quaternary tephrochronology helps define conditioning factors and triggering mechanisms of rock avalanches in nw argentina abstract ten previously defined quaternary tephra layers from northwestern argentina are described here in terms of electron microprobe analyses of glass shards in addition xrf analyses of three tephras alemania buey muerto and el paso ash were performed the alemania ash and buey muerto ash occur within 1 m of each other stratigraphically in several outcrops and exhibit identical glass chemistry as well xrf analyses show that both tephras have identical composition based on stratigraphic relations and age determinations these tephras can clearly be separated as two individual layers tephra ages were defined relatively by ams 14c dates of organic material by 10be surface exposure ages of landslides in stratigraphic association with these tephra layers or directly by 40ar 39ar ages of mineral separates or fission track dating of zircons ages of these tephra layers constrain the temporal distribution of large landslides rock avalanche type in northwestern argentina temporal distribution shows that these large collapses concentrated in deeply incised valleys during climate periods characterized by enhanced precipitation and run off in addition in the tonco valley tephra characteristics also suggest earthquake triggering of coeval landsliding in this valley four landslides one with a 10be age of 7820 830 yr lie in direct contact atop un redeposited paranilla ash showing the coeval age of these landslides the age of the dated landslide deposit coincides with the age of soft sediment deformation structures in nearby lake deposits with an ams 14c age of 7430 7570 cal yr bp

evolution of the western amazon lowland relief impact of andean foreland dynamics the congruency in the depositional origin and age of the uppermost sedimentary strata forming non flooded rainforest ground terra firme in the western and central amazon lowlands is a much debated subject here we conclude from the study of remote sensing imagery that active andean foreland dynamics have played a major role in the evolution of the plio pleistocene fluvial landscape in the western amazon foreland dynamics have resulted in a terra firme composed of late tertiary alluvium and younger alluvial terraces and plains in peru thermoluminescence and 14c dating show local aggradation of this younger alluvium between 180 and 30 ka the documented high age heterogeneity of the terra firme has implications for considerations of the biogeography of the amazon forest

hydrological modelling of a pleistocene landslide dammed lake in the santa maria basin nw argentina abstract around 30 000 14 c yr bp multiple landslides in the quebrada de cafayate 26 s 66 w dammed the rio de las conchas and caused a lake in the santa maria basin nw argentina since climate could have played a major role in landslide generation we used a lake balance model to compare the present day climate with the conditions during landsliding this model reveals that a hypothetical present day lake would stabilize around 70 m below the reconstructed paleolake level the volume of the water body would be one third of the corresponding paleolake volume however a minimum 10 15 increase in mean annual precipitation accompanied by at least a 3 4 c drop in temperature would result in a stable lake level at the reconstructed elevation this result implies wetter and cooler conditions at 30 000 14 c yr bp as compared to the present wetter climate in turn could help to explain enhanced landsliding in the quebrada de cafayate during this time

amazonia during the last glacial abstract new data on the vegetational history and quaternary geology of amazonia permit an improved reconstruction of past environments in amazonia during the last glacial period although limited data from rondonia carajas and guyana show that in certain areas savanna type vegetation and savanna forest had replaced the rain forest during the late pleniglacial ca 22 000 13 000 yr b p the amazonian forest may have been split up into one major west amazonian and several other medium size forest areas this suggests a decline in rainfall of 500 to 1000 mm a reduction of 25 to 40 temperatures may also have been 2 to 6 c 4 2 c lower than today possibly substancially influencing amazonian vegetation during the humid middle pleniglacial 55 000 26 000 yr b p rivers carried a lot of water and sediment resulting in the deposition of lower terrace sediments with one dry interruption around 40 000 yr b p carajas and katira in carajas there is evidence of dry periods ocurring at about 40 000 yr b p and during the early pleniglacial ca 60 000 yr b p rivers carried little water and incised into the low terrace sediments during the dry late pleniglacial water levels rose during the late glacial 13 000 10 000 yr b p or at the beginning of the holocene 10 000 yr b p sedimentation in and of the present inundation valleys commenced after that

endogenous growth of dacite dome at unzen volcano japan 1993 1994 a dacite dome at unzen volcano grew mainly exogenously when it was small and the effusion rate was high but endogenously when the dome became large and the effusion rate declined the endogenous dome that has grown since late 1993 shows a shape classified as pelean whereas the earlier stage had a low lava dome shape the carapace of the endogenous dome moved like the crust of a basaltic lava pillow although the dome is several hundred times larger than such pillows the surface carapace was carried from the inside of the dome where it had been produced and it thickened as it cooled the crater floor was strongly deformed by the advancing endogenous dome the movement and crater floor deformation can be compared to that of a tractor tread moving on unconsolidated ground

mid holocene climate and culture change in the atacama desert northern chile abstract twenty archaeological campsites intercalated between more than 30 debris flows caused by heavy rainfall events between 6200 and 310014c yr b p have recently been discovered at quebrada puripica in the atacama desert of northern chile this record provides detailed information about extreme short lived climatic events during the hyperarid mid holocene period for the first time we found evidence of continuous human occupation in this area filling the regional hiatus in the atacama basin silencio arqueologico between 8000 and 480014c yr b p the transformation of early archaic hunters into the complex late archaic cultural tradition was an adaptive process during this time the site was a local ecological refuge with abundant resources in a generally hostile environment

climate change and mass movements in the nw argentine andes abstract the chronology of multiple landslide deposits and related lake sediments in the eastern argentine cordillera suggests that major mass movements cluster in two time periods during the quaternary between 35 000 and 25 000 14 c yr bp and after 5000 14 c yr bp the older cluster may correspond to the minchin wet period 40 000 and 25 000 14 c yr bp identified in tropical and subtropical south america suggesting a causal relation between enhanced landslide activity and climate change the younger cluster predates the titicaca wet period that began at about 3900 14 c yr bp which also affected other regions in the andes and the amazon basin no landslide and associated lake sediments are documented during the tauca wet period between 16 000 and 8000 14 c yr bp however the two clusters correspond to periods where it assumed that the el nino southern oscillation enso and tropical atlantic sea surface temperature dipole tad were active the analysis of the present day precipitation patterns in nw argentina indicates significant spatial and temporal differences between the intra andean part of the study area and the andean foreland whereas the tad seems to consistently increase rainfall the intensity of precipitation during the el nino phase of the enso is reduced to only 25 of the mean annual average in the intra andean basins whereas the regions east of the andes receive more than 125 similar results but with an opposite sign characterize la nina events the comparison of this pattern with paleo precipitation data as inferred from varved lake sediments suggests that increased interannual climate variability and therefore increased fluctuations in rainfall and river discharge in narrow valleys may reduce landsliding thresholds

insolation values for the climate of the last 10 million years new values for the astronomical parameters of the earth s orbit and rotation eccentricity obliquity and precession are proposed for paleoclimatic research related to the late miocene the pliocene and the quaternary they have been obtained from a numerical solution of the lagrangian system of the planetary point masses and from an analytical solution of the poisson equations of the earth moon system the analytical expansion allows the direct determination of the main frequencies with their phase and amplitude numerical and analytical comparisons with the former astronomical solution ber78 are performed so that the accuracy and the interval of time over which the new solution is valid can be estimated the corresponding insolation values have also been computed this analysis leads to the conclusion that the new values are expected to be reliable over the last 5 ma in the time domain and at least over the last 10 ma in the frequency domain

transitions between explosive and effusive eruptions of silicic magmas water rich silicic magmas may erupt explosively giving rise to massive columns of fragmented ash1 5 or effusively as viscous bubbly lavas to form lava domes6 8 complex cycles of explosive and effusive eruptions are often observed8 12 although these two eruption mechanisms have been modelled independently1 8 the conditions under which a volcano exhibits a particular style of eruption are not known and the transitions between eruption styles are poorly understood by modelling the ascent of magma along a permeable conduit we show here that for small magma chamber overpressures the style of eruption explosive or effusive is dependent on the magma flow rate we also identify a critical overpressure above which only explosive eruptions occur complex eruption sequences follow naturally for example if a shallow magmatic system is supplied with magma at an intermediate flow rate the eruption will be slow and effusive until the chamber over pressure becomes too large an explosive eruption then relieves the overpressure and an effusive eruption style resumes we suggest that monitoring of temporal variations in chamber overpressure for example by measuring ground deformation can be used to assess whether a passively effusing volcano has the potential to erupt explosively

the last 50 000 years in the neotropics southern brazil evolution of vegetation and climate abstract in the lagoa campestre lake of salitre 19 s 46 46 w 970 m elev there are plant taxa belonging to many ecological groups that are encountered nowhere else at this latitude frequent incursions of polar advections causing cooling and humidity a cool and foggy climate in the middle of the depression and warmer temperatures on the surrounding slopes help to maintain all these groups within a fairly restricted area late pleistocene holocene climatic change has had a considerable impact on the flora and vegetation of salitre the pollen record of the 6 m deep core lc3 shows how cold forest trees such as araucaria angustifolia and drimys brasiliensis semi deciduous forest halophytic plants and peat bog started to develop on this site the initial period between c 50 000 and 40 000 yr b p was an arid phase not recorded in any other neotropical lowland site it was followed by a period of high moisture levels 40 000 to 27 000 yr b p with a maximum estimated at c 35 000 yr b p the late glacial maximum is missing because of a gap in sedimentation humidity gradually increased during the late pleistocene between 16 000 and 11 000 yr b p the early holocene 9500 to 5000 yr b p is characterized by a more marked seasonal pattern and higher temperatures reaching a maximum c 5000 yr b p the spread of semi deciduous forest between 4000 and 3000 yr b p attests to a return of humidity comparison with the serra negra section 19 s 46 45 46 w 1170 m elev not far from salitre confirms the high moisture rates recorded at c 35 40 000 yr b p although temperatures were cooler at the altitude of serra negra as is attested by the presence of araucaria forest and also confirms the strong impact of polar advections on the climate of southeastern brazil

groundmass crystallization of mount st helens dacite 1980 1986 a tool for interpreting shallow magmatic processes the 1980 1986 eruption of mount st helens volcano provides an unprecedented opportunity to observe the evolution of a silicic magma system over a short time scale groundmass plagioclase size measurements are coupled with measured changes in matrix glass plagioclase and fe ti oxide chemistry to document increasing groundmass crystallinity and thus to better constrain proposed physical models of the post may 18 1980 magmatic reservoir measurements of plagioclase microlite and microphenocryst sizes demonstrate that relatively rapid growth approximately 10 9 cm s of groundmass plagioclase occurred immediately subsequent to may 18 relatively rapid plagioclase growth continued through the end of 1980 at an average rate of 3x10 11 cm s plagioclase growth rates then decreased to 1x10 11 cm s through 1986 changes in groundmass crystallinity are reflected in changes in both matrix glass and plagioclase microphenocryst rim chemistry although the matrix glass composition appears to have remained approximately constant from 1981 1986 after a rapid compositional change from may 18 until the end of 1980 plagioclase microphenocrysts show increasingly more complex zoning patterns with time microphenocryst core compositions are commonly positively correlated with crystal size both of these observations indicate continuous groundmass plagioclase growth through 1986 magmatic temperatures estimated from fe ti oxide pairs are approximately constant through 1981 at eruption temperatures of 930 c and at log fo2 of 10 8 by 1985 1986 oxide temperatures decreased to 870 c chemical and textural changes can be explained by 1 rapid degassing and crystallization in response to the intrusion of magma into a shallow 4 5 km reservoir toward the end of the may 18 1980 eruption 2 continued crystallization at a much reduced rate through 1986 due to slow cooling of the shallow magma reservoir growth rates and consequent chemical changes appear to decrease at the end of 1980 this is coincident with the change in eruption style from explosive eruptions sometimes followed by dome growth to solely extrusive dome building events and can be explained by the expected viscosity increase of both degassing and increasing crystallinity the model of twostage crystallization of magma in a shallow reservoir is consistent with conclusions from gas studies casadevall et al 1983 gerlach and casadevall 1986 a b patterns of crater deformation chadwkck et al 1988 and post 1980 seismicity endo et al 1990 although it does not explain the experimental data of hill and rutherford 1989 on the growth rate of amphibole reaction rims textural measurements on mount st helens dacite can also be used to evaluate crystallization kinetics in silicic magmas systems for which experimental data is almost non existent plagioclase growth rates are 5 10 times slower than estimated plagioclase growth rates in basaltic systems a result consistent with the higher viscosity of a more silicic melt furthermore patterns of textural change both average crystal size and number density are similar to those observed during the 1984 mauna loa eruption by lipman and banks 1987 suggesting that the only modification to the crystallization behavior of plagioclase required in extrapolation from basaltic systems is a moderate decrease in rates such that the rate of crystallization scales with the melt viscosity

palaeoclimates in central and south america since 18 000 bp based on pollen and lake level records abstract the last 18 ka of palaeoclimates are discussed at 3 ka intervals based on palaeoenvironmental changes shown in pollen and lake level records from central and south america a total of 61 pollen records fulfill the requirements of continuity and chronology to be used in this synthesis 8 records from tropical and subtropical lowlands in central and south america 7 records from the central american highlands 26 records from the high andes north and south of the equator 2 records from the southern subtropics and 18 from the southern andes and subantarctic islands palaeoclimatic interpretation of the pollen records is semiquantitative based on bioclimatic parameters such as e g elevational temperature gradients or latitudinal precipitation gradients that can be related to vegetation zonation additional palaeoclimatic information is obtained from lake level records derived either from dated shorelines or from the limnological history of lake sediment sections large scale palaeoclimatic comparison of the proxy data records indicates that full and late glacial climates throughout central and south america were cooler than today by about 4 to 5 c but that moisture patterns showed regional differentiation thus in both the southern subtropics and high latitudes in southern chile conditions were substantially moister than today during full and late glacial times whereas everywhere else conditions were far drier than today in terms of palaeocirculation this scenario can be interpreted as a poleward shift of the subtropical as well as the westerly circulation a scenario that has been suggested by palaeoclimatic simulations from general circulation models gcm another major palaeoclimatic aspect predicted by the gcm simulations is the suppressed early holocene summer monsoon in the southern subtropics although proxy records are few from this zone their palaeoenvironmental history during that time shows a period of aridity that would support the model one of the most controversial aspects of south american palaeoclimates concerns the existence of a younger dryas equivalent a cold period between 11 and 10 ka bp based on the existing data from south america that show a great diversity of palaeoclimatic trends the concept of an interval that is characterized by the same climatic signal as documented for the north atlantic seems too simplistic moreover at this point only very few records have been analysed in sufficient detail to yield unambiguous climatic information for the interval in question the principal outcome of this palaeoenvironmental and palaeoclimatic synthesis is the better understanding of the type and quality of data available at present and the realization of regions and periods where much more and concerted effort is needed to address problems of intra and interhemispheric palaeoclimatic correlations only then can questions concerning the mechanism of climatic change be tackled successfully

tectonics and climate of the southern central andes the history of the southern central andes including the world s second largest plateau and adjacent intermontane basins and ranges of the eastern cordillera and the northern sierras pampeanas of argentina and bolivia impressively documents the effects of tectonics and topography on atmospheric circulation patterns the development of orographic barriers and their influence on erosion and landscape evolution at various timescales protracted aridity in the orogen interior has facilitated the creation and maintenance of the puna altiplano plateau contraction and range uplift filling of basins and possibly wholesale uplift of the plateau increased gravitational stresses in the orogen interior which caused the eastward migration of deformation into the foreland and successive aridification the uplift of the andean orogen has also had a far reaching influence on atmospheric and moisture transport patterns in south america this is documented by the onset of humid climate conditions on the eastern side of the andes in late miocene time which was coupled with the establishment of dramatic precipitation gradients perpendicular to the orogen and changes in tectonic processes in the andean orogenic wedge

landslides and quaternary climate changes the state of the art abstract climatic factors are considered major causes and triggers of slope instability but the palaeoclimatic implications of landslides have not yet been systematically reviewed as inferred from landslides dated to the late glacial and holocene glacial interglacial transitions are the major intervals of enhanced landsliding but the length of this interval varies between distinct types of landscapes mass movements in non glaciated temperate mountain ranges arid and tropical regions and coastal areas intensified soon after the onset of the holocene alongside the warming and wetting of the climate whereas large rock slides in areas within the limits of pleistocene glaciations mostly show a millennial scale time lag with respect to glacier withdrawal although the incidence of landslides was ubiquitous throughout the holocene the mid to late holocene transition 5 4 ka was especially favourable for the origin of landslides across distinct types of landscapes indicating a strengthened role of mass movements during the culmination phases of interglacial climate optima in addition growing evidence suggests that the enhanced activity of landslides occurred during warmer interstadial conditions which is best demonstrated in the temporal coincidence of arid zone landslides with quaternary pluvials future progress in the understanding of the relationships between landslides and quaternary climate changes should benefit from the i extension of datasets of dated landslides ii geographical expansion of dating studies to regions with a lack of chronologically determined landslides e g tropics iii improvement of landslide dating strategies involving the incorporation of independent palaeoclimatic proxies and iv wider incorporation of numerical modeling within landslide dating studies

the climate of the altiplano observed current conditions and mechanisms of past changes abstract the large scale controls on the climate of the south american altiplano are investigated using local observations reanalysis data and general circulation model experiments the objective is to gain understanding of causes of climate variability and climate change by relating mechanisms that operate on timescales ranging from the intraseasonal to the glacial interglacial our results suggest that on all timescales the climatic conditions on the altiplano are closely related to the upper air circulation with an easterly zonal flow aloft favoring wet conditions and westerly flow causing dry conditions different factors influence the upper air circulation on the different timescales intraseasonal variability is a reflection of the position and intensity of the bolivian high which is modulated by rossby waves emanating from the midlatitude south pacific the annual cycle of dry winter and wet summer conditions is caused by the seasonal expansion of the equatorial easterlies in the upper troposphere rather than direct insolation forcing over the altiplano or moisture changes in the source area interannual variability is primarily related to changes in the mean zonal flow over the altiplano reflecting changes in meridional baroclinicity between tropical and subtropical latitudes which in turn is a response to sea surface temperature changes in the tropical pacific orbitally forced changes in the land sea contrast drive continental scale circulation changes which significantly alter the zonal flow over the altiplano on glacial interglacial timescales the contrast in heating between northern and southern hemispheres during the glacial leads to upper air easterly anomalies throughout the tropics on modern timescales the marked submonthly seasonal and interannual changes of moisture over the altiplano cannot be accounted for by moisture changes in the humid tropical lowlands however the model experiments suggest that cooler conditions during a glacial reduce moisture availability from the tropical lowlands which counteracts the effect of the upper level circulation resulting in little overall change in precipitation this observational and modeling analysis provides a physical framework for relating the mechanisms of both internal and forced climate change on the altiplano on a wide range of timescales

el nino like climate teleconnections in new england during the late pleistocene a glacial varve chronology from new england spanning the 4000 year period from 17 500 to 13 500 calendar years before the present was analyzed for evidence of climate variability during the late pleistocene the chronology shows a distinct interannual 3 to 5 years band of enhanced variability suggestive of el nino southern oscillation enso teleconnections into north america during the late pleistocene when the laurentide ice sheet was near its maximum extent and climatic boundary conditions were different than those of today this interannual variability largely disappears by the young end of the 4000 year chronology with only the highest frequency components roughly 3 year period persisting this record provides evidence of enso like climate variability during near peak glacial conditions

15 k y paleoclimatic and glacial record from northern new mexico the southern sangre de cristo mountains new mexico contain evidence of glacial activity from the late pleistocene to late holocene sediment cores recovered from an alpine bog 3100 m trapped behind a pinedale age moraine 2 km downvalley from a high elevation cirque reached glacial age debris and recovered 6 m of lake clays overlain by gyttja accelerator mass spectrometry dating sedimentology variations in magnetic properties and organic carbon data reveal six distinct periods of glacial and or periglacial activity these include a late pleistocene pinedale glacial termination just before 12120 1 4 c yr b p a younger dryas chron cirque glaciation an early neoglacial periglacial event ca 4900 1 4 c yr b p a late holocene cirque glaciation 3700 1 4 c yr b p as well as late holocene periglacial events at 2800 1 4 c yr b p and the little ice age ca 120 1 4 c yr b p cold events in the middle to late holocene correlate with subtle ice rafting events in the north atlantic and records of cold events in north america and europe and were probably hemispheric in extent

intrusive and extrusive growth of the mount st helens lava dome lava domes grow by a combination of endogenous intrusive and exogenous extrusive addition of magma either as single continuous events1 2 or through a series of relatively small pulses which together build a larger construct3 evaluating the hazards that attend growth of a dome requires an assessment of the processes controlling whether lava is added to its interior or surface because many of these dangers depend on how volatiles are admitted to or released from hot magma if the volatile pressure inside an inflating dome becomes high enough sudden exposure provided by flow front slumping can lead to explosive decompression and the generation of pyroclastic flows4 5 on the other hand lava erupted on the oversteepened surface of a dome may collapse fragment and mix with snow or ice to form pyroclastic flows surges and mudflows6 7 here we provide a quantitative assessment of the partitioning of magma into endogenous intrusions and exogenous lobes using high resolution digital topographic maps of the mount st helens dome derived from aerial photographs taken periodically between 1980 and 1986 endogenous exogenous and total volume production rates follow distinct trends which provide important clues about the nature of eruption mechanisms calculating endogenous and exogenous components for active domes like those recently formed at redoubt volcano may help quantify magma chamber processes and provide another tool for understanding the re equilibration of shallow magmatic systems

silicic lava effusion controlled by the transition from viscous magma flow to friction controlled flow silicic magma forms a vertical fault in volcanic conduits via the brittle ductile transition the formation of the fault changes flow type from viscous flow to friction controlled flow here we investigate flow dynamics of magma by coupling a one dimensional conduit flow model with an experimentally calibrated brittle ductile transition the numerical simulation demonstrates that the length of magma plug at which friction controlled flow becomes the main flow type depends on magma flux because of crystallization kinetics and the ductile brittle transition at high flux a short plug forms because nonequilibrium crystallization inhibits an increase of magma viscosity this results in the effusion of less viscous lava and large shear stress at the shallow part of the conduit in contrast the long plug that forms under low flux cannot maintain large shear stress due to weakness of the magmatic fault which causes the extrusion of a solidified lava spine

dynamics of seismogenic volcanic extrusion at mount st helens in 2004 05 the 2004 05 eruption of mount st helens exhibited sustained near equilibrium behaviour characterized by relatively steady extrusion of a solid dacite plug and nearly periodic shallow earthquakes here we present a diverse data set to support our hypothesis that these earthquakes resulted from stick slip motion along the margins of the plug as it was forced incrementally upwards by ascending solidifying gas poor magma we formalize this hypothesis with a dynamical model that reveals a strong analogy between behaviour of the magma plug system and that of a variably damped oscillator modelled stick slip oscillations have properties that help constrain the balance of forces governing the earthquakes and eruption and they imply that magma pressure never deviated much from the steady equilibrium pressure we infer that the volcano was probably poised in a near eruptive equilibrium state long before the onset of the 2004 05 eruption

paleohydrology and paleoclimate of the past 33 000 years at the tamandua river central brazil abstract deposits of the tamandua river contain evidence for four major paleohydrologic stages in the last 33 000 years a wet period between 33 000 and 20 000 14 c yr b p produced a high water table that allowed organic rich deposition in the tamandua valley a dry interval 17 000 10 000 14 c yr b p produced sandy deposits of braided channels and alluvial fans river aggradation during this period probably resulted from a high sediment load promoted by intense slope erosion and from flash floods a wet period after 10 000 and before 6000 14 c yr b p was marked by reduced slope erosion and by high discharge that led to erosion of the valley fill during that time forests developed widely in brazil a drier climate after 6000 14 c yr b p caused a reduction of discharge but allowed a high water table to be maintained

signature of rhine valley sturzstrom dam failures in holocene sediments of lake constance germany abstract landslide dammed lakes that form upstream of large scale rockslide avalanches sturzstroms present an enormous potential hazard to downstream areas such failures are recorded in the early holocene sediments of upper lake constance germany by the presence of two clastic layers that were emplaced around 9400 cal ybp the sedimentology of these layers contrasts sharply with the typical late glacial and holocene lacustrine sediments present beneath the floor of the lake sedimentary structures and the magnetic fabric of these clastic deposits strongly suggest that they were emplaced by hyperpycnal underflows related to catastrophic floods that entered the lake from the rhine river from stratigraphic and geologic data it is inferred that the hyperpycnite deposits are directly related to the failure of two sturzstrom dams and the draining of the dammed lakes that developed upstream from them these lakes were dammed by the flims and the tamins sturzstrom deposits respectively sedimentologic data from lake constance indicate that the two successive events occurred within a very short time interval probably not exceeding several years consequently these lacustrine deposits provide essential information about the morphogenic evolution of the rhine river valley during the early holocene and about the sturzstrom events that occurred during the same time interval

conduit degassing and thermal controls on eruption styles at mount st helens abstract the explosivity of silicic eruptions depends on the interplay between magma rheology exsolution kinetics and degassing magma degassing is governed by the competing effects of vertical transport within the conduit and the lateral flux of gas out of the conduit diller et al 2006 jaupart and allegre 1991 we combine a simplified treatment of these degassing processes with thermodynamic modeling to examine the conditions present at mount st helens during the spine extruding eruption from 2004 to 2008 we find that two parameters are primarily responsible for controlling the eruptive style the magma chamber temperature and a dimensionless parameter that gauges the efficiency of lateral degassing together these parameters determine whether and where magma can solidify at depth to form a dense solid plug that is gradually extruded as a volcanic spine we show that the small 50 oc decrease in magma chamber temperature between eruptive activity in the 1980s and that of 2004 2008 combined with a modest increase in degassing efficiency associated with lower volumetric flux can explain the observed change in erupted material from viscous lava flows to solidified spines more generally we suggest that similar threshold behavior may explain observed abrupt transitions in effusive eruptive styles at other intermediate composition volcanoes finally we extrapolate our results to suggest that the increase in degassing efficiency accompanying decreasing magma supply rates may have caused the transition from explosive to effusive activity in late 1980

does a monsoon climate exist over south america the climatology and the basic state of the summertime circulation and rainfall over south america are studied using assimilation products from the data assimilation system of goddard earth observing system 1 geos1 and satellite derived rainfall results indicate the existence of a regional summer monsoon circulation regime induced by strong diabatic heating over the subtropical south american highland centered at the altiplano plateau sensitivity of the results to the assimilation scheme is tested by comparing that with the national centers for environmental prediction ncep reanalysis and with satellite rainfall estimates results show general agreement between the model produced rainfall anomaly and the satellite estimates as well as consistency between the basic circulation features in the geos 1 and the ncep reanalyses a case study of 1989 90 south american summer monsoon sasm reveals the following characteristics 1 in late spring the onset of sasm is signaled by an abrupt merging of the upper tropospheric double westerly jets one in the subtropics and the other in the subpolar region into a single jet in the midlatitudes this is followed by the establishment of a vortex to the southeast of altiplano and occurrence of heavy precipitation over subtropical eastern brazil 2 during the mature phase of sasm the heavy rainfall zone moves over the altiplano plateau and the southernmost brazilian highland the fully established sasm features are the following a an enhancement of equatorial north atlantic trade wind which emanates from the sahara high and crosses the equator over the south american continent b a buildup of strong northwesterlies along the eastern side of the tropical andes and c development of the south atlantic convergence zone in the southernmost position with strong convective activity meanwhile the upper tropospheric return flow emerges from an anticyclone formed over the altiplano plateau crosses the equator and sinks over northwestern africa 3 the withdrawal of sasm in late summer is signaled by the resplitting of the midlatitude westerly jet at the same time the low level northwest monsoon flow diminishes reducing the moisture supply and leading to the termination of heavy precipitation over the subtropical highland results also show that the above mentioned characteristics of sasm are clearly linked to the tropospheric temperature changes over the central south american highland sensible versus latent heating over the highland are bound to play an important role in the evolution of sasm to provide further support of presence of a monsoon climate over south america sasm is compared and contrasted to the classic east asian summer monsoon easm many similar features including evolution characteristics between the two systems have been identified contrasting aspects of the sasm from the easm are also discussed it is pointed out that a number of monsoonal characteristics of the climate of south america such as the seasonal reversal of the low level wind become apparent only when the strong annual mean wind is removed based on the characteristic features and their evolution the authors conclude that a monsoon climate does exist over south america

disorientation of corals in late ordovician lime mudstone a case for ephemeral biodegradable substrate abstract massive coralla representing the tabulate coral agetolites occur on a lime mudstone bed in the upper ordovician xiazhen formation of southeastern china other fossils include solitary rugose corals bryozoans trilobites and mollusks in addition abundant spicules and spicule networks suggest that sponges were widespread the occurrence of intact unabraded fossils in micritic matrix and the absence of high energy sedimentary structures indicate deposition in low energy conditions thin section analysis of disoriented specimens demonstrates that geopetal indicators are consistent with stratigraphic up and differ from the growth axes of the corals implying that geopetal infillings formed after disorientation the growth axes of coralla were not redirected during life suggesting that the corals were either dead at the time of disorientation or died as a result of disorientation an examination of cyclomorphism indicates that the corals died at different times rather than during a single event a close association between sponges and corals is suggested by the presence of spicule networks in calices and intracorallum spaces of agetolites it is hypothesized that disorientation of corals was related to growth on biodegradable substrates we suggest that many coral larvae settled on sponges that formed sponge meadows disorientation of the resulting corals may have occurred when 1 a host sponge could no longer support the weight of a coral and collapsed 2 increasing weight or imbalance of a coral caused it to fall off the sponge or 3 a coral became detached when the sponge died and decomposed if the coral was alive sudden deposition in a disoriented position within muddy sediment resulted in its death disorientation of massive skeletons such as colonial coralla is most commonly attributed to water energy this study suggests that ephemeral substrates may have been involved in some cases

kinematic evolution of fault ramps and its role in development of landslides and lakes in the northwestern argentine andes the quebrada del toro and quebrada de las conchas in the cordillera oriental of northwestern argentina are notable for repeated landsliding and damming of major drainages both areas correspond to displacement transfer zones along major thrust systems that developed during the neogene uplift of the central andes during this period displacement transfer zones were areas of subdued relief that localized where rivers cross the fault systems however neotectonic deformation converted the transfer zones into restraining bends along strike slip systems resulting in enhanced uplift at river crossings the antecedent streams responded by rapidly downcutting into the ascending hanging wall blocks producing precipitous local relief the extreme local relief that was controlled by the kinematic evolution of faulting in both areas provides an effective precondition for landsliding which may have been ultimately triggered by seismicity and gravitational failure aided by enhanced undercutting during past humid conditions

structural and lithological controls on large quaternary rock avalanches sturzstroms in arid northwestern argentina landsat thematic mapper tm analysis aerial photograph interpretation and field studies of the semiarid puna plateau adjacent eastern cordillera and sierras pampeanas of argentina lat 24 28 s have revealed the existence of at least 55 rock avalanche deposits with volumes larger than 10 6 m 3 that formed by the collapse of entire mountain fronts the spatial distribution of landslide deposits is not random but it clusters along mountain fronts bounded by active faults inspection in the field reveals five principal controls on the distribution of these events the source area of the rock avalanches has two topographic constraints 1 vertical relief contrasts between the breakaway zone and the mountain front must exceed a threshold of 400 m and 2 the slope inclinations must be steeper than 20 rock avalanches are restricted to three types of lithology granites low grade metamorphic rocks and coarse clastic sediments structural controls are very important rock avalanches are controlled by planar structures such as bedding planes exfoliation joints minor faults and cleavage that all dip toward the valley in addition major slide clusters occur along mountain fronts that underwent quaternary reverse fault reactivation of former transfer faults with strike slip kinematics the trigger mechanism for the majority of these landslides is interpreted to be seismic although the ages of some major slides are about 30 ka and they may correspond to a more humid interval in southern south america

paleoenvironmental insights into the quaternary evolution of the southern brazilian coast based on fossil and modern diatom assemblages abstract diatom assemblages provide a strong basis for detailed interpretations of paleoceanography and diatom paleoecology of the southern brazilian coast nine cores obtained in the coastal plain shelf and continental slope and thirteen surface sediment samples of the patos lagoon provide an excellent opportunity to use a paleoecological approach to study detailed quaternary environmental changes in the southern coasts in the interval studied the basin sedimentation occurred in a marine dominated environment related to humid periods prior to 43 500 year bp abundant diatom taxa are highlighted as excellent indicators of environmental changes including salinity sediment composition and transport by incised valleys providing additional insights into coastal evolution paralia sulcata is dominant in all analyzed cores occurring continuously from late pleistocene to holocene but is rare in the modern sediments non marine diatom assemblages occurred continuously but in very low densities in periods before and after the last glacial maximum abrupt changes in sedimentation have been recorded related to shifts from marine dominated to shallow estuarine deposition during the holocene sea level rose rapidly which is responsible for the progressive coastal flooding prior to 8420 7930 year bp the holocene barrier system became more continuous and patos lagoon developed consequently the secondary former inlets from the jacui and camaqua rivers are closed and only one single inlet rio grande channel remains active the spatial distribution of modern assemblages is controlled by salinity gradient wind action and climate forcing the bottom sediments of patos lagoon are dominated by freshwater diatom aulacoseira veraluciae aulacoseira sp 2 and marine estuarine species of cyclotella litoralis

late quaternary of the middle caqueta river area colombian amazonia the late quaternary history of the middle caqueta river area in colombia northwestern amazonia is described based on observations of river bank sections radiocarbon dates and palynological analyses of organic layers in floodplain and low terrace sediments of the caqueta river it is shown that the late pleistocene and holocene climatic changes that took place in the andean cordilleras were related to the depositional and erosional history of the caqueta river in the colombian amazonian lowlands the low terrace sediments consist of sandy and gravelly deposits covered by clays that sometimes contain lenses of peaty material from these organic low terrace sediments seven finite radiocarbon dates were obtained of middle pleniglacial age between 56 000 and 30 000 yr bp the coarse textured basal deposits of the low terrace apparently stem from the early part of the middle pleniglaciai period during which the effective rainfall in the andes was relatively high and the andean glaciers had a considerable extension palynological data from silty sediments with organic remains at one site show an interval when drier and more open types of vegetation on poor soils must have covered a larger area than today but amazonian forest was still the dominating type of vegetation this interval might correspond to one of the middle pleniglacial savanna intervals from eastern amazonia carajas no organic sediments from the upper pleniglacial period were found and hence radiocarbon dates were not obtained in the andes this period had a very cold climate with low effective rainfall and in the east amazonian carajas area it is characterised by the relative extension of open savanna vegetation the river run off and sediment transport must have been much lower than in the middle pleniglacial and the caqueta river cut itself down in its own sediments two late glacial radiocarbon datings obtained at one site ca 12 500 yr bp indicate the existence of a late glacial sedimentation phase separated from the holocene sequence by a minor erosional phase organic layers in the holocene floodplain sediments yielded 28 radiocarbon dates between 10 000 and 355 yr bp holocene sedimentation started with the rapid deposition of sandy clay possibly in a partly permanently inundated caqueta valley during the major part of the holocene silty clays were deposited with a dominant seasonal inundation cycle

late pleistocene climate change and erosion in the santa maria basin nw argentina present erosion and sediment flux in the semiarid intra montane santa maria basin nw argentina are compared with con ditions during a period of wetter and more variable climate at about 30 000 14 c yr ago the results suggest that the influence of climate change on the overall erosional sediment budget is significant mainly because of a change in the erosion regime coupled with an increase in mass movements the most effective mechanism to increase landslide activity in this environment is a highly variable climate on interannual timescales in contrast quaternary changes in erosional budgets due to variations in moisture regimes is small in the santa maria basin because the magnitude of a potential increase in background erosion as well as enhanced landsliding is smaller than typical levels of uncer tainty of erosional budgets for such large basins it is not likely that climate driven erosional unloading can influence tectonic style and rates in this semiarid environment on time scales of several thousands to a few tens of thousands of years

formation of landslide dammed lakes during a wet period between 40 000 and 25 000 yr b p in northwestern argentina abstract new ams radiocarbon data from the deposits of landslide dammed lakes in the santa maria basin 26 s 66 w and the quebrada del toro 25 s 66 w in the eastern argentine cordillera show that important mass movements occurred during a wet period between 40 000 and 25 000 yr b p in addition to increased humidity during this time detailed analysis of annual layered lacustrine strata suggests enhanced inter and intra annual fluctuations in precipitation as compared to the present variations in sediment characteristics and diatom assemblages with mean periodicities of 2 to 5 and 10 to 14 years respectively suggest an influence of the el nino southern oscillation enso and the atlantic sea surface temperature dipole the atlantic dipole slightly increased rainfall every 10 14 years as at present in contrast the weak modern enso influence in this area differs from the past situation where rainfall increased significantly with enso type periodicities both increased humidity and seasonality are believed to have reduced thresholds for the generation of catastrophic mass movements in northwestern argentina

a 15 000 year record of climate change in northern new mexico usa inferred from isotopic and elemental contents of bog sediments elemental c n pb and isotopic d 13 c d 15 n measurements of cored sediment from a small bog in northern new mexico reveal changes in climate during the late pleistocene and holocene abrupt increases in pb concentration and d 13 c values ca 14 420 cal ybp indicate significant runoff to the shallow lake that existed at that time weathering and transport of local volcanic rocks resulted in the delivery of pb bearing minerals to the basin while a 13 c enriched terrestrial vegetation source increased the d 13 c values of the sedimentary material wet conditions developed over a 300 a period and lasted for a few hundred years the younger dryas period ca 12 700 11 500 cal ybp caused a reduction in terrestrial productivity reflected in decreasing c n values d 15 n values consistently greater than 0 and low organic content by contrast aquatic productivity increased during the second half of this period evidenced by increasing d 13 c values at the time of highest abundance of algae dry conditions ca 8 000 6 000 cal ybp were characterised by low organic carbon content and high pb concentrations the latter suggesting enhanced erosion and aeolian transport of volcanic rock the range in d 13 c d 15 n and c n values in the sedimentary record fall within the range of modern plants except during the periods of runoff and drought the sedimentary record provides evidence of natural climate variability in northern new mexico including short multi centennial and long millennial term episodes during the late pleistocene and holocene copyright 2010 john wiley sons ltd

neogene to quaternary broken foreland formation and sedimentation dynamics in the andes of nw argentina 25 s 1 the northwest argentine andes constitute a premier natural laboratory to assess the complex interactions between isolated uplifts orographic precipitation gradients and related erosion and sedimentation patterns here we present new stratigraphic observations and age information from intermontane basin sediments to elucidate the neogene to quaternary shortening history and associated sediment dynamics of the broken salta foreland this part of the andean orogen which comprises an array of basement cored range uplifts is located at 25 s and lies to the east of the arid intraorogenic altiplano puna plateau in the salta foreland spatially and temporally disparate range uplift along steeply dipping inherited faults has resulted in foreland compartmentalization with steep basin to basin precipitation gradients sediment architecture and facies associations record a three phase 10 5 and 2 ma east directed yet unsystematic evolution of shortening foreland fragmentation and ensuing changes in precipitation and sediment transport the provenance signatures of these deposits reflect the trapping of sediments in the intermontane basins of the andean hinterland as well as the evolution of a severed fluvial network present day moisture supply to the hinterland is determined by range relief and basin elevation the conspiring effects of range uplift and low rainfall help the entrapment and long term storage of sediments ultimately raising basin elevation in the hinterland which may amplify aridification in the orogen interior

enso like forcing on oceanic primary production during the late pleistocene late pleistocene changes in oceanic primary productivity along the equator in the indian and pacific oceans are revealed by quantitative changes in nanoplankton communities preserved in nine deep sea cores we show that variations in equatorial productivity are primarily caused by glacial interglacial variability and by precession controlled changes in the east west thermocline slope of the indo pacific the precession controlled variations in productivity are linked to processes similar to the southern oscillation phenomenon and they precede changes in the oxygen isotopic ratio which indicates that they are not the result of ice sheet fluctuations the 30 000 year spectral peak in the tropical indo pacific ocean productivity records is also present in the antarctica atmospheric co2 record suggesting an important role for equatorial biological productivity in modifying atmospheric co2

late pleistocene climate and water budget of the south american altiplano abstract geomorphic evidence from the peruvian bolivian altiplano indicates that during episodes prior to 28 000 and around 12 500 11 000 yr b p lakes covered an area about six and four times as large as at present respectively within the constraints of the heat and water budget model calculations are used to estimate the precipitation rate that would allow hydrologic equilibrium on this basis it is suggested that rainfall on the altiplano during the episodes of enlarged lakes was respectively some 300 and 200 mm annum 1 larger than at present representing increases of about 75 and 50 respectively field evidence suggests that the episodes of enlarged lakes on the altiplano may have preceded or coincided with periods of maximum glaciation in the neigh boring andes in this region with high elevation of the ice equilibrium line increased precipitation is particularly conducive to glaciation

orbital controls on the el nino southern oscillation and the tropical climate the global synchroneity of glacial interglacial events is one of the major problems in understanding the link between milankovitch forcing and the climate of the late quaternary in this study we isolate a part of the climate system the tropical pacific and test its sensitivity to changes in solar forcing associated with changes in the earth s orbital parameters we use a simplified coupled ocean atmosphere model that is run for the past 150 000 years and forced with milankovitch changes in the solar insolation this system responds primarily to the precessional cycle in solar forcing and is capable of generating a mean response to the changes in the seasonal distribution of solar radiation even while the annual mean insolation is roughly constant the mean response to the precessional forcing is due to an interaction between an altered seasonal cycle and the el nifio southern oscillation enso changes in the enso behavior result in a mean tropical climate change the hypothesis is advanced that such a change in the tropical climate can generate a globally synchronous climate response to milankovitch forcing

climatic controls on debris flow activity and sediment aggradation the del medio fan nw argentina in the central andes several studies on alluvial terraces and valley fills have linked sediment aggradation to periods of enhanced sediment supply however debate continues over whether tectonic or climatic factors are most important in triggering the enhanced supply the del medio catchment in the humahuaca basin eastern cordillera nw argentina is located within a transition zone between sub humid and arid climates and hosts the only active debris flow fan within this intermontane valley by combining 10be analyses of boulder and sediment samples within the del medio catchment with regional morphometric measurements of nearby catchments we identify the surface processes responsible for aggradation in the del medio fan and their likely triggers we find that the fan surface has been shaped by debris flows and channel avulsions during the last 400 years among potential tectonic climatic and autogenic factors that might influence deposition our analyses point to a combination of several favorable factors that drive aggradation these are in particular the impact of occasional abundant rainfall on steep slopes in rock types prone to failure located in a region characterized by relatively low rainfall amounts and limited transport capacity these characteristics are primarily associated with the climatic transition zone between the humid foreland and the arid orogen interior which creates an imbalance between sediment supply and sediment transfer the conditions and processes that drive aggradation in the del medio catchment today may provide a modern analog for the conditions and processes that drove aggradation in other nearby tributaries in the past

permeability evolution during non isothermal compaction in volcanic conduits and tuffisite veins implications for pressure monitoring of volcanic edifices abstract explosive volcanic eruptions are destructive geological phenomena that pose hazards of significant socioeconomic impact and potential loss of life effective risk mitigation and decision making prior to and during volcanic crises require real time monitoring of gas overpressure the single most important driving force for explosive eruptions development and release of gas overpressure are regulated by gas loss through permeable pathways that are inherently transient here we use geometry dependent conductive cooling models in concert with the most up to date welding and permeability models to assess the potential for freezing in permeability within 1 conduit filling pyroclastic deposits and 2 tuffisite veins within the edifice we find that both geometry and dimension of each deposit dictate its thermal evolution and with that its transient outgassing capacity rapid cooling of thin sheet like tuffisite veins preserves high porosities and permeabilities in contrast wide cylindrical conduit filling deposits cool slowly and permeability is annihilated over a period of minutes to hours this highlights that conduit filling deposits lose their outgassing capacity through welding while tuffisite veins previously thought to rapidly seal can form long lived outgassing features we use the model results to calculate the time dependent gas flow partitioning between both degassing lithologies based on the reconstructed outgassing pattern we outline the potential to use the gas flow balance between the central conduit and distal fumaroles fed by tuffisite veins as a simple tool to monitor gas overpressure within a volcanic edifice

modeling climate shift of el nino variability in the holocene a coupled ocean atmosphere general circulation model is used to investigate climatic shift of el nino in the holocene the model simulates a reduced enso intensity in the early and mid holocene in agreement with paleoclimate record the enso reduction is proposed to be caused by both an intensified asian summer monsoon and a warm water subduction from the south pacific into the equatorial thermocline

the effects of solar irradiation changes on the migration of the congo air boundary and water levels of paleo lake suguta northern kenya rift during the african humid period 15 5 ka bp abstract the water level record from the 300 m deep paleo lake suguta northern kenya rift during the african humid period ahp 15 5 ka bp helps to explain decadal to centennial intensity variations in the west african monsoon wam and the indian summer monsoon ism this water level record was derived from three different sources 1 grain size variations in radiocarbon dated and reservoir corrected lacustrine sediments 2 the altitudes and ages of paleo shorelines within the basin and 3 the results of hydro balance modeling providing important insights into the character of water level variations abrupt or gradual in the amplifier paleo lake suguta the results of these comprehensive analyses suggest that the ahp highstand in the suguta valley was the direct consequence of a northeastwards shift in the congo air boundary cab which was in turn caused by an enhanced atmospheric pressure gradient between east africa and india during a northern hemisphere insolation maximum rapidly decreasing water levels of up to 90 m over less than a hundred years are best explained by changes in solar irradiation either reducing the east african indian atmospheric pressure gradient and preventing the cab from reaching the study area or reducing the overall humidity in the atmosphere or a combination of both these effects in contrast although not well documented in our record we hypothesize a gradual end of the ahp despite an abrupt change in the source of precipitation when a decreasing pressure gradient between asia and africa prevented the cab from reaching the suguta valley the abruptness was probably buffered by a contemporaneous change in precession producing an insolation maximum at the equator during october whether or not this is the case the water level record from the suguta valley demonstrates the importance of both orbitally controlled insolation variations and short term changes in solar irradiation as factors affecting the significant water level variations in east african rift lakes

bajada de rahue province of neuquen argentina an interstadial deposit in northern patagonia abstract pollen and diatom analyses of a radiocarbon dated lacustrine section in the temperate andean region of argentina rahue province neuquen suggest interglacial type climatic conditions between 27 000 and 33 000 yr b p with environments that resemble the modern conditions at the locality this finding correlates with a woodland record from central chile interpreted as reflecting conditions substantially warmer and drier prior to 27 000 yr b p than during the following full glacial period it also appears to relate to the global paleoclimatic scheme derived from deep sea records suggesting interhemispheric synchroneity of such broad scale palaeoclimatic phases

welding of pyroclastic conduit infill a mechanism for cyclical explosive eruptions vulcanian style eruptions are small to moderate sized singular to cyclical events commonly having volcanic explosivity indices of 1 3 they produce pyroclastic flows disperse tephra over considerable areas and can occur as precursors to larger e g plinian eruptions the fallout deposits of the 2360 b p eruption of mount meager bc canada contain bread crusted blocks of welded breccia as accessory lithics they display a range of compaction welding intensity and provide a remarkable opportunity to constrain the nature and timescales of mechanical processes operating within explosive volcanic conduits during repose periods between eruptive cycles we address the deformation and porosity permeability reduction within natural pyroclastic deposits infilling volcanic conduits we measure the porosity permeability and ultrasonic wave velocities for a suite of samples and quantify the strain recorded by pumice clasts we explore the correlations between the physical properties and deformation fabric based on these correlations we reconstruct the deformation history within the conduit model the permeability reduction timescales and outline the implications for the repressurization of the volcanic conduit our results highlight a profound directionality in the measured physical properties of these samples related to the deformation induced fabric gas permeability varies drastically with increasing strain and decreasing porosity along the compaction direction of the fabric but varies little along the elongation direction of the fabric the deformation fabric records a combination of compaction within the conduit and postcompaction stretching associated with subsequent eruption model timescales of these processes are in good agreement with repose periods of cyclic vulcanian eruptions

an upper ordovician sponge bearing micritic limestone and implication for early palaeozoic carbonate successions abstract a potentially new type of non reef sponge bearing micritic limestone is reported from the upper ordovician xiazhen formation of south eastern china the sponges are preserved as incomplete skeletons that consist of curved bifurcated and trifurcated spicules embedded in dark micrite and can only be recognized under a petrographic microscope the characteristics of the spicule networks suggest that the sponges are probably belonging to demosponges however based on the absence of features such as desma zygome a distinct dermal layer and a canal system they are not considered to be lithistids the majority of the sponges are found in a lime mudstone facies together with some micritic portions of wackestone to grainstone facies comprising approximately 13 of the 50 m thick micritic limestone successions it is interpreted that the non lithistid demosponges flourished on soft substrates over shallow marine carbonate platform such sponge bearing carbonates are similar to spiculites and spongolites in terms of being a major constituent of the sedimentary rocks and their potential contribution as sediment producers but affinity and modes of preservation of the xiazhen sponges are significantly different to those of the spiculites and spongolites in light of the present finding it is suggested that non lithistid demosponges may have been more widespread in early palaeozoic non reef carbonates than has previously been recognized thus indicating the critical need for further detailed studies if we are to understand their distributions and sedimentological contributions

mount st helens and santiaguito lava domes the effect of short term eruption rate on surface texture and degassing processes in order to assess the effect of eruption rate on the surface morphology and degassing mechanisms of silicic lava flows we studied surface characteristics and obtained water content and hydrogen isotopic values of samples from flows at the mount st helens and santiaguito lava domes we compared the surface textures and inferred degassing processes to short term extrusion rates and found that when domes are small and eruption rates are high lava will not completely degas in transit to the surface allowing additional volatile loss through surface vesiculation which results in the formation of a scoriaceous carapace when domes exceed a critical size and or their cooled crusts reach a critical strength emergence of new magma is impeded short term eruption rates decline and more thorough degassing can take place leading to smooth textured flows lacking scoria development at mount st helens this transition occurred during the dome s third year when it grew from 31 8 to 53 2 106 m3 santiaguito attained a comparable state after 2 3 years of growth and for most of its 70 year history has produced non vesicular lava degassing patterns that combine closed open and kinetic processes can be distinguished using isotope data obtained from samples whose positions on a flow and emplacement histories are well constrained evidence for these patterns is most clearly preserved in lavas erupted during early rapid stages of dome growth petrologists and volcanologists seeking to infer magma chamber conditions from the volatile contents of extruded lavas thus need to sample flows early in their emplacement while paying attention to surface texture position relative to the vent and flow front and time of emergence

cyclic shear zone cataclasis and sintering during lava dome extrusion insights from chaos crags lassen volcanic center usa abstract the ascent and extrusion of crystal rich magma is commonly facilitated by deformation partitioned within annular conduit parallel shear zones the physical properties and textures of the shear zone materials where exposed at surface provide a record of ascent and eruption dynamics we describe the shear zone developed in dome c part of chaos crags in the lassen volcanic center california usa the extruded shear zone comprises volcanic fault gouge and variably densified cataclasites the competent cataclasites evidence deep seated gouge production followed by gouge densification within the conduit on the timescale of lava dome ascent textural geochemical and mineralogical data identify solid state sintering as the densification mechanism at the temperatures and pressures in the volcanic conduit solid state sintering causes rapid porosity and permeability loss within the gouge and concomitant material strengthening longer dwell times i e slower ascent allow for more sintering producing stronger denser and less permeable cataclasites at chaos crags we use the extent of sintering quantified by residual porosity to recover minimum in conduit dwell times necessary to produce the observed cataclasites our analysis of the dome c cataclasites suggests a maximum linear ascent rate of 10 m day and a minimum ascent time of 100 days we evaluate the consequences of shear zone lithification by solid state sintering for the eruption of other crystal rich glass poor magmas chaos crags cataclasites preserve evidence of multiple cycles of fracturing cataclasis and re sintering suggesting a mechanism for transitions between effusive and explosive phases of dome building eruptions

crouching shells hidden sponges unusual late ordovician cavities containing sponges abstract marine cavities harbouring cryptic organisms have been ubiquitous throughout the phanerozoic however our knowledge of early cryptic communities is as yet insufficient and how metazoans began to utilize such habitats remains unknown in this study we document demosponge remains within intraskeletal cavities embedded in the micritic succession of a shallow carbonate platform in the upper ordovician katian xiazhen formation of south china molluscs gastropods bivalves and nautiloids and corals the solitary rugosan tryplasma and colonial agetolitids within the succession commonly contain patches of spicular demosponge remains 11 n 45 415 mainly occupying intraskeletal spaces with areas of 1 30 mm2 in thin section sponge occurrence varies according to sedimentary facies within lime mudstone facies sponges commonly occur both inside and outside intraskeletal cavities suggesting that sponges would have inhabited and become preserved within any available space in this environment in contrast when other sessile organisms co occur in wackestone to packstone facies there are fewer sponge occurrences both inside and outside cavities possibly due to competition in open habitats and or their poorer preservation in such environments overall this result suggests that sponges would have exploited cryptic habitats by normal expansion of the open surface biota in addition compared with coeval reef and hardground crypts the xiazhen intraskeletal cryptic biota is monotonous in composition suggesting decoupled occupation of cryptic habitats in different environments

multiple landslide clusters record quaternary climate changes in the northwestern argentine andes the chronology of multiple landslide deposits and related lake sediments in the semi arid eastern argentine cordillera suggests that major mass movements cluster in two time periods during the quaternary i e between 40 and 25 and after 5 14 c kyr bp these clusters may correspond to the minchin maximum at around 28 27 14 c kyr bp and titicaca wet periods after 3 9 14 c kyr bp the more humid conditions apparently caused enhanced landsliding in this environment in contrast no landslide related damming and associated lake sediments occurred during the coipasa 11 5 10 14 c yr bp and tauca wet periods 14 5 11 14 c yr bp the two clusters at 40 25 and after 5 14 c kyr bp may correspond to periods where the el nin o southern oscillation enso and tropical atlantic sea surface temperature variability tav were active this however was not the case during the coipasa and tauca wet periods lake balance modelling of a landslide dammed lake suggests a 10 15 increase in precipitation and a 3 4 c decrease in temperature at v30 14 c kyr bp as compared to the present in addition time series analysis reveals a strong enso and tav during that time the landslide clusters in northwestern argentina are therefore best explained by periods of more humid and more variable climates 2003 elsevier science b v all rights reserved

glacial chronology and production rate cross calibration of five cosmogenic nuclide and mineral systems from the southern central andean plateau abstract glacial deposits on the high altitude arid southern central andean plateau cap the puna in northwestern argentina document past changes in climate but the associated geomorphic features have rarely been directly dated this study provides direct age control of glacial moraine deposits from the central puna 24 s at elevations of 3900 5000 m through surface exposure dating with cosmogenic nuclides our results show that the most extensive glaciations occurred before 95 ka and an additional major advance occurred between 46 and 39 ka the latter period is synchronous with the highest lake levels in the nearby pozuelos basin and the minchin inca huasi wet phase on the altiplano in the northern cap none of the dated moraines produced boulder ages corresponding to the tauca wet phase 24 15 ka additionally the volcanic lithologies of the deposits allow us to establish production ratios at low latitude and high elevation for five different nuclide and mineral systems 10be 21ne and 26al from quartz 11 or 12 samples and 3he and 21ne from pyroxene 10 samples we present production ratios for all combinations of the measured nuclides and cross calibrated production rates for 21ne in pyroxene and quartz for the high sub tropical andes the production rates are based on our 10be normalized production ratios and a weighted mean of reference 10be production rates calibrated in the high tropical andes 4 02 0 12 at g 1 yr 1 these are 21neqtz 18 1 1 2 at g 1 yr 1 and 21nepx 36 6 1 8 at g 1 yr 1 en 88 94 scaled to sea level and high latitude using the lal stone scheme with 1 uncertainties as 3he and 26al have been directly calibrated in the tropical andes we recommend using those rates finally we compare exposure ages calculated using all measured cosmogenic nuclides from each sample including 11 feldspar samples measured for 36cl and a suite of previously published production rates

late quaternary spatio temporal limnological variations in the altiplano of bolivia and peru abstract between the western and eastern andean cordilleras in peru and bolivia there are three main lacustrine basins lake titicaca lake poopo and the group of coipasa uyuni for the past few millennia highly variable environmental conditions have been recorded in their sediments today a latitudinal meteorological gradient influences the lakes status leading to specific deposits and ostracod communities lake titicaca in the north is oligohaline whereas lake poopo further south is polyhaline in the south the coipasa uyuni depression is characterized by a 12 000 km2 surficial salt crust during the late pleistocene ca 40 000 to 25 000 yr b p the water depth and salinity in paleolake poopo fluctuated widely and paleolake titicaca was slightly larger than at present sedimentation was mostly biocarbonate in the shallower areas and it was detrital organic in the deepest zones during the holocene a dry period transformed lake poopo into a salar with evaporite precipitation lake titicaca registered a large decline in water level 8100 3600 yr b p initially inducing gypsum precipitation followed by short influxes of water with an ostracod faunal composition similar to that of the modern brines of lake poopo lake titicacas present condition only appeared between 2200 and 1500 yr b p

classifying past climate change in the chew bahir basin southern ethiopia using recurrence quantification analysis the chew bahir drilling project cbdp aims to test possible linkages between climate and evolution in africa through the analysis of sediment cores that have recorded environmental changes in the chew bahir basin in this statistical project we consider the chew bahir palaeolake to be a dynamical system consisting of interactions between its different components such as the waterbody the sediment beneath lake and the organisms living within and around the lake recurrence is a common feature of such dynamical systems with recurring patterns in the state of the system reflecting typical influences identifying and defining these influences contributes significantly to our understanding of the dynamics of the system different recurring changes in precipitation evaporation and wind speed in the chew bahir basin could result in similar but not identical conditions in the lake e g depth and area of the lake alkalinity and salinity of the lake water species assemblages in the water body and diagenesis in the sediments recurrence plots rps are graphic displays of such recurring states within a system measures of complexity were subsequently introduced to complement the visual inspection of recurrence plots and provide quantitative descriptions for use in recurrence quantification analysis rqa we present and discuss herein results from an rqa on the environmental record from six short 17 m sediment cores collected during the cbdp spanning the last 45 kyrs the different types of variability and transitions in these records were classified to improve our understanding of the response of the biosphere to climate change and especially the response of humans in the area

timing of past glaciation at the sierra de aconquija northwestern argentina and throughout the central andes abstract advances in cosmogenic nuclide exposure dating have made moraines valuable terrestrial recorders of palaeoclimate a growing number of moraine chronologies reported from the central andes show that tropical glaciers responded sensitively to past changes in precipitation and temperature over timescales ranging from 103 to 105 years however the causes of past glaciation in the central andes remain uncertain explanations have invoked insolation modulated variability in the strength of the south american summer monsoon teleconnections with the north atlantic ocean and or cooling in the southern hemisphere the driver for these past climate changes is difficult to identify partly due to a lack of dated moraine records especially in climatically sensitive areas of the southern central andes moreover new constraints are needed on precisely where and when glaciers advanced we use cosmogenic 10be produced in situ to determine exposure ages for three generations of moraines at the sierra de aconquija situated at 27 s on the eastern flank of the southern central andes these moraines record glacier advances at approximately 22 ka and 40 ka coincident with summer insolation maxima in the sub tropics of the southern hemisphere as well as at 12 5 ka and 13 5 ka during the younger dryas and the antarctic cold reversal respectively we also identify minor glaciation during bond event 5 also known as the 8 2 ka event these moraines register past climate changes with high fidelity and currently constitute the southernmost dated record of glaciation on the eastern flank of the central andes to contextualise these results we compile 10be data reported from 144 moraines in the eastern central andes that represent past glacier advances we re calculate exposure ages from these data using an updated reference production rate and we re interpret the moraine ages by taking the oldest clustered boulder age after the exclusion of outliers attributed to nuclide inheritance as closest to the timing of glacier advance an approach for which we provide empirical justification this compilation reveals that central andean glaciers have responded to changes in temperature and precipitation we identify cross latitude advances in phase with insolation cycles the last global glacial maximum and episodes of strengthened monsoonal moisture transport including the younger dryas and heinrich stadials 1 and 2 our results from the sierra de aconquija allow us to constrain the southerly limit of enhanced precipitation associated with heinrich stadials at 25 s more broadly our findings demonstrate at both local and regional scales that moraines record past climate variability with a fine spatial and temporal resolution

ordovician reef systems and settings in south china before the late ordovician mass extinction abstract ordovician reefs of the south china block occur chiefly in three stratigraphic units 1 the middle tremadoc fenhsiang formation and the upper part of the lunshan formation 2 the late tremadoc hunghuayuan formation and 3 the middle ashgill xiazhen sanjushan and daduhe formations the region therefore records part of the early palaeozoic reef expansion episode and permits a broader assessment of change in ordovician reef facies during middle tremadoc time lithistid sponge bryozoan calathium calcimicrobially dominated patch reefs of the fenhsiang formation occur in the high energy belt of the central yangtze platform columnar nonskeletal stromatolites of the lunshan formation coeval with fenhsiang formation occur on the southeastern margin of the platform where low diversity reef attached organisms and the generally fine grained character of the sediment are consistent with a depth below normal wave base calathium and lithistid sponges were the principal reef builders of late tremadoc reefs and were widespread in the areas of yichang hubei province and dongzhi anhui province from the platform centre to its margin settings bryozoan reef builders occur only in the platform centre with a high diversity of reef dwellers such as brachiopods trilobites and nautiloids however during the same time at the platform margins microbes played an important part in reef building together with calathium and lithistid sponges the middle late tremadoc lithistid sponge bryozoan calathium microbial community was replaced by a middle ashgill coral stromatoporoid community and shows that the succession of community replacement during the ordovician took a relatively long time restricted by the black shales of the main part of the yangtze region middle ashgill reef complexes can be found only on the northeast platform of cathaysian land between the yushan jiangxi and changshan zhejiang regions carbonate mudmounds are present on the western margin of the yangtze platform on the northeast platform of cathaysian land patch reefs some higher relief with talus and biostromes of the xiazhen formation consist of high diversity biotas of corals stromatoporoids calcimicrobes brachiopods and gastropods the sanjushan formation is age equivalent to the xiazhen formation and contains carbonate mudmounds that are composed of abundant calcareous algae and calcimicrobia uplift forced a northward extension of cathaysian land and caused a regional relative sea level fall eliminating this reef complex prior to the first extinction event of the late ordovician the daduhe formation carbonate mudmounds occur in the nearshore belt of kangdian land western margin of the yangtze platform and are paraconformably overlain by the hirnantian nancheng formation in total the ordovician reefs of south china show a range of habitats and controls on growth and demise

lacustrine chronology links late pleistocene climate change and mass movements in northern new mexico well dated lacustrine deposits in northern white rock canyon new mexico record damming of the rio grande by at least four separate failures of a slump complex between about 17 5 and 12 4 ka sup 14 c linking mass movements to a period of rapid climate change in the western united states failure of metastable slumps probably resulted from removal of lateral support during down cutting and erosion by the rio grande and from a decrease in resisting forces due to increased pore pressures our chronology suggests that the lake that formed between 17 5 and 15 0 ka may record effects of both glacial melt and pluvial activity mainly enhanced rainfall the youngest lake neary 12 4 ka may record pluvial runoff and the intermediate lakes 13 7 to 13 1 ka may record pluvial and minor melt water activity we have not found lacustrine deposits younger than about 12 4 ka the record of geomorphic response along the rio grande suggests that late pleistocene climatic changes may have triggered similar mass movements elsewhere in the southwest 27 refs 4 figs

strain induced outgassing of three phase magmas during simple shear a major factor determining the explosivity of silicic eruptions is the removal of volatiles from magma through permeability controlled outgassing we studied the microstructural development of permeability during deformation of highly viscous magma by performing simple shear experiments on bubble 0 12 0 36 volume fraction and crystal bearing 0 0 42 volume fraction silicate melts experiments were performed under torsion at high temperature and pressure 723 873 k and 150 200 mpa in a paterson deformation apparatus at bulk shear strains between 0 and 10 the experimental setup allows for gas escape if bubble connectivity is reached on the sample periphery three dimensional imaging and analysis of deformed bubbles was performed using x ray tomography the development of localized deformation in all samples enhanced by crystal content leads to brittle fracture at bulk strains 2 and sample wide fracturing in samples deformed to strains 5 a decrease in both bubble fraction and dissolved volatile content with increasing strain along with strain hardening rheological behavior suggests significant shear induced outgassing through the fracture networks applicable to shallow conduit degassing in magmas containing crystal fractions of 0 0 42 this study contributes to our understanding of highly viscous magma outgassing and processes governing the effusive explosive transition

fluvial perturbance in the western amazon basin regulation by long term sub andean tectonics haffer s refuge theory proposes that during the arid climatic phases of the late pleistocene tropical lowland forests of amazonia were reduced to isolated patches contributing to the high species richness of the present day forest the theory was developed because no obvious historic or modern geomorphic isolation barriers were recorded in amazonia analyses of radar images combined with stratigraphical data show that in the basinal forelands of the tectonically active andes the geological setting causes long term fluvial perturbance this leads to a temporally structured highly complex mosaic of fossil and present floodplains these dynamics have been present with varying activity and geographic range during the tertiary and quaternary providing site turnover that has not been recognized by the biogeographic tradition of the amazon basin

spatiotemporal trends in erosion rates across a pronounced rainfall gradient examples from the southern central andes abstract the tectonic and climatic boundary conditions of the broken foreland and the orogen interior of the southern central andes of northwestern argentina cause strong contrasts in elevation rainfall and surface process regimes the climatic gradient in this region ranges from the wet windward eastern flanks 2 m yr rainfall to progressively drier western basins and ranges 0 1 m yr bordering the arid altiplano puna plateau in this study we analyze the impact of spatiotemporal climatic gradients on surface erosion first we present 41 new catchment mean erosion rates derived from cosmogenic nuclide inventories to document spatial erosion patterns second we re evaluate paleoclimatic records from the calchaquies basin 66 w 26 s a large intermontane basin bordered by high 4 5 km mountain ranges to demonstrate temporal variations in erosion rates associated with changing climatic boundary conditions during the late pleistocene and holocene three key observations in this region emphasize the importance of climatic parameters on the efficiency of surface processes in space and time 1 first order spatial patterns of erosion rates can be explained by a simple specific stream power ssp approach we explicitly account for discharge by routing high resolution satellite derived rainfall this is important as the steep climatic gradient results in a highly nonlinear relation between drainage area and discharge this relation indicates that erosion rates er scale with er ssp1 4 on cosmogenic nuclide time scales 2 we identify an intrinsic channel slope behavior in different climatic compartments channel slopes in dry areas 0 75 m yr with equal drainage areas thus compensating lower amounts of discharge with steeper slopes 3 erosion rates can vary by an order of magnitude between presently dry 0 05 mm yr and well defined late pleistocene humid 0 5 mm yr conditions within an intermontane basin overall we document a strong climatic impact on erosion rates and channel slopes we suggest that rainfall reaching areas with steeper channel slopes in the orogen interior during wetter climate periods results in intensified sediment mass transport which is primarily responsible for maintaining the balance between surface uplift erosion sediment routing and transient storage in the orogen

viscosity of high crystal content melts dependence on solid fraction 1 the rheological properties of suspensions containing high solid fractions are investigated attention is focused on viscosity of silicate and magmatic melt systems a general empirical equation which describes the relative viscosity of suspensions as a function of suspended solid fraction is proposed in the limit of very dilute solid concentrations it reduces to the einstein equation the proposed relationship is satisfactorily applied to reproduce available experimental data relative to silicate melts moreover the extrapolation of the model to very high concentrations is compared with experimental observations on partially melted granite

uplift of the central andes of nw argentina associated with upper crustal shortening revealed by multiproxy isotopic analyses this study contributes to the uplift history of the andes which has received increasing attention in recent years because of its implications for geodynamic models and climate feedbacks shortening resulting in crustal thickening and removal of gravitationally unstable mantle lithosphere has been proposed to control deformation and uplift of cordillera type orogenic systems such as the puna plateau of the central andes and its eastern margin the eastern cordillera we present new clumped isotope 47 18o and 2 data from carbonate nodules marlstone spring deposits and volcanic ashes from the puna plateau and eastern cordillera of nw argentina when combined with other geological evidence our data indicate that the puna plateau was near its present elevation since at least 10 ma whereas the eastern cordillera rose 1 5 km between 14 and 7 ma this history of uplift correlates with active shortening in the eastern cordillera and with incorporation of a regional foreland into the propagating orogenic wedge our study suggests that the elevation of the puna plateau changed little during the miocene pliocene whereas the margin experienced significant uplift associated with active deformation and crustal thickening

variation in holocene el nino frequencies climate records and cultural consequences in ancient peru analysis of mollusks from archaeological sites on the north and central coasts of peru indicates that between ca 5800 and 3200 2800 cal yr b p el nino events were less frequent than today with modern rapid recurrence intervals achieved only after that time for several millennia prior to 5 8 ka el nino events had been absent or very different from today the phenomena called el nino have had severe consequences for the modern and colonial historically recorded inhabitants of peru and el nino events also influenced prehistoric cultural development the onset of el nino events at 5 8 ka correlates temporally with the beginning of monumental temple construction on the peruvian coast and the increase in el nino frequency after 3 2 2 8 ka correlates with the abandonment of monumental temples in the same region

comparing modern and pleistocene enso like influences in nw argentina using nonlinear time series analysis methods higher variability in rainfall and river discharge could be of major importance in landslide generation in the northwestern argentine andes annual layered varved deposits of a landslide dammed lake in the santa maria basin 26 s 66 w with an age of 30 000 14c years provide an archive of precipitation variability during this time the comparison of these data with present day rainfall observations tests the hypothesis that increased rainfall variability played a major role in landslide generation a potential cause of such variability is the el nino southern oscillation enso the causal link between enso and local rainfall is quantified by using a new method of nonlinear data analysis the quantitative analysis of cross recurrence plots crp this method seeks similarities in the dynamics of two different processes such as an ocean atmosphere oscillation and local rainfall our analysis reveals significant similarities in the statistics of both modern and palaeo precipitation data the similarities in the data suggest that an enso like influence on local rainfall was present at around 30 000 14c years ago increased rainfall which was inferred from a lake balance modeling in a previous study together with enso like cyclicities could help to explain the clustering of landslides at around 30 000 14c years ago

summertime precipitation variability over south america role of the large scale circulation abstract the observed large scale circulation mechanisms associated with summertime precipitation variability over south america are investigated particular attention is paid to the altiplano where a close relationship has been observed between rainfall and the position and intensity of the bolivian high empirical orthogonal function eof correlation and composite analyses suggest that on intraseasonal timescales typically 5 20 days rainy periods on the altiplano are associated with at least three types of circulation anomalies involving either extratropical cyclones cold core lows or the westward enhancement of the south atlantic high in each instance the primary support for high rainfall rates is a moist poleward flow at low levels along the eastern flank of the central andes in association with the south atlantic convergence zone sacz the warm low level flow along the sacz also inflates the overlying atmospheric column resulting in an intensification and southward shift of the bolivi

middle cambrian siliceous sponge calcimicrobe buildups daegi formation korea metazoan buildup constituents in the aftermath of the early cambrian extinction event abstract numerous decimetre to metre scale carbonate buildups dominated by siliceous sponges and the calcimicrobe epiphyton are reported from the middle cambrian series 3 daegi formation of korea these siliceous sponge epiphyton buildups consist predominantly of grey micritic boundstones with dark clots and or white clumps the boundstones contain sponge spicule networks interpreted as the calcified remains of siliceous sponges the white clumps and dark clots in the boundstones represent variously preserved epiphyton siliceous sponges form constructional pore space and are commonly encrusted by epiphyton the sponges were probably the primary frame builders providing substrates for the attachment and subsequent growth of epiphyton epiphyton is considered to be a binder when covering the surface of siliceous sponges and a subordinate frame builder when filling depositional voids created by siliceous sponges or growing on top of other epiphyton growth bundles the siliceous sponge epiphyton buildups of the daegi formation show similarities to previously described late cambrian furongian anthaspidellid calcimicrobe buildups from iran and the usa together with recently reported examples from the zhangxia formation of eastern china the sponge epiphyton buildups from korea represent some of the oldest metazoan calcimicrobe buildups after the extinction of most archaeocyaths at the end of the early cambrian series 2 this implies that the incorporation of metazoans in middle cambrian series 3 carbonate buildups occurred much earlier than previously known the buildup forming siliceous sponges described in this study demonstrate that their role in early phanerozoic carbonate buildups has been grossly underestimated

directional orientations of solitary rugose corals unattached solitary rugose corals lying parallel to bedding are common in many ordovician and younger paleozoic units but their directional orientations have seldom been examined interpretations based on occurrences in north american upper ordovician and lower silurian strata provide a foundation for such studies distinct directional patterns resulted from preferred orientation with respect to water motion they indicate transportation rather than biologic orientation if it can be shown that the corals were not preserved in life position or were abraded to a significant degree before burial transported slightly curved specimens having trochoid to ceratoid form were aligned parallel to currents with the apex pointing upstream and or were rolled nearly perpendicular to currents or almost parallel to crests of advancing waves with the apex facing either way but directed slightly upstream therefore unimodal orientation patterns bimodal patterns with equal peaks that are opposite one another but

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

effects of gas escape and crystallization on the complexity of conduit flow dynamics during lava dome eruptions 1 we investigated the coupled effects of gas escape and crystallization on the dynamics of lava dome eruptions using a one dimensional conduit flow model the relationship between chamber pressure pch and mass flow rate q for steady conduit flow commonly has a regime of negative differential resistance i e dpch dq 0 5 feedback 1 is the main mechanism that forms negative differential resistance in this case the transition from lava dome to explosive eruption occurs when the magma supply rate exceeds a fixed critical value for low phenocryst content magma volume fraction 0 5 feedback 2 plays a key role so that the transition is controlled by the permeability of the surrounding rocks the critical magma supply rate remarkably decreases with decreasing permeability transition due to feedback 2 is associated with a change in the chemical composition of volcanic gas a drastic increase in magma porosity from nearly 0 to greater than 0 8 and overpressure at a shallower level which can be detected from geochemical and geophysical field observations

mechanisms of lake level change at laguna potrok aike argentina insights from hydrological balance calculations abstract laguna potrok aike is an exceptional site in the southern hemisphere s mid latitudes because it records changes in the hydrological regime through lake level variations in a continuous high resolution sediment record back into the late pleistocene in this study driving forces of lake level changes at laguna potrok aike are evaluated by means of process studies using data from an extensive multi year monitoring in and around the lake lake volume changes were calculated with an energy budget bulk transfer approach and translated into lake level variations which were then compared to pressure sensor data calculated lake levels are in broad agreement with measured data we hypothesize that on short time scales lake level fluctuations are mainly driven by the precipitation to evaporation ratio apart from changing catchment conditions relative humidity precipitation temperature wind strength and wind direction have the most important influence on the hydrological balance of the lake lake level decreases during periods of persistently high wind speeds from westerly directions whereas during periods with more frequent occurrences of easterly winds it increases these situations are linked to a strengthening of the southern hemispheric westerlies in the first and more frequent blocking situations in the latter case although lake level changes at laguna potrok aike show some degree of similarity to variations of the el nino southern oscillation enso and the southern hemisphere annular mode sam a persistent correspondence remains to be documented water chemistry and sediment trap data suggest that lake internal carbonate precipitation is highly sensitive to short term changes in the lake water volume and thus to lake level variations however it becomes obvious that the sedimentary carbonate record representing changes on longer time scales is not linearly linked to lake level changes hence complicating quantitative lake level reconstructions back in time we suggest that short term changes in authigenic carbonate production are mainly driven by changing precipitation evaporation ratios probably superimposed on longer term changes in groundwater input that represent a buffered climate signal scenario calculations for a period of 60 years show that changes of the controlling meteorological parameters in the range of 15 17 can lead to lake level changes with a magnitude comparable to the reconstructed holocene and lateglacial extreme situations in addition modifications in the water retaining capacity of the lake can also produce large lake level changes it is hypothesized that the development or disappearance of permafrost in the catchment of laguna potrok aike during the last glacial interglacial transition may have changed the water retaining capacity tremendously the lake level reconstructions for laguna potrok aike might express some of the meridional climate variability observed in coupled general circulation model cgcm simulations for southern south america for the last glacial maximum lgm and the mid holocene

remotely sensed evapotranspiration estimates for an improved hydrological modeling of the early holocene mega lake suguta northern kenya rift abstract the actual evapotranspiration is an important but difficult to determine element in the water balance of lakes and their catchment areas reliable data on evapotranspiration are not available for most lake basins for which paleoclimate reconstructions and modeling have been performed particularly those in remote parts of africa we have used thermal infrared multispectral data for 14 aster scenes from the terra satellite to estimate the actual evapotranspiration in the 12 800 km2 catchment of the suguta valley northern kenya rift evidence from sediments and paleo shorelines indicates that during the african humid period ahp 14 8 to 5 5 kyrs bp this valley contained a large lake 280 m deep and covering 2200 km2 which has now virtually disappeared evapotranspiration estimates for the suguta basin were generated using the surface energy balance algorithm for land sebal climate data required for the model were extracted from a high resolution gridded dataset obtained from the climatic research unit east anglia uk results suggest significant spatial variations in evapotranspiration within the catchment area ranging from 450 mm yr in the basin to the north to 2000 mm yr in more elevated areas and precipitation that was 20 higher during the ahp than in recent times these results are in agreement with other estimates of paleo precipitation in east africa the extreme response of the lake system 280 m greater water depth than today and a lake surface area of 2200 km2 to only moderately higher precipitation illustrates the possible sensitivity of this area to future climate change

ascent driven crystallisation of dacite magmas at mount st helens 1980 1986 we introduce a novel scheme that enables natural silicic glasses to be projected into the synthetic system qz ab or h2o in order to relate variations in volcanic glass chemistry to changing pressure p and temperature t conditions in the sub volcanic magma system by this means an important distinction can be made between ascent driven and cooling driven crystallisation under water saturated or undersaturated conditions in samples containing feldspar and a silica phase quartz or tridymite quantitative p t estimates of the conditions of last equilibrium between crystals and melt can be made formation of highly silicic melts i e 77 wt sio2 is a simple consequence of the contraction of the silica phase volume with decreasing pressure such that high silica glasses can only form by crystallisation at low pressure resorption of quartz crystals appears to be a further diagnostic feature of decompression crystallisation groundmass and inclusion glasses in dacites from the 1980 1986 eruption of mount st helens volcano wa span a wide range in sio2 68 80 wt anhydrous the compositions of the least evolved sio2 poor inclusions in amphibole phenocrysts record entrapment of silicic liquids with 5 4 wt water corresponding to a water saturation pressure of 200 mpa at 900 c the compositions of more evolved higher sio2 plagioclase hosted inclusions and groundmass glasses are consistent with extensive ascent driven fractional crystallisation of plagioclase oxide and orthopyroxene phenocrysts and microlites to low pressures during this polybaric crystallisation plagioclase phenocrysts trapped melts with a wide range of dissolved water contents 3 5 5 7 wt magmas erupted during the plinian phase of the 18 may 1980 eruption were derived from a large reservoir at depths of 6 km subsequent magmas ascended to varying depths within the sub volcanic system prior to extraction from glass chemistry and groundmass texture two arrest levels have been identified at depths of 0 5 1 and 2 4 km a single dome sample from february 1983 contains groundmass plagioclase tridymite and quartz testifying to temperatures of at least 885 c at 11 mpa these shallow storage conditions are comparable to those in the cryptodome formed during spring 1980 the corresponding thermal gradient 0 2 c mpa 1 is consistent with near adiabatic magma ascent from 8 km we argue that the crystallisation history of mount st helens dacite magma was largely a consequence of decompression crystallisation of hot magma beyond the point of water saturation this challenges the conventional view that phenocryst crystallisation occurred by cooling in a large magma chamber prior to the 1980 1986 eruption because the crystallisation process is both polybaric and fractional it cannot be simulated directly using isobaric equilibrium crystallisation experiments however calculation of the phase proportions in water saturated 910 15 c experiments by rutherford et al 1985 over the pressure range 220 125 mpa reproduces the crystallisation sequence and phenocryst modes of mount st helens dacites from 18 may 1980 by allowing for the effects of fractional versus equilibrium crystallisation entrained residual source material and small temperature differences between nature and experiment phase compositions can also be matched to the natural samples we conclude that decompression of water saturated magma may be the dominant driving force for crystallisation at many other silicic volcanic centres

late quaternary intensified monsoon phases control landscape evolution in the northwest himalaya the intensity of the asian summer monsoon circulation varies over decadal to millennial time scales and is reflected in changes in surface processes terrestrial environments and marine sedi ment records however the mechanisms of long lived 2 5 k y intensified monsoon phases the related changes in precipitation distribution and their effect on landscape evolution and sedimen tation rates are not yet well understood the arid high elevation sectors of the orogen correspond to a climatically sensitive zone that currently receives rain only during abnormal i e strength ened monsoon seasons analogous to present day rainfall anom alies enhanced precipitation during an intensified monsoon phase is expected to have penetrated far into these geomorphic threshold regions where hillslopes are close to the angle of failure we as sociate landslide triggering during intensified monsoon phases with enhanced precipitation discharge and sediment flux leading to an increase in pore water pressure lateral scouring of rivers and ov ersteepening of hillslopes eventually resulting in failure of slopes and exceptionally large mass movements here we use lacustrine deposits related to spatially and temporally clustered large land slides 0 5 km 3 in the sutlej valley region of the northwest him alaya to calculate sedimentation rates and to infer rainfall patterns during late pleistocene 29 24 ka and holocene 10 4 ka inten sified monsoon phases compared to present day sediment flux measurements a fivefold increase in sediment transport rates re corded by sediments in landslide dammed lakes characterized these episodes of high climatic variability these changes thus em phasize the pronounced imprint of millennial scale climate change on surface processes and landscape evolution

late pleistocene landslide dammed lakes along the rio grande white rock canyon new mexico massive slump complexes composed of pliocene basaltic rocks and underlying miocene and pliocene sediments flank the rio grande along 16 km of northern white rock canyon new mexico the toe area of at least one slump complex was active in the late pleistocene damming the rio grande at least four times during the period from 18 to 12 sup 14 c ka and impounding lakes that extended 10 20 km upriver stratigraphic relationships and radiocarbon age constraints indicate that three separate lakes formed between 13 7 and 12 4 sup 14 c ka the age and dimensions of the ca 12 4 ka lake are best constrained it had an estimated maximum depth of approx 30 m a length of approx 13 km a surface area of approx 2 7 km sup 2 and an initial volume of approx 2 5 x 10 sup 7 m sup 3 the youngest landslide dammed lakes formed during a period of significantly wetter regional climate strongly suggesting that climate changes were responsible for reactivation of the slump complexes we are not certain about the exact triggering mechanisms for these landslides but they probably involved removal of lateral support due to erosion of the slope base by the rio grande during periods of exceptionally high flood discharge or rapid incision more increased pore pressures associated with higher water tables higher seepage forces at sites of ground water discharge or some combination of these processes seismic shaking could also have contributed to triggering of some of the landslides particularly if aided by wet antecedent conditions 54 refs 19 figs 3 tabs less

temperature and precipitation in the southern central andes during the last glacial maximum heinrich stadial 1 and the younger dryas abstract recent developments in terrestrial cosmogenic nuclide tcn exposure dating and the reinterpretation of tcn boulder ages from moraines have improved our understanding of the glacial chronology in the central andes according to these records glacial advances throughout the region correlate with insolation driven changes in the intensity of the south american summer monsoon and millennial scale climate events such as heinrich stadials and the younger dryas quantifying the temperature and precipitation shifts during these events helps to constrain past moisture pathways and associated changes in atmospheric circulation patterns yet particularly in the southern central andes where a wealth of glacigenic landforms attests to formerly cooler and or wetter conditions the magnitudes of past temperature and precipitation changes are only loosely constrained at a few sites here we present results from tcn dated moraines combined with the reconstruction of former glaciers and paleolakes within the eastern sector of the southern central andes 24 27 s for the last glacial maximum lgm heinrich stadial 1 hs1 and the younger dryas yd we performed monte carlo simulations with 2 dimensional models of ice flow and lake hydrology that are forced by a spatially distributed surface energy balance model our results indicate that temperatures were 3 0 4 2 c 2 0 3 3 c and 1 3 2 5 c cooler during the lgm hs1 and yd respectively in agreement with previous estimates elsewhere in the central andes we find that temperature changes during the late glacial are compatible with sea surface temperature anomalies derived from the tropical atlantic precipitation was only 5 27 greater than today which contrasts with larger anomalies reconstructed for the bolivian part of the andean plateau altiplano we attribute this discrepancy to the southerly position of our study region with respect to the bolivian high supporting the hypothesis that this atmospheric pressure system played a prominent role for south american summer monsoon dynamics during glacial episodes

multiscale analysis of the summertime precipitation over the central andes precipitation over the central andes in south america exhibits a marked annual march with most of the rainfall concentrated during the austral summer season december february when the atmospheric circulation favors the uplifting of moist air from the lowlands to the east of the mountain range within its rainy season the central andes experiences week long rainy and dry episodes the large scale and local conditions during these episodes are investigated using satellite imagery reanalyzed atmospheric fields and in situ data despite the deep layer of conditional instability prevalent during most summertime afternoons deep convection can occur only on those days in which the mixing ratio within the local boundary layer exceeds some threshold 7gk g 21 yielding saturation of near surface air parcels rising more than 600 m above ground convective cloudiness anomalies over the central andes extend southeastward and tend to be concurrent with anomalies of opposite sign over the eastern part of the continent rainy dry episodes are also associated with anticyclonic cyclonic anomalies centered over subtropical south america that extend through the depth of the troposphere accompanied by easterly westerly wind anomalies over the central andes these anomalies are presumably forced by planetary waves originating in the southern hemisphere extratropics to gain insight into the regional processes linking the large scale and local conditions the pennsylvania state university national center for atmospheric research mesoscale model version 5 2 was used to simulate contrasting rainy and dry episodes the most marked and relevant differences are the strength and extent of diurnally varying flow over the eastern slope of the andes during the rainy simulation strong easterly winds reach the upper part of the slope by midmorning initiating an intrusion of warm and moist air high ue air originating in the eastern lowlands into the central andes in the dry case the moisture transport from the east is restricted to the eastern slope of the andes and the central andes is flooded by low ue air from the western foothills that cannot support deep convection even in the presence of localized updrafts the momentum balance based on the model output indicates that turbulent momentum mixing from aloft determined by the large scale anomalies of the upper level flow into the convective boundary layer is the leading term causing the differences in the daytime upslope flow and hence moisture transport over the upper part of the eastern side of the andes between rainy and dry simulations

shell beds as tools in basin analysis the jurassic of kachchh western india skeletal concentrations are ubiquitous in bathonian oxfordian shallow water sediments of the pericratonic basins of kachchh and rajasthan western india by analysing the biofabric of the concentrations and the taphonomic signatures of individual components it is possible to distinguish between the final concentration process recorded by the biofabric and events taking place prior to final deposition recorded by taphonomic signatures the skeletal concentrations are grouped in nine genetic types ranging from fair weather wave concentrations to storm concentrations and condensed concentrations they mainly reflect biological and hydraulic processes and in some instances permit estimates of their duration and show a rough zonation along a bathymetric gradient plotted against the stratigraphic sections the skeletal concentrations show in accordance with other sedimentological data three orders of bathymetric trends the first two orders are shallowing deepening cycles which possibly correspond to eustatic changes in sea level the third order represents parasequences with a strongly asymmetric sedimentary record skeletal concentrations are a useful additional tool in basin analysis but are best used in combination with other data

geomorphological study of the cafayate dune field northwest argentina during the last millennium abstract the cafayate depression is a dry valley located in the pre andean region of northwest argentina the area shows the development of a large dune field its significance has been established from geomorphological and mineralogical evidence grain shape characteristics and chronological data osl dating archaeological remains and historical data the dating results were between 1000 1100 ad to1740 1830 ad showing that the aeolian dynamics remained active during the last millennium it was not possible to identify stabilised phases marked by ruptures or paleosol development nevertheless other regional and local proxies from northwest argentina archaeological documentary etc were used to complete the interpretative framework to propose the paleoenvironmental evolution of the area in addition human activity and land use must be considered as a complementary factor affecting aeolian dynamics during the most recent stage