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

a 3400 year paleolimnological record of prehispanic human environment interactions in the holmul region of the southern maya lowlands abstract the timing magnitude and drivers of late holocene environmental change in the holmul region of the southern maya lowlands are examined by combining paleoenvironmental and archeological data environmental proxy analyses on a 3350 cal yr lacustrine sediment record include pollen charcoal loss on ignition magnetic susceptibility and elemental geochemistry archeological evidence is derived from extensive settlement surveys conducted near the study site results indicate nearby settlement and agricultural activity taking place in an environment characterized by open forest from around 3350 to 950 cal yr bp the fire history shows a dramatic increase in burning during the classic period possibly reflecting changing agricultural strategies a distinct band of carbonate deposited from 1270 to 1040 cal yr bp suggests decreased hydrologic input associated with drier conditions abrupt changes in proxy data around 940 cal yr bp indicate a cessation of human disturbance and local abandonment of the area

persistent solar influence on north atlantic climate during the holocene surface winds and surface ocean hydrography in the subpolar north atlantic appear to have been influenced by variations in solar output through the entire holocene the evidence comes from a close correlation between inferred changes in production rates of the cosmogenic nuclides carbon 14 and beryllium 10 and centennial to millennial time scale changes in proxies of drift ice measured in deep sea sediment cores a solar forcing mechanism therefore may underlie at least the holocene segment of the north atlantic9s 1500 year cycle the surface hydrographic changes may have affected production of north atlantic deep water potentially providing an additional mechanism for amplifying the solar signals and transmitting them globally

links between salinity variation in the caribbean and north atlantic thermohaline circulation variations in the strength of the north atlantic ocean thermohaline circulation have been linked to rapid climate changes1 during the last glacial cycle through oscillations in north atlantic deep water formation and northward oceanic heat flux2 3 4 the strength of the thermohaline circulation depends on the supply of warm salty water to the north atlantic which after losing heat to the atmosphere produces the dense water masses that sink to great depths and circulate back south2 here we analyse two caribbean sea sediment cores combining mg ca palaeothermometry with measurements of oxygen isotopes in foraminiferal calcite in order to reconstruct tropical atlantic surface salinity5 6 during the last glacial cycle we find that caribbean salinity oscillated between saltier conditions during the cold oxygen isotope stages 2 4 and 6 and lower salinities during the warm stages 3 and 5 covarying with the strength of north atlantic deep water formation7 at the initiation of the bolling allerod warm interval caribbean surface salinity decreased abruptly suggesting that the advection of salty tropical waters into the north atlantic amplified thermohaline circulation and contributed to high latitude warming

the arid humid transition in the sahara and the sahel during the last deglaciation at the time of the last glacial maximum the sahara and sahel regions of north africa were extremely dry new records of rainfall show that during the subsequent deglaciation the transition from arid to humid conditions in these regions occurred synchronously in two main steps comparison with other records of palaeoclimate in europe and the north atlantic ocean shows that certain common factors controlled changes in ocean and atmosphere dynamics during the deglaciation

the preboreal climate reversal and a subsequent solar forced climate shift accurate chronologies are essential for linking palaeoclimate archives carbon 14 wiggle match dating was used to produce an accurate chronology for part of an early holocene peat sequence from the borchert the netherlands following the younger dryas preboreal transition two climatic shifts could be inferred around 11 400 cal yr bp the expansion of birch betula forest was interrupted by a dry continental phase with dominantly open grassland vegetation coeval with the pbo preboreal oscillation as observed in the grip ice core at 11 250 cal yr bp a sudden shift to a humid climate occurred this second change appears to be contemporaneous with i a sharp increase of atmospheric c 14 ii a temporary decline of atmospheric co2 and iii an increase in the grip be 10 flux the close correspondence with excursions of cosmogenic nuclides points to a decline in solar activity which may have forced the changes in climate and vegetation at around 11 250 cal yr bp copyright c 2004 john wiley sons ltd

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

quantification of soil erosion rates related to ancient maya deforestation we used seismic and sediment core data to quantify soil erosion rates for the past 6000 yr in the closed catchment of lake salpeten in the tropical lowlands of northern guatemala the region was affected by ancient maya land use from before ca 1000 b c to a d 900 this period of human impact coincided with deposition in the lake of a detrital unit maya clay as much as 7 m thick that contrasts sharply with the relatively organic rich gyttja deposited both before and after maya occupation of the watershed the greatest soil loss with mean sustained values of 1000 t km2yr 1 occurred in the middle and late preclassic periods 700 b c to a d 250 associated with relatively low maya population densities soil erosion slowed during the period of maximum population density in the late classic period a d 550 830 indicating a decoupling between human population density and soil erosion rate the most rapid soil loss occurred early during initial land clearance suggesting that even low numbers of people can have profound impacts on lowland tropical karst landscapes

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

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

nature and stability of atoll island shorelines gilbert island chain kiribati equatorial pacific islands rimming pacific atolls typically form narrow low lying lands that are commonly perceived to be particularly vulnerable to global changes such as sea level rise as these low islands form the only habitable land for many island nations understanding the character of shorelines and the rates and controls that operate to bring about changes is an issue of central importance the purpose of this study is to unravel the characteristics of coastal change on atoll islands of the gilbert island chain of the equatorial pacific nation of kiribati especially as they relate to autogenic shoreline processes and el nino southern oscillation variability integration of field observations differential global positioning system data historical aerial photographs and ultra high resolution remote sensing images demonstrates the nature spatial patterns and rates of change from 17 islands on maiana and aranuka atolls the results illustrate that between 2005 and 2009 ca 50 of the shorelines on these islands displayed a discernable shift in position some shorelines were accretionary at net rates up to ca 8 m year 1 and others were erosional up to ca 18 m year 1 long term net rates of change on maiana between 1969 and 2009 were lower than short term net rates measured between 2005 and 2009 both short term and long term observations illustrate some of the greatest change occur near terminations of the largest north south oriented islands associated with longshore movement of coarse sand and gravel direct hits by tropical depressions and marked seasonality factors interpreted as being essential in island growth and shoreline dynamics elsewhere do not directly impact these equatorial atolls and can be eliminated as fundamental controls on shoreline dynamics similarly observations over four years suggested that shoreline variability probably is not influenced directly by marked sea level change although a recent increase in the rates of shoreline change could reflect instability related to the cumulative effect of a long term increase in the rate of sea level rise within this framework of global change local anthropogenic effects autogenic shoreline processes and el nino southern oscillation influenced wind and wave variability control many aspects of these dynamic shorelines these results provide quantitative insights into the character and variability of rates of shoreline change information essential for evaluating and mitigating the vulnerability of island nations such as kiribati

calcite raft geochemistry as a hydrological proxy for holocene aquifer conditions in hoyo negro and ich balam sac actun cave system quintana roo mexico abstract two cores from calcite rafts deposits located in cenote ich balam and hoyo negro were dated and analyzed for 87sr 86sr 18o 13c sr ca and cl ca the geochemical records show changing aquifer salinity spanning the last 8 5 cal kyrs bp and interrelationships with holocene climate trends wet and dry periods during the wet mid holocene the salinity of the meteoric water mass wm at 7 8 8 3 cal kyrs bp was relatively high at 1 5 2 7 ppt and then became less saline 1 0 1 5 ppt during the last 7000 yrs as climate became progressively drier high salinity of the meteoric wm during the wet mid holocene is attributed to increased turbulent mixing between the meteoric and underlying marine wm increased precipitation in terms of amount frequency and intensity e g hurricanes causes higher flow of meteoric water towards the coast and mixing at the halocline a phenomenon recorded with recent instrumental monitoring of the aquifer conversely during dry periods reduced precipitation and flow in the meteoric wm would result in lower salinity karst properties and holocene sea level rise also seem to have an effect on the aquifer when the regionally extensive network of shallow cave passages 10 12 m water depth are flooded at 8000 cal yrs bp there is a rapid shift in salinity this study demonstrates that calcite raft deposits can be used as paleo environmental recorders documenting the effects of sea level and climate change on aquifer condition

glacial meltwater cooling of the gulf of mexico gcm implications for holocene and present day climates an atmospheric general circulation model the ncar ccm has been used to investigate the possible effects that reduced gulf of mexico sea surface temperatures sst could have on regional and hemispheric climates 18o records and terrestrial evidence indicate at least two major glacial meltwater discharges into the gulf of mexico subsequent to the last glacial maximum it is probable that these discharges reduced gulf of mexico sst we have conducted three numerical experiments with imposed gulf wide sst coolings of 3 c 6 c and 12 c and find in all three experiments significant reductions in the north atlantic storm track intensity along with a strong decrease in transient eddy water vapor transport out of the gulf of mexico surface pressures are higher over the north atlantic indicating a reduction of the climatological icelandic low the region is generally cooler and drier with a reduction in precipitation that agrees well with evidence from greenland ice cores other statistically significant changes occur across the northern hemisphere but vary between the three experiments in particular warmer wetter conditions are found over europe for both the 6 c and 12 c sst reductions but cooler conditions are found for the 3 c reduction this indicates a dependence in both the sign and magnitude of the model response on the magnitude of the imposed sst anomaly the results suggest that the present day north atlantic storm track is dependent on warm gulf of mexico sst for much of its intensity they also suggest that meltwater induced coolings may help account in part for some of the climatic oscillations that occurred during the last glacial interglacial transition

causes and timing of the 8200 yr bp event inferred from the comparison of the grip be 10 and the tree ring delta c 14 record we analyse the 8200 yr bp cold event by comparing the high resolution be 10 record from the grip ice core from central greenland with the well known tree ring delta 14 c record by transferring the absolute dated tree ring chronology to the ice core time scale we show that the coldest phase in the grip record occurred around 8150 yr bp furthermore this method allows us to disentangle production and climate effects on be 10 and c 14 with important implications for the reconstruction of past solar activity and changes in the carbon cycle and be 10 transport we show that in principle it is possible to infer changes in ocean circulation by comparing be 10 and c 14 records however the duration of the 8200 yr bp event is too short to assign unambiguously a significant change in atmospheric c 14 concentration to changes in the global ocean circulation based on the comparison of be 10 with climate records one could argue that the 8200 yr bp cold event is triggered by a change towards lower solar activity however this link is questioned by the fact that around this period there are other similar and even stronger changes in solar activity that have no apparent connection to climate changes c 2004 elsevier ltd all rights reserved less

holocene climatic instability a prominent widespread event 8200 yr ago the most prominent holocene climatic event in greenland ice core proxies with approximately half the amplitude of the younger dryas occurred 8000 to 8400 yr ago this holocene event affected regions well beyond the north atlantic basin as shown by synchronous increases in windblown chemical indicators together with a significant decrease in methane widespread proxy records from the tropics to the north polar regions show a short lived cool dry or windy event of similar age the spatial pattern of terrestrial and marine changes is similar to that of the younger dryas event suggesting a role for north atlantic thermohaline circulation possible forcings identified thus far for this holocene event are small consistent with recent model results indicating high sensitivity and strong linkages in the climatic system

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

the gisp2 δ18o climate record of the past 16 500 years and the role of the sun ocean and volcanoes measured 18o 16o ratios from the greenland ice sheet project 2 gisp2 ice core extending back to 16 500 cal yr b p provide a continuous record of climate change since the last glaciation high resolution annual 18o 16o results were obtained for most of the current millennium a d 818 1985 and record the medieval warm period the little ice age and a distinct 11 yr 18o 16o cycle volcanic aerosols depress central greenland annual temperature 1 5 c maximally and annual 18o 16o for about 4 yr after each major eruptive event on a bidecadal to millennial time scale the contribution of solar variability to holocene greenlandic temperature change is 0 4 c the role of thermohaline circulation change on climate problematic during the holocene is more distinct for the 16 500 10 000 cal yr b p interval analogous to 14c age calibration terminology we express time in calibrated cal yr b p a d 1950 0 cal yr b p the oldest dryas bolling older dryas allerod younger dryas sequence appears in great detail bidecadal variance in 18o 16o but not necessarily in temperature is enhanced during the last phase of lateglacial time and the younger dryas interval suggesting switches of air mass transport between jet stream branches the branched system is nearly instantaneously replaced at the beginning of the bolling and holocene at 14 670 and 11 650 cal yr b p respectively by an atmospheric circulation system in which 18o 16o and annual accumulation initially track each other closely thermodynamic considerations of the accumulation rate temperature relationship can be used to evaluate the 18 o 16o temperature relationship the gisp2 ice layer count years of major gisp2 climate transitions also support the use of coral 14c ages for age calibration

rate and processes of river network rearrangement during incipient faulting the case of the cahabon river guatemala deeply incised river networks are generally regarded as robust features that are not easily modified by erosion or tectonics although the reorganization of deeply incised drainage systems has been documented the corresponding importance with regard to the overall landscape evolution of mountain ranges and the factors that permit such reorganizations are poorly understood to address this problem we have explored the rapid drainage reorganization that affected the cahabon river in guatemala during the quaternary sediment provenance analysis field mapping and electrical resistivity tomography ert imaging are used to reconstruct the geometry of the valley before the river was captured dating of the abandoned valley sediments by the be 10 al 26 burial method and geomagnetic polarity analysis allow us to determine the age of the capture events and then to quantify several processes such as the rate of tectonic deformation of the paleovalley the rate of propagation of post capture drainage reversal and the rate at which canyons that formed at the capture sites have propagated along the paleovalley transtensional faulting started 1 to 3 million years ago produced ground tilting and ground faulting along the cahabon river and thus generated differential uplift rate of 0 3 0 1 up to 0 7 0 4 mm y 1 along the river s course the river responded to faulting by incising the areas of relative uplift and depositing a few tens of meters of sediment above the areas of relative subsidence then the river experienced two captures and one avulsion between 700 ky and 100 ky the captures breached high standing ridges that separate the cahabon river from its captors captures occurred at specific points where ridges are made permeable by fault damage zones and or soluble rocks groundwater flow from the cahabon river down to its captors likely increased the erosive power of the captors thus promoting focused erosion of the ridges valley fill formation and capture occurred in close temporal succession suggesting a genetic link between the two we suggest that the aquifers accumulated within the valley fills increased the head along the subterraneous system connecting the cahabon river to its captors and promoted their development upon capture the breached valley experienced widespread drainage reversal toward the capture sites we attribute the generalized reversal to combined effects of groundwater sapping in the valley fill axial drainage obstruction by lateral fans and tectonic tilting drainage reversal increased the size of the captured areas by a factor of 4 to 6 at the capture sites 500 m deep canyons have been incised into the bedrock and are propagating upstream at a rate of 3 to 11 mm y 1 deepening at a rate of 0 7 to 1 5 mm y 1 at this rate 1 to 2 million years will be necessary for headward erosion to completely erase the topographic expression of the paleovalley it is concluded that the rapid reorganization of this drainage system was made possible by the way the river adjusted to the new tectonic strain field which involved transient sedimentation along the river s course if the river had escaped its early reorganization and had been given the time necessary to reach a new dynamic equilibrium then the transient conditions that promoted capture would have vanished and its vulnerability to capture would have been strongly reduced

a giant underwater encrusted stalactite from the blue hole lighthouse reef belize revisited a complex history of biologically induced carbonate accretion under changing meteoric and marine conditions a late pleistocene to holocene submerged and encrusted speleothem exhibits a complex history including a meteoric phase and two marine phases a combined study using petrography mineralogy and inorganic and organic geochemistry as well as geochronology has shown that phototrophic and heterotrophic biological activity impacted carbonate precipitation during all phases of carbonate accretion the stalactite formed ca 30 m below modern sea level at a marginal overhang in the blue hole of lighthouse reef atoll unlike purely meteoric speleothems the belize example consists of a meteoric core a marine aragonite crust and a serpulid micrite rich outer crust as a result of postglacial flooding of the karst cave the core of the stalactite has a tufaceous texture containing algal or microbial remains and consists entirely of low magnesium calcite formed 19 55 10 68 kyr bp the texture suggests that the stalactite formed at the cave entrance and hence the former cave ceiling had apparently collapsed earlier during the pleistocene oxygen 18o and carbon 13c isotopes across the core suggest a trend towards drier conditions and reduced soil and plant cover after the last glacial maximum the marine aragonite crust consists of stacked botryoids in which individual crystals up to 700 lm have dark terminations enriched in high magnesium calcite this crust accreted from 10 82 to 9 95 kyr bp in warm shallow water during the early holocene thermal optimum carbonate accretion rates were considerable and averaged 125 m yr the crust has a dense laminated texture on one side and a porous shrubby texture on the other the presence of n c16 1 5 n c17 1 6 and 10me c16 fatty acids in the laminated crust suggests that sulfate reducing bacteria contributed to aragonite formation in an environment that was less open than the formation environment of the porous crust where these biomarkers are lacking n c16 1 5 n c17 1 6 or are less abundant 10me c16 enrichment of 34s and 18o in carbonate associated sulfate cas relative to seawater sulfate also suggests sulfate reduction during carbonate formation the greater contribution of heterotrophic processes to aragonite precipitation in the laminated crust is also reflected in 13c values as low as 1 3 whereas no such depletion is observed in the aragonite of the porous crust 13c values as low as 0 0 a pronounced isotopic variability and excursions to positive 13c values as high as 3 5 0 in the inner half of the laminated crust indicate an episodic local impact of photosynthesis on aragonite precipitation whereas the lack of such excursions in the porous crust 13c values as high as 1 5 0 is again best explained by a more open environment of formation after a ca 5 kyr hiatus from 4 39 kyr bp a biogenic crust of abundant serpulids and finely crystalline microbial and detrital carbonate consisting of high magnesium calcite and aragonite accreted on the outer surface of the stalactite outermost crust accretion was probably influenced by the inundation of the lighthouse reef lagoon that started to shed abundant fine grained carbonate sediment into the blue hole the stalactite broke off the cave ceiling either before or after the formation of the outermost crust likely due to seismic movements along the nearby plate boundary the study demonstrates that like speleothems from the terrestrial realm submerged stalactites may have had complex histories with great potential as paleoenvironmental archives

a 10 300 14c yr record of climate and vegetation change from haiti abstract pleistocene and holocene vegetation dynamics in the american tropics are inferred largely from pollen in continental lake sediments maritime influences may have moderated climate and vegetation changes on caribbean islands stable isotope 18 o study of a 7 6 m core from lake miragoane haiti provided a high resolution record of changing evaporation precipitation e p since 10 300 14 c yr b p the miragoane pollen record documents climate influences and human impacts on vegetation in hispaniola the 18 o and pollen data near the base of the core indicate cool dry conditions before 10 000 14 c yr b p lake miragoane filled with water in the early holocene as e p declined and the freshwater aquifer rose despite increasing early holocene moisture shrubby xeric vegetation persisted forest expanded 7000 14 c yr b p in response to greater effective moisture and warming the middle holocene 7000 3200 14 c yr b p was characterized by high lake levels and greatest relative abundance of pollen from moist forest taxa climatic drying that began 3200 14 c yr b p may have driven some mesophilic animal species to extinction the pollen record of the last millennium reflects pre columbian taino and european deforestation long term holocene vegetation trends in southern haiti are comparable to trends from continental lowland circum caribbean sites suggesting a common response to regional climate 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

a 2000 year record of caribbean and tropical north atlantic hydrographic variability 1 here we present near annually resolved oxygen isotope records from two species of planktic foraminifera from the cariaco basin that reflect sea surface temperature sst and intertropical convergence zone itcz precipitation related salinity variations over the caribbean and tropical north atlantic spanning the last 2000 years a strong broad spatial pattern of correlation exists between foraminiferal 18o and ssts over the period of instrumental overlap but the correlations weaken as they are extended back in time and instrumental sst records become discontinuous a long term trend in the globigerinoides ruber 18o record can be explained by two different but equally plausible scenarios first the increase in 18o may indicate that tropical summer fall ssts have cooled by as much as 2 c over the last 2000 years possibly as a result of a long term increase in upwelling intensity alternately comparisons to other studies of itcz and regional evaporation precipitation variability suggest that much of the 18o record is influenced by decadal to centennial scale variations in the mean annual position of the itcz and associated rainfall patterns similarities between the g bulloides 18o record and the 11 year sunspot cycle support prior studies that suggest solar variability plays a role in influencing the hydrologic balance of the circum caribbean

the preboreal oscillation around the nordic seas terrestrial and lacustrine responses the occurrence of an early preboreal climatic cooling oscillation pbo in lacustrine and glacial records from northwest europe iceland and greenland is reviewed and documented the often subtle response of the proxy records to this oscillation in combination with its short duration make it difficult to detect owing to its chronostratigraphic position between the 10 000 9900 and 9600 9500 14 c plateaux c 11 300 11 150 calendar yr bp it is also difficult to 14 c date with precision we find that the vegetation response to the pbo varies between sites and regions in contrast to the pioneer vegetation in iceland and southern sweden the expanding birch pine forest in germany denmark was more susceptible to deteriorating growing conditions the combined lacustrine tree ring and glacial records imply that the pbo was characterised by cool and humid conditions throughout northwestern and central europe this is documented by vegetation changes decreased aquatic production increased soil erosion increased 2 h and 13 c content in tree rings readvances or stillstands of the ice sheet in norway and finland and ingression of brackish water into the baltic icelandic proxy records from lake sediments and glacial moraines imply cooler conditions than during the previous preboreal period but not as extreme as during the younger dryas greenland records suggest that the early preboreal was characterised by ice readvances as an effect of cool climate and increased precipitation in relation to the younger dryas it was not until the end of the pbo that climate was warm enough to melt the land based ice sheet this preboreal oscillation found on both sides of the nordic seas is interpreted as an effect of increased freshwater forcing on the thermohaline circulation in the nordic seas which is implied by a simultaneous and distinct rise in the atmospheric 14 c 12 c ratio a slow down of the thermohaline circulation may tempor arily have pushed the polar front further south 1997 by john wiley sons ltd preboreal oscillation thermohaline circulation lacustrine records glacial records chronology

meltwater cooling of the gulf of mexico a gcm simulation of climatic conditions at 12 ka marine and terrestrial evidence indicate that at least two major glacial meltwater discharges into the gulf of mexico occurred during the last deglaciation based on radiocarbon dates the time interval between these two meltwater events appears to coincide with the younger dryas roughly 10 11 ka we have used an atmospheric general circulation model the national center for atmospheric research community climate model version 1 ncar ccm1 to investigate possible effects of reduced gulf of mexico sea surface temperatures sst assumed due to the meltwater discharges on regional and hemispheric climates we conducted six numerical experiments 1 perpetual january wintertime and 2 july summertime sensitivity experiments using present day boundary conditions except for an imposed gulfwide sst cooling of 6 c 3 perpetual january and 4 july control simulations using 12 ka boundary conditions for continental ice sea surface temperature no reduction due to meltwater cooling sea ice and radiative forcing and 5 perpetual january and 6 july 12 ka simulations with a 6 c sst cooling of the gulf of mexico in both of the january simulations with a cooled gulf of mexico a significant reduction in the north atlantic storm track intensity occurred with higher surface pressures over the north atlantic indicating a reduction of the icelandic low in the january 12 ka control there are large changes in surface pressure over the atlantic ocean and greenland relative to the present day control however these changes are almost completely offset when the gulf of mexico is cooled in the july simulations with a cool gulf of mexico the atlantic subtropical high is strengthened and expanded to the west this results in stronger trade winds across the caribbean and southwest atlantic there is also an enhanced monsoonal flow of moisture over northwest mexico and the american southwest and rocky mountain regions leading to increased precipitation unlike the winter situation in summer the gulf of mexico cooling serves to enhance the effects of the other 12 ka boundary conditions

a 43 ka record of paleoenvironmental change in the central american lowlands inferred from stable isotopes of lacustrine ostracods we present a continuous ostracod isotope 18o and 13c record from lake peten itza peten guatemala in the northern lowland neotropics that spans the last 43 cal ka bp variations in oxygen and carbon isotopes closely follow lithologic variations which consist of alternating gypsum and clay deposits that were deposited under relatively dry and wet climate respectively during the last glacial period the greatest 18o and 13c values coincide with gypsum deposited during lake lowstands under arid climate conditions that were correlated previously with north atlantic heinrich events in contrast interstadials and the entirety of the last glacial maximum 24 19 cal ka bp are marked by clay deposition and lower 18o and 13c values reflecting higher lake levels and relatively moister climate isotope results and pollen data along with independently inferred past water levels show the early deglacial period 19 15 cal ka bp was the time of greatest aridity and lowest lake stage of the past 43 ka this period occurred during heinrich stadial 1 hs 1 when an extensive tropical megadrought has been postulated stager et al 2011 heinrich stadial 1 is represented by two episodes of gypsum precipitation and high 18o and 13c values in peten itza interrupted by an intervening period of lower 18o and 13c and clay deposition centered on 17 cal ka bp the two periods of inferred maximum cold and or arid conditions at 17 5 and 16 1 cal ka bp coincide approximately with two pulses of ice rafted debris ird recorded off southern portugal bard et al 2000 at 15 cal ka bp coinciding with the start of the bolling allerod period 18o and 13c decrease and gypsum precipitation ceases indicating a transition to warmer and or wetter conditions gypsum precipitation resumed while 18o and 13c increased at the start of the younger dryas at 13 1 cal ka bp and continued until 10 4 cal ka bp near the onset of the holocene precipitation changes during the last glacial period in the northern hemisphere neotropics were closely linked with freshwater forcing to the high latitude north atlantic and sensitive to changes in the location of meltwater input climate was coldest driest when meltwater directly entered the high latitude north atlantic permitting sea ice expansion and weakening of atlantic meridional overturning circulation amoc which resulted in a more southerly position of the intertropical convergence zone itcz upon deglaciation when meltwater was directed to the gulf of mexico at 17 ka and during the bolling allerod period 15 13 ka precipitation increased in the northern hemisphere neotropics as north atlantic sea ice retreated and the itcz shifted northward results from lake peten itza offer some support for the meltwater routing hypothesis of clark et al 2001

recovery of the forest ecosystem in the tropical lowlands of northern guatemala after disintegration of classic maya polities we employed paleolimnological methods to investigate tropical forest recovery and soil stabilization that followed abandonment of agricultural systems associated with disintegration of classic maya polities ca a d 800 1000 we used lithological geochemical magnetic and palynological data from sediment cores of lake peten itza in the maya lowlands of northern guatemala sediment core chronology was developed using radiocarbon dates on terrestrial wood and charcoal fragments our results indicate that in the absence of large human populations and extensive farming activities peten forests recovered under humid climate conditions within a span of 80 260 yr soil stabilization postdates pollen evidence of forest regrowth stratigraphically and required between 120 and 280 yr we conclude that the tropical forest ecosystem in the watershed of lake peten itza had been reestablished by the early postclassic period a d 1000 1200

palaeomagnetic secular variation and rock magnetic studies of holocene sediments from a maar lake hoya de san nicolas in central mexico summary three up to 520 cm long sediment cores from hoya san nicolas in guanajuato mexico were analysed for various magnetic properties in order to better define a palaeomagnetic secular curve for central mexico the results magnetic susceptibility hysteresis cycles anhysteretic and isothermal remanent magnetization suggest that the remanent magnetization of the sediments is controlled by ferrimagnetic minerals pseudo single domain magnetite which are suitable recorders of the geomagnetic field the age depth model indicates average deposition rates of 0 32 interval 146 198 cm and 0 45 mm yr 1 interval 200 520 cm and a basal age of about 11 600 calibrated years bp cal yr bp we used magnetic susceptibility and natural remanent magnetization to correlate the three cores a composite palaeomagnetic secular variation sv record was obtained from the cores with a stretching and stacking process and a chronology established with accelerator mass spectrometer dates on microscopic charcoal and stratigraphic correlations with other well dated holocene records from mexico and guatemala the declination and inclination results show oscillatory behaviour varying in a narrow range although a distinctive directional swing is evident between 9060 and 9810 cal yr bp the san nicolas palaeomagnetic sv curve is similar to palaeomagnetic master curves from europe and north america in shape occurrence and synchronicity of directional features especially with respect to inclination

sensitivity of the atlantic intertropical convergence zone to last glacial maximum boundary conditions 1 recent paleoproxy records suggest that the mean latitude of the atlantic intertropical convergence zone itcz varied synchronously with north atlantic climate over a range of timescales throughout the holocene and last glacial maximum we show that the present day meridional mode of atmosphere ocean variability in the tropical atlantic is a potentially useful model for understanding these paleoclimate changes the tropical atlantic in a coupled atmospheric general circulation and slab ocean model responds to last glacial maximum conditions with a southward displacement of the itcz this response arises primarily through the land ice sheet that forces increased north atlantic trades analogous to the forcing on the present day meridional mode changes to sea ice coverage and to ocean heat transport associated with a weakened atlantic thermohaline circulation also cause a meridional mode response though through different mechanisms our results highlight the potential for tropical atlantic paleoclimate to be driven from the high latitude influences in particular land ice on glacial interglacial timescales

late quaternary climate induced lake level variations in lake peten itza guatemala inferred from seismic stratigraphic analysis abstract we used seismic images and sedimentary data from piston cores to conduct a sequence stratigraphic analysis of sediments in lake peten itza northern guatemala our results document lake level fluctuations in this lowland neotropical region that were related to glacial to interglacial climate change during the late pleistocene a bathymetric survey of lake peten itza area 100 km2 revealed a maximum water depth of 160 m and the existence of a deep cryptodepression that extends 50 m below modern sea level the great depth suggests that the basin held water even during arid conditions associated with full glacial periods lake peten itza may thus possess the only long continuous lacustrine sediment record of late pleistocene environmental and climate change in the lowland neotropics two seismic reflection campaigns imaged the subsurface basin sediments that overlie basement the sediment package was divided into four major seismic sequences t g r and b sequences are separated by unconformities that represent depositional cycles related to lake level fluctuations sediments of the uppermost sequence t were recovered and radiocarbon dated in kullenberg piston cores taken along a water depth transect seismic profiles reveal a basin wide paleoshoreline just below sequence t at 56 m below present lake level this constructional feature formed during a lowstand of the last glacial period when the lake was reduced to only 13 of its present volume in cores taken landward of the paleoshoreline late glacial age deposits consist of paleosols indicating subaerial exposure basinward of the shoreline sediments are composed of dense gypsum sands and interbedded silty clays reflecting authigenic gypsum formation under arid climate conditions the top of the soil horizon and cessation of gypsum precipitation are represented by a strong seismic reflection t it marks the base of the uppermost seismic sequence t and is dated in several cores between 11 1 and 10 2 cal kyr bp lake level rose quickly at this time in response to a shift from arid to humid climate conditions at the late glacial early holocene transition we infer a similar sediment response to climate variations in the older stratigraphic sequences g r and b related to earlier glacial to interglacial and stadial to interstadial cycles older sequences are also distinguished from one another by erosional unconformities that probably represent major lake level falls future recovery of the older stratigraphic record by drilling in lake peten itza will provide ages for these older units and enable us to test the depositional model inferred from seismic stratigraphy

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

environmental change in northern belize since the latest pleistocene cores from the new river lagoon in belize have been analysed using a range of palaeolimnological proxies to reconstruct change since the latest pleistocene the combination of diatom and stable isotope analyses supported by mineralogical and major element data indicate that the new river lagoon has been a freshwater system throughout the period of record for most of this time the lagoon has been hydrologically open but there are indications that it may have become closed in the latest pleistocene or very early holocene this may have been associated with a drier climate and lower sea level mid holocene records are consistent with those from elsewhere in the region indicating a stable and moist climate the late holocene was marked by greater variability but there is no clear evidence of any drying in the late classic coincident with the maya collapse ca ad 900 our results indicate that the continuity of occupation at the maya site of lamanai on the shores of the new river lagoon is consistent with continued access to freshwater during periods of regional climatic variability the importance of conditions at individual sites when considering both system response and human vulnerability to climate change is emphasised copyright 2009 john wiley sons ltd

relation between electrical resistivity and earthquake generation in the crust of west anatolia turkey abstract in this paper we present a relation between the earthquake occurrence and electric resistivity structures in the crust in west anatolia and the thrace region of turkey the relationship between magnetotelluric georesistivity models and crustal earthquakes in west anatolia during a period from 1900 to 2000 is investigated it is found that most of the large crustal earthquakes occurred in and around the areas of the highest electrical resistivity in the upper crust although rare small magnitude earthquakes are observed in some parts of the conductive lower crust in west anatolian extensional terrain the high resistivity zones may represent rocks that are probably mechanically strong enough to permit sufficient stress to accumulate for earthquakes to occur in western anatolia and the thrace region however some recent studies state that the generation of a large earthquake is not only a pure mechanical process but is closely related to fluid existence we also reviewed recent world wide researches including results from the anatolian data for the first time and discussed all general findings in combination our findings show that the boundary between the resistive upper crust and the conductive lower crust correlates well with the cutout depth of the seismicity in west anatolia and thrace this boundary is also attributed to the fluid bearing brittle ductile transition zone in world literature fluid migration from the conductive lower crust to the resistive upper crust may contribute the seismicity in resistive zones alternatively the upper lower crust boundary may act as a stress concentrator and fluids may help to release strain energy in brittle parts of lower crust by small magnitude earthquakes whereas they may help in focusing strain in mechanically strong and electrically resistive zones for large earthquakes to occur

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

millennial to century scale variability in gulf of mexico holocene climate records of the intertropical convergence zone itcz related to orbital forcing the d 18 o of the surface dwelling planktic foraminifer globigerinoides ruber show negative excursions between 14 and 10 2 ka radiocarbon years that reflect influx of meltwater into the western gom during melting of the laurentide ice sheet the relative abundance of the planktic foraminifer globigerinoides sacculifer is related to transport of caribbean water into the gom maximum transport of caribbean surface waters and moisture into the gom associated with a northward migration of the average position of the itcz occurs between about 6 5 and 4 5 ka in addition abundance variations of g sacculifer show century scale variability throughout most of the holocene the gom record is consistent with records from other areas suggesting that century scale variability is a pervasive feature of holocene climate the frequency of several cycles in the climate records is similar to cycles identified in proxy records of solar variability indicating that at least some of the century scale climate variability during the holocene is due to external solar forcing index terms 4267 oceanography general paleoceanography 9604 information related to geologic time cenozoic 3030 marine geology and geophysics micropaleontology

southward migration of the intertropical convergence zone through the holocene titanium and iron concentration data from the anoxic cariaco basin off the venezuelan coast can be used to infer variations in the hydrological cycle over northern south america during the past 14 000 years with subdecadal resolution following a dry younger dryas a period of increased precipitation and riverine discharge occurred during the holocene thermal maximum since approximately 5400 years ago a trend toward drier conditions is evident from the data with high amplitude fluctuations and precipitation minima during the time interval 3800 to 2800 years ago and during the little ice age these regional changes in precipitation are best explained by shifts in the mean latitude of the atlantic intertropical convergence zone itcz potentially driven by pacific based climate variability the cariaco basin record exhibits strong correlations with climate records from distant regions including the high latitude northern hemisphere providing evidence for global teleconnections among regional climates

multistage 8 2 kyr event revealed through high resolution xrf core scanning of cuban sinkhole sediments we use sediments from a flooded sinkhole cenote jennifer in northern cuba to provide new well dated high resolution evidence for the 8 2 kyr event from 7600 to 8700 cal yr b p the sinkhole contained shallow low salinity water which supported a marsh dominated by cattail and grass peaks in cl and br occurring at 8150 8200 and 8250 cal yr b p are attributable to increased evaporation due to regional drying associated with the 8 2 kyr event the three peaks in these elements also closely correspond to the greyscale record from the cariaco basin indicative of increased upwelling in the southern caribbean sea at this time supporting the notion of a multistage 8 2 kyr event our work provides new data that help to clarify the initiation behavior and impacts of the 8 2 kyr event in the northern tropics

tropical response to the 8200 yr b p cold event speleothem isotopes indicate a weakened early holocene monsoon in costa rica a 1 8 o monsoon rainfall proxy record from a u th dated costa rican stalagmite 8840 4920 yr b p documents an early holocene dry period correlative with the high latitude 8200 yr b p cold event high 1 8 o values between ca 8300 and 8000 yr b p demonstrate reduced rainfall and a weaker monsoon in central america a relatively wetter and more stable monsoon was established ca 7600 yr b p the early holocene dry event suggests a tropical extratropical teleconnection to the 8200 yr b p cold event and a possible association of isthmian rainfall anomalies with high latitude climate changes the likely source of such a tropical anomaly is a decrease in atlantic thermohaline circulation and atmospheric perturbations associated with drainage of proglacial lakes and freshwater discharge into the north atlantic a weaker monsoon at 8200 yr b p may be linked to wetland contraction and a decrease in methane observed in greenland ice cores

freshwater forcing of abrupt climate change during the last glaciation large millennial scale fluctuations of the southern margin of the north american laurentide ice sheet occurred during the last deglaciation when the margin was located between about 43 and 49 n fluctuations of the ice margin triggered episodic increases in the flux of freshwater to the north atlantic by rerouting continental runoff from the mississippi river drainage to the hudson or st lawrence rivers we found that periods of increased freshwater flow to the north atlantic occurred at the same time as reductions in the formation of north atlantic deep water thus providing a mechanism for observed climate variability that may be generally characteristic of times of intermediate global ice volume

triggering basaltic volcanic eruptions by bubble melt separation understanding the processes by which volcanic eruptions are triggered is crucial for volcanic hazard prediction and assessment in intermediate and silicic magmas where water is the dominant volatile phase1 2 second boiling provides an effective eruption trigger1 as the magma cools and crystallizes volatiles are increasingly concentrated in the residual melt where they eventually become saturated and are exsolved thereby raising the magma pressure to the point of eruption1 2 in contrast in basaltic magma chambers in which carbon dioxide is the dominant volatile phase the effect of second boiling is to decrease the chamber pressure but basaltic magmas are also relatively inviscid so volatile bubbles can separate from the turbulently convecting melt to produce a foam layer at the chamber roof3 4 decompression of the rising bubbles can increase the chamber pressure and so trigger an eruption5 6 here we model the complex competition between the processes of magma cooling and bubble ascent we find that in basalt of very low viscosity 1 10pas bubble separation dominates and can increase the reservoir pressure by as much as 10 mpa sufficient to trigger an eruption7 on timescales of 1 10 years comparable to the duration of eruptive episodes at kilauea volcano hawaii4 8

the midsummer drought over mexico and central america abstract the annual cycle of precipitation over the southern part of mexico and central america exhibits a bimodal distribution with maxima during june and september october and a relative minimum during july and august known as the midsummer drought msd the msd is not associated with the meridional migration of the intertropical convergence zone itcz and its double crossing over central america but rather with fluctuations in the intensity and location of the eastern pacific itcz during the transition from intense to weak weak to intense convective activity the trade winds over the caribbean strengthen weaken such acceleration in the trade winds is part of the dynamic response of the low level atmosphere to the magnitude of the convective forcing in the itcz the intensification of the trade winds during july and august and the orographic forcing of the mountains over most of central america result in maximum precipitation along the caribbean coast and minimum precipitation along the pacific

a 400 ka tephrochronological framework for central america from lake peten itza guatemala sediments highlights dating 400 ka paleoclimate record of neotropics revision and new eruptive volumes for large central american eruptions age models for peten itza sediments linking lacustrine ash inventory to eruptions from central america and mexico abstract lake peten itza northern guatemala lies within a hydrologically closed basin in the south central area of the yucatan peninsula and was drilled under the auspices of the international continental scientific drilling program icdp in 2006 at 16 55 n latitude the lake is ideally located for study of past climate and environmental conditions in the neotropical lowlands because of its great depth 160 m lake peten itza has a record of continuous sediment accumulation that extends well into the late pleistocene a key obstacle to obtaining long climate records from the region is the difficulty of establishing a robust chronology beyond 40 ka the limit of 14c dating tephra layers within the lake peten itza sediments however enable development of age depth relations beyond 40 ka ash beds from large magnitude pleistocene to holocene silicic eruptions of caldera volcanoes along the central american volcanic arc cava were found throughout drill cores collected from lake peten itza these ash beds were used to establish a robust chronology extending back 400 ka we used major and trace element glass composition to establish 12 well constrained correlations between the lacustrine tephra layers in lake peten itza sediments and dated deposits at the cava source volcanoes and with their marine equivalents in eastern pacific ocean sediments the data also enabled revision of eight previous determinations of erupted volumes and masses and initial estimates for another four eruptions as well as the designation of source areas for 14 previously unknown eruptions the new and revised sedimentation rates for the older sediment successions identify the interglacial of mis5a between 84 and 72 ka followed by a stadial between 72 and 59 ka that corresponds to mis4 we modified the age models for the lake peten itza sediment sequences extended the paleoclimate and paleoecological record for this neotropical region to 400 ka and determined the magnitude and timing of cava eruptions

synchroneity of tropical and high latitude atlantic temperatures over the last glacial termination a high resolution western tropical atlantic sea surface temperature sst record from the cariaco basin on the northern venezuelan shelf based on mg ca values in surface dwelling planktonic foraminifera reveals that changes in sst over the last glacial termination are synchronous within 30 to 90 years with the greenland ice sheet project 2 air temperature proxy record and atmospheric methane record the most prominent deglacial event in the cariaco record occurred during the younger dryas time interval when ssts dropped by 3 to 4 c a rapid southward shift in the atmospheric intertropical convergence zone could account for the synchroneity of tropical temperature atmospheric methane and high latitude changes during the younger dryas