bristlecone pine tree rings and volcanic eruptions over the last 5000 yr many years of low growth identified in a western usa regional chronology of upper forest border bristlecone pine pinus longaeva and pinus aristata over the last 5000 yr coincide with known large explosive volcanic eruptions and or ice core signals of past eruptions over the last millennium the agreement between the tree ring data and volcano ice core data is high years of ring width minima can be matched with known volcanic eruptions or ice core volcanic signals in 86 of cases in previous millennia while there is substantial concurrence the agreement decreases with increasing antiquity many of the bristlecone pine ring width minima occurred at the same time as ring width minima in high latitude trees from northwestern siberia and or northern finland over the past 4000 5000 yr suggesting climatically effective events of at least hemispheric scale in contrast with the ice core records the agreement between widely separated tree ring records does not decrease with increasing antiquity these data suggest specific intervals when the climate system was or was not particularly sensitive enough to volcanic forcing to affect the trees and they augment the ice core record in a number of ways by providing confirmation from an alternative proxy record for volcanic signals by suggesting alternative dates for eruptions and by adding to the list of years when volcanic events of global significance were likely including the mid 2nd millennium bc eruption of thera

controls on alluvial fan long profiles water and debris flows exiting confined valleys have a tendency to deposit sediment on steep fans on alluvial fans where water transport of gravel predominates channel slopes tend to decrease downfan from 0 10 0 04 to 0 01 across wide ranges of climate and tectonism some have argued that this pattern reflects grain size fining downfan such that higher threshold slopes are required just to entrain coarser particles in the waters of the upper fan whereas lower slopes are required to entrain finer grains downfan threshold hypothesis an older hypothesis is that slope is adjusted to transport the supplied sediment load which decreases downfan as deposition occurs transport hypothesis we have begun to test these hypotheses for alluvial fan long profiles using detailed hydraulic and particle size data in sediment transport models on four alluvial fans in the western u s we find that channel hydraulic radiiare largely 0 5 0 9 m at fan heads decreasing to 0 1 0 2 m at distal margins we find that median gravel diameter does not change systematically along the upper 60 80 of active fan channels as slope declines so downstream gravel fining cannot explain most of the observed channel slope reduction however as slope declines channel bed sand cover increases systematically downfan from areal fractions of 20 above fan heads to distal fan values in excess of 70 as a result entrainment thresholds for bed material might decrease systematically downfan leading to lower slopes however current models of this effect alone tend to underpredict downfan slope changes this is likely due to off channel gravel deposition calculations that match observed fan long profiles require an exponential decline in gravel transport rate so that on some fans approximately half of the load must be deposited off channel every 0 20 1 4 km downfan this leads us to hypothesize that some alluvial fan long profiles are statements about the rate of overbank deposition of coarse particles downfan a process for which there is currently no mechanistic theory

measuring alluvial fan sensitivity to past climate changes using a self similarity approach to grain size fining death valley california the effects of climate change on eroding landscapes and the terrestrial sedimentary record are poorly understood using mountain catchment alluvial fan systems as simple analogues for larger landscapes a wide range of theoretical studies numerical models and physical experiments have hypothesised that a change in precipitation rate could leave a characteristic signal in alluvial fan sediment flux grain size and down system fining rate however this hypothesis remains largely untested in real landscapes this study measures grain size fining rates from apex to toe on two alluvial fan systems in northern death valley california usa which each have well exposed modern and ca 70 ka surfaces and where the long term tectonic boundary conditions can be constrained between them these surfaces capture a well constrained temporal gradient in climate a grain size fining model is adapted based on self similarity and selective deposition for application to these alluvial fans this model is then integrated with cosmogenic nuclide constraints on catchment erosion rates and observed grain size fining data from two catchment fan systems to estimate the change in sediment flux from canyon to alluvial fan that occurred between mid glacial and modern interglacial conditions in a fan system with negligible sediment recycling an approximately 30 decrease in precipitation rate led to a 20 decrease in sediment flux and a clear increase in the down fan rate of fining supporting existing landscape evolution models consequently this study shows that small mountain catchments and their alluvial fan stratigraphy can be highly sensitive to orbital climate changes over 105 year timescales however in the second fan system it is observed that this sensitivity is completely lost when sediment is remobilised and recycled over a time period longer than the duration of the climatic perturbation these analyses offer a new approach to quantitatively reconstructing the effects of past climate changes on sedimentation using simple grain size data measured in the field this article is protected by copyright all rights reserved

age and paleoclimatic significance of late holocene lakes in the carson sink nv usa abstract new dating in the carson sink at the termini of the humboldt and carson rivers in the great basin of the western united states indicates that lakes reached elevations of 1204 and 1198 m between 915 and 652 and between 1519 and 1308 cal yr b p respectively these dates confirm morrison s original interpretation lake lahontan geology of the southern carson desert professional paper 40 u s geol survey 1964 that these shorelines are late holocene features rather than late pleistocene as interpreted by later researchers paleohydrologic modeling suggests that discharge into the carson sink must have been increased by a factor of about four and maintained for decades to account for the 1204 m lake stand the hydrologic effects of diversions of the walker river to the carson sink were probably not sufficient by themselves to account for the late holocene lake level rises the decadal long period of increased runoff represented by the 1204 m lake is also reflected in other lake records and in tree ring records from the western united states

frequency and initiation of debris flows in grand canyon arizona 1 debris flows from 740 tributaries transport sediment into the colorado river in grand canyon arizona creating rapids that control its longitudinal profile debris flows mostly occur when runoff triggers failures in colluvium by a process termed the fire hose effect debris flows originate from a limited number of geologic strata almost exclusively shales or other clay rich fine grained formations observations from 1984 through 2003 provide a 20 year record of all debris flows that reached the colorado river in grand canyon and repeat photography provides a 100 year record of debris flows from 147 tributaries observed frequencies are 5 1 events year from 1984 to 2003 and historic frequencies are 5 0 events year from 1890 to 1983 logistic regression is used to model historic frequencies based on drainage basin parameters observed to control debris flow initiation and transport from 5 to 7 of the 16 parameters evaluated are statistically significant including drainage area basin relief and the height of and gradient below debris flow source areas variables which reflect transport distance and potential energy the aspect of the river channel which at least partially reflects storm movement within the canyon is also significant model results are used to calculate the probability of debris flow occurrence at the river over a century for all 740 tributaries owing to the variability of underlying geomorphic controls the distribution of this probability is not uniform among tributaries of the colorado river in grand canyon

development of vegetation in the central mojave desert of california during the late quaternary vegetation analysis of 47 neotoma packrat middens from the core of the central mojave desert of california reveal changes in desert plant community composition over the last ca 24 000 years bp one of the lowest and most arid locations in north america habitat currently dominated by mojave desert larrea tridentata creosote bush was occupied prior to ca 11 500 years bp by pinus monophylla pinon juniperus osteosperma utah juniper purshia mexicana bitterbush cercocarpus ledifolius mountain mahogany and prunus fasciculata desert almond woodland above 1000 m p monophylla juniperus woodland was widespread over the southern and central mojave desert however less than 150 km north of the central mojave j osteosperma steppe dominated the landscape paleoecologic records spanning latitude 358n ca 378n document a biogeographic boundary between milder moister environments to the south and the colder drier environments to the north at elevations b1000 m desert scrub species of lycium cooperi wolfberry salvia mojavensis mojave sage and the mesophytic prosopis juliflora mesquite occurred near the shore of pluvial lake mojave from ca 8810 to 8430 years bp larrea is not recorded on the xeric carbonate substrates 605 m until ca 6990 years bp then took over 2000 years to dominate the regional landscape modern vegetation associates such as thamnosma montana desert rue and atriplex confertifolia shadscale were established by ca 4500 years bp late holocene vegetation changes document neoglacial cooling and wetter conditions than today between ca 3490 and 2640 years bp paralleling the regional pluvial lake highstands and marsh deposit records decreases of mesophytic mojave desert associates in the granite mountains suggest climates similar to today s aridity between ca 1970 and 1440 years bp mesophytic species increase in middens

late holocene sedimentary environments and lake level fluctuations at walker lake nevada usa walker lake is a terminal lake located at the end of the walker river in western nevada lake level lowering of 50 m during the past 100 yr has led to deep incision by the walker river into the former lake bed this incision has exposed a sedimentary record of lake level fluctuations spanning at least the past 4000 yr fourteen new radiocarbon dates on plant material collected from diverse sedimentary environments including fluvial deltaic beach and offshore settings have led to the refinement of the late holocene lake level curve for this basin this new curve documents at least four highstands that have occurred since ca 3500 cal yr b p between elevations of 1245 and 1255 m the lake has also experienced several lowstands during this same period comparison of this outcrop based record to existing lake level curves and to an oxygen isotope curve for the past 1200 yr reveals many similarities between the records but also some differences therefore research should be conducted to integrate outcrop studies where absolute water depth and lake volume can be estimated with isotopic or other types of proxies from cored sediments that may have higher temporal resolution the extensive and nearly continuous outcrops along the lower walker river also provide valuable information on the relationships between various sedimentary features and their formative water depths in clastic dominated lacustrine systems the paleo water depth relationships deciphered here can be applied in other basins where outcrops are typically more limited to arrive at better constrained lake level records

numerical dating of a late quaternary spit shoreline complex at the northern end of silver lake playa mojave desert california a comparison of the applicability of radiocarbon luminescence terrestrial cosmogenic nuclide electron spin resonance u series and amino acid racemization methods a late quaternary spit shoreline complex on the northern shore of pleistocene lake mojave of southeastern california usa was studied with the goal of comparing accelerator mass spectrometry ams radiocarbon luminescence electron spin resonance esr terrestrial cosmogenic radionuclide tcn surface exposure amino acid racemization aar and u series dating methods the pattern of ages obtained by the different methods illustrates the complexity of processes acting in the lakeshore environment and highlights the utility of a multi method approach tcn surface exposure ages mostly 20 30 ka record the initial erosion of shoreline benches whereas radiocarbon ages on shells determined in this and previous studies within the spit supported by aar data record its construction at fluctuating lake levels from 16 to 10 ka luminescence ages on spit sediment 6 7 ka and esr ages on spit shells 4 ka are anomalously young relative to radiocarbon ages of shells within the same deposits the significance of the surprisingly young luminescence ages is not clear the younger esr ages could be a consequence of post mortem enrichment of u in the shells high concentrations of detrital thorium in tufa coating spit gravels inhibited the use of single sample u series dating detailed comparisons such as this provide one of the few means of assessing the accuracy of quaternary dating techniques more such comparisons are needed

thermoluminescence dating of sand dunes in rajasthan india we report here a new application of thermoluminescence tl for dating sand dunes this application assumes that exposure to sunlight causes bleaching of the geological tl of a sediment to a residual value and that tl accumulation starts again when the sediment is buried under fresh deposit and is thus shielded from the sun although the tl method has been applied to loess1 2 and ocean sediments3 4 the application of this technique to sand dunes provides the first reliable dating control for dune dynamics palaeoclimatology and spread of deserts

luminescence ages for alluvial fan deposits in southern death valley implications for climate driven sedimentation along a tectonically active mountain front abstract controversy exists over whether alluvial fan sedimentation along tectonically active mountain fronts is driven by climatic changes or tectonics knowing the age of sedimentation is the key to understanding the relationship between sedimentation and its cause alluvial fan deposits in death valley and throughout the arid southwestern united states have long been the subjects of study but their ages have generally eluded researchers until recently most mapping efforts have recognized at least four major relative age groupings q1 oldest q2 q3 and q4 youngest using observed changes in surface soils and morphology relation to the drainage net and development of desert pavement obtaining numerical age determinations for these morphologic stages has proven challenging we report the first optically stimulated luminescence osl ages for three of these four stages deposited within alluvial fans along the tectonically active black mountains of death valley deposits showing distinct remnant bar and swale topography q3b have osl ages from 7 to 4 ka whereas those with moderate to poorly developed desert pavement and located farther above the active channel q3a have osl ages from 17 to 11 ka geomorphically older deposits with well developed desert pavement q2d have osl ages 25 ka using this osl based chronology we note that alluvial fan deposition along this tectonically active mountain front corresponds to both wet to dry and dry to wet climate changes recorded globally and regionally these findings underscore the influence of climate change on alluvial fan deposition in arid and semi arid regions

santa barbara basin diatom and silicoflagellate response to global climate anomalies during the past 2200 years santa barbara basin sbb diatom and silicoflagellate assemblages are quantified from a box core record spanning ad 1940 2001 and an ocean drilling program hole 893a record from 220 bc to ad 1880 the combined relative abundance of the diatoms fragilariopsis doliolus and nitzschia interrupteseriata from continuous two year sampling intervals in the box core varies with sea surface temperature sst suggesting its utility in sst reconstruction the assemblage data from the odp 893a record indicate a broad interval of generally cooler ssts between ad 800 and 1350 which corresponds to the medieval climate anomaly mca a period of generally warmer temperatures across other regions of the northern hemisphere the assemblages also indicate an interval of generally warmer ssts between ad 1400 and 1800 a period of otherwise global cooling referred to as the little ice age lia the changes in assemblages of diatoms and silicoflagellates support the hypothesis that the widespread droughts of the medieval climate anomaly in the western us were associated with cooler eastern north pacific sst the box core assemblages have higher percentages of tropical and subtropical compared to temperate and subpolar species than the odp samples reflecting a response of phytoplankton communities to an unusual 20th century warming pseudonitzschia australis a diatom linked with domoic acid production begins to become more common 3 of the diatom assemblage in the box core only after ad 1985 suggesting a link to anthropogenic activity

atmospheric circulation during holocene lake stands in the mojave desert evidence of regional climate change it is commonly thought that the climate conditions that supported lakes over a period of years in the mojave desert in southern california only existed before 8 000 yr bp and that the environment has been arid since1 2 here we look at a drill core in the silver lake playa at the terminus of the mojave river and find holocene lake deposits which indicate that shallow lakes existed for at least a few decades these deposits were radiocarbon dated at 3620 70 and 390 90 yr bp corresponding to the early neo glacial and the little ice age respectively3 to identify the conditions necessary to produce these holocene lake events we have examined the modern climate and hydrological patterns that produce ephemeral lakes in this usually arid watershed available data indicate that there is a link between anomalous winter atmospheric conditions over the north pacific and mojave river floods that produced ephemeral lakes in the silver lake playa and that the mojave river filters out small to medium floods and allows only the extreme floods to reach the terminal playa and leave a record of the anomalous conditions we suggest that the late holocene lakes may have resulted from persistent similar atmospheric circulation patterns and winter floods

reconciling the roles of tectonism and climate in quaternary alluvial fan evolution tectonism and climate are the primary variables considered in conceptual models of alluvial fan evolution and basin fill architecture yet their roles and relative importance are far from resolved the madison range and madison river valley southwestern montana were affected during late quaternary time by varying degrees of tectonic activity as well as by climate change through multiple glaciations we are thus able to evaluate the impact of tectonism and climate on fan evolution two primary fan deposits were correlated along the range front they were deposited as proglacial outwash fans during glacial periods of increased sediment and water discharge during the present interglacial period these deposits were entrenched and secondary fans formed on lower fan areas temporal and stratigraphic relations indicate that climate was largely responsible for driving both fan aggradation and entrenchment in contrast stratigraphic relations between fan deposits and surface faults do not support a direct relation either spatial or temporal between local faulting and fan deposition because tectonism produces and maintains the relief necessary for fans to form its primary role is long term controlling the duration over which fan deposition may occur along a mountain front if quaternary alluvial fan evolution is representative of ancient alluvial fan evolution this study has important implications for interpreting primary controls on deposition of the ancient alluvial fan sequences

holocene landscape response to seasonality of storms in the mojave desert abstract new optically stimulated and radiocarbon ages for alluvial fan and lake deposits in the mojave desert are presented which greatly improves the temporal resolution of surface processes the new mojave desert climate landscape record is particularly detailed for the late holocene evidence from ephemeral lake deposits and landforms indicates times of sustained stream flow during a wet interval of the latter part of the medieval warm period at ca ad 1290 and during the little ice age at ca ad 1650 the former lakes postdate megadroughts of the medieval warm period whereas the latter match the maunder minimum of the little ice age periods of alluvial fan aggradation across the mojave desert are 14 9 cal ka and 6 3 cal ka this timing largely correlates to times of increased sea surface temperatures in the gulf of california and enhanced warm season monsoons this correlation suggests that sustained alluvial fan aggradation may be driven by intense summer season storms these data suggest that the close proximity of the mojave desert to the pacific ocean and the gulf of california promotes a partitioning of landscape process responses to climate forcings that vary with seasonality of the dominant storms cool season pacific frontal storms cause river flow ephemeral lakes and fan incision whereas periods of intense warm season storms cause hillslope erosion and alluvial fan aggradation the proposed landscape process partitioning has important implications for hazard mitigation given that climate change may increase sea surface temperatures in the gulf of california which indirectly could increase future alluvial fan aggradation

implications of a freshwater radiocarbon reservoir correction for the timing of late holocene settlement of the elk hills kern county california abstract uncertainties regarding the magnitude of freshwater radiocarbon reservoir effects can introduce random errors into dates on archaeological freshwater carbonates as a result many archaeologists avoid dating freshwater shells unless no other datable materials are available the chronology of prehistoric occupation of the former naval petroleum reserve no 1 npr 1 at elk hills kern county has been established with 50 radiocarbon dates on freshwater mussels gonidea and anodonta sp characterization of any freshwater radiocarbon reservoir effect is crucial for the accurate interpretation of inferred settlement and subsistence changes on the elk hills paired charcoal and freshwater mussels sampled from closely associated contexts were dated to identify a freshwater reservoir effect paired anodonta and gonidea sp shells were dated to investigate interspecific differences in fractionation results indicate that a 340 20 14 c yr correction should be applied to conventional 14 c dates on freshwater carbonates in the buena vista basin before calendar calibration evidence of interspecific differences is inconclusive dates recalibrated with the reservoir correction indicate that widespread occupation of the elk hills is correlated with increasing precipitation towards the end of the medieval climatic anomaly and during the little ice age suggesting that slough resource exploitation may have been driven by regional population pressure rather than drought related declines in aquatic productivity

glacial interglacial climate changes recorded by debris flow fan deposits owens valley california abstract it is hotly debated whether and how climate changes are recorded by terrestrial stratigraphy basin sediments produced by catchment alluvial fan systems may record past climate over a variety of timescales and could offer unique information about how climate controls sedimentation unfortunately there are fundamental uncertainties about how climatic variables such as rainfall and temperature translate into sedimentological signals here we examine 35 debris flow fan surfaces in owens valley california that record deposition throughout the past 125 000 years during which climate has varied significantly we show that the last full glacial interglacial cycle is recorded with high fidelity by the grain size distributions of the debris flow deposits these flows transported finer sediment during the cooler glacial climate and became systematically coarser grained as the climate warmed and dried we explore the physical mechanisms that might explain this signal and rule out changes in sediment supply through time instead we propose that grain size records past changes in storm intensity which is responsible for debris flow initiation in this area and is decoupled from average rainfall rates this is supported by an exponential clausius clapeyron style scaling between grain size and temperature and also reconciles with climate dynamics and the initiation of debris flows the fact that these alluvial fans exhibit a strong sustained sensitivity to orbital climate changes sheds new light on how eroding landscapes and their sedimentary products respond to climatic forcing finally our findings highlight the importance of threshold controlled events such as storms and debris flows in driving erosion and sedimentation at the earth s surface in response to climate change

timing of surficial process changes down a mojave desert piedmont abstract we measured 10 be and 26 al in 29 sediment samples to infer the history and millennial scale rates of change down a low gradient piedmont a common but enigmatic landform that dominates the mojave desert nuclide data suggest that a large volume of sediment was deposited on the proximal east range road piedmont in fort irwin california 75 500 yr ago since then this material has been stable or eroding slowly in contrast on the distal piedmont 3 5 km from the upland source basins soil stratigraphy suggests that there have been alternating periods of surface stability erosion and deposition over the last 70 000 yr nuclide data from samples amalgamated along cross piedmont transects suggest that long term average down gradient sediment speeds range from 9 cm yr 1 near the uplands to 22 cm yr 1 6 km down piedmont these speeds are similar to 10 be estimated sediment speeds down three other piedmonts in the mojave desert suggesting that piedmont surface morphologies dominated by shallow migrating channels have similar sediment transport rates the timing of surface process change down the east range road piedmont is determined by a combination of sediment available in the source basins sediment transport rates and the size of the piedmont

the rambla highstand shoreline and the holocene lake level history of tulare lake california usa abstract the stratigraphy associated with a highstand wave cut shoreline and with sites farther out into the lake plain constrain the holocene lake level history of tulare lake california usa as follows seven to eight major fluctuations in lake level occurred during the past 11 500 yr lake level was generally higher during the early holocene prior to 6000 cal yr bp peaking in two highstands 65 70 masl at 9500 8000 cal yr bp and 6900 5800 cal yr bp thereafter it fluctuated at lower amplitude until reaching a major highstand during the most recent millennium between 750 and 150 cal yr bp two lake level rises of lower amplitude were centered on 3300 and 1600 cal yr bp at least three probably brief lowstands

paleoceanographic history of the guaymas basin gulf of california during the past 15 000 years based on diatoms silicoflagellates and biogenic sediments high resolution records of calcium carbonate biogenic opal diatoms and silicoflagellates from western guaymas basin gravity core ggc55 and piston core jpc56 and eastern guaymas basin dsdp site 480 reveal a complex paleoceanographic history of the central gulf of california during the past 15 000 years prior to 6 2 ka the eastern and western guaymas basin proxy records were remarkably similar after conditions similar to those of today during the bolling allerod the younger dryas yd saw a major drop in diatom production coincident with increased calcium carbonate and tropical microfossils suggestive of el nino like conditions biosiliceous productivity began increasing during the latter part of the yd but it was only during the earliest holocene 11 6 to 11 0 ka that conditions similar to those of the bolling allerod returned to the central gulf between around 11 0 and 6 2 ka tropical diatoms and silicoflagellates were virtually absent from the central gulf as relatively cooler and fresher surface waters resembling those of the modern northern gulf were present in the central gulf beginning at about 6 2 ka tropical diatoms and silicoflagellates began increasing in the central gulf and coccoliths returned to western gulf sediments the onset of modern day monsoon conditions in the american southwest required the presence of warm ssts in the northern gulf which probably did not occur until after about 5 4 ka when tropical diatoms and silicoflagellates became relatively common in the central gulf modern east west contrasts which arise from late winter early spring coastal upwelling on the mainland side and lower diatom productivity on the western side of the gulf commenced between 6 2 and 5 4 ka possibly due to a shift in the direction of late winter early spring winds more towards the southeast or down the axis of the gulf this proposed wind shift might have ultimately been due to a late holocene strengthening of enso like conditions in the eastern equatorial pacific published by elsevier b v

alpha decay of 184os revealed by radiogenic 180w in meteorites half life determination and viability as geochronometer abstract the decay of the rare nuclide 184os by alpha emission to 180w has been theoretically predicted but was previously never observed in experiments variable excesses of 180w were recently observed for iron meteorites but the contribution to these excesses by 184os decay was regarded as insignificant here we present combined 180w and os w concentration data for meteorites and terrestrial rocks now indicating that the 180w heterogeneities can be explained by decay of 184os a combined 184os 180w isochron for iron meteorites and chondrites yields a decay constant value of 184 os of 6 49 1 34 10 14 a 1 half life 1 12 0 23 10 13 yr in good agreement with theoretical estimates the 184os 180w decay system may constitute a viable tracer and chronometer for important geological processes like core formation silicate differentiation or late accretion processes this is illustrated by a measured 180w deficit in terrestrial basalts relative to chondrites by 1 16 0 69 parts in 10 000 consistent with core formation 4 5 ga ago

infra red stimulated luminescence dating of holocene reworked silty sediments abstract infra red stimulated luminescence irsl ages are reported for holocene colluvial lacustrine and alluvial sediments and late glacial loess for colluvial sediments irsl ages concur with independent ages age overestimation occur in the case of very poorly bleached sediments such as young less than 2000 years floodloams for which a high suspended sediment load was deduced from sedimentation rates and actually observed floods age underestimates are observed for late glacial loess probably due to inadequate preheat procedures

rates and processes of soil development on quaternary terraces in cajon pass california field and laboratory analyses of soils on 11 well dated fluvial terraces spanning the past 0 5 m y demonstrate that a threshold governs changes in several morphological and chemical characteristics of increasingly older soils correlations with respect to time among iron species soil morphology and soil silt and clay demonstrate that the chronosequence at cajon pass reflects primarily an evolutionary largely time dependent trend and does not reflect differences in external factors such as climate most of the soil development on holocene terraces of the cajon pass area is due to physical incorporation of eolian dust and organic material into initially very permeable gravels this process decreases soil permeability and is conducive for an increase in the magnitude of chemical weathering latest pleistocene and older pleistocene soils have developed clay and authigenic iron oxide rich b horizons at the expense of organic matter rich a horizons and color b horizons as the extent of chemical weathering has increased this conversion of the soil from a noncolloidal system to a much more colloidal system takes place over a relatively short period of time 4 000 yr and is herein defined as a type of pedologic threshold in the cajon pass area the attainment of the threshold and subsequent development of the argillic b horizon of soils on latest pleistocene terraces occurred during the holocene thus the absence of argillic horizons in soils on holocene terraces is attributable to simply their younger age rather than to the pleistocene to holocene climatic change the threshold is a function of several variables including influx rate of eolian dust and initial soil permeability therefore the time required to attain the threshold will vary in chronose quences characterized by geomorphic or geographic settings that are different from conditions found in cajon pass

influence of late holocene hillslope processes and landforms on modern channel dynamics in upland watersheds of central nevada stratigraphic geomorphic and paleoecological data were collected from upland watersheds in the great basin of central nevada to assess the relationships between late holocene climate change hillslope processes and landforms and modern channel dynamics these data indicate that a shift to drier warmer climatic conditions from approximately 2500 to 1300 ypb led to a complex set of geomorphic responses the initial response was massive hillslope erosion and the simultaneous aggradation of both side valley alluvial fans and the axial valley system the final response was fan stabilization and axial channel incision as fine grained sediments were winnowed from the hillslope sediment reservoirs and sediment yield and runoff processes were altered the primary geomorphic response to disturbance for approximately the past 1900 years has been channel entrenchment suggesting that the evolutionary history of hillslopes has produced watersheds that are prone to incision the magnitude of the most recent phase of channel entrenchment varies along the valley floor as a function of geomorphic position relative to side valley alluvial fans radial fan profiles suggest that during fan building fan deposits temporarily blocked the flow of sediment down the main stem of the valley commonly creating a stepped longitudinal valley profile stream reaches located immediately upvalley of these fans are characterized by low gradients and alternating episodes of erosion and deposition in contrast reaches coincident with or immediately downstream of the fans exhibit higher gradients and limited valley floor deposition thus modern channel dynamics and associated riparian ecosystems are strongly influenced by landforms created by depositional events that occurred approximately 2000 years ago

isotopic evidence for climatic influence on alluvial fan development in death valley california at least three semiarid to arid cycles are recorded by delta sup 13 c values of organic matter in layers of rock varnishes on surfaces of hanaupah canyon and johnson canyon alluvial fans death valley california these isotopic paleoenvironmental signals are interpreted as indicating major periods of fan aggradation during relatively more humid periods and fan entrenchment during subsequent lengthy arid periods

chronology sedimentology and microfauna of groundwater discharge deposits in the central mojave desert valley wells california during the late pleistocene emergent groundwater supported persistent and long lived desert wetlands in many broad valleys and basins in the american southwest when active these systems provided important food and water sources for local fauna supported hydrophilic and phreatophytic vegetation and acted as catchments for eolian and alluvial sediments desert wetlands are represented in the geologic record by groundwater discharge deposits which are also called spring or wetland deposits groundwater discharge deposits contain information on the timing and magnitude of past changes in water table levels and thus are a source of paleohydrologic and paleoclimatic information here we present the results of an investigation of extensive groundwater discharge deposits in the central mojave desert at valley wells california we used geologic mapping and stratigraphic relations to identify two distinct wetland sequences at valley wells which we dated using radiocarbon luminescence and uranium series techniques we also analyzed the sediments and microfauna ostracodes and gastropods to reconstruct the specifi c environments in which they formed our results suggest that the earliest episode of high water table conditions at valley wells began ca 60 ka thousands of calendar yr b p and culminated in peak discharge between ca 40 and 35 ka during this time cold 4 12 c emergent groundwater supported extensive wetlands that likely were composed of a wet sedge rush tussock meadow mixed with mesic riparian forest after ca 35 ka the water table dropped below the ground surface but was still shallow enough to support dense stands of phreatophytes through the last glacial maximum lgm the water table dropped further after the lgm and xeric conditions prevailed until modest wetlands returned briefl y during the younger dryas cold event 13 0 11 6 ka we did not observe any evidence of wet conditions during the holocene at valley wells the timing of these fl uctuations is consistent with changes in other paleowetland systems in the mojave desert the nearby great basin desert and in southeastern arizona near the border of the sonoran and chihuahuan deserts the similarities in hydrologic conditions between these disparate locations suggest that changes in groundwater levels during the late pleistocene in desert wetlands scattered throughout the american southwest were likely driven by synopticscale climate processes

luminescence dating of loess from the northwest united states thermoluminescence tl and infra red stimulated luminescence irsl ages have been determined for loess from the columbia plateau region of southeast washington and northeast oregon u s a the irsl ed values and ages appear to be tending towards a maximum level lower than that of the tl ed values and ages in general the tl ages are in stratigraphic order and agree well with the age control in the form of 14c and tl dated tephra units derived from various eruptions of mount st helens

debris flows as geomorphic agents in the huachuca mountains of southeastern arizona abstract numerous debris flows occurred in the huachuca mountains of southeastern arizona during the summer rainy season of 1988 in areas that were burned by a forest fire earlier in the summer debris flows occurred following a major forest fire in 1977 as well suggesting a causal link between fires and debris flows abundant evidence of older debris flows preserved along channels and in mountain front fans indicates that debris flows have occurred repeteadly during the late quaternary in this environment soil development in sequences of debris flow deposits indicates that debris flows probably recur over time intervals of several hundred to a thousand years in individual drainage basins in the study area surface runoff in the steep drainage basins of the huachuca mountains is greatly enhanced following forest fires as the hillslopes are denuded of their vegetative cover water and sediment eroded from the hillslope regolith are rapidly introduced into the upper reaches of tributary channels by widespread rilling and slope wash during rainfall events this influx of water and sediment destabilizes regolith previously accumulated in the channel triggering debris flows that scour the channel to bedrock in the upper reaches following a debris flow the scoured trapezoidally shaped channel gradually assumes a swale shape and the percentage of exposed bedrock declines as material is introduced from the slopes debris flows do a tremendous amount of work in a very short time however and are the major channel forming events where the tributary channels enter larger trunk channels the debris flows serve as the main source of very coarse sediment the local slope and coarse particle distribution of the trunk channel depend on the competence of water flows in the channel to transport the material introduced by debris flows where the smaller channels drain directly to the mountain front debris flows create extensive alluvial fans which dominate the morphology of the basin range boundary time intervals between debris flows in the drainage basins of the huachuca mountains are probably controlled by complex interactions among climate forest fires and slope processes fires destroy the protective vegetation that stabilizes the upper catchment slopes and inhibits erosion however not every fire that burns a catchment causes debris flows because sufficient weathered material must accumulate in the upper channel reaches to initiate a large debris flow if such accumulation has not occurred the material introduced to a channel following a forest fire will move only a short distance down the channel thus the episodic nature of debris flows probably depends on rates of slope weathering and erosion which are in turn controlled by climate both directly and through vegetation and forest fires

paleoseismology of the johnson valley kickapoo and homestead valley faults clustering of earthquakes in the eastern california shear zone paleoseismic data from 11 trenches at seven sites excavated across the southern johnson valley kickapoo and homestead valley faults that ruptured in the 1992 landers earthquake as well as the northern johnson valley fault which did not fail in 1992 indicate that the return period for large surface rupturing events in this part of the eastern california shear zone is in the range of 5 15 ka the inferred slip rates based on their respective recurrence intervals are in the range of 0 2 0 6 mm yr for each of the faults studied a previous large earthquake ruptured the southern johnson valley and kickapoo faults about 5 ka b p the northern johnson valley fault also failed at about this time at 5 8 ka b p and may have been part of the same rupture in contrast the penultimate large earthquake that we identify on the homestead valley fault occurred about 15 ka b p much earlier than other faults involved in the 1992 rupture from these observations combined with paleoseismic work by others after the 1992 earth quake it appears that previous events along the southern johnson valley and kick apoo faults were different than those of 1992 and may have involved other fault segments it has been over 5 ka since the most recent rupture on the northern johnson valley fault therefore it is surprising that it did not fail in the 1992 rupture from our observations dextral shear appears to be distributed across the entire eastern california shear zone with individual faults taking only a small proportion of the overall slip release of this regional strain appears to occur in temporal clusters of large earthquakes with the 1992 event apparently the most recent of a sequence of late holocene 0 1 ka earthquakes that have ruptured the nine faults we have trenched in the southwestern mojave desert previous clusters of earthquake activity occurred in the early 8 9 ka and middle 5 6 ka holocene and possibly the latest pleistocene 15 ka

forest clearing and regional landsliding the influence of forest clearing on landsliding is central to longstanding concern over the effects of timber harvesting on slope stability here we document a strong topographic control on shallow landsliding by combining unique ground based landslide surveys in an intensively monitored study area with digital terrain modeling using high resolution laser altimetry and a coarser resolution regional study of 3224 landslides as predicted by our digital terrain based model landslides occur disproportionately in steep convergent topography in terrain predicted to be at low risk of slope failure a random model performs equally well to our mechanism based model our monitoring shows that storms with 24 hr rainfall recurrence intervals of less than 4 yr triggered landslides in the decade after forest clearing and that conventional monitoring programs can substantially underestimate the effects of forest clearing our regional analysis further substantiates that forest clearing dramatically accelerates shallow landsliding in steep terrain typical of the pacific northwest

late quaternary stratigraphy and luminescence geochronology of the northeastern mojave desert abstract the chronology of the holocene and late pleistocene deposits of the northeastern mojave desert have been largely obtained using radiocarbon ages our study refines and extends this framework using optically stimulated luminescence osl to date deposits from valjean valley silurian lake playa red pass and california valley of particular interest are eolian fine silts incorporated in ground water discharge gwd deposits bracketed at 185 140 and 20 50 ka alluvial fan deposits proved amenable for osl by dating both eolian sand lenses and reworked eolian sand in a matrix of gravel that occurs within the fan stratigraphy lacustrine sand in spits and bars also yielded acceptable osl ages these osl ages fill gaps in the geochronology of desert deposits which can provide data relevant to understanding the responses of several depositional systems to regional changes in climate this study identifies the most promising deposits for future luminescence dating and suggests that for several regions of the mojave desert sediments from previously undated landforms can be more accurately placed within correct geologic map units

extracting holocene paleohydrology and paleoclimatology information from modern extreme flood events an example from southern california abstract the extraction of paleohydrological and paleoclimatological information from a modern hydrological system shown to represent unique and extreme hydroclimatological conditions is illustrated by an example from the mojave river drainage basin in southern california the mojave river allows only the most extreme floods to reach its terminal basin in the silver lake playa and to form ephemeral lakes all the other floods are lost by transmission into the alluvial aquifer along its 200 km channel this filtering out of regular floods by the river provides an essential tool in establishing a physical link between atmospheric and hydrologic conditions we demonstrate such a link between anomalous present day atmospheric circulation patterns over the north pacific ocean extreme storms in southern california that produced the heaviest precipitation on record the largest floods of record in the mojave river watershed and ephemeral lakes in its terminal playa this physical link determines the possible cause of the formations of perennial short duration shallow lakes in silver lake playa during the late holocene and characterizes the hydroclimatic conditions that prevailed during these lacustrine episodes hydrological simulations of this river and its filtering character demonstrate that these lakes could have formed only if the most extreme modern storms and floods were more frequent in at least an order of magnitude during specific time episodes we conclude that such extreme hydroclimatic conditions occurred more frequent in past episodes during which the holocene lakes formed in turn this conclusion indicates that the cause of these storms and floods i e the anomalous atmospheric circulation pattern must have been more frequent this research outlines a way to extract information on holocene climates in hydrologic settings that demonstrate a unique cause and effect relationship

solar forcing of gulf of california climate during the past 2000 yr suggested by diatoms and silicoflagellates abstract high resolution records of the past 2000 yr are compared in a north south transect 28 n to 24 n of three cores from the eastern slopes of the guaymas carmen and pescadero basins of the gulf of california hereafter referred to as the gulf evenly spaced samples from the varved sediments in each core allow sample resolution ranging from 16 to 37 yr diatoms and silicoflagellates capture the seasonal variation between a late fall to early spring period of high biosiliceous productivity that is driven by northwest winds and a summer period of warmer more stratified waters during which these winds slacken and or reverse direction monsoonal flow as these winds decrease tropical waters enter the gulf and spread northward individual samples represent a composite of 7 to 23 yr of deposition and are assumed to record the relative dominance of the winter vs summer floral components intervals of enhanced summer incursion of tropical waters alternating with periods of increased late fall to early spring biosiliceous productivity are recorded in all three cores regularly spaced cycles 100 yr duration of octactis pulchra a silicoflagellate proxy for lower sst and high productivity and azpeitia nodulifera a tropical diatom occur between a d 400 and 1700 in the more nearshore carmen basin core nh01 21 26 3 n suggesting a possible solar influence on coastal upwelling cores bam80 e 17 27 9 n and nh01 26 24 3 n contain longer duration cycles of diatoms and silicoflagellates the early part of medieval climate anomaly a d 900 to 1200 is characterized by two periods of reduced productivity warmer sst with an intervening high productivity cool interval centered at a d 1050 reduced productivity and higher sst also characterize the record of the last 100 to 200 yr in these cores solar variability appears to be driving productivity cycles as intervals of increased radiocarbon production sunspot minima correlate with intervals of enhanced productivity it is proposed that increased winter cooling of the atmosphere above southwest u s during sunspot minima causes intensification of the northwest winds that blow down the gulf during the late fall to early spring leading to intensified overturn of surface waters and enhanced productivity a new silicoflagellate species dictyocha franshepardii bukry is described and illustrated

directly dated mis 3 lake level record from lake manix mojave desert california usa abstract an outcrop based lake level curve constrained by 70 calibrated 14 c ages on anodonta shells indicates at least 8 highstands between 45 and 25 cal ka bp within 10 m of the 543 m upper threshold of lake manix in the mojave desert of southern california correlations of manix highstands with ice marine and speleothem records suggest that at least the youngest three highstands coincide with dansgaard oeschger d o stadials and heinrich events 3 and 4 the lake level record is consistent with results from speleothem studies in the southwest that indicate cool wet conditions during d o stadials notably highstands between 43 and 25 ka apparently occurred at times of generally low levels of pluvial lakes farther north as interpreted from core based proxies mojave lakes may have been supported by tropical moisture sources during oxygen isotope stage 3 perhaps controlled by southerly deflection of pacific storm tracks due to weakening of the sea surface temperature gradient in response to north atlantic climate perturbations

late holocene sea temperatures along the central california coast mussel shells from central california coastal archaeological sites record changes in sea surface temperatures in the past 2000 years water temperatures inferred from oxygen isotopes in the shells were about 1 c cooler than present and stable between 2000 and 700 yr ago between about 700 and 500 yr ago seasonal variation was greater than present with extremes above and below historic levels water temperatures were 2 3 c cooler than today 500 300 yr ago the interval of variable sea temperatures 700 500 yr ago partially coincided with an interval of drought throughout central california a coincident disruption in human settlement along the coast suggests movements of people related to declining water sources quantities of fish bone in central coast middens dating to this same period are high relative to other periods and the remains of northern anchovies a species sensitive to changing oceanographic conditions are also abundant the continued use of local fisheries suggests that changes in settlement and diet were influenced more by drought than by a decrease in marine productivity as fish provided a staple during an interval of low terrestrial productivity

environmental controls on the evolution of alluvial fans in buena vista valley north central nevada during late quaternary time abstract alluvial fans in buena vista valley north central nevada formed during at least four time periods during the late quaternary as evidenced by four temporally and spatially constrained fan deposits qf1 qf2 qf3 and qf4 from oldest to youngest the stratigraphic relation between qf3 and remnant beach ridge deposits of pluvial lake lahontan the latter of which forms a paleoclimatic proxy for this region indicate that climate and climatic change are primary controls on the evolution of fans qf3 deposition commenced during or immediately following the highstand of lake lahontan between 13 5 and 12 5 ka and continued as lake levels plummeted to levels below the sill elevation of the buena vista subbasin we interpret qf3 to be the result of a dynamic interplay between hillslope vegetation soils and hydrology as they are conditioned by climate and climatic change accelerated pedogenesis because of aerosolic silts and clays from exposed lake sediments coupled with decreased density of vegetation reduced infiltration capacity of the hillslope soils in interscrub areas in contrast to fans in more arid climates the increased runoff was not associated with increased coarse grained sediment yield and major fan aggradation discharge from sediment deficient basins entrenched older proximal fan deposits and transported the sediment downfan to form secondary fans the process response of qf3 to climatic change in north central nevada is especially illustrative of the impact of climate on landscapes in that 1 it results from changes in basin sediment and or water yield that occur in less than 500 years and 2 it differs from responses in arid climates to the south and more semiarid climates to the north the magnitude of vegetation change including type and density of vegetation and the relative proportion of interscrub area is particularly critical to water and sediment yield unit qf2 an early late pleistocene deposit may also have been deposited during a transition from a wetter to drier climate the synchronism of aggradation stability and entrenchment on all fans along the perimeter of buena vista valley in the late quaternary is further support for the dominance of climatic control on fan process on the other hand the impact of other extrinsic variables on fan process including tectonism base level and basin lithology all of which vary along the perimeter is negligible

use of multiparameter relative age methods for age estimation and correlation of alluvial fan surfaces on a desert piedmont eastern mojave desert california numerical and calibrated age determinations of the late quaternary alluvial fan deposits of the soda mountains piedmont in the mojave desert provide an opportunity to study the utility of the multiparameter relative age ra method for distinguishing and mapping geomorphic surfaces on a desert piedmont most ra parameters could not discriminate between deposits of holocene age although pavements have formed over locally significant parts of surfaces as young as middle holocene several parameters including lithologic composition particle size soil development and varnish cover permit distinguishing between holocene surfaces and late pleistocene surfaces statistically significant differences in initial particle size and lithology of the deposits inferred to be the result of complex interaction among hillslope alluvial fan and eolian processes and climatic change create conditions unfavorable for use of most ra techniques in contrast soil profile development and varnish cover data are successful in discrimination among deposits of holocene and pleistocene age this is attributed to the development of pedogenic features and varnish that are strongly dependent on dust influx and to the relatively minor dependence of these features on differences in the depositional character of the fan

integration of micro gravity and geodetic data to constrain shallow system mass changes at krafla volcano n iceland new and previously published micro gravity data are combined with insar data precise levelling and gps measurements to produce a model for the processes operating at krafla volcano 20 years after its most recent eruption the data have been divided into two periods from 1990 to 1995 and from 1996 to 2003 and show that the rate of deflation at krafla is decaying exponentially the net micro gravity change at the centre of the caldera is shown using the measured free air gradient to be 85 gal for the first and 100 gal for the second period after consideration of the effects of water extraction by the geothermal power station within the caldera the net gravity decreases are 73 17 gal for the first and 65 17 gal for the second period these decreases are interpreted in terms of magma drainage following a mogi point source model we calculate the mass decrease to be 2 1010 kg year reflecting a drainage rate of 0 23 m3 s similar to the 0 13 m3 s drainage rate previously found at askja volcano n iceland based on the evidence for deeper magma reservoirs and the similarity between the two volcanic systems we suggest a pressure link between askja and krafla at deeper levels at the lower crust or the crust mantle boundary after the krafla fires co rifting pressure decrease of a deep source at krafla stimulated the subsequent inflow of magma eventually affecting conditions along the plate boundary in n iceland as far away as askja we anticipate that the pressure of the deeper reservoir at krafla will reach a critical value and eventually magma will rise from there to the shallow magma chamber possibly initiating a new rifting episode we have demonstrated that by examining micro gravity and geodetic data our knowledge of active volcanic systems can be significantly improved

radiocarbon dating of minute gastropods and new constraints on the timing of late quaternary spring discharge deposits in southern arizona usa abstract gastropod shells are commonly preserved in quaternary sediments but are often avoided for radiocarbon dating because some taxa incorporate 14 c deficient carbon during shell formation recently brennan and quade 1997 quat res 47 329 336 found that some minute taxa vallonia pupilla and succineidae appear to yield reliable 14 c ages for late pleistocene samples a more rigorous evaluation of the 14 c inventory of minute gastropods is presented here which involved measuring the 14 c activity of specimens collected live in two geologic settings that maximize the potential for ingestion of old carbon 1 alluvium dominated by paleozoic carbonate rocks and 2 adjacent to extant springs with highly 14 c deficient water present at the surface we found that several minute taxa including vallonia incorporate significant and variable amounts of old carbon 2 to 30 during shell formation the 14 c activities of the land snails pupilla blandi and euconulus fulvus however are indistinguishable from the 14 c activity of live plants the 14 c activity of the semi aquatic gastropod catinella sp family succineidae deviates from modern values in the presence of 14 c deficient water by an amount equivalent to 10 of the local carbon reservoir effect these results imply that at least some minute gastropods can provide reliable 14 c ages even when 14 c deficient carbon is readily available to demonstrate an application of our findings we 14 c dated shells from p muscorum e fulvus and succinea sp family succineidae recovered from the coro marl a late pleistocene spring fed marsh deposit exposed at the murray springs paleoindian site in the san pedro valley of southern arizona usa radiocarbon ages obtained from the minute gastropods show that the unit was deposited between 25 000 and 13 000 14 c years ago the marl is situated 15 m above the modern water table at murray springs and is similarly positioned in discontinuous outcrops along a 150 km stretch of the san pedro valley thus the 14 c ages of minute gastropods presented here may be used to infer the timing of high water table levels throughout the valley

sedimentologic and geomorphic variations in storm generated alluvial fans howgill fells northwest england in june 1982 a storm with a return period greater than 100 yr but lasting less than 2 5 hr destabilized hillslopes and produced a suite of geomorphologically and sedimentologically diverse alluvial fans thirteen major fans were deposited at the tributary junctions between small 2 catchments and two north flowing headwater streams of the river lune northwest england storm generated fans spread over or became inset into older stable fans and produced localized vertical accretion of as much as 3 m and lateral accretion of as much as 100 m sedimentary processes operating during deposition involved debris flow transitional flow and streamflow six facies types are recognized on the basis of depositional topography sedimentary texture and fabric and matrix content viscous debris flow d1 dilute debris flow d2 transitional flow t1 fluvial bars and lobes s1 s2 and fluvial sheet gravels s3 regionally streamflow deposition prevails over debris flow deposition and type s3 facies has the greatest areal extent temporal and spatial variations in facies deposition during the storm however resulted from water sediment ratio variations fan deposition involved an early phase of debris flow to transitional flow due to large inputs of sediment from hillslope failures this was followed by a systematic change to more dilute conditions resulting in streamflow deposition and eventually in channel incision a significant amount of geomorphic work and complex variations in sedimentary processes during the storm resulted in part from extensive overland flow and hillslope destabilization discriminant analyses indicate that catchment size channel gradient and percentage of area eroded during the storm controlled whether debris flow or streamflow facies dominated a fan sequence smaller steeper catchments had a greater percentage of the area yielding sediment and are dominanted by debris flows whereas larger catchments produced more runoff resulting in dilution and streamflow the results indicate that the facies sequences and fan entrenchment in the howgill fells which are typically considered products of longer term climatic change or tectonics in other localities are here primarily affected by thresholds related to catchment geomorphology by the type of sediment available and by the position within the storm cell

an enhanced role for the tropical pacific on the humid pleistocene holocene transition in southwestern north america abstract climate effects on landscape evolution during the late pleistocene holocene transition 14 6 8 ka in southwestern north america traditionally are linked to the activity of the north american monsoon and to vegetation change related to a decrease in winter precipitation acting in response to orbital cyclicity we performed an integrated analysis of regional alluvial fan lacustrine and paleobotanical records for the area comparing them with hemispheric and regional paleoclimate proxies our focus was on the potential role the tropical pacific has as a synoptic pattern modulator and moisture source for hydrogeomorphic activity in the region our analysis indicates that the onset of alluvial fan aggradation in most of the region at 13 5 ka could have been a response to semi permanent el nino like conditions in the tropical pacific which enhanced the frequency of winter frontal storms as well as increased penetration of tropical cyclones in the region the north american monsoon was restricted in extent and intensity until 7 ka and probably was not a major factor in alluvial fan aggradation a second stage of alluvial fan aggradation from 11 5 to 9 ka was dominated by hyper concentrated flows and sheet flood sedimentation along with deposition in fluvial settings storms were probably were linked to landfall of enhanced water vapor bands in the leading edge of winter extra tropical cyclones with moisture advected directly from the tropical pacific at 8 ka favorable conditions for the occurrence of these storms waned and storm tracks shifted northward analysis of modern analogs for storm types described above as prevalent during this period indicates that changes in circulation patterns across the tropical pacific can affect storm properties enough to explain the observed geomorphic effects regardless of other factors traditionally considered of large impact like vegetation change our results suggest that the tropical pacific plays a larger role than currently thought in landscape evolution of the region

medieval drought in the upper colorado river basin 1 new tree ring records of ring width from remnant preserved wood are analyzed to extend the record of reconstructed annual flows of the colorado river at lee ferry into the medieval climate anomaly when epic droughts are hypothesized from other paleoclimatic evidence to have affected various parts of western north america the most extreme low frequency feature of the new reconstruction covering a d 762 2005 is a hydrologic drought in the mid 1100s the drought is characterized by a decrease of more than 15 in mean annual flow averaged over 25 years and by the absence of high annual flows over a longer period of about six decades the drought is consistent in timing with dry conditions inferred from tree ring data in the great basin and colorado plateau but regional differences in intensity emphasize the importance of basin specific paleoclimatic data in quantifying likely effects of drought on water supply

a 1000 year record of temperature and precipitation in the sierra nevada abstract tree ring data from subalpine conifers in the southern sierra nevada were used to reconstruct temperature and precipitation back to a d 800 tree growth of foxtail pine pinus balfouriana and western juniper juniperus occidentalis ssp australis is influenced by nonlinear interactions between summer temperature and winter precipitation reconstruction of the separate histories of temperature and precipitation is feasible by explicitly modeling species and site differences in climatic response using response surfaces the summer temperature reconstruction shows fluctuations on centennial and longer time scales including a period with temperatures exceeding late 20th century values from ca 1100 to 1375 a d corresponding to the medieval warm period identified in other proxy data sources and a period of cold temperatures from ca 1450 to 1850 corresponding to the little ice age precipitation variation is dominated by shorter period decadal scale oscillations the long term record presented here indicates that the 20th century is anomalous with respect to precipitation variation a tabulation of 20 and 50 yr means indicates that precipitation equaling or exceeding 20th century levels occurred infrequently in the 1000 yr record