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

neoproterozoic snowball earth simulations with a coupled climate ice sheet model ice sheets may have reached the equator in the late proterozoic era 600 800 myr ago according to geological and palaeomagnetic studies possibly resulting in a snowball earth but this period was a critical time in the evolution of multicellular animals posing the question of how early life survived under such environmental stress here we present computer simulations of this unusual climate stage with a coupled climate ice sheet model to simulate a snowball earth we use only a reduction in the solar constant compared to present day conditions and we keep atmospheric co2 concentrations near present levels we find rapid transitions into and out of full glaciation that are consistent with the geological evidence when we combine these results with a general circulation model some of the simulations result in an equatorial belt of open water that may have provided a refugium for multicellular animals

correction to rheology of the lithosphere inferred from postseismic uplift following the 1959 hebgen lake earthquake figure 2 elastic modulus and viscosity of the layered earth models gray layers are purely elastic a viscosity and depth of layers of the ve 1 model see text b shear modulus m bulk modulus k and density r in units of gpa gpa and kg m of the ve 1 model c viscosity and depth of layers of the ve 2 model see text journal of geophysical research vol 109 b02311 doi 10 1029 2003jb002798 2004

hydrohalite in cold sea ice laboratory observations of single crystals surface accumulations and migration rates under a temperature gradient with application to snowball earth 1 when nacl precipitates out of a saturated solution it forms anhydrous crystals of halite at temperatures above 0 11 c but at temperatures below this threshold it instead precipitates as the dihydrate hydrohalite nacl 2h2o when sea ice is cooled hydrohalite begins to precipitate within brine inclusions at about 23 c in this work hydrohalite crystals are examined in laboratory experiments their formation their shape and their response to warming and desiccation sublimation of a sea ice surface at low temperature leaves a lag deposit of hydrohalite which has the character of a fine powder the precipitation of hydrohalite in brine inclusions raises the albedo of sea ice and the subsequent formation of a surface accumulation further raises the albedo although these processes have limited climatic importance on the modern earth they would have been important in determining the surface types present in regions of net sublimation on the tropical ocean in the cold phase of a snowball earth event however brine inclusions in sea ice migrate downward to warmer ice so whether salt can accumulate on the surface depends on the relative rates of sublimation and migration the migration rates are measured in a laboratory experiment at temperatures from 2 c to 32 c the migration appears to be too slow to prevent formation of a salt crust on snowball earth

geological consequences of super sized earths 1 the discovery of terrestrial scale extrasolar planets and their calculated abundance in the galaxy has prompted speculation on their surface conditions and thermal structure both are dependent on the tectonic regime of a planet which is itself a function of the balance between driving forces and the resistive strength of the lithosphere here we use mantle convection simulations to show that simply increasing planetary radius acts to decrease the ratio of driving to resisting stresses and thus super sized earths are likely to be in an episodic or stagnant lid regime this effect is robust when associated increases in gravity are included as the more dominant effect is increased fault strength rather than greater buoyancy forces the thermo tectonic evolution of large terrestrial planets is more complex than often assumed and this has implications for the surface and conditions habitability of such worlds

intraplate extensional tectonics of the eastern basin range inferences on structural style from seismic reflection data regional tectonics and thermal mechanical models of brittle ductile deformation the general lack of a correlation between earthquakes and surface faulting in the interior of the western u s cordillera has motivated our efforts to evaluate the geometry structural style and mechanism of normal faulting characteristic of this region of intraplate extension to address the problem we have interpreted over 1500 km of seismic reflection data and constructed detailed cross sections of the upper crust rheological models of the continental crust were calculated to examine the possibility of shallow quasi plastic flow and its influence on faulting in the eastern basin range the western colorado plateau and the middle rocky mountains our data and interpretations have revealed the following styles of cenozoic deformation 1 steep to low angle dip normal faulting along the wasatch fault 2 low angle dip and listric normal faulting possibly associated with movement on preexisting thrusts 3 the occurrence of asymmetric mostly eastward tilted tertiary basins that are bounded by low to moderate dipping planar and listric faults and 4 at least three vertically stacked en echelon low angle reflections in the mid to upper crust that dip gently westward from 3 km beneath the wasatch plateau to over 15 km at the utah nevada border these reflections are interpreted as normal detachment faults the structural style of the pervasive low to moderate angle dipping faults cannot be easily reconciled with classic brittle failure theory but the interpreted termination of normal faults at or above the deeper low angle reflections suggests the presence of shallow zones of ductile deformation that may have accommodated slip an important observation based on interpretations of seismic reflection profiles is that normal fault zones dip more gently in the subsurface than their associated scarps in unconsolidated surficial deposits segment boundaries of the wasatch fault zone apparently coincide with the positions of east trending displacement transfer structures in thrust sheets and the position of the precambrian uinta aulacogen suggesting that preexisting crustal structure partly controls the geometry of extension to examine the influence of ductile deformation quasi plastic flow was modeled for appropriate geotherms of the basin range and colorado plateau crusts these models provide constraints on the depth to the frictional quasi plastic transition that occurs as shallow as 8 km in the eastern basin range this depth also corresponds to the approximate depth above which most accurately determined earthquake foci for small earthquakes m 5 5 appear to nucleate at 10 15 km in or near the brittle ductile transition the rheological modeling suggests that a thermally deforming continental lithosphere plays an important role in the evolution of this extending intraplate region large scale low angle reflections may represent decollements and normal faults at the top of upper crustal zones of reduced shearing strength and higher ductility

on the nature of transverse variability in a coastal plain estuary even though estuarine processes are inherently three dimensional most of the previous studies have focused primarily on the axial longitudinal and vertical variabilities recent surveys in the lower delaware bay indicate the presence of significant transverse variability in the salinity distribution and residual circulation pattern conceptual models are developed to examine the mechanisms which might explain the observed transverse structure results indicate that both the gravitational effect and wind forcing are capable of generating substantial transverse structure in drowned river valley estuaries with triangular cross sections

clouds and snowball earth deglaciation neoproterozoic and possibly paleoproterozoic glaciations represent the most extreme climate events in post hadean earth and may link closely with the evolution of the atmosphere and life according to the snowball earth hypothesis the entire ocean was covered with ice during these events for a few million years during which time volcanic co 2 increased enough to cause deglaciation geochemical proxy data and model calculations suggest that the maximum co 2 was 0 01 0 1 by volume but early climate modeling suggested that deglaciation was not possible at co 2 0 2 we use results from six different general circulation models gcms to show that clouds could warm a snowball enough to reduce the co 2 required for deglaciation by a factor of 10 100 although more work is required to rigorously validate cloud schemes in snowball like conditions our results suggest that snowball deglaciation is consistent with observations

in situ study of the spatiotemporal variability of sediment erodibility in a microtidal estuary abstract seabed erosion is a vital process for sediment transport in estuaries and coasts in this study we investigated the seabed erosion process and its spatiotemporal variability in the pearl river estuary china via in situ observations and laboratory experiments it is found that the seabed was more erodible in the channel than at the shoal in most cases due to the difference in sediment size in a neap spring neap tidal cycle a laminated seabed with different sediment ages was inferred to form due to deposition on flood tides and erosion on ebb tides the seabed was more erodible during spring tides than neap tides because of the difference in sediment availability at the unconsolidated fresh layer furthermore the erosion curve showed evident flood ebb asymmetry which was found to be related to the variations of sediment availability and bottom stress besides erosion experiments with different sediment consolidations revealed that sediment erodibility decreased with the growth of consolidation this study promotes our understanding on the spatiotemporal variability of sediment erodibility and has significant implications for simulation and prediction of estuarine sediment transport and morphological evolution

mudball surface dust and snowball earth deglaciation 1 recent modeling results have raised doubts about the ability to deglaciate from a global glaciation at atmospheric carbon dioxide levels that are realistic for a neoproterozoic snowball earth here we argue that over the lifetime of a snowball event ice dynamics should lead to the development of a layer of continental and volcanic dust at the ice surface in the tropics that would significantly lower the tropical surface albedo and encourage deglaciation this idea leads to the prediction that clay drapes found on top of neoproterozoic glaciations should be thicker in tropical than extratropical regions we test this idea by running the foam general circulation model gcm with an added tropical dust layer of different sizes and albedos and find that the tropical dust layer causes snowball deglaciation at pco2 0 01 0 1 bar in a reasonable regime of these parameters we find similar though more nuanced results from a limited number of test cases using national center for atmospheric research s cam gcm

integration of paleoseismic data from multiple sites to develop an objective earthquake chronology application to the weber segment of the wasatch fault zone utah we present a method to evaluate and integrate paleoseismic data from multiple sites into a single objective measure of earthquake timing and recurrence on discrete segments of active faults we apply this method to the weber segment ws of the wasatch fault zone using data from four fault trench studies completed between 1981 and 2009 after systematically reevaluating the stratigraphic and chro nologic data from each trench site we constructed time stratigraphic oxcal models that yield site probability density functions pdfs of the times of individual earth quakes we next qualitatively correlated the site pdfs into a segment wide earthquake chronology which is supported by overlapping site pdfs large per event displace ments and prominent segment boundaries for each segment wide earthquake we computed the product of the site pdf probabilities in common time bins which emphasizes the overlap in the site earthquake times and gives more weight to the narrowest best defined pdfs the product method yields smaller earthquake timing uncertainties compared to taking the mean of the site pdfs but is best suited to earth quakes constrained by broad overlapping site pdfs we calculated segment wide earthquake recurrence intervals and uncertainties using a monte carlo model five surface faulting earthquakes occurred on the ws at about 5 9 4 5 3 1 1 1 and 0 6 ka with the exception of the 1 1 ka event we used the product method to define the earthquake times the revised ws chronology yields a mean recurrence interval of 1 3 kyr 0 7 1 9 kyr estimated two sigma 2 range based on interevent recurrence these data help clarify the paleoearthquake history of the ws including the important question of the timing and rupture extent of the most recent earthquake and are essential to the improvement of earthquake probability assessments for the wasatch front region

voyager photometry of europa abstract ninety voyager images ranging in phase angle from 3 to 143 and covering the spectral range from 0 34 to 0 58 m were analyzed to derive the photometric properties of europa at small phase angles the disk integrated phase curve is remarkable in that it shows little or no evidence of an opposition effect in agreement with earlier earth based observations by millis and thompson icarus 26 408 1975 the phase integral determined in the voyager clear filter centered near 0 47 m is 1 09 0 11 in good agreement with previous estimates based on radiometry the bolometric bond albedo is 0 62 0 14 the scattering properties of europa in general and of the two major terrain types bright plains and darker mottled terrain in particular cannot be represented by a lunar like photometric law however an equation which is a linear superposition of a lunar like scattering law and a lambert component provides an adequate simple representation of the scattering properties the plains are photometrically more homogeneous than the darker mottled terrain in the voyager clear filter the average normal reflectance is 0 71 for the plains on both the leading and trailing hemispheres for the darker mottled terrain the values are 0 60 on the leading hemisphere and 0 48 on the trailing one

triggering global climate transitions through volcanic eruptions abstracta coupled climate model with idealized representations of atmosphere ocean sea ice and land is used to investigate transitions between global climate equilibria the model supports the p

strong water isotopic anomalies in the martian atmosphere probing current and ancient reservoirs we measured maps of atmospheric water h2o and its deuterated form hdo across the martian globe showing strong isotopic anomalies and a significant high deuterium hydrogen d h enrichment indicative of great water loss the maps sample the evolution of sublimation from the north polar cap revealing that the released water has a representative d h value enriched by a factor of about 7 relative to earth s ocean vienna standard mean ocean water vsmow certain basins and orographic depressions show even higher enrichment whereas high altitude regions show much lower values 1 to 3 vsmow our atmospheric maps indicate that water ice in the polar reservoirs is enriched in deuterium to at least 8 vsmow which would mean that early mars 4 5 billion years ago had a global equivalent water layer at least 137 meters deep

the effect of interacting downstream branches on saltwater intrusion in the modaomen estuary china abstract in recent years the increased pressure of saltwater intrusion in the modaomen estuary one of the outlets in the pearl river delta prd china has threatened the freshwater supply for the surrounding regions the estuary has complex geometry and bathymetry and branches into three waterways madaomen waterway hongwan waterway and hezhou waterway entering into the coastal sea in this study a three dimensional baroclinic model efdc was used to investigate salt transport among these branches and saltwater intrusion in the mainstem of the estuary the salt transport at selected cross sections was decomposed into three components advection steady shear and tidal oscillatory following the method of lerczak et al lerczak j a geyer w r chant r j 2006 mechanisms driving the time dependent salt flux in a partially stratified estuary journal of physical oceanography 36 2296 2311 results from this study indicate that the hongwan waterway serves as a salt source for the mainstem of the estuary especially during spring tides while the hezhou waterway mainly acts as a salt sink for the mainstem the down estuary wind increases the steady shear transport in the modaomen waterway and the advection transport in the hongwan waterway which alters the saltwater intrusion in the estuary closure of the hongwan and the hezhou waterways could result in a 20 decrease of saltwater intrusion in the estuary these results provide scientific basis for water resource management in the region

thermal inertia and bolometric bond albedo values for mimas enceladus tethys dione rhea and iapetus as derived from cassini cirs measurements abstract spectra taken by cassini s composite infrared spectrometer cirs between 10 and 600 cm 1 17 1000 m of surface thermal emission of mimas enceladus tethys dione rhea and iapetus have been used to derive the thermal inertia and bolometric bond albedo values only an upper limit for the bolometric bond albedo of iapetus dark leading side could be determined due to the insensitivity of the thermal model to albedo when albedos are very low the thermal inertia in this region however is better constrained the cirs coverage of enceladus is extensive enough that the latitudinal variation in these values from 60 s to 70 n has been determined in 10 wide bins the bolometric bond albedos determined here are consistent with literature values which show the surface of the saturnian icy moons to be covered in ice contaminated to varying degrees the thermal inertia of the moons is shown to be in the range 9 20 j m 2 k 1 s 1 2 approximately 2 6 times lower than that of the galilean satellites implying a less well consolidated and more porous surface the thermal inertias of iapetus and phoebe are somewhat higher suggesting that the very low thermal inertias of satellites from rhea inwards may be related to their probable coating of e ring material latitudinal variations on the surface of enceladus show that the bolometric bond albedo and thermal inertia increase towards the active plume source at the south pole

gravitational viscoelastic postseismic relaxation on a layered spherical earth viscoelastic relaxation of a ductile asthenosphere underlying a purely elastic plate is a strong candidate process for explaining anomalous rates of crustal deformation observed following large earthquakes the nongravitational treatment of pollitz 1992 which is valid on a global scale and includes the effects of compressibility is here extended to permit the calculation of gravitational viscoelastic relaxation for a specified spherically layered viscoelastic rheology following an earthquake in an elastic layer the simple approximations we adopt make the resulting treatment particularly suitable for near field calculations for an asthenosphere with a maxwell rheology the effect of gravitational coupling is manifested only many relaxation times after the earthquake as obtained by previous investigators its effect is generally to speed up the long wavelength component of the relaxation process and attenuate the overall vertical displacement pattern several subtle features common to the relaxation behavior from several different fault types thrust rift and strike slip are identified the effect of gravitational coupling on horizontal displacements is consistent with flexure of the upper elastic plate driven by the corresponding effects on the vertical displacement stress diffusion away from the source region generally exhibits pulse like behavior which is dispersive in both space and time if the asthenosphere is confined to a relatively narrow channel then the dispersion branches governing relaxation are radically altered and stress diffusion effects far from the coseismic rupture zone exhibit a complicated time dependence reflecting the competing tendencies of toroidal and spheroidal mode relaxation

effects of lithospheric viscoelastic relaxation on the contemporary deformation following the 1959 mw 7 3 hebgen lake montana earthquake and other areas of the intermountain seismic belt 1 the 1959 mw 7 3 hebgen lake mt normal faulting earthquake occurred in an extensional stress regime near the yellowstone volcanic field time dependent crustal deformation data following this major earthquake were acquired by precise trilateration and gps surveys from 1973 to 2000 around the hebgen lake fault zone modeling the changes of baseline lengths across and near the fault reveals a lateral variation of transient rheology in which the lithosphere is stronger near the hebgen lake fault zone than in the vicinity of the yellowstone volcano system the models also imply that the lower crust is stronger than the upper mantle in agreement with results from studies of postseismic and post lake filling relaxations 100 years in addition evaluations of the postseismic motion produced by the hebgen lake and the 1983 mw 6 9 borah peak id earthquakes indicate that horizontal transient motion of up to 1 mm yr contribute significantly to the contemporary regional crustal deformation near the epicentral areas for the eastern basin and range 500 km south of the hebgen lake fault similar rheologic models were derived from the observed uplift associated with the lake bonneville rebound and were used to evaluate the postseismic deformation associated with six most recent paleoearthquakes of the wasatch fault zone and three m 5 6 historic earthquakes of northern utah the results show 0 1 mm yr of horizontal postseismic motion at present time that are within the horizontal uncertainties of continuous gps velocity from the basin and range and significantly smaller than the contemporary extension of 1 3 mm yr in the wasatch front

decadal changes in bathymetry of the yangtze river estuary human impacts and potential saltwater intrusion abstract this study analyzed bathymetric changes of the 77 km yangtze river estuary in china over the past ten years in order to understand the impacts of recent human activities on the estuary of a large alluvial river morphological changes were assessed by analyzing digitized bathymetric data of the estuarine channels from 2002 to 2013 additionally multi beam bathymetric measurements made in 2012 2014 and 2015 were utilized to investigate microtophographic bedforms of the lower reach of the estuary our results showed that the middle and upper reaches of the yangtze river estuary experienced substantial channel bed erosion in the past 10 years and that the recent human activities have contributed to the change these included the construction of a 70 km2 reservoir along the yangtze river estuary the qingcaosha reservoir for drinking water supply for the city of shanghai which has caused progressive bed erosion in the north channel the net volume of channel erosion in the hengsha passage from 2002 to 2013 was 0 86 108 m3 a large amount of the eroded sediment was trapped downstream causing overall accretion in the upper reach of the north passage the middle and upper reaches of the south passage also experienced intense erosion 0 45 108 m3 in the past ten years while high accretion occurred in the lower reach because of the deepening waterway project the channel dredging left a large range of dredging marks and hollows in the north passage the increasing saltwater intrusion found in the yangtze river estuary may have been a consequence of either dredging or erosion or both combined

morphological evolution of the south passage in the changjiang yangtze river estuary china abstract estuarine morphology of the world s major large rivers have experienced great changes due to intensive anthropogenic activities and natural forcings especially in the changjiang estuary located at the end of the longest river in asia this study focuses on morphological changes of the south passage sp which is one of the 4 channels in the branching estuary at the mouth of the changjiang yangtze river china a multivariate analysis technique of empirical orthogonal eigen function eof method was used to examine the major modes of change in the long term over 26 years water depth data the results show that the morphological changes at the sp could be divided into two stages between 1987 and 1997 the sp had a single stable channel with closure of a cross channel between 1997 and 2012 sp displayed southeastward elongation of a spit into the main channel and westward shoal incision by a cross channel the opening of the sp developed a two channel morphology which stabilized and showed infilling during 2003 2012 the average deposition rate was 10 cm yr in the past 30 years the most dominant morphological changes of sp included the deposition around the upstream opening of the channel the second most important pattern of morphological change was related to the downstream elongation retreat and lateral migration of the spit of the jiangyanan shoal which resulted in the two channel configuration of the sp additionally these morphological changes were not triggered by the decline of the distal sediment source from the upstream but due to the input of proximal sources of the shoal at the upstream opening of the sp and spill over sediment from the north passage via a short cut channel

a reevaluation of the isostatic rebound of lake bonneville recent data pertaining to the uplift of the provo and bonneville shorelines of the late pleistocene lake bonneville are evaluated in a reanalysis using a rigorous model of deformation of a plane earth comprising an elastic layer overlying a viscoelastic channel the new observations constrain the wavelength and amplitude of the uplift of two shorelines formed at two different epochs and they partly resolve the ambiguities in the plausible mechanical parameters found in previous solutions the results indicate that the effective thickness of the elastic lithosphere is 28 to 30 km equal to the seismically determined crustal thickness in the region the estimate of the upper mantle viscosity in the region varies from 1 5 3 4 lo19 pa s to 2 1 5 8 l018 pa s depending on whether the upper mantle is assumed to be of uniform viscosity or the flow is confined to a narrow channel in the asthenosphere the predicted rate of present uplift at the center of the desiccated lake is 0 06 mm yr and 0 03 mm yr for the shallow channel and uniform mantle models respectively these values are beyond the first order leveling accuracies and the question of the existence of a narrow channel in the region cannot be resolved with the available observations

an extraordinary episode of yellowstone caldera uplift 2004 2010 from gps and insar observations 1 geodetic measurements of yellowstone ground deformation from 2006 to june 2010 reveal deceleration of the recent uplift of the yellowstone caldera following an unprecedented period of uplift that began in 2004 in 2006 2008 uplift rates decreased from 7 to 5 cm yr and 4 to 2 cm yr in the northern and southwest caldera respectively and in 2009 rates further reduced to 2 cm yr and 0 5 cm yr in the same areas elastic dislocation modeling of the deformation data robustly indicates an expanding sill at 7 10 km depth near the top of a seismically imaged crystallizing magma reservoir with a 60 decrease in the volumetric expansion rate between 2006 and 2009 reduction of hydrothermal volcanic recharge from beneath the northeast caldera and seismic moment release of the 2008 and 2010 large earthquake swarms are plausible mechanisms for decelerating the caldera uplift and may have influenced the change in recent caldera motion from uplift to subsidence

saltwater intrusion into the changjiang river a model guided mechanism study 1 the changjiang river cr is divided into a southern branch sb and a northern branche nb by chongming island as the river enters the east china sea observations reveal that during the dry season the saltwater in the inner shelf of the east china sea flows into the cr through the nb and forms an isolated mass of saltwater in the upstream area of the sb the physical mechanism causing this saltwater intrusion has been studied using the high resolution unstructured grid finite volume coastal ocean model fvcom the results suggest that the intrusion is caused by a complex nonlinear interaction process in relation to the freshwater flux upstream tidal currents mixing wind and the salt distribution in the inner shelf of the east china sea the tidal rectification resulting from the interaction of the convergence or divergence of tidal momentum flux and bottom friction over abrupt topography produces a net upstreamward volume flux from nb to sb with river discharge the net water transport in the nb is driven through a momentum balance of surface elevation gradient forcing horizontal advection and vertical diffusion in the dry season reducing the surface elevation gradient forcing makes tidal rectification a key process favorable for the saltwater intrusion a northerly wind tends to enhance the saltwater intrusion by reducing the seaward surface elevation gradient forcing rather than either the baroclinic pressure gradient forcing or the wind driven ekman transport a convergence experiment suggests that high grid resolution 100 m or less is required to correctly resolve the net water transport through the nb particularly in the narrow channel on the northern coast of chongming island

dynamics and rapid migration of the energetic 2008 2009 yellowstone lake earthquake swarm 1 yellowstone national park experienced an unusual earthquake swarm in december january 2008 2009 that included rapid northward migration of the activity at 1 km per day and shallowing of the maximum focal depths from 12 to 2 km beneath northern yellowstone lake the swarm consisted of 811 earthquakes 0 5 mw 4 1 aligned on a n s 12 km long vertical plane of hypocenters the largest earthquake of the swarm had a 50 tensile crack opening source determined by a full waveform inversion that we interpret as a magmatic expansion component in addition gps data revealed e w crustal extension coincident with the swarm modeling of gps and seismic data is consistent with e w opening of 10 cm on a n s striking vertical dike our interpretation is that the swarm was induced by magmatic fluid migration or propagation of a poroelastic stress pulse along a pre existing fracture zone

comment on snowball earth a thin ice solution with flowing sea glaciers by david pollard and james f kasting 1 pollard and kasting 2005 hereinafter referred to as pk have coupled an energy balance climate model to an ice shelf flow model to investigate the snowball earth episodes of the neoproterozoic 600 800 million years ago when the ocean apparently froze all the way to the equator hoffman and schrag 2002 pk s particular concern was to investigate the possibility that over a wide equatorial band where sublimation exceeded snowfall the bare ice may have been thin enough to permit transmission of sunlight to the water below providing an extensive refugium for the photosynthetic eukaryotes that survived the snowball events this possibility was first proposed by mckay 2000 whose model of radiative transfer and heat conduction predicted tropical ice only a few meters thick for ice albedo up to 0 7 under assumed conditions of sunlight and temperature at the snowball equator however when spectral resolution was incorporated into the radiative transfer model warren et al 2002 the ice albedo had to be reduced below 0 4 to obtain the thin ice solution under otherwise identical conditions it seemed unlikely that such dark sea ice could avoid melting under the equatorial sun but even if it could avoid melting the thin ice would risk being crushed by the inflow of kilometer thick sea glaciers from higher latitudes goodman and pierrehumbert 2003 to further investigate the feasibility of tropical thin ice it was therefore necessary to couple a climate model to models of sea ice thermodynamics and sea glacier flow this is what pk have done surprisingly they conclude that thin ice 3 m thick may have prevailed in a 20 degree latitude band centered on the equator we argue here that this conclusion is too optimistic because pk s thin ice solution apparently required that several controlling variables be set outside their measured ranges the albedo of cold glacier ice the depth of transition from snow to ice and the thermal conductivity of ice we then raise the more general question of how wide is the thin ice domain in the parameter space of model variables 2 choices of model variables that favor thin ice 2 1 albedo of cold glacier ice 2 as sea glaciers flowed equatorward into the tropical region of net sublimation their surface snow and subsurface firn would sublimate away exposing bare glacier ice to the atmosphere and solar radiation this ice would be freshwater meteoric ice which originated from compression of snow so it would contain numerous bubbles giving a high albedo the albedo of cold nonmelting glacier ice exposed by sublimation antarctic blue ice has been measured as 0 55 0 65 in four experiments in the atlantic sector of antarctica bintanja and van den broeke 1995 bintanja et al 1997 liston et al 1999 reijmer et al 2001 0 63 in the transantarctic mountains warren et al 2002 and 0 66 near the coast of east antarctica weller 1968 weller s blue ice albedo is often quoted as 0 69 e g weller 1980 king and turner 1997 but that is an average value which included a few times with patchy snow cover on the ice not all glacier ice is bubbly under several hundred meters of ice thickness the pressure becomes so great that each air bubble dissolves in the ice to form a clathrate crystal and the ice becomes relatively clear price 1995 however when this ice becomes depressurized as its cover sublimates away as in the antarctic blue ice areas where the albedos were measured the bubbles reform lipenkov 2000 so that the albedo of 0 55 0 66 is observed at the surface 3 in contrast to glacier ice which forms by compression of snow sea ice forms by freezing of liquid water in their model pk tried two values of scattering coefficient each of which they applied uniformly to both sea ice and glacier ice with albedo 0 47 a reasonable value for meter thick snowfree sea ice brandt et al 2005 figure 1 the sea glaciers terminated at 9 degrees latitude leaving an equatorial strip of thin ice but with albedo 0 64 i e within the measured range for glacier ice the sea glaciers advanced to the equator with a thickness of 1 km to add realism to the model it would be good to assign different values of scattering coefficient to these two very different types of ice

the snowball earth aftermath exploring the limits of continental weathering processes article i nfo carbonates capping neoproterozoic glacial deposits contain peculiar sedimentological features and geochemical anomalies ascribed to extraordinary environmental conditions in the snowball earth aftermath it is commonly assumed that post snowball climate dominated by co2 partial pressures several hundred times greater than modern levels would be characterized by extreme temperatures a vigorous hydrological cycle and associated high continental weathering rates however the climate in the aftermath of a global glaciation has never been rigorously modelled here we use a hierarchy of numerical models from an atmospheric general circulation model to a mechanistic model describing continental weathering processes to explore characteristics of the earth system during the supergreenhouse climate following a snowball glaciation these models suggest that the hydrological cycle intensifies only moderately in response to the elevated greenhouse indeed constraints imposed by the surface energy budget sharply limit global mean evaporation once the temperature has warmed sufficiently that the evaporation approaches the total absorbed solar radiation even at 400 times the present day pressure of atmospheric co2 continental runoff is only 1 2 times the modern runoff under these conditions and accounting for the grinding of the continental surface by the ice sheet during the snowball event the simulated maximum discharge of dissolved elements from continental weathering into the ocean is approximately 10 times greater than the modern flux consequently it takes millions of years for the silicate weathering cycle to reduce post snowball co2 levels to background neoproterozoic levels regarding the origin of the cap dolostones we show that continental weathering alone does not supply enough cations during the snowball melting phase to account for their observed volume

thermal convection and the convective regime diagram in super earths numerical models of bottom heated thermal convection of highly compressible fluid with strongly temperature dependent viscosity are presented to understand how the rayleigh number ra and the temperature dependence of viscosity exert control over the regimes of thermal convection in massive super earths thermodynamic properties of mantle materials are pressure dependent but other material properties including the viscosity are not a stagnant lid develops along the surface of the planet when the viscosity contrast across the mantle due to temperature dependence r exceeds 106 at high rayleigh number relevant to super earths the threshold in r which increases with increasing ra is higher than that expected for the earth from earlier boussinesq models the efficiency of convective heat transport measured by the nusselt number nu is considerably lower than that expected from boussinesq models nu depends on ra and r as nu 59 r 0 23 ra 109 0 27 when r 105 strong adiabatic compression significantly reduces the activity of hot ascending plumes especially at high r at r relevant for super earths hot ascending plumes lose their buoyancy on their way and hardly reach the surface boundary hot spot volcanism due to ascending plumes is probably suppressed on super earths the lithosphere is considerably thicker than that suggested by earlier boussinesq models and is unlikely to show a plate like behavior

axial wind effects on stratification and longitudinal salt transport in an idealized partially mixed estuary abstract a 3d hydrodynamic model regional ocean model system roms is used to investigate how axial wind influences stratification and to explore the associated longitudinal salt transport in partially mixed estuaries the model is configured to represent a straight estuarine channel connecting to a shelf sea the results confirm that wind straining of the along channel salinity gradient exerts an important control on stratification two governing parameters are identified the wedderburn number w defined as the ratio of wind stress to axial baroclinic pressure gradient force and the ratio of an entrainment depth to water depth hs h here w controls the effectiveness of wind straining which promotes increases decreases in stratification during down estuary up estuary wind the ratio hs h determines the portion of the water column affected by direct wind mixing while stratification is always reduced by up estuary wind stratification shows an increase then decrease transition when down estuar

mobility of continental mantle evidence from postseismic geodetic observations following the 1992 landers earthquake the crust around the rupture zone of the 1992 landers earthquake has continued to deform in the years following the earthquake at rates 3 times greater than pre earthquake rates we use a combination of global positioning system gps and synthetic aperture radar insar data collected during a 3 year epoch following the earthquake in order to investigate postseismic mechanisms responsible for the high transient velocities in order to maximize the potential signal from viscoelastic relaxation we evaluate and model postseismic relaxation following the first few months of documented accelerated deformation the combination of gps and insar data allows us to establish viscoelastic relaxation of the lower crust and upper mantle as the dominant postseismic process and to discriminate among possible viscoelastic models the data particularly require the presence of a highly ductile uppermost mantle beneath the central mojave domain with temperature between the wet and dry basalt solidus this is consistent with independent seismic and geochemical inferences of a regionally warm uppermost mantle further consideration of seismic velocity variations in conjunction with faulting patterns within the mojave desert suggests that the primary faulting characteristics of the mojave desert namely the pervasive late cenozoic deformation within the eastern california shear zone versus the near absence of faults in the western mojave domain are controlled by the rheology of the uppermost mantle

the importance of ice vertical resolution for snowball climate and deglaciation sea ice schemes with a few vertical levels are typically used to simulate the thermodynamic evolution of sea ice in global climate models here it is shown that these schemes overestimate the magnitude of the diurnal surface temperature cycle by a factor of 2 3 when they are used to simulate tropical ice in a snowball earth event this could strongly influence our understanding of snowball termination which occurs in global climate models when the midday surface temperature in the tropics reaches the melting point a hierarchy of models is used to show that accurate simulation of surface temperature variation on a given time scale requires that a sea ice model resolve the e folding depth to which a periodic signal on that time scale penetrates this is used to suggest modifications to the sea ice schemes used in global climate models that would allow more accurate simulation of snowball deglaciation

postseismic mantle relaxation in the central nevada seismic belt holocene acceleration of deformation and postseismic relaxation are two hypotheses to explain the present day deformation in the central nevada seismic belt cnsb discriminating between these two mechanisms is critical for understanding the dynamics and seismic potential of the basin and range province interferometric synthetic aperture radar detected a broad area of uplift 2 to 3 millimeters per year that can be explained by postseismic mantle relaxation after a sequence of large crustal earthquakes from 1915 to 1954 the results lead to a broad agreement between geologic and geodetic strain indicators and support a model of a rigid basin and range between the cnsb and the wasatch fault

crustal deformation in the new madrid seismic zone and the role of postseismic processes global navigation satellite system data across the new madrid seismic zone nmsz in the central united states over the period from 2000 through 2014 are analyzed and modeled with several deformation mechanisms including the following 1 creep on subsurface dislocations 2 postseismic frictional afterslip and viscoelastic relaxation from the 1811 1812 and 1450 earthquakes in the nmsz and 3 regional strain in agreement with previous studies a dislocation creeping at about 4 mm yr between 12 and 20 km depth along the downdip extension of the reelfoot fault reproduces the observations well we find that a dynamic model of postseismic frictional afterslip from the 1450 and february 1812 reelfoot fault events can explain this creep kinematic and dynamic models involving the cottonwood grove fault provide minimal predictive power this is likely due to the smaller size of the december 1811 event on the cottonwood grove fault and a distribution of stations better suited to constrain localized strain across the reelfoot fault regional compressive strain across the nmsz is found to be less than 3 10 9 yr if much of the present day surface deformation results from afterslip it is likely that many of the earthquakes we see today in the nmsz are aftershocks from the 1811 1812 new madrid earthquakes despite this conclusion our results are consistent with observations and models of intraplate earthquake clustering given this and the recent paleoseismic history of the region we suggest that seismic hazard is likely to remain significant

snowball earth ice thickness on the tropical ocean 1 on the tropical oceans of a neo proterozoic snowball earth snow free ice would have existed in regions of net sublimation photosynthesis could have continued beneath this bare ice if it was sufficiently thin and sufficiently clear the steady state ice thickness is determined by the necessity to balance the upward conduction of heat with three subsurface heating rates the heat flux from the ocean to the ice base the latent heat of freezing to the ice base and the solar energy absorbed within the ice a preliminary study using a broadband model for solar radiation and assuming a large freezing rate had indicated that tropical ice might be only a few meters thick here we show that the vertical throughput of ice by surface sublimation and basal freezing would be too slow to keep the ice thin and that the broadband model had exaggerated the absorption depth of sunlight we use a spectral model for solar absorption computing radiative transfer at 60 wavelengths considering absorption by the ice and scattering by bubbles with the spectral model the computed ice thickness is much greater for a solar flux of 320 w m 2 at the equatorial surface and expected albedo of 0 5 for bare sea ice we find that surface temperatures below 12 c generate ice layers too thick for photosynthesis 100 m if the albedo were as low as 0 4 thick ice would occur only for surface temperatures below 25 c but such low temperatures would be difficult to maintain with such low albedo for surface temperatures warmer than these limits the ice becomes thin 1 m and is unlikely to represent a coherent ice layer however glacial deformation of thick floating ice from nearby oceanic regions may preclude the existence of thin or ice free patches

effects of bubbles cracks and volcanic tephra on the spectral albedo of bare ice near the transantarctic mountains implications for sea glaciers on snowball earth 1 spectral albedo was measured along a 6 km transect near the allan hills in east antarctica the transect traversed the sequence from new snow through old snow firn and white ice to blue ice showing a systematic progression of decreasing albedo at all wavelengths as well as decreasing specific surface area ssa and increasing density broadband albedos under clear sky range from 0 80 for snow to 0 57 for blue ice and from 0 87 to 0 65 under cloud both air bubbles and cracks scatter sunlight their contributions to ssa were determined by microcomputed tomography on core samples of the ice although albedo is governed primarily by the ssa and secondarily by the shape of bubbles or snow grains albedo also correlates highly with porosity which as a proxy variable would be easier for ice sheet models to predict than bubble sizes albedo parameterizations are therefore developed as a function of density for three broad wavelength bands commonly used in general circulation models visible near infrared and total solar relevance to snowball earth events derives from the likelihood that sublimation of equatorward flowing sea glaciers during those events progressively exposed the same sequence of surface materials that we measured at allan hills with our short 6 km transect representing a transect across many degrees of latitude on the snowball ocean at the equator of snowball earth climate models predict thick ice or thin ice or open water depending largely on their albedo parameterizations our measured albedos appear to be within the range that favors ice hundreds of meters thick

the thermal equation of state of mg fe sio3 bridgmanite perovskite and implications for lower mantle structures the high pressure high temperature equation of state eos of synthetic 13 fe bearing bridgmanite mg silicate perovskite is measured using powder x ray diffraction in a laser heated diamond anvil cell with a quasi hydrostatic neon pressure medium we compare these results which are consistent with previous 300 k sound speed and compression studies with a reanalysis of fe free mg end member data from tange et al 2012 to determine the effect of iron on bridgmanite s thermoelastic properties eos parameters are incorporated into an ideal lattice mixing model to probe the behavior of bridgmanite at deep mantle conditions with this model a nearly pure bridgmanite mantle composition is shown to be inconsistent with density and compressibility profiles of the lower mantle we also explore the buoyant stability of bridgmanite over a range of temperatures and compositions expected for large low shear velocity provinces concluding that bridgmanite dominated thermochemical piles are more likely to be passive dense layers externally supported by convection rather than internally supported metastable domes the metastable dome scenario is estimated to have a relative likelihood of only 4 7 given the narrow range of compositions and temperatures consistent with seismic constraints if buoyantly supported such structures could not have remained stable with greater thermal contrast early in earth s history ruling out formation scenarios involving a large concentration of heat producing elements

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

the galilean satellites nasa s galileo mission to jupiter and improved earth based observing capabilities have allowed major advances in our understanding of jupiter s moons io europa ganymede and callisto over the past few years particularly exciting findings include the evidence for internal liquid water oceans in callisto and europa detection of a strong intrinsic magnetic field within ganymede discovery of high temperature silicate volcanism on io discovery of tenuous oxygen atmospheres at europa and ganymede and a tenuous carbon dioxide atmosphere at callisto and detection of condensed oxygen on ganymede modeling of landforms seen at resolutions up to 100 times as high as those of voyager supports the suggestion that tidal heating has played an important role for io and europa

response of sediment dynamics in the york river estuary usa to tropical cyclone isabel of 2003 tropical cyclone isabel of 2003 generated large storm surge strong waves and subsequent river flooding in the york river estuary usa during its passage across the chesapeake bay region a 3d model was used to investigate the changes of sediment concentration sediment flux and the recovery time of the york river estuary to its naturally evolved condition without the storm the results showed that two sediment concentration peaks appeared during the storm event the first one was induced by the large upstream flow and waves during the storm surge rising period and the later one was caused by the strong downstream flow during the descent of the storm surge the advection which was induced by the barotropic gradient dominated the sediment flux during the storm event the sediment fluxes increased by a factor of 100 during the rise and descent of the storm surge a large amount of sediment that was transported into the estuary and eroded from the seabed during the rising of the storm surge was quickly transported out of the estuary during the descent of the storm surge waves played a key role in stirring the seabed and increasing the sediment concentration during the storm subsequent high freshwater inflow changed the sediment loading and hydrodynamics in the estuary and thus influenced the estuarine turbidity maximum etm dynamics profoundly the etm moved downstream with the river flooding initially and returned upstream with the waning of river flooding and the re establishment of gravitational circulation the effect of river flooding on sediment concentration varied spatially and depended on the changes of etm locations and vertical mixing the model results suggest that a large amount of sediment was transported out of the estuary during the storm event and the subsequent river flooding had a larger impact on recovery time of the estuary

resolved snowball earth clouds abstractrecent general circulation model gcm simulations have challenged the idea that a snowball earth would be nearly entirely cloudless this is important because clouds would provide a strong warming to a high albedo snowball earth gcm results suggest that clouds could lower the threshold co2 needed to deglaciate a snowball by a factor of 10 100 enough to allow consistency with geochemical data here a cloud resolving model is used to investigate cloud and convection behavior in a snowball earth climate the model produces convection that extends vertically to a similar temperature as modern tropical convection this convection produces clouds that resemble stratocumulus clouds under an inversion on modern earth which slowly dissipate by sedimentation of cloud ice there is enough cloud ice for the clouds to be optically thick in the longwave and the resulting cloud radiative forcing is similar to that produced in gcms run in snowball conditions this result is robust to large changes in the clo