an 15 000 year record of el nino driven alluviation in southwestern ecuador debris flows have deposited inorganic laminae in an alpine lake that is 75 kilometers east of the pacific ocean in ecuador these storm induced events were dated by radiocarbon and the age of laminae that are less than 200 years old matches the historic record of el nino events from about 15 000 to about 7000 calendar years before the present the periodicity of clastic deposition is greater than or equal to 15 years thereafter there is a progressive increase in frequency to periodicities of 2 to 8 5 years this is the modern el nino periodicity which was established about 5000 calendar years before the present this may reflect the onset of a steeper zonal sea surface temperature gradient which was driven by enhanced trade winds

the last 7500 cal yr b p of westerly rainfall in central chile inferred from a high resolution pollen record from laguna aculeo 34 s abstract we report multiproxy analyses of a sediment core obtained from laguna aculeo that spans the past 7500 years laguna aculeo 33 50 s 70 55 w is one of the few natural inland lakes located in the mediterranean zone of central chile near the northern margin of the influence of the southern westerlies the record shows elevated pollen counts of halophytes and seasonally drying of the lake basin prior to 5700 cal yr b p indicating severe aridity and warmer than present conditions this was followed by the establishment of a fresh water lake along with an increase in arboreal and herbaceous plant diversity between 5700 and 3200 cal yr b p an intensification of this trend started at 3200 cal yr b p along with the abrupt decrease of halophytes until 100 cal yr b p within this humid period pollen accumulation rates show large amplitude fluctuations coeval with numerous turbidite layers suggesting a highly variable and torrential rainfall pattern this intense and variable precipitation regime is probably associated with the el nino southern oscillation enso phenomenon we suggest that the modern mediterranean climate of central chile was established at 3200 cal yr b p paleovegetation and paleolimnological changes starting at 100 cal yr b p correlate with documented human activity surrounding the lake

pollen evidence for variations in the southern margin of the westerly winds in sw patagonia over the last 12 600 years we report pollen and charcoal records from vega nandu 51 0 s 72 45 w a small mire located near the modern forest steppe ecotone in torres del paine national park southern chile the record shows an open landscape dominated by low shrubs and herbs between 12 600 and 10 800 cal yr bp under cold and relatively humid conditions nothofagus experienced frequent large amplitude oscillations between 10 800 and 6800 cal yr bp indicating recurrent transitions between shrubland parkland environments under warm and highly variable moisture conditions a sustained increase in nothofagus started at 6800 cal yr bp punctuated by step wise increases at 5100 and 2400 cal yr bp implying further increases in precipitation we interpret these results as indicative of variations in the amount of precipitation of westerly origin with prominent increases at 6800 5100 and 2400 cal yr bp these pulses led to peak precipitation regimes during the last two millennia in this part of sw patagonia our data suggest variations in the position and or strength of the southern margin of the westerlies most likely linked to variations in the extent and or persistence of sea ice and sea surface temperature anomalies in the southern ocean over the last two centuries the record shows a forest decline and expansion of rumex acetosella an exotic species indicative of european disturbance 2007 university of washington all rights reserved

sea ice and wind variability during the holocene in east antarctica insight on middle high latitude coupling abstract micropaleontological and biomarker data from two high accumulation marine sites from the coastal and continental shelf zone ccsz off east antarctica adelie land at 140 e and eastern prydz bay at 77 e are used to reconstruct holocene changes in sea ice and wind stress at the basin wide scale these data demonstrate congruent increase in sea ice concentration persistence and wind stress related sea surface turbulence in the two regions since 7 cal ka bp with a particularly strong signal since 4 5 3 5 cal ka bp comparison of these high latitude records with sea ice and turbulence records from the southern mid latitudes highlights distinctive climatic evolutions according to the different latitudinal bands sea ice persistence and turbulence increase in east antarctica ccsz are opposite to sea surface warming and sea ice retreat recorded after 4 5 3 5 cal ka bp in the east atlantic and indian sector between 55 and 45 s at the same period paleodata suggest sst cooling in all major coastal upwelling systems of the southern hemisphere caused by the northward transport of subpolar surface waters as a response to southern westerlies reinforcement we therefore propose as suggested for the northern hemisphere that holocene changes in the latitudinal insolation gradient primarily forced by obliquity and precession and amplified by sea ice and glacial ice expansions in the antarctic realm are responsible for the observed contrasted latitudinal patterns of southern latitudes

late holocene environmental changes as recorded in the sediments of high andean laguna chepical central chile 32 s 3050 m a s l abstract we present a reconstruction of environmental changes from sediments of high altitude laguna chepical in the subtropical andes of central chile 32 16 s 70 30 w 3050 m a s l for the past 3100 years based on subfossil pollen microscopic charcoal and diatoms we inferred changes in moisture related to precipitation and ice cover ice free season related to summer temperature at decadal to millennial scales sustained wetter and colder summer temperatures than today prevailed between 1100 bc and ca ad 1 afterward decreasing pollen accumulation rates and increased fire activity suggest drier conditions and possibly enhanced seasonality and or inter annual climate variability frequent changes between cold and warm summers were observed particularly for the last 1000 years about ad 1250 during the medieval climate anomaly wet years and early break up of ice cover occurred in central chile which is today typical for el nino like mean conditions conversely and with the exception of a few wet pulses a generally dry period with extended ice cover cool summers was observed between ad 1400 and ad 1850 little ice age this can be interpreted as a trend toward more la nina like mean conditions recent climate change and human disturbances during the last 100 years have prompted changes in diatom and plant communities that are unprecedented in the late holocene first planktonic diatoms increased as a result of hydraulic interventions in the lake during the late 19th century and secondly the andean vegetation shifted upward as result of recent warming and the frequency of arboreal taxa was significantly reduced at the same time peaks of fire activity were observed

mathematical modeling of graded river profiles numerical modeling of the longitudinal profiles of rivers at grade is accomplished using the basic equations of open channel flow sediment transport equations and empirical relations for downstream variation in flow discharge sediment discharge sediment caliber and channel width only in some cases are the computed stream profiles fit exactly by any one of the commonly supported mathematical function analogs to graded profile form exponential logarithmic or power function but in most cases any of these functions can provide a fit with a degree of error smaller than would be noted in treating field data profiles dominated by spatial change in fluid and sediment discharge are distinctly power functions while profiles dominated by sediment size reduction are not necessarily exponential in form other important controls on profile shape are the degree of downstream width change in response to increasing discharge and the general range of sediment size a dynamic model of a river s approach to grade

vegetation dynamics at raraku lake catchment easter island during the past 34 000 years abstract easter island is a paradigmatic example of human impact on ecosystems the role of climate changes in recent vegetation shifts has commonly been rejected without proper assessment a palynological study of a long sediment core from raraku lake documents the vegetation dynamics for the last 34 ka and investigates their driving forces particularly the effects of climate variability on vegetation changes significant relationships between pollen assemblage changes and sedimentary and geochemical proxies demonstrate the rapid response of vegetation to lake crater basin hydrology and climatic changes the lake surroundings were occupied by an open mixed palm grove during the last glacial period poaceae and sophora increased at the expense of palms and triumfetta and coprosma practically disappeared in response to slightly wetter and or colder climate during the last glacial maximum palms and triumfetta thrived in a warmer and or drier climate during the deglaciation minor vegetation changes a slight increase in sophora and a drop in asteraceae and poaceae occurred between 13 2 and 11 8 cal ka bp and can be related to rapid changes in the younger dryas chronozone the increase in herbaceous taxa indicates a gradual shallowing of the lake and development of a mire during the holocene caused by sediment infilling and warmer and drier climate relatively rapid vegetation changes in the holocene were caused by climate and by plant succession on the expanding mire the rates of vegetation change observed in the mire were similar to those at the initial stages of human impact identified in a previous study these results reveal significant vegetation changes prior to human presence due to the interplay of climate variations temperature and moisture changes in lake basin form by infilling and intrinsic dynamics of plant succession hence the potential contribution of these factors in vegetation shifts during the period of human presence should not be neglected

climate variability over the last 9900 cal yr bp from a swamp forest pollen record along the semiarid coast of chile abstract we present a fossil pollen analysis from a swamp forest in the semiarid coast of chile 32 05 s 71 30 w at the northern influence zone of southern westerly wind belt a 10 000 cal yr bp calendar years before 1950 palynological sequence indicates a humid phase characterized by dense swamp forest taxa dated between 9900 and 8700 cal yr bp the presence of pollen starved sediments with only scant evidence for semiarid vegetation indicates that extreme aridity ensued until 5700 cal yr bp the swamp forest recovered slowly afterwards helped by a significant increase in moisture at 4200 cal yr bp a new swamp forest contraction suggests that another slightly less intense drought occurred between 3000 and 2200 cal yr bp the swamp forest expansion begins again at 2200 cal yr bp punctuated by a highly variable climate comparisons between the record presented here with other records across the region imply major variations in the extent of the southern westerlies during the holocene this variability could have been caused either by latitudinal displacements from the present mean position of southern westerlies wind belt or by changes in the intensity of the south pacific subtropical anticyclone both of which affect winter precipitation in the region

moisture changes and fluctuations of the westerlies in mediterranean central chile during the last 2000 years the laguna aculeo record 33 50 s abstract the multi proxy study of lacustrine sediments from laguna aculeo 33 50 s provides detailed information about moisture changes in the lowlands of mediterranean central chile during the last 2000 years the lake lies just at the northern border of the strong westerlies influence with dry summers and humid winters geochemical sedimentological and diatom analyses provide evidence for an arid period between cal 200 bc and ad 200 and a subsequent increase in moisture after cal ad 200 abundant clastic layers in the core represent flood events periods with a higher frequency of flood events indicate an increased intensity of the westerlies more winter frontal system activity and possibly enso related variability probably comparable to modern conditions in mediterranean central chile periods of high clastic input occur around cal ad 200 400 500 700 especially around cal ad 1300 1700 and around ad 1850 1998 during the last 50 years at least eight flood events were detected correlating mainly with el nino years a very short drier period also occurred in the late 1960s in recent decades human impact has resulted in a eutrophication of the lake

mid holocene radiocarbon ages in the subtropical andes 29 35 s climatic change and implications for human space organization this article discusses the distribution of radiocarbon age signatures obtained from archeological sites between 29 and 35 s in central chile and midwest argentina the goal of this analysis is to establish bases from which to interpret regional trends in the distribution of the archaeological record that connect these areas which have been traditionally considered to be geographically decoupled we propose a standardized methodology for selecting ages that provide a reliable human signature variations in date frequencies in a regional scale are discussed with the use of summed probability distributions radiocarbon voids at the regional level previously identified in midwest argentina are explored regional chronological information is compared to the available paleoenvironmental records thereby emphasizing the possible role of climate pulses in the spatial organization of human populations significant arid conditions between 7800 and 5700 cal bp are coincidental with a focused occupation of the andes mountains an area which may have offered stable resources and thus was more effectively occupied than other environmental bands

numerical simulation of downstream fining by selective transport in gravel bed rivers model development and illustration a numerical model has been developed for the routing of gravel sized sediment along a river channel which is free to adjust both its long profile and surface texture hydraulic calculations use a step backwater approach and sediment transport is predicted with the method of parker 1990a which uses a low degree of size selectivity exchange of sediment between the surface and subsurface is described using the modified exner equation of parker and sutherland 1990 the model is applied to an idealized channel based on the highly concave allt dubhaig scotland in which fining by particle wear is minor the rapid downstream fining observed in this river is closely matched by model predictions after a time equivalent to 102 years under the present flow regime of the river the evolution of the fining pattern during the model run and associated changes in sediment transport and bed aggradation are described it is concluded that strong profile concavity can force rapid downstream fining even though bed load transport is only slightly size selective this run of the model serves as a basis for testing of the sensitivity of downstream fining to alternative choices of parameter values and boundary conditions which are summarized here and will be described in a subsequent paper

high resolution planktic foraminiferal record of the last 13 300 years from the upwelling area off chile abstract a high resolution sedimentary record spanning the last deglacial and holocene periods was obtained by studying the deep sea sediment core gik 17748 2 retrieved from 2545 m water depth in the valparaiso basin chilean continental slope southeast pacific 32 45 00 s 72 02 00 w ams 14 c measurements and oxygen isotope analyses of the planktic foraminifera species globigerina bulloides and neogloboquadrina pachyderma dex indicate an age of 13 3 ams 14 c kyr b p at the base of the core 383 cm and sedimentation rates ranging between 9 and 92 cm kyr foraminiferal assemblages and accumulation rates were analysed in order to investigate variations in paleoceanography and paleoproductivity in the southern peru chile current during the last 13 kyr changes in foraminiferal assemblages indicate a sequence of three major hydrographic regimes at the site of the core 1 deglacial sediments are characterised by variable faunal compositions generally reflecting strong upwelling and high productivity 2 during the early and middle holocene upwelling and thus productivity decreased as indicated by an elevated relative abundance of n pachyderma dex and the importance of the subtropical surface water of the peru chile countercurrent increased as indicated by higher relative amounts of n dutertrei 3 for the last 3 kyr variable faunal assemblages indicate variable competing environmental conditions characterised on one hand by increased upwelling reflected by an elevated relative abundance of n pachyderma sin and on the other hand by warmer conditions deduced from a higher relative abundance of n dutertrei probably due to more frequent and more intense el nino events in addition hints for slightly cooler conditions between 11 4 and 10 7 kyr b p might be related to a younger dryas event in this region

evidence of neogene wildfires in central chile charcoal records from the navidad formation abstract mediterranean type climate mtc ecosystems are characterized by recurrent wildfires although the majority of wildfires are human ignited non anthropogenic i e natural wildfires are common in all mtc regions except central chile the low frequency of natural wildfires in this chilean region is explained by the scarcity of non anthropogenic ignition sources basically thunderstorm induced lightning and volcanic activity however from a geological perspective the current relative absence of non anthropogenic wildfires in central chile is a recent phenomenon in the transition from the early to middle miocene the climate in the region was likely warm and seasonally dry such climate conditions would allow the growth of fuel during the spring becoming flammable during the dry season this fire prone landscape would likely have been ignited by the high volcanic activity that concomitantly occurred with the orogeny of the andes to evaluate this hypothesis we sampled rocks from the three locations at the navidad formation considering the fossil plant evidence deposited during this warm and seasonally dry period we found a high concentration of charcoal in playa navidad coinciding with the global warming event reported between 17 and 15 ma the predominance of microscopic charcoal particles between 125 and 250 m only allows us to infer the occurrence of neogene fires at a regional scale the fused cell walls preserved in the charcoal anatomy likely suggest that such fire events were highly severe the presence of pumice associated with the high charcoal concentrations supports the hypothesis of volcanic ignition sources very little charcoal was found in punta perro late oligocene early miocene or in cerro los pololos late middle miocene where fire may have been limited by fuel humidity and ignition sources respectively although changes in the depositional environment would also help explain differences in the fire record throughout the navidad formation in conclusion our results provide the first quantitative neogene charcoal record for south america the evidence of fire occurrence in central chile during the neogene will contribute to understanding the evolution of the mediterranean type flora

high resolution marine record of climatic change in mid latitude chile during the last 28 000 years based on terrigenous sediment parameters abstract marine sediment cores from the continental slope off mid latitude chile 33 s were studied with regard to grain size distributions and clay mineral composition the data provide a 28 000 yr14c accelerator mass spectrometry dated record of variations in the terrigenous sediment supply reflecting modifications of weathering conditions and sediment source areas in the continental hinterland these variations can be interpreted in terms of the paleoclimatic evolution of mid latitude chile and are compared to existing terrestrial records glacial climates 28 000 18 000 cal yr b p were generally cold humid with a cold semiarid interval between 26 000 and 22 000 cal yr b p the deglaciation was characterized by a trend toward more arid conditions during the middle holocene 8000 4000 cal yr b p comparatively stable climatic conditions prevailed with increased aridity in the coastal range the late holocene 4000 0 cal yr b p was marked by more variable paleoclimates with generally more humid conditions variations of rainfall in mid latitude chile are most likely controlled by shifts of the latitudinal position of the southern westerlies compared to the holocene the southern westerly wind belt was located significantly farther north during the last glacial maximum less important variations of the latitudinal position of the southern westerlies also occurred on shorter time scales

a 7000 year high resolution lake sediment record from coastal central chile lago vichuquen 34 s implications for past sea level and environmental variability we present a 7 ka environmental reconstruction based on sedimentological and geochemical data from lago vichuquen a coastal eutrophic lake in central chile 34 48 s 72 03 w 4 m a s l a relatively shallow and restricted marine environment with low productivity high detrital input and dominant anoxic conditions in the vichuquen basin occurred from 7 0 to 6 5 cal ka bp rapid onset of a holocene marine transgression at 6 5 cal ka bp favoured deeper and more biologically productive environments that existed until 2 8 cal ka bp bioproductivity changes during the mid holocene 6 5 4 2 cal ka bp were related to upwelling dynamics controlled by the intensity of the south east pacific anticyclone spa periods with lower organic productivity and dominant anoxic conditions reflect an increased intensity of spa increased upwelling and decreased precipitation a shift at 4 0 cal ka bp reflects the onset of modern southern westerly winds and el nino southern oscillation patterns tectonic uplifting and geormorphological activity dune advancement are possible reasons behind the vichuquen basin closure at 1 2 cal ka bp leading to a low bioproductivity lacustrine environment which has developed until the present

southern hemispheric westerlies control sedimentary processes of laguna azul south eastern patagonia argentina multiproxy investigations of lacustrine sediments from laguna azul 52 s document multi millennial holocene influences of southern hemispheric westerlies shw on the hydroclimatic variability of

the southern westerlies in central chile during the two last glacial cycles as documented by coastal aeolian sand deposits and intercalating palaeosols abstract changes in the position and intensity of the southern westerly winds sww and related causal processes during the quaternary are controversial and not well understood here we present a record from continental central chile based on coastal aeolian sand and dunes with intercalated palaeosols reaching back 190 ka in time sixteen samples for luminescence dating and additional samples for geochemical procedures were analysed from three locations in the norte chico la serena los vilos las ventanas besides the recent bw horizons four palaeosols btb1 btb2 btb3 btb4 are identified they formed in periods with stable surface conditions and a relatively dense vegetation cover whereas sand accumulation reflects increased aeolian activity under dry conditions and in parts glacial sea level lowering three of these soils are well bracketed by luminescence data to

late quaternary changes in ecosystems and carbon cycling on mt kenya east africa a landscape ecological perspective based on multi proxy lake sediment influxes abstract the degree to which different lakes within a landscape respond coherently in unison to external drivers such as climate change and soil development is uncertain presentation of multi proxy geochemical and palaeoecological data from individual lakes in the form of fluxes minimizes distortions resulting from variable sedimentation rates and changes in sediment composition we use the accumulation rates of magnetic minerals total organic c and n terrestrial and aquatic biomarkers graminoid epidermis pollen green algae diatoms and diatom c in four small lakes situated between 2350 and 4595 m a s l on the ne flank of mt kenya east africa to reconstruct changes in c cycling over the last 38 ka the results conflict with earlier models of landscape and lake development showing 1 that glacial interglacial changes in vegetation cannot be interpreted as simple altitudinal shifts in the modern vegetation belts and 2 that limnological changes were not coherent rapid variations in climate water level erosion and nutrient input overwhelmed long term successional trends in lake sedimentation and c accumulation since the last glacial maximum the results also reveal previously unrecognized features of the palaeoenvironmental record such as the rapid degradation of organic matter in diatoms and the occurrence of a productive fire prone montane grassland during the highly seasonal monsoonal climate of the lateglacial and early holocene

abrupt early to mid holocene climatic transition registered at the equator and the poles paleoclimatic records from equatorial east africa antarctica and greenland reveal that atmospheric circulation changed abruptly at the early to mid holocene transition to full postglacial conditions a climatic reorganization occurred at all three sites between 8200 and 7800 years ago that lasted 200 years or less and appears to have been related to abrupt transitions in both marine and terrestrial records around the world

enso tropical extratropical climate teleconnections and mechanisms for holocene debris flows along the hyperarid coast of western south america 17 24 s abstract el nino the warm phase of the enso cycle involves ocean climate anomalies in the tropical pacific ocean and in the extratropics which frequently result in heavy rainfall episodes along the equatorial and subtropical regions of western south america here we investigate meteorological mechanisms producing heavy rains floods and debris flows along the less known hyperarid coasts of southern peru and northern chile to evaluate the paleoclimate significance of holocene debris flow deposits our results reveal that heavy rainfall over the coast of southernmost peru occurs either during austral summer at the mature stage of el nino in connection with warmer sea surface temperatures and anomalous jet streams off northern chile or during the previous austral winter spring associated with equatorward shifted pacific south america psa atmospheric teleconnection patterns at antofagasta in northern chile such events occur almost exclusively in the latter season when deeper psa related anomaly poles extend their influence equatorwards beyond central chile during non el nino conditions short lived heavy rainfall episodes in southernmost peru can be associated with similar albeit weaker or less persistent circulation anomalies in addition to that a seasonally enhanced low level southerly flow provides orographic uplift rainfall favourable conditions by the andes at the 18 s coastal bend ultimately the trigger for rainfall events in all seasons and phases of the enso cycle was invariably connected with mid troposphere wave disturbances from mid latitudes the chronostratigraphy of debris flow deposits from both areas and its comparison with other paleoclimate records at the western side of the andes suggests that the enso related teleconnection patterns operated only during the second half of the holocene supporting an onset of modern el nino manifestations at 5300 5500 cal bp and increased frequency of major events during recent times we suggest that several debris flows dated between 12 900 and 8400 cal bp in southernmost peru previously interpreted as an indication of strong el nino events were associated with short and intense heavy rainfall episodes similar to those described here during the late winter spring season in non el nino conditions concomitant with stronger low level southerlies strengthened south eastern pacific subtropical anticyclone and intensified coastal upwelling

lake level stratigraphy and geochronology revisited at lago lake cardiel argentina and changes in the southern hemispheric westerlies over the last 25 ka abstract paleoshorelines around lago lake cardiel in southern argentina s48 9 w71 3 275 m record substantial changes in lake area over the past 25 ka our results combined with previous research show that during the last glacial maximum or lgm 23 21 ka the lake stood at near modern levels but had nearly dried up by 13 ka between 11 3 and 10 1 ka the lake reached its highest point 54 58 m and greatest extent in at least the last 40 ka lake levels dropped thereafter and experienced two lower lake periods 8 5 7 5 ka and 5 3 3 ka and two higher lake periods 7 4 6 and 5 2 ka in the last 3 5 ka the lake has remained generally near or slightly above its present level the depth and surface area of lago cardiel are controlled mainly by precipitation onto the lake and surrounding catchment air and water temperature and wind speed related to local strength of the southern hemispheric westerlies shw our lake level reconstruction combined with evidence from other studies suggest that on average the core of the shw was located well to the north 55 s during the hydrologic maximum of cardiel in the early holocene the lower phases of the lake at 20 0 11 5 8 5 7 5 and 5 0 3 3 ka generally correspond to cold conditions in other records when we infer that the shw were strongly focused around the latitudes of cardiel at 49 s

major hydrological regime change along the semiarid western coast of south america during the early holocene water availability in the semiarid western coast of chile 30 32 s is conditioned by high interannual precipitation variability reflecting the transition between arid subtropical and moist mid latitude climates in the southeastern pacific ocean a paleoclimate reconstruction based on the latest pleistocene holocene geological record from the quebrada santa julia archeological site in chile 31 50 s and on modern meteorological mechanisms producing alluvial episodes in this region indicates a major change in the rainfall regime shortly after 8600 cal yr bp this together with other paleoclimate proxies along the west coast of south america 34 14 s suggests la nina like conditions 13 000 8600 cal yr bp based on sedimentological and geomorphologic evidence we hypothesized that the absence of heavy rainfall events in northern chile and the new hydrological regime that prevailed ca 8600 5700 cal yr bp in north central chile resulted from an increase in the large scale westerly flow over central chile as expected in near neutral enso conditions this atmospheric circulation anomaly is compatible with an equatorward shift of the influence of the southeast pacific subtropical anticyclone relative to the early holocene prior to the onset of modern enso variability

chilean and southeast pacific paleoclimate variations during the last glacial cycle directly correlated pollen and δ18o records from odp site 1234 abstract joint pollen and oxygen isotope data from ocean drilling program site 1234 in the southeast pacific provide the first continuous record of temperate south american vegetation and climate from the last 140 ka located at 36 s 65 km offshore of concepcion chile site 1234 monitors the climatic transition zone between northern semi arid summer dry winter wet climate and southern year round rainy cool temperate climate dominance of onshore winds suggests that pollen preserved here reflects transport to the ocean via rivers that drain the region and integrate conditions from the coastal mountains to the andean foothills down hole changes in diagnostic pollen assemblages from xeric lowland deciduous forest characterized by grasses herbs ferns and trees such as deciduous beech nothofagus obliqua mesic valdivian evergreen forest including conifers such as the endangered prumnopitys andina and subantarctic evergreen rainforest comprised primarily of southern beech n dombeyi reveal large rapid shifts that likely reflect latitudinal movements in atmospheric circulation and storm tracks associated with the southern westerly winds during glacial intervals mis 2 4 and 6 rainforests and parkland dominated by nothofagus moved northward into the region at the mis 6 5e transition coeval with the rapid shift to lower isotopic values rainforest vegetation was rapidly replaced by xeric plant communities associated with mediterranean type climate an increased prominence of halophytic vegetation suggests that mis 5e was more arid and possibly warmer than mis 1 although rainforest pollen rises again at the end of mis 5e lowland deciduous forest pollen persists through mis 5d and 5c into mis 5b substantial millennial scale variations occur in both interglacial and glacial regimes attesting to the sensitivity of the southern westerly belt to climate change comparison of the cool mesic n dombeyi rainforest assemblage from site 1234 with 18 o in the byrd ice core shows that on time scales longer than 10 ka cool moist conditions in central chile were coherent with and occurred in phase with antarctic cooling this is also likely at millennial scales although rainforest pollen lags antarctic cooling with exponential response times of about 1000 years which plausibly reflects the ecological response time to regional climate change

numerical simulation of mid holocene tidal regime and storm tide inundation in the south yangtze coastal plain east china abstract coastal flooding is reported as one of the major causes for the interruptions of neolithic cultures inhabiting the south yangtze coastal plain china during the middle holocene the archaeologically sterile transgressive sedimentary layers could be the results of extreme events or relative sea level rise induced by the enlargement of the local tidal range or global regional sea level rise to clarify the mechanism of the transgressive deposits at typical neolithic sites and the termination of neolithic liangzhu culture in the south yangtze coastal plain this study reconstructed the paleo topography at 7 6 6 4 and 4 5 cal ka bp on the basis of a database including 107 sediment cores and 644 radiocarbon ages and simulated the paleo tidal regimes and the inundation areas induced by typhoon events under the paleo topography and sea levels using a hydrodynamic model of high resolution changjiang estuary finite volume community ocean model ce fvcom the simulation results of tidal regimes showed that the tidal amplitude increased from 1 6 m to 2 2 m in the head of hangzhou bay induced by the paleo topographic change during the period 7 6 4 5 cal ka bp by contrast it reduced from 1 6 m to 1 0 1 2 m in the ningbo plain southeast of the bay which was unable to explain the long lasting marine inundation there and supported our previous assumption of an event of abrupt sea level rise at 4 5 4 4 cal ka bp simulation of the storm induced inundation further suggested that coastal flooding mainly occurred along the coastline of the taihu plain under the combined effect of tidal amplification and typhoon events at 4 5 cal ka bp while the inundation almost covered half of the taihu plain if all factors including tidal amplification extreme event and the abrupt sea level rise were considered we therefore suggest that the obvious increase in flood risk under the backdrop of sea level rise was a major cause for the abandonment of the liangzhu culture in the south yangtze coastal plain at around 4 4 cal ka bp

subsidence control on river morphology and grain size in the ganga plain the ganga plain represents a large proportion of the current foreland basin to the himalaya the himalayan sourced waters irrigate the plain via major river networks that support approximately 10 percent of the global population however some of these rivers are also the source of devastating floods the tendency for some of these rivers to flood is directly linked to their large scale morphology in general the rivers that drain the east ganga plain have channels that are perched at a higher elevation relative to their floodplain leading to more frequent channel avulsion and flooding in contrast those further west have channels that are incised into the floodplain and are historically less prone to flooding understanding the controls on these contrasting river forms is fundamental to determining the sensitivity of these systems to projected climate change and the growing water resource demands across the plain here we present a new basin scale approach to quantifying floodplain and channel topography that identifies areas where channels are super elevated or entrenched relative to their adjacent floodplain we explore the probable controls on these observations through an analysis of basin subsidence rates sediment grain size data and sediment supply from the main river systems that traverse the plain yamuna ganga karnali gandak and kosi rivers subsidence rates are approximated by combining basement profiles derived from seismic data with known convergence velocities results suggest a more slowly subsiding basin in the west than the east grain size fining rates are also used as a proxy for relative subsidence rates along the strike of the basin the results also indicate higher fining rates and hence subsidence rates for given sediment supply in the east by integrating these observations we propose that higher subsidence rates are responsible for a deeper basin in the east with perched low gradient river systems that are relatively insensitive to climatically driven changes in base level in contrast the lower subsidence rates in the west are associated with a higher elevation basin topography and entrenched river systems recording climatically induced lowering of river base levels during the holocene

longitudinal profiles in simple alluvial systems theoretical considerations suggest that exponential or quadratic functions are apt for describing the longitudinal profiles of aggrading alluvial systems that are unaffected by significant lateral inputs of water or sediment a new set of field data confirms this for individual sedimentary links along a wandering gravel bed river in british columbia canada each link is viewed as a simple alluvial system without major water or sediment inputs within which grain size typically fines downstream in a relatively systematic manner such homogeneous reaches are suitable for the investigation of simple profile form it is found that quadratic approximations are the most flexible descriptor for link longitudinal profiles overall the river forms an irregularly cuspate long profile structured by these fundamental length scale units both link gradients and link fining rate reflect size selective aggradation associated with the bounding lateral inputs

geoarchaeological evidence from peru for a 5000 years b p onset of el nino for the tropical west coast of south america where el nino southern oscillation enso is most pronounced archaeological and associated paleontological deposits in northern peru revealed a major climate change at about 5000 years before the present yr b p the data implied the presence of stable warm tropical water as far south as 10 s during the early mid holocene about 8000 to 5000 yr b p these data suggest that enso did not occur for some millennia preceding 5000 yr b p when global and regional climate was warmer than today

glacial to holocene changes in sea surface temperature and coastal vegetation in north central chile high versus low latitude forcing abstract north central chile is located at an important climatic transition zone between humid conditions under the influence of the southern hemisphere westerly winds in the south and the extremely dry climate of the atacama desert in the north offshore equatorward flowing cold waters of the peru chile current converge with relatively warmer waters coming from the low latitudes based on terrestrial and marine proxies from the same archive geob 7139 2 30 12 s 71 59 w we show that during the glacial between 33 and 19 kyr cold sea surface temperatures paralleled enhanced humidity with high terrestrial input and abundant vegetation on the adjacent land suggesting a stronger influence of cold waters from the antarctic circumpolar current and the westerly winds while the deglacial sea surface temperature warming started at 19 kyr the humidity decrease occurred around 17 16 kyr bp the early to mid holocene is characterized by extremely warm and dry conditions we suggest that climate changes were driven by fluctuations in antarctic sea ice extent and the circulation of the hadley cell both ultimately linked to insolation changes our records further imply warm and dry conditions in north central chile during the northern hemisphere heinrich events

grain size along two gravel bed rivers statistical variation spatial pattern and sedimentary links a new set of field data facilitates a detailed analysis of variations in bed material grain size within two confluent gravel bed rivers in northeastern british columbia canada a preliminary assessment of grain size variability establishes a basis for examination of the spatial pattern of grain size change standard anova techniques are inappropriate because individual samples have unequal variances and are not normally distributed alternative tests for homoscedasticity and comparison of means are therefore utilized within site between sample variability is not significant the grain size distributions that were obtained at individual sites are therefore representative of the depositional environments that were sampled in both rivers mean grain size does vary significantly between sites and there is therefore a basis for examining the data for spatial patterns such as downstream fining textural variations along the two rivers studied here are complex and show negligible overall fining in over 100 km this is the consequence of a large number of tributary inputs and non alluvial sediment sources which are the legacy of late pleistocene glaciation the identification of lateral sources like these is fundamental for understanding textural changes within rivers the sedimentary link a channel reach between significant lateral sediment inputs provides a means of isolating fluvial maturation processes abrasion and sorting from contingent lateral inputs strong fining trends are apparent in most links and classification of grain size measurements according to their location within particular links greatly improves the statistical explanation of textural variation identification of sedimentary links provides a means of applying models of fluvial fining processes so isolation of link networks will aid the development of basin scale models of textural variation 1998 john wiley sons ltd

identifying the controls over downstream fining of river gravels abstract a wide range of rates of downstream fining in gravel bed rivers has been reported as a consequence explanations of the phenomenon range from those that consider only abrasion to others that consider only selective sediment transport this apparent confusion results in part from inconsistent definitions of downstream fining analysis of results from a small gravel bed stream is used to show how the method of sediment sampling and the percentile used to characterize fining rates can affect the results when fining is adequately defined a theoretical relationship between fining rate and drainage basin area is derived from established alluvial fan relations this relationship is remarkably consistent with a range of studies by numerous authors using often different methods such results suggest that fining is achieved through the mechanism of selective transport but that the overall control on fining patterns is the volume of sediment supplied from within the drainage basin as such the rate of size decline in sedimentary deposits such as river terraces has the potential to be used to infer past changes in climate tectonics and or relative base level models of this are presented and these are shown to be broadly consistent with observations made in an ancient sandstone body full use of changing rates of fining as paleo environmental indicators requires further work on the rate of adjustment of fining profiles with respect to the rates of change in the forcing environmental conditions

holocene environmental and climatic changes inferred from wulungu lake in northern xinjiang china abstract sedimentological geochemical and palynological data from wulungu lake in northern xinjiang china are used to reconstruct environmental and climate changes since 9550 cal yr bp high abundance of sparganium and poaceae low md median diameter and 13 c organic values indicate aridity between 9550 and 6730 cal yr bp high md and 13 c organic values and the prevalence of desert steppe and steppe vegetation between 4200 and 560 cal yr bp indicate that effective moisture increased after 6730 cal yr bp peaking at 4200 and 560 cal yr bp low md values a negative excursion of 13 c org and the transition from steppe to desert vegetation since 560 cal yr bp reflect a decrease in effective moisture during the latest holocene late holocene human activities were indicated by sharp increase in the abundance of pediastrum then variations in carbonate contents indicate that temperature was generally high between 9550 and 7740 cal yr bp low between 7740 and 6730 cal yr bp intermediate between 6730 and 560 cal yr bp and low during the last 560 yr regional comparison indicates that the asian monsoon did not extend to wulungu lake and westerlies were the main factor in determining the moisture availability during the holocene

a worldwide correlation for exponential bed particle size variation in subaerial aqueous flows the particle size of the bed sediments in or on many natural streams alluvial fans laboratory flumes irrigation canals and mine waste deltas varies exponentially with distance along the stream a plot of the available worldwide exponential bed particle size diminution coefficient data against stream length is presented which shows that all the data lie within a single narrow band extending over virtually the whole range of stream lengths and bed sediment particle sizes found on earth this correlation applies to both natural and artificial flows with both sand and gravel beds irrespective of either the solids concentration or whether normal or reverse sorting occurs this strongly suggests that there are common mechanisms underlying the exponential diminution of bed particles in subaerial aqueous flows of all kinds thus existing models of sorting and abrasion applicable to some such flows may be applicable to others a comparison of exponential laboratory abrasion and field diminution coefficients suggests that abrasion is unlikely to be significant in gravel and sand bed streams shorter than about 10 km to 100 km and about 500 km respectively copyright c 1999 john wiley sons ltd

southern westerlies during the holocene deduced from geomorphological and pedological studies in the norte chico northern chile 27 33 s abstract in the norte chico precipitation is controlled by atmospheric perturbations and frontal systems associated with the belt of westerly wind circulation the holocene climatic history of the region can be reconstructed from geomorphic and pedologic investigations in comparison to modern climatic conditions increased influence of the westerlies is deduced for the time periods prior to 7300 yr b p between 5000 3700 yr b p 3000 1800 yr b p and around 270 yr b p it remains uncertain whether the increased influence of the westerlies was the consequence of northward shifts of the westerlies as a whole of a meridional expansion of the westerly belt or only of intensity changes of the atmospheric dynamics almost synchronous with changes in the westerlies the coastal fogs in the region were apparently reduced or the fog belt in the coastal range was elevated probably due to a weakened upwelling and warmer water conditions in the eastern pacific in general the climate of the norte chico has become more variable since 5000 yr b p especially since 3000 yr b p with frequent changes of the geoecological conditions

ice age vegetation and climate of subtropical chile abstract vegetation and climate of subtropical chile over the past 50 000 yr are reconstructed from pollen spores and other microfossils contained in a 10 7 m section of lacustrine deposits of laguna de tagua tagua 34 30 s 71 10 w controlled by 14 radiocarbon age determinations five pollen assemblages covering stratigraphic zones in the section represent plants on the upland and species inhabiting the lake basin surface pollen spectra from 58 stations approximately 30 40 s form an adjunct for interpreting past vegetation and climate from the fossil record temperature semi humid woodland of southern beech nothofagus dombeyi type and podocarp prumnopitys andina was apparently established about laguna de tagua tagua during the pleistocene contrasting existing semi arid broad sclerophyllous vegetation the woodland was extensive about the lake at the time of the last glacial maximum 25 000 14 000 yr b p and earlier 43 000 34 000 yr b p pollen from the families chenopodiaceae and amaranthaceae which characterize intervals of the pleistocene centered around 33 000 and 43 000 yr b p and the holocene suggests intervening episodes of relative aridity lake level fluctuations appear to follow the late quaternary climatic pattern implied by pollen assemblage data water in the lake was relatively high during development of southern beech podocarp woodland whereas during intervals of chenopod amaranth dominance lake levels were comparatively low excess precipitation over evaporation which during the pleistocene favored development of woodland of beech and podocarp is attributed to greater storm frequency conditions were apparently governed by strengthening of atmospheric circulation in the belt of southern westerly winds in the late glacial the temperate and more humid ice age climate with limited seasonality underwent transition to a subtropical semi arid mediterranean type that identifies the postglacial fluctuations in intensity of the southern westerlies marked by latitudinal movement of the polar front can account for past vegetation changes at laguna de tagua tagua

worldwide correlations for subaerial aqueous flows with exponential longitudinal profiles it is shown that very strong worldwide correlations exist between the bed concavity coefficients of a wide range of subaerial aqueous flows with exponential longitudinal profiles and both the corresponding stream segment lengths and exponential bed particle size diminution coefficients the former correlation is complementary to an existing similar correlation for the exponential size diminution coefficients while the latter is consistent with earlier theoretical correlations based on very limited data the data supporting the correlations extend over virtually the whole range of stream lengths solids concentrations and bed sediment particle sizes found on earth this universality strongly suggests that there are underlying mechanisms common to all kinds of mobile bed subaerial aqueous flows however the scatter of the data for the correlations is significant and is mostly attributable to variations in hydraulic conditions and sediment properties rather than measurement errors some of the conditions and properties have been identified but others remain obscure copyright c 1999 john wiley sons ltd

early to mid holocene aridity in central chile and the southern westerlies the laguna aculeo record 34 s abstract central chile 32 35 s lies at the northern border of strong westerly influence and thus exhibits a steep precipitation gradient therefore the palaeoclimatic archives in the region are suitable for detecting past moisture changes the study of laguna aculeo 33 50 s 70 54 w presents a multiproxy holocene lake record including sedimentology geochemistry mineralogy pollen diatoms and radiocarbon dating 17 dates results indicate an arid early to mid holocene period about 9500 5700 cal yr b p after 5700 cal yr b p effective moisture increased progressively and around 3200 cal yr b p modern humid conditions were established numerous intercalated clastic layers reflect flood deposition during rainy winters a fluvial unit was deposited shortly before 9000 cal yr b p subsequently flood events were absent until 5700 but have become frequent since 3200 cal yr b p the frequency of flood layers possibly points to weak or no el nino activity during the early and mid holocene with a subsequent increase during the late holocene during the early and mid holocene the westerlies were probably blocked and hence deflected southward by the subtropical high pressure cell higher precipitation during the last 3200 yr seems strongly related to a weakened subtropical high pressure cell with intensified westerlies and possibly increased el nino activity

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

change in the mathematical function type describing the longitudinal profile of a river through an evolutionary process japanese rivers where a large amount of sediment accumulates along the middle reaches are today not at grade nevertheless most of their longitudinal profiles can be described by one of the mathematical functions proposed for graded rivers for these rivers the mathematical function should show an aspect of fluvial processes different from the explanations of grade theories the geomorphological significance of differences in the mathematical functions describing the longitudinal profiles of rivers is discussed here a dynamic change in the shape of longitudinal profile of a river accompanying the change in mathematical function type is proposed in relation to changes in fluvial processes and stage of evolution of a river most japanese rivers can be described by either exponential or power functions the fluvial processes of the rivers described by exponential functions are in the depositional state the front of depositional area of gravels increases in altitude and migrates downstream the rivers exp

on the functioning of the southern oscillation in the south american sector part i surface climate abstract the functioning of the southern oscillation so over south america and the surrounding tropical atlantic and eastern pacific oceans is studied through analyses of bimonthly patterns of correlation between an so index and the fields of pressure wind temperature and rainfall hydrometeorological anomalies being further ascertained from river discharge and lake level records and measurements of outgoing longwave radiation most pronounced is the anomalously low pressure over the eastern pacific during the negative so phase defined by anomalously low high pressure at tahiti darwin consistently positive so pressure correlations over the eastern tropical pacific extend eastward into the north atlantic region and southern south america during the respective winter semester during the austral winter the tendency for anomalously abundant rainfall in central chile during the negative so phase appears related to an anomalously weak and northward displaced south pacific subtropical high while relativ

the nature and controls on downstream fining within sedimentary links abstract at scales of 100 to 102 km grain size change in gravelly alluvial systems is characterized by punctuated downstream fining significant lateral sediment inputs define a sequence of sedimentary links within which fluvial processes produce fining trends using a new set of surface grain size data eighteen such links along two canadian gravel bed rivers are examined negative exponential functions are found to be most appropriate for describing within link grain size changes development of a pragmatic means of predicting grain size is then pursued by searching for a means to relate model diminution coefficients to link attributes consideration of lithologically differentiated fining trends indicates that lithology plays an insignificant role in controlling fining rate examination of within link longitudinal profiles indicates a reasonable relation between fining rate and the rate of change of channel slope the predictive value of this relation is limited by the regional nature of the data upon which it is based and the relative difficulty of obtaining accurate gradient information however as previously noted for alluvial fans although for different reasons channel gradient is correlated with link length and a reasonable predictive tool is apparent in a relation between link length and diminution rate the validity of the derived relation cannot be tested for want of a suitable data set several lines of evidence indicate that abrasion is unimportant as a fining mechanism on these rivers and it is argued that this may be typical of sedimentary links fining rate must then be a function of the efficiency with which sorting processes operate and it is suggested that this efficiency is dependent on aggradation rates associated with the lateral sediment influxes at the head and terminus of individual links

gualas glacier sedimentary record of climate and environmental change golfo elefantes western patagonia 46 5 s gualas glacier is an outlet glacier of the northern patagonian icefield one of the largest temperate ice bodies on earth golfo elefantes the depositional basin of gualas glacier has a sedimentary record that spans with some hiatuses at least the last 11 3 3 0 ka during this period the gulf remained free of ice as suggested by the absence of proximal glacimarine sediment and till in the sedimentary section this implies that the arcuate terminal moraines that occur along the edges of golfo elefantes were formed during the waning stages of the local glacial maxima late pleistocene or the early holocene between 11 3 3 0 ka and 4 2 0 3 1 4 ka the basin received low sediment input consisting of fine grained sediments in the form of low concentration density currents that filled bathymetric lows sediment discharge increased several times in the late holocene 1 4 ka and started with the accumulation of 0 5 km3 of sandy sediments in a relatively short time span 670 yr this high discharge

paleoenvironmental changes in the semiarid coast of chile 32 s during the last 6200 cal years inferred from a swamp forest pollen record pollen analysis of two sediment records from a coastal swamp forest site in the chilean semiarid region 31 50 s 71 28 w shows an alternation of dry and wet phases during the past 6100 cal yr b p the most prominent vegetation changes occur at 4200 cal yr b p with the expansion of the swamp forest taxa luma chequen and escallonia sp followed by a regression of the forest beginning at 3200 cal yr b p and ending with its replacement by a xerophytic scrub between 1800 and 1300 cal yr b p the swamp forest reexpanded after 1300 cal yr b p and persisted with minor variation until the present we interpret the establishment of the swamp forest at the study site to be the result of a rising watertable in response to increased rainfalls from 4200 cal yr b p onward our results indicate that in north central chile the second half of the holocene was climatically more variable than previously thought suggesting significant changes in the position and or intensity of the westerlies wind belt and possibly in the frequency of el nino southern oscillation events

covariability of the southern westerlies and atmospheric co2 during the holocene a suite of mechanisms has been proposed to account for natural variations in atmospheric co 2 during the holocene all of which have achieved limited success in reproducing the timing direction and magnitude of change recent modeling studies propose that changes in the latitudinal position and strength of the southern hemisphere westerly winds sww can greatly influence large scale ocean circulation and degassing of the deep ocean via changes in wind driven upwelling in the southern ocean the extent to which the hypothesized sww southern ocean coupled system could account for changes in atmospheric co 2 is uncertain because of a lack of observations on the behavior of the sww in the past the paucity of appropriate records of productivity changes in the southern ocean and our limited understanding of the sensitivity of the southern ocean biological and or physical system to sww forcing here we report a reconstruction of the behavior of the sww during the past 14 k y based on terrestrial ecosystem proxies from western patagonia south america the reconstructed variations in the intensity of zonal flow correspond to the timing and structure of atmospheric co 2 changes and are consistent with the modeled magnitude of co 2 changes induced by varying strengths of the sww the close match between data and models supports the view that the sww southern ocean coupled system underpins multimillennial co 2 variations during the current interglacial and possibly during glacial cycles over the past 800 k y

palynological evidence for increased aridity on the central chilean coast during the holocene abstract the late quaternary vegetation of the semiarid coast of central chile is inferred from the palynological analysis of profiles from quereo 31 55 s and quintero 32 47 s prior to 11 400 yr b p wet conditions are suggested by the abundance of pollen indicators of swamp and aquatic taxa such as cyperaceae and myriophyllum and by the presence of traces of arboreal pollen since ca 10 000 yr b p a trend toward increasingly drier conditions is implied by the almost complete absence of arboreal and aquatic taxa and a general decrease in the diversity of the semiarid shrubland indicators from 3000 yr b p onward the pollen records show the reappearance of swamp and aquatic taxa presumably associated with wetter conditions which led to recolonization by forest taxa at 1720 yr b p in quintero the drier climate detected along the semiarid coast of central chile during most of the holocene extended inland to the andean foot hills within the present mediterranean type climate zone of chile and also affected the distribution of the winter deciduous nothofagus forests and the northern boundary of the temperate rain forests

holocene sea surface temperature variability in the chilean fjord region here we provide three new holocene 11 0 cal ka bp alkenone derived sea surface temperature sst records from the southernmost chilean fjord region 50 53 s sst estimates may be biased towards summer temperature in this region as revealed by a large set of surface sediments the holocene records show consistently warmer than present day ssts except for the past 0 6 cal ka bp however they do not exhibit an early holocene temperature optimum as registered further north off chile and in antarctica this may have resulted from a combination of factors including decreased inflow of warmer open marine waters due to lower sea level stands enhanced advection of colder and fresher inner fjord waters and stronger westerly winds during the mid holocene pronounced short term variations of up to 2 5 c and a cooling centered at 5 cal ka bp which coincides with the first neoglacial glacier advance in the southern andes are recorded the latest holocene is characterized by two pronounced cold events centered at 0 6 and 0 25 cal ka bp i e during the little ice age these cold events have lower amplitudes in the offshore records suggesting an amplification of the sst signal in the inner fjords

marine radiocarbon reservoir effect along the northern chile southern peru coast 14 24 s throughout the holocene abstract through an extensive sampling and dating of pairs of associated shells and charcoal fragments combined with reanalysis of all the available previous data we reconstruct the evolution throughout the holocene of the regional marine radiocarbon reservoir effect r values along the northern chile southern peru area 14 24 s after elimination of the cases in which the terrestrial component yielded older ages than the marine shells to which they were associated the study is based upon data from 47 pairs of associated marine and terrestrial material our results suggest major changes in both the magnitude and variability range of r during the whole holocene period 1 between 10 400 and 6840 cal yr bp high values 511 278 yr probably result from a strengthened se pacific subtropical anticyclone and shoaling of equatorial subsurface waters during intensified upwelling events 2 between 5180 and 1160 cal yr bp lower values 226 98 yr may reflect a major influence of subtropical water and diminished coastal upwelling processes 3 during the last thousand years high values between 355 105 and 253 207 yr indicate an increased influence of 14 c depleted water masses and of enso for the early twentieth century a r value of 253 207 yr was calculated

causes of concavity in longitudinal profiles of rivers a smooth concave configuration represents the most common quasi equilibrium profile of alluvial rivers in the present paper a generalized unified set of models for the longitudinal profile of rivers is presented various independent effects that drive concavity in longitudinal rivers are considered and classified the driving mechanisms considered here are 1 horizontal wavelike progradation of the river profile 2 abrasion of bed particles 3 aggradation of the river profile balancing subsidence at a constant speed and 4 the effect of tributaries adding sediment and water to the main stem of the river for simplicity it is assumed that the long term supply of water and sediment is constant hence a single discharge that is bank full discharge is used as an approximate characterization of the hydrologic regime for each point along the river sediment sorting has been studied elsewhere and is excluded from this analysis the river is assumed to have uniform bed material although the mean size may vary downstream it is concluded that the quasi equilibrium longitudinal profile created by each of the first three driving mechanisms mentioned above is concave the degree of concavity varies however depending on a set of physical conditions that can be identified by a set of dimensionless numbers a comparison of these numbers reveals the relative importance of the driving mechanisms for concavity a condition for concavity driven by the fourth mechanism that is tributary input is delineated