the chronology of speleothem deposition in northern yukon and its relationships to permafrost in the northern yukon there is a significant group of limestone caves in an area that was not glaciated during the quaternary permafrost appears to have played an important role in restricting calcite speleothem deposition in the caves u th dating and stable isotopic studies indicate that the deposits form two distinct groups a first group younger than 350 ka and a second group older than the 350 ka bp limit of the conventional u th alpha spectrometric dating method two large samples from the second group yield sequences of magnetic declinations and inclinations with reversed and normal polarity perhaps from the tertiary the pollen content of five speleothems the modes of calcite deposition and stable isotopic analyses indicate that the speleothems were deposited under a cold regime it is suggested that the ancient speleothems were deposited at a time when permafrost was absent during the later tertiary or at the beginning of the quaternary period the establishment and maintenance of permafros

modeling the evolution of incised streams ii streambank erosion incision and ensuing widening of alluvial stream channels is widespread in the midsouth and midwestern united states and represents an important form of channel adjustment streambanks have been found to contribute as much as 80 of the total suspended load the location timing and magnitude of streambank erosion are difficult to predict results from field studies to characterize the resistance of fine grained materials to hydraulic and geotechnical erosion the impact of pore water pressures on failure dimensions and shearing resistance and the role of riparian vegetation on matric suction streambank permeability and shearing resistance are used to enhance the channel evolution model concepts conservational channel evolution and pollutant transport system this paper discusses the conceptualization of the above mentioned physical processes and demonstrates the ability of the derived model to simulate streambank failure processes the model is tested against observed streambank erosion of a bendway on goodwin creek miss between march 1996 and march 2001 where it accurately predicts the rate of retreat of the outside bank of the bendway the observed change in average channel width within the central section of the bendway is 2 96 m over the simulation period whereas a retreat of 3 18 m 7 4 larger is simulated the observed top bank retreat within the central section of the bendway is 3 54 m over the simulation period whereas a retreat of 3 01 m 15 smaller is simulated doi 10 1061 asce0733 94292008134 7905 ce database subject headings bank erosion limit equilibrium mathematical models sensitivity analysis numerical analysis

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

effects of changing channel morphology on vegetation groundwater and soil moisture regimes in groundwater dependent ecosystems abstract channel incision channel widening and excessive floodplain sedimentation are major causes of riparian ecosystem degradation across the country although the causes and consequences of these processes vary significantly the resulting morphology in all cases results in a lower stream stage relative to the floodplain surface for any given discharge this change in channel morphology alters surface water groundwater interactions between the stream and the riparian aquifer and affects the soil moisture and groundwater regimes differentially across the floodplain the distribution of vegetation is altered as the hydrologic regime shifts between the hydrologic niches of potential species we simulate the hydroecologic response of a groundwater dependent ecosystem on the floodplain of an archetypical watershed under three scenarios representing changes in channel morphology a base case a widened channel case and an incised channel case stochastically driven synthetic rainfall records are used to drive a rainfall runoff model of the archetype watershed the resulting hydrograph is transformed to stage via manning s equation for the three channel morphologies considered this stage record is used as a boundary condition for a finite element model simulating 2 dimensional variably saturated groundwater flow in the riparian aquifer the model predicts soil moisture and groundwater regimes lateral to the channel the 7 day moving average high water level is determined for ten growing seasons and used to predict the distribution of three species across the floodplain plant frequency is predicted for carex emoryi an obligate wetland species carex crawei facultative wetland species and carex duriuscula a facultative upland species for each of the channel morphologies considered the frequency is predicted using existing nonlinear parametric species response curves determined empirically for these species using direct gradient analysis results show the sensitivity of the distribution of vegetation on floodplains to channel morphologic changes our linked hydrologic and ecological model provides a solid framework for considering potential hydroecologic impacts of channel disturbance and or restoration on vegetation communities which are often affected by channel degradation and targeted in restoration efforts

environmental magnetic properties used to assess sediment sources and transport patterns in the muddy area along the zhejiang coast china abstract the muddy area along the zhejiang coast contains a record of historical environmental conditions and can facilitate a better understanding of the linkages among terrestrial sediment fluxes littoral sediment dynamics and climate change however the origin and transport of the sediments in this muddy area has been open to debate we present a detailed environmental magnetic study with high density sampling to provide magnetic evidence for the input and transport of the materials in this region our results show that magnetic minerals in the sediments are generally dominated by magnetite and titanomagnetite with minor amounts of maghemite and hematite the magnetic minerals are mainly pseudo single domain psd particles and decrease in concentration seawards based on the coastal current flow and magnetic characteristics we argue that the sediments in this area are mainly derived from the changjiang river the mountainous rivers in the surrounding islands and erosion by coastal currents notably the increase in sediment input from small mountainous rivers and coastal erosion in the zhejiang coastal area is primarily associated with decreased sediment discharge from the changjiang river however there is no evidence of the accumulation of taiwan derived sediment in this area

riparian vegetation and fluvial geomorphic processes abstract riparian vegetation and fluvial geomorphic processes and landforms are intimately connected parts of the bottomland landscape relations among vegetation processes and landforms are described here for representative streams of four areas of the united states high gradient streams of the humid east coastal plain streams great plains streams and stream channels of the southwestern united states vegetation patterns suggest that species distributions in the humid east are largely controlled by frequency duration and intensity of floods along channelized streams vegetation distribution is largely controlled by variation in fluvial geomorphic processes cycles of degradation and aggradation in response to increases in channel gradient associated with channelization similarly riparian vegetation of great plains streams may be controlled by fluxes in sediment deposition and erosion along braided streams patterns of riparian vegetation in semi arid regions may be most closely related to patterns of water availability unlike most other streams in more humid environments channel equilibrium conditions control stability of the coincident fluvial landform and attendant vegetation pattern throughout the continent in most situations riparian vegetation patterns are indicative of specific landforms and thus of ambient hydrogeomorphic conditions

preface multiscale feedbacks in ecogeomorphology 1 introductiongeomorphic systems are known to exhibit nonlinear responses tophysical biological feedbacks thornes 1985 baas 2002 reinhardtet al 2010 these responses make understanding and or predictingsystem response to change highly challenging with growingconcerns over ecosystem health a pressing need exists for researchthat tries to elucidate these feedbacks jerolmack 2008 darby 2010 national research council 2010 a session was convened at the fall2008 meeting of the american geophysical union agu to providean outlet for some of this truly interdisciplinary and original research which is central to understanding geomorphic and ecologicaldynamics the session attracted over 39 contributions which weredivided into two well attended oral sessions and a very busy postersession this special issue presents new research from the agu session which highlights clear physical biological feedbacks the aim is tobring together contrasting perspectives on biological and geomorphicfeedbacks in a diversity of physiographic settings ranging fromwetlands and estuaries through rivers to uplands these papershighlight biological and physical feedbacks which involve themodulation or ampli cation of geomorphic processes these paperswill be of interest to a core geomorphology audience and should alsodraw attention from the elds of ecohydraulics hydroecology ecohydrology ecomorphology biogeochemistry and biogeography and biogeomorphology as well as the more traditional elds ofhydrology ecology and biology in this preface to the special issue we a review past contributionsto the emerging eld of ecogeomorphology and related disciplines b provide some context for how this topical special issue came tofruition and c summarize the contributions to this special issue 2 what is ecogeomorphology examples of studies explicitly integrating ecology and geomor phology date back to at least the late 1800s e g cowles 1899 withmore examples appearing in the 1950s e g olson 1958 and 1960s e g hack and goodlet 1960 terms such as ecogeomorphology andbiogeomorphologyonlybecamepopularinthe1990s osterkampandhupp 2010 the 1990 british geomorphology research groupannual meeting thornes 1990 and the 1995 binghamton sympo sium on biogeomorphology hupp et al 1995b were two of the rstsymposiums to emphasize the theme of biogeomorphology ecogeo morphology and biogeomorphology are generally considered syno nyms hupp et al 1995a but they do sprout from slightly differentresearch interests both are part of a larger family of terms thatresearchers have used somewhat inconsistently to describe researchat the interface between some combination of hydrology geomor phologyand orecology biology fig 1 unlikesomeoftheothereco geo hydro combinations in fig 1 the eco or bio root is an adjectivemodifying the noun geomorphology based on the types of researchpublished under these two banners the biogeomorphology commu nity has generally focused more on how chemical and physicalweathering are ampli ed and modulated with biological feedbacks ecogeomorphology is commonly used to describe studies that focusmore on the ampli cation and modulation of erosion and depositionprocesses both are fundamentally concerned with the bidirectionalin uences of biota and landscapes on each other biogeomorphologyis concerned with both the in uence of landforms on plants animalsand micro organisms distribution and development as well as thein uence of those plants animals and micro organisms on earthsurfaceprocessesandthelandformsshapedbythoseprocesses viles 1988 naylor et al 2002 p 4 loosely de ne biogeomorphology asinvolving the cooperation between ecology and geomorphology tostudy two way linkages between ecological and geomorphologicalprocesses murray et al 2008 argue that biomorphodynamics is amore restrictive term than biogeomorphology or ecogeomorphology and that it highlights landscape dynamics in which an explicit twoway coupling i e feedback occurs between biotic and physicalprocesses therelatedzoogeomorphology e g butler 1995 termhasreceived less play but it is worth mentioning that research under thisbanner focuses on examples of animals in uencing geomorphicprocesses and landforms e g beaver salmon burrowing animals etc the term biogeomorphology has been more widely used sinceviles 1988 rst attempted to provide a comprehensive overview ofwhat was at that time an emerging discipline a review of web of

endostromatolites from permafrost karst yukon canada paleoclimatic proxies for the holocene hypsithermal a remarkable biogenic calcite precipitate forms carpets of finely laminated 1 mm diameter columns lining fissures within limestone bedrock in the permafrost regions of the northern yukon this material endostromatolite for its laminated morphology and growth hidden within the carbonate rock is ubiquitous within limestone terrains of the arctic and grew during the early holocene hypsithermal dissolution on the interior fissure faces is accompanied by biomineralization of the opposing faces a previously unrecognized weathering process in permafrost regions occurrence is restricted to outcrops with a southern orientation in permafrost regions in this case from the ogilvie mountains northern yukon growth occurs in water saturated talik during periods of permafrost degradation during insolation maxima their enriched 13c values 1 7 to 11 4 are generated in a methanogenic environment during anaerobic degradation of soil derived organic carbon a paleotemperature signal extracted from the 18o

micro morphological inorganic organic isotope geochemisty and microbial populations in endostromatolites cf fissure calcretes haughton impact structure devon island canada the influence of geochemical pathways on the preservation of isotope biomarkers abstract endostromatolites cf fissure calcretes which possess microbial evidence for a biogenic origin are also thought to preserve isotopic biosignatures in this study a multi proxy approach combining micro morphological geochemical and isotopic analyses of middle holocene age endostromatolites within sub horizontal fissures in dolomitic limestone outcrops in the haughton impact crater region devon island nu was used to determine their origin abiotic versus biogenic and to identify potential isotope biosignatures the micro morphologies of the endostromatolites revealed some structures typical of a physico chemical origin whereas the presence of rod shape particles and filamentous structures was more reminiscent of biologically induced forms the endostromatolites have 13 c and 18 o compositions reaching maximum values of 7 2 and 11 2 respectively positive relations between the elemental mg sr and isotopic 18 o and 13 c composition of the endostromatolites are indicative of an evaporative enrichment process of the meteoric water infiltrating the fissures prior to calcite precipitation however the positive 13 c toc 13 c caco3 relation in the endostromatolites is strongly indicative that they were microbially mediated in support of a microbial origin mostly aerobic heterotrophic bacteria that have been linked to both carbonate dissolution and mineralization were observed in the microbial diversity of the endostromatolites however the results are inconclusive to attribute the formation of the endostromatolites solely to a biologically induced mineralization but instead favor a more complex origin that involved abiotic evaporation and to some extent biological processes prior to and during calcite precipitation considering that the endostromatolites result from microbially influenced mineralization the effects of physico chemical processes on the geochemical and isotopic composition of the endostromatolites were much greater than the effect of the biological processes thus it seems that preservation of isotopic biosignatures in secondary carbonate precipitates is dependent on the sequence of processes generating chemical and isotopic modifications of the solution prior to calcite precipitation the mechanism and condition equilibrium or kinetic of formation and post fossilization alteration processes

modeling fluvial incision and transient landscape evolution influence of dynamic channel adjustment 1 channel geometry exerts a fundamental control on fluvial processes recent work has shown that bedrock channel width depends on a number of parameters including channel slope and is not solely a function of drainage area as is commonly assumed the present work represents the first attempt to investigate the consequences of dynamic gradient sensitive channel adjustment for drainage basin evolution we use the channel hillslope integrated landscape development child model to analyze the response of a catchment to a given tectonic perturbation using as a template the topography of a well documented catchment in the footwall of an active normal fault in the apennines italy that is known to be undergoing a transient response to tectonic forcing we show that the observed transient response can be reproduced to first order with a simple detachment limited fluvial incision law transient landscape is characterized by gentler gradients and a shorter response time when dynamic channel adjustment is allowed the differences in predicted channel geometry between the static case width dependent solely on upstream area and dynamic case width dependent on both drainage area and channel slope lead to contrasting landscape morphologies when integrated at the scale of a whole catchment particularly in presence of strong tilting and or pronounced slip rate acceleration our results emphasize the importance of channel width in controlling fluvial processes and landscape evolution they stress the need for using a dynamic hydraulic scaling law when modeling landscape evolution particularly when the relative uplift field is nonuniform

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

the impact of humans on continental erosion and sedimentation rock uplift and erosional denudation of orogenic belts have long been the most impor tant geologic processes that serve to shape continental surfaces but the rate of geomor phic change resulting from these natural phe nomena has now been outstripped by human activities associated with agriculture con struction and mining although humans are now the most important geomorphic agent on the planet s surface natural and anthro pogenic processes serve to modify quite dif ferent parts of earth s landscape in order to better understand the impact of humans on continental erosion we have examined both long term and short term data on rates of sediment transfer in response to glacio fl u vial and anthropogenic processes phanerozoic rates of subaerial denuda tion inferred from preserved volumes of sedimentary rock require a mean conti nental erosion rate on the order of 16 m per million years m m y resulting in the accumulation of 5 gigatons of sediment per year gt yr erosion irregularly increased over the 542 m y span of phanerozoic time to a pliocene value of 53 m m y 16 gt yr current estimates of large river sediment loads are similar to this late neogene value and require net denudation of ice free land surfaces at a rate of 62 m m y 21 gt yr consideration of the variation in large river sediment loads and the geomorphology of respective river basin catchments suggests that natural erosion is primarily confi ned to drainage headwaters 83 of the global river sediment fl ux is derived from the high est 10 of earth s surface subaerial erosion as a result of human activity primarily through agricultural practices has resulted in a sharp increase in net rates of continental denudation although less well constrained than estimates based on surviving rock volumes or current river loads available data suggest that present farmland denudation is proceeding at a rate of 600 m m y 75 gt yr and is largely confi ned to the lower elevations of earth s land surface primarily along passive continental margins 83 of cropland erosion occurs over the lower 65 of earth s surface the conspicuous disparity between natu ral sediment fl uxes suggested by data on rock volumes and river loads 21 gt yr and anthropogenic fl uxes inferred from measured and modeled cropland soil losses 75 gt yr is readily resolved by data on thicknesses and ages of alluvial sediment that has been deposited immediately downslope from erod ing croplands over the history of human agriculture accumulation of postsettlement alluvium on higher order tributary channels and fl oodplains mean rate 12 600 m m y is the most important geomorphic process in terms of the erosion and deposition of sedi ment that is currently shaping the landscape of earth it far exceeds even the impact of pleistocene continental glaciers or the cur rent impact of alpine erosion by glacial and or fl uvial processes conversely available data suggest that since 1961 global cropland area has increased by 11 while the global population has approximately doubled the net effect of both changes is that per capita cropland area has decreased by 44 over this same time interval 1 per year this is 25 times the rate of soil area loss antici pated from human denudation of cropland surfaces in a context of per capita food pro duction soil loss through cropland erosion is largely insignifi cant when compared to the impact of population growth

floodplain sedimentation in the upper mississippi valley natural versus human accelerated abstract understanding the time scales and pathways for response and recovery of rivers and floodplains to episodic changes in erosion and sedimentation has been a long standing issue in fluvial geomorphology floodplains are an important component of watershed systems because they affect downstream storage and delivery of overbank flood waters and they also serve as sources and temporary sinks for sediments and toxic substances delivered by river systems here 14c and 137cs isotopic dating methods are used along with ages of culturally related phenomena associated with mining and agriculture to determine rates of sedimentation and morphologic change for a reach of the upper mississippi river and adjacent tributaries in southwestern wisconsin and northwestern illinois the most important environmental change that influenced fluvial activity in this region during last 10 000 years involved the conversion of a late holocene mosaic of prairie and forest to a landscape dominated by cropland and pastureland associated with euro american settlement results presented herein for the upper mississippi valley umv show that the shift from pre agriculture natural land cover to landscape dominance by agricultural land use of the last 175 200 years typically increased rates and magnitudes of floodplain sedimentation by at least an order of magnitude accelerated overbank flooding led to increased bank heights on tributary streams and in turn contributed to more frequent deep flows of high energy these high energy flows subsequently promoted bank erosion and lateral channel migration and the formation of a historical meander belt whose alluvial surface constitutes a new historical floodplain inset against the earlier historical floodplain the new historical floodplain serves as a flume like channel that provides efficient downstream transport of water and sediment associated with moderate and large magnitude floods floodplains on lower tributaries however continue to experience rates of overbank sedimentation that are of anomalously high magnitude given improved land cover and land conservation since about 1950 this lower valley anomaly is explained by minimal development of historical agriculture period meander belts because of relatively low stream power in these channel and floodplain reaches of relatively low gradient in general long term pre agriculture rates of vertical accretion between about 10 000 and 200 years ago averaged about 0 2 mm yr 1 in tributary watersheds smaller than about 700 km2 and about 0 9 mm yr 1 on the floodplain of the upper mississippi river where the contributing watershed area increases to about 170 000 km2 on the other hand rates of historical vertical accretion during the period of agricultural dominance of the last 200 years average between 2 and 20 mm yr 1 with short episodes of even higher rates during times of particularly poor land conservation practices significant hydrologic effects of mining and agricultural started by the 1820s and became widespread in the study region by the mid 19th century the hydrologic and geomorphic influences of mining were relatively minor compared to those related to agriculture high resolution dating of floodplain vertical accretion deposits shows that large floods have frequently provided major increments of sedimentation on floodplains of tributaries and the main valley upper mississippi river the relative importance of large floods as contributors to floodplain vertical accretion is noteworthy because global atmospheric circulation models indicate that the main channel upper mississippi river should experience increased frequencies of extreme hydrologic events including large floods with anticipated continued global warming instrumental and stratigraphic records show that coincident with global warming a shift to more frequent large floods occurred since 1950 on the upper mississippi river and these floods generally contributed high magnitudes of floodplain sedimentation

origin age and paleoenvironmental significance of carbonate precipitates from a granitic environment akshayuk pass southern baffin island canada this study documents the discovery of calcite crusts on the upper surface of clasts within morainic complexes in akshayuk pass southern cumberland peninsula baffin island a region underlain by granitic and gneissic rocks of the precambrian canadian shield strontium isotope ratios 87 sr 86 sr indicate that the major source of calcium is derived from the local dissolution of fracture filling calcite and a minor source is derived from silicate weathering the calcite crusts have 13 c values between 1 6 and 12 0 and 18 o values in the 13 0 and 7 9 range these values are highly enriched over the predicted isotopic values based on the 13 cdic and 18 o of the local water and temperature at which calcite precipitation occurred the isotopic enrichments are attributed to a combination of both equilibrium and kinetic evaporation and were verified experimentally the series of evaporative experiments indicate that kinetic evapo ration produces a far from equilibrium isotope effect on both the 13 c and 18 o composition of the precipitating calcite 13 ckie caco3 co2 between 20 2 and 40 5 vpdb 18 okie averaging 34 9 3 7 vsmow based on these re sults the formation of the calcite crusts is ascribed to the evaporation of stagnating ephemeral lakes and streams fol lowing the retreat of valley glaciers given the 1500 m difference in heights between the highland and the valley the katabatic winds originating from the penny ice cap can act as a catalyst to increase the rate of evaporation in ad dition radiocarbon dating of ten of these crusts yielded holocene ages 7640 cal calibrated bp to modern these re sults suggest that the maximum expansion of holocene valley glacier in akshayuk pass previously thought to have occurred during the little ice age is probably much older

environmental setting micro morphologies and stable c o isotope composition of cold climate carbonate precipitates a review and evaluation of their potential as paleoclimatic proxies abstract given the growing interest in carbonate deposits from polar regions as paleoclimatic proxies this review paper first provides a classification of the various types of cold climate carbonate precipitates followed by a summary of the 13 c and 18 o composition of the carbonate deposits and parent water from which the carbonates precipitated the cold climate carbonate precipitates were classified into three broad categories powders crusts and speleothem the carbonate powders include those that precipitated in relation to aufeis aggradation cryogenic aufeis calcite and in relation to the growth of various annual perennial ice formations in freezing caves cryptocrystalline calcite and calcite pearls the carbonate crusts can be further subdivided based on their lithic environment those that precipitated on the upper surface of bedrock clasts i e subglacially precipitated calcite and evaporative calcite crusts those that are located on the underside of clasts i e pedogenic carbonates and those that precipitated in rock outcrop fissures i e endostromatolites the cold climate carbonate precipitates have a highly variable isotopic composition with 18 o values ranging between 6 5 and 28 vsmow and 13 c values in the 10 20 vpdb range however each type of carbonate precipitates has a specific 13 c and 18 o range suggesting that their environmental setting and the mechanism by which they formed controls their 13 c and 18 o signature it was found that carbonate deposits that precipitated under equilibrium physico chemical conditions had a 13 c value that is in equilibrium with that of the parent water while its 18 o composition was more variable as it is in part controlled by the temperature of reaction and by the 18 o and calcite saturation state of the parent water by contrast the 18 o composition of biologically precipitated carbonate deposits endostromatolites reflect that of the parent water while its 13 c composition was enriched over that of the parent water due to bacterial methanogenesis in the case of kinetically precipitated carbonate deposits the 18 o and 13 c values are out of equilibrium relative to that of the parent water due to the faster rate of reaction

natural streams and the legacy of water powered mills gravel bedded streams are thought to have a characteristic meandering form bordered by a self formed fine grained floodplain this ideal guides a multibillion dollar stream restoration industry we have mapped and dated many of the deposits along mid atlantic streams that formed the basis for this widely accepted model these data as well as historical maps and records show instead that before european settlement the streams were small anabranching channels within extensive vegetated wetlands that accumulated little sediment but stored substantial organic carbon subsequently 1 to 5 meters of slackwater sedimentation behind tens of thousands of 17th to 19th century milldams buried the presettlement wetlands with fine sediment these findings show that most floodplains along mid atlantic streams are actually fill terraces and historically incised channels are not natural archetypes for meandering streams

channel instability as a control on silting dynamics and vegetation patterns within perifluvial aquatic zones many authors have shown that the sedimentology of former channels and subsequent vegetation changes are controlled by temporal flood events and successional processes and spatial e g distance to the main channel factors river channel instability can disrupt these associations the ain river france has undergone a fluvial metamorphosis during the past 100 years its braided pattern being replaced by a sinuous single thread pattern as a consequence former channels have different geometrical characteristics and sediment trap efficiencies former meandering channels experience more frequent backflows and are more rapidly silted than the older former braided channels the recently abandoned channels are characterized by the development of large sized vegetation species with a relatively slow colonization rate whereas the older channels are colonized predominantly by flood tolerant aquatic plants the locally derived discharge of former channels from groundwater or from their own basin may reduce or prevent sediment entry during flood events and thus may decrease the sedimentation rate in such cases the oligotrophic component of the water from the hillslope aquifer is high and the former channel is usually nutrient poor characterized by oligotrophic species the main river channel also has experienced local incision aggradation and horizontal displacement during recent decades so that the dynamics of the former channels strongly depend on the dynamics of the reach in which they are located in degraded reaches former channels are often dry and helophyte species have been replaced by mesophytes the frequency and magnitude of flow connection between the river channel and the former channel can increase or decrease owing to the movement of the active river channel within the fluvial corridor inducing varying modifications of former channel vegetation patterns river channel instability at various time scales is a key factor controlling process diversity and thus biodiversity in the fluvial corridor it can modify the geometry of abandoned channels groundwater fluxes the amount mobilization and deposition of sediment within the corridor and consequently the vegetation community patterns this increases the complexity of successional patterns because an old former channel may be characterized by pioneer species whereas a younger one can become quickly filled and colonized by terrestrial species copyright c 2000 john wiley sons ltd

estimating 14c groundwater ages in a methanogenic aquifer this paper addresses the problem of 14c age dating of groundwaters in a confined regional aquifer affected by methanogenesis increasing ch4 concentrations along the groundwater flow system and 13c and 14c isotopic data for dissolved inorganic carbon dissolved organic carbon and ch4 clearly show the effect of methanogenesis on groundwater chemistry inverse reaction path modeling using netpath indicates the predominant geochemical reactions controlling the chemical evolution of groundwater in the aquifer are incongruent dissolution of dolomite ion exchange methanogenesis and oxidation of sedimentary organic matter modeling of groundwater 14c ages using netpath indicates that a significant part of groundwater in the alliston aquifer is less than 13 000 years old however older groundwater in the range of 15 000 23 000 years is also present in the aquifer this paper demonstrates that 14c ages calculated using netpath incorporating the effects of methanogenesis on the carbon pools provide reasonable groundwater ages that were not possible by other isotopic methods

reciprocal interactions and adjustments between fluvial landforms and vegetation dynamics in river corridors a review of complementary approaches abstract until recently one way relationships between flow dynamics geomorphology and plant ecology were considered dominantly when studying the functioning of river systems whereby fluvial landforms and hydrogeomorphic processes drive the evolution of riparian plant communities however biological communities may significantly control geomorphic processes and have strong impacts on landform dynamics in order to fully identify the processes linked to river dynamics changes in time and space of fluvial landforms and associated plant communities conceptual multidisciplinary progress is clearly needed to understand the mutual interactions and feedbacks between fluvial landforms and vegetation community dynamics this paper presents a detailed literature review of fluvial geomorphology riparian plant ecology and hydraulic engineering knowledge the historical and recent development of ecological plant succession theory toward the integration of hydrogeomorphic disturbances is discussed as well as the integration of vegetation within geomorphology as a significant landform control factor incorporating both hydrogeomorphic controls on riparian vegetation dynamics and mechanical impacts of vegetation structures on flow properties and sediment dynamics recent progress in ecology hydraulic engineering and fluvial geomorphology emphasises interdependence between biological and physical forms and processes based on this literature review a fluvial biogeomorphic succession concept is proposed to link fluvial landform and riparian vegetation community evolution within a bi directional model the succession of fluvial landforms and associated vegetation communities is composed of four main critical phases that represent a shift in the relative dominance of hydrogeomorphic and ecological processes as a response to biostabilisation and passive bioconstruction processes the positive feedbacks associated with this shift lead to the development of characteristic biogeomorphic structures such as vegetated banks islands or floodplains which are moderated by the biogeomorphic functional roles of ecosystem engineers that induce or reinforce the positive feedbacks this fluvial biogeomorphic succession concept relates the natural darwinian selection and ecological succession theories to fluvial geomorphology

variations chimiques et isotopiques de l eau du sol lors de la reprise en gel d une couche active sur pergelisol continu presqu ile de brogger svalbard the isotopic and chemical composition of soil waters in a periglacial environment was studied during a 2 month period 89 10 07 to 89 11 09 the experimental site chosen is located near ny alesund

fissure calcretes in the arctic a paleohydrologic indicator abstract the effects of modern climate warming are expected to be amplified in northern periglacial regions paleohydrological reconstructions in the arctic provide important insights into natural climatic variations during the pleistocene and holocene with which to compare predictions of modern impacts here the authors describe a new material calcrete found in fissures in arctic carbonate terrains which record hydrogeological and climatic conditions during climatic optimums in the north 234 u and 230 th activity ratios suggest growth during northern insolation maxima these fissure calcretes strongly resemble biogenic calcite precipitates endostromatolites and grow on the external face within fissures in carbonate bedrock in periglacial environments in thin section samples are finely laminated and are composed of diverse fabrics including a uniformly present basal layer with regularly spaced columns from several microns to 3 cm in length their surface morphology is often preserved in the relief of the adjacent fissure face in a micro lapies texture strongly suggestive of a dissolution reprecipitation mechanism whereby the fissure widens in lock step with column growth their highly enriched 13 c 0 to 8 4 composition suggests that methanogenenic bacteria supported by a substrate of soil derived organic matter control growth in a network of groundwater saturated near surface fissures extending to a depth of 3 to 5 m accordingly 18 o holds valuable paleotemperature information