spatial arrangement of soil mantle in glacis de buenavista mexico as a product and record of landscape evolution abstract factors controlling spatial soil organization have recently attracted the attention of soil scientists because they can contribute to regional management of soil resources we present here a case study in the glacis de buenavista mexico an extended volcanic piedmont in order to develop an integral understanding of its soil cover as a product and at the same time as a record of the main stages of landscape evolution three phenomena were considered luvisol and vertisol type soils and pedosediments all of them analyzed in terms of their development degree clay content macro and micromorphology selective extractions of fe fe d fe o silicon si o and aluminum al o weathering index and clay mineralogy luvisols represented in the ahuatenco mexicapa and buenavista sections are located in the northern and central part of the area they are red clayey polygenetic soils that show strong weathering with high kaolinitic clay content and high weathering index values clay illuviation and reductomorphic processes combined with vertic features in some parts of the profiles vertisols dominate the central and southern portion of the glacis they exhibit cracking angular blocks with wedge like shapes slickensides and stress cutans the pedogenesis of the vertisol type is clearly associated with the presence of smectitic minerals in the clay fraction as a product of neoformation factors controlling this pedodiversity are relief which affects bioclimatic differentiation within the studied landsurface and defines the lateral redistribution of moisture and dissolved substances and time luvisols represent a longer pedogenetic phase that started in the late pleistocene according to the age of buried paleosols while vertisols mainly originated in the late holocene according to the age of buried organic horizons pedosediments are located in the central area of the glacis here past and present geomorphic processes interact to produce the greatest soil diversity the recent human induced erosion partly destroyed polycyclic luvisols and exposed ancient subsurface pedosedimentary strata at the same time it produced patches of unconsolidated pedosediments consisting mostly of redeposited luvisol materials

paleosol sedimentary sequences of the glacis de buenavista central mexico interaction of late quaternary pedogenesis and volcanic sedimentation abstract two sections buenavista and ahuatenco of late quaternary paleosol sedimentary sequences of the glacis de buenavista morelos central mexico were studied and interpreted as a regional record of climatic change and interaction of pedogenesis and volcanic sedimentation buenavista is the deepest section and includes seven pedostratigraphic units with andosol surface unit and luvisol all underlying units types of pedogenesis both types indicate a humid forest environment the divergency being related to differences in pedogenesis duration albeluvisol with a bleached e horizon in unit 5 indicates cooler but still moist conditions in the final late pleistocene whereas strong vertic properties in unit 4 indicate a climate with contrasting seasonal variation of precipitation during the first part of the holocene in the upper part of the ahuatenco section bt horizons of different pedogenetic events form a welded luvisol profile instead of a set of separate paleosols due to less intensive sedimentation material of indurated cm horizons tepetates in the lower part of the ahuatenco section is a mixture of fresh coarse volcanoclastic components and re deposited luvisol clayey material formed by mass movement associated with volcanic events the resulting granulometric composition of these layers is favourable for structural collapse and hydroconsolidation which caused hardening together with subsequent pedogenic illuviation and redoximorphic processes

provenance of meso to neoproterozoic cover sediments at the ming tombs beijing north china craton an integrated study of u pb dating and hf isotopic measurement of detrital zircons and whole rock geochemistry abstract the north china craton ncc was subjected to an extensional regime after the luliang movement at 1 8 ga and then was covered by an extensive meso to neoproterozoic sedimentary succession namely the changcheng jixian and qingbaikou groups in ascending order we report age spectra for detrital zircons and monazites hf isotopic systematics of detrital zircons and whole rock chemical and nd isotopic compositions for sediments from the succession in the ming tombs area beijing one of the typical meso to neoproterozoic areas in the ncc detrital zircons of six sedimentary samples have two distinct age peaks at 2 52 ga and 1 85 ga there are some detrital zircons at 2 4 2 0 ga but none at 2 3 ga and only a few 2 7 ga the detrital zircon age spectra change with time sediments in the lower succession changcheng group and in the upper successions jixian and qinbaikou groups are dominated by significant detrital zircon populations of late neoarchean and late paleoproterozoic ages respectively the 2 5 ga detrital zircons of the changcheng group have ehf 2 5 ga values and tdm hf model ages mainly ranging from 2 to 7 and 2 8 to 2 7 ga respectively detrital monazites of a sample from the jixian group exhibit a major age peak between 1 95 and 1 80 ga with some data between 2 0 and 1 95 ga the sedimentary rocks of the changcheng group are characterized by high k2o contents mostly 7 09 15 20 and insignificant eu anomalies eu eu 0 71 1 16 they have tdm nd model ages ranging from 2 70 to 2 43 ga being older than the tdm nd ages 2 11 and 1 99 ga of sedimentary samples from the qingbaikou group based on a comparison with ages for the early precambrian 1 8 ga basement of the ncc it can be concluded that 1 the sediments of the meso to neoproterozoic cover were undoubtedly derived from the ncc itself or once neighboring terranes 2 variations in the detrital zircon age spectra from the lower to the upper successions reflect provenance evolution in that the lower crustal late paleoproterozoic rocks were exposed at the surface after the upper crustal late neoarchean rocks had already been eroded

a 1 78 ga large igneous province in the north china craton the xiong er volcanic province and the north china dyke swarm abstract the 1 78 ga xiong er volcanic province xvp and coeval north china giant mafic dyke swarm ncds are the most important magmatic events occurring after the amalgamation of the north china craton ncc the xvp consists of 3 7 km of extrusive volcanics and some feeder dykes sills located along the southern margin of the ncc and extending over an area 0 06 m km2 compositions vary from basalt to rhyolite but are predominantly intermediate in terms of silica content there are also minor sedimentary intercalations and pyroclastic units the sedimentary interlayers indicate an environment changing from continental facies to oceanic facies up section the xvp is characterized by fractional crystallization from an em i type mantle source and both continental arc andean type and rift environments have been proposed the ncds is widespread in the central ncc with an outcrop area 0 1 m km2 and are exposed at variable depths up to 20 km deepest in the north dyke compositions vary from basalt to andesite and dacite but are dominantly mafic and comprise two series of magmatism previous studies revealed that the ncds recorded assimilation and fractional crystallization of an em i type magma source with a minor dm contribution in the younger magmas both syn collisional and intra continental anorogenic environments have been proposed spatial and petrogenic correlations suggest a cogenetic relationship between the ncds and xvp and considered together they define a large igneous province lip of 0 1 m km2 in area and 0 1 m km3 in volume which is also notable for its continuous compositional range from mafic to felsic with no gap at intermediate compositions the petrology is explained by a common magma source that undergoes a silica poor and iron enriched fractionation trend at depth followed by a silica rich and iron poor fractionation trend in shallow level magma conduits dykes and surface lavas a mantle plume is favored as the cause of this 1 78 ga north china lip

interpretation of a few ice event transients this article discusses some of the observables important to the study of sound radiated into the water by ice fracturing events such studies are important in order to understand the mechanisms of arctic ocean ambient noise generation theoretical work for a homogeneous ice plate indicates that there are three important paths of elastic wave propagation over which energy travels away from an ice event source these are the flexural wave in the ice the longitudinal wave in the ice and the acoustic wave in the water each has a distinctive propagation speed attenuation and associated pressure field in the water the experimental work consisting of the interpretation of four ice events supports the theoretical description in each case the event is located and the contributing wave paths identified in all cases the acoustic wave dominates the work also demonstrates how event analysis can be used to determine ice properties

early mid pleistocene environments in the valsequillo basin central mexico a reassessment the valsequillo basin in central mexico has been of interest due to the presence of megafauna and evidence for early human occupation but research has been controversial it has been suggested that extensive and deep lakes characterised the early pleistocene environment but sediment exposure is highly fragmentary and reliable dating has been difficult here we report for the first time early pleistocene palaeoenvironmental reconstructions using stable isotopes diatoms tephra and pollen we studied several stratigraphic sections of mainly non volcanic rocks containing the 1 3 ma xalnene ash as a stratigraphic marker the isotope and other proxy data show that topographically low points in the basin were occupied by spring fed shallow water lakes during the early mid pleistocene with a trend to drier conditions the basin was a dynamic volcaniclastic environment during this period with the production of the toluquilla volcano sequence and other rhyolitic dacitic volcanic ashes interbedded with the lake sediments at the sections studied there is no evidence from the sections for extensive and deep lakes before or after the xalnene ash deposition the presence of lakes in the basin during the early pleistocene would have made it attractive for megafauna

inversion of pack ice elastic wave data to obtain ice physical properties geophones mounted on floating ice sheets can potentially provide data for a robust method to monitor mechanical properties of the ice the technique relies on the measurement and inversion of low frequency elastic waves propagating in the ice to test this hypothesis several geophone systems were deployed on the arctic ice as part of the office of naval research sea ice mechanics initiative simi including a winter over system of 20 triaxial geophones at the fall 1993 simi camp an inversion technique is discussed here through references to the literature and by analysis of geophone data specifically those data collected on clean first year ice off resolute bay canada and on highly irregular multiyear ice found at the fall simi camp for undeformed first year ice the inversion technique gave an estimate of ice properties that agreed well with known values although extensive work remains to determine the effects of anisotropy and inhomogeneity the geophone data for the nonuniform ice at the simi camp were very complex and difficult to analyze the inversion yielded only trends in what can at best be termed effective ice thickness and bending rigidity it is uncertain how these relate to actual ice properties the simi geophone system demonstrated the feasibility of performing autonomous measurements of the characteristics of propagating waves in the arctic ice with continued study it seems evident that such systems can be used in clean first year ice to monitor ice properties however extensive research is required to make this technique useful in multiyear or complex first year ice

ecosystem responses to climate and disturbances in western central mexico during the late pleistocene and holocene abstract central mexico is a complex area with late quaternary volcanic activity climatic diversity and a long history of human occupation pollen microcharcoal and magnetic susceptibility analyses of two sedimentary sequences from lake zirahuen in western central mexico spanning the last 17 000 years provide evidence of a highly dynamic environment during the late glacial pinus forest developed around the lake indicating cold and dry conditions during the last deglaciation climatic amelioration was recorded at 13 5 ka driving a strong and rapid change in vegetation composition and increase in lake levels from the latest pleistocene to early holocene a hiatus probably related to an erosive event is recognized in the northern sequence the central sequence spans the entire holocene and reveals three periods of important ecological changes during the early holocene a first episode between 9 5 and 9 0 ka with a decrease in pine forest seems to have been associated with summer insolation increases a second peak of forest change at 8 2 ka and was probably associated with the cold oscillation documented in the north atlantic a third abrupt change was evident from 7 5 to 7 1 ka with an anomalous plant community related to wetter holocene climates and possible to a volcanic event an episode of dry conditions was recorded from 4 5 to 4 2 ka which was related to an increase in enso activity human influence over the landscape was evident from 3 5 ka to the present the zirahuen records offer a complex history of a landscape characterized by short and long term vegetation changes associated with factors ranging from global climate to local disturbances

riphean basins of the central and western siberian platform abstract the siberian platform is unique by its volume of meso neoproterozoic sedimentary deposits for about one billion years 1650 650 ma several sedimentary basins were developed here resulting in the formation of several kilometers thickness of sedimentary cover the riphean mesoproterozoic lower neoproterozoic rocks are exposed mainly along platform peripheries the most complete sections are represented by several megacycles each megacycle contains terrigenous series at the base and carbonate formations in the upper part several isolated and anisochronous basins were created during the riphean on the territory of east siberia some of them were intracratonic others were developed on passive margins neoproterozoic orogeny along the platform boundaries resulted in re organization of the siberian basins with extensive faulting uplifting and erosion of the territories in eastern siberia riphean series contain large hydrocarbon accumulations the reservoirs were formed mainly due to fracturing and leaching of carbonate strata e g vugular carbonates of the pre vendian weathering crust the upper proterozoic deposits are overlain by thick clayey carbonate and saliferous carbonate series of the upper vendian and cambrian isolating them from the upper sedimentary cover the riphean basins contained thick organic rich clayey and clayey carbonate in some of them a hydrocarbon generation maximum took place at the end of the riphean the pre vendian erosion has removed a significant volume of riphean sediments during this time a majority of already formed hydrocarbon accumulations have been lost or degraded remaining riphean series have generated hydrocarbons during the paleozoic despite its complex history the riphean is still considered highly prospective with source rocks developing at multiple levels and reservoirs occurring in both carbonate and clastic rocks discoveries of new oil and gas fields in east siberia are likely but will depend on integration of detailed seismic data and a large volume of core data for the correct prognosis of riphean reservoir distribution

late pleistocene holocene environmental changes in ultra continental subarid permafrost affected landscapes of the terekhol basin south siberia abstract this study is an attempt to use closely related inter complementary paleo archives of a local landscape to access understanding of late pleistocene holocene environmental changes in the region the study site is a small intermountain basin in the sayan tuva upland 51 n 97 e 1300 m a s l paleo archives covering about 13 000 yrs were studied paleosol sedimentary sequences on a delta alluvial fan of a small river lacustrine sediments in bottom cores and on palsa islands and soils of palsa islands the following sequence of environmental changes was established the fluvial activity in the basin reached its maximum at the end of the late pleistocene the sharp decrease of the fluvial activity is terminated by two successive paleosols of pleistocene holocene transition the older paleosol indicates meadow steppe or tundra steppe conditions with a shallow permafrost table and impeded drainage the younger paleosol testifies on sharp aridization biological activity suppression contrasting water regime and warming dammed lake appeared in the midst of the trough about 11 000 cal yr bp sedimentation on delta alluvial fan was fairly inconsiderable in holocene the first part of holocene before 4000 cal yr bp was most balanced in annual distribution of precipitation runoff even being prominently enhanced in a fluvial and relatively warm sub phase 8000 6250 cal yr bp was canalized without giving seasonal floods sub phase 6250 3800 is characterized by increased continentality and relative aridization caused reduction of runoff lowering of the lake level and enhancement of cryogenic processes the next phase 3800 2000 cal yr bp was more humid it caused slight revival of fluvial processes and rise of lake level the last 2000 years climate was more continental and the most arid within the studied period the lake level and runoff values dropped again and fluvial activity totally decreased in contemporary soils aridization is reflected in widely spread natric features and progressive salinization

relating arctic ambient noise to thermally induced fracturing of the ice pack thermally induced stresses within pack ice floes are simulated using a viscoelastic model that accounts for the time variation of the atmospheric forcing parameters and the temporal and spatial variations of snow cover to relate the model predicted stresses to the observed 500 hz under ice noise variations a fracturing paradigm was developed the fracturing paradigm includes factors related to ice tensile strength given its salinity and temperature and stress relief as a result of fracturing a fracture count time history was produced using the thermal stress model and the fracturing paradigm for the fall of 1988 in the eastern arctic ocean the results were compared with the observed 500 hz noise the fracture count shows increases for 11 of 13 under ice noise episodes most of the observed under ice noise episodes were of the type identified in previous studies i e those associated with nightly cooling in addition this study identifies under ice noise episodes related to longer term occurrences a

on small scale horizontal variations of salinity in first year sea ice measurements of salinity over horizontal distances of 38 to 76 cm in a thick first year ice sheet have revealed significant differences a maximum salinity difference of 2 was observed between ice core segments from the same depth the mean standard deviation for 10 cm thickness increments through the 2 0 m ice sheet was 0 39 between the five closely spaced cores the most likely mechanism for these significant differences in salinity over short distances is differential brine drainage in the ice sheet due to varying locations of brine drainage channels a simple one dimensional model which assumes a normally distributed arrangement of brine drainage channels provides results consistent with the horizontal differences observed

the role of pedogenesis in palaeosols of mexico basin and its implication in the paleoenvironmental reconstruction abstract most of the paleoenvironmental information for the basin of mexico comes from sedimentary proxies which unfortunately are incomplete for the terminal pleistocene and the holocene in this paper we present a temporal and spatial reconstruction of past soil cover in the south and north of the basin of mexico during the mis 2 and 1 marine isotope stage 2 and 1 palaeosol profiles were selected from three locations cuicuilco copilco and avon in the south basin and compared with soils studied in the teotihuacan valley pachuca and texcoco at north of the basin the results show that the pleistocene soil cover is more homogeneous dominated by the presence of luvisol type soils with secondary carbonates dated to around 20 000 cal yr bp during the holocene the soil distribution changes with andosols developing in the south while vertisols and calcisols form in the north the new soil distribution reflects differing climate conditions during the holocene conditions which characterize the basin today the andosol cover in south basin however represents the result of short term pedogenesis andosols are considered high quality soils that were attractive to settlers and were a factor in the development of sedentary villages

a reevaluation of cosmogenic 36cl production rates in terrestrial rocks we have measured 36cl in a suite of 33 rock samples having well constrained exposure histories and ages the 36cl production parameters were estimated by minimizing the squared deviations between the 36cl and independent ages yielding the following production parameters spallation and muon production from ca 2940 200 atoms 36cl mole ca 1 yr 1 spallation from k 6020 400 atoms 36cl mole k 1 yr 1 and neutron production in air 586 40 fast neutrons g air 1 yr 1 the new production constants for spallation on ca and thermal neutron activation are in good agreement with previous results but that for spallation on k is about 50 larger the past decade has seen a rapid growth in the application of terrestrial cosmogenic nuclides produced by cosmic ray particles within rocks at the earth surface nuclear cross sections particularly for reactions initiated by neutrons and muons are not known with sufficient accuracy so production rates for these nuclides must be empirically calibrated using independently dated geological samples direct production estimates for 36cl were first obtained by zreda et al 1991 who measured 36c1 in mineral separates from granitic boulders on moraine crests at chiatovitch creek in the eastern white mountains of california and nevada this data set was the best available at the time but since then several factors have prompted a reevaluation of the 36cl production rates one is that zreda et al 1994 have shown that there can be significant scatter of the cosmogenic ages of individual glacial boulders toward ages younger than that of the moraine due to soil erosion a second factor is that a much wider range of samples is now available for this purpose finally recent theoretical advances in calculating production by thermal neutron absorption liu et al 1994 now permit more realistic calibration of this reaction

geochemistry geochronology and sr nd hf isotopes of two types of early cretaceous granite porphyry dykes in the sulu orogenic belt eastern china late mesozoic granitic rocks are widely distributed in the sulu orogenic belt but the source tectonic affinity and associated geodynamic setting that produced the respective magmas remain contro

thermomechanics of pack ice a thermodynamic formulation for pack ice is presented that includes a means of relating stresses in floes to thermally induced strain rates the formulation combines some basic thermodynamics as detailed in previous research the mechanics component includes viscous creep and mechanically induced strain rates due to typical floes being too large to bend or twist in response to spatially varying strain rates moreover the rheology quantifies the impact of existing cracks in the floe on the overall strain rate in the floe and the resulting stress state a paradigm is proposed which specifies tensile fracturing and the resulting stress relief as proportional to the amount of existing cracks in the floe and some measure of the tensile strength of the ice this allows the thermomechanical equations to be used for both first year and multiyear floes by differentiating the two by the extent of existing cracks the thermomechanical equations are applied in an inverse fashion to solve for flaw structure for a first year floe as well as a multiyear floe this is done by matching stresses predicted by the thermomechanical equations to observed stresses in the floes the results indicate that the top two thirds of the ice in the first year floe was relatively free of cracks while the bottom third of the floe had cracks covering 50 of the ice in the multiyear floe cracks covered 70 to 90 of the ice in the top two thirds of the floe while the bottom third of the floe had cracks covering 30 of the ice

late pleistocene and holocene paleoenvironments of chalco lake central mexico abstract in order to establish paleoenvironmental conditions during the late quaternary four cores from the basin of mexico central mexico were drilled in chalco lake located in the southeastern part of the basin the upper 8 m of two parallel cores were studied using paleomagnetic loss on ignition pollen and diatom analyses based on 11 14c ages the analyzed record spans the last 19 000 14c yr b p volcanic activity has affected microfossil abundances both directly and indirectly resulting in absence or reduction of pollen and diatom assemblages important volcanic activity took place between 19 000 and 15 000 yr b p when the lake was a shallow alkaline marsh and an increase of grassland pollen suggests a dry cold climate during this interval abrupt environmental changes with increasing moisture occurred from 15 000 until 12 500 yr b p the lake level increased and the pollen indicates wetter conditions the highest lake level is registered from 12 500 to ca 9000 yr b p the end of the pleistocene is characterized by an increase in humidity from 9000 until ca 3000 yr b p chalco lake was a saline marsh and the pollen record indicates warmer conditions after 3000 yr b p the lake level increased and human disturbance dominates the lacustrine record

shrimp u pb zircon geochronology of the fuping complex implications for formation and assembly of the north china craton abstract the fuping complex located within the central zone of the north china craton is composed of amphibolite to granulite facies orthogneisses interleaved with minor supracrustal rocks at similar metamorphic grade the oldest components recognised are hornblende gneiss enclaves within the predominant biotite orthogneiss which have a shrimp u pb zircon age of 2708 8 ma we consider these enclaves to represent fragments of 2 7 ga continental materials incorporated in the biotite gneiss the biotite gneiss has a shrimp u pb zircon age of 2513 12 ma interpreted to be time of magmatic crystallisation of the igneous precursor based on the igneous characteristics of the zircons this indicates a major magmatic episode at 2 52 ga in the fuping complex identical to the age of felsic volcanism within the low grade wutai complex which crops out immediately to the west a gneissic granite that intrudes the biotite gneisses has a poorly defined 207pb 206pb age of 2045 64 ma this is within error of the age of 2097 46 ma obtained from a fine grained gneiss interlayered with amphibolite of the wanzi supracrustal suite wss interpreted to be volcanic in origin zircons from both these samples have strong oscillatory zoning and provide the first indication of a palaeoproterozoic magmatic event in the area again similar in age to magmatic events recently recognised in the adjacent wutai complex these data indicate a comparable history for the fuping and wutai complexes and support geochemical evidence that they had a common origin and formed part of a late archaean arc affected by later palaeoproterozoic re activation low uranium zircons without oscillatory zoning separated from a sample of biotite gneiss yield data clustered at 1817 26 ma which is interpreted to reflect a period of new zircon growth during a major metamorphic event these data support the recently proposed tectonic model that amalgamation of the north china craton occurred due to collision of the eastern and western blocks along the central zone at 1 8 ga ago

sta cruz atizapan a 22 ka lake level record and climatic implications for the late holocene human occupation in the upper lerma basin central mexico abstract the upper lerma is a high altitude basin with three water bodies linked by the lerma river this basin has a long archaeological history characterised by the establishment of settlements within the lacustrine ecosystem itself man made islands during the late classic to epiclassic ad 550 900 which were abandoned by the end of the epiclassic the upper lerma is an ideal site to study climatic and environmental conditions during the period of human occupation as well as during the last full glacial interglacial cycle two sediment cores stcrz 9 54 m and almoloya del rio 5 12 m were recovered from the highest lake in the system chignahuapan ten radiocarbon dates provide chronologies for these sequences in which the tres cruces tephra c 8500 yr bp and the upper toluca pumice c 11 600 yr bp serve as stratigraphic markers magnetic properties loss on ignition and diatom analyses were used to infer lake level fluctuations during the last c 22 000 yr bp the late pleistocene environment was characterised by a freshwater lake high sediment input and variable lake levels are recorded during the last glacial maximum c 19 000 16 000 yr bp while slightly higher water levels and reduced sediment input are recorded during the late glacial c 16 000 11 000 yr bp a short episode of shallow conditions is inferred by c 12 400 yr bp holocene lake levels were generally shallower and three episodes of very shallow slightly alkaline waters are identified the first dates to the early holocene c 11 000 7000 yr bp the second is centred at c 4600 4500 yr bp the third occurred between c 2000 and 800 yr bp c 200 bc ad 1100 calibrated ages with very shallow water after c 1400 yr bp ad 550 calibrated age lake level increased after c 800 yr bp these three shallow water events are also recorded at other sites in central mexico indicating regional climatic trends rather than local events a deeper water phase occurred between 7000 and 6400 6200 yr bp the last shallow water phase correlates with the classic and epiclassic periods ad 200 900 and shallowest conditions occurred in the late classic to epiclassic c ad 550 900 when the construction of man made islands reached a peak an increase in lake level after c 800 yr bp ad 1100 calibrated age may have led to the abandonment of this life strategy

anisotropic internal thermal stress in sea ice from the canadian arctic archipelago results from an ice stress buoy deployed near the center of a multi year floe in the viscount melville sound of the canadian arctic archipelago between 10 october 2010 and 17 august 2011 are presented the position record indicates the landlocked season was approximately 5 months from 18 january to 22 june when the sea ice was fast to melville island and victoria island thermal stresses ranging from 84 to 66 kpa dominate the internal stress record with only a few dynamic stress events 50 kpa recorded before the landlocked season intriguingly the thermal stresses are isotropic before the landlocked ice onset and anisotropic during the landlocked season two possible causes to explain anisotropy in thermal stresses are considered preferred c axis alignment of the ice crystal and land confinement associated with the nearby coastline the orientation of the principal stresses indicates that land confinement is responsible for the anisotropy the stress record also clearly shows the presence of residual compressive stresses at the melt onset suggesting a viscous creep relaxation time constant of several days finally results show an interesting reversal in the sign of the correlation from negative to positive between surface air temperature and thermal stress after the onset of surface melt we attribute this to the onset of water infiltration within sea ice after which colder night temperature leads to refreezing and compressive stresses to the best of the authors knowledge this is the first time that anisotropic thermal stresses have been reported in sea ice

observations and modeling of thermally induced stresses in first year sea ice during spring 1992 ice property geophone meteorological and stress data were collected on first year ice southwest of cornwallis island within the canadian archipelago one of the goals of the study was to specify the average characteristics of the ice use these characteristics in a model of thermally induced stresses in the ice and examine the fracturing associated with the occurrence of those stresses the results of simulations with a thermal stress model indicate that stress variations within the ice can be reasonably approximated by a viscoelastic rheology the rheology takes into consideration thermally induced strains generated locally as well as strains generated elsewhere and then mechanically transmitted through the ice the geophone data showed both ice borne and water borne propagation paths for individual fracturing events the data imply a detection radius out to 500 600 m for the ice borne signatures of fractures an investigation of a region after fracturing showed that 1 fracturing occurred in an area with a 10 to 15 cm snow cover 2 the snow cover had been scored down to the surface of the ice and 3 cracks in the ice were found under each location where the snow had been scored the cracks were 5 6 m long and at least 15 cm deep a review of these and other experimental results draw us to the conclusion that the forces required to produce fractures in response to natural forcing is greater for first year floes than for multiyear floes

volcanomagnetic signals during the recent popocatepetl mexico eruptions and their relation to eruptive activity abstract an interdisciplinary approach correlating magnetic anomalies with composition of the ejecta in each eruption as well as with seismicity was used to study the effect of magmatic activity on the local magnetic record at popocatepetl volcano located 65 km southeast of mexico city eruptions began on december 1994 and have continued with dome growth and ash emissions since then the tlamacas tla geomagnetic total field monitoring station located 5 km away from popocatepetl s crater was installed in december 1997 in order to detect magnetic anomalies induced by this activity spatial correlation and weighted difference methods were applied to detect temporal geomagnetic anomalies using tla s record and the teoloyucan magnetic observatory as a reference station weighted differences were applied to cancel the effects of non vulcanogenic external field variations magnetic anomalies over a 2 year time span were classified into four types correlating them with geochemical seismic and visual monitoring of the volcanic activity magnetic anomalies are believed to be caused by magma injection and gas pressure build up which is sensitive to vent morphology and clearing during eruption although some anomalies appear to be thermally related changes in the stress field are very important most magnetic anomalies are short time signals that reverse to baseline level decreasing anomalies 0 5 to 6 8 nt precede eruptions by 1 8 days the presence of a mafic magmatic component was determined by mineral examination and silica and magnesium analyses on the ejecta from the 1997 1999 eruptions whole rock analyses ranged from dacitic 65 sio 2 to andesitic 57 sio 2 with 2 6 6 mgo the higher mgo lower silica samples contain forsteritic olivine fo90 sio 2 does not increase and mgo does not increase with time suggesting ascent of small magma pulses which are consistent with the magnetic data

interactions between human activity volcanic eruptions and vegetation during the holocene at garua and numundo west new britain png this paper reviews recent fossil phytolith analysis from wet tropical west new britain papua new guinea the holocene vegetation has been influenced by spatially and temporally diverse patterns of both prehistoric human settlement and catastrophic volcanic events we have hypothesized different landscape responses and recovery pathways to events during the last six millennia phytolith sequences on the coastal lowlands the willaumez peninsula and nearby island of garua provide details of vegetational change and human interactions at different landscape scales since c 5900 cal yr b p during this period four major volcanic eruptions c 5900 3600 1700 and 1400 cal yr b p have disrupted the landscape the evidence provides detailed descriptions of temporal and spatial patterning in the impacts and changes in the vegetation in particular vegetation responded differently from one event to another reflecting both forest recovery from seed bank and shooting and the influence of prehistoric people on recovering vegetation furthermore after some events landscape recovery was moderately uniform while after others there was considerable landscape partitioning although these differences largely relate to airfall tephra type distribution and magnitude the partitioning is more strongly influenced by human activity

final amalgamation of the central asian orogenic belt in ne china paleo asian ocean closure versus paleo pacific plate subduction a review of the evidence abstract the central asian orogenic belt caob evolved through complex closure of the paleo asian ocean from the neoproterozoic to the late phanerozoic this caused the chinese cratons to collide with eurasia and led to the formation of the world s largest phanerozoic orogenic belt ocean closure commenced in the west and was completed in the east near changchun closure of the paleo asian ocean in ne china was along the solonker xar moron changchun yanji suture and this was likely completed in the late permian although associated activity continued into the triassic there was an overlap in the latest permian early triassic between terminal activity associated with paleo asian ocean closure and the onset of tectonism associated with subduction of the paleo pacific plate this switch in geodynamic setting occurred at 260 250 ma and is reflected by a relaxing of north south directed compression and the onset of east west directed processes related to paleo pacific subduction by the early jurassic events associated with the westward advance of the paleo pacific plate dominated leading to extensive development of i type granites as far inland as the great xing an range from 140 ma the paleo pacific plate retreated eastward resulting in an extensional setting in the early cretaceous the effects of which were enhanced by regional thinning of the lithosphere commonly attributed to delamination throughout this period the eastern asian margin was tectonically complex the north south oriented jiamusi khanka bureya block was rifted away from the eastern margin of the caob in the late triassic but was then re united in the jurassic by westward advancing subduction that affected both the western and eastern margins of the block accretionary complexes continued to evolve in the cretaceous along the whole eastern margin of asia with final accretion of the nadanhada terrane part of the sikhote alin accretionary terrane with the caob at 130 ma followed by the emplacement of s type granites

tectono sedimentary evolution of the mesoproterozoic basins in the southern yan liao and mianchi queshan areas insights from stratigraphic pattern and detrital zircon geochronology the widespread mesoproterozoic successions around the world have recorded the breakup of the ancient supercontinent columbia in the north china craton ncc the mesoproterozoic basins in the yan liao and southern ncc are the important evidence to study the nature and evolution of the mesoproterozoic basins in this study we scrutinized the stratigraphic pattern and detrital zircon u pb geochronology in the southern yan liao and mianchi queshan areas in the southern ncc to investigate the evolutionary relationship between the mesoproterozoic basins in the north and south of ncc detrital zircons from the mesoproterozoic sequence in the southern yan liao areas showed monotonous source with age peak at ca 2500 ma corresponding well with the ages of the neoarchean complex in the central ncc however the mesoproterozoic sequence in the mianchi queshan areas which is represented by the ruyang group exhibited significantly different characteristics with influx of much younger zircons the southern yan liao and mianchi queshan areas showed distinct similarity in the stratigraphic pattern reflected by the mesoproterozoic successions in these two areas being both bracketed by two sequence boundaries and further divided into two sequences under a uniform sea level change the mesoproterozoic strata in these two areas probably possessed a correlative relationship in the stratigraphic patterns and basin evolutionary history

limnologic history of lago de patzcuaro michoacan mexico for the past 48 000 years impacts of climate and man abstract lago de patzcuaro lies at an elevation of 2035 m 19 36 n 101 39 w on the west part of the volcanic meseta central of mexico about 50 km east southeast of morelia pollen diatom and geochemical records from a 15 m core taken in 6 m of water continuously track vegetation and limnological changes in the patzcuaro basin for the past 48 ky calendar years pine oak and alder pollen dominate both the wisconsin and holocene vegetation record and indicate that this region did not witness strongly arid climates during that time before the holocene juniperus type and artemisia pollen document cooler climates of more effective moisture at least seasonally isoetes characterizes the full and late glacial interval 25 13 ka and implies along with the diatoms aulacoseira and stephanodiscus cool deep freshwater conditions at lago de patzcuaro the freshest and deepest lacustrine phase in the lake occurred 38 to 25 ka during this time lago de patzcuaro may have drained into the lerma river system between 30 and 10 ka diatom species that bloom in the winter and early spring imply moisture during those seasons evaluation of contemporaneous marine and terrestrial records west and east of lago de patzcuaro suggests that this moisture probably originated from westerly storm tracks shifted south by the laurentide ice sheet after 10 ka lago de patzcuaro became shallower and more eutrophic rooted aquatic plants prospered under a climate regime of negative hydrologic budgets increased seasonality and moisture arriving during the summer as it does today the lake became alkaline enough by the mid holocene to deposit ostracode rich marls at the core site for the first time in the past 48 ky indigenous agriculture in the patzcuaro drainage and around the lake appears about 4 ka with the appearance of zea and chenopodiineae pollen iron and potassium track increased soil erosion and planktic diatoms cyclostephanos record nutrients flushed into the lake from the drainage basin

a preliminary view of the coexistence of mammoth and early peoples in mexico abstract a progress report about human mammoth interactions in mexico is provided based primarily on a literature search more than 270 mammoth localities are known in mexico but only 17 of them have shown some evidence of an association between early peoples and mammoth however that number is even less when each locality is assessed in detail due to the lack of a supportable association or the loss of the actual specimens that precludes their analysis using current techniques only six localities have modified mammoth bone among them the greatest potential for demonstrating such a relationship is at santa isabel iztapa valsequillo villa de guadalupe and tocuila establishing an analysis methodology for the materials from those as well as new localities is warranted to provide the basis for interpreting the human mammoth relationship in mexico future research calls for detailed stratigraphic and radiometric control and an analytical protocol for bone analysis focused on taphonomy and biotechnology

lake levels since about 40 000 years ago at lake chalco near mexico city abstract diatoms magnetic susceptibility organic content and14c ages of sediments from a 26 m core suggest that lake chalco in the southern part of the basin of mexico went through a series of major fluctuations during the late pleistocene and the holocene before ca 39 00014c yr b p the lake was very deep about 8 10 m alkaline and saline it then became shallow

development of the north china craton during the late archaean and its final amalgamation at 1 8 ga some speculations on its position within a global palaeoproterozoic supercontinent abstract new data obtained over the past few years indicate that by the end of the archaean the north china craton consisted of two major crustal entities identified as the eastern and western blocks the formation of island and magmatic arcs along the western margin of the eastern block record the latest archaean events so far identified the location of the western block at this time is unknown however the occurrence of palaeoproterozoic passive margin sediments similar in age to those occurring in the eastern block indicates essentially similar conditions and possible separation by an ocean of unknown dimensions re initiation of east dipping subduction beneath the western margin of the eastern block resulted in closure of this palaeoproterozoic ocean bringing the western block into close proximity continued subduction resulted in a major continental continental collision and the western margin of the eastern block was again reactivated leading to extensive thrusting high pressure metamorphism and the generation of crustal melts this occurred between 1 88 and 1 79 ga ago resulting in the formation of the trans north china orogen and final amalgamation of the north china craton although information is sparse to non existent we have reviewed the possible location of the north china craton at the time of collision and note a similarity in terms of age lithology and distribution of major crustal components between the north china craton and baltica although speculative we present this model in the hope that it will stimulate debate and lead to further research in these areas

sequence stratigraphy of the precambrian rooihoogte timeball hill rift succession transvaal basin south africa third order sequence stratigraphic analysis is performed on the rooihoogte timeball hill second order rift succession of the paleoproterozoic transvaal basin south africa this provides a case study for systems tract and sequence development during a time of glacio eustatic fall when accommodation was generated by subsidence related to syn rift and post rift tectonic processes two third order depositional sequences have been identified separated by a basin wide subaerial unconformity the lower third order sequence includes the complete succession of lowstand transgressive and highstand systems tracts lst tst and hst whereas the upper third order sequence only preserves lowstand and transgressive systems tracts this indicates that the fall in base level associated with the upper second order boundary of the rooihoogte timeball hill sequence was of higher magnitude relative to the third order subaerial unconformity which is in agreement with the principles of boundary hierarchy based on the magnitude of base level changes the position of the lower boundary of the rooihoogte timeball hill second order sequence has been revised from the base of the chert breccias to the contact between the breccias and the overlying chert conglomerates this is because a major tilting event occurred between the deposition of the two facies which are genetically unrelated and which are separated by a subaerial unconformity the lithostratigraphic contact between the rooihoogte and timeball hill formations is interpreted as a diachronous transgressive surface of erosion in this interpretation the polo ground member of the rooihoogte formation may be coeval with the basal black shales of the timeball hill formation the two facies fluvial and marine respectively forming together a transgressive systems tract d 2002 elsevier science b v all rights reserved

melting of enriched archean subcontinental lithospheric mantle evidence from the ca 1760ma volcanic rocks of the xiong er group southern margin of the north china craton abstract the xiong er group is an important geologic unit in the southern margin of the north china craton it is dominated by the volcanic rocks dated at 1763 15 ma that have sio2 contents ranging from 52 10 wt to 73 51 wt these volcanic rocks are sub alkaline and can be classified into three subgroups basaltic andesites andesites and rhyolites they unexceptionally show enrichment of light rare earth elements lree and share similar trace element patterns depletions in nb ta sr p and ti relative to the adjacent elements are evident for all the samples the volcanic rocks are evolved with low mgo contents 0 29 5 88 wt and accordingly low mg values of 11 53 the nd isotopes are enriched and show a weak variation with nd t 7 12 to 9 63 zircon hf isotopes are also enriched with hf t 12 02 0 45 the volcanic rocks of the xiong er group are interpreted to represent fractional crystallization of a common mantle source the volcanic rocks might have been generated by high degree partial melting of a lithospheric mantle that was originally modified by the oceanic subduction in the late archean this brings a correlation with the subduction modified lithospheric mantle in an extensional setting during breakup of the columbia supercontinent in the late paleoproterozoic rather than in an arc setting the elevated sio2 contents and evolved radiogenic isotope features indicate the possible incorporation into their source of lower crustal materials that have similar nd isotopic characteristics to the subcontinental lithospheric mantle the existence of extensive xiong er volcanic rocks 60 000 km2 indicates an early large scale subduction related metasomatism in the area and probably suggest a flat subduction model for the plate margin magmatism in the late archean

paleoproterozoic microbially induced sedimentary structures from lagoonal depositional settings in northern china abstract microbially induced sand cracks crack fills occur extensively on the top surface of fine sandstone beds of the paleoproterozoic zhaojiazhuang formation changcheng group 1 7 ga around the cangyan mountain hebei province northern china detailed field and microscopic petrographical evidence reveal that these sand cracks crack fills possibly resulted from dehydration and desiccation of microbial mats the age and peculiar morphology of these microbially induced sedimentary structures do not allow comparison with trace fossils or purely physical desiccation cracks the fine sandstone beds on the surfaces of which these microbially induced sedimentary structures formed were deposited in a lagoon brackish depositional setting marine to non marine transitional setting with episodic injection of marine water as such these microbially induced sedimentary structures suggest the colonization of the marine to non marine transitional settings in the paleoproterozoic period and that the search for evidence of early microbial life should be sought in the marine to non marine transitional settings these broad habitats suggest these microbes could have been eurytopic organisms capable of adapting to varying extents of salt and oxygen content variations like their modern counterparts

accuracy of cosmogenic ages for moraines analyses of all published cosmogenic exposure ages for moraine boulders show an average age range of 38 between the oldest and youngest boulders from each moraine this range conflicts with the common assumption that ages of surface boulders are the same as the age of the landform the wide spread in boulder ages is caused by erosion of the moraine surface and consequent exhumation of fresh boulders a diffusion model of surface degradation explains the age range and shows that a randomly sampled small set of boulders n 3 7 will always yield a lower age limit for the moraine the model indicates that for identical dating accuracy six to seven boulders are needed from old and tall moraines 40 000 100 000 yr 50 100 m initial height but only one to four boulders from small moraines 20 000 100 000 yr 10 20 m by following these guidelines the oldest obtained boulder age will be 90 of the moraine age 95 probability this result is only weakly sensitive to a broad range of soil erosion rates our analysis of published boulder ages indicates that 3 of all moraine boulders have prior exposure and 85 of these boulders predate the dated moraine

fluvial processes and paleopedogenesis in the teotihuacan valley mexico responses to late quaternary environmental changes a paleosol sequence in the teotihuacan valley was studied in order to reconstruct landscape evolution during the late pleistocene holocene in a cut belonging to a river gully san pablo the sequence is constituted by 4 pedostratigraphic units labeled as sp1 sp4 separated by volcanogenic alluvial colluvial sediments sp4 at the base of the sequence has a prismatic structure clay cutans and vertic features sp3 shows abundant signs of zooturbation krotovinas and clay cutans both paleosols are interpreted to have formed in stable humid forest landscapes during marine isotope stage 3 mis3 however towards the end of mis3 these conditions alternated with the periods of contrasting seasonal climate the overlying sp2 unit is a pedocomplex demonstrating 2 petrocalcic horizons combined with redoximorphic features the pedocomplex merges laterally with gradually stratified gleyic channel deposits gravels in the base to fine sand and silt in the top the age of the lower caliche is 24 000 bp and thus sp2 formed during mis2 including the last glacial maximum lgm the environmental conditions during lgm are mostly dry with stages of high humidity evidenced by gleyic processes and fluvial activity at the end of the pleistocene the landscape becomes highly unstable and pedogenesis is replaced by sedimentation new paleochannels cut through the previously formed paleosols these channels are infilled by poorly sorted sediments with no gradation the configuration of all paleochannels is nne ssw nearly at 90 to the modern san pablo river flow geomorphic stability during the early and middle holocene allowed the development of sp1 in a dry environment the a horizon of this paleosol has been dated at 2900 bp it is well developed with strong vertic features this paleosol is thought to have formed the soil cover when the classic period teotihuacan culture flourished in the valley between the first and third centuries ad soils above sp1 are poorly developed and erosion deposition processes predominate over pedogenesis recent landscape instability is due to human activities that intensified erosion processes during the classic and especially the postclassic and colonial periods

a sedimentary overlap assemblage links australia to northwestern laurentia at 1 6 ga abstract the columbia nuna supercontinent existed from approximately 1 9 ga to 1 3 ga laurentia was part of columbia and the western edge of laurentia current coordinates was likely proximal to a large landmass during parts of this interval here we present detrital zircon ages of a paleoproterozoic sedimentary succession in northern yukon canada that bear on the evolution of columbia the sedimentary succession is preserved as clasts within 1 60 ga hydrothermal megabreccias analyses of detrital zircon reveal abundant 1 78 1 68 ga zircon with evolved hf isotope values 16 1

tepexpan revisited a multiple proxy of local environmental changes in relation to human occupation from a paleolake shore section in central mexico abstract building up a scenario of late pleistocene holocene environmental change and human landscape interactions in central mexico one of the key areas for the natural and cultural history of america requires development of local paleoenvironmental reconstructions we studied the tepexpan section at the edge of texcoco a paleolake near the mouth of teotihuacan valley as a multiproxy record of the environmental dynamics at the shore in the basin of mexico throughout the period of human occupation the section is located in an ecotone affected by intensive and variable geomorphic processes and includes lacustrine fluvial and volcanic sediments as well as paleosols our chronological scale is based on 6 ams 14c dates from pollen concentrates and paleosol organic matter the lower segment of the section dominated by the lacustrine sediments yielded pollen spectra in the upper segment the record is based on the pedogenetic characteristics of paleosols different proxies agree in demonstrating the general trend although some reversals are apparent of decreasing effective moisture since mis3 it is reflected first in the increase of herbaceous pollen after 27 ka bp the decrease of lake level the cessation of lacustrine sedimentation and beginning of marsh soil development at 10 ka bp and finally the shift from hydromorphic to dryland semiarid pedogenesis in the late holocene we assume that this trend was climatically controlled whereas the deposition of sedimentary layers enriched with tephra are related to the pulses of volcanic activity the pedosedimentary features associated with the upper soil are human induced comparing the proposed scheme of environmental change with the archaeological record we propose that the initial settlers late paleolithic hunters could have utilized the wet swampy meadows which expanded on the basin bottom as the lake level lowered and provided the niche for large herbivores during the pleistocene holocene transition development of agriculture and the network of permanent settlements are believed to be linked to the formation of well drained soils in the methodological perspective the study shows the potential of ecotones as highly dynamic and sensitive systems that may archive valuable paleoecological records

a conceptual model of the impact of flaws on the stress state of sea ice recent theoretical advances have lead to a rheology for sea ice and an increased understanding of how stresses are generated in an ice floe by thermally induced strain rates one of the underlying assumptions of the ice rheology is that floes in general order of 300 m or greater in diameter are too large to bend or twist in response to vertical or horizontal variations in strain rates in essence there are no nonzero bending moments a numerical model based on this rheology has tieen used to produce good predications of observed ice stresses and times of ice fracturing lewis 1994 lewis et al 1994 on the basis of these results this paper presents a conceptual model of how existing flaws in an ice floe can impact the stress state of the ice while still assuming no bending or twisting of the floe a modified ice rheology is developed which quantifies the extent of existing cracks in the ice and how they impact the overall strain rate within a floe

shrimp u pb zircon ages of the fuping complex implications for late archean to paleoproterozoic accretion and assembly of the north china craton the fuping complex is situated in the central part of the north china craton and consists of four major lithological assemblages fuping tonalitic trondhjemitic granodioritic gneisses longquanguan augen gneisses wanzi supracrustal assemblage and nanying granitic gneisses shrimp u pb geochronology combined with u th and cathodoluminescence cl imaging of zircon enables resolution of magmatic and metamorphic events that can be directed towards understanding the late archean to paleoproterozoic history of the fuping complex cl images reveal the coexistence of magmatic and metamorphic zircons in nearly all rock types of the fuping complex the metamorphic zircons occur as either single grains or overgrowth or recrystallization rims surrounding and truncating oscillatory zoned magmatic zircon cores and are all characterized by nebulous zoning or being structureless with extremely high luminescence and very low th contents these features make them distinct from magmatic zircons that are characterized by concentric oscillatory zoning comparatively low luminescence and high th and u contents shrimp u pb analyses on magmatic zircons reveal that the tonalitic trondhjemitic and granodioritic plutons of the fuping gneisses were emplaced at 2523 14 ma 2499 10 ma and 2486 8 ma respectively whereas the monzogranitic and granitic plutons of the longquanguan augen gneisses were intruded respectively at 2510 22 ma and 2507 11 ma prismatic and oscillatory zoned zircons dominate in the pelitic rocks of the wanzi supracrustal assemblage and are interpreted as detritus from igneous source rocks the concordant and discordant u pb ages of 2502 5 ma and 2507 14 ma obtained from two pelitic rock samples indicate these rocks must have been deposited no earlier than 2507 ma ago in addition a zoned zircon grain in one pelitic rock sample has a near concordant age of 2109 5 1 ma which may provide a maximum depositional age for the wanzi supracrustal rocks shrimp results also reveal that granitic magmatism assigned to the nanying granitic gneisses occurred over a protracted interval from 2077 13 ma to 2024 21 ma the nebulously zoned zircon grains and overgrowth recrystallization zircon rims from different rocks yielded similar concordant 207pb 206pb ages in the range 1875 to 1802 ma interpreted as approximating the age of regional metamorphism of the fuping complex timing of primary zircon crystallization and regional metamorphism of the fuping complex is in general agreement with recent u pb zircon ion probe results for the wutai and hengshan complexes that bound the fuping complex to the northwest these areas are characterized by the emplacement of major granitoid bodies at around 2 50 ga to 2 48 ga ago deposition of supracrustal rocks in the paleoproterozoic intrusion of paleoproterozoic granitic bodies at 2 1 to 2 0 ga and regional metamorphism at 1 875 to 1 802 ga these data indicate that the fuping and hengshan complexes do not represent an older crystalline basement to the wutai complex as suggested in previous tectonic models but together with the wutai complex represent elements of a single late archean to paleoproterozoic magmatic arc system that has been subsequently tectonically disrupted and juxtaposed during the collision of the eastern and western north china blocks at 1 85 ga which resulted in the final assembly of the north china craton

characteristics of pack ice stress in the alaskan beaufort sea ice stresses in a multiyear floe were continuously monitored over 6 months in the fall winter spring of 1993 1994 stresses sensors were installed at sites near the edge and at the center of the floe which was located in the pack ice of the alaskan beaufort sea compressive stresses in the major principal stress component varied significantly among the measurement sites being of greater magnitude and exhibiting more high frequency variations at the edge than at the center of the floe maximum compressive stresses measured at a site 400 m from the edge of the floe ranged from 100 to 300 kpa tensile stresses maximum of 50 kpa and the minor principal stress component 50 kpa were relatively constant at all measurement sites a cross correlation analysis indicates that the minor principal stress is strongly correlated to changes in the ice temperature this result suggests that the minor principal stress component provides a good first order approximation of thermally induced stresses correlation between the major principal stress component and the ice temperature at the center site was also high but weakened near the edges of the floe where the ice motion induced stresses became more significant attempts to filter the major principal stress signal to separate the thermal and ice motion induced stresses were unsuccessful this result implies that the ice motion induced stresses distinguished by variations in magnitude of the order of hours also have a significant low frequency content similar to the thermal stresses these low frequency changes occur over a period of days seasonal variations in the characteristics of the stress were also evident and are likely to reflect the developing continuity of the pack as the winter season progresses

paleosol organic matter and pedogenic carbonates and paleontological δ13c records applied to the paleoecology of late pleistocene holocene in mexico abstract stable isotopes of carbon 13 c in terrestrial geological materials paleosol humus and carbonates as well as fossil bones collagen and tooth enamel are established as highly specific records on paleoenvironmental characteristics it was assumed that comparing the results of different types of materials on regional or local scales provides more accurate and reliable reconstructions and even opens new aspects of interpretation this was tested by the comparison of the data obtained from the late pleistocene paleosols and or remains of pleistocene megafauna in various locations in northwestern mexico sonora paleosols formed in mis3 mis2 and mis1 central northern mexico san luis potosi teeth corresponding to mis2 and mis1 and central southern mexico puebla paleosols corresponding to mis3 and mis2 and teeth associated with these paleosols and state of mexico teeth corresponding to mis2 the results of isotopic studies are complemented with other paleoenvironmental proxies the 13 c values of paleosols of sonora is 19 8 to 16 95 in organic matter and shows little difference between paleosols and holocene soils 13 c values of pedogenic carbonates are 6 4 to 4 52 in san luis potosi the 13 c values of paleosol are 12 16 in organic matter in puebla they vary from 24 6 in paleosols to 15 16 in the modern surface soil pointing to contrasting vegetation change the 13 c values in fossils range from 10 7 to 1 2 in slp of 2 73 and 2 93 in puebla and 6 5 to 1 3 in state of mexico the 13 c values of paleosols and analysis of megafauna in northwest and central northern mexico matched each other indicating mixed c3 c4 vegetation while for central southern mexico paleosols indicated predominance of c3 plants and megafauna indicated mixed c3 c4 composition in general the results indicate that in northern mexico environmental conditions were slightly wetter than at present while for the central area there was a large change in the environmental conditions of high humidity in mis3 and mis2 with a trend to drier climates in mis1

forced regressive wedge in the mesoproterozoic koldaha shale vindhyan basin son valley central india abstract the present paper highlights the sequence development within the mesoproterozoic koldaha shale member of the kheinjua formation vindhyan supergroup which records the occurrence of a forced regressive wedge and associated discontinuity surfaces at the base of the wedge nine lithofacies have been identified within the study area that are grouped into three lithofacies associations varying in depositional setting from outer shelf through shoreface foreshore beach to continental braidplain the outer shelf sediments are aggradational to slightly progradational representing highstand systems tract the rapidly progradational wedge shaped shoreface to foreshore beach succession occurs sharply or erosively above the outer shelf sediments and is bounded by a regressive surface of marine erosion rsme at the base and by a subaerial unconformity at the top this along with its downstepping trajectory supports deposition of this sedimentary wedge during falling sea level a laterally extensive soft sediment deformation zone occurs at the base of the wedge the forced regressive wedge is incised by fluvial braidplain deposits that rest on an erosive surface representing a sequence boundary the thin braidplain deposits are the product of aggradation during a subsequent early rise in relative sea level and thus they are inferred to represent a lowstand systems tract the constituent architectural elements that characterize the braidplain deposits are downstream accretion elements and small channel elements further landward the base and top of the shoreface wedge merge to form an unconformity across deposits that rest directly on the outer shelf sediments the identification of forced regressive wedges has significant economic importance in view of the potential occurrence of hydrocarbons within the proterozoic formations

magnetic properties of lake sediments from lake chalco central mexico and their palaeoenvironmental implications lake chalco 99 0 w 19 5 n in the basin of mexico was formed during the pleistocene after the emplacement of the chichinautzin volcanic field that closed the former drainage system the lake sediment record has been influenced by a number of factors including glacial interglacial cycles local volcanism erosion of soils and anthropogenic disturbances the magnetic properties of the lake sediments and the associated tephra layers of the last 16500 yr have been studied it is found that the magnetic properties of the lake chalco sediments are very distinctive magnetic concentration varies by a factor of 1000 and magnetic stability also varies over an extremely wide range the predominant magnetic mineral is titanomagnetite in addition to an imperfect antiferromagnetic phase possibly goethite an unusually large range of coercivities is found in certain of the tephras down core variations in magnetic properties closely follow climatic environmental changes previously established by other proxy methods the late pleistocene and late holocene lake sediments display a higher concentration of magnetic minerals than the early middle holocene sediments in the non volcanic sediments fluctuations in the magnetic concentration reflect changes in both the intensity of erosion as represented by ferrimagnetic and paramagnetic minerals and by the maturing of soils as represented by geothite copyright 2000 john wiley sons ltd

cosmogenic 36cl accumulation in unstable landforms 2 simulations and measurements on eroding moraines cosmogenic 36cl surface exposure ages obtained for multiple boulders from single landforms are usually characterized by a variance larger than that of the analytical methods employed this excessive boulder to boulder variability progressively more profound with increasing age of landforms is due to removal of soil and gradual exposure of boulders at the surface in our gradual exposure model boulders are initially buried in moraine matrix with time erosion lowers the moraine surface and the boulders are gradually exposed to cosmic rays because the cosmic ray intensity changes with depth the boulders are subjected to variable production rates of the cosmogenic 36cl initial depth of boulders and their chemical composition are variable which results in different amounts of the accumulated cosmogenic 36cl and thus different apparent ages of boulders the shape of the resulting distribution of the apparent ages and the coefficient of variation depend on the erosion depth while the first moment is a function of the true surface age and the erosion depth these properties of the apparent age distributions permit calculation of the surface age the erosion depth and also the average erosion rate we tested the model calculations using 26 boulders from a late pleistocene moraine at bishop creek sierra nevada california the set exhibited a bimodal distribution of the 36cl surface exposure ages we interpreted the older mode as the result of gradual exposure and the younger one as the result of surficial processes other than soil removal the 10 samples that constitute the older mode produced a distribution which closely matches the modeled distribution calculated using an age of 85 kyr and erosion depth of 570 g cm 2 this age is the same as an independent estimate obtained from cation ratio studies and the calculated erosion depth is very close to the erosion depth of 600 g cm 2 based on a simple analytical model of soil erosion these results indicate that our statistical model adequately describes effects of soil erosion on accumulation of cosmogenic 36cl the approach can be used to simultaneously obtain the true landform age and the erosion rate from apparent 36cl ages and thereifore may help in evaluation of surface exposure ages of eroding landforms

reconstruction of late pleistocene climate in the valsequillo basin central mexico through isotopic analysis of terrestrial and freshwater snails we aim to reconstruct the climatic and environmental conditions in the valsequillo basin during the deposition of the valsequillo gravels between c 40 000 and 8000 years ago when large mega fauna and potentially humans occupied the basin fossil freshwater fossaria sp and sphaeriidae family and terrestrial polygyra couloni holospira sp and cerionidae family snail shells from sections within the barranca caulapan were collected for oxygen and carbon stable isotope analysis oxygen and carbon isotopes in terrestrial and freshwater snail shells relate to local climatic parameters and environmental conditions prevailing during the lifetime of the snail whole shell isotope analysis showed that c 35 000 years ago climate in the valsequillo basin was similar to the present day between c 35 000 and 20 000 bp conditions became increasingly dry after which conditions became wetter again although this record is truncated intra shell isotopic analyses show that the amount of precipitation varied seasonally during the late pleistocene if people did reach this part of the americas in the late pleistocene they would have experienced changing long term and seasonal climatic conditions and would have had to adapt their life strategies accordingly

under ice noise resulting from thermally induced fracturing of the arctic ice pack theory and a test case application a theory is presented that relates thermally induced fracturing of pack ice to under ice noise level variations it begins with the governing equations for the thermomechanics of pack ice the thermomechanics relates thermally induced strain rates to the stresses within various vertical layers of the floe in addition paradigms are developed which specify the relative quantity of fracturing and stress relief in the floe as the tensile yield strength of the ice is exceeded the thermomechanics is complemented by an acoustic propagation model that relates the number of fracture events at a given time to the acoustic levels at arbitrary frequency and depth below the ice the acoustic theory assumes that each fracture acts as a simple monopole source the fractures are evenly distributed horizontally and the energy of each fracture propagates through the ice and the water column on the basis of the governing equations for elastic waves in a horizontally stratified medium the results indicate that noise episodes resulting from fracturing occurring over the top 40 cm of a 160 cm thick floe will propagate over distances of up to 100 km however noise episodes associated with fracturing occurring in the lower 100 cm of the floe will only propagate over a range of 10 km the thermomechanics and acoustic propagation theories were used to develop a numerical model for predicting under ice noise levels for a given thermal forcing of floes within the arctic ice pack the model was used to simulate stresses in a multiyear floe and under ice noise levels at 500 hz at 305 m below the floe model predicted ice stresses and under ice noise levels compare quite well to observed stresses and noise variations during the fall of 1988 in the eastern arctic ocean the model predicts that most of the thermally induced under ice noise at 500 hz was a result of fracturing occurring between 5 and 30 cm below the ice surface for a 1 6 m thick multiyear floe

late quaternary lacustrine paleoenvironments in the cuenca de mexico abstract a late quaternary paleolimnological history from the mexican highlands has been obtained by diatom analysis of short cores and stratigraphic sections of lake and marsh sediments from the cuenca de mexico the large endorheic graben basin that holds mexico city the records dated by radiocarbon and tephrochronologic methods extend back to about 30 ka bp and document the presence of extensive saline lakes in the basin until 25 ka bp thereafter lake levels fell and marginal sites became shallow and fresh under the influence of surficial drainage and especially spring discharge a shallow saline lake existed 18 ka bp in texcoco the central and lowest basin in the system reflecting moderately increased effective moisture at that time by 16 ka bp lake texcoco had become so shallow that diatoms were no longer preserved however marginal sites nourished by spring flow recorded changes in the local hydrologic balance resulting from increased infiltration at higher elevations these changes appear to coincide with glacial advances between about 14 and 10 ka bp on the volcanic mountains surrounding the basin dry climates with reduced infiltration characterized the early holocene but by 5 ka bp a modest increase in precipitation established the modern climatic regime these lacustrine records offer important insights for evaluating the paleoenvironmental history of the cuenca de mexico based on other evidence they confirm glaciological stratigraphic and palynologic data that suggest dry climates and the absence of large pluvial lakes in the cuenca de mexico during and after the full glacial but document climates of significantly increased precipitation at least 10 ka prior to 18 ka bp

stratigraphic and sedimentary records of the rift to drift evolution of the northern north china craton at the paleo to mesoproterozoic transition article i nfo this paper presents a stratigraphic and sedimentary study of late paleoproterozoic to early mesoproterozoic successions of the northern margin of the north china craton including the changcheng and nankou groups a reconstruction of the tectono sedimentary history of these two groups demonstrates that the northern north china craton experienced a two stage evolution as recorded by the changcheng group early synrift subsidence and sedimentation were controlled by normal faults and accompanied by volcanism in contrast a late postrift stage was characterized by broad subsidence and widespread occurrence of shallow marine clastic and carbonate rocks a transgressive unconformity is identified and together with its time equivalent conformable contact clearly separates the synrift and postrift sequences the diachronous unconformity can be related to continental rifting and the separation of the north china craton from the adjacent india craton at roughly 1600 ma and is thus interpreted as a breakup unconformity this age of ca 1600 ma is constrained by newly reported precise shrimp u pb zircon ages of transgressive deposits in the overlying mesoproterozoic dahongyu and gaoyuzhuang formations just coincident with the paleoproterozoic to mesoproterozoic transition the 1600 ma time estimate for the tectonic departure of the north china craton from the india craton is comparable with the timing for the breakup of columbia a pre rodinia supercontinent containing almost all of the earth s continental blocks

marine redox conditions in the middle proterozoic ocean and isotopic constraints on authigenic carbonate formation insights from the chuanlinggou formation yanshan basin north china abstract to improve our understanding of ocean chemistry and biogeochemical cycling following the termination of large scale paleoproterozoic iron formation if deposition 1 85 billion years ago ga we conducted a fe s c mo geochemical study of the 1 65 ga chuanlinggou formation yanshan basin north china despite the cessation of if deposition our results suggest the presence of anoxic but non euxinic ferruginous conditions persisted below the surface mixed layer for the deepest portion of the continental rifting basin and that this pattern is apparently independent of the local organic carbon content however our paired s isotope data of carbonate associated sulfate and pyrite suggest presence of sulfate in pore fluids which is not consistent with insufficient sulfate for bacterial sulfate reduction in the water column despite evidence for deposition under anoxic conditions sedimentary molybdenum mo concentrations are mostly not enriched relative to average continental crust this relationship is consistent with the notion that sulfide dominated conditions in the water column and or the sediments are required for mo enrichment and validates past assertions that mo enrichment patterns in ancient shales track both the local presence and global distribution of euxinia specifically in addition we identified extensive diagenetic carbonate precipitation in the upper chuanlinggou formation with only moderately negative 13 c values 3 4 1 4 we propose with support from a numerical model that these diagenetic carbon isotope values were most likely derived from precipitation of carbonates dominantly in the methanic zone within the sediments diagenetic carbonate precipitation in the methanic zone is likely to have been more extensive in the proterozoic than the phanerozoic due to porewater oxidant limitation

reconstruction and interpretation of giant mafic dyke swarms a case study of 1 78 ga magmatism in the north china craton

the xiong er volcanic belt at the southern margin of the north china craton petrographic and geochemical evidence for its outboard position in the paleo mesoproterozoic columbia supercontinent abstract the xiong er volcanic belt covering an area of more than 60 000 km 2 along the southern margin of the north china craton has long been considered an intra continental rift zone and recently interpreted as part of a large igneous province formed by a mantle plume that led to the breakup of the paleo mesoproterozoic supercontinent columbia however such interpretations cannot be accommodated by lithology mineralogy geochemistry and geochronology of the volcanic rocks in the belt lithologically the xiong er volcanic belt is dominated by basaltic andesite and andesite with minor dacite and rhyolite different from rock associations related to continental rifts or mantle plumes which are generally bimodal and dominated by mafic components however they are remarkably similar to those rock associations in modern continental margin arcs in some of the basaltic andesites and andesites amphibole is a common phenocryst phase suggesting the involvement of h 2 o rich fluids in the petrogenesis of the xiong er volcanic rocks geochemically the xiong er volcanic rocks fall in the calc alkaline series and in most tectono magmatic discrimination diagrams the majority of the xiong er volcanic rocks show affinities to magmatic arcs in the primitive mantle normalized trace element diagrams the xiong er volcanic rocks show enrichments in the lile and lree and negative nb ta ti anomalies similar to arc related volcanic rocks produced by the hydrous melting of metasomatized mantle wedge nd isotope compositions of the xiong er volcanic rocks suggest that 5 15 older crust has been transferred into the upper lithospheric mantle by subduction related recycling during archean to paleoproterozoic time available shrimp and la icp ms u pb zircon age data indicate that the xiong er volcanic rocks erupted intermittently over a protracted interval from 1 78 ga through 1 76 1 75 ga and 1 65 ga to 1 45 ga though the major phase of the volcanism occurred at 1 78 1 75 ga such multiple and intermittent volcanism is inconsistent with a mantle plume driven rifting event but is not uncommon in ancient and existing continental margin arcs taken together the xiong er volcanic belt was most likely a paleo mesoproterozoic continental magmatic arc that formed at the southern margin of the north china craton similar paleo mesoproterozoic continental magmatic arcs were also present at the southern and southeastern margins of laurentia the southern margin of baltica the northwestern margin of amonzonia and the southern and eastern margins of the north australia craton which are considered to represent subduction related episodic outbuilding on the continental margins of the paleo mesoproterozoic supercontinent columbia therefore in any configuration of the supercontinent columbia the southern margin of the north china craton could not have been connected to any other continental block as proposed in a recent configuration but must have faced an open ocean whose lithosphere was subducted beneath the southern margin of the north china craton

shrimp and la icp ms zircon geochronology of the xiong er volcanic rocks implications for the paleo mesoproterozoic evolution of the southern margin of the north china craton the paleo mesoproterozoic xiong er volcanic rocks along the southern margin of the north china craton are lithologically and geochemically similar to those formed in subduction related continental margin volcanic arcs the volcanic rocks are primarily composed of basaltic andesites and andesites with minor dacites and dacitic rhyolites traditionally the xiong er volcanic rocks have been divided from lower to upper into the xushan jidanping and majiahe formations but the ages of volcanic rocks in these formations have not been well constrained which has hindered further understanding the tectonic significance of the xiong er volcanic belt at the southern margin of the north china craton shrimp and la icp ms u pb zircon analyses combined with cathodeluminescence cl images have enabled resolution of xenocrystic and magmatic zircons that can be directed toward determination of the ages of the xiong er volcanic rocks shrimp and la icp ms u pb analyses on magmatic zircons from two basaltic andesite samples one dacite sample and one rhyolite sample of the xushan formation known as the lowest sequence of the xiong er volcanic rocks indicate that the volcanic eruption of this formation occurred at 1 78 ga but most xenocrystic inherited zircons in these samples yielded 207pb 206pb ages ranging from 2 55 ga to 1 91 ga of three samples collected from the jidanping formation two rhyolite samples 05xe015 and 05xe100 yielded weighted mean 207pb 206pb ages of 1778 5 5 ma and 1751 14 ma respectively similar to the ages of the volcanic rocks in the xushan formation whereas one dacite sample 05xe066 gave a weighted mean 207 pb

archean blocks and their boundaries in the north china craton lithological geochemical structural and p t path constraints and tectonic evolution an examination of lithological geochemical geochronological structural and metamorphic p t path data suggests that the basement of the north china craton can be divided into eastern and western blocks separated by major crustal boundaries that roughly correspond with the limits of a 300 km wide zone called the trans north china orogen the eastern block consists predominantly of late archean domiform tonalitic trondhjemitic granodioritic ttg batholiths surrounded by anastomosing networks and linear belts of open to tight synforms of minor volcanic and sedimentary rocks metamorphosed from greenschist to granulite facies at 2 5 ga with anticlockwise p t paths some early to middle archean rocks are locally present in the eastern block but their tectonic history is unclear due to reworking by the 2 5 ga tectonothermal event the western block has a late archean assemblage structural style and metamorphic history similar to that of the eastern block but it differs in the absence of early to middle archean assemblages and in being overlain by and interleaved with paleoproterozoic khondalites which were affected by a 1 8 ga metamorphic event involving clockwise p t paths a mantle plume model is proposed for the formation and evolution of late archean basement rocks in the eastern and western blocks based on a combination of extensive exposure of ttg gneisses affinities of mafic rocks to continental tholeiitic basalts presence of voluminous komatiitic rocks dominant diaprism related domiform structures anticlockwise p t paths and a short time span from the primary emplacement of ttg and ultramafic to mafic rocks until the onset of regional metamorphism between the two blocks is the trans north china orogen which is bounded by two major fault systems and is composed of late archean to paleoproterozoic ttg gneisses and granitoids interleaved with abundant sedimentary and volcanic rocks that are geochemically interpreted as having developed in magmatic arc and intra arc basin environments these rocks underwent multiple phases of compressional deformation and peak high pressure metamorphism followed by rapid exhumation during the late paleoproterozoic at 1 8 ga as a result of collision between the eastern and western blocks resulting in the amalgamation of the north china craton

buried palaeosols of the nevado de toluca an alternative record of late quaternary environmental change in central mexico buried palaeosols of central mexico not previously analysed from a palaeopedological standpoint have helped to develop a reliable regional model of late quaternary climatic change this paper focuses upon morphological and micromorphological properties particle size distribution and extractable fe al and si of seven palaeosols named pt1 pt7 in the vicinity of the nevado de toluca volcano the characteristics of andic cambisol pt1 similar to those of modern soils in semi arid environments indicate a drier climate in the first half of the holocene humic andosols pt2 pt4 are analogous to modern volcanic soils of humid forest environments they show evidence that a moist palaeoclimate prevailed during marine oxygen isotope stages ois 2 and 3 luvisols pt5 and pt6 which are assumed to have formed at the end of marine ois 5 to marine ois 4 also indicate humid conditions we attribute the differences between andosols pt2 pt4 and luvisols pt5 pt7 to variations in the duration in the pedogenesis rather than to changes in palaeoclimate micromorphological features of andosol to luvisol transition confirm that these soils form an evolutionary sequence being for the most part consistent with lacustrine records the palaeosol properties do not reflect the episodes of drier climate during the last glacial maximum however as shown by diatom and palynological data from lake sediments mesoclimate variations and or incompleteness of soil memory reflecting mostly periods of humid pedogenesis probably provide the reason for this disagreement copyright 2001 john wiley sons ltd

detrital zircon u pb lu hf and o isotopes of the wufoshan group implications for episodic crustal growth and reworking of the southern north china craton abstract the paleoproterozoic orogens of the north china craton ncc provide important windows to evaluate the crustal evolution history during early precambrian here we report results from sims zircon geochronology and oxygen isotopes as well as la icpms zircon hf isotopic analyses on detrital zircons from mesoproterozoic neoproterozoic sandstones of the wufoshan group from the southern ncc to probe the paleoarchean paleoproterozoic crustal evolution our data show episodic magmatism during the paleoarchean neoarchean and paleoproterozoic as inferred from the 207pb 206pb age range of 3514 2751 ma and the well defined 207pb 206pb weighted mean ages of 2669 15 ma 2510 8 ma 2402 39 ma 2298 36 ma 2179 10 ma 2058 16 ma 1940 6 ma 1874 7 ma and 1819 5 ma the similarity in u pb geochronology and hf isotopes of the archean detrital zircons from the wufoshan group to those from the paleoproterozoic songshan group reveal that the 2669 ma zircons record a major phase of neoarchean crustal growth and that 2510 ma zircons correspond to the latest neoarchean crustal reworking the 18o depleted isotopic features displayed by some detrital zircons are absent in those from songshan group and together with the rarity of core rim textures which were widespread in archean zircons from the songshan group we suggest that the source materials for the mesoproterozoic neoproterozoic wufoshan group are different from the archean cratonic source for the paleoproterozoic songshan group we propose that the zircons were probably sourced from the newly formed trans north china orogen tnco our paleoproterozoic zircon data provide robust evidence for continuous and complex reworking of the mesoarchean to early neoarchean crust in the southern ncc during early paleoproterozoic at ca 2402 ma 2298 ma and 2179 ma the middle paleoproterozoic ages 2058 ma 1940 ma and 1874 ma mark a partial cycle of oceanic crust subduction to continental collision during the formation of the tnco the extreme oxygen isotopic variation with the fairly restricted range in hf isotopes demonstrates that the components from both high temperature and the low temperature rock water interaction induced by the release of hydrothermal fluids from the subducted altered oceanic crust must have played a significant role during the growth of the zircons the paleoproterozoic thermal event at 1819 ma records post collisional uplifting as evidenced by an apparent negative correlation in the hf t vs 18o plot possibly inherited from the mixing of the ancient crust and the juvenile input during the oceanic subduction and continental collision integrated with the information from granulite facies metamorphic rocks in the orogenic root of the tnco our data suggest that the episodic paleoproterozoic magmatism was mainly induced by the tectonics of plate subduction and subsequent continental collision leading to the final cratonization of the ncc following the mesoproterozoic neoproterozoic sedimentation of the paleoproterozoic zircons except for the pb loss during the subsequent thermal events there was no major alteration in the hf and oxygen isotopic compositions thus the remarkably large variation of oxygen isotopes from the lower than normal mantle to the supracrustal value 18o 2 28 10 09 in the paleoproterozoic detrital zircons further demonstrates that these zircons came from the uplifted and exposed upper part of the tnco our detrital zircon data provide convincing evidence for the repeated crustal reworking in early paleoproterozoic and the onset of modern style plate tectonics in the ncc in middle paleoproterozoic

provenance and paleogeographic reconstruction of a mesoproterozoic intracratonic sag basin upper espinhaco basin brazil we would like to thank the capes cnpq grant no 304045 2010 1 and petrobras for their financial support for the field work and analysis

palynological and magnetic susceptibility records of lake chalco central mexico abstract pollen and magnetic susceptibility data from a 11 27 m long core drilled in the central part of lake chalco 99 00 w 19 15 n located at the southeastern part of the basin of mexico are interpreted in terms of paleoenvironmental changes the record of this subtropical highland lake dated by radiocarbon methods spans approximately 24 000 yr b p and documents changes in climate vegetation lake level and volcanic activity seven palynozones were established by using the method of cluster analysis for correlation purposes zones 7 6 and 5 between 20 600 and 18 300 yr b p indicate reduced forests with extensive grasslands relatively high input of sediments by water discharge of surface streams and high lake level the evidences obtained in zone 4 between 18 300 and 17 500 yr b p suggest a dry and warm climate with xerophytic vegetation lower lake level and erosion rates zone 3 between 17 500 and 10 000 yr b p shows a trend to increasing moisture and cooler temperatures volcanic activity during this period is intense and strongly influences the natural environments at around 12 000 yr b p a clear expansion of forests and fulfillment of the lake recorded at the beginning of the holocene an oak forest expansion and a lowering of lake levels are initiated and by the mid holocene zone 2 subhumid temperate conditions are detected although lake level continues to be low the human impact is recorded in the last 4000 yr

a 1 5 d anisotropic sigma coordinate thermal stress model of landlocked sea ice in the canadian arctic archipelago we present a 1 5d thermal stress model that takes into account the effect of land confinement which causes anisotropy in thermal stresses to this end we fix the total strain in the direction perpendicular to the coastline to its value at landlocked ice onset this prevents thermal expansion in the direction perpendicular to the coastline and therefore induces larger thermal stresses in this direction the simulated stresses best match the observations when a young s modulus of 0 5 gpa and a relaxation time constant of 8 days are used this simulation gives root mean square errors of 13 0 kpa and 13 1 kpa 15 in the major and minor principal stresses respectively the simulated anisotropic component of thermal stress also generally agrees with observations the optimal young s modulus is in the low range of reported values in the literature and the optimal relaxation time constant 8 days is larger than the largest relaxation time constant reported in the literature 5 days a series of experiments are done to examine the model sensitivity to vertical resolution snow cover and the parameterizations of young s modulus and viscous creep results show that a minimum of one and three layers in the snow and ice respectively is required to simulate the thermal stresses within 15 error of the value assessed with the higher resolution control simulation this highlights the importance of resolving the internal snow and ice vertical temperature profile in order to properly model the thermal stresses of sea ice this article is protected by copyright all rights reserved

sulfur and carbon isotope records from 1700 to 800 ma carbonates of the jixian section northern china implications for secular isotope variations in proterozoic seawater and relationships to global supercontinental events abstract this study is a comprehensive stable isotope survey of the marine carbonate dominated upper paleo to lower neoproterozoic stratigraphy of jixian county china carbonate associated sulfate cas was extracted and measured for 34 s cas using the same samples analyzed for 13 c carbonate this integrated proxy approach is a step towards a more comprehensive picture of secular variation in the composition of proterozoic seawater we specifically sampled marine carbonate intervals from the lower section of the chuanlinggou formation changcheng group ca 1700 ma to the top of the jingeryu formation qingbaikou group ca 800 ma 13 c carbonate values are mostly negative in the upper paleoproterozoic changcheng group with an ascending trend from 3 to 0 we observed variation of approximately 0 1 in the mesoproterozoic jixian group and positive values of 2 2 characterize the lower neoproterozoic qingbaikou group stratigraphic variations in 34 s cas are more remarkable in their ranges and magnitudes including conspicuously high values exceeding 30 in the three intervals at ca 1700 ma 1300 1100 ma and 1000 900 ma in the changcheng group 34 s cas values are typically higher than 25 with only a few values of less than 15 in contrast most of the data spanning from the mesoproterozoic tieling formation of the jixian group to the lower neoproterozoic jingeryu formation of the qingbaikou group are highly variable between 10 and 25 with some values exceeding 25 in the late paleoproterozoic 1700 1600 ma a 10 decrease in 34 s cas and 3 increase in 13 c carbonate are coincident with and likely related to the breakup of columbia a supercontinent that predated rodinia carbon and sulfur isotope data from the mesoproterozoic when global tectonic activity was comparatively weaker fall mostly in the ranges of 15 10 and 0 1 respectively but fluctuations of 20 for 34 s cas and 3 for the 13 c carbonate at ca 1450 1400 ma may reflect subduction and large scale magmatic activity in island arcs marking the end of columbia breakup from the late mesoproterozoic ca 1300 1100 ma to the early neoproterozoic ca 800 ma the 13 c and 34 s of seawater increased gradually with increasing variability most impressive are 34 s cas values that exceed 30 in two intervals at ca 1300 1100 ma and ca 1000 900 ma which may reflect the assembly and early breakup of rodinia although gaps in the record remain and studies of even higher resolution are warranted our results suggest that changes in paleoceanographic conditions linked to global tectonics strongly influenced the biogeochemical cycles of c and s furthermore periods of the proterozoic previously noted for their isotopic invariability show clear isotopic expressions of this tectonic activity

geochemical characterization of late pleistocene and holocene tephra layers from the basin of mexico central mexico in order to aid palaeoenvironmental research of late pleistocene and holocene deposits of central mexico tephra layers collected from the sediments of the texcoco and chalco sub basins in the southern part of the basin of mexico are geochemically characterized and used as stratigraphic markers the tephra layers range in composition from basaltic andesites to rhyolites and are calc alkaline the tephras range in age from 34 000 to ca 260014c yr b p new names are used informally to designate correlated tephras the tlahuac tephra is present in chalco at a depth of 18 m in the southeastern part of texcoco at a depth of around 10 m and at the tlapacoya archaeological site where it had been mistakenly described as basaltic this basalt andesite tephra is dated to at least 34 00014c yr b p the tlapacoya 1 tephra is dated to between 15 020 450 and 14 430 190 yr b p and is present in all chalco sections the tlapacoya 2 tephra corresponds to the previously described pomez gruesa con fragmentos de andesita ca 14 400 yr b p and is present in all chalco and texcoco sections the likely source of these three tephras is the volcano popocatepetl tephra ii at chalco dates to 12 520 135 yr b p and correlates with the upper toluca pumice from nevado de toluca volcano these represent the first geochemical glass shard analysis of tephras from the basin of mexico and so further research is necessary before a reliable tephrochronology can be established

spatial variability of environment change in the teotihuacan valley during the late quaternary paleopedological inferences the teotihuacan valley mexico has been the object of considerable interest because of its importance in the context of human history in america although archeological research has been extensively conducted to understand causes of the decline of teotihuacan culture no sufficient scientific evidence has been provided this paper is focused on paleopedological evidence especially properties that constitute soil memory in order to reconstruct the spatial distribution of environment variability during the late pleistocene and holocene three profiles that include modern soils and buried paleosols were studied in different geomorphic positions uplands cerro gordo site cg lower talus slope transition to valley bottom maseca ma and the mouth of the valley at the ancient border of the former texcoco lake this last site is near tepexpan te where some of the oldest human remains in mexico were recovered the oldest paleosol is at cg at 3000 m a s l it is dated at 22 000 yr bp and exhibits a truncated profile with a very well developed bt horizon dominated by kaolinite with phytoliths of c3 plants all of its features are related to a humid probably warm environment it was classified as a luvisol this paleosol is buried by another luvisol dated at 13 000 yr bp with morphological characteristics similar to the previous soil although clay mineralogy consists mostly of halloysite and a higher percentage of c4 plant phytoliths is apparent in ma the oldest paleosol was not recognized it is probable that the two paleosols found in cg merge into one at ma which shows strong redoximorphic features but high percentages of c4 plant phytoliths and halloysitic clays at the te site paleosols are less developed fluvisols modern soil cover also varies in relation to its geomorphic position in cg it is a phaeozem while in ma and te present day soils have carbonate accumulation we interpret prevailing conditions during the last glacial maximum in the teotihuacan valley to be humid with a tendency towards drying at the end of pleistocene dry conditions prevail in the valley today

quantitative estimates of orbital and millennial scale climatic variability in central mexico during the last 40 000 years abstract diatom based transfer functions for salinity precipitation and temperature were developed using a training set that included data from 40 sites along central mexico these transfer functions showed good performance parameters and were subsequently applied to the previously published diatom record from lake chalco southern basin of mexico heinrich stadials hs 3 to hs 0 were reconstructed as cold and dry events with hs 2 representing the coldest and driest conditions while hs 1 shows a complex bimodal cooling pattern orbital scale variability is also recorded high and variable lake salinities from 34 to 29 ka bp marine isotope stage 3 mis 3 corresponded with times of maximum summer insolation and evidences of frequent fires mis 2 was identified as a cold period with low lake salinity associated with low evaporation with two cold intervals during the minima in spring 25 5 ka bp and summer 20 ka bp insolation the 20 ka bp cold event together with hs 2 24 23 ka bp defined a bimodal maximum cooling during the last glacial maximum lgm 24 5 19 5 ka bp within the range of pollen based estimates further supporting a maximum tropical cooling of 4 5 c for the lgm the deglacial 19 5 11 5 ka bp showed the highest precipitation anomalies with three peaks that closely correlated with glacier advances in the nearby mountains iztaccihuatl the early holocene marked a change towards high lake salinities and the highest positive temperature anomalies 3 5 c during a peak in summer insolation even though at orbital scale chalco showed a nearly opposite trend to the record from the more southerly lake peten itza guatemala at millennial scale it showed a common pattern of cold and dry conditions during hss comparable to other tropical latitude sites in the northern hemisphere and further supporting the idea that there is a strong coupling between tropical and higher latitudes climates in particular with the north atlantic

mineral and organic components of the buried paleosols of the nevado de toluca central mexico as indicators of paleoenvironments and soil evolution abstract results of earlier studies of quaternary tephra paleosol sequences in central mexico revealed contradictions between paleopedological and lacustrine records to settle the contradictions selected quantitative characteristics of mineral and organic components of the paleosols pt1 pt7 from the nevado de toluca sequence were studied as independent paleoclimate proxies mineralogical composition of sand and clay fractions allows assessment of weathering and secondary mineral formation both being climate dependent processes stable carbon isotope ratios of humus and phytolith morphology indicate paleovegetation in all paleosols the sand fractions consist mainly of unstable volcanogenic minerals although the grains have weathering features short durations of pedogenesis are responsible for limited weathering status the clay fraction is dominated by halloysite and dehydrated halloysite in units pt1 pt4 which suggests drier periods during paleosol formation whereas major amounts of kaolinite in the units pt5 pt7 indicate extended humid pedogenesis 13c values of pt2 pt4 humus are intermediate between those characteristic of c3 and c4 plants phytolith assemblages which include both forms of c3 dominant and c4 minor quantities grasses agree with carbon isotope data these data indicate plant cover changes driven by wet dry paleoclimate oscillations the new results showing drier stages during the formation of late pleistocene paleosols pt2 and pt3 correct the primary interpretation of these units as indicating only humid pedogenesis and settles the contradiction between paleopedological and lacustrine records

some problems in the late quaternary pollen records of central mexico basins of mexico and zacapu most pollen analyses of late quaternary sediments have been carried out in closed basins of the trans mexican neovolcanic belt in central mexico climatic changes have been inferred from these pollen data however some problems arise from reinvestigations the dominance of a few pollen types in general pinus quercus and alnus is almost constant among these records in spite of the importance of vulcanism and tectonism in the evolution of this region no effects of these factors are evident from the pollen records some of the problems are illustrated in the pollen records from lake texcoco and lake chalco in the basin of mexico and lake zacapu in michoacan

acoustical radiation from thermally stressed sea ice observations of the sound generated by cracking ice in the canadian arctic during a cooling period provide an opportunity for studying the response of different types of ice to thermal stress synthetic aperture radar sar images and air photographs of ice around the experimental site allow mapping of cracking events back to their points of origin thus providing a correlation between ice type and acoustical activity it is shown that there are only a few cracking events in the first year ice during periods of air temperature variation whereas multiyear ice produces a large number and is responsible for most of the ambient sound recorded analysis of several individual events including both natural and artificial sources reveals that failure processes cause acoustic emissions with an angularly dispersive radiation pattern in which higher frequencies tend to be radiated downward and lower frequencies radiated toward the horizontal this pattern can be explained in terms of two types of eigenmodes used in

diagenetic xenotime age constraints on the sanjiaotang formation luoyu group southern margin of the north china craton implications for regional stratigraphic correlation and early evolution of eukaryotes abstract eukaryote fossil bearing ruyang and luoyu groups are extensively exposed along the southern margin of the north china craton but their age has long been a matter of controversy due to the absence of reliable radiometric dates by dating detrital zircons and diagenetic xenotimes from the sanjiaotang formation of the luoyu group this paper confirms that the ruyang and luoyu groups were deposited during the period of 1750 1400 ma the new age constraints suggest that the sanjiaotang formation is coeval with the xiamaling formation of the jixian group on the northern margin of the north china craton correlation of the luoyu group with the jixian group is thus indicated the ruyang and luoyu groups contain abundant fossils of eukaryotic organisms such as acritarch and macroalgae accordingly eukaryotic proliferation extends back to the mesoproterozoic consistent with results of molecular phylogenetic analysis that a rapid morphological diversification of eukaryotic organisms occurred prior to 1400 ma

a high elevation holocene pollen record from iztaccihuatl volcano central mexico pollen data from 3860 m in the agua el marrano valley on the northwest side of iztaccihuatl volcano southeastern part of the basin of mexico 198n combined with data on the glacial chronology and tephrochronology document holocene climatic and vegetation changes near the present timberline glacial advances occurred between 12 000 and 10 500 cal yr bp milpulco 1 and between 8300 and 7300 cal yr bp milpulco 2 a 450 cm core was drilled in sediments accumulated in a milpulco 1 moraine depression the sequence covers the last 11 000 cal yr bp according to three 14 c dates and the presence of a distinctive pumice deposit dated elsewhere at 5700 cal yr bp the pollen record of agua el marrano shows barren conditions after glacier retreat followed by a gradual colonization by elements of the alpine grasslands around 10 900 cal yr bp this plant community remained until c 7200 cal yr bp between c 7200 cal yr bp and 6500 cal yr bp coniferous forest grew close to or at the site but alpine grasslands were re established between 6500 cal yr bp and c 5000 cal yr bp apparently in connection with mid holocene dry conditions recorded in central mexico the upper timberline attained its modern altitude 4000 m in the last 3000 years during this period no significant changes are observed in the pollen spectra comparison with other palaeoecological records from central mexico suggests that the timberline fluctuated in response to both temperature and precipitation changes during the holocene