climate stability of habitable earth like planets abstract the carbon silicate cycle regulates the atmospheric co 2 content of terrestrial planets on geological timescales through a balance between the rates of co 2 volcanic outgassing and planetary intake from rock weathering it is thought to act as an efficient climatic thermostat on earth and by extension on other habitable planets if however the weathering rate increases with the atmospheric co 2 content as expected on planets lacking land vascular plants the carbon silicate cycle feedback can become severely limited here we show that earth like planets receiving less sunlight than current earth may no longer possess a stable warm climate but instead repeatedly cycle between unstable glaciated and deglaciated climatic states this has implications for the search for life on exoplanets in the habitable zone of nearby stars

influence of cohesion on river bed scour in the wake region of piers abstractstudies on scour in the wake region of piers in cohesive sediments containing gravel were not reported in literature to the best of our knowledge results from the laboratory experiments based study on the process of wake scour are presented here the sediment used for the experiment were of two types 1 fine gravel mixed with clay in proportions varying from 20 to 60 by weight and 2 fine gravel and fine sand in equal proportion by weight mixed with clay in proportions varying from 20 to 60 by weight the experiments revealed that the process of scour as well as depth shape and geometry of scour hole developed in such cohesive sediment were significantly different from that of cohesionless sediments proportion of clay fraction and unconfined compressive strength of cohesive sediment mixture were found to be the most significant variables controlling the depth of scour in the wake of piers on the basis of dimensional considerations relationships have been proposed for the estimation

a comparison between flume and field bed load transport data and consequences for surface based bed load transport prediction 1 the ability of simple equations to predict bed load transport with limited knowledge of the bed surface material was investigated this was done using a data set consisting of 7 636 bed load transport values from the flume 1 317 data and from 84 river reaches 6 319 field data it was possible to collapse field and flume data by correcting the ratio between the shields number and its critical value with a very simple hiding function proposed as a power law of the d84 d50 ratio in so doing a surface based bed load transport formula was proposed it was successfully tested on an independent data set comprising sand and gravel bed rivers with slope in the range 0 0002 0 08 with 86 of the values predicted within a precision of 1 order of magnitude moreover the formula reproduced the low transport rates well contrary to the usual surface based formulas also tested and is particularly well suited for estimating low transport rates associated with near bankfull flow discharge this new formula is neither time consuming no fractionwise calculation nor data consuming the required parameters are the flow discharge the active width the slope and the surface grain diameters d50 and d84

from rifting to drifting africa is splitting apart down the east african rift an analysis of the relative motion of the tectonic plates involved allows the rate of continental rifting to be estimated

tectonics and magmatism in turkey and the surrounding area this volume contains 23 papers from a range of international contributors describing recent research into the tectonics and magmatism of turkey and its surroundings this region is sited at the collision zone between eurasia and afro arabia and as such provides an extraordinarily complete and well exposed record of the staged tectonic evolution of this sector of the alpine himalayan orogen the geological history of this area involves separation of continental fragments from the margin of gondwana their migration across the tethyan oceans the subsequent closure of these oceans and finally the development of the neotectonic regime which continues to evolve to the present day such a comprehensive record is relevant to the understanding of collisional zones worldwide the volume is divided into five sections tethyan evolution neotethyan ophiolites post tethyan basin evolution neotectonics and igneous activity the first two sections deal with tethyan oceans whose growth and subsequent closure dominated the geodynamic framework in the mesozoic and cenozoic the subsequent sections deal with more recent geological developments from the balkan peninsula in the west to the transcaucasus in the east that followed consumption of the tethyan oceans there is a broad mix of papers throughout the volume wide ranging review papers on ocean development and extensional tectonics are followed by detailed descriptions of petrology and geochemistry and geographically focused studies on basin evolution specific aspects of extensional and strike slip tectonics and discussions of the relationship of magmatic activity to the tectonic development of the area tectonics and magmatism in turkey and the surrounding area presents up to date results and ideas from a large number of international contributors on a wide range of current research activity in this region it is essential reading for all geoscientists with an interest in both academic and applied aspects of eastern mediterranean geology

sampling techniques for gravel sized sediments the methods commonly used for sampling the coarser surface layers of gravel bed streams are reviewed it is found that while an areal sample is biased toward the coarser grains compared with a volumetric sample of the same material the conversion formula suggested by a voidless cube model overcompensates for this effect a modified version of the cube model that accounts for porosity is proposed the modified cube model and tests conducted with areal sampling using wax indicate that area by weight analyses can be converted successfully to the equivalent bulk sieve analyses the average depth of the wax samples ranged from d65 d91 increasing with median grain size the equivalence of the grid by number and bulk sieve analyses is reaffirmed

marginal bed load transport in a gravel bed stream sagehen creek california marginal bed load transport describes the condition when relatively few bed particles are moving at any time bed particles resting in the shallowest bed pockets will move when the dimensionless shear stress exceeds a value of about 0 020 as increases the number of bed particles moving increases significant motion of bed particles i e when a substantial fraction of the bed particles are moving occurs when exceeds a value of about 0 060 thus marginal bed load transport occurs over the domain 0 020 0 060 marginal bed load transport rates and associated hydraulic characteristics of sagehen creek a small mountain gravel bed stream were measured on 55 days at discharges ranging from slightly less than one half of the bank full discharge to more than 4 times the bank full discharge dimensionless shear stress varied from 0 032 to 0 042 and bed particles as large as the 80th percentile of the bed surface were transported the relation between reference dimensionless shear stress and relative particle protrusion for sagehen creek was determined by varying ri to obtain the best fit of the parker bed load function to the measured transport rates during the period of record water years 1954 1991 the mean annual quantity of bed load transported past the sagehen creek gage was 24 7 tons forty seven percent of all bed load transported during the 38 years of record occurred in just 6 years during 10 of the 38 years of record essentially no bed load was transported the median diameter of bed load was 26 mm compared to 58 mm in the surface bed material

modeling bed load rates from fine grain size patches during small floods in a gravel bed river this study investigates the applicability of five bed load transport formulas the meyer peter and muller schoklitsch bagnold smart and jaeggi and rickenmann equations to predict bed load transport rates of frequent low magnitude flood events maximal bankfull discharge for a mountainous poorly sorted gravel bed river characterized by a bimodal sediment size distribution and spatially distributed patches for model parametrization special emphasis was placed on the spatial composition of the grain size distribution gsd to evaluate the impact of preferential removal of sediments from patches with finer sediments on bed load transport three parametrization approaches to the choice of an appropriate sediment size that considered the apparent bimodality of the gsd to varying degrees were tested the modeling study demonstrated that the incorporation of spatial structure of gsd and its bimodal character has an important impact on model performance a unimodal parametrization failed to reproduce meas

a general power equation for predicting bed load transport rates in gravel bed rivers 1 a variety of formulae has been developed to predict bed load transport in gravel bed rivers ranging from simple regressions to complex multiparameter formulations the ability to test these formulae across numerous field sites has until recently been hampered by a paucity of bed load transport data for gravel bed rivers we use 2104 bed load transport observations in 24 gravel bed rivers in idaho to assess the performance of eight different formulations of four bed load transport equations results show substantial differences in performance but no consistent relationship between formula performance and degree of calibration or complexity however formulae containing a transport threshold typically exhibit poor performance furthermore we find that the transport data are best described by a simple power function of discharge from this we propose a new bed load transport equation and identify channel and watershed characteristics that control the exponent and coefficient of the proposed power function we find that the exponent is principally a factor of supply related channel armoring transport capacity in excess of sediment supply whereas the coefficient is related to drainage area a surrogate for absolute sediment supply we evaluate the accuracy of the proposed power function at 17 independent test sites

bedload model for nonuniform sediment abstractin this study a bedload model for nonuniform sediment is developed considering the density function of sediment entrainment with no active layer the model is formulated based on physical considerations dimensional reasoning regression analysis and laboratory data four parameters are incorporated in the model formulation namely the shields stress g the critical shields stress cg the kramer coefficient of uniformity cu and the relative grain size size of the jth grain fraction to the geometric mean size dj dg the model collectively accounts for the hiding effects and the variation of the transported bedload material fractions from the surface and subsurface material the findings of the study show that at low values of g cg fractions within the range of 0 3 dj dg 3 0 are present with larger amounts in the transported bedload material than in the substrate whereas fractions finer or coarser than that range are present with larger amounts in the surface layer than in the subst

coupling fluvial hydraulic models to predict gravel transport in spatially variable flows this study investigated spatial temporal variations of shear stress and bed load transport at three gravel bed river reaches of the williams fork river colorado a two dimensional flow model was used to compute spatial distributions of shear stress for four discharge levels between one third of bankfull qbf and qbf results indicate that mean values are highly variable among sites however the properties of the mean normalized distributions of are similar across sites for all flows the distributions of are then used with a transport function to compute bed load transport rates of individual grain size fractions probability distributions of the instantaneous unit width transport rates qb indicate that most of the bed load is transported through small portions of the bed with high the mean normalized probability distributions of qb are different among sites for all flows except at qbf when the distributions overlap we also find that the grain size distribution gsd of the bed load adjusts with discharge to resemble the grain size distribution of the subsurface at qbf we extend these results to 13 locations in the basin using the mean normalized distributions of shear stress and measured subsurface grain sizes to compute bed load transport rates at qbf we found a remarkably similar shape of the qb distribution among sites highlighting the basin wide balance between flow forces and gsd at qbf and the potential to predict sediment flux at the watershed scale

tectonic setting origin and obduction history of the spontang ophiolite ladakh himalaya nw india abstract the spontang ophiolite forms the highest tectonic thrust slice above the mesozoic early tertiary continental margin of the north indian plate in the ladakh zanskar himalaya detailed field mapping combined with geochemical analysis has defined two major units a full ophiolite sequence spontang ophiolite overlain by an upper unit consisting of 500 m thick basalt andesite volcanic and volcano sedimentary rocks of island arc affinity spong arc the spontang ophiolite comprises a harzburgitic mantle sequence gabbroic and ultramafic cumulates isotropic gabbros and highly tectonized sheeted dikes feeding pillow lavas with a few uncommon highly fractionated plagiogranites a separate lherzolitic peridotite unit affected by possible transform related shearing was thrust over the harzburgites in the west possibly during the early stages of subduction initiation beneath the ophiolite whole rock geochemistry and u pb geochronology show that the ophiolite formed at a normal mid ocean ridge sp

thickness of tropical ice and photosynthesis on a snowball earth on a completely ice covered snowball earth the thickness of ice in the tropical regions would be limited by the sunlight penetrating into the ice cover and by the latent heat flux generated by freezing at the ice bottom the freezing rate would balance the sublimation rate from the top of the ice cover heat transfer models of the perennially ice covered antarctic dry valley lakes applied to the snowball earth indicate that the tropical ice cover would have a thickness of 10 m or less with a corresponding transmissivity of 0 1 this light level is adequate for photosynthesis and could explain the survival of the eukaryotic algae

validation and implications of an energy based bedload transport equation a recently developed bedload equation abrahams gao 2006 has the form ib xg 3 4 where ib is the immersed bedload transport rate x is the stream power per unit area g 1 hc h h is the dimensionless shear stress and hc is the associated threshold value for the incipient motion of bed grains this equation has a parsimonious form and provides good predictions of transport rate in both the saltation and sheetflow regimes i e flows with low and high h values respectively in this study the equation was validated using data independent of those used for developing it the data represent bedload of identical sizes transported in various steady uniform fully rough and turbulent flows over plane mobile beds the equation predicted ib quite well over five orders of magnitude this equation was further compared with six classic bedload equations and showed the best performance its theoretical significance was subsequently examined in two ways first based on collision theory the parameter g was related to the ratio of grain to grain collisions to the total collisions including both grain to grain and grain to bed collisions pg by pg g 2 suggesting that g characterizes the dynamic processes of bedload transport from the perspective of granular flow which partly accounts for the good performance of the equation moreover examining the ability of two common equations to predict bedload in gravel bed rivers revealed that g can also be used to simplify equations for predicting transport capacities in such rivers second a simple dimensionless form of the equation was created by introducing b ib x the theoretical nature of the term b was subsequently revealed by comparing this equation with both the bagnold model and two commonly used parameters representing dimensionless bedload transport rates

bedload transport in gravel bed streams with unimodal sediment bedload transport in many gravel bed streams becomes highly complicated because of the nonuniformity of the grain size and the vertical stratification of channel bed material a new relation for computing bedload transport rates in gravel bed streams is proposed here in an effort to account for the variation of the makeup of the surface material within a wide range of shields stresses the relation employs a two parameter approach one related to the material in the pavement and the other to that in the subpavement layers the mode is used to represent the grain sizes of each layer available bedload transport data from gravel bed streams with unimodal sediment are used to test the accuracy of the relation a comparison with other bedload transport relations is also considered

weathering plants and the long term carbon cycle the chemical weathering of ca mg silicate rocks is the principal process whereby co2 is removed from the atmosphere on long or multimillion year time scales at present vascular land plants exert an important influence on weathering by recycling water and by bringing about the secretion of acids and the buildup of high levels of co2 in soils before the spread of vascular plants on the continents during the mid paleozoic chemical weathering must have been achieved by higher levels of atmospheric co2 and or by the intercession of primitive land biota such as lichens and cyanobacteria from examination of several pre vascular weathering scenarios it is concluded that in order to prohibit unreasonably high levels of atmospheric co2 weathering during the precambrian and early paleozoic must have taken place under essentially closed system abiotic conditions linear feedback or via strong regulation by primitive biota preliminary examination of modern weathering by lichens over tens to thousands of years militates against the latter possibility however much more work needs to be done on the quantitative effect of both primitive biota and vascular plants on the rate of weathering and their response to changes in atmospheric co2 before anything more definitive can be said

snowball earth a thin ice solution with flowing sea glaciers 1 the late neoproterozoic era 600 800 myr ago was marked by at least two intervals of widespread cold that left glacial deposits at low paleolatitudes both snowball solutions with global ice cover and slushball solutions with ice free tropical oceans have been proposed to explain the paleomagnetic data the snowball model is best able to explain the auxiliary geological evidence particularly the existence of cap carbonates but it implies drastic survival pressure on the photosynthetic biota if the ice cover was everywhere thick enough to prevent sunlight from reaching the underlying ocean a thin ice solution that avoids this problem has been proposed but is thought to be inconsistent with realistic optical properties of sea ice and with the equatorward flow of sea glaciers here we use a coupled energy balance climate sea glacier model to argue that these apparent difficulties can be overcome and that thin tropical ice may have prevailed during these remarkable glacial episodes we also suggest that 1 some processes beyond the scope of zonal mean energy balance models may significantly affect the solutions and 2 thin ice could have prevailed on low latitude seas or large lakes protected from large scale sea glacier flow by surrounding land even if most of the earth was in a hard snowball state

co2 condensation is a serious limit to the deglaciation of earth like planets abstract it is widely believed that the carbonate silicate cycle is the main agent through volcanism to trigger deglaciations by co2 greenhouse warming on earth and on earth like planets when they get in a frozen state here we use a 3d global climate model to simulate the ability of planets initially completely frozen to escape from glaciation episodes by accumulating enough gaseous co2 the model includes co2 condensation and sublimation processes and the water cycle we find that planets with earth like characteristics size mass obliquity rotation rate etc orbiting a sun like star may never be able to escape from a glaciation era if their orbital distance is greater than 1 27 astronomical units flux 847 w m 2 or 62 of the solar constant because co2 would condense at the poles here the cold traps forming permanent co2 ice caps this limits the amount of co2 in the atmosphere and thus its greenhouse effect furthermore our results indicate that for 1 high rotation rates p rot 24 h 2 low obliquity obliquity 0 6 this critical limit could occur at a significantly lower equivalent distance or higher insolation for each possible configuration we show that the amount of co2 that can be trapped in the polar caps depends on the efficiency of co2 ice to flow laterally as well as its gravitational stability relative to subsurface water ice we find that a frozen earth like planet located at 1 30 au of a sun like star could store as much as 1 5 4 5 and 15 bars of dry ice at the poles for internal heat fluxes of 100 30 and 10 mw m 2 respectively but these amounts are in fact lower limits for planets with a significant water ice cover we show that co2 ice deposits should be gravitationally unstable they get buried beneath the water ice cover in geologically short timescales of 104 yrs mainly controlled by the viscosity of water ice co2 would be permanently sequestered underneath the water ice cover in the form of co2 liquids co2 clathrate hydrates and or dissolved in subglacial water reservoirs if any this would considerably increase the amount of co2 trapped and further reduce the probability of deglaciation

cohesion influences on erosion and bed load transport 1 studies on erosion and transport rates of cohesive sediment mixtures containing gravel are not reported so far in the literature to the best of our knowledge results from laboratory experiments on the process of erosion of cohesive sediment mixtures by open channel flow are presented the sediment used for experimentation consisted of fine gravel mixed with clay in proportions varying from 10 to 50 and fine gravel fine sand in equal proportion by weight mixed with clay proportions again varying from 10 to 50 by weight experimental observations have revealed that the process of detachment of cohesive sediment mixtures containing gravel is significantly different from that of cohesionless sediments also it is different from the detachment process of cohesive sediment mixtures not containing coarser size material experimental data on transport rates of the bed load of the cohesive sediment mixtures are presented on the basis of dimensional considerations a computational procedure is proposed to determine the bed load transport rates generated from erosion of the different size fractions of sediment mixtures clay percentage and unconfined compressive strength of the sediment bed were the main parameters controlling bed load transport rate of such sediments in the absence of cohesion the proposed method reduces to the patel and ranga raju 1996 method for bed load transport rate computation of nonuniform cohesionless sediments

laboratory flume experiments with the swiss plate geophone bed load monitoring system 2 application to field sites with direct bed load samples the swiss plate geophone is a bed load surrogate monitoring system that had been calibrated in several gravel bed streams through field calibration measurements field calibration measurements are generally expensive and time consuming therefore we investigated the possibility to replace it by a flume based calibration approach we applied impulse diameter relations for the swiss plate geophone obtained from systematic flume experiments to field calibration measurements in four different gravel bed streams the flume based relations were successfully validated with direct bed load samples from field measurements by estimating the number of impulses based on observed bed load masses per grain size class we estimated bed load transport mass by developing flume based and stream dependent calibration procedures for the swiss plate geophone system using an additional empirical function the estimated masses are on average in the range of 90 of measured bed load masses in the field but the accuracy is generally improved for larger transported bed load masses we discuss the limitations of the presented flume based calibration approach

approach to separate sand from gravel for bed load transport calculations in streams with bimodal sediment the bed material found in gravel bed streams is nonuniform in terms of grain size and can typically be classified as unimodal or bimodal the latter type of sediment distribution is usually represented by two modes one of sand size and another of gravel for this case the movement of one mode becomes nonlinearly influenced by the other as a result the presence of the two modes in a bimodal material complicates the calculation of bed load transport rates the present study proposes an approach to separate the calculation of bed load transport rates for bimodal materials into two independent fractions of sand and gravel thereby rendering the bed sediment into two unimodal components this approach is accomplished by decoupling the two fractions through scaling the reference shields stresses of the sand and gravel modes to match the value of the mode of unimodal materials consequently the contribution of each fraction to bed load can be estimated using a suitable relation derived for unimodal materials laboratory and field bed load data available in the literature are used to examine the validity of the overall approach

the concept of transport capacity in geomorphology the notion of sediment transport capacity has been engrained in geomorphological and related literature for over 50 years although its earliest roots date back explicitly to gilbert in fluvial geomorphology in the 1870s and implicitly to eighteenth to nineteenth century developments in engineering despite cross fertilization between different process domains there seem to have been independent inventions of the idea in aeolian geomorphology by bagnold in the 1930s and in hillslope studies by ellison in the 1940s here we review the invention and development of the idea of transport capacity in the fluvial aeolian coastal hillslope debris flow and glacial process domains as these various developments have occurred different definitions have been used which makes it both a difficult concept to test and one that may lead to poor communications between those working in different domains of geomorphology we argue that the original relation between the power of a flow and its ability to transport sediment can be challenged for three reasons first as sediment becomes entrained in a flow the nature of the flow changes and so it is unreasonable to link the capacity of the water or wind only to the ability of the fluid to move sediment secondly environmental sediment transport is complicated and the range of processes involved in most movements means that simple relationships are unlikely to hold not least because the movement of sediment often changes the substrate which in turn affects the flow conditions thirdly the inherently stochastic nature of sediment transport means that any capacity relationships do not scale either in time or in space consequently new theories of sediment transport are needed to improve understanding and prediction and to guide measurement and management of all geomorphic systems

the nahal yatir bedload database sediment dynamics in a gravel bed ephemeral stream the nahal yatir bedload transport database arises from the northern negev river sediment monitoring programme and represents the first body of bedload information to be collected during flash floods in desert gravel bed streams bedload flux was established automatically with three slot samplers of the birkbeck type this was complemented by hydraulic measurements that allow sediment transport to be rated against channel average shear stress and specific stream power bed material and bedload grain size distributions are also provided the database is given in both printed and electronic tabular format

bed load transport of coarse nonuniform sediment experiments involving measurements of the bed load transport rates of different fractions in a mixture are reported a conceptual model for the effect of a particular size of sediment on the transport rates of other sizes of sediment is proposed the experimental data are analyzed with the dual objectives of assessing the accuracy of existing methods of computation of bed load transport and of proposing a new method of computation

the jormungand global climate state and implications for neoproterozoic glaciations 1 geological and geochemical evidence can be interpreted as indicating strong hysteresis in global climate during the neoproterozoic glacial events 630 ma and 715 ma standard climate theory only allows such strong hysteresis if global climate enters a fully glaciated snowball state however the survival of photosynthetic eukaryotic marine species through these glaciations may indicate that there were large areas of open ocean a previously proposed slushball model for neoproterozoic glaciations could easily explain the survival of these organisms because it has open ocean throughout the tropics but there is only a small amount of hysteresis associated with the slushball state in this paper a new state of global climate the jormungand state is proposed in this state the ocean is very nearly globally ice covered but a very small strip of the tropical ocean remains ice free the low ice latitude of the jormungand state is a consequence of the low albedo of snow free bare sea ice if the ice latitude propagates into the subtropical desert zone it can stabilize without collapsing to the equator because subtropical ice covered regions have a relatively low top of atmosphere albedo as a result of the exposure of bare sea ice and relatively lower cloud cover moreover there is strong hysteresis associated with the jormungand state as greenhouse gas levels are varied because of the high albedo contrast between regions of bare and snow covered sea ice the jormungand state is illustrated here in two different atmospheric global climate models and in the budyko sellers model by offering a scenario that could explain both strong hysteresis in global climate and the survival of life the jormungand state represents a potential model for neoproterozoic glaciations although further study of this issue is needed

bedload transport in gravel bed streams field data obtained from oak creek are used to study the bedload transport in gravelbed streams a similarity approach is used to delineate a functional relationship obtained for the bedload transp

climate cycling on early mars caused by the carbonate silicate cycle abstract for decades scientists have tried to explain the evidence for fluvial activity on early mars but a consensus has yet to emerge regarding the mechanism for producing it one hypothesis suggests early mars was warmed by a thick greenhouse atmosphere another suggests that early mars was generally cold but was warmed occasionally by impacts or by episodes of enhanced volcanism these latter hypotheses struggle to produce the amounts of rainfall needed to form the martian valleys but are consistent with inferred low rates of weathering compared to earth here we provide a geophysical mechanism that could have induced cycles of glaciation and deglaciation on early mars our model produces dramatic climate cycles with extended periods of glaciation punctuated by warm periods lasting up to 10 myr much longer than those generated in other episodic warming models the cycles occur because stellar insolation was low and because co 2 outgassing is not able to keep pace with co 2 consumption by silicate weathering followed by deposition of carbonates while co 2 by itself is not able to deglaciate early mars in our model we assume that the greenhouse effect is enhanced by substantial amounts of h 2 outgassed from mars reduced crust and mantle our hypothesis can be tested by future mars exploration that better establishes the time scale for valley formation

tethyan ophiolites and tethyan seaways tethyan ophiolites have played a major role in the development of the ophiolite concept during the past 200 years and contributed significantly to the formulations of the suprasubduction zone ssz ophiolite paradigm and the plate tectonics theory their melt evolution and magmatic construction witnessed the dispersal of the supercontinent pangaea via continental rifting and seafloor spreading opening and closure of multiple seaways in an eastward widening latitudinal ocean basin neotethys and deep mantle recycling processes through subduction zone tectonics and plume activities neotethyan ophiolites are highly diverse in their crustal mantle structures and compositions pointing to major differences in their melt sources and tectonic settings of magmatic construction the papers in this thematic set demonstrate these differences in the neotethyan ophiolite record through new structural geochemical isotopic and geochronological data and interpretations the jurassic western alpine ophiolites formed in a narrow basin alpine tethys that developed between europe and north africa adria iberia their peridotites represent exhumed continental lithospheric mantle the triassic jurassic and cretaceous ophiolites east of adria formed in different neotethyan seaways and their ssz magmatic construction involved multiple episodes of melting depletion and refertilization of subduction modified oceanic mantle the late cretaceous peri arabian and peri indian ophiolites which are discontinuously exposed along a c 9000 km long belt from sw anatolia to se tibet formed above a trans tethyan subduction accretion system within southern neotethys diachronous collisions of arabia and india with this intraoceanic subduction accretion system in the latest mesozoic and the paleogene respectively resulted in diachronous ophiolite emplacement in the mediterranean and tibetan himalayan orogenic belts thematic collection this article is part of the tethyan ophiolites and tethyan seaways collection available at https www lyellcollection org cc tethyan ophiolites and tethyan seaways

methane and climate during the precambrian era abstract the sun was substantially less bright in the distant past yet earth s surface temperature remained above freezing higher concentrations of the greenhouse gases co 2 and ch 4 were likely responsible for keeping the early climate warm ch 4 concentrations of 1000 ppm or higher are predicted for the late archean paleoproterozoic atmosphere prior to the rise of o 2 photolysis of this ch 4 may have created an optically thin organic haze during much of this time the rise of o 2 at 2 3 ga eliminated most of the methane and probably triggered the paleoproterozoic glaciations ch 4 concentrations could have remained elevated throughout much of the proterozoic however as a consequence of low concentrations of dissolved o 2 and sulfate in the deep oceans and a corresponding increase in organic matter recycling by fermentation and methanogenesis

habitable planets with high obliquities earth s obliquity would vary chaotically from 0 degrees to 85 degrees were it not for the presence of the moon j laskar f joutel and p robutel 1993 nature 361 615 617 the moon itself is thought to be an accident of accretion formed by a glancing blow from a mars sized planetesimal hence planets with similar moons and stable obliquities may be extremely rare this has lead laskar and colleagues to suggest that the number of earth like planets with high obliquities and temperate life supporting climates may be small to test this proposition we have used an energy balance climate model to simulate earth s climate at obliquities up to 90 degrees we show that earth s climate would become regionally severe in such circumstances with large seasonal cycles and accompanying temperature extremes on middle and high latitude continents which might be damaging to many forms of life the response of other hypothetical earth like planets to large obliquity fluctuations depends on their land sea distribution and on their position within the habitable zone hz around their star planets with several modest sized continents or equatorial supercontinents are more climatically stable than those with polar supercontinents planets farther out in the hz are less affected by high obliquities because their atmospheres should accumulate co2 in response to the carbonate silicate cycle dense co2 rich atmospheres transport heat very effectively and therefore limit the magnitude of both seasonal cycles and latitudinal temperature gradients we conclude that a significant fraction of extrasolar earth like planets may still be habitable even if they are subject to large obliquity fluctuations

abrupt climate transition of icy worlds from snowball to moist or runaway greenhouse ongoing and future space missions aim to identify potentially habitable planets in our solar system and beyond planetary habitability is determined not only by a planet s current stellar insolation and atmospheric properties but also by the evolutionary history of its climate it has been suggested that icy planets and moons become habitable after their initial ice shield melts as their host stars brighten here we show from global climate model simulations that a habitable state is not achieved in the climatic evolution of those icy planets and moons that possess an inactive carbonate silicate cycle and low concentrations of greenhouse gases examples for such planetary bodies are the icy moons europa and enceladus and certain icy exoplanets orbiting g and f stars we find that the stellar fluxes that are required to overcome a planet s initial snowball state are so large that they lead to significant water loss and preclude a habitable planet specifically they exceed the moist greenhouse limit at which water vapour accumulates at high altitudes where it can readily escape or the runaway greenhouse limit at which the strength of the greenhouse increases until the oceans boil away we suggest that some icy planetary bodies may transition directly to a moist or runaway greenhouse without passing through a habitable earth like state icy planets and moons could become habitable as their host stars brighten and their ice melts climate simulations instead show a rapid transition from a snowball to an inhospitable greenhouse climate with significant water loss

sediment transport and resulting deposition in spawning gravels north coastal california incubating salmonid eggs in streambeds are often threatened by deposition of fine sediment within the gravel to relate sedimentation of spawning gravel beds to sediment transport infiltration of fine sediment 2 mm in diameter into clean gravel beds bed material size distributions scour fill depths and sediment transport during 10 storm flow events were measured in three streams of north coastal california although suspended sediment comprised most 75 94 of the clastic load during storm flows bed load material 0 25 2 mm accounted for most 70 78 of the fine sediment accumulated in experimental gravel implanted in the streambeds sand trapped in the interstices of the top several centimeters formed a seal that impeded deeper deposition of very fine sand and finer material the seal was responsible at least in part for a decrease in the rate of fine sediment accumulation with increasing cumulative bed load transport areas of the streambeds commonly scoured or filled 0 1 m or more during storm flows and thus scour and fill commonly created a sandy layer at least as thick as the seal formed by sediment infiltration scour could erode eggs laid in the bed and expose deeper levels of the bed to infiltration by fine sediment but at the same time could allow fine sediment to be winnowed away great temporal and spatial variation in sedimentation in these streams suggests that individual storms of moderate size pose a threat to eggs in many but not all areas selected by fish for spawning

anomalously fast convergence of india and eurasia caused by double subduction prior to collision with eurasia the indian plate rapidly accelerated northwards numerical simulations show that the combined pull of two slabs in two parallel north dipping subduction systems could have caused this pulse of rapid movement