palaeomagnetism of late cretaceous sediments from southern tibet evidence for the consistent palaeolatitudes of the southern margin of eurasia prior to the collision with india abstract twenty sites were drilled in the late cretaceous shexing formation for palaeomagnetic studies in the lhasa terrane near the locality of maxiang 29 9 n 90 7 e the stepwise thermal demagnetizations successfully isolated high unblocking temperature characteristic directions the tilt corrected mean direction is d i 350 8 32 1 with 95 8 1 and n 20 sites corresponding to a paleopole at 75 0 n 306 7 e with a 95 6 8 positive fold tests indicate a primary origin for the characteristic remanence based on previous cretaceous data mainly from the takena formation and paleocene data from the linzizong volcanic rocks near the city of lhasa the latitude of the southern margin of asia is located at about 15 n and yields a stable position of the lhasa terrane during cretaceous and paleocene compared with expected paleomagnetic directions from the stable india and eurasia blocks the collision palaeolatitude further implies the total latitudinal convergence was accommodated by 1700 800 km 16 2 7 6 between southern tibet and stable eurasia and 1500 830 km 14 4 7 9 between southern tibet and stable india since the collision of india and eurasia a collision age between c 54 and 47 ma was determined using the results for the southern margin of eurasia according to our new data and the extent of greater india

fatty acid composition as an indicator of the trophic position of abyssal megabenthic deposit feeders in the kuril basin of the sea of okhotsk abstract this study examines the fatty acid fa composition in eight species of deposit feeders including holothurians echiurans sipunculan polychaete and bivalve mollusc from the kuril basin of the sea of okhotsk most of the fas in the species studied have been described previously for abyssal deposit feeders but 20 2 n 12 22 4 n 8 and 22 5 n 5 were new among megabenthic invertebrates principal component analysis indicated sample separation of the deposit feeders into three groups the first group included subsurface deposit feeders and the other two consisted of surface deposit feeders all groups differed in the ratio of 20 5 n 3 to 20 4 n 6 this ratio was 0 2 0 4 in the subsurface deposit feeder molpadia musculus risso 1826 group i and 0 6 0 9 in the surface deposit feeding echiuran pseudoikedella achaeta zenkevitch 1958 and the elasipodid holothurian psychropotes raripes ludwig 1893 group ii in the second group of surface deposit feeders group iii this ratio was a little larger in the polychaete travisia sp 1 3 and was much larger 2 1 2 5 in the echiuran alomasoma nordpacificum zenkevitch 1958 the sipunculan golfingia margaritacea sars 1851 and the holothurian peniagone dubia d yakonov and savel eva in d yakonov et al 1958 unlike previous data we found that the most abundant megabenthos species in the basin are subsurface deposit feeders and two groups of surface deposit feeders

middle cretaceous calcareous nannofossil biogeography and preservation in the atlantic and indian oceans implications for paleoceanography abstract the quantitative study of calcareous nannofossil preservation and biogeographic distribution puts constraints on paleoceanographic models for the atlantic and indian oceans during the middle cretaceous we establish a dissolution ranking of middle cretaceous calcareous nannofossils based on susceptibility to dissolution a positive correlation of organic carbon content of the sediments and the relative abundance of watznaueria indicates that much of the carbonate dissolution was caused by the release of carbon dioxide during oxidation of organic matter the depth distribution of the most dissolution resistant species watznaueria barnesae shows relative abundance maxima at the deepest sites and a maximum where intermediate waters impinge on slopes this reflects stagnant oxygen poor bottom waters and possibly a mid water oxygen minimum zone with increased deposition and catabolic breakdown of organic matter factor analyses are performed on various data sets for three different time slices ranging in age from late aptian to early albian middle albian and late albian to cenomanian the resulting factors can be related to paleoceanographic conditions such as colder water at high latitudes high fertility upwelling conditions and neritic conditions for all three time intervals a wide tropical to subtropical zone was flanked by boreal and austral zones upwelling conditions probably existed along the north african margin and over the falkland plateau zonal upwelling is tentatively postulated for the northwestern and northeastern margin of the north atlantic in the middle albian and for the eastern margin of the north atlantic off western europe for the late albian to cenomanian we speculate on surface water current patterns location of upwelling regions and haline driven deep circulation for the middle cretaceous

calcareous nannofossils from the great australian bight iodp site u1512 as a record of the peak and early decline of the mid cretaceous super greenhouse abstract international ocean discovery program drilling in the great australian bight site u1512 provides a record of turonian to santonian hemipelagic sedimentation deposited in an elongate epicontinental basin that opened westward to the southern ocean during the peak and the early decline of the cretaceous hot greenhouse despite its relatively high paleolatitude at about 60 s the biostratigraphy of calcareous nannofossils from this succession is directly correlative to low latitude zonation schemes throughout the turonian to santonian these calcareous nannofossil assemblages are somewhat restricted in species composition lacking some elements generally found in late cretaceous assemblages e g holococcoliths in addition two biostratigraphically important high latitude taxa thiersteinia ecclesiastica and zeugrhabdotus kerguelenesis have biostratigraphic ranges notably shorter than elsewhere indicating range truncation through local extirpation quantitative paleoecological analysis of the calcareous nannofossil assemblages indicates that the major change in the paleoenvironment was the evolution from conditions that were quite warm and had relatively elevated surface water fertility in the turonian to cooler and possibly more oligotrophic conditions in the coniacian santonian the first cooling began at about 92 ma accelerated at about 91 2 ma and then continued to gradually cool during the coniacian santonian this paleotemperature scenario is consistent with those documented from other localities globally

sr isotope evolution of seawater the role of tectonics we use a high resolution seawater sr isotopic evolution curve for the last 100 m y in conjunction with modern riverine sr flux measurements and also geologic tectonic and geochronological data to make the case for a close relationship between seawater sr isotopic composition and the india asia continental collision using a simple seawater sr budget model we begin by showing that the sr flux associated with alteration of seafloor basalts is too small and does not have the right time evolution to account for much of the seawater sr isotopic curve of the last 100 m y the flux of dissolved sr carried by rivers originating in the himalaya tibet region on the other hand is presently a significant fraction of the global sr budget we calculate how this riverine flux would have had to change with time in order to match the observed seawater sr isotopic curve and find that the riverine flux remains relatively constant prior to the collision of india with asia but then increases very significantly after collision we note that the period of most rapid change in seawater sr isotopic ratio from 20 ma to 15 ma is also a period of exceptionally high erosion in parts of the himalayas and the tibetan plateau as further evidence that sr derived from the collision of india with asia plays a major role in the sr isotopic evolution of seawater we show that the total amount erosion of the himalaya tibetan plateau since collision which we calculate separately represents a total amount of sr that is very nearly the same as the cumulative amount required by the sr isotopic change of seawater since collision the relationship between erosion and riverine sr flux allows us to use the sr isotopic evolution of seawater to reconstruct a history of erosion since collision and we find that the erosion rate accelerates with time since collision with the present having the largest rate when we apply the sr budget model to the entire phanerozoic using a new compilation of deformed continental area versus time we find that we can account for the large scale structure of the seawater sr isotopic curve but fail to reproduce several local maxima and minima especially in the period 100 300 ma the present high s7sr s6sr of seawater and similar highs in the devonian and cambrian do correlate with extensive deformation on the continents

u pb and re os geochronological evidence for the jurassic porphyry metallogenic event of the xiongcun district in the gangdese porphyry copper belt southern tibet prc abstract two large copper gold porphyry deposits no i and no ii were discovered in the last decade within the xiongcun district which is located along the south margin of the gangdese porphyry copper belt these two deposits are hosted by two porphyries that were emplaced into the early jurassic volcano sedimentary rock sequences of the xiongcun formation the porphyries are both quartz diorite bodies with one of these distinguished by its large quartz eyes our new molybdenite re os and zircon u pb dates for no i and no ii deposits and host intrusions reveal the presence of two middle jurassic ore forming episodes in the xiongcun district the no ii deposit is related to the 181 175 ma quartz diorite porphyry and formed ca 172 6 2 1 ma the no i deposit is related to the 167 161 ma quartz diorite porphyry with large quartz eyes and formed ca 161 5 2 7 ma geochronological and geochemical features of the jurassic ore bearing porphyries in the xiongcun district indicate that the porphyry copper gold mineralization formed in an island arc setting which is related to the northward subduction of the neo tethys oceanic plate these observations in combination with geochronological data reported at gangdese porphyry copper belt indicated four major metallogenic events existed in the gangdese porphyry copper belt at 172 161 ma 65 48 ma 40 23 ma and 20 12 ma we have identified two mineralization episodes comprise middle jurassic metallogenic event 172 161 ma in the xiongcun district which provides new constrains for the middle jurassic metallogenic event in the gangdese porphyry copper belt

revision of the cretaceous paleogene stratigraphic framework facies architecture and provenance of the xigaze forearc basin along the yarlung zangbo suture zone abstract new field investigation structural analysis review of biostratigraphy stratigraphic framework facies architecture provenance and flow direction of the cretaceous paleogene sedimentary strata in southern tibet resulted in reinterpretation of the evolution of the xigaze forearc basin xfb the xfb is comprised by flysch dominant xigaze group consisting of ngamring chongdoi and sangzugang formations and of the shallow marine cuojiangding group padana qubeiya quxia and gyalaze formations with their ages herein revised as the albian coniacian and the santonian ypresian respectively the deep sea flysch represented by the ngamring formation constitutes the early sedimentary fill of the xfb it was subdivided into five megasequences ms1 ms5 in order the early submarine fan facies stage channel submarine fan stage the mature submarine fan stage the highstand submarine fan stage and outer fan pelagic stage the paleocurrent flows were eastwards and westwards during the early and late stages of basin development and northwards and southwards during its middle stage indicating typical forearc basin sedimentation with longitudinal supply and lateral filling provenance studies point to multiple sources with andesitic debris sourced from the jurassic lower cretaceous yeba formation and sangri group formed in the volcanic island arc chains in the southern gangdese the four periods of the xfb development are 1 the formation of the xfb basement by the xigaze ophiolite and completed before the aptian the coeval deposits on the shelf are the sangzugang formation carbonate 2 the earliest xfb deposits of the earliest aptian age ca 125 120 ma represented by the chongdoi formation related to the residual forearc 3 composite forearc developed represented by the late albian late cenomanian flysch megasequences ms1 ms2 and the turonian coniacian flysch megasequences ms3 ms5 respectively and 4 the final stage of the xfb development started the terminal forearc stage represented by the late late cretaceous early paleogene shallow marine cuojiangding group and terminated during middle ypresian when the gyalaze formation was deposited

the origin and pre cenozoic evolution of the tibetan plateau different hypotheses have been proposed for the origin and pre cenozoic evolution of the tibetan plateau as a result of several collision events between a series of gondwana derived terranes e g qiangtang lhasa and india and asian continent since the early paleozoic this paper reviews and reevaluates these hypotheses in light of new data from tibet including 1 the distribution of major tectonic boundaries and suture zones 2 basement rocks and their sedimentary covers 3 magmatic suites and 4 detrital zircon constraints from paleozoic metasedimentary rocks the western qiangtang amdo and tethyan himalaya terranes have the indian gondwana origin whereas the lhasa terrane shows an australian gondwana affinity the cambrian magmatic record in the lhasa terrane resulted from the subduction of the proto tethyan ocean lithosphere beneath the australian gondwana the newly identified late devonian granitoids in the southern margin of the lhasa terrane may represent an extensional magmatic event associated with its rifting which ultimately resulted in the opening of the songdo tethyan ocean the lhasa northern australia collision at similar to 263 ma was likely responsible for the initiation of a southward dipping subduction of the bangong nujiang tethyan oceanic lithosphere the yarlung zangbo tethyan ocean opened as a back arc basin in the late triassic leading to the separation of the lhasa terrane from northern australia the subsequent northward subduction of the yarlung zangbo tethyan ocean lithosphere beneath the lhasa terrane may have been triggered by the qiangtang lhasa collision in the earliest cretaceous the mafic dike swarms ca 284 ma in the western qiangtang originated from the panjal plume activity that resulted in continental rifting and its separation from the northern indian continent the subsequent collision of the western qiangtang with the eastern qiangtang in the middle triassic was followed by slab breakoff that led to the exhumation of the qiangtang metamorphic rocks this collision may have caused the northward subduction initiation of the bangong nujiang ocean lithosphere beneath the western qiangtang collision related coeval igneous rocks occurring on both sides of the suture zone and the within plate basalt affinity of associated mafic lithologies suggest slab breakoff induced magmatism in a continent continent collision zone this zone may be the site of net continental crust growth as exemplified by the tibetan plateau c 2012 international association for gondwana research published by elsevier b v all rights reserved

mesozoic radiolarian faunas from the antarctic peninsula age tectonic and palaeoceanographic significance new assemblages of radiolaria including some of the few occurrences of high southern latitude jurassic and cretaceous radiolarian faunas show that several localities in the lemay group of alexander island range in age from latest jurassic earliest cretaceous to at least albian by demonstrating that sedimentation and deformation in the lemay group was diachronous younging oceanwards to the northwest these new age assessments support the model of the lemay group as an accretionary complex the polarity of subduction beneath alexander island was not affected by arc collisions from at least the lower jurassic to the oligocene and such a long period of continuous accretion appears to be unusual deposition of the lemay group spans the kimmeridgian to albian sedimentation in the fossil bluff group fore arc basin thus making the earlier concept of the lemay group as pre jurassic basement untenable allochthonous latest jurassic earliest cretaceous radiolarian assemblages with some supposed tethyan affinities are present in the lemay group in contrast an in situ latest jurassic assemblage from the nordenskjold formation of the back arc basin and a further jurassic assemblage from a probable trench slope basin have characteristics believed diagnostic of high latitudes the biogeographic affinities of radiolarians from cherts in the lemay group accretionary complex suggest that both these cherts and associated basalts are far travelled slices of the phoenix plate rocks from the probable trench slope basin formerly assigned to the younger fossil bluff group fore arc basin sequence now appear to be part of a new previously unrecognized formation

jurassic u pb and re os ages for the newly discovered xietongmen cu au porphyry district tibet prc implications for metallogenic epochs in the southern gangdese belt the gangdese belt in southern tibet includes the cretaceous to early eocene gangdese continental magmatic arc a newly recognized middle jurassic magmatic arc and post gangdese arc magmatism of miocene age that is well known for its association with important copper molybdenum deposits although the tectonic history of the gangdese continental arc is relatively well constrained the tectonomagmatic setting of the jurassic and miocene magmatic events was poorly constrained previously geochronological analysis of samples from the xietongmen copper gold porphyry district has identified a previously unknown metallogenic event related to middle jurassic arc magmatism the occurrence of several metallogenic epochs within the southern gangdese belt highlights the importance of systematic metallogenic studies of this belt

paleoclimatic control on the composition of the paleoproterozoic serpent formation huronian supergroup canada a greenhouse to icehouse transition unlike other feldspathic arenites in the 12 km thick huronian supergroup strata of the 250 350 m thick serpent formation contain abundant plagioclase deposited in a distal alluvial setting the serpent formation consists of arenites with minor shale and siltstone the plagioclase abundance could reflect unroofing of a unique source relative to other huronian units or could be due to less intense chemical weathering major trace and rare earth element ree geochemistry of the serpent formation are used to evaluate these two possibilities serpent formation source rocks form part of the archean superior province which mainly consists of tonalitic granitic and supracrustal assemblages and their metamorphosed equivalents in al2o3cao na2ok2o acnk space shales define a linear array which indicates that the unweathered source contained plagioclase and k feldspar in the ratio of 5 1 this particular array indicates that serpent fluvial systems tapped a variably weathered carapace developed on the superior province rather than purely fresh bedrock a modelled mixture of 80 tonalite and 20 granite group rocks reproduces ree and trace element characteristics of the serpent formation sandstones and shales average eu eu 0 87 lan smn 4 4 gdnybn 1 8 and th sc 1 65 mass balance considerations indicate that less than 5 supracrustal group rocks can be added at the expense of the granite group a unique source for serpent detritus is rejected because underlying and overlying units have approximately the same provenance composition plagioclase k feldspar ratios in serpent sandstones range from 2 1 to 1 2 which achieves mass balance among source mudstone and sandstone compositions when plotted in acnk space achievement of mass balance implies that sandstones and mudstones were derived from the same weathering profiles these findings indicate that plagioclase preservation in the serpent formation is related to less intense compared with other huronian arenites paleoweathering conditions probably a harbinger of the widespread continental glaciation recorded in the overlying gowganda formation

large scale ocean circulation in the southern hemisphere with closed and open drake passage a laboratory minimal model approach abstract we report on a laboratory model of the large scale flow phenomena in the southern ocean with a closed drake passage imitating the situation before the eocene oligocene transition eot ca 34 million years ago in a differentially heated rotating annular wave tank an insulating meridional barrier is installed to block zonal circulation the obtained temperature fields and time series are compared to the ones from control runs with partially blocked and fully opened passages in the closed case a persistent azimuthal temperature gradient emerges whose magnitude scales linearly with the meridional temperature contrast the anomalous temperature distribution is accompanied with perturbations of the background flow yielding significantly larger low frequency variability than in the opened configuration the experimental findings have implications to the eot but at present validation of results is hindered by the lack of deep sea drilling data from the southeast pacific and adjacent parts of the southern ocean our model may inspire further paleoceanographic research in this largely unexplored region

geochemistry of archean shales from the witwatersrand supergroup south africa source area weathering and provenance with a few exceptions shales from the archean witwatersrand supergroup 2800 ma in south africa are depleted in na ca lile ree and hfse compared to phanerozoic shales cr co and ni are enriched in all witwatersrand shales and fe and mg are high in shales from the west rand groups wrg and lower central rand group crg shales from the crg and uppermost wrg are enriched in na al lile ree hfse and transition metals relative to shales from the lower wrg chondrite normalized ree patterns for all witwatersrand shales are enriched in light ree and exhibit small to moderate negative eu anomalies a positive correlation of ree and al2o3 contents in the shales suggests that ree are contained principally in clay minerals relative to shales from the crg shales from the wrg exhibit depletions of na ca and sr a feature probably reflecting intense chemical weathering of their source rocks cia indices in witwatersrand shales are variable chiefly 70 98 even within the same shale unit such variations reflect chiefly variable climatic zones or rates of tectonic uplift in source areas with perhaps some contribution from provenance and element remobilization during metamorphism compared to present day upper continental crust all but the orange grove roodepoort and k8 shales appear to have been derived from continental sources depleted in lile ree and hfse and enriched in transition metals computer mixing models based on six relatively immobile elements th hf yb la sc co and four source rocks indicate that the relative proportions of granite basalt and komatiite increased with time in sediment source areas at the expense of tonalite the contributions of basalt and komatiite appear to reach a maximum during deposition of the booysens shale and granite during deposition of the k8 shales and possibly during deposition of the orange grove shales

new morphologic information on cretaceous nannofossils from the niobrara formation upper cretaceous of kansas outcrops of the niobrara formation from northwestern kansas have yielded horizons of well preserved calcareous nannofossils scanning electron and light microscopy have revealed not only several new taxa but also an extraordinary cylindrical coccosphere not previously described from the fossil record this unusual cylindrical form of a species of biscutum black also differs from the characteristic spherical coccosphere by the arrangement of its coccoliths in an ordered and repetitive fashion a new term coccocylinder is proposed here as a more appropriate descriptive term than coccosphere for the cylinder a stagnant anoxic benthic environment prevalent during intervals of niobrara deposition enhanced the preservation of monospecific coccolith clusters which represent coccospheres that settled to the bottom often via fecal pellets before disarticulating these clusters representing 34 different species provide information on the minimum number of coccoliths originally present on the living cell such data may be valuable for determining the total biomass of the living populations on the basis of the abundances of individual coccoliths in the fossil record

a geologic perspective on the miocene evolution of the antarctic circumpolar current system abstract the dominant geologic feature of pelagic sediment accumulation patterns at intermediate water depths in the southern ocean is the sequential northward migration and replacement of a calcareous ooze sedimentary facies by a siliceous ooze facies following the late oligocene opening of the drake passage and the subsequent initiation of the antarctic circumpolar current closely spaced whole core measurements of bulk density are evaluated for four sites drilled in the southern ocean during ocean drilling program odp legs 113 sites 689 and 690 and 120 sites 747 and 751 these sites are all located on submarine rises in open ocean pelagic depositional environments south of the polar front a major oceanographie and sedimentologic boundary high resolution profiles of bulk density are used to correlate between adjacent drill holes on the maud rise where multiple overlapping sedimentary sequences were recovered using the advanced hydraulic piston core barrel apc stratigraphie age models have been applied to the bulk density data to provide a temporal framework for a discussion of the sedimentologic basis for paleoceanographic interpretations and also to constrain future acoustic research studies the early to middle miocene intervals at sites 689 and 690 are marked by distinct sequences of meter scale alternating carbonate and siliceous units separated by several proposed stratigraphie hiatuses at least six dissolution productivity events are identified in the middle to late miocene on the maud rise and others are noted at the odp sites on the kerguelen plateau the downhole profiles are used to identify the density signature and position of sediment facies changes and proposed stratigraphie hiatuses in sediment cores of late oligocene to late miocene age based on the reversals inflections and discontinuities in the downhole bulk density trends an analysis of the observed bulk density trends at these sites is combined with a review of other tectonic sedimentologic oceanographie and modeling studies to develop a hypothesis for explaining the observed sediment accumulation patterns at intermediate water depths of the southern ocean

tectonic forcings of maastrichtian ocean climate evolution a global compilation of deep sea isotopic records suggests that maastrichtian ocean climate evolution tvas tectonically driven during the early maastrichtian the atlantic intermediate deep ocean was isolated from the pacific indian and southern oceans deep water formed in the high latitude north atlantic and north pacific at the early late maastrichtian boundary a major reorganization of oceanic circulation patterns occurred resulting in the development of a thermohaline circulation system similar to that of the modem oceans a combination of isotopic and plate kinematic data suggests that this event was triggered by the final breaching of tectonic sills in the south atlantic and the initiation of north south flow of intermediate and deep water in the atlantic the onset of laramide tectonism during the mid maastrichtian led to the concurrent draining of major epicontinental seaways together these events caused cooling increased latitudinal temperature gradients increased ventilation of the deep ocean and affected a range of marine biota

faunal turnover at the end of the cretaceous in the north pacific region implications from combined magnetostratigraphy and biostratigraphy of the maastrichtian senpohshi formation in the eastern hokkaido island northern japan abstract a combined magnetostratigraphic and biostratigraphic study has been performed on the maastrichtian senpohshi formation in eastern hokkaido island northern japan which is an approximately 1300 m thick section mainly composed of hemipelagic mudstone the identification of magnetic polarity was possible at 51 horizons whereby four magnetozones were recognized these magnetozones were correlatable to geomagnetic polarity chrons c31r to c30n suggesting that the age of the senpohshi formation is spanning from middle to upper part of the maastrichtian ca 69 67 ma the magnetostratigraphy of the senpohshi formation established in this study enables a direct age correlation to the maastrichtian successions in other regions thus this detailed chronology of the formation contributes to paleontological studies of the maastrichtian in the north pacific region for instance this magnetostratigraphic age assessment implies the following 1 the stratigraphic range of the ammonite pachydiscus flexuosus contains polarity chrons from the lower part of c31r to the lower part of c31n 2 the first occurrence fo of the calcareous nannofossil nephrolithus frequens in the north pacific region is correlatable to polarity chron c30n or below and 3 the fo of the bivalve inoceramus awajiensis is located within polarity chrons from c31r to the upper part of c31n this suggests that the inoceramid extinction event in the north pacific region might have occurred during polarity chrons from c31r to the upper part of c31n ca 70 5 67 8 ma which is 2 3 5 0 myr prior to the cretaceous paleogene boundary the trend of the maastrichtian faunal turnover in the north pacific is well consistent with those of other regions brings a new evidence for understanding the global faunal turnover in the maastrichtian just before cretaceous paleogene mass extinction

paleobiogeography of campanian maastrichtian foraminifera in the southern high latitudes abstract distributions of planktonic and benthic foraminifera have revealed antarctic land sea distributions oceanic surface gyre configurations and changes in the configurations of the tethyan transitional and austral realms during campanian maastrichtian time poleward changes in the diversity of total and keeled planktonic species of the southern hemisphere are compared for the early and late campanian and early and late maastrichtian latitudinal diversity gradients were weakest during the early campanian and more pronounced during the late campanian and maastrichtian shallow marine seaways within west antarctica are suggested by similarity of late cretaceous nearshore benthic and open ocean planktonic assemblages from southern high latitude land and drill sites and by the occurrence of recycle cretaceous marine microfossils from antarctic interior and continental margin localities development of a major water mass boundary between cool surface waters south of about 50 s paleolatitude and warmer surface waters to the north in the south atlantic and indian ocean may have caused biogeographic isolation of austral realm assemblages during late campanian and early maastrichtian time several planktonic foraminifera show significant diachroneity in their latitudinal distributions magnetobiostratigraphic data suggest that abathomphalus mayaroensis migrated from high to low latitudes whereas pseudotextularia elegans globigerinelloides subcarinatus globotruncanella citae globotruncanella petaloidea globotruncana bulloides and globotruncana subcircumnodifer made time transgressive poleward migrations the distribution of a mayaroensis is in agreement with isotopic evidence for late maastrichtian climatic cooling while that of p elegans correlates with a latest maastrichtian warming event about 66 8 66 6 ma delayed first occurrences of the other taxa cannot be explained by paleoclimatic factors alone variations in global eustatic sea level and orogeny between southern south america and west antarctica may have also influenced the taxonomic composition of southern high latitude foraminiferal assemblages

early cretaceous gangdese retroarc foreland basin evolution in the selin co basin central tibet evidence from sedimentology and detrital zircon geochronology the selin co basin in the northern lhasa terrane includes more than 3000 m of upward coarsening lower cretaceous strata and the sedimentary sequence from the flysch to the molasse indicates the evolution of a foreland basin petrographic analysis shows that sandstones are rich in volcanic and sedimentary lithics and most of them fall into recycled orogen and magmatic arc uranium lead u pb ages were determined for 435 detrital zircons from the lower cretaceous strata in the selin co basin relative probability of detrital zircon ages shows the eshaerbu formation was rich in zircon grains with the age of 125 140 and 160 180 ma and the duoni formation was dominated by one main age cluster of 125 150 ma analysis of the potential provenances suggests the early cretaceous zircon grains were primarily derived from the gangdese magmatic arc to the south the youngest zircon ages in the lowermost exposure of the eshaerbru formation are c 130 ma providing a maximum depositional age of sediments in the selin

assembly of the lhasa and qiangtang terranes in central tibet by divergent double subduction abstract integration of lithostratigraphic magmatic and metamorphic data from the lhasa qiangtang collision zone in central tibet including the bangong suture zone and adjacent regions of the lhasa and qiangtang terranes indicates assembly through divergent double sided subduction this collision zone is characterized by the absence of early cretaceous high grade metamorphic rocks and the presence of extensive magmatism with enhanced mantle contributions at ca 120 110 ma two jurassic cretaceous magmatic arcs are identified from the caima duobuza rongma kangqiong amdo magmatic belt in the western qiangtang terrane and from the along tso yanhu daguo baingoin daru tso magmatic belt in the northern lhasa terrane these two magmatic arcs reflect northward and southward subduction of the bangong ocean lithosphere respectively available multidisciplinary data reconcile that the bangong ocean may have closed during the late jurassic early cretaceous most likely ca 140 130 ma through arc arc soft collision rather than continent continent hard collision subduction zone retreat associated with convergence beneath the lhasa terrane may have driven its rifting and separation from the northern margin of gondwana leading to its accretion within asia

late mesozoic tectonic evolution and growth of the tibetan plateau prior to the indo asian collision abstract the elevation of the tibetan plateau is a revolutionary event in the earth history which resulted in change not only of cenozoic regional and global climate but also of monsoon intensity a critical aspect for the development of the tibetan plateau that remains open to intense debate is whether the collision among the tibetan continental blocks or the subduction of the tethys prior to the indo asian collision in the cenozoic contributed to the plateau growth we propose here that an andean type orogen could have been present in southern tibet from the jurassic to early cretaceous but collapsed during the middle cretaceous based on a synthesis of existing structural sedimentary geochronological and geochemical data the orogeny is marked by north verging jurassic early cretaceous thrusting in the lhasa block a result of flat subduction of the yarlung zangpo neo tethys that is recorded by a narrow belt of 205 174 ma adakitic rocks along the southernmost margin of the lhasa block denudation of the orogen produced a thick up to 15 km sequence of jurassic turbidites and molasse type sediments on the northern lhasa block widespread middle cretaceous 135 100 ma postorogenic strongly peraluminous to calc alkaline magmatism and extensional deformation in the lhasa block and an extensive marine transgression in tibet indicate that the orogen broke down due to extension resulting from lithosphere delamination and asthenosphere upwelling meanwhile the north dipping subduction of the bangong meso tethys produced a wide jurassic middle cretaceous magmatic arc along the southern margin of the qiangtang block findings of 132 108 ma aged ophiolites rich with middle cretaceous radiolarians in central tibet indicate that the bangong meso tethys did not close until the late cretaceous repeated flat slab subduction of the yarlung zangpo neo tethys occurred during the late cretaceous 90 78 ma as indicated by the adakites in the southernmost lhasa block which together with the ongoing qiangtang lhasa collision could have contributed to the growth of the tibetan plateau

did the indo asian collision alone create the tibetan plateau it is widely believed that the tibetan plateau is a late cenozoic feature produced by the indo asian collision however because tibet was the locus of continental accretion and subduction throughout the mesozoic crustal thickening during that time may also have contributed to growth of the plateau this portion of the geologic history was investigated in a traverse through the central lhasa block southern tibet together with earlier studies our mapping and geochronological results show that the lhasa block underwent little north south shortening during the cenozoic rather our mapping shows that 60 crustal shortening perhaps due to the collision between the lhasa and qiangtang blocks occurred during the early cretaceous this observation implies that a significant portion of southern tibet was raised to perhaps 3 4 km elevation prior to the indo asian collision

mesozoic palaeoceanography of the north atlantic and tethys oceans summary the biogeography and preservation of calcareous nannofossil assemblages allow mapping of surface water fertility patterns and put constraints on carbonate dissolution in the global oceans of the middle cretaceous cyclic changes in carbonate dissolution and surface water biologic productivity in the cretaceous oceans were linked to oceanographic and climatic changes that had frequencies close to the milankovitch cycles tens of thousands to hundreds of thousand years evolution of calcareous nannoplankton during the jurassic and cretaceous periods occurred in two primary cycles of about 75 ma duration that were punctuated by major diversification events occurring every 15 30 ma that marked the beginning of periods of increased marine organic matter production and preservation stagnation events calcareous nannoplankton evolved largely in shallow seas during the rhaetian to kimmeridgian in the latest jurassic calcareous nannoplankton conquered the oceanic realm became major producers of pelagic carbonates and are thus responsible for the shift of carbonate deposition from shallow seas to the deep ocean in the late cretaceous chalk seas nannofossils replaced shallow water benthos as major carbonate producers warm climates due to high atmospheric co 2 concentrations and increased albedo during high sea level stands caused by global tectonic processes made these tiny creatures flourish during the late mesozoic

calcareous nannofossil paleoceanography of the cretaceous greenhorn sea two distinctive laterally traceable bentonite beds were used to construct three isochronous time slices through the marine sediments of the upper cretaceous greenhorn cyclothem of the western interior basin calcareous nannofossil assemblages from these time slices were analyzed from 40 localities to examine the paleoceanographic conditions in the basin deposition of the lower time slice x bentonite approximately corresponded to the time at which free communication between the basin and open oceanic systems first occurred the most striking trend is the presence of common to abundant nannofossils in the center of the basin and their absence from the eastern and western margins this indicates that open oceanic conditions conducive to large standing crops of calcareous nannoplankton existed only in a narrow centrally located channel conditions at the basin margins were probably unsuitable for large populations due to salinity constraints multivariate analyses indicate two distinct biofacies northern and southern which are interpreted to represent distinct surface water masses the geometry of the water mass boundary suggests that the northern surface water mass actively flowed southward into or over the relatively passive southern water mass the upper time slice hl 3 bentonite samples the nannofossil distributions at or near the maximum transgression of the greenhorn sea in this time slice nannofossils occur beyond the limits of the study area except in proximity to the frontier delta system diversities are significantly higher indicating water mass conditions more similar to those in open oceanic settings multivariate analyses indicate three distinct surface water masses northern southern and western the northern and southern water masses exhibit relationships similar to those during the x bentonite time slices the interaction between the northern and southern water masses promoted vertical stratification of the water column and resultant sediment anoxia the western biofacies may be indicative of a surface water mass with significant contributions from nearby fresh water influx

mesozoic cenozoic history of calcareous nannofossils in the region of the southern ocean abstract the mesozoic cenozoic evolution of high southern latitude calcareous nannofossil assemblages developed in parallel with that of the southern hemisphere ocean basins and the attendant progressive changes in global climate the equitable middle to late jurassic climates are reflected in a cosmopolitan zonation although distinctive high latitude taxa developed in both polar regions monospecific or low diversity provincial assemblages characterized the incipient early cretaceous south atlantic basins until the introduction of tethyan elements during the aptian bipolar or austral provincial taxa are useful in erecting a biostratigraphic subdivision for the remainder of the cretaceous sequence on the falkland plateau in contrast to the middle cretaceous the global sample coverage of dsdp holes for the campanian maestrichtian is sufficiently broad to demonstrate a distinct latitudinal distribution of species and assemblages provincial austral or bipolar taxa are relatively minor components of the paleocene eocene assemblages but quantitative methods document the existence of definable high latitude assemblages which shifted latitudinally in response to climatic changes indirect evidence suggests that calcareous nannofossils were deposited in the interior basins of antarctica during this interval species diversities declined progressively through the oligocene miocene epochs in response to the deterioration of neogene climates and glacial episodes on the antarctic continent finally resulting in low diversity assemblages generally dominated by high latitude taxa sharp migrations of these assemblages occurred in response to glacial climatic events plio pleistocene sediments deposited beneath the expanding antarctic water mass are essentially barren of calcareous nannofossils but coccoliths are widely distributed in the subantarctic region as a result of the amelioration of holocene climate and the quaternary descent of the carbonate compensation depth two new taxa are described stephanolithion bigotii brevispinus n ssp from the upper jurassic and gephyrobiscutum diabolum n g n sp from the lower campanian

the palaeoceanographic crisis of the early aptian oae 1a in the vocontian basin se france abstract the vocontian basin se france which presents lower aptian expanded successions characterized by major lithological changes is particularly suitable to determine palaeoenvironmental changes occurring across the oae 1a a multidisciplinary study sedimentology caco3 toc carbon and oxygen stable isotopes micropalaeontology cyclostratigraphy was carried out in the notre dame de rosans section in order to establish a detailed chronological framework of these changes the oae 1a corresponds to carbon isotope segments c3 c6 and it lasted for 1 2 myr a first drop in the carbonate production occurs 500 kyr before the oae 1a upper part of the carbon isotope segment c2 and could result from both onset of urgonian carbonate platform demise and associated reducing export of platform derived sediments into the basin and nannoconid crisis a second drastic drop crash in the carbonate production is recorded within the lower part of the oae 1a latter part of segment c3 to c5 and is explained by a strong dissolution this study then shows that the onset of the major carbonate crisis that occurs before the oae 1a could be due to both rise in sea level and in marine surface water fertility whereas its acme that occurs within the oae 1a could be related to co2 induced ocean acidification black shales of the niveau goguel occur in the upper part of the oae 1a and represent the lower part of segment c6 surface waters primary producers are principally represented by cyanobacteria whereas nannofossil primary productivity is reduced and deep water anoxia prevailed during the deposition of the niveau goguel the last 300 kyr of the oae 1a are characterized by a partial recovery of both nannofossil primary productivity and pelagic carbonate production which sharply increase just after the end of the event this study also shows that organic rich layers associated to the oae 1a are diachronous in the tethyan realm

southern ocean pliocene paleotemperature variation from high resolution silicoflagellate biostratigraphy abstract applying the modern biogeographic distribution of silicoflagellate genera dictyocha and distephanus and calcareous nannoplankton as an analog the biostratigraphic record of these phytoplankton can be used qualitatively to examine past changes in surface temperature in the southern ocean the antarctic polar frontal zone pfz is a major biogeographic barrier limiting dictyocha and calcareous nannoplankton to areas north of this zone ocean drilling project odp holes 748b and 751a on the southern kerguelen plateau 900 km south of the present location of the pfz contain expanded pliocene sections through four time intervals 4 5 to 4 2 ma hole 751a 3 7 to 3 1 ma hole 748b 3 6 to 3 5 ma hole 751a and 3 2 to 3 0 ma hole 751a three distinct peaks in dictyocha and calcareous nannofossil abundance reflect surface water warming of at least 4 c through either southward migration of the pfz or a weakening of the thermal gradient across this zone peak warming in the intervals studied is centered near 4 3 ma with other brief yet significant warming events near 4 5 4 2 and 3 6 ma cooling as a result of northward migration of the pfz or intensified thermal gradient is indicated by the absence of dictyocha and calcareous nannofossils in other stratigraphic intervals these data support previous studies indicating pliocene oceanic and climatic variability in the southern high latitudes silicoflagellate abundance patterns from hole 751a identify taxonomic relationships between several morphotypes the simultaneous appearance and proportional occurrence of five sided and four sided dictyocha specimens suggests a close relationship between these two forms distephanus crux s l and dictyocha aspera var pygmaea also show a near mutually exclusive abundance pattern in hole 751a and are similar in basal ring size and morphology indicating they are conspecific they differ only in apical structure whereby distephanus crux s l may have formed an apical bar under fluctuating environmental conditions in the early pliocene these variations in silicoflagellate architecture appear to be related to paleoenvironmental changes indicating that silicoflagellate morphology may represent a more useful paleoceanographic tool that has been noted previously

the takena formation of the lhasa terrane southern tibet the record of a late cretaceous retroarc foreland basin our understanding of the processes involved in the indo asian collision and the construction of the tibetan plateau are in part predicated on our understanding of the tectonic setting and crustal conditions of southern tibet in the time period immediately preceding the indo asian collision late cretaceous several hypotheses have been proposed that describe the middle to late cretaceous tectonic and paleogeographic evolution of southern tibet each with different implications for when and how the tibetan plateau was uplifted we examined the mid upper cretaceous takena formation of the lhasa terrane in southern tibet in order to reconstruct the middle to late cretaceous tectono sedimentary history of this area and test the competing tectonic hypotheses the takena formation consists of 2 km of sedimentary strata that include a lower marine limestone member penbo member and an upper member of clastic red beds lhunzhub member the aptian albian penbo member consists of 250 m of orbitolinid limestone beds that were deposited in a shallow marine seaway during a time when there was little regional subsidence the overlying lhunzhub member 1500 m consists primarily of fl strata that were deposited during a period of increased subsidence overall the lhunzhub member coarsens upward from meandering and anastomosing stream deposits interbedded with numerous paleosols near the base to multistory multilateral braided stream deposits near the top the fl uvial sandstone units are lithic rich and contain abundant plagioclase and volcanic grains and paleocurrent data record northwest directed transport these data indicate that the sediment was derived from the gangdese volcanic arc that developed along the southern margin of the lhasa terrane following deposition but prior to ca 70 ma the beds of the takena formation were folded and partially eroded the sedimentary and stratigraphic characteristics of the takena formation are most consistent with deposition in a retroarc foreland basin setting the limestone beds of the penbo member were deposited in a shallowmarine seaway that was eventually infi lled by clastic sediment derived principally from the gangdese volcanic arc the low subsidence rate recorded in the lowermost strata of the takena formation including the penbo member and the paleosol rich interval of the lhunzhub member is interpreted to be associated with the passage of a fl exural forebulge the overlying upward coarsening fl uvial strata were deposited in progressively more proximal locations within the foredeep of a foreland basin the late cretaceous folding of the takena formation indicates that the foreland basin strata were eventually incorporated into a fold and thrust belt the upper cretaceous sedimentary strata of the lhasa terrane of southern tibet predict that a north verging fold and thrust belt existed along the southern margin of the lhasa terrane during late cretaceous time prior to the indo asian collision collectively the evidence indicates that the southern margin of the lhasa terrane had thickened crust and was likely to have been at high elevations immediately before the indo asian collision

a review of tertiary climate changes in southern south america and the antarctic peninsula part 2 continental conditions abstract climate changes in southern south america and the antarctic peninsula during the tertiary show a strong correlation with ocean warming and cooling events which are in turn related to tectonic processes during periods of accelerated sea floor spreading and mid ocean ridge activity sea levels rose so that parts of the continents were flooded and forests were destroyed however this was balanced by the large scale release of co2 during volcanic outgassing and carbonate precipitation on the continental shelves which caused rising air temperatures and the poleward expansion of sub tropical and temperate forests cooling episodes generally caused an increase in the north south thermal gradient because of an equatorward shift in climate belts so that the westerly winds intensified and brought higher rainfall to the lower latitudes an increase in wind blown dust caused temperatures to drop further by reflecting sunlight back into space the rising andes range had a marked influence on climate patterns up to the middle miocene it was still low enough to allow summer rainfall to reach central and north central chile but after about 14 ma it rose rapidly and effectively blocked the spill over of moisture from the atlantic ocean and amazon basin at this time the cold humboldt current was also established which together with the andes helped to create the arid diagonal of southern south america stretching from the atacama desert to the dry steppes of patagonia this caused the withdrawal of subtropical forests to south central chile and the expansion of sclerophytic vegetation to central chile however at the same time it intercepted more rain from the northeast causing the effect of the south american monsoon to intensify in northwestern argentina and southern bolivia where forest communities presently occur in patagonia glaciation started as early as 10 5 ma but by 7 ma had become a prominent feature of the landscape and continued apparently uninterruptedly into the pleistocene the antarctic peninsula saw its first mountain glaciation between 45 and 41 ma with major ice sheet expansion commencing at about 34 ma isolated stands of nothofagus forests were still present in low lying areas suggesting that the glaciers were initially wet based but dry based glaciers were established at around 8 ma although temperatures rose briefly during the messinian pliocene transition causing sub antarctic flora to retreat to higher elevations of the transantarctic mountains the present cold polar conditions were finally established by about 3 ma

new latitude based nannofossil zonations for the campanian maastrichtian of the south atlantic ocean and their paleoceanographic implications the global evolution of calcareous nannofossils during the campanian maastrichtian interval is not fully understood and nannofossil zonations for the central and south atlantic ocean are yet to be constructed here we test the applicability of previous calcareous nannofossil zonations developed for other ocean basins for biostratigraphy in the south atlantic ocean and refine campanian maastrichtian biostratigraphy in the region we document calcareous nannofossil distributions at deep sea drilling project dsdp and ocean drilling program odp sites from the high mid and low latitudes in the south atlantic ocean dsdp sites 354 356 511 525a and 530a and odp sites 690c 700b 661a and 1258a combining nannofossil occurrences with magnetostratigraphic schemes previously constructed for some of the sites we propose three new zonations tied to the global magnetostratigraphic chrons with distinct bio horizons selected at different latitudinal regions in this study we present detailed definitions of new south atlantic ocean low latitude sal south atlantic mid latitude sam and south atlantic high latitude sah zonations we identified significant diachronism of some species between latitudes throughout the campanian and maastrichtian these species diachronism mainly between zonal markers from low and mid latitudes during the late maastrichtian was probably related to migration patterns due to fundamental paleoceanographic changes

middle eocene lower miocene calcareous nannofossil magnetobiochronology of odp holes 699a and 703a in the subantarctic south atlantic abstract the distribution of calcareous nannofossils is documented for the middle eocene through lowermost miocene cores from ocean drilling program holes 699a and 703a in the subantarctic south atlantic the detailed nannofossil biostratigraphies established in combination with published magnetostratigraphic data have provided a fairly detailed age model for each hole this study suggests that the middle eocene through lowermost miocene section from hole 699a is virtually complete a major hiatus has been identified in hole 703a in the earliest oligocene coincident with an abrupt cooling in the southern ocean comparison of the nannofossil datum ages calibrated with magnetostratigraphy in the two holes with those from mid and southern high latitudes demonstrates synchroneity or diachroneity for the following nannofossil datums 1 the last occurrence lo of reticulofenestra bisecta is a consistent and reliable biostratigraphic marker for the oligocene miocene boundary from mid to high latitudes but not in extreme high latitudes 2 similarly the lo of chiasmolithus altus has a consistent age of about 26 8 ma in the southern ocean except in the extreme high latitudes where the datum appears to be substantially younger 3 the lo of reticulofenestra umbilica is about 32 9 ma in the southern ocean 4 the lo of isthmolithus recurvus is reliable and consistent from mid through high latitudes and correlates with the lower part of subchron c12r 34 4 ma 5 the lo of reticulofenestra oamaruensis has a consistent age of 36 0 ma at all four southern ocean sites that have yielded a lower oligocene magnetostratigraphy 6 the first occurrence fo of r oamaruensis is at 38 4 ma in the southern ocean and 7 the fo of i recurvus shows some age variations from mid to high latitudes and the age range is 38 5 39 0 ma at the five southern ocean sites

deciphering latitudinal shifts in coccolith accumulation in the eastern tropical pacific ocean through the pleistocene abstract middle and late pleistocene coccolithophore assemblages from ocean drilling program odp sites 1241 and 1242 in the northeastern tropical pacific north etp were analyzed to reconstruct coccolithophore productivity and surface water conditions over the last 925 kyr stratigraphic control was provided by 18o data and geomagnetic time scales and the identification of synchronous calcareous nannofossil events formerly described in mid and low latitudes changes in sea surface coccolithophore productivity were inferred by estimates of the so called n ratio an independent proxy on conversion to accumulation rates and the total coccolith derived carbonate accumulation rate car car variations follow those of the n ratio but show larger amplitude changes fluctuations in the n ratio and total car values imply coccolithophore productivity maxima during glacial times most likely as a result of a shallower nutri thermocline defined as the boundary beneath the sea surface at which nutrient rich cool and nutrient poor warm waters are separated low interglacial values of the n ratio and car suggest lower coccolithophore productivity and a deeper nutri thermocline deep stratification of the mixed layer this result implies that coccolithophore productivity controlled cars and dissolution were driven by the ratio between caco3 production and organic matter oxidation and that effects of dissolution were enhanced when coccolith production fell the data suggest high coccolithophore productivity and a shallow nutri thermocline during mis 22 through mis 6 likely due to the reinforcement of northeastern wind driven advection of nutrient rich waters which is compatible with a la nina like state the dominance of gephyrocapsa caribbeanica during the mid brunhes event mbe is consistent with this eutrophic environment conversely data from mis 5 suggests a weakening of trade winds in the etp which would have reduced the normal upwelling of cool water and warmed sst and which is compatible with the oligotrophic setting of an el nino like state during mis 4 through mis 1 coccolithophore productivity was not significantly different from mis 5 and therefore not indicative of a prominent la nina like state as previously reported for the equatorial cold tongue ect comparison with coccolithophore data from site 1240 supports a reduction of coccolithophore productivity and nutri thermocline deepening from the ect to the costa rica margin and suggests that nutrient availability was the primary control on the distribution of the whole assemblage during the past 925 kyr

cretaceous tertiary shortening basin development and volcanism in central tibet the geologic map pattern of the qiangtang terrane in central tibet defines a 600 km long and up to 270 km wide east plunging structural culmination it is characterized by early mesozoic blueschist bearing melange and upper paleozoic strata in the core and mainly triassic jurassic strata along the limbs in the western qiangtang terrane 84 e the culmination is unconformably overlain by weakly deformed mid cretaceous volcanic flows and tuffs along the bangong suture to the south 32 n 84 e mid cretaceous nonmarine red beds and volcanic rocks lie unconformably on jurassic suture zone rocks these relationships demonstrate that west central tibet was above sea level during the mid cretaceous and experienced significant denudation prior to mid cretaceous time growth of the qiangtang culmination is inferred to have initiated during southward emplacement of a thrust sheet of early mesozoic melange and upper paleozoic strata during the early cretaceous lhasa qiangtang collision the north south width of the inferred thrust sheet provides a minimum slip estimate of 150 km at 84 e decreasing eastward to 70 km at 87 e paleogene deformation in the qiangtang terrane is characterized by widely distributed mainly north dipping thrust faults that cut eocene oligocene red beds and volcanic rocks in their footwall along the bangong suture the north dipping shiquanhe gaize amdo thrust system cuts 64 and 43 m y old volcanic tuffs in its footwall and accommodated 40 km of post mid cretaceous shortening the tertiary south dipping gaize siling co backthrust bounds the southern margin of the bangong suture and marks the northernmost limit of mid cretaceous marine strata in central tibet cretaceous deformation and denudation in central tibet is attributed to northward underthrusting of the lhasa terrane beneath the qiangtang terrane along the bangong suture this model implies that 1 cretaceous strata along the bangong suture and in the northern lhasa terrane were deposited in a flexural foreland basin system and derived at least in part from the qiangtang terrane and 2 the central tibetan crust was thickened substantially prior to the indo asian collision although its magnitude is poorly known tertiary shortening in the qiangtang terrane is more prevalent than in the lhasa terrane this difference may be attributed to the presence of underthrust melange in the deeper central tibetan crust which would have made it weaker than the lhasa terrane during the indo asian collision

hf isotopic compositions of the standard zircons and baddeleyites used in u pb geochronology abstract zircon and baddeleyite u pb geochronological dating is widely used in solid earth sciences and the advent of rapid in situ methods of analysis such sims and icp ms has re emphasized the importance of having uniform standards recently it has been shown that hf isotopic data can provide important information on these minerals since they contain high concentrations of hf but have low lu hf ratios which results in negligible age correction however the complex internal structures that result from multiple thermal events such as inherited cores and metamorphic overgrowths require that the hf isotopic data be measured with high spatial resolution however the isobaric interferences of 176 yb and 176 lu on 176 hf hamper the precise determination of the 176 hf 177 hf ratio during in situ laser ablation mc icpms analysis it is shown here that mass biases of yb yb and hf hf change with time during analyses and behave differently for solutions and solid material therefore it is suggested that the mean yb value of the individual spot be used to obtain the precise isotopic composition for in situ zircon and baddeleyite hf isotopic analyses for low yb hf 176 yb 177 hf yb value have little effect on the accuracy of the hf isotopic composition however the appropriate yb isotopic abundance is necessary for high yb hf 176 yb 177 hf 0 001 zircons and baddeleyites since the interference of 176 yb on 176 hf is significant using the mean yb value of the individual spot and newly published yb isotopic abundance data six standard zircons and two standard baddeleyites have been investigated using a neptune mc icpms with 193 nm laser for zircons the obtained 176 hf 177 hf ratios are 0 282307 31 2sd for 91500 0 282680 31 2sd for temora 0 281729 21 2sd for cz3 282177 17 2sd for cn92 1 0 282983 17 2sd for fm0411 and 281234 11 2sd for phalaborwa the baddeleyites from phalaborwa and sk10 2 have 176 hf 177 hf ratios of 0 281238 12 and 0 282738 13 2sd these results agree well with the values obtained by the solution method and indicate that these standards have different hf isotopic compositions in which the extremely low 176 lu 177 hf and 176 yb 177 hf values of cz3 zircon and phalaborwa baddeleyite make them excellent standards for machine calibration during in situ zircon hf isotopic measurement with the other standards being more suitable for the development of the correction method

origin of rhythmic albian black shales piobbico core central italy calcareous nannofossil quantitative and statistical analyses and paleoceanographic reconstructions 1 the mid cretaceous 120 90 ma was a time of excess atmospheric co2 greenhouse climate and widespread o2 deficiency in the ocean the albian is punctuated by brief intermittent episodes of anoxia dysoxia recorded as cyclic black shales in the western tethys and atlantic oceans the albian section of the piobbico core central italy contains 30 m 10 ma of rhythmic black shales that were sampled at a high resolution and examined for calcareous nannofossil assemblages and c and o stable isotopes unlike oceanic anoxic events productivity was not the primary factor controlling the deposition of albian rhythmic black shales it is suggested that during warm humid climatic cycles higher temperatures and or increased precipitation and runoff produced density stratification at a regional scale recurrent lowered salinity in the early albian and warming in the late albian are credited with causing development of a pycnocline resulting in slower rates of deep water renewal and consequent anoxia

palaeobiogeography of early cretaceous berriasian barremian calcareous nannoplankton in order to assess early cretaceous nannoplankton biogeography we studied a series of sites which provide a north south transect across the atlantic ocean supplemented by sections from the north sea basin barents sea falkland plateau weddell sea antarctica argo abyssal plain nw of australia and neuquen basin argentina quantitative assemblage data were gathered from each site for seven time slices within the berriasian to barremian interval each horizon being determined by a nannofossil datum trends in species relative abundance and measures of diversity evenness and richness provide revealing biogeographic information a broad low to mid palaeolatitude zone 50 degrees n 50 degrees s is flanked in both northern and southern hemispheres by distinct high palaeolatitude zones major changes in assemblage abundance and composition occur across a sharp biogeographic front at around 50 degrees n and s palaeolatitude high palaeolatitude assemblages are lower in species richness and diversity and characterised by the presence of abundant typically bipolar taxa e g crucibiscutum salebrosum a less distinct biogeographic boundary at similar to 40 degrees n is distinguished by the presence absence of rarer but biogeographically significant taxa many of which have previously been assigned to boreal or tethyan provinces continental shelf sites are characterised by lower diversity assemblages with common to dominant diagnostic taxa which vary with palaeolatitude nannoconus and micrantholithus at low palaeolatitudes and biscutum constans and zeugrhabdotus spp at higher palaeolatitudes the latter two taxa are considered to be indicative of elevated surface water fertility and the former two may have been similarly adapted the genus watznoueria is ubiquitously dominant giving the populations an unevenness which appears to be a common feature throughout coccolithophore history c 2000 published by elsevier science b v all rights reserved

geochemistry detrital zircon geochronology and hf isotope of the clastic rocks in southern tibet implications for the jurassic cretaceous tectonic evolution of the lhasa terrane abstract in order to reconstruct tectonic evolution history of the southern margin of asia i e lhasa terrane before the india asia collision here we present a comprehensive study on the clastic rocks in the southern lhasa terrane with new perspectives from sedimentary geochemistry detrital zircon geochronology and hf isotope clasts from the jurassic early cretaceous sedimentary sequences i e yeba and chumulong formations display high compositional maturity and experienced moderate to high degree of chemical weathering whereas those from the late early late cretaceous sequences ngamring and shexing formations are characterized by low compositional maturity with insignificant chemical weathering our results lead to a coherent scenario for the evolution history of the lhasa terrane during the early middle jurassic 192 168ma the lhasa terrane was speculated to be an isolated geological block the yeba formation is best understood as being deposited in a back arc basin induced by northward subduction of the neo tethys ocean with sediments coming from the interiors of the lhasa terrane the middle jurassic early cretaceous lhasa qiangtang collision resulted in the formation of a composite foreland basin with southward flowing rivers carrying clastic materials from the uplifted northern lhasa and or qiangtang terranes during the late early late cretaceous 104 72ma the gangdese magmatic arc was uplifted rapidly above the sea level forming turbidites ngamring formation in the xigaze forearc basin and fluvial red beds shexing formation on the retro arc side at the end of late cretaceous the lhasa terrane was likely to have been uplifted to high elevation forming an andean type margin resembling the modern south america before the india asia collision