geomorphological significance of at many stations hydraulic geometry the critically important at a station hydraulic geometry ahg relationships relate hydraulic variables depth width or velocity to discharge in power law form the recently discovered at many stations hydraulic geometry amhg states that ahg exponents and coefficients are strongly correlated removing one parameter and lending amhgs to remote sensing discharge estimation despite the excitement there is an urgent need to clarify amhg s geomorphological significance for different hydraulic variables using data from 57 rivers in the u s and perturbation experiments we show that the width amhg is weak arising mainly from the mathematical construct involving the exponent in both the regressor and the regressand in contrast the depth amhgs result from geomorphological coevolution the similar time mean condition as argued for in the literature is sufficient but not necessary for rating curve convergence and in turn amhg for depth and velocity the predictive accuracy of amhgs whose coefficient of determination is unit dependent is similar to the flow percentile based downstream hydraulic geometry

polymetallic epithermal fissure vein mineralization topia durango mexico part i district geology geochronology hydrothermal alteration and vein mineralogy the topia district lies on the western flank of the sierra madre occidental structural arch and tertiary volcanic plateau beneath unmineralized mid tertiary silicic ignimbrites lies a kilometer thick sequence of early tertiary andesitic lavas and pyroclastics resting on mesozoic limestones and marine clastic sedimentary rocks laramide geanticlinal uplift along the sierra produced regional tilting and major fracturing of the volcanic pile following an interval of major erosion the sequence was intruded by granodiorite stocks and a northeast trending set of faults was mineralized as ag zn pb au cu rich fissure veins mineralization was followed by an interval of tectonic quiescence and then by eruption of more than a kilometer of oligocene silicic ignimbrites k ar dating of igneous rocks and mineralization yields an age of 46 1 m y for one of the granodiorite stocks a date of 43 8 m y for vein adularia 43 5 m y for hydrothermal celadonite and 44 0 m y for an andesite lava the k ar age of which was reset by the mineralization thermal event a date of 37 9 m y on the lowermost rhyolite welded tuff corresponds to initiation of widespread silicic ignimbrite eruptions in the sierra fissure filling mineralization was accompanied by development of hydrothermal alteration comprising 1 a narrow 1 m quartz sericite pyrite alteration envelope adularia chamosite kaolinite hematite celadonite that grades outward into 2 a variably silicified oxidative alteration zone that occupies the entire interval between veins at elevations in and above the ote horizon the distal oxidative facies envelopes consist of an inner intense celadonite quartz hematite calcite illite clinochlore subfacies and an outer subfacies of hematite calcite veinlets in nearly fresh rock at elevations below the ore horizon the local oxidative alteration facies is absent and the inner silicification zone passes outward into conventional propylitic facies envelopes chemical gains and losses and porosity changes have been monitored quantitatively across the succession of alteration envelopes oblique tensional slip along faults having gentle convex and concave irregularities and steps in the slip surface produced a succession of alternating dilatant fissure and convergent shear zone intervals along each fault open fissure intervals were filled by more or less symmetric crustified precipitation on the walls whereas wall rock replacement mineralization characterizes narrow veins in shear zone intervals in each vein ore occupies a restricted vertical interval of 100 to 200 m the ore horizon which grades downward into barren coarsegrained quartz rich filling and grades upward into barren cherty quartz calcite barite rich vein filling the ore horizon commonly contains subequal amounts of sphalerite pyrite galena and quartz with minor arsenopyrite marcasite chalcopyrite ag rich sulfosalts electrum chamosite muscovite and carbonates locally massive sphalerite as millimeter to centimeterthick crustification bands dominates the paragenetic sequence crustification bands dominated by quartz calcite and barite are predominantly fine grained they precipitated from lowsalinity boiling fluids intercalated bands dominated by sulfides are characteristically very coarse grained to 10 cm and have high salinity fluid inclusions 7 23 wt no primary vapor dominant inclusions have been recognized in sphalerite or pyrargyrite precipitation of crustification bands dominated by coarse grained sulfides was punctuated by recurrent effervescence of ore fluids within the ore horizon accompanied by precipitation of electrum

multistage damming of the rhine river by tephra fallout during the 12 900 bp plinian laacher see eruption germany syn eruptive rhine damming i abstract the rhine the largest river in western europe was dammed during the plinian laacher see eruption lse 12 900 bp damming during the climactic plinian episode of lse occurred both upstream and downstream of the broad tectonic lower neuwied basin lnb that interrupts the narrow rhine canyon we here document details of the upstream damming at the bottleneck entrance to the lnb near the present city of koblenz our reconstruction is based on a high resolution analysis and correlation of the complex intercalation of primary fallout tephra relics with fluvially reworked laacher see tephra in the lnb tephra units representing complete eruptive cycles repeatedly fell on drained ground in between one minor and 4 major flooding events even at the base of side channels that had been active prior to the lse and that had been flooded by a preceding flooding event this demonstrates that flooding occurred generally during breaks and not during fallout events the repeated formation and breach of a dam at the upstream entrance of the lnb koblenz dam consisting of fallout components and driftwood washed together convincingly explains the multiple repetition of the drainage of the channels in the lnb followed by large magnitude flooding in rapid succession the strongly pulsating nature of the lse reflected in multiple interruptions of eruptive activity fundamentally controlled the damming and flooding dynamics the rhine became completely blocked during distinct fallout phases due to overloading with pumice that had fallen into the river and its major tributaries the temporary dam collapsed during eruptive breaks this is the first recorded example of tephra fallout damming a major watercourse the extremely low gradient of the rhine river allowed the repeated accumulation of large volumes of water in a long multi phase dammed up lake lake koblenz that extended along the upstream course of the river for up to c 30 km despite the low height damming of a major river while a large plinian eruption is in full progress represents an extraordinary challenge for hazard mitigation this is especially pertinent for an area close to and downwind from the vent and therefore simultaneously affected by massive plinian fall such as in the lnb

pedogenic carbonate proxies for amount and seasonality of precipitation in paleosols the depth to carbonate nodular bk horizon in soils d in cm is correlated with mean annual precipitation p in mm so that bk horizons are deep in subhumid regions but shallow in semiarid regions previous quantifications of this relationship are unchanged by this new compilation of 807 soils p 137 24 6 45 d 0 013 d 2 where r 2 0 52 and standard error s e 147 mm in most north american postglacial soils the bk horizon is thin and well defined whereas in monsoonal tropical soils of pakistan and kenya the bk horizon is thick and diffuse data from 675 modern soils define the relationship between the thickness of soil with nodules t in cm and mean annual range of precipitation m in mm difference between monthly means of wettest and driest months m 0 79 t 13 71 where r 2 0 58 and s e 22 mm the relationship between carbonate nodule size s in cm and soil age a in ka is quantified by 9 radiocarbon dated soils from las cruces new mexico a 3 92 s 0 34 where r 2 0 57 and s e 1 8 k y these transfer functions are applied to reconstructing paleoclimatic change from paleocene eocene paleosols of the north horn formation and flagstaff limestone of axhandle canyon utah the terminal paleocene spike of warmth recorded by fossil plants in nearby wyoming was coincident with a brief peak in both mean annual precipitation and mean annual range of precipitation in utah

microstructural and fabric studies from the rocks of the moine nappe eriboll nw scotland abstract microstructures and quartz c axis fabric diagrams from mylonites and psammitic moine schists collected in traverses across the lower levels of the moine nappe in the eriboll area are presented on approaching the moine thrust from the kyle of tongue the following microstructural sequence is encountered interlayered coarse grained biotite psammitic and schistose tectonites being in part mylonitic with two platy slide zones one containing biotite and the other only muscovite and chlorite and both showing quartz microstructures indicative of post tectonic relaxation these pass into more mylonitic rocks nearer the thrust zone which in turn passes into the main chlorite grade mylonite belt and finally adjacent to the moine thrust into reworked lower chlorite grade mylonites although there is some local variation the overall quartz c axis fabric is an incomplete asymmetric type i girdle the main variation is the development of type ii girdles in the reworked ultrafine grained mylonites the extent of the mylonitization is more extensive than previously reported studies of folds within the mylonite belt have revealed eye structures and small scale folds many are sheath folds they cannot be unequivocally correlated with large scale recumbent folds within the moine nappe the results presented indicate that mylonitization is not limited to a single phase and raises the possibility that there may be earlier caledonian or possibly precambrian structural elements present in the eriboll region moines prior to much of the mylonitization

ecoregions and stream morphology in eastern oklahoma abstract broad scale variables i e geology topography climate land use vegetation and soils influence channel morphology how and to what extent the longitudinal pattern of channel morphology is influenced by broad scale variables is important to fluvial geomorphologists and stream ecologists in the last couple of decades there has been an increase in the amount of interdisciplinary research between fluvial geomorphologists and stream ecologists in a historical context fluvial geomorphologists are more apt to use physiographic regions to distinguish broad scale variables while stream ecologists are more apt to use the concept of an ecosystem to address the broad scale variables that influence stream habitat for this reason we designed a study using ecoregions which uses physical and biological variables to understand how landscapes influence channel processes ecoregions are delineated by similarities in geology climate soils land use and potential natural vegetation in the fluvial system stream form and function are dictated by processes observed throughout the fluvial hierarchy recognizing that stream form and function should differ by ecoregion a study was designed to evaluate how the characteristics of stream channels differed longitudinally among three ecoregions in eastern oklahoma usa boston mountains ozark highlands and ouachita mountains channel morphology of 149 stream reaches was surveyed in 1st through 4th order streams and effects of drainage area and ecoregion on channel morphology was evaluated using multiple regressions differences existed 0 05 among ecoregions for particle size bankfull width and width depth ratio no differences existed among ecoregions for gradient or sinuosity particle size was smallest in the ozark highlands and largest in the ouachita mountains bankfull width was larger in the ozark highlands than in the boston mountains and ouachita mountains in larger streams width depth ratios of the boston mountains and ozark highlands were not statistically different significant differences existed however between the boston mountains and ozark highlands when compared individually to the ouachita mountains we found that ecoregions afforded a good spatial structure that can help in understanding longitudinal trends in stream reach morphology surveyed at the reach scale the hierarchy of the fluvial system begins within a broad relatively homogenous setting that imparts control on processes that affect stream function ecoregions provide an adequate regional division to begin a large scale geomorphic study of processes in stream channels

glacially conditioned specific stream powers in low relief river catchments of the southern laurentian great lakes abstract fluvial systems of the southern laurentian great lakes region are carved into a complex glacial landscape shaped by continental ice and meltwater of the late pleistocene these glacially conditioned river catchments are typically small with drainage areas 4 km 2 a 10 m digital elevation model dem is used to map the spatial distribution of stream gradient for 22 major river catchments of peninsular southern ontario which drain to base levels in the lower great lakes huron st clair erie and ontario raw data from the dem show stream gradients that exhibit multiscale variance from real and from artifact sources based on a vertical slice and multiple pass moving window averaging approach slope data are generalised to the river reach scale 1 2 km as a representative spatial scale for fluvial processes operating over holocene timescales models of specific stream power are then compared with glacial landform and surface geology mapping inherited glacial signatures in river slope appear as deviations in a stream length gradient index sl k index where river reaches are frequently oversteepened or understeepened based on a slope area analysis and complementary to theories of channel pattern discrimination constant stream power curves with power law exponent of 0 4 provide a first order approach to stratify river reaches in terms of glacial conditioning and expected planform morphologies however multiple channel planform types are rare and localised in southern ontario indicating that oversteepened reaches with high stream powers may often be moderated by 1 sediment calibre with cobble beds from inherited glacial sediments and or 2 relative bank strength with limited channel widening particularly in gravel and sand bed channels further discrimination of glacially conditioned fluvial process domains will ultimately require consideration of alluvial floodplain characteristics in addition to general observations of river morphology and channel pattern

mineral chemical studies of metamorphosed hydrothermal alteration in the kristineberg volcanogenic massive sulfide district sweden the massive sulfide deposits of the kristineberg area sweden occur within a 2 to 3 km thick succession of felsic volcaniclastic rocks belonging to the skellefte group the volcanic pile is intruded by a synvolcanic jorn type granitoid viterliden intrusive complex and is overlain by a thick sequence of metasedimentary rocks vargfors group mineralization occurs at two main stratigraphic levels at the base of the felsic volcanic succession and at the contact with the metasedimentary rocks of the vargfors group the kristineberg cu zn mine is the largest deposit approximately 21 mt and occurs at the base of the volcanic pile close to the contact with the viterliden intrusive complex four smaller deposits ravliden ravlidmyran horntrask and nyliden occur along the upper ore horizon these deposits are thought to be related to a late intrusive phase of the viterliden complex which cuts the altered volcanic rocks at the kristineberg deposit within an area of about 50 km2 surrounding the kristineberg deposit felsic volcanic rocks between the two ore horizons are affected by extensive albite destructive alteration sodium depletion and development of chlorite and muscovite strong co enrichment in magnesium and potassium the kristineberg deposit is enveloped by a large and partly transposed quartz chlorite alteration zone approximately 2 km in diameter and a distal but coherent pyrite quartz muscovite alteration zone extending as far as 4 km from the deposit chlorite talc in the mine area is notably magnesium rich and contains anomalous f ba zn and mn high fluorine is also present in coexisting muscovite and phlogopite the magnesium rich chlorite alteration contrasts sharply with the iron enrichment observed in many other felsic volcanic hosted precambrian massive sulfide deposits this may indicate fixation of iron by large amounts of pyrite in the section or entrainment of large amounts of seawater in the hydrothermal upflow zones kyanite is developed locally in the chlorite rich pipe at kristineberg in response to regional thermal metamorphism of highly aluminous alteration in the immediate foot wall rocks spectacular andalusite bearing quartz muscovite schists and quartz biotite cordierite schists also occur where the altered felsic volcanic rocks are intruded by the late revsund granite however similar metamorphic mineral growth is not observed where the volcanic rocks at the contact are less altered deposits near the top of the felsic volcanic succession are characterized by magnesium rich chlorite alteration in the foot wall and proximal calc silicate assemblages dolomite calcite tremolite garnet margarite where the host sedimentary rocks are carbonate rich in general the calc silicate alteration is restricted to the immediate hanging wall and zones lateral to the deposits and does not represent a regionally extensive exploration target the two main ore horizons in the kristineberg area are not linked by any obvious discordant structures or alteration zones however mineral chemical studies highlight several possible fluid flow pathways leading from the kristineberg deposit to the ravliden ore horizon more than 2 km upsection overprinting regional metamorphic minerals have inherited the hydrothermal signature of the ore related alteration electronic supplementary material is available at http dx doi org 10 1007 s00126 002 0299 y on that page frame on the left side a link takes you directly to the supplementary material

a model of channel response in disturbed alluvial channels dredging and straightening of alluvial channels between 1959 and 1978 in west tennessee caused a series of morphologic changes along modified reaches and tributary streams degradation occurred for 10 to 15 years at sites upstream of the area of maximum disturbance and lowered bed levels by as much as 6 1 m following degradation reaches upstream of the area of maximum disturbance experienced a secondary aggradation phase in response to excessive incision and gradient reduction aggradation downstream of the area of maximum disturbance reached 0 12 m per year with the greatest rates occurring near the stream mouths the adjustment of channel geometry and phases of channel evolution are characterized by six process oriented stages of morphologic development premodified constructed degradation threshold aggradation and restabilization down cutting and toe removal during the degradation stage causes bank failure by mass wasting when the critical height and angle of the bank material is exceeded threshold stage channel widening continues through the aggradation stage as the slough line develops as an initial site of lower bank stability the bank profile develops three dynamic elements 1 vertical face 70 to 90 2 upper bank 25 to 50 and 3 slough line 20 to 25 alternate channel bars form during the restabilization stage and represent incipient meandering of the channel

downstream hydraulic geometry relationships gathering reference reach scale width values from lidar abstract this paper examines the ability of lidar topography to provide reach scale width values for the analysis of downstream hydraulic geometry relationships along some streams in the dolomites northern italy multiple reach scale dimensions can provide representative geometries and statistics characterising the longitudinal variability in the channel improving the understanding of geomorphic processes across networks starting from the minimum curvature derived from a lidar dtm the proposed algorithm uses a statistical approach for the identification of the scale of analysis and for the automatic characterisation of reach scale bankfull widths the downstream adjustment in channel morphology is then related to flow parameters drainage area and stream power with the correct planning of a lidar survey uncertainties in the procedure are principally due to the resolution of the dtm the outputs are in general comparable in quality to field survey measurements and the procedure allows the quick comparison among different watersheds the proposed automatic approach could improve knowledge about river systems with highly variable widths and about systems in areas covered by vegetation or inaccessible to field surveys with proven effectiveness this research could offer an interesting starting point for the analysis of differences between watersheds and to improve knowledge about downstream channel adjustment in relation for example to scale and landscape forcing e g sediment transport tectonics lithology climate geomorphology and anthropic pressure

successful predictions of river characteristics across england and wales based on ordination predictions of river channel form under current conditions or in response to environmental or management changes and the rapid comparison of different channel reaches are important tasks in river management river classification is a common and valuable framework to address these aims but may suffer from the necessity to force a continuum of channel morphology into discrete groups more generally the scope to test the ability of predictive tools has been limited because of a shortage of field data in this study we used principal component analysis pca to identify the main sources of variation among river reaches in england and wales based on a set of 20 variables expected to correlate with channel morphology the pca scores were then used to predict the distributions of a wide range of hydromorphic features based on 4000 reaches surveyed in the river habitat survey baseline for comparison the predictive ability of three pairs of variables channel slope discharge slope catchment area and specific power catchment area was also tested the pca identified specific stream power channel size and groundwater input as the main sources of variation among reaches regression models using pca scores or paired variables were effective predictors of a range of channel characteristics including predominant substrate flow biotopes and channel vegetation channel cross sections and anthropogenic modifications were less predictable all of the approaches permitted simple plots of river reaches and quantitative comparisons of the dis similarity among individual reaches whilst the paired variables also minimised the data requirements our work reiterates the value of simple paired variables as a basis for rapidly comparing river reaches and for the first time quantifies the predictive ability of these approaches across a wide range of channel characteristics at a national scale principal component analysis provides a valuable exploratory tool for identifying the main sources of variation in complex multivariate data from which a simplified version e g specific power and area could be adopted

a systematic rare earth elements and yttrium study of archean cherts at the mount goldsworthy greenstone belt in the pilbara craton implications for the origin of microfossil bearing black cherts abstract a systematic ree y study of archean ca 3 0 ga cherts from the mount goldsworthy greenstone belt in the northeastern pilbara craton western australia was performed in order to understand their origin and depositional environment analyzed samples include microfossil bearing black cherts from the farrel quartzite and a black vein chert from the underlying warrawoona group and laminated to banded chert including carbonaceous chert jaspilite and banded iron formation from the overlying deepening upward cleaverville formation laminated to banded cherts from the cleaverville formation show a clear stratigraphic trend upsection of increasing y ho and positive eu anomalies with hree enrichment and positive la anomalies the data comprise a mixing array between two end member components on a newly proposed y ho eu anomaly diagram one end member is interpreted to be archean seawater with a super chondritic y ho ratio 100 and a weak positive eu anomaly 3 the other end member with a chondritic y ho ratio and a negligible eu anomaly is assumed to be non marine water such as continental run off ground water or geothermal water black cherts containing microfossils in the farrel quartzite are characterized by a positive la anomaly hree enrichment negligible to a slight positive eu anomaly and a chondritic to slightly super chondritic y ho ratio they are distinct from vein cherts with a distinct mree enrichment and contemporaneous hydrothermal cherts with pronounced eu anomaly and plot close to the supposed non marine end member component on the y ho eu anomaly diagram the black cherts were thus precipitated from a water mass influenced significantly by for example continental run off ground water and or geothermal water but not from high t hydrothermal solution increasing the credibility of microfossils contained in them

hierarchy of facies of pyroclastic flow deposits generated by laacher see type eruptions the upper quaternary pyroclastic flow deposits of laacher see volcano show compositional and structural facies variations on four different scales 1 eruptive units of pyroclastic flows composed of many flow units 2 depositional cycles of as many as five flow units flow units containing 3 regional intraflow unit facies and 4 local intraflow unit subfacies these facies can be explained by successively overlapping processes beginning in the magma column and ending with final deposition the pyroclastic flow deposits thus reflect major aspects of the eruptive history of laacher see volcano a drastic changes in eruptive mechanism due to increasing access of water to the magma chamber and b change in chemical composition and crystal and gas content as evacuation of a compositionally zoned magma column progressed the four scales of facies result from four successive sets of processes 1 differentiation in the magma column and external factors governing the mechanism of eruption 2 temporal variations of factors inducing eruption column collapse 3 physical conditions in the eruption column and the way in which its collapse proceeds and 4 interplay of flow inherent and morphology induced transport mechanics

geomorphologic evidence for the late pliocene onset of hyperaridity in the atacama desert the atacama desert has experienced a long and protracted period of hyperaridity that has resulted in what may be the most unusual biome on earth but the duration of this aridity is poorly constrained we reconstructed aspects of the fluvial and geochemical history of this region using integrated landscape features alluvial fans hillslope soils soil chemistry river profiles in the southern portion of the present desert topographic reconstructions of a large watershed 11 000 km 2 show deep incision and sediment removal between the late miocene and the end of the pliocene and modest to negligible incision in post pliocene times these changes in incision suggest an 50 280 reduction in river discharge which should reflect corresponding changes in precipitation changes in the nature of hillslope soils in the atacama desert indicate that in the pliocene or earlier hillslopes were mantled with silicate derived soil this mantle was stripped off and locally deposited as alluvial fans late pliocene to early pleistocene that now block or otherwise cause a rearrangement of pliocene and earlier river channels finally the hillslopes have largely accreted a soil mantle of dust and salt since the apparent late pliocene stripping suggesting a decline in annual precipitation of at least 125 mm yr 1 or more mean annual precipitation map is now 1 embedded in the long post pliocene era of salt accumulation there are a variety of features suggesting overland flow on hillslopes rills striped gravel deposits piping and water spouts and large infrequent storms that infiltrated gentle alluvial fans due to the depth of salt rich horizons despite evidence for episodes that punctuate the hyperaridity the magnitude and duration of these pluvial events have been insufficient to remove the regional accumulations of sulfate chloride and nitrate the late pliocene cessation of many fluvial features is coincident with recent research on the tropical pacific which shows that the pacific was in a permanent el nino state until ca 2 2 ma at which time sea surface temperatures offshore of south america declined greatly relative to those of the western pacific in turn setting up the present el nino southern oscillation enso climate system these observations indicate that the latest period of aridity has been prolonged and largely continuous and it appears to have occurred in step with the onset of the enso climate system beginning 2 m y ago

paleoproterozoic ca 1 9 ga megascopic life on land in western australia abstract controversial hairpin shaped trace fossils myxomitodes stirlingensis and discoid fossils cyclomedusa davidi are here reinterpreted in a reassessment of sedimentology and paleosols of the 1 9 0 1 ga stirling range formation of western australia paleosols in the stirling range formation were recognized from complex cracking patterns peds and cutans chemical and mineralogical zonation soil horizons and poikiloblastic crystals desert roses redox profiles of the paleosols are evidence of well drained profiles of a coastal plain under a low oxygen 433 116 ppmv and carbon dioxide 564 64 ppmv atmosphere gypsum pseudomorphs in paleosols that are deeply weathered chemically represent a persistence of acid sulfate weathering better known in archean paleosols before the great oxidation event at 2 45 ga chemical composition of the paleosols is evidence of a humid 1478 182 mm mean annual precipitation temperate 11 3 0 5 c mean annual temperature paleoclimate their paleoenvironmental setting was a coastal plain on a passive tectonic margin megafossils were found on paleosols of western red sandstones of the stirling range formation not in gray shales and sandstones of marine facies to the east trace fossils myxomitodes stirlingensis and discoid fossils are both found at the surface of well drained paleosols and so were not metazoan trails nor sea jellies myxomitodes was more likely a grex slug trail of soil slime molds and the cyclomedusa discoids were probably microbial colonies pervasive microfilamentous structures in the paleosol surface are additional evidence of paleoproterozoic microbial earth ecosystems

evolution of boudins under progressively decreasing pore pressure a case study of pegmatites enclosed in marble deforming at high grade metamorphic conditions naxos greece during ductile deformation of marbles under high grade metamorphic conditions on naxos greece pegmatites enclosed in the marbles were deformed in a brittle fashion forming blocky boudins with quartz crystallized in the interboudin zones we studied the three dimensional geometry of the boudins in the field and in two large samples the first sample was serially sectioned to observe the structural and microstructural evolution in the second sample we mapped the surface morphology of the deformed pegmatite in profile the boudins can be classified as symmetric torn type boudins evolving towards asymmetric boudins with a later domino boudin component in three dimensions however the morphology of the boudinaged pegma tite is a simple set of normal faults with mode i fractures in the fault tips and rotation of the fault blocks to accommodate the extension in the marble deformation history was constrained by petrology and microstructures in combi nation with simple order of magnitude calculations of both the cooling and pore fluid pressure evolution the dynamically recrystallized coarse grained calcite of the marble provides clear evidence that after the pegmatite intruded the marble and solidified it was deformed at peak conditions of m2b metamorphism 670 c and 0 6 gpa such pegmatitic melts contain 10 percent h2o which is released during crystalliza tion we infer that after crystallization of the pegmatite the pore fluid pressure in the pegmatite remained close to lithostatic due to the very low permeability of the surrounding marble and the pegmatite was deformed at very low effective stress which led to brittle deformation of feldspar and mode i fracturing of the pegmatite forming torn type boudins with time the pore fluid pressure slowly decreased increasing the effective stress ongoing n s extension thus resulted in slip along the quartz filled interboudin zones and in block rotation producing domino boudins

could bacteria have formed the precambrian banded iron formations banded iron formations bifs are prominent sedimentary deposits of the precambrian but despite a century of endeavor the mechanisms of their deposition are still unresolved interactions between microorganisms and dissolved ferrous iron in the ancient oceans offer one plausible means of mineral precipitation in which bacteria directly generate ferric iron either by chemolithoautotrophic iron oxidation or by photoferrotrophy on the basis of chemical analyses from bif units of the 2 5 ga hamersley group western australia we show here that even during periods of maximum iron precipitation most if not all of the iron in bifs could be precipitated by iron oxidizing bacteria in cell densities considerably less than those found in modern fe rich aqueous environments those ancient microorganisms would also have been easily supported by the concentrations of nutrients p and trace metals v mn co zn and mo found within the same iron rich bands these calculations highlight the potential importance of early microbial activity on ancient metal cycling

equation for compaction of paleosols due to burial existing empirically derived compaction curves for marine sediments fail to estimate the compaction of paleosols flood plain sediments and peats as indicated by deformation of clastic dikes dinosaur footprints and sandstone paleochannels yet compaction estimates are vital to reconstructing paleoclimate and geochemical profiles of paleosols to modeling sediment accumulation rates of nonmarine rocks and to tectonic flexural modeling of molasse facies this paper presents a compaction equation and physically derived constants that make geologically realistic estimates of compaction of paleosols and other nonmarine sediments a protocol for the application of the equation is suggested that would allow the following equation to solve for burial compaction c as a fraction of original thickness given depth of burial d in km as c s i f e d k 1 this equation can be applied to nonmarine sedimentary rocks or paleosols given appropriate empirical data on the physical constants in the equation such as initial solidity s i initial porosity f and the corresponding curve fitting constant k data on physical constants useful for this equation are compiled here for a range of paleosol and nonmarine sediment types

potassium metasomatism of precambrian paleosols abstract independent of substratum composition ancient regoliths formed by subaerial weathering are characterized by accumulation of k and other alkaline elements potassium enrichment due to the formation of clay minerals is a trend typically observed in regoliths developed by modern hypergene processes however the k content in precambrian paleosols is much higher than in modern weathering profiles k metasomatism affecting buried hypergen targets is a possible mechanism for k accumulation a novel reaction transport dissolution precipitation model designed as part of this work proved able to reproduce many essential features of post burial alterations of a subaerial weathering profile formed on basaltic rocks including potassium enrichment dehydration compaction and pore filling that impacted the buried regolith the phenomena of preservation of the primary basaltic structure the apparent selective alteration of soil levels and immobility of al ti and p could be explained by the model

active channel width as a proxy of sediment supply from mining sites in new caledonia although the channel morphology of upland fluvial systems is known to be strongly controlled by sediment supply from hillslopes it is still difficult to isolate this effect from the other controlling factors of channel forms such as the sediment transport capacity depending notably on the size of the catchment and local conditions e g confinement riparian vegetation valley floor slope the rivers in new caledonia offer an interesting field laboratory to isolate the morphological effect of contrasted sediment supply conditions some of these rivers are known to be highly impacted by the coarse sediment waves induced by the mining of nickel deposits that started in the early 1870s which was particularly intensive between the 1940s and 1970s the propagation of the sediment pulses from the mining sites can be traced by the presence of wide and aggraded active channels along the stream network of nickel rich peridotite massifs a first set of 63 undisturbed catchments in peridotite massifs distributed across the grande terre was used to fit a classic scaling law between active channel width and drainage area a second set of 86 impacted sites where the presence of sediment waves was clearly attested by recent aerial imagery showed systematically wider active channels with a width ratio around 5 established from the intercept ratio of width area power laws more importantly this second set of disturbed sites confirmed that the residual of active channel widths computed from the scaling law of undisturbed sites is statistically positively related to the catchment scale relative area of major mining sediment sources it is therefore confirmed that the characterization of sediment supply conditions is crucial for the understanding of spatial patterns of active channel width and this should be more thoroughly considered in morphological studies of rivers draining environments with contrasted geomorphic activities on hillslopes

setting and origin for problematic rocks from the 3 7 ga isua greenstone belt southern west greenland earth s oldest coarse clastic sediments abstract whether or not coarse detrital sedimentary rocks occur within the 3 7 ga isua greenstone belt igb southern west greenland has been debated for some time repeated regional metamorphic deformational and metasomatic events have obscured most protolith lithologies leading to misunderstandings about the stratigraphy and environments of deposition rocks here interpreted as meta conglomerate crop out in a fault bounded structural domain that is lower in strain relative to adjacent domains the meta conglomerate has a strike length of 1 km and is 10 m thick bed thickness ranges from 10 cm to more than 1 m and beds may be either framework or matrix supported a poorly sorted and variably rounded polymict assemblage of framework clasts consisting of meta chert bif and a variety of mafic volcanic rock fragments are set into a matrix of biotite quartz garnet schist clast compositions indicate reworking of adjacent stratigraphic units a lack of structural similarity in framework clasts range of grain sizes range of rounding and polymict composition demand a primary sedimentary origin for the deposit inferred depositional processes include traction and debris flow which would be consistent with subaerial or shallow subaqueous environments although the limited extent of the meta conglomerate warrants interpretational caution

low oxygen levels in earliest triassic soils an earliest triassic methane postapocalyptic greenhouse following the permian triassic p t extinction event was proposed on the basis of evidence of deeply weathered paleosols at high latitudes with features of low latitude soils and low stomatal index values of seed fern leaves reexamination of distinctive phyllosilicates unique to a single stratigraphic level in paleosols located just above the isotopically defined permian triassic boundary in australia and antarctica furnishes additional tests of this hypothesis illite is the dominant clay mineral in earliest triassic paleosols from antarctica but the paleosols also contain conspicuous green nodules of coarsely crystalline berthierine examples from the geologic record and from experimental studies indicate that the formation of berthierine is restricted to reducing conditions the occurrence of this unusual mineral in soils may indicate soil oxygen consumption by the influx of atmospheric methane to form carbon dioxide which in turn warmed the earliest triassic giving rise to a postapocalyptic greenhouse

an updated expanded universal definition of soil a universally accepted definition of soil is missing since the birth of soil science the boundary between what is soil and what is not soil is still a matter of debate can the loose surfaces of the other rocky bodies of our solar system be considered real soils a definition of soil that represents a reliable term also for outer space landscapes is needed more than ever an updated definition of soil that has a universal value is provided here

ediacaran life on land a new interpretation of fossilized soils palaeosols suggests that at least some ediacaran 625 542 million years ago organisms lived on land thus these ediacaran fossils were not animals but a fungus dominated terrestrial biota that predated vascular plants by about 100 million years ediacaran fossils 542 to 635 million years old occur worldwide in a variety of sedimentary deposits generally interpreted as shallow to deep marine origin they have been regarded as early animal ancestors of the cambrian evolutionary explosion of marine invertebrate phyla as giant marine protists and as lichenized fungi here gregory retallack raises doubts over the assumption that ediacaran organisms were marine a new interpretation of fossilized soils palaeosols from south australia suggests that at least some lived on land retallack s suggestion that some ediacaran fossils were large sessile organisms of cool dry soils is compatible with observations that ediacaran fossils are similar in appearance and preservation to lichens and other microbial colonies of biological soil crusts rather than marine animals or protists ediacaran 635 542 million years ago fossils have been regarded as early animal ancestors of the cambrian evolutionary explosion of marine invertebrate phyla1 as giant marine protists2 and as lichenized fungi3 recent documentation of palaeosols in the ediacara member of the rawnsley quartzite of south australia4 confirms past interpretations of lagoonal aeolian deposition based on synsedimentary ferruginization and loessic texture5 6 further evidence for palaeosols comes from non marine facies dilation cracks soil nodules sand crystals stable isotopic data and mass balance geochemistry4 here i show that the uppermost surfaces of the palaeosols have a variety of fossils in growth position including charniodiscus dickinsonia hallidaya parvancorina phyllozoon praecambridium rugoconites tribrachidium and old elephant skin ichnogenus rivularites7 these fossils were preserved as ferruginous impressions like plant fossils8 and biological soil crusts9 10 of phanerozoic eon sandy palaeosols sand crystals after gypsum11 and nodules of carbonate12 are shallow within the palaeosols4 even after correcting for burial compaction13 periglacial involutions and modest geochemical differentiation of the palaeosols are evidence of a dry cold temperate ediacaran palaeoclimate in south australia4 this new interpretation of some ediacaran fossils as large sessile organisms of cool dry soils is compatible with observations that ediacaran fossils were similar in appearance and preservation to lichens and other microbial colonies of biological soil crusts3 rather than marine animals1 or protists2

origin and emplacement of a pyroclastic flow and surge unit at laacher see germany abstract within the pyroclastic ejecta sequence of laacher see volcano west germany are many repeated phreatomagmatic depositional cycles a complete cycle begins with a thin low aspect ratio fines depleted breccia layer followed by a sequence with several coarse to fine grained cross bedded units in turn overlain by a massive pyroclastic flow layer each sequence is interpreted in terms of water magma interaction and effects upon the eruption column the breccia represents the triggering eruption with a high eruption column where fines were winnowed out and fallback from high altitudes produced a low aspect ratio layer the cross bedded sequence containing more fine grained ash resulting from an increase in amount of water represents a fluctuating eruption column of sufficient height to produce large antidunes within 700 meters of the rim the massive pyroclastic flow layer which contains more fine grained ash than the underlying layers represents the culmination of the eruption where an increase in amount of water interacting with the magma resulted in a dense and very low eruption column producing a dense laminar moving pyroclastic flow

the influence of riparian vegetation on stream width eastern pennsylvania usa we surveyed adjacent reaches with differing riparian vegetation to explain why channels with forested banks are wider than channels with nonforested banks cross sections and geomorphic mapping demonstrate that erosion occurs at cutbanks in curving reaches while deposition is localized on active floodplains on the insides of bends our data indicate that rates of deposition and lateral migration are both higher in nonforested reaches than in forested reaches two dimensionless parameters and e explain our observations represents the influence of grassy vegetation on rates of active floodplain deposition it is 5 times higher in nonforested reaches than in forested reaches e is proportional to rates of cutbank migration it is 3 times higher in nonforested reaches than in forested reaches differences in width between forested and nonforested reaches are proportional to e in forested reaches channels are wide with banks that are difficult to erode dense tree roots create a low value of e and the channel migrates slowly e is high however because is very low shade from trees inhibits the growth of grass on active floodplains in nonforested reaches channels are narrow with banks that are easy to erode e is high and the channel migrates rapidly e is low however due to a very large value of grass grows readily on nonforested convex bank floodplains thus differences in width between forested and nonforested reaches are related to a balance between rates of cutbank erosion and rates of deposition on active floodplains implying that equilibrium widths develop to equalize rates of cutbank erosion and vegetation mediated rates of deposition on active flood plains these results suggest that accurate models of width adjustment should consider the combined effects of bank erodibility and floodplain depositional processes rather than focusing on these processes in isolation from one another

co2 drawdown and cooling at the onset of the great oxidation event recorded in 2 45 ga paleoweathering crust abstract the archean atmosphere is thought to have been devoid of oxygen but instead containing high concentrations of greenhouse gases such as co2 and possibly ch4 that were required to keep the earth surface environments warm enough for maintenance of liquid water under faint young sun conditions earlier studies have suggested that the co2 concentrations must have been as high as over 1000 times present atmospheric levels pal to overcome the effects of weaker solar luminosity however more recent studies of precambrian paleosols imply existence of at least tenfold lower but still high pco2 levels the appearance of minute amounts of atmospheric o2 at the transition of archean and proterozoic is well documented however the bulk composition of the atmosphere and its dynamics at this crucial time is still widely debated different paleoclimate proxies of the 2 45 ga kuksha paleoweathering crust suggest its formation in a cool temperate climate paleoatmospheric pco2 estimates by geochemical and isotopic mass balance models suggest remarkably low paleoatmospheric co2 levels with the best guess estimates of atmospheric co2 between 1 and 10 pal we speculate that the kuksha paleoweathering crust either predates or partly overlaps with the onset of huronian glaciation and corresponds to a time interval of low co2 level during the first recorded large glaciation in earth history

icelandic volcanic ash from the late glacial open air archaeological site of ahrenshoft la 58 d north germany abstract cryptotephra of icelandic origin from the open air archaeological site of ahrenshoft la 58 d kr nordfriesland schleswig holstein northern germany overlies a late glacial havelte lithic assemblage hitherto dated by 14 c and biostratigraphy to the earliest part of the late glacial interstadial gi 1e to gi 1c 3 peaks in ash shards are observed in two profiles major and minor element geochemistry indicates volcanic ash originating in the katla system precise correlation to previously described tephra is uncertain due to overlapping chemical characteristics the ahrenshoft 14 c determinations litho and bio stratigraphy encompass a broad age span for the cryptotephra bearing sediments from the end of the allerod to the preboreal the most plausible volcanic eruption correlates are the vedde ash younger dryas already known from the european mainland tephra af555 late younger dryas and the suduroy tephra preboreal boreal hitherto recorded only in the north atlantic region these three ash horizons have been dated to respectively 12 171 57 yr b2k in the ngrip ice core c 11 500 cal bp in scotland and c 8000 cal bp by radiocarbon from the faroe isles ongoing research on deposits from the type sites for the tephra layers may in the future differentiate these markers leading to better discrimination of the chemistries and a resolution of this question

lateral variations in the taupo ignimbrite abstract the young non welded taupo ignimbrite shows remarkable lateral variations which are documented by granulometric and component analyses and studies of maximum clast size and density the grain size spans practically the entire known ignimbrite field the coarser proximal ignimbrite having a median diameter 100 times greater than the finest distal ignimbrite the content and maximum size of lithic fragments decrease also by a factor of 100 between proximal and distal parts the content of free crystals first rises to reach a peak but thereafter decreases to attain a very low value in far distal exposures the pumice maximum size decreases by a factor of about 10 and the most conspicuously coarse pumice rocks occur in a girdle nearly halfway out from vent to distal limit the pumice in each grain size class decreases in density to half of its near source value in distal ignimbrite the overall outward trend is towards an ignimbrite which consists wholly of fine vitric ash some distal exposures closely approach this condition these variations are accounted for by a combination of processes operating in the moving ash flow one is a continuous fragmentation of pumice leading to a rounding of the clasts a progressive decrease in maximum size the generation of much vitric dust and the liberation of crystals another is a continuous sedimentation of heavy constituents lithics and crystals and an antipathetic rise of lighter coarse pumice towards the top of the flow these processes operated in a moving flow whose upper layers travelled progressively farther from source it is the topmost layers strongly depleted in heavy constituents and enriched in light pumice which have travelled the farthest and constitute the far distal parts of the ignimbrite a number of ignimbrite facies are characterized the ignimbrite proper with its proximal distal and pumice concentration zone facies the deposits which form in the head and are then over ridden by the body of the flow including the fines depleted ignimbrite variant and the heavies enriched ground layer and the ignimbrite veneer deposits which are left behind by the flow which differ little from the ignimbrite except in their landscape mantling form and the occurrence in them of lee side coarse pumice lenses

climatic and geochemical implications of archean pedogenic gypsum in the moodies group 3 2 ga barberton greenstone belt south africa abstract lithic sandstones of braided fluvial to supratidal facies in the paleoarchean moodies group 3 22 ga barberton greenstone belt south africa include several regionally traceable units with common to abundant in places rock forming nodular concretions of megaquartz pseudomorphs after gypsum barite and calcite concretionary accumulations are stratiform and commonly associated with aqueously reworked fine grained tuffaceous sediment of originally rhyodacitic composition and can grow to fist sized agglomerates in crusts tens of m in lateral extent weathering of tuffaceous material and feldspar delivered alkali cations such as ca ba and k while carbonates were likely supplied by silicate weathering of mafic to ultramafic volcanic rocks during exposure to a co2 rich atmosphere sulfate ions were partly delivered by oxidative pyrite dissolution which may have included microbial and abiotic disproportionation of volcanic s or so2 concretionary growth apparently took place under pedogenic to early diagenetic conditions within unconsolidated granular sediment in the vadose zone dominated by seasonal fluctuations of the groundwater level under evaporitic conditions the concretions likely represent the oldest terrestrial evaporites known to date and form part of the oldest known compound paleosols their formation and composition constrain the local occurrence of sulfate in the archean atmo and hydrosphere their interaction with the emerging biosphere archean weathering regime local climate and vadose zone hydrodynamics

channel geometry median grain size and stream power in small mountain streams abstract small mountain streams are typically affected by the same variables as larger counterparts but small headwater channels are also coupled with hillslopes the added degree of system complexity limits extension of existing conceptual and quantitative models of stream response to small mountain streams our objective is to determine how the presence of multiple lithologies on channel beds and coarse sediments affect relationships between channel geometry and driving and resisting variables in small mountain streams when the variables are examined at the landscape scale we test for systematic scaling of channel geometry stream power indices and median bed grain size d 50 in headwater streams draining 2 and overlying multiple strata on a passive continental margin field data were collected from 157 reaches in 32 watersheds local strata can be divided into a thick sequence of resistant sandstones and a thick sequence of low resistance calcareous siltstones and shales with interbedded limestones both stratigraphic units are present in every watershed and all streams carry abundant coarse bed load derived from the resistant unit mean widths and gradients of the channels are correlated with upstream drainage area unit stream power is not statistically different as a function of rock unit but channel gradients are lower atop the low resistance unit d 50 is not correlated with drainage area but is positively correlated with unit stream power the correlation between unit stream power and d 50 is evidence for mutual adjustments among gradients widths and d 50 as a function of the primary driving force in the drainage area discharge the small effect of bed lithology on stream processes and strong relationship between channel geometry unit stream power and d 50 implies that available driving forces and grain size are the primary controls on stream processes in the study area we conclude that future research into system response in this and similar settings should focus on sediment characteristics production and flux

lithic enriched segregation bodies in pyroclastic flow deposits of laacher see volcano east eifel germany small volume ca 0 6 km3 pyroclastic flow deposits at laacher see contain lithic breccias and two types of ground layers that differ significantly in their structure and composition from the main body of flow units lithic breccia bodies up to 3 5 m thick containing up to 85 weight lithic blocks occur locally at various distances from the vent the deposition of these breccias was apparently governed by the strong influence of paleomorphology on the dynamics of the pyroclastic flows the breccias were deposited at three main changes in bottom gradient along the path of the pyroclastic flows the accumulation of large lithics is explained a by compression of flows on the rising bottom close to the vent b by thinning of flows accelerating over a steep incline c by deceleration of the pre concentrated lower part of flows in hydraulic jumps and d possibly by a stationary vortex at the inner bend of a valley curvature poorly sorted lithic rich ground layers laterally highly variable in internal structure and composition are restricted to marginal regions of the pyroclastic flow deposits within deep and narrow valleys they are interpreted as having formed due to the extreme roughness of the valley walls enforcing irregular turbulent flow and intense fluidization of the flow head in which density dominated segregation of lithics occurred wellsorted lapilli rich ground layers of constant lateral thickness were probably generated by a more regularly moving less intensely fluidized head of pyroclastic flows in which size dominated segregation was effective but density segregation was minor a model of the temporal and longitudinal evolution of a flow head is proposed close to the vent the head is exclusively erosive with increasing distance erosive power declines and erosion is paralleled by ground layer formation under strong fluidization further from the vent the head ceases to erode while fluidization is still sufficient for ground layer formation when fluidization declines to a level ineffective for segregation ground layers terminate while the head advances and only terminates when plug flow dominates

riverine mixing and fluvial iron formation a new type of precambrian biochemical sediment precambrian iron formations are biochemical sediments that record ocean chemistry and circulation on the early earth the appearance of large economically important continental margin iron formation reflects the creation of extensive continental shelves and oxygenation of the ocean atmosphere system near the end of the archean exhalative iron formation contains a record of hydrothermal vent chemistry through time we introduce here fluvial iron formation a new type of fe rich microbial biochemical sediment that formed by mixing river discharge and seawater in coastal environments the paleoproterozoic chiall formation ca 1 8 ga earaheedy basin western australia contains laminated and granular hematitic iron formation in delta channel deposits where mixing occurred in adjacent peritidal settings laminated iron formation and hematitic oncoids formed because fluvial iron formations precipitated at the interface between terrestrial and marine realms the locus of known fe precipitation processes is shifted landward into paleoestuarine settings and reflects fe derived from both terrestrial weathering and coastal upwelling providing a new window into ocean atmosphere evolution

land use floods and channel changes upper middle fork willamette river oregon 1936 1980 flow trends and channel characteristics from 1936 to 1980 were evaluated for the middle fork willamette river which drains a 668 km2 forested watershed in the cascade mountains of western oregon an inventory of aerial photographs from 1959 to 1972 shows that landslides associated with roads and in clearcuts were 27 and 23 times more frequent respectively than in forested areas numerous landslides unloaded sediments directly into the drainage system most landslides appear to have been initiated during a large flood return period greater than or equal to 100 years of december 22 1964 analysis of precipitation and peak flows greater than 100m3 s 1 from 1958 to 1980 by means of power function models suggests a trend of increasing flows as timber harvesting and road building expanded in the basin changes in channel pattern documented from aerial photographs show major increases in channel width from 1959 to 1967 and a trend of decreasing width from 1967 to 1980 during summer low flows in 1979 and 1980 65 cross sections of the channel were surveyed to provide detailed measurements of existing channel conditions channel widths of 62 of the aggraded reaches were significantly greater 0 05 than those for nonaggraded reaches

regional bankfull geometry relationships for southern california mountain streams and hydrologic applications abstract this study develops and intercompares regional relationships for bankfull channel width hydraulic depth and cross sectional area for southern california mountain streams based on several data sources surveyed streams us geological survey stream survey reports and existing literature although considerable uncertainty exists in estimating bankfull conditions the relationships developed from the varying data sources show significant agreement for small watersheds with drainage area ranging from 15 to 2000 km 2 the estimates of bankfull top width ranged from 7 2 to 44 5 m and hydraulic depth estimates ranged from 0 35 to 1 15 m the utility of the developed bankfull geometry regional curves is demonstrated for southern california catchments through a the computation of the bankfull discharge and b the estimation of the surface runoff response necessary to produce bankfull conditions in the streams at the outlet of these catchments for selected locations with instantaneous flow records the occurrence frequency of events exceeding bankfull flow was examined for the available 10 15 year span of observational records bankfull discharge estimates for all small watersheds in the region ranged from 1 3 to 74 m 3 s while the range at the selected gauged stream locations was from 2 6 to 16 4 m 3 s stream locations along the transverse mountains of southern california showed an average occurrence frequency of less than 1 year whereas along the peninsular mountains the average return period tended to be greater than 1 year the application of the regional curves to the estimation of the surface runoff response necessary to produce bankfull conditions at the channel outlets of small catchments may be used as an index for conditions of minor flooding with saturated soils this surface runoff response index ranges from 2 0 to 5 5 mm for a 3 hour rainfall duration for southern california watersheds greater than 15 km 2 in area differences between the values for the peninsular and transverse mountain ranges are linked to geological climatic and geomorphologic differences the developed regional geometry relationships are suitable for use in various hydrologic modeling applications including distributed modeling with high resolution pertinent to flash flood forecasting

a comparison of average rates of alluvial erosion between the south western and south eastern united states alluvial channel bed incision and bank widening have been reported in both the south western and south eastern us throughout the past century distinct regional differences in climate and landscape properties likely influence the rate of erosion this study discusses regional differences in hydraulic driving forces and substrate resistance and tests the hypothesis that regional differences exist in average rates of channel incision bank erosion and knickpoint retreat specifically we hypothesize that erosion rates are higher in south western us streams and reason that this is because of greater flood magnitudes and limited substrate resistance a review of the literature documenting incision bank erosion and knickpoint retreat however indicates that intra regional differences are larger than inter regional differences and that average rates in the south western us are either statistically similar or less than the rates in parts of the south eastern us this could either be a result of strong intra regional hydroclimatic and substrate variability or because average erosion rate may not be an appropriate metric for inter regional comparisons because of the variability between case studies associated with the field methods to measure erosion duration of study period and time since disturbance to the channel nevertheless these findings provide a basis for future evaluations of the relative importance of different controls on driving and resisting forces in these and other landscapes characterized by rapid channel incision and arroyo formation copyright 2010 john wiley sons ltd

field and laboratory tests for recognition of ediacaran paleosols abstract not a single paleosol had been described from rocks of ediacaran age until 2011 but 354 ediacaran paleosols have been described by 20 different authors since then some of these newly recognized paleosols have proven controversial so this paper reviews 20 distinct tests to determine whether a particular ediacaran bed could be a paleosol or not one problem has been that ediacaran paleosols are not precisely like modern soils because they lack root traces a diagnostic feature of silurian and geologically younger paleosols the principal problem for recognition of some ediacaran paleosols is the occurrence in them of megafossils assumed to have been marine although most of these fossils remain problematic for both biological and ecological affinities not all the tests discussed here are diagnostic of paleosols some are ranked permissive or persuasive permissive conditions for paleosols include ripple marks hummocky bedding pyritic limestones acritarchs or thalloid fossils low strontium isotopic ratios high 26 mg ratios and red color persuasive tests include loessites tsunamites desert playa minerals low boron content high 10 b isotopic ratios high carbon sulfur ratios and very low total reactive iron ratios diagnostic tests include matrix supported lapilli or crystal tuff parent materials ice wedges and other cryoturbation sepic birefringence fabrics evaporitic sand crystals and negative geochemical strain and mass transfer and highly correlated 13 c and 18 o like other geological periods the ediacaran is known from a variety of marine and non marine paleoenvironments

exogenous carbonaceous microstructures in early archaean cherts and bifs from the isua greenstone belt implications for the search for life in ancient rocks abstract the microstructure of hf etched samples of early archaean banded iron formations bifs and cherts from the 3 7 b y old isua greenstone belt southwestern greenland was investigated using high resolution scanning electron microscopy equipped with an electron diffraction system capable of analysing light elements the rocks contain both endogenous of internal origin and exogenous of external origin carbonaceous microstructures the former consist of inclusions of graphite and possibly small amorphous carbonaceous particles both embedded in metacherts however further in situ tem studies are needed to verify the endogeneity of the amorphous particles moreover these rocks also contain endolithic microorganisms i e inhabiting cracks in rocks as well as undifferentiated carbonaceous matter that occur in fractures and cracks between grains the microorganisms include cyanobacteria filamentous microorganisms such as fungal hyphae and possibly bacteria as well as large unidentified cells or spores most of the microorganisms appear to have been fossilised the endoliths are evidently younger than the host rock but must have infiltrated at different periods most likely after the inland ice retreated 8000 years ago the presence of endolithic carbonaceous matter in cracks and microfissures in these rocks will affect any analyses of bulk samples such as carbon isotopes and chemical biomarkers as well as analyses of acid macerated residues thus previous isotope measurements made on bifs and cherts from isua may reflect younger contamination rather than an endogenous original signal likewise some of the previously described isuan microorganisms probably represent recent endolithic contamination

climate dependent chemical weathering of volcanic soils in iceland abstract the major element mobility within soil profiles of brown to gleyic andosols that developed under diverse climatic conditions in iceland is assessed the volcanic soils were selected from areas with good monitoring of annual temperature and precipitation and the degree of weathering and elemental behavior is compared icelandic soils are subject to high fluxes of aeolian dust and at times to tephra ejecta from volcanic eruptions the source of dust input is assessed for each profile based on comparison of the chemical signatures found in the less weathered upper horizons with those of volcanic systems supplying material to source areas results show that tio 2 al 2 o 3 and fe 2 o 3 t and mno are the least mobile species and generally found enriched within more mature horizons the mobile base cations mgo cao and na 2 o are depleted in these horizons as a result of chemical weathering during pedogenesis soils developed in colder climatic conditions with mean annual temperature mat 1 c give values for the chemical index of weathering ciw of 37 45 that reflect only weak chemical weathering soils developed in milder climates mat 2 4 c are more strongly affected by weathering ciw 50 77 the parent material has ciw values of 37 temperature is demonstrated as the dominant variable exerting control on the extent of weathering with only minor mobilization following the incipient near surface weathering stage a robust linear relationship is found between ciw and model mat mat 0 21ciw 8 93 r 2 0 81 this climofunction can deliver proxy climate estimations from volcanic soils and paleosols of basaltic origin in cool to subarctic conditions 2 to 6 c

graphite and carbonates in the 3 8 ga old isua supracrustal belt southern west greenland we present a systematic study of abundance isotopic composition and petrographic associations of graphite in rocks from the ca 3 8 ga isua supracrustal belt isb in southern west greenland most of the graphite in the isb occurs in carbonate rich metasomatic rocks metacarbonates while sedimentary units including banded iron formations bifs and metacherts have exceedingly low graphite concentrations regardless of isotopic composition of graphite in metacarbonate rocks their secondary origin disqualifies them from providing evidence for traces of life stemming from 3 8 ga recognition of the secondary origin of isua metacarbonates thus calls for reevaluation of earlier interpretations that suggested the occurrence of 3 8 ga biogenic graphite in these rocks thermal decomposition of siderite 6feco3 2fe3o4 5co2 c is the process seemingly responsible for the graphite formation the cation composition fe mg mn and ca of the carbonate minerals carbon isotope ratios of carbonates and associated graphite and petrographic assemblages of a suite of metacarbonates support the conclusion that multiple pulses of metasomatism affected the isb causing the deposition of fe bearing carbonates and subsequent partial disproportionation to graphite and magnetite equilibrium isotope fractionation between carbonate and graphite in the rocks indicates peak metamorphic temperatures between 500 and 600 c in agreement with other estimates of metamorphic temperature for the isb

influence of bank vegetation on channel morphology in rural and urban watersheds stream bank vegetation significantly influences the morphology of streams in the piedmont region of the united states we surveyed the morphology of 26 paired stream reaches in southeastern pennsylvania northern maryland and delaware one member of each pair has a forested riparian zone whereas the other has a riparian zone composed pri marily of grass the paired reaches are nearly contiguous so all significant channel forming variables except riparian vegetation are held constant the extent of urban development of the watersheds upstream of the paired reaches also varies considerably allowing us to determine the combined influence of riparian vegetation and urbanization on channel morphology statistical analyses indicate that 1 channels with forested riparian zones are wider than channels with nonforested riparian zones 2 channels in urbanized watersheds are wider than channels in nonurbanized watersheds and 3 the effect of riparian vegetation is independent of the level of urbanization

precambrian paleosols and atmospheric co2 levels abstract precambrian atmospheric pco2 levels were previously estimated using a simple thermodynamic model based on the mineral assemblage in paleosols in addition to theoretical flaws with that model recalculation of pco2 levels using more recent thermodynamic data gives significantly lower values a new model based on the mass balance of weathering in paleosols gives consistent results from three separate 2 2 ga old paleosols the calculated pco2 value of 23 3 3 times present atmospheric levels is insufficient to overcome the faint young sun paradox suggesting that another greenhouse gas such as ch4 was present at elevated levels relative to the present atmosphere to account for the discrepancy applying this new approach to other precambrian paleosols a proterozoic pco2 curve is generated which indicates consistently high pco2 from 2 5 to 1 8 ga ago and a substantial drop in atmospheric pco2 at some time between 1 8 and 1 1 ga ago

downstream variation in bankfull width of wadeable streams across the conterminous united states abstract bankfull channel width is a fundamental measure of stream size and a key parameter of interest for many applications in hydrology fluvial geomorphology and stream ecology we developed downstream hydraulic geometry relationships for bankfull channel width w as a function of drainage area a w a dhg wa for nine aggregate ecoregions comprising the conterminous united states using 1588 sites from the u s environmental protection agency s national wadeable streams assessment wsa including 1152 sites from a randomized probability survey sample sampled stream reaches ranged from 1 to 75 m in bankfull width and 1 to 10 000 km 2 in drainage area the dhg wa exponent which expresses the rate at which bankfull stream width scales with drainage area fell into three distinct clusters ranging from 0 22 to 0 38 width increases more rapidly with basin area in the humid eastern highlands encompassing the northern and southern appalachians and the ozark mountains and the upper midwest great lakes region than for the west both mountainous and xeric areas the southeastern coastal plain and the northern plains the dakotas and montana stream width increases least rapidly with basin area in the temperate plains cornbelt and southern plains great prairies in the heartland the coefficient of determination r 2 was least in the noncoastal plains 0 36 0 41 and greatest in the appalachians and upper midwest 0 68 0 77 dhg wa equations differed between streams with dominantly fine bed material silt sand and those with dominantly coarse bed material gravel cobble boulder in six of the nine analysis regions where dhg wa equations varied by sediment size fine bedded streams were consistently narrower than coarse bedded streams within the western mountains ecoregion where there were sufficient sites to develop dhg wa relationships at a finer spatial scale and ranged from 1 23 to 3 79 and 0 23 to 0 40 respectively with r 2 0 50 for 10 of 13 subregions range 0 36 to 0 92 enhanced dhg equations incorporating additional data for three landscape variables that can be derived from gis mean annual precipitation elevation and mean reach slope significantly improved equation fit and predictive value in several regions most notably the western mountains and the temperate plains channel width was also related to human disturbance we examined the influence of human disturbance on channel width using several indices of local and basinwide disturbance contrary to our expectations the data suggest that the dominant response of channel width to human disturbance in the united states is a reduction in bankfull width in streams with greater disturbance particularly in the western mountains where population density road density agricultural land use and local riparian disturbance were all negatively related to channel width and in the appalachians and new england where urban and agricultural land cover and riparian disturbance were all negatively associated with channel width

rare earth elements in the early archaean isua iron formation west greenland abstract the rare earth element ree patterns in the 3 8 ga old isua iron formation are generally flat resembling those of some primitive basalts samples with positive negative or no europium anomaly were found it is shown that diagenesis and metamorphism did not significantly change the ree patterns the presence or absence of europium anomalies in iron formations cannot be used as an indicator of the presence or absence of oxygen in the atmosphere during the archaean and precambrian the ree contents cannot be used to distinguish algoma type from superior type iron formations there appears to be a striking similarity between the archaean submarine exhalations and modern submarine hydrothermal systems it is considered likely that archaean and early precambrian seawater had a chondritic ree pattern with a slight enrichment of light ree

extreme sulfur cycling in acid brine lake environments of western australia abstract regional scale extreme acid brines in southern western australia are an example of sulfur rich end member continental waters and are amongst the most physically and chemically dynamic settings in the world here aqueous sulfur compounds sulfur minerals and sulfur interacting microorganisms are abundant and diverse and sulfur cycling is intense lake waters and groundwaters of the yilgarn craton are na cl so 4 mg rich with ph as low as 1 7 salinity as high as 32 total dissolved solids and sulfuric acid as the dominant acid measured concentrations of dissolved total s range from 500 to 19 800 ppm and so 4 ranges from 692 to 35 169 ppm the difference between total s and so 4 suggests that other s species are present in these waters particularly in the most acid brines rainwaters which have previously been suggested as a source of sea spray aerosol so 4 to the lake systems yielded 7 to 8 ppm so 4 sulfur isotope analysis shows that 34 s for lake waters is 17 0 to 19 1 for groundwaters is 17 0 to 18 6 for gypsum is 20 2 to 20 4 and for alunite is 18 9 these values are consistent with a combination of s sources and sinks sulfur cycling processes include sulfide oxidation evapoconcentration mineral precipitation mineral dissolution and bacterial reduction and oxidation field petrographic and molecular analyses of microorganisms in these acid saline environments suggest that some are s oxidizing and s reducing bacteria sulfur bearing minerals in the area include both sulfides and sulfates sulfides including pyrite have been observed in veins and disseminated in surface exposures and shallow cores of archean igneous and metamorphic host rocks as well as in overlying sediments sedimentary sulfate minerals include gypsum bassanite anhydrite and rozenite that grow directly from lake waters and gypsum anhydrite jarosite natrojarosite alunite basaluminite hydrobasaluminite langbeinite and rozenite that grow diagenetically from shallow groundwaters these sedimentary minerals are subjected to physical reworking and chemical dissolution and re precipitation sulfur cycling among the lithosphere hydrosphere biosphere and atmosphere in acid saline environments is dynamic driving forces are rock weathering weather and climate which ultimately influence the water chemistry flooding evapoconcentration desiccation cycles accelerate short term local and regional sulfur cycling the combination of low ph highly saline waters and dynamic changes in environment make sulfur arguably the most important element in these environments

chemical divides and evaporite assemblages on mars assemblages of evaporite minerals record detailed physical and chemical characteristics of ancient surficial environments accordingly newly discovered regions of saline minerals on mars are high priority targets for exploration the chemical divide concept of evaporite mineral formation is used successfully to predict evaporite mineralogy and brine evolution on earth however basaltic weathering largely controls fluid compositions on mars and the robust predictive capabilities of terrestrial chemical divides cannot be used to interpret martian evaporites here we present a new chemical divide system that predicts evaporite assemblages identified in snc type meteorites ancient evaporites discovered on meridiani planum by the opportunity rover and mars express omega data we suggest that a common fluid type that has been buffered to different ph levels by basaltic weathering controls the variability among martian evaporite assemblages and that evaporite mineralogy and brine evolution is essentially established by the initial composition of the dilute evaporating fluid b 2005 elsevier b v all rights reserved

channel response to sediment wave propagation and movement redwood creek california usa redwood creek north coastal californa usa has experienced dramatic changes in channel confguation since the 1950s a series of large floods in 1955 1964 1972 and 1975 combined with the advent of widespread commercial timber harvest and road building resulted in extensive erosion in the basin and contributed high sediment loads to redwood creek since 1975 no peak flows have exceeded a 5 year recurrence interval twenty years of cross sectional survey data document the downstream movement of a sediment wave in the lower 26 km of this gravel bedded river at a rate of 800 to 1600 m a i durng this period of moderately low flows higher transit rates are associated with reaches of higher unit stream power the wave was initially deposited at a site with an abrupt decrease in channel gradient and increase in channel width the amplitude of the wave has attenuated more than i m as it moved downstream and the duration of the wave increased from eight years upstream to more than 20 years downstream channel aggradation and subsequent degradation have been accommodated across the entire channel bed channel width has not decreased significantly after initial channel widening from large 25 year recurrence interval floods three sets of longitudinal surveys of the streambed showed the highest increase in pool depths and frequency in a degrading reach but even the aggrading reach exhbited some pool development through time the aggraded channel bed switched from functioning as a sediment sink to a significant sediment source as the channel adjusted to high sediment loads from 1980 to 1990 sediment eroded from temporary channel storage represented about 25 per cent ofthe total sediment load and 95 per cent of the bedload exported from the basin

evidence for biogenic graphite in early archaean isua metasedimentary rocks the archaean rocks of isua west greenland contain graphite but its origins are debated geochemical and microscopic analyses suggest that the graphite was formed from biologically formed carbon that was deposited at least 3 7 billion years ago

regional impacts of urbanization on stream channel geometry a case study in semiarid southern california abstract urbanization often increases storm runoff peak discharges and rates of stream channel erosion coastal california has experienced rapid urbanization over the past several decades and has the potential for stream channel degradation several counties in california have implemented hydromodification management plans hmps to protect channels from erosion but few studies have quantified the impact of urbanization on channel geometry in diverse geological settings at the county scale a synoptic survey of field sites n 56 by the california environmental data exchange network ceden and additional field surveys n 24 were used to develop regional hydraulic geometry curves relating bankfull cross sectional area axs width w mean depth d and discharge qbf to watershed area aw in san diego county regional curves were compared for urban and reference sites and to other regional curves developed for southern california multiple regression models were used to identify dominant watershed and channel controls on geometry including aw percent impervious cover i mean annual precipitation underlying geology longitudinal slope hydrologic soil group and channel particle size for the reference streams regional curves were statistically significant for w and axs p 20 had significantly larger w d axs and qbf for a given watershed size a majority 68 of the urban channels and 78 of the small urban channels aw

the eruptive center of the late quaternary laacher see tephra about 5 km3 of phonolite magma were erupted 11 000 years ago in the laacher see area in plinian and vulcanian eruptions the eruptive vents for all tephra in dispute for over 200 years must have been located entirely within the laacher see basin because 1 all deposits are thickest in the tuffring surrounding the basin with one exception pyroclastic flow deposits are thickest 2 to 6 km away from the rim of the basin having ponded in radial valleys three plinian fallout layers show minor secondary thickness maxima 12 to 16 km east of laacher see 2 axes of all depositional fans and single tephra sheets converge in the laacher see basin 3 diameters of both lithic and pumice clasts reach their maxima in outcrops close to laacher see volcano 4 directions of asymmetric ballistic impact sags indicate the laacher see basin as source area 5 the dimension of laacher see crater roughly corresponds to the erupted volume of phonolitic tephra 5 3 km3 magma dre and lithic clasts 0 7 km3 6 xenolith types are consistent with an eruptive center within the laacher see basin 7 the systematic chemical and mineralogical zonation of the tephra deposits from highly differentiated aphyric phonolite at the base to highly phyric mafic phonolite at the top strongly suggests eruption from a single compositionally zoned magma column 8 several lines of evidence indicate migration of the main eruptive focus from the south to the north during the later part of the eruption both vents however being confined within the laacher see basin

non linearity of reach hydraulic geometry relations summary reach average hydraulic geometry rhg defines the relations between the average hydraulic variables estimated at river reach water depth velocity channel width and the discharge single log linear models are classically used by fluvial geomorphologists hydrologists and ecologists to represent these relations in this paper we show that these single log linear models are not relevant in most of cases and so can lead to significant errors rhg estimated on 15 alluvial river reaches the catchment sizes vary from 10 km 2 to 1700 km 2 were found non linear for two thirds of the river reaches when considering discharges ranging from low to bankfull flows so we propose using two power functions or log linear to model these relations i e the log piecewise linear model this model can detect a discharge qbreak i e a physical transition or break point between both relations from very low flows about 30 of the median discharge to qbreak a first relation is comparable to the rhg found in the literature for aquatic habitat analysis the second relation is relevant for flows ranging from qbreak to bankfull discharge and therefore may be more useful for sediment transport and flood routing analysis averaging along a reach including several pool riffle units makes it difficult to clearly ascribe the form of rhg to particular physical processes the non linearity of rhg may have multiple origins the presence of lateral gravel bars the hydraulic conditions of riffle cross sections a change in the resistance to flow with water depth vegetation sediment influences further research is needed to focus on the physical processes that govern the rhg non linearity

tephrostratigraphy of a lateglacial lake sediment sequence at wegliny southwest poland abstract this paper presents the first late quaternary locality in the present day territories of poland where multiple cryptotephra layers have been identified located near wegliny in southwest poland study of the lateglacial gyttja deposits reveals the presence of at least four non visible tephra horizons electron microprobe and laser ablation icpms analysis of glass shards suggests products from at least two icelandic volcanic centres katla and snaesfellsness of particular importance is the discovery of two eruptions believed to originate from the east eifel volcanic field within the allerod chronozone one correlates with the well documented laacher see tephra lst but the second horizon herein designated t642 t655 would appear to represent an earlier precursor eruption the chemical composition of the lst and the precursor tephra both appear to match to the upper laacher see tephra ulst phase which previously was thought to have dispersed not to the northeast but in a southerly direction towards the alpine foreland this indicates the eruption dynamics of the laacher see are more complex than hitherto recognised

channel pattern and river floodplain dynamics in forested mountain river systems channel pattern effectively stratifies the dynamics of rivers and floodplains in forested mountain river systems of the pacific northwest usa straight channels are least dynamic with relatively slow floodplain turnover and floodplains dominated by old surfaces braided channels are most dynamic with floodplain turnover as low as 25 years and predominantly young floodplain surfaces island braided and meandering channels have intermediate dynamics with moderately frequent disturbances erosion of floodplain patches maintaining a mix of old and young surfaces return intervals for the erosion of floodplains increase in the order of braided island braided meandering and straight 8 33 60 and 89 years respectively a threshold for the lateral migration of a channel occurs at a bankfull width of 15 20 m the most likely mechanism underlying this threshold is that larger channels are deep enough to erode below the rooting zone of bank vegetation above this threshold channels not confined between valley walls exhibit channel patterns distinguishable by slope and discharge and slope discharge domains can be used to predict channel patterns meandering and braided patterns are most consistently identified by the model and prediction errors are largely associated with indistinct transitions among channel patterns that are adjacent in plots of slope against discharge locations of straight channels are difficult to identify accurately with the current model the predicted spatial distribution of channel patterns reflects a downstream decline in channel slope which is likely correlated with a declining ratio of bed load to suspended load ecological theory suggests that biological diversity should be highest where the intermediate disturbance regime of island braided channels sustains high diversity of habitat and successional states through time

stable channels with mobile gravel beds regime type equations for mobile gravel bed rivers are presented based on data obtained from 62 stable gravel bed river reaches in the united kingdom multiple regression techniques are applied to derive equations relating reach average and riffle values of width mean and maximum depth slope velocity sinuosity and riffle spacing to bankfull discharge bed and bank material characteristics valley slope bank vegetation type and an independent estimate of bankfull bed load transport rates although discharge has a dominant control on channel geometry these equations indicate that bed load discharge also has a significant influence particularly with regard to channel slope bank vegetation has a major control on width and velocity while depth velocity and slope are storngly affected by bed material size reasons for these results are considered in terms of the physical processes controlling channel adjustment the application of these regime equations is discussed and particular consideration is given to the design of sinuous channels with a pool riffle bed topograph

carbon isotope fractionation during decomposition of organic matter in soils and paleosols implications for paleoecological interpretations of paleosols abstract stable isotope ratios reported as 13c of organic carbon preserved in paleosols have potential utility as a valuable paleoecological indicator if the isotopic composition of paleobiomass is preserved in the paleosol organic carbon without fractionation however soil organic matter is in a constant state of decomposition in an open system and frequently decomposes further during alteration after burial of the paleosol during this decomposition kinetic fractionation of 13c 12c in an open system results in concentration of 13c in the residual organic matter preserved in the soil and paleosol this paper summarizes studies of modern soils which show that this 13c enrichment follows a rayleigh distillation process whereby the 13c 12c ratio is a function of the natural logarithm of the fraction of remaining soil organic carbon leaving soil organic carbon 13c enriched by up to 6 with respect to the original biomass if the fraction of original biomass carbon remaining during these decomposition processes deviates significantly from 1 the soil organic carbon can be 13c enriched by several per mil and thus not a good paleoecological indicator of the 13c value of the original biomass therefore effective paleoecological studies of 13c preserved in paleosol organic carbon must therefore demonstrate that the measured concentration of organic carbon in the paleosol is near its maximum concentration during soil formation or that the paleosol organic matter does not include the 13c enriched products of microbial decomposition suggestions are made for interpreting the effects of carbon isotope fractionation of organic matter and for materials and methods most useful for paleoecological studies from bulk paleosol organic carbon

point counting pores and illuvial clay in thin section abstract six soil thin sections containing various amounts of illuvial clay previously estimated by ten operators were point counted four times by an additional experienced operator the results indicate that a count of 6000 points over a section area of 25 25 mm is sufficient to quantify satisfactorily the pore space of the section replicated results for illuvial clay however differed widely for some sections these differences are attributed both to difficulties in recognising illuvial clay and to orientation and variability of soil components variability of results for illuvial clay in this study was less than that reported for ten operators but nonetheless it is evident that for most sections the amount of illuvial clay cannot be estimated precisely by a single count of 6000 points suggestions are made for improving the reliability of estimates of illuvial clay in soil thin sections and an improved basis for reporting results is proposed

boron paleosalinity proxy for deeply buried paleozoic and ediacaran fossils abstract boron content is an indicator of paleosalinity for mesozoic and cenozoic clayey sediments but is compromised by clay diagenesis in deeply buried sediments of paleozoic and ediacaran age this study of north american russian and australian paleozoic and ediacaran fossils showed tight covariance of b ppm and k wt confirming common wisdom that boron is carried largely by illlite smectite clays ratios of b k 40 g g distinguish modern marine and freshwater sediments but this discrimination threshold declines to 21 g g for sequences with bituminous coal just below the anchimetamorphic zone and to 5 g g for sequences within the anchimetamorphic zone toward greenschist facies marine and non marine threshold b k values can be predicted from weaver and kubler indices of illite crystallinity as a measure of b depletion during deep burial and metamorphism deviations from that expectation b k weaver and b k kubler are positive in marine rocks and negative in non marine rocks trilobites stromatolites and other marine fossils were correctly identified as marine by this proxy and fossil land plants identified as non marine though in some cases by small margins because of declining b content with increased clay crystallinity cryogenian to devonian problematic quilted fossils dickinsonia arumberia rangea pteridinium ernietta aspidella rutgersella protonympha are indistinguishable from devonian to triassic plants using this proxy and significantly different from trilobites brachiopods ammonites and other securely marine fossils

a roughness corrected index of relative bed stability for regional stream surveys abstract quantitative regional assessments of streambed sedimentation and its likely causes are hampered because field investigations typically lack the requisite sample size measurements or precision for sound geomorphic and statistical interpretation we adapted an index of relative bed stability rbs for data calculated from a national stream survey field protocol to enable general evaluation of bed stability and anthropogenic sedimentation in synoptic ecological surveys rbs is the ratio of bed surface geometric mean particle diameter dgm divided by estimated critical diameter dcbf at bankfull flow based on a modified shield s criterion for incipient motion application of rbs to adequately depict bed stability in complex natural streams however has been limited because typical calculations of rbs do not explicitly account for reductions in bed shear stress that result from channel form roughness we modified the index rbs to incorporate the reduction in bed shear stress available for sediment transport that results from the hydraulic resistance of large wood and longitudinal irregularities in channel dimensions form roughness based on dimensional analysis we derived an adjustment to bankfull shear stress by multiplying the bankfull hydraulic radius rbf by the one third power of the ratio of particle derived resistance to total hydraulic resistance cp ct 1 3 where both resistances are empirically based calculations we computed cp using a keulegan equation relating resistance to relative submergence of bed particles we then derived an empirical equation to predict reach scale hydraulic resistance ct from thalweg mean depth thalweg mean residual depth and large wood volume based on field dye transit studies in which total hydraulic resistance ct was measured over a wide range of natural stream channel complexity including manipulation of large wood volumes we tested our estimates of ct and rbs by applying them to data from a summer low flow probability sample of 104 wadeable stream reaches in the coastal ecoregion of oregon and washington usa stream discharges calculated using these ct estimates compared favorably with velocity area measurements of discharge during summer low flow and with the range of 1 to 2 year recurrence floods scaled by drainage area at u s geological survey gauged sites in the same region log rbs ranged from 4 2 to 0 98 in the survey region dgm ranged from silt to boulders while estimated bankfull critical diameter d cbf ranged from very fine gravel to large boulders the median value of d cbf adjusted for form roughness influences averaged 40 inter quartile range 28 to 59 of the unadjusted estimate dcbf log rbs was consistently negatively related to human disturbances likely to produce excess sediment inputs or hydrologic alteration log rbs ranged from 1 9 to 0 5 in the streams within the lower quartile of human disturbance in their basin and riparian areas and was substantially lower 4 2 to 1 1 in streams within the upper quartile of human disturbance the synoptic survey methods and designs we used appear adequate to evaluate regional patterns in bed stability and sedimentation and their general relationship to human disturbances although the rbs concept also shows promise for evaluating sediment and bed stability in individual streams our approach is relatively coarse so site specific assessments using these rapid field methods might prudently be confined to identifying severe cases of sedimentation or channel alteration greater confidence to discern subtle differences in site specific assessments could be gained by calculating rbs using more precise field measurements of channel slope bed particle size and bankfull dimensions and by refining our adjustments for energy loss from channel form roughness

laacher see tephra a widespread isochronous late quaternary tephra layer in central and northern europe a late quaternary tephra layer widespread in central and northern europe resulted from explosive plinian and phreatomagmatic eruptions of the laacher see volcano 11 000 yr b p the tephra is distinguished from other late quaternary andesitic rhyolitic airfall tuff layers in northern europe and from basaltic or trachytic tuff deposits in southern europe by its phonolitic composition and abundance of sanidine plagioclase clinopyroxene amphibole and sphene the proximal tephia sequence at laacher see is divided into three main deposits the predominantly plinian deposits of lower and middle laacher see tephra llst and mlst and phreatomagmatic deposits of the upper laacher see tephra ulst the mlst member is further subdivided into beds a b and c1 c2 and c3 the chemical composition of the magma is highly differentiated phonolite in the llst to mlst b sections but mafic phonolite in the mlst c1 to ulst sections all deposits are considered to be isochronous the frequency maximum of 16 radiocarbon datings indicating an eruption about 11 000 50 yr b p distal ash was deposited in three main fans directed to the northeast lst traced up to 1 100 km distance south lst traced up to 600 km and southwest lst traced up to 100 km tephrostratigraphic correlation of the distal ash deposits is based on a the major element composition of glass shards b lithology and c heavy mineral analyses the northeastern fan consists of deposits from llst mlst b and mlst c1 eruptive phases the southern fan comprises mlst a mlst c2 and ulst deposits and the southwestern fan consists exclusively of ash from the ulst eruptive phase northeastern transport of ash during eruptive phases with high plinian eruption columns but southern and southwestern transport of ash along phases of relatively low eruption columns are interpreted in terms of prevailing southwesterly paleowinds at high altitudes tropopause level but northerly winds dominating in the lower atmosphere the laacher see eruption columns were emplaced into an atmosphere vertically zoned with respect to paleowind directions which also explains the near vent shifting of llst mlst b and mlst c1 iso pach axes from east southeast to northeast within the first 20 km of transport

boundary conditions for damming of a large river by fallout during the 12 900 bp plinian laacher see eruption germany syn eruptive rhine damming ii abstract the rhine river germany the largest river in western europe was dammed by pyroclastic material multiple times during the major plinian laacher see eruption 12 900 bp dams formed both upstream and downstream of the broad tectonic lower neuwied basin lnb which interrupts the narrow rhine canyon here we document upstream damming of the rhine river at the entrance to the lnb close to the present city of koblenz due to overloading with tephra fall into the rhine and its major tributaries the moselle and the lahn the dam was formed repeatedly during rapid pumiceous tephra fall events and breached during breaks in eruptive activity causing extensive high energy flooding throughout the entire basin the ephemeral koblenz dams differed significantly from normal volcanically induced dams by consisting principally of washed together pumice clasts and some driftwood the porous nature of pumice and its ability to absorb water were crucial factors thus a large volume percentage of the tephra that had fallen into the rhine floated submerged within the upper part of the water column or swam at the surface moreover the absorption of the river water by the pumice clasts increased the sediment water ratio of the two phase flow considerably we here present a model of dam formation resembling the formation of ice jams we visualize the koblenz dams to have been elongate partly floating and partly grounded permeable plugs many kilometers long and rising no higher than the flood plain damming was most plausibly initiated in the lnb within the area of maximum tephra loading and propagated upstream in a chain reaction comparable to the formation of traffic jams a major dam was finally accumulated at the bottleneck entrance to the lnb a site combining several favorable conditions the upstream multi channel rhine was confined to a single channel change of flow direction by 125 extremely low gradient 0 19 starting already 24 km upstream of the bottleneck constant decrease of flow velocity over many kilometers towards the bottleneck and the moselle river largest tributary of the rhine within the lnb and an important conveyor of additional tephra masses entered the rhine only 700 m upstream of the bottleneck we assume that the koblenz dams could only have formed and been stabilized by an extremely long foot region that extended many kilometers downstream and that was possibly connected to one or several low rise secondary jams dams the backwater of lake brohl that was dammed by pyroclastic flows 7 km downstream of the lnb about halfway through the eruption extended further and further upstream into the lnb during the second plinian stage of the laacher see eruption and was probably a major factor contributing to the formation and large size of koblenz dam no 4 the koblenz dams were probably not completely sealed most of the time this way the major pre eruptive rhine channel received some water an equilibrium condition was established that enabled the dams to remain stable as long as tephra fell into the rhine relatively continuously

the lateral facies of ignimbrites at laacher see volcano valley fill ignimbrites of the middle laacher see tephra mlst in the proximal depositional fan south of laacher see volcano are laterally continuous with fine grained overbank facies deposits 0 5 1 5 m thick covering higher elevations and interfluves between the paleovalleys the overbank deposits consist of up to 12 ash layers each 4 10 cm thick which show internal structures typical of ash flow transport such as poor sorting reverse size grading of pumice local normal grading of coarse ash sized lithics above a fine grained basal layer cross stratification behind obstacles and erosional unconformities thickness median grain size and number of individual layers decrease systematically with distance from the vent overbank ash layers thicken at the valley slopes and form discrete valley fill ignimbrite flow units in the paleochannels with median grain size increasing from 63 m to 350 m toward the center of paleochannels however the well defined overbank facies is obscured by mutual erosion of individual flow units overlapping data fields in ternary grain size variation diagrams indicate the overbank facies to have evolved from ash flows chiefly through depletion of lapilli and coarse ash overbank facies ash layers do not represent dust layers resulting from elutriation clouds of ash flows co ignimbrite ash or surge deposits developed on higher ground due to low concentration of solids they are similar in some parameters to taupo type ignimbrite veneer deposits but are interpreted differently the thin fine grained laacher see ash layers are thought to have been deposited from diluted portions of the flow proceeding directly from the eruption column while the main pyroclastic flows were confined to the paleovalleys radiating away from the eruptive center the wide distribution of the thin ash layers is attributed to the balance of deposition from various flow parts and turbulent entraining and heating of ambient air that sustained sufficient mobility of the diluted flows to spread across hills and level ground

ree and trace element patterns from organic rich rocks of the ediacaran cambrian transitional interval abstract it has been established that important changes in the marine environment and the biosphere occurred during the cambrian however the relationships between the so called cambrian explosion and the concomitant environmental changes are not yet fully understood this study presents new geochemical data from the black shale successions from different facies belts of the yangtze platform in south china variations in the concentrations of ree and trace elements varying ce ce th u v sc and v cr ratios in kerogen as well as in bulk rocks from different depositional environments along a transect from platform to basin indicate two oxidation events which led to the oxygenation of the water column in shallow marine environments and euxinic conditions weak correlation between toc v u and mo in the deeper sea during the first oxidation event in the late terreneuvian anoxic conditions in bottom waters rapidly changed to euxinic conditions subsequently the second oxidation event during the early epoch 2 of the cambrian led to oxic suboxic conditions in deeper seawater

are there ancient soils in the 3 7 ga isua greenstone belt greenland abstract a lens of black schist within 3 7 ga quartzites of greenland may be earth s oldest known alluvial paleosol the suspect metamorphic rock is a lens in orthoquartzite of berthierine schist with crystals of ripidolite but it has a truncated top above dark gray grading down to gray color ptygmatically folded surface cracks filled with silt grains and large sand crystals unusual for sedimentary or metamorphic rocks the paleosol hypothesis was tested with thin sections showing plausible mineral weathering trends and by chemical analysis showing molar weathering ratios and ree distribution like those of soils the schist is deeply weathered and at the culmination of weathering trends from analysis of other metasediments of the isukasia area the protolith can be reconstructed as a saponite clay with a salt rich horizon of kieserite like other acid sulfate paleosols of the early earth models for proton and electron consumption of paleosols applied to the profile reveal an atmosphere with only 36 510 ppm o2 and 820 201 ppm co2 and humid cool temperate paleoclimate the profile has organic 13cpdb consistently of 24 2 to 27 4 and modest raleigh distillation near the top similar consistent values and trends are produced by decay of organic matter in living soils but biotic carbon isotopic composition of sediments is erratic from bed to bed and abiotic carbon compounds of meteorites differ dramatically for each kerogen particle thus life in this very ancient soil is not precluded by our analyses but ultrastructural and geochemical testing of carbon particles would further test this hypothesis

effects of coarse woody debris on morphology and sediment storage of a mountain stream system in western oregon effects of coarse woody debris cwd on channel morphology and sediment storage were investigated at five sites representative of first order to fifth order streams in the steep and bedrock confined stream first second order interaction between the channel and cwd was limited except where breakage upon falling produced cwd pieces shorter than channel width channel widening steepening and sediment storage associated with cwd were observed predominantly in third to fifth order streams variation in channel width and gradient was regulated by cwd in the fifth order stream most of the cwd pieces derived from the riparian forest interacted directly with the channel without being suspended by sideslopes in this system cwd promoted lateral channel migration but sediment storage was temporary with annual release and capture

early life signatures in sulfur and carbon isotopes from isua barberton wabigoon steep rock and belingwe greenstone belts 3 8 to 2 7 ga

stable isotope relations in an open magma system laacher see eifel frg 18o 16o and d h ratios have been measured for matrix glasses and phenocrysts from the zoned phonolitic laacher see tephra sequence 11000 y b p east eifel volcanic field frg to study open system behaviour of the associated magma system mineral and glass 18o values appear to be largely undisturbed by low temperature secondary alteration record isotopic equilibrium and confirm previous conclusions based on radiogenic isotope evidence of early small scale crustal assimilation during differentiation of parental magmas in a crustal magma chamber one sanidine glass pair possibly documents the late stage influx of meteoric fluids into the topmost magma layer prior to eruption a sealing carapace of chilled magma which itself was strongly contaminated prevented large scale fluid exchange up to the point prior to eruption when this carapace was fractured and meteoric water gained access to parts of the magma system d h measurements of various glass types glass inclusions dense and pumiceous glass and amphiboles gave conflicting results suggesting a combination of degassing volatile exchange with country rocks and hydration stable isotope ratios for primitive parental magmas 18o 5 5 to 7 0 and mantle megacrysts 18o 5 5 to 6 0 d 21 to 38 for amphiboles and phlogopite resp suggest a rather variable fluid composition for the sub eifel mantle

stream channel erosion and change resulting from riparian forests forested stream banks compared to grassed ones can destabilize stream channels by promoting erosion four reaches of coon creek wisconsin each with long term grassed and forested subreaches were examined grassed reaches were narrower and had smaller channels bankfull cross sections than forested reaches suggesting that grassed channel reaches stored about 2100 to 8800 m 3 more sediment per kilometre than forested reaches available evidence suggests that conversion of riparian forests to grass would allow storage of sediment along channels possibly decreasing downstream sediment yields these findings are important as many grassed riparian corridors are rapidly reverting to forest because of economic conditions and governmental policies

lateritization and bauxitization events laterites and bauxites are produced in tropical soils by weathering which enriches iron of laterites and alu mina of bauxites as well as trace elements such as nickel gold phosphorus and niobium to ore grade la terites and bauxites can be redeposited into sedimentary sequences and remain as ores if not transported far and diluted with other materials the age of redeposited laterites and bauxites and of bauxitic and lateritic pa leosols can be established from the geologic age of overlying rocks an approach especially effective in pale osols within sequences of isotopically datable volcanic rocks lateritic profiles can also be dated by paleomag netic inclination in special cases in which land masses such as in australia and india drifted long distances northward during cenozoic time in addition cryptomelane and other k mn oxides can be dated by k ar and 40ar 39ar techniques to obtain multiple ages from different crystals in a single relict paleosol compilation of new and more accurate laterite and bauxite ages reveals unusually widespread and intense laterite and bauxite formation during events of less than 100 k y duration at 2 12 16 35 48 55 65 and 100 ma such events can also be inferred at times older than 100 ma from paleolatitudinal distribution of laterites and bauxites but these are poorly sampled laterite and bauxite peaks were coeval with times of global high warmth and pre cipitation elevated atmospheric carbon dioxide oceanic anoxia exceptional fossil preservation and mass ex tinction these co2 greenhouse events and attendant titration of carbonic acid with soils are interpreted as transient fluctuations in the atmosphere produced by meteorite impact flood basalt volcanism and methane outbursts concentration of bauxite and laterite resources in particular stratigraphic horizons formed during greenhouse crises suggests the usefulness of an event stratigraphic approach to exploration and exploitation of these and related ores

ediacaran and cambrian paleosols from central australia abstract ediacaran to cambrian red sandstones of the northern territory including arumbera sandstone and grant bluff and central mount stuart formations have been reexamined and sampled in order to reconstruct paleoenvironments from sedimentary facies and paleosols sedimentary facies include green gray calcareous sandstone member iii of arumbera sandstone only red siltstone with calcareous nodules also only member iii and also non calcareous red green siltstone interbedded red siltstone and sandstone and trough bedded sandstone the green gray facies has diverse and distinctive cambrian marine trace fossils not found in the red beds which have instead an assemblage of problematic body fossils including arumberia noffkarkys hallidaya ernietta trepassia and dickinsonia the red facies are interpreted as fluvial paleochannels trough cross bedded fluvial levees interbedded siltstone and sandstone poorly drained floodplain red green siltstone and well drained floodplain or low terrace calcareous nodular the carbonate nodules have micritic replacive fabrics and a significant positive correlation of 13c and 18o characteristic of pedogenic carbonate of calcid soils other red beds have pseudomorphous sand crystals after gypsum like those of gypsid soils individual red beds show negative strain and mass transfer for cationic bases typical of soils as well as gradational alteration down from fossiliferous tops characteristic of soils their varied degree of pedogenic differentiation is identified as psamment aquent and ochrept paleosols phosphorus depletion in the paleosols is evidence that organisms lived in the paleosols and were not introduced by transient marine incursions the problematic quilted fossils arumberia noffkarkys and hallidaya formed a distinctive inland terrestrial polsterland during the ediacaran and early cambrian

tracing the laacher see tephra in the varved sediment record of the trzechowskie palaeolake in central northern poland abstract tephrochronological studies of partly varved sediments of trzechowskie palaeolake in central northern poland led to the finding of the late allerod laacher see tephra lst from the eifel volcanic field for the first time in a very distal site ca 840 km ene from its volcanic source the detection of glass shards of the lst involved a comprehensive combination of techniques i e biostratigraphical constrains high resolution xrf core scanning and areal xrf mapping of impregnated sediment slabs as well as detailed visual inspection of sediments the major element chemistry of volcanic glass confirmed the laacher see tephra composition in trzechowskie palaeolake sediments suggesting a deposition from the middle laacher see tephra mlst c or upper laacher see tephra ulst dispersal fans the finding of the lst in this palaeolake enables direct synchronisation with other high resolution archives in north central europe i e lake meerfelder maar rehwiese palaeolake to investigate regional variations of environmental responses at the onset of the younger dryas along a west east transect through north central europe