radiocarbon dates on deglaciation cordillera central northern peruvian andes abstract three radiocarbon dates along with relative dating criteria place limits on the deglaciation history of manachaque valley cordillera central ice retreated from the late glacial maximum by at least 12 100 yr b p during ice retreat numerous moraines were deposited throughout the valley glacier cover was reduced to about half that of the last glacial maximum by at least 9700 yr b p and to less than a tenth by at least 6450 yr b p because all dates are minimum the dates and field data are consistent with little or no ice remaining by early holocene no unambiguous younger dryas moraines are present

material balance in alpine orogeny recent geophysical data are combined with older information for an updated picture of alpine kinematics using semiquantitative considerations of material balance in the alps a conventional pile of thin peel nappes is disrupted by a central longitudinal system of steep faults and steeply limbed folds both affecting disharmoniously the entire edifice of otherwise flat lying nappes it is particularly from late tertiary phases of deformation that structures of the second type the insubric system with the insubric line and the belt of late alpine windows acquired prominence for the present aspect of the alps these late structures formed as a dextrally transpressive intracontinental branch of the africa europa plate boundary mostly between areas of extension early motions in the late cretaceous to the late eocene were probably responsible for at least half of the 300 km dextral displacement required for the palinspastic restoration of the inner west carpathians and the dinarides during oligocene extension the late alpine batholiths intruded in a belt roughly along the insubric line while deep crustal and lithospheric roots were destroyed in the latest oligocene to middle miocene the adriatic plate moved dextrally to the west along the insubric transfer fault from its frontal insubric indenter a lower crust upper mantle flake probably first obducted in the cretaceous was detached and wedged into the penninic nappes of the western alps bird s head of the ivrea body both the northern and the southern transfer faults of the insubric indenter were disrupted and inactivated by late miocene to later events the giudicarie and neo north apennine events instead of the inactivated insubric fault the windows belt of en echelon folds assumed the role of dextral transpression all of these successive zones of motion are entangled in the ligurian knot largely hidden under the sea or young sediments a large part of the pre alpine crust and practically the whole mantle were subducted only in some cases were high pressure rocks of shallow continental origin re obducted the lithospheric root of the neo alps is largely contained in a vertical slab under the alps but some parts seem to have been removed and others disharmonically displaced with respect to the surface structures

geophysical geological transect and tectonic evolution of the swiss italian alps a complete alpine cross section integrates numerous seismic reflection and refraction profiles across and along strike with published and new field data the deepest parts of the profile are constrained by geophysical data only while structural features at intermediate levels are largely depicted according to the results of three dimensional models making use of seismic and field geological data the geometry of the highest structural levels is constrained by classical along strike projections of field data parallel to the pronounced easterly axial dip of all tectonic units because the transect is placed close to the western erosional margin of the austroalpine nappes of the eastern alps it contains all the major tectonic units of the alps a model for the tectonic evolution along the transect is proposed in the form of scaled and area balanced profile sketches shortening within the austroalpine nappes is testimony of a separate cretaceous age orogenic event west directed thrusting in these units is related to westward propagation of a thrust wedge resulting from continental collision along the meliata hallstatt ocean further to the east considerable amounts of oceanic and continental crustal material were subducted during tertiary orogeny which involved some 500 km of n s convergence between europe and apulia consequently only a very small percentage of this crustal material is preserved within the nappes depicted in the transect postcollisional shortening is characterized by the simultaneous activity of gently dipping north directed detachments and steeply inclined south directed detachments both detachments nucleating at the interface between lower and upper crust large scale wedging of the adriatic or apulian lower crust into a gap opening between the subduced european lower crust and the pile of thin upper crustal flakes alpine nappes indicates a relatively strong lower crust and detachment between upper and lower crust

glacier fluctuations in the southern peruvian andes during the late glacial period constrained with cosmogenic 3he the occurrence of pronounced climate reversals during the last glacial termination has long been recognised in palaeoclimate records from both hemispheres and from high to low latitudes accurate constraint of both the timing and magnitude of events such as the younger dryas and antarctic cold reversal is vital in order to test different hypotheses for the causes and propagation of abrupt climate change however in contrast to higher latitude regions well dated records from the tropics are rare and the structure of late glacial tropical climate remains uncertain as a step toward addressing this problem we present an in situ cosmogenic 3 he surface exposure chronology from nevado coropuna southern peru documenting a significant fluctuation of the ice margin during the late glacial period ten tightly clustered ages from a pair of moraines located halfway between the modern glacier and the last glacial maximum terminus range from 11 9 to 13 9ka and give an arithmetic mean age of 12 8 0 7ka 1s these data constitute direct evidence for a readvance or prolonged stillstand of glaciers in the arid andes of southwestern peru copyright 2011 john wiley sons ltd

15 000 yr pollen record of vegetation change in the high altitude tropical andes at laguna verde alta venezuela pollen analysis of sediments from a high altitude 4215 m neotropical 9 n andean lake was conducted in order to reconstruct local and regional vegetation dynamics since deglaciation although deglaciation commenced 15 500 cal yr b p the area around the laguna verde alta lva remained a periglacial desert practically unvegetated until about 11 000 cal yr b p at this time a lycopod assemblage bearing no modern analog colonized the superparamo although this community persisted until 6000 cal yr b p it began to decline somewhat earlier in synchrony with cooling following the holocene thermal maximum of the northern hemisphere at this time the pioneer assemblage was replaced by a low diversity superparamo community that became established 9000 cal yr b p this replacement coincides with regional declines in temperature and or available moisture modern more diverse superparamo assemblages were not established until 4600 cal yr b p and were accompanied by a dramatic decline in alnus probably the result of factors associated with climate humans or both pollen influx from upper andean forests is remarkably higher than expected during the late glacial and early to middle holocene especially between 14 000 and 12 600 cal yr b p when unparalleled high values are recorded we propose that intensification of upslope orographic winds transported lower elevation forest pollen to the superparamo causing the apparent increase in tree pollen at high altitude the association between increased forest pollen and summer insolation at this time suggests a causal link however further work is needed to clarify this relationship

spatial heterogeneities in a bioclastic shoal outcrop reservoir analogue architectural cyclical and diagenetic controls on lateral pore distribution abstract the distribution of pores in carbonate rocks is controlled by the synergy of different factors such as the initial sediment environment processes and diagenesis prediction of the texture related pore distribution to evaluate reservoir quality is still a challenging task in geothermal and hydrocarbon exploration in order to degrease these uncertainties 3d facies models are produced on the basis of facies types and their relationship to petrophysical properties however several studies have shown that these relationships are often rather controlled by diagenesis that is often described as a sequence for an entire reservoir two outcrops as part of the same bioclastic shoal reservoir analogue present two different architectural styles i layer cake architecture and ii high angle foreset architecture in order to identify petrophysical trends and related controlling factors both outcrops were mapped and studied in detail based on the investigation of 41 thin sections microfacies types were defined and diagenesis was analysed in addition permeability porosity was measured and pores were digitally characterized although both outcrop successions studied are associated with the same reservoir interval varied architecture leads to fundamental differences in the porosity permeability distribution petrophysical trends in the layer cake architecture are mainly controlled by cyclicity and channel features compared to layer cake architecture trends in the high angle foreset architecture are dominantly controlled by the depositional environment both outcrops show differences in their diagenesis while layer cake architecture suffered the entire sequence large parts of the high angle foreset architecture were not influenced by late burial and telogenetic processes as a consequence primary porosity is persevered within the high angle foreset architecture in addition results of this investigation indicate that primary porosity has a bigger influence on good permeability values compared to telogenetic associated vug enhancement this investigation therefore demonstrates that individual carbonate reservoirs can suffer different diagenetic pathways moreover the study highlights the potential of outcrop studies to improve the principal understanding of fundamental trends within potential subsurface reservoirs

late quaternary vegetation and climate of southern tierra del fuego vegetation and climate over approximately the past 13 000 yr are reconstructed from fossil pollen in a 9 4 m mire section at caleta robalo on beagle channel isla navarino 54 56 s 67 38 w southern tierra del fuego fifty surface samples reflecting modern pollen dispersal serve to interpret the record chronologically controlled by nine radiocarbon dates fossil pollen assemblages are empetrum gramineae gunnera tubuliflorae zone 3b 13 000 11 850 yr b p gramineae empetrum assorted minor taxa zone 3a 11 850 10 000 yr b p nothofagus gramineae tubuliflorae polypodiaceae zone 2 10 000 5000 yr b p nothofagus empetrum zone 1b 5000 3000 yr b p and empetrum nothofagus zone 1a 3000 0 yr b p assemblages show tundra under a cold dry climate zone 3 followed by open woodland zone 2 as conditions became warmer and less dry and later with greater humidity and lower temperatures by closed forest and the spread of mires zone 1 comparisons drawn with records from antarctica new zealand tasmania and the subantarctic islands demonstrate broadly uniform conditions in the circumpolar southern hemisphere the influences of continental and maritime antarctic air masses were apparently considerable in tierra del fuego during cold late glacial time whereas holocene climate was largely regulated by interplay between maritime polar and maritime tropical air

response of interior north america to abrupt climate oscillations in the north atlantic region during the last deglaciation several broad scale climatic oscillations during the last deglaciation are well documented in regions around the north atlantic ocean this paper reviews empirical evidence for these deglacial climatic oscillations from non coastal north america and discusses implications for testing climatic simulations and for understanding the cause and transmittal mechanisms paleoclimatic interpretation of oxygen isotope records from several small sites in the eastern great lakes region indicates a classic deglacial climatic sequence that is comparable with records from europe and greenland the climatic events as interpreted from crawford lake oxygen isotopes include the bolling allerod boa warming at 12 700 14c bp a warm boa at 12 500 10 920 14c bp an intra allerod cold period shortly before 11 000 14c bp a cold younger dryas yd climate reversal at 10 920 10 000 14c bp the holocene warming at 10 000 14c bp a brief preboreal oscillation pb at 9650 14c bp and an early holocene cold event at 7500 14c bp 8200 cal bp some of these events were also evident from changes in upland and aquatic vegetation and sediment lithology the pronounced yd climatic reversal has been documented from pollen records along the ecotones at this time and from glacier readvances in the great lakes region along the rocky mountains from alberta to colorado the yd event is indicated by alpine glacier advance and or shift in timberline vegetation in minnesota and upper new york early holocene climatic instability is also suggested by oxygen isotope records and or varve thickness the regional variations in evidence for the yd and other events in north america suggest that climatic oscillations may have different expressions in paleo records depending on geographic location and characteristics of a particular site the extent and magnitude of these climatic oscillations across north america suggest that these oscillations are an expression of climatic change that was probably widespread rather than locally induced by a nearby glacier the location of these sites implies that climatic signals were likely carried over the northern hemisphere through the atmosphere as indicated by general circulation models we hypothesize that the lack of evidence for a cold yd in interior north america west of the great lakes region and east of the rocky mountains was caused by the trapping of cold arctic air mass north of the laurentide ice sheet and by uninterrupted northward expansion of warm caribbean air this strongly contrasted climate also produced non analogous biological assemblages

gravity modelling of the lithosphere in the eastern alpine western carpathian pannonian basin region abstract gravity models illustrate changes in the degree of continental convergence in the eastern alpine western carpathian region and modifications to the lithosphere due to the plate convergence and subsequent pannonian basin extension analysis of the continental collision zone incorporates a kinematic model of ocean basin closure whereby gravity anomalies and topography are viewed as part of a continuum of continental crustal shortening erosion and isostatic rebound thick crust and high topography in the eastern alps along with a broad bouguer anomaly of 140 mgal amplitude are consistent with about 175 km of crustal shortening followed by 10 km of isostatic rebound eastward crustal thicknesses and gravity anomaly widths and amplitudes are less so that only about 50 km of continental crustal shortening and 4 km of rebound occurred in the western carpathians preservation of thick flysch deposits and small isostatic rebound are attributable to the high density shallow mantle of the intact continent ocean transition zone seismic delay time studies have suggested that relative to the average thickness of the region the lithosphere thickens by about 70 km beneath the eastern alps and thins by about 60 km under the pannonian basin in both regions gravity anomalies cannot be explained fully without considering this large relief on the lithosphere asthenosphere boundary the eastern alpine crustal root which extends 15 km below the average depth for the region overcompensates the topography and results in gravity anomalies that are 40 mgal lower than those observed the extra 70 km of lithosphere provides excess mass that achieves isostatic equilibrium and accounts for the 40 mgal difference observed gravity anomalies and local isostasy are also consistent with thin crust and thin lithosphere beneath the pannonian basin whereby the 60 km of extra asthenosphere provides a large part of the compensation for the elevated mantle regional cross sections suggest that shallowing of the lithosphere asthenosphere boundary associated with pannonian basin extension has propagated northward beyond the carpathians to within the european platform crustal thinning however appears to be confined to exotic terranes of the carpathian interior so that crustal structure in the eastern alps and outer carpathians is a remnant of the earlier collision orogen

reactivated basement structures affecting the sedimentary facies in a tectonically quiescent epicontinental basin an example from nw switzerland abstract the jurassic deposits in the southern part of central europe accumulated in a shallow epicontinental sea and their deposition has usually been believed to have corresponded to a phase of tectonic quiescence neither on conventional seismic records nor in outcrops obvious indications of synsedimentary tectonics were found nonetheless subtle variations in lithofacies and thickness occur above faults in the crystalline basement in fact preexisting structures became reactivated in the jurassic during major extensional phases when the tethys and the north atlantic opened this reactivation led to differential subsidence and or to rotation of fault bounded blocks but the sediments were deformed mainly flexurally as vertical movements in the basement were dissipated by triassic salt thus the depositional area was morphologically differentiated into swells and depressions in siliciclastic muddy environments swells were characterized by hardbottoms and hiatus beds depressions were filled by distal tempestites and gravity deposits in carbonate settings reefs nucleated on swells marls and muds were deposited in depressions many of them gravity deposits reactivation of faults occurred only during short episodes and was not synchronous throughout the study area and reactivation of individual faults was episodic and probably controlled by the palaeostress field and the faults orientation reactivation of deep rooted faults is also documented by hydrothermal activity which led to vein mineralization and alteration of minerals today exposed in nearby basement units the chronostratigraphic ages of the hydrothermal processes coincide with phases of enhanced subsidence during the sinemurian aalenian bajocian bathonian and oxfordian in turn facies changes of the sedimentary cover should be useful to predict basement structures when no seismic records are available

comments on c m clapperton s glacier readvances in the andes at 12500 10000 yr bp implications for mechanism of late glacial climatic change

triassic climates state of the art and perspectives abstract the climate of the triassic period was characterized by a non zonal pattern dictated by a strong global monsoon system with effects that are most evident in the tethys realm this strong monsoonal regime is related to the aggregation of the pangaean supercontinent which by triassic time was already completed climate oscillations existed within this framework the harsh hot house climatic conditions that characterized the late permian and perhaps precipitated the end permian mass extinction were probably maintained during the early triassic and may account for the impoverished but distinctive faunal and floral lower triassic associations although metazoan reef builders were probably the most affected group carbonate production remained high at least in the western tethys realm the middle triassic was characterized locally by humid episodes although their geographical distribution has yet to be clarified the carnian pluvial event marks an episode of increased rainfall documented worldwide was the most distinctive climate change within the triassic different hypotheses have been proposed for its causes changes of atmospheric or ocean circulation driven by plate tectonics a peak of the global monsoon due to maximum continent aggregation or triggering by the eruption of a large igneous province subsequently the late carnian and norian seem to have been climatically stable although minor climatic changes have recently been described even from this time period finally the end triassic extinction event is also associated with climate change specifically warming and increased rainfall but this evidence comes mostly from the northern parts of the central atlantic magmatic province and the global pattern of climate change at the triassic jurassic boundary has still to be resolved many facets of triassic climate are intriguing and deserve further research however palaeoclimate studies on the triassic have so far been carried out only locally with different proxies those proxies will require inter calibration in order to depict correctly the temporal and geographical patterns of triassic climate

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

integrated sequence stratigraphy facies stable isotope and palynofacies analysis in a deeper epicontinental carbonate ramp late jurassic sw germany abstract sequence stratigraphy in deeper water epicontinental carbonates such as in the upper jurassic of southern germany is difficult because the recognition of parasequences sequences and sequence boundaries is impeded by the paucity in diagnostic sedimentological criteria or stratal surfaces using the genetic stratigraphic approach and integrating facies stable isotope c o and palynofacies analysis two types of genetic depositional sequences can be discriminated small scale sequences are stacked into medium scale sequences which may record a 400 kyr milankovitch signal the medium scale sequences were correlated regionally using both gamma ray logs and stable isotope records regional correlations show that the depocentres are controlled by underlying palaeotectonic elements late palaeozoic troughs the rise fall turnarounds of medium scale sequences are marked by negative 18o peaks temperature maxima and reduced absolute palynoclast contents the fall rise turnarounds are marked by positive 18o peaks temperature minima and high absolute palynoclast contents the initiation and termination of sponge microbial mounds show characteristic patterns thrombolitic microbialites form during intervals of 1 reduced input of terrestrial palynomorphs interpreted as an increase in distality 2 decreasing 13c trends interpreted to be related to decreasing nutrient supply and 3 decreasing 18o values interpreted as phases of warming and rising relative sea level in contrast thrombolitic stromatolitic microbialites were found to occur during phases of 1 increasing input of terrestrial palynomorphs interpreted as an increase in proximality 2 increasing 13c values interpreted to reflect increasing terrestrial input and nutrient supply as well as increasing 18o values interpreted as phases of cooling and relative sea level falls isotopic and palynofacies evidence suggests that bioherms were terminated by sudden input of nutrients during relative sea level falls sedimentological criteria were clearly not sufficient to delineate a robust sequence stratigraphic framework

nature of environmental changes in south america at the last glacial maximum abstract a review is given of geomorphological evidence suggesting the nature of environmental changes in south america during the last glaciation maximum lgm the data are from glacial geocryogenic alluvial colluvial and aeolian systems and span 68 of latitude making the most complete land transect of major climatic systems in the southern hemisphere glaciers in the northern and central andes may have reached their maximal extent by 27 000 yr b p reduced precipitation at the lgm caused by lower temperatures and lower atmospheric humidity probably led to slight glacier recession in the tropical andes icefields in the southern andes were most expansive when global temperature and sea level were lowest because pacific westerlies supplied abundant moisture and western outlets advancing across the continental shelf were constrained by sea level low gradient river networks became incised by at least 100 m into narrow channels as global sea level fell by 120 m during the lgm draw down of water tables possibly impacted the forest cover enhancing the drying influence of reduced sea surface temperature and atmospheric humidity as forest and grass covers diminished in extent effecting greater atmospheric cooling because of reduced evapotranspiration and convective condensation colluvial and aeolian processes became more active and widespread almost 25 of the continent is covered with palaeo aeolian features presumed to have been active when airflow was stronger due to steeper atmospheric pressure and temperature gradients at the lgm substantial changes in precipitation totals and incidence and in ground water availability probably eliminated modern type tropical rainforest except in areas currently receiving ca 5000 mm of annual rainfall equivalent changes in vegetation cover spread across much of the guyana and brazilian highlands and throughout the southern temperate regions

triassic monsoonal climate and its signature in ladinian carnian carbonate platforms southern alps italy abstract paleoclimatic general circulation models suggest the existence of a monsoonal climate during the permo triassic over wide parts of the megacontinent pangea and its adjacent oceans this paper discusses how ladinian carnian sedimentary successions outcropping in the southern alps record the signature of this climate sedimentological associations of tepees capped by terra rossa paleokarst braided fluvial sediments capped by caliche soils and evaporite beds alternating with clay rich delta deposits all indicate that net precipitation values changed substantially over short geological time scales early diagenetic features including episodes of dissolution in stratigraphies of meteoric calcite cements corrosion and hematitization of siliciclastic detritus prior to deposition early eu edral tectosilicate cementation and dolomitization from evaporation concentrated seawater record frequently changing paleohydrological conditions sedimentological and early diagenetic data recording highly variable seemingly conflicting paleoclimate information can best be attributed to fluctuations in net precipitation intensities controlled by monsoonal climate ranging from seasons to 106 yr d18o values in early meteoric cements 5 2 to 6 4 reflect the presence of strongly depleted meteoric waters which are not compatible with the southern alps paleolatitude or paleotopography in the triassic and can be directly related to precipitation intensities associated with a monsoonal climate strong precipitation could have also resulted in decreased surface water salinities and depleted d18o in surface waters d18o time series from marine rocks and early meteoric cements indicate a trend from middle triassic values generally deplet d with respect to the expected marine signature to less depleted late triassic values this is interpreted to represent an unusually wet episode high net precipitation transitionally grading in the late triassic into a relatively add period or low net precipitation despite the fact that several factors controlled the intensity of monsoonal precipitation the effects of its variations through time are evident at different time scales in the middle and upper triassic record of the southern alps

geological interpretation of dekorp 2 s a deep seismic reflection profile across the saxothuringian and possible implications for the late variscan structural evolution of central europe abstract a 250 km long deep seismic section the first of its kind in west germany s dekorp project is interpreted in terms of late variscan collisional tectonics the structural model used is based on the intracontinental thrust tectonics concept of modern collision orogens the crustal profile across the saxothuringian zone seems to indicate north facing ramps antiformal stacks and culminations within the crystalline basement of the internal mid european variscides that have been only partially overprinted by permo mesozoic crustal stretching

a reevaluation of the evidence for a younger dryas climatic reversal in the tropical andes abstract a review of the evidence for a younger dryas equivalent climatic reversal in the tropical andes shows that a number of hypotheses need to be reassessed assumptions of the el abra cold and humid phase in colombia are compromised by insufficient dating control periods of glacier advance in peru and bolivia lack sufficient dating control to be correlated with the european younger dryas only glaciers on the quelccaya ice cap in peru may have advanced shortly before the onset of the european climatic reversal but even there a correlation is questionable a late glacial advance in the chimborazo area in ecuador is considerably older than previously suggested the same glacier advance has been dated to earlier than 12 000 bp in other areas of ecuador the emerging picture suggests two phases of glacier advances in the tropical andes 13 000 12 000 bp and 9000 10 000 bp both may have been caused by increasing precipitation unequivocal evidence for a cold younger dryas event in the tropical andes has yet to emerge

evidence for a younger dryas glacial advance in the andes of northwestern venezuela abstract deposits of push moraine outwash and glaciolacustrine sediments recovered from two areas in the northwestern venezuelan andes document the latest pleistocene advance of merida ice underlying peats provide maximum ages on till and outwash evidently emplaced during the younger dryas yd climatic event one example recovered from the humboldt massif where the farthest extent of yd ice buried peat in the surface of late glacial till provides a within glacier advance age of 12 4 ka cal bp the peat lies on moraine deposited during a stillstand event when the humboldt glacier retreated to the area of lago verde at 4000 m a s l approximately 0 5 km upvalley toward lago suero yd till buries peat deposited in glaciolacustrine sediments of presumed late glacial age farther north in the mucunuque mucubaji catchment of the eastern cordillera a push moraine of possible yd age buries older till of late glacial age 0 2 km upvalley outwash of yd age buries glaciolacustrine peat and organic rich alluvial sediment dated to 13 7 and 13 3 ka cal bp the latest merida advance documented here is approximately synchronous with the yd cold event of europe and the north atlantic region ca 11 6 12 7 ka cal bp the yd event in both areas of the northwestern venezuelan andes nearly reestablished earlier late glacial ice positions and termination appears to have been abrupt the valleys in both areas were evacuated of yd ice without emplacement of recessional moraines as during the main deglaciation at the humboldt site equilibrium line altitudes ela s for the late glacial were about 50 m lower than during the inferred yd in the mucunuque mucubaji catchment ela s for the late glacial 3900 m a s l are difficult to establish given the absence of lateral moraines

gravity interpretation of a unified 2 d acoustic image of the central alpine collision zone summary four individual deep seismic reflection profiles across the eastern and southern swiss alps have been combined along the trend of the bouguer gravity anomalies onto the european geotraverse the profiles were migrated ray theoretically with the migration velocity model derived from a smoothed reinterpretation of the seismic wide angle profiles running parallel to the strike of the alpine arc the contours of this velocity model were ray theoretically depth migrated together with the combined digitized line drawings of the seismic reflection profiles the resulting acoustic image depicts the subduction of parts of the lower crust of the european plate beneath the adriatic promontory of the african plate at a low angle of 15 based on the interpretation of the seismic data orogenic crustal thickening is attributed to the stacking of crystalline nappes onto the upper crust of the european plate and wedging of the european and adriatic middle and lower crusts the latter being ill constrained by the seismic evidence alone the south vergent thrusting of the southern alps can be accounted for by the observed downbending of the adriatic moho and the lower crust in conjunction with the inferred wedging at mid crustal levels using the geometric constraints provided by seismic data gravity modelling of the alpine lithosphere asthenosphere system relative to stable central european platform clearly favours a gently inclined subduction zone reaching down to at least 200 km depth such a gently dipping subduction zone is at odds with prominent models on alpine geodynamics which favour a near vertical orientation of the subducting lithosphere verschluckung in agreement with the wedging hypothesis indirectly inferred from the seismic data the short wavelength part of the alpine gravity anomaly requires a middle crust of anomalously high density in the axial zone of the orogen both seismic and gravimetric evidence therefore suggest that late orogenic lithospheric shortening and crustal thickening was governed by the mechanical decoupling of the upper middle and lower parts of the crust the amount of subducted lower crustal material cannot be constrained by gravity modelling since its gravity effect cannot be separated from the effect of the subducting slab

dolomite and dolomitization of the permian khuff c reservoir in ghawar field saudi arabia abstract the permian khuff c reservoir in saudi arabia is known for its lateral and vertical heterogeneity caused mainly by dolomitization detailed petrographic analysis of 600 thin sections from six cored wells revealed three main replacive dolomite fabrics 1 fabric preserving mimetic fpm 2 fabric preserving nonmimetic fpnm and 3 fabric destructive fd dolomites crystal sizes are mostly less than or equal to 20 m for fpm dolomite less than or equal to 50 m for fpnm dolomite and less than or equal to 100 m for fd dolomite the fpm dolomite decreases in abundance and fpnm dolomite increases in abundance with increasing grain content of the facies the 18 o values of dolostones although considered an obsolete term dolostone is used here to mean rock containing 80 dolomite indicate early dolomitization at low temperatures in permian seawater or evaporated seawater with landward facies mudstone and wackestone generally dolomitized by more evaporated waters and seaward grainy facies generally dolomitized by less evaporated more normal marine seawaters stratigraphic variations in the dolostones 18 o values track with facies variations through fourth order depositional sequences and indicate that different stratigraphic bodies of dolomite formed from seawaters with different degrees of evaporation the 13 c values of the dolostones exhibit temporal trends inherited from the precursor limestones variations in the lateral and vertical abundance of dolomite and dolomite fabrics in the propensity for each facies to be dolomitized and in the dolomites oxygen isotopic values all suggest that multiple dolomitization events occurred in the khuff c reservoir as depositional cycles accumulated with some dolostones overprinted by younger events average porosities of grain rich dolostones are greater than those of mud rich dolostones indicating that depositional facies preordained porosity distribution within the dolostones however the more evaporated the dolomitizing fluid the more likely dolomitization resulted in lower porosity regardless of facies

the paleogeography of paleozoic and mesozoic storm depositional systems present day hurricane belts are confined to 5 45 latitude along the western edges of oceans whereas winter storms occur ubiquitously above 25 latitude both hurricanes and winter storms occur between 25 and 45 latitude in some areas storm zones can be identified on global paleogeographic maps with the assumption that this latitudinal zonation of storm systems existed in the geological past evaluation of 69 previously recognized paleozoic and mesozoic storm depositional systems on such maps showed that only 70 of them occur in ancient hurricane mixed hurricane and winter storm and winter storm belts the remainder occur on the western sides of continents and or at low paleolatitudes and therefore well may be of different origin mesozoic storm depositional systems containing hummocky stratification tend to occur in counterpart paleo winter storm belts whereas hummocky stratification in suggested paleozoic examples occur in both winter storm and hurricane belts and in paleolatitudes where we in

climate of the supercontinent pangea numerous climate models predict that the geography of the supercontinent pangea was conducive to the establishment of a megamonsoonal circulation in general geologic evidence supports the hypothesis of a megamonsoon that reached maximum strength in the triassic pangea in the late carboniferous had widespread peat formation in what is now central and eastern north america and europe and relatively dry conditions on the colorado plateau the equatorial region of the continent became drier through the end of the carboniferous by the permian the equatorial region of pangea was dry and indicators of aridity and rainfall seasonality became more widespread wind directions from colorado plateau eolian sandstones are consistent with an increasing influence of monsoonal circulation at this time in the triassic climate in the colorado plateau region became relatively wet though still seasonal and the few eolian sandstones indicate a major shift in wind direction at that time in addition sedimentation

reactive transport modeling of brine reflux dolomitization anhydrite precipitation and porosity evolution abstract both geothermal convection and brine reflux drive circulation of sea water derived fluids through carbonate platforms during early burial but dynamic interactions between heat and solute transport and resulting diagenesis are at present poorly understood this paper describes high resolution reactive transport model rtm simulations that suggest that reflux of 85 ppt brines rapidly restricts geothermal convection to the platform margin with flow focused in the more permeable shallow carbonates in a baseline simulation involving an elongate 25 km wide grain dominated packstone platform brine reflux resulted in complete dolomitization beneath the 5 km wide brine pool in 335 ky the dolomite body then extends downward at c 22 m 100 ky into an underlying broad area of partial dolomitization this process enhances porosity at shallow depth but beneath the dolomite body precipitation of anhydrite occludes porosity and limits the depth of reflux in contrast geothermal convection at the platform margin forms a smaller partially dolomitized body over a longer time 60 dolomite after 1 my with very minor associated anhydrite cementation reflux diagenesis is sensitive to platform geometry with higher rates of fluid flow increasing the depth of alteration beneath the brine pool for a circular platform compared to the linear baseline fluid flow across thermal gradients enhances reaction rates and ignoring heat transport by 85 ppt brine reflux underestimates the extent of reflux dolomite by 25 and associated anhydrite by 90 the depth and rate of anhydritization is sensitive to the geothermal heat flux whereas platform top temperatures affect dolomitization rate reflux diagenesis is also sensitive to brine density which affects both fluid flow and reaction rates sediment permeability and reactive surface area rsa are key intrinsic controls on diagenesis where the permeability structure permits sufficient fluid flow diagenesis preferentially affects more reactive fine grained sediments however as flow rates decline reactions become focused in the more permeable but less reactive sediments simulations thus shed light on why in some settings reflux preferentially dolomitizes muddy sediments but elsewhere favors grainstones once active reflux ceases brines continue to flow in the subsurface but this latent reflux causes only minor dolomitization due to prior mg2 consumption at shallow depth

southern oscillation signal in south american palaeoclimatic data of the last 7000 years abstract during strong el nino events rainfall anomalies and changes in wind patterns are observed in different regions of south america along the central brazilian coast during the 1983 el nino year the frontal systems were blocked to the south provoking a reversal of the longshore sand transport long duration reversals of longshore transport were also recorded in holocene beach ridge terraces from the rio doce coastal plain this led to the formulation of a model relating these reversals of longshore transport to el nino like conditions el nino like conditions are past average climate situations that generate the same perturbations as the strong el nino events observed during the last decade they are likely to correspond to the long duration low phase of the southern oscillation to confirm this hypothesis we compared the holocene beach ridge record with other palaeoenvironmental records from regions where strong el nino events would have a substancial signal as well 1 water level fluctuations of lake titicaca 2 a pollen and sediment record in an eastern amazonian lake 3 changes of the rio xingu discharge in eastern amazonia and 4 variations of sand supply at the rio piura and rio chira outlets in the sechura desert the occurrences of el nino like conditions were numerous before 3900 3600 yr b p absent between 39003600 and 2800 2500 yr b p and infrequent after 2800 2500 yr b p

δ13c and δ18o values of triassic brachiopods and carbonate rocks as proxies for coeval seawater and palaeotemperature abstract a dataset of 160 isotope 13c and 18o values from anisian ladinian carnian and rhaetian articulate brachiopod shells complemented by 158 carbon and oxygen isotope values from whole rock carbonates define the first continuous stable isotope baseline trends for the triassic seawater the carbon isotope data suggest the existence of several short term high amplitude excursions in the early triassic followed by a predominance of values around 0 5 1 during the middle triassic a rise to 3 5 during the carnian plateau at this level during the late carnian to early norian and a 1 5 decline in the middle norian to values around 2 during the late norian rhaetian interval the causation scenarios for these rapid oscillations are at present equivocal but may in part reflect a biological instability of the carbon cycle following the recovery from the end permian extinction event and or an input of mantle derived co2 from enhanced volcanic activity the 18o values from well preserved brachiopods from the tethyan realm range from 3 9 to 0 6 v pdb these values require open marine triassic seawater 18o values close to 0 v smow for the calculated temperatures to be within the range of tolerance of the coexisting reef building corals a 2 18o increase within the uppermost cordevolian and early julian suggests either a distinct temperature decline in the southern alps during that time interval an increase in seawater salinity or their combination oxygen isotope values for the muschelkalk brachiopods range between 6 2 and 2 0 and likely reflect a strong influx of meteoric waters into the germanic basin that shifted the oxygen isotopes to more negative values

late quaternary chronology of the taitao peninsula southern chile six new radiocarbon ages and a pollen sequence are provided for laguna stibnite a small lake on the taitao peninsula chile latitude 46 25 s longitude 74 24 w the sediments record a late glacial to present sequence with a basal age of 14 335 145 yr bp q 2840 the radiocarbon ages provide the basis for a sound chronology in a region of chile where few palynological studies have been made and where previous radiocarbon ages are ambiguous the chronology from laguna stibnite supports the acceptance of a radiocarbon chronology based on wood fragments rather than bulk analysis at a site near puerto eden further south in the chilean channels 49 08 s these data from laguna stibnite provide evidence for an early deglaciation before 14 000 yr bp in this region of southern chile the sequence provides no evidence for a climatic reversal between 11 000 yr bp and 10 000 yr bp the so called younger dryas chron

a younger dryas icecap in the equatorial andes abstract morphologic and stratigraphic evidence shows that a late glacial ice cap existed on part of the eastern cordillera of ecuador lat 0 20 s on ground with a mean elevation of 4200 m where none exists now an outlet glacier from an ca 800 km 2 ice cap terminated at 3850 m altitude in the papallacta valley on the eastern side of the plateau radiocarbon dates show that moraines formed by this advance were ice free by 13 200 14 c yr b p tephras and the age of organic deposits at the plateau edge indicate ice free conditions before 11 800 14 c yr b p this interval was followed by the expansion of an ca 140 km 2 ice cap that discharged glaciers into adjacent valleys where terminal moraines were built at 3950 m altitude ams and conventional radiocarbon dates from macrofossils peat and gyttja above and below till of the readvance indicate that the ice cap formed between ca 11 000 and 10 000 14 c yr b p and was thus coeval with the european younger dryas event the ice cap developed in response to a surface temperature cooling of at least 3 c in the tropical andes a finding that is consistent with a coupled equatorial high latitude north atlantic climate system operating at the late glacial holocene transition these results are further evidence that younger dryas cooling may have been a global event

k ar dating of synkinematic clay gouges from neoalpine faults of the central western and eastern alps abstract with ongoing alpine convergence and deformation during exhumation there is a general progression from ductile to brittle behaviour on the periadriatic fault and kinematically related faults farther north e g the simplon rhone fault and a diffuse discontinuous zone of generally dextral strike slip movement on the southern boundary of the aar massif k ar dating of fine grained illite from clay fault gouges provides a reliable method for establishing the maximum age of this transition to brittle faulting and for short lived faults the approximate time of faulting itself the new results establish that brittle faulting on the northeastern segment of the canavese fault i e the part of the periadriatic fault southwest of lago maggiore occurred around 20 ma with south side up kinematics a k ar age of ca 17 ma for the crosscutting giudicarie fault in the eastern alps is effectively identical with an already published pseudotachylyte age and places a lower limit on major and continuous strike slip movements of the periadriatic fault the age of brittle faulting farther north on the southern border of the aar massif is from 13 6 0 3 to 8 3 1 1 ma consistent with the younger cooling and exhumation in this area the dominantly dextral brittle faulting becoming younger to the north reflects the continued indentation and anticlockwise rotation of adria as alpine units become exhumed and progressively welded to the southern block in contrast to the eastern alps there is no field evidence for westward lateral extrusion of alpine units relative to adria and the european foreland

the timing of the last deglaciation in cordillera oriental northern peru based on glacial geology and lake sedimentology the degree of soil development on moraines and radiocarbon ages indicate that the cordillera oriental a part of the eastern andes in northern peru 7 30 7 55 s 77 20 77 35 w was last glaciated during the last glacial maximum marine isotope stage 2 this glaciation was the most extensive late quaternary glaciation in the area radiocarbondated sedimentological changes in six sediment cores from three lakes and one bog reveal that the last deglaciation is marked by a two stepped transition from rhythmically laminated inorganic sediment to massive organic rich sediment the first step of this transition commenced before 12 1 0 2 ka 1 and probably by ca 13 5 ka glaciers covered less than 50 of their last glacial maximum area by 12 1 0 6 ka and cirques west of the main divide were probably ice free by this time a return to rhythmically laminated inorganic sediment in the lakes and the bog occurred between 12 1 0 6 and 10 3 0 4 ka this may reflect an episode of increased stream discharge and glacier expansion during which glaciers reoccupied cirques above 3 800 m this glacier expansion adds to widespread evidence of a late glacial readvance in the tropical andes all cirques have been ice free since between 10 3 0 4 and 9 7 0 5 ka

large scale strike slip displacement of the drauzug and the transdanubian mountains in early alpine history evidence from permo mesozoic facies belts abstract both the drauzug italy austria and the transdanubian mountains hungary show great differences in facies compared to the geological units that presently surround them i e proximal facies contrast with distal facies in the permo triassic lower liassic strata in the drauzug and the transdanubian mountains indicate an extensional regime causing typical structural features such as tilted blocks fault scarps and drowned carbonate platforms the permo mesozoic facies zones provide markers for the paleogeographic fitting of the drauzug and the transdanubian mountains with areas today lying some 300 400 km to the west the drauzug corresponds to the lombardian basin and the westernmost part of the northern calcareous alps while the transdanubian mountains correspond to the westernmost part of the lombardian basin the trento platform and the belluno trough the drauzug and the transdanubian mountains together with the south alpine realm were displaced to the east along strike slip faults during the middle jurassic to early cretaceous opening of the central atlantic and ligurian piemont oceans and the simultaneous subduction of the vardar ocean at the eastern margin of apulia finally in late oligocene and miocene times the southern alps were displaced back to the west along the dextral periadriatic fault system

late oligocene neogene evolution of europe adria collision new structural and geochronological evidence from the giudicarie fault system italian eastern alps the giudicarie fault system giudicarie sensu stricto meran mauls passeier thurnstein and jaufen faults represents a sharp break in the generally e w strike direction of the orogen scale periadriatic fault system and is a key element for understanding the late oligocene neogene evolution of the alpine chain the kinematics timing and magnitude of movements on the giudicarie fault system are presented here on the basis of a detailed structural study of the component fault segments combined with zircon and apatite fission track analysis from closely spaced samples two main tectonic phases are established 1 back thrusting of the austroalpine units over the southern alps around 32 ma recorded by basement and limestone mylonites along the giudicarie and meran mauls faults with transport directions toward 100 110 and 2 later sinistral transpressive displacement characterized by structures at the ductile brittle transition which overprinted the top to e ese thrust related mylonites but also partitioned into a major system of transcurrent faults in the southalpine domain it was during this later event that the periadriatic fault attained its present day geometry however the amount of sinistral displacement along the giudicarie system was only 15 20 km the magnitude is established here from the 15 km sinistral offset of the jaufen mylonites across the passeier brittle fault and the 20 km long gap in the otherwise continuous distribution of oligocene tonalitic lamellae along the giudicarie fault a direct structural connection is also established between the brenner and the jaufen faults this constrains the timing of the second phase since it must postdate the main exhumation phase of the tauern window at 20 18 ma the results of this study argue against an originally straight periadriatic fault the giudicarie fault formed a restraining bend in this part of the periadriatic fault system since at least the late miocene and probably since the late oligocene

influence of subtle paleo tectonics on facies and reservoir distribution in epeiric carbonates integrating stratigraphic analysis and modelling u muschelkalk sw germany abstract the middle triassic upper muschelkalk carbonate ramp deposits in sw germany provide an excellent laboratory scale analogue for vast epeiric basin fills such as in the middle east the upper muschelkalk and the entire mesozoic overburden in sw germany were deposited on a complex mosaic of three major differentially subsiding variscan paleo tectonic blocks the rhenohercynian saxothuringian and moldanubian subtly influencing the depositional facies pattern for long periods of time this study is based on detailed sedimentological logs of 41 boreholes and surface outcrop sections and 39 gamma ray logs lithofacies analysis and a 1 d and 2 d sequence stratigraphic analysis leading to a regional correlation integrating sequence architecture and marker beds using the subsurface 3d modelling tool petrel two nested geocellular reservoir models were generated i a basin scale 201 5 325 km lithofacies associations lfa model based on truncated gaussian simulation tgs ii a field scale 50 50 km lithofacies types lft model based on sequential indicator simulation sis the integrated 3d facies models reveal landward facies retrogradation with patchy isolated shoal bodies during the transgressive hemisequence and basinward facies progradation with well connected large shoals within the regressive hemisequence models combined with structural maps show the influence of large variscan block segments and smaller scale internal structural elements on the distribution and the size of reservoir analogues both shoal and dolomite bodies this relationship offers a new tool for exploration in epeiric carbonates using deep seated paleo tectonic patterns for the prediction of both hydrocarbon and geothermal reservoirs

forest changes in the amazon basin during the last glacial maximum we report here the discovery of forest beds in the amazon basin radiocarbon dated to the wisconsin wurm glaciation these are the first dates from the last glaciation determined on amazonian samples the locality is in ecuador at 1 100 m elevation near the upper limit of the present amazonian rainforest both pollen spectra and podocarpus wood in the deposits suggest that vegetation comparable to the present andean forest grew at least 700 m lower than it does now suggesting a temperature depression of at least 4 5 c for the amazon lowlands

late quaternary vegetation climate and fire dynamics and evidence of early to mid holocene polylepis forests in the jimbura region of the southernmost ecuadorian andes abstract the vegetation climate and fire history of high mountain ecosystems in the southern ecuadorian andes are documented in a continuous pollen and charcoal record from lagunas natosas forest bog at 3495 m a s l spanning the late pleistocene and holocene periods since 15 930 cal yr bp the chronology of this record is based on four radiocarbon dates paramo vegetation with abundant poaceae and plantago rigida characterized the late glacial lg period implying cold and wet conditions slight expansions of mountain forests suggest warming during the lg the upper forest line was lower than today and fire events were rare during the pleistocene during the early and mid holocene until 4300 cal yr bp the area of paramo was markedly smaller and the subparamo with woody vegetation melastomataceae was larger the upper mountain forest dominated and the upper forest line shifted to higher elevations the pollen record provides the first evidence of the development of polylepis forests in the southern part of the so called andean depression reflecting warmer and drier climatic conditions during early and mid holocene the late holocene after about 4300 cal yr bp is characterized by larger areas of paramo and subparamo promoted by moister conditions these moister conditions and frequent fires may have been the cause of the very rare occurrence or even extinction of polylepis populations in the jimbura region since ca 1000 cal yr bp human occupation has altered the landscape by frequent burning wood cutting grazing and cultivation

late quaternary vegetational change in the central peruvian andes abstract new fossil pollen records from the central peruvian andes at elevations above 4000 m including pollen concentration data and additional radiocarbon dates provide information on late quaternary vegetation change for the region a pollen assemblage typical of superpuna plus low pollen concentrations and organics in the late glacial section of laguna tuctua indicate a cold dry climate but strong evidence for a late glacial climate reversal i e the younger dryas is not presently confirmed an abrupt increase in urticales pollen as compositae and polylepis acaena type pollen decline signals the beginning of the holocene subsequent increases in urticales and plantago rigida type pollen point to increased moisture as well as higher temperatures from about 11 000 to 7000 yr b p a possible correlation exists between the rapid expansion of puna and closed continuous forest and the rise in importance of camelids in the early holocene in the mid holocene plantago rigida cushion bogs and continuous montane forest elements decline suggesting a drier climate grasses and cheno ams then increase reaching maxima about 4000 yr b p pastoralism and agriculture are thought to have developed at this time according to ethnobotanical studies from this area and pollen evidence seems to support these findings between 3000 and 2000 yr b p pollen percentages of poaceae plantago rigida type and other local taxa decline as glacial activity was renewed in the higher mountains zea mays is present by 2000 yr b p and pollen of weeds and secondary successional plants associated with increasing agricultural disturbance are evident

late glacial and late holocene moraines in the cerros cuchpanga central peru abstract small ice fields on the western cordillera northeast of lima were expanded to three times their present size in the recent past and the regional snow line was probably about 100 m lower than it is today outwash from the expanded glaciers formed deltas of silt in valley bottom lakes when the ice lobes retreated the reduced outwash was trapped behind recessional moraines and the clear meltwater infiltrated into the limestone bedrock and emerged at the heads of the deltas in spring pools the delta surfaces then became covered with peat and radiocarbon dates for the base of the peat 1100 70 and 430 70 yr b p for two different deltas indicate that the maximum ice advance was older than those dates and thus older than the little ice age of many north temperate regions much older moraines date from expansion of the same local summit glaciers to even lower levels in the main valleys which had previously been inundated by the cordilleran ice field the cordilleran deglaciation and this expansion of local glaciers probably occurred between 12 000 and 10 000 yr ago on the basis of slightly contradictory radiocarbon dates

outcrop analog for an oolitic carbonate ramp reservoir a scale dependent geologic modeling approach based on stratigraphic hierarchy considerable effort has been devoted to the development of simulation algorithms for facies modeling whereas a discussion of how to combine those techniques has not existed the integration of multiple geologic data into a three dimensional model which requires the combination of simulation techniques is yet a current challenge for reservoir modeling this article presents a thought process that guides the acquisition and modeling of geologic data at various scales our work is based on outcrop data collected from a jurassic carbonate ramp located in the high atlas mountain range of morocco the study window is 1 km 0 6 mi wide and 100 m 328 1 ft thick we describe and model the spatial and hierarchical arrangement of carbonate bodies spanning from largest to smallest 1 stacking pattern of high frequency depositional sequences 2 facies association and 3 lithofacies five sequence boundaries were modeled using differential global position system mapping and light detection and ranging data the surface based model shows a low angle profile with modest paleotopographic relief at the inner to middle ramp transition facies associations were populated using truncated gaussian simulation to preserve ordered trends between the inner middle and outer ramps at the lithofacies scale field observations and statistical analysis show a mosaiclike distribution that was simulated using a fully stochastic approach with sequential indicator simulation this study observes that the use of one single simulation technique is unlikely to correctly model the natural patterns and variability of carbonate rocks the selection and implementation of different techniques customized for each level of the stratigraphic hierarchy will provide the essential computing flexibility to model carbonate settings this study demonstrates that a scale dependent modeling approach should be a common procedure when building subsurface and outcrop models

continental crust under compression a seismic refraction study of south island geophysical transect i south island new zealand 1 the 1996 south island geophysical transect sight active source seismic survey was designed to show the style of lithospheric thickening due to late cenozoic oblique convergence across the australian pacific plate boundary in new zealand as part of this study two seismic refraction lines were shot across central south island and offshore extensions of the continental crust in the tasman sea and pacific ocean we present the data and a 603 km long seismic velocity profile of the crust and uppermost mantle along one of these seismic transects a tomographic inversion of 62 563 travel times from crustal and upper mantle refractions and wide angle reflections resulted in a model with a two layer crust upper crustal velocities were between 5 9 and 6 3 km s and lower crustal velocities were between 6 5 and 7 0 km s continental compression has locally reduced the seismic velocities in the pacific plate crust by 0 2 0 3 km s a possible effect of high strain and fluids in the crust the thickening of the crust from 28 km at the east coast of south island to 37 km beneath the southern alps can account for about 25 of the 80 110 km shortening of pacific plate crust while the rest must be accounted for by rapid erosion of mesozoic sedimentary rocks on the west side of the orogen in our model the lower crust forms a continuous 2 6 km thick layer beneath central south island the asymmetric topography of the southern alps is reflected in the crustal root which has a steeper flank at the west coast this observation is consistent with westward underthrusting of pacific lower lithosphere beneath south island that has been suggested in earlier studies

the influence of climate change on recent peat accumulation patterns of distichia muscoides cushion bogs in the high elevation tropical andes of colombia 1 extensive cushions of distichia muscoides found at high elevations in the andes form deep deposits of peat and function as peatland the location of distichia peatlands at the highest elevations makes them susceptible to the effects of global change including global warming accumulation rates of organic matter are the key function of peatland ecosystems and are highly susceptible to changes in climate this study used 210pb chronologies to infer the recent dynamics of three distichia muscoides peatlands located above 4000 m in elevation chronological markers from dated volcanic eruptions were correlated to the 210pb chronologies of distichia peat and validated the 210pb chronologies changes in 210pb activity with depth indicated rapid accumulation in recent times organic matter had a mean accumulation rate of 1400 g m 2 yr 1 during the past 10 years with values well above the observed regional accumulation rate of 300 400 g m 2 yr 1 the same pattern was observed in peat accretion rates with growth rates of 1 2 cm per year during the last 10 years cushion plants engineer their own environment allowing the system to be less susceptible to direct changes in climate however the high rates of peat accretion in recent times may be originated from increasing temperatures and excess water from nearby glacier melt however glaciers are predicted to disappear during the next 20 years in the northern andes reducing water inputs in tropical high elevation ecosystems

climatic change during the younger dryas chron in northern south america a test of the evidence abstract new ams and palynological data are presented from the colombian andes to assess vegetational and climatic change during the lateglacial holocene transition with special emphasis on the younger dryas yd chronozone the new evidence is compared and discussed with other colombian cores and with data from other countries in the region the lateglacial climatic reversal in colombia known as the el abra stadial has an estimated lower boundary of 11 200 conventional and 10 900 interpolated 14 c yr bp respectively although the el abra stadial is assumed to be the equivalent of the european yd the present data suggest that the el abra stadial is a biostratigraphical signal representing both the yd chronozone and the earliest holocene 11 000 9000 14 c yr bp on the basis of new ams dates and a re evaluation of the pollen zones we divided the colombian el abra biozone into two phases from ca 11 000 to ca 10 500 14 c yr bp there is a sharp increase of subparamo and paramo pollen reflecting a relatively cool phase during the yd chronozone zone y1 after ca 10 500 14 c yr bp a slight increase of arboreal pollen and the presence of cactaceae zone z1 point toward a relatively milder but drier phase extending to ca 9000 14 c yr bp in the earliest holocene our conclusions add detail to the concept of astronomical forcing of contrasting rainfall changes in northern south america between 12 400 and 8800 14 c yr bp we propose an environmental drought during the el abra biozone with subsequent erosion of the sediments deposited during that period as a major factor explaining the poor presence of yd evidence in northern south america we conclude that at hydrologically sensitive sites without a clear lithological change around 11 000 9000 14 c yr bp a hiatus during the yd chronozone can only be detected if other cores are available with bracketed time control we state that the start of the temperature decline in central america and northern south america is related to the global yd cooling event however much work is still required to understand the duration and amplitude of the climatic reversal during the yd chronozone in this part of the globe

a 28 000 7600 cal yr bp pollen record of vegetation and climate change from pukaki crater northern new zealand a pollen record of vegetation and climate change representing the interval ca 28 000 7600 cal yr bp was obtained from a 52 5 m sediment core taken from a volcanic explosion crater at present sea level in auckland new zealand 36 59 s this provides one of the few continuous terrestrial records covering marine isotope stage 2 and part of stage 1 from the mid latitudes of the southern hemisphere chronology of the record is underpinned by eight distal taupo volcanic zone rhyolitic tephra of known age augmented by thirteen ams radiocarbon dates the last glacial maximum cooling extended from at least c 28 000 cal yr bp to ca 18 000 cal yr bp when vegetation apparently consisted of patches of beech dominated forest embedded within shrubland and grassland forest patches were probably larger or more widespread than those in central north island to the south but less extensive than further to the north in northland climate warming commenced ca 18 000 cal yr bp marked by the replacement of beech by conifers and angiosperms and by the expansion of forest at the expense of shrubland and grassland no clear late glacial climatic reversal is evident at this site early holocene forest is characterised by the replacement of the dominant forest tree prumnopitys taxifolia by dacrydium cupressinum following breach of the crater rim ca 7600 cal yr bp by rising sea level the maar was rapidly infilled with marine sediments over a period of ca 1000 years

outcrop constrained hydrogeological simulations of brine reflux and early dolomitization of the permian san andres formation our hydrogeologic model tests the effectiveness of brine reflux as the mechanism for early dolomitization of the permian san andres formation brine circulation is constrained by sequence stratigraphic parameters and a heterogeneous distribution of petrophysical properties based on outcrop data the model simulates accumulation of the san andres platform and calculates fluid flow and solute transport in response to relative sea level fluctuations it tracks porosity loss caused by compaction and the concomitant permeability feedback the amount of dolomite potentially formed is calculated by means of a magnesium mass balance between brine and rock results show that 1 brine reflux is an effective mechanism to deliver magnesium to dolomitize large carbonate successions 2 relative sea level controlled transient boundary conditions result in intricate flow and salinity patterns that can generate irregular dolomite bodies with complex spatial distributions 3 pervasive dolomitization can result from several short lived reflux events by the amalgamation of brine plumes sourced in different locations and times and 4 the model successfully recreates the dolostone and limestone patterns observed in san andres outcrops

multiple scale facies and reservoir quality variations within a dolomite body outcrop analog study from the middle triassic sw german basin abstract this study focuses on a porous dolomite body in the proximal part of an epeiric carbonate ramp triassic muschelkalk south german basin analogous to hydrocarbon bearing dolomite reservoirs in the middle east e g khuff and arab formations the dolomite body is made up of shoal lagoonal and peritidal facies types and is built by a three fold hierarchy of cycles 3rd order to 5th order the best reservoir quality occurs in finely crystalline lagoonal dolo mudstones mean k 37 8 md 20 2 with micro intercrystalline and vuggy porosity coarsely crystalline shoal related dolo grainstones show variable reservoir potential mean k 30 1 md 4 4 due to late diagenetic modification reservoir quality significantly decreases in the non dolomitized foreshoal section mean k 4 5 md 3 6 stratigraphically the most porous dolomite occurs in the upper regressive portion of the 3rd order cycle reservoir quality progressively decreases towards the limy bottom of the section outcrop mapping showed high lateral continuity for the distribution of facies types and poroperm values on the scale of several 10s of meters within correlated 5th order cycles considerable facies and poroperm heterogeneity is already observed on the 100s of meters scale on a 1000s of meters scale the facies and lithology distribution is controlled by the geometry of a gently inclined carbonate ramp during regression a progradational pattern is recognized marked by a basinward migration of backshoal facies and a down dip decrease of dolomite body thickness lithology facies and poroperm data from outcrops and cores were used for 3d modeling with the software petrel the modeled variations in the poroperm distribution are consistent with the degree of heterogeneity expected for a combination of laterally persistent lithology and highly variable facies results of this study may help to better understand the reservoir heterogeneity of inner ramp dolomite bodies and can be applied to improve characterization and modeling of similar dolomite reservoirs

three dimensional facies modeling of carbonate sand bodies outcrop analog study in an epicontinental basin triassic southwest germany this article is the first part of an integrated study to characterize the anatomy and geometries of carbonate shoal reservoir bodies in epicontinental settings it is based on outcrop analog data from triassic layer cake carbonates in the south german basin which were deposited along an epicontinental very gently inclined carbonate ramp the database of this study consists of 56 measured sections supplemented by six shallow cores and wireline logs covering an area of 25 by 36 km 15 by 22 mi the potential reservoir bodies consist generally of midramp shoal and shoal fringe facies types which are composed of skeletal and oolitic carbonate packstones and grainstones with significant amounts of porosity the upper muschelkalk is composed of a lower transgressive and an upper regressive interval within these shoal bodies show not only similarities but also major differences in character geometry and distribution these reflect the conditions of an epeiric system which reacts strongly to small changes in accommodation the accommodation seems to be mainly triggered by the interaction of hierarchically organized large medium and small scale relative sea level changes and a subtle paleorelief at the level of medium scale cycles distinct two dimensional cyclicity styles can be differentiated which record the lateral facies distribution along the depositional gradient different styles of medium scale cycles include different types of shoal bodies 1 transgressive crinoidal shoal style shoal bodies mean width 21 km 13 mi mean length 37 km 23 mi mean thickness 4 2 m 13 7 ft with low facies differentiation deposited on top of subtle paleohighs 2 skeletal sheets style thin reservoir sheets mean width 5 1 km 3 1 mi mean length 11 1 km 6 8 mi mean thickness 0 3 m 0 9 ft of reservoir facies on top of the paleohighs 3 regressive oolitic shoal style shoal bodies mean width 14 6 km 9 0 mi mean length 20 8 km 12 9 mi mean thickness 0 69 m 2 2 ft with high facies differentiation on the flanks of paleohighs and 4 low accommodation style patchy and mosaic distribution of shoal bodies mean width 11 2 km 6 9 mi mean length 26 km 16 mi mean thickness 0 69 m 2 2 ft all data were loaded into three dimensional 3 d modeling software to distribute 14 facies types and model the 3 d stratigraphic architecture the resulting facies distribution implies that volume and dimensions of the shoal bodies are mainly controlled by the combination of stratigraphic cycles and a subtle paleorelief which is indicated by overall thickness changes of succession high energy shoal facies types occur only in and around areas with a reduced overall thickness whereas areas with a thicker development are dominated by low energy muddy facies types all observations combined point to the presence of a subtle paleorelief which could be induced by slight differential subsidence of inherited paleotectonic basement blocks on the kilometer to regional scale previous studies suggest simple layer cake stratal patterns however in the full 3 d view the apparent layer cake stratigraphy turned out to be a pseudolayer cake with very gentle 0 01 0 001 dip clinoform geometries these very subtle offlapping pinch out geometries may have been overlooked in epeiric reservoir systems elsewhere

the effects of brittle plastic transitions in basement involved foreland belts the central southern alps case n italy abstract the central southern alps csa a 100 km wide basement involved fold and thrust belt formed during the europe africa convergence from the cretaceous onward two main phases of deformation have been recognized in the central southern alps the first is a pre late oligocene thrusting of pre alpine basement units upon triassic cover sequences which is sealed by the adamello intrusion 42 30 ma in the northern sector of the belt the folds and thrust faults of this phase are locally overprinted by a post adamello deformation phase possibly corresponding to the middle late miocene 16 6 ma milano phase recorded by the oligo miocene sediments buried under the plio pleistocene padane plain several lines of evidence indicate that compressive deformations originating from the current relative convergence between eurasian and adriatic plates are still occurring within the southalpine wedge which can therefore be modelled as a critical wedge a simple brittle rheology is not adequate to explain the slope and plateau topographic features of the belt which compare more favourably with those predicted by a critical taper model including the effects of temperature dependent deformations low angle surface slopes typical of frontal sectors of wedges deforming with a purely brittle behaviour are interrupted by relatively steeper slopes where the wedge base crosses the brittle plastic b p transition and starts creeping while the basal decollement deforming at higher strain rates still behaves frictionally further to the rear where both the wedge base and the decollement are plastic the surface slope flattens plateau calculated brittle plastic transition depths along the basal decollement fit well with observed topographic changes the location of slope to plateau topographic changes in the csa is strongly influenced by surface heat flow distribution and the occurrence within the wedge and in particular at the wedge base of rock types characterised by different plastic rheological parameters is responsible for repeated topographic changes each related to a different b p transition the transition from brittle to plastic behaviour at the base of the wedge is responsible for a significant weakening within the basal decollement which in turn has dramatic consequences on the dynamics and hence on the kinematics of the foreland fold and thrust belt the maintenance of the critical state of a wedge with a weak plastic basal decollement necessitates a lower amount of internal deformation than that required by completely brittle wedges moreover these deformations are strongly localised in the lower weak portions of the wedge and do not pervasively affect the upper brittle portions these characteristics have been ascertained in the rear part of the csa b p transitions commonly develop not only within wide mountain belts e g the andes but also in narrow ones e g csa this means that b p transitions must be taken into account when modelling the mechanics of foreland fold and thrust belts at all scales the occurrence of b p transitions must be tested particularly when topographic features suggest their existence since only an appropriate rheological behaviour can account for the structural evolution and kinematics derived from field studies

sequence stratigraphy of middle triassic carbonates and terrigenous deposits muschelkalk and lower keuper in the sw germanic basin maximum flooding versus maximum depth in intracratonic basins abstract we analyze the sequence stratigraphy of the middle muschelkalk to the lower keuper strata middle triassic in the sw germanic basin the sequences are interpreted on the basis of a synthesis of facies in the neritic carbonates terrigenous sediments and evaporites along a basin margin to center transect continental sediments cs lower middle muschelkalk occur at the base of the middle upper muschelkalk sequence thick retrogradational marginal marine sediments overlain by open marine deposits middle middle muschelkalk to lower upper muschelkalk form the transgressive systems tract in the basin center only bathymetric criteria are available for recognition of this systems tract the maximum flooding surface mfs at the basin margin differs in age from the maximum depth interval in the basin center maximum depth surfaces or intervals mdi new term should be recognized in many deep basins where flooding is not recorded the highstand systems tract upper upper muschelkalk is strongly progradational the upper sequence boundary muschelkalk keuper boundary is characterized by subaerial exposure at the basin margin and submarine erosion in most other sections along this boundary the low paleo relief resulted in a lack of coarse grained clastic deposits in the basin center a strongly regressive succession lower lower keuper allows possibly for the first time in an intracratonic basin the distinction of an early or late lowstand systems tract elst or llst high frequency low amplitude sea level fluctuations resulted in parasequences in the open marine upper muschelkalk but in high frequency sequences in the coastal plain sediments of the overlying parts of the lower keuper

middle triassic shallow water limestones from the upper muschelkalk of eastern france the origin and depositional environment of some early mesozoic fine grained limestones abstract we have analyzed the sedimentary structures depositional geometries and petrography of a middle triassic fine grained limestone succession from the upper muschelkalk in the intracratonic germanic basin the limestones occur in a unit several metres thick that extends over an area of at least 2500 km2 in eastern france the geometry of specific wave ripples and small channels filled by lateral accretion bedded limestones and the cyclic sedimentation of the upper muschelkalk in eastern france indicate deposition on a very shallow subtidal shelf commonly under the influence of relatively strong currents the limestones mostly consist of microspar with a few skeletal grains the formation of current ripples in the fine grained limestones appear to be incompatible with an originally muddy grain size the major part of the microsparite may thus have originated from the recrystallization of silt size carbonate grains e g calcispheres or peloids

pangaean climates megamonsoons of the megacontinent the climate for a highly idealized geographic representation of the megacontinent of pangaea corresponding to the period 250 200 m y ago is explored using a low resolution atmospheric general circulation model that is coupled to a static mixed layer ocean of constant depth with a provision for sea ice formation and decay soil moisture is included as a predicted variable on the continent because there are large uncertainties about key factors that might influence the pangaean climate sensitivity experiments are used to examine the climate for two different mean elevations 0 and 1000 m two different snowcover prescriptions and two different levels of implied greenhouse heating and solar luminosity modern co2 concentration and modern solar luminosity fivefold increase in modern co2 concentration and a 1 decrease in modern solar luminosity in all of the experiments the large bipolar land masses of laurasia and gondwanaland joined near the equator in the west and separated by the tethys sea in the east exhibit extreme continentality with hot summers cold winters and large scale summer and winter monsoon circulations year round or seasonal aridity is simulated for all but the eastern coastal regions the tropical western coast and the regions poleward of 40 latitude the simulated climate of the idealized pangaean continent has many features that correspond qualitatively with geologic evidence for tropical warmth and aridity in the late permian and triassic however the model results serve mainly to indicate the importance of various climate forming factors geography topography heating detailed comparisons will require new experiments with better models and more accurate geologic boundary conditions

late pleistocene temperature depression and vegetation change in ecuadorian amazonia abstract paleoecological pollen phytolith and wood analyses of sediments radiocarbon dated 33 000 to 26 000 yr b p from two sites in ecuadorian amazonia provide data that suggest a cooling of ca 7 5 c below present in equatorial lowlands from 33 000 to 30 000 yr b p a period of warning followed in which novel species assemblages a blend of montane and lowland floral components persisted for at least 4000 years these data of forest community change from sites lying within the postulated glacial rain forest napo refugium provide the strongest paleoecological refutation of the refugial hypothesis yet obtained the large temperature depression at ca 30 000 yr b p allows the possibility that if maximum cooling at the equator was synchronous with the last glacial maximum lgm of the northern hemisphere freezing temperatures would have been experienced in parts of lowland amazonia between 25 000 and 18 000 b p

the ups and downs of tectonic quiescence recognizing differential subsidence in the epicontinental sea of the oxfordian in the swiss jura mountains abstract recent studies in northern switzerland have shown that epicontinental areas thought to have been tectonically stable during the mesozoic were not necessarily as rigid as presumed by comparing oxfordian facies boundaries and depocenters in their palinspastic position with known faults in the basement a direct relationship between the two can be demonstrated previously the lack of obvious synsedimentary tectonic features has lulled scientists into believing that the realm of the swiss jura was tectonically stable during the mesozoic however it can be shown that facies and sedimentary structures are largely influenced by tectonics subsurface data provide evidence for the presence of paleozoic troughs in the basement which apparently were prone to reactivation during the pan european stress field reorganization taking place in the late jurassic this led to differential subsidence along pre existing lineaments within the study area which can be recognized in the distribution of oxfordian epicontinental basins and their coeval shallow water counterparts eustatic sea level fluctuations played an important role in the development of shallow water facies patterns but a subordinate role in the control of accommodation space in basins while tectonic activity is often recorded in the sedimentary record in the form of platform break ups and associated sedimentary debris more subtle indicators may be overlooked or even misinterpreted sedimentary structures and isopach maps as well as subsurface data in the study area suggest that subtle synsedimentary tectonic movements led to the formation of two shallow diachronous epicontinental basins during the late jurassic it becomes possible to recognize and differentiate the combined effects of local and regional tectonism eustasy and sedimentation

early diagenesis driven by widespread meteoric infiltration of a central european carbonate ramp a reinterpretation of the upper muschelkalk abstract meteoric diagenesis of carbonate ramps is often difficult to interpret and can commonly be confused with other coinciding diagenetic processes the middle triassic upper muschelkalk of switzerland provides an insightful case in which the effects of several overprinting diagenetic environments including matrix dolomitization can be clearly unravelled previous studies suggested that diagenesis took place in connate marine waters with later meteoric waters being invoked to explain recrystallization of dolomite in this study diagenetic analyses c o stable isotope ratios thin section point counting cathodoluminescence and uv fluorescence microscopy of calcitic bioclastic samples have revealed that early diagenesis pre stylolitization and the accompanying porosity evolution did not occur exclusively in the presence of marine fluids five sequential stages of diagenesis have been identified marine shallow burial mixing zone meteoric and dolomitization marine diagenesis induced precipitation of bladed and inclusion rich syntaxial cements that fluoresce strongly under uv light both cements account for a mean 7 5 vol reduction in the porosity of bioclastic beds shallow burial diagenesis likely induced mouldic porosity and associated fluorescent dog tooth cementation based on light oxygen isotope and elevated strontium isotope ratios matrix aragonite calcite neomorphism is interpreted to have occurred in a mixture of marine and meteoric fluids the combination of shallow burial and mixing zone processes reduced porosity on average by 4 8 vol evidence for subsequent meteoric diagenesis is found in abundant dog tooth and blocky calcite cements that have mean 18 o vpdb of 9 36 and no signs of recrystallization these meteoric cements reduced porosity by a further 13 4 vol percolation of meteoric water through the ramp was driven by hydraulic gradients on an adjacent basement high which was exposed by a cycle of early ladinian regressions following meteoric diagenesis the upper muschelkalk was dolomitized by refluxing brines this complex history of diagenesis resulted in moderate porosities in dolomitized rocks up to 20 and low porosities

recent glacial history and paleoclimate of the peruvian bolivian andes abstract radiocarbon dates of glacial deposits in the peruvian bolivian andes are used to place the observed phases of glaciation in a common chronological framework the climatic conditions that may determine modern snowline are examined in order to understand the types of climatic forcing to which glaciers may have been sensitive in the past also considered are independent proxies of climatic change including pollen sequences lake level history and ice core data late pleistocene glaciation in central peru between 24 ka and 12 ka bp appears to have occurred during colder and perhaps drier conditions in bolivia a glacial maximum is dated to phases of late holocene glaciation in peru that correspond in part to the little ice age in the northern hemisphere appear to have been caused by a combination of colder and wetter conditions an overall warming trend may be the cause for the retreat of many glaciers in the region since the mid 19th century

palynology of the circum mediterranean triassic phytogeographical and palaeoclimatological implications following a tentative evaluation of palynological information from ladinian and more particularly karnian successions there seems to be every indication that qualitative and quantitative compositional differences of palynological assemblages could well be applied in testing concepts of triassic phytogeography and palaeoclimatology

pollen studies in the junin area central peruvian andes laguna junin is a large lake 675 km long but only 4 m deep located in the grass covered altiplano at an altitude of 4 100 m northeast of lima pollen analysis of a 30 m core indicates that from about 12 000 yr ago until some time after 3 000 yr ago about 30 of the pollen was carried in by easterly winds from the east andean forests which today are at least 600 m lower in elevation contemporaneous pollen deposition in lakes on moraines of the last glaciation west of the junin plain was dominated by local grassland types and had little contribution from this easterly source at some time after 3 000 yr ago perhaps about 1 200 yr ago the pollen influx to laguna junin from this distant forest source greatly diminished perhaps because of deforestation by increased human populations between about 12 000 and 24 000 yr ago in the junin core the sparse pollen concentration was dominated by grassland types supplemented by polylepis acaena alnus and compositae blown in from the sub puna shrublands that may have occupied much of the east andean slopes in place of the humid forest that occurs there today the silty sediment in this section of the core is interpreted as outwash from piedmont glaciers that invaded the plain especially from the cordillera to the west these glaciers left a distinctive pattern of moraines and outwash fans which served to dam tine lake basin an unconformity in the sediment covers the time from about 24 000 to 39 000 yr ago when the lake was probably dry before this time the silty sediment and the dominance of grassland and shrubland pollen types indicates an earlier interval of glaciation recorded on the landscape by an older set of moraines and outwash fans which initially dammed the lake basin

fault slip analysis and late exhumation of the tauern window eastern alps exhumation of the eastern alps from the early tertiary to the late miocene was localized mainly in the tauern window a thermal and structural dome located in front of the dolomites indenter stress inversions based on new structural investigations over the entire tauern window indicate a regional zoning of the paleostress field with a predominance of strike slip states of stress in the core of the investigated area and dominant extensional regimes in the eastern and western borders of the dome few inverse fault structures have been highlighted we propose a two stage deformation history in order to explain the different types of structures that characterise the ductile and the brittle domains during the first stage of exhumation of the tauern window corresponding to the folding event the brittle crust was probably dominated by n s shortening and compression the second stage of exhumation was marked by normal faulting at the borders of the dome and strike slip faulting in the core during the second stage the brittle part of the crust that was previously affected by compressive structures belonging to the first stage was eroded normal faulting associated to e w extension along the eastern and western borders of the tauern window was accommodated by strike slip faulting located in the core of the tauern window yielding e w extension and n s shortening the orientation of the extensional axes and the nature of the stress tensors are similar to the ones inferred for the late stage brittle overprint of the internal basement massifs of the central alps pointing to a possible common large scale state of stress acting well beyond the area of the eastern alps

late quaternary climatic history of the chilean channels based on fossil pollen and beetle analyses with an analysis of the modern vegetation and pollen rain fossil beetles and pollen were examined from an intermorainal bog at puerto eden isla wellington chile latitude 49 08 s longitude 74 25 w wood from near the base of the section has an age of 12 960 150 yr bp occurrence of flightless beetle species in the basal peat sample is evidence that some members of the biota survived the last glacial maximum in refugia the assumption that the chilean channels were entirely ice covered is incorrect plants and insects that invaded the deglaciated terrain were those of an empetrum heathland in which patches of nothofagus forest were restricted to sheltered locations the climate supporting the heathland is inferred to have been windier and probably drier than that of the present day from 13 000 yr bp to 9500 yr bp nothofagus forest expanded possibly in response to less windiness and more available moisture neither the fossil beetle nor pollen data support a return to significantly colder conditions between 11 000 and 10 000 yr bp at the time of the younger dryas stade from 9500 to 5500 yr bp the climate was as wet as that of the present day based on an increased representation of the pollen of moorland plants and of aquatic beetle species from 5500 to 3000 yr bp the climate was drier as indicated by the expansion of empetrum heath and the reduction in mesic habitats from 3000 yr bp to the present day mesic habitats dominated as the climate returned to a wetter mode the alternatively wetter and drier episodes are attributed to latitudinal shifts in the position of storm tracks in the belt of southern westerlies

a review of lateglacial pollen records from ecuador and peru with reference to the younger dryas event abstract published biostratigraphic records from ecuador spanning the lateglacial period of approximately 13 000 to 10 000 bp lack radiocarbon dates delineating the younger dryas and records with extrapolated dates do not exhibit evidence of a climatic reversal for peru fossil pollen data from four sites suggest one and possibly two climatic oscillations in the lateglacial but only one site laguna junin has dates that delimit a short term climatic change and this occurs between 12 000 and 11 000 bp pollen records from laguna baja and rio blanco pond give the strongest indication of a climatic reversal but bracketing dates are not available other pollen records from peru have tenuous dating or data and do not support a lateglacial climatic reversal at the time of the younger dryas

a 24 700 yr paleolimnological history from the peruvian andes abstract a new paleolimnological dataset from lake pacucha 13 s 3095 m elevation in the peruvian andes provides evidence of changes in lake level over the past 24 700 yr a late glacial highstand in lake level gave way to an early holocene lowstand this transition appears to have paralleled precessional changes that would have reduced insolation during the wet season the occurrence of benthic salt tolerant diatoms and caco3 deposition suggest that the lake had lost much of its volume by c 10 000 cal yr bp pronounced holocene oscillations in lake level included a second phase of low lake level and heightened volatility lasting from c 8300 to 5000 cal yr bp while a polymictic lake formed at c 5000 cal yr bp these relatively wet conditions were interrupted by a series of drier events the most pronounced of which occurred at c 750 cal yr bp paleolimnological changes in the holocene were more rapid than those of either the last glacial maximum or the deglacial period

a late glacial holocene pollen record from the eastern andes of northern peru abstract a pollen and sediment record of a core 4 2 m long from laguna baja 7 42 s 77 32 w 3575 m in the cordillera oriental of northern peru suggests several episodes of major vegetational and climatic change over the past 13 000 yr the oldest pollen assemblage consists of a mixture of paramo elements tropical alpine vegetation including high percentages of poaceae 40 that decline upward moist montane forest compositae and polylepis and wet montane forest e g hedyosmum and podocarpaceae organic carbon content range from 60 with high percentages of jamesonia a fern characteristic of paramo and decreasing values of plantago tubulosa and the wet montane forest elements hedyosmum and podocarpaceae charcoal percentages are at a maximum during this period magnetic susceptibility and sand percentages are high and percentages of organic matter are low several explanations for these changes are possible including a reduction in temperature and moisture more frequent periods of aridity with increased fires or natural succession the holocene record begins with pronounced increases in organic carbon and pollen of wet montane forest primarily hedyosmum podacarpaceae and urticales high values of podocarpaceae pollen 35 and a decline in charcoal suggest temperature and moisture levels above modern day values wet montane forest pollen remain high and charcoal values are low from about 10 000 to 6000 yr b p suggesting that warm and moist conditions prevailed for about 4000 yr subsequently podocarpaceae and urticales decline and for a brief time alnus is prominent in the pollen record following the alnus maximum at about 5000 yr b p poaceae ambrosia and chenopodiaceae amaranthaceae become frequent increased paramo and disturbance indicator pollen suggest increased anthropogenic activities in this region from the middle holocene to the present