penetration of atlantic westerly winds into central and east asia abstract the geographical position of the convergence zone where the western atlantic climate system meets the asian winter monsoonal system depends on their relative strength these individual strengths are determined by the intensities of the air pressure gradients over the north atlantic and the siberian high pressure cell respectively since the factors that forced those gradients varied throughout glacial interglacial cycles the location of the convergence zone should have moved accordingly both the grain size characteristics of wind blown sediments and climate modelling prove that the atlantic influence extended remarkably far eastward at times the northeastern tibetan plateau was at the cross road of western and monsoonal influences during the last glacial this region was dominated by an atlantic signal with millennial scale climatic oscillations that are similar to the north atlantic marine and greenland ice core records in contrast that same region was only under monsoonal influence during the last interglacial it implies that the atlantic winds penetrated more vigorously eastward in full glacial conditions than in interglacial conditions

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

the geochemistry of rhenium and osmium in recent sediments from the black sea abstract the os isotopic composition of recent sediments from the black sea are reported with the concentrations of re os and major and minor elements osmium concentrations range from 0 23 to 0 69 ppb and re concentrations range from 21 to 85 ppb concentrations of both elements are large relative to average crustal material and are positively correlated with organic carbon 187 os 186 os ratios range from 5 7 to 7 1 all of these values are less radiogenic than the estimated 187 os 186 os ratio of seawater and average continental crust material variations in the os isotopic composition of these sediments are attributed to mixing of a hydrogenous component 187 os 186 os ratio between 7 and 9 and a less radiogenic detrital component 187 os 186 os ratio between 1 and 4 a mixing model is used to determine the fraction of os in the sediment associated with each component and the burial fluxes of re and os in the black sea are calculated in addition the data are used to estimate the magnitude of systematic error which such initial os isotopic heterogeneity may introduce into re os age determinations of ancient organic rich sediments

upstream subtropical signals preceding the asian summer monsoon circulation in this study the authors address several issues with respect to the antecedent signals of the large scale asian summer monsoon that were earlier identified by webster and yang in particular they revisit the changes in the subtropical upper tropospheric westerlies preceding the monsoon depict the detailed structure of the monsoon s antecedent signals and investigate the physical processes from the signals to the monsoon they also explore the teleconnection of these signals to various large scale climate phenomena and emphasize the importance of the upstream location of the signals relative to the tibetan plateau and the monsoon before a strong weak asian summer monsoon the 200 mb westerlies over subtropical asia are weak strong during the previous winter and spring a significant feature of these signals is represented by the variability of the middle east jet stream whose changes are linked to the arctic oscillation north atlantic oscillation el

1 500 year cycle in the arctic oscillation identified in holocene arctic sea ice drift decadal to centennial scale variability has been identified in the arctic oscillation but less is known about variations on the millennial scale a record of sea ice drift from off the alaskan coast shows a 1 500 year cycle in the phase of the arctic oscillation

molybdenum behavior during early diagenesis insights from mo isotopes 1 we present molybdenum isotope and concentration data from 14 sites in the eastern pacific from the central california to the peru margin the environments studied have been chosen to represent a broad range in oxidation reduction redox potential which provide a framework for the behavior of this redox sensitive element manganese rich hemipelagic sediments from the eastern tropical pacific have a mean characteristic mo isotope signature 98 95mo 0 49 0 04 two times the standard deviation of the mean 2 sdom with n 14 that reflects fractionation between ocean water and authigenic mo associated with mn oxides authigenic fe mo s deposits from reducing continental margin settings also have a characteristic mo isotopic signature 98 95mo 1 64 0 04 2 sdom with n 136 both of these values are in contrast to highly sulfidic 11 m h2saq restricted basin environments which contain mo isotope values analytically indistinguishable from seawater in terms of the mo isotope composition the modern oceanic mo sink is dominated by continental margin type environments where 98 95mo 1 6 and mn rich sediments where 98 95mo approximately 0 5 with a minor contribution from euxinic settings where h2saq 11 m

the record of sea water 187os 186os variation through the cenozoic the variation of sea water 187os 186os during the cenozoic has been determined by analyzing the hydrogenous os released by acidic hydrogen peroxide leaching of layers in an abyssal north pacific pelagic clay core overall sea water 187os 186os has increased from 3 2 at 58 ma to the present day value of 8 6 this pattern is similar to the well known 87sr 86sr evolution of sea water the increase in the proportion of continental os 187os 186os 10 could be due to enhanced weathering rates of black shales associated with the uplift of the himalayas other elements associated with black shale weathering such as u and p should show effects in the sedimentary record

persistent solar influence on north atlantic climate during the holocene surface winds and surface ocean hydrography in the subpolar north atlantic appear to have been influenced by variations in solar output through the entire holocene the evidence comes from a close correlation between inferred changes in production rates of the cosmogenic nuclides carbon 14 and beryllium 10 and centennial to millennial time scale changes in proxies of drift ice measured in deep sea sediment cores a solar forcing mechanism therefore may underlie at least the holocene segment of the north atlantic9s 1500 year cycle the surface hydrographic changes may have affected production of north atlantic deep water potentially providing an additional mechanism for amplifying the solar signals and transmitting them globally

os isotope chemostratigraphy applied to organic rich marine sediments from the eocene oligocene transition on the west african margin odp site 959 1 the known temporal relationship between the benthic foraminiferal 18o record and the marine os isotope record is used to reinterpret the absolute chronology and paleoceanographic context of an episode of organic carbon burial on the west african margin ocean drilling program odp site 959 although organic rich sediments require significant corrections for in situ decay of 187re to 187os these results demonstrate the utility of the marine os isotope record for chemostratigraphic correlation of organic rich sediments that are devoid of age diagnostic microfossils with pelagic carbonate sequences revision of the odp site 959 chronology shifts the age assignment of an interval of biosiliceous organic rich sediment deposition from the oligocene to the late eocene and earliest oligocene likely culminating with the first major glaciation of the oligocene oi1 we speculate that enhanced organic carbon burial over much of the west african margin may have contributed to drawdown of atmospheric carbon dioxide before and during the oi1 event and suggest that os isotope chemostratigraphy provides a valuable tool for further exploring this possibility

climatic and local effects on stalagmite δ13c values at lianhua cave china abstract carbon isotopes in speleothems may serve as indicators of vegetative change climatic conditions and karst processes in many recent studies of chinese stalagmites however carbon isotopes have often been neglected or underutilized in interpreting paleoenvironments here we present a continuous decadal scale 13 c record 819 measurements of the mid to late holocene from a precisely dated 10 230 th dates aragonite stalagmite from lianhua cave hunan province china compared to coeval stalagmites from other chinese caves the average 13 c value 3 6 of stalagmite a1 is higher by 2 7 variations in the 13 c values 0 1 to 6 0 reflect changes in both vegetative productivity and inorganic processes which respond to climatic processes the 13 c record of stalagmite a1 can be subdivided into three intervals 1 warm humid stage 6 6 to 3 8 ka 2 transitional stage 3 8 to 1 6 ka and 3 cool arid stage 1 6 ka to present comparisons with other stalagmite and paleoclimatic records demonstrate that these intervals are generally consistent with changes in regional vegetation and climatic conditions

teleconnections between the subtropical monsoons and high latitude climates during the last deglaciation the major deglacial intensification of the southwest monsoon occurred at 11 450 150 calendar years before present synchronous with a major climate transition as recorded in greenland ice an earlier event of monsoon intensification at 16 000 150 calendar years before present occurred at the end of heinrich layer 1 in the atlantic and parallels the initial rise in global atmospheric methane concentrations and the first abrupt climate changes in the antarctic thus the evolution of the monsoonal and high latitude climates show teleconnections but hemispheric asymmetries superimposed on abrupt events the monsoonal climate shows high frequency variability of 1785 1450 and 1150 year oscillations and abrupt climate change seems to occur when at least two of these oscillations are in phase

natural mass dependent variations in the isotopic composition of molybdenum we present the first observations of natural mass dependent fractionation of the isotopic composition of molybdenum mo using multi collector inductively coupled plasma mass spectrometry variations in the isotopic composition of mo are reported as 97 95mo 97mo 95mo sample 97mo 95mo standard 1 1000 external analytical precision of 97 95mo is 1 between sediments deposited under anoxic conditions 97 95mo 1 02 to 1 52 relative to our in house standard and ferromanganese nodules 97 95mo 0 63 to 0 42 97 95mo of pacific ocean seawater 97 95mo 1 48 lies within the range of values for anoxic sediments closest to modern black sea anoxic sediments molybdenites from continental ore deposits have intermediate 97 95mo ranging from 0 26 to 0 09 variations in the abundances of 92mo 95mo 96mo 97mo and 98mo are consistent with mass dependent fractionation a sporadic unidentified interference occurs at mass 94 and 100mo is not measured we hypothesize that the 97 95mo offset between anoxic sediments and ferromanganese nodules results from mo isotope fractionation during inefficient scavenging of mo from seawater by mn oxides under oxic conditions the similarity in 97 95mo of anoxic sediments and seawater is consistent with the very efficient removal of mo from seawater under anoxic conditions in the presence of h2s the data can be interpreted in terms of a steady state mass balance between the mo flux into the oceans from the continents and the mo flux out of the oceans into oxic and anoxic sediments such an interpretation is quantitatively consistent with existing estimates of the removal fluxes of mo to anoxic and oxic sediments these findings suggest that 97 95mo in seawater may co vary with changes in the relative proportions of anoxic and oxic sedimentation in the oceans and that this variation may be recorded in 97 95mo of anoxic sediments hence the mo isotope system may be useful in paleoredox investigations

the osmium isotopic composition change of cenozoic sea water as inferred from a deep sea core corrected for meteoritic contributions a detailed study was made of the oceanic 187os 186os variation through the cenozoic by using a hydrogen peroxide leaching procedure on a pelagic clay core from the north pacific long lines 44 gpc3 a 6 h2o2 solution was used the range of 187os 186os obtained by this procedure was between 2 and 8 2 a milder leach 0 15 h2o2 in three out of four samples yielded a slightly higher 187os 186os value than the stronger leach implying that the stronger leach released some meteoritic os with 187os 186os of 1 using published 3he concentrations in gpc3 and cosmic dust 3he and os fluxes to deep sea sediments the extraterrestrial os concentration was estimated for each core segment and subtracted to yield corrected values of 187os 186os the results based on the milder leaching protocol and the 3he correction protocol yield similar values the corrected ocean water 187os 186os pattern for the past 25 million years is similar to that obtained by other workers and is compatible with other results for the entire cenozoic the variation with time in gpc3 uniquely provides a statigraphic signature for the paleogene

rhenium osmium and molybdenum isotope systematics of black shales from the lower cambrian niutitang formation sw china evidence of a well oxygenated ocean at ca 520 ma abstract the early cambrian was a key period in earth s history and the reconstruction of ancient ocean chemistry on spatial and temporal scales can contribute to a better understanding of events during that period new early cambrian data for re os and mo isotopic compositions and redox sensitive elements rse are reported here for the niutitang formation se chongqin sw china a re os age of 520 30 myr model 3 2 n 21 mswd 62 and a 187os 188os i ratio of 0 79 0 11 were obtained consistent with previously reported data for equivalent strata from other areas separate regression of samples with 187os 188os i of 0 72 0 79 and 0 81 0 89 produce more precise ages of 520 1 9 5 myr model1 2 n 7 mswd 1 0 and 513 10 myr model 1 2 n 8 mswd 0 96 respectively suggesting heterogeneity in the primary source of os as the dominant cause of scatter and uncertainty high initial 187os 188os of 0 79 0 11 may indicate a high os input from oxidative weathering of upper continental crust for early cambrian ocean based on rse data such as mo u v enrichment and ni co v cr v v ni moef uef re mo ratios two geochemically distinct zones upper and lower can be delineated in the sedimentary sequence as reflected in drillcore sections the upper section n 9 is characterized by low total organic carbon toc contents mean 1 8 1 5 wt low rse contents mo 0 008 0 006 10 3 g g v 0 22 0 15 10 3 g g u 0 008 0 004 10 3 g g and low v cr 2 0 1 7 ni co 5 9 3 5 moef uef 2 6 1 8 and mo toc 6 0 3 9 ratios in contrast the lower core section n 13 is characterized by high toc contents 4 4 2 2 wt high rse contents mo 0 10 0 08 10 3 g g v 1 0 1 0 10 3 g g u 0 05 0 04 10 3 g g high v cr 6 5 ni co 24 14 moef uef 6 3 2 2 and mo toc 21 9 ratios uncertainties are 1sd here significantly decreasing trends of those redox proxies from lower to upper sections suggest that the depositional conditions evolved from anoxic euxinic to oxic condition all samples display extremely low re mo ratios of 0 2 10 3 1 9 10 3 indicating the absence of intermediate reducing conditions possibly reflecting rapid ocean oxygenation the mo isotopic composition in the sedimentary succession of the yc9 core exhibits a large variation with 98 95mo values of 0 04 2 00 upper section 98 95mo values display an opposite trend to mn content and negative correlation with mo toc ratios indicating mo fractionation associated with adsorption on mn oxides or oxyhydroxides lower section 98 95mo values are positively correlated with mo toc ratios and negatively with re mo ratios suggesting incomplete conversion of molybdates to tetrathiomolybdates under weakly euxinic conditions high 98 95mo of 2 00 and high average mo toc ratios of 21 10 6 g g wt were observed in euxinic shales in the niutitang formation suggesting extensive ocean oxygenation may have taken place at ca 520 ma

uranium and molybdenum isotope systematics in modern euxinic basins case studies from the central baltic sea and the kyllaren fjord norway abstract recent investigations have revealed significant fractionation of 238u 235u between organic rich sediments of anoxic marginal seas and seawater indicating redox dependent u isotope fractionation this study explores the conditions controlling u isotope fractionation in selected modern anoxic basins baltic sea landsort and gotland deeps and the kyllaren fjord in norway and compares u with mo isotope fractionation therefore the concentrations and isotope compositions of dissolved u and mo from the water column and organic rich sediments from three currently anoxic basins have been analysed the water column samples from the kyllaren fjord display a moderate depletion of u and a strong depletion of mo with increasing depth these variations are associated with a decrease in 238u and an increase in 98mo with depth from 0 35 to 0 70 and from 2 4 to 2 6 respectively from the u isotope composition of the deep euxinic water column of the kyllaren fjord a minimum value of 238ured 0 7 for the u isotope fractionation during reduction is inferred due to the high sedimentation rate in the kyllaren fjord surface sediment samples are only moderately enriched in u and mo 4 g g and 6 g g to 37 g g respectively and display 238u and 98mo averages of 0 26 0 10 and 2 18 0 21 respectively water column samples from the baltic sea display a minor decrease in the u and mo concentrations and only negligible u and mo isotope fractionation compared to open seawater the sediment layers from the baltic sea landsort deep which were deposited under mostly anoxic conditions are moderately enriched in u up to 8 g g and highly enriched in mo up to 222 g g however u and mo isotopic compositions are significantly lighter with 238u of 0 36 0 08 and 98mo of 0 04 0 11 than those of typical organic rich sediments from anoxic basins these findings reveal that compared to mo the u isotopic composition of sediments in restricted and strongly euxinic basins like the kyllaren fjord is significantly more dependent on 1 the extent of u removal from the water column which is lower than that of mo and on 2 the sedimentation rate i e the fraction of authigenic u relative to detrital u in the sediment which is also lower compared to that of mo in more open and only temporarily euxinic basins such as the investigated basins of the baltic sea strong mo is coupled with weak u isotope fractionation between water and sediment these signatures were likely the result of isotope fractionation under weakly sulfidic conditions and the generation of isotopically light mo during frequently occurring flushing events with oxygen rich water from the open sea this implies that mo and u isotope signatures of sediments only record paleo water column redox conditions of restricted basins if the water column was permanently stratified

synchroneity of tropical and high latitude atlantic temperatures over the last glacial termination a high resolution western tropical atlantic sea surface temperature sst record from the cariaco basin on the northern venezuelan shelf based on mg ca values in surface dwelling planktonic foraminifera reveals that changes in sst over the last glacial termination are synchronous within 30 to 90 years with the greenland ice sheet project 2 air temperature proxy record and atmospheric methane record the most prominent deglacial event in the cariaco record occurred during the younger dryas time interval when ssts dropped by 3 to 4 c a rapid southward shift in the atmospheric intertropical convergence zone could account for the synchroneity of tropical temperature atmospheric methane and high latitude changes during the younger dryas

proxy based temperature reconstruction in china for the holocene abstract the temperature variation in china is closely related to the dynamics of global climate change in order to reveal its possible connections with global climate during the holocene we present a nationwide temperature reconstruction in china for the past 12 000 years based on multiple proxies with three statistical methods including empirical orthogonal function composite plus scale and simple arithmetic average our reconstruction shows that the late 20th century temperature in china is the highest at least over the past millennium although the low temperature in the period of 10 000 12 000 yr bp was mainly related to the north atlantic cool conditions the reconstructed china s temperature followed solar forcing variations more closely than climatic influence from the north atlantic the asian monsoon was also closely connected to temperature change in china over the holocene

influence of atlantic meridional overturning circulation on the east asian winter monsoon east asian summer monsoon precipitation varied on millennial timescales during the last glacial period sediment records and climate modelling suggest that the winter monsoon was also affected by millenial scale variability and that the abrupt changes were driven by changes in the strength of the atlantic meridional overturning circulation

the variation of summer monsoon precipitation in central china since the last deglaciation abstract two stalagmites c996 1 and c996 2 collected from the jiuxian cave in the shaanxi province in central china have been studied with u series dating and stable isotope analysis thirty eight 230 th dating results showed that the stalagmite c996 1 was continuously deposited through the last 8 5 ka bp thousand years before present present 1950 ad and c996 2 was deposited through the last 19 ka bp excluding two growth hiatuses between 8 3 and 5 7 ka bp and 15 4 and 11 9 ka bp with a relatively stable boundary condition we interpret the 18 o of speleothem calcite as most indicative of the amount of summer monsoon precipitation although temperature and other factors might have some minor impact the 18 o records show notable changes within the last glacial maximum lgm resembling other east asian monsoon records such as those from the hulu and sanbao caves suggesting that significant monsoonal climate changes occurred in eastern asia as far north as the qinling mountains during the lgm interval a comparison of our records to precisely dated contemporaneous speleothem records from other caves shows that the increasing trend of 18 o during the holocene commenced as early as 7 5 ka bp in the low latitude monsoonal area i e the hoti cave while at higher latitudes this shift occurred later such as 7 0 ka bp in the dongge cave 5 3 ka bp in the heshang cave 4 7 ka bp in the sanbao cave and 4 5 ka bp in the jiuxian cave these results imply an asynchronous change of the summer monsoon precipitation occurred in east asia during the holocene the asynchrony may be related to the responses of a coupled tropical and subtropical monsoon system to changes of the insolation and the differences in thermal forcing which result from the complex geographical configuration the variation of sea surface temperature sst in the western tropical pacific may also have important impacts on the summer monsoon precipitation changes in central and northern china because it affects the northwestern pacific subtropical high a monsoon front regulator more robust tests are needed to confirm this phenomenon and to evaluate the contribution of different factors in detail

paleotemperature variability in central china during the last 13 ka recorded by a novel microbial lipid proxy in the dajiuhu peat deposit the asian summer monsoon is a very important climatic component affecting the land ecosystem on the eastern asian continent here we assess microbe derived lipid biomarker evidence from a well date

mo total organic carbon covariation in modern anoxic marine environments implications for analysis of paleoredox and paleohydrographic conditions 1 sedimentary molybdenum mo s has been widely used as a proxy for benthic redox potential owing to its generally strong enrichment in organic rich marine facies deposited under oxygen depleted conditions a detailed analysis of mo s total organic carbon toc covariation in modern anoxic marine environments and its relationship to ambient water chemistry suggests that 1 mo s while useful in distinguishing oxic from anoxic facies is not related in a simple manner to dissolved sulfide concentrations within euxinic environments and 2 patterns of mo s toc covariation can provide information about paleohydrographic conditions especially the degree of restriction of the subchemoclinal water mass and temporal changes thereof related to deepwater renewal these inferences are based on data from four anoxic silled basins the black sea framvaren fjord cariaco basin and saanich inlet and one upwelling zone the namibian shelf representing a spectrum of aqueous chemical conditions related to water mass restriction in the silled basin environments increasing restriction is correlated with a systematic decrease in mo s toc ratios from 45 5 for saanich inlet to 4 5 1 for the black sea this variation reflects control of mo s by mo aq which becomes depleted in stagnant basins through removal to the sediment without adequate resupply by deepwater renewal the basin reservoir effect the temporal dynamics of this process are revealed by high resolution chemostratigraphic data from framvaren fjord and cariaco basin sediment cores which exhibit long term trends toward lower mo s toc ratios following development of water column stratification and deepwater anoxia mo burial fluxes peak in weakly sulfidic environments such as saanich inlet owing to a combination of greater mo aq availability and enhanced mo transport to the sediment water interface via fe mn redox cycling and are lower in strongly sulfidic environments such as the black sea and framvaren fjord these observations demonstrate that at timescales associated with deepwater renewal in anoxic silled basins decreased sedimentary mo concentrations and burial fluxes are associated with lower benthic redox potentials i e more sulfidic conditions these conclusions apply only to anoxic marine environments exhibiting some degree of water mass restriction e g silled basins and are not valid for low oxygen facies in open marine settings such as continent margin upwelling systems

contrasting impacts of the arctic oscillation on surface air temperature anomalies in southern china between early and middle to late winter abstractin the boreal winter the arctic oscillation ao evidently acts to influence surface air temperature sat anomalies in china this study reveals a large intraseasonal variation in the relationship between the winter ao and southern china sat anomalies specifically a weak in phase relationship occurs in december but a significant out of phase relationship occurs in january and february the authors show that the linkage between the ao and southern china sat anomalies strongly depends on the ao associated changes in the middle east jet stream mejs and that such an ao mejs relationship is characterized by a significant difference between early and middle to late winter in middle to late winter the azores center of high pressure anomalies in the positive ao phase usually extends eastward and yields a significantly anomalous upper level convergence over the mediterranean sea which can excite a rossby wave train spanning the arabian sea and intensify the mejs in early winter however the azo

reconstruction of early cambrian ocean chemistry from mo isotopes the neoproterozoic cambrian transition was a key time interval in the history of the earth especially for variations in oceanic and atmospheric chemical composition however two conflicting views exist concerning the nature of ocean chemistry across the precambrian cambrian boundary abundant geochemical evidence suggests that oceanic basins were fully oxygenated by the late ediacaran while other studies provide seemingly conflicting evidence for anoxic deep waters with ferruginous conditions fe ii enriched persisting into the cambrian here two early cambrian sedimentary platform and shelf slope sections in south china were investigated to trace early cambrian ocean chemistry from mo isotopes the results reveal that early cambrian sediments deposited under oxic to anoxic euxinic conditions have 98 95mo values ranging from 0 28 to 2 29 which suggests that early cambrian seawater may have had 98 95mo values of at least 2 29 similar to modern oceans the heaviest and relatively homogeneous 98 95mo values were recorded in siltstone samples formed under completely oxic conditions which is considered that mn oxide free shuttling was responsible for such heavy 98 95mo value further combined with fe species data and the accumulation extent of mo and u the variation of 98 95mo values in the two studied sections demonstrate a redox stratified ocean with completely oxic shallow water and predominantly anoxic even euxinic deeper water having developed early on which eventually became completely oxygenated this suggests that oceanic circulation at the time became reorganized and such changes in oceanic chemistry may have been responsible for triggering the cambrian explosion of biological diversity

the osmium riverine flux and the oceanic mass balance of osmium the osmium concentration os and isotopic composition were determined in a set of 17 of the largest rivers of the world os varies between 4 6 and 52 1 pg kg and the 187os 188os ratio varies between 0 64 and 2 94 measurement of rainwater samples shows that there is no input of oceanic os to the continent through rain assuming a negligible anthropogenic os input in the dissolved load the natural average river water has an os concentration of 7 9 pg kg and a mean 187os 188os ratio of 1 54 the total riverine flux of os to the ocean is estimated to be 295 kg yr the dissolved os flux from island arcs and oceanic islands represents less than 5 of the total riverine flux and is not further considered the continental os flux to the ocean is then represented by the riverine flux as dissolved os from eolian dust and glacial sediments is negligible assuming steady state it is possible to estimate a maximum unradiogenic flux to the ocean of 126 kg yr cosmic dust or mantle derived and an oceanic residence time between 2 5 104 and 5 4 104 with a mean of 3 5 104 year the estimation of the flux of dissolved cosmic particles shows that their contribution to the seawater os would be 14 of the contribution of the unradiogenic component which means that the mantle derived flux should contribute a major part the first results on water from high temperature axial hydrothermal systems indicate that their input is probably negligible which would necessitate that dominant contribution from the low temperature alteration of the oceanic crust and or of the ultramafic exposures contributes dominantly to the input of unradiogenic os to the seawater we show that it would be necessary to leach 1 3 of the os contained in the volume of ultramafic exposures accessible to seawater to account for all of the unradiogenic component contribution another simpler but less likely possibility is that the dissolved cosmic dust represents the only source of dissolved unradiogenic os to the ocean in which case the riverine input represents 94 of the total dissolved flux to the ocean instead of 70 the modern global dissolved os flux to the ocean would then have a 187os 188os ratio of 1 44 instead of 1 06 and the system would be far from steady state

asynchronous holocene optimum of the east asian monsoon abstract the spatial and temporal distribution of summer monsoon precipitation or effective moisture during the holocene has been reconstructed on the basis of geological data including lake levels pollen profiles and loess paleosol records in addition the summer july precipitation increment effective precipitation and monsoon strength index have been obtained from numerical modeling experiments both geological data and numerical modeling indicate that the holocene optimum as defined by peak east asian summer monsoon precipitation was asynchronous in central and eastern china reaching a maximum at different times in different regions e g ca 10 000 8000 yr ago in northeastern china 10 000 7000 yr ago in north central and northern east central china ca 7000 5000 yr ago in the middle and lower reaches of the yangtze river and ca 3000 yr ago in southern china in southwestern china the maximum appeared ca 11 000 yr ago but probably was related to the maximum landward extension of the indian summer monsoon the regional shift in the maximum precipitation belt from northwest to southeast over the past 10 000 yr is interpreted as a response to changing seasonality related to orbital forcing of the climate the southeastward shift of the east asian summer monsoon maximum is consistent with the progressive weakening of the summer monsoon as the summer solar radiation anomaly decreased progressively through the holocene and the east asian monsoon index declined while the early maximum in southwestern china matches the maximum of the indian monsoon index

determination of molybdenum isotope fractionation by double spike multicollector inductively coupled plasma mass spectrometry 1 molybdenum isotopic compositions are precisely determined by mc icp ms measurements using a mo double spike the double spike is added prior to chemical purification so that laboratory and instrumental mass fractionations are separated from natural mass dependent fractionation fractionation is determined on four mo mass ratios providing an internal consistency check the external standard reproducibility is at 0 06 per mil on the 98mo 95mo ratio 2 standard deviation using a normal microconcentric nebuliser with a cyclonic spray chamber the minimum quantity of mo is 1 g for high precision results a hydrothermal molybdenite shows fractionation of 0 3 per mil on the 98mo 95mo ratio relative to our standard johnson matthey 1000 g ml 0 3 icp standard solution lot 602332b fine grained sediments show fractionation of 0 1 and 0 3 per mil on the 98mo 95mo ratio the observed mo isotope fractionation is small but resolvable with the presented high resolution technique

evolution of the irminger and east icelandic current systems through the holocene revealed by diatom based sea surface temperature reconstructions abstract sediment core md99 2269 from the northern shelf of iceland has been studied to assess the stability of the east icelandic irminger current system during the holocene the 25 4 m long holocene section was studied with a 40 years average resolution the chronology is based on the combination of 24 14c ams dates and two tephra layers diatoms are utilized to quantify the development and variability of the august sea surface temperature sst for the last 11 5 cal kyr bp three different transfer function methods have been used to reconstruct august sst at the site weighted average partial least squares wa pls maximum likelihood ml and imbrie and kipp i k the combined use of the three methods allows the august sst reconstruction with a higher confidence level than with using only one statistical technique these three reconstructions generate very similar august sst trends through the holocene the onset of the holocene climate optimum is characterized at the site by a rapid august sst increase at 10 4 cal kyr bp of almost 4 c the holocene climate optimum is recorded between 10 4 and 4 7 cal kyr bp the august ssts were around 2 c warmer during the major part of the holocene compared with the august ssts for the later half of the holocene combining the diatom record with the benthic foraminifera records from the same core enables us to reconstruct the evolution of both surface and bottom water masses during the holocene this reconstruction shows dominance of the arctic waters throughout the water column at the onset of the holocene and subsequent increase of the influence of the atlantic water especially during the holocene climate optimum this strong dominance of atlantic water tends to decrease between 4 7 and 2 2 cal kyr bp especially in the upper part of the water column since 2 2 cal kyr bp the trend is reversed suggesting an increased influence of atlantic water at least in the upper water column

large holocene summer temperature oscillations and impact on the peopling of the northeastern tibetan plateau summer temperatures on the tibetan plateau tp significantly affect stability of glaciers that provide steady water resources to nearly half of the world population however lack of reliable long term proxy records greatly impedes understanding of regional temperature sensitivity to climate forcings here we present a 16ka long alkenone based summer temperature record from lake qinghai northeastern tp that demonstrates major regional temperature response to changes in summer insolation and atlantic meridional overturning circulation during the holocene and late glacial importantly we find a period of sustained summer temperature decline 4 degrees c between 5 and 3 5ka which coincides with expansion of barents sea ice coverage and is likely driven by intensification of the westerlies this unusually long and pronounced regional cooling event likely delayed permanent human settlements on the high altitude regions 3000m of the tp by at least 500years

holocene environmental and climatic changes inferred from wulungu lake in northern xinjiang china abstract sedimentological geochemical and palynological data from wulungu lake in northern xinjiang china are used to reconstruct environmental and climate changes since 9550 cal yr bp high abundance of sparganium and poaceae low md median diameter and 13 c organic values indicate aridity between 9550 and 6730 cal yr bp high md and 13 c organic values and the prevalence of desert steppe and steppe vegetation between 4200 and 560 cal yr bp indicate that effective moisture increased after 6730 cal yr bp peaking at 4200 and 560 cal yr bp low md values a negative excursion of 13 c org and the transition from steppe to desert vegetation since 560 cal yr bp reflect a decrease in effective moisture during the latest holocene late holocene human activities were indicated by sharp increase in the abundance of pediastrum then variations in carbonate contents indicate that temperature was generally high between 9550 and 7740 cal yr bp low between 7740 and 6730 cal yr bp intermediate between 6730 and 560 cal yr bp and low during the last 560 yr regional comparison indicates that the asian monsoon did not extend to wulungu lake and westerlies were the main factor in determining the moisture availability during the holocene

chemical weathering controls on variations in the molybdenum isotopic composition of river water evidence from large rivers in china abstract mo isotopic composition in large rivers is very important for understanding global mo cycle at present temporal variation signatures in the 98 95 mo in large rivers have not been investigated which hinder a comprehensive understanding on the mechanism for the variations of mo isotopic compositions in river water in this study we report a one year long time series march 2010 to march 2011 of the 98 95 mo of both the water and suspended particulates collected at guiping from the middle reaches of the xijiang river xjr and of the water 98 95 mo from the lower reaches of the huanghe river hhr or yellow river collected at lijin china the results indicate that the temporal variations in the concentration and 98 95 mo of dissolved mo in the xjr range from 4 32 to 10 5 nmol l mean 7 31 nmol l and 1 04 to 1 31 relative to nist 3134 mean 1 20 respectively but that the particulates have a lower 98 95 mo 0 18 to 0 58 analysis of the suspended particulates and other chemical parameters of the river water suggests that the weathering of silicates and sulfides is the main contributor to the dissolved mo content in the xjr subsequently the highly efficient preferential trapping of lighter 98 95 mo by weathering products such as clay minerals fe mn oxides and organic materials in soils and saprolites which are abundant in the tropical catchment is the key to the heavy mo isotope signatures in the xjr furthermore it seems that neither mo scavenging by suspended particulates during riverine transportation nor the weathering of sulfates significantly influences the 98 95 mo of the xjr water cross comparison among the 98 95 mo in the waters of the three largest rivers in china the changjiang cjr hhr and xjr indicates that the hhr has the highest mo concentrations but the lightest 98 95 mo the xjr has the lowest mo concentrations but the heaviest 98 95 mo and the values for the cjr are intermediate between those for the other two rivers this supports the conclusion that the efficiency of the selective trapping of mo by soils and saprolites is the main factor controlling 98 95 mo in these large rivers the tropical subtropical xjr catchment experiences intense chemical weathering the semi arid temperate hhr catchment experiences very little and the cjr catchment falls somewhere between the two such a relationship between water mo concentrations and 98 95 mo agrees with previous observations from many large rivers worldwide it is therefore suggested that chemical weathering on continents is the key to variations in the isotopic composition of mo in the waters of large rivers

molybdenum isotope fractionation during adsorption by manganese oxides the isotopic composition of mo 97 95mo in seawater is 2 heavier than mo in marine ferromanganese crusts and nodules barling et al earth planet sci lett 193 2001 447 457 siebert et al earth planet sci lett 211 2003 159 171 to explore this phenomenon we have conducted an experimental investigation into the mass dependent fractionation of mo isotopes during adsorption onto mn oxyhydroxide two series of experiments were carried out a time series in which adsorption proceeded for 2 96 h and a ph series in which ph varied from 6 5 to 8 5 the extent of mo adsorption by mn oxyhydroxides decreases with increasing ph a trend typical of anion adsorption and takes 48 h to reach steady state lighter mo isotopes are preferentially adsorbed experimentally determined fractionation factors soln mnox exhibit no systematic variation with either time or experimental ph the mean soln mnox for all experiments is 1 0018 0 0005 2 s d comparison of the mo isotopic data for experimental solutions and mo adsorbed to mn oxyhydroxide with predictions for closed system equilibrium and rayleigh fractionation models indicates that isotope fractionation occurs as a result of closed system equilibrium exchange between dissolved and adsorbed mo the isotopic offset between dissolved and adsorbed mo is comparable to that observed between mo in seawater and mo in ferromanganese nodules and crusts it is therefore likely that adsorption of mo to mn oxyhydroxides is a significant factor in the fractionation of mo isotopes in the oceans

evidence from wavelet analysis for a mid holocene transition in global climate forcing a strong mid holocene transition has been identified by wavelet analyses in several sea ice cover records from the circum antarctic area ice core records taylor dome byrd and tropical marine records the results are compared with those previously published in a synthesis of north atlantic records and with 4 new records from the norwegian and icelandic seas and from a coastal site in ireland these new records confirm the previous pattern for the north atlantic area extend this pattern nearly to the arctic circle and include a continental record we further tested the possibility of extending this scheme using continental records from south america the holocene pattern proposed here confirms the importance of external forcing during the early holocene solar activity 1000 years cyclicity and 2500 years during the entire holocene even if the signal is disturbed by meltwater fluxes the second part of the holocene is then marked by the gradual appearance of internal forcing thermohaline circulation around 1500 years associated with a stabilisation of the signal coupling between ocean and atmosphere seems to play a fundamental role in the observed frequencies which vary accordingly in the atlantic circum antarctic and pacific areas the north atlantic area seems to be the instigator of thermohaline circulation as shown by its sensitivity to meltwater discharges during the early holocene even though each sector is independent with regards to its frequency content around 1600 years for atlantic area around 1250 years for antarctica the holocene methane pattern still under debate ruddiman w f 2003a orbital insolation ice volume and greenhouse gases quaternary science review 22 1597 1629 ruddiman w f 2003b the anthropogenic greenhouse era began thousands of years ago climatic change 61 261 293 could be explained by a more efficient thermohaline circulation around the mid holocene with an anthropogenic effect initiated at not vert similar2500 bp as shown by the inter hemispheric gradient

coupled molybdenum iron and uranium stable isotopes as oceanic paleoredox proxies during the paleoproterozoic shunga event the paleoproterozoic era was a time of remarkable importance in the redox evolution of earth s atmosphere and oceans here we present a multi proxy study of mo u and fe isotopes together with fe speciation of black shales and siltstones from the upper zaonega formation of the onega basin in karelia we attempt to better understand oceanic redox conditions during the 2 05 ga shunga event as the next step following the great oxidation event goe and the lomagundi carbon isotope excursion event a cautious examination of the fe speciation data shows that the studied section was deposited under dominantly euxinic conditions anoxic and sulfidic and that the lower part of the section experienced metamorphism through which pyrite was altered to pyrrhotite during this episode the system was closed with respect to fe but not sulfur the mo and u isotopic compositions corrected for detrital input were not affected by the metamorphism and loss of s and are fairly uniform throughout the entire section the fe isotope compositions are exceptionally heavy in the lower part of the section up to 56 fe irmm 14 0 83 and become lighter towards the upper intervals which also show significant mo and u enrichments we suggest that this pattern reflects changes in the position of the deposition site relative to the redox structure of the water column the upper part was deposited within a locally euxinic portion of the basin where h 2s availability was highest removal of mo and u was more efficient and precipitated pyrite captured relatively non fractionated dissolved fe in other words quantitative uptake of fe was favored in contrast the lower interval was deposited on the lower margin of a euxinic wedge where h 2s availability was lower and removal of mo and u was less efficient pyrite precipitation in this part of the water column reflected a more fractionated dissolved fe reservoir due to more protracted non quantitative fe uptake because of less efficient pyrite formation under lower sulfide conditions and greater access to the large oceanic pool of fe the u isotopic signal was corrected for detrital contribution giving compositions similar to the riverine input and suggesting that co precipitation into carbonates was the main process of u removal at this time we estimate the mo isotope composition of the contemporaneous ocean to be 98 mo sw 0 85 0 21 this is the lowest value yet reported for the proterozoic ocean suggesting that the oceanic mo cycle was dominated by euxinic and anoxic sinks with negligible mo removal into oxic environments recent studies have proposed a sharp increase in ocean atmosphere oxygen levels during the lomagundi event followed by a dramatic crash our results from black shales of the 2 05 ga shunga event are consistent with a post lomagundi decrease in biospheric oxygen levels

general characteristics of temperature variation in china during the last two millennia three alternate china wide temperature composites covering the last 2000 years were established by combining multiple paleoclimate proxy records obtained from ice cores tree rings lake sediments and historical documents five periods of temperature variation can be identified a warm stage in ad 0 240 a cold interval between ad 240 and 800 a return to warm conditions from ad 800 1400 including the medieval warm period between ad 800 1100 the cool little ice age period between 1400 1920 and the present warm stage since 1920 regional temperature variation is found during ad 800 1100 when warm conditions occurred in eastern china and in the northeastern tibetan plateau and in ad 1150 1380 when the southern tibetan plateau experienced a warm interval in contrast evidence for cool conditions during the lia is more consistent among the proxy records the temperature reconstructions for china and the northern hemisphere show good agreement over the past millennium

mid to late holocene climate change an overview the last 6000 years are of particular interest to the understanding of the earth system because the boundary conditions of the climate system did not change dramatically in comparison to larger glacial interglacial changes and because abundant detailed regional palaeoclimatic proxy records cover this period we use selected proxy based reconstructions of different climate variables together with state of the art time series of natural forcings orbital variations solar activity variations large tropical volcanic eruptions land cover and greenhouse gases underpinned by results from general circulation models gcms and earth system models of intermediate complexity emics to establish a comprehensive explanatory framework for climate changes from the mid holocene mh to pre industrial time the redistribution of solar energy due to orbital forcing on a millennial timescale was the cause of a progressive southward shift of the northern hemisphere nh summer position of the intertropical convergence zone itcz this was accompanied by a pronounced weakening of the monsoon systems in africa and asia and increasing dryness and desertification on both continents the associated summertime cooling of the nh combined with changing temperature gradients in the world oceans likely led to an increasing amplitude of the el nin o southern oscillation enso and possibly increasingly negative north atlantic oscillation nao indices up to the beginning of the last millennium on decadal to multi century timescales a worldwide coincidence between solar irradiance minima tropical volcanic eruptions and decadal to multi century scale cooling events was not found however reconstructions show that widespread decadal to multi century scale cooling events accompanied by advances of mountain glaciers occurred in the nh e g in scandinavia and the european alps this occurred namely during the little ice age lia between ad w1350 and 1850 when the lower summer insolation in the nh due to orbital forcing coincided with solar activity minima and several strong tropical volcanic eruptions the role of orbital forcing in the nh cooling the southward itcz shift and the desertifi cation of the sahara are supported by numerous model simulations other simulations have suggested that the fingerprint of solar activity variations should be strongest in the tropics but there is also evidence that changes in the ocean heat transport took place during the lia at high northern latitudes with possible additional implications for climates of the southern hemisphere sh

a high resolution absolute dated late pleistocene monsoon record from hulu cave china oxygen isotope records of five stalagmites from hulu cave near nanjing bear a remarkable resemblance to oxygen isotope records from greenland ice cores suggesting that east asian monsoon intensity changed in concert with greenland temperature between 11 000 and 75 000 years before the present yr b p between 11 000 and 30 000 yr b p the timing of changes in the monsoon as established with 230th dates generally agrees with the timing of temperature changes from the greenland ice sheet project two gisp2 core which supports gisp2 s chronology in this interval our record links north atlantic climate with the meridional transport of heat and moisture from the warmest part of the ocean where the summer east asian monsoon originates

continental scale temperature variability during the past two millennia past global climate changes had strong regional expression to elucidate their spatio temporal pattern we reconstructed past temperatures for seven continental scale regions during the past one to two millennia the most coherent feature in nearly all of the regional temperature reconstructions is a long term cooling trend which ended late in the nineteenth century at multi decadal to centennial scales temperature variability shows distinctly different regional patterns with more similarity within each hemisphere than between them there were no globally synchronous multi decadal warm or cold intervals that define a worldwide medieval warm period or little ice age but all reconstructions show generally cold conditions between ad 1580 and 1880 punctuated in some regions by warm decades during the eighteenth century the transition to these colder conditions occurred earlier in the arctic europe and asia than in north america or the southern hemisphere regions recent warming reversed the long term cooling during the period ad 1971 2000 the area weighted average reconstructed temperature was higher than any other time in nearly 1 400 years

rhenium and osmium isotopes in black shales and ni mo pge rich sulfide layers yukon territory canada and hunan and guizhou provinces china rhenium and osmium abundances and osmium isotopic compositions were determined by negative thermal ionization mass spectrometry for samples of devonian black shale and an associated ni enriched sulfide layer from the yukon territory canada the same composition information was also obtained for samples of early cambrian ni mo rich sulfide layers hosted in black shale in guizhou and hunan provinces china this study was undertaken to constrain the origin of the pge enrichment in the sulfide layers samples of the ni sulfide layer from the yukon territory are highly enriched in re os and other pge with distinctly higher re 192os but similar pt re compared to the black shale host re os isotopic data of the black shale and the sulfide layer are approximately isochronous and the data plot close to reference isochrons which bracket the depositional age of the enclosing shales samples of the chinese sulfide layers are also highly enriched in re os and the other pge re 192os are lower than in the yukon sulfide layer re os isotopic data for the sulfide layers lie near a reference isochron with an age of 560 ma similar to the depositional age of the black shale host the osmium isotopic data suggest that re and pge enrichment of the brecciated sulfide layers in both the yukon territory and in southern china may have occurred near the time of sediment deposition or during early diagenesis during the middle to late devonian and early cambrian respectively

application of the 187re 187os system to black shale geochronometry the decay of 187re to 187os provides a tool for determining depositional ages of black shales re and os concentrations and os isotopic compositions of whole rock samples of the bakken shale a mississippian devonian boundary black shale yield a whole rock isochron with an age of 354 49 ma this age is in agreement with the accepted age of the bakken shale 360 ma the initial 187os186os ratio of the isochron is 6 2 3 3 this value is indistinguishable from the 187os186os ratios observed in modern black sea sediments the concentration of common os in bakken shale samples is strongly correlated with total nitrogen indicating that a large fraction of the os in these samples is associated with a hydrogenous component which overwhelms any os supplied to the sediment in association with detrital material or cosmic dust the dominance of the hydrogenous component imparts a relatively homogeneous initial 187os186os ratio to the sediment at the time of deposition the degree of scatter about the isochron exceeds the degree of scatter expected from either analytical error or from the estimated degree of initial isotopic heterogeneity the pattern of scatter is consistent with postdepositional mobilization of re and or os on a small spatial scale we suggest that this mobilization may be a consequence of petroleum formation and migration in the bakken shale

a model for uranium rhenium and molybdenum diagenesis in marine sediments based on results from coastal locations abstract the purpose of this research is to characterize the mobilization and immobilization processes that control the authigenic accumulation of uranium u rhenium re and molybdenum mo in marine sediments we analyzed these redox sensitive metals rsm in benthic chamber pore water and solid phase samples at a site in buzzards bay massachusetts usa that has high bottom water oxygen concentrations 230 300 mol l and high organic matter oxidation rates 390 mol c cm2 y the oxygen penetration depth varies from 2 to 9 mm below the sediment water interface but pore water sulfide is below detection release of rsm to pore waters during the remineralization of solid phases occurs near the sediment water interface at depths above the zone of authigenic rsm formation this release occurs consistently for mo at both sites but only in the winter for re in buzzards bay and intermittently for u at the buzzards bay site re removal to the solid phase extends to the bottom of the profile while the zone of removal is restricted to 2 9 cm for u and mo authigenic re formation is independent of the anoxic remineralization rate which is consistent with an abiotic removal mechanism the rate of authigenic u formation and its modeled removal rate constant increase with increasing anoxic remineralization rates and is consistent with u reduction being microbially mediated authigenic mo formation is related to the formation of sulfidic microenvironments the depth and extent of mo removal from pore water is closely associated with the balance between iron and sulfate reduction and the consumption of pore water sulfide via iron sulfide formation pore water rsm reach constant asymptotic concentrations in sulfidic sediments but only pore water re is constant at depth in buzzards bay the increases in pore water u at the buzzards bay site are consistent with addition via irrigation and subsequent upward diffusion to the removal zone deep pore water mo concentrations exceed its bottom water concentration due to irrigation induced oxidation and remobilization from the solid phase in sulfidic sediments there is no evidence for higher pore water u or mo concentrations at depth due to the absence of irrigation and or the presence of more stable authigenic rsm phases there are good correlations between benthic fluxes and authigenic accumulation rates for u and mo in sulfidic sediments however results from buzzards bay suggest irrigation ultimately results in the partial loss of u and mo from the solid phase with accumulation rates that are 20 30 of the modeled flux irrigation can augment re possibly u or compromise u mo authigenic accumulation in sediments and is important when determining burial rates in continental margin sediments

a high resolution stalagmite record of the holocene east asian monsoon from mt shennongjia central china high resolution oxygen isotope 18o profiles of six stalagmites from sanbao cave in hubei province central china established with 1413 oxygen isotope data and 65 230th ages provide a continuous history of east asian summer monsoon easm intensity for the period from 13 0 2 thousand years before present ky bp relative to ad 1950 the 18o record includes four distinct stages in the evolution of the easm 1 an abrupt transition 11 5 ky bp into the holocene 2 a period of gradual increase in monsoon intensity 11 5 9 5 ky bp 3 the maximum humid period 9 5 6 5 ky bp and 4 a period of gradual decline in monsoon intensity 6 5 0 2 ky bp comparison of sanbao with regional records of comparable resolution reveals that the timing of the beginning and end of the holocene optimum as defined by the minimum in 18 o was similar in the indian and east asian monsoon systems this supports the idea that shifts in the monsoon tied to shifts in the mean position of the inter tropical convergen

correlation between climate events in the north atlantic and china during the last glaciation episodes of massive iceberg release heinrich events 1 3 into the north atlantic ocean during the last glaciation were associated with recurring episodes of unusually cold north atlantic surface water bond cycles 4 and cold air temperatures over greenland dansgaard oeschger events 5 6 four of the youngest of these cold events have also been reported in climate records from sites outside the north atlantic region7 but until now the entire suite has been identified only in north atlantic marine sediments greenland ice core records and tentatively in french lake sediments8 here we examine grain size data from chinese loess and intercalated accretionary palaeosols of last glacial age for evidence of similar climate signals remote from the north atlantic region we see grain size maxima with ages that match those of the last six heinrich events which we interpret as an indication of the changing strength of the east asian winter monsoon which largely controls the transport and deposition of central asian aeolian dust thus it seems that these heinrich events have left their signature in the chinese loess record this is consistent with simulations of the glacial climate9 which imply that the climates of the north atlantic and china were linked by the effect of westerly winds

wind blown sand on beaches an evaluation of models five models for predicting rates of aeolian sand transport were evaluated using empirical data obtained from field experiments conducted in april 1994 at a beach on inch spit co kerry republic of ireland measurements were made of vertical wind profiles to derive shear velocity estimates beach slope and rates of sand transport sediment samples were taken to assess characteristics of grain size and surface moisture content estimates of threshold shear velocity were derived using grain size data after parsing the field data on the basis of the quality of shear velocity estimation and the occurrence of blowing sand 51 data sets describing rates of sand transport and environmental conditions were retained mean grain diameter was 0 17 mm surface slopes ranged from 0 02 on the foreshore to about 0 11 near the dune toe mean shear velocities ranged from 0 23 m s 1 just above the observed transport threshold to 0 65 m s 1 rates of transport ranged from 0 02 kg m 1 h 1 to more than 80 kg m 1 h 1 these data were used as input to the models of bagnold bagnold r a 1936 the movement of desert sand proc r soc london a157 594 620 kawamura kawamura r 1951 study of sand movement by wind translated 1965 as university of california hydraulics engineering laboratory report hel 2 8 berkeley zingg zingg a w 1953 wind tunnel studies of the movement of sedimentary material proc 5th hydraulics conf bull 34 iowa city inst of hydraulics pp 111 135 kadib kadib a a 1965 a function for sand movement by wind university of california hydraulics engineering laboratory report hel 2 8 berkeley and lettau and lettau lettau k and lettau h 1977 experimental and micrometeorological field studies of dune migration in k lettau and h lettau eds exploring the world s driest climate university of wisconsin madison ies report 101 pp 110 147 correction factors to adjust predictions of the rate of transport to account for the effects of slope and moisture content were calculated using the models of bagnold bagnold r a 1973 the nature of saltation and bed load transport in water proc r soc london ser a 332 473 504 and belly belly p y 1964 sand movement by wind u s army corps eng cerc tech mem 1 washington d c 38 pp respectively none of the models was able to produce a strong correspondence between measured and predicted rates of transport best results were obtained using the bagnold and zingg models and the kadib model was the least viable of this group the influence of sediment moisture content appeared to be the critical factor in degrading model viability overall none of the models is adequate for general applications to coastal aeolian environments where moisture content complications tend to override the predictive competence of the simple transport formulations c 1998 elsevier science b v

molybdenum isotope evidence for global ocean anoxia coupled with perturbations to the carbon cycle during the early jurassic relatively brief periods of severe paleoenvironmental change during the jurassic and cretaceous were associated with the widespread accumulation of organic rich marine deposits termed oceanic anoxic events oaes these intervals involved abrupt global warming of 5 10 c higher rates of continental weathering elevated extinction rates and large scale perturbations to the global carbon cycle the major oaes also overlapped temporally the emplacement of large igneous provinces however despite being known as oaes the extent of seawater anoxia at those times is undefined and the causative processes remain unclear here we show how changes in seawater molybdenum isotope ratios a proxy for seawater anoxia during the toarcian early jurassic oae define the onset and expansion of oxygen deficient conditions our data also place constraints on the areal extent of marine anoxia during the event and demonstrate that anoxia expanded and contracted periodically broadly in line with precession driven changes in 13 corg despite their intermittent occurrence over geological history oaes have an important contemporary relevance because the magnitude and high rates of environmental change then were broadly similar to those occurring at the present day