exchange of neodymium and its isotopes between seawater and small and large particles in the sargasso sea the concentrations and isotopic ratios of nd were measured in small 1 53 m and large 53 m in situ filtered particles sediment trap material sediment and aerosol samples collected during oligotrophic and productive conditions in the sargasso sea atmospheric input is the main source of exogeneous nd into the sargasso sea based on ndal ratios in trapped material we suggest that up to 50 of atmospheric nd could dissolve at the air sea interface the concentration and isotopic composition of nd vary with both depth and the particle size neodymium concentrations of small and large particles increase with depth from 2 9 12 g g and 5 8 11 8 g g respectively at 40 m the nd signature of large particles is significantly more negative nd 0 13 1 0 2 than that of the small ones nd 0 11 7 0 3 and the particles filtered deeper below 237 m the nd 0 values for small particles match the seawater whereas the nd 0 values of large particles remain less radiogenic these differences are related to the different behaviors of the particulate fractions small particles apparently exchange nd with seawater and then sink rapidly to depth as biologically formed aggregates large particles these aggregates isolate the small particles slowing the exchange with the water to produce different signatures between large particles and seawater since these exchange processes modify the particulate nd signature the shallow radiogenic nd 0 signature observed in the surface and subsurface waters in the nw atlantic is not due to external contributions and is likely of advective origin at depth below 800 m the magnitude of the aggregation disaggregation process varies significantly with season the fraction of small particles repackaged into large aggregates can vary from 0 8 to 80 100 in productive and oligotrophic conditions respectively this seasonal variation may correspond to sinking particles of different zooplanktonic origin vertical fluxes of nd into and out of the sargasso sea as well as exchange fluxes in the water column are examined about 50 of the particulate flux from the surface is remineralized at depth a net scavenging flux of 0 8 10 4 g m2 y is required this value corresponds to a scavenging nd residence time of 1900 y considerably longer than previous estimates of global oceanic residence time of nd on the order of 400 y

elemental carbon record of paleofire history on the chinese loess plateau during the last 420 ka and its response to environmental and climate changes an elemental carbon ec record covering the last 420 ka was reconstructed using chemical oxidation method on a loess and paleosol sequence from the lingtai section on the chinese loess plateau the ec record reveals the paleofire history and its relationship with climate and vegetation changes on the chinese loess plateau our results show that the ec abundance is generally higher in the paleosols than in the loess layers showing a glacial interglacial pattern which is coincident with biomass changes this variation pattern indicates that paleofires were intensified when biomass accumulated and climate changed abruptly especially from wet to dry conditions the ec abundance increases sharply at similar to 130 kab p indicating a dramatic change in the vegetation and climate variation patterns the occurrence of a peak value with the highest average ec abundance in the holocene may reflect the occurrence of a major climate event at similar to 6 kab p and may also be partly due to more frequent anthropogenic fire usages c 2007 elsevier b v all rights reserved

late quaternary aeolian activity in the mu us and otindag dune fields north china and lagged response to insolation forcing 1 dune fields in parts of northern china contain important stratigraphic records of late quaternary change in the east asian monsoon in this study 33 new optically stimulated luminescence osl ages and other measurements from aeolian sediment sections are used to reconstruct the timing of wet dry climate variation in the mu us and otindag dune fields of north china the results indicate dune activity and dry climate in the last few hundred years 14 ka to about 7 8 ka and 50 ka to 60 ka the dunes were mainly stable implying a wetter climate between about 7 8 ka and 2 4 ka these results imply a lag of several thousand years between peak summer insolation at 10 11 ka and high summer monsoon rainfall after 7 8 ka in the investigated regions the monsoon climate may not respond directly to orbital forcing over millennial time scales land surface feedbacks may account for lagged dune field response

cycling of dissolved rare earth elements in chesapeake bay the measurement of dissolved rare earth elements ree in the surface waters water column of anoxic basins and pore waters in chesapeake bay has provided new insights to the biogeochemistry and estuarine chemistry of ree all dissolved ree from the riverine source show large scale removal in the lower salinity zone 0 10 the shale normalized pattern of dissolved susquehanna river water is opposite in form to that of surface sediments in the bay the former is enriched in heavy ree hree while the latter are enriched in light ree lree neither component has a flat pattern normally assumed for river transported ree to the ocean the dissolved lree i e la ce nd sm and eu are enriched 3 to 9 times in the oxygen depleted deep waters over their concentrations in the oxic surface waters in contrast hree er yb and lu are slightly depleted all ree concentrations in the two surface 0 2 cm pore waters are greatly enriched 8 17 times 43 times for ce relative to oxygen depleted bottom waters and have relative abundances opposite to those of their sediments while ce has a positive anomaly in the pore waters negative anomalies exist in the oxygen depleted bottom waters the ree are participating in a set of complex biogeochemical cycles within the water column and surface sediments fractionation of ree during these cycles leads to the preferential enrichment of lree in the seasonally oxygen depleted bottom waters it is proposed that this fractionation is coupled to redox cycles of mn and fe and the interaction of dissolved ree with suspended particles and surficial sediments

multiple expansions of c4 plant biomass in east asia since 7 ma coupled with strengthened monsoon circulation the expansion of plants using the c4 photosynthetic pathway is one of the most prominent changes in the global ecosystem during the cenozoic era a significant late miocene expansion is well documented however the existence and cause of subsequent expansions are still not clear owing in part to the lack of long continuous climate vegetation records here we present two high resolution carbon isotope time series covering the past 7 m y derived from eolian deposits on the chinese loess plateau the data indicate three intervals of significant c4 plant expansions within the semiarid monsoonal region of east asia ca 2 9 2 7 ma 1 3 0 9 ma and 0 6 ma present these expansions covary with strengthened east asian summer monsoon circulation we conclude that in east asia large scale late miocene c4 c3 vegetation changes in semiarid areas have been primarily driven by warm seasonal precipitation and temperature variations associated with changes in monsoon circulation

a megalandslide in the northern caucasus foredeep uspenskoye russia geomorphology possible mechanism and age constraints abstract an extremely large landslide 2 8 km 3 in volume and 32 km 2 in area has been detected on the right bank of the kuban river in the northern caucasus foredeep russian federation the megalandslide has a very gently inclined slip surface

changes in fire regimes on the chinese loess plateau since the last glacial maximum and implications for linkages to paleoclimate and past human activity abstract a high resolution black carbon bc record from 27 5 kyr bp to present was reconstructed using a chemical oxidation method on loess and paleosol samples from the lijiayuan section of the chinese loess plateau the black carbon mass sedimentation rates bcmsr and carbon isotopic record reveal a paleofire history and its relationship with climate and vegetation changes at the study site the bcmsr record was decomposed into two components background bcmsr and the bcmsr peaks the background bcmsr represents regional fires and shows high fire activities occurred contemporaneous with the younger dryas older dryas heinrich events and greenland stadials as registered in the loess grain size record this suggests a rapid response of regional fires on the loess plateau to abrupt climate changes spectral analysis of background bcmsr showed two meaningful periodicities of 1620 and 1040 years close to the cyclicity of the east asian monsoon as recorded in the stalagmite 18o record in central china this indicates a tight control of millennial scale wet dry changes in the monsoonal climate on regional fires on the loess plateau by contrast the bcmsr peaks are considered to reflect local fire episodes the occurrences of local fires were more frequent during the last glacial period with a maximum frequency of 6 episodes 1000 years during the last glacial maximum lgm 22 3 to 14 6 kyr bp when the climate was drier and more continuous grassy fuels existed on the landscape during the last glacial interglacial transition lgit period 14 6 to 11 0 kyr bp fire frequency was largely reduced due to an increase in precipitation and more woody vegetation if the lgit period is taken as an analog for the projected near future then future global warming alone may not produce large wildfires in northwestern china wildfires remained infrequent during the early to middle holocene biomass burning increased after 4 0 kyr bp when the climate became drier and land use was more intensive bc carbon isotope ratios may well reflect changes in the vegetation being burnt i e grasses versus trees yielding results consistent with the associated pollen data in the region

a new approach to the nd residence time in the ocean the role of atmospheric inputs concentrations of rare earth elements ree and nd isotopic ratios were analyzed for seawater filtered suspension and sediment trap samples collected in the tropical atlantic ocean eumeli program eutrophic mesotrophic and oligotrophic sites 20 n 18 21 w this is the first ree nd dataset on solution and different sized particles collected at the same site we present direct evidence of the nd isotopic exchange between particulate lithogenic fraction and seawater without significant mass transfer this exchange is probably one of the main factors that simultaneously constrains the nd concentration and isotopic ratio budget we propose a new approach to estimate the residence time of nd in the ocean nd based on isotopic exchange 200 yr nd 1000 yr the exchange requires a partial dissolution of lithogenic nd we estimate that the fraction of soluble nd proportion in atmospheric dust is of the order of 20 based on the isotopic ratios we suggest that the partial dissolution of atmospheric fallout is probably one of the main ree sources of the ocean

chemical evolution of rare earth elements fractionation between colloidal and solution phases of filtered river water abstract within the dissolved fraction of connecticut river water there is large scale fractionation of rare earth elements between colloidal and solution phases colloidal particles carry a large part of the dissolved ree pool filtration through filters with progressively smaller pore size 0 45 0 22 and 0 025 m results in filtrates with lower absolute concentrations of ree and more fractionated compositions with decreasing pore size the filtrates have shale normalized abundances which become systematically more depleted in the lree relative to the hree hence both the solution and colloidal phases are significantly fractionated with respect to the dissolved fraction these conclusions are consistent with previous studies of ree in rivers estuaries and soils and with surface solution speciation models colloids play an important role in the aquatic geochemistry of rare earth elements they are responsible for the fractionation of ree during the estuarine removal of ree the removal of suspended and colloidal particles from river water leaves the solution phase of river water with compositions of ree as evolved as those of surface seawater hence the evolution of the composition of ree in river water and seawater is significantly influenced by colloid formation during chemical weathering on the continents

the rare earth element chemistry of estuarine surface sediments in the chesapeake bay abstract concentrations of rare earth elements ree were measured in the fine fraction of shallow surface sediments from the lower chesapeake bay as studies suggest that coagulation of colloids dominates the removal of ree from the dissolved load in low salinity regions of estuaries ree in sediments showed spatial heterogeneity both laterally and longitudinally we observed the influence of salinity on the spatial and temporal variability in total ree content and on eu anomalies lower total ree was found in sediments deposited along the eastern shore the chemistry of the mid bay islands was distinct with higher overall ree content and slightly more positive ce anomaly values positive ce ce in the mid bay are attributed to coriolis acceleration restricting freshwater flow towards the west away from the islands leading to oxidative removal of ce from the pore waters to the sediments the presence of positive eu eu in sediments deposited along the eastern shore is linked to salinity we suggest that unique local conditions along the eastern shore possibly including submarine groundwater discharge of nutrient rich reducing waters combined with organic matter decay and the predominance of oceanic water flow toward this region leads to the development of a strong localized salinity gradient this study demonstrates the utility of ree sediment chemistry in identifying and resolving local and estuarine wide geochemical processes

tracing the provenance of fine grained dust deposited on the central chinese loess plateau eolian dust deposits in north china provide an excellent means of determining past variations in continental paleoclimate and atmospheric circulation however debate still exists on which deserts in east asia are the dominant sources of chinese loess and whether the dust provenance has shifted significantly at different time scales here we present new constraints on the provenance of fine grained dust deposited on the central chinese loess plateau clp by combining electron spin resonance signal intensity and crystallinity index of fine grained quartz contained in samples from two loess paleosol sequences our results show that the fine grained dust deposits on the clp originate mainly from the gobi desert in southern mongolia and the sandy deserts in northern china primarily the badain juran and tengger deserts rather than from the taklimakan desert in western china at least during the last climatic cycle the dominant source of fine grained dust varied significantly from southern mongolia during cold periods to northern china during warm periods the glacial interglacial provenance fluctuations are strongly coupled with changes in the intensity of the near surface northwesterly winter monsoon

comparison of quantification methods to measure fire derived black elemental carbon in soils and sediments using reference materials from soil water sediment and the atmosphere black carbon bc the product of incomplete combustion of fossil fuels and biomass called elemental carbon ec in atmospheric sciences was quantified in 12 different materials by 17 laboratories from different disciplines using seven different methods the materials were divided into three classes 1 potentially interfering materials 2 laboratory produced bc rich materials and 3 bc containing environmental matrices from soil water sediment and atmosphere this is the first comprehensive intercomparison of this type multimethod multilab and multisample focusing mainly on methods used for soil and sediment bc studies results for the potentially interfering materials which by definition contained no fire derived organic carbon highlighted situations where individual methods may overestimate bc concentrations results for the bc rich materials one soot and two chars showed that some of the methods identified most of the carbon in all three materials as bc whereas other methods identified only soot carbon as bc the different methods also gave widely different bc contents for the environmental matrices however these variations could be understood in the light of the findings for the other two groups of materials i e that some methods incorrectly identify non bc carbon as bc and that the detection efficiency of each technique varies across the bc continuum we found that atmospheric bc quantification methods are not ideal for soil and sediment studies as in their methodology these incorporate the definition of bc as light absorbing material irrespective of its origin leading to biases when applied to terrestrial and sedimentary materials this study shows that any attempt to merge data generated via different methods must consider the different operationally defined analytical windows of the bc continuum detected by each technique as well as the limitations and potential biases of each technique a major goal of this ring trial was to provide a basis on which to choose between the different bc quantification methods in soil and sediment studies in this paper we summarize the advantages and disadvantages of each method in future studies we strongly recommend the evaluation of all methods analyzing for bc in soils and sediments against the set of bc reference materials analyzed here

rare earth elements in river waters we measured rare earth element ree concentrations in river waters to characterize the suspended and dissolved river flux of the ree to the oceans the ree pattern of river suspended materials is sensitive to drainage basin geology a positive correlation is observed between la yb ratios and nd model ages for the rivers studied major rivers have light ree enriched patterns relative to the north american shale composite nasc with la yb n 1 6 2 7 river water dissolved material 0 2 m is heavy ree enriched relative to suspended material and the most pronounced negative ce anomalies occur in rivers of high ph light ree concentrations vary by approximately 3 orders of magnitude and are inversely related to ph and major cation concentrations from these data we estimate that typical major river runoff has heavy ree depleted suspended material with la yb n 1 9 we conclude that the terrigenous input to the oceans from major rivers is heavy ree depleted relative to shales from the available data average river water dissolved material appears to be heavy ree enriched with la yb n 0 4 estuarine removal processes lower the dissolved ree river flux by approximately 60 and result in a flux that is more heavy ree enriched with la yb n 0 2 calculated oceanic residence times with respect to river input range from 2300 to 21 000 years are shortest for ce and greatest for the heavy ree and la such long residence times may suggest the presence of additional sources of ree in seawater

coherent changes in relative c4 plant productivity and climate during the late quaternary in the north american great plains evolution of the mixed and shortgrass prairie of the north american great plains is poorly understood because of limited proxies available for environmental interpretations buried soils in the great plains provide a solution to the problem because they are widespread both spatially and temporally with their organic reservoirs serving as a link to the plants than once grew on them through stable carbon isotopic analysis of soil organic carbon d

recharge to the inter dune lakes and holocene climatic changes in the badain jaran desert western china we present new estimates on evaporation and groundwater recharge in the badain jaran desert western inner mongolia of northwestern china based on a modified penman equation suitable for lakes in china geochemical data and water balance calculations suggest that local rainfall makes a significant contribution to groundwater recharge and that past lake level variations in this desert environment should reflect palaeoclimatic changes the chronology of lake level change established by radiocarbon and u series disequilibrium dating methods indicates high lake levels and a wetter climate beginning at ca 10 ka and lasting until the late mid holocene in the badain jaran desert the greatest extension of lakes in the inter dune depressions indicates that the water availability was greatest during the mid holocene relicts of neolithic tools and pottery of qijia culture 2400 1900 bc suggest relatively intensive human activity in the badain jaran desert during the early and middle holocene supporting our interpretation of a less harsh environment wetter climates during the holocene were likely triggered by an intensified east asian summer monsoon associated with strong insolation

magnetic susceptibility evidence of monsoon variation on the loess plateau of central china during the last 130 000 years the magnetic susceptibility of loess and paleosols in central china represents a proxy climate index closely related to past changes of precipitation and vegetation and thus to summer monsoon intensity time series of magnetic susceptibility constructed for three loess paleosol sequences in the southern part of the chinese loess plateau document the history of summer monsoon variation during the last 130 000 yr they correlate closely with the oxygen isotope record of stages 1 to 5 in deep sea sediments soils were forming during intervals of strong summer monsoon whereas loess units were deposited at times of reduced monsoon intensity the chinese loess paleosol sequence can thus be viewed as a proxy record of asian monsoon variability extending over the last 2 5 myr

the behaviour of the rare earth elements during mixing of river and sea waters rare earth element concentrations have been measured in organic rich luce river water and coastal sea water concentrations e g 350 1850 pmol kg nd in the water of luce and 45 350 pmol kg nd in luce bay are related to the presence of particles with 30 60 of the ree associated with 0 4 0 7 m particles and to riverine fe concentrations ree fractionation occurs in the river water the submicrometre river water is heavy ree enriched whereas the coarser fraction has a more shale like ree pattern laboratory experiments show that the ree in organic rich river waters are chiefly associated with feorganic matter colloids which flocculate during estuarine mixing preferential removal of heavy ree 95 relative to light ree 60 occurs but no ce anomaly is developed in contrast no ree removal occurs during estuarine mixing with organic poor river water

importance of vertical geochemical processes in controlling the oceanic profiles of dissolved rare earth elements in the northeastern indian ocean vertical profiles of dissolved rare earth elements rees were obtained in the bay of bengal and the andaman sea the ree concentrations at various depths in the bay of bengal are the highest in the indian ocean this is attributable ultimately to the large outflow of the ganges brahmaputra and irrawaddy rivers but the dissolved ree flux to surface waters alone cannot explain the large and near constant ree enrichment throughout the entire water column the underlying fan sediments serve as not a source but a sink for dissolved ree iii s absence of excess 228ra in the deep waters suggests that lateral input of dissolved rees from slope sediments is also small in these regions partial 0 3 dissolution of detrital particles which are carried by the rivers and lateral surface currents and subsequently settle through the water column appears to be a predominant source for the dissolved rees vertical profiles showing an almost linear increase with depth are common features for the light and middle rees everywhere but their concentration levels are variable from basin to basin and from element to element this suggests that their oceanic distributions respond quickly to the variation of particle flux and its ree composition through reversible exchange equilibrium with suspended and sinking particles much like the case for th the relative importance of the vertical geochemical processes of reversible scavenging over the horizontal basin scale ocean circulation with passive regeneration like nutrients decreases systematically from the light to the heavy rees using a model the mean oceanic residence times of rees in the bay of bengal are estimated to range from 37 years for ce to 140 1510 years for the strictly trivalent rees in the deep water of the andaman sea isolated from the bay of bengal by the andaman nicobar ridge maximum sill depth of 1800 m the ree concentrations are almost uniform presumably due to rapid vertical mixing the ree iii concentrations are similar to that of 1250 m depth water in the bay of bengal consistent with other oceanographic properties however the ree composition of the deep water appears to be altered slightly by preferential scavenging of the light ree iii at the bottom boundary of the basin

stable carbon isotope of black carbon in lake sediments as an indicator of terrestrial environmental changes an evaluation on paleorecord from daihai lake inner mongolia china abstract we measured the carbon isotope ratio of black carbon bc from the daihai lake sediment core dh99a in north central china with an objective to examine the effectiveness and sensitivity of the 13c values of bc 13cbc as a potential indicator of terrestrial environmental changes we first performed a statistical study on the available data regarding carbon isotope fractionation cif during the conversion of c3 and c4 vegetation to bc and observed that the mean cif for bc produced from c3 plants is 0 3 whereas that for bc from c4 plants is 1 7 this result provides a solid reference for reconstructing vegetation and environmental changes using the 13cbc values the 13cbc record in the dh99a sediment core spanning the last ca 10 000 years displayed large variations from 23 7 to 29 2 which suggests that c3 plants dominantly occupied the daihai lake region during the holocene the most negative 13cbc peaks coincided with high values of tree percentages and grain sizes which occurred under relatively wetter climatic conditions during the middle holocene ca 6500 3200 cal yr bp and an interval between 1700 and 1350 cal yr bp in contrast the least negative 13cbc values corresponded to low values of tree percentages and grain sizes during relatively drier phases of the early and late holocene the generally negative correlation of the 13cbc values with the tree percentages and grain sizes was thought to reflect a negative correlation of the 13cbc values with the monsoon precipitation this correlation is consistent with the response of carbon isotope in modern c3 plants to precipitation in north china therefore we developed a computational model to reconstruct the changes in annual precipitation over the daihai lake region using the 13cbc values the results indicated that the annual precipitation was highly variable ranging from 170 mm lower to 310 mm higher than that at present during the middle holocene whereas the annual precipitation was generally 70 mm lower than that at present during the early and late holocene the general features of the inferred precipitation changes using the 13cbc values are generally consistent with those reconstructed using pollen data of the same sediment core meanwhile the 13cbc values tend to register some extreme variations of monsoon precipitation which were not reflected in the pollen assemblages we conclude that the 13cbc values in the daihai lake sediments may serve as a sensitive and reliable proxy for monitoring monsoon precipitation

the comparative behaviors of yttrium and lanthanides in the seawater of the north pacific yttrium has long been recognized as an ekalanthanide because of its chemical contiguity relative to the chemistries of rare earth elements which are in recent years intensively utilized for elucidation of metal scavenging processes in the ocean here we present the first detailed depth profile of y iii in seawater together with the lanthanides in the north pacific ocean the concentrations of y iii range 70 290 pmol kg and show the nutrient like profile best resembling that of ho iii amongst the other rare earth elements the results agree well with an expectation based on the similarity in their ionic radii and hence stability constants of complexation with carbonate ions yet the ho iii y iii ratios in seawater systematically increase with depth suggesting that y and ho are fractionated during scavenging by natural marine particulates this is likely to result from the different complexation behavior in that y iii is more weakly complexed than ho iii with soft organic ligands on the surface of particulate matter during scavenging in the surface water but once released into seawater in the deep sea y iii is complexed with carbonate ions equally or stronger than ho the pattern of deep water enrichment in the lanthanide series appears to be consistent with the recent observation of partitioning between suspended particles and seawater our precise measurements also indicated that pr and tb best resemble nd and dy respectively in their oceanic behavior whereas ho and tm are intermediate between their neighboring rare earth elements

rare earth elemental concentrations in some pacific northwest rivers abstract soluble rare earth element ree concentrations have been measured for five pacific northwest rivers columbia fraser klamath rogue and sacramento using radiochemical neutron activation analysis the relative abundances of the ree in river water resemble the ree abundance pattern in average sedimentary type rocks the la material balance in the oceans is discussed

linkage between c4 vegetation expansion and dune stabilization in the deserts of ne china during the late quaternary abstract knowledge of the role of c4 vegetation expansion in regulating dune stabilization during the late quaternary is important for understanding the evolution of the environment and climate in the climatically sensitive margin of the east asian summer monsoon however due to the lack of both reliable vegetation records and chronological control during episodes of dune activity the relationship between c4 vegetation expansion and dune stabilization in the deserts of ne china during the late quaternary remains unclear here we present a new regional compilation of late quaternary stable carbon isotopic data from organic matter together with dated records from sub aerial sedimentary deposits in the region our compilation of vegetation development and dune activity in these deserts suggests that c4 vegetation expansion with a main contribution from chenopodiaceae and a secondary contribution from poaceae and cyperaceae may have played a critical role in dune semi stabilization with a northwestward trend from 13 to 9 ka and dune stabilization throughout the entire study region from 9 to 5 ka in addition during the late holocene intensified millet cultivation or overgrazing and or climatic deterioration at 1 5 ka likely caused an increase in c4 vegetation abundance in the deserts of ne china which is associated with a dune mobilization episode in contrast after 2 5 ka decreased vegetation cover and dune mobilization in the mu us and hobq deserts were likely caused by poor land use practices and historically documented droughts overall c4 vegetation expansion was the main cause of dune stabilization in these deserts during the late quaternary

dissolved rare earth elements and hydrography in the sulu sea because most southeast asian basins are flushed rapidly by waters from the western pacific the effects of respiration and silica dissolution within those basins are hardly discernible based on general hydrographic and geochemical observations broecker et al 1987 however a different situation is expected for the sulu sea because its deep water is isolated from other deep sea basins by shallow sills of 400 m depth hence any heterogeneity of elemental distributions within the basin must be ascribed to vertical physical and biogeochemical processes we have obtained detailed vertical profiles of dissolved rare earth elements rees together with conductivity temperature depth ctd and hydrographic measurements in the sulu sea during the 1996 97 hakuho maru cruise vertical profiles clearly indicate that the ree iii s are enriched in the deep water due to regeneration and are involved in vertical biogeochemical cycling the light ree iii s la pr and nd showed a subsurface maximum at 500 m and a minimum 1500 m in depth none of the hydrographic properties or nutrients shows such features therefore the ree iii s and their elemental ratios may be useful as tracers of water masses the relationship of ree iii s versus dissolved si suggests that the ree iii s are regenerated in a delayed fashion relative to si and it is likely that their dissolution largely occurs at the bottom interface the mean residence time of the deep water is estimated to be 300 6 150 yr consequently the calculated benthic flux of ree iii s becomes roughly comparable with the river and atmospheric fluxes because of oxidation to ce iv the ce profile shows a decrease from 6 pmol kg at the surface to a constant value at 3 9 pmol kg below 1500 m unlike the other rees the rees in the sulu sea are enriched in the middle rees and ce relative to those of the north pacific deep water and must derive from local sources around the sulu sea copyright 1999 elsevier science ltd

distribution patterns of the elements in the ocean a synthesis the observed partition patterns of elements between a marine solid phase e g pelagic clays deep sea manganese nodules seamount manganese crusts and plankton and seawater can be to the first approximation explained by the james and healy adsorption model except for ce co mn pb and tl which are concentrated in the solid phase through oxidative uptake processes in other words the composition of seawater is mainly controlled by the compositions of source materials i e the crustal igneous rocks and primary magmatic volatiles and the physicochemical principles regulating the adsorption processes of various chemical species in seawater onto oceanic pelagic clays

the geochemistry of rare earth elements in the amazon river estuary abstract the estuarine geochemistry of rare earth elements rees was studied using samples collected in the amazon river estuary from the amasseds amazon shelf sediment study cruise of august 1989 extensive removal of dissolved 0 22 m filtered trivalent rees from river water occurs in the low 0 6 salinity region removal by the salt induced coagulation of river colloids leads to fractionation among the ree iii series the order of removal is light rees middle rees heavy rees there also is the enhanced removal of ce relative to trivalent la and nd in the low salinity 0 6 zone and in the zone of high biological activity this is the first field observation of strong ce removal associated with coagulation of river colloids and biological productivity the argument is made that the decrease in the ce anomaly across a biological front is caused by biologically mediated oxidation of ce iii to ce iv coagulation of river colloids and biologically mediated oxidation of ce iii lead to fractionation of ree iii and redox modification of ce these processes result in the ree composition becoming fractionated relative to the amazon river water and crust and more evolved toward the ree composition of the oceans this study implies that reactions in estuaries play significant yet poorly understood roles in controlling the ree composition and ce anomaly of the oceans

new neodymium isotope data quantify nile involvement in mediterranean anoxic episodes the development of widespread anoxic conditions in the deep oceans is evidenced by the accumulation and preservation of organic carbon rich sediments but its precise cause remains controversial the two most popular hypotheses involve 1 circulation induced increased stratification resulting in reduced oxygenation of deep waters or 2 enhanced productivity in the surface ocean increasing the raining down of organic matter and overwhelming the oxic remineralization potential of the deep ocean in the periodic development of deep water anoxia in the pliocene pleistocene mediterranean sea increased riverine runoff has been implicated both as a source for nutrients that fuel enhanced photic zone productivity and a source of a less dense freshwater cap leading to reduced circulation basin wide stagnation and deep water oxygen starvation monsoon driven increases in nile river discharge and increased regional precipitation due to enhanced westerly activity two mechanisms that represent fundamentally different climatic driving forces have both been suggested as causes of the altered freshwater balance here we present data that confirm a distinctive neodymium nd isotope signature for the nile river relative to the eastern mediterranean providing a new tracer of enhanced nile outflow into the mediterranean in the past we further present nd isotope data for planktonic foraminifera that suggest a clear increase in nile discharge during the central intense period of two recent anoxic events our data also suggest however that other regional freshwater sources were more important at the beginning and end of the anoxic events taken at face value the data appear to imply a temporal link between peaks in nile discharge and enhanced westerly activity

organic stable carbon isotopic composition reveals late quaternary vegetation changes in the dune fields of northern china abstract vegetation changes during the late quaternary in the dune fields of northern china are not well understood we investigated organic carbon stable isotopic composition of surface soils related mainly to the ratio of c3 and c4 plants across a range of arid to subhumid climates in this region isotopic composition is weakly related to both temperature and moisture multiple r2 0 53 with the highest 13c greatest c4 abundance in the warm subhumid horqin dune field in late quaternary eolian stratigraphic sections of the mu us and horqin dune fields but not in the much colder otindag dune field 13c is higher in organic carbon from paleosols than in eolian sands this contrast most evident for paleosols recording a major early to middle holocene phase of dune stabilization is interpreted as evidence for expansion of c4 plants due to increased effective moisture high temperature because of high insolation and decreased disturbance related to eolian erosion and deposition

the estuarine geochemistry of rare earth elements and indium in the chao phraya river thailand abstract a new filtration method using a 0 04 m hollow fiber filter was applied to the river estuarine and coastal waters in the chao phraya estuary for geochemical investigation the filtered waters were analyzed for all the lanthanides y and in by using inductively coupled plasma mass spectrometry icpms the dissolved concentrations of rare earth elements rees are significantly lower than those reported previously for other rivers presumably because of effective removal of river colloids by the ultra filtration the variation of dissolved rees in the estuary is dependent on the season the light rees vary considerably in the low salinity s 3 show almost conservative trends being consistent with some of the previous works europium is strongly enriched in the river and estuarine waters compared to the south china sea waters thus the ree source of the chao phraya river must be fractionated and modified in entering to the south china sea dissolved in and ce in the high salinity s 20 25 zone of the estuary are lower than those of the offshore waters and therefore the dissolved flux of the chao phraya river cannot account for the higher concentrations of dissolved in and ce in the surface waters of the south china sea the negative ce anomaly is progressively developed with increasing salinity being consistent with continued oxidation of ce iii to ce iv in the estuary fractionation of the light to heavy rees seems to take place whereas the y ho fractionation is unclear in the estuarine mixing zone

evidence for wildfire in the meishan section and implications for permian triassic events polynuclear aromatic hydrocarbons pahs and black carbon bc in sediments are powerful tools in the identification of the combustion process throughout geologic history in this study combustion derived pahs and bc were carefully investigated in sediments from the global stratotype section and point of the permian triassic p tr boundary in meishan china quantitative analyses of combustion derived pahs and bc demonstrate anomalously high concentrations in the boundary event beds that coincide with the mass extinction horizon the prevalence of parent polynuclear aromatics e g phenanthrene in pahs together with non metric multidimensional scaling analysis confirms that the pahs are mainly derived from vegetation burning instead of having a coal and or oil origin bc detected in sediment occurs in various forms from large irregular charcoal particles to fine aciniform soot with an equivalent reflectance of up to 3 5 the results strongly suggest that a wildfire occurred during the p tr boundary which served as one of the possible triggers of mass extinction on land the wildfire occurrence indicates that the o2 concentration of the atmosphere during or before the p tr mass extinction was probably 17 the temporal coincidence of the mass extinction with intensive volcanic eruption marine anoxia and wildfire events in the region of the meishan section provides new insight into the mechanisms of the p tr biotic crisis our results show that wildfires could have played an important role in the collapse of the ecosystem in the meishan p tr events

chinese loess and the paleomonsoon abstract this paper presents a general review of the recent research advances of the east asian paleomonsoon based mainly on studies of the loess soil sequences in the chinese loess plateau in the last 2 6 million years the paleomonsoonal history may be divided into about 166 events on a time scale of the earth s orbital variations during the last glacial period millenial scale oscillations of the monsoon system were prominent which can be fairly well correlated with the dansgaard oeschger cycles recognized in the greenland ice cores the monsoonal rainfall belt has experienced a wide repeated advance retreat change during the glacial interglacial cycles of the pleistocene both temporal and spatial changes of the monsoon system in the quaternary could have been linked closely to global ice volume variations

summer monsoon intensity controls c4 c3 plant abundance during the last 35 ka in the chinese loess plateau carbon isotope evidence from bulk organic matter and individual leaf waxes the natural abundance of c3 and c4 plants is affected by multiple environmental factors including temperature moisture balance and atmospheric pco2 the relative importance of these factors is a subject of considerable debate and may vary in different natural ecosystems previous studies generally focus on single loess sequences in the chinese loess plateau and conflicting conclusions on c4 c3 have been reached when studying carbon isotope ratios in carbonate and organic matter in this paper we report a comprehensive carbon isotopic characterization of total organic carbon toc and individual higher plant leaf waxes from five loess sequences spanning the last 35 ka from the chinese loess plateau clp the five coring sites encompass large gradients of annual mean temperature 9 2 13 9 c and precipitation 402 673 mm allowing us to assess the controlling mechanisms on c4 c3 plant ratios glacial interglacial sequences provide carbon isotope data for comparison with other climatic and environmental proxies such as lithology and magnetic susceptibility our results demonstrate that increased c4 c3 ratios are positively correlated with higher temperature and increased summer rainfall which characterize stronger summer monsoon in all five sites we conclude c4 abundance increases from the last glacial to the holocene in response to greater monsoon activity and that the c4 expression is suppressed in the cold and drier intervals

black carbon in soils and sediments analysis distribution implications and current challenges this review highlights the ubiquity of black carbon bc produced by incomplete combustion of plant material and fossil fuels in peats soils and lacustrine and marine sediments we examine various definitions and analytical approaches and seek to provide a common language bc represents a continuum from partly charred material to graphite and soot particles with no general agreement on clear cut boundaries formation of bc can occur in two fundamentally different ways volatiles recondense to highly graphitized soot bc whereas the solid residues form char bc both forms of bc are relatively inert and are distributed globally by water and wind via fluvial and atmospheric transport we summarize chronologically the ubiquity of bc in soils and sediments since devonian times differentiating between bc from vegetation fires and from fossil fuel combustion bc has important implications for various biological geochemical and environmental processes as examples bc may represent a significant sink in the global carbon cycle affect the earth s radiative heat balance be a useful tracer for earth s fire history build up a significant fraction of carbon buried in soils and sediments and carry organic pollutants on land bc seems to be abundant in dark colored soils affected by frequent vegetation burning and fossil fuel combustion thus probably contributing to the highly stable aromatic components of soil organic matter we discuss challenges for future research despite the great importance of bc only limited progress has been made in calibrating analytical techniques progress in the quantification of bc is likely to come from systematic intercomparison using bcs from different sources and in different natural matrices bc identification could benefit from isotopic and spectroscopic techniques applied at the bulk and molecular levels the key to estimating bc stocks in soils and sediments is an understanding of the processes involved in bc degradation on a molecular level a promising approach would be the combination of short term laboratory experiments and long term field trials

indium and yttrium in north atlantic and mediterranean waters comparison to the pacific data abstract vertical profiles of dissolved indium and yttrium were determined in the eastern north atlantic and the mediterranean sea to compare with those of the north pacific reported earlier the y concentrations in the surface waters are 120 pmol kg in the north atlantic and 205 pmol kg in the mediterranean sea which are significantly higher than 80 pmol kg in the north pacific the difference may be attributable to the different strength of input of y to the oceans from fluvial and aeolian sources in contrast the deep water concentration of y increases in the order of north atlantic north north pacific exhibits an inverse trend of y and nutrients indium is highly particle reactive 47 association in the mediterranean sea and must have a short mean oceanic residence time however the featureless dissolved in profile in the mediterranean sea is clearly different from the profiles of dissolved al showing increase with depth hydes et al 1988 measures and edmond 1988 suggesting that significant fractionation of the two elements is taking place in the ocean the interoceanic variations of dissolved in and al may be ascribed to the different intensities of external input of which aeolian has been considered to be major rather than fluvial however the difference of in and al concentrations in the deep waters of the above oceanic basins are significantly greater than those of other refractory elements such as ce ti hf and zr whose major sources to the ocean are also considered to be aeolian furthermore the in al ratios in seawater are about two orders of magnitude greater than the average crustal ratio thus some additional sources though not yet certain may be required to explain the high concentrations of in in the atlantic and the mediterranean deep waters

climatic dependency of soil organic carbon isotopic composition along the s n transect from 34 n to 52 n in central east asia abstract we explored the relationships between surface soil 1 20 cm organic carbon isotopic signatures and associated climatic factors in central east asia in an attempt to develop transfer functions that can be used to retrieve the paleoclimatic information stored in the thick eolian paleosol sequences within the area our analysis shows that the negative correlation between the surface soil organic 13c values and the mean annual precipitation is robust r2 0 453 n 196 p

stable carbon isotopic composition of black carbon in surface soil as a proxy for reconstructing vegetation on the chinese loess plateau abstract black carbon bc is a continuum of carbon produced by incomplete combustion whether its stable carbon isotopic composition expressed as 13 c bc values can be used directly for paleovegetation reconstruction is still in debate in the present study we investigated surface soil from the chinese loess plateau and collected samples from the following three transects the ruicheng jingbian lantian huanxian and qin an dingxi from east to west the 13 c bc values for surface soil ranged from 27 9 to 21 9 the stable carbon isotopic composition of soil organic carbon expressed as 13 c soc values ranged from 26 8 to 20 9 a strong positive correlation was observed between 13 bc and 13 c soc r 0 88 p 0 000 13 bc and 13 c soc 13 soc bc values ranged from 1 5 to 1 3 with an average of 0 2 the characteristics of vegetation on the chinese loess plateau and its variation indicated by 13 bc and 13 c soc values were consistent with those of modern plants it suggests that 13 bc values for surface soil are controlled mainly by surface plants and that therefore 13 bc can indicate the vegetation effectively within a precision range of 1 5

onset of asian desertification by 22 myr ago inferred from loess deposits in china the initial desertification in the asian interior is thought to be one of the most prominent climate changes in the northern hemisphere during the cenozoic era1 2 3 4 but the dating of this transition is uncertain partly because desert sediments are usually scattered discontinuous and difficult to date here we report nearly continuous aeolian deposits covering the interval from 22 to 6 2 million years ago on the basis of palaeomagnetic measurements and fossil evidence a total of 231 visually definable aeolian layers occur as brownish loesses interbedded with reddish soils this new evidence indicates that large source areas of aeolian dust and energetic winter monsoon winds to transport the material must have existed in the interior of asia by the early miocene epoch at least 14 million years earlier than previously thought3 5 regional tectonic changes and ongoing global cooling are probable causes of these changes in aridity and circulation in asia

determination of black carbon by chemical oxidation and thermal treatment in recent marine and lake sediments and cretaceous tertiary clays abstract several sample treatments and analytical methods for the measurement of black carbon in a wide variety of sediments were considered in which the removal of organic carbon is a critical step in the procedure a comparison between chemical oxidation and thermal treatment was conducted and the latter was shown to be inappropriate due to the polymerisation of organic matter using acid dichromate oxidation reactive kerogen was separated from black carbon in a variety of lake and marine and recent and ancient sediments with relatively small and quantifiable losses 1 6 of black carbon under carefully controlled reaction conditions

neodymium isotopic composition of the mediterranean outflow and the eastern north atlantic the neodymium concentration and isotopic composition of the mediterranean outflow in the strait of gibraltar has been directly determined the outflow has a dissolved nd concentration of 23 pmoles kg with nd 0 9 4 these results are in clear disagreement with previous values inferred for the mediterranean this concentration and nd 0 do not sharply contrast with north atlantic deep water nadw values and can produce isotopic anomalies no larger than 1 unit in the north atlantic a new profile in the eastern north atlantic does not exhibit an nd 0 anomaly at the depth of the mediterranean water core in accordance with the above inferences while concentrations and nd 0 at this site indicate that nd is conservative in the deep eastern north atlantic a previously reported nd 0 anomaly in the eastern north atlantic originally associated with the mediterranean outflow has a different source the direct contribution of the mediterranean outflow to the nadw nd budget is small 10 of the high latitude sources however the outflow plus entrained water supply 30 of the total nd associated with northern latitude deep water sources concentrations of dissolved nd in the outflow are enriched compared to the atlantic inflow which has 16 pmoles kg with nd 0 11 8 the principal source of the excess dissolved nd is inferred to be partial dissolution of detrital particles simple mass balance calculations suggest that 0 6 of the nd associated with detrital river transported sediments and or 3 5 of the nd associated with eolian particles delivered to the mediterranean is mobilized during passage into the marine environment if general these mobilization efficiencies imply that the dissolution of detrital particles is a significant source of dissolved nd and other rare earth elements in seawater

dissolved rare earth elements in the south china sea geochemical characterization of the water masses we have measured the vertical profiles of dissolved rare earth elements rees and yttrium in the south china sea together with conductivity temperature depth and hydrographic measurements to compare with those in the western north pacific and the suiu sea although the south china sea is rapidly flushed by the pacific through the luzon strait with a sill depth of 2500 m broecker et al 1986 a unique ree pattern is developed within the sea the most striking difference exists in the dissolved ce profiles dissolved ce generally decreases from high values 6 9 pmol kg at the surface to a minimum of 3 pmol kg at around 300 500 m where the north pacific intermediate water penetrates in deepwaters of the north pacific and the sulu sea it remains at a relatively low and nearly constant concentration level of 5 pmol kg throughout the water column whereas in the south china sea it gradually increases with depth to a maximum of 12 9 pmol kg at 2500 m resembling the nutrient like profiles of other strictly trivalent rees and then sharply drops to a constant value of 6 pmol kg in the bottom water below 2900 m some lighter rees like pr nd and gd though to a much lesser extent also show similar concentration breaks at the sill depth but the other hydrographic properties like dissolved oxygen nutrients ph and alkalinity do not therefore dissolved rees may best be utilized to characterize the water masses two major sources for dissolved rees in the south china sea are fluvial and coastal input to the surface ocean and a bottom release into the deep water during the passage over the luzon strait redox chemistry including reduction of ce iv to ce iii in the pore water of hemipelagic sediments and subsequent release of dissolved ce iii to the overlying deep water may be involved in the latter the middle ree enriched patterns with a significant gd depression relative to that of the north pacific deep water are characteristic of the south china sea and prevail throughout the water column by physical circulation since the bottom water of the basin is also fed by the north pacific the dissolved ce iii must be oxidized through bacterial mediation and removed from the bottom water presumably by scavenging near at the sediment water interface along the slopes of the basin

black carbon concentration and isotopic composition of surface sand from deserts and dune fields in northern china abstract black carbon bc is a continuum of incomplete combustion products of vegetation and fossil fuels it can be used as a proxy for wildfires and paleovegetation reconstruction nevertheless bc can be transported by wind and water via atmospheric and fluvial transport to research the characteristics of bc in deserts and dune fields that are directly connected with the chinese loess plateau clp in northern china and to evaluate the possibility that bc in deserts and dune fields occurs as exogenous bc in the clp we investigated surface sand from the mu us dune field the tengger desert and the badain jaran desert and analyzed the stable carbon concentration and carbon isotopes of organic carbon soc 13 c soc and bc bc 13 c bc the 13 c soc 13 c bc values ranged from 27 4 to 22 5 and 28 2 to 23 0 respectively which mainly reflected 13 c changes in surface c 3 grasses a positive correlation was observed between 13 c bc and 13 c soc suggesting that local vegetation mainly controls 13 c bc the difference between 13 c bc and 13 c soc ranged from 0 2 to 3 5 and the difference is likely to be due to carbon isotope discrimination occurring during burning and soc decomposition the soc and bc in these deserts and dune fields are very small averaging 0 048 and 0 011 the bc of surface sand in these areas is about 5 5 of bc in the clp surface soils and the 13 bc value between surface sand in deserts and surface soils in clp ranged from 0 3 to 6 3 so the change of 13 bc in the clp surface soils deduced by exogenous bc from deserts is less than 0 35 all these findings suggest that bc in deserts has a limited impact on bc in the clp and is unlikely to affect interpretations of the bc record of wildfires and paleovegetation information in the clp

black carbon records in chinese loess plateau over the last two glacial cycles and implications for paleofires abstract to study the temporal and spatial changes in paleofires over the chinese loess plateau black carbon concentrations were analyzed on the loess paleosol samples from three sections along a north south transect using the orbitally tuned time scales of the sections the black carbon mass sedimentation rates bcmsr were calculated results show that in the last two glacial cycles the bcmsr values in glacial periods are 2 3 times higher than in interglacial periods and the bcmsr variability has a relatively strong precession associated 23 ky period suggesting that the glacial cold dry climate conditions were apt to induce natural fires over the loess plateau comparison of the bcmsr records among the three loess sections demonstrates that natural fire occurrence was much more intensive and frequent in the northern loess plateau than in the southern part coinciding with the previous conclusion that the northern plateau has experienced a drier climate regime in both glacial and interglacial periods the substantial increase in bcmsr of the upper s0 relative to the lower s0 at lingtai and weinan indicates that human activities have exerted a significant influence on fire regimes in the middle and southern loess plateau during late holocene due to the relatively intensive agricultural usage of land