effectiveness of natural riparian buffers to reduce subsurface nutrient losses to incised streams abstract perennial vegetation borders many incised perennial streams and rivers in southern iowa and is assumed to provide a natural riparian buffer against subsurface nutrient losses from shallow groundwater questions remain about the effectiveness of these systems near incised channels that cut through nutrient rich holocene alluvium underlying the riparian corridors in this study riparian groundwater nutrient concentrations were evaluated near an incised stream under four perennial land cover types common to southern iowa with the objectives to assess how groundwater quality differed by i distance away from an incised stream and ii perennial land cover type groundwater samples were collected from riparian wells installed 1 m 20 m and 40 m from an incised channel under four replicated land covers cool season grass warm season grass woods pasture on six occasions across the growing season results indicate that channel incision lowers water tables in the near stream zone and maintains more aerobic conditions but did not result in enhanced nitrogen mineralization or leaching to groundwater average concentrations of nitrate nitrogen phosphorus and dissolved organic carbon were 0 3 mg l 0 2 mg l and 6 mg l respectively with variations due mainly to land cover type and geomorphology study results suggest that nutrient losses from perennial buffers in the region do not greatly impact regional water n and p loads although average concentrations in riparian groundwater may exceed proposed stream nutrient criteria

tectonics of planetary loading a general model and results the tectonic response of a planet s lithosphere to an imposed load contains information on the mechanical properties of the lithosphere particularly its thickness from which more general planetary properties such as heat flow and rheology may be inferred traditionally mathematical models of the lithosphere s response to loads were constructed using either a flat plate or thin shell approximation and tectonic patterns were predicted as a function of the lithospheric thickness such predictions were then compared with the observed tectonics to determine the thickness of the lithosphere at the time of loading many of these models however are restricted in the size of features that can be treated by assumptions on load width and or lithospheric thickness additionally since membrane stresses dominate over bending stresses for broad laods many of these models consider only the membrane stresses we here develop a general model for planetary tectonics due to an axisymmetric load that is valid for arbitrary shell thicknesses and load widths the model predicts two distinct tectonic styles the first which results from narrow loads on thick lithospheres is the style familiar to terrestrial geologists successive regions of radial thrust strike slip and concentric normal faulting with increasing radial distance from the load we call this pattern the plate response the second style is the result of wide loads on thin lithospheres successive regions of disorganized thrust concentric thrust and strike slip faulting we call this pattern the membrane response while this second style of faulting may not be as familiar it agrees with the predictions of membrane theory the transition between these two styles is not abrupt but occurs over a range of lithospheric thicknesses it is marked by the development of bands of radial thrust and radial normal faults on mars if the tharsis rise is treated strictly as a load only this transitional regime can produce the observed radial normal faulting for reasonable lithospheric thicknesses since it occurs in the transitional regime it is not predicted by either the traditional flat plate or membrane approximations we use the model to determine the limits of shell thickness and load width where the use of the end member approximations is appropriate the overall style of tectonic response turns out to be a function of the interplay between shell thickness load width and the ratio of buoyancy to flexural support and cannot be predicted on the basis of any one of these for example some authors extend membrane theory to an arbitrarily chosen lithospheric thickness of 10 of the planetary radius our results show that such an extension is valid only for very broad loads on small icy moons for terrestrial planets the tectonic response predicted by membrane theory is realized only when the lithospheric thickness is less than 5 of planetary radius depending on the load width tectonic analysis on a planetary scale must take account of these different response styles and the limited range of load widths and lithospheric thicknesses for which they occur

patterns of fracture and tidal stresses due to nonsynchronous rotation implications for fracturing on europa this study considers the global patterns of fracture that would result from nonsynchronous rotation of a tidally distorted planetary body the incremental horizontal stresses in a thin elastic or viscous shell due to a small displacement of the axis of maximum tidal elongation are derived and the resulting stress distributions are applied to interpret the observed pattern of fracture lineaments on europa the observed pattern of lineaments can be explained by nonsynchronous rotation if these features formed by tension fracturing and dike emplacement tension fracturing can occur for a small displacement of the tidal axis so that the resulting lineaments may be consistent with other evidence suggesting a young age for the surface

reorientation and faulting of pluto due to volatile loading within sputnik planitia the location of sputnik planitia on pluto is shown to result from volatiles sequestered within the basin forcing the reorientation of the dwarf planet as supported by the planet wide fault network james keane et al report that the location of sputnik planitia the approximately 1 000 kilometre diameter topographic basin that dominates the surface of pluto is a natural consequence of the infill of volatile ices within the basin and the resulting reorientation true polar wander of the dwarf planet as sputnik planitia slowly loaded with volatiles the pull of tidal torques from charon would have resulted in the eventual alignment of the basin with the pluto charon tidal axis sputnik planitia probably formed northwest of its present location and was loaded with volatiles over million year timescales as a result of volatile transport cycles this is one of four papers on the geology of sputnik planitia in this issue of nature in news views amy barr puts these latest contributions into context pluto is an astoundingly diverse geologically dynamic world the dominant feature is sputnik planitia a tear drop shaped topographic depression approximately 1 000 kilometres in diameter possibly representing an ancient impact basin1 2 the interior of sputnik planitia is characterized by a smooth craterless plain three to four kilometres beneath the surrounding rugged uplands and represents the surface of a massive unit of actively convecting volatile ices n2 ch4 and co several kilometres thick1 2 3 4 5 this large feature is very near the pluto charon tidal axis here we report that the location of sputnik planitia is the natural consequence of the sequestration of volatile ices within the basin and the resulting reorientation true polar wander of pluto loading of volatile ices within a basin the size of sputnik planitia can substantially alter pluto s inertia tensor resulting in a reorientation of the dwarf planet of around 60 degrees with respect to the rotational and tidal axes the combination of this reorientation loading and global expansion due to the freezing of a possible subsurface ocean generates stresses within the planet s lithosphere resulting in a global network of extensional faults that closely replicate the observed fault networks on pluto sputnik planitia probably formed northwest of its present location and was loaded with volatiles over million year timescales as a result of volatile transport cycles on pluto6 7 pluto s past present and future orientation is controlled by feedbacks between volatile sublimation and condensation changing insolation conditions and pluto s interior structure

surface vector mapping of magnetic anomalies over the moon using kaguya and lunar prospector observations we have provided preliminary global maps of three components of the lunar magnetic anomaly on the surface applying the surface vector mapping svm method the data used in the present study consist of about 5 million observations of the lunar magnetic field at 10 45 km altitudes by kaguya and lunar prospector the lunar magnetic anomalies were mapped at 0 2 equi distance points on the surface by the svm method showing the highest intensity of 718 nt in the crisium antipodal region overall features on the svm maps indicate that elongating magnetic anomalies are likely to be dominant on the moon except for the young large basins with the impact demagnetization remarkable demagnetization features suggested by previous studies are also recognized at hertzsprung and kolorev craters on the farside these features indicate that demagnetized areas extend to about 1 2 radii of the basins craters there are well isolated central magnetic anomalies at four craters leibnitz aitken jules verne and grimaldi craters their magnetic poles through the dipole source approximation suggest occurrence of the polar wander prior to 3 3 3 5 ga when compared with high albedo markings at several magnetic anomalies such as the reiner gamma anomalies three dimensional structures of the magnetic field on near the surface are well correlated with high albedo areas these results indicate that the global svm maps are useful for the study of the lunar magnetic anomalies in comparison with various geological and geophysical data

water and magmas thermal effects of exsolution the temperature changes caused by water exsolution from magma have been determined through calorimetric measurements performed on phonolitic and albitic compositions the enthalpies of mixing of water with these melts have been derived from hf solution calorimetry made at 323 k on glass samples containing up to 5 wt water together with heat capacity data for the same series of samples mixing between aluminosilicate melts and water appears nearly ideal at magmatic temperatures with small enthalpies of mixing that are negative for both melts at low pressures but can become positive for albite at high pressure regardless of the endothermic or exothermic nature of the process water exsolution is associated with negligible temperature changes of only a few degrees even when 5 wt h2o is degassed however thermal effects might be greater for more depolymerized melts such as basalts and related compositions

east west faults due to planetary contraction abstract contraction expansion and despinning have been common in the past evolution of solar system bodies these processes deform the lithosphere until it breaks along faults their characteristic tectonic patterns have thus been sought for on all planets and large satellites with an ancient surface while the search for despinning tectonics has not been conclusive there is good observational evidence on several bodies for the global faulting pattern associated with contraction or expansion though the pattern is seldom isotropic as predicted the cause of the non random orientation of the faults has been attributed either to regional stresses or to the combined action of contraction expansion with another deformation despinning tidal deformation reorientation another cause of the mismatch may be the neglect of the lithospheric thinning at the equator or at the poles due either to latitudinal variation in solar insolation or to localized tidal dissipation using thin elastic shells with variable thickness i show that the equatorial thinning of the lithosphere transforms the homogeneous and isotropic fault pattern caused by contraction expansion into a pattern of faults striking east west preferably formed in the equatorial region by contrast lithospheric thickness variations only weakly affect the despinning faulting pattern consisting of equatorial strike slip faults and polar normal faults if contraction is added to despinning the despinning pattern first shifts to thrust faults striking north south and then to thrust faults striking east west if the lithosphere is thinner at the poles the tectonic pattern caused by contraction expansion consists of faults striking north south i start by predicting the main characteristics of the stress pattern with symmetry arguments i further prove that the solutions for contraction and despinning are dual if the inverse elastic thickness is limited to harmonic degree two making it easy to determine fault orientation for combined contraction and despinning i give two methods for solving the equations of elasticity one numerical and the other semi analytical the latter method yields explicit formulas for stresses as expansions in legendre polynomials about the solution for constant shell thickness though i only discuss the cases of a lithosphere thinner at the equator or at the poles the method is applicable for any latitudinal variation of the lithospheric thickness on iapetus contraction or expansion on a lithosphere thinner at the equator explains the location and orientation of the equatorial ridge on mercury the combination of contraction and despinning makes possible the existence of zonal provinces of thrust faults differing in orientation north south or east west which may be relevant to the orientation of lobate scarps

an integrated model of magma chamber conduit and column for the analysis of sustained explosive eruptions abstract explosive volcanic eruptions comprise a complex series of processes involving withdrawal from the magma chamber magma ascent along the conduit and eruption column dynamics numerous studies have modeled the different sub domains of a volcanic system but their interplay has seldom been analyzed to this end we developed c 3 c cubed that stands for chamber conduit and column a new integrated model that describes the dynamics of an explosive eruption as a series of steady state regimes and as a function of geometry and initial conditions of the magma reservoir we used global sensitivity analysis to quantify the role of the relevant model parameters and describe the interplay between the different volcanic sub domains in particular we analyzed the evolution of a sustained explosive eruption in order to identify the conditions for buoyant super buoyant and collapsing columns input data were based on field reconstructions of quaternary explosive eruptions in the vulsini volcanic district roman province central italy model results show that 1 the column regime although affected by complex interactions among several factors mostly depends on the conduit radius the volatile content i e supersaturation concentration at the top of the chamber and length of the conduit in decreasing level of importance 2 the amount of mass erupted is independent of the conduit radius and depends mostly on volatile supersaturation the radius of the magma chamber the length of the conduit and the overpressure at the conduit inlet 3 the mass flow rate column height and duration of the eruption are largely controlled by the conduit radius 4 the flow pressure and density at the conduit exit are mostly controlled by the conduit inlet overpressure at the onset of the eruption and by the length of the conduit at the end of the eruption 5 the exit velocity from the conduit is mostly controlled by the volatile content the length of the conduit and the inlet overpressure in this model framework and with specific reference to selected plinian events of the vulsini volcanic district simulation results show that column collapse is not achieved for reasonable eruption durations order of hours and conduit widths tens of meters this is consistent with field reconstructions suggesting that column collapse did not likely occur and that pyroclastic flows were therefore generated by independent mechanisms from ring fissures and or multiple vents concomitant to caldera collapse

global contraction of planetary bodies due to despinning application to mercury and iapetus abstract we extend previous work on the global tectonic patterns generated by despinning with a self consistent treatment of the isotropic despinning contraction that has been ignored we provide simple analytic approximations that quantify the effect of the isotropic despinning contraction on the global shape and tectonic pattern the isotropic despinning contraction of mercury is 93 m t 1 day 2 where t is the initial rotation period if we take into account both the isotropic contraction and the degree 2 deformations associated with despinning the preponderance of compressional tectonic features on mercury s surface requires an additional isotropic contraction 1 km t 1 day 2 presumably due to cooling of the interior and growth of the solid inner core the isotropic despinning contraction of iapetus is 9 m t 16 h 2 and it is not sensitive to the presence of a core or the thickness of the elastic lithosphere the tectonic pattern expected for despinning including the isotropic contraction does not explain iapetus ridge furthermore the ridge remains unexplained with the addition of any isotropic compressional stresses including those generating by cooling

is there evidence for polar wander on europa abstract the orientations of the albedo lineaments bands and lineations on europa s surface have been compared in previous studies with the global stress fields set up by orbital eccentricity orbital recession and nonsynchronous rotation of these orbital and rotational effects nonsynchronous rotation combined with an offsetting of the tidal bulge comes closest to providing agreement between the stress field generated and the lineation orientations if the lineations trace tension or extension fractures mcewen 1986 nature321 49 51 however inferred minimum principal stress directions for a broad region of wedge shaped bands near the anti jove point cannot satisfactorily be accounted for by any of the stress fields above but are consistent with the stresses resulting from a rotation of europa s ice shell about an axis through the sub and anti jove points clockwise as seen from the anti jove hemisphere p m schenk and w b mckinnon 1989 icarus79 75 100 calculations by ojakangas and stevenson 1989 icarus81 220 241 of the thermal state of europa s ice shell indicate that spatial variations in the thickness of the shell may cause it to undergo such a reorientation we have investigated whether any reorientation of the shell about an axis through the sub and anti jove points produces a stress field consistent with the full global set of prominent lineations on europa s surface we find that no such reorientation provides a good fit between the lineations and plausible fracture orientations derived from the principal stress trajectories topographic ridges identified in a limited zone south of the anti jove point are roughly consistent with compression due to clockwise polar wander but the orientations of these ridges may be strongly biased by illumination direction within the limitations of the presently available imagery nonsynchronous rotation is still the most likely cause of the prominant fractures on europa s surface and the best specific albeit regionally limited tectonic evidence consistent with recent polar wander remains the wedge shaped bands

evidence for a 20 tilting of the earth s rotation axis 110 million years ago true polar wander tpw the shift of the earth s rotation axis with respect to the entire globe is most probably due to mass redistribution in the earth s mantle as a result of convection using a new rigorously selected palaeomagnetic database gathering only directions obtained from magmatic rocks we find that tpw has been clearly intermittent over the last 200 ma with two long periods of strict standstill from the present to 80 ma and from approximately 150 to 200 ma a single period of shifting is observed between 80 and about 150 ma ago this period culminates around 110 ma ago in an 20 abrupt tilting during which an angular speed exceeding 5 ma 0 5m yr may have been reached assuming that the time averaged geomagnetic field is axial our results indicate that the changes in the position of the rotation axis and therefore in the inertia tensor of the earth are intermittent we suggest that a major reorganization of the mass distribution in the earth s mantle occurred in the lower cretaceous this event concomitant with plume hyperactivity at the earth s surface and probable drastic changes at the core mantle boundary attested by the inhibition of geomagnetic reversals suggests unmixing of upper and lower mantle by avalanching of upper mantle material down to the core mantle boundary the astonishingly strict stability of the time averaged position of the rotation axis before and after this episode of shifting implies the existence of some steady convection which does not modify the large scale distribution of mass within the mantle given the intermittence of mantle avalanching we suggest that these long periods of stability correspond to the temporary reestablishment of a basically two layered convection system within the mantle

holocene paleoenvironments in southeastern minnesota chasing the prairie forest ecotone this paper uses a multi proxy approach involving pollen plant macrofossils speleothem isotopes and alluvial history of streams to reconstruct the history of prairie expansion and contraction along the prairie forest border of southeastern minnesota usa early holocene forests were replaced by prairie along this border but eastward expansion of prairie stalled for 2000 yr when the prairie forest ecotone stabilized prairie invaded the area from the west 8000 9000 yr b p but mesic forest remained less than 100 km to the east until about 6000 yr b p changes in n 13 c values in speleothem calcite that reflect the rise of c4 grasses correlate well with the presence of c4 grass species identified in the plant macrofossil record periods of large floods correlate with speleothem evidence of dry summers increased cool season precipitation both resulting in less plant cover to absorb moisture and change to prairie vegetation fl 2002 elsevier science b v all rights reserved

streambank erosion rates and loads within a single watershed bridging the gap between temporal and spatial scales abstract the importance of streambank erosion to watershed scale sediment export is being increasingly recognized however few studies have quantified bank erosion and watershed sediment flux at the basin scale across temporal and spatial scales in this study we evaluated the spatial distribution extent and temporal frequency of bank erosion in the 5218 ha walnut creek watershed in iowa across a seven year period we inventoried severely eroding streambanks along over 10 km of stream and monitored erosion pins at 20 sites within the watershed annual streambank recession rates ranged from 0 6 cm yr during years of hydrological inactivity to 28 2 cm yr during seasons with high discharge rates with an overall average of 18 8 cm yr the percentage of total basin export attributed to streambank erosion along the main stem of walnut creek ranged from 23 to 53 large variations in individual site annual rates and percentage of annual load suggested that developing direct relationships between streambank erosion rates and total sediment discharge may be confounded by the timing and magnitude of discharge events storage of sediments within channel system and the remobilization of eroded material

complex response of a midcontinent north america drainage system to late wisconsinan sedimentation abstract the geomorphic evolution of mud creek basin in eastern iowa u s a serves to illustrate how geomorphic influences such as sediment supply valley gradient climate and vegetation are recorded in the alluvial stratigraphic record sediment supply to the fluvial system increased significantly during the late wisconsinan through a combination of periglacial erosion and loess accumulation subsequent evolution of the holocene alluvial stratigraphic record reflects long term routing of the late wisconsinan sediment through the drainage basin in a series of cut and fill cycles whose timing was influenced by hydrologic response to change in climate and vegetation when viewed in a regional context the alluvial stratigraphic record appears to reflect a long term complex response of the flu ial system to increased sediment supply during the late wisconsinan hydrologic and sediment supply changes accompanying the spread of euroamerican agriculture to the basin in the 1800s dramatically upset trends in sedimentation and channel behavior established during the holocene

vertical distribution of total carbon nitrogen and phosphorus in riparian soils of walnut creek southern iowa abstract subsurface lithology plays an important role in many riparian zone processes but few studies have examined how sediment nutrient concentrations vary with depth in this study we evaluated concentrations of nutrients n c and p with depth in a riparian zone of the glaciated midwest a total of 146 sediment samples were collected from 24 cores that extended to a maximum depth of 3 6 m at eight sites in the riparian zone of walnut creek subsurface deposits were predominantly silt loam becoming coarser and more variable with depth nitrogen and carbon content ranged from

measuring streambank erosion due to ground water seepage correlation to bank pore water pressure precipitation and stream stage limited information exists on one of the mechanisms governing sediment input to streams streambank erosion by ground water seepage the objective of this research was to demonstrate the importance of streambank composition and stratigraphy in controlling seepage flow and to quantify correlation of seepage flow erosion with precipitation stream stage and soil pore water pressure the streambank site was located in northern mississippi in the goodwin creek watershed soil samples from layers on the streambank face suggested less than an order of magnitude difference in vertical hydraulic conductivity ks with depth but differences between lateral ks of a concretion layer and the vertical ks of the underlying layers contributed to the propensity for lateral flow goodwin creek seeps were not similar to other seeps reported in the literature in that eroded sediment originated from layers underneath the primary seepage layer subsurface flow and sediment load quantified using 50 cm wide collection pans were dependent on the type of seep intermittent low flow lf seeps flow rates typically less than 0 05 l min 1 persistent high flow hf seeps average flow rate of 0 39 l min 1 and buried seeps which eroded unconsolidated bank material from previous bank failures the timing of lf seeps correlated to river stage and precipitation the hf seeps at goodwin creek began after rainfall events resulted in the adjacent streambank reaching near saturation i e soil pore water pressures greater than 5 kpa seep discharge from hf seeps reached a maximum of 1 0 l min 1 and sediment concentrations commonly approached 100 g l 1 buried seeps were intermittent but exhibited the most significant erosion rates 738 g min 1 and sediment concentrations 989 g l 1 in cases where perched water table conditions exist and persistent hf seeps occur seepage erosion and bank collapse of streambank sediment may be significant copyright 2007 john wiley sons ltd

reorientation of icy satellites by impact basins 1 large impact basins are present on many of the icy satellites of the outer solar system assuming that their present day topography is uncompensated such basins can cause significant poleward reorientations for slow rotating satellites this reorientation may have been accompanied by transient large amplitude wobble the largest basins on tethys rhea and titania are predicted to have caused reorientations of roughly 4 7 and 12 respectively resulting in global tectonic stresses up to 0 5 mpa the potential anomalies associated with the basins can be up to one third of those expected for a hydrostatic tidally and rotationally deformed body and may complicate interpretation of the satellite interior structure pluto and charon because of their slow rotation are also likely to have undergone reorientation of 10 20 if they possess impact basins of comparable sizes to those of the saturnian satellites

vesiculation in rhyolite at low h2o contents a thermodynamic model we present experimental data on the thermodynamics and kinetics of bubble nucleation and growth in weakly h2o oversaturated rhyolitic melts the high temperature 900 1100 c experiments involve heating of rhyolitic obsidian from hrafntinnuhryggur krafla iceland to above their glass transition temperature tg 690 c at 0 1 mpa for times of 0 25 24 h during experiments the rhyolite cores increase in volume as h2o vapor filled bubbles nucleate and expand the extent of vesiculation as tracked by porosity is mapped in temperature time t t space at constant temperature and for a characteristic dwell time the rhyolite cores achieve a maximum volume where the t t conditions reach thermochemical equilibrium for each t t snapshot of vesiculation we use 3 d analysis of x ray computed tomographic xct images of the quenched cores to obtain the bubble number density bnd and bubble size distribution bsd bnds for the experimental cores are insensitive to t and t indicating a single nucleation event all bsds converge to a common distribution independent of t melt viscosity or initial degree of saturation suggesting a common growth process we use these data to calibrate an empirical model for predicting the rates and amounts of vesiculation in rhyolitic melts as a function of and thermochemical affinity a two computable parameters that are dependent on t pressure and h2o content the model reproduces the experimental data set and data from the literature to within experimental error and has application to natural volcanic systems where bubble formation and growth are not diffusion limited e g lavas domes ignimbrites conduit infill

an estimate of motion between the spin axis and the hotspots over the past century 1 estimates of the motion of the spin axis over the past century either neglect plate motion or are relative to the mean lithosphere to understand the response of the spin axis to mass changes it is more useful to estimate spin axis motion relative to the hotspots because the hotspots well represent the solid earth the hotspots originate in the lower mantle which comprises 2 3 of the earth s volume relative to the lower mantle the plates and the upper mantle move in opposite directions to a large extent canceling each other out the mean lithosphere moves quickly relative to the hotspots and poorly represents the solid earth the spin axis has moved relative to the hotspots over the past century along the 68 4 w meridian at 4 03 milliarcsecond yr this motion is 11 counterclockwise of 0 5 milliarcsecond yr faster than and 25 different than spin axis motion relative to the mean lithosphere

oceans in the icy galilean satellites of jupiter abstract equilibrium models of heat transfer by heat conduction and thermal convection show that the three satellites of jupiter europa ganymede and callisto may have internal oceans underneath ice shells tens of kilometers to more than a hundred kilometers thick a wide range of rheology and heat transfer parameter values and present day heat production rates have been considered the rheology was cast in terms of a reference viscosity 0 calculated at the melting temperature and the rate of change a of viscosity with inverse homologous temperature the temperature dependence of the thermal conductivity k of ice i has been taken into account by calculating the average conductivity along the temperature profile heating rates are based on a chondritic radiogenic heating rate of 4 5 pw kg 1 but have been varied around this value over a wide range the phase diagrams of h2o ice i and h2o 5 wt nh3 ice have been considered the ice i models are worst case scenarios for the existence of a subsurface liquid water ocean because ice i has the highest possible melting temperature and the highest thermal conductivity of candidate ices and the assumption of equilibrium ignores the contribution to ice shell heating from deep interior cooling in the context of ice i models we find that europa is the satellite most likely to have a subsurface liquid ocean even with radiogenic heating alone the ocean is tens of kilometers thick in the nominal model if tidal heating is invoked the ocean will be much thicker and the ice shell will be a few tens of kilometers thick ganymede and callisto have frozen their oceans in the nominal ice i models but since these models represent the worst case scenario it is conceivable that these satellites also have oceans at the present time the most important factor working against the existence of subsurface oceans is contamination of the outer ice shell by rock rock increases the density and the pressure gradient and shifts the triple point of ice i to shallower depths where the temperature is likely to be lower then the triple point temperature according to present knowledge of ice phase diagrams ammonia produces one of the largest reductions of the melting temperature if we assume a bulk concentration of 5 wt ammonia we find that all the satellites have substantial oceans for a model of europa heated only by radiogenic decay the ice shell will be a few tens of kilometers thinner than in the ice i case the underlying rock mantle will limit the depth of the ocean to 80 100 km for ganymede and callisto the ice i shell on top of the h2o nh3 ocean will be around 60 to 80 km thick and the oceans may be 200 to 350 km deep previous models have suggested that efficient convection in the ice will freeze any existing ocean the present conclusions are different mainly because they are based on a parameterization of convective heat transport in fluids with strongly temperature dependent viscosity rather than a parameterization derived from constant viscosity convection models the present parameterization introduces a conductive stagnant lid at the expense of the thickness of the convecting sublayer if the latter exists at all the stagnant lid causes the temperature in the sublayer to be warmer than in a comparable constant viscosity convecting layer we have further modified the parameterization to account for the strong increase in homologous temperature and therefore decrease in viscosity with depth along an adiabat this modification causes even thicker stagnant lids and further elevated temperatures in the well mixed sublayer it is the stagnant lid and the comparatively large temperature in the sublayer that frustrates ocean freezing

exsolution enthalpy of water from silicate liquids abstract because water dissolves in silicate melts as at least two species h2o molecules and oh groups the exsolution evaporation enthalpy of the h2o component from a silicate melt depends on species proportions which in turn depends on h2o pressure in this short communication a formal analysis of the exsolution enthalpy of water from a silicate melt is presented taking into account of the role of speciation the exsolution enthalpy from a rhyolitic melt at 850 c is found to increase with pressure at 100 bar and this increase is mostly caused by the speciation reaction sahagian and proussevitch sahagian d l proussevitch a a 1996 thermal effects of magma degassing j volcanol geotherm res 74 19 38 also found an increase of the exsolution enthalpy with pressure up to 20 bar for an albitic melt however the rate of increase determined from this work is an order of magnitude less than that determined by sahagian and proussevitch it is concluded that the strong pressure dependence of the exsolution enthalpy at low pressures they inferred is probably an artifact of their treatment the method presented here is general and can be applied to other multi species components

coupled thermal orbital evolution of the early moon abstract coupled thermal orbital histories of early lunar evolution are considered in a simple model we consider a plagioclase lid overlying a magma ocean overlying a solid mantle tidal dissipation occurs in the plagioclase lid and heat transport is by conduction and melt migration we find that large orbital eccentricities can be obtained in this model we discuss possible consequences of this phase of large eccentricities for the shape of the moon and geochronology of lunar samples we find that the orbit can pass through the shape solution of garrick bethell et al garrick bethell i wisdom j zuber m 2006 science 313 652 but we argue that the shape cannot be maintained against elastic deformation as the orbit continues to evolve

porosity and salt content determine if subduction can occur in europa s ice shell motivated by recent evidence for subduction in europa s ice shell we explore the geophysical feasibility of this process here we construct a simple model to track the evolution of porosity and temperature within a slab that is forced to subduct we also vary the initial salt content in europa s ice shell and determine the buoyancy of our simulated subducting slab we find that porosity and salt content play a dominant role in determining whether the slab is nonbuoyant and subduction in europa s ice shell is actually possible generally we find that initially low porosities and high salt contents within the conductive lid are more conducive to subduction if salt contents are laterally homogenous and europa has a reasonable surface porosity of 0 0 1 the conductive portion of europa s shell must have salt contents exceeding 22 for subduction to occur however if salt contents are laterally heterogeneous with salt contents varying by a few percent subduction may occur for a surface porosity of 0 0 1 and overall salt contents of 5 thus we argue that under plausible conditions subduction in europa s ice shell is possible moreover assuming that subduction is actively occurring or has occurred in europa s recent past provides important constraints on the structure and composition of the ice shell

enceladus an active ice world in the saturn system enceladus one of the mid sized icy moons of saturn has an importance to planetary science far greater than its modest 504 km diameter would suggest intensive exploration of enceladus by the cassini saturn orbiter has revealed that it is the only known icy world in the solar system with ongoing deep seated geological activity active tectonic fractures at enceladus s south pole dubbed tiger stripes warmed by internal tidally generated heat spew supersonic jets of water vapor other gases and ice particles into circum saturnian space a subsurface saltwater sea probably exists under the south pole between the ice shell and the silicate core because of evidence that liquid water is probably present at the jet sources enceladus is also of great astrobiological interest as a potential habitat for life

multistage evolution and variable motion history of agenor linea europa abstract europa the second galilean moon of jupiter is composed of a silicate mantle and an ice shell which overlies a supposed subsurface ocean the surface of europa is scarred with fractures varying in morphology dimensions and geometry we focus on agenor linea a 1500 km bright band that extends across europa s antijovian to trailing southern hemisphere agenor linea is morphologically a band like strike slip fault however it experienced at least three evolutionary growth phases marked by three zones of varying albedo each with a different geological history structures within the band material and kinematic indicators are consistent with formation through oblique left lateral dilation followed by normal dilation and right lateral strike slip movement or combinations thereof these motions produced a cumulative maximum right lateral offset of 29 5 km and a maximum band width of 34 km we interpret agenor linea to have formed primarily in response to the combined effects of nonsynchronous rotation stress and diurnal tidal flexing if so its orientation is optimal for right lateral oblique opening in the current global stress field consistent with its most recent kinematic behavior a small amount of offset of relatively young tension fractures that postdate cryospheric disruption by lenticulae suggest the possibility of ongoing activity along agenor in contrast to a previously published model in which strike slip duplexing was the primary process of band formation this study shows that initial dilation of three distinct morphological zones under disparate stress orientations preceded strike slip motion that resulted in both localized and distributed deformation features within the zones across the width of the band

stresses generated in cooling viscoelastic ice shells application to europa 1 a cooling viscoelastic ice shell overlying an ocean develops stresses due to two effects thermal contraction of the ice due to cooling and the expansion of the shell due to the ice water volume change the former effect generates near surface compression and deeper extension the second effect generates extension only in both cases stresses are smaller at depth due to viscous creep the resulting combined stresses are extensional except at shallow 1 km depths in thin ice shells for ice shells thicker than 45 km stresses are extensional throughout the extensional stresses exceed 10 mpa for shells thicker than 20 km and thus dominate all other likely sources of stress as long as shell cooling occurs the dominantly extensional nature of the stresses may help to explain the puzzling lack of compression observed on europa and other large icy satellites however after 100 myr of conductive cooling the maximum theoretical elastic strains for europa are 0 35 which are probably insufficient to explain the total amount of observed extension

mechanics of evenly spaced strike slip faults and its implications for the formation of tiger stripe fractures on saturn s moon enceladus abstract we present the first mechanical analysis based on realistic rheology and boundary conditions on the formation of evenly spaced strike slip faults two quantitative models employing the stress shadow concept widely used for explaining extensional joint spacing are proposed in this study 1 an empirically based stress rise function model that simulates the brittle deformation process during the formation of evenly spaced parallel strike slip faults and 2 an elastic plate model that relates fault spacing to the thickness of the fault hosting elastic medium when applying the models for the initiation and development of the tiger stripe fractures tsf in the south polar terrain spt of enceladus the mutually consistent solutions of the two models as constrained by the mean spacing of the tsf at 35 km requires that the brittle ice shell thickness be 30 km the elastic thickness be 0 7 km and the cohesive strength of the spt ice shell be 30 kpa however if the brittle and elastic models are decoupled and if the ice shell cohesive strength is on the order of 1 mpa the brittle ice shell would be on the order of 10 km

reorientation of vesta gravity and tectonic predictions 1 vesta s large southern hemisphere impact basin is likely to have caused reorientation however because the basin is not centred at the south pole vesta likely also has a remnant rotational figure reorientation of 6 is predicted to have occurred based on the dimensions of the basin existing measurements of vesta s shape are consistent with 20 or less reorientation and 20 or less despinning both the remnant rotational figure and the basin contribute to the degree 2 gravity coefficients which will be measured by the dawn mission and will provide a test of the reorientation hypothesis reorientation and despinning also give rise to stresses vesta s stress state is likely to be dominated by isotropic contraction due to cooling which does not affect the gravity coefficients however the orientation of the resulting thrust features will be controlled by the amount of reorientation and despinning providing another observational test

evidence for increased cool season moisture during the middle holocene isotopic analyses of three stalagmites from cold water cave northeast iowa united states reveal nearly identical 13 c trends from ca 7 to 2 ka however 18 o patterns differ by as much as 3 from 5 7 to 3 2 ka these disparate 18 o values reflect evaporative 18 o enrichment in meteoric water prior to infiltration suggesting that previously calculated temperatures based on a single cold water cave stalagmite overestimated middle holocene warming the coincidence of elevated middle holocene growth rates in the stalagmites with the lowest oxygen isotopic compositions indicates that the middle holocene was marked by a predominance of coolweather precipitation during a period of increased overall aridity

thermal effects of magma degassing abstract the thermodynamics of diffusive bubble growth is dominated by latent heat of water exsolution vaporization and the work of gas expansion p dv work a numerical assessment of the effects of these on cooling of bubbly magma water in albite indicates that a magma can exsolve volatiles at equilibrium or with varying degrees of oversaturation depending on decompression history and degassing kinetics heat of water exsolution vaporization from magmatic melts has not previously been experimentally or analytically determined and we have evaluated it from others thermodynamic functions known for the albite water system heat of exsolution is small for pressures over 100 mpa but can reach 20 kj mole at 1 2 mpa 10 20 bar oversaturation degassing at 10 20 bar can cause cooling of albite by 8 k wt of exsolved water the thermal effect of equilibrium degassing depends on the starting pressure of decompression because it follows the solubility curve for a saturated melt at 100 mpa 4 wt water it can cool a magma at least by 35 k before it disrupts into spray as gas volume fraction exceeds 0 8 although we do not specifically consider the dynamics of bubble interactions in this study our results suggest that the dynamic cooling rate could be significant at the vent of an erupting magma column and under extreme eruption conditions can lead to glass formation around fast growing bubbles of magmatic foam this could cause fragmentation into fine ash by brittle failure conversely bubble wall cooling even before glass formation can serve to reduce diffusive volatile flux into bubbles decreasing the overall cooling of the system and lead to solidification of oversaturated magmas as occasionally observed in eruption products

eruptions arising from tidally controlled periodic openings of rifts on enceladus the cassini flyby of 14 july 2005 revealed plumes of water vapour and ice associated with the tiger stripe features on the surface of saturn s icy moon enceladus since then the challenge has been to explain the nature of the plumes and the forces driving them two papers this week offer an explanation that accounts for both the plume characteristics and the presence of hot spots without the need to assume the existence of near surface liquid water a requirement of some previous models nimmo et al identify tidally driven lateral fault motions near the tiger stripes as the most likely drivers of heat and vapour production and hurford et al show that as enceladus orbits saturn the parent planet s tides make the satellite s ice flex this may cause the tiger stripes to open and close periodically exposing volatile gases and allowing them to be released plumes of gas have been seen near the south polar region of enceladus a small icy satellite of saturn and observations revealed large rifts in the crust informally called tiger stripes hurford et al report that during each orbit every portion of each tiger stripe rift spends about half the time in tension which allows the rift to open exposing volatiles and allowing eruptions plume activity is expected to vary periodically in 2005 plumes were detected near the south polar region of enceladus1 a small icy satellite of saturn observations of the south pole revealed large rifts in the crust informally called tiger stripes which exhibit higher temperatures than the surrounding terrain and are probably sources of the observed eruptions2 models of the ultimate interior source for the eruptions are under consideration1 3 4 5 other models of an expanding plume6 require eruptions from discrete sources as well as less voluminous eruptions from a more extended source to match the observations no physical mechanism that matches the observations has been identified to control these eruptions here we report a mechanism in which temporal variations in tidal stress open and close the tiger stripe rifts governing the timing of eruptions during each orbit every portion of each tiger stripe rift spends about half the time in tension which allows the rift to open exposing volatiles and allowing eruptions in a complementary process periodic shear stress along the rifts also generates heat along their lengths7 8 9 which has the capacity to enhance eruptions plume activity is expected to vary periodically affecting the injection of material into saturn s e ring10 and its formation evolution and structure moreover the stresses controlling eruptions imply that enceladus icy shell behaves as a thin elastic layer perhaps only a few tens of kilometres thick

europa s ice tectonics new insights from physical wax experiments with implications for subduction initiation and global resurfacing processes abstract jupiter s moon europa has one of the youngest geological surfaces in our solar system with an age of 40 90 ma implying an intense history of resurfacing the surface of europa indeed shows abundant evidence of tectonic deformation related to extension strike slip and shortening however observed features related to shortening are scarce compared with pervasive extensive extensional features such as dilational bands and do not suffice as the sole mechanism for recycling aging terranes recently evidence for potential plate tectonics associated with subduction zones has been discovered on europa this could be responsible for recycling most of europa s surface however basic physical parameters needed to initiate subduction on europa such as thickness of the brittle layer deformation rates and orientation of pre existing zones of weakness at which subduction could start are not well understood here we aim to better understand the process and the conditions that could lead to initiation of subduction on europa through physical experiments using wax to simulate europa s two layered i e convective icy crust by deforming the wax strain on europa s surface possibly caused by diurnal tides or its nonsynchronous rotation is simulated our results indicate that subduction could initiate over a broad range of surface thicknesses and deformation rates above a minimum conductive layer thickness but is strongly dependent on the orientation of the pre existing zones of weakness very thin conductive layer experiments however result in a previously undescribed process that we term ductile rolldown which creates surface features similar to double ridges observed on europa thus subduction and ductile rolldown represent physically plausible mechanisms that could play a critical role in resurfacing europa throughout its geologic history these results could yield significant implications for europa s thermal history and evolution habitability and future spacecraft missions

shapes of the saturnian icy satellites and their significance the sizes and shapes of six icy saturnian satellites have been measured from cassini imaging science subsystem iss data employing limb coordinates and stereogrammetric control points mimas enceladus tethys dione and rhea are well described by triaxial ellipsoids iapetus is best represented by an oblate spheroid all satellites appear to have approached relaxed equilibrium shapes at some point in their evolution but all support at least 300 m of global wavelength topography the shape of enceladus is most consistent with a homogeneous interior if enceladus is differentiated its shape and apparent relaxation require either lateral inhomogeneities in an icy mantle and or an irregularly shaped core iapetus supports a fossil bulge of over 30 km and provides a benchmark for impact modification of shapes after global relaxation satellites such as mimas that have smoother limbs than iapetus and are expected to have higher impact rates must have relaxed after the shape of iapetus was frozen

reconciliation of experimental results on h2o speciation in rhyolitic glass using in situ and quenching techniques in order to resolve the controversy over how to interpret experimental h2o speciation results using in situ and quenching techniques we have carried out an infrared spectroscopic study to determine whether the molecular h2o 5230 cm 1 and oh 4520 cm 1 band intensity variation with measurement temperature below glass transition is owing to species interconversion or to the temperature dependence of molar absorptivities by comparing rhyolitic glasses with different total h2o content from 0 18 to 0 76 wt we show that the peak height of the 4520 cm 1 band increases by a similar relative amount about 2 if the baseline is fit with a flexicurve and 10 if the baseline is fit by a straight line from 25 to 400oc independent of the total h2o content the results show that 1 the molar absorptivities do indeed change with temperature and 2 in our experiments below the glass transition temperature species concentrations do not change noticeably with temperature and the band intensity variations are caused mainly by changes in the shape of the absorbance bands with temperature the absence of unquenchable species reaction in the glass state on our experimental time scale confirms that speciation data can be obtained using the quench technique from 400 to 600oc on the other hand the temperature dependence of the molar absorptivities must be quantified for the full potential of the in situ technique to be realised 1999 elsevier science b v all rights reserved

the volcanic and tectonic history of enceladus abstract enceladus has a protracted history of impact cratering cryovolcanism and extensional compressional and probable strike slip faulting it is unique in having some of the outer solar system s least and most heavily cratered surfaces enceladus cratering record tectonic features and relief elements have been analyzed more comprehensively than done previously like few other icy satellites enceladus seems to have experienced major lateral lithospheric motions it may be the only icy satellite with global features indicating probable lithospheric convergence and folding ridged plains 500 km across consist of a central labyrinthine ridge complex atop a broad dome surrounded by smooth plains and peripheral sinuous ridge belts the ridged plains have few if any signs of extension almost no craters and an average age of just 107to 108years ridge belts have local relief ranging from 500 to 2000 m and tend to occur near the bottoms of broad regional troughs between swells our reanalysis of peter thomas dermott s f and p c thomas 1994 the determination of the mass and mean density of enceladus from its observed shape icarus 109 241 257 limb profiles indicates that high peaks probably ridge belts also occur in unmapped areas sinuous ridges appear foldlike and are similar to terrestrial fold belts such as the appalachians if they are indeed folds it may require that the ridged plains are mechanically perhaps volcanically layered regional topography suggests that folding may have occurred along zones of convective downwelling the cratered plains in contrast to the ridged plains are heavily cratered and exhibit extensional structures but no obvious signs of compression cratered plains contain a possible strike slip fault isbanir fossa along which two pairs of fractures seem to have 15 km of right lateral offset the oldest cratered plains might date from shortly after the formation of the saturnian system or the impact disruption and reaccretion of enceladus another area of cratered plains has modified craters e g ali baba and aladdin which some workers have explained by anomalous heat flow and viscous relaxation lateral shear and shield building volcanism also may have been important a young rift like structure northern samarkand sulci has few craters and a concentration of cracks or grabens and flattened flooded and rifted craters pit chains and cratered domes suggest explosive volcanism smooth plains may have formed by cryovolcanic equivalents of flood basalt volcanism pure h2o would be difficult to extrude through an icy crust and is cosmochemically improbable as a cryovolcanic agent density relations rule out eutectic brine lavas on enceladus but nh3 h2o volcanism is possible current steady state tidal dissipation may cause melting of ammonia hydrate at a depth of 25 km if the crust is made of ammonia hydrate or 100 km if it is made of water ice

influence of exsolved volatiles on reheating silicic magmas by recharge and consequences for eruptive style at volcan quizapu chile the two most recent eruptions of volcan quizapu southern andes chile only 86 years apart were both triggered by magma recharge and extruded the same volume about 5 km3 of the same volatile rich dacitic magma but showed a remarkable shift from effusive 1846 47 to explosive 1932 behavior we demonstrate using a newly developed model that the presence or absence of an exsolved volatile phase in the reservoir strongly influences its mechanical and thermal response to new inputs of magma we propose that prior to the 1846 47 effusive eruption gas bubbles damped the build up of excess pressure and allowed recharge of a significant volume of magma before triggering the 1846 1847 eruption the strong temperature increase that resulted enhanced syn eruptive outgassing leading to an effusive eruption in contrast during the repose period between the 1847 and 1932 eruptions new recharges found a much less compressible host reservoir as the exsolved gas phase was largely removed in response to the prior eruption yielding rapid pressurization minor reheating and comparatively less syneruptive outgassing the combination of these effects culminated in an explosive eruption

tectonic patterns on reoriented and despun planetary bodies abstract several processes may produce global tectonic patterns on the surface of a planetary body the stresses associated with distortions of biaxial figures due to despinning or reorientation were first calculated by vening meinesz vening meinesz f a 1947 trans am geophys union 28 1 1 23 we adopt a mathematically equivalent but physically more meaningful treatment for distortions associated with rotation the new approach allows us to find analytic solutions for the general case of stresses associated with distortions of biaxial or triaxial planetary figures distortions of biaxial figures may be driven by variations in rotation rate rotation axis orientation or the combination of both distortions of triaxial figures may be driven by the same mechanisms and or variations in tidal axis orientation for tidally deformed satellites while the magnitude of the resulting stresses depends on the adopted elastic and physical parameters the expected tectonic pattern is independent of these parameters for these mechanisms reorientation of the rotation tidal axis alone is expected to produce normal thrust faulting provinces enclosing the initial rotation tidal poles and thrust normal faulting provinces enclosing the final rotation tidal poles reorientation of both the rotation and tidal axis results in a wide variety of tectonic patterns for different reorientation geometries on europa the tidal axis reorientation which generally accompanies rotation axis reorientations may provide an alternative explanation for tectonic features that have been interpreted as evidence for non synchronous rotation the observed tectonic pattern on enceladus is more easily explained by a large reorientation 90 of the rotation axis than by rotation rate variations

h2o in rhyolitic glasses and melts measurement speciation solubility and diffusion dissolved h2o in silicate melts and glasses plays a crucial role in volcanic eruptions on terrestrial planets and affects glass properties and magma evolution in this paper major progress on several aspects of the h2o melt or glass system is reviewed consistency among a variety of data is investigated discrepancies are evaluated and confusion is clarified on the infrared measurement of total h2o and species concentrations calibration for a variety of glasses has been carried out at room temperature the measurements for h2o in rhyolitic glasses have undergone the most scrutiny resulting in the realization that absorptivities for the near infrared bands depend on total h2o content although the variation of the absorptivities does not seem to significantly affect the determination of total h2o it does affect the determination of molecular h2o and oh species concentrations calibration of the infrared technique for h2o in rhyolitic glasses still needs much improvement especially at high total h2o furthermore it is now almost certain that the molar absorptivities also depend on the measurement temperature in in situ studies hence it will be necessary to carry out calibrations in situ at high temperatures on h2o speciation results from two experimental approaches the quench technique and the in situ technique are very different leading to controversy in our understanding of true speciation a solution is presented to reconcile this controversy it is almost certain that the quench technique does not suffer from a quench problem but interpretation of in situ results suffered from ignoring the dependence of the molar absorptivities on measurement temperature accurate calibration at high temperatures is necessary for the quantitative application of the in situ technique to h2o speciation in silicate melts and glasses on h2o solubility in silicate melts recent experimental work has significantly expanded the t p range of solubility measurements and recent solubility models fill a gap for predicting solubility for a wide range of melt compositions i present a solubility model for rhyolitic and quasi rhyolitic melts over a wide range of t and p 500 1350 c 0 8 kbar by incorporating the role of speciation the solubility model is able to recover the experimental solubility data and has extrapolative value although the partial molar volume of h2o derived from the solubility model differs from that derived from density measurements on h2o diffusion recent studies on h2o diffusion in a quasi rhyolitic melt at 800 1200 c 0 5 5 kbar and up to 7 total h2o not only provide important new diffusion data but are also challenging earlier understanding of h2o diffusion based on data in rhyolitic glasses at 400 550 c 1 bar and 0 2 1 8 total h2o a comparison between the earlier model and recent data is made the recent high temperature diffusivities at total h2o 2 can be predicted by the earlier model however at higher total h2o the earlier model fails new work is under way to understand the diffusion mechanisms at high h2o contents

pre eruptive magma mixing and crystal transfer revealed by phenocryst and microlite compositions in basaltic andesite from the 2008 eruption of kasatochi island volcano the august 7 8 2008 eruption of kasatochi island volcano located in the central aleutians islands alaska produced abundant compositionally heterogeneous basaltic andesite 52 55 wt sio2 that has been interpreted to result from pre eruptive magma mixing the basaltic andesite contains two populations of plagioclase phenocrysts the first volumetrically dominant population consists of oscillatory zoned phenocrysts with an overall normal zonation trend toward comparatively sodic rims an55 65 interrupted by dissolution features and spikes in calcium content up to an85 the second population consists of phenocrysts with highly calcic compositions an90 these phenocrysts contain sharp decreases in calcium content close to their rims reaching as low as an60 but are otherwise texturally and compositionally homogeneous groundmass plagioclase microlites are generally much more calcic than rims of the first phenocryst population with more than 50 of measured microlites containing an80 major minor and trace element concentrations of plagioclase microlites and phenocrysts indicate that oscillatory zoned phenocrysts derived from cooler 800 950 c more silicic mixing magma while unzoned calcic phenocrysts were associated with hotter 900 1050 c mafic magma the mixing of these magmas just prior to eruption followed by decompression during the eruption itself created high effective undercoolings in the mafic end member and lead to the nucleation of high an microlites mgo and feo concentrations of plagioclase microlites and high an phenocryst rims up to 0 4 and 1 3 wt respectively provide further evidence for high mixing and eruption induced effective undercoolings

cassini observes the active south pole of enceladus cassini has identified a geologically active province at the south pole of saturn s moon enceladus in images acquired by the imaging science subsystem iss this region is circumscribed by a chain of folded ridges and troughs at 55 s latitude the terrain southward of this boundary is distinguished by its albedo and color contrasts elevated temperatures extreme geologic youth and narrow tectonic rifts that exhibit coarse grained ice and coincide with the hottest temperatures measured in the region jets of fine icy particles that supply saturn s e ring emanate from this province carried aloft by water vapor probably venting from subsurface reservoirs of liquid water the shape of enceladus suggests a possible intense heating epoch in the past by capture into a 1 4 secondary spin orbit resonance

hydrogeologic controls on nitrate transport in a small agricultural catchment iowa 1 effects of subsurface deposits on nitrate loss in stream riparian zones are recognized but little attention has been focused on similar processes occurring in upland agricultural settings in this paper we evaluated hydrogeologic controls on nitrate transport processes occurring in a small 7 6 ha iowa catchment subsurface deposits in the catchment consisted of upland areas of loess overlying weathered pre illinoian till drained by two ephemeral drainageways that consisted of holocene age silty and organic rich alluvium water tables in upland areas fluctuated more than 4 m per year compared to less than 0 3 m in the drainageway water quality patterns showed a distinct spatial pattern with groundwater in the drainageways having lower nitrate concentrations 10 mg l 1 as wells as lower ph dissolved oxygen and redox and higher ammonium and dissolved organic carbon levels several lines of evidence suggested that conditions are conducive for denitrification of groundwater flowing from uplands through the drainageways field measured nitrate decay rates in the drainageways 0 02 day 1 were consistent with other laboratory studies and regional patterns results from modflow and mt3dms simulations indicated that soils in the ephemeral drainageways could process all upland groundwater nitrate flowing through them however model simulated tile drainage increased both water flux and nitrate loss from the upland catchment study results suggest that ephemeral drainageways can provide a natural nitrate treatment system in our upland glaciated catchments offering management opportunities to reduce nitrate delivery to streams

polar wander of an ice shell on europa an ice shell on europa that is decoupled from the silicate core by a layer of liquid water has a thermal equilibrium thickness profile that varies with position over its surface because of spatial variations in the surface temperature and tidal dissipation within the ice g w ojakangas and d j stevenson 1989 icarus 81 220 241 the second spherical harmonic degree components of these thickness variations and of any fossil rotational and tidal bulges present on the shell contribute to the inertia tensor of the body the problem is that of a planetary elastic lithosphere that is topographically loaded from below following r j willemann and d l turcotte 1981 proc lunar planet sci b 12 837 851 we develop equations describing the variations in the inertia tensor of a body when second harmonic degree topography is added to the base of the crust for an ice shell on europa it is found that a state of thermal equilibrium may involve an unusual orientation of the principal axes of inertia when hydrostatic bulges are ignored in which the intermediate and maximum principal moments are interchanged to reach the preferred orientation for synchronous satellites a thermal equilibrium shell must execute a net reorientation of 90 about the satellite planet direction we present a simple model of rigid synchronously rotating satellite in a circular orbit for which the principal moment difference b c increases with time becoming positive for t 0 the model demonstrates that the expected reorientation is indeed dynamically favored we then consider a more realistic model in which europa s shell and ocean are assumed to reorient as a single entity independently of the core hindered only by viscous dissipation within the shell such coupling of the shell with the ocean and lack of coupling with the core is suggested by g w ojakangas 1988 coupled thermal and dynamical evolution of planetary bodies ph d thesis california institute of technology the model suggests that friction in the shell eliminates the possibility of polar wander unless a low conductivity regolith increases the near surface temperature by a few tens of degrees so the ice just below the regolith behaves viscously on the polar wander time scale however if the equator to pole near surface temperature difference is decreased by more than a critical model dependent amount the shell s inertia tensor no longer has an unstable form in thermal equilibrium alternatively polar wander may occur if preexisting surface fractures e g due to tidal stresses extend to a depth where the ice behaves viscously these fractures must be lubricated perhaps by liquid water from below if the temperature t f at the base of the regolith or the surface fractures is 125 k the model suggests that polar wander occurs on a time scale 2 x 10 6 years decreasing as t f increases after b c becomes postive in the absence of dissipation polar wander would occur in few x 10 3 years large scale polar wander may occur episodically separated by periods on the order of the thermal diffusion time for the shell 10 7 years although a state of slow continuous drifting of the pole is also possible the time scales of viscous flow of topography at the base of the ice is also near 10 7 years polar wander is a very effective means for fracturing the ice and may have contributed to the observed global fracture systems in europa s ice

tectonic processes on europa tidal stresses mechanical response and visible features abstract europa s orbital eccentricity driven by the resonance with io and ganymede results in diurnal tides 3 5 day period and possibly in nonsynchronous rotation both diurnal variation and nonsynchronous rotation can create significant stress fields on europa s surface and both effects may produce cracking patterns and time sequences of apparent tectonic features on europa include lineaments that correlate with both sources of stress if we take into account nonsynchronous rotation after initial crack formation by amounts ranging up to several tens of degrees for example the crosscutting time sequence of features in the cadmus and minos linea region is consistent with a combined diurnal and nonsynchronous tensile stress field as it evolves during tens of degrees of nonsynchronous rotation constraints on the rotation rate from comparing voyager and galileo images show that significant rotation requires 104yr but could be fast enough to have allowed significant rotation since the last global resurfacing even if such resurfacing was as recent as a few million years ago once cracking is initiated diurnal tides work cracks so that they open and close daily although the daily effect is small over 105yr double ridges could plausibly be built along the cracks with sizes and morphologies consistent with observed structures according to a model in which underlying liquid water fills the open cracks partially freezes and is extruded during the daily closing of the cracks thus several lines of observational and theoretical evidence can be integrated if we assume nonsynchronous rotation and the existence of a liquid water layer

a fracture history on enceladus provides evidence for a global ocean 1 the region surrounding the south pole of saturn s moon enceladus shows a young pervasively fractured surface that emanates enough heat to be detected by the cassini spacecraft to explain the elevated heat and eruptive icy plumes originating from large cracks informally called tiger stripes in the surface many models implicitly assume a global liquid ocean beneath the surface here we show that the fracture patterns in the south polar terrain spt of enceladus are inconsistent with contemporary stress fields but instead formed in a temporally varying global stress field related to nonsynchronous rotation of a floating ice shell above a global liquid ocean this finding increase to at least three the number of outer planet satellites likely to possess a subsurface liquid water layer

polygonal impact craters on dione evidence for tectonic structures outside the wispy terrain abstract plan view impact crater geometries can be indicative of pre impact structures within the target material impact events that occur on a pre fractured surface generate craters exhibiting one or more straight rim segments termed polygonal impact craters pics impact craters that appear to be pics are abundant on the surface of saturn s icy satellite dione both within the wispy terrain a region with large visible fractures and also outside the wispy terrain the non wispy terrain where less evidence for fracturing has been observed in the non wispy terrain subtle lineaments are hypothesized to be ne sw nw se and e w trending fractures suggesting that tectonism may have been an important process in this terrain results of previous studies have shown that pic straight rim segment azimuths pic azimuths commonly parallel pre impact fracture azimuths although disagreements about this relationship exist in the literature we investigated the hypothesis that fractures either subtle or nonvisible with available spacecraft images are present within dione s non wispy terrain our first step was to assess the relationship between pics and pre existing fracture azimuths in the wispy terrain our results from this initial assessment show a parallel relationship between pic azimuths and fracture azimuths based on this correlation in the wispy terrain we find it likely that this relationship would hold true in the non wispy terrain if pics are present we tested for pics using crater rim azimuth data collected from randomly distributed study locations throughout the non wispy terrain from these data we identified widespread pics in this terrain which supports the hypothesis that subtle fractures are also present analysis of the pic azimuth data yield a pattern for these inferred fractures across dione s surface that is consistent with the hypothesized global deformation that would result from a combination of satellite despinning and volume expansion our results provide evidence for these previously hypothesized events in dione s history and demonstrate that mapping pics and their azimuths is a useful tool for investigating subtle fractures on solar system bodies

groundwater phosphate dynamics in a river riparian zone effects of hydrologic flowpaths lithology and redox chemistry this study examines the influence of riparian zone hydrology lithology and redox chemistry on groundwater phosphate dynamics patterns of soluble reactive phosphorus srp dissolved oxygen do and ferrous iron fe2 in combination with hydrologic data and sediment characteristics were studied in a forested floodplain connected to a large upland sand aquifer in an agricultural region of southern ontario canada groundwater discharge from the upland aquifer flowed laterally beneath peat in a 2 4 m thick zone of permeable sands across the floodplain to the river within the sands low srp concentrations 3 mg l 1 and fe2 concentrations 0 2 mg l 1 which extended for a horizontal distance of 100 140 m across the riparian zone high srp concentrations 50 950 g l 1 were associated with low do and high fe2 concentrations which exceeded 1 mg l 1 in buried channel sediments near the river bank sediment p fractionation indicated that the buried channel sediments contained a much higher pool of total p and fe al p than the sands groundwater srp concentrations at the river bank were 25 80 g l 1 compared to 10 g l 1 in river water indicating that the floodplain was a source of srp to the river areas of elevated srp and fe2 within the floodplain expanded in august when do concentrations in groundwater were lower than in late spring or autumn these data suggest that the microbial reduction of fe3 to soluble fe2 in anaerobic conditions influences groundwater srp concentrations in the riparian zone this study shows that well organized patterns of groundwater srp concentrations occur in riparian zones which reflect the interaction of hydrologic flowpaths and environments of different redox state internal sources of p associated with buried channel sediments can also influence subsurface srp transport and release to streams

soil properties controlling seepage erosion contributions to streambank failure the majority of sediment leaving catchments may be from streambank failure seepage erosion of unconsolidated sand above a restrictive layer is an important erosion process in incised streams that leads to streambank failure by undercutting banks the objective of this study was to determine the impact of soil properties on seepage erosion and the resulting streambank failure seepage flow and sediment concentrations were measured in situ at eight locations along the banks of a deeply incised stream in northern mississippi using field observations as a guide the soil profile conditions of a shallow 45 cm streambank consisting of 30 cm of topsoil a 10 cm conductive layer and a 5 cm restrictive layer were mimicked in laboratory lysimeter experiments to quantify the hydrologic properties controlling seepage erosion and bank failure under a 40 cm head the time to flow initiation and the flow rate were linearly related to the slope of the restrictive layer seepage erosion began within minutes of flow initiation and resulted in substantial 3 to 34 cm undercutting of the bank sediment concentrations of seeps were as high as 660 g l 1in situ and 4500 g l 1 in the lysimeters sediment concentrations were related to the layer slope thereby indicating the importance of detailed site characterization the usda ars streambank stability model demonstrated the increase in instability of banks due to undercutting by seepage erosion but failed to account for the sediment loss due to sapping for stable banks and overestimated the sediment loads for failed banks published in 2006 by john wiley sons ltd

true polar wander of enceladus from topographic data many objects in the solar system are suspected to have experienced reorientation of their spin axes as their rotation rates are slow and their shapes are nearly spherical the formation of mass anomalies by either endogenic or exogenic processes can change objects moments of inertia therefore the objects reorient to align their largest moment of inertia with their spin axis such a phenomenon is called true polar wander tpw here we report the discovery of a global series of topographic lows on saturn s satellite enceladus that we interpret to show that this synchronously locked moon has undergone tpw by 55 about the tidal axis we use improved topographic data from the spherical harmonic expansion of cassini limb and stereogrammetric measurements to characterize regional topography over the surface of enceladus we identify a group of nearly antipodal basins orthogonal to a basin chain tracing a non equatorial circumglobal belt across enceladus surface we argue that the belt and the antipodal regions are fossil remnants of earlier equator and poles respectively we argue that these lows arise from isostasic compensation and that their pattern reflects spatial variations in internal dynamics of the ice shell our hypothesis is consistent with a variety of geological features visible in cassini images

constraints on cooling and degassing of pumice during plinian volcanic eruptions based on model calculations during explosive volcanic eruptions pumice clasts are transported into the atmosphere and their thermal and degassing histories determine how much volatiles are lost syneruptively this process is studied by combining a steady state eruption column model with a model for the cooling and degassing of pumice in the model we investigate the impact of various parameters e g mass eruption rate biot number eruption temperature and geometry on the cooling and degassing of pumice and find that the biot number and the eruption temperature are the most influential during typical risetimes of pumices inside eruption columns 200 300 s those smaller than 0 5 cm in diameter are found to lose little of their volatiles syneruptively and remain in thermal equilibrium with the plume in the case of larger pumices the ratio of the cooling timescale r p 2 where rp is the pumice radius and is the thermal diffusivity to the degassing timescale w 2 d 0 where w is the vesicle wall thickness and d 0 is the species diffusivity controls degassing if this ratio is larger than 50 degassing of the pumice will be nearly complete with a few percent of the original volatiles left in the outer rind of the pumice for a ratio of less than 0 1 no volatiles escape the pumice typical values for thermal and species diffusivity as well as vesicle wall thickness indicate that in the case of degassing cl and h 2 o the transition from 0 1 to 50 will be in the pumice size range of 1 10 cm here volatile concentration is a strong function of position inside the pumice suggesting that values for volatile contents measured on picked matrix glass from crushed samples may not be the best way to estimate the volatile content of matrix glass a number which is frequently used to estimate the total volatile input of volcanic eruptions this effect is even more pronounced when looking at hydrogen isotopic fractionation with larger pumices tending toward d 80

disturbance stream incision and channel evolution the roles of excess transport capacity and boundary materials in controlling channel response abstract channel incision is part of denudation drainage network development and landscape evolution rejuvenation of fluvial networks by channel incision often leads to further network development and an increase in drainage density as gullies migrate into previously non incised surfaces large anthropogenic disturbances similar to large or catastrophic natural events greatly compress time scales for incision and related processes by creating enormous imbalances between upstream sediment delivery and available transporting power field examples of channel responses to antrhopogenic and natural disturbances are presented for fluvial systems in the mid continent and pacific northwest usa and central italy responses to different types of disturbances are shown to result in similar spatial and temporal trends of incision for vastly different fluvial systems similar disturbances are shown to result in varying relative magnitudes of vertical and lateral widening processes and different channel morphologies as a function of the type of boundary sediments comprising the bed and banks this apparent contradiction is explained through an analysis of temporal adjustments to flow energy shear stress and stream power with time numerical simulations of sand bed channels of varying bank resistance and disturbed by reducing the upstream sediment supply by half show identical adjustments in flow energy and the rate of energy dissipation the processes that dominate adjustment and the ultimate stable geometries however are vastly different depending on the cohesion of the channel banks and the supply of hydraulically controlled sediment sand provided by bank erosion the non linear asymptotic nature of fluvial adjustment to incision caused by channelization or other causes is borne out in similar temporal trends of sediment loads from disturbed systems the sediments emanating from incised channels can represent a large proportion of the total sediment yield from a landscape with erosion from the channel banks generally the dominant source disturbances that effect available force stream power or flow energy or change erosional resistance such that an excess of flow energy occurs can result in incision channel incision therefore can be considered a quintessential feature of dis equilibrated fluvial systems

kinetics of the reaction h2o o 2oh in rhyolitic and albitic glasses preliminary results the kinetics of homogeneous reactions are important in understanding the cooling history of rocks and in understanding experimental speciation data we have experimentally studied the kinetics of the interconversion reaction between h 2o molecules and oh groups in natural rhyolitic glasses 0 5 2 3 total water and a synthetic albitic glass l total water at 400 600 c the reaction rate increases with temperature and total water content equilibrium is not always approached monotonically the speciation may first depart from equilibrium and then come back to equilibrium experimental reaction rates agree with those inferred from previous speciation data of rhyolitic glasses quenched from 850 c the experimental data are modeled successfully by considering both the reaction and the diffusion of oh that brings oh groups together to react this study shows that species concentrations in glasses quenched from 600 c reflect those at experimental temperatures unless the water content is higher than that used in the present study species concentrations in glasses with total water contents 0 8 wt and which were rapidly quenched in water from 850 c do not represent their equilibrium concentrations in the melt at 850 c but record a lower apparent equilibrium temperature that depends on water content and quench rate natural rhyolitic glasses and glass inclusions do not record preeruptive melt speciation though total water content may be conserved the experimental data are used to infer cooling rates for natural obsidian glasses pyroclastic glass fragments from the bb site of mono craters have cooling rates similar to air cooled experimental charges 3 c s different types of glasses from the mono craters have different cooling rates which cover four orders of magnitude some natural obsidians appear to have had complex cooling histories the wide range of cooling rates and thermal histories is consistent with previous inferences that some obsidian clasts at the mono craters formed as glass selvages lining volcanic conduits or dikes that were subsequently caught up in the explosive eruption which led to variable degrees of transient heating followed by rapid cooling and deposition these experimental data reveal surprisingly rich detail in water speciation in volcanic glasses and show how at least in principle quantitative constraints on thermal histories can be extracted by experimentation and application of kinetic models

patterns of holocene environmental change in the midwestern united states abstract four pollen sequences along a transect from north central iowa to southeast wisconsin reveal the distribution of prairie and forest during the holocene and test the use of pollen isopolls in locating the holocene prairie forest border prairie was dominant in central iowa and climate was drier than present from about 8000 to 3000 yr b p during the driest part of this period in central iowa 6500 5500 yr b p mesic forest prevailed in eastern iowa and wisconsin suggesting conditions wetter than at present prairie replaced the mesic forest about 5400 yr b p in eastern iowa but did not extend much farther east mesic forests were replaced in southern wisconsin and northern illinois about 5400 yr b p by xeric oak forests this change from mesic to xeric conditions at 5400 yr b p was widespread and suggests that the intrusion of drier pacific air was blocked by maritime tropical air from the gulf of mexico until the late holocene in this area

reorientation of planets with elastic lithospheres abstract the orientation of a planet is controlled by the positions of the principal axes of the inertia tensor relative to the planetary surface using the theory for the deflection of thin elastic shells the principal axes are computed after emplacement of an arbitrary axisymmetrical load the partial compensation of the load and the partial relaxation of rotational flattening are included in the computation it is found that the amount of reorientation is independent of lithosphere thickness the parameters controlling the amount of reorientation are the location of the load and the size of the load compared to the rotational flattening the results indicate that the tharsis rise has probably reoriented mars by only 3 to 9 and certainly less that 18 the position of the caloris basin on mercury indicates that if the surrounding lava sheet controls the planetary orientation then the lava sheet is probably less than 2000 m thick

enceladus south polar sea recent observations of the south pole of saturn s moon enceladus by the cassini spacecraft have revealed an active world powered by internal heat in this paper we propose that localized subsurface melting on enceladus has produced an internal south polar sea evidence for this localized sea comes from the shape of enceladus which does not match a differentiated body at its current orbital position we show that melting induced by the observed heat flow at the south pole produces a large enough pit to match the shape of enceladus with a differentiated rock and ice interior numerical modeling of melting and ice flow shows that the sea produced beneath the south pole is stable against inflow of ductile ice from its surroundings for the duration of the heating the shape modification due to melting also produces a negative degree two gravity anomaly which can reorient the spin axis of enceladus in order to place the sea at the pole

diapir induced reorientation of saturn s moon enceladus the recent discovery of active water venting from the surface of saturn s icy moon enceladus has caused a ripple of excitement as it suggests the possibility of subsurface liquid water geologically enceladus poses problems one the reason why the hotspot of geological activity is centred precisely on the south pole has been tackled by francis nimmo and robert t pappalardo they identify reorientation of enceladus s rotation axis as the likely cause and an upwelling of low density material to the surface as the cause of that reorientation comparison of the distribution of impact craters on the leading and trailing hemispheres will be a good test of the reorientation part of the theory enceladus is a small icy satellite of saturn its south polar region consists of young tectonically deformed terrain and has an anomalously high heat flux1 2 this heat flux is probably due to localized tidal dissipation within either the ice shell3 or the underlying silicate core4 the surface deformation is plausibly due to upwelling of low density material diapirism5 as a result of this tidal heating here we show that the current polar location of the hotspot can be explained by reorientation of the satellite s rotation axis because of the presence of a low density diapir if the diapir is in the ice shell then the shell must be relatively thick and maintain significant rigidity elastic thickness greater than 0 5 km if the diapir is in the silicate core then enceladus cannot possess a global subsurface ocean because the core must be coupled to the overlying ice for reorientation to occur the reorientation generates large 10 mpa tectonic stress patterns6 that are compatible with the observed deformation of the south polar region2 we predict that the distribution of impact craters on the surface will not show the usual leading hemisphere trailing hemisphere asymmetry a low density diapir also yields a potentially observable negative gravity anomaly

transient two dimensional dynamics in the upper conduit of a rhyolitic eruption a comparison of closure models for the granular stress to elucidate the role of particle collisions in redistributing momentum after fragmentation a numerical study was performed comparing the behavior of an inviscid or collision less granular material with a granular material whose viscosity and pressure were modeled using kinetic theory the granular viscosity calculation is sensitive to particle size and three particle sizes were considered 0 0002 m 0 002 m and 0 02 m a critical volume fraction of gas 0 75 identifies the onset of fragmentation dividing the region of rhyolitic magma with dispersed bubbles from the region of turbulent gas with dispersed particles the transient simulations can be divided into two dynamic regimes an initial shock followed by a transition to steady state during the initial shock phase treatment of the granular pressure and viscosity led to greater particle velocities relative to the inviscid calculation due to the development of higher gas pressure at fragmentation however as steady state is approached the viscosity slows the particle phase relative to the inviscid counterpart for particles greater than a millimeter the modeled sub millimeter particle velocity was insensitive to the treatment of the granular pressure and viscosity centimeter scale particles have a much higher granular viscosity up to 10 1 pa s and the kinetic theory calculation is thus relevant in these conditions after reaching steady state the differential velocity between gas and particles at the conduit exit correlates with particle size for both the inviscid and granular viscosity calculations negligible differential velocity develops for sub millimeter particles but centimeter scale particles exit the conduit with 5 differential velocity between phases

true polar wander of ceres due to heterogeneous crustal density ceres is the largest body in the main asteroid belt it was recently explored by the dawn mission to uncover strong similarities with other icy bodies the morphological features observed on the surface of ceres indicate a relatively wide range of water ice concentrations leading us to investigate the magnitude and distribution of crustal density heterogeneities and to consider whether they could have caused a reorientation of ceres here we present three independent and corroborating lines of evidence for the true polar wander of ceres thanks to the global gravity inversion approach applied to the shape and gravity data of ceres we find crustal density heterogeneities up to approximately 0 3 g cm 3 with a prominent positive density anomaly aligned with the equator in the region of ahuna mons the topography shows the remnants of an equatorial ridge compatible with the position of the palaeo equator and indicates that ceres reoriented by approximately 36 with the palaeo pole following an indirect path to the current pole of ceres the tectonic patterns generated by the true polar wander are in close agreement with the location and orientation of the samhain catenae and uhola catenae crustal fractures these results highlight the complex interior structure and richness of processes taking place in ceres scale icy bodies the dwarf planet ceres may have reoriented in the past due to a heterogeneously dense crust a scenario consistent with gravity and topographic data and the distribution of crustal fractures

impact of euro american settlement on a riparian landscape in northeast iowa midwestem usa an integrated approach based on historical evidence floodplain sediments fossil pollen plant macrofossils and insects european settlement and attendant forest clearance and agricultural activities in northeastern iowa caused changes in the landscape vegetation insect fauna and water quality unequalled in rate and magnitude since the melting of wisconsinan glaciers historical documents show that the upper part of the roberts creek drainage basin was settled between ad 1840 and 1856 and the area was under intensive cultivation by 1880 extensive soil erosion beginning at this time resulted in increased runoff and more frequent flooding aggradation rates increased by one to two orders of magnitude over those in presettlement times and the entire floodplain was covered with up to 1 m of sediment channel widening between about 1880 and 1930 allowed the stream to accommodate greater floods overbank deposition decreased and further deposits were restricted mostly to the channel belt the presettlement vegetation was a stable mix of wet meadows and riparian shrubs on the floodplain a rich aquatic community in the stream