapid sea floor burial of logs containing numerous individuals resulted in the borings becoming completely or partially sealed by the plug like pallets in this exceptional mode of preservation valves pseudofaeces and pallets are found in life position and early cementation prevented compaction of the wood and borings the bored wood acted as centres of carbonate concretion formation cement compositions mineral associations and 13 c and 18 o stable isotope data indicate that most precipitation occurred in the iron reduction zone with sulphate reduction also being an important source of carbonate for the sheppey concretions fractures and their infilling cements are demonstrably contemporaneous with the cement zones in which they terminate it is likely that rapid changes in cement chemistry are a consequence of sudden release of fluid by dewatering events whilst more gradual shifts in chemistry were controlled be evolution through the oxic fe mn and s reduction zones teredinid boring linings and the cements which infill them may have recrystallized by a dissolution reprecipitation process which has preserved the chemistry of successive cements and has probably not significantly changed 18 o values

a comparison of atmospheric inputs and deep ocean particle fluxes for the sargasso sea trace metal distributions in ocean waters are dominated by internal recycling processes however fluxes out of the water column must balance inputs if steady state is to be maintained we use deep sea particle fluxes at 3200 m in the sargasso sea as measured by sediment traps to define the flux to the sediments beneath the sargasso sea and compare this flux to the atmospheric input for a group of trace metals despite the uncertainties associated with the comparison several important features are revealed for elements predominantly associated with crustal particles al mn fe and v lateral advection provides a significant part of the deep sea particle flux for cd cu ni pb and zn this lateral component is minor and atmospheric inputs to the sargasso sea exceed deep sea particle fluxes these excesses appear to be consistent with calculations of the export of these metals from the sargasso sea arising from ocean boundary scavenging pb and release from decomposing organic matter in the southward flowing north atlantic deep water cd cu ni and zn in the atmospheric deposition itself wet deposition exceeds dry deposition for cd cu mn ni pb and zn while wet and dry deposition are of similar magnitudes for v fe and al

marine vertebrate fauna from the late eocene samlat formation of ad dakhla southwestern moroccos zouhri and otherseocene vertebrate fauna from ad dakhla morocco

the early toarcian early jurassic and the cenomanian turonian late cretaceous mass extinctions similarities and contrasts abstract the early toarcian eto and cenomanian turonian c t mass extinctions are comparable from a wide range of scales and perspectives from a broad standpoint their similarities include virtually identical extinction intensity at the familial and generic levels widespread basinal facies deposited during sea level highstands an overall greenhouse climate and anoxia as an important causal mechanism the high resolution macroinvertebrate data analyzed here consisting of stratigraphic ranges diversity and abundance point to smaller scale similarities the two events resulted in significant ecological disruption and in both cases the biotic responses were very similar taxa inhabiting the upper water column were unaffected by anoxia and included ammonites and in the eto belemnites in addition epifaunal taxa adapted to low oxygen conditions such as the buchiids posidoniids and inoceramids flourished in the post extinction environment during the survival interval as conditions ameliorated the biota became more diverse and gradually began to resemble pre extinction biotas furthermore the 13c curves predict the end of the survival interval and suggest that the period characterized by carbon isotope excursions represent disrupted environmental conditions this points to the potential application of 13c as a tool for determining the repopulation modes and timing for other mass extinctions

mass transport in eocene pelagic chalk on the northwestern edge of the arabian platform shefela area israel glides rotational slumps debris and mud flows occur in the eocene pelagic chalk of the shefela region israel the pelagic lithofacies are present from the beginning of the eocene but mass transport is initiated only in the latest early eocene and the early middle eocene slight steepening of the palaeoslope coupled with increased sedimentation rate and early diagenetic changes lowering inherent strength led to gravity induced mass transport in the poorly consolidated pelagic sediments instability of the sediment pile was enhanced by underconsolidation and development of spacious packing associated with a relatively high rate of deposition and low initial strength due to high content of siliceous micro biota the palaeogeographic position indicated by the mass transport features is the steepened distal part of the ramp like continental margin of the arabian platform distal steepening of the ramp as part of regional downwarping is coincident with reactivation of syrian arc folding reflecting northward convergence of the arabian plate during the eocene

mineralogy petrology u pb geochronology and geologic evolution of the dabie sulu classic ultrahigh pressure metamorphic terrane east central china the dabie sulu triassic collisional orogen in eastern asia was created by northward subduction of the yangtze continental crust capped plate beneath the sino korean craton eclogites garnet peridotites and surrounding country rock gneisses and marbles were all subjected to in situ uhp metamorphism as indicated by the presence of rare but widespread coesite inclusions in eclogitic minerals and in zircon crystals in the country rocks as well as by virtually identical metamorphic ages of various uhp rock types metamorphic p t estimates combined with investigations of mineral exsolution textures and high p polymorphs indicate that recovered depths of continental subduction may have exceeded 200 km parageneses of mineral inclusions in zoned zircon domains combined with u pb ages delineate a well constrained p t time path suggesting exhumation rates of 5 10 km myr a similar p t time trajectory has been established for the microdiamond bearing kokchetav massif thus far however diamond inclusions have not been confirmed from coesite bearing zircon domains of dabie sulu uhp rocks despite numerous detailed studies oxygen isotopes of minerals from many outcrop samples and the chinese continental scientific drilling ccsd project main hole cores indicate that delta 18 o depletion took place in a volume of proterozoic protoliths exceeding 100 000 km 3 along the northern edge of the yangtze craton evidently passive margin sediments and bimodal igneous rocks that had formed during rifting and breakup of the supercontinent rodinia were subjected to extensive meteoric water rock interactions attending terminal neoproterozoic snowball earth conditions such hydrothermal alteration volatilized and depleted c from the relatively oxidized protoliths accounting for the rare occurrences of graphite and apparent lack of microdiamond in dabie sulu uhp rocks

a survey of palaeontological sampling biases in fishes based on the phanerozoic record of great britain fishes represent more than half of all living vertebrate species but patterns of fish diversity remain little explored in the fossil record a compendium of fossil occurrences from great britain was assembled in order to address a series of questions concerning the palaeontological record of fishes there are broad similarities between british richness trajectories and those compiled from global data including an initial peak in the mid palaeozoic devonian or carboniferous depending on the compilation with a late palaeozoic trough followed by a sharp rise in diversity in the late cretaceous and paleogene the british dataset is too small to reveal any significant differences in richness between time bins using subsampling but a modelling approach based on sampling and geological proxies consistently shows lower than predicted richness in the silurian devonian and higher than predicted richness in the late cretaceous and eocene this positive excursion is robust to the exclusion of data from the early eocene london clay lagerstatte chondrichthyans sharks rays and ratfishes and osteichthyans ray finned and lobe finned fishes show contrasting relationships with geological and sampling proxies possibly reflecting different taphonomic profiles or idiosyncratic variation in the relative proportion of freshwater and marine deposits over the british phanerozoic

marine biodiversity through the late cenomanian early turonian palaeoceanographic controls and sequence stratigraphic biases changes in the marine macro and microfauna sedimentary geochemistry and surface water palaeoproductivity through the last 500 000 years of the cenomanian and first 300 000 years of the turonian are documented these are based on the succession at eastbourne the thickest and most complete section through the late cenomanian and early turonian in the anglo paris basin two levels of rapid faunal and geochemical change are identified one coincident with a significant increase in siliciclastic input at the base of the plenus marls member and the other with a marked drop in surface water productivity near the top of the same unit faunal change is demonstrated to be largely a pattern of immigration emigration rather than true extinction and our sequence stratigraphical analysis shows that it was coincident with major sea level changes no evidence is found to support the hypothesis that reduced bottom water oxygenation developed and was responsible for extinctions amongst the benthos in mid shelf environments the onset of pure chalk facies is interpreted to mark the breakdown of shelf break fronts and the spread of oligotrophic oceanic waters over much of the continental shelf initiated by rising sea level the cenomanian turonian event far from recording a mass extinction of shelf fauna is most probably an artifact caused by a significant switch in the nature of the surviving sedimentary record as a result of a major but perfectly ordinary oceanographic change

a multiple proxy and model study of cretaceous upper ocean temperatures and atmospheric co2 concentrations foraminiferal d 18 o and mg ca suggests that the ratio of magnesium to calcium in the turonian coniacian ocean may have been lower than in the albian cenomanian ocean perhaps coincident with an ocean 87 sr 86 sr minimum the carbon isotopic compositions of distinct marine algal biomarkers were measured in the same sediment samples the d 13 c values of phytane combined with foraminiferal d 13 c and inferred temperatures were used to estimate atmospheric carbon dioxide concentrations through this interval estimates of atmospheric co2 concentrations range between 600 and 2400 ppmv within the uncertainty in the various proxies there is only a weak overall correspondence between higher lower tropical temperatures and more less atmospheric co2 the genesis climate model underpredicts tropical atlantic temperatures inferred from odp leg 207 foraminiferal d 18 o and mg ca when we specify approximate co2 concentrations estimated from the biomarker isotopes in the same samples possible errors in the temperature and co2 estimates and possible deficiencies in the model are discussed the potential for and effects of substantially higher atmospheric methane during cretaceous anoxic events perhaps derived from high fluxes from the oxygen minimum zone are considered in light of recent work that shows a quadratic relation between increased methane flux and atmospheric ch4 concentrations with 50 ppm ch4 genesis sea surface temperatures approximate the minimum upper ocean temperatures inferred from proxy data when co2 concentrations specified to the model are near those inferred using the phytane d 13 c proxy however atmospheric co2 concentrations of 3500 ppm or more are still required in the model in order to reproduce inferred maximum temperatures

cretaceous oceanic anoxic event 2 triggered by a massive magmatic episode oceanic anoxic events oaes were episodes of widespread marine anoxia during which large amounts of organic carbon were buried on the ocean floor under oxygen deficient bottom waters oae2 occurring at the cenomanian turonian boundary about 93 5 myr ago is the most widespread and best defined oae of the mid cretaceous although the enhanced burial of organic matter can be explained either through increased primary productivity or enhanced preservation scenarios the actual trigger mechanism corresponding closely to the onset of these episodes of increased carbon sequestration has not been clearly identified it has been postulated that large scale magmatic activity initially triggered oae2 refs 4 5 but a direct proxy of magmatism preserved in the sedimentary record coinciding closely with the onset of oae2 has not yet been found here we report seawater osmium isotope ratios in organic rich sediments from two distant sites we find that at both study sites the marine osmium isotope record changes abruptly just at or before the onset of oae2 using a simple two component mixing equation we calculate that over 97 per cent of the total osmium content in contemporaneous seawater at both sites is magmatic in origin a approximately 30 50 fold increase relative to pre oae conditions furthermore the magmatic osmium isotope signal appears slightly before the oae2 as indicated by carbon isotope ratios suggesting a time lag of up to approximately 23 kyr between magmatism and the onset of significant organic carbon burial which may reflect the reaction time of the global ocean system our marine osmium isotope data are indicative of a widespread magmatic pulse at the onset of oae2 which may have triggered the subsequent deposition of large amounts of organic matter

the onset of india asia continental collision early steep subduction required by the timing of uhp metamorphism in the western himalaya ultrahigh pressure uhp rocks in the nw himalaya are some of the youngest on earth and allow testing of critical questions of uhp formation and exhumation and the timing of the india asia collision initial collision of india with asia is widely cited as being at 55f1 ma based on a paleomagnetically determined slowdown of india s plate velocity and as being at ca 51 ma based on the termination of marine carbonate deposition even relatively small changes in this collision age force large changes in tectonic reconstructions because of the rapid india asia convergence rate of 134 mm a at the time of collision new u pb shrimp dating of zircon shows that indian rocks of the tso morari complex reached uhp depths at 53 3f0 7 ma given the high rate of indian subduction this dating implies that indian continental crust arrived at the asian trench no later than 57f1 ma providing a metamorphic age for comparison with previous paleomagnetic and stratigraphic estimates india s collision with asia may be compared to modern processes in the timor region in which initiation of collision precedes both the slowing of the convergence rate and the termination of marine carbonate deposition the indian uhp rocks must have traveled rapidly along a short hence steep path into the mantle early continental subduction was at a steep angle probably vertical comparable to modern continental subduction in the hindu kush despite evidence for modern day low angle subduction of india beneath tibet oceanic slab break off likely coincided with exhumation of uhp terranes in the western himalaya and led to the initiation of low angle subduction and leucogranite generation d 2005 elsevier b v all rights reserved

exhumation history of the nw indian himalaya revealed by fission track and 40ar 39ar ages new fission track and ar ar geochronological data provide time constraints on the exhumation history of the himalayan nappes in the mandi beas valley tso morari transect of the nw indian himalaya results from this and previous studies suggest that the sw directed north himalayan nappes were emplaced by detachment from the underthrusted upper indian crust by 55 ma and metamorphosed by ca 48 40 ma the nappe stack was subsequently exhumed to shallow upper crustal depths 10 km by 40 30 ma in the tso morari dome northern section of the transect and by 30 20 ma close to frontal thrusts in the baralacha la region from the oligocene to the present exhumation continued slowly metamorphism started in the high himalayan nappe prior to the late oligocene high temperatures and anatexis of the subducting upper indian crust engendered the buoyancy driven ductile detachment and extrusion of the high himalayan nappe in the zone of continental collision late extrusion of the high himalayan nappe started about 26 ma ago accompanied by ductile extensional shearing in the zanskar shear zone in its roof between 22 and 19 ma concomitant with thrusting along the basal main central thrust to the south the northern part of the nappe was then rapidly exhumed to shallow depth 10 km between 20 and 6 ma while its southern front reached this depth at 10 5 ma

how does the mid crust accommodate deformation in large hot collisional orogens a review of recent research in the himalayan orogen abstract the presence of hot weak crust is a central component of recent hypotheses that seek to explain the evolution of continent continent collisions and in particular may play an important role in accommodating the 3000 km of convergence within the himalaya tibetan collision over the last 55 myr models that implicate flow of semi viscous midcrustal rocks south toward the front of the himalayan orogen channel flow are able to account for many geologic observations in the himalaya while alternative models of collision particularly thrust wedge taper demonstrate that much of the observed geology could have formed in the absence of a low viscosity mid crustal layer several recent studies synthesized here have prompted a shift from initial assumptions that channel flow and thrust wedge taper processes are by definition mutually exclusive these new studies reveal the presence of several tectonometamorphic discontinuities in the midcrust that appear to reflect a continuum of deformation in which both channel and wedge type processes operate in spatially and temporally distinct domains within the orogen and further that the system may migrate back and forth between these types of behavior this continuum of deformation styles within the collisional system is of crucial importance for explaining the evolution of the himalayan orogen and hence for understanding the evolution of earth s many continent continent collision zones

interaction between bottom currents and slope failure in the late cretaceous of the southern danish central graben north sea the nw european chalk group was deposited in a deep epicontinental sea traditionally conceived as a quiet depositional setting that was only affected by redeposition along structural highs the chalk sea was however locally affected by powerful bottom currents during some periods three dimensional seismic reflection data from the southern central graben show abundant intra chalk morphological elements concentrated at two stratigraphic levels within the turonian campanian sequence both levels are topped by extensive unconformities the geometry of the elements produced by alongslope processes is interpreted as caused by se directed bottom currents flowing between the bo jens and adda ridges the highest current intensities led to the formation of the two unconformities judged by the occurrence of the largest current formed channels and drifts at these two stratigraphic levels this sea floor topography was locally modified by large scale downslope mass transport stratigraphical coincidence of the largest slumps and bottom current erosion suggests a coupling between downslope and alongslope processes current erosion impinged upon the slopes during periods of highest current speeds destabilizing the slope sediments and triggering slumping the smoothing of the bathymetry following local relaxation of the tectonic inversion accounted for the coeval deactivation of alongslope and downslope processes during the succeeding maastrichtian deposition

nautiloid shell taphonomy interpretations based on water pressure abstract nautiloid taphonomy is a difficult and widely neglected topic acting as a test of many of the assumptions made in nautiloid paleobiology phylogenetic interpretations and geological facies analysis the eocene nautiloids from the offshore london clay formation of england are considered from the perspective of flooding and implosion experiments on recent nautilus the deeper water facies are often characterized by body chambers in approximate life position one or two complete septa and various types of shell debris intermediate depth facies show intact and often vertically embedded phragmocone walls with zones of internally broken and spar cemented septa taphnomic characteristics of the former facies probably result from the implosion of shells at depths of 200 or 300 m on the seafloor while those of intermediate depth facies result from the sinking of gas filled shells to the seafloor and subsequent bending stress failure of septa left unsupported by sediment in the last whorl evidence for the removal of the body from the largely unbroken shell by octopus illustrates the potential for a biological influence on the taphonomic history of nautiloids through the phanerozoic tertiary nautiloids were probably unusually prone to extensive post mortem transport a process which tends to remove shells from the fossil record under most geological circumstances

subduction followed by collision alpine and himalayan examples abstract the high mountains of the himalaya and alps are both the result of ongoing continental collision following closure by subduction of neo tethys and related ocean basins during the mesozoic and tertiary the high topography is associated in both areas with a thickened crustal root of up to 70 km thickness as deduced from various geophysical investigations in the himalayan region where several thousand kilometres of ocean crust was consumed below the lhasa block southern margin of the asian continent both continental trans himalayan batholith and oceanic kohistan ladakh arc accreted to asia in the jurassic magmatic arcs are recognisable in the alps where much smaller oceanic rifts and basins were consumed such arc sequences are absent in the area of the high mountains regardless of this major difference in volume of subducted oceanic lithosphere seismic tomography in both regions has identified several anomalous regions in the mantle interpreted as the remnants of detached slabs of oceanic crust metamorphosed rocks of continental crust sequences in both the himalaya and alps record evidence of subduction to depths exceeding 90 km coesite stability field as well as rapid exhumation to shallower depths at rates of 4 mm a or more this subduction related metamorphism of the eclogite and blueschist facies is clearly distinguishable from subsequent younger lower pressure greenschist amphibolite facies metamorphic overprints related to imbricate stacking of continental crust slices thermal relaxation and final exhumation dragging of buoyant upper crust down to upper mantle depths requires an attached root of oceanic lithosphere but the rapid exhumation stage may well be a consequence of slab break off this process appears to have been equally effective in the alps and the nw himalaya despite the significant differences in duration of subduction and the volumes of oceanic lithosphere involved

transport of industrial lead in snow through soil to stream water and groundwater determination of isotopic compositions and concentrations of pb in stream water groundwater soil and bed rock in a subalpine watershed combined with concentrations of ca na k mg fe and mn and mineralogical data show that 90 of the industrial pb in snow is trapped in a subunit of the upper 2 cm of soil this is composed of humus plus fe hydroxides and mn oxides where industrial pb mixes isotopically with pb already accumulated there from previous inputs of industrial pb and natural rock pb the 206 pb 207 pb ratio of pb in the upper 2 cm soil accumulation reservoir is different from that of industrial pb added from snow each year because it is a mixture of natural rock pb derived from weathering 20 which has a 206 pb 207 pb ratio different from industrial pb and because the 206 pb 207 pb ratio of industrial pb accumulated from atmospheric inputs during the past century 80 has changed considerably during that time during spring melt when streams are fed mostly by surface runoff pb in the surface soil accumulation reservoir is released back into the runoff and into streams at this time the proportion of industrial pb in mountain stream waters is that in the surface soil accumulation reservoir 80 concentrations and isotopic composition of pb in soil profiles and the relation between pb and organic matter concentration in soil indicate little or no industrial pb accumulates in the reservoir below 2 cm depths these data combined with measured proportions of ca and pb and 206 pb 207 pb ratios in groundwater indicate that most of the pb in aquifers at depth below 30 cm originates by weathering from natural rock pb although aquifers are charged by snow melt industrial pb is removed from the water as it percolates downward through the soil accumulation reservoir therefore during times when streams are fed mainly from these aquifers stream waters contain mainly natural rock pb and some industrial pb in colloids originally contained in snow melt concentrations of pb in stream waters tend to remain constant both throughout different seasons when rates of stream discharge change 20 fold and over the last decade when pb inputs from snow decreased 6 fold because pb in those waters originates from a fixed reservoir in the soil

crustal channel flows 1 numerical models with applications to the tectonics of the himalayan tibetan orogen 1 plane strain thermal mechanical numerical models are used to examine the development of midcrustal channel flows in large hot orogens in the models radioactive self heating reduces the viscosity of tectonically thickened crust and increases its susceptibility to large scale horizontal flow channels can be exhumed and exposed by denudation focused on the high relief transition between plateau and foreland we interpret the himalaya to have evolved in this manner channel flows are poorly developed if the channel has a ductile rheology based on wet quartz flow laws and well developed if there is an additional reduction in viscosity to 1019 pa s this reduction occurs from 700 c to 750 c in the models and is attributed to a small percentage of in situ partial melt melt weakening model ht1 provides an internally consistent explanation for the tectonic evolution of many features of the himalayan tibetan orogenic system erosional exhumation exposes the migmatitic channel equivalent to the greater himalayan sequence ghs between coeval normal and thrust sense ductile shear zones corresponding to the south tibetan detachment and the main central thrust systems outward flow of unstable upper crust rotates these shears to low dip angles in the model both the ghs and the lesser himalayan sequence are derived from indian crust with the latter from much farther south similar models exhibit a range of tectonic styles including the formation of domes resembling north himalayan gneiss domes model results are relatively insensitive to channel heterogeneities and to variations in the behavior of the mantle lithosphere beneath the model plateau

mid cretaceous albian santonian sea surface temperature record of the tropical atlantic ocean paleoclimate records of geologic time periods characterized by extreme global warmth such as the mid cretaceous are important for a better understanding of the earth9s climate system operating in an exceptionally warm mode here we applied an organic geochemical proxy tex 86 on organic matter rich albian santonian sediments recovered from ocean drilling program leg 207 sites 1258 and 1259 on demerara rise to reconstruct sea surface temperatures ssts in the western equatorial atlantic preceded by a stepwise cenomanian warming trend 31 35 c the onset of the cretaceous thermal maximum coincided here with the cenomanian turonian boundary event once established this extreme warm climate regime characterized by averaged tropical ssts close to 35 c lasted up to the turonian coniacian transition two pronounced cooler intervals 2 3 c interrupt this otherwise remarkably stable record providing the first 18 o independent evidence for middle turonian cooling that previously has been attributed to glacioeustatic sea level lowering coniacian ssts decline stepwise reaching a minimum in the santonian 32 33 c where cooling is most pronounced presumably concomitant with the first progressive opening of a deep water passage through the equatorial atlantic gateway

paleolimnology and taphonomy of some fish deposits in fossil and uinta lakes of the eocene green river formation utah and wyoming abstract in four sections in the lacustrine eocene green river formation in utah lake uinta fish are preserved on carbonate mudflats along high energy carbonate shorelines in the littoral zone and in deeper profundal areas scattered bones and scales are found throughout dropped from disintegrating floating carcasses gar scales typify shallow water deposits partly due to habitat preference beach concentrations of greatly abraded bones and scales grade offshore into storm lag layers and lenses which grade to loosely packed horizons or patches of unworn remains littoral fish fossils range from dropped bones to good skeletons with much floating lifting and scavenging preservation improves away from the waterline not only into deep and cold profundal waters where the carcasses are protected from scavenging by anoxia and prevented from floating by temperature and pressure but also onto mudflats where stranding and burial of carcasses can also protect them a section through an isolated part of the green river formation in wyoming fossil lake also ranged from profundal to littoral with minor emergence dropped body parts were found throughout but preservation quality of the best specimens improved with presumed depth the beds with the best specimens seem to have accumulated at the bottom of a deep stratified warm monomictic lake our lake uinta sections were generally shallower and suggest more fluctuations in water level but this is not relevant to broad paleolimnological interpretations because they represent marginal zones which are necessarily more sensitive to changes in depth overall fish fossils are especially informative in deeper waters where sedimentary distinctions fail

temporal variability of lead in the western north atlantic consumption of alkyl leaded gasoline has decreased considerably in the united states over the last 15 years in order to evaluate the chemical response of the ocean to decreased atmospheric deposition of anthropogenic lead new 210pb and stable pb data have been obtained near bermuda june 1983 to december 1984 concentrations of these tracers in the mixed layer are highly variable during the period of observations 25 from june through september soluble eolian deposits accumulate in the shallow mixed layer because mixing and productivity are inhibited by the seasonal thermocline in the fall these enriched surface waters mix with upper thermocline waters and surface pb concentrations decrease in the winter storms from the east coast of north america cause local episodes of high pb and 210pb input into the ocean surface the resulting areally limited high pb concentrations are dispersed within less than 2 weeks by vertical and horizontal mixing pb 210pb ratios in surface waters are almost invariant over this period averaging 669 pmol dpm ranging from 564 to 718 pmol dpm surface pb 210pb appears to have declined by a factor of 2 since 1979 similar to the 2 6 fold decrease in u s leaded gasoline consumption during this period a substantial fraction on the order of 10 of gasoline pb is transported into the ocean pb concentrations in the upper thermocline may have declined by about 15 from 1979 to 1984 the helium tritium thermocline ventilation model of jenkins 1980 coupled with boundary conditions appropriate for pb and 210pb shows that the supply of pb and 210pb to the thermocline is dominated by lateral isopycnal ventilation the shape of the vertical profile is dominated by this physical ventilation near surface biological removal processes in situ scavenging and for stable lead time dependent input vertical transport by particles is responsible for minimal transport of dissolved lead into the upper ocean similar processes are likely to account for the plutonium maximum in the thermocline of the north pacific observations of pb and other reactive species in winter surface outcrops of thermocline isopycnals would provide valuable constraints on chemical processes within the thermocline

new late cretaceous macrobaenid turtle with asian affinities from the high canadian arctic dispersal via ice free polar routes a new turtle aurorachelys gaffneyi gen et sp nov is described on the basis of a carapace and plastron of late turonian coniacian age from axel heiberg island in the high canadian arctic this turtle is a member of the macrobaenidae a group that is thought to have originated in asia it differs from all other late cretaceous macrobaenids in its nearly circular shell the earliest record of macrobaenids in north america documented by this specimen together with the earliest records of other turtle clades that presumably originated in asia indicates that turtle dispersal was episodic with most first occurrences of asian taxa in north america clustering around the turonian the high global temperatures of the turonian are interpreted as facilitating this episode of dispersal of asian turtle groups into north america islands and seamounts in the young volcanically active arctic ocean during the late cretaceous may have acted as an alternative to an alaskan intercontinental route and may have allowed macrobaenids to enter the part of north america east of the interior continental seaway

high temperatures in the late cretaceous arctic ocean to understand the climate dynamics of the warm equable greenhouse world of the late cretaceous period it is important to determine polar palaeotemperatures the early palaeoceanographic history of the arctic ocean has however remained largely unknown because the sea floor and underlying deposits are usually inaccessible beneath a cover of floating ice a shallow piston core taken from a drifting ice island in 1970 fortuitously retrieved unconsolidated upper cretaceous organic rich sediment from alpha ridge a submarine elevated feature of probable oceanic origin a lack of carbonate in the sediments from this core has prevented the use of traditional oxygen isotope palaeothermometry here we determine arctic palaeotemperatures from these upper cretaceous deposits using tex86 a new palaeothermometer that is based on the composition of membrane lipids derived from a ubiquitous component of marine plankton crenarchaeota from these analyses we infer an average sea surface temperature of approximately 15 degrees c for the arctic ocean about 70 million years ago this calibration point implies an equator to pole gradient in sea surface temperatures of approximately 15 degrees c during this interval and by extrapolation we suggest that polar waters were generally warmer than 20 degrees c during the middle cretaceous approximately 90 million years ago

response of lead cycling in the surface sargasso sea to changes in tropospheric input lead and its stable isotopes have been analyzed in surface water samples 0 600 m and trapped particles collected at the bermuda atlantic time series station u s joint global ocean flux study in april and november 1989 these results are compared with wet atmospheric lead deposition as determined from precipitation continuously collected in bermuda since august 1988 as pan of the atmosphere ocean chemistry experiment program despite an expected seasonal variability lead concentrations in surface waters have clearly decreased by 30 to 40 since 1979 in response to a corresponding decline by a factor of 5 to 8 of the tropospheric deposition this result is corroborated by stable lead isotope measurements with 206pb 207pb ratios which are significantly less radiogenic 1 18 1 20 in the first 500 m of the 1989 profile than those measured in 1984 1 20 1 21 this isotopic shift reflects changes of lead ore supply in the united states as well as a relative increase of the eurafrican contribution to lead input in the northwest atlantic that is likely due to the reduction of lead emissions from gasoline consumption in north america this fast response of lead to interannual variations of the tropospheric input into surface waters is related to its efficient bioreactivity as demonstrated with a sediment trap deployed at 150 m in april 1989 sediment trap results show the rapid penetration of lead into the first 200 m associated with large particles during a period of high plankton activity retrospective isentropic air mass trajectories in three dimensions coupled with the precipitation events collected in bermuda in 1988 1989 show that 30 to 40 of the annual lead deposition originates from the trade easterly meteorological regime this input is clearly evidenced with lead isotopic signals observed in surface waters 0 100 m in april and november 1989 we show that lead accumulated in the seasonal mixed layer 0 50 m reflects this atmospheric input taking into account the isotopic signal measured in this mixed layer 1 178 0 001 as well as the respective contribution of the temperate westerlies and trade easterlies to atmospheric lead deposition to the sargasso sea we calculate that the isotopic signature from the eurafrican regions is 1 155 0 004 based on these results we calculate that the actual 206pb 207pb average ratio in surface waters of the sargasso sea is 1 188 0 004

evolutionary model of the himalaya tibet system geopoem based on new modelling geological and geophysical data a two dimensional thermo mechanical laboratory modelling of continental subduction was performed the subducting continental lithosphere includes a strong brittle upper crust a weak ductile lower crust and a strong upper mantle the lithosphere is underlain by a low viscosity asthenosphere subduction is produced by a piston push force and the pull force from the mantle lithospheric layer which is denser than the asthenosphere the lithospheric layers are composed of material whose strength is sensitive to and inversely proportional to temperature throughout the experiment the model surface was maintained under relatively low temperature and the model base at higher temperature the subduction rate satisfied the peclet criterion modelling confirms that the continental crust can be deeply subducted and shows that slab break off delamination and tectonic underplating are fundamental events with drastic consequences on the subsequent evolution of the convergent system combining these results with previous purely mechanical modelling we elaborate a new evolutionary model for the himalaya tibet convergent system the principal successive stages are 1 subduction of the indian continental lithosphere to 200 250 km depth following subduction of the tethys oceanic lithosphere 2 failure and rapid buoyancy driven uplift of the subducted continental crust from ca 100 km depth to some depth that varies along the mountain belt 20 30 km on average 3 break off of the indian subducted lithospheric mantle with the attached oceanic lithosphere 4 subduction underplating of the indian lithosphere under asia over a few to several hundred kilometers 5 delamination roll back and break off of the indian lithospheric mantle 6 failure of the indian crust in front of the mountain belt formation of the main central thrust and underthrusting of the next portion of indian lithosphere beneath tibet for a few hundred kilometers at the beginning of stage 6 the crustal slice corresponding to the crystalline himalayas undergoes erosion activated uplift and exhumation fl 2000 elsevier science b v all rights reserved

mass transport in european cretaceous chalk fabric criteria for its recognition although it is a pelagic sediment fine grained calcareous ooze may be mobilized prior to general lithification and redeposited as allochthonous units numerous occurrences of allochthonous chalk have been reported in recent years having been recognized by large scale bedding features seen in outcrop smaller scale internal features such as contorted laminae and larger features such as smeared burrows and imbricated flint nodules attest to a significant amount of soft sediment deformation and synsedimentary slumping in european chalk sections of late cretaceous age truly autochthonous chalks contain complex tiered ichnofabrics and in some cases exhibit a diagenetic nodular fabric that is undisturbed by transport in some situations such as stagnant water conditions autochthonous chalks may exhibit primary lamination although this is very uncommon in european chalk sequences different types of redepositional processes produce an array of varied allochthonous fabrics glide and slump units for example contain internal deformational features produced during sliding ooze flow causes plastic deformation of chalk units internally as well as externally resuspension and fluid flow of chalk sediment produces a deposit having a totally new fabric such as a conglomerate composed of detrital chalk clasts in this paper typical macroscopic sedimentary fabric types are illustrated and the means of identifying them are discussed in terms of bioturbation features in situ diagenetic nodules versus detrital clasts physical deformation structures and development of flints

petrochronology of himalayan ultrahigh pressure eclogite the timing and nature of the india asia collision earth s largest ongoing continent continent collisional orogen are unclear ultrahigh pressure metamorphism of indian continental margin rocks is used as a proxy for initial collision because it indicates subduction of india records of this metamorphism are preserved only at kaghan valley pakistan and tso morari ladakh india separated by 500 km and having published ages of peak pressure of 46 2 0 7 ma and 53 51 ma respectively the apparent 6 m y age difference may refl ect multiple subduction events a large promontory along the former indian margin or inadequate constraints on the time of peak pressure recrystallization at tso morari we present 108 coupled in situ u th pb and rare earth element ree analyses of zircons in two tso morari eclogites to obtain age and petrologic information the ages range from ca 53 ma to 37 ma and peak at ca 47 43 ma flat heavy ree slopes and the absence of an eu anomaly are compatible with eclogite facies zircon re crystallization this re crystallization probably occurred at ultrahigh pressure because 64 of the analyses are from zircon included in ultrahigh pressure garnet and omphacite these results are consistent with those from kaghan valley and suggest that a single protracted ultrahigh pressure metamorphic event occurred contemporaneously across much of the orogen following initial contact of the indian and asian continents at ca 51 ma or later

the bolca lagerstatten shallow marine life in the eocene the eocene limestones around the italian village of bolca occur in a series of distinct localities providing a unique snapshot of marine life in the early cenozoic famous for its fishes the localities of bolca also yield diverse invertebrate faunas and a rich but relatively understudied flora most fossils from bolca derive from the pesciara and monte postale sites which bear similar fossils but are characterized by slightly different taphonomic and environmental profiles although not precisely contemporaneous the age of these principal localities is well constrained to a narrow interval within the ypresian stage c 50 49 ma this places bolca at a critical time in the evolutionary assembly of modern marine fish diversity and of reef communities more generally

crustal channel flows 2 numerical models with implications for metamorphism in the himalayan tibetan orogen 1 results from a thermal mechanical model ht1 that includes midcrustal channel flow are compatible with many features of the himalayan tibetan system radioactive self heating and rheological weakening of thickened model orogenic crust lead to the formation of a hot low viscosity midcrustal channel and a broad plateau channel material corresponding to the greater himalayan sequence ghs flows outward from beneath the plateau in response to topographically induced differential pressure at the plateau flank it is exhumed by focused surface denudation and juxtaposed with cooler newly accreted material corresponding to the lesser himalayan sequence lhs the model channel is bounded by coeval thrust and normal sense ductile shear zones interpreted to represent the main central thrust mct zone and south tibetan detachment system respectively inverted metamorphism associated with the model mct zone results from distributed ductile shear along the mct and extrusion of the hot channel a variety of model p t t path styles resembling those observed in the ghs and lhs are produced for points traveling through contrasting tectonic regimes that coexist in different parts of the model predicted times of peak metamorphism cooling and erosion of metamorphic facies are generally compatible with observations although model ghs cooling ages are too young the times of m1 and m2 metamorphic events observed in the ghs correspond to model times of maximum burial and maximum heating respectively the results highlight the need to integrate tectonics and metamorphism in continental collision models and demonstrate the importance of lateral transport of both heat and material in large hot orogens

the age of deep steep continental subduction in the nw himalaya relating zircon growth to metamorphic history comment on the onset of india asia continental collision early steep subduction required by the timing of uhp metamorphism in the western himalaya by mary l leech s singh a k jain simon l klemperer and r m manickavasagam earth and planetary science letters 234 2005 83 97 abstract leech et al mary l leech s singh a k jain simon l klemperer and r m manickavasagam earth and planetary science letters 234 2005 83 97 present 3 clusters of ages for growth stages in zircon from quartzo feldspathic gneisses hosting coesite bearing eclogite from the tso morari complex nw india these age clusters from oldest to youngest are interpreted to represent the age of ultrahigh pressure metamorphism a subsequent eclogite facies overprint and a later amphibolite facies retrogression and require subduction of indian crust to have started earlier than previously accepted however no petrographic evidence such as inclusions in the zircons relating to particular metamorphic events is presented to substantiate the proposed sequence of metamorphic stages previously published data from eclogites of the same area indicate that coesite eclogite is not the first but at least the second eclogite facies stage in addition the newly proposed time interval between coesite eclogite and the amphibolite facies overprint is longer than previously indicated by diffusion modelling of natural garnet garnet couples in eclogite neither the age of ultrahigh pressure metamorphism nor the timing of initiation of subduction is reliably constrained by the presented data

carbon and oxygen isotope stratigraphy of the english chalk and italian scaglia and its palaeoclimatic significance a detailed carbon and oxygen isotope stratigraphy has been generated from upper cretaceous coastal chalk sections in southern england east kent culver cliff isle of wight eastbourne and seaford head sussex norfolk coast and the british geological survey bgs trunch borehole norfolk data are also presented from a section through the scaglia facies exposed near gubbio italian apennines wherever possible the sampling interval has been one metre or less both the chalk and scaglia carbon isotopic curves show minor positive excursions in the mid cenomanian mid and high turonian basal coniacian and highest santonian lowest campanian there is a negative excursion high in the campanian in chalk sections that span that interval the well documented cenomanian turonian boundary spike is also well displayed as is a broad positive excursion centred on the upper coniacian a number of these positive excursions correlate with records of organic carbon rich deposition in the atlantic ocean and elsewhere the remarkable similarity in the carbon isotope curves from england and italy enables cross referencing of macrofossil and microfossil zones and pinpoints considerable discrepancy in the relative positions of the turonian coniacian and santonian stages the oxygen isotope values of the various chalk sections although showing different absolute values that are presumably diagenesis dependent show nonetheless a consistent trend the east kent section which is very poorly lithified indicates a warming up to the cenomanian turonian boundary interval then cooling thereafter regional organic carbon burial documented for this period is credited with causing drawdown of co 2 and initiating climatic deterioration inverse greenhouse effect data from other parts of the world are consistent with the hypothesis that the cenomanian turonian temperature optimum was a global phenomenon and that this interval represents a major turning point in the climatic history of the earth

stratigraphy and structure of the indus suture in the lower swat pakistan nw himalaya abstract we present a geological map detailed structural description and new subdivisions of the indus suture zone lithologies in northeast pakistan lower swat region the indus suture zone is a dominantly fore arc related assemblage obducted onto the indian plate initial southward thrusting was followed by the reactivation of the indus suture as a ductile brittle normal fault significant strike slip faulting may have taken place at this stage later brittle reverse faults demonstrate that shortening outlasted normal faulting the comparison of the structural evolution of the indus suture zone and the indian plate shows that both terrains were sharing the deformational history possibly since the earliest stage of collision the lower swat area records a full orogenic cycle from early rifting through continent arc collision to orogenic collapse we present a geodynamic model that integrates all these elements

lead in corals reconstruction of historical industrial fluxes to the surface ocean twentieth century environmental lead chronologies for the western north atlantic pacific and indian oceans have been reconstructed from annually banded scleractinian corals measurements of lattice bound pb in sequential coral bands reveal temporal changes in surface water pb concentrations and pb isotopic distributions perturbations are observable in all specimens studied attesting to global augmentation of environmental pb by industrialization in the western north atlantic pb perturbations have occurred in direct response to the american industrial revolution and the subsequent introduction and phasing out of alkyl pb additives in gasoline surface ocean conditions near bermuda may be reliably reconstructed from the coral data via a lead distribution coefficient of 2 3 for the species diploria strigosa based on210pb measurements a similar distribution coefficient may be characteristic of corals in general surface pb concentrations in the pre industrial sargasso sea were about 15 20 pm concentrations rose to near 90 pm by 1923 as a result of metals manufacture and fossil fuel combustion beginning in the late 1940 s increased utilization of leaded gasoline eventually led to a peak concentration of 240 pm in 1971 representing an approximate 15 fold increase over background surface ocean concentrations are presently declining rapidly 128 pm in 1984 as a result of curtailed alkyl pb usage lead isotopic shifts parallel the concentration record indicating that characteristic industrial and alkyl pb source signatures have not changed appreciably in time industrial releases recorded in the florida keys reflect a weaker source and evidence of recirculated pb 5 6 years old from the north atlantic subtropical gyre an inferred background concentration of 38 pm suggests influence of shelf and or resuspended inputs of pb to these coastal waters in remote areas of the south pacific and indian oceans industrial signals are fainter and the corals studied much younger than their atlantic counterparts contemporary pb concentrations implied by coral measurements assumingkd 2 3 are 40 50 pm for surface waters near tutuila and galapagos in the south pacific and 25 29 pm near mauritius in the indian ocean a single coral band from fiji 1920 5yr implies a pre industrial surface water concentration of 16 19 pm pb for the south pacific in view of reported surface water measurements and the north atlantic coral data the pacific coral extrapolations may be slightly high this could be a result of small variations inkd among different coral genera or incorporation of diagenetic pb by corals sampled in coastal environments

high gradients of lead isotopic composition in north east pacific upwelling filaments substantial differences in stable lead isotopic composition in coastal waters of the north east pacific have been inferred from a single vertical profile taken near western canada1 here we present measurements of lead isotopes in two upwelling filaments off central california 39 n 124 w which confirm that such marked differences exist the distinct filamentary structures appear in association with surface jets which transport cold upwelled water hundreds of kilometres offshore2 surface water ratios of stable lead isotopes 204pb 206pb 207pb 208pb within the core of the first filament for example 206pb 207pb 1 17 contrasted with those of surrounding coastal waters 206pb 207pb 1 19 and those of industrial lead aerosols from the united states 206pb 207pb 1 22 contrasts in the isotopic composition indicated that the lead within the core was primarily derived from aeolian inputs of asian industrial lead 206pb 207pb 1 16 to remote waters which were then upwelled near the north american coastline high gradients of lead isotopic composition in coastal waters were confirmed by surface and sub surface measurements surrounding a second upwelling filament these gradients illustrate the complexity of trace element cycles in coastal waters and the offshore fluxes within the ephemeral filaments demonstrate significant influences of coastal dynamics on composition within the ocean gyre

plate velocity exhumation of ultrahigh pressure eclogites in the pakistan himalaya u pb ages of zircon and allanite from the coesite bearing ultrahigh pressure uhp units in the kaghan valley northern pakistan demonstrate that peak uhp metamorphism along the northern margin of the indian plate occurred at 46 4 0 1 ma at peak pressure temperature conditions of 27 5 kbar 100 km and 720 770 c much lower pressure retrogressive growth of titanite took place between 46 4 and ca 44 ma indicating that the eclogites were exhumed to 35 km depth at or before 44 ma implying very rapid exhumation rates within the mantle of 30 80 mm yr or more comparable to rapid plate velocities once entrained in the base of the crust the eclogites followed a slower cooling history from 45 ma similar to the amphibolite facies gneisses of the pakistan himalaya

exhumation of early tertiary coesite bearing eclogites from the pakistan himalaya early tertiary coesite bearing eclogites crop out in the internal zones of the indian plate in the kaghan valley of north pakistan field and geochronological data suggest that uhp metamorphism at c 46 ma accompanied subduction of the leading edge of continental india beneath kohistan a new u pb rutile age of 44 1 1 1 ma can be interpreted as either a growth or a cooling age given the uncertainty of the closure temperature of rutile for pb but clearly documents a very short lived metamorphic peak with cooling occurring between 44 and 40 ma eclogitic assemblages are preserved in permo triassic mafic rocks in both cover and basement sequences with the best preserved eclogites hosted in the anhydrous basement the eclogite facies rocks lie on the hanging wall of a major south directed ductile thrust zone which placed basement rocks that contain coesite eclogites southward onto lower pressure cover metasediments shear criteria show that those sequences that contain eclogite facies rocks are flanked above by a zone of pervasive north directed extensional shearing mineral assemblages contained within both the thrust and extensional shear zones are consistent with them both having operated synchronously during the amphibolite to greenschist facies transition amphibole ar ar cooling history data suggest that this was at c 40 42 ma as in the alps this implies exhumation from the metamorphic peak to greenschist facies conditions within a few million years this short time scale has important implications as it is likely that both the thrust and extensional shear zones represent the late stage of a deformation continuum that commenced at uhp and brought the uhp rocks back to mid crustal levels this late eocene extensional shearing and associated exhumation of hp or uhp rocks has not previously been recorded in the pakistan himalaya the thrusts and shears were later folded by major east trending folds themselves cut by miocene aged north vergent extensional faults that currently separate the indian plate from the structurally overlying kohistan island arc

extremely high sea surface temperatures at low latitudes during the middle cretaceous as revealed by archaeal membrane lipids the middle cretaceous 125 88 ma greenhouse world was characterized by high atmospheric co 2 levels the general absence of polar ice caps and much higher global temperatures than at present both 1 8 o based and model based temperature reconstructions indicate extremely high sea surface temperatures ssts at high latitudes however there are a number of uncertainties with sst reconstructions based on 1 8 o isotope data of foraminifera due to diagenetic overprinting effects and tenuous assumptions with respect to the 1 8 o value of cretaceous seawater the paleoecology of middle cretaceous marine organisms and seawater ph here we applied a novel sst proxy i e text tetraether index of 86 carbon atoms based on the membrane lipids of marine crenarchaeota derived from middle cretaceous sedimentary rocks deposited at low latitudes the text proxy indicates that tropical ssts in the proto north atlantic were at 32 36 c during the early albian and late cenomanian early turonian this finding agrees with sst estimates based on 1 8 o paleothermometry of well preserved foraminifera as well as global circulation model calculations the text proxy indicates cooler ssts 27 32 c for the equatorial pacific during the early aptian which is in agreement with sst estimates based on 1 8 o paleothermometry

northwestern european paleogene magnetostratigraphy biostratigraphy and paleogeography calcareous nannofossil evidence global calcareous nannofossil biostratigraphy is used to correlate the paleogene chronostratigraphic units of northwestern europe and standard nannofossil biozones such biostratigraphic correlations show that the lithostratigraphic sequence is discontinuous and allows interpretation of the paleogeography of northwestern europe during the paleogene and evaluation of the influence of global sea level eustatic changes on northwestern european stratigraphy a well documented unconformity occurs near the lower middle eocene boundary in the paris basin this unconformity which also can be traced in the london hampshire basin and in belgium correlates with a coastal offlap event presumed sea level of vail et al on the basis of magnetostratigraphic biostratigraphic correlations the duration of the hiatus associated with this unconformity can be estimated to be 0 6 to 1 5 m y in the hampshire basin and 2 to 4 m y in the paris basin

crustal structure a key constraint on the mechanism of ultra high pressure rock exhumation abstract the distribution of ultra high pressure uhp metamorphic rocks demonstrates that burial to 100 km and rapid exhumation 1 cm a 1 of continental crust is a normal part of early 10 ma continental collision currently there is no comprehensive model for this fundamental tectonic process that also satisfactorily explains the upper crustal structures resulting from early collisional uhp rock exhumation characteristic features requiring explanation include structural domes that are cored by uhp nappes associated medium to high pressure nappes displaying a distinct pressure gap overlying lower grade rocks including suture zone ophiolites and coeval foreland directed thrust faults and syn exhumation normal faults we present a geodynamical model involving crustal burial and exhumation in a subduction channel below an accretionary wedge competition between down channel shear traction and up channel buoyancy forces expressed as the exhumation number e controls burial and exhumation leading to rapid up channel flow when e 1 exhuming uhp material forms a nappe stack and structural dome as it penetrates and destabilises the overlying wedge driving thrusting and extension this solution is compelling because it explains both the geology and the petrology of the tso morari and other uhp complexes and because it demonstrates that pulse like buoyant exhumation from deep in the subduction channel creates observed upper crustal structures this places constraints on the exhumation mechanism and provides a test of alternative models other proposed mechanisms such as continuous circulation in a lithospheric scale wedge or overpressured subduction channel predict different types of upper crustal structures and are therefore unsatisfactory explanations for early collisional exhumation of uhp terranes

dating the indian continental subduction and collisional thickening in the northwest himalaya multichronology of the tso morari eclogites multichronometric studies of the low temperature eclogitic tso morari unit ladakh india place timing constraints on the early evolution of the northwest himalayan belt several isotopic systems have been used to date the eclogitization and the exhumation of the tso morari unit lu hf sm nd rb sr and ar ar a ca 55 ma age for the eclogitization has been obtained by lu hf on garnet omphacite and whole rock from mafic eclogite and by sm nd on garnet glaucophane and whole rock from high pressure metapelites these results agree with a previously reported u pb age on allanite and together these ages constrain the subduction of the indian continental margin at the paleocene eocene boundary during exhumation the tso morari rocks underwent thermal relaxation at about 9 3 kbar characterized by partial recrystallization under amphibolite facies conditions ca 47 ma as dated by sm nd on garnet calcic amphibole and whole rock from metabasalt rb sr on phengite apatite and whole rock and ar ar on medium si phengite from metapelites ar ar analyses of biotite and low si muscovite from metapelites which recrystallized at 5 kbar toward the end of the exhumation show that the tso morari unit was at upper crustal levels ca 30 ma these results indicate variable exhumation rates for the tso morari unit beginning with rapid exhumation while the indian margin subduction was still active and later proceeding at a slower pace during the crustal thickening associated with the himalayan collision

a review of selected engineering geological characteristics of english chalk abstract chalk is a variable material the properties of which are dependent upon its composition textural features and diagenetic history with the exception of certain horizons in the lower chalk that contain appreciable amounts of clayey material the english chalk is a remarkably pure micritic carbonate rock that generally can be divided into coarse and fine fractions the latter comprises 70 80 of chalk cementation took place more or less contemporaneously with deposition so that the sediment was able to support relatively high overburden pressures hence high values of porosity were retained chalk varies appreciably in density and hardness the harder chalks are the result of diagenetic processes and bioturbation that brought about densification in soft chalks the grains are only bound together at the points of contact by thin films of calcite the latest classification of chalk is based on an assessment of intact dry density discontinuity aperture and discontinuity spacing chalk tends to vary from moderately weak to moderately strong and its strength is significantly reduced on saturation under triaxial loading conditions diagonal shear failure tends to occur at lower confining pressures but at higher confining pressures barrel shaped failure occurs indicating plastic deformation and textural disaggregation similarly at low loading chalk exhibits low volume compressibility but much more significant consolidation occurs if the yield stress is exceeded chalk undergoes dissolution and so solution features are found throughout its outcrop mineworkings in the chalk extend back into the distant past the most ancient being those excavated in the neolithic age for flint several types of workings exist collapse of old mineworkings most of which are unrecorded is difficult to predict the potential for subsidence caused by the collapse of both mineworkings and dissolution features affects development and its occurrence can lead to the abandonment of property or worse the loss of lives

composite ichnofabrics and tiering of burrows infaunal communities in marine environments typically are tiered that is different taxa live at different depths within the sediment tiered suites of biogenic structures yield complex biogenic sedimentary fabrics ichnofabrics with the traces of deep burrowing organisms overprinted on those of shallow burrowing organisms careful analysis of crosscutting relationships of burrows in such composite ichnofabrics allows reconstruction of the tiered nature of fossil endobenthic communities it is important to recognize that the best preserved and most prominently displayed trace fossils in most assemblages usually represent the deepest tier thus they were farther removed from the sea floor and therefore less indicative of actual sea floor conditions than the more poorly preserved traces of the shallower tiers on which the deeper traces are juxtaposed

magnetic and calcareous nannofossil stratigraphy of the lower palaeogene formations of the hampshire and london basins a magnetostratigraphic study of the lower palaeogene formations exposed in the hampshire and london basins southern uk has led to the identification of eight stratigraphically distinct normal polarity magnetozones within an otherwise reverse polarity sedimentary sequence which spans the late palaeocene to middle eocene a complementary study of the calcareous nannofossil biostratigraphy of these sections and related sections elsewhere in nw europe allows a correlation of this succession of normal polarity magnetozones with the biostratigraphically calibrated geomagnetic polarity timescale based on marine magnetic anomalies it is concluded that the normal polarity chrons corresponding to anomalies 20 21 and 23 are recorded within the huntingbridge earnley and wittering divisions of the bracklesham group respectively that anomalies 24a and 24b are represented within divisions d to b2 of the london clay and anomaly 26 is represented in the lower part of the thanet beds an additional normal polarity magnetozone within the oldhaven beds is believed to represent a short normal polarity event between anomalies 24b and 25 anomalies 22 and 25 appear not to be represented in the uk lower palaeogene succession because of major unconformities at the early middle eocene boundary within the wittering division of the bracklesham group and within the late palaeocene respectively the former appears to correlate with a widespread erosional event in the oceans identified from seismic records vail et al 1979 and interpreted as reflecting a major eustatic sea level fall

timing of himalayan ultrahigh pressure metamorphism sinking rate and subduction angle of the indian continental crust beneath asia coesite relics were discovered as inclusions in clinopyroxene in eclogite and as inclusions in zircon in felsic and pelitic gneisses from higher himalayan crystalline rocks in the upper kaghan valley north west himalaya the metamorphic peak conditions of the coesite bearing eclogites are estimated to be 27 32 kbar and 700 770 c using garnet pyroxene phengite geobarometry and garnet pyroxene geothermometry respectively cathodoluminescence cl and backscattered electron bse imaging distinguished three different domains in zircon inner detrital core widely spaced euhedral oscillatory zones and thin broadly zoned outermost rims each zircon domain contains a characteristic suite of micrometre sized mineral inclusions which were identified by in situ laser raman microspectroscopy core and mantle domains contain quartz apatite plagioclase muscovite and rutile in contrast the rim domains contain coesite and minor muscovite quartz inclusions were identified in all coesite bearing zircon grains but not coexisting with coesite in the same growth domain rim domain 206 pb 238 u zircon ages reveal that the quartz bearing mantle domains and the coesite bearing rim were formed at c 50 ma and 46 2 0 7 ma respectively these facts demonstrate that the continental materials were buried to 100 km within 7 9 myr after initiation of the india asia collision palaeomagnetic data from the indian oceanic floor supports an initial india asia contact at 55 53 ma combination of the sinking rate of 1 1 1 4 cm year 1 with indian plate velocity of 4 5 cm year 1 suggests that the indian continent subducted to about 100 km depth at an average subduction angle of 14 19