reduction in northward incursions of the south asian monsoon since 1400 ad inferred from a mt everest ice core 1 a highly resolved mt everest ice core reveals a decrease in marine and increase in continental air masses related to relatively high summer surface pressure over mongolia and reduction in northward incursions of the summer south asian monsoon since 1400 ad previously published proxy records from lower sites south of the himalayas indicate strengthening of the monsoon since this time these regional differences are consistent with a south north seesaw in convective activity in the asian monsoon region and reflect a southward shift in the mean summer position of the monsoon trough since 1400 ad the change in monsoonal circulation at 1400 ad is synchronous with a reduction in solar irradiance and the onset of the lia this demonstrates a hemispheric scale circulation reorganization at this time and the potential for future large shifts in monsoonal circulation

chemical and isotopic characterization of water rock interactions in shales induced by the intrusion of a basaltic dike a natural analogue for radioactive waste disposal abstract disposal of nuclear waste in deep geological formations is expected to induce thermal fluxes for hundreds of years with maximum temperature reaching about 100 150 c in the nearfield argillaceous environment the long term behavior of clays subjected to such thermal gradients needs to be perfectly understood in safety assessment considerations in this respect a toarcian argillaceous unit thermally disturbed by the intrusion of a 1 1 m wide basaltic dike at the perthus pass herault france was studied in detail as a natural analogue the thermal imprint induced by the dike was evaluated by a mineralogical chemical and k ar study of the at the eastern contact with the dike the mineral and chemical compositions of both the shales and the basalt suggest water rock interactions subsequent to the intrusion with precipitation of palagonite and renewed but discrete deposition of carbonate a pencil cleavage developed in the shales during the dike emplacement probably favored water circulation along the contact strontium isotopic data suggest that the fluids of probable meteoric origin reacted with bathonian and bajocian limestones before entering the underlying toarcian shales by analogy with deep geological radioactive waste repositories the results report discrete mineralogical variations of the clays when subjected to temperatures of 100 150 c that are expected in deep storage conditions beyond 150 c significant mineralogical changes may alter the physical and chemical properties of the shales especially of the clay fraction also the development of structural discontinuities in the so called thermally disturbed zone might be of importance as these discontinuities might become zones for preferential fluid circulation finally the study emphasizes the use of rb sr and k ar isotopic systems as tracers of local circulating fluids related to low grade thermal imprints

the indian monsoon and its variability for over 300 years the monsoon has been viewed as a gigantic land sea breeze it is shown in this paper that satellite and conventional observations support an alternative hypothesis which considers the monsoon as a manifestation of seasonal migration of the intertropical convergence zone itcz with the focus on the indian monsoon the mean seasonal pattern is described and why it is difficult to simulate it is discussed some facets of the intraseasonal variation such as active weak cycles break monsoon and a special feature of intraseasonal variation over the region namely poleward propagations of the itcz at intervals of 2 6 weeks are considered vertical moist stability is shown to be a key parameter in the variation of monthly convection over ocean and land as well as poleward propagations special features of the bay of bengal and the monsoon brought out by observations during a national observational experiment in 1999 are briefly described

the continental margin of central east greenland in relation to north atlantic plate tectonic evolution the east greenland continental margin is inferred to have formed in three stages during the tertiary evolution of the adjacent north atlantic ocean the east greenland basaltic volcanism occurred over a period of 2 3 ma which was just prior to the probable date of onset of seafloor spreading in the early eocene shortly before magnetic anomaly 24b 52 ma the associated coastal flexure is interpreted as marking the edge of a zone of stretched and thinned continental crust beneath the inner denmark strait shelf and jan mayen microcontinent this represents a short lived rifting phase just prior to break up according to the evolutionary model the initial break up produced the margins south of the faeroe transform fault 68 5 n and north of the jan mayen fracture zone 72 n but in the intervening region the microcontinent remained attached to east greenland after break up the outer shelf of denmark strait formed from anomalously thick oceanic crust complementary to the iceland faeroe ridge the blosseville kyst margin 68 5 n to 70 n was initiated after anomaly 20 time 42 ma as the jan mayen microcontinent started to separate from greenland with spreading on both sides the new split occupied the line of the pre existing coastal dyke swarm and flexure the liverpool land margin 70 n to 72 n was also initiated at this time but the main separation of the northern part of the microcontinent only commenced after anomaly 7 time 26 ma when a single spreading axis was relocated west of the microcontinent it is suggested that the blosseville lavas subsided during deposition by loading and slight crustal stretching doming occurred near kangerdlugssuaq at the time of break up otherwise the topography of the lavas remained subdued until the relocation of the spreading axis at 26 ma after which the main regional uplift took place

grain size dependence of the magnetic behavior of pyrrhotite during its low temperature transition at 34 k the grain size dependent behavior of pyrrhotite during a magnetic lt transition at 34 k is suitable for granulometric purposes cycling down to liquid helium temperature suppresses soft md jrs more than hard sd jrs so that it may be effective in removing soft secondary nrm components the domain structure of large pyrrhotite grains changes from md at room temperature to sd or psd below the lt transition

pyrrhotite ptrm acquisition in metamorphic limestones in the light of microscopic observations abstract the potential of pyrrhotite as a recorder of successive independent ptrms in graphite bearing metacarbonates depends on the amount and composition of metamorphic fluids during its formation very low amounts of h2s dominated fluids will favour an in situ epitaxy of pyrrhotite at the expense of iron bearing minerals in its surroundings the resulting particle assemblage of pyrrhotite is within the single domain grain size range and exhibits a broad blocking temperature spectrum with increasing fluid content the desulfidation of pyrite controls the pyrrhotite formation and results in large crystals up to several millimeters where independent ptrms cannot be expected as both types are present in regional and contact metamorphic rocks the existence of pervasive fluids determines whether or not pyrrhotite will be able to record successive ptrms i e changes of the earth magnetic field emf during metamorphic cooling

source rock facies distribution predicted from oil geochemistry in the central sumatra basin indonesia the molecular and compound specific isotopic composition of the n alkanes in 30 oil samples from the central sumatra basin were analyzed with the purpose of determining facies variations of the source rock in this basin the oil samples were generated by the eocene oligocene lacustrine brown shale formation biomarkers in the oil samples indicate varying contributions from algal and terrigenous organic matter primarily under oxic or suboxic depositional conditions five source rock facies were identified based on the relative concentrations of n alkanes isoprenoids tricyclic terpanes and steranes the individual facies were also distinguished by the compound specific isotopic composition of the n alkanes source rock depositional environments vary from deep lacustrine shallow lacustrine and lacustrine saline to coal and mixed coal and lacustrine shale deep lacustrine facies predominate in the aman and bengkalis troughs whereas saline and shallow lacustrine facies predominate in the south aman and kiri troughs respectively oils derived from coal and mixed coal and lacustrine shale facies predominate in the northern part of the central sumatra basin in the tanjung medan and north of the balam grabens

the late diagenetic conversion of pyrite to magnetite by organically complexed ferric iron abstract the geochemistry of late stage diagenetic replacement of pyrite by magnetite was investigated by laboratory experiments of pyrite dissolution and magnetite precipitation involving organically complexed ferric iron the design of the experiments is based on the hypothesized geochemical conditions for in situ diagenesis in the pennsylvanian age belden formation an organic rich carbonate in northwestern colorado with pyrite grains rimmed with magnetite that contains a cretaceous paleopole position the geochemistry is applicable to both deep and near surface reactions involving pyrite magnetite and organic matter and bears upon the geologic interpretation of secondary chemical remanent magnetizations crm aqueous solutions of ferric ligand complexes and pyrite were heated 60 c for 60 days in the absence of light and at low partial pressures of oxygen additional experiments with each ligand were performed both in the presence of bentonite and without ferric iron ligand only raising the ph to 8 5 produced a ferrous hydroxide precipitate and subsequent heating 90 c resulted in the formation of magnetite ferric complexes of oxalate salicylate and acetylacetonate resulted in pyrite dissolution and magnetite formation the activity of strongly reducing ligands e g catechol or unsaturated fe iii exchange capacity particularly in experiments with bentonite or with solutions of uncomplexed ligands can result in an effective reduction of ferric iron concentration and prevent pyrite dissolution and or interfere with the crystallization of magnetite the bentonite acted to adsorb dissolved species and also underwent direct dissolution especially by the oxalate ligands although these reactions could occur at any depth a continuing supply of molecular oxygen more available in near surface settings would result in the formation of hematite or the production of acidic sulfate conditions whereas at greater depths or restricted porosity the redox gradation is halted at magnetite the results of this study thus provide a likely geochemical mechanism for the replacement of pyrite by magnetite during the latter stages of diagenesis

searching for single domain magnetite in the pseudo single domain sedimentary haystack implications of biogenic magnetite preservation for sediment magnetism and relative paleointensity determinations 1 magnetic hysteresis measurements of sediments have resulted in widespread reporting of pseudo single domain like magnetic properties in contrast the ideal single domain sd properties that would be expected to be responsible for high quality paleomagnetic records are rare determining whether sd particles are rare or common in sediments requires application of techniques that enable discrimination among different magnetic components in a sediment we apply a range of such techniques and find that sd particles are much more common than has been reported in the literature and that magnetite magnetofossils the inorganic remains of magnetotactic bacteria are widely preserved at depth in a range of sediment types including biogenic pelagic carbonates lacustrine and marine clays and possibly even in glaci marine sediments thus instead of being rarely preserved in the geological record we find that magnetofossils are widespread this observation has important implications for our understanding of how sediments become magnetized and highlights the need to develop a more robust basis for understanding how biogenic magnetite contributes to the magnetization of sediments magnetofossils also have grain sizes that are substantially smaller than the 1 15 m size range for which there is reasonable empirical support for relative paleointensity studies the different magnetic response of coexisting fine biogenic and coarser lithogenic particles is likely to complicate relative paleointensity studies this issue needs much closer attention despite the fact that sediments have been subjected to paleomagnetic investigation for over 60 years much remains to be understood about how they become magnetized

non residual sr of the sediments in daihai lake as a good indicator of chemical weathering abstract the rb sr ratio of lake sediments has been demonstrated to be a potential indicator of chemical weathering by increasing work however rb and sr in lake sediments are derived from both chemical weathering and physical erosion rb and sr of different forms in lake sediments may record different environmental processes and information in this study the variation patterns of rb and sr of different forms in sediments of daihai lake were investigated the results show that rb and sr of different forms display noticeably different variation patterns due to their different sources and associated environmental processes using the rb sr ratios of bulk lake sediments to reflect chemical weathering is not accurate the non residual sr of the sediments without detrital carbonates representing the sr leached from the catchment can be used as an index of chemical weathering because the weathering of sr minerals is very sensitive to climate change and the non residual sr content of the sediment is determined more by sr2 influx than by the physicochemical conditions of lake water the correspondence between the non residual sr and tic toc in daihai lake also indicates that the non residual sr of the sediments is a good indicator of chemical weathering in the catchment

productivity and paleoclimatic controls on source rock character in the aman trough north central sumatra indonesia abstract the central sumatra basin csb is a prolific oil producing basin in indonesia where a single source rock has been identified the brown shale formation of the paleogene pematang group despite the presence of a single source rock the oils in the central sumatra basin and particularly in the aman trough exhibit variable molecular and isotopic compositions previously described as reflective of lateral facies variations within the source rock analyses by gas chromatography gc gas chromatography mass spectrometry gc ms and gas chromatography isotope ratio mass spectrometry gc irms of 15 oil samples from the aman trough allowed identification of vertical and lateral facies changes within the brown shale formation as well as paleoproductivity and paleoclimatic changes that took place during deposition of this prolific source rock the molecular and isotopic data indicate the source rock for these oils was deposited in a fresh to brackish water stratified lake in which co 2 limiting conditions prevailed compound specific isotopic data also indicate that changes in paleoclimatic conditions possibly associated with the eocene oligocene paleoclimatic transition had an effect on the geochemical composition of the source rock in the aman trough

tibetan plateau summer monsoon northward extent revealed by measurements of water stable isotopes a program of individual precipitation events and river water sampling and of water isotopic measurements d 18o was carried out during summer 1996 along a northeast southwest transect of the tibetan plateau the spatial distribution of both 18o and deuterium excess d d 8 18o of the precipitation reveals three distinct regions simulations with a simple isotopic model and seasonal isotopic variations measured at two extreme south and north locations support our interpretation in terms of different summer moisture origins 1 south of the himalayan mountains the moisture provided by the indian monsoon has been recycled over the indian peninsula 2 between the himalayas and the tanggula mountains the oceanic moisture is directly transported from the bay of bengal along the brahmaptra river valley 3 north of the tanggula mountains the moisture is not provided by the monsoon anymore but by continental water recycling

holocene climate variability archived in the puruogangri ice cap on the central tibetan plateau two ice cores 118 4 and 214 7 m in length were collected in 2000 from the puruogangri ice cap in the center of the tibetan plateau tp in a joint us chinese collaborative project these cores yield paleoclimatic and environmental records extending through the middle holocene and complement previous ice core histories from the dunde and guliya ice caps in northeast and northwest tibet respectively and dasuopu glacier in the himalaya the high resolution puruogangri climate record since ad 1600 details regional temperature and moisture variability the post 1920 period is characterized by above average annual net balance contemporaneous with the greatest 18 o enrichment of the last 400 years consistent with the isotopically inferred warming observed in other tp ice core records on longer timescales the aerosol history reveals large and abrupt events one of which is dated 4 7 kyr bp and occurs close to the time of a drought that extended throughout the tropics and may have been associated with centuries long weakening of the asian indian african monsoon system the puruogangri climate history combined with the other tp ice core records has the potential to provide valuable information on variations in the strength of the monsoon across the tp during the holocene

palaeo moisture evolution in monsoonal central asia during the last 50 000 years abstract the late quaternary climate history of monsoonal central asia was inferred from 75 palaeoclimatic records which provide information about moisture conditions in the last 50 ka or part of this period wet conditions occurred during middle and late marine isotope stage 3 while the last glacial maximum lgm was characterized by dry climate conditions in the region a stepwise climate amelioration is suggested by the climate records following the lgm several climate signals of this period which were reported from high latitude ice core records are preserved in archives from monsoonal central asia as well during the early holocene high effective moisture was inferred from most records from the area dominated by the indian monsoon e g the tibetan plateau suggesting that holocene optimal climate conditions occurred there during this period in contrast areas which are dominated by the south east asian monsoon se monsoon and the westerlies in north western and north central china mongolia do not uniformly show an early holocene climate optimum for this area optimal conditions prevailed during the mid holocene these apparent contradictions can possibly be explained by the regional uplift and descent of air masses in the holocene during the early holocene strengthened insolation possibly caused an enhanced low level convergence over the tibetan plateau which led to the intensification of the summer monsoon the strong air uplift caused intensified precipitation and air divergence in the upper troposphere over the tibetan plateau the areas adjacent to the north therefore experienced an intensified descent of air masses and consequently increased aridity the majority of the palaeoclimatic records suggest reduced effective moisture since the late holocene in the region

applications of organic geochemistry to paleolimnological reconstructions a summary of examples from the laurentian great lakes the organic matter content of lake sediments contains information that helps to reconstruct past environmental conditions evaluate histories of climate change and assess impacts of humans on local ecosystems the elemental isotopic and molecular compositions of organic matter buried in sediment provide evidence of the biota that have lived in a lake and its catchment area and they serve as proxies of organic matter delivery and accumulation sedimentary records from the north american great lakes provide examples of applications of organic geochemistry to paleolimnological reconstructions the records of these lakes date from retreat of the laurentide ice sheet around 12 ka include the mid holocene hypsithermal and show consequences of recent human changes low corg ntotal ratios indicate that most of the sediment organic matter in the great lakes is from algal production yet changes in biomarker molecule compositions also show that varying amounts of land plant organic matter have been delivered to the lakes elevated algal productivity that accompanies nutrient enrichments of lake waters is recorded as excursions to less negative 13c values in the organic matter of sediments that were deposited in the 1960s and 1970s increased organic carbon mass accumulation rates mirror the isotopic excursions in most parts of the great lakes accumulations of petroleum residues and pyrogenic polycyclic aromatic hydrocarbons in sediments identify fluvial and eolian delivery of organic matter components to different parts of the great lakes emerging applications of multiple and novel organic geochemical proxies to paleolimnological reconstructions are promising yet some potentially important measurements remain underutilized

detecting the thermal aureole of a magmatic intrusion in immature to mature sediments a case study in the east greenland basin 73 n the cretaceous and triassic argillaceous rocks from the passive margin of greenland have been investigated in order to detect the thermal aureole of magmatic intrusions ranging from metric dyke to kilometric syenite pluton rock eval data tmax generally 0 3 all indicate a maximum of 5km burial for the argillaceous rocks whatever the distance to an intrusion the k ar dating of the clays 2 m fraction suggests that illites are mostly detrital except near magmatic intrusions where younger ages are recorded to get more information about the extent of the thermal aureole rock magnetism data were determined at distance away from the thermal aureole of the syenite intrusion triassic argillaceous rocks reveal a standard magnetic assemblage compatible with their burial r 0 4 percent it is constituted essentially by neoformed stoichiometric magnetite fe o in contrast within the thermal aureole of the magmatic intrusions the cretaceous argillaceous rocks contain micron sized pyrrhotite fe s firmly identified through the recognition of besnus transition at 35k the thermal demagnetization of natural remanence carried by this pyrrhotite shows a diagnostic square shouldered pattern indicating a narrow grain size distribution of pyrrhotite the extension of this diagnostic pyrrhotite maps a 10 km thick aureole around the syenitic pluton away from this aureole the magnetic assemblage is diagnostic of those found in argillaceous rocks where organic matter is mature

a comparison of different methods for determining the organic and inorganic carbon content of lake sediment from two lakes on the tibetan plateau this paper presents three parallel datasets of total organic carbon toc and total inorganic carbon tic as well as carbonate content caco3 of lake sediment from two lakes on the tibetan plateau in order to compare different methods and different machines the results indicate that two carbon analyzers a shimadzu toc vcph with solid sample module ssm 5000a and an elementar vario el ii provide very similar toc and tic content despite slightly different performance procedures and temperature controls a toc record determined by a wet oxidation method and a carbon analyzer shows consistent down core variation although it seems that the wet oxidation method may overestimate toc content due to the existence of some reduced compounds or minerals in the sediment tic content measured by a carbon analyzer and carbonate content determined by a chemical method exhibit similar fluctuation trends in both cores this suggests that tic content may be used as an environmental proxy roughly taking the place of carbonate content especially in sediment with simple carbonate compositions

spatial distribution and controlling factors of stable isotopes in meteoric waters on the tibetan plateau implications for paleoelevation reconstruction abstract debates persist about the interpretations of stable isotope based proxies for the surface uplift of the central northern tibetan plateau these disputes arise from the uncertain relationship between elevation and the 18 o values of meteoric waters based on modern patterns of isotopes in precipitation and surface waters we present a large river water data set 1 340 samples covering most parts of the tibetan plateau to characterize the spatial variability and controlling factors of their isotopic compositions compared with the amount weighted mean annual oxygen isotopic values of precipitation we conclude that river water is a good substitute for isotopic studies of precipitation in the high flat e g elevation 3 300 m interior of the tibetan plateau in the mean annual timescale we construct for the first time based on field data contour maps of isotopic variations of meteoric waters 18 o d and d excess on the tibetan plateau in the marginal mountainous regions of the plateau especially the southern through eastern margins the 18 o and d values of river waters decrease with increasing mean catchment elevation which can be modeled as a rayleigh distillation process however in the interior of the plateau northward increasing trends in 18 o and d values are pronounced and present robust linear relations d excess values are lower than the marginal regions and exhibit distinct contrasts between the eastern 8 12 and western

oxygen isotopes versus climap 18 ka temperatures a comparison from the tropical atlantic glacial interglacial differences in 18 o values of globigerinoides sacculifer have been mapped in the western and central tropical atlantic using data from surface and core sediments glacial interglacial differences in the western tropical atlantic if attributed to temperature changes only are at odds with temperature reconstructions performed by climap in the ceara rise region delta 18 o values exceeding 2 are recorded they can only be partly attributed to changes in the precipitation evaporation regime i e higher salinities and 18 o enriched precipitation an additional cooling of 1 2 c as compared to climap9s results is required to balance the 18 o record thus temperature changes from glacial to modern times amount to 2 3 c in the tropical atlantic off the northern coast of brazil

the transition from the continent to the ocean a deeper view on the norwegian margin we present a regional crustal scale 3d structural model of the norwegian continental margin integrating sedimentary and crustal layers from the continental and the oceanic domain the model includes six sedimentary units underlain on the continental side by a thinned crystalline crust and a lower crustal high velocity body in the oceanic domain three crustal layers 2ab 3a and 3b thickened at the continent ocean transition cot are modelled below the post breakup deposits two major rift phases with different rift axes late jurassic early cretaceous and late cretaceous early tertiary have caused post jurassic subsidence and post depositional deformation of the pre cretaceous units the modelled cot suggests that the pre breakup rifting event was related to differential stretching focused at the outer margin and that breakup took place in a base up magmatic process as a continuation of underplating for the earlier rift event stretching was distributed over the entire margin and led to accumulation of up to 12 km of cretaceous deposits the large sediment thickness of the cretaceous units requires deep water conditions and abundant sediment supply and thus coeval offshore subsidence and onshore uplift all layers indicate a sinistral offset along the jan mayen fracture zone and its continentward continuation

quaternary ecological responses and impacts of the indian ocean summer monsoon at nam co southern tibetan plateau the transition from the last glacial to the current interglacial the holocene represents an important period with climatic and environmental changes impacting ecosystems in this study we examined the interplay between the indian ocean summer monsoon iosm and the westerlies at lake nam co southern tibet to understand the climatic effects on the ecosystem different organic geochemical proxies n alkanes glycerol dialkyl glycerol tetraethers d 13corg 15n are applied to reconstruct the environmental and hydrological changes on one of the longest available paleorecords at the tibetan plateau based on our paleohydrological d proxies the aquatic signal lags the terrestrial one due to specific ecological thresholds which in addition to climatic changes can influence aquatic organisms the aquatic organisms response strongly depends on temperature and associated lake size as well as ph and nutrient availability because the terrestrial vegetation reacts faster and more sensitively to changes in the monsoonal and climatic system the d of n c29 and the reconstructed inflow water signal represent an appropriate iosm proxy in general the interplay of the different air masses seems to be primarily controlled by solar insolation in the holocene the high insolation generates a large land ocean pressure gradient associated with strong monsoonal winds and the strongest iosm in the last glacial period however the weak insolation promoted the westerlies thereby increasing their influence at the tibetan plateau our results help to elucidate the variable iosm and they illustrate a remarkable shift in the lake system regarding ph 13corg and 15n from the last glacial to the holocene interglacial period

monsoon source shifts during the drying mid holocene biomarker isotope based evidence from the core monsoon zone cmz of india abstract a better understanding of past variations of the indian summer monsoon ism that plays a vital role for the still largely agro based economy in india can lead to a better assessment of its potential impact under global climate change scenarios however our knowledge of spatiotemporal patterns of ism strength is limited due to the lack of high resolution continental paleohydrological records here we reconstruct centennial scale hydrological variability during the holocene associated to changes in the intensity of the ism based on a record of lipid biomarker abundances and compound specific stable isotopic composition of a 10 m long sediment core from saline alkaline lonar lake situated in the core monsoon zone of central india we identified three main periods of distinct hydrology during the holocene in central india the period between 10 1 and 6 cal ka bp was likely the wettest during the holocene lower average chain length acl index values 29 4 28 6 and negative 13 c wax values 34 8 to 27 8 of leaf wax n alkanes indicate the dominance of woody c 3 vegetation in the catchment and negative d wax values concentration weighted average 171 to 147 suggest a wet period due to an intensified monsoon after 6 cal ka bp a gradual shift to less negative 13 c wax values particularly for the grass derived n c 31 and appearance of the triterpene lipid tetrahymanol generally considered as a marker for salinity and water column stratification mark the onset of drier conditions at 5 1 cal ka bp an increasing flux of leaf wax n alkanes along with the highest flux of tetrahymanol indicate a major lowering of the lake level between 4 8 and 4 cal ka bp we find evidence for a transition to arid conditions indicated by high and strongly variable tetrahymanol flux in addition a pronounced shift to less negative 13 c wax values in particular for n c 31 25 2 to 22 8 during this period indicates a change of dominant vegetation to c 4 grasses in agreement with other proxy data such as deposition of evaporite minerals we interpret this period to reflect the driest conditions in the region during the last 10 1 ka this transition led to protracted late holocene arid conditions after 4 ka with the presence of a permanent saline lake supported by the sustained presence of tetrahymanol and more positive average d wax values 122 to 141 a late holocene peak of cyanobacterial biomarker input at 1 3 cal ka bp might represent an event of lake eutrophication possibly due to human impact and the onset of cattle livestock farming in the catchment a unique feature of our record is the presence of a distinct transitional period between 4 8 and 4 cal ka bp which was characterized by some of the most negative d wax values during the holocene up to 180 when all other proxy data indicate the driest conditions during the holocene these negative d wax values can as such most reasonably be explained by a shift in moisture source area and or pathways or rainfall seasonality during this transitional period we hypothesize that orbital induced weakening of the summer solar insolation and associated reorganization of the general atmospheric circulation as a possible southward displacement of the tropical rainbelt led to an unstable hydroclimate in central india between 4 8 and 4 ka our findings shed light onto the sequence of changes during mean state changes of the monsoonal system once an insolation driven threshold has been passed and show that small changes in solar insolation can be associated with major hydroclimate changes on the continents a scenario that may be relevant with respect to future changes in the ism system

dominant control of the south asian monsoon by orographic insulation versus plateau heating heat emitted from the tibetan plateau as dry heat and water vapour has long been assumed to be the main driver of the south asian summer monsoon but new work suggests that in fact it is the neighbouring mountains that are the major influence william boos and zhiming kuang use an atmospheric model to show that flattening the tibetan plateau has little effect on the monsoon so long as the himalayas and surrounding mountain ranges remain the plateau does boost rainfall locally along its southern edge but it is the build up of hot moist air over india insulated from colder drier air by the himalayas that drives large scale monsoon circulation the elevation of the tibetan plateau is thought to cause its surface to serve as a heat source that drives the south asian summer monsoon potentially coupling uplift of the plateau to climate changes on geologic timescales here however an atmospheric model is used to show that flattening of the tibetan plateau has little effect on the monsoon provided that the narrow orography of the himalayas and adjacent mountain ranges is preserved the tibetan plateau like any landmass emits energy into the atmosphere in the form of dry heat and water vapour but its mean surface elevation is more than 5 km above sea level this elevation is widely held to cause the plateau to serve as a heat source that drives the south asian summer monsoon potentially coupling uplift of the plateau to climate changes on geologic timescales1 2 3 4 5 observations of the present climate however do not clearly establish the tibetan plateau as the dominant thermal forcing in the region peak upper tropospheric temperatures during boreal summer are located over continental india south of the plateau here we show that although tibetan plateau heating locally enhances rainfall along its southern edge in an atmospheric model the large scale south asian summer monsoon circulation is otherwise unaffected by removal of the plateau provided that the narrow orography of the himalayas and adjacent mountain ranges is preserved additional observational and model results suggest that these mountains produce a strong monsoon by insulating warm moist air over continental india from the cold and dry extratropics these results call for both a reinterpretation of how south asian climate may have responded to orographic uplift and a re evaluation of how this climate may respond to modified land surface and radiative forcings in coming decades

a synthesis of abrupt changes in the asian summer monsoon since the last deglaciation we have compiled 36 previously published palaeoclimate records to determine the timing and spatial pattern of century scale abrupt changes in asian monsoon precipitation since the last deglaciation we identify abrupt events from 1 the interpretations of the authors of these records and 2 the more objective moving t test calculation our results indicate that abrupt climatic changes occurred at 11 5 cal ka 4 5 5 0 cal ka and ad 1300 at the start of the holocene 11 5 cal ka asian monsoon precipitation increased dramatically this climatic change is synchronous with an abrupt warming in the north atlantic during the middle holocene there was a time of preferred and widespread weakening in monsoon strength 4 5 5 0 cal ka this result contradicts previous notions of either a gradual trend towards drier conditions or a series of abrupt events that occurred in an unorganized fashion across space and time the middle holocene abrupt event could have been synchronous with an abrupt cooling event in the north atlantic as well as a warming and intensie cation of internannual variability in the tropical pacie c in contrast to previous periods precipitation changes at ad 1300 have a heterogeneous spatial pattern we e nd no conclusive evidence for a change in the asian monsoon at 8 2 cal ka as suggested by several previous studies more high resolution data may be needed to observe this short lived event overall our results attest to the potential for rapid and major shifts in asian monsoon precipitation that may be triggered by variations in other components of the climatic system

long term glacier melt fluctuations over the past 2500 yr in monsoonal high asia revealed by radiocarbon dated lacustrine pollen concentrates long term records of glacier mass changes are important for improving our understanding of glacier dynamics and for predicting the response of glaciers to future climate change in contrast to moraine sequences that only record isolated stages of glacier status proglacial lake sediments may record long term continuous glacier activities the melt of old glacier ice releases old pollen that may affect the radiocarbon ages of pollen in proglacial lake sediments we define the offset between the calibrated pollen 14 c ages and the sediment depositional age as the old pollen effect ope in small catchments dominated by glaciers the ope may record variations in glacier melt intensity and extent even though complex processes e g modern pollen flux to a glacier or a proglacial lake glacier flow velocities may also impact the ope using the sediments of a small proglacial lake on the southern tibetan plateau we found that over the past 2 5 k y a weakened ope occurred during three historical cool periods that coincided with regional glacier advances defined by moraine ages thus we interpret the ope as a new indicator of glacier melt intensity and its fluctuations our reconstructed glacier variability agrees well with glacier fluctuations in the european alps and the global average temperature record suggesting that hemispheric scale temperature variations and or mid latitude westerlies may have controlled the late holocene glacier variability in monsoonal high asia we also show that the 20 th century glacier melt intensity has exceeded that of two historical warm periods and is unprecedented over the past 2 5 k y this implies that current anthropogenic warming poses a serious threat to the survival of glaciers in monsoonal high asia

sedimentary response to mantle plumes implications from paleocene onshore successions west and east greenland the uplift history of the upper cretaceous paleocene sedimentary successions underneath the extensive tertiary flood basalts of west and east greenland supports recently described models for the generation of flood basalt provinces in response to rising hot mantle plumes however the sedimentary development indicates that the period of plume related uplift prior to eruption of volcanic rocks was very short 5 m y substantial regional uplift and faulting took place in west and east greenland in the early paleocene to earliest late paleocene resulting in a basinwide unconformity in west greenland the uplift was associated with fluvial peneplanation valley incision and catastrophic deposition in east greenland it was associated with fluvial peneplanation the cumulative uplift in west greenland is probably on the order of several hundred meters whereas in east greenland the observed uplift cannot be quantified in both west and east greenland this phase was shortly followed by rapid major subsidence associated with extensive volcanism it is suggested that this sequence of events reflects the arrival of the north atlantic mantle plume or plumes

rapid northward shift of the indian monsoon on the tibetan plateau at the end of the little ice age variations in the indian monsoon im and westerlies ws significantly affect the climate on the tibetan plateau tp and have widespread ecological and socioeconomic impacts on the whole of asian society so far however the rate and magnitude of changes in the im have still remained unclear here we report for the first time that the im rapidly shifted northward at the end of the little ice age lia we used sediment proxies for humidity and moisture sources from the taro co lake which is located in the transition zone between the ws and im our comprehensive survey of climate records for the tp and its peripheral mountain ranges revealed that the northern boundary of the im i e the southern boundary of the ws lay along the southern slope of the gandise range 29 5 n in the late lia in contrast it passed quickly over the gandise range by at least 1 5 in latitude at the end of the lia our results suggest that this rapid climatic shift was potentially triggered by the counteracting effects of blocking by the tp and its marginal orography which hindered the northward movement of the im and the pulling thermal gradient of the tp

authigenesis of magnetite in organic rich sediment next to a dike implications for thermoviscous and chemical remagnetizations rock magnetic results from organic rich sediment around a tertiary dike in scotland indicate changes in magnetic mineralogy and magnetite content as well as grain size in the contact zone sediment specimens within 85 cm of a 90 cm thick dike contain a tertiary magnetization that is equivalent in direction to the thermal remanent magnetization in the dike whereas the sediment at greater distances does not carry a stable remanence based on acquisition of isothermal remanent magnetization irm thermal decay of triaxial irms and s ratio determinations magnetite is the dominant magnetic phase in the sediment except near the dike 8 cm where hematite is abundant maximum laboratory unblocking temperatures tbmax for the magnetization are higher than predicted by a thermal cooling model at distances greater than 40 cm but the tbmax values are not corrected for gradual cooling of the dike and are therefore overestimates trends in rock magnetic parameters such as irm at 300 mt and anhysteretic remanence arm are consistent with an increasing contribution of single domain pseudosingle domain magnetite towards the dike an increase in saturation irm sirm along with a decrease in arm sirm values 25 cm from the dike also suggest an increase in multidomain magnetite low temperature experiments indicate the presence of abundant superparamagnetic magnetite changes in hysteresis parameters are consistent with changes in the relative amounts of the different magnetite size fractions towards the dike the onset of changes in the magnetite grain size budget starting at 85 cm or approximately one dike width suggests that chemical processes contributed to the remanence at temperatures as low as 220 c petrographic and geochemical studies suggest that the contact zone was not pervasively altered by hydrothermal fluids utilizing the dike as an analog for thermal processes that occur during burial the results of this study suggest that moderately elevated burial temperatures are sufficient to cause magneto chemical changes

magnetic properties of sedimentary greigite fe3s4 an update greigite fe3s4 is an authigenic ferrimagnetic mineral that grows as a precursor to pyrite during early diagenetic sedimentary sulfate reduction it can also grow at any time when dissolved iron and sulfide are available during diagenesis greigite is important in paleomagnetic environmental biological biogeochemical tectonic and industrial processes much recent progress has been made in understanding its magnetic properties greigite is an inverse spinel and a collinear ferrimagnet with antiferromagnetic coupling between iron in octahedral and tetrahedral sites the crystallographic c axis is the easy axis of magnetization with magnetic properties dominated by magnetocrystalline anisotropy robust empirical estimates of the saturation magnetization anisotropy constant and exchange constant for greigite have been obtained recently for the first time and the first robust estimate of the low field magnetic susceptibility is reported here the curie temperature of greigite remains unknown but must exceed 350 c greigite lacks a low temperature magnetic transition on the basis of preliminary micromagnetic modeling the size range for stable single domain behavior is 17 200 nm for cubic crystals and 17 500 nm for octahedral crystals gradual variation in magnetic properties is observed through the pseudo single domain size range we systematically document the known magnetic properties of greigite at high ambient and low temperatures and with alternating and direct fields and illustrate how grain size variations affect magnetic properties recognition of this range of magnetic properties will aid identification and constrain interpretation of magnetic signals carried by greigite which is increasingly proving to be environmentally important and responsible for complex paleomagnetic records including widespread remagnetizations

magnetism in rocks rock magnetism is the study of induced and remanent magnetization of ferrimagnetic mineral grains in rocks sediments soils and organisms its applications include environmental magnetism magnetic anisotropy sources of continental and oceanic magnetic anomalies records of geomagnetic field variations and polarity reversals and the paleomagnetic record of plate motions and the wilson cycle this paper reviews the beginnings of rock magnetism and then traces the development of six particularly interesting areas pseudo single domain behavior magnetic domains and micromagnetic structures diagnostic tests of the type and stability of remanent magnetization magnetic microanalysis thermoviscous remagnetization and chemical remanent magnetization other areas including sediment and soil magnetism are covered in a companion paper by verosub and roberts on environmental magnetism

alteration of a basaltic glass in an argillaceous medium the salagou dike of the lodeve permian basin france analogy with an underground nuclear waste repository abstract volcanic basaltic glasses are commonly considered to be very suitable natural analogs of vitrified waste packages intended for geological disposal the studied basaltic glass of the salagou dike lodeve basin france intruded into a permian argillaceous formation about 1 4 ma ago provides a means of assessing the long term behavior of nuclear waste glass in an argillaceous repository concept this study is based on combining chemical mineralogical and isotopic investigations the occurrence of a palagonite film no more than 1 2 mm thick characterizes the basaltic glass alteration in contact with the argillaceous host rock the chemical and structural properties of the palagonite suggest constant volume alteration the alteration rates estimated from palagonite thickness and age of the glass are comparable to those measured on natural glasses altered in nonargillaceous media the occurrence of the studied argillaceous material in contact with the basaltic glass did not result in long term alteration kinetics significantly different from those measured in simple glass water systems mineralogical chemical and isotopic data obtained on the basaltic and argillaceous components suggest that an aqueous fluid flowed primarily at the glass argillite interface and did not propagate in the argillaceous host rock beyond 5 cm from the basaltic dike the elements released by alteration of the basaltic glass notably strontium did not diffuse into the surrounding clay this conclusion is important from the perspective of a natural analog for a deep geological repository for nuclear waste and highlights the major role of the structural properties of the clayey barrier

hydroclimatic changes in china and surroundings during the medieval climate anomaly and little ice age spatial patterns and possible mechanisms abstract investigating hydroclimatic changes during key periods such as the medieval climate anomaly mca 1000 1300 ad and the little ice age lia 1400 1900 ad is of fundamental importance for quantifying the responses of precipitation to greenhouse gas induced warming on regional and global scales this study synthesizes the most up to date and comprehensive proxy moisture precipitation records during the past 1000 years in china and surroundings the proxy data collected include 34 records from arid central asia aca and 37 records from monsoonal asia our results demonstrate a pattern of generally coherent regional moisture variations during the mca and lia in mid latitude asia north of 30 n monsoonal northern china north china and northeast china was generally wetter while aca northwest china and central asia was generally drier during the mca than in the lia a west east mode the boundary between wetter northern china and drier aca was roughly consistent with the modern summer monsoon boundary in monsoonal china to the east of 105 e the northern part was generally wetter while the southern part was generally drier during the mca than in the lia a north south mode with a boundary roughly along the huai river at about 34 n these spatial patterns of moisture precipitation variations are also identified by instrumental data during the past 50 years in order to understand the possible mechanisms related to the moisture variations during the mca and lia we investigate the major sst and atmospheric modes e g the el nino southern oscillation enso the atlantic multidecadal oscillation amo and the north atlantic oscillation nao which affect the moisture precipitation variations in the study region using both the instrumental data and the reconstructed time series it is found that the enso may play an important role in determining hydroclimatic variability over china and surroundings on a multi centennial time scale and that the foregoing spatial patterns could be attributed to the la nina like el nino like condition during the mca lia in addition amo and nao may also have their own contributions

the nonlinear response of the atmosphere to large scale mechanical and thermal forcing abstract the subject of large scale mountain waves is reviewed briefly existing mountain wave theory based on a linear system is shown to give an inadequate description of the balance of angular momentum the response of the atmosphere to mechanical forcing in a nonlinear framework is then discussed using a two level quasi geostrophic long wave spectral model based on spherical coordinates including diabatic heating surface friction and mountains the nonlinear theory shows that there exists a critical mountain height hc which is a function of the frictional coefficient as well as the phase difference between the mountain and the surface pressure field if and only if the mountain height is less than this critical value can the deflection effect of the mountain be neglected and the response regarded as approximately linear this critical mountain height is only about 1 km thus most of the atmospheric response to large scale mountains must be nonlinear in the nonlinear case as the mountain heigh

paleoclimate changes of the last 1000 yr on the eastern qinghai tibetan plateau recorded by elemental isotopic and molecular organic matter proxies in sediment from glacial lake ximencuo abstract total organic carbon corg and nitrogen ntot concentrations and isotope compositions and n alkane and n alkan ol molecular biomarker compositions were measured in two parallel sediment cores from lake ximencuo a typical glacial lake on the eastern qinghai tibetan plateau qtp to reconstruct local climatic variations during the past 1000 yr concentrations of corg and ntot vary similarly with changes in precipitation recorded by tree rings in dulan northeastern qtp indicating their close relation to precipitation carbon preference index cpi values of c22 c33 n alkanes and c22 c28 n alkan 1 ols and average chain length acl values of c27 c33 n alkanes generally negatively correlate with 13corg and 15ntot and also vary comparably with the oxygen isotope temperature record from the dunde ice core northeastern qtp these patterns indicate that high cpi values and negative shifts of the 13corg and 15ntot correspond to warmer conditions that favor the growth of vascular land plants and that lower values and positive shifts correspond to cooler conditions in the lake ximencuo locality this observation contradicts previous interpretations from peat and modern soil studies that conclude that the cpi indices were controlled by microbial degradation under different climate conditions here we propose that the changes of organic matter sources under different climate regimes combined with physiological adjustments of vascular plants to different air temperatures might yield different cpi responses to climate changes in glacial plateau lakes air temperature and precipitation increases and decreases reconstructed in this 1000 yr study were generally independent of each other but they both occurred at centennial or multi centennial scales the interaction between subtropical asian monsoon and mid latitude westerly atmospheric circulation systems probably dominated the local precipitation variations in the lake ximencuo catchment whereas solar insolation volcanism and greenhouse gas variations were likely responsible for the air temperature changes that are recorded in the lake sediments and that are consistent with regional temperature variations in the eastern qtp

the influence of 14c reservoir age on interpretation of paleolimnological records from the tibetan plateau there is a great deal of controversy regarding the fate of glaciers and ice fields on the tibetan plateau in the face of continued anthropogenic global warming paleoclimate reconstructions and spatial analyses aimed at mapping past climate changes are the key to understanding the climatic response of the tibetan plateau to changing conditions specifically the numerous lakes distributed across the tibetan plateau can provide high resolution spatial and temporal climate reconstructions to investigate past changes in the climate system in this paper we review the primary limitation to exploiting these valuable paleoclimate archives errors in radiocarbon based age models we review the techniques that have been used to estimate 14 c reservoir ages on the tibetan plateau and compile the published 14 c reservoir ages to examine their spatial and temporal patterns and to assess the imposed chronological uncertainties using site specific evaluations of bangong co and lake qinghai we demonstrate that 14 c age model uncertainties permit equally probable and contrasting interpretations of existing paleoclimate records we also examine 14 c induced uncertainties in the spatial climatic response on the tibetan plateau to 1 the termination of the last glacial maximum and 2 the holocene thermal maximum we conclude with recommendations for reducing uncertainties in future lake based paleoclimate studies on the tibetan plateau

towards pyrrhotite magnetite geothermometry in low grade metamorphic carbonates of the tethyan himalayas shiar khola central nepal abstract in mesozoic metacarbonates of the tethyan himalayas shiar khola area central nepal two characteristic remanent magnetisations chrm1 and chrm2 were identified by their unblocking temperature spectra the chrm1 is carried by pyrrhotite unblocking temperature 270 360 c and the chrm2 by magnetite unblocking temperature spectra 430 580 c the temperature related formation of pyrrhotite at the expense of primary magnetite during low grade metamorphism in marly carbonates allows the determination of thermal gradients by the pyrrhotite magnetite ratio this new method can be used as a geothermometer for t 300 c in low grade metamorphic carbonates where other methods are not available this method is applied for the first time in the tethyan himalayas of central nepal in the shiar khola valley systematic variations in the ferrimagnetic content of the metacarbonates along an e w profile were detected by the ratio of remanence intensity of pyrrhotite to magnetite derived from natural remanent magnetisation rpyr mag and saturation magnetisation spyr mag over a stretch of 10 km the rpyr mag and spyr mag increase from w to e from 0 42 to 0 91 and 0 48 to 1 0 respectively based on temperature estimates the eastern part experienced upper anchizone epizone 250 300 c conditions while the western part underwent only diagenesis 200 c the temperature gradient and the temperature ranges suggested are consistent with the findings of the calcite twin lamellae geothermometry which is a non magnetic method

δd values of n alkanes in tibetan lake sediments and aquatic macrophytes a surface sediment study and application to a 16 ka record from lake koucha abstract a set of lake surface sediment samples and aquatic macrophytes from the eastern and central tibetan plateau and a sediment core from koucha lake northeastern tibetan plateau were analysed for hydrogen isotope signals of aliphatic lipid biomarkers the d values of n alkanes were correlated with those of meteoric water in the summer months and showed increasing pearson correlation coefficients with increasing n alkane chain length r 0 80 for n c 31 while in samples from the most humid regions southeastern tibetan plateau the apparent enrichment factors between meteoric water and lipids are close to those reported for other humid sites e 135 for mid chain and 125 for long chain n alkanes they become smaller e 118 to 41 mean value 86 at sites with a negative moisture balance northeastern and central tibetan plateau and qaidam basin this might be caused by enrichment in deuterium of the source water used for lipid synthesis as a result of lake water evaporation and soil and leaf water evapotranspiration in the core from lake koucha decreasing d values of all n alkanes between 15 and 10 cal ka bp give evidence for an intensification of the asian summer monsoon at ca 9 cal ka bp a ca 15 shift towards higher d values of mid chain n alkanes suggests enhanced evaporation of the lake water which outweighs higher precipitation amounts because of a temperature increase during the early holocene monsoonal maximum only a few surface sediment samples show a significant offset between d values of mid chain n alkanes and n c 31 d mid n c31 and no systematic dependence between this offset and environmental parameters such as mean annual precipitation was found in the core available data for most samples also do not show a significant offset only a slight increase 15 of d mid n c31 at ca 10 0 cal ka bp and a decrease 17 at ca 6 8 cal ka bp might indicate wetter and drier conditions respectively however interpretation of d mid n c31 throughout the record is difficult because of a lack of reliable data for n c 31 in key sections of the core so more records are necessary to further test the potential applicability of d mid n c31 as a proxy for palaeo hydrological conditions

deep structures and breakup along volcanic rifted margins insights from integrated studies along the outer voring basin norway three dimensional multichannel seismic survey and well data allows a deeper insight into rifting mechanisms and crustal structure of the norwegian volcanic rifted margin a surprising high velocity lower crustal dome marked by a strong amplitude reflection t reflection is described below the north gjallar ridge ngr near the volcanic plateau formed during the late paleocene early eocene breakup faulting along the ngr occurs during early campanian paleocene and is controlled by the crustal dome the t reflection was previously related to either the top of mafic underplated crust or soft lower crust updomed during the continental breakup the influence and timing of the crustal dome on the basin structure shows clearly that the dome predates the continental breakup based on the structural relationship and the results of the modelling it is therefore concluded that the t reflection does not necessarily originate from an anomalous tertiary magmatic event related to the breakup underplated material but could be also partly attributed to pre breakup mafic material or and inherited high pressure granulite eclogite rocks in the continental domain q 2004 elsevier ltd all rights reserved

holocene cyclic climatic variations and the role of the pacific ocean as recorded in varved sediments from northeastern china we present an n alkane and compound specific carbon isotope record of the past 9 ka from the annually laminated sedimentary sequence of lake xiaolongwan northeastern china the n alkane distribution suggests that lake xiaolongwan has undergone a shift from an oligotrophic state with low algal production and little emergent submerged macrophytes in the early holocene to a eutrophic state with high algal production and abundant emergent submerged macrophytes since the middle holocene the pattern of variation observed in the biomarker proxies acl the n alkane average chain length paq aquatic macrophyte versus aquatic macrophyte and terrestrial plant ratio and lptp lake productivity terrigenous organic production is throughout the record similar to that of the total organic carbon the variation of compound specific carbon isotopic values in the middle and short chain alkanes was mainly regulated by lake productivity and the accumulating organic pool through time in this forested region where the vegetation is dominated by c 3 plants the long chain n alkanes c27ec31 are predominantly derived from leaf wax lipids the compound specific d 13 c27e31 value is sensitive to effective precipitation and therefore represents a useful indicator of regional monsoonal precipitation spectral analysis on the d 13 c27e31 time series reveals significant periodicities of 87e89 205e212 1020e1050 and 1750e2041 years on the centennial timescale the quasi periodicities around 88 and 210 years suggest a strong link between solar activity and monsoon rainfall the millennial monsoon cycle in northeastern china is associated with sea surface temperature sst variations in two active centers of the summer monsoon the western pacific subtropical high wpsh and the okhotsk high increasing sst in the subtropical sea may cause a northwards shift of the wpsh which extends the monsoon rain band meiyu to northeastern china and thus increasing rainfall in that region meanwhile decreasing sst in the okhotsk sea may strengthen the okhotsk high bringing more moisture into northeastern china we suggest that the pacific ocean is a main regulator for summer monsoon rainfall in northeastern china at present and at different time scales during the holocene

tropical climate instability the last glacial cycle from a qinghai tibetan ice core an ice core record from the guliya ice cap on the qinghai tibetan plateau provides evidence of regional climatic conditions over the last glacial cycle 36 cl data suggest that the deepest 20 meters of the core may be more than 500 000 years old the 18 o change across termination i is 5 4 per mil similar to that in the huascaran peru and polar ice cores three guliya interstadials stages 3 5a and 5c are marked by increases in 18 o values similar to that of the holocene and eemian 124 000 years ago the similarity of this pattern to that of ch 4 records from polar ice cores indicates that global ch 4 levels and the tropical hydrological cycle are linked the late glacial stage record contains numerous 200 year oscillations in 18 o values and in dust nh 4 and no 3 levels

abrupt changes in indian summer monsoon strength during 33 800 to 5500 years b p speleothem proxy records from northeastern ne india reflect seasonal changes in indian summer monsoon strength as well as moisture source and transport paths we have analyzed a new speleothem record from mawmluh cave meghalaya india in order to better understand these processes the data show a strong wet phase 33 500 32 500 years b p followed by a weak dry phase from 26 000 to 23 500 years b p and a very weak phase from 17 000 to 15 000 years b p the record suggests abrupt increase in strength during the bolling allerod and early holocene periods and pronounced weakening during the heinrich and younger dryas cold events we infer that these changes in monsoon strength are driven by changes in temperature gradients which drive changes in winds and moisture transport into northeast india

behavior of major and trace elements including ree during paleoproterozoic pedogenesis and diagenetic alteration of an archean granite near ville marie quebec canada abstract this paper presents a complete set of chemical data from a pre 2 0 ga weathering profile that formed on the archean granite near ville marie quebec examination of data from two sections of the paleosol indicates that na ca p and sr were lost from the paleosol whereas ti nb th zr hf and ta remained immobile during chemical weathering and all subsequent alteration rare earth mobility during genesis of the ville marie paleosol closely follows the pattern of early stages of chemical weathering in modern profiles average concentration of ree in both profiles is 20 40 lower than what is found in the fresh granite suggesting mobilization during pedogenesis hrees seem to be less mobilized than the lrees la sm n ratios gradually decrease toward the top of the paleosol but the values remain very close to that of the protolith gd yb n values remain unchanged in the weathering profile la yb n values in the weathering profile are quite variable and generally lower than that of the protolith eu eu and ce ce values are essentially the same as those of the protolith but two samples from the uppermost part of the profile have developed positive cerium anomaly of 0 3 relative to the protolith

pematang sihapas petroleum system of central sumatra abstract the pematang sihapas petroleum system of central sumatra is one of the most important lacustrine oil systems in southeast asia the brown shale formation lacustrine of the pematang group has generated 60 109 barrels of oil sihapas early miocene sandstones are the principal reservoirs of this system giant fields e g minas and duri having sihapas marine sandstone reservoirs occur principally along the eastern margins of sub basins smaller fields having pematang nonmarine sandstone reservoirs are confined to the deeper troughs the pematang group was deposited in a series of small grabens it exhibits a tripartite stratal architecture basal lower red bed fluvial alluvial unit medial brown shale lacustrine unit and upper upper red bed fluvial alluvial unit the upper red bed unit is the main pematang reservoir source rock attributes of the brown shale are highly variable pyrolysis yields of samples containing 1 0 wt toc range up to 120 mg hc g rock with a mean of 25 3 mg hc g rock the oil proneness of the kerogen also varies the more oil prone portions of the unit appear in the upper portion of the stratigraphic unit and in the fully lacustrine facies i e poor source rock development occurred in lake margin deltaic and fan delta facies geochemical differences also exist among the different sub basins the pematang group is disconformably overlain by the menggala formation basal transgressive unit of sihapas group menggala strata consist of well sorted quartzose to subarkosic sandstones having an average porosity of 20 and an average permeability of 1500 md menggala sandstones are the most prolific hydrocarbon reservoirs in central sumatra many of the oil fields in central sumatra are associated with paleohighs drag folds and post mid miocene inversion hydrocarbon generation was initiated during the miocene and continues currently in some parts of the basin

control of wind strength and frequency in the aral sea basin during the late holocene changing content of detrital input in laminated sediments traced by xrf scanning and microfacies analyses reflect prominent variations in sedimentation processes in the aral sea a high resolution record of titanium from a core retrieved in the northwestern large aral sea allows a continuous reconstruction of wind strength and frequency in western central asia for the past 1500 yr during ad 450 700 ad 1210 1265 ad 1350 1750 and ad 1800 1975 detrital inputs bearing titanium are high documenting an enhanced early spring atmospheric circulation associated with an increase in intensity of the siberian high pressure system over central asia in contrast lower titanium content during ad 1750 1800 and ad 1980 1985 reflects a diminished influence of the siberian high during early spring with a reduced atmospheric circulation a moderate circulation characterizes the time period ad 700 1150 unprecedented weakened atmospheric circulation over western central asia are inferred during ca ad 1180 1210 and ad 1265 1310 with a considerable decrease in dust storm frequency sedimentation rates lamination thickness and detrital inputs screened at 40 m resolution our results are concurrent with changes in the intensity of the siberian high during the past 1400 yr as reported in the gisp2 ice core from greenland

sensitivity of east asian monsoon climate to the uplift of the tibetan plateau abstract using the cola center for ocean land atmosphere studies general circulation model a series of numerical experiments were conducted to examine the impact of uplift of the tibetan plateau on the evolution of the monsoon climate in east asia an attempt was made to isolate the effect of the plateau uplift from the changes in other forcing mechanisms we examined the spatial and temporal variation patterns of a variety of climatic variables and indices including monsoon intensity sea level pressure surface and upper air winds temperature at different levels precipitation air humidity and soil moisture our results suggest that the evolution of the east asia monsoon may be more sensitive to the uplift of the tibetan plateau than that of the south asia monsoon moreover the effect of the plateau uplift on the east asia winter monsoon is more significant than that on the summer monsoon in the northern part of east asia north china the formation of the monsoon climate is marked mainly by the establishment of the northerly winds in winter or the winter monsoon system corresponding to the uplift of the plateau when the height reaches approximately 50 of its current elevation on the other hand the establishment of the monsoon climate in the southern part of east asia the yangtze river valley and the area south of it may be much earlier than that in the northern part

magnetic transition at 30 34 kelvin in pyrrhotite insight into a widespread occurrence of this mineral in rocks abstract a characteristic magnetic transition at 30 34 k is shown to provide a powerful tool for the identification of pyrrhotite with concentration down to 10 ppm through the same low temperature techniques as applied to magnetite and hematite extended down to liquid helium temperature a review of rock magnetic and petrological data on pyrrhotite suggests that this mineral should be considered as a major carrier of paleomatnetic signals unblocking temperature up to 350 c and extreme resistance against af may be encountered in fine grained pyrrhotite

kar systematics of clay to mica minerals in a multi stage low grade metamorphic evolution the behavior of the ar isotopic system of clay to mica fractions was studied in cambrian greywacke samples along a transect of northwestern morocco these rocks were affected by a progressively increasing metamorphic overprint spanning from diagenetic to epizonal metamorphic conditions k ar isotopic data of 0 2 10 m size fractions recovered from seven samples allowed to recognized three episodes of illite crystallization 1 during an upper ordovician thermal event at 445 ma which is characterized by an illite chlorite mineral paragenesis in the northernmost part of the transect 2 during a carboniferous low grade overprint at 305 ma which is characterized by a similar mineral paragenesis and 3 during a triassic to liassic hydrothermal activity at 205 ma which generated a smectite dominated paragenesis the ages were examined in combining x ray mineralogic and k ar isotopic data in terms of linear arrays in both isochron and harper diagrams to differentiate true isochrons from mixing lines it could also be shown that mixtures of two generations of micaceous minerals can induce different trends in the k ar dates depending whether the first generation of minerals was either or not affected by the event inducing the later generation

cenozoic remagnetization of the paleozoic rocks in the kitakami massif of northeast japan and its tectonic implications abstract secondary remanent magnetization is identified in the paleozoic igneous and sedimentary rocks of the kitakami massif the secondary nature is shown by a negative fold test for the permo carboniferous sedimentary rocks the northwesterly paleomagnetic declination with moderate inclination d 321 2 i 56 5 95 5 2 n 18 of this secondary remanent magnetization is almost parallel to the primary magnetization reported for the cenozoic welded tuffs of northeast japan indicating that the paleozoic rocks were subjected to remagnetization at any period between 62 and 16 ma the secondary magnetization of the serpentinized ultramafic rocks is carried by magnetite which grew in veins and mesh rims of serpentine whereas the carrier of the magnetization in limestones is fine grained pyrrhotite combining this with the previously reported remagnetization of the kitakami granitic rocks it is suggested that rocks in the kitakami massif were subjected to crystallization remanent magnetization at low temperature conditions since serpentinization requires fluid migration one of the most likely events is the eastward lateral migration of water into the kitakami massif we postulate a cenozoic suturing of the kitakami massif with the asian continent as a plausible tectonic event for this fluid migration

paleomagnetism of the notch peak contact metamorphic aureole revisited pyrrhotite from magnetite pyrite under submetamorphic conditions 1 on the basis of new demagnetization and rock magnetic experiments a scattered two polarity remagnetization extending throughout the contact metamorphic aureole surrounding the jurassic notch peak granitic stock in western utah is now thought to reside in metamorphic pyrrhotite rather than in fine grained hematite in part the pyrrhotite formed by in situ reduction of magnetite by pyrite and organic compounds it could not have been formed by the large scale introduction of reducing fluids as isotopic evidence indicates that these metalimestones remained impermeable at higher grades some pyrrhotite probably also formed by the reaction of pyrite and fe bearing silicates as magnetite destruction and pyrrhotite formation extends into rocks that are not obviously metamorphosed the reactions responsible could proceed at very low less than 200 c temperatures and the feasibility of such low temperature reactions in a closed system is shown by detailed thermodynamic calculations this study demonstrates that under reducing conditions in the presence of sufficient pyrite pyrrhotite can form at the expense of magnetite under very mild conditions indeed magnetite may be destroyed merely by deep burial such a mechanism may help explain the paucity of primary magnetite in ancient sedimentary rocks particularly since the single domain and pseudo single domain grains capable of retaining a stable paleomagnetic signature over geologic time are extremely fine grained and hence particularly vulnerable to destruction

δd values of individual n alkanes from terrestrial plants along a climatic gradient implications for the sedimentary biomarker record abstract the compound specific hydrogen isotope composition d of sedimentary n alkanes is increasingly used as a palaeoclimate proxy to explore the origin of the hydrogen isotope signal of n alkanes from higher terrestrial plant biomass in sedimentary organic matter of small lakes we have analysed the n alkane composition and d values of 31 plant biomass samples birch betula beech fagus oak quercus alder alnus hornbeam carpinus and myrtle myrtus as well as non stomata containing sphagnum cladonia and moss from 14 sites along a climatic gradient from northern finland to southern italy eleven of the sites were previously investigated for the n alkane d values in adjacent lake sediments and are compared to the leaf wax n alkane d values the distribution of terrestrial n alkanes in lake sediments was not found to be characteristic of the dominant vegetation around the lake evidently n alkane distributions within the same species at different sites were found to be variable these findings complicate the use of certain n alkane distributions in sediments as a palaeoenvironmental indicator d values of plant biomass n alkanes show a significant positive correlation with the precipitation d values biomass n alkane d values are in general less negative than the associated sedimentary n alkanes of terrestrial origin possibly due to the stronger evapotranspiration during the summer when sampling took place this suggests that n alkanes in leaves show seasonal variations and the sedimentary organic matter receives the lighter isotope signal of the autumn when most leaves are incorporated into the sediment the variability of the d values from non stomata containing plants is much higher than for broadleaf trees since stomata can regulate water loss through evapotranspiration the average chain length acl of n alkanes from deciduous tree leaves increases from north to south with increasing acl the hydrogen isotopic fractionation e between source water and n alkanes also increases our results prove that terrestrial n alkane d values record the d value of precipitation modified by the amount of isotope enrichment in the leaf water due to plant anatomy stomata vs no stomata and site specific meteorological conditions such as evapotranspiration relative humidity and soil moisture therefore terrestrial n alkanes in lake sediments can deliver information on changes in these parameters over geological timescales in lake sediments where aquatic n alkanes such as n c 17 record the lake water d value the isotopic difference between terrestrial and aquatic n alkanes could therefore be used to reconstruct changes in evapotranspiration

effect of lake evaporation on δd values of lacustrine n alkanes a comparison of nam co tibetan plateau and holzmaar germany compound specific hydrogen isotope ratio values of lacustrine n alkanes from two contrasting ecosystems the semiarid to arid nam co central tibet and the humid holzmaar germany were compared in order to assess whether or not these environmental conditions are recorded in the isotopic signatures of biomarkers increased evaporation of lake water at nam co is recorded by the n alkanes of aquatic origin hence isotopic enrichment results in a di erence between terrestrial and aquatic n alkanes in the opposite direction 68 from that known for humid climate conditions 30 predominating at holzmaar based on this isotopic di erence between terrestrial and aquatic n alkanes evaporation to inflow ratio e i values were estimated and suggest that the isotopic di erence is indicative of the general hydroclimatic characteristics of a lake system moreover the comparison with e i values calculated with actual stable water isotope data showed that the dd values of aquatic and terrestrial lacustrine n alkanes serve as a proxy for the relative isotopic di erences between lake and inflow waters and can be used to assess the proportion of water undergoing evaporation relative to the inflow and thus to reconstruct the lake water balance in the past 2008 published by elsevier ltd

ice core records of climate variability on the third pole with emphasis on the guliya ice cap western kunlun mountains abstract records of recent climate from ice cores drilled in 2015 on the guliya ice cap in the western kunlun mountains of the tibetan plateau which with the himalaya comprises the third pole tp demonstrate that this region has become warmer and moister since at least the middle of the 19th century decadal scale linkages are suggested between ice core temperature and snowfall proxies north atlantic oceanic and atmospheric processes arctic temperatures and indian summer monsoon intensity correlations between annual scale oxygen isotopic ratios 18o and tropical western pacific and indian ocean sea surface temperatures are also demonstrated comparisons of climate records during the last millennium from ice cores acquired throughout the tp illustrate centennial scale differences between monsoon and westerlies dominated regions among these records guliya shows the highest rate of warming since the end of the little ice age but 18o data over the last millennium from tp ice cores support findings that elevation dependent warming is most pronounced in the himalaya this along with the decreasing precipitation rates in the himalaya region is having detrimental effects on the cryosphere although satellite monitoring of glaciers on the tp indicates changes in surface area only a few have been directly monitored for mass balance and ablation from the surface this type of ground based study is essential to obtain a better understanding of the rate of ice shrinkage on the tp

vitrinite reflectance and molecular geochemistry of jurassic sediments the influence of heating by tertiary dykes northwest scotland vitrinite reflectance measurements from sedimentary rocks adjacent to six dykes from sites across the inner hebrides indicate that the lateral extent of thermal alteration is reasonably similar and proportional to dyke thickness d t with alteration starting to take effect at 70 d t detailed sampling of a jurassic siltstone adjacent to a 0 9 m dyke reveals that the zone of maximum extractable organic matter eom wt g rock equivalent to the oil window is relatively narrow ca 25 of the d t the position of this oil window relative o the vitrinite reflectance profile is comparable with that from burial maturation gas chromatograms of the aliphatic hydrocarbon fractions reveal i a shift from branched and cyclic dominated hydrocarbons to an increasing predominance of straight chain components and ii the carbon number distribution of the n alkanes shifts towards the shorter chain homologues with decreasing distance from the dyke contact apart from the methylphenanthrene index mpi1 changes in each of the molecular maturity parameters are initiated by the the thermal effects of the dyke between 80 and 70 d t from the contact the mpi1 parameter was the most sensitive maturity indicator in that it responded to the dyke emplacement even in the distal samples i e distance from dyke contact 140 d t a significant reduction in toc at the dyke contact suggests that the synthesis gas reaction may have occurred i e the reaction of organic matter with steam resulting in the formation of carbon monoxide this reaction may be associated with the vitrinite reflectance reversal this study suggests that these particular dykes have had only a local effect on the maturity of the intruded sediments

metamorphic control of the magnetic mineralogy of black shales in the swiss alps toward the use of magnetic isogrades abstract the magnetic susceptibility of the helvetic jurassic black shales sampled around the aar and gothard massifs in the swiss alps is due to iron bearing silicates magnetite or pyrrhotite it exhibits a clear correlation with the alpine metamorphism from zeolite to amphibolite facies it firstly decreases due to the breakdown of original magnetite then sharply increases when pyrite is transformed into magnetic pyrrhotite using numerous field measurements of the susceptibility it was possible to carry out a fast and precise mapping of the pyrrhotite in isograde which appears to correspond closely to the stilpnomelane out isograde the matrix susceptibility obtained by means of high field measurements with a cryogenic magnetometer is due to iron bearing silicates and shows only smooth variations with metamorphism both monoclinic and hexagonal pyrrhotites are observed in the high grade area

petrogenesis of late stage magmatism at hold with hope east greenland the myggbukta caldera complex and a swarm of basic dykes constitute the latest tertiary magmatism in the hold with hope region east greenland the sr and nd isotope ratios of these rocks show coherent variations which extend to high 87sr 86sr and low 143nd 144nd values and require a contribution from continental lithosphere broad correlations with major element differentiation indices suggest that the continental component was incorporated during magmatic differentiation thereby favouring a crustal contamination process trace element concentrations are strongly correlated with isotopic compositions but display ranges for many incompatible elements which extend beyond likely crustal contaminant compositions this is readily modelled by afc processes in which the dominant cause of trace element enrichment is the concentration effect of fractional crystallisation rather than the composition of the contaminant the simplest such models still require unrealistically high degrees of fractional crystallisation to explain the ten fold enrichment of some trace elements this can be overcome if the primary magmas entering the crust already had highly variable trace element compositions such variability is readily achieved if melts from different parts of the melting column escape without thorough homogenization an afc model which incorporates variability in parental magma composition is then able to simulate the range of compositions observed at hold with hope this carries the implication that the variations observed are more readily attributed to changes in uncontaminated parental magma than to variations in the composition or amount of contaminant

a late diagenetic syn folding magnetization carried by pyrrhotite implications for paleomagnetic studies from magnetic iron sulphide bearing sediments paleomagnetic rock magnetic and sedimentary micro textural data from an early miocene mudstone sequence exposed in okhta river sakhalin russia indicate the presence of pyrrhotite and magnetite at different stratigraphic levels sites that contain only magnetite have a reversed polarity characteristic remanent magnetization chrm with a low coercivity overprint which coincides with the present day geomagnetic field direction pyrrhotite bearing sites have stable normal polarity chrms that are significantly different from the present day field direction after correction for bedding tilt the chrm data fail a reversals test however the normal polarity pyrrhotite chrm directions become antipodal to the tilt corrected magnetite chrm directions and are consistent with the expected geocentric axial dipole field direction at the site latitude after 40 partial unfolding these data suggest that the pyrrhotite magnetization was acquired during folding and after lock in of the magnetite remanences electron microscope observations of polished sections indicate that fluid associated halos surround iron sulphide nodules pyrrhotite is present in randomly oriented laths in and around the nodules and the nodules do not appear to have been deformed by sediment compaction this observation is consistent with a late diagenetic origin of pyrrhotite documentation of a late diagenetic magnetization in pyrrhotite bearing sediments here and in recent studies of greigite bearing sediments suggests that care should be taken to preclude a late origin of magnetic iron sulphides before using such sediments for geomagnetic studies where it is usually crucial to establish a syn depositional magnetization

late quaternary vegetation dynamics and hydro climate in the drakensberg south africa abstract a multi proxy study of a sediment sequence from braamhoek wetland covering the last c 16 000 years reveals a record of regional climate and vegetation dynamics in the drakensberg region south africa including signals from both the organic sediment fraction fossil pollen charcoal n alkane abundance n alkane 13c toc and the inorganic fraction mineral magnetic properties the reconstruction supported by a robust chronology indicates two major periods of increased regional wetness during the late pleistocene to early holocene phase c 13 800 12 600 cal yr bp c 10 200 8500 cal yr bp and one during the late holocene c 2000 cal yr bp to present drier conditions are recorded during the younger dryas c 12 600 11 300 cal yr bp and mid holocene c 7000 2000 cal yr bp a major decline in fynbos vegetation during the early holocene suggests a shift towards warmer temperatures and possibly towards less pronounced winter rains in eastern south africa from c 8500 cal yr bp comparison with records from interior of south africa show relatively high inter site variability however the braamhoek moisture proxies do co vary with the speleothem isotope records from makapansgat suggesting a similar hydro climate evolution in eastern and interior parts of the summer rainfall region during the studied period on multi millennial time scales an inverse hydro climatological pattern is evident between these two south african records and reconstructions from tropical locations in southeast africa such a rainfall dipole between eastern tropical and southern africa has previously been identified on shorter time scales i e on inter annual to millennial scales the braamhoek study suggests that a similar dipole pattern is acting also on a multi millennial perspective these long term precipitation anomalies are tentatively coupled to teleconnections from multi millennial changes in the dynamics of the indian ocean dipole iod and el nino southern oscillation enso

environmental changes since 8 4 ka reflected in the lacustrine core sediments from nam co central tibetan plateau china the tibetan plateau induces and enhances the asian monsoon that influences the plateau itself east china and even the whole of asia pursuing the changing monsoon history and its impact in this environment sensitive area is a key for understanding the climatic changes here we present the initial results from core sed iments in nam co central tibet to elucidate the holocene environment changes in this area the studied lake nam co is the second largest lake with an elevation of 4718 m water area of 2015 km 2 and catchment area of 10 610 km 2 by using echo seismic profiling and hydro echosounding we investigated the bathymetric dis tribution of the whole lake one 332 cm long piston core was successfully taken in the east part of the lake at 60 m water depth twelve ams 14 c dates from top to bottom of this core established a good time sequence toc tn n alkanes grain size clay minerals fe mn sr ba and caco3 content were analysed with sampling intervals from 1 cm to 10 cm results showed that since 8400 yr bp the environment changes in this region indicate three stages the early stage 8400 6900 yr bp was characterized by slight shifting from warm to cold with a cold dry interval at 8100 7800 yr bp the middle stage was from 6900 to 2900 yr bp during which cli mate changed from warm humid to cold dry however around 2900 yr bp the climate once was slightly cold but strongly dry the late stage from 2900 yr bp to present was characterized by a general tendency toward cold dry the first temperature decline was around 1700 1500 yr bp following which temperature increased once before decreasing again around 600 300 yr bp which might be the result of the little ice age

biomarker distribution in the lacustrine shales of the upper triassic lower jurassic kap stewart formation jameson land greenland abstract freshwater lacustrine shales with a good to excellent petroleum source potential from the upper triassic lower jurassic kap stewart formation in jameson land eastern greenland have been investigated by rock eval pyrolysis toc and sulphur determinations gas chromatography mass spectrometry and pyrolysis gas chromatography distributions of n alkanes isoprenoids steranes and hopanes together with rock eval data and sulphur contents have been used to describe depositional environment and thermal maturity the lacustrine shales are characterized by long chain n alkanes high cpi values high pristane phytane ratios high hopane sterane ratios and very low sulphur contents good potential for generation of liquid petroleum is confined to samples having the highest relative contents of c 27 steranes gammacerane and 17 h trisnorhopane ts regional maturity levels correspond to an immature stage perturbed by local heating from sills comparison with the time equivalent are formation on haltenbanken offshore mid norway has been made with respect to biomarker distribution and petroleum potential the biomarker distributions of the two formations are similar and can be ascribed to the clastic non marine nature of both depositional areas the petroleum potential of the are formation is poorer than that of the kap stewart formation probably due to a predominantly deltaic facies of the former with major terrestrial organic matter input

oxygen and hydrogen isotopes in the hydrologic cycle changes of the isotopic composition of water within the water cycle provide a recognizable signature relating such water to the different phases of the cycle the isotope fractionations that accompany the evaporation from the ocean and other surface waters and the reverse process of rain formation account for the most notable changes as a result meteoric waters are depleted in the heavy isotopic species of h and o relative to ocean waters whereas waters in evaporative systems such as lakes plants and soilwaters are relatively enriched during the passage through the aquifers the isotope composition of water is essentially a conservative property at ambient temperatures but at elevated temperatures interaction with the rock matrix may perturb the isotope composition these changes of the isotope composition in atmospheric waters surface water soil and groundwaters as well as in the biosphere are applied in the characterization of hydrological system as well as indicators of paleo climatological conditions in proxy materials in climatic archives such as ice lake sediments or organic materials

environmental conditions in the donggi cona lake catchment ne tibetan plateau based on factor analysis of geochemical data abstract we present a methodological approach for the characterization of spatial variations of environmental and transport conditions based on geochemical data we analyzed the geochemical characteristics of terrestrial sediments on an alluvial fan and adjacent areas in the eastern part of the donggi cona catchment the geochemical characteristics of the sediment samples were measured using x ray fluorescence spectroscopy a factor model based on nine elements sr rb zn fe mn v ti ca and k gave the best results using factor analysis our results show that factor analysis of geochemical data can explain spatial variations in the catchment even within a relatively small area with more or less constant climatological conditions the four most important factors of our model explain 68 4 of the total variance within the dataset the four factors represent a carbonate weathering and two redox condition signals respectively the spatial distribution of factor loadings points to regions with specific environmental conditions these regions show a certain carbonate input or production factor 1 specific weathering rates factor 2 or specific redox conditions factors 3 and 4 the factor loadings of the first and second factor are used to reclassify the data into four groups being dune loess lake and fluvial gravel sediments this reclassification confirms or improves the prior field classification in 70 of all cases for 23 of the samples the model gave a different interpretation the factor loadings in these cases represent the origin of the material rather than its present sediment type thereby giving valuable information about sediment provenance since the third and fourth factor seem more dependent on location they do not differentiate between different sediment types multivariate statistics of the geochemistry of terrestrial sediments i allows the subdivision of samples into different sediment types ii indicates dominant regions for weathering carbonate production and manganese washout and iii gives suggestions about the provenance of the sediments this is the first detailed study of geochemical parameters on the tibetan plateau not focusing on lake cores resulting in a spatial characterization of the local geochemistry this gives insight in the sediment transport connections between the catchment and the lake illustrating the value of terrestrial sediments as additional indicator of environmental variations

the magnetization of clay rich rocks in sedimentary basins low temperature experimental formation of magnetic carriers in natural samples abstract rocks are often magnetically altered during burial thus providing a useful method to study their thermo chemical history in order to improve the understanding of burial diagenetic magnetization we experimentally synthesized magnetite in natural samples lower jurassic argillites from the paris basin were heated to 150 c under a confined atmosphere of argon when a magnetic field was applied during the heating a chemical remanent magnetization was acquired rock magnetism studies reveal that most of the newly formed magnetic carriers were magnetite using anhysteretic remanent magnetization as a measure of the quantity of magnetite an increase of up to 150 after heating was observed ferrimagnetic iron sulphide and hematite are observed to have formed in samples composed of less than 3 calcite and new magnetite is sometimes completely absent in samples containing less than 0 5 calcite hysteresis measurements suggest that the new magnetite is in the pseudo single domain state scanning electron microscope studies indicate that the new magnetic carriers are associated with pre existing pyrite the magnetic transformations during the experiments involved a fluid which reacted with iron bearing and sulfur bearing minerals in ways depending on local redox conditions this study shows that magnetite can be produced in argillaceous sediments at the normal temperatures and pressures found in sedimentary basins without any external supply from long range fluid flow

new constraints on the thermal history of north east greenland from apatite fission track analysis apatite fission track analyses from the area north of jameson land east greenland indicate that the region has undergone at least three phases of cooling during the mesozoic and cenozoic two major periods of cooling during the tertiary have been identified a middle tertiary episode in which cooling began between 40 and 30 ma and a late tertiary episode in which cooling began between 10 and 5 ma the middle tertiary event is synchronous with emplacement of major intrusive bodies associated with continental rifting and may be due either to uplift and erosion or hydrothermal effects the late tertiary event appears to be related to erosion associated with uplift resulting from changes in the north atlantic spreading direction and associated events no paleothermal effects have been identified related to the onset of rifting in the early tertiary results from samples farthest from the continental margin reveal an earlier event in which cooling began between 225 and 165 ma the origin of this event is not clear but it may reflect uplift and erosion associated with recognized unconformities within the jurassic section

holocene lake level fluctuations and environmental changes at taro co southwestern tibet based on ostracod inferred water depth reconstruction lake level change is an important indicator of changes in the hydrological status of lakes triggered by the influence of atmospheric circulation systems previous studies show that the interaction

changes in remanence coercivity and domain state at low temperature in magnetite abstract submicron magnetite crystals with mean sizes of 0 037 0 10 and 0 22 m undergo major changes in hysteresis properties and domain states in crossing the verwey transition t v 120 k the 0 037 m crystals are single domain sd both in the cubic phase at room temperature t 0 and in the monoclinic phase below t v the 0 10 and 0 22 m crystals have a mixture of sd and two domain 2d states at room temperature t 0 but mainly sd structures below t v in agreement with micromagnetic calculations coercive force h c increases on cooling through t v by a factor 3 5 in the submicron magnetites and 40 in a 1 3 mm single crystal because of the high crystalline anisotropy and magnetostriction of monoclinic magnetite as a result domain walls and sd moments are so effectively pinned below t v that all remanence variations in warming or cooling are reversible however between 100 k and t 0 remanence behavior is variable saturation remanence sirm produced in monoclinic magnetite at 5 k drops by 70 100 in warming across t v with minor recovery in cooling back through t v ultimate levels at 5 k of 23 37 for the submicron crystals and 3 for the 1 3 mm crystal in contrast sirm produced in the cubic phase at 300 k decreases 5 35 submicron or 95 1 3 mm during cooling from 300 to 120 k due to continuous re equilibration of domain walls but there is little further change in cooling through t v itself however the submicron magnetites lose a further 5 15 of their remanence when reheated through t v these irreversible changes in cycling across t v and the amounts of the changes have potential value in determining submicron magnetite grain sizes the irreversibility is mainly caused by 2d sd transformations on cooling through t v which preserve or enhance remanence while sd 2d transformations on warming through t v cause remanence to demagnetize

spatial heterogeneity in lipid biomarker distributions in the catchment and sediments of a crater lake in central india the basin scale spatial variability in lipid biomarker proxies in lacustrine sediments which are established tools for studying continental environmental change has rarely been examined it is often implicitly assumed that a lake sediment core provides an average integral of catchment sources here we evaluated the distribution of lipid biomarkers in a modern ecosystem and compared it with the sedimentary record we analyzed lipid biomarkers in terrestrial and aquatic organisms and in lake surface sediments from 17 locations within the saline alkaline lonar crater lake in central india terrestrial vegetation and lake surface sediments were characterized by relatively high average chain length acl index values 29 6 32 8 of leaf wax n alkanes consistent with suggestions that plants in drier and warmer climates produce longer chain alkyl lipids than plants in cooler and humid areas a heterogeneous spatial distribution of acl values in lake surface sediments was found at locations away from the shore the values were highest 31 or more possibly indicating different sources and or transport of terrestrial biomarkers in floating benthic microbial mats and surface sediment n heptadecane carotenoids diploptene phytol and tetrahymanol occurred in large amounts interestingly these biomarkers of a unique bacterial community were found in substantially higher concentrations in nearshore sediment samples we suggest that human influence and subsequent nutrient supply resulted in increased primary productivity leading to an unusually high concentration of tetrahymanol in the nearshore sediments in summary the data showed that substantial heterogeneity existed within the lake but leaf wax n alkanes in a core from the center of the lake represented an integral of catchment conditions however lake level fluctuation may potentially affect aquatic lipid biomarker distributions in lacustrine sediments in addition to source changes

effect of yunnan guizhou topography at the southeastern tibetan plateau on the indian monsoon abstracttopographic insulation is one of the primary origins for the influence of the tibetan plateau tp on asian climate the yunnan guizhou yg plateau at the southeastern margin of the tp is known to block the northern branch of the indian monsoon circulation in summer however it is an open question whether this blocking feeds back to the monsoon in this study the effect of the yg topography on the indian monsoon and its comparison with that of the tp were evaluated using general circulation model experiments the results showed that the tp strengthens the monsoon precipitation especially during the onset however the yg topography significantly weakens the monsoon with the yg topography strengthened low level airflow around the yg plateau induces anomalous anticyclonic winds to the southwest and the changes remodulate the whole circulation structure over asia as a result the indian monsoon becomes weakened from the bay of bengal to the indian subcontinent and arabian sea as does the a

high precision three dimensional paleothermometry derived from paleomagnetic data in an alpine metamorphic unit in the dauphinoise zone of the french western alps a posttectonic thermoremanent magnetization carried by pyrrhotite has been isolated the paleomagnetic signature is interpreted as the record of successive partial thermoremanent magnetizations of opposite directions acquired during the early miocene slow cooling of the epimetamorphic marly limestones of the studied area a local vertical thermal gradient of 22 2 c km was derived from the unblocking temperature of a reversal recorded at different altitudes using this vertical gradient we have produced a paleotemperature map at the time of the reversal about 20 ma the map shows a horizontal regional thermal gradient of 7 c km the higher temperatures were toward the crystalline basement paleomagnetic directions demonstrate that this horizontal gradient was present during cooling its origin can be explained by large volume fluid circulation along the major fault that forms the boundary between the crystalline basement and its liassic cover

the jameson land basin east greenland a fission track study of the tectonic and thermal evolution in the cenozoic north atlantic spreading regime abstract the late mesozoic and cenozoic thermal history of the jameson land basin is constrained by new apatite and zircon fission track ft data of surface permian to jurassic sedimentary rocks the results show a general regional thermal evolution related to burial to temperatures close to and in excess of the maximum temperatures of the apatite annealing interval 125 c followed by cooling mainly due to cenozoic uplift and erosion faulting and differential movements in the basin generally occurred after cooling below the apatite partial annealing zone paz 75 125 c however in the northern part of the basin the data suggest a thicker sediment cover or localized heating related to an earlier fracture zone both apatite ft analysis and vitrinite reflectance values reveal a postmature signature for the studied rocks in the northeastern jameson land and premature to mature for the western central and southern jameson land rocks with respect to generation of hydrocarbons the chemical variations of apatite enhance the possibility of recognizing sample positions near maximum temperatures in the paz furthermore the pb zn mineralization pattern closely follows the tertiary maturity trend given by the ft data the type and distribution of mineralization suggest that it was influenced by the regional thermal evolution of the basin in the northeast domain circulating fluids may have overprinted the regional thermal record before ca 20 ma basaltic dyke and sill intrusions 55 45 ma locally caused resetting of apatite ft ages but generally the direct influence from upper crustal magmatic activity played only a minor role the thermal evolution in northeast jameson land is related to the late tectonic evolution of the northeast atlantic involving a change in ridge position at ca 25 ma which followed the passage of the proto icelandic mantle plume at 63 40 ma

holocene itcz and indian monsoon dynamics recorded in stalagmites from oman and yemen socotra abstract high resolution oxygen isotope 18o profiles of holocene stalagmites from four caves in northern and southern oman and yemen socotra provide detailed information on fluctuations in precipitation along a latitudinal transect from 12 n to 23 n 18o values reflect the amount of precipitation which is primarily controlled by the mean latitudinal position of the itcz and dynamics of the indian summer monsoon ism during the early holocene rapidly decreasing 18o values indicate a rapid northward displacement in the mean latitudinal position of the summer itcz and the associated ism rainfall belt with decadal to centennial scale changes in monsoon precipitation correlating well with high latitude temperature variations recorded in greenland ice cores during the middle to late holocene the summer itcz continuously migrated southward and monsoon precipitation decreased gradually in response to decreasing solar insolation a trend which is also recorded in other monsoon records from the indian and east asian monsoon domains importantly there is no evidence for an abrupt middle holocene weakening in monsoon precipitation although abrupt monsoon events are apparent in all monsoon records they are short lived and clearly superimposed on the long term trend of decreasing monsoon precipitation for the late holocene there is an anti correlation between ism precipitation in oman and inter monsoon spring autumn precipitation on socotra revealing a possible long term change in the duration of the summer monsoon season since at least 4 5 ka bp together with the progressive shortening of the ism season gradual southward retreat of the mean summer itcz and weakening of the ism the total amount of precipitation decreased in those areas located at the northern fringe of the indian and asian monsoon domains but increased in areas closer to the equator

pyrrhotite as a tracer for denudation of the taiwan orogen 1 the use of pyrrhotite as an index mineral to delineate metamorphic isograds has long been proposed however details of the occurrence of pyrrhotite in different metamorphic facies and its implications have rarely been explored here by simple characterization of magnetic properties and mineral identification we demonstrate that pyrrhotite is widely distributed in low grade metamorphic terranes of taiwan s central range and in sediments derived therefrom by coupling the distribution of pyrrhotite in rocks with that in dated sediment strata we have studied the denudation history of taiwan s orogen from a source to sink perspective we suggest that pyrrhotite is a potential tracer for studying surface processes in orogens with high denudation rates similar to that in taiwan

experimental evaluation of thermal recording of successive polarities during uplift of metasediments summary the discovery of a post tectonic natural remanent magnetization nrm carried by pyrrhotite in the dauphinoise zone of the western alps allows us to test the ability of these rocks to record a sequence of magnetic polarity versus temperature acquired during their slow cooling several laboratory experiments composite partial thermal remanent magnetizations ptrms thellier thellier experiments trm thermal demagnetization as well as nrm demagnetization were performed this shows that the pyrrhotite grains are single domain sized and that they are able to record successive independent ptrms the nrm thermal demagnetization analysis demonstrates the occurrence of several antiparallel components a comparison between nrm and trm thermal demagnetizations is attempted in order to highlight small ptrms naturally recorded in a temperature interval of only a few degrees we attempt to retrieve the temperature at which each reversal occurs during the post metamorphic cooling this method called thermopalaeomagnetism is discussed and developed in order to delineate the sequence of polarity chrons recorded versus temperature

continuous production of nanosized magnetite through low grade burial geological processes such as burial can lead to remagnetization in rocks due to neoformed magnetic minerals that have passed a critical volume called blocking volume in this study we designed a heating experiment for claystones obtained from the paris basin france in the 50 130 c temperature range in order to simulate 20 nm and 20 nm minerals respectively this result led us to propose a conceptual model of nucleation and growth process of magnetite during low grade burial from 2 to 4 km depth ultrafine magnetite 20 nm was predominant over single domain magnetite 20 nm for 4 km depth transposed to natural conditions our heating steps experiment suggested that claystone type rocks are remagnetized during burial for temperatures higher than 200 c the extrapolation of our results indicated that burial remagnetization due to the chemical remanent magnetization might be larger than the natural remanent magnetization 2012 american geophysical union all rights reserved

chemical weathering since the little ice age recorded in lake sediments a high resolution proxy of past climate because of the different geochemical behaviour of rubidium and strontium in earth surface processes variations of the rb sr ratios in lake sediments were used as a geochemical proxy of chemical weathering and past climate in a single watershed low magnetic susceptibility low caco3 low sr concentration and hence high rb sr ratio in the lake sediments indicate weak chemical weathering under a cold but wet climate during the little ice age lia in the closed daihai lake watershed the concordant change in both sr and caco3 concentrations with 18o values in the dunde ice core suggests that weak chemical weathering during the wet lia was controlled by air temperature after the lia however precipitation played a dominant role in chemical weathering copyright 2001 john wiley sons ltd

a synthesis of hydrogen isotope variability and its hydrological significance at the qinghai tibetan plateau abstract hydrogen isotope ratios of sedimentary biomarkers are known to record the climatic variability in terrestrial and marine environments however there is still a lack of calibration studies that can quantitatively retrace the driving forces especially at the tibetan plateau here we elaborate the actual influence of environmental parameters such as temperature evapotranspiration salinity and biosynthetic fractionation on d values of n alkanes we measured hydrogen isotope values d of n alkanes in recent sediment and plant samples as well as d values of different water sources lake and inflow water precipitation as well as leaf root and soil water from six tibetan lakes along a 10 spanning longitudinal transect covering an aridity gradient as expected the deuterium record can be used to distinguish two water pools at the plateau i precipitation water which supplies water for the lake inflow and terrestrial plants and ii enriched lake water having a clear evaporative signal based on significant correlations of the source water and d values of n alkanes d of alkane n c 23 record the lake water isotope composition and track the evaporative enrichment of the lake system in contrast d of alkane n c 29 can be used to retrace the isotope composition of the inflow displaying the integrated rainfall signal in the vegetation period modified by soil and leaf water evaporation while temperature changes are less pronounced across the tibetan transect the isotopic difference between c 23 and c 29 could potentially be used as a proxy to reconstruct effective moisture precipitation minus evaporation areas with lower precipitation amounts and higher evaporation rates are characterized by higher d c 23 c 29 values this relationship is more pronounced in regions with mean annual precipitation below 350 mm even in contrasting environments the application of the d proxies is very promising combining other tibetan studies it is possible to present a comprehensive picture of the usage of compound specific hydrogen isotopes of n alkanes on the tibetan plateau

intrusion history of the portrush sill county antrim northern ireland evidence for rapid emplacement and high temperature contact metamorphism the gabbroic portrush sill in northern ireland part of the north atlantic igneous province intruded lower jurassic mudstones and siltstones about 55 ma ago we used petrologic observations and geochemical analyses to study how the sill interacted with the sedimentary rocks field relationships show that an upper sill and numerous associated minor intrusions were emplaced in the sedimentary host rocks before intrusion of the main sill some 10 m above its upper contact geochemical analyses reveal two magma contamination processes nb and ta anomalies coupled with incompatible element enrichment record contamination by deep crustal rocks whereas li pb and ba anomalies reveal a superficial contamination through fluid circulation at the contact between magmatic and sedimentary rocks analysis of mineral assemblages and geochemical data from the contact aureole demonstrate uniform metamorphic conditions between the two main intrusions and an absence of a thermal gradient the identification of pyrrhotite by magnetization analyses and of orthopyroxene by microprobe analyses indicates very high temperatures up to 660 c thermal modelling explains these temperatures as the coupled effects of the main sill and the earlier intruded upper sill and minor intrusions even though the chemical composition of the main sill suggests another type of parental liquid all three units were emplaced in a very short time certainly less than five years

geochemistry of dust aerosol over the eastern pamirs abstract the pamirs are situated in the inner part of the asian continent a region which plays a critical role in asian dust emission and transport this study measured the elemental composition of atmospheric dust aerosol samples collected during the period between july 2004 and april 2006 at mt muztagata eastern pamirs at a high altitude 38 17 n 75 01 e 4430 m the muztagata aerosol samples show ca al 0 7 and fe al 0 7 ratios that are distinguishable from those from other sites the la th ratios averaging 2 30 2 36 and th u ratios averaging 2 75 3 11 indicate typical eolian deposition the dust aerosol samples have very similar rare earth element ree patterns with relative enrichment of light ree a slight depletion of heavy ree and a strongly negative europium anomaly eu values averaging 0 65 the muztagata dust shows homogeneity of composition during the sampling period with minor variations due to seasonal effects and possible different air mass suggesting the possibility of a well mixed atmospheric background dust on a regional scale the zonal westerlies dominate and the longitude circulations are relatively very weak for dust transport dynamics in the pamirs and tien shan regions our results demonstrate a distinct difference in elemental composition between dust over the eastern pamirs and that over inilchek central tien shan indicating that the asian dust emission regions have great variations in their chemical properties and provide a better constraint on the climatic impact of asian dust