paleobotanical evidence for near present day levels of atmospheric co2 during part of the tertiary understanding the link between the greenhouse gas carbon dioxide co 2 and earth9s temperature underpins much of paleoclimatology and our predictions of future global warming here we use the inverse relationship between leaf stomatal indices and the partial pressure of co 2 in modern ginkgo biloba and metasequoia glyptostroboides to develop a co 2 reconstruction based on fossil ginkgo and metasequoia cuticles for the middle paleocene to early eocene and middle miocene our reconstruction indicates that co 2 remained between 300 and 450 parts per million by volume for these intervals with the exception of a single high estimate near the paleocene eocene boundary these results suggest that factors in addition to co 2 are required to explain these past intervals of global warmth

cretaceous pco2 estimation from stomatal frequency analysis of gymnosperm leaves of patagonia argentina abstract stomatal frequency of fossil conifers and ginkgoals cuticles were considered as potential indicators of the p co 2 values in the patagonian palaeoatmosphere during the cretaceous samples are from continental deposits of early upper aptian age anfiteatro de tico and punta del barco formations and upper albian lower cenomanian age kachaike formation both outcropping in andean and extra andean portions of santa cruz province argentina it can be inferred that the co 2 atmospheric content was similar during both stages or was slightly higher toward the late albian early cenomanian the co 2 content estimated from the conifers is between 700 and 1400 ppmv or lower from the mid aptian ginkgoals this co 2 range is consistent with those predicted worldwide by previous studies stomatal indices biogeochemical models and carbon isotopes and implies a co 2 content 2 3 4 0 times higher than present the high atmospheric co 2 content during the middle aptian and late albian early cenomanian was probably linked to a warming process at high latitudes during these times this is consistent with the floristic development recorded during both stages in patagonia

the silicate weathering feedback in the context of ophiolite emplacement insights from an inverse model of global weathering proxies silicate weathering provides a stabilizing feedback in the global carbon cycle and has played a key role in keeping surface environments clement for the majority of earth9s history it was recently proposed that ophiolite emplacement at low latitude at 90 to 70 ma and 50 to 40 ma enhanced the rate of silicate weathering and led to global cooling that terminated the early eocene climatic optimum in this study we revisit this proposal with a series of models of varying complexities to investigate the likelihood that earth9s climate was driven largely by ophiolite emplacement in the paleogene our models are constrained by either marine os isotopic records or the combination of carbon cycle sr and os isotopic records and solid earth degassing rates the proposed changes in ophiolite weathering are inconsistent with our os isotope mass balance model with our coupled carbon cycle and weathering proxy model total global silicate weathering fluxes including ophiolite weathering slightly decreased or remained relatively constant from 52 to 42 ma this constancy in global silicate weathering fluxes is inconsistent with the idea that enhanced ophiolite weathering directly led to global climate cooling however our model results support the idea that the strength of the silicate weathering feedback weatherability has played a significant role in regulating the pco2 levels through the cenozoic further the modeling toolkit developed here can be used to explore the role of mafic rock weathering in driving climate change in other intervals through earth9s history

estimating paleoatmospheric pco2 during the early eocene climatic optimum from stomatal frequency of ginkgo okanagan highlands british columbia canada abstract estimates of p co 2 for the early paleogene vary widely from near modern day levels to an order of magnitude greater based on various proxy measures resolving the relationship between climate and p co 2 during this globally warm period is a key task in understanding climate dynamics in a warmer world here we use the stomatal frequency of fossil ginkgo adiantoides from the okanagan highlands of british columbia canada to estimate p co 2 during the early eocene climatic optimum eeco the interval of peak warmth in the cenozoic we also examine a dataset of modern ginkgo biloba leaves to critically assess the accuracy and precision of stomatal frequency as a proxy indicator of p co 2 early eocene fossil g adiantoides has significantly lower stomatal frequency than modern g biloba suggesting p co 2 levels 2 modern pre industrial values this result is in contrast to earlier studies using stomatal frequency of ginkgo that indicated near modern day levels of p co 2 in the early paleogene though not including samples from the eeco we also find that levels of p co 2 as indicated by stomatal frequency are correlated with trends in climate mean annual temperature over time at the falkland fossil locality suggesting that climate and p co 2 were coupled during the eeco hyperthermal

paleo co2 variation trends and the cretaceous greenhouse climate the cretaceous was one of the most remarkable periods in geological history with a greenhouse climate and several important geological events reconstructions of atmospheric co2 using proxies are crucial for understanding the cretaceous greenhouse in this paper we summarize the major approaches for reconstructing co2 based on paleobotanical or geochemical data and synthesize the co2 variations throughout the cretaceous the results show that atmospheric co2 levels remained relatively high throughout the cretaceous but were lower in the early cretaceous highest in the mid cretaceous and gradually declined during the late cretaceous however this overall trend was interrupted by several rapid changes associated with ocean anoxic events oaes and the end cretaceous catastrophic event new data on paleo co2 levels from paleobotanical and paleosol evidences support not only the overall trends indicated by geochemical models but provide more precise records of the short term fluctuations related to brief episodes of climate change temporal resolution within the long quiet magnetic period in the middle cretaceous is one of the obstacles preventing us from a more comprehensive understanding of the co2 climate linkage but new paleo co2 determinations and climatic data from stratigraphic sections of sediments intercalated with datable volcanic rocks will allow a better understanding of the relationships between fluctuations of atmospheric co2 climate change and geological events

reconstruction of atmospheric co2 during the early middle eocene by application of a gas exchange model to fossil plants from the messel formation germany abstract in this study atmospheric co 2 during the early middle eocene 47 ma is reconstructed using fossil plants from the messel formation close to darmstadt germany co 2 concentration is calculated using a mechanistic model of gas exchange which optimizes co 2 uptake by photosynthesis against water vapor loss by transpiration a strategy that is commonly realized in land plants input data for the model are stomatal data density and pore length paleoclimate data obtained by two different approaches and biochemical parameters of photosynthesis leaves from three taxa belonging to two different families were studied laurophyllum lanigeroides daphnogene crebrigranosa both lauraceae and rhodomyrtophyllum sinuatum myrtaceae the results suggest that co 2 was between 853 and 1033 ppm during the development of the messel formation stomatal data therefore indicate a co 2 concentration that was substantially higher than today the reconstructed range of co 2 concentration is close to data provided by geochemical modeling results and conforms to various data that indicate a warm and ice free climate during this time interval

inter and intracrystalline isotopic disequilibria techniques and applications in the last 20 years we have seen a significant expansion of techniques related to geochemical and isotopic microsampling of materials in the earth sciences from constraining pre eruptive histories of flood basalt magmas to identifying the natal rivers of origin of anadromous fishes these techniques have had significant impacts in a wide variety of scientific fields nowhere has the impact been greater than in identifying the sources processes and timing of processes involved in igneous magmatic systems both technique refinements and the development of new technologies have aided in advancing microsampling applications thus allowing for a better understanding of the sources and mechanisms responsible for changing geochemical and isotopic signatures in natural systems in this chapter we focus on the techniques and technologies associated with radiogenic isotope microsampling and review applications of these techniques as utilized in scientific investigations isotope microsamping is a logical extension of earlier studies that evaluated individual components of magmas and magma systems including melts and minerals from the use of petrographic microscopes and the later introduction of the electron microprobe the focus on internal chemical variations in melts and minerals is critical to assessing the petrogenetic histories of igneous rocks even today these technologies are used to ensure that further trace element and isotopic analyses are undertaken in a textural and major element context for trace elements and isotopes early studies e g cortini and van calsteren 1985 confirmed variations in the melt and mineral components of many igneous rocks but focused on mineral or glass separates potential information associated with isotopic variations retained by individual crystals or internal variations within individual crystals was lost later studies such as geist et al 1988 focused on isotope variations within single megacrysts to constrain mixing scenarios between basaltic andesitic and rhyolitic magmas and davidson et al 1990 used

stomatal response of swordfern to volcanogenic co2 and so2 from kilauea volcano 1 the experimentally determined relationship between atmospheric pco2 and plant stomata has been used to interpret large but transient changes in atmospheric composition such as may have resulted from the eruptions of flood basalt however this relationship has not been tested in the field i e in the vicinity of active volcanoes to examine the specific effects of volcanogenic emissions moreover the interpretation of paleoatmospheric pco2 from fossil stomatal data assumes that the stomatal response resulted solely from variation in pco2 and ignores the potential effect of outgassed so2 we hypothesize that volcanogenic so2 also has a significant effect on leaf stomata and test this hypothesis by measuring the stomatal index of the common swordfern nephrolepis exaltata in the plumes of the actively outgassing vents of kilauea volcano we find that compared to control locations stomatal index is lowest at sample sites in the plume of halema uma u crater where concentrations of both co2 and so2 are much higher than background however sites located directly in the plume of pu u o o where so2 levels are high but co2 levels are not also yield low values of stomatal index we propose that shifts in the stomatal index of fossil leaves may record transient atmospheric increases in both so2 and co2 such as may be caused by eruptions of flood basalts calculations of pco2 based on stomatal frequency are likely to be exaggerated

rapid determination of pb isotopes to define precambrian allochthonous domains an example from west greenland lacking provincial fauna establishing autochthoneity or allochthoneity of domains in precambrian orogens typically requires comparison of precollisional attributes across suspect terrane boundaries we have measured the pb isotopic compositions of k feldspar grains in epoxy mounts and rock slabs by a rapid laser ablation inductively coupled plasma mass spectrometry method with sufficient precision and accuracy to define two large pb isotopic domains in the paleoproterozoic nagssugtoqidian rinkian orogen of west greenland these domains are interpreted to correlate with and help delineate the rae and north atlantic cratons their boundary coincides with a recently proposed location of the main collisional suture of this orogen thus supporting this interpretation this study validates a rapid analytical pb isotopic method that will be useful in addressing a range of problems that requires knowledge of crustal affinity in precambrian terrains

basin evolution and destruction in an early proterozoic continental margin the rinkian fold thrust belt of central west greenland in central west greenland early palaeoproterozoic siliciclastic and carbonate sequences of the karrat group shelf sequences of the rae craton margin were deposited in sedimentary basins controlled by nw and sw trending linked extensional fault systems the shelf basins were later filled and overtopped by turbidite systems filling a foredeep advancing ahead of a thrust system the karrat fjord thrust system that propagated west to east deformation culminated in emplacement of basement cored nappes progressive deformation and high grade metamorphism at c 1 87 ga reactivation of lower plate growth faults formed dome and basin like folds and related thrusts that refolded the structure of the karrat fjord thrust system and inverted the shelf basins the southern karrat fjord thrust system was reworked in a belt of intense ductile nw directed thrusting the nunaarsussuaq thrust system formed at c 1 84 ga at the northern limit of the nagssugtoqidian orogen kinematics of these events are at odds with the consensus view that the rinkian fold thrust belt is a northward extension of the nagssugtoqidian orogen resulting from north south convergence between the rae and north atlantic cratons application of structural restoration techniques to basin analysis of palaeoproterozoic rocks has potential to provide new insights into proterozoic orogenic processes worldwide

influences of depositional environment and diagenesis on geophysical log response in the south carolina coastal plain effects of sedimentary fabric and mineralogy abstract interpretations of depositional environments and hydrologic units are made routinely from the study of geophysical well logs spontaneous potential sp and resistivity logs can be used as indicators of textural parameters gamma ray logs denote lithologic zones based on the presence of radioactive material particularly in fine grained sediments on the south carolina coastal plain surficial fluvially derived cobbles sands and clays are characterized by very low radioactivity weathered paleosurfaces enriched in hydroxy interlayered vermiculite hiv and associated mica produce higher gamma ray responses at discrete intervals the underlying sequence of siliciclastic shelf sands deposited under conditions of eustatic sea level rise has high concentrations of smectite and low gamma ray signatures small scale variability of gamma radiation reflects temporal changes of sub environments higher values are recorded in finer grained organic rich nearshore sediments calcareous ecocene sediments are characterized by highly variable signals on the gamma ray sp and resistivity logs phospharic material smectite and disseminated organic matter produced very high gamma ray values corresponding low sp readings are a function of low macro to microscale porosity resulting from the fine grain size and authigenic pore filling cement intervals that resemble massively bedded chalks at the macroscale are characterized by low gamma ray and resistivity values and high sp readings high sp values result from microposity preserved in biogenic tests rather than intergranular porosity that would be expected for a permeable sand these observations indicate the importance of incorporating sedimentologic techniques with well log data for a comprehensive evaluation of subsurface lithologic units

eocene continental climates and latitudinal temperature gradients global climate during the mesozoic and early cenozoic is thought to have been warmer than at present but there is debate about winter temperatures paleontological data indicate mild temperatures even at high latitudes and in mid latitude continental interiors whereas computer simulations of continental paleoclimates produce winter temperatures closer to modern levels foliar physiognomy and floristic composition of 23 eocene floras from the interior of north america and australia indicate cold month means generally 2 c even where the mean annual temperature mat was

changes in atmospheric co2 concentrations and climate from the late eocene to early miocene palaeobotanical reconstruction based on fossil floras from saxony germany abstract in the present study proxy data concerning changes in atmospheric co2 and climatic conditions from the late eocene to the early miocene were acquired by applying palaeobotanical methods fossil floras from 10 well documented locations in saxony germany were investigated with respect to 1 stomatal density index of fossil leaves from three different taxa eotrigonobalanus furcinervis laurophyllum pseudoprinceps and laurophyllum acutimontanum 2 the coexistence approach ca based on nearest living relatives nlr and 3 leaf margin analysis lma whereas the results of approach 1 indicate changes in atmospheric co2 concentration approaches 2 and 3 provide climate data the results of the analysis of stomatal parameters indicate that the atmospheric co2 concentration was higher during the late eocene than during the early oligocene and increased towards the late oligocene a lower atmospheric pco2 level after the late eocene is also suggested by an increase in marine palaeoproductivity at this time from the late oligocene onwards no changes in atmospheric co2 concentration can be detected with the present data for the considered sites the results of the coexistence approach and of the leaf margin analysis document a significant cooling event from the late eocene to the early oligocene the pco2 decrease from the late eocene to the early oligocene indicated by the stomatal data raised in this study was thus coupled to a temperature decrease which is reflected by the present datasets from the early oligocene onwards however no further fundamental climate change can be inferred for the considered locations the pco2 increase from the early oligocene to the late oligocene which is indicated by the present data is thus not accompanied by a climate change at the considered sites a warming event during the late oligocene is however recorded by marine climate archives according to the present data no change in pco2 occurred during the cooling event at the oligocene miocene boundary which is also indicated by marine data the quality and validity of stomatal parameters as sensors for atmospheric co2 concentration are discussed

geologic constraints on the glacial amplification of phanerozoic climate sensitivity the long term carbon cycle depends on many feedbacks silicate weathering consumes atmospheric co2 but is also enhanced by the increased temperatures brought about by this important greenhouse gas the long term sensitivity t2x of climate to co2 doubling modulates the strength of this negative feedback we update the model experiment of royer and others 2007 by estimating an empirical probability density function pdf of t2x for the phanerozoic by using an improved geocarbsulf carbon cycle model to predict a larger recalibrated set of proxy co2 measurements from the present day to 420 ma the new geocarbsulf parameterizes the rapid weathering of volcanic rocks relative to plutonic rocks updates to the carbon cycle model and the proxy co2 data set induce opposing model responses as a result our experiment maintains an agreement with t2x estimates based on numerical climate models and late cenozoic paleoclimate for a climate sensitivity t2x that is uniform throughout the phanerozoic the most probable value is 3 to 4 c geocarbsulf fits the proxy co2 data equally well and with far more parameter choices if t2x is amplified by at least a factor of two during the glacial intervals of the paleozoic 260 340 ma and cenozoic 0 40 ma relative to non glacial intervals of earth history for glacial amplification of two the empirical pdfs for glacial climate sensitivity predict t2x g 2 0 c with 99 percent probability t2x g 3 4 c with 95 percent probability and t2x g 4 4 c with 90 percent probability the most probable values are t2x g 6 to 8 c this result supports the notion that the response of earth9s present day surface temperature will be amplified by the millennial and longer term waxing and waning of ice sheets

assessing the mechanisms of common pb incorporation into titanite abstract common pb the portion of non radiogenic pb within a u bearing mineral needs to be accurately accounted for in order to subtract its effect on u pb isotopic ratios so that meaningful ages can be calculated the propensity to accommodate common pb during crystallization or later is different across the range of u bearing minerals used for geochronology titanite frequently accommodates significant amounts of common pb however the most appropriate method to correct for this requires knowledge on the mechanism and timing of common pb incorporation information that is commonly difficult to extract in this study the spatial and compositional distribution of trace elements including pb in metamorphic titanites from a greenland amphibolite is investigated on the grain to nano scale titanites have an isotopically similar signature for both common and radiogenic pb in all grains but significantly different quantities of the non radiogenic component microstructural and compositional examination of these grains reveals undeformed but high common pb f207 titanites have homogeneous element distributions on the atomic scale suggesting common pb is incorporated into titanite during its growth and not during later processes in contrast deformed titanite comprising low angle boundaries formed by subgrain rotation recrystallization comprise networks of dislocations that are enriched in mg al k and fe smaller cations may migrate due to elastic strain in the vicinity of the dislocation network yet the larger k cations more likely reflect the mobility of externally derived k along the orientation interface the absence of pb enrichment along the boundary indicates that either pb was too large to fit into migrating lattice dislocations or static low angle boundaries and or that there was no external pb available to diffuse along the grain boundary as the common pb composition is distinctly different to regional pb models the metamorphic titanite grew in a homogeneous pb reservoir dominated by the break down of precursor u bearing phases the different quantity of common pb in the titanite grains indicates a mineral driven element partitioning in an isotopically homogeneous metamorphic reservoir consistent with low u low total ree and flat lree signatures in high f207 analyses these results have implications for the selection of appropriate common pb corrections in titanite and other accessory phases

abrupt plant physiological changes in southern new zealand at the termination of the mi 1 event reflect shifts in hydroclimate and pco2 abstract a rise in atmospheric co2 is believed to be necessary for the termination of large scale glaciations although the antarctic ice sheet is estimated to have melted from 125 to 50 its modern size there is thus far no evidence for an increase in atmospheric co2 associated with the mi 1 glacial termination in the earliest miocene here we present evidence from a high resolution terrestrial record of leaf physiological change in southern new zealand for an abrupt increase in atmospheric co2 coincident with the termination of the mi 1 glaciation and lasting approximately 20 kyr quantitative p co 2 estimates made using a leaf gas exchange model suggest that atmospheric co2 levels may have doubled during this period from 516 111 ppm to 1144 410 ppm and subsequently returned back to 425 53 ppm the 20 kyr interval with high p co 2 estimates is also associated with a period of increased moisture supply to southern new zealand inferred from carbon and hydrogen isotopes of terrestrial leaf waxes the results provide the first high resolution record of terrestrial environmental change at the oligocene miocene boundary document a 20 kyr interval of elevated p co 2 and increased local moisture availability and provide insight into ecosystem response to a major orbitally driven climatic transition

late early cretaceous climate and pco2 estimates in the liupanshan basin northwest china abstract the extinct conifer family cheirolepidiaceae especially pseudofrenelopsis is commonly used to reconstruct the atmospheric palaeo co2 concentration pco2 during the cretaceous period in recent years many pseudofrenelopsis specimens have been collected from the uppermost stratum of the naijiahe formation in the liupanshan basin ningxia hui autonomous region northwest china the stomatal parameters such as stomatal density stomatal index stomatal pore length and stomatal pore depth and the stable carbon isotope composition of the present pseudofrenelopsis were analyzed the palaeo co2 levels of the liupanshan basin during the late early cretaceous late albian were reconstructed using stomatal ratio sr and mechanistic model approaches the sr method yielded a semi quantitative pco2 estimate of 620 ppmv on the basis of the recent standardization and 1030 ppmv on the basis of the carboniferous standardization which show similarities with other sr based and geochemical based reconstructions for the late early cretaceous the pco2 was about 670 ppmv on the basis of mechanistic model method similar to the estimated pco2 values recovered by the sr method corrected with the recent standardization based on comparative studies of previously published pco2 results of the early cretaceous pco2 levels varied during the early cretaceous and pco2 was relatively stable during the late early cretaceous furthermore environmental conditions of the liupanshan basin during the late early cretaceous were studied the stable carbon isotope composition of pseudofrenelopsis leaves ranged from 21 5 to 24 3 with an average of 22 8 indicating an arid semi arid and high evaporation climate during the late early cretaceous in the liupanshan basin these environmental conditions were also supported by palynological lithological and geochemical evidence

eocene biostratigraphy of south carolina and its relationship to gulf coastal plain zonations and global changes of coastal onlap an assimilation of mega invertebrate assemblages in south carolina with gulf coast biozones allows for the subdivision of the middle and late eocene into four regional zonations that can be compared with proposed global sequences controlled by eustatic changes of sea level the middle eocene can be divided in ascending stratigraphic order into the cubitostrea perplicata zone c lisbonensis zone and c sellaeformis zone the late eocene is represented by the crassatella alta chlamys cawcawensis zone these zones are separated by type 1 or type 2 unconformities that bracket global sequence boundaries unconformity types after vail and todd 1981 the following sequences are proposed for the middle and upper eocene te2 1 cubitostrea perplicata zone followed by a type 2 unconformity te2 2 c lisbonensis zone followed by a type 2 unconformity te2 3 c sellaeformis zone followed by a type 1 unconformity and te3 crassatella alta chlamys cawcawensis zone followed by a type 2 unconformity although not recognized in the downdip areas of south carolina the te3 sequence can be further subdivided into a te3 1 sequence and te3 2 sequence separated by a type 2 unconformity this regional break lies at the gosport sand lower moodys branch formation upper moodys branch formation contact in the gulf coastal plain and at the base of the tivola tonque in georgia

atmospheric co2 from the late oligocene to early miocene based on photosynthesis data and fossil leaf characteristics abstract the late oligocene represents a comparatively cool phase followed by a warming event the so called late oligocene warming that predates the mi 1 glacial event at the oligocene miocene transition there is evidence that these climate events were linked to level changes in atmospheric co2 in this study atmospheric co2 from the late oligocene to the early miocene is reconstructed by a terrestrial proxy stomatal parameters of angiosperm leaves fossil leaves of six plant species were considered laurophyllum pseudoprinceps l acutimontanum both lauraceae platanus neptuni platanaceae eotrigonobalanus furcinervis quercus rhenana and q praerhenana all fagaceae the plant material originates from various sites in germany late oligocene to the early miocene and austria early miocene a mechanistic theoretical approach based on stomatal data photosynthesis and gas exchange control was used since this approach also requires climate data as input parameters for calculating assimilation and transpiration rates palaeoclimate of these sites were also derived in detail considerable differences were found between results obtained by the two lauraceae taxa and all other taxa with the lauraceae indicating a high co2 level if lauraceae are included a co2 level of about 700 ppm for the late and latest oligocene is obtained and about 500 ppm for the early miocene all non lauraceae taxa belonging to different families indicate a consistently lower level of co2 of about 400 ppm for all considered sites and time slices a value of 400 ppm is also consistent with other proxy data that are available for that time period a steady level of 400 ppm would indicate a decoupling of co2 and cooling warming events from the late oligocene to the early miocene more data provided by different proxies are desirable to unravel the interactions between atmosphere and climate for the late oligocene and the early miocene

the amazon laurentian connection as viewed from the middle proterozoic rocks in the central andes western bolivia and northern chile middle proterozoic rocks underlying the andes in western bolivia western argentina and northern chile and early proterozoic rocks of the arequipa massif in southern peru form the arequipa antofalla craton these rocks are discontinuously exposed beneath mesozoic and cenozoic rocks but abundant crystalline clasts in tertiary sedimentary rocks in the western altiplano allow indirect samples of the craton near berenguela western bolivia the oligocene and miocene mauri formation contains boulders of granodiorite augen gneiss 1171 20 ma and 1158 12 ma u pb zircon quartzose gneiss and granofels that are inferred to have arkosic protoliths 1100 ma source region u pb zircon quartzofeldspathic and mafic orthogneisses that have amphibolite and granulite facies metamorphic mineral assemblages 1080 ma metamorphism u pb zircon and undeformed granitic rocks of phanerozoic age the middle proterozoic crystalline rocks from berenguela and elsewhere in western bolivia and from the middle proterozoic belen schist in northern chile generally have present day low 206pb 204pb 15 57 and elevated 208pb 204pb 37 2 to 50 7 indicative of high time averaged th u values the middle proterozoic rocks in general have higher present day 206pb 204pb values than those of the early proterozoic rocks of the arequipa massif 206pb 204pb between 16 1 and 17 1 but lower than rocks of the southern arequipa antofalla craton 206pb 204pb 18 5 a difference inferred to reflect grenvillian granulite metamorphism the pb isotopic compositions for the various proterozoic rocks lie on common pb isotopic growth curves implying that pb incorporated in rocks composing the arequipa antofalla craton was extracted from a similar evolving pb isotopic reservoir evidently the craton has been a coherent terrane since the middle proterozoic moreover the pb isotopic compositions for the arequipa antofalla craton overlap those of the amazon craton thereby supporting a link between these cratons and seemingly precluding part of the arequipa antofalla craton from being a detached fragment of another craton such as eastern laurentia which has been characterized by a different u pb history pb isotopic compositions for the arequipa antofalla craton are furthermore distinct from those of the proterozoic basement in the precordillera terrane western argentina indicating a pb isotopic and presumably a tectonic boundary between them the pb isotopic compositions for the precordillera basement are similar to those of eastern laurentia and support other data indicating that these rocks are a detached fragment of north america finally the distinct pb isotopic evolution history of the arequipa antofalla craton and eastern laurentia require minor modification to tectonic models linking eastern north america scotland to the oroclinal bend in western south america

mantle and crustal sources of archean anorthosite a combined in situ isotopic study of pb pb in plagioclase and lu hf in zircon isotopic analyses of ancient mantle derived magmatic rocks are used to trace the geochemical evolution of the earth s mantle but it is often difficult to determine their primary initial isotope ratios due to the detrimental effects of metamorphism and secondary alteration we present in situ analyses by la mc icpms for the pb isotopic compositions of igneous plagioclase an75 89 megacrysts and the hf isotopic compositions of bse imaged domains of zircon grains from two mantle derived anorthosite complexes from south west greenland fiskenaesset and nunataarsuk which represent two of the best preserved archean anorthosites in the world in situ la icpms u pb geochronology of the zircon grains suggests that the minimum crystallization age of the fiskenaesset complex is 2 936 13 ma 2 mswd 1 5 and the nunataarsuk complex is 2 914 6 9 ma 2 mswd 2 0 initial hf isotopic compositions of zircon grains from both anorthosite complexes fall between depleted mantle and a less radiogenic crustal source with a total range up to 5 ehf units in terms of pb isotopic compositions of plagioclase both anorthosite complexes share a depleted mantle end member yet their pb isotopic compositions diverge in opposite directions from this point fiskenaesset toward a high more radiogenic pb crustal composition and nunataarsuk toward low less radiogenic pb crustal composition by using hf isotopes in zircon in conjunction with pb isotopes in plagioclase we are able to constrain both the timing of mantle extraction of the crustal end member and its composition at fiskenaesset the depleted mantle melt interacted with an eoarchean 3 700 ma mafic crust with a maximum 176lu 177hf 0 028 at nunataarsuk the depleted mantle melt interacted with a hadean 4 200 ma mafic crust with a maximum 176lu 177hf 0 0315 evidence from both anorthosite complexes provides support for the long term survival of ancient mafic crusts that although unidentified at the surface to date could still be present within the fiskenaesset and nunataarsuk regions

natural and anthropogenic changes in atmospheric co2 over the last 1000 years from air in antarctic ice and firn a record of atmospheric co2 mixing ratios from 1006 a d to 1978 a d has been produced by analysing the air enclosed in three ice cores from law dome antarctica the enclosed air has unparalleled age resolution and extends into recent decades because of the high rate of snow accumulation at the ice core sites the co2 data overlap with the record from direct atmospheric measurements for up to 20 years the effects of diffusion in the firn on the co2 mixing ratio and age of the ice core air were determined by analyzing air sampled from the surface down to the bubble close off depth the uncertainty of the ice core co2 mixing ratios is 1 2 ppm 1 preindustrial co2 mixing ratios were in the range 275 284 ppm with the lower levels during 1550 1800 a d probably as a result of colder global climate natural co2 variations of this magnitude make it inappropriate to refer to a single preindustrial co2 level major co2 growth occurred over the industrial period except during 1935 1945 a d when co2 mixing ratios stabilized or decreased slightly probably as a result of natural variations of the carbon cycle on a decadal timescale

the southeastern churchill province synthesis of a paleoproterozoic transpressional orogen the paleoproterozoic southeastern churchill province secp is located in the northeastern canadian shield of labrador and quebec the secp formed through the oblique collisions of the archean nain and superior cratons with a third intervening archean block the core zone the belt has a tripartite structure comprising the torngat orogen to formed by nain craton core zone collision in the east the core zone in the centre and the new quebec orogen nqo formed by superior craton core zone collision in the west the secp thus records transpressional development on the flanks of the superior and nain cratons as they indented northwards to form the larger trans hudson nagssugtoqidian orogenic belt to the north principal stages of tectonic development were 1 2 2 2 1 ga crustal rifting of nain and superior cratons 2 ca 1 9 ga subduction under eastern nain craton 3 ca 1 87 1 85 ga collision of nain craton and core zone to form the to 4 1 845 1 820 ga sinistral transpression in the to and subduction under the western core zone and 5 1 82 1 77 ga collision of superior craton and core zone to form the nqo in association with dextral transpression crustal scale cross sections of the secp have been developed from reflection and refraction seismic data the western part of the nqo is dominantly west vergent and associated with an imbricate thick skinned thrust stack that ramps from the base of the crust the core zone is characterized by a 35 40 km thick crust and pervasive east dipping fabrics related to westerly thrusting the to is a narrow doubly vergent belt associated with a 48 km thick crust that forms a crustal root with a moho relief of 12 km the root is interpreted to result from attempted subduction of the core zone under the nain craton possibly as a result of mid crustal wedging by the nain craton the to was the site of intense convergence that resulted in excision of juvenile crust possibly including tectonic removal of the axial magmatic arc as a result the middle to lower levels of the secp consist largely of refractory archean lithosphere this may account for the lack of widespread post collisional plutonism in the secp and the preservation of the to root

moderate levels of eocene pco2 indicated by southern hemisphere fossil plant stomata reducing the uncertainty in predictions of future climate change is one of today s greatest scientific challenges with many significant problems unsolved including the relationship between pco2 a

a model for atmospheric co 2 over phanerozoic time a new model has been constructed for calculating the level of atmospheric co sub 2 over phanerozoic time which is much simpler mathematically than the blag model but more complex geologically and biologically results suggest that there has been a notable pattern of varying atmospheric co sub 2 level over the past 570 my with high levels during the mesozoic and early paleozoic and low levels during the permo carboniferous and late cenozoic sensitivity analysis shows that within reasonable limits of the principal governing parameters this qualitative trend of varying co sub 2 is relatively insensitive to the actual values chosen for these parameters causes of the co sub 2 variation are multiple and no single geological or biological process can be called upon to explain all co sub 2 variation for all time the calculated trend of co sub 2 over time agrees well with independent estimations of paleoclimates thus the greenhouse theory of paleoclimate on a long geological time scale is supported by the results of the present study

cenozoic tectonic and paleoenvironmental evolution of northwestern china evidence from two deep boreholes in the jartai basin abstract to study the cenozoic tectonic and paleoenvironmental evolution of northwestern china we drilled two deep boreholes jlt1 and jlt2 in the jartai basin magnetostratigraphy stratigraphy studies were conducted to establish a chronological framework sedimentary facies were reconstructed using lithology geophysical logs gamma ray gr and resistivity r and measurements of grain size sediment color l a and b and magnetic susceptibility and its frequency dependence the stratigraphic sequence of borehole jlt2 is similar to that of paleogene sediments in basins along the northeastern tibetan plateau which may be the result of the far field effect of the uplift of the tibetan plateau during the paleocene via faulting activity the results of borehole jlt1 indicate that the basin was occupied by alternating desert lacustrine and alluvial fan environments since ca 6 7 ma during ca 6 7 3 6 ma alternations of aeolian sand and lacustrine facies may reflect climatic cooling and drying and at that time the area may have been an important sediment source for the aeolian red clay of the loess plateau during ca 3 6 2 1 ma alluvial fan development also reflected the uplift of the northeastern tibetan plateau and the area acted as a source region for dust supplied to the loess plateau and the shanxi graben the alternations of aeolian sand and lacustrine deposits since ca 2 1 ma may have resulted from glacial interglacial variations of the east asian monsoon since ca 1 7 ma lacustrine deposition developed because of the enormous discharge of the yellow river into the jartai basin a tectonic movement at ca 0 8 ma is reflected by lacustrine red clay at the top of borehole jlt1 after the mid pleistocene transition the modern landscape of the jartai basin and its adjacent regions were formed

drainage reorganization during breakup of pangea revealed by in situ pb isotopic analysis of detrital k feldspar pb isotopes in detrital k feldspar grains provide a powerful provenance tracer for feldspathic sandstones common pb isotopic compositions show broad hundred kilometer scale regional variation and this signature can survive weathering transport and diagenesis the feldspar pb signature can be measured rapidly using laser ablation multicollector inductively coupled plasma mass spectrometry la mc icp ms and careful targeting can avoid inclusions and altered regions within grains here we combine a new pb domain map for the circum north atlantic with detrital k feldspar pb isotopic data from triassic and jurassic sandstones from basins on the irish atlantic margin the pb isotopic compositions reveal otherwise cryptic feldspar populations that constrain the evolving drainage pattern triassic sandstones originated from distant archean and paleoproterozoic rocks probably in green land labrador and the rockall bank to the nw implying long 500 km transport across a nascent rift system later jurassic sandstones had a composite paleo and mesoproterozoic source in more proximal sources to the north

pb isotopes during crustal melting and magma mingling a cautionary tale from the miki fjord macrodike central east greenland abstract pb isotopic data are presented for hybrid rocks formed by mingling between mantle derived tholeiitic magma of the eocene miki fjord macrodike east greenland and melt derived from the adjacent precambrian basement bulk mixing and afc processes between end members readily identified in the field fail to model the pb isotope systematics selective contamination during diffusional exchange which can explain the complex sr and nd isotope compositions of the hybrid rocks blichert toft et al 1992 cannot fully account for the variability of the pb isotopic data using the identified crustal end members the crustal anatectic end member although similar in sr and nd isotope composition has a markedly different pb isotopic composition than its source gneiss the differences are consistent with preferential incorporation of radiogenic pb from accessory phases such as metamict zircon or loosely bound pb from grain boundaries during disequilibrium melting of the host rock by the mafic magma the crustal melt involved in magma interactions was therefore heterogeneous with respect to pb isotopes on a metre scale these results illustrate the difficulties inherent in interpreting isotopic variations in contaminated mafic magmas even when the end members are well constrained by field relations we show that the pb isotopic composition of the crustal contaminants and contamination trajectories for the miki fjord hybrid magmatic lithologies are markedly different from regional basement gneisses and contaminated lavas of the associated flood basalt province that were contaminated by characteristically unradiogenic pb and sr lithologies resembling lewisian basement not exposed locally

leaf stomatal frequency in the australian tropical rainforest tree neolitsea dealbata lauraceae as a proxy measure of atmospheric pco2 a putative relationship has been demonstrated for european and north american woody dicots and gymnosperms between leaf stomatal frequency and historical levels of atmospheric co2 however hitherto no such study has been presented for australian tropical broadleaved evergreen trees in this study variation of stomatal index si along environmental gradients is examined for the broadleaved evergreen tropical rainforest tree neolitsea dealbata lauraceae historical herbarium samples from natural populations in northeastern and southeastern queensland were analysed leaf si for neolitsea dealbata is shown to be insensitive to mean annual rainfall or seasonal totals or to temperature variables indicating the climatic factors that influence the water budget of the plants were not a factor controlling si contrary to the pattern most commonly observed in temperate areas si decreased with elevation for collections within a single year for 3 out of 4 years surveyed a pattern consistent with environmental factors other than pco2 acting as the control over si however an overall decrease in si was recorded for samples collected in queensland 30 17 s over the period 1899 1988 corresponding to a southern hemisphere increase in pco2 from 295 to 350 ppm restricting the analysis to sites within an altitudinal band of 640 1120 m demonstrated a significant relationship between si and pco2 r2 0 8942 f11 84 5 p 0 001 this data set was used to estimate atmospheric pco2 from fossil leaf cuticle for the closely related genus litsea giving pco2 values comparable to those estimated using ginkgo from northern hemisphere fossil sites

eocene atmospheric co2 from the nahcolite proxy estimates of the atmospheric concentration of co 2 co 2 atm for the hothouse climate of the early eocene climatic optimum eeco vary for different proxies extensive beds of the mineral nahcolite nahco 3 in evaporite deposits of the green river formation piceance creek basin colorado usa previously established co 2 atm for the eeco to be 1125 ppm by volume ppm here we present experimental data that revise the sodium carbonate mineral equilibria as a function of co 2 and temperature co precipitation of nahcolite and halite nacl now establishes a well constrained lower co 2 atm limit of 680 ppm for the eeco paleotemperature estimates from leaf fossils and fluid inclusions in halite suggest an upper limit for co 2 atm in the eeco from the nahcolite proxy of 1260 ppm these data support a causal connection between elevated co 2 atm and early eocene global warmth but at significantly lower co 2 atm than previously thought which suggests that ancient climates on earth may have been more sensitive to a doubling of co 2 atm than is currently assumed

declining atmospheric co2 during the late middle eocene climate transition the transition from the extreme greenhouse of the early paleogene 52 ma to the present day icehouse is the most prominent change in earth s cenozoic climate history during the late middle eocene climate transition 42 38 ma which preceded the onset of long lived continental scale ice sheets there is concor dant evidence for brief pulses 1 m y in length of global warmth and ice sheet growth but few constraints on atmospheric co 2 here we estimate the concentration of atmospheric co 2 during this critical interval using stomatal indices of fossil metasequoia needles from ten levels in an exceptionally well preserved core from the giraffe kimberlite locality in northwestern canada reconstructed co 2 concentra tions are mainly between 700 to 1000 ppm but include a secular decline to 450 ppm towards the top of the investigated section because the co 2 threshold for nucleating continental ice sheets at this time was 500 to 750 ppm the co 2 decline is compatible with a rapid 10 4 yrs transition from warm largely ice free conditions to cooler climates with ice sheets these fossils provide direct evidence that high latitude deciduous forests thrived in the geological past under co 2 concentrations that will likely be reached within the 21 st century 500 1000 ppm