relation between depositional environment and the elemental composition of early diagenetic siderite early diagenetic siderites from marine and fresh water depositional environments are characterized by distinctive compositional trends siderite from fresh water environments is often relatively pure i e greater than 90 mol feco sub 3 and commonly attains end member composition siderite from marine environments however is always extremely impure and has extensive substitution of mg up to 41 mol and to a lesser extent ca up to 15 mol for fe in the siderite lattice in addition marine siderite generally contains less mn and has a higher mg ca ratio than fresh water siderite this compositional variation appears to result from differences in the chemistry of early marine and meteoric pore waters inasmuch as early marine pore waters generally have a higher mg sup 2 ca sup 2 ratio and contain less mn sup 2 and fe sup 2 and more ca sup 2 and mg sup 2 than meteoric waters

ankerite cementation in deeply buried jurassic sandstone reservoirs of the central north sea abstract upper jurassic fulmar formation shelf sandstones of the high temperature high pressure franklin and elgin fields south central graben north sea contain abundant disseminated and concretionary ankerite in contrast most jurassic north sea reservoirs contain only minor amounts of dispersed ankerite disseminated ankerite cement in the franklin and elgin fields has a fairly uniform isotopic composition 18o 10 to 12 5o oo pdb 13c 3 to 5o oo pdb ankerite concretions have 18o values similar to disseminated cements but a wider range of 13c values 1 to 5 5o oo pdb they also have highly variable intergranular volumes which together with the 13c data are interpreted as a combination of pore filling cementation and in situ replacement of comminuted bioclastic debris by ankerite fluid inclusion 18o and paragenetic evidence suggests that ankerite formed during deep burial c 3 5 to 4 5 km 140 170 c after the onset of overpressuring but before hydrocarbon emplacement in the reservoirs the regionally consistent 18o data suggest that ankerite formed via a temperature influenced mechanism and the relatively uniform 13c cement value indicates that organic matter and marine bioclastic carbonate contributed to the dissolved carbon reservoir in constant proportion this can be explained by calcite dissolution in response to ph decrease during thermal breakdown of organic acids such acids were derived from adjacent mudrocks undergoing hydrocarbon maturation and clay mineral transformations and are likely to have been transported in pore fluids with mg2 and fe2 the presence of these cations in solution upon thermal decarboxylation is inferred to have stabilized ankerite at the expense of calcite a relative paucity of ankerite in other fulmar formation reservoirs may reflect different sedimentological compositions less bioclastic debris and or lower burial temperatures less advanced decarboxylation

influences of quaternary climatic changes on processes of soil development on desert loess deposits of the cima volcanic field california summary soils formed in loess are evidence of both relict and buried landscapes developed on pliocene to latest pleistocene basalt flows of the cima volcanic field in the eastern mojave desert california the characteristics of these soils change systematically and as functions of the age and surface morphology of the lava flow four distinct phases of soil development are recognized phase 1 weakly developed soils on flows less than 0 18 m y old phase 2 strongly developed soils with thick argillic horizons on 0 18 0 7 m y old flows phase 3 strongly developed soils with truncated argillic horizons massively impregnated by carbonate on 0 7 to 1 1 m y old flows and phase 4 degraded soils with petrocalcic rubble on pliocene flows a critical aspect of the development of stage 1 soils is the evolution of a vesicular a horizon which profoundly affects the infiltration characteristics of the loess parent materials laboratory studies show that secondary gypsum and possibly other salt accumulation probably occurred during the period of phase 1 soil development slight reddening of the interiors of peds from vesicular a horizons of phase 1 soils and presence of weakly developed b horizons indicates a slight degree of in situ chemical alteration however clay and fe oxide contents of these soils show that these constituents as well as carbonates and soluble salts are incorporated as eolian dust in contrast to phase 1 soils chemical and mineralogical analysis of argillic horizons of phase 2 soils indicate proportionally greater degrees of in situ chemical alteration these data the abundant clay films and the strong reddening in the thick argillic horizons suggest that phase 2 and phase 3 soils formed during long periods of time and periodically were subjected to leaching regimes more intense than those that now exist flow age data and soil stratigraphic evidence also indicate that several major loess deposition events occurred during the past 1 0 m y loess events are attributed to past changes in climate such as the pleistocene to holocene climatic change that periodically caused regional desiccation of pluvial lakes reduction of vegetational density and exposure of loose unconsolidated fine materials during times of warmer interglacial climates precipitation infiltrates to shallower depths than during glacial periods extensive saline playas which developed in the mojave desert during the holocene are a likely source of much of the carbonates and soluble salts that are accumulating at shallow depths both in phase 1 soils and in the formerly noncalcareous nongypsiferous argillic horizons of phase 2 and 3 soils

desert pavements and associated rock varnish in the mojave desert how old can they be desert pavements are common features of arid landscapes and have been widely used as a relative age indicator of the geomorphic surfaces upon which they are developed in this study i examined the patterns of pavement development as a function of elevation in the mojave desert as well as the causes for the gradual disappearance of pavement at high elevations pavement density as measured by percentage of pebble coverage decreases systematically with elevation gain by 3 per 100 m from 95 coverage below 500 m to less than 60 at 1700 m plants appear to be the main agent of pavement disruption plant density decreases as pavement density increases burrowing by rodents and crusting by cryptobiota also disrupt pavement development at higher elevation during the last glacial maximum plant communities were displaced 1000 1400 m downward in the mojave desert pavements today generally do not survive above the blackbush coleogyne ramossisma sagebrush artemisia tridentata zone evidence from packrat middens shows that these and other plants typical of high elevations today grew as low as 300 400 m during the last glacial maximum i suggest that during the last glacial maximum desert pavements were confined to the lowest alluvial fans of death valley and adjoining low valleys no alluvial desert pavements above 400 m in the region are older than the latest pleistocene by the same reasoning desert varnish on desert pavements above 400 m may all be holocene in age except where developed on stable boulders

a tectonic boundary for the southern colorado plateau abstract a tectonic boundary should be defined by changes in tectonic elements tectonic elements would include such parameters as structural style stress orientations volcanism heat flow seismicity and changes in crustal thickness examination of these tectonic elements for the southern colorado plateau suggests that the southwestern part of the physiographic plateau appears to be tectonically part of the basin and range province i therefore propose a boundary at 112 w longitude the locus of seismicity and recent volcanism and also the site of crustal thickening and a change of structural style the tectonic boundary of the southern colorado plateau is suggested to form part of the eastern boundary of the southern basin and range seismotectonic region this seismotectonic region is bounded on the north by the las vegas shear zone nevada seismic zone and has a different stress orientation than the northern basin and range from which it began to evolve separately 13 m y ago the present principal stress orientation of the southern basin and range of arizona appears to be the result of compound movements of the san andreas transform and garlock las vegas shear zone resulting in a shear couple affecting the crust in arizona and an associated ne sw least principal stress orientation

cosmic ray labeling of erosion surfaces in situ nuclide production rates and erosion models a number of in situ cosmogenic radionuclides and stable nuclides have been measured in natural exposed rock surfaces with a view to study their in situ production and rock erosion rates 1 the in situ radionuclides can be used for a high resolution tomography of the erosional history of an exposed surface two stable nuclides 3he 21ne and five radionuclides 10be 26al 36cl 14c 39ar having half lives in the range of 300 1 5 106 yr half life are measurable in many rock types a prerequisite for the application of the in situ nuclides for the study of erosional histories of surfaces is a knowledge of their production rates under different irradiation conditions altitude latitude irradiation geometry and shielding relative nuclide production rates can be determined fairly accurately using the extensive available data on cosmic ray neutrons 2 absolute nuclide production rates cannot generally be predicted with any accuracy because of lack of data on excitation functions of nuclides unless some normalization is possible as was done in the case of several cosmic ray produced isotopes in the atmosphere 3 based on a recent natural calibration experiment in which erosion free surfaces exposed to cosmic radiation for 11 000 yrs were sampled the absolute production rates of 10be and 26al in quartz have been accurately estimated for mountain altitudes in sierra nevada 4 the absolute production rates of 10be and 26al in quartz can therefore be estimated fairly accurately for any given latitude and altitude some measurements of 14c in rocks of low erosion rate 5 similarly allow an estimate of its production rate attempts made to measure the in situ production rates of 3he in rocks have not yet led to a convergent production rate in view of the importance of knowing the production rates of isotopes of he ne and ar i present here theoretical estimates of their production rates based on available cross section data i discuss the information that can be extracted from the study of the in situ nuclides in rocks useful parameters characterizing the exposure history of a rock surface are 1 the effective surface exposure age and 2 the time averaged erosion rate the implications of these parameters for single and multiple nuclide studies are discussed in terms of the erosion models considered

cosmogenic 3he production rates from 39 n to 46 n latitude western usa and france the in situ production rate of cosmogenic 3he was determined in seven different olivine or pyroxene bearing rocks between the latitudes 39 n and 46 n the integrated production rate over the last 17 500 years is about 115 4 atoms gram yr for high latitude at sea level the variation in the integrated production rate between 2000 and 18 000 years is less than 30 and is probably on the order of 20 or less

variability in categories of arctic sea ice in fram strait an attempt to quantify the temporal variability in the volume composition of arctic sea ice is presented categories of sea ice in the transpolar drift in fram strait are derived from monthly ice thickness distributions obtained by moored sonars 1990 2011 the inflection points on each side of the old ice modal peak are used to separate modal ice from ice which is thinner and thicker than ice in the modal range the volume composition is then quantified through the relative amount of ice belonging to each of the three categories thin modal and thick ice in the monthly ice thickness distributions the trend of thin ice was estimated to be negative at 8 8 per decade relative to the long term mean which was compensated for by increasing trends in modal and thick ice of 7 9 and 4 7 per decade respectively a 7 8 year cycle is apparent in the thin and thick ice records which may explain a loss of deformed ice since 2007 we also quantify how the categories contribute to the mean ice thickness over time thick predominantly deformed ice dominates the mean ice thickness constituting on average 66 of the total mean following the loss of deformed ice since 2007 the contribution of thick ice to the mean decreased from 75 to 52 at the end of the record thin deformed ice did not contribute to this reduction it was pressure ridges thicker than 5 m that were lost and hence caused the decrease in mean ice thickness

effects of keels on ice bottom turbulence exchange 1 the effects of ice keels on the upper ocean are examined using a combination of turbulence measurements and output from a large eddy simulation les model two cases are examined one during the winter when the under ice boundary layer is relatively deep 20 m and near the freezing point and a second during the summer when the ice is melting and the boundary layer consists of a shallow 0 5 m highly stratified fresh layer in the winter case measurements show that flow disruption by a 10 m deep keel causes enhanced vertical mixing increasing the heat flux from a background value of 5 w m 2 to values averaging 25 w m 2 simulations using the les model are in good agreement with the measurements and indicate that the keel generates a turbulent wake region extending hundreds of meters downstream from the keel elevated heat fluxes in the wake region are generated by increased entrainment of warmer water from beneath the mixed layer simulations of summer cases demonstrate that shallow keels 0 5 m generate strong turbulence that is able to rapidly mix the fresh layer in the lee of keels however this effect decreases quickly as the fresh layer accelerates to match the ice velocity deeper keels 1 m follow a similar pattern but generate more mixing as the fresh layer is forced under the keel simulated ice melt heat fluxes are similar to estimates made from ice balance measurements taken during the surface heat budget of the arctic ocean summer field program

turbulent stress at the ice ocean interface and bottom surface hydraulic roughness during the sheba drift 1 upper ocean turbulence and mean current data from the 1997 1998 surface heat budget of the arctic ocean sheba drift in the western arctic ocean were used to estimate turbulent shear stress at the ice ocean interface and hydraulic roughness of the ice undersurface techniques for determining interfacial stress from velocity covariance measurements in the ice ocean boundary layer iobl are complicated by buoyancy flux and the heterogeneity of the ice undersurface where protrusions corresponding to relatively small upper surface features become major obstacles when compared to the scale of the iobl in addition ice deformation forced relocation of the main sheba oceanographic program about midway through the experiment hence there were two different measurement sites to consider three different methods were used to compute the undersurface roughness characterizing the undeformed multiyear floe occupied by the sheba project the first was straightforward naive application of the law of the wall to turbulent stress and mean velocity measurements from the turbulence instrument cluster nearest to the interface assuming a logarithmic velocity profile and constant stress for the second method local turbulence closure was used to model the entire iobl in order to estimate the impact of coriolis stress attenuation and buoyancy forces on stress and shear near the boundary a third method proposed here for the first time estimated mixing length and eddy viscosity from a scale inversely proportional to the wave number at the peak of the weighted vertical velocity spectrum the last method appears most robust in that it shows no overall difference in z0 between the two sites and only minor seasonal and directional differences the best estimate for the undersurface roughness of undeformed multiyear ice at the sheba site is 0 0048 m z0 0 007 i e about 6 mm specification of a regional aggregate under ice roughness requires additional consideration of the added drag from isolated pressure keels and floe edges along with reduced drag from smoother newly frozen ice and open water

laboratory measurements of the drag force on a family of two dimensional ice keel models in a two layer flow abstract a review of the upper structure of the arctic ocean and its overlying ice cover suggests that significant potential exists for the generation of internal wave fields by the deeper drafts of the pressure ridge keels laboratory measurements are presented of the drag force on two dimensional ice keel models of varying degrees of slenderness in both homogeneous and two layer fluid systems the experiments were performed in a towing tank 10 m long and covered the range from subcritical to fully supercritical flows mw results show that the two layer stratification increases the drag dramatically over that in a corresponding homogeneous flow reaching a maximum in the transcritical flow regime as a result of the establishment of a system of internal waves the increase in drag was greatest for the most slender obstacle approaching the values to be expected from simple hydraulic theory for the more steeply sloped obstacles the wave growth is limited by dispersion with a corresponding reduction in t

cosmogenic 3he exposure ages of pleistocene debris flows and desert pavements in capitol reef national park utah abstract the quaternary history of the capitol reef area utah is closely linked to the basaltic andesite boulder deposits that cover much of the landscape understanding the age and mode of emplacement of these deposits is crucial to deciphering the quaternary evolution of this part of the colorado plateau using cosmogenic 3 he exposure age dating we obtained apparent exposure ages for several key deposits in the capitol reef area coarse boulder diamicts capping the johnson mesa and carcass creek terraces are not associated with the bull lake glaciation as previously thought but were deposited 180 15 to 205 17 ka minimum age and are the result of debris flow deposition desert pavements on the johnson mesa surface give exposure ranging from 97 8 to 159 14 ka and are 34 96 kyears younger than the boulder exposure ages the offset between the boulder and pavement exposure ages appears to be related to a delay in pavement formation until the penultimate glacial interglacial transition or periodic burial and exposure of pavement clasts since debris flow deposition incision rates for the capitol reef reach of the fremont river calculated from the boulder exposure ages range from 0 40 to 0 43 m kyear 1 maximum rates and are some of the highest on the colorado plateau

high resolution spectrometric analysis of rare earth elements activated cathodoluminescence in feldspar minerals abstract cathodoluminescence cl investigations of igneous metamorphic and sedimentary feldspars indicate that rare earth elements ree activated cl in feldspars is more common than previously assumed hot cathode cl microscopy combined with high resolution spectrometric analysis of cl emission allow to detect some ree below the detection limits of electron microprobe and proton induced x ray emission analysis pixe and reveal variations in the ree distribution within single feldspar crystals differently luminescing zones can reflect changes during feldspar crystallization and or element fluctuations during secondary alteration processes which are not discernible using conventional polarizing microscopy the results of the study document eu 2 sm 3 dy 3 tb 3 and nd 3 activated cl in feldspars of different origin the influence of the crystal field on shape and position of ree luminescence spectra significantly differs for divalent and trivalent ree ions whereas eu 2 shows a broad band emission 420 nm which is influenced by the local crystal field trivalent ions of the rare earth show narrow emission lines which reflect the transitions between excited state wave functions lying inside closed electronic shells the positions of these peaks and the characteristic energies are described for the different ree 3

the isotopic composition of radiogenic helium and its use to study groundwater movement in confined aquifers abstract the 3 he 4 he ratio for radiogenic he is shown to be primarily determined by the abundance of li and neutron absorbing elements in the host formations and is almost independent of the radioelement abundance the 3 he 4 he ratios have been computed for various li contents in rock matrices with average abundances of other elements the 3 he 4 he ratio is specific for each formation and may therefore be used to study crustal diffusion of he it is shown however that both the 3he and 4he contents of continental basements are likely to exceed those of the mantle so that diffusion of mantle he through continental crust is not likely to occur changes in the 3 he 4 he ratio for dissolved he have potential for studying groundwater movement and mixing processes atmospheric he which was dissolved at recharge is progressively diluted by radiogenic he with an isotopic ratio characteristic of the aquifer as the groundwater migrates down dip mixing proportions for formation he and atmospheric he in aquifers for which there is admixture of fracture borne water and interstitially stored water can also be estimated from isotopic measurements these processes are discussed in relation to two aquifers in eastern england

microseismicity of the kaibab plateau northern arizona and its tectonic implications abstract a microearthquake investigation of the kaibab plateau in northern arizona revealed swarms of small to very small magnitude earthquakes a 12 station network of portable seismographs recorded 296 events with magnitudes from 0 5 m l t axes from these fault plane solutions yields an average least principal horizontal stress direction of ene wsw the inferred orientation of principal stresses found in this study indicates the kaibab plateau lies within a region of predominantly normal faulting which extends from central arizona northward into southwestern utah although data are sparse in the region surrounding the kaibab plateau evidence for the location of a basin range colorado plateau stress boundary through the kaibab plateau is not supported by the results of this study

burial dolomitization and porosity development in a mixed carbonate clastic sequence an example from the bowland basin northern england widespread dolomitization and leaching occur in the asbian to brigantian dinantian sequence of the bowland basin within this mudrock dominated succession dolomite is developed in calcarenites and limestone breccia conglomerates deposited in a carbonate slope environment pendleside limestone and also within graded quartz wackes deposited by density currents in a generally starved basin environment pendleside sandstone the dolomitized intervals range in thickness from less than one metre to several tens of metres and have a stratabound nature all stages of calcite cement pre date dolomitization and calcite veins are dolomitized dolomite crystals replace neomorphic spar and may also contain insoluble residues that were concentrated along stylolites thus dolomitization was a late stage process within the carbonate diagenetic sequence a late stage diagenetic origin is also indicated within the sandstones with dolomite post dating the development of quartz overgrowths six main textural styles of dolomite are observed 1 scattered 2 mosaic 3 subhedral to euhedral rhombic 4 microcrystalline 5 single crystal and 6 saddle the style of dolomite developed is dependent on the host rock mineralogy on whether it is space filling or replacive and also on temperature chemically the dolomite varies from near stoichiometric compositions to ankeritic varieties containing up to 20 mole feco3 generally the dolomites have isotopic compositions depleted in 18o compared to the host limestone with similar or lighter 13c values initial dolomite was of the scattered type but with progressive replacement of the host a mosaic dolostone with a sucrosic texture was produced there was a general increase in the fe and mn content and reduction in 18o ratio of the crystals during dolomitization leaching is restricted to partly dolomitized horizons where calcite feldspars micas clays and to some extent dolomite have been leached this has produced biomouldic and vuggy secondary porosity within the carbonates whereas in the sandstones honeycombed corroded and floating grains associated with oversized pores occur porosity within both carbonates and sandstones is reduced by ferroan dolomite ankerite cements field petrographic and chemical characteristics indicate that dolomitizing solutions were predominantly derived from the enclosing mudrocks bowland shales during intermediate deep burial fluid migration out of the mudrocks would have been sided by dehydration reactions and overpressure the fluids migrating along the most permeable horizons the coarse grained carbonates and sandstones that are now dolomitized and contain secondary porosity

development of seasonal pack ice in the beaufort sea during the winter of 1991 1992 a view from below subsea sonars moored in the beaufort sea acquired a spatial section of draft across 941 km of sea ice during the winter of 1991 1992 these observations document the development of seasonal sea ice from an open sea surface in october to an average accumulation exceeding 2 5 m draft by early april initially level ice occupied 85 of the profile but continued ridging reduced this fraction to 50 in late winter three modes of level ice were tracked that of seasonal ice whose growth was initiated at freeze up and two others created during openings of the ice field in january and in march the growth of these modes can be closely matched by calculations based on a slab model of ice formation forced using surface meteorological data initially ridge keels were small and widely spaced by midwinter an exponential dependence of keel frequency on draft was obvious e folding scale 2 16 m and the frequency and mean draft of keels had stabilized the maximum keel draft was a modest 17 4 m the low incidence of deep keels at all times is a statistically significant departure from the exponential dependence valid at lesser drafts the truncation point of the exponential relation is related to the draft of the thinnest level ice present in late winter the formation of deeper keels from thick first year ice was apparently precluded by the presence of younger thinner ice which limited the force available for ridge building through calculation of the seaward transport of ice over the sonars the total production of ice in the coastal flaw lead during 1991 1992 was determined to be about 60 of earlier indirect estimates in general the observations revealed winter ice conditions significantly less severe than those found on the periphery of the polar pack only a few tens of kilometers to the north but more so than conditions in a marginal first year ice zone at the same latitude in davis strait

cosmogenic 36cl and 10be ages of quaternary glacial and fluvial deposits of the wind river range wyoming we measured cosmogenic 36 cl in 56 samples from boulders on moraines and fluvial terraces in the vicinity of the wind river range wyoming we also measured 10 be in 10 of the same samples most of the 10 be ages were in good agreement with the 36 cl ages indicating that rock surface erosion rates were very low the oldest moraine investigated the type sacagewea ridge site yielded only a limiting minimum age of 232 ka the oldest moraines in the type bull lake complex also could be constrained only to 130 ka the main sequence of type bull lake moraines yielded age distributions indicating deposition within the intervals 130 to 100 ka and 120 to 100 ka the best estimates are closer to the upper limits of these ranges and associated uncertainties are in the range of 10 to 15 these uncertainties could permit deposition in either marine isotope stage 6 or stage 5d we found no evidence of glacial deposits dating to marine isotope stage 4 both bull lake age moraines from fremont lake on the opposite side of the wind river range and boulders on a fluvial terrace above the wind river gave age distributions very similar to that of the second oldest bull lake advance ca 130 to 100 ka the distribution of boulder ages for pinedale moraines at bull lake indicated deposition between 23 and 16 ka nearly identical to the distribution of 10 be ages previously reported for the type pinedale moraines at fremont lake

pedimentation versus debris flow origin of plateau side desert terraces in southern utah plateau side terraces in arid areas around the world are commonly described as pediment remnants although in many cases they may have been formed by debris flow deposition pediments do exist in the area of the aquarius and kaiparowits plateaus of southern utah however many alluvial terraces that were classified by previous workers as pediments are actually formed of thick deposits of sediment released into valleys by episodic landslide events pedimentation may have implications of base level stability and aridity that are very different from the implications of landsliding so that the identification of pediments where none exist as has occurred for some places in the aquarius and kaiparowits areas may result in erroneous interpretations of geologic history although both pediment and debris flow depositional terraces have smooth alluvium covered upper surfaces the two can be distinguished by the shape of the underlying bedrock surface and the process of formation the distinction is that pedim

the effect of structural porosity on the ablation of sea ice ridges 1 observations reveal that the decrease in ice thickness through melting in summer is much more rapid for ridges than for surrounding level ice a physical model that represents internal melting within ridge keels has been developed to explain this observed draft dependent ablation for first year pack ice in the beaufort sea the porous structure of a ridge keel permits percolation of a substantial fraction of the oncoming oceanic flow up to 20 for a feature with 30 porosity and 9 m draft the percolating flow delivers oceanic heat to a large surface area deep within the keel and increases melt rates relative to surrounding level ice by a factor of 5 when seawater temperatures are 0 18 degrees above freezing melt rates are sensitive to the internal geometry of ridges through keel porosity and block dimensions characteristics that vary widely between ridge features however the average rate of melting as a function of draft calculated for a realistic population of keels with average cross sectional shape and differing draft has the same draft dependence as the observations this concurrence suggests that the process of internal melting may be dominant in the ablation of ridged ice in addition internal melting during the summer may well hasten structural consolidation of surviving ridge keels through freezing during the following winter it appears that the evolution of the thickest ice within the arctic ice pack is dependent on the small scale structural characteristics of the ridged ice and its interaction with the upper layer of the ocean

the influence of calcite dissolution on reservoir porosity in miocene sandstones picaroon field offshore texas gulf coast abstract middle miocene sandstones occur in picaroon field corsair trend offshore texas gulf coast at depths of approximately 4 9 to 5 2 km 16 17 000 ft these deltaic sandstones contain evidence of the following sequence of diagenetic events 1 formation of chlorite coatings on detrital grains 2 partial dissolution of detrital feldspar 3 quartz cementation 4 calcite cementation 5 dissolution of calcite cement and grains 6 ankerite cementation the reservoir quality of the sandstones is largely a function of porosity enhancement by calcite dissolution calcite cement was emplaced at depths of approximately 1 8 to 2 6 kin the calcite has 87 86sr values of 0 7083 0 7086 eliminating coeval 15 myr seawater and marine carbonate 0 70873 0 70885 as the primary source of strontium the 87 86sr composition of calcite cement implies that mass transfer of calcite from older marine sources to younger sediments has occurred fluid inclusion measurements indicate that ankerite cement formed at minimum temperatures of 120 188 c 3 0 5 1 km oxygen isotope modeling predicts that at these depths shales would expel waters with 18osmow of 5 to 9 during smectite illite conversion ankerite 18opdb 7 8 would be in isotopic equilibrium with these waters at temperatures similar to those derived from fluid inclusions ankerite cements have relatively radiogenic 87 86sr ratios 0 7097 which are consistent with their formation from shale derived fluids calcite dissolution occurs between the precipitation of calcite and ankerite it is therefore concluded that calcite cement dissolution occurred at burial depths of 2 6 to 3 0 km 107 120 c

dating geomorphologic surfaces using cosmogenic 3he cosmogenically produced 3he in rocks can be used to date geomorphologic surfaces using the late pleistocene bonneville flood event the subsequent development of the provo shoreline and associated volcanic rocks a production rate of 432 atoms g yr in olivine is determined for 38 56 n latitude present geomagnetic latitude 46 5 at 1445 m altitude integrated over the last 14 400 years measurements of cosmogenic helium in flood deposits on river scoured surfaces and lava flows show that it is possible to date directly large scale events such as volcanic eruptions and catastrophic floods olivine and pyroxene are both satisfactory minerals for cosmogenic dating by the 3he method however in most cases plagioclase and quartz are unsuitable because they do not quantitatively retain helium isotopes in their lattices

cosmogenic noble gas studies in the oldest landscape on earth surface exposure ages of the dry valleys antarctica extraordinarily high surface exposure ages have been determined for sirius group tillites of mt fleming and mt feather as well as at localities in the inner dry valleys using cosmogenic helium and neon ages of 10 ma at mt fleming 5 3 ma at mt feather and 6 5 ma at insel mountain are among the highest nominal exposure ages published so far these values are minimal ages as they are based on the assumption of zero erosion and uplift the mt feather sample independently confirms the pre pliocene age of the sirius group sediments in the dry valleys as previously determined at mt fleming the insel mountain samples provide evidence for a landscape formation of the inner dry valleys not later than late miocene time assuming conservatively low values of 2 5 cm ma 1 for erosion rate and 50 m ma 1 for uplift rate we infer that the sirius group tillites at mt fleming were deposited earlier than 20 ma ago this indicates that the overriding of the dry valleys block of the transantarctic mountains by the east antarctic ice sheet occurred not later than the early miocene maximum long term erosion rates in the inner dry valleys must be 15 cm ma 1 down to altitudes 1000 m since such low erosion rates require permanently cold and hyperarid conditions the response of antarctica to the pliocene warm climatic episode must have been small cosmogenic nuclide data from both the inner dry valleys and the sirius group sediment localities support the hypothesis of a stable east antarctic ice sheet since at least late miocene time implying that the climate of antarctica was decoupled from that of lower southern latitudes we present also new elemental 21ne production rates of p21 mg 196 atoms g 1 yr 1 and p21 al 55 atoms g 1 yr 1 at sea level and high geomagnetic latitude these figures are consistent with a 3he production rate of p3 110 atoms g 1 yr 1 similar to previously published values this consistency provides evidence that pyroxene is retentive for both helium and neon over at least 10 ma cosmogenic ne in quartz and pyroxene has a 22ne 21ne cos ratio of 1 266 0 040 and 1 159 0 040 respectively

cosmic ray exposure dating with in situ produced cosmogenic 3he results from young hawaiian lava flows in an effort to determine the in situ production rate of spallation produced cosmogenic 3 he and evaluate its use as a surface exposure chronometer we have measured cosmogenic helium contents in a suite of hawaiian radiocarbon dated lava flows the lava flows ranging in age from 600 to 13 000 years were collected from hualalai and mauna loa volcanoes on the island of hawaii because cosmic ray surface exposure dating requires the complete absence of erosion or soil cover these lava flows were selected specifically for this purpose the 3 he production rate measured within olivine phenocrysts was found to vary significantly ranging from 47 to 150 atoms g 1 yr 1 normalized to sea level although there is considerable scatter in the data the samples younger than 10 000 years are well preserved and exposed and the production rate variations are therefore not related to erosion or soil cover data averaged over the past 2000 years indicate a sea level 3 he production rate of 125 30atoms g 1 yr 1 which agrees well with previous estimates the longer record suggests a minimum in sea level normalized 3 he production rate between 2000 and 7000 years 55 15atoms g 1 yr 1 as compared to samples younger than 2000 years 125 30 atoms g 1 yr 1 and those between 7000 and 10 000 years 127 19atoms g 1 yr 1 the minimum in production rate is similar in age to that which would be produced by variations in geomagnetic field strength as indicated by archeomagnetic data however the production rate variations a factor of 2 3 0 8 are poorly determined due to the large uncertainties in the youngest samples and questions of surface preservation for the older samples calculations using the atmospheric production model of o brien 1979 35 and the method of lal and peters 1967 11 predict smaller production rate variations for similar variation in dipole moment a factor of 1 15 1 65 because the production rate variations archeomagnetic data and theoretical estimates are not well determined at present the relationship between dipole moment and production rate will require further study precise determination of the production rate is an important uncertainty in the surface exposure technique but the data demonstrate that it is feasible to date samples as young as 600 years of age providing that there has been no erosion or soil cover therefore the technique will have important applications for volcanology glacial geology geomorphology and archaeology

meteoric phreatic diagenesis in cyclic late dinantian carbonates northwest england abstract luminescent non luminescent zoned syntaxial overgrowths in carbonate grainstones of late asbian late dinantian age of the southern lake district represent precipitation in barely saturated relatively shallow 15 120 m meteoric phreatic pore waters the zonation reflects repeated establishment of meteoric lenses in response to eustatic effects within cyclic carbonate platform sediments isolated from sources of laterally fed groundwaters geochemistry and petrology suggest that there was a consistent positive covariance between eh of pore waters and precipitation of calcite during establishment of these lenses such that cements only precipitated from suboxic and oxic meteroic waters mixing zone and stagnant marine phreatic diagenetic environments are thought to have been sites of non precipitation to slight dissolution cement cycles can be correlated up to 30 km across the platform and decrease in number in a predictable fashion up through the sequence the stratigraphic resolution they offer within this platform is more sensitive than that of microfossils in the same sequence cementation patterns are more complex in the outer 5 km of the platform and increase in complexity basinwards reflecting the influences of lowstand sea levels locally developed erosional and depositional slopes lateral components within meteoric flow patterns rapid lateral facies variations and possible trace element partitioning the base of the meteoric lenses from which these cements precipitated was controlled by the presence of a shale aquitard this style of cementation ceased after an early brigantian platform drowning event

modeling thermodynamic ice ocean interactions at the base of an ice shelf abstract models of ocean circulation beneath ice shelves are driven primarily by the heat and freshwater fluxes that are associated with phase changes at the ice ocean boundary their behavior is therefore closely linked to the mathematical description of the interaction between ice and ocean that is included in the code an hierarchy of formulations that could be used to describe this interaction is presented the main difference between them is the treatment of turbulent transfer within the oceanic boundary layer the computed response to various levels of thermal driving and turbulent agitation in the mixed layer is discussed as is the effect of various treatments of the conductive heat flux into the ice shelf the performance of the different formulations that have been used in models of sub ice shelf circulation is assessed in comparison with observations of the turbulent heat flux beneath sea ice formulations that include an explicit parameterization of the oceanic boundary layer give results that

using in situ produced cosmogenic isotopes to estimate rates of landscape evolution a review from the geomorphic perspective the application of in situ produced cosmogenic isotopes to problems in geomorphology has increased rapidly over the past decade at least 57 papers and numerous abstracts have been published since the mid 1980s when the first mass spectrometric measurements of terrestrially produced cosmogenic isotopes were made taken at face value these studies provide quantitative information about rates of landscape evolution and landform age however the significance of calculated erosion rates and exposure ages depends strongly on the models used to interpret isotopic data the validity of assumptions inherent to these models and the geologic surroundings in which the samples were collected this paper attempts to place cosmogenic isotope studies in a geomorphic context by reviewing fundamentals of the method and evaluating the validity of assumptions under which these data have been interpreted at present the establishment of high precision cosmogenically based glacial and alluvial chronologies is stymied by the evolution of geomorphic surfaces the erosion of rock from sampled boulders the potential for isotope inheritance from previous exposure and the uncertainty of isotopic measurements uncertainties in isotope production rates and the observed variability of exposure ages on individual geomorphic surfaces limit the confidence with which cosmogenic ages can be correlated reliably with those obtained by other techniques estimation of erosion rates at single points on the landscape gives useful small scale information extrapolation of these rates over longer time and larger spatial scales is less sure and most likely biased toward lower erosion rates by die inadvertent selection of resistant sample sites however because erosion rates are so poorly constrained at present even estimates to within a factor of 2 may be of significant value to geomorphologists and tectonicists

middle and late wisconsin paleobotanic and paleoclimatic records from the southern colorado plateau usa the colorado plateau is a distinct physiographic province in western north america which presently straddles the transition between summer wet and summer dry climatic regimes to the south and northwest respectively in addition to climate the diversity of environments and plant communities on the colorado plateau has resulted from extreme topographic diversity desert lowlands as low as 360 m elevation are surrounded by forested plateaus and even higher peaks greater than 3800 m elevation this environmental diversity provides a unique opportunity to study the history of biotic communities in an arid region of north america although the colorado plateau harbours numerous potential sites the paleoecological record of the plateau is poorly known potential deposits for analysis include packrat middens alluvial and cave sites at lower elevations and lake bog and wetland sites at higher elevations forty six sites have been analysed across the nearly 337 000 km2 region of which 27 contain records that span marine oxygen isotope stage is 2 data with is 3 information coming from only 12 sites most is 2 and 3 sites are clustered along the lowland regions of the colorado river corridor and the uplands of the mogollon rim area we compiled selected data from long paleoecological records to examine patterns of vegetation and climate change across the southern colorado plateau for the middle and late wisconsin during the middle wisconsin mixed conifers covered middle elevations presently dominated by ponderosa pine pinus ponderosa and juniper juniperus woodland grew at elevations today covered by blackbrush coleogyne and sagebrush artemisia desert during the late wisconsin boreal conifers primarily engelmann spruce picea engelmannii replaced the mixed conifer association estimates of mean annual temperatures mat during is 3 were at least 3 4 c cooler than today whereas is 2 mat estimates are at least 5 c colder our investigation of millennial scale climatic variability within the region provided equivocal results the packrat midden sequence could not distinguish vegetation changes that might be associated with heinrich events in the north atlantic from the lake records however many heinrich events were associated with generally drier intervals often with elevated sagebrush pollen concentrations future paleoecological investigations should concentrate on the northern colorado plateau as well as the eastern and western margins additional sites along with closer spaced sampling in regions already studied will be important in determining the history of important climatic phenomena such as the timing of the arizona monsoon

duration and structure of the past four interglaciations reanalysis and additional dating of the devils hole 18o paleotemperature record confirm that the last interglaciation in the great basin the continental analog of marine isotopic substage 5e lasted 22 000 yr consistent with the vostok paleotemperature record which suggests a duration of 19 000 yr for this event in antarctica the three preceding interglaciations in the devils hole record analogs of marine isotopic substages 7e 9c and 11c range from 20 000 to 26 000 yr in duration a 20 000 yr duration for the last interglaciation is consistent with tims uranium series dated sea level high stands thus the widely held view that interglaciations were of 11 000 to 13 000 yr duration and constituted only about 10 of mid to late pleistocene climatic cycles needs reexamination the warmest portion of each interglaciation in the devils hole time series is marked by a 18o plateau signifying apparent climatic stability for periods of 10 000 to 15 000 yr duration

cosmogenic 3he ages and frequency of late holocene debris flows from prospect canyon grand canyon usa abstract lava falls rapid which was created and is maintained by debris flows from prospect canyon is the most formidable reach of whitewater on the colorado river in grand canyon and is one of the most famous rapids in the world debris flows enter the colorado river at tributary junctures creating rapids the frequency of debris flows is an important consideration when management of regulated rivers involves maintenance of channel morphology we used cosmogenic 3 he 14 c and historical photographs to date 12 late holocene and historic debris flows from prospect canyon the highest and oldest deposits from debris flows on the debris fan yielded a 3 he date of about 3 ka which indicates predominately late holocene aggradation of one of the largest debris fans in grand canyon the deposit which has a 25 m escarpment caused by river reworking crossed the colorado river and raised its base level by 30 m for an indeterminate although likely short period we mapped depositional surfaces of 11 debris flows that occurred after 3 ka two deposits inset against the highest deposit yielded 3 he ages of about 2 2 ka and at least two others followed shortly afterwards at least one of these debris flows also dammed the colorado river the most recent prehistoric debris flow occurred no more than 0 5 ka the largest historic debris flow which constricted the river by 80 occurred in 1939 five other debris flows occurred after 1939 these debris flows constricted the colorado river by 35 80 assuming the depositional volumes of late holocene debris flows can be modeled using a lognormal distribution we calculated recurrence intervals of 15 to more than 2000 years for debris flows from prospect canyon

genetic relationship between faults and folds and determination of laramide and neotectonic paleostress western colorado plateau transition zone central utah faults in the western part of the colorado plateaus and the adjacent eastern part of the structural transition zone to the basin and range in central utah represent two stages of deformation laramide compressional faults on the plateau eastern side and late cenozoic extensional faults on the transition zone western side the faults range in displacement from a few centimeters to several hundred meters this study of minor structures provides strong evidence for regional ne sw horizontal maximum compressive stress as a dynamic component of monoclinal flexure the results further suggest that it is inaccurate to interpret the monoclines simply as passive drape folds formed during vertical block up lift the laramide faults on the plateau side cut two types of large scale folds 1 a broad warp called the teasdale anticline and 2 two monoclines that flank the teasdale anticline called the waterpocket and cocks comb monoclines the faults that cut the monoclines dip steeply and have mostly strike slip displacements strike azimuths of dextral and sinistral faults tend to occupy separate sectors suggesting that they are conjugate shears activated in a stress field with a subhorizontal maximum compressive stress 1 oriented northeast southwest this orientation is confirmed by computations of mean paleostress characteristics by inversion of fault slip data the monoclines differ from one another in trend by 30 so it is possible to compare these trends with the results of separate paleostress computations for faults from each monocline four separate computed 1 axes three from the cocks comb monocline are approximately horizontal and cluster within 10 of the n 65 e normal to the axial trace of the waterpocket monocline suggesting that the waterpocket at the latitude of our study formed approximately perpendicular to the local laramide compressive stress direction the maximum compressive stress 1 is located clockwise about 30 from the normal to the cocks comb monocline we interpret this discordance as evidence that the cocks comb trend was influenced by and in large part inherited from the trend of an ancient buried fault rooted in the basement whereas the exposed faults from which 1 was computed are structures that formed in and are probably restricted to the sedimentary cover rocks the exposed faults probably formed fresh in the cover rocks during laramide compression and are generally more reliable indicators of compressional kinematics than fold orientations part of the cocks comb monocline shows a 13 distortion of its axial trace by cumulative dextral slip on hundreds of small displacement transverse faults we interpret this distortion as a late stage brittle compressional strain 20 related to the buttressing by mechanically resistant rocks in the most structurally elevated part of the adjacent teasdale anticline fault slip data were gathered from the thousand lake fault the easternmost main fault zone of the transition zone faults that parallel the main fault show mainly dip slip but many faults in the zone strike oblique to it and show strike slip or dip slip analysis of the predominantly dip slip faults indicates a west northwest east southeast 285 least principal stress strain axis that we interpret as the neotectonic extension direction for the area

thinning of arctic sea ice observed in fram strait 1990 2011 1 time series 1990 2011 of sea ice thickness observed by moored sonars in the transpolar drift in fram strait are examined contrasting the post 2007 years against the 1990s three remarkable changes in the monthly ice thickness distributions are highlighted 1 the thickness of old level ice modal thickness is reduced by 32 2 the old ice modal peak width is reduced by 25 and 3 the fraction of ridged ice thicker than 5 m is reduced by 50 the combined effect on the mean ice thickness is a reduction from an annual average of 3 0 m during the 1990s to 2 2 m during 2008 2011 most of the thinning took place after 2005 2006 while the old ice modal thickness and peak width show signs of recovery after 2008 the decreasing trend in fraction of ridged ice and mean ice thickness persists until the end of the record in 2011 the ice observed in fram strait carries an integrated signal of arctic change due to the advection of ice from many sites in the arctic based on concurrence in timing we conclude that much of the thinning quantified here is reflecting recent change in the age composition of the arctic ice cover toward younger ice the old level ice remains thin and as such the ice cover remains preconditioned for new summers of very low sea ice extent

time variability in the annual cycle of sea ice thickness in the transpolar drift the annual cycle of modal and mean sea ice thickness was derived from upward looking sonar ice thickness observations 1990 2011 in fram strait the average annual peak to trough amplitude of the mode of 0 54 m is superimposed on interannual variability with peak to trough amplitudes of 0 73 m on time scales of 6 8 years which again is superimposed on a long term trend of 0 55 m decade over the observation period the long term trend is stronger for april than for august the average months of maximum and minimum modal thickness as a result the annual peak to trough modal thickness amplitude was reduced by 30 between the 1990s and the 2000s the average annual peak to trough amplitude of the mean ice thickness of 1 20 m is also superimposed on interannual variability with as much as 0 97 m thickness change over only 3 years these two modes of variability are superimposed on a long term trend of 0 35 m decade through the entire data set in contrast to the modal thickness the long term trend is weaker for the average month of maximum mean thickness june than for the average month of minimum september therefore the annual peak to trough amplitude of the mean ice thickness increased by 14 between the 1990s and the 2000s

dating soils and alluvium with cosmogenic 21ne depth profiles case studies from the pajarito plateau new mexico usa abstract 21 ne depth profiles measured from amalgamated samples of sand sized quartz from the quaternary bandelier tuff were used to estimate inheritance corrected exposure ages in stream terraces and an alluvial fan on the pajarito plateau stable alluvium and associated soils display a downward decreasing type i nuclide profile pattern bioturbated colluvium with uniform type iii nuclide concentrations has buried these alluvial surfaces another possible profile type with downward increasing nuclide concentrations type ii is described but was not found on the pajarito plateau we present mathematical models for each of the profile patterns relating nuclide concentration to inheritance time averaged bulk density and either exposure time or burial rate curve fits of type i profiles are used to estimate the age of alluvial landforms a burial correction technique for deposits buried by younger colluvium is also presented inheritance and burial corrected 21 ne surface exposure ages of 144 17 ka and 63 8 ka for two pajarito plateau stream terraces agree within error with independent soil profile development index dates of 160 ka and 68 78 ka a soil overlying the 50 60 ka el cajete pumice yields an inheritance corrected 21 ne exposure age of 44 11 ka that also agrees closely with a 14 c age on soil charcoal of 41 ka these results demonstrate that reliable cosmogenic 21 ne exposure ages can be obtained for fine grained alluvial surfaces with the depth profile technique

influences of eolian and pedogenic processes on the origin and evolution of desert pavements well developed desert pavements are present above eolian deposits that mantle flows of the cima volcanic field located in the mojave desert california soil stratigraphic data and geochemical data demonstrate that eolian and pedogenic processes play major roles in the evolution of these pavements eolian dust 1 accelerates mechanical fragmentation of flow rock providing the source material for pavements and 2 accumulates slowly below basaltic colluvium in flow depressions eventually promoting development of cumulate soils below the evolving stone pavement an increase in dust flux during the holocene has raised ancient pleistocene pavements as much as 20 cm above the former land surface the results of our studies demonstrate for the first time that most desert pavements do not form by deflation by overland flow or by upward migration of stones through a slowly formed clayey argillic horizon desert pavements are born and maintained at the surface

carbonate cements and grains in submarine fan sandstones the cergowa beds oligocene carpathians of poland recorded by cathodoluminescence the cathodoluminescence cl observations with cold cathode supplemented by reconnaissance scanning electron microscope analyses bring new data on petrology provenance and diagenesis of the oligocene age cergowa sandstones from the outer carpathians se poland the sandstones represent a variety of mass gravity flow sediments deposited on a submarine fan which now forms a lenticular lithosome a part of the menilite beds krosno beds suite important for the hydrocarbons industry the most common components of the cergowa sandstones observed under the cl are carbonates cement and grains that are mainly represented by lithoclasts carbonate cement is represented by five generations brown cb orange co yellow cy zoned cz and black ck pore filling cb and co calcite cements are interpreted as genetically related to eo and mesodiagenetic phases the mesodiagenetic phase is characterised by randomly distributed relatively large monocrystalline zoned rhombs of dolomite cement cz and ankerite ferroan dolomite ck the telodiagenetic phase is represented by pore filling yellow calcite cy that crystallised under the influence of suboxic meteoric waters lithoclasts represent six microfacies of carbonate rocks eroded in the source area i e microbreccia tectonised immature calcarenite wacke microsparite sparite biomicrosparite packstone and dolostone pronounced indentations of terrigenous sand grains into intraclasts of packstone biomicrosparite coupled with commonly present similar packstone type matrix suggest that a significant part of matrix resulted from compaction of soft biomicrosparite grains terrigenous grains bound by calcite cement are commonly corroded by acidic diagenetic fluids and partial or even complete replacement of silicates by calcite and clay minerals is illustrated here by feldspar grains substantial carbonate cementation has resulted in both the significant hardness and abrasion resistance of the cergowa sandstones as well giving rise to their very low porosity and permeability

sea ice thickness distribution in the greenland sea and eurasian basin may 1987 upward sonar data from a 3400 km track in the greenland sea and the eurasian basin of the arctic ocean obtained by a british submarine in may 1987 have been statistically analyzed mean ice thicknesses in the greenland sea show significant melting with downstream distance at an average rate of 0 38 m per degree of latitude equivalent to about 2140 km3 yr 1 of freshwater input between 80 n and 75 n in the region north of fram strait between 82 n and 80 n there is remarkable consistency between mean ice drafts in 1987 and in three previous cruises 1976 1979 1985 while between greenland and the north pole there was a significant thinning in 1987 relative to 1976 in fram strait the mean ice draft shows evidence of thinning within the region of influence of a semipermanent eddy ice draft distributions show that the probability density function of deformed ice follows a negative exponential distribution and that the change of shape of the distribution with downstream distance in the greenland sea is due to preferential melting of thicker ice pressure ridge draft distributions also fit a negative exponential while their spacings fit a lognormal distribution lead width distribution follows a power law with an exponent of 1 45 for narrow leads and 2 50 for wider leads if leads are defined as having a 1 m maximum draft 1 67 and 2 76 for a 0 5 m maximum draft lead spacings follow a negative exponential distribution for moderate spacings 400 1500 m but with an excess of lead pairs at small and very large spacings

sea ice of the northern canadian arctic archipelago 1 existing information concerning the pack ice and relevant climate variables of the canadian arctic archipelago north of parry channel is summarized this knowledge is enhanced by newly available data on ice thickness derived from 123 703 drill holes completed during the 1970s pack ice in this area is a mix of multiyear second year and first year ice types with the latter subordinate except in the southeast ice remains land fast for more than half the year and summertime ice concentration is high 7 9 tenths in a typical year less than 20 of the old ice and 50 of the first year ice melt there are large interannual fluctuations in ice coverage and some suggestion of a decadal cycle the average ice thickness in late winter is 3 4 m but subregional means reach 5 5 m the pack is a mix of two populations one consisting largely of multiyear ice imported from the zone of heavy ridging along the periphery of the beaufort gyre and the other consisting of a mix of relatively undeformed first year second year and multiyear ice types that grow and age within the basin the ice of the sverdrup basin is strongly influenced by a flux of heat approximately 10 w m 2 that originates in the atlantic derived waters of the arctic ocean the drift of ice through the basin is controlled in the present climate by the formation of stable ice bridges across connecting channels the drift is episodic relaxation of these controls in a warmer climate may cause deterioration in ice conditions in canadian arctic waters

beryllium 10 dating of the duration and retreat of the last pinedale glacial sequence accurate terrestrial glacial chronologies are needed for comparison with the marine record to establish the dynamics of global climate change during transitions from glacial to interglacial regimes cosmogenic beryllium 10 measurements in the wind river range indicate that the last glacial maximum marine oxygen isotope stage 2 was achieved there by 21 700 700 beryllium 10 years and lasted 5900 years ages of a sequence of recessional moraines and striated bedrock surfaces show that the initial deglaciation was rapid and that the entire glacial system retreated 33 kilometers to the cirque basin by 12 100 500 beryllium 10 years

geometrical constraints on the evolution of ridged sea ice 1 a numerical model of the evolving draft distribution of seasonal pack ice is driven by freezing and ice field compression in one dimension spatial transects of sea ice draft acquired during winter in the beaufort sea are used to evaluate the model histograms obtained by ice profiling sonar on subsea moorings reveal changes in the draft distribution while observations of ice velocity by doppler sonar allow calculation of the strain to which the draft distribution is responding numerical diffusion in thermal ice growth is controlled using a remapping algorithm mechanical redistribution algorithms in common use generate much more deep ridged ice than is observed geometric constraints on ridge keel development that reflect the finite extent of level floes available for ridge building and the true average shape of keels produce more realistic results in the seasonal pack ice of the beaufort sea 75 of all floes are too small to provide a volume of ice sufficient to construct a keel of draft equal to that commonly assumed in ice dynamics modeling on average the distribution of draft within keels has a negative exponential form implying a cusped keel shape with more area on the thinner flanks than at the crest models commonly assume a uniform redistribution of ice into a keel of triangular shape clearly the spatial organization of ice within seasonal pack or equivalently the existence of ridges and floes should be an acknowledged factor in redistribution theory for pack ice thickness

the underside topography of sea ice over the continental shelf of the beaufort sea in the winter of 1990 a spatial section of draft across 348 km of sea ice was obtained by moored subsea sonars in the beaufort sea in the winter of 1990 the ice field was a mixture of first year and multiyear types with mean draft of the order of 3 m level ice of 0 to 4 m draft occupied approximately 50 of the section within this fraction draft values of 0 and 1 5 m were most common while at 2 to 3 m draft a noticeable contribution was made by rafted first year ice deformed ice was observed to a maximum draft of nearly 27 m keels within the section were identified using both the rayleigh criterion and a new criterion based on level ice both criteria defined exponentially distributed populations but with different e folding scales the observations revealed an ice cover whose statistical properties are consistent with other data from the periphery of the polar pack in the beaufort sea but which are much more severe than those of ice found in the seasonal ice zone only a few tens of kilometres to the south the cross sectional shape of individual keels was highly variable but on average the distribution of ice within a keel was an exponential function of draft the topography of both first year and old ice subsections of the data was fractal in character with a dimension of about 1 5 for both ice types the average mass of sea ice per unit area was estimated as 2540 2970 kg m 2 with 68 73 of this mass contributed by deformed ice the large uncertainties in these estimates arise because the porosity of keels and the burden of snow or melt water upon the ice are not known

toward the modeling of enhanced basal melting in ridge keels observations from sonar data have suggested enhanced melting of thick ridged ice relative to level ice there are several mechanisms that may account for this intensified melting in this paper we examine the effects of two dimensional 2 d heat conduction and enlarged basal surface area due to the sloping sides of the keel on heat conduction and melt rates the cross section of the 2 d ridge is taken to be an isosceles triangle with a rounded crest this is roughly the shape observed and allows a convenient numerical representation in polar coordinates for comparison ridges of similar shape are also represented as a collection of 1 d columns of varying thickness similar to what is implicit in typical ice thickness distribution models the results show that 2 d ridges inhibit the heat conduction compared to 1 d ridges owing to the dominating effect of weaker temperature gradients the slope of the keel is the dominant factor in determining the temperature gradient a size distribution of 2 d ridges reduces heat transfer to the atmosphere by 3 w m 2 compared to a similar distribution of 1 d ridges over an annual cycle basal ablation along the keel is insignificant for 2 d ridges with small slopes whereas ridges with large slopes show ablation rates determined by the ice ocean heat flux these melt rates imply a transition from a triangular to a more rounded shape the 1 d ridge geometry is not adequate to simulate the net melting at the keel base over an annual cycle melt rates are calculated along the ridge keel and for level ice over a 40 day period for comparison with observations for 1 d ridges all ice thicker than 5 m melts more slowly than the corresponding level ice the inclusion of 2 d heat conduction increases the amount of ablation in the thicker ice relative to the 5 m ice especially for ridges with larger slopes however this increase explains only a small fraction of the enhanced basal melting seen in the observations these results suggest that other mechanisms are important in determining the mass loss from thicker ice

recent seismicity near capitol reef national park utah and its tectonic implications on the basis of a limited historical record the seismicity of the colorado plateau appears to be of very low level with diffusely distributed earthquakes of small to moderate magnitude m l 5 5 from december 1978 to january 1980 a spatially and temporally isolated swarm of earthquakes was observed near capitol reef national park in south central utah this sequence of at least 38 earthquakes m l 1 0 to 3 6 represents the highest level of seismicity historically observed within the interior of the colorado plateau physiographic province with the exception of coal mining induced seismicity in the book cliffs of east central utah the earthquakes occurred in a structurally complex area near the juncture of waterpocket fold and the caineville monocline just west of the henry mountains the location of the swarm suggests a possible association with the laramide basement faults that controlled the development of the fold and monocline a focal mechanism for one of the events exhibited 1 normal faulting trending northwest and 2 crustal extensional stress in a northeast southwest direction these observations are inconsistent with the expected reverse or strike slip faulting and the compressional tectonic stress field of the colorado plateau interior the observed capitol reef seismicity may therefore 1 represent an occurrence of basin and range extensional tectonism along a reactivated laramide structure and 2 refine zoback and zoback9s boundary of the basin and range colorado plateau transition zone

how to overcome imaging problems associated with carbonate minerals on sem based cathodoluminescence systems on sem based cathodoluminescence systems scanned cl methods are needed to overcome image quality problems caused by persistent luminescence of carbonate minerals an effective solution to the persistence problem is to acquire images using only the shorter wavelengths most easily done by using a broadband short wavelength uv blue range filter the filter used provides 80 to 90 transmissivity in the range of 385 to 495 nm and some transmissivity as low as 350 nm the filter allows transmission of the relatively nonpersistent uv violet luminescence present in most carbonates in the range of 350 to 425 nm but blocks the common orange red wavelength luminescence found in carbonates the lack of imaging problems in subsequent images shows that persistent luminescence in carbonates is primarily in the orange red wavelengths cathodoluminescence images produced using a short wavelength filter are comparable in detail to those obtained from conventional light microscope based cathodoluminescence systems in almost all examples features visible in the orange red wavelengths show corresponding variations in luminescence in the shorter wavelengths

four stages of pressure ridging the pressure ridging process is simulated using a two dimensional particle model blocks are broken from an intact sheet of relatively thin lead ice pushed against a thick multiyear floe at a constant speed the blocks of ice rubble accumulate to form the ridge sail and keel during the simulations the energy consumed in ridge growth including dissipation is explicitly calculated on the basis of the results of simulations performed with the model the ridging process can be divided into four distinct stages the first stage begins with an intact sheet of lead ice impacting a floe and ends when the sail reaches its maximum height in the second stage the ridge keel deepens and widens the stage ends when the maximum keel draft is reached in the third stage the direction of growth is leadward creating a rubble field of more or less uniform thickness the third stage ends when the supply of thin ice is exhausted in the fourth stage the rubble field is compressed between converging floes the results of simulations establish the dependence of ridging energetics in the first and second stages on the thickness of the ice sheet and the amount of ice pushed into the ridge the average profiles of the simulated ridges delineate the growth process in the first second and third stages the energetics and profiles of the fourth stage were described by hopkins et al 1991 lead ice extents of up to 1300 m are pushed into ridges to determine maximum sail heights keel drafts and ridging forces