response characteristics of a short range high resolution digital sonar altimeter abstract the datasonics model asa 920 digital sonar altimeter dsa is a compact high frequency 1 mhz short range 0 5 5 m underwater sonar device originally designed as an altimeter for submersibles the dsa has previously been successfully used on a bottom mounted tripod to measure changes in relative bed elevation at a point on the shoreface prior to and during a storm fixed to a rigid mounting on the seabed the dsa produces a digital output that is proportional to the transducer elevation above the bed the purpose of this report is to describe the response characteristics of the dsa tests using a flat steel plate and a flat sand bed as targets showed that the dsa is capable of resolving differences in elevation of at least 0 5 cm a laboratory calibration demonstrated that the instrument response is linear because the acoustic pulse emitted by the transducer has a finite width the dsa is incapable of delineating undulating sand beds when the bedslope taken over the beamwidth is significant this limits the instrument s use over a bed of wave generated ripples in general the altimeter does not see into the troughs of ripples when its height above the bed is more than eight times the ripple wavelength the instrument stability is good over 12 operating hours 14 days of burst sampling the indicated elevation drifted by 0 3 cm in the field the acoustic pulse is susceptible to interference by suspended particulate matter and bubbles during deployment in a water depth of 8 m on the shoreface two problems occurred pulse scattering that caused a loss of echo detection which was manifested as spikes in the record and premature echoes that obscured the actual bed surface a second altimeter with an operating frequency of 300 khz datasonics model psa 900 also gave a linear response but with a reduced accuracy of approximately 1 cm

east asian monsoon variations during oxygen isotope stage 5 evidence from the northwestern margin of the chinese loess plateau abstract we present the results of high resolution multi proxy climate studies of the s1 palaeosol corresponding to oxygen isotope stage ois 5 from the northwestern margin of the chinese loess plateau area here s1 is much thicker ca 6 8 m than in the central loess plateau areas ca 2 m where most previous studies have been conducted hence much higher resolution stratigraphic studies are possible yielding more insight into the temporal variations of the east asian monsoon during mis 5 the frequency dependent magnetic susceptibility as well as the concentration of secondary carbonate is used as an indicator of the summer monsoon intensity and the median particle size as an indicator of the winter monsoon intensity the results suggest that the northwestern margin of the chinese loess plateau experienced the strongest summer monsoon intensity in sub stage oiss 5e and the weakest in oiss 5a among the three warmer periods during stage 5 the summer monsoon was weaker in oiss 5b than in oiss 5d a dusty interval interrupted the second warmer period 5c and a soil forming event interrupted the first colder period 5d the results also suggest that the directions of changes in the intensities of summer and winter monsoons may not always have been proportionately opposite for example the weakest summer monsoon occurred in oiss 5a during which the winter monsoon was not the strongest we further conclude that the winter monsoon during the last interglacial was probably driven by global ice volume fluctuations while the summer monsoon was primarily controlled by the northern hemisphere solar insolation and was probably modified by a feedback mechanism that is the climatic buffering effect of low latitudinal oceans may have distorted the response of the summer monsoon to insolation variations finally our results do not show the degree of climatic instability comparable to that recorded in the grip ice core for the last interglacial oiss 5e even though the study area is situated in a region which has been sensitive to climatic changes

land surface feedbacks and palaeomonsoons in northern africa we ran a sequence of climate model experiments for 6000 years ago with land surface conditions based on a realistic map of palaeovegetation lakes and wetlands to quantify the effects of land surface feedbacks in the saharan region vegetation induced albedo and moisture flux changes produced year round warming forced the monsoon to 17 25 n two months earlier and shifted the precipitation belt 300 km northwards compared to the effects of orbital forcing alone the addition of lakes and wetlands produced localised changes in evaporation and precipitation but caused no further extension of the monsoon belt diagnostic analyses with biome and continental hydrology models showed that the combined land surface feedbacks although substantial could neither maintain grassland as far north as observed 26 n nor maintain lake megachad 330 000 km

high resolution holocene environmental changes in the thar desert northwestern india sediments from lunkaransar dry lake in northwestern india reveal regional water table and lake level fluctuations over decades to centuries during the holocene that are attributed to changes in the southwestern indian monsoon rains the lake levels were very shallow and fluctuated often in the early holocene and then rose abruptly around 6300 carbon 14 years before the present 14c yr b p the lake completely desiccated around 4800 14c yr b p the end of this 1500 year wet period coincided with a period of intense dune destabilization the major harrapan indus civilization began and flourished in this region 1000 years after desiccation of the lake during arid climate and was not synchronous with the lacustral phase

rotary sidescan images of nearshore bedform evolution during a storm abstract high resolution acoustic images are presented of bedform development on the crest of a nearshore bar during a storm the images were obtained using a 2 25 mhz rotating sidescan sonar the sonar system provides a continuous record of the evolution of the bedform field through time within a 10 m diameter field of view the images document the occurrence of ripples cross ripples and megaripples during the waning stages of the storm transitions among ripple types occurred on time scales of 1 3 h beginning with flat bed and culminating in highly 3 d short crested ripples

tidal asymmetry in sediment resuspension on a macrotidal beach in northwestern australia abstract a field experiment was conducted on a macrotidal beach in northwestern australia to investigate the relation between sediment resuspension and sea bed morphology continuous measurements of waves currents and suspended sediment concentrations were carried out over a period of several hours around high tide concurrent with half hourly visual observations of the sea bed morphology the incident waves had a significant wave height of 0 35 m and period 11 s most of the data were collected outside the surf zone where sediment resuspension was mainly by wave groups and suspended sediment transport was primarily accomplished by mean longshore and cross shore currents the sediment resuspension process was further characterised by a pronounced tidal asymmetry with suspended sediment concentrations and transport rates during the falling tide significantly larger than during the rising tide the asymmetry is attributed to the combined effects of a larger bed roughness and stronger mean nearshore flows during the falling tide the increase in bed roughness occurred during the rising tide under the influence of decreasing bed shear stresses when post vortex ripple morphology developed on a previously plane bed around high tide the ripples were fully developed and had attained a ripple height of approximately 0 005 m and a ripple length of 0 06 0 08 m application of the van rijn van rijn l c 1989 handbook sediment transport by currents and waves delft hydraulics report h461 sediment transport model further confirmed that enhanced sediment resuspension during the falling tide was primarily the result of the development of ripple morphology with increased mean current strengths playing a secondary role the results have major implications for modelling suspended sediment transport on macrotidal beaches

tidal cycle changes in oscillation ripples on the inner part of an estuarine sand flat abstract oscillation ripples form on subaqueous sand beds when wave generated near bottom water motions are strong enough to move sand grains the threshold of grain motion is the lower bound of the regime of oscillation ripples and the onset of sheet flow is the upper bound based on the relation between ripple spacing and orbital diameter three types of symmetrical ripples occur within the ripple regime in the lower part of the ripple regime orbital ripples spacing is proportional to orbital diameter in the upper part anorbital ripples spacing is independent of orbital diameter between these regions occurs a transitional region suborbital ripples oscillation ripples develop on a sandy tidal flat in willapa bay washington as a result of waves traversing the area when it is submerged because wave energy is usually low within the bay the ripples are primarily orbital in type this means that their spacing should respond in a systematic way to changes in wave conditions during the high water parts of some tidal cycles ripples near the beach decrease in spacing during the latter stage of the ebb tide while ripples farther offshore do not change observations made over several tidal cycles show that the zone of active ripples shifts on or offshore in response to different wave conditions detailed bed profiles and current measurements taken during the high water part of spring tides show the manner in which the oscillation ripples change with changes in orbital diameter changes in ripple spacing at the study site could be correlated with changes in orbital diameter in the manner suggested by the criterion for orbital ripples however there appeared to be a lag time between a decrease in orbital diameter and the corresponding decrease in ripple spacing absence of change during a tidal cycle could be attributed to orbital velocities below the threshold for grain motion that negated the effects of changes in orbital diameter because changes in sand flat ripples depend both upon changes in orbital diameter and upon the magnitude of the orbital velocity exposed ripples were not necessarily produced during the preceding high tide in fact some ripples may have been just produced while others farther offshore may have been produced an unknown number of tides earlier therefore when interpreting past wave conditions over tidal flats from low tide ripples one must remember that wave periods have to be short enough to produce velocities greater than the threshold velocity for the orbital diameters calculated from the observed ripple spacings

environmental change during the holocene in western china on a millennial timescale recent data on the evolution of the inland lakes loess palaeosol sequences and glacier variations in western china are briefly reviewed the lake history shows three distinct stages in the holocene i e low water levels before 7500 bp a stage of expansion during the climatic optimum between 7500 and 3500 bp and contraction after 3500 bp three stages wooded grassland mixed woodland wooded grassland becoming steppe also appear indicated by the holocene loess palaeosol sequence in the lanzhou region the optimal period for soil development being between 7500 and 3500 bp there were seven periods of glacier advance the continental glaciers advanced at about 8300 5700 4000 and 400 bp in northwestern china and the maritime glaciers advanced at about 3000 2000 1000 and 200 bp it is suggested that the climatic changes show two periodicities during the holocene namely at intervals of about 2500 and 1000 years

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

late quaternary monsoon patterns on the loess plateau of china china s loess plateau was formed under special conditions the tectonic movement topographical characteristics and monsoon patterns combined to create a favourable environment for the accumulation of thick loessic deposits the loess plateau itself is part of the monsoon triangle of china a region very susceptible to climatic changes throughout the upper pleistocene the palaeoenvironment on the loess plateau alternated from steppe to deciduous forest and coniferous forest in response to shifts in the atmospheric circulation three monsoon patterns appear to be indicated 1 a full glacial monsoon pattern 18000 15000 yr bp which induced a cold and dry climate favouring loess accumulation in steppe conditions 2 an interglacial monsoon pattern last interglacial and holocene in which a warm humid climate prevailed with deciduous forests leaving palaeosols interbedded within the loess sequence and 3 a transitional or interstadial monsoon pattern 50 000 23 000 yr bp in which the climate was cold and humid in the loess plateau encouraging the development of coniferous forest

lacustrine and eolian records of holocene climate changes in the mongolian plateau preliminary results abstract this study compares two pairs of adjacent lacustrine and eolian sections at sites in the southern and northern mongolian plateaus in order to test spatial climate variability during the holocene based on the lithology proxy data and 14 c dated and the interpolated ages the following observations can be made in the northern mongolian plateau a best developed holocene paleosol dated at 8672 14 c yr bp at the shaamar section and the carbonate rich laminated layer in the gun nuur lake core mark the interval of warmer and dryer climate during the early holocene younger paleosols at the shaamar section and corresponding organic rich layers in the gun nuur core were formed under distinctly cooler and more humid conditions the baahar nuur lake core in the southern mongolian plateau and the dingxi type section in the northern part of the western chinese loess plateau appear to indicate that a prolonged interval of maximum humidity prevailed in this region during the early and mid holocene 9000 4000 14 c yr bp by contrast in the northern mongolian plateau the most humid conditions seem to have occurred from 4500 to 2500 possibly to 1650 14 c yr bp this discrepancy implies that the concept of the holocene climatic optimum has limitations and may have to be reconsidered if it is intended to have a large scale connotation

wave formed structures and paleoenvironmental reconstruction clifton h e and dingier j r 1984 wave formed structures and paleoenvironmental reconstruction in b greenwood and r a davis jr editors hydrodynamics and sedimentation in wave dominated coastal environments mar geol 60 165 198 wave formed sedimentary structures can be powerful interpretive tools because they reflect not only the velocity and direction of the oscillatory currents but also the length of the horizontal component of orbital motion and the presence of velocity asymmetry within the flow several of these aspects can be related through standard wave theories to combinations of wave dimensions and water depth that have definable natural limits for a particular grain size threshold of particle movement and that of conversion from a rippled to fiat bed indicate flow velocity limits the ratio of ripple spacing to grain size provides an estimate of the length of the near bottom orbital motion the degree of velocity asymmetry is related to the asymmetry of the bedforms though it presently cannot be estimated with confidence a plot of water depth versus wave height h h diagram provides a convenient approach for showing the combination of wave parameters and water depths capable of generating any particular structure in sand of a given grain size natural limits on wave height and inferences or assumptions regarding either water depth or wave period based on geologic evidence allow refinement of the paleoenvironmental reconstruction the assumptions and the degree of approximation involved in the different techniques impose significant constraints inferences based on wave formed structures are most reliable when they are drawn in the context of other evidence such as the association of sedimentary features or progradational sequences

the history of south american tropical precipitation for the past 25 000 years long sediment cores recovered from the deep portions of lake titicaca are used to reconstruct the precipitation history of tropical south america for the past 25 000 years lake titicaca was a deep fresh and continuously overflowing lake during the last glacial stage from before 25 000 to 15 000 calibrated years before the present cal yr b p signifying that during the last glacial maximum lgm the altiplano of bolivia and peru and much of the amazon basin were wetter than today the lgm in this part of the andes is dated at 21 000 cal yr b p approximately coincident with the global lgm maximum aridity and lowest lake level occurred in the early and middle holocene 8000 to 5500 cal yr b p during a time of low summer insolation today rising levels of lake titicaca and wet conditions in amazonia are correlated with anomalously cold sea surface temperatures in the northern equatorial atlantic likewise during the deglacial and holocene periods there were several millennial scale wet phases on the altiplano and in amazonia that coincided with anomalously cold periods in the equatorial and high latitude north atlantic such as the younger dryas

modern pollen based interpretations of mid holocene palaeoclimate 8500 to 3000 cal bp at the southern margin of the tengger desert northwestern china the focus of this paper is on the reconstruction of the mid holocene climatic changes based on the pollen records contained in a 6 2 m thick section in the southern part of the tengger desert northwestern china to understand the modem analogues of the mid holocene changes modem pollen spectra across different vegetation zones from the qilian mountains to the lowland tengger desert were studied first four pollen combinations were identified corresponding to four different modem ecological environments 1 a picea artemisia combination in a spruce forest and its vicinity 2 an artemisia chenopodiaceae gramineae salix combination in a mixture belt of montane grassland and poplar woodland 3 a chenopodiaceae artemisia nitraria combination in a desert steppe belt and 4 an elaeagnus artemisia chenopodiaceae nitraria combination in a desert riparian forest our study also shows that a relatively low percentage of picea pollen 30 10 seems to be associated with water transportation alluvial depos

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

late quaternary environmental change of yellow river basin an organic geochemical record in bohai sea north china abstract bulk geochemical characterization total organic carbon grain size distribution carbon isotope composition and molecular biomarkers lignin phenols straight chain aliphatic hydrocarbons glycerol dialkyl glycerol tetraethers were analyzed for a 21 m core from the bohai sea north china spanning ca 21 ka bp these paleo proxies presented remarkable differences between the late glacial period and the holocene reflecting continental and coastal environments respectively two peat layers were deposited during the period of ca 9000 8460 yr bp thereafter the core site has been consistently covered by seawater until recent reclamation of land from the sea the occurrence of a total organic carbon maximum from ca 6000 3800 yr bp was attributed to delivery of organic carbon enriched sediments via the yellow river consistent with increased vegetation density and higher development of soil under warm and humid mid holocene climate conditions the distributions of lignin phenol compositions and c 31 c 29 n alkane ratio suggested the largest expansion of woody plants between ca 5300 and 4000 yr bp corresponding to the extremely favorable climatic conditions since ca 3800 yr bp an abrupt increase in the c 31 c 29 n alkane ratio suggested higher abundance of grasses consistent with a drying climate trend after the mid holocene since our coastal sediments close to the yellow river outflow contain catchment integrated environmental signals of the river basin molecular proxies demonstrate that the variability of vegetation distributions in the holocene is a widespread phenomenon in those areas adjacent to yellow river basin

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

acoustic imaging of sea bed geometry a high resolution remote tracking sonar hrrts ii abstract a high resolution remote tracking sonar hrrts ii is described which is capable of measuring a 2 d profile of the sea bed over a maximum distance of 5 m under conditions of significant sediment transport the backscattered signal from a pulsed 30 s 5 mhz sonar provides acoustic range measurements of the small scale topography generated by bedforms at any point in time as well as changes in this small scale topography and the average elevation of sections of the sea bed over time bedform height and spacing can be determined accurately although bedform shape is distorted by smoothing of large slope changes as a result of the finite size of the footprint of this narrow beamed 0 74 sonar the system operates on command from a shore based micro computer and topographic resolution of the order of a few millimetres is possible resolution deteriorates as wave energy increases to levels where vibration of the transducer and large concentrations of sediment in the water column result in signal attenuation and a decrease in the signal to noise ratio

response of the african monsoon to orbital forcing and ocean feedbacks in the middle holocene simulations with a climate model that asynchronously couples the atmosphere and the ocean showed that the increased amplitude of the seasonal cycle of insolation in the northern hemisphere 6000 years ago could have increased tropical atlantic sea surface temperatures in late summer the simulated increase in sea surface temperature and associated changes in atmospheric circulation enhanced the summer monsoon precipitation of northern africa by more than 25 percent compared with the middle holocene simulation with prescribed modern sea surface temperatures and provided better agreement with paleorecords of enhanced monsoons

nonstationary phase of the plio pleistocene asian monsoon paleoclimate records indicate that the strength of the asian summer monsoon is sensitive to orbital forcing at the obliquity and precession periods 41 000 and 23 000 years respectively and the extent of northern hemisphere glaciation over the past 2 6 million years the timing phase of strong monsoons has changed by 83 degrees in the precession and 124 degrees in the obliquity bands relative to the phase of maximum global ice volume inferred from the marine oxygen isotope record these results suggest that one or both of these systems is nonstationary relative to orbital forcing

teleconnection of climatic events between east asia and polar high latitude areas during the last deglaciation abstract from observations of continuous aeolian and swamp sediment sequences augmented by a detailed 14 c chronology 13 c and organic c analyses we can demonstrate century to millennium scale fluctuations in east asian monsoon palaeoclimatic events we also infer significant precipitation variability within the last deglaciation the major climatic zones that have been recognised in europe and find counterparts in east asia are the bolling allerod 14 750 12 800 cal yr b p and the younger dryas 12 800 11 600 cal yr b p the last deglaciation sediment sequences are characterised by frequent facies changes reflecting climate instability these frequent abrupt climatic events correlate well with fluctuations recorded in high latitude and polar areas as represented by the greenland ice core gisp 2 and a core from the north sea this indicates a palaeoclimate teleconnection between the high latitude and polar areas and the east asian monsoon areas through cold air mass activity and the related atmospheric pressure system

platinum group element geochemistry of continental tholeiites analysis of the long range dyke swarm newfoundland canada abstract the rift related late proterozoic long range continental dykes of western newfoundland have platinum group element pge pd pt rh ru and ir determined concentrations that correlate with indicators of magma evolution e g mg mg mg 0 9 fe tot and magma alkalinity la sm suggesting that metamorphism had a minimal effect on the pge the low melting point lmp pge pd pt and rh and cu behaved incompatibly as magma evolved indicating that sulphide fractionation did not occur and that the dykes hold little economic promise hypotheses that augite a major fractionating phase has an affinity for the pge are not supported primitive rocks tend to have lower ir than evolved rocks and all samples display a negative high melting point hmp pge ru and ir anomaly on mantle normalized diagrams olivine and early chromite fractionation may have decreased the concentrations of these elements the long range dykes confirm that pd ir ratios and absolute lmp pge concentrations tend to decrease with increasing hot spot magma alkalinity the lower pge concentrations in most morb compared with hot spot basalts imply that there are at least two mantle reservoirs for the pge lithosphere overlying mantle sources continental or oceanic has little effect on pge concentrations the apparently similar source region pge concentrations for hot spot basalts suggest that differences in pge concentrations between morb and hot spot basalts reflect incomplete segregation of the pge into the core during its formation and rise of the homogeneously dispersed elements to upper mantle depths in deep mantle plumes

holocene climatic and environmental changes in the arid and semi arid areas of china a review this paper reviews recently published literature most of which was published in chinese and searches for regional patterns of holocene changes useful in depicting global patterns the holocene in the xinjiang region can be divided into three stages a warming and dry early stage from 11 similar to 10 to 8 similar to 7 ka bp a warm and wet middle stage from 8 similar to 7 to 4 5 similar to 3 ka bp and a fluctuating cool and dry late stage since 4 5 similar to 3 ka bp the holocene in the northern tibetan plateau can also be divided into three stages a warming and wet stage from 10 5 similar to 10 to 5 similar to 4 ka bp followed by a variable drying and probably warm stage 5 similar to 4 to 3 ka bp and ending with a cool and dry stage since 3 ka bp in the inner mongolian plateau the early holocene from 10 5 similar to 9 5 to 8 similar to 7 5 ka bp was warming and dry and a warm and wet climate occurred from 7 5 to 3 5 during which the best time was 6 3 3 8 ka bp the climate has been variably drying and probably cooling since 3 5 ka bp in the northwestern part of the loess plateau several holocene palaeosols have been identified 10 9 7 5 5 4 3 and 2 7 2 ka bp with the 7 5 5 ka bp palaeosol being most strongly expressed the best developed palaeosol equivalent in major valleys is a swamp wetland facies deposited between 8885 and 3805 c 14 yr bp under an extremely wet regime the climate has fluctuated significantly at least three times around a dry and probably cool regime after the swamp wetland facies depositional period our summary shows that the holocene climatic optimum occurred nearly contemporaneously 8 5 ka bp at all sites in the xinjiang region in the inner mongolian plateau and in the northwestern part of the loess plateau a warming and wet early holocene 10 8 ka bp in the northern tibetan plateau is most likely related to high effective soil moisture resulting from snow and ice melting we propose here that the middle holocene climatic optimum 8 5 ka bp in arid to semi arid china was primarily a delayed response of the low latitude oceans to high latitude peak insolation 9 8 ka bp

loess particle size data indicative of stable winter monsoons during the last interglacial in the western part of the chinese loess plateau abstract particle size data from the western part of the chinese loess plateau do not support the last interglacial correlation suggested by the data from the central part of the loess plateau between the high dust influx events in the loess plateau and the cool events in the high northern latitudes the lack of marked oscillations in the particle size of the last interglacial paleosol s1 in the western part of the plateau is interpreted as an indication of stable winter monsoons it is thus inferred that the teleconnection between the northern high latitudes and the chinese loess plateau evident in the last glacial might not have existed during the last interglacial it is argued that the western part of the plateau is a better place to establish synchroneity between parent material eolian deposition and s1 paleosol formation 6 8 m thick under such cooler and drier conditions however to assume synchroneity between parent material eolian deposition and s1 paleosol formation only 2 5 m thick under the warmer and moister interglacial conditions in the central part of the plateau is a questionable practice thus the high resolution correlation with the data from the north atlantic area remains problematic

el nino like pattern in ice age tropical pacific sea surface temperature sea surface temperatures ssts in the cold tongue of the eastern equatorial pacific exert powerful controls on global atmospheric circulation patterns we examined climate variability in this region from the last glacial maximum lgm to the present using a sst record reconstructed from magnesium calcium ratios in foraminifera from sea floor sediments near the galapagos islands cold tongue sst varied coherently with precession induced changes in seasonality during the past 30 000 years observed lgm cooling of just 1 2 c implies a relaxation of tropical temperature gradients weakened hadley and walker circulation southward shift of the intertropical convergence zone and a persistent el nino like pattern in the tropical pacific this is contrasted with mid holocene cooling suggestive of a la nina like pattern with enhanced sst gradients and strengthened trade winds our results support a potent role for altered tropical pacific sst gradients in global climate variations

magnetic properties of modern soils and quaternary loessic paleosols paleoclimatic implications the magnetic properties of paleosols developed in quaternary sequences of loess have been used for stratigraphic definition correlation with other terrestrial and deep sea sequences and paleoclimatic paleorainfall reconstruction in some loess paleosol sequences including those of the chinese loess plateau tajikistan and the czech republic maxima in magnetic susceptibility values correspond with the paleosol horizons and minima with the least weathered loess layers in other loess soil sequences including those of siberia alaska and argentina the relationship is completely opposite with susceptibility minima associated with the most developed paleosols to account for these opposite relationships the respective roles of 1 magnetic enhancement and 2 magnetic depletion and or dilution in determining soil magnetic properties are investigated for a range of modern soil types most magnetic enhancement is seen in the upper horizons of well drained cambisols absence or loss of magnetic iron oxides is apparent in acid podsol profiles and waterlogged soils for the cambisol profiles significant correlation is found between susceptibility and organic carbon cation exchange capacity and clay content the mineralogy morphology and grain size of soil magnetic carriers extracted from three modern enhanced soils and a paleosol from the chinese loess plateau are also identified by independent petrographic means microscopy and x ray diffraction of magnetic extracts magnetite and maghemite of ultrafine grain size from 0 4 to 0 001 i m are the major contributors to the magnetically enhanced soils weathering can concentrate detrital magnetic grains especially in the fine silt size fractions of soils the magnetic data from the modern soils indicate that interpretation of paleosol magnetic properties must be done on a site specific basis taking into account the possibilities of pedogenic enhancement pedogenic dilution or depletion and allochthonous inputs of magnetic minerals excessively arid wet or acid soils are unable to form significant amounts of pedogenic ferrimagnets well drained intermittently wet dry soils with reasonable buffering capacity and a substrate source of fe show most magnetic enhancement for those soils which favour magnetic enhancement processes correlation has been found between the maximum value of the pedogenic susceptibility and the annual rainfall the almost unique pedogenic system in the chinese loess plateau where variation in soil forming factors other than climate is reduced to a global minimum allows use of the paleo susceptibility values as proxy paleorainfall values to identify the mechanism of any link between magnetic properties for example susceptibility and climate change the mineralogy and grain size of soil magnetic carriers requires detailed investigation given the prospect of 1 quantitative paleorainfall records from continental loess records 2 predictions of present and future climate change and 3 the value of paleoclimate data for testing of numerical climate models such magnetic investigations can contribute significantly to our understanding of past and future environmental change

holocene vegetation variations and the associated environmental changes in the western part of the chinese loess plateau abstract the western part of the chinese loess plateau has experienced a series of environmental changes during the holocene a desert steppe of late glacial was succeeded by a forest steppe from 8850 to 7540 14 c years bp and a pinus dominated forest occupied the landscape from 7540 to 6560 14 c years bp a deciduous forest of high density and diversity existed from 6560 to 5790 14 c years bp afterwards the vegetation changed to a pinus dominated forest steppe 5790 4950 14 c years bp and then to an ulmus dominated forest steppe 4950 to 4000 14 c years bp the vegetation subsequently changed to a steppe from 4000 to 3120 14 c years bp and further to a desert steppe from 3120 to 2900 14 c years bp after a period of vegetation improvement steppe from 2900 to 2460 14 c years bp a desert steppe resumed 2460 2020 14 c years bp steppe vegetation was re established around 2020 14 c years bp and seems to have then deteriorated again around 1000 14 c years bp the general trend of aforementioned climatic changes is proposed here to have been modulated by the insolation changes specifically the insolation started to increase around 15 000 years bp and peaked around 9000 years bp when the obliquity driven seasonality reached a maximum with the summer insolation being about 8 more than the present as a result tropical holocene sst increased steadily from 10 000 to 6 000 years bp thus effectively strengthening the east asian summer monsoon in addition the documented high temperatures from 10 000 to 4000 years bp in high northern latitudes might have weakened the strength of the winter monsoon and thus enhanced the strength of the summer monsoon probably via lengthening of the rainy season directly contributing to the megahumid climate between 10 000 and 4000 years bp in the western part of the chinese loess plateau

holocene moisture evolution across the mongolian plateau and its surrounding areas a synthesis of climatic records abstract based on the review of 26 high standard holocene climatic reconstructions mainly pollen based from the mongolian plateau and its surrounding areas temporal and spatial patterns of the holocene moisture evolution are synthesized the regionally averaged moisture history from the summer monsoon influenced semiarid belt in china i e region a demonstrates that the moisture index curve is broadly in agreement with the synthesized east asian monsoon strength curve both following the general trend of the west tropical pacific sst that is in turn the delayed response to the northern hemispheric summer solar insolation the regionally averaged moisture indices from the winter monsoon dominated southern siberia including lake baikal area and the altai mountains i e region b exhibit a general declining trends since 10 6 9 6 cal kyr bp being largely consistent with the trends of the annual precipitation and the warm season temperature in the russian plain the consistency might be attributable to the holocene declining trend of the warm season temperature in north atlantic region the predominant feature of the regionally averaged moisture index from the westerlies affected northern xinjiang i e region c is a persistent increasing trend since 8 cal kyr bp the wetting trend of northern xinjiang during the past 8000 years might be attributable to the increasing trend of winter insolation and to the associated increasing trend of cold season temperature in northwestern europe the chronological correspondences between dry phases and warm intervals in the arid areas of the mongolian plateau i e northern mongolian plateau within mongolia and southern mongolian plateau within china region d lend a support to the proposal that the mid holocene dry phase was most likely the result of mid holocene high warm season temperature

performance of a sonar altimeter in the nearshore abstract a 1 mhz sonar altimeter with automatic gain control is shown to provide accurate estimates of the distance between the instrument and the seafloor laboratory experiments indicate that distance estimates degrade slightly when the bottom is rough or sloped and when sediment is suspended in the water column results from field tests both within and seaward of the surf zone show some degradation owing to a combination of suspended sediment and bubbles bed undulations and perhaps the dynamic nature of the sand bottom under waves seaward of the surf zone the bottom can be located within 2 cm nearly continuously whereas inside the surf zone the bottom can be located only intermittently and the accuracy decreases to 3 cm a 300 m long cross shore transect of 16 altimeters was deployed from the shoreline to about 4 m depth for 3 months in summer fall 1994 near duck nc results show that the altimeters are robust and can usually provide estimates of the seafloor position every few minutes even in the surf zone during large storms

acoustic measurements of suspended sand on the shoreface and the control of concentration by bed roughness acoustic backscatter measurements of the concentrations of sand in suspension on the shoreface seawards of the breaking zone during a mild storm event show that sand concentrations increase initially but then rapidly decrease as the wave energy increases it is suggested that the bed roughness is a major control on suspended sand concentration and that the decrease in concentration is due to decreasing ripple steepness after the break off bed shear stress is exceeded no direct measurements of bedforms were available and the combined wave current interaction model of grant and madsen j geophys res vols 84 1979 and 87 1982 with extensions to movable bed roughness ripple dimensions from carstens et al s 1969 laboratory data u s army corps eng cerc tech mem 28 and to sediment induced self stratification glenn and grant j geophys res vol 92 1983 was used to predict the current and suspended sand concentration profile matching the measured and modeled concentrations at 2 cm above the bed the values of the resuspension coefficient 0 were found to decrease as excess skin friction increased in a manner similar to that suggested by drake and cacchione cont shelf res vol 9 1989 but were an order of magnitude larger using the much smaller ripple dimensions of nielsen j geophys res vol 86 1981 changed the model results very little significant differences were found between the time averaged suspended sand profiles and those predicted by the model sand transport fluxes have also been computed using the instantaneous products of measured suspended concentrations and currents the current being the sum of steady and wave induced components the wave induced fluxes show considerable variability from run to run but the general pattern is of a transport profile having a shoreward maximum at 5 10 cm above the bed and offshore transport in the few centimetres closest to the bed little net transport occurred above 15 cm these fluxes show a weak dependence on the wave energy becoming more shoreware and the height of the maximum shoreward transport decreasing as the wave energy increased again consistent with the influence of the ripple steepness

synchronous tropical south china sea sst change and greenland warming during deglaciation the tropical ocean plays a major role in global climate it is therefore crucial to establish the precise phase between tropical and high latitude climate variability during past abrupt climate events in order to gain insight into the mechanisms of global climate change here we present alkenone sea surface temperature sst records from the tropical south china sea that show an abrupt temperature increase of at least 1 c at the end of the last glacial period within the recognized dating uncertainties this sst increase is synchronous with the bolling warming observed at 14 6 thousand years ago in the greenland ice sheet project 2 ice core

holocene treeline history and climate change across northern eurasia radiocarbon dated macrofossils are used to document holocene treeline history across northern russia including siberia boreal forest development in this region commenced by 10 000 yr b p over most of russia forest advanced to or near the current arctic coastline between 9000 and 7000 yr b p and retreated to its present position by between 4000 and 3000 yr b p forest establishment and retreat was roughly synchronous across most of northern russia treeline advance on the kola peninsula however appears to have occurred later than in other regions during the period of maximum forest extension the mean july temperatures along the northern coastline of russia may have been 2 5 to 7 0 c warmer than modern the development of forest and expansion of treeline likely reflects a number of complimentary environmental conditions including heightened summer insolation the demise of eurasian ice sheets reduced sea ice cover greater continentality with eustatically lower sea level and extreme arctic penetration of warm north atlantic waters the late holocene retreat of eurasian treeline coincides with declining summer insolation cooling arctic waters and neoglaciation