computer analysis of regional groundwater flow and boundary conditions in the basin of mexico abstract mexico city is situated in the valley of mexico on the extensive lacustrine clays that overlay highly productive aquifers of both volcanic and sedimentary origin the valley is closed by volcanic mountains the natural hydraulic boundary conditions associated withe mountain ranges and their relationship to the important aquifers were studied using a two dimensional steady state finite element model in cross section four cross sections were analysed under hydrologic conditions existing prior to the large scale pumping of the aquifers factors such as bulk hydraulic conductivities and regional infiltration rates were obtained from field observations and the literature to assess location of the associated groundwater divides and the water table in the mountains the modeled flow patterns are consistent with the historical hydrologic records piezometric characteristics and observed surface features of the groundwater in the basin of mexico from the modeling results the groundwater recharge in the mountains is 30 50 of the mean average precipitation higher and lower rates result in a flow regime that is not compatible with field observations in general the location of the divides in the mountains is displaced towards the valley of mexico which influences the groundwater budget of the valley the water table in places is several hundred metres below ground surface in accordance with field observations of a very thick unsaturated zone before major aquifer exploitation began about 50 years ago 40 50 of the total discharge into the valley was by upward flow through the lacustrine deposits the best results were obtained using a subsurface distribution of hydrostratigraphic units based on recently published geological interpretations

high resolution stalagmite climate record from the yucatan peninsula spanning the maya terminal classic period the decline of the classic maya civilization was complex and geographically variable and occurred over a 150 year interval known as the terminal classic period tcp c e 800 950 paleoclimate studies based on lake sediments from the yucatan peninsula lowlands suggested that drought prevailed during the tcp and was likely an important factor in the disintegration of the classic maya civilization the lacustrine evidence for decades of severe drought in the yucatan peninsula however does not readily explain the long 150 year socio political decline of the classic maya civilization here we present a new absolute dated highresolution stalagmite 18o record from the northwest yucatan peninsula that provides a much more detailed picture of climate variability during the last 1500 years direct calibration between stalagmite 18o and rainfall amount offers the first quantitative estimation of rainfall variability during the terminal classic period our results show that eight severe droughts lasting from 3 to 18 years occurred during major depopulation events of classic maya city states during these droughts rainfall was reduced by 52 to 36 the number and short duration of the dry intervals help explain why the tcp collapse of the mayan civilization occurred over 150 years

late holocene hydroclimate variability in costa rica signature of the terminal classic drought and the medieval climate anomaly in the northern tropical americas abstract in the northern tropical americas neotropics notable hydrological variability has been documented in multi proxy paleoclimate records between 600 and 1200 ce this interval generally overlaps with the maya terminal classic drought tcd 770 1100 ce and the medieval climate anomaly mca 950 1250 ce there is however limited paleoclimate data that provide reliable estimates of terrestrial thermal variability in southern central america during this time in this study we present chironomid based temperature reconstructions developed from sediment cores from two lakes 1 lago morrenas 3c a high elevation glacial lake located on the crest of the atlantic slope of the cordillera talamanca costa rica and 2 laguna zoncho a mid elevation lake located on the pacific slope in southern costa rica distinctive shifts in the chironomid assemblages occurred at both sites between 610 and 1230 ce these changes are inferred to reflect an 600 year interval characterized by depressed mean annual temperatures at lago morrenas 3c and dry conditions and lake level decline at laguna zoncho taken together the multi proxy paleoclimate records from these two lakes including stable carbon isotope 13c and charcoal data suggest that middle elevations on the pacific slope and high elevations on the atlantic slope of costa rica experienced sustained drought and possible cooling coeval with the tcd mca interval the timing of hydroclimate variability in these records provides support for the hypothesis that cooling and drought in the northern tropical americas during the tcd and early mca were linked to thermal changes in the tropical north atlantic

optimizing subsampling strategies for u th dating and geochemical proxies in carbonate speleothems abstract since they provide the chronological frame on which paleoclimate and paleoenvironmental reconstructions are built radiometric ages of carbonate speleothems especially u th dating are of fundamental importance to speleothem and paleoclimate sciences based on 17 aragonite calcite speleothems from the pont de ratz cave s france we propose an integrated petrographical and geochemical approach for optimizing subsampling in speleothems portable energy dispersive x ray fluorescence was used as a fast and non destructive in situ trace element screening of the speleothems these geochemical results are combined with petrographical and mineralogical information to produce distribution maps showing the growth discontinuities and the identified primary and secondary diagenetic products on the speleothem slabs because they provide a clear and precise spatial distribution of primary and secondary carbonate phases at the sample scale these maps constitute a valuable tool for selecting the position of subsamples for radiometric dating or environmental proxy analysis the efficiency and reliability of our integrative approach is convincingly demonstrated by the remarkable coherence of u th ages both within and between speleothem samples

shelf circulation induced by an orographic wind jet the dynamical response to cross shelf wind jet episodes is investigated the study area is located at the northern margin of the ebro shelf in the northwestern nw mediterranean sea where episodes of strong northwesterly wind occur in this case the wind is channeled through the ebro valley and intensifies upon reaching the sea resulting in a wind jet the wind jet response in terms of water circulation and vertical density structure is investigated using a numerical model the numerical outputs agree with water current observations from a high frequency radar additionally temperature sea level and wind measurements are also used for the skill assessment of the model for the wind jet episodes the numerical results show a well defined two layer circulation in the cross shelf direction with the surface currents in the direction of the wind this pattern is consistent with sea level set down due to the wind effect the comparison of the vertical structure response for different episodes revealed that the increase of stratification leads to an onshore displacement of the transition from inner shelf to mid shelf in general the cross shelf momentum balance during a wind jet episode exhibits a balance between the frictional terms and the pressure gradient in shallow waters shifting to a balance between the coriolis force and the wind stress terms in deeper waters

advancements in the use of speleothems as climate archives abstract speleothems have become a cornerstone of the approach to better understanding earth s climatic teleconnections due to their precise absolute chronologies their continuous or semicontinuous deposition and their global terrestrial distribution we review the last decade of speleothem related research building off a similar review by mcdermott 2004 in three themes i investigation of global teleconnections using speleothem based climate reconstructions ii refinement of climate interpretations from speleothem proxies through cave monitoring and iii novel technical methods of speleothem based climate reconstructions speleothem records have enabled critical insight into the response of global hydroclimate to large climate changes this includes the relevant forcings and sequence of climatic responses involved in glacial terminations and recognition of a global monsoon response to climate changes on orbital and millennial time scales we review advancements in understanding of the processes that control speleothem 13 c values and introduce the idea of a direct atmospheric pco 2 influence we discuss progress in understanding kinetic isotope fractionation which with further advances may help quantify paleoclimate changes despite non equilibrium formation of speleothems this feeds into the potential of proxy system modeling to consider climatic hydrological and biogeochemical processes with the objective of quantitatively interpreting speleothem proxies finally we provide an overview of emerging speleothem proxies and novel approaches using existing proxies most recently technical advancements made in the measurement of fluid inclusions are now yielding reliable determinations of paleotemperatures

satellite observations of the wind jets off the pacific coast of central america part i case studies and statistical characteristics measurements of near surface winds by the nasa scatterometer nscat from october 1996 through june 1997 are analyzed to investigate the three major wind jets along the pacific coast of central america that blow over the gulfs of tehuantepec papagayo and panama each jet is easily identifiable as locally intense offshore winds in the lee of low elevation gaps through the sierra madre mountain range the jets have relatively narrow cross stream width but often extend several hundred kilometers or more into the pacific the tehuantepec and papagayo jets sometimes merge with the northeast trade winds of the pacific the tehuantepec jet was highly energetic with characteristic timescales of about 2 days events were triggered by high pressures associated with cold surges into the gulf of mexico that originated over the great plains of north america the papagayo and panama jets were much more persistent than the tehuantepec jets the winds at both of these lower latitude locations exhibited a strong seasonal variation with almost exclusively offshore flow from late november 1996 through late may 1997 and periods of onshore flow in october and november during the late stages of the 1996 central american monsoon season superimposed on this low frequency seasonal variation were events with characteristic timescales of a few days based on nscat data the spatial and temporal evolution of major wind events is described in detail for three representative case studies in december 1996 the jets developed sequentially from north to south consistent with the notion that wind events in the two lower latitude jets are associated with cold air outbreaks that trigger the tehuantepec jet a day or so earlier in november 1996 and march 1997 the papagayo and panama jets were strongly influenced by tropical phenomena that had little apparent association with the tehuantepec jet these latter two case studies together with the distinction between the statistical characteristics of the three jets suggest that the papagayo and panama jets are predominantly controlled by a mechanism that is very different from the across gap pressure gradients associated with high pressure systems of midlatitude origin that control the tehuantepec jet

late holocene volcanic activity and environmental change in highland guatemala abstract we present a record of late holocene volcanic eruptions with elemental data for a sequence of sampled tephras from lake amatitlan in highland guatemala our tephrochronology is anchored by a bayesian p sequence age depth model based on multiple ams radiocarbon dates we compare our record against a previously published study from the same area to understand the record of volcanism and environmental changes this work has implications for understanding the effects of climate and other environmental changes that may be related to the emission of volcanic aerosols at local regional and global scales

holocene hydroclimate and environmental change inferred from a high resolution multi proxy record from lago ditkebi chirripo national park costa rica abstract multi proxy analysis of a sediment core recovered from lago ditkebi in chirripo national park costa rica was undertaken to develop a multi decadal to sub centennial scale reconstruction of holocene hydroclimate and environmental change for the region analyses of sub fossil chironomid assemblages macroscopic charcoal and bulk sediment geochemistry suggest that the glacial highlands in chirripo national park experienced notable hydroclimate variability periodical burning by wildfires and climate related vegetation change during the last 8100 years a single chironomid taxon procladius most commonly associated with cold glacial lakes in costa rica dominates the holocene sub fossil chironomid assemblage in lago ditkebi inferred from the proxy records the interval between 8100 and 5270 cal yr bp at the glacial highlands was relatively cold and dry with low effective moisture and limited fire activity cool and dry conditions were also observed between 2820 cal yr bp and present but co occurred with more frequent low severity fires the highest fire frequency occurred between 3300 and 1600 cal yr bp the shifts in the chironomid assemblage and the low 13c values detected between 5270 and 2820 cal yr bp suggest a warm and wet climate and a decrease in abundance of muhlenbergia a c4 grass during that time concurrent maxima in c n charcoal accumulation and the abundance of thermophilous chironomid taxon polypedilum n type at 5200 cal yr bp are indicative of an abrupt climate change event that was characterized by rapid warming quickly increased effective moisture and intense wildfires in the glacial highlands of costa rica

terminal classic drought in the northern maya lowlands inferred from multiple sediment cores in lake chichancanab mexico we present new density records from sediment cores taken along a depth transect in lake chichancanab mexico the data reveal in great detail the climatic events that comprised the terminal classic drought tcd and coincided with the demise of classic maya civilization between ca 750 and 1050 ad in shallow water cores the tcd is marked by a condensed gypsum horizon in deeper water sections it is expanded and represented by numerous interbedded gypsum and organic rich strata the tcd was not a single two century long megadrought but rather consisted of a series of dry events separated by intervening periods of relatively moister conditions we estimate the tcd occurred between ca 770 and 1100 ad and included an early phase ca 770 870 ad and late phase ca 920 1100 ad the intervening 50 year period ca 870 920 ad was relatively moister each dry phase is represented by multiple gypsum density bands interbedded with organic rich sediment that indicate alternating dry and wet conditions spectral analysis revealed significant periods around 213 50 and 27 years despite uncertainty regarding the ages of these events their pattern is robust and generally consistent with other proxy records under the same climate regime such as the marine cariaco basin off northern venezuela science 299 2003 1731

isotope amount effects in hydrologic and climate reconstructions of monsoon climates implications of some long term data sets for precipitation abstract many studies of quaternary climate make use of terrestrial stable isotope records which are interpreted based on seasonal patterns of stable isotopes in modern precipitation multi decade records of isotopes in rainfall allow testing of the assumed behavior of isotope signals used for this interpretation on multi year to decadal scales a 32 year record of stable o and h isotopes in precipitation in tucson arizona permits a detailed examination of stable isotope amount effects at time scales ranging from individual events to decades in a location with summer monsoonal and winter frontal rainy seasons amount effects are weak to non existent in tucson at seasonal and longer time scales and are not useful for discriminating either wetter or drier rainy seasons or wetter or drier decades amount effects are also weak to non existent in published data for annual and multi year amount weighted averages for monsoonal precipitation in new delhi and hong kong but an annual amount effect appears to be present on guam u s territory in addition site specific amount effects do not correlate with measures of regional monsoon intensity this data analysis challenges the general validity of paleoclimate reconstructions based on short term sub annual relationships observed in precipitation isotope data when applied to long term records such as speleothem studies

late holocene erosion events in the valley of teotihuacan central mexico insights from a soil geomorphic analysis of catenas abstract this paper addresses the timing of accelerated soil erosion and landscape degradation in central mexico it shows data on erosion sequences in the valley of teotihuacan an area where agricultural dates back to at least since 1100 bce and an ideal setting to test ideas on the effect of preconquest versus post conquest anthropogenic impacts on the environment the methods include stratigraphy detailed description of soil profiles and soil micromorphology along with analysis of pottery sherds and radiocarbon dating a chronology of events for two catenas focusing on periods of landscape stability and change establish the sequence of erosion events from pre hispanic to modern times six erosion phases are recognized over the last 2000 years in teotihuacan although results suggest that the most intense erosion occurred after spanish conquest in the 16th century several erosion phases took place during pre hispanic times and also in the postcolonial period including the 20th century probable causes for these impacts are related to settlement dynamics population variability and land use changes thus it provides new information for the discussion on the timing and causes of erosion sedimentation and landscape modification in the central mexican highlands during the late holocene

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

oxygen isotope values of precipitation and surface waters in northern central america belize and guatemala are dominated by temperature and amount effects abstract an understanding of the climatic controls on precipitation 18 o is required to interpret isotopic records of paleoclimate and paleoaltimetry however variations in precipitation 18 o in time and space are only poorly known in northern central america to test the hypothesis that precipitation and surface water 18 o values are dominated by temporal and spatial amount effects we analyzed 18 o in surface waters collected from guatemala and belize and in precipitation from the global network for isotopes in precipitation database for veracruz mexico and san salvador el salvador herein we show that the dominant controls on 18 o values of precipitation and surface waters are fairly simple temporally the dominant control on precipitation 18 o values is the amount effect whereby there is an inverse correlation between rainfall amount and 18 o precipitation 18 o values decrease by 1 24 per 100 mm increase of monthly rainfall spatially only two variables distance from the coast and mean catchment altitude explain 84 of the surface water 18 o variability surface water 18 o values show an altitude effect of 1 9 to 2 4 km 1 and a continental effect of 0 69 per 100 km once corrected for altitude effects a decrease in surface water 18 o by 3 to 4 from the caribbean sea to the pacific ocean is evident as an isotopic rain shadow on the pacific slope our data also show that river waters in this humid tropical environment are good proxies for 18 o values of precipitation in northern central america the guatemala belize surface water line is defined as d 8 0 18 o 8 7 which is similar to the meteoric water line at san salvador of d 8 1 18 o 10 9 spatial variability in 18 o values is interpreted to reflect 1 progressive rainout of caribbean sourced air masses upon traverse of central america and 2 the temperature dependent equilibrium fractionation between vapor and condensate related to the altitude effect the data show that the northeast trade winds are the dominant moisture source to central america mixing with pacific derived moisture west of the cordilleran divide

late holocene forest contraction and fragmentation in central africa during the warmer holocene period two major climatic crises affected the central african rainforests the first crisis around 4000 cal yr bp caused the contraction of the forest in favor of savanna expansion at its northern and southern periphery the second crisis around 2500 cal yr bp resulted in major perturbation at the forest core leading to forest disturbance and fragmentation with a rapid expansion of pioneer type vegetation and a marked erosional phase the major driver of these two climatic crises appears to be rapid sea surface temperature variations in the equatorial eastern atlantic which modified the regional atmospheric circulation the change between ca 2500 to 2000 cal yr bp led to a large increase in thunderstorm activity which explains the phase of forest fragmentation ultimately climatic data obtained recently show that the present day major rise in thunderstorms and lightning activity in central africa could result from some kind of solar influence and hence the phase of forest fragmentation between ca 2500 to 2000 cal yr bp may provide a model for the present day global warming related environmental changes in this region

control of insolation on stalagmite growth rainfall and migration of the tropical rain belt in northern namibia over the last 100 kyr as suggested by a rare mis 5b 5c stalagmite from dante cave abstract dating by the 230th method indicates that stalagmite danf from dante cave in northern namibia in the summer rainfall zone srz of southern africa formed about 92 ka at the boundary of marine isotope stages mis 5b and 5c in this dominantly semiarid zone stalagmite growth may be evidence of a relatively wetter period and trends in danf s mineralogy and its c o and u isotope ratios further support formation in a period wetter than those before or after its formation danf s time of formation at 92 ka like other wet periods in the srz of southern africa over the most recent 100 kyr coincides with a maximum in january insolation at 30 s that brought the srz and more generally the tropical rain belt farther south than before or after

a 2400 yr mesoamerican rainfall reconstruction links climate and cultural change droughts are a recurring feature of mexican climate but few high resolution data are available to test for climate change forcing of mesoamerican civilizations we present a quantitative 2400 yr rainfall reconstruction for the basin of mexico from a precisely dated and highly resolved speleothem that documents highly variable rainfall over the past 2400 yr dry conditions peaked during a 150 yr long late classic ca 600 900 ce common era mega drought that culminated at 770 ce which followed centuries of climatic drying that spanned the fall of the city of teotihuacan ca 550 ce the wettest conditions in the 1450s ce were associated with fl ooding in the basin of mexico our data suggest that rainfall variability was likely forced by the el nino southern oscillation and impacts on spring fed irrigation agriculture may have been a stressor on mesoamerican civilizations

atmospheric forcing of the eastern tropical pacific a review abstract the increase in marine land surface atmospheric and satellite data during recent decades has led to an improved understanding of the air sea interaction processes in the eastern tropical pacific this is also thanks to extensive diagnoses from conceptual and coupled ocean atmosphere numerical models in this paper mean fields of atmospheric variables such as incoming solar radiation sea level pressure winds wind stress curl precipitation evaporation and surface energy fluxes are derived from global atmospheric data sets in order to examine the dominant features of the low level atmospheric circulations of the region the seasonal march of the atmospheric circulations is presented to depict the role of radiative forcing on atmospheric perturbations especially those dominating the atmosphere at low levels in the tropics the trade winds constitute an important north south energy and moisture exchange mechanism as part of the low level branch of the hadley circulation that determines to a large extent the precipitation distribution in the region i e that associated with the inter tropical convergence zone itcz monsoonal circulations also play an important role in determining the warm season precipitation distribution over the eastern tropical pacific through a large variety of air sea land interaction mechanisms westward traveling waves tropical cyclones low latitude cold air intrusions and other synoptic and mesoscale perturbations associated with the itcz are also important elements that modulate the annual rainfall cycle the low level jets of the gulf of california the intra americas sea gulf of mexico and caribbean sea and choco colombia are prominent features of the eastern tropical pacific low level circulations related to sub regional and regional scale precipitation patterns observations show that the intra americas low level jet intensity varies with el nino southern oscillation enso phases however its origin and role in the westward propagation and development of disturbances that may hit the eastern tropical pacific such as easterly waves and tropical cyclones are still unclear changes in the intensity of the trade winds in the caribbean sea and the gulf of mexico associated with eastern tropical pacific wind jets exert an important control on precipitation by means of wind topography interactions gaps in the mountains of southern mexico and central america allow strong wind jets to pass over the continent imprinting a unique signal in sea surface temperatures and ocean dynamics of the eastern tropical pacific the warm pools of the americas constitute an important source of moisture for the north american monsoon system the northeastern tropical pacific is a region of intense cyclogenetic activity just west of the coast of mesoamerica over the oceanic regions large scale properties of key variables such as precipitation moisture surface energy fluxes and wind stress curl are still uncertain which inhibits a more comprehensive view of the region and stresses the importance of regional field experiments progress has been substantial in the understanding of the ocean and atmospheric dynamics of the eastern tropical pacific however recent observational evidence such as that of a shallow meridional circulation cell in that region in contrast to the classic concept of the hadley type deep meridional circulation suggests that more in situ observations to validate theories are still necessary this paper is part of a comprehensive review of the oceanography of the eastern tropical pacific ocean

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

global patterns of enso induced precipitation although there have been many analyses of el nino southern oscillation enso induced precipitation anomalies global patterns from these analyses remain incomplete here we combine recent satellite estimates of oceanic precipitation and historical rain gauge records to derive a global climatology of enso induced precipitation anomalies using empirical orthogonal function eof analyses the patterns suggest that the re arrangement of convection centers of the walker circulation during enso events induces large precipitation anomalies in the tropics while associated changes in the monsoon systems through the hadley cell over the pacific indian and atlantic oceans and their interactions with midlatitude westerlies generate coherent anomaly patterns over the extratropics our results can be used to evaluate climate models and forecast enso induced precipitation anomalies

stable isotope characterisation of recent aragonite travertine deposits associated with the fitero thermal waters spain the travertines of the fitero thermal springs with more than 98 of aragonite in most of the samples are studied in this paper the main objective is to improve the general understanding of aragonite precipitation since the deposits of almost pure aragonite are very scarce this study presents a complete mineralogical and isotopic characterisation including the evaluation of the 18o and 13c fractionation during precipitation as valuable information for paleoclimate and paleoenvironmental studies samples of a laminated travertine deposit constituted by almost pure aragonite were taken from a pipe discharging water at 40 c waters suffered an important co2 outgassing as suggested by the geochemical calculations and the 13c values of travertines and waters this outgassing triggers the oversaturation and precipitation of carbonate phases temperature seems to be the main factor controlling the precipitation of aragonite or calcite as checked by studying another travertine sample with higher calcite content various 18o isotope fractionation equations for aragonite and calcite were used the results indicate that precipitation took place close to equilibrium according to some of these equations the fact that the equilibrium is maintained in a natural system with an important co2 loss is surprising however it can be explained by an hco3 water oxygen isotopic equilibrium and a direct transfer of the hco3 isotope signal to the carbonate without fractionation due to the fast co2 loss and precipitation finally considering other natural aragonite samples a fractionation equation is defined for natural aragonite in the temperature range between 23 and 80 c

human impact on natural systems modeled through soil erosion in geowepp a comparison between pre hispanic periods and modern times in the teotihuacan valley central mexico abstract anthropogenic changes during the past 2000 years in the teotihuacan valley imply that intensity of soil degradation varies depending on the land management practices and the intensity of soil use as a part of a broader effort to reconstruct erosion dynamics in the teotihuacan valley through geoarchaeological approaches our study applies a process based watershed hydrology and upland erosion model the water erosion prediction project wepp the geo spatial interface for wepp geowepp was used to characterize locations of soil detachment and sediment deposition predicted in the watershed through time based on current and reconstructed conditions in the valley climate topography soil and land use were used as inputs to wepp to estimate surface runoff and soil loss rates for periods with different environmental and anthropogenic conditions the teotihuacan period 1 650 ce the aztec period 1325 1521 ce and for modern times after 1970 ce over a simulated and established timeframe for those periods surface runoff and rate of soil loss were estimated to be higher during the aztec period when the area devoted to agriculture dominated the landscape land use change had a major impact on soil erosion comparison of the pre hispanic periods with current conditions shows that wepp is useful in showing the roles of management and climate in environmental degradation the results contribute to the scientific debate about the antiquity and causes of erosion in central mexico the research shows that land use is one of the foremost factors affecting soil erosion both in ancient and modern periods with particular impact during the aztec period

the combined influence of pacific decadal oscillation and atlantic multidecadal oscillation on central mexico since the early 1600s abstract periodic droughts have been one of the most serious environmental issues in central mexico since the earliest times the impacts of future droughts are likely to become even more severe as the current global warming trend increases potential evaporation and moisture deficits a full understanding of the mechanism underlying climate variability is imperative to narrow the uncertainty about future droughts and predict water availability the climatic complexity generated by the combined influence of both atlantic and pacific forcings however causes considerable difficulty in interpreting central mexican climate records also the lack of high resolution information regarding the climate in the recent past makes it difficult to clearly understand current drought mechanisms our new high resolution 18 o record from hoya rincon de parangueo in central mexico provides useful information on climate variations since the early 1600s according to our results the central mexican climate has been predominantly controlled by the combined influence of the 20 year pacific decadal oscillation pdo and the 70 year atlantic multidecadal oscillation amo however the amo probably lost much of its influence in central mexico in the early 20th century and the pdo has mostly driven climate change since marked dryness was mostly associated with co occurrence of highly positive pdo and negative amo between 1600 and 1900

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

stable isotope proxy records in tropical terrestrial environments abstract the tropics refers to an area that is loosely equatorial characterized by relatively high temperatures at low altitude comparatively low seasonal contrast in temperatures and generally high or highly seasonal rainfall the stable isotopes of carbon oxygen and hydrogen are increasingly important tools of relevance to understanding trajectories and mechanisms of past environmental change in the tropics in the case of carbon this is because at the broadest level the carbon isotope composition of organic and carbon containing inorganic proxy materials formed in the tropics can be read as being derived from the balance between c3 tree and c4 grass photosynthesis hence providing a relative measure of vegetation type and indirectly climate likewise changes in the oxygen and hydrogen isotope composition of many proxy materials again at the broadest level can be read as indicative of changes in hydrological conditions and or altitude in the past this review provides an entry point into the literature relating to the use of stable isotopes as terrestrial environmental proxies for past environmental change with a focus on the measurement and interpretation of stable isotope data from the wide range of proxy materials preserved in tropical environments

testing the tropical storm hypothesis of yucatan peninsula climate variability during the maya terminal classic period abstract we examine the tropical storm hypothesis that precipitation variability in the yucatan peninsula yp was linked to the frequency of tropical cyclones during the demise of the classic maya civilization in the terminal classic period tcp ad 750 950 evidence that supports the hypothesis includes 1 a positive relationship between tropical storm frequency and precipitation amount over the yp today proof of feasibility 2 a statistically significant correlation between a stalagmite chaac quantitative precipitation record from the yp and the number of named tropical cyclones affecting this region today 1852 2004 calibration sensu lato and 3 correlations between the stalagmite chaac precipitation record and an atlantic basin tropical cyclone count record and two proxy records of shifts in macro scale climate and ocean states that influence atlantic tropical cyclongenesis at face value regional paleotempestology proxy records suggest that tropical storm activity in the yp was either similar or significantly lower than today during the tcp the tropical storm hypothesis has implications for our understanding of the role the hydrological cycle played in the collapse of classic maya polities and the role of tropical storms in possibly ameliorating future drought in the yp and other tropical regions

are aragonite stalagmites reliable paleoclimate proxies tests for oxygen isotope time series replication and equilibrium the use of aragonite stalagmites as paleoclimate proxies has become increasingly common because of their often exceptional uranium series dating precision and fast growth rates which allow near annual climate resolution however aragonite is known to be susceptible to open system behavior and may recrystallize to calcite under typical cave conditions as a result the fidelity of the oxygen isotopic variations preserved in aragonite stalagmites is not yet as firmly established as for calcite stalagmites and few studies have investigated oxygen isotope replication and equilibrium with cave drip waters i present new data and review findings from the literature that show that aragonite stalagmites may pass both the equilibrium and replication tests in some cases but that other aragonites are clearly precipitated out of equilibrium with cave drip waters as a case study 18 o data for three exceptionally well dated stalagmites from juxtlahuaca cave mexico are presented two of the stalagmites are aragonite and the third was measured on a diagenetically calcitized aragonite stalagmite these results show that the modern aragonite precipitates in apparent oxygen isotopic equilibrium with drip waters and the two aragonite speleothems replicate strongly over the past 530 yr thus confirming a common paleoclimate signal observations of 18 o systematics in previously published aragonite stalagmite 18 o records from china and belize show that at least two samples clearly precipitated modern aragonite out of isotopic equilibrium with cave drip waters because the equilibrium test only applies to modern precipitation for which temperature and drip water 18 o values are known both equilibrium and replication tests are required for confident interpretation of aragonite stalagmite 18 o time series and studies that fail or do not attempt such tests should be considered cautiously

geodetic measurements near parkfield california 1959 1984 trilateration measurements have been made by various agencies since 1959 near parkfield california in the transition zone between the central creeping section and the southern locked section of the san andreas fault within the transition zone there are several creep meters and alignment arrays four small aperture trilateration networks and a broad trilateration network extending 70 km southwest from the san andreas fault to the pacific coast line length changes observed after the 1966 ml 5 6 parkfield earthquake indicate a geodetic moment of 4 5 1 0 1025 dyn cm the data are consistent with either 91 8 cm slip on a rupture zone which is 30 km long and extends from 3 to 8 km below the surface or 59 5 cm on a 30 km long zone 2 10 km deep this moment estimate includes some postseismic slip and is a factor of 2 4 greater than the moment determined from seismic surface waves the post 1966 data set is well suited to discriminate between the effects of surface and deep slip surface slip rates determined by creep meters alinement arrays and small aperture trilateration networks decrease from 29 mm yr at slack canyon to zero south of highway 46 a gradient of roughly 0 48 0 01 mm yr km deformation of a subset of the trilateration network between cholame and san luis obispo is consistent with shallow slip of 2 9 1 2 mm yr from the surface to a depth of 16 km and 33 4 5 5 mm yr below 16 km the trilateration data alone do not uniquely constrain both the transition depth from shallow to deep slip and the deep slip rate a transition depth near 16 km is preferred because it provides the best fit to the data and because the predicted slip rate is consistent with independent estimates of the total slip rate across the san andreas fault this simple two dimensional model neglects along strike variations in the deformation the data show no evidence for significant strike slip deformation across faults between the san andreas and the pacific coast

high resolution chronology for the mesoamerican urban center of teotihuacan derived from bayesian statistics of radiocarbon and archaeological data article i nfo 14 c chronology for the teopancazco archaeological site in the teotihuacan urban center of mesoamerica was generated by bayesian analysis of 33 radiocarbon dates and detailed archaeological information related to occupation stratigraphy pottery and archaeomagnetic dates the calibrated intervals obtained using the bayesian model are up to ca 70 shorter than those obtained with individual calibrations for some samples this is a consequence of plateaus in the part of the calibration curve covered by the sample dates 2500 to 1450 14 c yr bp effects of outliers are explored by comparing the results from a bayesian model that incorporates radiocarbon data for two outlier samples with the same model excluding them the effect of outliers was more significant than expected inclusion of radiocarbon dates from two altered contexts 500

a 6 000 year record of environmental change from the eastern pacific margin of central mexico abstract the transition from the mid to late holocene in mesoamerica saw increasing complexity in spatial patterns of change records from the western part of the region are sparse with lacustrine sequences affected by long term anthropogenic disturbance or lacking chronological resolution here we present a continuous palaeoecological and geochemical record from laguna de juanacatlan a remote lake in the mountains of the western tmvb diatom assemblages xrf scanning data and bulk organic geochemistry from a well dated 7 25 m laminated sequence were combined with summary pollen data from a 9 m partially laminated core to provide a continuous record of catchment and lake ecosystem changes over the last c 6000 years relatively humid conditions prevailed prior to c 5 1 cal ka which supported dense oak pine forest cover around a deep stratified lake a trend towards drier conditions began c 5 1 cal ka intensifying after 4 0 cal ka consistent with weakening of the north american monsoon between 3 0 and 1 2 cal ka lower lake levels and variable catchment run off are consistent with increasing enso influence observed in the late holocene in the neotropics from 1 2 to 0 9 cal ka a marked change to catchment stability and more intense stratification reflected drier conditions and or reduced rainfall variability and possibly warmer temperatures after 0 9 cal ka conditions were wetter with an increase in catchment disturbance associated with the combined effects of climate and human activity in recent decades the lake ecosystem has changed markedly possibly in response to recent climate change as well as local catchment dynamics

climatic and environmental controls on speleothem oxygen isotope values variations in speleothem oxygen isotope values d 18 o result from a complicated interplay of environmental controls and processes in the ocean atmosphere soil zone epikarst and cave system as such the controls on speleothem d 18 o values are extremely complex an understanding of the processes that control equilibrium and kinetic fractionation of oxygen isotopes in water and carbonate species is essential for the proper interpretation of speleothem d 18 o as paleoclimate and paleoenvironmental proxies and is best complemented by study of site specific cave processes such as infiltration flow routing drip seasonality and saturation state and cave microclimate among others this review is a process based summary of the multiple controls on d 18 o in the atmosphere soil epikarst and speleothem calcite illustrated with case studies primary controls of d 18 o in the atmosphere include temperature and relative humidity through their role in the multiple isotope effects variability and modifications of water d 18 o values in the soil and epikarst zones are dominated by evaporation mixing and infiltration of source waters the isotopically effective recharge into a cave system consists of those waters that participate in precipitation of caco3 resulting in calcite deposition rates which may be biased to time periods with optimal dripwater saturation state recent modeling experimental and observational data yield insight into the significance of kinetic fractionation between dissolved carbonate phases and solid caco3 and have implications for the hendy test to assist interpretation of speleothem d 18 o time series quantitative and semi quantitative d 18 o climate calibrations are discussed with an emphasis on some of the difficulties inherent in using modern spatial and temporal isotope gradients to interpret speleothems as paleoclimate proxy records finally several case studies of globally significant speleothem paleoclimate records are discussed that show the utility of d 18 o to reconstruct past climate

the 1993 superstorm cold surge frontal structure gap flow and tropical impact in the wake of the eastern united states cyclone of 12 14 march 1993 a cold surge originating over alaska and western canada brought northerlies exceeding 20 m s21 and temperature decreases up to 158c over 24 h into mexico and central america this paper addresses the multiscale aspects of the surge from the planetary scale to the mesoscale focusing on 1 the structure and evolution of the leading edge of the cold surge 2 the reasons for its extraordinary intensity and equatorward extent and 3 the impact of the surge on the tropics specifically on the strength of the trade winds and on the sea surface temperature in the eastern pacific the cold surge was initiated as a developing cyclone over the gulf of mexico and an upstream anticyclone east of the rockies caused an along barrier pressure gradient to form forcing topographically channeled northerlies along the rocky and sierra madre mountains to transport cold air equatorward on the mesoscale the leading edge of the cold surge possessed nonclassical frontal structure for example as the cold surge entered mexico the coldest air and the strongest wind arrived at about 900 hpa before affecting the surface suggestive of a tipped forward leading edge to the surge also satellite imagery and surface observations indicate that the leading edge appeared to be successively regenerated in the warm presurge air the cold surge had characteristics reminiscent of a kelvin wave a tipped forward cold front a pressure jump line a bore and a gravity current but none of these conceptual dynamical models was fully applicable associated with the cold surge gap winds up to 25 m s21 were observed in the gulfs of tehuantepec a tehuantepecer fonseca papagayo and panama owing to the strong cross mountain pressure gradient in the case of the tehuantepecer a rope cloud emanated from the isthmus of tehuantepec and turned anticyclonically consistent with an inertial oscillation on the synoptic and planetary scales the extraordinary equatorward extent of the cold surge was aided by topographic channeling similar to cold air damming by a low latitude upper tropospheric trough and by the lower branch of the secondary circulation associated with a confluent jet entrance region aloft the cold surge also impacted the tropical atmosphere and ocean by contributing to the strengthening of the northeast trade winds over the eastern pacific ocean and by inducing local cooling of the sea surface temperature in the gulfs of tehuantepec and papagayo by about 48 88c

wetland geomorphology and paleoecology near akab muclil rio bravo floodplain of the belize coastal plain abstract to understand wetland geomorphology and paleoecology we collected a 2 6 m sediment core from a flooded swamp adjacent to the maya archaeological site of akab muclil in the maya lowlands of northwestern belize the site of akab muclil has a known occupation that persisted from early maya classic 1700 1350 bp through the terminal maya classic 1180 1050 bp and into the postclassic 1050 450 bp and lies near a vast network of ancient maya canal and field systems we analyzed this core using a combination of paleoecological and geochemical techniques to determine the history of land use and natural change over time within this wetland accelerator mass spectrometry dating pollen charcoal analysis micropaleontology geochemical analysis and magnetic susceptibility provide a suite of methods from which we interpret the geomorphic and ecological history of this wetland system four ams dates from the length of the core provide us with an age model that runs from 1675 cal bp through the maya classic onward to the present at the base of this system soil composition and chemistry provide evidence that the system changed from a seasonally wet terrestrial soil to a perennially wet swamp as the basal mollisol soil lies buried by peats and calcareous sediments this shift to a perennial wetland could be related to ancient maya water management or a natural geomorphic change though we suspect the former because of nearby ancient maya large scale geomorphic and hydrological manipulation in the form of intensive canalization and agriculture evidence of ancient maya uses and impacts including sedimentation zea mays pollen and high charcoal counts occur from the lowest levels of the sequence through the classic and into the postclassic period above this level the strata change to stable peats laminated deposits of light gray dark gray gypsum authigenic carbonate and layers of fibrist peat with little evidence of human impact until recent increases in charcoal and phosphorous this study compared with other regional studies indicates a later transition from terrestrial to wetland later human impacts in the postclassic and a geomorphic impact record closely tied to the history of the adjacent site rather than broader land use trends