el nino la nina and sahel precipitation during the middle holocene simulations with a synchronously coupled atmo sphere land ocean sea ice model indicate el nino la nina like events in the eastern tropical pacific ocean during the middle holocene 6 ka with similar intensities and frequencies as at present 0 ka july august september sahel precipitation shifts northward as expected from proxy data for present day interannual decadal variability of western sahel precipi tation is correlated with both pacific el nino la nina sea sur face temperature sst anomalies and the atlantic dipole teleconnections at 6 ka between sahel precipitation and el nino la nina sst anomalies are absent with tropical atlantic ssts asserting a dominant influence these results illustrate potential problems with using present day teleconnection patterns in interpreting past climate variability from proxy data

linking the 8 2 ka event and its freshwater forcing in the labrador sea 1 the 8 2 ka event was the last deglacial abrupt climate event a reduction in the atlantic meridional overturning circulation amoc attributed to the drainage of glacial lake agassiz may have caused the event but the freshwater signature of lake agassiz discharge has yet to be identified in 18o of foraminiferal calcite records from the labrador sea calling into question the connection between freshwater discharge to the north atlantic and amoc strength using mg ca paleothermometry we demonstrate that 3 c of near surface ocean cooling masked an 1 0 decrease in western labrador sea 18o of seawater concurrent with lake agassiz drainage comparison with north atlantic 18o of seawater records shows that the freshwater discharge was transported to regions of deep water formation where it could perturb amoc and force the 8 2 ka event

estudio sismico de la corteza iberica norte 3 3 a seismic image of the variscan crust in the hinterland of the nw iberian massif an offshore vertical incidence reflection seismic study with simultaneous on land wide angle recording has been conducted as part of the estudio sismico de la corteza iberica norte escin project

quantification of holocene asian monsoon rainfall from spatially separated cave records abstract a reconstruction of holocene rainfall is presented for southwest china an area prone to drought and flooding due to variability in the east asian monsoon the reconstruction is derived by comparing a new high resolution stalagmite 18 o record with an existing record from the same moisture transport pathway the new record is from heshang cave 30 27 n 110 25 e 294 m and shows no sign of kinetic or evaporative effects so can be reliably interpreted as a record of local rainfall composition and temperature heshang lies 600 km downwind from dongge cave which has a published high resolution 18 o record wang y j cheng h edwards r l he y q kong x g an z s wu j y kelly m j dykoski c a li x d 2005 the holocene asian monsoon links to solar changes and north atlantic climate science 308 854 857 by differencing co eval 18 o values for the two caves secondary controls on 18 o e g moisture source moisture transport non local rainfall temperature are circumvented and the resulting 18 o signal is controlled directly by the amount of rain falling between the two sites this is confirmed by comparison with rainfall data from the instrumental record which also allows a calibration of the 18 o proxy the calibrated 18 o record provides a quantitative history of rainfall in southwest china which demonstrates that rainfall was 8 higher than today during the holocene climatic optimum 6 ka but only 3 higher during the early holocene significant multi centennial variability also occurred with notable dry periods at 8 2 ka 4 8 4 1 ka 3 7 3 1 ka 1 4 1 0 ka and during the little ice age this holocene rainfall record provides a good target with which to test climate models the approach used here of combining stalagmite records from more than one location will also allow quantification of rainfall patterns for past times in other regions

milankovitch forcing of fluctuations in the level of tropical lakes from 18 to 0 kyr bp an atmospheric general circulation model has been used to simulate the climates of january and july at 3 000 yr intervals from 15 kyr bp onwards the model simulates a strengthened monsoon circulation and increased precipitation in the northern hemisphere tropics culminating at 9 6 kyr bp the computed hydrological budgets were used to estimate the area of closed lakes between 8 9 and 26 6 n giving with minor exceptions results in good agreement with geological evidence

reconstruction of lake level changes in lake xinias central greece during the last 40 000 years abstract the investigation of lake xinias greece deals with lake level changes since about 40 000 b p particular effort was made to obtain a reconstruction of lake level during the last glacial maximum at about 18 000 b p the reconstruction was based on a transect of sediment cores the cores were pollen stratigraphically correlated and the interpretation of the sediment stratigraphy was based on sediment chemical and mineral magnetic analyses the chronology was based on accelerator and conventional radiocarbon datings the most striking change was a lake level lowering at about 32 000 b p resulting in an overgrowing of the whole lake the lake level probably remained generally low until somewhat before 15 000 b p the period around the last glacial maximum must have been characterized by a low lake level which is in contrast to the hitherto common opinion additional investigations are required to demonstrate that the lake level changes reconstructed in lake xinias are regionally significant and record climatic changes however it may already be suggested that the conflict between pollen and lake level evidence which hitherto has produced controversy about the glacial climate perhaps does not exist the low lake level in lake xinias around the last glacial maximum is in accordance with the pollen evidence of steppe vegetation both recording a dry climate another distinct lowering occurred around the late wurmian holocene boundary at about 10 000 b p some minor lowerings are recorded also during the holocene e g at about 6000 5000 and 3000 b p

depositional evidence of palaeofloods during 4 0 3 6 ka bp at the jinsha site chengdu plain china abstract at the jinsha site it has been discovered through field surveys and observations of the geomorphology and sedimentology that the direction of flow of ancient rivers obtained through debris fabric measurement was consistent with the direction in which buried palaeotrees had been broken indicating that the jinsha site possibly suffered palaeofloods by sampling the suspected palaeoflood sediments at the jinsha site and analysing their grain size distribution and zr rb and ba nb ratios the paper discovered that the sediments tend to be composed of coarse particles with a relatively high percentage of traction load components and saltation load components various samples had relatively great differences in particle size distribution the change in the percentage content of coarse sand particles being very obvious and the sedimentary particles exhibiting upward coarsening within the same layer the zr rb ratios and ba nb ratios of the sediments reflect the changes in hydrodynamic forces within the sedimentary environment when the hydrodynamic force was relatively weak the zr rb ratios are relatively low and the ba nb ratios relatively high whereas when the hydrodynamic force was strengthened the zr rb ratios increase and the ba nb ratios decrease on account of these sedimentological and geochemical characteristics these sediments are judged to be palaeoflood sediments the ams 14 c dating results of the palaeoflood sediments shows that the period when sudden flooding occurred frequently in the chengdu plain was 4 0 3 6 ka bp contemporaneous with an ancient global flooding event this palaeoflood event probably brought about the extinction of the baodun culture

the mohorovicic discontinuity beneath the continental crust an overview of seismic constraints abstract the seismic signature of the moho from which geologic and tectonic evolution hypotheses are derived is to a large degree a result of the seismic methodology which has been used to obtain the image seismic data of different types passive source earthquake broad band recordings and controlled source seismic refraction densely recorded wide angle deep seismic reflection and normal incidence reflection using vibroseistm explosives or airguns have contributed to the description of the moho as a relatively complex transition zone of critical importance for the quality and resolution of the seismic image are the acquisition parameters used in the imaging experiments a variety of signatures have been obtained for the moho at different scales generally dependent upon bandwidth of the seismic source this variety prevents the development of a single universally applicable interpretation in this way source frequency content and source and sensor spacing determine the vertical and lateral resolution of the images respectively in most cases the different seismic probes provide complementary data that gives a fuller picture of the physical structure of the moho and its relationship to a petrologic crust mantle transition in regional seismic studies carried out using passive source recordings the moho is a relatively well defined structure with marked lateral continuity the characteristics of this boundary change depending on the geology and tectonic evolution of the targeted area refraction and wide angle studies suggest the moho to be often a relatively sharp velocity contrast whereas the moho in coincident high quality seismic reflection images is often seen as the abrupt downward decrease in seismic reflectivity the origin of the moho and its relation to the crust mantle boundary is probably better constrained by careful analysis of its internal details which can be complex and geographically varied unlike the oceanic moho which is formed in a relatively simple well understood process the continental moho can be subject to an extensive variety of tectonic processes making overarching conclusions about the continental moho difficult speaking very broadly 1 in orogenic belts still undergoing compression and active continental volcanic arcs the moho evolves with the mountain belt 2 in collapsed phanerozoic orogenic belts the moho under the collapse structure was formed during the collapse often by a combination of processes 3 in regions having experienced widespread basaltic volcanism the moho can result from underplated basalt and basaltic residuum in precambrian terranes the moho may be as ancient as the formation of the crust in others precambrian tectonic and magmatic processes have reset it we note that seismic reflection data in phanerosoic orogens as well as from precambrian cratonic terranes often show thrust type structures extending as deep as the moho and suggest that even where crust and mantle xenoliths provide similar age of formation dates the crust may be semi allochothonous

global climate instability reflected by eastern mediterranean marine records during the late holocene abstract extremely high sediment accumulation rates in the southeastern se mediterranean off israel make this region ideal for high resolution paleoceanographic reconstructions two cores were examined and foraminiferal oxygen and carbon isotope composition as well as physical and geochemical properties of the sediments were used to reconstruct the climatic and environmental changes of the se mediterranean basin during the late holocene 3 6 ka the 18o values of globigerinoides ruber suggest that the freshwater balance changed several times during this time interval humid phases took place between 3 5 3 0 ka bp and 1 7 1 0 ka bp the later was accompanied by enhanced preservation of terrestrial organic matter within the sediment ors event relatively more arid conditions prevailed in this area between 3 0 and 1 7 ka bp during the last millennia at ca 0 8 and 0 27 ka two climatic events occurred which are probably correlative to both the medieval warm period and the cooling global event known as the little ice age respectively a long term slight increase in planktonic foraminiferal 18o values occurs together with a gradual decrease in the 13c values of both g ruber and the benthic foraminifera uvigerina mediterranea this trend is associated with an increase in sedimentation rates ti al ratio magnetic susceptibility and color index of the sediments we suggest that this trend shown by various independent proxies seems to be related to the aridification process that started ca 7 0 ka in the mid low latitude desert belt and the se mediterranean region and continuous until the present the long term 13c decrease shown by surface and bottom water foraminifera reflects a gradual change in the 13c of the dissolved co2 of the entire se mediterranean water column this follows the global co2 rise in the late holocene as a result of the terrestrial biomass destruction during the aridification process moreover the gradual reduction of the vegetation cover in east africa led to an increased erratic flood related sediment flux via the nile river up to the present this is reflected by the general change in the local sediment composition at 3 6 ka the saharan eolian input reached 65 whereas at about 0 3 ka 70 of the se mediterranean sediment was composed of nile particulate matter

variation of rb sr ratios in the loess paleosol sequences of central china during the last 130 000 years and their implications for monsoon paleoclimatology abstract rb sr and magnetic susceptibility have been measured in the last interglacial glacial loess profiles at luochuan and huanxian central china a high degree of similarity between the parameters in both profiles suggests that variations of rb sr ratios in the sequence can be regarded as an indicator of east asian summer monsoon strength matching the rb sr record with the specmap 18o curve suggests that the rb sr ratio responds sensitively to changes of the east asian monsoon induced by global ice volume variation

the post collisional stabilization of thickened crust in the southern urals abstract the urals are characterized by a depression of the moho to a depth of 57 km this structure is interpreted as a relic orogenic root which has been conserved because no significant post collisional processes occurred however there is evidence that voluminous post collisional magmatism affected the lower crust in this paper we use thermal finite element models to quantify the influence of the post collisional magmatism on the stabilization of the root we show that at least 70 of the heat producing elements migrated in granitic melts from the lower crust to the upper crust as a result the crustal heat flow reduced and the lithosphere could stabilize at a thickness of 180 km furthermore we propose that a granulite metamorphic event during the thermal relaxation of the collision zone prevented the 57 km thick crust from delamination these results strongly indicate that post collisional processes were necessary for the stabilization of the uralian crust and lithosphere

what is the timing of orbital scale monsoon changes abstract a major but little noted divergence of opinion has developed among climate scientists over the orbital scale periodicity and phasing of tropical monsoon variations kutzbach 1981 monsoon climate of the early holocene climate experiment with earth s orbital parameters for 9000 years ago science 214 59 61 proposed that monsoons are driven by northern summer insolation at the precession period but clemens and prell 1990 late pleistocene variability of arabian sea summer monsoon winds and continental aridity eolian records from the lithogenic component of deep sea sediments paleoceanography 5 109 145 2003 a 350 000 year summer monsoon multi proxy stack from the owen ridge northern arabian sea marine geology 201 35 51 inferred a more complicated response tied to latent heat transfer from the southern hemisphere because tropical monsoons affect climate over a vast area resolving this divergence is an important task for the climate community the purpose of this note is to highlight definitive evidence from high resolution dating of speleothem calcite that provides unambiguous support for the kutzbach hypothesis

on the river lake relationship of the middle yangtze reaches abstract the geological and meteorological setting of the jianghan dongting lake area leads to high precipitation and siltation but poor water discharge thus the area is frequently flooded in the past the river lake relationship of the middle yangtze has been variable but has deteriorated recently under increasing human influence the jianghan plain of the middle yangtze becomes a waterlogged lowland under the constant threat of flooding from the perched jingjiang river due to siltation and land reclamation the dongting lake has lost most of its regulatory function for the river and has become increasingly vulnerable to flood disasters the middle yangtze river has been undergoing siltation resulting from a downstream decline of sediment transport capacity resulting in the elevation of the flood level above the lowlands heightening of the levees has caused further siltation of the channel the three gorges reservoir will provide a buffering period of 50 80 years during which much of the silt will be trapped in the reservoir and scouring downstream may occur we should utilize this period to work out an overall resolution to the problem construction of a water and silt diversion project in the honghu lake and surrounding areas may resolve this problem in the dangerous chenglingji wuhan segment of the yangtze widening the canals connecting the middle yangtze and han rivers may function as a discharge dividing channel of the yangtze which may prove to be beneficial

environmental humidity changes inferred from multi indicators in the jianghan plain central china during the last 12 700 years abstract time sequence of the jz 2010 profile derived from ams 14 c dating and the content of the sediment elements ti rb and sr the rb sr values as well as the granularity and the magnetic susceptibility are utilized to reconstruct the environmental humidity changes in the jianghan plain since 12 700 cal bp the comprehensive analyses of multiple alternative indicators indicate that the regional moisture turned from relatively dry to wet after the late glacial period and reached its optimum until the mid holocene however the trend was punctuated by several decreased humidity phases beginning with the severe dry event approximately 4400 4100 cal bp the environment was dry as a whole with modest humidity from approximately 3900 cal bp humidity changes in the study area are dominated by the east asian monsoon system which is under the influence of the gradual southward migration of the itcz driven by the summer solar insolation changes in the northern hemisphere due to orbital forcing as the first attempt to apply rb sr value and ti concentration in tracing moisture evolution in a region with typical subtropical humid climate this study highlights the different effects and consequences of the proxies throughout the palaeoenvironmental change process and the impacts of regional topography that sustained tectonic subsidence since the cretaceous as well as the southeast tilt that opened the plain to the summer monsoon moreover the fluctuation of magnetic susceptibility is associated with the granularity feature the reduction oxidation conditions and the impact of human factors on the natural sedimentary environments during the historic period in the jianghan plain

paleohydrological changes over the last 4000 years in the middle and lower reaches of the yangtze river evidence from particle size and n alkanes from longgan lake we have reconstructed the history of late holocene paleohydrological changes in the middle and lower reaches of the yangtze river using grain size and n alkane data from a sediment core retrieved f

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

a late weichselian stable isotope stratigraphy compared with biostratigraphical data a case study from southern sweden late weichselian lake sediments from a site in southern sweden were analysed for stable carbon and oxygen isotopes as well as plant macrofossils and insect remains by comparison of independent d

global monsoons in the mid holocene and oceanic feedback the response of the six major summer monsoon systems the north american monsoon the northern africa monsoon the asia monsoon the northern australasian monsoon the south america monsoon and the southern africa monsoon to mid holocene orbital forcing has been investigated using a coupled ocean atmosphere general circulation model foam with the focus on the distinct roles of the direct insolation forcing and oceanic feedback the simulation result is also found to compare well with the ncar csm the direct effects of the change in insolation produce an enhancement of the northern hemisphere monsoons and a reduction of the southern hemisphere monsoons ocean feedbacks produce a further enhancement of the northern africa monsoon and the north american monsoon however ocean feedbacks appear to weaken the asia monsoon although the overall effect direct insolation forcing plus ocean feedback remains a strengthened monsoon the impact of ocean feedbacks on the south american and southern african monsoons is relatively small and therefore these regions especially the south america experienced a reduced monsoon regime compared to present however there is a strong ocean feedback on the northern australian monsoon that negates the direct effects of orbital changes and results in a strengthening of austral summer monsoon precipitation in this region a new synthesis is made for mid holocene paleoenvironmental records and is compared with the model simulations overall model simulations produce changes in regional climates that are generally consistent with paleoenvironmental observations

the origin of monsoons abstract the monsoon is interpreted as an intertropical convergence zone itcz substantially away more than 10 from the equator and the existence of the itcz does not have to rely on land sea contrast land sea contrast can provide a favorable longitudinal location for the itcz but this role can be replaced by sea surface temperature contrast in the longitudinal direction thus the interpretation of the monsoon presented herein differs from the long held fundamental belief that its basic cause is land sea thermal contrast on the continental scale in the sense that the existence of landmass is not considered a necessary condition for monsoons through general circulation model experiments support has been found for this interpretation the asian and australian summer monsoon circulations are largely intact in an experiment in which asia maritime continent and australia are replaced by ocean with sea surface temperature sst taken from that of the surrounding oceans thus in these areas land sea c

precambrian moho offset and tectonic stability of the east european platform from the urseis deep seismic profile deep seismic reflection data from the 1995 urals seismic experiment and integrated studies urseis transect of the southern urals document a distinct 5 km vertical offset of the moho beneath the uralian foreland here named the makarovo fault zone this offset does not disrupt the overlying riphean sedimentary section nor does it correlate with any major surface lineaments or known phanerozoic subduction features it is thus interpreted to be an archean or early proterozoic fault zone possibly a terrane boundary that disrupts an even older moho most important is the implication that this structure and the moho it offsets were not significantly modified by two subsequent cycles of continental collision that affected this region the baikalian 700 570 ma and the late paleozoic uralian 350 230 ma orogenic phases similarly the pronounced reflectivity of the moho beneath the uralian foreland must also be a precambrian signature not a young feature as would be implied by certain recent thermo rheological and reflectivity models that associate a reflective lower crust and moho with areas that underwent pronounced synorogenic or postorogenic collapse the makarovo fault zone provides yet another demonstration that relicts of ancient tectonic processes can survive for long periods of time in the lower crust and that the moho can be a key marker horizon in delineating the tectonic evolution of the lithosphere

mid holocene climates of the americas a dynamical response to changed seasonality simulations of the climatic response to mid holocene 6 ka bp orbital forcing with two coupled ocean atmosphere models foam and csm show enhancement of monsoonal precipitation in parts of the american southwest central america and northernmost south america during northern hemisphere summer the enhanced onshore flow that brings precipitation into central america is caused by a northward displacement of the inter tropical convergence zone driven by cooling of the equatorial and warming of the northern subtropical and mid latitude ocean ocean feedbacks also enhance precipitation over the american southwest although the increase in monsoon precipitation there is largely driven by increases in land surface temperature the northward shift in the equatorial precipitation band that causes enhanced precipitation in central america and the american southwest has a negative feedback effect on monsoonal precipitation in northern south america the simulations demonstrate that mid holocene aridity in the mid continent of north america is dynamically linked to the orbitally induced enhancement of the summer monsoon in the american southwest with a spatial structure wet in the southwest and dry in the mid continent similar to that found in strong monsoon years today changes in winter precipitation along the west coast of north america in central america and along the gulf coast caused by southward displacement of the westerly storm tracks indicate that changes in the northern hemisphere winter monsoon also play a role in regional climate changes during the mid holocene although the simulations with foam and csm differ in detail the general mechanisms and patterns are common to both the model results thus provide a coherent dynamical explanation for regional patterns of increased or decreased aridity shown by vegetation lake status and aeolian data from the americas

influence of the intertropical convergence zone on the east asian monsoon a palaeoclimate record spanning the past 16 000 years with nearly annual time resolution has been obtained from lake huguang maar in china the magnetic properties and titanium content of the lake sediments are thought to reflect changes in the east asian winter monsoon strength thus providing a complement to palaeoclimate archives that record the strength of the rain bearing summer monsoon the records point to an anti correlation between winter and summer monsoon strength best explained by migration of the intertropical convergence zone itcz a belt of low pressure air at the equator interestingly the decline of china s tang dynasty and the mayan civilization broadly coincide with drought periods recorded in lake huguang maar and cariaco basin sediments respectively did major shifts in the position of the itcz catalyse simultaneous events in civilizations on opposite sides of the pacific ocean the asian australian monsoon is an important component of the earth s climate system that influences the societal and economic activity of roughly half the world s population the past strength of the rain bearing east asian summer monsoon can be reconstructed with archives such as cave deposits1 2 3 but the winter monsoon has no such signature in the hydrological cycle and has thus proved difficult to reconstruct here we present high resolution records of the magnetic properties and the titanium content of the sediments of lake huguang maar in coastal southeast china over the past 16 000 years which we use as proxies for the strength of the winter monsoon winds we find evidence for stronger winter monsoon winds before the bolling allerod warming during the younger dryas episode and during the middle and late holocene when cave stalagmites suggest weaker summer monsoons1 2 3 we conclude that this anticorrelation is best explained by migrations in the intertropical convergence zone similar migrations of the intertropical convergence zone have been observed in central america for the period ad 700 to 900 refs 4 6 suggesting global climatic changes at that time from the coincidence in timing we suggest that these migrations in the tropical rain belt could have contributed to the declines of both the tang dynasty in china and the classic maya in central america

early holocene climatic oscillations recorded by lake level fluctuations in west central europe and in central italy abstract on the basis of lake level data the period of the preboreal oscillation pbo at 11 300 11 150 cal yr bp as defined by the grip oxygen isotope record appears to correspond to wetter climatic conditions in west central europe and to a marked drying in north central italy additional short lived phases of higher lake level have been identified before the pbo at ca 11 450 11 400 and 11 350 cal yr bp in west central europe and at ca 11 500 cal yr bp in italy such multiple climatic oscillations around the pbo have been observed in various records in northwestern europe on the basis of various proxies a map of changes in moisture over western europe during the pbo indicates a mid latitude zone between 58 and 43 n characterised by wetter conditions while a drier climate developed in southern and northern europe this wet middle zone shows a larger extension and suggests a more meandering weaker atlantic westerly jet during the pbo than during the 8 2 kyr event a comparison of lake level records in west central europe with 1 outbursts from north american and north european proglacial lakes and 2 variations in solar activity as reflected by 14 c and the 10 be records supports the hypothesis that the pbo was a response to successive meltwater pulses at 11 300 11 250 11 200 and 11 170 cal yr bp and to a sudden decrease in solar activity at 11 250 cal yr bp this study points to the necessity of developing integrated multi proxy approaches to construct more robust regional event stratigraphies and of better documenting palaeohydrological changes in the mediterranean area

u pb zircon age for a tuff in the campbell group griqualand west sequence south africa an ion microprobe u pb age of 2552 or 11 ma has been obtained on zircon separated from a regional banded tuff horizon in the nauga formation beukes 1980b upper campbell group griqualand west sequence south africa this age permits time constraints to be placed on lithologically correlated units within the adjacent transvaal sequence and correlations to be made with the further removed hamersley group in australia calculated rock accumulation rates of 2 to 4 m m y for these strata of mainly shale and banded iron formation suggest that sedimentation rates were significantly slower in the late archean early proterozoic than is generally assumed

holocene itcz and indian monsoon dynamics recorded in stalagmites from oman and yemen socotra abstract high resolution oxygen isotope 18o profiles of holocene stalagmites from four caves in northern and southern oman and yemen socotra provide detailed information on fluctuations in precipitation along a latitudinal transect from 12 n to 23 n 18o values reflect the amount of precipitation which is primarily controlled by the mean latitudinal position of the itcz and dynamics of the indian summer monsoon ism during the early holocene rapidly decreasing 18o values indicate a rapid northward displacement in the mean latitudinal position of the summer itcz and the associated ism rainfall belt with decadal to centennial scale changes in monsoon precipitation correlating well with high latitude temperature variations recorded in greenland ice cores during the middle to late holocene the summer itcz continuously migrated southward and monsoon precipitation decreased gradually in response to decreasing solar insolation a trend which is also recorded in other monsoon records from the indian and east asian monsoon domains importantly there is no evidence for an abrupt middle holocene weakening in monsoon precipitation although abrupt monsoon events are apparent in all monsoon records they are short lived and clearly superimposed on the long term trend of decreasing monsoon precipitation for the late holocene there is an anti correlation between ism precipitation in oman and inter monsoon spring autumn precipitation on socotra revealing a possible long term change in the duration of the summer monsoon season since at least 4 5 ka bp together with the progressive shortening of the ism season gradual southward retreat of the mean summer itcz and weakening of the ism the total amount of precipitation decreased in those areas located at the northern fringe of the indian and asian monsoon domains but increased in areas closer to the equator

thermotectonic evolution of the western fold and thrust belt southern uralides russia as revealed by apatite fission track data abstract the uralides a linear n s trending palaeozoic fold belt reveals an intact well preserved orogen with a deep crustal root within a stable continental interior in the western fold and thrust belt of the southern uralides devonian to carboniferous siliciclastic and carbonate rocks overlay mesoproterozoic to neoproterozoic sedimentary rocks deformation in the devonian carboniferous and permian caused thick skinned tectonic features in the western and central parts of the western fold and thrust belt a stack of several nappes characterizes the deformation in the eastern part along the e w transect ac ts 96 that crosses the western fold and thrust belt apatite fission track data record various stages of the geodynamic evolution of the uralide orogeny such as basin evolution during the palaeozoic synorogenic movements along major thrusts synorogenic to postorogenic exhumation and a change in the regional stress field during the upper jurassic and lower cretaceous the palaeozoic sedimentary cover and the neoproterozoic basement of the ala tau anticlinorium never exceed the upper limit of the paz since the devonian a temperature gradient similar to the recent one 20 c km would account for the ft data reactivation of the neoproterozoic zilmerdak thrust was time equivalent to the onset of the devonian and carboniferous collision related deformation in the east west directed movement along the tashli thrust occurred in the lower permian the devonian and carboniferous exhumation path of the neoproterozoic siliciclastic units of the tirlyan synclinorium mirrors the onset of the uralian orogeny the emplacement of the tirlyan nappe and the continuous west directed compression the five main tectonic segments inzer synclinorium beloretzk terrane ala tau anticlinorium yamantau anticlinorium and zilair synclinorium were exhumed one after another to a stable position in the crust between 290 and 230 ma each segment has its own t t path but the exhumation rate was nearly the same final denudation of the western fold and thrust belt and exhumation to the present surface probably began in late tertiary in jurassic and cretaceous south directed movements along w e trending normal faults indicate a change in the tectonic regime in the southern uralides

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

the tectono metamorphic evolution of gneiss complexes in the middle urals russia a reappraisal abstract the middle urals are characterized by a major virgation in the linear trend of the urals orogen and represent the most highly contracted part of the late palaeozoic collisional belt this part of the orogen is dominated by metamorphic complexes and major fault and shear zones the main uralian fault zone muf the east dipping suture of the orogen containing low grade metamorphic rocks separates the sysert complex in the east from the ufaley complex in the west the sysert complex in the hanging wall of the muf consists of intensely deformed gneisses granitic intrusions and a metamorphosed melange zone tectonic and isotopic investigations suggest the following stages for the evolution of the sysert complex a pre orogenic rifting and magmatism during ordovician and silurian times b oceanic closure island arc formation related to convergence and subduction during devonian times c major ductile deformation under amphibolite facies conditions related to nw directed thrusting associated with crustal stacking during collision in carboniferous times d exhumation and contractional intracontinental tectonics during permian times and e closing of isotope systems related to cooling and the end of orogenic shortening through triassic times the ufaley complex in the footwall of the muf is interpreted as an east dipping crustal stack that records an amphibolite facies uralian metamorphism lithologically the complex can be divided into pre orogenic european basement west ufaley and intensely deformed palaeozoic metasediments and amphibolites east ufaley high pressure relics in the east ufaley complex are interpreted to be the result of subduction whereas intense ductile deformation is related to overthrusting onto west ufaley the west ufaley complex is composed of gneisses amphibolites migmatites and granitic intrusions and has been thrust onto devonian limestones along a major shear zone in both sysert and ufaley complexes nw trending stretching lineations and top to the nw kinematic indicators suggest an oblique plate convergence with a significant sinistral component the muf is interpreted as a major normal fault that developed congruent with continental subduction and that compensated lithospheric thickening and the rapid exhumation of subducted crust in the footwall

sedimentology geochemistry and palaeogeographic implications of volcanic rocks in the upper archaean campbell group western kaapvaal craton south africa abstract tuffs and lava interbedded in campbell group carbonates and shales are investigated for their sedimentological and geochemical properties the most proximal tuffs occur at the southwestern margin of the kaapvaal craton in griqualand west they were deposited in a shallow marine to tidal carbonate environment from hydroclastic eruptions the tuffs along this margin are subdivided into two main facies types 1 graded fallout lapilli to ash tuffs containing accretionary lapilli and 2 current reworked laminated and rippled lapilli to ash tuffs calculation of water depth from settling velocities of tuff particles indicates deposition in depth of a maximum of some 30 m the tuff beds generally thin out towards the north and northwest but distal tuff layers are widespread in the campbell group and allow for some lateral correlation within regionally narrow limits single zircon age dates suggest that correlation over the entire basin is unreliable two geochemical groups of tuffs are tentatively distinguished one group includes andesites and basalts and tends to concentrate in higher stratigraphic levels than the other which consists exclusively of basalts volcanism was of a tholeiitic within plate character situated in a rifted cratonic setting that failed to develop an oceanic basin geochemical similarity to some karoo volcanic rocks is striking a cratonic rift basin model for the griqualand west depository in which carbonate sedimentation in shallow environments prevailed is discussed the sedimentation of carbonates was periodically interrupted by hydroclastic eruptions along a rifting graben or half graben system

isotopic constraints on the late archean carbon cycle from the transvaal supergroup along the western margin of the kaapvaal craton south africa few existing studies illuminate the operation of the carbon cycle before the rise of atmospheric oxygen circa 2400 million years ago stable carbon isotopic measurements of shallow stromatolitic carbonates 0 vpdb and basinal carbonate minerals 6 in iron formation have been used to infer a strong isotopic depth gradient in archean ocean basins from new diamond drill cores obtained by the agouron drilling project from the griqualand west structural basin in the northern cape province south africa we present 13 c data from carbonates and organic matter that offer fresh insights into the late archean carbon cycle three drill cores cover the development progradation and ultimate demise by drowning of the campbellrand carbonate platform ca 2590 2500 ma one captures the platform top shallow marine and intertidal paleoenvironments the other two run through slope and basinal sections deposited adjacent to the platform margin increasing inwater depth likely to 1 km both shallowand deep water carbonates precipitated on the seafloor consistently show 13 c values around 0 5 incompatible with a strong late archean isotopic depth gradient a mathematical model suggests that these isotopic data are consistent with a reduced biological pump increased dissolved inorganic carbon in seawater due to higher atmospheric p co2 or both certain horizons do show distinct isotopic variability such areas are commonly shaly and they tend to be organic and or iron rich strong c isotopic variations occur on a cm scale and most likely stem from diagenetic remineralization of organic matter in sediment starved areas where iron formation developed siderite tends to be 13 c depleted sometimes by as much as 14 these observations suggest a carbon cycle in which iron respiration played a conspicuous role carbon isotope ratios from organic matter in shales are commonly 1 lighter than stratigraphically contiguous carbonates but there is no clear water depth trend in the organic carbon isotopic data taken as a whole the 13 c of organic matter can be explained by several non unique sets of processes including different autotrophic mechanisms of carbon fixation heterotrophic recycling including fermentation and methanotrophy and post depositional diagenesis the most striking feature is the occurrence of organic 13 c values 40 a feature that appears to be commonplace in late archean successions framed in the context of carbon cycle isotopic mass balance both organic and carbonate carbon isotopic data suggest that the proportion of carbon buried as organic matter was not radically different before the appearance of free environmental oxygen

impacts of the ancient maya on soils and soil erosion in the central maya lowlands abstract many studies across the central and southern maya lowlands of belize guatemala honduras and mexico have produced records of land degradation mostly sedimentation and soil erosion during the ancient maya period from before 1000 bc to the maya collapse of c ad 900 this paper provides new data from two sites blue creek and cancuen synthesizes more than a decade of the authors research in guatemala belize and mexico and synthesizes other findings from this region these research projects analyzed more than 100 excavations in upland and depression sites cored lakes and wetland sediments and studied sediments in the field and laboratory using radiocarbon dating a battery of soil chemistry tests stratigraphic analysis magnetic susceptibility elemental analyses and artifact identification our objective was to date when sedimentation and soil erosion occurred identify stable surfaces and correlate them with the state of knowledge about past land use these findings indicate three general epochs of accelerated soil erosion and identified two major paleosols the three waves of soil erosion occurred in the preclassic period c 1000 bc to ad 250 the late classic ad 550 to 900 and in the last several decades the major paleosol eklu um in these sites is a well developed mollisol or vertisol that started forming in the early holocene and was buried in either the preclassic or classic periods ad 250 to 900 at some sites the eklu um paleosol lies beneath sediments with a fainter paleosol which in turn lies buried below classic period and later sediments this picture shows higher than expected soil erosion linked to the region s first pioneer farmers in the preclassic and less than expected soil erosion in the late classic when population peaked and land use was the most intensive in other regions like cancuen guatemala however most soil erosion occurred during the maya late classic ad 550 830 erosion here was intense but short lived depressions record 1 3 m of aggradation in two centuries a third epoch of accelerated soil loss and aggradation arose with the rapid land use changes brought by new pioneers during the last several decades

the structural architecture of the footwall to the main uralian fault southern urals abstract the uralide orogen is a linear collisional belt formed during the upper paleozoic as a result of convergence between the east european craton and outboard terranes and accretion of intervening island arcs and obduction of oceanic crust onto the east european craton this paper presents a new structural subdivision of the southern urals to the west of the main suture zone that puts this part of the mountain belt in a context similar to that of other collisional orogenic belts this subdivision consists of a west vergent thrust stack that is made up of an autochthonous to parautochthonous foreland basin and a parautochthonous foreland thrust and fold belt flanked to the east by a metamorphic thrust stack the latter units are structurally overlain by allochthonous rocks of an accretionary complex slope sediments oceanic crust and high grade eclogite blueschist bearing gneiss where possible these units are defined on the basis of their internal stratigraphy and structure and on the nature of their bounding faults

peat records of human impacts on the atmosphere in northwest china during the late neolithic and bronze ages abstract man has played a major role in shaping the landscape in inland china during the second half of the holocene however little is known about the impact on atmospheric environments here we present a high resolution peat record of atmospheric variations in northwest china which covers the late neolithic and bronze ages ten heavy metallic elements pb zn fe mn cu co ni ti v and al were analyzed and the abundances of cu pb and zn were found to be significantly above their background levels during some periods while intensified human activities occurred in the adjacent gansu qinghai area modern meteorological observations and sedimentological analyses indicate that the element bearing aerosol was transported by the strengthened northeast wind four elements ti v co and ni exhibit coherent variations with pb prior to 5500 cal a b p and thus they were used to reconstruct the background level of pb the human induced pb signal correlates well with the population inferred from archaeological excavations in the liuwan tombs in qinghai province suggesting that human activities may have a great impact on the atmospheric environment in this area

sudden changes in north atlantic circulation during the last deglaciation sudden changes in the flow of warm atlantic surface waters into the norwegian sea occurred frequently during the last deglaciation typically involving shifts in sea surface temperature of 5 c in fewer than 40 years these led to equally large and rapid changes in atmospheric temperatures and to shifts in atlantic deep thermo haline circulation and ice sheet melting rates

lithofacies control on multiple sulfur isotope records and neoarchean sulfur cycles abstract triple sulfur isotope ratios 32 s 33 s 34 s were measured for 141 bulk rock samples from two agouron scientific drill cores gkp01 and gkf01 that recovered neoarchean successions of the transvaal supergroup south africa these two deep time cores are correlated to each other with 14 tie lines using volcanic and impact spherule layers in a sequence stratigraphic framework allowing us to evaluate both lithofacies and temporal controls over multiple sulfur isotope systematics the ca 2 5 ga giga annum before present basinal klein naute formation and the ca 2 6 ga peritidal boomplaas and vryburg formations yield an array of data characterized by 33 s 1 4 34 s these linear trends are found in both shallow water and deepwater facies but are characteristic to rocks with high iron content suggesting these may reflect isotopic compositions of aqueous sulfide elemental sulfur reservoirs in the neoarchean oceans data that deviate from this linear array are interpreted as resulting from additional inputs of sulfide from microbial sulfate reduction with or without contribution from sulfur disproportionation sulfate derived sulfur evolved to be either enriched or depleted in 34 s depending on the local depositional environment for example the reivilo formation in core gkf01 is characterized by abundant microbialite textures and shows an isotopic signature of closed system sulfate reduction rapid cementation of these carbonate fabrics may have attenuated the supply of sulfate to pore waters resulting in the progressive 34 s enrichments during bacterial sulfate reduction below the sediment water interface in contrast signatures of open system sulfate reduction are associated with slope facies dominated by granular dolostones preserved in the upper nauga formations in gkf01 and gkp01 the two different sulfur isotope patterns interpreted to reflect closed versus open system sulfate reduction are both found in the lower reivilo formation in both cores this lateral variation of isotope signals documents that observed 34 s shifts can be controlled locally and may not have temporal significance this study demonstrates critical importance of recovering sulfur isotope data from stratigraphically correlated drill cores to evaluate both geographical and temporal shifts of sulfur cycles and their links to the great oxidation event at the end of the archean eon

holocene east asian monsoonal precipitation pattern revealed by grain size distribution of core sediments of daihai lake in inner mongolia of north central china abstract two sediment cores recovered in the central part of daihai lake in north central china were analysed at 2 to 4 cm intervals for grain size distribution grain size distributions of the lake sediments are inferred to be a proxy for past changes in east asian monsoon precipitation such that greater silt size percentage and higher median grain size reflect increased monsoonal precipitation rates the grain size record of daihai lake sediments spanning the last ca 10 000 yr indicates that the monsoonal precipitation in the lake region can be divided into three stages the early middle and late holocene during the early holocene before ca 7900 cal yr bp the median grain size md and the silt fraction content were relatively low and constant suggesting relatively low precipitation over the lake region the middle holocene between ca 7900 and 3100 cal yr bp was marked by intensified and highly variable monsoonal precipitation as indicated by high and variable md values and silt contents of the lake sediments during this period average precipitation rate gradually increased from ca 7900 to 6900 cal yr bp displayed intense oscillations between ca 6900 and 4400 cal yr bp and exhibited a decreasing trend while fluctuating from ca 4400 to 3100 cal yr bp although generally high during the middle holocene both the md and the silt content assumed distinctly low values at the short intervals of ca 6500 6400 6000 5900 5700 5600 4400 4200 cal yr bp implying that monsoonal precipitation might have been significantly reduced during these intervals during the late holocene since ca 3100 cal yr bp grain size values suggest that precipitation decreased however during the late holocene relatively higher md values and silt contents occurring between ca 1700 to 1000 cal yr bp may denote an intensification of hydrological cycles in the lake area changes in the east asian monsoonal precipitation were not only directly linked with the changing seasonality of solar insolation resulting from progressive changes in the earth s orbital parameters but also may have been closely related to variations in the temperature and size of the western pacific warm pool in the intensity of the el nino southern oscillation and in the path and strength of the north equatorial current in the western pacific

controls on the east asian monsoon during the last glacial cycle based on comparison between hulu cave and polar ice core records previous studies have suggested a sound chronological correlation between the hulu cave record east asian monsoon and greenland ice core records which implies a dominant control of northern hemisphere climate processes on monsoon intensity we present an objective straightforward statistical evaluation that challenges this generally accepted paradigm for sub orbital variability we propose a more flexible global interpretation which takes into account a broad range of variability in the signal structures in the hulu cave and polar ice core records rather than a limited number of major transitions our analysis employs the layer counted greenland ice core chronology 2005 gicc05 which was developed for greenland records and has since been applied via methane synchronisation to the high resolution 18oice series from epica dronning maud land edml the gicc05 chronology allows these ice core records to be compared to the u th dated hulu cave record within relatively narrow 3 bounds of age uncertainty following previous suggestions our proposed interpretation suggests that the east asian monsoon is influenced by a combination of northern hemisphere pull which is more intense during boreal warm periods and southern hemisphere push which is more intense monsoon during austral cold periods our analysis strongly suggests a dominant control on millennial scale monsoon variability by southern hemisphere climate changes during glacial times when the monsoon is weak overall and control by northern hemisphere climate changes during deglacial and interglacial times when the monsoon is strong the deduced temporally variable relationship with southern hemisphere climate records offers a statistically more plausible reason for the apparent coincidence of major east asian monsoon transitions with northern hemisphere dansgaard oeschger do climate events during glacial times than the traditional a priori interpretation of strict northern hemisphere control

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

timing and spatial distribution of mid holocene drying over northern china response to a southeastward retreat of the east asian monsoon 1 to determine the timing and spatial distribution of mid holocene drying over northern china the mineralogical and oxygen isotopic composition of authigenic carbonate from a closed lake at bayanchagan southern inner mongolia were measured further analysis and synthesis of the spatial geological data were performed results from lake bayanchagan show a significant drying at 6000 calendar years cal years b p indicated by dolomite precipitation and a striking rise in 18o values the synthesis of spatial data reveals a zonal distribution for timing of drying over northern china in the mid holocene which began at 9000 7000 cal years b p in deserts of north central china at 7000 5500 cal years b p drying extended into the desert steppe transitional zone and at 4500 cal years b p into northeastern and south central china this pattern indicates that the east asian summer monsoon has significantly retreated southeastward since the mid holocene which may be related to orbitally induced northern hemisphere insolation changes a retreat of 400 550 km is inferred for the front of the summer monsoon from 6500 to 4500 cal years b p

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

holocene changes in eastern tropical pacific climate inferred from a galapagos lake sediment record abstract paleoclimate records from the tropical pacific suggest the early to mid holocene was a period of reduced el nino southern oscillation enso variability with a transition to modern increased enso frequency occurring some time in the last few thousand years however the nature and timing of this shift remains uncertain due to the discontinuous nature and or coarse resolution of many enso proxies as well as a lack of agreement between previously published records a new continuous climate record from el junco crater lake in the galapagos islands reveals several abrupt changes in lake level and precipitation through the holocene hydroclimatic model simulations suggest that el junco lake level responds sensitively to increases in precipitation associated with el nino events rising during wet el nino events and falling during the intervening dry periods grain size data from el junco sediment cores indicate past lake level variability likely associated with changing seasonal precipitation and enso frequency the grain size data suggest increased precipitation intensity prior to 9000 120 cal years bp and after 4200 130 cal years bp as well as a two step increase in precipitation at 3200 160 and 2000 100 cal years bp maximum holocene precipitation and inferred enso variability occurred between 2000 100 and 1500 70 cal years bp during the same period that six other independent proxy records suggest higher enso frequency and longer stronger el nino events decreasing sediment carbon nitrogen c n ratios in el junco sediments indicate rising lake levels from the early holocene to present corroborating the grain size data the inferred increase in precipitation at 4200 130 cal years bp and at 2000 100 cal years bp coincides with decreasing southwest asian and east asian monsoon intensity suggesting tropical pacific climate and the asian monsoon were interconnected systems at centennial to millennial timescales during the holocene a weakening trend in the asian monsoon and the trend toward wetter conditions at el junco also coincide with a trend toward cooler and drier conditions inferred from cariaco basin sediment proxies from the mid holocene to present suggesting the migration of the intertropical convergence zone itcz likely influenced hydrological changes in both the eastern tropical pacific and the asian monsoon region during the holocene

osl chronology of the liena archeological site in the yarlung tsangpo valley throws new light on human occupation of the tibetan plateau recent environmental archeological evidence has started to throw light on both the timing and processes of human colonization of the tibetan plateau tp yarlung tsangpo yt valley a very import

simulation of holocene cooling events in a coupled climate model three potential mechanisms behind centennial scale holocene cooling events are studied in simulations performed with the coupled climate model ecbilt clio 1 internal variability 2 solar forcing and 3 freshwater forcing in experiments with constant preindustrial forcings three centennial scale cooling events occur spontaneously in 15 000 years these rare events represent an unstable internal mode of variability that is characterised by a weaker thermohaline circulation a more southward location of the main site of deep water formation expanded sea ice cover and cooling of 101c over the nordic seas this mode is visited more frequently when the climate is cooled by abruptly reducing the solar constant by 5 or 3 wm 2 prescribing a solar forcing of the same magnitude but following a sinusoidal function with a period of 100 or 1000 years does not result in any centennial scale cooling events the latter forcing does however result in more frequent individual cold years in the north atlantic region that are related to local weakening of the deep convection and sea ice expansion adding realistic freshwater pulses to the labrador sea is also able to trigger centennial scale cooling events with temperature anomalies resembling proxy evidence for the cooling event at 8 2 kyr bp suggesting that freshwater forcing is a valid explanation for early holocene cooling events r 2007 elsevier ltd all rights reserved

lithosphere scale seismic image of the southern urals from explosion source reflection profiling explosive source deep seismic reflection data from the southern ural mountains of central russia provided a lithosphere scale image of the central eurasian plate that reveals deep reflections 35 to 45 seconds in travel time 130 to 170 kilometers deep from the mantle the data display laterally variable reflectivity at the base of the crust that deepens beneath the central part of the profile documenting a crustal thickness of 55 to 60 kilometers beneath the axis of the orogen these data provide an image of the structure of the crust and underlying mantle lithosphere in a preserved collisional orogen perhaps to the base of the lithosphere

contrasting tectonic history of the arc continent suture in the southern and middle urals implications for the evolution of the orogen the main uralian fault has been considered the original arc continent suture for 2000 km along the uralide orogen the symmetry of the tectonic units across it suggested a consistent east dipping polarity for the palaeosubduction zone which together with its topographic and aeromagnetic signature supported the idea of a single suture however several characteristics vary at different latitudes in the middle urals it is a strike slip fault zone with moderately deformed and metamorphosed volcanic arc fragments in its hanging wall and low grade metamorphic rocks of the east european craton in its footwall here it has a prominent nnw trending magnetic signature which cross cuts north trending anomalies in its hanging wall and a pronounced reflection seismic signature that can be traced to the top of the middle crust at c 5 s twt in the southern urals it is a serpentinite melange zone of ambiguous kinematics with a weakly deformed and metamorphosed volcanic arc in its hanging wall and moderately metamorphosed to high pressure rocks of the east european craton in its footwall in this part of the orogen it has a weak reflection seismic character and a magnetic signature that parallels that of its hanging wall on the basis of an integrated analysis of these different data sets we suggest that the main uralian fault as it is currently defined is not a single entity but rather the original arc continent suture in the south and the western strand of a strike slip fault system that reworked the original suture in the middle urals

improved methods for isolating and validating indigenous biomarkers in precambrian rocks hydrocarbon biomarkers in highly mature precambrian rocks have the potential to provide important information about the diversity ecology and evolution of early life but studying them presents special analytical challenges extractable hydrocarbons are present in archean and most paleoproterozoic sedimentary rocks in such trace concentration that even slight contamination from petroleum derived materials in situ or during drilling storage sampling handling and laboratory analysis would compromise the results and thereby any consequent inferences here we report protocols that we have developed for the analysis of cores from several recently completed deep time scientific drilling initiatives by paying special attention to cutting cleaning crushing and extraction it is possible to significantly reduce laboratory blanks to acceptable levels when these methods are utilized meaningful variations in the patterns of biomarkers over stratigraphic and lithologic boundaries provide compelling evidence for syngeneity

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

preserved collisional crustal structure of the southern urals revealed by vibroseis profiling the vibroseis reflection profiling component of the urseis 95 experiment provides a high resolution crustal scale image of the unextended southern uralide orogen a marked lateral and vertical variation of reflectivity throughout the entire crust differentiates the former margin of the east european craton to the west from accreted terranes to the east between these regions is a less reflective zone corresponding to the magnitogorsk volcanic arc and the crustal root zone continuous reflections and reflective domains that correlate at the surface with major tectonic features define a bivergent orogen in which the paleozoic collisional structure has been largely preserved

mudrock sequence stratigraphy a multi proxy sedimentological paleobiological and geochemical approach devonian appalachian basin abstract delineation of stratigraphic sequences and their component systems tracts in mudrock dominated facies is generally difficult due to the relatively homogenous fine grained nature of the strata in this study we apply a multi proxy analytical approach to a thick devonian mudrock dominated succession through detailed analysis of sedimentologic paleobiologic and geochemical data through 600 m of mudrock rich facies varied combinations of proxies prove to be most useful in delineating sequence development in anoxic dysoxic and oxic dominated mudrock settings and in mixed mudrock carbonate and mixed mudrock sandstone successions these interpretations are tested against an established sequence stratigraphic framework for 11 middle to upper devonian mid eifelian to lower famennian sequences in the appalachian basin the sequences presented here further detail and refine global devonian t r cycles id to iie of the well known johnson sandberg and klapper sea level curve the usefulness of proxies in delineating depositional sequences and systems tracts varies dependent on depositional paleoceanographic paleoecologic and early diagenetic conditions those proxies that show a range of variations in specific settings such as grain size degree of bioturbation and concentrations of toc and elements elemental ratios e g caco3 al ti mg sc si mo ni v si al and ti al may help delineate depositional dynamics related to redox conditions condensation dilution and clastic biologic and or authigenic sediment sources in fine grained anoxic dominated facies interpreted to represent basinal settings sequences and systems tracts are best delineated by anoxic related proxies toc and mo in intermediate dysoxic dominated settings toc mo bioturbation and al remain effective indicators of sequence development in relatively oxygenated mudrock rich and carbonate poor sequences bioturbation may function as the most effective proxy for recognizing systems tracts for mixed fine grained siliciclastic carbonate successions concentration and type of caco3 e g benthic macroskeletal pelagic styliolinid dacryoconarid and micritic calcisilt are useful in identifying position within cycles in more proximal carbonate poor successions fine and coarse grained fractions become increasingly differentiated these can be distinguished by relatively high si al ratios si al ca 5 elemental ratios indicative of coarser clastic input e g si al zr al and ti al are applicable to identifying position with a sequence but they may also be affected by input from eolian volcanogenic or biogenic sources in addition fluxes of siliciclastic carbonate and toc sediment types may dilute the concentration of the others multiple lines of evidence should be examined in interpreting relative depth and position within a sequence