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

energy budgets of atlantic hurricanes and changes from 1970 1 on the basis of the current observational record of tropical cyclones and sea surface temperatures ssts in the atlantic estimates are made of changes in surface sensible and latent heat fluxes and hurricane precipitation from 1970 to 2006 the best track data set of observed tropical cyclones is used to estimate the frequency that storms of a given strength occur after 1970 empirical expressions for the surface fluxes and precipitation are based on simulations of hurricane katrina in august 2005 with the advanced weather and research forecasting wrf model at 4 km resolution without parameterized convection the empirical relationships are computed for the surface fluxes and precipitation within 400 km of the eye of the storm for all categories of hurricanes based upon the maximum simulated wind and the observed sea surface temperature and saturation specific humidity strong trends are not linear but are better depicted as a step function increase from 1994 to 1995 and large variability reflects changes in ssts and precipitable water modulated by el nino events the environmental variables of sst and water vapor are nonetheless accompanied by clear changes in tropical cyclone activity using several metrics

a 4000 year lacustrine record of environmental change in the southern maya lowlands peten guatemala a 4000 yr sediment core record from lake salpeten guatemala provides evidence for maya induced forest clearance and consequent soil erosion between 1700 cal yr b c and 850 cal yr a d radiocarbon ages of wood seeds and charcoal support an age depth model with average errors of 110 cal yr relatively low carbonate 18o values between 1300 and 400 cal yr b c coincide with pollen evidence for forest loss consistent with increased surface and groundwater flow to the lake minimum 18o values between 400 cal yr b c and 150 cal yr a d suggest a high lake level as do 14c dated aquatic gastropods as much as 7 5 m above the present lake stage high lake levels resulted from reduced evaporation to precipitation ratios increased hydrologic input caused by anthropogenic deforestation or both the preclassic abandonment 150 a d and early classic late classic boundary 550 a d are marked by relatively high 18o values indicating reduced lake levels oxygen isotope composition increased further coincident with the terminal classic maya demographic decline between 800 and 900 a d this period of high 18o may have been caused by the greater aridity that has been documented in northern yucatan lakes or by decreased hydrologic input to the lake as a consequence of forest recovery reduced soil erosion after 850 cal yr a d coincided with the terminal classic maya demographic decline and permitted forest recovery and resumption of organic sedimentation

stalagmite evidence from belize indicating significant droughts at the time of preclassic abandonment the maya hiatus and the classic maya collapse abstract paleoenvironmental data from a stalagmite from western belize provide a 3300 year record of droughts that impacted the maya civilization at least four times across a span of 1500 years and the most sustained period of drought coincided with the collapse of classic maya civilization the stalagmite which comes from macal chasm in the vaca plateau provides reliably dated reflectance color luminescence and c and o stable isotope records for the period from 1225 b c to the present the record thus encompasses the maya preclassic classic and postclassic periods the maya civilization peaked in population density and socioeconomic complexity during the classic period extending from a d 25 to 900 but it declined abruptly over the years from a d 750 and 900 the stalagmite record indicates that a series of droughts which collectively form the most prolonged dry interval in the 3300 year record lasted from a d 700 to 1135 and thus coincided with the collapse of the maya civilization in addition two earlier droughts evident in the stalagmite record coincided with the preclassic abandonment and the maya hiatus two earlier declines in maya civilization a drought in the mid 1400s recorded in post classic documents is also evident in the stalagmite record collectively these findings illustrate the dependence of mayan civilization on water supplies and the impact of declining water resources on a vibrant civilization

low stable isotope ratios of tropical cyclone rains tropical cyclone rains have distinctly lower stable isotope ratios than rains in other tropical and summer precipitation systems with a mean value slightly above that of water vapor near the sea surface the isotope ratios also decrease radially inward to the eye wall but appear to be anomalously low even near the periphery of the rain shield these findings indicate that tropical cyclones are highly efficient precipitation systems and suggest the use of stable isotope ratios as dynamic tracers of a tropical cyclone s water and energy budgets

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

corals record persistent multidecadal sst variability in the atlantic warm pool since 1775 ad 1 accurate low latitude sea surface temperature sst records that predate the instrumental era are needed to put recent warming in the context of natural climate variability and to evaluate the persistence of lower frequency climate variability prior to the instrumental era and the possible influence of anthropogenic climate change on this variability here we present a 235 year long sst reconstruction based on annual growth rates linear extension of three colonies of the atlantic coralsiderastrea sidereasampled at two sites on the northeastern yucatan peninsula mexico located within the atlantic warm pool awp awp ssts vary in concert the atlantic multidecadal oscillation amo a basin wide quasiperiodic 60 80 years oscillation of north atlantic ssts we demonstrate that the annual linear growth rates of all three coral colonies are significantly inversely correlated with sst we calibrate annual linear growth rates to sst between 1900 and 1960 ad the linear correlation coefficient over the calibration period is r 0 77 and 0 66 over the instrumental record 1860 2008 ad we apply our calibration to annual linear growth rates to extend the sst record to 1775 ad and show that multidecadal sst variability has been a persistent feature of the awp and likely of the north atlantic over this time period our results imply that tropical atlantic ssts remained within 1 c of modern values during the past 225 years consistent with a previous reconstruction based on coral growth rates and with most estimates based on the mg ca of planktonic foraminifera from marine sediments

the sedimentary record of glacial retreat marinelli fjord patagonia regional correlations and climate ties abstract the inland passage of chile comprises the most continuous latitudinal transect of ice fields ice caps and alpine glaciers in the southern hemisphere providing an excellent natural laboratory for the study of glacial response to climate change during the holocene here we report results from a marine geological investigation of marinelli fjord tierra del fuego 54 25 s 69 35 w and ainsworth fjord which merges with marinelli fjord data acquired during july 2005 onboard the rv ib nathaniel b palmer include high resolution seismic data multibeam swath bathymetry data and sediment cores post cruise analyses include detailed seismic facies and lithofacies analysis and radiocarbon dating the results indicate that a major transition in the glacimarine record from ice proximal facies to ice distal facies occurred in the outer basins of the fjord at 15 5 kyr to 12 5 kyr the ice proximal facies consists of pebbly mud with abundant laminations and is characterized by high average sedimentation rates 28 mm yr in contrast an ice distal facies is dominantly bioturbated mud with low average sedimentation rates 0 56 mm yr this transition marks the retreat of marinelli glacier from the outer basins with no evidence for glacial re advance subsequently during the mid to late holocene ainsworth glacier retreated and exposed ainsworth fjord our results are consistent with the climatic trends for southern south america where the final deglaciation of the region ended at 12 5 ka during the holocene stable ice conditions persisted until the mid 1960s when marinelli glacier began rapidly retreating to its modern day position

high frequency holocene glacier fluctuations in new zealand differ from the northern signature understanding the timings of interhemispheric climate changes during the holocene along with their causes remains a major problem of climate science here we present a high resolution 10be chronology of glacier fluctuations in new zealand s southern alps over the past 7000 years including at least five events during the last millennium the extents of glacier advances decreased from the middle to the late holocene in contrast with the northern hemisphere pattern several glacier advances occurred in new zealand during classic northern warm periods these findings point to the importance of regional driving and or amplifying mechanisms we suggest that atmospheric circulation changes in the southwest pacific were one important factor in forcing high frequency holocene glacier fluctuations in new zealand

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

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 satellite derived climatology of the itcz abstract this paper presents fundamental climatological characteristics of the intertropical convergence zone itcz in a simple concise manner using the highly reflective cloud hrc dataset this satellite derived dataset uses both visible and infrared observations to measure the frequency of occurrence of large scale convective systems over the global tropics at a 1 spatial resolution these dataset characteristics make the hrc particularly well suited for climatological analysis of the itcz because the dataset is based on estimates of organized deep convective cloud systems rather than observations of clouds as a whole and it provides the spatial resolution needed to identify thew large wale convective structures furthermore the dataset covers a time period extending nearly two decades which provides for a fairly robust climatology and the opportunity to examine seasonal and interannual variability of both the convection and the latitude of the itcz

holocene glacier fluctuations on the northern flank of cordillera darwin southernmost south america abstract records of past variability afford context for evaluating present day glacier behavior and for testing hypotheses of climate change here we use 69 radiocarbon dates of wood and other organic materials in association with glacial deposits to document the behavior of ventisqueros marinelli and brooks in cordillera darwin over the past 17 000 years recession from the last glacial maximum was early with most occurring prior to 17 000 yr bp any glacial resurgence during the antarctic cold reversal must have terminated within the bounds of holocene ice fluctuations by early holocene time ventisquero marinelli had retreated such that it was no more extensive than it was in ad 1992 we identify several subsequent glacier readvances but also note long periods of restricted ice extent particularly in the mid holocene there were times when holocene glaciers must have been smaller than at present our new record from marinelli shows similarities to other reconstructions of holocene glacier variation from southern south america such as at mt sarmiento and the south patagonian icefield suggesting an underlying climate signal the overall implication is of substantial early holocene deglaciation followed by repeated advances interspersed with periods of recession when ice extent was smaller than at present this general pattern of glacier behavior appears to differ from that of new zealand s southern alps and points to the value of a geographic spread of datasets to elucidate the pattern of southern hemisphere climate during the holocene

the last glacial termination a major puzzle of paleoclimatology is why after a long interval of cooling climate each late quaternary ice age ended with a relatively short warming leg called a termination we here offer a comprehensive hypothesis of how earth emerged from the last global ice age a prerequisite was the growth of very large northern hemisphere ice sheets whose subsequent collapse created stadial conditions that disrupted global patterns of ocean and atmospheric circulation the southern hemisphere westerlies shifted poleward during each northern stadial producing pulses of ocean upwelling and warming that together accounted for much of the termination in the southern ocean and antarctica rising atmospheric co2 during southern upwelling pulses augmented warming during the last termination in both polar hemispheres

holocene sea level changes along the strait of magellan and beagle channel southernmost south america abstract radiocarbon dated marine sediments from five coastal sites along the strait of magellan and beagle channel in southernmost chile permit construction of a curve of relative sea level fluctuations during the holocene morphologic and stratigraphic data point to coastal submergence during the early holocene as the sea rose to a maximum level at least 3 5 m higher than present about 5000 yr ago progressive emergence then followed during the late holocene data from widely separated localities define a smooth curve the form of which is explainable in terms of isostatic and hydroisostatic deformation of the crust resulting from changing ice and water loads apparently anomalous data from one site located more than 100 km behind the outer limit of the last glaciation may reflect isostatic response to deglaciation the sea level curve resembles one derived by clark and bloom 1979 in proceedings of the 1978 international symposium on coastal evolution in the quaternary sao paulo brasil pp 41 60 sao paulo using a spherical earth model both in amplitude and in the timing of the maximum submergence

holocene glacier history of the lago argentino basin southern patagonian icefield abstract we present new geomorphic stratigraphic and chronologic data for holocene glacier fluctuations in the lago argentino basin on the eastern side of the southern patagonian andes chronologic control is based on 14c and surface exposure 10be dating after the lateglacial maximum at 13 000 cal yrs bp the large ice lobes that filled the eastern reaches of lago argentino retreated and separated into individual outlet glaciers this recession was interrupted only by a stillstand or minor readvance at 12 200 cal yrs bp the eight largest of these individual outlet glaciers are from north to south upsala agassiz onelli spegazzini mayo ameghino perito moreno and grande formerly frias holocene recession of upsala glacier exposed brazo cristina more than 10 115 100 cal yrs bp and reached inboard of the holocene moraines in agassiz este valley by 9205 85 cal yrs bp ice remained in an inboard position until 7730 50 cal yrs bp several subsequent glacier readvances are well documented for the upsala and frias glaciers the upsala glacier readvanced at least seven times the first being a relatively minor expansion documented only in stratigraphic sections between 7730 50 and 7210 45 cal yrs bp the most extensive holocene advances of upsala glacier resulted in the deposition of the pearson 1 moraines and related landforms which are divided into three systems the pearson 1a advance occurred about 6000 5000 cal yrs bp and was followed by the slightly less extensive pearson 1b and 1c advances dated to 2500 2000 and 1500 1100 cal yrs bp respectively subsequent advances of upsala glacier resulted in deposition of the pearson 2 moraines and corresponding landforms also separated into three systems pearson 2a 2b and 2c constructed respectively at 700 400 and

hells bells unique speleothems from the yucatan peninsula mexico generated under highly specific subaquatic conditions abstract we here report on a type of meter sized pendant speleothem growing under water in the submerged el zapote sinkhole cenote west of puerto morelos on the mexican yucatan peninsula these conical mantle shaped downward expanding and diverging calcareous structures here termed as hells bells are yet unreported in the scientific literature they are characterized by bell or trumpet shaped longitudinal and circular elliptical or horse shoe like horizontal cross sections hells bells grow downward based on the downward divergence of the structures and the horizontally laminated internal texture of both blade shaped spar calcite and microspar laminae age dating confirms that hells bells are young

atlantic warm pool acting as a link between atlantic multidecadal oscillation and atlantic tropical cyclone activity 1 multidecadal variability of atlantic tropical cyclone activity is observed to relate to the atlantic multidecadal oscillation amo a mode manifesting primarily in sea surface temperature sst in the high latitudes of the north atlantic in the low latitudes of the north atlantic a large body of warm water called the atlantic warm pool awp comprises the gulf of mexico the caribbean sea and the western tropical north atlantic awp variability occurs on both interannual and multidecadal timescales as well as with a secular variation the awp multidecadal variability coincides with the signal of the amo that is the warm cool phases of the amo are characterized by repeated large small awps since the climate response to the north atlantic sst anomalies is primarily forced at the low latitudes and the awp is in the path of or a birthplace for atlantic tropical cyclones the influence of the amo on atlantic tropical cyclone activity may operate through the mechanism of the awp induced atmospheric changes the awp induced changes related to tropical cyclones that we emphasize here include a dynamical parameter of tropospheric vertical wind shear and a thermodynamical parameter of convective instability more specifically an anomalously large small awp reduces enhances the vertical wind shear in the hurricane main development region and increases decreases the moist static instability of the troposphere both of which favor disfavor atlantic tropical cyclone activity this is the most plausible way in which the amo relationship with atlantic tropical cyclones can be understood

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

holocene glacier culminations in the western alps and their hemispheric relevance the natural variability of holocene climate defi nes the baseline to assess ongoing climate change greenland ice core records indicate warming superimposed by abrupt climate oscillations in the early holocene followed by a general cooling trend throughout the middle and late holocene that culminated during the little ice age lia tropical precipitation changes correlate with these patterns throughout the holocene here we use mountain glaciers in the european alps to reconstruct the regional holocene climate evolution and to test for a link between mid latitude north atlantic and tropical climate our precise 10 be chronology from tsidjiore nouve glacier western swiss alps indicates a glacier culmination during the earliest holocene 11 4 k y ago likely related to the preboreal oscillation based on our data no holocene glacier advance of similar amplitude occurred until 3 8 k y ago when the glacier reached lia limits the 10 be ages between 500 and 170 yr correspond to the lia while the youngest 10 be ages overlap with the historically recorded post lia glacier positions integrating our data with existing records we propose a hemispheric climate link between the alps north atlantic temperature and tropical precipitation patterns for the holocene supporting the concept of a pervasive climate driver these fi ndings from northern mid latitudes are consistent with the hypothesis formulated for the tropics that the earth s thermal equator responding to north atlantic temperature changes might have migrated southward throughout the holocene reaching the southern turning point toward the end of the lia

seismic stratigraphy of the blue hole lighthouse reef belize a late holocene climate and storm archive abstract five seismic units may be identified in the 8 m thick holocene sediment package at the bottom of the blue hole a 120 m deep sinkhole located in the atoll lagoon of lighthouse reef belize these units may be correlated with the succession of an existing 5 85 m long sediment core that reaches back to 1 385 kyrs bp the identification of seismic units is based on the fact that uniform fine grained background sediments show weak reflections while alternating background and coarser grained event storm beds exhibit strong reflections in the seismic profiles the main source of sediments is the marginal atoll reef and adjacent lagoon area to the east and north northeasterly winds and storms transport sediment into the blue hole as seen in the eastward increase in sediment thickness i e the eastward shallowing of the blue hole previous assumptions of much thicker holocene sediment packages in the blue hole could not be confirmed so far close to 6 m long cores were retrieved from the blue hole but the base of the sedimentary succession remains to be recovered the nature of the basal sediments is unknown but mid holocene and possibly older pleistocene sinkhole deposits can be expected the number of event beds identified in the blue hole n 37 during a 1 385 kyr long period and the number of cyclones listed in historical databases suggest that only strong hurricanes categories 4 and 5 left event beds in the blue hole sedimentary succession storm beds are numerous during 1 3 0 9 kyrs bp and 0 8 0 5 kyrs bp

two millennia of tropical cyclone induced mud layers in a northern yucatan stalagmite multiple overlapping climatic hazards during the maya terminal classic megadroughts an annually laminated stalagmite from the northern yucatan peninsula contains mud layers from 256 cave flooding events over 2240 years this new conservative proxy for paleotempestology recorded cave flooding events with a recurrence interval of 8 3 years during the twentieth century with the greatest frequency during the twentieth century and the least frequent during the seventeenth century tropical cyclone tc events are unlikely to flood the cave during drought when the water table is depressed applying tc masking to the chaac paleorainfall reconstruction suggests that the severity of the maya megadroughts was underestimated without a high resolution radiometric geochronology of individual local tc events speleothem isotope records cannot resolve whether the terminal classic period in the northern maya lowlands was punctuated by several brief drought breaks with normal tcs or whether the region was very dry and peppered by unusually severe and frequent hurricane seasons

synchronous acceleration of ice loss and glacial erosion glaciar marinelli chilean tierra del fuego to contribute to the understanding of the influence of climate on glacial erosion and on orogenic processes we report contemporary glacial erosion rates from a calving glacier in the southern andes and elucidate the influence of ice dynamics on erosion using seismic profiles of sediments collected in a proglacial fjord and a documented history of retreat we determine the time varying sediment flux of glaciar marinelli as a measure of basin wide erosion rates and compare these rates with the annual ice budget reconstructed using ncep ncar reanalysis climate data from 1950 to 2005 the rate of erosion of the largest tidewater glacier in tierra del fuego averaged 39 16 mm a 1 during the latter half of the 20th century with an annual maximum approaching 130 mm a 1 following a decade of rapid retreat a strong correlation emerges between the variable rate of ice delivery to the terminus and the erosion rate providing quantitative insight into the relationship between ice fluxes and glacial erosion rates for glaciar marinelli as for other calving glaciers for which suitable data exist the marked retreat and thinning over the past 50 years have resulted in a period of accelerated basal sliding and unusually rapid erosion

an extreme wave event in eastern yucatan mexico evidence of a palaeotsunami event during the mayan times the yucatan peninsula mexico has typically been considered a tectonically stable region without significant seismic activity in contrast the region is one that is regularly affected by hurricanes a detailed survey of ca 100 km of the eastern yucatan and cozumel coast identified the presence of ridges containing boulders measuring 1 m with up to three ridges at some locations the ridges reach five metres in height and their origin is associated with extreme wave event ewe activity previously modelled tsunami waves from known seismically active zones in the region muertos thrust belt and south caribbean deformed belt are not of sufficient at the yucatan peninsula to have produced the boulder ridges recorded in this study the occurrence of hurricanes in this region is more common but two of the most destructive hurricane gilbert 1988 and hurricane wilma 2005 produced coastal waves too small to have created the ridges recorded here in this paper a new tsunami model has been generated that indicates a tsunami event may have caused the ewe that resulted in the deposition of the boulders ridges

an 85 ka record of climate change in lowland central america drill cores obtained from lake peten itza peten guatemala contain a 85 kyr record of terrestrial climate from lowland central america that was used to reconstruct hydrologic changes in the northern neotropics during the last glaciation sediments are composed of alternating clay and gypsum reflecting relatively wet and dry climate conditions respectively from 85 to 48 ka sediments were dominated by carbonate clay indicating moist conditions during marine isotope stages mis 5a 4 and early 3 the first gypsum layer was deposited at 48 ka signifying a shift toward drier hydrologic conditions and the onset of wet dry oscillations during the latter part of mis 3 peten climate varied between wetter conditions during interstadials and drier states during stadials the pattern of clay gypsum wet dry oscillations during the latter part of mis 3 48 23 ka closely resembles the temperature records from greenland ice cores and north atlantic marine sediment cores and precipitation proxies from the cariaco basin the most arid periods coincided with heinrich events when cold sea surface temperatures prevailed in the north atlantic meridional overturning circulation was reduced and the intertropical convergence zone itcz was displaced southward a thick clay unit was deposited from 23 to 18 ka suggesting deposition in a deep lake and pollen accumulated during the same period indicates vegetation consisted of a temperate pine oak forest this finding contradicts previous inferences that climate was arid during the last glacial maximum lgm chronozone 21 2 ka at 18 ka peten climate switched from moist to arid conditions and remained dry from 18 to 14 7 ka during the early deglaciation moister conditions prevailed during the warmer bolling allerod 14 7 12 8 ka with the exception of a brief return to dry conditions at 13 8 ka that coincides with the older dryas and meltwater pulse 1a the onset of the younger dryas at 12 8 ka marked the return of gypsum and hence dry conditions the lake continued to precipitate gypsum until 10 3 ka when rainfall increased markedly in the early holocene

internal bores and breaking internal tides on the oregon continental slope abstractobservations of breaking internal tides on the oregon continental slope during a 40 day deployment of 5 moorings along 43 12 n are presented remotely generated internal tides shoal onto the slope steepen break and form turbulent bores that propagate upslope independently of the internal tide a high resolution snapshot of a single bore is captured from lowered acoustic doppler current profilers ladcp ctd profiles in a 25 h time series at 1200 m the bore is cold salty over 100 m tall and has a turbulent head where instantaneous dissipation rates are enhanced e 10 6 w kg 1 and sediment is resuspended at the two deepest slope moorings 1452 and 1780 m similar borelike phenomena are observed in near bottom high resolution temperature time series mean dissipation rates and diapycnal diffusivities increase by a factor of 2 when bores are present w kg 1 and m s 1 and observed internal tides are energetic enough to drive these enhanced dissipation rates globally the authors estimate

the alps with little ice evidence for eight holocene phases of reduced glacier extent in the central swiss alps glacially deformed pieces of wood organic lake sediments and clasts of reworked peat have been collected in front of alpine glaciers since ad 1990 the palaeoglaciological interpretation of these organic materials is related to earlier phases of glacier recession surpassing that of today s shrunken glaciers and to tree growth and peat accumulation in the valleys now occupied by the glaciers glacial transport of the material is indicated by wood anatomy incorporated silt sand and gravel particles missing bark and deformed treerings a total of 65 samples have been radiocarbon dated so far and clusters of dates provide evidence of eight phases of glacier recession 9910 9550 9010 7980 7250 6500 6170 5950 5290 3870 3640 3360 2740 2620 and 1530 1170 calibrated years bp allowing for the timelag between climatic fluctuations glacier response and vegetation colonization these recession phases may lag behind climatic changes by 100 200 years