holocene landscape evolution and geoarcheology of low order streams in the rio grande basin san juan mountains colorado usa abstract this geoarcheological study investigates soil stratigraphy and geochronology of alluvial deposits to determine holocene landscape evolution within the hot creek la jara creek and alamosa river drainage basins in the san juan mountains of colorado geomorphic mapping and radiocarbon dating indicate synchronicity in patterns of erosion deposition and stability between drainage basins in all three basins the maximum age of mapped alluvial terraces and fans is 3300 cal yr bp a depositional period seen at both hot creek and the alamosa river begins 3300 to 3200 cal yr bp based on soil development short periods of stability followed by alluvial fan aggradation occur in the alamosa river basin 2200 cal yr bp a period of landscape stability at hot creek before 1100 cal yr bp is followed by a period of rapid aggradation within all three drainages between 1100 and 850 cal yr bp a final aggradation event occurred between 630 and 520 cal yr bp at la jara creek these patterns of landscape evolution over the past 3300 yr provide the framework for an archeological model that predicts the potential for buried and surficial cultural materials in the research area

the effects of soluble salts on the threshold shear velocity of fine sand wind tunnel tests were carried out to investigate the effects of two soluble salts nacl and kci on the threshold shear velocity of a well sorted fine sand a sensitive laser monitoring system was used to detect initial grain motion during the tests results indicate that even small amounts of soluble salt can significantly increase the threshold velocity of the sand because of cement like bonds formed between grains that tend to hold individual particles in place however with increasing concentration the salt crystal growth causes the initially smooth sand surface to develop an irregular frothy texture which also directly affects the threshold shear velocity this is indicated by increased scatter of data on the threshold shear velocity surface salt concentration plots for both nacl and kcl although data scatter is evident significant exponential relationships do exist between threshold shear velocity and surface salt concentration in each case

modeling the role of sodic alteration in the genesis of iron oxide copper gold deposits eastern mount isa block australia liberation of iron and potassium by widespread postmetamorphic albitization of country rocks was one of the likely contributing processes in the formation of both barren and mineralized magnetite chalcopyrite biotite gold hematite clinopyroxene actinolite apatite ironstones in the cloncurry district of the proterozoic mount isa block whole rock geochemical data indicate nearly immobile al ga ti zr during transformation of a variety of least altered rocks toward albitite the data indicate that the addition of na from a brine to the rock accompanied the loss of fe k ba rb ca sr co v mn pb and zn from those altered rocks and enrichment in the brine but that cu was not systematically stripped from a variety of wall rocks during albitization conversely the formation of metasomatic ironstones the immediate hosts to some cu au ores involves addition of most of the same elements that were lost during albitization the correlation between intensity of alteration its distribution and timing e g in breccias cored by ca 1530 1500 ma granitoids and convergence of all rock types toward magmatic stable isotope values provides strong evidence for a substantial component of igneous derived fluid simulations of the albitization process were carried out isothermally at 550 c and 350 mpa and polythermally from these conditions down to 400 c and 200 mpa using the gibbs minimization method with hch software and the unitherm database both simple fluid rock mixing models and more complicated reactor style algorithms used a range of input fluids from fluids equilibrated fluid with two feldspar granite through to na k ratios consistent with fluid inclusion results and geochemical data for initially unaltered wall rocks the apparent paradox of widespread albitization resulting from fluid released by two feldspar granites can be explained by relatively small shifts away from the k feldspar albite equilibrium curve and even isothermal model fluids derived from two feldspar granites produce albitites in calc silicate rocks outboard of granite proximal k feldspar clinopyroxene skarns matching field patterns those models with fluid na k ratios similar to those of fluid inclusions produced the most realistic alteration assemblages dominated by albite for both isothermal conditions and decreasing temperature which approximate those observed in the field pixe and microthermometric data on fluid inclusions from quartz in two feldspar quartz monzonite and pegmatite at the top of the mount angelay pluton indicate bulk na k molar ratios in the fluid of between 10 and 20 considerably higher than our thermodynamically calculated values for fluid in equilibrium with two feldspar granite of around 3 at 550 c 350 mpa such shifts may have been attained by admixture of magmatic hydrothermal fluid with small amounts of nacl brines trapped along grain boundaries in scapolite bearing calc silicate wall rocks by fluid immiscibility due to high initial co2 contents in the felsic intrusions by contributions from mafic magmas or from dissolution of salt rich layers into the intrusions prior to crystallization and fluid release with increasing amounts of fluid rock interaction in the models the fluids were enriched in k fe and ca approaching compositions observed in fluid inclusions in the ore deposits these fluids reacted with pelitic rocks which are common ore hosts would produce magnetite clinopyroxene biotite actinolite alteration at high temperature similar to the proximal alteration around ore deposits we infer that precipitation of sulfides in the cu au deposits was the result of mixing of cu bearing brine of ultimately magmatic origin but modified extensively via albitization with sulfur bearing fluids or reaction of the brine with sulfur bearing rocks when cu was absent from the initial magmatic fluid barren ironstones may have been the result

the lightning creek sill complex cloncurry district northwest queensland a source of fluids for fe oxide cu au mineralization and sodic calcic alteration the lightning creek cu au prospect is hosted by a cogenetic suite of plutonic i type granitoids the dominant rock type is a porphyritic quartz monzodiorite that is intruded by more fractionated rocks including monzogranite and alkali feldspar granite a series of flat lying sills are interpreted to be late stage differentiates based on their timing mineralogy and chemistry in parts of the prospect there is pervasive sodic calcic alteration pyroxene after amphibole albite after k feldspar and oligoclase of the plutonic rocks this alteration predates sill emplacement and is unrelated to veining or fracturing of any kind the presence of small amounts of carbonate in the altered rocks suggests that the fluids were co2 bearing quartz and feldspar separates from these altered rocks have oxygen isotope compositions similar to those from fresh quartz monzodiorite suggesting that the fluids were hot and of magmatic composition sodium and ca were added and k fe cl and cu were stripped during what is interpreted as an autometasomatic event the sills display considerable textural and mineralogical complexity and evolved from equigranular quartzofeldspathic rocks aplites with magmatic chemistry to unusual fe rich rocks albite magnetite quartz that exhibit a range of bizarre spherulitic textures some of the albite and magnetite in the sills is secondary albite forms pseudomorphs after k feldspar na fe ca alteration along sill margins and within sills at the contacts between different textural zones halos of disseminated magnetite clinopyroxene fe ca na alteration are developed adjacent to early magnetite veins fluid inclusion studies indicate that these rocks crystallized at temperatures in excess of 500 c and at pressures in excess of 1 5 kbar the range of spherulitic textures is taken to indicate crystallization under hydrous conditions with the episodic release of a fluid phase this magmatic fluid phase was dominated by h2o co2 and chlorine and underwent phase separation into a co2 rich vapor and a hypersaline brine 33 55 wt nacl equiv the hypersaline fluid was enriched in fe 10 wt and cu 1 wt pixe analysis in addition to na k and ca where this fluid was retained within fe rich portions of the sills it caused ca fe na alteration pyroxene albite magnetite growth at the expense of quartz where the fluid was expelled from the sills it produced quartz magnetite clinopyroxene albite veins broadly coeval with the early magnetite veins although rich in cu these granitoid derived magmatic fluids did not generate significant cu au mineralization perhaps because of the high temperatures involved and or a lack of reduced sulfur in the fluids or host rock however the amount of iron present is estimated from the aeromagnetic anomaly to be in excess of 2 000 million tonnes mt a later generation of calcite chlorite pyrite chalcopyrite veins contain traces of cu au mineralization fluid inclusion and stable isotope work indicate that these veins probably crystallized from cooler 200 c more dilute 15 28 wt nacl equiv fluids perhaps generated by the admixture of a meteoric component the conclusions reached in this study have implications for understanding the genesis of fe oxide cu au deposits and related sodic calcic alteration the study indicates the potential for co2 rich granitoid magmas to evolve hypersaline fe and cu rich fluids capable of causing intense magnetite veining and cu au mineralization autometasomatic sodic calcic alteration of the granitoids may be an important precursor to mineralization contributing fe k cu and cl to the magmatic fluids

osl chronology and possible forcing mechanisms of dune evolution in the horqin dunefield in northern china since the last glacial maximum the evolution processes and forcing mechanisms of the horqin dunefield in northern china are poorly understood in this study systematic osl dating of multiple sites is used together with pollen analysis of a representative section in order to reconstruct the evolution of the dunefield since the last glacial maximum lgm our results show that there was extensive dune mobilization 25 10 ka transition to stabilization 10 8 ka considerable dune stabilization 8 3 ka and multiple episodes of stabilization and mobilization after 3 ka comparison of dune evolution of the dunefields in northern china during the holocene showed that asian monsoon and resultant effective moisture have played an important role in the evolution of dunefields at the millennial time scale further analysis indicated that the dune evolution in the horqin dunefield before 3 ka was synchronous with climatic changes however increasing human activity has impacted dune evolution during the last 3 ka c 2012 university of washington published by elsevier inc all rights reserved the evolution processes and forcing mechanisms of the horqin dunefield in northern china are poorly understood in this study systematic osl dating of multiple sites is used together with pollen analysis of a representative section in order to reconstruct the evolution of the dunefield since the last glacial maximum lgm our results show that there was extensive dune mobilization 25 10 ka transition to stabilization 10 8 ka considerable dune stabilization 8 3 ka and multiple episodes of stabilization and mobilization after 3 ka comparison of dune evolution of the dunefields in northern china during the holocene showed that asian monsoon and resultant effective moisture have played an important role in the evolution of dunefields at the millennial time scale further analysis indicated that the dune evolution in the horqin dunefield before 3 ka was synchronous with climatic changes however increasing human activity has impacted dune evolution during the last 3 ka c 2012 university of washington published by elsevier inc all rights reserved

tourmaline b isotopes fingerprint marine evaporites as the source of high salinity ore fluids in iron oxide copper gold deposits carajas mineral province brazil the carajas mineral province in northern brazil contains a variety of world class 100 mt ore iron oxide copper gold iocg deposits including the only archean examples of this deposit class e g the igarape bahia alemao and salobo deposits tourmaline of schorldravite composition a common gangue mineral in these deposits precipitated shortly prior to and after the ore assemblage a boron isotope study of texturally different tourmaline from three iocg deposits igarape bahia salobo and sossego using secondary ion mass spectrometry sims provides new evidence in the long standing debate of magmatic versus non magmatic sources for the high salinity up to 50 wt nacl equiv of ore fl uids in these deposits values of 11 b from 14 to 26 5 for the igarape bahia and salobo deposits confi rm marine evaporite derived brines in the ore fl uids whereas lower 11 b values for the igarape bahia deposit 5 8 to 8 8 suggest that these fl uids may have mixed with an isotopically different hydrothermal fl uid or one that had a mixed boron source more variable and isotopically lighter boron in tourmaline from the sossego deposit 8 to 11 is attributed to mixed sources including light boron leached from felsic intrusive and volcanic host rocks and heavy boron derived from marine evaporites the boron isotope data indicate that the characteristic high salinity of the ore fl uids in the carajas mineral province was acquired by the interaction of hydrothermal fl uids with marine evaporites for iocg deposits that contain tourmaline as a common gangue mineral boron isotopes offer a valuable tool to constrain the high salinity source problem which is a critical issue in metallogenesis of iocg deposits worldwide

the rate of granule ripple movement on earth and mars abstract the rate of movement for 3 and 10 cm high granule ripples was documented in september of 2006 at great sand dunes national park and preserve during a particularly strong wind event impact creep induced by saltating sand caused 24 granules min 1 to cross each cm of crest length during wind that averaged 9 m s 1 at a height well above 1 m which is substantially larger than the threshold for saltation of sand extension of this documented granule movement rate to mars suggests that a 25 cm high granule ripple should require from hundreds to thousands of earth years to move 1 cm under present atmospheric conditions

pliocene and pleistocene geologic and climatic evolution in the san luis valley of south central colorado rogers k l larson e e smith g katzman d smith g r cerling t wang y baker r g lohmann k c repenning c a patterson p and mackie g 1992 pliocene and pleistocene geologic and climatic evolution in the san luis valley of south central colorado palaeogeogr palaeoclimatol palaeoecol 94 55 86 sediments of the alamosa formation spanning the upper part of the gauss and most of the matuyama chrons were recovered by coring in the high 2300 m san luis valley of south central colorado the study site is located at the northern end of the rio grande rift lithologic changes in the core sediments provide evidence of events leading to integration of the san luis drainage basin into the rio grande the section which includes the huckleberry ridge ash 2 02 ma and spans the entire matuyama chron contains pollen and invertebrate and vertebrate fossils stable isotope analyses of inorganic and biogenic carbonate taken over most of the core indicate substantially warmer temperatures than occur today in the san luis valley at the end of the olduvai subchron summer precipitation decreased summer pan evaporation increased and temperatures increased slightly compared to the earlier climate represented in the core by the end of the jaramillo subchron however cold wet and warm dry cycles become evident and continue into the cold wet regime associated with the deep sea oxygenisotope stage 22 glaciation previously determined from outcrops at the same locality correspondence between the hansen bluff climatic record and the deep sea oxygen isotope record oxygen isotope stages from about 110 18 is apparent indicating that climate at hansen bluff was responding to global climatic changes

new data for late pleistocene pinedale alpine glaciation from southwestern colorado abstract new cosmogenic surface exposure ages of moraine crest boulders from southwestern colorado are compared with published surface exposure ages of boulders from moraine complexes in north central colorado and in west central fremont lake basin wyoming 10be data sets from the three areas were scaled to a single 10be production rate of 5 4 at g yr at sea level and high latitude slhl which represents the average 10be production rate for two high altitude mid latitude sites in the western united states us and austria multiple nuclide ages on single boulders indicate that this 10be production rate yields ages comparable to those calculated with a commonly used 36cl production scheme the average age and age range of moraine crest boulders on terminal moraines at the southwestern colorado and wyoming sites are similar indicating a retreat from their positions 16 8 36cl ka cosmogenic ages in this paper are labeled 10be or 36cl ka or just ka when both 10be or 36cl ages are being discussed radiocarbon ages are labeled 14c ka calibrated radiocarbon are labeled cal ka and calendar ages are labeled calendar ka errors 1 associated with ages are shown in tables radiocarbon ages were calibrated using the data of hughen et al science 303 2004 202 this suggests a near synchronous retreat of pinedale glaciers across a 470 km latitudinal range in the middle and southern rocky mountains hypothetical corrections for snow shielding and rock surface erosion shifts the time of retreat to between 17 2 and 17 5 10be ka at pinedale wyoming and between 16 3 and 17 3 36cl ka at hogback mountain colorado

the dynamic effects of moisture on the entrainment and transport of sand by wind abstract there is an increasing awareness of the influence of surface moisture on aeolian entrainment and transport of sediment existing wind tunnel studies have shown the impact of a limited range of moisture contents on entrainment thresholds but similar investigations are lacking in the field the research reported here investigated the influence of changes in surface moisture content on sand entrainment and transport on a meso tidal beach in anglesey north wales high frequency 1 hz wind velocities measured with hot wire anemometers were combined with grain impact data from a sensit monitor and mass flux measurements from a standard sand trap surface and near surface moisture contents were assessed gravimetrically from surface sand scrapes and also directly by using a thetaprobe critical threshold values for entrainment were specified using a modified form of the time fraction equivalence method stout j e zobeck t m 1996a establishing the threshold condition for soil movement in wind eroding fields proceedings of the international conference on air pollution from agricultural operations mwps c 3 kansas city 7 9 february 1996 pp 65 71 results indicate a time dependent change in dominant control of the sand transport system from moisture to wind speed dependent upon the moisture content of the surface sediment this interchange between controlling parameters on both entrainment and transport was very sensitive to prevailing moisture conditions and took place over a period of minutes to hours under conditions experienced in the experiments presented here the critical moisture threshold for sediment entrainment was determined to be between 4 and 6 higher than the 1 4 specified in previous wind tunnel experiments furthermore a moisture content of nearly 2 where moisture was adhered to transported sediment appeared to have little or no impact on the rate of sand flux

variable wind ripple migration at great sand dunes national park and preserve observed by timelapse imaging abstract granule ripples at great sand dunes national park and preserve gsdnpp were observed using inexpensive digital timelapse cameras over a 70 day period in winter 2010 2011 the ripples migrated during a handful of discrete events visible ripple movement occurred on only 11 days during the observation period the movement conditions are documented with hourly and 15 minute records from two nearby weather stations and by a cup anemometer at the site itself during the most prominent movement episode when local winds averaged 10 m s ripples of several sizes were observed simultaneously and a reciprocal relationship of ripple size and propagation speed was seen with small 10 cm ripples moving at 1 4 cm min and larger 80 cm ripples at 0 15 cm min ripple sizes and morphologies evolve throughout the observation period

stable isotope evidence for the origin of the mesoproterozoic starra au cu deposit cloncurry district northwest queensland new stable isotope data are consistent with a metasomatic origin for the controversial mesoproterozoic ironstone hosted au cu deposit at starra in the cloncurry district of northwest queensland this supports textural and mineralogical evidence that the ore bearing ironstones formed after the peak of metamorphism and were strongly controlled by brittle ductile deformation three dominant paragenetic stages are recognized these are 1 early widespread na ca metasomatism quartz albite scapolite actinolite 2 localized k fe metasomatism biotite magnetite hematite quartz pyrite and 3 mineralization quartz anhydrite barite hematite calcite gold pyrite chalcopyrite bornite chalcocite chlorite muscovite crosscutting anhydrite veins associated with carbonate or hematite are rare in the upper parts of the system but more common at depths greater than 700 m stage 3 minerals demonstrate that the ore fluid was highly oxidized a restricted range of fe oxide delta 18 o implies isotopically similar fluids were responsible for the magnetite ironstones and later hematite alteration associated with gold and sulfides between 0 2 to 3 3ppm except for one magnetite at 5 4ppm sulfur isotope compositions suggest that pyrite 0 2 to 3 9ppm chalcopyrite 5 8 to 0 7 and 14 6ppm bornite 4 7ppm chalcocite 0 1ppm and anhydrite 1 1 2 4ppm all formed from the same sulfur source anhydrite delta 34 s falls within the sulfide range suggesting that the sulfate inherited its delta 34 s through hydrolytic processes similar to those of some porphyry environments delta 13 c of ore stage carbonates range from 7 3 to 2 2 per mil temperatures based on stage 2 quartz magnetite pairs imply 400 degrees to 500 degrees c for ironstone formation and stage 3 calcite hematite pairs indicate 180 degrees to 330 degrees c for mineralization stage 2 fluid inclusions have homogenization temperatures between 345 degrees and 615 degrees c and salinities of 34 to 52 wt percent nacl equiv stage 3 fluid inclusions homogenized between 225 degrees and 360 degrees c and have salinities of 30 to 42 wt percent nacl equiv calculated delta 18 o fluid compositions for magnetite quartz and hematite calcite average 7 8 per mil and 9 5 per mil respectively fluid compositions for stages 2 and 3 fall within the range for a magmatic or metamorphic fluid however combined stable isotope and fluid inclusion data and thermodynamic considerations suggest a magmatic metasomatic origin for both the magnetite ironstones and the highly oxidized mineral assemblage associated with high grade gold deposition

cross sectional profiles of sand ripples megaripples and dunes a method for discriminating between formational mechanisms cross sectional profiles of sand ripples megaripples and sand dunes provide a useful tool for discriminating between formation by ripple and dune processes feature width defined as the basal break in slope along the profile to either side of the crest represents a good standard for comparison of profile attributes across more than three orders of magnitude aspect ratio height width as a function of log width separates measurements into clusters representing differing mechanisms of formation scaling both height and distance for individual profiles by feature width facilitates comparison of profile shapes across three orders of magnitude in width the data presented here should prove useful for evaluating possible mechanisms of origin for aeolian features observed remotely including on planetary bodies copyright 2012 john wiley sons ltd

palaeodunes intercalated in loess strata from the western chinese loess plateau timing and palaeoclimatic implications abstract dune systems represent important palaeoenvironmental archives and therefore provide valuable information on past climate conditions of dryland regions this study reports on several palaeodunes thickness ca 10 20 m intercalated in thick loess accumulations located in the lanzhou area of the western chinese loess plateau the intercalated dune sands are well sorted and have a typical aeolian dune structure with distinct cross bedded sand layers the chronology for these palaeodunes and the related loess sediments was established by quartz optically stimulated luminescence osl dating of 19 samples the dating results indicate that sand dune accumulation occurred at ca 35 25 ka which implies a period of increased aridity during dune formation this interpretation is in contrast to previously retrieved data from lake archives of the nearby deserts e g tengger and badain jaran desert indicating a humid episode during marine isotope stage 3 mis 3 the previously proposed humid mis 3a may not be universal over all of northwest china

the stabilizing role of bonding agents on the entrainment of sediment by wind well sorted fine sand pretreated with varying concentrations of two soluble salts mgcl2 and cacl2 was tested for threshold shear velocity in a recirculating wind tunnel these tests demonstrate that even relatively low surface salt concentrations can significantly increase threshold shear velocity however the relationships between surface salt concentration and threshold shear velocity for the two salts were not significantly different which is similar to the results for monovalent chlorides presented by nickling ecclestone a general relationship between threshold shear velocity and total surface soluble salt content is presented

on the origin and age of the great sand dunes colorado abstract over the past 100 yr several hypotheses have been proposed for the origin and age of the great sand dunes these hypotheses differ widely in the descriptions of dune morphometry the immediate source of eolian sand and when sand transport occurred the primary purpose of this paper is to evaluate these hypotheses and where warranted to present new ideas about the origin and age of the great sand dunes to evaluate the previous hypotheses we had to develop more detailed information about the surficial geology of the northern san luis valley thus we mapped the surficial geology of an area extending several tens of kilometers north south and west of the great sand dunes and examined subsurface stratigraphy in more than 200 wells and borings in addition we used relative dating criteria and several radiocarbon and osl ages to establish the chronology of surficial deposits and we determined the u pb ages of detrital zircons to obtain information about the sources of the sand in the great sand dunes the first principal finding of this study is that the lower part of the closed basin north of the rio grande referred to here as the sump is the immediate source of the sand in the great sand dunes rather than the late pleistocene flood plain of the rio grande the most widely accepted hypothesis a second principal finding is that the great sand dunes are older than late pleistocene they postdate the draining of lake alamosa which began 440 ka and predate the time when streams draining the west flank of the sangre de cristo mountains were deflected by incipient dunes that formed near the mountain front geomorphic and stratigraphic evidence indicate that this deflection occurred prior to the end of the next to last glaciation bull lake i e prior to 130 ka

an analysis of the current stability of the dune field at great sand dunes national monument using temporal tm imagery 1984 1998 abstract research conducted at the great sand dunes national monument 37 46 5 n 105 32 5 w thus far has focused on dune composition in order to explain formation dune migration to monitor aeolian processes and dune extent to detect encroachment in the eastern periphery a region that contains several visitor services these studies used a series of field techniques in conjunction with aerial photography however modern satellite data such as temporal tm imagery were not used to track changes of the dunes in this study i used two tm images from 1984 and 1998 to examine changes in dune extent a series of image enhancements including edge enhancements were applied and a supervised classification was conducted overall classification accuracies based on ground truth data ranged from 89 1998 to 91 1984 over the study period the amount of sand in the entire complex decreased in area by 416 4 ha dune grass decreased by 487 5 ha and a semidesert scrub category containing a mixture of bushes taller grasses and desert pavement increased by 1457 3 ha few changes in the extent and orientation of the main dune complex were observed however notable changes were observed in the periphery on the western side of the complex dune grass is slowly being converted to scrub as a result the extent of exposed sand is decreasing on the eastern side dune grass and scrub are increasing succeeding upon the dunes these results demonstrate the stability of the main dune complex and suggest that dune migration in the periphery is no longer a concern

interpretation of the complex dune morphology on mars dune activity modelling and a terrestrial analogue in this work we analyze a dark erg on mars that could be considered a mega dune draa where secondary dunes of different morphology are superposed over a main crescent shaped bedform primary dune the presence of a complex multi directional wind regime is indicated as one of the main causes for the accumulation of a tall draa presenting an analogy to the great sand dunes in colorado in both cases main regional winds from the sw blow in opposition to winds from the ne which are enhanced by the topography such a complex wind regime leads to the development of star and reversing dunes and is accurately predicted by atmospheric models on a regional and local scale signs of activity in the form of grainflow scars are also noted over the slip faces of many dunes suggesting that easterly winds are actively shaping the study draa in the present day climatic setting the presence of this draa on mars suggests a complex interaction between regional and local topographically controlled flows and a consistent availability of sand the future study of an analogue terrestrial site such as the great sand dunes could be fundamental for understanding the evolution of similar martian dune fields copyright 2012 john wiley sons ltd

luminescence dating of anthropogenic features of the san luis valley colorado from stone huts to stone walls the snake nest wall site and the crestone stone huts are in the northern san luis valley colorado and provide a unique opportunity to date high altitude archeological sites of unknown age and origin using optically stimulated luminescence osl we sampled sediment underlying foundation stones of these structures to establish a chronological framework for each site s construction osl dating of the quartz grains directly under the snake nest wall suggest that the stones and therefore the structure was most recently emplaced between 1855 and 1890 a d dating of the sediment beneath the crestone stone huts suggests the construction time of these huts is between 1860 and 1890 a d analysis of the equivalent dose de dispersion of the osl samples at snake nest wall and the crestone huts shows that the majority of sediments were fully bleached prior to deposition and the low scatter suggests that short term or shallow alluvial processes were the dominant transport for sediments in both cases the osl ages show that the construction was during very recent historical times although it is likely that the snake nest wall was rebuilt in the late 19th century further study is warranted at the snake nest wall since it shows signs of greater antiquity and a continued presence of human use the crestone huts are shown to be a product of railroad building during the boomtown days of lucky and crestone

middle pleistocene formation of the rio grande gorge san luis valley south central colorado and north central new mexico usa process timing and downstream implications abstract the rio grande is the fourth longest river in north america extending over 3 000 km from the rocky mountains to the gulf of mexico the pleistocene evolution of this river from individual subbasins into a coalesced fluvial system has been long debated herein we constrain the middle pleistocene evolution of the northernmost and largest rio grande basin the san luis basin and the timing of incision of the rio grande gorge based on new geologic mapping 3he surface exposure dating and u series dating on pedogenic carbonates 3he dating of shoreline deposits of closed basin lake alamosa and fluvially scoured bedrock along the rio grande gorge rim indicate the san luis basin was connected with southerly rio grande basins and progressively incised since 400 ka waning tectonic activity coupled with the hydrologic response to intensified middle pleistocene glaciation drove southward spillover of lake alamosa 400 ka expanding the river s drainage basin by nearly 24 000 km2 and adding recharge areas in the high altitude glaciated san juan tusas and sangre de cristo mountains geologic barriers to streamflow on the taos plateau were breached and basin tributary drainages were integrated and inset into their present canyons 250 200 ka with approximately 120 m of incision during 400 200 ka previous work demonstrates that southerly streamflow through basins south of the slb terminated into a large rift related bolson complex until the middle pleistocene and most axial rio grande river incision has occurred since 630 ka we propose that middle pleistocene integration of the slb was the major geomorphic event that eroded plio pleistocene tectonic and volcanic barriers and re established flow of the rio grande from the rocky mountains to the gulf of mexico

constraints on the age of the great sand dunes colorado from subsurface stratigraphy and osl dates abstract the age of the great sand dunes has been debated for nearly 150 yr seven ages ranging from miocene to late holocene have been proposed for them this paper presents new information chiefly subsurface stratigraphic data osl dates and geomorphic evidence that indicates that the great sand dunes began to form in the latter part of the middle pleistocene the dunes overlie a thick wedge of piedmont slope deposits which in turn overlies sediment of lake alamosa a paleolake that began to drain about 440 ka the wedge of piedmont slope deposits extends westward for at least 23 km and is as much as 60 m thick at a distance of 10 km from the sangre de cristo range ostracodes from one well indicate that the eastern shoreline of lake alamosa extended to within 4 3 km of where the great sand dunes eventually formed the time represented by the wedge of piedmont slope deposits is not known exactly but the wedge post dates 440 ka and was in place prior to 130 ka because by then the dunes overlying it were sufficiently close and tall enough to obstruct streams draining from the sangre de cristo range

episodic late holocene dune movements on the sand sheet area great sand dunes national park and preserve san luis valley colorado usa abstract the great sand dunes national park and preserve gsdnpp in the san luis valley colorado contains a variety of eolian landforms that reflect holocene drought variability the most spectacular is a dune mass banked against the sangre de cristo mountains which is fronted by an extensive sand sheet with stabilized parabolic dunes stratigraphic exposures of parabolic dunes and associated luminescence dating of quartz grains by single aliquot regeneration sar protocols indicate eolian deposition of unknown magnitude occurred ca 1290 940 715 80 320 30 and 200 120 yr ago and in the 20th century there are 11 drought intervals inferred from the tree ring record in the past 1300 yr at gsdnpp potentially associated with dune movement though only five eolian depositional events are currently recognized in the stratigraphic record there is evidence for eolian transport associated with dune movement in the 13th century which may coincide with the great drought a 26 yr long dry interval identified in the tree ring record and associated with migration of anasazi people from the four corners areas to wetter areas in southern new mexico this nascent chronology indicates that the transport of eolian sand across san luis valley was episodic in the late holocene with appreciable dune migration in the 8th 10 13th and 19th centuries which ultimately nourished the dune mass against the sangre de cristo mountains

late glacial and holocene record of climatic change in the southern rocky mountains from sediments in san luis lake colorado usa abstract large rapid climate changes occurred over the last glacial cycle in the southwestern united states and elsewhere in many regions of the world some of these changes were attributed to alternations between stadial and interstadial conditions in the north atlantic but intense debate exists on how climate anomalies in the north atlantic transmit to the southwest here we report a sediment record from san luis lake in southern colorado through analyses of grain size magnetic susceptibility mg ca total inorganic carbon 18 o and 13 c to indicate climatic and environmental changes in the southern rocky mountains over the last 16 5 ka we found that san luis lake remained hydrologically closed most of the time but overflowed during the second half of the mystery interval the big wet 15 7 14 9 ka and the latter part of the mid holocene the neopluvial 4 3 ka over the course of the last deglaciation san luis lake underwent a series of large millennial scale hydroclimatic changes such as the big dry 16 5 15 7 ka the big wet the bolling allerod dry 14 9 12 7 ka and the younger dryas wet 12 8 11 6 ka corresponding to warm cold phases in the high latitude northern hemisphere the north american monsoon waxed during the pre boreal interval 11 6 10 5 ka and waned through the holocene in phase with northward and southward displacement of the intertropical convergence zone itcz the san luis lake basin was relatively dry in the early holocene 10 5 6 7 ka wet and fluctuating in the mid holocene 6 7 2 6 ka and dry and less variable in the late holocene 2 6 0 ka we found evidence that extreme pluvial episodes of the southern rocky mountains and elsewhere in the american southwest were coeval with cold phases of the north pacific our results highlight the role of the north pacific in modulating atmospheric circulations over the region on millennial timescales

fine scale isotopic heterogeneities and fluids in the deep crust a40ar 39ar laser ablation and tem study of muscovites from a granulite eclogite transition zone spectacular exposures on holsnoy island western norway reveal that eclogites formed in situ from adjacent anhydrous granulites as a result of the localized infiltration of fluids stepwise and laser40ar 39ar experiments on muscovite from the eclogites have been used in conjunction with electron microprobe and transmission electron microscopy tem analysis to examine the behavior of argon under high pressure metamorphism and subsequent cooling muscovites with 0 10 0 17na na k yield40ar 39ar plateau and integrated ages in the range 450 465 ma 20 35 myr older than40ar 39ar muscovite plateau ages from adjacent eclogite samples laser40ar 39ar analyses on 75 m spots across two single muscovite grains from these samples are approximately uniform at 444 2 1 9 21 spots and 443 3 1 7 ma 26 spots however incremental heating of high sodic 0 18 0 32 na na k muscovite failed to yield a plateau laser40ar 39ar analyses of 100 areas from two single grains from this sample reveal complex two dimensional patterns with variable apparent ages in the range of 445 625 ma electron microprobe measurements obtained from one of the grains reveal that the paragonite content is also highly variable both parallel and perpendicular to 001 all the tem bright field images obtained from this mica yield information consistent with a single homogeneous phyllosilicate phase yet several of the selected area electron diffraction saed analyses document locally intergrown muscovite and sodic muscovite possibly in an exsolution relationship on a unit cell scale analytical electron microscopy aem analyses obtained over a 5 100 nm scale on the same sample reveal great range and heterogeneity inna k including pure muscovite although no separate paragonite domains were found in contrast muscovite grains with consistent laser spot fusion and plateau ages show little heterogeneity inna k at the microprobe level these data provide evidence of variable fluid mediated incorporation of excess40ar under high pressure eclogite facies conditions at the time of the development of the sodic muscovite compositions the data indicate that fine scale heterogeneities in argon isotopic compositions were generated during eclogite facies equilibration and were preserved as the rocks were cycled back to the surface despite the influx of free fluids in the rocks at high pressure the heterogeneities are best reconciled with local buffering and limited mobility of the fluids at least during the final stages of equilibration

carbon and oxygen isotope constraints on fluid sources and fluid wallrock interaction in regional alteration and iron oxide copper gold mineralisation eastern mt isa block australia the source of metasomatic fluids in iron oxide copper gold districts is contentious with models for magmatic and other fluid sources having been proposed for this study 18o and 13c ratios were measured from carbonate mineral separates in the proterozoic eastern mt isa block of northwest queensland australia isotopic analyses are supported by petrography mineral chemistry and cathodoluminescence imagery marine meta carbonate rocks ca 20 5 18o and 0 5 13c calcite and graphitic meta sedimentary rocks ca 14 18o and 18 13c calcite are the main supracrustal reservoirs of carbon and oxygen in the district the isotopic ratios for calcite from the cores of na ca alteration systems strongly cluster around 11 18o and 7 13c with shifts towards higher 18o values and higher and lower 13c values reflecting interaction with different hostrocks na ca rich assemblages are out of isotopic equilibrium with their metamorphic hostrocks and isotopic values are consistent with fluids derived from or equilibrated with igneous rocks however igneous rocks in the eastern mt isa block contain negligible carbon and are incapable of buffering the 13c signatures of co2 rich metasomatic fluids associated with na ca alteration in contrast plutons in the eastern mt isa block have been documented as having exsolved saline co2 rich fluids and represent the most probable fluid source for na ca alteration intrusion proximal skarn like cu au orebodies that lack significant k and fe enrichment e g mt elliott display isotopic ratios that cluster around values of 11 18o and 7 13c calcite indicating an isotopically similar fluid source as for na ca alteration and that significant fluid wallrock interaction was not required in the genesis of these deposits in contrast k and fe rich intrusion distal deposits e g ernest henry record significant shifts in 18o and 13c towards values characteristic of the broader hostrocks to the deposits reflecting fluid wallrock equilibration before mineralisation low temperature low salinity low 18o 10 calcite and co2 poor fluids are documented in retrograde metasomatic assemblages but these fluids are paragenetically late and have not contributed significantly to the mass budgets of cu au mineralisation

the noble gas systematics of late orogenic h2o co2 fluids mt isa australia abstract the noble gases he ne ar kr and xe are powerful geochemical tracers because they have distinctive isotopic compositions in the atmosphere crust and mantle this study illustrates how noble gases can be used to trace fluid origins in high temperature metamorphic and mineralising environments and at the same time provides new information on the composition of noble gases in deeper parts of the crust than have been sampled previously we report data for h2o and co2 fluid inclusions trapped at greenschist to amphibolite facies metamorphic conditions associated with three different styles of mineralisation and alteration in the proterozoic mt isa inlier of australia sulphide fluid inclusions are dominated by crustal 4he however co variations in fluid inclusion 20ne 22ne 21ne 22ne 40ar 36ar and 136xe 130xe indicate noble gases were derived from three or more reservoirs in most cases the fluid inclusions elemental noble gas ratios e g ne xe are close to the ranges expected in sedimentary and crystalline rocks however the elemental ratios have been modified in some of the samples providing evidence for independent pulses of co2 and interaction of co2 with high salinity aqueous fluids compositional variation is attributed to mixing of i magmatic fluids or deeply sourced metamorphic fluids characterised by basement derived noble gases with 20ne 22ne 8 4 21ne 22ne 0 4 40ar 36ar 40 000 and 136xe 130xe 8 ii basinal metamorphic fluids with a narrow range of compositions including near atmospheric values and iii noble gases derived from the meta sedimentary host rocks with 20ne 22ne 8 9 8 21ne 22ne these data provide the strongest geochemical evidence available for the involvement of fluids from two distinct geochemical reservoirs in mt isa s largest ore deposits in addition the data show how noble gases in fluid inclusions can provide information on fluid origins the composition of the crust s major lithologies fluid rock interactions and fluid fluid mixing or immiscibility processes

subseafloor phase equilibria in high temperature hydrothermal fluids of the lucky strike seamount mid atlantic ridge 37 17 n abstract as part of an integrated study conducted at the lucky strike seamount mid atlantic ridge 37 17 n in 2008 gas tight sampling devices were used to collect high temperature 300 c hydrothermal fluids issuing from sulfide structures distributed throughout the vent field located in the summit depression compared with previous observations from 1993 to 1997 the most substantial changes in vent fluid compositions are dramatically increased co2 concentrations 5 up to 133 mmol l and the observation of vent fluids enriched in dissolved chloride relative to seawater combined with an increase in 13 c co 2 values by 4 in 2008 the elevated co2 indicates replenishment of the magmatic heat source and may be indicative of a recent magmatic event the additional supporting fluid chemistry is however similar to that of the previous sampling intervals necessitating a reassessment of the subseafloor controls on vent fluid chemistry at lucky strike in the context of recently obtained geophysical data that provides the depth extent of a steady state magma chamber two phase behavior is indicated by the chloride variability in the vent fluids and comparison with experimental data for the associated chloride dependent partitioning of minor trace elements suggests the possibility of a similar source fluid for all the vent structures while limiting the likelihood of shallow phase separation and subseafloor mixing for the hydrothermal end members a recently calibrated fe mn geothermometer indicates minimum subseafloor equilibration temperatures of 350 385 c however constraints imposed by dissolved si cl in conjunction with geophysical observations are consistent with peak reaction conditions at temperatures of 430 475 c and pressures near the top of the axial magma chamber 410 480 bars where magmatic co2 becomes entrained in the circulating fluids the distance between the magma chamber and the seafloor at lucky strike is substantially greater than at most faster spreading ridges and we propose the resulting increased residence time in the up flow zone leads to the re equilibration of temperature sensitive transition metals at conditions less extreme than those associated with peak reaction agreement between experimental data thermodynamic model calculations and dissolved concentrations of fe cu zn h2 and h2s in the lucky strike fluids reinforce the hypothesis of ph redox equilibria for transition metals at relatively oxidizing conditions and temperatures predicted by the empirical fe mn geothermometer in situ ph measurements of the high temperature fluids exiting the seafloor are also consistent with the model calculations

inverse maximum gross bedform normal transport 1 how to determine a dune constructing wind regime using only imagery abstract it has been a goal of aeolian science to use bedforms as indicators of local and regional sediment transport and atmospheric circulation but even with the application of the rule of maximum gross bedform normal transport mgbnt the underdetermined nature of the problem has precluded its application in all but the most simple cases we present a method to apply the rule of mgbnt and its inverse imgbnt from analysis of aeolian dune crestlines derived from aerial imagery although the solutions to imgbnt analysis are non unique the possible transport vectors influencing bedform morphology can often be constrained by making inferences regarding bedform type e g transverse oblique or longitudinal resultant drift direction and the ratio of transport vector magnitudes the technique is demonstrated on the great sand dunes located in colorado usa this dune field has a wide array of dune morphologies eight crestline sets were identified and mapped imgbnt analysis and the subsequent constraint of possible solutions suggests that transport vectors from the southeast and southwest with a se sw transport ratio of 1 2 produce oblique north south oriented dunes that dominate the main dune field these results compare favorably with mgbnt analysis of meteorologic measurements from three stations located adjacent to the great sand dunes which predict dune types and orientations similar to those observed in their vicinity

dune mobility and vegetation cover in the southwest kalahari desert as part of a wider project investigating the palaeoenvironmental significance of partially vegetated linear dunes in the southwest kalahari data collected in the latter part of 1992 concerning dune movement and vegetation cover suggest that sediment transport is occurring on some dune surfaces and that the majority of surface activity occurs on the crests and upper slopes of the dunes the data suggest that the limiting variables on surface sediment movement vary on different parts of a dune on interdunes and lower dune slopes the primary limiting variable is available wind energy while on dune crests and upper slopes it is vegetation cover ground cover by litter has much greater importance in protecting the surface sediment from erosion than rooted vegetation from individual data points no evidence is found to support a threshold vegetation cover below which sediment movement occurs rather a gradient of activity is suggested whereby a reduction in vegetation cover increases the potential for sediment movement and surface change however dunes with differing amounts of mean vegetation cover display differing degrees of surface activity and at this scale a vegetation cover threshold in the region of 14 per cent may be recognized

twentieth century dune migration at the great sand dunes national park and preserve colorado relation to drought variability abstract the great sand dunes national park and preserve in southern colorado contains a large dune mass banked against the sangre de cristo mountains the dune mass is bounded for hundreds of kilometers by a vegetated low relief sand sheet with abundant active parabolic and barchanoid dunes the clear morphology of these dunes formed by winds mostly from the southwest and the unambiguous identification of these forms in remotely sensed images provide straightforward targets to assess temporal changes in dune position a digital database of georeferenced remote sensing images from 1936 to 1999 is used to quantify parabolic and barchan dune migration rates and surface reflectance an indicator of vegetation cover with evolving drought conditions in the twentieth century the total net migration of 13 parabolic dunes in 63 years is 0 31 to 0 66 km whereas 11 barchan dunes moved an average of 0 12 to 0 47 km compared to intervening wet year there is at least a three fold increase in average parabolic dune migration during well documented droughts in the 1930s 1950s and 1990s with a concomitant reduction in vegetation cover and surface water resources the landscape response to the most recent drought in the late twentieth century is documented by an average parabolic dune migration rate of 30 m year which is a six fold acceleration over prior wet years and similar to dune response during the 1930s a nonlinear threshold for parabolic dune migration is indicated with lower quartile palmer drought severity index values of