holocene climatic and human influences on lakes of the yucatan peninsula mexico an interdisciplinary palaeolimnological approach we used palaeolimnological techniques to examine effects of holocene climate change and human influence on lakes of the yucatan peninsula mexico the three study lakes are located along a west east transect that represents a gradient of increasing modern precipitation and density of former maya settlement at lake coba an 880 cm sediment core yields a complete record of lacustrine sedimentation that began when the lake first filled 8000 bp as groundwater level rose in response to rising sea level and increased precipi tation diatom ostracod and 18o evidence indicate that lake coba was initially shallow and saline coba presently in the region of greatest rainfall showed more episodic water level changes than lake san jose chulchaca or lake sayaucil high lake level and fresh water were evident at 440 cm 2600 bp followed by a decline in water levels and an increase in total ionic salinity to the present time in a 613 cm core from sayaucil in the intermediate precipitation zone total salini

climate change in the lake valencia basin venezuela 12600 yr bp to present we present a palaeoclimate record for northern south america extending from the latest pleistocene 12600 14c yr bp to present climate reconstruction for the valencia basin venezuela was base

preliminary results from u th dating of glacial interglacial deposition cycles in a silica speleothem from venezuela abstract recent explorations in cueva charles brewer a large cave in a sandstone tepui se venezuela have revealed silica biospeleothems of unprecedented size and diversity study of one a sub spherical mass of opaline silica reveals a complex laminated internal structure consisting of three narrow dark bands alternating with two wider light bands uranium thorium dating has produced 3 stratigraphically correct dates on the light bands from 298 6 mis 9 to 390 33 ka mis 11 u concentration is only 30 110 ppb initial 234u 238u ratios are high and increase over time from 1 8 to 5 3 growth rate is very low the fastest at 0 37 0 23 mm ka in mis 9 trace element and heavy metal content of the dark bands is distinctly higher than that of the light bands it is hypothesized that the dark and light bands correlate with drier glacial and wetter interglacial periods respectively and that this sample probably began to grow in mis 13 the cave is in a region that straddles a regionally important ecotone the speleothem isotopic and trace element variations may preserve a useful paleoclimatic signal this is the first published suite of u th dates from a single silica speleothem and the longest quaternary record for this region

carbon and oxygen isotope variations among lacustrine ostracods implications for palaeoclimatic studies large ranges of 13c 12c and 18o 16o ratios are found among modem ostracods from lakes in jamaica and mexico if their calcite carapaces grew in isotopic equilibrium with the lake water then the main period of carapace formation was probably in the cooler and or wetter months differences in 13c and 18 o values between ostracods from different sites relate to spatial variations in the composition of the lake water and differences between species at the same site probably reflect differences in organic photosynthesis or decay among the micro habitats occupied by the species there nevertheless remain significant variations in both 13c and 18o among modern valves of the same species at the same site a consequence of the temporal and spatial variation in lake water and the fact that ostracods form their carapace over a short period individual valves from a 1 cm section of a sediment core show that the isotopic effects of changes in the rainfall versus evaporation status of a lake over a period of c

reconstructing past climates using carbon isotopes from fulvic acids in cave sediments little is known about the potential of organic substances found within cave sediments to provide useful reconstructions of past climatic changes here we present a study of the carbon isotopes of fulvic acids extracted from sediments from a cave in west central florida fulvic acids are the most contemporaneous representation of soil humic substances derived from vegetation growing above the cave radiocarbon dating of these sediments constrains their depositional period to be late holocene the range of the fulvic acid 13c record shows the dominance of c3 vegetation during this period but with changes in abundance and the shifting presence of c4 plants the 13c values record both the little ice age and medieval climate anomaly which produce drier and wetter conditions for florida respectively our conclusion that the stable carbon isotopic values of the cave sediment record as a proxy for vegetation change as driven by variability in precipitation is strongly supported by a comparison with the 13c and 18o values of two uranium series dated speleothems from this part of florida with both being proxies for precipitation change

palaeovegetational and palaeoenvironmental trends in the summit of the guaiquinima massif venezuelan guayana during the holocene the summits of the table mountains tepuis from the neotropical guayana region are remote environments suitable for palaeoecological studies with evolutionary biogeographical and palaeoclimatic implications here using palynological analyses of two radiocarbon dated peat bogs from a tepui summit the holocene palaeovegetational trends are reconstructed and related to possible forcing factors because of the pristine character of the guaiquinima summit the recorded palaeoenvironmental changes are probably due to natural causes which makes them valuable archives of the natural component of climatic change at a millennial time scale the sequence begins with pioneer communities or meadows similar to present day ones between about 8 4 and 4 5 ky bp after this date and until about 2 kyr bp the expansion of gallery forests suggests an increase in precipitation documented also at regional neotropical level between ca 2 kyr bp and the last century gallery forests are replaced by forests characteristic of the upper guaiquinima altitudes coinciding with a regional phase of reduced moisture the present day meadows established relatively quickly during the last century substituted the former upland forests in the locality studied the main controlling factor of the vegetation during the holocene seems to have been the moisture balance in contrast to other tepui summits there is no clear evidence for changes linked to temperature oscillations this could be due to the elevation of the site far from any characteristic ecological boundary that makes it insensitive to this parameter copyright 2005 john wiley sons ltd

climate drying and associated forest decline in the lowlands of northern guatemala during the late holocene abstract palynological studies document forest disappearance during the late holocene in the tropical maya lowlands of northern guatemala the question remains as to whether this vegetation change was driven exclusively by anthropogenic deforestation as previously suggested or whether it was partly attributable to climate changes we report multiple palaeoclimate and palaeoenvironment proxies pollen geochemical sedimentological from sediment cores collected in lake peten itza northern guatemala our data indicate that the earliest phase of late holocene tropical forest reduction in this area started at 4500 cal yr bp simultaneous with the onset of a circum caribbean drying trend that lasted for 1500 yr this forest decline preceded the appearance of anthropogenically associated zea mays pollen we conclude that vegetation changes in peten during the period from 4500 to 3000 cal yr bp were largely a consequence of dry climate conditions furthermore palaeoclimate data from low latitudes in north africa point to teleconnective linkages of this drying trend on both sides of the atlantic ocean

late quaternary deglacial history of the merida andes venezuela radiocarbon dated sediment cores from seven lakes and two bogs spanning the cordillera de merida in the venezuelan andes were used to identify and date the regional history of late pleistocene and holocene glacial activity coring sites were selected at different elevations across a pronounced rain shadow from southeast wet to northwest dry sediment lithostratigraphy and magnetic susceptibility in conjunction with ams radiocarbon dates on macrofossils and char coal were used to constrain deglaciation the local expression of the last glacial maximum occurred between 22 750 and 19 960 cal yr bp on the wetter southeastern side of the cordillera de merida glaciers had significantly retreated by 15 700 cal yr bp followed by several minor glacial advances and retreats between 14 850 and 13 830 cal yr bp at least one major glacial readvance occurred between 13 830 and 10 000 cal yr bp in the wetter southeastern sector of the region the drier northwest side of the cordillera de merida records initial glacial retreat by 14 240 cal yr bp multiple sites on both sides of the merida andes record a further phase of extensive deglaciation approximately 10 000 cal yr bp however the north northwest facing mucubajo catchment remained partially glaciated until ca 6000 cal yr bp deglacial ages from the venezuelan andes are consistently younger than those reported from the southern hemisphere andes suggesting an inter hemispheric deglacial lag in the northern tropics of the order of two thousand years copyright 2005 john wiley sons ltd

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

the tropical extratropical interaction between high frequency transients and the madden julian oscillation abstract the interaction between high frequency transient disturbances and convection and the madden julian oscillation mjo is investigated using ncep ncar reanalysis and satellite outgoing longwave radiation data for 15 northern winters during the phase of the mjo with enhanced convection over the east indian ocean and indonesia and suppressed convection over the south pacific convergence zone both the asian pacific jet and the region of upper tropospheric tropical easterlies over the warm pool are displaced westward these changes in the basic state lead to a weaker or leakier waveguide in the asian pacific jet with a westward displaced forbidden region of tropical easterlies such that high frequency transient waves propagate equatorward into the deep tropics over the central pacific near the date line as these waves induce convection in the region of ascent and reduced static stability ahead of the upper level cyclonic disturbances there is an enhancement of high frequency convective var

disentangling abrupt deglacial hydrological changes in northern south america insolation versus oceanic forcing paleoenvironmental studies and climate models demonstrate that fl uvial runoff and moisture availability in the caribbean hinterland react very sensitively to climatic variations late pleistocene and holocene climate records document pronounced dry and wet periods over tropical south america mainly caused by shifts of the intertropical convergence zone itcz however forcing mechanisms for changes in the itcz position remain controversial here we present high resolution foraminiferal ba ca and 18oseawater records from a core located within the orinoco river outfl ow documenting abrupt hydrological changes in the orinoco catchment area during the deglacial and holocene our data obtained from the surface dwelling foraminifera globigerinoides ruber pink show an abrupt increase in ba ca ratios in the early holocene starting 600 yr after the end of the younger dryas yd cold interval at ca 10 8 ka and suggesting a massive reorganization of moisture sources in northern south america in contrast the salinity dependent 18oseawater from the same samples shows a gradual decrease starting at the end of the yd the offset of our ba ca peak excludes meltwater release in conjunction with the northern andean glacier retreat well before the end of the yd as a forcing mechanism we suggest that the ba ca record documents an abrupt increase in ba rich waters of a northern andean source caused by the insolation driven shift of the itcz and or enhanced monsoon activity

proglacial lake sediment records reveal holocene climate changes in the venezuelan andes lake sediment records from the cordillera de merida in the northern venezuelan andes document the history of local glacial variability and climate changes during the holocene w12 ka to the present the valleys that contain these lakes have similar bedrock compositions and hypsometries but have different headwall elevations and aspects which makes them useful for investigating the magnitude of past glaciations there was widespread glacial retreat in the venezuelan andes during the early holocene after which most watersheds remained ice free and thus far only valleys with headwalls higher than w4400 m asl contain evidence of glaciation during the last w10 ka there was a pronounced shift in sediment composition for the montos headwall w4750 m asl and los anteojos headwall w4400 m asl records during the middle holocene from w8 0 to 7 7 ka when conditions appear to have become ice free and drier there is tentative evidence that the glacier in the mucubaji valley headwall w4609 m asl advanced from w8 1 to 6 6 ka and then retreated during the latter stages of the middle holocene clastic sediment accumulation in other nearby lake basins was either low or decreased throughout most of the middle holocene as watersheds stabilized under warmer and or drier conditions in the montos record there was another major shift in sediment composition that occurred from w6 5 to 5 7 ka similar to other regional records that suggest conditions were drier during this period overall the late holocene was a period of warmer and wetter conditions with ice extent at a minimum in the northern tropical andes there were also punctuated decadal to multi centennial periods of higher clastic sediment accumulation during the last w4 ka likely in response to periods of cooling and or local precipitation changes in watersheds with headwalls above 4600 m asl there is evidence of glacial advances during the little ice age w0 6e0 1 ka the pattern of glacial variability is generally similar in both the northern and southern tropics during the little ice age suggesting that ice margins in both regions were responding to colder and wetter conditions during the latest holocene the observed pattern of holocene climate variability in the venezuelan andes cannot be explained by insolation forcing alone and tropical ocean influences were likely associated with the observed glacial and lake level changes

late holocene paleoenvironmental change at mid elevation on the caribbean slope of the cordillera central dominican republic a multi site multi proxy analysis abstract high resolution proxy records from the circum caribbean region indicate significant variation in late holocene climate especially precipitation attributed primarily to shifts in the mean annual position of the intertropical convergence zone itcz the paleoenvironmental and cultural impacts of this late holocene climate variability have been analyzed intensively in the western caribbean and to a lesser extent in the southern caribbean however the occurrence and impacts of late holocene climate shifts in the eastern caribbean especially in island interiors has not been well documented here we present sediment records of late holocene paleoenvironmental change from two lakes located on the caribbean slope of the cordillera central in the dominican republic that span the last 3000 years sediment characteristics pollen charcoal biogenic carbonate assemblages and isotopic composition and bulk sedimentary carbon isotope values in laguna castilla and laguna de salvador indicate extreme shifts in hydrology vegetation and disturbance regimes in response to climate change and human activity in the lake watersheds close correspondence between the hydrological histories of the lakes and trace metal concentrations in sediments of the cariaco basin indicate that precipitation variability here responds to the same controls and may similarly reflect shifts in the mean annual position of the itcz human occupation of the watersheds appears to be closely linked to severe dry periods and may indicate larger scale cultural responses to precipitation variability on the island of hispaniola prehistoric human populations strongly affected vegetation and disturbance regimes in the lake watersheds impacts may have lasted several centuries and may have been more severe than impacts of modern populations

climate variability on the yucatan peninsula mexico during the past 3500 years and implications for maya cultural evolution climate variability on the yucatan peninsula during the past 3500 yrs is reconstructed from the measurement of 18o in monospecific ostracods and gastropods in a 6 3 m sediment core from lake punta laguna mexico this late holocene record is divided into three periods based on changes in mean 18o values from 3310 to 178514c yr b p period i low mean 18o values indicate relatively wet conditions i e low evaporation to precipitation ratio e p mean oxygen isotopic values increased 178514c yr b p and the interval between 1785 and 93014c yr b p period ii was distinctly drier than the periods before or after the climate during the latter part of period ii was persistently dry with exceptionally arid events centered at 1171 1019 and 94314c yr b p equivalent to 862 986 and 1051 a d this interval of frequent drought was recorded at several other localities in mexico and central america and coincided with the collapse of classic maya civilization following the last arid event 18o values decreased abruptly at 93014c yr b p beginning of period iii signaling a return to wetter conditions that have generally prevailed to the present with the exception of a dry episode centered at 55914c yr b p 1391 a d the paleoclimatic record from punta laguna provides evidence that multi decadal and millennial scale changes ine poccurred on the yucatan peninsula during the late holocene these wet dry episodes may have influenced cultural evolution in mesoamerica

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

climate change in lowland central america during the late deglacial and early holocene the transition from arid glacial to moist early holocene conditions represented a pro found change in northern lowland neotropical climate here we report a detailed record of changes in moisture availability during the latter part of this transition 11 250 to 7500 cal yr bp inferred from sediment cores retrieved in lake peten itza northern guatemala pollen assemblages demon strate that a mesic forest had been largely established by 11 250 cal yr bp but sediment properties indicate that lake level was more than 35 m below modern stage from 11 250 to 10 350 cal yr bp during the preboreal period lithologic changes in sediments from deep water cores 50 m below modern water level indicate several wet dry cycles that suggest distinct changes in effective moisture four dry events designated pbe1 4 occurred centred at 11 200 10 900 10 700 and 10 400 cal yr bp and correlate with similar variability observed in the cariaco basin titanium record and glacial meltwater pulses into the gulf of mexico after 10 350 cal yr bp multiple sediment proxies suggest a shift to a more persistently moist early holocene climate comparison of results from lake peten itzawith other records from the circum caribbean demonstrates a coherent climate response during the entire span of our record furthermore lowland neotropical climate during the late deglacial and early holocene period appears to be tightly linked to climate change in the high latitude north atlantic we speculate that the observed changes in lowland neotropical precipitation were related to the intensity of the annual cycle and associated displacements in the mean latitudinal position of the intertropical convergence zone and azores bermuda high pressure system this mechanism operated on millennial to submillennial timescales and may have responded to changes in solar radiation glacial meltwater north atlantic sea ice and the atlantic meridional overturning circulation moc copyright 2005 john wiley sons ltd

paleoecological and archaeological implications of a late pleistocene early holocene record of vegetation and climate from the pacific coastal plain of panama abstract a phytolith record from monte oscuro a crater lake located 10 m above sea level on the pacific coastal plain of panama shows that during the late pleistocene the lake bed was dry and savanna like vegetation expanded at the expense of tropical deciduous forest the modern potential vegetation a significant reduction of precipitation below current levels was almost certainly required to effect the changes observed core sediment characteristics indicate that permanent inundation of the monte oscuro basin with water occurred at about 10 500 14c yr b p pollen and phytolith records show that deciduous tropical forest expanded into the lake s watershed during the early holocene significant burning of the vegetation and increases of weedy plants at ca 7500 to 7000 14c yr b p indicate disturbance which most likely resulted from early human occupation of the seasonal tropical forest near monte oscuro and the development of slash and burn methods of cultivation

climate change in the circum north atlantic region during the last deglaciation a survey of new and published palaeoclimate data indicates that both the high and low latitude north atlantic regions were characterized by at least three synchronous periods of abrupt climate change during the last glacial to interglacial transi tion climate model results suggest that changes in the melting history of the laurentide ice sheet may explain much of this nonlinear response of the climate system to astronomical milankovitch forcing

two dimensional resistivity imaging in the kizildere geothermal field by mt and dc methods abstract magnetotelluric mt and schlumberger dc surveys were conducted to identify conductive zones inside metamorphic complex in the easternmost part of b menderes graben in this study using these surveying data creation of some two dimensional images of the shallow and deeper parts of the area was aimed broadband 200 0 002 hz mt and schlumberger data were acquired along four profiles crossing the b menderes graben with a total of 18 mt and 19 dc stations these data were modeled using two dimensional inverse techniques the main findings are 1 presence of wide conductive regions with very low resistivity

paleoenvironments and human occupation in late glacial panama the first pollen and phytolith data covering the entire pleistocene holocene transition from the lowlands of the central american isthmus indicate that the forests of late glacial panama at an altitude of 650 m resembled those currently found at ca 1500 1800 m a temperature depression of ca 5 c and reduced precipitation evaporation ratios in the late glacial period are suggested forest composition from ca 14 000 to 10 500 yr b p although primarily montane in character contained a low biomass of species today segregated in lowland forests and hence a floral assemblage with no modern analog the sudden appearance of carbon and burnt weedy plant material at ca 11 000 yr b p is attributed to the earliest human impact yet recorded from tropical america and may perhaps have been associated with the first human occupation of the region

evidence for a prolonged retroflection of the north brazil current during glacial stages the importance of the role played by the tropics in driving and propagating climate change between hemispheres has long been the focus of attention in a bid to evaluate ocean atmosphere interactions on glacial interglacial timescales the amazon fan and ceara rise in the western equatorial atlantic ocean lie directly in the flowpath of the north brazil current nbc which as a conduit for the cross equatorial transport of heat and salinity is a key component for the heat budget of the north atlantic mg ca palaeothermometry and stable oxygen isotope analysis of planktonic foraminifera sampled from 15 sites across the amazon fan and ceara rise reveal variations in oceanic surface currents and of the dispersal of freshwater from the river amazon over five timeslices modern early holocene younger dryas late glacial and marine isotope stage 3 sea surface temperature reconstructions reveal progressive climatic amelioration over the last similar to 30 ka indicating a temperature increase of similar to 3 2 1 1 degrees c since the late glacial in conjunction with this warming values of delta delta o 18 a proxy for water column stratification indicates increased vertical mixing of the glacial ocean the spatial distribution of values of delta 18 w the isotopic composition of ambient seawater is used to infer surface current variations and demonstrates an oceanward shift in the river outflow plume in cold climates representing a prolongation or possibly permanent continuation in the duration of the seasonal retroflection of the nbc causing the curtailment of cross equatorial transport a prolongation of this retroflection could have resulted from a mean southward migration of the southern boundary of the itcz c 2011 elsevier b v all rights reserved

topography weather and human activity effects on the behavior of metallic elements in a tropical catchment abstract the geochemical behavior of metals in 11 rivers of the lake valencia drainage system north central venezuela was investigated by repeated sampling pb zn ni cu and cr discharges from domestic and industrial activities have resulted in concentration increases of 2 16 above background levels depending on the metal as well as an increase of 13 27 of the labile fraction lf no apparent effect on the metals grain size distribution was observed evidence suggests that chemical reactions coupled with transport of sediments from the mountainous areas to lowland areas produce 1 an increase in metal concentration and lf which are concomitant with the pollution effect and 2 a metals grain size redistribution toward fine fractions analysis of major elements i e na k mg ca al and fe provides a suitable approach to discriminate the two effects mentioned above the wet dry season variability in metal concentration appeared to be controlled mainly by the input of flushed material from soil this process can explain the large areal variability of concentrations compared with sampling and analytical variability a physical dilution of zn with less contaminated flushed soil material was also observed

a holocene vegetation history from lowland guatemala a 5 45 m core from lake peten itza lowland guatemala contains a near complete record of holo cene sedimentation the age depth relationship for the core is based on ams 14c dating of terrestrial wood fragments and provides a reliable chronology in this karst region where hard water lake error has typically confounded sediment geochronology in the basal part of the sequence pollen of the moraceae urticaceae group dominate indicating the presence of widespread tropical forest during the early holocene c 8600 5600 yr bp relative abundance of pollen of high forest taxa declined beginning as early as 5600 yr bp indicating climatic drying or perhaps initial land clearance deforestation by prehistoric mayan inhabitants is documented clearly in the pollen record beginning about 2000 yr bp 106 cal bc 122 cal ad 95 4 2 sigmas by the appearance of disturbance taxa e g ambrosia and poaceae and presence of zea forest regrowth occurred following the classic maya collapse c ad 900 as reflected by

a cooling event during the younger dryas chron in costa rica abstract a fossil pollen record from the costa rican cordillera de talamanca presents the first ams radiocarbon dated evidence of a temperature decrease during the younger dryas chron this cooling event is named la chonta stadial after the bog at 2310 m altitude located at the actual lower to upper montane forest boundary high resolution pollen analysis revealed that between 11 070 130 ams 14c age and 10 400 yr b p interpolated age vegetation comparable to present day subalpine forest occurred about 300 400 m lower than in the previous warmer interval and 600 to 700 m lower than at present time the radiocarbon dates are close to the earlier estimated ages of 11 080 10 500 yr b p for this interval based on interpolation and pollen concentration rates hooghiemstra et al 1992 the downslope shift of the upper forest line indicates an estimated temperature drop of 2 3 c during the la chonta stadial the local vegetation development is indicative of a drier climate from 10 400 to 9800 120 yr b p ams 14c age subalpine rain forest was replaced by upper montane forest a transitional period to holocene environmental conditions

an optical luminescence chronology for late pleistocene aeolian activity in the colombian and venezuelan llanos the lowland savannas llanos of colombia and venezuela are covered by extensive aeolian landforms for which little chronological information exists we present the first optically stimulated luminescence osl age constraints for dunes in the llanos orientales of lowland colombia and new ages for dunes in the venezuelan llanos the sampled dunes are fully vegetated and show evidence of post depositional erosion ages range from 4 5 0 4 to 66 4 ka with the majority dating to 27 10 ka marine isotope stage 2 some dunes accumulated quickly during the last glacial maximum although most were active 16 10 ka accretion largely ceased after 10 ka all dunes are elongated downwind from rivers parallel with dry season winds and are interpreted as source bordering features as they are presently isolated from fluvial sediments by gallery forest it is proposed that activity was associated with a more prolonged dry season which restricted gallery forest leading to greater sediment availability on river shorelines such variability in dry season duration was potentially mediated by the mean latitude of the itcz the cessation of most dune accretion after ca 10 ka suggests reduced seasonality and a more northerly itcz position consistent with evidence from the cariaco basin

a high resolution late quaternary upwelling record from the anoxic cariaco basin venezuela results are presented of a high resolution study of the planktonic foraminiferal faunas from two piston cores recovered from the cariaco basin in the southern caribbean sea the cariaco basin is a small anoxic marine basin on the northern continental margin of venezuela in an area today characterized by both seasonal trade wind induced upwelling and pronounced dry and wet seasons our data indicate that large changes in the intensity of upwelling and hence trade wind strength occurred in this region during the last glacial interglacial transition and throughout the holocene during the last glacial lowstand of sea level the cariaco basin was effectively isolated from the open caribbean along its northern margin by the then largely emergent tortuga bank oxic conditions existed in the deep cariaco basin at this time and surface productivity was low about 12 600 years ago the abrupt initiation of strong upwelling over the basin and the onset of permanent anoxia in the deep waters are coincident with the rapid rise of sea level that accompanied the peak interval of meltwater discharge from the laurentide ice sheet into the gulf of mexico strong upwelling between 12 600 and about 10 000 years ago may be related to intensified trade winds resulting in part from cooler sea surface temperatures in the caribbean and gulf of mexico after about 10 000 years ago upwelling intensity was reduced though highly variable a preliminary frequency domain analysis of the holocene portion of the cariaco basin time series suggests that solar forcing may explain a significant component of the century scale variability observed in the record of upwelling and trade wind strength

an 8700 year paleoclimate reconstruction from the southern maya lowlands abstract analysis of a sediment core from lago puerto arturo a closed basin lake in northern peten guatemala has provided an 8700 cal year record of climate change and human activity in the southern maya lowlands stable isotope magnetic susceptibility and pollen analyses were used to reconstruct environmental change in the region results indicate a relatively wet early to middle holocene followed by a drier late holocene which we interpret as reflecting long term changes in insolation precession higher frequency variability is more likely attributable to changes in ocean atmosphere circulation in both the north atlantic and the pacific oceans pollen and isotope data show that most of the period of prehispanic agricultural settlement i e 5000 1000 cal yr bp was characterized by drier conditions than previous or subsequent periods the presence of zea corn pollen through peak aridity during the terminal classic period 1250 1130 cal yr bp suggests that drought may not have had as negative an impact as previously proposed a dramatic negative shift in isotope values indicates an increase in precipitation after 950 cal yr bp hereafter bp