a record of changing redox conditions in the northern peru basin during the late quaternary deduced from mn fe and growth rate variations in two diagenetic manganese nodules abstract two diagenetic manganese nodules from the peru basin were investigated by thermal ionization mass spectrometry and high resolution alpha spectrometry for uranium and thorium the tims concentrations for nodule 62kd 63kg vary as follows 0 12 1 01 ppb 0 06 0 59 230th 0 51 1 98 ppm 0 43 1 40 232th 0 13 0 80 ppb 0 09 0 49 234u and 1 95 13 47 ppm 1 66 8 24 238u both nodules have average growth rates of 110 mm per million years however from the variations of excess 230th with depth we estimate partial accumulation rates which range from 50 to 400 mm per million years the 234u dating method cannot be applied due to remobilization of u from the sediment and subsequent incorporation into the nodules crystal lattice reflected by decay corrected 234u values far above the ocean water value sections of fast nodule growth are related to those layers having high mn fe ratios up to 200 and higher densities as a possible explanation we develop a scenario that describes similar glacial interglacial trends in both nodules as a record of regional changes of sediment and or deep water chemistry

geochemical characteristics of deep sea sediments from the arabian sea a high resolution study abstract five deep sea cores from the arabian sea covering a time interval of 170 ky were sampled at high resolution and analysed for major and trace elements the stratigraphy of the cores was obtained by comparing the ba al ratio with the specmap data this method is based on the close correlation between ba concentrations or ba al ratios and oxygen isotope ratios 18o of foraminifera in arabian sea sediments ba al ratios used as a productivity proxy are variable but high in all deep sea cores of the arabian sea indicating a basin wide influence of nutrient rich water masses compared to glacial intervals the warmer periods are characterised by higher element al ratios of proxies which are directly ba ca sr p i e hard parts of organisms or indirectly u 230thex related to biological productivity 230thex provides evidence for intense boundary scavenging caused by high productivity due to enhanced upwelling and terrigenous input from the indus fan the mn distribution in a core from the western arabian sea shows enrichments during interglacial periods and may indicate mn export owing to the presence of a stronger oxygen minimum zone several processes limit the applicability of proxies intercalated turbidites in the deep sea cores can be identified by means of si al ti al and zr al ratios versus depth due to the enrichment of quartz ti minerals and zircon in the basal layer these elements are also important for the reconstruction of the history and extent of the eolian dust input diagenetic redistribution of redox sensitive elements occurs at the boundaries between turbidites and normal pelagic sedimentation and may mask or destroy primary signals

aeolian sediments evolution controlled by fluvial processes climate change and human activities since lgm in the qaidam basin qinghai tibetan plateau abstract aeolian sediments are widely distributed in the eastern qaidam basin qb the main habitats for human in the hyper arid basin during the holocene especially since 3 ka the evolution of aeolian sediments is an important factor for the environmental change and influence human activities and migration however many questions e g when the aeolian sediments start to accumulate what controls their initiation and how their evolution responds to climate change and human activities still need further studies in this study we present a detailed optically stimulated luminescence osl chronology for both aeolian and underlying fluvial sediments from the tiekui desert in the eastern qb to discuss aeolian and fluvial processes climatic changes desert evolution and human activities dating results show that 1 underlying fluvial sediments were formed during 23 9 12 1 ka and the erosion caused by the fluvial process may provide an explanation for the absence of aeolian sediments during and before this period 2 aeolian sediments accumulated from ca 9 8 ka to modern times with loess development from 9 8 ka to 1 6 ka and 1 4 0 7 ka and dune sand accumulation at 1 6 1 4 ka and 0 7 0 ka through the comparison with the local and global palaeoclimatic records we suggest that desert evolution in this region was sensitive to climatic changes induced by the asian summer monsoon detailed local historical records of the past 2000 years suggest that desert evolution was also influenced by human activities

a record of holocene climate change in the guanzhong basin china based on optical dating of a loess palaeosol sequence optical dating was used to obtain a chronology for a loess palaeosol sequence in the guanzhong basin on the loess plateau in china the sedimentary sequence recorded two dry episodes 3 7 2 4 ka and 1 2 0 8 ka ago in the holocene as shown by the presence of two layers of loess within the holocene deposits these two arid events are consistent with records of global climate change in the holocene and also coincident with major historic events in china this finding suggests that loess paleosol sequences in china can record global climate changes and implies that rapid climate change affected the progress of human civilization in addition changes in the sedimentation rate through the section suggest that agricultural expansion may have accelerated the rate of dust accumulation

holocene climatic evolution at the chinese loess plateau testing sensitivity to the global warming cooling events abstract a high resolution petromagnetic analysis demonstrates that pedogenic alterations in holocene loess sand sequences from the regions of the guanzhong basin and the mu us desert on the chinese loess plateau clp were affected by climatic variations in temperature and precipitation three warm humid intervals 8 4 3 7 2 4 1 2 and 0 81 0 48 ka associated with soil formation and relatively high values of petromagnetic parameters occurred during the holocene a substantial paleosol development from 8 4 to 3 7 ka and higher petromagnetic parameter values indicate a generally strong warm humid phase in the mid holocene which can be attributed to the holocene climatic optimum our study demonstrates that the holocene climate in the eastern part of the clp is sensitive to warming and cooling events linked to global changes in temperature and precipitation a complete holocene climate record is constructed correlating well with the other regional climate records along the south to north profile on eastern clp and suggesting that similar climatic changes occurred in eastern monsoonal china during the holocene results are supported by other climate records in different regions of the world and imply that the holocene climatic optimum in the northern hemisphere probably took place at the same time

the u series toolbox for paleoceanography the geochemistry of marine sediments is a major source of information about the past environment of the many measurements that provide such information those of the u series nuclides are unusual in that they inform us about the rate and timescales of processes oceanic processes such as sedimentation productivity and circulation typically occur on timescales too short to be assessed using parent daughter isotope systems such as rb sr or sm nd so the only radioactive clocks that we can turn to are those provided by cosmogenic nuclides principally 14c or the u series nuclides this makes the u series nuclides powerful allies in the quest to understand the past ocean climate system and has led to their widespread application over the last decade as in other applications of the u series those in paleoceanography rely on fractionation of the nuclides away from secular equilibrium in the oceanic setting this fractionation is generally due to differences in the solubility of the various nuclides the behavior of the u series nuclides in the ocean environment was widely researched in the middle decades of the twentieth century this work established knowledge of the concentrations of the nuclides in the various compartments of the ocean system and of their fluxes between these compartments such understanding was comprehensively summarized in the ivanovich and harmon u series volume 1992 particularly by cochran 1992 understanding of the behavior of the u series nuclides has not advanced very dramatically in the decade since that summary but a major theme of research has been the use of this geochemical understanding to develop u series tools to assess the past environment table 1 view this table table 1 summary of seawater data for u series nuclides with paleoceanographic applications s soluble i insoluble full descriptions of the paleoceanographic uses and references are provided in the text further details of the half

238u234u230th disequilibrium in hydrogenous oceanic femn crusts palaeoceanographic record or diagenetic alteration abstract a detailed tims study of 234uexc 238u 230th 232th and th u ratios have been performed on the outermost margin of ten hydrogenous fe mn crusts from the equatorial pacific ocean and west central indian ocean th u concentration ratios generally decrease from the crust s surface down to 0 5 1 mm depth and growth rates estimated by uranium and thorium isotope ratios are significantly different in fe mn crusts from the peru basin and the west central indian ocean fe mn crusts from the same geographical area define a single trend in plots of ln 234uexc 238u vs ln 230th 232th and th u ratios vs age of the analysed fractions results suggest that l hydrogenous fe mn crusts remain closed systems after formation and consequently 2 the discrepancy observed between the 230th and 234u chronometers in fe mn crusts and the variations of the th u ratios through the margin of femn crusts are not due to redistribution of uranium and thorium isotopes after oxyhydroxide precipitation but rather to temporal variations of both th u and initial thorium activity ratios recorded by the fe mn layers implications of these observations for determination of fe mn crust growth rates are discussed variations of both th u and initial th activity ratios in fe mn crusts might be related to changes in particle input to seawater and or changes in ocean circulation during the last 150 ka

growth rate variations of manganese nodules and crusts induced by paleoceanographic events time markers at about 3 ma 6 5 ma and 14 ma are common to 16 manganese nodules and crusts from different localities in the world oceans these time markers correspond to variations in the growth rates in the inner structure and in the chemistry of the samples the observed time markers could be related to 1 changes of the bottom water velocity and 2 variations in the manganese and iron contents in the deep ocean the time markers were determined by high resolution 10be dating applying the accelerator mass spectrometry facility at the eidgenossische technische hochschule zurich

226 ra excess ba growth rates and u th ra ba systematic of baltic mn fe crusts abstract we analyzed 238u 234u 232th 230th and 226ra by thermal ionization mass spectrometry tims and ba by inductively coupled plasma optical emission spectrometry icp oes on eight mn fe crusts from the mecklenburg bay sw baltic and on one from the bothnian bay n baltic to test the 226raex ba ratio as potential geochronometer 226raex ba ratios decrease as a function of depth within the concretions in all analyzed profiles calculated diffusion coefficients are relatively low 9 10 7 cm2 yr for ra and 5 10 7 cm2 yr for ba and suggest that diffusion is negligible for the ra and ba record in addition 226raex ba ages are consistent and independent from the growth rate and growth direction within a crust thus the decline in 226raex ba ratio is most likely due to radioactive decay of 226raex although the influence of varying oxic conditions has still to be evaluated 226raex ba growth rates range from 0 021 to 0 0017 mm yr and tend to be lower than those calculated and based on stratigraphic methods 1 to 0 013 mm yr 226raex ba ages of concretions from shallow water environment 20 m depth mecklenburg bay sw baltic cover a time interval from 990 140 yr to 4310 310 yr bp corresponding to the stabilization of the sea level close to the present position about 5500 to 4500 yr ago one sample from greater depth 70 m bothnian bay n baltic showed a higher 226raex ba age of 6460 520 yr bp

nd sr isotopic and ree geochemical compositions of late quaternary deposits in the desert loess transition north central china implications for their provenance and past wind systems abstract the nd sr isotopic and ree geochemical compositions of the dishaogouwan profile in the desert loess transition between the ordos desert and the chinese loess plateau were investigated to identify the provenance of the sediments and to reconstruct the regional wind systems of the past the 87 sr 86 sr ratios of 0 7158 0 7175 nd 0 values of 8 9 to 16 8 ree values between 133 g g and 277 g g and roughly flat paas normalized ree patterns which differs from the 75 m fractions in the dishaogouwan profile this is most likely due to mineral sorting and or source features isotopic and geochemical comparisons of the dishaogouwan profile with potential sources show that 1 coarse eolian deposits resulted from wind erosion of the highlands in the adjacent west central ordos and the coarse fluvio lacustrine deposits originated chiefly from water erosion of the underlying eolian deposits in the dishaogouwan profile 2 fine particles of eolian deposits with absolute nd 0 values of 1 originated completely from the highland erosion in west central ordos and the fining of in situ coarse particles but the fine particles of the eolian deposits with absolute nd 0 values of 1 have more complex sources 3 fine fluvio lacustrine deposits were fed from the central loess plateau by water for the chengchuan and middle lower salawusu formations but for the upper salawusu formation they were significantly fed from the east central ordos desert by wind according to the source identification of the dishaogouwan profile the wind systems have been re established over the ordos and surrounding areas in the past 200 ka 1 the northwesterly was the dominant wind for eolian deposits in the dishaogouwan profile and the northeasterly simultaneously affected fine eolian deposits in certain periods the middle part of mis 6 and the lower part of mis 4 2 the summer monsoon was a predominant wind in periods during which most of the fluvio lacustrine deposits developed in the salawusu river valley but was thoroughly displaced by the cold dry northeasterly wind during the transition between mis 5 and 4 this study is helpful for further understanding the evolution of the desert loess transition and the related climate change

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

climatic influences on the growth rates of mn crusts during the late quaternary abstract the concentration of 230 th in mn crust va13 2 146 w 9 25 s 4830 m from the pacific ocean was measured with high depth resolution 0 02 mm to a depth of 1 4 mm corresponding to an age of 226 ka b p the mean growth rate estimated from these results is 6 2 0 4 mm ma however the exponential curve obtained shows two drastic events in the 230 th concentration record with durations of 41 4 ka and 37 6 ka which are interpreted as growth standstills the decay corrected 230 th concentration is inversely correlated with those of mn fe and co indicating that the 230 th flux into the crust has been constant throughout time and that the observed fluctuations in 230 th are due to dilution by the main components by applying a constant 230 th flux model high resolution growth rates were derived from every sample investigated a detailed comparison of the growth pattern with the specmap time table reveals that the va13 2 growth rates tend to be higher during interglacial than during glacial periods in particular the observed growth standstills are associated with glacial stages 6 and 8 further investigation of mn crust 72 dk 9 taken from shallower water 170 35 w 20 13 n 1550 m tend to confirm that there is a link between the growth rates of hydrogenetic crusts and the climate of the late quaternary

holocene sediment accumulation rates in the western loess plateau china and a 2500 year record of agricultural activity revealed by osl dating high resolution optically stimulated luminescence osl dating of a terraced loess section in the western margins of the chinese loess plateau provides evidence of continuous and varying accumulation of dust throughout the holocene from 12030 to 2500 years ago the sediment accumulation rate was approxi mately 0 2 mm year after this time it increased to approximately 0 8 mm year during a historically docu mented period of agricultural expansion in adjacent areas from 680 years ago a further increase in accumu lation rate to approximately 3 4 mm year is evident particle size analysis indicates that this increase in accumulation rate was associated with anthropogenic addition of sandy sediment probably for soil improve ment the osl dating also identifies the period when the terrace was first cut for agricultural use between 2500 and 2070 years ago

holocene environment changes around the sara us river northern china revealed by optical dating of lacustrine aeolian sediments abstract the sara us river is located along the boundary of the mu us desert and the chinese loess plateau in northern china the river has cut down through quaternary sediments creating 70 80 m deep valleys with thick lacustrine aeolian sequences exposed we applied optical stimulated luminescence on sediments from a holocene section of aeolian sand lacustrine deposits in the top of the river valley the dating results show that a humid period existed from 7 1 to 2 0 ka ago as evidenced by two layers of peat and lacustrine sediments however compared to other published holocene sections in the sara us river valleys close to the section under studying the local environment experienced very complicated changes during the holocene all of the sections recorded a period with drought and or cold before the holocene at around 13 ka and an episode of aridity after about 2 ka ago as evidenced by the layers of aeolian sand however the ages of the lacustrine and peat layers in these sections are substantially different geomorphological analysis by digital elevation models does not support the existence of a mega lake covering the study area at 2 ka the intricate environmental changes may have been caused by the meandering of the sara us river environmental changes also strongly affected human migration in this area which is documented by chinese historical records

uptake of elements from seawater by ferromanganese crusts solid phase associations and seawater speciation marine fe mn oxyhydroxide crusts form by precipitation of dissolved components from seawater three hydrogenetic crust samples one phosphatized and two hydrothermal mn oxide samples were subjected to a sequential leaching procedure in order to determine the host phases of 40 elements those host phase associations are discussed with respect to element speciation in seawater the partitioning of elements between the two major phases mn oxide and fe oxyhydroxide can in a first order approximation be explained by a simple sorption model related to the inorganic speciation of the elements in seawater as has been proposed in earlier models free and weakly complexed cations such as alkali and alkaline earth metals mn co ni zn tl i and partly y are sorbed preferentially on the negatively charged surface of the mno2 in hydrogenetic crusts the driving force is a strong coulombic interaction all neutral or negatively charged chloro cd hg tl carbonate cu y pb and u and hydroxide be sc ti fe zr nb in sn sb te hf ta bi th and tl iii complexes and oxyanions v cr as se mo and w bind to the slightly positively charged surface of the amorphous feooh phase while coulombic interaction can explain the sorption of the negatively charged species the binding of neutral species is based on specific chemical interaction organic complexation of elements in deep ocean water seems to be at most of minor importance surface oxidation can explain some strong metal associations e g of co and tl with the mno2 and te with the feooh sorption reactions initially driven by coulombic forces are often followed by the formation of specific bonds between the adsorbate and the atoms of the oxide surface differences in the associations of some metals between the non phosphatized and phosphatized hydrogenetic crusts and between the hydrogenetic and the hydrothermal samples reflect the different physico chemical environments of formation and speciations in oxic seawater vs less oxic fluids especially for the redox sensitive metals such as mo and v these environmental related differences indicate that the methodology of chemical speciation used here in combination with spectroscopic methods may allow for the detection of changes in paleoceanographic conditions recorded during the several tens of millions of years of crust growth

late holocene eolian activity in the mineralogically mature nebraska sand hills abstract the age of sand dunes in the nebraska sand hills has been controversial with some investigators suggesting a full glacial age and others suggesting that they were last active in the late holocene new accelerator mass spectrometry radiocarbon ages of unaltered bison bones and organic rich sediments suggest that eolian sand deposition occurred at least twice in the past 300014c yr b p in three widely separated localities and as many as three times in the past 80014c yr at three other localities these late holocene episodes of eolian activity are probably the result of droughts more intense than the 1930s dust bowl period based on independent great plains climate records from lake sediments and tree rings however new geochemical data indicate that the nebraska sand hills are mineralogically mature eolian sands in nebraska have lower k feldspar and k2o rb and ba contents than most possible source sediments and lower k feldspar contents than dunes of similar age in colorado the most likely explanation for mineralogical maturity is reduction of sand sized k feldspar to silt sized particles via ballistic impacts due to strong winds over many cycles of eolian activity therefore dunes of the nebraska sand hills must have had a long history probably extending over more than one glacial interglacial cycle and the potential for reactivation is high with or without a future greenhouse warming

thermoluminescence chronology of sand profiles in the thar desert and their implications thermoluminescence chronology of three sand profiles in western rajasthan indicates a peak in sand accretion rate at ca 14 ka and a period of dormancy during 13 6 ka bp this observation shows a general correlation with pollen and geochemical records from lakes in thar and demonstrates the potential of desert sands in paleoclimatic reconstruction thermoluminescence ages of 36 ka and 40 ka on two samples suggest that the aeolian sand mobilization activity in this regions is at least as old

advance retreat history of the east asian summer monsoon rainfall belt over northern china during the last two glacial interglacial cycles abstract in northern china the history of desert migration provides a good measure of the east asian summer monsoon variations since the desert margin environment is very sensitive to changes in summer monsoon rainfall desert margin migration can be well documented in the downwind loess deposits since the grain size of chinese loess is principally controlled by source to sink distance and hence the loess grain size is an effective proxy for reconstructing the history of the east asian summer monsoon in order to investigate the pattern and migration of the summer monsoon rainfall belt over northern china during the past two glacial interglacial cycles contour maps of loess grain size were constructed for mis 2 3 4 5 6 and 7 all the grain size isolines show a near north south gradient for both glacials and interglacials indicating a dominant dust source region in north and an approximately north south dust transport pathway for dust deposits in the main body of the loess plateau at least for the past two glacial interglacial cycles the reconstructed southeast desert margin for mis 2 3 4 5 6 and 7 roughly parallels the present isohyets implying that the pattern of the east asian summer monsoon rainfall belt during the past two glacial interglacial cycles is similar to the present i e a northeast southwest trend for the isohyets of the summer monsoon despite the glacial interglacial cycles using the grain size isolines of mis 2 as a reference we derive a northerly desert retreat of 160 km 19 km 225 km and 223 km for mis 3 4 5 and 7 respectively and a southerly desert advance of 29 km for mis 6 that could be preliminarily regarded as the relative migration distance for the east asian summer monsoon rainfall belt in addition the inferred climate conditions in mis 3 are cool and humid in northern china while the driest and coldest interval of the last two glacial periods occurs in mis 6

lateglacial and holocene dune evolution in the horqin dunefield of northeastern china based on luminescence dating abstract the horqin dunefield is located in the northern frontier of east asian monsoon eam zone and its activation and stabilization is sensitive to the ebb and flow of the eam however the activity of dunes in the horqin dunefield is still poorly understood due to limited numerical dating the purpose of the present study is to establish the chronology using luminescence dating in order to understand dune evolution of the horqin dunefield and to explore the relative role of climate and human activity eighteen samples were collected from seven sections following a southwest northeast transect through the dunefield and dated using quartz optically stimulated luminescence osl combining evidence from stratigraphy and the luminescence chronology dune evolution within the horqin dunefield since the lateglacial can be summarized as before 9 6 ka aeolian sand dunes were mobilized during the period 9 6 3 ka dunes were stabilized at around 3 ka dunes were mobilized again abruptly since 3 ka there were multiple phases of dune stabilization mobilization dune activity was dominated by climate before 3 ka however since 3 ka both climate variability and human impact have influenced dune evolution in the horqin dunefield

deposits and soils of the past 130 000 years at the desert loess transition in northern china the desert loess transitional zone in north central china has long been thought sensitive to quaternary climatic change however reconstruction of quaternary climates in this area has been hindered by incompleteness of geological sections here we report the analytical results of two recently found sand loess soil sections both sections have thick eolian deposits from the last interglacial glacial cycle and can be correlated with one another field observations thermoluminescence dating and other laboratory analyses show that the last interglacial period produced three paleosols and two intercalated loess layers loess from the last glacial period is interbedded with three sand horizons that represent desert extension the expansion and contraction of desert in northern china may have been forced by the east asia monsoon

origin of eolian sand mobilization during the past 2300 years in the mu us desert china abstract eolian sand dunes and sand sheets are extensive in the semiarid regions of northern central china previous studies indicated that these eolian sands mainly formed during historical times however the sand sources and their reworking processes have not been well studied and even the reasons for their occurrence are still controversial field investigation and sedimentary evidence indicate that both the inner mu us desert and its southern marginal region both sources of modern eolian sand are largely associated with reworking of sands of the last glaciation based on geological documentary and archaeological data three phases of land cultivation during the past 2300 years together with historical droughts high wind energy and the easily reworked sand sources largely account for the occurrence of active sand dunes and sand sheets in the mu us desert

the influence of the carbonate dissolution rate on the growth and composition of co rich ferromanganese crusts from central pacific seamount areas abstract the metal composition of oceanic ferromanganese deposits occurring in seamount regions line islands chain and mid pacific mountains varies with water depth and age the results of metal determinations of carbonate plankton samples suggest that carbonate dissolution in the water column might have an important influence on the accretion and composition of hydrogenetic precipitates two ferromanganese crust generations of different age have been observed the precipitation of the older crust took probably place during early oligocene the younger crust began to form during middle miocene between the two crust generations periods of carbonate sedimentation and of phosphorite deposition occur the hydrogenetic formation of the crusts is controlled by the metal supply from the water column according to the laws of colloidal surface chemistry dissolution experiments with carbonate plankton samples show that the main fe source for the hydrogenetic crust formation are colloidal fe hydroxide particles being released in the water column from the dissolution of carbonate plankton skeletons in the case of mn maximum dissolved mn occurs in the oxygen minimum zone as the result of in situ break down of organic matter and the in situ reduction of mn bearing solid phases closely beneath the oxygen minimum zone a fe supply mobilized within the oxygen minimum zone has also to be taken into account in the water column below the oxygen minimum zone a mixture of colloidal particles of mn fe oxyhydroxide and colloidal al fe silicate precipitate together on the surface of substratum rocks the mixing ratio of these colloidal phases controlling the metal composition of the ferromanganese precipitates is depth dependent and shows also temporal variations in general mn fe ratio ni and co contents decrease with depth down to the calcite compensation depth the most probable mechanism for the ultimate removal of co and ni from the water column might be a surface reaction mno2 is specifically able to absorb hydrous co2 and ni2 ions because of the surface enrichment of co and the strong electrical field of mn iv a subsequent oxidation of co2 to co3 takes place leading to higher enrichment of co in comparison to ni the most important factor governing the high co enrichment in the ferromanganese crusts is the growth rate the lower the growth rate the higher the co content maximum values of up to 2 co occurring in samples from water depths between 1500 and 1100 m 1 are related to lower carbonate dissolution rates and corresponding lower fe supply the metal supply from the water column is strongly related to distinct environmental factors such as bio productivity range of lysocline and calcite compensation depth rate of carbonate dissolution and activity of the antarctic bottom water thus our model shows that the growth periods and the metal composition of hydrogenetic seamount crusts are controlled by changes in the paleoceanography and reflect distinct environmental conditions

osmium isotope variations in the oceans recorded by fe mn crusts this study presents osmium os isotope data for recent growth surfaces of hydrogenetic ferromanganese fe mn crusts from the pacific atlantic and indian oceans in general these data indicate a relatively uniform os isotopic composition for modern seawater but suggest that north atlantic seawater is slightly more radiogenic than that of the pacific and indian oceans the systematic difference in the os isotopic composition between the major oceans probably reflects a greater input of old continental material with a high re os ratio in the north atlantic ocean consistent with the distribution of nd and pb isotopes this spatial variation in the os isotope composition in seawater is consistent with a residence time for os of between 2 and 60 kyr indian ocean samples show no evidence of a local source of radiogenic os which suggests that the present day riverine input from the himalaya tibet region is not a major source for os recently formed fe mn crusts from the tag hydrothermal field in the north atlantic yield an os isotopic composition close to that of modern seawater which indicates that in this area the input of unradiogenic os from the hydrothermal alteration of oceanic crust is small however some samples from the deep pacific 4 km possess a remarkably unradiogenic os isotope composition 187 os 186 os ratios as low as 4 3 the compositional control of os incorporation into the crusts and mixing relationships suggest that this unradiogenic composition is most likely due to the direct incorporation of micrometeoritic or abyssal peridotite particles rather than indicating the presence of an unradiogenic deep water mass moreover this unradiogenic signal appears to be temporary and local and has had little apparent effect on the overall evolution of seawater these results confirm that input of continental material through erosion is the dominant source of os in seawater but it is not clear whether global os variations are due to the input of mantle or meteoritic material or simply indicate that the continental source itself is not uniform 1999 elsevier science b v all rights reserved

u th chronology and paleoceanographic record in a fe mn crust from the ne atlantic over the last 700 ka hydrogenetic ferromanganese crusts fe mn crusts provide a secular record of the variations of seawater composition responding to changes in ocean circulation and erosion processes in this respect the acquisition of an absolute and reliable chronology in fe mn crusts is a prerequisite here we combine four different and complementary chronometers 10 be 230 thex 230 thex 232 th 234 u 238 u in a fe mn crust dredged at 2000 m depth in the east atlantic to first establish a reliable chronology over the quaternary period then we use eds chemical analysis to look for correlation between major element chemistry and climate changes 230 thex 230 thex 232 th and be data give very consistent growth rates in particular the good match between 230 thex and 230 thex 232 th data indicates that at the location of crust 121dk 230 th and 232 th fluxes in the water column change simultaneously and suggests that the normalization of 230 thex to 232 th makes 230 thex 232 th a better chronometer our best fit model suggests that crust 121dk experi enced changes in growth rates at 122 and 312 ka and a growth with a constant 230 th initial flux this chronology returns an age of 680 ka for the uppermost 1 5 mm the 234 u 238 u depth profile however was clearly affected by diffusion of 234 u in the porous crust and can therefore not be used to derive a reliable chronology one part of the crust seems isolated from pore water diffusion and can be physically recognized as a zone of very small porosity on the basis of the 230 thex 232 th chronology major element chemistry is shown to be linked to climate change mn fe variations compare well with those in a fe mn crust from the pacific showing systematic maxima during glacial stages 2 and 4 high mn fe are tentatively interpreted to reflect expansion of the oxygen minimum zone during glacial periods resulting from higher bioproductivity in addition we note that the surface 230 th 232 th activity ratio of crust 121dk is entirely consistent with advection of deep water from the western toward the eastern atlantic basin copyright 2005 elsevier ltd

sr nd isotopic and ree geochemical constraints on the provenance of fine grained sands in the ordos deserts north central china surface sands and sandstones in the ordos region alluvial sands from the surrounding mountains and fluvial sands in the yellow river are systemically collected for granulometric mineralogical geochemical and isotopic analyses fine grained fractions are small for the ordos desert sands as well as alluvial sands of the surrounding mountains whereas they are abundant for fluvial sands of the yellow river quartz feldspar and muscovite are common minerals but their relative proportions are variable for various sediments ree geochemical compositions are roughly identical but isotopic compositions particularly end 0 values of fine grained sands vary significantly in the ordos deserts based on the integrated sr nd isotopic data the possible source areas are isotopically divided into four regions for fine grained sands in the ordos deserts region a the helan yin mountain ranges region b the ordos sandstone areas region d the tarim basin the alxa plateau the hexi corridor the northern qinghai xizang plateau and the yellow river drainage and region e the jungger basin and the southern mongolia through geochemical and particularly isotopic comparisons in combination with expert meteorological and geographical knowledge fine grained sands in the ordos deserts originated both from weathering of local sandstones and from blowing of fine sediments in region d the input from region d is subordinate to the local source for fine grained sands in the ordos deserts contributions from weathering of local sandstones might be roughly homogeneous from the west to the east of the ordos region as well as the input of fine grained sediments from remote areas such as the tarim basin the qinghai xizang plateau and the alxa plateau geographical nd isotopic variations of fine grained sands in the ordos deserts might be caused by the input from adjacent places such as the yellow river drainage due to the near surface winds

processes controlling the heavy metal distribution in pacific ferromanganese nodules and crusts the ferromanganese precipitates existing in deep sea waters of the pacific consist of two types of deposits 1 nodules mainly are distributed in pelagic basins beneath the ccd calcite compensation depth where the rate of sedimentation is low 2 polymetallic encrustations are formed on exposed seamount rocks where currents prevent normal sediment accumulation nodules being formed in areas bordering the equatorial zone of high biological productivity grow by two different processes a early diagenetic growth by supply of metals and metal compounds from pore water and b hydrogenetic growth by supply of colloidal particles from near bottom seawater these processes lead to different kinds of oxide and different metal contents

spatial and temporal patterns of late quaternary eolian deposition eastern colorado u s a abstract eolian sediment covers about 60 of colorado east of the rocky mountains about 30 of the sediment is sand and 70 is loess initially flood plains were the principal sources of eolian sediment but during the holocene dunes formed from older eolian sand and alluvium on uplands since latest pleistocene time dominant dune forming winds have been northwesterly in the northern part of the region and southwesterly in the southern part at present sand sheets and dunes mainly parabolic types are stable and covered with vegetation in dunes sand is commonly 20 30 m thick but elsewhere averages

large scale stabilized dunes on the high plains of colorado understanding the landscape response to holocene climates with the aid of images from space principal component analysis of landsat thematic mapper images from eastern colorado reveals stabilized dune fields that are composed of single and compound parabolic dunes some longer than 10 km these dunes have been reactivated at least four times in the past 10 000 yr at ca 9500 to 5500 yr b p 5500 to gt 4800 yr b p 4800 to gt 1000 yr b p and lt 1000 yr b p during droughts that probably exceeded the dry conditions during the 1930s dune orientations indicate that these landforms were molded by winds from the northwest which now dominate during the winter and spring with the passage of pacific air masses this study underscores the sensitivity of the landscape of the high plains to drought conditions which may become more prevalent because of the anticipated effects of greenhouse warming

luminescence dating of sand loess sequences and response of mu us and otindag sand fields north china to climatic changes the sand loess transition zone in north china is sensitive to climate change and is an ideal place to investigate past environmental changes however past climate change at millennial centennial timescales in this region has not been well reconstructed because of limited numerical dating alternations of sandy loam soils with aeolian sand layers in the mu us and otindag sand fields which lie along the sand loess transition zone indicate multiple intervals of dune activity and stability this change is probably a response to variations of the east asian monsoon climate during the late quaternary the single aliquot regeneration sar optically stimulated luminescence osl dating protocol which has been successfully applied to aeolian deposits worldwide is applied to these two sand fields in this study the osl ages provide reliable constraints for reconstruction of past climate changes at suborbital timescale sections in both sand fields contain aeolian sand beds recording millennial scale episodes of dry climate and widespread dune activation including episodes at about the same time as heinrich event 5 and the younger dryas in the north atlantic region these results demonstrate the potential of aeolian sediments in semi arid north china to record millennial scale climatic events and also suggest that dry wet climate variation at the desert margin in china may be linked to climatic change elsewhere in the northern hemisphere through atmospheric circulation this article was published online on 27 november 2008 an error was subsequently identified this notice is included in the online and print versions to indicate that both have been corrected 16 december 2008 copyright 2008 john wiley sons ltd

correlation of co concentration and growth rate a method for age determination of ferromanganese crusts abstract based on the analysis and comparison of metal content and growth rate determination by the 10be method an empirical law has been developed to calculate the growth rate evaluating after 520 chemical analyses we have shown that the flux of co into ferromanganese seamount crusts has been constant over the entire growth period of all crusts investigated the amount of co supplied per unit of time and seabottom area is nearly constant in the oceanic water column the layered texture of ferromanganese crusts reflects the change of phases and hydrogenetically precipitated amounts as well as their different growth rates the most important indicator revealing the growth rate is the co enrichment of each crust layer the higher the growth rate the lower the co concentration therefore it is possible to recalculate the age and growth rate of each ferromanganese crust layer by g mm ma 1 1 28 co 0 24 based on the intense phosphorite impregnation of the lower part of the ferromanganese crust which has grown within the time period 18 12 ma so called older crust two additional effects have to be taken into account a the intense phosphorite impregnation causing dilution and b the chemical dissolution of the ferromanganese crusts caused by the phosphate rich pore water both reduce the co concentration and the relating co contents co x and co x have thus to be compensated a co x co x m mn co x mn co b b co x co x 1 0 05 p 1 where co x double corrected co concentration of the layer x of the older crust generation co x corrected co concentration of the layer x of the older crust generation concerning the phosphate dilution co x m measured co concentration of the layer x of the older crust generation mn co x mn co ratio measured of the layer x of the older crust generation mn co b mn co ratio measured of the boundary layer between younger and older crust generation and p difference between the apatite fraction of the older crust of the layer x and the average value of the younger crust generation this simple cobalt calculation model makes it possible to recalculate the growth rate and age of each simple layer of ferromanganese crusts

beryllium 10 thorium 230 and protactinium 231 in galapagos microplate sediments implications of hydrothermal activity and paleoproductivity changes during the last 100 000 years biogenic particle fluxes from highly productive surface waters boundary scavenging and hydrothermal activity are the main factors influencing the deposition of radionuclides in the area of the galapagos microplate eastern equatorial pacific in order to evaluate the importance of these three processes throughout the last 100 kyr concentrations of the radionuclides 10be 230th and 231pa and of mn and fe were measured at high resolution in sediment samples from two gravity cores klh 068 and klh 093 high biological productivity in the surface waters overlying the investigated area has led to 10be and 231pa fluxes exceeding production during at least the last 30 kyr and probably the last 100 kyr however during periods of high productivity at the up welling centers off peru and extension of the equatorial high productivity zone a relative loss of 10be and 231pa may have occurred in these sediment cores because of boundary scavenging the effects of hydrothermal activity were investigated by comparing the 230thex concentrations to the mn fe ratios and by comparing the fluxes of 230th and 10be which exceed production the results suggest an enhanced hydrothermal influence during isotope stages 4 and 5 and to a lesser extent during isotope stage 1 in core klh 093 during isotope stages 2 and 3 the hydrothermal supply of mn was deposited elsewhere probably because of changes in current regime or deep water oxygenation a strong increase of the mn fe ratio at the beginning of climatic stage 1 which is not accompanied by an increase of the 230thex concentration is interpreted to be an effect of mn remobilization and reprecipitation in the sediment

evidence for a late holocene warm and humid climate period and environmental characteristics in the arid zones of northwest china during 2 2 1 8 kyr b p 1 natural archives like ice cores tree rings river and lake sediments lake terraces and paleosols and also historical documents witness aspects of climate change in northwestern china during the western and eastern han dynasties 206 b c 220 a d reconstructions of decadal averages of annual mean temperature and precipitation fluctuations were derived from variations of 18o and net accumulation rates in the guliya ice core they revealed a period of higher temperatures and higher precipitation than today which affected vast areas of northwestern china during the period of interest until the fifth century a d these conditions resulted in a marked increase in the discharge of big endorheic river systems such as the tarim the keriya and the manas rivers as a consequence water levels in appendant terminal lakes rose e g at lop nor manas lake and baijian hu lake surface areas expanded and lake desalting occurred also at lakes in intermontane basins such as balikun hu and qinghai lake the warm and moist conditions during the western and eastern han dynasties might have been responsible for the large scale agricultural production and the local socioeconomic boom that is documented by the occurrence of the famous ruin groups of loulan niya and keriya the following desiccation phase led to a deterioration of water resources and most oases tended to dry out and were finally abandoned the appearance development flourishing and final abandonment of each great ruin group are closely associated with regional climate change at that time

major episodes of quaternary activity in the lower neales river northwest of lake eyre central australia abstract the lake eyre basin australia s largest internal drainage system represents a key site in unravelling the complexities of terrestrial climate change the basin covering one sixth of the australian continent spans a number of climatic zones including the tropical monsoon system to the north and the mid latitude westerly circulation to the south this study describes four major episodes of quaternary fluvial lacustrine and aeolian activity in the neales river a western catchment of lake eyre the first phase is represented by coarse grained fluvial aggradation which dates to at least 170 ka the depocentre of lake eyre is believed to have been located further to the east during this phase the second episode is a phase of high lake level lacustrine deposition which occurred before 103 ka the deltaic margin of the lake at this time was approximately 20 km west of the present playa the third phase was characterised by significant base level lowering and channel incision after 50 ka but before 31 ka lake level lowering induced fluvial incision of up to 9 m scouring several metres into the basal silicified miocene sediments some time after 50 ka the final phase was a period of aeolian and ephemeral fluvial deposition which peaked between 20 ka and 18 ka coincident with the last glacial maximum these episodes are compared with chronostratigraphic data from the monsoon dominated catchments of the cooper and diamantina rivers the nature and record of fluvial and lacustrine deposition are correlated throughout the basin during the penultimate and last interglacial cycles the late quaternary record is more ambiguous and further studies are required to elucidate the precise nature of climate change in the basin over the last 30 ka

optical dating of dune sands in the northeastern deserts of china abstract optical dating has been used to obtain the ages of fossil stabilized sand dunes from four sections in the northeastern deserts of china our results indicate that the optically stimulated luminescence ages of the four sections correlate well even though the samples were collected from different deserts about 600 km apart our results also indicate that active dune formation in this region lasted from the last glacial maximum to about 10 ka and that the warm climate of the holocene was interrupted by a cold dry dune formation episode about 3 5 1 7 ka the holocene optimum in this region is between 10 and 3 6 ka and a later warm humid dune stabilization phase lasted from at least 1 6 to 1 0 ka the youngest age on the uppermost sand unit yielded an age of only 40 yr supporting the previous argument that the existence of modern active eolian sands in the regions with a mean annual precipitation of up to 450 mm is not mainly due to drought but to extensive land cultivation over historic time from the luminescence properties of the quartz grains it is hypothesized that the sands in most of the sections are probably derived from more than one source with a minor source of quartz having a different thermal history before deposition

238u234u230th chronometry of femn crusts growth processes and recovery of thorium isotopic ratios of seawater abstract comparison of 234 u excess 238 u and 230 th 232 th activity ratios in oceanic fe mn deposits provides a method for assessing the closed system behaviour of 238u 234u 230th as well as variations in the initial uranium and thorium isotopic ratios of the precipitated metal oxides this approach is illustrated using a fe mn crust from lotab seamount marshall islands west equatorial pacific here we report uranium and thorium isotopic compositions in five subsamples from the surface of one large 5 cm diameter botryoid of this crust and from two depth profiles of the outermost rim of the same botryoid the decrease of 234 u excess 238 u and 230 th 232 th activity ratio with depth in the two profiles gives mean growth rates for the last 238u 212th 150 ka of 7 8 2 mm ma and 6 6 1 mm ma respectively all data points surface and core samples but one define a linear correlation in the ln 230 th 232 th ln 234 u excess 238 u diagram this correlation indicates that for all points the u th system remained closed after the fe mn layer precipitated and that the different samples possessed the same initial uranium and thorium isotope ratios furthermore these results show that the preserved surface of this fe mn crust may not be the present day growth surface and that the thorium and uranium isotopic ratios of seawater in west equatorial pacific have not changed during the past 150 ka the initial thorium activity ratio is estimated from the correlation obtained between ln 230 th 232 th ln 234 u excess 238 u

thorium and uranium isotopes in a manganese nodule from the peru basin determined by alpha spectrometry and thermal ionization mass spectrometry tims are manganese supply and growth related to climate thorium and uranium isotopes were measured in a diagenetic manganese nodule from the peru basin applying alpha and thermal ionization mass spectrometry tims alpha counting of 62 samples was carried out with a depth resolution of 0 4 mm to gain a high resolution230thexcess profile in addition 17 samples were measured with tims to obtain precise isotope concentrations and isotope ratios we got values of 0 06 0 59 ppb 230th 0 43 1 40 ppm 232th 0 09 0 49 ppb 234u and 1 66 8 24 ppm 238u the uranium activity ratio in the uppermost samples 1 6 mm and in two further sections in the nodule at 12 5 1 0 mm and 27 3 33 5 mm comes close to the present ocean water value of 1 144 0 004 in two other sections of the nodule this ratio is significantly higher probably reflecting incorporation of diagenetic uranium the upper 25 mm section of the mn nodule shows a relatively smooth exponential decrease in the230thexcess concentration tims the slope of the best fit yields a growth rate of 110 mm ma up to 24 5 mm depth the section from 25 to 30 3 mm depth shows constant230thexcess concentrations probably due to growth rates even faster than those in the top section of the nodule from 33 to 50 mm depth the growth rate is approximately 60 mm ma two layers in the nodule with distinct laminations 11 15 and 28 33 mm depth probably formed during the transition from isotopic stage 8 to 7 and in stage 5e respectively the mn fe ratio shows higher values during interglacials 5 and 7 and lower ones during glacials 4 and 6 a comparison of our data with data from adjacent sediment cores suggests a a variable supply of hydrothermal mn to sediments and mn nodules of the peru basin or b suboxic conditions at the water sediment interface during periods with lower mn fe ratios

natural and anthropogenic sources of east asian dust nd sr isotopic signatures of loess modern dust and their potential source materials are systematically investigated to discriminate the natural and anthropogenic sources of eastern asian dust chinese loess is an eolian deposit that has accumulated over the past few million years and is characterized by a negative nd sr isotopic correlation that implies binary sources for the natural background of eastern asian dust loess in northeastern china shows the highest end 0 value while loess in western china has the lowest end 0 considering the nd sr isotopic signatures of the potential sources we propose that the high end 0 end member is derived from the arid lands around the northern boundary of china while the low end 0 end member originates from the deserts on the northern margin of the tibetan plateau chinese loess plateau deposits are dominated by northern margin tibetan plateau dust suggesting a strong link between the thick eolian deposit in this region and the evolution of the tibetan plateau the long range transported spring dust in both nanjing south china and japan has an nd sr isotopic composition similar to that of loess however spring dust in beijing north china has a much lower end 0 value indicating an additional anthropogenic contribution of low end 0 material possibly from the sandy lands in the adjacent north and west

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

holocene aeolian activity and climatic change in qinghai lake basin northeastern qinghai tibetan plateau abstract aeolian deposits in the qinghai lake basin on the northeastern qinghai tibetan plateau netp were studied in order to reconstruct changes in aeolian activity related to climatic changes over the last 12 5 ka we used 20 optically stimulated luminescence and conventional radiocarbon dates to construct chronological frameworks for four aeolian sections the results indicate that aeolian activity occurred episodically at 12 5 ka 11 2 ka 9 0 ka 8 5 ka 5 5 ka 2 6 2 4 ka and 1 0 9 ka paleosols developed mainly during 9 5 4 ka the field stratigraphy of the sections together with measurements of grain size magnetic susceptibility ms and total organic carbon toc indicate that aeolian activity was strong during the last glacial period following the younger dryas yd holocene interglacial conditions began to develop in the qinghai lake basin at 12 ka with the strengthened asian summer monsoon asm and the decrease in insolation the climate became warm and humid according to the record of aeolian activity the early holocene can be divided into two intervals before and after 9 5 ka aeolian activity remained strong from 12 ka 9 5 ka although the climate was more humid than that of the last glacial period between 9 5 ka and 8 5 ka the aeolian activity weakened and the climate became warm and wet but variable following the mid holocene climatic optimum the climate became significantly drier at 4 ka and aeolian activity strengthened again overall the asm dominated aeolian activity in the qinghai lake basin during the holocene with enhanced aeolian activity corresponding to a weakened asm paleosol development exhibited a lagged response to the strengthened asm and therefore aeolian sand layers in the section do not always indicate a cold and dry climate

rhythmic growth of pacific ferromanganese nodules and their milankovitch climatic origin the internal microstructure of a ferromanganese nodule 2392 from 154 37 52 w 9 37 56 n at water depth 5194 m was examined in detail on polished sections and radiometrically dated 230thex 232th along a high resolution 0 1 mm depth profile 0 1 3 mm spanning approximately 271 ka the fabric shows typical stromatolithic structure and exhibits four orders of basic cyclic growth pattern namely laminae bands laminae zones laminae groups and laminae pairs having average thicknesses of 402 454 185 206 58 67 and 15 18 m respectively a profile from the depth of 200 2661 m was selected to obtain the geochemical series using line scanning electron microprobe analyses which provide a record of paleoceanographic oscillations during the growth period power spectral analysis of the geochemical series for al mn fe and fe mn from the depth of 200 1220 m where no obvious discontinuity could be observed display conspicuous cyclicities the cycles of laminae zones laminae groups and laminae pairs are reflected in the spectral patterns as well the significant spectral peaks are located at 186 108 66 m together with the cycle of laminae bands the ratios of these cyclicities are close to those of milankovitch orbital cycles through tuning to orbital cycles a net growth rate of 4 5 mm ma is derived for the profile which is in perfect agreement with the growth rate of 4 6 mm ma determined by 230thex 232th dating therefore the rhythmic growth of ferromanganese nodules appears definitely associated with milankovitch cycles and the growth cyclicities may offer a new tool for estimating growth rates of ferromanganese nodules and paleoenvironmental reconstruction at substage resolution when supported by radiometric dating

holocene changes in fire frequency in the daihai lake region north central china indications and implications for an important role of human activity abstract black carbon bc content in a sediment core from daihai lake inner mongolia was analyzed to reconstruct a high resolution history of fires occurring in northern china during the holocene and to examine the impacts of natural changes and human activities on the fire regime the black carbon mass sedimentation rate bcmsr was disintegrated into two components the background bcmsr and the bcmsr peak with the bcmsr peak representing the frequency of fire episodes both the background bcmsr and the magnitude of the bcmsr peak display a close relation with the percentage of tree pollen from the same sediment core suggesting that regional vegetation type would be a factor controlling the intensity of fires the inferred fire episode frequency for the holocene exhibits two phases of obvious increases i e the first increase from

palaeoceanographic conditions during the formation of a ferromanganese crust from the afanasiy nikitin seamount north central indian ocean geochemical evidence abstract a ferromanganese crust dredged from the summit of the afanasiy nikitin seamount in the north central indian ocean ncio has recorded neogene oceanographic events the substrate of the crust is composed of fresh water phreatic calcite cement terebratulinae casts rounded and ferruginised basalt clasts and weathered coralline algal fragments suggesting subaerial exposure during the oligocene 30 ma global sealevel drop the mineralogy major trace and ree element geochemistry and co model age estimates suggest three distinct accretionary environments during the crust growth 1 a period of contemporary precipitation of fe mn oxide and carbonate fluorapatite cfa indicate an intensified intermediate water oxygen minimum zone omz in the late miocene 2 a pulse of very high cfa deposition and detrital input in addition to fe mn oxide accretion at the close of miocene reflect a more intense omz and the erosion of the himalayas and 3 more oxidizing conditions of the ambient seawater due to contraction of the late miocene omz facilitated the accretion of almost pure fe mn oxide since the pliocene these interpretations solely depend upon the chronology based on an empirical relationship and are subjected to the confirmation by radiometric dating as a consequence of significant deposition of cfa in addition to fe mn oxide during the intensified omz the normally coherent behaviour of trivalent rare earth elements 3 ree is not observed in the crust la yb and lu show a positive association with the cfa phase elements ca and p and pr nd and sm with the mn oxide phase elements mn co and ni this leads to an abnormal incoherent behaviour of the 3 ree in multimineral authigenic system

variability of monsoon climate in east asia at the end of the last glaciation abstract high resolution paleomonsoon proxy records from peat and eolian sand paleosol sequences at the desert loess transition zone in china denote a rapid oscillation from cold dry conditions 11 200 10 60014c yr b p to cool humid conditions 10 600 10 20014c yr b p followed by a return to cold dry climate 10 200 10 00014c yr b p variations in precipitation proxies suggest that significant climatic variability occurred in monsoonal eastern asia during the younger dryas interval late glacial climate in the chinese desert loess belt that lies downwind from europe was strongly influenced by cold air from high latitudes and from the north atlantic via the westerlies the inferred precipitation variations were likely caused by variations in the strength of the siberian high which influenced the pressure gradient between land and ocean and therefore influenced the position of the east asian monsoon front

60 myr records of major elements and pb nd isotopes from hydrogenous ferromanganese crusts reconstruction of seawater paleochemistry we compare the time series of major element geochemical and pb and nd isotopic composition obtained for seven hydrogenous ferromanganese crusts from the atlantic indian and pacific oceans which cover the last 60 myr average crust growth rates and age depth relationships were determined directly for the last about 10 myr using be 10 be 9 profiles in the absence of other information these were extrapolated to the base of the crusts assuming constant growth rates and constant initial be 10 be 9 ratios due to the lack of additional information co contents have also been used previously to estimate growth rates in co rich pacific and atlantic seamount crusts puteanus and halbach 1988 a comparison of be 10 be 9 and co based dating of three co rich crusts supports the validity of this approach and confirms the earlier chronologies derived from extrapolated be 10 be 9 based growth rates back to 60 ma our data show that the flux of co into co poor crusts has been considerably lower the relationship between growth rate and co content for the co poor crusts developed from these data is in good agreement with a previous study of a wider range of marine deposits manheim 1986 the results suggest that the co content provides detailed information on the growth history of ferromanganese crusts particularly prior to 10 12 ma where the be 10 based method is not applicable the distributions of pb and nd isotopes in the deep oceans over the last 60 myr are expected to be controlled by two main factors a variations of oceanic mixing patterns and flow paths of water masses with distinct isotopic signatures related to major paleogeographic changes and b variability of supply rates or provenance of detrital material delivered to the ocean linked to climate change glaciations or major tectonic uplift the major element profiles of crusts in this study show neither systematic features which are common to crusts with similar isotope records nor do they generally show coherent relationships to the isotope records within a single crust consequently any interpretation of time series of major element concentrations of a single crust in terms of paleoceanograghic variations must be considered with caution this is because local processes appear to have dominated over more basin wide paleoceanographic effects in this study co is the only element which shows a relationship to pb and nd isotopes in pacific crusts a possible link to changes of pacific deep water properties associated with an enhanced northward advection of antarctic bottom water from about 14 ma is consistent with the pb but not with the nd isotopic results the self consistent profiles of the pb and nd isotopes suggest that postdepositional diagenetic processes in hydrogenous crusts including phosphatization events have been insignificant for particle reactive elements such as pb be and nd isotope time series of pb and nd show no systematic relationships with major element contents of the crusts which supports their use as tracers of paleo seawater isotopic composition

optical dating of holocene dune sands from the hulun buir desert northeastern china aeolian deposits from the hulun buir desert of northeastern china are studied with optically stimulated luminescence dating methods to establish the chronology of dune building phases and climatic changes since the last deglaciation our results indicate that wet climate marked by dune stabilization and soil development in the hulun buir desert commenced at 11 ka ago and this early episode of dune stabilization lasted until 4 4 ka ago this optimum climate between 11 and 4 4 ka ago is mostly the response to the strengthened monsoon circulation and increased precipitation in the northern hemisphere the environment generally became arid after 4 4 ka ago but the dry climate was interrupted by three phases of weak soil development occurring at 1 8 1 4 1 2 1 0 and 0 84 0 5 ka ago respectively such short events of dune stabilization were associated with the warm and humid climate in historical time however the present dune mobilization in the hulun buir desert is mainly the result of poor la

10 be dating of a manganese crust from central north pacific and implications for ocean palaeocirculation one of the most promising applications of the new 10be detection technique using accelerator mass spectrometry is the determination of growth rates of mn nodules and crusts1 5 because the half life of cosmogenic 10be 1 5 myr is of the same order as the time needed for a few millimetres of nodule material to grow 10be is a useful tool for unravelling such evolution during the late tertiary when most nodules are assumed to have started their growth6 mass spectrometric measurement of a 10be profile across the manganese crust va 13 2 from the central pacific using a tandem van de gr a ff accelerator yields growth rates of 2 7 and 4 8 mm myr 1 for the layers of the crust accumulated between recent and 6 myr bp and between 6 and 11 myr respectively using these measurements as well as 230th dating we have been able to distinguish boundaries between zones in the crust with different structures and chemical compositions and to assign ages to them these ages are 0 12 2 9 3 4 5 7 6 7 7 9 10 12 and 13 16 myr bp all of these boundaries apparently coincide with the times reported for quaternary and late tertiary palaeoclimatic events suggesting that the crust growth has been strongly influenced by palaeoclimate

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 dry holocene megathermal in inner mongolia the paleoclimate since 14 kyr bp 14c age was reconstructed based on a 16 22 m long sediment core collected from lake yanhaizi a saline lake located near the northern limit of the east asian summer monsoon in inner mongolia coarse sediments were deposited there during a shrinkage phase of the lake when sand dunes reactivated these sediments have low organic carbon contents but high maturity indices indicating that they were deposited in an arid environment by contrast based on high organic contents and low maturity indices fine sediments were deposited during periods of high lake stand in a humid environment it was in general dry between 8 0 and 4 3 kyr bp the above dry and wet phases are consistent with those recovered from the arid semiarid transition zone elsewhere but are unlike the widely perceived humid holocene megathermal reported in east china and the newly reconstructed record in the alpine retreat lake in taiwan the discrepancy may be due to a relative insensitivity to humidity changes in these two areas since they have both been under the total influence of the summer monsoon on the other hand much enhanced evaporation over higher monsoon precipitation at lake yanhaizi reduces the effective humidity in the warm climate near the northern boundary of the summer monsoon this also accounts for the fact the high temperature holocene megathermal as revealed in the okinawa trough and the northern south china sea is correlated to the dry phases at lake yanhaizi conversely the 4 2 kyr bp coldest period in the holocene corresponds to a wet phase at lake yanhaizi

late quaternary paleoenvironments in the southwestern kalahari abstract landforms of the southwestern kalahari in southern africa include linear dune systems pans and associated lunette dunes and the valleys of ephemeral rivers present day climates in the region are transitional from arid to semi arid making the area sensitive to climatic change the linear dunes are part of an extensive system of fixed dunes developed in the surface kalahari sands the dunes were in existence prior to 32 000 yr b p but have been reactivated on a number of occasions since then in conditions of increased wind velocities more nw winds and reduced rainfall especially in northern and eastern parts of the region periods of dune activity probably occurred at 19 000 17 000 10 000 6000 and 4000 3000 yr b p the development of pans in the region has been strongly influenced by changes in groundwater conditions deflation from pans and formation of lunettes occurred in periods when groundwater levels were higher and fluctuated seasonally more than they do today such periods probably occurred 25 000 20 000 yr b p around 10 000 yr b p and just prior to 1000 yr b p still higher groundwater discharges promoted by increased regional rainfall resulted in the formation of spring tufas and seasonal or permanent lakes in the pans which were also fed by surface runoff during the intervals between 33 000 28 000 23 000 21 000 17 000 15 000 and 12 000 11 000 yr b p the headwaters of the major ephemeral streams lie outside the region but within the same climatic zone increased flow in the molopo river occurred prior to 19 000 and between 15 000 and 13 000 yr b p

aeolian and fluvial evidence of changing climate and wind patterns during the past 100 ka in the western simpson desert australia abstract sediments near finke in central australia provide evidence of late quaternary evolution and the interaction of aeolian and fluvial systems in response to changing climate in the western simpson desert thermoluminescence tl dating is used to develop a chronology of aeolian and alluvial activity and to identify differences in sand provenance quaternary alluviation in the finke valley at this location occurred at or prior to c 90 ka and with no surviving evidence of subsequent activity until the holocene in contrast to rivers in the eastern part of the lake eyre basin substantial alluviation took place here in the early to mid holocene probably due to reactivation of the northern monsoon and its partial penetration into central australia the regional dunefield near finke consists of linear dunes largely reworked and aligned during the last glacial 30 12 ka as part of the great anticlockwise whorl of dunes in central australia the oldest dated source bordering dunes from the finke river are bright red in colour were deposited at c 100 ka and are now buried beneath paler source bordering dunes the latter consist of two exposed units a lower one of unknown orientation that dates at 17 9 ka and an upper unit aligned to the northwest and parallel to the prevailing winds that dates at 5 0 ka the tl signature and sediment texture of the oldest source bordering dunes show them to have probably been derived from weathered alluvial red beds of similar or greater age nearby and to have contributed abundant sand to the regional dunefield immediately north of the river in contrast sand from the younger paler source bordering dunes appears to be from a different source and to have only recently extended into the regional field the finke region would seem to be in a pivotal position for the study of palaeowind patterns that have created and modified australia s whorl of continental dunes the 30 18 ka regional linear dunes near finke are oriented almost due north their cross sectional asymmetry steeper eastern slopes suggests a response to southwesterly or westerly sand transporting winds between 18 and 10 ka these winds appear to have shifted to their present southeast orientation during the past 5 ka from dune ages alignments and asymmetry it is proposed here that there was a northward shift in the wind pattern at finke by about 100 150 km 1 1 5 of latitude during the last glacial but that through this period the regional dune pattern has remained remarkably stable

the age of the koputaroa dunes southwest north island new zealand abstract four recent dune phases have previously been identified in the manawatu and horowhenua regions of southwest north island new zealand the oldest of these dune phases the koputaroa dunes occur over a restricted area near the town of levin previous estimates of the age of these dunes have largely been based upon the occurrence of aokautere ash an airfall member of the kawakawa tephra ca 22 14 c ka b p interbedded within the dune sands nine samples of koputaroa dune sand were collected from five sites and dated using infrared stimulated luminescence irsl the results suggest that the dunes were originally deposited at or before 40 50 ka and that they were then re mobilised in the period from 24 to 10 ka these periods of dune activity are compared with records of loess deposition in wanganui basin aeolian influx of quartz in taranaki and dust flux in the vostok ice core

archaeology of the anthropocene in the yellow river region china 8000 2000 cal bp although archaeological analysis emphasizes the importance of climatic events as a driver of historical processes we use a variety of environmental and archaeological data to show that human modif

growth response of a deep water ferromanganese crust to evolution of the neogene indian ocean abstract a deep water ferromanganese crust from a central indian ocean seamount dated previously by 10 be and 230 th excess was studied for compositional and textural variations that occurred throughout its growth history the 10 be 9 be dated interval upper 32 mm yields an uniform growth rate of 2 8 0 1 mm ma frank m o nions r k 1998 sources of pb for indian ocean ferromanganese crusts a record of himalayan erosion earth planet sci lett 158 pp 121 130 which gives an extrapolated age of 26 ma for the base of the crust at 72 mm and is comparable to the maximum age derived from the co model based growth rate estimates this study shows that fe mn oxyhydroxide precipitation did not occur from the time of emplacement of the seamount during the eocene 53 ma until the late oligocene 26 ma this paucity probably was the result of a nearly overlapping palaeo ccd and palaeo depth of crust formation increased early eocene productivity instability and reworking of the surface rocks on the flanks of the seamount and lack of oxic deep water in the nascent indian ocean crust accretion began older zone with the formation of isolated cusps of fe mn oxide during a time of high detritus influx probably due to the early miocene intense erosion associated with maximum exhumation of the himalayas op cit this cuspate textured zone extends from 72 mm to 42 mm representing the early miocene period intense polar cooling and increased mixing of deep and intermediate waters at the close of the oligocene might have led to the increased oxygenation of the bottom water in the basin a considerable expansion in the vertical distance between the seafloor depth and the ccd during the early miocene in addition to the influx of oxygenated bottom water likely initiated fe mn crust formation pillar structure characterises the younger zone which extends from 40 mm to the surface of the crust i e 15 ma to present this zone is characterised by 25 higher content of oxide bound elements than in the older zone possibly corresponding to further increased oxygenation of bottom waters increased stability of the seamount slope and gradually reduced input of continental detritus from the erosion of the himalayas middle miocene antarctic glaciation which peaked 12 13 ma ago increased the oxic bottom water influx to the basin resulting in accretion of the crust with low detritus therefore the younger crust started to accrete in response to a shift in bottom water circulation towards the contemporary pattern which produced a uniform growth rate and pillar structure up to the present

palaeoclimatic evidence from sand seas abstract sand seas can be an important source of information on the quaternary history of many deserts because sand seas appear to react sensitivity to climatic changes the quality of palaeoenvironmental information obtained from them is frequently much better than that from adjacent rocky desert areas patterns of dune alignments may be used to reconstruct the wind directions that formed currently inactive or fixed dunes and thus can provide information on palaeocirculation patterns the climatic conditions under which these dunes formed can be examined using palaeoclimatic models to explore the combinations of wind velocity rainfall and evaporation rates required to reactivate them choice of appropriate values for these parameters may be hampered by the problems of dating episodes of dune activity in many sand seas the best palaeoclimatic information comes from studies of interdune lacustrine or playa deposits which may reflect increased local rainfall or higher groundwater discharge or greater penetration of the sand sea by rivers with headwaters outside the desert caution should be taken in the interpretation of such deposits for juxtaposition of aquatic and aeolian environments can occur even in modern hyperarid and seas the record of climatic changes preserved in the deposits of modern sand seas suggests that periods of dune formation may have been interrupted by long periods of stability which may also have included deposition of interdune lacustrine deposits for example within the past 40 000 years active dune formation in many sand seas appears to have been restricted to the interval between about 20 000 and 12 000 yr b p and to the late holocene approximately 1 4 to 1 3 rd of the time

luminescence dating of late holocene dunes showing remnants of early settlement in cuddalore and evidence of monsoon activity in south east india abstract in this study dunes from south east india were dated using optically stimulated luminescence osl to reconstruct the depositional history a belt of dunes has developed parallel to the coast between pondicherry and karikal in tamil nadu south east india in the area between cuddalore and porto novo the dune belt is 5 km wide a transect from the coast to the most western dune inland was investigated changes in the environmental conditions are recorded in the dunes they show features including unconformities changes in the direction of bedding erosional features water escape structures and remnants of human settlements the osl results show that strong changes in the environmental conditions occurred about 100 and 300 years ago the latter event marks also the termination of settlement in this place the settlement period started about 1500 years previously the periods of sand mobility and stabilisation of the land surface by soil formation correlate with changes in the precipitation record of india the investigated dunes very likely reflect fluctuations in the monsoon activity during the last 3500 years in south east india

holocene environmental changes in central inner mongolia revealed by luminescence dating of sediments from the sala us river valley luminescence dating of the fluvial and lacustrine sediments from the sala us river valley at the south edge of the mu us desert central inner mongolia is reported the study region lies in the northwestern marginal zone of the east asian summer monsoon and is sensitive to climate change the dating results combined with environmental proxies indicate that the holocene climate optimum period took place from 8 5 to 5 ka ago and was marked by lake development after 5 ka ago the region became arid as inferred from lake regression and fluvial activity deposition of fluvial sediments lasted from 5 ka to 2 ka ago at about 2 ka ago incision of the sala us river was initiated into the underlying sediments with a down cutting rate of 3 4 cm yr since 2 ka ago human activities also played an important role in causing environmental change in the region

wetting and drying of australia over the past 300 ka alternately dry and wet climatic episodes across central and eastern australia during the past 300 ka have greatly affected australia9s rivers lakes and dune fields evidence of widespread climate and flow regime changes has been provided by 75 thermoluminescence tl dates and 18 u th dates from alluvial and eolian sediments fluvial conditions dominated part of the last two interglacials stage 5 and 7 resulting in large sand loads in rivers in the present simpson desert and southeastern australia during the last interglacial fluvial activity in central australia peaked at 110 ka stage 5 pluvial probably 5 10 ka behind world temperature and sea level maxima following the last late interglacial wet phase aridity associated with dune building spread from central australia toward its margins achieving greatest intensity during the last glacial maximum a less widespread wet phase identified at about 55 35 ka stage 3 subpluvial is associated with high lake levels and paleochannel activity in southeastern australia this tl record of variable continental aridity in australia correlates well with global changes including the variable eolian dust flux into central china the northern pacific ocean and antarctica

nd isotope signature of holocene baltic mn fe precipitates as monitor of climate change during the little ice age neodymium nd isotope profiles were analyzed on two baltic mn fe precipitates 99 2 and tl1 from shallow water 20 m of the mecklenburg bay the age range of these mn fe precipitates determined by 226raex ba dating reaches from recent growth back to 4300 and 1000 yr bp respectively over this time range the nd isotope composition varies from end 0 13 1 to 17 5 in the selected baltic precipitates indicating substantial changes in the nd isotope composition of the baltic sea the lowest end values were recorded during the time interval of the little ice age lia ad 1350 to 1850 these minimum values indicate either an increase of the input of less radiogenic nd from scandinavian archean proterozoic sources end about 22 to the baltic sea or a decrease of the input of more radiogenic nd from continental european sources end about 12 and or north sea water end about 10 variations of both erosive continental input and north sea inflow may indicate a direct response of the nd isotope signal in the baltic sea to climate changes during the lia and be related to cyclic shifts in the atmospheric circulation triggered by the north atlantic oscillation nao another aspect that possibly influenced the input of trace elements and nd isotopes into the baltic sea is the population development in the circum baltic area during the lia the lowest end values also correspond to the medieval demographic crises that led to a significant decrease of agricultural activity and farmland the reduction of soil erosion and enhanced regrowth of natural vegetation may have changed the amount and proportions of dissolved and suspended particulate matter transported into the baltic sea by rivers which in turn may have resulted in a change of the nd isotope composition of baltic sea water

the hf isotope composition of global seawater and the evolution of hf isotopes in the deep pacific ocean from fe mn crusts this paper presents a revised chemical purification method of hf for the measurement of hf isotope ratios of fe mn crusts on a new generation of double focusing multiple collector plasma source mass spectrometer mc icp ms by measuring surface scrapings of hydrogenetic fe mn crusts distributed over the three major ocean basins the present day hf isotope distribution of oceanic deep water is assessed in detail the results show an ehf provinciality correlated with those of other radiogenic isotope tracers such as nd and pb in agreement with previous studies this supports the use of hf isotopes as tracer of element source provenance and water masses fe mn crusts display the same present day hf nd isotope array as given before for mn nodules the smaller isotopic variability of hf compared with nd may either be caused by a more efficient mixing of hf than nd in the ocean due to a longer residence time or may be a consequence of a systematically more radiogenic hf than nd isotope signature delivered to the oceans by weathering of continental crust a hf isotope time series was measured on crust va13 2 to assess the hf isotope compositions of the central pacific deep water over the past 26 ma no consistency is observed between the hf and pb isotope time series in contrast hf and nd isotope time series display similar patterns which are however apparently offset by 2 ma prior to 14 ma differential diffusion of hf and nd does not explain this offset the smaller amplitude in the hf isotope variations compared with the nd isotopes rather argues for more efficient mixing of hf in the ocean we suggest that both isotope systems have responded in a similar way to the processes affecting the dissolved radiogenic isotope composition of pacific deep water during this time interval the parallel increase in ehf and end observed between 14 and 3 ma may probably be attributed to the increased inputs of hf and nd into the central pacific ocean derived from the weathering of the pacific islands arcs over the past 3 ma an increased aeolian continental input derived from asia most likely caused negative shifts in end and ehf recorded by va13 2 the fact that hf and nd isotope compositions plot along the present day array for fe mn crusts and mn nodules over the entire past 26 ma suggests that aeolian supply of hf to the pacific ocean has been a long term important feature

thermoluminescence chronology of sand profiles in the mu us desert china abstract thermoluminescence tl is used to calculate the ages of two sand loess profiles in the marginal zone of the mu us desert china alternating units of dune sand and loess with palaeosols in the two profiles imply episodes of dune formation separated by periods of loess accumulation and pedogenesis our results indicate that there are three layers of dune sand intercalated in sediments of the last full glacial cycle suggesting the occurrence of three arid episodes during this time the first arid episode within this period occurred between

comparison of different distributed hydrological models for characterization of catchment spatial variability most physically based hydrological models rely on either spatially distributed or lumped characterizations of topography and other spatial variations three disturbed hydrological models with different treatments of topography namely the mike she topmodel and gb model geomorphology based hydrological model are discussed in this paper a regular square grid system is used by mike she for representation of catchment spatial variability and as the fundamental computation units topmodel characterizes the topography using a distributed topographic index ln a tan the areas with the same ln a tan are assumed to be hydrologically similar then the catchment is divided into a number of segments corresponding to the topographic index intervals the gb model employs the catchment area function and width function to lump the topography and divide the catchment into a series of flow interval hillslopes the catchment spatial variations are averaged over each flow interval and represented by one dimensional distribution functions with respect to the flow distance from the catchment outlet the hillslope elements are the fundamental computational units in the gb model the above three models are applied to the seki river in japan the model structure and performance are compared and discussed in the paper copyright 2000 john wiley sons ltd

grass habitat analysis and phytolith based quantitative reconstruction of asian monsoon climate change in the sand loess transitional zone northern china we investigated climate niches of grasses at regional scales and quantitatively reconstruct asian monsoon precipitation at the sand loess transitional zone in northern china our results provide direct evidence that certain grass lineages have been specialized in specific habitats pooideae grasses stand out and occupy a much cooler environment than all other subfamilies pooideae aristidoideae and chloridoieae occupy dry environments pooideae grasses occupy the coldest and driest environments compared to all other subfamilies with a mean annual temperature mat and precipitation map of 13 6 to 15 3 c and 224 to 1674 mm respectively at a regional scale we built a database for grasses and their corresponding climate parameters based on this database past climate parameters at the margin of the asian summer monsoon since 70 ka were quantitatively reconstructed by phytolith assemblages they show that this area was dominated by cold and dry adapted grasses since 70 ka with a mat and map of 3 3 to 11 0 c and 442 to 900 mm respectively generally consistent with the results of phytolith based transfer function reconstructions and with the results of previous nearby pollen based quantitative reconstructions with the improvement of the species climate and ecosystem dataset our database based method is a promising quantitative reconstruction approach to past climatic change in the monsoon region

holocene environmental changes in the central inner mongolia based on single aliquot quartz optical dating and multi proxy study of dune sands abstract alternating units of dune sands and paleosols in the central inner mongolia imply multiple episodes of dune building and stabilization in response to the waxing and waning of the east asian monsoon such eolian deposits were dated by using the single aliquot quartz optical dating method combined with the multi proxy study on the deposits the past environmental changes during the holocene have been reconstructed our results indicate that widespread eolian sand mobilization occurred in the studied region during the beginning of the early holocene from 11 5 ka to 9 ka the climate became warm and humid during the period between 9 ka and 5 6 ka holocene optimum after 5 6 ka the region again became arid as inferred from dune building however the environmental changes during the late holocene have been affected by both climate and human impacts and the presence of desert environment in such semiarid region is not only the result of climatic drought of the late holocene but also related to poor land use practices

historical desertification of the mu us desert northern china a multidisciplinary study abstract the problem of historical desertification of the mu us desert northern china has been paid considerable attention during the past several decades through the analysis of stratigraphic evidence historical records and archaeological discoveries this paper studied the problem using a multidisciplinary approach the results show that two phases of serious desertification occurred during the last two millennia one is during the mid to late tang dynasty about a d 800 and the other during the late ming dynasty about a d 1500 1600 further investigation of the cause of each phase of desertification demonstrates that the former phase of desertification is closely related to abrupt climate change occurring in the mid eighth century but the latter which started during the ming dynasty and stopped at late 1980s is not consistent with climatic causation therefore we suggest that human activities contributed to the development of the latter phase of desertification

sedimentary manganese metallogenesis in response to the evolution of the earth system abstract the concentration of manganese in solution and its precipitation in inorganic systems are primarily redox controlled guided by several earth processes most of which were tectonically induced the early archean atmosphere hydrosphere system was extremely o2 deficient thus the very high mantle heat flux producing superplumes severe outgassing and high temperature hydrothermal activity introduced substantial mn2 in anoxic oceans but prevented its precipitation during the late archean centered at ca 2 75 ga the introduction of photosystem ii and decrease of the oxygen sinks led to a limited buildup of surface o2 content locally initiating modest deposition of manganese in shallow basin margin oxygenated niches e g deposits in india and brazil rapid burial of organic matter decline of reduced gases from a progressively oxygenated mantle and a net increase in photosynthetic oxygen marked the archean proterozoic transition concurrently a massive drawdown of atmospheric co2 owing to increased weathering rates on the tectonically expanded freeboard of the assembled supercontinents caused paleoproterozoic glaciations 2 45 2 22 ga the spectacular sedimentary manganese deposits at ca 2 4 ga of transvaal supergroup south africa were formed by oxidation of hydrothermally derived mn2 transferred from a stratified ocean to the continental shelf by transgression episodes of increased burial rate of organic matter during ca 2 4 and 2 06 ga are correlatable to ocean stratification and further rise of oxygen in the atmosphere black shale hosted mn carbonate deposits in the birimian sequence ca 2 3 2 0 ga west africa its equivalents in south america and those in the francevillian sequence ca 2 2 2 1 ga gabon are correlatable to this period tectonically forced doming up attenuation and substantial increase in freeboard areas prompted increased silicate weathering and atmospheric co2 drawdown causing glaciation on the neoproterozoic rodinia supercontinent tectonic rifting and mantle outgassing led to deglaciation dissolved mn2 and fe2 concentrated earlier in highly saline stagnant seawater below the ice cover were exported to shallow shelves by transgression during deglaciation during the sturtian glacial interglacial event ca 750 700 ma interstratified mn oxide and bif deposits of damara sequence namibia was formed the varangian marinoan ca 600 ma cryogenic event produced mn oxide and bif deposits at urucum jacadigo group brazil the datangpo interglacial sequence south china liantuo nantuo varangian event contains black shale hosted mn carbonate deposits the early paleozoic witnessed several glacioeustatic sea level changes producing small mn carbonate deposits of tiantaishan early cambrian and taojiang mid ordovician in black shale sequences china and the major mn oxide carbonate deposits of karadzhal type central kazakhstan late devonian the mesozoic period of intense plate movements and volcanism produced greenhouse climate and stratified oceans during the early jurassic oae organic rich sediments host many mn carbonate deposits in europe e g urkut hungary in black shale sequences the late jurassic giant mn carbonate deposit at molango mexico was also genetically related to sea level change mn carbonates were always derived from mn oxyhydroxides during early diagenesis large mn oxide deposits of cretaceous age at groote eylandt australia and imini tasdremt morocco were also formed during transgression regression in greenhouse climate the early oligocene giant mn oxide carbonate deposit of chiatura georgia and nikopol ukraine were developed in a similar situation thereafter manganese sedimentation was entirely shifted to the deep seafloor and since ca 15 ma b p was climatically controlled glaciation deglaciation assisted by oxygenated polar bottom currents aabw nadw the changes in climate and the sea level were mainly tectonically forced

evidence of active dune sand on the great plains in the 19th century from accounts of early explorers eolian sand is extensive over the great plains of north america but is at present mostly stabilized by vegetation accounts published by early explorers however indicate that at least parts of dune fields in nebraska colorado kansas new mexico and texas were active in the 19th century based on an index of dune mobility and a regional tree ring record the probable causes for these periods of greater eolian activity are droughts accompanied by higher temperatures which greatly lowered the precipitation to evapotranspiration ratio and diminished the cover of stabilizing vegetation in addition observations by several explorers and previous historical studies indicate that rivers upwind of great plains dune fields had shallow braided sandy channels as well as intermittent flow in the 19th century wide braided sandy rivers that were frequently dry would have increased sand supplies to active dune fields we conclude that dune fields in the great plains are extremely sensitive to climate change and that the potential for reactivation of stabilized dunes in the future is high with or without greenhouse warming

cobalt in ferromanganese crusts as a monitor of hydrothermal discharge on the pacific sea floor ferromanganese oxide crusts which accumulate on unsedimented surfaces in the open ocean1 6 derive most of their metal content from dissolved and particulate matter in ambient bottom water7 8 in proportions modified by the variable scavenging efficiency of the oxide phase for susceptible ions9 they differ in this respect from abyssal nodules much of whose metals are remobilized from host sediments here we present maps of cobalt concentration and inferred accumulation rate of ferromanganese crusts from the pacific ocean we propose that depletion of cobalt in pacific crusts measures the location and intensity of submarine hydrothermal discharge use of the cobalt chronometer an algorithm inversely relating cobalt content and crust growth rate permits mapping of the accumulation rate of ferromanganese crusts with only indirect recourse to radioactivity based dating methods these maps show that crusts in hydrothermal areas grow from two to more than four orders of magnitude faster than in the central pacific ocean cobalt enriched crusts are found where water masses are most isolated from continental coastal and hydrothermal sources of metals now and in the past this relationship can resolve the problem of cobalt enrichment in crusts without recourse to hypotheses invoking special cobalt sources or enrichment mechanisms

isochron dating of sediments using luminescence of k feldspar grains sponsorship research grant council of the hong kong special administrative region china project 7106 02p 7032 05 and 7035 06

magnetostratigraphy of hydrogenetic manganese crusts from northwestern pacific seamounts abstract fine scale dating is crucial to understanding the growth and environments of formation of marine manganese deposits the paleomagnetic method of dating of manganese nodules and crusts has been attempted but with no success so far we measured remanent magnetization rm on up to 75 mm thick hydrogenetic crusts at intervals of 2 5 mm after mineralogical and chemical examination careful separation and alternative field demagnetization stable sharp rm and well correlated polarity change patterns between four samples made it possible to identify the pliocene quaternary magnetic chrons in them these data show that the three crusts have grown continuously at rates of 14 17 mm m y since the early pliocene this is three or four times faster than those for central pacific seamount hydrogenetic crusts estimates of growth rate based on determination of co flux are within this range magnetostratigraphy therefore appears to be a powerful method for dating hydrogenetic manganese crusts when supported by conventional dating methods these results encourage finer scale investigations which promise more detailed paleoceanographic reconstruction

the evolution of dry lands in northern china and in the republic of mongolia since the last glacial maximum abstract the dry lands in china and mongolia include sand seas gravel deserts stable sand dunes desert steppe steppe and scrub woodlands environments distributed in a wide range of geomorphological and tectonic settings from 155 m below sea level to more than 5000 m asl in china the sandy deserts and gravel deserts are located in the arid regions mainly west of the helan mountains 106 e whereas the stable dune regions are distributed in the semi arid areas east of the helan mountains in mongolia deserts cover more than 40 of the country they are mainly distributed in the southern and southwestern part of the country in inter mountain valleys and basins of the altai mountains and in the uvs nuur basin in northwestern mongolia the desert regions enclose mainly gravel desert gobi and comprise some areas with sand fields and dune fields especially in western mongolia relict frost wedges and moraines in the chaidamu and tarim basins of china indicate that the coldest time of the last glaciation occurred at 30 25 ka in these two areas the stratigraphy of the dunes in the badain jaran desert high lake levels in the areas near and within the large deserts in china and in mongolia suggest a much more humid climate in western china and in mongolia during marine isotope stage three however the period after 24 ka to the end of the pleistocene was mostly characterised by increased temperature and reduced precipitation in the chaidamu and tarim basins paleoenvironmental conditions in the eastern part of the chinese desert belt were marked by an extensive widening of dune formation at 21 13 ka reflecting a weaker intensity of east asian summer monsoon during the holocene higher lake levels stratigraphy of dunes and results of palynological isotopical and sedimentological analyses of lacustrine sediments in the western china and mongolia suggest a few wetter periods especially in the earlier part of the holocene the holocene climatic conditions in the eastern part of the chinese deserts have enabled stabilisation of dunes and formation of soils the presently active dunes in this region are a result of desertification due to human influences recent investigations in the dry lands of china and mongolia demonstrate broad regional differences that must be given attention for the understanding of palaeoclimates

the timing and environmental significance of late quaternary linear dune development in western zambia optical luminescence dates for 19 samples from the degraded linear dune field of western zambia indicate multiple periods of regional dune building for the late quaternary 32 000 27 000 16 000 13 000 10 000 8000 and 5000 4000 yr ago these dates show that the last glacial maximum was not the only time when dune construction commonly linked to marked aridity occurred in central southern africa during the late quaternary whereas rainfall significantly less than today s ca 1400 mm yr 1 is a prerequisite for dune construction in the area adequate sediment supply also determines dune construction and preservation so that dune building cannot be simply and singularly linked to marked aridity the zambezi river system is proposed as an important source of dune sediments with the nature of linear dune activity explaining why stacked sediments preserve several phases of dune formation chronologies of dune construction in western zimbabwe and the southwest kalahari are in broad agreement with our zambian chronology and support a model of rainfall shifts along a sw ne gradient with some notable disparities these are probably a function of interregional sediment supply differences the number of samples used to delimit constructional periods and the multicausal nature of forcing mechanisms