quantification of magnetic coercivity components by the analysis of acquisition curves of isothermal remanent magnetisation abstract a new method of analysing isothermal remanent magnetisation irm acquisition curves based on cumulative log gaussian analysis robertson and france phys earth planet inter 82 1994 223 234 is proposed it is based on the curve fitting of the irm acquisition curve versus the logarithm of the applied field with i the acquisition curve on a linear scale ii the acquisition curve expressed as a gradient and iii the acquisition curve on a probability scale even when a sample is not saturated its magnetic properties can be defined although with less certainty the number of magnetic components required for an optimal fit to a measured irm acquisition curve is evaluated statistically the method discriminates on the basis of different mineral coercivity therefore additional rock magnetic tests are still required to separate minerals with similar coercivities

grain size of loess palaeosol and red clay deposits on the chinese loess plateau significance for understanding pedogenic alteration and palaeomonsoon evolution abstract to assess pedogenic modification to grain size distributions of loess palaeosol and red clay deposits on the chinese loess plateau and to understand long term evolution of the east asian palaeomonsoon since the late miocene we investigate a continuous loess palaeosol red clay sequence at lingtai south chinese loess plateau and another parallel red clay sequence at zhaojiachuan central chinese loess plateau by analyzing the grain size distributions of bulk samples and chemically isolated quartz samples the relative intensity of pedogenic alteration of the loess palaeosol and red clay deposits can be quantified comparisons of the grain size distributions and contents of different grain size fractions between the bulk and the quartz samples suggest that pedogenic alteration is apparently stronger in the red clay sequence than in the overlying loess palaeosol deposits furthermore grain size parameters derived from the bulk and the quartz samples such as mean grain size coarse fraction content and u ratio exhibit similar variations in the loess palaeosol sequence quaternary 0 2 6 ma whereas these grain size parameters of quartz samples show different variability from those of bulk samples in the red clay sequence late miocene to pliocene 2 6 7 ma grain size of quartz from the loess palaeosol and red clay deposits exhibits distinct and persistent oscillations from the late miocene to the pleistocene implying that significant fluctuations of the palaeomonsoon climate in east asia might have occurred at least since late miocene time

high resolution analysis of early diagenetic effects on magnetic minerals in post middle holocene continental shelf sediments from the korea strait 1 two studied sediment cores from the korea strait contain mud sequences 14 m and 32 62 m in thickness that were deposited during the last 6 000 years the sediments have uniform lithology and geochemical properties however marked down core changes in magnetic properties suggest that diagenesis has significantly impacted the magnetic properties an expanded view of early diagenetic reactions that affect magnetic mineral assemblages is evident in these rapidly deposited continental shelf sediments compared to deep sea sediments the studied sediments are divided into four descending intervals based on magnetic property variations interval 1 is least affected by diagenesis and has the highest concentrations of detrital magnetite and hematite and the lowest solid phase sulfur contents interval 2 is characterized by the presence of paramagnetic pyrite and sharply decreasing magnetite and hematite concentrations which suggest active reductive dissolution of detrital magnetic minerals interval 3 is marked by a progressive loss of hematite with depth and coincides with the minimum magnetite concentration interval 4 has an increasing down core enhancement of authigenic greigite which apparently formed at depths of 3 30 m below the sediment water interface due to arrested pyritization reactions these results indicate delays of thousands of years for acquisition of magnetizations carried by greigite which suggests that studies of geomagnetic field behavior from greigite bearing continental shelf sediments should be conducted with care also virtually complete destruction of detrital magnetic minerals at depth suggests that magnetic studies of rapidly deposited shelf sediments are unlikely to provide a meaningful signature associated with syn depositional environmental processes

permanent el nino like conditions during the pliocene warm period during the warm early pliocene 4 5 to 3 0 million years ago the most recent interval with a climate warmer than today the eastern pacific thermocline was deep and the average west to east sea surface temperature difference across the equatorial pacific was only 1 5 0 9 c much like it is during a modern el nino event thus the modern strong sea surface temperature gradient across the equatorial pacific is not a stable and permanent feature sustained el nino like conditions including relatively weak zonal atmospheric walker circulation could be a consequence of and play an important role in determining global warmth

an integrated interpretation for the origin of the patrick draw oil field sage anomaly 1 the patrick draw oil field has been the site of research into surface and remote sensing methods for hydrocarbon exploration for nearly 10 yr the oil field is a stratigraphic trap with conspicuous sage vegetation on the surface and an associated tonal anomaly within that sage visible via landsat overlying the field s gas cap this paper attempts to resolve the controversy of whether the patrick draw sage anomaly is related to oil field factors or to other independent factors geology and production history of the field show the sage die out anomaly results from upward migration of injected gases and waters used to maintain reservoir pressures in the field these gases and waters produced anoxic low eh oxidation potential high ph and high salinity soils that are toxic to the overlying sage population stunted sage associated with the anomaly commonly occurs in marginal terrains in the western united states and is not tied uniquely to oil fields a velocity for the seeping gases of 250 1000 ft yr 76 305 m yr was calculated from the reservoir to the surface failure to account for the lag time associated with movement of seepage to the surface has been a source of misinterpretation

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

monsoon influence on planktic δ18o records from the south china sea abstract while the benthic 18 o records from many south china sea sites mimic the specmap lr04 standard their paired planktic 18 o curves differ by comparable values at negative peaks corresponding to interstadials especially between mis 5 1 5 3 5 5 6 5 and 7 3 similar planktic 18 o records also occur in some other low latitude oceans under monsoon influence because neither temperature nor salinity effects can fully account for the variations of such 18 o records after derivation from the specmap lr04 standard variations in the rain water 18 o affected by regional hydroclimate changes are considered to have played an important role in contrast to the specmap lr04 standard 18 o curve prevailed by 100 kyr cycles these regional planktic 18 o curves are distinguished by 20 kyr precession signal showing partial similarity to the 18 o records of atmospheric oxygen in polar ice core bubbles and in stalagmite calcite we speculate that the common features of these three independent 18 o records are indicative of the effect of evolving hydrological cycling driven by monsoon circulation in low latitudes

catastrophic drought in east asian monsoon region during heinrich event 1 abstract heinrich event 1 h1 is an important millennial climate event during the last deglaciation the substantial decreasing of monsoon strength in the east asian monsoon region during the h1 as shown by stalagmite 18o records has been attributed to the southward shift of the intertropical convergence zone itcz which is caused by the slowdown collapse of the atlantic meridional overturning circulation amoc however records from different asian monsoon regions show various trends in precipitation changes during the h1 and these trends cannot be solely interpreted by the southward shift of the itcz in the present study we reconstructed time series of east asian monsoon precipitation between 25 000 and 10 000 a bp from floodplain sediments in the huai river basin a white sediment layer distinct from other layers in the profile contains significantly low toc tree pollen and fern spore contents and more positive 13corg and it is deposited during the h1 event the determined toc pollen and 13corg time series together with previously reported stalagmite 18o indicate a catastrophic severe drought in jianghuai region one of the east asian monsoon regions during the h1 the la nina condition in tropical pacific likely also contributes to the catastrophic drought in jianghuai region and the precipitation variations in the asian monsoon region during the h1

seven million years of glaciation in greenland glacial till glaciomarine diamictites and ice rafted detritus found in marine cores collected off the shore of southeast greenland record multiple late cenozoic glaciations beginning in the late miocene distinct rock assemblages and seismic stratigraphic control correlate the diamictites with glaciation of the southeast greenland margin glaciers advanced to the sea during several intervals in the pliocene and pleistocene north atlantic glaciation may have nucleated in southern greenland rather than further north because of the high mountains and the high levels of precipitation in this region

towards an orbital time scale for chinese loess deposits abstract in this study we analyzed the grain size of the baoji loess soil sequence at 10 cm intervals results show that grain size variations are very sensitive to loess soil alternations with loess units being much coarser than soils such a change in loess soil grains is interpreted as the result of the glacial interglacial cyclic variations in the intensity of the winter monsoon winds out of siberia the grain size record is thus employed as a proxy indicator of the winter monsoon circulation and tuned to the orbital records calculated recently by berger and loutre 1991 under the control of major magnetic reversals the tuning is independent of any correlation with 18o signals in the deep sea sediments the resulting grain size time scale is tightly constrained as suggested by the following facts 1 the filtered obliquity and precession components from the grain size data on the orbital time scale closely match the theoretical orbital records 2 ages of the major magnetic reversals estimated from the grain size time scale are in good agreement with the k ar dated ages 3 there is close coherence between the baoji grain size time series and the orbital variations at the orbital frequency bands over the entire 0 2 5 ma period and 4 the grain size record on the orbital time scale shows a close similarity to the orbitally tuned dsdp site 607 and odp site 677 18o records examination of time dependent characteristics of the grain size record suggests that there are two major shifts of dominant periodicities in the long term monsoonal cycles one occurring at about 0 8 ma bp and the other around 1 7 ma bp

an extensive middle to late pleistocene tephrochronologic record from east central alaska abstract chester bluff cb in yukon charley rivers national preserve ycnp alaska contains an extensive middle to late pleistocene tephra record within thick loess deposited on a terrace the regionally prominent old crow oc tephra 140 10 ka gi tephra 560 80 ka of fairbanks the preido hill prh tephra of the klondike and vt tephra 77 8 4 1 ka of both the klondike and fairbanks were found at cb the presence of these tephra beds will aid in correlating disparate paleoenvironmental records across eastern beringia there are 15 new tephra beds 11 from the wrangell volcanic field wvf and or hayes volcano one from the aleutian arc alaska peninsula aaap and three unclassified beds cb is normally magnetized indicating the entire sequence is of bruhnes age

late cenozoic eolian deposition in the north pacific asian drying tibetan uplift and cooling of the northern hemisphere a newly constructed record of eolian dust accumulation from the central north pacific shows that dust deposition increased by an order of magnitude quite rapidly at 3 6 ma we associate this sudden drying with the uplift of at least the northern portion of the tibetan plateau shutting off the indian ocean moisture source to central and western china this ten fold increase in atmospheric dust loading appears to be associated with the 1 m y long shift toward heavy 18o values that occurred at 3 6 2 6 ma the dust grain size record of wind intensity begins its late cenozoic coarsening a million years before the drying event at 4 5 ma the northern hemisphere cooling that results in intensification of the subpolar westerly winds may have as its ultimate cause the drawdown of atmospheric co2 in the latest miocene and the early pliocene closing of the panamanian seaway

grain size magnetic susceptibility and geochemical characteristics of the loess in the chaohu lake basin implications for the origin palaeoclimatic change and provenance abstract rare studies on the aeolian deposit located in north bank of the yantgze river are documented recently it is found in the field investigations and in bore sections that the loess in the chaohu lake basin has the largest thickness of over 40 m in this study the probability cumulative curves frequency distribution the grain size distributions and the discriminant function of the grain size suggest that the loess in the chaohu lake basin is of eolian origin the magnetic susceptibility curves of the loess in the basin coincide perfectly with those of the loess in the northern china and the marine isotope stages mis and show that paleoclimatic cycles and sub cycles were documented since l3 during middle late pleistocene in the basin the ms curve of paleosol s1 paleosol s2 and loess l3 in the basin coincide perfectly with mis5 mis 7 and mis 8 respectively the good correspondence indicates that the loess in the basin has given a sensitive response to the globe paleoclimatic change since l3 on the other hand the climate changes in some stages recorded by the loess has regional characteristics obviously which might be the result of the dual effect of globe climate changes and east asia monsoon climate changes the result of geochemical characteristics suggests that the loess in the basin has undergone moderate to strong chemical weathering most elements are mobilized during chemical weathering na and ca of the loess are markedly lost and the removal of k is also evident and chemical weathering doesn t evidently turn into the si removal stage the chemical weathering of the loess is more intensive than that of the loess deposits in northwestern china and the upper reaches of the yantgze river the intensive chemical weathering has been documented by the loess might be related to strong monsoon climate in chaohu lake basin the provenance of the loess also differs from that in northern china and is discussed firstly with the lithofacies palaeogeography the well developed alluvial lacustrine deposits in the huaihe floodplain seem to be the major source materials of the loess

mid holocene glacier peak and mount st helens we tephra layers detected in lake sediments from southern british columbia using high resolution techniques abstract a glacier peak tephra has been found in the mid holocene sediment records of two subalpine lakes frozen lake in the southern coast mountains and mount barr cirque lake in the north cascade mountains of british columbia canada the age depth relationship for each lake suggests an age of 5000 5080 14 c yr b p 5500 5900 cal yr b p for the eruption which closely approximates the estimated age 5100 5500 14 c yr b p of the dusty creek tephra assemblage found near glacier peak the tephra layer which has not been reported previously from distal sites and was not readily visible in the sediments was located using contiguous sampling magnetic susceptibility measurements wet sieving and light microscopy the composition of the glass in pumice fragments was determined by electron microprobe analysis and used to confirm the probable source of this mid holocene tephra layer using the same methods the a d 1481 1482 mount st helens we tephra layer was identified in sediments from dog lake in southeastern british columbia suggesting the plume drifted further north than previously thought this high resolution method for identifying tephra layers in lake sediments which has worldwide application in tephrachronologic paleoenvironmental studies has furthered our knowledge of the timing and airfall distribution of holocene tephras from two important cascade volcanoes

the history and variability of the east asian paleomonsoon climate abstract changes in the east asian paleomonsoon reflect interactions between the global atmosphere ocean land and ice systems and are also an expression of their combined effect within the boundary conditions imposed by the east asian continent and solar radiation the history of the east asian monsoon is an alternation between dominance by the dry cold winter and warm humid summer monsoons high resolution eolian sequences preserved in the chinese loess plateau reveal that the east asian monsoon may have commenced at least 7 2 ma ago they also provide evidence indicating that the pulsed uplift of the tibetan plateau at about 3 4 and 7 2 ma may have played an important role in inducing climate change the palaeoclimatic records of the last glacial cycle show high frequency variability on time scales of 1000 year or even shorter and instability of the east asian paleomonsoon system the high frequency variability could be due to a non linear response to orbital forcing or a result of the coupling processes between different components of the global system cold air activity in northern high latitudes the trans equatorial air streams from the southern hemisphere and possibly enso may have played an important role in east asian monsoon variability the synchroneity of all the palaeoclimatic events along the polar equator polar pep transect is still an open question correlation of limited palaeoclimatic records for the last 30 kyr obtained from east asia and australia suggests that the trans equatorial air streams driven by the monsoon and trade winds may have had an influence on opposite hemispheres

x ray fluorescence core scanning records of chemical weathering and monsoon evolution over the past 5 myr in the southern south china sea with benthic foraminiferal d 18 o reveal that phases of the east asian summer monsoon abruptly changed by more than 90 at 4 0 ma 2 75 ma 1 0 ma and 0 6 ma relative to global ice volume at the obliquity and the precession bands over the past 5 myr strong 400 kyr and 100 kyr cycles in the k al and ti al records consistently exist over the past 5 myr particularly these cycles are highly coherent with the long and short eccentricity cycles in the truncated insolation at 65 n indicating an eccentricity forcing of the east asian summer monsoon the chemical weathering recorded in the elemental records of odp site 1143 also shows highly coherent relationship with the ocean carbon reservoir at the eccentricity the obliquity and the precession bands over the late pliocene and pleistocene

neogene growth of the sedimentary organic carbon reservoir we develop a recycling model using 13c 12c mass balance for net growth loss of the sedimentary organic carbon corg reservoir and apply it to the neogene bulk marine carbonate 13c record the model allows for variations in photosynthetic fractionation factors carbon cycling rates and the isotopic composition of riverine carbon inputs to the oceans the sign of the net flux term is controlled by the difference between fractional corg burial and fractional corg weathering independent of any variations in carbon cycling rate these terms are in turn estimated from the carbon isotope mass balance of newly deposited and weathered sediments respectively the magnitude of the net flux is sensitive to the global carbon cycling erosion deposition rate which may be estimated by the use of the records of radiogenic isotopic variations nd sr in paleoseawater a key observation and input to the model is that photosynthetic carbon isotope fractionation by both marine algae and terrestrial plants has decreased during the cenozoic incorporating time dependent carbon isotope fractionation into the model shows that the sedimentary corg reservoir has grown throughout most of the neogene even as marine 13c values fell after 14 ma a similar result is obtained if the variation in the marine 13c record is largely caused by changes in the carbon isotopic composition of river fluxes to the oceans rather than changes in the organic inorganic ratio of output to the burial sink the growth of the sedimentary organic carbon reservoir requires that the neogene sedimentary carbon cycle was a net source of o2 and a net sink of co2 to the atmosphere at least until the plio pleistocene as a consequence neogene co2 consumption by silicate weathering cannot be balanced by oxidation of sedimentary corg placing a significant constraint on global carbon balance models a related prediction of our model is that atmospheric o2 levels rose during the neogene

theory and application of the day plot mrs ms versus hcr hc 2 application to data for rocks sediments and soils 1 new theoretical curves relating the hysteresis parameters mrs ms and hcr hc for single domain sd superparamagnetic sp pseudo single domain psd and multidomain md grains and their mixtures are applied to published data for natural materials the day plot of mrs ms versus hcr hc has been used to crudely classify samples into box like sd psd and md or sometimes incorrectly md sp regions with arbitrary boundaries new type curves for md psd sd md and sd sp grains and mixtures permit more subtle and precise modeling the predicted md trend and its junction with the psd trend are observed in two data sets for magnetite spherules from carbonate rocks and for temperature varying hysteresis results spanning the verwey transition the latter data are the basis of a suggested new method for pinpointing the psd md threshold size selected data for pottery clays soils and paleosols generally follow sd md type curves and indicate intermediate size psd magnetite with narrow to broad size distributions a lake sediment section with known grain size progression tracks in the predicted sense along the sd md trend selected data for glaciomarine and pelagic sediments are also generally compatible with sd md trends examples of remagnetized carbonate rocks submarine basaltic glasses and glassy rims of pillow basalts all follow predicted sp sd or sp psd mixing curves with a large range in volume fraction of sp grains 0 75 but a narrow range of sp particle sizes 10 2 nm larger sp grains spanning the range to sd size 25 30 nm are absent for unknown reasons oceanic dolerites gabbros and serpentinized peridotites in some cases fall in a novel region of the day plot parallel to but below magnetite sd md mixing curves

evidence for a new geomagnetic reversal from lava flows in idaho discussion of short polarity reversals in the brunhes and late matuyama polarity chrons k ar ages and paleomagnetic data for basalt samples from a new core hole site e at the idaho national engineering laboratory inel indicate that the age of the reversed polarity event recorded in snake river plain lavas is older than 465 50 ka 1000 years before present reported previously by champion et al 1981 nine basalt flows eight with normal polarity and one with reversed polarity were recognized in the site e core hole the flows above and below the reversed flow have ages of 491 80 ka and 580 93 ka respectively the inclination of the paleomagnetic field direction of the reversed flow at site e agrees with the inclination of reversed flows elsewhere at inel which have an age of 565 14 ka these reversed flows were previously thought to be correlative with the emperor event we suggest that this polarity event is an older event which we name the big lost reversed polarity subchronozone and subchron a review of data documenting short reversal records from volcanic and sedimentary rocks shows that there is evidence for eight polarity subchrons in the brunhes and two besides the jaramillo in the late matuyama these 10 short subchrons begin to indicate the many short events that cox 1968 hypothesized must exist if polarity interval lengths have a poisson distribution these events are true subchrons not excursions and may or may not have low associated paleointensities although low field strengths might explain why the reversal process aborts the mean sustained polarity interval length since late matuyama chron time is 90 000 years the similarity of this number with the 105 year period of the earth s orbital eccentricity suggests anew that linkage between geomagnetic paleoclimatic and possible underlying earth orbital parameters should be evaluated

development of asian monsoon revealed by marked ecological shift during the latest miocene in northern pakistan carbon isotopes from soil carbonate1 4 and soil organic matter5 6 yield palaeoecological information because the carbon in the soil carbonate forms in isotopic equilibrium with local soil co2 refs 1 7 the isotopic composition of which is in turn determined by local plant cover siwalik group sediments in northern pakistan contain a well exposed palaeosol record spanning the past 18 myr here we report on stable carbon isotope results from associated pedogenic carbonate which indicate a dramatic ecological shift from to dominated floodplain biomass beginning 7 4 7 0 myr ago the earlier c3 floodplain biomasses were probably mainly composed of trees and shrubs whereas c4 grasslands dominated in the plio pleistocene oxygen isotopes also exhibit a shift in the latest miocene probably corresponding to a major climate change which may have induced the forest to grassland transition this dramatic ecological shift in the latest miocene may mark the inception or a marked strengthening of the asian monsoon system

magnetostratigraphy and paleoenvironmental events recorded in a late cenozoic sedimentary succession in huaibei plain east china abstract the thick late cenozoic deposits of huaibei plain provide a record of paleoenvironmental changes in the northern and southern transitional zone of eastern china and of the evolution of the asian monsoon system a detailed magnetostratigraphic study of the huainan hn 481 45 m deep drill core from the center of huaibei plain reveals a magnetic polarity sequence from chron c4n 2n to chron c1n spanning the interval from 8 ma to the present it is the first reliable magnetostratigraphic chronology for the late miocene to present for huaibei plain the stratigraphic sequence contains three intervals characterized by major peaks in magnetic susceptibility at depths of 58 8 m 108 0 m and 312 4 m which can be used as isochronous marker beds for regional stratigraphic correlations in huaibei plain major changes in sediment grain size occur at 7 0 and 1 7 ma corresponding to pronounced environmental shifts which we consider reflect the respective strengthening of the south asian summer monsoon and east asian winter monsoon at these times

p t grids for silica undersaturated granulites in the systems mas n 4 and fmas n 3 tools for the derivation of p t paths of metamorphism abstract considering the minerals cordierite cd sapphirine sa hypersthene hy garnet ga spinel sp sillimanite si and corundum co in the system feo mgo al2o3 sio2 fmas the stable invariant points are co ga cd and sa constraints imposed by experimental data for the system mas indicate that under low ph2o conditions the invariant points occur at high temperature 900 c and intermediate pressure 7 10 kbar this temperature is higher than that commonly advocated for granulite facies metamorphism in granulites fe mg exchange geothermometers may yield temperatures of 100 150 c below peak metamorphic conditions and evidence for peak temperatures is best preserved by relict high temperature assemblages and by al rich cores in orthopyroxene application of the fmas grid to some well documented granulite occurrences introduces important constraints on their p t histories rocks of different bulk compositions occurring in close proximity in the field may record distinct segments of their p t paths this applies particularly to rocks with evidence for reaction in the form of coronas symplectites and zoned minerals consideration of curved reaction boundaries and xms isopleths may explain apparently contradictory results for the stability of cordierite obtained from low temperature experiments and thermochemical calculations on the one hand and hightemperature experimental data on the other

magnetostratigraphic age of the xiantai paleolithic site in the nihewan basin and implications for early human colonization of northeast asia timing of the early habitation and stone technologies in the nihewan basin north china has provided insights into our understanding of early human adaptability to high northern latitudes in east asia here we contribute to this topic with detailed magnetostratigraphic investigation coupled with mineral magnetic measurements and palynological analyses on a lacustrine sequence in this basin which contains the xiantai paleolithic site magnetite and hematite were identified as the main carriers for the characteristic remanent magnetizations magnetostratigraphic results show that the xiantai lacustrine sequence recorded the brunhes chron the jaramillo and the olduvai subchrons and successive reverse polarity portions of the intervening matuyama chron stratigraphic correlation in terms of lithology magnetic susceptibility and magnetic polarity sequences between the xiantai and xiaochangliang sections indicates that the xiantai artefact layer is readily contemporary with the xiaochangliang artefact layer which has been previously estimated to be about 1 36 ma r x zhu k a hoffman r potts c l deng y x pan b guo c d shi z t guo b y yuan y m hou w w huang earliest presence of humans in northeast asia nature 413 2001 413 417 early humans of the xiantai paleolithic site lived in a steppe paleoenvironment indicated from fossil pollens furthermore the combined evidence of our magnetostratigraphy and previously published magnetochronology and paleoclimate data documents that early humans of north china were able to adjust to an increasing variability of paleoclimates and paleoenvironments over the early pleistocene

old crow tephra across eastern beringia a single cataclysmic eruption at the close of marine isotope stage 6 abstract old crow tephra is the largest and most widespread quaternary eruption presently known in eastern beringia its major and trace element geochemistry fe ti oxides and stratigraphic and paleoecological context indicate that it is the result of a single cataclysmic eruption the proximal region may well have experienced tephra fallout from small eruptions just prior to or after the old crow event but there is no evidence to indicate that the distal area was affected we recalculate the glass fission track age at 124 10 ka which coupled with stratigraphic and paleoenvironmental reconstructions indicates that deposition occurred prior to development of the last interglacial boreal forest which suggests a latest marine isotope stage mis 6 age the bulk tephra volume erupted is estimated by three different approaches that are in broad agreement at 200 km 3 but this result must be considered as tentative given the poor controls on definition of isopachs over such a large area the source caldera although presently unrecognized is located in the eastern aleutian arc possibly at or near the emmons lake volcanic center

late neogene history of deepwater ventilation in the southern ocean we compiled carbon isotope records from the north atlantic south atlantic and pacific oceans to estimate changes in vertical and interbasinal 13c gradients for the last 9 myr benthic 13c values of deep water in the south atlantic decreased in a series of steps at 7 2 75 and 1 55 ma away from the north atlantic and toward the pacific the benthic 13c of intermediate water in the south atlantic evolved differently from deep waters resulting in an increase in the intermediate to deep 13c gradient 13cid the 13cid increased in steps at 2 75 and 1 55 ma marking the development and intensification of a chemical divide in the atlantic ocean between well ventilated intermediate water and poorly ventilated deep water we suggest these changes in interbasinal and vertical gradients resulted from decreasing northern component water and reduced ventilation of southern component water scw the latter was caused by several interrelated processes in antarctic sources areas including increased sea ice cover enhanced surface water stratification decreased ekman induced upwelling and reduced vertical mixing across the thermocline because antarctic surface water processes and deepwater ventilation rates ultimately influence atmospheric co2 concentration these processes may have acted as a positive feedback that contributed to the major cooling steps in late neogene climate

paleoclimatic context of geomagnetic dipole lows and excursions in the brunhes clue for an orbital influence on the geodynamo the hypothesis of an influence of the astronomical precession on the geodynamo energy budget was recently reappraised by theoreticians paleomagnetic indications of such an influence remain controversial because reconstructions of paleointensity variations from sediments are suspected to be contaminated by lithological paleoclimatically induced influences three sets of complementary indicators are however available 1 records of the direction of magnetization in sediments 2 records of magnetic anomalies of the deep sea floor basalts and 3 records of production variations of cosmogenic isotopes from sediment and ice cores these records confirm the genuine geomagnetic origin of paleointensity lows and their narrow link with excursions or short reversals recorded in various materials and often dated by radiometric methods the analysis of these time series and their comparison with 18o records of the paleoclimate suggest that such globally recorded geomagnetic dipole lows have preferentially occurred in the context of interglacial or transitional paleoclimates at the time of low or decreasing obliquity the dominant periods extracted with the complex continuous wavelet transform technique range from 40 to 125 ka further suggesting a link with orbital parameters these results encourage future efforts of research to improve the precision the resolution and the dating of the time series of geomagnetic dipole low in order to better decipher orbital signatures and understand their origin an important implication of this topic is that the next geomagnetic dipole low should be related with the present interglacial

iron geochemistry of loess and red clay deposits in the chinese loess plateau and implications for long term asian monsoon evolution in the last 7 0 ma abstract recently some groups of authors have demonstrated that the tertiary red clay deposits underlying the pleistocene loess paleosol sequence in the chinese loess plateau are wind blown in origin thus extending the eolian record in the plateau from 2 6 ma back to about 7 0 ma and providing a good opportunity to reconstruct long term monsoon changes as magnetic susceptibility the widely used paleomonsoon proxy of loess deposits is problematic in reconstructing the monsoon history recorded in the red clay the development of other proxies is urgently needed in this study we analyzed the ratios of citrate bicarbonate dithionite cbd extractable fe2o3 to total fe2o3 concentrations in four loess sections along a north south transect of the loess plateau and in the lingtai loess red clay sequence of 305 m thick the fe2o3 ratio in the loess sections clearly shows higher values in soil units than in loess horizons and a southward increase in this ratio is also remarkable along the transect these are in good agreement with the temporal spatial variations in the east asia summer monsoon intensity identified by other monsoon proxies and soil development observations the fe2o3 ratio thus attests to being a good proxy indicator of the summer monsoon evolution the lingtai fe2o3 ratio record shows high values over three time intervals 4 8 4 1 ma 3 4 2 6 ma and during the interglacial periods of the past 0 8 ma the increase in summer monsoon intensity over the three intervals also coincides with the well developed soil characteristics it is therefore concluded that the east asia summer monsoon has experienced a non linear evolution since the late miocene the occurrence of a monsoon prime at about 4 8 4 1 ma implies that the tibetan plateau one of the most important boundary conditions in maintaining the monsoon circulation must have been uplifted to a critical height at that time

evolution of the south asian monsoon wind system since the late middle miocene abstract the evolution and timing of the inception of the south asian monsoon sam has been one of the most debated climatic mysteries amongst climate workers the himalaya tibetan plateau htp complex has been implicated as the main driver for sam circulation however there are intense debates as to whether there was a critical elevation of the htp complex that drove the sam and at what time such a critical elevation was attained also the role of the htp complex in driving the sam has been questioned in recent climate model studies the model simulation suggests a major intensification of the sam as early as 30 ma whereas marine records from the arabian sea indicate a major strengthening of the south asian summer or southwest monsoon winds between 10 and 8 ma the continental vegetation on the other hand captured a major transition from c 3 to c 4 type during 8 7 ma but whether this change was local or global and whether this transition was driven by monsoonal precipitation is not unanimously accepted our new record from ocean drilling program holes located off the oman margin and on the owen ridge western arabian sea shows appearance of monsoon wind proxy planktic foraminifer globigerina bulloides a significant increase in total organic carbon and a negative shift in stable carbon isotope record of benthic foraminifera at 12 9 ma these proxies indicate that present day south asian monsoon wind system began to develop during the late middle miocene 12 9 ma and summer monsoon was in its full strength in the late miocene 7 ma from 11 to 7 ma the summer monsoon was weaker when winter monsoon was stronger

magnetostratigraphy of a greigite bearing core from the south yellow sea implications for remagnetization and sedimentation sediments from the continental shelf are sensitive to sea level climatic changes and local tectonic history in this study we carried out a high resolution magnetostratigraphic investigation on the longest core nhh01 125 64 m recovered from the south yellow sea sys an abnormal interval dominated by negative inclinations was discovered by applying alternating field demagnetization afd on samples from a greigite bearing layer 44 90 51 80 m in contrast the inclinations of most greigite bearing samples changed from negative to positive when heated to 360 c this strongly indicates that this inclination anomaly revealed by the afd alone is not a true negative subchron after neglecting the effects of greigite bearing layers the straightforward correlation of the interpreted magnetostratigraphy defines the matuyama brunhes boundary 781 ka and the jaramillo top 990 ka at 68 64 m and 101 54 m respectively the linearly extrapolated basal age of the core is 1 10 ma in addition several short lived inclination anomalies can be tentatively assigned to magnetic excursions which indicates that the sedimentation could be continuous even at the millennial time scale at depth intervals bracketing these fast geomagnetic events moreover the excellent correspondence between clay content variations of the core and the marine oxygen isotope record indicates the potential of clay content as a paleoclimatic proxy in the studied region in the past 1 ma in brief our study provides not only a robust age model in the sys but also a methodological guide for paleomagnetic studies in continental shelf region

sediment characteristics and transportation dynamics of the ganga river abstract understanding of river systems that have experienced various forcing mechanisms such as climate tectonics sea level fluctuations and their linkages is a major concern for fluvial scientists the 2525 km long ganga river derives its fluvial flux from northern part of the indian subcontinent and drops in the ganga brahmaputra delta and the bengal fan regions this paper presents a study of the ganga river sediments for their textural properties grainsize characteristics and transportation dynamics a suite of recently deposited sediments 189 bedload samples and 27 suspended load samples of the river and its tributaries was collected from 63 locations dry and wet sieve methods of grainsize analysis were performed and folk and ward s parameters were calculated transportation dynamics of the sediment load was assessed by means of channel hydrology flow sediment rating curves bedform mechanics grainsize images and cumulative curves textural properties of the bedload sediments of the ganga river tributaries originating from the himalaya orogenic belt the northern indian craton and the ganga alluvial plain regions are characterised by the predominance of fine to very fine sand medium to fine sand and very fine sand to clay respectively downstream textural variations in the bedload and suspended load sediments of the ganga river are therefore complex and are strongly influenced by lateral sediment inputs by the tributaries and channel slope at the base of the himalaya a very sharp gravel sand transition is present in which median grainsize of bedload sediments decreases from over 0 16 to 2 46 within a distance of 35 km downstream decline in mean grainsize of bedload sediments in the upper ganga river within the alluvial plain can be expressed by an exponential formula as mean grainsize in 0 0024 distance in kilometres from the himalayan front 1 29 it is a result of selective transport phenomena rather than of abrasion the condition attributed to channel competence with low gradient of 70 to 25 cm km coarsening of the bedload sediments in the middle segment of the river develops due to the coarse bedload sediments mean grainsize 1 8 2 2 supply from the tributaries of the northern indian craton region and increased unit stream power the ganga river sediments show a wide range of grainsize parameters bedload sediments mean grainsize 1 5 3 8 standard deviation 0 3 0 7 skewness 0 2 0 5 and kurtosis 0 8 2 4 suspended load sediments mean grainsize 2 9 5 0 standard deviation 0 6 1 6 skewness 0 6 0 4 and kurtosis 0 7 4 6 mean grainsize of the bedload sediments is 2 6 which is 1 3 units coarser than the suspended load sediments the cumulative grainsize curves of the bedload sediments show a distinct break in slope representing nearly 80 of the bedload sediment transported as graded suspension during the peak monsoon discharge most of the bedload sediments are transported as the discontinuous fine sandy graded suspension by the unidirectional unsteady shooting flow at a high transport rate about 8 106 tons d a model of sedimentation transportation at river channel is proposed the ganga river mobilises a total of 729 106 tons of sediments annually through a narrow zone within its river valley under the present hydrogeological conditions the river sedimentation is climatically controlled and is predicted to produce a 2000 km long 2 to 40 km wide and 25 to 50 m thick ribbon shaped well sorted symmetrically skewed fine sand body

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

grain size distribution function of polymodal sediments in hydraulic and aeolian environments and numerical partitioning of the sedimentary components most continental sediments are polymodal composed of overlapping components of which the grain size generally obeys some type of natural distribution the grain size components and their function types can be determined from frequency and cumulative curve plots in order to define the function formula of the grain size distribution the function parameters can be estimated by fitting a defined function formula to the measured grain size data of the sample which simultaneously achieves numerical partitioning of the sedimentary components genetic analysis of grain size components of hydraulic and aeolian sediments demonstrates the following environmental implications fluvial sediment is composed of isolated saltation and suspension components the sediments in closed lake basins are dominated by a suspension silt clay component with a small proportion of saltation sand the fine sand component makes up the majority of desert sand overlapping with a small proportion of fine dust aeolian loess is composed of two overlapping components a short suspension time silt component and a long suspension time fine component aeolian material in the north pacific deep sea sediments is dominated by long suspension time fine dust the fine component in aeolian sediments shows a consistent grain size distribution and genetic connection from the desert sand loess of northern china to the north pacific ocean which is mainly transported by westerly winds and is dispersed in the atmosphere forming a background dust c 2002 elsevier science b v all rights reserved

geochemistry of sediments from the huaibei plain east china implications for provenance weathering and invasion of the yellow river into the huaihe river abstract the sediments of the huaibei plain in semi humid mid eastern china represent valuable geological records with respect to eolian fluvial interactions depositional environments and climate change in this region provenance and weathering conditions are often reconstructed using sedimentary geochemistry methods in this study an 8 m core from huainan and a set of loess samples from northern and southern china were analyzed for major trace and rare earth elements rees results were compared to determine the samples provenance the major trace rare earth elements contents and grain size distribution were found to fluctuate widely in the 2 8 m section of the huainan core and more narrowly closer to the surface 0 2 m this suggests a provenance shift at a depth of 2 m the tio 2 al 2 o 3 sio 2 al 2 o 3 th nb la nb values and ree patterns in the upper core 0 2 m are similar to those found in samples from the chinese loess plateau clp these results suggest that the clp in northern china is likely to be the primary origin of the upper part 0 2 m of the huainan core compared with clp samples the upper part of the huainan core exhibits lower k 2 o al 2 o 3 values and higher chemical alteration indices this is indicative of the material s substantial weathering during transportation and re deposition and implies that these sediments could reasonably be classified as typical recycled loess the sediments may have been transported from the clp to huainan as yellow river flood events probably during the last deglaciation 13 2 ka as a result of increased precipitation along with glacier and snow melt in the upper reaches of the yellow river catchment during this period this suggests that the yellow river may have migrated into the huaihe river catchment much earlier than the earliest historical records 361 bce suggest the implications of this would be profound with respect to chinese history

towards rapid assay of cryptotephra in peat cores review and evaluation of various methods abstract peat bogs are highly effective archives for the preservation and detection of cryptotephra but the conventional methods used to detect these hidden diminutive layers are destructive and can be time consuming we briefly review methods that have been used for cryptotephra detection and evaluate the potential of a range of alternative reconnaissance methods both non destructive and destructive to provide for more rapid examination of continuous cryptotephra content in peat cores the techniques evaluated magnetic susceptibility ms spectrophotometry and x ray fluorescence are used to pick out compositional contrasts between tephra deposits and peat measurements of organic content are also evaluated as a potential guide to tephra content based on an inverse relationship although we find limitations to each method particularly where deployed at the distal most end of tephra dispersal there is potential for all methods to be used in the detection of cryptotephra where time or material is limited these methods can also provide additional sedimentological and stratigraphic information for studies of peat cores however where a reliable cryptotephra profile is required we conclude that there is no quick or easy substitute for the conventional extraction microscopy method