late pleistocene pinon pines in the chihuahuan desert abstract examination of late pleistocene packrat middens from the northern and central chihuahuan desert disclosed macrofossils of colorado pinon pinus edulis and texas pinon p remota radiocarbon dating indicates that texas pinon was widespread in trans pecos texas and northeastern mexico between 30 000 and 11 000 yr b p today it is found in small refugia east of its former range in the late pleistocene colorado pinon occurred at lower elevations on the northern edge of the chihuahuan desert both species occurred in the hueco mountains near el paso texas no clear evidence was found of the presence of mexican pinon p cembroides though today it is abundant in the davis and chisos mountains a paleoclimate is postulated that had the following characteristics increased winter precipitation from pacific frontal sources reduced summer temperatures and precipitation and milder winter temperatures due to a reduced frequency of arctic airmass incursion winter precipitation appears to have decreased from north to south while winter temperatures and possibly summer precipitation increased from north to south during the late pleistocene the northern chihuahuan desert was dominated by woodlands of pinon pines junipers and oaks the desert scrub communities that characterize the area today are a holocene phenomenon

vegetation east of the rocky mountains 18 000 years ago abstract the various lobes and segments of the southern periphery of the laurentide ice sheet reached their maximum extension at different times between 21 000 and 14 000 yr ago but the climap date of 18 000 yr ago is taken as a reference level to review the distribution of major vegetational formations in central and eastern united states tundra was apparently confined to a narrow belt peripheral to the ice margin only in the minnesota area and from northern pennsylvania to new england with extensions down the crest of the appalachian highlands at least as far as maryland some areas south of the great lakes may later have been marked by treeless vegetation briefly as the ice retreated the boreal forest to the south in the central united states was dominated by spruce the jack pine that had prevailed during previous times was apparently eliminated by the time the ice reached its maximum in the appalachian highlands and the atlantic coastal plain however jack pine occurred along with spruce which decreased in importance southward the southern limit of the boreal forest in the southeast was perhaps somewhere in southern georgia and alabama oak and other temperate deciduous trees were minor components of the boreal coniferous forests especially in the southern appalacchians but there is no evidence yet in the southeastern states for a relic mixed mesophytic forest 18 000 yr ago similar to the rich modern deciduous forests of the region except possibly in the lower mississippi valley the climate in much of the southeast was apparently dry as well as cool at that time in florida oak pine scrub and prairie like openings prevailed and all but the deepest lakes dried up

sources of glacial moisture in mesoamerica paleoclimatic records from mesoamerica document the interplay between atlantic and pacific sources of precipitation during the last glacial stage and holocene today and throughout much of the holocene the entire region receives its principal moisture in the summer from an interaction of easterly trade winds with the equatorial calms glacial records from sites east of 95 w in guatemala florida northern venezuela and colombia record dry conditions before 12 ka however west of 95 w glacial conditions were moister than in the holocene for example pollen and diatom data show that lake patzcuaro in the central mexican highlands was cool deep and fresh during this time and fossil pinyon needles in packrat middens in chihuahua sonora arizona and texas indicate cooler glacial climates with increased winter precipitation cold gulf of mexico sea surface temperatures and reduced strength of the equatorial calms can explain arid full and late glacial environments east of 95 w whereas an intensified pattern of winter westerly air flow dominated hydrologic balances as far south as 20 n overall cooler temperatures may have increased effective moisture levels during dry summer months in both areas

quaternary glacial stratigraphy and chronology of mexico the volcano iztaccihuatl in central mexico was glaciated twice during the middle pleistocene once probably in pre illinoian or pre bull lake time and once in late illinoian or bull lake time glaciation during the late pleistocene was restricted to the late wisconsin or pinedale a maximum advance and one readvance are recorded in the early part and one readvance in the latter part three or four small neoglacial advances occurred during the holocene two other volcanoes nearby ajusco and malinche have a partial record of late pleistocene and holocene glaciations three others popocatepetl pico de orizaba and nevado de toluca have a full holocene record of three to five glacial advances during neoglaciation

palaeolimnological approaches to climate change with special regard to the biological record abstract lakes systems respond physically chemically and biologically to changes in climate and these responses are registered in many various ways in lake sediment records this paper focuses principally on the biological record and describes recent research that aims to reconstruct i past variability in effective moisture and ii past variability in temperature changes in effective moisture are manifested especially by changes in lake water level in closed lake basins water level change can cause significant changes in water salinity that is registered in sediments by changes in the composition of fossil assemblages especially diatoms in open freshwater lake basins changes in water level are best reconstructed from the aquatic macrofossil record and potentially from the ratio of open water to littoral species of cladocera ostracods and diatoms the most promising biological method for reconstructing past temperature is chironomid analysis and a range of new chironomid temperature transfer functions are being developed however biological responses to changing temperature are usually indirect as a result of temperature effects on water column stratification nutrient cycling and alkalinity generation and in most cases a correct interpretation of the climatic significance of changes in the fossil record requires an understanding of the processes that are operating in the water column a multi proxy whole lake approach is advocated as the best way forward not only as a means of reconstructing past climate but also as a means of assessing the impact of climate change on lake systems

climatic variability in the northern sector of the american tropics since the latest mis 3 abstract we inferred millennial scale climate variations and paleohydrological conditions in the northern sector of the american tropics for 30 3 5 5 cal ka bp using geochemical characteristics of sediments from lake chalco in central mexico the sediment sequence is chronologically constrained with three tephra and nine radiocarbon dates temporal variations in titanium total inorganic carbon total organic carbon titanium ratio carbon nitrogen ratio and silica titanium ratio indicate changes in runoff salinity productivity and sources higher concentrations of ti indicate more runoff during latest marine isotope stage mis 3 30 3 28 6 cal ka bp runoff was lower during the last glacial maximum lgm 23 19 cal ka bp than during the heinrich 2 event 26 24 cal ka bp the interval of reduced runoff continued up to 17 5 cal ka bp but increased during the bolling allerod trends of decreasing runoff and increasing salinity are observed throughout mis 1 lake chalco received less runoff during the lgm compared to deglaciation opposite the trend of other north american tropical records different amounts of rainfall at different sites are possibly due to shifts in the position of the intertropical convergence zone changes in the size of the altlantic warm pool and varying sea surface temperatures of the atlantic and pacific oceans

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

paleoclimatic changes during the late pleistocene holocene in laguna babicora near the chihuahuan desert mexico the paleoclimatic variability of northern mexico since the late wisconsinan is determined from sedimentological geochemical and stratigraphic studies of the lacustrine sequence of laguna babicora 29 4 n 107 7 w elevation 2100 m asl chronological control is based on 13 radiocarbon dates 6 from this study and 7 previously reported from four stratigraphic profiles that cover a range from 16 342 to 2800 bp two major periods of increased humidity are recognized by lake level variation during the late wisconsinan and early holocene the late wisconsinan humid period may be related to jet stream migration to southerly latitudes and the influence of the mobile polar high this period has been identified in the sedimentary record of el diablo profile with a radiocarbon date of 16 342 201 bp the early holocene humid period is recorded in el diablo 10 976 115 bp and cano profiles 9614 130 bp it is interpreted as a result of the younger dryas climatic anomaly characterized by a temperature decrease and cold paleoclimatic conditions similar to those for the late wisconsinan the early holocene was followed by an overall gradual drying that reached a maximum around 6000 bp during the middle holocene for the late holocene evidence of widespread erosion sedimentation changes flooded surfaces and paleosoils indicates that laguna babicora was subjected to short term climatic changes

orbital and internal forcing of climate on the yucatan peninsula for the past ca 36 ka abstract a 19 6 m sedimentary sequence from lake quexil guatemala is presented as a proxy record of climatic change on the low lying yucatan peninsula over the last ca 36 ka long term climatic fluctuations are attributed to orbital forcing of insolation while abrupt changes during the late glacial period are the result of internal forcing mechanisms that remain imperfectly modelled gcms simulating meltwater cooling of the gulf of mexico may predict some but not all climatic responses in the circum caribbean lake quexil did not experience greater aridity than predicted by orbital forcing during late glacial zones q lg 1 and 2 which were drier only in comparison with holocene conditions q lg 1 appears to have been colder than expected based on insolation alone and is contemporary with the younger dryas event between meltwater pulses 1a and 1b thus other feedbacks also may have cooled surface temperatures in the gulf of mexico such as unique late glacial circulation patterns increased albedo for the florida and yucatan peninsulas or surface temperatures on the north american continent additional terrestrial sequences are needed to improve the chronology and confirm the distribution of sub regional climatic variability in the circum caribbean

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

quantification of salt dust pathways from a groundwater fed lake implications for solute budgets and dust emission rates 1 emissions of salt dust from the shores of saline lakes significantly impact lake chemistry air quality transportation human health and climate quantitative methods for assessing these emissions however are still in the developmental stage we investigate salt pathways from groundwater to dust using an approach that takes advantage of opportune conditions at a groundwater fed saline lake in the nebraska sand hills region the mass of salt in the lakeshore surface crust and soil was measured as well as in the dust on the surrounding dune field these data together with information on the lake hydrology show that dust emission is an important mechanism controlling lake salinity even though a mere fraction of the salt crust is deflated each year under extant climatic conditions wind data collected at the lake site indicate high wind speeds capable of dust mobilization therefore the physical and chemical bonding of salts in the crust is offered as the primary limiting factor for dust emission rates

holocene environmental change in the zacapu basin mexico a diatom based record the zacapu basin michoacan mexico is one of a series of internally drained lake basins in the central highlands of mexico the basin has a history of occupation spanning at least 2000 years and before the arrival of the spanish was a major centre for the purepecha culture results are presented of diatom studies from four lacustrine sediment sequences which have holocene radiocarbon dates cemca point 1 cemca point 4 the zacapu core and zacapu section 1 these lie along a transect from the former marsh floor up to the western slopes of the basin zacapu section 1 indicates a deep lake in the basin at some time but the date on this section is thought to be unreliable and the sediments are probably pleistocene in age the other sequences record few major changes in water level in the basin although dry episodes occurred about 4500 bp and more severely about 1000 bp a period of marked catchment disturbance about 7000 to 6000 bp is believed to be the result of volcanic activity on the western flank

palaeoclimates in central and south america since 18 000 bp based on pollen and lake level records abstract the last 18 ka of palaeoclimates are discussed at 3 ka intervals based on palaeoenvironmental changes shown in pollen and lake level records from central and south america a total of 61 pollen records fulfill the requirements of continuity and chronology to be used in this synthesis 8 records from tropical and subtropical lowlands in central and south america 7 records from the central american highlands 26 records from the high andes north and south of the equator 2 records from the southern subtropics and 18 from the southern andes and subantarctic islands palaeoclimatic interpretation of the pollen records is semiquantitative based on bioclimatic parameters such as e g elevational temperature gradients or latitudinal precipitation gradients that can be related to vegetation zonation additional palaeoclimatic information is obtained from lake level records derived either from dated shorelines or from the limnological history of lake sediment sections large scale palaeoclimatic comparison of the proxy data records indicates that full and late glacial climates throughout central and south america were cooler than today by about 4 to 5 c but that moisture patterns showed regional differentiation thus in both the southern subtropics and high latitudes in southern chile conditions were substantially moister than today during full and late glacial times whereas everywhere else conditions were far drier than today in terms of palaeocirculation this scenario can be interpreted as a poleward shift of the subtropical as well as the westerly circulation a scenario that has been suggested by palaeoclimatic simulations from general circulation models gcm another major palaeoclimatic aspect predicted by the gcm simulations is the suppressed early holocene summer monsoon in the southern subtropics although proxy records are few from this zone their palaeoenvironmental history during that time shows a period of aridity that would support the model one of the most controversial aspects of south american palaeoclimates concerns the existence of a younger dryas equivalent a cold period between 11 and 10 ka bp based on the existing data from south america that show a great diversity of palaeoclimatic trends the concept of an interval that is characterized by the same climatic signal as documented for the north atlantic seems too simplistic moreover at this point only very few records have been analysed in sufficient detail to yield unambiguous climatic information for the interval in question the principal outcome of this palaeoenvironmental and palaeoclimatic synthesis is the better understanding of the type and quality of data available at present and the realization of regions and periods where much more and concerted effort is needed to address problems of intra and interhemispheric palaeoclimatic correlations only then can questions concerning the mechanism of climatic change be tackled successfully

holocene climate change and human impact central mexico a record based on maar lake pollen and sediment chemistry abstract this paper presents multiproxy pollen magnetic susceptibility sediment chemistry records from two maar crater lakes in the valle de santiago of guanajuato mexico hoya san nicolas and hoya rincon de parangueo sediment cores from the two sites have basal dates of ca 11 600 and 9600 cal yr bp respectively the san nicolas results show that the lake desiccated several times during the holocene and this resulted in the selective destruction of the less resistant pollen types a comparative analysis of data from both sites indicates that during the earliest holocene ca 11 600 9000 cal yr bp there were short term oscillations of wetter and drier climate during the early holocene ca 9000 5700 cal yr bp it was on average wetter during the middle holocene ca 5700 3800 cal yr bp drier and during the middle to late holocene ca 3800 2200 cal yr bp wetter increasing human disturbance during the late holocene of environment obscures the climatic record agricultural activities in the area began as early as ca 5700 cal yr bp and intensified around 2400 cal yr bp

the last glacial maximum in the basin of mexico the diatom record between 34 000 and 15 000 years bp from lake chalco lacustrine sediments from a section of a core extracted from the central part of the former lake chalco were analysed for diatom content radiocarbon dates and tephra correlations bracket the period under study between 34 and 15 ka bp according to the diatom stratigraphy chalco was an alkaline saline lake during the interval 34 31 5 ka bp after 31 5 ka bp it went through a shallower and more concentrated phase until ca 23 5 ka bp at ca 23 5 ka bp a major environmental change took place during which lake chalco was transformed from an saline alkaline pond into a freshwater lake previous archaeological work at tlapaocya indicates that humans entered the basin slightly after this environmental change by ca 22 ka bp at 19 ka bp another reduction in water level occurred with the onset of a period of quick fluctuations between slightly acidic and neutral water these variations were probably associated with intensified volcanic activity that affected the area between 19 and 15 ka bp

using pedotransfer functions in vadose zone models for estimating groundwater recharge in semiarid regions 1 process based vadose zone models are becoming common tools for evaluating spatial distributions of groundwater recharge gr but their applications are restricted by complicated parameterizations especially because of the need for highly nonlinear and spatially variable soil hydraulic characteristics shcs in an attempt to address the scarcity of field shc data pedotransfer functions ptf were introduced in earlier attempts to estimate shcs however the accuracy of this method is rarely questioned in spite of significant uncertainties of ptf estimated shcs in this study we investigated the applicability of coupling vadose zone models and ptfs for evaluating gr in sand and loamy sand soils in a semiarid region and also their sensitivity to lower boundary conditions first a data set containing measured shcs was used in the simulations a second data set contained correlated shcs drawn from the covariance matrix of the first data set the third shc data set used was derived from a widely used ptf although standard deviations for individual parameters were known for this ptf no covariance matrix was available hence we assumed that the parameters of this ptf were uncorrelated thereby potentially overestimating the volume of the parameter space results were summarized using histograms of gr for various sets of input parameters under the unit gradient flow lower boundary condition the distributions of gr for sand and loamy sand significantly overlap values of gr based on mean shcs or gr generally lie off the mode of the gr distribution this indicates that the routinely used method of taking gr as a regional representation may not be viable more importantly the computed gr largely depends in a nonlinear fashion on the shape factor n in the van genuchten model under the same meteorological conditions a coarser soil with a larger n generally produces a higher gr therefore the uncertainty in computed gr is largely determined by the uncertainty in estimated n by ptfs e g mean and standard deviation under the constant head lower boundary condition upward soil moisture flux may exist from the lower boundary especially for regions with shallow water tables where upward flux exists choosing an appropriate lower boundary condition is more important than selecting shc values for calculating gr the results show that the distribution of gr is less scattered and gr is more intense if the constant head lower boundary is located at deeper depths

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

eolian sediments generated by anthropogenic disturbance of playas human impacts on the geomorphic system and geomorphic impacts on the human system abstract in many of the earth s arid and semiarid lands saline lakes playas and similar landforms are disturbed as a result of human activity diversion and or consumptive use of surface or groundwaters has created the effect of a climate change in numerous drainage basins resulting in the desiccation of lakes and reactivation of eolian processes at many locations playas are natural sites for extensive eolian activity because of the deposition of clastic and chemical sediments in basins by surface water via fluvial transport and groundwater via efflorescence wind erosion and deposition of playa sediments has had a major role in the development of landforms and sedimentary units in the present lunette fields worldwide simpson desert australia and geological past from the triassic mercia mudstone england to the quaternary lahontan basin and cima volcanic field usa anthropogenic disturbance or desiccation of playa systems has resulted in the eolian transport of sand e g lop nor china konya basin turkey rajasthan india kappakoola australia several sites in west africa and or dust e g aral sea kazakhstan uzbekistan old wives lake canada kara bogaz gol ex ussr lake texcoco mexico owens dry lake mono lake and other playas usa typically this is accomplished by abstraction of water and or removal of vegetation from terminal lake basins an extensive review of the literature documents many examples and or potential examples of such phenomena in numerous nations the reactivation of eolian processes from closed basins produces air pollution in the form of fugitive dust naturally occurring compounds released into the atmosphere by human actions and has significant environmental and economic impacts on human activities in the surrounding areas restoration or mitigation of degraded land on or surrounding playas has been accomplished at lake texcoco kara bogaz gol and the konya basin and is being actively implemented at mono lake owens dry lake and the aral sea

evidence of the younger dryas in central america abstract three sites provide records that address whether the younger dryas was manifest in central america marine records from barbados and the cariaco basin are discussed as well as possible climatic forcing mechanisms sequences from montane la chonta bog costa rica and lowland lake quexil guatemala record a temperature decline of 1 5 2 5 c dated between 11 070 10 400 14c years bp and ca 12 000 10 300 14c years bp respectively the record from lake la yeguada panama does not include cooling although significant reorganization of local communities occurred during the younger dryas chronozone colder tropical ssts throughout deglaciation contributed to sub regional climatic variation more intense monsoonal activity and cooler air temperatures however the temperature reversals are not associated with the coldest tropical ssts which occurred during meltwater pulses the temperature reversals appear to be a response to extra tropical conditions presumably in the north atlantic which suggests that younger dryas cooling extended as far south as costa rica in panama monsoonal conditions may have blocked cooler temperatures precipitation continued to increase throughout deglaciation due to strong forcing by seasonal solar insolation in central america while desiccation events occurred to the north and south

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

the timing of late pleistocene mammalian extinctions in north america more than 375 14c dates from 150 fossil sites in north america have been analyzed to evaluate the question of extinction of late pleistocene megafauna when critically evaluated no 14c ages for any extinct pleistocene genera are younger than 10 000 yr b p

comment on pleistocene glaciation of volcano ajusco central mexico and comparison with the standard mexican glacial sequence by sidney e white and salvatore valastro jr

long and short term change in the patzcuaro basin central mexico abstract data are presented from four new cores from the patzcuaro basin in the volcanic highlands of central mexico which provide new insights into climate change and human impact over the last 23 000 cal yr 19 000 14c yr bp the cores have been analysed for a range of proxies including mineral magnetic properties loss on ignition diatoms and stable isotope composition 18o and 13c high lake levels are recorded in the late pleistocene and early holocene with a probable shift from winter precipitation from the pacific in the late pleistocene to the modern summer regime in the early holocene gradual drying and increased climatic variability are indicated from the early holocene but particularly since 4000 cal bp consistent with southward displacement of the itcz the changing chemical composition and depth of the lake led to the accumulation of ostracod layers the timing of the late holocene dry intervals corresponds to global periods of rapid change driven by bond and or solar cycles by combining a range of proxies we have been able to identify climatic change even within a period of significant human disturbance the use of multiple cores indicates that a coherent climatic signal is recorded even in a basin subject to tectonic disturbance this study also provides new data to support the interpretation of major human impact on the basin in both the pre and post hispanic periods refuting continued suggestions that basin wide erosion occurred only after the spanish conquest a new model of sediment un mixing has been applied to identify different source materials contributing to deposition

tocuila mammoths basin of mexico late pleistocene early holocene stratigraphy and the geological context of the bone accumulation abstract we report new stratigraphic tephrochronology and dating results from the tocuila mammoth site in the basin of mexico at the site there is evidence for a thin meteorite airburst layer dated between 10 878 and 10 707 cal bc at the onset of the younger dryas yd cool period the upper toluca pumice utp tephra marker caused by a plinian eruption of the nevado de toluca volcano dated from 10 666 to 10 612 cal bc is above that layer the eruption must have caused widespread environmental disruption in the region with evidence of extensive reworking and channelling by the lake texcoco shoreline and contributed to the widespread death and or extinction of megafaunal populations as suggested by earlier authors but the new work reinforces the view that both catastrophic events must have caused large environmental disruption in a short time period of around two hundred years there is no evidence for megafauna mammoths sabre toothed cats camels bison glyptodonts after the utp volcanic event and subsequent lahars in the basin of mexico at tocuila although there are some in situ tephra markers in nearshore lake sediments such as the great basaltic ash gba and the utp ash there is evidence of much reworking of several tephra populations in various combinations the mammoth bone accumulation is reworked in a lahar sequence volcanic mudflow derived from several source sediments but associated with the major utp plinian eruption paleoindian populations were also present in the basin of mexico during the yd period where several paleoindian skeletons were found associated with the utp ash deposits e g metro man chimalhuacan man and tlapacoya man

orbital pacing and ocean circulation induced collapses of the mesoamerican monsoon over the past 22 000 y the dominant controls on global paleomonsoon strength include summer insolation driven by precession cycles ocean circulation through its influence on atmospheric circulation and sea surface temperatures however few records from the summer north american monsoon system are available to test for a synchronous response with other global monsoons to shared forcings in particular the monsoon response to widespread atmospheric reorganizations associated with disruptions of the atlantic meridional overturning circulation amoc during the deglacial period remains unconstrained here we present a high resolution and radiometrically dated monsoon rainfall reconstruction over the past 22 000 y from speleothems of tropical southwestern mexico the data document an active last glacial maximum 18 24 cal ka b p monsoon with similar 18o values to the modern and that the monsoon collapsed during periods of weakened amoc during heinrich stadial 1 ca 17 ka and the younger dryas 12 9 11 5 ka the holocene was marked by a trend to a weaker monsoon that was paced by orbital insolation we conclude that the mesoamerican monsoon responded in concert with other global monsoon regions and that monsoon strength was driven by variations in the strength and latitudinal position of the intertropical convergence zone which was forced by amoc variations in the north atlantic ocean the surprising observation of an active last glacial maximum monsoon is attributed to an active but shallow amoc and proximity to the intertropical convergence zone the emergence of agriculture in southwestern mexico was likely only possible after monsoon strengthening in the early holocene at ca 11 ka

foraminiferal faunal evidence for glacial interglacial variations in the ocean circulation and the upwelling of the gulf of tehuantepec mexico abstract three marine sediment cores from the gulf of tehuantepec eastern tropical north pacific were used to determine variations in upwelling intensity and surface circulation patterns over the last 26 kyr bp changes in foraminiferal assemblages at glacial interglacial timescale indicate a sequence of three major palaeoceanographic events in the study area 1 the late pleistocene including the last glacial maximum lgm dominated by a neogloboquadrina dutertrei globorotalia menardii assemblage indicative of a stable thermocline stratified water column and greater persistence of warm equatorial waters in the region 2 the termination 1 dominated by a globigerinita glutinata assemblage which indicates mixed surface water and transitional conditions between upwelling and stratified water column episodes changes in sedimentation rates and a clear reduction of the faunal abundances at this time could be the result of eustatic sea level change 3 the holocene dominated by a globigerina bulloides globigerinoides ruber assemblage indicates periods of elevated productivity intense upwelling and the establishment of the modern ocean conditions these findings are consistent with climate model results that suggest that regional surface ocean and upwelling conditions are mostly linked to changes in atmospheric circulation over the laurentide ice sheet and shifts in the latitudinal position of the intertropical convergence zone

prehispanic and colonial landscape change and fluvial dynamics in the chalco region mexico abstract the late quaternary deposits of the lakeshore plain and western edge of lake chalco in the southeastern basin of mexico were explored in order to understand the influence of alluvial sedimentation on archeological visibility and preservation as well as to seek out records that would permit direct comparison of erosion and sedimentation with the demographic record derived from the archeological survey the chalco lakeshore plain is traversed by two of the largest rivers in the region and the pattern of human settlement recorded here by archeological survey is partly an artifact of alluvial sedimentation where the most prominent settlements were observed in areas with little or very slow sedimentation detailed radiocarbon dating of the alluvial deposits indicates that prior to the establishment of a sedentary lifestyle alluvial sedimentation was slow catastrophic soil erosion is inferred from a period of widespread rapid alluviation and mass movements during late preceramic early and middle formative periods which were periods of rapid population growth mass movements appear to coincide also with periods of wetter climate no evidence of sedimentation was noted during the population collapse that occurred at the end of the formative the next phase of alluvial sedimentation occurred during the early classic to late toltec periods which were also periods of population expansion during a gradually drying climate the postclassic period which was the largest pre columbian demographic expansion was paradoxically accompanied by minor evidence of alluvial activity the last and most widespread period of alluvial activity occurs in the post conquest period and is difficult to date with precision but appears to occur primarily during the seventeenth and eighteenth centuries

lipid biomarkers in lacustrine sediments of subtropical northeastern mexico and inferred ecosystem changes during the late pleistocene and holocene abstract evolution of ecosystems in the northeastern mexico since the last glacial maximum lgm and through the holocene are yet to be fully characterised creating a gap in our understanding of the ecological response to climate change in this water scarce region here we explore the response of lacustrine and surrounding terrestrial habitats of this region to hydrological changes of the late pleistocene and especially the holocene biomarker based proxy data from the el potosi basin indicates dynamic and variable input of organic matter om into the basin with sources being the surrounding vegetation bacterial biomass and aquatic microfauna changes in these inputs reveal distinct stages of ecosystem development over the last 20 000 years overall low lipid abundances as a proportion of toc from 20 to 19 cal kyr bp mainly from aquatic in situ microbial biomass and degraded terrestrial om are attributed to relatively low runoff increasing lipid proportions during the deglaciation from 19 to 15 cal kyr bp were mostly derived from terrestrial sources like woodland vegetation soil and wetland vegetation and we attribute them to more rainfall and runoff subsequently the wetland biomarkers disappeared and woodland input began to dominate the om assemblage from 15 to 11 7 cal kyr bp during the early and mid holocene after about 11 7 cal kyr bp putative woodland and grass inputs decreased and the dominant vegetation appeared to shift to subtropical desert shrub which could have been due to increasing regional aridity the onset of the dry late holocene was characterised by gradual environmental transition and ecosystem adaption especially from 4 2 to 2 8 cal kyr bp the terrestrial vegetation comprised increasing proportions of c4 grasses documented by changes in leaf wax carbon isotopic compositions and potentially more open vegetation while the in lake bacterial production increased with increasing desertification from 2 8 cal kyr bp the terrestrial productivity decreased substantially the lake became shallower ephemeral and the surrounding soils became more alkaline changes of the limnic habitat from algal towards microbial communities occurred 300 years after the onset of terrestrial habitat changes during the last millennium the c3 components of the vegetation again dominated this implies an enhanced moisture supply possibly from agricultural activities overall this work reveals the marked sensitivity of northeastern mexico ecosystems to the climatic shifts that occurred since the lgm and this perhaps indicates their vulnerability to future global warming

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 late quaternary palaeolimnological record from jamaica based on trace element chemistry of ostracod shells abstract the trace element chemistry of ostracod shells provides one of the most promising methods for palaeolinmological reconstruction in this paper molar sr ca and mg ca ratios in shells of the ostracod cypretta brevisaepta furtos 1934 were used to reconstruct the late quaternary palaeolimnology of wallywash great pond a small freshwater coastal lake in jamaica analyses of modern jamaican ostracods were undertaken to establish molar distribution coefficients kd d m s for this species k d sr 0 306 0 069 k d mg 0 0142 0 0088 these values were used to reconstruct past levels of sr ca water and mg ca water for a 9 23 m lake sediment core from wallywash great pond the results were coupled with previously published stable isotope 18 o and 13 c determinations on fine grained calcite to reconstruct the salinity palaeohydrology and palaeoproductivity of the lake over the last 125 ka b p 10 3 calendar years before present three major highstands with slightly elevated salinity occurred during marine isotope stage 5 under elevated sea level and a humid climate the lake dried out 93 5 ka b p as sea level felll and the climate became drier it refilled around 10 7 ka b p three separate highstands during the holocene were sustained by humid conditions and high relative sea level although there were small increases in salinity during the holocene when sea level was close to the deepest part of the lake significant saltwater intrusion appears to have been prevented by freshwater discharge from inland throughout the late quaternary the salinity of the great pond showed low absolute variations from 0 3 to 0 6 o the long term trends in lake level reflect both orbitally induced changes in insolation and eustatic sea level water level fluctuations during the holocene however reflect climatic changes on the millennial timescale rather than orbital forcing or sea level change

effects of pleistocene environmental changes on the distribution and community structure of the mammalian fauna of mexico article i nfo biological communities in mexico experienced profound changes in species composition and structure as a consequence of the environmental fluctuations during the pleistocene based on the recent and fossil mexican mammal checklists we determine the distribution composition diversity and community structure of late pleistocene mammalian faunas and analyze extinction patterns and response of individual species to environmental changes we conclude that 1 differential extinctions occurred at family genus and species level with a major impact on species heavier than 100 kg including the extinction all proboscideans and several ruminants 2 pleistocene mammal communities in mexico were more diverse than recent ones and 3 the current assemblages of species are relatively young furthermore pleistocene relicts support the presence of biogeographic corridors important refugia existed as well as centers of speciation in isolated regions we identified seven corridors eastern usa sierra madre oriental corridor rocky mountains sierra madre occidental corridor central united states northern mexico corridor transvolcanic belt sierra madre del sur corridor western usa baja california corridor tamaulipas central america gulf lowlands corridor and sonora central america pacific lowlands corridor our study suggests that present mammalian assemblages are very different than the ones in the late pleistocene

algae influence the hydrophysical parameters of a sandy soil abstract article history received 2 august 2011received in revised form 31 january 2012accepted 3 february 2012available online xxxxkeywords algaebiological soil crustsoil water repellencysand dunesorptivityhydraulic conductivity biological soil crusts have a major effect on water ow in soils two study sites located at a pine forest gladecovered with a biologicalsoil crust formed the basis of our study the sand soil atthe surface glade soil wascompared to a control soil pure sand with limited impact of vegetation or organic matter occurring at50 cm depth beneath a glade area to assess the in uence ofalgae in the biological soil crust on the propertiesof pure sand a coccal green alga choricystis minor lamentous green alga klebsormidium subtile and stra menopile alga tribonema minus were isolated from the top layer of glade soil and grown in the lab in petridishes on sterile pure sand as monoalgal and bialgal c minor and k subtile crusts for 3 5 7 9 11 13 15 17 19 and 21 days atthe end ofeach growth stage the hydrophysicalparametersof crusted sand were estimat ed after drying at 50 c for 15 h equivalent to a 3 day hot spell and compared to the parameters of puresand the hydrophysical parameters were substantially different between the two surfaces the glade soilhad an index of water repellency about 18 times that of pure sand and the persistence of water repellencyalmost 54 times that of pure sand both sorptivity and hydraulic conductivity in the glade soil were about7 those of the pure sand respectively the growth of arti cial algal crusts characterized by an increase inorganic carbon content from 0 16 to 0 33 resulted in an increase in water drop penetration time of thedried crusts up to 14 times that of the pure sand and a decrease in the water sorptivity of the dried crustsup to 10 that of the pure sand whereas k subtile crusts both monoalgal and bialgal with c minor hadup to a 9 decrease in hydraulic conductivity compared to pure sand there was no impact of monoalgalc minor and t minus crusts on hydraulic conductivity k subtile was possibly limited to surface growth whereas c minor and t minus penetrated to depth consequently k subtile may have clogged the toppores more effectively than the other two strains the water repellency cessation time increased with anincrease in water drop penetration time for all the dried monoalgal and bialgal crusts used in this study agreater impact of k subtile on the shifts in hydraulic behaviour could in uence water capture and storage potentially decreasing evaporation during dry periods but enhancing overland ow diminishing leachingduring wet periods 2012 elsevier b v all rights reserved

space weathering induced via microparticle impacts 1 modeling of impact velocities and flux of micrometeoroids from cometary asteroidal and interstellar origin in the main asteroid belt and the near earth environment

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

taphonomy pathology and paleoecology of the terminal pleistocene marmes rockshelter 45fr50 big elk cervus elaphus southeastern washington state usa in 1968 remains of what were reported to be a larger than modern elk cervus elaphus were recovered from terminal pleistocene sediments associated with the marmes rockshelter archaeological site in southeastern washington state originally thought to have been butchered by humans it is associated with radiocarbon dates suggesting an age of about 9800 14 c years b p taphonomic analysis in 2009 indicates the elk likely died of natural causes during winter months it was lightly scavenged by carnivores prior to burial from silt rich spring runoff the elk suffered from two pathological conditions one resulting in fusion of the fourth and fifth cervical vertebrae and the other resulting in exces sive bone tissue on the proximal ends of the first ribs seventh cervical and first and second thoracic vertebrae the marmes elk is larger than modern rocky mountain elk c e nelsoni and is on the large end of the size range of modern roosevelt elk c e roosevelti it is also larger than the similarly aged elk skeleton from three hills alberta a single elk bone from the sentinel gap archaeological site in central washington state dated to about 10 200 14 c years bp and located 130 km west of marmes rockshelter is the same size as the same bone of the marmes elk terminal pleistocene elk in eastern washington likely grew to exceptionally large size as a result of abundant grass at the time forage that de creased in abundance as holocene climatic conditions developed

late pleistocene paleohydrology near the boundary of the sonoran and chihuahuan deserts southeastern arizona usa abstract ground water discharge gwd deposits form in arid environments as water tables rise and approach or breach the ground surface during periods of enhanced effective precipitation where preserved these deposits contain information on the timing and elevation of past ground water fluctuations here we report on the investigation of a series of gwd deposits that are exposed in discontinuous outcrops along a 150 km stretch of the san pedro valley in southeastern arizona near the boundary of the sonoran and chihuahuan deserts chronologic isotopic geochemical faunal assemblage ostracodes and gastropods and sedimentological evidence collectively suggest that the elevation of the regional water table in the valley rose in response to a change in climate 50 ka ago and remained relatively high for the next 35 ka before falling during the bolling allerod warm period rebounding briefly during the younger dryas cold event and falling again at the onset of the holocene the timing of these hydrologic changes coincides closely with variations in 18o values of calcite from a nearby speleothem to the west and changes in lake levels at pluvial lake cochise to the east thus in southeastern arizona the assumption that changes in climate are reflected in all aspects of the hydrologic cycle of a region simultaneously is validated the timing of these changes also broadly coincides with variations in the gisp2 18o record which supports the hypothesis that atmospheric teleconnections existed between the north atlantic and the deserts of the american southwest during the late pleistocene

solar wind as the origin of rapid reddening of asteroid surfaces a comparison of the laboratory reflectance spectra of meteorites with observations of asteroids revealed that the latter are much redder with the spectral difference explained by space weathering though the actual processes and timescales involved have remained controversial a recent study of young asteroid families concluded that they suffered only minimal space weathering here we report additional observations of those families revealing that space weathering must be a very rapid process the final colour of a silicate rich asteroid is acquired shortly after its birth within 10 6 years of undergoing a catastrophic collision this rapid timescale favours solar wind implantation as the main mechanism of space weathering as laboratory experiments have shown that it is the most rapid of several competing processes we further demonstrate the necessity to take account of composition when evaluating weathering effectiveness as both laboratory and asteroid data show an apparent dependence of weathering on olivine abundance the rapid colour change that we find implies that colour trends seen among asteroids are most probably due to compositional or surface particle size properties rather than to different relative ages apparently fresh surfaces most frequently seen among small near earth asteroids may be the result of tidal shaking that rejuvenates their surfaces during planetary encounters

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

holocene paleohydrology of the etzatlan magdalena basin in western central mexico and evaluation of main atmospheric forcings abstract sediments deposited in the etzatlan magdalena basin were analyzed to reconstruct the paleohydrological variations and generate a new holocene paleoclimate record for the less studied western part of central mexico concentration of ti abundance of caco 3 zr ti ratio and toc ti ratio were compared with their average values to infer variations in runoff lake water salinity transportation of clastic minerals by aeolian activity and total organic productivity in general conditions were wetter during 9 6 5 7 ka early middle holocene and 2 2 0 7 ka late holocene compared to 5 7 2 2 ka middle late holocene transportation of above average amounts of zr bearing clastic minerals from an arid watershed and deposition of the highest abundance of caco 3 at 3 5 2 8 ka occurred within the drier interval of middle late holocene except for the interval of 2 2 0 7 ka the el nino southern oscillation enso and inter tropical convergence zone itcz controlled summer precipitation forced the hydrological variations the etzatlan magdalena basin remained wet and some sites in central mexico were dry over a large part of 2 2 0 7 ka the activity of tropical cyclones and frontal systems may have increased in the northeastern pacific during this interval of enhanced enso either of them or the combination of both possibly contributed more autumn and winter precipitation to western central mexico

spatial trends in saturated hydraulic conductivity of vegetated dunes in the nebraska sand hills effects of depth and topography summary in the nebraska sand hills nsh the largest vegetated dune field in the western hemisphere dunes have relatively high hydraulic conductivities and surficial aquifers can partially retain a significant fraction of precipitation as groundwater recharge and return it for preservation of plants this important mechanism for maintaining the stability of the ecosystem in the nsh requires knowledge of saturated hydraulic conductivity k fs in the vadose zone a unique 3d dataset for the distribution of k fs was collected within the framework of the grassland destabilization experiment gdex at the barta brothers ranch site nebraska using direct in situ measurements the results show that k fs increases with depth towards the lower boundary of the root zone up to 2 m for the topographic effects highlands generally present higher k fs values than lowlands and there exists a significant correlation between k fs and absolute elevation it is found that the effect of short term vegetation disturbance on k fs was minimal that may not hold in the long run

52 000 years of environmental history in zacapu basin michoacan mexico the magnetic record abstract mineral magnetic and total organic carbon toc measurements were performed on a 10 9 m long core from zacapu basin central mexico the sequence dated by ams 14 c spans the last ca 52 kyr curie temperature measurements low temperature susceptibility and low temperature remanence indicate that the magnetic fraction is characterized by oxidized titanomagnetites with variable ti content low s 300 ratios reflect the existence of high coercivity iron oxide phases these characteristics are considered to reflect the presence of goethite rather than hematite formed in relatively wet organic conditions where soil forming processes were intense comparison between magnetic and toc results suggests that the sediments record relatively humid conditions before 35 kyr bp and a later trend towards drier conditions a probable hiatus is recorded before 25 kyr and relatively dry conditions after this time that persisted throughout the late glacial maximum and mid holocene at 4 8 kyr these results contrast with previous observations of deep lake phases in the neighboring lake patzcuaro considered to be result of increased precipitation between 34 and 21 kyr bp

preliminary results from laguna minucua a potentially annually resolved record of climate and environmental change for the past 5000 years in the mixteca alta of oaxaca mexico abstract despite several decades of research focusing on prehispanic human ecology debate continues over the impact of climatic and anthropogenic landscape change on human populations in mesoamerica one problem in identifying the cause of this change is the lack of high resolution paleo environmental data from many regions the southern mexican highlands in particular have yielded few paleoenvironment data yet have a rich and diverse cultural history the sedimentary record from laguna minucua located within the sierra madre del sur oaxaca offers an exceptional opportunity to address human and environmental interactions in the region minucua is a small 0 25 ha currently shallow pond with no apparent inlets or outlets we retrieved two sediment cores from the site 3 5 m and 5 6 m long the cores are highly laminated core chronology was developed with paleomagnetic secular variation data and compared with couplet counts and limited radiometric measurements these data indicate that the minucua record at least spans the last 4500 100 years we discuss preliminary results that assess long term environmental change for the region through examination of geochemical and magnetic susceptibility data in particular we discuss in more detail a 500 year time slice which encompasses the period known as the classic collapse the record indicates overall dry conditions but with two extended wet periods experienced between 1160 and 1120 cal yr bp and 1060 1000 cal yr bp we place our findings in the context of current archaeological and paleoclimatological research in oaxaca

pollen and non pollen palynomorphs of lake chalco as indicators of paleolimnological changes in high elevation tropical central mexico since mis 5

holocene aged human footprints from the cuatrocienegas basin ne mexico two sets of well preserved human footprints have been found in tufa sediments in the cuatrocienegas basin ne mexico and here we present their u series dates of 10 55 0 03 ka and 7 24 0 13 ka the former are the oldest known footprints in mexico although their exact location is unknown the latter form part of a trackway with eleven in situ human footprints oxygen and to a lesser extent carbon isotope data from the sediments suggest that the tufa with in situ footprints formed during a transition to a wetter less arid period while pollen evidence indicates the basin floor presence of pecan carya and willow salix sp before the onset of regional chihuahuan desert aridity these footprints confirm the presence of humans possibly nomadic hunter gatherer groups which persisted until the 18th century ad

tepexpan palaeoindian site basin of mexico multi proxy evidence for environmental change during the late pleistocene late holocene the tepexpan palaeoindian skeleton was discovered in 1947 close to the former lake texcoco margin in the basin of mexico the find has been the object of considerable interest and discussion over the last 60 years regarding its real age and archaeological interpretation here we report new ams radiocarbon dates associated with the sedimentary succession at tepexpan with ages between 19 110 90 and 612 22 14c years bp and a new uranium series date for the skeleton with an age of 4700 200 years bp that indicates a mid holocene age the sedimentary succession was studied in detail using stable isotopes diatoms organic geochemistry and tephrochronology the multi proxy evidence suggests large changes around the margins of lake texcoco in terms of the balance between aquatic and terrestrial plants c3 and c4 plants saline alkaline and freshwater conditions volcanic activity marginal reworking of lake sediments and input from the drainage basin through the late pleistocene late holocene these changes had large impacts on the prehistoric human populations living by the lake shores since the late pleistocene in the basin of mexico

glacial interglacial variability in upwelling and bioproductivity off nw mexico implications for quaternary paleoclimate sedimentary accumulation of biogenic components organic carbon opal and biogenic barium on the northwestern mexican margin declined during every glacial interval of the past 140 kyr indicating decreases in upwelling induced productivity during cold periods the glacial interglacial contrasts in upwelling on this margin are attributed to reversals in land ocean thermal contrast the waxing and waning of the laurentide ice sheet and consequent responses of the western hemisphere wind fields this scenario is consistent with three independent lines of evidence terrestrial paleoclimatic data general circulation model results and our marine records this pattern of glacial interglacial variability in upwelling off nw mexico is opposite to that observed in other low latitude and midlatitude upwelling areas such as the eastern equatorial pacific these results add to a growing pool of observations that the response of oceanic upwelling to glacial climatic forcing has been regionally variable

soil organic carbon within the vadose zone of a floodplain past studies have focused on carbon variation in the upper 1 m of the soil profile however there is limited information on carbon variation at deeper depths e g 0 4 m and mathematical functions to extrapolate carbon content at these depths the objective of this study was therefore to assess the vertical variation in soc reached 4 m of the tarim river floodplain in northwestern china the vertical distribution in soc was described by exponential and power functions based on 1 soil depth 2 soil depth and silt content 3 soil depth and soc at the shallowest and deepest depths 4 soil depth silt content and soc at the shallowest and deepest depths and 5 soil depth and soc at the shallowest depth we found soc content decreased with depth from 6 82 g kg 1 at 0 0 2 m to 1 0 g kg 1 below 3 2 3 4 m averaged across five locations along the floodplain both the power and exponential functions provided a good fit to the measured data in the upper 1 m of the soil profile whereas the power function provided a better fit to the data when extrapolating to a depth of 3 4 m the power function describing soc as a function of soil depth silt content and soc at the shallowest and deepest depths best portrayed the distribution in soc with depth considering the cost and labor in measuring soil properties our results suggest that soc at the shallowest depth can provide good estimates of the vertical distribution in soc in a floodplain

vegetation history along the eastern desert escarpment of the sierra san pedro martir baja california mexico abstract plant macrofossils from 38 packrat middens spanning the last 33 000 cal yr bp record vegetation between 650 and 900 m elevation along the eastern escarpment of the sierra san pedro martir northern baja california the middens span most of the holocene with a gap between 4600 and 1800 cal yr bp but coverage in the pleistocene is uneven with a larger hiatus between 23 100 and 14 400 cal yr bp the midden flora is relatively stable from the pleistocene to holocene exceptions include pinus californiarum juniperus californica and other chaparral elements that were most abundant 23 100 cal yr bp and declined after 14 400 cal yr bp despite being near the chaparral woodland desertscrub ecotone during glacial times the midden assemblages reflect none of the climatic reversals evident in the glacial or marine record and this is corroborated by a nearby semi continuous pollen stratigraphy from lake sediments regular appearance of c4 grasses and summer flowering annuals since 13 600 cal yr bp indicates occurrence of summer rainfall equivalent to modern jas average of 80 90 mm this casts doubt on the claim based on temperature proxies from marine sediments in the guaymas basin that monsoonal development in the northern gulf and arizona was delayed until after 6200 cal yr bp

the taxonomic distribution of asteroids from multi filter all sky photometric surveys abstract the distribution of asteroids across the main belt has been studied for decades to understand the current compositional distribution and what that tells us about the formation and evolution of our solar system all sky surveys now provide orders of magnitude more data than targeted surveys we present a method to bias correct the asteroid population observed in the sloan digital sky survey sdss according to size distance and albedo we taxonomically classify this dataset consistent with the bus and binzel bus s j binzel r p 2002 icarus 158 146 177 and bus demeo et al demeo f e binzel r p slivan s m bus s j 2009 icarus 202 july 160 180 systems and present the resulting taxonomic distribution the dataset includes asteroids as small as 5 km a factor of three in diameter smaller than in previous work such as by mothe diniz et al mothe diniz t carvano j m a lazzaro d 2003 icarus 162 march 10 21 because of the wide range of sizes in our sample we present the distribution by number surface area volume and mass whereas previous work was exclusively by number while the distribution by number is a useful quantity and has been used for decades these additional quantities provide new insights into the distribution of total material we find evidence for d types in the inner main belt where they are unexpected according to dynamical models of implantation of bodies from the outer solar system into the inner solar system during planetary migration levison h f bottke w f gounelle m morbidelli a nesvorn d tsiganis k 2009 nature 460 july 364 366 we find no evidence of s types or other unexpected classes among trojans and hildas albeit a bias favoring such a detection finally we estimate for the first time the total amount of material of each class in the inner solar system the main belt s most massive classes are c b p v and s in decreasing order excluding the four most massive asteroids 1 ceres 2 pallas 4 vesta and 10 hygiea that heavily skew the values primitive material c p types account for more than half main belt and trojan asteroids by mass most of the remaining mass being in the s types all the other classes are minor contributors to the material between mars and jupiter

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

biogeographic implications of a packrat midden sequence from the sacramento mountains south central new mexico abstract thirteen packrat neotoma spp and two porcupine erethizon dorsatum middens from 1555 to 1690 m elevation from the sacramento mountains new mexico provide an 18 000 yr vegetation record in the northern chiuahuan desert the vegetation sequence is a mesic wisconsin fullglacial 18 000 16 000 yr b p pinyon juniper oak woodland a xeric early holocene ca 11 000 8000 yr b p juniper oak woodland a middle holocene ca 8000 4000 yr b p desert grassland and a late holocene ca 4000 yr b p to present chihuahuan desertscrub the frequency of spring freezes and summer droughts in the late wisconsin probably set the northern limits of pinus edulis and juniperus monosperma at about 34 n or 6 south of today s limit rising summer tempratures in the early holocene eliminated pinyon and other mesic woodland plants from the desert lowlands and allowed the woodland to move upslope at this time pinyon juniper woodland and pine forest dominated by pinus ponderosa probably began their spectacular holocene expansions to the north continued warming in the middle holocene led to very warm summers with strong monsoons relatively dry cold winters and widespread desert grasslands desertscrub communities in the northern chihuahuan desert did not develop until the late holocene when the biseasonal rainfall shifted slightly back toward the winter catastrophic winter freezes decreased and droughts in all seasons increased the creosote bush desertscrub corridor across the continental divide between the chihuahuan and sonoran deserts was probably connected for the first time since the last interglaciation

climatic variability over the last 30 000 years recorded in la piscina de yuriria a central mexican crater lake the trans mexican volcanic belt provides an excellent setting for reconstruction of late quaternary climate from different natural archives moreover human impact on the landscape since the mid holocene provides a good opportunity to investigate the complex interplay of natural and anthropogenic forcing of landscape change however despite the wealth of records understanding of the environmental history of the region and its wider significance for climate change across the northern neotropics remains incomplete we present a radiocarbon dated multiple proxy sedimentology sedimentary geochemistry ostracods diatoms stable isotopes record of climatic and environmental change based on the lacustrine sediments from la piscina de yuriria a hydrologically closed volcanic crater in the northern tmvb much of the last glacial interval was characterised by low effective moisture associated with a weakened north american monsoon nam although the interval from 30 000 to 27 500 abp experienced abrupt changes in rainfall the period corresponding to the late glacial stadial was also dry and the lake may have dried out at this time there was a change to wetter but variable conditions during the early holocene as the nam strengthened progressive drying during the later holocene was accompanied by phases of catchment disturbance which were partly the result of human impact

big game extinction caused by late pleistocene climatic change irish elk megaloceros giganteus in ireland abstract megaloceros giganteus the largest eurasian deer inhabited ireland from ca 12 000 yr b p to the time of its extinction ca 10 600 yr b p the archaeologic record documents that people arrived on the island no earlier than 9000 yr b p so they could not have caused the extinction in ireland close stratigraphic association of the geologically youngest elk fossils with sediments indicating the onset of the nahanagan stadial approximately younger dryas implicates climatic change as the exterminator palynologic data support the idea that extinction probably resulted when forage quantity and quality along with length of the spring green up decreased during the nahanagan stadial for m giganteus this meant that the energy intake required to sustain large bodies grow enormous antlers and build fat reserves for winter was increasingly difficult to maintain until deaths primarily by winterkill outnumbered births

paleoecological and climatic changes of the upper lerma basin central mexico during the holocene the record of almoloya lake in the upper lerma basin starts with the deposition of the late pleistocene upper toluca pumice layer the data from this interval indicate a period of climatic instability that lasted until 8500 cal yr b p when temperature conditions stabilized although moisture fluctuations continued until 8000 cal yr b p between 8500 and 5000 cal yr b p a temperate climate is indicated by dominance of pinus from 5000 to 3000 cal yr b p quercus forest expanded suggesting a warm temperate climate a first indication of drier environmental conditions is an increase in grassland between 4200 and 3500 cal yr b p during the late holocene 3300 to 500 cal yr b p the increase of pinus and grassland indicates temperate dry conditions with a considerable increase of pinus between 1100 and 950 cal yr b p at the end of this period humidity increased the main tendency during the holocene was a change from humid to dry conditions during the early holocene almoloya lake was larger and deeper the changing humidity regime resulted in a fragmented marshland with the presence of aquatic and subaquatic vegetation types

paleoecological studies at lake patzcuaro on the west central mexican plateau and at chalco in the basin of mexico abstract a 1520 cm sediment core from lake patzcuaro michoacan mexico is 44 000 yr old at the base all parts of the core have abundant pollen of pinus pine alnus alder and quercus oak with frequent abies fir the interval dated from 44 000 to 11 000 yr ago has a homogeneous flora characterized by abundant juniperus juniper pollen and frequent artemisia sagebrush it is believed to represent an appreciably drier and colder climate than at present the holocene at lake patzcuaro is characterized by a moderate increase in pinus pollen and the loss of juniperus pollen as the modern type of climate succeeded alnus was abundant until about 5000 yr ago its abrupt decrease with the first appearance of herbaceous weed pollen may reflect the cutting of lake shore and stream course alder communities for agricultural purposes or it may simply reflect a drying tendency in the climate pollen of zea corn appears at lake patzcuaro along with low peaks of chenopod and grass pollen at 3500 yr b p apparently recording a human population large enough to modify the natural environment as well as the beginning of agriculture a rich aquatic flora in this phase suggests eutrophication of the lake by slope erosion in the most recent period corn is absent from the sediments perhaps reflecting a change in agricultural practices the environment changes at lake patzcuaro are similar to and correlate with those in the cuenca de mexico where diatom stratigraphy from the chalco basin indicates fluctuations in lake levels and lake chemistry in response to variations in available moisture before 10 000 yr ago climates there were cool and dry and the chalco basin was occupied by a shallow freshwater marsh that drained north to lake texcoco where saline water accumulated by evaporation increases in effective moisture and possible melting of glaciers during the holocene caused lake levels to rise throughout the cuenca de mexico and lake texcoco flooded the chalco basin with brackish water after 5000 yr ago such flooding decreased and shallow freshwater ponds and marshes were restored in the chalco basin this environmental change coincides with the appearance of zea pollen and suggests cultural control of lake levels and salinity

late pleistocene woodlands in the bolson de mapimi a refugium for the chihuahuan desert biota abstract packrat middens radiocarbon dated at 12 280 345 and 12 700 165 yr b p record expansions of junipers and papershell pinyon pinus remota into the desert lowlands of durango and coahuila mexico 26 n extralocal trees and shrubs presently occur 24 580 km in nearly all directions including more subtropical areas to the northeast and southeast an equable late wisconsin climate marked by mild winters with increased precipitation and by cool summers with reduced summer monsoons is proposed the extensive playas of the bolson de mapimi probably held water at that time the bolson de mapimi was not a geographical refugium unaffected by glacial climates although many chihuahuan desert plants and animals probably remain in situ as members of equable woodlands equable climates low extinction rates and repeated rapid glacial interglacial climatic fluctuations may have been important in the evolution and accumulation of species at lower latitudes

accelerator radiocarbon dating at the molecular level molecular level 14c dating is the isolation of specific classes of molecules for their 14c dating by accelerator mass spectrometry ams complex matrices such as fossil bone are difficult to date due to their extreme chemical heterogeneity by isolating individual amino acids contaminants humates are removed and crystalline amino acids result bones with 0 1 0 2 n and collagenous compositions can be dated accurately because structural collagen is present contaminants are removable with xad resin bones with 0 1 n and non collagenous compositions yield dates hundreds to thousands of years too young because most of the preserved organic matter is exogenous accelerator 14c dates on collagenous and non collagenous bones are not comparable due to intrinsic dating inaccuracies ams 14c dating of amino acids demonstrated that 1 post 10 800 year ages for north american megafauna are due to sample contamination not holocene ages on extinct fauna 2 a clovis age 10 900 years was established for a human fossil from the anzick site montana 3 holocene ages cannot be established absolutely for many north american human fossils because the bones were non collagenous 4 accurate ages are attainable on vertebrate fossils as small as passerine birds from pacific island localities 5 well preserved bones are datable without their destruction by extracting protein with water at high temperatures and 6 stratigraphic anomalies to 45 000 years in european upper paleolithic rock shelters are recognizable by dating bone directly

lake level and salinity reconstruction from diatom analyses in quillagua formation late neogene central andean forearc northern chile abstract a sedimentary and micropaleontological study of the quillagua formation provides a detailed paleohydrological reconstruction of the lacustrine system which occupied the present day hyperarid quillagua llamara fore arc basin northern chile from lattermost miocene 5 8 0 4 ma to early pliocene times diatom and lithofacies analyses were carried out in two correlated stratigraphic sections of the lacustrine system the quebrada temblor section is located at the southern margin of the ancient lake and is directly influenced by the freshwater inputs of a northward flowing fluvial system draining the precordillera and calama regions the cerro mogote section occupies a western marginal location in the northern zone sheltered from direct fluvial inputs from the south but subjected to the activity of the alluvial fan systems of the coastal range a mostly shallow oligosaline waterbody occupied the basin during the interval studied though with fluctuations in salinity and the extent of the inner open waters the southern margin sector represented by quebrada temblor had in general terms a palustrine oligosaline character with almost freshwater conditions during certain periods which favoured the establishment of semi permanent to permanent freshwater plumes overlying a saline waterbody development of shoreline facies subjected to desiccation events was also characteristic at the top of this section the innermost shallow lacustrine areas represented by cerro mogote maintained more homogeneous oligosaline characteristics and more persistent open waters they were also subjected to freshwater pulses mediated in this case by the activity of the terminal alluvial fan zones paleohydrological evolution of the basin was strongly conditioned not only by shifts in the climatic tectonic system but by variations in the local hydrological parameters four hierarchical orders of variability in the lake level of the basin were distinguished both by stratigraphic analysis of lithofacies and from changes undergone by the diatom record strong intrasample mixing of diatoms of incompatible salinity spectra and the presence of fine laminated lacustrine facies in some terms define the highest order short term intra or inter annual pulses experienced by the lacustrine system higher than 6th order diatom based punctuated interruptions of the minor order bathymetrical trends highlight the high frequency variability in the basin probably 6th order 0 001 0 01 ma while the deepening shallowing facies defined by the arrangement of the decimetre thick lithological sequences revealed a lower order of variability 5th order 0 01 0 1 ma combined lithofacies and diatom analyses delineate the lowest order of variability 4th order 0 1 1 ma allowing the reconstruction of two well established highstand and lowstand situations which implied the existence of a regressive trend between two transgressive trends this order of variability is related to the interplay of tectonic events in the calama basin and the regional climatic evolution during late neogene to pliocene times

23 000 yr of vegetation history of the upper lerma a tropical high altitude basin in central mexico abstract pollen analysis on a 9 54 m sediment core from lake chignahuapan in the upper lerma basin the highest intermontane basin in central mexico 2570 m asl documents vegetation and limnological changes over the past 23 000 14c yr the core was drilled near the archaeological site of santa cruz atizapan a site with a long history of human occupation abandoned at the end of the epiclassic period ca 900 ad six radiocarbon ams dates and two well dated volcanic events the upper toluca pumice with an age of 11 600 14c yr b p and the tres cruces tephra of 8500 14c yr b p provide the chronological framework for the lacustrine sequence from ca 23 000 14c yr b p to ca 11 600 14c yr b p the plant communities were woodlands and grasslands based on the pollen data the glacial advances mii 1 and mii 2 correlate with abundant non arboreal pollen mainly grasses from ca 21 000 to 16 000 14c yr b p and at ca 12 600 14c yr b p during the late pleistocene lake chignahuapan was a shallow freshwater lake with a phase of lower level between 19 000 and 16 000 14c yr b p after 10 000 14c yr b p tree cover in the area increased and a more variable lake level is documented late holocene ca 3100 14c yr b p deforestation was concurrent with human population expansion at the beginning of the formative period 1500 b c agriculture and manipulation of the lacustrine environment by human lakeshore populations appear at 1200 14c yr b p 550 a d with the appearance of zea mays pollen and abundant charcoal particles

diatom inferred salinity changes from the yushe paleolake indicate an aridification during the pliocene pleistocene transition in north china abstract the global climate changed significantly during the pliocene pleistocene transition and a worldwide aridification occurred in order to investigate the influence of the aridification during the pliocene pleistocene transition in the east asian interior diatom assemblages from the upper zhangcun formation north china deposited during 2 7 2 5 ma bp were analyzed and the lake salinity was reconstructed using diatom environment transfer functions a total of 74 diatom taxa were found in the sediments belonging to 21 families and 32 genera four zones were established from the bottom to the top based on the changes in the diatom assemblages over the 15 7 m sediment sequence in zone 1 cyclotella iris dominated in assemblage while caloneis westii and staurosira construens were less presented among representative species in zone 2 c iris dominated which indicates carbonate rich water but in zone 3 benthic pennate taxa such as c westii and anomoeoneis sphaerophora became more abundant indicating a significant increase in water salinity from carbonate type to sulfate type caused by climatic change in zone 4 c iris became dominant again with abundant gomphonema intricatum and rare pinnularia rathsbergiana and martyana martyi species the salinity inferred by conductivity reconstruction increased from 1 9 ppt in zone 2 to 4 1 ppt in zone 3 the significant changes of anion ratio alk cl so42 indicate an abrupt change from carbonate rich to a sulfatic type water the increase of salinity reflects changes in the precipitation evaporation balance the change from carbonate to sulfatic water inferred from the diatom assemblages could indicate a significant aridification around ca 2 6 ma bp on the eastern edge of the loess plateau this is the most easterly record of aridification across the eastern asian interior and may be related with the global cooling and the development of the northern hemisphere glaciation

late quaternary glaciations of teyotl volcano central mexico geomorphic and stratigraphic evidence of three glacial advances were found on the late pleistocene lavas of teyotl volcano 4660 m located in the iztaccihuatl volcanic complex ca 19 n the first glaciation terminal moraines at 3400 3200 m took place prior to 14 500 years bp the second one terminal moraines at 3800 4000 m between 14 500 and 4800 years bp apparently at the end of the pleistocene the third one terminal moraines at 4060 4240 m occurred after 4800 years bp most likely around 2000 years bp small rock glaciers developed above 4300 m probably during the little ice age correlations are suggested with the glacial advances of other mountains of central mexico assuming that moist and cool cold climates are responsible for glacial advances the timing of glaciations is hypothesized in the light of existing palaeoecological records

the pretoria saltpan a 200 000 year southern african lacustrine sequence abstract the pretoria saltpan is a circular crater 1130 m in diameter and is situated some 40 km n of pretoria lat 25 34 30 long 28 04 59 e a recent tube sampling and core drilling programme has revealed an infilling consisting of some 90 m of fine lacustrine sediments chiefly organic muds underlain below 30 m depth by micrites which rest upon a further 61 m of coarse clastic debris granite bedrock was encountered at 151 m broad sedimentary zones correspond with major phases in the evolution of the crater lake superimposed cyclical patterns of accumulation reflect environmental changes on millenial to seasonal timescales 14c age determinations on algal debris from the upper 20 m of the core indicate a mean rate of sedimentation of about 1 m 2000 yr suggesting that the lacustrine sequence may span almost 200 000 yr over this period major environmental changes are apparent from sedimentological chemical mineralogical and isotopic analyses of the core and studies of the pollen spectra and diatom assemblages present within it this long continental sequence is therefore providing a high resolution palaeoenvironmental record for southern mid latitudes over much the same period as is covered by the vostok ice core in antarctica

the last 30 000 years of faunal history within the grand canyon arizona the vertebrate fauna of the last 30 000 radiocarbon years in the grand canyon is reviewed faunas accompanied with 92 14c dates have been analyzed from nine cave sites four systematically excavated and 50 packrat middens reasonably precise chronological and environmental data of late pleistocene and holocene age were obtained through dung studies in rampart muav and stanton s caves from the numerous packrat middens and from a ringtail refuse deposit in vulture cave the desert tortoise 8 species of lizards 12 species of snakes 68 species of birds and 33 species of mammals are identified extinct animals include the avian carrion feeder teratornis merriami and the mammalian herbivores oreamnos harringtoni camelops cf hesternus equus sp and nothrotheriops shastense there is no apparent abrupt end to the late pleistocene as observed in the grand canyon fossil faunal or floral record animal and plant taxa of the grand canyon responded individually to the changes in climate of the last 30 000 yr both animal and plant fossil assemblages indicate that a pre full glacial a full glacial and a late glacial woodland community with many less dominant desert taxa were slowly replaced by a holocene desert community all woodland taxa were absent from the lower elevations of the grand canyon by 8500 yr b p

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

the eruptive history of the patzcuaro lake area in the michoacan guanajuato volcanic field central mexico field mapping c 14 and 40ar 39ar geochronology abstract monogenetic volcanic fields occur in many tectonic settings representing an unpredictable and uncertain volcanic hazard the patzcuaro lake is located in the central part of the michoacan guanajuato volcanic field mgvf field mapping of an area of 560 km2 stratigraphy and new radiometric ages 40ar 39ar and 14c aided in the identification of 45 individual volcanic landforms formed during the past 3 9 ma these landforms comprise five medium size shield volcanoes 25 scoria cones 10 lava domes two lavas associated to scoria cones one fissural lava flow and two volcanic complexes these volcanic edifices erupted a minimum volume of 20 km3 since pliocene with an output rate of 0 02 km3 ka chemical composition of magmas ranges from 47 6 to 65 3 wt silica being dominantly andesites 76 vol followed by basaltic andesites 12 vol dacites 9 vol and basalts 3 vol volcanic products vary from mainly andesitic during the pliocene basaltic to dacitic during the pleistocene and basaltic andesitic to andesitic during the holocene all these rocks are the result of low dipping slab subduction related magmatism alignment of cones and structures follow ne sw n s and e w directions the youngest faults are associated with the e w morelia acambay fault system with historic earthquake activity the volcanic complexes el estribo and la muela have cone collapses to the north that correspond with the direction of maximum regional extension associated with the e w fault system the 3 9 ma history of monogenetic volcanism around the patzcuaro lake demonstrates that monogenic fields can be long lived volcanic features

paleoindian sites from the basin of mexico evidence from stratigraphy tephrochronology and dating we present new data on the stratigraphy dating and tephrochonology at the most important paleoindian sites in the basin of mexico these include a penon woman iii with the oldest directly radiocarbon dated human remains 10 755 75 bp b tlapacoya with two human crania dated to just over 10 ka bp c tocuila an important mammoth site with incorporation of fossils and suggested bone tools within the upper toluca pumice utp lahar volcanic mudflow the tocuila site also includes potential evidence for a layer associated with the younger dryas meteorite airburst with charcoal iron microspherules micro tektites melted glass and volcanic ash dated to 10 800 50 bp and d the santa isabel iztapan mammoths i and ii with lithics of scottsbluff lerma and angostura types and obsidian prismatic blades but lacking the characteristic fluted clovis type points normally associated with mammoth kills and butchering and dated after the pumice with andesite pwa layer between 14 500 bp and 10 900 bp before the younger dryas interval these results show that these lithic traditions in central mexico are older than in the great plains of usa several tephra markers are recognised in the sites that help to constrain the stratigraphy and dating of the archaeological sequences however tephra reworking in marginal lake sites is present and has been carefully considered especially for the pwa tephra

field evidence of a negative correlation between saturated hydraulic conductivity and soil carbon in a sandy soil this study was performed as a part of the grasslanddestabilization experiment at the university of nebraska sbarta brothers ranch site bbrs in the eastern nsh figure 1 which investigates the ecological and geomor phic stability of the nsh from an interdisciplinary perspec tive and focuses on atmosphere land surface groundwaterinteractions wang et al 2008 the nebraska sand hills isthe largest native grassland stabilized sand dune area inthe western hemisphere and an important groundwaterrecharge source for the high plains aquifer loope andswinehart 2000 mean annual temperature at the site is8 1 c and mean annual precipitation is 576 mm about90 of the landscape at the site is composed by uplanddunes and dry interdunal areas covered by native warm season grasslands while the remaining 10 consists of wetmeadows and wetlands holocene dune sands overlie qua ternary and or pliocene alluvial sand and silt with low somcontent throughout the nsh