middle miocene ocean climate transition high resolution oxygen and carbon isotopic records from deep sea drilling project site 588a southwest pacific high resolution stable isotopic records are presented for the epi benthic foraminifer cibicidoides the inferred shallow dwelling planktonic globigerinoides quadrilobatus and the inferred deep dwelling planktonic globoquadrina dehiscens from the middle miocene 16 12 ma of deep sea drilling project site 588a lord howe rise southwest pacific high resolution multiple species oxygen and carbon isotopic data define the timing and character of the well known middle miocene climatic oceanographic transition with a resolution comparable to quaternary records the benthic foraminiferal 18o record is marked by several large fluctuations from 16 to 14 8 ma followed by a series of rapid 50 kyr 18o increases that suggest a new state of the ocean climate system after 14 8 ma the total middle miocene benthic oxygen isotopic increase of 1 2 is largely incorporated in two steps an increase of 0 8 from 14 5 to 14 0 ma and a second increase of 0 7 from 13 45 to 12 45 ma each step is comprised of a series of marked 18o increases indicative of rapid east antarctic ice sheet growth and contemporaneous deepwater cooling a strong covariance of 0 7 between the benthic and deep dwelling planktonic species from 14 5 to 14 0 ma including a rapid increase from 14 1 to 14 05 ma suggests a 0 7 increase in the 18o composition of seawater 18osw because of east antarctic ice sheet growth comparison of the 18o record of gs quadrilobatus suggests that surface waters warmed at this site by 3 c from 14 1 to 13 6 ma carbon isotopic time series for each species generally covary throughout the early to middle miocene interval 16 12 ma confirming that 13c variations in this interval largely represent reservoir changes high resolution 13c data allow improved resolution of the latter five of six 13c maxima within the well known early to middle miocene carbon isotopic excursion the monterey carbon isotopic excursion from 17 0 to 13 5 ma this is useful for global correlation the last of these maxima ends with a 1 decrease centered from 13 9 to 13 7 ma 300 kyr after the 18o increase considered to reflect east antarctic ice growth covariance between benthic 18o and 13c from 16 to 13 8 ma suggests a sensitive relation between global carbon cycling and the ocean climate system prior to 13 8 ma episodic increases in organic carbon burial may have contributed to deep sea benthic 13c maxima and synchronous global cooling the positive relationship ended at 13 8 ma indicative of changing relations between global carbon cycling and the ocean climate system brought on by the increased stability of the east antarctic ice sheet after a major growth phase from 14 5 to 14 0 ma

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

measuring the sea ice floe size distribution sea ice is broken into floes whose diameters range from meters to a hundred kilometers this fragmentation affects the resistance of the ice cover to deformation and the melting at floe sidewalls in summer floes are broken by waves and swell near the ice edge and throughout the pack by isostatic imbalances thermal cracking winds and currents in winter they are welded together by freezing floe size can be measured by several properties p for instance area or mean caliper diameter two definitions of floe size distribution seem useful f p the fraction of area covered by floes no smaller than p and n p the number of floes per unit area no smaller than p f p can be measured by sets other than areas such as the fraction of a line or of a point set covered by floes no smaller than p if n behaves like for small where is mean caliper diameter must be greater than 2 so that the small floes occupy finite area if 2 1 the perimeter of small floes is infinite several summertime distributions have been measured on a log log graph their slopes local values of range from 1 7 to 2 5 one distribution follows a power law the others have steeper slopes for larger floes and more gradual slopes for smaller floes another sampling strategy is to measure the lengths of line segments on floes the distribution of these chord lengths is equivalent to the distribution of floe diameters the variance of an estimate of the fraction g of area covered by floes in any size range is g 1 g k 1 where k is the equivalent number of independent samples k can be found from the autocovariance of the indicator function for the chosen size range for line sampling of a narrow range of floe diameters k is the ratio of the sample length to the floe diameter

middle miocene paleoceanography of the western equatorial pacific dsdp site 289 and the evolution ofgloborotalia fohsella abstract evolution of the planktic foraminiferal lineagegloborotalia fohsella occurred during the miocene between 23 7 and 11 8 ma and forms the basis for stratigraphic subdivision of the early middle miocene zones n10 through n12 important morphologic changes within theg fohsella lineage included a marked increase in test size a transition from a rounded to an acute periphery and the development of a keel in later forms we found that the most rapid changes in morphology ofg fohsella occurred between 13 and 12 7 ma and coincided with an abrupt increase in the 18o ratios of shell calcite comparison of isotopic results ofg fohsella with other planktic foraminifers indicate that 18o values of the lineage diverge from surface dwelling species and approach deep dwelling species after 13 0 ma indicating a change in depth habitat from the surface mixed layer to intermediate depth near the thermocline isotopic and faunal evidence suggests that this change in depth stratification was associated with an expansion of the thermocline in the western equatorial pacific after adapting to a deeper water habitat at 13 0 ma theg fohsella lineage became extinct abruptly at 11 8 ma during a period when isotopic and faunal evidence suggest a shoaling of the thermocline following the extinction ofg fohsella the ecologic niche of the lineage was filled by thegloborotalia menardella group which began as a deep water form and later evolved to an intermediate water habitat we suggest that the evolution ofg fohsella andg menardella were tightly linked to changes in the structure of the thermocline in the western equatorial pacific

probable errors in width distributions of sea ice leads measured along a transect estimating the distribution of actual widths measured perpendicular to the local lead orientation knowing the apparent width distribution measured along the transect and vice versa the distribution of errors defined as the difference between the actual and apparent lead width can be estimated from the two width distributions and all moments of this distribution can be determined the problem isustrated with landsat imagery and the procedure is applied to a submarine sonar trarmect results are determined for a range of geometries and indicate the importance of orientation information ff data sampled along a transect are to be used for the description of lead geometries while the application here is to sea ice leads the methodology can be applied to measurements of any linear feature error for much of the archived sonar data an important polar sea ice is an important factor in the complex question concerns whether or not the data can be used for interaction of ocean and atmosphere reduction in the lead geometry statistics and if so what is the magnitude extent and thickness of sea ice due to global warming and of the maximum error in this paper a probabilistic the consequent increase in the number of cracks in the ice determination of this error is described providing a hereafter leads is expected to further increase global starting point for the application of stochastic geometry temperatures this positive feedback is a result of theorems in the analysis of lead geometries errors in reduced albedo and the increase in heat transfer from the ocean to the atmosphere model estimates indicate that an increase of 4 in the area covered by leads during winter could produce a hemisphere wide warming of 1 degree kelvin ledley 1988 understanding lead forma tion processes as well as the geographical and temporal distribution of lead networks is therefore important to studies of global climate measurements of ice thickness and lead coverage are commonly made along transects using upward looking submarine sonar the footprint of the sonar beam used by submarines to observe ice conditions is of sufficiently high resolution often 3 m or less as opposed to 25 m or more for satellite sensors to make it a potentially useful instrument for gathering lead statistics in fact sonar data have shown that the largest number of lead widths are in the 10 20 m range lead width statistics derived from submarine sonar data have been reported in the literature e g mclaren 1989 wadhams 1981 wadhams and horne 1950 but when lead orientation and width statistics derived for other lead and keel features are

late quaternary palaeoecology and palaeoclimatology from pollen records of the savannas of the llanos orientales in colombia abstract pollen analysis of sediment cores from laguna angel 4 28 n 70 34 w 200 m elevation and laguna sardinas 4 58 n 69 28 w 80 m a s l in the savannas of the llanos orientales of colombia are presented the pollen record of laguna angel includes 7 radiocarbon dates and shows the holocene vegetation and climatic history since 10 030 yr b p the record of laguna sardinas includes 7 radiocarbon dates and shows the late glacial and holocene history since 11 570 yr b p both pollen records show that the present day savanna vegetation dominated by grassland poaceae with only few woody savanna taxa e g curatella bryrsonima was present since the late glacial indicating dry climatic conditions with a marked dry season shrubs and trees of forest and or gallery forest e g mauritia mauritiella moraceae urticaceae melastomataceae alchornea and myrtaceae are well represented during the recorded period the proportions between forest and savanna were subject to change in the past but also the composition of the forest and savanna ecosystems changed laguna sardinas shows a greater abundance during the late glacial holocene transition than today for alchornea especially from 10 680 to 10 070 yr b p for this interval synchronous to the el abra stadial younger dryas equivalent in colombia more humid climatic conditions have been inferred which are primarily the result of higher moisture levels under colder conditions during the early and middle holocene from 9730 to 5260 yr b p in laguna angel and from 9390 to 6390 yr b p in laguna sardinas the maximum expansion of grassland occurred indicating very dry climatic conditions the following period from 5260 to 3890 yr b p in laguna angel and from 6390 to 3680 yr b p in laguna sardinas shows a decrease of savanna herbs and an increase of forest and gallery forests reflecting a wetter period probably with a shorter annual dry season in both lake records the period after ca 3500 yr b p is incomplete but is characterized by a high representation of the palm genera mauritiella in laguna angel and mauritia in laguna sardinas this marked change has been interpreted as evidence of strong a anthropogenic influence on the vegetation but also wetter climatic conditions than during the late glacial and early and middle holocene periods

non pollen palynomorphs from surface sediments along an altitudinal transect of the venezuelan andes palynological studies including records of non pollen palynomorphs npp are uncommon in the neotropics in spite of their demonstrated usefulness in other regions modern analog studies to improve palaeoecological interpretations of npp are even more scarce here we report the npp assemblages recorded in modern surface samples from an altitudinal transect of the venezuelan andes ranging from about 2300 and 4600 m we compared the assemblages with the results of previous pollen analyses of the same samples the variables considered to explain npp patterns along the transect are altitude and the local habitat of the sampling site sample type a total of 65 npp taxa classified into algal and zoological remains and fungal spores have been found unidentified taxa 23 have been named with a code depicted and described for further reference fungal spores are well represented along the whole transect whereas algal and zoological remains are absent or very scarce in the lower and the uppermost ranges the altitudinal zonation of fungal spores matches with that of pollen and the corresponding vegetation belts suggesting a close relationship the known environmental requirements of some of the fungi identified allowed inferences on particular ecological features in agreement with previous palynological interpretations both elevation and sample type are needed to explain the observed differences in the assemblages of the three groups the elevation being more decissive for fungal spores and the local habitat of the sampling site for algal and zoological remains the sample type effect is minimised when the npp studied are considered altogether thus increasing their usefulness as palaeoecological proxies this study reinforces the utility of modern analog surveys of npp with palaeoecological purposes and encourages further research particularly in poorly known areas as for example tropical regions

amazonian paleoecological histories one hill three watersheds abstract data from the hill of six lakes in the northwestern brazilian amazon region provide three records of paleoclimatic and vegetation change in lowland amazonia that span the last 170 000 years three lakes verde pata and dragao which occupy separate watersheds on the hill provide the most detailed image yet obtained of ice age conditions in lowland amazonia well dated sedimentary records for fossil palynological charcoal cation and pigment data are presented the data indicate the continuous presence of mesic forest throughout the last ice age oscillations of lake level are recorded and the lowstands are attributed to reduced precipitation inputs to systems delicately balanced between water loss leakage through the floor of the basin and gain precipitation gross stratigraphy algal remains and paleochemistry suggest that the stands of high and low lake level were cyclic apparently correlating precessional orbital variation times of lake lowstand coincide with wet season december january february djf insolation minima the strongest of eight lowstand cycles occurred ca 35 000 to 27 000 cal bp even during lowstand episodes pollen is well preserved and provides a clear signal of uninterrupted forest cover the principal lowland elements are continuously present in the record suggesting the long term quaternary availability of the lowland forest biome in this region however during the late pleistocene the forest changed in composition with the expansion or invasion of montane floral elements creating communities of the mesic forest biome without modern analogs while precipitation cycles were causing lake levels to rise and fall the principal influence on vegetation appears to have been cooling in the late pleistocene the population expansion of montane elements at lower elevations is consistent with a cooling of 4 5 c

evolution of depth ecology in the planktic foraminifera lineage globorotalia fohsella after over 15 m y of evolution in near surface waters of the western pacific and caribbean planktic foraminifera in the lineage globorotalia fohsella abruptly invaded thermocline habitats at 13 ma stable isotope data demonstrate that this habitat shift took 50 ka to complete a duration similar to the fastest measured rates of speciation in marine protistans the ecological change occurred near the end of a long series of gradual morphological transitions and demonstrates that at least some of the chronospecies in this lineage had distinctive ecologies and were probably biological species other than this one shift in depth distribution skeletal evolution is largely disconnected from depth changes in globorotalia fohsella morphological trends of globorotaliid foraminifera may not record changes in depth ecology contrary to existing models for foraminiferal diversification

palynological signal of the younger dryas in the tropical venezuelan andes the occurrence or not of the younger dryas cold reversal in the tropical andes remains a controversial topic this study reports a clear signal for this event in the venezuelan andes employing high resolution palynological analysis of a well dated sediment core from laguna de los anteojos situated around 3900 m elevation within grass paramo vegetation the lake is surrounded by some polylepis forests which are close to their upper distribution limit the section of the core discussed here is 150 cm long and dated between about 14 68 and 9 35 cal kyr bp using a polynomial age depth model based on six ams radiocarbon dates between 12 86 and 11 65 cal kyr bp an abrupt shift occurred in the pollen assemblage manifested by a decline of podocarpus polylepis and huperzia combined with an increase in poaceae and asteraceae the aquatic pteridophyte isoetes also decreased and disappeard and the algae remains show their minimum values pollen assemblages from the younger dryas interval show maximum dissimilarity values compared with today s pollen assemblage and are more similar to modern analogs from superparamo vegetation growing at elevations 400 500 m higher a lowering of vegetation zones of this magnitude corresponds to a temperature decline of between 2 5 and 3 8 c during this colder interval lake levels may have been lower suggesting a decrease in available moisture the vegetation shift documented in anteojos record between 12 86 and 11 65 cal kyr bp is comparable to the el abra stadial in the colombian andes but it differs in magnitude the anteojos shift is better dated and coincides with the younger dryas chron as recorded in the cariaco basin sea surface temperature reconstructions and records of continental runoff as well as in the oxygen isotope measurements from the greenland ice cores when compared to other proxies of quasi immediate response to climate the time lag for the response of vegetation to climate is found to be negligible at a centennial scale the polylepis pollen curve is especially noteworthy as it reproduces the overall pollen trends and matches well with paleoclimatic reconstructions based on other proxies hence polylepis might be used as a reliable paleoclimatic indicator in lake sediments close to its uppermost distribution boundary

forest savanna morichal dynamics in relation to fire and human occupation in the southern gran sabana se venezuela during the last millennia the southern gran sabana se venezuela holds a particular type of neotropical savanna characterized by the local occurrence of morichales mauritia palm swamps in a climate apparently more suitable for rain forests we present a paleoecological analysis of the last millennia of lake chonita 4 39 n 61 0 w 884 m elevation based on biological and physico chemical proxies savannas dominated the region during the last millennia but a significant vegetation replacement occurred in recent times the site was covered by a treeless savanna with nearby rainforests from 3640 to 2180 cal yr bp water levels were higher than today until about 2800 cal yr bp forests retreated since about 2180 cal yr bp onwards likely influenced by a higher fire incidence that facilitated a dramatic expansion of morichales the simultaneous appearance of charcoal particles and mauritia pollen around 2000 cal yr bp supports the potential pyrophilous nature of this palm and the importance of fire for its recent expansion the whole picture suggests human settlements similar to today in which fire is an essential element since around 2000 yr ago therefore present day southern gran sabana landscapes seem to have been the result of the synergy between biogeographical climatic and anthropogenic factors mostly fire

early human occupation and land use changes near the boundary of the orinoco and the amazon basins se venezuela palynological evidence from el pauji record this paper shows a holocene paleoecological reconstruction based on a peat bog sequence el pauji 4 28 n 61 35 w 865 m elevation located in the transition zone between the gran sabana se venezuela savannas and the amazon rainforests paleoecological trends are based on the analysis of pollen and pteridophyte spores algal and animal remains fungal spores and charcoal particles the whole record embraces the last ca 8000 cal yr bp and was subdivided into five pollen zones representing the following vegetation succession savanna rainforest mosaic 8250 7715 yr bp dense rainforests 7715 5040 yr bp savanna rainforest mosaic 5040 2690 yr bp secondary dry forests 2690 1440 yr bp and peat bog in an open savanna landscape 1440 yr bp present these vegetation changes have been attributed to the action of climate and or land use changes as well as the corresponding synergies between them fire has been determinant in the landscape evolution based on the reconstructed fire and vegetation shifts a changing land use pattern could have been recognized between the early and the mid holocene ca 8 3 5 0 kyr bp land use practices seem to have been more linked to shifting agriculture in a rainforest landscape as is usual in amazon cultures with medium fire incidence affecting only local forest spots or surrounding savannas more extensive forest burning was recorded between ca 5 0 and 2 7 kyr bp followed by land abandonment and the dominance of drier climates between 2 7 and 1 4 yr bp the modern indigenous culture which prefers open environments and makes extensive use of fire thus preventing forest re expansion seem to have established during the last 1500 yr therefore a significant cultural replacement has been proposed for the region leading to the present day situation changing human activities have been instrumental for ecological evolution in this savanna rainforest transitional region as well as for the shaping of modern landscapes

holocene fires in the northern amazon basin abstract the ubiquitous occurrence of charcoal in the forest soils of the upper rio negro region of colombia and venezuela indicates the presence of frequent and widespread fires in the amazon basin possibly associated with extremely dry periods or human disturbances charcoal ranged from 3 12 to 24 76 mg cm 3 in the upper 50 cm of soil and was more abundant in oxisols and ultisols than in other soil types charcoal dates range from 6260 yr b p to the present several dates coincide with dry phases recorded during the holocene ceramic shards were found at several sites and thermoluminescence analysis indicates that their ages range from 3750 to 460 yr b p the age of charcoal and shards confirms that this region has been subjected to fire and human disturbances during the past 6000 yr

human settlement of central america and northernmost south america 14 000 8000 bp abstract a human presence in central america colombia and venezuela outside amazonia before the late glacial stage 14 000 10 000 bp requires substantiation in venezuela hunter foragers who used elongate thick bifacial projectile points preyed upon megamammals e g gomphotheres before the onset of a 12 000 11 000 bp warming stage particularly visible in highland colombia guantiva interstadial some archaeologists argue for a distinct pre 12 000 bp population centered on the sabana de bogota colombia 2500 m their abriense stone tool kit lacked bifacial reduction and continued to be used well into the holocene in central america no human cultural remains yet pre date the widespread but poorly documented palaeoindian horizon this coincides chronologically with the 11 000 10 000 bp cold stage equivalent to the younger dryas in colombia el abra stadial makers of lanceolate clovis and stemmed fish tail fluted points carefully trimmed scrapers burins and perforators paleoindians have also been identified in northwest south america palaeoenvironmental data suggest that they moved around in many different vegetation types in general these were unlike modern potential vegetation because temperatures and annual rainfall were still lower than those of today human material culture at the onset of the holocene 10 500 700 bp depending on region does not seem not to have changed as rapidly as climate and vegetation but the disappearance of fluted projectile points and the late pleistocene megafauna may be causally related presently it is difficult to distinguish widespread and short lived diachronic changes in human culture from culturally diverse populations living synchronously in different geographic areas some archaeologists argue that early holocene forest expansion and foraging territory contraction were responsible for socioeconomic diversification and a gradual shift from mobile hunting and gathering to a more sylvan way of life this emphasized the intensive collection and incipient domestication of forest products e g palms and root crops and the hunting of deer and rodents

the middle miocene climatic transition east antarctic ice sheet development deep ocean circulation and global carbon cycling abstract the middle miocene represents a major change in state in cenozoic climatic evolution following the climax of neogene warmth in the late early miocene at 16 ma the early stage of this climatic transition from 16 to 14 8 ma was marked by major short term variations in global climates east antarctic ice sheet eais volume sea level and deep ocean circulation in the later stage from 14 8 to 12 9 ma climatic developments included major growth of the eais and associated antarctic cooling a distinct increase in the meridional temperature gradient large fluctuations in sea level followed by a global sea level fall and important changes in deep water circulation including increased production of southern component water east antarctic ice sheet growth and polar cooling also had large effects on global carbon cycling and on the terrestrial biosphere including aridification of mid latitude continental regions increased stability of the eais after 14 8 ma represents a crucial step in the establishment of late neogene global climate systems what controlled these changes in polar climates and the east antarctic ice sheet deep ocean circulation changes probably played a major role in the evolution and variation in polar climates as they have throughout the cenozoic oxygen and carbon isotopic evidence for warm saline deep water production in the eastern tethyan northern indian ocean indicates that meridional heat transport to the antarctic inhibited cenozoic polar cooling and eais growth during the early middle miocene from 16 to 14 8 ma inferred competition between warm low latitude sources derived from the eastern tethyan northern indian ocean and a cold high latitude source southern component water from 16 to 14 8 ma may have been associated with instability in the antarctic climate and cryosphere reduction of warm saline deep water flow to the southern ocean at 14 8 ma may have decreased meridional heat transport to the antarctic cooling the region and leading to increased production of southern component water these middle miocene climatic and cryospheric changes in the antarctic had profound effects on marine and terrestrial climates as the meridional surface temperature gradient increased boundaries between climatic zones strengthened leading to increased aridification of mid latitude continental regions in australia africa and north and south america enhancing the development of grasslands and stimulating the evolution of grazing mammals

a 5 year satellite climatology of winter sea ice leads in the western arctic the distribution of openings leads and polynyas in polar sea ice is not well known this study estimates the large scale distribution and variability of leads in the arctic of the western hemisphere in winter using a 5 year record of defense meteorological satellite program thermal and visible band imagery the occurrence density and orientation of leads are derived from gridded maps made at 10 day intervals their mean value and interannual seasonal and monthly variabilities are estimated lead densities are observed to be highest in early winter decreasing 20 from november through april the highest densities are observed in the central canada basin and the lowest are in the east siberian sea there is limited interannual variability in the positions of maximum and minimum densities preferred lead orientations are identified as generally north south in the beaufort sea sector and east west in the east siberian sea sector with transitional orientations in the intermediate area the mean distributions of lead density and orientation are observed to be associated with large scale mean fields of ice divergence and shear respectively

review and revision of cenozoic tropical planktonic foraminiferal biostratigraphy and calibration to the geomagnetic polarity and astronomical time scale planktonic foraminifera are widely utilized for the biostratigraphy of cretaceous and cenozoic marine sediments and are a fundamental component of cenozoic chronostratigraphy the recent enhancements in deep sea drilling recovery multiple coring and high resolution sampling both offshore and onshore has improved the planktonic foraminiferal calibrations to magnetostratigraphy and or modified species ranges this accumulated new information has allowed many of the planktonic foraminiferal bioevents of the cenozoic to be revised and the planktonic foraminiferal calibrations to be reassessed we incorporate these developments and amendments into the existing biostratigraphic zonal scheme in this paper we present an amended low latitude tropical and subtropical cenozoic planktonic foraminiferal zonation we compile 187 revised calibrations of planktonic foraminiferal bioevents from multiple sources for the cenozoic and have incorporated these recalibrations into a revised cenozoic planktonic foraminiferal biochronology we review and synthesize these calibrations to both the geomagnetic polarity time scale gpts of the cenozoic and astronomical time scale ats of the neogene and late paleogene on the whole these recalibrations are consistent with the previous work however in some cases they have led to major adjustments to the duration of biochrons recalibrations of the early middle eocene first appearance datums of globigerinatheka kugleri hantkenina singanoae guembelitrioides nuttalli and turborotalia frontosa have resulted in large changes in the durations of biochrons e7 e8 and e9 we have introduced upper oligocene zone o7 utilizing the biostratigraphic utility of paragloborotalia pseudokugleri for the neogene period major revisions are applied to the fohsellid lineage of the middle miocene and we have modified the criteria for recognition of zones m7 m8 and m9 with additional adjustments regarding the globigerinatella lineage to zones m2 and m3 the revised and recalibrated datums provide a major advance in biochronologic resolution and a template for future progress of the cenozoic time scale

vegetation changes in the neotropical gran sabana venezuela around the younger dryas chron the occurrence of the younger dryas cold reversal in northern south america midlands and lowlands remains controversial we present a palaeoecological analysis of a late glacial lacustrine section from a midland lake lake chonita 4 6501 8n 61 0157 8w 884 m elevation located in the venezuelan gran sabana based on physical and biological proxies the sediments were mostly barren from 15 3 to 12 7 k cal a bp probably due to poor preservation a ligneous community with no clear modern analogues was dominant from 12 7 to 11 7 k cal a bp younger dryas chronozone at present similar shrublands are situated around 200 m elevation above the lake suggesting a cooling driven downward shift in vegetation during that period the interval from 11 7 to 10 6 k cal a bp is marked by a dramatic replacement of the shrubland by savannas and a conspicuous increase in fire incidence the intensification of local and regional fires at this interval could have played a role in the vegetation shift a change to wetter and probably warmer conditions is deduced after 11 7 k cal a bp coinciding with the early holocene warming these results support the hypothesis of a mixed origin climate and fire of the gran sabana savannas and highlight the climatic instability of the neotropics during the late glacial copyright 2011 john wiley sons ltd

estimating the area fraction of geophysical fields from measurements along a transect methods are presented for estimating the fractional area coverage of geophysical phenomena from measurements taken along a transect these methods are most useful for assessing potential errors in sampling strategies but may also be used for the analysis of data the procedure provides a means to compute confidence interval estimates of the true area fraction when the autocovariance function for the geophysical field is known or assumed another approach that does not require a priori knowledge of the underlying autocovariance function is described for the special case of linear features modeled as a poisson line process

two histories of environmental change and human disturbance in eastern lowland amazonia two long palaeoecological records pollen diatoms 14c chemistry and charcoal provide a detailed record of holocene environmental conditions in the lowlands of the lower amazon changes in the composition of the terra firme forest are evident but a forest cover is maintained throughout fluctuations in lake level and changes in seasonality are tracked in both sedimentary sequences rising early holocene sea levels and the consequent rise of local water tables probably caused the flooding of one of the sites a stepped increase in precipitation is suggested with increases evident at 8800 yr bp 7400 bp and between 6100 and 5800 yr bp increased storminess is suggested between 7400 yr bp and 6100 yr bp human disturbance is manifested in the palaeoecological records at one study site as early as 5500 yr bp c 6500 years ago and an intensifi cation or onset of agriculture is evident at c 3350 yr bp the terminating phase of the long record of disturbance is a period of forest regrowth that may correlate to the depopulation of amazonia following euro pean contact

clinal variation in aperture shape in southwest pacific globorotalia partimlabiata and its ancestor about the time of the middle miocene climatic transition abstract to clarify the taxonomy and biostratigraphy of southwest pacific populations currently identified as fohsella peripheroronda a morphometric study is reported on the shape of its aperture in collections from dsdp sites 588a and 593 and from the stratotype of the new zealand lillburnian stage this material is compared with globorotalia partimlabiata from serravallian strata in sicily shape analyses use 50 equally spaced coordinates along the margin of the apertural re entrant of the last preserved chamber data are aligned by the procrustes algorithm and projected onto principal component axes canonical discriminant analysis of these scores shows that convexity of the margin increases clinally the cline is interpreted as primary evidence for a previously unrecognized lineage for which a provisional taxonomy is advanced globorotalia partimlabiata which has individuals with strongly convex margins represents the climax of the cline the highest occurrence of its ancestor recognized here informally as globorotalia cf peripheroronda is determined by the highest sample whose mean shape does not differ significantly from its earliest representative at site 593 there and at site 588a that datum is close to or within miocene isotope event mi 3b the architectural history of the lineage differs from the tropical fohsellids and its relation to them is unresolved

geomorphology of the gran sabana guayana shield southeastern venezuela abstract the forms of the gran sabana have been developed mainly on rocks of the gently folded precambrian roraima group a sequence of quartzites and siliceous conglomerates with intercalated jasper chert arkose siltstone and shale locally intruded by diabase dikes and sills two planation surfaces are preserved on the gran sabana the auyan tepui 2000 2900 m elevation and the wonken 900 1200 m surfaces with a speculative mesozoic initial age they are still extending table mountains dominate the landscape they comprise pediments foothills precipitous scarps hundreds of meters high a rim and a flattish summit the table mountain topography is controlled by joint systems water percolates along joints and bedding planes dissolves the siliceous cement the quartzite disaggregates and large blocks are isolated and collapse accumulating at the foot of the scarps and on top of the table mountain summits the percolating water forms large and intricate cave systems which frequently emerge in the scarp zone as high waterfalls these mark the lowest level of erosion of the table mountains karstic structures are prominent in the summit zone the evolution of the table mountain topography has been controlled by a combination of tectonic structural stratigraphic climatic and temporal factors

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